Conditions Restricting Depolarization-Dependent Calcium Influx in Synaptosomes Reveal a Graded Response of P96 Dephosphorylation and a Transient Dephosphorylation of P65

Temporal changes in the phosphorylation level of synaptosomal phosphoproteins following depolarization of synaptosomes were investigated under conditions restricting calcium influx. High-K+ depolarization in media of low [Na+]o (32 mM during preincubation and depolarization) at pH 6.5 resulted in a pronounced fall in the cytosolic free calcium concentration transient, and in a reduction in the initial K(+)-stimulated 45Ca2+ uptake and endogenous acetylcholine release relative to the values obtained with control synaptosomes (preincubated and depolarized in Na(+)-based media). This reduction was paralleled by a decrease in the rate of dephosphorylation of the synaptosomal protein P96. A slower dephosphorylation of P96 also was observed on exposure to 20 microM veratridine at 0.5 mM external calcium. Our results indicate that, similar to synapsin I phosphorylation, P96 dephosphorylation shows a graded response to the amount of calcium entering the presynaptic terminal. Depolarization of synaptosomes under conditions restricting the influx of calcium revealed a transient dephosphorylation (reversed within 10 s) of the phosphoprotein P65. The possible significance of this finding to the process of neurotransmitter release is discussed.