Abstract

Direct, voltage-integrated and frequency-analyzed cortical electroencephalograms (EEG's) as well as integrated electromyograms were obtained from control, morphine-dependent and abstinent rats prepared with chronically implanted cortical electrodes and i.v. cannulas. Morphine sulfate, 1.25 mg/kg, was injected automatically every hour for 24 hours during the first day. This dose was increased on successive days to 2.5, 5, 10 and 20 mg/kg/hr, resulting in the induction of a state of morphine dependence in the rats. When morphine injections were discontinued, the behavioral manifestations of the abstinence syndrome became evident. During this period, a decline in the mean integrated EEG voltage prevailed throughout the sleep-awake cycle. This drop of the EEG voltage output reached levels 35 to 70% below those of control. The lowered voltage output of the awake state EEG endured for two to three days. The reduction of the EEG voltage output of the sleep state, however, was more pronounced and longer lasting, with recovery reached by the fourth or fifth day of abstinence. Morphine injections given during the abstinence syndrome produced an immediate rise in the low voltage output; this rise was accompanied by a marked decrease in the irritability and arousal of the abstinent rat. It is suggested that the lowered voltage output of the EEG during morphine abstinence is the product of a central nervous system mechanism developed to counteract the acute effects of morphine as to synchronization of the EEG.