N-methyl-D-aspartate receptors on neurons that synthesize nitric oxide in rat nucleus tractus solitarii.

The aim of this study was to determine whether neuronal nitric oxide synthase and N-methyl-D-aspartate receptors are co-localized in the rat nucleus tractus solitarii. Such co-localization would support the hypothesis that nitric oxide participates in nucleus tractus solitarii-mediated functions, such as cardiovascular regulation, by a link to N-methyl-D-aspartate receptors. We used double fluorescent immunohistochemistry using antibodies against neuronal nitric oxide synthase and N-methyl-D-aspartate receptor subunit 1, the fundamental subunit for functional N-methyl-D-aspartate receptors. Labeled brainstem sections were examined with confocal laser scanning microscopy. Most of the N-methyl-D-aspartate receptor subunit 1 immunoreactivity was in cell bodies and proximal dendrites of the numerous labeled cells in the brainstem. High levels of N-methyl-D-aspartate receptor subunit 1 immunoreactivity were present in the dorsal motor nucleus of vagus, hypoglossal nucleus and nucleus ambiguus. All subnuclei of the nucleus tractus solitarii contained moderate levels of N-methyl-D-aspartate receptor subunit 1 immunoreactivity. The distribution of neuronal nitric oxide synthase immunoreactivity in the nucleus tractus solitarii was similar to that described in earlier reports. Superimposition of images revealed that almost all neuronal nitric oxide synthase immunoreactive neurons in the nucleus tractus solitarii contained N-methyl-D-aspartate receptor subunit 1 immunoreactivity, but a lesser portion of N-methyl-D-aspartate receptor subunit 1-immunoreactive cells contained neuronal nitric oxide synthase immunoreactivity. Although all nucleus tractus solitarii subnuclei contained double-labeled neurons, the central subnucleus exhibited the highest density of double-labeled neurons.Co-localization of neuronal nitric oxide synthase and N-methyl-D-aspartate receptor subunit 1 in the nucleus tractus solitarii provides anatomical support for the hypothesis that N-methyl-D-aspartate receptor activation can affect nucleus tractus solitarii-controlled functions via actions on neurons that synthesize nitric oxide.