Selective uptake and transport of label within three identified neuronal systems after injection of 3H-GABA into the pigeon optic tectum: an autoradiographic and Golgi study.

After injection of tritiated gamma-aminobutyric acid (GABA) into the pigeon optic tectum and thalamus it was possible to correlate certain aspects of the autoradiographic labeling pattern with observations made from Golgi material. Three neuronal, GABA specific systems were identified both from the uptake of the amino acid and from the subsequent and bidirectional in tracellular transport of labe. The first system derives from cell bodies within sublayer IIi the axons of which could be selectively labelled throughout their course within layer I and to the areas of termination within the pretectum and ventral thalamus. The radially ascending dendrites and axon collaterals of these neurons arbourised within sublayer IIf, and could be labelled in a retrograde fashion after tectal or thalamic injections. The second system was represented by small perikarya within sublayer IIc with locally and superficially directed dendrites and with a radially and deep directed axon from which an extensive axon collateral system arose. It was found possible to label these perikarya either directly or indirectly after tangential tectal injections which preferentially labelled the axons and terminals of these neurons within the deeper regions of the tectal cortex and resulted in the retrograde axonal movement of label to theoverlying cell bodies. A third system was found within sublayer IId, was horizontally organized and from a correlation with degeneration, other autoradiographic and Golgi preparations thought to be mainly dendritic in nature. The biochemical and anatomical implications of specific GABA uptake and subsequent transport of label are discussed and a model of the tectal cortex, based on the three proposed inhibitory systems and their relation to a number of tectal afferent inputs, considered.