Responses of spinal cord dorsal horn neurones to non-noxious and noxious cutaneous temperature changes in the spinal rat

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Abstract
(1) Lumbar dorsal horn units characterized by their mechanical cutaneous sensitivities were tested for their responses to temperature changes of the skin in the decerebrate spinal rats. (a) Class 1 units (i.e. driven by non-noxious mechanical stimuli) were rarely thermally sensitive. (b) Nearly all class 2 units (i.e. driven by both non-noxious and noxious mechanical stimuli) and 4 of the 5 class 3 units (i.e. driven by noxious mechanical stimuli) were sensitive to temperature changes. (2) According to their thermal response threshold and their response range, these units were divided into 3 groups. (a) Warming units whose response threshold and maximum response were below 42.5 degrees C. Such units were rarely encountered. (b) Warming/noxious heat units whose response threshold was below 42.5 degrees C but with a maximum response above this temperature. They represented approximatively one-third of the radiant heat-sensitive units. (c) Noxious heat units whose response threshold was above 42.5 degrees C and maximum discharge several degrees above it. Approximately 50% of units activated by radiant heat belonged to this group. (3) Responses to radiant heat stimulation were frequently affected by a first noxious heat application. It consisted: --in a threshold decrease and/or an increase of their cellular discharge for a same temperature range. Only observed for warming/noxious heat units and noxious heat units, this sensitization phenomenon predminantly affected noxious heat units. --in a decrease of cellular discharge for a same temperature range. This desensitization phenomenon was observed for the 3 groups of units driven by radiant heat but mainly for warming units. (4) Supramaximal transcutaneous electrical stimulation revealed that nearly all the thermally sensitive units received A delta and/or C inputs. These units were largely distributed throughout the dorsal horn (laminae I, IV and V). Ten of the 12 lamina I units responded to noxious thermal stimuli. (5) These data indicate that an increase in skin temperature is coded at the level of the rat spinal cord dorsal horn by both an increase in discharge of low threshold temperature sensitive units and a progressive recruitment of high threshold units.