Alzheimer's patients engage an alternative network during a memory task

Drug development for CNS disorders: strategies for balancing risk and reducing attrition

Disorders of the central nervous system (CNS) are some of the most prevalent, devastating and yet poorly treated illnesses. The development of new therapies for CNS disorders such as Alzheimer's disease has the potential to provide patients with significant improvements in quality of life, as well as reduce the future economic burden on health-care systems. However, few truly innovative CNS drugs have been approved in recent years, suggesting that there is a considerable need for strategies to enhance the productivity of research and development in this field. In this article, using illustrative examples from neurological and psychiatric disorders, we describe various approaches that are being taken to discover CNS drugs, discuss their relative merits and consider how risk can be balanced and attrition reduced.

Preserved metamemorial ability in patients with mild Alzheimer's disease: shifting response bias

Patients with mild Alzheimer's disease (AD) display a greater tendency to endorse unstudied items as "old" on memory tests than healthy older adults. This liberal response bias may result in mistaken beliefs about the completion of common tasks. This research attempted to determine whether it was possible to shift the response bias of mild AD patients to be more conservative on a recognition memory test through behavioral intervention. Patients with mild AD and matched controls were evaluated with two almost identical paradigms, separated by about one week. For each session, 30 words were studied and 60 words (half studied, half novel) were shown at test. During one session participants were told that 30% of words were old, and at the other session that 70% were old. We found that both groups were able to shift their response bias between the two conditions. That patients with mild AD were able to successfully shift their response bias demonstrates that--despite their overall liberal response bias and poor memory relative to controls--one component of metamemorial ability is preserved in patients with mild AD.

Use of functional magnetic resonance imaging in the early identification of Alzheimer's disease

A growing body of evidence suggests that a preclinical phase of Alzheimer's disease (AD) exists several years or more prior to the overt manifestation of clinical symptoms and is characterized by subtle neuropsychological and brain changes. Identification of individuals prior to the development of significant clinical symptoms is imperative in order to have the greatest treatment impact by maintaining cognitive abilities and preserving quality of life. Functional magnetic resonance imaging (fMRI) offers considerable promise as a non-invasive tool for detecting early functional brain changes in asymptomatic adults. In fact, evidence to date indicates that functional brain decline precedes structural decline in preclinical samples. Therefore, fMRI may offer the unique ability to capture the dynamic state of change in the degenerating brain. This review examines the clinical utility of blood oxygen level dependent (BOLD) fMRI in those at risk for AD as well as in early AD. We provide an overview of fMRI findings in at-risk groups by virtue of genetic susceptibility or mild cognitive decline followed by an appraisal of the methodological issues concerning the diagnostic usefulness of fMRI in early AD. We conclude with a discussion of future directions and propose that BOLD-fMRI in combination with cerebral blood flow or diffusion techniques will provide a more complete accounting of the neurovascular changes that occur in preclinical AD and thus improve our ability to reliably detect early brain changes prior to disease onset.

Functional MRI studies of associative encoding in normal aging, mild cognitive impairment, and Alzheimer's disease

Functional magnetic resonance imaging (fMRI) is a noninvasive neuroimaging technique that can be used to study the neural correlates of complex cognitive processes, and the alterations in these processes that occur in the course of normal aging or superimposed neurodegenerative disease. Our studies have focused on the neural substrates of successful associative encoding, particularly of face-name associations. We have found that the specific regions of the hippocampus and prefrontal cortices are critical for successful memory in both young and healthy older subjects. Our fMRI studies, as well as those of several other groups, have consistently demonstrated that, compared to cognitively intact older subjects, patients with clinical Alzheimer's disease (AD) have decreased fMRI activation in the hippocampus and related structures within the medial temporal lobe during the encoding of new memories. More recently, fMRI studies of subjects at risk for AD, by virtue of their genetics or evidence of mild cognitive impairment (MCI), have yielded variable results. Some of these studies, including our own, suggest that there may be a phase of paradoxically increased activation early in the course of prodromal AD. Further studies to validate fMRI in these populations are needed, particularly longitudinal studies to investigate the pattern of alterations in functional activity over the course of prodromal AD and the relationship to AD pathology.

Reelin depletion in the entorhinal cortex of human amyloid precursor protein transgenic mice and humans with Alzheimer's disease

Reelin regulates nervous system development and modulates synaptic plasticity in the adult brain. Several findings suggest that alterations in Reelin signaling may contribute to neuronal dysfunction associated with Alzheimer's disease (AD). Cell surface receptors for Reelin, including integrins and very-low-density lipoprotein receptor/apolipoprotein E2 receptor, may be targets of amyloid-beta (Abeta) peptides presumed to play key roles in the pathogenesis of AD. Reelin also regulates the extent of tau phosphorylation. Finally, increased amounts of Reelin fragments have been found in CSF from AD patients, suggesting altered processing of Reelin. We therefore hypothesized that Reelin levels might be altered in the brains of human amyloid precursor protein (hAPP) transgenic mice, particularly in brain regions vulnerable to AD such as hippocampus and entorhinal cortex. Compared with nontransgenic controls, hAPP mice had significantly fewer Reelin-expressing pyramidal cells in the entorhinal cortex, the major population of glutamatergic neurons expressing Reelin in the brain. Western blot analysis of the hippocampus, which receives projections from the entorhinal cortex, revealed significant reductions in Reelin levels. In contrast, the number of Reelin-expressing GABAergic interneurons was not altered in either the entorhinal cortex or the hippocampus. Thus, neuronal expression of hAPP/Abeta is sufficient to reduce Reelin expression in a specific population of entorhinal cortical pyramidal neurons in vivo. Underscoring the relevance of these findings, we found qualitatively similar reductions of Reelin-expressing pyramidal neurons in the entorhinal cortex of AD brains. We conclude that alterations in Reelin processing or signaling may be involved in AD-related neuronal dysfunction.

A network dysfunction perspective on neurodegenerative diseases

Patients with Alzheimer's disease or other neurodegenerative disorders show remarkable fluctuations in neurological functions, even during the same day. These fluctuations cannot be caused by sudden loss or gain of nerve cells. Instead, it is likely that they reflect variations in the activity of neural networks and, perhaps, chronic intoxication by abnormal proteins that the brain is temporarily able to overcome. These ideas have far-reaching therapeutic implications.

[Brain profile of hypometabolism in early Alzheimer's disease: relationships with cognitive deficits and atrophy]

While accurate and early prediction of patients that will develop Alzheimer's disease (AD) in the near future is urgently needed, the amnestic Mild Cognitive Impairment (MCI) state is of particular interest since it most conveniently represents the pre-dementia stage of AD. Consistently, the profile of brain functional alteration constantly evidenced in resting-state SPECT and PET studies is similar to that observed in mild AD, mainly involving the posterior cingulate and temporo-parietal regions. While the former is a characteristic feature of MCI, since it is present in each patient at this stage, the latter seems specifically associated with the future conversion to AD. Moreover, right temporo-parietal hypometabolism has been found to be the best predictor of subsequent global cognitive decline, over and above neuropsychological and MRI volumetric measurements. This review also presents a discussion on the relationships between the brain profile of hypometabolism on the one hand, and cognitive impairment as well as cerebral structural alterations on the other. Thus, firstly, while functional impairment in the posterior cingulate region seems to be associated with deficits in retrieval of episodic memories in MCI, the relationship between right temporo-parietal hypometabolism and cognitive impairment is still obscure. However, several arguments point to its relation with visuo-spatial deficits, which are often associated with future conversion to AD. Secondly, the discordance between brain areas of major functional changes, and those of highest structural alterations, leads to some relevant questions about the relations between both pathological manifestations and their underlying mechanisms. More specifically, additional hypometabolism-inducing factors could occur in areas of highest hypometabolism compared to atrophy, i.e. mainly in posterior associative cortical regions, leading to genuine functional perturbation in early AD before the development of real atrophy and perhaps of disease as well. By contrast, the hippocampus is the main site of atrophy while its functional alteration is still debated, suggesting that compensation/protective mechanisms probably specifically occur in this structure to maintain a high level of metabolism relative to its structural alteration.

Increased fMRI responses during encoding in mild cognitive impairment

Structural and functional magnetic resonance imaging (fMRI) was performed on 21 healthy elderly controls, 14 subjects with mild cognitive impairment (MCI) and 15 patients with mild Alzheimer's disease (AD) to investigate changes in fMRI activation in relation to underlying structural atrophy. The fMRI paradigm consisted of associative encoding of novel picture-word pairs. Structural analysis of the brain was performed using voxel-based morphometry (VBM) and hippocampal volumetry. Compared to controls, the MCI subjects exhibited increased fMRI responses in the posterior hippocampal, parahippocampal and fusiform regions, while VBM revealed more atrophy in MCI in the anterior parts of the left hippocampus. Furthermore, the hippocampal volume and parahippocampal activation were negatively correlated in MCI, but not in controls or in AD. We suggest that the increased fMRI activation in MCI in the posterior medial temporal and closely connected fusiform regions is compensatory due to the incipient atrophy in the anterior medial temporal lobe.

Pain sensitivity and fMRI pain-related brain activity in Alzheimer's disease

People with Alzheimer's disease are administered fewer analgesics and report less clinical pain than cognitively intact peers with similar painful diseases or injuries, prompting speculation about the likely impact of neurodegeneration on central pain processing. The present study measured pain ratings and functional MRI (fMRI) brain responses following mechanical pressure simulation in 14 patients with Alzheimer's disease and 15 age-matched controls. Contrary to the prevailing hypothesis that this disease is likely to differentially reduce emotional responses to pain, we show that activity in both medial and lateral pain pathways is preserved. Moderate pain was evoked with similar stimuli in both groups, and was associated with a common network of pain-related activity incorporating cingulate, insula and somatosensory cortices. Between-group analyses showed no evidence of diminished pain-related activity in Alzheimer's disease patients compared with controls. In fact, compared with controls, patients showed greater amplitude and duration of pain-related activity in sensory, affective and cognitive processing regions consistent with sustained attention to the noxious stimulus. The results of this study show that pain perception and processing are not diminished in Alzheimer's disease, thereby raising concerns about the current inadequate treatment of pain in this highly dependent and vulnerable patient group.