Effects of anesthesia and recovery from ketamine racemate and enantiomers on regional cerebral glucose metabolism in rats

Background: Unlike most anesthetics, ketamine racemate (S,R()-ketamine) induces heterogenous changes in cerebral metabolism. S,R()-ketamine is an equimolar mixture of two enantiomers, S()-ketamine and R()-ketamine, which differ in affinity for neuroreceptors and pharmacologic activities. This study investigated comparatively the effects of ketamine racemate and enantiomers on cerebral metabolism. Methods: Regional cerebral metabolic rates for glucose (rCMRglc) were determined with the quantitative, autoradio- graphic [14C]2-deoxy-D-glucose technique in 40 brain regions of Fischer-344 rats. rCMRglc were measured in three groups of rats during equimolar anesthesia, 10 min after intraperitoneal injection of 170 mg/kg S,R()-ketamine, S()-ketamine, or R()-ketamine; in three groups of rats during recovery from equivalent anesthesia, 20 min after intravenous injection of 20, 12.5, and 30 mg/kg S,R()-ketamine, S()-ketamine, or R()-ketamine; and in two groups of sa line-injected control rats. Results: S,R()-ketamine and S()-ketamine induced a sus-tained anesthesia; deep rCMRglc decreases in 22 and 14 cortical, thalamic, cerebellar, and brainstem regions; and rCMRglc in-creases in two limbic regions (average decreases, 23 and 15%). R()-ketamine determined a shorter anesthesia, lesser rCMRglc decreases in 11 brain areas, and marked rCMRglc increases in 14 basal ganglia and limbic regions (average decrease, 4%). S,R()-ketamine, S()-ketamine, and R()-ketamine all produced postanesthetic behavioral activation; widespread rCMRglc in-creases in 28, 16, and 20 cortical, thalamic, basal ganglia, limbic, and brainstem regions; and rCMRglc decreases in few auditory and limbic regions (average increases, 35, 13, and 20%). Conclusions: S,R()-ketamine and S()-ketamine anesthesia but not R()-ketamine anesthesia induced widespread rCMRglc reductions that were unreported but are typical of gaseous and intravenous general anesthetics. Postanesthetic recovery led to divergent, sharp behavioral and rCMRglc activations. The relation to dose of behavioral and rCMRglc effects differs from those of aminergic agents and resembles those of N-methyl-D-aspartate receptor antagonists, suggesting that ketamine racemate and enantiomers may preferentially interact with this receptor type.