Current knowledge on the melatonin system in teleost fish

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.

Melatonin induces follicle maturation in Danio rerio

Most organisms modulate their reproductive activity responding to day length by the nocturnal release of melatonin by the pineal gland. This hormone is also responsible for synchronizing reproduction with specific external environment stimuli in order to optimize reproductive success.The aim of this study was to establish the effect of melatonin on zebrafish reproduction.Adult females were daily exposed, via water, to two different doses (100 nM and 1 µM) of melatonin. Melatonin led to an increase of the Gonado Somatic Index (GSI) associated with the increase of eggs production, and the raise of gene and protein levels of vitellogenin (VTG) and estradiol receptor α (ERα) in the liver. The ability of melatonin to increase fecundity was consistent with a significant increase of gene transcription of kiss 1, kiss 2, gnrh3, in the brain, and lh in the pituitary, while in the ovary (in class IIIB follicles), with a significant decrease of two genes codifying for intra-ovarian regulators of premature oocyte maturation, the tgfβ1 and the bmp15. The reduction in the expression of these two genes was concomitant with the increase of lhr and a modulation of mprα and mprβ gene transcription, whose proteins are involved in oocyte maturation. Melatonin also exerted a direct action on follicles as shown by the increase of the oocytes undergoing to germinal vesicle break down (GVBD) and modulated mpr α and β gene expression in the in vitro exposure.These data highlight the effects of melatonin in promoting zebrafish reproduction exerting its effects either in the brain-pituitary and in the gonads.

Molecular evolution of multiple arylalkylamine N-acetyltransferase (AANAT) in fish

Arylalkylamine N-acetyltransferase (AANAT) catalyzes the transfer of an acetyl group from acetyl coenzyme A (AcCoA) to arylalkylamines, including indolethylamines and phenylethylamines. Multiple aanats are present in teleost fish as a result of whole genome and gene duplications. Fish aanat1a and aanat2 paralogs display different patterns of tissue expression and encode proteins with different substrate preference: AANAT1a is expressed in the retina, and acetylates both indolethylamines and phenylethylamines; while AANAT2 is expressed in the pineal gland, and preferentially acetylates indolethylamines. The two enzymes are therefore thought to serve different roles. Here, the molecular changes that led to their specialization were studied by investigating the structure-function relationships of AANATs in the gilthead seabream (sb, Sperus aurata). Acetylation activity of reciprocal mutated enzymes pointed to specific residues that contribute to substrate specificity of the enzymes. Inhibition tests followed by complementary analyses of the predicted three-dimensional models of the enzymes, suggested that both phenylethylamines and indolethylamines bind to the catalytic pocket of both enzymes. These results suggest that substrate selectivity of AANAT1a and AANAT2 is determined by the positioning of the substrate within the catalytic pocket, and its accessibility to catalysis. This illustrates the evolutionary process by which enzymes encoded by duplicated genes acquire different activities and play different biological roles.

Melatonin effects on gut motility are independent of the relaxation mediated by the nitrergic system in the goldfish

Melatonin is a key neuroendocrine transducer in the circadian organization of vertebrates. However, its role in gastrointestinal physiology has not been explored in depth. In goldfish, a role for melatonin as a modulator of intestinal motility has been reported, whereby it attenuates the cholinergic contraction. The aim of the present work was to investigate this relaxation induced by melatonin in the gut smooth muscle of the goldfish, studying the possible involvement of nitric oxide. An in vitro model of isolated goldfish intestine was used to test the effects on intestinal motility. The addition of melatonin (10 pM-100 μM) to the organ bath relaxed acetylcholine- and serotonin-stimulated gut strips, but no effect was observed on KCl-contracted preparations. The addition of L-NAME (nitric oxide synthase inhibitor) increased the amplitude of the spontaneous slow waves, while sodium nitroprusside (SNP, nitric oxide donor) abolished them. All these results support a role for the nitrergic system in goldfish gut motility. However, neither L-NAME, nor SNP nor the nitric oxide precursor, l-arginine, modified the melatonin relaxing effect. These results highlight the existence of a basal nitrergic tone in the gut of goldfish, where melatonin would exert a calcium-dependent, nitric oxide-independent relaxing effect on serotonergic and cholinergic contraction.

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.

The contribution of lower vertebrate animal models in human reproduction research

Many advances have been carried out on the estrogens, GnRH and endocannabinoid system that have impact in the reproductive field. Indeed, estrogens, the generally accepted female hormones, have performed an unsuspected role in male sexual functions thanks to studies on non-mammalian vertebrates. Similarly, these animal models have provided important contributions to the identification of several GnRH ligand and receptor variants and their possible involvement in sexual behavior and gonadal function regulation. Moreover, the use of non-mammalian animal models has contributed to a better comprehension about the endocannabinoid system action in several mammalian reproductive events. We wish to highlight here how non-mammalian vertebrate animal model research contributes to advancements with implications on human health as well as providing a phylogenetic perspective on the evolution of reproductive systems in vertebrates.

Diurnal expression of clock genes in pineal gland and brain and plasma levels of melatonin and cortisol in Atlantic salmon parr and smolts

In Atlantic salmon, the preadaptation to a marine life, i.e., parr-smolt transformation, and melatonin production in the pineal gland are regulated by the photoperiod. However, the clock genes have never been studied in the pineal gland of this species. The aim of the present study was to describe the diurnal expression of clock genes (Per1-like, Cry2, and Clock) in the pineal gland and brain of Atlantic salmon parr and smolts in freshwater, as well as plasma levels of melatonin and cortisol. By employing an out-of-season smolt production model, the parr-smolt transformation was induced by subjecting triplicate groups of parr to 6 wks (wks 0 to 6) under a 12 h:12 h light-dark (LD) regime followed by 6 wks (wks 6 to 12) of continuous light (LL). The measured clock genes in both pineal gland and brain and the plasma levels of melatonin and cortisol showed significant daily variations in parr under LD in wk 6, whereas these rhythms were abolished in smolts under LL in wk 12. In parr, the pineal Per1-like and Cry2 expression peaked in the dark phase, whereas the pineal Clock expression was elevated during the light phase. Although this study presents novel findings on the clock gene system in the teleost pineal gland, the role of this system in the regulation of smoltification needs to be studied in more detail.

Effect of continuous light on daily levels of plasma melatonin and cortisol and expression of clock genes in pineal gland, brain, and liver in atlantic salmon postsmolts

Continuous light is a common practice in salmon farming, where it is used to enhance growth, induce smoltification, and regulate puberty. However, knowledge about how different tissues receive information about daylength is limited. The aim of the present study was to evaluate the daily expression of clock (Per1-like, Cry2, and Clock), the nuclear transcription factor (peroxisome proliferator-activated receptor, PPAR; CCAAT/enhancer binding protein, C/EBP), and the endoplasmic reticulum (ER) stress (protein disulfide isomerase associated 3, PDIA3) genes in the pineal gland, brain, and liver of Atlantic salmon postsmolts reared under 12-h light:12-h dark (LD) regimes or under continuous light (LL) for 6 wks following transfer to seawater. All measured clock mRNAs displayed daily variations in one or more organs under LD, as well as plasma levels of melatonin. Similar variations were noted in the liver c/ebpα, pineal c/ebpδ, and pdia3 mRNAs. Under LL, the clock and nuclear transcription factor mRNAs did not show any daily variation in the studied organs, with the exception of pineal pdia3. Furthermore, LL had the opposite effect on the levels of melatonin and cortisol, as observed by the increase in pineal Clock, Per2, pparα, and c/ebpα and c/ebpδ mRNAs and decrease in liver Clock, Per2, and pparα mRNAs compared to those under LD. The present findings show that the expression of clock genes is affected by the light across organs and that there is a relation between PPAR, C/EBP, and clock mRNAs; however, the functional role of the individual nuclear transcription factors related to this observation remains to be established in the pineal gland and liver. (Author correspondence: Tihu@nifes.no ).

Pharmaceuticals as neuroendocrine disruptors: lessons learned from fish on Prozac

Pharmaceuticals are increasingly detected in a variety of aquatic systems. One of the most prevalent environmental pharmaceuticals in North America and Europe is the antidepressant fluoxetine, a selective serotonin reuptake inhibitor (SSRI) and the active ingredient of Prozac. Usually detected in the range below 1 μg/L, fluoxetine and its active metabolite norfluoxetine are found to bioaccumulate in wild-caught fish, particularly in the brain. This has raised concerns over potential disruptive effects of neuroendocrine function in teleost fish, because of the known role of serotonin (5-HT) in the modulation of diverse physiological processes such as reproduction, food intake and growth, stress and multiple behaviors. This review describes the evolutionary conservation of the 5-HT transporter (the therapeutic target of SSRIs) and reviews the disruptive effects of fluoxetine on several physiological endpoints, including involvement of neuroendocrine mechanisms. Studies on the goldfish, Carassius auratus, whose neuroendocrine regulation of reproduction and food intake are well characterized, are described and represent a reliable model to study neuroendocrine disruption. In addition, fish studies investigating the effects of fluoxetine, not only on reproduction and food intake, but also on stress and behavior, are discussed to complement the emerging picture of neuroendocrine disruption of physiological systems in fish exposed to fluoxetine. Environmental relevance and key lessons learned from the effects of the antidepressant fluoxetine on fish are highlighted and may be helpful in designing targeted approaches for future risk assessments of pharmaceuticals disrupting the neuroendocrine system in general.

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

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

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

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

Dopaminergic activity in the brain of a tropical wrasse in response to changes in light and hydrostatic pressure

Many tropical wrasses show a daily pattern of spawning with gamete release typically near daytime high tide. The environmental cues the fish obtains from day-night and tidal cycles to ensure spawning synchrony and how those cues are transduced, however, are not fully understood. To gain insight into these issues, the involvement of monoamines in mediating endogenous day-night and tidal rhythms in the threespot wrasse, Halichoeres trimaculatus, were examined. Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC, a metabolite of DA), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of 5-HT) in the brain of the fish were measured with high-performance liquid chromatography and electrochemical detection. DOPAC and the metabolic rate of DA activity (DOPAC/DA) were found to increase during the day and decrease during the night for fish held under a natural photoperiod. Fish acclimated to a 12:12 light-dark cycle and to constant dark conditions exhibited similar changes, whereas fish acclimated to constant light conditions exhibited little or no change. Intraperitoneal injection of melatonin resulted in a significant reduction in DOPAC/DA. Furthermore, DOPAC/DA was significantly lower in fish held at 3m compared to 0m depth, suggesting that hydrostatic pressure influences DA metabolic rate. These results indicate that light and hydrostatic pressure control dopaminergic turnover in the brain of threespot wrasse. Day-night and tidal changes in these two factors therefore may be the main environmental cues the fish uses to synchronize its spawning activity.

The Highly conserved gonadotropin-releasing hormone-2 form acts as a melatonin-releasing factor in the pineal of a teleost fish, the european sea bass Dicentrarchus labrax

With the exception of modern mammals, most vertebrate species possess two GnRH genes, GnRH-1 and GnRH-2. In addition, in many teleost fish, there is a third gene called GnRH-3. If the main function of GnRH-1 is unambiguously to stimulate gonadotropin release, the other two GnRH forms still lack clear functions. This is particularly true for the highly conserved GnRH-2 that encodes chicken GnRH-II. This GnRH variant is consistently expressed in neurons of the dorsal synencephalon in most vertebrate groups but still has no clear functions supported by anatomical, pharmacological, and physiological data. In this study performed on a perciform fish, the European sea bass, we show for the first time that the pineal organ receives GnRH-2-immunoreactive fibers originating from the synencephalic GnRH-2 neurons. This was shown through a combination of retrograde tracing and immunohistochemistry, using highly specific antibodies. Supporting the presence of GnRH-2 functional targets, RT-PCR data together with the in situ hybridization studies showed that the sea bass pineal gland strongly expressed a GnRH receptor (dlGnRHR-II-2b) with clear selectivity for GnRH-2 and, to a lesser extent, the dlGnRHR-II-1a subtype. Finally, in vitro and in vivo experiments demonstrate stimulatory effects of GnRH-2 on nocturnal melatonin secretion by the sea bass pineal organ. Altogether, these data provide, for the first time in a vertebrate species, converging evidence supporting a role of GnRH-2 in the modulation of fish pineal functions.