SULPHUR METABOLISM IN THE PITUITARY AND HYPOTHALAMUS OF THE RAT: A STUDY OF RADIOISOTOPE-UPTAKE AFTER THE INJECTION OF 35S dl-CYSTEINE, METHIONINE, AND SODIUM SULPHATE
SUMMARY: 1. The relative radioactivity of different parts of the pituitary and hypothalamus has been assessed with a flow counter, and, more satisfactorily, both visually and by granule-counts in autoradiographs. These experiments have involved 103 rats killed between 15 sec and 97 hr after the administration of 35S dl-cysteine, dl-methionine and sodium sulphate. 2. Subarachnoid injections proved more satisfactory than intraperitoneal or intracarotid ones. They were followed by the rapid localization of radioisotope in the adenohypophysis as well as in nervous tissue. 3. The early and marked uptake of radioisotope shown by the cell bodies of neurones in various nuclear regions, and in particular in the supraoptic nuclei, has been interpreted as evidence of active protein synthesis; this pattern of uptake was observed after the injection of labelled cysteine and methionine, but not sodium sulphate. 4. A similar, early, but less marked uptake of radioisotope was noted in the pars distalis after the injection of both cysteine and methionine. Only after the injection of methionine was there a marked uptake in the pars intermedia, and this was confined to its lateral border. 5. Uptake by the infundibular process of the neurohypophysis became greater than that in pars distalis or in the superjacent hypothalamus 9½ hr and longer after injection of labelled cysteine, but not methionine. This pattern of uptake was confirmed by granule counts in twenty-five animals. 6. It is suggested that the late neurohypophysial uptake of radioisotope reflects the storage in the nerve-terminals of the gland of slowly metabolizing proteins or polypeptides synthesized in the supraoptic and paraventricular nuclear regions. These substances probably include the posterior pituitary principles, since the latter are rich in cystine, but lack methionine.