Depolarization-induced release of ethanolamine from brain synaptic preparations in vitro

The release of ethanolamine from mouse brain synaptosomes and synaptoneurosomes has been investigated. The depolarizing agents veratridine (50 microM), KCl (35 mM) and 4-aminopyridine (2 mM) enhanced the release of [3H]ethanolamine from preloaded synaptosomes under superfusion conditions. Tetrodotoxin (2 microM) strongly inhibited veratridine- and 4-aminopyridine-stimulated release of [3H]ethanolamine but had no effect on KCl-evoked or resting release. In the absence of calcium, a reduction in the resting release of [3H]ethanolamine occurred and release evoked by veratridine, and KCl was markedly reduced. Exposure of preloaded synaptosomes to 5 mM ethanolamine (but not 5 mM serine or 5 mM choline) calcium-dependently increased the efflux of [3H]ethanolamine, however, this was not accompanied by membrane depolarization. When these experiments were performed using synaptoneurosomes, qualitatively similar results were obtained. The resting and evoked release of [3H]ethanolamine was however approximately 2.5-fold higher compared to synaptosomes on a brain equivalent basis, suggesting that uptake and release occur at sites in addition to the nerve ending. Our data are consistent with the idea that a significant amount of ethanolamine accumulates presynaptically and undergoes calcium-dependent release upon depolarization possibly via classical exocytosis. In contrast, ethanolamine-induced release of [3H]ethanolamine likely involves mostly diffusional exchange across the neuronal membrane rather than base exchange. The present results add support to the concept that ethanolamine may play a role as a synaptic signaling molecule in mammalian brain.