Abstract
BackgroundThe availability of GABAA receptor binding sites in the brain can be assessed by positron emission tomography (PET) using the radioligand, [18F]flumazenil. However, the brain uptake and binding of this PET radioligand are influenced by anesthetic drugs, which are typically needed in preclinical imaging studies and clinical imaging studies involving patient populations that do not tolerate relatively longer scan times. The objective of this study was to examine the effects of anesthesia on the binding of [18F]flumazenil to GABAA receptors in mice.MethodsBrain and whole blood radioactivity concentrations were measured ex vivo by scintillation counting or in vivo by PET in four groups of mice following administration of [18F]flumazenil: awake mice and mice anesthetized with isoflurane, dexmedetomidine, or ketamine/dexmedetomidine. Dynamic PET recordings were obtained for 60 min in mice anesthetized by either isoflurane or ketamine/dexmedetomidine. Static PET recordings were obtained at 25 or 55 min after [18F]flumazenil injection in awake or dexmedetomidine-treated mice acutely anesthetized with isoflurane. The apparent distribution volume (VT*) was calculated for the hippocampus and frontal cortex from either the full dynamic PET scans using an image-derived input function or from a series of ex vivo experiments using whole blood as the input function.ResultsPET images showed persistence of high [18F]flumazenil uptake (up to 20 % ID/g) in the brains of mice scanned under isoflurane or ketamine/dexmedetomidine anesthesia, whereas uptake was almost indiscernible in late samples or static scans from awake or dexmedetomidine-treated animals. The steady-state VT* was twofold higher in hippocampus of isoflurane-treated mice and dexmedetomidine-treated mice than in awake mice.ConclusionsAnesthesia has pronounced effects on the binding and blood-brain distribution of [18F]flumazenil. Consequently, considerable caution must be exercised in the interpretation of preclinical and clinical PET studies of GABAA receptors involving the use of anesthesia.
Highlights
The availability of GABAA receptor binding sites in the brain can be assessed by positron emission tomography (PET) using the radioligand, [18F]flumazenil
This rapid decline suggests that there is no specific binding in the myocardium. [18F]flumazenil uptake in ket/dex anesthetized mice peaked at 2 min post injection with 9.7 ± 1.3 %Injected dose per gram (ID/g) in hippocampus, and at 5 min with 8.4 ± 0.9 %ID/g in frontal cortex, whereas the heart signal peaked at 1 min with 7.9 ± 0.3 %ID/g and declined rapidly as in the isofluraneanesthetized mice
In the dex animals, [18F]flumazenil uptake peaked at 5 min post injection with 18.2 ± 6.9 %ID/g in the hippocampus, and at 17.5 ± 5.2%ID/g in frontal cortex, whereas the whole blood concentration peaked at 1 min with 5.4 ± 0.1 %ID/g
Summary
The availability of GABAA receptor binding sites in the brain can be assessed by positron emission tomography (PET) using the radioligand, [18F]flumazenil. Many radioligands have been developed for molecular imaging by positron emission tomography (PET), which target different sites of the GABAA receptors ([6]). Flumazenil (Ro 15-1788, Fig. 1b), a GABAA receptor antagonist, has found the widest application in PET studies. Both [11C]flumazenil and the longer-lived isotopologue [18F]flumazenil have been used to measure the in vivo receptor availability in humans [7, 8], monkeys [9, 10], and rodents [11, 12]
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