Inhibition by opioids of phagocytosis in peritoneal macrophages

We have tested the effect of prototypic opioid agonists on phagocytosis of sheep erythrocytes by mouse peritoneal macrophages. It was found that morphine and all the opioid peptides tested inhibited phagocytosis by a biphasic, naloxone-reversible mechanism. Delta agonists were the most effective inhibitors, suggesting that the response is mediated by a delta receptor. Chronic exposure to morphine apparently results in the development of tolerance since under these conditions the inhibitory effect of the opiate is abolished. These results are similar to previously reported effects of opioids on endocytosis in other systems, which suggests that this inhibition is part of a basic regulatory mechanism that has been conserved in evolution.