INHIBITORS OF NITRIC-OXIDE SYNTHESIS ANTAGONIZE NITROUS-OXIDE ANTINOCICEPTION IN MICE AND RATS

Administration of the anesthetic gas N2O evoked an antinociceptive effect in two rodent antinociception paradigms. In the mouse abdominal constriction test, pretreatment with the nitric oxide synthase (NOS) inhibitor L-N-G-nitroarginine (L-NOARG) caused dose-related antagonism of the antinociceptive effect of N2O but not of either morphine or trans(+)3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide methane sulfonate. This antinociceptive effect was also antagonized by systemic pretreatment with the NOS inhibitors L-N-G-nitroarginine methyl ester (L-NAME) and L-N-G-monomethylnitroarginine. The antagonism of N2O by L-NOARG and L-NAME was completely reversed by i.c.v. administration of L-arginine but not D-arginine. In the absence of NOS inhibition, N2O antinociception was potentiated by i.c.v. treatment with L-arginine but not D-arginine. The i.c.v. pretreatment with L-NAME also reduced N2O antinociception; this antagonism was also stereospecifically reversed by L-arginine. In the rat hot plate test, the antinociceptive response to 70% N2O was antagonized in dose-related manner by i.c.v. pretreatment with L-NOARG or L-NAME. N2O antinociception was restored by i.c.v. treatment with L-arginine but not D-arginine. However, neither L-arginine nor D-arginine alone affected N2O antinociception. These results implicate a key role for NO in the mediation of the antinociceptive effects of N2O in both mice and rats.