MELATONIN FACILITATES SYNCHRONIZATION OF SPARROW CIRCADIAN-RHYTHMS TOLIGHT

We recorded circadian locomotor activity rhythms of house sparrows (Pa sser 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 r hythms of the majority of the birds were not synchronized by the light cycle but either free-ran or showed relative coordination. However, w hen melatonin was administered continuously via subcutaneous silastic implants the rhythms became synchronized. It is proposed that melatoni n facilitates synchronization either by weakening the circadian oscill atory system thereby in creasing its range of entrainment, or by enhan cing 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 e ase with which circadian systems can be entrained.