Maintenance of high-frequency transmission at purkinje to cerebellar nuclear synapses by spillover from boutons with multiple release sites.

Cerebellar Purkinje neurons maintain high firing rates but their synaptic terminals depress only moderately, raising the question of how vesicle depletion is minimized. To identify mechanisms that limit synaptic depression, we evoked 100 Hz trains of GABAergic inhibitory postsynaptic currents (IPSCs) in cerebellar nuclear neurons by stimulating Purkinje axons in mouse brain slices. The paired-pulse ratio (IPSC(2)/IPSC(1)) of the total IPSC was approximately 1 and the steady-state ratio (IPSC(20)/IPSC(1)) was approximately 0.5, suggesting a high response probability of postsynaptic receptors, without an unusually high release probability. Three-dimensional electron microscopic reconstructions of Purkinje boutons revealed multiple active zones without intervening transporters, suggestive of "spillover"-mediated transmission. Simulations of boutons with 10-16 release sites, in which transmitter from any site can reach all receptors opposite the bouton, replicated multiple-pulse depression during normal, high, and low presynaptic Ca influx. These results suggest that release from multiple-site boutons limits depletion-based depression, permitting prolonged, high-frequency inhibition at corticonuclear synapses.