Vesicle availability partly determines the efficacy of synaptic communication in the CNS. The authors recently found that some hippocampal glutamate vesicles exhibit reluctance to exocytose during short, high-frequency action potential trains. These same vesicles can be "coaxed" into exocytosis by increased Ca2+ entry, by direct depolarization of synaptic terminals, or by challenge with hypertonic sucrose, a tool used to cause fusion of the population of release-ready synaptic vesicles. Interestingly, the authors did not find evidence of reluctance at hippocampal GABA synapses, suggesting that vesicle reluctance might be a negative feedback mechanism to prevent runaway excitation. It is also possible that synapses exhibit reluctance to retain a dormant population of readily accessible vesicles, ready to respond to triggers such as enhanced Ca2+ influx or neuromodulators. Recent work from the calyx of Held synapse suggests that reluctance might arise from inactivation of Ca2+ channels. The authors review this work, along with several other potential mechanisms of reluctance.

Original languageEnglish
Pages (from-to)11-15
Number of pages5
Issue number1
StatePublished - Feb 2006


  • Calcium
  • Glutamate
  • Hippocampus
  • Readily releasable pool of vesicles
  • Release probability
  • Synaptic vesicle


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