Differential labelling of bulk endocytosis in nerve terminals by FM dyes

Neurochem Int. 2008 Sep;53(3-4):51-5. doi: 10.1016/j.neuint.2008.06.002. Epub 2008 Jun 11.


Bulk endocytosis is triggered in central nerve terminals during intense physiological stimulation. This endocytosis pathway can be labelled by the dye FM1-43 but not its more hydrophilic counterpart FM2-10. This selective labelling was proposed to be due to the retention of FM1-43, but not FM2-10, in slowly retrieving structures after washout of the dye. However, this explanation assumed that bulk endocytosis was a slow process that persisted after stimulation. We have recently shown that the great majority of bulk endocytosis occurs during stimulation, therefore another explanation for the specific labelling of this pathway by FM1-43 must be found. In this paper we show that the ability of FM dyes to label bulk endocytosis is dependent on the concentration of dye used and not their washout properties. When the loading concentration of FM1-43 was reduced 10-fold, its ability to label bulk endocytosis was lost. Conversely when the loading concentration of FM2-10 was increased 10-fold, it now labelled the pathway. This suggests that a difference in affinity of bulk endosome membranes for FM1-43 and FM2-10 underlies the disparity in labelling.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Axonal Transport / physiology
  • Cells, Cultured
  • Central Nervous System / physiology*
  • Central Nervous System / ultrastructure
  • Dose-Response Relationship, Drug
  • Endocytosis / physiology*
  • Endosomes / drug effects
  • Endosomes / metabolism
  • Fluorescent Dyes / metabolism*
  • Fluorescent Dyes / pharmacokinetics
  • Intracellular Membranes / drug effects
  • Intracellular Membranes / metabolism
  • Neurons / physiology
  • Neurons / ultrastructure
  • Presynaptic Terminals / physiology*
  • Presynaptic Terminals / ultrastructure
  • Pyridinium Compounds / metabolism
  • Pyridinium Compounds / pharmacokinetics
  • Quaternary Ammonium Compounds / metabolism
  • Quaternary Ammonium Compounds / pharmacokinetics
  • Rats
  • Rats, Sprague-Dawley
  • Staining and Labeling / methods*
  • Synaptic Transmission / physiology
  • Transport Vesicles / drug effects
  • Transport Vesicles / metabolism


  • FM1 43
  • FM2 10
  • Fluorescent Dyes
  • Pyridinium Compounds
  • Quaternary Ammonium Compounds