Pineal Melatonin And Locomotor-Activity Of Rats Under Gradual Illuminance Transitions

The locomotor activity and pineal melatonin patterns of adult male rats were compared under two different lighting regimes. The animals were kept 8 days under 12/12 h light/dark cycles with abrupt or slowly decreasing and increasing transitions (twilight periods about 2 h). The onsets of high activity and melatonin rise were phase-locked in the two conditions and related to about half-maximal illuminance level of the gradual dusk. The high activity of the control rats stopped 30-60 min before the abrupt light onset and the rats under the gradual lighting transitions ceased the locomotor activity at about 1 hour before the half-maximal illuminance. The melatonin peak levels were found 4 h before the abrupt lights-on time. Under the slow illuminance transitions the average melatonin peak was related to the illuminance level between maximum and minimum in the morning. Thus, both the melatonin rhythm and the rest-activity rhythm under the gradual dawn and dusk were adjusted according to about half-maximal illuminances in the present conditions.