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Previous workers have noted that analgesia produced by ketamine can be antagonized by the narcotic antagonist, naloxone. In order to elaborate further the apparent similarity between ketamine- and narcotic-induced analgesia, the authors examined the effects of ketamine in three standard test systems for the opiate receptor. In a radioligand binding assay using 3H-dihydromorphine, ketamine stereospecifically bound to opiate receptors in rat brain homogenate, (+) ketamine being 2–3 times more potent than the (—) enantiomer of ketamine. In a bioassay for the opiate receptor, using the longitudinal muscle-myenteric plexus of the guinea pig ileum, ketamine inhibited the (witch-like muscular contractions, as do narcotics. However, only the inhibitory effects of (+) ketamine, which in this system also was twice as potent as (—) ketamine, could be partially antagonized by naloxone, suggesting that this enantiomer is responsible for the opiate receptor-related effects of ketamine. In vivo, the authors found that ketamine displaces 3H-etorphine, a potent narcotic, from opiate receptors in regional areas of the mouse brain, especially in the thalamic region, but not in the cortex. The results suggest that a significant mechanism of ketamine-induced analgesia is mediated by opiate receptors.