The influence of the γ2L subunit on the modulation of responses to GABAA receptor activation

Abstract

Whole‐cell patch clamp recordings were made from L‐cells transfected with 2 combinations of subunits of the GABAA receptor. Log concentration‐response curves were constructed to γ‐aminobutyric acid (GABA) on α1,β1,γ2L containing cells and compared to those from α1,β1 containing cells. The effects of flunitrazepam, pentobarbitone and alphaxalone on the concentration‐response relationships were also examined. From the log concentration‐response curves, GABA had a mean (± s.e.mean) pEC50 = 5.2 ± 0.09 and slope factor = 1.7 ± 0.02 on α1β1,γ2L cells which were significantly different from the values obtained from α1,β1 cells where the pEC50 = 5.6 ± 0.02 and the slope = 1.5 ±0.02. Flunitrazepam produced a parallel leftward shift of GABA concentration‐response curves on α1,β1,γ2L cells. The EC50 for flunitrazepam = 6.3 ± 2.7 nm. No increase in the maxima of the GABA concentration‐response curves was found in the presence of flunitrazepam. Flunitrazepam did not potentiate responses from α1,β1 cells. The log concentration‐response curves from both populations of cells were shifted to the left by equal amounts by pentobarbitone. A significant increase in the maximal response to GABA was also produced by pentobarbitone. This occurred at lower concentrations of pentobarbitone on α1,β1 cells. Alphaxalone produced leftward shifts of GABA log concentration‐response curves of similar magnitudes in both populations of cells. Significant increases in the maxima were found at 100 nm in α1,β1 cells but not up to 1 μm in α1,β1,γ2L cells. These results provide further evidence of the modulatory role of the γ2L subunit of the GABAA receptor containing α1 and β1 subunits. As well as influencing the apparent affinity of GABA and conferring benzodiazepine modulation, it also appeared to regulate the increase in maximal response produced in the presence of barbiturates and steroids. This latter effect may imply that barbiturates and steroids increase the channel open‐state probability in the presence of GABA and that this effect is diminished by the presence of the γ2L subunit. 1993 British Pharmacological Society