Hdc−/− mice10 are viable but have altered mast-cell morphology and subtle abnormalities in locomotion, arousal, learning, and memory.13-15 In addition, they have several traits relevant to features of Tourette's syndrome. The mice have exaggerated locomotor responses to methamphetamine and enhanced behavioral sensitization: the progressive and persistent augmentation of locomotor and stereotypic behaviors resulting from repeated administration of dopamine agonists.14 Such stimulant-induced stereotypic movements, including rearing, sniffing, and biting, have previously been proposed as a model of human tics.16 In addition, multiple studies have confirmed that Hdc−/− mice display behavior more consistent with anxiety than littermate controls,15,17 similar to the phenotype recently observed in mice deficient in Slitrk1.5
