Entrainment of Circadian Activity Rhythms in Rats to Melatonin Administered at T Cycles Different from 24 Hours

Entrainment of running wheel activity in DD was studied in adult male Long Evans rats exposed to cycles of a constant dose of melatonin (MEL; 100 µg/h) infused subcutaneously. The period (T) of the MEL cycle was initially kept at 24 h until stable entrainment was established; T was then changed in a stepwise manner, and each new T value was maintained for at least 20 cycles. Entrainment by phase advance occurred near circadian time 12 (activity onset), and the range of entrainment was between 30 and 35 min. The negative phase angle difference between activity onset and MEL onset increased as T values approached the entrainment limit, whereas no change in the duration of daily activity periods was found. No difference was observed between pre- and posttreatment values of the endogenous circadian period; hence, no aftereffects were found for any T value. These results indicate that the functional properties of entrainment to MEL are similar to those of entrainment to light, suggesting that both zeitgebers share a common timing mechanism.