Striatal Cholinergic Interneurons Express a Receptor-Insensitive Homomeric TASK-3–Like Background K+ Current

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Abstract
Large aspiny cholinergic interneurons provide the sole source of striatal acetylcholine, a neurotransmitter essential for normal basal ganglia function. Cholinergic interneurons engage in multiple firing patterns that depend on interactions among various voltage-dependent ion channels active at different membrane potentials. Leak conductances, particularly leak K+ channels, are of primary importance in establishing the prevailing membrane potential. We have combined molecular neuroanatomy with whole cell electrophysiology to demonstrate that TASK-3 (K2P9.1, Kcnk9) subunits contribute to leak K+ currents in striatal cholinergic interneurons. Immunostaining for choline acetyltransferase was combined with TASK-3 labeling, using nonradioactive cRNA probes or antisera selective for TASK-3, to demonstrate that striatal cholinergic neurons universally express TASK-3. Consistent with this, we isolated a pH-, anesthetic-, and Zn2+-sensitive current with properties expected of TASK-3 homodimeric channels. Surprisingly, activation of Gαq-linked receptors (metabotropic glutamate mGluR1/5 or histamine H1) did not appear to modulate native interneuron TASK-3–like currents. Together, our data indicate that homomeric TASK-3–like background K+ currents contribute to establishing membrane potential in striatal cholinergic interneurons and they suggest that receptor modulation of TASK channels is dependent on cell context.