Differential effects of constant light on circadian clock resetting by photic and nonphotic stimuli in Syrian hamsters

Sep 20, 2005Brain research

Constant light changes how light and non-light cues reset the body clock in Syrian hamsters

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Abstract

Phase shifts in circadian rhythms of Syrian hamsters can be significantly enhanced by 2 days of constant light exposure.

Behavioral arousal during rest periods can shift circadian rhythms in Syrian hamsters.
Exposure to constant light for 1 or 2 cycles can greatly potentiate these phase shifts.
Significant phase delays were observed with NMDA microinjections, mimicking light exposure effects.
Phase shifts induced by NMDA did not vary based on prior lighting conditions.
A nonphotic stimulus resulted in greater phase shifts after exposure to constant light.
Results suggest that constant light affects the response to nonphotic inputs more than the amplitude of the circadian pacemaker.

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Circadian rhythms in Syrian hamsters can be phase shifted by behavioral arousal during the usual rest phase of the circadian rest-activity cycle. Phase shifts can be greatly potentiated by exposing the animals to constant light for 1 or 2 cycles. This could reflect a change in a specific nonphotic input pathway to the suprachiasmatic nucleus (SCN) circadian pacemaker, or it could be caused by a change in the amplitude of the pacemaker. If the latter, then phase shifts to any stimulus, including those activating the photic input pathway, should be potentiated. This hypothesis was tested by measuring phase shifts induced by microinjections of NMDA (500 nl, 10 mM) into the SCN area of hamsters exposed to constant light or dark for 2 days. NMDA induced significant phase delay shifts that mimicked those induced by light exposure early in the night. The magnitude of these shifts did not differ by prior lighting condition. Shifts induced by NMDA (200 nl, 10 mM) microinjections on day 3 and 13 of LL also did not differ. Phase shifts induced by a nonphotic stimulus (3 h of running stimulated by confinement to a novel wheel) were significantly potentiated by 2 days of exposure to constant light. These results indicate that exposure to constant light for 2 circadian cycles differentially affects phase resetting responses to photic and nonphotic inputs to the circadian pacemaker, suggesting that potentiation of shifts to nonphotic stimuli reflect changes in a nonphotic input pathway rather than in an amplitude dimension of the circadian pacemaker.

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Differential effects of constant light on circadian clock resetting by photic and nonphotic stimuli in Syrian hamsters

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