Introduction: Intra-hippocampal CA1 injections of L-arginine, a nitric oxide precursor and NG-Nitro-L-arginine Methyl Ester (L-NAME), a nitric oxide synthase inhibitor, on morphine-induced antinociception in rat formalin test were investigated. Method: To induce inflammation pain, male Wistar rats received subcutaneous (s.c.) injections of formalin (50 μl at 2.5%) once prior to testing. Morphine (3-9 mg/kg) was injected intraperituneally (i.p.) 10 min before injection of formalin. Present study show that pre-administration (5 min) of L-arginine (0.15, 0.3, 1.0 and 3.0 µg/rat), but not L-NAME (0.15, 0.3 and 1.0 µg/rat) to the administration of formalin, during testing at the early phase reversed morphine-induced response. But at the late phase both agents' injections caused a significant effect. The response to L-arginine was blocked by L-NAME pre-administration. On the other hand, L-arginine or L-NAME by itself did induce pain behavior at the late phase. According to data interactions between morphine and L-arginine was statistically significant. Conclusion: Based on the finding, NO in the rat hippocampal CA1 area is involved in morphine-induced antinociception.

Keywords: morphine, antinociception, CA1, L-arginine, formalin test, rat, L-NAME

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