In vivo Release of Prolactin-Releasing Peptide in Rat Hypothalamus in Association with Luteinizing Hormone and Prolactin Surges

Abstract

Prolactin (PRL)-releasing peptide (PrRP) is a novel hypothalamic peptide reported to be a potent and specific stimulator of PRL secretion. This author recently reported that PrRP might play a significant role in mediating the steroid-induced PRL surge in the rat. In order to examine the secretory profile of PrRP in the rat hypothalamus before and during the luteinizing hormone (LH) and PRL surges, this study employed the push-pull perfusion technique and determined the in vivo release of PrRP and also of gonadotropin-releasing hormone (GnRH) in ovariectomized rats primed with estradiol and progesterone. In the medial preoptic area (MPOA) where the GnRH neuronal perikarya exist, GnRH release was increased prior to the initiation of the LH surge, and PrRP also started rising even earlier than GnRH. In the median eminence-arcuate nucleus complex (ME-ARC), where GnRH neuronal fibers terminate, GnRH secretion started increasing before the commencement of the LH surge, but the release of PrRP did not change significantly. These results suggest that PrRP may play a role in mediating the steroid-induced LH surge by activating GnRH neurons in the MPOA. A possible involvement of PrRP in the PRL surge was not suggested from the present data. The lack of a significant alteration in PrRP release in the ME-ARC may argue against a direct hypophysiotropic action of the peptide.

Preserved inhibitory effect of recurrent hypoglycaemia on the male gonadotrophic axis

“…Consequently, it is precluded that the observed effects on LH and PRL are based on a concurrent control by hypothalamic GnRH 24 or PRL-releasing peptide. 25 This fits with the data of Bogacka et al ., who found that GnRH did not affect PRL secretion in vitro . 26 Rather than being secondary to responses of PRL and the HPA axis, the decrease in LH and testosterone during hypoglycaemia may be a direct consequence of reduced glucose supply, and therefore may also be less adaptive to repetitions of this state.…”

“…Moreover, the adaptation of PRL to recurrent hypoglycaemia leads us to suspect that gonadotrophins and PRL are differentially regulated following separate mechanisms. Consequently, it is precluded that the observed effects on LH and PRL are based on a concurrent control by hypothalamic GnRH 24 or PRL‐releasing peptide 25 . This fits with the data of Bogacka et al ., who found that GnRH did not affect PRL secretion in vitro 26 .…”

RFamide-Related Peptide-3, a Mammalian Gonadotropin-Inhibitory Hormone Ortholog, Regulates Gonadotropin-Releasing Hormone Neuron Firing in the Mouse

“…The PRPs are unlikely to be PRPs (31) but do, nevertheless, regulate LH secretion through likely hypothalamic mechanisms (32,33), and GnRH neurons are reported to express immunoreactivity for G protein-coupled receptor 10, the PRP receptor (34). We find here that neither PRP-20 nor PRP-31 has any discernable effects on GnRH neuron firing rate at 1 M concentrations.…”

Functions of Two Distinct “Prolactin-Releasing Peptides” Evolved from a Common Ancestral Gene

“…PrRP gene expression is directly regulated by gonadal steroid hormones because PrRP neurons in the medulla oblongata are co-localized with receptors for estrogen or progesterone (53). In addition, administering estrogen or progesterone to ovariectomized rats induces PrRP mRNA expression in the medulla oblongata, as well as release of PrRP in the medial POA (53, 68, 91). …”