Ultrastructural evidence for monoamine uptake by vesicles of pineal sympathetic nerves immediately after their stimulation.

Bilateral electrical stimulation of the preganglionic fibers to the superior cervical ganglia of the rat markedly reduces the number of osmiophilic dense cores present in the nerve vesicles of the sympathetic fibers in the pineal gland. These cores owe their density to the presence of noradrenaline and serotonin in the vesicles. When sympathetic nerves of the pineal organ are exposed immediately after stimulation for a brief period to the false neurotransmitter 5-hydroxydopamine, either in vitro or in vivo, dense precipitates reappear in the electron-lucent vesicles. On the basis of these observations, it is concluded that the vesicles remaining in the nerves after releasing their neurotransmitter content have the capacity to take up and store monoamines. This provides a morphological correlate for the recent biochemical evidence suggesting that the vesicles in sympathetic nerves are reused after neurotransmitter release.