The Role of Endogenous Opioids in Moderate Exercise Training-Induced Enhancement of the Secondary Antibody Response in Mice

BACKGROUND AND PURPOSE: Moderate exercise training (60%-80% of maximal oxygen uptake) enhances the secondary antibody response. The mechanism underlying this enhancement, however, has not been determined. In moderate doses, endogenous opioids such as enkephalins enhance antibody response. Furthermore, serum concentrations of endogenous opioids increase in response to exercise, and training programs augment this effect. Therefore, the enhancement of the secondary antibody response induced by moderate exercise may be brought about, in part, by endogenous opioids. The purpose of this study was to examine the effects of naltrexone (an opioid antagonist) on the enhancement of secondary antibody response induced by moderate exercise in young mice. SUBJECTS AND METHODS: C57BL/6 mice immunized to human serum albumin (HSA) were randomly assigned to 1 of 3 groups: naltrexone, placebo, or control (received no intervention). Then, the mice in each group were randomly assigned to either an exercise group (treadmill running at 15 m/min, 0ø slope, 5 days per week for 8 weeks) or a non-exercise group. At the end of 8 weeks, booster immunization was given, and the mice in the exercise group continued to exercise. Ten days later, when high levels of antibodies are produced in secondary antibody response, anti-HSA antibodies in serum were measured by enzyme-linked immunosorbent assay (ELISA). RESULTS: With naltrexone implantation, mice that exercised showed a depression of secondary antibody response as compared with mice that exercised and either received a placebo or did not receive any intervention. DISCUSSION AND CONCLUSION: Endogenous opioids may play a role in the enhancement of the secondary antibody response observed after moderate exercise.