The properties of GABA_C receptors on ferret retinal bipolar cells

Purpose. The pharmacological properties of bipolar cell GABA_C receptors are different in rats and in cold-blooded vertebrates. Picrotoxin blocks GABA_C receptor-mediated responses in the latter, but not in the former. GABA_C receptors were characterized in ferret bipolar cells to determine which subtype predominated. Methods. Whole-cell, voltage-clamp recordings were made from bipolar cells in the retinal slice preparation. Currents were evoked in bipolar cells by puffing GABA focally in the IPL. GABA receptor antagonists were applied by locally perfusing the slice. Results. GABA currents recorded from P24, P28 and adult bipolar cells reversed polarity near ECl. The selective GABA_A receptor antagonist bicuculline (100 μM) reduced the amplitude of the bipolar cell GABA currents by 25-80% of control values, indicating that both type A and C receptors mediated the responses. The fraction of bicuculline-sensitive current depended on the strength of the GABA stimulus: currents evoked by short-duration puffs were less sensitive to bicuculline, indicating that type C receptors mediated proportionately more of these responses. In contrast, ganglion and amacrine cell GABA currents were always completely blocked by bicuculline, indicating that GABA_A receptors predominated on these cells. Picrotoxin (100μM), which was relatively ineffective at rat GABA_C receptors, completely blocked GABA currents in bipolar cells. Conclusions. GABA_C receptors on ferret bipolar cells are more similar to those in lower vertebrate retinas than to those in rat retina. GABA currents evoked by short-duration puffs were mediated predominantly by type C receptors while GABA currents evoked by long-duration puffs were mediated by both type A and C receptors.