[The effect of light of various wave lengths on suppression of nocturnal serotonin N-acetyltransferase activity in chick retina]

Abstract

Purpose: Effects of white and monochromatic light of various wave length on the night-time retinal serotonin N-acetyltransferase (NAT) activity were examined in chicks.

Material and methods: Male chicks (white leghorn; 3-4 weeks old) were used. Chicks 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, 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 performed 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 per 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 experiments 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 luxes). Retinas were isolated and frozen on dry ice. Exposure of animal 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 filtered with glass, narrow band interference filters, 7 nm half-peak band-width. The spectral wave length analysis for each interference filter was performed with the aid of Diode-Spectrophotometer and the light irradiance the three wave lengths used was measured with YSI Radiometer. To measure NAT activity, retinas were sonicated in 10 vols (w/v) in the proportion of 100 microliters of ice-cold 0.05 M Sodium phosphate buffer (pH 6.8). The enzyme activity was determined in supernatants of tissue homogenates with the radioisotopic method of Steinlechner with modifications of Nowak. Data were expressed as mean +/- standard error of mean (SEM) and analysed for statistical significance by analysis of variance and Student-Newman-Keuls test.

Results: The potency of the tested lights to suppress NAT activity of the retina had the following rank order. white > or = green (548 nm) > > blue (434 nm) > red (614 nm).

Conclusion: In chicks, the suppression of the nocturnal NAT activity produced by a short 5-min pulse of monochromatic light was partially reversible in the retina. The studied chick tissues were far less sensitive to pulses of monochromatic light than of white light.