NMDA-R1 SUBUNIT OF THE CEREBRAL-CORTEX COLOCALIZES WITH NEURONAL NITRIC-OXIDE SYNTHASE AT PRE-SYNAPTIC AND POSTSYNAPTIC SITES AND IN SPINES

The majority of nitric oxide's (NO) physiologic and pathologic actions in the brain has been linked to NMDA receptor activation. In order to determine how the NO-synthesizing enzyme within brain, neuronal NO sy nthase (nNOS), and NMDA receptors are functionally linked, previous st udies have used in situ hybridization techniques in combination with l ight microscopic immunocytochemistry to show that the two are expresse d within single neurons. However, this light microscopic finding does not guarantee that NMDA receptors are distributed sufficiently close t o nNOS within single neurons to allow direct interaction of the two. T hus, in this study, dual immune-electron microscopy was performed to d etermine whether nNOS and NMDA receptors co-exist within fine neuronal processes. We show that nNOS and the obligatory subunit of functional NMDA receptors, i.e. the NMDA-R1, co-exist within dendritic shafts, s pines and terminals of the adult rat visual cortex. Axon terminals for m asymmetric synaptic junctions with the dually labeled dendrites, sug gesting that the presynaptic terminals release glutamate. Axons and de ndrites expressing one without the other also are detected. These resu lts indicate that it is possible for the generation of NO to be tempor ally coordinated with glutamatergic synaptic transmission at axo-dendr itic and axo-axonic junctions and that NO may be generated independent ly of glutamatergic synaptic transmission. Together, our observations point to a greater complexity than previously recognized for glutamate rgic neurotransmission, based on the joint versus independent actions of NO relative to NMDA receptors at pre- versus postsynaptic sites.