Glycine-containing terminals in the rat dorsal vagal complex.

The present study examined the distribution of glycine and glycine-receptors in the dorsal vagal complex using pre-embedding immunocytochemistry. Glycine-immunoreactive terminals were present in moderate densities in the medial, intermediate, interstitial, commissural and ventrolateral subnuclei of the nucleus tractus solitarii. The dorsolateral nucleus tractus solitarii and the dorsal vagal motor nucleus contained only very few, scattered glycine-containing terminals. Glycine terminals appeared to be concentrated in regions of the dorsal vagal complex receiving primary vagal afferents, though previous studies have suggested that glycine is not present in these afferents. A conspicuously high concentration of glycine terminals was observed in the medial nucleus tractus solitarii where a population of cholinergic neurons has been identified previously. Ultrastructurally glycine immunoreactivity was principally associated with terminals containing flattened, pleomorphic vesicles and forming symmetrical synaptic contacts, mostly with dendrites. Glycine receptor immunoreactivity was present throughout the dorsal vagal complex with little evidence of subnuclear localization. With electron-microscopic examination, glycine receptor immunoreactivity was associated with dendritic membranes and was associated presynaptically with terminals containing flattened pleomorphic vesicles. Overall, the present data provide evidence consistent with a neurotransmitter role for glycine in the dorsal vagal complex. The presence of glycine in regions of the dorsal vagal complex receiving vagal afferents suggests a prominent role for this neurotransmitter in autonomic regulation.