Ethanol enhances both action potential-dependent and action potential-independent GABAergic transmission onto cerebellar Purkinje cells

Ethanol (EtOH) modulates synaptic efficacy in various brain areas, including the cerebellum, which plays a role in motor coordination. Previous studies have shown that EtOH enhances tonic inhibition of cerebellar granule cells, which is one of the possible reasons for the alcohol-induced motor impairment. However, the effects of EtOH on molecular layer interneurons (MLIs) in the mouse cerebellum have remained unknown. Here we found that MLIs were depolarized by EtOH through enhancement of hyperpolarization-activated cationic currents (Ih). Under physiological conditions, a low EtOH concentration (3–50 mM) caused a small increase in the firing rate of MLIs, whereas, in the presence of blockers for ionotropic glutamate and GABA receptors, EtOH (≥10 mM) robustly enhanced MLI firing, suggesting that synaptic inputs, which seem to serve as the phasic inhibition, could suppress the EtOH-mediated excitation of MLIs and Purkinje cells (PCs). Even in the absence of synaptic blockers, a high EtOH concentration (100 mM) markedly increased the firing rate of MLIs to enhance GABAergic transmission. Furthermore, 100 mM EtOH-facilitated miniature IPSCs via a mechanism that depended on intracellular cyclic AMP, voltage-dependent Ca2+ channels, and intracellular Ca2+ stores, but was independent of Ih or PKA. The two distinct effects of a high EtOH concentration (≥100 mM), however, failed to attenuate the EtOH-induced strong depolarization of MLIs. These results suggest that acute exposure to a low EtOH concentration (≤50 mM) enhanced GABAergic synaptic transmission, which suppressed the EtOH-evoked excitation of MLIs and PCs, thereby maintaining precise synaptic integration of PCs.