TY - JOUR T1 - Proportional downscaling of glutamatergic release sites by the general anesthetic propofol at <em>Drosophila</em> motor nerve terminals JF - eneuro JO - eNeuro DO - 10.1523/ENEURO.0422-19.2020 SP - ENEURO.0422-19.2020 AU - Shanker Karunanithi AU - Drew Cylinder AU - Deniz Ertekin AU - Oressia H. Zalucki AU - Leo Marin AU - Nickolas A. Lavidis AU - Harold L. Atwood AU - Bruno van Swinderen Y1 - 2020/01/31 UR - http://www.eneuro.org/content/early/2020/01/31/ENEURO.0422-19.2020.abstract N2 - Propofol is the most common general anesthetic used for surgery in humans, yet its complete mechanism of action remains elusive. In addition to potentiating inhibitory synapses in the brain, propofol also impairs excitatory neurotransmission. We use electrophysiological recordings from individual glutamatergic boutons in male and female larval Drosophila melanogaster motor nerve terminals to characterize this effect. We recorded from two bouton types, which have distinct presynaptic physiology and different average numbers of release sites or active zones. We show that a clinically relevant dose of propofol (3μM) impairs neurotransmitter release similarly at both bouton types, by decreasing the number of active release sites by half, without affecting release probability. In contrast, an analog of propofol has no effect on glutamate release. Co-expressing a truncated syntaxin1A protein in presynaptic boutons completely blocked this effect of propofol. Overexpressing wild-type syntaxin1A in boutons also conferred a level of resistance, by increasing the number of active release sites to a physiological ceiling set by the number of active zones or T-bars, and in this way counteracting the effect of propofol. These results point to the presynaptic release machinery as a target for the general anesthetic. Proportionally equivalent effects of propofol on the number of active release sites across the different bouton types suggests that smaller glutamatergic boutons with fewer release sites may be more vulnerable to the presynaptic effects of the drug.Significance statement Over 200 million surgeries are performed worldwide under general anesthesia every year, and the anesthetic of choice is increasingly the intravenous agent propofol. Sedation produced by propofol is understood to result from postsynaptic activation of inhibitory mechanisms in the brain. We have identified a presynaptic effect of propofol, on excitatory synapses. Recording from individual glutamatergic boutons in fly larvae, we found that a clinically relevant dose of propofol impairs glutamate release, by proportionally decreasing the participation of release sites across different bouton types. This suggests that propofol anesthesia involves presynaptic as well as postsynaptic mechanisms, and a proportional effect in large and small boutons may explain why some circuits are more vulnerable than others to the effects of the drug. ER -