Effects of topically applied capsaicin cream on neurogenic inflammation and thermal sensitivity in rats

The effects of capsaicin cream on neurogenic inflammation and thermal nocic eptive threshold were investigated in rats. Firstly, for topical applicatio n of capsaicin cream to hind paw, we shaped boots from dental cement to pre vent the animals from licking off the drug. Capsaicin cream (1%) led to sig nificant increases in the amounts of Evans blue and substance P (SP) releas ed into the perfusate, and the former response ws significantly suppressed by pretreatment with RP67580, an NK1-receptor antagonist, but not by treatm ent with an NK2-receptor antagonist. Subsequent electrical stimulation of t he sciatic nerve resulted in a significant reduction in Evans blue and SP e xtravasation 24 h after topical application of capsaicin cream. On the othe r hand, when capsaicin cream was repeatedly applied to both hind paws once a day, withdrawal latency for noxious heat stimulation decreased after 24 h , and this thermal hyperalgesia was reversed 3 days later. These results su ggest that capsaicin cream initially affects neurogenic inflammation mechan isms and then blocks the pain transmission mechanism.