Gonadotrophin-releasing activity of neurohypophysial hormones: I. Potential for modulation of pituitary hormone secretion in rats

Abstract

Neurohypophysial hormones have been implicated in the control of anterior pituitary function, and oxytocin has been shown to stimulate gonadotrophin excretion and ovarian follicular development in certain species. To determine the role of neurohypophysial peptides in the control of gonadotrophin release, their actions on LH and FSH secretion were analysed in rats in vivo and in vitro. In adult female rats, administration of oxytocin during early pro-oestrus advanced the spontaneous LH surge and markedly increased peripheral LH levels at 15.00 h compared with control animals. In cultured pituitary cells from adult female rats, oxytocin and vasopressin elicited dose-related increases in LH and FSH release. Such responses were not affected by a potent gonadotrophin-releasing hormone (GnRH) antagonist that abolished GnRH agonist-induced release of LH and FSH. Oxytocin did not enhance GnRH agonist-stimulated gonadotrophin release to the same extent as it increased basal secretion, but at low concentrations of GnRH agonist the effects were additive. The gonadotrophin responses to oxytocin and vasopressin were inhibited by the specific neurohypophysial hormone antagonists, [d(CH2)5D-Ile2,Ile4,Arg8]vasopressin and [d(CH2)5Tyr (Me),Arg8]vasopressin. These results provide direct evidence that neurohypophysial hormones can stimulate gonadotrophin secretion through a receptor system distinct from the GnRH receptor. Such a mechanism could represent a complementary hypothalamic control system for long-term modulation of LH and FSH secretion by exerting a basal or tonic influence on gonadotrophin production.

Pup removal suppresses estrogen-induced surges of LH secretion and activation of GnRH neurons in lactating rats

“…There is strong evidence indicating that oxytocin plays a role in the regulation of LH secretion at both the anterior pituitary and hypothalamic levels. Oxytocin stimulates basal pituitary secretion of LH (Evans et al 1989) and augments the response of LH secretion to GnRH (Evans et al 1995). Furthermore, peripheral administration of oxytocin antagonists prevents the estrogen-induced LH surge normally observed on the afternoon of proestrus in cyclic rats (Johnston & Negro-Vilar 1988, Robinson & Evans 1990).…”

Regulation of C-fos Protein in Gonadotrope Cells by Oxytocin and Gonadotropin-Releasing Hormone

“…It is known that oxytocin increases release of LH from dispersed anterior pituitary cells [17, 33]and from tissue pieces [34], and thus effects on stores may have taken place. This study noted that oxytocin stimulated c-fos and LH expression within cells as detected by immunocytochemistry.…”

“…Female rats at proestrous which receive oxytocin antagonist do not produce the normally observed pre-ovulatory LH surge [14, 15], indicating a crucial involvement of oxytocin in the process. In vitro, oxytocin increases secretion of LH from dispersed pituitary cells and stimulates an increase in intracellular Ca 2+ in both dispersed normal pituitary cells and the αT3-1 cell line [16, 17, 18]. Additionally, oxytocin induces synthesis of LH in pituitary cells [19].…”

The effect of oxytocin on the time of ovulation, conception, and embryonic survival in swine

“…(Wathes et al 1992). rather than with direct effects on the process of or.ulation as observed in the rat (Evans et al 1989;Robinson and Evans 1991). It is possible that the quantify of OT was not sufficient to elicit the expected response.…”

“…These reports suggest that improved sperm transport may be the reason for the increase in conception rate and litter size. In the rat, OT can influence the time of orulation (Evans et al 1989). In sheep (Wathes et al 1989, 1992\ and caffle (Voss and Fortune, 1991) (Soede er.al.…”