EEG changes with different levels of morphine self-administration.

Rats with chronically implanted jugular catheters and cortical EEG electrodes were allowed 24 hr per day access to morphine infusions (30 micrograms/kg) contingent on lever-pressing. Yoked control subjects received the same number and pattern of infusions as contingent subjects in an adjacent cage. Six subjects were studied, with data analyzed over 84 days of contingent and 63 days of yoked control (noncontingent) treatment. Self-administration rates were positively correlated with previous exposure to morphine. EEG total power was reduced in both contingent and yoked subjects during periods of self-administration. Averaging of trends in EEG power over time across multiple episodes of self-administration showed a greater reduction in power for contingent subjects that preceded the majority of morphine infusions. Desynchronization (diminished EEG amplitude) for contingent subjects at the onset of self-administration is probably related to lever-pressing activity. Ultradian describes biological rhythms in which the duration of one cycle (period) ranges from several minutes to values less than 24 hr. In both contingent and yoked subjects at intermediate or high levels of self-administration, increases occurred in the period and amplitude of ultradian cycles in EEG total power. Responsiveness to the light-dark cycle was also diminished at these levels of self-administration. In conclusion, morphine self-administration at intermediate or high levels disrupts both diurnal and ultradian rhythms in EEG total power for both contingent and yoked subjects. The ultradian EEG pattern associated with greater levels of morphine self-administration resembles diminished variation in EEG power that occurs normally during the inactive rest phase of the diurnal cycle.