Living organisms display an array of molecular, physiological and behavioral rhythms synchronized with natural environmental cycles. Understanding these interactions gains power when the complexity of natural habitats and the diversity of behavioral and physiological adaptations are taken into account. Brachyhypopomus gauderio, a South American weakly electric sh, are characterized by the emission of electrical discharges (EOD), with a very stable rate that is modulated by social and environmental cues. The nocturnal arousal in B. gauderio coincides with a melatonin-dependent EOD rate increase. Here we rst show a daily cycle in both the EOD-BR and EOD-BR variability of B. gauderio in nature. We approached the understanding of the role of melatonin on this natural behavior through both behavioral pharmacology and in vitro assays. We report, for the rst time in gymnotiformes, a direct effect of melatonin on the PN in vitro preparation. Melatonin treatment lowered EOD-BR in freely moving sh and PN-BR, while increasing the variability of both. These results show that melatonin plays a key role in modulating the electric behavior of B. gauderio through its effect on rate and variability, both of which must be under a tight temporal regulation to prepare the animal for a challenging nocturnal environment.

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Living organisms display an array of molecular, physiological and behavioral rhythms synchronized with natural environmental cycles. Understanding these interactions gains power when the complexity of natural habitats and the diversity of behavioral and physiological adaptations are taken into account. Brachyhypopomus gauderio, a South American weakly electric fish, are characterized by the emission of electrical discharges (EOD), with a very stable rate that is modulated by social and environmental cues. The nocturnal arousal in B. gauderio coincides with a melatonin-dependent EOD rate increase. Here we first show a daily cycle in both the EOD-BR and EOD-BR variability of B. gauderio in nature. We approached the understanding of the role of melatonin on this natural behavior through both behavioral pharmacology and in vitro assays. We report, for the first time in gymnotiformes, a direct effect of melatonin on the PN in vitro preparation. Melatonin treatment lowered EOD-BR in freely moving fish and PN-BR, while increasing the variability of both. These results show that melatonin plays a key role in modulating the electric behavior of B. gauderio through its effect on rate and variability, both of which must be under a tight temporal regulation to prepare the animal for a challenging nocturnal environment.