GABA-B RECEPTORS IN THE VERTEBRATE RETINA

The experiments summarized here indicate that baclofen affects almost all classes of retinal neuron. Photoreceptors, bipolar cells and ganglion cells all possess voltage-dependent calcium channels which are down-regulated when GABAb receptors are activated. This has ramifications for transmitter release, as well as for the movement of other ions through calcium-dependent channels. In both photoreceptor cones and bipolar cells in the amphibian retina, only about 20% of the cells possess putative GABAb receptors. Thus, these receptors modulate very specific visual pathways. In amacrine and ganglion cells, there also appear to be potassium channels activated by GABAb receptors. These receptors are more ubiquitous, almost all third-order cells are affected by GABAb agonists. Under standard experimental conditions, only a small portion of the available GABAb receptor population is synaptically regulated. Whether this represents a potential that can be stimulated under unique circumstances, or whether this reserve simply reflects non-synaptic receptors, is uncertain. GABAb receptors clearly modulate the balance between sustained and transient signals in the retina. This has been observed in the calcium channel kinetics of goldfish ganglion cells, in the light responses of amacrine and ganglion cells and in the feedback pathway to bipolar cells. The functional significance of this transition is uncertain, although it has been incorporated into models of directional selectivity and selective attention. The models are intriguing but as yet unsubstantiated. The GABAb receptor is finding a place within an integrative view of GABA function. There are several GABA receptor classes in the retina. Each cell type discussed in this review has at least two of these receptors. They may work in parallel, for example GABAb receptors may suppress presynaptic transmitter release while GABAa receptors shunt the postsynaptic neurons ability to respond. Or they may work serially, for example low extracellular GABA concentrations may only activate GABAb receptors, thereby reducing sustained, but not transient, light responses. Higher GABA concentrations would activate the GABAa receptor, providing a powerful shunt which would suppress all light responses. The actual significance of GABA receptor diversity will no doubt be much richer than this simple introduction, but this retinal receptor is clearly a work in progress. © 1995.