Light Intensity and the Control of Melatonin Secretion in Rats

The effect of varying ambient light intensity on the phase and amplitude of urinary melatonin rhythms was studied in rats housed individually in metabolism cages. For 17 days one group (D) was exposed to alternating 12-hour periods of dim light (0.1-0.3 micro/cm2) and total darkness; a second group (L) was exposed alternately to dim light and bright ligh (45-110 micro/cm2). All animals were then exposed to constant dim light for 15 days, after which they were returned to their original lighting regimens (D or L). 18 days later, half of each group was killed at the midpoint of the dim light phase, and the other half 12 h later. Both groups excreted melatonin rhythmically when exposed to daily cycles in light intensity; the L animals excreted 69% of the total daily melatonin output during the dim light phase and the D group of rats excreted 70% during the dark phase. When placed under continuous dim light, L animals continued to excrete melatonin as before, but D rats excreted significantly less, and the melatonin rhythm was dampened. When returned to a diurnal light cycle, both groups again exhibited rhythms in melatonin excretion that were entrained to the light cycle. Animals killed at the time of day coinciding with diminished melatonin excretion had lower pineal and serum melatonin levels (0.2 +/- 0.1 ng/pineal; 30 +/- 8 pg/ml serum) than those killed 12 h later (2.0 +/- 0.4 ng/pineal; 57 +/- 20 pg/ml serum). These observations provide additional evidence that measurement of urinary melatonin levels gives an accurate index of melatonin secretion from the rat pineal. They also show that a given light intensity presented for a part of the 24-hour day (e.g., dim light; 0.1-0.3 micro/cm2) can be interpreted by the mammalian pineal as light or dark, depending on the light intensity available during the rest of the day.