The distribution of labeled norepinephrine within sympathetic nerve terminals studied with electron microscope...

The distribution of radioactivity in association with sympathetic nerve terminals and intraneuronal organelles 30 min after the administration of tritiated norepinephrine (NE-(3)H) was studied by electron microscope radioautography with recently developed quantitative methods of analysis reported in the accompanying paper (Salpeter et al., 1969). Nerves from the pineal body and the adrenal capsule were examined. It was found that nerve terminals containing vesicles were heavily labeled. (These terminals were not necessarily in contact with some innervated structure.) There was no selective labeling of either the intraneuronal mitochondria or the relatively small population of large ( approximately 1000 A) dense core granules. Small vesicles ( approximately 500 A), some of which have a dense internal granule, could not be analysed separately because they are closely packed and occupy approximately 60% of the volume in terminals. Because of the extensive distribution of these small granular and agranular vesicles in the radioactive terminals, they remain the most likely site for norepinephrine binding. Yet although the vesicles were uniformly distributed within the nerve terminals, it appears that the radioactivity was not. There appeared to be a somewhat higher concentration of radioactivity at the periphery of the nerve terminals than in the center. The usefulness of the method of analysis used in this study for determining the location of bound H(3)NE pools in the nerve is discussed.