Long-term sensitization of primary afferents in adult rats exposed to neonatal colon pain.

We have previously shown that colon irritation (CI) in neonates results in chronic visceral hypersensitivity in adult rats, associated with central neuronal sensitization in the absence of identifiable peripheral pathology. The purpose of this study is to assess the relative contribution of peripheral mechanisms to chronic visceral hypersensitivity by examining the changes in responses of primary afferents at thoracolumbar (TL) and lumbosacral (LS) spinal segments to graded colorectal distension (CRD). Afferent discharges were recorded at the cut distal ends of spinal dorsal roots (DRs) in adult control and CI rats. We found that: (1). the average threshold of activation of LS afferents decreased significantly in CI rats; (2). the responses of TL and LS afferents to CRD in CI rats were significantly greater than those in control; (3). the spontaneous activity of LS afferents in CI rats was significantly stronger than that in control; (4). in CI rats, the average responses to graded CRD of LS DRs were significantly higher than those of TL DRs; and (5). the number of both LS DRs and TL DRs activated by CRD in CI rats was significantly larger than control. In summary, the results show that chronic visceral hypersensitivity is associated with peripheral sensitization, as well as central sensitization. TL visceral afferents projecting seem to be more involved in the processing of sensitized nociceptive input from the colon than acute nociceptive input. However, LS afferents seem to be equally important in both sensitized and acute pain states.