Effects of melatonin treatment and environmental lighting on the ultrastructural appearence, melatonin synthesis, norepinephrine turnover and microtubule protein content of the rat pineal gland.

Melatonin administration or exposure of rats to darkness for two weeks induced comparable changes in pineal ultrastructure, compatible with a generalized organ's activation. These include an increased number of ribosomes, procentrioles and microtubules, prominent nucleoli and Golgi apparatus, and annulate lamellae. Melatonin treatment resulted in a dose-dependent increase of hydroxyindole-O-methyl transferase and serotonin-N-acetyltransferase activities. In addition it increased by 85% the colchicine binding capacity of pineal homogenates, an estimation of the microtubule protein content of the gland. Pineal norepinephrine turnover was not affected by melatonin treatment. These data indicate that the pineal itself is a target organ for exogenously administered melatonin. Key words: Pineal gland, melatonin, norepinephrine, tubulin.