Behavioral Neuroscience Introduction to Neuroscience:

Sexual Dimorphism in Brain and Animal Behavior (Hormonal mechanism of behavior)

Tali Kimchi Department of Neurobiology Tali.kimchi@weizmann.ac.il

Sexual Dimorphism

Sexual dimorphism is the difference in form between male and female members of the same species

Sexual dimorphism in body character

Sexual Dimorphism in Animal Behavior

Sexual Dimorphism in Social and Sexual Behavior

Territoriality (aggressive) Behavior

Pup Nursing and Maternal aggression

Courtship Behavior Sexual Behavior

Nature versus Nurture

“Males Toys” “Females” Toys

Tim

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The role of sex hormones in regulation of

sexual dimorphism in Behavior

Arnold A. Berthold (1803-1861)

In 1849, Berthold conducted the first formal experiment in behavioral endocrinology

Hypothesis: Intact testes are necessary for the development of male-typical morphological and behavioral characters.

Castration Castration +

Transplantation

in the same body

Castration +

Transplantation

in other body

Findings summary: Males that were castrated as juveniles later showed deficits as adults in males-typical body characters and in behaviors such as aggression, mating and crowing. -All of these effects could be reversed if the subject’s testes, or the testes of another male, were implanted into the body cavity.

Conclusion: Testes influence morphology and behavior not by the actions of nerves, but by secreting a substance into the bloodstream (i.e. hormones).

The first experiment in Neuroendocrinology (Berthold, 1849)

Ernest Henry Starling (1866-1922), English Psiologist

Starling (1905); Lancet

The first to use the term hormone. “Hormones” from Greek “ to excite”

Hormones: Blood borne chemical communication molecules

Sexually dimorphic social and sexual behaviors in rodents are all regulated by hormones

Aggressive behavior

Sexual behavior Maternal behavior

Maternal behavior in postpartum female rats

Pup retrieval Pup licking

Nest building Pup nursing

Terkel and Rosenblatt (1968)

Lactating female

Virgin female

Blood was transfused from a parturient female (one that had given birth within 30 min of the onset of the transfusion) into a virgin female.

The maternal behavior of the virgin toward newborn pups was facilitated when compared to the response of a virgin female that was transfused with virgin blood.

Prolactin level during pregnancy and postpartum

• Removal of litters from mother rats in the beginning of postpartum resulted in a rapid decline in serum prolactin, reaching pregnancy levels 3 hr later.

• Pregnancy levels: 8.3 ±0.1 ng/ml.

• 8-14 Postpartum days: 65.5 ±19.0 ng/ml

• ~15 Postpartum days: 25.7 ±5.5 ng/ml

• When litters of 10 pups each were returned to their mothers for 3 hr of suckling after 12 hr of non-suckling, serum prolactin increased precipitously to 130.3 ±19.6 ng/ml,

Amenomori et al 1970; Endocrinology

The role of prolactin in maternal behavior

It was showed that hypophysectomy (removing the pituitary gland) delayed the onset of maternal behavior in estrogen-treated females.

When the hypophysectomized females were injected

with prolactin or were implanted with a pituitary gland in the kidney capsule, where it secretes large amount of prolactin, short-latency maternal behavior was restored in females that had been primed with estrogen.

Bridges et al. 1990; PNAS

Vasopression Oxytocin

Adrenocorticotropic hormone (ACTH) Thyroid-stimulating hormone (TSH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Growth Hormone (GH) Prolactin (PRL)

The Hypothalamus-Pituitary-Gonadal Axis

The Hypothalamus-Pituitary-Gonadal Axis

• The brain is the overall controller of circulating gonadal steroids

• Gonatopropin Releasing Hormone release by hypothalamus to stimulate anterior pituitary

• Gonatoproph cells in anterior pituitary release Luteinizing Hormone (LH) & Follicle- Stimulating Hormone (FSH).

• LH and FSH stimulates the gonads (Testes and Ovaries).

• Sex hormones (testosterone, estrogen, progesterone) release from the gonads feedbacks to influence brain function, particularly those relating to reproduction

The Sex Hormones

“Male” (andrerogenic) sex hormone

-Also being secreted by

the adrenal gland

“Female” (estrogenic) sex hormone

* Both are steroid hormones and secreted in both sexes

Steroids are lipophilic, low-molecular weight compounds derived from cholesterol that are synthesized in the endoplasmic reticulum of cells in the gonads and adernal cortices and are then released into the blood circulation.

Steroid Hormones

Males: The Hypothalamus-Pituitary-Gonadal Axis

Intact males

Castrated males

Testosterone treatment

Effect of castration & testosterone treatment on male rodent (guinea pigs)

•In all rodents (mammals), gonadectomy decreases (abolish) male courtship and sexual behavior.

•Testosterone replacement reinstates sexual behavior in males.

Females: The Hypothalamus-Pituitary-Gonadal Axis

Estrous cycle begins with secretion of gonadotropins from the hypothalamus, which stimulate the growth of ovarian follicles, and ovulation; the ruptured ovarian follicle becomes a corpus luteum and produces estrodiol and progesterone.

The Hypothalamus-Pituitary-Gonadal Axis and estrous cycle of female rat

Hormonal activation of female-typical sexual behavior

•In all rodents, gonadectomy decreases (abolish) female sexual receptivity.

•Estrogen and progesterone replacement reinstates sexual behavior of females.

William C. Young (1899-1965)

Endocrinology, 1959

• Young demonstrated that perinatal exposure of female guinea pigs to elevated androgens permanently suppressed their capacity to display feminine sexual behavior (defeminization) and significantly enhanced their display of masculine sexual behavior (masculinization).

• It was suggested that the exposure to prenatal androgens had permanently altered the brain tissues underlying sexual behavior and that, similarly to the peripheral sexual organs, androgens ‘organized’ the developing nervous system at a critical period of early development.

The organization/activation hypothesize

•Sex hormones act during prenatal stage to permanently (irreversibly) organize the nervous system in a sex-specific manner •During adult life, the same hormones have activation effects, causing it to function sex-typical manner in adulthood

Organization and activating effects of hormones

Sex hormones can have the following effects:

1. Organizing effects- occur mostly at sensitive stages of development.

-Determine whether the brain and body will develop male or female characteristics

2. Activating effects- occur at any time of life and temporarily activate a particular response.

- Genetic disease (CAH)Congential Adrenal Hyperplasia

Elevated exposure to testosterone during development

Male-typical games

ק שח

מן מז

)%(

Girl with CAH

Healthy Girls

Healthy Boys

Boys with CAH

Female-typical games

Embryonic gonads

-

SRY

+ Testes Ovary

XX ♂ XY

FEMALE MALE

Organizing hormonal effects (development)

Activating hormonal effects (adult)

Feminization Masculinization

The organization and activation prevailing model (morphology characterization)

XY XX Sry XY XX XX Sry

Genotypically Female Mice Transgenic for SRY are Phenotypically Male

The Klinefelter syndrome, also known as the XXY genetic disorder

symptoms include:

1. reduced fertility or full infertility to some

extent.

2. Female-typical body charectristics

The organization and activation prevailing model of the brain

The default sex in mammals is female. The differences between male and female behaviors are almost entirely a consequence of early-age exposure to testosterone.

Testosterone (Estrogen)

Low Estrogen

XY XX

Feminization Masculinization

Gender-specific phenotype

Testosterone

Brain Differentiation

Brain Activation

Estradiol

Sex Chromosome Genes P

erinatal

Adult

Organizing hormonal effects

Activating hormonal effects

SRY

♂ ♀

Hormones Hormones

Organization Activation

How sexually dimorphic behaviors are encoded in the brain of males and females?

SEX

Beer

Commitment

neuron

NOTHING

LOBE

SPEECH

SKILLS

Male Brain Female Brain

Bed Nuclei of the Stria Terminalis (BNST) Sexual Dimorphic-Nucleus of Preoptic Area (SDN-POA) Posterodorsal Medial Amygdala (MePD)

Sexual Dimorphic Brain Nuclei in Rodents (rat/hamster)

Anteroventral Periventricular Nucleus (AVPV)

Larger in male

Larger in Female

AVPV

Axonal projection from the BNST

♂ ♀

♂ ♀

Sexual Dimorphism in Brain Morphology

Cell number in the AVPV

♂ ♀

Tyrosine Hydroxylase

Estrogen Receptor β

AVPV

POA

BNST

Androgen Receptor

♂ ♀

Vasopression fiber in the Lateral Septum (LS)

Sexual Dimorphism in Brain Gene Expression

Xu et al 2012; Cell

Xu et al 2012; Cell

Steroids are lipophilic, low-molecular weight compounds derived from cholesterol that are synthesized in the endoplasmic reticulum of cells in the gonads and adernal cortices and are then released into the blood circulation.

Steroid Hormones

• Testosterone treatment in neonatal rats is blocked by prior administration of specific estrogen receptor antagonist.

• DHT does not mimic the effect of testosterone.

• Radio-labeled testosterone is recovered from the brain as radio-labeled estradiol.

• Aromatase inhibitors counteract the effect of testosterone administration.

Estradiol masculinizes the brain Testosterone, Estradiol or DHT masculinizes the brain ?

Estradiol production by the fetal ovaries is minimal

Circulation of α-fetoprotein (AFP) is present at high levels in embryos

AFP = Fetal plasma protein that binds estrogens with high affinity and prevents it’s passage through the placenta.

Why female brain is not masculinized by estrogen?

Alpha-fetoprotein (AFP) role in female’s brain development

Tyrosine Hydroxylase (TH) gene expression in the hypothalamus (AVPV)

ATD= Aromatase inhibitor

Baker et al 2005; Nature neuroscience

Female-typical behavior Male-typical behavior

Cell death and sexually dimorphic brain nucleus (SDN-POA)

Cell number in the AVPV

♂ ♀

Cell death (Bax gene) is involved in brain developmental organization

Forger et al 2004; PNAS Jyotika et al 2007; Dev. Neurobiol.

Cell Number in AVPV TH gene expression in AVPV

Female-typical sexual behavior

* Gonadectomized+ estrogen+progesterone treatment in adulthood

• Prenatal testosterone treatment increased SDN volume in female rats but do NOT lead to increase in masculine sexual behavior.

• Treating males prenatally with aromatase inhibitor reduced SDN volume but do NOT (little) effect male sexual behavior or lead to increase in feminine sexual behavior.

De Vries et al 2002 J Neuroscience

Nu

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Evidence for the affect of Y-linked genes on sexual dimorphism of the brain

♂ ♀

♂ ♀

Chemical (odor) signals that are emitted by animals to communicate information to their own species

What are pheromones?

What are pheromones?

Pheromone signals are largely involved in the regulation of social and reproductive behaviors in most animals (including in human)

Attracting mate partner Pups recognition Male territoriality

Primer effects: induce sequence of slow long-lasting physiological and neuroendocrine responses

Releaser effects: induce relatively rapid, fixed, behavioral responses

Ultrasonic vocalization in the presence of female

Aggressive behavior toward intruder male

Mating behavior

Pheromone effects in rodents

Aggressive behavior of lactating female

Maternal behavior (e.g. pups retrieval)

Bruce effect: Recently mated female will return to estrous if exposed to strange male (pregnancy block)

Whitten effect: Induction of estrous in group-housed females by exposing to male (urine)

Lee-Boot effect: Grouping several (8-12 individuals) females in a cage results in suppression of their estrous cycles

Vandenbergh effect: Puberty acceleration caused by exposure to male, during female development.

Puberty-delay caused by group-housed females.

Endocrine effects: Intact male exhibit LH surge Following exposure to female mice. Female exhibit LH surge in response to male or its bedding

The olfactory systems

Liman et al. 1999

TRPC2 expression in the VNO

VNO

OB

Detection of chemosensory signals in mice

Typical male-female reproductive behaviors

♂

♀

Aggressive behavior Sexual behavior

TRPC2 mutant female (brown) with normal male (black)

Sexual behavior of TRPC2-KO lab female

Kimchi et al. 2007; Nature

♂

♀ WT

WT

TRPC2-KO

Male-typical sexual behavior in TRPC2-KO lab females

An

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(%

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ou

nti

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tim

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se

c)

Pe

lvic

th

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♀

Kimchi et al. 2007; Nature

Female

Male

Social behavior under semi-natural environment

control

mutant

Maternal Behavior

Failure to discriminate between male and female

?

Male-typical sexual behavior (courtship and mounting behaviors)

Female mutant

Normal male

Female-typical behavior (maternal behavior)

Summary-part 1: Behavioral phenotype of TRPC2-KO lab females

4 mutant males

2 control (normal) males

Pheromone inputs repress neuronal circuit for female-typical nursing behavior in males

♂

Catherine Dulac

Social and sexual behaviors of male mutant mice

Failure to discriminate between male and female

?

Aggressive behavior

Normal testosterone level

Female-typical behavior (pup caring / nursing behavior)

Model: Pheromone inputs repress neuronal circuit for male-typical sexual behavior in females

♂ Sex-specific

pheromone signals

♀ Sex-specific pheromone signals

Dulac and Kimchi, 2007; CONB

♂