Search for Seasonal Rhythmicity of Pineal Melatonin Production in Rats Under Constant Laboratory Conditions: Spectral Chronobiological Analysis, and Relation to Solar and Geomagnetic Variables

Abstract

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. (Author correspondence: [email protected])

Keywords:

• Circannual
• Pineal
• Seasonal Geomagnetic field
• 6-Sulfatoxymelatonin
• Sunspots
• Urine

ACKNOWLEDGMENTS

This work is dedicated to Prof. Dr. Lutz Vollrath on his 75th birthday who has been supporting our work in many beneficial ways and thus helped us to remain on this difficult scientific track.

We express our special gratitude to Mrs. Ilse Herget (T-Systems International GmbH, Darmstadt, Germany) and Dr. Hugo Lehmann (Swisscom Innovation Competence Center, Swisscom AG, Bern, Switzerland) for constant help and support during the experiments as well as for constructive discussions when preparing the manuscript. We thank Mrs. Katalin Neumann for her persistent and reliable care of our animals throughout most of the experiments as well as for her help during urine collections, and Mr. Thomas Loppnow who helped maintaining the exposure units in our studies.

We thank Mr. H. Desneuves and Mr. L. Gouthière for their help to be familiarized with their chronobiological software TSA Cosinor 6.3 as well as for reading the manuscript.

Declaration of Interest: The authors report no conflicts of interest. The authors are alone responsible for the contents and writing of the paper. These investigations were funded by the Former Center of Technology of Deutsche Telekom AG, Darmstadt, Germany designing and providing the experimental exposure setup and funding the initial two long-term studies (1997–2000), Swisscom Innovation Competence Center of Swisscom (Schweiz) AG, Bern, Switzerland, funding the third and part of the fourth long-term study (2002–2006), and IBL International GmbH, Hamburg, Germany funding part of the fourth long-term study (2005–2006).