GABA_c 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 GABA_A receptors or the metabotropic GABA_B receptors. Over the past 10 yr, studies have shown that a third class, called the GABA_C receptor, also exists. GABA_C receptors are found primarily in the vertebrate retina and to some extent in other parts of the CNS. Although GABA_A and GABA_C receptors both gate chloride channels, they are pharmacologically, molecularly, and functionally distinct. The ρ subunit of the GABA_C receptor, which has about 35% amino acid homology to GABA_A receptor subunits, was cloned from the retina and, when expressed in Xenopus oocytes, has properties similar to retinal GABA_C receptors. There are probably distinct roles for GABA_C receptors in the retina, because they are found on only a subset of neurons, whereas GABA_A receptors are ubiquitous. This article reviews recent electrophysiological and molecular studies that have characterized the unique properties of GABA_C receptors and describes the roles that these receptors may play in visual information processing in the retina.