Daily cycles in plasma melatonin levels under long or short photoperiod in the common carp, Cyprinus carpio

Harbours as unique environmental sites of multiple anthropogenic stressors on fish hormonal systems

Fish development and acclimation to environmental conditions are strongly mediated by the hormonal endocrine system. In environments contaminated by anthropogenic stressors, hormonal pathway alterations can be detrimental for growth, survival, fitness, and at a larger scale for population maintenance. In the context of increasingly contaminated marine environments worldwide, numerous laboratory studies have confirmed the effect of one or a combination of pollutants on fish hormonal systems. However, this has not been confirmed in situ. In this review, we explore the body of knowledge related to the influence of anthropogenic stressors disrupting fish endocrine systems, recent advances (focusing on thyroid hormones and stress hormones such as cortisol), and potential research perspectives. Through this review, we highlight how harbours can be used as "in situ laboratories" given the variety of anthropogenic stressors (such as plastic, chemical, sound, light pollution, and invasive species) that can be simultaneously investigated in harbours over long periods of time.

Circadian rhythms and environmental disturbances – underexplored interactions

Biological rhythms control the life of virtually all organisms, impacting numerous aspects ranging from subcellular processes to behaviour. Many studies have shown that changes in abiotic environmental conditions can disturb or entrain circadian (∼24 h) rhythms. These expected changes are so large that they could impose risks to the long-term viability of populations. Climate change is a major global stressor affecting the fitness of animals, partially because it challenges the adaptive associations between endogenous clocks and temperature – consequently, one can posit that a large-scale natural experiment on the plasticity of rhythm–temperature interactions is underway. Further risks are posed by chemical pollution and the depletion of oxygen levels in aquatic environments. Here, we focused our attention on fish, which are at heightened risk of being affected by human influence and are adapted to diverse environments showing predictable changes in light conditions, oxygen saturation and temperature. The examined literature to date suggests an abundance of mechanisms that can lead to interactions between responses to hypoxia, pollutants or pathogens and regulation of endogenous rhythms, but also reveals gaps in our understanding of the plasticity of endogenous rhythms in fish and in how these interactions may be disturbed by human influence and affect natural populations. Here, we summarize research on the molecular mechanisms behind environment–clock interactions as they relate to oxygen variability, temperature and responses to pollutants, and propose ways to address these interactions more conclusively in future studies.

Effects of Moonlight Exposure on Plasma Melatonin Rhythms in the Seagrass Rabbitfish, Siganus Canaliculatus

Influences of light-dark (LD) cycle and moonlight exposure on plasma melatonin rhythms in the seagrass rabbitfish, Siganus canaliculatus, a lunar synchronized spawner, were determined by time-resolved fluoroimmunoassay (TR-FIA). When the fish were exposed to a natural LD (12:12) cycle, plasma melatonin levels exhibited a clear daily rhythm, with higher levels at midnight and lower levels during the day. These rhythms were not evident under either constant light (LL) or constant dark (DD) conditions. Plasma melatonin levels under LL condition were low and high under DD condition. These results indicate that plasma melatonin rhythms are driven by LD cycle in this species. When the fish were exposed to the 4 lunar phases, plasma melatonin levels around the new moon were significantly higher than during the first quarter moon and the full moon. Exposure to experimentalnewmoonand full moonconditions caused significant increases and decreases of plasma melatonin levels, respectively. The synchronous rhythmicity of melatonin levels in the plasma support the hypothesis that the seagrass rabbitfish perceives moonlight intensity and responds with secretion of melatonin into the bloodstream.

Constant illumination reduces circulating melatonin and impairs immune function in the cricket Teleogryllus commodus

Exposure to constant light has a range of negative effects on behaviour and physiology, including reduced immune function in both vertebrates and invertebrates. It is proposed that the associated suppression of melatonin (a ubiquitous hormone and powerful antioxidant) in response to the presence of light at night could be an underlying mechanistic link driving the changes to immune function. Here, we investigated the relationship between constant illumination, melatonin and immune function, using a model invertebrate species, the Australian black field cricket, Teleogryllus commodus. Crickets were reared under either a 12 h light: 12 h dark regimen or a constant 24 h light regimen. Circulating melatonin concentration and immune function (haemocyte concentration, lytic activity and phenoloxidase (PO) activity) were assessed in individual adult crickets through the analysis of haemolymph. Constant illumination reduced melatonin and had a negative impact on haemocyte concentrations and lytic activity, but its effect on PO activity was less apparent. Our data provide the first evidence, to our knowledge, of a link between exposure to constant illumination and variation in haemocyte concentration in an invertebrate model, while also highlighting the potential complexity of the immune response following exposure to constant illumination. This study provides insight into the possible negative effect of artificial night-time lighting on the physiology of invertebrates, but whether lower and potentially more ecologically relevant levels of light at night produce comparable results, as has been reported in several vertebrate taxa, remains to be tested.

Influence of Light Intensity on Plasma Melatonin and Locomotor Activity Rhythms in Tench

Melatonin production by the pineal organ is influenced by light intensity, as has been described in most vertebrate species, in which melatonin is considered a synchronizer of circadian rhythms. In tench, strict nocturnal activity rhythms have been described, although the role of melatonin has not been clarified. In this study we investigated daily activity and melatonin rhythms under 12:12 light-dark (LD) conditions with two different light intensities (58.6 and 1091 microW/cm2), and the effect of I h broad spectrum white light pulses of different intensities (3.3, 5.3, 10.5, 1091.4 microW/cm2) applied at middarkness (MD) on nocturnal circulating melatonin. The results showed that plasma melatonin in tench under LD 12:12 and high light conditions displayed rhythmic variation, where values at MD (255.8 +/- 65.9 pg/ml) were higher than at midlight (ML) (70.7 +/- 31.9 pg/ml). Such a difference between MD and ML values was reduced in animals exposed to LD 12: 12 and low light intensity. The application of 1 h light pulses at MD lowered plasma melatonin to 111.6 +/- 3.2 pg/ml (in the 3.3-10.5 microW/cm2 range) and to 61.8 +/- 18.3 pg/ml (with the 1091.4 microW/cm2 light pulse) and totally suppressed nocturnal locomotor activity. These results show that melatonin rhythms persisted in tench exposed to low light intensity although the amplitude of the rhythm is affected. In addition, it was observed that light pulses applied at MD affected plasma melatonin content and locomotor activity. Such a low threshold suggests that the melatonin system is capable of transducing light even under dim conditions, which may be used by this nocturnal fish to synchronize to weak night light signals (e.g., moonlight cycles).

Lack of circadian regulation of in vitro melatonin release from the pineal organ of salmonid teleosts

In many teleost species, the photoreceptive pineal organ harbors the circadian clock that regulates melatonin release in the pineal organ itself. However, the pineal organ of three salmonids (rainbow trout Oncorhynchus mykiss, masu salmon Oncorhynchus masou, and sockeye salmon Oncorhynchus nerka) did not exhibit circadian rhythms in melatonin release when maintained under constant darkness (DD) in vitro, suggesting that the pineal organs of all salmonids lack the circadian regulation of melatonin production. To test this hypothesis, the pineal organ of seven salmonids (common whitefish Coregonus lavaretus, grayling Thymallus thymallus, Japanese huchen Hucho perryi, Japanese charr Salvelius leucomaenis pluvius, brook trout Salvelius fontinalis, brown trout Salmo trutta and chum salmon Oncorhynchus keta) and closely related osmerids (ayu Plecoglossus altivelis altivelis and Japanese smelt Hypomesus nipponensis) were individually maintained in flow-through culture at 15 degrees C under several light conditions. Under light-dark cycles, the pineal organ of all species showed a rhythmic melatonin release with high rates during the dark phase. Under DD, the osmerid pineal organs exhibited circadian rhythms in melatonin release with high rates only during the subjective-night but the salmonid pineal organs constantly released melatonin at high rates. Under constant light, melatonin release was suppressed in all species. The pineal organ of rainbow trout maintained at different temperature (15, 20 or 25 degrees C) under DD released melatonin with high rates but the amount of melatonin released was temperature-sensitive (highest at 20 degrees C). Thus, melatonin release from the pineal organ of osmerids is regulated by both light and circadian clock but the circadian regulation is lacking in salmonids. These results indicate that ancestral salmonids lost the circadian regulation of melatonin production after the divergence from osmerid teleosts.

Lack of Circadian Regulation of Melatonin Rhythms in the Sockeye Salmon (Oncorhynchus nerka)in vivoandin vitro

Melatonin profiles were determined in the plasma in vivo and in the pineal organ in vitro of the sockeye salmon (Oncorhynchus nerka) under various light conditions to test whether they are under circadian regulation. When serial blood samples were taken at 4-h intervals for 3 days via a cannula inserted into the dorsal aorta, plasma melatonin exhibited significant fluctuation under a light-dark cycle, with higher levels during the dark phase than during the light phase. No rhythmic fluctuations persisted under either constant dark or constant light, with constant low and high levels, respectively. Melatonin release from the pineal organ in flow-through culture exhibited a similar pattern in response to the change in light conditions, with high and low release associated with the dark and light phases, respectively. These results indicate that melatonin production in the sockeye salmon is driven by light and darkness but lacks circadian regulation.

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.

Locomotor, feeding and melatonin daily rhythms in sharpsnout seabream (Diplodus puntazzo)

Sharpsnout seabream is a marine teleost of increasing interest for Mediterranean aquaculture, but there is still a lack of information regarding its circadian organization. In this study, we have investigated sharpsnout seabream locomotor activity, feeding and plasma melatonin daily rhythms under a 12:12-h LD cycle, as well as the persistence of locomotor activity circadian rhythmicity under constant light (LL) conditions. When submitted to an LD cycle, most sharpsnout seabream displayed a diurnal locomotor pattern, with an average 74% of activity recorded during daytime. However, along the experiment 40% of fish spontaneously changed their locomotor rhythm phasing and became nocturnal. Feeding behaviour, nevertheless, remained strictly diurnal in all cases, with 97% of food demands being made during the light period. Free-running locomotor rhythms were recorded in one third of the fish kept under LL. Daily plasma melatonin levels displayed a rhythmic profile, with low daytime values (111 pg/ml) and high nighttime concentrations (791 pg/ml). Taken together, these results evidence a high degree of plasticity for sharpsnout seabream activity patterns, as well as phasing independence of locomotor and feeding rhythms. Finally, the existence of a well-defined daily rhythm of plasma melatonin was found.

A direct influence of moonlight intensity on changes in melatonin production by cultured pineal glands of the golden rabbitfish, Siganus guttatus

Rabbitfish are a restricted lunar-synchronized spawner that spawns around a species-specific lunar phase. It is not known how the fish perceive changes in cues from the moon. One possible explanation is that rabbitfish utilize changes in moonlight intensity to establish synchrony. The purpose of the present study was to examine whether or not the pineal gland of the golden rabbitfish can directly perceive changes in moonlight intensity. Isolated pineal glands were statically cultured under natural or artificial light conditions and melatonin secreted into the culture medium was measured using a time-resolved fluoroimmunoassay. Under an artificial light/dark cycle, melatonin secretion significantly increased during the dark phase. Under continuous light conditions, melatonin secretion was suppressed, while culture under continuous dark conditions seemed to duplicate melatonin secretion corresponding to the light/dark cycle in which the fish were acclimated. When cultured pineal glands were kept under natural light conditions on the dates of the full and the new moon, small amounts of melatonin were secreted at night. Moreover, exposure of cultured pineal glands to artificial and natural light conditions resulted in a significant decrease of melatonin secretion within 2 hr. These results suggest that the isolated pineal gland of golden rabbitfish responds to environmental light cycles and that 'brightness' of the night moon has an influence on melatonin secretion from the isolated pineal gland.

Indoleamines and 5-methoxyindoles in trout pineal organ in vivo: daily changes and influence of photoperiod

This study describes the diel rhythms in several indoleamines, melatonin, and related 5-methoxyindoles in the pineal organ of rainbow trout in vivo. In addition, the effect of different photoperiod conditions was evaluated. Melatonin levels displayed clear daily rhythms in the pineal organ of rainbow trout kept experimentally under long (LD 16:08), neutral (LD 12:12), and short (LD 08:16) photoperiods. Duration of melatonin signal was dependent on the night length of prevailing photoperiod, while peak amplitude was higher when lengthening the photoperiod. Significant daily rhythms in 5-HT content, the precursor of melatonin synthesis, were found in neutral and short photoperiod with increases of the amine content just after the light-dark interphase and decreases in the middle of the night, which were more important under short photoperiod. In contrast, no significant 24-h cyclic variation was found in pineal 5-HT content under long photoperiod. Daily profiles in the content of the main 5-HT oxidative metabolite, the 5-hydroxyindoleacetic acid (5-HIAA), outlined those of the amine precursor. The chronograms of both aminergic compounds contrast with those of 5-hydroxytryptophan content, which displayed a net tendency to increase at night. This study also provides evidence for the existence of daily cyclic changes in the content of 5-methoxytryptamine (5-MT), 5-methoxyindoleacetic acid (5-MIAA), and 5-methoxytryptophol (5-MTOL) in trout pineal organ, which were also dependent on photoperiod. The 24-h profiles in 5-MT content correlated well with those of 5-HT, showing a peak at the first hour of darkness in all photoperiodic conditions, and a decay at midnight only in both neutral and long photoperiods. Similarly, the content of 5-MTOL also displayed high values during the day-night transition in trout kept under neutral and long photoperiods, followed by a slow decay all along the night. Finally, levels of 5-MIAA increased in all photoperiods when lights were turned off, being this nocturnal increase maximal in fish kept under LD 16:08. These results suggest that light-dark cycle modulates daily rhythms in pineal indoles and non-melatonin 5-methoxyindoles by acting mainly through the melatonin synthesis activity, which limits the availability of 5-HT for the oxidative and direct methylation pathways. In addition, it seems that a nocturnally increased synthesis of 5-HT might be a requirement for the optimal formation of melatonin and other 5-methoxyindoles in the pineal organ when trout remain under short photoperiods.

Daily and Circadian Variations of the Pineal and Ocular Melatonin Contents and their Contributions to the Circulating Melatonin in the Japanese Newt, Cynops pyrrhogaster

Daily and circadian variations of melatonin contents in the diencephalic region containing the pineal organ, the lateral eyes, and plasma were studied in a urodele amphibian, the Japanese newt (Cynops pyrrhogaster), to investigate the possible roles of melatonin in the circadian system. Melatonin levels in the pineal region and the lateral eyes exhibited daily variations with higher levels during the dark phase than during the light phase under a light-dark cycle of 12 h light and 12 h darkness (LD12:12). These rhythms persisted even under constant darkness but the phase of the rhythm was different from each other. Melatonin levels in the plasma also exhibited significant day-night changes with higher values at mid-dark than at mid-light under LD 12:12. The day-night changes in plasma melatonin levels were abolished in the pinealectomized (Px), ophthalmectomized (Ex), and Px+Ex newts but not in the sham-operated newts. These results indicate that in the Japanese newts, melatonin production in the pineal organ and the lateral eyes were regulated by both environmental light-dark cycles and endogenous circadian clocks, probably located in the pineal organ and the retina, respectively, and that both the pineal organ and the lateral eyes are required to maintain the daily variations of circulating melatonin levels.

Seasonal changes in brain melatonin concentration in the three-spined stickleback (Gasterosteus aculeatus): towards an endocrine calendar

Pineal organ and its hormone melatonin (N-acetyl-5-methoxytryptamine) is likely involved in timing and synchronisation of many internal processes, such as reproduction, with annual changes in environmental cues, i.e., photoperiod and water temperature. The seasonal changes in melatonin profile in stickleback brains related to the following reproductive phases were examined, and the link between melatonin concentrations and the stages of spawning cycle was analysed. Two wild populations of sticklebacks were exposed to annual environmental changes in their natural habitats. Brains, gonads, kidneys and livers were collected over 2 years. Melatonin was measured using RIA and the indices, gonadosomatic (GSI), nephrosomatic (NSI) and hepatosomatic (HSI), were calculated. The role of melatonin, as a component of internal calendar engaged in the control of seasonal breeding in this species, is discussed. The extremely high melatonin levels observed in early spring (March) and autumn (October) seem to mark out a time frame for spawning in sticklebacks. The seasonal pattern of melatonin production and identified development stages of gonads suggests the potential inhibitory effect of the hormone on stickleback reproduction in shortening photoperiod and stimulatory effect in lengthening photoperiod.

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.

Perception and possible utilization of moonlight intensity for reproductive activities in a lunar-synchronized spawner, the golden rabbitfish

Rabbitfishes are known to spawn synchronously around the species-specific lunar phase. It is considered that they perceive and utilize cues from the moon in order to be synchronized gonadal development and spawning with the lunar cycle. Using the golden rabbitfish, Siganus guttatus, which spawns synchronously around the first quarter moon during the reproductive season, we measured the fluctuation of melatonin levels and examined the response of the fish to moonlight intensity. Daily fluctuation of melatonin concentration in the blood of golden rabbitfish showed low levels during daytime and high levels during night-time, suggesting that melatonin functions in the perception and utilization of photoperiod. Plasma melatonin concentration at the new moon was higher than that at the full moon. When the fish were exposed to moonlight at midnight of the both moon phases, the melatonin concentrations decreased to the control levels. These results show that the fish possibly perceive moonlight intensity and plasma melatonin fluctuates according to 'lightness' at a point of night. At the first spawning period (experiment was started one month before the spawning), the fish reared under natural conditions spawned at the expected spawning dates, whereas the fish reared under the constant darkness and lightness of night did not spawn. At the second spawning period (experiment was started 2 weeks before the spawning), the fish reared under the conditions of natural and constant darkness of night spawned but not that of constant lightness of night. It is possible that night conditions are related to synchronous gonadal development and spawning in the golden rabbitfish.

Characterization and maturational differences of melatonin binding sites in the masu salmon brain

To obtain a better understanding of the roles of melatonin in the mediation of photoperiodic signaling, we have examined the pharmacological characteristics, guanine nucleotide modulation, and maturational differences of melatonin binding sites in the brain of masu salmon Oncorhynchus masou by radioreceptor assay using 2-[125I]iodomelatonin as the radioligand. The specific binding of 2-[125I]iodomelatonin was rapid, stable, saturable, and reversible. Saturation experiments demonstrated that 2-[125I]iodomelatonin binds to a single class of receptor sites with an affinity constant (K(d)) of 6.3+/-0.5 pM and a total binding capacity (B(max)) of 15.18+/-0.22 fmol/mg protein in underyearling precocious males in July. Competition experiments revealed that the binding sites are highly specific for melatonin and related analogues. Treatment with guanosine 5(')-O-(3-thiotriphosphate) significantly reduced the specific binding, indicating that melatonin binding sites in the masu salmon brain are coupled to G protein. Significant differences were seen in B(max), but not K(d), among the fish groups differing in maturity. In the underyearling fish in July, the B(max) of precocious males and immature males was significantly higher than that of immature females. Then, the B(max) of precocious males decreased in October, when the fish spermiated. In the 2-year-old fish, B(max) was significantly higher in spermiating males than ovulated females. These results indicate that melatonin plays neuromodulatory roles in the central nervous system through specific receptors. Furthermore, gonadal maturation affects the density of melatonin binding sites in the masu salmon brain by an unknown mechanism.

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.

Roles of melatonin in gonadal maturation of underyearling precocious male masu salmon

Testicular maturation of underyearling precocious male masu salmon (Oncorhynchus masou) is affected by photoperiod. It is accelerated by a short photoperiod (light-dark cycles of 8:16 h; LD 8:16) and delayed by a long photoperiod (LD 16:8). Circulating melatonin levels are high during the night and low during the day:the duration of the nocturnal elevation is longer under a short than under a long photoperiod, suggesting mediation of photoperiodic signals by melatonin. This study examined whether melatonin administration mimics short photoperiodic effects and whether it accelerates the testicular development of underyearling male masu salmon reared under a long photoperiod. Fish were randomly selected in June and were divided into two groups. They were reared under LD 16:8 (lights on 04:00-20:00 h) and fed pellets sprayed with melatonin (0.5 mg melatonin/kg body weight/day) or vehicle once a day at 11:00 h until October. The plasma melatonin profile of the melatonin-treated group was similar to that expected under a short photoperiod. Melatonin treatment had a stimulatory effect on the gonadosomatic index and pituitary gonadotropin (GTH) I contents. Plasma testosterone levels were significantly higher in the melatonin-treated group than in the control group in August. However, spermiation was observed in October in both groups and no significant differences were observed in GTH II contents in the pituitary in the two groups throughout the experiment. These results suggest that mimicking a short photoperiod by melatonin administration stimulated testicular development but did not completely activate the brain-pituitary-gonadal axis in precocious male masu salmon. Thus, melatonin is suggested to be one of the factors that mediates the transduction of photoperiodic information to the brain-pituitary-gonadal axis.

Cellular circadian clocks in the pineal

Daily rhythms are a fundamental feature of all living organisms; most are synchronized by the 24 hr light/dark (LD) cycle. In most species, these rhythms are generated by a circadian system, and free run under constant conditions with a period close to 24 hr. To function properly the system needs a pacemaker or clock, an entrainment pathway to the clock, and one or more output signals. In vertebrates, the pineal hormone melatonin is one of these signals which functions as an internal time-keeping molecule. Its production is high at night and low during day. Evidence indicates that each melatonin producing cell of the pineal constitutes a circadian system per se in non-mammalian vertebrates. In addition to the melatonin generating system, they contain the clock as well as the photoreceptive unit. This is despite the fact that these cells have been profoundly modified from fish to birds. Modifications include a regression of the photoreceptive capacities, and of the ability to transmit a nervous message to the brain. The ultimate stage of this evolutionary process leads to the definitive loss of both the direct photosensitivity and the clock, as observed in the pineal of mammals. This review focuses on the functional properties of the cellular circadian clocks of non-mammalian vertebrates. How functions the clock? How is the photoreceptive unit linked to it and how is the clock linked to its output signal? These questions are addressed in light of past and recent data obtained in vertebrates, as well as invertebrates and unicellulars.

Properties of the melatonin-generating system of the sailfin molly, Poecilia velifera

The properties of the melatonin-generating system of a tropical teleost, the sailfin molly (Poecilia velifera), were investigated in vitro in a series of experiments using static or perifusion culture techniques. The properties examined included photic entrainment, circadian rhythmicity under continuous light (LL) and continuous darkness (DD), functionality of the melatonin-generating system at birth, and presence of multiple circadian oscillators in the molly pineal. Pineal glands or skull caps with the pineal gland firmly attached were dissected from adult and new-born fishes, respectively, and placed into static or perifusion culture at constant temperature (27 degrees C) depending upon the experiment. Melatonin release in samples was quantified by RIA. Rhythmic melatonin release was observed from isolated adult pineals under 12L:12D and 14L:10D, with low amounts of melatonin released during the light and high amounts during the dark. Melatonin release was inhibited by LL. However, under DD, melatonin release was robust and rhythmic with a circadian period (Tau) that ranged between 21.3 and 27.0 h (n = 21). Pineals from new-born (1-day old) mollies released melatonin rhythmically under a light:dark cycle and DD in both static and perifusion culture. Melatonin release from half and quarter pineals of adult mollies under DD was robust and rhythmic with circadian periods that ranged between 22.5 and 29.0 h (n = 19). Taken together, these data show that the molly pineal is photosensitive, fully functional from birth, and contains multiple circadian oscillators (at least four) regulating melatonin production.

The effect of photoperiod on diel rhythms in serum melatonin, cortisol, glucose, and electrolytes in the common dentex, Dentex dentex

Diel rhythms in serum concentrations of melatonin, cortisol, glucose, sodium, chloride, and potassium were studied in the common dentex, Dentex dentex, under different photoperiods (DD, 8L:16D, 12L:12D, 16L:8D). Photoperiod affected both the diel rhythms and the absolute values of the estimated blood components. Regardless of the photoperiod, melatonin titers were elevated during the scotophase (384.3 +/- 13.9 pg/ml) compared with a mean baseline level of 54.4 +/- 2.7 pg/ml during the photophase. Serum melatonin concentrations reflected the prevailing photoperiod and constantly elevated melatonin levels with no diel rhythmicity were evident in fish held in the DD protocol. A circadian-like pattern in serum cortisol was observed in fish that were kept at the DD and 8L:16D protocols with cortisol peak at 18:00 h in the night. Fish exposed to the 16L:8D regime showed highest cortisol levels at 10:00 h, while no rhythmicity was evident under the 12L:12D protocol. A phase shift of 4 h between the peaks of cortisol and glucose was evident in fish exposed to the DD, 8L:16D, and 12L:12D regimes. Diel patterns of changes in serum Na+ and Cl- were observed only in the fish held in the DD protocol. Serum K+ values were lowest during the first part of the scotophase under all regimes, except the 16L:8D where no diel rhythmicity was detected. During the photophase, cortisol was positively correlated with glucose, Na+, and Cl- and negatively with K+. During the scotophase, melatonin was positively correlated with glucose and electrolytes. Results indicated that cortisol may be responsible for the observed rhythmicity of glucose and that melatonin may play a role in glucose and ion regulation in common dentex.

Melatonin does not prevent long photoperiod stimulation of secondary sexual characters in the male three-spined stickleback Gasterosteus aculeatus

Breeding in the three-spined stickleback is stimulated by long but not by short photoperiods in many seasons. The aim of the present study was to test the hypothesis that melatonin plays a role in the inhibitory effect of short photoperiod in this species. Adult nonbreeding males were kept either under constant light (Experiment 1) or under a stimulatory long photoperiod (16L 8D, Experiment 2), in water containing 0, 20, or 80 microg/liter melatonin for 16 hr/day for 28 days during the spring. These melatonin treatments were intended to simulate the daily melatonin pattern of a nonstimulatory short photoperiod. In the second experiment, fish were also kept under a nonstimulatory short photoperiod (8L 16D). In the natural breeding season the only germ cells found in the stickleback testes are spermatozoa and spermatogonia, a condition found in many fish under all treatments. In the first experiment, spermatogenesis was not influenced by melatonin. However, testes also containing spermatocytes and spermatids were more common in fish kept under 8L 16D and fish treated with 80 microg/liter melatonin than in 16L 8D controls in Experiment 2. Kidney hypertrophy, an androgen-dependent male secondary sexual characteristic in the stickleback, appeared in most males kept under constant light or 16L 8D and was not influenced by melatonin treatment. In contrast, control males kept under 8L 16D in Experiment 2 did not display kidney hypertrophy. Therefore, the presence of an extended period of elevated melatonin did not prevent the stimulatory effects of long photoperiod on development of this secondary sexual characteristic in the stickleback.

Characterization, guanosine 5'-O-(3-thiotriphosphate) modulation, daily variation, and localization of melatonin-binding sites in the catfish (Silurus asotus) brain

Characteristics, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) modulation, daily variation, and localization of melatonin-binding sites in the brain of a nocturnal teleost, the catfish Silurus asotus, were studied by radioreceptor assay using 2-[125I]iodomelatonin as the radioligand. The specific binding was rapid, stable, saturable, and reversible. The radioligand binds to a single class of receptor site with an affinity (Kd) of 30.7 +/- 7.3 pM and total binding capacity (Bmax) of 9.76 +/- 0.79 fmol/mg protein (mean +/- SE, n = 5). The binding sites were highly specific for 2-iodomelatonin and melatonin. The specificity was almost identical to that of functional melatonin receptors in the dermal and epidermal melanophores in this species and that of ML-1 subtype melatonin receptors in vertebrates, including melatonin-binding sites in the goldfish brain. GTPgammaS treatment altered both the Kd and Bmax values, indicating that melatonin-binding sites in the catfish brain are coupled to G protein. The Bmax values exhibited no daily variation under light-dark cycles of 12 hr light:12 hr dark whereas plasma melatonin levels and Kd fluctuated in a rhythmic fashion. The density of melatonin-binding sites in discrete brain areas was determined to be highest in optic tectum-thalamus and hypothalamus, intermediate in telencephalon, cerebellum, and medulla oblongata, and lowest in olfactory bulbs. These results suggest that melatonin secreted from the pineal organ and/or retina plays neuromodulatory roles in the catfish brain via G protein-coupled melatonin receptors. Characteristics of melatonin receptors seem to be highly conserved during evolution, although the density of melatonin receptors is not regulated by melatonin itself in this species.

Melatonin levels in the gastrointestinal tissues of fish, amphibians, and a reptile

Melatonin was detected by radioimmunoassay in the gastrointestinal tract (GIT) of several species of fish (sturgeon, rainbow trout, carp), amphibians (axolotl, leopard frog, bullfrog), and one reptile (red-sided garter snake), which were sacrificed during the daytime. The highest levels of melatonin were detected in the snake [means = 1018 pg/g stomach, 328 pg/g proximal gut (PG), 511 distal gut (DG)] and carp (means = 102 pg/g stomach, 146 pg/g PG and 141 pg/g DG). Lowest levels were found in the axolotl (means = 44 pg/g stomach and PG, 92 pg/g DG) and the bullfrog (means = 73 pg/g esophagus, 78 pg/g stomach, 20 pg/g PG, and 152 pg/g DG). In most cases there were no statistically significant differences in the melatonin levels among various GIT tissues of the same species but there were differences in tissue levels between different species.

Ocular melatonin rhythms in the goldfish, Carassius auratus

Ocular melatonin rhythms in the goldfish were studied and compared to those in the pineal organ and plasma. Under light:dark (LD) of 12 h light:12 h dark, melatonin contents in the eye as well as the pineal organ and plasma exhibited clear day-night changes with higher levels at mid-dark than at mid-light. However, melatonin contents in the eye at mid-light and mid-dark were approximately 100 and 9 times greater than those in the pineal organ, respectively. Day-night changes of ocular melatonin persisted after pinealectomy, which abolished those in plasma melatonin under LD 12:12. Ocular melatonin contents in the pinealectomized fish at mid-light were significantly higher than those in the sham-operated control. Under constant darkness (DD), circadian melatonin rhythms were observed in the eye but damped on the 3rd day, whereas plasma melatonin rhythms generated by the pineal organ persisted for at least 3 days. Under constant light, ocular melatonin contents exhibited a significant fluctuation with a smaller amplitude than that under DD, whereas plasma melatonin remained at low levels. These results indicate the involvement of LD cycles, a circadian clock, and the pineal organ in the regulation of ocular melatonin rhythms in the goldfish.

Comparative aspects of the pineal/melatonin system of poikilothermic vertebrates

The pineal gland of poikilothermic vertebrates originates as an evagination from the diencephalic roof between the habenular and the posterior commissures, and associates with a parapineal organ to form the so-called pineal complex. The pinealocytes may be photosensitive, secretory or intermediate cells between both. Melatonin, the indoleamine secreted by the pineal, exhibits a circadian secretory rhythm that conveys environmental information to the organism. The peak melatonin secretion occurs during the night, although there are a few examples of an increase in indoleamine secretion during the day. Melatonin is also synthesized in other sites such as the retina, and it has been found in many invertebrates and unicellular organisms. The rhythmic secretory pattern of melatonin is responsible for many biological rhythms exhibited by lower vertebrates. These rhythms are abolished by pinealectomy in some species, but not in others, suggesting the existence of an extra-pineal pacemaker. The photoperiod and the temperature (especially in reptiles) are the main environmental factors affecting the secretory rhythm of melatonin. Poikilothermic vertebrates exhibit a circadian rhythmic color change, with nocturnal blanching, usually related to melatonin secretion. In amphibians, melatonin exhibits a potent skin lightening activity. However, in fishes and reptiles the melatonin effects vary with the species, the developmental stage, and the pigment cell location. Melatonin also exerts inhibitory or excitatory activity on the amphibian reproductive system, regulation of circadian locomotory activity in reptiles, and modulation of the amphibian metamorphosis. Melatonin has also a modulatory effect on the response of target cells to different hormones and high concentrations or prolonged exposure to the indoleamine may cause autodesensitization in various tissues. Binding sites of melatonin have been detected in the central nervous system and peripheral tissues of various vertebrates. The relative potencies of melatonin analogues demonstrated two subtypes of melatonin receptors (ML-1 and ML-2). A transmembrane melatonin receptor has been cloned from Xenopus laevis melanophores; it belongs to the family of the G protein-coupled receptors and exhibits 85% homology with the mammalian nervous system receptor. Melatonin binding sites in the nucleus of many cell types and its potent intracellular anti-oxidant action suggest mechanisms of action other than through the G-protein coupled receptor.

Influences of season and of temperature, photoperiod, and subcutaneous melatonin infusion on the glomerular filtration rate of ewes

Influences of season and of temperature, photoperiod, and subcutaneous melatonin infusion on glomerular filtration rate (GFR) were measured in ewes. There was a seasonal difference of GFR between summer (June-August) and winter (December-February) in Tokyo (35 degrees, 35' N); GFR was significantly (P < 0.05) higher in summer (4.2 +/- 0.3 ml/min/kg) than in winter (3.0 +/- 0.2 ml/min/kg). GFR was measured after exposure to three types of photoperiod, 24L:0D, 12L:12D, and 0L:24D, for 7 to 8 days. The value for GFR obtained at 20 degrees C was significantly lower (P < 0.05) with 0L:24D than with the other two photoperiodic conditions. GFR obtained during subcutaneous melatonin infusion (20 micrograms/hr for 16 hr/day for 7 days) with 24L:0D conditions was significantly (P < 0.05) lower (2.5 +/- 0.1 ml/min/kg) than without infusion (3.8 +/- 0.3 ml/min/kg) and was about the same as that obtained in animals under 0L:24D conditions. At 30 degrees C, GFR exhibited no difference between the 3 photoperiodic conditions and was always lower than that found at 20 degrees C. Possible influences of melatonin on GFR are discussed.

Effects of pinealectomy and constant light exposure on day-night changes of melatonin binding sites in the goldfish brain

Effects of pinealectomy and constant light exposure on day-night changes of melatonin binding sites in the goldfish brain were examined. The density and affinity of binding sites were higher at mid-day than at mid-night in sham-pinealectomized goldfish under light-dark cycles. The rhythms disappeared after pinealectomy, or constant light exposure both of which abolish plasma melatonin rhythms. The effects of pinealectomy and constant light exposure were not additive. These results indicate that diel changes of melatonin binding sites in the goldfish brain are regulated by endogenous melatonin of pineal origin.

Effects of season, temperature, and photoperiod on plasma melatonin rhythms in the goldfish, Carassius auratus

Effects of season, environmental temperature, and photoperiod on plasma melatonin concentrations were studied in the goldfish, Carassius auratus. When goldfish were reared under natural conditions, melatonin levels at mid-dark exhibited seasonal changes, with higher levels obtained in June and September than in December and March. When fish were kept under light:dark (LD) cycle of 12:12 at 5, 15, or 25 degrees C during March-April, temperature-dependent increases in melatonin levels at mid-dark were observed. When animals were maintained under LD 16:8 or LD 8:16 in combination with temperature changes (5, 15, and 25 degrees C) during January-February, the duration of nocturnal elevation in melatonin was controlled by the length of the scotophase while the amplitude was influenced by environmental temperature. These results indicate that plasma melatonin profiles in the goldfish exhibit seasonal changes that are regulated by both photoperiod and temperature.

Melatonin desensitizing effects on the in vitro responses to MCH, alpha-MSH, isoproterenol and melatonin in pigment cells of a fish (S. marmoratus), a toad (B. ictericus), a frog (R. pipiens), and a lizard (A. carolinensis), exposed to varying photoperiodic regimens

Melatonin is a weak dose-independent lightening agonist in fish skin, a moderate dose-dependent lightening agonist in toad skin and a potent lightening agent in frog and lizard skins (reversing in a dose-dependent manner the darkening caused by alpha-melanocyte-stimulating hormone). In frog skins, previous exposure to melatonin reduced further lightening actions of the indoleamine, and in toad skins, increasing concentrations of melatonin elicited decreasing lightening responses, suggesting an autodesensitizing action of the hormone. Various concentrations of melatonin diminished the responses to the lightening agonist melanin-concentrating hormone (MCH) in fish skins and to the darkening agonists alpha-MSH in toad, frog and lizard skins and isoproterenol in frog skins. In vitro inhibitory actions of melatonin are mimicked in the absence of the hormone in skin preparations from toads kept in continuous darkness for 48 hr. The lipophylic nature of the indoleamine associated with the results herein described suggests intracellular actions of melatonin on vertebrate pigment cells.

Serotonin N-acetyltransferase activity as a target for temperature in the regulation of melatonin production by frog retina

The adaptive mechanisms of serotonin N-acetyltransferase (NAT) activity in the regulation of melatonin synthesis in frog retina in the face of chronic and acute temperature changes have been investigated. We performed thermal acclimation experiments to test different environmental temperatures at two seasons of the year (summer and winter), followed by the set-up of an eyecup culture system to investigate the acute effects of temperature on NAT activity and melatonin production daily rhythms. Low temperature induced a significant increase in NAT activity, independent of both the time of the photocycle (midday or midnight) and the season of the year (winter or summer). Acute cold-induced stimulation of NAT activity may be associated with lower decreases in the enzyme synthesis rate, rather than decreases in the degradation rate. In contrast, acclimation to warm temperature (25 degrees C) stimulated ocular melatonin production. Nocturnal melatonin production in eyecups cultured at 25 degrees C was significantly higher than in eyecups cultured at 5 degrees C. We suggest that this discrepancy in thermal regulation of melatonin synthesis can be justified by a seasonal variation in serotonin content within the photoreceptor cells, which determines the thermal response of melatonin production through changes in NAT kinetics.

Characteristics, day-night changes, subcellular distribution and localization of melatonin binding sites in the goldfish brain

Melatonin binding sites in the goldfish brain were characterized by radioreceptor assay using 2-[125I]iodomelatonin as the radioligand. Specific binding of 2-[125I]iodomelatonin was rapid, stable, saturable and reversible. Saturation experiments demonstrated that 2-[125I]iodomelatonin binds to a single class of receptor site with an affinity constant (Kd) of 29.8 +/- 0.7 pM and a total binding capacity (Bmax) of 11.47 +/- 0.33 fmol/mg protein at mid-light. At mid-dark, the Bmax value decreased significantly to 7.90 +/- 0.23 fmol/mg protein (P < 0.01) with no significant variation in the Kd value (33.8 +/- 1.5 pM). Competition experiments revealed the following order of pharmacological affinities: 2-iodomelatonin > melatonin > 6-hydroxymelatonin > N-acetyl-5-hydroxytryptamine > 5-methoxytryptamine > 5-methoxytryptophol > 5-methoxyindole-3-acetic acid. 5-Hydroxytryptamine, 5-hydroxytryptophol, 5-hydroxyindole-3-acetic acid, norepinephrine and acetylcholine exhibited no inhibition. Subcellular distribution of melatonin binding sites was demonstrated to be greatest in the P2 and P3 fractions as compared with the P1 fraction. Localization of melatonin binding sites in discrete brain areas was determined to be highest in the optic tectum-thalamus and hypothalamus, intermediate in the telencephalon, cerebellum and medulla oblongata, and lowest in the olfactory bulbs and pituitary gland. These results suggest that characteristics of melatonin receptors are highly conserved during evolution and that in this species melatonin plays neuromodulatory roles in the central nervous system through specific receptors.

Melatonin signal transduction in the goldfish, Carassius auratus

Generation and reception of melatonin signals in the goldfish, Carassius auratus, are reviewed. The photoreceptive pineal gland of the goldfish generates circulating melatonin rhythms according to a given photoperiod under light-dark cycles and in a circadian manner under continuous dark conditions. Melatonin is also produced in the retina in a similar fashion. Melatonin produced in the pineal gland and retina is considered to act as internal zeitgeber in the brain and retina, respectively, controlling various physiological events via specific melatonin binding sites that are coupled with G protein. The goldfish exhibit clear diurnal locomotor activity rhythms under light-dark cycles and free-running rhythms under constant conditions. However, the relationship between melatonin and locomotor activity rhythms in the goldfish remains unclear. Further studies should be required to demonstrate the roles of melatonin in the circadian system in this species.

The reproductive brain in fish

In fish as in other vertebrates, the brain is actively involved in the control of reproduction, first by participating, under the influence of external factors, in the establishment of an appropriate endocrine status, but also by allowing synchronization of the partners by the time of spawning. It is now well established that the pituitary gonadotropic function is controlled by multiple stimulatory and inhibitory factors, originating mainly from the preoptic region and the mediobasal hypothalamus, both target regions for sexual steroids. Little is known about the mechanisms involved in the mediation of external and internal factors, however there is indication that internal factors, such as androgens and melatonin, known to trigger particular behavioural and endocrine responses, act both at the level of neuroendocrine territories, but also on sensorial systems, which are the actual sites of action for external factors. This paper represents an attempt to summarize and integrate the recent literature devoted to the different aspects of the brain as a major participant in the complex endocrine and behavioural mechanisms of reproduction in fish.

Effects of photoperiod and temperature on rhythmic melatonin secretion from the pineal organ of the white sucker (Catostomus commersoni) in vitro

The secretion rate of melatonin from cultured pineal organs of the white sucker was examined for several days under either a 12:12-hr light:dark (LD) cycle or continuous darkness (DD) at either 10 degrees or 20 degrees. The incubation medium was changed at 3-hr intervals and secreted melatonin was measured by RIA. Under a 12:12-hr LD cycle (0800 light on, 2000 light off) melatonin secretion was suppressed during the day and highly active at night, with larger amplitudes at 20 than at 10 degrees. In DD at 10 degrees no circadian rhythmicity in secretion was found in October or January, whereas at 20 degrees a circadian-like pattern was detected in pineals which were derived from animals reared at either 10 degrees or 20 degrees for 1 week prior to the experiment in October or January. The pineals in the DD experiment still responded to an additional 24-hr LD cycle at both temperatures even after 6 or more days. These results clearly reveal the influence of photoperiod and temperature on melatonin secretion of organ-cultured pineal glands. The existence of a circadian oscillator for melatonin secretion in the pineal gland of the white sucker is suggested.

Pineal-gonadal relationship in the teleost Channa punctatus (Bloch): evidence for possible involvement of hypothalamic serotonergic system

In Channa punctatus, pinealectomy results in an accelerated growth (stimulatory effect) of the ovary in preparatory phase (March), but had no significant effect in prespawning (May-June) or postspawning (September) phases. Administration of melatonin (25 micrograms/fish or 0.4 mg/kg BW at 2-day intervals for 30 days) inhibits the stimulatory effect of a long photoperiod and high temperature (16L:8D; 22 +/- 1 degree C) on the ovary in the early preparatory phase (February). In the late preparatory phase (April), the administration of both melatonin (0.5 mg/kg BW at 2-day intervals) and parachlorophenylalanine (pCPA, a serotonin synthesis blocker; 100 mg/kg BW at 3-day intervals) for 30 days inhibits ovarian activity in comparison to that of the saline-treated control group. In the dose-response study, greater than or equal to 0.5 mg/kg BW of melatonin induced a significant increase in hypothalamic 5-HT content and greater than or equal to 10 mg/kg BW of pCPA decreased it significantly. In the time-course study, melatonin (2.0 mg/kg BW) elevates the 5-HT content significantly after an hr of the injection and maintained it up to 48 hrs. The administration of pCPA (200 mg/kg BW) had significantly inhibited the 5-HT content which was sustained for 72 hr. In another study, a single injection of melatonin (0.5 mg/kg BW) increased the 5-HT content significantly. A single injection of pCPA (100 mg/kg BW) decreased significantly both the content and activity of 5-HT. It is inferred that hypothalamic 5-HT may play a central role in photosexual mechanisms and mediate long photoperiodic effects on neuroendocrine-reproductive axis.

Circadian rhythms of melatonin secretion from superfused goldfish (Carassius auratus) pineal glands in vitro

A flow-through, whole-organ culture (superfusion) system was developed, and goldfish pineal glands were maintained at 25 degrees under light-dark (LD) 12:12 cycles, reversed LD 12:12 cycles, continuous dark (DD), or continuous light (LL) conditions for 48 hr. Under LD 12:12 and reversed LD 12:12 cycles, superfused pineal glands showed a rhythmic melatonin secretion: Scotophase was associated with high titers and photophase with low titers. The melatonin secretion rhythms persisted for two cycles under DD conditions, whereas nocturnal rises were suppressed under LL conditions. After the transition from LL to DD conditions on the third day, melatonin showed a nocturnal increase. These results indicate that melatonin secretion from the superfused goldfish pineal gland is directly photosensitive and that the goldfish pineal gland harbors a circadian oscillator which generates melatonin secretion rhythms.

Thermoperiodic influences on plasma melatonin rhythms in the lizard Tiliqua rugosa: effect of thermophase duration

Rhythms of plasma melatonin levels were determined in lizards (Tiliqua rugosa) subjected to a 12 h photocycle (12 h light: 12 h dark) at constant 33 degrees C, and at 7 different thermoperiods (33 degrees C thermophase and 15 degrees C cryophase) whose thermophase duration ranged from 1.5 to 21 h. The melatonin secretion rate, as measured by the amplitude and duration of elevated melatonin levels and the area under the curve, was maximal at thermoperiods whose thermophase was between 9 and 18 h in duration. The results indicate that in ectothermic vertebrates the prevailing thermoperiod as well as the photoperiod may influence melatonin rhythms and hence the timing of annual physiological cycles.

Phylogenetic distribution and function of arylalkylamine N-acetyltransferase

Amine acetylation is a diverse topic with importance to the regulation of several physiological processes as well as the metabolism of drugs and environmental chemicals. Arylalkylamine N-acetyltransferase is widely distributed in several species, where this enzyme plays an important role in the seasonal regulation of reproduction and photoperiodism in vertebrates through the pathway of melatonin formation. In insects, this enzyme is involved in monoamine neurotransmitter inactivation and the formation of catecholamine intermediates necessary for sclerotization of the insect cuticle.

Melatonin secretion from goldfish pineal gland in organ culture

Pineal glands were removed from goldfish reared under 12L-12D at 25 degrees for 2 weeks. These were incubated for 6 days under (1) normal 12L-12D (lights on 0600-1800 hr), (2) reversed 12L-12D (lights on 1800-0600 hr), (3) continuous dark, or (4) continuous light condition at 25 degrees. The incubation medium was changed at 12-hr intervals (0600-1800 and 1800-0600 hr) and secreted melatonin (MLT) was measured by RIA. Under 12L-12D or reversed 12L-12D, MLT secretion was active in the dark phase and was suppressed in the light phase of a given photoperiod. Under a continuous dark condition, a large amount of MLT was secreted into the medium, although the amount gradually decreased. The MLT secretion was more active in the period corresponding to the dark phase of the acclimatory photoperiod than in the period corresponding to the light phase. This pattern in secretion remained for 4 days. Under a continuous light condition, MLT secretion was suppressed, but the secretion was rapidly increased after changing the photoperiod from the light to the dark condition. These findings clearly indicate that MLT secretion in the organ-cultured pineal gland is photosensitive. It is active under dark and inactive under light conditions. The existence of a circadian rhythm in MLT secretion is also suggested.