After injection of its radioactive precursor, 5-hydroxytryptophan, radioactive serotonin was biosynthesized and bound in the myenteric plexus of the mouse intestine. Addition of nonradioactive serotonin to preparations in vitro caused a net release of radioactive serotonin from the plexus. This release appeared to result from activity in the intramural nervous system of the intestine. A neurotransmitter role between sensory and motor neurons in the peristaltic reflex pathway is suggested as a working hypothesis to explain the action of serotonin.

Thin slices of the finger pads of six individuals were fixed in buffered 1 per cent osmic acid, embedded in deaerated, nitrogcnated methacrylate, and cut into thin sections for electron microscopic study. Before embedding, the slices were trimmed so as to include several digital tactile corpuscles. Some thin sections were stained in l0 per cent aqueous phosphotungstic acid solution. The principal part of Meissner's corpuscle is made up of flattened laminar cells stretching across the corpuscle in irregular layers. The perinuclcar cytoplasm of these cells contains numerous small mitochondria, a sparse granular endoplasmic reticulum, and a large number of small vesicles. Nerve fibers enter the side or base of the corpuscle, lose their myelin sheaths, and follow a meandering course between the laminar cell plates. The nerve endings enter into a close appositional relationship with the flattened portions of the laminar cells. In some areas the apposed axolemma and cell membranes are slightly thickened with small vesicles located along the cell membranc or on both surfaces. These regions are interpreted as synapses. The most prominent feature of the nerve endings is an extraordinary accumulation of small mitochondria which vary in size and internal density. The nerve endings also contain vacuoles, groups of dense concentric membranes, and small dense vesicles of irregular distribution. The laminar cells are separated from one another by a dense intercellular substance of uniform thickness which also envelops the entire corpuscle. This material contains randomly oriented collagen fibers and fine fibrils bound together by a dense material at nodal points recurring at regular intervals of approximately 120 rap. These findings are discussed in relation to the problems of the function of Meissncr's corpuscle, neural material loss and rcplaccment, and the presence of synapses.

SUMMARY1. A technique for the radioautographic identification, localization, and study of the turnover of cellular depots of 5-hydroxytryptamine (5-HT) has been evaluated. A light microscopic survey was made of the uptake and turnover of 5-HT, synthesized in vivo from its administered tritium-labelled precursor, 5-hydroxytryptophan (5-HTP).2. 5-HT was taken up rapidly and retained for long periods of time by adrenal medullary chromaffin cells, gastric enterochromaffin cells, blood platelets, thyroid parafollicular cells, beta cells of pancreatic islets, mast cells, and septal cells of the lung. Reticulo-endothelial cells of liver and spleen took up radioactive 5-HT more slowly but also retained it for days.3. Specific uptake, and a rapid turnover of radioactive 5-HT, was found in pancreatic exocrine cells, neurones of the superior cervical ganglion, terminal axons of gastro-intestinal myenteric plexus, carotid body cells, and pinealocytes.4. A supranuclear localization of radioactive 5-HT in renal proximal tubular cells may represent tubular secretion of 5-HT.5. The localization of 5-HT by radioautography generally corresponds to that found by formaldehyde-induced fluorescence. It makes possible the study of 5-HT turnover and is capable of greater resolution than the fluorescence technique. Freeze-drying may also be avoided.

SUMMARY1. The synthesis, distribution, storage, and subsequent metabolism of 5-hydroxytryptamine (5-HT) produced in mice from the administration of its radioactive precursor, 5-hydroxytryptophan, has been investigated to form the basis for a similar study to be conducted by radioautography.2. Intravenous injection of the radioactive material was found to be essential for significant uptake of radioactivity by tissue. The duration of the period during which radioactive material was available for uptake by the tissue was 2 hr.3. The relative distribution of radioactivity in individual organs was studied and the radioactive compounds present in each were identified and quantitatively assayed. No unrelated routes of metabolism of the labelled material were found and radioautographic results may be interpreted in terms of the metabolic picture which emerged.4. Radioactive 5-HT was bound in tissues in preference to both its precursor, and metabolite, 5-hydroxytryptamine-0-glucuronide, and the radioactive 5-HT produced in vivo after the injection of labelled 5-hydroxytryptophan probably entered and labelled the endogenous 5-HT pool.5. Fixatives prepared with buffer solutions made hypertonic by the addition of sucrose could rapidly and effectively halt the movement of radioactive 5-HT out of tissues and prevent its subsequent extraction during histological processing.6. It is concluded that localization of sites concerned with 5-HT metabolism by means of radioautography is feasible and the 5-HT so localized will probably reflect the physiological compartmentalization of the amine.Physiol. i86 29