Paired-pulse facilitation of multivesicular release and intersynaptic spillover of glutamate at rat cerebellar granule cell-interneurone synapses

A simple form of presynaptic plasticity, paired-pulse facilitation (PPF), has been explained as a transient increase in the probability of vesicular release. Using the whole-cell patch-clamp technique to record synaptic activity in rat cerebellar slices, we found different forms of presynaptically originated short-term plasticity during glutamatergic excitatory neurotransmission from granule cells (GCs) to molecular-layer interneurones (INs). Paired-pulse activation of GC axons at short intervals (30-100 ms) elicited not only a facilitation in the peak amplitude (PPF_amp), but also a prolongation in the decay-time constant (PPP_decay) of the EPSCs recorded from INs. The results of pharmacological tests and kinetics analyses suggest that the mechanisms underlying the respective types of short-term plasticity were different. PPF_amp was elicited by a transient increase in the number of released vesicles. On the other hand, PPP_decay was caused not only by delayed release as has been reported but also by extrasynaptic spillover of the GC transmitter and the subsequent intersynaptic pooling. Both PPF_amp and PPP_decay closely rely on repetitive-activation-induced multivesicular release. Using a dynamic clamp technique, we further examined the physiological significance of different presynaptic plasticity, and found that PPF_amp and PPP_decay can differentially encode and process neuronal information by influencing the total synaptic charge transferred to postsynaptic INs to reflect activation frequency of the presynaptic GCs.