Modulation of pineal activity during the 23rd sunspot cycle: melatonin rise during the ascending phase of the cycle is accompanied by an increase of the sympathetic tone.

In two groups of female CD-rats nocturnal urine (19-23 h, 23-3 h, 3-7 h) was collected at monthly intervals over 658 days (I: 1997-1999) and 494 days (II: 1999-2000) coinciding with the ascending limb (1996-2000) of the 23rd sunspot cycle (1996-2008). The excretion of 6-sulfatoxymelatonin (aMT6s: I, II) was determined as well as the ratio of noradrenaline/adrenaline (NA/A: I) reflecting the activity of the sympathetic nervous system. AMT6s was higher in II than I (19-7 h: +24%; P < 0.001; 23-3 h: +30% and 3-7 h: +17%, P < 0.001), and progressively increased (19-23 h) showing linear regressions (1: R = +0.737, P = 0.003; II: R = +0.633, 0.008) which correlated (I) with the Planetary Index (Ap: R = +0.598, P = 0.020), an established estimate of geomagnetic disturbances due to solar activity. NA/A rose at all intervals (I: 46-143%) correlating with Ap (R = +0.554-0.768; P = 0.0399-0.0013). These results indicate that melatonin secretion rises as solar activity increases during the ascending limb of a sunspot cycle accompanied by growing geomagnetic disturbances (Ap) which elevate the sympathetic tone and thus affect the pineal gland, initially stimulating the activity of arylalkylamine N-acetyltransferase and subsequently fostering the expression of N-acetylserotonin O-methyltransferase (rate-limiting enzyme for melatonin biosynthesis) if Ap increases further. The potential (patho) physiological significance of these findings is discussed and the need for a systematic continuation of such studies is emphasized.