Perfusion of Naloxone Through the Fourth Cerebral Ventricle Reverses the Circulatory and Hypnotic Effects of Halothane in Dogs

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Abstract
In an attempt to determine whether naloxone, when perfused through the cerebroventricular system, would modify the circulatory and hypnotic effects of halothane, its effect was studied in 6 trained dogs. Naloxone (20 .mu.g/ml) was perfused through the 4th cerebral ventricle in 3 and through the 3rd cerebral and lateral ventricles in 3 other dogs when awake and during halothane anesthesia (0.75-0.82 vol% in O2). Blood pressure, heart rate and circulatory responses to bilateral occlusion of the carotid arteries were measured. The state of consciousness was evaluated by the animals'' behavior and the EEG. Arterial hypotension, bradycardia, depressed baroreceptor responses and EEG synchronization associated with halothane anesthesia were reversed when naloxone was perfused through the 4th ventricle. To maintain comparable depths of anesthesia the halothane concentration had to be doubled during naloxone perfusion. No change in the circulatory or hypnotic effects of halothane occurred when the 3rd ventricle was perfused with naloxone. Opiate receptors in structures bordering the 4th cerebral ventricle may be important modulators of inhalational anesthesia.