Synapse-specific trapping of SNARE machinery proteins in the anesthetized Drosophila brain

Abstract

General anesthetics disrupt brain network dynamics through multiple pathways, in part through post-synaptic potentiation of inhibitory ion channels as well as pre-synaptic inhibition of neuroexocytosis. Common clinical general anesthetic drugs, such as propofol and isoflurane, have been shown to interact and interfere with core components of the exocytic release machinery to cause impaired neurotransmitter release. Recent studies however suggest that these drugs do not affect all synapse subtypes equally. We investigated the role of the presynaptic release machinery in multiple neurotransmitter systems under isoflurane general anesthesia in the adult female Drosophila brain using live-cell super resolution microscopy and optogenetic readouts of exocytosis and neural excitability. We activated neurotransmitter-specific mushroom body output neurons (MBONs) and imaged presynaptic function under isoflurane anesthesia. We found that isoflurane impaired synaptic release and presynaptic protein dynamics in excitatory cholinergic synapses. In contrast, isoflurane had little to no effect on inhibitory GABAergic or glutamatergic synapses. These results present a distinct inhibitory mechanism for general anesthesia, whereby neuroexocytosis is selectively impaired at excitatory synapses, while inhibitory synapses remain functional. This suggests a presynaptic inhibitory mechanism that complements the other inhibitory effects of these drugs.Significance Statement General anesthetics are promiscuous drugs that act on a variety of pre-synaptic and post-synaptic proteins. Yet they produce a common endpoint - loss of behavioral responsiveness - in all animals. Using optogenetic techniques to measure functional readouts in identified neurons in the Drosophila brain, we have found that the volatile anesthetic isoflurane impairs neurotransmitter release from excitatory synapses, and that this is associated with immobilization of release machinery proteins. Inhibitory synapses were unaffected. This suggests a level of presynaptic specificity to the anesthetic's mechanism of action which complements the other known effects on synaptic function, and potentially explains how some of these drugs might work to produce the common endpoint termed general anesthesia.