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ClockΔ19 mice displayed greater motivation to obtain cocaine infusions in a progressive ratio schedule of reinforcement. a Number of lever presses at which the breakpoint occurred. b Dose–response lever discrimination (mean number of active and inactive lever presses). Open bars denote ClockΔ19 mice and gray bars denote WT mice. *p<0.05; **p<0.01; ****p<0.0001. N=4–5/genotype
