Synaptic organization of excitatory and inhibitory boutons associated with spinal neurons which project through the dorsal columns of the cat.

The cell bodies and proximal dendrites of postsynaptic dorsal column neurons were examined for synaptic boutons which displayed immunoreactivity for the principal excitatory and inhibitory neurotransmitters, glutamate and GABA. The neurons were labelled by retrograde transport of horseradish peroxidase and GABA or glutamate-containing boutons were revealed by performing postembedding immunogold reactions on electron microscope sections. Five neurons were examined and all of them were postsynaptic to boutons which contained either GABA or glutamate. Quantitative analysis of two of the cells revealed that more than 90% of the synaptic profiles associated with them displayed immunogold reactions for these transmitters. Analysis of series of alternate sections, which were reacted for either GABA or glutamate, showed that there was no overlap in the populations of immunoreactive boutons. Furthermore, GABA and glutamate immunoreactions were associated with boutons which had different morphological characteristics. In addition, some large glutamate-enriched boutons were postsynaptic to small boutons which displayed immunogold reactions for GABA. This study demonstrates morphological bases for direct excitation, postsynaptic inhibition and presynaptic inhibition of postsynaptic dorsal column cells.