Previous studies have shown that inhibition of voltage gated

Previous studies have shown that inhibition of voltage-gated Na+ ω-Agatoxin TK has a reduced effect on GABAergic boutons compared with other neurotransmitter phenotypes,46, 47 and voltage-gated Na+ channel subtypes are differentially expressed in hippocampus. In addition, SV exocytosis evoked by elevated extracellular K+, which bypasses Na+ channels activation and activates VGCCs directly, is relatively insensitive to isoflurane.31, 50 Taking these findings into account, voltage-gated Na+ channel activation upstream of VGCC activation is a plausible mechanism for isoflurane inhibition of SV exocytosis.51, 52, 53 Isoflurane inhibition of the mitochondrial respiratory chain has also been implicated in its presynaptic effects. Isoflurane directly inhibits mitochondrial complex I to limit synaptic ATP production in the excitatory pre-synapse which, in turn, inhibits excitatory SV endocytosis and exocytosis. Given that GABAergic neurone-specific complex I defects do not alter the sensitivity of isoflurane, the role of complex I function in vesicle cycling might be different between excitatory and inhibitory neurones. Future studies of the contributions of voltage-gated Na+ channels, mitochondrial complex I, or other targets to isoflurane-induced inhibition of SV exocytosis are necessary to clarify further the bouton selectivity of presynaptic isoflurane action. In conclusion, we show that SV exocytosis linked to N-type and P/Q-type Ca2+ channels, the principal neuronal VGCC subtypes that mediate the Ca2+ entry coupled to SV exocytosis in the hippocampus, have similar sensitivities to isoflurane. As presynaptic inhibition of SV exocytosis by isoflurane was independent of relative VGCC subtype expression in hippocampal synaptic boutons, we conclude that VGCCs are unlikely to determine the selective sensitivity to inhibition of SV exocytosis by isoflurane in specific neuronal phenotypes.
Authors' contributions
Declarations of interests HCH is the editor-in-chief of the British Journal of Anaesthesia. The other authors have no competing interests.
Funding National Institutes of Health, Bethesda, MD, USA (grant number GM58055 to HCH).
Acknowledgements
Introduction Molecules such as neurotransmitters and hormones are secreted by calcium-regulated exocytosis [1,2]. In neuroendocrine cells, exocytosis implies the recruitment and subsequent fusion of secretory granules at specific sites of the plasma membrane. Annexin A2 (AnxA2) was the first protein identified at these exocytotic sites in chromaffin cells releasing catecholamine [3]. Since then, AnxA2 was shown to bind two major actors of exocytosis, actin and phospholipids. Electron tomography of chromaffin cells revealed that actin filaments bundled by AnxA2 contribute to the formation of GM1-enriched lipid micro-domains required for the spatial organization of fusion sites at the plasma membrane [4,5]. Yet, the mechanism(s) regulating AnxA2 activity during exocytosis is an important aspect that remains poorly understood. The N-terminal domain of AnxA2 has many potential regulatory sites. These include phosphorylation sites at Tyr23 and Ser25 that have been proposed to control the activity and functions of AnxA2. In chromaffin cells, AnxA2-Ser25 phosphorylation by protein kinase C occurs in response to nicotine stimulation and has been linked to granule exocytosis [[6], [7], [8], [9]]. AnxA2 has also been identified as the major substrate for the Src family of tyrosine kinases [10]. Src-dependent phosphorylation of AnxA2-Tyr23 has been reported to modulate actin and membrane binding properties of AnxA2 without affecting the tetramer formation with S100A10 [[11], [12], [13]]. Moreover, AnxA2-Tyr23 phosphorylation induces a conformational change that stabilizes AnxA2 at the membrane and promotes the binding of AnxA2 with phosphatidic acid (PA) [14], a lipid described as an essential component for exocytosis [14]. Chromaffin cells are enriched in tyrosine kinases such as c-Src [15] and Fyn [16]. Additionally, nicotine-induced stimulation of chromaffin cells and catecholamine secretion is accompanied by the activation of pp60c-src kinase [17,18] and rapid tyrosine phosphorylation of multiple cellular proteins [19].