Different types of 3H-GABA accumulating neurons in the visual cortex of the rat. Characterization by combined autoradiography and Golgi impregnation.

Labelled neurons were identified by autoradiography following tangential intracortical injection of [3H]-gamma-aminobutyrate (GABA). The addition of cis-1,3-aminocyclohexane carboxylic acid to the GABA solution prevented perikaryal labelling. Labelled neurons were found in each injected layer and in addition they were always present directly above the injection track. The labelling of neurons in layer II, and upper III, following injections in layers V. and VI. can be explained by retrograde axonal transport and indicates that some GABA-ergic neurons project vertically. Ninety neurons of different types were Golgi impregnated and examined for selective [3H]-GABA uptake. Sixteen of these were labelled. On the basis of dendritic characteristics they were classified as aspiny multipolar neurons with small, medium or large dendritic fields, sparsely spiny multipolar neurons and one neuron was a bipolar cell. Thus Golgi impregnation of their processes reveals that cortical GABA-ergic neurons are a heterogeneous population. A [3H]-GABA accumulating, aspiny neuron with profoundly branching, "bushy" dendrites and locally arborizing axon in layer VI, was studied in the electron microscope. Its fine structural characteristics were similar to those of other identified non-pyramidal neurons. The existence of several types of cortical GABA-ergic neurons differing in their synaptic connections is discussed.