Continuous melatonin administration accelerates resynchronization following phase shifts of a light-dark cycle

Circadian rhythms of most passerine birds and some reptiles depend to a considerable extent on the presence of periodic melatonin, which is considered to be part of the central pacemaking system. Recent results on house sparrows have suggested that melatonin, apart from its periodic effects on the circadian system, may also exert effects derived from a constant action: Continuous melatonin applied through subcutaneously implanted silastic tubing enhanced the synchronization response to a low-amplitude light-dark zeitgeber, indicating some kind of sensitization to zeitgeber stimuli. In the present study we tested the prediction derived from these results, that speed of resynchronization after a phase shift of a light-cycle should be enhanced if melatonin is continuously administered. We found that, indeed, house sparrows required less time to resynchronize to an 8 h advance or delay phase shift of a low-amplitude light-dark cycle while carrying a silastic implant filled with melatonin than while carrying an empty implant. The effect is suggested to result from either one or a combination of the following mechanisms: (i) An increased circadian visual sensitivity, (ii) a diminished amplitude of the circadian oscillation, iii) an altered feedback of the locomotor activity to the oscillatory system.