effect of simultaneous injection of l-arginine in dorsal hippocampus and laterodorsal periaqueductal gray matter on morphine-induced analgesia in rat’s formalin test

Background and objective: injection of l-arginine, a precursor of nitric oxide, in the rat’s hippocampus or periaqueductal gray matter reduces the analgesic effect of morphine on formalin-induced pain, but the effect of simultaneous injection of the substance in both areas have not been shown as our purpose of this research. materials and methods: wistar rats were used as control, morphine, and l-arginine groups. the rats were simultaneously cannulated in two areas of the dorsal hippocampus and laterodorsal pag. one week later, the control animals received 50 μl of 2.5% formalin in the paw of the left foot under restrainer. the morphine group 10 min before formalin received the opioid (6 mg/kg, intraperitoneally). other groups took l-arginine (0.25-2 μg/rat) in only one area (d hippocampus or ld pag), prior to morphine administration. the effective dose of l-arginine (0.5 μg/rat) simultaneously was injected in both areas. the findings were analyzed by analysis of variance (anova) under α=0.05. results: morphine induced analgesic response. injection of no precursor both separately and simultaneously in the two nuclei reduced morphine-induced analgesia. conclusion: increasing levels of no due to exclusive or concurrent injection of l-arginine in the areas likely antagonize the morphine response.