Inhibition of neuronal and inducible nitric oxide synthase does not affect the analgesic effects of NMDA antagonists in visceral inflammatory pain

Previously we described the antinociceptive effect of magnesium sulfate and dizocilpine (MK-801) in the visceral and somatic rat models of pain. In the somatic model of pain, we established the influence of selective inhibitors of neuronal and inducible nitric oxide synthase on the antihyperalgesic effects of magnesium sulfate and dizocilpine. Therefore, the objective of the present study was to determine in the rat model of visceral pain whether same mechanisms are involved in the antinociceptive action of magnesium sulfate and dizocilpine. Analgesic activity was assessed using the acetic acid-induced writhing test in rats. Subcutaneous injection of either magnesium sulfate (15 mg/kg) or dizocilpine (0.01 mg/kg) decreased the number of writhes by about 60 and 70%, respectively. The role of nitric oxide on the effects of magnesium sulfate and dizocilpine was evaluated using selective inhibitor of neuronal [N-ω-Propyl-L-arginine hydrochloride (L-NPA)] and inducible [S-methylisothiourea (SMT)] nitric oxide synthase, which per se did not affect the number of writhes. We observed that the antinociceptive effect of magnesium sulfate or dizocilpine did not change in the presence of L-NPA (2 and 10 mg/kg, i.p.) and SMT (0.015 and 10 mg/kg, i.p.). We conclude that, nitric oxide produced by neuronal and inducible nitric oxide synthase does not modulate the effects of magnesium sulfate and dizocilpine in the visceral inflammatory model of pain in the rat