Release of endogenous dopamine from tuberoinfundibular neurons

Release of endogenous dopamine(DA) from arcuate-periventricular nucleus-median eminence fragments has been analyzed in an in vitro static incubation system. Exposure of these hypothalamic fragments to increasing concentrations of K+ ions produced a dose-dependent release of endogenous DA. The highest rate of K+-stimulated DA efflux occurred in the first 10 minutes, thereafter it progressively declined reaching prestimulated levels at 30 minutes. If two consecutive depolarizing stimuli of 40 mM KCl were applied to the same hypothalamic fragment, after a 40 minutes rest period, an equivalent release of endogenous DA occurred. Removal of Ca++ ions from the incubation medium containing the Ca++ chelator EGTA caused a decrease of basal DA efflux and completely prevented the K+-induced release of DA. Furthermore when verapamil, a blocker of Ca++ entrance, was added to the incubation medium in a concentration of 50 microM, the K+-induced DA efflux was completely counteracted, whereas spontaneous release was unmodified. Finally nomifensine, a potent blocker of DA uptake, added in vitro in a final concentration of 10 microM, significantly reinforced K+-induced release of endogenous DA. Since nomifensine did not modify basal DA release, this study confirmed its prevalent uptake blocking property rather than its releasing action on DA.