An Ionotropic GABA Receptor with Novel Pharmacology at Bullfrog Cone Photoreceptor Terminals

Characteristics of ionotropic gamma-aminobutyric acid (GABA) receptors at bullfrog cone terminals were studied by patch clamp techniques in isolated cell and retinal slice preparations. GABA-induced inward currents from isolated cones reversed in polarity at a potential, very close to the chloride equilibrium potential, and they were completely suppressed by picrotoxin. Unexpectedly, the GABA current was dose-dependently potentiated by the well-known GABA(A) receptor antagonist bicuculline (BIC), but was suppressed by gabazine, another GABA(A) antagonist, and imidazole-4-acetic acid (I4AA), a GABA(C) receptor antagonist. Similarly, currents induced by both GABA(A) agonist muscimol and GABA(C) agonist cis-4-aminocrotonic acid (CACA) were also potentiated by BIC. Furthermore, currents induced from cones by GABA and kainate-caused depolarization of horizontal cells in retinal slice preparations were both potentiated by BIC. All these results suggest that the ionotropic GABA receptor at the bullfrog cone terminal exhibits novel pharmacology, distinct from both traditional GABA(A) and GABA(C) receptors.