Effects of Ion Channel Blockers and Phorbol Ester Treatments on [³H]Dopamine Release and Neurotensin Facilitation of [³H]Dopamine Release from Rat Mesencephalic Cells in Primary Culture

In this work, we tested the effect of ion channel blockers and of phorbol ester treatments on [ 3 H]dopamine ([ 3 H]DA) release and neurotensin (NT)‐induced facilitation of [ 3 H]DA release from cultures of rat fetal mesencephalic cells. The potassium channel blockers tetraethylammonium and 4‐aminopyridine increased basal [ 3 H]DA release and decreased K + ‐evoked [ 3 H]DA release, whereas apamin was without effect. K + ‐evoked [ 3 H]DA release was decreased by ω‐conotoxin and nifedipine, totally suppressed by cadmium, and unaffected by amiloride. These results show the differential sensitivity of [ 3 H]DA release to blockade of various ion channels and suggest the involvement of N‐type, L‐type, and non‐L‐non‐N‐type, but not T‐type, voltage‐sensitive calcium channels in K + ‐evoked release. Phorbol 12‐myristate 13‐acetate increased both spontaneous and K + ‐evoked [ 3 H]DA release, suggesting a modulatory action of protein kinase C on DA release in this system. Unexpectedly, however, the effects of the phorbol ester were not counteracted by the protein kinase C inhibitors H7, staurosporine, or polymyxin B. NT‐induced facilitation of K + ‐evoked [ 3 H]DA release was insensitive to most of the ion channel blockers, except cadmium (64% decrease in NT effect), suggesting that the corresponding potassium' and calcium channels were not involved in the effect of NT on [ 3 H]DA release in this system. The NT effect was totally suppressed by phorbol ester treatments, indicating a possible desensitization of the corresponding transduction mechanisms after protein kinase C activation.