Correlation between a bicuculline-resistant response to GABA and GABA(A) receptor rho 1 subunit expression in single rat retinal bipolar cells

Using patch-clamp recording in combination with reverse transcriptase-polymerase chain reaction (RT-PCR), we show in individual bipolar cells acutely dissociated from the adult rat retina a correlation between the expression of the GABA(A) receptor rho 1 subunit mRNA and a bicuculline-resistant, diazepam-insensitive component of the GABA-activated whole-cell current response. This ''GABA(C)-like'' response, contributing to approximately 42% of the GABA-activated whole-cell current and displaying variable sensitivity to picrotoxin, was found in bipolar cells but not in any of the ganglion cells examined. Expression profiling of GABA(A) receptor subunit mRNAs in individual electrophysiologically tested retinal neurons revealed that, while both bipolar cells and ganglion cells may express numerous GABA(A) receptor subunit isoforms, including that of rho 2, the expression of the rho 1 subunit was strictly limited to bipolar cells. We propose a possible link between the presence of a receptor with GABA(C)-like pharmacological profile and the expression of the retina-specific rho 1 subunit isoform. The results presented in this study constitute the first direct demonstration of such a correlation at the single-cell level.