Release of Luteinizing Hormone from the Anterior Pituitary Gland in vitro Can Be Concurrently Regulated by at Least Three Peptides: Gonadotropin-Releasing Hormone, Oxytocin and Neuropeptide Y

Abstract

Normal action of the female ovulatory cycle is dependent on a surge of luteinizing hormone (LH) from the pituitary. Regulation of the levels of LH is therefore vital to reproductive function. It has long been established that gonadotropin-releasing hormone (GnRH) is an important component of the regulatory processes. Other peptides, including oxytocin and neuropeptide Y (NPY), have also been observed to affect LH activities. However, the possibility of the concurrent actions of these peptides has rarely been considered despite their documented presence in the hypophyseal blood during pro-oestrus. In this study, the direct effects of oxytocin and NPY on LH release were studied, as well as the effects of both peptides simultaneously. Also, pituitaries were stimulated with GnRH, and the effects of pre-exposure of the pituitary tissue to oxytocin or NPY were investigated. Further, the effect of oxytocin and NPY together on GnRH stimulation of LH release was determined. Anterior pituitaries were collected from adult female rats on the morning of pro-oestrus. Hemipituitaries were cut in two and placed in a chamber of a perifusion system. The pituitary tissue was perifused with medium alone, oxytocin and NPY, separately or in combination, for 2 h after an initial 100-min equilibration period with no peptide present. Fractions of eluate were collected and LH was measured by radioimmunoassay. LH output was stimulated by both oxytocin (p < 0.01) and NPY (p < 0.02). Furthermore, the addition of NPY to oxytocin during the perifusion elicited a further increase in LH release (p < 0.05). The pituitary tissue was exposed to a 4-min pulse of GnRH after 220 min. Stimulation of LH release by GnRH was synergistically augmented by exposure of the tissue to either oxytocin (p < 0.01) or NPY (p < 0.05) for the immediately preceding 2 h. When NPY was added to oxytocin in the perifusion medium, stimulation of LH release by GnRH was increased even further (p < 0.05). Oxytocin also synergistically enhanced the effect of a second, primed GnRH pulse, whereas the effect of NPY was less robust. This study therefore demonstrated that LH release is modified in the presence of oxytocin, NPY or GnRH alone, and also that combinations of the peptides produce further variations in LH output. Therefore, the concentrations of LH that are present in vivo are likely to be at least partly the result of the co-ordinated effects of combinations of peptides acting on the pituitary.

Peptides: Basic determinants of reproductive functions

“…Members of the vasopressin/oxytocin family influence egg-laying in invertebrates [113]. 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].…”

Effects of Galanin and Leptin on Gonadotropin-Releasing Hormone-Stimulated Luteinizing Hormone Release from the Pituitary

“…We have previously observed separate and conjoint effects of neuropeptide Y and oxytocin on GnRH-stimulated LH release [31, 54]. All of these observations, taken as a whole, suggest that particular peptides may act at the levels of the hypothalamus and the pituitary to meticulously modulate LH release in response to physiological circumstances.…”

“…The multi-column perifusion system used has been described previously [31, 32]. Both hemi-anterior pituitary pieces were placed into a single column.…”

Regulation of Gonadotropin Releasing Hormone Release by Neuropeptide Y at the Median Eminence during the Preovulatory Period in Ewes

“…Previous evidence for a ME site of action for NPY on GnRH terminals was indirect, since it was based on in vitro studies and/or on LH data alone [43, 50, 51, 52]. In the latter case, an anterior pituitary site of action for hypophysiotropic NPY needs to be ruled out [15, 48, 52, 53, 54]. Work done in ovariectomized rats primed with P4 and E2 (synchronized rat follicular phase model) had shown that exogenous infusion of a combination of GnRH and NPY induced a transient increased release of plasma LH that was greater than that induced by the same dose of GnRH alone [50].…”

“…Neuropeptide Y (NPY) stimulates gonadotropin-releasing hormone (GnRH) secretion during the follicular period in many species including laboratory rats, ewes and non-human primates [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. NPY neurons approach median eminence (ME)-GnRH neuronal terminals [4, 12, 16, 17, 18]suggesting the possibility that interaction between these neurons may be important in controlling timing and magnitude of the GnRH release during the preovulatory period.…”