Identification of two Lynx proteins in Nilaparvata lugens and the modulation on insect nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission in the insect brain and are targets for neonicotinoid insecticides. Some proteins, other than nAChRs themselves, might play important roles in insect nAChRs function in vivo and in vitro , such as the chaperone, regulator and modulator. Here we report the identification of two nAChR modulators (Nl-lynx1 and Nl-lynx2) in the brown planthopper, Nilaparvata lugens . Analysis of amino acid sequences of Nl-lynx1 and Nl-lynx2 reveals that they are two members of the Ly-6/neurotoxin superfamily, with a cysteine-rich consensus signature motif. Nl-lynx1 and Nl-lynx2 only increased agonist-evoked macroscopic currents of hybrid receptors Nlα1/β2 expressed in Xenopus oocytes, but not change the agonist sensitivity and desensitization properties. For example, Nl-lynx1 increased I _max of acetylcholine and imidacloprid to 3.56-fold and 1.72-fold of that of Nlα1/β2 alone, and these folds for Nl-lynx2 were 3.25 and 1.51. When the previously identified Nlα1^Y151S mutation was included (Nlα1^Y151S/β2), the effects of Nl-lynx1 and Nl-lynx2 on imidacloprid responses, but not acetylcholine response, were different from that in Nlα1/β2. The increased folds in imidacloprid responses by Nl-lynx1 and Nl-lynx2 were much higher in Nlα1^Y151S/β2 (3.25-fold and 2.86-fold) than in Nlα1/β2 (1.72-fold and 1.51-fold), which indicated Nl-lynx1 and Nl-lynx2 might also serve as an influencing factor in target-site insensitivity in N. lugens . These findings indicate that nAChRs chaperone, regulator and modulator may be of importance in assessing the likely impact of the target-site mutations such as Y151S upon neonicotinoid insecticide resistance.     

Activation of skeletal muscle nicotinic acetylcholine receptors

Summary and Conclusions Work over the past ten years has greatly increased our understanding of both the structure and function of the muscle nicotinic acetylcholine receptor. There is a strongly supported general picture of how the receptor functions: agonist binds rapidly to sites of low affinity and channel opening occurs at a rate comparable to the agonist dissociation rate. Channel closing is slow, so the channel has a high probability of being open if both agonist-binding sites are occupied by ACh. Results of expression studies have shown that each subunit can influence AChR activation and have given a structural basis for the major physiological change known for muscle AChR, the developmental change in AChR activation. These general statements notwithstanding, there are still major areas of uncertainty which limit our understanding. We have emphasized these areas of uncertainty in this review, to indicate what needs to be done.First, the quantitative estimates of rate constants are not as strongly supported as they should be. The major reasons are twofold—uncertainties about the interpretation of components in the kinetic data and difficulties of resolving brief events. As a result, any inferences about the functional consequences of structural alterations must remain tenuous.Second, the functional behavior of individual AChRs is not as well understood as it should be. The kinetic behavior of an individual receptor clearly can be complex (section II). In addition, there is evidence that superimposed on this complexity there may be stable and kinetically distinguishable populations of receptors (section III). Until the basis for the kinetically defined populations is clarified, kinetic parameters for receptors of defined structure cannot be unambiguously obtained.Finally, it is not surprising that the studies of AChR of altered structure have not given definitive results. Two reasons should be apparent from the preceding points: there is not a fully supported approach for kinetic analysis, and the normal population may not be clearly defined. An additional complication is also emerging, in that the available data support the idea that specific residues distributed over all subunits may influence AChR activation. This possibility renders the task of analysis that much more difficult.The muscle nicotinic AChR has served as a prototype for the family of transmitter-gated membrane channels, which includes the muscle and neuronal nicotinic receptors, the GABA_A, the glycine and possibly the non-NMDA excitatory amino acid receptor (Stroud et al., 1990). It is interesting to note that the functional properties of the GABA_A receptor, probably the best-studied of the other members of the family are rather similar. In particular, opentime and burst durations show multiple components interpreted as reflecting openings of singly and doubly liganded receptors (Mathers & Wang, 1988; Macdonald et al., 1989), the distribution of gaps indicates a relatively complex gating scheme (Twyman et al., 1990; Weiss & Magleby, 1989), and multiple kinetic modes are likely to exist (Newland et al., 1991). The situation with regards to the effects of GABA_A receptor subunit stoichiometry is more complex than for muscle AChR (e.g., Luddens & Wisden, 1991), perhaps similar to that found for neuronal nicotinic AChR (Papke et al., 1989; Luetje et al., 1990; Luetje & Patrick, 1991). Overall, it appears that the unresolved questions about the muscle nicotinic AChR are not indications that this is an exceptionally complicated transmitter-gated channel. Rather, it appears to be a relatively straightforward member of the family, and the lessons we learn from studying it are likely to be directly applicable to other receptors.We thank many friends for discussion, including Tony Auerbach, Paul Brehm, Jim Dilger, Meyer Jackson, and Chuck Stevens who told us about data before publication. Research in the authors' laboratories is supported by grants from the NIH (CL and JHS) and the AHA (CL).     

Agonists cause endocytosis of nicotinic acetylcholine receptors on cultured myotubes

Regulated trafficking of neurotransmitter receptors in excitable cells may play an important role in synaptic plasticity. In addition, agonist‐induced endocytosis of nicotinic acetylcholine receptors (nAChRs) in particular might be involved in nicotine tolerance and addiction. The existing evidence concerning regulated internalization of cell‐surface nAChRs is indirect and equivocal, however. In the present study, radioligand binding and fluorescence microscopy were used to show that agonists cause substantial endocytosis of nAChRs on cultured myotubes. Exposure to carbachol or nicotine caused a decrease in the intensity of fluorescent labeling of clusters of cell‐surface nAChRs that was blocked by low temperature. Overall, myotubes exposed to carbachol or nicotine bound 50–70% less [¹²⁵I]‐α‐bungarotoxin on the cell surface than untreated cells. The effect of carbachol was significant within 5 min, increased progressively for at least 4 h, and had a sensitivity of 100 nM or less. Exposure to carbachol caused the appearance or dramatic expansion of an intracellular pool of nAChRs, which were localized to discrete, largely perinuclear structures. A pulse‐chase labeling protocol allowed the selective labeling and localization of nAChRs that had been internalized from the cell surface. In untreated cells, very little internalization of nAChRs occurred over a period of 3 h, indicating that constitutive endocytosis of receptors over this period was minimal. Exposure to carbachol, however, caused a dramatic increase in the endocytosis of nAChRs. These results provide direct evidence that agonists, including the tobacco alkaloid nicotine, can cause substantial endocytosis of cell‐surface nAChRs. © 2001 John Wiley & Sons, Inc. J Neurobiol 49: 212–223, 2001     

Desensitization of the nicotinic acetylcholine receptor by diisopropylfluorophosphate

The interaction of diisopropylfluorophosphate (DFP) with the nicotinic acetylcholine (ACh) receptor of Torpedo electric organ was studied, using [³H]-phencyclidine ([³H]-PCP) as a reporter probe. Phencyclidine binds with different kinetics to resting, activated, and desensitized receptor conformations. Although DFP did not inhibit binding of [³H]-ACh or ¹²⁵I-α-bungarotoxin (BGT) to the receptor recognition sites and potentiated in a time-dependent manner [³H]-PCP binding to the receptor's high-affinity allosteric site, it inhibited the ACh or carbamylcholine-stimulated [³H]-PCP binding. This suggested that DFP bound to a third kind of site on the receptor and affected receptor conformation. Preincubation of the membranes with DFP increased the receptor's affinity for carbamylcholine by eightfold and raised the pseudo-first-order rate of [³H]-PCP binding to that of an agonist-desensitized receptor. Accordingly, it is suggested that DFP induces receptor desensitization by binding to a site that is distinct from the recognition or high-affinity noncompetitive sites.     

Non-neuronal nicotinic acetylcholine receptors: Cholinergic regulation of the immune processes

Nicotinic acetylcholine receptors (nAChRs) were initially discovered and studied as mediators of fast synaptic transmission in neuromuscular junctions and autonomic ganglia. Later on, they were found in the brain and in many nonexcitable tissues where they regulate vital cellular functions and the activity of other receptors. Primary immune organs, the bone marrow and thymus, are innervated with cholinergic nerves, which mediate the control of lymphopoiesis provided by the autonomic nervous system. In addition, lymphocytes are able to produce endogenous acetylcholine that can regulate the immune processes in an auto/paracrine way. Correspondingly, both T and B lymphocytes express functional nAChRs involved in the regulation of development and activation of these cells. This review describes the structure and roles of nAChRs in the immune system with regard to its potential regulation by the autonomic nervous system, as well as by self sources of endogenous agonists. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 307–314, July–October, 2007.     

Experimental determination of the vertical alignment between the second and third transmembrane segments of muscle nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) are members of the Cys‐loop ligand‐gated ion channel superfamily. Muscle nAChR are heteropentamers that assemble from two α, and one each of β, γ, and δ subunits. Each subunit is composed of three domains, extracellular, transmembrane and intracellular. The transmembrane domain consists of four α‐helical segments (M1–M4). Pioneering structural information was obtained using electronmicroscopy of Torpedo nAChR. The recently solved X‐ray structure of the first eukaryotic Cys‐loop receptor, a truncated (intracellular domain missing) glutamate‐gated chloride channel α (GluClα) showed the same overall architecture. However, a significant difference with regard to the vertical alignment between the channel‐lining segment M2 and segment M3 was observed. Here, we used functional studies utilizing disulfide trapping experiments in muscle nAChR to determine the spatial orientation between M2 and M3. Our results are in agreement with the vertical alignment as obtained when using the GluClα structure as a template to homology model muscle nAChR, however, they cannot be reconciled with the current Torpedo nAChR model. The vertical M2–M3 alignments as observed in X‐ray structures of prokaryotic Gloeobacter violaceus ligand‐gated ion channel and GluClα are in agreement. Our results further confirm that this alignment in Cys‐loop receptors is conserved between prokaryotes and eukaryotes.     

Emerging concepts: novel integration of in vivo approaches to localize the function of nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are important targets of the neuromodulator acetylcholine (ACh) and the drug nicotine. The role of their different subunits has been analysed for a decade by the creation of knock-out (KO) mice using homologous recombination. This technique shows that a given subunit is necessary for a given function. However, for ubiquitously expressed genes, it cannot demonstrate the localization for a given subunit in which its expression is sufficient, especially for behavioural phenotypes. Sufficient in this context means that the brain region requiring the expression of the gene product has been localized. Novel strategies have therefore been developed to re-express, region specifically, nAChR subunits on a KO background using lentiviral vectors. Localized regeneration of fully functional high-affinity nAChRs in defined brain regions has proven that these receptors are sufficient to restore a variety of functions: nicotine-induced dopamine release, nicotine self-administration in mice, dopamine neuron firing patterns, and exploratory and locomotor behaviours in a sequential locomotor task testing executive function were thus defined as depending exclusively on the 'knock-back' of beta2*-nAChRs into the ventral tegmental area. These analyses highlight the important role of endogenous cholinergic regulation of a variety of functions. The novel integrated use of restricted re-expressed nAChR subunits with in vivo electrophysiology and automated quantitative behavioural analysis enables the further analysis of defined neuronal circuits in nicotine addiction and higher cognitive function.     

NIDA522131, a new radioligand for imaging extrathalamic nicotinic acetylcholine receptors: in vitro and in vivo evaluation

A novel radioligand, 6-chloro-3-((2-( S )-azetidinyl)methoxy)-5-(2-fluoropyridin-4-yl)pyridine (NIDA522131), for imaging extrathalamic nicotinic acetylcholine receptors (nAChRs) was characterized in vitro and in vivo using positron emission tomography. The K_d and T_1/2 of dissociation of NIDA522131 binding measured at 37°C in vitro were 4.9 ± 0.4 pmol/L and 81 ± 5 min, respectively. The patterns of radioactivity distribution in monkey brain in vivo was similar to that of 2-[¹⁸F]fluoro-3-(2( S )-azetidinylmethoxy)pyridine (2FA), a radioligand that has been successfully used in humans, and matched the α₄β₂* nAChRs distribution. Comparison between [¹⁸F]NIDA522131 and 2FA demonstrated better in vivo binding properties of the new radioligand and substantially greater radioactivity accumulation in brain. Consistent with [¹⁸F]NIDA522131 elevated affinity for nAChRs and its increased lipophilicity, both, the total and non-displaceable distribution volumes were substantially higher than those of 2FA. Estimated binding potential values in different brain regions, characterizing the specificity of receptor binding, were 3–4 fold higher for [¹⁸F]NIDA522131 than those of 2FA. Pharmacological evaluation in mice demonstrated a toxicity that was comparable to 2FA and is in agreement with a 2300 fold higher affinity at α₄β₂* versus α₃β₄* nAChRs. These results suggest that [¹⁸F]NIDA522131 is a promising positron emission tomography radioligand for studying extrathalamic nAChR in humans.     

Regulation of nicotinic acetylcholine receptor function by adenine nucleotides

1. Nicotinic acetylcholine receptors (nAChR)4 from BC3H1 cells (which express a skeletal muscle-type receptor) and from Torpedo californica electric organ were expressed in Xenopus laevis oocytes and studied with a voltage-clamp technique. 2. We found that bath application of ATP in the micromolar to millimolar range increased the ACh-elicited current in both muscle and electrocyte receptors. The effect of ATP increased with successive applications. This "use-dependent" increase in potentiation was Ca2+ dependent, while the potentiation itself was not. 3. Four other nucleotides were tested on muscle nAChR: ADP, AMP, adenosine, and GTP. Of these, only ADP was a potentiator, but its effect was not use dependent. Neither ATP nor ADP affected the resting potential of the oocyte membrane. 4. ADP potentiated the response to suberyldicholine and nicotine, as well as ACh. 5. Finally, ADP reversed the phencyclidine-induced block of ACh currents in oocytes expressing muscle nAChR.     

褐飞虱对吡虫啉的抗性机理和靶标分子毒理学

褐飞虱Nilaparvata lugens是水稻最重要的害虫之一,长期依赖化学防治导致了该害虫对不同类型杀虫剂抗性的产生,对新烟碱类杀虫剂吡虫啉高水平抗性的产生更是造成了巨大的粮食生产损失。近年来在褐飞虱对吡虫啉抗性机理,以及在抗药性机理研究推动下吡虫啉作用靶标褐飞虱神经系统烟碱型乙酰胆碱受体（nicotinic acetylcholine receptors, nAChRs）毒理学等方面取得了许多研究进展。nAChRs是昆虫神经系统中最重要的神经递质受体,是几类重要杀虫剂的作用靶标,其中以新烟碱类杀虫剂为代表。通过对比敏感品系和室内连续筛选获得的高抗吡虫啉品系,在褐飞虱两个nAChRs亚基Nlα1和Nlα3中均发现了抗性相关点突变Y151S,该突变导致了受体与吡虫啉结合亲和力的显著下降,而对内源神经递质乙酰胆碱的亲和力影响很小。Nlα1与褐飞虱另外两个亚基Nlα2和Nlβ1共聚成一个受体,构成吡虫啉低亲和力结合位点;Nlα3与褐飞虱另外两个亚基Nlα8和Nlβ1共聚成一个受体,构成吡虫啉高亲和力结合位点。不仅褐飞虱nAChRs与吡虫啉抗性相关,某些nAChRs附属蛋白也直接影响褐飞虱对吡虫啉的抗性,如Lynx蛋白。关于褐飞虱nAChRs组成、抗药性相关变异、受体附属蛋白对抗药性的影响等方面的研究,均为国内外前沿报道,不仅有助于对新烟碱类杀虫剂抗性机理的理解,对昆虫nAChRs毒理学同样具有很大的推动作用。     

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

The effects of the acetylcholinesterase inhibitors physostigmine and tacrine on alpha4beta2 and alpha4beta4 subtypes of neuronal nicotinic acetylcholine (ACh) receptors, expressed in Xenopus laevis oocytes, have been investigated. In voltage-clamp experiments low concentrations of physostigmine and tacrine potentiate ion currents induced by low concentrations of ACh, whereas at high concentrations they inhibit ACh-induced ion currents. These dual effects result in bell-shaped concentration-effect curves. Physostigmine and tacrine, by themselves, do not act as nicotinic receptor againsts. The larger potentiation is observed with 10 microM: physostigmine on alpha4beta4 nicotinic receptors and amounts to 70% at 1 microM: ACh. The mechanism underlying the effects of physostigmine on alpha4beta4 ACh receptors has been investigated in detail. Potentiation of ACh-induced ion current by low concentrations of physostigmine is surmounted at elevated concentrations of ACh, indicating that this is a competitive effect. Conversely, inhibition of ACh-induced ion current by high concentrations of physostigmine is not surmounted at high concentrations of ACh, and this effect appears mainly due to noncompetitive, voltage-dependent ion channel block. Radioligand binding experiments demonstrating displacement of the nicotinic receptor agonist (125)I-epibatidine from its recognition sites on alpha4beta4 ACh receptors by physostigmine confirm that physostigmine is a competitive ligand at these receptors. A two-site equilibrium receptor occupation model, combined with noncompetitive ion channel block, accounts for the dual effects of physostigmine and tacrine on ACh-induced ion currents. It is concluded that these acetylcholinesterase-inhibiting drugs interact with the ACh recognition sites and are coagonists of ACh on alpha4-containing nicotinic ACh receptors.     

A nicotinic acetylcholine receptor transmembrane point mutation (G275E) associated with resistance to spinosad in Frankliniella occidentalis

High levels of resistance to spinosad, a macrocyclic lactone insecticide, have been reported previously in western flower thrips, Frankliniella occidentalis, an economically important insect pest of vegetables, fruit and ornamental crops. We have cloned the nicotinic acetylcholine receptor (nAChR) α6 subunit from F. occidentalis (Foα6) and compared the nucleotide sequence of Foα6 from susceptible and spinosad‐resistant insect populations (MLFOM and R1S respectively). A single nucleotide change has been identified in Foα6, resulting in the replacement of a glycine (G) residue in susceptible insects with a glutamic acid (E) in resistant insects. The resistance‐associated mutation (G275E) is predicted to lie at the top of the third α‐helical transmembrane domain of Foα6. Although there is no direct evidence identifying the location of the spinosad binding site, the analogous amino acid in the C. elegans glutamate‐gated chloride channel lies in close proximity (4.4 Å) to the known binding site of ivermectin, another macrocyclic lactone pesticide. The functional consequences of the resistance‐associated mutation have been examined in the human nAChR α7 subunit. Introduction of an analogous (A272E) mutation in α7 abolishes the modulatory effects of spinosad whilst having no significant effect upon activation by acetylcholine, consistent with spinosad having an allosteric mechanism of action.     

The effects of amantadine on liposomally reconstituted nicotinic acetylcholine receptor

The effects of amantadine on liposomally reconstituted nicotinic acetylcholine receptor function were studied. At 1 × 10^?4M, the drug blocked 85% of the carbamylcholine-induced cation influx into liposomes, but left 90% of the αbungarotoxin binding intact. In addition, amantadine was shown to be a non-competitive inhibitor of membrane bound acetylcholinesterase. These experiments are relevant to the mechanism of action of amantadine at the motor end plate, where it produces electrophysiological changes compatible with an inhibition of cholinergic agonist mediated ion flux.     

Photoaffinity labeling of insect nicotinic acetylcholine receptors with a novel [(3)H]azidoneonicotinoid

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel in the insect CNS and a target for major insecticides. Here we use photoaffinity labeling to approach the functional architecture of insect nAChRs. Two candidate 5-azido-6-chloropyridin-3-yl photoaffinity probes are evaluated for their receptor potencies: azidoneonicotinoid (AzNN) with an acyclic nitroguanidine moiety; azidodehydrothiacloprid. Compared to their non-azido parents, both probes are of decreased potencies at Drosophila (fruit fly) and Musca (housefly) receptors but AzNN retains full potency at the Myzus (aphid) receptor. [(3)H]AzNN was therefore radiosynthesized at high specific activity (84 Ci/mmol) as a novel photoaffinity probe. [(3)H]AzNN binds to a single high-affinity site in Myzus that is competitively inhibited by imidacloprid and nicotine and further characterized as to its pharmacological profile with various nicotinic ligands. [(3)H]AzNN photoaffinity labeling of Myzus and Homalodisca (leafhopper) detects a single radiolabeled peak in each case displaceable with imidacloprid and nicotine and with molecular masses corresponding to approximately 45 and approximately 56 kDa, respectively. The photoaffinity-labeled receptor in both Drosophila and Musca has imidacloprid- and nicotine-sensitive profiles and migrates at approximately 66 kDa. These photoaffinity-labeled polypeptides are considered to be the insecticide-binding subunits of native insect nAChRs.     

Assembly and trafficking of nicotinic acetylcholine receptors (Review)

Nicotinic acetylcholine receptors (nAChRs) are members of an extensive super-family of neurotransmitter-gated ion channels. In humans, nAChRs are expressed within the nervous system and at the neuromuscular junction and are important targets for pharmaceutical drug discovery. They are also the site of action for neuroactive pesticides in insects and other invertebrates. Nicotinic receptors are complex pentameric transmembrane proteins which are assembled from a large family of subunits; seventeen nAChR subunits (α1-α10, β1-β4, γ, δ and ε) have been identified in vertebrate species. This review will discuss nAChR subunit diversity and factors influencing receptor assembly and trafficking.     

神经元烟碱受体在全身麻醉机制中的作用

作为配体－门控离子通道超家族成员的神经元烟碱受体分布于中枢和外周神经系统,包括多种亚型,具有广泛的生理作用,可以成为多种疾病的药物治疗靶点。它在全身麻醉原理中的作用也被日益重视。部分全身麻醉药物（挥发性吸入、气体吸入麻醉药、硫喷妥钠、氯胺酮等）在低于临床麻醉剂量时能够明显抑制该受体功能,神经元烟碱受体可能参与了这些药物的临床作用机制。     

Functional divergence analysis of vertebrate neuronal nicotinic acetylcholine receptor subunits

Nicotinic acetylcholine receptors (nAChRs) are pentamers formed by subunits from a large multigene family and are highly variable in kinetic, electrophysiological and pharmacological properties. Due to the essential roles of nAChRs in many physiological procedures and diversity in function, identifying the function-related sites specific to each subunit is not only necessary to understand the properties of the receptors but also useful to design potential therapeutic compounds that target these macromolecules for treating a series of central neuronal disorders. By conducting a detailed function divergence analysis on nine neuronal nAChR subunits from representative vertebrate species, we revealed the existence of significant functional variation between most subunit pairs. Specifically, 44 unique residues were identified for the α7 subunit, while another 22 residues that were likely responsible for the specific features of other subunits were detected. By mapping these sites onto the 3?D structure of the human α7 subunit, a structure-function relationship profile was revealed. Our results suggested that the functional divergence related sites clustered in the ligand binding domain, the β2–β3 linker close to the N-terminal α-helix, the intracellular linkers between transmembrane domains, and the “transition zone” may have experienced altered evolutionary rates. The former two regions may be potential binding sites for the α7* subtype-specific allosteric modulators, while the latter region is likely to be subtype-specific allosteric modulations of the heteropentameric descendants such as the α4β2* nAChRs.

Communicated by Ramaswamy H. Sarma     

Structural determinants for interaction of partial agonists with acetylcholine binding protein and neuronal α7 nicotinic acetylcholine receptor

The pentameric acetylcholine‐binding protein (AChBP) is a soluble surrogate of the ligand binding domain of nicotinic acetylcholine receptors. Agonists bind within a nest of aromatic side chains contributed by loops C and F on opposing faces of each subunit interface. Crystal structures of Aplysia AChBP bound with the agonist anabaseine, two partial agonists selectively activating the α7 receptor, 3‐(2,4‐dimethoxybenzylidene)‐anabaseine and its 4‐hydroxy metabolite, and an indole‐containing partial agonist, tropisetron, were solved at 2.7–1.75 Å resolution. All structures identify the Trp 147 carbonyl oxygen as the hydrogen bond acceptor for the agonist‐protonated nitrogen. In the partial agonist complexes, the benzylidene and indole substituent positions, dictated by tight interactions with loop F, preclude loop C from adopting the closed conformation seen for full agonists. Fluctuation in loop C position and duality in ligand binding orientations suggest molecular bases for partial agonism at full‐length receptors. This study, while pointing to loop F as a major determinant of receptor subtype selectivity, also identifies a new template region for designing α7‐selective partial agonists to treat cognitive deficits in mental and neurodegenerative disorders.     

Evaluation of 5-(2-(4-pyridinyl)vinyl)-6-chloro-3-(1-methyl-2-(S)-pyrrolidinylmethoxy)pyridine and its analogues as PET radioligands for imaging nicotinic acetylcholine receptors

A novel series of compounds derived from the high-affinity nicotinic acetylcholine receptor (nAChR) ligand, 5-(2-(4-pyridinyl)vinyl)-6-chloro-3-((1-methyl-2-(S)-pyrrolidinyl)methoxy)pyridine (Me-p-PVC), originally developed by Abbott Laboratories, was characterized in vitro in nAChR binding assays at 37 degrees C to show K(i) values in the range of 9-611 pm. Several compounds of this series were radiolabeled with (11)C and evaluated in vivo in mice and monkeys as potential candidates for PET imaging of nAChRs. [(11)C]Me-p-PVC (K(i) =56 pm at 37 degrees C; logD = 1.6) was identified as a radioligand suitable for the in vivo imaging of the alpha 4 beta 2* nAChR subtype. Compared with 2-[(18)F]FA, a PET radioligand that has been successfully used in humans and is characterized by a slow kinetic of brain distribution, [(11)C]Me-p-PVC is more lipophilic. As a result, [(11)C]Me-p-PVC accumulated in the brain more rapidly than 2-[(18)F]FA. Pharmacological evaluation of Me-p-PVC in mice demonstrated that the toxicity of this compound was comparable with or lower than that of 2-FA. Taken together, these results suggest that [(11)C]Me-p-PVC is a promising PET radioligand for studying nAChR occupancy by endogenous and exogenous ligands in the brain in vivo.