Effects of voluntary exercise on hippocampal long-term potentiation in morphine-dependent rats

This study was designed to examine the effect of voluntary exercise on hippocampal long-term potentiation (LTP) in morphine-dependent rats. The rats were randomly distributed into the saline-sedentary (Sal/Sed), the dependent-sedentary, the saline-exercise (Sal/Exc), and the dependent-exercise (D/Exc) groups. The Sal/Exc and the D/Exc groups were allowed to freely exercise in a running wheel for 10 days. The Sal/Sed and the morphine-sedentary groups were kept sedentary for the same extent of time. Morphine (10 mg/kg) was injected bi-daily (12 h interval) during 10 days of voluntary exercise. On day 11, 2h after the morphine injection, the in vivo LTP in the dentate gyrus of the hippocampus was examined. The theta frequency primed bursts were delivered to the perforant path for induction of LTP. Population spike (PS) amplitude and the field excitatory post-synaptic potentials (fEPSP) slope were measured as indices of increase in synaptic efficacy. Chronic morphine increased the mean basal EPSP, and augmented PS-LTP. Exercise significantly increased the mean baseline EPSP and PS responses, and augmented PS-LTP in both saline and morphine-treated groups. Moreover, the increase of PS-LTP in the morphine-exercise group was greater (22.5%), but not statistically significant, than that of the Sal/Exc group. These results may imply an additive effect between exercise and morphine on mechanisms of synaptic plasticity. Such an interaction between exercise and chronic morphine may influence cognitive functions in opiate addicts.