Release of cholecystokinin from rat midbrain slices and modulatory effect of D2DA receptor stimulation

Cholecystokinin octapeptide (CCK-8) is colocalized within a majority of dopamine (DA)-containing neurons of the rat midbrain. Exogenous CCK-8 can modulate the electrophysiological activity of DA neurons, at least in part, by direct actions on the somatodendritic region of these cells. If CCK-8 is somatodendritically released from DA neurons, it may influence DA cell function as has been shown for DA itself. In the present study, radioimmunoassay was used to determine if CCK-8 is released in vitro from slices of rat midbrain under basal and depolarizing (30 mM potassium) conditions. Low levels of CCK-8 were detected in the basal incubation medium. Thirty mM potassium caused about a 3-fold increase in the release of CCK-8. This stimulated release was abolished in calcium-free medium. The D2 receptor agonist quinpirole, but not the D1 agonist SKF 38393, attenuated the potassium-stimulated release of CCK-8 but did not affect basal release. These results show that CCK-8, like DA, can be released from midbrain slices, presumably from DA/CCK-8-containing neurons. This finding is in accordance with the possibility that CCK-8 plays a role in the regulation of DA neuronal function at the level of the cell body, where it might influence the excitability of the DA cell membrane.