Reassessing morphine effects in cats: II. Protracted effects on sleep-wakefulness and the EEG

Adult cats were implanted with standard electrodes to record EEG, EOG, and EMG. After 15 days, morphine sulphate or saline placebo was given IP at 0.5, 1.0, 2.0, 3.0 mg/kg, at least 15 days apart. Cats were continuously recorded for 72 hr postinjection. Wakefulness, drowsiness, NREM and REM sleep percentages were scored from polygraphic features and statistically analysed. There was a dose-dependent suppression of NREM and REM sleep for at least 6 hours postmorphine, with a progressive sleep recovery thereafter. During the insomnia period there was an EEG/behavioral dissociation where bursts of high-voltage waves were seen over a background of desynchrony; meanwhile the animal was first aroused although quiet and later showed stereotypic behavior. There was a prolonged NREM sleep rebound which started later at the higher doses. A significant, relatively brief REM sleep rebound was seen only at the lowest dose. The latency for NREM and REM sleep onset was also dose-dependent. Possible brain sites of morphine actions and similarities with effects in other species are discussed.