Involvement of substance P as a mediator in capsaicin-induced mouse ear oedema

We examined the involvement of substance P (SP) in mouse ear oedema induced by topical application of capsaicin (250 micrograms/ear). Reapplication of capsaicin at 4 h, 24 h, and 48 h after initial treatment did not induce a second oedema response. Oedema induced after the second application was significantly (p < 0.01 or p < 0.001) suppressed for up to 30 days but was observed when capsaicin was applied 40 days after initial treatment. Topical pretreatment of ears with capsaicin at 4 h, 24 h and 48 h before i.v. injection of SP (5 micrograms/kg) did not cause a significant inhibition of plasma extravasation in ear skin. NK1 receptor antagonists such as RP 67580 (ED50:0.19 mg/kg, i.v.), spantide II (ED50:0.33 mg/kg, i.v.), and GR 82334 (ED50:0.26 mg/kg, i.v.), inhibited capsaicin-induced ear oedema, whereas SR 48968 (2.0 mg/kg, i.v.), a NK2 receptor antagonist, had no effect. Furthermore, RP 67580 (0.5 kg/mg, i.v.) inhibited the oedema response induced by reapplication of capsaicin at 50 days after initial treatment. These results indicate that tachyphylaxis of capsaicin-induced oedema is reversible and suggest that this response may be due mainly to a reduction of SP in sensory neurones but not to any loss of responsiveness of NK1 receptors. We also conclude that SP and NK1 receptors are involved predominantly in the development of capsaicin-induced mouse ear oedema.