Table 2. Influence of 100 mg/kg Morphine and Saline on Respiration in μ-Opioid Receptor Knockout (μOR^−/−) Mice and Their Wild-type Littermates (μOR^+/+)

Values are mean of the animal means ± SD.

* P values. Treatment effects (least significant differences test):† P < 0.02;

‡ P < 0.01;

§ P < 0.001.

ANOVA = analysis of variance; NS = not significant; V^T= tidal volume; RR = breathing frequency; V̇= minute ventilation; S = slope of the hypercapnic ventilatory response.