The Effects of Gonadal Steroids on Gonadotropin Secretion in Hamsters with a Lesion of the Arcuate Nucleus of the Hypothalamus*

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Abstract
The purpose of this investigation was to study the response of the hypothalamo-pituitary axis to gonadal steroids in animals with a chemically induced lesion of the arcuate nucleus (ARC). A lesion of the ARC was induced by the administration of 8 mg monosodium glutamate (MSG)/g BW to hamsters on day 8 of the neonatal period; control animals received saline. Treatment with MSG resulted in a destruction of 85–92% of all of the neuron cell bodies in the ARC. All animals were ovariectomized or orchidectomized at 60 days of age and given daily sc injections of sesame oil, 5 μg estradiol benzoate (EB; to females), or 1 mg testosterone propionate (TP; to males) for 7 days. The plasma and pituitary levels of LH and FSH were measured by RIA. In the castrate male, elevated levels of plasma LH and FSH in both control and MSG-treated animals were suppressed after treatment with TP. Similarly, in the ovariectomized female, elevated levels of plasma LH in both groups were suppressed by EB. However, in contrast to LH, plasma FSH concentrations in MSG-treated females were significantly lower than those in control animals. EB suppressed the elevated levels of FSH in control animals but did not alter the lower FSH levels in animals with a lesion of the ARC. To determine whether EB would induce a surge of LH and FSH in animals with a lesion of the ARC, control and MSGtreated female hamsters were given 50 μg EB 3 weeks after ovariectomy; plasma levels of LH and FSH were measured at 0900 and 1600 h 3 days later. The levels of LH were similar in control and MSG-treated animals, with elevated levels observed at 1600 h. Although plasma FSH levels were significantly lower than control values at 0900 and 1600 h in the MSG-treated hamsters, both groups exhibited significant elevations of FSH at 1600 h when compared with levels found at 0900 h These results suggest that neonatal treatment of male and female hamsters with MSG will produce a discrete lesion of the ARC. The destruction of the majority of cell bodies in the ARC does not appear to alter the negative feedback actions of TP on LH and FSH secretion in the male hamster or the positive and negative feedback actions of EB on LH secretion in the female hamster. However, in the female hamster, a chemically induced lesion of the ARC results in a removal of steroid-sensitive elements necessary for the tonic regulation of FSH secretion.

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