Spinal glycinergic and GABAergic neurons expressing C-fos after capsaicin stimulation are increased in rats with contralateral neuropathic pain.

There is increasing evidence that pain transmission on one side of the body is influenced by a painful state on the other side. We have investigated this phenomenon by studying the activation pattern (using C-fos labeling) of spinal glycinergic and GABAergic (Gly/GABA) neurons after capsaicin injection in the ipsilateral hind paw of rats that were preconditioned with an acute or chronic pain stimulus in the contralateral hind paw or rats that were not preconditioned (control). For this purpose, fluorescent in situ hybridization with GlyT2 and GAD67 mRNA probes was combined with fluorescent C-fos immunohistochemistry. Rats were preconditioned with acute (capsaicin, Complete Freund's Adjuvant (CFA) 1.5 h), chronic inflammatory (CFA 20 h and 4 days), neuropathic (spared nerve injury (SNI) 2 weeks), or control pain stimuli (saline 20 h and 4 days; sham-SNI 2 weeks). We found that after capsaicin injection in rats preconditioned with CFA inflammation (4 days), sham-SNI or with SNI neuropathic pain, the numbers (27 ± 3, 21 ± 2, and 21 ± 2, respectively) and percentages (55% ± 4, 43% ± 2, and 42% ± 2, respectively) of C-fos activated neurons that were Gly/GABA increased significantly as compared with control (10 ± 1 and 25% ± 2). The increase in the total number of C-fos activated Gly/GABA neurons was present primarily in the superficial dorsal horn (laminae I and II; control: 9%; CFA 4 days: 56%; SNI 2 weeks: 42%). This increase in C-fos activation of Gly/GABA neurons occurred without significant changes in the total number of C-fos activated neurons, and without any significant changes in the mechanical thresholds in the hind paws after capsaicin injection. The results showed that one-sided chronic pain, especially inflammation, significantly increases the C-fos activation pattern of spinal Gly/GABA neurons on the other side of the spinal cord. This further underlines the existence of a dynamic interaction between ipsi- and contralateral spinal neurons in the processing of nociceptive information.