In primary cultures of mesencephalic neurons the expression of the dopamine transport and release system as well as specific binding sites of dopamine uptake blockers are developmentally linked to axonal outgrowth. In neurons cultured for five days the potassium-evoked dopamine release is mediated through activation of N-type calcium channel, while L- or T-type channels appear to be inactive. In contrast, veratridine-evoked dopamine release was not altered by L-, N- or T-type calcium channel blockers, but was inhibited by tetrodotoxin and zinc. Since veratridine elicits membrane depolarization by prolonging sodium channel opening it is suggested that the veratridine-evoked dopamine release is not mediated through voltage-dependent calcium channels, but that during depolarization calcium may enter the cells through the sodium transport system. In addition, in cultured mesencephalic neurons cholecystokinin-specific mRNA is expressed, supporting the hypothesis that cholecystokinin may play the role of a cotransmitter in dopaminergic neurons.
