Nitric oxide enhances substance P‐induced itch‐associated responses in mice

Substance P (SP) elicits itch and itch‐associated responses in humans and mice, respectively. In mice, NK 1 tachykinin receptors are involved in SP‐induced itch‐associated responses, scratching, and mast cells do not play a critical role. The present study was conducted to elucidate the role of nitric oxide (NO) on SP‐induced scratching in mice. An intradermal injection of SP (100 nmol site −1 ) elicited scratching in mice, and it was suppressed by an intravenous injection of the NO synthase (NOS) inhibitor N G ‐nitro‐ L ‐arginine methyl ester ( L ‐NAME), but not by its inactive enantiomer D ‐NAME. Intradermal injections of L ‐NAME (100 nmol site −1 ), another NOS inhibitor 7‐nitroindazole (10 nmol site −1 ) and the NO scavenger haemoglobin (0.01–10 nmol site −1 ) also inhibited SP‐induced scratching.L ‐NAME (100 nmol site −1 ) did not affect scratching induced by an intradermal injection of 5‐hydroxytryptamine (100 nmol site −1 ). Intradermal injections of L ‐arginine (300 nmol site −1 ) and the NO donor (±)‐(E)‐4‐ethyl‐2‐[(E)‐hydroxyimino]‐5‐nitro‐3‐hexenamide (NOR3; 100 nmol site −1 ) increased scratching induced by SP. Intradermal injections of L ‐arginine (1–1000 nmol site −1 ) or NOR3 (1–100 nmol site −1 ) alone were without effects on scratching. Intradermal injections of SP (10–100 nmol site −1 ) increased the intradermal concentration of NO in a dose‐dependent manner in mice. An increase in NO levels induced by SP was inhibited by L ‐NAME and the NK 1 tachykinin receptor antagonist L‐668,169, but not by the NK 2 tachykinin receptor antagonist L‐659,877. SP (1–10 μ M ) elicited NO production in cultured human keratinocytes and the SP‐induced NO production was inhibited by L ‐NAME and L‐668,169. We conclude that intradermal SP increases NO in the skin, possibly through the action on NK 1 tachykinin receptors on the epidermal keratinocytes and that NO enhances SP‐induced itch‐associated responses.British Journal of Pharmacology (2003) 138 , 202–208. doi: 10.1038/sj.bjp.0705004