Responses of cutaneous A-fiber nociceptors to noxious cold

Responses of cutaneous nociceptors to natural stimuli, particularly mechanical and heat stimuli, have been well documented. Although nociceptors are excited by noxious cold stimuli, there have been few studies of their stimulus-response functions for cold stimuli over a wide range of stimulus temperatures. Furthermore, the proportion of nociceptors excited by noxious cold is not clear. In the present study, we examined responses of mechanosensitive Aδ-nociceptors and low-threshold mechanoreceptors to a wide range of cold stimuli that included stimulus temperatures <0°C. Electrophysiological recordings were made from single primary afferent fibers in the saphenous nerves of anesthetized rats. Cutaneous sensory receptors were classed according to their conduction velocity and subgrouped functionally according to their responses evoked by mechanical, heat, and cold stimuli (0°C). Responses evoked by a wide range of cold stimulus intensities that included stimuli considered innocuous and noxious (painful) were then assessed. Stimuli of 20 to -20°C were delivered to the receptive held via a 1-cm² contact thermode from a base temperature of 32°C. Stimuli were applied in descending order of 2°C decrements. Stimulus ramp rate was 5°C/s, and stimulus temperatures were applied for a duration of 10 s. A total of 90 A fibers was studied, of which 61 were nociceptors and had conduction velocity in the Aδ-range (2-30 m/s). Nociceptors were classed initially as mechanical, mechanoheat, and mechanocold nociceptors. The remaining 29 fibers were low-threshold mechanoreceptors with conduction velocity in the Aδ- or Aβ-range (>30 m/s). These were subgrouped according to their adaptive properties as slowly or rapidly adapting, and according to whether they were excited by hair movement (hair follicle afferent fibers). All nociceptors were excited by noxious cold. Only 30% of nociceptors were considered sensitive to cold on initial classification with the use of a cold stimulus of 0°C. However, all nociceptors were excited by stimulus intensities <0°C. Response thresholds for cold ranged from 14 to -18°C (- 4.6 ± 1.07°C, mean ± SE). The total number of impulses, discharge rate, and peak discharge increased monotonically as intensity of cold stimuli increased. Power functions were used to determine the rate at which the number of impulses increased its stimulus intensity increased. The slopes of power functions ranged from 0.12 to 2.28 (mean 1.07 ± 0.13). Most mechanoreceptors were not excited by cold stimuli. The only types of mechanoreceptors that responded reliably to cold stimuli were the slowly adapting mechanoreceptors. Responses usually occurred during the temperature ramp when the skin temperature was decreasing. There was no evidence that mechanoreceptors encoded the intensity of cold stimuli at intensities above or below 0°C, because evoked responses did not increase with intensity of cold stimuli. It is concluded that the proportion of cutaneous Aδ- nociceptors excited by noxious cold stimuli has been underestimated in previous studies. All nociceptors were excited by stimulus temperatures <0°C and encoded the intensity of cold stimuli. It is therefore likely that cutaneous Aδ-nociceptors contribute to the sensation of cold pain, particularly pain produced by stimulus temperatures <0°C.