Colocalization of GluR1 and neuronal nitric oxide synthase in rat nucleus tractus solitarii neurons

Previously we demonstrated that glutamate and neuronal nitric oxide synthase (nNOS) containing neuronal elements are frequently apposed in subnuclei of the rat nucleus tractus solitarii. It is known that glutamate receptors (GluRs) of the alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) subtype participate in cardiovascular regulation by the nucleus tractus solitarii and that responses to AMPA receptor activation may be linked to NO. Therefore, in the present study, we further tested the hypothesis that the calcium-permeable subunit GluR1 of AMPA type GluRs and nNOS are colocalized in neurons of the nucleus tractus solitarii. Distribution of GluR1 and nNOS in rat nucleus tractus solitarii was investigated by double fluorescent immunohistochemistry combined with confocal laser scanning microscopy. Numerous GluR1 immunoreactive cells and fibers were present in subnuclei of the nucleus tractus solitarii. The staining intensity of GluR1 immunoreactive cells varied among subnuclei. Cells in the interstitial subnucleus contained the highest GluR1 staining intensity. A moderate intensity of staining was present in the intermediate, dorsolateral, ventral, and commissural subnuclei. A slightly lower level of GluR1 immunoreactivity was present in cells of the medial subnucleus. Cells in the central subnucleus contained a low level of GluR1 immunoreactivity. The staining intensity of GluR1 immunoreactive fibers also varied among subnuclei. Distribution of nNOS immunoreactivity in the nucleus tractus solitarii and other brain stem areas was the same as in our earlier reports. Superimposition of confocal images of nNOS immunoreactivity and GluR1 immunoreactivity allowed us to identify double-labeled structures. Nearly all neurons that were immunoreactive for nNOS contained GluR1 immunoreactivity, but only a proportion of GluR1 immunoreactive cells contained nNOS immunoreactivity. Double-labeled neurons were present in all subnuclei of the nucleus tractus solitarii. The percentages of GluR1 immunoreactive cells that also contained nNOS immunoreactivity differed among subnuclei of the nucleus tractus solitarii. Fibers that labeled for nNOS alone, GluR1 alone or both were present among labeled cells in these subnuclei. These data support the hypothesis that GluR1 and nNOS are colocalized in neurons of nucleus tractus solitarii. The demonstration of this anatomical relationship provides further anatomical support for the hypothesis that activation of AMPA receptors on neurons that synthesize NO in the nucleus tractus solitarii contributes to autonomic regulation. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.