Does pain damage spinal cord neurons? Transsynaptic degeneration in rat following a surgical incision

In prior studies, rats with a chronic constriction injury to the sciatic nerve have been found to have small- to medium-sized, pyknotic, and hyperchromatic neurons ('dark neurons'; DNs) in spinal dorsal horn laminae I-III. It has been proposed that DNs are produced by an excitotoxic insult involving N-methyl-D-aspartate receptor activation subsequent to ectopic nociceptor discharge, and that at least some DNs are inhibitory interneurons whose functional impairment or death contributes to a central state of hyperexcitability that underlies neuropathic hyperalgesia and allodynia. We show here that laminae I-III DNs are also present 2 days after a surgical procedure that does not include major nerve damage. We propose that this is also the result of a nociceptor-driven excitotoxic insult and that the functional impairment of the affected neurons may contribute to postoperative pain and tenderness.