Loaded dopamine is preferentially stored in the halo portion of PC12 cell dense core vesicles

Large dense core vesicles in rat pheochromocytoma cells are morphologically distinct from dense core vesicles in mast and chromaffin cells in that the dense core occupies a much smaller fraction of the vesicular volume, allowing for a much larger vesicular clear space, or halo. In this work, we present evidence indicating that upon treatment with L-DOPA the majority of the dopamine loaded into these vesicles is preferentially compartmentalized into the halo portion of the vesicle. Amperometry was used to monitor release of loaded neurotransmitter from cells in both isotonic and hypertonic extracellular conditions, with the latter condition causing inhibition of dense core dissociation. In combination with this we have used transmission electron microscopy to determine the morphological characteristics of dense core vesicles before and after treatment with L-DOPA in solutions of varied osmolarity. The results provide a more complete understanding of the complex interaction of molecules within dense core vesicles, suggesting that newly loaded dopamine is located in the halo of the vesicle. This finding has fundamental significance for studies of neurotransmitter release from dense core vesicles, as the core appears to have a function involving more than simple storage of neurotransmitter and associated molecules, and the often overlooked vesicular halo appears to be an important storage compartment for neurotransmitter.