Electron microscopic radioautographic localization of tritiated serotonin, synthesized from tritiated 5-hydroxytryptophan, showed serotonin to be stored within calcitonin granules in parafollicular cells of the prehibernating bat. Kinetic studies of serotonin storage indicated that serotonin is added to the granules after the concentration of calcitonin. Serotonin is probably synthesized in the cytosol of the cell but is then taken up by mature prosecretory granules. The uptake of serotonin seems to be associated with the acquisition of a membrane by the granules in the Golgi apparatus and is not fully operative in granules within the cisternae of the rough endoplasmic reticulum. Granules which have lost their uniform dense core during the inactivation of parafollicular cells that occurs in early hibernation have also lost the ability to take up serotonin. Secretion of labeled granules begins in active cells within 24 hr after isotope injection and leads to the labeling of follicular cells and luminal colloid. Biochemically, the level of serotonin in the thyroid gland was high (9.32 ng/gland) and showed seasonal fluctuation. An elevation (to about 22 ng/gland) at the start of hibernation, with a subsequent slow decline (to about 3.5 ng/gland), was associated with inactivation of the parafollicular cells, the retention of granules and their slow intracellular removal by autophagy. A brief rise on arousal (to 49 ng/ gland) was probably associated with the resumption of synthesis of serotonin occurring somewhat before new granules began to be released after hibernation. These studies indicate that serotonin and calcitonin are linked by storage in the same granules. The release of one substance would therefore probably be associated with release of the other. The role of serotonin is most likely a local one. (Endocrinology90: 1008, 1972)
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Nunez et al. (1972) studied this question.