Integrated phylogeny of the human brain and pathobiology of Alzheimer's disease: A unifying hypothesis

Nodal degree centrality in the default mode-like network of the TgF344-AD Alzheimer's disease rat model as a measure of early network alterations

This study investigates brain network alterations in the default mode-like network (DMLN) at early stages of disease progression in a rat model of Alzheimer's disease (AD) with application in the development of early diagnostic biomarkers of AD in translational studies. Thirteen male TgF344-AD (TG) rats, and eleven male wild-types (WT) littermates underwent longitudinal resting-state fMRI at the age of 4 and 6 months (pre and early-plaque stages of AD). Alterations in connectivity within DMLN were characterized by calculating the nodal degree (ND), a graph theoretical measure of centrality. The ND values of the left CA2 subregion of the hippocampus was found to be significantly lower in the 4-month-old TG cohort compared to the age-matched WT littermates. Moreover, a lower ND value (hypo-connectivity) was observed in the right prelimbic cortex (prL) and basal forebrain in the 6-month-old TG cohort, compared to the same age WT cohort. Indeed, the ND pattern in the DMLN in both TG and WT cohorts showed significant differences across the two time points that represent pre-plaque and early plaque stages of disease progression. Our findings indicate that lower nodal degree (hypo-connectivity) in the left CA2 in the pre-plaque stage of AD and hypo-connectivity between the basal forebrain and the DMLN regions in the early-plaque stage demonstrated differences in comparison to healthy controls. These results suggest that a graph-theoretical measure such as the nodal degree, can characterize brain networks and improve our insights into the mechanisms underlying Alzheimer's disease.

Basal forebrain cholinergic signalling: development, connectivity and roles in cognition

Acetylcholine plays an essential role in fundamental aspects of cognition. Studies that have mapped the activity and functional connectivity of cholinergic neurons have shown that the axons of basal forebrain cholinergic neurons innervate the pallium with far more topographical and functional organization than was historically appreciated. Together with the results of studies using new probes that allow release of acetylcholine to be detected with high spatial and temporal resolution, these findings have implicated cholinergic networks in 'binding' diverse behaviours that contribute to cognition. Here, we review recent findings on the developmental origins, connectivity and function of cholinergic neurons, and explore the participation of cholinergic signalling in the encoding of cognition-related behaviours.

Progression of regional cortical cholinergic denervation in Parkinson's disease

Cortical cholinergic deficits contribute to cognitive decline and other deficits in Parkinson's disease. Cross-sectional imaging studies suggest a stereotyped pattern of posterior-to-anterior cortical cholinergic denervation accompanying disease progression in Parkinson's disease. We used serial acetylcholinesterase PET ligand imaging to characterize the trajectory of regional cholinergic synapse deficits in Parkinson's disease, testing the hypothesis of posterior-to-anterior progression of cortical cholinergic deficits. The 16 Parkinson's disease subjects (4 females/12 males; mean age: 64.4 ± 6.7 years; disease duration: 5.5 ± 4.2 years; Hoehn & Yahr stage: 2.3 ± 0.6 at entry) completed serial 11C-methyl-4-piperidinyl propionate acetylcholinesterase PET scans over a 4-8 year period (median 5 years). Three-dimensional stereotactic cortical surface projections and volume-of-interest analyses were performed. Cholinergic synapse integrity was assessed by the magnitude, k 3, of acetylcholinesterase hydrolysis of 11C-methyl-4-piperidinyl propionate. Based on normative data, we generated Z-score maps for both the k 3 and the k 1 parameters, the latter as a proxy for regional cerebral blood flow. Compared with control subjects, baseline scans showed predominantly posterior cortical k 3 deficits in Parkinson's disease subjects. Interval change analyses showed evidence of posterior-to-anterior progression of cholinergic cortical deficits in the posterior cortices. In frontal cortices, an opposite gradient of anterior-to-posterior progression of cholinergic deficits was found. The topography of k 3 changes exhibited regionally specific disconnection from k 1 changes. Interval-change analysis based on k 3/k 1 ratio images (k 3 adjustment for regional cerebral blood flow changes) showed interval reductions (up to 20%) in ventral frontal, anterior cingulate and Brodmann area 6 cortices. In contrast, interval k 3 reductions in the posterior cortices, especially Brodmann areas 17-19, were largely proportional to k 1 changes. Our results partially support the hypothesis of progressive posterior-to-cortical cholinergic denervation in Parkinson's disease. This pattern appears characteristic of posterior cortices. In frontal cortices, an opposite pattern of anterior-to-posterior progression of cholinergic deficits was found. The progressive decline of posterior cortical acetylcholinesterase activity was largely proportional to declining regional cerebral blood flow, suggesting that posterior cortical cholinergic synapse deficits are part of a generalized loss of synapses. The disproportionate decline in regional frontal cortical acetylcholinesterase activity relative to regional cerebral blood flow suggests preferential loss or dysregulation of cholinergic synapses in these regions. Our observations suggest that cortical cholinergic synapse vulnerability in Parkinson's disease is mediated by both diffuse processes affecting cortical synapses and processes specific to subpopulations of cortical cholinergic afferents.

Estrogen pendulum in schizophrenia and Alzheimer's disease: Review of therapeutic benefits and outstanding questions

Although produced largely in the periphery, gonadal steroids play a key role in regulating the development and functions of the central nervous system and have been implicated in several chronic neuropsychiatric disorders, with schizophrenia and Alzheimer's disease (AD) most prominent. Despite major differences in pathobiology and clinical manifestations, in both conditions, estrogen transpires primarily with protective effects, buffering the onset and progression of diseases at various levels. As a result, estrogen replacement therapy (ERT) emerges as one of the most widely discussed adjuvant interventions. In this review, we revisit evidence supporting the protective role of estrogen in schizophrenia and AD and consider putative cellular and molecular mechanisms. We explore the underlying functional processes relevant to the manifestation of these devastating conditions, with a focus on synaptic transmission and plasticity mechanisms. We discuss specific effects of estrogen deficit on neurotransmitter systems such as cholinergic, dopaminergic, serotoninergic, and glutamatergic. While the evidence from both, preclinical and clinical reports, in general, are supportive of the protective effects of estrogen from cognitive decline to synaptic pathology, numerous questions remain, calling for further research.