Responses of rat spinal neurones to natural and electrical stimulation of colonic afferents: effect of inflammation

Single unit electrical activity has been recorded from 107 neurones excited by electrical stimulation of the pelvic nerve in or around lamina X of the L6-S1 spinal cord in anaesthetised rats. Responses to colorectal distensio n (CRD; 30 s, 5-80 mmHg) and to somatic electrical and mechanical stimulati on were characterised. Of 107 neurones excited by pelvic nerve stimulation, 58 (54%) were affected by CRD: 46 neurones were excited (39 with a sustain ed response and 7 with an on-off response) and 12 neurones were inhibited. The vast majority of the neurones affected by CRD (54/58) had nociceptive s omatic receptive fields. Neurones excited by CRD showed graded stimulus res ponse functions in the noxious range (20-80 mmHg), except for two neurones which only encoded stimulus intensity below 20 mmHg. Neurones inhibited by CRD had significantly larger somatic receptive fields, and more superficial recording sites than those excited by CRD. A group of 12 neurones with sus tained excitatory responses to CRD were characterised before and 45 min aft er intracolonic instillation of 1 ml 0.6% acetic acid. Colon inflammation p rovoked a significant increase in responses to CRD and to pelvic nerve stim ulation (n=12), but no significant change in responses to pinch of their so matic receptive field (n=10). We conclude that of these neurones, the popul ation with excitatory sustained responses to CRD are those likely responsib le for processing information leading to acute pain sensations from the col on, and also show central sensitisation after colon inflammation. suggestin g they play an important role in development of colonic hyperalgesia. (C) 2 000 Published by Elsevier Science B.V. All rights reserved.