PROLONGED RESPONSES IN RAT CEREBELLAR PURKINJE-CELLS FOLLOWING ACTIVATION OF THE GRANULE CELL LAYER - AN INTRACELLULAR IN-VITRO AND IN-VIVOINVESTIGATION

We obtained intracellular recordings of 84 Purkinje cells in vitro fro m guinea pig slices and of 35 cells in vivo from ketamine-anesthetized rats in order to assess detailed properties of synaptic responses in Purkinje cells following granule cell activation. In vitro, electrical stimulation of the granule cell layer underlying recorded Purkinje ce lls was used in sagittal slices to predominantly activate synapses on ascending granule cell axons. In vivo, stimulation of the upper lip wa s used to activate Purkinje cells overlying the upper lip patch in the granule cell layer of crus IIa. In the presence of a GABA(A) antagoni st, Purkinje cells at resting membrane potential responded to both ele ctrical stimulation in vitro and peripheral stimulation in vivo, with a depolarization of 1-10 mV amplitude that lasted for 100-300 ms in th e absence of climbing fiber input. Similar prolonged depolarizations c ould also be induced by brief depolarizing current pulses delivered th rough the recording electrode, demonstrating that either synaptic or d irect depolarization may activate inward currents leading to a sustain ed response. In support of this hypothesis we found that prolonged dep olarizations were shortened significantly when stimulation in the gran ule cell layer or intracellular current pulses were delivered during h yperpolarizing current steps. Stimulation in the granule cell layer or intracellular current pulses delivered during periods of spontaneous somatic spiking resulted in prolonged depolarizations in dendritic rec ordings, which were accompanied by an increase in somatic spiking freq uency. Following upper lip stimulation in vivo, this increase in somat ic spiking was interrupted by an inhibition of 10-50 ms duration. In a majority of recordings, this inhibition did not completely abolish pr olonged depolarizations, however, and a delayed increase in somatic sp ike frequency was still observed. These results suggest that prolonged increases in Purkinje cell spike frequency following peripheral stimu lation are due to an underlying prolonged dendritic depolarization ind uced by granule cell input. Further, a single, short burst of input vi a ascending granule cell axons appears to be sufficient to induce thes e responses.