Hypothalamic-pituitary function in the old irregularly cycling rat.

Older irregularly cycling and younger regularly cycling rats exhibited similar increases in LH levels 1, 2 and 3 wks after ovariectomy, but FSH reached higher levels in the older than in the younger rats after ovariectomy. Neither the ability of 5 microgram/kg estrogen to suppress gonadotropin levels nor the capability of larger doses of estrogen (250 micrograms/kg) and/or progesterone (10 mg/kg to trigger an LH release were altered with age. LH release from the pituitary in response to exogenous gonadotropin releasing hormone (GnRH) was attenuated in ovariectomized older rats, but was partially restored after estrogen and/or progesterone priming. The FSH levels after GnRH treatment were greater in the estrogen primed aged that in estrogen-primed younger rats. The disruption of regular cycles in older rats may be due to a lack of a neural signal for gonadotropin release rather than a failure of the hypothalamus to respond to the positive feedback effect of estrogen or the failure of the pituitary to respond to GnRH.