EFFECTS OF DAILY MELATONIN ADMINISTRATION ON CIRCADIAN ACTIVITY RHYTHMS IN THE DIURNAL INDIAN PALM SQUIRREL (FUNAMBULUS-PENNANTI)

Exogenous melatonin induces phase shifts in circadian rhythms accordin g to a phase response curve in nocturnal rodents, several nonmammalian diurnal species, and humans. Daily administration of melatonin entrai ns rhythms within a narrow circadian window of sensitivity in these sp ecies. Entrainment to exogenous melatonin has not previously been demo nstrated in a (nonhuman) diurnal mammal. The authors examined the effe cts of daily melatonin administration (via food) in the diurnal Indian palm squirrel, Funambulus pennanti. The effects of melatonin or vehic le were examined at two times of day: zeitgeber time 0 (ZT 0: light on set time) and ZT 12 (dark onset time). In addition to melatonin-and ve hicle-treated squirrels, there was a third group of squirrels that rec eived no treatment. Squirrels were held initially under 12:12 light-da rk (LD) cycles, and melatonin (1 mg/kg) or vehicle was administered in food (a raisin) at either ZT 0 or ZT 12 for a total of 17 days. On th e third day of treatment, constant lighting (LL) was imposed. Treatmen t continued at the same ZTs for a further 14 days. The number of days before free-running commenced under constant conditions was assessed f or squirrels in each treatment group. Results showed that regardless o f the ZT of administration, the number of days before free-running com menced was significantly greater in melatonin-treated squirrels than i n vehicle-treated and untreated squirrels, and there was no difference between vehicle-treated and untreated squirrels. Although there was n ot a significant difference in the number of days before free-running commenced between the two times of administration, the results showed a trend for greater sensitivity to melatonin at ZT 12. This study has therefore demonstrated that the palm squirrel circadian system is entr ainable to melatonin at both times of day tested, ZTs 0 and 12. This f inding is in contrast to previous melatonin entrainment studies in oth er species, where entrainment generally occured at only one phase, aro und circadian times 10 to 12. Interspecies differences in response to melatonin were discussed.