The parafollicular (PF) cell of the mammalian thyroid gland is a neural crest derivative that produces calcitonin and 5-hydroxytryptamine (5-HT). The cells remain plastic, even in adult animals, and express neuron-like properties (extension of neurofilament bearing neurites; production of calcitonin gene related peptide [CGRP] instead of calcitonin) when they are removed from the thyroid and exposed to nerve growth factor (NGF) or are grown in co-culture with bowel. PF cells are found within the basement membrane of thyroid follicles. It is proposed that the close relationship between thyroid follicular and PF cells facilitates development and maintenance of an endocrine phenotype in PF cells. It is also suggested that the mechanisms involved in the uptake and storage of 5-HT in the secretory granules of PF cells are analogous to those of the synaptic vesicles of serotonergic neurons. A specific 5-HT binding protein, SBP is present in both of these organelles, and is not found in the 5-HT-storing subcellular organelles of cells that are not embryologically derived from neurectoderm. PF cells also decrease the internal pH of their secretory granules by increasing the Cl- conductance of the granular membranes (providing for the movement of a counterion for the inward translocation of H+) in response to stimulation of the cells by secretogogues (increased ((Ca2+)i) or TSH). Specific aims of the current proposal include determination of the role of cells in the acquisition and/or maintenance of the endocrine phenotype of PF cells. PF cells will be chromatographically purified and cultured in the presence and absence of NGF. The effect of thyroid hormones, co-culture with F cells, and co-culture with bowel on the phenotype (and phenotypic plasticity) of PF cells will be ascertained. The plasticity of ultimobranchial cells, the avian counterparts of PF cells, will be analyzed, and if found to be analogous to the mammalian system, used to study the physiological significance of the microenvironment in determining the expression of this phenotype. The role of acidification of the interior of PF granules in the ability of the granules to take up and retain 5-HT will be studied. The role of acidification in the TSH-stimulated release of 5-HT will also be examined. Mechanisms responsible for opening the Cl- channel in the membranes of PF granules following stimulation of the cells with TSH will be investigated. In particular, the roles of ((Ca2+)i) and/or specific protein kinases in the process will be studied. These studies are intended to help in the understanding of the potential of PF cells to contribute to the pathogenesis of thyroid diseases, and may also aid in the development of means of preventing or treating the PF cell-derived, medullary thyroid carcinoma.