Ultrastructural Relationships Between GABAergic Terminals and Cardiac Vagal Preganglionic Motoneurons and Vagal Afferents in the Cat: A Combined HRP Tracing and Immunogold Labelling Study

The ultrastructural relationships between gamma-aminobutyric acid-immunoreactive (GABA-ir) neurons and other neurons of the nucleus tractus solitarius (NTS) and motoneurons of the nucleus ambiguus (NA) and dorsal motor vagal nucleus (DMVN), were examined by electron microscopic (EM) immunogold labelling with an anti-GABA antiserum on brain stem sections in which vagal motoneurons and vagal afferent fibres were labelled with horseradish peroxidase (HRP). HRP was applied to the cervical vagus or the cardiac vagal branch of anaesthetized cats. After 24 - 48 h survival, brains were glutaraldehyde-fixed and a stable HRP-tetramethylbenzidine reaction product compatible with EM processing was revealed on 250 microm vibratome sections. Following osmium postfixation, dehydration and resin embedding, GABA-ir was localized on ultrathin sections by an immunogold technique. GABA-ir axon terminals, heavily and specifically labelled with gold particles, were very numerous within NTS, DMVN and NA. All terminals contained small, clear, pleomorphic vesicles and a few also contained larger dense cored vesicles. The density of gold particles over clear vesicles, dense cored vesicles and mitochondria was significantly greater than over the cytoplasm of these terminals. GABA-ir synapses were found on the soma and dendrites of neurons, but rarely on other axon terminals within NTS, where GABA-ir cell bodies and dendrites were also seen. These received synaptic contacts from both GABA-ir terminals and from HRP-labelled vagal afferents. In both the DMVN and NA, similar GABA-ir synapses were present on both the soma and dendrites of HRP-labelled motoneurons. GABA synapses were also present on other cell types in DMVN. These observations provide an anatomical basis for a GABAergic inhibition of neurons forming the central pathways of cardiovascular and other autonomic reflexes.