Hormone by Reserpine and Potentiation of Synthetic Gonadotropin-Releasing Hormone-Induced Release of Luteinizing Hormone by Estrogen in the Ovariectomized Ewe

Abstract

These experiments tested the effects of reserpine on estrogen-induced LH release and the effects of estrogen on gonadotropin-releasing hormone-induced LH release in ovariectomized ewes. Injection of 50, 200, or 500 mug of estradiol benzoate (EB) into progesterone-treated ewes, or of 50 mug of EB into non-treated ewes induced a large surge in serum LH levels approximately 15 h later. Injection of 5 mg/ewe of reserpine 6 h prior to the EB, blocked the LH surge. Reserpine reduced, but did not block the release of LH induced by injection of 20 mug or 40 mug of synthetic gonadotropin-releasing hormone (GnRH). Injection of graded doses (5, 10, or 20 mug) of GnRH into progesterone-treated ewes that had been treated with reserpine, induced increases in serum LH levels that were proportional to the logarithm of the dose. In the final series of experiments both progesterone-treated and non-treated ewes were injected with reserpine and then with either EB or oil. Fifteen hours after receiving EB or oil they were injected with either GnRH or the diluent. GnRH elevated serum LH levels significantly higher in ewes given EB than in those given oil. LH levels in ewes given oil and diluent were not elevated. These data suggest that reserpine blocked EB-induced LH release by an action on the central nervous system and that EB acted directly on the anterior pituitary to increase the response to GnRH.

Gonadotropin-Releasing Hormone Requirements for Ovulation1

“…For example, the follicular phase rise in estradiol stimulates synthesis of GnRH receptors and enhances pituitary responsiveness to GnRH [24][25][26][27]44]. These are needed for the LH surge regardless of whether the deterministic or permissive model applies.…”

“…Early studies to assess neural inputs required for the LH surge and ovulation in sheep utilized pharmacological agents to suppress hypothalamic neuroendocrine activity and thereby inhibit GnRH secretion [43][44][45]. Such agents clearly interrupted the LH surge and blocked ovulation.…”

Biphasic Effect of Estradiol on Mechanisms Regulating LH Release in Ovariectomized Sheep

“…On the other hand, neither the timing nor the magnitude of estrogen-induced LH surges was altered by continuous i.v. infusion of estradiol [G o d in g et al, 1969] or by multiple estrogen injections [Jackson et al. 1975].…”

Progesterone Receptor, Estrogen Receptor α, and the Type II Glucocorticoid Receptor Are Coexpressed in the Same Neurons of the Ovine Preoptic Area and Arcuate Nucleus: A Triple Immunolabeling Study1

“…In ewes, estradiol exerts either a facilitatory or inhibitory effect on the reproductive axis by affecting both GnRH and LH release [1]. In contrast, progesterone is a powerful inhibitor of GnRH secretion and thereby LH release [2,3], with little or no effect in the pituitary [3].…”