GABAC Receptors in the Vertebrate Retina

In the central nervous system (CNS), the inhibitory transmitter GABA interacts with three subtypes of GABA receptors, type A, type B, and type C. Historically, GABA receptors have been classified as either the inotropic GABAA receptors or the metabotropic GABAB receptors. Over the past 10 yr, studies have shown that a third class, called the GABAC receptor, also exists. GABAC receptors are found primarily in the vertebrate retina and to some extent in other parts of the CNS. Although GABAA and GABAC receptors both gate chloride channels, they are pharmacologically, molecularly, and functionally distinct. The rho subunit of the GABAC receptor, which has about 35% amino acid homology to GABAA receptor subunits, was cloned from the retina and, when expressed in Xenopus oocytes, has properties similar to retinal GABAC receptors. There are probably distinct roles for GABAC receptors in the retina, because they are found on only a subset of neurons, whereas GABAA receptors are ubiquitous. This article reviews recent electrophysiological and molecular studies that have characterized the unique properties of GABAC receptors and describes the roles that these receptors may play in visual information processing in the retina.