MORPHINE STIMULATES NITRIC-OXIDE RELEASE FROM INVERTEBRATE MICROGLIA

Morphine stimulates nitric oxide (NO) release in human endothelial cel ls. To determine whether this mechanism also occurs in invertebrates, the mussel Mytilus edulis was studied. Exposure of excised ganglia to morphine for 24 h resulted in a significant dose-dependent decrease in microglial egress that was naloxone sensitive. In coincubating the ex cised ganglia with morphine and the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl eater (L-NAME), an increase in microgli al egress was observed, suggesting that morphine may stimulate microgl ia to release NO. Morphine exposure to these cells in vitro resulted i n NO release (39.4 +/- 4.9 nM), a phenomenon found to be naloxone sens itive (10(-6) M; NO level = 5.9 +/- 2.6 nM) and t-NAME sensitive (10(- 4) M; NO level = 2.8 +/- 1.8 nM). Opioid peptides did not stimulate NO release, indicating that the process was mediated by the opiate alkal oid selective mu(3) receptor. Coincubation of microglia with L-arginin e or the superoxide scavenger, superoxide dismutase, resulted in signi ficantly higher NO levels observed following morphine stimulation. Tak en together, the data demonstrate that morphine can stimulate NO relea se in cells obtained from an invertebrate that represents an animal 50 0 million years divergent in evolution from man, underscoring the sign ificance of this process and further substantiating the critical impor tance of morphine as a naturally occurring signal molecule.