Demonstration of glutamate immunoreactivity in vagal sensory afferents in the nucleus tractus solitarius of the rat.

To investigate whether glutamate is a neurotransmitter in vagus nerve sensory afferents terminating in the nucleus tractus solitarius, these terminals were identified by the anterograde transport and their glutamate content examined using the post-embedding immunogold technique. After injection of horseradish peroxidase into the nodose ganglion anterogradely labelled axonal boutons were visualized throughout the nucleus of the solitary tract (nTS), the dorsal motonucleus of the vagus nerve (DVN), predominantly ipsilateral to the injection, and to a lesser extent in the area postrema. Electron microscopic analysis of 47 anterogradely labelled boutons in the nTS following post-embedding immunocytochemistry for glutamate revealed that 43 of these boutons (> 91%) contained a level of glutamate immunoreactivity significantly greater (P < 0.001%) than that observed in the surrounding tissue. The observed enrichment of glutamate immunoreactivity in boutons identified as vagus nerve sensory afferents indicate that glutamate may be a transmitter in these neurones.