Cobalt-sensitive and dihydropyridine-insensitive stimulation of dopamine release from tuberoinfundibular neurons by high extracellular concentrations of barium ions

Recently, it has been demonstrated that Ca²⁺ entrance into the neuronal cytoplasm can occur upon the activation of 3 different types of specific voltage-dependent channels which can be characterized according to the following criteria: (1) voltage threshold for activation; (2) tendency to inactivation; (3) bivalent cation permeability; and (4) drug sensitivity. In this study we investigated, in tuberoinfundibular dopaminergic (TIDA) hypothalamic neurons, the biochemical and pharmacological properties of Ca²⁺ channels, by comparing the effects of high extracellular concentrations of Ba²⁺ and Ca²⁺ ions on [³H]dopamine (DA) release from TIDA neurons. The results obtained show that extracellular Ba²⁺ ion concentrations dose-dependently (10-20 mM) stimulated [³H]DA release from superfused TIDA neurons and that this effect was prevented by Co²⁺ ions (2 mM). In addition, superfusion of TIDA neurons with a concentration of Ca²⁺ ions equimolar to that by Ba²⁺ ions (20 mM) failed to modify [³H]DA release. The fact that tetraethylammonium (10 mM), a blocker of K⁺ currents in excitable cells, did not mimick the stimulatory action of Ba²⁺ ions on [³H]DA release, seems to exclude that the effect of Ba²⁺ ions was dependent on the inhibition of K⁺ channels in TIDA neurons. The ommission of Ca²⁺ ions from the extracellular medium did not prevent the stimulatory effect on [³H]DA release elicited by elevated concentrations of Ba²⁺ ions, but rather reinforced this effect. Finally, nitredipine (50 μM) did not modify the stimulatory effect of high extracellular Ba²⁺ ions on [³H]DA release from TIDA neurons.