Etomidate evokes synaptic vesicle exocytosis without increasing miniature endplate potentials frequency at the mice neuromuscular junction

• Etomidate has pre and post-synaptic action at the mice neuromuscular junction.
• Etomidate evokes synaptic vesicle exocytosis by a Na+ independent manner.
• Etomidate acts on Ca2+ channels mainly on P-subtype.
• Etomidate interferes with post-synaptic nicotinic receptors.

Etomidate is an intravenous anesthetic used during anesthesia induction. This agent induces spontaneous movements, especially myoclonus after its administration suggesting a putative primary effect at the central nervous system or the periphery. Therefore, the aim of this study was to investigate the presynaptic and postsynaptic effects of etomidate at the mouse neuromuscular junction (NMJ). Diaphragm nerve muscle preparations were isolated and stained with the styryl dye FM1-43, a fluorescent tool that tracks synaptic vesicles exo-endocytosis that are key steps for neurotransmission. We observed that etomidate induced synaptic vesicle exocytosis in a dose-dependent fashion, an effect that was independent of voltage-gated Na+ channels. By contrast, etomidate-evoked exocytosis was dependent on extracellular Ca2+ because its effect was abolished in Ca2+-free medium and also inhibited by omega-Agatoxin IVA (30 and 200 nM) suggesting the participation of P/Q-subtype Ca2+ channels. Interestingly, even though etomidate induced synaptic vesicle exocytosis, we did not observe any significant difference in the frequency and amplitude of miniature end-plate potentials (MEPPs) in the presence of the anesthetic. We therefore investigated whether etomidate could act on nicotinic acetylcholine receptors labeled with α-bungarotoxin-Alexa 594 and we observed less fluorescence in preparations exposed to the anesthetic. In conclusion, our results suggest that etomidate exerts a presynaptic effect at the NMJ inducing synaptic vesicle exocytosis, likely through the activation of P-subtype voltage gated Ca2+ channels without interfering with MEPPs frequency. The present data contribute to a better understanding about the effect of etomidate at the neuromuscular synapse and may help to explain some clinical effects of this agent.