Respiratory effects of morphine in awake unrestrained rats

This report describes a systematic analysis of opiate drug effects on ventilation and its components tidal volume and frequency in intact, awake and unrestrained rats. A whole-body plethysmographic method was used to measure these parameters of respiration while animals breathed air or various concentrations of CO2 in air. Subcutaneous doses of morphine lower than 40 mg/kg exerted little or no apparent effect in rats breathing air; in rats breathing 4 to 8% of CO2 these doses of morphine also failed to depress any of the ventilatory parameters below the level of saline controls breathing air. Doses (0.16 to 160 mg/kg) of morphine blunted the frequency response to CO2 in a biphasic manner. The effects of morphine on tidal volume consisted of a slight increase at 0.16 and 0.63 mg/kg, a dose-dependent decrease at 2.5 to 40 mg/kg and a paradoxical rise at 160 mg/kg. These complex effects of morphine on tidal volume and frequency resulted in a simple sigmoid depression of minute volume. The slope of this sigmoid dose-response curve varied with the inspirate; it increased as the concentration of CO2 was higher. Naloxone antagonized the frequency depression produced by 40 mg/kg of morphine in a dose-dependent manner at doses ranging from 0.01 to 0.16 mg/kg, but frequency decreased again at 0.63 mg/kg. The effects of naloxone on the tidal volume depression consisted of a paradoxical further decrease at 0.01 mg/kg, a dose-dependent antagonism of depression at 0.04 to 0.16 mg/kg and a stimulation above the normal control level at 0.63 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)