LOW BUT NOT HIGH-RATE NOXIOUS RADIANT SKIN HEATING EVOKES A CAPSAICIN-SENSITIVE INCREASE IN SPINAL-CORD DORSAL HORN RELEASE OF SUBSTANCE-P

Some kinds of nociception appear to be partially mediated by the relea se of substance P (SP) in the spinal cord dorsal horn from terminals o f primary afferent nociceptors. Only some nociceptors contain and rele ase SP however. Specifically, SP appears to be released by unmyelinate d (C) nociceptive afferents when activated by noxious stimulation to t he skin, but does not appear to be contained in cutaneous myelinated ( A delta) nociceptive afferents. We have proposed a model of nociceptio n in rats that uses different rates of noxious skin heating to allow f or differential assessment on behavioral responses mediated by the act ivation of A delta or C fiber nociceptors. As one means of testing the validity of this model we have examined the effects of using high and low rate noxious skin heating on the dorsal horn release of substance P-like immunoreactivity (SPLI) in decerebrate/spinal transected anima ls. Consistent with the model, low rate skin heating evokes a signific ant increase in dorsal horn SPLI release indicating C fiber mediation, whereas high rate skin heating did not evoke SP release, indicating m ediation by afferents other than C afferents, i.e. A delta nociceptive afferents. Also consistent with behavioral effects, topical applicati on of capsaicin, which sensitizes C nociceptors, increased the SPLI re lease evoked by low but not high rate skin heating. These data provide additional evidence that foot withdrawals evoked by low rate skin hea ting are mediated by C fiber activation, whereas foot withdrawals evok ed by high rate skin heating are evoked by A delta fiber activation.