INVITRO DOPAMINE BIOSYNTHESIS IN THE MEDIAN-EMINENCE OF RAT HYPOTHALAMIC SLICES - INVOLVEMENT OF VOLTAGE-DEPENDENT CA-2+ CHANNELS

It was investigated whether Ca2+ is involved in the regulation of basal and depolarization-induced dopamine biosynthesis in tuberoinfundibular dopaminergic neurons. The rate of dopamine biosynthesis was estimated by in vitro dihydroxyphenylalanine (DOPA) synthesis in the median eminence following incubation of rat hypothalamic slices with a DOPA decarboxylase inhibitor. Depolarizing agents such as K+ and veratridine increased the synthesis rate of DOPA in the medium eminence in a dose-dependent manner with a maximal synthesis rate obtained at concentration of 50 mM and 50 .mu.M, respectively. Removal of Ca2+ and addition of EGTA (1 mM) into the medium did not influence basal DOPA synthesis in the medium eminence but blocked the K+-and veratridine-induced DOPA synthesis. The Ca2+ channel blockers verapamil (100 .mu.M) and Co2+ (4 mM) were effective in reducing the depolarization-induced DOPA synthesis A23187 (10 .mu.M), a Ca2+ ionophore, stimulated basal DOPA synthesis in the median eminence. On the other hand, tetrodotoxin (2 .mu.M), a Na+ channel blocker, did not change the basal and K+-induced DOPA synthesis in the median eminence whereas it completely inhibited the veratridine-induced DOPA synthesis. These results suggest that depolarization-induced synthesis of dopamine in tuberoinfundibular neurons requires Ca2+ influx through voltage-dependent Ca2+ channels.