Prolactin-releasing peptide (PrRP) was first isolated from bovine hypothalamus, and was found to act as an endogenous ligand at the G-protein-coupled receptor 10 (GPR10 or hGR3). Although originally named as it can affect the secretion of prolactin from anterior pituitary cells, the potential functions for this peptide have been greatly expanded over the past decade. Anatomical, pharmacological, and physiological studies indicate that PrRP, signaling via the GPR10 receptor, may have a wide range of roles in neuroendocrinology; such as in energy homeostasis, stress responses, cardiovascular regulation, and circadian function. This review will provide the current knowledge of the PrRP and GPR10 signaling system, its putative functions, implications for therapy, and future perspectives.
Seven-transmembrane-domain receptors (7TMRs) make up a receptor superfamily related by common signaling features and a structure that spans the cell membrane seven times. All 7TMRs are coupled to guanine nucleotide binding proteins (G-proteins) and, as such, are more commonly referred to as G-protein-coupled receptors (GPCRs; Probst et al., 1992). In the human genome, over 800 GPCRs have been annotated (>4% of the genome), many of which since have been implicated in diverse physiological roles from photoreception to olfaction, and from mood to appetite (Fredriksson et al., 2003). This diverse functionality infers immense therapeutic potential for the treatment of disease and, in fact, as many as half of the currently marketed drugs target GPCRs (Flower, 1999). Advances in genomics over the last century, that have allowed genome-wide homology analysis, have facilitated the discovery of so many new GPCRs. Currently the GenBank/EMBL database has over 1000 clones of eukaryotic GPCRs recorded, and many of the predicted receptors have no known ligand. These are termed “orphan” GPCRs. Although many of the GPCR genes probably correspond to homologs of sensory olfactory receptors, which are predicted to exist in considerable number in the genome, the remainder could encode for diverse unknown receptors, which may play important physiological roles (Buck and Axel, 1991). Due to the undoubted therapeutic potential for the treatment of different pathologies, the discovery of ligands by the “de-orphanization” of GPCRs and an understanding of their physiological function is the focus of an intense research effort that has far reaching implications for both frontier and translational science.
One of the first GPCRs to be de-orphanized was G-protein-coupled receptor 10 (GPR10; also known as hGR3 or UHR-1). GPR10 was originally cloned in hypothalamic tissue using low stringency PCR primers designed against to the highly conserved GPCR transmembrane domains 2 and 6 (Welch et al., 1995). The cloned receptor showed sequence similarity to the neuropeptide Y (NPY) receptor (31% overall and 46% in the transmembrane regions), however, it could not be activated by either NPY or pancreatic polypeptide (Marchese et al., 1995). This presented the scientific community with a novel problem, in that this represented the first GPCR for which its discovery preceded that of its endogenous ligand. Initial GPR10 localization studies indicated high mRNA expression in the anterior pituitary (Fujii et al., 1999). As hypothalamus derived factors frequently play important roles in regulating anterior pituitary function, it seemed intuitive that the natural ligand for GPR10 might exist in the hypothalamus. Using this insight, GPR10 was finally de-orphanized by Hinuma et al. (1998), using a novel reverse pharmacology approach. For reasons described below, the receptor ligand was termed prolactin-releasing peptide (PrRP). Later studies, using other in vitro heterologous expression systems, demonstrated that PrRP shows some promiscuous binding to another RFamide peptide family receptor, neuropeptide FF receptor 2 (NPFF-2R) (Engstrom et al., 2003; Ma et al., 2009). However, to date, PrRP is the only ligand known to have significant affinity for GPR10.
Initial studies showed that PrRP could stimulate prolactin secretion from dispersed anterior pituitary cells; hence, the peptide’s name (Hinuma et al., 1998). However since its discovery, the importance of PrRP in the physiological regulation of prolactin secretion has been put in doubt (see below). Instead, the PrRP-GPR10 signaling pathway has been implicated in a range of other physiological systems. For example, central administration of PrRP inhibits food intake and increases energy expenditure in rats and mice (Lawrence et al., 2000, 2004), suggesting that PrRP plays roles in the regulation of energy balance. It also elevates circulating plasma levels of adrenocorticotropic hormone (ACTH) level, suggesting an association of PrRP with stress responses (Takayanagi and Onaka, 2010). Moreover, PrRP also can affect the cardiovascular system (Samson et al., 2000) and circadian cyclicity (Zhang et al., 2000, 2001; Lin et al., 2002a). This article aims to review the current understanding of the physiological roles for PrRP and GPR10 signaling in the mammalian system, and to highlight future directions for research.
Determining the expression patterns of both receptor and ligand gives key insight into physiological function. In situ hybridization histology, RT-PCR, and immunohistochemical studies indicate that PrRP is expressed in neurons of the nucleus tractus solitarius (NTS), the ventrolateral medulla (VLM), and in the caudal portion of the dorsomedial hypothalamic nucleus (DMN) (Figure 1) (Chen et al., 1999; Maruyama et al., 1999; Ibata et al., 2000; Lee et al., 2000). PrRP mRNA has also been found in a number of peripheral tissues, including the adrenal gland, pancreas, placenta, and testis (Fujii et al., 1999; Matsumoto et al., 1999a; Kalliomaki et al., 2004).
The co-localization of PrRP with tyrosine hydroxylase (TH) in the caudal NTS and VLM, suggests that these PrRP cells are a subset of A2 and A1 noradrenergic neurons, respectively (Chen et al., 1999). The highest numbers of PrRP cell bodies are found within the NTS, and interestingly as the hypothalamus shows the highest levels of PrRP fiber immunoreactivity, this suggested the possible projection of PrRP from the brainstem to the hypothalamus (Hinuma et al., 1998; Fujii et al., 1999; Matsumoto et al., 1999a). PrRP-immunoreactive fibers are visible in many areas of the brain, such as the DMN, area postrema (AP), pontine parabrachial area, preoptic areas, bed nucleus of the stria terminalis (BNST), amygdala, mediodorsal nucleus of the thalamus, septal nucleus, and ependymal linings of the ventricles and blood vessels (Lin, 2008). One of the major projection sites is the paraventricular hypothalamus (PVN), where PrRP neurons appear to synapse directly on corticotrophin-releasing hormone (CRH) (Matsumoto et al., 1999a) and oxytocin neurons (Maruyama et al., 1999). Cell-specific connections also have been identified on magnocellular oxytocin/vasopressin neurons of the hypothalamic supraoptic nucleus (Maruyama et al., 1999), somatostatin neurons in the hypothalamic periventricular nucleus (Iijima et al., 2001), and on catecholaminergic cells of the adrenal medulla (Fujiwara et al., 2005).
Distribution of the GPR10 receptor has been investigated using autoradiography, in situ hybridization, and RT-PCR (Fujii et al., 1999; Roland et al., 1999; Ibata et al., 2000). The relative level of expression is high in the anterior pituitary, reticular nucleus of the thalamus (Rt), periventricular hypothalamus, DMN, AP, and NTS; with moderate expression in the BNST, PVN, medial preoptic area and nucleus, ventrolateral hypothalamus, stomach, femur, and adrenal gland (Roland et al., 1999).
There is good complementarity in the localization of GPR10 receptor immunoreactive PrRP fiber staining in many brain areas (BNST, supraoptic nucleus, PVN, DMN, and NTS). However, it is interesting to note discrepancies in localization, which might be surprising if GPR10 is the only receptor for PrRP. In fact, many peptide systems have significant mismatches between the distribution of the ligand and their respective cognate receptors. Much of this mismatch might be explained by redundancy in function, that is a receptor will not respond if it is not in contact with the ligand. It may be energetically convenient not to lose the expression of a receptor if there is no evolutionary pressure to do so. Furthermore, peptides often have permissive actions and may not function as classical transmitters at tightly regulated synaptic junctions. For instance, PrRP may be released from neuronal fibers terminating at the ventricular zones, and may enter and diffuse within the cerebral spinal fluid (Iijima et al., 1999); or as seen with substance P, PrRP may diffuse through the neuronal tissue to reach distant receptor sites (Duggan et al., 1990). Although GPR10 is considered to be the cognate receptor for PrRP, others (perhaps currently unknown) may exist. For example, PrRP has significant affinity at neuropeptide FF receptor 2 (NPFF-R2) in in vitro studies, and there is potential for overlap between the presence of PrRP and NPFF-R2 particular in the hypothalamus and adrenal gland (Gouarderes et al., 2004). Nevertheless, the diverse distribution profile of receptors and ligand may underlie the diverse physiological roles played by PrRP-GPR10 signaling, and each function needs careful investigation. In the absence of receptor-selective antagonists, this is probably best achieved in receptor knockout mice.
As high expression of GPR10 is seen in the anterior pituitary, initial studies investigating the physiological action of PrRP focused on hypophysiotropic secretion (Hinuma et al., 1998; Lin et al., 2002b). Preliminary in vitro studies, which gave rise to the name of the peptide, described an action of PrRP on prolactin secretion from anterior pituitary tumor cell lines and primary cell cultures (Hinuma et al., 1998). Subsequent studies investigating the relevance of PrRP in vivo as a central mediator of prolactin release were controversial, with posit
