Resting plasma concentrations of LH, FSH, estradiol, and progesterone and medial basal hypothalamic concentrations of LHRH (MBH-LHRH) were measured by RIA in 8- to 12-month-old female rats which had begun to exhibit constant estrous (CE) or prolonged diestrous (PD) vaginal smear patterns and compared to young cycling rats on proestrus, estrus, or diestrus. In addition, we examined the effect of ovariectomy on these hormonal profiles. Old CE rats have normal plasma LH, FSH and progesterone concentrations, but exhibit elevated estradiol levels and decreased MBH-LHRH concentrations compared to young cycling rats on the day of estrus. Ovariectomy results in an attenuated rise in plasma LH and FSH and a much lesser decrease in MBH-LHRH when compared to young rats, despite comparable steroid changes. Old PD rats have normal LH and FSH levels, but have elevated estradiol and progesterone concentrations and decreased MBH-LHRH levels when compared to young rats on the day of diestrus. Ovariectomy causes a normal decrease in MBH-LHRH; however, the increased gonadotropin levels are significantly less than seen in young controls.

When rat anterior pituitary (AP) was incubated at 37.5 C in a Dubnoff metabolic shaker for 2 hr, 169% more prolactin was found in the combined medium and AP than in nonincubated AP. When AP was incubated together with homogenate or acid extract of rat hypothalamus, prolactin levels in the medium and AP were markedly decreased (36–75%), indicating inhibition of synthesis and release. Acid extract of rat cerebral cortex had no effect on prolactin synthesis or release. Incubation of ovine or rat prolactin, with or without hypothalamus, did not decrease prolactin activity, demonstrating that hypothalamic inhibition of AP prolactin production was not due to hormone inactivation. Acetylcholine, epinephrine, norepinephrine, serotonin, histamine, substance P, oxytocin, and arginine or lysine vasopressin had no effect on AP prolactin release. These results indicate that the hypothalamus contains a factor(s) which inhibits synthesis and release of prolactin by the rat AP in vitro, and this factor(s) is not any of the recognized neurohumors in the hypothalamus.

No abstract

Electrical stimulation of the superior ovarian nerve of intact anaesthetized dioestrous rats for 30 min reduced ovarian progesterone levels, even when papaverine and propranolol were also given. The administration of phentolamine (an alpha receptor antagonist) before stimulation reversed this effect. The results suggest that a neural control of ovarian steroidogenesis may be either excitatory through the stimulation of beta receptors or inhibitory through the stimulation of alpha receptors.