[The effect of the acute and chronic administration of the opiate receptor antagonist naloxone on the development of fatigue during physical loading in rats].

Abstract

In chronic experiments with rats during the exercises on a treadmill the ECG was registered and duration of the development of exhaustion was determined under control and under acute (1 mg/kg intraperitoneally 1 hour before the experiment) or chronic (1 mg/kg intraperitoneally, twice a day during 5 days) treatment with the opioid receptor antagonist naloxone. Chronic but not acute naloxone action resulted in increase of the fatiguability: the time of achievement of exhaustion decreased by 55.3% (P < 0.05). In this case the exhaustion developed at lower degree of heart rate than in control. Comparison of data obtained with the results of chronic treatment with the opioid antagonist permits to conclude that the chronic blockade increases the fatiguability to a great extent than chronic activation of opioid system. Possible mechanisms causing this difference are under discussion.