Measuring metabolic rate in single flies during sleep and waking states

Abstract

Drosophila melanogaster is a leading genetic model for studying the neural regulation of sleep. Sleep is associated with changes in behavior and physiological state that are largely conserved across species. The investigation of sleep in flies has predominantly focused on behavioral readouts of sleep because physiological measurements, including changes in brain activity and metabolic rate are less accessible. We have previously used stop-flow indirect calorimetry to measure whole body metabolic rate in single flies and have shown that in flies, like mammals, metabolic rate is reduced during sleep. Here, we describe a modified version of this system that allows for efficient and highly sensitive acquisition of CO2 output from single flies. We also describe a modification that allows for simultaneous acquisition of CO2 and O2 levels, providing a respiratory quotient that quantifies how metabolic stores are utilized. Finally, we show that sleep-dependent changes in metabolic rate are diminished in aging flies, supporting the notion that sleep quality is reduced as flies age. Taken together, the use of indirect calorimetry provides a physiological measure of sleep with broad applications to genetic studies in flies.