Mu and delta receptors mediate morphine effects on phagocytosis by murine peritoneal macrophages.

Studies with selective opioid agonists show that mu- and delta(2)-opioid receptors, but not kappa, are involved in opioid inhibition of phagocytosis in elicited murine macrophages. All mu and delta(2) agonists tested had similar maximal effects on phagocytosis, and all dose-response curves suggest positive cooperativity. In addition, mu and delta antagonists antagonized the effect of both mu and delta agonists. Furthermore, in mu-opioid receptor knockout mice (MORKO), we observed a decrease in potency and maximal effect for a delta agonist. These data suggest that mu and delta receptors are not only involved in the modulation of phagocytosis in macrophages, but they also affect each other's activity by an unknown cooperative mechanism.