SDZ ENA 713 facilitates central cholinergic function and ameliorates spatial memory impairment in rats

Evaluation of rivastigmine in Alzheimer's disease

Dementia is the major cause of mortality and morbidity in older adults, with Alzheimer's disease (AD) being the most common cause. AD has a significant impact on economic and psychosocial status. Cholinesterase inhibitors (ChEIs) are currently the mainstay in the management of AD. Rivastigmine is the only ChEI that inhibits both acetylcholinesterase and butyrylcholinesterase enzymes in the brain. This dual inhibition makes it potentially more effective for AD patients. Its availability as both a transdermal formulation and oral capsule, may improve adherence rates and care giver satisfaction compared with other ChEIs. To date, the data from randomized clinical trials and post marketing observational studies have shown evidence for an impact on cognitive functions in AD with good safety and tolerability.

Anti-dementia drugs and hippocampal-dependent memory in rodents

Abnormalities in hippocampal structure and function are characteristics of early Alzheimer's disease (AD). Behavioral tests measuring hippocampal-dependent memory in rodents are often used to evaluate novel treatments for AD and other dementias. In this study, we review the effects of drugs marketed for the treatment of AD, such as the acetylcholinesterase inhibitors, donepezil, rivastigmine, galantamine and the N-methyl-D-aspartic acid antagonist, memantine, in rodent models of memory impairment. We also briefly describe the effects of novel treatments for cognitive impairment in rodent models of memory impairment, and discuss issues concerning the selection of the animal model and behavioral tests. Suggestions for future research are offered.

Cholinergic system, rearing environment and trajectory learning during aging in mice

Three, 12- and 20-month-old C57BL6/J mice, reared in standard conditions or in enriched environments, were administered subcutaneously either scopolamine hydrobromide, 0.6 or 1.2 mg kg(-1), or physiological saline (control mice) 15 min before testing their abilities to find an invisible platform in a modified version of the Morris water maze, the starting point being kept unchanged throughout the experiment to allow the aged animals to solve the task. The results demonstrated that: 1) All control mice, whatever their age, were able to learn the platform location, but the number of trials needed to reach the learning criterion (3 consecutive trials in less than 8 s) increased with age; 2) All the scopolamine-treated mice, whatever their age, were also able to learn the platform location. However, compared to age-matched controls, the number of trials needed to reach the learning criterion was greater; 3) Rearing the animals in an enriched environment antagonized the effect of scopolamine, but only in the youngest (3 month-old) mice. All control and scopolamine-treated mice, whatever their age and their rearing environment, remembered, 7 days later, the platform location.

Medicinal chemistry strategies to 5-HT6 receptor ligands as potential cognitive enhancers and antiobesity agents

Although the 5-hydroxytryptamine(6) (5-HT(6)) receptor was discovered only recently, its almost exclusive distribution in the brain makes it a promising, novel, target for central nervous system (CNS)-mediated diseases such as Alzheimer's disease (cognitive function), schizophrenia, anxiety and obesity. In the past few years a significant research interest has advanced the understanding of the functional roles and the pharmacophore requirements of this receptor. Two 5-HT(6) receptor antagonists have already entered Phase II clinical trials for the enhancement of cognitive function. Since the first discovery of selective ligands for the 5-HT(6) receptor by HTS in 1998, several medicinal-chemistry-driven approaches have delivered highly selective lead structures with well-defined functionalities, starting from either the endogenous ligand 5-HT or the chemical structures identified by HTS. The concept of 'scaffold hopping' has been employed to expand the variability of the available chemical scaffolds and to generate patentable ligands. Supported by pharmacophore models, which have been established recently, the binding and functionality (structure-activity relationships) of the lead structures have been optimized further.

Symptomatic effect of donepezil, rivastigmine, galantamine and memantine on cognitive deficits in the APP23 model

RATIONALE

APP23 mice are a promising model of Alzheimer's disease, expressing several histopathological, cognitive and behavioural hallmarks of the human condition. A valid animal model should respond to therapeutic interventions in an equivalent manner as human patients.

OBJECTIVES

To further validate the APP23 model, we examined whether cognitive deficits could be antagonised by donepezil, rivastigmine, galantamine or memantine, which are approved drugs for symptomatic treatment of dementia.

METHODS

Animals were tested at an age at which untreated APP23 mice display severe deficits in visual-spatial learning. Four-month-old APP23 mice and control littermates were administered donepezil (0.3 or 0.6 mg kg(-1)), rivastigmine (0.5 or 1.0 mg kg(-1)), galantamine (1.25 or 2.5 mg kg(-1)), memantine (2 or 10 mg kg(-1)) or saline through daily i.p. injections. After 1 week of treatment, acquisition phase commenced, with daily treatment continuing during cognitive testing.

RESULTS

All cholinesterase inhibitors reduced cognitive deficits with the following optimal daily doses: galantamine 1.25 mg kg(-1), rivastigmine 0.5 mg kg(-1) and donepezil 0.3 mg kg(-1). Higher dosages often did not exert beneficial effects in accordance with inverted U-shaped dose-response curves described for cholinomimetics. Symptomatic efficacy of memantine on cognition was mild, with significant amelioration manifesting during probe trial.

CONCLUSIONS

This is the first study to simultaneously evaluate the efficacy of therapeutically relevant doses of these four compounds in one particular learning and memory paradigm, being the Morris water maze. The fact that symptomatic intervention was able to diminish cognitive impairment, substantially adds to the validity of the APP23 model as a valuable tool to evaluate future therapeutic approaches.

Cognitive deficit associated with cholinergic and nerve growth factor down-regulation in experimental allergic encephalomyelitis in rats

Clinical symptoms in multiple sclerosis include cognitive dysfunction. Difficulties in learning and remembering new information represent the most common cognitive deficit and are associated with a general and progressive brain pathology. Possible pathogenetic mechanisms for neuronal damage such as neuroprotective strategies are under active investigation also in experimental allergic encephalomyelitis, the most widely used experimental model for multiple sclerosis. In this paper we demonstrate that a selective deficit in learning and memory performance, as investigated by the Morris water maze test, is a consistent feature in rat encephalomyelitis, which correlates with a decline in choline acetyltransferase activity and nerve growth factor mRNA level in cerebral cortex, hippocampus, and basal forebrain. Treatment aimed to restore acetylcholine content through chronic administration of selective acetylcholinesterase inhibitors (rivastigmine and donepezil) restores cognitive performance, choline acetyltransferase activity, and nerve growth factor mRNA expression.

Effect of Treatment with the Cholinesterase Inhibitor Rivastigmine on Vesicular Acetylcholine Transporter and Choline Acetyltransferase in Rat Brain

A decline of cholinergic neurotransmission probably contributes to cognitive dysfunction occurring in Alzheimer's disease (AD) and vascular dementia (VaD). Acetylcholinesterase (AChE)/cholinesterase (ChE) inhibitors are the only drugs authorized for symptomatic treatment of AD and are also under investigation for VaD. The present study has investigated the influence of two doses of the AChE inhibitor rivastigmine (0.625 mg/Kg/day and 2.5 mg/Kg/day) on vesicular acetylcholine transporter (VAChT) and on choline acetyltransferase (ChAT) expression in frontal cortex, hippocampus, striatum and cerebellum of normotensive and spontaneously hypertensive rats (SHR). Cholinergic markers were assessed by immunochemical (Western blotting) and immunohistochemical techniques. In frontal cortex and striatum of normotensive rats, treatment with the lower dose (0.625 mg/Kg/day) of rivastigmine had no effect on VAChT immunoreactivity and increased slightly ChAT protein immunoreactivity. The higher dose (2.5 mg/Kg/day) of the compound increased significantly VAChT and ChAT protein immunoreactivity. In hippocampus rivastigmine induced a concentration-dependent increase of VAChT protein expression and no significant changes of ChAT protein expression. A similar pattern of VAChT and ChAT protein expression was observed in control SHR, whereas treatment of SHR with rivastigmine induced a more pronounced increase of VAChT protein immunoreactivity in frontal cortex, hippocampus and striatum compared to normotensive rats. Our data showing an increase of VAChT after treatment with rivastgmine further support the notion of an enhancement of cholinergic neurotransmission by AChE/ChE inhibitors. The observation of a greater expression of this cholinergic marker in SHR suggest that AChE inhibition may provide beneficial effects on cholinergic neurotransmission in an animal model of VaD.

Asymmetric hydrogenation of enamides with Rh-BisP and Rh-miniPHOS catalysts. scope, limitations, and mechanism

The asymmetric hydrogenation of aryl- and alkyl-substituted enamides catalyzed by Rh-BisP complex affords optically active amides with very high ee values. The Rh-MiniPHOS catalyst gives somewhat less satisfactory results. Hydrogenation of the aryl-substituted enamides with (S,S)-BisP-Rh catalyst gives R-amides, whereas the t-Bu- and 1-adamantyl-substituted enamides give S-products with 99% ee. Reaction of [Rh(BisP)(CD(3)OD)(2)]BF(4) (11) with CH(2)=C(C(6)H(5))NHCOCH(3) (5) gives two diastereomers of the catalyst-substrate complex (12a,b), which interconvert reversibly by both intra- and intermolecular pathways as shown by EXSY data. Only one isomer in equilibrium with solvate complex 11 was detected for each of the catalyst-substrate complexes 17 and 18 obtained from CH(2)=C(t-Bu)NHCOCH(3) (6) or CH(2)=C(1-adamantyl)NHCOCH(3) (7). Hydrogenation of these equilibrium mixtures at -100 degrees C gave monohydride intermediates 19 and 20, respectively. In these monohydrides the Rh atom is bound to the beta-carbon. A new effect of the significant decrease of ee was found for the asymmetric hydrogenation of CH(2)=C(C(6)H(4)OCH(3)-o)NHCOCH(3) (21), when H(2) was substituted for HD or D(2).

Memory-related acetylcholine efflux from rat prefrontal cortex and hippocampus: a microdialysis study

To investigate the relationship between the prefrontal and hippocampal acetylcholine (ACh) systems and working memory, an in vivo microdialysis study was conducted. A group of rats was trained to perform a working memory task, delayed alternation, in an operant chamber for food reinforcement. The rats had to choose one of two response levers in an alternative manner in each trial, with a certain interval between trials. They had to remember which lever they chose in the previous trial without the assistance of external cues. Another group was trained to perform a reference memory task, cued alternation, in which the behavioral sequence was identical, but an external cue was provided. After stable behavior was established, a dialysis probe was implanted into the prefrontal cortex or the hippocampus of each rat. The extracellular concentration of ACh in the dialysates from the prefrontal cortex increased during performance of the delayed alternation task, while the hippocampal ACh showed a more distinct increase during performance of the cued alternation task. These results suggest that the prefrontal ACh is mainly related to working memory, whereas the hippocampal ACh is mainly related to reference memory.

Mild cognitive impairment: emerging therapeutics

OBJECTIVE

To present a general overview of the etiology, definition, and prevalence of mild cognitive impairment (MCI), as well as outline possible treatment strategies.

DATA SOURCES

A MEDLINE search was conducted for relevant references generated from 1990 to 2000 concerning MCI, mild to moderate Alzheimer disease (AD), and therapeutic strategies. Several books were also used in the compilation of data for this review, as well as the authors' experience in designing and conducting MCI trials.

DATA EXTRACTION

All of the references listed were assessed, and all relevant information was included in this review.

DATA SYNTHESIS

Forgetful individuals most likely to develop AD have a condition known as MCI previous to their development of dementia. This condition is hallmarked by memory impairment that is abnormal for the individual's age and educational level. While not all individuals with MCI develop AD, it is apparent that the condition can serve as a potential marker for early onset of AD.

CONCLUSIONS

As many clinicians can attest, occasional forgetfulness is a common aspect of the aging process. Eventually, however, a large portion of forgetful individuals, especially those with MCI, will be diagnosed with AD or some other form of dementia. Indeed, many researchers have suggested that MCI should be regarded as incipient AD and that these individuals would benefit from drug therapy. Thus, MCI screening may be beneficial in terms of both early AD intervention and perhaps even AD prevention.

Raloxifene and estradiol benzoate both fully restore hippocampal choline acetyltransferase activity in ovariectomized rats

Selective estrogen receptor modulators (SERMs) demonstrate tissue-specific estrogen receptor (ER) agonist or antagonist properties. Raloxifene, a prototypical SERM, has ER agonist properties in bone and on cholesterol metabolism but full antagonist properties in the uterus and breast. To characterize the ER agonist/antagonist profile of raloxifene in the brain, we have examined its effect on the activity of a known estrogen-responsive gene product, choline acetyltransferase (ChAT), in the hippocampus and other brain regions of 6-month-old ovariectomized (OVX) Sprague-Dawley rats. Three weeks post-ovariectomy, animals received estradiol benzoate (EB, 0.03 mg or 0.3 mg kg(-1) day(-1) for 3 or 10 days); raloxifene HCl (3.0 mg kg(-1) day(-1) for 3 or 10 days), tamoxifen (3.0 mg kg(-1) day(-1) for 10 days) or vehicle (20% CDX). As previously reported, ChAT activity decreased by approximately 20%-50% in the hippocampus of OVX compared with SHAM-operated control rats with no change in ChAT activity observed in the hypothalamus. Raloxifene or EB reversed the OVX-induced decrease in ChAT activity in the hippocampus but did not change ChAT activity in the hypothalamus. Animals that received combined EB (0.03 mg/kg) plus raloxifene (1 mg/kg) or tamoxifen alone (3.0 or 10 mg/kg) also showed increased hippocampal ChAT activity. Raloxifene failed to increase uterine weight and blocked the estrogen-induced increase in uterine weight, while another SERM, tamoxifen, increased uterine weight. These data demonstrate that raloxifene has estrogen-like properties on hippocampal ChAT activity in vivo, and suggest that benzothiophene SERMs may exert estrogen-like beneficial effects on cholinergic neurotransmission in brain without producing peripheral stimulation of breast or uterine tissue.

Effect of donepezil hydrochloride (E2020) on basal concentration of extracellular acetylcholine in the hippocampus of rats

The effects of oral administration of the centrally acting acetylcholinesterase (AChE) inhibitors, donepezil hydrochloride (donepezil: E2020: (+/-)-2-[(1-benzylpiperidin-4-yl)methyl]-5,6-dimethoxy-indan-1-one monohydrochloride), tacrine (9-amino-1,2,3,4-tetrahydroacridine hydrochloride) and ENA-713 (rivastigmine: (S)-N-ethyl-3-[(1-dimethyl-amino)ethyl]-N-methyl-phenylcarbamate hydrogentartrate), which have been developed for the treatment of Alzheimer's disease, on the extracellular acetylcholine concentration in the hippocampus of rats were evaluated by using a microdialysis technique without adding cholinesterase inhibitor to the perfusion solution. We also compared the inhibition of brain AChE and the brain concentrations of these drugs. Donepezil at 2.5 mg/kg and tacrine at 5 mg/kg showed significant effects for more than 6 h. At these doses, the maximum increases were observed at about 1.5 h after administration of donepezil, and at about 2 h with tacrine, and were 499% and 422% of the pre-level, respectively. ENA-713 produced significant effects at doses of 0.625, 1.25 and 2.5 mg/kg, which lasted for about 1, 2 and 4 h, respectively. The maximum increases produced by these doses at about 0.5 h after administration were 190, 346 and 458% of the pre-level, respectively. The time courses of brain AChE inhibition with donepezil at 2.5 mg/kg, tacrine at 10 mg/kg and ENA-713 at 2.5 mg/kg were mirror images of the extracellular acetylcholine-increasing action at the same doses. The time courses of the brain concentrations of drugs after oral administration of donepezil at 2.5 mg/kg and tacrine at 10 mg/kg were consistent with those of brain AChE inhibition at the same doses, and there was a linear relation between these parameters. Brain concentration of ENA-713 at 2.5 mg/kg was below the limit of quantification at all time points measured. These results suggest that oral administration of donepezil, tacrine and ENA-713 increases acetylcholine concentration in the synaptic cleft of the hippocampus mostly through AChE inhibition, and that donepezil has a more potent activity than tacrine and a longer-lasting effect than ENA-713 on the central cholinergic system.

Ameliorating effects of SDZ ENA 713 on age-associated decreases in learning performance and brain choline acetyltransferase activity in rats

In the present study, we have investigated the effects of SDZ ENA 713 on spatial learning deficits in aged rats. Using the same animals, the effect of SDZ ENA 713 on choline acetyltransferase was simultaneously studied to obtain a basis for the behavioral study. In the aged rats, the spatial learning and choline acetyltransferase activity in the frontal cortex were significantly deteriorated compared with young adult rats. SDZ ENA 713 (0.2 mg/kg) significantly shortened the time to reach a hidden platform without affecting swim rates in the water maze task. SDZ ENA 713 (0.1 and 0.2 mg/kg) inhibited aging-induced decreases in choline acetyltransferase activity in the frontal cortex. These results suggest that SDZ ENA 713 ameliorates aging-induced learning deficits and cholinergic dysfunction in rats.