Diurnal changes in the expression of genes encoding for arginine vasotocin and pituitary pro-opiomelanocortin in the rainbow trout (Oncorhynchus mykiss): correlation with changes in plasma hormones

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.

Diel variation in cortisol, glucose, lactic acid and antioxidant system of black sea bass Centropristis striata under natural photoperiod

Diel rhythm in activity of antioxidant enzymes, as well as contents of glutathione and lipid peroxides, has been intensively investigated in Mammalia and Aves, however, the relevant studies about fish are few. In the present study, we examined variation in contents of cortisol, glucose and lactic acid in plasma of black sea bass Centropristis striata under natural photoperiod during a 24-h period. In addition, variation in activity of antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-PX), catalase (CAT) and glutathione reductase (GR) as well as contents of total glutathione (T-GSH), reduced glutathione (GSH), oxidized glutathione (GSSG) and malondialdehyde (MDA) in liver and plasma of the fish were also determined. The plasma and liver samples were collected from the test fish at 3 h intervals during a 24-h cycle, with the first sampling time set at 03:00 h. No significant differences were found in glucose content and activities of GSH-PX and GR in plasma, as well as activities of SOD and GR in liver among different sampling times. In contrast, apparent variation was observed in contents of cortisol, lactic acid and MDA in plasma, activities of SOD and CAT in plasma, contents of MDA, T-GSH, GSH and GSSG in liver and activities of GSH-PX and CAT in liver between different sampling times. Moreover, contents of cortisol and MDA in plasma, SOD activity in plasma, and contents of MDA, GSH and GSSG in liver exhibited circadian rhythm, and their acrophases occurred at 06:08 h, 18:38 h, 15:09 h, 09:57 h, 23:36 h and 07:30 h, respectively. The present study indicates that some physiological parameters relating to stress response, such as cortisol and MDA contents in plasma, MDA, GSH and GSSG contents in liver and SOD activity in plasma changed at different time throughout a day in black sea bass. Therefore, caution should be taken when evaluating stress response in fish with these physiological parameters measured at different times.

Environmental Cycles, Melatonin, and Circadian Control of Stress Response in Fish

Fish have evolved a biological clock to cope with environmental cycles, so they display circadian rhythms in most physiological functions including stress response. Photoperiodic information is transduced by the pineal organ into a rhythmic secretion of melatonin, which is released into the blood circulation with high concentrations at night and low during the day. The melatonin rhythmic profile is under the control of circadian clocks in most fish (except salmonids), and it is considered as an important output of the circadian system, thus modulating most daily behavioral and physiological rhythms. Lighting conditions (intensity and spectrum) change in the underwater environment and affect fish embryo and larvae development: constant light/darkness or red lights can lead to increased malformations and mortality, whereas blue light usually results in best hatching rates and growth performance in marine fish. Many factors display daily rhythms along the hypothalamus-pituitary-interrenal (HPI) axis that controls stress response in fish, including corticotropin-releasing hormone (Crh) and its binding protein (Crhbp), proopiomelanocortin A and B (Pomca and Pomcb), and plasma cortisol, glucose, and lactate. Many of these circadian rhythms are under the control of endogenous molecular clocks, which consist of self-sustained transcriptional-translational feedback loops involving the cyclic expression of circadian clock genes (clock, bmal, per, and cry) which persists under constant light or darkness. Exposing fish to a stressor can result in altered rhythms of most stress indicators, such as cortisol, glucose, and lactate among others, as well as daily rhythms of most behavioral and physiological functions. In addition, crh and pomca expression profiles can be affected by other factors such as light spectrum, which strongly influence the expression profile of growth-related (igf1a, igf2a) genes. Additionally, the daily cycle of water temperature (warmer at day and cooler at night) is another factor that has to be considered. The response to any acute stressor is not only species dependent, but also depends on the time of the day when the stress occurs: nocturnal species show higher responses when stressed during day time, whereas diurnal fish respond stronger at night. Melatonin administration in fish has sedative effects with a reduction in locomotor activity and cortisol levels, as well as reduced liver glycogen and dopaminergic and serotonergic activities within the hypothalamus. In this paper, we are reviewing the role of environmental cycles and biological clocks on the entrainment of daily rhythms in the HPI axis and stress responses in fish.

Effects of temperature and melatonin on day-night expression patterns of arginine vasotocin and isotocin mRNA in the diencephalon of a temperate wrasse Halichoeres tenuispinis

Most wrasses are protogynous species that swim to feed, reproduce during the daytime, and bury themselves under the sandy bottom at night. In temperate and subtropical wrasses, low temperature influences emergence from the sandy bottom in the morning, and induces a hibernation-like state in winter. We cloned and characterized the prohormone complementary DNAs (cDNAs) of arginine vasotocin (AVT) and isotocin (IT) in a temperate wrasse (Halichoeres tenuispinis) and examined the effects of day/night and temperature on their expression in the diencephalon, because these neurohypophysial peptides are related to the sex behavior of wrasses. The full-length cDNAs of pro-AVT and pro-IT were 938 base pairs (154 amino acids) and 759 base pairs (156 amino acids) in length, respectively. Both pro-peptides contained a signal sequence followed by the respective hormones and neurophysin connected by a Gly-Lys-Arg bridge. Reverse-transcription polymerase chain reaction (RT-PCR) revealed that pro-AVT mRNA expression was specifically observed in the diencephalon, whereas pro-IT mRNA expression was seen in the whole brain. Quantitative RT-PCR revealed that the mRNA abundance of pro-AVT and pro-IT was higher at midday (zeitgeber time 6; ZT6) than at midnight (ZT18) under 12 h light and 12 h darkness (LD 12:12) conditions, but not under constant light. Intraperitoneal injection of melatonin decreased the mRNA abundance of pro-AVT, but not of pro-IT. When fish were reared under LD 12:12 conditions at 25, 20, and 15 °C, day high and night low mRNA expressions of pro-AVT and pro-IT were maintained. A field survey revealed seasonal variation in the number of swimming fish at observatory sites; many fish emerged from the sandy bottom in summer, but not in winter, suggesting a hibernation-like state under the sandy bottom under low temperature conditions. We conclude that the day-night fluctuation of pro-AVT and pro-IT mRNA abundance in the brain is not affected by temperature and repeated under the sandy bottom in winter.

Interplay between the endocrine and circadian systems in fishes

The circadian system is responsible for the temporal organisation of physiological functions which, in part, involves daily cycles of hormonal activity. In this review, we analyse the interplay between the circadian and endocrine systems in fishes. We first describe the current model of fish circadian system organisation and the basis of the molecular clockwork that enables different tissues to act as internal pacemakers. This system consists of a net of central and peripherally located oscillators and can be synchronised by the light-darkness and feeding-fasting cycles. We then focus on two central neuroendocrine transducers (melatonin and orexin) and three peripheral hormones (leptin, ghrelin and cortisol), which are involved in the synchronisation of the circadian system in mammals and/or energy status signalling. We review the role of each of these as overt rhythms (i.e. outputs of the circadian system) and, for the first time, as key internal temporal messengers that act as inputs for other endogenous oscillators. Based on acute changes in clock gene expression, we describe the currently accepted model of endogenous oscillator entrainment by the light-darkness cycle and propose a new model for non-photic (endocrine) entrainment, highlighting the importance of the bidirectional cross-talking between the endocrine and circadian systems in fishes. The flexibility of the fish circadian system combined with the absence of a master clock makes these vertebrates a very attractive model for studying communication among oscillators to drive functionally coordinated outputs.

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.

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.

Changes in vasotocin levels in relation to ovarian development in the catfish Heteropneustes fossilis exposed to altered photoperiod and temperature

Photoperiod and temperature are the major proximate factors that activate the brain-pituitary-gonadal-endocrine axis stimulating gonadal recrudescence. Vasotocin (VT), the basic nonapeptide hormone, is secreted by the nucleus preopticus in the hypothalamus and released from the pituitary into circulation as a neurohormone for physiological actions. Additionally, VT is secreted de novo in the ovary of the catfish and has been implicated in ovarian functions. In the present study, we evaluated the changes in VT secretion during altered photoperiod and temperature exposure. The ovarian changes were monitored over gonadosomatic index (GSI) and plasma steroid hormone levels. Exposure of the catfish to long photoperiod (LP, 16L:08D) daily, alone or in combination with high temperature (HT, 28 ± 2 °C), for 14 or 28 days resulted in a decrease in brain-pituitary VT level with a concomitant increase in plasma and ovarian VT levels. The changes were greater in the LP + HT group on day 28. Concurrently, the treatments stimulated the GSI and plasma estradiol-17β (E2), testosterone (T) and progesterone (P4) levels with higher more responses in the LP + HT group. Exposure of the catfish to short photoperiod (SP, 08L:16D) daily or total darkness (TD, 24L:00D) daily, with or without changing the ambient temperature, for 14 or 28 days produced a depressing effect on VT, GSI and steroid hormone levels, the range of the response varied with the temperature. The brain VT level was low except in the TD + NT group. Plasma and ovarian VT levels decreased more in the SP and TD groups under ambient temperature than in the groups at the raised temperature. The GSI and plasma steroid hormones (E2, T and P4) responded in a similar manner. Plasma cortisol level registered a significant increase in all the groups compared to the initial control groups, and the increase was significantly higher on day 28. The simultaneous activation of VT secretion and ovarian recrudescence by photoperiod and temperature suggests the peptide's involvement in the hormonal control of gametogenesis.

Neurohypophyseal hormones manipulation modulate social and anxiety-related behavior in zebrafish

RATIONALE

Oxytocin (OT) and arginine-vasopressin (AVP) regulate social behavior in mammals. Zebrafish (Danio rerio) allows higher throughput and ease in studying human brain disorders.

OBJECTIVES

This study investigated in zebrafish the effect of non-mammalian homologs isotocin (IT) and vasotocin (AVT) in comparison with OT/AVP on social behavior and fear response to predator. The mechanism was studied using the most human selective OT and AVP receptor antagonists.

METHODS

Zebrafish were injected i.m. with increasing doses (0.001-40 ng/kg) of the neuropeptides. DesGly-NH(2)-d(CH(2))(5)-[D-Tyr(2),Thr(4)]OVT) for OT receptor, SR 49059 for V1a subtype receptor, and SSR-149415 for V1b subtype receptor were injected i.m. 10 min before each agonist.

RESULTS

All the peptides increased social preference and reduced fear to predator response in a dose-dependent manner interpolated by symmetrical parabolas. AVT/AVP were more potent to elicit anxiolytic than social effect while IT and OT were equally potent. All the antagonists dose-dependently inhibited both the effects induced by the neuropeptides. The ratio between the ED50 obtained for blocking the OT-induced effects on social preference and fear response to predator was very high only for desglyDTTyrOVT (160). SR49059 showed the highest ratio in blocking AVP-induced effects (807). The less selective antagonist appeared to be SSR149415.

CONCLUSIONS

For the first time, IT/AVT and OT/AVP were found to modulate in zebrafish, social behavior, unrelated to sex, and fear to predator response through at least two different receptors. Zebrafish is confirmed as a valid, reliable model to study deficit in social behavior characteristic of some psychiatric disorders.

Diurnal rhythms in hypothalamic/pituitary AVT synthesis and secretion in rainbow trout: evidence for a circadian regulation

Arginine vasotocin (AVT) and isotocin (IT) are two neurohypophysial peptide hormones for which a role in adaptation to environmental changes has been suggested in fish. In teleosts, there are only a few available studies about circadian changes of AVT and IT levels, and a role of those peptides in the circadian system has been mainly suggested on the basis of the role of the homologous hormone AVP in mammals. Herein, we evaluated the diurnal rhythms in plasma AVT, pituitary AVT and IT content and the hypothalamic pro-vasotocin (pro-VT) expression in rainbow trout kept under a natural photoperiod, as well as their persistence in constant darkness as a tool for defining circadian dependence. Trout kept under a natural light cycle showed clear diurnal rhythms in both circulating and pituitary AVT levels with peak values around the last hours of the light phase. Hypothalamic pro-VT mRNA was also rhythmically expressed with similar peak characteristics. These rhythms persisted in fish kept under constant darkness for nearly two consecutive days, although peaks were progressively attenuated and phase-advanced. An IT rhythm was also found in pituitary of the trout maintained under a natural photoperiod, but not in those kept under continuous darkness. These results suggest that rhythms of hypothalamic AVT synthesis might be regulated by endogenous circadian mechanisms, and these rhythms contribute to maintain a similar fluctuation in pituitary AVT secretion into the blood. A potential role for AVT in the circadian and seasonal time-keeping system of teleost fish, either as a component of the neural machinery that participates in the adaptation to cyclic environmental changes, or as a circadian/seasonal output signal, is also discussed.

Diversity of the hypothalamo-neurohypophysial system and its hormonal genes

The hypothalamic neurosecretory cells (NSCs) which produce and release neurohypophysial hormones are involved in controls of diverse physiological phenomena including homeostatic controls of unconscious functions and reproduction. The far and wide distribution of neurosecretory processes in the discrete brain loci and the neurohypophysis is appropriate for coordination of neural and endocrine events that are required for the functions of NSCs. The presence of dye couplings and intimate contacts among NSCs supports harmonious production and release of hormone to maintain the plasma level within a certain range which is adequate for a particular physiological condition. Neurosecretory cells integrate diverse input signals from internal and external sources that define this particular physiological condition, although reactions of NSCs vary among different species, and among different cell types. An input signal to NSC is received by specific receptors and transduced as unique intracellular signals, important for the various functions of neurohypophysial hormones. Orchestration of multiple intracellular signaling systems, activities of which are individually modulated by input signals, determines the rates of synthesis and release of hormone through regulation of gene expression. The first step of gene expression, i.e., transcription, is amenable for diverse reaction of NSCs, because the 5' upstream regions of genes encoding neurohypophysial hormones are highly variable.

Immunohistochemical localization of glucokinase in rainbow trout brain

In mammals, glucosensing neurons reside in brain areas known to play a critical regulatory role in energy homeostasis and neuroendocrine function. In fish, although no glucosensing neurons have been characterized to date, glucokinase (GCK; the main glucosensing marker in mammals) activity and expression were found in hypothalamus and hindbrain of rainbow trout where they related to food intake regulation and glucose homeostasis. However, there are no available studies in literature regarding GCK distribution in brain areas as well as the cell types expressing that protein. In the present study we hypothesize that, as occurs in mammals, GCK would be localized where glucosensing areas have been described. In this sense, we have found GCK immunoreactivity in several areas of trout hypothalamus, of which some of them are related to glucose metabolism, energy homeostasis and food intake, including the lateral hypothalamus, anterior tuberal nucleus, posterior tuberal nucleus and lateral tuberal nucleus. On the other hand, GCKimmunoreactivity was also observed in other areas where the glucosensor system is probably functional,such as the preoptic area and the oculomotor nucleus. Therefore, in this study using immunoreactive techniques, we have demonstrated in those specific areas of the rainbow trout brain previously described as glucosensor the presence of GCK in different cell types.

Characterization and distribution of an arginine vasotocin receptor in mouse

A cDNA, which has a high homology with teleost Platichthys flesus [Arg8] vasotocin (AVT) receptor (GenBank: AK033957), was found in mouse genome database. Analyses of the deduced amino acid sequence revealed that a cDNA has several features of AVT receptor. We tentatively named it as a mouse vasotocin receptor (MVTR). A two-electrodes voltage clamp technique was applied to characterize the MVTR expressed in Xenopus laevis oocytes. AVT induced Ca2+-dependent Cl- currents in Xenopus oocytes injected with MVTR cRNA. On the other hand, [Arg8] vasopressin, oxytocin and isotocin did not induce such currents. RT-PCR showed that MVTR mRNA was specifically expressed in the brain. In situ hybridization analysis demonstrated significant expression of MVTR mRNA in suprachiasmatic nucleus, arcuate nucleus and medial habenular nucleus of mouse brain. These results suggest that MVTR may mediate a variety of physiological functions in mouse.

Arginine vasotocin a key hormone in fish physiology and behaviour: a review with insights from mammalian models

The arginine vasotocin (AVT) neuroendocrine system clearly provides integrative regulation of many aspects of fish physiology and behaviour, including circadian and seasonal biology, responses to stress, metabolism, reproduction, cardiovascular function, and osmoregulation. These are all considered here providing an important context for the design of experiments and interpretation of results for investigations of specific aspects of AVT function. Salt and water balance is a consistent function from fish to mammals and is examined in more detail. Both AVT and AVP secretion is sensitive to hyperosmotic stimuli and associated cellular dehydration, while hypovolaemia would appear less important. AVT and AVP both mediate renal water conservation, though actions involve different receptors and precise targets in fish (V1) and mammals (V2). The actions of AVT to promote gill NaCl extrusion in fish are conserved in the AVP-induced natriuresis in mammalian kidney to support restoration of plasma osmolality. The AVT/AVP regulatory mechanisms involve both altered neurohypophysial peptide secretion and changes in target-tissue receptor expression/modulation of action. Both mechanisms importantly afford integration with the actions of other related hormone systems.

Arginine vasotocin (AVT) and isotocin (IT) in fish brain: Diurnal and seasonal variations

An HPLC assay with solid-phase extraction and fluorescence derivatization was developed for measurement of arginine vasotocin (AVT) and isotocin (IT) in the neural tissues of fish. The efficiency and usefulness of the method have been verified in experiments by examination of peptides concentrations in brains of three fish species. The day-night changes in neuropeptides levels have been studied in brains of adult sea bream (Sparus aurata) and juvenile Atlantic salmon (Salmo salar). Seasonal fluctuations have been investigated in brains of three-spined sticklebacks (Gasterosteus aculeatus). The AVT and IT biosynthesis in brain seems to be controlled independently and probably each neuropeptide plays a different role in a circadian time-keeping system and an endocrine calendar in fish.

Exogenous vasotocin alters aggression during agonistic exchanges in male Amargosa River pupfish (Cyprinodon nevadensis amargosae)

Pupfishes in the Death Valley region have rapidly differentiated in social behaviors since their isolation in a series of desert streams, springs, and marshes less than 20,000 years ago. These habitats can show dramatic fluctuations in ecological conditions, and pupfish must cope with the changes by plastic physiological and behavioral responses. Recently, we showed differences among some Death Valley populations in brain expression of arginine vasotocin (AVT). As AVT regulates both hydromineral balance and social behaviors in other taxa, these population differences may indicate adaptive changes in osmoregulatory and/or behavioral processes. To test whether AVT is relevant for behavioral shifts in these fish, here we examined how manipulations to the AVT system affect agonistic and reproductive behaviors in Amargosa River pupfish (Cyprinodon nevadensis amargosae). We administered exogenous AVT (0.1, 1, and 10 microg/g body weight) and an AVP V1 receptor antagonist (Manning compound, 2.5 microg/g body weight) intraperitoneally to males in mixed-sex groups in the laboratory. We found that AVT reduced the initiation of aggressive social interactions with other pupfish but had no effect on courtship. The effects of AVT were confirmed in males in the wild where AVT (1 microg/g body weight) reduced the aggressive initiation of social interactions and decreased aggressive responses to the behavior of other males. Combined, these results show that AVT can modulate agonistic behaviors in male pupfish and support the idea that variation in AVT activity may underlie differences in aggression among Death Valley populations.

Attenuation of diurnal rhythms in plasma levels of melatonin and cortisol, and hypothalamic contents of vasotocin and isotocin mRNAs in pre-spawning chum salmon

In the present study, diurnal changes in plasma levels of melatonin and cortisol, and hypothalamic contents of neurohypophysial hormone mRNAs were examined in pre-spawning chum salmon, Oncorhynchus keta. From late November to early December, homing fish were captured at two sites along their migratory pathway on the Sanriku coast, Japan. Fish captured in the seawater (SW) environment were transferred to SW aquaria, and fish captured in the freshwater (FW) environment were to FW aquaria. They were maintained under natural photoperiod of approximately 10L:14D and sacrificed at 4-h interval through 24-h period. Plasma levels of melatonin were determined by radioimmunoassay, while cortisol levels were determined by enzyme immunoassay. Hypothalamic contents of vasotocin and isotocin mRNAs were determined by quantitative dot-blot hybridization assay. The melatonin levels showed weak nocturnal elevations in the SW and FW males, and FW females. The levels were maximal at 22:00 and minimal at 10:00 or 14:00, however the amplitudes were smaller than those reported in the previous studies using immature salmonids. The levels of vasotocin and isotocin mRNAs were higher in the males at all time points. The mRNA levels, however, did not show any diurnal variations in either of group. The same applied to plasma cortisol levels. These results indicate that the diurnal endocrine rhythms were attenuated in pre-spawning chum salmon, in contrast to the prominent diurnal rhythms in immature salmonids.

Ontogeny of a novel decapeptide derived from POMC-A in the brain and pituitary of the rainbow trout

Trout POMC-A exhibits a unique C-terminal extension of 25-amino acids which is processed in the pituitary and hypothalamus to generate two novel decapeptides, EQWGREEGEE and ALGERKYHFQ-NH(2). The fibers containing these two decapeptides are widely distributed in the brain, suggesting that they may exert neurotransmitter or neuromodulator activities. In the present study, we have investigated the ontogeny of the decapeptide EQWGREEGEE in the trout pituitary and brain. In the pituitary of 29-day embryos and 33-day alevins, EQWGREEGEE-immunoreactive material was observed in a cluster of cells located in the central and rostral region of the gland, respectively. In 47-day alevins, a second group of cells exhibiting EQWGREEGEE-like immunoreactivity was detected in the caudal region of the pituitary and the intensity of labeling in these cells increased in 61-day fry. In the brain, EQWGREEGEE immunoreactivity was detected in 47-day alevins. In 47- and 61-day larvae, immunoreactive elements were mainly detected in the diencephalon. Characterization of the immunoreactive material by reversed-phase high-performance liquid chromatographic analysis combined with radioimmunoassay detection revealed the existence of two major forms which exhibited different retention times than synthetic EQWGREEGEE. The present study indicates that EQWGREEGEE-related peptides are present in the trout pituitary early during ontogeny and appear in the brain only later, and that processing of the C-terminal extension of POMC-A generates distinct molecular species at different developmental stages. These data suggest that alternative processing of the C-terminal domain of POMC-A gives rise to various peptide products that may exert specific activities during trout development.

Day-night variations in plasma melatonin and arginine vasotocin concentrations in chronically cannulated flounder (Platichthys flesus)

Chronically catheterised, free swimming flounder (Platichthys flesus) have been used in experiments examining the day-night variations in circulating levels of melatonin (Mel) and arginine vasotocin (AVT). Under normal photoperiod (16 h light/8 h dark) serial blood samples taken from individual fish demonstrated a Mel rhythm with daytime levels at 09.00 and 15.00 h (238+/-14 and 179+/-12 fmol x ml(-1), respectively) lower than those at 23.00 h (1920+/-128 fmol x ml(-1)). Maintenance of fish in 24-h light abolished the light/dark Mel rhythm and circulating levels were comparable to those measured during the day in fish under normal photoperiod illumination. In fish maintained under 24 h dark, although a daily rhythm was still apparent, at the time when it would be normally dark, plasma Mel concentration was reduced and at times when it would be normally light, levels were higher than in fish maintained under normal light/dark illumination. Plasma AVT concentrations were higher in fish during the day (4.4+/-0.8 fmol x ml(-1)) than those at night (1.5+/-0.4 fmol x ml(-1)), the opposite to that seen with Mel. During acute study infusion of AVT resulted in reduced levels of plasma Mel, although this did not achieve statistical significance. Infusion of Mel did not alter circulating AVT concentration.

Do circulating plasma AVT and/or cortisol levels control pulsatile urea excretion in the gulf toadfish (Opsanus beta)?

Previous work has shown that pulsatile urea excretion at the gills of the gulf toadfish is due to periodic activation of a facilitated diffusion transport system with molecular and pharmacological similarity to the UT-A transport system of the mammalian kidney. In mammals, AVP and glucocorticoids are two important endocrine regulators of this system. The present study focused on the potential role of circulating AVT (the teleost homologue of AVP) and cortisol levels as possible triggers for urea pulses. Long-term (34-84 h) monitoring of plasma levels by repetitive sampling at 2-h intervals from chronic cannulae in individual toadfish demonstrated that circulating AVT concentrations are low (10(-12)-10(-11) M), and show no relationship to the occurrence of natural urea pulses. In contrast, plasma cortisol levels decline greatly prior to natural pulses and rise rapidly thereafter. AVT injections into the caudal artery or ventral aorta elicited pulse events, but these were extremely small (1-10%) relative to natural pulses, and occurred only at unphysiological dose levels (10(-9) M in the plasma). AVP was a partial agonist, but isotocin, insulin-like growth factor-1, and atrial natriuretic peptide were without effect at the same concentration. Artificially raising plasma cortisol levels by cortisol injection tended to reduce responsiveness to AVT. Pharmacological reduction of plasma cortisol levels by metyrapone injection elicited small pulses similar to those caused by AVT. Following such pulse events, AVT was ineffective in inducing pulses. We conclude that decreases in circulating cortisol play an important permissive role in urea pulsing, but that circulating AVT levels are not involved.

Cloning and characterization of an arginine vasotocin receptor from the euryhaline flounder Platichthys flesus

A sequence coding for an arginine vasotocin (AVT) receptor has been identified by the screening of a hepatic cDNA library from the teleost Platichthys flesus. The 2701-bp receptor sequence is predicted to yield a 384-amino acid peptide, analysis of which indicates a seven-transmembrane spanning sequence typical of G-protein-coupled receptors with the N terminus on the outer surface of the cell membrane. Sequence analysis showed this sequence to have a high homology with the Catostomus commersoni AVT receptor (76%) and mammalian vasopressin V(1)-type receptor (62%), but only 55% homology with the C. commersoni isotocin receptor. A two-electrode voltage clamp was used to characterize the receptor expressed in Xenopus laevis oocytes. AVT induced an inward current which was dose dependent over the range 16.7 fmol to 5 pmol; isotocin was without effect over the same dose range. The mammalian vasopressin V(1)-type receptor agonist ([Phe(2), Orn(8)] oxytocin)() induced an inward current but was less potent than AVT, whereas the mammalian vasopressin V(2)-type receptor agonist ([Deamino(1), Val(4), D-Arg(8)] AVP) was without effect. Injection of oocytes with heparin or BAPTA suppressed the response to AVT, indicating receptor linkage to the phospholipase C-phosphatidylinositol pathway. Northern analysis demonstrated the presence of this AVT receptor mRNA in the brain, kidney, and gill of flounder.

Differences in arginine vasotocin gene transcripts and cortisol secretion in trout with high or low endogenous melanin-concentrating hormone secretion

Previous studies on trout suggest that melanin-concentrating hormone (MCH) acts at both hypothalamic and pituitary levels to restrain the release of adrenocorticotropic hormone and hence cortisol during stress. Using in situ hybridization, the present work examined whether high rates of MCH secretion were associated with changes in the synthesis of arginine vasotocin (AVT), one of the corticotropin secretogogues. It also examined whether high endogenous MCH secretion restrains cortisol secretion during intense as well as mild stress, and how exogenous MCH affects the rise in plasma cortisol following injection stress. Trout were reared in black- or white-coloured tanks for 1 year or more to achieve maximal differences in MCH secretion. Following a mild stress, cortisol secretion was greater in black-reared fish with low MCH secretion which is in line with previous findings but, following a more severe stress, plasma cortisol concentrations were similar in the two groups. Injection of MCH into black-adapted fish restrained the stress-induced rise in plasma cortisol concentration during the first hour but did not affect final cortisol values. In two separate experiments, AVT mRNA levels were significantly lower in the hypothalamus of black-reared fish. Possible explanations for this include a greater negative-feedback restraint by cortisol, which is likely to rise higher in black-adapted fish during the moderate, daily stresses of aquarium life; or the possibility that exposure to a white background may be psychologically stressful, stimulating AVT transcription. The possibility that MCH directly stimulates AVT transcription cannot be excluded but seems less likely. The results suggest that while MCH may restrain the release of hypothalamo-pituitary stress hormones under moderately stressful conditions, it does not restrain AVT synthesis.

The effects of acute and chronic stresses on vasotocin gene transcripts in the brain of the rainbow trout (Oncorhynchus mykiss)

Secretion of adrenocorticotropic hormone (ACTH) from the fish pituitary, which occurs in times of stress, is stimulated by several hypothalamic neuropeptides, one of which is arginine vasotocin (AVT). This study investigates whether gene expression for AVT is up-regulated during acute or chronic stress. Rainbow trout (Oncorhynchus mykiss) were subjected to one of two forms of acute stress-either 2 h confinement followed by 2 h recovery, or capture and transfer to low water for 2 min followed by 4 h recovery in their home tank before autopsy. In other experiments, these stresses were repeated daily for 5 or 6 days (chronic stress). Quantification of AVT transcript prevalence in the parvocellular and magnocellular neurones of the preoptic nucleus after in situ hybridization was used as a monitor of the AVT gene response to stress. The results showed that acute confinement, but apparently not brief low-water stress, significantly increased AVT transcript prevalence in a group of parvocellular perikarya. When applied repeatedly, both forms of stress caused habituation, such that the AVT hybridization signal remained at control or even lower levels despite elevated pro-opiomelanocortin transcripts in the corticotropes and raised plasma cortisol concentrations. The AVT hybridization signal in the magnocellular perikarya showed no significant response to either acute or chronic stress. The results support the idea that these parvocellular AVT neurones are involved in ACTH stimulation during acute stress, and that the system habituates to chronic stresses.