Regional cholinergic denervation of cortical microvessels and nitric oxide synthase-containing neurons in Alzheimer's disease

Cerebral autoregulation in Alzheimer's disease

Cerebral autoregulation aims to stabilize blood flow to the brain during variations in perfusion pressure, thus protecting the brain against the risks of low or high systemic blood pressure. This vital mechanism is severely impaired in the transgenic mouse model of Alzheimer's disease (AD) that abundantly produces amyloid-β peptide β(1-42). These observations have been extrapolated to human AD, wherein impairment of autoregulation could have important implications for the clinical management and prevention of AD. Research on cerebral autoregulation in human AD, however, has only recently become available. Contrary to the animal models, preliminary studies suggest that cerebral autoregulation is preserved in patients with AD. Further research is urgently needed to elucidate this discrepancy in the current literature, given the accumulating evidence that implicates cerebrovascular pathology in AD.

Neurovascular function in Alzheimer's disease patients and experimental models

The ability of the brain to locally augment glucose delivery and blood flow during neuronal activation, termed neurometabolic and neurovascular coupling, respectively, is compromised in Alzheimer's disease (AD). Since perfusion deficits may hasten clinical deterioration and have been correlated with negative treatment outcome, strategies to improve the cerebral circulation should form an integral element of AD therapeutic efforts. These efforts have yielded several experimental models, some of which constitute AD models proper, others which specifically recapture the AD cerebrovascular pathology, characterized by anatomical alterations in brain vessel structure, as well as molecular changes within vascular smooth muscle cells and endothelial cells forming the blood-brain barrier. The following paper will present the elements of AD neurovascular dysfunction and review the in vitro and in vivo model systems that have served to deepen our understanding of it. It will also critically evaluate selected groups of compounds, the FDA-approved cholinesterase inhibitors and thiazolidinediones, for their ability to correct neurovascular dysfunction in AD patients and models. These and several others are emerging as compounds with pleiotropic actions that may positively impact dysfunctional cerebrovascular, glial, and neuronal networks in AD.

Role of M1, M3, and M5 muscarinic acetylcholine receptors in cholinergic dilation of small arteries studied with gene-targeted mice

Acetylcholine regulates perfusion of numerous organs via changes in local blood flow involving muscarinic receptor-induced release of vasorelaxing agents from the endothelium. The purpose of the present study was to determine the role of M₁, M₃, and M₅ muscarinic acetylcholine receptors in vasodilation of small arteries using gene-targeted mice deficient in either of the three receptor subtypes (M1R(-/-), M3R(-/-), or M5R(-/-) mice, respectively). Muscarinic receptor gene expression was determined in murine cutaneous, skeletal muscle, and renal interlobar arteries using real-time PCR. Moreover, respective arteries from M1R(-/-), M3R(-/-), M5R(-/-), and wild-type mice were isolated, cannulated with micropipettes, and pressurized. Luminal diameter was measured using video microscopy. mRNA for all five muscarinic receptor subtypes was detected in all three vascular preparations from wild-type mice. However, M(3) receptor mRNA was found to be most abundant. Acetylcholine produced dose-dependent dilation in all three vascular preparations from M1R(-/-), M5R(-/-), and wild-type mice. In contrast, cholinergic dilation was virtually abolished in arteries from M3R(-/-) mice. Deletion of either M₁, M₃, or M₅ receptor genes did not affect responses to nonmuscarinic vasodilators, such as substance P and nitroprusside. These findings provide the first direct evidence that M₃ receptors mediate cholinergic vasodilation in cutaneous, skeletal muscle, and renal interlobar arteries. In contrast, neither M₁ nor M₅ receptors appear to be involved in cholinergic responses of the three vascular preparations tested.

Intact memory in TGF-β1 transgenic mice featuring chronic cerebrovascular deficit: recovery with pioglitazone

The roles of chronic brain hypoperfusion and transforming growth factor-beta 1 (TGF-β1) in Alzheimer's disease (AD) are unresolved. We investigated the interplay between TGF-β1, cerebrovascular function, and cognition using transgenic TGF mice featuring astrocytic TGF-β1 overexpression. We further assessed the impact of short, late therapy in elderly animals with the antioxidant N-acetyl-L-cysteine (NAC) or the peroxisome proliferator-activated receptor-γ agonist pioglitazone. The latter was also administered to pups as a prophylactic 1-year treatment. Elderly TGF mice featured cerebrovascular dysfunction that was not remedied with NAC. In contrast, pioglitazone prevented or reversed this deficit, and rescued the impaired neurovascular coupling response to whisker stimulation, although it failed to normalize the vascular structure. In aged TGF mice, neuronal and cognitive indices--the stimulus-evoked neurometabolic response, cortical cholinergic innervation, and spatial memory in the Morris water maze--were intact. Our findings show that impaired brain hemodynamics and cerebrovascular function are not accompanied by memory impairment in this model. Conceivably in AD, they constitute aggravating factors against a background of aging and underlying pathology. Our data further highlight the ability of pioglitazone to protect the cerebrovasculature marked by TGF-β1 increase, aging, fibrosis, and antioxidant resistance, thus of high relevance for AD patients.

Effect of ApoE epsilon4 allele on visual evoked potentials and resultant flow coupling in patients with Alzheimer

The apolipoprotein E epsilon4 (ApoE epsilon4) allele is a strong susceptibility factor for Alzheimer disease, which promotes neurodegeneration and cerebrovascular dysfunction. To address this issue in more detail, we simultaneously obtained visual evoked potentials and resultant hemodynamic responses in newly diagnosed Alzheimer patients without signs of vascular lesions on a cerebral magnetic resonance imaging (MRI) scan. Patients were grouped according to ApoE genotype (n = 19 ApoE epsilon4 carrier and n = 12 noncarrier). ApoE epsilon4 carrier had significantly longer peak latencies and a trend to higher interpeak latencies of late potential components. Potential amplitudes and hemodynamic responses were similar in both groups. At the incidental stage of disease process, it appears that the ApoE epsilon4 allele mainly promotes neuronal dysfunction rather than aggravates neurovascular dysfunction. Studies with larger patient samples are warranted to corroborate the first findings.

The Nuclear Receptor PPARgamma as a Therapeutic Target for Cerebrovascular and Brain Dysfunction in Alzheimer's Disease

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear transcription factors that regulate peripheral lipid and glucose metabolism. Three subtypes make up the PPAR family (alpha, gamma, beta/delta), and synthetic ligands for PPARalpha (fibrates) and PPARgamma (Thiazolidinediones, TZDs) are currently prescribed for the respective management of dyslipidemia and type 2 diabetes. In contrast to the well characterized action of PPARs in the periphery, little was known about the presence or function of these receptors in the brain and cerebral vasculature until fairly recently. Indeed, research in the last decade has uncovered these receptors in most brain cell types, and has shown that their activation, particularly that of PPARgamma, is implicated in normal brain and cerebrovascular physiology, and confers protection under pathological conditions. Notably, accumulating evidence has highlighted the therapeutic potential of PPARgamma ligands in the treatment of brain disorders such as Alzheimer's disease (AD), leading to the testing of the TZDs pioglitazone and rosiglitazone in AD clinical trials. This review will focus on the benefits of PPARgamma agonists for vascular, neuronal and glial networks, and assess the value of these compounds as future AD therapeutics in light of evidence from transgenic mouse models and recent clinical trials.

The cerebrovascular role of the cholinergic neural system in Alzheimer's disease

The intrinsic cholinergic innervation of the cortical microvessels contains both subcortical pathways and local cortical interneurons mediated by muscarinic and nicotinic acetylcholine receptors. Stimulation of this system leads to vasodilatation. In the extrinsic innervation, choline acts as a selective agonist for the α7-nicoticinic acetylcholine receptor on the sympathetic nerves to cause vasodilatation, and through this mechanism, cholinergic modulation may affect this sympathetic vasodilatation. Alzheimer's disease is characterized by a cerebral cholinergic deficit and cerebral blood flow is diminished. Cholinesterase inhibitors, important drugs in the treatment of Alzheimer's disease, could influence the cerebral blood flow through stimulation of the intrinsic cholinergic cerebrovascular innervation. Indeed, cholinesterase inhibitors improve cerebral blood flow in Alzheimer patients who respond to treatment. Further, cerebrovascular reactivity and neurovascular coupling are impaired in Alzheimer's disease and both can be improved by cholinesterase inhibitors. Conversely, cholinesterase inhibitors inhibit the α7-nicoticinic acetylcholine receptor on extrinsic sympathetic nerves and thus may impair vasodilatation. The net outcome of these opposing effects in clinical practice remains unknown. Moreover, it is uncertain whether the regulation of cerebral blood flow during blood pressure changes (cerebral autoregulation) is impaired in patients with Alzheimer's disease. Technological developments now allow us to dynamically measure blood pressure, cerebral blood flow, and cerebral cortical oxygenation. Using simple maneuvers like single sit-stand and repeated sit-stand maneuvers, the regulation of cerebral perfusion in patients with Alzheimer's disease can easily be measured. Sit-stand maneuvers can be considered as a provocation test for cerebral autoregulation, and provide excellent opportunities to study the cerebrovascular effects of cholinesterase inhibitors.

Mouse models of neurodegenerative diseases: criteria and general methodology

The major symptom of Alzheimer's disease is rapidly progressing dementia, coinciding with the formation of amyloid and tau deposits in the central nervous system, and neuronal death. At present familial cases of dementias provide the most promising foundation for modelling neurodegeneration. We describe the mnemonic and other major behavioral symptoms of tauopathies, briefly outline the genetics underlying familiar cases and discuss the arising implications for modelling the disease in mostly transgenic mouse lines. We then depict to what degree the most recent mouse models replicate pathological and cognitive characteristics observed in patients.There is no universally valid behavioral test battery to evaluate mouse models. The selection of individual tests depends on the behavioral and/or memory system in focus, the type of a model and how well it replicates the pathology of a disease and the amount of control over the genetic background of the mouse model. However it is possible to provide guidelines and criteria for modelling the neurodegeneration, setting up the experiments and choosing relevant tests. One should not adopt a "one (trans)gene, one disease" interpretation, but should try to understand how the mouse genome copes with the protein expression of the transgene in question. Further, it is not possible to recommend some mouse models over others since each model is valuable within its own constraints, and the way experiments are performed often reflects the idiosyncratic reality of specific laboratories. Our purpose is to improve bridging molecular and behavioural approaches in translational research.

Hypertension and cognitive function

Cumulative evidence implicates hypertension in the pathogenesis of Alzheimer disease. Although it may not presently be possible to completely differentiate the effects of treatment and control of hypertension itself from those of the medication used to achieve such treatment goals, efforts directed at the treatment and control of hypertension can have significant public health impact.

Complete rescue of cerebrovascular function in aged Alzheimer's disease transgenic mice by antioxidants and pioglitazone, a peroxisome proliferator-activated receptor gamma agonist

Accumulating evidence suggests that cerebrovascular dysfunction is an important factor in the pathogenesis of Alzheimer's disease (AD). Using aged ( approximately 16 months) amyloid precursor protein (APP) transgenic mice that exhibit increased production of the amyloid-beta (Abeta) peptide and severe cerebrovascular and memory deficits, we examined the capacity of in vivo treatments with the antioxidants N-acetyl-L-cysteine (NAC) and tempol, or the peroxisome proliferator-activated receptor gamma agonist pioglitazone to rescue cerebrovascular function and selected markers of AD neuropathology. Additionally, we tested the ability of pioglitazone to normalize the impaired increases in cerebral blood flow (CBF) and glucose uptake (CGU) induced by whisker stimulation, and to reverse spatial memory deficits in the Morris water maze. All compounds fully restored cerebrovascular reactivity of isolated cerebral arteries concomitantly with changes in proteins regulating oxidative stress, without reducing brain Abeta levels or Abeta plaque load. Pioglitazone, but not NAC, significantly attenuated astroglial activation and improved, albeit nonsignificantly, the reduced cortical cholinergic innervation. Furthermore, pioglitazone completely normalized the CBF and CGU responses to increased neuronal activity, but it failed to improve spatial memory. Our results are the first to demonstrate that late pharmacological intervention with pioglitazone not only overcomes cerebrovascular dysfunction and altered neurometabolic coupling in aged APP mice, but also counteracts cerebral oxidative stress, glial activation, and, partly, cholinergic denervation. Although early or combined therapy may be warranted to improve cognition, these findings unequivocally point to pioglitazone as a most promising strategy for restoring cerebrovascular function and counteracting several AD markers detrimental to neuronal function.

Blood-brain barrier tight junction permeability and ischemic stroke

The blood-brain barrier (BBB) is formed by the endothelial cells of cerebral microvessels, providing a dynamic interface between the peripheral circulation and the central nervous system. The tight junctions (TJs) between the endothelial cells serve to restrict blood-borne substances from entering the brain. Under ischemic stroke conditions decreased BBB TJ integrity results in increased paracellular permeability, directly contributing to cerebral vasogenic edema, hemorrhagic transformation, and increased mortality. This loss of TJ integrity occurs in a phasic manner, which is contingent on several interdependent mechanisms (ionic dysregulation, inflammation, oxidative and nitrosative stress, enzymatic activity, and angiogenesis). Understanding the inter-relation of these mechanisms is critical for the development of new therapies. This review focuses on those aspects of ischemic stroke impacting BBB TJ integrity and the principle regulatory pathways, respective to the phases of paracellular permeability.

Longitudinal cerebral blood flow and amyloid deposition: an emerging pattern?

Although cerebral amyloid deposition may precede cognitive impairment by decades, the relationship between amyloid deposition and longitudinal change in neuronal function has not, to our knowledge, been studied. The aim of this article was to determine whether individuals without dementia with high and low amyloid burden show different patterns of longitudinal regional cerebral blood flow (rCBF) changes in the years preceding measurement of amyloid deposition.

METHODS

Twenty-eight participants without dementia (mean age+/-SD, 82.5+/-4.8 y; 6 mildly impaired) from the Baltimore Longitudinal Study of Aging underwent yearly resting-state (15)O-H(2)O PET scans for up to 8 y. (11)C-PIB images of amyloid deposition were acquired on average 10.8+/-0.8 y after the first CBF scan. (11)C-PIB distribution volume ratios of regions of interest were estimated by fitting a reference-tissue model to the measured time-activity curves. On the basis of mean cortical distribution volume ratios, participants were divided into groups with high or low (11)C-PIB retention. Differences in longitudinal rCBF changes between high- and low-(11)C-PIB groups were investigated by voxel-based analysis.

RESULTS

Longitudinal rCBF changes differed significantly between high- (n=10) and low- (n=18) (11)C-PIB groups (P <or= 0.001). Greater longitudinal decreases in rCBF in the high-(11)C-PIB group than in the low-(11)C-PIB group were seen in right anterior to middle cingulate, right supramarginal gyrus, left thalamus, and midbrain bilaterally. Greater increases in rCBF over time in the high-(11)C-PIB group were found in left medial and inferior frontal gyri, right precuneus, left inferior parietal lobule, and left postcentral gyrus.

CONCLUSION

In this group of older adults without dementia, those with high (11)C-PIB show greater longitudinal declines in rCBF in certain areas, representing regions with greater decrements in neuronal function. Greater longitudinal increases in rCBF are also observed in those with higher amyloid load and may represent an attempt to preserve neuronal function in these regions.

Neurovascular coupling in Alzheimer patients: effect of acetylcholine-esterase inhibitors

Dualistic effects of acetylcholine-esterase inhibitors on neuronal as well as vasoregulative function have been debated. This study investigated for the first time effects of medication on both components. Visually evoked potentials and resultant hemodynamic responses were assessed in Alzheimer patients (n=31) without vascular lesions in a MRI scan and compared to controls (n=20). After baseline recordings (AD0) tests were repeated under 2x1.5 to 2x3mg (AD1) and 2x4.5 to 2x6mg (AD2) rivastigmine/d. Long-term effects were investigated under 6 months of medication (AD2L). The ADAS, MMSE and DEMTECT were used to assess cognitive function at AD0, AD2 and AD2L. Improvement in vasoregulative function was independent from changes in evoked potentials. Acetylcholine-esterase inhibitors demonstrate substantial vascular effects in humans, which are independent from changes in neuronal function.

NO-mediated cGMP synthesis in cultured cholinergic neurons from the basal forebrain of the fetal rat

Previously, using brain slices, we reported NO-mediated cGMP synthesis in all cholinergic fibers in the rat neocortex. In order to answer the question whether this property of cholinergic fibers was present before or developed after birth, we investigated properties of NO-responsiveness of cultured cholinergic forebrain neurons. Basal forebrain neurons of E16 fetal rat were cultured. Under the conditions chosen and after one day of culturing, all cells had attained a cholinergic phenotype using choline acetyltransferase or the vesicular acetylcholine transporter molecule as markers. Between 95-99% of the cells also expressed neuronal NOS. In the presence of 1 mM IBMX, a non-selective phosphodiesterase (PDE) inhibitor, 10 microM of the NO donor diethylamine-NONOate (DEANO) increased cGMP synthesis in 80% of the cells. cGMP levels in the cultured forebrain neurons were also increased when cells were stimulated with DEANO in the presence of the selective PDE inhibitors BAY 60-7550 (PDE2), sildenafil (PDE5), or the mixed type inhibitor papaverine (PDE2,5,10). Subpopulations of cells from the basal forebrain expressed mRNA for PDE2, PDE5, and PDE9. Atropine increased cGMP levels in an NO-dependent manner in a small population of cultured forebrain cells in the presence of IBMX. In conclusion, cultured cholinergic basal forebrain neurons present a heterogeneous cell population in the magnitude of their response to NO. NO-responsiveness of the cultured cholinergic neurons is already detectable after one day of culturing and indicates that NO-sensitivity of the cholinergic neurons of the rat basal forebrain is present well before birth.

Postnatal development of the cholinergic innervation in the dorsal hippocampus of rat: Quantitative light and electron microscopic immunocytochemical study

Choline acetyltransferase (ChAT) immunocytochemistry was used to examine the distribution and ultrastructural features of the acetylcholine (ACh) innervation in the dorsal hippocampus of postnatal rat. The length of ChAT-immunostained axons was measured and the number of ChAT-immunostained varicosities counted, in each layer of CA1, CA3, and dentate gyrus, at postnatal ages P8, P16, and P32. At P8, an elaborate network of varicose ChAT-immunostained axons was already visible. At P16, the laminar distribution of this network resembled that in the adult, but adult densities were reached only by P32. Between P8 and P32, the mean densities for the three regions increased from 8.4 to 14 meters of axons and 2.3 to 5.7 million varicosities per cubic millimeter of tissue. At the three postnatal ages, the ultrastructural features of ChAT-immunostained axon varicosities from the strata pyramidale and radiatum of CA1 were similar between layers and comparable to those in adult, except for an increasing frequency of mitochondria (up to 41% at P32). The proportion of these profiles displaying a synaptic junction was equally low at all ages, indicating an average synaptic incidence of 7% for whole varicosities, as previously found in adult. The observed junctions were small, usually symmetrical, and made mostly with dendritic branches. These results demonstrate the precocious and rapid maturation of the hippocampal cholinergic innervation and reveal its largely asynaptic nature as soon as it is formed. They emphasize the remarkable growth capacities of individual ACh neurons and substantiate a role for diffuse transmission by ACh during hippocampal development.

The potential role of nutritional components in the management of Alzheimer's Disease

Epidemiological evidence linking nutrition to the incidence and risk of Alzheimer Disease is rapidly increasing. The specific nutritional deficiencies in Alzheimer patients may suggest a relative shortage of specific macro- and micronutrients. These include omega-3 fatty acids, several B-vitamins and antioxidants such as vitamins E and C. Recent mechanistic studies in cell systems and animal models also support the idea that nutritional components are able to counteract specific aspects of the neurodegenerative and pathological processes in the brain. In addition, it has been shown that several nutritional components can also effectively stimulate membrane formation and synapse formation as well as improve behavior and cerebrovascular health. The suggested synergy between nutritional components to improve neuronal plasticity and function is supported by epidemiological studies as well as experimental studies in animal models. The ability of nutritional compositions to stimulate synapse formation and effectively reduce Alzheimer Disease neuropathology in these preclinical models provides a solid basis to predict potential to modify the disease process, especially during the early phases of Alzheimer Disease.

Oxidative stress and cerebrovascular dysfunction in mouse models of Alzheimer's disease

Several factors have been implicated in Alzheimer's disease (AD) but there is no definite conclusion as to the main pathogenic agents. Mutations in the amyloid precursor protein (APP) that lead to increased production of amyloid beta peptide (A beta) are associated with the early-onset, familial forms of AD. However, in addition to ageing, the most common risk factors for the sporadic, prevalent form of AD are hypertension, hypercholesterolaemia, ischaemic stroke, the ApoE4 allele and diabetes, all characterized by a vascular pathology. In AD, the vascular pathology includes accumulation of A beta in the vessel wall, vascular fibrosis, and other ultrastructural changes in constituent endothelial and smooth muscle cells. Moreover, the ensuing chronic cerebral hypoperfusion has been proposed as a determinant factor in the accompanying cognitive deficits. In transgenic mice that overexpress mutated forms of the human APP (APP mice), the increased production of A beta results in vascular oxidative stress and loss of vasodilatory function. The culprit molecule, superoxide, triggers the synthesis of other reactive oxygen species and the sequestration of nitric oxide (NO), thus impairing resting cerebrovascular tone and NO-dependent dilatations. The A beta-induced cerebrovascular dysfunction can be completely abrogated in aged APP mice with antioxidant therapy. In contrast, in mice that overproduce an active form of the cytokine transforming growth factor-beta1 and recapitulate the vascular structural changes seen in AD, antioxidants have no beneficial effect on the accompanying cerebrovascular deficits. This review discusses the beneficial role and limitations of antioxidant therapy in AD cerebrovascular pathology.

Activation-flow coupling differentiates between vascular and Alzheimer type of dementia

The activation-flow coupling describes a mechanism, which adapts local cerebral blood flow in accordance with the underlying neuronal activity. It was suggested that the mechanism helps in differentiation between Alzheimer and vascular type of dementia. We combined EEG and Doppler techniques and assessed integrity of the activation-flow coupling in the occipital cortex utilizing a visual stimulation task. Alzheimer patients (MMSE: 18+/-8 points, DemTect 5+/-4 points) without signs of vascular lesions on a MRI scan and vascular demented patients (MMSE: 20+/-6 points, DemTect 6+/-3 points; MRI Fazekas score 7+/-3 points) were compared with data from an age-matched control group. Evoked flow velocity responses in the posterior cerebral artery were analysed according to a control system model specifying the parameters gain, attenuation, natural frequency and rate time. Evoked potentials were analysed for the N(75)-P(100) amplitude difference. Vascular demented patients exhibited a significant decreased gain parameter and increased attenuation parameter indicating severe cerebrovascular dysfunction. Also, the potential amplitudes were significantly decreased indicating neuronal damage due to the vascular disease process. Alzheimer patients did not differ in parameters as compared to the control group supporting other reports of intact occipital function at this stage of disease. Simultaneous assessment of electrical as well as vascular integrity might help in differentiating the most frequent forms of dementia.

Attenuation of the blood flow response to physostigmine in the rat cortex deafferented from the basal forebrain

Previous functional investigations in rats failed to demonstrate that the classical cholinesterase inhibitor, physostigmine, can compensate for cortical cholinergic deficit induced by deafferentation from the nucleus basalis magnocellularis (NBM). As these studies were carried out shortly after NBM lesion (1-2 weeks), we sought to determine whether compensatory effects of physostigmine would appear at a longer postlesion time (3-5 weeks). Cerebral blood flow was used as a quantitative measure of brain function. At 3-5 weeks after unilateral NBM lesion, interhemispheric comparisons in resting conditions showed that the cortical cholinergic deficit was still present and that blood flow was lower in cortical areas on the lesion side, similarly to what was observed after 1-2 weeks, while basal blood flow in intact hemispheres remained unchanged. In contrast, under physostigmine, blood flow became significantly lower in deafferented cortical areas at 3-5 weeks postlesion time, whereas there were no significant interhemispheric differences in the short term. Comparisons with saline-infused rats showed reduced blood flow responses to physostigmine in forebrain regions, e.g. in the parietal cortex from 83% to 25% at 1-2 and 3-5 weeks postlesion, respectively. These changes cannot be ascribed to a global loss of reactivity, since responses in brainstem regions (medulla, cerebellum) remained unchanged statistically. The results demonstrate a reduced responsiveness to physostigmine at the longer postlesion time, and support the existence of a cholinosensitive mechanism antagonizing NBM influence. This mechanism may limit the activating effects of cholinergic agonists in the forebrain after NBM deafferentation.

Vascular dementia and the cholinergic pathways

Vascular cognitive impairment/vascular dementia have been the subject of a large number of studies, due to their high prevalence and broad preventive and compensatory therapeutic potential. The knowledge of the cerebral anatomy correlated to the vascular territories of irrigation enables understanding of clinical manifestations, as well as classification into the several types of syndromic presentations. The central cholinergic system exercises important neuromodulatory functions on cerebral circuits related to cognitive and behavioral integration, as well as on vasomotor control related to cerebral blood flow adjustments. The acquisition of data on the anatomy of the cholinergic pathways, including the localization of the nuclei of the basal prosencephalon and the routes of their projections, established an important milestone. The knowledge of the vascular distribution and of the trajectories of the cholinergic pathways allows identification of the strategic points where a vascular lesion can cause interruption. The ensuing denervation leads to cholinergic hypofunction in the involved territories. This information proves important to better evaluate the sites of vascular lesions, emphasizing their strategic localizations in relation to the cholinergic pathways, and offering more robust foundations for treatment aiming at enhancing cholinergic activity.

Vascular determinants of cholinergic deficits in Alzheimer disease and vascular dementia

Alzheimer's disease (AD) and vascular dementia (VaD) are widely accepted as the most common forms of dementia. Cerebrovascular lesions frequently coexist with AD, creating an overlap in the clinical and pathological features of VaD and AD. This review assembles evidence for a role for cholinergic mechanisms in the pathogenesis of VaD, as has been established for AD. We first consider the anatomy and vascularization of the basal forebrain cholinergic neuronal system, emphasizing its susceptibility to the effects of arterial hypertension, sustained hypoperfusion, and ischemic cerebrovascular disease. The impact of aging and consequences of disruption of the cholinergic system in cognition and in control of cerebral blood flow are further discussed. We also summarize preclinical and clinical evidence supporting cholinergic deficits and the use of cholinesterase inhibitors in patients with VaD. We postulate that vascular pathology likely plays a common role in initiating cholinergic neuronal abnormalities in VaD and AD.

Loss of M5 muscarinic acetylcholine receptors leads to cerebrovascular and neuronal abnormalities and cognitive deficits in mice

The M5 muscarinic acetylcholine receptor (M5R) has been shown to play a crucial role in mediating acetylcholine-dependent dilation of cerebral blood vessels. We show that male M5R-/- mice displayed constitutive constriction of cerebral arteries using magnetic resonance angiography in vivo. Male M5R-/- mice exhibited a significantly reduced cerebral blood flow (CBF) in the cerebral cortex, hippocampus, basal ganglia, and thalamus. Cortical and hippocampal pyramidal neurons from M5R-/- mice showed neuronal atrophy. Hippocampus-dependent spatial and nonspatial memory was also impaired in M5R-/- mice. In M5R-/- mice, CA3 pyramidal cells displayed a significantly attenuated frequency of the spontaneous postsynaptic current and long-term potentiation was significantly impaired at the mossy fiber-CA3 synapse. Our findings suggest that impaired M5R signaling may play a role in the pathophysiology of cerebrovascular deficits. The M5 receptor may represent an attractive novel therapeutic target to ameliorate memory deficits caused by impaired cerebrovascular function.

Blood-brain barrier: structural components and function under physiologic and pathologic conditions

The blood-brain barrier (BBB) is the specialized system of brain microvascular endothelial cells (BMVEC) that shields the brain from toxic substances in the blood, supplies brain tissues with nutrients, and filters harmful compounds from the brain back to the bloodstream. The close interaction between BMVEC and other components of the neurovascular unit (astrocytes, pericytes, neurons, and basement membrane) ensures proper function of the central nervous system (CNS). Transport across the BBB is strictly limited through both physical (tight junctions) and metabolic barriers (enzymes, diverse transport systems). A functional polarity exists between the luminal and abluminal membrane surfaces of the BMVEC. As a result of restricted permeability, the BBB is a limiting factor for the delivery of therapeutic agents into the CNS. BBB breakdown or alterations in transport systems play an important role in the pathogenesis of many CNS diseases (HIV-1 encephalitis, Alzheimer's disease, ischemia, tumors, multiple sclerosis, and Parkinson's disease). Proinflammatory substances and specific disease-associated proteins often mediate such BBB dysfunction. Despite seemingly diverse underlying causes of BBB dysfunction, common intracellular pathways emerge for the regulation of the BBB structural and functional integrity. Better understanding of tight junction regulation and factors affecting transport systems will allow the development of therapeutics to improve the BBB function in health and disease.

Vascular remodeling versus amyloid beta-induced oxidative stress in the cerebrovascular dysfunctions associated with Alzheimer's disease

The roles of oxidative stress and structural alterations in the cerebrovascular dysfunctions associated with Alzheimer's disease (AD) were investigated in transgenic mice overexpressing amyloid precusor protein (APP+) or transforming growth factor-beta1 (TGF+). Age-related impairments and their in vitro reversibility were evaluated, and underlying pathogenic mechanisms were assessed and compared with those seen in AD brains. Vasoconstrictions to 5-HT and endothelin-1 were preserved, except in the oldest (18-21 months of age) TGF+ mice. Despite unaltered relaxations to sodium nitroprusside, acetylcholine (ACh) and calcitonin gene-related peptide-mediated dilatations were impaired, and there was an age-related deficit in the basal availability of nitric oxide (NO) that progressed more gradually in TGF+ mice. The expression and progression of these deficits were unrelated to the onset or extent of thioflavin-S-positive vessels. Manganese superoxide dismutase (SOD2) was upregulated in pial vessels and around brain microvessels of APP+ mice, pointing to a role of superoxide in the dysfunctions elicited by amyloidosis. In contrast, vascular wall remodeling associated with decreased levels of endothelial NO synthase and cyclooxygenase-2 and increased contents of vascular endothelial growth factor and collagen-I and -IV characterized TGF+ mice. Exogenous SOD or catalase normalized ACh dilatations and NO availability in vessels from aged APP+ mice but had no effect in those of TGF+ mice. Increased perivascular oxidative stress was not evidenced in AD brains, but vascular wall alterations compared well with those seen in TGF+ mice. We conclude that brain vessel remodeling and associated alterations in levels of vasoactive signaling molecules are key contributors to AD cerebrovascular dysfunctions.

Exercise, cognition and Alzheimer's disease: More is not necessarily better

Regional hypoperfusion, associated with a reduction in cerebral metabolism, is a hallmark of Alzheimer's disease (AD) and contributes to cognitive decline. Cerebral perfusion and hence cognition can be enhanced by exercise. The present review describes first how the effects of exercise on cerebral perfusion in AD are mediated by nitric oxide (NO) and tissue-type plasminogen activator, the release of which is regulated by NO. A conclusion of clinical relevance is that exercise may not be beneficial for the cognitive functioning of all people with dementia if cardiovascular risk factors are present. The extent to which cardiovascular risk factors play a role in the selection of older people with dementia in clinical studies will be addressed in the second part of the review in which the effects of exercise on cognition are presented. Only eight relevant studies were found in the literature, emphasizing the paucity of studies in this field. Positive effects of exercise on cognition were reported in seven studies, including two that excluded and two that included patients with cardiovascular risk factors. These findings suggest that cardiovascular risk factors do not necessarily undo the beneficial effects of exercise on cognition in cognitively impaired people. Further research is called for, in view of the limitations of the clinical studies reviewed here.

Microsphere embolism-induced cortical cholinergic deafferentation and impairments in attentional performance

Ischemic events have been hypothesized to play a critical role on the pathogenesis of dementia and the acceleration of cognitive impairments. This experiment was designed to determine the consequences of microvascular ischemia on the cortical cholinergic input system and associated attention capacities. Injections of microspheres ( approximately 50 microm diameter; approximately 5000 microspheres/100 microL) into the right common carotid artery of rats served as a model of microvascular ischemia and resulted in decreases in the density of cholinergic fibers in the ipsilateral medial prefrontal cortex and frontoparietal areas. Furthermore, dense astrogliosis, indicated by glial fibrillary acidic protein (GFAP) immunohistochemistry, was observed in the globus pallidus, including the areas of origin of cholinergic projections to the cortex. Fluoro-Jade B staining indicated that loss of neurons in the cortex was restricted to areas of microsphere-induced infarcts. Attentional performance was assessed using an operant sustained attention task; performance in this task was previously demonstrated to reflect the integrity and activity of the cortical cholinergic input system. Embolized animals' performance was characterized by a decrease in the animals' ability to detect signals. Their performance in non-signal trials remained unaffected. The residual density of cholinergic axons in prefrontal and frontoparietal areas correlated with the animals' performance. The present data support the hypothesis that microvascular ischemia results in loss of cortical cholinergic inputs and impairs associated attentional performance. Microsphere embolism represents a useful animal model for studying the role of interactions between microvascular disorder and impaired forebrain cholinergic neurotransmission in the manifestation of cognitive impairments.

Acetylcholine esterase inhibitor donepezil improves dynamic cerebrovascular regulation in Alzheimer patients

BACKGROUND

Alzheimer's disease (AD) leads to a degeneration of the nucleus basalis of Meynert and thus to decreased cholinergic tonus in the brain. The transcription of endothelial nitric oxide synthase depends on an adequate cholinergic innervation of microvessels and vasoregulative abnormalities have been reported in AD. We investigated activation-flow coupling to study the role of acetylcholine esterase inhibition (AChEI) on vasoregulative function.

METHODS

A functional transcranial Doppler approach was used to measure the visually evoked flow velocity response in the posterior cerebral artery in AD patients who had no vascular risk factors. The diagnosis of AD was made according to the ICD10/DSMIIIR-criteria. After baseline recording the effect of four weeks 5mg donepezil and then four weeks 10 mg was investigated. Doppler data were evaluated with a control system approach to obtain dynamic properties of vasoregulation and were compared with a healthy control group.

RESULTS

AD patients showed an increased damping (0.64 +/- 0.2; p = 0.007 vs. control) in evoked responses and lower resting flow velocity levels (40 +/- 13 cm/s; p = 0.06 vs. control), which were restored in a dose-dependent manner under AChEI (0.4 +/- 0.2; 44 +/- 11 cm/s).

CONCLUSIONS

AD is associated with a functional vasoregulative deficit possibly due to decreased levels of the endothelial nitric oxide synthase. Augmenting levels with AChEI normalized flow regulation possibly leading to a better blood supply to active neurons.

The blood-brain barrier/neurovascular unit in health and disease

The blood-brain barrier (BBB) is the regulated interface between the peripheral circulation and the central nervous system (CNS). Although originally observed by Paul Ehrlich in 1885, the nature of the BBB was debated well into the 20th century. The anatomical substrate of the BBB is the cerebral microvascular endothelium, which, together with astrocytes, pericytes, neurons, and the extracellular matrix, constitute a "neurovascular unit" that is essential for the health and function of the CNS. Tight junctions (TJ) between endothelial cells of the BBB restrict paracellular diffusion of water-soluble substances from blood to brain. The TJ is an intricate complex of transmembrane (junctional adhesion molecule-1, occludin, and claudins) and cytoplasmic (zonula occludens-1 and -2, cingulin, AF-6, and 7H6) proteins linked to the actin cytoskeleton. The expression and subcellular localization of TJ proteins are modulated by several intrinsic signaling pathways, including those involving calcium, phosphorylation, and G-proteins. Disruption of BBB TJ by disease or drugs can lead to impaired BBB function and thus compromise the CNS. Therefore, understanding how BBB TJ might be affected by various factors holds significant promise for the prevention and treatment of neurological diseases.

Selective cholinergic denervation, independent from oxidative stress, in a mouse model of Alzheimer’s disease

Alzheimer's disease (AD) is characterized by increases in amyloid-beta (Abeta) peptides, neurofibrillary tangles, oxidative stress and cholinergic deficits. However, the selectivity of these deficits and their relation with the Abeta pathology or oxidative stress remain unclear. We therefore investigated amyloidosis-related changes in acetylcholine (ACh) and serotonin (5-HT) innervations of hippocampus and parietal cortex by quantitative choline acetyltransferase (ChAT) and 5-HT immunocytochemistry, in 6, 12/14 and 18 month-old transgenic mice carrying familial AD-linked mutations (hAPP(Sw,Ind)). Further, using manganese superoxide dismutase (MnSOD) and nitrotyrosine immunoreactivity as markers, we evaluated the relationship between oxidative stress and the ACh deficit in 18 month-old mice. Thioflavin-positive Abeta plaques were seen in both regions at all ages; they were more numerous in hippocampus and increased in number (>15-fold) and size as a function of age. A majority of plaques exhibited or were surrounded by increased MnSOD immunoreactivity, and dystrophic ACh or 5-HT axons were seen in their immediate vicinity. Counts of immunoreactive axon varicosities revealed significant decreases in ACh innervation, with a sparing of the 5-HT, even in aged mice. First apparent in hippocampus, the loss of ACh terminals was in the order of 20% at 12/14 months, and not significantly greater (26%) at 18 months. In parietal cortex, the ACh denervation was significant at 18 months only, averaging 24% across the different layers. Despite increased perivascular MnSOD immunoreactivity, there was no evidence of dystrophic ACh varicosities or their accentuated loss in the perivascular area. Moreover, there was virtually no sign of tyrosine nitration in ChAT nerve terminals or neuronal cell bodies. These data suggest that aggregated Abeta exerts an early, non-selective and focal neurotoxic effect on both ACh and 5-HT axons, but that a selective, plaque- and oxidative stress-independent diffuse cholinotoxicity, most likely caused by soluble Abeta assemblies, is responsible for the hippocampal and cortical ACh denervation.

Developmental origins of the age-related decline in cortical cholinergic function and associated cognitive abilities

Ontogenetic abnormalities in the regulation of the cortical cholinergic input system are hypothesized to mediate early-life cognitive limitations (ECL) that later escalate, based on reciprocal interactions between a dysregulated cholinergic system and age-related neuronal and vascular processes, to mild cognitive impairment (MCI) and, subsequently, for a majority of subjects, senile dementia. This process is speculated to begin with the disruption of trophic factor support of the basal forebrain ascending cholinergic system early in life, leading to dysregulation of cortical cholinergic transmission during the initial decades of life and associated limitations in cognitive capacities. Results from neurochemical and behavioral experiments support the possibility that aging reveals the vulnerability of an abnormally regulated cortical cholinergic input system. The decline of the cholinergic system is further accelerated as a result of interactions with amyloid precursor protein metabolism and processing, and with cerebral microvascular abnormalities. The determination of the developmental variables that render the cortical cholinergic input system vulnerable to age-related processes represents an important step toward the understanding of the role of this neuronal system in the age-related decline in cognitive functions.

Muscarinic acetylcholine receptor knockout mice: novel phenotypes and clinical implications

Muscarinic acetylcholine receptors (mAChRs; M1-M5) play key roles in regulating the activity of many important functions of the central and peripheral nervous system. Because of the lack of ligands endowed with a high degree of receptor subtype selectivity and the fact that most tissues or cell types express two or more mAChR subtypes, identification of the physiological and pathophysiological roles of the individual mAChR subtypes has proven a difficult task. To circumvent these difficulties, several laboratories recently employed gene-targeting techniques to generate mutant mouse strains deficient in each of the five mAChR subtypes. Phenotyping studies showed that each mutant mouse line displayed characteristic physiological, pharmacological, behavioral, biochemical, or neurochemical deficits. The novel insights gained from these studies should prove instrumental for the development of novel classes of muscarinic drugs.

Abnormal cholinergic and GABAergic vascular innervation in the hypothalamic arcuate nucleus of obesetub/tubmice

Tubby and tubby-like proteins (TULPs) are encoded by members of a small gene family. An autosomal recessive mutation in the mouse tub gene leads to blindness, deafness, and maturity-onset obesity. The mechanisms by which the mutation causes the obesity syndrome has not been established. We compared obese tub/tub mice and their lean littermates in order to find abnormalities within the mediobasal hypothalamus, a region intimately associated with the regulation of body weight. Using an antiserum to the vesicular acetylcholine transporter (VAChT), a marker for cholinergic neurons, many unusually large VAChT-immunoreactive (-ir) nerve terminals, identified by colocalization with the synaptic vesicle protein synaptophysin, were demonstrated in the hypothalamic arcuate nucleus of obese tub/tub mice. Double-labeling showed that VAChT-ir nerve endings also contained glutamic acid decarboxylase (GAD), a marker for gamma-aminobutyric acid (GABA) neurons. The VAChT- and GAD-ir nerve terminals were in close contact with blood vessels, identified with antisera to platelet endothelial cell adhesion molecule-1 (PECAM; also called CD31), laminin, smooth muscle actin (SMA), and glucose transporter-1 (GLUT1). Such large cholinergic and GABAergic nerve terminals surrounding blood vessels were not seen in the arcuate nucleus of lean tub/+ mice. The presence of abnormal cholinergic/GABAergic vascular innervation in the arcuate nucleus suggests that alterations in this region, which contains neurons that receive information from the periphery and which relays information about the energy status to other parts of the brain, may be central in the development of the obese phenotype in animals with an autosomal recessive mutation in the tub gene.

Novel insights into M5 muscarinic acetylcholine receptor function by the use of gene targeting technology

Until recently, little was known about the possible physiological functions of the M(5) muscarinic acetylcholine receptor subtype, the last member of the muscarinic receptor family (M(1)-M(5)) to be cloned. To learn more about the potential physiological roles of this receptor subtype, we generated and analyzed M(5) receptor-deficient mice (M5 -/- mice). Strikingly, acetylcholine, a potent dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5 -/- mice, suggesting that endothelial M(5) receptors mediate this activity in wild-type mice. This effect was specific for cerebral blood vessels, since acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5 -/- mice. In addition, in vitro neurotransmitter release experiments indicated that M(5) receptors located on dopaminergic nerve terminals play a role in facilitating muscarinic agonist-induced dopamine release in the striatum, consistent with the observation that the dopaminergic neurons innervating the striatum almost exclusively express the M(5) receptor subtype. We also found that the rewarding effects of morphine, the prototypical opiate analgesic, were substantially reduced in M5 -/- mice, as measured in the conditioned place preference paradigm. Furthermore, both the somatic and affective components of naloxone-induced morphine withdrawal symptoms were significantly attenuated in M5 -/- mice. It is likely that these behavioral deficits are caused by the lack of mesolimbic M(5) receptors, activation of which is known to stimulate dopamine release in the nucleus accumbens. These results convincingly demonstrate that the M(5) muscarinic receptor is involved in modulating several important pharmacological and behavioral functions. These findings may lead to novel therapeutic strategies for the treatment of drug addiction and certain cerebrovascular disorders.

Cholinergic modulation of the cortical microvascular bed

Cortical microvessels receive a cholinergic input that originates primarily from basal forebrain neurons which, upon stimulation, induce significant increases in cortical perfusion together with a dilation of intracortical microvessels. Heterogeneous mAChRs have been detected in cortical microvessels with expression of M2 and M5 subtypes in endothelial cells, while M1 and M3, and possibly M5 mAChR subtypes, were expressed in smooth muscle cells. Application of ACh to isolated and pressurized microarterioles, whether at basal tone or pharmacologically preconstricted, elicited only a dilation. This response was dependent on NO production, and was mediated by a mAChR, the pharmacology of which correlated best with the M5 receptor subtype. ACh afferents also project to intracortical neurons that synthesize NO and VIP. These correspond to distinct sub-populations of GABA interneurons which were found to send numerous projections to local microvessels. Preliminary results suggest expression of the VPAC1 receptor in the smooth muscle cells of intracortical arterioles, where it could mediate dilation as it does in cerebral arteries. Together these results indicate that basal forebrain ACh fibers can directly affect the cortical microvascular bed, but further suggest that specific populations of GABA interneurons could serve as a functional relay to adapt perfusion to locally increased neuronal activity. In confirmed cases of Alzheimer's disease, we found a severe ACh denervation of both cortical microvessels and NO neurons, suggesting that two important regulators of cortical perfusion are dysfunctional in this pathology.

Novel insights into muscarinic acetylcholine receptor function using gene targeting technology

Muscarinic acetylcholine receptors (mAChRs) modulate the activity of an extraordinarily large number of physiological functions. Individual members of the mAChR family (M(1)-M(5)) are expressed in a complex, overlapping fashion in most tissues and cell types. However, the identification of the precise physiological roles of individual mAChR subtypes remains a challenging task because, with the exception of a few snake toxins, mAChR ligands that can activate or inhibit specific mAChR subtypes with a high degree of selectivity are not yet available. Knowledge of the specific roles of mAChR subtypes is of considerable interest for the development of novel, clinically useful mAChR ligands. In this article, recent studies of mutant mouse strains developed, using gene targeting techniques, to be deficient in one of the three G(q)-coupled mAChR subtypes (M(1), M(3) and M(5)) are discussed. These investigations have led to many important new insights into the physiological roles of these receptor subtypes.

Vaccines for Alzheimer's disease: how close are we?

Alzheimer's disease is a neurodegenerative disorder characterised by a progressive loss of cognitive function. Despite the considerable progress being made, a complete description of the molecular pathology of this disease has yet to be elucidated. The evidence indicates that abnormal processing and extracellular deposition of the longer form of the beta-amyloid (Abeta) peptide (Abeta(1-42), a proteolytic derivative of the amyloid precursor protein [APP]) is implicated in the pathogenesis of Alzheimer's disease. In this respect, recent use of experimental mouse models, in which the mice develop some aspects of Alzheimer's disease in a reproducible fashion, has provided a new opportunity for a multidisciplinary and invasive analysis of mechanisms behind the amyloid pathology and its role in Alzheimer's disease. It has been demonstrated, using a single transgenic mouse model system that overexpresses the human mutated APP gene, that an immunisation against Abeta(1-42) causes a marked reduction in the amyloid burden in the brain. The follow-up research provided more evidence that both active and passive Abeta immunisation also reduces cognitive dysfunction in transgenic mouse models of Alzheimer's disease. Other studies using different approaches - such as secretase, cholesterol and Abeta metalloprotein inhibitors or NSAIDs - but all targeting the abnormal metabolism of Abeta have confirmed in each case that a significant reduction of amyloid plaque burden can be achieved in transgenic mouse models of Alzheimer's disease. This research strongly supports the notion that abnormal Abeta processing is essential to the pathogenesis of Alzheimer's disease and provides a crucial platform for the development and detailed testing of potential treatments in experimental models before each of these approaches can be proposed as a therapy for Alzheimer's disease. Although the first clinical trial of active immunisation with a pre-aggregated synthetic Abeta(42) preparation (AN-1792 vaccine) met with some setbacks and was discontinued after several patients experienced meningoencephalitis, the follow-up analysis of the effect of immunisation against Abeta in humans revealed a powerful effect of vaccination in the clearance of amyloid plaques from the cerebral cortex.

Alzheimer's disease: its diagnosis and pathogenesis

A hypothesis has been presented that links many of the identified and putative risk factors for AD and suggests a mechanism for their action. Crawford (1996, 1998) proposes an association between AD and cerebral blood flow (CBF) by citing evidence that many of the factors that are linked with an increased risk of AD also decrease CBF (e.g., old age, depression, underactivity, head trauma). Similarly, it is suggested factors that increase CBF are associated with a decreased risk of AD (e.g., education, exercise, smoking, NSAIDs). Although the authors acknowledge that reduced CBF is not sufficient to cause AD, the reported positive and negative associations provide tantalizing evidence for a common mode of action for many of the equivocal risk factors reported to date. This hypothesis is also consistent with other data that links microvascular damage and impaired blood flow (de la Torre, 1997, 2000) and low education with increased cerebrovascular disease (Del Ser et al., 1999). Gaining a better understanding of the interaction between AD and vascular disease is of great importance. Not only will it provide insights into the pathogenesis of AD, but it may also provide us with a rare opportunity for the treatment and possible prevention of AD. A great many risk factors for vascular disease have been identified and intervention programs have successfully reduced the incidence of heart disease and stroke. The potential exists to provide the same level of success with AD.

Cholinergic dilation of cerebral blood vessels is abolished in M(5) muscarinic acetylcholine receptor knockout mice

The M(5) muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M(1)-M(5)) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M(5) receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M(5) receptor-deficient mice (M5R(-/-) mice). M5R(-/-) mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M(5) receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M(5) receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M(5) receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R(-/-) mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5R(-/-) mice. Our findings provide direct evidence that M(5) muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimer's disease and focal cerebral ischemia, cerebrovascular M(5) receptors may represent an attractive therapeutic target.

Cerebral microvascular pathology in aging and Alzheimer's disease

The aging of the central nervous system and the development of incapacitating neurological diseases like Alzheimer's disease (AD) are generally associated with a wide range of histological and pathophysiological changes eventually leading to a compromised cognitive status. Although the diverse triggers of the neurodegenerative processes and their interactions are still the topic of extensive debate, the possible contribution of cerebrovascular deficiencies has been vigorously promoted in recent years. Various forms of cerebrovascular insufficiency such as reduced blood supply to the brain or disrupted microvascular integrity in cortical regions may occupy an initiating or intermediate position in the chain of events ending with cognitive failure. When, for example, vasoconstriction takes over a dominating role in the cerebral vessels, the perfusion rate of the brain can considerably decrease causing directly or through structural vascular damage a drop in cerebral glucose utilization. Consequently, cerebral metabolism can suffer a setback leading to neuronal damage and a concomitant suboptimal cognitive capacity. The present review focuses on the microvascular aspects of neurodegenerative processes in aging and AD with special attention to cerebral blood flow, neural metabolic changes and the abnormalities in microvascular ultrastructure. In this context, a few of the specific triggers leading to the prominent cerebrovascular pathology, as well as the potential neurological outcome of the compromised cerebral microvascular system are also going to be touched upon to a certain extent, without aiming at total comprehensiveness. Finally, a set of animal models are going to be presented that are frequently used to uncover the functional relationship between cerebrovascular factors and the damage to neural networks.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder, characterized by a progressive loss of cognitive function. Despite considerable progress, a complete description of the molecular pathology of this disease has yet to be elucidated. In this respect, the need for an animal model that develops some or all aspects of this uniquely human disease in a reproducible fashion is crucial for the development and testing of potential treatments. A valid animal model for AD should exhibit (1) progressive AD-like neuropathology and (2) cognitive deficits, and (3) should be verified in several laboratories. Transgenic models should be able to (4) discern pathogenic effects of familial forms (FAD) mutations from those of transgene overexpression. Models derived from microinjection of FAD mutant alleles should (5) encompass more than one Tg line. At present, however, no model that replicates all of these desirable features exists. In this review, we discuss transgenic mouse models with well-characterized AD-like neuropathology that show some form of cognitive impairment. We argue that conclusions drawn from a limited selection of cross-sectional experiments should be verified in longitudinally designed experiments. Future studies should attempt to establish a closer relationship between molecular pathology and the degree of cognitive impairment. While exact replication of AD in mice may not attainable (due to phylogenetic differences and fundamental differences in behavioral ecology), rigorous comparative analysis of cognitive behavior observed in various mouse models of AD should provide a framework for better understanding of molecular mechanisms underlying cognitive impairment observed in AD patients.

Histochemical study of acute and chronic intraperitoneal nicotine effects on several glycolytic and Krebs cycle dehydrogenase activities in the frontoparietal cortex and subcortical nuclei of the rat brain

The effects of nicotine on the activity of different dehydrogenases in frontoparietal regions and subcortical nuclei of the rat brain have been studied using histochemical methods. Nicotine sulphate was intraperitoneally administered in acute (4 mg/kg/day x 3 days) or chronic (ALZET osmotic pump providing 2 mg/kg/day x 15 days) doses. The enzymes analyzed were glyceraldehyde-3-phosphate, lactate, malate and succinate dehydrogenases (gly3PDH, LDH, MDH, and SDH, respectively). The results demonstrate that chronic as well as acute administration of nicotine produced strong increases in all these enzymatic activities in the superior layers (I, II and III) of the frontoparietal cortex (cingulate, motor and somatosensory regions); but high increases were not seen in the deeper layers of the cortex or in the subcortical nuclei (substantia nigra, caudate-putamen, nucleus accumbens or nucleus basalis magnocellularis). These hyperactivities were produced in brain regions with normally low enzymatic activity (cortex), but not in those with great intensity (subcortical nuclei). The results are in rough agreement with previous reports on nicotine-induced increases in glucose utilization, gly3PDH genic expression and neuronal hyperactivity in the brain cortex; but significant discrepancies between the cortical enzymatic maps and those obtained both in these studies and others on nicotine(N)-receptor localization have been appreciated. The results support the hypothesis that nicotinic cholinergic drugs can have metabolic, long-lasting stimulant effects on cortical neurons at specific points (probably layer III pyramidal cells and structures with alpha7-N-receptors) of the cortical circuits that could be of great interest in improving altered cognitive functions that are present in Alzheimer disease, as well as in other less severe mental disturbances. Mitochondrial hyperfunction should also be evaluated as a possible side-effect (as an oxidative stress inductor) of these kinds of drugs.

Fibrillary beta-amyloid deposits are closely associated with atrophic nitric oxide synthase (NOS)-expressing neurons but do not upregulate the inducible NOS in transgenic Tg2576 mouse brain with Alzheimer pathology

Transgenic mice (Tg2576) that express the Swedish double mutation of human amyloid precursor protein and develop Alzheimer-like beta-amyloid deposits in the aged brain, were used to study the effect of beta-amyloid deposition on expression of both neuronal (nNOS) and inducible nitric oxide synthase (iNOS) in cells surrounding beta-amyloid plaques. Nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry and double immunofluorescent labeling revealed that most of the fibrillary, thioflavine-S-positive cortical beta-amyloid deposits in 13-, 17-, and 21-month-old transgenic animals were closely associated with dystrophic nNOS-positive neurons, while nNOS-bearing neurons located more distal to plaques appeared to be unaffected. There was no significant expression of iNOS in transgenic mouse brain. The data suggest enhanced vulnerability of nNOS-containing neocortical neurons to beta-amyloid toxicity. Alternatively, expression of nNOS may also be a response to plaque-mediated damage of neurons, consistent with a neuroprotective role of nitric oxide.

Beta-amyloid plaques induce neuritic dystrophy of nitric oxide-producing neurons in a transgenic mouse model of Alzheimer's disease

A causative role for nitric oxide has been postulated in a number of neurodegenerative diseases. Using histochemical and immunohistochemical methods, we examined the effect of beta-amyloid plaques on nitric oxide-producing cells in transgenic mice which overexpress a mutant human amyloid precursor protein (APP). In 14-month-old animals, nitric oxide synthase (NOS)-positive dystrophic neurites were observed frequently in the cerebral cortex and hippocampus of all of 16 plaque-bearing transgenic animals and in none of 16 wild-type animals. Double labeling of NOS and beta-amyloid revealed that 90% of beta-amyloid plaques were associated with NOS-containing dystrophic neurites. In 7-month-old animals, beta-amyloid plaques were very rare, but those present were frequently associated with NOS-positive neuritic dystrophy. We conclude that beta-amyloid plaques induce neuritic dystrophy in cortical neurons containing NOS in this model of AD, and hypothesize that this finding may be relevant to the mechanism of beta-amyloid neurotoxicity in human AD.

Donepezil dose-dependently inhibits acetylcholinesterase activity in various areas and in the presynaptic cholinergic and the postsynaptic cholinoceptive enzyme-positive structures in the human and rat brain

In the symptomatic treatment of mild to moderately severe dementia associated with Alzheimer's disease, donepezil (E2020) has been introduced for the inhibition of acetylcholinesterase activity in the human brain. However, there is no morphological evidence as to how this chemical agent affects the acetylcholinesterase-positive structures in the various areas of the human and the rat CNS. This study demonstrates by histochemical means that donepezil exerts a dose-dependent inhibitory effect in vitro on acetylcholinesterase activity. The most sensitive areas were the cortex and the hippocampal formation. Within the different layers of the cortex, the cholinoceptive acetylcholinesterase-positive postsynaptic pyramidal cell bodies were more sensitive than the presynaptic cholinergic axonal processes. In the cortex, the cell body staining was already abolished by even 2 x 10(-8)M donepezil, whereas the axonal staining could be eliminated only by at least 5 x 10(-8)M donepezil. In the hippocampus, the axonal acetylcholinesterase reaction end-product was eliminated by 5 x 10(-7)M donepezil. The most resistant region was the putamen, where the staining intensity was moderately reduced by 1 x 10(-6)M donepezil. In the rat brain, the postsynaptic cholinoceptive and presynaptic cholinergic structures were inhibited by nearly the same dose of donepezil as in the human brain. These histochemical results provide the first morphological evidence that, under in vitro circumstances, donepezil is not a general acetylcholinesterase inhibitor in the CNS, but rather selectively affects the different brain areas and, within these, the cholinoceptive and cholinergic structures. The acetylcholinesterase staining in the nerve fibers (innervating the intracerebral blood vessels of the human brain and the extracerebral blood vessels of the rat brain) and at the neuromuscular junction in the diaphragm and gastrocnemius muscle of rat, was also inhibited dose dependently by donepezil. It is concluded that donepezil may be a valuable tool with which to influence both the pre- and the postsynaptic acetylcholinesterase-positive structures in the human and rat central and peripheral nervous systems.