Morphine-induced Nitric Oxide Production in PC12 Cells

Background: The role of nitric oxide (NO) in many well-known effects of morphine is well defined. NO is involved in the signaling pathway of the N-methyl-D-aspartate (NMDA) receptor, which is proposed to mediate some of morphine s effects. This research studies the effect of morphine and NMDA on lipopolysaccharide (LPS)-stimulated NO production by clonal rat pheochromocytoma (PC12) cells. Methods: We used the Griess reaction to measure NO concentrations in cell culture medium. Results: PC12 cells that were incubated for 24 h with varying concentrations of morphine (0.1, 1, 10, 100, and 1000μM) plus LPS (1 μg/ml) did not significantly alter the concentration of NO in the medium. However, NO production increased when cells were treated for both 48 h with 100 and 1000 μM morphine and for 72 h with 10,100, and 1000 μM of morphine. After 72 h, 1 μM naloxone significantly decreased NO concentration. Naloxone, at doses of 0.1, 1, and 10μM prevented NO production by 1000 μM of morphine. NMDA (0.1, 1, and 10 μM) did not alter NO concentrations after 24, 48 or 72 h. Morphine (1 μM)-induced NO production was inhibited by 10 μM NMDA after 48 h. Inhibition of NO was also noted with1 and 10 μM concentrations of morphine after 72 h. Conclusion: Chronic treatment of PC12 cells with morphine significantly increases LPS-stimulated NO production via naloxone-sensitive receptors.The cells seem to release endogenous morphine in medium. NMDA does not affect NO production, which may be due to the lack of functional NMDA receptor expression in PC12 cells.