Distribution of GABA(c)-like responses among acutely dissociated rat retinal neurons

GABAergic responses of acutely dissociated rat retinal neurons, including b oth bipolar cells (BCs) and other, morphologically round cells (RCs), were assayed with the fluorescent (FL), voltage-sensitive probe oxonol DiBaC(4)( 5). Using intensified video microscopy and simultaneous recording, GABA res ponses were identified in one-third of cells in a typical microscope field; of these 85% hyperpolarized (0.05-0.3 log unit FL decreases) while the rem ainder depolarized (0.05-0.2 log unit FL increases). GABA-sensitive cells w ere also TACA-sensitive (trans-4-Aminocrotonic acid), and these ligands app eared interchangeable in ability to evoke responses. In RCs, an asymmetric co-responsive pattern was observed between GABA- and muscimol-evoked events . Muscimol-sensitive RCs responded well to GABA, but not all GABA-sensitive RCs responded to muscimol. In GABA-sensitive BCs, muscimol responses were typically weak or absent. Few BCs or RCs responded to CACA (cis-4-Aminocrot onic acid). Bicuculline-resistant GABA responses occurred in similar to 80% of GABA-responsive RCs and BCs. Both bicuculline-sensitive (GABA(A)-like) and bicuculline-insensitive (GABA(C)-like) responses were resistant to picr otoxin. Although a small minority of GABA-sensitive cells hyperpolarized in response to R(+)baclofen, bicuculline-insensitive responses were not antag onized by 2-hydroxysaclofen, and were abolished in low [Cl-](o). Results su ggested (1) that bicuculline-insensitive, Cl--dependent, GABA(C)-like respo nses were broadly distributed and predominant among dissociated rat retinal neurons; (2) that muscimol was a particularly weak agonist for rat retinal BCs; and (3) that oxonol was a sensitive probe for retinal GABA responses.