Volatile anesthetics bind rat synaptic snare proteins

Background: Volatile general anesthetics (VAs) have a number of synaptic actions, one of which is to inhibit excitatory neurotransmitter release; however, no presynaptic VA binding proteins have been identified. Genetic data in Caenorhabditis elegans have led to the hypothesis that a protein that interacts with the presynaptic protein syntaxin 1A is a VA target. Motivated by this hypothesis, the authors measured the ability of syntaxin 1A and proteins that interact with syntaxin to bind to halothane and isoflurane. Methods: Recombinant rat syntaxin 1A, SNAP-25B, VAMP2, and the ternary SNARE complex that they form were tested. Binding of VAs to these proteins was detected by ¹⁹F-nuclear magnetic resonance relaxation measurements. Structural alterations in the proteins were examined by circular dichroism and ability to form complexes. Results: Volatile anesthetics did not bind to VAMP2. At concentrations in the clinical range, VAs did bind to SNAP-25B; however, binding was detected only in preparations containing SNAP-25B homomultimers. VAs also bound at clinical concentrations to both syntaxin and the SNARE complex. Addition of an N-terminal His₆ tag to syntaxin abolished its ability to bind VAs despite normal secondary structure and ability to form SNARE complexes; thrombin cleavage of the tag restored VA binding. Thus, the VA binding site(s) has structural requirements and is not simply any α-helical bundle. VAs at supraclinical concentrations produced an increase in helicity of the SNARE complex; otherwise, VA binding produced no gross alteration in the stability or secondary structure of the SNARE complex. Conclusion: SNARE proteins are potential synaptic targets of volatile anesthetics.