Binding characteristics and daily rhythms of melatonin receptors are distinct in the retina and the brain areas of the European sea bass retina (Dicentrarchus labrax)

Melatonin and osmoregulation in fish: A focus on Atlantic salmon Salmo salar smoltification

Part of the life cycle of several fish species includes important salinity changes, as is the case for the sea bass (Dicentrarchus labrax) or the Atlantic salmon (Salmo salar). Salmo salar juveniles migrate downstream from their spawning sites to reach seawater, where they grow and become sexually mature. The process of preparation enabling juveniles to migrate downstream and physiologically adapt to seawater is called smoltification. Daily and seasonal variations of photoperiod and temperature play a role in defining the timing of smoltification, which may take weeks to months, depending on the river length and latitude. Smoltification is characterised by a series of biochemical, physiological and behavioural changes within the neuroendocrine axis. This review discusses the current knowledge and gaps related to the neuroendocrine mechanisms that mediate the effects of light and temperature on smoltification. Studies performed in S. salar and other salmonids, as well as in other species undergoing important salinity changes, are reviewed, and a particular emphasis is given to the pineal hormone melatonin and its possible role in osmoregulation. The daily and annual variations of plasma melatonin levels reflect corresponding changes in external photoperiod and temperature, which suggests that the hormonal time-keeper melatonin might contribute to controlling smoltification. Here, we review studies on (i) the impact of pinealectomy and/or melatonin administration on smoltification; (ii) melatonin interactions with hormones involved in osmoregulation (e.g., prolactin, growth hormone and cortisol); (iii) the presence of melatonin receptors in tissues involved in osmoregulation; and (iv) the impacts of salinity changes on melatonin receptors and circulating melatonin levels. Altogether, these studies show evidence indicating that melatonin interacts with the neuroendocrine pathways controlling smoltification, although more information is needed to clearly decipher its mechanisms of action.

Seasonal simulated photoperiods influence melatonin release and immune markers of pike perch Sander lucioperca

Melatonin is considered as the time-keeping hormone acting on important physiological functions of teleosts. While the influence of melatonin on reproduction and development is well described, its potential role on immune functions has little been considered. In order to better define an immune modulation by the melatonin hormone, we hypothesized that natural variations of photoperiod and subsequent changes in melatonin release profile may act on immune status of pikeperch. Therefore, we investigated during 70 days the effects of two photoperiod regimes simulating the fall and spring in western Europe, on pikeperch physiological and immune responses. Samples were collected at 04:00 and 15:00 at days 1, 37 and 70. Growth, plasma melatonin levels, innate immune markers and expression of immune-relevant genes in head kidney tissue were assessed. While growth and stress level were not affected by the seasonal simulated photoperiods, nocturnal levels of plasma melatonin were photoperiod-dependent. Innate immune markers, including lysozyme, complement, peroxidase and phagocytic activities, were stimulated by the fall-simulated photoperiod and a significant correlation was made with plasma melatonin. In addition to bring the first evidence of changes in fish immunocompetence related to photoperiod, our results provide an additional indication supporting the immunomodulatory action of melatonin in teleosts.

Are cortisol and melatonin involved in the immune modulation by the light environment in pike perch Sander lucioperca?

The pineal gland is the main organ involved in the transduction process converting environmental light information into a melatonin response. Since light environment was described as an important factor that could affect physiology of teleosts, and because melatonin is a crucial hormone regulating numerous physiological processes, we hypothesized that environmental light may act on both stress and circadian axes which in turn could influence the immune status of pike perch. Therefore, we investigated the effects of two light spectra (red and white) and two light intensities (10 and 100 lx) with a constant photoperiod 12L_(8:00-20:00) /12D on pike perch physiological and immune responses. Samples were collected at 04:00 and 16:00 at days 1 and 30 of the experiment. Stress markers, plasma melatonin levels, humoral innate immune markers, and expression of key immune genes in the head kidney were assessed. Light intensity clearly affected pike perch physiology. This included negative growth performances, increase in stress status, decrease in plasma melatonin levels, and immune depression. Light spectrum had only little influences. These results demonstrate that high stress status may have impacted melatonin production and secretion by the pineal organ. The drop in circulating melatonin and the increase in stress status may both be involved in the immune suppression.

Melatonin modulates monochromatic light-induced melatonin receptor expression in the hypothalamus of chicks

To study the mechanism of the effect of monochromatic light on physiological function in chicken, a total of 192 newly hatched chicks were randomly divided into intact, sham-operated and pinealectomy groups then exposed to white light (WL), red light (RL), green light (GL) and blue light (BL) using a light-emitting diode (LED) system for two weeks. At P14, the hypothalami were immediately collected for immunohistochemical staining of melatonin receptor subtypes (Mel1a and Mel1b) and detection of Mel1a and Mel1b expressions using RT-PCR and western blot. Immunohistochemical staining of the hypothalamus showed that the Mel1a-ir cells were distributed in the preoptic area (POA), nucleus preopticus periventricularis (POP) and suprachiasmatic nuclei (SCN), and the Mel1b-ir cells were presented in the POA and SCN. Analysis of RT-PCR and western blot showed that the mRNA and protein levels of Mel1a and Mel1b in the hypothalamus of chick exposed to GL were increased by 10.7-29.3%, 9.18-35.9% and 8.97-27.3% compared to those in the chicks exposed to WL (P=0.029-0.002), RL (P=0.027-0.001) and BL (P=0.038-0.007) in the intact group, respectively. After pinealectomy, however, these parameters decreased and there were no significant differences among the WL, RL, GL and BL groups. These findings suggested that melatonin plays a critical role in GL illumination-enhanced Mel1a and Mel1b expressions in the hypothalamus of chicks.

Influence of melatonin on the immune system of fish: a review

Endocrine-immune system interactions have been widely demonstrated in mammals, whereas in fish, these relationships remain unclear. Of the organs that constitute the endocrine system, the pineal gland and its secretory product melatonin act in the synchronization of daily and seasonal rhythms in most vertebrates, including fish. Seasonal differences in immunocompetence and disease prevalence have been well documented in humans. Seasonality also strongly influences the life history of fish by controlling the timing of physiological events, such as reproduction, food intake, locomotor activity, and growth performance. Apart from its synchronizing capabilities, the role of melatonin in physiological processes in fish is not thoroughly understood. The purpose of this review is to summarize current studies on the effects of melatonin on the fish immune system. These studies suggest that melatonin represents an important component of fish endocrine-immune system interactions. The elucidation of the defense mechanisms of fish will facilitate the development of health management tools to support the growing finfish aquaculture industry as well as address questions concerning the origins and evolution of the immune system in vertebrates.

Melatonin inhibits GnRH-1, GnRH-3 and GnRH receptor expression in the brain of the European Sea Bass, Dicentrarchus labrax

Several evidences supported the existence of melatonin effects on reproductive system in fish. In order to investigate whether melatonin is involved in the modulation of GnRH systems in the European sea bass, we have injected melatonin (0.5 μg/g body mass) in male specimens. The brain mRNA transcript levels of the three GnRH forms and the five GnRH receptors present in this species were determined by real time quantitative PCR. Our findings revealed day–night variations in the brain expression of GnRH-1, GnRH-3 and several GnRH receptors (dlGnRHR-II-1c, -2a), which exhibited higher transcript levels at mid-light compared to mid-dark phase of the photocycle. Moreover, an inhibitory effect of melatonin on the nocturnal expression of GnRH-1, GnRH-3, and GnRH receptors subtypes 1c, 2a and 2b was also demonstrated. Interestingly, the inhibitory effect of melatonin affected the expression of hypophysiotrophic GnRH forms and GnRH receptors that exhibit day–night fluctuations, suggesting that exogenous melatonin reinforce physiological mechanisms already established. These interactions between melatoninergic and GnRH systems could be mediating photoperiod effects on reproductive and other rhythmic physiological events in the European sea bass.

Characterization of two different melatonin binding sites in peripheral tissues of the teleost Tinca tinca

The aim of the present study was to localize and characterize 2-iodo-melatonin ([(125)I]Mel) binding sites in peripheral tissues of the teleost Tinca tinca. A wide distribution of [(125)I]Mel binding sites in peripheral locations of the tench is found, with highest densities being measured in the heart, gills and kidney, and low density of [(125)I]Mel binding sites in gastrointestinal tract, spleen, liver and gonads. Saturation, kinetics, and pharmacological approaches revealed the presence of, at least, two different [(125)I]Mel binding sites in the tench peripheral tissues. The unique characterized subtype in the heart fulfils all the criteria for a canonical melatonin receptor belonging to MT(1) family (the binding is saturable, reversible, and inhibited by GTP analogs), and gives support for the presence of a functional melatonin receptor in the heart of the tench. In contrast, kinetic and pharmacological studies in the kidney revealed the preponderance of a melatonin binding site belonging to the MT(3)-like receptor subtype. Moreover, the decrease of specific binding in both, heart and kidney membranes, and the decrease of affinity in the kidney, produced by the addition of a non-hydrolysable GTP analog, and sodium cations suggest the presence of G(i/o)-proteins (that mediate inhibition of cAMP formation) coupled to such melatonin binding sites. Our results also point to different G(i/o)-proteins involved in the underlying mechanism of melatonin binding sites activation in the kidney. Additionally, the kinetics of [(125)I]Mel binding in kidney membrane preparations is a highly thermosensitive process, being necessary to perform the assays at 4 °C since the equilibrium was not reached at 25 °C assay temperature. The time needed to complete association of [(125)I]Mel at such low temperature is only 15s, whereas 100s is required to displace [(125)I]Mel specific binding by the unlabeled melatonin in kidney membranes. Present results support previous reports on melatonin effects in the regulation of different physiological functions in teleost (as cardiovascular physiology and osmoregulation) acting through peripheral specific receptors.

Variation in melatonin receptors (Mel(1a) and Mel(1b)) and androgen receptor (AR) expression in the spleen of a seasonally breeding bird, Perdicula asiatica

Daily variation in the peripheral level of melatonin plays a major role in integrating reproduction and environmental information for seasonally breeding birds. However, the variation in immunity and reproduction has never been assessed in any avian species on a 24 h time scale. Therefore, to understand the relationship between immune function and reproductive phases in a seasonally breeding bird, Perdicula asiatica, the Indian jungle bush quail, we studied the daily variation of melatonin and testosterone levels along with expression of their receptors Mel(1a), Mel(1b), and androgen receptor in the spleen during the reproductively active phase. Immunocytochemistry for the melatonin receptors Mel(1a) and Mel(1b) presented a differential distribution pattern. Western blot of splenic protein suggested a daily rhythm of melatonin receptors, while acrophases for the two melatonin receptors Mel(1a) and Mel(1b) differed by 4 h, suggesting that the expression of the receptors may peak at different times, causing more of either Mel(1a) or Mel(1b) to be available at a particular time to mediate function. The circulatory melatonin level correlated with percentage stimulation ratio of splenocytes and plasma interleukin-2 level, but did not correlate with testosterone or androgen receptor, suggesting that melatonin could be a major hormone imparting a time-of-day effect on the modulation of immune function in a seasonally breeding bird during the reproductively active phase.

Continuous light and melatonin: daily and seasonal variations of brain binding sites and plasma concentration during the first reproductive cycle of sea bass

The present study reports on the daily and seasonal variations in plasma melatonin concentration, and also in optic tectum and hypothalamus melatonin binding sites, in male European sea bass maintained under natural photoperiod (NP) or continuous light (LL) from early stages of development. Samples were collected on a 24-h cycle, at four physiological phases of their first annual reproductive cycle, i.e., pre-spermatogenesis, spermatogenesis, spermiation and post-spermiation. Under NP, (1) plasma melatonin levels were higher at night than during the day regardless of the year period, and the duration of the signal matched the duration of the dark phase; (2) daily variations in Kd and Bmax were found in the optic tectum, but only during spermiation, with the acrophase being 180° out of phase with the plasma melatonin variations; and (3) significant seasonal Kd and Bmax changes were seen in the hypothalamus. Under LL, (1) plasma melatonin showed no elevation during the subjective night; and (2) Kd and Bmax exhibited seasonal variations in the hypothalamus. These results led to the conclusion that long-term exposure to LL affected both plasma melatonin and receptor oscillations; particularly, LL disrupted the receptor density circadian oscillation found in the optic tectum during spermiation under NP. This oscillation appears to be important for sea bass to pursue gametogenesis until full spermiation. The persistence of both daily and seasonal variation of receptor affinity and density in the hypothalamus under LL indicates that these variations are controlled by internal circadian and circannual clocks that do not involve melatonin.

Melatonin receptors in a pleuronectiform species, Solea senegalensis: Cloning, tissue expression, day-night and seasonal variations

Melatonin receptors are expressed in neural and peripheral tissues and mediate melatonin actions on the synchronization of circadian and circannual rhythms. In this study we have cloned three melatonin receptor subtypes (MT1, MT2 and Mel1c) in the Senegalese sole and analyzed their central and peripheral tissue distribution. The full-length MT1 (1452 nt), MT2 (1728 nt) and Mel1c (1980 nt) cDNAs encode different proteins of 345, 373, 355 amino acids, respectively. They were mainly expressed in retina, brain and pituitary, but MT1 was also expressed in gill, liver, intestine, kidney, spleen, heart and skin. At peripheral level, MT2 expression was only evident in gill, kidney and skin whereas Mel1c expression was restricted to the muscle and skin. This pattern of expression was not markedly different between sexes or among the times of day analyzed. The real-time quantitative PCR analyses showed that MT1 displayed higher expression at night than during the day in the retina and optic tectum. Seasonal MT1 expression was characterized by higher mRNA levels in spring and autumn equinoxes for the retina, and in winter and summer solstices for the optic tectum. An almost similar expression profile was found for MT2, but differences were less conspicuous. No day-night differences in MT1 and MT2 expression were observed in the pituitary but a seasonal variation was detected, being mRNA levels higher in summer for both receptors. Mel1c expression did not exhibit significant day-night variation in retina and optic tectum but showed seasonal variations, with higher transcript levels in summer (optic tectum) and autumn (retina). Our results suggest that day-night and seasonal variations in melatonin receptor expression could also be mediating circadian and circannual rhythms in sole.

Melatonin attenuates the acetylcholine-induced contraction in isolated intestine of a teleost fish

The present study investigates the possible direct actions of melatonin (N-acetyl-5-methoxytryptamine) on intestinal motility in goldfish (Carassius auratus) using an in vitro system of isolated intestine in an organ bath engaged to an isometric transducer. The longitudinal strips from goldfish intestine in the organ bath showed a resting spontaneous myogenic rhythmic activity which is not altered by melatonin. The addition of acetylcholine (1 nmol l(-1)-10 mmol l(-1)) to the organ bath induces a significant contraction of the intestinal strips in a concentration-dependent manner. The addition of melatonin and its agonist, 2-iodomelatonin, induced a concentration-dependent attenuation of acetylcholine-induced contractile response. The specificity of this effect is tested by the preincubation of the intestine strips in the presence of two melatoninergic antagonists, luzindole (a non-selective MT(1)/MT(2) melatonin receptor antagonist) and 4-P-PDOT (preferred antagonist of MT2 receptor subtype), which counteracted the melatonin-induced relaxation in a concentration-dependent manner. Finally, present results demonstrate that this melatoninergic effect on intestinal strips is a process highly dependent on extracellular calcium. In conclusion, this is the first study demonstrating the role of melatonin in the control of gut motility in a non-mammalian vertebrate. The melatonin effects on isolated intestine from goldfish are mediated by melatoninergic membrane receptors, and could suggest a delay in food transit time, supporting its anorectic effect reported on in vivo studies.

Localization and dynamics of Mel(1a) melatonin receptor in the ovary of carp Catla catla in relation to serum melatonin levels

We studied the localization, sub-cellular distribution and daily rhythms of a 37 kDa melatonin receptor (Mel(1a)R) in the ovary to assess its temporal relationship with the serum melatonin levels in four different reproductive phases in carp Catla catla. Our immunocytochemical study accompanied by Western blot analysis of Mel(1a)R in the ovary revealed that the expression of this 37-kDa protein was greater in the membrane fraction than in the cytosol. Ovarian Mel(1a)R protein peaked at midnight and fell at midday in each reproductive phase. Conversely, serum melatonin levels in the same fish demonstrated a minimum diurnal value at midday in all seasons, but a peak at midnight (during pre-spawning, spawning, and post-spawning phases) or at late dark phase (during preparatory phase). In an annual cycle, band intensity of Mel(1a)R protein showed a maximum at night in the spawning phase and a minimum in the post-spawning phase, demonstrating an inverse relationship with the levels of serum melatonin. Our data provide first evidence of the presence of Mel(1a) melatonin receptor in carp ovary and offer interesting perspectives especially for the study of the mechanisms of the control of its rhythmicity and its response to external factors.

Melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca: characterization and effect of temperature

The aim of the present study was to characterize the central melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca. We investigated the temperature-dependence of 2-iodo-melatonin ([(125)I]Mel) binding in the optic tectum-tegmentum area and the neural retina. The binding of [(125)I]Mel showed a widespread distribution in brain and ocular tissues, with the highest density in the optic tectum-thalamus and the lowest in hindbrain. The [(125)I]Mel affinity was similar in all the studied tissues, and it was on the order of the low pM range. Saturation, kinetic and pharmacological studies showed the presence of a unique MT(1)-like melatonin binding site. In addition, the non-hydrolysable GTP analog, the GTPgammaS, and sodium cations induced a specific binding decrease in the optic tectum and neural retina, suggesting that such melatonin binding sites in the tench are coupled to G protein. Thus, these melatonin binding sites in optic tectum and neural retina fulfil the requirements of a real hormone receptor, the specific binding is rapid, saturable, and reversible, and is inhibited by GTP analogs. Additionally, a clear effect of temperature on such central melatonin receptors was found. Temperature did not modify the B(max) and K(d), but the kinetics of [(125)I]Mel binding resulted in a highly thermosensitive process in both tissues. Both association and dissociation rates (K(+1) and K(-1)) significantly increased with assay temperature (15-30 degrees C), but the K(d) constant (estimated as K(-1)/K(+1)) remained unaltered. In conclusion, this high thermal dependence of the melatonin binding to its receptors in the tench central nervous system supports the conclusion that temperature plays a key role in melatonin signal transduction in fish.

Expression of the melatonin receptor Mel1c in neural tissues of the reef fish Siganus guttatus

The golden rabbitfish, Siganus guttatus, is a reef fish exhibiting a restricted lunar-related rhythm in behavior and reproduction. Here, to understand the circadian rhythm of this lunar-synchronized spawner, a melatonin receptor subtype-Mel(1c)-was cloned. The full-length Mel(1c) melatonin receptor cDNA comprised 1747 bp with a single open reading frame (1062 bp) that encodes a 353-amino acid protein, which included 7 presumed transmembrane domains. Real-time PCR revealed high Mel(1c) mRNA expression in the retina and brain but not in the peripheral tissues. When the fish were reared under light/dark (LD 12:12) conditions, Mel(1c) mRNA in the retina and brain was expressed with daily variations and increased during nighttime. Similar variations were noted under constant conditions, suggesting that Mel(1c) mRNA expression is regulated by the circadian clock system. Daily variations of Mel(1c) mRNA expression with a peak at zeitgeber time (ZT) 12 were observed in the cultured pineal gland under LD 12:12. Exposure of the cultured pineal gland to light at ZT17 resulted in a decrease in Mel(1c) mRNA expression. When light was obstructed at ZT5, the opposite effect was obtained. These results suggest that light exerts certain effects on Mel(1c) mRNA expression directly or indirectly through melatonin actions.

Diurnal and circadian regulation of a melatonin receptor, MT1, in the golden rabbitfish, Siganus guttatus

The golden rabbitfish Siganus guttatus is a reef fish with a restricted lunar-synchronized spawning rhythmicity and releases gametes simultaneously around the first quarter moon period during the spawning season. In order to understand the molecular aspects of the "circa" rhythms in this species, the full-length melatonin receptor (MT1) cDNA was cloned, and its diurnal/circadian regulation was examined. The full-length MT1 cDNA (1257 bp) contained an open reading frame that encodes a protein of 350 amino acids; this protein is highly homologous to MT1 of nonmammalian species. A high expression of MT1 mRNA with a day-night difference was observed in the whole brain, retina, liver, and kidney. When diurnal variations in MT1 mRNA expression in the retina and whole brain were examined using real-time quantitative RT-PCR, an increase in the mRNA expression was observed during nighttime in both tissues under conditions of light/dark, constant darkness, and constant light. This suggests that MT1 mRNA expression is under circadian regulation. The expression of MT1 mRNA in the cultured pineal gland also showed diurnal variations with high expression levels during nighttime; this suggests that the increased expression level observed in the whole brain is partially of pineal origin. Alternation of light conditions in the pineal gland cultures resulted in the changes in melatonin release into the culture medium as well as MT1 mRNA expression in the pineal gland. The present results suggest that melatonin and its receptors play an important role in the exertion of daily and circadian variations in the neural tissues.

Effect of the pineal hormone melatonin on teleost fish phagocyte innate immune responses after in vitro treatment

Although neuroendocrine-immune system interaction has been shown in teleost fish, no study has evaluated the role of melatonin (Mel) on fish immune response even considering that it is affected by the photoperiod. Gilthead seabream (Sparus aurata L.) and sea bass (Dicentrarchus labrax L.) head-kidney leucocytes were incubated with Mel (0-control-, 20 pM-400 microM) and leucocyte viability and main innate cellular immune parameters were evaluated. Overall, seabream and sea bass head-kidney leucocytes incubated with low (similar to physiological) doses of Mel unchanged the innate immune response, whereas very high (pharmacological) dosages did. Phagocytosis was not affected by any Mel treatment while the peroxidase activity was significantly inhibited with the highest Mel concentration. In contrast, the sea bass respiratory burst activity was increased in a dose-dependent manner with 400 nM Mel or higher. Further studies are needed to clarify whether there are interactions between the fish pineal gland, and its hormone Mel, and the fish immune system.

Ocular Melatonin Rhythms in Teleost Fish

Melatonin (N-acetyl-5-methoxytryptamine) is synthesized in the pineal organ and the retina of vertebrates. In some teleost species, ocular melatonin levels can exhibit a circadian periodicity with elevated levels during the dark phase under light-dark (LD) cycles and this periodicity can persist even under constant dark (DD) cycles. However, reversed melatonin profiles and an absence of circadian ocular melatonin rhythms have also been reported. In this study, we investigated the daily rhythms of ocular melatonin in 32 teleost species under LD cycles. The melatonin profiles could be classified into three types: (1) normal profiles, with higher melatonin levels during the dark phase than the light phase; (2) reversed profiles, with higher levels during the light phase than the dark phase; (3) no significant differences in melatonin levels. We also studied whether ocular melatonin exhibits circadian rhythms under DD in selected species. Our results showed that ocular melatonin exhibited circadian rhythms in some but not all of the species examined. These results indicate that ocular melatonin rhythms in teleost fish exhibit species-specific variations as a result of the changes in the regulatory mechanisms during the course of evolution.

Molecular cloning and daily variations of the Period gene in a reef fish Siganus guttatus

As the first step in understanding the molecular oscillation of the circa rhythms in the golden rabbitfish Siganus guttatus--a reef fish with a definite lunar-related rhythmicity--we cloned and sequenced a Period gene (rfPer). The rfPer gene contained an open reading frame that encodes a protein consisting of 1,452 amino acids; this protein is highly homologous to PER proteins of vertebrates including zebrafish. Phylogenetic analyses indicated that the rfPER protein is related to the zebrafish PER1 and PER4. The expression of rfPer mRNA in the whole brain, retina, and liver under light/dark (LD) conditions increased at 06:00 h and decreased at 18:00 h, suggesting that its robust circadian rhythm occurs in neural and peripheral tissues. When daily variation in the expression in rfPer mRNA in the whole brain and cultured pineal gland were examined under LD conditions, similar expression patterns of the gene were observed with an increase around dawn. Under constant light condition, the increased expression of rfPer mRNA in the whole brain disappeared around dawn. The present results demonstrate that rfPer is related to zPer4 and possibly zPer1. The present study is the first report on the Period gene from a marine fish.

Melatonin receptor of a reef fish with lunar-related rhythmicity: cloning and daily variations

Melatonin receptors are expressed in neural and peripheral tissues and mediate melatonin actions on the regulation of circadian rhythms in various species. For overall understanding of 'circa' rhythms in the golden rabbitfish, Siganus guttatus, which exhibits restricted lunar-related rhythms and spawns synchronously around the first quarter moon, the aim of the present study was to clone a melatonin receptor (Mel(lb)) cDNA and examine daily variations of Mel(lb) mRNA expression in certain tissues of the rabbitfish. The full-length Mel(lb) cDNA (1808 bp) contained an open reading frame to encode a protein with a length of 354 amino acids, which was highly homologous to a protein of nonmammalian species. Northern blot analysis showed transcripts of Mel(lb) in the brain and retina. Real-time quantitative polymerase chain reaction analysis also revealed expression of Mel(lb) in all tissues tested. Significantly high expression of the gene during daytime was evident in the liver and kidney. When the expression of Mel(lb) was examined in the brain and retina under conditions of light/dark cycles or constant darkness, daily and circadian variations of gene expression with two increases during daytime and nighttime for the brain and a single increase during nighttime for the retina were recognized. Moreover, daily variations in the expression of Mel(lb) were observed in the cultured pineal gland. These results suggest that the melatonin receptor plays a role in integration of melatonin actions in various tissues and that daily variations of Mel(lb) in the neural tissues may be related to regulation of circadian clock.

Changes in melatonin binding sites under artificial light–dark, constant light and constant dark conditions in the masu salmon brain

To test whether the affinity (Kd) and total binding capacity (Bmax) of melatonin receptors exhibit daily and circadian changes in teleost fish whose melatonin secretion is not regulated by intra-pineal clocks, we examined the changes in melatonin binding sites in the brains of underyearling masu salmon Oncorhynchus masou under artificial light-dark (LD), constant light (LL) and constant dark (DD) conditions. In Experiment 1, fish were reared under a long (LD 16:8) or short (LD 8:16) photoperiod for 69 days. Blood and brains were sampled eight times at 3 h intervals. Plasma melatonin levels were high during the dark phase and low during the light phase in both photoperiodic groups. The Bmax exhibited no daily variations. Although the Kd slightly, but significantly, changed under LD 8:16, this may be of little physiological significance. In Experiment 2, fish reared under LD 12:12 for 27 days were exposed to LL or DD from the onset of the dark phase under LD 12:12. Blood and brains were sampled 13 times at 4 h intervals for two complete 24 h cycles. Plasma melatonin levels were constantly high in the DD group and low in the LL group. No significant differences were observed in the Kd and the Bmax between the two groups, and the Kd and the Bmax exhibited no circadian variation either in the LL or DD groups. These results indicate that light conditions have little effect on melatonin binding sites in the masu salmon brain.

Circadian rhythms of ocular melatonin in the wrasse Halichoeres tenuispinnis, a labrid teleost

Using in vivo and in vitro methods we studied the regulation of ocular melatonin rhythms in the wrasse Halichoeres tenuispinnis, by either light or the circadian clock. Rhythmic changes in ocular melatonin levels under light-dark (LD) cycles were persistent under constant darkness (DD), and had a circadian periodicity of approximately 24h. However, ocular melatonin levels remained low under constant light conditions. When wrasse were exposed to a single 6-h light pulse at three different circadian phases under DD, phase-dependent phase shifts in the circadian rhythms of ocular melatonin were observed. When eyecups were prepared during mid-light periods or at the onset of darkness, and incubated in vitro in either light or dark periods, both time and light conditions affected melatonin release. These results indicate that the melatonin rhythms in the wrasse eye are driven by an ocular circadian clock that is entrained to LD cycles via local photoreceptors.

Day–night specific binding of 2-[125I]Iodomelatonin and melatonin content in gill, small intestine and kidney of three fish species

Some of melatonin's (Mel) well-established physiological effects are mediated via high-affinity cell-membrane receptors belonging to the superfamily of G-protein-coupled receptors. Specific binding of ligand 2-[(125)I]iodomelatonin, using membrane preparations from osmoregulatory tissues of flounder, rainbow trout and sea bream, together with Mel concentrations in the tissues and plasma were studied. The kidney, gill and small intestine samples were collected during the day and at night. The dissociation constants (K (d)) and maximal binding densities (B (max)) were calculated for each tissue at 11:00 and 23:00 h. The binding sites with K (d) values in the tissues in the picomolar range indicated the high affinity. K (d) and B (max) values were tissue- and species-dependent. The GTP analogue [Guanosine 5'-O-(3-thiotriphosphate)] treatment significantly reduced the B (max) value, indicating that the 2-[(125)I]iodomelatonin-binding sites are probably coupled to a G-protein. No daily variations in K (d) and B (max) values were observed. These are the first studies of the presence of 2-[(125)I]iodomelatonin-binding sites in the small intestine, kidney tubule and gill of fish. The data strongly suggest new potential targets for Mel action and the influence of Mel on water/ion balance in fish. The intestine seems to be a site of Mel synthesis and/or an active accumulation of the hormone.