PURPOSE: Rat pineal gland synthesizes melatonin in circadian rhythm, with peak values in a dark phase of an imposed light-dark illumination cycle. Light is the most important environmental factor regulating the melatonin-generating system in this gland. Exposure to light causes a dramatic decline of the night-time levels of these melatoninergic parameters. Effect of white and monochromatic lights of various wavelength on the night-time pineal gland serotonin N-acetyltransferase (NAT) activity was examined in rats.
MATERIAL AND METHODS: Wistar rats (12 weeks old) were used. All animals were offered ad libitum access to standard food and water, maintained under an ambient temperature of 21 +/- 2 degrees C, 60 +/- 5% humidity, and exposed to 12 hr light: 12 hr dark illumination cycle for a minimum of 10 days before experiments. The day-time light intensity at the surface of the animals' cages was about 150 luxes. Each experiment was performed at least twice. During the fifth hour of the dark phase of the light-dark illumination cycle individually housed rats were exposed to either white or monochromatic light for 15 sec., 1, 5 or 15 min, and then killed by decapitation. Control animals were quickly decapitated under dim red light (2 luxes). Pineal glands were dissected out and frozen on dry ice. Five rats were sacrificed at each time point. Exposure of animals to light took place in 25 x 21 cm white plastic chamber. Light produced by 5 W 14 bulb (Osram) was passed through a cotton filter or narrow band interference filters, filtered with glass, +/- 7 nm half-peak band-width. The spectral wavelength analysis for each interference filter was performed with the aid of Diode-Spectrophotometer, and irradiance of the light of the three used wavelengths was measured with YSI Radiometer. The estimated peak wavelengths (lambda max) of the filters were: 434 nm (blue), 548 nm (green) and 614 nm (red). The NAT activity was determined in supernatants of tissue homogenates by the radioisotopic method of Steinlechner with Nowak's modifications.
RESULTS: NAT of rat pineal gland is very sensitive to the inhibition by light and the marked decline of the night-time NAT activity was observed after 15-sec. pulse of either white or green light (by 44%-white light and 37%-green light). Exposure of rats to white, green, and blue lights for 1 min. decreased NAT pineal activity by 56%, 46%, and 21%, respectively, while the 1 min. pulse of red light did not significantly alter the enzyme activity.
CONCLUSION: Interestingly, exposure of rats to any tested lights for as long as 15 min. suppressed NAT activity of rat pineal gland to similar extent, reaching 8-9% of the dark control value.
