Amyloid ß, glutamate, excitotoxicity in Alzheimer's disease: are we on the right track?

Alzheimer's disease (AD) has a devastating impact on aged people worldwide. Although sophisticated and advanced molecular methods have been developed for its diagnosis since early phases, pharmacological treatment still represents an unresolved topic. The more the disease progresses, the more the uneffectiveness of antidementia drugs emerges. New and encouraging results from experimental works indicate that glutamate pathway may play a substantial role in the pathogenesis since early stages of the disease. Several experimental data together with the clinical use of the uncompetitive N-methyl-d-aspartate (NMDA) antagonist memantine strengthen this idea. Unfortunately, definitive data on the glutamatergic transmission involvement in AD are still incomplete. Moreover, clinical results indicate only temporarily limited effects of memantine. Currently, memantine is indicated for moderate-to-severe cases of AD, an indication that may limit its efficacy and impact on Alzheimer's dementia. The association of memantine with the acetylcholinesterase inhibitor drugs used to treat dementia symptoms appears to be beneficial, in both experimental and clinical studies. Because cholinergic and glutamatergic dysfunction occurs early in AD, the coadministration of appropriate treatment in early stages of the disease might represent a valid option from the beginning of cognitive decline. Moreover, to better evaluate drug efficacy, the association of the recently introduced biomarkers with a clinical AD profile should be considered an aim to pursue.

Dopamine D2-agonist rotigotine effects on cortical excitability and central cholinergic transmission in Alzheimer's disease patients.

Dopamine is a neurotransmitter involved in several brain functions ranging from emotions control, movement organization to memory formation. It is also involved in the regulation of mechanisms of synaptic plasticity. However, its role in Alzheimer's disease (AD) pathogenesis is still puzzling. Several recent line of research instead indicates a clear role for dopamine in both amyloid ß formation as well as in cognitive decline progression. In particular it has been shown that dopamine D2-like receptors (namely D3 and D2) could be mostly responsible for dopamine dysfunction in AD. Here we aimed to study the effects of the dopamine agonist Rotigotine on cortical excitability and on central cholinergic transmission in cases of AD. Rotigotine is a dopamine agonist with a pharmacological profile with high affinity for D3 and D2 receptors. We used paired pulse protocols assessing short intracortical inhibition (SICI) and intracortical facilitation (ICF) to asses cortical excitability over the primary motor cortex and Short Latency Afferent Inhibition (SLAI) protocols, to verify the effects of the drug on central cholinergic transmission in a group of AD patients compared to age-matched controls. We observed that rotigotine induces unexpected changes in both cortical excitability (increased) and central cholinergic transmission (restored) of AD patients. These unexpected effects might depend on the dopamine D2-like receptors dysfunction previously described in AD brains. The current findings could indicate that future strategies aimed to ameliorate symptoms of the related AD cognitive decline could also involve some dopaminergic drugs. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The load of amyloid-ß oligomers is decreased in the cerebrospinal fluid of Alzheimer's disease patients.

Amyloid-ß (Aß) oligomers are heterogeneous and instable compounds of variable molecular weight. Flow cytometry and fluorescence resonance energy transfer (FRET)-based methods allow the simultaneous detection of Aß oligomers with low and high molecular weight in their native form. We evaluated whether an estimate of different species of Aß oligomers in the cerebrospinal fluid (CSF) with or without dilution with RIPA buffer could be more useful in the diagnosis of Alzheimer's disease (AD) than the measurement of Aß42 monomers, total tau (t-tau), and phosphorylated tau (p-tau). Increased t-tau (p < 0.01) and p-tau (p < 0.01), and decreased Aß42 (p < 0.01), were detected in the CSF of patients with AD (n = 46), compared to patients with other dementia (OD) (n = 35) or with other neurological disorders (OND) (n = 56). In native CSF (n = 137), the levels of Aß oligomers were lower (p < 0.05) in AD than in OD and OND patients; in addition, the ratio Aß oligomers/p-tau was lower in AD than in OD (p < 0.01) and OND (p < 0.05) patients, yielding a sensitivity of 75% and a specificity of 64%. However, in CSF diluted with RIPA (n = 30), Aß oligomers appeared higher (p < 0.05) in AD than in OND patients, suggesting they become partially disaggregated and more easily detectable after RIPA. In conclusion, FRET analysis in native CSF is essential to correctly determine the composition of Aß oligomers. In this experimental setting, the simultaneous estimate of low and high molecular weight Aß oligomers is as useful as the other biomarkers in the diagnosis of AD. The low amount of Aß oligomers detected in native CSF of AD may be inversely related to their levels in the brain, as occurs for Aß monomers, representing a biomarker for the amyloid pathogenic cascade.

Cerebrospinal fluid levels of Aß42 relationship with cholinergic cortical activity in Alzheimer's disease patients.

The dysfunction of cholinergic neurons is a typical hallmark in Alzheimer's disease (AD). In animal models of AD, fragments of amyloid beta protein (Aß) and Tau protein are thought to interfere with central cholinergic transmission, specifically with synthesis and release of acetylcholine. Thus, we aimed to investigate whether the cerebrospinal fluid (CSF) levels of Aß42 and Tau proteins in AD patients could influence physiological central cholinergic activity. In AD patients (n = 19), central cholinergic function was evaluated in vivo by using short afferent latency inhibition (SLAI), and compared to age-matched healthy controls. In the same AD patients, CSF samples were collected through lumbar puncture to obtain individual levels of Aß42, total Tau (t-Tau) and phosphorylated Tau (p-Tau) (Thr181). SLAI was decreased in AD patients in comparison to age-matched healthy controls. We found that in patients there was a negative correlation between the individual amount of cholinergic activity assessed by SLAI and the CSF levels of Aß42. On the other hand, there was a positive correlation between the levels of SLAI and CSF p-Tau. No correlation was found when SLAI was analysed together with t-Tau. These results demonstrate that mechanisms of cortical cholinergic activity are altered in patients bearing a pathological CSF hallmark of AD, suggesting that these peptides may have some influence on the cholinergic dysfunction in AD. We suggest that coupling of CSF biomarkers with neurophysiological parameters of central cholinergic function could be important to better detect ongoing mechanisms of neural degeneration in vivo.

Plasmin system of Alzheimer's disease patients: CSF analysis.

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder characterized by the extracellular deposit of Amyloid beta (Aß), mainly of the Amyloid beta(1-42) (Aß(1-42)) peptide in the hippocampus and neocortex leading to progressive cognitive decline and dementia. The possible imbalance between the Aß production/degradation process was suggested to contribute to the pathogenesis of AD. Among others, the serine protease plasmin has shown to be involved in Aß(1-42) clearance, a hypothesis strengthened by neuropathological studies on AD brains. To explore whether there is a change in plasmin system in CSF of AD patients, we analyzed CSF samples from AD and age-matched controls, looking at plasminogen, tissue plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI-1) protein levels and t-PA and urokinase plasminogen activator (u-PA) enzymatic activities. We also measured Aß(1-42), total-tau and phospho-tau (181) CSF levels and sought for a possible relationship between them and plasmin system values. Our findings showed that t-PA, plasminogen and PAI-1 levels, as t-PA enzymatic activity, remained unchanged in AD with respect to controls; u-PA activity was not detected. We conclude that CSF analysis of plasminogen system does not reflect changes observed post-mortem. Unfortunately, the CSF detection of plasmin system could not be a useful biomarker for either AD diagnosis or disease progression. However, these findings do not exclude the possible involvement of the plasmin system in AD.

CSF tau levels influence cortical plasticity in Alzheimer's disease patients.

Alzheimer's disease (AD) is a neurodegenerative process characterized by progressive neuronal degeneration, reduced levels of neurotransmitters, and altered forms of synaptic plasticity. In animal models of AD, amyloid-ß (Aß) and tau proteins are supposed to interfere with synaptic transmission. In the current study, we investigated the correlation between motor cortical plasticity, measured with 1 Hz repetitive transcranial magnetic stimulation (rTMS), and the levels of Aß1₋42, total tau (t-Tau), and phosphorylated tau (p-Tau) detected in cerebrospinal fluid (CSF) of AD patients. We found that the overall rTMS after effects were milder in AD patients in comparison with controls. In AD patients the amount of rTMS-induced inhibition correlated with CSF t-Tau, but not with Aß1₋42 CSF levels. Surprisingly, higher CSF t-Tau levels were associated to a stronger inhibition of the motor evoked potentials, implying that the expected effects of the 1 Hz rTMS protocol were more evident in patients with more pathological t-Tau CSF levels. These data could be interpreted as the consequence of CSF t-Tau mediated abnormal excitatory activity and could suggest that CSF t-Tau may impact mechanisms of cortical plasticity.

Altered dopamine modulation of LTD-like plasticity in Alzheimer's disease patients.

OBJECTIVE: Mechanisms of synaptic plasticity like long term depression (LTD) are altered in experimental models of Alzheimer's disease (AD). LTD-like plasticity mechanisms has not been yet fully investigated in AD patients. METHODS: Here we studied the effects of low frequency (1 Hz) repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex in a group of patients with a diagnosis of probable AD, compared to healthy age-matched controls (HS). Moreover, we tested the effects of a single dose of orally administered L-dopa, one of the key neurotransmitters in modulating synaptic plasticity mechanisms, on rTMS induced plasticity. RESULTS: We found that in AD patients the 1 Hz rTMS protocol did not induce the expected inhibitory effect, while a long lasting inhibition of MEP was observed in HS. In addition, L-dopa induced a clear form of reversal of the direction of plasticity in HS that was not evident in AD. CONCLUSIONS: Dopamine modulation of LTD-like plasticity is impaired when tested in AD patients. SIGNIFICANCE: These findings provide evidence of possible dysfunction of dopaminergic transmission in AD patients.

Beyond the cholinergic hypothesis: do current drugs work in Alzheimer's disease?

Alzheimer's disease (AD) is a neurodegenerative disease characterized by memory and cognitive loss, and represents the leading cause of dementia in elderly people. Besides the complex biochemical processes involved in the neuronal degeneration (formation of senile plaques containing Abeta peptides, and development of neurofibrillary tangles), other molecular and neurochemical alterations, like cholinergic deficit due to basal forebrain degeneration, also occur. Because acetylcholine has been demonstrated to be involved in cognitive processes, the idea to increase acetylcholine levels to restore cognitive deficits has gained interest (the so-called cholinergic hypothesis). This has led to the development of drugs able to prevent acetylcholine hydrolysis (acetylcholinesterase inhibitors). However, the analysis of clinical efficacy of these drugs in alleviating symptoms of dementia showed unsatisfactory results. Despite such critical opinions on the efficacy of these drugs, it should be said that acetylcholinesterase inhibitors, and for some aspects memantine also, improve memory and other cognitive functions throughout most of the duration of the disease. The pharmacological activity of these drugs suggests an effect beyond the mere increase of acetylcholine levels. These considerations are in agreement with the idea that cognitive decline is the result of a complex and not fully elucidated interplay among different neurotransmitters. The role of each of the neurotransmitters implicated has to be related to a cognitive process and as a consequence to its decline. The current review aims to highlight the positive role of cholinergic drugs in alleviating cognitive deficits during wake as well as sleep. Moreover, we suggest that future therapeutic approaches have to be developed to restore the complex interplay between acetylcholine and other neurotransmitters systems, such as dopamine, serotonin, noradrenaline, or glutamate, that are likely involved in the progressive deterioration of several cognitive functions such as attention, memory, and learning.

Abeta1-42 Detection in CSF of Alzheimer's disease is influenced by temperature: indication of reversible Abeta1-42 aggregation?

Amyloid-beta 1-42 (Abeta1-42), peptide detectable in cerebrospinal fluid (CSF), has been extensively studied as diagnostic marker for Alzheimer's disease; however, results are variable. We investigated whether Abeta1-42 detection in CSF may be affected by handling temperature after lumbar puncture. CSF was collected from patients affected by probable AD (n=27), other dementias (OD) (n=24), or other neurological disorders without cognitive impairment (OND) (n=23). After lumbar puncture, CSF samples were either maintained at 37 degrees C, or handled according to standard procedures and centrifuged at 4 degrees C for 10 min; thereafter, one aliquot was further stored at 4 degrees C and another at 37 degrees C, before freezing all samples 90 min later at -80 degrees C, pending analysis. Abeta1-42 and total tau were determined using a commercially available sandwich enzyme-linked immunosorbent assay ELISA. Reduced Abeta1-42 and increased total tau CSF levels were confirmed as characteristic hallmarks of the OD and AD groups, providing standard measurement in samples stored at 4 degrees C before freezing. However, avoiding cooling or reheating CSF from 4 to 37 degrees C before freezing strikingly increased the Abeta1-42 concentration detectable in the AD group (P<0.01), but not in control groups. The results indicate that a pool of Abeta1-42 cannot be detectable in the CSF of AD patients, because standard preanalytical cooling masks in some ways the epitope recognized by Abeta1-42 specific antibodies. Moreover, our study suggests that low temperature could induce Abeta1-42 conformational changes and multimeric aggregates in probable AD, but, more importantly, Abeta1-42 aggregation could be reversible.

Dopamine modulates cholinergic cortical excitability in Alzheimer's disease patients.

In Alzheimer's disease (AD) patients dysfunction of cholinergic neurons is considered a typical hallmark, leading to a rationale for the pharmacological treatment in use based on drugs that enhance acetylcholine neurotransmission. However, besides altered acetylcholine transmission, other neurotransmitter systems are involved in cognitive dysfunction leading to dementia. Among others, dopamine seems to be particularly involved in the regulation of cognitive processes, also having functional relationship with acetylcholine. To test whether cholinergic dysfunction can be modified by dopamine, we used short latency afferent inhibition (SLAI) as a neurophysiological tool. First, we tested the function of the cholinergic system in AD patients and in healthy subjects. Then, we tested whether a single L-dopa challenge was able to interfere with this system in both groups. We observed that SLAI was reduced in AD patients, and preserved in normal subjects. L-dopa administration was able to restore SLAI modification only in AD, having no effect in healthy subjects. We conclude that dopamine can modify SLAI in AD, thus confirming the relationship between acetylcholine and dopamine systems. Moreover, it is suggested that together with cholinergic, dopaminergic system alteration is likely to occur in AD, also. These alterations might be responsible, at least in part, for the progressive cognitive decline observed in AD patients.

L-dopa modulates motor cortex excitability in Alzheimer's disease patients.

In Alzheimer's disease (AD), transcranial magnetic stimulation (TMS) studies have shown abnormalities of motor cortical excitability, such as a decreased intra-cortical inhibition (ICI) and changes in resting motor threshold (rMT). We studied the effects of L-dopa on rMT and ICI in a cohort of moderate AD patients after paired-pulse TMS. Results were compared with a control group of healthy subjects. As expected, AD patients showed a significant reduction in ICI and a lower rMT. L-dopa administration (soluble form, melevodopa 200 mg) promptly reversed the ICI impairment up to normalization. This effect was specific, since it was not mimicked in control subjects. These results indicate a possible role of dopamine in modulating AD cortical excitability, thus suggesting an interaction between dopaminergic ascending pathways and endogenous intracortical transmitters. In addition, considering that L-dopa showed a pharmacological profile similar to the one of cholinomimetics, L-dopa might represent a reliable tool to study new therapeutic perspective and strategies for AD.