Action Spectra for Effects of Light on Hydroxyindole-O-Methyl Transferases in Rat Pineal, Retina and Harderian Gland

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Abstract
The relations between the spectrum emitted by a particular light source and the modifications in pineal, retina and harderian gland hydroxyindole-O-methyl transferases (HIOMT) were examined by exposing rats previously maintained for 7 days in darkness to each of 7 different light environments for 0 to 96 hr. Exposure to green, blue or yellow bulbs caused time-related decreases in pineal HIOMT activity; green light was most effective. No significant changes in pineal HIOMT were observed among rats exposed to red light; UV produced a transient stimulation of the pineal enzyme. Common fluorescent light (cool-white) was more effective than a light source that simulates natural sunlight (Vita-Lite) in decreasing pineal HIOMT. Retinal HIOMT was significantly inhibited in rats exposed to green, cool-white, and Vita-Lite bulbs for up to 96 hr; blue light produced a biphasic effect, i.e., inhibition for the first 48 hr and stimulation afterwards. Harderian gland HIOMT activity was not modified by any of the light sources. The exposure of rats to continuous green light for 17 days resulted in almost total loss of pineal HIOMT and in significant stimulation of the retinal enzyme; harderian gland HIOMT was not modified. When similar rats were subsequently placed in darkness for 7 days, retinal HIOMT activity remained stimulated and pineal HIOMT rose significantly. A second period of exposure to green light again depressed pineal HIOMT. These results indicate that the action spectrum for the photic control of rat pineal is similar to the absorption spectrum of rhodopsin, and that the “neuroendocrine” photoreceptor cells in the rat retina continue to function even after animals are continuously exposed for 17 days to green light. (Endocrinology91: 877, 1972)

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