Antinociceptive activity of NK1 receptor antagonists: non‐specific effects of racemic RP67580

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Abstract
1 Release of substance P in the dorsal horn is considered a primary event in the perception of pain. The profile of racemic RP67580, a non-peptide antagonist at the NK1 (substance P) receptor, was examined in a range of antinociception tests on rodents. 2 At doses up to 30 mg kg−1, s.c. racemic RP67580 exhibited antinociceptive activity in writhing and formalin paw tests in mice and gerbils. Acetic acid induced writhing and the licking response to formalin were reduced to 40–50% of the level observed in vehicle-treated animals (P < 0.05). However, this agent was not active in mouse tail flick, rat paw pressure or rat and guinea-pig formalin paw tests. 3 Like racemic RP67580, the calcium channel blockers nifedipine (30 mg kg−1, i.p.) and verapamil (10 or 20 mg kg−1, s.c.) inhibited the response to formalin by approximately 60% in gerbils (P < 0.05 compared with vehicle-treated animals). 4 Evidence for calcium channel antagonist activity of RP67580 was obtained in vitro. Racemic RP67580 inhibited calcium entry into depolarized strips of guinea-pig ileum longitudinal muscle myenteric plexus (apparent KB = 587 ± 115 nm), inhibited [3H]-diltiazem binding to rabbit skeletal membranes (IC50 = 298 nm) and depressed high threshold calcium currents in neurones cultured from rat cortex (10% inhibition at 10 μm). 5 These findings indicate that the acute antinociceptive effects of RP67580 may not be attributable to a specific interaction with NK1 receptors and may be mediated via calcium channel blockade.