On organisational effects of ovarian hormones being later than testicular

Shamim Khaliq shamim at khaliq.intensive.co.uk
Wed Dec 13 10:46:17 EST 2000


You may have already seen this. "A Role For Ovarian Hormones In Sexual
Differentiation Of The Brain"
http://www.cogsci.soton.ac.uk/bbs/Archive/bbs.fitch.html
I thought it had a clear summary of organisational effects of the gonadal
steroids. Below I'll summarise a few salient points of their lovely
introduction.

The authors seem sure that we know of more anatomical areas susceptible to
organisational effects of aromatised testosterone than the sexually
dimorphic nuclei of the preoptic area of the hypothalamus. Unmentioned were
the dubious results (because of the small number of participants) regarding
a different sized bed nucleus of the stria terminalis of transsexuals. They
support this by saying the volume of the encapsulated region of the bed
nucleus of the stria terminalis is twice as large in male rats than female.

As far as I know, a couple of interstitial nuclei in the anterior
hypothalamus are larger in men than women. They mention that the number and
pattern of synapses on dendritic spines in the preoptic area of the
hypothalamus show sex differences in rats, hamsters and macaque monkeys.
They mention that in vitro, testosterone influences neuronal outgrowth and
survival of cultured rat preoptic cells (so some organisational effects may
not require the aromatisation of testosterone to oestrodiol?).

There's a slightly larger posterior fifth of the corpus callosum in women,
the planum temporale in Wernicke's area, and a thicker right cerebral cortex
in male rats given an enriched environment (an activational effect?). They
mention, the sexually dimorphic preoptic area of the (substantia nigra?)
pars compacta & sexually dimorphic suprachiasmatic nucleus, whose size can
be increased to male levels in neonatally ovariectomised female gerbils with
testosterone implants, and the size of the left sexually dimorphic preoptic
area of the pars compacta which is associated with gerbil courtship
vocalisations.

I've not summarised activational effects in terms of adult behaviour and
cognition though they're also mentioned. I found it confusing because
organisational effects of a steroid may often be in the opposite direction
to activational effects of that same steroid, making it hard to remember.
And, oh, it's just confusing, e.g. one thing that differentiates a female
brain from a male brain is its fluctuation in specific measures in response
to female hormonal cyclicity. I think they have this down as a permanent
change, hence organisational. It gets even more confusing when one realises
that the time range they're implying for organisational effects of oestrogen
may be as late as post puberty, so I can't use the early/late criteria to
distinguish the two.

Their main point, as given away by the title, is that oestrogen and
progesterone too have organisational effects. I've not read this part too
deeply, feeling (probably incorrectly) that my undergraduate course
emphasises the efficacy of alpha-foeto-protein in mopping oestrogen up.
Their main argument seems superficially to be (1) that male behaviours are
more likely, and female behaviours less likely, in female adult rats exposed
to testosterone perinatally if they've had their ovaries removed neonatally
than if they haven't, so oestrogen must inhibit the masculinising effects of
testosterone, and (2) neonatally castrated male rats with prepubertal
ovarian implants or receiving low doses of oestrogen will exhibit female
behaviours as adults, or neonatally ovariectomised female rats will show
fewer female characteristics than intact female rats in the absence of
testosterone.

They then went on to do their experiment, which I think they summarise
clearly themselves on page 19.

I haven't read the discussion yet, but look forward to doing so on the way
to my lecture now.

Yours,
an undergraduate psychology degree student







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