GABA(A) AND GABA(C) RECEPTORS ON HYBRID BASS RETINAL BIPOLAR CELLS

1. gamma-Aminobutyric acid (GABA) responses from solitory hy brid bass retinal bipolar cells were studied with the use of conventional and perforated whole cell patch-clamp recording. 2. GABA elicited a chloride current in bipolar cells that had both transient and sustained components. The transient component was sensitive to bicuculline and resembled GABA(A)-mediated currents, whereas the more sustained component was resistant to bicuculline and resembled the responses mediated by GABA(C) receptors. 3. The bicuculline-resistant GABA responses recorded from the bipolar cells could not be modulated by either diazepam or pentobarbital sodium, and they were unaffected by phaclofen and 2-hydroxysaclofen, GABA(B) receptor antagonists. On the other hand, the bicuculline-resistant GABA responses could be blocked substantially by imidazole-4-acetic acid (I4AA), a competitive antagonist of GABA(C) receptors. 4. Noise analysis of the GABA-elicited currents suggested a different single channel conductance for GABA(A) (10.1 pS) and GABA(C) receptors (3.6 pS). 5. Zinc, a putative modulator of synaptic transmission, strongly inhibited the GABA(C) responses on bipolar cells, whereas the GABA(A) responses were not significantly affected by zinc. 6. The proportion of the GABA(C) to GABA(A) responses varied widely between bipolar cells. Local application of GABA onto dendrites or axon terminals showed that both types of GABA receptors are present on both regions of the cell. 7. The distinct properties of these two GABA receptor types suggest that they play different roles in retinal function.