Running promotes wakefulness and increases cataplexy in orexin knockout mice

Abstract

Study objective: People with narcolepsy and mice lacking orexin/hypocretin have disrupted sleep/wake behavior and reduced physical activity. Our objective was to identify physiologic mechanisms through which orexin deficiency reduces locomotor activity.

Design: We examined spontaneous wheel running activity and its relationship to sleep/wake behavior in wild type (WT) and orexin knockout (KO) mice. Additionally, given that physical activity promotes alertness, we also studied whether orexin deficiency reduces the wake-promoting effects of exercise.

Measurements and results: Orexin KO mice ran 42% less than WT mice. Their ability to run appeared normal as they initiated running as often as WT mice and ran at normal speeds. However, their running bouts were considerably shorter, and they often had cataplexy or quick transitions into sleep after running. Wheel running increased the total amount of wakefulness in WT and orexin KO mice similarly, however, KO mice continued to have moderately fragmented sleep/wake behavior. Wheel running also doubled the amount of cataplexy by increasing the probability of transitioning into cataplexy.

Conclusions: Orexin KO mice run significantly less than normal, likely due to sleepiness, imminent cataplexy, or a reduced motivation to run. Orexin is not required for the wake-promoting effects of wheel running given that both WT and KO mice had similar increases in wakefulness with running wheels. In addition, the clear increase in cataplexy with wheel running suggests the possibility that positive emotions or reward can trigger murine cataplexy, similar to that seen in people and dogs with narcolepsy.

Figure 1

Wild type (WT) and orexin knockout (KO) mice acclimate similarly to running wheels, however, KO mice consistently run less than WT mice. Running wheels were locked on days 13 through 16. Sleep/wake EEG behavior was recorded on days 12 and 16. Wheels were unlocked on day 17, and running returned to prior levels.

Figure 2

Orexin KO mice run less than WT mice. A) Over 24 hours, KO mice run 42% less than WT mice. B) Orexin KO mice spend less time running than WT mice. Data shown here and on subsequent figures are from day 12, when mice were fully acclimated to wheels. *P < 0.05; **P < 0.01.

Figure 3

A) Orexin KO mice run less than WT mice, however, the temporal pattern of running is preserved. In both groups, 98% of running occurs during the dark period. B) In orexin KO mice, cataplexy is more frequent with running wheels unlocked, but the distribution of cataplexy across the dark period is similar to that seen with locked wheels. *P < 0.05; **P < 0.01.

Figure 4

Orexin KO mice initiate but do not sustain bouts of running. A) Orexin KO mice begin running as frequently as WT mice. B and C) The average running speed and maximal speed (top 10% of running epochs) in KO mice are similar to WT littermates. D) The mean duration of running bouts is much shorter in orexin KO mice. **P < 0.01.

Figure 5

The patterns of locomotor activity around sleep and cataplexy. Both WT and orexin KO mice have gradual reductions in wheel running prior to sleep, and then wheel running gradually increases after sleep. In contrast, prior to cataplexy, orexin KO mice have larger amounts of running that abruptly stop 10 to 20 seconds before cataplexy, and then they resume running within a minute after cataplexy. Graphs show the mean amount of wheel running in each 10-sec wake epoch, aligned to the end or beginning of a bout of wakefulness. (Many wake bouts contain no running, so this does not represent running speed.)

Figure 6

Wheel running consolidates sleep/wake behavior during the dark period, especially in WT mice. Wheel running in WT mice produces more wakefulness during the dark period, and wakefulness occurs in considerably longer bouts. A similar pattern occurs in orexin KO mice, though the increase in wake bout duration is smaller. Additionally, wheel running doubles the number of cataplexy episodes in orexin KO mice. *P < 0.05; **P < 0.01.

Figure 7

Wheel running increase the probability of cataplexy. A) With running wheels locked, the cumulative number of transitions into cataplexy gradually increases over the first 15 min of wakefulness. With wheels unlocked, the number of transitions accumulates more rapidly. In both conditions, a few additional episodes of cataplexy occur after 15 min of wakefulness. B) The probability of transitioning into cataplexy in any minute of wakefulness is higher with running wheels unlocked. This difference is most apparent in the first 10 min of wakefulness and disappears by 15 min, probably because of a time-dependent increase in cataplexy.

Supplemental Figure 1

Running wheels consolidate wakefulness during the dark period. These time-weighted frequency histograms show that with running wheels unlocked, less wakefulness occurs in very short bouts and more wakefulness occurs in long to very long bouts. Note that even with unlocked running wheels, the wakefulness of orexin KO mice mainly occurs in mid-length bouts. *, p < 0.05; **, p < 0.01.