Interindividual differences in the pattern of melatonin secretion of the Wistar rat

In vivo trans-pineal microdialysis was performed in male Wistar rats mainta ined under a 12 hr light:12 hr dark (LD 12:12) cycle. Collected dialysates were assayed by radioimmunoassay for melatonin concentrations. A non-linear regression was fitted through the obtained datapoints to determine the tim e points at which a 50% increase (IT50) and decrease (DT50) of the nocturna l melatonin peak were reached. In a first experiment, the nocturnal melaton in profiles of four animals were determined throughout 5 consecutive days. In a second experiment, we analysed the melatonin profiles during the night in rats originating from three different breeding colonies (Depre, Harlan, and Iffa-Credo). A low intraindividual variability was found on the phase markers IT50 and DT50, as on peak duration of melatonin rhythms estimated o ver 5 subsequent days in the same animal. In contrast, animals showed a lar ge interindividual variability in their profile phase markers and the value s were dependent on the origin of the breeding colony. Each rat colony was characterized by early or late IT50 and DT50 as long or short peak length. It is concluded from experiment 1 that the melatonin rhythm is a very stabl e circadian marker. Nevertheless, great caution must be taken in the choice of animal groups while studying circadian rhythms due to the large interin dividual variability observed in experiment 2. Therefore, as the technique allows the use of the animal as its own control, the present study demonstr ated that the use of the microdialysis technique is of interest in studies on the circadian system.