β-Endorphin regulates diverse functions of splenic phagocytes through different opioid receptors in freshwater fish Channa punctatus (Bloch): An in vitro study

In this in vitro study, the role of β-endorphin in the control of phagocytic and cytotoxic activities of fish splenic phagocytes was investigated. Further, the involvement of specific opioid receptor was explored. β-Endorphin stimulated phagocytosis, whereas inhibited nitric oxide production as assessed by nitrite release. However, it had concentration-related biphasic effects on superoxide production, stimulatory at low and inhibitory at high concentration. Naltrexone, non-selective opioid receptor antagonist, antagonized the effect of β-endorphin on phagocyte functions. Moreover, CTAP, selective μ-receptor antagonist, completely blocked the effect of β-endorphin on phagocytosis and nitrite release. With regard to superoxide production, CTAP blocked the stimulatory effect of β-endorphin at low concentration, while the inhibitory effect at high concentration was completely antagonized by selective δ-receptor antagonist, NTI. In conclusion, β-endorphin acting via μ-receptor stimulated phagocytosis and inhibited nitric oxide production, while its biphasic effect on superoxide production seems to be mediated by μ- and δ-receptors.