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.