An ionotropic GABA receptor with novel pharmacology at bullfrog cone photoreceptor terminals

Abstract

Characteristics of ionotropic γ-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 GABAA receptor antagonist bicuculline (BIC), but was suppressed by gabazine, another GABAA antagonist, and imidazole-4-acetic acid (I4AA), a GABAC receptor antagonist. Similarly, currents induced by both GABAA agonist muscimol and GABAC 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 GABAA and GABAC receptors. Copyright © 2006 S. Karger AG.