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Web Topic 5.5: Ovulation and the Brain

[Referenced on textbook p. 140]

The crucial event permitting ovulation is the reversal of the effect of estradiol on LH secretion, from negative feedback at low blood concentrations to positive feedback at high concentrations, which causes an LH surge late in the follicular phase of the menstrual cycle (Johnson, 2007). The switch is mediated by processes both in the pituitary gland and within the hypothalamus. High, sustained levels of estrogen cause the LH-secreting pituitary cells to express more GnRH receptors, and therefore to secrete more LH for a given pulse of GnRH. In addition, the high estradiol levels act directly on the hypothalamus to cause the GnRH cells to release more of their hormone into the portal vessels. The mechanism of this effect is not well understood.

In some animals, such as rats, only females are capable of showing a positive LH response to estrogens. One cannot turn a male rat into an “endocrinological female” by castrating it and implanting ovaries; it won’t show an LH surge and therefore won’t ovulate or cycle normally. It appears that the brains of female rats are imprinted during fetal life with the capacity to sustain the ovarian cycle, whereas the brains of male rats are not.

In primates, on the other hand, there seems not to be any sex difference of this kind. A castrated male monkey shows the same LH surge in response to sustained high estrogen levels as a female monkey. And if ovarian tissue is transplanted into a castrated male monkey, the animal’s hormones will cycle very much like those of a normal female. In humans, too, the LH response to estrogen seems to be similar in both sexes. Thus, with respect to this particular aspect of sexuality, the brains of men and women seem to be undifferentiated. (Later in this book, we’ll encounter other aspects of brain organization that are indeed different between men and women.)

References

Johnson, M. A. (2007). Essential Reproduction (6th ed.). Blackwell.

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