Unmyelinated nociceptive units in two skin areas of the rat

Responses of unmyelinated afferent fibres were investigated in two skin nerves of Sprague-Dawley rats. The units were tested as to whether they responded to mechanical probing of the skin, to controlled radiant heat stimulation, and/or to cooling of the skin (to less than or equal to 5 degrees C). Ninety-six units in a n. saphenaus and 129 units in a n. coccygealis were studied, which were identified as afferents by means of the above-mentioned stimuli. In both nerves mechano- and heat-sensitive units (CMHs) were most frequent (56% in n. saphenaus and 74% in n. coccygealis). There were, however, significantly more purely mechanoceptive units (CMs) in n. saphenaus than in n. coccygealis (30% vs 5%). In contrast cold-sensitive units (CCs) were more frequent in n. coccygealis. They constituted 25% of the afferent C-fibres in this nerve. When testing heat sensitivity of CMHs with ramp stimuli raising the temperature to 55 degrees C at a rate of 0.8 degree C/s, heat thresholds had a wide range of between 30 and 55 degrees C. Since CMHs with low heat thresholds had the highest discharge rates and the greatest dynamic sensitivity in the range of noxious temperatures, they most probably also had nociceptive functions. It was shown that the low heat thresholds of some CMHs were not due to sensitization by preceding heat stimuli. It is argued that low-frequency discharges (less than or equal to 2 Hz) observed in some nociceptive CMHs of the rat at non-noxious temperatures are insignificant for nociception. When comparing discharges during a first ramp heat stimulus to 50 degrees C (rise time 1 degree C/s) with those during a second stimulus of identical time course delivered 5-10 min later, 44% of the CMHs were sensitized, 24% were desensitized and the remainder were not clearly influenced. We did not find a significant correlation between initial heat thresholds and tendency to sensitization or desensitization.