Synaptophysin is phosphorylated in rat cortical synaptosomes treated with botulinum toxin A

Phosphorylation and dephosphorylation of neuronal proteins have been implicated in regulation of synaptic transmission. Studies were performed to determine if synaptophysin was phosphorylated or dephosphorylated during exposure of synaptosomes to botulinum toxin A (BoTX/A). Cholinergic-enriched synaptosomes were preincubated in the presence of 3H-choline to label newly synthesized acetylcholine (3H-ACh). This was followed by incubation with low or high potassium to stimulate release of newly synthesized 3H-ACh. BoTX/A inhibited total Ach release by 15-19% and inhibited release of newly synthesized 3H-ACh by 35%. A 165% increase in synaptophysin phosphorylation occurred in a dose-dependent manner over a range of doses (0.2 nM, 2 nM, 20 nM, 100 nM) of BoTX/A. When 4-Aminopyridine was added to synaptosomes that were BoTX/A treated, synaptophysin was dephosphorylated to control levels. Synaptosomes incubated with BoTX/A exhibited an inhibition of potassium stimulated ACh release and an increase in synaptophysin phosphorylation. Synaptophysin phosphorylation may be involved in inhibition of acetylcholine release.