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

In vivo trans-pineal microdialysis was performed in male Wistar rats maintained 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 time points at which a 50% increase (IT50) and decrease (DT50) of the nocturnal melatonin peak were reached. In a first experiment, the nocturnal melatonin 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 over 5 subsequent days in the same animal. In contrast, animals showed a large interindividual variability in their profile phase markers and the values 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 stable circadian marker. Nevertheless, great caution must be taken in the choice of animal groups while studying circadian rhythms due to the large interindividual variability observed in experiment 2. Therefore, as the technique allows the use of the animal as its own control, the present study demonstrated that the use of the microdialysis technique is of interest in studies on the circadian system.