EEG, EEG power spectral and behavioral profiles of two inbred rat strains upon acute and chronic opiate exposure

Abstract

Utilizing EEG, EEG power spectra and behavioral parameters, two inbred rat strains, Lewis (LEW) and Fischer 344 (F344) were compared following acute morphine, chronic morphine and acute ethylketocyclazocine (EKC) treatment. Following acute morphine administration, the LEW rats exhibited greater duration of morphine-induced EEG slow-wave bursts and associated behavioral stupor. For both LEW and F344 rats, suppression of onset to slow-wave sleep increased in a dose-related manner. Regarding morphine dose, all spectral parameters differed except peak frequency. For the chronic morphine study, LEW and F344 inbred rat strains were exposed to morphine (iv) over a period of seven days in order to discern differences in tolerance development and physical dependence. Following morphine injection, the LEW group exhibited a greater mean total amount, as well as a greater rate of reduction, of stuporous behavior across the seven days tested. The LEW rats exhibited a positive linear profile of opiate-induced hyperexcitability, while a quadratic profile emerged for the F344 group. Over days, differences in patterns of latency to onset of slow-wave sleep between the two strains were also revealed; the F344 rats exhibited a greater change (as reduction of suppression) across the seven days tested. Naloxone was administered (iv) following the seven days of morphine treatment in order to delineate differences in dependence. The LEW animals reflected a greater amount of withdrawal-type behavioral responses, e.g., wet dog shakes, diarrhea, body stretch, sluggish behavior. However, assessment of power spectral parameters pre- and post-naloxone treatment revealed a greater withdrawal response for the F344 group. Acute administration of EKC also revealed differences between LEW and F344 inbred rat strains. EKC-induced EEG slow-wave bursts and associated behavioral stupor increased in a dose-related manner, and was greater in duration, for the LEW animals; a less robust, quadratic trend over doses was displayed by the F344 group. Separation of the phases revealed rat strain differences; overall, the LEW animals displayed a dose-related EEG response. Except for total power, the F344 group displayed little variation across the three doses tested in the burst phase; the interburst phase reflected dose-related differences for this group but to a lesser extent in comparison to the LEW rats. (Abstract shortened with permission of author.)