Complementary DNA cloning of a mu-opioid receptor from rat peritoneal macrophages.

Treatment with opioid agonists in vitro and in vivo has been shown to affect the function of the immune system. Several investigators have suggested that immune cells may express opioid receptors, but it had been very difficult to demonstrate their presence on these cells by direct binding assays. Our earlier studies have shown that macrophage progenitor cells are highly sensitive to morphine treatment in vitro and in vivo. In the current investigation, we determined, unequivocally, the expression of mu-opioid receptor related transcripts in rat peritoneal macrophages by reverse transcriptase-polymerase chain reaction (RT-PCR) studies. In order to further characterize the transcript, the RT-PCR product was cloned and sequenced. The sequence analyses indicate that the transcripts from rat peritoneal macrophages are identical to those for the mu-opioid receptor described in the rat brain. To further confirm the presence of mu-opioid receptors, immunoreactivity to an antiserum raised against the carboxyl terminal fifteen amino acid residues of the mu-opioid receptor was determined. These studies show for the first time that rat peritoneal macrophages express a mu-opioid receptor.