Nitric oxide synthase (NOS) coexists with activated neurons by skeletal muscle contraction in the brainstem of cats.

Contraction of skeletal muscle evokes increases in arterial blood pressure and heart rate. Some regions of the brainstem have been implicated for expression of the cardiovascular responses to muscle contraction. Previous studies have reported that static muscle contraction induced c-Fos protein in the nucleus of tractus solitarii (NTS), lateral reticular nucleus (LRN), lateral tegmental field (FTL), subretrofacial nucleus (SRF), A1 region and periaqueductal gray (PAG) of the brainstem. Furthermore, neuronal NADPH-diaphorase (NADPH-d), which is considered as a marker of neuronal nitric oxide synthase (nNOS), has been localized in those same regions. In this study, static muscle contraction was induced by electrical stimulation of the L7 and S1 ventral roots in anaesthetized cats. Distribution of c-Fos protein within neurons containing nNOS was evaluated by double labeling methods in order to determine if nNOS containing neurons in the brainstem were activated during muscle contraction. The results indicate that c-Fos protein colocalized with NADPH-d positive staining within the neurons of the SRF and PAG, but not within the NTS neurons. Distinct number of neurons with c-Fos protein was in close proximity to NADPH-d positive staining in the NTS, SRF, and PAG. Coexisting of c-Fos protein and NADPH-d positive staining was not observed in the LRN, FTL and A1 region. These findings demonstrate that nNOS containing neurons were activated by muscle contraction in the selective regions of the brainstem, and nNOS positive staining had close anatomic contacts with the neurons activated by contraction. This result provides neuroanatomic evidence suggesting that nitric oxide modulates the cardiovascular responses to muscle contraction within the NTS, SRF and PAG of the brainstem.