Physiological roles of endocytosis and presynaptic scaffold in vesicle replenishment at fast-signaling and slow-plastic synapses

Abstract

AbstractAfter exocytosis, release sites are cleared of vesicular residues to be replenished with transmitter-filled vesicles. Endocytic and scaffold proteins are thought to underlie this mechanism. However, physiological significance of the site-clearance mechanism among diverse central synapses remains unknown. Here, we tested this using action-potential evoked EPSCs at fast-signaling calyx and slow-plastic hippocampal CA1 synapses in post-hearing mice brain slices at physiological temperature and Ca2+. Pharmacological block of endocytosis enhanced synaptic depression at the calyx synapse, whereas it attenuated synaptic facilitation at the hippocampal synapse. Block of scaffold protein activity likewise enhanced synaptic depression at the calyx but had no effect at the hippocampal synapse. At the calyx, enhancement of synaptic depression by blocking endocytosis or scaffold activity occurred at nearly identical time courses with a time constant of several milliseconds starting immediately after the stimulation onset. Neither endocytic nor scaffold protein inhibitors prolonged the recovery from short-term depression. We conclude that endocytic release-site clearance can be a universal phenomenon supporting vesicle replenishment across fast-signaling and slow-plastic synapses, whereas presynaptic scaffold mechanism likely plays a specialized role in vesicle replenishment predominantly at fast synapses.