Short‐term exposure to constant light promotes strong circadian phase‐resetting responses to nonphotic stimuli in Syrian hamsters

May 19, 2004The European journal of neuroscience

Short-term constant light exposure increases circadian resetting to non-light cues in Syrian hamsters

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Abstract

Phase advances to 8-OH-DPAT and sleep deprivation during constant light were 11 hours and 3 hours, respectively.

Circadian rhythms can be shifted by behavioral stimuli through serotonin and neuropeptide Y inputs to the suprachiasmatic nucleus.
Constant light exposure suppressed locomotor activity and eliminated the daily rhythm of serotonin release.
Phase advances in response to treatments were significantly greater in constant light compared to light/dark cycles.
Type 0 phase resetting was observed with constant light treatments, indicated by phase-transition curves with slopes close to 0.
Re-establishing serotonin levels in the suprachiasmatic nucleus reduced the phase advances induced by serotonin agonists.

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Behavioral (nonphotic) stimuli can shift circadian rhythms by serotonin (5-HT) and/or neuropeptide Y (NPY) inputs to the suprachiasmatic nucleus (SCN) circadian clock. Based on the idea that behavioral phase resetting is modulated by endogenous changes in postsynaptic sensitivity to such transmitters, hamsters were exposed to constant light (LL; approximately 250 lx) for 1-3 days, which suppresses locomotor activity and eliminates the daily rhythm of SCN 5-HT release measured by microdialysis. Groups subjected to brief LL or maintained under a light/dark cycle (LD) received phase-resetting treatments with the 5-HT(1A,7) agonist (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) or sleep deprivation (SD). Animals were released to constant darkness at the start of the treatments. Phase advances to 8-OH-DPAT and SD during the day were 11 and 3 h for LL vs. 2 and 1 h for LD, respectively. Phase delays during the night were -12 and -5 h for LL vs. no responses for LD, respectively. Phase-transition curves for both LL treatments had slopes approximating 0, indicative of Type 0 phase resetting. For all treatments, the degree of locomotor suppression by LL was not correlated with the phase shift magnitude. Re-establishing locomotor activity by overnight food deprivation did not prevent potentiated shifting to SD. However, re-establishing peak extracellular 5-HT levels by intra-SCN 5-HT reverse microdialysis perfusion in LL did significantly reduce potentiated 8-OH-DPAT phase advances. Constant light also enhanced intra-SCN NPY-induced phase advances during the day (6 vs. 2 h for LD). These results suggest that LL promotes Type 0 phase resetting by supersensitizing 5-HT and/or NPY postsynaptic responses and possibly by attenuating the amplitude of the circadian pacemaker, thus enhancing circadian clock resetting nonspecifically.

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Short‐term exposure to constant light promotes strong circadian phase‐resetting responses to nonphotic stimuli in Syrian hamsters

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