Decreased carbon-11-flumazenil binding in early Alzheimer's disease

Neuronal loss in Alzheimer's disease, a better correlate of cognitive impairment than amyloid deposition, is currently gauged by the degree of regional atrophy. However, functional markers, such as GABAA receptor density, a marker of neuronal integrity, could be more sensitive. In post-mortem hippocampus, GABAA messenger RNA expression is reduced even in mild cognitive impairment. We measured whole-brain GABAA binding potential in vivo using [ ¹¹C]-flumazenil positron emission tomography and compared GABAA binding with metabolic and volumetric measurements. For this purpose, we studied 12 subjects, six patients with early Alzheimer's disease and six healthy controls, with [¹¹C]-flumazenil and [¹⁸F]- fluorodeoxyglucose positron emission tomography, as well as with high-resolution magnetic resonance imaging. Data were evaluated with both voxel-based parametric methods and volume of interest methods. We found that in early Alzheimer's disease, with voxel-based analysis, [¹¹C]-flumazenil binding was decreased in infero-medial temporal cortex, retrosplenial cortex and posterior perisylvian regions. Inter-group differences reached corrected significance when using an arterial input function. Metabolism measured with positron emission tomography and volumetric measurements obtained with magnetic resonance imaging showed changes in regions affected in early Alzheimer's disease, but, unlike with [¹¹C]-flumazenil binding and probably due to sample size, the voxel-based findings failed to reach corrected significance in any region of the brain. With volume of interest analysis, hippocampi and posterior cingulate gyrus showed decreased [¹¹C]-flumazenil binding. In addition, [¹¹C]-flumazenil hippocampal binding correlated with memory performance. Remarkably, [¹¹C]-flumazenil binding was decreased precisely in the regions showing the greatest degree of neuronal loss in post-mortem studies of early Alzheimer's disease. From these data, we conclude that [¹¹C]-flumazenil binding could be a useful marker of neuronal loss in early Alzheimer's disease.