The behavior of rats in response to changes in the light regime and administration of melatonin.

This report describes studies of the effects of shifts in the daily light/dark rhythm and administration of different doses of melatonin on the behavior of rats with different activity in the open field test. Experiments were performed on 172 Wistar rats kept in a natural or an inverted light/dark regime. Groups of rats received 1 ml of physiological saline or melatonin (1 or 2 mg/kg, i.p.) or were not injected. Inversion of the light/dark regime led to a shift in the rhythm of movement activity and eliminated differences between measures of daytime and night-time activity in control rats not given injections. The effect of melatonin on the movement activity of rats in conditions of natural and inverted light regimes depended on the dose given, the time of administration, and the initial characteristics of the animals' behavior. The data obtained here show that use of melatonin to correct impairments induced by the shift in the light/dark cycle should take consideration of the individual behavioral characteristics of the subjects.