Effects of melatoninergic agonists on light-suppressed circadian rhythms in rats

This study addressed the question whether light-suppressed circadian rhythm s in cardiovascular parameters in rats could be restored by melatonin and a synthetic analogue. Blood pressure, heart rate, and locomotor activity wer e monitored by radiotelemetry in six Sprague-Dawley rats. After synchroniza tion to a 12:12 light/dark (LD) schedule (lights on at 0700 hours, 100 lux) , rats were kept in constant light (LL) of low intensity (5-10 lux) for II weeks. After 3 weeks of LL, rats received daily intraperitoneal (i.p.) inje ctions at 1900 hours of vehicle, the melatonin agonist S-21767 (5 mg/kg) an d melatonin (I mg/kg). Spectral power, 24-h amplitudes and the differences between day and night means were calculated as measures of circadian rhythm icity. During LL a lengthening of the endogenous period to 26 h was observe d, which was accompanied by a continuous decrease in circadian amplitude in all parameters monitored until, in the third week of LL, circadian rhythmi city was almost abolished. Neither vehicle, S-21767 nor melatonin were able to restore circadian rhythms in blood pressure and locomotor activity. In contrast, both agonists induced circadian rhythmicity in heart rate in two out of six rats. The day/night difference in heart rate of all animals was significantly increased by S-21767 and, to a smaller extent, by melatonin, whereas the circadian amplitude was not affected. In conclusion, melatonin and the synthetic agonist were able to partially synchronize circadian rhyt hmicity in heart rate during constant light, but could not restore circadia n rhythms in blood pressure. (C) 1998 Elsevier Science Inc.