Few cell lines with GABAA mRNAs have functional receptors

In the preceding companion article (Tyndale et al., 1994) we used the PCR to investigate the occurrence of 13 GABAA receptor subunit mRNAs in several cell lines, including those derived from brain (B65, B104, and NB41A3), cerebellum (C17), glia (C6), pituitary (AtT-20), adrenal medulla (PC12), and the endocrine pancreas (RINm5F and beta TC3). In the present study we used the whole-cell configuration of the patch-clamp technique to determine which of these cell lines express functional GABAA receptors. All of the cell lines contain detectable levels of at least one GABAA receptor subunit mRNA (Tyndale et al., 1994); however, only RINm5F and beta TC3 cells exhibited GABA-evoked currents. GABA activated currents in all RINm5F cells, but currents were only barely detectable in 50% of beta TC3 cells tested. Many of the cell lines that failed to respond to GABA were derived from cell types with functional GABAA receptors. For example, the failure of PC12 cells to respond to GABA contrasts with the observation of GABA responses recorded from all primary cultured adrenomedullary chromaffin cells tested. GABA-evoked currents recorded from beta TC3 cells were too small (< 10 pA) to characterize pharmacologically. However, GABA activated robust currents recorded from RINm5F cells. These currents reversed at a holding potential similar to the equilibrium potential for Cl-, were blocked by the antagonist bicuculline methiodide (10 microM), and were potentiated by pentobarbital (100 microM). RINm5F cell GABAA receptors were insensitive to diazepam (10 microM) and were inhibited by Zn2+ (10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)