Phorbol Ester Enhancement of Neurotransmitter Release from Rat Brain Synaptosomes

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Abstract
Neurotransmitter release from rat brain synaptosomes was measured following pretreatment with various phorbol esters. Ca2+‐dependent, evoked neurotransmitter release was increased by phorbol esters that were active in stimulating protein kinase C. Protein kinase C activation was demonstrated by increased incorporation of 32P into 87‐kilodalton phosphoprotein, a specific substrate for that kinase. Inactive phorbol esters had no effect on neurotransmitter release or on the phosphorylation of 87‐kilodalton phosphoprotein. The increased release was observed in either crude cortical synaptosomal fractions (P2) or purified cortical synaptosomal fractions. The enhancement was found for all neurotransmitters (norepinephrine, acetylcholine, γ‐aminobutyric acid, serotonin, dopamine, and aspartate), all brain regions (cerebral cortex, hippocampus, and corpus striatum), and all secretagogues (elevated extracellular K+ level, veratridine, or A23187) examined. It was also observed at all calcium concentrations present during stimulation of release. The phorbol ester enhancement of Ca2+‐dependent release occurred whether or not calcium was present during pretreatment. These results indicate that stimulation of protein kinase C leads to an enhanced sensitivity of the stimulus‐secretion coupling processes to calcium within the nerve terminal. The results support the possibility that presynaptic activation of protein kinase C modulates nerve terminal neurotransmitter release in the CNS.

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