A novel agonist binding site on nicotinic acetylcholine receptors

Galantamine is not a positive allosteric modulator of human α4β2 or α7 nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

The alkaloid galantamine was originally isolated from the green snowdrop Galanthus woronowii and is currently marketed as a drug for treatment of mild to moderate dementia in patients with Alzheimer's disease. In addition to a well-documented proficiency to inhibit acetylcholinesterase, galantamine has been reported to increase neuronal nicotinic ACh (nACh) receptor function by acting as a positive allosteric modulator. Yet there remains controversy regarding these findings in the literature. To resolve this conundrum, we evaluated galantamine actions at α4β2 and α7, which represent the nACh receptors most commonly associated with mammalian cognitive domains.

EXPERIMENTAL APPROACH

α4β2 [in (α4)₃ (β2)₂ and (α4)₂ (β2)₃ stoichiometries] and α7 nACh receptors were expressed in Xenopus laevis oocytes and subjected to two-electrode voltage-clamp electrophysiological experiments. Galantamine (10 nM to 100 μM) was evaluated for direct agonist effects and for positive modulation by co-application with sub-maximally efficacious concentrations of ACh. In addition, similar experiments were performed with α7 nACh receptors stably expressed in HEK293 cells using patch-clamp electrophysiology.

KEY RESULTS

In concentrations ranging from 10 nM to 1 μM, galantamine did not display direct agonism nor positive modulatory effects at any receptor combination tested. At concentrations from 10 μM and above, galantamine inhibited the activity with a mechanism of action consistent with open-channel pore blockade at all receptor types.

CONCLUSION AND IMPLICATIONS

Based on our data, we conclude that galantamine is not a positive allosteric modulator of α7 or α4β2 receptors, which represent the majority of nACh receptors in mammalian brain.

Axo-axonal interaction in autonomic regulation of the cerebral circulation

Noradrenaline (NE) and acetylcholine (ACh) released from the sympathetic and parasympathetic neurones in cerebral blood vessels were suggested initially to be the respective vasoconstricting and dilating transmitters. Both substances, however, are extremely weak post-synaptic transmitters. Compelling evidence indicates that nitric oxide (NO) which is co-released with ACh from same parasympathetic nerves is the major transmitter for cerebral vasodilation, and its release is inhibited by ACh. NE released from the sympathetic nerve, acting on presynaptic β2-adrenoceptors located on the neighbouring parasympathetic nitrergic nerves, however, facilitates NO release with enhanced vasodilation. This axo-axonal interaction mediating NE transmission is supported by close apposition between sympathetic and parasympathetic nerve terminals, and has been shown in vivo at the base of the brain and the cortical cerebral circulation. This result reveals the physiological need for increased regional cerebral blood flow in 'fight-or-flight response' during acute stress. Furthermore, α7- and α3β2-nicotinic ACh receptors (nAChRs) on sympathetic nerve terminals mediate release of NE, leading to cerebral nitrergic vasodilation. α7-nAChR-mediated but not α3β2-nAChR-mediated cerebral nitrergic vasodilation is blocked by β-amyloid peptides (Aβs). This may provide an explanation for cerebral hypoperfusion seen in patients with Alzheimer's disease. α7- and α3β2-nAChR-mediated nitrergic vasodilation is blocked by cholinesterase inhibitors (ChEIs) which are widely used for treating Alzheimer's disease, leading to possible cerebral hypoperfusion. This may contribute to the limitation of clinical use of ChEIs. ChEI blockade of nAChR-mediated dilation like that by Aβs is prevented by statins pretreatment, suggesting that efficacy of ChEIs may be improved by concurrent use of statins.

A transgenic mouse model reveals fast nicotinic transmission in hippocampal pyramidal neurons

The relative contribution to brain cholinergic signaling by synaptic- and diffusion-based mechanisms remains to be elucidated. In this study, we examined the prevalence of fast nicotinic signaling in the hippocampus. We describe a mouse model where cholinergic axons are labeled with the tauGFP fusion protein driven by the choline acetyltransferase promoter. The model provides for the visualization of individual cholinergic axons at greater resolution than other available models and techniques, even in thick, live, slices. Combining calcium imaging and electrophysiology, we demonstrate that local stimulation of visualized cholinergic fibers results in rapid excitatory postsynaptic currents mediated by the activation of α7-subunit-containing nicotinic acetylcholine receptors (α7-nAChRs) on CA3 pyramidal neurons. These responses were blocked by the α7-nAChR antagonist methyllycaconitine and potentiated by the receptor-specific allosteric modulator 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxanol-3-yl)-urea (PNU-120596). Our results suggest, for the first time, that synaptic nAChRs can modulate pyramidal cell plasticity and development. Fast nicotinic transmission might play a greater role in cholinergic signaling than previously assumed. We provide a model for the examination of synaptic properties of basal forebrain cholinergic innervation in the brain.

Calcium signalling mediated through α7 and non-α7 nAChR stimulation is differentially regulated in bovine chromaffin cells to induce catecholamine release

BACKGROUND AND PURPOSE

Ca(2+) signalling and exocytosis mediated by nicotinic receptor (nAChR) subtypes, especially the α7 nAChR, in bovine chromaffin cells are still matters of debate.

EXPERIMENTAL APPROACH

We have used chromaffin cell cultures loaded with Fluo-4 or transfected with aequorins directed to the cytosol or mitochondria, several nAChR agonists (nicotine, 5-iodo-A-85380, PNU282987 and choline), and the α7 nAChR allosteric modulator PNU120596.

KEY RESULTS

Minimal [Ca(2+) ](c) transients, induced by low concentrations of selective α7 nAChR agonists and nicotine, were markedly increased by the α7 nAChR allosteric modulator PNU120596. These potentiated responses were completely blocked by the α7 nAChR antagonist α-bungarotoxin (α7-modulated-response). Conversely, high concentrations of the α7 nAChR agonists, nicotine or 5-iodo-A-85380 induced larger [Ca(2+) ](c) transients, that were blocked by mecamylamine but were unaffected by α-bungarotoxin (non-α7 response). [Ca(2+) ](c) increases mediated by α7 nAChR were related to Ca(2+) entry through non-L-type Ca(2+) channels, whereas non-α7 nAChR-mediated signals were related to L-type Ca(2+) channels; Ca(2+) -induced Ca(2+) -release contributed to both responses. Mitochondrial involvement in the control of [Ca(2+) ](c) transients, mediated by either receptor, was minimal. Catecholamine release coupled to α7 nAChRs was more efficient in terms of catecholamine released/[Ca(2+) ](c) .

CONCLUSIONS AND IMPLICATIONS

[Ca(2+) ](c) and catecholamine release mediated by α7 nAChRs required an allosteric modulator and low doses of the agonist. At higher agonist concentrations, the α7 nAChR response was lost and the non-α7 nAChRs were activated. Catecholamine release might therefore be regulated by different nAChR subtypes, depending on agonist concentrations and the presence of allosteric modulators of α7 nAChRs.

Cholinesterase inhibitor blockade and its prevention by statins of sympathetic alpha7-nAChR-mediated cerebral nitrergic neurogenic vasodilation

Cholinesterase inhibitors (ChEIs) have been used to treat Alzheimer's disease (AD). The efficacy of these drugs, however, is less than satisfactory. The possibility that ChEIs may have effects unrelated to ChE activity, such as negatively modulate neuronal nicotinic acetylcholine receptors (nAChRs) was evaluated. Since alpha7-nAChRs on cerebral perivascular sympathetic neurons mediate cerebral parasympathetic-nitrergic vasodilation, effects of physostigmine, neostigmine, and galantamine on alpha7-nAChR-mediated dilation in isolated porcine basilar arterial rings denuded of endothelium was examined using in vitro tissue bath technique. The results indicated that these ChEIs blocked vasodilation induced by choline (0.3 mmol/L), nicotine (0.1 mmol/L), and transmural nerve stimulation (TNS). The ChEI inhibition of dilation induced by TNS but not by choline or nicotine was prevented by atropine (0.1 micromol/L) pretreatment. Furthermore, using confocal microscopy, significant calcium influx induced by choline and nicotine in cultured porcine superior cervical ganglion (SCG) cells was attenuated by ChEIs. In alpha7-nAChR-expressed Xenopus oocytes, nicotine-induced inward currents were attenuated by alpha-bungarotoxin and ChEIs. Moreover, ChEI inhibition of nicotine- and choline-induced dilation was prevented by pretreatment with mevastatin and lovastatin (10 micromol/L), which did not affect ChEI inhibition of TNS-induced relaxation. These findings suggest that ChEIs inhibit the alpha7-nAChRs located on postganglionic sympathetic nerve terminals of SCG origin, causing a decreased release of nitric oxide in the neighboring nitrergic nerves and cerebral vasodilation. Inhibition of alpha7-nAChRs leading to a potential cerebral hypoperfusion may contribute to the limitation of ChEIs and question the validity of using a ChEI alone in treating AD. The efficacy of ChEIs may be improved by concurrent use of statins.

Physostigmine modulation of acetylcholine currents in COS cells transfected with mouse muscle nicotinic receptor

Physostigmine (Phy), a reversible inhibitor of acetylcholine (ACh) esterase (AChE), may also act as a low potency agonist and a modulator of the nicotinic receptor. The actions of Phy on mouse muscle nicotinic receptors in the COS-7 cell line were studied by the patch-clamp technique. Currents were recorded in the whole-cell mode 3-7 days after cell transfection by plasmids coding alphabetagammadelta combination of receptor subunits. The application of ACh to cells clamped at -10 mV produced inward currents which displayed desensitization. The application of Phy in concentrations up to 1 x 10(-3) M did not give reliable specific whole-cell membrane responses. The application of Phy in concentrations of 10(-6)-10(-4) M together with ACh modulated the amplitude; accelerated desensitization of currents induced by ACh and increased the final extent of desensitization in a concentration-dependent manner. This finding is in contrast to the suppression and slowing down of desensitization by Phy and 1-methyl-galanthamine observed in Torpedo receptors.

Differential effects of TAK-802, a selective acetylcholinesterase inhibitor, and carbamate acetylcholinesterase inhibitors on contraction of the detrusor smooth muscle of the guinea pig

The aim of this study was to compare the effects of TAK-802, a novel acetylcholinesterase (AChE) inhibitor, and carbamate AChE inhibitors on the detrusor smooth muscle contractility in vitro using isometric tension measurements. The effects of drugs on the nicotine-induced contractions and basal tone of the isolated detrusor muscle of the guinea pig were examined. All of the drugs, namely, TAK-802, distigmine, neostigmine and pyridostigmine, enhanced the nicotine-induced contractions of the muscle strips in a concentration-dependent manner. On the other hand, while neostigmine and pyridostigmine markedly increased the basal tone, and distigmine slightly but significantly increased the basal tone, TAK-802 had no influence on the basal tone of the muscle strips at all. However, following co-treatment with tetraisopropyl pyrophosphoramide, a selective butyrylcholinesterase (BuChE) inhibitor, TAK-802 also did increase the basal tone. The increase of the basal tone by all of the above treatments was completely abolished by atropine. These results reveal that while all the four AChE inhibitors enhanced endogenous acetylcholine-induced contractions, their effects on the basal tone were clearly different. The effect of carbamate AChE inhibitors of increasing the basal tone could be partly attributed to their dual inhibition of both AChE and BuChE, because both cholinesterases may play a critical role in maintaining the resting tension of the urinary bladder. TAK-802, however, did not increase the basal tone of the detrusor muscle strips, probably because of its selective inhibitory effect against AChE. The effect of carbamate AChE inhibitors on the basal tone of the detrusor muscle may explain the decrease of bladder compliance observed in our previous study on guinea pigs as well as the deterioration of the bladder-storage function reported with their clinical use.

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.

Novel acetylcholinesterase inhibitor as increasing agent on rhythmic bladder contractions: SAR of 8-{3-[1-(3-fluorobenzyl)piperidin-4-yl]propanoyl}-1,2,5,6-tetrahydro-4H-pyrrolo[3,2,1-ij]quinolin-4-one (TAK-802) and related compounds

As part of an on-going investigation to develop an increasing agent on rhythmic bladder contractions, 1-aryl-3-(1-benzylpiperidin-4-yl)propanones were synthesized and examined as noncarbamate acetylcholinesterase (AChE) inhibitors. Among compounds with various aryl groups, 1,2,5,6-tetrahydro-4H-pyrrolo[3,2,1-ij]quinolin-4-one derivative 9c was found to possess a potent AChE inhibition activity with an IC(50) value of 1.3nM. The compound 9c increased rhythmic bladder contractions in Guinea pigs and rats without affecting the basal intravesical pressure, which suggests that 9c may be useful for the treatment of voiding dysfunction caused by detrusor underactivity.

Effects of TAK-802, a novel acetylcholinesterase inhibitor, and various cholinomimetics on the urodynamic characteristics in anesthetized guinea pigs

In the present study, we investigated the effects of cholinomimetic drugs on the urodynamic characteristics in anesthetized guinea pigs. 8-[3-[1-[(3-fluorophenyl)methyl]-4-piperidinyl]-1-oxopropyl]-1,2,5,6-tetrahydro-4H-pyrrolo[3,2,1-ij]quinolin-4-one (TAK-802), a novel acetylcholinesterase inhibitor, (0.003-0.03 mg/kg, i.v.) increased the voided volume and the maximum flow rate without affecting either the intravesical pressure or the bladder compliance. Distigmine (0.03-0.3 mg/kg, i.v.) and neostigmine (0.01-0.1 mg/kg, i.v.), both carbamate acetylcholinesterase inhibitors, while not increasing the maximum flow rate, increased the intravesical pressure at the maximum flow rate. They also decreased the bladder compliance. Bethanechol (0.1-1 mg/kg, i.v.), a muscarinic receptor agonist, decreased the voided volume and the bladder compliance but did not affect the maximum flow rate. TAK-802 did not affect the intraurethral pressure at doses of up to 0.03 mg/kg in anesthetized guinea pigs. Distigmine increased the intraurethral pressure when administered at the dose of 0.3 mg/kg, and the effect was completely abolished by pretreatment with d-tubocurarine. These results suggest that TAK-802 reinforces the bladder-voiding functions by increasing the bladder contractility without decreasing the storage function. On the other hand, carbamate acetylcholinesterase inhibitors not only deteriorate the voiding function by inducing contraction of the external urethral sphincter muscle, resulting in increasing the urethral resistance, but also cause deterioration of the storage function. Bethanechol obviously decreased the bladder capacity, possibly due to a direct contractile effect on the detrusor smooth muscle. TAK-802 may therefore be a more useful drug than either carbamate acetylcholinesterase inhibitors or muscarinic receptor agonists in the treatment of voiding dysfunction associated with impaired detrusor contractility.

Effects of TAK-802, a novel acetylcholinesterase inhibitor, on distension-induced rhythmic bladder contractions in rats and guinea pigs

In the present study, we investigated the effects of 8-[3-[1-[(3-fluorophenyl)methyl]-4-piperidinyl]-1-oxopropyl]-1,2,5,6-tetrahydro-4H-pyrrolo[3,2,1-ij]quinolin-4-one (TAK-802), a novel acetylcholinesterase inhibitor, on distension-induced rhythmic bladder contractions in urethane-anesthetized rats and guinea pigs. TAK-802 potently inhibited human-erythrocyte-derived acetylcholinesterase activity with an IC(50) value of 1.5 nM, which represented a potency 30 and 250 times greater than that of the two carbamate acetylcholinesterase inhibitors, neostigimine and distigmine, respectively. Unlike the carbamate acetylcholinesterase inhibitors, TAK-802 exhibits high selectivity for acetylcholinesterase inhibition over butyrylcholinesterase inhibition. In an assay conducted to measure the muscarinic and nicotinic actions, TAK-802 was found to exhibit higher selectivity for muscarinic actions over nicotinic actions in comparison to distigmine. Both TAK-802 and distigmine increased isovolumetric bladder contractions in rats and guinea pigs in a dose-dependent manner, with a minimum effective dose (MED) of 0.01 and 0.03 mg/kg i.v., respectively, in rats, and 0.01 and 0.1 mg/kg i.v., respectively, in guinea pigs. The effects of both the drugs were completely abolished by atropine. These results suggest that TAK-802 and other acetylcholinesterase inhibitors can effectively increase reflex bladder contractions by increasing the efficacy of acetylcholine released by nerve impulses. On the other hand, bethanechol, a muscarinic agonist, markedly changed the pattern of distension-induced bladder contractions when administered at the dose of 1 mg/kg i.v., and it did not necessarily augment well-coordinated bladder contractions. Thus, considering that it has some selectivity for muscarinic action, TAK-802 might be expected to be useful in the treatment of voiding dysfunction caused by impaired detrusor contractility.

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.

Localization of agonist and competitive antagonist binding sites on nicotinic acetylcholine receptors

Identification of all residues involved in the recognition and binding of cholinergic ligands (e.g. agonists, competitive antagonists, and noncompetitive agonists) is a primary objective to understand which structural components are related to the physiological function of the nicotinic acetylcholine receptor (AChR). The picture for the localization of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are located mainly on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are identical, the observed high and low affinity for different ligands on the receptor is conditioned by the interaction of the alpha subunit with other non-alpha subunits. This molecular interaction takes place at the interface formed by the different subunits. For example, the high-affinity acetylcholine (ACh) binding site of the muscle-type AChR is located on the alphadelta subunit interface, whereas the low-affinity ACh binding site is located on the alphagamma subunit interface. Regarding homomeric AChRs (e.g. alpha7, alpha8, and alpha9), up to five binding sites may be located on the alphaalpha subunit interfaces. From the point of view of subunit arrangement, the gamma subunit is in between both alpha subunits and the delta subunit follows the alpha aligned in a clockwise manner from the gamma. Although some competitive antagonists such as lophotoxin and alpha-bungarotoxin bind to the same high- and low-affinity sites as ACh, other cholinergic drugs may bind with opposite specificity. For instance, the location of the high- and the low-affinity binding site for curare-related drugs as well as for agonists such as the alkaloid nicotine and the potent analgesic epibatidine (only when the AChR is in the desensitized state) is determined by the alphagamma and the alphadelta subunit interface, respectively. The case of alpha-conotoxins (alpha-CoTxs) is unique since each alpha-CoTx from different species is recognized by a specific AChR type. In addition, the specificity of alpha-CoTxs for each subunit interface is species-dependent. In general terms we may state that both alpha subunits carry the principal component for the agonist/competitive antagonist binding sites, whereas the non-alpha subunits bear the complementary component. Concerning homomeric AChRs, both the principal and the complementary component exist on the alpha subunit. The principal component on the muscle-type AChR involves three loops-forming binding domains (loops A-C). Loop A (from mouse sequence) is mainly formed by residue Y(93), loop B is molded by amino acids W(149), Y(152), and probably G(153), while loop C is shaped by residues Y(190), C(192), C(193), and Y(198). The complementary component corresponding to each non-alpha subunit probably contributes with at least four loops. More specifically, the loops at the gamma subunit are: loop D which is formed by residue K(34), loop E that is designed by W(55) and E(57), loop F which is built by a stretch of amino acids comprising L(109), S(111), C(115), I(116), and Y(117), and finally loop G that is shaped by F(172) and by the negatively-charged amino acids D(174) and E(183). The complementary component on the delta subunit, which corresponds to the high-affinity ACh binding site, is formed by homologous loops. Regarding alpha-neurotoxins, several snake and alpha-CoTxs bear specific residues that are energetically coupled with their corresponding pairs on the AChR binding site. The principal component for snake alpha-neurotoxins is located on the residue sequence alpha1W(184)-D(200), which includes loop C. In addition, amino acid sequence 55-74 from the alpha1 subunit (which includes loop E), and residues gammaL(119) (close to loop F) and gammaE(176) (close to loop G) at the low-affinity binding site, or deltaL(121) (close to the homologous region of loop G) at the high-affinity binding site, are i

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.

Expression and renaturation of the N-terminal extracellular domain of torpedo nicotinic acetylcholine receptor alpha-subunit

The N-terminal extracellular region (amino acids 1-209) of the alpha-subunit of the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue was expressed as inclusion bodies in Escherichia coli using the pET 3a vector. Employing a novel protocol of unfolding and refolding, in the absence of detergent, a water-soluble globular protein of 25 kDa was obtained displaying approximately 15% alpha-helical and 45% beta-structure. The fragment bound alpha-[3H]bungarotoxin in 1:1 stoichiometry with a KD value of 0.5 nM as determined from kinetic measurements (4 nM from equilibrium binding). The kinetics of association of toxin and fragment were of second order, with a similar rate constant (8.2 x 10(5) M-1 s-1) as observed previously for the membrane-bound heteropentameric nAChR. Binding of small ligands was demonstrated by competition with alpha-[3H]bungarotoxin yielding the following KI values: acetylcholine, 69 microM; nicotine, 0.42 microM; anatoxin-a, 3 miroM; tubocurarine, 400 microM; and methyllycaconitine, 0.12 microM. The results demonstrate that the N-terminal extracellular region of the nAChR alpha-subunit forms a self-assembling domain that functionally expresses major elements of the ligand binding sites of the receptor.

Effects of extracellular acetylcholine on muscarinic receptor binding assessed by [125I]dexetimide and a simple probe

New pharmacologic approaches to enhance brain cholinergic function focus on increasing intrasynaptic acetylcholine. We examined the usefulness of a simple probe and [125I]dexetimide to evaluate in vivo the effects of extracellular acetylcholine on muscarinic receptor binding in the mouse brain. After radiotracer injection continuous time/activity curves were generated over 330 min. [125I]Dexetimide reached a plateau at 90 min post-injection. To increase extracellular acetylcholine, the anticholinesterase physostigmine was administered at 120 min, producing a reversible decrease in [125I]dexetimide specific binding (23%) for 30 min. These findings demonstrate that dynamic changes in extracellular acetylcholine can be evaluated by displacement of [125I]dexetimide binding in vivo using a simple probe system.

Inhibitory effects of tacrine and physostigmine on catecholamine secretion and membrane currents in guinea-pig adrenal chromaffin cells

The effects of tacrine and physostigmine on catecholamine secretion induced by veratridine and high K+, and on voltage-dependent Na+ and Ca2+ currents, were investigated in guinea-pig adrenal chromaffin cells. In perfused adrenal glands, tacrine (100 microM) caused an inhibition of veratridine-induced catecholamine secretion, but physostigmine (100 microM) did not. In dispersed cells, both tacrine (1 microM-1 mM) and physostigmine (1 microM-1 mM) decreased catecholamine secretion induced by veratridine in a dose-dependent manner. The inhibitory effect of tacrine was much greater than that of physostigmine. Tacrine alone at a high concentration (such as 1 mM) caused a substantial increase in catecholamine secretion by itself and completely abolished the veratridine-induced secretory response in dispersed cells. High-concentration physostigmine showed a similar effect, but to a much lesser extent. The high K+ (46.2 mM)-evoked catecholamine secretion from dispersed cells was not affected by tacrine (1-100 microM) or physostigmine (1 microM-1 mM). In fura-2 loaded cells, tacrine (100 microM) almost abolished [Ca2+]i rise induced by veratridine, but only slightly reduced that evoked by high K+. In voltage-clamped cells, tacrine (300 microM) depressed the voltage-dependent Na+ and Ca2+ current by about 93% and 69%, and physostigmine (300 microM) depressed them by about 30% and 17%, respectively. These results suggest that tacrine decreases the veratridine-induced catecholamine secretion primarily by inhibiting the voltage-dependent Na+ channels rather than the Ca2+ channels. Physostigmine acts in a manner similar to tacrine, but its potency is much lower than that of tacrine.

Brain nicotinic receptors: structure and regulation, role in learning and reinforcement

The introduction, in the late sixties, of the concepts and methods of molecular biology to the study of the nervous system had a profound impact on the field, primarily through the identification of its basic molecular components. These structures include, for example, the elementary units of the synapse: neurotransmitters, neuropeptides and their receptors, but also ionic channels, intracellular second messengers and the relevant enzymes, cell surface adhesion molecules, or growth and trophic factors [21,78,81, 52,79]. Attempts to establish appropriate causal relationships between these molecular components, the actual organisation of neural networks, and a defined behavior, nevertheless, still must overcome many difficulties. A first problem is the recognition of the minimum levels of organisation, from the molecular, cellular, or multicellular (circuit) to the higher cognitive levels, that determine the given physiological and/or behavioral performance under investigation. A common difficulty (and potential source of errors of interpretation) is to relate a cognitive function to a network organization which does not possess the required structural complexity and vice-versa. Another problem is to distinguish, among the components of the system, those which are actually necessary and those which, taken together, suffice for a given behavior to take place. Identification of such a minimal set of building blocks may receive decisive insights from the elaboration of neurally plausible formal models that bring together, within a single and coherent 'artificial organism', the neuronal network, the circulating activity, and the behavior they determine (see [42,43,45,72,30]). In this communication, we shall attempt, still in a preliminary fashion, to bring together: (1) our recent knowledge on the molecular biology of brain nicotinic receptors (nAChRs) and their allosteric properties and (2) integrated behaviors, such as cognitive learning, investigated for instance with delayed-response or passive avoidance tasks that are likely to involve nAChRs in particular at the level of reinforcement (or reward) mechanisms (see [18,29,135]).

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Direct actions of anticholinesterases on the neuronal nicotinic acetylcholine receptor channels

Recent studies have suggested that anticholinesterases including organophosphates and carbamates act directly on the nicotinic acetylcholine receptor (AChR) channel. We performed whole-cell and single-channel patch-clamp experiments to elucidate the mechanism of action of anticholinesterases on the nicotinic AChR in rat clonal phaeochromocytoma (PC12) cells. Neostigmine and carbaryl showed a biphasic effect; enhancement and suppression of carbachol-induced whole-cell currents. The currents induced by 100 microM carbachol was enhanced by the first co-application with 10 or 100 microM neostigmine, and the current was eventually suppressed below the control level during repeated co-applications. The decay phase of current was accelerated by neostigmine. Carbaryl at 0.1 microM greatly potentiated the carbachol-induced current, and at higher concentrations (0.3-3 microM), current was suppressed. In single-channel experiments, these compounds increased the short closures or gaps during channel opening without changing the single-channel conductance. Mean open time and burst duration were decreased in the presence of neostigmine and carbaryl. These results indicate that neostigmine and carbaryl directly block the nicotinic AChR channel.

Screening structural-functional relationships of neuropharmacologically active organic compounds at the nicotinic acetylcholine receptor

The mechanisms of action and pharmacological effects on the nicotinic cholinoceptor of a large database of organic compounds were analyzed using a new computational procedure. This procedure is a screening method based on comparison of the molecular structures (shape and charge) of the putative active organic compounds. The resulting predictions can be used as an exploratory tool in the design of experiments aimed at testing the effects of several compounds on a target macromolecule. Unlike a conventional database search for structural similarities, the present method is able to circumscribe objectively the results to the most statistically significant molecules.

Nicotinic acetylcholine receptors on hippocampal neurons: distribution on the neuronal surface and modulation of receptor activity

The recent development of a technique that uses infrared microscopy for the visualization of well-defined areas on the surface of neurons, and a computerized system of micromanipulators led to the discovery that functional nicotinic acetylcholine receptors (nAChRs) are expressed at higher density on the dendrites than on the soma of rat hippocampal neurons. The finding that the expression of alpha-bungarotoxin-sensitive, alpha 7-bearing, nAChRs and dihydro-beta-erythroidine-sensitive, alpha 4 beta 2 nAChRs tends to increase along the dendritic length suggests that these receptors may be highly involved in the integration of synaptic functions in hippocampal neurons. The present report also discusses the finding that ligands such as the anticholinesterase galanthamine can modulate the nAChR activity by binding to a novel receptor site, and that 5-hydroxytryptamine (5-HT) may serve as an endogenous ligand for this site. The ability of 5-HT to modulate the nAChR function in vivo supports the concept that the overall CNS function is determined not only by the neuronal network established by the neuronal wiring, but also by a chemical network established by the ability of a single substance to act as the primary neurotransmitter in one system and as a co-transmitter in another system.

Pharmacology of the nicotinic acetylcholine receptor from fetal rat muscle expressed in Xenopus oocytes

The fetal rat muscle nicotinic acetylcholine receptor was expressed in Xenopus oocytes. Using the voltage-clamp technique, the response to a range of agonists was measured, listed in order of (decreasing) activity efficacy: anatoxin > or = epibatidine > acetylcholine > DMPP (1,1-dimethyl-4-phenylpiperazinium) > > cytisine > pyrantel > nicotine > coniine > tubocurare > lobeline. The agonist responses were compared with the steric and electrostatic properties of the molecules, using molecular modelling. Single-channel current were measured in outside-out patches for acetylcholine, nicotine, cytisine, anatoxin and epibatidine. The conductance of the single channels was independent of the type of agonist. The mean open times were characteristic of the agonist applied. Tubocurare, better known for its antagonist properties, was also a partial agonist. Single-channel currents were also observed for tubocurare, and for methyllycaconitine in patches with a very high density of the muscle nicotinic acetylcholine receptor, and these were blocked by alpha-bungarotoxin. The agonist properties of physostigmine, galanthamine and their methyl derivatives were also investigated. The conductance of the channels observed in outside-out patches was similar to that obtained for the classical agonists. The single-channel currents observed for physostigmine, galanthamine and their methyl derivatives were blocked by alpha-bungarotoxin, methyllycaconitine and mecamylamine, in contrast to previously reported studies on neuronal and adult muscle nicotinic acetylcholine receptors.

The emerging three-dimensional structure of a receptor. The nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is the neurotransmitter receptor with the most-characterized protein structure. The amino acid sequences of its five subunits have been elucidated by cDNA cloning and sequencing. Its shape and dimensions (approximately 12.5 nm x 8 nm) were deduced from electron-microscopy studies. Its subunits are arranged around a five-fold axis of pseudosymmetry in the order (clockwise) alpha H gamma alpha L delta beta. Its two agonist/competitive-antagonist-binding sites have been localized by photolabelling studies to a deep gorge between the subunits near the membrane surface. Its ion channel is formed by five membrane-spanning (M2) helices that are contributed by the five subunits. This finding has been generalized as the Helix M2 model for the superfamily of ligand-gated ion channels. The binding site for regulatory non-competitive antagonists has been localized by photolabelling and site-directed-mutagenesis studies within this ion channel. Therefore a three-dimensional image of the nicotinic acetylcholine receptor is emerging, the most prominent feature of which is an active site that combines the agonist/ competitive-antagonist-binding sites, the regulatory site and the ion channel within a relatively narrow space close to and within the bilayer membrane.

The pharmacology of (-)-nicotine and novel cholinergic channel modulators

Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Physostigmine, galanthamine and codeine act as 'noncompetitive nicotinic receptor agonists' on clonal rat pheochromocytoma cells

The acetylcholine esterase inhibitor (-)-physostigmine has been shown to act as agonist on nicotinic acetylcholine receptors from muscle and brain, by binding to sites on the alpha-polypeptide that are distinct from those for the natural transmitter acetylcholine (Schröder et al., 1994). In the present report we show that (-)-physostigmine, galanthamine, and the morphine derivative codeine activate single-channel currents in outside-out patches excised from clonal rat pheochromocytoma (PC12) cells. Although several lines of evidence demonstrate that the three alkaloids act on the same channels as acetylcholine, the competitive nicotinic antagonist methyllycaconitine only inhibited channel activation by acetylcholine but not by (-)-physostigmine, galanthamine or codeine. In contrast, the monoclonal antibody FK1, which competitively inhibits (-)-physostigmine binding to nicotinic acetylcholine receptors, did not affect channel activation by acetylcholine but inhibited activation by (-)-physostigmine, galanthamine and codeine. The three alkaloids therefore act via binding sites distinct from those for acetylcholine, in a 'noncompetitive' fashion. The potency of (-)-physostigmine and related compounds to act as a noncompetitive agonist is unrelated to the level of acetylcholine esterase inhibition induced by these drugs. (-)-Physostigmine, galanthamine and codeine do not evoke sizable whole-cell currents, which is due to the combined effects of low open-channel probability, slow onset and slow inactivation of response. In contrast, they sensitize PC12 cell nicotinic receptors in their submaximal response to acetylcholine. While the abundance of nicotinic acetylcholine receptor isoforms expressed in PC12 cells excludes identification of specific nicotinic acetylcholine receptor subtypes that interact with noncompetitive agonists, the identical patterns of single-channel current amplitudes observed with acetylcholine and with noncompetitive agonists suggested that all PC12 cell nicotinic acetylcholine receptor subtypes that respond to acetylcholine also respond to noncompetitive agonist. The action of noncompetitive agonists therefore seems to be highly conserved between nicotinic acetylcholine receptor subtypes, in agreement with the high level of structural conservation in the sequence region harboring major elements of this site.

Nicotinic receptor function in the mammalian central nervous system

The diversity of neuronal nicotinic receptors (nAChRs) in addition to their possible involvement in such pathological conditions as Alzheimer's disease have directed our research towards the characterization of these receptors in various mammalian brain areas. Our studies have relied on electrophysiological, biochemical, and immunofluorescent techniques applied to cultured and acutely dissociated hippocampal neurons, and have been aimed at identifying the various subtypes of nAChRs expressed in the mammalian central nervous system (CNS), at defining the mechanisms by which CNS nAChR activity is modulated, and at determining the ion permeability of CNS nAChR channels. Our findings can be summarized as follows: (1) hippocampal neurons express at least three subtypes of CNS nAChRs--an alpha 7-subunit-bearing nAChR that subserves fast-inactivating, alpha-BGT-sensitive currents, which are referred to as type IA, and alpha 4 beta 2 nAChR that subserves slowly inactivating, dihydro-beta-erythroidine-sensitive currents, which are referred to as type II, and an alpha 3 beta 4 nAChR that subserves slowly inactivating, mecamylamine-sensitive currents, which are referred to as type III; (2) nicotinic agonists can activate a single type of nicotinic current in olfactory bulb neurons, that is, type IA currents; (3) alpha 7-subunit-bearing nAChR channels in the hippocampus have a brief lifetime, a high conductance, and a high Ca2+ permeability; (4) the peak amplitude of type IA currents tends to rundown with time, and this rundown can be prevented by the presence of ATP-regenerating compounds (particularly phosphocreatine) in the internal solution; (5) rectification of type IA currents is dependent on the presence of Mg2+ in the internal solution; and (6) there is an ACh-insensitive site on neuronal and nonneuronal nAChRs through which the receptor channel can be activated. These findings lay the groundwork for a better understanding of the physiological role of these receptors in synaptic transmission in the CNS.

Identification of purine binding sites on Torpedo acetylcholine receptor

Electrophysiological studies from this and other laboratories have suggested a direct action of ATP on nicotinic acetylcholine receptors (nAChR). To determine the site of binding of this purine derivative, we have covalently modified the nAChR from Torpedo marmorata electrocytes employing 2-[3H]-8-azido-ATP as a photoactivable affinity label. Covalently attached radioactivity was predominantly found in the beta-polypeptide of the receptor. Based on the results of protection studies with several nAChR ligands whose target sites at the receptor are known, we conclude that the purine site(s) differ from those of acetylcholine and of physostigmine, galanthamine and related ligands, and those of local anesthetics.

Blockade of nicotinic responses by physostigmine, tacrine and other cholinesterase inhibitors in rat striatum

1. The acetylcholinesterase inhibitors physostigmine, neostigmine, tetrahydroaminoacridine (tacrine; THA) and diisopropylfluorophosphate (DFP) were tested for possible direct nicotinic actions in rat striatal synaptosomes preloaded with [3H]-dopamine. In this preparation, nicotinic cholinoceptor activation evoked [3H]-dopamine release. 2. Antagonist activity was examined by giving a brief nicotine (1 microM) challenge after 30 min superfusion with an acetylcholinesterase (AChE) inhibitor (0.3-300 microM). Physostigmine, neostigmine and tacrine produced a concentration-dependent blockade. Physostigmine and tacrine were particularly potent (IC50S approx. 10 microM and 1 microM, respectively). DFP reduced nicotinic responses only at the highest concentration tested (300 microM). 3. Nicotinic blockade produced by superfusion with physostigmine (30 microM) was insurmountable when tested against nicotine (0.1-100 microM). 4. Physostigmine (30 microM) also reduced responses to the nicotinic agonists 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) and cytisine, but did not alter responses to high K+ or (+)-amphetamine. A higher concentration of physostigmine (300 microM) completely blocked responses to nicotine, somewhat reduced responses to amphetamine, and did not alter responses to high K+. Tacrine (3 microM) reduced responses to nicotine and to high K+ but did not affect responses to amphetamine. 5. Physostigmine (0.3-300 microM), given as a brief pulse, did not produce a nicotinic agonist-like effect. 6. Physostigmine, neostigmine, tacrine and DFP (all at 30 microM) each produced near-total (> 96%) inhibition of AChE activity. However, DFP at a concentration (60 microM) that produced a degree of AChE inhibition equal to that of physostigmine 30 microM, did not significantly reduce nicotine-induced dopamine release. 7. It thus appears that physostigmine blocks CNS nicotinic receptors in an insurmountable and pharmacologically selective manner, independent of its ability to inhibit acetylcholinesterase. Tacrine reduced nicotinic responses, quite possibly by an indirect mechanism. The possibility of direct or indirect blockade of nicotinic receptor-mediated actions may complicate the interpretation of preclinical studies that have employed physostigmine and tacrine.