Abstract

The present investigation describes the antinociceptive effect of capsaicin in the acetic acid-induced abdominal stretch assay and its mediation by substance P(1-7) fragment [SP(1-7)] and nitric oxide (NO). When injected intrathecally 24 hr before testing, SP(1-7) produced a dose-related decrease in the number of abdominal stretches induced by an i.p. injection of acetic acid. The antinociceptive effect of SP(1-7) (10 nmol) persisted for 62 hr after its injection, a time course that was similar to that produced by a dose of capsaicin (2.6 nmol) that produced an effect of similar magnitude. Antinociception induced by 10 nmol of SP(1-7) was completely reversed by coadministration of 10 nmol of D-SP(1-7); the equivalent antinociception produced by capsaicin was reversed by as small a dose as 1 nmol of D-SP(1-7). The guanylate cyclase inhibitor, methylene blue, at a dose of 10 nmol, prevented both SP(1-7)- and capsaicin-induced antinociception. Capsaicin-induced, but not SP(1-7)-induced, antinociception was prevented by Nw-nitro-L-arginine methyl ester, an NO synthase inhibitor. This inhibition of capsaicin was reversed by coadministration of 120 nmol of L-arginine. Reduced hemoglobin did not prevent capsaicin-induced antinociception. These findings suggest NO is produced and acts within capsaicin-sensitive primary afferent fibers in the dorsal spinal cord to mobilize substance P, resulting in N-terminal induced-antinociception.