The cortical topography of temporal lobe hypometabolism in early Alzheimer's disease

In vivo PET of synaptic density as potential diagnostic marker for cognitive disorders: prospective comparison with current imaging markers for neuronal dysfunction and relation to symptomatology - study protocol

BACKGROUND

18F-FDG brain PET is clinically used for differential diagnosis in cognitive dysfunction of unclear etiology and for exclusion of a neurodegenerative cause in patients with cognitive impairment in late-life psychiatric disorders. 18F-FDG PET measures regional glucose metabolism, which represents a combination of neuronal/synaptic activity but also astrocytic activity and neuroinflammation. Recently, imaging of synaptic vesicle protein 2 A (SV2A) has become available and was shown to be a proxy of synaptic density. This prospective study will investigate the use of 18F-SynVesT-1 for imaging SV2A and its discriminative power for differential diagnosis in cognitive disorders in a head-to-head comparison to 18F-FDG PET. In addition, simultaneous PET/MR allows an evaluation of contributing factors and the additional value of advanced MRI imaging to FDG/SV2A PET imaging will be investigated. In this work, the study design and protocol are depicted.

METHODS

In this prospective, multimodal imaging study, 110 patients with uncertain diagnosis of cognitive impairment who are referred for 18F-FDG PET brain imaging in their diagnostic work-up in a tertiary memory clinic will be recruited. In addition, 40 healthy volunteers (HV) between 18 and 85 years (M/F) will be included. All study participants will undergo simultaneous 18F-SynVesT-1 PET/MR and an extensive neuropsychological evaluation. Amyloid status will be measured by PET using 18FNAV4694, in HV above 50 years of age. Structural T1-weighted and T2-weighted fluid-attenuated inversion recovery MR images, triple-tagging arterial spin labeling (ASL) and resting-state functional MRI (rs-fMRI) will be obtained. The study has been registered on ClinicalTrials.gov (NCT05384353) and is approved by the local Research Ethics Committee.

DISCUSSION

The main endpoint of the study will be the comparison of the diagnostic accuracy between 18F-SynVesT-1 and 18F-FDG PET in cognitive disorders with uncertain etiology and in exclusion of a neurodegenerative cause in patients with cognitive impairment in late-life psychiatric disorders. The strength of the relationship between cognition and imaging data will be assessed, as well as the potential incremental diagnostic value of including MR volumetry, ASL perfusion and rs-fMRI.

Cerebrospinal Fluid α-Calcitonin Gene-Related Peptide: A Comparison between Alzheimer’s Disease and Multiple Sclerosis

Alzheimer’s disease (AD) and Multiple Sclerosis (MS) represent an emerging health problem on a global scale, as they are responsible for a significant contribution to the burden of disability in Western countries. Limited numbers of cerebrospinal fluid (CSF) diagnostic markers are available for each disease (amyloid and tau deposition markers for AD and oligoclonal bands for MS) representing mostly state markers that provide few, if any, clues about the severity of the clinical phenotype. α-CGRP is a neuropeptide implied in nociception, vasodilation, synaptic plasticity and immune functions. This neuropeptide is expressed in encephalic regions connected to memory, attention, autonomic and behavioral functions and is also expressed by spinal motor neurons. The present work confronted α-CGRP levels between 19 AD, 27 MS and 17 control subjects using an ELISA/EIA assay. We measured higher CSF α-CGRP contents in control subjects with respect to AD, as shown in previous studies, as well as in MS patients in comparison to AD. The control subjects and MS patients did not significantly differ between each other. We did not observe a relationship between CSF protein content, albumin quotient and α-CGRP. We also describe, retrospectively, an association between higher CSF CGRP content and higher MRI overall lesion count in MS and between lower α-CGRP and worse attention and visuo-perceptual skills in AD. We speculate that α-CGRP could be differentially involved in both disabling diseases.

Increased Contact System Activation in Mild Cognitive Impairment Patients with Impaired Short-Term Memory

An activated plasma contact system is an abnormality observed in many Alzheimer's disease (AD) patients. Since mild cognitive impairment (MCI) patients often develop AD, we analyzed the status of contact system activation in MCI patients. We found that kallikrein activity, high molecular weight kininogen cleavage, and bradykinin levels- measures of contact system activation- were significantly elevated in MCI patient plasma compared to plasma from age- and education-matched healthy individuals. Changes were more pronounced in MCI patients with impaired short-term recall memory, indicating the possible role of the contact system in early cognitive changes.

Alzheimer's disease alters oligodendrocytic glycolytic and ketolytic gene expression

INTRODUCTION

Sporadic Alzheimer's disease (AD) is strongly correlated with impaired brain glucose metabolism, which may affect AD onset and progression. Ketolysis has been suggested as an alternative pathway to fuel the brain.

METHODS

RNA-seq profiles of post mortem AD brains were used to determine whether dysfunctional AD brain metabolism can be determined by impairments in glycolytic and ketolytic gene expression. Data were obtained from the Knight Alzheimer's Disease Research Center (62 cases; 13 controls), Mount Sinai Brain Bank (110 cases; 44 controls), and the Mayo Clinic Brain Bank (80 cases; 76 controls), and were normalized to cell type: astrocytes, microglia, neurons, oligodendrocytes.

RESULTS

In oligodendrocytes, both glycolytic and ketolytic pathways were significantly impaired in AD brains. Ketolytic gene expression was not significantly altered in neurons, astrocytes, and microglia.

DISCUSSION

Oligodendrocytes may contribute to brain hypometabolism observed in AD. These results are suggestive of a potential link between hypometabolism and dysmyelination in disease physiology. Additionally, ketones may be therapeutic in AD due to their ability to fuel neurons despite impaired glycolytic metabolism.

Arterial spin labeling blood flow MRI: its role in the early characterization of Alzheimer's disease

Arterial spin labeling (ASL) enables the noninvasive, quantitative imaging of cerebral blood flow using standard magnetic resonance imaging (MRI) equipment. Because it requires no contrast injection, ASL can add resting functional information to MRI studies measuring atrophy and signs of ischemic injury. Key features of ASL technology that may affect studies in Alzheimer's disease are described. The existing literature describing ASL blood flow imaging applied to Alzheimer's disease and related dementia is reviewed, and the potential role of ASL in treatment and prevention studies of early Alzheimer's disease is discussed.

Hippocampal hyperperfusion in Alzheimer's disease

Many of the regions with the earliest atrophy in Alzheimer's Disease (AD) do not show prominent deficits on functional imaging studies of flow or metabolism. This paradox may provide unique insights into the pathophysiology of AD. We sought to examine the relationship between function and atrophy in AD using MRI blood flow and anatomic imaging. 22 subjects diagnosed with AD, mean Mini Mental State Exam (MMSE) score 22.2, and 16 healthy elderly controls were imaged with a volumetric arterial spin labeling blood flow MRI technique and an anatomical imaging method using the identical spatial resolution, image orientation, and spatial encoding strategy. Cerebral blood flow (CBF) and gray matter (GM) maps derived from the imaging were transformed to a standard anatomical space. GM and CBF maps were tested for significant differences between groups. Additionally, images were tested for regions with significant mismatch of the CBF and GM differences between groups. CBF was significantly lower in the bilateral precuneus, parietal association cortex and the left inferior temporal lobe but was non-significantly increased in the hippocampus and other medial temporal structures. After correction for GM loss, CBF was significantly elevated in the hippocampus and other medial temporal structures. The hippocampus and other regions affected early in AD are characterized by elevated atrophy-corrected perfusion per cm(3) of tissue. This suggests compensatory or pathological elevation of neural activity, inflammation, or elevated production of vasodilators.

Hippocampal hyperperfusion in Alzheimer's disease

Many of the regions with the earliest atrophy in Alzheimer's Disease (AD) do not show prominent deficits on functional imaging studies of flow or metabolism. This paradox may provide unique insights into the pathophysiology of AD. We sought to examine the relationship between function and atrophy in AD using MRI blood flow and anatomic imaging. 22 subjects diagnosed with AD, mean Mini Mental State Exam (MMSE) score 22.2, and 16 healthy elderly controls were imaged with a volumetric arterial spin labeling blood flow MRI technique and an anatomical imaging method using the identical spatial resolution, image orientation, and spatial encoding strategy. Cerebral blood flow (CBF) and gray matter (GM) maps derived from the imaging were transformed to a standard anatomical space. GM and CBF maps were tested for significant differences between groups. Additionally, images were tested for regions with significant mismatch of the CBF and GM differences between groups. CBF was significantly lower in the bilateral precuneus, parietal association cortex and the left inferior temporal lobe but was non-significantly increased in the hippocampus and other medial temporal structures. After correction for GM loss, CBF was significantly elevated in the hippocampus and other medial temporal structures. The hippocampus and other regions affected early in AD are characterized by elevated atrophy-corrected perfusion per cm(3) of tissue. This suggests compensatory or pathological elevation of neural activity, inflammation, or elevated production of vasodilators.

Functional imaging of cognition in Alzheimer's disease using positron emission tomography

Positron emission tomography in Alzheimer's disease (AD) demonstrates a metabolic decrease, predominantly in associative posterior cortices (comprising the posterior cingulate cortex), and also involving medial temporal structures and frontal regions at a lesser degree. The level of activity in this wide network is roughly correlated with dementia severity, but several confounds (such as age, education or subcortical ischemic lesions) may influence the brain-behaviour relationship. Univariate analyses allow one to segregate brain regions that are particularly closely related to specific neuropsychological performances. For example, a relationship was established between the activity in lateral associative cortices and semantic performance in AD. The role of semantic capacities (subserved by temporal or parietal regions) in episodic memory tasks was also emphasized. The residual activity in medial temporal structures was related to episodic memory abilities, as measured by free recall performance, cued recall ability and recognition accuracy. More generally, AD patients' performance on episodic memory tasks was correlated with the metabolism in several structures of Papez's circuit (including the medial temporal and posterior cingulate regions). Multivariate analyses should provide complementary information on impaired metabolic covariance in functional networks of brain regions and the consequences for AD patients' cognitive performance. More longitudinal studies are being conducted that should tell us more about the prognostic value of initial metabolic impairment and the neural correlates of progressive deterioration of cognitive performance in AD.

Imaging biomarkers and their role in dementia clinical trials

There are five potential major roles for neuroimaging with respect to dementia; (1) as a cognitive neuroscience research tool, (2) for prediction of which normal or slightly impaired individuals will develop dementia and over what time frame, (3) for early diagnosis of Alzheimer's disease (AD) in demented individuals, (sensitivity) and separation of AD from other forms of dementia (specificity), (4) for monitoring of disease progression, and (5) for monitoring response to therapies. Focusing on the last role, no single imaging approach is yet ideal, as all trade-off speed, cost, and accuracy. Functional imaging (SPECT and PET) is best suited to tracking symptomatic therapy response, and anatomic (MRI volumetric) imaging or amyloid PET are more suited to reflect dementia modulation studies. The potential for imaging with respect to pharmacological studies of dementia--to provide surrogate markers for drug studies, to improve diagnosis, to speed evaluation of outcomes, and to decrease sample sizes--is huge. At the present time, however, no single measure has sufficient proven reliability, replicability, or robustness, to replace clinical primary outcome measures.

Mapping of temporal and parietal cortex in progressive nonfluent aphasia and Alzheimer's disease using chemical shift imaging, voxel-based morphometry and positron emission tomography

Little and controversial evidence is available from neuroimaging studies in progressive nonfluent aphasia (PNA). The goal of this study was to combine information from different imaging modalities in PNA compared with Alzheimer's disease (AD). Chemical shift imaging (CSI), voxel-based morphometry (VBM) and fluorodeoxyglucose positron emission tomography (FDG-PET) were used in 5 PNA, 10 AD patients and 10 normal subjects. Group comparisons revealed left anterior lateral temporal abnormalities (BA20/21) in PNA using CSI, VBM and PET in comparison to normal subjects. AD patients showed more limited hypometabolism within the same area. In addition left lateral parietal (BA40) abnormalities were demonstrated in our PNA as well as our AD group using PET and VBM (AD group only). Combining information from all imaging modalities on a single case basis revealed pathology within the left anterior lateral temporal and lateral parietal lobe both in PNA and AD. PNA and AD patients differed significantly, however, with respect to the frequency of medial temporal lobe and posterior cingulate/precuneus involvement. Although our results might not be generalizable to all subgroups of PNA, we conclude that medial temporal and posterior cingulate/precuneus cortex pathology as assessed by CSI and VBM or PET distinguish PNA from AD, whereas lateral temporal and parietal areas are involved in both conditions.

Brain glucose metabolism in the early and specific diagnosis of Alzheimer's disease. FDG-PET studies in MCI and AD

The demographics of aging suggest a great need for the early diagnosis of dementia and the development of preventive strategies. Neuropathology and structural MRI studies have pointed to the medial temporal lobe (MTL) as the brain region earliest affected in Alzheimer's disease (AD). MRI findings provide strong evidence that in mild cognitive impairments (MCI), AD-related volume losses can be reproducibly detected in the hippocampus, the entorhinal cortex (EC) and, to a lesser extent, the parahippocampal gyrus; they also indicate that lateral temporal lobe changes are becoming increasingly useful in predicting the transition to dementia. Fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) imaging has revealed glucose metabolic reductions in the parieto-temporal, frontal and posterior cingulate cortices to be the hallmark of AD. Overall, the pattern of cortical metabolic changes has been useful for the prediction of future AD as well as in distinguishing AD from other neurodegenerative diseases. FDG-PET on average achieves 90% sensitivity in identifying AD, although specificity in differentiating AD from other dementias is lower. Moreover, recent MRI-guided FDG-PET studies have shown that MTL hypometabolism is the most specific and sensitive measure for the identification of MCI, while the utility of cortical deficits is controversial. This review highlights cross-sectional, prediction and longitudinal FDG-PET studies and attempts to put into perspective the value of FDG-PET in diagnosing AD-like changes, particularly at an early stage, and in providing diagnostic specificity. The examination of MTL structures, which has so far been exclusive to MRI protocols, is then examined as a possible strategy to improve diagnostic specificity. All told, there is considerable promise that early and specific diagnosis is feasible through a combination of imaging modalities.

Advances in the early detection of Alzheimer's disease

The combination of an aging population and the promise, possibly in the near future, of disease-modifying therapies have made the characterization of the early stages of Alzheimer's disease (AD) a topic of major research interest. In this article we review recent progress in our understanding of the evolution of early AD with particular reference to the symptomatic pre-dementia stage designated 'mild cognitive impairment', emphasizing work on the early cognitive profile and associated neuroimaging studies.

Imaging Alzheimer's disease: clinical applications

Extensive PET imaging research on AD has been conducted since PET scanners became available in the early 1980s. PET imaging using FDG, now commercially available, can detect early metabolic changes in AD and differential metabolic features of various dementing disorders. Image analysis techniques have also advanced in the field of functional brain imaging and permit accurate and consistent scan interpretation. PET studies that involve autopsy-confirmed cases suggest that the PET diagnosis of AD is no worse or may even be better than clinical diagnosis. Limited prospective studies demonstrated the effects of PET imaging in dementia management, which precludes the approval of FDG PET for more widespread, reimbursable use. Further evidence for the efficacy of PET imaging through well-organized clinical studies, as well as continuing efforts in technologic development and basic research to characterize functional alterations in dementing disorders in living patients, are equally important to achieve the goal of better dementia care.

Les substrats cérébraux des troubles de la mémoire épisodique dans la maladie d’Alzheimer

Resting state PET measurement is useful to unravel brain regions whose dysfunction is responsible for impairment of episodic memory in Alzheimer's disease. First, the consistent hypometabolism of posterior cingulate cortex, temporo-parietal cortex and frontal cortex contrasts with the frequent lack of hippocampal hypometabolism, although it is first to be concerned by neurofibrillary tangles. Several hypotheses are proposed to explain this paradoxical result. Second, the correlative approach (correlations between memory performances and metabolic values on a voxel basis) shows that dysfunction of the hippocampal region is responsible for the earliest deficits of episodic memory, and then suggests the recruitment of neocortical temporal areas normally involved in semantic memory, perhaps as a form of a compensatory mechanism. When applied to the study of Mild Cognitive Impairment, this approach is also very fruitful.

Working memory in mild Alzheimer's disease and early Parkinson's disease

Alzheimer's disease (AD) and Parkinson's disease (PD) impair working memory (WM). It is unclear, however, whether the deficits seen early in the course of these diseases are similar. To address this issue, the authors compared the performance of 22 patients with mild AD, 20 patients with early PD and without dementia, and 112 control participants on tests of inhibition, short-term memory, and 2 commonly administered tests of WM. The results suggest that although mild AD and early PD both impair WM, the deficits may be related to the interruption of different processes that contribute to WM performance. Early PD disrupted inhibitory processes, whereas mild AD did not. The WM deficits seen in patients with AD may be secondary to deficits in other cognitive capacities, including semantic memory.

Clinical application of positron emission tomography for diagnosis of dementia

Clinical applications of PET studies for dementia are reviewed in this paper. At the mild and moderate stages of Alzheimer's disease (AD), glucose metabolism is reduced not only in the parietotemporal region but also in the posterior cingulate and precuneus. At the advanced stage of AD, there is also a metabolic reduction in the frontal region. In AD patients, glucose metabolism is relatively preserved in the pons, sensorimotor cortices, primary visual cortices, basal ganglia, thalamus and cerebellum. In patients with dementia with Lewy bodies, glucose metabolism in the primary visual cortices is reduced, and this reduction appears to be associated with the reduction pattern in AD patients. In patients with frontotemporal dementia, reduced metabolism in the frontotemporal region is the main feature of this disease, but reduced metabolism in the basal ganglia, and/or parietal metabolic reduction can be associated with the frontotemporal reduction. When corticobasal degeneration is associated with dementia, the reduction pattern of dementia is similar to the reduction pattern in AD and the hallmarks of diagnosing corticobasal degeneration associated with dementia are a reduced metabolism in the primary sensorimotor region and/or basal ganglia and an asymmetric reduction in the two hemispheres. FDG-PET is a very useful tool for the diagnosis of early AD and for the differential diagnosis of dementia. I also describe clinical applications of PET for the diagnosis of dementia in Japan.

Effects of hormone replacement therapy on cognitive and brain aging

Recent reports suggest that hormone therapy may be associated with a reduced risk for Alzheimer's disease and may offer some protection against age-associated declines in specific cognitive functions. The majority of these reports are based on observational studies, which are confounded by the "healthy user" bias--the tendency for women receiving hormone therapy to be younger, better educated, and have fewer medical problems. In one attempt to address these limitations, we conducted a series of studies examining effects of hormone therapy on cognitive and brain functioning in nondemented postmenopausal women in the Baltimore Longitudinal Study of Aging (BLSA). In this sample, women receiving hormone therapy and women who never received hormone therapy were comparable with respect to educational attainment, general medical health, and performance on a test of verbal knowledge. Despite these similarities, women receiving hormone therapy performed better on tests of verbal and visual memory compared to never-treated women. The two groups also differed in the patterns of regional brain activation evoked during performance of delayed verbal and figural memory tasks. Furthermore, longitudinal comparisons revealed greater relative blood flow increases over two years in women receiving hormone therapy for the hippocampus and other mesial temporal lobe structures that subserve memory. These observational findings from our studies in the BLSA have led to the development of a large-scale randomized clinical trial of hormone therapy and cognitive aging, the ancillary Women's Health Initiative Study of Cognitive Aging (WHISCA), and have important implications for studies of the effects of SERM's on cognitive and brain functioning.

Relations between hypometabolism in the posterior association neocortex and hippocampal atrophy in Alzheimer's disease: a PET/MRI correlative study

OBJECTIVES

Hippocampal atrophy and hypometabolism in the posterior association neocortex are two well known features of Alzheimer's disease. A correlation between these two features was reported twice previously, suggesting intriguing relations. This question has been reassessed, this time controlling for severity of dementia as well as assessing each side of the brain separately and using a voxel based image analysis in addition to the previously employed regions of interest (ROIs).

PATIENTS AND METHODS

Eleven patients were studied with probable Alzheimer's disease and mild to moderate dementia in whom both volume MRI and PET assessed cerebral glucose consumption (CMRGlc) were available. Hypothesis driven correlations between hippocampal width (an index of atrophy) and CMRGlc were performed for two posterior association regions, the superior temporal and the inferior parietal (angular gyrus) cortices, using ROIs set separately for each hemisphere. To confirm significant correlations from the ROIs approach, if any, and to assess their specificity for the posterior association neocortex, CMRGlc image voxel based analysis of correlations with hippocampal width was then carried out.

RESULTS

There was a significant correlation, in the positive-neurobiologically expected-direction, between right hippocampal width and right angular gyrus metabolism (p< 0.01, Spearman), which remained significant with Kendall partial correlation controlling for dementia severity (estimated by mini mental state scores). Statistical non-parametric mapping (SnPM) confirmed this correlation (p< 0.025), and showed a single additional correlation in the right middle temporal gyrus (p< 0.005), which is also part of the posterior association cortex.

CONCLUSION

The findings with both ROIs and voxel based mapping replicate earlier reports of a relation between hippocampal atrophy and ipsilateral association cortex hypometabolism in Alzheimer's disease, and for the first time document that this relation is both region specific and independent of the dementing process itself. Why the correlation was significant only for the right hemisphere is unclear but may be related to the limited sample. Hippocampal-neocortical disconnection due to early and severe medial temporal lobe pathology may at least partly explain the posterior association cortex hypometabolism found in Alzheimer's disease.

Clinical correlative evaluation of an iterative method for reconstruction of brain SPECT images

BACKGROUND

Brain SPECT and PET investigations have showed discrepancies in Alzheimer's disease (AD) when considering data deriving from deeply located structures, such as the mesial temporal lobe. These discrepancies could be due to a variety of factors, including substantial differences in gamma-cameras and underlying technology. Mesial temporal structures are deeply located within the brain and the commonly used Filtered Back-Projection (FBP) technique does not fully take into account either the physical parameters of gamma-cameras or geometry of collimators. In order to overcome these limitations, alternative reconstruction methods have been proposed, such as the iterative method of the Conjugate Gradients with modified matrix (CG). However, the clinical applications of these methods have so far been only anecdotal. The present study was planned to compare perfusional SPECT data as derived from the conventional FBP method and from the iterative CG method, which takes into account the geometrical and physical characteristics of the gamma-camera, by a correlative approach with neuropsychology.

METHODS

Correlations were compared between perfusion of the hippocampal region, as achieved by both the FBP and the CG reconstruction methods, and a short-memory test (Selective Reminding Test, SRT), specifically addressing one of its function. A brain-dedicated camera (CERASPECT) was used for SPECT studies with 99mTc-hexamethylpropylene-amine-oxime in 23 consecutive patients (mean age: 74.2 +/- 6.5) with mild (Mini-Mental Status Examination score > or =15, mean 20.3 +/- 3), probable AD. Counts from a hippocampal region in each hemisphere were referred to the average thalamic counts.

RESULTS

Hippocampal perfusion significantly correlated with the MMSE score with similar statistical significance (p < 0.01) between the two reconstruction methods. Correlation between hippocampal perfusion and the SRT score was better with the CG method (r = 0.50 for both hemispheres, p < 0.01) than with the FBP method (r = 0.37 and 0.43, respectively for the right and left hemisphere, p < 0.05 and p < 0.02). The bootstrap procedure showed that such correlation indexes were statistically different both in the right (p < 0.01) and in the left (p < 0.05) hemisphere.

CONCLUSION

These results are interpreted as a better performance of the CG reconstruction method in correctly detecting counts from hippocampal ROI. By using the same gamma-camera or collimator, alternative methods for brain SPECT reconstruction may improve quality of data and then help SPECT diagnostic accuracy.

Hippocampal formation glucose metabolism and volume losses in MCI and AD

We used MRI volume sampling with coregistered and atrophy corrected FDG-PET scans to test three hypotheses: 1) hippocampal formation measures are superior to temporal neocortical measures in the discrimination of normal (NL) and mild cognitive impairment (MCI); 2) neocortical measures are most useful in the separation of Alzheimer disease (AD) from NL or MCI; 3) measures of PET glucose metabolism (MRglu) have greater diagnostic sensitivity than MRI volume. Three groups of age, education, and gender matched NL, MCI, and AD subjects were studied. The results supported the hypotheses: 1) entorhinal cortex MRglu and hippocampal volume were most accurate in classifying NL and MCI; 2) both imaging modalities identified the temporal neocortex as best separating MCI and AD, whereas widespread changes accurately classified NL and AD; 3) In most between group comparisons regional MRglu measures were diagnostically superior to volume measures. These cross-sectional data show that in MCI hippocampal formation changes exist without significant neocortical changes. Neocortical changes best characterize AD. In both MCI and AD, metabolism reductions exceed volume losses.

Olfactory-evoked regional cerebral blood flow in Alzheimer's disease

Olfaction is impaired in Alzheimer's disease (AD). It was hypothesized that AD would reduce olfactory-evoked perfusion in mesial temporal olfactory (piriform) cortex, where neuropathology begins. Seven AD patients and 8 elderly controls (ECs) underwent olfactory threshold and identification tests and olfactory stimulation during positron emission tomography. Odor identification was impaired in AD, but threshold was not. Olfactory stimulation in ECs activated right and left piriform areas and right anterior ventral temporal cortex. AD patients had less activation in right piriform and anterior ventral temporal cortex but not in the left piriform area. Although orbital cortex did not activate in ECs, there was a significant between-groups difference in this area. Right piriform activation correlated with odor identification. Impaired odor identification likely reflects sensory cortex dysfunction rather than cognitive impairment. Given olfactory bulb projections to the mesial temporal lobe, olfactory stimulation during functional imaging might detect early dysfunction in this region.

Hippocampal perfusion in mild Alzheimer's disease

Perfusion and metabolic studies in patients with Alzheimer's disease (AD) have so far yielded conflicting results on the functional status of the hippocampal region, whose deep location in the brain makes it critical to optimize the image-reconstruction technique employed in emission tomography. We used a brain-dedicated device (CERASPECT) to perform single photon emission computed tomography (SPECT) studies with 99mTc-hexamethylpropylene-amine-oxime in 22 consecutive patients (mean age: 74+/-6.5 years) with mild [mini-mental status examination (MMSE) score > or =15, mean 20.8+/-3.2], probable AD. The control subjects were 11 healthy elderly people (mean age: 70.5+/-6.5 years). In patients, the total score on the selective reminding test (SRT) was used as an index of memory function. Counts from a hippocampal and a temporoparietal region of interest in each hemisphere were referred to the average thalamic counts. To optimize SPECT images, we used conventional filtered back-projection (FBP) reconstruction and a new iterative method of conjugate gradients (CG), which takes into account the geometrical and physical characteristics of the gamma-camera. Hippocampal perfusion in the two hemispheres was significantly lower in patients than in control subjects, regardless of which reconstruction method was used, and correlated with the MMSE score. The correlation between hippocampal perfusion and the SRT score was significantly (bootstrap procedure) higher with the CG method than with the FBP method (CG: r=0.52 and 0.54; FBP: r=0.39 and 0.47, for the right and left hemisphere, respectively). These results show hippocampal hypoperfusion in patients with mild AD, a correlation between hippocampal perfusion and the severity of cognitive impairment, and enhanced identification of these subtle perfusional changes with the use of an alternative image-reconstruction method that improves the spatial resolution of SPECT images.

Early diagnosis of Alzheimer disease with positron emission tomography

The emergence of drugs that may slow progression of Alzheimer disease, if administered early during its course, has necessitated early diagnosis of the disease itself. Among the functional imaging methods that could assist in early diagnosis, positron emission tomography has an important role in providing quantitative measures of various aspects of brain function affected by the disease. Positron emission tomography studies in patients with Alzheimer disease have revealed a typical pattern of metabolic deficits in the temporal and parietal lobes. Additionally, converging evidence from numerous studies indicates that a similar pattern of deficits can be observed in nondemented subjects who are at risk of developing the disease, such as those with recognized genetic traits such as familial Alzheimer disease with mutations in chromosomes 21 and 14, Down syndrome, subjects with the epsilon4 allele of the apolipoprotein E gene, and individuals with mild cognitive impairment. These findings might have implications for the selection of patients for clinical trials, defining the outcome measures and evaluation of treatment efficacy and responder characteristics, but should be confirmed by prospective studies comprising larger samples and include clinicopathologic correlations.

Greater metabolic rate decreases in hippocampal formation and proisocortex than in neocortex in Alzheimer's disease

Neuropathological studies of Alzheimer's disease (AD) have found pathological changes in some cytoarchitectural regions and relative sparing in others. Positron emission tomography (PET) studies have also shown selective decreases in glucose metabolic rates but have generally focused on whole brain lobes or geometrically derived regions of interest. In this report, a template of Brodmann areas, derived from a whole brain histological section atlas, was used to analyze PET findings from 34 AD patients and 16 control subjects matched for age, sex, and educational level. AD patients had lowest glucose metabolic rates in limbic areas of the temporal lobe and other proisocortical areas higher rates in frontal lobe and unimodal association areas, and relative sparing of parietal/occipital lobes and motor/sensory cortices. Analysis of variance revealed larger effect sizes when AD and control subjects were compared on metabolic rate for cortical type than for lobe. These findings, which parallel neuropathological studies of regional distribution of neurofibrillary tangles in AD, suggest that vulnerability is greatest in cortical areas that are in closer synaptic contact with limbic areas.

Striatal dopaminergic transmission and neocortical glucose utilization in Alzheimer's disease: a triple-tracer positron emission tomography study

The central dopaminergic neuron system is related to cognitive function and behavior. For Alzheimer's disease (AD), using PET and the ligands of dopa, D2 receptor antagonist, and deoxyglucose, we previously found that dopa uptake into the striatum (the Ki value) was correlated with the Mini-Mental State (MMS) score. AD patients exhibiting wandering had an increased level of D2 receptor (the k3/k4 value). The Ki value was correlated with glucose metabolism (CMRglc) of the frontal and temporal lobes, those of wanderers being lower than nonwanderers. We investigated the relationships between the Ki value and the k3/k4 value with reference to cognitive impairment and wandering. Using the three-tracer technique for the single subject, the Ki value, the k3/k4 value, and CMRglc in 12 AD patients were measured. For the wanderers, there was a reverse correlation between the Ki and the k3/k4 values. Multiple regression analysis showed that the score on the verbal fluency test was explained by the Ki value and CMRglc of the frontal and temporal lobes, while MMS score was related to more global areas.

Method to correlate 1H MRSI and 18FDG-PET

The in vivo neuronal contribution to human cerebral metabolic rate of glucose (CMRglc), measured by 18FDG-PET, is unknown. Examining the effect of 1H MRSI-derived N-acetyl aspartate (NAA) concentration on positron emission tomography (PET) measures of metabolic activity might indicate the relationship of CMRglc to neuron density. In a population of 19 demented, cognitively impaired, and control subjects, the Miller-Gartner algorithm was applied to whole-brain PET data to isolate the PET signal originating in cortical gray matter alone (GMPET). An analogous procedure applied to multislice proton MRSI data yielded the N-acetyl aspartate concentration in cortical gray matter (GMNAA). In 18 of 19 subjects, a significant linear regression (P < 0.05) resulted when GMPET was plotted against GMNAA, whereby GMPET was higher for higher GMNAA. This suggests that CMRglc rises linearly with increasing neuron density in gray matter. This method may be used to investigate the relationship of CMRglc to neurons in various conditions.

Low cerebral glucose extraction rates in the human medial temporal cortex and cerebellum

Previous studies have reported that there exist different regional sensitivities to acute hypoxia. To better understand these differences, we estimated regional differences of cerebral blood flow (CBF), cerebral glucose metabolism (CMRglc) and kinetic constants (K(1), k(2), k(3)) in the human cortex under resting conditions. CBF, CMRglc, kinetic rate constants and glucose extraction rate (GER) were measured in eight normal male subjects (mean age: 26.1+/- 4.9 years) using the 15O-water autoradiographic technique and subsequently the dynamic and the static [18F]2-fluoro-2-deoxy-D-glucose technique with positron emission tomography (PET). Of all the brain structures investigated, the medial temporal lobe showed the lowest CBF (46.0 ml/100 g/min) and lowest CMRglc (3.97 mg/100 g/min). The medial temporal GER was lowest (8.9%), followed by the cerebellar GER (9.3%). While the cerebellar blood flow (64.0 ml/100 g/min) was the highest, the cerebellar metabolic rate for glucose (5.79 mg/100 g/min) was relatively low. The cerebellum showed the highest K(1) value (0.13) and k(2) value (0.16), and the lowest k(3) value (0.05). In the medial temporal cortices and cerebellum, CMRglc and GER were lower than those in the neocortices. These results indicate that there are great perfusional/metabolic differences between the medial temporal lobe, cerebellum and other brain regions in the normal human brain under resting conditions.

Mitochondrial genome lesions in the pathogenesis of sporadic Alzheimer's disease

BACKGROUND

The recent, magnificent results of molecular biology concerning beta-amyloid (betaA) metabolism in early onset Alzheimer's disease (AD) have generated a series of new findings and, in turn, a new etiological concept. Attention on the early events in the pathogenesis of AD has been shifted from the chromosomal abnormalities in the nucleus of nerve cells onto genetic changes in the mitochondrial genome. This offers a new pathogenetic approach which also opens new pharmacological challenges particularly for the episodic forms of AD.

OBJECTIVE

Alterations occurring at the mitochondrial genome result in major consequences of oxidative phosphorylation and, if a specific threshold is exceeded, they may constitute important causative events in the apoptosis of selected nerve cells. The fact that the main source of mitochondrial metabolism is its glucose turnover allows monitoring brain changes in glucose metabolism by 18F-2 deoxyglucose positron emission tomography. In the demented brain, a low glucose turnover causes a cholinergic deficit by decreasing the synthetic rate of acetyl coenzyme A (AcCoA). AcCoA represents the key substrate for the acetylation of choline to acetylcholine by choline acetyltransferase. The consistent energy need for AcCoA synthesis appears obvious when considering that 1 molecule of glucose generates just 2 molecules of AcCoA, but 38 molecules of ATP. In the brain, AcCoA is exclusively synthesized in the glycolitic pathway. Generation of betaA is increased if the synthetic rate of ATP drops below a critical threshold: under these conditions, the betaA precursor protein (betaAPP) is inserted only in part into synaptic membranes which have the highest betaAPP turnover. In conditions of short ATP supply, betaAPP is not split at the beta region by an ATP-activated protease and this results in a substantial increase in uncleaved betaA molecules.

CONCLUSION

Peroxidative alterations in mitochondrial DNA are of importance in degenerative diseases of postmitotic tissues, particularly in degenerative diseases. This offers a new pharmacological approach for the treatment of AD. Neurotrophic factors and estrogen seem to be the first pharmacological leads.

Neocortical and hippocampal glucose hypometabolism following neurotoxic lesions of the entorhinal and perirhinal cortices in the non-human primate as shown by PET. Implications for Alzheimer's disease

Temporoparietal glucose hypometabolism, neuronal loss in the basal forebrain cholinergic structures and preferential accumulation of neurofibrillary tangles in the rhinal cortex (i.e. in the entorhinal and perirhinal cortices) are three early characteristics of Alzheimer's disease. Based on studies of the effects of neurotoxic lesions in baboons, we previously concluded that damage to the cholinergic structures plays, at best, a marginal role in the association neocortex hypometabolism of Alzheimer's disease. In the present study, we have assessed the remote metabolic effects of bilateral neurotoxic lesions of both entorhinal and perirhinal cortices. Using coronal PET coregistered with MRI, the cerebral metabolic rate for glucose (CMR(glc)) was measured before surgery and sequentially for 2-3 months afterward (around days 30, 45 and 80). Compared with sham-operated baboons, the lesioned animals showed a significant and long-lasting CMR(glc) decline in a small set of brain regions, especially in the inferior parietal, posterior temporal, posterior cingulate and associative occipital cortices, as well as in the posterior hippocampal region, all of which also exhibit glucose hypometabolism in Alzheimer's disease. Remarkably, the degree of CMR(glc) decline in four of these regions significantly correlated with the severity of histologically determined damage in the rhinal cortex, strongly supporting the specificity of the observed metabolic effects. There were also differences between the metabolic pattern observed in the lesioned animals and that classically reported in Alzheimer's disease; for instance, the hypometabolism we found in the stratum has not been reported in early Alzheimer's disease, although this structure can be affected in late stages of the disease and has direct anatomical connections with the rhinal cortex. Nevertheless, this study shows for the first time that the temporoparietal and hippocampal hypometabolism found in Alzheimer's disease may partly result from neuroanatomical disconnection with the rhinal cortex. This, in turn, further strengthens the hypothesis that neuronal damage and dysfunction in the rhinal cortices play a major role in the expression of Alzheimer's disease.

Altered brain glucose metabolism in transgenic-PFKL mice with elevated L-phosphofructokinase: in vivo NMR studies

The gene for the liver-type subunit of phosphofructokinase (PFKL) resides on chromosome 21 and is overexpressed in Down syndrome (DS) patients. Transgenic PFKL (Tg-PFKL) mice with elevated levels of PFKL were used to determine whether, as in DS, overexpression of PFKL was also associated with altered sugar metabolism. We found that Tg-PFKL mice had an abnormal glucose metabolism with reduced clearance rate from blood and enhanced metabolic rate in brain. Transgenic-PFKL mice exhibited elevated activity of phosphofructokinase in both blood and brain, as compared to control non-transgenic (ntg) mice. Following glucose infusion, the rate of glucose clearance from the blood of Tg-PFKL mice was significantly slower than that of control ntg mice, although the basal blood glucose levels were similar. However, unlike the slower rate of glucose metabolism in blood, the initial rate of glucose utilization in the brain of the transgenic mice, was 58% faster than in control ntg mice. This was determined by infusion of [1-13C]-glucose followed by in vivo nuclear magnetic resonance (NMR) measurements of brain glucose metabolism. The faster utilization of glucose in Tg-PFKL brain is similar to the increased rate of cerebral glucose metabolism found in the brain of young adult DS patients, which may play a role in the etiology of their cognitive disabilities.

Relatively preserved hippocampal glucose metabolism in mild Alzheimer's disease

The purpose of this study was to clarify the changes in hippocampal glucose metabolism in mild Alzheimer's disease (AD) using positron emission tomography (PET) and 2-(18F)fluoro-2-deoxy-D-glucose (FDG). Forty-one patients with probable mild AD (age: 69.0 +/- 8.0 years; MMSE: 22.6 +/- 2.1) and 22 normal volunteers (age: 67.7 +/- 7.1 years) were studied. The regional cerebral metabolic rate for glucose (CMRglc) was measured using FDG and PET. Although the mean CMRglc in the parietal region was significantly lower in the AD group (right: 6.35 +/- 1.26 mg/100 g/min; left: 6.37 +/- 1.21 mg/100 g/min) than in the control group (right: 7.73 +/- 1.02 mg/100 g/min; left: 7.63 +/- 0.95 mg/100 g/min), the mean CMRglc in the hippocampus did not show a significant difference between the AD group (right: 4.58 +/- 0.70 mg/100 g/min; left: 4.63 +/- 0.67 mg/100 g/min) and the control group (right: 5.22 +/- 0.65 mg/100 g/min; left: 5.22 +/- 0.67 mg/100 g/min) by analysis of variance and post-hoc Tukey's test. The magnitude of the hippocampal CMRglc reduction was not as large as that of parietal CMRglc reduction. Statistical parametric maps (SPM) analysis also did not significantly demonstrate reduced hippocampal CMRglc in AD patients, although it did show a significant reduction in parietal CMRglc in AD patients. Hippocampal CMRglc was not significantly decreased in mild AD. This was unexpected in view of previous studies that have shown atrophy and clinical dysfunction concerning hippocampus in AD, and suggests that the pathophysiology of the hippocampus in AD may be more complex than was previously thought.

Re-evaluating the role of vascular changes in the differential diagnosis of Alzheimer's disease and vascular dementia

Alzheimer's disease (AD) and vascular dementia (VaD) are the two most common causes of dementia, and much effort has been devoted to their differential diagnosis. However, current epidemiological, clinical and neuropathological evidence points to a substantial overlap between AD and VaD and suggests that vascular pathology, the traditional cornerstone of the differential diagnosis between the two entities, may not represent as clear a line of demarcation as originally believed. It may be time to reevaluate the dichotomy between AD and VaD.

Clinical studies of cerebral blood flow in Alzheimer's disease

Studies of Alzheimer's disease using single-photon emission computed tomography (SPECT) and positron emission tomography (PET) have found reductions in blood flow and glucose metabolism in temporal and parietal cortex. In 50 AD patients who underwent neuropsychological testing and SPECT perfusion imaging, we found significant correlations between perfusion and performance on the Mini-Mental Status Examination in the frontal and parietal lobes. In addition, specific correlations between perfusion in the frontal lobes and performance on tests of frontal lobe ability were noted. These findings, while suggesting the importance of perfusion measures in determining clinical features of the disease, do not clearly define perfusion changes as primary, since similar findings have been seen when metabolism is studied. In a separate group of 5 AD patients and 16 controls, we used PET with the perfusion tracer HIPDM and examined cerebrovascular reactivity to carbon dioxide inhalation. We found that in multiple brain regions, including the temporal lobes, AD patients showed robust and significant increases in perfusion in response to carbon dioxide that did not differ from the response seen in the controls. Taken together, these results show that while perfusion changes are important in AD, they are not clearly either primary or limiting.

Metabolic reduction in the posterior cingulate cortex in very early Alzheimer's disease

This study investigated cerebral glucose metabolism in very early Alzheimer's disease, before a clinical diagnosis of probable Alzheimer's disease is possible, using [18F]fluorodeoxyglucose positron emission tomography. First, 66 patients with probable Alzheimer's disease with a spectrum of dementia severity (Mini-Mental State Examination score, 0-23) were recruited and studied. Cortical metabolic activity was analyzed topographically using three-dimensional stereotactic surface projections. Regression analysis was performed for each brain pixel to predict metabolic patterns of very early disease. Predictions were tested prospectively in a group of 8 patients who complained only of memory impairment without general cognitive decline (Mini-Mental State Examination score, 25 +/- 1) at the time of scanning but whose condition later progressed to probable Alzheimer's disease. Both results were compared to cerebral metabolic activity in 22 age-similar normal control subjects. Prediction and analysis of actual patients consistently indicated marked metabolic reduction (21-22%) in the posterior cingulate cortex and cinguloparietal transitional area in patients with very early Alzheimer's disease. Mean metabolic reduction in the posterior cingulate cortex was significantly greater than that in the lateral neocortices or parahippocampal cortex. The result suggests a functional importance for the posterior cingulate cortex in impairment of learning and memory, which is a feature of very early Alzheimer's disease.

Double dissociation of semantic categories in Alzheimer's disease

Data that demonstrate distinct patterns of semantic impairment in Alzheimer's disease (AD) are presented. Findings suggest that while groups of mild-moderate patients may not display category specific impairments, some individual patients do show selective impairment of either natural kinds or artifacts. We present a model of semantic organization in which category specific impairments arise from damage to distributed features underlying different types of categories. We incorporate the crucial notions of intercorrelations and distinguishing features, allowing us to demonstrate (1) how category specific impairments can result from widespread damage and (2) how selective deficits in AD reflect different points in the progression of impairment. The different patterns of impairment arise from an interaction between the nature of the semantic categories and the progression of damage.

The ratio of mesial to neocortical temporal lobe blood flow as a predictor of dementia

OBJECTIVE

The hypothesis tested was that an increased ratio of cerebral blood flow (rCBF) in the mesocortical temporal lobe to that of the neocortical temporal lobe (MES/ NEO ratio) would be related to clinical measures of dementia severity and would help distinguish Alzheimer's patients from normal controls.

DESIGN

The rCBF of nine Alzheimer's patients (5 males and 4 females; age = 65.9 +/- 6.0 years, range 55-71; Folstein Mini-Mental Status Examination = 18.6 +/- 7.4, range 9-28) and 10 age-matched normal controls (7 males, 3 females; age = 66.0 +/- 5.7 years, range 58-75) was determined by positron emission tomography (PET) using H2(15)0 and the method of Alpert et al.

RESULTS

Alzheimer's disease patients had a significantly higher MES/NEO ratio (1.19 +/- .17) than the age-matched normal controls (.854 +/- .14; t = .-4.74, df = 17, P = .0002). Using a MES/NEO ratio of 1 as the cutoff point for discrimination between Alzheimer's patients and normal controls, the ratio demonstrated 100% sensitivity (no. of correctly identified Alzheimer's patients/no. of Alzheimer's patients) and 90% specificity (no. of correctly identified unaffected subjects/no. of unaffected subjects). Further, those patients with the highest MES/NEO ratios had the lowest overall measures of cognitive function (Folstein Mini-Mental Status Examination: r = -.75, P < .02, 1-tail; Mattis Dementia Rating Scale: r = -0.655, P = .028, 1-tail) scores.

CONCLUSIONS

The findings are consistent with other in vivo and postmortem studies, suggesting that functional and structural changes of the lateral temporal lobe in Alzheimer's disease occur relatively early in the disease process and appear to be distinguishable from those changes accompanying normal aging. In contrast, the memory loss and pathology of the mesial temporal lobe that is characteristic of the early stages of Alzheimer's patients do not appear to be associated with a reduction in cerebral blood flow in the resting Alzheimer's patient. Nevertheless, the clinical significance of the results must await findings of longitudinal studies of larger numbers of Alzheimer's patients and controls.

Functional imaging predicts cognitive decline in Alzheimer's disease

Eighteen patients with Alzheimer's disease were studied with positron emission tomographic measurements of regional cerebral metabolism of glucose. All patients were initially diagnosed and evaluated, underwent positron emission tomography, and then were followed with annual reevaluations, at which time the Mini-Mental State Examination (MMSE) was performed. Patients were followed for an average of 2.5 years, and the rate of cognitive decline was calculated by determining the rate of change in the MMSE score defined as the MMSE score at the initial evaluation minus the MMSE score at the last examination, divided by the number of months between testing. The regional cerebral metabolic rates for glucose determined at the time of the first MMSE were then regressed on these changes in scores. Results showed that glucose metabolic rates in posterior temporal and primary visual cortex regions were significantly correlated with the subsequent rate of cognitive deterioration. These associations were not confounded by age, length of follow-up, baseline MMSE score, or education. Stratification on gender suggested that these associations were much stronger in women than in men. These results replicate previous findings showing that functional brain imaging is predictive of the rate of cognitive decline in Alzheimer's disease.

Neuropsychology of memory and SPECT in the diagnosis and staging of dementia of Alzheimer type

We studied the role of neuropsychology and SPECT imaging in the diagnosis and staging of dementia of Alzheimer type (DAT) in 33 patients with mild disease and 30 matched controls. Working, episodic, semantic and remote memory were assessed. For diagnosis, it was determined by logistic regression analysis that one of the memory tests (delayed verbal recall of the Doors and People Test) could correctly classify subjects as DAT or controls in 97% of cases. For staging, stepwise regression analysis using five of the memory tests could predict 70% of the variance in Mini Mental State Examination (MMSE) scores. The measures most useful for staging were tests of immediate recall, and tests of working, semantic and autobiographical memory. In a separate experiment, SPECT imaging on 31 of the above 33 patients and 24 different controls was used to address the issue of diagnosis and staging. 99Tc-HMPAO SPECT scans were analysed quantitatively to obtain measures of regional cerebral blood flow (rCBF). Logistic regression analysis showed that three of the SPECT regions of interest (left posterior temporal, right high frontal and right posterior temporal) could correctly classify subjects in 75% of cases. Of note was the fact that 39% of the DAT patients had normal SPECT scans. SPECT data were of limited use in modelling disease severity; only 38% of the variance in MMSE scores could be predicted from SPECT data. In addition, we found that the pattern of rCBF in DAT was much more heterogeneous than previously described.

Functional imaging patterns in Alzheimer's disease. Relationships to neurobiology

Brain imaging with functional techniques, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) have been widely applied to the study of dementia. While the pattern of temporal and parietal hypometabolism and hypoperfusion have often been suggested to be of diagnostic utility in ascertaining that a dementia is due to Alzheimer's disease (AD), the exact sensitivity and specificity of this pattern in clinically important situations is unclear. These imaging findings have been of considerable interest, however, in describing the regional patterns of predilection in the disease. Evidence supports the contention that the earliest sites of functional impairment in AD are in the temporal lobes. Surprisingly, however, mesial temporal lobe hypometabolism was difficult to detect in a group of mildly demented AD patients in comparison to a group of healthy older subjects. These results suggest that simple use of mesial temporal lobe metabolic rates as a diagnostic for AD may not be fruitful, and that evaluation of the earliest stages of AD can be most productively studied by investigating healthy older individuals.

The significance of glucose turnover in the brain in the pathogenetic mechanisms of Alzheimer's disease

This paper presents a comprehensive survey of the pathogenesis and pathophysiology of Alzheimer's disease (AD). Two mechanisms are of etiological importance in the development of a degenerative dementing brain disease: 1. Lesions in the mitochondrial genome that are caused by free radicals. Primary degenerative AD is characterized by a tendency to acquire random lesions within mitochondrial DNA that are produced by free radicals. The consequence of these lesions is a decrease in glucose turnover and a decline in oxidative phosphorylation. Point mutations on chromosome 21 are hypothesized to increase the susceptibility of mitochondrial DNA to lesions created by free radicals. 2. Ischemic brain lesions as well as traumatic brain damage cause an increase in the release of excitotoxic amino acids (glutamate, aspartate, etc.). These neurotransmitters increase CA(+2) influx into the nerve cell and significantly lower energy production. From a pathogenetic point of view, AD is characterized by a decrease in glucose turnover in the brain. The progression of AD can be monitored by F18- deoxyglucose PET studies. This technique also allows the recognition of patients who are prone to develop AD. The actual development of a cognitive deficit is a threshold phenomenon that occurs if glucose turnover in the hippocampus or temporoparietal cortex drops below a critical level of about 40% of the level of age-matched controls. The low glucose turnover in AD causes a cholinergic deficit by decreasing the synthesis of AcCoA, which is used by choline acetyltransferase in the acetylation of choline to acetylcholine. The decrease in glucose turnover also reduces oxidative phosphorylation. The resulting decrease in ATP triggers the hyperphosphorylation of tau protein by activating protein kinase 40erk. The hyperphosphorylation leads to the development of paired helical filaments. The generation of beta amyloid and the loss of neuronal synapses are also caused by a decrease in oxidative phosphorylation, since beta amyloid precursor proteins are not inserted into the membranes of nerve cells in the absence of a sufficient amount of ATP. This results in the generation of intact beta amyloid molecules and leads to amyloidosis in the brains of patients with Alzheimer's disease.

Reduced temporal lobe glucose metabolism in aging

The results of a positron emission tomography study of regional cerebral metabolic rates of glucose are reported for 8 healthy old subjects (mean age, 66 yr; standard deviation [SD], 5) and 9 young subjects (mean age, 27 yr; SD, 4.6) using a high-resolution positron emission tomograph and the glucose metabolic tracer 18F-fluorodeoxyglucose. Older subjects showed significantly lower cerebral metabolic rates than did the young subjects, in anterior, middle, and posterior temporal neocortex and in mesial temporal cortex, with the largest differences occurring in anterior temporal cortex (temporal pole). The current findings may reflect either decreases in regional cerebral metabolic rates for glucose that occur with normal aging, or early indications of cognitive dysfunction that is associated with age-related disorders.