GABA transporters and GABAc-like receptors on catfish cone- but not rod-driven horizontal cells

Abstract

The effects of GABA and related agents were studied in solitary rod- and cone-driven horizontal cells, acutely isolated from the catfish retina using enzymatic and mechanical treatment. Both types of horizontal cells, which normally receive glutamatergic input from photoreceptors, responded to pressure ejection of the glutamate analog kainate (50 μM) with an inward current of 300-800 pA when voltage-clamped at -70 mV using the whole-cell patch-clamp technique. But pressure ejection of GABA (500 μM) elicited an inward current only in cone-driven horizontal cells. This current, ranging between 100 and 400 pA, consisted of two components: (1) a GABA receptor-gated chloride current that reversed near the chloride equilibrium potential and was blocked by bath application of picrotoxin (100-500 μM), and (2) a GABA transporter-mediated current that was picrotoxin resistant but was blocked by NO-711 (1 μM) and cis-4-hydroxynipecotic acid (250 μM), two potent GABA transporter blockers. The GABA transporter current could also be eliminated when sodium was replaced by either choline or lithium in the bathing medium. The picrotoxin-sensitive receptor-gated current could not be elicited by the GABAB receptor agonist baclofen, nor could it be blocked by the potent GABAB receptor antagonist 2-hydroxysaclofen. The picrotoxin-sensitive current could be divided into two components based on their sensitivity to the specific GABAA receptor antagonist bicuculline methiodide. The bicuculline-sensitive component was found only in some cells, whereas the bicuculline-resistant, picrotoxin-sensitive component was found in all cells tested. The bicuculline-resistant current was insensitive to pentobarbital, an allosteric modulator of GABAA receptor. To confirm the effectiveness of the specific batch of bicuculline methiodide and pentobarbital, we tested both drugs in ganglion cells in the salamander retinal slice preparation, where the GABA-elicited current is almost exclusively mediated by GABAA receptors. Bicuculline methiodide almost completely blocked, while pentobarbital significantly enhanced, the GABA current recorded in ganglion cells. Thus, in catfish cone horizontal cells the bicuculline-resistant GABA receptor current is most likely mediated by the GABAC receptor based on the above pharmacological profile. The relative effectiveness of GABA, muscimol, trans- and cis-4-aminocrotonic acid (TACA and CACA) was determined at this GABAC receptor site after cells were bathed in choline Ringer to eliminate the transporter current and in the presence of 100 μM bicuculline methiodide to block GABAA receptor current. The order of effectiveness was GABA > TACA > muscimol ≫ CACA. Our results demonstrate the presence of GABA transporters as well as GABAA and GABAC receptors in catfish cone- but not rod-driven horizontal cells. This suggests that the GABA-mediated autofeedback and chemical coupling described recently in an amphibian retina may also exist in the catfish cone horizontal cell network.