Morphine-like effects of clonidine on the EEG, slow wave sleep and behavior in the dog

EEG, behavioral and autonomic effects of morphine and clonidine were compared in the unrestrained beagle dog placed in a dimly-lit, sound-attenuated chamber equipped with video monitors. Intravenous (i.v.) morphine (0.5, 1 and 2 mg/kg) and clonidine (11, 33 and 100 μg/kg) caused parallel dose-related increases in NREM sleep with clonidine being 43 times more potent than morphine. Other similar effects were: an initial transient EEG-behavioral dissociation, increases in spectral power (8-16 Hz); decreases in temperature and heart and respiratory rates; emesis, miosis and salivation. Intraventricular (i.v.t.) morphine (33, 100 and 300 μg) and clonidine (10 and 30 μg) caused qualitatively similar EEG, sleep and behavioral effects. Naloxone (30 μg/kg i.v.) prevented EEG synchrony and behavioral effects of i.v. morphine (1 mg/kg) but not those of i.v. clonidine (100 μg/kg). Yohimbine pretreatment (0.1 mg/kg i.v.) was more effective in antagonizing clonidine than morphine. These results suggest that morphine and clonidine induce similar EEG, behavioral and autonomic effects through actions upon different receptors on the same or parallel neural pathways. The results further emphasize the importance of an α₂-adrenergic-opiod interaction to regulate sleep in the dog.