Serotonin’s role in how constant light changes daily activity patterns in Syrian hamsters without using light signals

Short-term constant light exposure (1-3 days) potentiates phase shifts induced by serotonin stimulation, leading to approximately 12-hour phase-advance shifts.

• Brief light exposure alters the effect of serotonin agonists on the body's internal clock.
• Phase-advance shifts occur during the early morning when serotonin normally has no effect.
• Lower dosages of the serotonin agonist can produce significant phase shifts when combined with brief light exposure.
• The observed phase shifts are mediated by 5-HT1A receptors, as shown by the blocking effects of specific antagonists.
• Intra-SCN microinjection of the serotonin agonist produces effects similar to systemic administration under brief light conditions.
• The phase shifts are not due to increased serotonin receptor binding in the suprachiasmatic nucleus after light exposure.

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