Physiological roles of endocytosis and presynaptic scaffold in vesicle replenishment at fast and slow central synapses

Abstract

After 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 in mouse brainstem and hippocampal slices in physiologically optimized condition. Pharmacological block of endocytosis enhanced synaptic depression at brainstem calyceal fast synapses, whereas it attenuated synaptic facilitation at hippocampal CA1 slow synapses. Block of scaffold protein activity likewise enhanced synaptic depression at calyceal synapses but had no effect at hippocampal synapses. At calyceal synapses, 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 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 and slow synapses, whereas presynaptic scaffold mechanism likely plays a specialized role in vesicle replenishment predominantly at fast synapses.