Alteration in nicotine binding sites in Parkinson's disease, Lewy body dementia and Alzheimer's disease: possible index of early neuropathology

Acetylcholine, aging, and Alzheimer's disease

A substantial body of literature has suggested that the memory and learning deficits associated with Alzheimer's disease and aging are attributable to degeneration of the cholinergic magnocellular neurons of the nucleus basalis of Meynert (nbM). Subsequently, lesion-induced damage to the cholinergic projections from the nbM to the neocortex has been utilized extensively as an animal model of dementia. In addition, the effect of the normal aging process on deterioration of these neurons and on cognitive function has also been examined. Such studies have revealed, for example, that many of the learning and memory impairments traditionally attributed to the cholinergic corticopetal system are not due to degeneration of the cholinergic neurons of the nbM, but instead may be due to damage of more rostral elements of the cholinergic basal forebrain system. This review will examine the contribution of behavioural animal and human studies to out understanding of the role of the basal forebrain cholinergic neurons in age-related cognitive impairments.

Human alpha4beta2 neuronal nicotinic acetylcholine receptor in HEK 293 cells: A patch-clamp study

The cloning and expression of genes encoding for the human neuronal nicotinic acetylcholine receptors (nAChRs) has opened new possibilities for investigating their physiological and pharmacological properties. Cells (HEK 293) stably transfected with two of the major brain subunits, alpha4 and beta2, were characterized electrophysiologically using the patch-clamp technique. Fast application of the natural ligand ACh can evoke currents up to 3500 pA, with an apparent affinity (EC50) of 3 microM and a Hill coefficient of 1.2. The rank order of potency of four nAChR ligands to activate human alpha4beta2 receptors is (-)-nicotine > ACh > (-)-cytisine > ABT-418. At saturating concentrations, the efficacy of these ligands is ABT-418 >> (-)-nicotine > ACh >> (-)-cytisine > GTS-21 (previously named DMXB). Coapplication of 1 microM ACh with known nAChR inhibitors such as dihydro-beta-erythroidine and methyllycaconitine reversibly reduces the current evoked by the agonist with respective IC50 values of 80 nM and 1.5 microM. The current-voltage relationship of human alpha4beta2 displays a strong rectification at positive potentials. Experiments of ionic substitutions suggest that human alpha4beta2 nAChRs are permeable to sodium and potassium ions. In the "outside-out" configuration, ACh evokes unitary currents (main conductance 46 pS) characterized by a very fast rundown. Potentiation of the ACh-evoked currents is observed when the extracellular calcium concentration is increased from 0.2 to 2 mM. In contrast, however, a reduction of the evoked currents is observed when calcium concentration is elevated above 2 mM.

Structural basis of dementia in neurodegenerative disorders

Progressive dementia syndromes in adults are caused by a number of conditions associated with different structural lesions of the brain. In most clinical and autopsy series, senile dementia of the Alzheimer type is the most common cause of mental decline in the elderly accounting for up to 90%, whereas degenerative non-Alzheimer dementias range from 7 to 30% (mean 8-10%). They include a variety of disorders featured morphologically by neuron and synapse loss and gliosis, often associated with cytopathological changes involving specific cortical and subcortical circuits. These neuronal/glial inclusions and neuritic alterations show characteristic immunoreactions and ultrastructure indicating cytoskeletal mismetabolism. They are important diagnostic sign posts that, in addition to the distribution pattern of degenerative changes, indicate specific vulnerability of neuronal populations, but their pathogenic role and contribution to mental decline are still poorly understood. In some degenerative disorders no such cytopathological hallmarks have been observed; a small number is genetically determined. While in Alzheimer's disease (AD) mental decline is mainly related to synaptic and neuritic pathologies, other degenerative disorders show variable substrates of dementia involving different cortical and/or subcortical circuits which may or may not be superimposed by cortical Alzheimer lesions. In most demented patients with Lewy body disorders (Parkinson's disease, Lewy body dementia), they show similar distribution as in AD, while in Progressive Supranuclear Palsy (PSP), mainly prefrontal areas are involved. Lobar atrophies, increasingly apparent as causes of dementia, show fronto-temporal cortical neuron loss, spongiosis and gliosis with or without neuronal inclusions (Pick bodies) and ballooned cells, while dementing motor neuron disease and multisystem atrophies reveal ubiquitinated neuronal and oligodendroglial inclusions. There are overlaps or suggested relationships between some neurodegenerative disorders, e.g. between corticobasal degeneration, PSP and Pick's atrophy. In many of these disorders with involvement of the basal ganglia, degeneration of striatofrontal and hippocampo-cortical loops are important factors of mental decline which may be associated with isocortical neuronal degeneration and synapse loss or are superimposed by cortical AD pathology.