Melatonin effects on brain. Interaction with microtubule protein, inhibition of fast axoplasmic flow and induction of crystaloid and tubular formations in the hypothalamus.

[3-H]Melatonin administered in vivo in the rat cisterna magna became associated with a vinblastine-precipitable protein. Melatonin treatment decreased microtubule protein content by 44% in the arcuate-median eminence region and by 19% in the remaining hypothalamic block, being without significant effect on the cerebral cortex. Superior cervical gangliectomy but not pinealectomy increased microtubule protein content of the rat hypothalamus. Norepinephrine brought about a significantly greater decrease in hypothalamic microtubule protein levels of ganglionectomized rats than in sham-operated or in ganglionectomized-pinealectomized animals. Melatonin treatment induced in most of the axons ending in the pericapillary zone of the rat median eminence crystaloid and tubular formations. Rapid axonal transport in retinal ganglion cells of rabbits was inhibited to the extent of 71.9 and 87.2% by previous exposure to 1.5 of 15 mu g of melatonin intravitreally; melatonin did not affect retinal protein synthesis in this experimental model. These results suggest that melatonin interacts significantly with microtubule or actin-like protein in brain.