Diurnal variation in phagocytic activity of splenic phagocytes in freshwater teleost Channa punctatus: melatonin and its signaling mechanism

Abstract

The aim of the present study was to understand the rhythmic changes in innate immune response in freshwater fish Channa punctatus. Furthermore, the putative role of melatonin as the zeitgeber was explored. The phagocytic activity of splenic phagocytes assessed at 6-h intervals showed higher phagocytic activity during light phase than dark phase. The increased phagocytic activity during light phase was diminished by melatonin administration at 09:00 h. Implication of melatonin in control of diurnal variation in phagocytic activity was substantiated by administering irreversible tryptophan hydroxylase inhibitor, para-chlorophenylalanine (pCPA) at 18:00 h. pCPA abrogated the decrease of phagocytosis observed during dark phase, and the same was restored after melatonin administration. The direct involvement of melatonin in modulation of phagocytosis was demonstrated following in vitro experiments. Melatonin suppressed the phagocytic activity in a concentration-dependent manner without affecting the viability of phagocytes. The existence of functional membrane-bound melatonin receptors on fish phagocytes was pharmacologically demonstrated. Luzindole, melatonin membrane receptor antagonist, completely blocked the inhibitory effect of melatonin on phagocytosis. Further receptor-coupled adenylate cyclase-protein kinase A (PKA) pathway was implicated in transducing the melatonin effect as both adenylate cyclase and PKA inhibitor completely nullified the melatonininduced suppression. An increased intracellular cAMP level in response to melatonin ascertained the second messenger status of cAMP for downstream signaling. However, manipulation of phospholipase C/PKC failed to influence the effect of melatonin on phagocytic activity. These observations in C. punctatus evidenced the diurnal rhythmicity in phagocytic activity that is regulated by melatonin following membrane-bound receptorcoupled cAMP-PKA pathway.

Influence of Melatonin on the Immune System of Fish: A Review

“…One study reported evidence of diurnal rhythmicity in splenic phagocytic activity in a freshwater fish species ( Channa punctatus ) characterized by increased phagocytic activity during the light phase relative to the dark phase; the rhythmicity of the phagocytic activity was inhibited by exogenous melatonin administration [85]. Furthermore, the direct involvement of melatonin in the modulation of phagocytosis was demonstrated in in vitro experiments in which an irreversible tryptophan hydroxylase inhibitor (para-chlorophenylalanine, pCPA) known to diminish melatonin biosynthesis [86] was administered to fish.…”

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

“…Earlier Roy et al (2008) suggested that melatonin modulates the activity of Channa punctatus spleen phagocytes via a membrane receptor that is linked to the cAMP-PKA pathway, but no direct evidence for melatonin receptor expression was shown. In contrast, Confente et al (2010) did not find gene expression of MT1 receptors in blood cells of the Senegalese sole.…”

Dynorphin regulates the phagocytic activity of splenic phagocytes in wall lizards: involvement of a κ-opioid receptor-coupled adenylate-cyclase–cAMP–PKA pathway

“…In our earlier in vitro study in the wall lizard, dibutyryl cAMP was shown to inhibit the percentage phagocytosis and phagocytic index at higher concentrations (Roy and Rai, 2004). Also, an increased intracellular cAMP level in cortisoltreated (Roy and Rai, 2009) and melatonin-treated (Roy et al, 2008) splenic phagocytes is reported to be directly correlated with inhibition of phagocytosis in the freshwater fish Channa punctatus. In light of these studies, we speculated that dyn A (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) would have inhibited the phagocytic activity of lizard splenic phagocytes by activating the  s subunit of the G-protein coupled to -opioid receptors and, consequently, by increasing the intracellular cAMP level.…”