Neuroanatomical evidence that vagal afferent nerves do not possess a high affinity uptake system for glutamate

The ability of vagal and glossopharyngeal afferent neurons to retrogradely transport 3H-D-aspartate from the nucleus tractus solitarius to the nodose and petrosal ganglia was examined. Injections of 3H-D-aspartate centered in the medial NTS at the rostral-caudal level of the area postrema failed to consistently label cells in the nodose and petrosal ganglia. In 5 of the 10 rats studied no retrogradely labeled neurons were observed in these ganglia ipsilateral to the injection site, while in the other 5 rats a small number of cells (less than 3%) were labeled. Injections of 3H-D-aspartate into the NTS consistently produced retrograde labeling of neurons in the ipsilateral paratrigeminal area. In addition, many heavily labeled neurons were observed in the injected as well as the contralateral NTS. Injections of 3H-D-asparate into the spinal trigeminal nucleus consistently labeled neurons in the trigeminal ganglion. Since the uptake and retrograde transport of 3H-D-aspartate appears to be characteristic of neurons that use glutamate or aspartate as a neurotransmitter, these results suggest that vagal and glossopharyngeal afferents are not glutamatergic or aspartatergic.