Synaptic Organization of GABAergic Neurons and GABA A Receptors in the Lateral Geniculate Nucleus and Visual Cortex

Gamma-aminobutyric acid (GAllA), acting on type-A and type-B receptors, is the main inhibitory transmitter in the mammalian visual system. The organization of circuits, the localization of GABAergic synaptic terminals, and the expression of GABA A receptors show selective distribution according to neuronal classes and visual channels. For example, it has been shown that in the dorsal lateral geniculate nucleus (dLGN) of the cat, a class of small relay cells, tentatively correlated with the "lagged" X cells, particularly strongly expresses the receptor complex. In the dLGN and in the cortex, the GABA A receptor complex is present both at synaptic and at nonsynaptic sites in the plasma mem brane of neurons. It is suggested that this broad distribution enables the GABAergic system to keep the dynamic range of the postsynaptic cell optimal at widely varying intensities of excitatory drive. In the visual cortex GABAergic neurons show great diversity in their input and output selectivity and neurochemical characteristics, reflecting the different roles of subsets of cells using the same transmitter. The possible neuronal basis of two receptive-field properties, directional selectivity and end inhibition, is examined in light of new data on connectivity. Data are presented. for the hypothesis that the action of different cortical afferents involve subsets of GABAergic neurons selectively regulating the gain of particular inputs on common target cells. The requirements of the particular pathways and the separation of physiologically different streams in the same pathway explain the diversity of GABAergic neuronal circuits.