Effect of the pineal hormone melatonin on teleost fish phagocyte innate immune responses after in vitro treatment

Neuroendocrine-immune interaction: Evolutionarily conserved mechanisms that maintain allostasis in an ever-changing environment

It has now become accepted that the immune system and neuroendocrine system form an integrated part of our physiology. Immunological defense mechanisms act in concert with physiological processes like growth and reproduction, energy intake and metabolism, as well as neuronal development. Not only are psychological and environmental stressors communicated to the immune system, but also, vice versa, the immune response and adaptation to a current pathogen challenge are communicated to the entire body, including the brain, to evoke adaptive responses (e.g., fever, sickness behavior) that ensure allocation of energy to fight the pathogen. This phenomenon is evolutionarily conserved. Hence it is both interesting and important to consider the evolutionary history of this bi-directional neuroendocrine-immune communication to reveal phylogenetically ancient or relatively recently acquired mechanisms. Indeed, such considerations have already disclosed an extensive "common vocabulary" of information pathways as well as molecules and their receptors used by both the neuroendocrine and immune systems. This review focuses on the principal mechanisms of bi-directional communication and the evidence for evolutionary conservation of the important physiological pathways involved.

A role for melatonin in maintaining the pro- and anti-inflammatory balance by influencing leukocyte migration and apoptosis in carp

Melatonin is responsible for the synchronization of many physiological processes, including the immune response. Here we focus on the expression of melatonin MT1 receptors in/on leukocytes, and on the effects of melatonin administration on the inflammatory processes of carp. For the first time, we showed that fish leukocytes express MT1 receptors, implicating direct responsiveness to melatonin stimulation. Moreover, both in vitro and in vivo, melatonin modulated the immune response. The most potent effects of melatonin concerned the regulation of leukocyte migration. Melatonin reduced chemotaxis of leukocytes towards CXC chemokines in vitro. In vivo, during zymosan induced peritonitis, i.p. administration of melatonin reduced the number of neutrophils. This correlated with a melatonin-induced decrease of gene expression of the CXCa chemokine. Moreover, melatonin induced a decrease of the respiratory burst in inflammatory leukocytes. Although these data do suggest a potent anti-inflammatory function for this hormone, melatonin-induced inhibition of leukocyte apoptosis clearly indicates towards a dual function. These results show that also in carp, melatonin performs a pleiotropic and extra-pineal function that is important in maintaining the delicate pro- and anti-inflammatory balance during infection. They furthermore demonstrate that neuroendocrine-immune interaction via melatonin is evolutionary conserved.

Influence of melatonin on the immune system of fish: a review

Endocrine-immune system interactions have been widely demonstrated in mammals, whereas in fish, these relationships remain unclear. Of the organs that constitute the endocrine system, the pineal gland and its secretory product melatonin act in the synchronization of daily and seasonal rhythms in most vertebrates, including fish. Seasonal differences in immunocompetence and disease prevalence have been well documented in humans. Seasonality also strongly influences the life history of fish by controlling the timing of physiological events, such as reproduction, food intake, locomotor activity, and growth performance. Apart from its synchronizing capabilities, the role of melatonin in physiological processes in fish is not thoroughly understood. The purpose of this review is to summarize current studies on the effects of melatonin on the fish immune system. These studies suggest that melatonin represents an important component of fish endocrine-immune system interactions. The elucidation of the defense mechanisms of fish will facilitate the development of health management tools to support the growing finfish aquaculture industry as well as address questions concerning the origins and evolution of the immune system in vertebrates.

Nodavirus infection induces a great innate cell-mediated cytotoxic activity in resistant, gilthead seabream, and susceptible, European sea bass, teleost fish

Viral nervous necrosis (VNN) virus produces great mortalities in fish having susceptible and reservoir species between the most important marine aquaculture species. Cell-mediated cytotoxicity (CMC) is considered, towards the interferon (IFN), the most important mechanism of the immune response to fight against viral infections but it has been very scarcely evaluated. We aimed to evaluate the effects of VNNV infection in the reservoir gilthead seabream (Sparus aurata) and susceptible European sea bass (Dicentrarchus labrax). Firstly, after experimental infection we found mortalities in the sea bass (55%) but no in the seabream. Moreover, VNN virus replicates in the brain of both species as it was reflected by the high up-regulation of the Mx gene expression. Interestingly, the head-kidney leucocyte cell-mediated cytotoxic activity was significantly increased in both species reaching highest activity at 7 days: 3.65- and 2.7-fold increase in seabream and sea bass, respectively. This is supported by the significant up-regulation of the non-specific cytotoxic cell receptor (NCCRP-1) in the two fish species. By contrast, phagocytosis was unaffected in both species. The respiratory burst was increased in seabream 7 days post-infection whilst in sea bass this activity was significantly decreased at days 7 and 15. Our results demonstrate the significance of the CMC activity in both gilthead seabream and European sea bass against nodavirus infections but further studies are still needed to understand the role of cytotoxic cells in the antiviral immune response and the mechanisms involved in either reservoir or susceptible fish species.

In vivo actions of melatonin on the innate immune parameters in the teleost fish gilthead seabream

Melatonin, a molecule produced in the pineal gland and retina of vertebrates, plays a major role in day-night circadian rhythms and other physiological processes, including the immune responses. Because little is known in this respect in fish, we have evaluated the in vivo role of melatonin in the main innate immune activities and the expression of immune-relevant genes in a teleost fish, the gilthead seabream (Sparus aurata L.). An intraperitoneal injection of 1 or 10 mg melatonin/kg of body weight produced a dose-time dependent increase in circulating melatonin serum levels. Several innate immune responses such as peroxidase, phagocytic, reactive oxygen intermediates and cell-mediated cytotoxic activities were significantly enhanced by the administration of melatonin at different sampling times. The expression of immune-relevant genes such as interleukin-1beta, major histocompatibility complex, virus-related response (interferon-regulatory factor-1 and Mx) and lymphocyte markers (immunoglobulin M and T-cell receptor for B and T lymphocytes, respectively), as analysed by semi-quantitative reverse transcription-polymerase chain reaction, was up-regulated in the head-kidney of melatonin-injected fish 1 and 3 days postinjection and decreased after 7 days. These data, together with our previous observations describing how photoperiod and in vitro melatonin treatment may affect seabream and sea bass immunology, confirm melatonin as a regulator of fish immunology. However, further studies are still needed to reveal the mechanisms underlying the direct or indirect interactions of melatonin with the fish immune system.