Morphology of dorsal raphe neurons in the rat: Golgi and intracellular HRP filling study

Abstract

Taste neurons in the nucleus of the solitary tract (NST) not only send axons to the parabrachial nuclei (PbN), but also receive descending projections from gustatory nuclei in the forebrain in rodents. The parvicellular portion of the ventroposteromedial nucleus of the thalamus (VPMpc) receives projections from the bilateral PbN and transmits taste information to the gustatory cortex. Here, we examined the influence of bilateral stimulation of the VPMpc on taste-responsive neurons in the NST. Extracellular single unit activity was recorded from the urethane-anesthetized hamster. Taste responses were confirmed by delivery of four basic tastants to the anterior tongue. After identifying a taste neuron in the NST, the VPMpc was stimulated bilaterally. Thirty seven out of 83 neurons were orthodromically activated following VPMpc stimulation: 30 were excited and seven were inhibited. Among these cells, seven were excited and one was inhibited bilaterally. In addition, four NST neurons were antidromically invaded from the ipsilateral VPMpc. The effect of VPMpc activation on taste-driven responses was tested on 8 of 30 cells that were excited, and all seven cells that were inhibited by the VPMpc stimulation. The VPMpc stimulation enhanced responses to the effective taste stimuli or suppressed the taste-evoked activities in all eight and seven cells tested, respectively, parallel to the type of the inputs which they received from the VPMpc. These results suggest that a subset of taste neurons in the NST is under the influence from the bilateral VPMpc and that the VPMpc activation modulates taste responses of these cells.