Melatonin partially minimizes the adverse stress effects in Senegalese sole (Solea senegalensis)

Senegalese sole is a highly valued marine teleost introduced in aquaculture since the early 1980s, but growth and survival from juvenile to market-size fish still reveal deficiencies, with stress being one of the possible explanations for this issue. High circulating levels of catecholamines and cortisol are involved in the integrated response to stress in fish and, thus triggering tissue alterations in order to counter the impact of the stressor and satisfy the increased energy demand. Chronic exposure to stressors can lead to an allostatic overload (or distress) affecting negatively food intake, reproduction, growth and immune functions, leading to increased incidence of diseases and reduced animal welfare. Melatonin is mainly synthesized in pineal organ and plays a critical role in the regulation of seasonal and circadian rhythms of physiology and behavior. However, a role as an anti-stress molecule at both central and peripheral levels has been also proposed for the hormone. Based on that, the aim of the present study was to evaluate the putative role of melatonin in reducing stress-related neuroendocrine and metabolic effects in Senegalese sole exposed to different and representative chronic stress conditions such as high stocking density (HSD) and low water replacement (LWR). Our results show that stressing Senegalese sole by high stocking density or low water replacement induce typical changes in parameters related to stress response in teleosts (increased plasma cortisol levels, enhanced liver glycogenolytic potential, and increased dopaminergic and serotonergic activities in the hypothalamus). Whereas melatonin treatment in non-stressed fish increases the dopaminergic activity in the hypothalamus, the opposite effect is observed after the addition of the hormone to tank water before and during stress exposure, thus attenuating the effect of stress on plasma cortisol and liver glycogen levels. The dopaminergic and serotonergic metabolism was also affected by melatonin treatment only in the hypothalamus on stressed fish. Thus, the presence of melatonin at night in tank water was able to counteract several of the effects induced by stress especially those related to cortisol synthesis and release, which is in support of an anti-stress role for melatonin in Senegalese sole. ► Growth and survival of the Senegalese sole reveal deficiencies. Stress might explain this issue. ► Fish physiology is negatively affected by high stocking density or low water re