Serum opioid activity after physical exercise in rats

In order to study the effect of exercise on the total serum opioid activity, female rats were trained for 3 weeks on a motor-driven treadmill and the experiment was ended by a final strenuous run until exhaustion. The serum samples were taken immediately after the final run and were analyzed by radioreceptor assay. Despite considerable interindividual variations, serum opioid activity, expressed in met-enkephalin equivalents (ME eq +/- S.D.), was significantly higher in the exercising group (74.5+/-50.5 pmol ME eq/ml) than in the control group (35.7+/-20.2 pmol ME eq/ml). Because of the much lower molar levels of beta endorphin and met-enkephalin, this result suggests that many other opioid peptides might be involved in that increase.