Fos expression by glutamatergic neurons of the solitary tract nucleus after phenylephrine-induced hypertension in rats.

The baroreflex pathway might include a glutamatergic connection between the nucleus of the solitary tract (NTS) and a segment of the ventrolateral medulla (VLM) called the caudal ventrolateral medulla. The main goal of this study was to seek direct evidence for such a connection. Awake rats were subjected to phenylephrine- (PE-) induced hypertension (N=5) or received saline (N=5). Neuronal activation was gauged by the presence of Fos-immunoreactive (Fos-ir) nuclei. Fos-ir neurons that contained vesicular glutamate transporter 2 mRNA (glutamatergic neurons) or glutamic acid decarboxylase mRNA (GABAergic neurons) were mapped throughout the medulla oblongata. Saline-treated rats had very few Fos-ir neurons. In PE-treated rats, Fos-ir neurons were detected in both NTS and VLM. In NTS, 72% of Fos-ir neurons were glutamatergic and 26% were GABAergic. In the VLM, 41% of Fos-ir neurons were glutamatergic and 56% were GABAergic. In VLM, Fos-ir glutamatergic neurons were evenly distributed and were often catecholaminergic, whereas Fos-ir GABAergic cells were clustered around Bregma -13.0 mm. This region of the VLM was injected with Fluoro-Gold (FG) in eight rats, four of which received PE and the rest saline. Fos-ir NTS neurons retrogradely labeled with FG were detected only in PE-treated rats. These cells were exclusively glutamatergic and were concentrated within the NTS subnuclei that receive the densest inputs from arterial baroreceptors. In conclusion, PE, presumably via baroreceptor stimulation, induces Fos in glutamatergic and GABAergic neurons in both NTS and VLM. At least 29% of the Fos-ir glutamatergic neurons of NTS project to the vicinity of the VLM GABAergic interneurons that are presumed to mediate the sympathetic baroreflex.