RFamide peptides inhibit the expression of melanotropin and growth hormone genes in the pituitary of an Agnathan, the sea lamprey, Petromyzon marinus

Effect of the administration of prolactin-releasing peptide2 on feeding activity in the intertidal blenny Rhabdoblennius nitidus (Günther, 1861)

Prolactin-releasing peptide2 (PrRP2) was administered intraperitoneally to male intertidal blenny Rhabdoblennius nitidus, a species with male uniparental care of eggs, to investigate the effect on their feeding activity. A significant inhibitory effect on appetite was observed in the breeding season, but not in the nonbreeding season. These results suggest that PrRP2 and PrRP2 receptors are more active during the breeding season. The presence of a mechanism to inhibit feeding activity while parents take care of their offspring may be important for the success of parental care.

Immunohistochemical detection of prolactin-releasing peptide2 in the brain of the inshore hagfish Eptatretus burgeri

Prolactin-releasing peptide2 (PrRP2) belongs to the RFamide peptide group and is a paralog of prolactin-releasing peptide (PrRP). Recent studies demonstrated that PrRP2, but not PrRP, regulates prolactin release in teleosts. The evolutionary origin of PrRP and PrRP2 dates back to at least early vertebrates because homologs of PrRP/PrRP2 were identified in lampreys, one of the earliest branch of vertebrates class Agnatha. However, PrRP/PrRP2 remains to be identified in hagfish, another representative species of class Agnatha. Here, we examined the distribution of PrRP2 in the brain and pituitary of the inshore hagfish Eptatretus burgeri to obtain further understanding of the neuroendocrine system of PrRP2. PrRP2-immunoreactive (ir) cell bodies were detected in the infundibular nucleus of hypothalamus (HYinf). PrRP2-ir fibers were restricted around PrRP2-ir cell bodies and were not detected in the dorsal wall of the neurohypophysis compared to the abundant PrRP2-ir fiber distribution in the brain and innervation to the pituitary in other vertebrates. To examine possible reciprocal connections of PrRP2 and other neuropeptides, we further conducted dual-label immunohistochemistry of PrRP2 and the PQRFamide (PQRFa) peptide or corticotropin-releasing hormone (CRH). Reciprocal connections are suggested between PrRP2 and PQRFa neurons as well as between PrRP2 and CRH neurons. The present study demonstrates, for the first time, that PrRP2 is expressed in the brain of inshore hagfish. The restricted distribution of PrRP2-ir fibers in the HYinf suggests that PrRP2 does not directly regulate the pituitary gland, but regulates the function of the HYinf where PQRFa and CRH are expressed.

Review: Structure, function and evolution of GnIH

Neuropeptides that possess the Arg-Phe-NH₂ motif at their C-termini (i.e., RFamide peptides) have been characterized in the nervous system of both invertebrates and vertebrates. In vertebrates, RFamide peptides make a family and consist of the groups of gonadotropin-inhibitory hormone (GnIH), neuropeptide FF (NPFF), prolactin-releasing peptide (PrRP), kisspeptin (kiss1 and kiss2), and pyroglutamylated RFamide peptide/26RFamide peptide (QRFP/26RFa). It now appears that these vertebrate RFamide peptides exert important neuroendocrine, behavioral, sensory, and autonomic functions. In 2000, GnIH was discovered as a novel hypothalamic RFamide peptide inhibiting gonadotropin release in quail. Subsequent studies have demonstrated that GnIH acts on the brain and pituitary to modulate reproductive physiology and behavior across vertebrates. To clarify the origin and evolution of GnIH, the existence of GnIH was investigated in agnathans, the most ancient lineage of vertebrates, and basal chordates, such as tunicates and cephalochordates (represented by amphioxus). This review first summarizes the structure and function of GnIH and other RFamide peptides, in particular NPFF having a similar C-terminal structure of GnIH, in vertebrates. Then, this review describes the evolutionary origin of GnIH based on the studies in agnathans and basal chordates.

RFamide peptides in agnathans and basal chordates

Since a peptide with a C-terminal Arg-Phe-NH2 (RFamide peptide) was first identified in the ganglia of the venus clam in 1977, RFamide peptides have been found in the nervous system of both invertebrates and vertebrates. In vertebrates, the RFamide peptide family includes gonadotropin-inhibitory hormone (GnIH), neuropeptide FF (NPFF), prolactin-releasing peptide (PrRP), pyroglutamylated RFamide peptide/26RFamide peptide (QRFP/26RFa), and kisspeptins (kiss1 and kiss2). They are involved in important functions such as the release of hormones, regulation of sexual or social behavior, pain transmission, reproduction, and feeding. In contrast to tetrapods and jawed fish, the information available on RFamide peptides in agnathans and basal chordates is limited, thus preventing further insights into the evolution of RFamide peptides in vertebrates. In this review, we focus on the previous research and recent advances in the studies on RFamide peptides in agnathans and basal chordates. In agnathans, the genes encoding GnIH, NPFF, and PrRP precursors and the mature peptides have been identified in lamprey (Petromyzon marinus) and hagfish (Paramyxine atami). Putative kiss1 and kiss2 genes have also been found in the genome database of lamprey. In basal chordates, namely, in amphioxus (Branchiostoma japonicum), a common ancestral form of GnIH and NPFF genes and their mature peptides, as well as the ortholog of the QRFP gene have been identified. The studies revealed that the number of orthologs of vertebrate RFamide peptides present in agnathans and basal chordates is greater than expected, suggesting that the vertebrate RFamide peptides might have emerged and expanded at an early stage of chordate evolution.

Functions of two distinct "prolactin-releasing peptides" evolved from a common ancestral gene

Prolactin-releasing peptide (PrRP) is one of the RF-amide peptides and was originally identified in the bovine hypothalamus as a stimulator of prolactin (PRL) release. Independently, another RF-amide peptide was found in Japanese crucian carp and named Carassius-RFa (C-RFa), which shows high homology to PrRP and stimulates PRL secretion in teleost fish. Therefore, C-RFa has been recognized as fish PrRP. However, recent work has revealed that PrRP and C-RFa in non-mammalian vertebrates are encoded by separate genes originated through duplication of an ancestral gene. Indeed, both PrRP and C-RFa are suggested to exist in teleost, amphibian, reptile, and avian species. Therefore, we propose that non-mammalian PrRP (C-RFa) be renamed PrRP2. Despite a common evolutionary origin, PrRP2 appears to be a physiological regulator of PRL, whereas this is not a consistent role for PrRP itself. Further work revealed that the biological functions of PrRP and PrRP2 are not limited solely to PRL release, because they are also neuromodulators of several hypothalamus-pituitary axes and are involved in some brain circuits related to the regulation of food intake, stress, and cardiovascular functions. However, these actions appear to be different among vertebrates. For example, central injection of PrRP inhibits feeding behavior in rodents and teleosts, while it stimulates it in chicks. Therefore, both PrRP and PrRP2 have acquired diverse actions through evolution. In this review, we integrate the burgeoning information of structures, expression profiles, and multiple biological actions of PrRP in higher vertebrates, as well as those of PrRP2 in non-mammals.

The Evolution and Variety of RFamide-Type Neuropeptides: Insights from Deuterostomian Invertebrates

Five families of neuropeptides that have a C-terminal RFamide motif have been identified in vertebrates: (1) gonadotropin-inhibitory hormone (GnIH), (2) neuropeptide FF (NPFF), (3) pyroglutamylated RFamide peptide (QRFP), (4) prolactin-releasing peptide (PrRP), and (5) Kisspeptin. Experimental demonstration of neuropeptide-receptor pairings combined with comprehensive analysis of genomic and/or transcriptomic sequence data indicate that, with the exception of the deuterostomian PrRP system, the evolutionary origins of these neuropeptides can be traced back to the common ancestor of bilaterians. Here, we review the occurrence of homologs of vertebrate RFamide-type neuropeptides and their receptors in deuterostomian invertebrates - urochordates, cephalochordates, hemichordates, and echinoderms. Extending analysis of the occurrence of the RFamide motif in other bilaterian neuropeptide families reveals RFamide-type peptides that have acquired modified C-terminal characteristics in the vertebrate lineage (e.g., NPY/NPF), neuropeptide families where the RFamide motif is unique to protostomian members (e.g., CCK/sulfakinins), and RFamide-type peptides that have been lost in the vertebrate lineage (e.g., luqins). Furthermore, the RFamide motif is also a feature of neuropeptide families with a more restricted phylogenetic distribution (e.g., the prototypical FMRFamide-related neuropeptides in protostomes). Thus, the RFamide motif is both an ancient and a convergent feature of neuropeptides, with conservation, acquisition, or loss of this motif occurring in different branches of the animal kingdom.

Study of the role of novel RF-amide neuropeptides in affecting growth hormone secretion in a representative non-human primate (Macaca mulatta)

RF amide peptide family with distinctive terminal -Arg-Phe-NH(2) signature is evolutionarily conserved from invertebrates to mammals. These neuropeptides have been shown to affect diverse functions in invertebrates and vertebrates including influencing pituitary hormone secretion. More recently, two members of this family 26-amino acid and 43-amino acid RF amide peptide (26RFa and 43RFa, respectively) originally isolated from frog have been cloned in rats and humans. Actions of these peptides on hormone secretion have not been studied in primates. In the present study, effect of iv administration of three different doses of human 26RFa and 43RFa on GH secretion was studied in a representative higher primate, the rhesus monkey. As control against these two peptides, normal saline and a scrambled sequence of 26RFa was administered. A set of four intact adult male monkeys received the administration in a random order. Peripheral blood samples were obtained from the chairrestrained but fully conscious animals for a period of 30 min before and 240 min after the administration at 15-min intervals. For quantitative measurement of GH concentration, a human GH chemiluminescent immunometric assay was used. Peripheral administration of 38 and 76 nmol doses of 26RFa significantly (P < 0.05) stimulated GH AUC during a 0-120 min period after injection of 26RFa. In contrast to 26RFa, administration of 43RFa appeared to suppress GH levels during the later stages of the sampling i.e. from 120 to 240 min period. Mean AUC during the period was significantly (P < 0.05) reduced by 76 nmol dose of 43RFa, while 38 nmol dose of 43RFa also had similar effect but lacked full statistical significance (P = 0.058). To our knowledge present study reports for the first time-specific stimulatory effect of 26RFa on the GH secretion and a novel inhibitory and delayed effect of 43RFa on the GH secretion in higher primates. In conclusion, present findings extend evidence for endocrine actions of RF amides in primates and suggest differential effect of these peptides on GH secretion in primates.

Identification of the receptors for prolactin-releasing peptide (PrRP) and Carassius RFamide peptide (C-RFa) in chickens

Prolactin-releasing peptide (PrRP) and its structurally related peptide, Carassius Arg-Phe-amide peptide (C-RFa), have been reported to play similar roles in regulating food intake and pituitary functions in vertebrates. However, the identity, functionality, and expression of the receptor(s) for PrRP and C-RFa remain largely unknown in nonmammalian vertebrates, including birds. In this study, three receptors homologous to mammalian PrRP receptor (PrRPR), named cPrRPR1, cPrRPR2, and cC-RFaR, respectively, were cloned from chicken brain by RT-PCR. Using a pGL3-NFAT-RE-luciferase reporter system, we demonstrated that cPrRPR1 and cPrRPR2 expressed in Chinese hamster ovarian cells could be activated by cPrRP₂₀ and cC-RFa₂₀ potently, whereas cC-RFaR could only be activated effectively by cC-RFa₂₀ (EC₅₀, 0.11 nM), indicating that cPrRPR1 and cPrRPR2 can function as common receptors for PrRP and C-RFa, whereas cC-RFaR is a receptor specific to C-RFa. Using a pGL3-CRE-luciferase reporter system, cPrRPR1, cPrRPR2, and cC-RFaR expressed in Chinese hamster ovarian cells were also shown to activate intracellular protein kinase A signaling pathway upon cC-RFa₂₀ treatment (100 nM). Moreover, RT-PCR assay revealed that cPrRPR1, cPrRPR2, and cC-RFaR were widely expressed in most adult chicken tissues examined, including various regions of brain. These findings, together with evidence of PrRP and C-RFa encoded by separate genes in chicken, Xenopus, and zebrafish, and the differential expression of PrRP and C-RFa genes in chicken tissues, strongly suggest that PrRP and C-RFa may play similar yet distinctive roles in nonmammalian vertebrates, including chicken, and their actions are mediated by common receptor(s) or a specific C-RFa receptor.

Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions

Relative to mammals, the neuroendocrine control of pituitary growth hormone (GH) secretion and synthesis in teleost fish involves numerous stimulatory and inhibitory regulators, many of which are delivered to the somatotrophs via direct innervation. Among teleosts, how multifactorial regulation of somatotroph functions are mediated at the level of post-receptor signalling is best characterized in goldfish. Supplemented with recent findings, this review focuses on the known intracellular signal transduction mechanisms mediating the ligand- and function-specific actions in multifactorial control of GH release and synthesis, as well as basal GH secretion, in goldfish somatotrophs. These include membrane voltage-sensitive ion channels, Na(+)/H(+) antiport, Ca(2+) signalling, multiple pharmacologically distinct intracellular Ca(2+) stores, cAMP/PKA, PKC, nitric oxide, cGMP, MEK/ERK and PI3K. Signalling pathways mediating the major neuroendocrine regulators of mammalian somatotrophs, as well as those in other major teleost study model systems are also briefly highlighted. Interestingly, unlike mammals, spontaneous action potential firings are not observed in goldfish somatotrophs in culture. Furthermore, three goldfish brain somatostatin forms directly affect pituitary GH secretion via ligand-specific actions on membrane ion channels and intracellular Ca(2+) levels, as well as exert isoform-specific action on basal and stimulated GH mRNA expression, suggesting the importance of somatostatins other than somatostatin-14.

Characterization of novel RFamide peptides in the central nervous system of the brown hagfish: isolation, localization, and functional analysis

RFamide (RFa) peptides play various important roles in the central nervous system in both invertebrates and vertebrates. However, there is no evidence of the existence of any RFamide peptide in the brain of hagfish, one of the oldest lineages of vertebrates. In this study, we sought to identify novel RFamide peptides from the brains of hagfish (Paramyxine atami). We identified four novel RFamide peptides, which had the C-terminal Pro-Gln-Arg-Phe-NH2 structure. cDNA cloning revealed that the identified RFamide peptides are encoded in two types of cDNA. Molecular phylogenetic analysis of the two precursors indicated that the hagfish RFamide peptides belong to the PQRFamide peptide group that includes mammalian neuropeptide FF and AF. Based on immunohistochemistry and in situ hybridization, hagfish PQRFamide peptide precursor mRNA and its translated peptides were localized in the infundibular nucleus of the hypothalamus. Immunoreactive fibers were terminated on blood vessels in the infundibular nucleus. Dense immunoreactive fibers were also observed in other brain regions. We further showed that one of the hagfish PQRFamide peptides significantly stimulated the expression of gonadotropin-β mRNA in the cultured hagfish pituitary. These results indicate that the control mechanism of gonadotropin expression by a hypothalamic neuropeptide evolved in the agnathan brain. This is the first evidence describing the identification of RFamide peptides in the hagfish brain. This is also the first report showing the regulation of gonadotropin expression by a homolog of neuropeptide FF that belongs to the PQRFamide peptide group in any vertebrate.

Isolation and characterisation of prolactin-releasing peptide in chicks and its effect on prolactin release and feeding behaviour

Prolactin (PRL)-releasing peptides (PrRP) have been identified in mammals, amphibians and fishes, and these animals have several PrRPs that consist of different numbers of amino acids such as 20, 31 and 37. In the present study, we identified the cDNA encoding chicken prepro-PrRP, which can generate putative PrRPs, and cloned and sequenced it. Sequences for the coding region suggested the occurrence of putative PrRPs of 20, 31 and 32 amino acid residues. The amino acid sequence of chicken PrRP20 showed 100%, 95% and 70% identity with those of PrRP20s from teleosts, Xenopus laevis and mammals, respectively. On the other hand, chicken PrRP31 showed approximately 90% and 52-55% homology to PrRP31s of X. laevis and mammals, respectively. Native chicken PrRPs were purified from an acid extract of chick brain by a Sep-Pak C18 cartridge (Waters Corp., Milford, MA, USA), affinity chromatography using anti-salmon PrRP serum, and reverse phase high-performance liquid chromatography (HPLC) on an ODS-120T column (TOSOH, Tokyo, Japan). The existence of chicken PrRP20 and PrRP31 in the brain was demonstrated by comparing them with the synthetic peptides using HPLC and matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. Chicken PrRP31 increased plasma PRL concentration when administered peripherally, whereas central administration decreased the concentration, suggesting that chicken PrRP31 has a distinct effect on PRL secretion between tissues in chicks. On the other hand, plasma growth hormone concentration decreased with both peripheral and central administrations of chicken PrRP31. Furthermore, central administration of chicken PrRP31 increased food intake in chicks compared to those observed in mammals and fishes. Taken together with the results indicating that chicken PrRP20 did not show endocrine and behavioural effects, we showed that chicken PrRP has a similar amino acid sequence to teleosts, Xenopus laevis and mammals, although the actions were variable among vertebrates.