Characterization of neurons responsive to noxious colorectal distension in the T13-L2 spinal cord of the rat

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Abstract
1. One-hundred thirty-two neurons responsive to colorectal distension in the dorsal horn of the T13-L2 spinal segments of 35 spinalized and 7 intact, deeply pentobarbital-sodium-anesthetized rats were characterized for convergent cutaneous receptive fields, long ascending projections and responses to the intra-arterial administration of the algesic peptide bradykinin. All but 9 neurons had an identifiable excitatory cutaneous receptive field; all receptive fields were located on the lower abdomen, flank, and dorsal body surface. Electrical stimulation in the cutaneous fields of 28 neurons demonstrated that neurons responsive to colorectal distension receive afferent information carried by A- and C-fibers. Stimulus-response functions (SRFs) of 52 neurons excited by graded colorectal distension (20-100 mmHg, 20 s) were monotonic and accelerating, allowing extrapolation of threshold distending pressures to neuronal response. Neurons were subdivided into four classes based upon their response to an 80-mmHg, 20-s colorectal distension search stimulus. 2. Short-latency abrupt [SL-A] neurons (spinalized, n = 46; intact, n = 9) were excited at short latency; activity abruptly returned to base line on termination of distension. Six of 9 neurons in intact rats had long ascending projections as demonstrated by antidromic invasion from the contralateral, ventrolateral caudal medulla. Responses of SL-A neurons to colorectal distension were significantly greater in spinalized than in intact rats. Fifty-three of 55 SL-A neurons had convergent excitatory cutaneous receptive fields and most were responsive to both noxious and nonnoxious stimuli. Ten of 13 neurons tested were excited by intra-arterial bradykinin. The threshold distending pressure, determined from the SRFs of 29 neurons in both the spinalized and intact states, extrapolated to near 0 mmHg. 3. Short-latency sustained (SL-S) neurons (spinalized, n = 31; intact, n = 11) were also excited at short latency in response to colorectal distension, but responses were sustained for 4-50 s following termination of the distending stimulus. Nine of 11 SL-S neurons in intact rats had long ascending projections. All 42 SL-S neurons were spontaneously active and 41 of 42 had convergent excitatory cutaneous receptive fields, excited exclusively by noxious stimuli (n = 29) or excited by both noxious and nonnoxious stimuli (n = 12). Responses to colorectal distension and spontaneous activity were significantly greater in spinalized rats. Twelve of 12 neurons tested were excited by intra-arterial bradykinin.(ABSTRACT TRUNCATED AT 400 WORDS)