Central Stimulatory Influence of Oxytocin on Preovulatory Gonadotropin-Releasing Hormone Requires More than the Median Eminence

Abstract

The study was designed to determine whether the ability of central oxytocin (OT) to stimulate gonadotropin-releasing hormone (GnRH) on the afternoon of proestrus (PE) in the cycling female rat is mediated at the level of GnRH terminals within the median eminence (ME), or at higher hypothalamic levels where GnRH cell bodies and axons are located. Determining the location of this OT effect in vivo has proven difficult. Therefore, an in vitro system utilizing ME or basal hypothalamic (BH) explants containing GnRH terminals, or GnRH neurons including the cell bodies, axons and terminals, respectively, were harvested from regular cycling female rats at 15:00 h on PE or diestrus (DI). The explants were allowed to preincubate in Krebs Ringer Bicarbonate Buffer containing glucose, ascorbic acid, calcium, and a metalloprotease inhibitor (KRBG) and enriched with 95% O₂/5% CO₂ at 37°C until a stable baseline release of GnRH was achieved (30 min). The 0.05 level of probability was used as the minimum criterion of significance in all experiments. The ability of OT (10^–15–10^–9M) to stimulate the release of GnRH was determined in both ME and BH explants on PE and DI. The results demonstrated a sensitive, dose-dependent ability of OT to stimulate GnRH release from PE BH explants which was observed only in PE. Furthermore, OT failed to significantly stimulate GnRH release from ME explants on either PE or DI. The data indicate that the PE BH explant paradigm can be used to examine the manner and mechanisms by which OT influences GnRH release on the afternoon of PE. Furthermore, the results indicate for the first time that the stimulatory action of OT by itself on preovulatory GnRH release in cycling female rats is not mediated at the level of the GnRH terminals within the ME, but requires neuronal interactions and mechanisms within the BH explants.

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

“…However, at least, suppression of LH surges in pup-removed lactating rats is not attributable to blockade of oxytocin action at the pituitary level, because the primary cause of the suppression of LH surges by pup removal is at the hypothalamic level, i.e., it is attributable to the failure of activation of the GnRH surge generator in the hypothalamus as evidenced by the GnRH/c-Fos immunohistochemical studies. On the other hand, several studies have demonstrated the central action of oxytocin on GnRH release (Rettori et al 1994, Selvage & Johnston 2001. These findings may support our conclusion regarding the involvement of Photomicrographs showing c-Fos-positive GnRH neurons in lactating rats.…”

“…This hypothesis is based on the finding that suckling induces oxytocin release and oxytocin regulates LH secretion by acting at the hypothalamic level (Rettori et al 1994, Selvage & Johnston 2001. To mimic oxytocin release in response to the suckling stimulus, oxytocin was intracerebroventricularly injected into nonsuckled and, therefore, LH surge-suppressed rats.…”

Role of Glia in the Regulation of Gonadotropin-Releasing Hormone Neuronal Activity and Secretion

“…Oxytocin is a potent stimulator of GnRH secretion from hypothalamic explants of sexually mature animals [34,35], and accelerates the pulsatile GnRH secretion required for sexual maturation in both males and females [33,36]. In line with these data, the expression of oxytocin [37] and its receptor [38] increases in the hypothalamus at the time of puberty.…”

Peptides: Basic determinants of reproductive functions

“…Oxytocin has been shown to potentiate GnRH-stimulated LH release [41]. Similarly, oxytocin also induces GnRH release from hypothalamic explants [136], and oxytocin antagonists block the pro-oestrus LH surge [75]. Administration of exogenous oxytocin in the late follicular phase prompted the onset of the LH surge [67].…”