Capsaicin and sensory neurones — a review

In the late fifties and sixties Jancso published a series of papers on ‘the peculiar pharmacological effect of capsaicin Capsaicin or 8-methyl-N-vanillyi-6-nonenamide is the irritant compound in the capsicum plant (red pepper, chilli pepper, etc.). After initial violent irritation, capsaicin application renders animals and man insensitive to further noxious chemical stimuli. This desensitization can last for weeks or months following systemic administration in rats [47]. Despite this powerful effect, at that time, no clear-cut morphological lesion could be found to accompany it. In 1977, however, Jancso’s son and colleagues reported on the effects of capsaicin administered to neonatal rats. This was followed by a life-long insensitivity to chemical irritants accompanied by destruction of the small ‘B-type’ dorsal root ganglion cells [41]. It was this discovery that began a great surge of interest in the effects of capsaicin, in laboratories all over the world, producing the considerable amount of information that we have about its actions today. Its potential as a specific toxin for peripheral C fibres has made it of particular interest to neurobiologists concerned with pain mechanisms. This review was written as a result of a meeting on capsaicin in November 1981 at the Medical Research Council in London. At this meeting, neuroscientists of different disciplines who had used capsaicin in their research or studied its mode of action came together to discuss problems. Questions that arise out of the work so far, include: (1) Is the action of capsaicin on the peripheral nerve restricted to C fibres? (2) Does capsaicin have a direct effect on central nervous tissue? (3) Is capsaicin an axonal transport blocker? (4) What is the effect of capsaicin on nerve membrane? (5) Does capsaicin treatment result in analgesia? The following review shows to what extent we can answer these and other important questions about capsaicin. It is important too, in a general sense, to decide how useful a tool capsaicin is in understanding the nervous system and how much we have learnt from it so far. The review will be restricted almost completely to the somatosensory system. The effects of capsaicin on cardiovascular, respiratory, thermoregulatory and gastroin-