[Effect of melatonin on mean blood pressure and heart rate in morphine withdrawal rats].

The model of morphine dependent rats was established by gradually increasing the doses of morphine by subcutaneous injection(from 5 mg.kg-1 to 50 mg.kg-1, 5 days). The effects of melatonin(MT) on the mean arterial blood pressure(mABP) and heart rate(HR) were studied in naloxone-precipitated rats. MT was found to inhibit the hypertention and tachycardiac syndromes in morphine withdrawal rats in dose-dependent manner. The concentration of serum norepinephrine(NE) was shown to be decreased by MT. Moreover, regitine was shown to inhibit the increase of mABP in morphine withdrawal rats, but can not inhibit the increase of HR. These results suggest that the inhibitory effect of MT on hypertension and tachycardiac syndromes in morphine withdrawal rats may be mediated by the inhibitory effect of the drug on the sympathetic system.