Thyroid glands were removed from active bats, caught in their natural habitat during the prehibernating phase of their annual life cycle. Following freeze-drying and formaldehyde treatment, occasional parafollicular cells exhibited an intense greenish-yellow fluorescence indicative of endogenous monoamine. Comparison of the color of fluorescence with serotonin-containing enterochromafnn cells and norepinephrine-containing adrenergic nerve fibers indicated the fluorescence was probably due to the presence of serotonin. The number of fluorescent cells was increased following injection of the serotonin precursor, 5-hydroxytryptophan. Uptake of 5-hydroxytryptophan by parafollicular cells was confirmed by radioautography, although uptake was limited to a minority of the cells. Parafollicular cells did not take up serotonin itself. Metachromatic, interfollicular cells, probably mast cells, did not fluoresce nor did they incorporate 5-hydroxytryptophan. Since a minority of the parafollicular cells of prehibernating bats exhibit large, atypical, secretory granules and a similar minority appeared to contain endogenous serotonin, it is suggested that the large granule-containing cells might correspond to those with endogenous monoamine. (Endocrinology86: 160, 1970)
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Gershon et al. (1970) studied this question.