Effects of chronic morphine administration and naloxone on EEG, EEG power spectra, and associated behavior in two inbred rat strains.

Utilizing behavioral and electroencephalographic (EEG) assessments, two inbred rat strains, Lewis (LEW) and Fischer 344 (F344), were exposed to morphine (IV) over a period of 7 days to discern differences in tolerance development. Following morphine injection, the LEW group demonstrated a greater mean total amount, as well as a greater rate of reduction, of stuporous behavior across the 7 days tested. Differences in patterns of latency to onset of slow-wave sleep between the two strains were also exposed. EEG analysis of spectral parameters utilizing an analysis of variance with repeated measures revealed that peak frequency, mean frequency, and edge frequency differed as a function of inbred rat strain. All spectral parameters differed as a function of duration of morphine injection; linear trends were indicated for both strains. Naloxone was administered (IV) following the 7 days of morphine to delineate dependence differences. LEW animals reflected a greater amount of behavioral responses, for example, wet-dog shakes, diarrhea, body stretch, and sluggish behavior. However, F344 rats demonstrated a greater alteration in two spectral parameters assessed: peak frequency and total power. Genetic variability appears to play a major role in both morphine tolerance and dependence as indicated by differences in EEG and behavioral responses.