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 p erforated whole cell patch-clamp recording. 2. GABA elicited a chlorid e current in bipolar cells that had both transient and sustained compo nents. The transient component was sensitive to bicuculline and resemb led GABA(A)-mediated currents, whereas the more sustained component wa s resistant to bicuculline and resembled the responses mediated by GAB A(C) receptors. 3. The bicuculline-resistant GABA responses recorded f rom the bipolar cells could not be modulated by either diazepam or pen tobarbital sodium, and they were unaffected by phaclofen and 2-hydroxy saclofen, GABA(B) receptor antagonists. On the other hand, the bicucul line-resistant GABA responses could be blocked substantially by imidaz ole-4-acetic acid (I4AA), a competitive antagonist of GABA(C) receptor s. 4. Noise analysis of the GABA-elicited currents suggested a differe nt single channel conductance for GABA(A) (10.1 pS) and GABA(C) recept ors (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 pro portion of the GABA(C) to GABA(A) responses varied widely between bipo lar cells. Local application of GABA onto dendrites or axon terminals showed that both types of GABA receptors are present on both regions o f the cell. 7. The distinct properties of these two GABA receptor type s suggest that they play different roles in retinal function.