Sensitization of unmyelinated nociceptive afferents in monkey varies with skin type.

Abstract

C-fiber nociceptive afferents (96) responsive to both mechanical and heat stimuli (CMH) were studied in the monkey [Macaca mulatta, M. fascicularis] in an effort to determine what stimuli cause sensitization. Of the fibers, 32 innervated glabrous skin (G-CMHS), while 64 innervated hairy skin (H-CMH). Single-unit recording techniques were used. The response to heat stimuli was studied using a laser thermal stimulator that provided stepped increases in skin temperature over a 7.5-mm-diameter area with rise times to the desired temperature near 100 ms for each stimulus. Changes in sensitivity were studied with a thermal test sequence (TTS), which consisted of 10 3-s stimuli presented with a 27-s interstimulus interval. The first stimulus was always 45.degree. C. The remaining 9 stimuli ranged from 41 to 49.degree. C in 1.degree. C increments and were presented in random order. The effects of stimulation with a more intense stimulus, 53.degree. C for 30 s, were also determined. The TTS stimuli were presented multiple times to the same fiber with a 10-min stimulus-free interval between runs. The H-CMH were sensitized by the TTS stimuli, while the G-CMH were not. The difference between H-CMH and G-CMH cannot be explained by a difference in the initial sensitivity of the 2 types of fibers. The mean responses to the initial 45.degree. C stimulus of the first TTS run were similar: 10.3 .+-. 1.3 (SE) impulses for G-CMH, and 9.8 .+-. 1.8 impulses for H-CMH. The thermal thresholds, as measured by the response to the first TTS run, were also similar 44.3 .+-. 0.3.degree. C for G-CMH, and 44.6 .+-. 0.2.degree. C for H-CMH. The increased response to the TTS stimuli over successive runs for the H-CMH tended to reach a plateau by the 4th run. In 10 H-CMH, the duration of the TTS stimuli was changed from 3 to 1 s and runs were repeated after 10-min stimulus-free intervals. In contrast to the increased response evoked by 3-s stimuli, the mean response of the H-CMH to the 1-s stimuli did not change significantly over successive runs. Although the H-CMH showed sensitization from one run to the next, within a run only suppression was evident. The conduction velocity and receptive-field size of the H-CMH and G-CMH did not differ, but the mechanical threshold of the H-CMH (2.51 .+-. 0.11 bars) was significantly less than that of the G-CMH 4.27 .+-. 0.32 bars, (P < 0.001). Evidently, C-fiber nociceptive afferents do not play an important role in hyperalgesia in glabrous skin but may play an important role in hyperalgesia in hairy skin. The hypothesis that A fiber nociceptive afferents play an important role in explaining the marked hyperalgesia produced by a substantial thermal injury (53.degree. C for 30 s) to the glabrous skin of the hand (22), is supported.