Vesicular hypothesis
Vesicles are site of neurotransmitter storage and release into synaptic cleft
Quantal release
Vesicular hypothesis requires that neurotransmitter be released in discrete packets corresponding to contents of one vesicle, synaptic vesicles of a particular type in neuron are about same size
Miniature end plate potentials (mepps)
Evidence for quantal release from frog neuromuscular junction
· Highly amplified intracellular recordings from muscles at neuromuscular junction
· Minute, spontaneous postsynaptic potentials, amplitude varies in 0.5 mV steps, suggesting release of discrete numbers of quanta
Mepps do not prove that full sized EPPs are quantal
· At neuromuscular junction estimated several hundred vesicles involved in EPP, small variation in size make it impossible to observe effects of adding one or two vesicles
Experiment demonstrating quantal nature of EPP
· Reduce number of quanta in EPP by partially blocking neurotransmitter release with high levels of extracellular Mg2+ (Mg2+ blocks Ca2+ channels)
· If Mg2+ levels are high, only a little neurotransmitter is released and release is quantal as shown above
· An estimated 200 quanta involved in normal neuromuscular junction EPP
Some synapses release smaller number of quanta, in some cases only 1
Mechanism of vesicular release
1) Mobilization of vesicles
· Vesicles bound to cytoskeleton, often actin filaments, and held firmly in place, must be freed for release
· Process requires Ca2+ influx
Storage pool anchored vesicles
Releasable pool vesicles attached to membrane and ready for release
· Vesicles that are mobilized are not the same ones as those released for given AP, delay between opening of Ca2+ channels and release of neurotransmitter only 0.1 to 0.2 msec, not enough time for cascade of biochemical events required to mobilize vesicle
Influx of Ca2+ has 2 distinct effects:
1. release of presynaptic vesicles already docked at membrane
2. mobilization of vesicles from storage pool to releasable pool by promoting docking with membrane, mechanism by which vesicle moves and docks not yet understood
vesicles mobilized in one event of synaptic transmission are released in subsequent event