Neurosecretory granules: evidence from an aging process within the neurohypophysis.

When cysteine labeled with sulfur-35 is injected into the third ventricle of the rat brain, it is first incorporated into only one of the two populations of neurosecretory granules that can be isolated on an isosmotic gradient. The second population of granules is labeled much later. Stimulation of hormone release from isolated labeled neural lobes and subsequent isolation of neurosecretory granules at different times after the injection of labeled cysteine shows that the radioactivity decreases in only one population of granules. One of the fractions of the gradient represents the granules found near the release site; the second population is probably located deeper in the nerve endings or in the nerve swellings. Whereas neurophysins are found in both populations, smaller proteins can only be detected in one. Thus it appears that neurosecretory granules undergo an aging process and that isosmotic gradients can separate the aged granules from those newly formed.