Seasonality of pineal melatonin production in the rat: possible synchronization by the geomagnetic field.

Pineal melatonin production was estimated by means of urinary 6-sulfatoxymelatonin (aMT6s) determination in two groups of female rats for 1 year each. Seasonal changes of nocturnal aMT6s excretion were found with peak levels in summer despite constant photoperiods. We hypothesize that the horizontal component H of the geomagnetic field may act as a seasonal zeitgeber because H shows a similar seasonal rhythm, and changes in the direction and intensity of H can affect pineal activity. The observed seasonal changes of pineal melatonin production stress that despite constant environmental conditions, endocrine experiments require consideration of season, neglect of which may lead to contradictory results.