NOCICEPTIVE RESPONSES TO HIGH AND LOW RATES OF NOXIOUS CUTANEOUS HEATING ARE MEDIATED BY DIFFERENT NOCICEPTORS IN THE RAT - BEHAVIORAL EVIDENCE

Several lines of evidence suggest that different classes of nociceptiv e afferents mediate the responses produced by different rates of noxio us skin heating. More specifically, low skin heating rates evoke nocic eptive responses that appear to be mediated by the activation of capsa icin-sensitive C-fiber nociceptors, whereas high skin heating rates ap pear to produce responses mediated by the activation of other nocicept ors. This hypothesis was examined by both electrophysiological and beh avioral experiments. This report describes the results of experiments designed to determine whether pharmacologic treatments that selectivel y alter the activity of C-fiber nociceptive afferents also produce sel ective effects on foot withdrawal responses to either high or low rate s of noxious foot heating. The results of these experiments demonstrat e that: (1) topical application of a low concentration of capsaicin, w hich sensitizes-C-fiber nociceptors, selectively decreased the latency of responses to low heating rates; (2) topical application of a high concentration of capsaicin, that desensitizes C-fiber nociceptors, sel ectively increased the latency of responses to low heating rates; (3) low doses of systemic morphine, which selectively attenuate nociceptio n produced by the activation of C-fiber nociceptors, selectively incre ased response latencies for low skin heating rates. These results supp ort the conclusion that foot withdrawal responses evoked by low skin h eating rates are mediated by the activation of capsaicin-sensitive C-f iber nociceptors and foot withdrawal responses evoked by high skin hea ting rates are mediated by the activation of other nociceptors. This c onclusion is supported by the results of the accompanying electrophysi ological study which provides direct evidence that low rates of skin h eating preferentially activate C-fiber nociceptors while high rates of skin heating preferentially activate A delta nociceptors.