Effects of restraint and naltrexone on the biphasic heart rate response to morphine in rats

The effect of several doses of morphine (0, 0.5, 2, 5 or 10 mg/kg, i.v.) on heart rate was assessed in restrained and freely-moving rats. Morphine produced a dose-dependent bradycardia followed by tachycardia. The magnitude and duration of bradycardia were greater in restrained rats, whereas the magnitude and duration of tachycardia were greater in unrestrained rats. Naltrexone (5 mg/kg) pretreatment completely blocked the biphasic heart rate response to morphine (8 mg/kg). When naltrexone was given after the bradycardic portion of the response, tachycardia declined to baseline levels. These results suggest that one or both components of the biphasic response are mediated by opioid receptors.