Examining size-strength relationships at hippocampal synapses using an ultrastructural measurement of synaptic release probability

J Struct Biol. 2010 Nov;172(2):203-10. doi: 10.1016/j.jsb.2009.10.014. Epub 2009 Nov 4.


Release probability (p(r)) is a fundamental presynaptic parameter which is critical in defining synaptic strength. Knowledge of how synapses set and regulate their p(r) is a fundamental step in understanding synaptic transmission and communication between neurons. Despite its importance, p(r) is difficult to measure directly at single synapses. One important strategy to achieve this has relied on the application of fluorescence-based imaging methods, but this is always limited by the lack of detailed information on the morphological and structural properties of the individual synapses under study, and thus precludes an investigation of the relationship between p(r) and synaptic anatomy. Here we outline a powerful methodology based on using FM-styryl dyes, photoconversion and correlative ultrastructural analysis in dissociated hippocampal cultured neurons, which provides both a direct readout of p(r) as well as nanoscale detail on synaptic organization and structure. We illustrate the value of this approach by investigating, at the level of individual reconstructed terminals, the relationship between release probability and defined vesicle pools. We show that in our population of synapses, p(r) is highly variable, and while it is positively correlated with the number of vesicles docked at the active zone it shows no relationship with the total number of synaptic vesicles. The lack of a direct correlation between total synaptic size and performance in these terminals suggests that factors other than the absolute magnitude of the synapse are the most important determinants of synaptic efficacy.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 3,3'-Diaminobenzidine / chemistry
  • Animals
  • Cells, Cultured
  • Electric Stimulation
  • Fluorescent Dyes / chemistry
  • Fluorescent Dyes / metabolism
  • Hippocampus / cytology*
  • Membrane Fusion / physiology*
  • Microscopy, Electron, Transmission / methods*
  • Microscopy, Fluorescence
  • Neurons / physiology
  • Neurons / ultrastructure
  • Photochemical Processes
  • Presynaptic Terminals / physiology
  • Presynaptic Terminals / ultrastructure
  • Probability
  • Pyridinium Compounds / chemistry
  • Pyridinium Compounds / metabolism
  • Quaternary Ammonium Compounds / chemistry
  • Quaternary Ammonium Compounds / metabolism
  • Rats
  • Synapses / physiology*
  • Synapses / ultrastructure*
  • Synaptic Vesicles / physiology*
  • Synaptic Vesicles / ultrastructure*


  • FM1 43
  • Fluorescent Dyes
  • Pyridinium Compounds
  • Quaternary Ammonium Compounds
  • 3,3'-Diaminobenzidine