Mechanisms of Naltrexone-induced Reduction of Ethanol Preference in Drosophila Melanogaster

Naltrexone is an opioid antagonist used to treat alcohol dependence in human beings ever since its approval by Food and Drug Administration in 1994. Naltrexone exerts its action by blocking on central opioid receptors that mediate the drinking or reward behaviours, thus reducing the alcohol consumption. Although various animal and clinical studies have demonstrated the efficacy of Naltrexone, its action on reducing the preferential ethanol consumption in Drosophila melanogaster has not been illustrated so far. So it was of our interest to demonstrate the effect of Naltrexone on the drinking behaviour in fruit flies and to further explore the molecular mechanisms underlying this effect. In our study, we have employed the well-established CAFE methodology to test the preference of flies to consume alcohol food over normal food. 1-3 day old male flies (wild type) were used for all the experiments which were exposed or unexposed to 15% ethanol to examine the preferential consumption. Preference assays were conducted with or without Naltrexone treatment to demonstrate its effect under various experimental conditions. In addition to the behavioural assay, we have attempted a biochemical estimation to observe the changes in the phosphorylation patterns of protein kinase C (PKC) using an ELISA-based PKC kinase activity assay in order to explore the mechanism of action of Naltrexone in relation to PKC which has been identified to mediate alcohol addiction processes. To further explore any PKC-mediated mechanism of Naltrexone effect, preference assays were conducted in Drosophila PKC mutant line-20790. Our results showed that Drosophila pre-exposed to ethanol, prefers to consume ethanol food over non-ethanol food and for the first time we have demonstrated that Naltrexone reverses this preference to consume ethanol food. Our data also shows that mechanism of Naltrexone effect appears to be independent of PKC-mediated pathway and we propose that Naltrexone might be operating through a different system (eg; pathways or receptors or signalling molecules associated with neural circuitry such as Dopamine) and more research is needed to explore these mechanisms in detail to develop new hypotheses on potential therapeutic targets.