Fig. 2. Representative sample tracings of respiratory responses to two levels of inspired carbon dioxide in a wild-type mouse (top ) and a μ-opioid receptor (OR) knockout mouse (bottom ) after intraperitoneal administration of 100 mg/kg morphine. The vertical bars represent a tidal volume of 100 μl. Note the decrease in breathing frequency in the wild-type animal. Breathing frequency in the μOR−/−mouse was identical to control values. VT= tidal volume.