Molecular cloning and functional characterization of a vasotocin receptor subtype that is expressed in the shell gland and brain of the domestic chicken

The Evolution of Oxytocin and Vasotocin Receptor Genes in Jawed Vertebrates: A Clear Case for Gene Duplications Through Ancestral Whole-Genome Duplications

The neuronal and neuroendocrine peptides oxytocin (OT) and vasotocin (VT), including vasopressins, have six cognate receptors encoded by six receptor subtype genes in jawed vertebrates. The peptides elicit a broad range of responses that are specifically mediated by the receptor subtypes including neuronal functions regulating behavior and hormonal actions on reproduction and water/electrolyte balance. Previously, we have demonstrated that these six receptor subtype genes, which we designated VTR1A, VTR1B, OTR, VTR2A, VTR2B and VTR2C, arose from a syntenic ancestral gene pair, one VTR1/OTR ancestor and one VTR2 ancestor, through the early vertebrate whole-genome duplications (WGD) called 1R and 2R. This was supported by both phylogenetic and chromosomal conserved synteny data. More recently, other studies have focused on confounding factors, such as the OTR/VTR orthologs in cyclostomes, to question this scenario for the origin of the OTR/VTR gene family; proposing instead less parsimonious interpretations involving only one WGD followed by complex series of chromosomal or segmental duplications. Here, we have updated the phylogeny of the OTR/VTR gene family, including a larger number of vertebrate species, and revisited seven representative neighboring gene families from our previous conserved synteny analyses, adding chromosomal information from newer high-coverage genome assemblies from species that occupy key phylogenetic positions: the polypteriform fish reedfish (Erpetoichthys calabaricus), the cartilaginous fish thorny skate (Amblyraja radiata) and a more recent high-quality assembly of the Western clawed frog (Xenopus tropicalis) genome. Our analyses once again add strong support for four-fold symmetry, i.e., chromosome quadruplication in the same time window as the WGD events early in vertebrate evolution, prior to the jawed vertebrate radiation. Thus, the evolution of the OTR/VTR gene family can be most parsimoniously explained by two WGD events giving rise to the six ancestral genes, followed by differential gene losses of VTR2 genes in different lineages. We also argue for more coherence and clarity in the nomenclature of OT/VT receptors, based on the most parsimonious scenario.

Arginine vasotocin (AVT)/mesotocin (MT) receptors in chickens: Evidence for the possible involvement of AVT-AVPR1 signaling in the regulation of oviposition and pituitary prolactin expression

Two structurally related peptides, arginine vasotocin (AVT) and mesotocin (MT), are reported to regulate many physiological processes, such as anti-diuresis and oviposition in birds, and their actions are likely mediated by four AVT/MT receptors (AVPR1A, AVPR1B, MTR and AVPR2b), which are orthologous/paralogous to human AVPR1A, AVPR1B, OXTR and AVPR2 respectively. However, our knowledge regarding the functions of these avian AVT/MT receptors has been limited. Here, we examined the functionality and expression of these receptors in chickens and investigated the roles of AVT in the anterior pituitary. Our results showed that 1) AVPR1A, AVPR1B and AVPR2b could be preferentially activated by AVT, monitored by cell-based luciferase reporter assays and/or Western blot, indicating that they are AVT-specific receptors (AVPR1A; AVPR1B) or AVT-preferring receptor (AVPR2b) functionally coupled to intracellular calcium, MAPK/ERK and cAMP/PKA signaling pathways. In contrast, MTR could be activated by AVT and MT with similar potencies, indicating that MTR is a receptor common for both peptides; 2) Using qPCR, differential expression of the four receptors was found in chicken tissues including the oviduct and anterior pituitary. In particular, only AVPR1A is abundantly expressed in the uterus, suggesting its involvement in mediating AVT-induced oviposition. 3) In cultured chick pituitary cells, AVT could stimulate ACTH and PRL expression and secretion, an action likely mediated by AVPR1B and/or AVPR1A abundantly expressed in anterior pituitary. Collectively, our data helps to elucidate the roles of AVT/MT in birds, such as the 'oxytocic action' of AVT, which induces uterine muscle contraction during oviposition.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

The emergence of the vasopressin and oxytocin hormone receptor gene family lineage: Clues from the characterization of vasotocin receptors in the sea lamprey (Petromyzon marinus)

The sea lamprey (Petromyzon marinus) is a jawless vertebrate at an evolutionary nexus between invertebrates and jawed vertebrates. Lampreys are known to possess the arginine vasotocin (AVT) hormone utilized by all non-mammalian vertebrates. We postulated that the lamprey would possess AVT receptor orthologs of predecessors to the arginine vasopressin (AVP)/oxytocin (OXT) family of G protein-coupled receptors found in mammals, providing insights into the origins of the mammalian V1A, V1B, V2 and OXT receptors. Among the earliest animals to diverge from the vertebrate lineage in which these receptors are characterized is the jawed, cartilaginous elephant shark, which has genes orthologous to all four mammalian receptor types. Therefore, our work was aimed at helping resolve the critical gap concerning the outcomes of hypothesized large-scale (whole-genome) duplication events. We sequenced one partial and four full-length putative lamprey AVT receptor genes and determined their mRNA expression patterns in 15 distinct tissues. Phylogenetically, three of the full-coding genes possess structural characteristics of the V1 clade containing the V1A, V1B and OXT receptors. Another full-length coding gene and the partial sequence are part of the V2 clade and appear to be most closely related to the newly established V2B and V2C receptor subtypes. Our synteny analysis also utilizing the Japanese lamprey (Lethenteron japonicum) genome supports the recent proposal that jawless and jawed vertebrates shared one-round (1R) of WGD as the most likely scenario.

Neuropeptide Arginine Vasotocin Positively Affects Neurosteroidogenesis in the Early Brain of Grouper, Epinephelus coioides

The neuropeptide arginine vasotocin (AVT) has versatile physiological functions in non-mammalian vertebrates. However, the functional association between AVT and neurosteroidogenesis in the early brain of teleosts remains elusive. We thus studied the developmental expression patterns of the avt gene and their V1 type receptor (avt-rv1 ) at various stages of development [90-150 days after hatching (dah)] in relation to neurosteroidogenesis and oestrogen signalling in the early brain of the orange-spotted grouper (Epinephelus coioides). avt and avt-rv1 mRNAs displayed a significantly increase in expression at 110 dah in the telencephalon and diencephalon. Further, avt mRNAs were localised in three magnocellular neuronal populations of the preoptic area, such as parvocellular, magnocellular and gigantocellular preoptic neurones. Intriguingly, the avt transcripts in those neurones were more abundant in 110 dah compared to other ages. Subsequently, dual fluorescence in situ hybridisation analysis showed that the avt and avt-rv1 genes were highly coexpressed with cyp11a1, hsd3b1, cyp17a1, erα, erβ and gpr30, which indicates their potential for functional association. Cyp19a1b-immunoreactive positive fibres were found in close proximity to avt-expressing neurones. Moreover, our results showed that exogenous Avt caused a significant increase in the cellular and gene levels of steroidogenic enzymes and oestrogen receptors (ers), whereas the administration of an Avt-rv1 antagonist caused a decrease in the expression of both steroidogenic enzymes and ers genes in the brain. Furthermore, exogenous oestradiol (E2 ) strongly up-regulated avt mRNAs in the grouper brain. Taken together, the present studies suggest that avt and steroidogenesis may positively work together to increase both E2 biosynthesis and early brain development.

Molecular cloning, sequencing and phylogeny of vasotocin receptor genes in the air-breathing catfish Heteropneustes fossilis with sex dimorphic and seasonal variations in tissue expression

Vasotocin (VT) is the ortholog of vasopressin (VP) in non-mammalian vertebrates and is known for multiple functions. Teleost fishes have a complete repertoire of known VP/VT receptor subtypes (vasopressin type, VR): two V1A subtypes (V1Aa and V1Ab or V1a1 and V1a2) and five V2 subtypes (V2A1, V1A2, V2B1, V2B2 and V2C). Full-length cDNAs of v1a1, v1a2 and v2 (v2a1) with ORFs of 1,308, 1,137 and 1,527 bp, respectively, were cloned and characterized in the catfish Heteropneustes fossilis (Siluriformes, Ostariophysi). BLAST analysis revealed that the genes encoded three VT receptors, V1a1, V1a2 and V2 of 436, 379 and 509 amino acid residues, respectively. The predicted proteins showed typical features of the seven-transmembrane domain receptor core structure with hallmark triplets Asp-Arg-Tyr/Asp-Arg-His (DRY/DRH) and the variable intracellular loop III of vertebrate neurohypophysial hormone receptors. Phylogenetic analysis of the deduced protein sequences revealed that they clustered with the V1Aa, V1Ab and V2A1, respectively, of other teleosts. The V2R has a sequence identity of 70-76% with V2A1 than with the V2B type (sequence identity 43-49%). Semiquantitative PCR analysis showed that the receptor gene transcripts were expressed ubiquitously in the tissues examined (brain, pituitary, gonads, liver, muscle, kidney and gills) and displayed sex and seasonal fluctuations in a tissue-specific manner. The results form a basis for functional studies on the VT receptors in the catfish.

Species, sex and individual differences in the vasotocin/vasopressin system: relationship to neurochemical signaling in the social behavior neural network

Arginine-vasotocin (AVT)/arginine vasopressin (AVP) are members of the AVP/oxytocin (OT) superfamily of peptides that are involved in the regulation of social behavior, social cognition and emotion. Comparative studies have revealed that AVT/AVP and their receptors are found throughout the "social behavior neural network (SBNN)" and display the properties expected from a signaling system that controls social behavior (i.e., species, sex and individual differences and modulation by gonadal hormones and social factors). Neurochemical signaling within the SBNN likely involves a complex combination of synaptic mechanisms that co-release multiple chemical signals (e.g., classical neurotransmitters and AVT/AVP as well as other peptides) and non-synaptic mechanisms (i.e., volume transmission). Crosstalk between AVP/OT peptides and receptors within the SBNN is likely. A better understanding of the functional properties of neurochemical signaling in the SBNN will allow for a more refined examination of the relationships between this peptide system and species, sex and individual differences in sociality.

Hormonal regulation of vasotocin receptor mRNA in a seasonally breeding songbird

Behaviors associated with breeding are seasonally modulated in a variety of species. These changes in behavior are mediated by sex steroids, levels of which likewise vary with season. The effects of androgens on behaviors associated with breeding may in turn be partly mediated by the nonapeptides vasopressin (VP) and oxytocin (OT) in mammals, and vasotocin (VT) in birds. The effects of testosterone (T) on production of these neuropeptides have been well-studied; however, the regulation of VT receptors by T is not well understood. In this study, we investigated steroid-dependent regulation of VT receptor (VTR) mRNA in a seasonally breeding songbird, the white-throated sparrow (Zonotrichia albicollis). We focused on VTR subtypes that have been most strongly implicated in social behavior: V1a and oxytocin-like receptor (OTR). Using in situ hybridization, we show that T-treatment of non-breeding males altered V1a and OTR mRNA expression in several regions associated with seasonal reproductive behaviors. For example, T-treatment increased V1a mRNA expression in the medial preoptic area, bed nucleus of the stria terminalis, and ventromedial hypothalamus. T-treatment also affected both V1a and OTR mRNA expression in nuclei of the song system; some of these effects depended on the presence or absence of a chromosomal rearrangement that affects singing behavior, plasma T, and VT immunolabeling in this species. Overall, our results strengthen evidence that VT helps mediate the behavioral effects of T in songbirds, and suggest that the chromosomal rearrangement in this species may affect the sensitivity of the VT system to seasonal changes in T.

The vertebrate ancestral repertoire of visual opsins, transducin alpha subunits and oxytocin/vasopressin receptors was established by duplication of their shared genomic region in the two rounds of early vertebrate genome duplications

BACKGROUND

Vertebrate color vision is dependent on four major color opsin subtypes: RH2 (green opsin), SWS1 (ultraviolet opsin), SWS2 (blue opsin), and LWS (red opsin). Together with the dim-light receptor rhodopsin (RH1), these form the family of vertebrate visual opsins. Vertebrate genomes contain many multi-membered gene families that can largely be explained by the two rounds of whole genome duplication (WGD) in the vertebrate ancestor (2R) followed by a third round in the teleost ancestor (3R). Related chromosome regions resulting from WGD or block duplications are said to form a paralogon. We describe here a paralogon containing the genes for visual opsins, the G-protein alpha subunit families for transducin (GNAT) and adenylyl cyclase inhibition (GNAI), the oxytocin and vasopressin receptors (OT/VP-R), and the L-type voltage-gated calcium channels (CACNA1-L).

RESULTS

Sequence-based phylogenies and analyses of conserved synteny show that the above-mentioned gene families, and many neighboring gene families, expanded in the early vertebrate WGDs. This allows us to deduce the following evolutionary scenario: The vertebrate ancestor had a chromosome containing the genes for two visual opsins, one GNAT, one GNAI, two OT/VP-Rs and one CACNA1-L gene. This chromosome was quadrupled in 2R. Subsequent gene losses resulted in a set of five visual opsin genes, three GNAT and GNAI genes, six OT/VP-R genes and four CACNA1-L genes. These regions were duplicated again in 3R resulting in additional teleost genes for some of the families. Major chromosomal rearrangements have taken place in the teleost genomes. By comparison with the corresponding chromosomal regions in the spotted gar, which diverged prior to 3R, we could time these rearrangements to post-3R.

CONCLUSIONS

We present an extensive analysis of the paralogon housing the visual opsin, GNAT and GNAI, OT/VP-R, and CACNA1-L gene families. The combined data imply that the early vertebrate WGD events contributed to the evolution of vision and the other neuronal and neuroendocrine functions exerted by the proteins encoded by these gene families. In pouched lamprey all five visual opsin genes have previously been identified, suggesting that lampreys diverged from the jawed vertebrates after 2R.

Sex-specific activity and function of hypothalamic nonapeptide neurons during nest-building in zebra finches

Vertebrate species from fish to humans engage in a complex set of preparatory behaviors referred to as nesting; yet despite its phylogenetic ubiquity, the physiological and neural mechanisms that underlie nesting are not well known. We here test the hypothesis that nesting behavior is influenced by the vasopressin-oxytocin (VP-OT) peptides, based upon the roles they play in parental behavior in mammals. We quantified nesting behavior in male and female zebra finches following both peripheral and central administrations of OT and V1a receptor (OTR and V1aR, respectively) antagonists. Peripheral injections of the OTR antagonist profoundly reduce nesting behavior in females, but not males, whereas comparable injections of V1aR antagonist produce relatively modest effects in both sexes. However, central antagonist infusions produce no effects on nesting, and OTR antagonist injections into the breast produce significantly weaker effects than those into the inguinal area, suggesting that antagonist effects are mediated peripherally, likely via the oviduct. Finally, immunocytochemistry was used to quantify nesting-induced Fos activation of nonapeptide neurons in the paraventricular and supraoptic nuclei of the hypothalamus and the medial bed nucleus of the stria terminalis. Nest-building induced Fos expression within paraventricular VP neurons of females but not males. Because the avian forms of OT (Ile(8)-OT; mesotocin) and VP (Ile(3)-VP; vasotocin) exhibit high affinity for the avian OTR, and because both peptide forms modulate uterine contractility, we hypothesize that nesting-related stimuli induce peptide release from paraventricular vasotocin neurons, which then promote female nesting via peripheral feedback from OTR binding in the oviduct uterus.

AVT is involved in the regulation of ion transport in the intestine of the sea bream (Sparus aurata)

The intestine of marine fish plays a crucial role in ion homeostasis by selective processing of ingested fluid. Although arginine vasotocin (AVT) is suggested to play a role in ion regulation in fish, its action in the intestine has not been demonstrated. Thus, the present study investigated in vitro the putative role of AVT in intestinal ion transport in the sea bream (Sparus aurata). A cDNA encoding part of an AVT receptor was isolated and phylogenetic analysis revealed it clustered with the V1a2-type receptor clade. V1a2 transcripts were expressed throughout the gastrointestinal tract, from esophagus to rectum, and were most abundant in the rectum regardless of long-term exposure to external salinities of 12, 35 or 55p.p.t. Basolateral addition of AVT (10(-6)M) to the anterior intestine and rectum of sea bream adapted to 12, 35 or 55p.p.t. mounted in Ussing chambers produced rapid salinity and region dependent responses in short circuit current (Isc), always in the absorptive direction. In addition, AVT stimulation of absorptive Isc conformed to a dose-response curve, with significant effects achieved at 10(-8)M, which corresponds to physiological values of plasma AVT for this species. The effect of AVT on intestinal Isc was insensitive to the CFTR selective inhibitor NPPB (200μM) applied apically, but was completely abolished in the presence of apical bumetanide (200μM). We propose a role for AVT in the regulation of ion absorption in the intestine of the sea bream mediated by an absorptive bumetanide-sensitive mechanism, likely NKCC2.

Neuroendocrine regulation of stress in birds with an emphasis on vasotocin receptors (VTRs)

The neuroendocrine stress response of vertebrates, particularly mammals, comprises at least two types of neuropeptide containing neurons, corticotropin-releasing hormone (CRH) and vasopressin (VP) neurons, and four receptors [CRH receptor one (CRH-R1) and two (CRH-R2) and VP receptor 1a (V1aR) and 1b (V1bR)]. The avian neuropeptide CRH, a 41-amino acid peptide, has been shown to have the same amino acid sequence as humans while nonapeptide neurohormone arginine-vasotocin (AVT) is regarded as highly conserved having a single amino acid substitution compared to mammalian arginine vasopressin. Similar to mammals, birds have two receptor subtypes (CRH-R1 and CRH-R2) for CRH, however, four vasotocin receptors have been identified. Less is known about the functions of the four avian vasotocin receptors compared to homologous ones found in mammals and other vertebrate classes. Recently, chicken vasotocin receptor two (VT2R) and four (VT4R) have been characterized utilizing immunocytochemistry and an imposed stress test. The purpose of this review is to present evidence that the VT2R and VT4R are involved in the avian stress response and that the cephalic lobe of the anterior pituitary appears specialized for this function as it contains the major population of corticotropes and necessary neuroendocrine receptors to respond to stressors impacting avian species.

A possible mechanism contributing to the synergistic action of vasotocin (VT) and corticotropin-releasing hormone (CRH) receptors on corticosterone release in birds

Arginine vasotocin (AVT) and corticotropin-releasing hormone (CRH) are two neuronal regulators in the hypothalamic-pituitary-adrenal (HPA) axis that modulate biological responses to stress in avian species. When AVT and CRH are administered together in vitro or in vivo, levels of adrenocorticotropic hormone (ACTH) or plasma corticosterone (CORT) are released, respectively, in a synergistic manner. The underlying mechanism of this greater than additive stress response was investigated by expressing the vasotocin receptor type 2 (VT2R) and CRH receptor type 1 (CRH-R1), both G-protein coupled receptors, in HeLa cells. Fluorescence resonance energy transfer (FRET) analysis provided the evidence for heterodimerization of the VT2R/CRH-R1 in the presence of their respective ligands, AVT and CRH. The VT2R and CRH-R1 were tagged at the C-terminal ends with either cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP), and a VT2R chimera was constructed by replacing the fourth transmembrane region (TM4) of the VT2R with TM-IV of the β2-adrenergic receptor (β2AR). When VT2R/β2AR chimera and CRH-R1 were expressed in HeLa cells, heterodimerization was partly disrupted. Taken together, these data indicate that TM-IV of the VT2R may provide an important interface for effective receptor dimerization, suggesting that direct molecular interaction between VT2R and CRH-R1 receptors plays a role in mediating an enhanced interaction between these two receptors. Their interaction at the anterior pituitary level may potentiate the endocrine output of the avian HPA system.

Roles of arginine vasotocin receptors in the brain and pituitary of submammalian vertebrates

This chapter reviews the functions of arginine vasotocin (AVT) and its receptors in the central nervous system (CNS) of primarily submammalian vertebrates. The V1a-type receptor, which is widely distributed in the CNS of birds, amphibians, and fish, is one of the most important receptors involved in the expression of social and reproductive behaviors. In mammals, the V1b receptor of arginine vasopressin, an AVT ortholog, is assumed to be involved in aggression, social memory, and stress responses. The distribution of the V1b-type receptor in the brain of submammalian vertebrates has only been reported in an amphibian species, and its putative functions are discussed in this review. The functions of V2-type receptor in the CNS are still unclear. Recent phylogenetical and pharmacological analyses have revealed that the avian VT1 receptor can be categorized as a V2b-type receptor. The distribution of this newly categorized VT1 receptor in the brain of avian species should contribute to our knowledge of the possible roles of the V2b-type receptor in the CNS of other nonmammalian vertebrates. The functions of AVT in the amphibian and avian pituitaries are also discussed, focusing on the V1b- and V1a-type receptors.

The fifth neurohypophysial hormone receptor is structurally related to the V2-type receptor but functionally similar to V1-type receptors

The neurohypophysial peptides of the vasopressin (VP) and oxytocin (OT) families regulate salt and water homeostasis and reproduction through distinct G protein-coupled receptors. The current thinking is that there are four neurohypophysial hormone receptors (V1aR, V1bR, V2R, and OTR) in vertebrates, and their evolutionary history is still debated. We report the identification of a fifth neurohypophysial hormone receptor (V2bR) from the holocephalan elephant fish. This receptor is similar to conventional V2R (V2aR) in sequence, but induced Ca(2+) signaling in response to vasotocin (VT), the non-mammalian VP ortholog; such signaling is typical of V1-type receptors. In addition, V1aR, V1bR and OTR were also isolated from the elephant fish. Further screening revealed that orthologous V2bRs are widely distributed throughout the jawed vertebrates, and that the V2bR family is subdivided into two subfamilies: the fish specific type-1, and a type-2 that is characteristically found in tetrapods. Analysis suggested that the mammalian V2bR may have lost its function. Based on molecular phylogenetic, synteny and functional analyses, we propose a new evolutionary history for the neurohypophysial hormone receptors in vertebrates as follows: the first duplication generated V1aR/V1bR/OTR and V2aR/V2bR lineages; after divergence from the V2bR lineage, the V2aRs evolved to use cAMP as a second messenger, while the V2bRs retained the original Ca(2+) signaling system. Future studies on the role of V2bR in the brain, heart, kidney and reproductive organs, in which it is highly expressed, will open a new research field in VP/VT physiology and evolution.

Evolving nonapeptide mechanisms of gregariousness and social diversity in birds

Of the major vertebrate taxa, Class Aves is the most extensively studied in relation to the evolution of social systems and behavior, largely because birds exhibit an incomparable balance of tractability, diversity, and cognitive complexity. In addition, like humans, most bird species are socially monogamous, exhibit biparental care, and conduct most of their social interactions through auditory and visual modalities. These qualities make birds attractive as research subjects, and also make them valuable for comparative studies of neuroendocrine mechanisms. This value has become increasingly apparent as more and more evidence shows that social behavior circuits of the basal forebrain and midbrain are deeply conserved (from an evolutionary perspective), and particularly similar in birds and mammals. Among the strongest similarities are the basic structures and functions of avian and mammalian nonapeptide systems, which include mesotocin (MT) and arginine vasotocin (VT) systems in birds, and the homologous oxytocin (OT) and vasopressin (VP) systems, respectively, in mammals. We here summarize these basic properties, and then describe a research program that has leveraged the social diversity of estrildid finches to gain insights into the nonapeptide mechanisms of grouping, a behavioral dimension that is not experimentally tractable in most other taxa. These studies have used five monogamous, biparental finch species that exhibit group sizes ranging from territorial male-female pairs to large flocks containing hundreds or thousands of birds. The results provide novel insights into the history of nonapeptide functions in amniote vertebrates, and yield remarkable clarity on the nonapeptide biology of dinosaurs and ancient mammals. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

The oxytocin/vasopressin receptor family has at least five members in the gnathostome lineage, inclucing two distinct V2 subtypes

The vertebrate oxytocin and vasopressin receptors form a family of G-protein-coupled receptors (GPCRs) that mediate a large variety of functions, including social behavior and the regulation of blood pressure, water balance and reproduction. In mammals four family members have been identified, three of which respond to vasopressin (VP) named V1A, V1B and V2, and one of which is activated by oxytocin (OT), called the OT receptor. Four receptors have been identified in chicken as well, but these have received different names. Until recently only V1-type receptors have been described in several species of teleost fishes. We have identified family members in several gnathostome genomes and performed phylogenetic analyses to classify OT/VP-receptors across species and determine orthology relationships. Our phylogenetic tree identifies five distinct ancestral gnathostome receptor subtypes in the OT/VP receptor family: V1A, V1B, V2A, V2B and OT receptors. The existence of distinct V2A and V2B receptors has not been previously recognized. We have found these two subtypes in all examined teleost genomes as well as in available frog and lizard genomes and conclude that the V2A-type is orthologous to mammalian V2 receptors whereas the V2B-type is orthologous to avian V2 receptors. Some teleost fishes have acquired additional and more recent gene duplicates with up to eight receptor family members. Thus, this analysis reveals an unprecedented complexity in the gnathostome repertoire of OT/VP receptors, opening interesting research avenues regarding functions such as regulation of water balance, reproduction and behavior, particularly in reptiles, amphibians, teleost fishes and cartilaginous fishes.

Neural distribution of vasotocin receptor mRNA in two species of songbird

The neurohypophyseal hormones vasopressin and oxytocin are produced and released within the mammalian brain, where they act via multiple receptor subtypes. The neural distributions of these receptors, for example, V1a and oxytocin receptors, have been well described in many mammals. In birds, the distribution of binding sites for the homologous neuropeptides, vasotocin (VT) and mesotocin, has been studied in several species by using synthetic radioligands designed to bind to mammalian receptors. Such binding studies, however, may not reveal the specific distributions of each receptor subtype. To identify and map the receptors likely to bind VT and mesotocin, we generated partial cDNA sequences for four VT receptor subtypes, VT1, VT2 (V1b), VT3 (oxytocin-like), and VT4 (V1a), in white-throated sparrow (Zonotrichia albicollis) and zebra finch (Taeniopygia guttata). These genes shared high sequence identity with the homologous avian and mammalian neurohypophyseal peptide receptors, and we found evidence for VT1, VT3, and VT4 receptor mRNA expression throughout the brains of both species. As has been described in rodents, there was striking interspecific and intraspecific variation in the densities and distribution of these receptors. For example, whereas the VT1 receptor mRNA was more widespread in zebra finch brain, the VT3 (oxytocin-like) receptor mRNA was more prevalent in the sparrow brain. Although VT2 (V1b) receptor mRNA was abundant in the pituitary, it was not found in the brain. Because of their association with brain regions implicated in social behavior, the VT1, VT3, and VT4 receptors are all likely candidates for mediating the behavioral effects of VT.

Vasotocin induces final oocyte maturation and ovulation through the production of a maturation-inducing steroid in the catfish Heteropneustes fossilis

The study reports for the first time vasotocin (VT) induction of final oocyte maturation and ovulation through the production of the maturation-inducing steroid 17, 20β-dihydroxy-4-pregnen-3-one (MIS, 17, 20β-DP). Post-vitellogenic follicles of the catfish Heteropneustes fossilis were incubated with different concentrations of VT (1, 10, 100 and 1000 nM) for different time periods. Germinal vesicle breakdown [GVBD, as a marker of final oocyte maturation (FOM)] and ovulation were scored. In another series of experiments, the follicles were incubated with VT alone or in combination with VT receptor (V(1) and V(2)) antagonists, and GVBD and ovulation were increased with progesterone, 17-hydroxy-4-pregnene-3, 20-dione (17-P) and 17, 20β-DP levels. VT stimulated both GVBD and ovulation in a concentration and time-dependent manner, and the responses were inhibited to varying degrees in groups incubated with the VT receptor antagonists. The V(1) antagonist inhibited the responses by 2- to 3-fold and more than the V(2) antagonist, and the combination was more potent than the separate incubation. Progestins increased time-dependently in the VT groups and the fold increase was greater for the MIS. The VT-induced steroid stimulation was significantly inhibited to near the control levels in co-incubations with both V(1) and V(2) receptor antagonists, in the order 17, 20β-DP > 17-P > P(4). The inhibition by the V(1) receptor antagonist was greater than that with the V(2) blocker, and followed the same order of inhibition described above. The results suggest that VT induces FOM and ovulation mainly through the V(1) receptors.

Identification and characterization of mesotocin and V1a-like vasotocin receptors in a urodele amphibian, Taricha granulosa

The cDNA sequences encoding the mesotocin receptor (MTR) and vasotocin 1a receptor (VTR-1a) were identified in a urodele amphibian, the rough-skinned newt, Taricha granulosa. Saturation binding of [(3)H]oxytocin (OT) to the Taricha MTR (tMTR) was best fit by a two-state model; a high affinity-low abundance site and a lower affinity-high abundance site. Competition-binding studies found the following rank-order affinities for the tMTR: mesotocin (MT)>OT≈vasotocin (VT)>vasopressin (VP)>isotocin (IT). Inositol phosphate (IP) accumulation studies demonstrated functional activity of both the tMTR and Taricha VTR-1a (tVTR-1a) in a heterologous cell culture system. The rank-order potencies for the tMTR were MT>OT>VT≈VP>IT. The combined binding and IP results indicate that VT may act as a partial agonist of the tMTR. Rank-order potencies for the tVTR-1a were VT>VP>MT≈OT>IT. For both receptors, stimulation of IP accumulation was blocked by d(CH(2))(5)[Tyr(Me)(2)]AVP (Manning compound) and d(CH(2))(5)[Tyr(Me)(2),Thr(4),Tyr-NH(2)]OVT (OTA). OTA was a more potent antagonist for the transiently expressed tMTR while Manning compound was relatively more potent at inhibiting IP accumulation in tVTR-1a expressing cells. In contradiction to earlier assumptions, the absolute IC(50) of Manning compound was lower for the tMTR (27nM±13) than the tVTR-1a (586nM±166) indicating its potential higher affinity for the tMTR, a finding with special relevance to interpretation of comparative studies investigating the behavioral and physiological actions of neurohypophysial peptides in non-mammalian species.

Identification of multiple vasotocin receptor cDNAs in teleost fish: sequences, phylogenetic analysis, sites of expression, and regulation in the hypothalamus and gill in response to hyperosmotic challenge

Vasopressin and its homolog vasotocin regulate hydromineral balance, stress responses, and social behaviors in vertebrates. In mammals, the functions of vasopressin are mediated via three classes of membrane-bound receptors: V1a-type, V1b-type and V2-type. To date, however, only a single class of vasotocin receptor has been identified in teleost fish. Here, cDNAs encoding three putative vasotocin receptors - two distinct V1a-type receptor paralogs (V1a1 and V1a2) and a previously undescribed V2-type receptor (V2) - and a single isotocin receptor were isolated and sequenced from the Amargosa pupfish (Cyprinodon nevadensis amargosae). RT-PCR revealed that mRNAs for these receptors differed in expression patterns with V1a1 mRNAs abundant in the brain, pituitary and testis, V1a2 transcripts at greatest levels in brain, heart and muscle, V2 transcripts most common in the gills, heart and kidney, and isotocin receptor mRNAs abundant in the midbrain, pituitary and gonads. In response to an acute hyperosmotic challenge, pro-vasotocin and V2 mRNA levels in the hypothalamus decreased, while transcripts of V1a1 in the hypothalamus and V1a2 in the gills increased. Partial transcripts for structurally related V2-type, as well as multiple V1a-type, receptors were also identified in other teleosts, suggesting that multiple vasotocin receptors may be present in many Actinopterygii fishes.

New hypotheses on the function of the avian shell gland derived from microarray analysis comparing tissue from juvenile and sexually mature hens

Activation of the shell gland region of the avian oviduct is mediated by ovarian steroids. To understand more extensively how shell glands are maintained and function, we have compared gene expression in the shell glands from juvenile and laying hens using a chicken cDNA microarray. Average expression profiles of juvenile and sexually mature shell glands were compared resulting in the identification of 266 differentially regulated genes. Reverse transcription quantitative polymerase chain reaction confirmed expression differences. The differentially expressed genes included several with known involvement in shell gland function, including ion transport and shell matrix proteins. There were also many unpredicted differentially expressed genes, and for some we propose hypotheses for their functions. These include those encoding (a) osteoprotegerin, a decoy death receptor for receptor activator of nuclear factor NFkB ligand (RANKL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), that in the shell gland, may prevent apoptosis and/or may have an endocrine effect by preventing RANKL's action on bone osteoclasts that mobilize stored calcium; (b) prostatic acid phosphatase (ACPP) and prostate stem cell antigen (PSCA) that could play a role in sperm physiology within the shell gland; (c) urea transporter (SLC14A2) that could provide a novel anti-microbial defence; (d) bactericidal/permeability-increasing protein-like 2 (BPIL2), and other potential anti-microbials that have not previously been documented in the chicken. These new hypotheses, if borne out experimentally, will lead to a greater understanding of shell gland function including the processes involved in eggshell formation and anti-microbial activity.

Neural distribution of nonapeptide binding sites in two species of songbird

Vasotocin (VT) and its mammalian homologue, vasopressin (VP), modulate many social behaviors in a variety of vertebrate species. In songbirds, the effects of centrally administered VT vary according to species, which may reflect species-specific distributions of VT binding sites. Different radioligands used to map receptors in previous autoradiographical studies have revealed nonoverlapping distributions of VT binding, suggesting a heterogeneous population of more than one type of VT receptor. For two model songbird species, the white-throated sparrow (Zonotrichia albicollis) and zebra finch (Taeniopygia guttata), we labeled putative VT receptors with two radioligands, [(125)I]ornithine vasotocin analog ([(125)I]OVTA) and [(125)I]linear VP antagonist ([(125)I]HO-LVA). Competitive binding assays in the lateral septum showed that both ligands were effectively displaced by both VT and a related nonapeptide, mesotocin (MT), showing that these radioligands, which were developed to label mammalian nonapeptide receptors, label at least one population of related receptors in songbirds. [(125)I]OVTA labeled receptors throughout the telencephalon, diencephalon, midbrain, and brainstem, with a similar distribution in both species. In contrast, the binding of [(125)I]HO-LVA was restricted to the septal area, dorsal arcopallium, and optic tectum in sparrow and was essentially undetectable in zebra finch. Because the avian brain is likely to express multiple types of VT receptors, we hypothesize that the binding patterns of these radioligands represent a heterogeneous receptor population.

Transmembrane domain IV of the Gallus gallus VT2 vasotocin receptor is essential for forming a heterodimer with the corticotrophin releasing hormone receptor

Corticotropin releasing hormone receptor (CRHR) and the VT2 arginine vasotocin receptor (VT2R) are vital links in the hypothalamic-pituitary-adrenal axis that enable a biological response to stressful stimuli in avian species. CRHR and VT2R are both G-protein coupled receptors (GPCRs), and have been shown by us to form a heterodimer via fluorescent resonance energy transfer (FRET) analysis in the presence of their respective ligands, corticotrophin releasing hormone (CRH) and arginine vasotocin (AVT). The dimerization interface of the heterodimer is unknown, but computational analyses predict transmembrane domains (TMs) as likely sites of the interaction. We constructed chimerical VT2Rs, tagged at the C-terminal ends with either cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP), by replacing the fourth transmembrane region (TM4) of VT2R with TM4 of the beta2-adrenergic receptor (beta2AR). The VT2R/beta2AR chimeras were expressed in HeLa cells and proper trafficking is confirmed by observing cell membrane localization using confocal microscopy. VT2R/beta2AR-YFP chimera functionality was confirmed with a Fura-2 acetoxymethyl ester (Fura-2AM) assay. FRET analysis was then performed on VT2/beta2AR-chimera/CRHR pairs, and the calculated distance was observed to be >10 nm apart, indicating that heterodimerization was partly disrupted by mutating TM4 of the VT2R. Therefore, TM4 may form one region of the possible dimerization interfaces between the VT2R and CRHR.

Effect of photoperiod and estrogen on expression of arginine vasotocin and its oxytocic-like receptor in the shell gland of the Japanese quail

The avian neurohypophysial hormone arginine vasotocin (AVT) is an important regulatory hormone involved in many physiological processes including fluid balance, blood pressure regulation, stress responses and reproductive events including oviposition. The mechanisms by which AVT stimulates myometrial contraction during oviposition are not well established in birds. In the present study, immunohistochemistry and in situ hybridization were used to localize AVT and the oxytocin-like VT3 receptor in the shell gland of Japanese quail (Coturnix coturnix japonica). Using an AVT-specific antibody, immunoreactive AVT (ir-AVT) was observed in the myometrium of both photosensitive and photorefractory birds. Similarly, VT3 receptor gene transcripts were detected in the myometrial layer of the shell gland of both photosensitive and photorefractory birds. Body mass, shell gland mass and length of mucosal folds of the shell gland of photosensitive birds was higher compared to that of photorefractory birds. Treatment of photorefractory birds with estrogen increased shell gland activity (mass and length of mucosal folds) and levels of both AVT and VT3 receptor mRNA, whereas treatment of photosensitive birds with the estrogen antagonist tamoxifen decreased shell gland activity and levels of both AVT and VT3 receptor mRNA. Our results suggest that shell gland contractility in response to AVT may be regulated during the reproductive cycle of the Japanese quail and that, in part, this regulation is estrogen-dependent.

Molecular evolution of neuropeptide receptors with regard to maintaining high affinity to their authentic ligands

Recently, we cloned many of the bullfrog neuropeptide G protein-coupled receptors (GPCRs), including receptors for vasotocin (VT), mesotocin, gonadotropin-releasing hormone (GnRH), neurotensin, apelin, and metastin. Bullfrog GPCRs usually have high affinity for bullfrog ligands but relatively low affinity for mammalian ligands. Reciprocally, synthetic agonists and antagonists developed based upon mammalian ligands display lower affinity at bullfrog receptors. Studies using chimeric or domain-swapped receptors indicate that the motifs responsible for differential ligand selectivity usually reside within transmembrane domain 6 (TMD6)-extracellular loop 3 (ECL3)-transmembrane domain 7 (TMD7). Triple mutation of mammalian V1aR (Phe(6.51) to Tyr, Ile(6.53) to Thr, and Pro(7.33) to Thr) increases VT affinity but greatly reduces arginine vasopressin affinity. This binding profile is similar to that of bullfrog VT1R. Changing just three amino acids in the bullfrog GnRH receptor-1 (i.e. Ser-Gln-Ser in the ECL3) to those found in the type-I mammalian GnRH receptor (i.e. Ser-Glu-Pro) reverses GnRH selectivity. In conclusion, specific receptor motifs that govern ligand selectivity can be determined by comparative molecular analyses of GPCRs and their ligands. Such analysis provides clues for understanding how GPCRs maintain high affinity to their authentic ligands.

Vasopressin-induced translocation and proteolysis of protein kinase Cα in an amphibian brain: Modulation by corticosterone

In urodele amphibians, the hypothalamic neuropeptide arginine vasotocin and the adrenal steroid corticosterone interact to regulate reproductive behavior by actions in the brain. The present study investigated signal transduction pathways underlying acute effects of vasotocin and corticosterone, presumably mediated via "non-genomic" steroid action, in an amphibian brain. We used Western blot to examine the effects of corticosterone and the vasotocin receptor agonist arginine vasopressin, alone and in combination, on the subcellular localization and proteolytic processing of protein kinase C-alpha (PKCalpha) in tiger salamander brain tissue. Treatment of whole brain minces with vasopressin or vasotocin led to increases in PKCalpha in membrane fractions and concurrent decreases in PKCalpha in cytosolic fractions. Vasopressin or vasotocin treatment also induced the appearance in membrane and cytosolic fractions of a PKCalpha-immunoreactive band that corresponds to PKMalpha, the proteolytically generated, free catalytic subunit of PKCalpha. Treatment with corticosterone alone had no consistent effect on either PKCalpha or PKMalpha in either fraction. However, pretreatment with corticosterone reliably blocked vasopressin-induced increases in cytosolic PKMalpha. These data provide new information about the cellular mechanisms of action of vasopressin and corticosterone in the vertebrate brain and suggest a cellular mechanism by which the two hormones interact to regulate neuronal physiology and behavior.

Charged extracellular residues, conserved throughout a G-protein-coupled receptor family, are required for ligand binding, receptor activation, and cell-surface expression

For G-protein-coupled receptors (GPCRs) in general, the roles of extracellular residues are not well defined compared with residues in transmembrane helices (TMs). Nevertheless, extracellular residues are important for various functions in both peptide-GPCRs and amine-GPCRs. In this study, the V(1a) vasopressin receptor was used to systematically investigate the role of extracellular charged residues that are highly conserved throughout a subfamily of peptide-GPCRs, using a combination of mutagenesis and molecular modeling. Of the 13 conserved charged residues identified in the extracellular loops (ECLs), Arg(116) (ECL1), Arg(125) (top of TMIII), and Asp(204) (ECL2) are important for agonist binding and/or receptor activation. Molecular modeling revealed that Arg(125) (and Lys(125)) stabilizes TMIII by interacting with lipid head groups. Charge reversal (Asp(125)) caused re-ordering of the lipids, altered helical packing, and increased solvent penetration of the TM bundle. Interestingly, a negative charge is excluded at this locus in peptide-GPCRs, whereas a positive charge is excluded in amine-GPCRs. This contrasting conserved charge may reflect differences in GPCR binding modes between peptides and amines, with amines needing to access a binding site crevice within the receptor TM bundle, whereas the binding site of peptide-GPCRs includes more extracellular domains. A conserved negative charge at residue 204 (ECL2), juxtaposed to the highly conserved disulfide bond, was essential for agonist binding and signaling. Asp(204) (and Glu(204)) establishes TMIII contacts required for maintaining the beta-hairpin fold of ECL2, which if broken (Ala(204) or Arg(204)) resulted in ECL2 unfolding and receptor dysfunction. This study provides mechanistic insight into the roles of conserved extracellular residues.

Neuropeptide binding reflects convergent and divergent evolution in species-typical group sizes

Neuroendocrine factors that produce species differences in aggregation behavior ("sociality") are largely unknown, although relevant studies should yield important insights into mechanisms of affiliation and social evolution. We here focused on five species in the avian family Estrildidae that differ selectively in their species-typical group sizes (all species are monogamous and occupy similar habitats). These include two highly gregarious species that independently evolved coloniality; two territorial species that independently evolved territoriality; and an intermediate, modestly gregarious species that is a sympatric congener of one of the territorial species. Using males and females of each species, we examined binding sites for (125)I-vasoactive intestinal polypeptide (VIP), (125)I-sauvagine (SG; a ligand for corticotropin releasing factor, CRF, receptors) and a linear (125)I-V(1a) vasopressin antagonist (to localize receptors for vasotocin, VT). VIP, CRF and VT are neuropeptides that influence stress, anxiety and/or various social behaviors. For numerous areas (particularly within the septal complex), binding densities in the territorial species differed significantly from binding in the more gregarious species, and in most of these cases, binding densities for the moderately gregarious species were either comparable to the two colonial species or were intermediate to the territorial and colonial species. Such patterns were observed for (125)I-VIP binding in the medial bed nucleus of the stria terminalis, medial septum, septohippocampal septum, and subpallial zones of the lateral septum; for (125)I-SG binding in the infundibular hypothalamus, and lateral and medial divisions of the ventromedial hypothalamus; and for the linear (125)I-V(1a) antagonist in the medial septum, and the pallial and subpallial zones of the caudal lateral septum. With the exception of (125)I-SG binding in the infundibular hypothalamus, binding densitites are positively related to sociality.

Regulation of the avian kidney by arginine vasotocin

Nonapeptides secreted by the neurohypophysis have important roles in vertebrate cardio-fluid homeostasis. In birds, those peptides include mesotocin (the representative of the neutral, or oxytocin-like, nonapeptide family) and vasotocin (the representative of the basic, or vasopressin-like, hormones). The function of mesotocin is not well defined, but it does appear to have osmoregulatory functions. Vasotocin is considered the primary avian antidiuretic hormone. Receptors for AVT in avian kidney-either on renal vasculature or on the tubules-have yet to be localized or identified. However, AVT quite certainly effects antidiuresis via both vascular and tubular mechanisms. The former entail a reduction in the rate of glomerular filtration, apparently via constriction of afferent arterioles. Evidence for the latter (direct tubular action of AVT) has accumulated in recent years and includes enhanced fractional tubular water reabsorption, activation of second messenger (cAMP) in thick ascending limbs and collecting ducts, and modest AVT-stimulated water permeability in collecting ducts associated with expression of aquaporins. The relative importance of the renal vascular vs. tubular actions in birds likely depend on the dose of the hormone, the physiological condition of the animal, and the species of bird.

Identification of roughskin newt medullary vasotocin target neurons with a fluorescent vasotocin conjugate

Arginine8 vasotocin (AVT), a neurohypophyseal peptide in nonmammalian vertebrates, plays a key role in the regulation of social behaviors related to reproduction. In male roughskin newts (Taricha granulosa), AVT is an important facilitator of several reproductive behaviors, including courtship clasping of females. Although AVT is known to act in certain brain regions and AVT receptors have been localized to some extent, specific target neurons for this peptide have not been identified in any species. Internalization of a receptor-specific conjugate of AVT and the fluorescent dye Oregon green was used to identify AVT target cells in the medulla of male roughskin newts. Medullary neurons are of interest because they appear to mediate facilitation of clasping by AVT. Direct application of AVT-Oregon green to the fourth ventricular surface of the medulla in vivo resulted in conjugate internalization by a widespread population of medullary neurons, particularly in the medial reticular formation and nuclei of cranial nerves V, VII, VIII, IX, and X. Some fourth-ventricle ependymal cells were also labeled. Reticulospinal neurons, which play an important role in clasping, were identified by retrograde labeling with tetramethylrhodamine dextran amine. AVT-Oregon green was internalized by 72% of these neurons. These results show that AVT can directly affect a very large and diverse medullary neuronal population, which may underlie the peptide's role in multiple neuroendocrinological processes, including autonomic and behavioral regulation. Selectivity of the AVT action may arise through interactions between AVT and steroids such as corticosterone.

Molecular cloning of an oxytocin-like receptor expressed in the chicken shell gland

The avian homologs of arginine vasopressin (AVP) and oxytocin (OT) are arginine vasotocin (AVT) and mesotocin (MT), respectively. In birds, AVT shares many of the functions of AVP including regulation of fluid balance, blood pressure regulation and the stress response. AVT also plays an oxytocin-like reproductive role in birds by stimulating uterine (shell gland) contraction during oviposition. The role of MT in avian reproduction is not clear. Here, we report the cloning of a third neuropeptide receptor in the chicken (Gallus gallus). Parsimony analysis reveals that the new receptor has highest homology to mammalian OT receptors and the MT receptors of non-mammalian vertebrates. Moreover, the receptor bears far less homology to the two avian VT receptors that have been cloned. Reverse transcription-polymerase chain reaction and in in situ hybridization analyses reveal the receptor is expressed in both the endometrium and myometrium of the shell gland. The expression pattern and high homology to OT receptors suggest that the receptor may stimulate myometrial contraction and therefore play a critical role in oviposition.

The cloned avian neurohypophysial hormone receptors

Arginine vasotocin (AVT), a neurohypophysial hormone, has many essential functions in birds including the regulation of salt and fluid balance, blood pressure, the stress response and a variety of behaviors. In addition, AVT controls reproductive functions in birds that are served by oxytocin in mammals. In the following review, we examine the functions of AVT in birds with an emphasis on the present state of knowledge concerning the cloned receptors for this important hormone. Receptor and gene structure, signal transduction mechanisms and expression pattern are all discussed. Finally, we explore the phylogenetic relationships between the cloned avian receptors and other vertebrate and invertebrate neurohypophysial hormone receptors.

Characterization and immunohistochemical visualization of the vasotocin VT2 receptor in the pituitary gland of the chicken, Gallus gallus

Arginine vasotocin is a neurohypophysial hormone in lower vertebrates including birds. Its actions are mediated through G-protein coupled receptors that belong to the vasopressin/oxytocin receptor family. Our laboratory recently cloned a vasotocin receptor, designated the VT2 receptor (VT2R), which shares high sequence identity at both the nucleotide and amino acid level with the mammalian V1b vasopressin receptor (V1bR). In the present study, we report development and use of an antibody to the VT2R to obtain anatomical evidence for testing the hypothesis that the VT2R is the avian homolog of the mammalian V1bR. Results verified the specificity of the antibody and demonstrated a receptor distribution occurring predominantly in the cephalic lobe of the pars distalis and co-localizing with adrenocorticotropin in corticotrophs. With respect to VT2R distribution and cell-type localization in pituitary gland, evidence presented support its similarity with the mammalian V1bR. In contrast to the mammalian V1bR, VT2R expression was not observed in chicken brain. Further research will be required to determine which receptor/s in the arginine vasotocin/mesotocin family are expressed in brain and mediate regulatory functions of vasotocin in the central nervous system.

Identification of Amino Acid Residues That Direct Differential Ligand Selectivity of Mammalian and Nonmammalian V1a Type Receptors for Arginine Vasopressin and Vasotocin

Arginine vasotocin (VT) is the ortholog in all nonmammalian vertebrates of arginine vasopressin (AVP) in mammals. We have previously cloned an amphibian V1atype vasotocin receptor (VT1R) that exhibited higher sensitivity for VT than AVP, while the mammalian V1a type receptor (V1aR) responded better to AVP than VT. In the present study, we identified the amino acid residues that confer differential ligand selectivity for AVP and VT between rat V1aR and bullfrog VT1R (bfVT1R). A chimeric rat V1aR having transmembrane domain (TMD) VI to the carboxyl-terminal tail (C-tail) of bfVT1R showed a reverse ligand preference for AVP and VT, whereas a chimeric VT1R with TMD VI to the C-tail of rat V1aR showed a great increase in sensitivity for AVP. A single mutation (Ile(315(6.53)) to Thr) in TMD VI of V1aR increased the sensitivity for VT, while a single mutation (Phe(313(6.51)) to Tyr or Pro(334(7.33)) to Thr) reduced sensitivity toward AVP. Interestingly the triple mutation (Phe(313(6.51)) to Tyr, Ile(6.53) to Thr, and Pro(7.33) to Thr) of V1aR increased sensitivity to VT but greatly reduced sensitivity to AVP, behaving like bfVT1R. Further, like V1aR, a double mutant (Tyr(306(6.51)) to Phe and Thr(327(7.33)) to Pro) of bfVT1R showed an increased sensitivity to AVP. These results suggest that Phe/Tyr(6.51), Ile/Thr(6.53), and Pro/Thr(7.33) are responsible for the differential ligand selectivity between rat V1aR and bfVT1R. This information regarding the molecular interaction of VT/AVP with their receptors may have important implications for the development of novel AVP analogs.

Changes in expression of AVT and AVT receptor (VT1) gene in hypothalamus and shell gland in relation to egg laying in white leghorn hen

Oviposition is a complex phenomenon involving various regulatory mechanisms at the neuroendocrine levels. Present study was designed to access the changes in arginine vasotocin (AVT) and its receptor (VT1) gene expression in relation to the time of egg laying of white leghorn hen. The expression of AVT gene (Northern blot analysis and in situ hybridization) in the hypothalamus and localization of ir-AVT in the magnocellular neurons of paraventricular nuclei was studied 2 h before (-2 h), immediately after (0 h) and 2 h after (+2 h) egg laying. Simultaneous changes in the AVT and VT1 receptor gene in the shell gland, which finally responds to AVT for smooth muscle contraction and expulsion of egg, were also determined by semi-quantitative reverse transcriptase-polymerase chain reaction. The findings indicated increased hypothalamic AVT gene expression immediately after egg laying (0 h) when compared to 2 h before and 2 h after egg laying. AVT receptor gene expression in the shell gland also followed the same pattern. However, AVT gene expression in the shell gland, unlike that of hypothalamus was higher at -2 h compared to 0 and +2 h of oviposition. While highly significant increase was noted in plasma AVT concentration at the time of egg laying, other parameters such as plasma osmolality and ionic concentration (Na(+), K(+), Ca(2+), and Cl(-)) did not show any change. It is suggested that in addition to increased hypothalamic AVT transcript and peripheral release, local synthesis of AVT in the shell gland (paracrine release) may contribute to the contraction of shell gland smooth muscles during egg laying. Moreover, these findings clearly indicate temporal correlation of AVT and its receptor gene expression in different tissues during oviposition.

Comparison of brain arginine-vasotocin and corticotrophin-releasing factor for physiological responses in chicks

Arginine-vasotocin (AVT), a non-mammalian homologue of mammalian arginine-vasopressin, is a stress-related peptide in the brain of birds. The aim of the present study was to determine the effects of intracerebroventricular (ICV) injection of AVT on feeding behavior, body temperature, corticosterone release and several behavioral parameters in chicks. These effects were compared with those of corticotrophin-releasing factor (CRF), another stress-related peptide. The ICV injection of AVT inhibited feeding behavior, increased rectal temperature, and increased plasma corticosterone concentrations, but these effects were weaker than those of CRF. AVT induced hypoactivity as evidenced by decreased vocalization and stepping while CRF induced hyperactivity. The present results demonstrate that some functions of brain AVT are similar to those of CRF, although these effects are weaker than those induced by CRF. However, some AVT-induced behaviors were different from CRF, indicating that the physiological roles of AVT in the regulation of stress behavior are different from those of CRF in chicks.

Molecular cloning of an anuran V(2) type [Arg(8)] vasotocin receptor and mesotocin receptor: functional characterization and tissue expression in the Japanese tree frog (Hyla japonica)

In most amphibians, [Arg(8)] vasotocin (VT) has an antidiuretic effect that is coupled to the activation of adenylate cyclase. In contrast, mesotocin (MT) has a diuretic effect and acts via the inositol phosphate/calcium signaling pathway in amphibians. To further clarify the mechanisms of VT and MT activation, we report the molecular cloning of a VT receptor (VTR) and a MT receptor (MTR) from the Japanese tree frog, Hyla japonica. Tree frog VTR or MTR cDNA encoded 363 or 389 amino acids, and their amino acid sequences revealed close similarity to the mammalian vasopressin V(2) (51-52% identity) or toad MT (94% identity) receptors, respectively. Using CHO-K1 cells transfected with tree frog VTR, we observed elevated concentrations of intracellular cAMP following exposure of the cells to VT or other neurohypophysial hormones, whereas the cells transfected with MTR did not exhibit altered cAMP concentrations. The cells transfected with VTR exhibited the following efficiency for cAMP accumulation: VT = hydrin 1 > or = vasopressin > or = hydrin 2 > MT = oxytocin > isotocin. VTR or MTR mRNA exhibits a single 2.2- or 5.5-kb transcription band, respectively, and both are expressed in various tissues. VTR mRNA is clearly expressed in brain, heart, kidney, pelvic patch of skin, and urinary bladder, whereas brain, fat body, heart, kidney, and urinary bladder express MTR mRNA. Specifically, VTR mRNA in the pelvic patch or MTR mRNA in the dorsal skin is present at elevated levels in the skin. Characteristic distribution of VTR and MTR on osmoregulating organs indicates the ligands for these receptors would mediate a variety of functions. Further, the distribution of VTR in the skin would make the regional difference on cutaneous water absorption in response to VT in the Japanese tree frog.

Molecular cloning and functional characterization of a vasotocin receptor subtype expressed in the pituitary gland of the domestic chicken (Gallus domesticus): avian homolog of the mammalian V1b-vasopressin receptor

The neurohypophysial hormone arginine vasotocin (AVT) stimulates adrenocorticotropin hormone (ACTH) secretion from the avian anterior pituitary gland resulting in increased adrenal secretion of corticosterone in response to stress. Here, we report molecular cloning and functional characterization of a gene encoding an AVT receptor subtype, designated the VT2 receptor, that may mediate the stimulatory effect of AVT on ACTH secretion in birds. The open reading frame predicts a 425 amino acid polypeptide that includes seven segments of 19 to 24 hydrophobic amino acids, typical of guanine nucleotide-protein coupled receptors. Phylogenetic analysis revealed that the VT2 receptor shares highest identity with the mammalian V1b-vasopressin receptor subtype. Expressed VT2 receptors in COS7 cells mediate AVT-induced phosphatidylinositol turnover and Ca(2+) mobilization. In the domestic chicken, expression of VT2 receptor gene transcripts is limited to the pituitary gland. Based on similarities in sequence, site of expression and coupled signal transduction pathways, we conclude that the VT2 receptor is the avian homolog of the mammalian V1b-vasopressin receptor, and therefore may play an important role in the avian stress response.