Galantamine prevents long-lasting suppression of excitatory synaptic transmission in CA1 pyramidal neurons of soman-challenged guinea pigs.

Galantamine, a drug currently approved for the treatment of Alzheimer's disease, has recently emerged as an effective pretreatment against the acute toxicity and delayed cognitive deficits induced by organophosphorus (OP) nerve agents, including soman. Since cognitive deficits can result from impaired glutamatergic transmission in the hippocampus, the present study was designed to test the hypothesis that hippocampal glutamatergic transmission declines following an acute exposure to soman and that this effect can be prevented by galantamine. To test this hypothesis, spontaneous excitatory postsynaptic currents (EPSCs) were recorded from CA1 pyramidal neurons in hippocampal slices obtained at 1h, 24h, or 6-9 days after guinea pigs were injected with: (i) 1×LD50 soman (26.3µg/kg, s.c.); (ii) galantamine (8mg/kg, i.m.) followed 30min later by 1×LD50 soman, (iii) galantamine (8mg/kg, i.m.), or (iv) saline (0.5ml/kg, i.m.). In soman-injected guinea pigs that were not pretreated with galantamine, the frequency of EPSCs was significantly lower than that recorded from saline-injected animals. There was no correlation between the severity of soman-induced acute toxicity and the magnitude of soman-induced reduction of EPSC frequency. Pretreatment with galantamine prevented the reduction of EPSC frequency observed at 6-9 days after the soman challenge. Prevention of soman-induced long-lasting reduction of hippocampal glutamatergic synaptic transmission may be an important determinant of the ability of galantamine to counter cognitive deficits that develop long after an acute exposure to the nerve agent.

Short-term effects of pilocarpine on rat hippocampal neurons in culture.

PURPOSE: Status epilepticus (SE) has been considered an epileptogenic factor in humans. In the pilocarpine (PILO) model, after a brief period marked by SE, the rats exhibit recurrent spontaneous seizures, mimicking the clinical features of temporal lobe epilepsy. The aim of our study was to identify the molecular actions of PILO that could account for its ability to induce SE. METHODS: Whole-cell mode of the patch-clamp technique was applied to cultured hippocampal neurons (2-3 weeks old) in the absence and in the presence of PILO (1-10 microM), to study the spontaneous activity, the evoked, and the miniature postsynaptic currents. The postsynaptic currents were isolated pharmacologically. RESULTS: PILO (1 and 10 microM) caused an initial increase followed by a decrease in the frequency of spontaneous activity. The increase in the frequency of excitatory postsynaptic currents (EPSCs) and inhibitory PSCs (IPSCs) was blocked by atropine (1 microM), indicating that this effect is mediated through muscarinic receptors. PILO also promoted a brief increase of the amplitude of IPSCs indirectly evoked by stimulation of a neuron synaptically connected to the neuron under study. Conversely, PILO promoted a sustained increase on the amplitude of electrically evoked EPSCs. In presence of tetrodotoxin (TTX; 300 nM), PILO (1 microM) increased the frequency of miniature EPSCs and IPSCs without changing their amplitude during the first 3 min of application. CONCLUSIONS: These results indicate that PILO acting through muscarinic receptor causes an imbalance between excitatory and inhibitory transmission that can result in the generation of SE observed in animals acutely treated with PILO.

Nicotinic acetylcholine receptor alpha7 and alpha4beta2 subtypes differentially control GABAergic input to CA1 neurons in rat hippocampus.

The hippocampus, a limbic brain region involved in the encoding and retrieval of memory, has a well-defined structural network assembled from excitatory principal neurons and inhibitory interneurons. Because the GABAergic interneurons form synapses onto both pyramidal neurons and interneurons, the activation of nicotinic acetylcholine receptors (nAChRs) present on certain interneurons could induce either inhibition or disinhibition in the hippocampal circuitry. To understand the role of nAChRs in controlling synaptic transmission in the hippocampus, we evaluated the magnitude of nAChR-modulated GABAergic postsynaptic currents (PSCs) in pyramidal neurons and various interneurons of the CA1 region. Using whole cell patch-clamp recording and post hoc identification of neuronal types in rat hippocampal slices, we show that brief (12-s) nAChR activation by ACh (1 mM) or choline (10 mM) enhances the frequency of GABAergic PSCs in both pyramidal neurons and CA1 interneurons. The magnitude of alpha7 nAChR-mediated GABAergic inhibition, as assessed by the net charge of choline-induced PSCs, was highest in stratum lacunosum moleculare interneurons followed by pyramidal neurons and s. radiatum interneurons. In contrast, the magnitude of alpha4beta2 nAChR-mediated GABAergic inhibition, as assessed by the difference between the net charge of PSCs induced by ACh and choline, was highest in pyramidal neurons followed by s. lacunosum moleculare and s. radiatum interneurons. The present results suggest that cholinergic cues transmitted via specific subtypes of nAChRs modify the synaptic function in the hippocampus by inducing a differential degree of GABAergic inhibition in the target neurons.

Human bronchial epithelial and endothelial cells express alpha7 nicotinic acetylcholine receptors.

The epithelial or endothelial cells that line the human bronchi and the aorta express nicotinic acetylcholine receptors (nAChRs) of alpha3 subtypes. We report here that human bronchial epithelial cells (BEC) and aortic endothelial cells (AEC) express also the nAChR alpha7 subunit, which forms functional nAChRs. Polymerase chain reaction and in situ hybridization experiments detected alpha7 subunit mRNA in cultured human BEC and AEC and in sections of rat trachea. The binding of radiolabeled alpha-bungarotoxin revealed a few thousand binding sites per cell in cultured human BEC and human and bovine AEC. Western blot and immunohistochemistry experiments demonstrated that cultured BEC and AEC express a protein(s) recognized by anti-alpha7 antibodies. Whole-cell patch-clamp studies of cultured human BEC demonstrated the presence of fast-desensitizing currents activated by choline and nicotine that were blocked reversibly by methyllycaconitine (1 nM) and irreversibly by alpha-bungarotoxin (100 nM), consistent with the expression of functional alpha7 nAChRs. In some cells, choline activated also slowly decaying currents, confirming previous reports that BEC express functional alpha3beta4 nAChRs. Exposure of cultured BEC to nicotine (1 microM) for 3 days up-regulated functional alpha7 and alpha3 nAChRs, as indicated by the increased number of cells responding to acetylcholine and choline, with both fast-desensitizing currents, which were blocked irreversibly by alpha-bungarotoxin, and with slowly desensitizing currents, which are alpha-bungarotoxin-insensitive currents. The presence of alpha7 nAChRs in BEC and AEC suggests that some toxic effects of tobacco smoke could be mediated through these nicotine-sensitive receptors.

Modulation of nicotinic receptor activity in the central nervous system: a novel approach to the treatment of Alzheimer disease.

Impaired cholinergic function in the central nervous system is an early feature of Alzheimer disease (AD). Currently, cholinergic deficit is usually corrected by increasing the amount of acetylcholine in the synapse by inhibiting acetylcholinesterase (AChE). One of the most consistent cholinergic deficits in AD is the reduced expression of nicotinic acetylcholine receptors (nAChR) in the brain. Since these receptors are essential for learning and memory, restoring nicotinic cholinergic function is a promising approach to treating AD. Allosteric modulation of nAChR is a novel approach, which circumvents development of tolerance through long-term use of conventional nicotinic agonists. Allosteric modulators interact with receptor-binding sites distinct from those capable of recognizing the natural agonist. Positive allosteric modulation of nAChR activity has no effect on conductance of single channels; instead, by facilitating channel opening, it potentiates responses evoked by the interaction of the natural agonist with presynaptic and postsynaptic nAChR. Allosteric modulation of nAChR activity could therefore potentially produce a significant benefit in AD. One such allosteric modulator is galantamine. In addition to increasing nAChR activity, galantamine also inhibits AChE. This novel, dual mechanism of action distinguishes galantamine from many other AChE inhibitors. Galantamine has been shown to improve cognitive and daily function for at least 6 months in placebo-controlled trials, and to maintain these functions at baseline levels for at least 12 months in a 6-month open-label extension study. Galantamine has positive effects on nAChR expression, which are likely to contribute to its sustained efficacy in the treatment of AD patients.

A novel thienylhydrazone, (2-thienylidene)3,4-methylenedioxybenzoylhydrazine, increases inotropism and decreases fatigue of skeletal muscle.

This study was designed to investigate the effects on single skeletal muscle fibers of a novel thienylhydrazone, referred to as LASSBio-294, which is a bioisoster of pyridazinone compounds that inhibit the cyclic AMP-specific phosphodiesterase (PDE) 4. Twitch and fatigue were analyzed in single skeletal muscle fibers isolated from either the semitendinous or the tibialis anterior muscles dissected from the frog Rana pipiens. LASSBio-294 (12.5-100 microM) increased twitch tension, accelerated the maximal rate of tension decay during relaxation, and had very little effect in the maximal rate of tension development of muscle fibers directly stimulated at < or =30 Hz. The positive inotropic effect of LASSBio-294 developed slowly, reaching its maximum at 40 min and was inversely proportional to the frequency of stimulation, becoming negligible at 60 and 90 Hz. The concentration-response relationship for LASSBio-294-induced potentiation of twitch tension was bell-shaped, with maximal effect occurring at 25 microM. In addition, LASSBio-294 reduced development of fatigue induced by tetanic stimulation of the muscle fibers and reduced the time needed for 80% prefatigue tension recovery after fatigue had developed to 50% of the maximal pretetanic force. These effects of LASSBio-294 can be fully explained by stimulation of the sarcoplasmic reticulum Ca2+ pump and could be ascribed to an increase in cellular levels of cyclic AMP due to PDE inhibition. The novel thienylhydrazone LASSBio-294 may be useful for treatment of patients suffering from conditions in which muscle fatigue is a debilitating symptom (e.g., chronic heart failure).

The brain metabolite kynurenic acid inhibits alpha7 nicotinic receptor activity and increases non-alpha7 nicotinic receptor expression: physiopathological implications.

The tryptophan metabolite kynurenic acid (KYNA) has long been recognized as an NMDA receptor antagonist. Here, interactions between KYNA and the nicotinic system in the brain were investigated using the patch-clamp technique and HPLC. In the electrophysiological studies, agonists were delivered via a U-shaped tube, and KYNA was applied in admixture with agonists and via the background perfusion. Exposure (>/=4 min) of cultured hippocampal neurons to KYNA (>/=100 nm) inhibited activation of somatodendritic alpha7 nAChRs; the IC(50) for KYNA was approximately 7 microm. The inhibition of alpha7 nAChRs was noncompetitive with respect to the agonist and voltage independent. The slow onset of this effect could not be accounted for by an intracellular action because KYNA (1 mm) in the pipette solution had no effect on alpha7 nAChR activity. KYNA also blocked the activity of preterminal/presynaptic alpha7 nAChRs in hippocampal neurons in cultures and in slices. NMDA receptors were less sensitive than alpha7 nAChRs to KYNA. The IC(50) values for KYNA-induced blockade of NMDA receptors in the absence and presence of glycine (10 microm) were approximately 15 and 235 microm, respectively. Prolonged (3 d) exposure of cultured hippocampal neurons to KYNA increased their nicotinic sensitivity, apparently by enhancing alpha4beta2 nAChR expression. Furthermore, as determined by HPLC with fluorescence detection, repeated systemic treatment of rats with nicotine caused a transient reduction followed by an increase in brain KYNA levels. These results demonstrate that nAChRs are targets for KYNA and suggest a functionally significant cross talk between the nicotinic cholinergic system and the kynurenine pathway in the brain.

A galantamina no tratamento da doença de Alzheimer's: da pesquisa ao paciente / Galantamine in the treatment of Alzheimer disease: from research to the patient

A doença de Alzheimer (DA) é uma doença neurodegenerativa progressiva, que representa um sério problema para saúde pública, particularmente nos países industrializados, onde a população de idosos vem crescendo de modo acentuado. O tratamento da doença, além de sintomático, visa retardar a progressão das deterioração mental dos pacientes. Recentemente, com bases no seu mecanismo de ação duplo, a galantamina foi adicionada ao arsenal terapêutico para tratamento da DA. Além de atuar como um anticolinesterásico fraco, a galantamina, associando-se diretamente a um sítio de ligação localizado nas subunidades alfa dos receptores nicotínicos, também age como "ligante potencializador alostérico" (LPA/APL) da atividade desses receptores. Esta revisão tem como objetivo central discutir as bases fisiopatológicas que levaram à introdução da galantamina na clínica para o tratamento de pacientes portadores da DA. Para tanto, apresenta uma abordagem geral dos clínicos da doença, incluído não somente uma descrição breve dos sintomas e diagnósticos da doença, como também um resumo dos processos celulares e moleculares alterados durante o curso da mesma. Ênfase é dada à hipótese colinérgica, que assume que a progressão da doença está associada à hipoatividade das funções colinérgicas, especialmente daquelas mediadas pelos receptores nicotínicos neuronais do cérebro. Estudos recentes indicam que pode haver uma relação causal entre a hipofunção colinérgica nicotínica no cérebro e a doença de Alzheimer. É com base nessa hipótese e na descoberta laboratorial da ação LPA/APL da galantamina sobre os receptores nicotínicos, que este alcalóide, originalmente isolado do bulbo da Galanthus nivalis, foi recentemente introduzido em vários países para o tratamento da DA

Allosteric sensitization of nicotinic receptors by galantamine, a new treatment strategy for Alzheimer's disease.

Cholinesterase inhibitors are the only approved drug treatment for patients with mild to moderately severe Alzheimer's disease. Interestingly, the clinical potency of these drugs does not correlate well with their activity as cholinesterase inhibitors, nor is their action as short lived as would be expected from purely symptomatic treatment. A few cholinesterase inhibitors, including galantamine, produce beneficial effects even after drug treatment has been terminated. These effects assume modes of action other than mere esterase inhibition and are capable of inducing systemic changes. We have recently discovered a mechanism that could account, at least in part, for the above-mentioned unexpected properties of some cholinesterase inhibitors. We have found that a subgroup of cholinesterase inhibitors, including galantamine but excluding tacrine, directly interacts with nicotinic acetylcholine receptors. These compounds, named allosterically potentiating ligands, sensitize nicotinic receptors by increasing the probability of channel opening induced by acetylcholine and nicotinic agonists and by slowing down receptor desensitization. The allosterically potentiating ligand action, which is not necessarily associated with cholinesterase inhibition, has been demonstrated by whole-cell patch-clamp recordings to occur in natural murine and human neurons and in murine and human cell lines expressing various subtypes of neuronal nicotinic acetylcholine receptors.

Synthesis and preliminary pharmacological evaluation of coumestans with different patterns of oxygenation.

Five coumestans with different patterns of oxygenation in rings A and D were synthesized from resorcinol and aromatic aldehydes, and screened for their antimyotoxic activity. The most potent compound (2b, IC50 = 1 microM) was selected for study of its pharmacological profile.

Choline and acetylcholine have similar kinetic properties of activation and desensitization on the alpha7 nicotinic receptors in rat hippocampal neurons.

The alpha7-type nicotinic acetylcholine receptor (nAChR) was recently found to be both fully activated and desensitized by choline, in addition to ACh. In order to understand the combined effects of the two agonists on alpha7 nAChR-mediated neuronal signaling, the kinetics of the receptor-channel's interaction with ACh and choline was examined. To this end, whole-cell and single-channel currents evoked by fast-switching pulses of the agonists were recorded in rat hippocampal neurons in culture. Currents evoked by equieffective concentrations of choline and ACh were very similar, except that choline-evoked currents decayed more quickly to the baseline after removal of the agonist, and that recovery from desensitization was faster with choline. The conductance of channels activated by choline and ACh was 91.5+/-8.5 and 82.9+/-11.6 pS, respectively. The mean apparent channel open times were close to 100 micros, with both agonists. After a 4-s exposure to concentrations up to 80 microM ACh or 600 microM choline, the extent of desensitization and the cumulative charge flow carried by the channels increased in the same proportion, until reaching a maximum. At higher concentrations of either agonist, the cumulative charge started decreasing with concentration, reflecting further desensitization. Kinetic modeling suggested that alpha7 nAChRs have at least two non-equivalent paths to desensitized states, and that choline dissociates faster than ACh from the receptor. Our results established that the main difference between choline and ACh is of affinity, and support the concept that the switching of endogenous agonist may change the desensitization-resensitization dynamics of alpha7 nAChRs.

Nicotine at concentrations found in cigarette smokers activates and desensitizes nicotinic acetylcholine receptors in CA1 interneurons of rat hippocampus.

Behavioral effects of cigarette smoking are attributed to the interactions of nicotine with brain nicotinic acetylcholine receptors (nAChRs). However, the mechanisms by which nAChR function in developing and mature brain is affected by a smoker's level of nicotine (50-500 nM) remain unclear. Thus, the objective of this study was to determine the concentration- and time-dependent effects of nicotine on alpha7 and alpha4beta2 nAChRs, the two major brain subtypes, natively expressed in CA1 interneurons of rat hippocampal slices. Only at concentrations > or =5 microM did nicotine (applied for 6-60 s) elicit action potentials or measurable whole-cell currents (EC(50)=158 microM) in stratum radiatum interneurons that express alpha7 nAChRs. Continuous exposure for 10-15 min of the neurons to nicotine (0.5-2.5 microM) inhibited alpha7 nAChR-mediated currents (IC(50)=640 nM) evoked by choline (10 mM). Nicotine (> or =0.125 microM) applied to the neurons for 1-5 min induced slowly desensitizing whole-cell currents (EC(50)=3.2 microM) in stratum lacunosum moleculare interneurons; this effect was mediated by alpha4beta2 nAChRs. Also via activation of alpha4beta2 nAChRs, nicotine (0.125-0.5 microM) increased the frequency and amplitude of GABAergic postsynaptic currents (PSCs) in stratum radiatum interneurons. However, exposure of the neurons for 10-15 min to nicotine (0.25-0.5 microM) resulted in desensitization of alpha4beta2 nAChRs. It is suggested that nanomolar concentrations of nicotine after acute intake suppress inhibitory inputs to pyramidal cells through a disinhibitory mechanism involving activation of alpha4beta2 nAChRs and desensitization of alpha7 nAChRs, and after chronic intake leads to up-regulation of both receptor subtypes via desensitization. These findings have direct implications to the actions of nicotine in cigarette smokers.

The opioid antagonist naltrexone inhibits activity and alters expression of alpha7 and alpha4beta2 nicotinic receptors in hippocampal neurons: implications for smoking cessation programs.

This study was designed to investigate whether naltrexone, an opioid antagonist that has been evaluated clinically as a co-adjuvant in smoking cessation programs, affects function and expression of neuronal nicotinic receptors (nAChRs). Whole-cell current recordings from rat hippocampal neurons in culture and in slices demonstrated that alpha7 nAChRs can be inhibited non-competitively by naltrexone (IC(50) approximately 25 microM). The voltage dependence of the effect suggested that naltrexone acts as an open-channel blocker of alpha7 nAChRs. Naltrexone also inhibited activation of alpha4beta2 nAChRs in hippocampal neurons; however its IC(50) was higher ( approximately 141 microM). At a concentration as high as 300 microM (which is sufficient to block by 100% and 70% the activity of alpha7 and alpha4beta2 nAChRs, respectively), naltrexone had no effect on kainate and AMPA receptors, blocked by no more than 20% the activity of NMDA and glycine receptors, and reduced by 35% the activity of GABA(A) receptors. A 3-day exposure of cultured hippocampal neurons to naltrexone (30 microM) or nicotine (10 microM, a concentration that fully desensitized alpha7 nAChRs) resulted in a 2-fold increase in the average amplitude of alpha7 nAChR-subserved currents. Naltrexone did not augment the maximal up-regulation of alpha7 nAChRs induced by nicotine, indicating that both drugs act via a common mechanism. In addition to increasing alpha7 nAChRs-mediated responses per neuron, nicotine increased the number of neurons expressing functional non-alpha7 nAChRs (probably alpha4beta2 nAChRs); this effect was blocked by naltrexone (0.3 and 30 microM). Therefore, naltrexone may affect dependence on cigarette smoking by differentially altering function and expression of alpha7 and alpha4beta2 nAChRs in the central nervous system.

Ca(2+)-sensitive inhibition by Pb(2+) of alpha7-containing nicotinic acetylcholine receptors in hippocampal neurons.

In the present study the patch-clamp technique was applied to cultured hippocampal neurons to determine the kinetics as well as the agonist concentration- and Ca(2+)-dependence of Pb(2+)-induced inhibition of alpha7 nicotinic receptors (nAChRs). Evidence is provided that more than two-thirds of the inhibition by Pb(2+) (3-30 microM) of alpha7 nAChR-mediated whole-cell currents (referred to as type IA currents) develops rapidly and is fully reversible upon washing. The estimated values for tau(onset) and tau(recovery) were 165 and 240 ms, respectively. The magnitude of the effect of Pb(2+) was the same regardless of whether acetylcholine or choline was the agonist. Pre-exposure of the neurons for 800 ms to Pb(2+) (30 microM) decreased the amplitude and accelerated the decay phase of currents evoked by moderate to high agonist concentrations. In contrast, only the amplitude of currents evoked by low agonist concentrations was reduced when the neurons were exposed simultaneously to Pb(2+) and the agonists. Taken together with the findings that Pb(2+) reduces the frequency of opening and the mean open time of alpha7 nAChR channels, these data suggest that Pb(2+) accelerates the rate of receptor desensitization. An additional reduction of type IA current amplitudes occurred after 2-min exposure of the neurons to Pb(2+). This effect was not reversible upon washing of the neurons and was most likely due to an intracellular action of Pb(2+). Pb(2+)-induced inhibition of alpha7 nAChRs, which was hindered by the enhancement of extracellular Ca(2+) concentrations, may contribute to the neurotoxicity of the heavy metal.

Neuronal nicotinic receptors in synaptic functions in humans and rats: physiological and clinical relevance.

The present report describes the participation of nicotinic receptors (nAChRs) in controlling the excitability of local neuronal circuitries in the rat hippocampus and in the human cerebral cortex. The patch-clamp technique was used to record responses triggered by the non-selective agonist ACh and the alpha7-nAChR-selective agonist choline in interneurons of human cerebral cortical and rat hippocampal slices. Evidence is provided that functional alpha7- and alpha4beta2-like nAChRs are present on somatodendritic and/or preterminal/terminal regions of interneurons in the CA1 field of the rat hippocampus and in the human cerebral cortex and that activation of the different nAChR subtypes present in the preterminal/terminal areas of the interneurons triggers the tetrodotoxin-sensitive release of GABA. Modulation by nAChRs of GABAergic transmission, which can result either in inhibition or disinhibition of pyramidal neurons, depends both on the receptor subtype present in the interneurons and on the agonist acting upon these receptors. Not only do alpha7 nAChRs desensitize faster than alpha4beta2 nAChRs, but also alpha7 nAChR desensitization induced by ACh lasts longer than that induced by choline. These mechanisms, which appear to be retained across species, might explain the involvement of nAChRs in cognitive functions and in such neurological disorders as Alzheimer's disease and schizophrenia.

Allosterically potentiating ligands of nicotinic receptors as a treatment strategy for Alzheimer's disease.

One of the most prominent cholinergic deficit in Alzheimer's disease (AD) is the reduced number of nicotinic acetylcholine receptors (nAChR) in the hippocampus and cortex of AD patients, as compared to age-matched controls. This deficit results in reduced nicotinic cholinergic excitation which may not only impair postsynaptic depolarization but also presynaptic neurotransmitter release and Ca2+-dependent intracellular signaling, including transcriptional activity. Presently, the most common approach to correct the nicotinic cholinergic deficit in AD is the application of cholinesterase inhibitors. Due to the resulting increase in synaptic acetylcholine levels, both in concentration and time, additional nAChR molecules, e.g. those more distant from the ACh release sites, could be activated. As an obvious disadvantage, this approach affects cholinergic neurotransmission as a whole, including muscarinic neurotransmission. As a novel and alternative approach, a treatment strategy which exclusively targets nicotinic receptors is suggested. The strategy is based on a group of modulating ligands of nicotinic receptors, named allosterically potentiating ligands (APL), which increase the probability of channel opening induced by ACh and nicotinic agonists, and in addition decrease receptor desensitization. The action of APL on nicotinic receptors is reminiscent of that of benzodiazepines on GABA(A) receptors and of that of glycine on the NMDA-subtype of glutamate receptor. Representative nicotinic APL are the plant alkaloids physostigmine, galanthamine and codeine, and the neurotransmitter serotonin (5HT). The potentiating effect of APL on nicotinic neurotransmission has been shown by whole-cell patch-clamp studies in natural murine and human neurons, and in murine and human cell lines expressing various subtypes of neuronal nAChR.

alpha7 nicotinic acetylcholine receptors and modulation of gabaergic synaptic transmission in the hippocampus.

The present report provides new findings regarding modulation of gamma-aminobutyric acid (GABA) transmission by alpha7 nicotinic receptor activity in CA1 interneurons of rat hippocampal slices. Recordings were obtained from tight-seal cell-attached patches of the CA1 interneurons, and agonists were delivered to the neurons via a modified U-tube. Application for 6 s of the alpha7 nicotinic receptor-selective agonist choline (> or =1 mM) to all CA1 interneurons tested triggered action potentials that were detected as fast current transients. The activity triggered by choline terminated well before the end of the agonist pulse, was blocked by the alpha7 nicotinic receptor antagonist methyllycaconitine (50 nM) and was concentration dependent; the higher the concentration of choline the higher the frequency of events and the shorter the delay for detection of the first event. In 40% of the neurons tested, choline-triggered action potentials decreased in amplitude progressively until no more events could be detected despite the presence of the agonist. Primarily, this finding could be explained by Na(+)-channel inactivation associated with membrane depolarization induced by alpha7 nicotinic receptor activation. In 60% of the neurons, the amplitude of choline-induced action potentials was sustained at the intial level, but again the activity did not last as long as the agonist pulse, in this case apparently because of agonist-induced receptor desensitization. These results altogether demonstrate that agonists interacting with alpha7 nicotinic receptors, including the natural transmitter acetylcholine and its metabolite choline, influence GABAergic transmission, not only by activating these receptors, but also by controlling the rate of Na(+)-channel inactivation and/or by inducing receptor desensitization.

Allosteric modulation of nicotinic acetylcholine receptors as a treatment strategy for Alzheimer's disease.

The basic symptoms of Alzheimer's dementia, i.e., a loss in cognitive function, are due to impaired nicotinic cholinergic neurotransmission. To compensate for this impairment by drug treatment, blockers of the acetylcholine-degrading enzyme acetylcholinesterase are applied, even though this approach obviously is prone to many side-effects, including those of muscarinic nature. We have recently described a novel class of nicotinic acetylcholine receptor ligands which, similar to the action of benzodiazepines on GABA(A) receptors, allosterically potentiate submaximal nicotinic responses. The sensitizing effect is a consequence of facilitated channel opening in the presence of allosterically potentiating ligand (APL). Representative members of this class of ligands are the plant alkaloids physostigmine, galanthamine, and codeine. Because APLs could enhance nicotinic neurotransmission under conditions of reduced secretion and/or increased degradation of acetylcholine or reduced acetylcholine-sensitivity of nicotinic acetylcholine receptors, they could have a preventive and corrective action on impaired but still functioning nicotinic neurotransmission.

Nicotinic receptor activation in human cerebral cortical interneurons: a mechanism for inhibition and disinhibition of neuronal networks.

Cholinergic control of the activity of human cerebral cortical circuits has long been thought to be accounted for by the interaction of acetylcholine (ACh) with muscarinic receptors. Here we report the discovery of functional nicotinic receptors (nAChRs) in interneurons of the human cerebral cortex and discuss the physiological and clinical implications of these findings. The whole-cell mode of the patch-clamp technique was used to record responses triggered by U-tube application of the nonselective agonist ACh and of the alpha7-nAChR-selective agonist choline to interneurons visualized by means of infrared-assisted videomicroscopy in slices of the human cerebral cortex. Choline induced rapidly desensitizing whole-cell currents that, being sensitive to blockade by methyllycaconitine (MLA; 50 nM), were most likely subserved by an alpha7-like nAChR. In contrast, ACh evoked slowly decaying whole-cell currents that, being sensitive to blockade by dihydro-beta-erythroidine (DHbetaE; 10 microM), were most likely subserved by an alpha4beta2-like nAChR. Application of ACh (but not choline) to the slices also triggered GABAergic postsynaptic currents (PSCs). Evidence is provided that ACh-evoked PSCs are the result of activation of alpha4beta2-like nAChRs present in preterminal axon segments and/or in presynaptic terminals of interneurons. Thus, nAChRs can relay inhibitory and/or disinhibitory signals to pyramidal neurons and thereby modulate the activity of neuronal circuits in the human cerebral cortex. These mechanisms, which appear to be retained across species, can account for the involvement of nAChRs in cognitive functions and in certain neuropathological conditions.

Galantamine is an allosterically potentiating ligand of the human alpha4/beta2 nAChR.

Galantamine (Reminyl) is a novel drug treatment for mild to moderate Alzheimer's disease (AD). Originally established as a reversible inhibitor of the acetylcholine-degrading enzyme acetylcholinesterase (AChE), galantamine also acts as an allosterically potentiating ligand (APL) on nicotinic acetylcholine receptors (nAChR). Having previously established this second mode of action on nAChRs from murine brain, we demonstrate here the same action of galantamine on the most abundant nAChR in the human brain, the alpha4/beta2 subtype. This nAChR-sensitizing action is not a common property of all, or most, AChE inhibitors, as is shown by the absence of this effect for other therapeutically applied AChE inhibitors including tacrine, metrifonate, rivastigmine and donepezil. The possible benefits for therapy of AD of an APL action on nicotinic receptors is discussed.