Concurrent inactivation of calcium dependent phosphoryiation and neurotransmitter release in cultured rat brain neurons

Depolarization of cultured rat brain neurons preloaded with ³H-dopamine provokes a transient (t_1/2= 9.6 sec), Ca²⁺-dependent release of the labeled neurotransmitter from cells. In parallel, the amount of ³²Pi incorporated into a protein of apparent molecular weight of 43000 increased whereas the phosphorylation of a protein with an apparent molecular weight of 55000 daltons decreased. The time course of the change in phosphorylation pattern was examined. The depolarization-induced phosphorylation of the 45000 protein and dephosphorylation of the 55000 dalton protein consisted of an initial, rapidly terminating phase (t_1/2= 5 sec), and of a slow, Ca²⁺-independent phosphorylation of both proteins which persisted during maintained depolarization. The depolarization-evoked changes in the neuronal protein phosphorylation were dependent on the extracellular Ca²⁺ concentration (half saturation at 0.4-0.5 mM Ca²⁺). These data indicate that the entry of Ca²⁺into the depolarized cells induces rapid phosphorylation-dephosphorylation activities. These processes terminate within 10 sec, concurrently with the depression of neurotransmitter release.