The Neurotransmitters Involved in Drosophila Alcohol-Induced Behaviors

Abstract

Alcohol is a widely used and abused substance with numerous negative consequences for human health and safety. Historically, alcohol's widespread, non-specific neurobiological effects have made it a challenge to study in humans. Therefore, model organisms are a critical tool for unraveling the mechanisms of alcohol action and subsequent effects on behavior. Drosophila melanogaster is genetically tractable and displays a vast behavioral repertoire, making it a particularly good candidate for examining the neurobiology of alcohol responses. In addition to being experimentally amenable, Drosophila have high face and mechanistic validity: their alcohol-related behaviors are remarkably consistent with humans and other mammalian species, and they share numerous conserved neurotransmitters and signaling pathways. Flies have a long history in alcohol research, which has been enhanced in recent years by the development of tools that allow for manipulating individual Drosophila neurotransmitters. Through advancements such as the GAL4/UAS system and CRISPR/Cas9 mutagenesis, investigation of specific neurotransmitters in small subsets of neurons has become ever more achievable. In this review, we describe recent progress in understanding the contribution of seven neurotransmitters to fly behavior, focusing on their roles in alcohol response: dopamine, octopamine, tyramine, serotonin, glutamate, GABA, and acetylcholine. We chose these small-molecule neurotransmitters due to their conservation in mammals and their importance for behavior. While neurotransmitters like dopamine and octopamine have received significant research emphasis regarding their contributions to behavior, others, like glutamate, GABA, and acetylcholine, remain relatively unexplored. Here, we summarize recent genetic and behavioral findings concerning these seven neurotransmitters and their roles in the behavioral response to alcohol, highlighting the fitness of the fly as a model for human alcohol use.

Keywords: AUD; Drosophila; alcohol abuse; alcohol behavior; genetics; neurotransmitter.

Figure 1

The biphasic alcohol activity response in Drosophila, and neurotransmitter involvement in fly and mammalian alcohol responses. (A) A sample activity plot shows the change in locomotion of a fly across time following exposure to alcohol vapor. Immediately after ethanol vapor is delivered, the fly has an initial startle response, significantly increasing locomotion from baseline. This startle response quickly drops off and the fly's activity returns to a level close to baseline. As absorption of alcohol takes place, the fly's locomotion gradually increases as nervous system stimulation occurs. Eventually, intoxication peaks, and the fly enters the sedative phase associated with nervous system depression, and activity declines over time until the fly is completely sedated (Bainton et al., ; Singh and Heberlein, ; Wolf et al., 2002). EtOH = ethanol (B) The stimulatory and sedative phases involve distinct neurotransmitter actions. The biphasic alcohol response (nervous system stimulation in green and nervous system depression in blue) is very similar in Drosophila and mammals, and some of the same neurotransmitter actions have been implicated in these responses. Arrows indicate increase or decrease in activity for the specified neurotransmitter. See the main text for further details and references.

Figure 2

The development of alcohol tolerance in Drosophila. (A) Schematic of alcohol vapor exposure assay. Blue-dyed alcohol is applied to the vial plug, and following the first exposure, flies recover for 4 hours in fresh air. In the second exposure, at the same time point, fewer flies are sedated that in the first exposure, indicating that tolerance has developed. (B) Sample data shows ST50 (time it takes for 50% of the flies to become sedated) for a first and second alcohol vapor exposure for wildtype flies and flies with manipulations of different neurotransmitters. These manipulations are either genetic (“Reduced OA and TA” and “Reduced OA, Elevated TA”) or pharmacological via drug feeding (GABA agonist).

Figure 3

Alcohol consumption preference and related neurotransmitter circuitry in Drosophila. (A) Schematics of common assays for alcohol preference. The capillary feeder (CAFÉ) and fluorometric reading assay of preference primed by ethanol (FRAPPE) assays are consumption assays in which flies have the choice between two food sources, one with and one without ethanol. (B) Known circuitry for neurotransmitters mediating alcohol preference and avoidance. DAergic projections from the PAM cluster to the mushroom body (Ojelade et al., 2019) and OAergic VMI-VMIII neurons (Schneider et al., 2012) promote alcohol preference. DAergic projections from the PPL cluster to the fan-shaped body (Ojelade et al., 2019) and serotonergic signaling in CSD interneurons (Kasture et al., 2019) promote alcohol avoidance. FSB, fan-shaped body; LH, lateral horn; MB, mushroom body. See the text for further details and additional references.