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

Hypothalamic peptide hormones regulate the secretion of most of the anterior pituitary hormones, that is, growth hormone, follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone and adrenocorticotropin. These peptides do not regulate the secretion of prolactin, at least in a specific manner, however. The peptides act through specific receptors, which are referred to as seven-transmembrane-domain receptors or G-protein-coupled receptors. Although prolactin is important in pregnancy and lactation in mammals, and is involved in the development of the mammary glands and the promotion of milk synthesis, a specific prolactin-releasing hormone has remained unknown. Here we identify a potent candidate for such a hormone. We first proposed that there may still be unknown peptide hormone factors that control pituitary function through seven-transmembrane-domain receptors. We isolated the complementary DNA encoding an 'orphan' receptor (that is, one for which the ligand is unknown). This receptor, hGR3, is specifically expressed in the human pituitary. We then searched for the hGR3 ligand in the hypothalamus and identified a new peptide, which shares no sequence similarity with known peptides and proteins, as an endogenous ligand. We show that this ligand is a potent prolactin-releasing factor for rat anterior pituitary cells; we have therefore named this peptide prolactin-releasing peptide.

The integration of 'long-term' adiposity signaling with the 'short-term' meal-related signal cholecystokinin (CCK) is proposed to involve descending hypothalamic projections to areas of the caudal brainstem (CBS) that regulate the amount of food consumed during a single meal. One such projection extends from cell bodies in the hypothalamic paraventricular nucleus (PVN) to the nucleus tractus solitarius (NTS), where cells that respond to peripheral CCK are concentrated. Candidate neuronal cell types that may comprise this PVN-NTS projection includes those expressing corticotropin-releasing hormone (CRH) or oxytocin. We therefore sought to determine whether oxytocin or CRH axons are preferentially located in close anatomical proximity to neurons of the NTS that are activated by peripheral administration of CCK, as determined by immunocytochemical staining for Fos protein. Rats received injections of either an anorexic dose of CCK (8 nmol/kg, i.p.) or vehicle and were perfused 2 h later with 4% paraformaldehyde. Immunocytochemistry was performed on cryostat sections (14 microm) of caudal brainstem, using a polyclonal antibody to Fos protein and either a monoclonal antibody to oxytocin or a polyclonal antibody to CRH. As expected, CCK administration significantly increased the numbers of Fos-positive neurons by 489% (p<0.01) and 400% (p<0.01), respectively, in the medial and gelatinosus subdivisions of the NTS. These same regions received dense oxytocin axon innervation, whereas CRH immunoreactivity was not as prevalent in these areas. In areas outside the NTS, such as the dorsal motor nucleus of the vagus (DMV), Fos activation was absent despite a dense oxytocin and CRH innervation. To investigate whether CCK-induced reductions of food intake require intact oxytocin signaling, we performed a separate study in which CCK injection was preceded by injection into the fourth ventricle of an oxytocin receptor antagonist [d(CH(2))(5), Tyr (Me)(2), Orn(8)]-vasotocin (OVT). This study showed CCK was 23% and 22% less effective at inhibiting food intake at 30 min (p<0.05) and 1 h (p<0.05) food intake, respectively, in the presence of OVT. Taken together, the data indicate that oxytocin axons within the descending pathway from the PVN to the NTS are anatomically positioned to interact with NTS neurons that respond to vagally mediated peripheral CCK signals such as those that occur following ingestion of a meal. These findings support the hypothesis that oxytocin exerts a tonic stimulatory effect on the response of key neurons within the NTS to CCK and further reduce meal size.

Oxytocin is released from the pituitary gland in response to a variety of stressful stimuli, including noxious stimuli, conditioned fear and exposure to novel environments. These responses are believed to be mediated, at least in part, by noradrenergic projections from the medulla oblongata, and some of these noradrenergic neurones also contain prolactin-releasing peptide (PrRP). Central administration of either PrRP or noradrenaline stimulates oxytocin secretion into the circulation. Stressful stimuli activate PrRP-containing noradrenergic neurones in the medulla oblongata, and it is thus possible that PrRP/noradrenergic projections to the hypothalamus mediate oxytocin responses to stressful stimuli. Here, the roles of brainstem PrRP/noradrenergic projections to the hypothalamus in oxytocin responses to different kinds of stressful stimuli are reviewed, with a particular emphasis on conditioned fear. Roles of dendritic oxytocin release during stress and metabolic factors affecting stress pathways are also discussed.