Reentrainment of Rat Circadian Activity Rhythms: Effects of Melatonin

Experiments were conducted to determine whether daily melatonin injections could influence reentrainment of circadian locomotor activity rhythms in male Long‐Evans rats after phase shifts of the light dark (LD) cycle. When exposed to a 5‐h advance of the LD cycle, six out of 15 rats injected daily with melatonin at the new dark onset reentrained by a phase delay of the activity rhythm. The remaining nine melatonin‐injected rats and 14 of the 15 control injected rats phase advanced. Daily melatonin injections administered at the old dark onset did not alter direction of reentraining transients but decreased latency to reach steady‐state entrainment. In contrast, the effects of melatonin injections given at the end of both the old and new dark periods are difficult to interpret due to the effects of the injection procedure itself at these times. Following an 8‐h advance of the LD cycle, all melatonin‐injected rats phase advanced while all vehicle and uninjected control rats phase delayed when injections were given at the old dark onset. As has been found in other research areas, the time of day of melatonin administration appears to be a critical factor in its mode of action. It is yet to be established whether these results from the use of pharmacological doses of melatonin are indicative of endogenous melatonin's involvement in the circadian reentrainment mechanism.