Time-intensity profiles of cutaneous pain in normal and hyperalgesic skin: a comparison with C-fiber nociceptor activities in monkey and human

Abstract

Contributions of evoked discharge in nociceptors with C-fibers to the temporal profiles of magnitude judgments of pain by humans were determined for heat stimulations of the skin before and after the development of hyperalgesia (increased sensitivity to pain) produced by a mild heat injury. Human subjects continuously rated the magnitude of pain evoked by short-duration heat stimuli of 39-51 degrees C delivered to the hairy skin of the arm or leg (calf or foot) before and after the development of hyperalgesia produced by a conditioning stimulus (CS) of either 50 degrees C for 100 s or 48 degrees C for 360 s. During heat stimulations of the leg in humans, magnitude judgments of pain were obtained simultaneously with recordings of evoked discharges in single C-fiber mechanoheat (CMH) nociceptive afferent fibers. Seven fibers were studied before and after the CS. In other experiments, magnitude ratings of pain evoked by heat stimulations of the arm were compared with heat-evoked discharges in 21 CMH nociceptive afferent fibers innervating the hairy skin of the wrist or hand in anesthetized monkeys. From CMH responses obtained in each species, median response latencies were calculated as well as poststimulus time (PST) histograms--the latter plotting mean frequency of discharge versus time during each stimulus. In these analyses, the times of action potentials in CMHs were calculated as they would occur at entry to the lumbar or cervical spinal cord in humans, taking into account the temporal dispersion that should occur because of differing conduction velocities. These results were then compared with response latencies for pain and the temporal profiles of pain ratings made by individual subjects. Comparisons were made for data obtained before the CS (normal skin) and those obtained 10 min after the CS in heat-sensitized (hyperalgesic) skin. For normal skin, PST histograms of mean frequencies of discharge were similar for CMHs with similar heat thresholds (41-43 degrees) in the anesthetized monkey and the awake human. Despite minor discrepancies, there were similarities in the changes in these histograms for monkey and human CMHs following heat sensitization after the CS. It was concluded that CMHs in monkeys and humans have similar response magnitudes and temporal profiles of response to heat. The major differences in the temporal profiles of CMH responses and human pain ratings were the latencies at which CMH responses and pain ratings began, reached maximum, and ended.(ABSTRACT TRUNCATED AT 400 WORDS)