GABA(C) RECEPTORS IN THE VERTEBRATE RETINA

In the central nervous system (CNS), the inhibitory transmitter GABA i nteracts with three subtypes of GABA receptors, type A, type B, and ty pe C. Historically, GABA receptors have been classified as either the inotropic GABA(A) receptors or the metabotropic GABA(B) receptors. Ove r the past 10 yr, studies have shown that a third class, called the GA BA(C) receptor, also exists. GABA(C) receptors are found primarily in the vertebrate retina and to some extent in other parts of the CNS. Al though GABA(A) and GABA(C) receptors both gate chloride channels, they are pharmacologically, molecularly, and functionally distinct. The rh o subunit of the GABA(C) receptor, which has about 35% amino acid homo logy to GABA(A) receptor subunits, was cloned from the retina and, whe n 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, wh ereas GABA(A) receptors are ubiquitous. This article reviews recent el ectrophysiological and molecular studies that have characterized the u nique properties of GABA(C) receptors and describes the roles that the se receptors may play in visual information processing in the retina.