Glycine-immunoreactive synaptic terminals in the nucleus tractus solitariiof the cat: Ultrastructure and relationship to GABA-immunoreactive terminals

Postembedding immunogold labeling methods applied to ultrathin and semithin sections of cat dorsomedial medulla showed that neuronal perikarya, dendri tes, myelinated and nonmyelinated axons, and axon terminals in the nucleus tractus solitarii contain glycine immunoreactivity. Light microscopic obser vations on semithin sections revealed that these immunoreactive structures were unevenly distributed throughout the entire nucleus. At the electron mi croscopic level, synaptic terminals with high levels of glycine-immunoreact ivity, assumed to represent those releasing glycine as a neurotransmitter, were discriminated from terminals containing low, probably metabolic levels of glycine-immunoreactivity, by a quantitative analysis method. This compa red the immuno-labeling of randomly sampled terminals with a reference leve l of labeling derived from sampling the perikarya of dorsal vagal neurones. The vast majority of these "glycinergic" terminals contained pleomorphic v esicles, formed symmetrical synaptic active zones, and targeted dendrites. They appeared to be more numerous in areas of the nucleus tractus solitarii adjoining the tractus solitarius, but rather scarce caudally, medially, ve ntrally, and in the dorsal motor vagal nucleus. In a random analysis of the entire nucleus tractus solitarii, 26.2% of sampled terminals were found to qualify as glycine-immunoreactive, In contrast, boutons immunoreactive for gamma-aminobutyric acid (GABA) were more evenly distributed throughout the dorsal vagal complex and accounted for 33.7% of the synaptic terminals sam pled. A comparison of serial ultrathin sections suggested three subpopulati ons of synaptic terminals: one containing high levels of both GABA- and gly cine-immunoreactivities (21% of all terminals sampled), one containing only GABA-immunoreactivity (12.7%), and relatively few terminals (5.2%) that we re immunoreactive for glycine alone. These results were confirmed by dual l abeling of sections using gold particles of different sizes. This study rep orts the first analysis of the ultrastructure of glycinergic nerve terminal s in the cat dorsal vagal complex, and the pattern of coexistence of glycin e and GABA observed provides an anatomical explanation for our previously r eported inhibitory effects of glycine and GABA on neurones with cardiovascu lar and respiratory functions in the nucleus tractus solitarii. (C) 1999 Wi ley-Liss, Inc.