Melatonin as a signaling molecule for metabolism regulation in response to hypoxia in the crab Neohelice granulata

A brief and updated introduction to the neuroendocrine system of crustaceans

The neuroendocrine system of crustaceans is complex and regulates many processes, such as development, growth, reproduction, osmoregulation, behavior, and metabolism. Once stimulated, crustaceans' neuroendocrine tissues modulate the release of monoamines, ecdysteroids, and neuropeptides that can act as hormones or neurotransmitters. Over a few decades, research has unraveled some mechanisms governing these processes, substantially contributing to understanding crustacean physiology. More aspects of crustacean neuroendocrinology are being comprehended with molecular biology, transcriptome, and genomics analyses. Hence, these studies will also significantly enhance the ability to cultivate decapods, such as crabs and shrimps, used as human food sources. In this review, current knowledge on crustacean endocrinology is updated with new findings about crustacean hormones, focusing mainly on the main neuroendocrine organs and their hormones and the effects of these molecules regulating metabolism, growth, reproduction, and color adaptation. New evidence about vertebrate-type hormones found in crustaceans is included and discussed. Finally, this review may assist in understanding how the emerging chemicals of environmental concern can potentially impair and disrupt crustacean's endocrine functions and their physiology.

CasEcR and CasMIH Genes in the Blue Crab, Callinectes sapidus: A Temporal Evaluation and Melatonin Effects

Environmental cues synchronize endogenous rhythms of many physiological processes such as hormone synthesis and secretion. Little is known about the diurnal pattern of hormones and gene expression of the Callinectes sapidus molt cycle. We aimed to investigate in the eyestalk and hepatopancreas of premolt and intermolt C. sapidus the following parameters: 1) the diurnal expression of the ecdysteroid receptor CasEcR isoforms, and the molt inhibiting hormone CasMIH; 2) the diurnal hemolymph ecdysteroid and melatonin levels; and 3) melatonin effects on the transcripts of the above-mentioned genes in intermolt C. sapidus. Ecdysteroid levels were higher in the premolt than the intermolt animals at all time points evaluated (ZTs). Premolt crabs displayed a variation of ecdysteroid concentration between time points, with a reduction at ZT17. No difference in the melatonin level was seen in either molt stage or between stages. In the eyestalk of intermolt animals, CasEcR expression oscillated, with a peak at ZT9, and premolt crabs have a reduction at ZT9; CasMIH transcripts did not vary along 24 h in either molt stage. Moreover, the evaluated eyestalk genes were more expressed at ZT9 in the intermolt than the premolt crabs. In the hepatopancreas, CasEcR expression showed a peak at ZT9 in premolt crabs. Exogenous melatonin (10⁻⁷ mol/animal) reduced the expression of both genes in the eyestalk at ZT17. In the hepatopancreas, melatonin markedly increased the expression of the CasEcR gene at ZT9. Taken altogether, our results are pioneer in demonstrating the daily oscillation of gene expression associated to molt cycle stages, as well as the daily ecdysteroid and melatonin levels and the remarkable influence of melatonin on the molt cycle of C. sapidus.

The protective effects of melatonin on survival, immune response, digestive enzymes activities and intestinal microbiota diversity in Chinese mitten crab (Eriocheir sinensis) exposed to glyphosate

Glyphosate is one of the most widely used pesticides, which can cause toxicity to aquatic animals. In this study, the survival rate, immune response, digestive enzyme activities, and the intestinal microbiota diversity of Chinese mitten crab (Eriocheir sinensis) were evaluated after 14 days of exposure to glyphosate (48.945 mg/L from 50% 96 h LC50 value) and melatonin feeding (80 mg/kg). The results showed that MT significantly improved the survival rate, antibacterial capacity of E. sinensis (P < 0.05). After exposure to glyphosate, the expression of Hsp60, Hsp70 and Hsp90 in cranial ganglia and thoracic ganglia was decreased significantly, but MT significantly raised the expression of these proteins (P < 0.05). Glyphosate significantly decreased lipase activity compared with the control group (P < 0.05), while melatonin significantly increased the lipase, amylase and trypsin activities (P < 0.05). Melatonin significantly increased the Chao1 and Shannon index and the relative abundance of Proteobacteria and Bacteroidetes (P < 0.05). This study shows that melatonin has a protective effect on the glyphosate exposed E. sinensis.

Melatonin Synthesis and Function: Evolutionary History in Animals and Plants

Melatonin is an ancient molecule that can be traced back to the origin of life. Melatonin's initial function was likely that as a free radical scavenger. Melatonin presumably evolved in bacteria; it has been measured in both α-proteobacteria and in photosynthetic cyanobacteria. In early evolution, bacteria were phagocytosed by primitive eukaryotes for their nutrient value. According to the endosymbiotic theory, the ingested bacteria eventually developed a symbiotic association with their host eukaryotes. The ingested α-proteobacteria evolved into mitochondria while cyanobacteria became chloroplasts and both organelles retained their ability to produce melatonin. Since these organelles have persisted to the present day, all species that ever existed or currently exist may have or may continue to synthesize melatonin in their mitochondria (animals and plants) and chloroplasts (plants) where it functions as an antioxidant. Melatonin's other functions, including its multiple receptors, developed later in evolution. In present day animals, via receptor-mediated means, melatonin functions in the regulation of sleep, modulation of circadian rhythms, enhancement of immunity, as a multifunctional oncostatic agent, etc., while retaining its ability to reduce oxidative stress by processes that are, in part, receptor-independent. In plants, melatonin continues to function in reducing oxidative stress as well as in promoting seed germination and growth, improving stress resistance, stimulating the immune system and modulating circadian rhythms; a single melatonin receptor has been identified in land plants where it controls stomatal closure on leaves. The melatonin synthetic pathway varies somewhat between plants and animals. The amino acid, tryptophan, is the necessary precursor of melatonin in all taxa. In animals, tryptophan is initially hydroxylated to 5-hydroxytryptophan which is then decarboxylated with the formation of serotonin. Serotonin is either acetylated to N-acetylserotonin or it is methylated to form 5-methoxytryptamine; these products are either methylated or acetylated, respectively, to produce melatonin. In plants, tryptophan is first decarboxylated to tryptamine which is then hydroxylated to form serotonin.

Melatonin Uptake by Cells: An Answer to Its Relationship with Glucose?

Melatonin, N-acetyl-5-methoxytryptamine, is an indole mainly synthesized from tryptophan in the pineal gland and secreted exclusively during the night in all the animals reported to date. While the pineal gland is the major source responsible for this night rise, it is not at all the exclusive production site and many other tissues and organs produce melatonin as well. Likewise, melatonin is not restricted to vertebrates, as its presence has been reported in almost all the phyla from protozoa to mammals. Melatonin displays a large set of functions including adaptation to light: dark cycles, free radical scavenging ability, antioxidant enzyme modulation, immunomodulatory actions or differentiation–proliferation regulatory effects, among others. However, in addition to those important functions, this evolutionary ‘ancient’ molecule still hides further tools with important cellular implications. The major goal of the present review is to discuss the data and experiments that have addressed the relationship between the indole and glucose. Classically, the pineal gland and a pinealectomy were associated with glucose homeostasis even before melatonin was chemically isolated. Numerous reports have provided the molecular components underlying the regulatory actions of melatonin on insulin secretion in pancreatic beta-cells, mainly involving membrane receptors MTNR1A/B, which would be partially responsible for the circadian rhythmicity of insulin in the organism. More recently, a new line of evidence has shown that glucose transporters GLUT/SLC2A are linked to melatonin uptake and its cellular internalization. Beside its binding to membrane receptors, melatonin transportation into the cytoplasm, required for its free radical scavenging abilities, still generates a great deal of debate. Thus, GLUT transporters might constitute at least one of the keys to explain the relationship between glucose and melatonin. These and other potential mechanisms responsible for such interaction are also discussed here.

Nanoencapsulated Melaleuca alternifolia essential oil exerts anesthetic effects in the brachyuran crab using Neohelice granulate

The aim of this study was to evaluate the efficacy and safety of several anesthetics in the brachyuran crab Neohelice granulata, an emergent experimental model. The essential oils (EOs) of Lippia alba, Aloysia tryphilla, and Melaleuca alternifolia (tea tree oil; TTO), the isolated compounds eugenol, menthol, terpinen-4-ol, and the nanoencapsulated form of TTO, were administered in one or more of the following ways: added to the water (immersion), through an arthrodial membrane (injected), or by oral gavage. Unexpectedly, most EOs did not produce an anesthetic effect after immersion. Only TTO and eugenol induced anesthesia by immersion, with very long induction and recovery times compared to anesthesia of other crustaceans. However, a good anesthetic effect was observed with the injection of terpinen-4-ol and nanoencapsulated TTO in N. granulata; both demonstrated ideal induction and recovery times. These substances appear to be promising anesthetic alternatives for crustaceans.

Melatonin Promotes Cheliped Regeneration, Digestive Enzyme Function, and Immunity Following Autotomy in the Chinese Mitten Crab, Eriocheir sinensis

In the pond culture of juvenile Eriocheir sinensis, a high limb-impairment rate seriously affects the culture success. Therefore, it is particularly important to artificially promote limb regeneration. This study evaluated the effects of melatonin on cheliped regeneration, digestive ability, and immunity, as well as its relationship with the eyestalk. It was found that the injection of melatonin significantly increased the limb regeneration rate compared with the saline group (P < 0.05). The qRT-PCR results of growth-related genes showed that the level of EcR-mRNA (ecdysteroid receptor) and Chi-mRNA (chitinase) expression was significantly increased following the melatonin injection, while the expression of MIH-mRNA (molt-inhibiting hormone) was significantly decreased (P < 0.05). Melatonin significantly increased lipase activity (P < 0.05). We observed that the survival rates of limb-impaired and unilateral eyestalk-ablated crabs were substantially improved following melatonin treatment, whereas the survival of the unilateral eyestalk-ablated crabs was significantly decreased compared with the control group (P < 0.05). Furthermore, the results of serum immune and antioxidant capacity revealed that melatonin significantly increased the total hemocyte counts (THC), hemocyanin content, total antioxidant capacity (T-AOC), acid phosphatase (ACP), and glutathione peroxidase activity (GSH-Px), whereas the immune-related parameters were significantly decreased in eyestalk-ablated crabs (P < 0.05). Therefore, these findings indicate that melatonin exerts a protective effect on organism injury, which could promote limb regeneration by up-regulating the expression of growth-related genes, improve digestive enzyme activity, and strengthen the immune response, particularly antioxidant capacity.

The Hyperglycemic Effect of Melatonin in the Chinese Mitten Crab, Eriocheir sinensis

Melatonin has been identified in a variety of invertebrate species, but its function is not as well understood as in crustaceans. The effects of melatonin on hemolymph glucose levels and tissue carbohydrate metabolism in the Chinese mitten crab, Eriocheir sinensis, were fully investigated in this study. Moreover, whether the eyestalk (an important endocrine center in invertebrate species) involves in this process or not, also were clarified. Analysis revealed that eyestalk ablation, especially bilateral, caused a significant decrease in the hemolymph glucose level. Moreover, injection of melatonin induced hyperglycemia in a dose-dependent manner both in intact and ablated crabs. Based on the expression of CHH mRNA in the 10 different tissues, eyestalk, thoracic ganglion, intestinal tract and hemolymph were selected to estimate the effect of melatonin on the expression of CHH mRNA. Bilateral eyestalk ablation caused a significant increase in the expression of CHH mRNA in the thoracic ganglion, intestinal tract and hemolymph compared with the controls. In addition, injection of melatonin into intact or ablated crabs elevated the CHH mRNA level in the eyestalk, thoracic ganglion and intestinal tract tissues compared with controls. The hemolymph CHH mRNA after melatonin injection was elevated only in ablated crabs. Administration of melatonin resulted in a significant decrease in total carbohydrates and glycogen levels with an increase in phosphorylase activity levels in the hepatopancreas and muscle in intact and ablated crabs. Our findings demonstrated that melatonin can induce hyperglycemic effects in both intact and ablated crabs, suggesting that this effect is probably not mediated solely via eyestalk.