Effects of D-amino acids on sleep in Drosophila
Introduction
Sleep is related to various physiological phenomena and known to be closely related to metabolism and thus food intake. Under fasting conditions, arousal is induced, but after eating, sleepiness is induced. There are two distinct effects of food intake: pre-ingestive influence such as smell, taste and sweetness by direct food consumption, and post-ingestive influence such as post-prandial hyperglycemia due to nutrition absorption. We have used a fruit fly, Drosophila melanogaster for investigating sleep, metabolism and longevity [[1], [2], [3], [4], [5], [6]]. We and others reported sugar with both sweetness and nutritional value induced sleep while sugar without sweetness did not induce sleep [[7], [8], [9]]. In addition, we reported that the high concentration of artificial sweeteners without nutritional values, such as sucralose had also sleep-inducing effects in Drosophila [9]. Recently, the regulation of sleep by amino acids has been reported in mammals and insects. It has been reported that oral administration of glycine induces sleep in rats caused by decreased core body temperature [10]. Also, in Drosophila, threonine induces sleep via enhancing GABA signal [11] and genes involved in the metabolic pathway of serine are involved in the reduction of sleep during fasting [12]. Although most amino acids in the body are present as the L-form, the importance of D-form amino acids has been increasingly notified. In addition to glycine, some D-amino acids such as D-Ser, D-Ala, and D-Asp, which are ligands for N-methyl-D-aspartic acid type glutamate receptor channel (NMDAR), have been reported to be involved in physiological phenomena [13,14]. D-Ser is abundant in the mammalian brain and regulates memory and other functions [[15], [16]]. However, there are still limited knowledge about the physiological functions of D-amino acids, especially sleep regulation. In this study, we comprehensively examined the effects on sleep of 19 D-amino acids and approached their physiological role in sleep regulation using Drosophila.
Section snippets
Fly stocks
Flies were maintained on 12 h; 12 h light and dark cycles at 25 °C and about 60% humidity. Flies were cultured on standard food which include cornmeal, yeast, glucose, wheat germ and agar as described [1]. As for fly stocks, dEAAT1-GAL4 (#8849), Daao1 hypo (PBac{PB}Daao1c05692, #17722) were obtained from the Bloomington Drosophila Stock Center (BDSC), Indiana University. UAS-Dicer-2 (#60008), UAS-Srr RNAi (#24201) and w1118 (#60000: genetic background of RNA interference line) were obtained
Effects of D-amino acids on sleep
To investigate whether D-amino acids have any effects on sleep and locomotor activity of Drosophila, we measured locomotor activities of the flies fed with 1% D-amino acids using the DAMS. Flies were kept in tubes with agar-sucrose food without amino acids for 1 day, and at ZT 0 on the next day, they were transferred to other tubes with agar-sucrose food containing 1% D-amino acid. We found that D-Ser and D-Gln significantly increased the amount of sleep (Fig. 1A). In addition to them, D-Asp,
Discussion
In this study, we showed significant effects of D-amino acids on sleep in Drosophila and their possible involvement in the sleep regulation. We especially examined D-Ser, which is known to be synthesized in Drosophila, and found its dose-dependent increasing effects on sleep, which were evident in flies with different genetic backgrounds. Since D-Ser is known to bind to NMDAR and D-Ser did not have significant effects on sleep and locomotor activity in dNR1 hypomorphic mutants, we presumed
Funding
This study was supported by the Japan Society for the Promotion of Science (JSPS), Japan: Kazuhiko Kume 18H02481, 21H02529; Jun Tomita 20K06744.
Author contributions
HN, SN, GB, JT and KK designed the experiments. HN, SN, GB conducted all experiments and data analysis. HN and KK drafted and all authors wrote the manuscript.
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Kazuhiko Kume reports financial support was provided by Japan Society for the Promotion of Science. Jun Tomita reports financial support was provided by Japan Society for the Promotion of Science.
Acknowledgments
We thank BDSC, and VDRC stock centers for fly stocks, and the members of Kume lab for discussions.
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These authors have contributed equally to this work.