[The effect of light with various wavelengths and impulse times on nocturnal suppression of N'acetyltransferase activation by serotonin in the pineal gland of the chick]

Abstract

Purpose: Chick pineal gland synthesise melatonin in circadian rhythm, with peak values in the 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 the melatoninergic parameters. Effect of white and monochromatic lights of various wavelength on the night-time pineal gland serotonin N-acetyltransferase (NAT) activity were examined in chicks.

Material and methods: Male chicks (white leghorn: 3-4-week old) were used. They were purchased on the day of hatching. All animals were offered ad libitum access to standard food and water, maintained under an ambient temperature of 27 +/- 2 degrees C (chick), 60 +/- 5% humidity, and exposed to 12 hr light: dark illumination cycle for a minimum of 8-10 days before experiments. The daytime light intensity at the surface of the animals' cages was about 150 luxes. Each experiment was done at least twice. At the beginning of the fourth hour of the dark phase of the light-dark illumination cycle groups of chicks (four animals/group) were exposed to either white or monochromatic light for 5, 10, 30 or 60 min, and then killed by decapitation. In another set of experiment birds were illuminated for 5 min, returned to darkness for additional 5, 15, 60 or 120 min, and then decapitated. Decapitation was done quickly under dim red light (2 lux). Pineal glands were isolated and frozen on dry ice. The exposure of an animal to light took place in 25 x 21 cm white plastic chamber. Light produced by 5 W 14 bulbs (Osram) was passed through a cotton filter or filtered with glass, narrow band interference filters, 7 nm half-peak band-width. The spectral wavelength analysis for each interference filter was performed with the aid of Diode-Spectrophotometer, and the irradiance of the light of the three used wavelengths was measured with YSI Radiometer. The estimated peak wavelengths (Imax) of the filters were: 434 nm (blue), 548 nm (green) and 614 nm (red). The NAT activity was determined in supernatants of tissue homogenates with the radioisotopic method of Steinlechner with modifications by Nowak.

Results: Significant decrease of the night-time NAT activity of the chick pineal gland was observed after a 5-min pulse of either white or green light. In birds illuminated by blue light a marked reduction (by about 22%) in the pineal NAT activity was found after 10-min exposure, whereas in the group of animals that were exposed to red light a significant decline of the enzyme activity in pineal gland was produced by a 30-min pulse. The enzyme activity started to increase after 15 min in the dark and reached the control values by 1 hr (red light) or 2 hrs (blue and green lights). Two hrs and 5 min after the exposure to white light (5 min light and 120 min dark) pineal NAT activity was still significantly lower (by 16%) than the activity found in the dark control group.

Conclusion: Suppressive effect of monochromatic (but not white) light on NAT activity was completely reversible in the pineal gland of chick.