A subpopulation of neurons in the rat rostral nucleus of the solitary tract that project to the parabrachial nucleus express glutamate-like immunoreactivity

In rodents, gustatory information is transmitted from second order neurons in the rostral nucleus of the solitary tract (rNST) to the parabrachial nuc leus (PBN) in the pens. The chemical nature of this projection is unknown. Therefore, the goal of the current study was to determine if rNST neurons t hat project to the PEN express glutamate-like immunoreactivity. Projection neurons were retrogradely labeled following stereotaxic injection of rhodam ine-filled latex microspheres into the right PEN of seven rats while glutam ate-immunoreactive (GLU-IR) structures were visualized in the same tissue u sing an immunoperoxidase procedure. The number of single- and double-labele d neurons located in the right (ipsilateral) and left rNST, in each of the nuclear subdivisions as well as their position along the rostral-caudal axi s of the rNST was determined. GLU-IR cell bodies were located throughout th e rNST. Although the rostral central subdivision contained the highest perc entage (33.8%) of GLU-IR perikarya, immunolabeled neurons were most concent rated (number/area of subdivision) within the medial subnucleus. The rostra l third of the rNST contained the fewest (20.5%) and lowest density of GLU- IR cell bodies. The highest percentage of rNST neurons retrogradely labeled from the PEN were located ipsilateral (85.4%) to the pontine injection sit e, in the middle third of the nucleus (44.2%) and within the rostral centra l subdivision (52.4%). Overall, 18% of the labeled rNST projection neurons were GLU-IR. The distribution of double-labeled neurons mirrored that of th e projection neurons with the largest number located in the ipsilateral rNS T (84.5%), middle third of the nucleus (40.5%) and rostral central subdivis ion (64.7%). These results indicate that glutamate may be a main component of the ascending pathway from the rNST to the PEN. In addition, since GLU-I R neurons were located throughout the rNST and most were not retrogradely-l abeled, the current results suggest that glutamate may be an important neur otrans-mitter within the medulla. (C) 1999 Elsevier Science B.V. All rights reserved.