Microdosing ketamine in Drosophila does not inhibit SERT like SSRIs, but causes behavioral changes mediated by glutamate and serotonin receptors

Recently, the FDA approved microdosing ketamine for treatment resistant depression. Traditional antidepressants, like serotonin selective reuptake inhibitors (SSRIs), block serotonin reuptake, but it is not clear if ketamine blocks serotonin reuptake. Here, we tested the effects of feeding ketamine and SSRIs to Drosophila melanogaster larvae, which has a similar serotonin system to mammals, and is a good model to track depression behaviors, such as locomotion and feeding. Fast-scan cyclic voltammetry (FSCV) was used to measure optogenetically-stimulated serotonin changes, and locomotion tracking software and blue dye feeding to monitor behavior. We fed larvae various doses (1-100 mM) of antidepressants for 24 hours and found that 1 mM ketamine did not affect serotonin, but increased locomotion and feeding. Low doses ([≤] 10 mM) of escitalopram and fluoxetine inhibited dSERT and also increased feeding and locomotion behaviors. At 100 mM, ketamine inhibited dSERT and increased serotonin concentrations, but decreased locomotion and feeding due to its anesthetic properties. Since microdosing ketamine causes behavioral effects, we also investigated behavior changes with low doses of other NMDA receptor antagonists and 5-HT1A and 2 agonists, which are other possible sites for ketamine action. NMDA receptor antagonism increased feeding, while serotonin receptor agonism increased locomotion, which could explain these effects with ketamine. Ultimately, this work shows that Drosophila is a good model to discern antidepressant mechanisms, and that ketamine does not work on dSERT like SSRIs at microdoses, but affects behavior with other mechanisms. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=176 HEIGHT=200 SRC="FIGDIR/small/566121v1_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@1660e62org.highwire.dtl.DTLVardef@13a636borg.highwire.dtl.DTLVardef@5542f4org.highwire.dtl.DTLVardef@19eaead_HPS_FORMAT_FIGEXP M_FIG C_FIG Microdosing ketamine is a novel depression treatment, but it is not clear how it changes serotonin in real-time. Drosophila melanogaster (fruit fly) is a good model to study depression behaviors. Here, we used fast-scan cyclic voltammetry, video tracking, and feeding assays to measure serotonin and behavior after feeding ketamine and SSRIs to larvae. At microdoses, ketamine did not affect serotonin, which was different from SSRIs. However, higher doses inhibited dSERT. Locomotion and feeding changes were also dose-dependent, and we saw separate effects with NMDA and serotonin receptor drugs. This work facilitates future behavioral and pharmacological testing with ketamine using Drosophila.