Cholinergic toxins and Alzheimer's disease

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Experimental and clinical methods in the development of anti-Alzheimer drugs

Methodology used for the development of anti-Alzheimer's disease (AD) drugs raises specific problems which are rarely examined in the literature. While the general development scheme is similar to that required for most drugs, some specific aspects must be analyzed, highly dominated by the dual goal of pharmacology, i.e., to obtain both symptomatic and etiopathogenic drugs. During preclinical studies, aged or lesioned animals are mainly useful for symptomatic drugs, whereas transgenic models or neurodegeneration-induced techniques would probably lead to etiopathogenic drugs potentially slowing down the process of AD. The first administrations of a new compound to human beings raise the question of the activity measurement techniques. Psychometry remains the most informative procedure to detect and analyze the activity of the drugs on the different components of cognition. Electrophysiology and neuroimaging need some complementary studies before they can be proposed as surrogate criteria in phase III trials. At this stage of development, American and the recently published European guidelines are of great help while insisting on long-term (6 months) placebo controlled trials with the use of the triple efficacy criterion: an objective cognition scale, a global assessment, and the opinion of the caregiver. In the long term, pharmacoepidemiology and pharmacoeconomy will have to confirm the rationale of this recent progress in the methodology of anti-AD drug development.

Alternating lever cyclic-ratio schedule analysis of the effects of atropine sulfate

Cholinergic dysfunction has been implicated in the behavioral and memory impairment that is the hallmark of conditions such as delirium and Alzheimer's Disease. Anticholinergic drugs have been widely used in procedures designed to mimic aspects of the pathology of these conditions in rats. Procedures in use vary widely in sensitivity and behavioral specificity and may be confounded by administration of high drug doses that may not be physiologically relevant. The current study proposes the use of rats responding on an alternating lever cyclic-ratio schedule to study the effects of the anticholinergic compound atropine sulfate. This procedure enables simultaneous measurement of anticipatory ratio tracking (postreinforcement pause durations), perseverations (lever switching errors), and nonspecific peripheral drug effects (running response rates). Results of this study suggest that the schedule is sensitive to low drug doses (0.1-1.0 mg/kg atropine), measures the ability to track changing ratio conditions and to execute lever alternation, and allows for monitoring of peripheral drug effects during behavioral testing. The procedure's sensitivity and low effective dose range may make it useful in the study of behaviors related to anticholinergic effects.

Tolcapone, an inhibitor of catechol O-methyltransferase, counteracts memory deficits caused by bilateral cholinotoxin lesions of the basal nuclei of Meynert

Repeated administration of tolcapone, an inhibitor of catechol-O-methyltransferase, was able to partially restore the memory deficits caused by bilateral cholinotoxin (AF64A) lesions in the basal magnocellular nuclei of Meynert. The 2-week tolcapone treatment (3 mg kg-1, once a day) was started 24 h before toxin infusion and the last injection was given 24 h before the first avoidance test. The beneficial action of tolcapone may be related to antioxidant properties of nitrocatechols.

Effects of bilateral cholinotoxin infusions on the behavior and brain biochemistry of the rats

We examined behavioral and biochemical specificity and the general usefulness of the proposed rat model of Alzheimer's disease. Bilateral infusions of ethylcholine aziridinium (AF64A) into the basal magnocellular nuclei caused a deterioration of learning in passive and active avoidance tests, increased emotional reactivity, and decreased motoric activity. Choline acetyltransferase activity was decreased by 22% in the frontal cortex but increased by 8-10% in the hippocampus and hypothalamus. Noradrenaline and dopamine levels in the frontal cortex were decreased by 20%. In striatum, dopamine and its metabolites were strongly suppressed (by 50-60%). Also striatal noradrenaline (-48%) and 5-hydroxytryptamine (-34%) were significantly decreased. Hypothalamic 5-hydroxytryptamine was increased (+25%). Bilateral AF64A lesions decreased significantly (by 14-20%) activities of prolyl endopeptidase, dipeptidyl peptidase II and IV in hippocampal and frontal cortical brain homogenates. These results show that AF64A can be used to induce long-term learning deficits in the rat. However, striatal amine levels are also strongly suppressed, and are reflected as hypomotility and increased emotional reactivity. These changes may limit the usefulness of the rat model. Universally decreased peptidase activities offer interesting views regarding the role of peptidase inhibitors in amnestic disorders.