DISTINCT POOLS OF SYNAPTIC VESICLES IN NEUROTRANSMITTER RELEASE

NERVE terminals are unique among cellular secretory systems in that th ey can sustain vesicular release at a high rate. Although little is kn own about the mechanisms that account for the distinctive features of neurotransmitter release, it can be assumed that neuron-specific prote ins are involved. One such protein family, the synapsins, are believed to regulate neurotransmitter release through phosphorylation-dependen t interactions with synaptic vesicles and cytoskeletal elements(1). He re we show that clusters of vesicles at synaptic release sites are com posed of two pools, a distal pool containing synapsin and a proximal p ool devoid of synapsin and located adjacent to the presynaptic membran e. Presynaptic injection of synapsin antibodies resulted in the loss o f the distal pool, without any apparent effect on the proximal pool. D epletion of this distal pool,vas associated with a marked depression o f neurotransmitter release evoked by high-frequency (18-20 Hz) but not by low-frequency (0.2 Hz) stimulation. Thus the availability of the s ynapsin-associated pool of vesicles seems to be required to sustain re lease of neurotransmitter in response to high-frequency bursts of impu lses.