Glial cell mechanisms regulate alcohol sedation in Drosophila melanogaster

Approximately 16 million people in America are diagnosed with Alcohol Use Disorder (AUD) but no efficacious medical treatments exist. Alcohol-related behaviors can be studied in model organisms, and changes in these behaviors can be correlated with either (i) a risk for alcohol dependence or (ii) a symptom/feature of AUD itself. Although AUD is a disease of the central nervous system, a majority of research has focused on the neuronal underpinnings, leaving glial contributions largely undescribed. We used Drosophila melanogaster (fruit fly) to identify genes whose expression in glia regulates alcohol sedation. Mammals and Drosophila have conserved behavioral responses to alcohol and functionally similar adult glial cells, especially astrocytes. Since previous research in mammals and flies has demonstrated that glia respond to alcohol administration, we hypothesized that glia are important regulators of alcohol-related behaviors. To pursue this, we characterized a pan-glial steroid-inducible GeneSwitch transgenic fly, which allows gene manipulation within glia during adulthood. We performed a targeted screen and manipulated genes that were known to be expressed within Drosophila glia and measured their alcohol sedation sensitivity using the ethanol sedation assay. We identified the genes Cysteine proteinase 1 (Cp1) and Tyramine decarboxylase 2 (Tdc2). Knocking down Cp1 in cortex glia, as well as all glia during adulthood, increased alcohol sedation sensitivity and may also enhance rapid tolerance development. We could not identify what pathway Cp1 was functioning within to mediate this response, suggesting that Cp1 may have a unique function within glia. Knockdown or overexpression of Tdc2 in glia increased or decreased alcohol sedation sensitivity, respectively. Tdc2 functions upstream of the vesicular monoamine transporter (VMAT) and the SNARE complex to regulate alcohol sedation. These results were specific to astrocytes, as well as all glia during adulthood. These results suggest that tyramine synthesis via Tdc2 and its release via vesicular exocytosis regulates alcohol sedation. Taken together, these results suggest that glia are important regulators of alcohol-related behaviors in flies. Interestingly, fly cortex glia and astrocytes are functionally similar to mammalian astrocytes, indicating that these results may be translatable to mammals