Characterization of superficial T 13-L 2 dorsal horn neurons encoding for colorectal distension in the rat: comparison with neurons in deep laminae

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Fifty-five neurons responsive to colorectal distension located in the superficial spinal dorsal horn (0.0–0.3 mm ventral to the cord dorsum) of the T 13-L 2 spinal segments of pentobarbital-anesthetized, physiologically intact or spinalized (C 1 transection), decerebrate rats were characterized. These neurons could be separated into three groups based upon their response to an 80 mm Hg, 20 s colorectal distension: (1) short latency-abrupt (SL-A) neurons ( n = 22) that were excited by colorectal distension at a short latency (<1s) and abruptly terminated responses following the termination of the distending stimulus; (2) short latency-sustained (SL-S) neurons ( n = 26) that were excited by colorectal distension at a short latency (<1s) and demonstrated sustained responses (<4s) following termination of the distending stimulus; and (3) INHIB neurons ( n = 7) that were spontaneously active and were inhibited by colorectal distension. All 55 neurons had convergent cutaneous receptive fields (i.e. were ‘viscerosomatic’), exhibiting excitatory responses to noxious (pinch/heat) and/or non-noxious (brush) stimuli. Neurons excited by colorectal distension also demonstrated monotonic, accelerating responses to graded colorectal distension, were excited by the intraarterial administration of bradykinin, could be antidromically activated by electrical stimulation in the caudal ventrolateral medulla and were subject to tonic descending inhibitory modulation as evidenced by more vigorous responses to distension when rats were reversibly spinalized using a cold block. A comparison of these 55 superficial dorsal horn neurons with 90 neurons located in the deep spinal dorsal horn (0.4–1.3 mm ventral to the cord dorsum) demonstrated quantitative differences between SL-A neurons located superficially and those located at deeper sites; no quantitative differences between respective groups of SL-S and INHIB neurons were apparent.