The growth cones of dissociated rat sympathetic neurons developing in culture were fixed with potassium permanganate to visualize vesicular stores of norepinephrine through the formation of granular precipitates. It was found that growth cones contain numerous small granular vesicles (SGV) 40-60 nm in diameter. The majority of the SGV was present in the varicosity of the growth cone but SGV also occurred in filopodia. The SGV appeared in clusters or scattered throughout the varicosity. Treatment of the cultured neurons, before fixation, with reserpine, which depletes catecholamine stores by blocking uptake into vesicles, resulted in the presence of small clear vesicles. In contrast, growth cones of nonadrenergic sensory neurons dissociated from dorsal root ganglia and fixed with permanganate lacked SGV and possessed small clear vesicles. These observations indicate that the growth cones of cultured sympathetic neurons contain norepinephrine, suggest that the norepinephrine is stored in synaptic vesicles, and raise the question whether this transmitter plays a role in early axon-target cell interactions during synapse formation.

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