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Synaptically released glutamate evokes slow IPSPs mediated by metabotropic glutamate receptors (mGluRs) in midbrain dopamine neurons. These mGluR IPSPs are caused by release of Ca2 from intracellular stores and subsequent activation of small-conductance Ca2-activated K channels (SK channels). To further investigate the intracellular mechanisms involved, the effect of photolyzing intracellular caged inositol 1, 4, 5-triphosphate (InsP3) on membrane conductance and intracellular Ca2 concentration ([Ca2] i) was examined in rat midbrain slices. Photolytic release of InsP3 elicited a transient outward current and a sharp rise in [Ca2] i that lasted for 5 sec. Apamin, a blocker of SK channels, abolished the InsP3-induced outward current without affecting the rise in [Ca2] i. Depleting intracellular Ca2 stores with cyclopiazonic acid completely blocked both the outward current and the Ca2 transient elicited by …