Potentiation of N-methyl-D-aspartate-induced currents by the nootropic drug nefiracetam in rat cortical neurons

L-type voltage-gated calcium channel agonists mitigate hearing loss and modify ribbon synapse morphology in the zebrafish model of Usher syndrome type 1

The mariner (myo7aa^−/−) mutant is a zebrafish model for Usher syndrome type 1 (USH1). To further characterize hair cell synaptic elements in myo7aa^−/− mutants, we focused on the ribbon synapse and evaluated ultrastructure, number and distribution of immunolabeled ribbons, and postsynaptic densities. By transmission electron microscopy, we determined that myo7aa^−/− zebrafish have fewer glutamatergic vesicles tethered to ribbon synapses, yet maintain a comparable ribbon area. In myo7aa^−/− hair cells, immunolocalization of Ctbp2 showed fewer ribbon-containing cells in total and an altered distribution of Ctbp2 puncta compared to wild-type hair cells. myo7aa^−/− mutants have fewer postsynaptic densities – as assessed by MAGUK immunolabeling – compared to wild-type zebrafish. We quantified the circular swimming behavior of myo7aa^−/− mutant fish and measured a greater turning angle (absolute smooth orientation). It has previously been shown that L-type voltage-gated calcium channels are necessary for ribbon localization and occurrence of postsynaptic density; thus, we hypothesized and observed that L-type voltage-gated calcium channel agonists change behavioral and synaptic phenotypes in myo7aa^−/− mutants in a drug-specific manner. Our results indicate that treatment with L-type voltage-gated calcium channel agonists alter hair cell synaptic elements and improve behavioral phenotypes of myo7aa^−/− mutants. Our data support that L-type voltage-gated calcium channel agonists induce morphological changes at the ribbon synapse – in both the number of tethered vesicles and regarding the distribution of Ctbp2 puncta – shift swimming behavior and improve acoustic startle response.

Summary: We quantified behavioral and synaptic morphology differences between wild-type zebrafish larvae and the mariner (myo7aa^−/−) mutant, finding that these differences can be modified by L-type voltage-gated calcium channel agonists.

Postreactivation mifepristone impairs generalization of strongly conditioned contextual fear memories

The efficacy of pharmacological disruption of fear memory reconsolidation depends on several factors, including memory strength and age. We built on previous observations that systemic treatment with the nootropic nefiracetam potentiates cued fear memory destabilization to facilitate mifepristone-induced reconsolidation impairment. Here, we applied nefiratecam and mifepristone to strongly conditioned, 1-wk-old contextual fear memories in male rats. Unexpectedly, the combined treatment did not result in impairment of contextual fear expression. However, mifepristone did reduce freezing to a novel context. These observations suggest that strong and established contextual fear memories do undergo destabilization without the need for pharmacological facilitation, and that impairments in strong context fear memory reconsolidation can manifest as a reduction in generalization.

Dopaminergic D1 receptor signalling is necessary, but not sufficient for cued fear memory destabilisation

RATIONALE

Pharmacological targeting of memory reconsolidation is a promising therapeutic strategy for the treatment of fear memory-related disorders. However, the success of reconsolidation-based approaches depends upon the effective destabilisation of the fear memory by memory reactivation.

OBJECTIVES

Here, we aimed to determine the functional involvement of dopamine D1 receptors in cued fear memory destabilisation, using systemic drug administration.

RESULTS

We observed that direct D1 receptor agonism was not sufficient to stimulate tone fear memory destabilisation to facilitate reconsolidation disruption by the glucocorticoid receptor antagonist mifepristone. Instead, administration of the nootropic nefiracetam did facilitate mifepristone-induced amnesia, in a manner that was dependent upon dopamine D1 receptor activation. Finally, while the combined treatment with nefiracetam and mifepristone did not confer fear-reducing effects under conditions of extinction learning, there was some evidence that mifepristone reduces fear expression irrespective of memory reactivation parameters.

CONCLUSIONS

The use of combination pharmacological treatment to stimulate memory destabilisation and impair reconsolidation has potential therapeutic benefits, without risking a maladaptive increase of fear.

Radix Puerariae modulates glutamatergic synaptic architecture and potentiates functional synaptic plasticity in primary hippocampal neurons

ETHNOPHARMACOLOGICAL RELEVANCE

Neurologic disorders are frequently characterized by synaptic pathology, including abnormal density and morphology of dendritic spines, synapse loss, and aberrant synaptic signaling and plasticity. Therefore, to promote and/or protect synapses by the use of natural molecules capable of modulating neurodevelopmental events, such as, spinogenesis and synaptic plasticity, could offer a preventive and curative strategy for nervous disorders associated with synaptic pathology. Radix Puerariae, the root of Pueraria monatana var. lobata (Willd.) Sanjappa&Pradeep, is a Chinese ethnomedicine, traditionally used for the treatment of memory-related nervous disorders including Alzheimer's disease. In the previous study, we showed that the ethanolic extracts of Radix Puerariae (RPE) and its prime constituent, puerarin induced neuritogenesis and synapse formation in cultured hippocampal neurons, and thus could improve memory functions.

AIMS OF THE STUDY

In the present study, we specifically investigated the abilities of RPE and puerarin to improve memory-related brain disorders through modulating synaptic maturation and functional potentiation.

MATERIALS AND METHODS

Rat embryonic (E19) brain neurons were cultured in the absence or presence of RPE or puerarin. At predetermined times, cells were live-stained with DiO or fixed and immunostained to visualize neuronal morphologies, or lysed for protein harvesting. Morphometric analyses of dendritic spines and synaptogenesis were performed using Image J software. Functional pre- and postsynaptic plasticity was measured by FM1-43 staining and whole-cell patch clamping, respectively. RPE or puerarin-mediated changes in actin-related protein 2 were assessed by Western blotting. Neuronal survivals were measured using propidium iodide exclusion assay.

RESULTS

RPE and puerarin both: (1) promoted a significant increase in the numbers, and maturation, of dendritic spines; (2) modulated the formation of glutamatergic synapses; (3) potentiated synaptic transmission by increasing the sizes of reserve vesicle pools at presynaptic terminals; (4) enhanced NMDA receptor-mediated postsynaptic currents, and (5) increased cell viability against naturally occurring cell death. Moreover, upregulation of actin-related protein 2 (ARP2) in RPE and puerarin treated brain neurons suggest that RPE and puerarin induced synaptic plasticity might be associated, at least in part, with ARP2-mediated actin-dependent regulation of spinogenesis.

CONCLUSIONS

Our findings indicate that RPE and puerarin might play a substantial role in the morphological and functional maturation of brain neurons and suggest that RPE and puerarin are potentially valuable preventative therapeutics for memory-related nervous disorders.

Multi-Target Directed Donepezil-Like Ligands for Alzheimer's Disease

HIGHLIGHTS ASS234 is a MTDL compound containing a moiety from Donepezil and the propargyl group from the PF 9601N, a potent and selective MAO B inhibitor. This compound is the most advanced anti-Alzheimer agent for preclinical studies identified in our laboratory.Derived from ASS234 both multipotent donepezil-indolyl (MTDL-1) and donepezil-pyridyl hybrids (MTDL-2) were designed and evaluated as inhibitors of AChE/BuChE and both MAO isoforms. MTDL-2 showed more high affinity toward the four enzymes than MTDL-1.MTDL-3 and MTDL-4, were designed containing the N-benzylpiperidinium moiety from Donepezil, a metal- chelating 8-hydroxyquinoline group and linked to a N-propargyl core and they were pharmacologically evaluated.The presence of the cyano group in MTDL-3, enhanced binding to AChE, BuChE and MAO A. It showed antioxidant behavior and it was able to strongly complex Cu(II), Zn(II) and Fe(III).MTDL-4 showed higher affinity toward AChE, BuChE.MTDL-3 exhibited good brain penetration capacity (ADMET) and less toxicity than Donepezil. Memory deficits in scopolamine-lesioned animals were restored by MTDL-3.MTDL-3 particularly emerged as a ligand showing remarkable potential benefits for its use in AD therapy. Alzheimer's disease (AD), the most common form of adult onset dementia, is an age-related neurodegenerative disorder characterized by progressive memory loss, decline in language skills, and other cognitive impairments. Although its etiology is not completely known, several factors including deficits of acetylcholine, β-amyloid deposits, τ-protein phosphorylation, oxidative stress, and neuroinflammation are considered to play significant roles in the pathophysiology of this disease. For a long time, AD patients have been treated with acetylcholinesterase inhibitors such as donepezil (Aricept®) but with limited therapeutic success. This might be due to the complex multifactorial nature of AD, a fact that has prompted the design of new Multi-Target-Directed Ligands (MTDL) based on the "one molecule, multiple targets" paradigm. Thus, in this context, different series of novel multifunctional molecules with antioxidant, anti-amyloid, anti-inflammatory, and metal-chelating properties able to interact with multiple enzymes of therapeutic interest in AD pathology including acetylcholinesterase, butyrylcholinesterase, and monoamine oxidases A and B have been designed and assessed biologically. This review describes the multiple targets, the design rationale and an in-house MTDL library, bearing the N-benzylpiperidine motif present in donepezil, linked to different heterocyclic ring systems (indole, pyridine, or 8-hydroxyquinoline) with special emphasis on compound ASS234, an N-propargylindole derivative. The description of the in vitro biological properties of the compounds and discussion of the corresponding structure-activity-relationships allows us to highlight new issues for the identification of more efficient MTDL for use in AD therapy.

Nefiracetam attenuates post-ischemic nonconvulsive seizures in rats and protects neuronal cell death induced by veratridine and glutamate

AIMS

Stroke patients are at a high risk of developing post-ischemic seizures and cognitive impairment. Nefiracetam (NEF), a pyrrolidone derivative, has been shown to possess both anti-epileptic and cognitive-enhancing properties. In this study the anti-seizure effects of NEF were evaluated in a rat model of post-ischemic nonconvulsive seizures (NCSs). Its potential mechanisms were investigated in neuronal cell culture assays of neurotoxicity associated with ischemic brain injury and epileptogenesis.

MAIN METHODS

In the in vivo study, rats received 24h permanent middle cerebral artery occlusion. NEF was administered intravenously either at 15 min post-injury but prior to the first NCS event (30 mg/kg, pre-NCS treatment) or immediately after the first NCS occurred (30 or 60 mg/kg, post-NCS treatment). In the in vitro study, neuronal cell cultures were exposed to veratridine or glutamate and treated with NEF (1-500 nM).

KEY FINDINGS

The NEF pre-NCS treatment significantly reduced the NCS frequency and duration, whereas the higher NEF dose (60 mg/kg) was required to achieve similar effects when given after NCS occurred. The NEF treatment also dose-dependently (5-500 nM) protected against neuronal cell death induced by veratridine as measured by MTT cell viability assay, but higher doses (250-500 nM) were required against glutamate toxicity.

SIGNIFICANCE

The anti-seizure property of NEF was demonstrated in a clinically relevant rat model of post-ischemic NCS. The preferential effects of NEF against in vitro veratridine toxicity suggest the involvement of its modulation of sodium channel malfunction. Future studies are warranted to study the mechanisms of NEF against ischemic brain injury and post-ischemic seizures.

Pharmacological study on Alzheimer's drugs targeting calcium/calmodulin-dependent protein kinase II

In the brain of Alzheimer's disease patients, down-regulation of both cholinergic and glutamatergic systems have been found and is thought to play an important role in impairment of cognition, learning, and memory. Nefiracetam is a pyrrolidine-related nootropic drug exhibiting various pharmacological actions such as a cognitive-enhancing effect. The present study was undertaken to elucidate mechanisms underlying the action of nefiracetam on glutamatergic receptors and intracellular protein kinases. N-Methyl-D-aspartate (NMDA)-evoked currents were recorded from rat cortical neurons in long-term cultured primary neurons using the whole-cell patch-clamp technique. NMDA-evoked currents were greatly and reversibly potentiated by bath application of nefiracetam, resulting in a bell-shaped dose-response curve. The maximum potentiation of 170% relative to the control was produced at 10 nM. Treatment with an inhibitor of the glycine binding site of the NMDA receptor, 7-chlorokynurenic acid, at 1 µM prevented augmentation of NMDA-evoked currents by nefiracetam. In rat hippocampal CA1 slices, field excitatory postsynaptic potentials were recorded by stimulation of Schaffer collateral/commissural pathways. Nefiracetam treatment significantly enhanced long-term potentiation (LTP) with the same bell-shaped dose-response curve. Furthermore, nefiracetam-induced LTP enhancement was closely associated with calcium/calmodulin-dependent protein kinase II (CaMKII) activation with concomitant increase in phosphorylation of AMPA-type glutamate receptor subunit 1 (GluA1) (Ser-831) as a postsynaptic CaMKII substrate. In conclusion, nefiracetam enhances NMDA-receptor function through stimulation of its glycine binding site and nefiracetam-induced CaMKII activation likely contributes to improvement of cognition, learning, and memory.

Apathy following stroke

OBJECTIVE

We will review the available evidence on the frequency, clinical correlates, mechanism, and treatment of apathy following stroke.

METHODS

We have explored relevant databases (that is, PubMed, MEDLINE, and PsycINFO) using the following key words and their combinations: apathy, motivation, abulia, stroke, cerebrovascular disease, basal ganglia, prefrontal cortex, anterior cerebral infarction, and thalamus.

RESULTS

The frequency of apathy following stroke has been consistently estimated between 20% and 25%. It appears to be associated with the presence of cognitive impairment, a chronic course characterized by progressive functional decline, and with disruption of neural networks connecting the anterior cingulate gyrus, the dorsomedial frontal cortex, and the frontal pole with the ventral aspects of the caudate nucleus, the anterior and ventral globus pallidus, and the dorsomedian and intralaminar thalamic nuclei. Published treatment studies have been mostly limited to anecdotal case reports, generally using dopamine agonists or stimulant medications. Cholinesterase inhibitors and nefiracetam may significantly reduce apathetic symptoms. However, their efficacy was examined in relatively small clinical trials that require replication.

CONCLUSION

Apathy is a frequent neuropsychiatric complication of stroke that, although often associated with depression and cognitive impairment, may occur independently of both. Its presence has been consistently associated with greater functional decline. However, there is no conclusive evidence about which is the best treatment for this condition.

Nefiracetam and galantamine modulation of excitatory and inhibitory synaptic transmission via stimulation of neuronal nicotinic acetylcholine receptors in rat cortical neurons

The cholinergic and glutamatergic systems are known to be downregulated in the brain of Alzheimer's disease patients. Galantamine and nefiracetam have been shown to potentiate the phasic activity of nicotinic acetylcholine receptors (nAChRs) in the brain. Stimulation of nAChRs is also known to cause release of various neurotransmitters including glutamate and gamma-aminobutyric acid (GABA). We have previously reported that nefiracetam and galantamine potentiate the activity of nAChRs. Therefore, nefiracetam and galantamine are hypothesized to cause stimulations of the glutamate and GABA systems via stimulation of nAChRs. The present study was set out to test this hypothesis by measuring the effects of these drugs on spontaneous miniature excitatory postsynaptic currents (mEPSCs) and spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded by the whole-cell patch clamp technique from rat cortical neurons in primary cultures. Acetylcholine (ACh) at 30 nM generated a steady inward current and increased the frequency of mEPSCs and mIPSCs. Nefiracetam at 10 nM plus 30 nM ACh increased the frequency of mEPSCs and mIPSCs beyond the levels increased by ACh alone. The potentiating action of nefiracetam was abolished by dihydro-beta-erythroidine. None of these treatments affected the amplitude of mEPSCs or mIPSCs. Galantamine at 1 muM plus ACh did not significantly potentiate the frequency. Nefiracetam at 10 nM had no effect on neurons that did not respond to 30 nM ACh. It was concluded that the nefiracetam released glutamate via stimulation of the alpha4beta2 nAChRs.

Improvement of depressive behaviors by nefiracetam is associated with activation of CaM kinases in olfactory bulbectomized mice

Olfactory bulbectomized (OBX) mice exhibit depressive-like behaviors as assessed by the tail suspension test (TST) and the forced swim test (FST). Interestingly, chronic intraperitoneal administration (1 mg/kg/day) of nefiracetam (DM-9384), a prototype cognitive enhancer, significantly improved depressive-like behaviors as well as spatial reference memory assessed by Y-maze task. As previously reported (Moriguchi, S., Han, F., Nakagawasai, O., Tadano, T., Fukunaga, K., 2006. Decreased calcium/calmoculin-dependent protein kinase II and protein kinase C activities mediate impairment of hippocampal long-term potentiation in the olfactory bulbectomized mice. J. Neurochem. 97, 22-29), decreased activities of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) in the hippocampal CA1 region and amygdala were observed in OBX mice. Nefiracetam treatment (1 mg/kg/day) significantly elevated CaMKII but not ERK activities in the amygdala, prefrontal cortex and hippocampal CA1 regions. In addition, we found an elevation of cAMP response element-binding protein (CREB) phosphorylation in the amygdala and prefrontal cortex but not in the hippocampal CA1 region. Increased CREB phosphorylation was associated with activation of CaMKI and CaMKIV as well as CaMKII in these regions. Taken together, in addition to CaMKII, CaMKI and CaMKIV activation mediated by nefiracetam treatment might mediate CREB phosphorylation following chronic nefiracetam treatment, thereby eliciting an anti-depressive and cognition-enhancing effect on OBX mice.

CaM kinase II and protein kinase C activations mediate enhancement of long-term potentiation by nefiracetam in the rat hippocampal CA1 region

Nefiracetam is a pyrrolidine-related nootropic drug exhibiting various pharmacological actions such as cognitive-enhancing effect. We previously showed that nefiracetam potentiates NMDA-induced currents in cultured rat cortical neurons. To address questions whether nefiracetam affects NMDA receptor-dependent synaptic plasticity in the hippocampus, we assessed effects of nefiracetam on NMDA receptor-dependent long-term potentiation (LTP) by electrophysiology and LTP-induced phosphorylation of synaptic proteins by immunoblotting analysis. Nefiracetam treatment at 1-1000 nM increased the slope of fEPSPs in a dose-dependent manner. The enhancement was associated with increased phosphorylation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor through activation of calcium/calmodulin-dependent protein kinase II (CaMKII) without affecting synapsin I phosphorylation. In addition, nefiracetam treatment increased PKCalpha activity in a bell-shaped dose-response curve which peaked at 10 nM, thereby increasing phosphorylation of myristoylated alanine-rich protein kinase C substrate and NMDA receptor. Nefiracetam treatment did not affect protein kinase A activity. Consistent with the bell-shaped PKCalpha activation, nefiracetam treatment enhanced LTP in the rat hippocampal CA1 region with the same bell-shaped dose-response curve. Furthermore, nefiracetam-induced LTP enhancement was closely associated with CaMKII and PKCalpha activation with concomitant increases in phosphorylation of their endogenous substrates except for synapsin I. These results suggest that nefiracetam potentiates AMPA receptor-mediated fEPSPs through CaMKII activation and enhances NMDA receptor-dependent LTP through potentiation of the post-synaptic CaMKII and protein kinase C activities. Together with potentiation of nicotinic acetylcholine receptor function, nefiracetam-enhanced AMPA and NMDA receptor functions likely contribute to improvement of cognitive function.

Multifunctional neuroprotective drugs targeting monoamine oxidase inhibition, iron chelation, adenosine receptors, and cholinergic and glutamatergic action for neurodegenerative diseases

A new paradigm is emerging in the targeting of multiple disease aetiologies that collectively lead to neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease, post-stroke neurodegeneration and others. This paradigm challenges the widely held assumption that 'silver bullet' agents are superior to 'dirty drugs' when it comes to drug therapy. Accumulating evidence in the literature suggests that many neurodegenerative diseases have multiple mechanisms in their aetiologies, thus suggesting that a drug with at least two mechanisms of action targeted at multiple aetiologies of the same disease may offer more therapeutic benefit in certain disorders compared with a drug that only targets one disease aetiology. This review offers a synopsis of therapeutic strategies and novel investigative drugs developed in the authors' own and other laboratories that modulate multiple disease targets associated with neurodegenerative diseases.

Nefiracetam potentiates N-methyl-D-aspartate (NMDA) receptor function via protein kinase C activation and reduces magnesium block of NMDA receptor

Nicotinic acetylcholine receptors and N-methyl-D-aspartate (NMDA) receptors are known to be down-regulated in the brain of Alzheimer's disease patients. We have previously demonstrated that the nootropic drug nefiracetam potentiates the activity of both nicotinic acetylcholine and NMDA receptors and that nefiracetam modulates the glycine binding site of the NMDA receptor. Because the NMDA receptor is also modulated by Mg2+ and protein kinases, we studied their roles in nefiracetam action on the NMDA receptor by the whole-cell patch-clamp technique and immunoblotting analysis using rat cortical or hippocampal neurons in primary culture. The nefiracetam potentiation of NMDA currents was inhibited by the protein kinase C (PKC) inhibitor chelerythrine, but not by the protein kinase A (PKA) inhibitor N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H89). In immunoblotting analysis, nefiracetam treatment increased the PKCalpha activity with a bell-shaped dose-response relationship peaking at 10 nM, thereby increasing phosphorylation of PKC substrate and NMDA receptor. Such an increase in PKCalpha-mediated phosphorylation was prevented by chelerythine. Nefiracetam treatment did not affect the PKA activity. Analysis of the current-voltage relationships revealed that nefiracetam at 10 nM largely eliminated voltage-dependent Mg2+ block and that this action of nefiracetam was sensitive to PKC inhibition. It was concluded that nefiracetam potentiated NMDA currents not by acting as a partial agonist but by interacting with PKC, allosterically enhancing glycine binding, and attenuating voltage-dependent Mg2+ block.

Multifunctional drugs with different CNS targets for neuropsychiatric disorders

The multiple disease etiologies that lead to neuropsychiatric disorders, such as Parkinson's and Alzheimer's disease, amyotrophic lateral sclerosis, Huntington disease, schizophrenia, depressive illness and stroke, offer significant challenges to drug discovery efforts aimed at preventing or even reversing the progression of these disorders. Transcriptomic tools and proteomic profiling have clearly indicated that such diseases are multifactorial in origin. Further, they are thought to be initiated by a cascade of molecular events that involve several neurotransmitter systems. In response to this complexity, a new paradigm has recently emerged that challenges the widely held assumption that 'silver bullet' agents are superior to 'dirty drugs' in therapeutic approaches aimed at the prevention or treatment of neuropsychiatric diseases. A similar pattern of drug development has occurred in strategies for the treatment of cancer, AIDS and cardiovascular diseases. In this review, we offer an overview of therapeutic strategies and novel investigative drugs discovered or developed in our own and other laboratories, that address multiple CNS etiological targets associated with an array of neuropsychiatric disorders.

Modulation of N-Methyl-d-aspartate Receptors by Donepezil in Rat Cortical Neurons

Nicotinic acetylcholine receptors and N-methyl-D-aspartate (NMDA) receptors are known to be down-regulated in the brain of patients with Alzheimer's disease. It was previously shown that the nootropic drugs nefiracetam and galantamine potentiate the activity of both nicotinic and NMDA receptors. We hypothesized that donepezil, a nootropic with a potent anticholinesterase activity, might also affect the NMDA system. NMDA-induced currents were recorded from rat cortical neurons in primary culture using the whole-cell patch-clamp technique at a holding potential of -70 mV in Mg2+-free solutions. In multipolar neurons, NMDA currents were decreased by bath and U-tube applications of 1 to 10 microM donepezil but were increased by 30 to 100 microM donepezil. Donepezil suppression occurred in a manner independent of NMDA concentrations ranging from 3 to 1000 microM. The donepezil suppression of NMDA currents was prevented by inhibition of protein kinase C (PKC) but unaffected by protein kinase A (PKA) and G proteins. In bipolar neurons, however, NMDA currents were potently augmented by bath and U-tube applications of 0.01 to 100 microM donepezil. Donepezil potentiation occurred at high NMDA concentrations that evoked the saturating responses and in a manner independent of NMDA concentrations ranging from 3 to 1000 microM. The potentiation of NMDA currents by donepezil was decreased by inhibition of PKC and abolished by modulation of G proteins but not by PKA inhibition. It was concluded that donepezil at low therapeutic concentrations (0.01-1 microM) potentiated the activity of the NMDA system and that this action together with cholinesterase inhibition would contribute to the improvement of learning, memory, and cognition in patients with Alzheimer's disease.

Mechanism of Action of Galantamine onN-Methyl-d-Aspartate Receptors in Rat Cortical Neurons

Galantamine, a new Alzheimer's drug approved in the United States, is known to inhibit acetylcholinesterase and potentiate acetylcholine-induced currents in brain neurons. However, because both cholinergic and N-methyl-D-aspartate (NMDA) systems are down-regulated in the brain of Alzheimer's patients, we studied the effects of galantamine on NMDA receptors. NMDA-induced whole-cell currents were recorded from the rat multipolar cortical neurons in primary culture. NMDA currents recorded in Mg2+-free media without addition of glycine were reversibly potentiated by bath and U-tube applications of galantamine at 10 to 10,000 nM, showing a bell-shaped dose-response relationship. However, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate currents were not affected by galantamine. The maximum potentiation of NMDA currents to approximately 130% of the control was obtained at 1 microM galantamine. The potentiation was due to a shift of the NMDA dose-response curve in the direction of lower NMDA concentrations. Glycine at 1 to 3000 nM enhanced NMDA currents, and potentiation by 1 microM galantamine and 1 to 300 nM glycine was additive. The glycine site antagonist 7-chlorokynurenic acid did not prevent the galantamine action. These results suggested that galantamine did not interact with the glycine binding site. Experiments with various concentrations of Mg2+ indicated that galantamine did not affect the Mg2+ blocking site of the NMDA receptor. PKC was involved in galantamine potentiation of NMDA currents, but protein kinase A, Gi/Go proteins, and Gs proteins were not involved. Potentiation of the activity of NMDA receptors is deemed partially responsible for the improvement of cognition, learning, and memory in Alzheimer's patients.

Unique mechanism of action of Alzheimer's drugs on brain nicotinic acetylcholine receptors and NMDA receptors

While a variety of hypotheses have been proposed for the cause of Alzheimer's disease, our knowledge is far from complete to explain the disease making it difficult to develop the methods for treatment. In the brain of Alzheimer's patients, both neuronal nicotinic acetylcholine (nACh) receptors and NMDA receptors are known to be down-regulated. Thus four anticholinesterases have been developed and approved for the treatment in the U.S.A. However, these are not ideal drugs considering their side effects and limited effectiveness. Nefiracetam is being developed for the treatment of Alzheimer's and other patients with dementia, and has unique actions in potentiating the activity of both nACh and NMDA receptors as demonstrated by in vitro patch clamp experiments using rat cortical neurons in primary culture. Nefiracetam potentiated alpha4beta2-like ACh- and NMDA-induced currents at nanomolar concentrations forming bell-shaped dose-response curves with the maximum potentiation occurring at 1 and 10 nM, respectively. Nefiracetam potentiated nACh receptor currents via G(s) proteins, but not G(i)/G(o) proteins, PKA or PKC. Nefiracetam potentiation of NMDA currents occurred via interactions with the glycine binding site of the NMDA receptor. The nefiracetam potentiation of both nACh and NMDA receptors in a potent and efficacious manner is deemed responsible for its cognitive enhancing action.

Mechanisms of Action of Cognitive Enhancers on Neuroreceptors

No strategies for curing Alzheimer's disease have been developed yet as we do not know the exact cause of the disease. The only therapy that is available for patients is symptomatic treatment. Since Alzheimer's disease is associated with downregulation of the cholinergic system in the brain, its stimulation is expected to improve the patients' cognition, learning, and memory. Four anticholinesterases have been approved in the U.S.A. for the treatment of Alzheimer's disease patients. However, because of the inhibition of cholinesterases, these drugs have side effects and their effectiveness does not last long. Thus new approaches are needed. One approach is to stimulate directly nicotinic acetylcholine (nACh) receptors in the brain, and another is to stimulate NMDA receptors which are also known to be downregulated in Alzheimer's patients. Nefiracetam has been shown to potentiate ACh currents in the alpha4beta2 receptor of rat cortical neurons with a bell-shaped dose-response relationship and the maximum effect at 1 nM. This effect was exerted via G(s) proteins. The alpha7 receptor was almost unaffected by nefiracetam. Nefiracetam also potentiated NMDA currents with the maximum effect at 10 nM via interaction with the glycine-binding site of the receptor. Galantamine had a moderate potentiating effect on the alpha4beta2 receptor and potentiated NMDA currents with the maximum effect at 1 microM. However, galantamine did not interact with the glycine-binding site. Donepezil, a potent anticholinesterase, also potentiated NMDA currents at 1-10000 nM. In conclusion, these three drugs potentiate the activity not only of the cholinergic system but also of the NMDA system, thereby stimulating the downregulated nACh receptors and NMDA receptors to improve patients' learning, cognition, and memory.