Neuropeptides and Thyroid Function and Growth II. Intrathyroidal Peptidergic Nerves and Neuropeptides Located in Parafollicular (C) Cells

Numerous intrathyroidal factors of neuropeptide nature are believed to directly influence thyroid hormone secretion, thyroid blood flow and growth processes of this gland. Apart from adrenergic and cholinergic innervation also peptidergic nerve fibers, containing vasoactive intestinal peptide (VIP), peptide histidine isoleucine amide (PHI), neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP) and gastrin/cholecystokinin (CCK-4, CCK-8) have been localized in the thyroid gland. In contrast, somatostatin (SS) has not been found in thyroid nerve fibers, however, its presence has been ascertained in C cells. Besides calcitonin (CT) and SS, these cells contain CGRP and katacalcin, while in tumors derived from C cells, VIP, neurotensin, bombesin, pancreatic polypeptide (PP) and gastrin-releasing peptide can additionally be demonstrated. Moreover, TRH immunoreactivity has been reported in the thyroid gland, suggesting that this neuropeptide acts locally. Out of all the afore mentioned neuropeptides VIP, PHI, TRH, and perhaps SP, seem to be the stimulators of thyroid follicular cell (TFC) function. In turn, SS has an inhibitory action on TSH-induced thyroid hormone secretion and NPY potentiates norepinephrine (NE)-suppressing action on TSH-induced thyroid hormone secretion. Furthermore, VIP and PHI markedly increase thyroid blood flow, whereas NPY does the opposite. Studies dealing with the influence of neuropeptides on thyroid growth are rather scarce. We have recently shown a definite involvement of SS in a local (paracrine ?) control of thyroid follicular cell proliferation; SS inhibited both basal and TSH-stimulated thyroid follicular cell proliferation, as well as 3[H]-thymidine incorporation into DNA of thyroid follicular cells in the organ cultured rat thyroid gland. Also CT has been suggested to inhibit thyroid growth.