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Abstract: The in vivo binding of N-[18F]fluoroethyl-piperidinyl benzilate ([18F]FEPB) to the muscarinic cholinergic receptor was measured in awake and anesthetized rats. Studies were done using an equilibrium infusion technique to provide estimates of specific binding as distribution volume ratios. Anesthesia with either isoflurane or sodium pentobarbital produced a significant (65–90%) increase of radiotracer binding in receptor-rich brain regions (striatum, cortex, hippocampus) relative to awake controls. Pretreatment of anesthetized animals with the acetylcholinesterase inhibitor phenserine produced no further increases in radioligand binding, in contrast to the large (>70%) increases previously observed in awake animals following drug treatment. These studies demonstrate that anesthesia can produce significant changes in baseline biochemical measures that can obscure even very large effects of pharmacological challenges. [Copyright &y& Elsevier]