PHARMACOLOGY OF GABA RECEPTOR CL- CHANNELS IN RAT RETINAL BIPOLAR CELLS

Gamma-AMINOBUTYRIC acid (GABA), a major inhibitory neurotransmitter in the mammalian nervous system, is known to operate bicuculline-sensitive Cl- channels through GABA(A) receptors and bicuculline-insensitive cation channels through GABA(B) receptors1,2. Recent observations indicate that the retina may contain GABA receptors with unusual pharmacological properties3-5. 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 GABA(B) 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 GABA(A) receptor channels recorded in other neurons of the same preparation, and in other parts of the brain1,6-8. The bicuculline- and baclofen-insensitive GABA receptors were activated selectively by the GABA analogue cis-4-aminocrotonic acid (CACA)9. Hence they may be similar to those receptors termed GABA(C) receptors10.