GABA TRANSPORTERS AND GABA(C)-LIKE RECEPTORS ON CATFISH CONE-DRIVEN BUT NOT ROD-DRIVEN HORIZONTAL CELLS

The effects of GABA and related agents were studied in solitary rod- a nd cone-driven horizontal cells, acutely isolated from the catfish ret ina 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 m u M) with an inward current of 300-800 pA when voltage-clamped at -70 mV using the whole-cell patch-clamp technique. But pressure ejection o f GABA (500 mu M) elicited an inward current only in cone-driven horiz ontal cells. This current, ranging between 100 and 400 pA, consisted o f two components: (1) a GABA receptor-gated chloride current that reve rsed near the chloride equilibrium potential and was blocked by bath a pplication of picrotoxin (100-500 mu M), and (2) a GABA transporter-me diated current that was picrotoxin resistant but was blocked by NO-711 (1 mu M) and cis-4-hydroxynipecotic acid (250 mu M), two potent GABA transporter blockers. The GABA transporter current could also be elimi nated when sodium was replaced by either choline or lithium in the bat hing medium. The picrotoxin-sensitive receptor-gated current could not be elicited by the GABA(B) receptor agonist baclofen, nor could it be blocked by the potent GABA(B) receptor antagonist 2-hydroxysaclofen. The picrotoxin-sensitive current could be divided into two components based on their sensitivity to the specific GABA(A) receptor antagonist bicuculline methiodide. The bicuculline-sensitive component was found only in some cells, whereas the bicuculline-resistant, picrotoxin-sen sitive component was found in all cells tested. The bicuculline-resist ant current was insensitive to pentobarbital, an allosteric modulator of GABA(A) receptor. To confirm the effectiveness of the specific batc h 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 GABA(A) recep tors. Bicuculline methiodide almost completely blocked, while pentobar bital significantly enhanced, the GABA current recorded in ganglion ce lls. Thus, in catfish cone horizontal cells the bicuculline-resistant GABA receptor current is most likely mediated by the GABA, receptor ba sed on the above pharmacological profile. The relative effectiveness o f GABA, muscimol, trans- and cis-4-aminocrotonic acid (TACA and CACA) was determined at this GABA(C) receptor site after cells were bathed i n choline Ringer to eliminate the transporter current and in the prese nce of 100 mu M bicuculline methiodide to block GABA(A) receptor curre nt. The order of effectiveness was GABA > TACA > muscimol >> CACA. Our results demonstrate the presence of GABA transporters as well as GABA (A) and GABA(C) receptors in catfish cone- but not rod-driven horizont al cells. This suggests that the GABA-mediated autofeedback and chemic al coupling described recently in an amphibian retina may also exist i n the catfish cone horizontal cell network.