DO NITRIC-OXIDE SYNTHASE, NMDA RECEPTOR SUBUNIT R1 AND CYTOCHROME-OXIDASE CO-LOCALIZE IN THE RAT CENTRAL-NERVOUS-SYSTEM

The activation of nitric oxide synthase (NOS) has been linked to excit atory input via NMDA receptors. We hypothesized that NOS-positive neur ons that have NMDA receptors on their surface would have high levels o f cytochrome oxidase (C.O.) as energy generator for membrane repolariz ation. In order to compare the distribution of these markers on the sa me section, we reacted rat brain sections for C.O. histochemistry foll owed by NOS immunogold silver staining (IGSS). Adjacent sections were reacted for NOS IGSS followed by indirect immunoperoxidase for NMDA re ceptor subunit R1 (NMDAR1). We found that the staining pattern varied among regions but were consistent within each region examined. There a re three types of NOS immunoreactive (NOS-ir) cells: (1) NOS-ir neuron s that had moderate to high levels of both NMDAR1 and C.O. staining, s uch as the pontine reticular nuclei, motor and mesencephalic nuclei of the trigeminal nerve, and some motor neurons in the spinal cord. (2) NOS-ir neurons that were immunoreactive for NMDAR1 (NMDAR1-ir) but had low levels of C.O. activity in their somata. Their dendrites, however , were both NMDAR1-ir and rich in C.O. Examples of this type include n eurons in the caudate and putamen, and periglomerular cells in the olf actory bulb. (3) We also found that some NOS-ir neurons were not NMDAR 1-ir and had low C.O. activity. In addition to postsynaptic neurons, C .O. and NOS levels were both high in the inner segments of retinal pho toreceptor cells, where energy-demanding active ion transport maintain s the dark current and where NO presumably activates guanylate cyclase for the production of cGMP, which keeps the Na+ channels open in the dark. Our findings suggest that NMDA receptors are available for the m ajority of NOS-ir neurons, which comprise a heterogenous population wi th varying energy demands.