Melatonin and peripheral circuitries: insights on appetite and metabolism in Danio rerio

Zebrafish Feed Intake: A Systematic Review for Standardizing Feeding Management in Laboratory Conditions

Zebrafish are one of the most used animal models in biological research and a cost-effective alternative to rodents. Despite this, nutritional requirements and standardized feeding protocols have not yet been established for this species. This is important to avoid nutritional effects on experimental outcomes, and especially when zebrafish models are used in preclinical studies, as many diseases have nutritional confounding factors. A key aspect of zebrafish nutrition is related to feed intake, the amount of feed ingested by each fish daily. With the goal of standardizing feeding protocols among the zebrafish community, this paper systematically reviews the available data from 73 studies on zebrafish feed intake, feeding regimes (levels), and diet composition. Great variability was observed regarding diet composition, especially regarding crude protein (mean 44.98 ± 9.87%) and lipid content (9.91 ± 5.40%). Interestingly, the gross energy levels of the zebrafish diets were similar across the reviewed studies (20.39 ± 2.10 kilojoules/g of feed). In most of the reviewed papers, fish received a predetermined quantity of feed (feed supplied). The authors fed the fish according to the voluntary intake and then calculated feed intake (FI) in only 17 papers. From a quantitative point of view, FI was higher than when a fixed quantity (pre-defined) of feed was supplied. Also, the literature showed that many biotic and abiotic factors may affect zebrafish FI. Finally, based on the FI data gathered from the literature, a new feeding protocol is proposed. In summary, a daily feeding rate of 9-10% of body weight is proposed for larvae, whereas these values are equal to 6-8% for juveniles and 5% for adults when a dry feed with a proper protein and energy content is used.

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

Background

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

Results

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

Conclusions

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

Melatonin as a Mediator of the Gut Microbiota-Host Interaction: Implications for Health and Disease

In recent years, the role played by melatonin on the gut microbiota has gained increasingly greater attention. Additionally, the gut microbiota has been proposed as an alternative source of melatonin, suggesting that this antioxidant indoleamine could act as a sort of messenger between the gut microbiota and the host. This review analyses the available scientific literature about possible mechanisms involved in this mediating role, highlighting its antioxidant effects and influence on this interaction. In addition, we also review the available knowledge on the effects of melatonin on gut microbiota composition, as well as its ability to alleviate dysbiosis related to sleep deprivation or chronodisruptive conditions. The melatonin-gut microbiota relationship has also been discussed in terms of its role in the development of different disorders, from inflammatory or metabolic disorders to psychiatric and neurological conditions, also considering oxidative stress and the reactive oxygen species-scavenging properties of melatonin as the main factors mediating this relationship.

Circadian Rhythm and Endocrinological Control on the Swimming and Sand Burrowing Behaviors of Japanese Sand Lances Ammodytes spp. (Uranoscopiformes, Ammodytidae)

In diurnal and nocturnal organisms, daily activity is regulated by the perception of environmental stimuli and circadian rhythms, which enable organisms to maintain their essential behaviors. The Japanese sand lances genus Ammodytes are coastal marine fish that exhibit unique nocturnal sand burrowing behavior. To elucidate the extrinsic and intrinsic regulation of this behavior and its endocrinological basis, we conducted a series of rearing experiments under various light conditions and hormone administrations. Under a light-dark photoperiod, the fish showed three types of behavior: sand buried, head-exposed from sand, and swimming/feeding. During the transition from dark to light periods, the fish first showed head exposure, followed by swimming and foraging, and buried themselves in the sand immediately after shifting to the dark period. Under constant light conditions, fish exhibited swimming behavior during the period corresponding to the acclimated light period. In addition, swimming did not occur under constant dark conditions but head exposure was observed at the time of the dark-light transition during acclimation. These observations indicate that the essential behavior of sand lances is regulated by both light and circadian rhythms. Subsequently, a melatonin-containing diet promoted the onset of burrowing in 10 to 120 min in a dose-dependent manner at 0.3-128 µg/g-diet, suggesting the direct behavioral regulation by this hormone. These findings suggest that the behavior of sand lances is strictly regulated by an intrinsic mechanism and that melatonin is a regulatory endocrine factor that induces burrowing behavior.

Duplicated zebrafish (Danio rerio) inositol phosphatases inpp5ka and inpp5kb diverged in expression pattern and function

One hurdle in the development of zebrafish models of human disease is the presence of multiple zebrafish orthologs resulting from whole genome duplication in teleosts. Mutations in inositol polyphosphate 5-phosphatase K (INPP5K) lead to a syndrome characterized by variable presentation of intellectual disability, brain abnormalities, cataracts, muscle disease, and short stature. INPP5K is a phosphatase acting at position 5 of phosphoinositides to control their homeostasis and is involved in insulin signaling, cytoskeletal regulation, and protein trafficking. Previously, our group and others have replicated the human phenotypes in zebrafish knockdown models by targeting both INPP5K orthologs inpp5ka and inpp5kb. Here, we show that inpp5ka is the more closely related orthologue to human INPP5K. While both inpp5ka and inpp5kb mRNA expression levels follow a similar trend in the developing head, eyes, and tail, inpp5ka is much more abundantly expressed in these tissues than inpp5kb. In situ hybridization revealed a similar trend, also showing unique localization of inpp5kb in the pineal gland and retina indicating different transcriptional regulation. We also found that inpp5kb has lost its catalytic activity against its preferred substrate, PtdIns(4,5)P2. Since most human mutations are missense changes disrupting phosphatase activity, we propose that loss of inpp5ka alone can be targeted to recapitulate the human presentation. In addition, we show that the function of inpp5kb has diverged from inpp5ka and may play a novel role in the zebrafish.

The Zebrafish, an Outstanding Model for Biomedical Research in the Field of Melatonin and Human Diseases

The zebrafish has become an excellent model for the study of human diseases because it offers many advantages over other vertebrate animal models. The pineal gland, as well as the biological clock and circadian rhythms, are highly conserved in zebrafish, and melatonin is produced in the pineal gland and in most organs and tissues of the body. Zebrafish have several copies of the clock genes and of aanat and asmt genes, the latter involved in melatonin synthesis. As in mammals, melatonin can act through its membrane receptors, as with zebrafish, and through mechanisms that are independent of receptors. Pineal melatonin regulates peripheral clocks and the circadian rhythms of the body, such as the sleep/wake rhythm, among others. Extrapineal melatonin functions include antioxidant activity, inducing the endogenous antioxidants enzymes, scavenging activity, removing free radicals, anti-inflammatory activity through the regulation of the NF-κB/NLRP3 inflammasome pathway, and a homeostatic role in mitochondria. In this review, we introduce the utility of zebrafish to analyze the mechanisms of action of melatonin. The data here presented showed that the zebrafish is a useful model to study human diseases and that melatonin exerts beneficial effects on many pathophysiological processes involved in these diseases.

New Eco-Sustainable Feed in Aquaculture: Influence of Insect-Based Diets on the Content of Potentially Toxic Elements in the Experimental Model Zebrafish (Danio rerio)

According to the concept of circular economy, insects represent good candidates as aquafeed ingredients. Nevertheless, there are some potential chemical risks linked with insect consumption. In this study, we reared the teleost Danio rerio, used as an experimental model, with five experimental diets characterized by increasing levels (0%, 25%, 50%, 75%, and 100%) of full-fat Hermetia illucens (Hi) prepupae, substituting for fish meal (FM) and fish oil (FO). We investigated the presence of potentially toxic elements (PTEs) Cd, Pb, Ni, As, and Hg in larval (20 days), juvenile (2 months), and adult (6 months) fish. Quantitative determinations of Cd, Pb, Ni, and As were made with an atomic absorption spectrometer; the total mercury content was determined by a direct mercury analyzer. The substitution of FM and FO with Hermetia illucens meal led to a reduction in the content of some PTEs, such as Pb, As, and Ni, in fishfeed, leading to concentrations below the legal limit of undesirable substances in animal feed. By increasing the Hi meal dietary content, we observed in the Danio rerio specimens an increase in Cd, Pb, and Ni content and a reduction in As content for all life stages. Moreover, a general increase in the content of Cd, Pb, Hg, and Ni from larvae to juvenile was measured, while the shift of Danio rerio from the juvenile to the adult stage involved a significant increase in the content of Pb, Hg, and Ni. Larvae had a reduced ability to bioaccumulate metal(loid)s compared to juveniles and adults. In conclusion, the content of PTEs in Danio rerio is influenced both by the type of diet administered and by the life stage of the animal itself. This research demonstrates the possibility of using Hi prepupae as an aquafeed ingredient without exposing fish to a chemical risk and, in perspective, allows applying these eco-sustainable diets for the breeding of edible fish species, without endangering human health.

The probiotic Lactobacillus rhamnosus mimics the dark-driven regulation of appetite markers and melatonin receptors' expression in zebrafish (Danio rerio) larvae: Understanding the role of the gut microbiome

The use of probiotics has been recently considered a novel therapeutic strategy to prevent pathologies such as obesity; however, the specific mechanisms of action by which probiotics exert their beneficial effects on metabolic health remain unclear. The aim of the present study was to investigate the short-term effects of a probiotic Lactobacillus rhamnosus supplementation (PROB) on appetite regulation, growth-related markers, and microbiota diversity in zebrafish (Danio rerio) larvae, compared to a group subjected to a constant darkness photoperiod (DARK), as well as to evaluate the effects of both treatments on melatonin receptors' expression. After a 24 h treatment, both PROB and DARK conditions caused a significant increase in leptin a expression. Moreover, mRNA abundances of leptin b and proopiomelanocortin a were elevated in the PROB group, and DARK showed a similar tendency, supporting a negative regulation of appetite markers by the treatments. Moreover, both PROB and DARK also enhanced the abundances of melatonin receptors transcript (melatonin receptor 1 ba and bb) and protein (melatonin receptor 1) suggesting a potential involvement of melatonin in mediating these effects. Nevertheless, treatments did not exhibit a significant effect on the expression of most of the growth hormone/insulin-like growth factor axis genes evaluated. Finally, only the DARK condition significantly modulated gut microbiota diversity at such short time, altogether highlighting the rapid effects of this probiotic on modulating appetite regulatory and melatonin receptors' expression, without a concomitant variation of gut microbiota.

Effect of Melatonin on the Reduction of Hepatic Steatosis and Intestinal Leptin Expression in Zebrafish Exposed to Fructose

Melatonin is a hormone related to circadian rhythms and has potential clinical applications. Our objectives were to verify the effect of melatonin on the liver of zebrafish exposed to fructose and evaluate the expression of appetite-related genes (leptin, ghrelin, and melanocortin receptor 4 [MC4R]). Animals were divided into three groups: control (CG, n = 25), fructose (FG, n = 25), and fructose+melatonin (FMG, n = 25). The study was carried out in 8 weeks. FG and FMG were exposed to 2% fructose and FMG treated with 1 μM of melatonin. Histological liver studies and gene expression analyses of Leptin, Ghrelin, and MC4R (liver and intestines) were performed. FG developed hepatic steatosis, which did not occur with CG and FMG. Genetic expression of hepatic leptin and MC4R did not show significant difference among the groups. Animals exposed to fructose (FG) presented an increased expression of intestinal leptin compared to those administered with melatonin. Animals exposed to fructose gained weight and developed an important hepatic steatosis, but melatonin reduced significantly the hepatic damage. Intestinal leptin showed increased expression in the group exposed to fructose.

Melatonin for an obese child with MC4R gene variant showing epilepsy and disordered sleep: A case report

BACKGROUND

Abnormalities in the melanocortin receptor 4 (MC4R) gene often lead to obesity, but are rarely associated with other conditions such as epilepsy and sleep disorder.

CASE SUMMARY

Here, we present a case of a male obese child with a heterozygous variant in MC4R (c.494G>A, p.Arg165Gln) inherited from his father, who presented with disordered sleep and abnormal facial movements. Examination through melatonin rhythm testing and electroencephalography led to a diagnosis of sleep disorder and epilepsy, as his melatonin rhythm was markedly distorted and the electroencephalography revealed epileptic discharges. He received treatment with an antiepileptic drug; however, the therapy was ineffective and the sleep disorder appeared to be deteriorating. Subsequently, we initiated adjuvant treatment with melatonin. Upon re-examination, his body mass index had decreased, the sleep disturbance had resolved, and his seizures were well controlled. Electro-encephalography review was normal, and a typical melatonin rhythm was restored.

CONCLUSION

We concluded that, in addition to causing obesity, abnormalities in the MC4R gene may contribute to the development of sleep disorders and epilepsy, and that melatonin can be used as an adjuvant therapy to alleviate these symptoms.

Gut melatonin: A potent candidate in the diversified journey of melatonin research

After being discovered from the bovine pineal gland by Aaron Lerner and co-workers in the year 1958, various distinguished researchers have reported melatonin (5-methoxy-N-acetyl-tryptamine) from several extra-pineal sources, including the gastrointestinal tract (GIT). In the year 1974, Raikhlin and Kvetnoy first detected this molecule in the gastrointestinal tissue. Later, within the last 45 years, many renowned investigators found that the GIT is a rich source of melatonin, in addition to the pineal gland. In the carp gut, the estimation of Arylalkylamine-N-acetyltransferase (AANAT) mRNA/protein levels, which is the rate-determining enzyme for melatonin biosynthesis in the pineal gland, confirmed the endogenous synthesis of melatonin. The remarkable feature of the pineal gland melatonin is its rhythmic synthesis with a peak at dark-phase and lowest at light-phase in synchronization with seasonal environmental light-dark (LD) cycle. Recent studies on carp demonstrated that the melatonin concentrations and the AANAT protein intensities in different gut segments underwent significant daily fluctuations. However, compared to the melatonin rhythm in the pineal gland, the melatonin profiles in gut tissue displayed daily rhythm in parallel with the feeding cycle of the carp, irrespective of LD conditions of the environment. Notably, in carp, the temporal pattern of the gut melatoninergic system found to vary with the environmental non-photic signal(s), such as food entrainment factors (viz. availability of food, timing of food supply, number(s) of feed per day, quality of food) those act as the most dependable synchronizer(s) in daily rhythm characteristics of gut melatonin and AANAT. Thereby in this review, it appears meaningful to highlight the existing data on the mode of synthesis of melatonin in cells of the digestive tract, and most importantly, the regulation of its synthesis. Finally, in comparison with the dynamic actions of melatonin derived from the pineal gland, this review will lead to underline the role of gut-derived melatonin in a variety of physiological functions.

Size-selective mortality induces evolutionary changes in group risk-taking behaviour and the circadian system in a fish

Intensive and trait-selective mortality of fish and wildlife can cause evolutionary changes in a range of life-history and behavioural traits. These changes might in turn alter the circadian system due to co-evolutionary mechanisms or correlated selection responses both at behavioural and molecular levels, with knock-on effects on daily physiological processes and behavioural outputs. We examined the evolutionary impact of size-selective mortality on group risk-taking behaviour and the circadian system in a model fish species. We exposed zebrafish Danio rerio to either large or small size-selective harvesting relative to a control over five generations, followed by eight generations during which harvesting was halted to remove maternal effects. Size-selective mortality affected fine-scale timing of behaviours. In particular, small size-selective mortality, typical of specialized fisheries and gape-limited predators targeting smaller size classes, increased group risk-taking behaviuor during feeding and after simulated predator attacks. Moreover, small size-selective mortality increased early peaks of daily activity as well as extended self-feeding daily activity to the photophase compared to controls. By contrast large size-selective mortality, typical of most wild capture fisheries, only showed an almost significant effect of decreasing group risk-taking behaviour during the habituation phase and no clear changes in fine-scale timing of daily behavioural rhythms compared to controls. We also found changes in the molecular circadian core clockwork in response to both size-selective mortality treatments. These changes disappeared in the clock output pathway because both size-selected lines showed similar transcription profiles. This switch downstream to the molecular circadian core clockwork also resulted in similar overall behavioural rhythms (diurnal swimming and self-feeding in the last hours of darkness) independent of the underlying molecular clock. To conclude, our experimental harvest left an asymmetrical evolutionary legacy in group risk-taking behaviour and in fine-scale daily behavioural rhythms. Yet, the overall timing of activity showed evolutionary resistance probably maintained by a molecular switch. Our experimental findings suggest that size-selective mortality can have consequences for behaviour and physiological processes.

Zebrafish (Danio rerio) physiological and behavioural responses to insect-based diets: a multidisciplinary approach

Black Soldier Fly (BSF) meal is considered as an alternative, emerging and sustainable ingredient for aquafeed production. However, results on fish physiological responses are still fragmentary and often controversial, while no studies are available on fish behavior in response to these new diets. The present work represents the first comprehensive multidisciplinary study aimed to investigate zebrafish physiological and behavioural responses to BSF-based diets. Five experimental diets characterized by increasing inclusion levels (0, 25, 50, 75 and 100% respect to fish meal) of full fat BSF prepupae meal were tested during a 2-months feeding trial. Prepupae were cultured on coffee silverskin growth substrate enriched with a 10% Schizochytrium sp. to improve insects’ fatty acids profile. The responses of zebrafish were assayed through biometric, histological, gas chromatographic, microbiological, spectroscopic, molecular and behavioural analyses. Results evidenced that BSF-based diets affected fish fatty acid composition, while behavioural tests did not show differences among groups. Specifically, a 50% BSF inclusion level diet represented the best compromise between ingredient sustainability and proper fish growth and welfare. Fish fed with higher BSF inclusions (75 and 100%) showed hepatic steatosis, microbiota modification, higher lipid content, fatty acid modification and higher expression of immune response markers.

Mechanisms for Temperature Modulation of Feeding in Goldfish and Implications on Seasonal Changes in Feeding Behavior and Food Intake

In fish models, seasonal change in feeding is under the influence of water temperature. However, the effects of temperature on appetite control can vary among fish species and the mechanisms involved have not been fully characterized. Using goldfish (Carassius auratus) as a model, seasonal changes in feeding behavior and food intake were examined in cyprinid species. In our study, foraging activity and food consumption in goldfish were found to be reduced with positive correlation to the gradual drop in water temperature occurring during the transition from summer (28.4 ± 2.2°C) to winter (15.1 ± 2.6°C). In goldfish with a 4-week acclimation at 28°C, their foraging activity and food consumption were notably higher than their counterparts with similar acclimation at 15°C. When compared to the group at 28°C during summer, the attenuation in feeding responses at 15°C during the winter also occurred with parallel rises of leptin I and II mRNA levels in the liver. Meanwhile, a drop in orexin mRNA along with concurrent elevations of CCK, MCH, POMC, CART, and leptin receptor (LepR) transcript expression could be noted in brain areas involved in feeding control. In short-term study, goldfish acclimated at 28°C were exposed to 15°C for 24 h and the treatment was effective in reducing foraging activity and food intake. The opposite was true in reciprocal experiment with a rise in water temperature to 28°C for goldfish acclimated at 15°C. In parallel time-course study with lowering of water temperature from 28 to 15°C, short-term exposure (6-12 h) of goldfish to 15°C could also increase leptin I and II mRNA levels in the liver. Similar to our seasonality study, transcript level of orexin was reduced along with up-regulation of CCK, MCH, POMC, CART, and LepR gene expression in different brain areas. Our results, as a whole, suggest that temperature-driven regulation of leptin output from the liver in conjunction with parallel modulations of orexigenic/anorexigenic signals and leptin responsiveness in the brain may contribute to the seasonal changes of feeding behavior and food intake observed in goldfish.

Neuroendocrine-Metabolic Dysfunction and Sleep Disturbances in Neurodegenerative Disorders: Focus on Alzheimer's Disease and Melatonin

Alzheimer's disease (AD) is associated with altered eating behavior and metabolic disruption. Amyloid plaques and neurofilament tangles are observed in many hypothalamic nuclei from AD brains. Some of these areas (suprachiasmatic nuclei, lateral hypothalamic area) also play a role in the regulation of the sleep/wake cycle and may explain the comorbidity of eating and sleep disorders observed in AD patients. Inadequate sleep increases the neurodegenerative process, for example, the decrease of slow-wave sleep impairs clearance of β-amyloid peptide (Aβ) and tau protein from cerebral interstitial fluid. Cerebrospinal fluid (CSF) melatonin levels decrease even in preclinical stages (Braak-1 stage) when patients manifest no cognitive impairment, suggesting that reduction of melatonin in CSF may be an early marker (the cause for which is still unknown) of oncoming AD. Melatonin administration augments glymphatic clearance of Aβ and reduces generation and deposition of Aβ in transgenic animal models of AD. It may also set up a new equilibrium among hypothalamic feeding signals. While melatonin trials performed in the clinical phase of AD have failed to show or showed only modest positive effects on cognition, in the preclinical stage of dementia (minimal cognitive impairment) the effect of melatonin is demonstrable with significant improvement of sleep and quality of life. In this review, we discuss the main aspects of hypothalamic alterations in AD, the association between interrupted sleep and neurodegeneration, and the possible therapeutic effect of melatonin on these processes.

Melatonin treatment suppresses appetite genes and improves adipose tissue plasticity in diet-induced obese zebrafish

PURPOSE

Overweight and obesity are important risk factors for diabetes, cardiovascular diseases, and premature death in modern society. Recently, numerous natural and synthetic compounds have been tested in diet-induced obese animal models, to counteract obesity. Melatonin is a circadian hormone, produced by pineal gland and extra-pineal sources, involved in processes which have in common a rhythmic expression. In teleost, it can control energy balance by activating or inhibiting appetite-related peptides. The study aims at testing effects of melatonin administration to control-fed and overfed zebrafish, in terms of expression levels of orexigenic (Ghrelin, orexin, NPY) and anorexigenic (leptin, POMC) genes expression and morphometry of visceral and subcutaneous fat depots.

METHODS

Adult male zebrafish (n = 56) were divided into four dietary groups: control, overfed, control + melatonin, overfed + melatonin. The treatment lasted 5 weeks and BMI levels of every fish were measured each week. After this period fishes were sacrificed; morphological and morphometric studies have been carried out on histological sections of adipose tissue and adipocytes. Moreover, whole zebrafish brain and intestine were used for qRT-PCR.

RESULTS

Our results demonstrate that melatonin supplementation may have an effect in mobilizing fat stores, in increasing basal metabolism and thus in preventing further excess fat accumulation. Melatonin stimulates the anorexigenic and inhibit the orexigenic signals.

CONCLUSIONS

It seems that adequate melatonin treatment exerts anti-obesity protective effects, also in a diet-induced obesity zebrafish model, that might be the result of the restoration of many factors: the final endpoint reached is weight loss and stabilization of weight gain.

Circadian dysrhythmia-linked diabetes mellitus: Examining melatonin’s roles in prophylaxis and management

Diabetes mellitus is a chronic, life-threatening metabolic disorder that occurs worldwide. Despite an increase in the knowledge of the risk factors that are associated with diabetes mellitus, its worldwide prevalence has continued to rise; thus, necessitating more research into its aetiology. Recent researches are beginning to link a dysregulation of the circadian rhythm to impairment of intermediary metabolism; with evidences that circadian rhythm dysfunction might play an important role in the aetiology, course or prognosis of some cases of diabetes mellitus. These evidences thereby suggest possible relationships between the circadian rhythm regulator melatonin, and diabetes mellitus. In this review, we discuss the roles of the circadian rhythm in the regulation of the metabolism of carbohydrates and other macronutrients; with emphasis on the importance of melatonin and the impacts of its deficiency on carbohydrate homeostasis. Also, the possibility of using melatonin and its analogs for the “prophylaxis” or management of diabetes mellitus is also considered.

The role of melatonin in the onset and progression of type 3 diabetes

Alzheimer’s disease (AD) is defined by the excessive accumulation of toxic peptides, such as beta amyloid (Aβ) plaques and intracellular neurofibrillary tangles (NFT). The risk factors associated with AD include genetic mutations, aging, insulin resistance, and oxidative stress. To date, several studies that have demonstrated an association between AD and diabetes have revealed that the common risk factors include insulin resistance, sleep disturbances, blood brain barrier (BBB) disruption, and altered glucose homeostasis. Many researchers have discovered that there are mechanisms common to both diabetes and AD. AD that results from insulin resistance in the brain is termed “type 3 diabetes”. Melatonin synthesized by the pineal gland is known to contribute to circadian rhythms, insulin resistance, protection of the BBB, and cell survival mechanisms. Here, we review the relationship between melatonin and type 3 diabetes, and suggest that melatonin might regulate the risk factors for type 3 diabetes. We suggest that melatonin is crucial for attenuating the onset of type 3 diabetes by intervening in Aβ accumulation, insulin resistance, glucose metabolism, and BBB permeability.

Dietary administration of EDC mixtures: A focus on fish lipid metabolism

Many man-made chemical compounds are recognized as endocrine disruptors and once released into the environment are likely to spread and bioaccumulate in wild species. Due to their lipophilic nature, these substances pass through the cell membrane or bind to specific receptors activating physiological responses that in the long run can cause reproductive impairment, physiological disorders, including the occurrence of metabolic syndromes. One significant source of contamination is represented by the consumption of polluted food. As a consequence, different environmental pollutants, with similar or different modes of action, can accumulate in organisms and biomagnify along the food web, finally targeting humans. The aim of this study was to analyze, under controlled conditions, the effects induced by the consumption of contaminated diets, focusing on the effects exerted at hepatic level. Juvenile seabream were fed for 21days a diet enriched with different combinations of pollutants, nonylphenol (NP), tert-octylphenol (t-OP) and bisphenol A (BPA). The different diets containing 5mg/kg bw of each contaminant, were formulated as follows: NP+tOP, BPA+NP, BPA+tOP and NP+BPA+tOP (NBO). EDCs, at the doses administered, showed low biomagnification factor (BMF), suggesting that these pollutants hardly accumulate in muscles. The results obtained at hepatic level pinpointed the steatotic effect of all the administered diets, associated to a modulation of the expression of genes involved in lipid metabolism (ppars, fas, lpl, and hsl). Results were compared to those obtained in previous studies in which fish were fed single pollutants evidencing that the administration of mixture of contaminants exerts a milder lipogenic effect, highlighting the contrasting/antagonistic interaction establishing among chemicals. Noteworthy was the setup of a new chromatographic method to detect the presence of the selected chemical in fish muscle and the application of Fourier Transform Infrared (FT-IR) analysis to evaluate pollutant-induced changes in the liver macromolecular building.

Genetically Blocking the Zebrafish Pineal Clock Affects Circadian Behavior

The master circadian clock in fish has been considered to reside in the pineal gland. This dogma is challenged, however, by the finding that most zebrafish tissues contain molecular clocks that are directly reset by light. To further examine the role of the pineal gland oscillator in the zebrafish circadian system, we generated a transgenic line in which the molecular clock is selectively blocked in the melatonin-producing cells of the pineal gland by a dominant-negative strategy. As a result, clock-controlled rhythms of melatonin production in the adult pineal gland were disrupted. Moreover, transcriptome analysis revealed that the circadian expression pattern of the majority of clock-controlled genes in the adult pineal gland is abolished. Importantly, circadian rhythms of behavior in zebrafish larvae were affected: rhythms of place preference under constant darkness were eliminated, and rhythms of locomotor activity under constant dark and constant dim light conditions were markedly attenuated. On the other hand, global peripheral molecular oscillators, as measured in whole larvae, were unaffected in this model. In conclusion, characterization of this novel transgenic model provides evidence that the molecular clock in the melatonin-producing cells of the pineal gland plays a key role, possibly as part of a multiple pacemaker system, in modulating circadian rhythms of behavior.

Zebrafish dives into food research: effectiveness assessment of bioactive compounds

Zebrafish ease of use and characteristics reveal it to be an interesting and underused model in food and nutrition research.

Daily rhythms of lipid metabolic gene expression in zebra fish liver: Response to light/dark and feeding cycles

Despite numerous studies about fish nutrition and lipid metabolism, very little is known about the daily rhythm expression of lipogenesis and lipolysis genes. This research aimed to investigate the existence of daily rhythm expressions of the genes involved in lipid metabolism and their synchronization to different light/dark (LD) and feeding cycles in zebra fish liver. For this purpose, three groups of zebra fish were submitted to a 12:12 h LD cycle. A single daily meal was provided to each group at various times: in the middle of the light phase (ML); in the middle of the dark phase (MD); at random times. After 20 days of acclimation to these experimental conditions, liver samples were collected every 4 h in one 24-h cycle. The results revealed that most genes displayed a significant daily rhythm with an acrophase of expression in the dark phase. The acrophase of lipolytic genes (lipoprotein lipase - lpl, peroxisome proliferator-activated receptor - pparα and hydroxyacil CoA dehydrogenase - hadh) was displayed between ZT 02:17 h and ZT 18:31 h. That of lipogenic genes (leptin-a - lepa, peroxisome proliferator-activated receptor - pparγ, liver X receptor - lxr, insulin-like growth factor - igf1, sterol regulatory element-binding protein - srebp and fatty acid synthase - fas) was displayed between ZT 15:25 h and 20:06 h (dark phase). Feeding time barely influenced daily expression rhythms, except for lxr in the MD group, whose acrophase shifted by about 14 h compared with the ML group (ZT 04:31 h versus ZT 18:29 h, respectively). These results evidence a strong synchronization to the LD cycle, but not to feeding time, and most genes showed a nocturnal acrophase. These findings highlight the importance of considering light and feeding time to optimize lipid metabolism and feeding protocols in fish farming.

Melatonin regulates aging and neurodegeneration through energy metabolism, epigenetics, autophagy and circadian rhythm pathways

Brain aging is linked to certain types of neurodegenerative diseases and identifying new therapeutic targets has become critical. Melatonin, a pineal hormone, associates with molecules and signaling pathways that sense and influence energy metabolism, autophagy, and circadian rhythms, including insulin-like growth factor 1 (IGF-1), Forkhead box O (FoxOs), sirtuins and mammalian target of rapamycin (mTOR) signaling pathways. This review summarizes the current understanding of how melatonin, together with molecular, cellular and systemic energy metabolisms, regulates epigenetic processes in the neurons. This information will lead to a greater understanding of molecular epigenetic aging of the brain and anti-aging mechanisms to increase lifespan under healthy conditions.

Anti-inflammatory activities of melatonin derivatives in lipopolysaccharide-stimulated RAW 264.7 cells and antinociceptive effects in mice

Preclinical Research This study describes the anti-inflammatory activities of two semisynthesized melatonin (MT) derivatives, benzoyl-melatonin (BMT) and acetyl-melatonin (AMT), on the production of pro-inflammatory mediators in lipopolysaccharide (LPS)-stimulated macrophage cells (RAW 264.7) and their antinociceptive effects in mice. The MT derivatives inhibited production of nitric oxide NO and prostaglandin E2 in LPS-stimulated RAW264.7 cells in a dose-dependent manner with IC50 values lower than those of MT. BMT produced increased tail flick latency time, decreased number of writhes, and reduced nociceptive response in mice when compared with AMT and MT. BMT and AMT had enhanced anti-inflammatory effects in LPS-stimulated RAW264.7 compared with MT. However, in mouse studies BMT exhibited the highest potency as an anti-inflammatory agent and was longer-acting as an antinociceptive compound compared with AMT or MT, suggesting that BMT has potential as an anti-inflammatory and analgesic compound.