Depolarisation‐Dependent Protein Phosphorylation in Rat Cortical Synaptosomes Is Inhibited by Fluphenazine at a Step After Calcium Entry

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Abstract
The sequence of molecular events linking depolarization-dependent Ca influx to Ca-stimulated protein phosphorylation is unknown. The effect of the neuroleptic drug fluphenazine on depolarization-dependent protein phosphorylation was investigated using an intact postmitochondrial pellet isolated from rat cerebral cortex. Fluphenazine, in a dose-dependent manner, completely inhibited the increases in protein phosphorylation observed previously. The concentration of fluphenazine required for 50% inhibition varied for different phosphoproteins but for synapsin I was 123 .mu.M. Other neuroleptics produced effects similar to fluphenazine with their order of potency being thioridazine > haloperidol > trifluoperazine > fluphenazine > chlorpromazine. Fluphenazine also increased the phosphorylation of proteins in nondepolarized controls at concentrations of 20 and 60 .mu.M. The inhibition of depolarization-dependent phosphorylation was apparently not due to a loss of synaptosomal integrity or viability, a decrease in Ca uptake, a change in substrate availability, or to a change in protein phosphatase activity. The data are most consistent with an inhibition of protein kinase activity by blockade of calmodulin or phospholipid activation.