Entrainment of locomotor activity rhythm in pinealectomized adult Syrian hamsters by daily melatonin infusion.

Melatonin entrains circadian rhythms in several species of rodents, but a role for melatonin as a Zeitgeber in the adult Syrian hamster is debated. The aim of this study was to define the conditions of daily programmed melatonin infusion in which an entrainment of the locomotor activity rhythm is obtained in adult male Syrian hamsters. The animals were pinealectomized, cannulated with a subcutaneous infusion system and submitted to dim red light conditions. They were initially daily infused with vehicle until free-running was established. Then, the animals were divided into three experimental groups, each group corresponding to a specific melatonin dose and infusion duration: (1) 10 microg melatonin/h for 5 h; (2) 30 microg melatonin/h for 5 h; and (3) 50 microg melatonin/h for 1 h. Of the total 64 hamsters, 37 hamsters fully entrained to the melatonin infusion regardless of whether the animals expressed during pre-treatment a free-running period (tau)< or >24 h, 20 animals presented a transient entrainment and seven did not entrain. Of the 37 animals entrained, withdrawal of melatonin re-established free-running rhythms, although often with a different tau compared with that observed during pre-treatment. These results indicate that after a long time of daily infusion, melatonin is able to entrain the free-running rhythm in adult Syrian hamster. The mechanism involved is not known, but the change in tau observed after melatonin treatment in some animals suggests that melatonin, directly or indirectly, affects the functioning of the clock.