GABA- and glycine-activated currents in the rod bipolar cell of the rabbit retina.
1. Voltage- and ligand-gated currents were recorded from solitary rabbit rod bipolar cells using the whole cell patch-clamp technique. The rod bipolar cell forms a single, stereotypical physiological and morphological class of cells that was easily identified from other neurons and support cells after enzymatic and mechanical dissociation from isolated retina. Protein kinase C immunoreactivity confirmed the validity of using a purely morphological identification of this cell type. 2. Voltage steps in 15-mV increments from a holding potential of -45 mV elicited a large outward current activated near -30 mV. These voltage-gated currents were eliminated by using equimolar substitutions of Cs+ and tetraethylammonium+ for K+ in the pipette, indicating that they represent a mixture of K+ currents. 3. The putative inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine activated inward Cl- currents when pressure-applied from pipettes placed near the axon terminals of rod bipolar cells, which were voltage-clamped at -45 mV. With changes in intracellular or extracellular Cl- concentration, the reversal potential of these ligand-gated currents changed as predicted by the Nernst equation for Cl- activity. The dose-response curves for GABA and glycine were sigmoidal with saturating concentrations of 100 and 300 microM, respectively. 4. GABA-activated currents were 1) reversibly reduced by the allosteric inhibitor picrotoxin and the competitive antagonist bicuculline; 2) potentiated by the benzodiazepine diazepam and the barbiturate barbital sodium; and 3) indistinguishable from muscimol-activated currents. There was no response to the GABAB agonist baclofen. Collectively, these data strongly suggest that the GABA-activated currents in rabbit rod bipolar cells are mediated by the GABAA receptor. This is similar to the GABA-activated currents in other mammalian rod bipolar cells. 5. Application of the conformationally restricted GABA analogue cis-4-aminocrotonic acid (CACA) failed to elicit a response, whereas the conformationally extended GABA analogue trans-4-aminocrotonic acid (TACA) elicited a response similar to that of GABA. Although bicuculline appeared to suppress the GABA-activated current slightly more than the TACA-activated current (not significant using Student's t-distribution), GABA- and TACA-activated currents were equally suppressed by picrotoxin and equally enhanced by diazepam and barbital sodium. These data, coupled with the inefficacy of CACA, argue against the existence of a GABAC-type channel in the rod bipolar cell of the rabbit and suggest that GABA and TACA were activating the same GABAA receptor-channel complex.(ABSTRACT TRUNCATED AT 400 WORDS)
D000818:Animals; D002478:Cells, Cultured / Q000187:drug effects; D018118:Chloride Channels / Q000187:drug effects*; D005998:Glycine / Q000494:pharmacology*; D007150:Immunohistochemistry; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D011817:Rabbits; D012160:Retina / Q000187:drug effects / Q000276:immunology*; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000276:immunology*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
D018118:Chloride Channels; D005680:gamma-Aminobutyric Acid; D005998:Glycine