The Role of Melatonin as a Hormone and an Antioxidant in the Control of Fish Reproduction

Can photoperiod improve growth performance and antioxidant responses of pacu (Piaractus mesopotamicus) reared in recirculation aquaculture systems?

The present study investigated the best photoperiod for culturing pacu (Piaractus mesopotamicus) in recirculation aquaculture systems (RAS) based on its growth performance and hematological and oxidative stress responses. Juveniles (∼ 5 g) were subjected to five treatments (in triplicate): 24 L (light):0D (dark), 15 L: 09D, 12 L:12D, 9 L:15D, and 0 L:24D for 45 days. A total of 225 pacu individuals were randomly distributed among 15 tanks of 210 L (n = 15 per tank). Zootechnical, hematological (glucose, lactate, hematocrit, and hemoglobin), and antioxidant and oxidative stress parameters (glutathione S-transferase (GST), total antioxidant capacity against peroxyl radicals (ACAP), and lipid peroxidation (LPO) were analyzed. The zootechnical parameters (e.g., weight gain, Fulton's condition factor, and specific growth rate) were better and worse with 9 L:15D and 24 L:0D photoperiods, respectively. The hepatosomatic index was higher and lower in the 0 L:24D and 9 L:15D photoperiods. Blood lactate levels and antioxidant and oxidative stress responses were increased in the longest photoperiods (15 L:9D and 24 L:0D). In contrast, the treatments that showed lower oxidative damage (liver, gills, brain, and muscle) were 9 L:15D and 12 L:12D. In conclusion, manipulating artificial light is one way to improve fish growth and health, where the best photoperiod for pacu farming in RAS is 9 L:15D.

Effects of dietary melatonin supplementation on growth performance and intestinal health of rice field eel (Monopterus albus)

The objective was to assess the impact of melatonin supplementation on the growth performance and intestinal health of rice field eel, Monopterus albus. Three hundred and sixty fish (28.46 ± 0.24 g) were fed five diets supplemented with melatonin of 0, 30, 60, 120, and 240 mg/kg for 70 days. The study found that the variables FBW, WGR, SGR, and FCR exhibited a statistically significant quadratic relationship (P < 0.05) with the dietary melatonin concentrations, and the highest FBW, WGR and SGR as well as lowest FCR were observed in the 120 mg/kg melatonin group, digestive enzymes activities (such as amylase, trypsin, and lipase) also had significant quadratic relationship (P < 0.05), and the highest intestinal villus height and goblet cells were found in the 120 mg/kg diet (P < 0.01), melatonin in diets significantly increased SOD and CAT activities in serum, up-regulated the expression of anti-inflammatory factors (IL-10) and tight junction protein (ZO-1), and down-regulated the expression of pro-inflammatory factors (IL-1β, IL-8, IL-15, and TNF-α) in the gut, dietary melatonin improved the intestinal microflora compositions, in the group that supplementation a dosage of 120 mg/kg, there was a noticeable rise in the abundance of Firmicutes and the ratio of Firmicutes/Bacteroidota, compared with control group (P < 0.1). Conclusively, dietary supplementation of melatonin promoted growth performance, enhanced intestinal immune capacity and serum antioxidant level, and improved intestinal morphology properties and intestinal flora composition in M. albus. In conclusion, based on quadratic broken-line regression analysis of WGR and FCR, the optimal concentration of melatonin to be supplied is predicted to be 146-148 mg/kg.

Zinc modulates hypothalamo-pituitary-gonadal-liver axis to impair reproduction in female Mystus vittatus (Bloch, 1794)

The present study investigated the effects of zinc on the hypothalamo-pituitary-gonadal-liver (HPGL) axis of the bagrid catfish Mystus vittatus. Female fish (pre-ovulatory and ovulatory phases) were exposed to zinc sulphate at 1/10th of LC50 (5.62 mg/L) for 60 days and sacrificed at every 15-day interval to collect tissues. Zinc concentration in all tissues was significantly higher in the metal-exposed group at all exposure durations compared to control for both phases. Metallothionein (MT) levels increased in the brain, liver and ovary of fish from both phases with exposure duration. Reactive oxygen species (ROS) generation in the brain, liver and ovary tissues increased with exposure duration at both reproductive phases while serum cortisol levels in ovulatory fish increased significantly compared to pre-ovulatory. Condition factor, gonadosomatic index and hepatosomatic index decreased in Zn-exposed fish. Brain GnRH and kisspeptin levels decreased significantly in the Zn-exposed group for both phases. GnIH was significantly higher in Zn-exposed fish. Serum FSH levels in pre-ovulatory and LH levels in ovulatory fish decreased gradually with an increase in the duration of exposure. Zn exposure reduced vitellogenin (Vtg) and estradiol (E2) in the liver and ovary with an increase in duration from both phases. Ovary maturation-inducing hormone (MIH) levels showed a decrease with exposure duration in ovulatory fish. Moreover, Zn-exposed ovulatory fish showed a degenerated oocyte nucleus due to the disintegration of the nuclear membrane. It might be inferred that Zn altered the HPGL regulatory system of M. vittatus reproduction at both the pre-ovulatory and ovulatory phases.

Decoding the effect of photoperiodic cues in transducing kisspeptin-melatonin circuit during the pubertal onset in common carp

This study unravels the intricate interplay between photoperiod, melatonin, and kisspeptin to orchestrate the pubertal onset of Common carp. Female fingerlings exposed to long days (LD) exhibited a hormonal crescendo, with upregulated hypothalamic-pituitary-ovarian (HPO) axis genes (kiss1, kiss1r, kiss2, gnrh2, gnrh3) and their downstream targets (lhr, fshr, ar1, esr1). However, the expression of the melatonin receptor (mtnr1a) diminished in LD, suggesting a potential inhibitory role. This hormonal symphony was further amplified by increased activity of key transcriptional regulators (gata1, gata2, cdx1, sp1, n-myc, hoxc8, plc, tac3, tacr3) and decreased expression of delayed puberty genes (mkrn1, dlk1). In contrast, short days (SD) muted this hormonal chorus, with decreased gnrh gene and regulator expression, elevated mtnr1a, and suppressed gonadal development. In in-vitro, estradiol mimicked the LD effect, boosting gnrh and regulator genes while dampening mtnr1a and melatonin-responsive genes. Conversely, melatonin acted as a conductor, downregulating gnrh and regulator genes and amplifying mtnr1a. Our findings illuminate the crucial roles of melatonin and kisspeptin as opposing forces in regulating pubertal timing. LD-induced melatonin suppression allows the kisspeptin symphony to flourish, triggering GnRH release and, ultimately, gonadal maturation. This delicate dance between photoperiod, melatonin, and kisspeptin orchestrates common carp's transition from juvenile to reproductive life.

The invisible Threat: Assessing the reproductive and transgenerational impacts of micro- and nanoplastics on fish

Micro- and nanoplastics (MNPs), emerging as pervasive environmental pollutants, present multifaceted threats to diverse ecosystems. This review critically examines the ability of MNPs to traverse biological barriers in fish, leading to their accumulation in gonadal tissues and subsequent reproductive toxicity. A focal concern is the potential transgenerational harm, where offspring not directly exposed to MNPs exhibit toxic effects. Characterized by extensive specific surface areas and marked surface hydrophobicity, MNPs readily adsorb and concentrate other environmental contaminants, potentially intensifying reproductive and transgenerational toxicity. This comprehensive analysis aims to provide profound insights into the repercussions of MNPs on fish reproductive health and progeny, highlighting the intricate interplay between MNPs and other pollutants. We delve into the mechanisms of MNPs-induced reproductive toxicity, including gonadal histopathologic alterations, oxidative stress, and disruptions in the hypothalamic-pituitary-gonadal axis. The review also underscores the urgency for future research to explore the size-specific toxic dynamics of MNPs and the long-term implications of chronic exposure. Understanding these aspects is crucial for assessing the ecological risks posed by MNPs and formulating strategies to safeguard aquatic life.

The Role of Melatonin to Ameliorate Oxidative Stress in Sperm Cells

It is widely accepted that oxidative stress (OS) coming from a wide variety of causes has detrimental effects on male fertility. Antioxidants could have a significant role in the treatment of male infertility, and the current systematic review on the role of melatonin to ameliorate OS clearly shows that improvement of semen parameters follows melatonin supplementation. Although melatonin has considerable promise, further studies are needed to clarify its ability to preserve or restore semen quality under stress conditions in varied species. The present review examines the actions of melatonin via receptor subtypes and its function in the context of OS across male vertebrates.

Heat hardening enhances metabolite-driven thermoprotection in the Mediterranean mussel Mytilus galloprovincialis

Introduction: Temperature affects organisms' metabolism and ecological performance. Owing to climate change, sea warming constituting a severe source of environmental stress for marine organisms, since it increases at alarming rates. Rapid warming can exceed resilience of marine organisms leading to fitness loss and mortality. However, organisms can improve their thermal tolerance when briefly exposed to sublethal thermal stress (heat hardening), thus generating heat tolerant phenotypes. Methods: We investigated the "stress memory" effect caused by heat hardening on M. galloprovincialis metabolite profile of in order to identify the underlying biochemical mechanisms, which enhance mussels' thermal tolerance. Results: The heat hardening led to accumulation of amino acids (e.g., leucine, isoleucine and valine), including osmolytes and cytoprotective agents with antioxidant and anti-inflammatory properties that can contribute to thermal protection of the mussels. Moreover, proteolysis was inhibited and protein turnover regulated by the heat hardening. Heat stress alters the metabolic profile of heat stressed mussels, benefiting the heat-hardened individuals in increasing their heat tolerance compared to the non-heat-hardened ones. Discussion: These findings provide new insights in the metabolic mechanisms that may reinforce mussels' tolerance against thermal stress providing both natural protection and potential manipulative tools (e.g., in aquaculture) against the devastating climate change effects on marine organisms.

Melatonin Improves Turbot Oocyte Meiotic Maturation and Antioxidant Capacity, Inhibits Apoptosis-Related Genes mRNAs In Vitro

High-quality eggs are essential for the sustainability of commercial aquaculture production. Melatonin is a potent candidate for regulating the growth and maturation of oocytes. Therefore, research on the effect of melatonin on marine fish oocytes in vitro has been conducted. The present study successfully established a culture system of turbot (Scophthalmus maximus) oocytes in vitro and investigated the effect of melatonin on oocyte meiotic maturation, antioxidant capacity, and the expression of apoptosis-related genes. The cultures showed that turbot Scophthalmus maximus late-vitellogenic denuded oocytes, with diameters of 0.5-0.7 mm, had a low spontaneous maturation rate and exhibited a sensitive response to 17α, 20β-dihydroxyprogesterone (DHP) treatment in vitro. Melatonin increased by four times the rate of oocyte germinal vesicle breakdown (GVBD) in a concentration- and time-dependent manner. The mRNA of melatonin receptor 1 (mtnr1) was significantly upregulated in the oocyte and follicle after treatment with melatonin (4.3 × 10^-9 M) for 24 h in vitro, whereas melatonin receptor 2 (mtnr2) and melatonin receptor 3 (mtnr3) remained unchanged. In addition, melatonin significantly increased the activities of catalase, glutathione peroxidase, and superoxide dismutase, as well as the levels of glutathione, while decreasing the levels of malondialdehyde and reactive oxygen species (ROS) levels in turbot oocytes and follicles cultures in vitro. p53, caspase3, and bax mRNAs were significantly downregulated in oocytes and follicles, whereas bcl2 mRNAs were significantly upregulated. In conclusion, the use of turbot late-vitellogenesis oocytes (0.5-0.7 mm) is suitable for establishing a culture system in vitro. Melatonin promotes oocyte meiotic maturation and antioxidative capacity and inhibits apoptosis via the p53-bax-bcl2 and caspase-dependent pathways, which have important potential to improve the maturation and quality of oocytes.

From metabolism to behaviour - Multilevel effects of environmental methamphetamine concentrations on fish

Methamphetamine (METH) is a concerning drug of abuse that produces strong psychostimulant effects. The use of this substance, along with the insufficient removal in the sewage treatment plants, leads to its occurrence in the environment at low concentrations. In this study, brown trout (Salmo trutta fario) were exposed to 1 μg/L of METH as environmental relevant concentration for 28 days in order to elucidate the complex effects resulting from the drug, including behaviour, energetics, brain and gonad histology, brain metabolomics, and their relations. Trout exposed to METH displayed lowered activity as well as metabolic rate (MR), an altered morphology of brain and gonads as well as changes in brain metabolome when compared to controls. Increased activity and MR were correlated to an increased incidence of histopathology in gonads (females - vascular fluid and gonad staging; males - apoptotic spermatozoa and peritubular cells) in exposed trout compared to controls. Higher amounts of melatonin in brain were detected in exposed fish compared to controls. Tyrosine hydroxylase expression in locus coeruleus was related to the MR in exposed fish, but not in the control. Brain metabolomics indicated significant differences in 115 brain signals between control and METH exposed individuals, described by the coordinates within the principal component analyses (PCA) axes. These coordinates were subsequently used as indicators of a direct link between brain metabolomics, physiology, and behaviour - as activity and MR varied according to their values. Exposed fish showed an increased MR correlated with the metabolite position in PC1 axes, whereas the control had proportionately lower MR and PC1 coordinates. Our findings emphasize the possible complex disturbances in aquatic fauna on multiple interconnected levels (metabolism, physiology, behaviour) as a result of the presence of METH in aquatic environments. Thus, these outcomes can be useful in the development of AOP's (Adverse Outcome Pathways).

Photoperiod Induces DNA Methylation Changes in the Melatonin Receptor 1A Gene in Ewes

Research has shown that MTNR1A plays an essential role in the estrus cycle and seasonal reproduction changes in sheep. However, few people have focused on the DNA methylation of MTNR1A by season or photoperiod. In this study, using qPCR and Western blotting, we measured the MTNR1A expression in the hypothalamus of ovariectomized and estradiol-treated (OVX + E2) sheep under different photoperiod treatment conditions. Subsequently, the core promoter of the MTNR1A gene was identified, and its methylation level in sheep exposed to different photoperiod treatments was measured by pyrosequencing. The results showed that MTNR1A gene expression significantly differed between the short 42-day photoperiod (SP42) and the LP42 or combined SP-LP42 treatment groups (p < 0.05). In addition, we determined that the core MTNR1A promoter region was within 540 bp upstream of the transcriptional start site (TSS) and that the DNA methylation levels at CpG sites in the SP42 vs. LP42 and SP42 vs. SP-LP42 groups significantly differed. Finally, a significant negative correlation (p < 0.001) between gene expression and DNA methylation levels was revealed, suggesting that photoperiod may induce DNA methylation of the MTNR1A gene and thus change its expression. The findings provide valuable bases for the further study of seasonal reproduction in sheep.

Activity-Rest Circadian Rhythm of the Pearly Razorfish in Its Natural Habitat, before and during Its Mating

Recent technological advances in marine biotelemetry have demonstrated that marine fish species perform activity-rest rhythms that have relevant ecological and evolutionary consequences. The main objective of the present report is to study the circadian rhythm of activity-rest of the pearly razorfish, Xyrichtys novacula in its own habitat, before and during the reproduction season using a novel biotelemetry system. This fish species is a small-bodied marine species that inhabits most shallow soft habitats of temperate areas and has a high interest for commercial and recreational fisheries. The activity of free-living fish was monitored by means of high-resolution acoustic tracking of the motor activity of the fish in one-minute intervals. The obtained data allowed the definition of the circadian rhythm of activity-rest in terms of classical non-parametric values: interdaily stability (IS), intradaily variability (IV), relative amplitude (RA), average activity during the most-active period of consecutive 10 h (M10), and average activity during the least-active period of consecutive 5 h (L5). We observed a well-marked rhythm, with little fragmentation and good synchrony with the environmental cycle of light-darkness, regardless of sex and the period studied. However, the rhythm was found to be slightly more desynchronized and fragmented during reproduction because of variations in the photoperiod. In addition, we found that the activity of the males was much higher than that of the females (p < 0.001), probably due to the peculiar behavior of the males in defending the harems they lead. Finally, the time at which activity began in males was slightly earlier than it was in females (p < 0.001), presumably due to the same fact, as differences in activity or for the individual heterogeneity of this species in the time of awakening are considered to be an independent axis of the fish's personality. Our work is novel, as it is one of the first studies of activity-rest rhythm using classical circadian-related descriptors in free-living marine fish using locomotory data facilitated by novel technological approaches.

Melatonin effective to reduce the microscopic symptoms of polycystic ovary syndrome-related infertility: An experimental study

Polycystic ovary syndrome (PCOS) is an endocrine disorder seen in women of reproductive age and has been gradually increasing over the years. The mechanism of the syndrome has still not been clearly understood. In this study, the possible effects of exogenously administrated melatonin on melatonin (MT1) receptor, Growth Differentiation Factor-9 (GDF9), and Bone Morphogenetic Protein-15 (BMP15) in experimental PCOS were investigated. Thirty-two 6-8-week-old Sprague-Dawley rats were randomly divided into four groups (n = 8 in each) as Sham control (Group 1), Melatonin (Group 2), PCOS (Group 3), and PCOS + Melatonin (Group 4) groups. At the end of the 21st day, the experiment was terminated, the ovary tissues were taken, and Hematoxylin-Eosin staining, MT1, GDF9, BMP15 immunohistochemical labeling, western blot, and quantitative real-time polymerase chain reaction (qPCR) analyses were performed. Serum Luteinizing Hormone (LH)/Follicle Stimulating Hormone (FSH) levels and colpo-cytological examinations were also carried out. The results revealed that melatonin administration increased the expression levels of the MT1 receptor, GDF9, and BMP15 in PCOS at protein and mRNA levels. It was determined that melatonin administration reduced the microscopic symptoms of PCOS. Melatonin was found to be effective via the MT1 receptor in the pathogenesis of PCOS, and it suppressed the transport pathways of GDF9 to granulosa cells in antral follicles.

Meiosis-mediated reproductive toxicity by fenitrothion in Caenorhabditis elegans from metabolomic perspective

Fenitrothion (FNT), an organophosphorus insecticide, is widely detected in the living environment. The reproductive and endocrine toxicity of FNT to biological communities has been ever reported, but potential mechanism and reproductive toxicity dose effect remain unclear. In our study, we constructed Caenorhabditis elegans model to analyze the reproductive toxicity mechanism of FNT based on metabolomics and evaluated its reproductive toxicity dose effect using benchmark dose (BMD)method. Our results showed that FNT exposure significantly reduced brood size, number of germ cells, and delayed gonadal development in nematodes. Non-targeted metabolomics revealed that FNT exposure caused significant metabolic disturbances in nematodes, leading to a significant reduction in the synthesis of cortisol and melatonin, and the latter played a mediating role in the effects of FNT on number of germ cells. We further found that the levels of these two hormones were significantly negative correlated with the expression of the androgen receptor nhr-69 and affected the meiosis of germ cells by regulating the nhr-69/ fbf-1/2 /gld-3 /fog-1/3 pathway. Meanwhile, the study found the BMDL10s for N2 and him-5 mutant were 0.411 μg/L by number of germ cells and 0.396 μg/L by number of germ cells in the meiotic zone, respectively, providing a more protective reference dose for ecological risk assessment of FNT. This study suggested that FNT can affect androgen receptor expression by inhibiting cortisol and melatonin secretion, which further mediate the meiotic pathway to affect sperm formation and exert reproductive toxicity, and provides a basis for setting reproductive toxicity limits for FNT.

Transcriptomic response to GnRH down regulation by RNA interference in clam Ruditapes philippinarum, suggest possible role in reproductive function

Gonadotropin-releasing hormone (GnRH) plays a key role in the control of the reproductive axis in vertebrates, however, little is known about its function in reproductive endocrine regulation in molluscs. In the present study, RNA-seq was used to construct transcriptomes of Ruditapes philippinarum testis and ovaries of control and GnRH suppressed individuals using RNA interference. GnRH suppression caused 112 and 169 enriched KEGG pathways in testis and ovary, with 92 pathways in common in both comparisons. The most enriched KEGG pathways occurred in the "Oxidative phosphorylation", "Dorso-ventral axis formation", "Thyroid hormone synthesis" and "Oxytocin signaling pathway" etc. A total of 1838 genes in testis and 358 genes in ovaries were detected differentially expressed in GnRH suppressed clams. Among the differentially expressed genes, a suit of genes related to regulation of steroid hormones synthesis and gonadal development, were found in both ovary and testis with RNAi of GnRH. These results suggest that GnRH may play an important role in reproductive function in bivalves. This study provides a preliminary basis for studying the function and regulatory mechanism of GnRH in bivalves.

Utility of melatonin in mitigating ionizing radiation-induced testis injury through synergistic interdependence of its biological properties

Background

Ionizing radiations (IR) have widespread useful applications in our daily life; however, they have unfavorable effects on reproductive health. Maintaining testicular health following IR exposure is an important requirement for reproductive potential. The current study explored the role of melatonin (MLT) in mitigating IR-induced injury in young adult rat testis.

Methods

Rats were given daily MLT (25 mg/kg) for 3 and 14 days after receiving 4 Gy γ-radiation.

Results

Serum MLT levels and other antioxidants, including glutathione content, and the activity of glutathione peroxidase and glutathione reductase in the testis of the irradiated rats were remarkably maintained by MLT administration in irradiated rats. Hence, the hydrogen peroxide level declined with remarkably reduced formation of oxidative stress markers, 4-hydroxynonenal, and 8-Hydroxy-2′-deoxyguanosine in the testis of irradiated animals after MLT administration. The redox status improvement caused a remarkable regression of proapoptotic protein (p53, Cyto-c, and caspase-3) in the testis and improved inflammatory cytokines (CRP and IL-6), and anti-inflammatory cytokine (interleukin IL-10) in serum. This is associated with restoration of disturbed sex hormonal balance, androgen receptor upregulation, and testicular cell proliferation activity in irradiated rats, explaining the improvement of sperm parameters (count, motility, viability, and deformation). Consequently, spermatogenic cell depletion and decreased seminiferous tubule diameter and perimeter were attenuated by MLT treatment post irradiation. Moreover, the testis of irradiated-MLT-treated rats showed well-organized histological architecture and normal sperm morphology.

Conclusions

These results show that radiation-induced testicular injury is mitigated following IR exposure through synergistic interdependence between the antioxidant, anti-inflammatory, anti-apoptotic, and anti-DNA damage actions of MLT.

Expression and distribution of MTNR1A in the hypothalamus-pituitary-gonadal axis of Tibetan sheep during the estrous cycle

The melatonin 1A receptor (MTNR1A) is a membrane receptor distributed across the mammalian gonadal axis-associated membrane. Melatonin (MT) can specifically bind with MTNR1A on the cell membrane and regulates mammalian reproductive activities. However, the role of MTNR1A in regulating the reproductive physiological activities of sheep in the Tibetan Plateau remains unclear. In this study, the MT content in Tibetan sheep blood during the estrous cycle was detected by ELISA. The distribution of MTNR1A in the hypothalamus-pituitary-gonadal axis (HPGA) was analyzed by immunohistochemistry and immunofluorescence. Western blot and qRT-PCR were used to detect dynamic changes of MTNR1A mRNA and protein expression, and the protein distributions in the HPGA. The results showed that the average secretion level of MT in Tibetan sheep blood was highest occurred during diestrus and the lowest during proestrus. Additionally, the secretion of MT at night was significantly higher than during the day. The immunopositive products of MTNR1A were primarily distributed around the glial cells in the dorsal hypothalamic nucleus region, chromophobe cells, and eosinophilic cytoplasm in the pituitary gland, follicular granular layer, follicular adventitia, tubal mucosa, cilia, endometrium, interstices, and glands in the uterus. The expression trends of MTNR1A mRNA and proteins in the HPGA during the estrous cycle were the same. The relative expression levels of MTNR1A mRNA and proteins in the hypothalamus and ovaries were the highest during proestrus and the lowest during metestrus; the highest during diestrus in the pituitary and oviducts; the highest during metestrus in the uterus. Collectively, the differences in the secretion of MT in Tibetan sheep blood and the expression of MTNR1A in HPGA suggest that they may be affected by steroid hormone secretion during the estrous cycle of Tibetan sheep, which has a potential impact on the regulation of animal estrous cycle.

Melatonin and cortisol as components of the cutaneous stress response system in fish: Response to oxidative stress

The skin being a passive biological barrier that defends the organism against harmful external factors is also a site of action of the system responding to stress. It appears that melatonin (Mel) and its biologically active metabolite AFMK (N1-acetyl-N2-formyl-5-methoxykynuramine), both known as effective antioxidants, together with cortisol, set up a local (cutaneous) stress response system (CSRS) of fish, similar to that of mammals. Herein we comment on recent studies on CSRS in fish and show the response of three-spined stickleback skin to oxidative stress induced by potassium dichromate. Our study indicates that exposure of the three-spined stickleback to K₂Cr₂O₇ affects Mel and cortisol levels and pigment dispersion in melanophores in the skin. In our opinion, an increased concentration of Mel and cortisol in the skin may be the strategy to cope with oxidative stress, where both components act locally to prevent damage caused by active oxygen molecules. Furthermore, the pigment dispersion may be a valuable, easy-to-observe mark of oxidative stress, useful in the evaluation of fish welfare.

Evolutionary Genomics Reveals Multiple Functions of Arylalkylamine N-Acetyltransferase in Fish

As an important hormone, melatonin participates in endocrine regulation of diverse functions in vertebrates. Its biosynthesis is catalyzed by four cascaded enzymes, among them, arylalkylamine N-acetyltransferase (AANAT) is the most critical one. Although only single aanat gene has been identified in most groups of vertebrates, researchers including us have determined that fish have the most diverse of aanat genes (aanat1a, aanat1b, and aanat2), playing various potential roles such as seasonal migration, amphibious aerial vision, and cave or deep-sea adaptation. With the rapid development of genome and transcriptome sequencing, more and more putative sequences of fish aanat genes are going to be available. Related phylogeny and functional investigations will enrich our understanding of AANAT functions in various fish species.

The Protective Role of Melatonin in Sperm Cryopreservation of Farm Animals and Human: Lessons for Male Fish Cryopreservation

Simple Summary

In recent years, cryopreservation of fish sperm has been a rapidly evolving technique that contributes both to the improvement of genetic reproduction programs and the proper management of broodstock as well as to ensuring the viability of endangered species. However, this technique can cause significant damage to sperm, making the use of cryoprotectants and antioxidants in cryopreservation solutions imperative. The hormone melatonin has demonstrated positive effects on the cryopreservation of sperm in both farm animals and humans. Therefore, the plethora of research that has been conducted on animals and humans could be expanded to fish cryopreservation, making melatonin potentially a very promising alternative cryoprotectant.

Abstract

Cryopreservation is a technique that offers various advantages, especially in fish, among others, that makes the reproduction of species easier through a constant supply of sperm, synchronization of the gamete availability of both sexes, storage of semen for genetic improvement programs, reduction in the cost by eliminating the need to maintain male broodstock, and conserving the gametes of endangered species. However, freezing and warming procedures for cryopreservation lead to a reduction in the quality and viability of cryopreserved sperm because of oxidative stress. For this reason, the enrichment of extender media with antioxidants is a common method of cryopreservation of the semen of several fish species. Recently, many studies have been published for the protective role of antioxidants and especially of melatonin on male fertility preservation both in farm animals and humans, demonstrating the beneficial effects of melatonin as a sperm cryoprotectant. On the other hand, very few studies were conducted using melatonin as an antioxidant in different male fish species for semen cryopreservation. We conclude that the use of moderate concentrations of melatonin are beneficial to semen preservation, and the mechanisms through which melatonin acts positively on spermatozoa need to be further investigated to establish improvement protocols for cryopreservation in fish species.

Photoperiodic light pulse induces ovarian development in the catfish, Mystus cavasius: Possible roles of dopamine and melatonin in the brain

In the freshwater catfish, Mystus cavasius, locally known as gulsha, ovarian maturation is triggered by long-day conditions. Using dopaminergic neuronal activity in the brain, the purpose of this study was to identify the brain's detection of a nocturnal light pulse that induced ovarian development. Since direct inhibition of pituitary gonadotropin release is exerted by dopamine (DA), it may serve as a neuromodulator of photoperiodic stimulation in teleosts. We studied functional effects of photoperiodicity on dopaminergic rhythmicity in gulsha brain. Nocturnal illumination and Nanda-Hamner photocycles revealed that ovarian development is induced by a 1 h light pulse between zeitgeber time (ZT) 12 and 13. Daily fluctuations in DA, 3, 4-dihydroxyphenylacetic acid (DOPAC) and DOPAC/DA were observed under a 12L:12D photoperiod. Fish exhibited increased levels during the daytime and decreased levels at night. Rhythmic patterns of dopaminergic activity also showed clear circadian oscillations under constant light, but not constant dark conditions. After 7 days of exposure to long photoperiod (14L:10D), DA, DOPAC and DOPAC/DA in the brain at ZT12 and ZT16 were significantly higher than during a short photoperiod (10L:14D). Melatonin-containing water inhibited the release of DA and DOPAC 6 h and 24 h after treatment, respectively, and DOPAC/DA 6 h after treatment. This inhibition was blocked by the melatonin receptor antagonist, luzindole. These results suggest that a 1 h nocturnal light pulse induces ovarian development through alteration of dopaminergic neuronal excitability in the brain, via oscillation in melatonin triggered by photic stimuli, which may interfere with the reproductive endocrine axis in gulsha.

Intraperitoneal kisspeptin-10 administration ameliorates sodium arsenite-induced reproductive toxicity in adult male mice

The current study investigated the protective ameliorative effect of intraperitoneally administered kisspeptin-10 (50 nmol/day) against reproductive toxicity in adult male mice challenged with 35 days of exposure to sodium arsenite in drinking water. Mice were divided into tap water control, sodium arsenite-alone (4 ppm and 10 ppm), kisspeptin-alone (intermittent and continuous) and combined (sodium arsenite +kisspeptin-10 intermittent and continuous) treatment groups. Results revealed protective effect of both intermittent and continuous kisspeptin doses on reproductive organs against sodium arsenite-induced toxicity. This was indicated by an increase (p < 0.001) in the activity of antioxidant enzymes and a decrease (p < 0.001) in the levels of oxidative stress biomarkers. Concomitant significant increase was noticeable in the relative organ weight (p < 0.01), and serum testosterone and seminal fructose (p < 0.001), and a significant improvement in sperm parameters was also observed. A significant downregulation of lactate dehydrogenase concentration demonstrated further the protective effect of kisspeptin against tissue damage. Histologically, both treatment regimens of kisspeptin combined with sodium arsenite exposure prevented massive germ cell loss and tissue damage, a condition prominent in sodium arsenite-alone-treated mice. The study demonstrates for the first time kisspeptin's potential to mitigate the biochemical and histotoxic effects of arsenic on male reproductive system.

Adaptation of life-history traits and trade-offs in marine medaka (Oryzias melastigma) after whole life-cycle exposure to polystyrene microplastics

Microplastics are ubiquitous in marine environments and may cause unexpected ecological effects. This study adopted a whole life-cycle exposure to illuminate the impact of polystyrene microplastics on life-history strategies of marine medaka (Oryzias melastigma), including the hatching of embryos, growth and reproduction of F₀ generation, and embryonic and larval development of F₁ offspring. Microplastics accumulated on the eggshell and reduced embryonic hatching rate and larval body length and weight. Similarly, 150 days of microplastic exposure decreased body mass and gonadosomatic index of adult fish, but accelerated sexual maturity of female fish, showing a trade-off between growth and reproduction. Microplastic exposure also caused obvious histopathological damages to gonads and decreased egg productions and fertilization rates. Moreover, parental microplastic exposure induced elevated heartbeats, premature hatching, and slow growth in F₁ offspring. Anti-oxidative stress response, sex hormone disruption, and disturbed transcription of steroidogenic genes in the reproductive axis could partially explain the reproduction impairment and transgenerational trade-offs. Furthermore, transcriptome analysis revealed that the steroid hormone biosynthesis and cytochrome P450 pathways in the testes of male fish were significantly affected after 20 μg/L microplastic exposure. These findings suggest that microplastic pollution may be an emerging threat to the sustainability of marine fish population.

Dietary supplementation with melatonin: influence on growth performance, oxidative stress status, and amelioration of silver nanoparticles-induced toxicity in Nile tilapia (Oreochromis niloticus)

Excessive use of silver nanoparticles (AgNPs) due to antibacterial properties can raise concerns about their release into environment and potential toxicity in aquatic organisms. Melatonin has several physiological functions especially antioxidant potential against oxidative stress. The current study was conducted to investigate the potential effects of two doses of dietary melatonin on growth performance, plasma biochemistry, and liver enzyme activity in Nile tilapia (Oreochromis niloticus) juveniles. We also investigated the potential ameliorative effect of melatonin in AgNPs-induced biochemical alterations in tilapia fish. The results showed that melatonin-supplemented diets had no significant effect on growth performance of fish (P>0.05). The liver GPx activity increased in fish fed melatonin-supplemented diets (P<0.05), but the SOD activity showed no significant difference in comparison with the control (P>0.05). The administration of melatonin-supplemented diets reduced the activity of liver MDA compared to the control (P<0.05). Feeding fish with high melatonin-supplemented diet (200 mg kg^-1 of diet) decreased the plasma glucose, total protein, and AST levels (P<0.05). The liver GPx and SOD activities were higher in high melatonin-treated fish exposed to AgNPs than the control group (P<0.05). Dietary melatonin decreased the liver MDA activity in AgNPs-exposed fish. The plasma glucose, AST, and ALT levels in melatonin-treated fish exposed to AgNPs decreased compared to the untreated exposed fish (P<0.05). Melatonin-treated fish exposed to 0.05 and 0.5 mg L^-1 of AgNPs had lower plasma LDH level than the control group (P<0.05). The results showed that consumption of melatonin-supplemented diets could modulate some of the biochemical indices of plasma and liver in Nile tilapia. The findings also indicated the ameliorative effect of dietary melatonin on AgNPs-induced toxicity in Nile tilapia.

Opsins and gonadal circadian rhythm in the swordfish (Xiphias gladius) ovary: Their potential roles in puberty and reproductive seasonality

Circadian rhythm is well known to play a pivotal role in reproduction but the presence of a gonadal circadian rhythm is opening a lot of questions about a local regulation of reproduction. In the present study, we first set to identify the key genes driving circadian rhythmicity from the gonadal transcriptome of the swordfish, a commercially relevant species undergoing overfishing, and we then investigated whether their transcriptional activity was influenced by the maturation stage. Finally, we explored whether seasonality had the ability to modulate the expression of these genes. Interestingly, we identified a wide number of circadian rhythm related genes in the transcriptome of the swordfish gonad including, among the others, clock, bmal1, cry2 and per2, which have been found to be differentially expressed between sexually immature and mature individuals sampled during the breeding season. This differential modulation was also found for melatonin biosynthesis genes (mel1b, asmt and tph1) as well as opsin genes (opsin4, tmt opsin, parapinopsin, VA opsin, rho and sws), known to be the primary receptors of light stimuli. These differences were not observed between mature and immature individuals when considering the non-breeding season suggesting that, despite the molecular machinery of mature gonads is able to respond to signals driving ovarian maturation, these signals are not present hence highlighting the potential role of seasonality in modulating the gonadal circadian rhythm. These results confirm the presence of a gonadal circadian rhythm in the swordfish and open new interesting questions about its role in driving puberty onset.

Roles of melatonin in the teleost ovary: A review of the current status

Melatonin, the neurohormone mainly synthesized in and secreted from the pineal gland of vertebrates following a circadian rhythm, is an important factor regulating various physiological processes, including reproduction. Recent data indicate that melatonin is also synthesized in the ovary and that it acts directly at the level of the ovary to modulate ovarian physiology. In some teleosts, melatonin is reported to affect ovarian steroidogenesis. The direct action of melatonin on the ovary could be a possible factor promoting oocyte maturation in teleosts. A role for melatonin in follicle rupture during ovulation in the teleost medaka has recently emerged. In addition, melatonin is suggested to affect oocyte maturation by its antioxidant activity. However, the molecular mechanisms underlying these direct effects of melatonin are largely unknown.

Antioxidants in Fish Sperm and the Potential Role of Melatonin

In recent years, the effects of novel antioxidants have played an important role in the research focusing on fish cell protection. As food demand grows, aquaculture production becomes more intensive, and fish are more exposed to oxidative stress conditions, like high densities, temperature shifting, frequent fish handling and samplings, and prophylactic or disease treatments, which expose fish to a different environment. Particularly in reproduction, germ cells lose antioxidant capacity with spermatogenesis, as spermatozoa are more prone to oxidative stress. Antioxidants have been used in a variety of fish physiological problems including in reproduction and in the establishment of cryopreservation protocols. From the most used antioxidants to natural plant food and herbs, and endogenously produced antioxidants, like melatonin, a review of the literature available in terms of their effects on the protection of fish spermatozoa is presented here in a classified structure. Several direct and indirect approaches to improve gamete quality using antioxidants administration are mentioned (through feed supplementation or by adding in cryopreservation media), as well as factors affecting the efficiency of these molecules and their mechanisms of action. Special attention is given to the unclear melatonin pathway and its potential scavenger activity to prevent and counteract oxidative stress damage on fish spermatozoa.

Effectiveness of melatonin to restore fish brain activity in face of permethrin induced toxicity

Present study demonstrates permethrin induced oxidative damage in fish brain and explores effectiveness of melatonin to ameliorate brain function. Adult female Notopterus notopterus were exposed to nominal permethrin concentrations at 1/20th (0.34 μg/l) and 1/10th (0.68 μg/l) of LC₅₀ for 15 days. The measured permethrin concentrations using gas chromatography (GC-ECD) were 0.28 μg/l and 0.57 μg/l, respectively. Some fish were sacrificed to collect brain tissue after 15 days of exposure. Remaining fish from both groups were administered exogenous melatonin (50 μg/kg, 100 μg/kg body weight) for 7 days and brain tissues were collected. Brain enzymes, ntioxidant factors, HSP70, HSP90, nuclear factor-kappa binding (NFkB), melatonin receptor (MT1R) proteins were measured. Permethrin treatment significantly (P < 0.05) decreased the levels of glutathione and brain enzymes. Malondialdehyde (MDA), xanthine oxidase (XO), HSPs increased at each concentration of permethrin. However, superoxide dismutase, glutathione s-transferase levels increased at low permethrin concentration followed by sharp decrease at higher concentration. Expression of NFkB and MT1R increased significantly (P < 0.05). Melatonin administration reinstated activity of brain enzymes, reduced MDA, XO levels and modulated HSPs. Melatonin also increased expression of NFkB and MT1R. Exogenous melatonin improves oxidative status in permethrin stressed fish brain. Melatonin modulates expression of HSPs that enables brain to become stress tolerant and survive by initiating NFkB translocation. Melatonin could act through melatonin receptor protein to induce synthesis of antioxidant proteins. Therefore the study successfully evaluates the potential of melatonin application for better culture and management of fish against pesticide toxicity.

Effect of short-term intermittent exposure to waterborne estradiol on the reproductive physiology of the round goby (Neogobius melanostomus)

The objective of this study was to determine how the short-term exposure to a supraphysiological concentration of waterborne 17β-estradiol (E₂) influences on melatonin (Mel) and thyroxine (T₄) concentrations in plasma and E₂ and 11-ketotestosterone (11-KT) concentrations in plasma and gonads in both sexes of round goby (Neogobius melanostomus) during the pre-spawning, spawning, late spawning and non-spawning phases. The experimental protocol was based on short-term, repeated exposures of fish to a supraphysiological dose of waterborne E₂. Mel level was unchanged on exposure to E₂ during the investigated phases, and its role in determining a time frame for spawning in both sexes of round goby seems to be stable in those conditions. T₄ and sex steroids (E₂ and 11-KT) were sensitive to the exposure of E₂, and those changes influence gonads by accelerating oocyte development, ovulation and regression and inhibiting spermatogenesis in this species. The results demonstrate that the physiological responses of fish in all investigated phases were altered over a short window of exposure, indicating that short-term exposure to a supraphysiological dose of E₂ may impact fish in the wild. Furthermore, round goby can be recommended as a very suitable model for studying endocrine disruptors, which is sensitive to even short exposure to E₂.

Melatonin protects sperm cells of Capoeta trutta from toxicity of titanium dioxide nanoparticles

In this study, it was aimed to determine the protective effects of melatonin (0.01, 0.1, and 1 mM) against 10 mg/L titanium dioxide nanoparticles (TiO₂-NPs) on kinematic and oxidative indices in the sperm cells of Capoeta trutta. Therefore, TiO₂ nanoparticles were synthesized primarily within the scope of the study. The synthesized nanoparticles were characterized by structurally different techniques. Then, melatonin and TiO₂ were applied to Capoeta trutta sperm cells by in vitro. According to our data, all doses of melatonin showed protective effects on all velocities of sperm cells such as the straight line velocity (VSL), the curvilinear velocity (VCL), and the angular path velocity (VAP) against TiO₂-NPs, while 0.1 and 1 mM doses of melatonin improved the VSL value. Although TiO₂-NPs increased total glutathione (tGSH), malondialdehyde (MDA) lipid peroxidation, and superoxide dismutase (SOD) compared to the control group, there were positive treatment effects for all doses of melatonin on antioxidant capacity of sperm cells. At the end of this research, it is suggested that over 0.1 mM dose of melatonin improves the velocity of sperm cells and it plays a protective role against the toxic effects of TiO₂-NPs.

Effect of Light–Dark Cycle on Skin Mucosal Immune Activities of Gilthead Seabream (Sparus aurata) and European Sea Bass (Dicentrarchus labrax)

Changes in different immune activities in the skin mucus of gilthead seabream (Sparus aurata L.) and European sea bass (Dicentrarchus labrax L.) specimens exposed to a constant light–dark photoperiod (12 h L:12 h D) were studied. Samples were collected at 08:00 (light on), 14:00, 20:00 (light off), 02:00, and again at 08:00 to determine immunoglobulin M (IgM) levels, several enzymes related to the immune system, and bactericidal activity. IgM levels were higher during the day in seabream and reached a minimum value at 20:00, but it was hardly affected in sea bass. No significant variations were recorded in the levels of protease and antiprotease. Peroxidase reached its maximum level in seabream at 02:00, the same time that it reached its minimum level in sea bass. Lysozyme showed little variation in seabream, but it was significantly lower at 14:00 than during the rest of the cycle in sea bass. Finally, different interspecific variations on bactericidal activity against Vibrio harveyi were recorded. The findings demonstrate that the immune parameters present in skin mucus of these important fish species are affected by the light–dark cycle and that there are substantial interspecies differences.

Melatonin effects on ovarian follicular cells: a systematic review

Melatonin is known for its effects on both the sleep and reproductive system of mammals. The latter has melatonin receptors type 1 and 2, which act to regulate, among other things, cyclic AMP. Notwithstanding all the literature data, there is still no sound knowledge or a clear understanding of the hormone's action on the physiology of ovarian follicular cells. OBJECTIVE To review and evaluate studies about melatonin action on the ovarian granulosa/theca interna cells from the literature. METHODS The systematic review was carried out according to the PRISMA recommendations. The MEDLINE and Cochrane primary databases were consulted with the use of specific terms. There was no limitation on language or publication year. RESULTS Seven papers about melatonin action on granulosa cells were selected. The following can be attributed to the hormone's effects: a) progesterone increase in culture medium; b) increased estrogen production; c) antagonistic action on estrogen; d) improvement in cell quality resulting in improved embryo and higher pregnancy rates; e) improved cell proliferation via MAPK; f) reduction of free radicals. Nevertheless, there are contrarian papers reporting a reduction in progesterone production. Melatonin interferes in sex steroid production, boosting progesterone output. Such action may help improve oocyte quality.

Formation of an antioxidant profile in the sea trout (Salmo trutta m. trutta L.) from the Slupia River

Using a stage- and sex-based multivariate significance tests on the sea trout Salmo trutta m. trutta L. model, we show dependencies in the balance between lipid peroxidation processes, levels of carbonyl derivatives, and activity of antioxidant enzymes (superoxide dismutase SOD, catalase CAT, glutathione reductase GR, and peroxidase GPx) in the processes of antioxidant profile formation during the fish growing process. The study was aimed at examination of the relationships between the biomarkers of oxidative stress estimated by the total antioxidant status as well as the dependencies between the sex (male, female) and developmental stage of the wild sea trout from the Slupia River and its catchment area rivers. Functioning of the pro/antioxidant balance of the liver tissue reflected the course of the individual developmental stages of the trout and was associated with significant intensification of lipoperoxidation, oxidative modification of proteins, and reduction of the total antioxidant capacity of fish along with age. Formation of a holistic model for the analysis of the involvement of all parameters of antioxidant protection in all stages of development and sex allowed us to obtain the following rank order for the level of lipoperoxidation processes, modified proteins, and antioxidant enzyme complex: CAT > SOD > GPx > GR and TBARS > OMP KD > TAC > OMP AD.

The genetic program of oocytes can be modified in vivo in the zebrafish ovary

Oocytes, the irreplaceable gametes for generating a new organism, are matured in the ovary of living female animals. It is unknown whether any genetic manipulations can be applied to immature oocytes inside the living ovaries. As a proof-of-concept, we here demonstrate genetic amendments of zebrafish immature oocytes within the ovary. Oocyte microinjection in situ (OMIS) stimulates tissue repair responses, but some of the microinjected immature oocytes are matured, ovulated and fertilizable. By OMIS-mediated Cas9 approach, ntla and gata5 loci of oocytes arrested at prophase I of meiosis are successfully edited before fertilization. Through OMIS, high efficiency of biallelic mutations in single or multiple loci using Cas9/gRNAs allows immediate manifestation of mutant phenotypes in F0 embryos and multiple transgenes can co-express the reporters in F0 embryos with patterns similar to germline transgenic embryos. Furthermore, maternal knockdown of dnmt1 by antisense morpholino via OMIS results in a dramatic decrease of global DNA methylation level at the dome stage and causes embryonic lethality prior to segmentation period. Therefore, OMIS opens a door to efficiently modify the genome and provides a possibility to repair genetically abnormal oocytes in situ.

Response of gastrointestinal melatonin, antioxidants, and digestive enzymes to altered feeding conditions in carp (Catla catla)

The purpose of present study was to ascertain whether the response of gastrointestinal (gut) melatonin to altered feeding conditions was related to the levels of different antioxidants and digestive enzymes in the same gut tissues of a sub-tropical carp (Catla catla). Accordingly, the fish were subjected to food deprivation for 4 or 8 days and separately to re-feeding for 4 or 8 or 12 days after deprivation of food for 8 days, and their gut tissue homogenates were used to measure the levels of melatonin, both enzymatic [superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST)] and non-enzymatic [reduced glutathione (GSH)] antioxidants, as well as different digestive enzymes (α-amylase, cellulase, protease, and lipase). Notably, the gut levels of melatonin, SOD, CAT, GPx, and GST underwent gradual increase with the progress of food deprivation, but a sudden fall after restoration of food supply for 4 days and a rise thereafter. Conversely, the activity of all the digestive enzymes significantly decreased after deprivation of food, but started increasing when food supply was reinforced. Gut melatonin concentrations by showing a positive correlation with the titers of different antioxidants (in both food-deprived and re-fed fish groups) and a negative (in food-deprived fish) or a positive (in re-fed fish) correlation with the activity of each digestive enzyme underlined possible physiological interplay between them. Collectively, our findings lend support to the hypothesis that gut melatonin response to altered feeding conditions in carp might be associated with the oxidative status as well as the digestive functions of the gastrointestinal tissues itself.

Seasonal Reproduction in Vertebrates: Melatonin Synthesis, Binding, and Functionality Using Tinbergen's Four Questions

One of the many functions of melatonin in vertebrates is seasonal reproductive timing. Longer nights in winter correspond to an extended duration of melatonin secretion. The purpose of this review is to discuss melatonin synthesis, receptor subtypes, and function in the context of seasonality across vertebrates. We conclude with Tinbergen's Four Questions to create a comparative framework for future melatonin research in the context of seasonal reproduction.

The effects of melatonin on colonization of neonate spermatogonial mouse stem cells in a three-dimensional soft agar culture system

BACKGROUND

Melatonin is a pleiotropic hormone with powerful antioxidant activity both in vivo and in vitro. The present study aimed to investigate the effects of melatonin on the proliferation efficiency of neonatal mouse spermatogonial stem cells (SSCs) using a three-dimensional soft agar culture system (SACS) which has the capacity to induce development of SSCs similar to in vivo conditions.

METHODS

SSCs were isolated from testes of neonate mice and their purities were assessed by flow cytometry using PLZF antibody. Isolated testicular cells were cultured in the upper layer of the SACS in αMEM medium in the absence or presence of melatonin extract for 4 weeks.

RESULTS

The identity of colonies was confirmed by alkaline phosphatase staining and immunocytochemistry using PLZF and α6 integrin antibodies. The number and diameter of colonies of SSCs in the upper layer were evaluated at days 14 and 28 of culture. The number and diameter of colonies of SSCs were significantly higher in the melatonin group compared with the control group. The levels of expression of ID-4 and Plzf, unlike c-kit, were significantly higher in the melatonin group than in the control group.

CONCLUSIONS

Results of the present study show that supplementation of the culture medium (SACS) with 100 μM melatonin significantly decreased reactive oxygen species (ROS) production in the treated group compared with the control group, and increased SSC proliferation.

Eye and heart morphogenesis are dependent on melatonin signaling in chick embryos

Calmodulin is vital for chick embryos morphogenesis in the incubation time 48-66 h when the rudimentary C-shaped heart attains an S-shaped pattern and the optic vesicles develop into optic cups. Melatonin is in the extraembryonic yolk sac of the avian egg; melatonin binds calmodulin. The aim of this study was to investigate the function of melatonin in the formation of the chick embryo optic cups and S-shaped heart, by pharmacological methods and immunoassays. Mel1a melatonin receptor immunofluorescence was distributed in the optic cups and rudimentary hearts. We separated embryonated chicken eggs at 48 h of incubation into basal, control and drug-treated groups, with treatment applied in the egg air sac. At 66 h of incubation, embryos were excised from the eggs and analyzed. Embryos from the basal, control (distilled water), melatonin and 6-chloromelatonin (melatonin receptor agonist) groups had regular optic cups and an S-shaped heart, while those from the calmidazolium (calmodulin inhibitor) group did not. Embryos from the luzindole (melatonin receptor antagonist) and prazosin (Mel1c melatonin receptor antagonist) groups did not have regular optic cups. Embryos from the 4-P-PDOT (Mel1b melatonin receptor antagonist) group did not have an S-shaped heart. Previous application of the melatonin, 6-chloromelatonin or forskolin (adenylate cyclase enhancer) prevented the abnormal appearance of chick embryos from the calmidazolium, luzindole, prazosin and 4-P-PDOT groups. However, 6-chloromelatonin and forskolin only partially prevented the development of defective eye cups in embryos from the calmidazolium group. The results suggested that melatonin modulates chick embryo morphogenesis via calmodulin and membrane receptors.