Effect of taurine and caffeine on sleep-wake activity in Drosophila melanogaster

Caffeine and taurine are two major neuromodulators present in large quantities in many popular energy drinks. We investigated their effects on sleep-wake control in constant darkness using the fruit fly Drosophila as a model system. It has been shown that caffeine, as the most widely used psychostimulant, can boost arousal through the dopamine pathway in the mushroom bodies of flies. Taurine is a GABA receptor agonist, which is inhibitory to neuronal firing. We show here that flies receiving a low dose of caffeine (0.01%) increase locomotor activity by 25%, and decrease total sleep by 15%. Treatment with taurine at 0.1% to 1.5% reduces locomotor activity by 28% to 86%, and shifts it from diurnal to nocturnal. At 0.75%, taurine also increases total sleep by 50%. Our results show that taurine increases sleep, while caffeine, as previously reported, attenuates sleep. Flies treated with both caffeine and taurine exhibit two differential effects which depend upon the ratio of taurine to caffeine. A high taurine:caffeine ratio promotes sleep, while a low ratio of taurine:caffeine inhibits sleep to a greater extent than the equivalent amount of caffeine alone. This intriguing enhancement of caffeine action by low doses of taurine may account for the presence of both compounds in energy-promoting drinks such as Red Bull® and Monster®.