Pharmacology of GABA receptor CI− channels in rat retinal bipolar cells

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Abstract
Gamma-Aminobutyric acid (GABA), a major inhibitory neurotransmitter in the mammalian nervous system, is known to operate bicuculline-sensitive Cl- channels through GABAA receptors and bicuculline-insensitive cation channels through GABAB receptors. Recent observations indicate that the retina may contain GABA receptors with unusual pharmacological properties. Here we report that GABA gates bicuculline-insensitive Cl- channels in rod bipolar cells of the rat retina, which were not modulated by flunitrazepam, pentobarbital and alphaxalone and were only slightly blocked by picrotoxinin. Moreover, the GABAB receptor agonist baclofen, and the antagonist 2-hydroxysaclofen had no effect. The underlying single-channel conductance was 7 pS and the open time 150 ms. These values are clearly different from those obtained for GABAA receptor channels recorded in other neurons of the same preparation, and in other parts of the brain. The bicuculline- and baclofen-insensitive GABA receptors were activated selectively by the GABA analogue cis-4-aminocrotonic acid (CACA). Hence they may be similar to those receptors termed GABAC receptors.