Pattern of distribution of serotonergic ﬁbers to …vertes/sert-thalamus.pdfORIGINAL ARTICLE...

ORIGINAL ARTICLE

Pattern of distribution of serotonergic fibers to the thalamusof the rat

Robert P. Vertes • Stephanie B. Linley •

Walter B. Hoover

Received: 31 December 2009 / Accepted: 16 March 2010 / Published online: 13 April 2010

� Springer-Verlag 2010

Abstract It is well established that serotonergic

(5-hydroxytryptamine, 5-HT) fibers, mainly originating

from the dorsal and median raphe nuclei of the brainstem,

distribute throughout the forebrain, most heavily to ‘lim-

bic’ forebrain structures. Few reports have examined the

distribution of 5-HT fibers to the thalamus and none to our

knowledge using immunoprocedures for the detection of

the serotonin transporter (SERT)—a very sensitive marker

for 5-HT fibers. Using immunohistochemical methods for

SERT, we examined the pattern of distribution of 5-HT

fibers to the thalamus in the rat. We show that serotonergic

fibers are heavily concentrated in midline, intralaminar and

association nuclei of the thalamus, and with the exception

of the lateral geniculate complex, weakly distributed to

principal nuclei of thalamus. Specifically, we demonstrate

that 5-HT fibers are densely concentrated in the antero-

ventral, anteromedial and interanteromedial nuclei of the

anterior thalamus, the paraventricular, rhomboid and

reuniens nuclei of the midline thalamus, the central medial

and central lateral nuclei of the intralaminar thalamus, the

intermediodorsal nucleus, the lateral dorsal nucleus, and

the dorsal and ventral lateral geniculate nuclei and inter-

geniculate leaflet of the LGN complex. Less densely

innervated sites include the mediodorsal, paracentral,

parafascicular, lateral posterior and submedial nuclei of

thalamus. Remaining regions of the thalamus, largely

consisting of principal nuclei, contained few 5-HT fibers.

This pattern of 5-HT innervation indicates that serotonin/

serotonergic fibers mainly affect thalamic nuclei with

connections to ‘non-principal’ or limbic regions of the

cortex (or forebrain). This suggests that serotonergic fibers

to the thalamus may exert a significant influence on

affective and cognitive functions, possibly complementing

the actions of 5-HT fibers to other parts of the brain

involved in emotional and cognitive behaviors.

Keywords Midline thalamus � Intralaminar thalamus �Lateral geniculate complex � Limbic forebrain �Nucleus reuniens � Paraventricular nucleus of thalamus �Affective behavior

Abbreviations

5-HT 5-Hydroxytryptamine, serotonin

AD Anterodorsal nucleus of the thalamus

AM Anteromedial nucleus of thalamus

APN Anterior pretectal nucleus

AV Anteroventral nucleus of thalamus

CL Central lateral nucleus of the thalamus

CM Central medial nucleus of thalamus

COM Commissural nucleus, periaqueductal gray

cpd Cerebral peduncle

DR Dorsal raphe nucleus

ec External capsule

EC Entorhinal cortex

fr Fasciculus retroflexus

fx Fornix

HD Head direction

HF Hippocampus

IAD Interanterodorsal nucleus of thalamus

R. P. Vertes (&) � W. B. Hoover

Center for Complex Systems and Brain Sciences,

Florida Atlantic University, Boca Raton, FL 33431, USA

e-mail: [email protected]

S. B. Linley

Department of Psychology, Florida Atlantic University,

Boca Raton, FL 33431, USA

123

Brain Struct Funct (2010) 215:1–28

DOI 10.1007/s00429-010-0249-x

IAM Interanteromedial nucleus of thalamus

ic Internal capsule

IGL Intergeniculate leaflet

IL Intralaminar thalamus

IMD Intermediodorsal nucleus of thalamus

LD Lateral dorsal nucleus of thalamus

LGNd Dorsal lateral geniculate nucleus

LGNv,m,l Ventral lateral geniculate nucleus, medial and

lateral divisions

LH Lateral habenula

LHy Lateral hypothalamus

LPl,m Lateral posterior nucleus of thalamus, lateral

and medial divisions

MB Mammillary bodies

MDc,1,m Mediodorsal nucleus of thalamus, central,

lateral, and medial divisions

MGN Medial geniculate nucleus

MH Medial habenula

ml Medial lemniscus

MPT Medial pretectal nucleus

MR Median raphe nucleus

MRF Mesencephalic reticular formation

mt Mammillothalamic tract

NOT Nucleus of optic tract

NPC Nucleus of posterior commissure

OP Olivary pretectal nucleus

PAG Periaqueductal gray

pc Posterior commissure

PCN Paracentral nucleus of thalamus

PF Parafascicular nucleus of thalamus

PFC Prefrontal cortex

PH Posterior hypothalamus

PO Posterior nucleus of thalamus

PR Peri-reuniens nucleus

PT Paratenial nucleus of thalamus

PVa,p Paraventricular nucleus of thalamus, anterior

and posterior divisions

RE Nucleus reuniens of thalamus

RH Rhomboid nucleus of thalamus

RSC Retrosplenial cortex

RT Reticular nucleus of thalamus

SC Superior colliculus

SCN Suprachiasmatic nucleus

SERT Serotonin transporter

sm Stria medullaris

SMT Submedial nucleus of thalamus

SPF Subparafascicular nucleus

st Stria terminalis

VAL Ventroanterior lateral complex of thalamus

VB Ventrobasal complex of the thalamus

VM Ventral medial nucleus of thalamus

ZI Zona incerta

Introduction

It is well recognized that serotonin-containing (5-hydroxy-

tryptamine, 5-HT) fibers are widely distributed throughout

the neuroaxis. Although 5-HT fibers reach virtually all

areas of the forebrain, they are concentrated in limbic

regions of the forebrain (Steinbusch 1981; Jacobs and

Azmitia 1992; Halliday et al. 2004; Vertes and Linley

2007, 2008; Lowry et al. 2008a).

The majority of ascending 5-HT fibers originate from

the dorsal (DR) and median raphe (MR) nuclei of the

brainstem. Several reports have described patterns of dis-

tribution of DR/MR fibers to the forebrain. With respect to

the thalamus, early studies indicated rather limited DR/MR

projections to the thalamus (Azmitia and Segal 1978;

Moore et al. 1978; Vertes and Martin 1988) but subsequent

reports, using improved tracing techniques, showed rela-

tively substantial DR and MR afferents to the thalamus.

DR/MR fibers mainly target ‘non-specific’ (or limbic)

nuclei of the thalamus as well as parts of the ‘visual thal-

amus’. This would primarily include the anterior nuclei, the

mediodorsal nucleus, the midline and intralaminar nuclei,

the habenula, the laterodorsal nucleus, and the lateral

geniculate (LGN) complex. With a few exceptions, specific

(principal) nuclei of the thalamus appear to lack input from

DR/MR (Vertes 1991; Morin and Meyer-Bernstein 1999;

Vertes et al. 1999).

In addition to 5-HT cells, the DR and MR contain other

types of ‘projection’ neurons including dopaminergic,

GABAergic, glutamatergic and various peptide containing

cells (Trulson et al. 1985; Melander et al. 1986; Austin

et al. 1997; Charara and Parent 1998; Day et al. 2004;

Waselus and Van Bockstaele 2007; Lowry et al. 2008a).

Accordingly, the extent to which DR/MR projections to the

thalamus originate specifically from 5-HT DR/MR cells

remains to be determined. Although no report has exam-

ined the overall distribution of serotonergic DR/MR fibers

to the thalamus, a few studies have described 5-HT DR/MR

projections to some nuclei of the thalamus. Specifically,

studies combining retrograde tracing with 5-HT immuno-

staining have demonstrated 5-HT DR or MR projections to

the anterodorsal and anteroventral nuclei of the anterior

thalamus (Gonzalo-Ruiz et al. 1995), to the lateral genic-

ulate nucleus and intergeniculate leaflet (Villar et al. 1988;

Meyer-Bernstein and Morin 1996; Harrington 1997; Morin

and Blanchard 1999) and to the paraventricular nucleus

(Otake and Ruggiero 1995; Hsu and Price 2009).

The pattern of distribution of serotonergic dorsal and

median raphe fibers to most nuclei of the thalamus remains

largely unknown. Despite this, the finding of two early

immunohistochemical analyses, one in rats (Cropper et al.

1984) and the other in monkeys (Lavoie and Parent 1991),

2 Brain Struct Funct (2010) 215:1–28

123

showed that 5-HT fibers spread widely throughout the

thalamus. These early studies, however, used immuno-

staining procedures for the detection of serotonin in cells/

fibers (Steinbusch 1981), and while still a widely utilized

technique, newer immunostaining procedures have been

developed for the identification of the serotonin transporter

protein (SERT) in 5-HT fibers (Sur et al. 1996; Zhou et al.

1996). Although both methods are useful and each has its

unique advantages, Aznar and colleagues (Nielsen et al.

2006) recently reported that SERT was preferable to 5-HT

as a marker for serotonergic fibers.

In particular, they showed that serotonin only began to

approach the quality of SERT for identifying 5-HT fibers

when 5-HT immunohistochemistry was combined with

pre-treatments, particularly the use of monoamine oxidase

inhibitors (MAOIs). For instance, they reported: (1) an

approximately 200% increase in the detection of 5-HT

immunolabeled fibers in rats pre-treated with MAOIs and

(2) *90% correspondence in the co-expression of 5-HT?

and SERT? fibers in MAOI-treated rats compared to only

a 30% correspondence in non-treated rats. The latter dif-

ference (30% correspondence) was attributed to the loss (or

inability to detect) of immunostained 5-HT fibers in

untreated animals.

Consistent with Nielsen et al. (2006), we found that

5-HT and SERT immunoreactive procedures labeled a

comparable set of 5-HT fibers in the thalamus, and that

SERT produced a stronger signal than did 5-HT. Accord-

ingly, we used SERT immunohistochemical techniques to

characterize the pattern of distribution of serotonergic

fibers to the thalamus of the rat.

In brief, we show that serotonergic fibers are densely

concentrated in midline nuclei, rostral intralaminar nuclei,

most of the anterior nuclei, the laterodorsal nucleus, and

the LGN complex. Of the midline group, 5-HT labeling

was most pronounced in the paraventricular, rhomboid and

reuniens nuclei. Although no region of the thalamus was

devoid of 5-HT fibers, labeling was very light in sensory

and motor nuclei of the thalamus and in the medial

geniculate nucleus.

Materials and methods

Ten (5 male, 5 female) naıve Sprague-Dawley rats (Harlan,

Indianapolis, IN) weighing 275–300 g were housed in pairs

on a 12:12 light dark cycle for 7 days during which food

and water were available ad libitum. Rats were then deeply

anesthetized with an intraperitoneal injection of sodium

pentobarbital (Nembutal, 75 mg/kg) and perfused tran-

scardially with 30 ml of cold heparinized 0.1 M phosphate

buffer saline (PBS), followed by 200–300 ml chilled 4%

paraformaldehyde in 0.1 M phosphate buffer (PB) at pH of

7.4. The brains were removed and postfixed overnight in

4% paraformaldehyde in 0.1 M PB. Brains were then

placed in a 30% sucrose solution for another 48 h. Fol-

lowing this, 50 lm coronal sections were taken on a

freezing microtome in a one in three series: one series of

sections through the thalamus were prepared for SERT

immunohistochemistry and a second series was stained

with cresyl violet. The experiments were approved by the

Florida Atlantic University Institutional Animal Care and

Use Committee and conform to all federal regulations and

National Institutes of Health guidelines for the care and use

of laboratory animals.

Immunohistochemistry

SERT

Sections were initially treated with a 30-min sodium

borohydride incubation (1% in 0.1 M PB) to remove

excess aldehydes. Following a copious PB wash, sections

were incubated for 1 h in 0.5% BSA in 0.1 M Tris buffered

saline (TBS) (pH 7.6). Sections were rinsed in 0.1 M PB

and then incubated in the primary antibody, rabbit anti-

SERT (Immunostar, Hudson, WI) at a concentration of

1:10,000 in a diluent of 0.1% bovine serum albumin (BSA)

in TBS containing 0.25% Triton X-100 for 48 h. Following

a 0.1 M PB wash, sections were placed in a secondary

antibody solution of biotinylated goat anti-rabbit immu-

noglobulin (Vector Labs) at a 1:500 concentration in dil-

uent for 2 h. This was followed by another PB wash.

Sections were then incubated for 2 h in a tertiary antibody

solution of biotinylated horse anti-goat immunoglobulin

(Vector Labs) at a 1:500 concentration in diluent. After

washing the tissue in 0.1 M PB, sections were incubated

for 60 min in a 1:200 dilution of a peroxidase–avidin

complex using the Vector Elite kit. Following a final 0.1 M

PB wash, SERT immunoreactive fibers were visualized by

placing them for approximately 3–4 min in a solution

containing 0.022% DAB (Aldrich, Milwaukee, WI) and

0.003% hydrogen peroxide in TBS. Sections were mounted

onto chrome-alum gelatin-coated slides, dehydrated using

graded methanols and coverslipped with permount.

Photomicroscopy

For depiction of SERT and 5-HT fibers, lightfield photo-

micrographs at 100 times magnification were captured

throughout the extent of the thalamus from representative

cases using a Nikon DXM1200 camera mounted onto a

Nikon Eclipse E600 microscope. The individual captures

were then compiled using Image Pro-Plus 4.5 (Media

Cybernetics, Silver Springs, MD) and imported into Adobe

Photoshop (CS 2.0; Mountain View, CA) where they were

Brain Struct Funct (2010) 215:1–28 3

123

adjusted for brightness and contrast. Representative sec-

tions throughout the thalamus were captured and illus-

trated. Files were imported into Adobe Illustrator (CS 2.0)

where borders were drawn around thalamic nuclei by

comparing immunostained sections to an adjacent series

of Nissl-stained sections and with the aid of the rat atlas

of Swanson (2003). Particularly noteworthy patterns of

labeling were illustrated with high magnification photo-

micrographs. Patterns of SERT labeling are described as

light, moderate and dense, with ‘light’ referring to a few

labeled fibers widely dispersed throughout a nucleus,

‘dense’ as a heavy concentration of labeled fibers generally

occupying a significant portion (or most) of a nucleus, and

‘moderate’ lying between these two patterns.

Results

The pattern of distribution of SERT immunoreactive fibers

in the thalamus of the rat is depicted with a series of 14

rostral to caudal transverse sections through the thalamus

(plates 1–14). We first describe patterns of labeling at each

rostral to caudal section and then compare patterns across

anatomical/functional groups of the thalamus.

A note on categories of thalamic nuclei

An early categorization of thalamic nuclei essentially

divided the thalamus into ‘relay’ nuclei and ‘non-specific’

nuclei (Dempsey and Morison 1942, 1943; Morison and

Dempsey 1942). The relay nuclei generally referred to

nuclei that transfer modality-specific information to dis-

crete regions and layers of the cortex, while ‘non-specific’

nuclei referred to nuclei that received multimodal infor-

mation and distribute to several regions of the cortex as

well as to subcortical sites. This categorization has been

revised in large part on the basis that ‘‘non-specific’’ nuclei

target very specific regions of the cortex and exhibit unique

functions (Bentivoglio et al. 1991; Groenewegen and

Berendse 1994).

Based on recent formulations (Price 1995; Groenewegen

and Witter 2004), thalamic nuclei will be categorized as

follows: (1) principal nuclei, consisting of the ventrobasal

complex (ventral posteromedial and ventral posterolateral

nuclei), the ventroanterior lateral complex, the ventrome-

dial nucleus, the posterior nucleus, the dorsal and ventral

lateral geniculate nuclei, the intergeniculate leaflet, and the

medial geniculate nucleus; (2) association nuclei, consist-

ing of the mediodorsal and intermediodorsal nuclei, the

submedial nucleus, the anterior nuclei (anterodorsal, ante-

roventral, anteromedial and interanteromedial), the lateral

dorsal nucleus and the lateral posterior nucleus; (3) midline

and intralaminar nuclei, consisting of nucleus reuniens,

rhomboid nucleus, paraventricular nucleus, paratenial

nucleus, the central medial nucleus, paracentral nucleus,

central lateral nucleus and the parafascicular nucleus; (4)

the reticular nucleus; and (5) the epithalamus, consisting of

the medial and lateral habenula.

Plate 1 (Fig. 1a)

At the very anterior thalamus, SERT? fibers spread widely

throughout the thalamus. Prominent numbers of labeled

fibers were present in the anterior paraventricular nucleus

(PVa) and nucleus reuniens (RE) of the midline thalamus

and in the anteroventral nucleus (AV) of the anterior

thalamus whereas fewer, but still significant numbers, were

visible in the anteromedial (AM), interanterodorsal (IAD),

paratenial (PT) and reticular nuclei (RT) of thalamus. The

anterodorsal nucleus of the anterior thalamus was lightly

labeled.

Plate 2 (Fig. 1b)

As seen rostrally (Fig. 1a), PVa and RE were densely and

uniformly labeled. Within the anterior nuclei of thalamus,

AV was heavily labeled, AM, IAD and the interantero-

medial nucleus (IAM) moderately labeled, and AD lightly

labeled (Fig. 2). PT and RT contained modest numbers of

SERT? fibers.

Plate 3 (Fig. 3a)

Labeled fibers continued to be heavily distributed

throughout AV, PVa, and RE—and nearly as densely within

the rostral pole of the lateral dorsal nucleus (LD). Figure 4

depicts the pattern of midline labeling, showing dense

aggregates of SERT? fibers in PVa, central medial nucleus

(CM), IAM and RE. The dorsal subnucleus of RE (arrow)

(Risold et al. 1997) contained fewer fibers than other parts

of RE (Figs. 3a, 4). Moderate numbers of SERT? fibers

were present in AM, IAD, rostral aspects of the mediodorsal

(MD) and the ventral anterior lateral complex (VAL) of

thalamus. AD, PT and RT were lightly labeled.

Plate 4 (Fig. 3b)

Significant numbers of SERT? axons were visible in AV,

PVa (strongest medially), RE and the rostral pole of LD.

Other heavily labeled sites were CM and the rhomboid

nucleus (RH) and to a lesser extent IAM. Whereas rostrally

there was a clear difference in density of labeling between

AV and the medially bordering AM and IAD, the gradients

between them were less pronounced at this level, owing to

stronger labeling of AM and IAD. Labeled fibers were

moderately packed within ventral parts of the ventral


123

thalamus in the ventral medial nucleus (VM), but consid-

erably less so in dorsal aspects of the ventral thalamus in

VAL. AD, PT and RT contained few SERT? fibers. There

was a marked decrease in RT labeling from that of rostral

levels.

Plate 5 (Fig. 5a)

A distinctive pattern of labeling was observed at this level.

SERT? fibers were heavily distributed along the midline

within PVa, IMD, CM, IAM, rhomboid nucleus (RH) and

RE, and two bilateral bands of labeled fibers stretched

dorsolaterally from the midline to the dorsal surface of the

brain appearing as a ‘Y’-shaped pattern of labeling. The

arms of the ‘‘Y’’ contained prominent numbers of SERT?

fibers in AM, AV and LD. On the whole, labeling was less

pronounced in MD than in the anterior or midline groups.

There was a progressive decrease in labeling from the

dorsolateral to ventromedial MD, resulting in stronger

labeling in the lateral (MDl) and central (MDc) subnuclei

of MD than in the medial MD (MDm). In a similar manner,

there is a gradual reduction in density of SERT? fibers in

the ventral thalamus, extending outward from dorsal

aspects of VAL (bordering LD/AV) to the lateral surface of

the thalamus. Accordingly, inner regions of VAL (and VM)

were moderately labeled, whereas outer parts of VAL and

the ventrobasal complex (VB) were lightly labeled. As

rostrally, RT contains few SERT? axons.

Plate 6 (Fig. 5b)

With the exception of IAM which was moderately

labeled, SERT? fibers were densely concentrated dorso-

ventrally along the midline in PVa, CM, RH and RE.

Labeling was about as pronounced in the paracentral (PC)

and central lateral (CL) nuclei of the intralaminar com-

plex and in LD. This pattern of labeling is depicted in

Fig. 6 showing dense aggregates of labeled fibers in CL,

Fig. 1 Photomicrographs of

rostrocaudally aligned

transverse sections through the

diencephalon showing the

pattern of distribution of SERT

immunopositive fibers at two

levels of the anterior thalamus,

plate 1 (a) and plate 2 (b). As

depicted, the anterodorsal (AD)

and paratenial (PT) nuclei were

lightly labeled, the anteromedial

(AM) nucleus was moderately

labeled, and the anteroventral

(AV), paraventricular (PV) and

reuniens (RE) nuclei were

heavily labeled. This illustrates

the general range of labeling at

these and more caudal levels

(plates 3–14) of the thalamus.

See list for abbreviations.

Scale bar a, b 400 lm


123

PC and LD—considerably more than present dorsomedi-

ally in MD or ventrolaterally in VAL. As observed ros-

trally, there was a gradual dorsal to ventral decline in

labeling from deep (inner) to superficial aspects of the

ventral thalamus (VAL to VB). VM and the medially

adjacent submedial nucleus (SMT) were moderately and

uniformly labeled. RT contained few SERT immunore-

active fibers.

Plate 7 (Fig. 7)

Three major clusters of SERT? fibers were visible at this

level: (1) a midline group, with labeling heaviest in PVa,

RH and RE and less pronounced in the intermediodorsal

nucleus (IMD); (2) an intralaminar group, with labeling

strongest in CM followed by CL and PC; and (3) the lateral

dorsal nucleus. Moderate numbers of SERT immunoreac-

tive fibers were also present in the medial (MH) and lateral

habenula (LH) and in SMT, but relatively few were seen

throughout the vast expanse of ventral thalamus (VM,

VAL, VB) and in RT. MD was lightly to moderately

labeled which contrasted with much stronger labeling of

adjacent regions, medially (PVa, IMD, CM) and laterally

(CL, PC). As depicted in Fig. 8, there were subtle differ-

ences in density of labeling across subdivisions of MD such

that the central nucleus (MDc) was more heavily labeled

than the medial (MDm) or lateral MD (MDl).

Plate 8 (Fig. 9)

SERT? fibers heavily invaded the midline and LD, but

were sparsely distributed over a large expanse of the ven-

tral thalamus in VB, VAL and RT. Within midline nuclei,

labeling was densest in PVp, RH and RE, but was also

pronounced in CM. There was a gradual decline in labeling

medially to laterally in the ventral thalamus, from VM to

VAL to VB. SMT contained moderate numbers of SERT?

fibers, intermediate between those of the laterally adjacent

VM and medially bordering RH/RE. MD was moderately

labeled, with slightly more labeled fibers in MDl and MDc

than in MDm.

Plate 9 (Fig. 10)

There was a general reduction in labeling at caudal levels

of the thalamus due to the fact that a greater part of the

Fig. 2 High magnification

photomicrograph of a transverse

section through the rostral

diencephalon (from left side of

plate 2, Fig. 1b) showing the

fine detail of SERT

immunopositive fibers in the

anterior thalamus. Note dense

fiber labeling in the

anteroventral (AV)

paraventricular (PV) and

reuniens (RE) nuclei of the

thalamus. See list for

abbreviations. Scale bar150 lm


123

caudal thalamus is occupied by principal as opposed to

non-principal nuclei. Although labeling was still prominent

along the midline in PVp, IMD, CM, RH and RE, it was

less robust than seen rostrally within these nuclei and fairly

confined to a narrow band along the midline. Figure 11

shows a dense collection of labeled fibers within RE and

the dorsally adjacent RH at this level. Comparable to

midline labeling, LD was densely labeled and CL, PC and

MD (or lateral parts of MD) were moderately labeled.

Relatively few SERT? fibers were present throughout the

vast expanse of thalamus, mainly comprised of the ventral/

ventrolateral thalamus, i.e., within the posterior nucleus

(PO), VAL, VB and VM. The habenula was lightly labeled

(MH [ LH).

Plate 10 (Fig. 12)

Whereas rostrally in the thalamus labeling was described as

forming a ‘Y’-shaped configuration (plate 5, Fig. 5a), it

largely resembled a ‘T’-shaped pattern at this level. In

effect, this consisted of a vertical column of midline

labeling in MH, PVp, IMD, CM, RE, and lateral extensions

from the midline across the dorsal surface of the brain

within CL, the lateral posterior nucleus (LP), LD and the

Fig. 3 Photomicrographs of rostrocaudally aligned transverse sections through the diencephalon showing the pattern of distribution of SERT

immunopositive fibers at two levels of the anterior thalamus, plate 3 (a) and plate 4 (b). See list for abbreviations. Scale bar a, b 400 lm


123

dorsal lateral geniculate nucleus (LGNd). Of these sites,

labeling was densest in PVp, RE, LD and LGNd. Addi-

tionally, moderate numbers of SERT? fibers were present

in MD, PC, VM and SMT, but few were visible in most

remaining regions of the thalamus including PO, VAL, VB

and RT.

Plate 11 (Fig. 13)

As discussed, associated with the progressive switch from

midline/intralaminar and association nuclei to principal

nuclei (anterior to posterior thalamus), there was a corre-

sponding gradual reduction in numbers of SERT? fibers.

This is further exemplified at this level. Whereas the LGN

complex (LGNd, LGNv), PVp and RE were densely

labeled, relatively few SERT? fibers were present

throughout most of the thalamus in MH, IMD, CL, LP,

MD, CM (moderately labeled) as well in PO, VB, VM and

RT (lightly labeled). PO labeling was slightly stronger at

this level than rostrally—a trend that continued caudally in

the thalamus.

Plate 12 (Fig. 14)

Although labeling was still present along the midline

within PVp, IMD and CM, it was less pronounced at this

level than rostrally. The parafascicular nucleus (PF) lateral

to IMD was relatively densely labeled which set it off from

surrounding nuclei which were less heavily labeled. With

the exception of the LGN complex (LGNd and LGNv)

which was strongly labeled, SERT? fibers spread moder-

ately (and homogeneously) throughout the dorsal thalamus

to MH, LH, caudal CL, and LP. By contrast with the dorsal

thalamus, the ventral thalamus (mainly composed of PO

and VB) contained few SERT? fibers—more in PO than in

VB.

Plate 13 (Fig. 15)

At the caudal pole of the thalamus (or juncture between the

diencephalon and midbrain), thalamic nuclei are mainly

located laterally/dorsolaterally, lateral to the emerging

pretectal area. Within the dorsolateral thalamus, LGNd,



section through the rostral

diencephalon (from plate 3,

Fig. 3a) showing the fine detail

of SERT immunopositive fibers

in midline and adjacent nuclei at

the level of the anterior

thalamus. Note dense fiber

labeling in the paraventricular

(PV) and reuniens (RE) nuclei

of the midline thalamus and the

anteromedial (AM) and

interanteromedial nuclei of the

anterior thalamus. See list for



123

LGNv and the intergeniculate leaflet (IGL) were densely

labeled, PO, LP and PF were moderately labeled and VB

was lightly labeled. Figure 16 shows a dense collection of

SERT? fibers throughout the LGN complex and particu-

larly strong within IGL and the lateral division of the

ventral LGN.

Plate 14 (Fig. 17)

The caudal extent of the thalamus consists of visual and

auditory structures. Of these, labeling was pronounced in

the LGNd, LGNv and IGL, moderate in LP and light in the

medial geniculate nucleus (MGN).

Fig. 5 Photomicrographs of rostrocaudally aligned transverse sections through the diencephalon showing the pattern of distribution of SERT

immunopositive fibers at two levels of the mid thalamus, plate 5 (a) and plate 6 (b). See list for abbreviations. Scale bar a, b 400 lm


123

Functional/anatomical groups of the thalamus

Anterior nuclei

The anterior nuclei consist of the anterodorsal (AD),

anteroventral (AV), anteromedial (AM), interanterodorsal

(IAD) and interanteromedial (IAM) nuclei. On the whole,

the anterior group contained dense concentrations of 5-HT

fibers. AV was the most heavily labeled nucleus of the

anterior group and one of the most densely labeled sites of

the thalamus. Labeling was almost as prominent in AM,

IAD and IAM. By contrast, AD was lightly labeled. For the

most part, 5-HT fibers were homogeneously distributed

throughout the anterior thalamic nuclei.

Midline nuclei

The midline nuclei consist of the paraventricular (PV),

paratenial (PT), rhomboid (RH) and reuniens (RE) nuclei.

Other nuclei lying along the midline of the thalamus which

are not considered part of the midline nuclei, per se,

include the intermediodorsal (IMD), interanteromedial

(IAM), and central medial (CM) nuclei. Whereas the entire

dorsoventral extent of the midline thalamus (including

IMD, IAM and CM) was densely labeled, labeling was

heaviest within PV, RH and RE. In marked contrast to

other midline groups, PT was lightly labeled. Although

5-HT fibers generally distributed homogeneously through-

out the midline nuclei, there was some variation. Specifi-

cally, labeling was heavier in rostral than caudal regions of

PT, PV and RE, and somewhat denser in medial than

lateral aspects of PV and RE, particularly within the central

core compared to the lateral wings of RE.

Mediodorsal (MD) and intermediodorsal (IMD) nuclei

IMD lies on the midline and as a ‘midline-localized’ group

contained a dense concentration of 5-HT fibers—consid-

erably more than the laterally adjacent MD. On the whole,

MD contained moderate numbers of 5-HT fibers; fewer

Fig. 6 High magnification photomicrograph of a transverse section

through the diencephalon (from the right side of plate 5, Fig. 5a)

showing the fine detail of SERT immunopositive fibers in the

intralaminar and laterodorsal nuclei of the thalamus. Note the

relatively dense fiber labeling in the paracentral (PC), central lateral

(CL) and laterodorsal (LD) nuclei and comparatively lighter labeling

in the mediodorsal (MD) and ventral anterior lateral (VAL) nuclei of

thalamus. See list for abbreviations. Scale bar 300 lm


123

Fig

.7

Ph

oto

mic

rog

rap

ho

fa

tran

sver

sese

ctio

nth

rou

gh

the

die

nce

ph

alo

nsh

ow

ing

the

pat

tern

of

dis

trib

uti

on

of

SE

RT

imm

un

op

osi

tiv

efi

ber

sat

the

mid

thal

amu

s,p

late

7.

See

list

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

than in bordering nuclei, medially (PV, IMD), laterally

(CL, PC), and perhaps dorsally (LH). There was a distinct

dorsolateral to ventromedial gradient in density of labeling

within the caudal two-thirds of MD resulting in denser

labeling in MDl and MDc than in MDm.

Intralaminar nuclei

The intralaminar (IL) nuclei consist of the central lateral (CL),

paracentral (PC) and central medial (CM) nuclei, rostrally,

and the parafascicular nucleus (PF), caudally. On the whole,

5-HT fibers were quite densely distributed throughout the

rostral intralaminar (IL) complex. As discussed with respect

to IMD, and perhaps owing to its position on the midline, CM

was the most heavily labeled nucleus of the rostral IL,

followed in order by CL and PC. Labeled fibers were

homogenously distributed rostrocaudally throughout CL, but

were more densely concentrated in lateral (adjacent to LD)

than in medial aspects of CL. In an analogous but reverse

manner, labeling was slightly denser medially in PC (on the

border with CM) than laterally in PC. PF was moderately to

heavily labeled—equivalent in density to that to CL.

Reticular nucleus

Somewhat surprisingly, with the exception of the rostral

pole of RT which was moderately labeled, RT contained

relatively few 5-HT fibers. In fact, labeling throughout

approximately the caudal two-thirds of RT was roughly

equivalent to that of the dorsally adjacent ventrobasal

complex which was among the most sparsely labeled sites

of the thalamus.


through the diencephalon (from the right side of plate 7, Fig. 7)

showing the fine detail of SERT immunopositive fibers in mediodor-

sal nucleus (MD) and surrounding regions of the thalamus. Note the

slightly denser labeling in the central (MDc) than the medial (MDm)

or lateral (MDl) divisions of MD. See list for abbreviations. Scale bar300 lm


123

Fig

.9

Ph

oto

mic

rog

rap

ho

fa

tran

sver

sese

ctio

nth

rou

gh

the

die

nce

ph

alo

nsh

ow

ing

the

pat

tern

of

dis

trib

uti

on

of

SE

RT

imm

un

op

osi

tiv

efi

ber

sat

the

mid

thal

amu

s,p

late

8.

See

list

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

Fig

.1

0P

ho

tom

icro

gra

ph

of

atr

ansv

erse

sect

ion

thro

ug

hth

ed

ien

cep

hal

on

sho

win

gth

ep

atte

rno

fd

istr

ibu

tio

no

fS

ER

Tim

mu

no

po

siti

ve

fib

ers

atth

ele

vel

of

the

mid

thal

amu

s,p

late

9.

See

list

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

Medial and lateral habenula (epithalamus)

The medial habenula was moderately and homogeneously

labeled. A rather narrow dorsomedial region of the lateral

habenula (LH), bordering MD, was moderately labeled

while remaining aspects of LH were lightly labeled.

Lateral dorsal and lateral posterior nuclei

LD was one of the most heavily labeled nuclei of the

thalamus. The entire rostrocaudal extent of LD was densely

and quite homogeneously labeled. There was, however, a

slight dorsal to ventral gradient in intensity of LD labeling,

favoring the dorsal two-thirds of LD. Considerably, fewer

5-HT fibers were present in LP than in LD; LP was

nonetheless moderately labeled.

Submedial nucleus (or nucleus gelatinosus)

On the whole, SMT contained moderate numbers of 5-HT

fibers. There was, however, a distinct rostral–caudal gra-

dient in SMT labeling: the rostral two-thirds of SMT was

moderately labeled and the caudal one-third was lightly

labeled. SMT labeling was considerably less pronounced

than that of medially adjacent midline nuclei.

Ventral nuclei (VAL, VB, VM) and the posterior nucleus

(PO)

VAL is composed of the ventral anterior and ventral

lateral nuclei and VB consists of the ventral postero-

medial (VPM) and ventral posterolateral (VPL) nuclei.

VB, VAL, PO, and VM are major principal nuclei of the


through the diencephalon (from plate 9, Fig. 10) showing the fine

detail of SERT immunopositive fibers in the reuniens (RE) and

rhomboid (RH) nuclei of the midline thalamus. Note the dense fiber

labeling in the RE and RH compared to lighter labeling in the laterally

adjacent submedial nucleus (SMT) of the thalamus. See list for

abbreviations. Scale bar 300 lm


123

Fig

.1

2P

ho

tom

icro

gra

ph

of

atr

ansv

erse

sect

ion

thro

ug

hth

ed

ien

cep

hal

on

sho

win

gth

ep

atte

rno

fd

istr

ibu

tio

no

fS

ER

Tim

mu

no

po

siti

ve

fib

ers

atth

eca

ud

alth

alam

us,

pla

te1

0.

See

list

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

Fig

.1

3P

ho

tom

icro

gra

ph

of

atr

ansv

erse

sect

ion

thro

ug

hth

ed

ien

cep

hal

on

sho

win

gth

ep

atte

rno

fd

istr

ibu

tio

no

fS

ER

Tim

mu

no

po

siti

ve

fib

ers

atth

eca

ud

alth

alam

us,

pla

te1

1.

See

list

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

Fig

.1

4P

ho

tom

icro

gra

ph

of

atr

ansv

erse

sect

ion

thro

ug

hth

ed

ien

cep

hal

on

sho

win

gth

ep

atte

rno

fd

istr

ibu

tio

no

fS

ER

Tim

mu

no

po

siti

ve

fib

ers

atth

eca

ud

alth

alam

us,

pla

te1

2.S

eeli

st

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

Fig

.1

5P

ho

tom

icro

gra

ph

of

atr

ansv

erse

sect

ion

thro

ug

hth

ed

ien

cep

hal

on

sho

win

gth

ep

atte

rno

fd

istr

ibu

tio

no

fS

ER

Tim

mu

no

po

siti

ve

fib

ers

atth

eca

ud

alth

alam

us,

pla

te1

3.

See

list

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

thalamus. With the possible exception of parts of VM

which were moderately labeled, the ventral thalamus and

PO were lightly labeled. 5-HT fibers were heteroge-

neously distributed throughout VM, with a marked var-

iation in density found rostrocaudally in VM. The rostral

half of VM was moderately labeled; the caudal half

was sparsely labeled. Additionally, labeling was heavier

medially than laterally in VM. Overall, VAL was lightly

labeled, with a stronger concentration of 5-HT fibers

rostrally than caudally as well as dorsally than ventrally

in VAL. VB was one of the most sparsely innervated

sites of the thalamus, and unlike other regions of the

ventral thalamus, showed little diversity (heterogeneity)

in strength of labeling across the expanse of the nucleus.

PO labeling was also generally light but more pro-

nounced than that of VB.

Medial and lateral geniculate nuclei

The LGN complex was among the most densely labeled

sites of the thalamus. Significant numbers of 5-HT fibers

were present in all nuclei of the LGN complex: the dorsal

and ventral LGN and the intergeniculate leaflet. Labeling

was, however, denser in IGL and the lateral division of

LGNv than in LGNd. By contrast with the LGN complex,

MGN was lightly and homogeneously labeled.

Discussion

The present report describes the pattern of distribution of

serotonergic fibers to the thalamus in the rat using antisera

for SERT. As demonstrated, there is significant variation in



section through the caudal

diencephalon (from just rostral

to the left side of plate 13,

Fig. 15) showing the fine detail

of SERT immunopositive fibers

in the lateral geniculate nucleus

(LGN) complex. Note the strong

fiber labeling in all three

divisions of the LGN complex,

the dorsal (LGNd) and ventral

(LGNv) nuclei and the

intergeniculate leaflet (IGL) and

particularly dense labeling in

the lateral part of LGNv

(LGNvl) and IGL. See list for



123

Fig

.1

7P

ho

tom

icro

gra

ph

of

atr

ansv

erse

sect

ion

thro

ug

hth

ed

ien

cep

hal

on

sho

win

gth

ep

atte

rno

fd

istr

ibu

tio

no

fS

ER

Tim

mu

no

po

siti

ve

fib

ers

atth

eca

ud

alth

alam

us,

pla

te1

4.

See

list

for

abb

rev

iati

on

s.S

cale

ba

r4

00

lm


123

the density of 5-HT-labeled fibers across nuclei of the

thalamus. On the whole, the thalamus could be partitioned

into regions of high and low density 5-HT innervation

corresponding on the one hand to midline/intralaminar and

association nuclei, and on the other, to principal nuclei of

the thalamus.

Serotonergic fibers distribute: (1) densely to the ante-

roventral, anteromedial and interanteromedial nuclei of

the anterior thalamus, the paraventricular, rhomboid and

reuniens nuclei of the midline thalamus, the central medial

and central lateral nuclei of the intralaminar thalamus, the

intermediodorsal nucleus, the lateral dorsal nucleus, and

the dorsal and ventral lateral geniculate nuclei and inter-

geniculate leaflet of the LGN complex; (2) moderately to

the paratenial, mediodorsal, paracentral, parafascicular,

lateral posterior and submedial nuclei; and (3) lightly to

remaining regions of the thalamus, largely consisting of

principal nuclei of the thalamus. The latter sites include

the posterior nucleus, the ventral anterior lateral complex,

the ventral medial nucleus, the ventral basal complex, the

reticular nucleus, and the medial geniculate nucleus. This

pattern of 5-HT innervation suggests that serotonin/sero-

tonergic fibers primarily affect anterior, midline and

intralaminar nuclei of thalamus—and consequently their

limbic forebrain targets.

Comparisons with previous examinations of 5-HT

innervation of the thalamus

Two previous reports, one in the rat (Cropper et al. 1984)

and the other in the monkey (Lavoie and Parent 1991),

described patterns of distribution of 5-HT fibers to the

thalamus. As discussed, these studies used immunostaining

procedures for serotonin which are reportedly less sensitive

than those for SERT for the identification of 5-HT fibers

(Nielsen et al. 2006).

Cropper et al. (1984) described a similar pattern of 5-HT

innervation of the thalamus as demonstrated here—with

some notable exceptions. Consistent with present findings,

they described relatively dense concentrations of 5-HT

fibers in the anteroventral nucleus, the midline thalamus

(PV, RH and RE), the lateral dorsal nucleus and the ventral

part of LGN, but few in somatomotor (VAL and VB) and

auditory (MGN) regions of the thalamus. Unlike our

results, however, they indicated that labeling was light in

several nuclei presently shown to be heavily labeled

including MD, AM, CM, SMT (nucleus gelatinosus) and

LGNd. For instance, they reported that MD and AM con-

tained few immunoreactive fibers which contrast with our

demonstration of pronounced labeling in these cell groups.

Finally, several sites were omitted from their description of

labeling such as IAM, IMD, the intergeniculate leaflet, and

the medial habenula—all of which were moderately to

densely labeled in the present study. These differences

could, in part, involve the superior sensitivity of immu-

noprocedures for SERT compared to those for serotonin

in the detection of 5-HT fibers (see ‘‘Introduction’’ and

Nielsen et al. 2006).

The pattern of distribution of 5-HT fibers to the thala-

mus in the monkey (Lavoie and Parent 1991) was in some

respects very similar, but in others, quite different than

presently shown for the rat. Specifically, in accord with us,

Lavoie and Parent (1991) demonstrated that 5-HT fibers in

monkeys strongly targeted the midline and intralaminar

nuclei of thalamus. They stated: ‘‘The densest 5-HT

innervation of the thalamus was observed in nuclei located

directly on the midline’’ (Lavoie and Parent 1991). By

comparison with the rat, midline nuclei of the squirrel

monkey consist of three main groups: the paraventricular,

central and reuniens nuclei. The central nucleus is located

between PV and RE, and depending on rostrocaudal levels,

this region would be comparable to IAM, IMD, or the

rhomboid nucleus in the rat. The central medial nucleus

(CM) in monkeys lies ventral to the central nucleus. All

cell groups lying along the midline (PV, central, CM and

RE) contained dense concentrations of 5-HT fibers in the

monkey.

Like here, Lavoie and Parent (1991) drew particular

attention to marked differences in overall density of

labeling between ‘non-specific’ and ‘relay’ nuclei of thal-

amus, noting that the ‘‘non-specific nuclei received the

heaviest innervation’’ and ‘‘by comparison the more spe-

cific relay nuclei and associated nuclei were less densely

labeled’’.

By contrast with present findings, however, Lavoie and

Parent (1991) reported that the reticular nucleus was one of

the most densely labeled sites of the thalamus (with

labeling comparable to that of midline and intralaminar

nuclei) and that AV, LD, LGNd, and the habenular com-

plex were weakly labeled. The medial and lateral habenula

were described as the most poorly innervated sites of the

thalamus. This contrasts with our demonstration that RT

(or the caudal RT) was lightly labeled and AV, LD, MH

and LGNd were heavily labeled. These discrepancies likely

involve species differences.

In a report primarily devoted to an examination of

ascending MR and DR projections, Morin and Meyer-

Bernstein (1999) also described patterns 5-HT innervation

of the forebrain (including the thalamus) in hamsters.

Consistent with present results, Morin and Meyer-Bern-

stein (1999) described a sharp decreasing gradient of

labeling in the transition from non-principal to principal

nuclei of the thalamus—or in their terms, a dense seroto-

nergic ring of labeling surrounding ‘‘a sparsely innervated

core consisting of the posterior and ventral nuclear com-

plexes and the reticular nucleus’’. More specifically, Morin


123

and Meyer-Bernstein (1999) reported that labeling was

prominent along the midline within PV, IMD, CM, RH,

and RE, in MH, in parts of the intralaminar thalamus (CM

and PF), and in the LGN complex. By contrast with them,

we observed considerably denser labeling in the anterior

nuclei, LD, LP and RH, but lighter labeling in PT.

The foregoing indicates, then, that 5-HT fibers of vari-

ous species (hamster, rat and monkey) strongly target

midline/intralaminar and association nuclei of the thala-

mus, and excluding the LGN complex, distribute lightly to

principal nuclei of the thalamus. Some species differences

exist with respect to the innervation of the visual-associ-

ated lateral nuclei (LD and LP), the habenula and RT, but

for the rat we found that LD was densely labeled, LP, MH

and LH moderately labeled and RT lightly labeled.

Correspondence between 5-HT innervation

of the thalamus and DR/MR projections to the thalamus

Generally consistent with present findings, previous

examinations of DR and MR projections (Vertes 1991;

Jacobs and Azmitia 1992; Morin and Meyer-Bernstein

1999; Vertes et al. 1999; Vertes and Linley 2007, 2008)

demonstrated that DR/MR fibers: (1) predominantly target

non-principal nuclei of the thalamus; (2) generally avoid

the central core of the thalamus including VAL, PO, VB

and VM; and (3) distribute lightly to RT.

Despite this correspondence, several regions of the

thalamus presently shown to contain relatively dense con-

centrations 5-HT fibers do not appear to receive significant

projections from DR or MR. These primarily include the

anterior nuclei, LP, LD, and parts of the LGN complex. It

seems possible that the anterograde tracers of earlier

reports were not optimally placed within subregions of DR/

MR giving rise to 5-HT projections to these thalamic sites.

In this regard, DR projections to LGN and LP appear to

almost entirely arise from the lateral wings of DR (Villar

et al. 1988; Waterhouse et al. 1993), those to IGL from

rostral aspects of DR (Meyer-Bernstein and Morin 1996)

and those to the anterior thalamus (AD and AV) from

ventromedial and ventrolateral regions of the rostral DR

(Gonzalo-Ruiz et al. 1995).

It could also be the case that the serotonergic innerva-

tion of thalamus may, at least in part, originate from 5-HT

neurons lying outside of DR/MR within rostral raphe nuclei

such as the supralemniscal nucleus (Vertes and Crane

1997) or caudal raphe nuclei (Peschanski and Besson 1984;

Krout et al. 2002). Finally, for some thalamic groups such

as CL and PF, DR/MR projections appear to exceed the

5-HT innervation. This suggests non-serotonergic DR/MR

projections to these sites.

Serotonergic innervation of nuclei lying along

the midline of the thalamus (PV, PT, IMD, IAM,

CM, RH, RE)

As described, the midline nuclei of the thalamus (PV, PT,

RH, RE), as well as other groups lying along the midline

but not classified as midline nuclei (IMD, IAM, CM),

contained dense concentrations of 5-HT fibers. Of these

nuclei, PV, CM and RE were the most heavily labeled; by

comparison PT was lightly labeled.

Unlike principal nuclei of the thalamus that target spe-

cific sensory/motor regions of the cortex, the midline nuclei

distribute almost exclusively to limbic subcortical and

cortical sites (Berendse and Groenewegen 1990, 1991; Su

and Bentivoglio 1990; Wouterlood et al. 1990; Bentivoglio

et al. 1991; Groenewegen and Berendse 1994; Van der

Werf et al. 2002; Vertes 2006; Vertes et al. 2006; Li and

Kirouac 2008; Vertes and Hoover 2008). Accordingly, the

midline nuclei occupy a pivotal position within limbic

forebrain circuitry; i.e., they receive a diverse array of

afferent projections (Krout et al. 2002; Van der Werf et al.

2002; McKenna and Vertes 2004; Kirouac et al. 2005,

2006) and distribute to restricted sets of (limbic) forebrain

structures which appear largely unique for each midline

nucleus (for review, Vertes 2006).

Among its functions, the midline thalamus is thought to

exert an activating or arousing effect on the limbic fore-

brain (Van der Werf et al. 2002; Vertes 2006). Regarding

the possible functional role of serotonergic afferents to the

midline thalamus, 5-HT fibers may represent a source of

excitatory drive to the midline thalamus in arousal/atten-

tion or alternatively may gate (enhance or impede) the flow

of information through the midline thalamus to other parts

of the limbic forebrain (Vertes 2006).

With respect to activating influence on the forebrain, the

dorsal raphe nucleus (and possibly MR) reportedly forms

part of a widespread ‘waking system’ of the brain that also

includes other monoaminergic nuclei, cholinergic cells of

the dorsolateral pontine tegmentum, and orexinergic neu-

rons of the lateral hypothalamus (Saper et al. 2005a, b;

Datta and MacLean 2007). A primary, but not sole, fore-

brain target for DR effects on arousal/wakefulness appears

to be the paraventricular nucleus of the midline thalamus.

PV is reciprocally connected with the suprachiasmatic

nucleus (SCN) (Moga et al. 1995; Novak et al. 2000a; Peng

and Bentivoglio 2004; Li and Kirouac 2008; Vertes and

Hoover 2008) and distributes strongly to the dorsomedial

nucleus of the hypothalamus (Vertes and Hoover 2008)—a

critical hub in sleep–wake control (Saper et al. 2005a, b).

PV cells show elevated levels of c-fos expression during

wakefulness (Peng et al. 1995; Novak et al. 2000b).


123

Anterior nuclei of the thalamus: anterodorsal,

anteroventral, anteromedial and interanteromedial

nuclei

Serotonergic fibers were densely concentrated in the ante-

rior nuclei of the thalamus. The anteroventral nucleus of

thalamus was most heavily labeled, followed by AM and

IAM and then AD. Comparatively, AD was lightly labeled.

It is well documented that lesions of the anterior thala-

mus in rats produce deficits in spatial learning, or specifi-

cally in tasks utilizing allocentric cues—or allocentric

spatial learning (Aggleton et al. 1996; Byatt and Dalrymple-

Alford 1996; Warburton et al. 1997; van Groen et al.

2002a). By contrast, lesions of structures bordering the

anterior thalamus including the mediodorsal or intralaminar

nuclei fail to produce spatial deficits (Mitchell and

Dalrymple-Alford 2005). The involvement of the anterior

nuclei in spatial behavior has been attributed to its close

connections with the hippocampus (Amaral and Witter

1995; Aggleton and Brown 1999). Aggleton and colleagues

have, in fact, proposed that the anterior thalamus forms part

of an extended network subserving hippocampal-dependent

memory, notably episodic memory in humans (Aggleton

and Brown 1999, 2006). Supporting this, alterations of the

anterior thalamus in humans produce the same severe def-

icits in episodic memory (diencephalic amnesia) as found

with hippocampal damage (von Cramon et al. 1985; Graff-

Radford et al. 1990; Van der Werf et al. 2000, 2003a, b).

Two parallel but separate sub-circuits have been iden-

tified within interconnected nuclei that were originally

described as forming Papez’s circuit (Papez 1937): a head

direction (HD) circuit and a theta rhythm circuit (Vann and

Aggleton 2004; Vertes et al. 2001, 2004; Vann 2009).

Within the anterior thalamus, HD cells are present in AD,

and ‘theta cells’ mainly in AV (Taube 1998, Vann and

Aggleton 2004; Vertes et al. 2004). It has been suggested

that theta bursting AV neurons may promote the transfer of

head direction information from AD to the retrosplenial

cortex thereby supporting spatial navigation/learning

(Vertes et al. 2004). As described, AV contains a dense

concentration of 5-HT fibers—among the heaviest in the

thalamus. Serotonergic input to AV could amplify the

effects of theta on HD circuitry. In this regard, theta

rhythmically firing neurons have been identified in DR and

MR, and may excite/drive ‘theta cells’ of AV (Kocsis and

Vertes 1992, 1996; Kocsis et al. 2006; Vertes 2010).

The rostral intralaminar nuclei (CL, PC and CM),

and the intermediodorsal (IMD) and mediodorsal (MD)

nuclei

As a group, the rostral intralaminar nuclei contain dense

concentrations of 5-HT fibers, but fewer than present in the

midline nuclei or AV. The central medial nucleus was

heavily labeled; PC and CL were moderately labeled.

There was a mediolateral gradient in density of labeling in

the intralaminar complex (CM [ PC/CL) as well as in

IMD and MD (IMD [ MD). MD contained moderate

numbers of 5-HT fibers.

Consistent with dense labeling of midline nuclei, the

midline ‘located’ CM and IMD contained strong concen-

trations of 5-HT fibers—denser than the laterally situated

cell groups of these complexes. This could involve the fact

that these medial nuclei (CM and IMD) mainly target

limbic forebrain structures, whereas the lateral cell groups

(PC/CL and central/lateral MD) primarily distribute to

motor regions of the forebrain (Van der Werf et al. 2002;

Groenewegen and Witter 2004; Vertes et al. 2006).

The intralaminar (IL) and MD complexes differ from

the anterior thalamus by their virtual lack of connections

with the hippocampus and parahippocampal structures

(Berendse and Groenewegen 1991; Van der Werf et al. 2002;

Groenewegen and Witter 2004). As such, the IL thalamus

(as well as MD/IMD) do not appear to participate in hip-

pocampal-dependent functions, but rather are involved

in prefrontal cortical-associated behaviors. Specifically, IL

or MD lesions produce little or no alteration on tasks

involving spatial memory (Hunt and Aggleton 1998a;

Bailey and Mair 2005; Mitchell and Dalrymple-Alford

2005; Wolff et al. 2008), but severely disrupt performance

on ‘prefrontally associated’ tasks, or those requiring shifts

in strategy or behavioral flexibility (Beracochea et al. 1989;

McAlonan et al. 1993; Hunt and Aggleton 1998b; Lacroix

et al. 2002; Floresco et al. 2008; Ghods-Sharifi et al. 2008;

Dolleman-van der Weel et al. 2009). The pronounced 5-HT

input to IL and MD/IMD could serve to coordinate the

activity of medial affective (CM and IMD) and lateral

motor (PC, CL and lateral MD) components of these sys-

tems, thereby providing emotional drive for complex motor

acts.

Lateral dorsal nucleus (LD) and the lateral geniculate

complex

Serotonergic fibers were densely concentrated in the lateral

dorsal nucleus and in the LGN complex: the dorsal and

ventral lateral geniculate nuclei and the intergeniculate

leaflet. Labeling was heaviest in IGL and LGNlv of the

LGN complex.

5-HT fibers were more heavily distributed within the

rostral than in the caudal LD. Rostral LD borders the

anterior nuclei and has been anatomically and functionally

linked with the anterior group (Groenewegen and Witter

2004; Jones 2007). Like the anterior nuclei, rostral LD is

strongly reciprocally connected with the retrosplenial cor-

tex and subiculum (of hippocampus) (van Groen and Wyss


123

1992; Groenewegen and Witter 2004; Shibata and Naito

2007), contains head direction cells (Mizumori and

Williams 1993), and participates in spatial learning/memory

(Mizumori et al. 1994; Wilton et al. 2001; van Groen et al.

2002b). Unlike the anterior nuclei, however, LD receives

fairly substantial input from subcortical visual structures

(Thompson and Robertson 1987; Kolmac et al. 2000). The

convergence of limbic and visual information in the lateral

dorsal nucleus has led to the suggestion that LD partici-

pates in visually guided spatial navigation/learning

(Mizumori et al. 1994; van Groen et al. 2002b; Bezdudnaya

and Keller 2008). The prominent serotonergic input to LD,

particularly to the rostral LD, may sharpen visuospatial

processing during conditions requiring focused attention.

The LGN complex in the rat consists of LGNd, LGNv

and IGL. LGNd is the main relay nucleus of the LGN

complex, conveying visual information from the retina to

the visual cortex (Price 1995; Groenewegen and Witter

2004). By comparison, the LGNv and IGL receive retinal

inputs (as well as afferents from several other nuclei of the

visual system) and do not project to visual cortices, but

rather to various visual and ‘non-visual’ structures of the

forebrain and brainstem (Kolmac and Mitrofanis 2000;

Moore et al. 2000; Vrang et al. 2003; Horowitz et al. 2004).

Among its sites of distribution, the LGNv projects signif-

icantly to midline (PV, RH and RE) and lateral nuclei (LP

and LD) of the thalamus (Kolmac et al. 2000; Moore et al.

2000). The visual information reaching these structures

would appear to participate in visually guided spatial

behavior (LP/LD) or visually elicited shifts in attention

(PV/RH/RE).

The LGNv consists of a lateral, magnocellular part and a

medial parvicellular division. Based in part on differential

sets of inputs and outputs (Kolmac and Mitrofanis 2000;

Kolmac et al. 2000), the lateral and medial LGNv have

been characterized, respectively, as visual (lateral) and non-

visual (medial) divisions of the LGNv complex (Kolmac

et al. 2000). As described, we showed that 5-HT fibers

were much more densely concentrated in the lateral than in

the medial LGNv, suggesting a greater serotonergic influ-

ence on LGNv-mediated visual than non-visual functions.

LGNd and LGNv receive afferent projections from the

dorsal raphe nucleus (Villar et al. 1988; Kolmac and Mi-

trofanis 2000; Horowitz et al. 2004). The serotonergic

input to LGNd may modulate the transfer of signals from

the retina to the visual cortex, whereas that to LGNv (and

hence to its targets in the midline and lateral thalamus) may

amplify the effect of visual information on spatial and

attentional processing.

The IGL receives afferents from photosensitive mela-

nopsin containing neurons of the retina and in turn

projects to the suprachiasmatic nucleus (SCN) of the

hypothalamus (Moore et al. 1995, 2000; Vrang et al.

2003; Horowitz et al. 2004). This indirect route from the

retina to SCN via IGL appears to fine-tune the effects of

light on circadian rhythmicity (Moore et al. 2000). For

instance, Morin and Pace (2002) demonstrated that neu-

rotoxic lesions of IGL in hamsters significantly attenuated

(by 50%) the lengthened circadian period produced by

constant light.

IGL receives significant 5-HT (and non-5-HT) projec-

tions from the dorsal raphe nucleus (Meyer-Bernstein and

Morin 1996; Horowitz et al. 2004), and DR stimulation has

been shown to produce circadian phase shifts, mediated by

the IGL (Glass et al. 2000). Glass et al. (2000) proposed

that based on its (DR) ‘‘functional linkages to the SCN and

intergeniculate leaflet, the DR could serve to provide

behavior/arousal state information to various sites com-

prising the brain circadian system’’.

Summary and conclusions

In summary, serotonergic fibers were found to be densely

concentrated in the midline/intralaminar and association

nuclei of the thalamus, and with the exception of the LGN

complex, sparsely distributed within principal nuclei and

the reticular nucleus of the thalamus. Specifically, sub-

stantial numbers of 5-HT fibers were present in the midline

nuclei (PV, RH, RE), the anterior nuclei (AV, AM, IAM),

the intralaminar nuclei (CM, PC, CL), mediodorsal

nucleus, lateral nuclei (LD, LP) and the medial and lateral

habenula. With a few exceptions, these structures might be

appropriately classified as the ‘limbic thalamus’; i.e., a

constellation of thalamic nuclei that predominantly target

limbic forebrain structures that subserve affective/cogni-

tive functions. Accordingly, through actions on the ‘limbic

thalamus’, serotonin/serotonergic axons may exert a sig-

nificant modulatory influence on emotional and cognitive

aspects of behavior, complementing 5-HT effects on other

forebrain structures involved in these functions (Cassel and

Jeltsch 1995; Cools et al. 2008).

This is consistent with the role of serotonin in a host of

‘limbic’ functions as well as its well recognized involve-

ment in affective disorders (Jacobs and Azmitia 1992;

Cools et al. 2008; Lowry et al. 2008b). Although the pre-

cise role served by serotonin in emotional and cognitive

behaviors remains to be fully determined, the present

findings showing that 5-HT fibers distribute densely to the

midline nuclei of thalamus suggest that serotonin may play

an important role in functions associated with the midline

thalamus such arousal/attention and response selection

(Vertes 2006).

Acknowledgments This research was supported by National Sci-

ence Foundation grant IOS 0820639 to RPV.


123

References

Aggleton JP, Brown MW (1999) Episodic memory, amnesia, and the

hippocampal-anterior thalamic axis. Behav Brain Sci 22:425–

444

Aggleton JP, Brown MW (2006) Interleaving brain systems for

episodic and recognition memory. Trends Cogn Sci 10:455–463

Aggleton JP, Hunt PR, Nagle S, Neave N (1996) The effects of

selective lesions within the anterior thalamic nuclei on spatial

memory in the rat. Behav Brain Res 81:189–198

Amaral DG, Witter MP (1995) Hippocampal formation. In: Paxinos G

(ed) The rat nervous system, 2nd edn. Academic Press, London,

pp 443–493

Austin MC, Rhodes JL, Lewis DA (1997) Differential distribution of

corticotropin-releasing hormone immunoreactive axons in

monoaminergic nuclei of the human brainstem. Neuropsycho-

pharmacology 17:326–341

Azmitia EC, Segal M (1978) An autoradiographic analysis of the

differential ascending projections of the dorsal and median raphe

nuclei in the rat. J Comp Neurol 179:641–667

Bailey KR, Mair RG (2005) Lesions of specific and nonspecific

thalamic nuclei affect prefrontal cortex-dependent aspects of

spatial working memory. Behav Neurosci 119:410–419

Bentivoglio M, Balercia G, Kruger L (1991) The specificity of the

nonspecific thalamus: the midline nuclei. Prog Brain Res 87:53–80

Beracochea DJ, Jaffard R, Jarrard LE (1989) Effects of anterior or

dorsomedial thalamic ibotenic lesions on learning and memory

in rats. Behav Neural Biol 51:364–376

Berendse HW, Groenewegen HJ (1990) Organization of the thalamo-

striatal projections in the rat, with special emphasis on the

ventral striatum. J Comp Neurol 299:187–228

Berendse HW, Groenewegen HJ (1991) Restricted cortical termina-

tion fields of the midline and intralaminar thalamic nuclei in the

rat. Neuroscience 42:73–102

Bezdudnaya T, Keller A (2008) Laterodorsal nucleus of the thalamus:

a processor of somatosensory inputs. J Comp Neurol 507:1979–

1989

Byatt G, Dalrymple-Alford JC (1996) Both anteromedial and

anteroventral thalamic lesions impair radial-maze learning in

rats. Behav Neurosci 110:1335–1348

Cassel JC, Jeltsch H (1995) Serotonergic modulation of cholinergic

function in the central nervous system: cognitive implications.

Neuroscience 69:1–41

Charara A, Parent A (1998) Chemoarchitecture of the primate dorsal

raphe nucleus. J Chem Neuroanat 15:111–127

Cools R, Roberts AC, Robbins TW (2008) Serotoninergic regulation

of emotional and behavioural control processes. Trends Cogn Sci

12:31–40

Cropper EC, Eisenman JS, Azmitia EC (1984) An immunocyto-

chemical study of the serotonergic innervation of the thalamus of

the rat. J Comp Neurol 224:38–50

Datta S, Maclean RR (2007) Neurobiological mechanisms for the

regulation of mammalian sleep–wake behavior: reinterpretation

of historical evidence and inclusion of contemporary cellular and

molecular evidence. Neurosci Biobehav Rev 31:775–824

Day HE, Greenwood BN, Hammack SE, Watkins LR, Fleshner M,

Maier SF, Campeau S (2004) Differential expression of 5HT-1A,

alpha 1b adrenergic, CRF-R1, and CRF-R2 receptor mRNA in

serotonergic, gamma-aminobutyric acidergic, and catecholam-

inergic cells of the rat dorsal raphe nucleus. J Comp Neurol

474:364–378

Dempsey EW, Morison RS (1942) The production of rhythmically

recurrent cortical potentials after localized thalamic stimulation.

Am J Physiol 135:293–300

Dempsey EW, Morison RS (1943) The electrical activity of a

thalamocortical relay system. Am J Physiol 138:283–298

Dolleman-van der Weel MJ, Morris RG, Witter MP (2009) Neuro-

toxic lesions of the thalamic reuniens or mediodorsal nucleus in

rats affect non-mnemonic aspects of watermaze learning. Brain

Struct Funct 213:329–342

Floresco SB, Block AE, Tse MT (2008) Inactivation of the medial

prefrontal cortex of the rat impairs strategy set-shifting, but not

reversal learning, using a novel, automated procedure. Behav

Brain Res 190:85–96

Ghods-Sharifi S, Haluk DM, Floresco SB (2008) Differential effects

of inactivation of the orbitofrontal cortex on strategy set-shifting

and reversal learning. Neurobiol Learn Mem 89:567–573

Glass JD, DiNardo LA, Ehlen JC (2000) Dorsal raphe nuclear

stimulation of SCN serotonin release and circadian phase-

resetting. Brain Res 859:224–232

Gonzalo-Ruiz A, Lieberman AR, Sanz-Anquela JM (1995) Organi-

zation of serotoninergic projections from the raphe nuclei to the

anterior thalamic nuclei in the rat: a combined retrograde tracing

and 5-HT immunohistochemical study. J Chem Neuroanat

8:103–115

Graff-Radford NR, Tranel D, Van Hoesen GW, Brandt JP (1990)

Diencephalic amnesia. Brain 113:1–25

Groenewegen HJ, Berendse HW (1994) The specificity of the

‘nonspecific’ midline and intralaminar thalamic nuclei. Trends

Neurosci 17:52–57

Groenewegen HJ, Witter MP (2004) Thalamus. In: Paxinos G (ed)

The rat nervous system, 3rd edn. Academic Press, New York, pp

407–453

Halliday G, Harding A, Paxinos G (2004) The serotonin and

tachykinin systems. In: Paxinos G (ed) The rat nervous system,

3rd edn. Academic Press, New York, pp 1205–1256

Harrington ME (1997) The ventral lateral geniculate nucleus and the

intergeniculate leaflet: interrelated structures in the visual and

circadian systems. Neurosci Biobehav Rev 21:705–727

Horowitz SS, Blanchard JH, Morin LP (2004) Intergeniculate leaflet

and ventral lateral geniculate nucleus afferent connections: an

anatomical substrate for functional input from the vestibulo-

visuomotor system. J Comp Neurol 474:227–245

Hsu DT, Price JL (2009) Paraventricular thalamic nucleus: subcor-

tical connections and innervation by serotonin, orexin, and

corticotropin-releasing hormone in macaque monkeys. J Comp

Neurol 512:825–848

Hunt PR, Aggleton JP (1998a) An examination of the spatial working

memory deficit following neurotoxic medial dorsal thalamic

lesions in rats. Behav Brain Res 97:129–141

Hunt PR, Aggleton JP (1998b) Neurotoxic lesions of the dorsomedial

thalamus impair the acquisition but not the performance of

delayed matching to place by rats: a deficit in shifting response

rules. J Neurosci 18:10045–10052

Jacobs BL, Azmitia EC (1992) Structure and function of the brain

serotonin system. Physiol Rev 72:165–229

Jones EG (2007) The thalamus, 2nd edn. Cambridge University Press,

Cambridge, UK

Kirouac GJ, Parsons MP, Li S (2005) Orexin (hypocretin) innervation

of the paraventricular nucleus of the thalamus. Brain Res

1059:179–188

Kirouac GJ, Parsons MP, Li S (2006) Innervation of the paraventric-

ular nucleus of the thalamus from cocaine- and amphetamine-

regulated transcript (CART) containing neurons of the hypo-

thalamus. J Comp Neurol 497:155–165

Kocsis B, Vertes RP (1992) Dorsal raphe neurons: synchronous

discharge with the theta rhythm of the hippocampus in the freely

behaving rat. J Neurophysiol 68:1463–1467


123

Kocsis B, Vertes RP (1996) Midbrain raphe cell firing and hippo-

campal theta rhythm urethane-anaesthetized rats. Neuroreport

7:2867–2872

Kocsis B, Varga V, Dahan L, Sik A (2006) Serotonergic neuron

diversity: identification of raphe neurons with discharges time-

locked to the hippocampal theta rhythm. Proc Natl Acad Sci

USA 2103:1059–1064

Kolmac C, Mitrofanis J (2000) Organization of brain stem afferents to

the ventral lateral geniculate nucleus of rats. Vis Neurosci

17:313–318

Kolmac CI, Power BD, Mitrofanis J (2000) Dorsal thalamic

connections of the ventral lateral geniculate nucleus of rats.

J Neurocytol 29:31–41

Krout KE, Belzer RE, Loewy AD (2002) Brainstem projections to

midline and intralaminar thalamic nuclei of the rat. J Comp

Neurol 448:53–101

Lacroix L, White I, Feldon J (2002) Effect of excitotoxic lesions of rat

medial prefrontal cortex on spatial memory. Behav Brain Res

133:69–81

Lavoie B, Parent A (1991) Serotoninergic innervation of the thalamus

in the primate: an immunohistochemical study. J Comp Neurol

312:1–18

Li S, Kirouac GJ (2008) Projections from the paraventricular nucleus

of the thalamus to the forebrain, with special emphasis on the

extended amygdala. J Comp Neurol 506:263–287

Lowry CA, Evans AK, Gasser PJ, Hale MW, Staub DR, Shekhar A

(2008a) Topographic organization and chemoarchitecture of the

dorsal raphe nucleus and the median raphe nucleus. In: Monti

JM, Pandi-Perumal SR, Jacobs BL, Nutt DJ (eds) Serotonin and

sleep: molecular, functional and clinical aspects. Birkhauser,

Basel, Switzerland, pp 25–67

Lowry CA, Hale MW, Evans AK, Heerkens J, Staub DR, Gasser PJ,

Shekhar A (2008b) Serotonergic systems, anxiety, and affective

disorder: focus on the dorsomedial part of the dorsal raphe

nucleus. Ann N Y Acad Sci 1148:86–94

McAlonan GM, Robbins TW, Everitt BJ (1993) Effects of medial

dorsal thalamic and ventral pallidal lesions on the acquisition of

a conditioned place preference: further evidence for the

involvement of the ventral striatopallidal system in reward-

related processes. Neuroscience 52:605–620

McKenna JT, Vertes RP (2004) Afferent projections to nucleus

reuniens of the thalamus. J Comp Neurol 480:115–142

Melander T, Hokfelt T, Rokaeus A, Cuello AC, Oertel WH,

Verhofstad A, Goldstein M (1986) Coexistence of galanin-like

immunoreactivity with catecholamines, 5-hydroxytryptamine,

GABA and neuropeptides in the rat CNS. J Neurosci 6:3640–

3654

Meyer-Bernstein EL, Morin LP (1996) Differential serotonergic

innervation of the suprachiasmatic nucleus and the intergenic-

ulate leaflet and its role in circadian rhythm modulation.

J Neurosci 16:2097–2111

Mitchell AS, Dalrymple-Alford JC (2005) Dissociable memory

effects after medial thalamus lesions in the rat. Eur J Neurosci

22:973–985

Mizumori SJ, Williams JD (1993) Directionally selective mnemonic

properties of neurons in the lateral dorsal nucleus of the thalamus

of rats. J Neurosci 13:4015–4028

Mizumori SJ, Miya DY, Ward KE (1994) Reversible inactivation of

the lateral dorsal thalamus disrupts hippocampal place represen-

tation and impairs spatial learning. Brain Res 644:168–174

Moga MM, Weis RP, Moore RY (1995) Efferent projections of the

paraventricular thalamic nucleus in the rat. J Comp Neurol

359:221–238

Moore RY, Halaris AE, Jones BE (1978) Serotonin neurons of the

midbrain raphe: ascending projections. J Comp Neurol 180:417–

438

Moore RY, Speh JC, Card JP (1995) The retinohypothalamic tract

originates from a distinct subset of retinal ganglion cells. J Comp

Neurol 352:351–366

Moore RY, Weis R, Moga MM (2000) Efferent projections of the

intergeniculate leaflet and the ventral lateral geniculate nucleus

in the rat. J Comp Neurol 420:398–418

Morin LP, Blanchard JH (1999) Forebrain connections of the hamster

intergeniculate leaflet: comparison with those of ventral lateral

geniculate nucleus and retina. Vis Neurosci 16:1037–1054

Morin LP, Meyer-Bernstein EL (1999) The ascending serotonergic

system in the hamster: comparison with projections of the dorsal

and median raphe nuclei. Neuroscience 91:81–105

Morin LP, Pace L (2002) The intergeniculate leaflet, but not the visual

midbrain, mediates hamster circadian rhythm response to

constant light. J Biol Rhythms 17:217–226

Morison RS, Dempsey EW (1942) A study of thalamocortical

relations. Am J Physiol 135:281–292

Nielsen K, Brask D, Knudsen GM, Aznar S (2006) Immunodetection

of the serotonin transporter protein is a more valid marker for

serotonergic fibers than serotonin. Synapse 59:270–276

Novak CM, Harris JA, Smale L, Nunez AA (2000a) Suprachiasmatic

nucleus projections to the paraventricular thalamic nucleus in

nocturnal rats (Rattus norvegicus) and diurnal nile grass rats

(Arviacanthis niloticus). Brain Res 874:147–157

Novak CM, Smale L, Nunez AA (2000b) Rhythms in Fos expression

in brain areas related to the sleep–wake cycle in the diurnal

Arvicanthis niloticus. Am J Physiol Regul Integr Comp Physiol

278:R1267–R1274

Otake K, Ruggiero DA (1995) Monoamines and nitric oxide are

employed by afferents engaged in midline thalamic regulation.

J Neurosci 15:1891–1911

Papez JW (1937) A proposed mechanism for emotion. Arch Neurol

Psychiatry 38:725–743

Peng ZC, Bentivoglio M (2004) The thalamic paraventricular nucleus

relays information from the suprachiasmatic nucleus to the

amygdala: a combined anterograde and retrograde tracing study

in the rat at the light and electron microscopic levels.

J Neurocytol 33:101–116

Peng ZC, Grassi-Zucconi G, Bentivoglio M (1995) Fos-related

protein expression in the midline paraventricular nucleus of the

rat thalamus: basal oscillation and relationship with limbic

efferents. Exp Brain Res 104:21–29

Peschanski M, Besson JM (1984) Diencephalic connections of the

raphe nuclei of the rat brainstem: an anatomical study with

reference to the somatosensory system. J Comp Neurol 224:509–

534

Price JL (1995) Thalamus. In: Paxinos G (ed) The rat nervous system,

2nd edn. Academic Press, New York, pp 629–648

Risold PY, Thompson RH, Swanson LW (1997) The structural

organization of connections between hypothalamus and cerebral

cortex. Brain Res Rev 24:197–254

Saper CB, Cano G, Scammell TE (2005a) Homeostatic, circadian,

and emotional regulation of sleep. J Comp Neurol 493:92–98

Saper CB, Scammell TE, Lu J (2005b) Hypothalamic regulation of

sleep and circadian rhythms. Nature 437:1257–1263

Shibata H, Naito J (2007) Organization of anterior cingulate and

frontal cortical projections to the anterior and laterodorsal

thalamic nuclei in the rat. Brain Res 1059:93–103

Steinbusch HW (1981) Distribution of serotonin-immunoreactivity in

the central nervous system of the rat-cell bodies and terminals.

Neuroscience 6:557–618

Su HS, Bentivoglio M (1990) Thalamic midline cell populations

projecting to the nucleus accumbens, amygdala, and hippocam-

pus in the rat. J Comp Neurol 297:582–593

Sur C, Betz H, Schloss P (1996) Immunocytochemical detection of

the serotonin transporter in rat brain. Neuroscience 73:217–231


123

Swanson LW (2003) Brain maps: structure of the rat brain. Elsevier,

New York

Taube JS (1998) Head direction cells and the neurophysiological basis

for a sense of direction. Prog Neurobiol 55:225–256

Thompson SM, Robertson RT (1987) Organization of subcortical

pathways for sensory projections to the limbic cortex. II.

Afferent projections to the thalamic lateral dorsal nucleus in

the rat. J Comp Neurol 265:189–202

Trulson ME, Cannon MS, Raese JD (1985) Identification of

dopamine-containing cell bodies in the dorsal and median raphe

nuclei of the rat brain using tyrosine hydroxylase immunochem-

istry. Brain Res Bull 15:229–234

Van der Werf YD, Witter MP, Uylings HB, Jolles J (2000)

Neuropsychology of infarctions in the thalamus: a review.

Neuropsychologia 38:613–627

Van der Werf YD, Witter MP, Groenewegen HJ (2002) The

intralaminar and midline nuclei of the thalamus. Anatomical

and functional evidence for participation in processes of arousal

and awareness. Brain Res Rev 39:107–140

Van der Werf YD, Jolles J, Witter MP, Uylings HB (2003a)

Contributions of thalamic nuclei to declarative memory func-

tioning. Cortex 39:1047–1062

Van der Werf YD, Scheltens P, Lindeboom J, Witter MP, Uylings

HB, Jolles J (2003b) Deficits of memory, executive functioning

and attention following infarction in the thalamus; a study of 22

cases with localised lesions. Neuropsychologia 41:1330–1344

van Groen T, Wyss JM (1992) Projections from the laterodorsal

nucleus of the thalamus to the limbic and visual cortices in the

rat. J Comp Neurol 324:427–448

van Groen T, Kadish I, Wyss JM (2002a) Role of the anterodorsal and

anteroventral nuclei of the thalamus in spatial memory in the rat.

Behav Brain Res 132:19–28

van Groen T, Kadish I, Wyss JM (2002b) The role of the laterodorsal

nucleus of the thalamus in spatial learning and memory in the

rat. Behav Brain Res 136:329–337

Vann SD (2010) Re-evaluating the role of the mammillary bodies in

memory. Neuropsychologia (in press)

Vann SD, Aggleton JP (2004) Deficits of memory, executive

functioning and attention following infarction in the thalamus;

a study of 22 cases with localised lesions. Nat Rev Neurosci

5:35–44

Vertes RP (1991) A PHA-L analysis of ascending projections of the

dorsal raphe nucleus in the rat. J Comp Neurol 313:643–668

Vertes RP (2006) Interactions among the medial prefrontal cortex,

hippocampus and midline thalamus in emotional and cognitive

processing in the rat. Neuroscience 142:1–20

Vertes RP (2010) Serotonergic regulation of rhythmical activity of the

brain, concentrating on the hippocampus. In: Muller C, Jacobs B

(eds) Handbook of the behavioral neurobiology of serotonin.

Academic, London, pp 277–292

Vertes RP, Crane AM (1997) Distribution, quantification, and

morphological characteristics of serotonin-immunoreactive cells

of the supralemniscal nucleus (B9) and pontomesencephalic

reticular formation in the rat. J Comp Neurol 378:411–424

Vertes RP, Hoover WB (2008) Projections of the paraventricular and

paratenial nuclei of the dorsal midline thalamus in the rat.

J Comp Neurol 508:212–237

Vertes RP, Linley SB (2007) Comparisons of projections of the dorsal

and median raphe nuclei, with some functional considerations.

In: Takai K (ed) The interdisciplinary conference on tryptophan

and related substances: chemistry, biology, and medicine.

International Congress Series, 1304. Elsevier, Oxford, pp 98–

120

Vertes RP, Linley SB (2008) Efferent and afferent connections of the

dorsal and median raphe nuclei in the rat. In: Monti JM, Pandi-

Perumal SR, Jacobs BL, Nutt DJ (eds) Serotonin and sleep:

molecular, functional and clinical aspects. Birkhauser, Basel,

Switzerland, pp 69–102

Vertes RP, Martin GF (1988) Autoradiographic analysis of ascending

projections from the pontine and mesencephalic reticular

formation and the median raphe nucleus in the rat. J Comp

Neurol 275:511–541

Vertes RP, Fortin WJ, Crane AM (1999) Projections of the median

raphe nucleus in the rat. J Comp Neurol 407:555–582

Vertes RP, Albo Z, Viana Di Prisco G (2001) Theta-rhythmically

firing neurons in the anterior thalamus: implications for

mnemonic functions of Papez’s circuit. Neuroscience 104:619–

625

Vertes RP, Hoover WB, Viana Di Prisco G (2004) Theta rhythm of

the hippocampus: subcortical control and functional significance.

Behav Cogn Neurosci Rev 3:173–200

Vertes RP, Hoover WB, Do Valle AC, Sherman A, Rodriguez JJ

(2006) Efferent projections of reuniens and rhomboid nuclei of

the thalamus in the rat. J Comp Neurol 499:768–796

Villar MJ, Vitale ML, Hokfelt T, Verhofstad AA (1988) Dorsal raphe

serotoninergic branching neurons projecting both to the lateral

geniculate body and superior colliculus: a combined retrograde

tracing-immunohistochemical study in the rat. J Comp Neurol

277:126–140

von Cramon DY, Hebel N, Schuri U (1985) A contribution to the

anatomical basis of thalamic amnesia. Brain 108:993–1008

Vrang N, Mrosovsky N, Mikkelsen JD (2003) Afferent projections to

the hamster intergeniculate leaflet demonstrated by retrograde

and anterograde tracing. Brain Res Bull 59:267–288

Warburton EC, Baird AL, Aggleton JP (1997) Assessing the

magnitude of the allocentric spatial deficit associated with

complete loss of the anterior thalamic nuclei in rats. Behav Brain

Res 87:223–232

Waselus M, Van Bockstaele EJ (2007) Co-localization of corticotro-

pin-releasing factor and vesicular glutamate transporters within

axon terminals of the rat dorsal raphe nucleus. Brain Res

1174:53–65

Waterhouse BD, Border B, Wahl L, Mihailoff GA (1993) Topo-

graphic organization of rat locus coeruleus and dorsal raphe

nuclei: distribution of cells projecting to visual system structures.

J Comp Neurol 336:345–361

Wilton LA, Baird AL, Muir JL, Honey RC, Aggleton JP (2001) Loss

of the thalamic nuclei for ‘‘head direction’’ impairs performance

on spatial memory tasks in rats. Behav Neurosci 115:861–869

Wolff M, Loukavenko EA, Will BE, Dalrymple-Alford JC (2008)

The extended hippocampal-diencephalic memory system:

enriched housing promotes recovery of the flexible use of spatial

representations after anterior thalamic lesions. Hippocampus

18:996–1007

Wouterlood FG, Saldana E, Witter MP (1990) Projection from the

nucleus reuniens thalami to the hippocampal region: light and

electron microscopic tracing study in the rat with the anterograde

tracer Phaseolus vulgaris-leucoagglutinin. J Comp Neurol

296:179–203

Zhou FC, Xu Y, Bledsoe S, Lin R, Kelley MR (1996) Serotonin

transporter antibodies: production, characterization, and locali-

zation in the brain. Mol Brain Res 43:267–278


123

Pattern of distribution of serotonergic ﬁbers to …vertes/sert-thalamus.pdfORIGINAL ARTICLE...

Documents

Transcript of Pattern of distribution of serotonergic ﬁbers to …vertes/sert-thalamus.pdfORIGINAL ARTICLE...