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Minute ventilation and breathing frequency (means ± SD) following the administration of 1 mg/kg of fentanyl at D1 and D15–D20 in the control and β-arr2 deficient animals. At D1, 1 mg/kg fentanyl was administered following two lower doses of fentanyl (0.1 and 0.5 mg/kg), while at D15–D20, 1 mg/kg was administered first (before 1.5 and 5 mg/kg). There was no significant difference between the two injections. Arrb2tm1Rjl, Arrb2 knockout mice; β-arr2, β arrestin 2.
