The effects of the general anaesthetic propofol on Drosophila larvae

Abstract

Although general anaesthetics have been in use since the mid-19th century, the mechanism by which these drugs induce reversible loss of consciousness is still poorly understood. Previous research has indicated that general anaesthetics activate endogenous sleep pathways by potentiating GABAA receptors in wake-promoting neurons. However, more recent studies have demonstrated that general anaesthetics also inhibit synaptic release through interactions with the SNARE complex, an integral part of presynaptic neurotransmitter release machinery in all neurons. The presynaptic and postsynaptic mechanisms may thus be linked in a two-step process: at low doses, general anaesthetics activate sleep-promoting circuits, thereby producing unconsciousness, while at the higher doses necessary for surgery, general anaesthetics inhibit presynaptic release machinery brain-wide, thereby causing a total loss of behavioural responsiveness. While this hypothesis remains speculative, it is testable in animal models. This study develops larval Drosophila melanogaster as an animal model to test this hypothesis in the context of a common intravenous GABA-acting general anaesthetic, propofol. Although presynaptic effects of general anaesthetics have been studied in larval neuromuscular junction preparations, there is not much data for how these drugs affect larval behaviour or brain activity. General anaesthesia is easily addressed in animal models because it can be described as a state of decreased responsiveness which can be assessed using diverse behavioural endpoints. In this study, a series of behavioural assays were designed and tested to assess the effect of GABA-acting general anaesthetics and sedative drugs on Drosophila larvae. These assays were then used to explore key parts of the pre- and postsynaptic mechanisms thought to underlie general anaesthesia. It was found that knockdown of the RDL subunit, the main anaesthetic target in the GABA receptor, did not confer resistance to propofol. However, deletion of the N-terminus of the H3 domain in syntaxin1A, a key component of the SNARE complex, resulted in resistance to propofol and isoflurane. These results suggest that propofol and isoflurane act through similar mechanisms and that this mechanism involves a presynaptic component that transcends life stage or brain size. Larval Drosophila thus provide a valuable counterpart to adult flies for investigating presynaptic mechanisms of anaesthesia.