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

Behavioral nociceptive responses evoked by relatively high rates of no xious radiant skin heating appear to be mediated by A delta nociceptor activation, whereas responses evoked by low rates of skin heating app ear to be mediated by the activation of C-fiber nociceptors. This hypo thesis was confirmed by the results of single unit recordings of A del ta and C nociceptive afferent fibers isolated from the saphenous nerve s of pentobarbital anesthetized rats. Heating the hind paw skin of the rat at a relatively high rate of 6.5 degrees C/sec activated A delta units within 2 sec after the onset of the stimulus. This response late ncy is similar to the 2.5 sec latency of the foot withdrawal response to a similar stimulus. In contrast, C-fibers were only slightly activa ted at a longer latency of 5-6 sec. Conversely, heating the hind paw s kin at a relatively low rate of 0.9 degrees C/sec activated C-fibers, but evoked only a few action potentials in A delta nociceptors. C-fibe rs began firing at a rate less than 1 Hz between 8 and 10 sec after th e onset of heating and fired at a mean rate of 1.5 Hz between 10 and 1 2 sec, which corresponds to the latency of the foot withdrawal respons e. Topical application of capsaicin to the hind paw skin decreased the latency of C-fiber responses from control values of 8-12 sec to appro ximately 4 sec after topical capsaicin treatment. The mean latency of the foot withdrawal response to skin heating at the low rate is also r educed from control values of 12-14 sec to 4-5 sec after capsaicin tre atment. In contrast, capsaicin treatment did not significantly affect the responses of A delta nociceptors. These results support the conclu sion that nociceptive foot withdrawal responses to a low rate of skin heating are mediated predominantly by the activation of C-fiber nocice ptors. These results provide direct evidence that, under the condition s of these experiments, nociceptive foot withdrawal responses evoked b y high rates of skin heating are primarily mediated by A delta nocicep tors, and foot withdrawal responses evoked by low rates of skin heatin g are primarily mediated by C-fiber nociceptors.