Effects of capsaicin on articular afferents of the cat's knee joint

This study on the effects of capsaicin on primary afferents from normal knee joints of the cat was performed to further elucidate the mechanisms of articular pain evoked by an acute arthritis and by chemical irritants. It showed that close i.a. bolus injection of capsaicin (10(-7)-10(-4) M) excites most fine articular afferents (conduction velocity less than or equal to 11.3 m/s) whereas fast units are not excited. Fine afferents with low to medium thresholds to knee joint movement are less readily excited by capsaicin than high threshold ones. The response to capsaicin is usually a rapid burst of impulses of a very short latency. This response pattern differs considerably from that seen after application of endogenous substances produced in inflammation. Thus capsaicin seems to differ in its mode of action from that of endogenous algesic substances.