Competitive potentiation of acetylcholine effects on neuronal nicotinic receptors by acetylcholinesterase-inhibiting drugs

Berberine and palmatine, acting as allosteric potential ligands of α7 nAChR, synergistically regulate inflammation and phagocytosis of microglial cells

Berberine and palmatine are isoquinoline quaternary alkaloids derived from Chinese medicinal herbs. These alkaloids have shown promising synergy in inhibiting acetylcholinesterase (AChE), indicating their potential in treating Alzheimer's disease (AD). Besides, the anti-inflammatory effects of berberine and palmatine have been widely reported, although the underlying mechanism remains unclear. Here, we found that berberine and palmatine could induce calcium ion (Ca2+) influx via activating α7 nicotinic acetylcholine receptor (α7 nAChR) in cultured microglial cells, possibly serving as its allosteric potential ligands. Furthermore, we examined the synergistic anti-inflammatory effects of berberine and palmatine in the LPS-induced microglia, that significantly suppressed the production of TNF-α and iNOS. Notably, this suppression was reversed by co-treatment with a selective antagonist of α7 nAChR. Moreover, the alkaloid-induced microglial phagocytosis was shown to be mediated by the induction of Ca2+ influx through α7 nAChR and subsequent CaMKII-Rac1-dependent pathway. Additionally, the combination of berberine and palmatine, at low concentration, protected against the LPS-induced endoplasmic reticulum stress and mitochondrial dysfunction in microglia. These findings indicate the potential of berberine and palmatine, either individually or in combination, in contributing to anti-AD drug development, which provide valuable insights into the mechanisms by which natural products, such as plant alkaloids, exert their anti-AD effects.

The lesion site of organophosphorus-induced central apnea and the effects of antidotes

Organophosphorus poisoning kills individuals by causing central apnea; however, the underlying cause of death remains unclear. Following findings that the pre-Bötzinger complex impairment alone does not account for central apnea, we analyzed the effect of paraoxon on the brainstem-spinal cord preparation, spanning the lower medulla oblongata to phrenic nucleus. Respiratory bursts were recorded by connecting electrodes to the ventral 4th cervical nerve root of excised brainstem-spinal cord preparations obtained from 6-day-old Sprague-Dawley rats. We observed changes in respiratory bursts when paraoxon, neostigmine, atropine, and 2-pyridine aldoxime methiodide were administered via bath application. The percentage of burst extinction in the paraoxon-poisoning group was 50% compared with 0% and 18.2% in the atropine and 2-pyridine aldoxime methiodide treatment groups, respectively. Both treatments notably mitigated the paraoxon-induced reduction in respiratory bursts. In the neostigmine group, similar to paraoxon, bursts stopped in 66.7% of cases but were fully reversed by atropine. This indicates that the primary cause of central apnea is muscarinic receptor-mediated in response to acetylcholine excess. Paraoxon-induced central apnea is hypothesized to result from neural abnormalities within the inferior medulla oblongata to the phrenic nucleus, excluding pre-Bötzinger complex. These antidotes antagonize central apnea, suggesting that they may be beneficial therapeutic agents.

Acetylcholine waves and dopamine release in the striatum

Striatal dopamine encodes reward, with recent work showing that dopamine release occurs in spatiotemporal waves. However, the mechanism of dopamine waves is unknown. Here we report that acetylcholine release in mouse striatum also exhibits wave activity, and that the spatial scale of striatal dopamine release is extended by nicotinic acetylcholine receptors. Based on these findings, and on our demonstration that single cholinergic interneurons can induce dopamine release, we hypothesized that the local reciprocal interaction between cholinergic interneurons and dopamine axons suffices to drive endogenous traveling waves. We show that the morphological and physiological properties of cholinergic interneuron - dopamine axon interactions can be modeled as a reaction-diffusion system that gives rise to traveling waves. Analytically-tractable versions of the model show that the structure and the nature of propagation of acetylcholine and dopamine traveling waves depend on their coupling, and that traveling waves can give rise to empirically observed correlations between these signals. Thus, our study provides evidence for striatal acetylcholine waves in vivo, and proposes a testable theoretical framework that predicts that the observed dopamine and acetylcholine waves are strongly coupled phenomena.

Supplemental treatment to atropine improves the efficacy to reverse nerve agent induced bronchoconstriction

Exposure to highly toxic organophosphorus compounds causes inhibition of the enzyme acetylcholinesterase resulting in a cholinergic toxidrome and innervation of receptors in the neuromuscular junction may cause life-threatening respiratory effects. The involvement of several receptor systems was therefore examined for their impact on bronchoconstriction using an ex vivo rat precision-cut lung slice (PCLS) model. The ability to recover airways with therapeutics following nerve agent exposure was determined by quantitative analyses of muscle contraction. PCLS exposed to nicotine resulted in a dose-dependent bronchoconstriction. The neuromuscular nicotinic antagonist tubocurarine counteracted the nicotine-induced bronchoconstriction but not the ganglion blocker mecamylamine or the common muscarinic antagonist atropine. Correspondingly, atropine demonstrated a significant airway relaxation following ACh-exposure while tubocurarine did not. Atropine, the M3 muscarinic receptor antagonist 4-DAMP, tubocurarine, the β₂-adrenergic receptor agonist formoterol, the Na⁺-channel blocker tetrodotoxin and the K_ATP-channel opener cromakalim all significantly decreased airway contractions induced by electric field stimulation. Following VX-exposure, treatment with atropine and the Ca²⁺-channel blocker magnesium sulfate resulted in significant airway relaxation. Formoterol, cromakalim and magnesium sulfate administered in combinations with atropine demonstrated an additive effect. In conclusion, the present study demonstrated improved airway function following nerve agent exposure by adjunct treatment to the standard therapy of atropine.

Concatemers to re-investigate the role of α5 in α4β2 nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are pentameric ion channels expressed in the central nervous systems. nAChRs containing the α4, β2 and α5 subunits are specifically involved in addictive processes, but their functional architecture is poorly understood due to the intricacy of assembly of these subunits. Here we constrained the subunit assembly by designing fully concatenated human α4β2 and α4β2α5 receptors and characterized their properties by two-electrodes voltage-clamp electrophysiology in Xenopus oocytes. We found that α5-containing nAChRs are irreversibly blocked by methanethiosulfonate (MTS) reagents through a covalent reaction with a cysteine present only in α5. MTS-block experiments establish that the concatemers are expressed in intact form at the oocyte surface, but that reconstitution of nAChRs from loose subunits show inefficient and highly variable assembly of α5 with α4 and β2. Mutational analysis shows that the concatemers assemble both in clockwise and anticlockwise orientations, and that α5 does not contribute to ACh binding from its principal (+) site. Reinvestigation of suspected α5-ligands such as galantamine show no specific effect on α5-containing concatemers. Analysis of the α5-D398N mutation that is linked to smoking and lung cancer shows no significant effect on the electrophysiological function, suggesting that its effect might arise from alteration of other cellular processes. The concatemeric strategy provides a well-characterized platform for mechanistic analysis and screening of human α5-specific ligands.

Botanical monoterpenes synergise the toxicity of azamethiphos in the vector of Chagas disease, Triatoma infestans (Hemiptera: Reduviidae)

OBJECTIVE

To investigate what toxicological interactions occur when binary combinations of azamethiphos and botanical monoterpenes (eugenol, menthol or menthyl acetate) are applied to Triatoma infestans.

METHODS

The toxicity of binary mixtures of azamethiphos and sublethal doses of a monoterpene (eugenol, menthol or menthyl acetate) was evaluated in nymphs of the first stage of T. infestans. Experiments using exposure to filter papers and topical application were carried out. Values of Lethal Concentration 50% (LC50) were calculated in the first case, and values of Lethal Dose 50% (LD50) in the second.

RESULTS

The LC50 of azamethiphos applied on filter paper was 50.3 µg/cm² . However, when it was simultaneously applied with a sublethal concentration of monoterpene, its toxicity increased (LC50 with eugenol = 11.20 µg/cm² , LC50 with menthyl acetate = 5.30 µg/cm² , LC50 with menthol = 7.26 µg/cm² ). When applied topically, the LD50 of azamethiphos was 7.85 µg/insect, but its toxicity drastically increased when it was applied together with sublethal doses of menthol (LD50 = 0.00016 µg/insect) or menthyl acetate (LD50 = 0.00051 µg/insect). The simultaneous application with eugenol did not significantly change azamethiphos toxicity (LD50 = 12.79 µg/insect).

CONCLUSIONS

The toxicity of azamethiphos in T. infestans was synergised when it was applied together with eugenol, menthol or menthyl acetate on a filter paper. However, only menthol and menthyl acetate synergysed azamethiphos when mixtures were topically applied. The drastic effects of menthol and menthyl acetate in topical application experiments should be further studied as they could be the basis for developing more efficient triatomicidal products with a lower content of conventional insecticides than those currently used for controlling T. infestans.

Design, Synthesis, and Functional Evaluation of a Novel Series of Phosphonate-Functionalized 1,2,3-Triazoles as Positive Allosteric Modulators of α7 Nicotinic Acetylcholine Receptors

The α7 nicotinic acetylcholine receptor is a pentameric ligand-gated ion channel widely distributed in the central nervous system, mainly in the hippocampus and cortex. The enhancement of its activity by positive allosteric modulators (PAMs) is a promising therapeutic strategy for cognitive deficits and neurodegenerative disorders. With the aim of developing novel scaffolds with PAM activity, we designed and synthesized a series of phosphonate-functionalized 1,4-disubstituted 1,2,3-triazoles using supported copper nanoparticles as the cycloaddition reaction catalyst and evaluated their activity on α7 receptors by single-channel and whole-cell recordings. We identified several triazole derivatives that displayed PAM activity, with the compound functionalized with the methyl phosphonate group being the most efficacious one. At the macroscopic level, α7 potentiation was evidenced as an increase of the maximal currents elicited by acetylcholine with minimal effects on desensitization, recapitulating the actions of type I PAMs. At the single-channel level, the active compounds did not affect channel amplitude but significantly increased the duration of channel openings and activation episodes. By using chimeric and mutant α7 receptors, we demonstrated that the new α7 PAMs share transmembrane structural determinants of potentiation with other chemically nonrelated PAMs. To gain further insight into the chemical basis of potentiation, we applied structure-activity relationship strategies involving modification of the chain length, inversion of substituent positions in the triazole ring, and changes in the aromatic nucleus. Our findings revealed that the phosphonate-functionalized 1,4-disubstituted 1,2,3-triazole is a novel pharmacophore for the development of therapeutic agents for neurological and neurodegenerative disorders associated with cholinergic dysfunction.

The wonderland of neuronal nicotinic acetylcholine receptors

Nearly 30 years of experimental evidence supports the argument that ligands of nicotinic acetylcholine receptors (nAChRs) have potential as therapeutic agents. However, as in the famous Lewis Carroll novel "Alice in Wonderland", there have been many unexpected adventures along the pathway of development, and few nAChR ligands have been approved for any clinical condition to date with the exception of nicotine dependence. The recent failures of nAChR ligands in AD and schizophrenia clinical trials have reduced enthusiasm for this therapeutic strategy and many pharmaceutical companies have now abandoned this field of research. As with other clinical failures, multiple questions arise as to the basis for the failure. More generic questions focus on a potential translational gap between the animal models used and the human clinical condition they are meant to simulate, or the clinical trial mindset that large Ns have to be achieved for statistical power (often requiring multiple trial sites) as opposed to smaller patient cohorts at limited sites where conditions can be better controlled and replicated. More specific to the nAChR field are questions about subtype selectivity, dose selection, whether an agonist, antagonist, or allosteric modulator strategy is best, etc. The purpose of this review is to discuss each of these questions, but also to provide a brief overview of the remarkable progress that has been made over the last three decades in our understanding of this unique ligand-gated ion channel and how this new knowledge may help us improve drug development successes in the future.

Effects of the antidepressant mirtazapine and zinc on nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) and zinc are associated with regulation of mood and related disorders. In addition, several antidepressants inhibit muscle and neuronal nAChRs and zinc potentiates inhibitory actions of them. Moreover, mirtazapine (a noradrenergic, serotonergic and histaminergic antidepressant) inhibits muscarinic AChRs and its effects on nAChRs are unknown. Therefore, we studied the modulation of muscle α1β1γd nAChRs expressed in oocytes and native α7-containing nAChRs in hippocampal interneurons by mirtazapine and/or zinc, using voltage-clamp techniques. The currents elicited by ACh in oocytes (at -60 mV) were similarly inhibited by mirtazapine in the absence and presence of 100 μM zinc (IC₅₀ ∼15 μM); however, the ACh-induced currents were stronger inhibited with 20 and 50 μM mirtazapine in the presence of zinc. Furthermore, the potentiation of ACh-induced current by zinc in the presence of 5 μM mirtazapine was 1.48 ± 0.06, and with 50 μM mirtazapine zinc potentiation did not occur. Interestingly, in stratum radiatum interneurons (at -70 mV), 20 μM mirtazapine showed less inhibition of the current elicited by choline (Ch) than at 10 μM (0.81 ± 0.02 and 0.74 ± 0.02 of the Ch-induced current, respectively). Finally, the inhibitory effects of mirtazapine depended on membrane potential: 0.81 ± 0.02 and 0.56 ± 0.05 of the control Ch-induced current at -70 and -20 mV, respectively. These results indicate that mirtazapine interacts with muscle and neuronal nAChRs, possibly into the ion channel; that zinc may increase the sensitivity of nAChRs to mirtazapine; and that mirtazapine decreases the sensitivity of nAChRs to zinc.

Determination of the Residues in the Extracellular Domain of the Nicotinic α Subunit Required for the Actions of Physostigmine on Neuronal Nicotinic Receptors

Physostigmine can potentiate and inhibit neuronal nicotinic receptors, in addition to inhibiting the activity of acetylcholinesterase. We found that receptors containing three copies of the α2 subunit are inhibited by low concentrations of physostigmine in contrast to receptors containing three copies of the α4 subunit that are potentiated. We exploited this observation to determine the regions required for the actions of physostigmine. Chimeric constructs of the α2 and α4 subunits located two regions in the extracellular amino-terminal domain of the subunit: the E loop (a loop of the transmitter-binding domain) and a region closer to the amino-terminus that collectively could completely determine the different effects of physostigmine. Point mutations then identified a single residue, α2(I92) versus α4(R92), that, when combined with transfer of the E loop, could convert the inhibition seen with α2 subunits to potentiation and the potentiation seen with α4 subunits to inhibition. In addition, other point mutations could affect the extent of potentiation or inhibition, indicating that a more extensive set of interactions in the amino-terminal domain plays some role in the actions of physostigmine.

The E Loop of the Transmitter Binding Site Is a Key Determinant of the Modulatory Effects of Physostigmine on Neuronal Nicotinic α4β2 Receptors

Physostigmine is a well known inhibitor of acetylcholinesterase, which can also activate, potentiate, and inhibit acetylcholine receptors, including neuronal nicotinic receptors comprising α4 and β2 subunits. We have found that the two stoichiometric forms of this receptor differ in the effects of physostigmine. The form containing three copies of α4 and two of β2 was potentiated at low concentrations of acetylcholine chloride (ACh) and physostigmine, whereas the form containing two copies of α4 and three of β2 was inhibited. Chimeric constructs of subunits indicated that the presence of inhibition or potentiation depended on the source of the extracellular ligand binding domain of the subunit. Further sets of chimeric constructs demonstrated that a portion of the ACh binding domain, the E loop, is a key determinant. Transferring the E loop from the β2 subunit to the α4 subunit resulted in strong inhibition, whereas the reciprocal transfer reduced inhibition. To control the number and position of the incorporated chimeric subunits, we expressed chimeric constructs with subunit dimers. Surprisingly, incorporation of a subunit with an altered E loop had similar effects whether it contributed either to an intersubunit interface containing a canonical ACh binding site or to an alternative interface. The observation that the α4 E loop is involved suggests that physostigmine interacts with regions of subunits that contribute to the ACh binding site, whereas the lack of interface specificity indicates that interaction with a particular ACh binding site is not the critical factor.

Neuronal α7 Nicotinic Receptors as a Target for the Treatment of Schizophrenia

Schizophrenia is a lifelong disease, the burden of which is often underestimated. Characterized by positive (e.g., hallucinations) and negative (e.g., avolition, amotivation) symptoms, schizophrenia is also accompanied with profound impairments in cognitive function that progress throughout the development of the disease. Although treatment with antipsychotic medications can effectively dampen some of the positive symptoms, these medications largely fail to reverse cognitive deficits or to mitigate negative symptoms. With a worldwide prevalence of approximately 1%, schizophrenia remains a large unmet medical need that stands to benefit greatly from (1) continued research to better understand the biological underpinnings of the disease and (2) the targeted development of novel therapeutics to improve the lives of those affected individuals. Improvements in our understanding of the neuronal networks associated with schizophrenia as well as progress in identifying genetic risk factors and environmental conditions that may predispose individuals to developing the disease are advancing new strategies to study and treat it. Herein, we review the evidence that supports the role of α7 nicotinic acetylcholine receptors in the central nervous system and why these receptors constitute a promising target to treat some of the prominent symptoms of schizophrenia.

Functional characterization of AT-1001, an α3β4 nicotinic acetylcholine receptor ligand, at human α3β4 and α4β2 nAChR

INTRODUCTION

Genome-wide association studies linking the α3, β4, and α5 nicotinic acetylcholine receptor (nAChR) subunits to nicotine dependence suggest that α3β4* nAChR may be targets for smoking cessation pharmacotherapies. We previously reported that AT-1001, a selective α3β4* nAChR ligand binds with high affinity to rat α3β4 and human α3β4α5 nAChR, antagonizes epibatidine-induced activation of rat α3β4 nAChR in HEK cells and potently inhibits nicotine self-administration in rats.

METHODS

Two-electrode voltage clamp was used for functional characterization of AT-1001 at recombinant human α3β4 and α4β2 nAChR expressed in Xenopus oocytes.

RESULTS

Concentration-response curves show that AT-1001 is a partial agonist at human α3β4 nAChR, evoking up to 35% of the maximal acetylcholine (ACh) response (50% effective concentration [EC50] = 0.37 μM). AT-1001 showed very little agonist activity at the α4β2 nAChR, evoking only 6% of the ACh response (EC50 = 1.5 μM). Pre- and co-application of various concentrations of AT-1001 with 50 μM ACh revealed a complex pattern of activation-inhibition by AT-1001 at α3β4 nAChR, which was best fitted by a 2-site equation. At α4β2 nAChR, co-exposure of AT-1001 with ACh only showed inhibition of ACh current with a shallower curve.

CONCLUSIONS

AT-1001 is a partial agonist at the human α3β4 nAChR and causes desensitization at concentrations at which it evokes an inward current, resulting in an overall functional antagonism of α3β4 nAChR. AT-1001 does not significantly activate or desensitize α4β2 nAChR at the same concentrations as at the α3β4 nAChR, but does inhibit ACh responses at α4β2 nAChR at higher concentrations. A combination of these mechanisms may underlie the inhibition of nicotine self-administration by AT-1001, suggesting that AT-1001 and compounds from this class may have clinical potential for smoking cessation pharmacotherapy.

Different effects of lobeline on neuronal and muscle nicotinic receptors

Lobeline is a plant alkaloid known to interact with cholinergic system. The effect of lobeline on neuronal α3β4 receptors expressed in COS cells and muscle embryonic αβγδ receptors naturally expressed in TE671 cells was studied using a patch-clamp technique. Our results show that lobeline inhibited responses to acetylcholine in human embryonic muscle nicotinic receptor in a pseudo-noncompetitive manner. The responses of rat neuronal α3β4 receptors to a low concentration of acetylcholine were potentiated by a mixed occupation mechanism that corresponds to "competitive potentiation". This potentiation turned into voltage-dependent inhibition for α3β4 receptors was activated by a high concentration of acetylcholine.

EVP-6124, a novel and selective α7 nicotinic acetylcholine receptor partial agonist, improves memory performance by potentiating the acetylcholine response of α7 nicotinic acetylcholine receptors

EVP-6124, (R)-7-chloro-N-quinuclidin-3-yl)benzo[b]thiophene-2-carboxamide, is a novel partial agonist of α7 neuronal nicotinic acetylcholine receptors (nAChRs) that was evaluated here in vitro and in vivo. In binding and functional experiments, EVP-6124 showed selectivity for α7 nAChRs and did not activate or inhibit heteromeric α4β2 nAChRs. EVP-6124 had good brain penetration and an adequate exposure time. EVP-6124 (0.3 mg/kg, p.o.) significantly restored memory function in scopolamine-treated rats (0.1 mg/kg, i.p.) in an object recognition task (ORT). Although donepezil at 0.1 mg/kg, p.o. or EVP-6124 at 0.03 mg/kg, p.o. did not improve memory in this task, co-administration of these sub-efficacious doses fully restored memory. In a natural forgetting test, an ORT with a 24 h retention time, EVP-6124 improved memory at 0.3 mg/kg, p.o. This improvement was blocked by the selective α7 nAChR antagonist methyllycaconitine (0.3 mg/kg, i.p. or 10 μg, i.c.v.). In co-application experiments of EVP-6124 with acetylcholine, sustained exposure to EVP-6124 in functional investigations in oocytes caused desensitization at concentrations greater than 3 nM, while lower concentrations (0.3-1 nM) caused an increase in the acetylcholine-evoked response. These actions were interpreted as representing a co-agonist activity of EVP-6124 with acetylcholine on α7 nAChRs. The concentrations of EVP-6124 that resulted in physiological potentiation were consistent with the free drug concentrations in brain that improved memory performance in the ORT. These data suggest that the selective partial agonist EVP-6124 improves memory performance by potentiating the acetylcholine response of α7 nAChRs and support new therapeutic strategies for the treatment of cognitive impairment. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Allosteric modulators of the α4β2 subtype of neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are ligand-gated ion conducting transmembrane channels from the Cys-loop receptor super-family. The α4β2 subtype is the predominant heteromeric subtype of nicotinic receptors found in the brain. Allosteric modulators for α4β2 receptors interact at a site other than the orthosteric site where acetylcholine binds. Many compounds which act as allosteric modulators of the α4β2 receptors have been identified, with both positive and negative effects. Such allosteric modulators either increase or decrease the response induced by agonist on the α4β2 receptors. Here we discuss the concept of allosterism as it pertains to the α4β2 receptors and summarize the important features of allosteric modulators for this nicotinic receptor subtype.

Localization by site-directed mutagenesis of a galantamine binding site on α7 nicotinic acetylcholine receptor extracellular domain

Galantamine is an approved drug treatment for Alzheimer's disease. Initially identified as a weak cholinesterase inhibitor, we have established that galantamine mainly acts as an 'allosterically potentiating ligand (APL)' of nicotinic acetylcholine receptors (nAChR). Meanwhile other 'positive allosteric modulators (PAM)' of nAChR channel activity have been discovered, and for one of them a binding site within the transmembrane domain has been proposed. Here we show, by performing site-directed mutagenesis studies of ectopically expressed chimeric chicken α7/mouse 5-hydroxytryptamine 3 receptor-channel complex, in combination with whole-cell current measurements, in the presence and absence of galantamine, that the APL binding site is different from the proposed PAM binding site. We demonstrate that residues T197, I196, and F198 of ß-strand 10 represent major elements of the galantamine binding site. Residue K123, earlier suggested as being 'close to' the APL binding site, is not part of this site but rather appears to play a role in coupling of agonist binding to channel opening and closing. Our data confirm our earlier results that the galantamine binding site is different from the ACh binding site. Both sites are in close proximity and hence may influence each other in a synergistic fashion. Other interesting areas identified in the present study are a 'hinge' region around and containing residues F122, K123, and K143 possibly being involved in relaying the signal of agonist binding to gating of the transmembrane channel, and a 'folding centre', with P119 as the dominating residue, that crucially positions the agonist binding site with respect to the hinge region.

Parazoanthoxanthin A blocks Torpedo nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are implicated in different nervous system-related disorders, and their modulation could improve existing therapy of these diseases. Parazoanthoxanthin A (ParaA) is a fluorescent pigment of the group of zoanthoxanthins. Since it is a potent acetylcholinesterase inhibitor, it may also bind to nicotinic acetylcholine receptors (nAChRs). For this reason its effect on Torpedo nAChR (alpha1(2)betagammadelta) transplanted to Xenopus laevis oocytes was evaluated, using the voltage-clamp technique. ParaA dose-dependently reduced the acetylcholine-induced currents. This effect was fully reversible only at lower concentrations. ParaA also reduced the Hill coefficient and the time to peak current, indicating a channel blocking mode of action. On the other hand, the combined effect of ParaA and d-tubocurarine (d-TC) on acetylcholine-induced currents exhibited only partial additivity, assuming a competitive mode of action of ParaA on nAChR. These results indicate a dual mode of action of ParaA on the Torpedo AChR.

Upregulation of alpha7 Nicotinic Receptors by Acetylcholinesterase C-Terminal Peptides

Background

The alpha-7 nicotinic acetylcholine receptor (α7-nAChR) is well known as a potent calcium ionophore that, in the brain, has been implicated in excitotoxicity and hence in the underlying mechanisms of neurodegenerative disorders such as Alzheimer's disease. Previous research implied that the activity of this receptor may be modified by exposure to a peptide fragment derived from the C-terminal region of the enzyme acetylcholinesterase. This investigation was undertaken to determine if the functional changes observed could be attributed to peptide binding interaction with the α7-nAChR, or peptide modulation of receptor expression.

Methodology/Principal Findings

This study provides evidence that two peptides derived from the C-terminus of acetylcholinesterase, not only selectively displace specific bungarotoxin binding at the α7-nAChR, but also alter receptor binding properties for its familiar ligands, including the alternative endogenous agonist choline. Of more long-term significance, these peptides also induce upregulation of α7-nAChR mRNA and protein expression, as well as enhancing receptor trafficking to the plasma membrane.

Conclusions/Significance

The results reported here demonstrate a hitherto unknown relationship between the α7-nAChR and the non-enzymatic functions of acetylcholinesterase, mediated independently by its C-terminal domain. Such an interaction may prove valuable as a pharmacological tool, prompting new approaches for understanding, and combating, the process of neurodegeneration.

Acteylcholinesterase inhibitor rivastigmine enhances cellular defenses in neuronal and macrophage-like cell lines

Neuroprotection mediated by the cellular heat shock response offers 1 clinical strategy to prevent, stabilize, and possibly reverse neurodegenerative processes. Although damaged proteins are thought to be the primary stimulus for the heat shock response, several studies indicate that pharmaceutical agents can either directly induce the heat shock transcription factor (Hsf1) or enhance its activation during different forms of cellular stress. Because Hsf1 is now known to combat the proteotoxicity of aging and has a central role in modulating amyloid aggregation, pharmacologic interventions to strengthen Hsf1 action may have important implications for preventing neurodegeneration linked to altered and damaged proteins such as observed in Alzheimer's disease. Given reports that some agents for the treatment of Alzheimer's disease have neuroprotective properties, this project investigated whether rivastigmine, which is an acetyl and butaryl cholinesterase inhibitor, mediates the neuroprotection of the neuronal-like cell line SH-SY5Y. The cells were exposed to various concentrations of rivastigmine to determine whether the drug protected cells from toxic injury and induced the 1st phase of the cellular heat shock response. In all, 100-micromol/L rivastigmine decreases cell death by 40% compared with untreated cells. This concentration enhances Hsf1 activation by strengthening both its multimerization and its phosphorylation, which leads to increased messenger RNA (mRNA) for hsp70. Therefore, one of the putative neuroprotective mechanisms of rivastigmine seems to be mediated through the heat shock response. These results also are observed in cultured macrophage-like cells, which suggests a future clinical tool for monitoring pharmacologically improved stress responses in peripheral blood mononuclear cells during treatment of Alzheimer disease.

Activation and block of the adult muscle-type nicotinic receptor by physostigmine: single-channel studies

The plant-derived acetylcholinesterase inhibitor physostigmine has previously been shown to act on the nicotinic acetylcholine receptor (nAChR) causing either direct activation or potentiation of currents elicited by low concentrations of nicotinic agonists, or, at higher concentrations, channel block. We examined mouse adult-type muscle nAChR activation by physostigmine and found that channel activation by physostigmine exhibits many characteristics common with channel activity elicited by nicotinic agonists. Single-channel conductance was indistinguishable, and mutants known to slow channel closing in the presence of nicotinic agonists had a similar effect in the presence of physostigmine. However, physostigmine is a very inefficacious agonist. The presence of physostigmine did not alter the effective opening rate for a subsaturating dosage of carbachol, suggesting that physostigmine does not interact with the nicotinic agonist binding site. Mutations to a residue (alphaLys125) previously identified as part of the putative binding site for physostigmine reduced the duration of openings elicited by physostigmine, but the effects were generally small and, in most cases, nonsignificant. At higher concentrations, physostigmine blocked channel activity. Block manifested as a reduction in the mean open time and the emergence of a closed state, with a mean duration of 3 to 7 ms. The properties of block were consistent with two equivalent blocking sites per receptor with microscopic binding and unbinding rate constants for physostigmine of 20 microM(-1) s(-1) and 450 s(-1) (K(D) = 23 microM). These observations indicate that physostigmine is able to activate muscle nAChR by interacting with a site other than the nicotinic ligand binding site.

Alzheimer's disease drugs: an application of the hormetic dose-response model

This article provides an evaluation of the dose-response features of drugs that are intended to improve memory, some of which have been used in the treatment of Alzheimer's disease (AD). A common feature of these drugs is that they act via an inverted U-shaped dose response, consistent with the hormetic dose response model. This article assesses historical foundations that lead to the development of AD drugs, their dose-response features and how the quantitative features of such dose responses affected drug discovery and development, and the successes and possible failures of such agents in preclinical and clinical settings. This story begins about 150 years ago with the discovery of an active agent in the Calabar bean plant called physostigmine, its unfolding medical applications, and its implications for dose-response relationships, memory enhancement, and improved drug discovery activities. The article also demonstrates the occurrence of U-shaped dose responses for memory with numerous endogenous agonists including neurosteroids, various peptides (e.g., vasopressin, CCK-8, neuropeptide Y), and other agents (e.g., epinephrine, antagonists for platelet activity factor and nicotinic receptors), supporting the generalizability of the hormetic biphasic dose response. Finally, the significance of the U-shaped dose response is critical for successful clinical application, since it defines the therapeutic window.

Morantel allosterically enhances channel gating of neuronal nicotinic acetylcholine alpha 3 beta 2 receptors

We studied allosteric potentiation of rat alpha3beta2 neuronal nicotinic acetylcholine receptors (nAChRs) by the anthelmintic compound morantel. Macroscopic currents evoked by acetylcholine (ACh) from nAChRs expressed in Xenopus laevis oocytes increase up to 8-fold in the presence of low concentrations of morantel (< or =10 microM); the magnitude of the potentiation depends on both agonist and modulator concentrations. It is noteworthy that the potentiated currents exceed the maximum currents achieved by saturating (millimolar) concentrations of agonist. Studies of macroscopic currents elicited by prolonged drug applications (100-300 s) indicate that morantel does not increase alpha3beta2 receptor activity by reducing slow (> or =1 s) desensitization. Instead, using outside-out patch-clamp recordings, we demonstrate that morantel increases the frequency of single-channel openings and alters the bursting characteristics of the openings in a manner consistent with enhanced channel gating; these results quantitatively explain the macroscopic current potentiation. Morantel is a very weak agonist alone, but we show that the classic competitive antagonist dihydro-beta-erythroidine inhibits morantel-evoked currents noncompetitively, indicating that morantel does not bind to the canonical ACh binding sites.

An analysis of acetylcholinesterase sequence for predicting mechanisms of its non-catalytic actions

The enzyme acetylcholinesterase (AChE) which belongs to the family of α/β hydrolases is well known for hydrolyzing the neurotransmitter acetylcholine (ACh). In addition to its catalytic function, AChE appears to play a significant non-catalytic role in development, regeneration and modulation of properties of neurons. However the mechanisms underlying these important actions of AChE are unknown. It prompted the analysis of the sequence of fetal bovine serum AChE to seek clue(s) for the mechanisms of AChE non-catalytic actions. The searches for motifs, finger prints and domains demonstrated the presence of a highly conserved carboxylesterase type B signature in AChE from slime molds to human. Interestingly, the presence of finger prints for a metabotropic glutamate receptor, gap junction protein connexin, a calcium binding motif, several phosphorylation sites, and a motif similar to nicotinic acetylcholine receptor were also found in the sequence of AChE suggesting a role for AChE in cell signalling. The similarity search using gapped BLAST retrieved previously known similar proteins, namely neurotactin, neuroligin and thyroglobulin. When aligned neurotactin, neuroligin and bovine AChE showed the presence of three stretches of consensus, one of which is carboxylesterase B signature. The identity of the other two are not known. Amino acids in these stretches in AChE may be involved in possible growth factor like action and awaits experimental verification. Thus sequence analysis can be very useful for biologists while searching for a possible mechanism of action of a protein. Further, these results underscore the significance of performing both local and global searches to identify possible functional domains as well as consensus sequence.

Activation of nicotinic ACh receptors with alpha4 subunits induces adenosine release at the rat carotid body

The effect of ACh on the release of adenosine was studied in rat whole carotid bodies, and the nicotinic ACh receptors involved in the stimulation of this release were characterized. ACh and nicotinic ACh receptor agonists, cytisine, DMPP and nicotine, caused a concentration-dependent increase in adenosine production during normoxia, with nicotine being more potent and efficient in stimulating adenosine release from rat CB than cytisine and DMPP. D-Tubocurarine, mecamylamine, DHbetaE and alpha-bungarotoxin, nicotinic ACh receptor antagonists, caused a concentration-dependent reduction in the release of adenosine evoked by hypoxia. The rank order of potency for nicotinic ACh receptor antagonists that inhibit adenosine release was DHbetaE>mecamylamine>D-tubocurarine>alpha-bungarotoxin. The effect of the endogenous agonist, ACh, which was mimicked by nicotine, was antagonized by DHbetaE, a selective nicotinic receptor antagonist. The ecto-5'-nucleotidase inhibitor AOPCP produces a 72% inhibition in the release of adenosine from CB evoked by nicotine. Taken together, these data indicate that ACh induced the production of adenosine, mainly from extracellular ATP catabolism at the CB through a mechanism that involves the activation of nicotinic receptors with alpha4 and beta2 receptor subunits.

Acetylcholine nicotinic receptors: finding the putative binding site of allosteric modulators using the "blind docking" approach

Allosteric potentiation of acetylcholine nicotinic receptors is considered to be one of the most promising approaches for the treatment of Alzheimer's disease. However, the exact localization of the allosteric binding site and the potentiation mechanism at the molecular level are presently unknown. We have performed the "blind docking" of three known allosteric modulators (galanthamine, codeine and eserine) with the Acetylcholine Binding Protein and models of human alpha7, alpha3beta4 and alpha4beta2 nicotinic receptors, created by homology modeling. Three putative binding sites were identified in the channel pore, each one showing different affinities for the ligands. One of these sites is localized opposite to the agonist binding site and is probably implicated in the potentiation process. On the basis of these results, a possible mechanism for nicotinic acetylcholine receptor (nAChRs) activation is proposed. The present findings may represent an important advance for understanding the allosteric modulation mechanism of nAChRs. [Figure: see text].

Galantamine activates muscle-type nicotinic acetylcholine receptors without binding to the acetylcholine-binding site

Galantamine (Reminyl; Janssen Pharmaceutica, Titusville, NJ) belongs to a class of acetylcholinesterase inhibitors approved for symptomatic treatment of Alzheimer's disease. The drug presumably acts by raising and prolonging the profile of acetylcholine (ACh) via an inhibitory effect on the esterase. However, there is also evidence demonstrating that galantamine can activate the nicotinic ACh receptor or modulate its activation by ACh. In this study, we have examined the ability of galantamine to directly activate the muscle-type nicotinic ACh receptor or to modulate receptor activation by selected nicotinic agonists. Studies of direct activation by galantamine demonstrated that this ligand is a low-efficacy agonist of the muscle-type ACh receptor. Point mutations in the M2-M3 linker (alphaS269I) and the M2 transmembrane domain (epsilonT264P) had similar effects on receptor activation by galantamine and nicotinic agonists, suggesting that the general features of receptor activation by galantamine are similar to that in the presence of ACh. Experiments performed in the simultaneous presence of galantamine and various nicotinic ligands showed that channel activation by the nicotinic ligands studied (ACh, carbachol, and choline) was not affected by the presence of galantamine at concentrations up to 100 microm. In addition, galantamine did not reduce the initial rate of binding for 125I-alpha-bungarotoxin. These results demonstrate that galantamine does not interfere with the occupation of the nicotinic agonist binding site by ACh, carbachol, or choline. We conclude that galantamine activates the muscle-type ACh receptor by interacting with a binding site that is distinct from the site for nicotinic agonists.

The acetylcholinesterase inhibitor BW284c51 is a potent blocker of Torpedo nicotinic AchRs incorporated into the Xenopus oocyte membrane

This work was aimed to determine if 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide (BW284c51), the most selective acetylcholinesterase inhibitor (AchEI), affects the nicotinic acetylcholine (Ach) receptor (AchR) function. Purified Torpedo nicotinic AchRs were injected into Xenopus laevis oocytes and BW284c51 effects on Ach- and carbamylcholine (Cch)-elicited currents were assessed using the voltage-clamp technique.BW284c51 (up to 1 mM) did not evoke any change in the oocyte membrane conductance. When BW284c51 (10 pM-100 microM) and Ach were co-applied, Ach-evoked currents (I(Ach)) were reversibly inhibited in a concentration-dependent manner (Hill coefficient, 1; IC(50), 0.2-0.5 muM for 0.1-1000 microM Ach). Cch-elicited currents showed a similar inhibition by BW284c51.I(Ach) blockade by BW284c51 showed a strong voltage dependence, being only apparent at hyperpolarising potentials. BW284c51 also enhanced I(Ach) desensitisation.BW284c51 changed the Ach concentration-dependence curve of Torpedo AchR response from two-site to single-site kinetics, without noticeably affecting the EC(50) value. The BW284c51 blocking effect was highly selective for nicotinic over muscarinic receptors. BW284c51 inhibition potency was stronger than that of tacrine, and similar to that of d-tubocurarine (d-TC). Coapplication of BW284c51 with either tacrine or d-TC revealed synergistic inhibitory effects. Our results indicate that BW284c51 antagonises nicotinic AchRs in a noncompetitive way by blocking the receptor channel, and possibly by other, yet unknown, mechanisms. Therefore, besides acting as a selective AchEI, BW284c51 constitutes a powerful and reversible blocker of nicotinic AchRs that might be used as a valuable tool for understanding their function.

Cholinergic neurotransmission in the preBötzinger Complex modulates excitability of inspiratory neurons and regulates respiratory rhythm

We investigated whether there is endogenous acetylcholine (ACh) release in the preBötzinger Complex (preBötC), a medullary region hypothesized to contain neurons generating respiratory rhythm, and how endogenous ACh modulates preBötCneuronal function and regulates respiratory pattern. Using a medullary slice preparation from neonatal rat, we recorded spontaneous respiratory-related rhythm from the hypoglossal nerve roots (XIIn) and patch-clamped preBötC inspiratory neurons. Unilateral microinjection of physostigmine, an acetylcholinesterase inhibitor, into the preBötC increased the frequency of respiratory-related rhythmic activity from XIIn to 116+/-13% (mean+/-S.D.) of control. Ipsilateral physostigmine injection into the hypoglossal nucleus (XII nucleus) induced tonic activity, increased the amplitude and duration of the integrated inspiratory bursts of XIIn to 122+/-17% and 117+/-22% of control respectively; but did not alter frequency. In preBötC inspiratory neurons, bath application of physostigmine (10 microM) induced an inward current of 6.3+/-10.6 pA, increased the membrane noise, decreased the amplitude of phasic inspiratory drive current to 79+/-16% of control, increased the frequency of spontaneous excitatory postsynaptic currents to 163+/-103% and decreased the whole cell input resistance to 73+/-22% of control without affecting the threshold for generation of action potentials. Bath application of physostigmine concurrently induced tonic activity, increased the frequency, amplitude and duration of inspiratory bursts of XIIn motor output. Bath application of 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, 2 microM), a M3 muscarinic acetylcholine receptor (mAChR) selective antagonist, increased the input resistance of preBötC inspiratory neurons to 116+/-9% of control and blocked all of the effects of physostigmine except for the increase in respiratory frequency. Dihydro-beta-erythroidine (DH-beta-E; 0.2 microM), an alpha4beta2 nicotinic receptor (nAChR) selective antagonist, blocked all the effects of physostigmine except for the increase in inspiratory burst amplitude. In the presence of both 4-DAMP and DH-beta-E, physostigmine induced opposite effects, i.e. a decrease in frequency and amplitude of XIIn rhythmic activity. These results suggest that there is cholinergic neurotransmission in the preBötC which regulates respiratory frequency, and in XII nucleus which regulates tonic activity, and the amplitude and duration of inspiratory bursts of XIIn in neonatal rats. Physiologically relevant levels of ACh release, via mAChRs antagonized by 4-DAMP and nAChRs antagonized by DH-beta-E, modulate the excitability of inspiratory neurons and excitatory neurotransmission in the preBötC, consequently regulating respiratory rhythm.

Pharmacology and toxicology of cholinesterase inhibitors: uses and misuses of a common mechanism of action

Cholinesterase inhibitors have been used in the treatment of human diseases, the control of insect pests, and more notoriously as chemical warfare agents and weapons of terrorism. Most uses of cholinesterase inhibitors are based on a common mechanism of action initiated by inhibition of acetylcholinesterase (AChE). Extensive inhibition of this enzyme leads to accumulation of the neurotransmitter acetylcholine and enhanced stimulation of postsynaptic cholinergic receptors. This action is beneficial in cases where a reduction in cholinergic transmission contributes to clinical symptoms, e.g., low muscle tone in the autoimmune disorder myasthenia gravis due to loss of nicotinic receptors. Under normal conditions, however, extensive inhibition of AChE leads to excess synaptic acetylcholine levels, over-stimulation of cholinergic receptors, alteration of postsynaptic cell function and consequent signs of cholinergic toxicity. This biochemical cascade forms the basis for the use of anticholinesterase insecticides in pest control as well as for nerve agents in chemical warfare. Paradoxically, the short-acting cholinesterase inhibitor pyridostigmine, an important therapeutic agent in the treatment of myasthenia gravis, was used during the Persian Gulf War to prevent the long-term clinical consequences of possible organophosphate nerve agent exposure. As shown in the attacks in Matsumoto and Tokyo, these same nerve agents can be effectively used to inflict urban terror. Cholinesterase inhibitors thus share a common mechanism of pharmacological or toxicological action, ultimately modifying cholinergic signaling through disruption of acetylcholine degradation. While the use of cholinesterase inhibitors relies on their interaction with AChE, a variety of reports indicate that a number of cholinesterase inhibitors have additional sites of action that may have pharmacologic or toxicologic relevance. A variety of esterase and non-esterase enzymes, neurotransmitter receptors and elements of cell signaling pathways are targeted by some anticholinesterases. In some cases, these actions may occur at concentrations/dosages below those affecting cholinergic transmission. Studies of interactive toxicity of binary mixtures of common organophosphorus insecticides indicate that non-cholinesterase targets may be important in cumulative toxicity. Exposure to multiple anticholinesterases having selective effects on other macromolecules could confound the assumption of additivity in cumulative risk assessment. Knowledge of such selective additional targets may aid, however, in the optimization of strategies for poisoning therapy and in the further elucidation of mechanisms of toxicity for this class of compounds.

Acute effects of physostigmine and galantamine on the binding of [18F]fluoro-A-85380: A PET study in monkeys

2-[18F]fluoro-3-[2S-2-azetidinylmethoxy]pyridine ([18F]fluoro-A-85380) is an alpha4beta2 subtype selective nicotinic cholinergic agonist with potential suitability for studying changes in endogenous acetylcholine synaptic concentration. Physostigmine, a potent AChE inhibitor, and galantamine, an allosteric modulator of nAChRs, are widely used for the treatment of Alzheimer's disease. Before studying patients with this neurodegenerative disease, positron emission tomography (PET) studies in monkeys were performed to assess the impact of these two compounds on the radiotracer distribution volumes. Physostigmine was administered i.v. at two dosages: 150 microg/kg/h and 37.5 microg/kg/h for 160 min. Galantamine was administered i.v. at two dosages: 2 or 4 mg over 20 min. For PET data analysis, a model with one tissue (radioactivity of the parent compound in plasma and radioactivity in brain tissue) compartment was chosen because reliable parameter estimates could not be obtained with a more complex model. The higher dose of physostigmine produced a 40%, 23%, and 30% reduction of distribution volumes in the putamen, the temporal, and frontal cortices, respectively. The lower dose of physostigmine produced a reduction of 33%, 31%, and 24% in the same structures, respectively. Galantamine (4 mg or 2 mg) produced no significant change of distribution volumes in the basal ganglia, the temporal and frontal cortex. The effects of physostigmine, a more potent AChE inhibitor than galantamine, could be interpreted as a desensitization of nAChRs, due to a prolonged exposure to high synaptic concentration of acetylcholine or as a competition with acetylcholine.

Activation of heteroliganded mouse muscle nicotinic receptors

The activation of the mouse muscle-type nicotinic acetylcholine receptor was studied in the presence of carbachol, and in the simultaneous presence of carbachol and choline. The channel currents were recorded under steady-state conditions using cell-attached single-channel patch clamp, and during transient exposures to the agonists using a piezo-driven fast application system. The presence of choline resulted in inhibition of currents elicited by carbachol. The inhibitory effect of choline manifested as a reduction in the effective opening rate (increase in the mean intracluster closed time duration) in single-channel recordings. In the fast application experiments, the peak current amplitude was reduced and the current rise time increased when choline was co-applied with carbachol. The data were analysed according to a model in which receptor interactions with carbachol and choline resulted in three types of ligation: receptors occupied by two carbachol molecules, receptors occupied by two choline molecules, and receptors in which one agonist binding site was occupied by carbachol and the other by choline, i.e. heteroliganded receptors. All three agonist-bound receptor populations could open albeit with different efficacies. The affinity of the resting receptor to choline was estimated to be 1-2 mm, and heteroliganded receptors opened with an opening rate constant of approximately 3000 s(-1). The results of the analysis suggest that the presence of choline in the neuromuscular junction in vivo has little effect on the time course of synaptic currents. Nevertheless, the contribution of heteroliganded receptors should be taken into consideration when the receptor is exposed simultaneously to two or more agonists.

Cholinergic drugs potentiate human nicotinic α4β2 acetylcholine receptors by a competitive mechanism

Effects of cholinergic drugs on human alpha4beta2 nicotinic acetylcholine receptors expressed in Xenopus oocytes have been investigated in electrophysiological and ligand binding experiments. Atropine, scopolamine, physostigmine, and tacrine combine potentiation of ion current induced by low concentrations of acetylcholine with inhibition of ion current evoked by high concentrations of acetylcholine. Rivastigmine, galanthamine, and dichlorvos cause only inhibition of ion current evoked by low concentrations of acetylcholine. Binding experiments show that the potentiating cholinergic drugs atropine, scopolamine, and physostigmine are competitive ligands of human alpha4beta2 nicotinic acetylcholine receptors. Conversely, the inhibitory cholinergic drugs galanthamine and rivastigmine are non-competitive. The non-competitive drugs are not allosteric, since they do not affect the saturation curve of the radioligand [3H]cytisine. Effects of potentiating cholinergic drugs on nicotinic acetylcholine receptors are consistent with and predicted by a model comprising competitive drug effects at two equivalent agonist recognition sites on the nicotinic acetylcholine receptor combined with non-competitive ion channel block.

Regulation of nicotinic acetylcholine receptor channel function by acetylcholinesterase inhibitors in rat hippocampal CA1 interneurons

Neuronal nicotinic acetylcholine receptors (nAChRs) are involved in cognition and may play a role in Alzheimer's disease (AD). Known inhibitors of acetylcholinesterase (AChE) are used to treat AD and are known cognitive enhancers; however, their mechanism of action relating to AD is not fully understood. We tested several AChE inhibitors, including huperzine A, tacrine, and 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide (BW284c51), on nAChRs in rat hippocampal CA1 interneurons in slices using patch-clamp techniques. These interneurons express both alpha7 and non-alpha7 subunit-containing nAChRs and were activated with pressure applications of acetylcholine (ACh), choline, or carbachol. These AChE inhibitors had no significant effect on either the amplitude or kinetics of alpha7 nAChRs activated by ACh, but they slowed the rate of recovery from desensitization through an indirect mechanism; responses activated with either choline or carbachol were unaffected. For non-alpha7 receptors, these inhibitors significantly increased the amplitude and decay phase for responses induced by ACh (but not carbachol), also through an indirect mechanism. Slices preincubated with diisopropylflurophosphate (to permanently inactivate AChE) mimicked the effect of these AChE inhibitors on both alpha7 and non-alpha7 nAChRs. In addition, galantamine, which is both an inhibitor of AChE and an allosteric potentiator of nAChRs, had similar effects. Therefore, various AChE inhibitors are having significant and indirect effects on nAChRs through direct inhibition of AChE; this results in an enhanced amount and/or duration of ACh in slices, with no effect on the levels of choline or carbachol. Therefore, drugs that target AChE are likely to be important regulators of cholinergic signaling in the hippocampus.

Endogenous ouabain-like factor (OLF) secretion is modulated by nicotinic mechanisms in rat adrenocortical cells

This study tested the hypothesis that rat adrenocortical secretion of endogenous ouabain-like factor (OLF) is regulated by nicotinic mechanisms. OLF secreted by dispersed cell suspensions of zona glomerulosa (ZG) and fasciculata/reticularis (ZFR) cells was found to co-elute with authentic ouabain by reverse phase HPLC; OLF concentrations in cell supernatants were measured by radioimmunoassay. Nicotine (10(-6) - 10(-3) M) stimulated significant OLF secretion in rat adrenocortical cells. Acetylcholine (10(-7) - 10(-4) M) and eserine (10(-7) - 10(-3) M) stimulated OLF secretion in ZG cells at lower concentrations and stimulated at higher concentrations. Acetylcholine had no effect on ZFR secretion of OLF, but eserine stimulated OLF secretion. ACTH (10(-8) M) strongly potentiated the OLF stimulatory effect of nicotine in ZG cells; however significant interactions between nicotine and ACTH or angiotensin II on OLF secretion in ZFR cells were not apparent. The ganglionic blockers hexamethonium and mecamylamine further potentiated the effect of nicotine, implicating nicotinic acetylcholine receptors (nAChRs) in regulation of OLF secretion. The alpha7-receptor antagonist methyllycaconitine (MLA) dose-dependently inhibited the effect of nicotine in the ZG cells, and in ZFR cells MLA potentiated nicotine-induced OLF secretion. These data suggest that nicotinic regulation may underlie OLF secretion by rat adrenocortical cells, and strongly suggest presence of functional nicotinic acetylcholine receptors on these cells.

Selective effects of carbamate pesticides on rat neuronal nicotinic acetylcholine receptors and rat brain acetylcholinesterase

Effects of commonly used carbamate pesticides on rat neuronal nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes have been investigated using the two-electrode voltage clamp technique. The potencies of these effects have been compared to the potencies of the carbamates to inhibit rat brain acetylcholinesterase. The potency order of six carbamates to inhibit alpha4beta4 nicotinic receptors is fenoxycarb > EPTC > carbaryl, bendiocarb > propoxur > aldicarb with IC50 values ranging from 3 microM for fenoxycarb to 165 microM for propoxur and >1 mM for aldicarb. Conversely, the potency order of these carbamates to inhibit rat brain acetylcholinesterase is bendiocarb > propoxur, aldicarb > carbaryl > EPTC, fenoxycarb with IC50 values ranging from 1 microM for bendiocarb to 17 microM for carbaryl and > mM for EPTC and fenoxycarb. The alpha4beta2, alpha3beta4, and alpha3beta2 nicotinic acetylcholine receptors are inhibited by fenoxycarb, EPTC, and carbaryl with potency orders similar to that for alpha4beta4 receptors. Comparing the potencies of inhibition of the distinct subtypes of nicotinic acetylcholine receptors shows that the alpha3beta2 receptor is less sensitive to inhibition by fenoxycarb and EPTC. The potency of inhibition depends on the carbamate as well as on a combination of alpha and beta subunit properties. It is concluded that carbamate pesticides affect different subtypes of neuronal nicotinic receptors independently of acetylcholinesterase inhibition. This implicates that neuronal nicotinic receptors are additional targets for some carbamate pesticides and that these receptors may contribute to carbamate pesticide toxicology, especially after long-term exposure.

The allosteric potentiation of nicotinic acetylcholine receptors by galantamine is transduced into cellular responses in neurons: Ca2+ signals and neurotransmitter release

Neuronal nicotinic acetylcholine receptors (nAChR) modulate a variety of cellular responses, including Ca2+ signals and neurotransmitter release, which can influence neuronal processes such as synaptic efficacy and neuroprotection. In addition to receptor activation through the agonist binding site, an allosteric modulation of nAChR has also been described for a novel class of allosteric ligands. Of these, the acetylcholinesterase inhibitor and Alzheimer drug galantamine represents the prototypical allosteric ligand, based on its potentiation of nAChR-evoked single-channel and whole-cell currents. The aim of this study was to establish whether the allosteric potentiation of nAChR currents is transduced in downstream cellular responses to nAChR activation, namely increases in intracellular Ca2+ and [3H]noradrenaline release. In SH-SY5Y cells, galantamine potentiated nicotine-evoked increases in intracellular Ca2+ and [3H]noradrenaline release with a bell-shaped concentration-response profile; maximum enhancement of nicotine-evoked responses occurred at 1 muM galantamine. This potentiation was blocked by mecamylamine, whereas galantamine had no effect on these measures in the absence of nicotine. Galantamine did not compete for radioligand binding to the agonist binding sites of several nAChR subtypes, consistent with an allosteric mode of action. Unlike galantamine, the acetylcholinesterase inhibitors rivastigmine and donepezil did not potentiate nAChR-mediated responses, whereas donepezil was a reasonably potent inhibitor of nicotine- and KCl-evoked increases in Ca2+. nAChR-mediated [3H]noradrenaline release from hippocampal slices was also potentiated by galantamine, with an additional component attributable to acetylcholinesterase inhibition and subsequent increase in acetylcholine. These results indicate that the allosteric regulation of nAChR results in the potentiation of receptor-dependent cellular processes relevant to many of the physiological consequences of nAChR activation.

The anthelmintic levamisole is an allosteric modulator of human neuronal nicotinic acetylcholine receptors

L-[-]-2,3,5,6-Tetrahydro-6-phenylimidazo[2,1b]-thiazole hydrochloride (levamisole) is an anthelmintic that targets the nicotinic acetylcholine receptors of parasitic nematodes. We report here the effects of levamisole on human neuronal alpha 3 beta 2 and alpha 3 beta 4 nicotinic receptors, heterologously expressed in Xenopus oocytes and studied with the voltage clamp method. Applied alone, levamisole was a very weak partial agonist for the two subunit combinations. When co-applied with acetylcholine, micromolar concentrations of levamisole potentiated responses, while millimolar concentrations inhibited them; these effects were complex functions of both acetylcholine and levamisole concentrations. The differences in the levamisole effects on the two receptor combinations suggest that the effects are mediated by the beta subunit. Several combinations of agonist and anthelmintic gave the dual potentiation/inhibition behavior, suggesting that the modulatory effects are general. Levamisole inhibition showed macroscopic characteristics of open channel block. Several results led us to conclude that levamisole potentiation occurs through noncompetitive binding to the receptor. We propose pseudo-site binding for noncompetitive potentiation by levamisole.

Galantamine is an allosterically potentiating ligand of neuronal nicotinic but not of muscarinic acetylcholine receptors

Galantamine (Reminyl), an approved treatment for Alzheimer's disease (AD), is a potent allosteric potentiating ligand (APL) of human alpha 3 beta 4, alpha 4 beta 2, and alpha 6 beta 4 nicotinic receptors (nAChRs), and of the chicken/mouse chimeric alpha 7/5-hydroxytryptamine3 receptor, as was shown by whole-cell patch-clamp studies of human embryonic kidney-293 cells stably expressing a single nAChR subtype. Galantamine potentiates agonist responses of the four nAChR subtypes studied in the same window of concentrations (i.e., 0.1-1 microM), which correlates with the cerebrospinal fluid concentration of the drug at the recommended daily dosage of 16 to 24 mg. At concentrations >10 microM, galantamine acts as an nAChR inhibitor. The other presently approved AD drugs, donepezil and rivastigmine, are devoid of the nicotinic APL action; at micromolar concentrations they also block nAChR activity. Using five CHO-SRE-Luci cell lines, each of them expressing a different human muscarinic receptor, and a reporter gene assay, we show that galantamine does not alter the activity of M1-M5 receptors, thereby confirming that galantamine modulates selectively the activity of nAChRs. These studies support our previous proposal that the therapeutic action of galantamine is mainly produced by its sensitizing action on nAChRs rather than by general cholinergic enhancement due to cholinesterase inhibition. Galantamine's APL action directly addresses the nicotinic deficit in AD.

5-Hydroxyindole potentiates human alpha 7 nicotinic receptor-mediated responses and enhances acetylcholine-induced glutamate release in cerebellar slices

The effects of 5-hydroxyindole (5-HI) have been investigated on human alpha 7 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes and GH4 cells, on native alpha 7 nAChRs expressed by IMR-32 cells and on alpha 7 nAChR-mediated events in mossy fibre-granule cell synapses in rat cerebellar slices. In oocytes expressing alpha 7 nAChRs, 5-HI potentiated sub-maximal, 60 micro M ACh-induced ion currents in a concentration-dependent manner, the threshold effective concentration being 30 micro M. 5-HI itself did not act as an agonist on alpha 7 nAChRs. A maximum potentiation of 12 times the control was observed at 20 mM 5-HI. The effect of 1 mM 5-HI on the concentration-response curve for ACh revealed that 5-HI increased the potency as well as the efficacy of ACh on alpha 7 nAChRs. 5-HI also potentiated alpha 7-mediated increases in intracellular free calcium levels in both mammalian cells heterologously expressing human alpha 7 nAChRs and in human IMR-32 neuroblastoma cells expressing native alpha 7 nAChRs. At mossy fibre-granule cell synapses, application of 1 mM ACh induced glutamate-evoked excitatory post-synaptic currents (EPSCs). Co-application of 1 mM 5-HI with 1 mM ACh further increased the frequency of the EPSCs. The ACh-induced release, as well as the 5-HI-induced enhancement of release, were blocked by 1-10 nM methyllycaconitine or 200 nM alpha-bungarotoxin, demonstrating that both effects were mediated by presynaptic alpha 7 nAChRs. The results demonstrate that responses mediated by alpha 7 nAChRs are strongly potentiated by 5-HI.

Unconventional ligands and modulators of nicotinic receptors

Evidence gathered from epidemiologic and behavioral studies have indicated that neuronal nicotinic receptors (nAChRs) are intimately involved in the pathogenesis of a number of neurologic disorders, including Alzheimer's disease, Parkinson's disease, and schizophrenia. In the mammalian brain, neuronal nAChRs, in addition to mediating fast synaptic transmission, modulate fast synaptic transmission mediated by the major excitatory and inhibitory neurotransmitters glutamate and GABA, respectively. Of major interest, however, is the fact that the activity of the different subtypes of neuronal nAChR is also subject to modulation by substances of endogenous origin such as choline, the tryptophan metabolite kynurenic acid, neurosteroids, and beta-amyloid peptides and by exogenous substances, including the so-called nicotinic allosteric potentiating ligands, of which galantamine is the prototype, and psychotomimetic drugs such as phencyclidine and ketamine. The present article reviews and discusses the effects of unconventional ligands on nAChR activity and briefly describes the potential benefits of using some of these compounds in the treatment of neuropathologic conditions in which nAChR function/expression is known to be altered.

Mechanism of tacrine block at adult human muscle nicotinic acetylcholine receptors

We used single-channel kinetic analysis to study the inhibitory effects of tacrine on human adult nicotinic receptors (nAChRs) transiently expressed in HEK 293 cells. Single channel recording from cell-attached patches revealed concentration- and voltage-dependent decreases in mean channel open probability produced by tacrine (IC(50) 4.6 microM at -70 mV, 1.6 microM at -150 mV). Two main effects of tacrine were apparent in the open- and closed-time distributions. First, the mean channel open time decreased with increasing tacrine concentration in a voltage-dependent manner, strongly suggesting that tacrine acts as an open-channel blocker. Second, tacrine produced a new class of closings whose duration increased with increasing tacrine concentration. Concentration dependence of closed-times is not predicted by sequential models of channel block, suggesting that tacrine blocks the nAChR by an unusual mechanism. To probe tacrine's mechanism of action we fitted a series of kinetic models to our data using maximum likelihood techniques. Models incorporating two tacrine binding sites in the open receptor channel gave dramatically improved fits to our data compared with the classic sequential model, which contains one site. Improved fits relative to the sequential model were also obtained with schemes incorporating a binding site in the closed channel, but only if it is assumed that the channel cannot gate with tacrine bound. Overall, the best description of our data was obtained with a model that combined two binding sites in the open channel with a single site in the closed state of the receptor.

Nicotinic cholinergic modulation: galantamine as a prototype

Nicotinic acetylcholine receptor pharmacology is becoming increasingly important in the clinical symptomatology of neurodegenerative diseases in general and of cognitive and behavioral aspects in particular. In addition, the concept of allosteric modulation of nicotinic acetylcholine receptors has become a research focus for the development of therapeutic agents. In this review the scientific evidence for changes in nicotinic acetylcholine receptors in Alzheimer's disease is described. Within this context, the pharmacology of galantamine, a recently approved drug for cognition enhancement in Alzheimer's disease, is reviewed along with preclinical studies of its efficacy on learning and memory. Galantamine modestly inhibits acetylcholinesterase and has an allosteric potentiating ligand effect at nicotinic receptors. The data collected in this review suggest that the unique combination of acetylcholinesterase inhibition and nicotinic acetylcholine receptor modulation offers potentially significant benefits over acetylcholinesterase inhibition alone in facilitating acetylcholine neurotransmission.

Regulation of GABA release by nicotinic acetylcholine receptors in the neonatal rat hippocampus

1. The whole-cell configuration of the patch-clamp technique was used to study the modulation of giant depolarizing potentials (GDPs) by nicotinic acetylcholine receptors (nAChRs) in CA3 hippocampal neurons in slices from postnatal day (P) 2-6 rats. 2. Bath application of nicotine increased GDP frequency in a concentration-dependent manner. For example, nicotine (0.5-1 microM) enhanced GDP frequency from 0.05 +/- 0.04 to 0.17 +/- 0.04 Hz. This effect was prevented by the broad-spectrum nicotinic receptor antagonist dihydro-beta-erythtroidine (DHbetaE, 50 microM) and partially antagonized by methyllycaconitine (MLA, 50 nM) a competitive antagonist of alpha7 nAChRs. GDP frequency was also enhanced by AR-17779 (100 microM), a selective agonist of alpha7 nAChRs. 3. The GABA(A) receptor antagonist bicuculline (10 microM) and the non-NMDA glutamate receptor antagonist DNQX (20 microM) blocked GDPs and prevented the effects of nicotine on GDPs. In the presence of DNQX, nicotine increased GABA-mediated synaptic noise, indicating that this drug may have a direct effect on GABAergic interneurons. 4. Bath application of edrophonium (20 microM), a cholinesterase inhibitor, in the presence of atropine (1 microM), increased GDP frequency, indicating that nAChRs can be activated by ACh released from the septo-hippocampal fibres. This effect was prevented by DHbetaE (50 microM). 5. In the majority of neurons tested, MLA (50 nM) and DHbetaE (50 microM) reduced the frequency of GDPs with different efficacy: a reduction of 98 +/- 11 and 61 +/- 29 % was observed with DHbetaE and MLA, respectively. In a subset of cells (40 % in the case of MLA and 17 % in the case of DHbetaE) these drugs induced a twofold increase in GDP frequency. 6. It is suggested that, during development, nAChRs modulate the release of GABA, assessed as GDPs, through distinct nAChRs. The rise of intracellular calcium via nAChRs would further strengthen GABA-mediated oscillatory activity. This can be crucial for consolidation of synaptic contacts and for the fine-tuning of the developing hippocampus.

Activity of cytisine and its brominated isosteres on recombinant human alpha7, alpha4beta2 and alpha4beta4 nicotinic acetylcholine receptors

Effects of cytisine (cy), 3-bromocytisine (3-Br-cy), 5-bromocytisine (5-Br-cy) and 3,5-dibromocytisine (3,5-diBr-cy) on human (h) alpha7-, alpha4beta2- and alpha4beta4 nicotinic acetylcholine (nACh) receptors, expressed in Xenopus oocytes and cell lines, have been investigated. Cy and its bromo-isosteres fully inhibited binding of both [alpha-(125)I]bungarotoxin ([alpha-(125)I]BgTx) to halpha7- and [(3)H]cy to halpha4beta2- or halpha4beta4-nACh receptors. 3-Br-cy was the most potent inhibitor of both [alpha-(125)I]BgTx and [(3)H]cy binding. Cy was less potent than 3-Br-cy, but 5-Br-cy and 3,5-diBr-cy were the least potent inhibitors. Cy and 3-Br-cy were potent full agonists at halpha7-nACh receptors but behaved as partial agonists at halpha4beta2- and halpha4beta4-nACh receptors. 5-Br-cy and 3,5-diBr-cy had low potency and were partial agonists at halpha7- and halpha4beta4-nACh receptors, but they elicited no responses on halpha4beta2-nACh receptors. Cy and 3-Br-cy produced dual dose-response curves (DRC) at both halpha4beta2- and halpha4beta4-nACh receptors, but ACh produced dual DRC only at halpha4beta2-nACh receptors. Low concentrations of cy, 3-Br-cy and 5-Br-cy enhanced ACh responses of oocytes expressing halpha4beta2-nACh receptors, but at high concentrations they inhibited the responses. In contrast, 3,5-diBr-cy only inhibited, in a competitive manner, ACh responses of halpha4beta2-nACh receptors. It is concluded that bromination of the pyridone ring of cy produces marked changes in effects of cy that are manifest as nACh receptor subtype-specific differences in binding affinities and in functional potencies and efficacies.