Enzyme histochemical profile of immunohistochemically identified Renshaw cells in rat lumbar spinal cord

Activity levels of cytochrome oxidase, acid phosphatase, and NADPH diaphorase were examined in the perikarya of immunohistochemically identified Renshaw cells from sections of rat lumbar spinal cord. Renshaw cell profiles were identified on the basis of their characteristic anti-gephyrin-immunofluorescent labelling. Intrasomatic densities of enzyme histochemical reaction product were employed as indicators of relative mitochondrial activity (cytochrome oxidase), intracytoplasmic digestion (acid phosphatase), or putative nitrergic signalling (NAPDH-diaphorase). Approximately half of the Renshaw cell somata examined displayed moderate levels of cytochrome oxidase reaction product (142 of 262 Renshaw cells) or low levels of acid phosphatase activity (156 of 243 Renshaw cells). A majority (160 of 202 cells) of Renshaw cells contained low intrasomatic levels of NADPH-diaphorase activity but most of these cells were closely apposed by at least one NADPH-diaphorase reactive axonal varicosity. Our findings suggest that moderate levels of perikaryal oxidative metabolism and low levels of intracytoplasmic digestion are sufficient for, and support, the unique physiological capabilities of Renshaw cells. The presence of NADPH-diaphorase containing somatic close contacts indicate that nitric oxide may have at least a minor role in the regulation of Renshaw cell activity. These results are complementary and consistent with previous morphological and pharmacological demonstrations of Renshaw cell heterogeneity.