Biochemical evidence that l-glutamate is a neurotransmitter of primary vagal afferent nerve fibers

Abstract

To determine in rat if vagal afferent fibers projecting into the intermediate one third of the nucleus tractus solitarius (NTS), the site of termination of baroafferents, utilize glutamate as a neurotransmitter, the high-affinity uptake of [3H]L-glutamate and content of glutamate were analyzed in micropunches of rat brain stem. The intermediate NTS contains a high-affinity synaptosomal uptake system for [3H]L-glutamate that is greater in capacity than that in areas adjacent to the NTS; it is almost two-fold higher than uptake in medial septum and nucleus accumbens and equal to that of hippocampal regions purportedly containing a rich glutamatergic innervation. Unilateral ablation of the nodose ganglion (i.e. cells of origin of vagal afferents) resulted, within 24 h in a prolonged significant reduction, to 56% of control, of [3H]L-glutamate uptake, bilaterally in the NTS. The reduction of Na+-dependent synaptosomal uptake of [3H]L-glutamate, resulted from a decrease in Vmax without change in the Km of the process, was anatomically restricted to the intermediate NTS, and was not associated with changes in [3H]GABA uptake. The content of glutamate in the NTS was significantly (P less than 0.01) decreased by 30% 7 days following unilateral extirpation of the nodose ganglion without changes in the concentrations of aspartate, glycine, glutamine, or GABA. A population of vagal afferent fibers projecting to NTS are glutamatergic. The results are consistent with the hypothesis obtained by physiological and pharmacological techniques that glutamate is a neurotransmitter of baroafferents.