Search for seasonal rhythmicity of pineal melatonin production in rats under constant laboratory conditions: spectral chronobiological analysis, and relation to solar and geomagnetic variables.

Earlier we reported that in a number of experiments pineal melatonin production in rats under constant laboratory conditions displayed seasonal rhythms but subsequently were not always able to confirm this. Since there was no indication under which conditions such rhythms may be present, we performed four consecutive identical experiments with untreated female Sprague-Dawley rats within the same animal room during 1997-2006. Nocturnal urine samples (19-23, 23-3, 3-7 h) were collected at monthly intervals over 494-658 d with 12 animals each in experiments I and II (1997-1999, 1999-2000), 30 animals in experiment III (2002-2004), and 15 in experiment IV (2005-2006). 6-Sulfatoxymelatonin (aMT6s) was measured by ELISA. The excreted aMT6s at each time interval as well as total nocturnal aMT6s-excretion (19-7 h) was submitted to standard statistical analyses as well as to a spectral chronobiological analysis to determine the period lengths of the components involved which was followed by processing with the single cosinor method. Seasonal rhythm components (circannual period length: 360 ± 60 d) were detected in experiment III (2002-2004) for the overall nocturnal excretion as well as for two sub-intervals (23-3 and 3-7 h) and in one night interval of experiment II (23-3 h). Multiple components with mostly short period lengths of around 100 d and some long ones of 500-650 d were found in the other experiments. Systematic MESOR and amplitude variations were observed during the experiments, being highest in experiment II (19-7 h, also 23-3 h and 3-7 h) and lowest in experiments I and IV. These results illustrate that seasonal melatonin rhythms are not a general phenomenon in female laboratory rats indicating an involvement of unknown environmental cues. As an extension of our earlier hypothesis regarding a seasonal Zeitgeber function of the horizontal intensity H of the geomagnetic field showing circannual variations, we assume further modulation by the 11-yrs' sunspot cycle which leads to geomagnetic disturbances and could facilitate seasonal aMT6s rhythmicity during specific years.