Low doses of ethanol and a neuroactive steroid positively interact to modulate rat GABA(A) receptor function

Abstract

Fast inhibitory responses in the central nervous system are mediated by the GABA(A) receptor. The activation and function of the GABA(A) receptor can be modulated by a variety of compounds including benzodiazepines, barbiturates and neuroactive steroids. Modulation of the GABA(A) receptor function by ethanol has been observed in some but not all studies. We have studied the effect of ethanol at concentrations corresponding to light intoxication on the function of the recombinant GABA(A) receptor containing alpha1beta2gamma2 subunits. The experiments were performed both in the absence and presence of low, subthreshold concentrations of a neuroactive steroid. The results demonstrate that, in the presence of the steroid, 0.05 % (9 mM) ethanol potentiates the GABA(A) receptor function by increasing the channel mean open duration. No effect was observed on the channel closed time durations. The data suggest that ethanol influences channel closing with no effect on the affinity of the receptor for GABA or the channel opening rate constant.

Figure 2. The increase in cluster open probability in the presence of ethanol and ACN is caused by an increase in the open time durations

The intracluster mean open duration (A) and open probability (B) of clusters elicited by 50 µm GABA and various concentrations of ethanol (EtOH) in the absence (○) or presence (•) of 10 nm ACN. For comparison, data representing mean open times and open probability at 50 µm GABA (continuous line) are presented in both graphs. The dotted lines show error limits for the 50 µm GABA data. The mean open duration was calculated as a weighted mean from the open time values presented in Table 2. The cluster open probability (P_o) was calculated from the open and closed time durations presented in Table 2 according to: P_o = mean open duration/(mean open duration + mean closed duration). Each point represents the mean ±s.e.m. of data from 3 to 4 patches (*P < 0.05 GABA vs. GABA + ACN + ethanol).

Figure 1. Coapplication of ethanol and ACN increases cluster open probability

Single-channel clusters obtained in the presence of 50 µm GABA and in the absence or presence of 0.1 % ethanol, 10 nm ACN, 0.1 % ethanol + 10 nm ACN or 1 µm ACN. Channel openings are shown downward. The intracluster closed and open time histograms for each set of conditions are given next to the representative cluster. In the presence of GABA and 1 µm ACN, the mean closed durations were 0.12 (67 %), 1.1 (17 %), 21.2 (11 %) and 91 ms (5 %). The mean open durations were 0.42 (26 %), 3.4 (33 %) and 27.8 (42 %). For other sets of data, the parameters of the fit are given in Tables 1 and 2.