α5GABAA receptors regulate the intrinsic excitability of mouse hippocampal pyramidal neurons

GABAA receptors generate both phasic and tonic forms of inhibition. In hippocampal pyramidal neurons, GABAA receptors that contain the α5 subunit generate a tonic inhibitory conductance. The physiological role of this tonic inhibition is uncertain, although α5GABAA receptors are known to influence hippocampal-dependent learning and memory processes. Here, we provide evidence that α5GABAA receptors regulate the strength of the depolarizing stimulus that is required to generate an action potential in pyramidal neurons. Neurons from α5 knock-out (α5−/−) and wildtype (WT) mice were studied in brain slices and cell cultures using whole-cell and perforated-patch clamp techniques. Membrane resistance was 1.6-fold greater in α5−/− than in WT neurons, but the resting membrane potential and chloride equilibrium potential were similar. Membrane hyperpolarization evoked by an application of exogenous GABA was greater in WT neurons. Inhibiting the function of α5GABAA receptor with non-selective (picrotoxin) or α5 subunit-selective (L-655,708) compounds depolarized WT neurons by approximately 3 mV, whereas no change was detected in α5−/− neurons. The depolarizing current required to generate an action potential was 2-fold greater in WT than in α5−/− neurons whereas the slope of the input-output relationship for action potential firing was similar. We conclude that shunting inhibition mediated by α5GABAA receptors regulates the firing of action potentials and may synchronize network activity that underlies hippocampal-dependent behavior. Page 2 of 49