NEUROTRANSMITTERS IN THE SPINAL-CORD DORSAL HORN IN A MODEL OF PAINFUL NEUROPATHY AND IN NERVE CRUSH

We tested the hypothesis that neurochemical changes in the spinal cord dorsal horn associated with neuropathic pain states differ from those seen in association with non-painful neuropathies. Immunohistochemistry was performed on spinal cord sections from rats with a chronic constriction injury (CCI), which develop hyperalgesia, and from animals with a nerve crush injury, which do not develop hyperalgesia or other signs of a painful syndrome. Immunohistochemistry was quantified by computer-assisted densitometry. Calcitonin gene-related peptide (CGRP) immunoreactivity and substance P (SP) immunoreactivity were decreased from 1 to 4 weeks after injury in CCI and from 2 to 6 weeks in crush. Gamma-aminobutyric acid immunoreactivity was unchanged in both conditions at all time points. Met-enkephalin (Met-enk) immunoreactivity was increased in CCI and unchanged in crush. Although SP and CGRP are involved in pain transmission, we conclude that their decrease in immunoreactivity is not specific for the CCI model, but rather a more general event in nerve de- and regeneration, The increase in immunoreactivity for the opioid peptide Met-enk, however, was only seen in the late phase of CCI, and may be specific for conditions associated with neuropathic pain and its resolution.