INTERACTIVE EFFECTS OF MK-801 AND MORPHINE ON EEG, EEG POWER SPECTRA AND BEHAVIOR IN RATS .1. MORPHINE-TOLERANCE DEVELOPMENT

This study investigated a potential interaction between MK-801 and mor phine tolerance using electroencephalography (EEG), EEG spectral param eters and behavior in the rat. Rats were treated for 7 days with morph ine alone or with morphine and MK-801. Control groups received chronic MK-801 alone or saline. On day 8 all rats received morphine alone. Co -treatment significantly accelerated the development of tolerance to m orphine-induced total power and latency to sleep onset. Co-treatment, but not morphine alone, produced tolerance to morphine-induced complex ity and edge frequency, and produced tolerance equal to that from morp hine alone in duration of EEG bursting. MK-801 co-treatment decreased, where chronic morphine alone increased, the excitatory response to mo rphine. Our results support recent reports that chronic morphine treat ment produces supersensitivity to glutamate in the rat cortex and alte ration in mesolimbic dopamine levels that are modulated by glutamaterg ic activity.