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This work was supported in part by grant MH-40159 from NIH and unrestricted gifts from Bristol Myers Squibb and Zeneca Pharmaceuticals (to M.G.C.). M.G.C. is an Investigator of the Howard Hughes Medical Institute. R.R.G. was a recipient of a fellowship from the Tourette Syndrome Association and is a visiting researcher from the Institute of Pharmacology, Russian Academy of Medical Sciences, Baltiyskaya 8, 125315 Moscow, Russia.