Clinical studies of the cholinergic deficit in Alzheimer's disease. II. Psychopharmacologic studies

Mitochondrial Dysfunction and Synaptic Transmission Failure in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder, in which multiple risk factors converge. Despite the complexity of the etiology of the disease, synaptic failure is the pathological basis of cognitive impairment, the cardinal sign of AD. Decreased synaptic density, compromised synaptic transmission, and defected synaptic plasticity are hallmark synaptic pathologies accompanying AD. However, the mechanisms by which synapses are injured in AD-related conditions have not been fully elucidated. Mitochondria are a critical organelle in neurons. The pivotal role of mitochondria in supporting synaptic function and the concomitant occurrence of mitochondrial dysfunction with synaptic stress in postmortem AD brains as well as AD animal models seem to lend the credibility to the hypothesis that mitochondrial defects underlie synaptic failure in AD. This concept is further strengthened by the protective effect of mitochondrial medicine on synaptic function against the toxicity of amyloid-β, a key player in the pathogenesis of AD. In this review, we focus on the association between mitochondrial dysfunction and synaptic transmission deficits in AD. Impaired mitochondrial energy production, deregulated mitochondrial calcium handling, excess mitochondrial reactive oxygen species generation and release play a crucial role in mediating synaptic transmission deregulation in AD. The understanding of the role of mitochondrial dysfunction in synaptic stress may lead to novel therapeutic strategies for the treatment of AD through the protection of synaptic transmission by targeting to mitochondrial deficits.

Pharmaceutical treatment for cognitive deficits in Alzheimer's disease and other neurodegenerative conditions: exploring new territory using traditional tools and established maps

RATIONALE

Over 30 years ago, we began to develop a nonhuman primate model to study cognitive deficits of age-related neurodegenerative diseases and their neuroanatomical-neurochemical underpinnings for purposes of translating this work toward first pharmacotherapies. This effort produced several notable findings that eventually received consensus support, which we have been asked to review.

OBJECTIVES

A discussion of these findings, in the context of issues and obstacles confronted and principles applied, might facilitate the development of even more effective models and treatments, not only for Alzheimer's disease (AD) but for many other disorders involving cognitive deficits.

RESULTS

Collectively, our research provided first evidence of the following: aged primates can be used as 'models' for human age-related neurodegenerative diseases; key cognitive deficits in early AD share important conceptual similarities to deficits in both aged monkeys as well as non-demented humans (e.g., age-associated memory impairment and mild cognitive impairment); pharmacological intervention can reduce age-related cognitive impairments in animals that are conceptually similar to those seen in human diseases, including AD; cholinergics would likely be the first approved therapeutics for AD; and that many other classes of drugs would not likely succeed.

CONCLUSIONS

Despite the early promise shown by behavioral/functional approaches to develop treatment strategies, the dramatic shift in focus away from behavioral outcomes in animal neurodegenerative research that began 20 years ago has compromised further progress and continues to impede our ability to understand how these diseases impair human cognition and what pathways might lead to effective therapies. Principles applied successfully in the past should provide guidance for facilitating efforts in the future.

Hormesis and medicine

Evidence is presented which supports the conclusion that the hormetic dose-response model is the most common and fundamental in the biological and biomedical sciences, being highly generalizable across biological model, endpoint measured and chemical class and physical agent. The paper provides a broad spectrum of applications of the hormesis concept for clinical medicine including anxiety, seizure, memory, stroke, cancer chemotherapy, dermatological processes such as hair growth, osteoporosis, ocular diseases, including retinal detachment, statin effects on cardiovascular function and tumour development, benign prostate enlargement, male sexual behaviours/dysfunctions, and prion diseases.

Alzheimer's disease drugs: an application of the hormetic dose-response model

This article provides an evaluation of the dose-response features of drugs that are intended to improve memory, some of which have been used in the treatment of Alzheimer's disease (AD). A common feature of these drugs is that they act via an inverted U-shaped dose response, consistent with the hormetic dose response model. This article assesses historical foundations that lead to the development of AD drugs, their dose-response features and how the quantitative features of such dose responses affected drug discovery and development, and the successes and possible failures of such agents in preclinical and clinical settings. This story begins about 150 years ago with the discovery of an active agent in the Calabar bean plant called physostigmine, its unfolding medical applications, and its implications for dose-response relationships, memory enhancement, and improved drug discovery activities. The article also demonstrates the occurrence of U-shaped dose responses for memory with numerous endogenous agonists including neurosteroids, various peptides (e.g., vasopressin, CCK-8, neuropeptide Y), and other agents (e.g., epinephrine, antagonists for platelet activity factor and nicotinic receptors), supporting the generalizability of the hormetic biphasic dose response. Finally, the significance of the U-shaped dose response is critical for successful clinical application, since it defines the therapeutic window.

Nerve growth factor in treatment and pathogenesis of Alzheimer's disease

The etiology of Alzheimer's disease (AD) is still unknown. In addition, this terrible neurodegenerative disease will increase exponentially over the next two decades due to longer lifespan and an aging "baby-boomer" generation. All treatments currently approved for AD have moderate efficacy in slowing the rate of cognitive decline in patients, and no efficacy in halting progression of the disease. Hence, there is an urgent need for new drug targets and delivery methods to slow or reverse the progression of AD. One molecule that has received much attention in its potential therapeutic role in AD is nerve growth factor (NGF). This review will demonstrate data from humans and animals which promote NGF as a potential therapeutic target by (1) outlining the hypothesis behind using NGF for the treatment of AD, (2) reviewing both the normal and AD altered signaling pathways and effects of NGF in the central nervous system (CNS), and (3) examining the results of NGF treatment obtained from animal models of AD and AD patients.

CR 2249: a new putative memory enhancer. Behavioural studies on learning and memory in rats and mice

The effects of S-4-amino-5-[4,4-dimethylcyclohexyl)amino]-5-oxopentanoic acid (CR 2249), a new entity selected from a new series of glutamic acid derivatives, has been investigated in different paradigms for screening nootropics. CR 2249 ameliorated the memory retention deficit produced by scopolamine in step-through-type passive avoidance in rats and by electroconvulsive shock in step-down-type passive avoidance in mice. CR 2249 was also capable of improving performance in behavioural tests of learning and memory in the absence of cholinergic hypofunction or cognitive deficit. The activity was determined using different passive and active avoidance behavioural test procedures on rats. CR 2249 was active only when given 45 min before training and did not show any effect when administered immediately after the learning training or before the retention trial. No changes in the general behaviour or motor activity of the animals were observed, indicating that CR 2249 effects cannot be attributed to sensory-motor deficit. Microdialysis experiments have shown that CR 2249 significantly increased noradrenaline release in the hippocampus of freely moving rats and reduced 3,4-dihydroxyphenylglycol efflux. These effects have led us to hypothesize that CR 2249 memory effect might be mediated by a direct or indirect action on noradrenergic transmission. These behavioural results suggest that this new agent has clinical application in memory disorders.

Effects of the centrally acting cholinesterase inhibitors tetrahydroaminoacridine and E2020 on the basal concentration of extracellular acetylcholine in the hippocampus of freely moving rats

The effects of the centrally acting cholinesterase (ChE) inhibitors, tetrahydroaminoacridine (THA) and E2020 (1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl] methylpiperidine hydrochloride), potential drugs for the treatment of senile dementia, on the basal extracellular acetylcholine (ACh) concentration in the hippocampus of freely moving rats, were determined using a microdialysis technique without the use of a ChE inhibitor in the perfusion fluid and a sensitive RIA. The mean (+/- SEM) basal ACh content in the perfusate was 103.1 +/- 3.6 fmol/sample collected over 30 min when microdialysis probes with a length of 3 mm dialysis membrane were used. The content of ACh decreased to an almost undetectable level upon perfusion of magnesium, suggesting that, in the present study, most of the ACh detected in the perfusates was due to cholinergic neuronal activity. THA (1.65 mg/kg, i.p.) produced an insignificant increase in the extracellular ACh concentration, but a dose of 5 mg/kg, i.p. caused a prolonged and significant 5.5-fold increase from the control value. E2020 (0.65 and 2 mg/kg, i.p.) produced significant, prolonged and dose-dependent increases (4 and 12 times the control value, respectively), the peak effect occurring within 1 h. Perfusion with 10 mumol/l physostigmine produced an about 30-fold increase of ACh output, suggesting that the basal extracellular ACh concentration is highly dependent on ChE activity. When ChE was inhibited locally by perfusion with physostigmine, THA (5 mg/kg) produced a transient and, at its maximum, a 1.42-fold increase in extracellular ACh concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenserine: a physostigmine derivative that is a long-acting inhibitor of cholinesterase and demonstrates a wide dose range for attenuating a scopolamine-induced learning impairment of rats in a 14-unit T-maze

Phenserine ((-)-N-phenylcarbamoyl eseroline), a carbamate analog of physostigmine (Phy), is a long-acting inhibitor of cholinesterase. We have assessed the potential clinical value of phenserine for cholinomimetic therapy of cognitive impairments associated with aging and Alzheimer's disease by evaluating its duration of in vivo activity against rat plasma acetylcholinesterase (AChE) and its effect on attenuating a scopolamine-induced impairment in learning performance of young rats in a shock-motivated 14-unit T-maze. Phenserine achieved maximum AChE inhibition of 73.5% at 5 min and maintained a high and relatively constant inhibition for more than 8 h. For analysis of effects on learning performance, 69, 3-month-old male Fischer-344 rats were pretrained in a straight runway to avoid electric footshock. On the following day, each animal received 15 trials in the 14-unit T-maze. Sixty minutes prior to the maze training, each rat received the first IP injection of either vehicle (Tween 80, ethanol and 0.9% NaCl) or phenserine at 1.5, 3.0, 4.0, 5.0, 7.5, or 10.0 mg/kg. Then 30 min prior to the training, each animal received a second IP injection of either 0.9% NaCl or scopolamine hydrochloride (0.75 mg/kg; SCOP). Compared to the vehicle-SCOP group, all but the 7.5 mg/kg dose of phenserine significantly ameliorated error performance, runtime, shock frequency and shock duration in SCOP-treated rats at the final block of three trials. Appearing to have a long effect and a wide therapeutic window, phenserine deserves further study as a cognitive enhancer.

The long-acting cholinesterase inhibitor heptyl-physostigmine attenuates the scopolamine-induced learning impairment of rats in a 14-unit T-maze

Heptyl-physostigmine (heptyl-Phy), a new carbamate derivative of physostigmine (Phy), has been assessed for potential clinical value by evaluating its in vitro activity against human erythrocyte acetylcholinesterase (AChE) and plasma butyrylcholinesterase (BChE), its duration of in vivo activity against rat plasma AChE, and its effects on attenuating a scopolamine-induced impairment in learning performance of young rats in a 14-unit T-maze. Heptyl-Phy demonstrated potent cholinesterase inhibition, with activity similar to that of Phy against AChE, IC50 values 21.7 +/- 2.0 nM and 27.9 +/- 2.4 nM, respectively, and significantly greater than that of Phy against BChE, IC50 values 5.0 +/- 0.1 nM and 16.0 +/- 2.9 nM, respectively. Heptyl-Phy achieved maximum AChE inhibition of 92.5% at 60 min and maintained a high and relatively constant inhibition for more than 8 h. For analysis of effects on learning performance, heptyl-Phy at 1.0, 1.5, 2.0 or 3.0 mg/kg, or vehicle was administered i.p. to 52 3-month-old male Fischer-344 rats 60 min prior to maze training. Thirty minutes prior to training, each animal received either 0.9% NaCl or scopolamine hydrochloride (0.75 mg/kg). Only a 2.0 mg/kg dose of heptyl-Phy significantly reduced the number of errors in scopolamine-treated rats. The other doses did not improve any aspect of maze performance. Although the therapeutic window of heptyl-Phy did not appear wide enough for clinical use, the longer duration of action of heptyl-Phy would appear beneficial.

Effects of acute infusion of the muscarinic cholinergic agonist arecoline on verbal memory and visuo-spatial function in dementia of the Alzheimer type

1. Treatment of patients with dementia of the Alzheimer type (DAT) with arecoline, a muscarinic cholinergic receptor agonist, reportedly improves performance on a picture recognition memory task, but not on other memory measures. To examine further possible performance improvements following arecoline treatment, patients with DAT were treated with a 30 min intravenous infusion of arecoline (5 mg). 2. Psychometric testing was done at five time points (two before and three following the infusion). Patients were tested on a memory task (Buschke selective reminding) and a test of visuo-spatial performance (figure copying). 3. No net change from baseline was seen in mean scores following arecoline infusion. However, the changes in performance on the two tasks were correlated (p less than 0.02) over subjects at 10 min but not at 1.5 or 5.5 hr following the infusion. 4. This result suggests that although individual patients vary in their response to a given dose of arecoline, their responses are consistent across types of tasks. Thus the lack of a mean drug effect may be due to individual differences in response rather than to a lack of response.

Effects of physostigmine on operant serial discrimination/reversal learning in rats

Two experiments examined the effects of physostigmine on acquisition and performance of operant serial reversals by rats. In Experiment 1, four groups of rats (n = 6/group) were injected with either vehicle or 0.03 mg/kg physostigmine five minutes prior to each session, or vehicle or 0.5 mg/kg physostigmine immediately after each session of a three-stimulus (bright, dim or flashing light) repeated discrimination/reversal procedure. Rats treated with physostigmine pre- or postsession learned significantly more reversals over 50 sessions than animals injected with vehicle. Experiment 2 used only two discriminative stimuli, a light and a 2,500 Hz tone. Following establishment of a stable daily reversal baseline, postsession injections of physostigmine significantly increased the number of trials to criterion on the next session compared to each subject's control baseline. Results are attributed to enhanced between-session transfer of previously learned discriminated instrumental responses by physostigmine-treated animals.

The dexamethasone suppression test in dementia: a review of the literature

To examine the utility of the dexamethasone suppression test (DST) in the differential diagnosis of depression in elderly demented patients, we reviewed the literature and focused on four components of this question: (1) cortisol nonsuppression rates in dementia; (2) cortisol nonsuppression and dementia severity; (3) cortisol nonsuppression in demented versus depressed patients; and (4) cortisol nonsuppression following antidepressant treatment. A combined analysis of 27 articles showed cortisol nonsuppression in 60% of patients with concurrent dementia and depression, in 47% of patients with depression only, in 41% of patients with dementia only, in 46% of patients with multi-infarct dementia, in 36% of patients with primary degenerative dementia, and in 10% of controls. The abnormal DST rate in demented patients was not significantly different from the abnormal DST rate in depressed patients. Eight of 12 studies (67%) did not find a significant relationship between DST results and dementia severity dementia patients without depression. Twelve of 13 studies (92%) did not find a relationship between age and DST outcome. The data we reviewed do not support the use of the DST in discriminating between depression and dementia or between dementia subtypes.

Multiple effects of tetrahydroaminoacridine on the cholinergic system: biochemical and behavioural aspects

9-Amino-1,2,3,4-tetrahydroaminoacridine (THA) in combination with lecithin has been reported to improve the memory of Alzheimer's disease patients. We have examined some properties of THA in vitro and in vivo so as to define some of the mechanism(s) by which THA might produce its therapeutic effects. In vitro, THA was more potent at inhibiting human plasma cholinesterase (IC50 = 0.03 microM) than human erythrocyte acetylcholinesterase (IC50 = 0.3 microM) and rat brain acetylcholinesterase (IC50 = 0.32 microM). Radioligand binding studies indicated that THA binds reversibly and competitively to primary M1 and M2 human cortical muscarinic receptors with similar affinities. Moreover, THA showed similar affinity for temporal cortices muscarinic receptors from Alzheimer and non-Alzheimer (control) brains. In vivo, subcutaneous administration of THA (1-8 mg/kg body weight) to adult rats (6 months old) produced a dose dependent decrease in general activity compared to saline-treated rats. However, at a concentration of 0.5 mg/kg body weight, the general activity of the rats was increased compared to saline-treated rats. The cognitive function of the THA-treated adult rats (subcutaneously 2 mg/kg body weight) was not significantly improved compared to saline-treated rats. It is concluded that the mechanisms of action of THA on the cholinergic system involve reversible inhibition of cholinesterases and reversible and competitive interaction with muscarinic acetylcholine receptors. These effects might be of therapeutic value in the treatment of Alzheimer's disease.

Drug treatments of dementia

Present drug treatments for the more common types of dementia are largely palliative or symptomatic, but the promise of drugs to prevent the development of dementia does not seem unrealistic. Neuropharmacological strategies that aim to improve behavioural or cognitive symptoms in dementia are subdivided into drugs that modify central cholinergic, serotonergic, dopaminergic, GABAnergic, and peptidergic transmission. There are no clear clinical guidelines for the management of demented patients who are also hypertensive. The early identification of patients with incipient dementia, the detection of slight but theoretically important beneficial effects, the optimum method of drug administration, more soundly based treatment of behavioural symptoms, and the evaluation of drug combinations are all identified as areas requiring further research.

The cholinergic pharmacology of tetrahydroaminoacridine in vivo and in vitro

1. The effect of tetrahydroaminoacridine (THA) on cholinergically mediated behaviour in the rat and mouse has been investigated. In addition the actions of this compound on cholinesterase activity and on muscarinic and nicotinic receptors has also been examined. 2. Administration of THA (5-20 mg kg-1, i.p.) produced a dose-dependent increase in tremor, hypothermia and salivation in both rats and mice. A similar profile of activity was seen following physostigmine (0.1-0.6 mg kg-1) administration. 3. THA was approximately fifty fold less potent than physostigmine in inducing behavioural change but its effects persisted for over twice as long as those of physostigmine. For example THA-induced hypothermia was still present at 4 h in the mouse and 8 h in the rat. 4. In vitro THA was a potent non-competitive inhibitor of rat brain cholinesterase (IC50: 57 +/- 6 nM) and bovine erythrocyte acetylcholinesterase (IC50: 50 +/- 10 nM) but was a more potent inhibitor of horse serum butyrylcholinesterase (IC50: 7.2 +/- 1.4 nM). 5. Radioligand binding studies indicated that THA binds non-selectively but with moderate potency to both M1 (Ki: 600 nM) and M2 (Ki: 880 nM) muscarinic receptors. THA also interacted with the allosteric site present on cardiac M2 receptors. 6. It is concluded that THA is a reversible non-competitive inhibitor of cholinesterase with a long half life (compared with physostigmine). It also may antagonize muscarinic receptors at high doses. The long half life may account for its reported efficacy in the treatment of Alzheimer's disease.

Pharmacokinetics and pharmacodynamics of physostigmine in the rat after oral administration

The distribution, metabolism, and pharmacokinetics of physostigmine (Phy) and the time course of butyrylcholinesterase (BuChE) in plasma and cholinesterase (ChE) activity in brain and muscle and their relationship to Phy concentration were described after oral administration of 3H-Phy (650 micrograms kg-1) to rats. Physostigmine concentration vs time data was analyzed by nonlinear computer fitting program using one-compartment model. The absorption rate constant (ka) and elimination rate constant (ke) were found to be 0.1 +/- 0.07 min-1 and 0.036 +/- 0.024 min-1, respectively. Cpmax and tmax were 3.3 ng ml-1 and 16 min. The clearance (C1) was found to be 80.9 ml min-1kg-1. Half-life of Phy in brain, muscle, and liver were 33.4 min, 22.5, and 28 min, respectively. The bioavailability (F) was calculated to be 0.02 and the extraction ratio was found to be 0.98 indicating the 'first pass' effect. Butyrylcholinesterase activity in plasma was 76 per cent at 15 min and this activity did not change significantly up to 120 min. However, Phy concentration in plasma was very low; 2.89 ng ml-1 at 15 min and declined to 0.71 ng ml-1 at 90 min. Physostigmine concentration in brain peaked at 22 min to 2.85 +/- 1.09 ng g-1 and declined to 0.33 +/- 0.11 ng g-1 at 60 min. Cholinesterase activity in brain was 96 per cent, 82 per cent and 89 per cent at 10, 45, and 120 min, respectively. Physostigmine concentration in muscle was very low and the ChE activity in the muscle was 66.4 per cent of control at 45 min. The time course of Phy metabolism indicated that at 5 min most of the RA in the tissues was due to metabolites accounting for 94.6 per cent in plasma, 90 per cent in liver, 79.8 per cent in brain and 86.3 per cent in muscle. M1 appeared to be the major metabolite followed by eseroline. The results showed extremely low concentrations of Phy (200 times less in plasma and 350 times less in brain) after oral administration compared to our previous studies with the same dose after i.m. administration.

Potential pharmacotherapy of Alzheimer disease. A comparison of various forms of physostigmine administration

This paper reviews clinical trials with physostigmine administered to Alzheimer patients using three different routes of administration: oral, i.v. and intracerebroventricular (i.c.v.). It compares results obtained with three different routes by the authors as well as by other authors. Particular emphasis is given to a novel type of physostigmine administration, the i.c.v. route. Advantages and disadvantages, as well as side effects of each route are presented and discussed.