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

Hypo-Osmoregulatory Roles of Vasotocinergic and Isotocinergic Systems in the Intestines of Two European Sea Bass Lineages

European sea bass (Dicentrarchus labrax) are a major aquaculture species that live in habitats with fluctuating salinities that are sometimes higher than in seawater (SW). Atlantic and West-Mediterranean genetic lineages were compared regarding intestinal neuropeptide receptor expression in SW (36%) and following a two-week transfer to hypersalinity (HW, 55%). Phylogenetic analysis revealed seven neuropeptide receptors belonging to the arginine vasotocine (AVTR) family and two isotocin receptors (ITR). Among AVTR paralogs, the highest mRNA levels were recorded for v1a2, with a two- to fourfold upregulation in the European sea bass intestinal sections after transfer of fish to HW. Principal component analysis in posterior intestines showed that v1a2 expression grouped together with the expression and activity of main ion transporters and channels involved in solute-coupled water uptake, indicating a possible role of this receptor in triggering water absorption. v1a1 expression, however, was decreased or did not change after transfer to hypersaline water. Among ITR paralogs, itr1 was the most expressed paralog in the intestine and opposite expression patterns were observed following salinity transfer, comparing intestinal sections. Overall, different expression profiles were observed between genetic lineages for several analyzed genes which could contribute to different osmotic stress-related responses in D. labrax lineages.

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 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.

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.

Acquisition of germ plasm accelerates vertebrate evolution

Primordial germ cell (PGC) specification occurs either by induction from pluripotent cells (epigenesis) or by a cell-autonomous mechanism mediated by germ plasm (preformation). Among vertebrates, epigenesis is basal, whereas germ plasm has evolved convergently across lineages and is associated with greater speciation. We compared protein-coding sequences of vertebrate species that employ preformation with their sister taxa that use epigenesis and demonstrate that genes evolve more rapidly in species containing germ plasm. Furthermore, differences in rates of evolution appear to cause phylogenetic incongruence in protein-coding sequence comparisons between vertebrate taxa. Our results support the hypothesis that germ plasm liberates constraints on somatic development and that enhanced evolvability drives the evolution of germ plasm.

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.

Distribution of nonapeptide systems in the forebrain of an African cichlid fish, Astatotilapia burtoni

Nonapeptides and their receptors have important functions in mediating social behavior across vertebrates. Where these nonapeptides are synthesized in the brain has been studied extensively in most vertebrate lineages, yet we know relatively little about the neural distribution of nonapeptide receptors outside of mammals. As nonapeptides play influential roles in behavioral regulation in all vertebrates, including teleost fish, we mapped the distributions of the receptors for arginine vasotocin (AVT; homolog of arginine vasopressin) and isotocin (IST; homolog of oxytocin/mesotocin) throughout the forebrain of Astatotilapia burtoni, an African cichlid fish with behavioral phenotypes that are plastic and reversible based on the immediate social environment. We characterized the distribution of the AVT V1a2 receptor (V1aR) and the IST receptor (ITR) using both immunohistochemistry for protein detection and in situ hybridization for mRNA detection, as well as AVT and IST using immunohistochemistry. Expression of the neuropeptide receptors was widely distributed throughout the fore- and midbrain, including the proposed teleost homologs of the mammalian amygdala complex, striatum, hypothalamus, and ventral tegmental area. We conclude that although the location of nonapeptide synthesis is restricted compared to tetrapod vertebrates, the distribution of nonapeptide receptors is highly conserved across taxa. Our results significantly extend our knowledge of where nonapeptides act in the brains of teleosts to mediate social transitions and behavior.

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.

The distribution of an AVT V1a receptor in the brain of a sex changing fish, Epinephelus adscensionis

The present study describes the distribution of an arginine vasotocin (AVT) V1a receptor (AVTr) throughout the brain of a sex-changing grouper, rock hind Epinephelus adscensionis. The objectives of this study were to describe the AVTr distribution in the brain of rock hind for potential linkages of the AVT hormone system with sex-specific behaviors observed in this species and to examine sex-specific differences that might exist. An antibody was designed for rock hind AVTr against the deduced amino acid sequence for the third intracellular loop. Protein expression, identified with immunohistochemistry showed high concordance with mRNA expression, identified with in situ hybridization. AVTr protein and mRNA expression was widely distributed throughout the brain, indicating that AVT may act as a neuromodulator via this V1a receptor subtype. AVTr protein and mRNA were present in regions associated with behavior, reproduction and spatial learning, as well as sensory functions such as vision, olfaction and lateral line sensory processing. We observed high AVTr expression in granular cell formations in the internal cellular layer of olfactory bulbs, torus longitudinalis, granular layer of the corpus cerebellum, valvula of the cerebellum, nuclei of the lateral and posterior recesses, and granular eminence. High protein and mRNA expression was also observed in the preoptic area, anterior hypothalamus, and habenular nucleus. No obvious sex differences were noted in any region of the rock hind brain.

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.

Molecular cloning and characterization of V2-type receptor in two ray-finned fish, gray bichir, Polypterus senegalus and medaka, Oryzias latipes

In tetrapods, vasopressin (VP) and vasotocin (VT) are involved in various aspects of physiology and behavior including osmoregulation, cardiovascular function, reproduction, stress response and social behavior. Pharmacological and molecular studies have identified three types of VP/VT receptors, V1a-type (V1aR), V1b-type (V1bR) and V2-type (V2R). On the other hand, only V1aR has so far been identified in teleosts. In the present study, we successfully cloned V2Rs from two ray-finned fish, gray bichir and medaka. Phylogenetic analysis showed that the cloned receptors belong to the V2R group of lobe-finned fish and tetrapods. The amino acid sequences of bichir V2R and medaka V2R were high identity (60-65.5% and 53.2-80.9%, respectively) with other known V2R members, respectively. Reverse transcriptase PCR revealed that ray-finned fish V2R transcripts have been detected in various tissues including brain, gill, heart, liver, kidney and reproductive organs, suggesting that ray-finned fish V2R might mediate multiple functions of VT. In functional analysis, the cells transfected with the cloned receptors responded with the accumulation of intracellular cAMP in a concentration-dependent manner following VT stimulation, but not respond with [Ca(2+)]i. Furthermore, pretreatment with mammalian V2R antagonist (OPC-31260) to the cells transfected with medaka V2R significantly inhibited an increase of the VT-induced intracellular cAMP. These results suggest that ray-finned fish possess a functional V2R linked to adenylate cyclase and the cAMP signaling pathway as well as V2Rs of lobe-finned fish and tetrapods. Thus, the present study suggests that functional V2R evolved prior to the divergence of the ray- and lobe-finned fish lineages.

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.

Prolactin increases the synthesis of 7alpha-hydroxypregnenolone, a key factor for induction of locomotor activity, in breeding male Newts

We recently found that the Japanese red-bellied newt, Cynops pyrrhogaster, actively produces 7alpha-hydroxypregnenolone, a previously undescribed amphibian neurosteroid. 7alpha-Hydroxypregnenolone stimulates locomotor activity of male newts. Locomotor activity of male newts increases during the breeding period as in other wild animals, but the molecular mechanism for such a change in locomotor activity is poorly understood. Here we show that the adenohypophyseal hormone prolactin (PRL) stimulates 7alpha-hydroxypregnenolone synthesis in the brain, thus increasing locomotor activity of breeding male newts. In this study, cytochrome P450(7alpha) (CYP7B), a steroidogenic enzyme catalyzing the formation of 7alpha-hydroxypregnenolone, was first identified to analyze seasonal changes in 7alpha-hydroxypregnenolone synthesis. Only males exhibited marked seasonal changes in 7alpha-hydroxypregnenolone synthesis and CYP7B expression in the brain, with a maximum level in the spring breeding period when locomotor activity of males increases. Subsequently we identified PRL as a key component of the mechanism regulating 7alpha-hydroxypregnenolone synthesis. Hypophysectomy decreased 7alpha-hydroxypregnenolone synthesis in the male brain, whereas administration of PRL but not gonadotropins to hypophysectomized males caused a dose-dependent increase in 7alpha-hydroxypregnenolone synthesis. To analyze the mode of PRL action, CYP7B and the receptor for PRL were localized in the male brain. PRL receptor was expressed in the neurons expressing CYP7B in the magnocellular preoptic nucleus. Thus, PRL appears to act directly on neurosteroidogenic magnocellular preoptic nucleus neurons to regulate 7alpha-hydroxypregnenolone synthesis, thus inducing seasonal locomotor changes in male newts. This is the first report describing the regulation of neurosteroidogenesis in the brain by an adenohypophyseal hormone in any vertebrate.

7α-hydroxypregnenolone, a new key regulator of locomotor activity of vertebrates: identification, mode of action, and functional significance

Steroids synthesized de novo by the central and peripheral nervous systems are called neurosteroids. The formation of neurosteroids from cholesterol in the brain was originally demonstrated in mammals by Baulieu and colleagues. Our studies over the past two decades have also shown that, in birds and amphibians as in mammals, the brain expresses several kinds of steroidogenic enzymes and produces a variety of neurosteroids. Thus, de novo neurosteroidogenesis from cholesterol is a conserved property that occurs throughout vertebrates. However, the biosynthetic pathways of neurosteroids in the brain of vertebrates was considered to be still incompletely elucidated. Recently, 7α-hydroxypregnenolone was identified as a novel bioactive neurosteroid stimulating locomotor activity in the brain of newts and quail through activation of the dopaminergic system. Subsequently, diurnal and seasonal changes in synthesis of 7α-hydroxypregnenolone in the brain were demonstrated. Interestingly, melatonin derived from the pineal gland and eyes regulates 7α-hydroxypregnenolone synthesis in the brain, thus inducing diurnal locomotor changes. Prolactin, an adenohypophyseal hormone, regulates 7α-hydroxypregnenolone synthesis in the brain, and may also induce seasonal locomotor changes. This review highlights the identification, mode of action, and functional significance of 7α-hydroxypregnenolone, a new key regulator of locomotor activity of vertebrates, in terms of diurnal and seasonal changes in 7α-hydroxypregnenolone synthesis, and describes some of their regulatory mechanisms.

African lungfish, Protopterus annectens, possess an arginine vasotocin receptor homologous to the tetrapod V2-type receptor

In tetrapods, arginine vasopressin and its counterpart, arginine vasotocin (AVT), are involved in renal water conservation through vascular V1a-type and tubular V2-type receptors, and only the former has thus far been cloned in fish. We successfully cloned the V1a-type and V2-type AVT receptor from the kidney of the African lungfish, Protopterus annectens, and the deduced amino acid sequences exhibited high homology with amphibian V1a- and V2-type receptors, respectively. Functional analysis showed that AVT addition to CHO cells transfected with lungfish V1a-type receptor increased [Ca2+]i in a concentration-dependent manner, whereas CHO cells transfected with lungfish V2-type receptor responded with cAMP accumulation after AVT stimulation. Lungfish V2-type receptor mRNA was strongly expressed in the heart and kidney, while V1a-type receptor mRNA was ubiquitously expressed in all the tissues examined. In the kidney, immunohistochemistry using a specific antibody to lungfish V2-type receptor showed localization in the basolateral area of the cells in the late part of the distal tubules. Artificial estivation (EST) for 90 days significantly increased plasma osmolality and sodium and urea concentrations. There was no significant difference in the V2-type receptor mRNA and protein expression levels in the kidney between the freshwater and EST lungfish, while the AVT precursor mRNA level in the hypothalamus was remarkably higher in the EST lungfish. Our results indicate that African lungfish possess a functional V2-type receptor similar to that in tetrapods, suggesting that elevated plasma AVT during estivation exerts a renal tubular antidiuretic effect through the V2-type receptor expressed in the distal segments of lungfish kidney.

Pheromone communication in amphibians and reptiles

This selective review considers herpetological papers that feature the use of chemical cues, particularly pheromones involved in reproductive interactions between potential mates. Primary examples include garter snake females that attract males, lacertid lizards and the effects of their femoral gland secretions, aquatic male newts that chemically attract females, and terrestrial salamander males that chemically persuade a female to mate. Each case study spans a number of research approaches (molecular, biochemical, behavioral) and is related to sensory processing and the physiological effects of pheromone delivery. These and related studies show that natural pheromones can be identified, validated with behavioral tests, and incorporated in research on vomeronasal functional response.

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.

Vasotocin has the potential to inhibit basolateral Na(+)/K (+)-pump current across isolated skin of tree frog in vitro, via its V(2)-type receptor/cAMP pathway

Adult frog skin transports Na(+) from the apical to the basolateral side across the skin. Antidiuretic hormone (ADH) is involved in the regulation of Na(+) transport in both mammals and amphibians. We investigated the effect of arginine vasotocin (AVT), the ADH of amphibians, on the short-circuit current (SCC) across intact skin and on the basolateral Na(+)/K(+)-pump current across apically nystatin-permeabilized skin of the tree frog, Hyla japonica, in which the V(2)-type ADH receptor is expressed in vitro. In intact skin, 1 pM AVT had no effect on the SCC, but 10 nM AVT was sufficient to stimulate the SCC since 10 nM and 1 microM of AVT increased the SCC 3.2- and 3.4-fold, respectively (P > 0.9). However, in permeabilized skin, AVT (1 microM) decreased the Na(+)/K(+)-pump current to 0.79 times vehicle control. Similarly, 500 microM of 8Br-cAMP increased the SCC 3.2-fold, yet 1 mM of 8Br-cAMP decreased the Na(+)/K(+)-pump current to 0.76 times vehicle control. Arachidonic acid (10(-5) M) tended to decrease the Na(+)/K(+)-pump current. To judge from these in vitro experiments, AVT has the potential to inhibit the basolateral Na(+)/K(+)-pump current via the V(2)-type receptor/cAMP pathway in the skin of the tree frog.

Effects of hypertonic stimuli and arginine vasotocin (AVT) on water absorption response in Japanese treefrog, Hyla japonica

Anuran amphibians do not drink orally but absorb water osmotically through the highly permeable ventral skin. In this cutaneous water absorption, roles of the putative cerebral osmoreceptors and functions of arginine vasotocin (AVT) were examined in the central nervous system of the Japanese treefrog, Hyla japonica. Intracerebroventricular (ICV) or intralymphatic sac (ILS) administration of various hypertonic solutions (NaCl, mannitol and urea) significantly extended the residence time in water in a dose-dependent manner, suggesting facilitation of water absorption in frogs. ICV injection of AVT also increased significantly the residence time in a dose-dependent manner. The water absorption effect of AVT was significantly inhibited by pretreatment of ICV OPC-21268, a vasopressin V(1) receptor antagonist. But pre-ICV injection of OPC-31260, a vasopressin V(2) receptor antagonist, did not block the water absorption effect of AVT. Extension of the residence time induced by hyperosmotic NaCl (1000 mOsm) ICV injection was significantly inhibited by pretreatment of ICV OPC-21268. The present results showed that increases of osmotic pressure in plasma and/or cerebrospinal fluid stimulate water absorption response, suggesting that osmoreceptors are certainly present in the central nervous system and AVT may directly stimulate water absorption in the treefrog. It is also suggested that AVT activates cellular mechanisms via V(1)-like but not V(2)-like receptors in the central nervous system and facilitates water absorption response in the treefrog.