Sexual Differentiation of Brain & Physiology:
Pfeiffer (1933)
• Known: The pituitary and ovary were important for ovulation.
• Question: Are there differences between males and females in ability to support ovulation?
• Concluded: Males and female rats differ in their ability to support ovulation.
• Concluded (although incorrectly): Male rat’s pituitary was unable to support ovulation.
ovarian fragments(able to ovulate)
eyesof female
rats
showovulation
eyesof male
rats
could notshow
ovulation
Sexual Differentiation of Brain & Physiology:
Pfeiffer (1933)
• Question: Are testicular secretions during first few days of life
involved in making male rats unable to support ovulation?
• Concluded: Testicular secretions act early in development to make male rats (and even female rats) unable to support ovulation.
newborn male castratedat birth
male couldshow
ovulation
as adult,w/own ovariesor implanted
ovaries
femalecould not
showovulation
implantedw/ovariesas adults
newbornfemale
implantedw/testicular
tissueat birth
Sexual Differentiation of Brain & Physiology:Harris (1952)
• Known:
– species that are “induced ovulators”--ovulation is triggered by copulation (e.g.,
rabbit, cat); implied that the NS was involved in the process of ovulation (via
sensory input associated with copulation)
– hypothalamus (in the brain) was the likely source of control
• Question: Are differences in the ability to support ovulation between male
and female rats the result of differences in the pituitary or brain?
• Concluded: The difference between male and female rats in the ability to
support ovulation was not due to differences in the pituitary.
• Instead, differences must lie within the brain--hypothalamus.
female rat with amale pituitary
could showovulation
removedpituitary
from female rat
implantedpituitary
frommale rat
Ovulation and GnRH Surge in Rats:
Ovulation:
• as follicles develop in ovary
– increasing levels of estrogen
are released
– in female rats, increases in
estrogen lead to a GnRH
surge (positive feedback)
– GnRH surge leads to LH
surge
– LH surge leads to ovulation
• male rats are unable to show
a GnRH surge in response to
increases in estrogen
GnRH Neuron
HYPO
ANTPIT
OVARY
FSHLH
Estrogen
GnRH+
GnRH: gonadotropin-releasing hormoneFSH: follicle stimulating hormoneLH: luteinizing hormone
Sexual Differentiation of Brain & Behavior:
Males & females of a variety of species show differences in behavior.
• One example is the display of lordosis by female rodents during mating.
• Lordosis--posture shown by female rodents in which the female arches her
back to elevate the rump and head.
• Background:
– estrogen and progesterone have activational effects on female sex behavior
– ovariectomize (OVX) adult female rats-->females don’t show lordosis
– OVX adult female rats + [estrogen followed by progesterone]-->female will
show lordosis
– castrate adult male rats + [estrogen followed by progesterone]-->don’t see
lordosis
• Basic observation: the nervous system of adult males and females are
different in their behavioral responsiveness to hormones.
Sexual Differentiation of Brain & Behavior:Phoenix and associates (1959)
• Studied female sex behavior in guinea pigs--display of lordosis.
• Question: Does exposure to androgens early in development alter the
display of female sex behavior in the adult?
• Concluded: Testicular secretions act early in development to make male rats (and androgenized females) unable to show female sex behavior (lordosis). Instead, androgen exposure “organizes” the brain so that males will show male sex behavior in the adult.
newborn male
castratedat birth
male canshow lordosisin response to
mounting
givenestrogen &
progesteroneas an adult
female cannotshow lordosisin response to
mounting
givenestrogen &
progesteroneas an adult
newbornfemale
givenandrogens
at birth
Process of Sexual Differentiation:
Role of androgens in males:• permanently masculinize the brain
– male specific responses (sex behavior)
• permanently defeminize the brain– inability to support ovulation (no GnRH surge in male rats)– inability to show female sex behavior in presence of ovarian hormones
What about females?• Dogma: no androgens-->brain & behavior becomes feminized (passive)• However, recent evidence suggests that some estrogen is needed for
development of female brain and display of female-specific responses.– block interaction of estrogen with ER (antagonist) during development-->
individual that shows no female sex behavior nor ovulation
MALES
feminized
demasculinized
FEMALES
masculinized
defeminized
“active”
Critical Periods:
• brief “windows of time” when steroids can alter development of body and nervous system
• multiple “windows” are seen during development
– prenatal, perinatal and postnatal periods
• these “windows” reflect transient changes in steroid levels and require the presence of steroids receptors (ARs or ERs) and possibly converting enzymes (aromatase or 5α-reductase)
Rats:
• injections of testosterone on day of birth or up to around the 10th day of life can render a female anovulatory and unlikely to display lordosis
• however, injections of testosterone by day 12th (or after) has little effect on these measures
Paradox:
Early exposure of newborn females to elevated levels of estrogen could
masculinize the brain, behavior & physiology of these individuals as adults.
Estrogens are largely formed by the ovary and ovaries are found in females.
• Why would an ovarian hormone cause masculinization of the brain and behavior?
• Aromatization is important for masculinizing the brain in some species (rats).
– testosterone estrogen estrogen-ERs produces an effect
– need testosterone, aromatase (which will produce estrogen), and ERs
Testosterone(precursor)
estrogen
aromatase
• If estrogens can masculinize the brain, how do females normally escape
masculinization?
– ovaries of female fetuses (in utero) secrete very little steroid
– both male and female fetuses see high levels of estrogen (source: maternal ovaries,
adrenal glands)
– normally, the brains of male fetuses are masculinized/defeminized but the brains of
females are not--why?
– presence of a protein: -fetoprotein (AFP)
– AFP binds to estrogen and appears to block estrogen’s ability to reach the brain
– however, AFP does not bind to testosterone; therefore, testosterone can enter the
brain and be converted to estrogen via aromatase--masculinization of brain
– the ability of exogenous estrogen to masculinize the brain requires--high levels--
which presumably swamps the buffering capacity of AFP allowing some estrogen
to reach the brain
– estrogen and sexual differentiation:
• no estrogen ”unisex brain”
• some estrogen feminized brain
• testosterone or lots of estrogen masculinized brain
Species Differences:
There are species differences in what hormones masculinize and defeminize the
brain and behavior.
• In guinea pigs and primates:– testosterone or other androgens (dihydrotestosterone) must interact with ARs to
masculinize/defeminize the brain
– of interest, a homologue to AFP has been identified in primates but does not bind to estrogen (in rats, the AFP does bind to estrogen)
rats/hamsters
male sexbehavior
guinea pigs/primates
dependenton
aromatization
not dependenton
aromatization
Species Differences:
There are species differences in what processes are masculinized and
defeminized.
male rats
Masculinized:
male primates
male sexbehavior
true
Defeminized: no positivefeedback response
to estrogen; nosupport of ovulation
not true
Sexual Dimorphisms within Adult Nervous System:
Sexual dimorphisms have been observed in the following parameters:
• number of neurons
• size of neurons (large or small)
• number and shape of synapses
• length and branching of dendrites
• amount and type of neurotransmitters, enzymes and receptors that are expressed
Examples (shown in class):
• SDN-POA (sexually-dimorphic nucleus of the preoptic area)--sex differences in the size of nucleus
• SNB (spinal nucleus of the bulbocaveronosus)--sex differences in the presence/absence of brain nucleus
How do hormones affect sexual differentiation?
• drive neuronal cell differentiation (number of cells born), cell migration
and/or cell survival
– Ex. SDN-POA
• promote outgrowth of dendrites and axons of specific neurons
• provide target-derived neurotrophic action
– Ex. SNB
• regulate the expression of specific molecules--neurotransmitters, enzymes
and receptors
Hormones--Cell Survival:Ex. Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA) (in rats)
• SDN-POA is 3-5 times larger in males than in females
• aromatization of testosterone to estrogen is important for masculinization
• originally thought--androgens were important in stimulating the number of
neurons born in males that will migrate to, and form, the SDN-POA (book)
• however--more recent data suggest that exposure to androgens perinatally act
to increase the number of neurons that survive in males than in females
• how can we follow cell birth/survival? tritiated thymidine autoradiography
• current view: (following injection of 3H-thymidine on day 18 of gestation)
– PN4--# neurons in SDN-POA: males=androgenized females= females
– PN7--# neurons in SDN-POA: [males=androgenized females]>females
– neurons are lost in females at PN7 and PN10; exposure to testosterone (from E20
to PN10) can prevent this loss
– males have larger SDN-POA because more neurons survive into adulthood, and
also because of an increase in volume not associated with addition of more
neurons--increase in cell size (larger) and/or more connections
Tritiated Thymidine Autoradiography:• neuroblasts in the ventricular
zone will divide, differentiate into neurons, and migrate to specific areas in the brain
• during cell division, DNA is being synthesized
• 3H-thymidine will be incorporated into newly synthesized DNA
• if 3H-thymidine is injected on day 18 of gestation, then all neurons “born” on day 18 will have radioactive DNA with cell’s nucleus
• can identify “birthdate” of neurons by exposing brain sections to X-ray film or by dipping sections in a photographic emulsion
day 17
VentricularZone
day 18 day 19
DevelopingBrain
[3H]-thymidine
Gestation
• radioactivity will expose the photographic emulsion
• can learn: how many neurons are born on a given day, where they migrate, and if they survive into adulthood
• Ex. Spinal Nucleus of the Bulbocavernosus (SNB) (in rats)
• SNB is present in males and absent in females
• SNB neurons are motoneurons that innervate muscles attached to the penis (perineal muscles)
• early in development: androgens increase survival of muscles which leads to survival of motoneurons innervating the muscles
• later in development: androgens act subsequently to increase size of neurons (larger); ARs are expressed in SNB motoneurons at later time than ARs expressed in muscle
• administration of estrogen cannot masculinize SNB motoneurons (aromatization is not important); thus, androgens act directly at ARs to masculinize SNB system
SNB
Perinealmuscles
Hormones--Target-Derived Neurotrophic Function:
duringcritical period
of development,muscles can bind
androgens while themotoneurons
cannot
secrete a“retrograde factor”
that leads to survival
of neurons
Example Question:
What structures within the nervous system would be masculinized or feminized
in a male rat with testicular feminization mutation?
SDN-POA
normalmale
SNB
masculinized(large)
male withTFM
masculinized(large)
Sexual Differentiation of the Human Nervous System:Sexually dimorphic nuclei have been described within preoptic area of humans:
• INAH-1 (intermediate nucleus of the anterior hypothalamus--cell group #1)– nucleus is larger in males than in females
– sex difference develops postnatally (not present at birth)
– after 4 years of age, the number of neurons in nucleus die in females, but remain the same in males; androgens are believed important for survival of these neurons
– function is not currently known
• INAH-3 (interstitial nucleus of the anterior hypothalamus--cell group #3)– nucleus is larger in males than in females
– not clear how hormones affect its development
– nucleus is also larger in heterosexual men than homosexual men; suggested that this nucleus might be important for sexual orientation
– sexual orientation can be viewed as a sexually dimorphic response: masculine preference is for female partners, and feminine preference is for male partners
– smaller INAH-3 in homosexual men=feminine preference for male partners
– elevated androgens in utero may act to masculinize sexual orientation
– evidence that 37% women with CAH rate themselves as bisexual or homosexual while only 7% women without the disorder rate themselves similarly
– cautionary note: sex behavior in humans can be affected by many factors
Sexual Differentiation of the Human Nervous System:Onuf’s nucleus is also sexually dimorphic in humans:
• Onuf’s nucleus is the homologue to the rat SNB
• motoneurons within Onuf’s nucleus innervate roughly the same group of
muscles within the perineum: the bulbocavernosus (BC) and ischiocavernosus
(IC) muscles; humans (and other higher mammals) lack levator ani muscle
• men have larger BC and IC muscles and more neurons within Onuf’s nucleus
than females
• subtle effect: sex difference between men and women is smaller than the
difference reported between male and female rats; in female rats, these
muscles are lost and so are the motoneurons
Sexual Differentiation of the Human Nervous System:Sex differences have also been reported in cognitive function:
• men are lateralized in auditory function: most men can hear better with their
right ear than with their left
• in contrast, women tend to be less lateralized in auditory processing, hearing
equally well with right and left ears
Women exposed to a synthetic estrogen in utero show higher levels of
lateralization in auditory function:
• in 1950s and 1960s, diethylstilbestrol (DES--synthetic estrogen) was given to
pregnant women to prevent miscarriages
• women exposed to DES in utero were more lateralized in word detection than
their sisters that were not exposed to the drug
• in females, exposure to exogenous estrogen can masculinize auditory function
• in males, testosterone is most likely aromatized to estrogen which leads to
lateralization of auditory function