Repetitive firing of rat cerebellar parallel fibres after a single stimulation.

The excitatory postsynaptic currents (EPSCs) evoked in Purkinje cells (PCs) by stimulating parallel fibres (PFs) usually show a single peak, but EPSCs with multiple peaks (polyphasic EPSCs) can be observed in slices from animals older than 15 days. The EPSCs remain polyphasic when the postsynaptic current is reduced (either by reducing the intensity of the PF stimulation or by adding AMPA receptor antagonists) and when the PC membrane potential is made positive. Thus the late peaks are not due to postsynaptic active currents generated in the imperfectly clamped PC, and must arise from repetitive action potentials in the PF. Extracellular recordings from granule cell (GC) somata showed that a single PF stimulation can elicit a doublet or a train of action potentials. Both the late action potentials recorded in the GCs and the late peaks of the polyphasic EPSCs recorded in the PCs were reduced or abolished by paired-pulse stimulation of the PF or by bath application of the GABA(A) agonist muscimol. The late action potentials in the GCs were also suppressed by local application of muscimol around the cell body. We propose that after a single stimulation of a PF, the antidromic invasion of the ascending axon and the granule cell can trigger a doublet or a burst of action potentials which back-propagate into the PF (except for the first, which finds the PF still in its refractory period). The repetitive activation of the PF by a single stimulation could play a role in the induction of long-term depression.