Exploring the immune‐modulatory role of melatonin in head kidney and spleen cells of freshwater snakehead teleost, <i>Channa punctatus</i>

Melatonin, the ubiquitous hormone marks its presence from plants to animals with remarkable antioxidant and anti-inflammatory properties. In the teleost, melatonin is secreted from the pineal gland, which functions as a modulator of circadian rhythm, being homologous to the suprachiasmatic nuclei of mammals. The current study revolves around evaluating the role of in vitro melatonin in the immune mechanism of the snakehead fish, Channa punctatus. Reactive oxygen and nitrogen species production was assayed by respiratory burst and nitrite assay. Mitogen-stimulated lymphocyte proliferation was done using concanavalin A, phytohemagglutinin and lipopolysaccharide. Macrophage phagocytosis was also assessed using yeast cells. Melatonin administration to the cultures of spleen and head kidney cells enhanced phagocytic activity, superoxide and nitric oxide production. Direct action of melatonin was confirmed by using the melatonin receptor antagonist luzindole, which resulted in suppression of melatonin-mediated elevation of immune responses. Further, the use of adenylate cyclase inhibitor, SQ 22536, caused suppression of immune activity, signifying the involvement of cAMP in the downstream signalling cascade. This study concludes the immuno-stimulatory role and elucidates the signalling pathway of melatonin. The findings of the present study may help understand the melatonin-mediated immune modulation, which may result in altered disease resistance and mortality.