Opioid modulation of feeding behavior following repeated exposure to forced swimming exercise in male rats

Patterns of normal and stimulated food intake (FI) as well as its possible endogenous opioid (EO) modulation were investigated in male rats given regular swimming exercise (trained; TR) and compared with nonexercised sedentary (SED) controls. Rats in the TR group had lower body weights as well as reduced 24 hr FI due to lower nocturnal FI. TR rats also ate less food in response to injections of 2-deoxy-D-glucose (2-DG) but not insulin (INS) when injections were given during the first 4-5 weeks of training. However, this difference between TR and SED rats in the 2-DG induced feeding was not demonstrable after 10 or more weeks of training. Plasma concentrations of immunoreactive B-endorphin (IR-B-ep) were elevated, as expected, in TR rats (10-12 weeks) during nocturnal sampling whereas the nocturnal increase of IR-B-ep was absent in SED controls. However, these SED rats did increase daytime IR-B-ep in response to 2-DG and acute exercise, albeit somewhat less in magnitude when compared to TR rats. Injection of naltrexone (NTX) decreased feeding in TR rats (10-12 weeks) but not in contemporary SED controls. In summary, exercise training modified feeding behavior, and at 4 weeks of training, TR rats ate less in response to opioid-related feeding stimulus of 2-DG, but responded similarly to insulin (relatively opioid independent) treatment. At later stages of training this difference between TR and SED rats disappeared. Moreover, SED rats had atypical profiles of IR-B-ep and reduced hypophagic responses to NTX suggesting that TR rats might have greater EO modulation of feeding at this stage.