Direct observations of synapses between GABA-immunoreactive boutons and identified spinocervical tract neurons in the cat's spinal cord.

Three spinocervical tract neurons in adult cats were physiologically characterized and intracellularly labelled with horseradish peroxidase. The neurons were reconstructed and examined with the light microscope and were prepared for postembedding immunochemical analysis by using an antiserum which specifically recognizes GABA in glutaraldehyde-fixed tissue. Semithin sections were tested and examined with the light microscope. Somata, proximal, and distal dendrites of all three cells were associated with numerous punctate GABA-immunoreactive structures. Immunoreactive perikarya of small neurons in the vicinity of spinocervical tract cells were also observed. Ultrastructural analysis, with the immunogold technique, revealed that somata and proximal dendrites of all three neurons received synaptic contacts (about 37% of total synapses) from GABA-immunoreactive boutons and that distal dendrites were also associated with substantial numbers of immunoreactive structures (about 27% of synapses). Immunoreactive boutons were small (about 1 micron in diameter), contained irregularly shaped agranular vesicles, and formed symmetrical synaptic junctions with identified neurons. An additional group of immunoreactive boutons was observed to be associated with one of the cells only; these contained many large dense-core vesicles in addition to small agranular vesicles. Boutons containing round agranular vesicles and flattened agranular vesicles were not observed to be immunoreactive. The evidence supports the idea that much of the postsynaptic inhibition observed in spinocervical tract neurons is mediated by GABA and that even the most distal dendrites of these neurons receive inhibitory inputs.