13.3 - Hormones and Sexual Development of Brain and Behaviour Flashcards
Are there differences between the male and female brain?
yes
what are the x differences referenced between the male and female brain (5)
- male brains are 15% larger
- difference inn average volume across areas, nuclei, fibre tracts
- number and types of neural and glial cells that compose certain structures
- plasticity of certain structures
- number and types of synapses that connect the cells in various structures
Explain the Pfiffer study on castrated neonatal rats and their gonadotropin release
neonatal rats were castrated, others were not, some received transplants, others did not
- the castrated rats of either sex developed into adults w the female cyclic pattern of gonadotropin release
- transplantation of testes into any female neonatal (castrated or not) caused them to develop steady gonadotropin release
- transplantation of ovaries had no effect on patters
what did Pfeiffer conclude from his studies on castrated neonatal rats
female cyclic pattern of gonadotropin release develops unless it is overidden by testosterone
Why was Pfeiffer incorrect?
concluded that testosterone influenced pituitary development, but we now know that the release of gonadotropins from the ant pit are controlled by the hypo thal
What did the knowledge that gonadotropin release from the ant pit is determined by the hyp. thal contribute to Pfeiffer’s findings on castrated neonatal rats
the first evidence for the role of perinatal androgens in overriding programmed female cyclic gonadotropin release
what is aromatization
Gonadal and adrenal sex hormones are steroid hormones
- since all steroid hormones are derived from cholesterol, they have very similar structures and can be converted into one another
changing the testosterone molecule by an enzyme (aromatase) converts it to estradiol - called aromatization
what is the aromatization hypothesis
perinatal testosterone doesn’t masculinize the brain, it is masculine by estradiol that has been aromatized form perinatal testosterone
what are the two strongest pieces of evidence for the aromatization hypothesis
(2 on 2)
- findings that show masculinizing effects on the brain of early injections of estradiol
- a) masculinization of the Brian does not occur in response to testosterone when it is administered alongside agents that block aromatization
b) masculinization also doesn’t occur in the presence of other androgens that cannot be aromatized (dihydrotestosterone)
How do females prevent being masculinized by estradiol which their mothers circulate through fetal blood supply?
Alpha fetoprotein, which is present in the blood during the perinatal period - deactivates circulating estradiol by binding to it
- solid finding in rodents, not so sure in humans
How does estradiol masculinize the brain of male rodent fetus even in the presence of deactivating alpha fetoprotein?
testosterone is immune to it, can travel unaffected from testes to brain, where it can be aromatized to estradiol
- estradiol can function in the brain because alpha fetoprotein can’t really cross the BBB
what was the original assumption about the aromatization hypothesis?
is this correct?
that it was the sole mechanism that produced sex differences in the Brain
no
Are there ay brain differences with clear behavioural significance?
is it likely that there are not such differences?
No
no
what is the primary principle when considering sex differences in the brain
they are never masculinized or feminized wholesale, they develop independently in different parts of the brain at different points in time and by different mechanisms
what is an example of brain-area dependent sex differences
aromatase is found in only a few areas of the rat brain (hypthal) and only in these areas is armozatization critical for testosterones mascluinizing effects
are all brain differences developed perinatally?
no, some do not arise until puberty, so probs not due to aromatization or whatever
what are the two factors that play little or no tole in the sexual differentiation of repro organs that do influence sex diff in the brain?
- sex chromosomes - influence brain dev independent of their effects on hormones
- although female program of repro organs proceeds in the absence of gonadal steroids, evidence suggests that estradiol plays and active role
- knockout mice without the gene that forms estradiol receptors do not develop typical brains
what is an example of the fact that sex chromosomes influence brain dev independent of their effects on hormones
different patterns of gene expression exist in the brains of male and female mice even before gonads become functional
what complicates the study of brain differences in sexes?
the mechanisms that cause it are different in different mammals - aromatization is less important in primates than in rats and mice
Explain the Pheonix et al (1959) study on the effects of perinatal testosterone
- injected pregnant guinea pigs w T
- when born, ovariwctomized the females
- when the ovariectomized females matured, they injected them w T to test their copulatory behaviour
- females exposed to perinatal T displayed more male mounting behaviours in response to T injections in adulthood
- when injected with progesterone and estradiol and mounted by males, they displayed less lordosis
Describe the Grady, Pheonix and Young study on the effects of perinatal testosterone
- lack of early experience to T both feminizes ad demasculiizes their repro behaviour as adults
- when castrated right after birth, males dont display mounting, intromission (dick insert) and ejaculationeven if given T as adults and a prime lady to make love to
- when injected w estrogen and progesterone as adults, they exhibited more lordosis than uncastrated conntrols
What process is important for the defeminizing and masculinizing of copulatory behaviour
In what species?
Is this universal?
aromatization of perinatal T to estradiol
rodents
monkeys
Is there a critical period for the effects of perinatal testosterone? if yes, when?
yes, the ability of single injections of T to masculinize and defeminizie rat repro behaviour is restricted to the first 11 days of life
what is a preceptive behaviour
a solicitation behaviour for sex