Synaptic vesicle recycling in synaptosomes in vitro

Abstract

PINCHED-OFF presynaptic nerve terminals (synaptosomes) prepared from rat brain homogenates^1,2 retain many of the functional properties of intact neurones^3,4. They can accumulate K⁺ and extrude Na⁺ against concentration gradients^5,6, and their membranes seem to develop K⁺ diffusion potentials which behave like the membrane potentials of most neurones⁷. Furthermore, the isolated terminals exhibit an increased rate of Ca²⁺ accumulation⁸ and Ca²⁺-dependent transmitter release^8–10 when exposed to depolarising agents such as veratridine or an increased external K⁺ concentration ([K]₀). The latter findings are consistent with the ‘calcium hypothesis’ of depolarisation–secretion coupling¹¹. The calcium which enters the terminals during depolarisation seems to trigger the release of transmitter from intraterminal vesicles by an exocytotic process involving transient fusion of the vesicle membrane with the surface membrane^12–14. This hypothesis is supported by experiments with electron-opaque extracellular markers; the appearance of the markers within intraterminal vesicles, after stimulation, has been taken as evidence for vesicle recycling^15–17. Here we report similar observations on synaptosomes incubated in vitro, showing that synaptosomes function like intact terminals in this respect as well.