NMDAR1 GLUTAMATE-RECEPTOR SUBUNIT ISOFORMS IN NEOSTRIATAL, NEOCORTICAL, AND HIPPOCAMPAL NITRIC-OXIDE SYNTHASE NEURONS

Nitric oxide (NO), an unconventional and diffusible neurotransmitter, is synthesized by nitric oxide synthase (NOS). NMDA glutamate receptor s are potent regulators of NO synthesis. We have used dual-label immun ofluorescence and confocal microscopy to examine forebrain neurons in the rat that contain high levels of neuronal NOS (nNOS) for the presen ce of the NMDAR1 receptor subunit protein and regions of this protein encoded by three alternative spliced segments of the NMDAR1 mRNA: N1, C1, and C2. In the neostriatum, neocortex, and hippocampus, nNOS-label ed neurons exhibit strong NMDAR1 immunoreactivity (-ir). In all three of these regions, nNOS-positive neurons are characterized by the absen ce of immunoreactivity for the C1 segment of NMDAR1, whereas C1-ir is abundant in most nNOS-negative neurons, In addition, nNOS-ir neurons e xhibit selective staining for the alternative C2' terminus of NMDAR1 t hat is produced when the C2 segment is absent. These results demonstra te directly that neurons with abundant nNOS-ir contain NMDAR1 receptor subunit proteins and that the NMDAR1 isoforms present in these cells differ from those of most other neurons in these regions. The distinct NMDA receptor phenotype of these nNOS-positive neurons is likely to c ontribute to both the physiological regulation of NO release by glutam ate as well as to NO-mediated excitotoxic injury.