Prolonged noxious mechanical stimulation of the rat's tail: responses and encoding properties of dorsal horn neurones.

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Abstract
Single-unit electrical activity has been recorded from dorsal horn neurones in the sacral (S1-S2) segments of the spinal cord of barbiturate-anaesthetized rats. Fifty-two neurones responding to a manually applied pinch of their receptive fields in the tail were selected. They were subsequently tested for their responses to four successive 2 min pinches at noxious intensities delivered by a feed-back-controlled mechanical device. Neurones were tested with both innocuous (i.e. brushing and stroking) and noxious (i.e. pinching, pin-prick, and in some cases heating about 45.degree. C) stimulation of their cutaneous receptive fields. Three of the tested cells were driven exclusively by innocuous skin stimulation (mechanoreceptive or class 1), thirty-six were driven by both innocuous and noxious skin stimulation (multireceptive or class 2) and thirteen were driven exclusively by noxious skin stimulation (nocireceptive or class 3). All of the multireceptive and nocireceptive neurones responded to the 2 min noxious pinch with an initial phasic discharge followed by sustained firing that showed little evidence of adaptation throughout the stimulus period. The three mechanoreceptive neurones responded to the 2 min noxious pinch with a short discharge at the stimulus onset, but were silent for the remainder of the stimulus period. Thirty-one cells were tested with successive 2 min pinches of 4, 6 and 8 N (and in some cases, a further 4 N pinch) applied at 10 min intervals. Different encoding properties were observed during the sustained part of the neuronal response according to: (i) the afferent fiber input characteristics of the cell; (ii) whether or not the tail had received a test series of pinches earlier in the same experiment. None of the multireceptive cells with only an A-fiber afferent input encoded the stimulus strength. However, the multireceptive cells with both an A- and a C-fiber afferent input and all nocireceptive cells did encode the stimulus strength, providing that no previous noxious test stimuli had been applied to the tail. The encoding nocireceptive neurones had in general a steeper stimulus-response curve than the encoding multireceptive neurones, though the two groups overlapped to some extent. Three encoding cells (two multireceptive and one nocireceptive) were tested with a second series of pinches (4, 6, 8 and 4 N), 40 min subsequent to the initial test series. These cells did not encode this second test series, but were more excitable, producing a greater response to a given test force. These results demonstrate that nocireceptive and multireceptive dorsal neurones respond throughout a 2 min noxious pinch of their receptive fields and suggest that nocireceptive neurones are better suited to encoding the intensity of this type of stimulus than multireceptive neurones. However, these encoding properties appear to be disrupted by a previous noxious stimulus.