Molecular determinants of subtype-selective efficacies of cytisine and the novel compound NS3861 at heteromeric nicotinic acetylcholine receptors

Cytisine: State of the art in pharmacological activities and pharmacokinetics

Cytisine is a naturally occurring bioactive compound, an alkaloid mainly isolated from legume plants. In recent years, various biological activities of cytisine have been explored, showing certain effects in smoking cessation, reducing drinking behavior, anti-tumor, cardiovascular protection, blood sugar regulation, neuroprotection, osteoporosis prevention and treatment, etc. At the same time, cytisine has the advantages of high efficiency, safety, and low cost, has broad development prospects, and is a drug of great application value. However, a summary of cytisine's biological activities is currently lacking. Therefore, this paper summarizes the pharmacological action, mechanism, and pharmacokinetics of cytisine by referring to numerous databases, and analyzes the new and core targets of cytisine with the help of computer simulation technology, to provide reference for doctors.

Acetylcholine waves and dopamine release in the striatum

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

Pyrrolidinyl benzofurans and benzodioxanes: Selective α4β2 nicotinic acetylcholine receptor ligands with different activity profiles at the two receptor stoichiometries

A series of racemic benzofurans bearing N-methyl-2-pyrrolidinyl residue at C(2) or C(3) has been synthesized and tested for affinity at the α4β2 and α3β4 nicotine acetylcholine receptors (nAChRs). As previously reported for the benzodioxane based analogues, hydroxylation at proper position of benzene ring results in high α4β2 nAChR affinity and α4β2 vs. α3β4 nAChR selectivity. 7-Hydroxy-N-methyl-2-pyrrolidinyl-1,4-benzodioxane (2) and its 7- and 5-amino benzodioxane analogues 3 and 4, which are all α4β2 nAChR partial agonists, and 2-(N-methyl-2-pyrrolidinyl)-6-hydroxybenzofuran (12) were selected for functional characterization at the two α4β2 stoichiometries, the high sensitivity (α4)₂(β2)₃ and the low sensitivity (α4)₃(β2)₂. The benzene pattern substitution, which had previously been found to control α4β2 partial agonist activity and α4β2 vs. α3β4 selectivity, proved to be also involved in stoichiometry-selectivity. The 7-hydroxybenzodioxane derivative 2 selectively activates (α4)₂(β2)₃ nAChR, which cannot be activated by its 5-amino analogue 4. A marginal structural modification, not altering the base pyrrolidinyl benzodioxane scaffold, resulted in opposite activity profiles at the two α4β2 nAChR isoforms providing an interesting novel case study.

Striatal Cholinergic Signaling in Time and Space

The cholinergic interneurons of the striatum account for a small fraction of all striatal cell types but due to their extensive axonal arborization give the striatum the highest content of acetylcholine of almost any nucleus in the brain. The prevailing theory of striatal cholinergic interneuron signaling is that the numerous varicosities on the axon produce an extrasynaptic, volume-transmitted signal rather than mediating rapid point-to-point synaptic transmission. We review the evidence for this theory and use a mathematical model to integrate the measurements reported in the literature, from which we estimate the temporospatial distribution of acetylcholine after release from a synaptic vesicle and from multiple vesicles during tonic firing and pauses. Our calculations, together with recent data from genetically encoded sensors, indicate that the temporospatial distribution of acetylcholine is both short-range and short-lived, and dominated by diffusion. These considerations suggest that acetylcholine signaling by cholinergic interneurons is consistent with point-to-point transmission within a steep concentration gradient, marked by transient peaks of acetylcholine concentration adjacent to release sites, with potential for faithful transmission of spike timing, both bursts and pauses, to the postsynaptic cell. Release from multiple sites at greater distance contributes to the ambient concentration without interference with the short-range signaling. We indicate several missing pieces of evidence that are needed for a better understanding of the nature of synaptic transmission by the cholinergic interneurons of the striatum.

Determinants for α4β2 vs. α3β4 Subtype Selectivity of Pyrrolidine-Based nAChRs Ligands: A Computational Perspective with Focus on Recent cryo-EM Receptor Structures

The selectivity of α4β2 nAChR agonists over the α3β4 nicotinic receptor subtype, predominant in ganglia, primarily conditions their therapeutic range and it is still a complex and challenging issue for medicinal chemists and pharmacologists. Here, we investigate the determinants for such subtype selectivity in a series of more than forty α4β2 ligands we have previously reported, docking them into the structures of the two human subtypes, recently determined by cryo-electron microscopy. They are all pyrrolidine based analogues of the well-known α4β2 agonist N-methylprolinol pyridyl ether A-84543 and differ in the flexibility and pattern substitution of their aromatic portion. Indeed, the direct or water mediated interaction with hydrophilic residues of the relatively narrower β2 minus side through the elements decorating the aromatic ring and the stabilization of the latter by facing to the not conserved β2-Phe119 result as key distinctive features for the α4β2 affinity. Consistently, these compounds show, despite the structural similarity, very different α4β2 vs. α3β4 selectivities, from modest to very high, which relate to rigidity/extensibility degree of the portion containing the aromatic ring and to substitutions at the latter. Furthermore, the structural rationalization of the rat vs. human differences of α4β2 vs. α3β4 selectivity ratios is here proposed.

Cytisine and cytisine derivatives. More than smoking cessation aids

Cytisine, a natural bioactive compound that is mainly isolated from plants of the Leguminosae family (especially the seeds of Laburnum anagyroides), has been marketed in central and eastern Europe as an aid in the clinical management of smoking cessation for more than 50 years. Its main targets are neuronal nicotinic acetylcholine receptors (nAChRs), and pre-clinical studies have shown that its interactions with various nAChR subtypes located in different areas of the central and peripheral nervous systems are neuroprotective, have a wide range of biological effects on nicotine and alcohol addiction, regulate mood, food intake and motor activity, and influence the autonomic and cardiovascular systems. Its relatively rigid conformation makes it an attractive template for research of new derivatives. Recent studies of structurally modified cytisine have led to the development of new compounds and for some of them the biological activities are mediated by still unidentified targets other than nAChRs, whose mechanisms of action are still being investigated. The aim of this review is to describe and discuss: 1) the most recent pre-clinical results obtained with cytisine in the fields of neurological and non-neurological diseases; 2) the effects and possible mechanisms of action of the most recent cytisine derivatives; and 3) the main areas warranting further research.

In Silico Finding of Key Interaction Mediated α3β4 and α7 Nicotinic Acetylcholine Receptor Ligand Selectivity of Quinuclidine-Triazole Chemotype

The selective binding of six (S)-quinuclidine-triazoles and their (R)-enantiomers to nicotinic acetylcholine receptor (nAChR) subtypes α3β4 and α7, respectively, were analyzed by in silico docking to provide the insight into the molecular basis for the observed stereospecific subtype discrimination. Homology modeling followed by molecular docking and molecular dynamics (MD) simulations revealed that unique amino acid residues in the complementary subunits of the nAChR subtypes are involved in subtype-specific selectivity profiles. In the complementary β4-subunit of the α3β4 nAChR binding pocket, non-conserved AspB173 through a salt bridge was found to be the key determinant for the α3β4 selectivity of the quinuclidine-triazole chemotype, explaining the 47-327-fold affinity of the (S)-enantiomers as compared to their (R)-enantiomer counterparts. Regarding the α7 nAChR subtype, the amino acids promoting a however significantly lower preference for the (R)-enantiomers were the conserved TyrA93, TrpA149 and TrpB55 residues. The non-conserved amino acid residue in the complementary subunit of nAChR subtypes appeared to play a significant role for the nAChR subtype-selective binding, particularly at the heteropentameric subtype, whereas the conserved amino acid residues in both principal and complementary subunits are essential for ligand potency and efficacy.

Stoichiometry of the Heteromeric Nicotinic Receptors of the Renshaw Cell

Neuronal nicotinic acetylcholine receptors (nAChRs) are pentamers built from a variety of subunits. Some are homomeric assemblies of α subunits, others heteromeric assemblies of α and β subunits which can adopt two stoichiometries (2α:3β or 3α:2β). There is evidence for the presence of heteromeric nAChRs with the two stoichiometries in the CNS, but it has not yet been possible to identify them at a given synapse. The 2α:3β receptors are highly sensitive to agonists, whereas the 3α:2β stoichiometric variants, initially described as low sensitivity receptors, are indeed activated by low and high concentrations of ACh. We have taken advantage of the discovery that two compounds (NS9283 and Zn) potentiate selectively the 3α:2β nAChRs to establish (in mice of either sex) the presence of these variants at the motoneuron-Renshaw cell (MN-RC) synapse. NS9283 prolonged the decay of the two-component EPSC mediated by heteromeric nAChRs. NS9283 and Zn also prolonged spontaneous EPSCs involving heteromeric nAChRs, and one could rule out prolongations resulting from AChE inhibition by NS9283. These results establish the presence of 3α:2β nAChRs at the MN-RC synapse. At the functional level, we had previously explained the duality of the EPSC by assuming that high ACh concentrations in the synaptic cleft account for the fast component and that spillover of ACh accounts for the slow component. The dual ACh sensitivity of 3α:2β nAChRs now allows to attribute to these receptors both components of the EPSC.SIGNIFICANCE STATEMENT Heteromeric nicotinic receptors assemble α and β subunits in pentameric structures, which can adopt two stoichiometries: 3α:2β or 2α:3β. Both stoichiometric variants are present in the CNS, but they have never been located and characterized functionally at the level of an identified synapse. Our data indicate that 3α:2β receptors are present at the spinal cord synapses between motoneurons and Renshaw cells, where their dual mode of activation (by high concentrations of ACh for synaptic receptors, by low concentrations of ACh for extrasynaptic receptors) likely accounts for the biphasic character of the synaptic current. More generally, 3α:2β nicotinic receptors appear unique by their capacity to operate both in the cleft of classical synapses and at extrasynaptic locations.

Radiosynthesis of (S)-[18F]T1: The first PET radioligand for molecular imaging of α3β4 nicotinic acetylcholine receptors

Recent pharmacologic data revealed the implication of α3β4 nicotinic acetylcholine receptors (nAChRs) in nicotine and drug addiction. To image α3β4 nAChRs in vivo, we aimed to establish the synthesis of a [¹⁸F]-labelled analog of the highly affine and selective α3β4 ligand (S)-3-(4-(4-fluorophenyl)-1H-1,2,3-triazol-1-yl)quinuclidine ((S)-T1). (S)-[¹⁸F]T1 was synthesized from ethynyl-4-[¹⁸F]fluorobenzene ([¹⁸F]5) and (S)-azidoquinuclidine by click reaction. After a synthesis time of 130min (S)-[¹⁸F]T1 was obtained with a radiochemical yield (non-decay corrected) of 4.3±1.3%, a radiochemical purity of >99% and a molar activity of >158 GBq/μmol. The brain uptake and the brain-to-blood ratio of (S)-[¹⁸F]T1 in mice at 30min post injection were 2.02 (SUV) and 6.1, respectively. According to an ex-vivo analysis, the tracer remained intact (>99%) in brain. Only one major radiometabolite was detected in plasma and urine samples. In-vitro autoradiography on pig brain slices revealed binding of (S)-[¹⁸F]T1 to brain regions associated with the expression of α3β4 nAChRs, which could be reduced by the α3β4 nAChR selective drug AT-1001. These findings make (S)-[¹⁸F]T1 a potential tool for the non-invasive imaging of α3β4 nAChRs in the brain by PET.

Enhanced Sensitivity of α3β4 Nicotinic Receptors in Enteric Neurons after Long-Term Morphine: Implication for Opioid-Induced Constipation

Opioid-induced constipation is a major side effect that persists with long-term opioid use. Previous studies demonstrated that nicotine-induced contractions are enhanced after long-term morphine exposure in guinea pig ileum. In the present study, we examined whether the increased sensitivity to nicotine could be observed in single enteric neurons after long-term morphine exposure, determined the subunits in mouse enteric neurons, and examined the effect of nicotine in reversing opioid-induced constipation. Nicotine (0.03-1 mM) dose-dependently induced inward currents from a holding potential of -60 mV in isolated single enteric neurons from the mouse ileum. The amplitude of the currents, but not the potency to nicotine, was significantly increased in neurons receiving long-term (16-24 h) but not short-term (10 min) exposure to morphine. Quantitative mRNA analysis showed that nicotinic acetylcholine receptor (nAChR) subunit expression in the mouse ileum was α3 ≥ β2 > β4 > α5 > α4 > β3 > α6. Nicotine-induced currents were obtained in neurons from α7, β2, α5, and α6 knockout mice. The currents were, however, inhibited by mecamylamine (10 μM) and the α3β4 blocker α-conotoxin AuIB (3 μM), suggesting that nicotine-induced currents were mediated by the α3β4 subtype of nAChRs on enteric neurons. Conversely, NS3861, a partial agonist at α3β4 nAChR, enhanced fecal pellet expulsion in a dose-dependent manner in mice that received long-term, but not short-term, morphine treatment. Overall, our findings suggest that the efficacy of nAChR agonists on enteric neurons is enhanced after long-term morphine exposure, and activation of the α3β4 subtype of nAChR reverses chronic, but not acute, morphine-induced constipation.

From shellfish poisoning to neuroscience

In this issue of Structure, Bourne et al. (2015) report X-ray structures of acetylcholine binding protein with two fast-acting phycotoxins from the pinnatoxin family. The results may pave the way for development of new CNS-penetrant and subtype-selective nAChR antagonists.

Engineered α4β2 nicotinic acetylcholine receptors as models for measuring agonist binding and effect at the orthosteric low-affinity α4-α4 interface

The nicotinic acetylcholine receptor α4β2 is important for normal mammalian brain function and is known to express in two different stoichiometries, (α4)2(β2)3 and (α4)3(β2)2. While these are similar in many aspects, the (α4)3(β2)2 stoichiometry differs by harboring a third orthosteric acetylcholine binding site located at the α4-α4 interface. Interestingly, the third binding site has, so far, only been documented using electrophysiological assays, actual binding affinities of nicotinic receptor ligands to this site are not known. The present study was therefore aimed at determining binding affinities of nicotinic ligands to the α4-α4 interface. Given that epibatidine shows large functional potency differences at α4-β2 vs. α4-α4 interfaces, biphasic binding properties would be expected at (α4)3(β2)2 receptors. However, standard saturation binding experiments with [(3)H]epibatidine did not reveal biphasic binding under the conditions utilized. Therefore, an engineered β2 construct (β2(HQT)), which converts the β(-) face to resemble that of an α4(-) face, was utilized to create (α4)3(β2(HQT))2 receptors harboring three α4-α4 interfaces. With this receptor, low affinity binding of epibatidine with a Kd of ∼5 nM was observed in sharp contrast to a Kd value of ∼10 pM observed for wild-type receptors. A strong correlation between binding affinities at the (α4)3(β2(HQT))2 receptor and functional potencies at the wild-type receptor of a range of nicotinic ligands highlighted the validity of using the mutational approach. Finally, large differences in activities at α4-β2 vs. α4-α4 interfaces were observed for structurally related agonists underscoring the need for establishing all binding parameters of compounds at α4β2 receptors.

Recent developments in novel antidepressants targeting α4β2-nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) have been investigated for developing drugs that can potentially treat various central nervous system disorders. Considerable evidence supports the hypothesis that modulation of the cholinergic system through activation and/or desensitization/inactivation of nAChR holds promise for the development of new antidepressants. The introductory portion of this Miniperspective discusses the basic pharmacology that underpins the involvement of α4β2-nAChRs in depression, along with the structural features that are essential to ligand recognition by the α4β2-nAChRs. The remainder of this Miniperspective analyzes reported nicotinic ligands in terms of drug design considerations and their potency and selectivity, with a particular focus on compounds exhibiting antidepressant-like effects in preclinical or clinical studies. This Miniperspective aims to provide an in-depth analysis of the potential for using nicotinic ligands in the treatment of depression, which may hold some promise in addressing an unmet clinical need by providing relief from depressive symptoms in refractory patients.

Non-equivalent ligand selectivity of agonist sites in (α4β2)2α4 nicotinic acetylcholine receptors: a key determinant of agonist efficacy

The α4β2 nicotinic acetylcholine receptor (nAChR) is the most abundant nAChR type in the brain, and this receptor type exists in alternate (α4β2)2α4 and (α4β2)2β2 forms, which are activated by agonists with strikingly differing efficacies. Recent breakthroughs have identified an additional operational agonist binding site in the (α4β2)2α4 nAChR that is responsible for the signature sensitivity of this receptor to activation by agonists, yet the structural mechanisms determining agonist efficacy at this receptor type are not yet fully understood. In this study, we characterized the ligand selectivity of the individual agonist sites of the (α4β2)2α4 nAChR to determine whether differences in agonist selectivity influence agonist efficacy. Applying the substituted cysteine accessibility method to individual agonist sites in concatenated (α4β2)2α4 receptors, we determined the agonist selectivity of the agonist sites of the (α4β2)2α4 receptor. We show that (a) accessibility of substituted cysteines to covalent modification by methanesulfonate reagent depends on the agonist site at which the modification occurs and (b) that agonists such as sazetidine-A and TC-2559 are excluded from the site at the α4/α4 interface. Given that additional binding to the agonist site in the α4/α4 interface increases acetylcholine efficacy and that agonists excluded from the agonist site at the α4/α4 interface behave as partial agonists, we conclude that the ability to engage all agonist sites in (α4β2)2α4 nAChRs is a key determinant of agonist efficacy. The findings add another level of complexity to the structural mechanisms that govern agonist efficacy in heteromeric nAChRs and related ligand-gated ion channels.

Selective ligand behaviors provide new insights into agonist activation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are a diverse set of ion channels that are essential to everyday brain function. Contemporary research studies selective activation of individual subtypes of receptors, with the hope of increasing our understanding of behavioral responses and neurodegenerative diseases. Here, we aim to expand current binding models to help explain the specificity seen among three activators of α4β2 receptors: sazetidine-A, cytisine, and NS9283. Through mutational analysis, we can interchange the activation profiles of the stoichiometry-selective compounds sazetidine-A and cytisine. In addition, mutations render NS9283--currently identified as a positive allosteric modulator--into an agonist. These results lead to two conclusions: (1) occupation at each primary face of an α subunit is needed to activate the channel and (2) the complementary face of the adjacent subunit dictates the binding ability of the agonist.

Functional characterization improves associations between rare non-synonymous variants in CHRNB4 and smoking behavior

Smoking is the leading cause of preventable death worldwide. Accordingly, effort has been devoted to determining the genetic variants that contribute to smoking risk. Genome-wide association studies have identified several variants in nicotinic acetylcholine receptor genes that contribute to nicotine dependence risk. We previously undertook pooled sequencing of the coding regions and flanking sequence of the CHRNA5, CHRNA3, CHRNB4, CHRNA6 and CHRNB3 genes and found that rare missense variants at conserved residues in CHRNB4 are associated with reduced risk of nicotine dependence among African Americans. We identified 10 low frequency (<5%) non-synonymous variants in CHRNB4 and investigated functional effects by co-expression with normal α3 or α4 subunits in human embryonic kidney cells. Voltage-clamp was used to obtain acetylcholine and nicotine concentration–response curves and qRT-PCR, western blots and cell-surface ELISAs were performed to assess expression levels. These results were used to functionally weight genetic variants in a gene-based association test. We find that there is a highly significant correlation between carrier status weighted by either acetylcholine EC₅₀ (β = −0.67, r² = 0.017, P = 2×10⁻⁴) or by response to low nicotine (β = −0.29, r² = 0.02, P = 6×10⁻⁵) when variants are expressed with the α3 subunit. In contrast, there is no significant association when carrier status is unweighted (β = −0.04, r² = 0.0009, P = 0.54). These results highlight the value of functional analysis of variants and the advantages to integrating such data into genetic studies. They also suggest that an increased sensitivity to low concentrations of nicotine is protective from the risk of developing nicotine dependence.

Molecular recognition of the neurotransmitter acetylcholine by an acetylcholine binding protein reveals determinants of binding to nicotinic acetylcholine receptors

Despite extensive studies on nicotinic acetylcholine receptors (nAChRs) and homologues, details of acetylcholine binding are not completely resolved. Here, we report the crystal structure of acetylcholine bound to the receptor homologue acetylcholine binding protein from Lymnaea stagnalis. This is the first structure of acetylcholine in a binding pocket containing all five aromatic residues conserved in all mammalian nAChRs. The ligand-protein interactions are characterized by contacts to the aromatic box formed primarily by residues on the principal side of the intersubunit binding interface (residues Tyr89, Trp143 and Tyr185). Besides these interactions on the principal side, we observe a cation-π interaction between acetylcholine and Trp53 on the complementary side and a water-mediated hydrogen bond from acetylcholine to backbone atoms of Leu102 and Met114, both of importance for anchoring acetylcholine to the complementary side. To further study the role of Trp53, we mutated the corresponding tryptophan in the two different acetylcholine-binding interfaces of the widespread α4β2 nAChR, i.e. the interfaces α4(+)β2(−) and α4(+)α4(−). Mutation to alanine (W82A on the β2 subunit or W88A on the α4 subunit) significantly altered the response to acetylcholine measured by oocyte voltage-clamp electrophysiology in both interfaces. This shows that the conserved tryptophan residue is important for the effects of ACh at α4β2 nAChRs, as also indicated by the crystal structure. The results add important details to the understanding of how this neurotransmitter exerts its action and improves the foundation for rational drug design targeting these receptors.

Two distinct allosteric binding sites at α4β2 nicotinic acetylcholine receptors revealed by NS206 and NS9283 give unique insights to binding activity-associated linkage at Cys-loop receptors

Positive allosteric modulators (PAMs) of α4β2 nicotinic acetylcholine receptors have the potential to improve cognitive function and alleviate pain. However, only a few selective PAMs of α4β2 receptors have been described limiting both pharmacological understanding and drug-discovery efforts. Here, we describe a novel selective PAM of α4β2 receptors, NS206, and compare with a previously reported PAM, NS9283. Using two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes, NS206 was observed to positively modulate acetylcholine (ACh)-evoked currents at both known α4β2 stoichiometries (2α:3β and 3α:2β). In the presence of NS206, peak current amplitudes surpassed those of maximal efficacious ACh stimulations (Emax(ACh)) with no or limited effects at potencies and current waveforms (as inspected visually). This pharmacological action contrasted with that of NS9283, which only modulated the 3α:2β receptor and acted by left shifting the ACh concentration-response relationship. Interestingly, the two modulators can act simultaneously in an additive manner at 3α:2β receptors, which results in current levels exceeding Emax(ACh) and a left-shifted ACh concentration-response relationship. Through use of chimeric and point-mutated receptors, the binding site of NS206 was linked to the α4-subunit transmembrane domain, whereas binding of NS9283 was shown to be associated with the αα-interface in 3α:2β receptors. Collectively, these data demonstrate the existence of two distinct modulatory sites in α4β2 receptors with unique pharmacological attributes that can act additively. Several allosteric sites have been identified within the family of Cys-loop receptors and with the present data, a detailed picture of allosteric modulatory mechanisms of these important receptors is emerging.

Biophysical and pharmacological characterization of α6-containing nicotinic acetylcholine receptors expressed in HEK293 cells

Nicotinic acetylcholine receptors (nAChR's) containing the α6 subunit (α6) are putative drug targets of relevance to Parkinson's disease and nicotine addiction. However, heterologous expression of α6 receptors has proven challenging which has stifled drug discovery efforts. Here, we investigate potential new avenues for achieving functional α6 receptor expression. Combinations of chimeric and mutated α6, β2 and β3 subunits were co-expressed in the human HEK293 cell line and receptor expression was assessed using Ca(2+)-imaging (FLIPR™) and whole-cell patch-clamp electrophysiology. Transient transfections of a chimeric α6/α3 subunit construct in combination with β2 and β3(V9'S) gave rise to significant acetylcholine-evoked whole-cell currents. Increasing the β3(V9'S):β2:α6/α3 cDNA ratio, resulted in a significantly higher fraction of cells with robust current levels. Using an excess of wild-type β3, significant functional expression of α6/α3β2β3 was also demonstrated. Comparing the acetylcholine concentration-response relationship of α6/α3β2β3(V9'S) to that of α6/α3β2β3 revealed the β3 point mutation to result in decreased current decay rate and increased ACh agonist potency. Ca(2+)-imaging experiments showed preservation of basic α6 receptor pharmacology. Our results establish that α6/α3β2β3(V9'S) replicate several basic features of native α6 receptors but also highlight several caveats associated with using this construct and may therefore provide guidance for future drug hunting efforts.

Elucidation of molecular impediments in the α6 subunit for in vitro expression of functional α6β4* nicotinic acetylcholine receptors

Explorations into the α6-containing nicotinic acetylcholine receptors (α6* nAChRs) as putative drug targets have been severely hampered by the inefficient functional expression of the receptors in heterologous expression systems. In this study, the molecular basis for the problem was investigated through the construction of chimeric α6/α3 and mutant α3 and α6 subunits and functional characterization of these co-expressed with β4 or β4β3 subunits in tsA201 cells in a fluorescence-based assay and in Xenopus oocytes using two-electrode voltage clamp electrophysiology. Substitution of a small C-terminal segment in the second intracellular loop or the Phe(223) residue in transmembrane helix 1 of α6 with the corresponding α3 segment or residue was found to enhance α6β4 functionality in tsA201 cells significantly, in part due to increased cell surface expression of the receptors. The gain-of-function effects of these substitutions appeared to be additive since incorporation of both α3 elements into α6 resulted in assembly of α6β4* receptors exhibiting robust functional responses to acetylcholine. The pharmacological properties exhibited by α6β4β3 receptors comprising one of these novel α6/α3 chimeras in oocytes were found to be in good agreement with those from previous studies of α6* nAChRs formed from other surrogate α6 subunits or concatenated subunits and studies of other heteromeric nAChRs. In contrast, co-expression of this α6/α3 chimera with β2 or β2β3 subunits in oocytes did not result in efficient formation of functional receptors, indicating that the identified molecular elements in α6 could be specific impediments for the expression of functional α6β4* nAChRs.

The effects of noncontingent and self-administered cytisine on body weight and meal patterns in male Sprague-Dawley rats

RATIONALE

Increased appetite and weight gain after cessation are deterrents for quitting smoking. Pharmacotherapies that can reduce this weight gain in ex-smokers would be invaluable, and yet are not well studied in this context.

OBJECTIVE

To examine the effects of extended daily exposure to intravenous cytisine, an alpha4beta2 nAChR partial agonist used for smoking cessation in some European countries, on body weight and patterns of food intake in rats.

METHODS

In the first experiment, programmed infusions of cytisine were administered over 15 h per day. Food intake, meal patterns, and weight change were examined relative to a vehicle-infused group during treatment, and in a post-cytisine phase. The second experiment examined the effects of cytisine on food intake, meal patterns, and weight change when substituted for nicotine in a self-administration protocol. Rats self-administered nicotine and cytisine during alternating four day periods, and changes in body weight, drug infusions, and meal patterns were compared between drugs and during an extinction phase.

RESULTS

In the first experiment, cytisine-treated rats ate less and gained less weight than those that received the vehicle. This occurred primarily by a reduced frequency of meals. In the 12 day post-cytisine phase, animals maintained a lower body weight relative to controls throughout. In the second experiment, total pellet intake increased during cytisine substitution relative to nicotine and animals self-administered cytisine significantly less than nicotine. However, cytisine substitution maintained decreases in food intake and weight gain compared to baseline via decreases in total pellet intake and meal size.

CONCLUSION

Cytisine administration results in decreased weight gain and changes in meal patterns dependent upon mode and pattern of administration and a previous history of nicotine administration.