See this (from the Wikipedia article on dopamine):
Dopamine, like most amines, is an organic base. At neutral or acidic pH levels it is generally protonated. The protonated form is highly water-soluble and relatively stable, though it is capable of oxidizing if exposed to oxygen or other oxidants. At basic pH levels, dopamine becomes deprotonated. In this free base form it is less soluble and also highly reactive and easily oxidized
I'm very curious about this since I'm curious about dopamine auto-oxidation, as it is often a cause of neurodegeneration observed in those who take amphetamines (and seems more likely in the cytosol, than the vesicles it would otherwise be in). Though I'm also curious how much auto-oxidation would happen for extracellular dopamine as well.