Quantitative analysis of the ultrastructural distribution of GABA-like immunoreactivity in the intermediate and medial part of hyperstriatum ventrale of chick

The intermediate and medial part of the hyperstriatum ventrale of the chick telencephalon plays a crucial role in the learning processes of imprinting. The distribution within the intermediate and medial part of the hyperstriatum ventrale of the neurotransmitter γ-amino butyric acid was studied with light and electron microscopy using an antibody against this amino acid. The antibody labelled 18.4% of neuronal somata. GABA-labelled terminals made symmetrical synapses onto somata and dendrites of labelled and unlabelled neurons. Labelled somata received about three times as many synaptic boutons as unlabelled somata. Approximately 21% of synaptic terminals on labelled somata were themselves labelled; unlabelled somata received a higher proportion (37.6%) of such terminals. Most labelled terminals synapsing with dendrites were confined to the shafts; very few labelled terminals contributed to axospinous synapses. Synaptic contacts made on dendritic shafts by labelled boutons were intermingled with symmetrical and asymmetrical contacts from non-immunoreactive terminals. The proportion of labelled terminals received by labelled dendrites (33.1%) was approximately twice that received by unlabelled dendrites (15.9%). Labelled neurons therefore received a higher proportion of labelled terminals on their dendrites and a lower proportion on their somata compared with unlabelled neurons. No immunoreactivity was seen in glial cells or ependyma