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

Stress, health and the welfare of laying hens

It is essential to understand responses to stress and the impact of stress on physiological and behavioural functioning of hens, so as to assess their welfare. The current understanding of stress in laying hens is comprehensively reviewed here. Most research on stress in hens has focussed on the activity of the adrenal glands, with the most common approach being to measure corticosterone, which is the predominant glucocorticoid produced by birds in response to stress. While these measures are useful, there is a need to understand how the brain regulates stress responses in hens. A greater understanding of the sympathoadrenal system and its interaction with the hypothalamo–pituitary–adrenal axis is required. There is also a lack of knowledge about the many other peptides and regulatory systems involved in stress responses in hens. The usefulness of understanding stress in hens in terms of assessing welfare depends on appreciating that different stressors elicit different responses and that there are often differences in responses to, and impacts of, acute and chronic stress. It is also important to establish the actions and fate of stress hormones within target tissues. It is the consequences of these actions that are important to welfare. A range of other measures has been used to assess stress in hens, including a ratio of heterophils to lymphocytes and haematocrit:packed cell-volume ratio and measures of corticosterone or its metabolites in eggs, excreta, feathers and the secretions of the uropygial gland. Measures in eggs have proffered varying results while measures in feathers may be useful to assess chronic stress. There are various studies in laying hens to indicate impacts of stress on the immune system, health, metabolism, appetite, and the quality of egg production, but, generally, these are limited, variable and are influenced by the management system, environment, genetic selection, type of stressor and whether or not the birds are subjected to acute or chronic stress. Further research to understand the regulation of stress responses and the impact of stress on normal functioning of hens will provide important advances in the assessment of stress and, in turn, the assessment of welfare of laying hens.

The vasotocinergic system and its role in the regulation of stress in birds

The regulation of stress in birds includes a complex interaction of neural systems affecting the hypothalamic-pituitary-adrenal (HPA) axis. In addition to the hypothalamic paraventricular nucleus, a structure called the nucleus of the hippocampal commissure likewise affects the output of pituitary stress hormones and appears to be unique to avian species. Within the anterior pituitary, the avian V1a and V1b receptors were found in corticotropes. Based on our studies with central administration of hormones in the chicken, corticotropic releasing hormone (CRH) is a more potent ACTH secretagogue than arginine vasotocin (AVT). In contrast, when applied peripherally, AVT is more efficacious. Co-administration of AVT and CRH peripherally, resulted in a synergistic stimulation of corticosterone release. Data suggest receptor oligomerization as one possible mechanism. In birds, vasotocin receptors associated with stress responses include the V1a and V1b receptors. Three-dimensional, homology-based structural models of the avian V1aR were built to test agonists and antagonists for each receptor that were screened by molecular docking to map their binding sites on each receptor. Additionally, binding affinity values for each available peptide antagonist to the V1aR and V1bR were determined. An anterior pituitary primary culture system was developed to determine how effective each antagonist blocked the function of each receptor in culture when stimulated by a combination of AVT/CRH administration. Use of an antagonist in subsequent in vivo studies identified the V1aR in regulating food intake in birds. The V1aR was likewise found in circumventricular organs of the brain, suggesting a possible function in stress.

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.

Arginine vasotocin is the major adrenocorticotropic hormone-releasing factor in the bullfrog Rana catesbeiana

In a previous study, we showed that corticotropin-releasing factor (CRF) is the major thyroid-stimulating hormone (TSH)-releasing factor in the bullfrog (Rana catesbeiana) hypothalamus. Our findings prompted us to ascertain whether CRF or arginine vasotocin (AVT), a known adrenocorticotropic hormone (ACTH) secretagogue in several vertebrates, is the main stimulator of the release of ACTH from the bullfrog pituitary. Both the frog CRF and AVT stimulated the release of immunoassayable ACTH from dispersed anterior pituitary cells in vitro in a concentration-dependent manner. AVT, however, exhibited far more potent ACTH-releasing activity than CRF. Although CRF by itself weakly stimulated ACTH release, it acted synergistically with AVT to enhance the release of ACTH markedly. Mesotocin and AVT-related peptides such as hydrin 1 and hydrin 2 showed relatively weak ACTH-releasing activity. Subsequently, cDNAs encoding the bullfrog AVT V1a-type and V1b-type receptors were molecularly cloned. Reverse transcriptase-PCR using specific primers revealed that the anterior lobe of the pituitary predominantly expressed AVT V1b-type receptor mRNA but scarcely expressed AVT V1a-type receptor mRNA. Abundant signals for V1b-type receptor mRNA in the corticotropes were also detected by in situ hybridization. The results obtained by the experiments with the bullfrog pituitary indicate that AVT acts as the main ACTH-releasing factor through the AVT V1b-type receptor and that CRF acts synergistically with AVT to enhance the release of ACTH.

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.

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.

Regulation of gene expression of vasotocin and corticotropin-releasing hormone receptors in the avian anterior pituitary by corticosterone

The effect of chronic stress (CS) on gene expression of the chicken arginine vasotocin (AVT) and corticotropin-releasing hormone (CRH) receptors [VT2R, VT4R, CRH-R1, and CRH-R2] was examined by measuring receptor mRNA levels in the anterior pituitary gland of the chicken after chronic immobilization stress compared to acute stress (AS). Radioimmunoassay results showed that blood circulating corticosterone (CORT) levels in the CS group were significantly decreased compared to that of birds in the AS group (P<0.05). The VT2R and CRH-R2 mRNA in CS birds were significantly decreased to that of controls. The VT4R mRNA was significantly decreased compared to controls in AC birds and was further decreased in the CS group compared to controls (P<0.05). The CRH-R1 mRNA was significantly decreased in the AS birds compared to controls. However, there was no significant difference of CRH-R1 mRNA between acute stress and chronic stress birds. Using primary anterior pituitary cell cultures, the effect of exogenous CORT on VT/CRH receptor gene expression was examined. Receptor mRNA levels were measured after treatment of CORT followed by AVT/CRH administration. The CORT pretreatment resulted in a dose-dependent decrease of proopiomelanocortin heteronuclear RNA, a molecular marker of a stress-induced anterior pituitary. Without CORT pretreatment of anterior pituitary cell cultures, the VT2R, VT4R and CRH-R1mRNA levels were significantly increased within 15 min and then decreased at 1 h and 6 h by AVT/CRH administration (P<0.05). Pretreatment of CORT in anterior pituitary cells induced a dose-dependent increase of VT2R, VT4R and CRH-R2 mRNA levels, and a significant decrease of CRH-R1 mRNA levels at only the high dose (10 ng/ml) of CORT (P<0.05).Taken together, results suggest a modulatory role of CORT on the regulation of VT/CRH receptor gene expression in the avian anterior pituitary gland dependent upon CORT levels.

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.

Distribution of the Vasotocin Subtype Four Receptor (VT4R) in the Anterior Pituitary Gland of the Chicken, Gallus gallus, and its Possible Role in the Avian Stress Response

The neurohormone arginine vasotocin (AVT) in non mammalian vertebrates is homologous to arginine vasopressin (AVP) in mammals. Its actions are mediated via G protein-coupled receptors that belong to the vasotocin/mesotocin family. Because of the known regulatory effects of nonapeptide hormones on anterior pituitary functions, receptor subtypes in that family have been proposed to be located in anterior pituitary cells. Recently, an avian vasotocin receptor subtype designated VT4R has been cloned, which shares 69% sequence homology with a human vasopressin receptor, the V1aR. In the present study, a polyclonal antibody to the VT4R was developed and validated to confirm its specificity to the VT4R. The antibody was used to test the hypothesis that the VT4R is present in the avian anterior pituitary and is specifically associated with certain cell types, where its expression is modulated by acute stress. Western blotting of membrane protein extracts from pituitary tissue, the use of HeLa cells transfected with the VT4R and peptide competition assays all confirmed the specificity of the antibody to the VT4R. Dual-labelling immunofluorescence microscopy was utilised to identify pituitary cell types that contained immunoreactive VT4R. The receptor was found to be widely distributed throughout the cephalic lobe but not in the caudal lobe of the anterior pituitary. Immunoreactive VT4R was associated with corticotrophs. Approximately 89% of immunolabelled corticotrophs were shown to contain the VT4R. The immunoreactive VT4R was not found in gonadotrophs, somatotrophs or lactotrophs. To determine a possible functional role of the VT4R and previously characterised VT2R, gene expression levels in the anterior pituitary were determined after acute immobilisation stress by quantitative reverse transcriptase-polymerase chain reaction. The results showed a significant increase in plasma corticosterone levels (three- to four-fold), a significant reduction of VT4R mRNA and an increase of VT2R mRNA (P < 0.05) in acutely immobilised chicks compared to controls. The data suggest a role of the VT4R in the avian stress response.

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.

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.

Genetic variation in the C-terminal domain of arginine vasotocin receptor in avian species

Arginine vasotocin (AVT) is a neurohypophysial hormone that plays an essential role in various social behaviours. We investigated the degree of polymorphisms in the C-terminal domain of the AVT V2-type receptor (AVT2R) among avian species to determine the mechanism by which genetic polymorphisms in the neuropeptide receptor may contribute to different levels of signal transduction. In passerine birds, AVT2R was characterised by 2 variable regions, both of which were managed by insertion/deletion (indel); however, indels were rarely found in other avian taxa. The presence or absence of deletions in passerines largely affected the properties of the predicted palmitoylation sites at the proximal part of the C-terminal tail. Moreover, we detected intraspecific polymorphisms in estrildid finches based on the number of tri-amino acid (GHQ/EHQ/EHR) repeats in another variable region. Our results indicate that amino acid substitutions and length variation at the C-terminus may impact subsequent signal transduction and affect behavioural traits in wild birds.

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.

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.

Testosterone modulates pituitary vasotocin receptor expression and adrenal activity in osmotically stressed chicken

Regulation of arginine vasotocin (AVT), avian neurohypophyseal hormone, is an important component of the hypothalamo-pituitary-adrenal axis. Changes in plasma osmolality levels and sex steroids are known to affect AVT gene expression. The present study reports the effect of water deprivation and testosterone treatment independently, as well as simultaneously, on the pituitary vasotocin receptor VT2R expression and adrenal steroidogenic activity in sexually immature male chicken (Gallus gallus). Birds were divided into four groups- control, water deprived (WD), testosterone injected (TE) and TE treated water deprived (TE+WD). WD decreased and TE treatment alone or in combination with WD (TE+WD) increased VT2R expression compared to the control. Expression of pro-opiomelanocortin (POMC) was also studied since this gene is a polypeptide precursor of ACTH and is under the negative feedback of adrenal corticoids. TE treatment as well as WD separately or when coupled together decreased the POMC mRNA expression in the pituitary but stimulated adrenal steroidogenic activity. Further, VT2R expression decreased in TE+WD compared to TE group, but it was not different from the vehicle treated control group suggesting that the suppressive effect of WD on VT2R expression was inhibited by the stimulatory effect of testosterone. Similarly, although both TE and WD decreased POMC expression and increased steroidogenic activity, no further decrease or increase in these parameters was observed when these two (WD and TE) treatments were combined together. Although, the exact mechanism is not clear, data indicate a stimulatory action of testosterone on VT2R expression and adrenal function despite a decreased expression of POMC mRNA. Results also suggest that testosterone treatment to sexually immature birds, in addition to its effect on hypothalamic AVT neurons (earlier study) and pituitary VT2R expression (present study), masks or inhibits osmotic stress-induced alterations in pituitary-adrenal activity.

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.

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.

Corticosterone- or metapyrone-induced alterations in adrenal function and expression of the arginine vasotocin receptor VT2 in the pituitary gland of domestic fowl, Gallus gallus

The avian neurohypophyseal hormone arginine vasotocin (AVT) is known to be involved in the regulation of adrenocorticotropin (ACTH) release by interacting with the VT2 receptor (VT2R), which is homologous to the mammalian vasopressin V1b receptor (V1bR). To study the role of glucocorticoid in the expression and regulation of the VT2R, corticosterone (1 or 5mg/bird/day) or metapyrone (10 or 50mg/kg body weight/day) were administered daily for 8 days to white leghorn chickens. While low doses were ineffective, a high dose of corticosterone upregulated, while metapyrone downregulated, pituitary VT2R mRNA expression and ir-VT2 in the cephalic lobe of the anterior pituitary. Further, although no change was observed in the expression of POMC mRNA, adrenal activity (as judged by the changes in total cholesterol, 3beta HSD, cortical cord width and cortico-medullary ratio) exhibited suppression or stimulation following treatment with corticosterone or metapyrone, respectively. In view of the classical negative feedback effect of glucocorticoids at the level of hypothalamic CRH neurons and pituitary corticotrophs, high corticosterone level-induced suppression of the CRH-ACTH axis may have been masked by VT2R-mediated stimulation of corticotrophs, and hence the POMC mRNA level did not change. The same argument could be used for metapyrone. It is concluded that expression of the VT2 receptor is regulated by glucocorticoids in chicken. These findings confirm a role for AVT, mediated by the VT2 receptor, in regulating ACTH secretion during stress and suggest that corticotroph VT2 receptor levels may be dynamically regulated depending on the HPA axis activity.

Osmotic stress induced alteration in the expression of arginine vasotocin receptor VT2 in the pituitary gland and adrenal function of domestic fowl

The role of arginine vasotocin in the regulation of the pituitary-adrenal axis of domestic fowl was analyzed by studying the expression of its recently cloned pituitary receptor VT2 and adrenal activity following osmotic stress. Four days of water deprivation induced an increase in plasma osmolality-a known stimulator of AVT synthesis and release from hypothalamic magnocellular neurons. Water deprivation also decreased pituitary mRNA levels for both the VT2 receptor and for pro-opiomelanocortin (POMC). Despite a decrease in the expression of VT2 mRNA, immunoreactive-VT2 receptor levels in the pituitary increased, suggesting a possible role for post-transcriptional mechanisms in the regulation of this receptor. Further, adrenal activity (as judged by adrenal weight, cholesterol content, 3beta hydroxysteroid dehydrogenase, cortical cord width and cortico-medullary ratio) showed stimulation in water-deprived chicken as compared to control. On the basis of present findings, it is concluded that water deprivation down regulates the mRNA expression of AVT receptor VT2 as well as POMC but stimulates adrenal function. It is also suggested that in addition to the release of magnocellular AVT into the neurohypophysis to act as antidiuretic hormone following water deprivation, AVT may also modulate HPA axis to cope with the osmotic stress.

Changes in vasotocin immunoreactivity of paraventricular nuclei and adrenal function of Japanese quail in relation to different phases of sexual development

In relevance to osmoregulatory and reproductive functions, activity of hypothalamic neurosecretory neurons may also vary seasonally. The current study was performed to determine annual changes in ir-AVT neurons of hypothalamus and adrenal gland function. We examined changes in ir-AVT neuron by immunohistochemical method and plasma testosterone was measured by enzyme immunoassay. The steroidogenic interrenal activity was studied by histological and biochemical methods. Birds were sampled in February (quiescent), April (recrudescent), June (breeding) and November (regressive). A significant and gradual increase in the number of ir-AVT neurons was observed from quiescent to breeding phase which decreased during regressive phase of annual gonadal cycle. The gradual increase in ir-AVT neurons along with annual gonadal activity of quail were accompanied by increase in plasma levels of testosterone. These results indicate a functional interaction between sex steroid and AVT synthesizing neurons. Adrenal activity (as judged by weight, ascorbic acid content, cortical cord width and cortico-medullary ratio) was also maximum during breeding phase. It is thus postulated that domesticated quail when exposed to natural day length (NDL), exhibits seasonal/annual cyclicity in vastocinergic activity and adrenal function which may be due to difference in sex steroid hormone.

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.

Molecular cloning of three types of arginine vasotocin receptor in the newt, Cynops pyrrhogaster

Three types of cDNA encoding the arginine vasotocin (AVT) receptors from the newt, Cynops pyrrhogaster were cloned and the gene expression of each receptor analyzed in the organs and tissues of the newt. The deduced amino acid sequence of one type of AVT receptor, consisting of 418 amino acid residues, showed a high degree of sequence identity with the mammalian arginine vasopressin (AVP) V1a receptors (61-68%). The second type of cDNA, encoding an amino acid sequence consisting of 367 amino acid residues, exhibited a relatively high sequence identity with mammalian AVP V2 receptors (50-51%). The third cDNA, encoding a sequence of 415 amino acid residues, possessed high sequence identity with mammalian AVP V3/V1b receptors (59-63%). Phylogenetic analysis revealed that the first, second and third types of receptor were close to mammalian AVP V1a, V2 and V3/V1b receptors, respectively, and RT-PCR using gene specific primers for each type of receptor indicated that the first and second types of receptor mRNA were expressed in various organs and tissues, including the circulatory, osmoregulatory, and reproductive organs of both male and female newts. In contrast, mRNA expression of the third cDNA was mainly detected in the brain and pituitary, and its expression pattern was distinctly different from that of the other two. We suggest that the first, second and third types of newt AVT receptor obtained in the present study are counterparts of mammalian AVP V1a, V2 and V3/V1b receptors, respectively.

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.

Effect of long and short photoperiod on vasotocin neurons of paraventricular nuclei and adrenal function of water deprived Japanese quail

The responses of magnocellular neurons of paraventricular nuclei (PVN) and changes to adrenal activity to water deprivation in Japanese quail maintained under gonado-inhibitory and stimulatory photoperiods were examined. Water deprivation of 4 days resulted in a 12% decrease in body weight of sexually regressed short day (SD, 6L:18D) quail, while the decrease was more (18%) in sexually stimulated long day (LD, 16L:8D) quail. The increase in plasma osmolality following water deprivation was also more (47%) in LD than to SD quail (36%). Under the LD condition, quail had increased numbers, sizes and immunostaining of ir-AVT neurons of PVN compared to SD condition. A significant increase in the number of ir-AVT neurons was observed following 4 days of water deprivation in both SD and LD quail compared to their respective fully hydrated controls. However, the degree of response was more under the LD compared to the SD condition suggesting that gonado-stimulatory long days increase the activity/response of the AVT system. Increased adrenal ascorbic acid content (i.e., activity) was also observed to quail of LD when compared to SD treatment. However, osmotic stress led to adrenal hypertrophy and hyperactivity of quail of both of the photoperiodic regimes. Our findings indicate that not only osmotic stress but also photo-gonadal stimulation upregulates the expression of hypothalamic AVT genes and increases the localization of ir-AVT in many neurons of PVN. The above results support the existence of a parallel adrenal-gonad relationship and increase in adrenal function during osmotic stress, which also leads to simultaneous increase in AVT system. We conclude that photo-sexual conditions alter hypothalamic vasotocinergic and adrenal activity in Japanese quail and the degree of stimulation of the two systems following osmotic stress is higher under gonado-stimulatory LD conditions.

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.

Inhibitory effect of ghrelin on food intake is mediated by the corticotropin-releasing factor system in neonatal chicks

It is known that, in rats, central and peripheral ghrelin increases food intake mainly through activation of neuropeptide Y (NPY) neurons. In contrast, intracerebroventricular (ICV) injection of ghrelin inhibits food intake in neonatal chicks. We examined the mechanism governing this inhibitory effect in chicks. The ICV injection of ghrelin or corticotropin-releasing factor (CRF), which also inhibits feeding and causes hyperactivity in chicks. Thus, we examined the interaction of ghrelin with CRF and the hypothalamo-pituitary-adrenal (HPA) axis. The ICV injection of ghrelin increased plasma corticosterone levels in a dose-dependent or a time-dependent manner. Co-injection of a CRF receptor antagonist, astressin, attenuated ghrelin-induced plasma corticosterone increase and anorexia. In addition, we also investigated the effect of ghrelin on NPY-induced food intake and on expression of hypothalamic NPY mRNA. Co-injection of ghrelin with NPY inhibited NPY-induced increase in food intake, and the ICV injection of ghrelin did not change NPY mRNA expression. These results indicate that central ghrelin does not interact with NPY as seen in rodents, but instead inhibits food intake by interacting with the endogenous CRF and its receptor.

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.