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

Abstract

Studies with selective opioid agonists show that μ- and δ₂-opioid receptors, but not κ, are involved in opioid inhibition of phagocytosis in elicited murine macrophages. All μ and δ₂ agonists tested had similar maximal effects on phagocytosis, and all dose–response curves suggest positive cooperativity. In addition, μ and δ antagonists antagonized the effect of both μ and δ 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 μ and δ 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.

Introduction

Morphine and other opioids modulate macrophage functions Roy and Loh, 1996, Gomez-Flores and Weber, 1998, such as nitric oxide production (Magazine et al., 1996), cytokine secretion Alicea et al., 1996, Bussiere et al., 1993, superoxide formation (Peterson et al., 1989), chemotaxis (Grimm et al., 1998), and phagocytosis Casellas et al., 1991, Rojavin et al., 1993, Sowa et al., 1997, and these effects may be a contributing factor to the susceptibility to infections observed in drug addicts Haverkos and Lange, 1990, Donahoe et al., 1991. We have reported that, while acute morphine results in inhibition of Fc-mediated phagocytosis by murine macrophages, subchronic morphine results in a tolerant-like state (Tomei and Renaud, 1997). Furthermore, drug withdrawal from putatively tolerant cells results in an inhibition of phagocytosis, suggesting a dependent-like state (Lázaro et al., 2000). However, the identity of the opioid receptor mediating these effects is unknown at the moment. Pharmacological studies suggest that the mu-opioid receptor (MOR) is the major target site for opiates Iversen, 1996, Loh and Smith, 1990. However, morphine can also bind to δ- and κ-opioid receptor subtypes. Since all opioid receptors have been reported in immune cells Chuang et al., 1994, Chuang et al., 1995, Gavériaux et al., 1995, Chao et al., 1996, a role for δ and κ receptors cannot be excluded in the modulation of macrophage phagocytosis. We intend to answer this question by studying the effect of receptor-selective agonists and antagonists on phagocytosis by murine macrophages.

Opioid receptor involvement in modulation of phagocytosis has recently become amenable to genetic analysis. Exons 2 and 3 of the mu-opioid receptor (MOR) gene were deleted in mice, resulting in the loss of six out of seven transmembrane domains (Loh et al., 1998), making the development of mu-opioid receptor knockout mice (MORKO) possible. In vivo studies show that the analgesic effects of morphine and its major metabolites, as well as morphine-induced lethality, are drastically reduced in these knockout mice, suggesting that both μ₁ and μ₂ receptors had been affected (Loh et al., 1998). In macrophages, morphine modulation of non-opsonic phagocytosis was not observed in MORKO mice (Roy et al., 1998). However, no studies have been performed to date showing the effect of deletion of μ receptors on morphine modulation of Fc-mediated phagocytosis, and these studies are one of the aims of this work.