GABA Receptor Binding

GABA (γ-aminobutyric acid) receptors are among the most ubiquitous in the brain, having been identified (neurophysiologitally) in all regions and at all levels (e.g., Iversen, 1978 ). All neuronal cells respond to the iontophoretic application of GABA, usually with a decrease in firing rate associated with hyperpolarization (Curtis, 1979 ). However, well-known instances of GABA-induced membrane depolarization occur, and the same cells may respond with a local depolarization or hyperpolarization, depending on where the GABA is applied (dendrite vs axon hillock vs cell body, and so on) (Andersen et al., 1980 ). This electrophysiological evidence indicates that all cells possess GABA receptors (most likely, GABA_A receptors, see below), but it does not at all provide evidence for the existence of GABA synapses at these same cells. Indeed, the dorsal root ganglion (rat or cat) possesses GABA_A receptors linked to chloride ion channels, activation of which results in membrane depolarization; there is, however, no GABA-ergic input to the dorsal root ganglion. This is therefore an example of an extrasynaptic GABA receptor (Desarmenien et al., 1980 ; Gallagher et al., 1983 ).