HISTOCHEMISTRY OF THE MAGNOCELLULAR NEURONS IN THE POSTERIOR HYPOTHALAMUS, WITH SPECIAL REFERENCE TO MAO ACTIVITY AND ABILITY OF 5HTP UPTAKE AND DECARBOXYLATION

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Morphological study at the light and electron microscopic levels was carried out on the posterior group of the magnocellular neurons in the hypothalamus of the rat as revealed by monoamine oxidase (MAO) histochemistry. A conspicuously high activity of type B MAO was found in the perikarya and dendritic processes of these neurons. The dendritic processes ramified into fine varicose fibers which ran in all directions throughout the hypothalamus. Axodendritic synapses were frequently observed on the MAO-positive dendritic trunks and varicosities, some of which were filled with the small round vesicles. The ventrally directed dendrites richly branched into a dense network at the basal surface of the posterior hypothalamus and penetrated the limiting astrocytes where they appeared to have a close association with cerebrospinal fluid. In addition, these neurons showed an ability to synthesize serotoin after administration of the immediate precursor, 5-hydroxytryptophan. The possible neuronal and humoral functions of the magnocellular neurons in the posterior hypothalamus is discussed.