The objectives of the proposed research for the final two years of the project period are to complete the work already begun and are as follows: (1) Test the hypothesis that 5-hydroxytryptamine (5-HT) is involved as a neurotransmitter in the myenteric plexus of the mammalian gut. (2) Determine the properties of 5-HT mechanisms in the myenteric plexus such as transport, storage and synthesis of the amine. (3) Compare 5-HT mechanisms in the myenteric plexus with those of non-neural peripheral cells which also store 5-HT, such as thyroid parafollicular cells, and mast cells. (4) Determine the function of 5-HT in the thyroid and its relationship to the polypeptide calcitonin, which may share storage vesicles with 5-HT. (5) Study the role of 5-HT in the regulation of electrical activity, characteristic of REM in the oculomotor and visual systems (PGO waves). Morphologic, biochemical and physiological methods will continue to be used. Three newer approaches, not outlined in the initial application will be expanded. These include the growth of the muscularis externa and myenteric plexus in organotypic tissue culture, the immunohistochemical localization of tryptophan hydroxylase, and the isolation of a serotonin binding protein from the myenteric plexus. Other approaches to be continued include electron microscopic radioautography for the identification of axons by their transmitter content and an ontogenetic study of the development of the putative 5-HT neurons of the myenteric plexus. BIBLIOGRAPHIC REFERENCES: Nunez, E.A. and Gershon, M.D. (1976) Appearance and disappearance of multiply ciliated follicular cells during development of the dog's thyroid gland. Anat. Rec. 184:133-146. Dreyfus, Cheryl F., M.D. Gershon, A. Haymovitz and E. Nunez (1976) Calcitonin: Antagonism at intestinal muscarinic receptors. Brit. J. Pharmacol. 57:155-157.