Central depressant and nootropic effects of daytime melatonin in mice

BACKGROUND Effects of orally administered daytime melatonin on novelty induced behaviors and spatial working memory in mice were evaluated using the open field, the Y maze and the radial arm maze. PURPOSE To ascertain the possible nootropic and/or central excitatory or inhibitory effects of daytime oral melatonin in mice. METHODS Adult male mice from our colony, assigned to three and four groups for open field tests and memory tests respectively were given vehicle (normal saline), a standard drug Scopolamine at 0.5 mg/kg i.p, single dose, 30 minutes before behavioral study) or one of two doses of melatonin (5 and 10 mg/kg daily for a period of 30 days). All administrations were done between 8.00 a.m. and 9.00 a.m. daily. Behavioral tests were carried out on day 30 after administration. Results were analysed using a one-way ANOVA followed by a posthoc test (Student-Newman-Keul's) and expressed as mean ± S.E.M. RESULTS Open field tests revealed a significant reduction in rearing and grooming behaviors at both doses tested while no significant changes in horizontal locomotion were seen. Y maze studies showed an improvement in spatial memory in mice that received 5 mg/kg of melatonin when compared to scopolamine control. At 10 mg/kg, no significant improvement was seen. A significant increase in the radial arm maze spatial working memory following melatonin administration was seen at 5 mg/kg and 10 mg/kg compared to scopolamine control. Radial arm maze exploration was also significantly reduced. CONCLUSION The study demonstrates the ability of exogenously administered melatonin, to affect both central excitation and spatial working memory in mice even when given orally.