MECAMYLAMINE INHIBITS NICOTINE BUT NOT CAPSAICIN IRRITATION ON THE TONGUE - PSYCHOPHYSICAL EVIDENCE THAT NICOTINE AND CAPSAICIN ACTIVATE SEPARATE MOLECULAR RECEPTORS

Using a two-alternative forced-choice (2-AFC) discrimination test coup led with category intensity ratings, we investigated the effect of mec amylamine, an antagonist of neuronal nicotinic acetylcholine receptors (nAchRs), on oral irritation elicited by nicotine or capsaicin. Mecam ylamine (0.075%) was first delivered to one side of the tongue with di stilled H2O delivered to the other side. After 10 min either capsaicin (1 ppm) or nicotine (0.12%) was applied bilaterally to the tongue, an d subjects were asked to choose which side yielded a stronger sensatio n (2-AFC) as well as to provide a rating of the irritation intensity d ifference between the two sides of the tongue. When nicotine was given after mecamylamine, a significant proportion of subjects chose the me camylamine-untreated side as yielding stronger irritation. When capsai cin was given after mecamylamine, both sides of the tongue were chosen in equal numbers. These data indicate that mecamylamine reduced irrit ation elicited by nicotine but not capsaicin, and provide further evid ence that nicotine oral irritation is mediated via a neuronal nAchR wh ile capsaicin activates trigeminal fibers via a separate molecular rec eptor. (C) 1998 Elsevier Science Ireland Ltd.