Opiate Receptor Mediation of Ketamine Analgesia

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Abstract
Analgesia produced by ketamine can be antagonized by the narcotic antagonist naloxone. To elaborate the apparent similarity between ketamine- and narcotic-induced analgesia, the effects of ketamine were examined in 3 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 twitch-like muscular contractions, as do narcotics. 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, 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. Evidently, a significant mechanism of ketamine-induced analgesia is mediated by opiate receptors.

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