EXOGENOUS MELATONIN ACCELERATES REENTRAINMENT - ATTENUATION OF THE CIRCADIAN-RHYTHM OF METABOLIC-RATE IN THE CANARY, SERINUS-CANARIA

Administration of melatonin in the drinking water (200 mu g/ml in 1% e thanol) decreased the time of re-entrainment of the circadian rhythm o f the metabolic rate (measured as oxygen-uptake) of domestic canaries (Serinus canaria) after 10-h delay phase shifts of the light-dark (LD) cycle by 1.3 days on average. Associated with faster re-entrainment, the amplitude of the metabolic rhythm was attenuated by 46% on average on the first day after the shift as compared with about 25% in the co ntrols. After re-entrainnent, the amplitude of the metabolic rhythm du ring melatonin administration was about 23% lower than in the controls . The minimum resting metabolic rate increased by ca 5% on average dur ing treatment with melatonin. The results are consistent with the hypo thesis that constant high plasma levels of melatonin act on higher lev els of the circadian oscillatory system rather than by directly affect ing peripheral or central photoreceptors.