Cholinergic receptors in aging and Alzheimer's disease

Acetylcholinesterase and Butyrylcholinesterase – Important Enzymes of Human Body

The serine hydrolases and proteases are a ubiquitous group of enzymes that is fundamental to many critical lifefunctions. Human tissues have two distinct cholinesterase activities: acetylcholinesterase and butyrylcholinesterase. Acetylcholinesterase functions in the transmission of nerve impulses, whereas the physiological function of butyrylcholinesterase remains unknown. Acetylcholinesterase is one of the crucial enzymes in the central and peripheral nerve system. Organophosphates and carbamates are potent inhibitors of serine hydrolases and well suited probes for investigating the chemical reaction mechanism of the inhibition. Understanding the enzyme’s chemistry is essential in preventing and/or treating organophosphate and carbamate poisoning as well as designing new medicaments for cholinergic-related diseases like as Alzheimer’s disease.

Scopolamine disrupts place navigation in rats and humans: a translational validation of the Hidden Goal Task in the Morris water maze and a real maze for humans

RATIONALE

Development of new drugs for treatment of Alzheimer's disease (AD) requires valid paradigms for testing their efficacy and sensitive tests validated in translational research.

OBJECTIVES

We present validation of a place-navigation task, a Hidden Goal Task (HGT) based on the Morris water maze (MWM), in comparable animal and human protocols.

METHODS

We used scopolamine to model cognitive dysfunction similar to that seen in AD and donepezil, a symptomatic medication for AD, to assess its potential reversible effect on this scopolamine-induced cognitive dysfunction. We tested the effects of scopolamine and the combination of scopolamine and donepezil on place navigation and compared their effects in human and rat versions of the HGT. Place navigation testing consisted of 4 sessions of HGT performed at baseline, 2, 4, and 8 h after dosing in humans or 1, 2.5, and 5 h in rats.

RESULTS

Scopolamine worsened performance in both animals and humans. In the animal experiment, co-administration of donepezil alleviated the negative effect of scopolamine. In the human experiment, subjects co-administered with scopolamine and donepezil performed similarly to subjects on placebo and scopolamine, indicating a partial ameliorative effect of donepezil.

CONCLUSIONS

In the task based on the MWM, scopolamine impaired place navigation, while co-administration of donepezil alleviated this effect in comparable animal and human protocols. Using scopolamine and donepezil to challenge place navigation testing can be studied concurrently in animals and humans and may be a valid and reliable model for translational research, as well as for preclinical and clinical phases of drug trials.

Molecular imaging of neuropsychiatric symptoms in Alzheimer's and Parkinson's disease

Neuropsychiatric symptoms (NPS) are very common in neurodegenerative diseases and are a major contributor to disability and caregiver burden. There is accumulating evidence that NPS may be a prodrome and/or a "risk factor" of neurodegenerative diseases. The medications used to treat these symptoms in younger patients are not very effective in patients with neurodegenerative disease and may have serious side effects. An understanding of the neurobiology of NPS is critical for the development of more effective intervention strategies. Targeting these symptoms may also have implications for prevention of cognitive or motor decline. Molecular brain imaging represents a bridge between basic and clinical observations and provides many opportunities for translation from animal models and human post-mortem studies to in vivo human studies. Molecular brain imaging studies in Alzheimer's disease (AD) and Parkinson's disease (PD) are reviewed with a primary focus on positron emission tomography studies of NPS. Future directions for the field of molecular imaging in AD and PD to understand the neurobiology of NPS will be discussed.

Microdosing of scopolamine as a "cognitive stress test": rationale and test of a very low dose in an at-risk cohort of older adults

BACKGROUND

Abnormal β-amyloid (Aβ) is associated with deleterious changes in central acetylcholinergic tone in the very early stages of Alzheimer's disease (AD), which may be unmasked by a cholinergic antagonist. We aimed to establish an optimal "microdose" of scopolamine for the development of a "cognitive stress test."

METHODS

Healthy older adults (n = 26, aged 55-75 years) with two risk factors for AD, but with low cortical Aβ burden, completed the Groton Maze Learning Test (GMLT) at baseline and then received scopolamine (0.20 mg subcutaneously). Participants were reassessed at 1, 3, 5, 7, and 8 hours postinjection.

RESULTS

There were significant differences, of a moderate magnitude, in performance between baseline and 3 hours postinjection for total errors, rule break errors, and the GMLT composite (d ≈ 0.50) that were all unrelated to body mass.

CONCLUSIONS

A very low dose of scopolamine leads to reliable cognitive impairment at 3 hours postdose (Tmax) and full cognitive recovery within 5 hours, supporting its use as a prognostic test paradigm to identify individuals with potential preclinical AD. This paradigm is being implemented in a larger cohort of healthy adults, with high or low Aβ, to identify pharmacodynamic differences between groups.

Animal models in the drug discovery pipeline for Alzheimer's disease

With increasing feasibility of predicting conversion of mild cognitive impairment to dementia based on biomarker profiling, the urgent need for efficacious disease-modifying compounds has become even more critical. Despite intensive research, underlying pathophysiological mechanisms remain insufficiently documented for purposeful target discovery. Translational research based on valid animal models may aid in alleviating some of the unmet needs in the current Alzheimer's disease pharmaceutical market, which includes disease-modification, increased efficacy and safety, reduction of the number of treatment unresponsive patients and patient compliance. The development and phenotyping of animal models is indeed essential in Alzheimer's disease-related research as valid models enable the appraisal of early pathological processes - which are often not accessible in patients, and subsequent target discovery and evaluation. This review paper summarizes and critically evaluates currently available animal models, and discusses their value to the Alzheimer drug discovery pipeline. Models dealt with include spontaneous models in various species, including senescence-accelerated mice, chemical and lesion-induced rodent models, and genetically modified models developed in Drosophila melanogaster, Caenorhabditis elegans, Danio rerio and rodents. Although highly valid animal models exist, none of the currently available models recapitulates all aspects of human Alzheimer's disease, and one should always be aware of the potential dangers of uncritical extrapolating from model organisms to a human condition that takes decades to develop and mainly involves higher cognitive functions.

Cholinesterase inhibitors improve both memory and complex learning in aged beagle dogs

Similar to patients with Alzheimer's disease (AD), dogs exhibit age-dependent cognitive decline, amyloid-β (Aβ) pathology, and evidence of cholinergic hypofunction. The present study sought to further investigate the role of cholinergic hypofunction in the canine model by examining the effect of the cholinesterase inhibitors phenserine and donepezil on performance of two tasks, a delayed non-matching-to-position task (DNMP) designed to assess working memory, and an oddity discrimination learning task designed to assess complex learning, in aged dogs. Phenserine (0.5 mg/kg; PO) significantly improved performance on the DNMP at the longest delay compared to wash-out and partially attenuated scopolamine-induced deficits (15 μg/kg; SC). Phenserine also improved learning on a difficult version of an oddity discrimination task compared to placebo, but had no effect on an easier version. We also examined the effects of three doses of donepezil (0.75, 1.5, and 6 mg/kg; PO) on performance of the DNMP. Similar to the results with phenserine, 1.5 mg/kg of donepezil improved performance at the longest delay compared to baseline and wash-out, indicative of memory enhancement. These results further extend the findings of cholinergic hypofunction in aged dogs and provide pharmacological validation of the canine model with a cholinesterase inhibitor approved for use in AD. Collectively, these studies support utilizing the aged dog in future screening of therapeutics for AD, as well as for investigating the links among cholinergic function, Aβ pathology, and cognitive decline.

Impaired attention in the 3xTgAD mouse model of Alzheimer's disease: rescue by donepezil (Aricept)

Several mouse models of Alzheimer's disease (AD) with abundant β-amyloid and/or aberrantly phosphorylated tau develop memory impairments. However, multiple non-mnemonic cognitive domains such as attention and executive control are also compromised early in AD individuals. Currently, it is unclear whether mutations in the β-amyloid precursor protein (APP) and tau are sufficient to cause similar, AD-like attention deficits in mouse models of the disease. To address this question, we tested 3xTgAD mice (which express APPswe, PS1M146V, and tauP301L mutations) and wild-type control mice on a newly developed touchscreen-based 5-choice serial reaction time test of attention and response control. The 3xTgAD mice attended less accurately to short, spatially unpredictable stimuli when the attentional demand of the task was high, and also showed a general tendency to make more perseverative responses than wild-type mice. The attentional impairment of 3xTgAD mice was comparable to that of AD patients in two aspects: first, although 3xTgAD mice initially responded as accurately as wild-type mice, they subsequently failed to sustain their attention over the duration of the task; second, the ability to sustain attention was enhanced by the cholinesterase inhibitor donepezil (Aricept). These findings demonstrate that familial AD mutations not only affect memory, but also cause significant impairments in attention, a cognitive domain supported by the prefrontal cortex and its afferents. Because attention deficits are likely to affect memory encoding and other cognitive abilities, our findings have important consequences for the assessment of disease mechanisms and therapeutics in animal models of AD.

Mechanisms of alpha7-nicotinic receptor up-regulation and sensitization to donepezil induced by chronic donepezil treatment

alpha7-nicotinic acetylcholine receptors are one of the most abundant subtypes of nicotinic receptors in the brain and have been shown to be involved in the neuroprotective effect of donepezil. Recently, we showed that in primary culture of rat cortical neurons, chronic donepezil treatment (10 muM, 4 days) (1) induces the up-regulation of alpha7-nicotinic receptors, (2) enhances the nicotine-induced increase in [Ca(2+)](i) and (3) enhances the sensitivity to the neuroprotective effect of donepezil. Here we demonstrate the involvement of alpha7-nicotinic receptors in these three effects. Concomitant treatment with nicotinic receptor antagonist inhibited the up-regulation of alpha7-nicotinic receptor, enhancement of the increase in [Ca(2+)](i) induced by nicotine, and enhancement of sensitivity to the neuroprotective effect of donepezil. Next, using inhibitors of phosphatidylinositol 3-kinase and mitogen-activated protein kinase signaling pathways, we demonstrate the involvement of these pathways in the up-regulation of alpha7-nicotinic receptors and in making the neurons more sensitive to the neuroprotective effects of donepezil. Concomitant chronic donepezil treatment with inhibitors of phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways inhibited nicotinic receptor up-regulation and enhancement of the response to nicotine, and enhanced the sensitivity to donepezil. This study increases understanding of the less-studied mechanism of chronic donepezil treatment-induced nicotinic receptor up-regulation and increased sensitivity to donepezil.

Impaired muscarinic regulation of excitatory synaptic transmission in the APPswe/PS1dE9 mouse model of Alzheimer's disease

Cholinergic hypothesis and amyloid cascade hypothesis are mainly proposed for Alzheimer's disease; however, the relationship between these hypotheses is poorly understood. To address the question of whether amyloid beta-peptide pathology affects cholinergic neurotransmission, we examined the effect of a cholinesterase inhibitor, physostigmine, on field excitatory postsynaptic potentials (EPSPs) evoked by single-pulse stimulation in the CA1 region of the hippocampus of various APPswe/PS1dE9 transgenic mice with different degrees of amyloid beta-peptide pathology. Reduced field EPSPs by physostigmine in transgenic mice at 3 months of age, when the mice had negligible amyloid beta-peptide levels and no amyloid beta-peptide deposits, were indistinguishable from those in age-matched wild-type mice. In contrast, reduced field EPSPs by physostigmine in transgenic mice at 5 months of age, when the mice had low amyloid beta-peptide levels and subtle amyloid beta-peptide deposits, were significantly lower than those in age-matched wild-type mice. Next, we characterized acetylcholine receptors, which play important roles in cholinergic neurotransmission, because physostigmine resulted in increased acetylcholine levels in the synaptic cleft. Different reductions of field EPSPs by physostigmine between transgenic and wild-type mice at 5 months of age were not affected by a nicotinic receptor antagonist, mecamylamine; however, reduced field EPSPs by physostigmine in both transgenic and wild-type mice were restored to basal levels by a muscarinic receptor antagonist, atropine. These results indicate that cholinergic modulation of glutamatergic transmission is already impaired at the onset of the formation of amyloid beta-peptide deposits, and muscarinic receptor dysfunction is one of the causes of this impairment.

Neuromodulation by glutamate and acetylcholine can change circuit dynamics by regulating the relative influence of afferent input and excitatory feedback

Substances such as acetylcholine and glutamate act as both neurotransmitters and neuromodulators. As neuromodulators, they change neural information processing by regulating synaptic transmitter release, altering baseline membrane potential and spiking activity, and modifying long-term synaptic plasticity. Slice physiology research has demonstrated that many neuromodulators differentially modulate afferent, incoming information compared to intrinsic and recurrent processing in cortical structures such as piriform cortex, neocortex, and the hippocampus. The enhancement of afferent (external) pathways versus the suppression at recurrent (internal) pathways could cause cortical dynamics to switch between a predominant influence of external stimulation to a predominant influence of internal recall. Modulation of afferent versus intrinsic processing could contribute to the role of neuromodulators in regulating attention, learning, and memory effects in behavior.

Transcriptional and translational regulation of BACE1 expression--implications for Alzheimer's disease

The proteolytical processing of the amyloid precursor protein (APP) gives rise to beta-amyloid peptides, which accumulate in brains of Alzheimer's disease (AD) patients. Different soluble or insoluble higher molecular weight forms of beta-amyloid peptides have been postulated to trigger a complex pathological cascade that may cause synaptic dysfunction, inflammatory processes, neuronal loss, cognitive impairment, and finally the onset of the disease. The generation of beta-amyloid peptides requires the proteolytical cleavage of APP by an aspartyl protease named beta-site APP-cleaving enzyme 1 (BACE1). The expression and enzymatic activity of BACE1 are increased in brains of AD patients. Here we discuss the importance of a number of recently identified transcription factors as well as post-transcriptional modifications and activation of intracellular signaling molecules for the regulation of BACE1 expression in brain. Importantly, some of these factors are known to be involved in the inflammatory and chronic stress responses of the brain, which are compromised during aging. Moreover, recent evidence indicates that beneficial effects of non-steriodal anti-inflammatory drugs on the progression of AD are mediated--at least in part--by effects on the peroxisome proliferator-activated receptor-gamma response element present in the BACE1 promoter. The identification of the cell type-specific expression and activation of NF-kappaB, Sp1 and YY1 transcription factors may provide a basis to specifically interfere with BACE1 expression and, thereby, to lower the concentrations of beta-amyloid peptides, which may prevent neuronal cell loss and cognitive decline in AD patients.

Chronic but not acute nicotine treatment reverses stress-induced impairment of LTP in anesthetized rats

Stress impairs long-term potentiation (LTP) and is a major cause for starting or increasing tobacco smoking. We have previously shown that chronic concurrent nicotine treatment prevents stress-induced LTP impairment. Nicotine reduces stress-induced impairment of LTP, probably, through activation of nicotinic acetylcholine receptors in the hippocampus. Herein, we investigated the effects of acute and chronic nicotine treatments on the chronic-stress-induced impairment of LTP in area CA1 of the hippocampus of urethane-anesthetized rats. Extracellular in vivo recording from the hippocampal area CA1 showed that pre-treatment with nicotine (1 mg/kg; sc twice/day for 2 weeks prior to stress) protected LTP from the inhibitory effect of subsequent chronic psychosocial stress (4 additional weeks concurrently with nicotine). In another series of experiments, 2 weeks of psychosocial stress was followed by 4 weeks of nicotine treatment concurrently with continuing stress. Nicotine treatment reversed established stress-induced impairment of LTP. However, acute nicotine treatment of rats (a single dose of 1 mg/kg; sc.) did not reverse chronic-stress-induced impairment of LTP. The results show that the impairment of LTP during chronic stress can be blocked by chronic, but not acute, nicotine treatment.

Nicotinic modulation of glutamatergic synaptic transmission in region CA3 of the hippocampus

Cholinergic modulation of synaptic transmission in the hippocampus appears to be involved in learning, memory and attentional processes. In brain slice preparations of hippocampal region CA3, we have explored the effect of nicotine on the afferent connections of stratum lacunosum moleculare (SLM) vs. the intrinsic connections of stratum radiatum (SR). Nicotine application had a lamina-selective effect, causing changes in synaptic transmission only in SLM. The nicotinic effect in SLM was characterized by a transient decrease in synaptic potential size followed by a longer period of enhancement of synaptic transmission. The effect was blocked by gamma-aminobutyric acid (GABA)ergic antagonists, indicating the role of GABAergic interneurons in the observed nicotinic effect. The biphasic nature of the nicotinic effect could be due to a difference in receptor subtypes, as supported by the effects of the nicotinic antagonists mecamylamine and methyllycaconitine. Nicotinic modulation of glutamatergic synaptic transmission could complement muscarinic suppression of intrinsic connections, amplifying incoming information and providing a physiological mechanism for the memory-enhancing effect of nicotine.

The developing cholinergic system as target for environmental toxicants, nicotine and polychlorinated biphenyls (PCBs): implications for neurotoxicological processes in mice

During neonatal life, offspring can be affected by toxic agents either by transfer via mother's milk or by direct exposure. In many mammalian species the perinatal period is characterized by a rapid development of the brain - "the brain growth spurt" (BGS). This period in the development of the mammalian brain is associated with numerous biochemical changes that transform the feto-neonatal brain into that of the mature adult. In rodents, the cholinergic transmitter system undergoes a rapid development during the neonatal period, a time when spontaneous motor behaviour also reaches peak activity. We have observed that low-dose exposure to environmental toxicants such as nicotine, polychlorinated biphenyls (PCBs) and polybrominated diphenylethers (PBDE, flame retardants) during the "BGS" can lead to irreversible changes in adult brain function in the mouse. The induction of persistent effects on behaviour and cholinergic nicotinic receptors in the adult animal appears to be limited to a short period during neonatal development. Furthermore, the neurotoxic effects were shown to develop over time, indicating a time-response/time-dependent effect. This indicates that environmental toxicants, such as nicotine, PCBs and probably PBDEs, might be involved in the slow, implacable induction of neurodegenerative disorders and/or interfere with normal aging processes.

Neuronal nicotinic receptors in the human brain

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand gated ion channels which are widely distributed in the human brain. Multiple subtypes of these receptors exist, each with individual pharmacological and functional profiles. They mediate the effects of nicotine, a widely used drug of abuse, are involved in a number of physiological and behavioural processes and are additionally implicated in a number of pathological conditions such as Alzheimer's disease, Parkinson's disease and schizophrenia. The nAChRs have a pentameric structure composed of five membrane spanning subunits, of which nine different types have thus far been identified and cloned. The multiple subunits identified provide the basis for the heterogeneity of structure and function observed in the nAChR subtypes and are responsible for the individual characteristics of each. A substantial amount of information on human nAChR structure and function has come from studies on neuroblastoma cell lines which naturally express nAChRs and from recombinant nAChRs expressed in Xenopus oocytes. In vitro brain nAChR distribution can be mapped with a number of appropriate agonist and antagonist radioligands and subunit distribution may be mapped by in situ hybridization using subunit specific mRNA probes. Receptor distribution in the living human brain can be studied with noninvasive imaging techniques such as PET and SPECT, with a significant reduction in nAChRs in the brains of Alzheimer's patients having been identified with [11C] nicotine in PET studies. Despite the significant body of knowledge now accumulated about nAChRs, much remains to be elucidated. This review will attempt to describe the current knowledge on the nAChR subtypes in the human brain, their functional roles and neuropathological involvement.

The M1 muscarinic agonist CI-1017 facilitates trace eyeblink conditioning in aging rabbits and increases the excitability of CA1 pyramidal neurons

The M1 muscarinic agonist CI-1017 was administered intravenously to aging rabbits on a daily basis before and during hippocampally dependent trace eyeblink conditioning sessions. Circulating levels of CI-1017 were significantly related to the drug dose. The drug was found to significantly increase the rate and amount of learning in a dose-dependent manner with no significant effects on the amplitude, area, or latency of conditioned responses. There was no evidence of pseudoconditioning at the highest drug concentration, and the minimally effective dose produced only mild and temporary hypersalivation as a side effect. CI-1017 (10 microM) was also found to increase the excitability of CA1 pyramidal neurons recorded from hippocampal slices from young and aging naive rabbits as measured by changes in spike-frequency adaptation and the postburst afterhyperpolarization. These biophysical changes were reversed with either atropine (1 microM) or pirenzepine (1 microM). These results suggest that M1 agonists ameliorate age-related learning and memory impairments at least in part by reducing the afterhyperpolarization and spike-frequency adaptation of hippocampal pyramidal neurons and that M1 agonists may be an effective therapy for reducing the cognitive deficits that accompany normal aging and/or Alzheimer's disease.

Physostigmine challenge before and after chronic cholinergic blockade in elderly volunteers

BACKGROUND

As a test of possible muscarinic up-regulation, the cortisol response to intravenous (i.v.) physostigmine (an anticholinesterase) was measured in 9 elderly volunteers before and after chronic cholinergic blockade with the muscarinic cholinergic antagonist scopolamine.

METHODS

Each of the 9 elderly control subjects was given two physostigmine (0.5 mg i.v.) infusions separated by 21 doses of nightly scopolamine (1.2 mg p.o.). No scopolamine was administered the night before infusions, and glycopyrrolate (0.2 mg i.v.) was administered prior to physostigmine, to block its peripheral effects. Vital signs were monitored and blood samples were collected at six time points surrounding the physostigmine infusion (-10, +10, +20, +30, +50, and +70 min). Behavioral measures and cognitive tests were administered prior to and 30 min after the physostigmine.

RESULTS

The cortisol response to physostigmine was greater after the second (post-chronic scopolamine) infusion study compared to the first (p < .05) as measured by an area under the curve analysis of all time points. When individual time points were compared, the mean cortisol response was significantly increased after the second physostigmine infusion at the +50- and +70-min time points (p < .05). There were no significant changes in behavioral rating scales, cognitive tests, or vital signs between the two physostigmine infusion study days.

CONCLUSIONS

This study demonstrates increased hypothalamic-pituitary-adrenocortical axis responsivity to a central nervous system cholinergic stimulus after chronic muscarinic blockade in 9 elderly control subjects. It also gives further evidence to support previous suggestions of muscarinic plasticity, specifically postsynaptic up-regulation, in the aging brain following exposure to chronic anticholinergic treatment.

Phase I clinical trials with WAL 2014, a new muscarinic agonist for the treatment of Alzheimer's disease

The safety, tolerability and pharmacological activity of WAL 2014, a new centrally-acting M1 agonist were examined in two clinical studies (0.5-80 mg and 100-160 mg). Single increasing p.o. doses were administered to groups of 8 volunteers (6 verum, 2 placebo) each. Both studies were placebo controlled with single-blind observation within the respective dose groups. Vital functions (BP, HR, resp. rate) did not reveal any clinically significant substance-induced changes up to a dose level of 60 mg. A slight, but obvious increase in HR was measured with a dose of 80 mg and higher; a slight increase in systolic BP was registered at the dose levels of 120 and 160 mg. No substance-related alterations were observed in the laboratory tests (exception: a significant, reversible increase of the salivary fraction of alpha-amylase in 3 volunteers at the dose levels 100 mg-140 mg). The majority of volunteers reported an increased salivary secretion with doses of 40 mg and higher; this was confirmed by the greater volume of measured saliva. Furthermore, with doses of 100 mg upwards there were isolated reports of side effects such as a desire to urinate, a burning sensation on urination, increased lacrimation and nasal secretion, disturbances of accommodation, heartburn, rumbling of the stomach as well as cramps, nausea, diarrhoea, excessive sweating and palpitation. WAL 2014 did not cause any abnormal changes in the EEG. Dose dependent central effects were observed with 40, 60, 80, 100 and 140 mg treatments. Pharmacokinetic data indicate a rapid and good absorption and an absolute bioavailabitlity > or = 70%. The pharmacodynamic and side effects observed in both studies are regarded as being drug-dependent and might be due to the cholinergic activity of the compound and a weak sympathetic activation via M1 receptors. In summary, the substance did not produce any effects in the dose range tested to suggest further use in man might be inadvisable.

Differential expression of muscarinic subtype mRNAs after exposure to neurotoxic pesticides

We have recently reported an increase in the density of muscarinic cholinergic receptors in mice neonatally exposed to a persistent environmental agent, dichlorodiphenyltrichloroethane (DDT), and a subsequent exposure as adults to nonpersistent toxicants, such as bioallethrin or paraoxon. Here we have examined the effects of an exposure like this on muscarinic receptor mRNA expression. Ten-day-old Naval Medical Research Institute mice received a single oral dose of DDT (0.5 mg/kg body weight). When aged 5 months, they received bioallethrin (0.7 mg/kg body weight per day for 7 days) or paraoxon (1.4 mg/kg body weight every second day for 7 days). mRNA expression of subtypes m1, m3, and m4 was studied in 7-month-old animals. Changes could only be discovered in the DDT-bioallethrin treated mice, where expression of subtype m4 was elevated in cortex and caudate putamen. Moreover, the expression pattern of the subtypes m1, m3, and m4 in mouse brains was found to be very similar to that seen in rats, except for slight differences in the pyramidal cell layer of the hippocampus, where the outermost part of the CA3 region did not show any m4 hybridization. The present study indicates that the earlier observed increase in muscarinic receptor density in mice exposed as neonates to DDT and as adults to bioallethrin can be attributed to changes in the expression of m4.

Changes in behavior and muscarinic receptor density after neonatal and adult exposure to bioallethrin

Throughout life, mammals are exposed to environmental toxicants, some of which have acute effects on the nervous system. Early, low-dose exposure in combination with later re-exposure and possible interference with normal aging have been little studied. The present study revealed increased susceptibility in adult mice, exposed neonatally to a low dose of the insecticide bioallethrin, to renewed exposure to bioallethrin as adults. Ten-day-old Naval Medical Research Institute male mice received bioallethrin orally (0.7 mg per kg body weight per day for 7 days). When aged 5 months they were given the same dose of bioallethrin by gavage. Twenty-four hours after the last administration, a spontaneous motor activity test revealed significant aberrations in mice exposed both neonatally and as adults to bioallethrin. The density of muscarinic receptors was significantly increased. When aged 7 months, spontaneous behavioral disturbances and muscarinic receptor changes persisted and learning and memory deficits had developed. These results indicate that neonatal exposure to bioallethrin has the potential to increase susceptibility of the adult mouse to a new exposure at a dosage that does not have any effect in animals treated neonatally with vehicle.

A bridging study of LU 25-109 in patients with probable Alzheimer's disease

Lu 25-109 is a functionally selective partial M1 agonist with M2/M3 antagonist properties. This double-blind, placebo-controlled, two-part, inpatient bridging study was designed to evaluate the safety and tolerability of multiple oral doses of Lu 25-109 in patients with Alzheimer's Disease(AD), and to determine the maximum tolerated dose (MTD) in this population. In the first part of the study, the fixed-dose MTD was to be determined in five consecutive panels of 6 patients each (4 Lu 25-109/2 placebo). Doses for the five panels were 100, 125, 150, 200, and 225 mg tid for 7 days. Cholinergic adverse events such as increased salivation, dizziness, and gastrointestinal symptoms were observed at all doses studied. The dosing of fixed-dose panels was discontinued after 3 days at 200 mg tid due to unacceptable gastrointestinal adverse events. Thus, 150 mg tid was defined as the fixed-dose MTD. The second part of the study, conducted in a single panel of 8 patients (6 Lu 25-109/2 placebo), was designed to determine if patients could tolerate higher doses of Lu 25-109 when administered on a titration regimen. Patients were to receive doses that were 50%, 75%, 100%, 125%, and 150% of the fixed dose MTD, with dose increases every five days. The first dose, 75 mg tid, was very well-tolerated; however, as in the first phase of the study, patients did not tolerate the 200 mg tid dose. Thus, the titration regimen employed did not improve the overall tolerability of Lu 25-109.

Cholinergic immunolesions by 192IgG-saporin--useful tool to simulate pathogenic aspects of Alzheimer's disease

Alzheimer's disease, the most common cause of senile dementia, is characterized by intracellular formation of neurofibrillary tangles, extracellular deposits of beta amyloid as well as cerebrovascular amyloid accumulation and a profound loss of cholinergic neurons within the nucleus basalis Meynert with alterations in cortical neurotransmitter receptor densities. The use of the cholinergic immunotoxin 192IgG-saporin allows for the first time study of the impact of cortical cholinergic deafferentation on cortical neurotransmission, learning, and memory without direct effects on other neuronal systems. This model also allows the elucidation of contributions of cholinergic mechanisms to the establishment of other pathological features of Alzheimer's disease. The findings discussed here demonstrate that cholinergic immunolesions by 192IgG-saporin induce highly specific, permanent cortical cholinergic hypoactivity and alterations in cortical neurotransmitter densities comparable to those described for Alzheimer's disease. The induced cortical cholinergic deficit also leads to cortical/hippocampal neurotrophin accumulation and reduced amyloid precursor protein (APP) secretion, possibly reflecting the lack of stimulation of postsynaptic M1/M3 muscarinic receptors coupled to protein kinase C. This immunolesion model should prove useful to test therapeutic strategies based on stimulation of cortical cholinergic neurotransmission or amelioration of pathogenic aspects of cholinergic degeneration in the basal forebrain. Application of the model to animal species that can develop beta-amyloid plaques could provide information about the contribution of cholinergic function to amyloidogenic APP processing.

The cholinergic system in Alzheimer's disease

The past decade has witnessed an enormous increase in our knowledge of the variety and complexity of neuropathological and neurochemical changes in Alzheimer's disease. Although the disease is characterized by multiple deficits of neurotransmitters in the brain, this overview emphasizes the structural and neurochemical localization of the elements of the acetylcholine system (choline acetyltransferase, acetylcholinesterase, and muscarinic and nicotinic acetylcholine receptors) in the non-demented brain and in Alzheimer's disease brain samples. The results demonstrate a great variation in the distribution of acetylcholinesterase, choline acetyltransferase, and the nicotinic and muscarinic acetylcholine receptors in the different brain areas, nuclei and subnuclei. When stratification is present in certain brain regions (olfactory bulb, cortex, hippocampus, etc.), differences can be detected as regards the laminar distribution of the elements of the acetylcholine system. Alzheimer's disease involves a substantial loss of the elements of the cholinergic system. There is evidence that the most affected areas include the cortex, the entorhinal area, the hippocampus, the ventral striatum and the basal part of the forebrain. Other brain areas are less affected. The fact that the acetylcholine system, which plays a significant role in the memory function, is seriously impaired in Alzheimer's disease has accelerated work on the development of new drugs for treatment of the disease of the 20th century.

Functional and neurobiological similarities of aging in monkeys and humans

Aging in humans may be accompanied by alterations in several functional abilities. However, there is a great deal of individual variability in the functions that may be altered with age within and across aged people. One potential source of age-related behavioral variation may lie in a differential vulnerability of neurobiological systems to the aging process in particular individuals. Aged monkeys demonstrate behavioral and brain alterations that have many parallels with those observed in aged humans and are valuable animal models in which to investigate the interrelationships between age, behavior and neurobiological measures. This review outlines the similarities of functional and neurobiological aging in monkeys and humans, notes the variability that exists in both behavioral and neural systems in aging, and identifies some of the areas of aging that are in need of further investigation.

Increased regional cerebral glucose metabolism and semantic memory performance in Alzheimer's disease: a pilot double blind transdermal nicotine positron emission tomography study

Nicotinic receptor dysfunction and impaired semantic memory occur early in Alzheimer's disease patients (AD). Previous research implied that nicotine's ability to enhance alertness, arousal, and cognition in a number of nonclinical populations was a function of its ability to stimulate CNS nicotinic cholinergic receptors. In this study it was hypothesized that transdermal administration of nicotine would increase both regional cerebral glucose metabolism (rCMRglc) and semantic memory (as assessed by verbal fluency). Two mild AD and two elderly controls underwent positron emission tomography scanning during a double blind nicotinic agonist verbal fluency challenge procedure. rCMRglc increases occurred in both AD patients, but not controls. In the two AD patients, verbal fluency scores increased by an average of 17%. One elderly control's verbal fluency increased, and the other decreased. These findings suggest that nicotine's effect on metabolism and verbal fluency is due to its ability to stimulate the cholinergic system.

In vivo quantification of cerebral muscarinic receptors in normal human aging using positron emission tomography and [11C]tropanyl benzilate

Regional cerebral muscarinic cholinergic receptor binding was quantified in normal young and elderly subjects employing the muscarinic antagonist radioligand [11C]tropanyl benzilate (TRB). Binding was determined by kinetic analyses of positron emission tomographic (PET) determinations of cerebral activity in conjunction with radial arterial blood sampling following intravenous radiotracer injection. A significant, but minor (8%), loss of frontal cortical receptors relative to whole brain average receptor density was found with advancing age. Parametric estimates of binding suggest small reductions in cerebral cortex binding as well as increases in brain stem and cerebellar binding underlying the observed pattern difference. However, these latter changes did not achieve statistical significance. We conclude that cerebral muscarinic receptor availability, as depicted by antagonist binding, does not undergo a major decline during normal aging of the adult human brain. The cerebral cortical cholinergic dysfunction in elderly subjects, suggested by prior clinical evidence, is not attributable to major loss of total muscarinic cholinoceptive capacity.

Differential cholinergic regulation in Alzheimer's patients compared to controls following chronic blockade with scopolamine: a SPECT study

The effects of low-dose chronic scopolamine on measures of cerebral perfusion and muscarinic receptors were tested in eight Alzheimer's disease (AD) subjects and eight elderly controls. Single photon emission computed tomography (SPECT) scans using technetium-labelled hexamethypropylene amine oxide (99mTc-HMPAO) to measure cerebral perfusion before and after chronic scopolamine revealed a significant 12% increase in the normal controls (P < 0.01) while the AD subjects showed no significant change. In contrast, the controls showed decreased muscarinic binding as evidenced by 123I-quinuclidinyl-4-iodobenzilate (123I-QNB) labelling after chronic drug (-10%, P < 0.01) whereas the AD subjects showed increased 123I-QNB labelling (+8%, P < 0.05). The difference between AD and control subjects was even more marked when the ratio of I-QNB to HMPAO uptake was compared, pointing to a double dissociation in the SPECT results. These data cannot be explained by group differences in cerebral perfusion alone and suggest a differential sensitivity between AD and elderly controls to chronic cholinergic blockade.

Scopolamine reversal of nicotine enhanced delayed matching-to-sample performance in monkeys

The basis for the memory enhancing action of nicotine was evaluated in five adult monkeys (Macaca fascicularis) well trained in the performance of a delayed matching-to-sample (DMTS) paradigm. Nicotine (1.25-20 micrograms/kg, IM) produced a dose-dependent improvement in performance of the task. The optimal dose of nicotine for each monkey also improved performance when the animals were tested 24 h later in the no-drug situation. In the same animals, low doses of scopolamine produced a dose-dependent decrement in DMTS performance. A subthreshold dose (defined by DMTS performance decrement) of scopolamine was administered 20 min prior to the optimal dose of nicotine. Scopolamine pretreatment completely blocked the enhanced performance observed earlier with nicotine. The results of this study are consistent with the hypothesis that the enhanced cognitive performance associated with nicotine is due to central acetylcholine release and subsequent muscarinic receptor stimulation.

Comparison of the concentration of messenger RNA encoding four muscarinic receptor subtypes in control and Alzheimer brains

We determined the concentration of the messenger RNA species which encode four (m1-m4) of the five cloned muscarinic receptors in brains of Alzheimer's disease patients as compared to age-matched controls. Assays were performed using the quantitative method of DNA-excess solution hybridization in the cerebral cortex (frontal, temporal and occipital), hippocampus, nucleus basalis of Meynert and brainstem. The results suggest a statistically significant decrease in the m1 muscarinic receptor message in the temporal and occipital cortex, with no change in other regions. There was no change in the level of mRNA encoding the m2, m3 or m4 receptors in any of the brain regions studied.

Comparison of the level of mRNA encoding m1 and m2 muscarinic receptors in brains of young and aged rats

We compared the concentration of mRNA encoding the m1 and m2 muscarinic receptors in several brain regions obtained from young (5-8 months) and aged (24-28 months) male Fischer 344 rats. DNA-excess solution hybridization was employed as a quantitative measure of mRNA concentration. The results indicate the absence of changes in the m1 receptor message with aging in the cerebral cortex, hippocampus and striatum. While there was no statistically significant aging-associated alteration in the concentration of the message encoding the m2 receptor in the thalamus, midbrain, cerebellum and brainstem, there was a decrease in the message level in the hypothalamus.

Increased cognitive sensitivity to scopolamine with age and a perspective on the scopolamine model

18 older normal volunteers (mean age = 66.5 +/- 7.9 years) and 46 younger volunteers (mean age = 27.0 +/- 6.1 years) were administered the anticholinergic drug scopolamine (0.5 mg i.v.) followed by a battery of cognitive tests evaluating attention, learning and memory. The older subjects were significantly more impaired than the younger by scopolamine on some tests of learning and memory. This increased sensitivity of the older group to scopolamine is consistent with studies in animals and humans showing decreased cholinergic system function with age. The findings also indicate that age is an important variable to consider in using the scopolamine model of memory impairment. The cognitive impairment caused by scopolamine in younger subjects in this and prior studies is similar to some, but not all aspects of the impairment which occurs in normal aging. Scopolamine also caused impairments on digit span and word fluency tasks, which are not consistent with normal aging changes. In the older group of subjects, scopolamine produced aspects of the cognitive impairment which occurs in AD on tests of episodic memory and learning, vigilance-attention, category retrieval, digit span, and number of intrusions. Other areas of cognition that are of relevance to aging and AD such as psychomotor speed, praxis, concept formation and remote memory were not evaluated in this study. Some of these are being evaluated in ongoing studies, along with additional and more specific tests of retrieval from knowledge memory, implicit memory and attention. The scopolamine model has provided a fruitful pharmacologic starting point for the study of a number of cognitive operations. The idea of dissecting apart aspects of memory systems pharmacologically depends on the availability of neurochemically specific drugs and on the specificity and sensitivity of neuropsychological tests for distinct cognitive operations or domains. Further studies using such tools will aid not only in the understanding of the impairments which occur in aging and in AD, but also of the conceptualization of memory and other cognitive operations and ultimately the physiological mechanisms involved in memory and learning.

Age-related deficits of central muscarinic cholinergic receptor function in the mouse: partial restoration by chronic piracetam treatment

The effect of aging on muscarinic cholinergic receptor function in dissociated cell aggregates of the mouse brain was investigated using two biochemical models, i.e., carbachol-induced accumulation of inositol monophosphates and carbachol-induced desensitization of muscarinic cholinergic receptors as measured by the sequestration of specific 3H-N-methyl-scopolamine binding. While aging strongly reduced carbachol-induced inositol monophosphate accumulation, desensitization was not affected in the brains of aged animals. Chronic treatment of aged mice with the nootropic drug piracetam (500 mg/kg daily PO) significantly elevated the agonist-induced accumulation of inositol monophosphates possibly by increasing the available number of muscarinic cholinergic receptors not being in a desensitized state. The results support the hypothesis that nootropics like piracetam might act in part by restoring age-related deficits of central muscarinic cholinergic receptor function.

Alterations in cortical muscarinic receptors following cholinotoxin (AF64A) lesion of the rat nucleus basalis magnocellularis

Cortical choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), muscarinic receptors and sodium-dependent, high-affinity, choline uptake (SDHACU) sites were examined in the rat brain following unilateral stereotaxic injection of the cholinotoxin, AF64A, into the nucleus basalis magnocellularis (NBM). Injection of AF64A resulted in a significant loss of presynaptic cholinergic markers in the cortex without alteration in TH and TPH activity. The binding to SDHACU sites was reduced to background values in the NBM and increased in the central amygdala (Ce) and cortex. The increase in cortical [3H]QNB binding was the result of a change in muscarinic receptor number (BMAX) and not a change in receptor affinity (KD). Examination of muscarinic receptor subtypes demonstrated a reduction of M1 receptor binding in the cortex and NBM without any alteration in the Ce. Non-M1 binding was significantly increased in all the laminae of the cortex and in the Ce, but decreased in the NBM. These data suggest that there exists a population of M1 receptors on NBM projections to the cortex and that NBM projections influence a population of postsynaptic receptors in the cortex and Ce which are not of the M1 subtype.

Chronic treatment with phosphatidylserine restores muscarinic cholinergic receptor deficits in the aged mouse brain

Chronic treatment (21 days) with phosphatidylserine (BC-PS) partially restored the reduced density of muscarinic cholinergic receptors in several regions of the aged (18 months) mouse brain. The effect was similar whether 3H-QNB or 3H-NMS was used as radioligand. The affinity of both radioligands was not altered by BC-PS treatment. Similar treatment of young (3 months) animals was without any effect on muscarinic cholinergic receptor density in all brain regions investigated. The effect was dose-dependent with elevations of receptor density between 15 and 28% for daily IP doses between 10 and 40 mg/kg, respectively. Similar treatment of aged mice with phosphatidylcholine (40 mg/kg) was without any effect. The data give further evidence that chronic treatment of aged animals with BC-PS reverses a variety of aged-related deficits of brain function.

The effects of thyrotropin-releasing hormone and scopolamine in Alzheimer's disease and normal volunteers

Thyrotropin-releasing hormone (TRH), a neuromodulator and possibly a neurotransmitter in the central nervous system, was shown in a prior study of young normal volunteers to attenuate the memory impairment induced by the anticholinergic drug scopolamine. In the present study, the cognitive, behavioral and physiologic effects of high dose TRH (0.5 mg/kg), both alone and following administration of scopolamine, were examined in 10 Alzheimer's disease (AD) patients (mean age±SD=63.5 years) and 12 older normal volunteers (mean age=64.9±8.8 years). On the day AD subjects received TRH alone, modest but statistically significant improvement from baseline performance was documented on some tests of learning and memory, especially in those with mild dementia severity. In comparing cognitive test performance between the scopolamine alone and scopolamine+TRH conditions, only two test scores were significantly higher in the latter condition. In the group of older volunteers, TRH did not attenuate scopolamine-induced cognitive impairment, contrary to prior findings in a group of younger controls. In fact, older subjects performed worse after receiving scopolamine followed by TRH than after receiving scopolamine alone. In addition, no change from baseline cognitive performance was detected after subjects received TRH alone. These findings raise several questions and speculations on possible age-related changes in the cholinergic system, as well as on the mechanism of the interaction of TRH with the cholinergic system.

High affinity acylating antagonists for muscarinic receptors

The muscarinic antagonists pirenzepine and telenzepine were derivatized as alkylamino derivatives at a site on the molecules corresponding to a region of bulk tolerance in receptor binding. The distal primary amino groups were coupled to the cross-linking reagent meta-phenylene diisothiocyanate, resulting in two isothiocyanate derivatives that were found to inhibit muscarinic receptors irreversibly and in a dose-dependent fashion. Preincubation of rat forebrain membranes with an isothiocyanate derivative followed by radioligand binding using [3H]N-methylscopolamine diminished the Bmax value, but did not affect the Kd value. The receptor binding site was not restored upon repeated washing, indicating that irreversible inhibition had occurred. IC50 values for the irreversible inhibition at rat forebrain muscarinic receptors were 0.15 nM and 0.19 nM, for derivatives of pirenzepine and telenzepine, respectively. The isothiocyanate derivative of pirenzepine was non-selective as an irreversible muscarinic inhibitor, and the corresponding derivative prepared from telenzepine was 5-fold selective for forebrain (mainly m1) vs. heart (m2) muscarinic receptors.

Central cholinergic functioning and aging

Normal aging in experimental animals and humans has been demonstrated to affect various aspects of central cholinergic functions. Although deficits at the levels of the number of cholinergic neurons, the acetylcholine synthesis, and the number of muscarinic cholinergic receptors are probably less relevant, deficits at the levels of acetylcholine release, muscarinic cholinergic receptor plasticity, as well as muscarinic cholinergic receptor function are fairly pronounced and seem to justify the assumption that the functioning of the central cholinergic system is impaired by aging. However, whether these cholinergic deficits of normal aging are the sole neurochemical basis to explain age-associated memory impairment or whether other transmitter systems also play a role is still a matter of controversy.

Effect of [1,3-di-n-butyl-7-(2-oxopropyl)-xanthine] (denbufylline), a low Km phosphodiesterase inhibitor, on striatal acetylcholine release in the rat: analysis using cerebral microdialysis

Effect of oral and intraperitoneal administrations of [1,3-di-n-butyl-7-(2-oxopropyl)-xanthine] (denbufylline) on acetylcholine (ACh) content and release in the rat striatum was investigated. Denbufylline (3, 10 and 30 mg/kg, p.o.) decreased striatal ACh contents in a dose-dependent manner. Denbufylline administration (30 mg/kg, i.p.) produced no significant change in the spontaneous release of ACh, while it increased a high potassium-evoked ACh release in the striatum. These results suggest that denbufylline may be a drug inducing the increased release of ACh in the brain.

Aging does not alter muscarinic receptor-mediated inhibition of cyclic AMP formation in the striatum and hippocampus

The effects of aging on the inhibition of forskolin-stimulated cyclic AMP formation by muscarinic receptors were investigated. There were no detectable changes in the magnitude of maximal inhibition by carbamylcholine or the potency of the agonist in inducing this response in either the striatum or hippocampus obtained from young or old Fisher 344 rats.

Nicotine enhances delayed matching-to-sample performance by primates

The non-human primate provides an excellent model for studies of learning and memory, and one particular test, the delayed matching-to-sample task, is performed in a similar manner by both humans and non-human primates. Five young adult macaques were employed in this study, displaying variable capacities for retention in the task. Baseline performance was very consistent and three levels of performance difficulties (95-100%, 80-85% and 65-75% correct choices) were employed by including several delay intervals (0-60 sec) in each session. A reproducible enhancement in performance by nicotine in macaques performing a delayed matching-to-sample task was demonstrated. Nicotine enhanced performance with an average increase of 10% at the longest retention delay interval. This beneficial effect of nicotine was abolished in animals pretreated with a low dose (0.5 mg/kg) of mecamylamine to block central nicotinic receptors. Selective blockade of peripheral nicotinic receptors with hexamethonium was without effect on the nicotine response. A high dose (2 mg/kg) of mecamylamine itself induced a marked inhibition of performance, while an equivalent dose of hexamethonium was without effect. These experiments point to the possibility that central nicotinic receptors may be exploited pharmacologically to enhance memory performance. In this respect it is interesting that nicotine was most effective at enhancing performance when recall was more difficult, that is, on the longer retention interval delays. This could signify that nicotine might be particularly effective in the most impaired individuals. Lastly, it is encouraging that the mecamylamine induced decrease in cognitive performance might provide a new model of memory impairment from which to study the pathogenesis and develop new pharmacological strategies for the dementias.

New approaches to quantitative neuropathology: multivariate analysis of morphologic and neurochemical measures

The excellent review by Coleman and Flood on neuropathological changes in normal aging and Alzheimer's disease highlights the need for development and application of computer-assisted image analysis to the study of neurons in these conditions. The morphological and neurochemical changes in normal and pathological aging require quantitation and statistical analysis that can be best performed with the assistance of the image and data processing capabilities of the computer. Advanced image processing systems are being developed to identify and classify neurons according to several intelligently chosen visual features, apply discriminant analysis and population statistics to this data, and correlate this information to other neurochemical measurements as well as the clinical history of the patient. Techniques such as immunocytochemistry, receptor autoradiography and in situ hybridization produce information-rich images of the distribution of proteins and nucleic acids in tissue slices that can be analyzed by this approach.