Macrophage-derived nitric oxide is involved in the depressed concanavalin A responsiveness of splenic lymphocytes from rats administered morphine in vivo.

The present study examined the role of macrophage-derived nitric oxide in the suppressive effect of in vivo morphine administration on Con A-stimulated proliferation of splenic lymphocytes in rats. The results showed that concentrations of nitrite are significantly greater in Con A-stimulated splenocyte cultures from morphine-treated rats than in cultures from saline-treated rats, and that the depletion of macrophages from splenocyte cultures abolishes the suppressive effect of morphine on Con A-stimulated proliferation. Moreover, the addition of NG-monomethyl-L-arginine (NMMA) to Con A-stimulated splenocyte cultures attenuates the suppressive effect of morphine on mitogenic responsiveness. The addition of excess L-arginine to splenocyte cultures containing NMMA reverses the effect of NMMA and restores morphine's suppressive effect on Con A-stimulated proliferation, but the addition of D-arginine to splenocyte cultures containing NMMA does not restore the suppressive effect of morphine. Taken together, these findings demonstrate that the suppressive effect of in vivo morphine administration on Con A-stimulated proliferation of splenic lymphocytes involves macrophage-derived nitric oxide.