DAILY INJECTIONS OF MELATONIN ENTRAIN THE CIRCADIAN ACTIVITY RHYTHMS OF NOCTURNAL RATS BUT NOT DIURNAL CHIPMUNKS

It is well known that daily injections of melatonin entrain the free-r unning rhythms of nocturnal rodents (rats and hamster) and diurnal sau ropside (birds and lizard). Here, we asked whether daily injections of melatonin entrain the free-running rhythm of the chipmunk, a diurnal rodent, and, if they do, is the phase relationship between the time of injection and onset of the activity interval similar to that in sauro pside rather than that of nocturnal rodents? Contrary to our expectati ons, daily injections of melatonin did not entrain the free-running rh ythm in 9 of 10 chipmunks, even when a high dose of melatonin (1 mg/kg , b.wt.) was used. These results indicate that the entraining effect o f daily injections of melatonin on free-running rhythm varies among ma mmalian species. (C) 1997 Elsevier Science B.V.