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Heroin self-administration maintained by different unit doses of heroin under fixed-ratio (A, B) or progressive-ratio (C-F) reinforcement schedules. Deletion of D3Rs in D3-KO mice shifted heroin self-administration dose-response curves upward (A) and heroin intake dose-response curves upward (B), as well as increased heroin infusions (C, D) and break-points for heroin reward (E, F) under PR reinforcement. *p < 0.05; **p < 0.01, compared with WT group.
