MELATONIN FACILITATES SYNCHRONIZATION OF SPARROW CIRCADIAN-RHYTHMS TO LIGHT

We recorded circadian locomotor activity rhythms of house sparrows (Passer domesticus) exposed to low-amplitude light-dark cycles (2:1 lux) with periods of 22.5 or 24.5 h. Under these conditions the circadian rhythms of the majority of the birds were not synchronized by the light cycle but either free-ran or showed relative coordination. However, when melatonin was administered continuously via subcutaneous silastic implants the rhythms became synchronized. It is proposed that melatonin facilitates synchronization either by weakening the circadian oscillatory system thereby in creasing its range of entrainment, or by enhancing circadian sensitivity to the light zeitgeber. In general, the re suits suggest that melatonin, besides its well-known phasic effects on the circadian system also has important tonic effects modifying the ease with which circadian systems can be entrained.