Cerebellar basket cell filtering of Purkinje cell responses elicited by low frequency parallel fibre transmission.

Basket cells (BC) are inhibitory interneurons of the cerebellar molecular layer (ML) forming peri-somatic synapses on Purkinje cells (PC). BC physiological and computational properties remained poorly understood and not clearly differentiated from those of stellate cells (SC). We identified BCs in acute mouse cerebellar slices and measured their intrinsic excitability and synaptic responsiveness. BCs and SCs were similar in some respects, although BCs showed stronger and faster synaptic excitation in response to parallel fibre (pf) bursts. The analysis of BC inhibition of PCs was extended over a broad parameter space using accurate multi-compartmental computational models. During pf bursts, the BC reduced the PC response at low-frequency, while SCs did it at high-frequency. BC filtering was explained by the engagement of HCN1 channels, which activated slowly during low-frequency BC-PC GABAergic transmission. The increase of input conductance caused by HCN1 channels in the PC soma, by shunting excitatory currents elicited by pfs and travelling toward the axon initial segment (AIS), reduced the PC output frequency. These simulations predict that BC and SC operate in tandem, setting the frequency band of PC transmission through the regulation of PC frequency/response curves.