Calcium release from internal stores is required for the generation of spontaneous hyperpolarizations in dopaminergic neurons of neonatal rats

We recently have demonstrated the existence of spontaneous hyperpolarizations in midbrain dopaminergic neurons of neonatal but not adult rats. These events are mediated by the opening of apamin-sensitive K+ channels after a rise in the intracellular concentration of Ca2+ They are resistant to tetrodotoxin in most cases and are probably endogenous (i.e., not synaptically activated). Here their mechanism was investigated. Cyclopiazonic acid (10 mu M), a specific inhibitor of endoplasmic reticulum Ca2+ ATPases, reversibly abolished the events. Caffeine, which promotes Ca2+ release from intracellular stores, had concentration-dependent effects. At 1 mM, it markedly and steadily increased the frequency and the amplitude of the hyperpolarizations. At 10 mM, it induced a transient increase in their frequency followed by their cessation. All these effects were quickly reversible. Ryanodine (10 mu M), which decreases the conductance of Ca2+ release channels, irreversibly blocked the spontaneous hyperpolarizations. Dantrolene (100 mu M), a blocker of Ca2+ release from sarcoplasmic reticulum of striated muscle, did not affect the events. On the other hand, Cd2+ (100-300 mu M), a broad antagonist of membrane voltage-gated Ca2+ channels, significantly reduced the amplitude and the frequency of the hyperpolarizations. However, when the frequency of the events was increased by 1 mM, caffeine, Cd2+ affected them to a smaller extent, whereas cyclopiazonic acid still abolished them. We conclude that internal stores are the major source of Ca2+ ions that induce the K+ channel openings underlying the spontaneous hyperpolarizations of these neurons.