Changes in cerebral neurotransmitters and metabolites induced by acute donepezil and memantine administrations: A microdialysis study

Movement disorders associated with acetylcholinesterase inhibitors in Alzheimer's dementia: A systematic review

BACKGROUND

Acetylcholinesterase inhibitors (AChEIs) are widely used in Alzheimer's disease (AD). This study aims to systematically review the literature about movement disorders (MDs) associated with AChEIs for AD, which include donepezil, galantamine, rivastigmine, tacrine, and ipidacrine.

METHODOLOGY

Two reviewers conducted a comprehensive review of relevant studies across six databases, without language restrictions, covering publications from 1992 to 2024.

RESULTS

Overall, 74 studies containing 92 cases were found of MDs related to ACHEIs. The MDs found were Pisa syndrome in 33 patients, parkinsonism in 31, myoclonus in 11, dystonia in 10, dyskinesia in 6, and extrapyramidal symptoms in 1. Regarding the medications, the abnormal movements were associated with donepezil in 62 cases, rivastigmine in 15, galantamine in 10, and tacrine in 5. No case of ipidacrine-induced MD was found. Overall, the most commonly affected sex was the female, accounting for 61.9% of the cases. The mean and median age was 74.1 (standard deviation: 8.9) and 75 years (range: 49-93 years). The MD occurred within 6 months of the starting of AChEI in approximately 70% of the patients. Furthermore, the full recovery of the MD after the main management was noticed within 6 months in about 80% of the patients. About 86.3% of the individuals fully recovered after treatment, which included AChEI discontinuation, dose adjustment, and prescription of additional therapy.

CONCLUSIONS

The occurrence of tacrine-induced tremor indicated a potential predisposition to movement disorders associated with AChEI therapy. Based on the drug class side effect profile, it is possible that future studies may observe abnormal movements with other AChEIs.

The Effect of an α-7 Nicotinic Allosteric Modulator PNU120596 and NMDA Receptor Antagonist Memantine on Depressive-like Behavior Induced by LPS in Mice: The Involvement of Brain Microglia

Nicotinic acetylcholine receptors (nAChRs), particularly the α7 nAChR, play a critical role in neuroinflammation and microglial activation associated with major depressive disorder (MDD). Microglial quinolinic acid (QUIN), which is synthesized by 3-hydroxyanthranilic acid dioxygenase (HAAO), is an N-methyl-D-aspartate (NMDA) receptor agonist and has been implicated in the development of MDD-related symptoms. In the present study, we assessed the effects of PNU120596, an α7 nAChR positive allosteric modulator (PAM), on HAAO expression and QUIN formation in the hippocampus and prefrontal cortex. We also investigated the effects of memantine, an NMDA receptor antagonist, alone and in combination with PNU120596 on cognitive deficit and depressive-like behaviors induced by lipopolysaccharide (LPS) in mice using the Y-maze and forced swim test, respectively. LPS (1 mg/kg, i.p.) elevated HAAO expression and QUIN formation in the hippocampus and prefrontal cortex, which were reduced with pretreatment with PNU120596 (4 mg/kg, i.p.). Furthermore, memantine (1 or 3 mg/kg, i.p.) prevented the cognitive deficit and depressive-like behaviors induced by LPS in mice. Together, these results suggest that the antidepressant-like effects of PNU120596 are mediated by attenuation of LPS-induced QUIN formation. Therefore, α7 nAChR PAM could be a potential therapeutic candidate for MDD associated with neurotoxic glutamatergic transmission.

Effects of Donepezil Treatment on Brain Metabolites, Gut Microbiota, and Gut Metabolites in an Amyloid Beta-Induced Cognitive Impairment Mouse Pilot Model

Accumulated clinical and biomedical evidence indicates that the gut microbiota and their metabolites affect brain function and behavior in various central nervous system disorders. This study was performed to investigate the changes in brain metabolites and composition of the fecal microbial community following injection of amyloid β (Aβ) and donepezil treatment of Aβ-injected mice using metataxonomics and metabolomics. Aβ treatment caused cognitive dysfunction, while donepezil resulted in the successful recovery of memory impairment. The Aβ + donepezil group showed a significantly higher relative abundance of Verrucomicrobia than the Aβ group. The relative abundance of 12 taxa, including Blautia and Akkermansia, differed significantly between the groups. The Aβ + donepezil group had higher levels of oxalate, glycerol, xylose, and palmitoleate in feces and oxalate, pyroglutamic acid, hypoxanthine, and inosine in brain tissues than the Aβ group. The levels of pyroglutamic acid, glutamic acid, and phenylalanine showed similar changes in vivo and in vitro using HT-22 cells. The major metabolic pathways in the brain tissues and gut microbiota affected by Aβ or donepezil treatment of Aβ-injected mice were related to amino acid pathways and sugar metabolism, respectively. These findings suggest that alterations in the gut microbiota might influence the induction and amelioration of Aβ-induced cognitive dysfunction via the gut–brain axis. This study could provide basic data on the effects of Aβ and donepezil on gut microbiota and metabolites in an Aβ-induced cognitive impairment mouse model.

Multiple cholinesterase inhibitors have antidepressant-like properties in the mouse forced swim test

There is high clinical interest in improving the pharmacological treatment of individuals with Major Depressive Disorder (MDD). This neuropsychiatric disorder continues to cause significant morbidity and mortality worldwide, where existing pharmaceutical treatments such as selective serotonin reuptake inhibitors often have limited efficacy. In a recent publication, we demonstrated an antidepressant-like role for the acetylcholinesterase inhibitor (AChEI) donepezil in the C57BL/6J mouse forced swim test (FST). Those data added to a limited literature in rodents and human subjects which suggests AChEIs have antidepressant properties, but added the novel finding that donepezil only showed antidepressant-like properties at lower doses (0.02, 0.2 mg/kg). At a high dose (2.0 mg/kg), donepezil tended to promote depression-like behavior, suggesting a u-shaped dose-response curve for FST immobility. Here we investigate the effects of three other AChEIs with varying molecular structures: galantamine, physostigmine, and rivastigmine, to test whether they also exhibit antidepressant-like effects in the FST. We find that these drugs do exhibit therapeutic-like effects at low but not high doses, albeit at lower doses for physostigmine. Further, we find that their antidepressant-like effects are not mediated by generalized hyperactivity in the novel open field test, and are also not accompanied by anxiolytic-like properties. These data further support the hypothesis that acetylcholine has a u-shaped dose-response relationship with immobility in the C57BL/6J mouse FST, and provide a rationale for more thoroughly investigating whether reversible AChEIs as a class can be repurposed for the treatment of MDD in human subjects.

Protective effect of nanocurcumin against neurotoxicity induced by doxorubicin in rat's brain

Cognitive impairment is one of the serious side effects that cancer-treated patients suffer from after treatment by doxorubicin (DOX). Investigating the mechanisms underlying this impairment is crucial for its treatment or prevention. The current study investigates the cortical and hippocampal neurochemical changes induced by an acute dose of DOX (20 mg/kg, i.p.) and evaluates the neuroprotective effect of nanocurcumin (NC) (50 mg/kg, p.o.) against these changes. Animals were randomly divided into four groups, control, rats treated with either NC or DOX, and the fourth group treated with NC prior to DOX. Cortical dopamine level has significantly increased (71.88 %) after DOX injection. This was associated with a significant rise in the levels of lipid peroxidation (183.99 %, 201.4 %) and nitric oxide (36.54 %, 55 %) and a significant reduction in reduced glutathione (13 %, 21.44 %) in the cortex and hippocampus, respectively. In addition, DOX inhibited the cortical and hippocampal activities of acetylcholinesterase (94.82 %, 62.75 %) and monoamine oxidase (64.40 %, 68.84 %), respectively. Protection with NC mitigates the changes induced in the oxidative stress parameters by DOX. However, the effect on the activities of AchE and MAO was insignificant. This was reflected in the level of dopamine that showed non-significant changes in comparison to control and DOX-treated rats. The present findings indicate that oxidative stress, inhibition in AchE, MAO, and the subsequent elevation in dopamine could have a crucial role in mediating the chemo-brain adverse effects induced by DOX. In addition, protection with NC mitigated some of these adverse effects thus rendering DOX more tolerable.

Recent Developments in New Therapeutic Agents against Alzheimer and Parkinson Diseases: In-Silico Approaches

Neurodegenerative diseases (ND), including Alzheimer's (AD) and Parkinson's Disease (PD), are becoming increasingly more common and are recognized as a social problem in modern societies. These disorders are characterized by a progressive neurodegeneration and are considered one of the main causes of disability and mortality worldwide. Currently, there is no existing cure for AD nor PD and the clinically used drugs aim only at symptomatic relief, and are not capable of stopping neurodegeneration. Over the last years, several drug candidates reached clinical trials phases, but they were suspended, mainly because of the unsatisfactory pharmacological benefits. Recently, the number of compounds developed using in silico approaches has been increasing at a promising rate, mainly evaluating the affinity for several macromolecular targets and applying filters to exclude compounds with potentially unfavorable pharmacokinetics. Thus, in this review, an overview of the current therapeutics in use for these two ND, the main targets in drug development, and the primary studies published in the last five years that used in silico approaches to design novel drug candidates for AD and PD treatment will be presented. In addition, future perspectives for the treatment of these ND will also be briefly discussed.

Evaluation of the Adverse Effects of Chronic Exposure to Donepezil (An Acetylcholinesterase Inhibitor) in Adult Zebrafish by Behavioral and Biochemical Assessments

Donepezil (DPZ) is an acetylcholinesterase inhibitor used for the clinical treatment of mild cognitive impairment. However, DPZ has been reported to have adverse effects, including causing abnormal cardiac rhythm, insomnia, vomiting, and muscle cramps. However, the existence of these effects in subjects without Dementia is unknown. In this study, we use zebrafish to conduct a deeper analysis of the potential adverse effects of DPZ on the short-term memory and behaviors of normal zebrafish by performing multiple behavioral and biochemical assays. Adult zebrafish were exposed to 1 ppm and 2.5 ppm of DPZ. From the results, DPZ caused a slight improvement in the short-term memory of zebrafish and induced significant elevation in aggressiveness, while the novel tank and shoaling tests revealed anxiolytic-like behavior to be caused by DPZ. Furthermore, zebrafish circadian locomotor activity displayed a higher reduction of locomotion and abnormal movement orientation in both low- and high-dose groups, compared to the control group. Biomarker assays revealed that these alterations were associated with an elevation of oxytocin and a reduction of cortisol levels in the brain. Moreover, the significant increases in reactive oxygen species (ROS) and malondialdehyde (MDA) levels in muscle tissue suggest DPZ exposure induced muscle tissue oxidative stress and muscle weakness, which may underlie the locomotor activity impairment. In conclusion, we show, for the first time, that chronic waterborne exposure to DPZ can severely induce adverse effects on normal zebrafish in a dose-dependent manner. These unexpected adverse effects on behavioral alteration should be carefully addressed in future studies considering DPZ conducted on zebrafish or other animals.

Potential of Low Dose Leuco-Methylthioninium Bis(Hydromethanesulphonate) (LMTM) Monotherapy for Treatment of Mild Alzheimer's Disease: Cohort Analysis as Modified Primary Outcome in a Phase III Clinical Trial

Background:

LMTM is being developed as a treatment for AD based on inhibition of tau aggregation.

Objectives:

To examine the efficacy of LMTM as monotherapy in non-randomized cohort analyses as modified primary outcomes in an 18-month Phase III trial in mild AD.

Methods:

Mild AD patients (n = 800) were randomly assigned to 100 mg twice a day or 4 mg twice a day. Prior to unblinding, the Statistical Analysis Plan was revised to compare the 100 mg twice a day as monotherapy subgroup (n = 79) versus 4 mg twice a day as randomized (n = 396), and 4 mg twice a day as monotherapy (n = 76) versus 4 mg twice a day as add-on therapy (n = 297), with strong control of family-wise type I error.

Results:

The revised analyses were statistically significant at the required threshold of p < 0.025 in both comparisons for change in ADAS-cog, ADCS-ADL, MRI atrophy, and glucose uptake. The brain atrophy rate was initially typical of mild AD in both add-on and monotherapy groups, but after 9 months of treatment, the rate in monotherapy patients declined significantly to that reported for normal elderly controls. Differences in severity or diagnosis at baseline between monotherapy and add-on patients did not account for significant differences in favor of monotherapy.

Conclusions:

The results are consistent with earlier studies in supporting the hypothesis that LMTM might be effective as monotherapy and that 4 mg twice a day may serve as well as higher doses. A further suitably randomized trial is required to test this hypothesis.

A Role for Prefrontal Cortical NMDA Receptors in Murine Alcohol-Heightened Aggression

Alcohol is associated with nearly half of all violent crimes committed in the United States; yet, a potential neural basis for this type of pathological aggression remains elusive. Alcohol may act on N-methyl-d-aspartate receptors (NMDARs) within cortical circuits to impede processing and to promote aggression. Here, male mice were characterized as alcohol-heightened (AHAs) or alcohol non-heightened aggressors (ANAs) during resident-intruder confrontations after self-administering 1.0 g/kg alcohol (6% w/v) or water. Alcohol produced a pathological-like pattern of aggression in AHAs; these mice shifted their bites to more vulnerable locations on the body of a submissive animal, including the anterior back and ventrum after consuming alcohol. In addition, through immunoblotting, we found that AHAs overexpressed the NMDAR GluN2D subunit in the prefrontal cortex (PFC) as compared to ANAs while the two phenotypes expressed similar levels of GluN1, GluN2A and GluN2B. After identifying several behavioral and molecular characteristics that distinguish AHAs from ANAs, we tested additional mice for their aggression following preferential antagonism of GluN2D-containing NMDARs. In these experiments, groups of AHAs and ANAs self-administered 1.0 g/kg alcohol (6% w/v) or water before receiving intraperitoneal (i.p.) doses of ketamine or memantine, or infusions of memantine directly into the prelimbic (PLmPFC) or infralimbic medial PFC (ILmPFC). Moderate doses of IP ketamine, IP memantine, or intra-PLmPFC memantine increased aggression in AHAs, but only in the absence of alcohol. Prior alcohol intake blocked the pro-aggressive effects of ketamine or memantine. In contrast, only memantine, administered systemically or intra-PLmPFC, interacted with prior alcohol intake to escalate aggression in ANAs. Intra-ILmPFC memantine had no effect on aggression in either AHAs or ANAs. In sum, this work illustrates a potential role of GluN2D-containing NMDARs in the PLmPFC in alcohol-heightened aggression. GluN2D-containing NMDARs are highly expressed on cortical parvalbumin-containing interneurons, suggesting that, in a subset of individuals, alcohol may functionally alter signal integration within cortical microcircuits to dysregulate threat reactivity and promote aggression. This work suggests that targeting GluN2D-NMDARs may be of use in reducing the impact of alcohol-related violence in the human population.

In Vivo Systems Response Profiling and Multivariate Classification of CNS Active Compounds: A Structured Tool for CNS Drug Discovery

This paper describes the application of in vivo systems response profiling in CNS drug discovery by a process referred to as the Integrative Screening Process. The biological response profile, treated as an array, is used as major outcome for selection of candidate drugs. Dose-response data, including ex vivo brain monoaminergic biomarkers and behavioral descriptors, are systematically collected and analyzed by principal component analysis (PCA) and partial least-squares (PLS) regression, yielding multivariate characterization across compounds. The approach is exemplified by assessing a new class of CNS active compounds, the dopidines, compared to other monoamine modulating compounds including antipsychotics, antidepressants, and procognitive agents. Dopidines display a distinct phenotypic profile which has prompted extensive further preclinical and clinical investigations. In summary, in vivo profiles of CNS compounds are mapped, based on dose response studies in the rat. Applying a systematic and standardized work-flow, a database of in vivo systems response profiles is compiled, enabling comparisons and classification. This creates a framework for translational mapping, a crucial component in CNS drug discovery.

Memantine improves memory impairment and depressive-like behavior induced by amphetamine withdrawal in rats

Amphetamine (AMPH) induces deficits in cognition, and depressive-like behavior following withdrawal. The aim of the present study was to investigate whether pre-treatment with memantine (5mg/kg, i.p.), a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, attenuates memory impairment induced by withdrawal from a 1 day binge regimen of AMPH (2mg/kg, four times every 2h, i.p.), in the novel object recognition test in rats. Herein, the influence of scopolamine (0.1mg/kg), an antagonist of the muscarinic cholinergic receptors, and the impact of MK-801 (0.1mg/kg), an antagonist of the NMDA receptors, on the memantine effect, were ascertained. Furthermore, the impact of memantine (5; 10; 20mg/kg, i.p.) was measured on depression-like effects of abstinence, 14 days after the last AMPH treatment (2mg/kg×1×14 days), in the forced swim test. In this test, the efficacy of memantine was compared to that of tricyclic antidepressant imipramine (10; 20; 30mg/kg, i.p.). Our study indicated that withdrawal from a binge regimen of AMPH impaired recognition memory. This effect was attenuated by administration of memantine at both 72h and 7 days of withdrawal. Moreover, prior administration of scopolamine, but not MK-801, decreased the memantine-induced recognition memory improvement. In addition, memantine reversed the AMPH-induced depressive-like behavior in the forced swim test in rats. The antidepressant-like effects of memantine were stronger than those of imipramine. Our study indicates that memantine constitutes a useful approach towards preventing cognitive deficits induced by withdrawal from an AMPH binge regimen and by depressive-like behavior during AMPH abstinence.

Donepezil-associated manic episode with psychotic features: a case report and review of the literature

OBJECTIVE

Reports of manic episodes associated with the use of cholinesterase inhibitors (including donepezil) are limited. Despite the previous notion of procholinergic drugs potentially inducing depression, the contemporary evidence for cholinesterase inhibitors appears to also indicate a trend for elevated mood (in patients with or without a history of depressive disorder).

METHOD

Case report.

RESULTS

The authors report a case of a manic episode with psychotic features associated with the up-titration of donepezil in a patient with Alzheimer's disease and a distant history of major depression but without a preexisting bipolar disorder.

CONCLUSION

Pathophysiology of donepezil-induced mania appears to contradict the traditional cholinergic-adrenergic hypothesis. Donepezil-associated mania should be suspected after donepezil initiation/dose up-titration when correlated to new onset of mania. Donepezil should be used more cautiously in patients with current or previous mood episodes or in those who are otherwise at high risk for manic episodes (e.g., cerebrovascular disease). Although this requires further investigation in different patient populations, there may be subtypes of older patients with neurocognitive disorders who are particularly vulnerable to activation effects of cholinesterase inhibitors.

Aggression and increased glutamate in the mPFC during withdrawal from intermittent alcohol in outbred mice

RATIONALE

Disrupted social behavior, including occasional aggressive outbursts, is characteristic of withdrawal from long-term alcohol (EtOH) use. Heavy EtOH use and exaggerated responses during withdrawal may be treated using glutamatergic N-methyl-D-aspartate receptor (NMDAR) antagonists.

OBJECTIVES

The current experiments explore aggression and medial prefrontal cortex (mPFC) glutamate as consequences of withdrawal from intermittent access to EtOH and changes in aggression and mPFC glutamate caused by NMDAR antagonists memantine and ketamine.

METHODS

Swiss male mice underwent withdrawal following 1-8 weeks of intermittent access to 20 % EtOH. Aggressive and nonaggressive behaviors with a conspecific were measured 6-8 h into EtOH withdrawal after memantine or ketamine (0-30 mg/kg, i.p.) administration. In separate mice, extracellular mPFC glutamate after memantine was measured during withdrawal using in vivo microdialysis.

RESULTS

At 6-8 h withdrawal from EtOH, mice exhibited more convulsions and aggression and decreased social contact compared to age-matched water controls. Memantine, but not ketamine, increased withdrawal aggression at the 5-mg/kg dose in mice with a history of 8 weeks of EtOH but not 1 or 4 weeks of EtOH or in water drinkers. Tonic mPFC glutamate was higher during withdrawal after 8 weeks of EtOH compared to 1 week of EtOH or 8 weeks of water. Five milligrams per kilogram of memantine increased glutamate in 8-week EtOH mice, but also in 1-week EtOH and water drinkers.

CONCLUSIONS

These studies reveal aggressive behavior as a novel symptom of EtOH withdrawal in outbred mice and confirm a role of NMDARs during withdrawal aggression and for disrupted social behavior.

Donepezil-Induced Myoclonus in a Patient With Alzheimer Disease

Objective: To report an uncommon adverse event in an elderly patient related to the cholinesterase inhibitor, donepezil. Case Summary: An 80-year-old Greek woman with Alzheimer disease was admitted to the hospital with a sudden onset of myoclonus in both upper and lower extremities after receiving 30 mg of donepezil daily for 25 days. After 36 hours of donepezil remission, the frequency of the myoclonic jerks was sharply reduced. The patient remains asymptomatic after 6 months of follow-up. Discussion: Our literature search yielded only 2 cases of myoclonus in relation to memantine, which has a different action with donepezil; to our knowledge, this is the first report of generalized myoclonus induced by donepezil overdosing. Based on the Naranjo probability scale it was probable that the patient’s myoclonus was related to donepezil because of clear temporal proximity and lack of alternative explanations. Conclusions: Clinicians should take into consideration therapeutic drug monitoring in the event of a potentially rare serious adverse reaction. Further studies are needed to clarify the possible role of donepezil in the pathophysiology of myoclonus.

Effects of the adenosine A2A receptor antagonist on cognitive dysfunction in Parkinson's disease

Parkinson's disease (PD) is primarily characterized by motor abnormalities, but cognitive changes also occur in the early and late stages of the disease process. In PD patients, cognitive dysfunction is associated with reduced quality of life, as well as increased morbidity and mortality, resulting in increases in caregiver burden, and health-related costs. Therefore, safe and effective approaches are needed to treat cognitive dysfunction in PD patients. The underlying pathophysiology of cognitive dysfunction is complex and not fully understood, however. α-Synuclein, amyloid-related proteins, and cholinergic deficits have been reported to partially contribute to cognitive dysfunction. Changes in cortical dopamine (DA) content may also be responsible for early cognitive changes in patients with PD. Certainly, dopaminergic afferents to the frontal cortex degenerate in PD, and there is a reduction of DA content in the prefrontal cortex (PFC). It has also been reported that PFC dopaminergic input plays an important role in working memory performance. Moreover, PFC DA levels and working memory performance are significantly reduced by a 6-hydroxydopamine lesion in the PFC of a rat. Recent findings in the areas of pharmacological manipulation and genetic ablation suggest that the adenosine A2A receptor is also related to cognitive functions, especially working memory. In addition, the blockade of adenosine A2A receptors reverses cognitive dysfunction in PFC-lesioned rats, and this blocking effect may be due to an increase in PFC DA content. Therefore, adenosine A2A receptor antagonists not only improve motor performance, but they may also lead to improved cognitive function in those with PD.

Possible synergistic effect of direct angiotensin II type 2 receptor stimulation by compound 21 with memantine on prevention of cognitive decline in type 2 diabetic mice

Type 2 diabetes mellitus (T2DM) is known to be associated with increased risk of cognitive impairment including Alzheimer disease. Recent studies have suggested an interaction between angiotensin II and N-methyl-d-aspartic acid (NMDA) glutamate receptors. We previously reported that stimulation of the angiotensin II type 2 (AT2) receptor exerts brain protective effects. A newly developed AT2 receptor agonist, compound 21 (C21), has enabled examination of the direct effect of AT2 receptor stimulation in vivo. Accordingly, we examined the possible synergistic effect of C21 and memantine on cognitive impairment in T2DM mice, KKAy. KKAy were divided into four groups; (1) control, (2) treatment with C21 (10 μg/kg/day), (3) treatment with memantine (20mg/kg/day), and (4) treatment with both for 4 weeks, and subjected to Morris water maze tasks. Treatment with C21 or memantine alone at these doses tended to shorten escape latency compared to that in the control group. C21 treatment increased cerebral blood flow (CBF), but memantine did not influence CBF. Treatment with C21 or C21 plus memantine increased hippocampal field-excitatory postsynaptic potential (f-EPSP). Moreover, treatment with memantine or C21 increased acetylcholine level, which was lower in KKAy than in wild-type mice, and C21 plus memantine treatment enhanced memantine or C21-induced acetylcholine secretion. This study provides an insight into new approaches to understand the interaction of angiotensin II and neurotransmitters. We can anticipate a new therapeutic approach against cognitive decline using C21 and memantine.

Effects of the adenosine A2A antagonist istradefylline on cognitive performance in rats with a 6-OHDA lesion in prefrontal cortex

RATIONALE

Altered cognitive function is a common feature of both the early and later stages of Parkinson's disease (PD) that involves alterations in cortical dopamine content. Adenosine A2A antagonists, such as istradefylline, improve motor function in PD, but their effect on cognitive impairment has not been determined.

OBJECTIVE

The present study investigated whether impairment of working memory due to the loss of dopaminergic input into the prefrontal cortex (PFC) is reversed by administration of istradefylline. We also evaluated whether A2A antagonist administration modulates dopamine levels in the PFC.

METHODS

Bilateral lesions of the dopaminergic input to the PFC were produced in rats using 6-hydroxydopamine (6-OHDA). Cognitive performance was evaluated using an object recognition task and delayed alternation task. The effects of istradefylline, donepezil and methamphetamine on cognitive performance were examined. In addition, the effect of istradefylline on extracellular dopamine levels in the PFC was studied.

RESULTS

PFC dopamine levels and cognitive performance were significantly reduced by 6-OHDA lesioning. Istradefylline, donepezil and methamphetamine improved cognitive performance of PFC-lesioned rats. Istradefylline increased dopamine levels in the PFC in both normal and PFC-lesioned rats.

CONCLUSIONS

PFC dopaminergic input plays an important role in working memory performance. Blockade of A2A receptors using istradefylline reverses the changes in cognitive function, and this may be due to an increase in PFC dopamine content. Adenosine A2A receptor antagonists not only improve motor performance in PD but may also lead to improved cognition.

Alterations in metabolic pathways and networks in Alzheimer's disease

The pathogenic mechanisms of Alzheimer's disease (AD) remain largely unknown and clinical trials have not demonstrated significant benefit. Biochemical characterization of AD and its prodromal phase may provide new diagnostic and therapeutic insights. We used targeted metabolomics platform to profile cerebrospinal fluid (CSF) from AD (n=40), mild cognitive impairment (MCI, n=36) and control (n=38) subjects; univariate and multivariate analyses to define between-group differences; and partial least square-discriminant analysis models to classify diagnostic groups using CSF metabolomic profiles. A partial correlation network was built to link metabolic markers, protein markers and disease severity. AD subjects had elevated methionine (MET), 5-hydroxyindoleacetic acid (5-HIAA), vanillylmandelic acid, xanthosine and glutathione versus controls. MCI subjects had elevated 5-HIAA, MET, hypoxanthine and other metabolites versus controls. Metabolite ratios revealed changes within tryptophan, MET and purine pathways. Initial pathway analyses identified steps in several pathways that appear altered in AD and MCI. A partial correlation network showed total tau most directly related to norepinephrine and purine pathways; amyloid-β (Ab42) was related directly to an unidentified metabolite and indirectly to 5-HIAA and MET. These findings indicate that MCI and AD are associated with an overlapping pattern of perturbations in tryptophan, tyrosine, MET and purine pathways, and suggest that profound biochemical alterations are linked to abnormal Ab42 and tau metabolism. Metabolomics provides powerful tools to map interlinked biochemical pathway perturbations and study AD as a disease of network failure.

The potent effects of ginseng root extract and memantine on cognitive dysfunction in male albino rats

The study determined the maximum intraperitoneal (ip) scopolamine dose inducing memory impairment in rats (2 mg/kg) compared to 0.5 or 1 mg/kg dose. The effect reflected by significant increase from normal in the latency time required for rats to find the hidden platform in water maze task and acetylcholinesterase (AChE) activities in cortex, hippocampus and striatum. The dose-related histopathological effect via the hemorrhage, vacuolation and gliosis in cortex and hippocampus is assessed. Then the study investigated the potency of Panax ginseng root extract on scopolamine cognitive dysfunction rat model compared to memantine hydrochloride as reference Food and Drug Administration approved. Ginseng extract was administered at dose 100 or 200 mg/kg/day and memantine at 20 mg/kg/day orally for 2 weeks. All treatments showed improvement in the water maze task, however, ginseng (200 mg/kg) group acquired the advantage without statistical difference control. Scopolamine (2 mg/kg ip) group showed significant increase in AChE reactivity and glutamate level and reduced monoamines (norepinephrine, dopamine and serotonin) and γ-aminobutyric acid contents in cortex, hippocampus and striatum. Ginseng extract in a dose-dependent manner appears effective as memantine and can improve memory impairment through the retrieved homeostasis via neurotransmitter levels and AChE activities in rat brain areas with partial effect on the histological feature of the brain tissue.

Effects of prolonged treatment with memantine in the MRL model of CNS lupus

OBJECTIVES

Neuropsychiatric manifestations and brain atrophy of unknown etiology are common and severe complications of systemic lupus erythematosus (SLE). An autoantibody that binds to N-methyl-D-aspartate (NMDA) receptor NR2 has been proposed as a key factor in the etiology of central nervous system (CNS) SLE. This hypothesis was supported by evidence suggesting memantine (MEM), an uncompetitive NMDA receptor antagonist, prevents behavioral dysfunction and brain pathology in healthy mice immunized with a peptide similar to an epitope on the NR2 receptor. Given that SLE is a chronic condition, we presently examine the effects of MEM in MRL/lpr mice, which develop behavioral deficits alongside SLE-like disease.

METHODS

A broad behavioral battery and 7-Tesla MRI were used to examine whether prolonged treatment with MEM (~25 mg/kg b.w. in drinking water) prevents CNS involvement in this spontaneous model of SLE.

RESULTS

Although MEM increased novel object exploration in MRL/lpr mice, it did not show other beneficial, substrain-specific effects. Conversely, MEM was detrimental to spontaneous activity in control MRL +/+ mice and had a negative effect on body mass gain. Similarly, MRI revealed comparable increases in the volume of periventricular structures in MEM-treated groups.

CONCLUSIONS

Sustained exposure to MEM affects body growth, brain morphology, and behavior primarily by pharmacological, and not autoimmunity-dependant mechanisms. Substrain-specific improvement in exploratory behavior of MEM-treated MRL/lpr mice may indicate that the NMDA system is merely a constituent of a complex pathogenenic cascade. However, it was evident that chronic administration of MEM is unable to completely prevent the development of a CNS SLE-like syndrome.

Anti-dyskinetic mechanisms of amantadine and dextromethorphan in the 6-OHDA rat model of Parkinson's disease: role of NMDA vs. 5-HT1A receptors

Amantadine and dextromethorphan suppress levodopa (L-DOPA)-induced dyskinesia (LID) in patients with Parkinson's disease (PD) and abnormal involuntary movements (AIMs) in the unilateral 6-hydroxydopamine (6-OHDA) rat model. These effects have been attributed to N-methyl-d-aspartate (NMDA) antagonism. However, amantadine and dextromethorphan are also thought to block serotonin (5-HT) uptake and cause 5-HT overflow, leading to stimulation of 5-HT(1A) receptors, which has been shown to reduce LID. We undertook a study in 6-OHDA rats to determine whether the anti-dyskinetic effects of these two compounds are mediated by NMDA antagonism and/or 5-HT(1A) agonism. In addition, we assessed the sensorimotor effects of these drugs using the Vibrissae-Stimulated Forelimb Placement and Cylinder tests. Our data show that the AIM-suppressing effect of amantadine was not affected by the 5-HT(1A) antagonist WAY-100635, but was partially reversed by the NMDA agonist d-cycloserine. Conversely, the AIM-suppressing effect of dextromethorphan was prevented by WAY-100635 but not by d-cycloserine. Neither amantadine nor dextromethorphan affected the therapeutic effects of L-DOPA in sensorimotor tests. We conclude that the anti-dyskinetic effect of amantadine is partially dependent on NMDA antagonism, while dextromethorphan suppresses AIMs via indirect 5-HT(1A) agonism. Combined with previous work from our group, our results support the investigation of 5-HT(1A) agonists as pharmacotherapies for LID in PD patients.

Disease-specific expression of the serotonin-receptor 5-HT(2C) in natural killer cells in Alzheimer's dementia

Alzheimer's dementia (AD) is a degenerative brain disorder characterized mainly by cholinergic failure, but other neuro-transmitters are also deficient especially at late stages of the disease. Misfolded β-amyloid peptide has been identified as a causative agent, however inflammatory changes also play a pivotal role. Even though the most prominent pathology is seen in the cognitive functions, specific abnormalities of the central nervous system (CNS) are also reflected in the periphery, particularly in the immune responses of the body. The aim of this study was to characterize the dopaminergic and serotonergic systems in AD, which are also markedly disrupted along with the hallmark acetyl-choline dysfunction. Peripheral blood mono-nuclear cells (PBMCs) from demented patients were judged against comparison groups including individuals with late-onset depression (LOD), as well as non-demented and non-depressed subjects. Cellular sub-populations were evaluated by mono-clonal antibodies against various cell surface receptors: CD4/CD8 (T-lymphocytes), CD19 (B-lymphocytes), CD14 (monocytes), and CD56 (natural-killer (NK)-cells). The expressions of dopamine D(3) and D(4), as well as serotonin 5-HT(1A), 5-HT(2A), 5-HT(2B) and 5-HT(2C) were also assessed. There were no significant differences among the study groups with respect to the frequency of the cellular sub-types, however a unique profound increase in 5-HT(2C) receptor exclusively in NK-cells was observed in AD. The disease-specific expression of 5-HT(2C), as well as the NK-cell cyto-toxicity, has been linked with cognitive derangement in dementia. These changes not only corroborate the existence of bi-directional communication between the immune system and the CNS, but also elucidate the role of inflammatory activity in AD pathology, and may serve as potential biomarkers for less invasive and early diagnostic purposes as well.

Effect of a CNS-Sensitive Anticholinesterase Methane Sulfonyl Fluoride on Hippocampal Acetylcholine Release in Freely Moving Rats

Anticholinesterases (antiChEs) are used to treat Alzheimer's disease. The comparative effects of two antiChEs, methanesulfonyl fluoride (MSF) and donepezil, on the extracellular levels of ACh in the hippocampus were investigated by in vivo microdialysis in freely moving rats. MSF at 1 and 2 mg/kg produced a dose-dependent increase in ACh efflux from 10 min to at least 3 hrs after injection. At 2 mg/kg, the increase was still present at 24 hr. Donepezil at 1 mg/kg showed a similar but smaller effect, and, paradoxically, 2 mg/kg showed no consistent effect. MSF at 1 and 2 mg/kg decreased acetylcholinesterase activity in the hippocampus to 54.8 and 20.1% of control, respectively. These results suggest that MSF is a suitable candidate for the treatment of Alzheimer's disease.

Memantine partly rescues behavioral and cognitive deficits in an animal model of neurodegeneration

Memantine, a non-competitive NMDA receptor antagonist, is used for the treatment of Alzheimer's disease (AD) and off-label as an anti-depressant. Here we investigated possible anti-depressant, cognitive enhancing and neuroprotective effects of memantine in the olfactory bulbectomized (OBX) rat. OBX is used as a screening model for antidepressants and shows cognitive disturbances. In Experiment I, memantine treatment started 14 days after OBX surgery (this setup is similar to what we use for screening of potential antidepressants) and 2 days before surgery in experiment II. In both experiments, memantine (20 mg/kg, p.o) was administered once daily for 28 days. Animals were tested in the open field (locomotor activity), passive avoidance (fear learning and memory), and holeboard (spatial acquisition and memory) before and after the bulbectomy. Memantine, when administered before surgery, prevented OBX-induced hyperactivity and partly fear memory loss. These behavioral effects were present for at least 3 weeks after cessation of treatment. Memantine, however did not improve spatial memory. When administered 2 weeks after OBX surgery, memantine was ineffective in normalizing open field hyperactivity and improving cognitive deficits. Interestingly, after the animals were retrained in passive avoidance, memantine- treated OBX rats (both in experiment I and II) showed improved fear learning and memory. Our findings suggest that memantine has both neuroprotective and cognitive enhancing effects without antidepressant-like properties in the OBX rat. Based on our results, we propose that memantine may be more beneficial to AD patients when administered early in the disease process.

Bipolar affective disorder, type II, apparently precipitated by donepezil

There is considerable evidence that pro-cholinergic agents can cause depressed mood. However, there are also published case reports of a rare association between cholinesterase inhibitors and mood elevation in patients with pre-existing major functional psychiatric disorders, or organic disorders other than dementia. This report adds to the literature by describing a case of mood elevation in a patient without pre-existing psychiatric disorder.

Oral administration of glycine increases extracellular serotonin but not dopamine in the prefrontal cortex of rats

AIM

Glycine, one of the non-essential amino acids, has been reported to be effective in reducing negative symptoms of schizophrenia. Recently, we found that glycine improves subjective sleep quality in humans. The aim of this study was to investigate the effects of oral glycine administration on endogenous 5-hydroxytryptamine (5-HT) and dopamine in the prefrontal cortex (PFC) of living rats.

METHODS

Microdialysis probes were inserted stereotaxically into the rat prefrontal cortex. Cortical levels of 5-HT and dopamine were measured following oral administration of 1 or 2 g/kg glycine, 2 g/kg d-serine, or 2 g/kg L-serine.

RESULTS

Both glycine and d-serine significantly increased extracellular 5-HT levels for 10 min, whereas dopamine levels remained unchanged. L-serine, in contrast, had no significant effects on 5-HT levels.

CONCLUSIONS

It is possible that the increase in 5-HT in response to glycine and d-serine was mediated by N-methyl-D-aspartate receptors. The transient increase in 5-HT in the PFC might be associated with the alleviation of negative symptoms in patients with schizophrenia and the amelioration of sleep quality in patients with insomnia.

Activation of brain histaminergic neurotransmission: a mechanism for cognitive effects of memantine in Alzheimer's disease

We previously reported that some N-methyl-D-aspartate (NMDA)-receptor antagonists enhanced histamine neuron activity in rodents. Here, we have investigated the effects of memantine, an NMDA-receptor antagonist used for the treatment of Alzheimer's disease, on histaminergic neurotransmission. In vitro, memantine antagonized native NMDA receptors with a micromolar potency but had no effect at recombinant human histamine receptors. In vivo, a single administration of memantine increased histamine neuron activity, as shown by the 60% increase of tele-methylhistamine (t-MeHA) levels observed in the brain of mice. This increase occurred with an ED(50) of 0.3 ± 0.1 mg/kg, similar to that found on inhibition of ex vivo [(3)H]dizocilpine maleate (MK-801) binding (1.8 ± 1.3 mg/kg). Two days after pretreatment of mice with memantine at 5 mg/kg twice daily for 5 days, t-MeHA levels were enhanced by 50 ± 7% (p < 0.001), indicating a long-lasting activation of histamine neurons. Quantitative polymerase chain reaction analysis was used to explore genes involved in this persistent effect. H(3) receptor mRNAs were strongly increased, but the density of H(3) receptor binding sites was increased solely in hypothalamus (by 141 ± 24%). Up-regulations of brain-derived neurotrophic factor and NMDA-receptor 1 subunit mRNAs were also found but were restricted to hippocampus. mRNA expression of α7-nicotinic receptors remained unchanged in any region. Considering the well established cognitive effects of histamine neurons, the increase in brain t-MeHA levels after single or repeated administration of therapeutic doses of memantine suggests that the drug exerts its beneficial effects on cognitive deficits of Alzheimer's disease, at least partly, by activating histamine neurons.

A new strategy for antidepressant prescription

From our research and literature search we propose an understanding of the mechanism of action of antidepressants treatments (ADTs) that should lead to increase efficacy and tolerance. We understand that ADTs promote synaptic plasticity and neurogenesis. This promotion is linked with stimulation of dopaminergic receptors. Previous evidence shows that all ADTs (chemical, electroconvulsive therapy, repetitive transcranial magnetic stimulation, sleep deprivation) increase at least one monoamine neurotransmitter serotonin (5-HT), noradrenaline (NA) or dopamine (DA); this article focuses on DA release or turn-over in the frontal cortex. DA increased dopaminergic activation promotes synaptic plasticity with an inverted U shape dose–response curve. Specific interaction between DA and glutamate is mediated by D1 receptor subtypes and Glutamate (NMDA) receptors with neurotrophic factors likely to play a modulatory role. With the understanding that all ADTs have a common, final, DA-ergic stimulation that promotes synaptic plasticity we can predict that (1) AD efficiency is related to the compound strength for inducing DA-ergic stimulation. (2) ADT efficiency presents a therapeutic window that coincides with the inverted U shape DA response curve. (3) ADT delay of action is related to a “synaptogenesis and neurogenesis delay of action.” (4) The minimum efficient dose can be found by starting at a low dosage and increasing up to the patient response. (5) An increased tolerance requires a concomitant prescription of a few ADTs, with different or opposite adverse effects, at a very low dose. (6) ADTs could improve all diseases with cognitive impairments and synaptic depression by increasing synaptic plasticity and neurogenesis.

The potent inhibitory effect of tipepidine on marble-burying behavior in mice

Our previous study revealed that centrally acting non-narcotic antitussives inhibited G-protein-coupled inwardly rectifying K(+) (GIRK) channel currents in brain neurons, and that the tipepidine antitussives had a novel antidepressive-like effect on rats. Furthermore, the antitussives revealed multiplexed ameliorating actions on intractable brain disease models. This study evaluated the therapeutic potential of tipepidine in obsessive-compulsive disorder (OCD) subjects using marble-burying behavior (MBB) tests in mice. In fact, OCD is classified as an anxiety disorder characterized by obsession or compulsion. Although selective 5-HT reuptake inhibitors (SSRIs) are considered first choice agents for the pharmacological treatment of OCD, 50% of patients with OCD failed to respond to SSRIs. The burying of harmless objects such as marbles by mice might reflect the formation of compulsive behavior. The results show that tipepidine reduced MBB in a dose-dependent manner. The effect of tipepidine was significant even at a dosage as small as 5 mg/kg. The tipepidine at 10 mg/kg s.c. nearly abolished MBB without reducing the locomotor activity in mice. It is particularly interesting that the dopamine D₂ antagonist or 5-HT(1A) antagonist partly inhibited the effect of tipepidine on MBB. The results suggest that tipepidine has more of a potent inhibitory effect on MBB, compared with known drugs used for the treatment of OCD, and that the tipepidine action mechanism might differ from that of known drugs.

Pharmacodynamics of memantine: an update

Memantine received marketing authorization from the European Agency for the Evaluation of Medicinal Products (EMEA) for the treatment of moderately severe to severe Alzheimer s disease (AD) in Europe on 17(th) May 2002 and shortly thereafter was also approved by the FDA for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist with strong voltage-dependency and fast kinetics. Due to this mechanism of action (MOA), there is a wealth of other possible therapeutic indications for memantine and numerous preclinical data in animal models support this assumption. This review is intended to provide an update on preclinical studies on the pharmacodynamics of memantine, with an additional focus on animal models of diseases aside from the approved indication. For most studies prior to 1999, the reader is referred to a previous review [196].In general, since 1999, considerable additional preclinical evidence has accumulated supporting the use of memantine in AD (both symptomatic and neuroprotective). In addition, there has been further confirmation of the MOA of memantine as an uncompetitive NMDA receptor antagonist and essentially no data contradicting our understanding of the benign side effect profile of memantine.

Confocal Analysis of Cholinergic and Dopaminergic Inputs onto Pyramidal Cells in the Prefrontal Cortex of Rodents

Cholinergic and dopaminergic projections to the rat medial prefrontal cortex (mPFC) are both involved in cognitive functions including attention. These neuronal systems modulate mPFC neuronal activity mainly through diffuse transmission. In order to better understand the anatomical level of influence of these systems, confocal microscopy with triple-fluorescent immunolabeling was used in three subregions of the mPFC of rats and Drd1a-tdTomato/Drd2-EGFP transgenic mice. The zone of interaction was defined as a reciprocal microproximity between dopaminergic and cholinergic axonal segments as well as pyramidal neurons. The density of varicosities, along these segments was considered as a possible activity-dependant morphological feature. The percentage of cholinergic and dopaminergic fibers in microproximity ranged from 12 to 40% depending on the layer and mPFC subregion. The cholinergic system appeared to have more influence on dopaminergic fibers since a larger proportion of the dopaminergic fibers were within microproximity to cholinergic fibers. The density of both cholinergic and dopaminergic varicosities was significantly elevated within microproximities. The main results indicate that the cholinergic and dopaminergic systems converge on pyramidal cells in mPFC particularly in the layer V. In transgenic mice 93% of the pyramidal cells expressed the transgenic marker for Drd2 expression, but only 22% expressed the maker for Drd1ar expression. Data presented here suggest that the modulation of mPFC by dopaminergic fibers would be mostly inhibitory and localized at the output level whereas the cholinergic modulation would be exerted at the input and output level both through direct interaction with pyramidal cells and dopaminergic fibers.

Beyond the cholinergic hypothesis: do current drugs work in Alzheimer's disease?

Alzheimer's disease (AD) is a neurodegenerative disease characterized by memory and cognitive loss, and represents the leading cause of dementia in elderly people. Besides the complex biochemical processes involved in the neuronal degeneration (formation of senile plaques containing Abeta peptides, and development of neurofibrillary tangles), other molecular and neurochemical alterations, like cholinergic deficit due to basal forebrain degeneration, also occur. Because acetylcholine has been demonstrated to be involved in cognitive processes, the idea to increase acetylcholine levels to restore cognitive deficits has gained interest (the so-called cholinergic hypothesis). This has led to the development of drugs able to prevent acetylcholine hydrolysis (acetylcholinesterase inhibitors). However, the analysis of clinical efficacy of these drugs in alleviating symptoms of dementia showed unsatisfactory results. Despite such critical opinions on the efficacy of these drugs, it should be said that acetylcholinesterase inhibitors, and for some aspects memantine also, improve memory and other cognitive functions throughout most of the duration of the disease. The pharmacological activity of these drugs suggests an effect beyond the mere increase of acetylcholine levels. These considerations are in agreement with the idea that cognitive decline is the result of a complex and not fully elucidated interplay among different neurotransmitters. The role of each of the neurotransmitters implicated has to be related to a cognitive process and as a consequence to its decline. The current review aims to highlight the positive role of cholinergic drugs in alleviating cognitive deficits during wake as well as sleep. Moreover, we suggest that future therapeutic approaches have to be developed to restore the complex interplay between acetylcholine and other neurotransmitters systems, such as dopamine, serotonin, noradrenaline, or glutamate, that are likely involved in the progressive deterioration of several cognitive functions such as attention, memory, and learning.

A novel procedure for assessing the effects of drugs on satiation in baboons: effects of memantine and dexfenfluramine

RATIONALE

Procedures for studying the effects of medications on satiation will assist the development of obesity medications.

OBJECTIVES

Develop a procedure for measuring satiation during consumption of bland and highly palatable food and determine the effect of acute intramuscular administration of dexfenfluramine (DFEN), which increases serotonin levels, and memantine (MEM), which blocks N-methyl-D: -aspartate receptors.

MATERIALS AND METHODS

A modified progressive ratio (PR) procedure was used to track changes in reinforcing strength when a food was consumed. The response requirement increased after each reinforcement, and reinforcing strength was estimated using the breakpoint (BP), which was the last completed response cost. There was one preferred food (sweet candy) and one chow pellet PR session per week. During each session, four male and four female adult baboons experienced three 1-h PR trials, separated by 30 min. Chow pellets were available at all other times. We examined the BP for one to 20 candies or chow pellets. Drug effects were examined when baboons had access to one and ten candies or chow pellets.

RESULTS

BPs for candy were greater than for pellets. Varying the pellet/candy pieces per delivery produced an inverted U-shaped function on the first trial, i.e., maximal BP was observed for three items, and the BP for multiple items, but not a single item, decreased across trials, i.e., BP decreased with food intake and satiation. DFEN and MEM decreased responding with the greatest effects at ten deliveries, suggesting that DFEN and MEM enhanced satiation.

CONCLUSION

Drugs that enhance satiation for several types of food may be particularly effective for decreasing food intake.

Coactivation of M(1) muscarinic and alpha1 adrenergic receptors stimulates extracellular signal-regulated protein kinase and induces long-term depression at CA3-CA1 synapses in rat hippocampus

Intact cholinergic innervation from the medial septum and noradrenergic innervation from the locus ceruleus are required for hippocampal-dependent learning and memory. However, much remains unclear about the precise roles of acetylcholine (ACh) and norepinephrine (NE) in hippocampal function, particularly in terms of how interactions between these two transmitter systems might play an important role in synaptic plasticity. Previously, we reported that activation of either muscarinic M(1) or adrenergic alpha1 receptors induces activity- and NMDA receptor-dependent long-term depression (LTD) at CA3-CA1 synapses in acute hippocampal slices, referred to as muscarinic LTD (mLTD) and norepinephrine LTD (NE LTD), respectively. In this study, we tested the hypothesis that mLTD and NE LTD are independent forms of LTD, yet require activation of a common Galphaq-coupled signaling pathway for their induction, and investigated the net effect of coactivation of M(1) and alpha1 receptors on the magnitude of LTD induced. We find that neither mLTD nor NE LTD requires phospholipase C activation, but both plasticities are prevented by inhibiting the Src kinase family and extracellular signal-regulated protein kinase (ERK) activation. Interestingly, LTD can be induced when M(1) and alpha1 agonists are coapplied at concentrations too low to induce LTD when applied separately, via a summed increase in ERK activation. Thus, because ACh and NE levels in vivo covary, especially during periods of memory encoding and consolidation, cooperative signaling through M(1) and alpha1 receptors could function to induce long-term changes in synaptic function important for cognition.

Cognitive performance of healthy young rats following chronic donepezil administration

RATIONALE

Experimental studies have investigated the effects of chronic donepezil treatment on the behavioral deficits elicited by reduced activity or the loss of cholinergic neurons that occurs in aging or in models of dementia. However, few studies have analyzed the effects of chronic donepezil treatment on the cognitive functions of intact animals.

OBJECTIVES

The cognitive functions of healthy young rats treated chronically with the acetylcholinesterase inhibitor donepezil were evaluated using a wide behavioral test battery.

RESULTS

Chronic treatment with donepezil ameliorated memory functions and explorative strategies, speeded up the acquisition of localizing knowledge, augmented responsiveness to the context, and reduced anxiety levels. However, it did not affect spatial span, modify motivational levels, or influence associative learning.

CONCLUSIONS

The present findings show the specific profile of donepezil action on cognitive functions in the presence of unaltered cholinergic neurotransmission systems.

Nicotine-induced monoamine neurotransmitter changes in the brain of young rats

A number of studies in various species including man indicated a greater risk of drug preference and addictive behavior in young as compared to adults. Such age dependent preference was also found with nicotine. To examine possible mechanisms for this difference in our continuing study of reward mechanisms, we compared nicotine-induced neurotransmitter changes in the brain regions of adult and young Sprague-Dawley rats, assaying the transmitters via microdialysis in conscious freely moving animals. In general, nicotine-induced changes were significantly less in the regions measured in the young. Nicotine-induced effects on dopamine in the dorsal and ventral hippocampus (VH), prefrontal and medial temporal cortex, and superior cerebral peduncle were lower in the young than in adult, the same in the ventral tegmental area (VTA) and lateral septal nucleus (LS), and somewhat higher in the nucleus accumbens shell (NAccS). Norepinephrine levels in the young were lower in all areas except in the VH where they were the same, and serotonin levels were lower except in the VTA and LS where they remained the same, and higher in the NAccS. Age-dependent differences in the metabolites measured were more mixed. We conclude that the greater nicotine preference in young is not paralleled by a greater effect of nicotine on the release of monoamines at least in most of the brain areas assayed. Thus, increases of nicotine reward are not likely due to increases of monoamines in reward and cognitive areas. The small increase of dopamine (DA) and more significant increase of serotonin (5-HT) only in the NAccS are of significance, and would indicate a more significant role of 5-HT than of DA at least in the age difference in nicotine preference. Developmental changes in receptor composition and distribution involving several transmitter systems and other components such as neuropeptides are also likely to play a role.