Effect of zolpidem on miniature IPSCs and occupancy of postsynaptic GABA(A) receptors in central synapses

GABA(A)-mediated miniature IPSCs (mIPSCs) were recorded from layer V pyrami dal neurons of the visual cortex using whole-cell patch-clamp recording in rat brain slices. At room temperature, the benzodiazepine site agonist zolp idem enhanced both the amplitude (to 138 +/- 26% of control value at 10 mu M) and the duration (163 +/- 14%) of mIPSCs. The enhancement of mIPSC ampli tude was not caused by an increase of the single-channel conductance of the postsynaptic receptors, as determined by peak-scaled non-stationary fluctu ation analysis of mIPSCs. The effect of zolpidem on fast, synaptic-like (1 msec duration) applications of GABA to outside-out patches was also investi gated. The EC50 for fast GABA applications was 310 mu M. In patches, zolpid em enhanced the amplitude of currents elicited by subsaturating GABA applic ations (100-300 mu M) but not by saturating applications (10 mM). The incre ase of mIPSC amplitude by zolpidem provides evidence that the GABA(A) recep tors are not saturated during miniature synaptic transmission in the record ed cells. By comparing the facilitation induced by 1 mu M zolpidem on outsi de-out patches and mIPSCs, we estimated the concentration of GABA seen by t he postsynaptic GABA(A) receptors to be similar to 300 mu M after single ve sicle release. We have estimated a similar degree of receptor occupancy at room and physiological temperature. However, at 35 degrees C, zolpidem did not enhance the amplitude of mIPSCs or of subsaturating GABA applications o n patches, implying that, in these neurons, zolpidem cannot be used to prob e the degree of receptor occupancy at physiological temperature.