Sensory effects of capsaicin congeners I. Relationship between chemical structure and pain-producing potency of pungent agents.

A quantitative method for measuring the efficiency of pungent agents of capsaicin-type by the pain reaction elicited on the eye of rats is described. About 50 derivatives -- most of them amides or esters of homovanillic acid -- were tested by this method and the share of different chemical groups of the molecule in the pungent action was analyzed. It turned out that the presence of an acylamide linkage or alkyl chain is not essential for pungency, since acylamide linkage can be replaced by an esteric group and the alkyl chain by cycloalkyl rings. The importance of OH group on the aromatic ring is confirmed. On the basis of the findings a hypothetical pharmacological receptor for capsaicin on pain sensory nerve endings is presented.