The regulatory role of nitric oxide in morphine-induced analgesia in the descending path of pain from the dorsal hippocampus to the dorsolateral periaqueductal gray.

BACKGROUND: Nitric oxide (NO) levels in brain nuclei, such as the hippocampus and brainstem, are involved in morphine analgesia, but the relationship between the dorsal hippocampus (dH) and the dorsolateral periaqueductal gray matter (dlPAG) needs to be clarified, which is our goal. METHODS: Wistar rats were simultaneously equipped with a stereotaxic device with unilateral guide cannula at dH and dlPAG. After recovery, they were divided into control and experimental groups. Formalin (50 µl of 2.5%) was inoculated into the left hind paw of the rat. Morphine (6 mg/kg) was administered intraperitoneally (i.p.) 10 min before formalin injection. L-Arginine (0.25, 0.5, 1 and 2 µg/rat), and L-NAME (0.25, 0.5, 1 and 2 µg/rat), unrelatedly or with respect in the order of injection were used in the nuclei before morphine injection (i.p.). Activation of the neuronal NO synthase (nNOS) in the brains of all animals was measured using NADPH-diaphorase, a selective biochemical marker of nNOS. RESULTS: Morphine reduced inflammatory pain in the early and late stages of the rat formalin test. The morphine response was attenuated before injection of single L-arginine but not L-NAME in the two target areas. However, the acute phase result was stopped due to L-NAME pretreatment. When L-NAME was injected into dlPAG before injecting L-arginine at dH, the morphine response did not decrease at all, indicating a modulatory role of NO in dlPAG, which was confirmed by NADPH-d staining. CONCLUSIONS: High levels of NO in dlPAG may regulate the pain process in downward synaptic interactions. SIGNIFICANCE: Nitric oxide is involved in the dH and dlPAG in morphine-induced analgesia in the rat formalin test. Morphine has analgesic effects in both phases of the rat formalin test. The morphine response is reduced in two stages by injection of the NO precursor L-arginine but not the nNOS inhibitor L-NAME in the dH and dlPAG. By injecting L-NAME before L-arginine in both nuclei, the morphine-induced response returns in the early stages. Due to the initial injection of L-NAME into dlPAG and the subsequent injection of L-arginine at dH, morphine analgesia is not reduced at all, indicating NO modulation in the pain pathway from dH to dlPAG.

Role of nitric oxide in the rat hippocampal CA1 in morphine antinociception.

In the present study, the effects of intra-hippocampal 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 antinociception in rat formalin test were investigated. To induce inflammation pain, formalin (50 microl at 2.5%) was injected into the right hind-paw of male Wistar rats prior to testing. Morphine (3-9 mg/kg) was injected intraperitoneally (i.p.) 10 min before injection of formalin. The present study shows that administration of L-arginine (0.08, 0.15, 0.3, 1.0 and 3.0 microg/rat), but not L-NAME (0.15, 0.3 and 1.0 microg/rat), 5 min before formalin injection reversed morphine-induced antinociception at the early phase of formalin test. However, both drugs blocked morphine antinociception at the late phase of the test, but none of these drugs elicited any response by themselves at the tonic phase when injected alone. Moreover, the response to l-arginine was potentiated by L-NAME pre-treatment. It should be noted that a single injection of both L-arginine and L-NAME showed nociceptive effect at the early phase of the test. The present study reveals an expression of NADPH-diaphorase in the rat brain samples administered by L-arginine. Expression of NADPH-d is decreased in the samples which were pre-injected with L-NAME. This study suggests NO participation in the rat hippocampal CA1 area in morphine-induced antinociception.