Benzylidene anabaseines act as high-affinity agonists for insect nicotinic acetylcholine receptors

Benzylidene anabaseines are agonists selective for vertebrate alpha7-nicotinic acetylcholine receptor (nAChR), while they exhibit antagonist activity toward vertebrate alpha4beta2-nAChR. To investigate the effects of benzylidene anabaseines on insect nAChRs, we performed [3H]epibatidine-binding assays and neurophysiological experiments using American cockroach (Periplaneta americana) nerve-cord preparations. Of the six compounds tested, 3-benzylideneanabaseine (BA) and 6'-chloro-3-benzylideneanabaseine (CBA) displayed the highest potency in the binding assays, with K(i)s of 35.0 and 21.2nM, respectively. The introduction of a nitro group at the 4-position of the phenyl group led to a decrease in affinity by two orders of magnitude, while that of a chlorine atom at the 6'-position had little effect on affinity. In neurophysiological experiments, BA at 3.3 microg/ml increased the spike frequency observed with the nerve preparation, as observed with nicotine at 16.6 microg/ml. These findings suggest that benzylidene anabaseines act as high-affinity agonists in P. americana nAChRs and that they might therefore prove useful as probes for insect nAChRs.

Upregulation of neuronal nicotinic receptor subunits alpha4, beta2, and alpha7 in transgenic mice overexpressing human acetylcholinesterase

Neuronal nicotinic receptor binding sites as well as mRNA levels encoding for subunits alpha4, beta2, and alpha7 were analysed in 3-mo-old transgenic mice generated with a neuronal overexpression of human acetylcholinesterase and in age-matched controls. The acetylcholinesterase transgenic mice display progressive cognitive impairment in spatial learning and memory. We here report a significantly increased [3H]epibatidine and [125I]alphabungarotoxin binding in the cortex and the caudate putamen of these mice. Quantitativein situ hybridization showed significant upregulation of mRNA corresponding to the nicotinic receptor subunits alpha4, beta2, and alpha7 in various brain regions in the transgenic mice compared to nontransgenic controls. Our results suggest that disruption of balanced cholinergic transmission by constitutive overexpression of acetylcholinesterase is accompanied by variable upregulation of several nicotinic receptor subtypes, in particular these associated with cholinergic terminals participating in compensatory response.

Synthesis and pharmacological evaluation of some (pyridyl)cyclopropylmethyl amines and their methiodides as nicotinic receptor ligands

A series of 3- and (4-pyridyl)cyclopropylmethyl amines and their quaternary ammonium derivatives have been synthesized; they can be considered as rigid analogues of nicotine. The compounds have been tested on rat cerebral cortex to measure the affinity for the central nicotinic receptor. Only the methiodides show affinity in the micromolar range. The results obtained can provide useful information on the topography of the nicotinic receptor-binding site.

Neonatal exposure to the brominated flame retardant 2,2',4,4',5-pentabromodiphenyl ether causes altered susceptibility in the cholinergic transmitter system in the adult mouse

Polybrominated diphenyl ethers (PBDEs) are used as flame-retardants and have recently been shown to be increasing in the environment and in human mother's milk. We have recently reported that neonatal exposure to 2,2',4,4',5-pentaBDE (PBDE 99) can induce persistent aberrations in spontaneous behavior and also affect learning and memory functions in the adult animal. The present study indicates that the cholinergic system, in its developing stage, may be a target of and sensitive to PBDEs. Neonatal exposure of male NMRI mice on postnatal day 10, to 2,2',4,4',5-pentaBDE (8 mg/kg bw) was shown to alter the response to a cholinergic agent, nicotine, at an adult age. The nicotine-induced behavior test revealed a hypoactive response to nicotine in PBDE 99-treated animals, whereas the response of controls was an increased activity. These findings show similarities to observations made from neonatal exposure to PCBs and nicotine, compounds shown to affect cholinergic nicotinic receptors. This indicates that PBDE 99 can affect the cholinergic system and might thereby interact with other environmental toxicants.

[3H]Epibatidine binding to bovine adrenal medulla: evidence for alpha3beta4* nicotinic receptors

In these studies, [3H]epibatidine is used as the radioligand to characterize nicotinic acetylcholine receptors (nAChRs) from bovine adrenal medulla. Specific binding reaches equilibrium within 30 min, and is saturable with a Kd value of 0.5 nM. The affinities of several cholinergic agents were determined, including nicotine (Ki, 0.2 microM), cytisine (Ki, 0.4 microM), carbachol (Ki, 4.7 microM), dihydro-beta-erythrodine (Ki, 33.6 microM), D-tubocurarine (Ki, 0.4 microM), 1,1-dimethyl-4-phenyl-piperazinium (Ki, 0.8 microM), decamethonium (Ki, 234 microM) and methyllycaconitine (Ki, 1.3 microM). These values are similar to reported values for recombinant alpha3beta4 nAChRs in transfected cell lines. These studies demonstrate [3H]epibatidine binding to an easily obtainable adrenal membrane preparation and support the characterization of adrenal nAChRs as alpha3beta4* nAChRs.

Formation of the quaternary ammonium-linked glucuronide of nicotine in human liver microsomes: identification and stereoselectivity in the kinetics

The formation of the N1-glucuronide metabolite of each nicotine enantiomer was studied in pooled human liver microsomes (n = 6). The metabolite formed from natural S(-)-nicotine was identified by comparison of the high-pressure liquid chromatography (HPLC) retention time and positive ion electrospray ionization-mass spectral characteristics with a synthetic reference standard. A radiometric HPLC method was used to quantify the metabolite. The specificity of the assay method was demonstrated by experiments in which beta-glucuronidase treatment of incubated assay samples resulted in elimination of the peak due to the N1-glucuronide metabolite. The glucuronides of S(-)- and R(+)-nicotine were formed by one-enzyme kinetics, with K(m) values of 0.11 and 0.23 mM and V(max) values of 132 and 70 pmol/min/mg of protein, respectively. There is marked stereoselectivity in the apparent intrinsic clearance values (V(max)/K(m)) in that the value for S(-)-nicotine is 4 times greater than for the R(+)-isomer (1.2 versus 0.31 microl/min/mg of protein).

Role of neuronal nicotinic receptors in the transmission and processing of information in neurons of the central nervous system

The properties of nicotinic acetylcholine receptors (nAChRs) were studied following exogenous expression in a host system or using whole-cell recordings in brain slices, autoradiography and immunohistochemistry. When expressed in HEK-293 cells, alpha 4 beta 2 nAChRs displayed both a high and a low affinity component. The ratio of these two states was modified by chronic nicotine exposure, resulting in an enhanced sensitivity and a marked reduction in desensitization. Mutations in the gene coding for the alpha 4 subunit are responsible for a particular form of nocturnal epilepsy. When expressed in Xenopus oocytes, alpha 4 beta 2 nAChRs containing these mutations displayed distinct alterations in agonist affinity, desensitization and calcium permeability. Magnocellular endocrine neurons in the supraoptic (SO) nucleus of the hypothalamus were found to express functional alpha 7-containing nAChRs, which could play a role in regulating neurohypophysial peptide secretion. Facial (VII), hypoglossal (XII) and vagal (X) motoneurons of young rats responded to ACh by a fast inward current. The nAChRs present in VII and XII nuclei were of the non-alpha 7-containing type, whereas those present in the X nucleus contained the alpha 7 subunit. In Bcl-2 transgenic mice, facial nerve axotomy caused nAChRs downregulation by interfering negatively with the expression of the alpha 4 subunit. Binding sites corresponding to alpha 7-containing nAChRs were also detected in spinal motor nuclei and axotomy provoked a reduction of the binding. Together, these data indicate that long-term exposure to nicotine can promote neuroadaptive changes in nAChRs and that genetic alterations of neuronal nAChRs can result in transmissible neurological diseases. They also suggest that these receptors probably play a role in the central regulation of autonomic functions, as well as in motor control.

Determinants of agonist binding affinity on neuronal nicotinic receptor beta subunits

The alpha and beta subunits of heteromeric neuronal nicotinic acetylcholine receptors (nAChRs) are thought to contribute "principal" and "complementary" components to the agonist binding site, respectively. At least six loops of amino acid sequence (A, B, and C from alpha; D, E, and F from beta) are involved. We demonstrated previously that receptors containing the beta2 subunit had consistently higher affinities for a variety of agonists than beta4-containing receptors. For example, the affinity of the alpha2beta2 receptor for epibatidine, ACh, nicotine, and dimethylphenylpiperazinium (DMPP) exceeds that of alpha2beta4 by 9-, 61-, 87-, and 120-fold, respectively. Using saturation and competition analysis of receptors formed by chimeric beta subunits coexpressed with alpha2 in Xenopus laevis oocytes, we have now identified sequence segment 54-63 (corresponding to loop D) as a major determinant of affinity for epibatidine, ACh, nicotine, and DMPP. We then analyzed a series of mutant beta2 subunits in which each residue that differs between beta2 and beta4 in this region was changed from what occurs in beta2 to what occurs in beta4. The N55S, V56I, and E63T mutations each resulted in a loss of affinity for ACh and nicotine of 3- to 4-fold, whereas the T59K mutation resulted in a 7-fold loss of ACh and nicotine affinity. These mutations had little or no effect on epibatidine and DMPP affinity. The positive charge introduced by the T59K mutation does not appear to underlie loss of agonist affinity, because a similar loss of affinity was observed when a negative charge (T59D) was introduced at this position.

Aromatics at the murine nicotinic receptor agonist binding site: mutational analysis of the alphaY93 and alphaW149 residues

1. Two aromatic residues of the muscle nicotinic receptor putative agonist binding site, a tyrosine in position alpha93 and a tryptophan in position alpha149, were mutated to phenylalanine and the effects of the mutations on receptor properties were investigated using single-channel patch clamp. 2. The alphaY93F mutation reduced the receptor affinity by approximately 4-fold and the channel opening rate constant by 48-fold. The alphaW149F mutation reduced the receptor affinity by approximately 12-fold and the channel opening rate constant by 93-fold. 3. The kinetic properties of hybrid receptors that contained one wild-type and one mutated alpha subunit were also examined. Only one type of hybrid receptor activity was detected. The hybrid receptors had a channel opening rate constant intermediate to those of the wild-type and mutant receptors. It was concluded that the ligand binding sites in the mutated muscle nicotinic receptor contributed equally to channel gating. In the case of the alphaW149F mutation, the presence of the mutation in one of the binding sites had no effect on the binding properties of the other, non-mutated, site. 4. The mutant channel opening and closing rate constants were also estimated in the presence of tetramethylammonium. The data suggested significant interaction between the acetyl group of acetylcholine and the alphaY93 residue.

Synthesis and in vivo studies of the stereoisomers of N-[11C]methyl-homoepibatidine

The carbon-11 labeled enantiomers of nicotinic acetylcholine receptor (nAChR) ligand N-[11C]methyl-homoepibatidine have been synthesized to study the neuronal nicotinic acetylcholine receptors (nAChRs). In vivo evaluations were performed in mice and pig using positron emission tomography (PET). The radioligands displayed a strong enantioselectivity. The (-)-enantiomer showed high uptake in the brain while the (+)-enantiomer was rapidly washed out. In metabolite studies in mice >65% unchanged ligand was found in the blood after 60 minutes. No metabolites were found in the brain. After intravenous application of N-[11C]methyl-(-)-homoepibatidine in the pig specific accumulation in the thalamus was seen. Blocking experiments with cytisine showed specific binding consistent with labeling of the alpha4beta2-nAChR-subtype in the brain. Quantitative kinetic modeling of radiotracers in the pig brain was performed using the arterial input function. The brain uptake of the (-)-isomer was best fitted by a three-compartment model. High distribution volumes were found in the thalamus (DV(TOT) = 66.617, DV(S) = 59.910) versus a low uptake in the cerebellum (DV(TOT) = 8.605m, DV(S) = 1.898). The binding characteristics suggest N-[11C]methyl-(-)-homoepibatidine to be suited for PET imaging studies, but high toxicity prevents routine use in humans.

Multiple roles of the conserved key residue arginine 209 in neuronal nicotinic receptors

We have examined the role of a highly conserved arginine (R209), which flanks the M1 transmembrane segment of nAChRs, in the biogenesis and function of neuronal nAChRs. Point mutations revealed that, in alphaBgtx-sensitive neuronal alpha7 nAChRs, the conserved arginine is required for the transport of assembled receptors to the cell surface. By contrast, R209 does not play any role in the transport of assembled alpha-Bgtx-insensitive neuronal alpha3beta4 nAChRs to the cell surface. However, a basic residue at this position of alpha3 and beta4 subunits is necessary for either synthesis, folding, or assembly of alpha3beta4 receptors. Moreover, electrophysiological experiments revealed that in alpha3beta4 receptors the conserved arginine of the alpha3 subunit is involved in either coupling agonist binding to the channel or regulating single channel kinetics.

Dahl salt-sensitive and salt-resistant rats: examination of learning and memory performance, blood pressure, and the expression of central nicotinic acetylcholine receptors

Substantial human and animal data suggest a correlation between hypertension and memory impairment that may appear prior to overt manifestations of cerebrovascular pathology. It is unclear, however, whether hypertension plays a causal role in these memory deficits, whether hypertension and cognitive impairment are each based in family history and not interdependent, or whether a combination of these factors is important. The purpose of this study was to assess whether deficits in memory performance and nicotinic acetylcholine receptors were present in Dahl salt-sensitive rats (as observed previously in spontaneously hypertensive rats) and whether the presence of hypertension per se (induced with an 8% Na(+) diet) contributed to the deficits. Memory was assessed in a passive avoidance task, an eight-arm radial arm maze and in a water maze task, and nicotinic receptors were measured via quantitative receptor autoradiography utilizing [125I]alpha-bungarotoxin and [3H]epibatidine. Salt-sensitive rats exhibited impaired performance in both spatial learning tasks, but not the passive avoidance task, compared to controls (salt-resistant strain) and they exhibited reductions in nicotinic receptors labeled by [125I]alpha-bungarotoxin but not [3H]epibatidine in some brain regions, including some areas important for memory (e.g. the hippocampus and amygdala). In the radial arm maze, the degree of memory impairment and in binding studies the reduced expression of nicotinic receptors each failed to correlate with the highest blood pressures, and the salt-sensitive animals were impaired relative to controls whether or not the high Na(+) diet was administered. In contrast, higher blood pressures did correlate with inferior task performance in the water maze. These findings may suggest that the genetics of the subjects were critical for performance when appetitive drives were involved, but diet (and perhaps hypertension) were key to performance when memory did not involve appetitive drives or mechanisms. Overall, the data obtained from Dahl rats appear to support the role of family history (selective breeding in rats) as underlying the reductions in central nicotinic acetylcholine receptors, whereas both family history and hypertension may contribute to poor cognitive performance.

Agonists at the alpha4beta2 nicotinic acetylcholine receptors: structure-activity relationships and molecular modelling

Agonists of the alpha4beta2 nicotinic acetylcholine receptors have been synthesised as potential drugs for treatment of a variety of diseases. In this review, the published nicotinic agonists are presented and, on the basis of the molecular structure, the compounds are divided into three compound classes, nicotinoids (structurally close to nicotine), bicyclic compounds (structurally close to epibatidine and anatoxin-a), and analogues of imidacloprid (structurally close to the insecticide imidacloprid). The structure-activity relationships are discussed within and in between the classes. On the basis of computational studies of ligands for the nicotinic acetylcholine receptors the structure-activity relationships are discussed and a possible binding mode suggested. The binding mode encompasses: (A) an interaction between an anionic site in the receptor and the protonated nitrogen atom in the ligand, (B) a hydrogen bond between a hydrogen bond donor in the receptor and a hydrogen bond acceptor in the ligand, (C) an interaction between a pi-system (heteroaromatic ring, carbonyl bond) in the ligand and another pi-system or a positively charged amino acid residue in the binding site, (D) a pi-cation interaction between aromatic residues in the receptor binding site and the protonated nitrogen atom in the ligand, and (E) steric interactions of positive and negative character around the aliphatic and the heteroaromatic part of the ligand.

Neurotoxicity of channel mutations in heterologously expressed alpha7-nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) composed of chick alpha7 subunits mutated to threonine at amino acid valine-251 in the putative channel-lining M2 domain were expressed heterologously in several neuron-like and non-neuronal mammalian cell lines. Expression of mutant alpha7-nAChR is toxic to neuron-like cells of the human neuroblastoma cell lines SH-SY5Y and IMR-32, but not to several other cell types. Growth in the presence of the alpha7-nAChR antagonist methyllycaconitine (MLA) protects against neurotoxicity, as does gradual downregulation of functional, mutant alpha7-nAChR in surviving transfected SH-SY5Y cells. Relative to wild-type alpha7-nAChR, functional alpha7-nAChR mutants show a higher affinity for agonists, slower rates of desensitization, and sensitivity to dihydro-beta-erythroidine (DHbetaE) as an agonist, but they retain sensitivity to MLA as a competitive antagonist. These findings demonstrate that expression of hyperfunctional, mutant forms of Ca2+-permeable alpha7-nAChR is toxic to neuron-like cells.

Syntheses and evaluation of halogenated cytisine derivatives and of bioisosteric thiocytisine as potent and selective nAChR ligands

We have developed one-step syntheses of halogenated derivatives of (-)-cytisine featuring a halogen substituent at positions 3, 5 or 3 and 5 of the 2-pyridone fragment, and prepared the novel bioisosteric thiocytisine by oxygen-sulphur exchange. The affinities of these pyridone-modified analogs of (-)-cytisine for (alpha 4)(2)(beta 2)(3) and alpha 7* nAChRs in rat forebrain membranes were determined by competition with (+/-)-[(3)H]epibatidine and [(3)H]MLA, respectively. The 3-halocytisines 7 possess subnanomolar affinities for (alpha 4)(2)(beta 2)(3) nAChRs, higher than those found for (-)-cytisine as well as for the 5-halocytisines 8 and 3,5-dihalocytisines 6. In contrast to the parent alkaloid the 3-halogenated species display much a higher affinity for the alpha 7* nAChR subtype. The most potent molecule was 3-bromocytisine (7b) with preferential selectivity (200-fold) for the (alpha 4)(2)(beta 2)(3) subtype [K(i)=10 pM (alpha 4 beta 2) and 2.0 nM (alpha 7*)]. Replacement of the lactam with a thiolactam pharmacophore to thiocytisine (12) resulted in a subnanomolar affinity for the (alpha 4)(2)(beta 2)(3) nAChR subtype (K(i)=0.832 nM), but in a drastic decrease of affinity for the alpha 7* subtype; thiocytisine (12) has a K(i) value of 4000 nM (alpha 7*), giving a selectivity of 4800-fold for the neuronal (alpha 4)(2)(beta 2)(3)-nAChR and thus displaying the best affinity-selectivity profile in the series under consideration.

Lobeline and structurally simplified analogs exhibit differential agonist activity and sensitivity to antagonist blockade when compared to nicotine

In the present study, lobeline and two structurally simplified analogs were evaluated for activity in muscarinic and nicotinic binding assays, a functional assay for nicotinic receptor activation (86Rb+ efflux from striatal synaptosomes) and an acetylcholinesterase (AChE) assay. Lobeline displaced [3H]cytisine binding to rat cortical membranes with a mean inhibition constant (KI) value of 16.0 nM, while the lobeline analogs CRM-I-13-1 and CRM-I-32-1 exhibited values of 15.0 and 5.4 microM, respectively. [3H]methylscopolamine was displaced by lobeline with a mean KI value of 37.0 microM while CRM-I-13-1 and CRM-I-32-1 exhibited values of 55.0 and 16.0 microM, respectively. While nicotine stimulated 86Rb+ efflux from striatal synaptosomes in a mecamylamine reversible manner at each concentration tested, lobeline slightly increased 86Rb+ efflux at lower concentrations and reduced efflux at higher concentrations. Further, none of the lobeline effects were reversed with mecamylamine. Although less potent, the two lobeline analogs exhibited a similar pattern of activity. These data may suggest that lobeline and structurally similar compounds bind with different subtype selectivity than nicotine, or exert their agonists effects through non-nicotinic mechanisms. All of the compounds tested were at least several hundred times less potent than physostigmine as AChE inhibitors. While some differences were apparent between the lobeline analog which contained the 2-keto-ethyl portion of lobeline and the analog which contained the phenyl 2-hydroxy-ethyl moiety, each compound was much less active than lobeline in most parameters assessed.

Synthesis and structure-activity relationships of 5-substituted pyridine analogues of 3

In an effort to probe the steric influence of C5 substitution of the pyridine ring on CNS binding affinity, analogues of 1 substituted with a bulky moiety--such as phenyl, substituted phenyl, or heteroaryl-were synthesized and tested in vitro for neuronal nicotinic acetylcholine receptor binding affinity. The substituted analogues exhibited Ki values ranging from 0.055 to 0.69 nM compared to a Ki value of 0.15 nM for compound 1. Assessment of functional activity at subtypes of neuronal nicotinic acetylcholine receptors led to identify several agonists and antagonists.

Differential alterations in nicotinic receptor alpha6 and beta3 subunit messenger RNAs in monkey substantia nigra after nigrostriatal degeneration

Our previous studies showed that alpha4, alpha6, alpha7, beta2, beta3 and beta4 nicotinic acetylcholine receptor messenger RNAs are present in monkey substantia nigra, with a particularly intense and localized labelling of the alpha6 and beta3 subunit messenger RNAs to this brain region. Because loss of nigrostriatal neurons is a central feature of Parkinson's disease and evidence suggests that nicotinic agonists potentiate antiparkinsonian effects of L-dopa, experiments were done to determine whether nicotinic receptor subunit messenger RNAs and binding sites were altered in the basal ganglia after nigrostriatal degeneration. Squirrel monkeys (Saimiri sciureus) were rendered parkinsonian by systemic injection of the selective dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine. Behavioral studies showed that this treatment decreased baseline motor activity to 36+/-11% of control. One month after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration, caudate and putamen dopamine levels were reduced to 51+/-15% and 43+/-6% of control, respectively, while the number of tyrosine hydroxylase-positive neurons in the substantia nigra was 75+/-6% of control. Despite the reduction in nigral cell number after nigrostriatal degeneration, there were no changes in alpha4, alpha7, beta2 and beta4 messenger RNA levels in the substantia nigra. In contrast, alpha6 mRNA levels were significantly increased (143+/-10%) and the beta3 transcript decreased (62+/-6%) in the substantia nigra after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Declines were also observed in [125I]epibatidine binding in both the caudate-putamen and substantia nigra, with no change in alpha7 receptor binding. These results may suggest a dissociation in the regulation of receptor messenger RNA and binding sites, and/or that there are differential alterations in the different receptor subtypes measured using [125I]epibatidine. The changes in the two nicotinic receptor subunit messenger RNAs, alpha6 and beta3, which exhibit a selective localization to the substantia nigra, may indicate that nicotinic receptors containing these subunits are altered after nigrostriatal degeneration.

The effect of nicotine and haloperidol co-treatment on nicotinic receptor levels in the rat brain

Genetic and biological data have suggested a role for the neuronal nicotinic acetylcholine receptors in the neuropathophysiology of schizophrenia. Studies in human postmortem brain demonstrate dose-dependent increases in nicotinic receptor binding in normal smokers. We found this upregulation to be reduced in schizophrenic smokers, many of whom had taken typical neuroleptics during their lifetime. The present study examined the hypothesis that typical antipsychotic drug treatment might modulate nicotinic receptor upregulation in a rat model. Nicotine, administered alone or in combination with haloperidol, increased both high and low affinity neuronal nicotinic receptors in a region specific manner. Haloperidol had no generalized effect on basal levels of nicotinic receptor binding or nicotine induced upregulation of nicotinic receptors. However, haloperidol attenuated high affinity nicotinic receptor upregulation in thalamus and low affinity receptor upregulation in hippocampus. These results suggest that haloperidol is not likely to affect nicotinic receptor regulation by smoking in most brain regions.

Contributions of Torpedo nicotinic acetylcholine receptor gamma Trp-55 and delta Trp-57 to agonist and competitive antagonist function

Results of affinity-labeling studies and mutational analyses provide evidence that the agonist binding sites of the nicotinic acetylcholine receptor (nAChR) are located at the alpha-gamma and alpha-delta subunit interfaces. For Torpedo nAChR, photoaffinity-labeling studies with the competitive antagonist d-[(3)H]tubocurarine (dTC) identified two tryptophans, gammaTrp-55 and deltaTrp-57, as the primary sites of photolabeling in the non-alpha subunits. To characterize the importance of gammaTrp-55 and deltaTrp-57 to the interactions of agonists and antagonists, Torpedo nAChRs were expressed in Xenopus oocytes, and equilibrium binding assays and electrophysiological recordings were used to examine the functional consequences when either or both tryptophans were mutated to leucine. Neither substitution altered the equilibrium binding of dTC. However, the deltaW57L and gammaW55L mutations decreased acetylcholine (ACh) binding affinity by 20- and 7,000-fold respectively. For the wild-type, gammaW55L, and deltaW57L nAChRs, the concentration dependence of channel activation was characterized by Hill coefficients of 1.8, 1.1, and 1.7. For the gammaW55L mutant, dTC binding at the alpha-gamma site acts not as a competitive antagonist but as a coactivator or partial agonist. These results establish that interactions with gamma Trp-55 of the Torpedo nAChR play a crucial role in agonist binding and in the agonist-induced conformational changes that lead to channel opening.

The tethered agonist approach to mapping ion channel proteins--toward a structural model for the agonist binding site of the nicotinic acetylcholine receptor

BACKGROUND

The integral membrane proteins of neurons and other excitable cells are generally resistant to high resolution structural tools. Structure-function studies, especially those enhanced by the nonsense suppression methodology for unnatural amino acid incorporation, constitute one of the most powerful probes of ion channels and related structures. The nonsense suppression methodology can also be used to incorporate functional side chains designed to deliver novel structural probes to membrane proteins. In this vein, we sought to generalize a potentially powerful tool - the tethered agonist approach - for mapping the agonist binding site of ligand-gated ion channels.

RESULTS

Using the in vivo nonsense suppression method for unnatural amino acid incorporation, a series of tethered quaternary ammonium derivatives of tyrosine have been incorporated into the nicotinic acetylcholine receptor. At three sites a constitutively active receptor results, but the pattern of activation as a function of chain length is different. At position alpha149, there is a clear preference for a three-carbon tether, while at position alpha93 tethers of 2-5 carbons are comparably effective. At position gamma55/delta57 all tethers except the shortest one can activate the receptor. Based on these and other data, a model for the receptor binding site can be developed by analogy to the acetylcholine esterase crystal structure.

CONCLUSION

Through the use of nonsense suppression techniques, the tethered agonist approach has been made into a general tool for probing receptor structures. When applied to the nicotinic receptor, the method places new restrictions on developing models for the agonist binding site.

Modes of allosteric interactions with free and [3H]N-methylscopolamine-occupied muscarinic M2 receptors as deduced from buffer-dependent potency shifts

Muscarinic M2 acetylcholine receptors contain an allosteric site that is probably located at the entrance of the ligand binding pocket above the orthosteric binding site. With the orthosteric area not occupied, allosteric agents might gain access to this site. The interaction of allosteric agents with orthoster-occupied receptors is known to depend on the buffer conditions in an alloster-specific fashion. Utilizing the buffer-dependent potency shift as an indicator, we aimed to find out for two rod-like shaped and flexible allosteric agents whether or not there is evidence for a switch in the site of attachment in free compared with [3H]N-methylscopolamine ([3H]NMS)-occupied porcine heart M2 receptors. These agents are the bispyridinium compounds WDuo3 (1,3-bis[4-(phthalimidomethoxyimino-methyl)-pyridinium-1-yl] propane dibromide) and Duo3 (4,4'-bis-[(2,6-dichloro-benzyloxy-imino)-methyl]-1,1'-propane-1,3-diyl-bis-pyridinium dibromide). The prototype allosteric agents gallamine and alcuronium were included. Inhibition of [3H]NMS association was taken to reflect alloster interaction with free receptors, inhibition of [3H]NMS dissociation indicated binding to [3H]NMS-occupied receptors. In Na,K,Pi buffer (4 mM Na2HPO4, 1 mM KH2PO4, pH 7.4 at 23 degrees C) compared with Mg,Tris,Cl,Pi buffer (45 mM Tris-HCl, 2.6 mM MgHPO4, pH 7.3 at 37 degrees C) WDuo3 underwent the same loss of potency for the interaction with either free or [3H]NMS-liganded receptors. The loss of potency was quantified by a potency ratio (PR), i.e. the ratio between the concentrations of the modulator leading to a half-maximal delay of [3H]NMS association or dissociation, respectively, in Mg,Tris,Cl,Pi compared with Na,K,Pi. For WDuo3 the ratios were PRass=27 and PRdiss=22, respectively. For Duo3, the interaction with free and [3H]NMS-occupied receptors only slightly depended on the composition of the incubation medium: PRass=1.3, PRdiss=2.8. In contrast to the other agents, the concentration-effect curves of which had slope factors nH not different from unity, the curves of Duo3 were steep (nH about -1.6). For alcuronium the shift factors amounted to PRass=29 and PRdiss=25, for gallamine to PRass=216 and PRdiss=159. In conclusion, there was a wide variation between the allosteric agents with regard to the respective buffer dependence of action. Yet, for a given allosteric agent, the interaction with either free or [3H]NMS-occupied receptors was always characterized by the same buffer-dependent shift. Thus, even the applied rod-shaped allosteric agents do not appear to switch to the orthosteric site in free compared with orthoster-occupied M2 receptors.

(-)-Spiro[1-azabicyclo[2.2.2]octane-3,5'-oxazolidin-2'-one], a conformationally restricted analogue of acetylcholine, is a highly selective full agonist at the alpha 7 nicotinic acetylcholine receptor

Neuronal nicotinic acetylcholine receptors are members of the ligand-gated ion channel receptor superfamily and may play important roles in modulating neurotransmission, cognition, sensory gating, and anxiety. Because of its distribution and abundance in the CNS, the alpha 7 nicotinic receptor is a strong candidate to be involved in some of these functions. In this paper we describe the synthesis and in vitro profile of AR-R17779, (-)-spiro[1-azabicyclo[2.2. 2]octane-3,5'-oxazolidin-2'-one] (4a), a potent full agonist at the rat alpha 7 nicotinic receptor, which is highly selective for the rat alpha 7 nicotinic receptor over the alpha 4 beta 2 subtype. Preliminary SAR of AR-R17779 presented here indicate that there is little scope for modification of this rigid molecule as even minor changes result in significant loss of the alpha 7 nicotinic receptor affinity.

Alpha and beta nicotinic acetylcholine receptors subunits and synaptophysin in putamen from Parkinson's disease

It is well established that nicotinic receptors in the mammalian striatum are involved in modulation of the release of several neurotransmitters, including dopamine. In addition, nicotinic receptors with high affinity for agonists have generally been found to be reduced in the striatum in Parkinson's disease. In the present study antibodies have been used to examine which subunits contribute to the striatal nicotinic receptor loss in Parkinson's disease, and whether the reduction in [(3)H]nicotine binding correlates with synaptic loss. Autopsy tissue from the putamen of 12 Parkinson's disease cases and 12 age-matched control subjects was analysed by immunoblotting using antibodies against recombinant peptides specific for alpha3, alpha4, alpha7, beta2 and beta4 nicotinic acetylcholine receptor (nAChR) subunits and the synaptic marker synaptophysin, in conjunction with assessment of [(3)H]nicotine binding by autoradiography. The data indicate that there is no loss of alpha3, alpha4, alpha7 and beta2 immunoreactivity in the putamen in Parkinson's disease, despite a highly significant reduction in [(3)H]nicotine binding. An intense signal of beta4 immunoreactivity was found in human dorsal root ganglia, but not in temporal cortex or putamen samples. Synaptophysin immunoreactivities were also similar in Parkinson's disease and control cases. These results suggest that the loss of nicotine binding in the putamen in Parkinson's disease may involve an nAChR subunit (e.g., alpha5 and/or alpha6) other than those investigated. Alternatively, the results could reflect impaired subunit assembly at the plasma membrane.

Identification of a novel nicotinic binding site in mouse brain using [(125)I]-epibatidine

[(125)I]-Epibatidine binds to multiple nicotinic acetylcholine receptor (nAChR) subtypes with high affinity. In this study, [(125)I]-epibatidine was used to label and characterize a novel nAChR subtype found in mouse brain inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates. Binding of [(125)I]-epibatidine was saturable and apparently monophasic in each brain region (K:(D:)=71+/-12 pM mean+/-s.e.mean across regions) but inhibition of [(125)I]-epibatidine binding (200 pM) by A85380, cytisine and (-)-nicotine was biphasic, indicating the presence of multiple binding sites. The sites with lower agonist affinity comprised 30.0+/-2.2, 58.6+/-0.1 and 48.7+/-3.3% of specific [(125)I]-epibatidine (200 pM) binding in inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates, respectively. The affinity difference between A85380-sensitive and -resistant binding sites was particularly marked (approximately 1000 fold). Thus A85380 was used to differentiate agonist-sensitive and -resistant sites. The pharmacological profiles of the A85380-resistant sites in each region were assessed with inhibition binding experiments, using 14 agonists and five antagonists. The profiles were indistinguishable across regions, implying that A85380-resistant [(125)I]-epibatidine binding sites in inferior colliculus, interpeduncular nucleus, and olfactory bulb represent a single nAChR subtype. The pharmacological profile of the A85380-resistant sites is very different from that previously reported for high affinity (-)-[(3)H]-nicotine-, [(125)I]-alpha-bungarotoxin-, or [(125)I]-alpha-conotoxin MII-binding sites, suggesting that they represent a novel nAChR population in mouse brain.

Binding properties of agonists and antagonists to distinct allosteric states of the nicotinic acetylcholine receptor are incompatible with a concerted model

Recent work has shown that the nicotinic acetylcholine receptor (nAChR) can be fixed in distinct conformations by chemical cross-linking with glutardialdehyde, which abolishes allosteric transitions in the protein. Here, two conformations that resemble the desensitized and the resting states were compared with respect to their affinities for different classes of ligands. The same ligands were tested for their ability to convert the nAChR from a conformation with low affinity to a conformation with high affinity for acetylcholine. As expected, agonists were found to bind with higher affinity to the desensitized state-like conformation and to induce a shift of the nAChR to this high affinity state. In contrast, although most antagonists tested bound preferentially to the desensitized receptor as well they failed to induce a change of the affinity for acetylcholine. These observations sharply contradict basic predictions of the concerted model, including the postulate of a preformed equilibrium between the different states of the nAChR in the absence of agonist. With a similar approach we could show that the non-competitive inhibitor ethidium is displaced in a non-allosteric manner by other well characterized channel blockers from the cross-linked nAChR. These results require revision of current models for the mechanisms underlying non-competitive antagonism at the nAChR.

Mechanism for increase in expression of cerebral diazepam binding inhibitor mRNA by nicotine: involvement of L-type voltage-dependent calcium channels

We investigated the mechanisms underlying the increase in diazepam binding inhibitor (DBI) and its mRNA expression induced by nicotine (0.1 microM) exposure for 24 h using mouse cerebral cortical neurons in primary culture. Nicotine-induced (0.1 microM) increases in DBI mRNA expression were abolished by hexamethonium, a nicotinic acetylcholine (nACh) receptor antagonist. Agents that stabilize the neuronal membrane, including tetrodotoxin (TTX), procainamide (a Na(+) channel inhibitor), and local anesthetics (dibucaine and lidocaine), dose-dependently inhibited the increased expression of DBI mRNA by nicotine. The nicotine-induced increase in DBI mRNA expression was inhibited by L-type voltage-dependent Ca(2+) channel (VDCC) inhibitors such as verapamil, calmodulin antagonist (W-7), and Ca(2+)/calmodulin-dependent protein kinase II (CAM II kinase) inhibitor (KN-62), whereas P/Q- and N-type VDCC inhibitors showed no effects. In addition, nicotine exposure for 24 h induced [3H]nicotine binding to the particulate fractions of the neurons with an increased B(max) value and no changes in K(d). Under these conditions, the 30 mM KCl- and nicotine-induced 45Ca(2+) influx into the nicotine-treated neurons was significantly higher than those into non-treated neurons. These results suggest that the nicotine-stimulated increase in DBI mRNA expression is mediated by CAM II kinase activation resulting from the increase in intracellular Ca(2+) through L-type VDCCs subsequent to the neuronal membrane depolarization associated with nACh receptor activation.

Nicotinic receptor losses in dementia with Lewy bodies: comparisons with Alzheimer's disease

We sought to delineate differences between alpha7 nicotinic acetylcholine receptor (nAChR) levels in Alzheimer's disease (AD), dementia with Lewy bodies (DLB) and age matched controls, as well as the correlations between alpha7 or non-alpha7 nAChR levels and synaptophysin (Syn) or choline acetyltransferase (ChAT) in DLB. Mean bungarotoxin (Bgt) binding was 2.7 - 1.1 for controls, 2.4 +/- 1.0 for AD and 1.4 +/- 0.5 for DLB. There were significant decreases in Bgt binding for the DLB group compared to either controls or AD. Mean epibatidine (Epi) binding was 14.8 +/- 3.2 for controls, 6.3 +/- 3.2 for AD and 7.1 +/- 2.4 fmoles/mg protein for DLB. Epi binding in both the AD and DLB groups was significantly lower than in the controls. Although Syn loss correlated with the decrease in Epi binding in both diseases, declining ChAT levels correlated with Epi binding only in DLB. These data demonstrate a different pattern of nAChR loss in AD and DLB that may, in part, explain some of the differences in the two phenotypes.

Exploring the nature of molecular recognition in nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are the subject of ever increasing interest because of their presumed involvement in the etiology of numerous clinical disorders. Unfortunately, the absence of atomic-level structural data, as well as the pharmacological complexity of these receptors leaves many fundamental questions unanswered. An understanding of how ligands interact with the receptor and, in-turn, how these interactions lead to pharmacological effect is vital in the advancement of nAChR-based therapeutics. We will first explore physico-chemical themes that are evidenced to be of particular importance in nAChR molecular recognition; these are- pi-cation interaction, conformational entropy and stereochemistry. The second objective of this review is an interpretive encapsulation of the extensive and disparate body of structure-activity data that now exists for nAChRs. Finally, this review will advocate a re-investigation of distance geometry based methods as well as the need for additional approaches in nicotinic receptor pharmacophore generation.

Inhibition of cytochrome P450 2A6 increases nicotine's oral bioavailability and decreases smoking

BACKGROUND

Nicotine establishes and maintains tobacco dependence. Individuals with genetically deficient CYP2A6 nicotine metabolism are at lower risk to become smokers and, if dependent, will smoke fewer cigarettes. Hepatic CYP2A6 accounts for nicotine's low systemic bioavailability, precluding oral nicotine replacement to treat dependence.

OBJECTIVE

We sought to determine whether CYP2A6 inhibition via oral methoxsalen decreases nicotine clearance, increases nicotine bioavailability, and decreases smoking.

METHODS

Two within-subject designs in healthy tobacco-dependent volunteers were conducted: a singleblind kinetic study (n = 17) of methoxsalen 30, 10, or 3.5 mg or placebo given with nicotine 4 mg orally to abstinent smokers; and a double-blind randomized crossover study (n = 11) of methoxsalen 30 mg or placebo crossed with nicotine 4 mg given orally or placebo before 60 minutes' abstinence and 90 minutes' free smoking.

RESULTS

Placebo plus nicotine 4 mg orally increased the mean 3-hour plasma nicotine level by 4 ng/mL over residual baseline nicotine level, whereas methoxsalen 10 or 30 mg plus nicotine increased it by 9 ng/mL (P<.01), demonstrating in vivo inhibition of CYP2A6 nicotine metabolism. Methoxsalen 30 mg plus nicotine 4 mg given orally decreased breath carbon monoxide concentration at the end of free smoking by 47% (4.6 versus 8.7 ppm; P<.01) and cigarettes smoked by 24% (3.1 versus 4.1, P<.01) compared with placebo plus placebo.

CONCLUSIONS

Methoxsalen inhibits nicotine first-pass metabolism of orally administered nicotine, and the combination directly reduces smoking in a laboratory setting. CYP2A6 inhibitors may have an important role in smoking cessation and tobacco exposure reduction.

Cotinine effects on bile flow and biliary NNK elimination

Nicotine and its major metabolite cotinine inhibit alpha-hydroxylation of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) suggesting that an alternative pathway of NNK metabolism and elimination, biliary excretion of the O-glucuronide of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL-Gluc) may be enhanced. To verify the possible role of cotinine on biliary elimination of NNK and its metabolites, bile duct cannulated rats were administered a single i.p. dose of 50 mg/kg [56sup;-3 H]-NNK with or without i.p. co-administration of 5 mg/kg cotinine or nicotine. Cotinine significantly reduced cumulative bile flow and biliary elimination of NNK-derived radioactivity within six hours to 42 and 27 percent, respectively. The pattern of NNK metabolites in bile was unchanged. Nicotine had a similar inhibitory effect on bile flow. This result constitutes the first experimental evidence that cotinine inhibits bile flow. In rats, biliary elimination of NNK is reduced accordingly which may lead to an increased carcinogen burden in the body. In humans, inhibition of bile flow by tobacco alkaloids may contribute to the appetite suppressing effect of tobacco products.

Structural differences in the two agonist binding sites of the Torpedo nicotinic acetylcholine receptor revealed by time-resolved fluorescence spectroscopy

The nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata carries two nonequivalent agonist binding sites at the alphadelta and alphagamma subunit interfaces. These sites have been characterized by time-resolved fluorescence with the partial nicotinic agonist dansyl-C(6)-choline (Dnscho). When bound to the detergent-solubilized receptor, the fluorescence lifetime distribution of Dnscho displays a characteristic signature, with four separable components at 0.2, 1.8, 7.2, and 18.3 ns, respectively. Competition experiments with the antagonist d-tubocurarine (dTC), known to bind preferentially to the alphagamma site, result in substantial changes of this signature, associated with a strong decrease in average fluorescence lifetime. Comparisons with two other competitive antagonists, alpha-conotoxin M1 and alpha-bungarotoxin, demonstrate that Dnscho binds with a similar affinity to the two sites but that the microenvironment of the probe is different for each site. Using a two-site binding model together with published equilibrium constants to describe the competitive binding of dTC and Dnscho, we reach a satisfactory description of the changes in fluorescence lifetimes and propose characteristic fluorescence parameters of the probe bound to each type of site. This analysis indicates that Dnscho at the alphadelta site is principally associated with a 8.7 ns lifetime, while it has a 20.2 ns major lifetime at the alphagamma site. Therefore, the observed fluorescence heterogeneity arises in large part from the structural differences of the two binding sites. As a result, this signal can be used to identify the binding preferences of competitive ligands of unknown pharmacology.

Influence of lipid peroxidation on the nicotinic acetylcholine receptors in PC12 cells

The expression of neuronal nicotinic acetylcholine receptors (nAChRs) influenced by lipid peroxidation has been investigated. The results showed that exposure of PC12 cells to a free radical insult (FeSO(4)) induced concentration-dependent increase in level of MDA (malondialdehyde), and a significant decline in MTT [3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide] reduction. In cultured cells exposed to FeSO(4), a significant dose-dependent decrease in [(3)H] epibatidine binding sites was detected and a significant reduction in B(max) value was observed in the high affinity epibatidine binding site. These results demonstrate that lipid peroxidation in cellular membranes can induce a reduction in numbers of neuronal nAChRs.

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

Synthesis and binding of 3-aminopyridine derivatives at central nicotinic receptors

A series of secondary and tertiary pyridyl amides as potential central nicotinic acetylcholine receptors (nAChRs) ligands were prepared. Amides displayed negligible or very low affinity, whereas two amines achieved by reduction of corresponding secondary amides, showed affinity in the nanomolar range for nAChRs.

Synthesis and structure-activity relationship of novel pyridyl ethers for the nicotinic acetylcholine receptor

The preparation of novel pyridyl ethers as ligands for the nicotinic acetylcholine receptor (nAChR) is described. Variations of the ring size of the azacycle and substitution on the pyridine had dramatic effects on receptor binding affinity with IC50s at the alpha4beta2 nAChR ranging from 22 to >10,000 nM. The most potent molecule was (R)-2-chloro-3-(4-cyanophenyl)-5-((3-pyrrolidinyl)oxy)pyridine 27f with an IC50 of 22 nM.

In vitro metabolism of (S)-(-)-[2'-14C]nicotine, using various tissue preparations of marmoset

The nicotine metabolite profile produced by marmoset liver, lung and kidney preparations was investigated after 30 minutes incubation of (S)-(-)-[2'-14C]nicotine. Cation-exchange high performance liquid radiochromatography was employed to separate and quantify nicotine and its metabolites. Cotinine-N-oxide (CNO, 0.7%), 3'-hydroxy-cotinine (3'-OH-C, 0.2%), norcotinine (NORC, 0.9%) and nornicotine (NORN, 0.4%) were formed in the incubates of marmoset lung homogenates; when marmoset kidney homogenates were used, CNO, 0.4%; 3'-OH-C, 0.2%; NORC, 0.7%; NORN, 0.7%; and cotinine (COT, 0.4%) were detected in the incubates. These nicotine metabolites constituted only approximately 2.2% and 2.4% of the original nicotine substrate used by lung and kidney homogenates respectively. When marmoset hepatic homogenates and microsomes were used, both COT and NORN were detected as the major nicotine metabolites. In addition, traces of CNO and 3'-OH-C were also detected in both incubates. The amounts of COT (6.4%) and NORN (1.8%) in the hepatic homogenates were approximately twice that of those formed by hepatic microsomes (3.8% and 0.9%, respectively). Nicotine-1'-N-oxide (NNO, 1.1%) was only detected in the latter preparation. Under the experimental conditions, these nicotine metabolites constituted only 8.2% and 5.8% of the substrate nicotine used in the respective incubates. The present results showed that both primary C-oxidation pathways, i.e. cotinine formation and N-demethylation of nicotine, occurred in the lung, kidney and liver of marmoset in vitro. However, N-oxidation of nicotine was only observed when a marmoset hepatic microsomal preparation was used.

Mecamylamine prevents tolerance but enhances whole brain [3H]epibatidine binding in response to repeated nicotine administration in rats

RATIONALE

Chronic administration of nicotine in rats results in upregulation of neuronal nicotinic receptors. Upregulation has been proposed to reflect receptor desensitization, which may underlie functional tolerance to nicotine's effects. However, evidence indicates that tolerance and upregulation do not always parallel each other, suggesting that either upregulation does not always reflect desensitization, or mechanisms other than receptor desensitization account for tolerance to nicotine.

OBJECTIVES

The present studies examined tolerance to nicotine-induced antinociception and changes in receptor binding after two regimens of intermittent nicotine injections in rats. The role of receptor activation in upregulation and tolerance was also examined by co-administering nicotine with the non-competitive antagonist, mecamylamine.

METHODS

Sprague-Dawley rats were administered a short (once-daily, s.c. for 6 days (0.35 mg/kg)) or long (twice-daily for 11 days (0.66 mg/kg)) series of injections and tolerance to nicotine-induced antinociception and [3H]epibatidine binding in whole brain were measured.

RESULTS

The short series of injections resulted in tolerance to nicotine-induced antinociception, but failed to increase [3H]epibatidine binding. In contrast, the long series of injections resulted in both tolerance and increased receptor binding. Once-daily pairings of mecamylamine (1 mg/kg, s.c.) with nicotine (0.35 mg/kg) for 6 days blocked the development of tolerance, indicating receptor activation is necessary for tolerance to occur. Pairing mecamylamine with nicotine (0.66 mg/kg) twice daily for 11 days blocked tolerance but produced a greater increase in [3H]epibatidine binding than nicotine alone.

CONCLUSIONS

A dissociation of tolerance from receptor upregulation was observed in the present study. The finding that receptor activation may be necessary for tolerance but not upregulation is discussed within the context of possible mechanisms controlling tolerance to nicotine.

Comparative cholinergic neurotoxicity of oral chlorpyrifos exposures in preweanling and adult rats

Chlorpyrifos (CPF) is a common organophosphorus (OP) pesticide. Previous studies have demonstrated that neonatal rats are more sensitive than adults to the acute toxicity of high dosages of CPF. The present study examined lethality and age-related differences in neurochemical indicators and functional signs of neurotoxicity following a broad range of acute and repeated oral CPF exposures. There was about a 9-fold difference in sensitivity to the acute-dose lethality of chlorpyrifos among neonatal (7 days-of-age) and adult (90 days-of-age) rats (LD(10): neonates = 15 mg/kg; adults = 136 mg/kg), while juvenile rats (21 days-of-age) exhibited intermediate sensitivity (LD(10) = 47 mg/kg). Neonatal and adult rats (n = 5-7/treatment/age group/time point) were given CPF (0, 0.15, 0.45, 0. 75, 1.5, 4.5, 7.5, or 15 mg/kg/day) for 14 days and sacrificed 4 h after either the first or 14th dose for neurochemical measurements (cholinesterase activity in frontal cortex, plasma and RBC, and muscarinic ([(3)H]QNB) and nicotinic ([(3)H]epibatidine) receptor binding in frontal cortex. No overt signs of functional toxicity (involuntary movements, SLUD signs) were noted in either age group by 4 h after the first dose. With repeated CPF exposures, however, signs of cholinergic toxicity were noted in both age groups at the higher dose levels [no observed effect levels (NOELs): neonate = 4.5 mg/kg/day; adult = 7.5 mg/kg/day]. Similar degrees of ChE inhibition were noted in neonatal brain and blood fractions following acute exposure, but substantial ChE inhibition was only noted in adult plasma and RBC 4 h after the first treatment. Following repeated CPF exposures, similar degrees of ChE inhibition were again noted in tissues from immature animals, but a wide range of sensitivity to inhibition was noted in adult tissues. NOELs based on ChE inhibition for adults were about 1->/=10-fold higher than in neonates with acute exposure but only 0.2-2 times higher with repeated dosing. Moreover, dose-related inhibition of brain ChE was similar between age groups, and similar reductions in both QNB and epibatidine binding were noted between the age groups after repeated dosing, even though by the end of the dosing period young animals (juveniles) were still about 3 times more sensitive than adults, based on acute lethality. We conclude that while immature animals can be markedly more sensitive to lethal effects of high doses of CPF, lesser or no age-related differences are apparent, based on non-lethal endpoints, in particular with repeated exposures.

Synthesis and in vivo evaluation of (E)-N-[(11)C]Methyl-4- (3-pyridinyl)-3-butene-1-amine ([(11)C]metanicotine) as a nicotinic receptor radioligand

(E)-N-[(11)C]Methyl-4-(3-pyridinyl)-3-butene-1-amine ([(11)C]metanicotine), a high affinity (K(i) = 16 nM) CNS-selective nicotinic agonist, was prepared by the [(11)C]alkylation of the desmethyl precursor with [(11)C]methyl trifluoromethanesulfonate. In vivo distribution studies in mice demonstrated good blood brain permeability but essentially uniform regional brain distribution and no evidence of specific binding to nicotinic cholinergic receptors. Identical results were obtained in an imaging study performed in a monkey brain. Therefore, despite literature reports supporting the use of metanicotine as a cognition enhancing nicotinic agonist, (E)-N-[(11)C]methyl-4-(3-pyridinyl)-3-butene-1-amine does not appear to be a suitable candidate for in vivo imaging studies of nicotinic acetylcholine receptors in the mammalian brain.

UB-165: a novel nicotinic agonist with subtype selectivity implicates the alpha4beta2* subtype in the modulation of dopamine release from rat striatal synaptosomes

Presynaptic nicotinic acetylcholine receptors (nAChRs) on striatal synaptosomes stimulate dopamine release. Partial inhibition by the alpha3beta2-selective alpha-conotoxin-MII indicates heterogeneity of presynaptic nAChRs on dopamine terminals. We have used this alpha-conotoxin and UB-165, a novel hybrid of epibatidine and anatoxin-a, to address the hypothesis that the alpha-conotoxin-MII-insensitive subtype is composed of alpha4 and beta2 subunits. UB-165 shows intermediate potency, compared with the parent molecules, at alpha4beta2* and alpha3-containing binding sites, and resembles epibatidine in its high discrimination of these sites over alpha7-type and muscle binding sites. (+/-)-Epibatidine, (+/-)-anatoxin-a, and (+/-)-UB-165 stimulated [(3)H]-dopamine release from striatal synaptosomes with EC(50) values of 2.4, 134, and 88 nM, and relative efficacies of 1:0.4:0.2, respectively. alpha-Conotoxin-MII inhibited release evoked by these agonists by 48, 56, and 88%, respectively, suggesting that (+/-)-UB-165 is a very poor agonist at the alpha-conotoxin-MII-insensitive nAChR subtype. In assays of (86)Rb(+) efflux from thalamic synaptosomes, a model of an alpha4beta2* nAChR response, (+/-)-UB-165 was a very weak partial agonist; the low efficacy of (+/-)-UB-165 at alpha4beta2 nAChR was confirmed in Xenopus oocytes expressing various combinations of human nAChR subunits. In contrast, (+/-)-UB-165 and (+/-)-anatoxin-a were similarly efficacious and similarly sensitive to alpha-conotoxin-MII in increasing intracellular Ca(2+) in SH-SY5Y cells, a functional assay for native alpha3-containing nAChR. These data support the involvement of alpha4beta2* nAChR in the presynaptic modulation of striatal dopamine release and illustrate the utility of exploiting a novel partial agonist, together with a selective antagonist, to dissect the functional roles of nAChR subtypes in the brain.

Photoaffinity labeling of Torpedo nicotinic receptor with the agonist [3H]DCTA: identification of amino acid residues which contribute to the binding of the ester moiety of acetylcholine

Torpedo marmorata acetylcholine binding sites were photolabeled using 360 nm light, at equilibrium in the desensitized state, with the agonist [3H]DCTA utilizing the CeIV/glutathione procedure described previously (Grutter, et al. (1999) Biochemistry 38, 7476-7484). Photoincorporation of [3H]DCTA was concentration-dependent with a maximum of 7.5% specific labeling on the alpha-subunit and 1.2% on the gamma-subunit. The apparent dissociation constants for labeling of the alpha- and gamma-subunits were 2.2 +/- 1.1 and 3.6 +/- 2.8 microM, respectively. The alpha-chains isolated from receptor-rich membranes photolabeled in the absence or in the presence of carbamylcholine were cleaved with CNBr using an efficient "in gel" procedure. The resulting peptide fragments were purified by HPLC and further submitted to trypsinolysis. The digest was analyzed by HPLC leading to a single radioactive peak which, by microsequencing, revealed two sequences extending from alpha Lys-179 and from alpha His-186, respectively. Radioactive signals could be unambiguously attributed to positions corresponding to residues alpha Tyr-190, alpha Cys-192, alpha Cys-193, and alpha Tyr-198. These four identified [3H]DCTA-labeled residues, which have been also labeled with other affinity and photoaffinity probes including the agonist [3H]nicotine, belong to loop C of the ACh binding site. The chemical structure of [3H]DCTA, together with its well-defined and powerful photochemical reactivity, provides convincing evidence that loop C-labeled residues are primarily involved in the interaction with the ester moiety of acetylcholine.

The influence of nicotinic receptor subunit composition upon agonist, alpha-bungarotoxin and insecticide (imidacloprid) binding affinity

A series of cell lines stably expressing recombinant nicotinic acetylcholine receptors (nAChRs) has been established by transfection of mammalian (rat) and insect (Drosophila) nicotinic subunit cDNAs. By equilibrium radioligand binding, we have examined the influence of individual subunits upon the affinity of two nicotinic agonists (epibatidine and methylcarbamylcholine), an antagonist (the snake neurotoxin, alpha-bungarotoxin) and a recently developed chloronicotinyl insecticide (imidacloprid). Imidacloprid bound with very low affinity to the rat alpha4/beta2 nAChR but did so with high affinity to hybrid nAChRs containing Drosophila alpha subunits co-assembled with rat beta2. Of the subunit combinations examined, imidacloprid showed highest affinity binding to nAChRs containing the recently identified Drosophila alpha subunit, D alpha3, co-assembled with beta2. In contrast, no specific binding of imidacloprid was detected when D alpha3 was co-expressed with the mammalian neuronal beta4 subunit, or with the muscle-type (gamma or delta) subunits. However, despite the absence of imidacloprid binding to D alpha3/beta4, D alpha3/gamma or D alpha3/delta, these subunit combinations all exhibited high affinity binding of other nicotinic radioligands. Epibatidine showed substantially higher affinity binding to subunit combinations containing neuronal (beta2 or beta4) subunits than it did to combinations containing muscle-type (gamma or delta) subunits. In contrast, alpha-bungarotoxin bound with higher affinity to combinations containing muscle-type subunits. Our results demonstrate that both alpha and non-alpha subunits exert a profound influence upon the affinity of nicotinic ligands for recombinant nAChRs.

Synthesis and binding studies of some epibatidine analogues

A series of epibatidine analogues and their positional isomers bearing an 8-azabicyclo[3.2.1]octane moiety is described. Some of the compounds, especially those containing 8-azabicyclo[3.2.1]oct-2-ene moiety show high affinity for the nicotinic cholinergic receptor.

Deficient cotinine formation from nicotine is attributed to the whole deletion of the CYP2A6 gene in humans

Nicotine is mainly metabolized to cotinine by cytochrome P450 (CYP) 2A6. Large interindividual differences in nicotine metabolism have been reported in humans. The purpose of this study was to clarify the relationship between the poor metabolism of nicotine and the existence of the CgammaP2A6v1 and CgammaP2A6v2 alleles, and a whole deletion allele of the CgammaP2A6 gene. The plasma concentrations of nicotine and cotinine were measured in 10 healthy subjects after each smoked one cigarette or chewed one piece of nicotine gum. One subject showed no detectable cotinine level in plasma when smoking and the lowest cotinine level when receiving nicotine gum. The subject was regarded as a poor metabolizer of nicotine by a probit analysis and was found to carry a homozygous whole deletion allele of the CgammaP2A6 gene. This is the first report to show that deficient cotinine formation in humans is attributed to the whole deletion of the CgammaP2A6 gene.

Ethnic differences in N-glucuronidation of nicotine and cotinine

We previously reported that the metabolism of cotinine, the proximate metabolite of nicotine, is significantly slower in black than in white cigarette smokers. To understand why the metabolism of nicotine and cotinine might differ between blacks and whites, we studied the pattern of nicotine metabolism in blacks and whites. One hundred eight healthy smokers (51 blacks and 57 whites), of similar age, gender distribution, and smoking history, received an i.v. infusion of deuterium-labeled nicotine and cotinine. The clearance of cotinine, the fractional conversion of nicotine to cotinine, and the metabolic clearance of nicotine to cotinine were significantly lower in blacks than in whites. Blacks excreted significantly less nicotine as nicotine-N-glucuronide and less cotinine as cotinine-N-glucuronide than whites, but there was no difference in the excretion of 3'-hydroxycotinine-O-glucuronide. Nicotine and cotinine glucuronidation appeared to be polymorphic, with evidence of slow and fast N-glucuronide formers among blacks but was unimodal with fast conjugators only among whites. Other findings of note included the demonstration of a significant correlation between the distribution volumes of nicotine and cotinine with lean body mass: there was a smaller distribution volume and a shorter half-life for cotinine in women than in men and a smaller volume of distribution of cotinine in blacks than in whites. We conclude that the metabolism of cotinine is slower in blacks than in whites because of both slower oxidative metabolism of nicotine to cotinine (presumably via cytochrome P-450 2A6) and slower N-glucuronidation. Ethnic differences in the metabolism of other drugs undergoing N-glucuronidation should be studied.

Gating of alpha3beta4 neuronal nicotinic receptor can be controlled by the loop M2-M3 of both alpha3 and beta4 subunits

Previous studies have shown that the gating mechanism of alpha3beta4 neuronal nicotinic receptors is affected by a residue in the middle of the M2-M3 loop of the beta4 subunit. We have extended the study of the same location to the alpha3 subunit. Bovine alpha3beta4 receptors were mutated in position 268, substituting the residue present in wild-type receptors, i.e. leucine in alpha3 and asparagine in beta4, for an aspartate. Wild-type and mutated alpha3 and beta4 subunits were combined to form four different receptors. We have measured macroscopic currents in Xenopus oocytes elicited by nicotine, and related them to surface receptor expression measured with an epibatidine-binding essay. We also obtained single-channel recordings of the receptors to study their kinetic behaviour. The results were analysed in terms of an allosteric model with three states. We found that the effect of the mutation in the alpha3 subunit on the gating of the receptor was similar to the corresponding mutation in the beta4 subunit. The effect when both subunits were mutated was additive, suggesting that the contribution of each subunit to the gating mechanism is independent.

Agonist-induced release of nitric oxide during acute exposure to nicotine

While we have shown that acute infusion of nicotine impairs agonist-induced dilatation of resistance arterioles (Am. J. Physiol. 272:H2337-H2342, 1997), no studies have examined the release of nitric oxide in response to these agonists before and during treatment with nicotine. Thus, the first goal of the present study was to examine agonist-induced release of nitric oxide by the hamster cheek pouch microcirculation under control conditions and during acute infusion of nicotine. We measured the release of nitric oxide (Sievers NO analyzer) in response to repeated topical application of acetylcholine (1.0 microM) and 5'-adenosine diphosphate (ADP; 1.0 microM) during infusion of vehicle and during infusion of nicotine (2.0 microg/kg/min i.v. for 30 minutes followed by a maintenance dose of 0.35 microg/kg/min). In hamsters treated with vehicle, topical application of acetylcholine and ADP elicited reproducible increases in nitric oxide release. In contrast, in hamsters treated with nicotine, there was a marked inhibition of nitric oxide release in response to acetylcholine and ADP. In a previous study (J. Appl. Physiol. 85:1292-1298, 1998) we found that treatment of the hamster cheek pouch microcirculation with superoxide dismutase restored impaired agonist-induced vasodilatation during acute infusion of nicotine. Thus, our second goal was to examine whether superoxide dismutase would restore agonist-induced release of nitric oxide during infusion of nicotine. We found that treatment of the hamster cheek pouch microcirculation with superoxide dismutase prior to infusion of nicotine prevented nicotine-induced impairment of nitric oxide release in response to acetylcholine and ADP. We suggest that nicotine alters dilatation of arterioles via an increased release of superoxide anion and subsequent inactivation of nitric oxide.