The Effect of Injection of L-Arginine and L-NAME in the Rat's Intra-Hippocampal (CA1) in Morphine-Induced Antinociception in Formalin Test

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. 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 morphineinduced response. But at the late phase both agent's 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. Based on the finding, NO in the rat hippocampal CA1 area is involved in morphine-induced antinociception.