Release of Cholecystokinin-Like Immunoreactivity in the Frontal Cortex of Conscious Rats as Assessed by Transcerebral Microdialysis: Effects of Different Depolarizing Stimuli

Abstract: The release of cholecystokinin-like immunoreactivity (CCK-LI) from the frontal cortex of freely moving rats has been studied using a transcerebral microdialysis technique coupled to a radioimmunoassay procedure. Basal levels of CCK-LI in the dialysate were above detection limits (2.4 ± 0.7 pg/20 min; n = 8). High-K⁺ media evoked CCK-LI overflow in a concentration-dependent manner. The threshold concentration was 50 mM KCI. The peak overflow evoked by 100 mM K⁺ amounted to 42.7 ± 2.8 pg/20 min (n = 6); it was totally Ca²⁺ dependent but insensitive to 1 μM tetrodotoxin. Infusion of 4-aminopyridine (1 mM ; 20 min) evoked an overflow of CCK-LI (32 ± 2.3 pg/ 20 min; n = 4), wnich was totally Ca²⁺ dependent and tetrodotoxin sensitive. Depolarization with 100 μg/ml of veratrine (20 min) provoked a CCK-LI overflow (62.2 ± 10 pg/20 min; n = 6), which was also blocked by tetrodotoxin or by the absence of Ca²⁺ ions. The CCK-LI material collected under basal conditions or during veratrine infusion consisted essentially of CCK octapeptide sulfate. The veratrine-induced CCK-LI overflow did not change significantly when the infusion time was prolonged to 100 min. A second 20-min stimulus with 100 μg/ml of veratrine applied 200 min after a first 20-min stimulus evoked a barely significant CCK-LI overflow. These data suggest that one single 20-min stimulus with 100 μg/ml of veratrine may be sufficient to deplete the CCK-LI releasable stores and that >200 min are required to replenish the depleted CCK-containing vesicles. Taken together the data allow us to conclude that the physiology and the pharmacology of CCK release can be adequately studied in vivo by brain microdialysis.