Abstract
Input from the light/dark (LD) cycle constitutes the primary synchronizing stimulus for the suprachiasmatic nucleus (SCN) circadian clock. However, the SCN can also be synchronized by non-photic inputs. Here, we hypothesized that the vestibular system, which detects head motion and orientation relative to gravity, may provide sensory inputs to synchronize circadian rhythmicity. We investigated the resynchronization of core temperature (Tc) circadian rhythm to a six-hour phase advance of the LD cycle (LD + 6) using hypergravity (2 G) as a vestibular stimulation in control and bilateral vestibular loss (BVL) rats. Three conditions were tested: an LD + 6 exposure alone, a series of seven 2 G pulses without LD + 6, and a series of seven one-hour 2 G pulses (once a day) following LD + 6. First, following LD + 6, sham rats exposed to 2 G pulses resynchronized earlier than BVL rats (p = 0.01), and earlier than sham rats exposed to LD + 6 alone (p = 0.002). Each 2 G pulse caused an acute drop of Tc in sham rats (−2.8 ± 0.3 °C; p < 0.001), while BVL rats remained unaffected. This confirms that the vestibular system influences chronobiological regulation and supports the hypothesis that vestibular input, like physical activity, should be considered as a potent time cue for biological rhythm synchronization, acting in synergy with the visual system.
Highlights
In most organisms, this process of entrainment occurs through the permanent exposure to light and darkness alternance induced by the Earth’s rotation[3]
We evaluated the effect of a repeated one-hour (1 h) hypergravity 2 G pulse on the timing of circadian rhythm resynchronization after a six-hour LD phase shift in bilateral vestibular loss (BVL) and sham rats
The main results show that stimulating the vestibular system through short periods of 2 G hypergravity affects the circadian rhythm of Tc after a six-hour LD cycle phase shift
Summary
This process of entrainment occurs through the permanent exposure to light and darkness alternance induced by the Earth’s rotation[3]. A vestibular stimulation through chronic hypergravity (2 G) paradigm into a centrifuge[9,10] caused a transient loss of circadian rhythm of core temperature (Tc) for seven to 15 days. The vestibular system could assist the visual system in entraining circadian rhythms and the sleep/wake rhythmicity. To confirm and extend previous work[12], the present study focused on vestibular stimulation as a potential synchronizer of circadian rhythms, which can accelerate the resynchronization of Tc after visually induced desynchronization. Our aim was to test whether a daily 1 h hypergravity vestibular stimulation could improve the timing of Tc rhythm resynchronization of sham rats, using vestibular sensors. We hypothesized that the Tc rhythm resynchronization pattern in BVL rats with no functional gravity receptors should not be affected by the vestibular stimulation
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