Introduction to Neuroscience:
Behavioral Neuroscience
Sexual Dimorphism in Brain and Animal Behavior
(Hormonal mechanism of behavior)
Tali Kimchi
Department of Neurobiology
Tali.kimchi@weizmann.ac.il
Outline
•Introduction:
- Sexual dimorphism (Anatomically, Neurologically and Behaviorally)
- Brief history of behavioral neuroendocrinology field
- The organization/activation model of sexual dimorphism
•Can sex differences in brain structure explain sexually dimorphic behavior?
•Hormonal activating effects:
-Sexual behavior in males and females
- Maternal behavior
• Pheromones and the regulation of sexually dimorphic reproductive behaviors
Further reading:
An Introduction to Behavioral Endocrinology; Randy J. Nelson, 2005
Behavioral Endocrinology; Jill B. Becker, Marc Breedlove et al. 2002
Sexual Dimorphism
Sexual dimorphism is the difference in form between
male and female members of the same species
Sexual dimorphism in body character
Sexual Dimorphic Brain Nuclei in Rodents (rat/hamster)
Bed Nuclei of the Stria Terminalis (BNST)
Sexual Dimorphic-Nucleus of Preoptic Area (SDN-POA)
Posterodorsal Medial Amygdala (MePD)
Anteroventral Periventricular Nucleus (AVPV)
AVPV
Larger in male
Larger in Female
Sexual Dimorphism in Brain Morphology
♂
♂
♀
♀
Cell number in the AVPV
♂
♀
Axonal projection from the BNST
Sexual Dimorphism in Brain Gene Expression
Vasopression fiber in
the Lateral Septum (LS)
Tyrosine
Hydroxylase
Estrogen
Receptor β
AVPV
♂
♀
Androgen
Receptor
POA
BNST
Re
pe
rt
o
ire
s
Sexual Dimorphism in Social and Sexual Behavior
Pup Nursing and
Maternal aggression
In
na
te
Be
ha
vio
ra
l
Territoriality (aggressive)
Behavior
Courtship Behavior
Sexual 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 characters.
Findings: Males that were castrated as juveniles later showed deficits as adults in males-typical body
characters (e.g. plumage) 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).
Ernest Henry Starling (1866-1922)
The first to use the term hormone.
“Hormones” from Greek “ to excite”
Starling (1905); Lancet
Hormones: Blood borne chemical communication molecules
Sexually dimorphic social and sexual behaviors in rodents
Aggressive behavior
Sexual behavior
Maternal behavior
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 tissues underlying sexual behavior and that, similarly to the
peripheral sex 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.
The organization and activation prevailing model
XY
XX
Perinatal
Sex Chromosome
Genes
SRY
Testosterone
(Estrogen)
Masculinization
Low Estrogen
Brain
Differentiation
Testosterone
Feminization
Organizing hormonal
effects
Estradiol
Adult
Brain
Activation
Activating hormonal
effects
Gender-specific phenotype
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.
Outline
•Introduction:
- Sexual dimorphism (Anatomically, Neurologically and Behaviorally)
- Brief history of behavioral neuroendocrinology field
- The organization/activation model of sexual dimorphism
•Can sex differences in brain structure explain sexually dimorphic behavior?
•Hormonal activating effects:
-Sexual behavior in males and females
- Maternal behavior
• Pheromones and the regulation of sexually dimorphic reproductive behaviors
Hormones
Hormones
♂
Organization
♀
Activation
Steroid Hormones
Steroids are lipophilic, low-molecular weight compounds derived from
cholesterol that are synthesized in the endoplasmic reticulum of the
gonads and adernal cortices and are then released into the blood
circulation.
Estradiol masculinizes the brain
• 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.
Why female brain is not masculinized by estrogen?
 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.
Alpha-fetoprotein (AFP) role in female’s brain development
Tyrosine Hydroxylase (TH) gene expression in the hypothalamus (AVPV)
ATD=
Aromatase inhibitor
Female-typical behavior
Male-typical behavior
Baker et al 2005; Nature neuroscience
Cell death and sexually dimorphic brain nucleus (SDN-POA)
♂
♀
Cell number in the AVPV
Cell death (Bax gene) is involved in brain developmental organization
Cell Number in AVPV
TH gene expression in AVPV
Female-typical sexual behavior
* Gonadectomized+
estrogen+progesterone treatment in adulthood
Forger et al 2004; PNAS
Jyotika et al 2007; Dev. Neurobiol.
• 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 and do NOT lead to increase in
feminine sexual behavior.
The Medial Preoptic Area (MPOA) is activated by testosterone
and is essential to the activation of male sexual behavior
♀
♂
Anderogen Receptor expression in the MPOA
Sexual behavior
(pheromone inputs)
Induce c-fos in MPOA of both males and females
(c-fos is immediate early gene, indirect molecular marker
of neuronal activity)
Sexual behavior
(pheromone inputs)
Increase of neuronal firing rate in the MPOA
Castration
Castration
Abolish of
sexual behavior
Microinjection of
testosterone
into the MPOA
Reinstate
sexual behavior
Abolish of c-fos and neuronal activity
in the MPOA in response to reproductive stimuli
Outline
•Introduction:
- Sexual dimorphism (Anatomically, Neurologically and Behaviorally)
- Brief history of behavioral neuroendocrinology field
- The organization/activation model of sexual dimorphism
•Can sex differences in brain structure explain sexually dimorphic behavior?
•Hormonal activating effects:
-Sexual behavior in males and females
- Maternal behavior
• Pheromones and the regulation of sexually dimorphic reproductive behaviors
Activating (adult) effects of hormones
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) & FollicleStimulating 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 Hypothalamus-Pituitary-Gonadal Axis
Adrenocorticotropic hormone (ACTH)
Thyroid-stimulating hormone (TSH)
Follicle-stimulating hormone (FSH)
Luteinizing hormone (LH)
Growth Hormone (GH)
Prolactin (PRL)
Vasopression
Oxytocin
Activation of male-typical sexual behavior
Male: Production of testosterone from the testes is controlled by the release of
luteinizing hormone (LH) from the anterior lobe of the pituitary gland, which is in
turn controlled by the release of gonadotropin releasing hormone (GnRH) from
the hypothalamus.
Hypothalamus
External
stimuli

GnRH
(-) feedback

Pituitary
(-)

LH

Testes

Testosterone
Effect of castration & testosterone treatment
on male rodent (guinea pigs)
Intact
males
Castrated
males
Testosterone
treatment
•In all rodents (mammals), gonadectomy decreases (abolish) male courtship
and sexual behavior.
•Testosterone replacement reinstates sexual behavior in males.
T
Activation of female-typical sexual behavior
Female: The ovaries of sexually-mature females secrete a mixture of three
estrogens, of which 17β -estradiol is the most abundant (and most potent).
The synthesis and secretion of estrogens is stimulated by follicle-stimulating
hormone (FSH), which is, in turn, controlled by the hypothalamic GnRH.
Hypothalamus

GnRH
External
stimuli

Pituitary
(-)
(-) feedback

FSH

Follicle

Estrogens
The Hypothalamus-Pituitary-Gonadal Axis
and estrous cycle of female rat
ual
Sex tivity
ep
Rec
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.
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.
Gonadal Steroid Hormones
Gonadal steroids influence the sexual differentiation of the genitalia, secondary sexual
characteristics and of the brain, and contribute to the maintenance of their functional
state in adulthood and control or modulate sexual behavior of males and females.
Maternal behavior in postpartum female rats
Pup licking
In
na
te
Nest building
Be
ha
vio
ra
l
Re
pe
rt
o
ire
s
Pup retrieval
Pup nursing
Terkel and Rosenblatt (1968)
Virgin
female
Lactating
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
• 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
• 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.
• 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
Behav. Neurosci. 2001
Studying Maternal Behavior Motivation using
Conditioned Place Preference (CPP) Test



The CPP procedure assesses the preference for or the motivation to seek a
reinforcing stimulus, including a variety of natural reinforces, as well as drugs
of abuse.
Animals are given pairings of an unconditioned reinforcing stimulus with a set
of unique environmental cues that serve as the conditioned stimulus
(Pavlov’s Classical Conditioning).
The CPP method allows assessment of the preference for a reinforcing stimulus
in its absence.
CPP procedure


A
Dams were conditioned for 4 days at the early, middle or later postpartum
period. Females were exposed to unconditioned stimuli (3 pups or 10 mg/kg
cocaine) in the presence of conditioned stimuli cues for 2 h.
On postpartum day 8 (A), 10 (B) or 16 (C), the time the dams spent in each
chamber and their behavior were recorded for 1 h.
B
C
The early postpartum females preferred the pup cues, whereas the
middle and late postpartum females preferred the cocaine cues.
Mattson et al. 2001; Behav. Neurosci.
Summary


Dams in the early postpartum period (high level of prolactin) can be
considered to have a high level of motivation for maternal behavior.
Dams in the middle and late postpartum periods (mid-low level of
prolactin) can be considered to be more susceptible to the
reinforcing effect of cocaine and less motivated for maternal
behavior.
Outline
•Introduction:
- Sexual dimorphism (Anatomically, Neurologically and Behaviorally)
- Brief history of behavioral neuroendocrinology field
- The organization/activation model of sexual dimorphism
•Can sex differences in brain structure explain sexually dimorphic behavior?
•Hormonal activating effects:
-Sexual behavior in males and females
- Maternal behavior
• Pheromones and the regulation of sexually dimorphic reproductive behaviors
Pheromone effects in rodents
Releaser effects: induce relatively rapid,
fixed, behavioral responses
Ultrasonic vocalization in the presence
of female
Aggressive behavior toward intruder male
Mating behavior
Aggressive behavior of lactating female
Maternal behavior (e.g. pups retrieval)
Primer effects: induce sequence of slow
long-lasting physiological and neuroendocrine
responses
Bruce effect: Recently mated female will return
to estrous if exposed to strange male
(pregnancy block)
Lee-Boot effect: Grouping several (8-12 individuals)
females in a cage results in suppression of their
estrous cycles
Whitten effect: Induction of estrous
in group-housed females by exposing to male (urine)
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
VNO
TRPC2
Mutant female (Brown) + normal male (Black)
Male-typical sexual behavior in TRPC2 mutant female
Time (sec)
Female intruder
25
TRPC2-/- mutant female
20
Control male
15
10
5
0
Monting behavior
Pelvic thrusting
Male intruder
25
20
Time (sec)
Control female
15
10
5
0
Monting behavior
Pelvic thrusting
Semi-natural enclosure
Mutant female (Brown) + normal male (Black)
Maternal Behavior
control
mutant
Social and sexual behaviors of female mutant mice
Male-typical sexual behavior
(courtship and mounting behaviors)
Female mutant
Normal male
?
Failure to discriminate between
male and female
Female-typical behavior
(pup caring / nursing behavior)
2 control (normal) males
4 mutant males
Pheromone inputs repress neuronal circuit for
female-typical nursing behavior in males
♂
Catherine Dulac
Social and sexual behaviors of male mutant mice
Aggressive behavior
Failure to discriminate between
male and female
Female-typical behavior
(pup caring / nursing behavior)
Normal testosterone level
?
Pheromone inputs repress neuronal circuit for male-typical sexual behavior
in females, while in male it represses female-typical neuronal circuit.
Sex-specific
pheromone signals
Sex-specific
pheromone signals
♀
♂
♀