Role of nitric oxide in the rat hippocampal CA1 area on morphine-induced conditioned place preference

Effects of intrahippocampal CA1 injections of L-arginine, a nitric oxide (NO) precursor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, on morphine-induced conditioned place preference in male Wistar rats were investigated. Animals received subcutaneous (s.c.) injections of saline (1.0 ml/kg) or morphine (0.5-7.5 mg/kg) once daily for 3 days to induce conditioned place preference. The administration of L-arginine (0.3, 1.0, and 3.0 microg/rat), but not L-NAME (0.3, 1.0, and 3.0, microg/rat), prior to administration of morphine (5.0 mg/kg) during acquisition of morphine-induced conditioned place preference increased morphine-induced conditioned place preference, but the interaction between the response to morphine and/or L-arginine was not statistically significant. The response to L-arginine was blocked by L-NAME pre-administration. L-Arginine or L-NAME by itself did not induce conditioned place preference. The administration of L-arginine but not L-NAME, 1 min before conditioned place preference testing, increased the expression of morphine-induced conditioned place preference. Pre-administration of L-NAME blocked the L-arginine response. It is concluded that NO in the rat hippocampal CA1 area may be involved in morphine-induced conditioned place preference.