Excitation of rat cutaneous nociceptors by noxious cold

Electrophysiological recordings were made from mechanosensitive nociceptors innervating hairy skin of the rat hindpaw, and responses evoked by a wide range of noxious cold stimuli (20 to -12 degrees C) were determined. All A delta and C nociceptors sampled were excited by noxious cold. Response thresholds of A delta nociceptors were significantly higher (colder stimulus temperatures) than thresholds of C fibers, and many exhibited response thresholds below 0 degree C. Responses of both classes of nociceptors increased as stimulus temperature decreased. These data demonstrate that A delta and C nociceptors are excited by a wide range of cold stimuli and suggest that the proportion of cutaneous nociceptors excited by noxious cold has been underestimated in previous studies.