Effect of cortisol on melatonin production by the pineal organ of tilapia, Oreochromis mossambicus

Melatonin as an anti-stress signal: effects on an acute stress model and direct actions on interrenal tissue in goldfish

Background

Melatonin is a key hormone in regulation of circadian rhythms, and involved in many rhythmic functions, such as feeding and locomotor activity. Melatonin reportedly counteracts stress responses in many vertebrates, including fish. However, targets for this action of melatonin and underlying mechanisms remain unknown.

Results

This study reports potential anti-stress properties of melatonin in goldfish (Carassius auratus), with a focus on its effect on plasma cortisol, food intake, and locomotor activity, all of them involved in the responses to stress exposure. Indeed, acute injection of melatonin counteracted stress-induced hypercortisolinemia and reduced food intake. The reduced locomotor activity following melatonin treatment suggests a possible sedative role in fish. To assess whether this anti-stress effects of melatonin involve direct actions on interrenal tissue, in vitro cultures of head kidney (containing the interrenal cortisol-producing tissue) were carried out in presence of ACTH, melatonin, and luzindole, an antagonist of melatonin receptors. Melatonin in vitro reduced ACTH-stimulated cortisol release, an effect attenuated by luzindole; this suggests the presence of specific melatonin receptors in interrenal tissue.

Conclusions

Our data support a role for melatonin as an anti-stress signal in goldfish, and suggest that the interrenal tissue of teleosts may be a plausible target for melatonin action decreasing cortisol production.

Effects of Extreme Light Cycle and Density on Melatonin, Appetite, and Energy Metabolism of the Soft-Shelled Turtle (Pelodiscus sinensis)

Constant darkness and constant light exposure often disturb the circadian rhythm in the behavior and energy metabolism of vertebrates. Melatonin is known as the hormonal mediator of photoperiodic information to the central nervous system and plays a key role in food intake and energy balance regulation in vertebrates. The popularly cultured soft-shelled turtle Pelodiscus sinensis has been reported to grow better under constant darkness; however, the underlying physiological mechanism by which darkness benefits turtle growth is not clear yet. We hypothesized that increased melatonin levels induced by darkness would increase appetite and energy metabolism and thus promote growth in P. sinensis. In addition, in order to elucidate the interaction of photoperiod and density, juvenile turtles were treated under three photoperiods (light/dark cycle: 24L:0D, 12L:12D, 0L:24D, light density 900 lux) and two stocking densities (high density: 38.10 ind./m2, low density: 6.35 ind./m2) for 4 weeks, and then the blood and brain tissues of turtles were collected during the day (11:00–13:00) and at night (23:00–1:00) after 2 days of fasting. We examined changes in plasma melatonin levels, food intake (FI), and appetite-related hormones (plasma ghrelin and leptin), as well as growth and energy metabolism parameters such as specific growth rate (SGR), standard metabolic rate (SMR), plasma growth hormone (GH), and thyroid hormone/enzyme activity (plasma triiodothyronine T3, thyroxine T4, and T45′-deiodinase activity). Moreover, we also assessed the responses of mRNA expression levels of food intake-related genes (kisspeptin 1 (Kiss1); cocaine amphetamine-regulated transcript (CART); neuropeptide Y (NPY)) in the brain. The results showed that under high density, SGR was the lowest in 24L:0D and the highest in 0L:24D. FI was the highest in 0L:24D regardless of density. Plasma melatonin was the highest in 0L:24D under high density at night. SMR increased with decreasing light time regardless of density. Most expressions of the measured appetite-related genes (Kiss1, CART, and NPY) were not affected by photoperiod, nor were the related hormone levels, such as plasma leptin, ghrelin, and GH. However, thyroid hormones were clearly affected by photoperiod. T3 level in 0L:24D under high density during the day was the highest among all treatment groups. T4 in 24L:0D under high density during the day and T45′-deiodinase activity in 24L:0D under low density at night were significantly reduced compared with the control. Furthermore, the energy metabolism-related hormone levels were higher under higher density, especially during the day. Together, melatonin secretion is not only modulated by light but also likely to be regulated by unknown endogenous factors and density. Altered plasma melatonin induced by constant darkness and density seems to be involved in the modulation of energy metabolism rather than appetite in the soft-shelled turtle.

The fasted and post-prandial physiological responses of the Patagonian blennie Eleginops maclovinus

Eleginops maclovinus is a native species with potential for Chilean aquaculture. Understanding the variations between the post-prandial and fasted metabolic responses can contribute to improving the aquaculture of this species. This study aimed to characterize variations in intermediate metabolism during the course of the day in the liver, serum, and gills of fed and unfed fish. For this, 72 fish were assigned to two experimental groups, "fed" and "fasted". The first group was fed "ad libitum" at 8.30, while the fasted group was not fed for 24 h. Samples were taken from both groups at 9:00, and every 2 h: 11:00, 13:00, 15:00, 17:00, and 19:00. In the fed group, food spent a long time in the gastrointestinal tract, with a large increase in stomach size and without evidence of complete emptying of the stomach at 19:00 (10.5 h post-feeding). In serum, the levels of amino acids, glucose, and triglycerides presented significant differences with peak levels at different times of day in the fed group. The cortisol in the fasted group presented a diurnal pattern with high levels during the morning and very low levels after 13:00, while in the fed group, the high cortisol variability did not allow a clear pattern to be established. In the liver, the effect of time on the enzymatic activity of the intermediary metabolism was greater compared to the effect of feeding. In the liver, enzyme activity decreased at later hours of the day, while glycogen levels increased at later hours of the day in both groups: but its levels were higher in the fed group. In gills, as well as in the liver, time had a greater effect than feeding on intermediate metabolism, since feeding only had a significant effect on the levels of hexokinase, lactate, and amino acids, suggesting an effect on carbohydrate metabolism. Meanwhile, time significantly affected the levels of Na⁺, K⁺-ATPase, glutamate dehydrogenase, aspartate aminotransferase, amino acids, and proteins, suggesting an effect on amino acid metabolism. In conclusion, the intermediate metabolism of E. maclovinus presents variations according to the time of day, with an increased metabolism during the morning and decreased metabolism as the day progresses, especially at the hepatic level. The gill tissue, despite not being a metabolic organ, presents feeding-dependent variations in its metabolism. Additional studies will be required to corroborate if coordinating a feeding strategy during the first hours of the day when metabolism is greater would improve the growth of E. maclovinus.

Alterations of Cortisol and Melatonin Production by the Theca Interna Cells of Porcine Cystic Ovarian Follicles

Simple Summary

The mechanism of follicular cyst formation is largely unknown but changes in follicular composition are known to be involved. In particular, there is abnormal hormone secretion in cystic follicles. Here, we found there was disruption of hormone secretion in the fluid of cystic follicles in sows. The glucocorticoid receptor was highly expressed, and the melatonin receptor was weakly expressed in cystic follicles compared with control follicles. Thus, secretion of steroid hormones in cystic follicles is disrupted and disturbances in signaling via cortisol and melatonin are involved in the development of follicular cysts in sows.

Abstract

(1) Background: Cortisol and melatonin (MT) act in regulating follicular development. We hypothesized that abnormal levels of cortisol, MT, and steroids in theca interna cells might be involved in the development of follicular cysts in sows. (2) Methods: To test this hypothesis, we measured the mRNA levels of enzymes involved in steroid hormone synthesis, the glucocorticoid receptor (GR), and melatonin receptors (MTRs) in theca interna cells of cystic and normal porcine follicles. (3) Results: The concentrations of estradiol, progesterone, and cortisol were greater in cystic follicles than in control ones (p = 0.034, p = 0.020, p = 0.000), but the concentration of MT was significantly lower (p = 0.045). The levels of GR, 11β-HSD1, and 11β-HSD2 were higher in cystic follicles than in control l follicles. MT types 1 and 2 were significantly lower in cystic follicles (p < 0.05). The mRNA expression levels of genes encoding the steroid hormone synthesis enzymes, steroidogenic acute regulatory protein (StAR), recombinant cytochrome P45011A1 (CYP11A1), and 3β-hydroxysteroid dehydrogenase (3β-HSD) in theca interna cells of cystic follicles were significantly higher than in control follicles. Thus, there was disruption of hormone secretion in the fluid of cystic follicles in sows. (4) Conclusions: The levels of steroid hormones, cortisol and MT are disrupted in porcine cystic follicles.

The interplay of pineal hormones and socioeconomic status leading to colorectal cancer disparity

Colorectal cancer (CRC) is the third leading cause of cancer-related deaths in the United States. Despite increased screening options and state-of-art treatments offered in clinics, racial differences remain in CRC. African Americans (AAs) are disproportionately affected by the disease; the incidence and mortality are higher in AAs than Caucasian Americans (CAs). At the time of diagnosis, AAs more often present with advanced stages and aggressive CRCs, primarily accounting for the racial differences in therapeutic outcomes and mortality. The early incidence of CRC in AAs could be attributed to race-specific gene polymorphisms and lifestyle choices associated with socioeconomic status (SES). Altered melatonin-serotonin signaling, besides the established CRC risk factors (age, diet, obesity, alcoholism, and tobacco use), steered by SES, glucocorticoid, and Vitamin D status in AAs could also account for the early incidence in this racial group. This review focuses on how the lifestyle factors, diet, allelic variants, and altered expression of specific genes could lead to atypical serotonin and melatonin signaling by modulating the synthesis, secretion, and signaling of these pineal hormones in AAs and predisposing them to develop more aggressive CRC earlier than CAs. Crosstalk between gut microbiota and pineal hormones and its impact on CRC pathobiology is addressed from a race-specific perspective. Lastly, the status of melatonin-focused CRC treatments, the need to better understand the perturbed melatonin signaling, and the potential of pineal hormone-directed therapeutic interventions to reduce CRC-associated disparity are discussed.

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.

Effects of a temperature rise on melatonin and thyroid hormones during smoltification of Atlantic salmon, Salmo salar

Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the salmon's life cycle, including increased gill Na⁺/K⁺-ATPase activity (NKA) and plasma hormone levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish have adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ + 4° C) combinations, on gill NKA, plasma free triiodothyronine (T3) and thyroxine (T4), and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gill NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormone levels displayed day-night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered the MEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on the endocrine system of S. salar, and indicate that climate change might compromise salmon's time "deciphering" during smoltification, downstream migration and seawater residence.

Melatonin receptor agonist Piper betle L. ameliorates dexamethasone-induced early life stress in adult zebrafish

Early life exposure to stress has been suggested to be a crucial factor for the development of the brain and its functions. It is well documented that childhood stress is a risk factor for sleep problems in adulthood. Piper betle L. leaf extract (PB) has been used in several traditional medicines to cure various ailments. Recently, PB has been proved to have antidepressant activity. The literature suggests that antidepressants affect the synthesis and release of melatonin through several mechanisms. Thus, this study investigated the potential role of PB for the treatment of sleep disruption after early life stress exposure. Firstly, dexamethasone (DEX) (2 and 20 mg/l for 24 h) was administered to zebrafish larvae on the 4th day post-fertilization (dpf) to induce early life stress. The effects of stress on behaviour during adulthood, melatonin level and stress-related gene expression (nfkb) in the brain were then studied. Next, the possible role of PB (10 and 30 mg/Kg) was studied by measuring its effect on behaviour and by quantifying the expression levels of several melatonin-related (MT1, MT2, aanat1, aanat2) and stress-related (nfkb) genes by qPCR. DEX-treated zebrafish exhibited anxious behaviour, along with a lower level of melatonin and a higher mRNA expression of nfkb. After treatment with PB, a similar effect on behaviour and gene expression levels as the melatonin treatment group (10 mg/kg; positive control) was seen in adult zebrafish. These molecular confirmations of the observed behavioural effects of the PB indicate a possible role in the treatment of early life stress-induced sleep disruption.

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

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

Cortisol and melatonin in the cutaneous stress response system of fish

The stress hormone cortisol, together with antioxidants, melatonin (Mel) and its biologically active metabolites, 5-methoxykynuramines, including AFMK, set up a local stress response system in mammalian skin. Our in vitro study of the European flounder (Platichthys flesus) was designed to examine whether Mel and AFMK would respond to cortisol while a glucocorticoid is added to the incubation medium. The concentrations of cortisol in the incubation medium mimic plasma cortisol levels seen in fish exposed to different types of stresses such as handling, confinement, high density, food-deprivation or air-exposure. We measured Mel and AFMK in skin explants and culture media using high-performance liquid chromatography (HPLC) with fluorescence detection. We also analysed melanosome response (dispersion/aggregation) in the explants subjected to the different treatments. Cortisol stimulated the release of Mel and AFMK from skin explants in a dose-dependent manner. Melanosome dispersion and a darkening of the skin explants were observed after incubation with cortisol. This study is the first to demonstrate the interrelationship between cortisol and Mel/AFMK in fish skin. Our data strongly suggest that the cutaneous stress response system (CSRS) is present in fish. The question remains whether Mel, AFMK or cortisol are synthetized locally in fish skin and/or transported by the bloodstream. The presence of the CSRS should be taken into account during elaboration of new indicators of fish welfare both in aquaculture and in the wild.

Analyses of the cellular clock gene expression in peripheral tissue, caudal fin, in the Japanese flounder, Paralichthys olivaceus

Understanding the systems for maintaining the circadian rhythms that give organisms the flexibility to adapt to environmental changes is important in both aquaculture and fish chronobiology, because nursery lighting conditions can affect the survival and growth rates of larvae. We previously demonstrated that in flounder, the suprachiasmatic nucleus (SCN) exhibits daily rhythm in per2 expression, in sharp contrast to zebrafish, in which the SCN does not exhibit clear per2 expression rhythm. To examine whether a hierarchy exists in systems that maintain the expression rhythm of peripheral clock genes in flounder, in the present study we analyzed the in vivo and in vitro expression of three clock genes, per2, per1, and cry1, in the caudal fin and the effects of cortisol and melatonin administration on the expression of each clock gene. In vivo, the fin maintained a daily expression rhythm of all three genes, even in 24-h darkness (DD) when shifted from 12-h light:12-h dark (LD) conditions, but fin explants lost the expression rhythm after a short time of tissue culture, even under LD conditions. Cortisol, but not melatonin, significantly upregulated the expression of the three clock genes in fin both in vitro and in vivo. Therefore, we hypothesize that the SCN-pituitary-adrenal cortex pathway plays a role in the oscillation of the peripheral clock in flounder. However, in vivo, peak expression of per2 and cry1 was shifted 2-4h earlier under DD conditions, and their expression was upregulated in response to short exposures to light when larvae were kept under DD conditions. Therefore, we also hypothesize that in addition to the SCN, a light-responsive coordinating factor also functions in photo-entrainment of the peripheral clock in flounder.

Interplay between the endocrine and circadian systems in fishes

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

Mianserin affects alarm reaction to conspecific chemical alarm cues in Nile tilapia

In this study, I show that mianserin, a chemical with serotonin and adrenoceptor antagonist activities, increases fish vulnerability to a potential predator threat, when prey fish must deal with this threat based on conspecific chemical alarm cues. For that, I evaluated whether mianserin, diluted in the water, influences the behavioral responses of Nile tilapia (Oreochromis niloticus) to conspecific skin extract (chemical alarm cues). I found that, while mianserin did not abolished antipredator responses, this drug mitigates some components of this defensive reaction. Thus, a potential decrease in serotonin and adrenergic activities reduces the ability of dealing with predators when perceiving conspecific chemical alarm cues.

Sexual plasticity: A fishy tale

Teleost fish exhibit remarkably diverse and plastic patterns of sexual development. One of the most fascinating modes of plasticity is functional sex change, which is widespread in marine fish including species of commercial importance; however, the regulatory mechanisms remain elusive. In this review, we explore such sexual plasticity in fish, using the bluehead wrasse (Thalassoma bifasciatum) as the primary model. Synthesizing current knowledge, we propose that cortisol and key neurochemicals modulate gonadotropin releasing hormone and luteinizing hormone signaling to promote socially controlled sex change in protogynous fish. Future large-scale genomic analyses and systematic comparisons among species, combined with manipulation studies, will likely uncover the common and unique pathways governing this astonishing transformation. Revealing the molecular and neuroendocrine mechanisms underlying sex change in fish will greatly enhance our understanding of vertebrate sex determination and differentiation as well as phenotypic plasticity in response to environmental influences. Mol. Reprod. Dev. 84: 171-194, 2017. © 2016 Wiley Periodicals, Inc.

Innate immune defenses exhibit circadian rhythmicity and differential temporal sensitivity to a bacterial endotoxin in Nile tilapia (Oreochromis niloticus)

The present study investigated the daily dynamics of humoral immune defenses and the temporal influence in the sensitivity of these responses to a bacterial endotoxin in Nile tilapia (Oreochromis niloticus). The first experiment subjected the fish to two photoperiod conditions, 12L:12D (LD) and 0L:24D (DD), for 20 days to characterize the rhythms of humoral immunity. Serum alkaline phosphatase (ALP), lysozyme (LYZ), peroxidase (PER) and protease (PRO) exhibited significant rhythmicity under LD but not in DD. No significant rhythms were observed in esterase (ESA) and anti-protease (ANTI) in both photoperiod conditions. Fish reared under LD were subsequently subjected to DD while the group previously under DD was exposed to LD, and this carried on for 3 days before another set of samples was collected. Results revealed that the rhythms of LYZ, PER and PRO but not ALP persisted when photoperiod was changed from LD to DD. Nonetheless, immune parameters remained arrhythmic in the group subjected from DD to LD. Cluster analysis of the humoral immune responses under various light conditions revealed that each photic environment had distinct daily immunological profile. In the second experiment, fish were injected with bacterial endotoxin lipopolysaccharide (LPS) either at ZT3 (day) or at ZT15 (night) to evaluate the temporal sensitivity of humoral immunity to a pathogen-associated molecular pattern. The results demonstrated that responses to LPS were gated by the time of day. LPS significantly modulated serum ALP and ANTI activities but only when the endotoxin was administered at ZT3. Serum LYZ and PER were stimulated at both injection times but with differing response profiles. Modulated LYZ activity was persistent when injected at ZT3 but transient when LPS was applied at ZT15. The magnitude of LPS-induced PER activity was higher when the endotoxin was delivered at ZT3 versus ZT15. It was further shown that plasma cortisol was significantly elevated but only when LPS was administered at ZT3. On the other hand, plasma melatonin was significantly affected by LPS injection but only when exposed at ZT15. Taken together, this study shows that several key components of humoral immunity in tilapia exhibit circadian rhythms and adapt to photoperiodic changes. Further, results of the bacterial endotoxin challenge suggest that responsiveness of serum humoral factors to a biological insult is likely mediated by the time of day, highlighting the importance of circadian rhythm in the immunological functions of fish.

Development of a label-free immunosensor system for detecting plasma cortisol levels in fish

Fishes display a wide variation in their physiological responses to stress, which is clearly evident in the plasma corticosteroid changes, chiefly cortisol levels in fish. In the present study, we describe a novel label-free immunosensor for detecting plasma cortisol levels. The method is based on immunologic reactions and amperometric measurement using cyclic voltammetry. For the immobilization of the antibody on the surface of sensing electrode, we used a self-assembled monolayer of thiol-containing compounds. Using this electrode, we detect the CV signal change caused by the generation of antigen-antibody complex. The immunosensor showed a response to cortisol levels, and the anodic peak value linearly decreased with a correlation coefficient of 0.990 in diluted plasma. The specificity of the label-free immunosensor system was investigated using other steroid hormones, such as 17α, 20β-dihydroxy-4-pregnen-3-one, progesterone, estriol, estradiol, and testosterone. The specific detection of cortisol was suggested by a minimal change from -0.32 to 0.51 μA in the anodic peak value of the other steroid hormones. The sensor system was used to determine the plasma cortisol levels in Nile tilapia (Oreochromis niloticus), and the results were compared with those of the same samples determined using the conventional method (ELISA). A good correlation was obtained between values determined using both methods (correlation coefficient 0.993). These findings suggest that the proposed label-free immunosensor could be useful for rapid and convenient analysis of cortisol levels in fish plasma samples.

Diel patterns of baseline glucocorticoids and stress responsiveness in a teleost fish (bluegill, Lepomis macrochirus)

Little is known about whether glucocorticoids (GC) and GC responsiveness vary on a diel basis in the wild, especially for fish. Using bluegill (Lepomis macrochirus Rafinesque, 1819) as a model freshwater teleost fish, we tested whether baseline concentration and stress responsiveness of GCs (i.e., plasma glucose and cortisol) varied over a 24 h period. Blood samples from lake-dwelling wild bluegill were obtained across six periods representing a complete circadian cycle to determine GC levels in newly captured fish (i.e., within 3 min of capture; baseline), the maximum value (maximum) 45 min following exposure to a standardized aerial exposure stressor, and determining responsiveness (by subtracting minimum from maximum). Our results revealed that baseline glucose concentration did not vary on a diel basis, whereas baseline cortisol concentration did. Maximum and stress-induced glucose responsiveness varied significantly among several time periods with lowest values recorded at midnight and higher values at mid-day. Maximum and stress-induced cortisol responsiveness were consistent across time periods. Collectively, these data suggest that baseline concentrations and stress-induced values of GCs in a freshwater temperate teleost fish tend to be consistent across diel periods such that there is apparently an absence of strong GC diel patterns.

Stress inhibition of melatonin synthesis in the pineal organ of rainbow trout (Oncorhynchus mykiss) is mediated by cortisol

Cortisol has been suggested to mediate the effect of stress on pineal melatonin synthesis in fish. Therefore, we aimed to determine how pineal melatonin synthesis is affected by exposing rainbow trout to different stressors, such as hypoxia, chasing and high stocking density. In addition, to test the hypothesis that cortisol is a mediator of such stress-induced effects, a set of animals were intraperitoneally implanted with coconut oil alone or containing cortisol (50 mg kg(-1) body mass) and sampled 5 or 48 h post-injection at midday and midnight. The specificity of such effect was also assessed in cultured pineal organs exposed to cortisol alone or with the general glucocorticoid receptor antagonist, mifepristone (RU486). Stress (in particular chasing and high stocking density) affected the patterns of plasma and pineal organ melatonin content during both day and night, with the greatest reduction occurring at night. The decrease in nocturnal melatonin levels in the pineal organ of stressed fish was accompanied by increased serotonin content and decreased AANAT2 enzymatic activity and mRNA abundance. Similar effects on pineal melatonin synthesis to those elicited by stress were observed in trout implanted with cortisol for either 5 or 48 h. These data indicate that stress negatively influences the synthesis of melatonin in the pineal organ, thus attenuating the day-night variations of circulating melatonin. The effect might be mediated by increased cortisol, which binds to trout pineal organ-specific glucocorticoid receptors to modulate melatonin rhythms. Our results in cultured pineal organs support this. Considering the role of melatonin in the synchronization of daily and annual rhythms, the results suggest that stress-induced alterations in melatonin synthesis could affect the availability of fish to integrate rhythmic environmental information.

A short note on how tied fights affect cortisol levels

Previous work exploring the interrelationships between sex steroids (e.g. androgens, testosterones and 11-ketotestosterones) and social behavior in teleosts suggest that mirror-elicited aggression in cichlid fish may not trigger a hormonal response. Using the Mozambique tilapia (Oreochromis mossambicus) to analyze immune responses as a result of social stress, we measured levels of cortisol and melatonin using Enzyme-Linked Immunosorbent Assay (ELISA) assays. In this work, we demonstrated that cortisol concentrations are significantly lower yet the levels of melatonin remain unchanged in tilapia that are fighting their mirror image. Our results suggested that in tied fights, certain hormone levels remain unchanged (e.g. androgens) due to the lack of melatonin induction.

The effects of low levels of light at night upon the endocrine physiology of western scrub-jays (Aphelocoma californica)

Florida scrub-jays (Aphelocoma coerulescens) in the suburbs breed earlier than jays in native habitat. Amongst the possible factors that influence this advance (e.g., food availability, microclimate, predator regime, etc.), is exposure to artificial lights at night (LAN). LAN could stimulate the reproductive axis of the suburban jays. Alternatively, LAN could inhibit pineal melatonin (MEL), thus removing its inhibitory influence on the reproductive axis. Because Florida scrub-jays are a threatened species, we used western scrub-jays (Aphelocoma californica) to investigate the effects of LAN upon reproductive hormones and melatonin. Jays were held under conditions in which the dark-phase of the light:dark cycle was without illumination and then under low levels of LAN. Under both conditions, birds were exposed first to short-days (9.5L:14.5D) that were gradually increased to long-days (14.5L:9.5D). At various times, blood samples were collected during the light part of the cycle to measure reproductive hormones (luteinizing hormone, LH; testosterone, T; and estradiol, E2 ). Similarly, samples to assess melatonin were collected during the dark. In males, LAN caused a depression in LH levels and levels were ∼4× greater under long- than short-days. In females, there was no effect of LAN or photoperiod upon LH. LAN resulted in depressed T levels in females, although there was no effect on T in males. E2 levels in both sexes were lower under LAN than under an unlighted dark-phase. Paradoxically, MEL was higher in jays under LAN, and under long-days. MEL did not differ by sex. LAN disrupted the extraordinarily strong correlation between T and E2 that existed under unlighted nocturnal conditions. Overall, our findings fail to support the hypothesis that LAN stimulates the reproductive axis. Rather, the data demonstrate that LAN tends to inhibit reproductive hormone secretion, although not in a consistent fashion between the sexes.

Glucocorticoid receptor blockade inhibits brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus

When animals are under stress, glucocorticoids commonly inhibit adult neurogenesis by acting through glucocorticoid receptors (GRs). However, in some cases, conditions that elevate glucocorticoids promote adult neurogenesis, and the role of glucocorticoid receptors in these circumstances is not well understood. We examined the involvement of GRs in social enhancement of brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus. In this species, long-term social interaction simultaneously elevates plasma cortisol, enhances brain cell addition and increases production of aggressive electrocommunication signals ("chirps"). We implanted isolated and paired fish with capsules containing nothing (controls) or the GR antagonist, RU486, recorded chirp production and locomotion for 7d, and measured the density of newborn cells in the periventricular zone. Compared to isolated controls, paired controls showed elevated chirping in two phases: much higher chirp rates in the first 5h and moderately higher nocturnal rates thereafter. Treating paired fish with RU486 reduced chirp rates in both phases to those of isolated fish, demonstrating that GR activation is crucial for socially induced chirping. Neither RU486 nor social interaction affected locomotion. RU486 treatment to paired fish had a partial effect on cell addition: paired RU486 fish had less cell addition than paired control fish but more than isolated fish. This suggests that cortisol activation of GRs contributes to social enhancement of cell addition but works in parallel with another GR-independent mechanism. RU486 also reduced cell addition in isolated fish, indicating that GRs participate in the regulation of cell addition even when cortisol levels are low.

Neural regulation of dark-induced abundance of arylalkylamine N-acetyltransferase (AANAT) and melatonin in the carp (Catla catla) pineal: an in vitro study

In all the vertebrates, synthesis of melatonin and its rhythm-generating enzyme arylalkylamine N-acetyltransferase (AANAT) reaches its peak in the pineal during the night in a daily light-dark cycle, but the role of different neuronal signals in their regulation were unknown for any fish. Hence, the authors used specific agonist and antagonists of receptors for different neuronal signals and regulators of intracellular calcium (Ca(2+)) and adenosine 3',5'-cyclic monophosphate (cAMP) in vitro to study their effects on the abundance of AANAT and titer of melatonin in the carp (Catla catla) pineal. Western blot analysis followed by quantitative analysis of respective immunoblot data for AANAT protein, radioimmunoassay of melatonin, and spectrophotometric analysis of Ca(2+) in the pineal revealed stimulatory effects of both adrenergic (α(1) and β(1)) and dopaminergic (D(1)) agonists and cholinergic (both nicotinic and muscarinic) antagonists, inhibition by both adrenergic and dopaminergic antagonists and cholinergic agonists, but independent of the influence of any agonists or antagonists of α(2)-adrenergic receptors. Band intensity of AANAT and concentration of melatonin in the pineal were also enhanced by the intracellular calcium-releasing agent, activators of both calcium channel and adenylate cyclase, and phophodiesterase inhibitor, but suppressed by inhibitor of calcium channel and adenylate cyclase as well as activator of phophodiesterase. Moreover, an inhibitory effect of light on the pineal AANAT and melatonin was blocked by both cAMP and proteasomal proteolysis inhibitor MG132. Collectively, these data suggest that dark-induced abundance of AANAT and melatonin synthesis in the carp pineal are a multineuronal function, in which both adrenergic (α(1) and β(1), but not α(2)) and dopaminergic signals are stimulatory, whereas cholinergic signals are inhibitory. This study also provides indications, though arguably not conclusive evidence, that in either case the neuronal mechanisms follow a signal-transduction pathway in which Ca(2+) and cAMP may act as the intracellular messengers. It also appears that proteasomal proteolysis is a conserved event in the regulation of AANAT activity in vertebrates.

Changes in plasma melatonin levels and pineal organ melatonin synthesis following acclimation of rainbow trout (Oncorhynchus mykiss) to different water salinities

Melatonin has been suggested to play a role in fish osmoregulation, and in salmonids has been related to the timing of adaptive mechanisms during smolting. It has been described that acclimation to different environmental salinities alters levels of circulating melatonin in a number of fish species, including rainbow trout. However, nothing is known regarding salinity effects on melatonin synthesis in the pineal organ, which is the main source of rhythmically produced and secreted melatonin in blood. In the present study we have evaluated, in rainbow trout, the effects of acclimation to different salinities on day and night plasma melatonin values and pineal organ melatonin synthesis. Groups of freshwater (FW)-adapted rainbow trout were placed in tanks with four different levels of water salinity (FW, 6, 12, 18 p.p.t.; parts per thousand) and maintained for 6 h or 5 days. Melatonin content in plasma and pineal organs, as well as the pineal content of serotonin (5-HT) and its main oxidative metabolite (5-hydroxyindole-3-acetic acid; 5-HIAA) were measured by high performance liquid chromatography. In addition, day–night changes in pineal organ arylalkylamine N-acetyltransferase (AANAT2) activity and aanat2 gene expression were studied. Plasma osmolalities were found to be higher in rainbow trout exposed to all salinity levels compared with the control FW groups. A salinity-dependent increase in melatonin content was found in both plasma and pineal organs. This effect was observed during the night, and was related to an increase in aanat2 mRNA abundance and AANAT2 enzyme activity, both of which also occurred during the day. Also, the levels of indoles (5-HT, 5-HIAA) in the pineal organ were negatively affected by increasing water salinity, which seems to be related to the higher recruitment of 5-HT as a substrate for the increased melatonin synthesis. A stimulatory effect of salinity on pineal aanat2 mRNA expression was also identified. These results indicate that increased external salinity promotes melatonin synthesis in the pineal organ of rainbow trout by enhancing synthesis of AANAT protein independently of its regulation by light. The possibility that pineal melatonin is a target for hormones involved in the response of fish to osmotic challenge is discussed, as well as the potential role of melatonin in the timing of osmoregulatory processes.