CHARACTERIZATION OF THE RELEASE OF CHOLECYSTOKININ-8 FROM ISOLATED NERVE-TERMINALS AND COMPARISON WITH EXOCYTOSIS OF CLASSICAL TRANSMITTERS

In the present study, the release of the neuropeptide cholecystokinin-8 (CCK) from purified nerve terminals (synaptosomes) of the rat hippocampus was characterized with respect to the subcellular distribution, the release upon addition of various agents, the release kinetics, the Ca2+ and ATP dependence of release, and the relationship between CCK release and elevations of intraterminal free Ca2+ concentration ([Ca]i). These characteristics were compared with those for the release of classical transmitters in similar preparations. CCK-like immunoreactivity (CCK-LI) is enriched in the purified synaptosomal fraction of hippocampus homogenates and released in a strictly Ca2+-dependent manner upon chemical depolarization, addition of 4-aminopyridine, or stimulation with the Ca2+ ionophore ionomycin. The presence of Ca2+ in the medium significantly stimulates the basal efflux of CCK-LI from synaptosomes. The release upon stimulation develops gradually in time with no significant release in the first 10 s and levels off after 3 min of depolarization. At this time, a large amount of CCK-LI is still present inside the synaptosomes. A correlation exists between the release of CCK-LI and the elevations of [Ca]i. The release of CCK-LI is decreased, but not blocked, upon ATP depletion. These characteristics markedly differ from those for classical transmitters, which show a fast component of Ca2+-dependent (exocytotic) release, an absolute dependence on cellular ATP and no marked stimulation of basal efflux in the presence of Ca2+. Furthermore, the relationship between the volume average [Ca]i (measured with fura-2) and the extent of release is more or less linear for the release of CCK-LI, whereas this relationship is clearly nonlinear for the release of endogenous glutamate and gamma-aminobutyric acid in similar preparations. We hypothesize that these differences between the neuropeptide CCK-8 and classical transmitters are caused by three differences: (a) vesicle type; (b) Ca2+ sensitivity of the release mechanism; and (c) release site.