Journal of Neuroscience, Vol 7, 4017-4033, Copyright © 1987 by Society for Neuroscience
Serotonin-storing secretory granules from thyroid parafollicular cells
JM Barasch, H Tamir, EA Nunez and MD Gershon
Department of Anatomy and Cell Biology, Columbia University College of Physicians and Surgeons, New York, NY 10032.
The subcellular storage of 5-HT was studied in sheep thyroid parafollicular
cells. These cells are neural crest derivatives and were investigated as a
serotonergic model system. Light and electron microscopic
immunocytochemistry was used to examine the distributions of 5-HT, 45 and
56 kDa forms of 5-HT binding protein (SBP), and calcitonin. A single type
of parafollicular cell was found that contained calcitonin, 5-HT, and 45
kDa SBP but not 56 kDa SBP. The secretory granules of parafollicular cells
all displayed calcitonin immunoreactivity, and many were also
immunoreactive for 5-HT and 45 kDa SBP. Granules were isolated, first by
size and then by density, on successive metrizamide gradients. These
provided a granular fraction that was enriched in calcitonin, endogenous
5-HT, and 45 kDa SBP. Immunoblots confirmed the presence of 45 kDa SBP in
the isolated granules and in suspensions of parafollicular cells that were
purified by an affinity chromatographic technique. Parafollicular cell
granules did not appear to contain substantial stores of ATP. Granules
isolated on Percoll gradients were morphologically homogeneous and took up
3H-5- HT. The specificity of this uptake was confirmed by quantitative
electron microscopic radioautography. The granular uptake of 3H-5-HT was
inhibited by reserpine (10 microM). It is concluded that parafollicular
cell granules are different from other amine-storing vesicles that do
contain ATP; nevertheless, since parafollicular cell granules store 5-HT
and have the same 45 kDa SBP as is found in serotonergic axon terminals,
parafollicular cell granules may be analogous to the synaptic vesicles of
serotonergic neurons.