Measuring metabolic rate in single flies during sleep and waking states via indirect calorimetry

Background: 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. New method: Here, we describe a modified version of this system that allows for efficient and highly sensitive acquisition of CO2 output from single flies. Results: In this modified system, 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. We also describe a modification that allows for simultaneous acquisition of CO2 and O-2 levels, providing a respiratory quotient that quantifies how metabolic stores are utilized. We find that the respiratory quotient identified in flies on an all-sugar diet is suggestive of lipogenesis, where the dietary sugar provided to the flies is being converted to fat. Comparison with existing methods and conclusions: Taken together, the measurement of metabolic rate via indirect calorimetry not only provides a physiological readout of sleep depth, but also provides insight the metabolic regulation of nutrient utilization, with broad applications to genetic studies in flies.

Automated summary

This study presents a modified method for efficient and sensitive acquisition of CO2 output from single flies, and demonstrates that sleep-dependent changes in metabolic rate are diminished in aging flies. The study also suggests that flies on an all-sugar diet may undergo lipogenesis.