Minute ventilation and breathing frequency (means ± SD) in baseline condition and following fentanyl in control and in β-arr2-deficient mice. In contrast to Fig. 7, data obtained during periods of automatic locomotion (circling) and immobility are presented separately. The periods of immobility could be computed in baseline and only above 1 mg/kg fentanyl, there was, however, no period of automatic locomotion in baseline condition. Again, no difference was observed between the two groups. *P < 0.05 when compared with baseline. β-arr2, β arrestin 2.