Are endogenous opiates involved in potentiation of field potentials in the hippocampus of the rat?

Changes in hippocampal CA1 field potentials during and after tetanic stimulation were studied in rats pretreated with 5 mg/kg s.c. naloxone or saline. Stimuli consisted of successive tetani of increasing frequency; responses at threshold, midrange and asymptotic intensities were sampled. A several fold increase in response amplitude occurred during low frequency stimulation, with no significant differences between the groups. Post-tetanic responses showed depression at the lower frequencies but increases ranging up to 400% at higher frequencies, with the greatest potentiation at threshold; again there were no significant differences between naloxone and saline groups, suggesting that endogenous opiates are not involved.