Profiles enriched in GABA-like immunoreactivity participate in serial synapses in the pontine nuclei of baboon (Papio anubis).

Using a postembedding immunogold procedure with an antiserum against glutaraldehyde-fixed GABA, we demonstrate GABA-like immunoreactivity in two classes of synaptic profiles in the pontine nuclei of baboon. One is an axon terminal in symmetrical synaptic contact with small or medium-sized GABA-immunonegative dendrites, the other is a pale, vesicle-containing profile resembling a dendrite or dendritic process which participates in serial synaptic arrangements. These synaptic arrangements, or triads, consist of a GABA-like immunoreactive, pale vesicle-containing profile being postsynaptic to a GABA-immunonegative axon terminal, and presynaptic to a small or medium-sized GABA-immunonegative dendrite. In at least some of these triads, the GABA-immunonegative axon terminals also contact the GABA-immunonegative dendrite directly.