GABAergic structures in the chick telencephalon: GABA immunocytochemistry combined with light and electron microscope autoradiography, and Golgi impregnation.

The distribution of gamma-aminobutyric acid (GABA)-ergic elements in 3 forebrain regions (medial mid-telencephalic hyperstriatum ventrale; paleostriatum augmentatum; lobus parolfactorius) of two-day-old domestic chicks was investigated using (1) light and electron microscope autoradiography following [3H]GABA uptake in vitro in combination with pre-embedding GABA immunocytochemistry and (2) Golgi impregnation and 'gold-toning' combined with postembedding GABA immunocytochemistry. In both the paleostriatal regions and the medial (mid-telencephalic) hyperstriatum ventrale, GABA immunolabelling was demonstrated with the pre-embedding technique. Radiolabelling with [3H]GABA was also shown in these regions, co-localised in many cases with the immunolabelling. In the paleostriatal regions, the majority of perikaryal labelling was found in ovoid, elongated or fusiform cell bodies of 6-7 micron diameter whereas in the medial (mid-telencephalic) hyperstriatum ventrale, larger (10-15 micron) multipolar and smaller (5-6 micron) bipolar neurons were found labelled. In the latter region, Golgi impregnated neurons of similar morphology were found to be immunopositive to GABA using the postembedding technique. The ultrastructure of [3H]GABA accumulating cells is characterised by pale or moderately granular nuclei with small invaginations, few mitochondria and a prominent Golgi apparatus. Astrocytes and ependymal cells are also labelled with [3H]GABA. GABA-labelled axon terminals represent 29-36% of the total in the 3 brain regions studied. They appear as electron-lucent boutons with few and often scattered synaptic vesicles and in most cases they form symmetrical axo-dendritic junctions.