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

Social regulation of arginine vasopressin and oxytocin systems in a wild group-living fish

The neuropeptides arginine vasopressin (AVP) and oxytocin (OXT) are key regulators of social behaviour across vertebrates. However, much of our understanding of how these neuropeptide systems interact with social behaviour is centred around laboratory studies which fail to capture the social and physiological challenges of living in the wild. To evaluate relationships between these neuropeptide systems and social behaviour in the wild, we studied social groups of the cichlid fish Neolamprologus pulcher in Lake Tanganyika, Africa. We first used SCUBA to observe the behaviour of focal group members and then measured transcript abundance of key components of the AVP and OXT systems across different brain regions. While AVP is often associated with male-typical behaviours, we found that dominant females had higher expression of avp and its receptor (avpr1a2) in the preoptic area of the brain compared to either dominant males or subordinates of either sex. Dominant females also generally had the highest levels of leucyl-cystinyl aminopeptidase (lnpep)-which inactivates AVP and OXT-throughout the brain, potentially indicating greater overall activity (i.e., production, release, and turnover) of the AVP system in dominant females. Expression of OXT and its receptors did not differ across social ranks. However, dominant males that visited the brood chamber more often had lower preoptic expression of OXT receptor a (oxtra) suggesting a negative relationship between OXT signalling and parental care in males of this species. Overall, these results advance our understanding of the relationships between complex social behaviours and neuroendocrine systems under natural settings.

Fluctuating and Stable High Temperatures Differentially Affect Reproductive Endocrinology of Female Pupfish

For many fishes, reproductive function is thermally constrained such that exposure to temperatures above some upper threshold has detrimental effects on gametic development and maturation, spawning frequency, and mating behavior. Such impairment of reproductive performance at elevated temperatures involves changes to hypothalamic-pituitary-gonadal (HPG) axis signaling and diminished gonadal steroidogenesis. However, how HPG pathways respond to consistently high versus temporally elevated temperatures is not clear. Here, sexually mature Amargosa River Pupfish (Cyprinodon nevadensis amargosae) were maintained under thermal regimes of either stable ∼25°C (low temperature), diurnal cycling temperatures between ∼27 and 35°C (fluctuating temperature), or stable ∼35°C (high temperature) conditions for 50 days to examine effects of these conditions on HPG endocrine signaling components in the pituitary gland and gonad, ovarian and testicular gametogenesis status, and liver gene expression relating to oogenesis. Female pupfish maintained under stable high and fluctuating temperature treatments showed reduced gonadosomatic index values as well as a lower proportion of oocytes in the lipid droplet and vitellogenic stages. Females in both fluctuating and stable 35°C conditions exhibited reduced ovarian mRNAs for steroid acute regulatory protein (star), cholesterol side chain-cleavage enzyme, P450scc (cyp11a1), and 3β-hydroxysteroid dehydrogenase (3bhsd), while ovarian transcripts encoding 11β-hydroxysteroid dehydrogenase (11bhsd) and sex hormone-binding globulin (shbg) were elevated in females at constant 35°C only. Ovarian aromatase (cyp19a1a) mRNA levels were unaffected, but circulating 17β-estradiol (E2) was lower in females at 35°C compared to the fluctuating temperature condition. In the liver, mRNA levels for choriogenins and vitellogenin were downregulated in both the fluctuating and 35°C conditions, while hepatic estrogen receptor 2a (esr2a) and shbg mRNAs were elevated in 35°C females. Taken together, these results demonstrate the potential for elevated temperatures to impair ovarian steroidogenesis and reduce egg envelope and vitellogenin protein production in female C. n. amargosae pupfish, while also shedding light on how thermal regimes that only intermittently reach the upper thermal range for reproduction have differential impacts on reproductive endocrine pathways than constantly warm conditions.

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.

Localization of the Neuropeptide Arginine Vasotocin and Its Receptor in the Osmoregulatory Organs of Black Porgy, Acanthopagrus schlegelii: Gills, Kidneys, and Intestines

The neurohypophysial hormone arginine vasotocin (avt) and its receptor (avtr) regulates ions in the osmoregulatory organs of euryhaline black porgy (Acanthopagrus schlegelii). The localization of avt and avtr transcripts in the osmoregulatory organs has yet to be demonstrated. Thus, in the present study, we performed an in situ hybridization analysis to determine the localization of avt and avtr in the gills, kidneys, and intestines of the black porgy. The avt and avtr transcripts were identified in the filament and lamellae region of the gills in the black porgy. However, the basal membrane of the filament contained more avt and avtr transcripts. Fluorescence double tagging analysis revealed that avt and avtr mRNAs were partially co-localized with α-Nka-ir cells in the gill filament. The proximal tubules, distal tubules, and collecting duct of the kidney all had positive hybridization signals for the avt and avtr transcripts. Unlike the α-Nka immunoreactive cells, the avt and avtr transcripts were found on the basolateral surface of the distal convoluted tubule and in the entire cells of the proximal convoluted tubules of the black porgy kidney. In the intestine, the avt and avtr transcripts were found in the basolateral membrane of the enterocytes. Collectively, this study provides a summary of evidence suggesting that the neuropeptides avt and avtr with α-Nka-ir cells may have functions in the gills, kidneys, and intestines via ionocytes.

Accustomed to the heat: Temperature and thyroid hormone influences on oogenesis and gonadal steroidogenesis pathways vary among populations of Amargosa pupfish (Cyprinodon nevadensis amargosae)

Many fish experience diminished reproductive performance under atypically high or prolonged elevations of temperature. Such high temperature inhibition of reproduction comes about in part from altered stimulation of gametogenesis by the hypothalamic-pituitary-gonadal (HPG) endocrine axis. Elevated temperatures have also been shown to affect thyroid hormone (TH) signaling, and altered TH status under high temperatures may impact gametogenesis via crosstalk with HPG axis pathways. Here, we examined effects of temperature and 3'-triiodo-_L-thyronine (T₃) on pathways for gonadal steroidogenesis and gametogenesis in Amargosa pupfish (Cyprinodon nevadensis amargosae) from two allopatric populations: 1) the Amargosa River - a highly variable temperature habitat, and 2) Tecopa Bore - an invariably warm groundwater-fed marsh. These populations were previously shown to differ in TH signaling profiles both in the wild and under common laboratory conditions. Sexually-mature pupfish from each population were maintained at 24 °C or 34 °C for 88 days, after which a subset of fish was treated with T₃ for 18-24 h. In both populations, mRNA abundances for follicle-stimulating hormone receptor and luteinizing hormone receptor were higher in the ovary and testis at 24 °C compared to 34 °C. Females from Tecopa Bore - but not from the Amargosa River - also had greater ovarian transcript abundances for steroidogenic enzymes cytochrome P450 aromatase, 3β-hydroxysteroid dehydrogenase, and 17β-hydroxysteroid dehydrogenase at 24 °C compared to 34 °C, as well as higher liver mRNA levels of vitellogenins and choriogenins at cooler temperature. Transcript abundances for estrogen receptors esr1, esr2a, and esr2b were reduced at 34 °C in Amargosa River females, but not in Tecopa Bore females. T₃ augmented gonadal gene transcript levels for steroid acute regulatory protein (StAR) transporter in both sexes and populations. T₃ also downregulated liver estrogen receptor mRNAs in females from the warmer Tecopa Bore habitat only, suggesting T₃ modulation of liver E₂ sensitivity as a possible mechanism whereby temperature-induced changes in TH status may contribute to shifts in thermal sensitivity for oogenesis.

Circulating Isotocin, not Angiotensin II, is the Major Dipsogenic Hormone in Eels

Angiotensin II (AngII) is generally known as the most important dipsogenic hormone throughout vertebrates, while two other neurohypophysial hormones, vasopressin and oxytocin, are not dipsogenic in mammals. In this study, we found that systemic isotocin, but not vasotocin, is the potent dipsogenic hormone in eels. When injected intra-arterially into conscious eels, isotocin, vasotocin and AngII equally increased ventral aortic pressure dose-dependently at 0.03-1.0 nmol/kg, but only isotocin induced copious drinking. The dipsogenic effect was dose-dependent and occurred significantly at as low as 0.1 nmol/kg. By contrast, a sustained inhibition of drinking occurred after AngII, probably due to baroreflexogenic inhibition. No such inhibition was observed after isotocin despite similar concurrent hypertension. The baroreceptor may exist distal to the gill circulation because the vasopressor effect occurred at both ventral and dorsal aorta after AngII but only at ventral aorta after isotocin. By contrast, intra-cerebroventricular (i.c.v.) injection of isotocin had no effect on drinking or blood pressure, but AngII increased drinking and aortic pressure dose-dependently at 0.03-0.3 nmol/eel. Lesioning of the area postrema (AP), a sensory circumventricular organ, abolished drinking induced by peripheral isotocin, but not i.c.v. AngII. Collectively, isotocin seems to be a major circulating hormone that induces swallowing through its action on the AP, while AngII may be an intrinsic brain peptide that induces drinking through its action on a different circumventricular site, possibly a recently identified blood-brain barrier-deficient structure in the antero-ventral third ventricle of eels, as shown in birds and mammals.

Air sac and gill vasotocin receptor gene expression in the air-breathing catfish Heteropneustes fossilis exposed to water and air deprivation conditions

Heteropneustes fossilis is a facultative air-breathing freshwater catfish and inhabits ponds, ditches, swamps, marshes and rivers that dry up in summers. It possesses a pair of unique tubular accessory respiratory organ (air sac), which is a modification of the gill chamber and enables it to live in water-air transition zones. In the catfish, three vasotocin (Vt) receptor gene paralogs viz., v1a1, v1a2 and v2a were identified for Vt actions. In the present study, the receptor gene transcripts were localized in the gill and air sac by in situ hybridization, and their expression levels in relation to water and air deprivation conditions were investigated by quantitative RT-PCR. The catfish were exposed to 1 h and 2 h in gonad inactive (resting) and gonad active (prespawning) phases. The gene paralogs showed overlapping distribution in the respiratory epithelium of primary and secondary lamellae of gills and reduced lamellae of the air sacs. In water deprivation (forced aerial mode of respiration) experiment, v2a expression showed a high fold increase in the air sac, which was unchanged or inhibited in the gill. Both v1a1 and v1a2 expression was significantly upregulated in the air sac but showed varied responses in the gill. The gill v1a1 expression was unchanged in the resting phase and modestly upregulated in the prespawning phase. The gill v1a2 expression was modestly upregulated at 1 h in both phases but unchanged at 2 h. In the air deprivation experiment (forced aquatic respiration), the v2a expression in the air sac was inhibited except for a mild stimulation at 1 h in the prespawning phase. In the gill, the v2a expression was stimulated with a steep upregulation at 2 h in the prespawning phase. Both v1a1 and v1a2 expression was significantly high in the gill but only modestly increased or unchanged in the air sac. The expression patterns point to a functional distinction; the V2 type receptor expression was higher in the air sac during forced aerial respiration, and the V1 type receptor expression was highly prominent in the gill during forced aquatic respiration. Water and air deprivation treatments caused a significant increase in plasma cortisol level, and the stimulation was higher in the water deprivation fish in the resting phase but equally prominent in the water and air deprivation groups in the prespawning phase. The results indicate that the changes in the expression patterns of Vt receptor genes may be a sequel to stress (hypoxic, metabolic and osmotic), and both Vt and cortisol may interact to counter the stress responses. This study shows that Vt has a new role in the control of air sac functions.

Role of Habenula in Social and Reproductive Behaviors in Fish: Comparison With Mammals

Social behaviors such as mating, parenting, fighting, and avoiding are essential functions as a communication tool in social animals, and are critical for the survival of individuals and species. Social behaviors are controlled by a complex circuitry that comprises several key social brain regions, which is called the social behavior network (SBN). The SBN further integrates social information with external and internal factors to select appropriate behavioral responses to social circumstances, called social decision-making. The social decision-making network (SDMN) and SBN are structurally, neurochemically and functionally conserved in vertebrates. The social decision-making process is also closely influenced by emotional assessment. The habenula has recently been recognized as a crucial center for emotion-associated adaptation behaviors. Here we review the potential role of the habenula in social function with a special emphasis on fish studies. Further, based on evolutional, molecular, morphological, and behavioral perspectives, we discuss the crucial role of the habenula in the vertebrate SDMN.

Neuroendocrine Regulation of Plasma Cortisol Levels During Smoltification and Seawater Acclimation of Atlantic Salmon

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue-adrenocorticotropic hormone (ACTH)-as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts.

Social effects on AVT and CRF systems

Stress and aggression have negative effects on fish welfare and productivity in aquaculture. Thus, research to understand aggression and stress in farmed fish is required. The neuropeptides arginine-vasotocin (AVT) and corticotropin-releasing factor (CRF) are involved in the control of stress and aggression. Therefore, we investigated the effect of agonistic interactions on the gene expression of AVT, CRF and their receptors in juvenile rainbow trout (Oncorhynchus mykiss). The social interactions lead to a clear dominant-subordinate relationship with dominant fish feeding more and being more aggressive. Subordinate fish had an upregulation of the AVT receptor (AVT-R), an upregulation of CRF mRNA levels, and higher plasma cortisol levels. The attenuating effect of AVT on aggression in rainbow trout is proposed to be mediated by AVT-R, and the attenuating effect of the CRF system is proposed to be mediated by CRF.

Neurohypophysial Hormones Associated with Osmotic Challenges in the Brain and Pituitary of the Euryhaline Black Porgy, Acanthopagrus schlegelii

Our study showed differential expression of the arginine vasotocin (avt)/isotocin (it) in the brain and pituitary gland of the euryhaline black porgy (Acanthopagrus schlegelii) during osmotic stress. A decrease in serum osmolality and increased cortisol levels were observed after acute transfer from seawater (SW) to freshwater (FW). The increased expressions of avt, avt receptor (avtr: v1a), and isotocin receptor (itr: itr1) transcripts on day 1 and it and itr transcripts on days 7 and 30 were found in the brains and pituitary glands of FW fish. Increased levels of avt mRNA in the diencephalon and avtr mRNA in the pituitary together with serum cortisol on day 1 of FW exposure indicated activation of the hypothalamic-pituitary-interrenal (HPI) axis. The expression levels of avtr and itr after FW transfer were increased in the pituitary on days 7 and 30. Furthermore, in situ hybridization demonstrated spatially differential expression of avt and itr transcripts in nucleus preopticus parvocellularis of pars gigantocellularis (PMgc), magnocellularis (PMmc), and parvocellularis (PMpc) of the preoptic area (POA). Positive signals for avt and it were highly abundant in PMpc after FW exposure. The data suggest involvement of neurohypophysial hormones in the brain (telencephalon and diencephalon) and pituitary for osmotic stress.

Widespread alterations to hypothalamic-pituitary-gonadal (HPG) axis signaling underlie high temperature reproductive inhibition in the eurythermal sheepshead minnow (Cyprinodon variegatus)

Fish experiencing abnormally high or prolonged elevations in temperature can exhibit impaired reproduction, even for species adapted to warm water environments. Such high temperature inhibition of reproduction has been linked to diminished gonadal steroidogenesis, but the mechanisms whereby hypothalamic-pituitary-gonadal (HPG) axis signaling is impacted by high temperature are not fully understood. Here, we characterized differences in HPG status in adult sheepshead minnow (Cyprinodon variegatus), a eurythermal salt marsh and estuarine species of eastern North America, exposed for 14 d to temperatures of 27 °C or 37 °C. Males and females at 37 °C had lower gonadosomatic index (GSI) values compared to fish at 27 °C, and females at 37 °C had fewer spawning capable eggs and lower circulating 17β-estradiol (E₂). Gene transcripts encoding gonadotropin-inhibitory hormone (gnih) and gonadotropin-releasing hormone-3 (gnrh3) were higher in relative abundance in the hypothalamus of both sexes at 37 °C. While pituitary mRNAs for the β-subunits of follicle-stimulating hormone (fshβ) and luteinizing hormone (lhβ) were lowered only in males at 37 °C, Fsh and Lh receptor mRNA levels in the gonads were at lower relative levels in both the ovary and testis of fish at 37 °C. Females at 37 °C also showed reduced ovarian mRNA levels for steroid acute regulatory protein (star), P450 side-chain cleavage enzyme (cyp11a1), 3β-hydroxysteroid dehydrogenase (3βhsd), 17β-hydroxysteroid dehydrogenase (hsd17β3), and ovarian aromatase (cyp19a1a). Females at the higher 37 °C temperature also had a lower liver expression of mRNAs encoding estrogen receptor α (esr1) and several vitellogenin and choriogenin genes, but elevated mRNA levels for hepatic sex hormone-binding globulin (shbg). Our results substantiate prior findings that exposure of fish to high temperature can inhibit gonadal steroidogenesis and oogenesis, and point to declines in reproductive performance emerging from alterations at several levels of HPG axis signaling including increased hypothalamic Gnih expression, depressed gonadal steroidogenesis, and reduced egg yolk and egg envelope protein production in the liver.

The oxytocin/vasopressin-like peptide receptor mRNA in the central nervous system and ovary of the blue swimming crab, Portunus pelagicus

The authors recently reported the presence and distribution of oxytocin/vasopressin-like peptide in Portunus pelagicus as well as demonstrated its function to inhibit ovarian steroid release (Saetan et al., 2018). Here, the full-length receptor of this peptide, namely oxytocin/vasopressin-like peptide receptor (PpelOT/VP-like peptide receptor) is reported. The coding region of the PpelOT/VP-like peptide receptor contained 1497 bp which translationally corresponded to 499 amino acids. Sequence analysis revealed its seven transmembrane characteristics, with -two N-linked glycosylation residues located before the first transmembrane domain (TM I). The phylogenetic tree revealed that the PpelOT/VP-like peptide receptor was placed in the group of invertebrate OT/VP-like receptors, and was clearly distinguishable from the V1R, V2R and OTR of vertebrates. Also, this receptor gene transcript was detected in several organs of the blue swimming crab with highest abundance found in brain tissue. In situ hybridization exhibited its distribution in all neuronal clusters of the eyestalk, brain, ventral nerve cord (VNC), as well as in the ovary. Comparative gene expressions between this receptor and its corresponding peptide in immature and mature female crabs revealed no significant difference of the PpelOT/VP-like peptide receptor gene expression in the central nervous system (CNS) and ovary. In contrast, the PpelOT/VP-like peptide gene was shown to significantly express higher in the VNC of immature crabs and in the ovary of mature crabs. Changes in expression of this peptide gene, but not its receptor, might result in ovarian steroid release inhibition. However, the detailed mechanism of this peptide in reproduction regulation will be included in our further studies.

Fish as a model in social neuroscience: conservation and diversity in the social brain network

Mechanisms for fish social behaviours involve a social brain network (SBN) which is evolutionarily conserved among vertebrates. However, considerable diversity is observed in the actual behaviour patterns amongst nearly 30000 fish species. The huge variation found in socio-sexual behaviours and strategies is likely generated by a morphologically and genetically well-conserved small forebrain system. Hence, teleost fish provide a useful model to study the fundamental mechanisms underlying social brain functions. Herein we review the foundations underlying fish social behaviours including sensory, hormonal, molecular and neuroanatomical features. Gonadotropin-releasing hormone neurons clearly play important roles, but the participation of vasotocin and isotocin is also highlighted. Genetic investigations of developing fish brain have revealed the molecular complexity of neural development of the SBN. In addition to straightforward social behaviours such as sex and aggression, new experiments have revealed higher order and unique phenomena such as social eavesdropping and social buffering in fish. Finally, observations interpreted as 'collective cognition' in fish can likely be explained by careful observation of sensory determinants and analyses using the dynamics of quantitative scaling. Understanding of the functions of the SBN in fish provide clues for understanding the origin and evolution of higher social functions in vertebrates.

In situ localization of vasotocin receptor gene transcripts in the brain-pituitary-gonadal axis of the catfish Heteropneustes fossilis: a morpho-functional study

In the catfish Heteropneustes fossilis, three vasotocin (VT) receptor subtype genes, v1a1, v1a2, and v2a, were cloned and characterized previously. In the present study, using RNA probes, we localized the distribution of the gene transcripts in the brain-pituitary-gonadal (BPG) axis. The V1a-type receptor, v1a1 and v1a2, genes showed similar and overlapping distribution in the brain. The gene paralogs are distributed in the radial glial cells (RGCs) of the telencephalic ventricle and around the third ventricle in the hypothalamus and thalamus, olfactory tract, nucleus preopticus, nucleus lateralis tuberis, nucleus recessus lateralis and posterioris, nucleus saccus vasculosi, thalamic nuclei, habenular nucleus, habenular commissure, basal part of pineal stalk, accessory pretectal nucleus, optic tectum, corpus and valvula of the cerebellum, and facial and vagal lobes. The V2a receptor gene (v2a) has restricted distribution and is largely confined to the anterior subependymal region of the telencephalon. The localization pattern shows that the V1a-type receptors are distributed in major sensorimotor processing centers and the neuroendocrine/reproductive centers of the brain. In the pituitary, the receptor genes were localized differentially in the three divisions with the V1a-type receptor genes strongly expressed in the rostral pars distalis compared to the v2a paralog. In the ovary, the V1a-type receptor genes were localized in the follicular layer while v2a was localized in the oocyte membrane. In the testis, v1a2 and v2a are densely distributed in the interstitial tissue and seminiferous epithelium but the v1a1 is lowly expressed. The results suggest that the VT receptor genes have an extensive but differential distribution in the BPG axis. Future experimental studies are required to correlate the cellular localizations with specific functions of VT in the BPG axis.

Evidence for a role of arginine vasotocin receptors in the gill during salinity acclimation by a euryhaline teleost fish

The nonapeptide arginine vasotocin (AVT) regulates osmotic balance in teleost fishes, but its mechanisms of action are not fully understood. Recently, it was discovered that nonapeptide receptors in teleost fishes are differentiated into two V1a-type, several V2-type, and two isotocin (IT) receptors, but it remains unclear which receptors mediate AVT's effects on gill osmoregulation. Here, we examined the role of nonapeptide receptors in the gill of the euryhaline Amargosa pupfish (Cyprinodon nevadensis amargosae) during osmotic acclimation. Transcripts for the teleost V1a-type receptor v1a2 were upregulated over fourfold in gill 24 h after transferring pupfish from 7.5 ppt to seawater (35 ppt) or hypersaline (55 ppt) conditions and downregulated after transfer to freshwater (0.3 ppt). Gill transcripts for the nonapeptide degradation enzyme leucyl-cystinyl aminopeptidase (LNPEP) also increased in fish acclimating to 35 ppt. To test whether the effects of AVT on the gill might be mediated by a V1a-type receptor, we administered AVT or a V1-type receptor antagonist (Manning compound) intraperitoneally to pupfish before transfer to 0.4 ppt or 35 ppt. Pupfish transferred to 35 ppt exhibited elevated gill mRNA abundance for cystic fibrosis transmembrane conductance regulator (cftr), but that upregulation diminished under V1-receptor inhibition. AVT inhibited the increase in gill Na⁺/Cl^- cotransporter 2 (ncc2) transcript abundance that occurs following transfer to hypoosmotic environments, whereas V1-type receptor antagonism increased ncc2 mRNAs even without a change in salinity. These findings indicate that AVT acts via a V1-type receptor to regulate gill Cl^- transport by inhibiting Cl^- uptake and facilitating Cl^- secretion during seawater acclimation.

Dynamics of Gene Expression Responses for Ion Transport Proteins and Aquaporins in the Gill of a Euryhaline Pupfish during Freshwater and High-Salinity Acclimation

Pupfishes (genus Cyprinodon) evolved some of the broadest salinity tolerances of teleost fishes, with some taxa surviving in conditions from freshwater to nearly 160 ppt. In this study, we examined transcriptional dynamics of ion transporters and aquaporins in the gill of the desert Amargosa pupfish (Cyprinodon nevadensis amargosae) during rapid salinity change. Pupfish acclimated to 7.5 ppt were exposed to freshwater (0.3 ppt), seawater (35 ppt), or hypersaline (55 ppt) conditions over 4 h and sampled at these salinities over 14 d. Plasma osmolality and Cl^- concentration became elevated 8 h after the start of exposure to 35 or 55 ppt but returned to baseline levels after 14 d. Osmolality recovery was paralleled by increased gill Na⁺/K⁺-ATPase activity and higher relative levels of messenger RNAs (mRNAs) encoding cystic fibrosis transmembrane conductance regulator (cftr) and Na⁺/K⁺/2Cl^- cotransporter-1 (nkcc1). Transcripts encoding one Na⁺-HCO₃^- cotransporter-1 isoform (nbce1.1) also increased in the gills at higher salinities, while a second isoform (nbce1.2) increased expression in freshwater. Pupfish in freshwater also had lower osmolality and elevated gill mRNAs for Na⁺/H⁺ exchanger isoform-2a (nhe2a) and V-type H⁺-ATPase within 8 h, followed by increases in Na⁺/H⁺ exchanger-3 (nhe3), carbonic anhydrase 2 (ca2), and aquaporin-3 (aqp3) within 1 d. Gill mRNAs for Na⁺/Cl^- cotransporter-2 (ncc2) also were elevated 14 d after exposure to 0.3 ppt. These results offer insights into how coordinated transcriptional responses for ion transporters in the gill facilitate reestablishment of osmotic homeostasis after changes in environmental salinity and provide evidence that the teleost gill expresses two Na⁺-HCO₃^- cotransporter-1 isoforms with different roles in freshwater and seawater acclimation.

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.

Reproductive stage- and sex-dependant effects of neurohypophyseal nonapeptides on gonadotropin subunit mRNA expression in the catfish Heteropneustes fossilis: An in vitro study

In the present study, in vitro effects of synthetic vasotocin (VT), isotocin (4Ser, 8Ile- oxytocin; ITb) and the recently cloned IT gene paralog product (8Val-Isotocin, ITa) were studied on the expression of pituitary gonadotropin (GtH) subunit mRNA levels. In male pituitaries of early (preparatory phase) and late (prespawning phase) recrudescing catfish, Heteropneustes fossilis, VT (10 nM, 100 nM and 1000 nM) stimulated fshβ expression dose-dependently. But in females, the dose-dependent effect was found only in the preparatory phase. In males, VT stimulated lhβ expression only at higher doses. In females, VT produced a significant dose-dependent increase of the lhβ expression only in the prespawning phase. VT stimulated the expression of gpα, dose-dependently in the preparatory phase in males and in the prespawning phase in females. The incubation of the pituitaries with ITb did not alter the fshβ expression in either sex in both preparatory and prespawning phases. In males, ITb stimulated the expression of lhβ and gpα only at the highest concentration (1000 nM) in both phases. In females, ITb stimulated both lhβ and gpα expression only at 1000 nM in the preparatory phase and dose-dependently in the prespawning phase. The incubation of the pituitaries with ITa produced effects similar to ITb on the expression of fshβ, lhβ, and gpα. The results show that the basic peptide VT modulates both fshβ and lhβ expressions, which are influenced by the sex and reproductive stage. The neutral peptide ITA/ITb exerts an insignificant effect on the fshβ expression regardless of sex or season. Both VT and ITa/ITb elicit a significant effect on the lhβ expression in late recrudescent phase especially in females.

The nonapeptide isotocin in goldfish: Evidence for serotonergic regulation and functional roles in the control of food intake and pituitary hormone release

Nonapeptides are a highly conserved family of peptides synthesized in the neuroendocrine brain and acting on central and peripheral receptors to regulate physiological functions in vertebrates. While the evolution of the two gene families of oxytocin-like and vasopressin-like nonapeptides and their receptors, as well as the neuroanatomy of their independent neuronal circuits have been well-characterized across vertebrate species, comparative studies on the physiological roles across vertebrates are lagging behind. In the current study, we focused on the comparative neuroendocrine functions and regulation of isotocin, the teleost homologue of mammalian oxytocin. Specifically, we address the hypothesis that isotocin exerts opposing effects on food intake and reproduction, which are well-established effects of its homologue oxytocin in mammalian species. Using goldfish, a well-characterized model of neuroendocrine regulation of both food intake and reproduction, we here showed that isotocin acts as an anorexigenic factor while exerting stimulatory effects on pituitary luteinizing hormone and growth hormone release. Given the dual inhibitory and stimulatory roles of serotonin on food intake and pituitary release of reproductive hormone in goldfish, we also investigated the potential crosstalk between both systems using immunohistochemistry and pharmacological approaches. Results provide neuroanatomical and pharmacological evidence for serotonergic regulation of magnocellular isotocinergic neurons in the preoptic area and pituitary. Together, these findings firstly provide the basis to investigate neuroendocrine cross-talk between serotonergic and nonapeptidergic systems in the regulation of both food intake and reproduction in goldfish, and secondly point to a conserved function of oxytocin-like peptides in the differential neuroendocrine control of both physiological processes in vertebrates.

Identification of an oxytocinase/vasopressinase-like leucyl-cystinyl aminopeptidase (LNPEP) in teleost fish and evidence for hypothalamic mRNA expression linked to behavioral social status

The vasotocin/vasopressin and isotocin/mesotocin/oxytocin family of nonapeptides regulate social behaviors and physiological functions associated with reproductive physiology and osmotic balance. While experimental and correlative studies provide evidence for these nonapeptides as modulators of behavior across all classes of vertebrates, mechanisms for nonapeptide inactivation in regulating these functions have been largely overlooked. Leucyl-cystinyl aminopeptidase (LNPEP) - also known as vasopressinase, oxytocinase, placental leucine aminopeptidase (P-LAP), and insulin-regulated aminopeptidase (IRAP) - is a membrane-bound zinc-dependent metalloexopeptidase enzyme that inactivates vasopressin, oxytocin, and select other cyclic polypeptides. In humans, LNPEP plays a key role in the clearance of oxytocin during pregnancy. However, the evolutionary diversity, expression distribution, and functional roles of LNPEP remain unresolved for other vertebrates. Here, we isolated and sequenced a full-length cDNA encoding a LNPEP-like polypeptide of 1033 amino acids from the ovarian tissue of Amargosa pupfish, Cyprinodon nevadensis. This deduced polypeptide exhibited high amino acid identity to human LNPEP both in the protein's active domain that includes the peptide binding site and zinc cofactor binding motif (53.1% identity), and in an intracellular region that distinguishes LNPEP from other aminopeptidases (70.3% identity). Transcripts encoding this LNPEP enzyme (lnpep) were detected at highest relative abundance in the gonads, hypothalamus, forebrain, optic tectum, gill and skeletal muscle of adult pupfish. Further evaluation of lnpep transcript abundance in the brain of sexually-mature pupfish revealed that lnpep mRNAs were elevated in the hypothalamus of socially subordinate females and males, and at lower abundance in the telencephalon of socially dominant males compared to dominant females. These findings provide evidence of an association between behavioral social status and hypothalamic lnpep transcript abundance and suggest that variation in the rate of VT/IT peptide inactivation by LNPEP may be a contributing component in the mechanism whereby nonapeptides regulate social behavior.

Effects of the fish spawning inducer ovaprim on vasotocin receptor gene expression in brain and ovary of the catfish Heteropneustes fossilis with a note on differential transcript expression in ovarian follicles

Ovaprim (OVP), a commercial formulation of a salmon GnRH analogue and the dopamine receptor-2 blocker domperidone, is a successful spawning inducer for fish breeding. It induces a preovulatory surge in LH, which stimulates the synthesis of a maturation-inducing steroid (MIS, 17,20β-dihydroxy-4-pregnen-3-one) that initiates germinal vesicle breakdown (GVBD) and ovulation. Coincidently, the OVP treatment also stimulates vasotocin (VT) secretion in the brain and ovary of the catfish Heteropneustes fossilis that also stimulates the synthesis of the MIS. VT mediates its effect through V1- and V2-type receptors. In the present study in the catfish, we report that OVP stimulates the expression of VT receptor genes v1a1, v1a2 and v2a in the brain and ovary. A single intraperitoneal administration of OVP (0.5μL/g body weight) or incubation of post-vitellogenic ovarian follicles with 5μL/mL OVP, for 0, 4, 8, 12, 16, and 24h stimulated ovulation and GVBD, respectively, in a time-dependent manner. The OVP treatment in vivo stimulated brain VT receptor transcript levels 4h onwards. The peak expression was noticed at 12h (v1a1), 8 and 12h (v1a2), and 8, 12 and 16h (v2a), coinciding with FOM and ovulation. The VT receptor genes are expressed in the ovarian follicles compartmentally; both v1a1 and v1a2 are expressed in the isolated follicular layer (theca and granulosa) but absent in denuded oocytes. V2a is expressed in the denuded oocytes and not in the follicular layer. The OVP injection stimulated the v1a1 and v1a2 expression from 4h onwards in both intact follicle and isolated follicular layer, the peak expression was observed at 16h. The v2a expression was up-regulated in both intact follicles and denuded oocytes at 4h (denuded oocytes) or 8h (intact follicle) onwards with the peak expression at 12h and 16h (denuded oocytes) or at 16h (intact follicles). Under in vitro conditions, the OVP incubations elicited similar pattern of changes with the peak stimulation at 16h for all the genes. In conclusion, the VT receptor genes are differentially expressed in the ovarian follicles and OVP induced periovulatory stimulation of the VT receptor genes, coinciding with FOM and ovulation.

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 Non-Peptide Arginine-Vasopressin v1a Selective Receptor Antagonist, SR49059, Blocks the Rewarding, Prosocial, and Anxiolytic Effects of 3,4-Methylenedioxymethamphetamine and Its Derivatives in Zebra Fish

3,4-Methylenedioxymethamphetamine (MDMA) and its derivatives, 2,5-dimethoxy-4-bromo-amphetamine hydrobromide (DOB) and para-methoxyamphetamine (PMA), are recreational drugs whose pharmacological effects have recently been attributed to serotonin 5HT_2A/C receptors. However, there is growing evidence that the oxytocin (OT)/vasopressin system can modulate some the effects of MDMA. In this study, MDMA (2.5-10 mg/kg), DOB (0.5 mg/kg), or PMA (0.005, 0.1, or 0.25 mg/kg) were administered intramuscularly to adult zebra fish, alone or in combination with the V_1a vasopressin antagonist, SR49059 (0.01-1 ng/kg), before carrying out conditioned place preference (CPP), social preference, novel tank diving, and light-dark tests in order to evaluate subsequent rewarding, social, and emotional-like behavior. The combination of SR49059 and each drug progressively blocked: (1) rewarding behavior as measured by CPP in terms of time spent in drug-paired compartment; (2) prosocial effects measured on the basis of the time spent in the proximity of a nacre fish picture; and (3) anxiolytic effects in terms of the time spent in the upper half of the novel tank and in the white compartment of the tank used for the light-dark test. Antagonism was obtained at SR49059 doses which, when given alone, did not change motor function. In comparison with a control group, receiving vehicle alone, there was a three to five times increase in the brain release of isotocin (the analog of OT in fish) after treatment with the most active doses of MDMA (10 mg/kg), DOB (0.5 mg/kg), and PMA (0.1 mg/kg) as evaluated by means of bioanalytical reversed-phase high-performance liquid chromatography. Taken together, these findings show that the OT/vasopressin system is involved in the rewarding, prosocial, and anxiolytic effects of MDMA, DOB, and PMA in zebra fish and underline the association between this system and the behavioral alterations associated with disorders related to substance abuse.

Arginine Vasotocin Preprohormone Is Expressed in Surprising Regions of the Teleost Forebrain

Nonapeptides play a fundamental role in the regulation of social behavior, among numerous other functions. In particular, arginine vasopressin and its non-mammalian homolog, arginine vasotocin (AVT), have been implicated in regulating affiliative, reproductive, and aggressive behavior in many vertebrate species. Where these nonapeptides are synthesized in the brain has been studied extensively in most vertebrate lineages. While several hypothalamic and forebrain populations of vasopressinergic neurons have been described in amniotes, the consensus suggests that the expression of AVT in the brain of teleost fish is limited to the hypothalamus, specifically the preoptic area (POA) and the anterior tuberal nucleus (putative homolog of the mammalian ventromedial hypothalamus). However, as most studies in teleosts have focused on the POA, there may be an ascertainment bias. Here, we revisit the distribution of AVT preprohormone mRNA across the dorsal and ventral telencephalon of a highly social African cichlid fish. We first use in situ hybridization to map the distribution of AVT preprohormone mRNA across the telencephalon. We then use quantitative real-time polymerase chain reaction to assay AVT expression in the dorsomedial telencephalon, the putative homolog of the mammalian basolateral amygdala. We find evidence for AVT preprohormone mRNA in regions previously not associated with the expression of this nonapeptide, including the putative homologs of the mammalian extended amygdala, hippocampus, striatum, and septum. In addition, AVT preprohormone mRNA expression within the basolateral amygdala homolog differs across social contexts, suggesting a possible role in behavioral regulation. We conclude that the surprising presence of AVT preprohormone mRNA within dorsal and medial telencephalic regions warrants a closer examination of possible AVT synthesis locations in teleost fish, and that these may be more similar to what is observed in mammals and birds.

Differential activation of vasotocin neurons in contexts that elicit aggression and courtship

Despite continued study on the neurobiological bases of aggressive and sexual behaviors, it is still not well understood how the brain integrates social information with physiological and neural states to produce context-specific behavioral outcomes. In fishes, manipulation of endogenous levels of arginine vasotocin (AVT) through peripheral and intracerebroventricular pharmacological injections results in significant changes in social behaviors, including aggressive and reproduction-related behaviors. In addition, many features of AVT neurons have been shown to correlate with social status and associated behavioral phenotypes. In this study, we used the immediate early gene egr-1 as a marker for neuronal activity and quantified the number of AVT neurons that were positive for egr-1 mRNA by in situ hybridization in Astatotilapia burtoni males that were exposed to either a social context that would elicit aggression or to one that would elicit courtship. In these social settings, focal males readily displayed context- appropriate bouts of aggression (towards the opponent) or bouts of courting (towards females). We found that males that fought had higher levels of egr-1 expression in the preoptic area compared to courting males. A greater proportion of AVT cells was positive for egr-1 after a fight than after a bout of courting. We mapped mRNA distribution of AVT V1a receptor subtypes v1a1 and v1a2 in the brain and identified overlapping areas of expression in nuclei in the ventral telencephalon, hypothalamus and thalamus as key areas for AVT signaling in males.

Species-specific patterns of nonapeptide brain gene expression relative to pair-bonding behavior in grouping and non-grouping cichlids

Comparative studies have revealed that vasopressin-oxytocin pathways are associated with both pair bonding and grouping behavior. However, the relationship between pair bonding and grouping behavior remains unclear. In this study, our aim was to identify whether two species that differ in grouping behavior display a corresponding difference in their pair bonds, and in the underlying vasopressin-oxytocin hormonal pathways. Using two species of cichlid fishes, the highly social Neolamprologus pulcher and the non-social Telmatochromis temporalis, we measured proximity of pairs during pair bond formation, and then measured social behaviors (proximity, aggression, submission, affiliation) and brain gene expression of isotocin and arginine vasotocin (the teleost homologues of oxytocin and vasopressin, respectively), as well as their receptors, after a temporary separation and subsequent reunion of the bonded pairs. Pairs of the social species spent more time in close proximity relative to the non-social species. Rates of aggression increased in both species following the separation and reunion treatment, relative to controls that were not separated. Overall, whole brain expression of isotocin was higher in the social species relative to the non-social species, and correlated with proximity, submission, and affiliation, but only in the social species. Our results suggest that both a social and a non-social cichlid species have similar behavioral responses to a temporary separation from a mate, and we found no difference in the brain gene expression of measured hormones and receptors based on our separation-reunion treatment. However, our results highlight the importance of isotocin in mediating submissive and affiliative behaviors in cichlid fishes, and demonstrate that isotocin has species-specific correlations with socially relevant behaviors.

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.

Arginine vasotocin V1a2 receptor and GnRH-I co-localize in preoptic neurons of the sex changing grouper, Epinephelus adscensionis

The arginine vasotocin/vasopressin (AVT/AVP) and gonadotropin releasing hormone (GnRH) systems are known to control sexual behaviors and reproduction, respectively, in different vertebrate groups. However, a direct functional connection between these two neuroendocrine systems has not been demonstrated for any vertebrate species. Therefore, the objective of this research was to test the hypothesis that AVT acts on the GnRH system via an AVT V1a receptor in a sex changing grouper species, the rock hind, Epinephelus adscensionis. AVT V1a2 receptors were co-localized with GnRH-I on neurons in the preoptic anterior hypothalamus identifying a structural linkage between the AVT system and GnRH-I. Transcripts for avt, gnrh-I, and two AVT receptor subtypes (v1a1 and v1a2) were isolated and characterized for E. adscensionis and their expression was measured in males and females by q-RT-PCR. Translation of V1a-type cDNA sequences revealed two distinct forms of the AVT V1a receptor in E. adscensionis brain similar to those reported for other species. The observation of significantly higher gnrh-I mRNA in the POA+H of rock hind males as compared to females suggests differential regulation of the gnrh-I transcripts in the two sexes of this protogynous species. In male E. adscensionis, but not in females, a negative relationship was seen between plasma 11-ketotestosterone (11-KT) and the v1a1 receptor mRNA levels in the POA+H, while a positive trend was observed between 11-KT and v1a2 receptor mRNA levels, indicating that these receptor forms may be differentially regulated.

Vasotocin--A new player in the control of oocyte maturation and ovulation in fish

In this article, the physiological role of ovarian vasotocin (VT) on fish final oocyte maturation (FOM) and ovulation is reviewed based on the studies mainly available in the catfish Heteropneustes fossilis. The VT system is characterized in the follicular layer of the oocytes by both immunocytochemical and in situ hybridization techniques. The distribution was confirmed in isolated follicular layer preparations by HPLC characterization and quantification. Three VT receptor subtype genes are identified: V1a1 and V1a2 subtypes are distributed in the follicular layer and V2 subtype is present along the granulosa-oocyte membrane junction. The expression of peptide, VT precursor gene and VT receptor genes shows seasonal and periovulatory changes in the ovary. VT secretion is modulated by E2 differentially in a season-specific manner, and by progestin steroids positively. VT modulates E2 in a biphasic manner in early recrudescent phase and induces a steroidogenic shift inhibiting E2 and stimulating progestin steroid (P4, 17P4 and 17,20β-DP) pathways in the late recrudescent phase. VT stimulates prostaglandin secretion, germinal vesicle breakdown (GVBD), oocyte hydration and ovulation. VT acts through different receptors to stimulate these processes. It uses the V1 type receptor to stimulate GVBD and ovulation, and the V2 type to stimulate oocyte hydration. VT acts as an important link in the cascade of gonadotropin control of FOM and ovulation. More research is required in other species.

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.

Is there convergence in the molecular pathways underlying the repeated evolution of sociality in African cichlids?

Despite wide variation in the complexity of social interactions across taxa, the basic behavioral components of sociality appear to be modulated by conserved hormone pathways. Specifically, the nonapeptide hormones oxytocin and vasopressin and their receptors have been implicated in regulating diverse social behaviors across vertebrates. Here, we took advantage of the repeated evolution of cooperative breeding in African cichlids to investigate whether there are consistent brain gene expression patterns of isotocin and arginine vasotocin (teleost homologues of oxytocin and vasopressin), as well as their receptors, between four closely related pairs of social (cooperative) and non-social (non-cooperative) species. We first found that the coding sequences for the five genes studied were highly conserved across the eight species. This is the first study to examine the expression of both isotocin receptors, and so we performed a phylogenetic analysis that suggests that these two isotocin receptors are paralogues that arose during the teleost genome duplication. When we then examined brain gene expression patterns relative to social system, we found that there were whole-brain gene expression differences between the social and non-social species in many of the species pairs. However, these relationships varied in both the direction and magnitude among the four species pairs. In conclusion, our results suggest high sequence conservation and species-specific gene expression patterns relative to social behavior for these candidate hormone pathways in the cichlid fishes.

Integrating resource defence theory with a neural nonapeptide pathway to explain territory-based mating systems

The ultimate-level factors that drive the evolution of mating systems have been well studied, but an evolutionarily conserved neural mechanism involved in shaping behaviour and social organization across species has remained elusive. Here, we review studies that have investigated the role of neural arginine vasopressin (AVP), vasotocin (AVT), and their receptor V1a in mediating variation in territorial behaviour. First, we discuss how aggression and territoriality are a function of population density in an inverted-U relationship according to resource defence theory, and how territoriality influences some mating systems. Next, we find that neural AVP, AVT, and V1a expression, especially in one particular neural circuit involving the lateral septum of the forebrain, are associated with territorial behaviour in males of diverse species, most likely due to their role in enhancing social cognition. Then we review studies that examined multiple species and find that neural AVP, AVT, and V1a expression is associated with territory size in mammals and fishes. Because territoriality plays an important role in shaping mating systems in many species, we present the idea that neural AVP, AVT, and V1a expression that is selected to mediate territory size may also influence the evolution of different mating systems. Future research that interprets proximate-level neuro-molecular mechanisms in the context of ultimate-level ecological theory may provide deep insight into the brain-behaviour relationships that underlie the diversity of social organization and mating systems seen across the animal kingdom.

Neurohypophysial Hormones Regulate Amphibious Behaviour in the Mudskipper Goby

The neurohypophysial hormones, arginine vasotocin and isotocin, regulate both hydromineral balance and social behaviors in fish. In the amphibious mudskipper, Periophthalmus modestus, we previously found arginine-vasotocin-specific regulation of aggressive behavior, including migration of the submissive subordinate into water. This migration also implies the need for adaptation to dehydration. Here, we examined the effects of arginine vasotocin and isotocin administration on the amphibious behavior of individual mudskippers in vivo. The mudskippers remained in the water for an increased period of time after 1-8 h of intracerebroventricular (ICV) injection with 500 pg/g arginine vasotocin or isotocin. The 'frequency of migration' was decreased after ICV injection of arginine vasotocin or isotocin, reflecting a tendency to remain in the water. ICV injections of isotocin receptor antagonist with arginine vasotocin or isotocin inhibited all of these hormonal effects. In animals kept out of water, mRNA expression of brain arginine vasotocin and isotocin precursors increased 3- and 1.5-fold, respectively. Given the relatively wide distribution of arginine vasotocin fibres throughout the mudskipper brain, induction of arginine vasotocin and isotocin under terrestrial conditions may be involved also in the preference for an aquatic habitat as ligands for brain isotocin receptors.

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.

Vasotocin and isotocin regulate aquaporin 1 function in the sea bream

Aquaporins (AQPs) are specific transmembrane water channels with an important function in water homeostasis. In terrestrial vertebrates, AQP2 function is regulated by vasopressin (AVP) to accomplish key functions in osmoregulation. The endocrine control of aquaporin function in teleosts remains little studied. Therefore, in this study we investigated the regulatory role of vasotocin (AVTR) and isotocin (ITR) receptors in Aqp1 paralog gene function in the teleost gilthead sea bream (Sparus aurata). The complete coding regions of Aqp1a, Aqp1b, AVTR V1a2-type, AVTR V2-type and ITR from sea bream were isolated. A Xenopus oocyte-swelling assay was used to functionally characterize AQP1 function and regulation by AVT and IT through their cognate receptors. Microinjection of oocytes with Aqp1b mRNA revealed regulation of water transport via PKA (IBMX+forskolin sensitive), whereas Aqp1a mRNA injection had the same effect via PKC signaling (PDBU sensitive). In the absence of expressed receptors, AVT and IT (10−8 mol l−1) were unable to modify AQP1 function. AVT regulated AQP1a and AQP1b function only when the AVTR V2-type was co-expressed. IT regulated AQP1a function, but not AQP1b, only when ITR was present. Considering that Aqp1a and Aqp1b gene expression in the sea bream intestine is highly salinity dependent in vivo, our results in ovo demonstrate a regulatory role for AVT and IT in AQP1 function in the sea bream in the processing of intestinal fluid to achieve osmoregulation.

An essential role of the arginine vasotocin system in mate-guarding behaviors in triadic relationships of medaka fish (Oryzias latipes)

To increase individual male fitness, males of various species remain near a (potential) mating partner and repel their rivals (mate-guarding). Mate-guarding is assumed to be mediated by two different types of motivation: sexual motivation toward the opposite sex and competitive motivation toward the same sex. The genetic/molecular mechanisms underlying how mate presence affects male competitive motivation in a triadic relationship has remained largely unknown. Here we showed that male medaka fish prominently exhibit mate-guarding behavior. The presence of a female robustly triggers male-male competition for the female in a triadic relationship (2 males and 1 female). The male-male competition resulted in one male occupying a dominant position near the female while interfering with the other male's approach of the female. Paternity testing revealed that the dominant male had a significantly higher mating success rate than the other male in a triadic relationship. We next generated medaka mutants of arginine-vasotocin (avt) and its receptors (V1a1, V1a2) and revealed that two genes, avt and V1a2, are required for normal mate-guarding behavior. In addition, behavioral analysis of courtship behaviors in a dyadic relationship and aggressive behaviors within a male group revealed that avt mutant males displayed decreased sexual motivation but showed normal aggression. In contrast, heterozygote V1a2 mutant males displayed decreased aggression, but normal mate-guarding and courtship behavior. Thus, impaired mate-guarding in avt and V1a2 homozygote mutants may be due to the loss of sexual motivation toward the opposite sex, and not to the loss of competitive motivation toward rival males. The different behavioral phenotypes between avt, V1a2 heterozygote, and V1a2 homozygote mutants suggest that there are redundant systems to activate V1a2 and that endogenous ligands activating the receptor may differ according to the social context.

Arginine vasotocin regulates social ascent in the African cichlid fish Astatotilapia burtoni

Neuropeptides modulate many aspects of behavior and physiology in a broad range of animals. Arginine vasotocin (AVT) is implicated in mediating social behavior in teleost fish, although its specific role varies between species, sexes, life stages, and social context. To investigate whether the effects of AVT on behavior depend on social context, we used the African cichlid fish Astatotilapia burtoni, which is well-known for its remarkable behavioral plasticity. We pharmacologically manipulated the AVT system in established socially dominant and subordinate A. burtoni males, as well as in males ascending to dominance status in a socially unstable environment. Our results show that exogenous AVT causes a stress response, as evidenced by reduced behavioral activity and increased circulating levels of cortisol in established dominant and subordinate males. Administration of the AVT antagonist Manning compound, on the other hand, did not affect established subordinate or dominant males. However, AVT antagonist-treated males ascending from subordinate to dominant status exhibited reduced aggressive and increased courtship behavior compared to vehicle-treated animals. Finally, we measured circulating cortisol levels and brain gene expression levels of AVT and its behaviorally relevant V1a2 receptor in all three social phenotypes and found that plasma cortisol and mRNA levels of both genes were increased in ascending males compared to dominant and subordinate males. Our results provide a more detailed understanding of the role of the AVT system in the regulation of complex behavior in a dynamically changing social environment.

Arginine vasotocin, isotocin and nonapeptide receptor gene expression link to social status and aggression in sex-dependent patterns

Nonapeptide hormones of the vasopressin/oxytocin family regulate social behaviours. In mammals and birds, variation in behaviour also is linked to expression patterns of the V1a-type receptor and the oxytocin/mesotocin receptor in the brain. Genome duplications, however, expand the diversity of nonapeptide receptors in actinopterygian fishes, and two distinct V1a-type receptors (v1a1 and v1a2) for vasotocin, as well as at least two V2-type receptors (v2a and v2b), have been identified in these taxa. The present study investigates how aggression connected to social status relates to the abundance patterns of gene transcripts encoding four vasotocin receptors, an isotocin receptor (itr), pro-vasotocin (proVT) and pro-isotocin (proIT) in the brain of the pupfish Cyprinodon nevadensis amargosae. Sexually-mature pupfish were maintained in mixed-sex social groups and assessed for individual variation in aggressive behaviours. Males in these groups behaved more aggressively than females, and larger fish exhibited higher aggression relative to smaller fish of the same sex. Hypothalamic proVT transcript abundance was elevated in dominant males compared to subordinate males, and correlated positively with individual variation in aggression in both social classes. Transcripts encoding vasotocin receptor v1a1 were at higher levels in the telencephalon and hypothalamus of socially subordinate males than dominant males. Dominant males exhibited elevated hypothalamic v1a2 receptor transcript abundance relative to subordinate males and females, and telencephalic v1a2 mRNA abundance in dominant males was also associated positively with individual aggressiveness. Transcripts in the telencephalon encoding itr were elevated in females relative to males, and both telencephalic proIT and hypothalamic itr transcript abundance varied with female social status. Taken together, these data link hypothalamic proVT expression to aggression and implicate forebrain expression of the V1a-type receptor v1a2 as potentially mediating the effects of vasotocin on behaviour in male fish. These findings also illustrate how associations between social status, aggression and gene expression within the VT and IT nonapeptide systems can be contingent on behavioural context.

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.

Cortisol modulates vasotocinergic and isotocinergic pathways in the gilthead sea bream

In the present study, we assessed the responses of the vasotocinergic and isotocinergic systems to chronic stress induced by cortisol administration in the gilthead sea bream (Sparus aurata). Pituitary and plasma arginine vasotocin (AVT) and isotocin (IT) levels, as well as hypothalamic pro-vasotocin (pro-VT) and pro-isotocin (pro-IT) mRNA expression levels, were analysed. In addition, the mRNA levels of three receptors, AVTR type V1a2, AVTR type V2 and ITR, were analysed in several target organs associated with the following physiological processes: (i) integration and control (hypothalamus), (ii) metabolism and its control (liver and hypothalamus), (iii) osmoregulation (gills) and (iv) stress response (head kidney). Specimens were injected intraperitoneally with slow-release implants (5 μL g(-1) body mass) containing coconut oil alone (control group) or with cortisol (50 μg g(-1) body mass; cortisol group). Both AVT and IT synthesis and release were correlated with plasma cortisol values, suggesting a potential interaction between both hormonal systems and cortisol administration. Our results suggest that the activation of hepatic metabolism as well as the hypothalamic control of metabolic processes provide the energy necessary to overcome stress, which could be partly mediated by AVTRs and ITR. Upregulation of branchial AVT and IT receptor expression following cortisol treatment suggests an involvement of the vasotocinergic and isotocinergic systems in the regulation of ion channels/transporters during stressful situations. Finally, changes in AVT and IT receptor mRNA expression in the head kidney suggest these nonapeptides participate in feedback mechanisms that regulate the synthesis/release of cortisol. Our results indicate a relationship between cortisol and both the vasotocinergic and isotocinergic systems during simulated chronic stress in S. aurata.

The endocrine regulation of cichlids social and reproductive behavior through the eyes of the chanchita, Cichlasoma dimerus (Percomorpha; Cichlidae)

Sociobiology, the study of social behavior, calls for a laboratory model with specific requirements. Among the most obvious is the execution of social interactions that need to be readily observable, quantifiable and analyzable. If, in turn, one focuses on the neuroendocrinological basis of social behavior, restrictions grow even tighter. A good laboratory model should then allow easy access to its neurological and endocrine components and processes. During the last years, we have been studying the physiological foundation of social behavior on what we believe fits all the aforementioned requirements: the so called "chanchita", Cichlasoma dimerus. This Neotropical cichlid fish exhibits biparental care of the eggs and larvae and presents a hierarchical social system, established and sustained through agonistic interactions. The aim of the current article is to review new evidence on chanchita's social and reproductive behavior.