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

Abstract

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®.

Keywords: GABA (γ-aminobutyric acid); caffeine (trimethylxanthine); energy drinks; locomotor activity; sleep; taurine (2-aminoethanesulfonic acid).

Figure 1

Locomotor activity in taurine- (T) and caffeine- (C) treated flies. A) Taurine treatment decreases locomotor activity. Data shown are from control (n = 22), 0.1%T (n = 19), 0.5%T (n = 12), and 1.5%T (n = 10). Average movements in a 30 minute window were calculated over 5 days, and indicated on the y axis. Value represents mean ± standard error of the mean (SEM). Asterisks (* or **) indicate P < 0.05 and P < 0.005 compared to the control, respectively. B) Representative activity record (actogram) of a single fly recording from each treatment group: control, 0.1%T, 0.5%T, and 1.5%T. The x axis indicates a 24 hour timeline on top of each actogram. The shaded areas represent activity which is measured as described in Methods. Activity profiles from day 2 to day 9 are shown (y axis). The amplitude of activity is adjusted to the same scale (from 0–5) in each activity record. C) Caffeine increases locomotor activity at higher concentrations. Shown here is representative activity record of a single fly recording from each treatment group (control, 0.01% C, 0.05% C, and 0.1% C), with sucrose/agar media (S/A) in the first three days (day 2 to 4 on y axis) then transferred to either S/A only (eg, control), or sucrose/agar with various concentrations of caffeine (days 4–9). The arrow indicates time of transfer on day 4.

Figure 1

Locomotor activity in taurine- (T) and caffeine- (C) treated flies. A) Taurine treatment decreases locomotor activity. Data shown are from control (n = 22), 0.1%T (n = 19), 0.5%T (n = 12), and 1.5%T (n = 10). Average movements in a 30 minute window were calculated over 5 days, and indicated on the y axis. Value represents mean ± standard error of the mean (SEM). Asterisks (* or **) indicate P < 0.05 and P < 0.005 compared to the control, respectively. B) Representative activity record (actogram) of a single fly recording from each treatment group: control, 0.1%T, 0.5%T, and 1.5%T. The x axis indicates a 24 hour timeline on top of each actogram. The shaded areas represent activity which is measured as described in Methods. Activity profiles from day 2 to day 9 are shown (y axis). The amplitude of activity is adjusted to the same scale (from 0–5) in each activity record. C) Caffeine increases locomotor activity at higher concentrations. Shown here is representative activity record of a single fly recording from each treatment group (control, 0.01% C, 0.05% C, and 0.1% C), with sucrose/agar media (S/A) in the first three days (day 2 to 4 on y axis) then transferred to either S/A only (eg, control), or sucrose/agar with various concentrations of caffeine (days 4–9). The arrow indicates time of transfer on day 4.

Figure 2

Effect of taurine on locomotion is reversible. Locomotor activity of the sham control and taurine-treated flies (1.5T and 1.5T reversal) were monitored for 5 days. On day 6, flies were transferred to fresh tubes containing either sucrose/agar media (control and 1.5 reversal groups), or 1.5% taurine (1.5T), and returned to the monitor for a further four days. A) Data shown are average of 4-day locomotor activity after switching. Values represent mean ± S.E.M. from each group (n = 32 in control, and n = 16 in both 1.5T and 1.5T reversal groups). No significant difference was observed between control and the 1.5T reversal groups. B) Representative activity records of individual flies. The arrows indicate the time of transfer on day 6.

Figure 3

Taurine alone or in combination with caffeine attenuates locomotor activity. A) Average locomotor activity from flies treated with sucrose/agar (control), 0.01% caffeine (0.01C) alone, or 0.01C with taurine ranging from 0.125% to 0.75% (0.125T, 0.25T, 0.5T, and 0.75T) with between 12 and 16 flies in each group. Asterisks (* or **) indicate P values that are smaller than 0.05 and 0.005, respectively. B) Representative activity records of flies from groups in (A). The arrow indicates the beginning of recording on day 1. The shaded rectangle at the top of the actogram indicates subjective day (10:00 to 22:00) and solid dark rectangle indicates subjective night (22:00 to 10:00).

Figure 4

Taurine and caffeine have opposite effects on sleep. A) Total sleep is shown as minutes of sleep in a 24 hour period. Between 12 and 16 flies were used in each group. B) Number of sleep bouts from nighttime sleep (22:00 to 10:00) or daytime sleep (10:00 to 22:00) are collected from three consecutive days or nights (day 2–4), and converted to minutes of sleep per hour. All caffeine-only or taurine-only groups were compared to the control, whereas the taurine/caffeine combined groups were compared to corresponding caffeine or taurine only treatments at the same concentration.