Differential processing of noxious colonic input by thoracolumbar and lumbosacral dorsal horn neurons in the rat.

Previous studies suggest the lumbosacral (LS) spinal cord processes acute colorectal stimuli whereas the thoracolumbar (TL) and LS spinal segments process inflammatory stimuli. In this study, the effects of colorectal distention (CRD) on TL and LS dorsal horn neuronal activity were recorded in Nembutal-anesthetized male rats both with and without colonic inflammation. Both single cells (before and after inflammation) and populations (multiple cells from noninflamed or inflamed rats) were studied. CRD-responsive neurons had excitatory Abrupt (on-off with stimulus) or Sustained (prolonged after discharge) responses or were Inhibited by CRD. In noninflamed rats, a significantly greater percentage of LS neurons (63% Abrupt, 27% Sustained) were excited by CRD than TL neurons (61% Abrupt, 3% Sustained). The remaining cells were Inhibited (10% LS, 36% TL). LS Abrupt neurons had lower thresholds and greater response magnitudes to CRD compared with TL Abrupt neurons. After colonic inflammation, TL neurons became more excitable: the percentage of Inhibited neurons decreased, the response magnitude of Abrupt neurons increased, and the threshold decreased. In contrast, in single-cell recordings, the response of LS Sustained neurons increased, whereas LS Abrupt neurons decreased. These data suggest that in noninflamed rats, the net response to CRD of TL visceroceptive spinal sensory neurons is less than that of LS neurons. Colonic inflammation increases the net response of TL neurons and differentially modulates the response of LS neurons. These differences may contribute to the functional dichotomy between the TL and LS spinal segments in processing acute and inflammatory colorectal pain.