Presynaptic Modulation of Cholecystokinin Release by Protein Kinase C in the Rat Hippocampus

The role of protein kinase C (PKC) in modulating the release of the octapeptide cholecystokinin (CCK-8) was investigated in rat hippocampal nerve terminals (synaptosomes). The PKC-activating phorbol ester 4β-phorbol 12,13-dibutyrate (β-PDBu) dose dependently (5-5,000 nM) increased CCK-8 release in a strictly Ca 2+ -dependent way. This effect was observed only when synaptosomes were stimulated with the K + A channel blocker 4-aminopyridine (4-AP; 1 mM) but not with KCI (10-30 mM). The PDBu-induced exocytosis of CCK-8 was completely blocked by the two selective PKC inhibitors chelerythrine and calphostin-C and was not mimicked by α-PDBu, an inactive phorbol ester. In addition, an analogue of the endogenous PKC activator diacylglycerol, oleoyl-acetylglycerol, dose dependently increased CCK-8 exocytosis. β-PDBU (50-100 nM) also stimulated the 4-AP-evoked Ca 2+ -dependent release of the classic transmitter GABA, which co-localizes with CCK-8 in hippocampal interneurons. As a possible physiological trigger for PKC activation, the role of the metabotropic glutamate receptor was investigated. However, the broad receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid did not stimulate, but instead inhibited, both the CCK-8 and the GABA exocytosis. In conclusion, presynaptic PKC may stimulate exocytosis of distinct types of co-localizing neurotransmitters via modulation of presynaptic K + channels in rat hippocampus.