Chapter 9 · Fig. 9.10 Ocular dominance properties of visual cortex neurons in normally reared...

© 2019 Elsevier Inc. All rights reserved.

Chapter 9

Refinement of Synaptic Connections

© 2019 Elsevier Inc. All rights reserved. 2

Fig. 9.1 Three kinds of immature afferent projections. (A) The projection of oneafferent is shown with its arborization centered at the

topographically correct position in the target. However, the arbor initially extends to far, and some of its local branches are eliminated

during development. (B) A single neuron is shown to receive input from three afferents initially, and two of these inputs are eliminated

during development. (C) A projection is shown to innervate the soma and dendrite of a postsynaptic neuron initially, but the somatic

innervation is eliminated during development.


Fig. 9.2 Functional synapses are eliminated during development. (A) An electrophysiological method for determining the number of inputs

converging onto a neuron. A stimulating electrode is placed on the afferent population while an intracellular recording is obtained from the

postsynaptic cell. As the stimulation current is increased, the afferent inputs are recruited to become active. When a single afferent is

active (top), the postsynaptic potential (PSP) is small. When two (middle), and then three afferents are activated (bottom), the PSP

become quantally larger. One can estimate the number of inputs by counting the number of quantal increases in PSP amplitude, in this

case three. (B) Three examples of decreased convergence as measured electrophysiologically, as described in panel A. On the left are

shown the increases in afferent-evoked PSP amplitude recorded in immature postsynaptic cells of the neuromuscular junction (top), the

chick cochlear nucleus (middle), and the rat autonomic ganglion (bottom). There are 3–5 quantal increases in PSP amplitude. On the right

are shown the increases in afferent-evoked PSP amplitude in mature neurons. There are 1–2 quantal increases in PSP amplitude,

indicating the functional elimination of inputs during development.


Fig. 9.3 Elimination of commissural axons during development. (A) The schematic shows the location of the anterior commissure, a

bundle of fibers that connect the cortical temporal lobes. (B) Electron micrographs of the anterior commissure of rhesus macaques at two

postnatal ages. There are more axon profiles at P21 than at P90, but myelination is greater at P90. (C) The graph plots the total number

of anterior commissure axons, and the number of myelinated axons, as a function of age. The red and blue dots signify the ages for which

micrographs are shown in panel B. The number of axons reach a maximum at birth, and declines threefold over the next few months.

Myelination begins during this period, but continues for almost 9 months.


Fig. 9.4 Developmental emergence of specific innervation patterns in the cat visual pathway. (A) When 3H-proline is injected into one eye,

it is transported to the visual thalamus (lateral geniculate nucleus, LGN), and reveals an eye-specific innervation pattern (contralateral

eye, red layers; ipsilateral eye, blue layer). The proline moves transynaptically into thalamic neurons and is transported to the cortex. LGN

neurons project to eye-specific stripes in cortex Layer IV (contralateral eye, red stripes; ipsilateral eye, blue stripes. (B) Images showing

the emergence of eye-specific stripes in the visual cortex. The first column of images (left) shows autoradiograms of 3H-proline in cross

sections. The terminal fields continue to segregate for several weeks of development. The second column of images (right) shows the

activity pattern recorded at the surface of the cortex in response to stimulation of each eye (contralateral, black; ipsilateral, white).

Segregation of afferent can first be observed by about 14 days postnatal.


Fig. 9.5 Development of retinal ganglion cell terminals in the cat lateral geniculate nucleus (LGN). (A) When individual retinal fibers are

labeled with horseradish peroxidase (HRP) at embryonic days 43–55, they have many side branches in the inappropriate layer of LGN

(arrows). (B) By birth, most of the side branches have been eliminated, and terminals arborizations have been restricted to the correct

layer. However, the terminal zone remains wider in the eye-specific lamina at 3–4 weeks postnatal (arrows). (C) When fibers of retinal

X-cells are filled in adult cats, they are found to have retracted (black arrows).


Fig. 9.6 Competition and synapse elimination at the neuromuscular junction and at cerebellar Purkinje cells. (A) In vivo imaging of the same multiply

innervated mouse neuromuscular junction from P11 to P14. In these transgenic mice, motor neurons expressed either of two different fluorescent

proteins. One of the motor terminals (blue) occupies a larger percentage of the postsynaptic territory at P11. It gradually withdraws from the junction

(arrows) over the next 4 days, and the other terminal (green) takes over the postsynaptic site. The withdrawing axon is marked by an asterisk at P14.

Scale bar is 10 μm. (B) In vivo imaging of the same two inferior olivary nucleus climbing fibers (CF) innervating a cerebellar Purkinje cell from P11 to

P17. The CFs of a transgenic mouse line expressed enhanced green fluorescent protein (EGFP). A subset of CFs was labeled with a second, red

fluorescent molecule, by injecting the inferior olive with tetramethylrhodamine (TMR). Therefore, all CF fibers fluoresced green, but a subset of axons

could be distinguished because they fluoresced orange (i.e., the combination of green and red). Both CFs innervate a Purkinje cell body at P11.

However, the green CF gradually translocates to the Purkinje cell dendrite by P17, while the orange CF is eliminated. Scale bar is 10 μm. (C) The larger

of two motor axons (orange) innervating a muscle cell is damaged with laser-targeted ablation (circle, arrowhead), and is no longer present by 1 h.

When the same region was imaged 24 h later, the smaller motor axon (green) had come to occupy the entire NMJ. Scale bar is 20 mm. (D) The larger

of two CFs (orange, arrowhead) innervating a Purkinje cell is selectively photo-ablated at P11, and gradually dies away. When the same region is

imaged over the next few days, the smaller CF (green) takes over the Purkinje cell and comes to occupy the dendrite on P15 and P17 (arrowhead).


Fig. 9.7 Decreased activity prevents synapse elimination. At the rat nerve-muscle junction, polyneuronal innervation declines between 10

and 15 days postnatal (black line). When a TTX cuff is placed around the motor nerve root from postnatal day 9–19 and action potentials

are eliminated, polyneuronal innervation remains high (red circles).


Fig. 9.8 Increased muscle activity accelerates synapse elimination. Most rat soleus muscles are polyneuronally innervated between

postnatal days 8–10 (black line). When a stimulating electrode is implanted in the leg to activate the sciatic nerve and muscle from

postnatal days 6–8, there is a severe decline in the number of polyneuronally innervated muscle cells (red circles).


Fig. 9.9 Synapse elimination depends on distance between contacts. Two motor axons were positioned on the same muscle, either close

to one another or at a distance, and intracellular recordings were made to monitor polyneuronal innervation, as shown in Fig. 9.2. When

the synapses are close together (blue circles), synaptic elimination occurs within a few weeks. When the synapses are far apart (red

circles), synaptic elimination fails to occur.


Fig. 9.10 Ocular dominance properties of visual cortex neurons in normally reared cats. (A) The visual system received normal

stimulation until the time of recording. (B) Single neuron recordings were made with an extracellular electrode that passed tangentially

through the cortex. Neurons respond to only one eye in Layer IV (monocular). In Layers I–III and V–VI, neurons respond to both eyes

(binocular) due to convergent connections. In normal cats, the terminal stripes from each eye-specific layer of the LGN occupy a similar

amount of space. (C) Each neuron was characterized as responding to a single eye (monocular), or responding to both eyes (binocular).

In normal animals, most visual cortical neurons are binocular.


Fig. 9.11 Thalamic innervation and ocular dominance properties of visual cortex neurons in monocularly deprived cats. (A) The visual

system received normal stimulation through one eye, and the other eye was kept closed until time of recording. (B) The terminal stripes

from the deprived contralateral eye became much narrower (red), and the visual response of neurons outside of Layer IV was more

responsive to the eye that remained open during development (blue). (C) In monocularly deprived cats, the vast majority of cortical

neurons respond to the open eye only (blue).


Fig. 9.12 Thalamic innervation and ocular dominance properties of visual cortex neurons in binocularly deprived cats. (A) Both eyes were

kept closed until time of recording. (B) The terminal stripes from each eye occupied a similar amount of space. (C) The majority of visually

responsive neurons were driven by both eyes.


Fig. 9.13 Ocular dominance properties of visual cortex neurons in cats reared with artificial strabismus. (A) One eye is surgically

deflected, such that a visual stimulus activates an inappropriate position on the misaligned eye. That is, individual cortical neurons are not

activated by both eyes at the same time. (B) The terminal stripes from each eye occupied a similar amount of space. However, neurons

outside of Layer IV did not receive convergent input from both eyes. (C) In strabismic cats, the vast majority of cortical neurons responded

to either one eye or the other.


Fig. 9.14 Visual experience influences intrinsic cortical projections. (A) When a dye injection (green) is made into superficial layers of the

visual cortex of normal cats, the label is retrogradely transported by neurons in ocular dominance columns from both eyes. (B) In cats

reared with artificial strabismus, the label is retrogradely transported only by neurons that share the same ocular dominance.


Fig. 9.15 Visual experience influences the refinement of retinal ganglion cell (RGC) connections in the Xenopus visual midbrain. (A) The

schematic illustrates that the direction of visual motion will sequentially activate RGCs from posterior to anterior retina. RGC axons form a

topographic map in the optic tectum, with anterior retina projecting to caudal tectum. In this experiment, RGC terminals were labeled with

a fluorescent molecule (tdTomato) and synaptic terminals were identified with fluorescent labeling for Synaptophysin. (B) Tadpoles were

reared in either of two conditions: visual stimuli moved in a biologically normal direction from front to back (left, top) or in an abnormal

direction from back to front (right, top). The rearing stimuli were present for 4 days at 10 h per day. When animals were reared with the

normal direction of motion, the RGC soma position was well-correlated with their appropriately positioned terminals along the rostrocaudal

axis of the tectum (left, bottom). In contrast, when animals were reared in an abnormal direction of motion, the RGC projections did not

refine their terminal arbors properly along the tectum (right, bottom), yielding poor retinotopic maps. (C) A conceptual model of visual

activity-dependent refinement of RGC terminals suggests that small terminal branches are added randomly along the rostrocaudal axis,

but are selectively retracted in an activity-dependent fashion. When two RGC terminals compete, the later activated terminal displays a

selective retraction of rostral branches.


Fig. 9.16 The sound rearing environment influences tonotopic maps. (A) Developing animals were exposed to a specific acoustic

environment during development. (B) The two lobes of the inferior colliculus (IC) are shown in green, as is the approximate region of the

right auditory cortex (AC). (C) The IC tonotopic map was reorganized when mice were reared in a two-tone environment. Three-

dimensional maps of the inferior colliculus show the region activated by 16 kHz (green) and 40 kHz (red) tones at two postnatal ages, P13

and P19. When animals aged P9–17 were exposed to a synchronous presentation of the two tones, 16 and 40 kHz, a significant volume

of IC became responsive to both rearing frequencies. (D) Representative cortical tonotopic maps from a naïve rat and a rat that had been

exposed to 7.1-kHz tone pulses from P9–30. The average frequency to which each area responded is represented by color. The tone-

exposed animal displayed an expanded region near the rearing tone frequency (green).


Fig. 9.17 Visual experience influences the alignment of visual and auditory maps in the ban owl midbrain. (A) Immature barn owls were reared with

prismatic goggles that shifted the visual field by 23 degrees. In control animals, a squeaking mouse in front of the owl would appear at 0 degree, and the

sound would reach both ears simultaneously (0 μs interaural time difference, black arrows). With prisms in place, a squeaking mouse at 23 degrees to the

left would appear at 0 degree (blue mouse), but the squeak would reach the left ear first (60 μs interaural time difference, black arrows). (B) A tracer was

injected into the auditory nucleus (ICC, central nucleus of the inferior colliculus) that encodes the ITD of the sound stimulus (left). The IC relays this

information to the ICX where the map of auditory space is assembled. ICX then projects to the optic tectum where visual and auditory information is first

integrated (right). (C) In control animals, the projection from a region in ICC that responds to a 20 μs interaural time difference projects to a narrow region

in the ICX (and ICX then projects to an optic tectal location that responds to visual cues about 8 degrees from the midline). In prism-reared animals, ICC

projected to a more rostral region of ICX. Thus, a new projection is formed within the auditory space map which compensates during prism rearing such

that sound and sight can once again be integrated.


Fig. 9.18 The visual environment influences orientation and motion processing. (A) Kittens were reared with goggles that permitted visual

experience with only vertically oriented stimuli. The response to oriented stimuli was then obtained from single visual cortex neurons. The cell

shown (middle) responds best to vertical stimuli. In goggle-reared cats, 87% of neurons were selective for vertical stimuli, compared to only

50% in controls. (B) Kittens were reared in stroboscopic light that permitted visual experience, but only with stationary objects. The response to

moving stimuli was then obtained from single visual cortex neurons. The cell shown (middle) responds best to stimuli moving upward. In strobe-

reared cats, only 10% of neurons were motion selective, compared to 66% in controls.


Fig. 9.19 The selective response of visual cortex neurons to direction of motion emerges after eye opening, and requires visual experience. (A) Stimulus evoked cortical

activity is obtained by monitoring changes in the reflectance of light by the cortex (top). The response of visual cortex to oriented stimuli is shown at two ages, and in a visually

deprived animal (middle). There are already specific responses to vertical (white regions) and horizontal objects (black regions) at the time of eye opening, postnatal day 30

(blue boxes). Specific response to orientation can emerge in the absence of visual experience (dark-rearing). The response of visual cortex to moving stimuli is also shown at

two ages, and in a visually deprived animal (red boxes). There is little response specificity to vertical motion at the time of eye opening, and it emerges over the next several

days. Furthermore, visual deprivation prevents the emergence of direction-specific responses. The graph shows the average amount of response selectivity for orientation

and direction of motion during development (bottom). Orientation selectivity begins to emerge prior to eye opening, and direction-selectivity emerges afterwards. Orientation

selectivity is intact, but somewhat reduced in dark-reared animals (blue square), while direction selectivity is abolished (red circle). (B) Direction selectivity was monitored in

the visual cortex of ferrets that had less than 1 day of visual experience, and moving visuals were presented over the course of several hours. As illustrated by this example,

there was little evidence of direction selectivity in the first 8 h, but strong selectivity began to emerge after 10–12 h of stimulation.


Fig. 9.20 Spontaneous activity in the developing visual and auditory systems. (A) Retinal explants were obtained from neonatal ferrets and placed on an array

of electrodes to record bursts of spontaneous activity from several retinal locations at the same time (for clarity, only three locations are shown). These bursts of

activity moved across the retina, as shown by the increasing response latency in each oscilloscope trace (left). Electrode array recordings are plotted for mouse

retina from postnatal day 9–15 (right). Each square shows the spatial pattern of activity across the two dimensions of the flattened retinal, with circles

representing individual neurons (circle radius is proportional to firing rate, with a maximum of 20 spikes/sec). By P15, the spontaneous activity no longer travels

across the retina. The spatial activity patterns were acquired every 4 s. (B) Spontaneous retinal waves drive activity in the superior colliculus (SC) and visual

cortex (V1). Wide-field calcium imaging was used to measure fluorescence from a calcium indicator dye in both the SC and V1 (left). A sequence of images is

shown for a P9 mouse (right), and the white numbers show time in seconds. Retinal-driven activity progresses across both the SC and V1 (arrows). (C) The

cochlea is removed prior to the onset of hearing and placed in vitro to visualize inner hair cells (IHC) and auditory neurons (AN). The schematic illustrates that

supporting cells release ATP which leads to the activation of adjacent IHCs. (D) Recordings obtained simultaneously from an IHC (blue trace) and a nearby AN

fiber (black trace) displayed a correlated bursting response (left). When an antagonist of ATP receptors was added to the bath, spontaneous activity was

eliminated (right).


Fig. 9.21 Spontaneous activity in the developing cortex. (A) Hindlimb twitching during sleep drives activity in motor cortex (M1).

Recordings were obtained from M1 of anaesthetized rats at P8–10 as they cycled between sleep and waking states. The activity of leg

muscles was monitored with an electromyogram (EMG). While animals were awake, the hindlimbs moved freely (red line) and EMG

activity was continuous, but little activity was recorded in M1. In contrast, when animals were asleep, the hindlimbs twitched (red tick

marks), there were brief bouts of EMG activity, and M1 was very high. The histogram of M1 activity (right) shows that discharge rate

increased immediately after each twitch. (B) Brain slices were obtained from rats during the first postnatal week, and intracellular calcium

was monitored using a Ca-sensitive indicator (top). Waves of spontaneous activity swept across the cortex from caudal to rostral (bottom).


Fig. 9.22 Spontaneous retinal activity regulates the formation of stripes. (A) Cats were reared in the dark, and 3H-proline was injected into

one eye to visualize ocular dominance columns. Although the columns were slightly degraded, they did form. (B) Bilateral intraocular TTX

injections were performed beginning at postnatal day 14. When 3H-proline was injected into one eye to visualize ocular dominance

columns, it was found that segregation of geniculate afferents into stripes failed to occur. Thus, spontaneous retinal activity is sufficient to

influence stripe formation.


Fig. 9.23 Critical periods for the effects of monocular deprivation. One eye was sutured shut in macaque monkeys at different postnatal

ages. The sutures were removed after several months, and ocular dominance columns were examined with anatomical (left) and

electrophysiological techniques (right). When deprivation began between 0 and 2 months of age, most neurons subsequently responded

to the open eye only, and the open eye occupied much more of Layer IV. The effects of deprivation declined with age. The anatomical

effect of deprivation on ocular dominance columns declined more rapidly than the physiological effect on ocular dominance histograms.


Fig. 9.24 Synaptic inhibition regulates the critical period for ocular dominance plasticity. (A) In normal mice, recordings from the binocular

region of visual cortex (top) yield an ocular dominance histogram that is biased towards the contralateral eye (bottom left). When juvenile

mice were monocularly deprived (MD) between 19–32 days, the ocular dominance histogram was shifted towards the nondeprived eye

(bottom right, blue-shaded bars). The distribution of cells in the control histogram is shown for comparison (dashed line). (B) When adult

animals were monocularly deprived, there was no change to the ocular dominance histogram. In contrast, mice lacking the GABA-

synthesizing enzyme, GAD65, continued to display an effect of MD. (C) Embryonic inhibitory neurons can be obtained from their zone of

proliferation (top), the medial ganglionic eminence (MGE), and transplanted into the visual cortex at P10 (bottom). These animals also

continued to display an effect of MD after the normal critical period ended.


Fig. 9.25 NMDA receptors mediate developmental plasticity. (A) The NMDA receptor is activated by a combination of glutamate binding and

membrane depolarization. A positively charged magnesium ion blocks the NMDA receptor channel at rest. Depolarization causes the

magnesium ion to be ejected, and this permits sodium and calcium ions to flow in. (B) When a third eye was implanted into a frog embryo, the

tectum became coinnervated by two eyes. The afferents from each eye segregated into stripes (top left). An image of the dorsal surface of the

tectum shows the pattern of innervation when the retinal ganglion cell afferents from one eye were labeled (bottom left). When three-eyed frogs

were treated with a NMDAR antagonist, the afferents did not segregate, and stripe formation was prevented (middle). When three-eyed frogs

were treated with NMDA, stripe formation was enhanced (right), and the border between stripes became sharper.


Fig. 9.26 Heterosynaptic depression at the developing neuromuscular synapse in vivo and vitro. (A) The lumbrical muscle in the rat foot is innervated by

two physically separate motor nerve roots, LP and SN. In control animals, stimulation of the SN motor nerve root (red) elicits a robust contraction of the

lumbrical muscle (left). However, when the LP motor nerve root (blue) was repetitively stimulated during the developmental period when synapse

elimination occurs, then SN nerve root stimulation elicited a much weaker contraction of the lumbrical muscle (right) when it is subsequently tested.

Therefore, stimulation of one set of synapses leads to a decrease in the strength of a second set of synapses, referred to as heterosynaptic depression.

(B) Whole-cell recordings were made from Xenopus muscle cells that were coinnervated by two neurons (left). Evoked synaptic currents were first

measured in response to stimulation of each neuron (top left). A strong stimulus was then applied to neuron 2 (blue), and the strength of each neuron

tested again. Stimulation of neuron 2 led to smaller evoked synaptic currents from the unstimulated neuron 1 (red) but did not change the neuron 2-

evoked response (top right). The graph of ESC amplitude before and after stimulation of neuron 2 shows that the depression was long-lasting (bottom

right).


Fig. 9.27 Transient expression of long-term depression may mediate synapse elimination (A) Long-term depression (LTD) declines with age in Layer IV of visual

cortex. At postnatal day 6, low-frequency stimulation of the thalamic afferents induced long-lasting depression of excitatory synaptic currents. By P90, the same

treatment has no effect. The probability of detecting LTD is nearly 90% in tissue from juvenile animals but is negligible in young adults. (B) Contacts between

presynaptic terminals (red) and postsynaptic spines (green) were monitored before and after the induction of LTD in a hippocampal slice (top). The presynaptic

contact withdrew following LTD induction. When this experiment was repeated many times, it was found that those synapses displaying the greatest depression were

associated with a decrease in colocalization of pre- and postsynaptic elements (red shading, bottom left). In contrast, those synapses displaying the greatest stability

of response were equally likely to increase their colocalization (green shading, top right). (C) LTD was impaired in animals lacking major histocompatibility class I

(MHCI) proteins. Optically evoked LTD is ordinarily observed at retinogeniculate synapses in wild-type (WT) mice. However, when two MHCI genes were deleted

(KbDb −/−), the LTD was eliminated. (D) During normal development, ipsilateral retinal ganglion cell (RGC) projections to the lateral geniculate nucleus (LGN) initially

innervate territory belonging to the contralateral eye and are subsequently eliminated (top). Images show ipsilateral (green) and contralateral (red) RGC projections

to the LGN in WT mice, and those missing two MHCI genes (KbDb −/−). The ipsilateral projection failed to complete its normal period of elimination and refinement in

KbDb −/− mice.


Fig. 9.28 Synapse depression depends on postsynaptic calcium. (A) A whole-cell recording was made from an innervated muscle cell

that was filled with “caged” calcium. Intracellular calcium is elevated when UV light releases the calcium from its “cage” (left). Under these

conditions, nerve-evoked synaptic currents became depressed (middle). Baseline nerve-evoked synaptic currents were recorded from the

muscle cell during a control period (black downward deflections beneath each dot which represent the stimuli). When intracellular calcium

was elevated by exposure to UV light, the nerve-evoked synaptic currents (red) became depressed within seconds. This suggests a

model in which elevated calcium triggers the release of a retrograde factor that leads to reduced presynaptic transmitter release (right).

(B) When the presynaptic nerve was stimulated during the UV-evoked rise in calcium (left), synaptic depression was prevented (middle).

In this case, the model suggests that presynaptic activity can interfere with the retrograde signal (right).


Fig. 9.29 Heterosynaptic depression is associated with a loss of postsynaptic AChRs. (A) The schematic shows muscle cells that are coinnervated by

two neurons in vitro. When one neuron is stimulated for 1–2 h, the nonstimulated neuron displays heterosynaptic depression and a loss of AChRs (red)

beneath its synapse. The images show staining for nerve (green) and AChR clusters (red) beneath the unstimulated synapse before (left) and after

(right) heterosynaptic depression. AChR aggregates are stained with rhodamine-bungarotoxin. (B) Model of activity-dependent synapse elimination. The

active terminal increases PKA and PKC locally. PKC phosphorylates AChRs beneath the inactive terminal, leading to their loss. However, PKA activity

can protect the AChRs beneath the active terminal. PKC may become activated by calcium influx, whereas PKA may become activated by a ligand that

is released along with ACh. (C) Synapse elimination is induced by proBDNF. Immunostaining of the nerve-muscle junction reveals axons and terminals

(green) and postsynaptic AChR clusters (red). At postnatal day 6, the nerve terminals were induced to retract during the in vivo application of proBDNF.

(D) A model of synapse elimination suggests that postsynaptic release of proBDNF can act through its receptor complex (p75NTR and sortillin) on

presynaptic terminals to induce elimination. However, when proBDNF is proteolytically converted, the mature BDNF can act via its receptor (TrkB) to

stabilize presynaptic terminals.


Fig. 9.30 Activity-dependent synapse formation and cell adhesion molecules. (A) In wild-type flies (left), muscles 6 and 7 receive about

180 boutons, and the expression of FasII (blue) is relatively high. In an eag Shaker mutant strain of flies (right) with increased activity,

FasII levels are lower and about 70% more boutonal endings are formed. (B) A model illustrates that synaptic activity leads to calcium

influx and activation of the kinase, CaMKII (left). The active CaMKII phosphorylates a protein that mediates clustering of synaptic proteins

(DLG), including the cell adhesion molecule, FasII. The phosphorylated DLG is not as restricted to the synaptic complex, and FasII may

no longer be located at the synapses, leading to less adhesion and sprouting of new boutons (right). Synaptic sprouting is induced by

Glass bottom boat (Gbb), a TGF-β/BMP ligand, via an interaction with a presynaptic TGF-β receptor, Wishful thinking (Wit).


Fig. 9.31 GABA transmission is required for normal inhibitory synapse formation. The axonal projections of cortical inhibitory neurons was

analyzed in control mice, and compared to animals in which the GABA synthesizing enzyme, GAD67, was inactivated. The axonal

projections (green) of control inhibitory cells (top left) were much denser compared to those in GAD67 −/− mice (bottom left). On average,

the control axons targeted about twice as many postsynaptic cell bodies. The number of inhibitory terminal boutons per postsynaptic

pyramidal cell also declined by about 50%.


Fig. 9.32 Activity-dependent elimination of inhibitory connections during development. (A) Animals can locate a sound source based on the sound intensity

difference between the two ears (top). A nucleus in the auditory brainstem, the lateral superior olive (LSO), encodes intensity differences by integrating

excitatory input driven by the ipsilateral ear with inhibition driven by the contralateral ear (bottom). This inhibition is mediated by MNTB neurons which map

tonotopically onto the LSO. (B) The development of two MNTB arbors is shown schematically (top). During the period prior to hearing, the number of functional

connections between MNTB and LSO decreases dramatically (P13 control, dashed lines). During the period after hearing onset, MNTB axonal arbors are

eliminated anatomically (P21 control). The anatomical elimination was assessed by measuring individual MNTB terminals arbors and presynaptic boutons at

each age (bottom). In AChR α9 knockout mice, the spontaneous activity pattern is altered, but the absolute amount of activity remains unchanged. This

manipulation had no effect on MNTB innervation through P13, but interfered with the anatomical refinement of MNTB terminals during the third postnatal week.

(C) MNTB terminals are also eliminated in a second postsynaptic target, the MSO. At birth, inhibitory terminals are located on the soma and dendrites of MSO

neurons. However, most of the dendritic synapses are eliminated during postnatal development (top left). The micrograph (top right) shows stained MSO

neurons and glycine receptors from an adult animal. The glycine receptors (yellow) are largely restricted to the soma, and very few remain on the dendrites

(blue). When animals were deafened unilaterally during development, the elimination of inhibitory synapses failed to occur (bottom left). The micrograph shows

that, following deafening, significant glycine receptor staining (yellow) remained on the dendrites (bottom right). Scale bars are 20 μm.


Fig. 9.33 Homeostatic plasticity of excitatory and inhibitory synapse function in the developing auditory cortex. (A) The schematic shows the orientation

of a brain slice containing the project from auditory thalamus (MG) to auditory cortex (ACx). Stimuli delivered to the MG activate excitatory afferents

(green) and excitatory synaptic currents can be recorded in the ACx. Stimuli delivered within the ACx activate inhibitory afferents (red) and inhibitory

synaptic currents can be recorded. (B) A comparison is made between evoked synaptic currents in control animals (Ctl) and those raised with bilateral

hearing loss (HL). Electrical stimuli are delivered just above threshold to activate a single excitatory or inhibitory afferent. The traces and bar graph

illustrate that developmental HL induced an increase in the amplitude of evoked excitatory synaptic currents (green), yet caused a decrease in the

amplitude of inhibitory currents (red). (C) To determine whether the onset of visual experience influences auditory cortex development, the eyelids were

either opened prematurely (left) or eyelid opening was delayed (right). Normal eyelid opening occurs at ~ postnatal day 18 in gerbils. (D) The effect of

eyelid opening was tested by determining whether the auditory cortex remained vulnerable to hearing loss induced with bilateral earplugs. When the

eyelids were opened prematurely, the auditory cortex lost its sensitivity to hearing loss by P15. However, when the eyelids were glued shut, the auditory

cortex remained vulnerable to hearing loss through P19, after the critical period would ordinarily have closed.


Fig. 9.34 Increased auditory activity during development influences cortical vascular development. When mice were reared with 10 h of

daily auditory stimulation from P15 to 25, there was a significant reduction of blood vessels in auditory cortex (A1). (A) The images show

the microvasculature in A1 of sound-reared and control mice. The blood vessels were visualized with an immunohistochemical stain for

collagen IV. (B) The vasculature was quantified by measuring blood vessel branch points. A significant reduction was observed in A1

animals exposed to sound from P15–25. However, blood vessels in a control region, the cingulate cortex, were not affected. Furthermore,

when 9-week old adults were exposed to the same treatment, the A1 vasculature was not affected. Scale bar, 200 μm.


Fig. 9.35 Synaptic activity regulates dendrite morphology. (A) Neurons of the chick nucleus laminaris (NL) receive afferents from

ipsilateral NM to their dorsal dendrites and from the contralateral NM to their ventral dendrites (left). The ventral dendrites of NL neurons

were denervated by cutting NM afferent axons at the midline. The dorsal dendrites remained fully innervated. When NL neurons were

stained with the Golgi technique, the ventral dendrites were found to be significantly shorter than those on the dorsal side (right). (B) The

ventral dendrites shrunk by almost 40% by 96 h after the lesion, and this effect can be measured within 1 h of the manipulation. (C) Two-

photon imaging of dendritic spine and filopodia motility in vivo shows the age-dependent effect of visual deprivation. Two time-lapse

sequences are shown (acquired over 2 h). In the first, a spine retracts (top), and in the second a spine elongates (bottom). (D) Binocular

deprivation from P13 significantly increases spine motility at P28 by 60%. However, there is no change in motility when animals are

imaged at P21, and only a slight reduction in motility in mice imaged at P42.

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