Neuronal nicotinic acetylcholine receptors as targets for drug discovery

Selective changes in expression of different nicotinic receptor subtypes in brain and adrenal glands of mice carrying human mutated gene for APP or over‐expressing human acetylcholinestrase

In this study, we investigated regulatory mechanisms and plasticity of the nicotinic acetylcholine receptors (nAChRs) in the brain and adrenal glands of two transgenic mice models over-expressing human beta-amyloid precursor protein (APP(SWE)Tg) and human AChE enzyme (hAChE-Tg), respectively. All animals were studied at 3 months of age. Binding studies showed higher (125)I-alpha-bungarotoxin (alpha7 nAChRs) and (3)H-epibatidine (alpha3 and alpha4 nAChRs) binding in the brain cortex and adrenal glands of hAChE-Tg mice compared to control mice. The APP(SWE)Tg mice showed a significantly lower relative level for the alpha4 mRNA in the brain cortex as well as a lower level of alpha3 mRNA, and higher level of alpha7 mRNA in the adrenal glands compared to control mice. A higher relative mRNA level of alpha3 and alpha4 nAChRs was observed in the brain as well as of alpha3 and alpha7 nAChRs in the adrenal glands of hAChE-Tg mice compared to control mice. Different nicotinic receptor plasticity is revealed in the brain cortex and adrenal glands in two transgenic mice models with different underlying pathophysiological mechanisms. Deposition of beta-amyloid (Abeta) may impair neurotransmitter activity in brain as well as in the adrenal gland.

Carbamoylcholine homologs: synthesis and pharmacology at nicotinic acetylcholine receptors

In a recent study, racemic 3-(N,N-dimethylamino)butyl-N,N-dimethylcarbamate (1) was shown to be a potent agonist at neuronal nicotinic acetylcholine receptors with a high selectivity for nicotinic over muscarinic acetylcholine receptors [Mol. Pharmacol. 64 (2003) 865-875]. Here we present the synthesis and pharmacological characterization of a series of analogs of, where the methyl group at C-3 has been replaced by different alkyl substituents. Ring systems have been incorporated into the carbon backbone of some of the molecules, or the amino group has been build into ring systems. Furthermore, the (+)- and (-)-enantiomers of have been separated, and X-ray crystallography has revealed that (-)-1 possesses (S)-configuration. The compounds have been characterized pharmacologically at recombinant nicotinic receptor subtypes. The structure-activity relationship study has provided valuable insight into the mode of interactions of and its analogs with neuronal nicotinic acetylcholine receptors.

Interaction of Noncompetitive Inhibitors with an Immobilized α3β4 Nicotinic Acetylcholine Receptor Investigated by Affinity Chromatography, Quantitative−Structure Activity Relationship Analysis, and Molecular Docking

A large number of drug substances act as noncompetitive inhibitors (NCIs) of the nicotinic acetylcholine receptor (nAChR) by blocking the ion flux through the channel. An affinity chromatography technique has been developed for investigating the interactions between NCIs and the alpha3beta4 subtype of neuronal nAChR. The data obtained from the chromatographic study were used to construct QSAR models of the NCI-nAChR binding with both electronic and steric parameters observed as important descriptors. A molecular model of the transmembrane domain of the alpha3beta4 subtype of nAChR was constructed and used to simulate the docking of a series of NCIs. A key aspect of the model was the discovery of the cleft produced by the incorporation of the bulky phenylalanine moiety into the nonpolar section of the lumen by the beta4 subunit. Quantitatively, the results of docking simulations modeled the experimental affinity data better than QSAR results. The computational approach, combined with the modeling of NCI-nAChR interaction by affinity chromatography, can be used to predict possible toxicities and adverse interactions.

Synthesis of fused tetrazole derivatives via a tandem cycloaddition and N-allylation reaction and parallel synthesis of fused tetrazole amines

A method for the synthesis of novel fused tricyclic tetrazoles from allylic bromides generated by the recently discovered DiazAll reaction has been developed. This new tandem reaction comprises a cycloaddition between a nitrile and (TMS)N(3) followed by an intramolecular N-allylation. The variation of functionalities in the benzene moiety was well-tolerated, and only a moderate difference in yield and degree of purity was noticed. An exo-methylene group in these new compounds permitted further derivatization. Structural resemblance with substances which possess important pharmacological properties motivated the synthesis of a series of ketones and a small library of amines.

Synthesis and pharmacological characterization of bivalent ligands of epibatidine at neuronal nicotinic acetylcholine receptors

A series of bivalent ligands 6a-d of epibatidine were synthesized. All four ligands showed nanomolar binding affinities at six neuronal nicotinic acetylcholine receptor (nAChR) subtypes in competition binding assays. In contrast to epibatidine, these bivalent ligands are weak partial agonists at the alpha3beta4 nAChR as shown by functional assays.

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.

Synthesis and nicotinic binding of novel phenyl derivatives of UB-165. Identifying factors associated with alpha7 selectivity

Four racemic phenyl-substituted analogues 3-6 of the potent nicotinic agonist UB-165 1 have been synthesised and evaluated against the alpha(4)beta(2), alpha(3)beta(4), and alpha(7) neuronal nicotinic receptors. The 2'-phenyl derivative 3 shows no activity at these major receptor subtypes, while the 4'-phenyl analogue 4 shows an enhanced level of alpha(7) selectivity as compared to UB-165 and deschloro UB-165 2. These results are discussed within the context of recent pharmacophore models.

Mecamylamine reduces nicotine cross-desensitization of trigeminal caudalis neuronal responses to oral chemical irritation

We investigated the role of neuronal nicotinic acetylcholine receptors (nAChRs) in nicotine cross-desensitization of chemonociceptive responses of trigeminal subnucleus caudalis (Vc) neurons in rats. Vc responses to lingually applied pentanoic acid were significantly reduced following nicotine, and this was prevented when the nAChR antagonist mecamylamine was applied before or after nicotine. A peripheral site of nicotine cross-desensitization is suggested via a nAChR-mediated reduction in acidic excitation of lingual nociceptors that project to Vc.

Total Enantioselective Synthesis of (−)-Cytisine

[reaction: see text] The first total enantiosynthesis of the biologically active alkaloid (-)-cytisine is reported, featuring a ruthenium-catalyzed RCM reaction as the key step. The approach relies on readily available cis-piperidine-3,5-dimethanol monoacetate as the chiral building block, and it is suited for achieving the target compound in both enantiomeric forms.

Nicotinic acetylcholine receptor distribution in Alzheimer's disease, dementia with Lewy bodies, Parkinson's disease, and vascular dementia: in vitro binding study using 5-[(125)i]-a-85380

Nicotinic acetylcholine receptors (nAChRs) have been implicated in a number of neurological disorders. 5-Iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380) is a novel nAChR marker, binding predominantly to the alpha4beta2 subtype. This in vitro autoradiography study describes the distribution of 5-[(125)I]-A-85380 binding in post-mortem brain tissue from normal elderly individuals and from cases with age-associated dementias of both neurodegenerative and vascular types. The binding distribution of 5-[(125)I]-A-85380 in normal brain tissue was found to be consistent with the reported distribution of other high-affinity nicotinic ligands. In addition to high thalamic and moderate striatal and temporal cortex density, moderate 5-[(125)I]-A-85380 binding was also seen in white matter tracts in cingulate, occipital, and temporal areas, indicating the presence of nAChRs along nerve fiber tracts, which has not been reported in other high-affinity nicotinic agonist distribution studies. In Parkinson's disease (PD), loss of striatal 5-[(125)I]-A-85380 binding closely parallels the loss of nigrostriatal dopaminergic markers previously observed. In dementia with Lewy bodies (DLB) reduced striatal 5-[(125)I]-A-85380 binding density, comparable to that in PD, may be a marker of early degeneration in nigrostriatal inputs, while in Alzheimer's disease (AD) reduced striatal 5-[(125)I]-A-85380 binding could be related to reduced cortical inputs. The reductions of nAChRs seen in AD, DLB, and PD were not apparent in vascular dementia (VaD). In conclusion, 5-I-A-85380 is clearly a useful ligand for both in vitro and in vivo single photon emission tomography human studies investigating disease symptoms and progression, response to acetylcholinesterase-inhibiting drugs and in differentiating primary degenerative dementia from VaD.

Neurochemical and behavioral studies on ethanol and nicotine interactions

The most commonly abused drugs, alcohol and nicotine, are likely also the most costly drugs in terms of health and societal costs. A large body of evidence from epidemiological studies indicate that smoking and alcohol-intake are positively correlated. The mesocorticolimbic dopamine system has been implicated in mediating some of the reinforcing effects of ethanol, however, the mechanism(s) of action remains to be elucidated; consideration as to ethanol's ability to interact with ligand-gated ion channels should be considered. Accumulating evidence from electrophysiological, pharmacological and neurochemical studies suggest that ethanol may interact with the nicotinic acetylcholine receptor (nAChR). Thus, it has been shown that the ethanol-induced stimulation of the mesolimbic dopamine system and of locomotor activity as well as ethanol intake and preference in rodents may involve central nicotinic acetylcholine receptors. Additionally, data has been presented that nAChRs located in the ventral tegmental area may be of particular importance for these effects of ethanol. Studies aimed at defining the nAChR subpopulation(s) involved in mediating ethanol-induced locomotor stimulation and accumbal dopamine overflow as well as ethanol-intake have revealed that alpha(3)beta(2) or alpha(6) (using alpha-Conotoxin MII) but not alpha(4)beta(2) (using dihydro-beta-erythroidine) or alpha(7) (using methyllycaconitine), could represent targets for developing new drugs in the treatment of alcoholism. These results do not allow any conclusion as to whether the involvement nAChRs in mediating the effects of ethanol is direct and/or indirect. With regard to an indirect effect, evidence has accumulated indicating that the cholinergic excitatory input to the dopaminergic neurons in the ventral tegmental area may be an important part of the neuronal circuits mediating natural as well as drug-rewarded behavior. The possibility may thus be considered that ethanol activates the cholinergic afferents causing a release of acetylcholine in the ventral tegemental area leading to a stimulation of nAChRs and thereby excite the mesocorticolimbic dopamine system.

Synthesis and analgesic activity of hydrochlorides and quaternary ammoniums of epibatidine incorporated with amino acid ester

Hydrochloride derivatives 5a-c and quaternary ammonium derivatives 6a-c of epibatidine incorporated with amino acid ester were synthesized and evaluated for their in vivo analgesic activity and toxicity. Among all tested compounds, compound 6c has the most potent analgesic activity. The quaternary ammonium salts 6a and 6c showed better analgesic activity than the corresponding hydrochlorides 5a and 5c. Both 5a-c and 6a-c showed significantly lower toxicity than epibatidine itself.

Neuroprotection by nicotine against hypoxia-induced apoptosis in cortical cultures involves activation of multiple nicotinic acetylcholine receptor subtypes

Activation of neuronal nicotinic acetylcholine receptors (nAChR) by nicotine has been suggested to protect neurons against a hypoxic insult. The objective of this study was to examine the nature of cell death induced by acute hypoxia in rat primary cortical cultures and the neuroprotective potential of nicotine in ameliorating these processes. Neuronal cell death induced by a 4-h exposure to hypoxia (0.1% O(2)) was apoptotic, as shown by TUNEL staining and assays monitoring DNA strand breaks and caspase-3/7 activity. The presence of nicotine (10 microM) during the hypoxic insult protected a subpopulation of susceptible neurones against DNA damage and apoptosis induced by oxygen deprivation. This protective effect of nicotine was prevented by a 30-min pre-incubation with either 100 nM alpha-bungarotoxin or 1 microM dihydro-beta-erythroidine, but not 1 microM atropine, suggesting that activation of at least two subtypes of nAChR, alpha7 and beta2* nAChR, is involved in mediating nicotine neuroprotection.

Carbamoylcholine homologs: novel and potent agonists at neuronal nicotinic acetylcholine receptors

The classic muscarinic acetylcholine receptor (mAChR) agonist carbamoylcholine (carbachol) does not seem to be the most obvious lead for the development of selective ligands at nicotinic acetylcholine receptors (nAChRs). In the past, however, N-methylations of carbachol have provided N-methylcarbamoylcholine and N,N-dimethylcarbamoylcholine (DMCC), which predominantly display nicotinic activity. In this study, 12 homologous analogs of DMCC and its corresponding tertiary amine, N,N-dimethylcarbamoyl-N,N-dimethylaminoethanol, were synthesized and their binding affinities to native mAChR and nAChR sites estimated. One of the compounds in the series, 3-N,N-dimethylaminobutyl-N,N-dimethylcarbamate (7), displayed low nanomolar binding affinity to nAChRs and a 400-fold selectivity for nAChRs over mAChRs. Hence, a new series of compounds was synthesized in which alkyl and aryl groups and different ring systems were introduced in the carbamate moiety of 7. In a [3H]epibatidine binding assay, the Ki values of 7 and its analogs at rat alpha2beta2, alpha4beta2, alpha2beta4, alpha3beta4, and alpha4beta4 nAChRs, stably expressed in mammalian cell lines, ranged from low nanomolar to midmicromolar concentrations, whereas all of the compounds displayed weak binding to an alpha7/5-HT3 chimera and to native mAChRs. Compound 7 and its analogs were determined to be agonists at the alpha3beta4 nAChR subtype. This series includes the most potent and selective nicotinic agonists structurally derived from ACh to date. Furthermore, the compounds are tertiary amines, implying some advantages in terms of bioavailability pertinent to future in vivo pharmacological studies. Finally, observations made in the study hold promising perspectives for future development of ligands selective for specific nAChR subtypes.

Bioassay-guided isolation of epiquinamide, a novel quinolizidine alkaloid and nicotinic agonist from an Ecuadoran poison frog, Epipedobates tricolor

Analytical HPLC fractionation, combined with an off-line 96-well fluorescent bioassay screen, has been developed and used for the separation and screening of a natural product extract. This method was used to guide the isolation of a novel quinolizidine alkaloid from the methanolic skin extracts of an Ecuadoran frog, Epipedobates tricolor. The structure was determined on the basis of MS, IR, and NMR analysis as (1R,10R)-1-acetamidoquinolizidine (alkaloid 196). We have named this compound epiquinamide, reflecting its origin and structure. The activity of the isolated compound was determined in five cell lines expressing various nicotinic acetylcholine receptor subtypes. The bioactivity of epiquinamide was evaluated on the basis of membrane potential fluorescence and was found to be beta2 selective. This compound represents a new structural class of nicotinic agonists and a potential lead compound for the development of new therapeutics and pharmacological probes for nicotinic receptors. The off-line screening technique was found to be very sensitive for the detection of compounds active at nicotinic receptors.

The Nicotinic Pharmacophore: Thermodynamics of the Hydrogen-Bonding Complexation of Nicotine, Nornicotine, and Models

The thermodynamics of the hydrogen-bonding complexation of the acetylcholine agonists nicotine and nornicotine and of model pyridines, pyrrolidines, and N-methylpyrrolidines has been measured in CCl(4) by FTIR spectrometry toward a reference hydrogen-bond donor, 4-fluorophenol. Various methods are devised for measuring separately the hydrogen-bond acceptor strength of each nitrogen of nicotine and nornicotine: variation of the stoichiometry of complexation; correlations with electrostatic potentials on nitrogens and with substituent constants in the series of 3-substituted pyridines, 2-substituted pyrrolidines, and 2-substituted N-methylpyrrolidines; and linear free energy relationships between 4-fluorophenol and hydrogen fluoride hydrogen-bonded complexes. It is consistently found that nicotine and nornicotine have two active hydrogen-bond acceptor sites, the pyridine and pyrrolidine nitrogens, and that ca. 90% (for nicotine) and 80% (for nornicotine) of the 1:1 hydrogen-bonded complexes are formed to the pyridine nitrogen, although the pyrrolidine nitrogen is the first protonation site of nicotine and nornicotine in water. The low hydrogen-bond basicity of the pyrrolidine nitrogen in nicotine is mainly explained by the inductive electron-withdrawing and steric effects of the 2-(3-pyridyl) substituent. The partition of the Gibbs energy of the isomerism of complexation (AH...Nsp(2) <==> AH...Nsp(3)) into enthalpic and entropic contributions shows that the selectivity in favor of the pyridine nitrogen is driven by entropy. It is important to recognize the bifunctionality of nicotine in hydrogen bonding for understanding its lipophilicity and molecular recognition in non protonic media. When monoprotonated on their sp(3) nitrogen, nicotine and nornicotine keep, through their sp(2) nitrogen, a significant hydrogen-bond basicity which is greater than that of the ester group of acetylcholine.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Nitrogen substitution modifies the activity of cytisine on neuronal nicotinic receptor subtypes

Cytisine very potently binds and activates the alpha 3 beta 4 and alpha 7 nicotinic subtypes, but only partially agonises the alpha 4 beta 2 subtype. Although with a lower affinity than cytisine, new cytisine derivatives with different substituents on the basic nitrogen (CC1-CC8) bind to both the heteromeric and homomeric subtypes, with higher affinity for brain [3H]epibatidine receptors. The cytisine derivatives were tested on the Ca(2+) flux of native or transfected cell lines expressing the rat alpha 7, or human alpha 3 beta 4 or alpha 4 beta 2 subtypes using Ca(2+) dynamics in conjunction with a fluorescent image plate reader. None elicited any response at doses of up to 30-100 microM, but all inhibited agonist-induced responses. Compounds CC5 and CC7 were also electrophysiologically tested on oocyte-expressed rat alpha 4 beta 2, alpha 3 beta 4 and alpha 7 subtypes. CC5 competitively antagonised the alpha 4 beta 2 and alpha 3 beta 4 subtypes with similar potency, whereas CC7 only partially agonised them with maximum responses of respectively 3% and 11% of those of 1 mM acetylcholine. Neither compound induced any current in the oocyte-expressed alpha 7 subtype, and both weakly inhibited acetylcholine-induced currents. Adding chemical groups of a different class or size to the basic nitrogen of cytisine leads to compounds that lose full agonist activity on the alpha 3 beta 4 and alpha 7 subtypes.

Influence of acetylcholine levels on the binding of a SPECT nicotinic acetylcholine receptor ligand [123I]5-I-A-85380

Although in vitro theory indicates that ligand binding is sensitive to competition with neurotransmitters, only some imaging ligands have shown such competition in vivo. The purpose of this study was to determine whether increases in acetylcholine (ACh) levels induced by an acetylcholinesterase inhibitor, physostigmine, inhibit in vivo binding of [(123)I]5-iodo-3-(2(S)-2-azetidinyl-methoxy) pyridine (5-I-A-85380), a single photon emission computed tomography ligand for the high-affinity type nicotinic ACh receptor (nAChR). Baboons were used for seven studies with a bolus plus constant infusion equilibrium paradigm. After achieving equilibrium at 5 h, physostigmine (0.02 (n = 1), 0.067 (n = 3), and 0.2 (n = 3) mg/kg) was administered intravenously and data were acquired for up to 8 h. To confirm equilibrium conditions, [(123)I]5-I-A-85380 plasma levels were measured in four studies, including all studies with 0.2 mg/kg physostigmine. Prior to physostigmine administration, thalamic activities were stable, with changes of 1.1%/h or less, except in one study with a gradual increase of 4.2%/h. Thalamic activities were decreased by 15% in one study with 0.067 mg/kg and 14-17% in all studies with 0.2 mg/kg physostigmine administration (P = 0.009). In these studies with 0.2 mg/kg physostigmine administration, [(123)I]5-I-A-85380 plasma levels showed a transient or a sustained increase after physostigmine administration that would have increased thalamic activities. These results suggest that elevated ACh levels induced by physostigmine can effectively compete in vivo with [(123)I]5-I-A-85380 binding at nAChRs. However, decreased thalamic activities could have been caused by other mechanisms, including internalization of the receptor with an associated decreased affinity for radioligand.

3D QSAR analyses-guided rational design of novel ligands for the (alpha4)2(beta2)3 nicotinic acetylcholine receptor

Three-dimensional quantitative structure-activity relationship methods, the comparative molecular field analysis (CoMFA) and the comparative molecular similarity indices analysis (CoMSIA), were applied using a training set of 45 ligands of the (alpha4)2(beta2)3 nicotinic acetylcholine receptor (nAChR). All compounds are related to (-)-epibatidine, (-)-cytisine, (+)-anatoxin-a, and (-)-ferruginine, and additionally, novel diazabicyclo[4.2.1]nonane- and quinuclidin-2-ene-based structures were included. Their biological data have been determined by utilizing the same experimental protocol. Statistically reliable models of good predictive power (CoMFA r2 = 0.928, q2 = 0.692, no. of components = 3; CoMSIA r2 = 0.899, q2 = 0.701, no. of components = 3) were achieved. The results obtained were graphically interpreted in terms of field contribution maps. Hence, physicochemical determinants of binding, such as steric and electrostatic and, for the first time, hydrophobic, hydrogen bond donor, and hydrogen bond acceptor properties, were mapped back onto the molecular structures of a set of nAChR modulators. In particular, changes in the binding affinity of the modulators as a result of modifications in the aromatic ring systems could be rationalized by the steric, electrostatic, hydrophobic, and hydrogen bond acceptor properties. These results were used to guide the rational design of new nAChR ligands such as 48-52 and 54, which were subsequently synthesized for the first time and tested. Key steps of our synthetic approaches were successfully applied Stille and Suzuki cross-coupling reactions. Predictive r2 values of 0.614 and 0.660 for CoMFA and CoMSIA, respectively, obtained for 22 in part previously unknown ligands for the (alpha4)2(beta2)3 subtype, demonstrate the high quality of the 3D QSAR models.

Unique spirocyclopiperazinium salt I: synthesis and structure-activity relationship of spirocyclopiperazinium salts as analgesics

Based on the structure of compound 3, two series of spirocyclopiperazinium derivatives 7a-n and 10a-h were synthesized and evaluated for their in vivo analgesic and sedative activities. Compounds 7f and 10c were discovered to exhibit excellent analgesic activity. Structure-activity relationships revealed that anion of the quaternary salt affected the analgesic and sedative activity significantly; the allyl group is a most effective group among the compounds 7a-n; the electron-released substitute on the aromatic ring is favorable to increase the analgesic activity.

Membrane potential fluorescence: a rapid and highly sensitive assay for nicotinic receptor channel function

Seven cell lines expressing native and transfected nicotinic receptor subtypes were evaluated functionally by using fluorescent assays based on membrane potential and calcium dynamics with "no-wash" dye systems. Both assays provided the same rank orders of potency for (+/-)-epibatidine, 2S-(-)-nicotine, 7R,9S-(-)-cytisine, and 1,1-dimethyl-4-phenylpiperazinium in a cell line expressing rat alpha 3 beta 4 receptors. Nicotinic antagonists mecamylamine and dihydro-beta-erythroidine inhibited responses in both assays. Both agonist and antagonist activity were assessed within the same experiment. Agonists seemed more potent in the membrane potential assay than in the calcium assay, whereas the converse was true for antagonists. The membrane potential assay afforded robust responses in K-177 cells expressing human alpha 4 beta 2 receptors, in IMR-32 and SH-SY5Y cells expressing human ganglionic receptors, and in TE-671 cells expressing human neuromuscular receptors. These lines gave weak to modest calcium responses. Moreover, membrane potential responses were obtained in cell lines expressing rat alpha 4 beta 2 and alpha 4 beta 4 receptors, which were devoid of calcium responses. Thus, membrane potential serves as a sensitive measure of nicotinic activity, and the resulting depolarization may be as important as calcium in cell signaling.

Synthesis of two fluoro analogues of the nicotinic acetylcholine receptor agonist UB-165

Two racemic fluoropyridine analogues 4 and 5 of the potent nicotinic agonist UB-165 have been synthesized. Halogenated pyridines 7 and 12 provided the organometallic reagents needed for the Negishi and Suzuki coupling reactions used for the preparation of 4 and 5, and the N-vinyloxycarbonyl protecting group of 8 and 15 was cleaved using a novel trifluoroacetic acid-mediated deprotection protocol. Analogue 4 retained high binding affinity at rat brain alpha4beta2 and alpha7 nicotinic receptors.

Synthesis, nicotinic acetylcholine receptor binding affinities, and molecular modeling of constrained epibatidine analogues

Conformationally constrained epibatidine analogues 20a,b and 23a,b were synthesized using a radical cyclization as the key step. Radioligand displacement assays to six defined rat nicotinic acetylcholine receptor (nAChR) subtypes showed that 20a,b bind with moderate affinities, while 23a,b have low affinities. 20a exhibits higher affinity for the beta2 containing subtype than for the beta4 containing counterpart, while 20b possesses reversed selectivity. Modeling studies suggest that the spatial distribution of the ligand's atoms around the pharmacophore elements may control their nAChR subtype selectivity.

[(3-Chlorophenyl)piperazinylpropyl]pyridazinones and analogues as potent antinociceptive agents

A number of [(3-chlorophenyl)piperazinylpropyl]pyridazinones and the corresponding isoxazolopyridazinones, showing the arylpiperazinyl substructure present in very potent antinociceptive agents reported in the literature, were synthesized and tested for their analgesic activity. The investigated compounds showed antinociceptive properties in the mouse hot-plate test (thermal nociceptive stimulus) after systemic administration with an efficacy similar to that exerted by morphine. The increase of the pain threshold induced by the compounds labeled 5a, 7, 8, and 11 was prevented by reserpine, suggesting the involvement of the noradrenergic and/or serotoninergic system in their mechanism of action. Among them, 7 and 11 showed the highest analgesic potency and efficacy together with a good ratio (133 and 200, respectively) of the minimal nontoxic dose (MNTD) to the minimal analgesic dose (MAD). Furthermore, they were also active after icv administration and in the presence of a chemical, painful stimulus (abdominal constriction test).

Cytisine derivatives as ligands for neuronal nicotine receptors and with various pharmacological activities

Neuronal nicotinic acetylcholine receptors (nAChRs) form a family of ACh-gated cation channels made up of different subtypes. They are widely distributed in peripheral and central nervous systems and are involved in complex cerebral processes as learning, memory, nociception, movement, etc. The possibility that subtype-selective ligands be used in the treatment of CNS disorders promoted the synthesis of a large number of structural analogues of nicotine and epibatidine, two very potent nAChR agonists. Pursuing our long standing research on the structural modification of quinolizidine alkaloids, we devoted our attention to cytisine, another very potent ligand for many nAChR subtypes. Thus a systematic structural modification of cytisine was undertaken in order to obtain compounds of potential therapeutic interest at peripheral as well as central level, with a particular concern for achieving nAChR subtype selective ligands. Up to the present more than 80 cytisine derivatives, mainly of N-substitution and a few by modifying the pyridone ring, have been prepared. The biological results, which concern so far about an half of the prepared compounds, indicate that the introduction of a nitro group in position 3 of the pyridone nucleus further enhances the high affinity of cytisine, while the introduction of substituents on the basic nitrogen, though reducing in different degrees the affinity, gives rise to compounds with a higher selectivity for central (alpha(4)beta(2)) versus gangliar (alpha(3)-containing) receptor subtype. On the other hand, the analgesic, antihypertensive and inotropic activities found in some N-substituted cytisines, represent an attractive starting point for the development of more active compounds.

Binding of nicotine and homoazanicotine analogues at neuronal nicotinic acetylcholinergic (nACh) receptors

A total of 20 substituted analogues of nicotine (1a) and homoazanicotine (3a) were examined in order to determine whether or not they might bind in a similar manner at alpha4beta2 nicotinic acetylcholinergic (nACh) receptors. It was found that parallel structural changes in the two series resulted in parallel shifts in affinity. Evidence suggests that the two series are binding in a comparable fashion.

5-Azidoepibatidine: an exceptionally potent photoaffinity ligand for neuronal alpha 4 beta 2 and alpha 7 nicotinic acetylcholine receptors

Racemic 5-azidoepibatidine [(+/-)-1] was synthesized via 5-aminoepibatidine as a candidate photoaffinity ligand with exceptionally high affinity at the mammalian neuronal nicotinic receptors (K(i) values of 0.027 nM for alpha 4 beta 2 and 9.7 nM for alpha 7) and excellent photoreactivity.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Immobilized nicotinic receptor stationary phases: going with the flow in high-throughput screening and pharmacological studies

The nicotinic acetylcholine receptor (nAChR) subtypes alpha3beta4-nAChR and alpha4beta2-nAChR have been immobilized and the resulting stationary phases used to determine binding affinities. The alpha3beta4-nAChR column was coupled to a C(18) column and a mixture of 18 compounds was sorted into ligands and non-ligands for the alpha3beta4-nAChR. The results demonstrate that the nAChR stationary phases can be used for on-line high-throughput screening (HTS).

N,N-disubstituted piperazines: synthesis and affinities at alpha4beta2(*) and alpha7(*) neuronal nicotinic acetylcholine receptors

A series of N,N-disubstituted piperazines were prepared and evaluated for binding to alpha4beta2(*) and alpha7(*) neuronal nicotinic acetylcholine receptors using rat striatum and whole brain membrane preparations, respectively. This series of compounds exhibited selectivity for alpha4beta2(*) nAChRs and did not interact with the alpha7(*) nAChRs subtype. The most potent analogues were compounds 8b and 8f (K(i)=32 microM). Thus, linking together a pyridine pi-system and a cyclic amine moiety via a piperazine ring affords compounds with low affinity, but good selectivity for alpha4beta2(*) nicotinic receptors.

Synthesis and binding affinities of 5-(3-pyridinyl)- and 5-(3-quinolinyl)-4-azahomoadamantanes to alpha7 nicotinic acetylcholine receptors

A general synthetic route that can lead to nicotinic ligands bearing a variety of bulky aza-ring systems was developed. This methodology was applied to obtain 5-(3-pyridinyl)- and 5-(3-quinolinyl)-4-azahomoadamantanes 2a, 3a and 2b, 3b. The parent 5-(3-pyridinyl)-4-azahomoadamantane 2a (Ki = 5.0 microM) binds with about 100 times lower affinity than (+)-epibatidine 1 (Ki = 0.045 microM) to alpha7 nicotinic acetylcholine receptors (nAChRs). N-methyl substitution of 2a gives compound 3a which has about nine times lower binding affinity. The replacement of pyridinyl with a quinolinyl ring (compounds 2b, 3b) results in a dramatic reduction in potency (Ki > 1000 microM).

bis-Azaaromatic quaternary ammonium analogues: ligands for alpha4beta2* and alpha7* subtypes of neuronal nicotinic receptors

A series of bis-nicotinium, bis-pyridinium, bis-picolinium, bis-quinolinium and bis-isoquinolinium compounds was evaluated for their binding affinity at nicotinic acetylcholine receptors (nAChRs) using rat brain membranes. N,N'-Decane-1,12-diyl-bis-nicotinium diiodide (bNDI) exhibited the highest affinity for [(3)H]nicotine binding sites (K(i)=330 nM), but did not inhibit [(3)H]methyllycaconitine binding (K(i) >100 microM), indicative of an interaction with alpha4beta2*, but not alpha7* receptor subtypes, respectively. Also, bNDI inhibited (IC(50)=3.76 microM) nicotine-evoked (86)Rb(+) efflux from rat thalamic synaptosomes, indicating antagonist activity at alpha4beta2* nAChRs. N,N'-Dodecane-1,12-diyl-bis-quinolinium dibromide (bQDDB) exhibited highest affinity for [(3)H]methyllycaconitine binding sites (K(i)=1.61 microM), but did not inhibit [(3)H]nicotine binding (K(i)>100 microM), demonstrating an interaction with alpha7*, but not alpha4beta2* nAChRs. Thus, variation of N-n-alkyl chain length together with structural modification of the azaaromatic quaternary ammonium moiety afforded selective antagonists for the alpha4beta2* nAChR subtype, as well as ligands with selectivity at alpha7* nAChRs.

Upregulation of [3H]methyllycaconitine binding sites following continuous infusion of nicotine, without changes of alpha7 or alpha6 subunit mRNA: an autoradiography and in situ hybridization study in rat brain

It is well established that exposure of experimental animals to nicotine results in upregulation of the alpha4beta2-subtype of neuronal nicotinic acetylcholine receptors (nAChRs). The aim of this study was to determine the effect of nicotine on the levels of alpha7-nAChRs in rat brain, for which only partial information is available. Rats were infused with nicotine (3 mg/kg/day) or saline for 2 weeks and their brains processed for receptor autoradiography with [3H]methyllycaconitine (MLA), a radioligand with nanomolar affinity for alpha7-nAChRs. In control rats binding was high in hippocampus, intermediate in cerebral cortex and hypothalamus, and low in striatum, thalamus and cerebellum. There was high correlation between the distribution of [3H]MLA binding sites and alpha7 subunit mRNA (r = 0.816). With respect to saline-treated controls, nicotine-treated rats presented higher [3H]nicotine binding in 11 out of 15 brain regions analysed (average increase 46 +/- 6%). In contrast, only four regions showed greater [3H]MLA binding, among which the ventral tegmental area (VTA) and cingulate cortex (mean increase 32 +/- 3%). No changes in alpha7 mRNA levels were observed after nicotine treatment. Similarly, there was no variation of alpha6 subunit transcript in the VTA, a region which may contain MLA-sensitive (non-alpha7)-alpha6*-nAChRs (Klink et al., 2001). In conclusion, nicotine increased [3H]MLA binding, although to a smaller extent and in a more restricted regional pattern than [3H]nicotine. The enhancement of binding was not paralleled by a significant change of alpha7 and alpha6 subunit transcription. Finally, the present results provide the first anatomical description of the distribution of [3H]MLA binding sites in rat brain.

Role of different nicotinic acetylcholine receptors in mediating behavioral and neurochemical effects of ethanol in mice

Ethanol and nicotine are the most abused drugs, and it is well known that co-abuse of ethanol and nicotine is frequent in human beings. We have previously obtained results indicating that the ethanol-induced stimulation of both the mesolimbic dopamine system and locomotor activity may involve activation of central nicotinic acetylcholine receptors (nAChRs), especially those located in the ventral tegmental area. Different subpopulations of nAChRs have been identified, and, in the present series of experiments, we have studied the effects of various nAChR antagonists on the stimulation of dopamine overflow in the nucleus accumbens and on locomotor activity induced by ethanol in male mice. Ethanol (2.0 g/kg, i.p.) enhanced dopamine overflow in the nucleus accumbens by approximately 40%, measured by means of in vivo microdialysis in awake, freely moving mice. Mecamylamine (negative allosteric modulator of nAChR; 2.0 mg/kg, i.p.) blocked the ethanol-induced stimulation of both locomotor activity and accumbal dopamine overflow. Methyllycaconitine citrate (alpha(7) antagonist; 2.0 mg/kg, i.p.) and dihydro-beta-erythroidine (competitive and selective alpha(4)beta(2) antagonist; 0.5 mg/kg, s.c.), in doses that had no marked effects per se, did not significantly reduce the behavioral and neurochemical stimulation caused by ethanol. The present results support the suggestion that the stimulatory effects of ethanol on locomotor activity and dopamine release do not involve the alpha(4)beta(2) or alpha(7) subunit compositions of the nAChR and that the effects of mecamylamine are mediated through a site not directly associated with the alpha(4)beta(2) or alpha(7) nAChR subunits.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 2-exo-2-(2',3'-disubstituted 5'-pyridinyl)-7-azabicyclo[2.2.1]heptanes: epibatidine analogues

A number of 2',3'-disubstituted epibatidine analogues were synthesized and evaluated in vitro for potency at nicotinic acetylcholine receptors (nAChRs) and in vivo for antinociception activity in the tail-flick and hot-plate models of acute pain and for their ability to affect core body temperature. Compounds that possessed electron-withdrawing groups (F, Cl, Br, and I) in both the 2'- and the 3'-positions showed affinities at the nAChR similar to epibatidine. However, in vivo efficacy did not correlate with affinity. 2-exo-(3'-Amino-2'-chloro-5'-pyridinyl)-7-azabicyclo[2.2.1]heptane (2i), an epibatidine analogue possessing an electron-releasing amino group in the 3'-position, produced the highest affinity. Compound 2i was also the most selective epibatidine analogue with a K(i) of 0.001 nM at alphabeta nAChRs, which is 26 times greater than that of epibatidine, and a alphabeta/alpha(7) K(i) ratio of 14,000, twice that of epibatidine. In vivo testing revealed that this compound potently inhibited nicotine-induced antinociception with AD(50) values below 1 microg/kg. Surprisingly, this same compound was also an agonist at higher doses (ED(50) approximately 20 microg/kg). Thus, the addition of the 3'-amino group to epibatidine confers potent antagonist activity to the compound with little effect on agonist activity. 2,3-Disubstituted epibatidine analogues possessing a 2'-amino group combined with a 3'-bromo or 3'-iodo group showed in vitro and in vivo nAChR properties similar to nicotine.

Homoazanicotine: a structure-affinity study for nicotinic acetylcholine (nACh) receptor binding

We have recently identified 3-[(1-methyl)-4,5-dihydro-1H-imidazol-2-yl)methyl]pyridine (homoazanicotine, 8) as a novel nicotinic acetylcholinergic (nACh) receptor ligand. In the present investigation, after we determined that 8 binds selectively at nicotinic (K(i) = 7.8 nM) vs muscarinic (K(i) > 10,000 nM) acetylcholinergic receptors, we examined its structure-affinity relationships for nACh receptor binding. The features investigated included the influence of (i) the composition of connector that separates the two rings, (ii) the N-methyl group, (iii) the ring opening of the imidazoline ring, (iv) the pyridine nitrogen atom, and (v) the aromatization of the imidazoline ring on nACh receptor affinity. As with nicotine, the parent structure seems optimal and most structural changes reduce nACh receptor affinity. Also, as with nicotine analogues, alteration of the spacer group influences affinity in a manner that is somewhat different than that seen with the parent structure.

Ligand-receptor interaction at the neural nicotinic acetylcholine binding site: a theoretical model

Recent mutagenesis experiments have identified some of the functional amino acids that are essential in the interaction of nicotinic agents with the binding site of the neural nicotinic acetylcholine receptor (nAChR). Although this receptor is one of the best studied and characterized the lack of detailed experimental information regarding its quaternary structure has turned it into a challenge for computational chemistry. We have previously reported [J. Comput. Aided Mol. Design 13 (1999) 57-68] a computational protocol based on molecular mechanics and molecular dynamics (MD) where SER82, ASP83, TRP86, ASP89, TYR93, TYR190, TYR198 and ARG209 were placed around selected agonists and antagonists aided by stereoelectronic criteria. Explicit water molecules were used with the double goal of simulating aqueous environment and keeping the system from falling apart. The protocol was stable enough to allow the ligands to evolve to their thermodynamically most probable structure while maintaining the key interactions. In this communication we use the average model for the agonists (one average structure for each agonist) to calculate quantum mechanically the interactions of the binding site with one neurotransmitter acetylcholine (ACh, 1), as well as with two of the most potent agonists described so far [nicotine (2) and epibatidine (3)] and the modeled binding site. A wide variety of methods as well as basis sets were used in order to rationalise the best way to treat the problem. In this limited set of compounds, a good correlation between total interaction energies and biological affinity is observed.

(−)6-n-Propylnicotine antagonizes the antinociceptive effects of (−)Nicotine

Several 6-alkyl analogues of nicotine were examined in radioligand binding and in vivo functional assays. Although (-)6-ethylnicotine (3) binds with high affinity at nACh receptors (Ki=5.6 nM) and produces nicotine-like actions, its n-propyl homologue (-)4 (Ki=22 nM) failed to produce such effects. In fact, (-)4 antagonized the antinociceptive effects of (-)nicotine in the tail-flick assay in mice, but not the spontaneous activity or discriminative stimulus effects of (-)nicotine. Compound (-)4 appears to selectively antagonize only one of the three effects examined and is an interesting cholinergic agent for subsequent investigation.