Release of dopamine from human neocortex nerve terminals evoked by different stimuli involving extra- and intraterminal calcium

The release of [(3)H]-dopamine ([(3)H]-DA) from human neocortex nerve terminals was studied in synaptosomes prepared from brain specimens removed in neurosurgery and exposed during superfusion to different releasing stimuli. Treatment with 15 mM KCl, 100 microM 4-aminopyridine, 1 microM ionomycin or 30 mM caffeine elicited almost identical overflows of tritium. Removal of external Ca(2+) ions abolished the overflow evoked by K(+) or ionomycin and largely prevented that caused by 4-aminopyridine; the overflow evoked by caffeine was completely independent of external Ca(2+). Exposure of synaptosomes to 25 microM of the broad spectrum calcium channel blocker CdCl(2) strongly inhibited the 4-aminopyridine-induced tritium overflow while that evoked by ionomycin remained unaffected. The Ca(2+) chelator, 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N' tetraacetic acid (BAPTA), reduced significantly the K(+)- and the caffeine-induced tritium overflow. The effect of caffeine was attenuated by exposure to the ryanodine receptor blocker dantrolene or when the membrane-impermeant inositol trisphosphate receptor antagonist, heparin, was entrapped into synaptosomes; the combined treatment with dantrolene and heparin abolished the release elicited by caffeine. Tetanus toxin, entrapped into human neocortex synaptosomes to avoid prolonged incubation, inhibited in a concentration-dependent manner the K(+)- or the 4-aminopyridine-evoked tritium overflow; in contrast, the release stimulated by ionomycin and by caffeine were both totally insensitive to the same concentrations of tetanus toxin. Western blot analysis showed about 50% reduction of the content of the vesicular protein, synaptobrevin, in synaptosomes poisoned with tetanus toxin. In conclusion, the release of dopamine from human neocortex nerve terminals can be triggered by Ca(2+) ions originating from various sources. It seems that stimuli not leading to activation of voltage-sensitive Ca(2+) channels elicit Ca(2+)-dependent, probably exocytotic, release that is insensitive to tetanus toxin.