Diversity of vertebrate nicotinic acetylcholine receptors

Biosynthesis of ionotropic acetylcholine receptors requires the evolutionarily conserved ER membrane complex

The number of nicotinic acetylcholine receptors (AChRs) present in the plasma membrane of muscle and neuronal cells is limited by the assembly of individual subunits into mature pentameric receptors. This process is usually inefficient, and a large number of the synthesized subunits are degraded by endoplasmic reticulum (ER)-associated degradation. To identify cellular factors required for the synthesis of AChRs, we performed a genetic screen in the nematode Caenorhabditis elegans for mutants with decreased sensitivity to the cholinergic agonist levamisole. We isolated a partial loss-of-function allele of ER membrane protein complex-6 (emc-6), a previously uncharacterized gene in C. elegans. emc-6 encodes an evolutionarily conserved 111-aa protein with two predicted transmembrane domains. EMC-6 is ubiquitously expressed and localizes to the ER. Partial inhibition of EMC-6 caused decreased expression of heteromeric levamisole-sensitive AChRs by destabilizing unassembled subunits in the ER. Inhibition of emc-6 also reduced the expression of homomeric nicotine-sensitive AChRs and GABAA receptors in C. elegans muscle cells. emc-6 is orthologous to the yeast and human EMC6 genes that code for a component of the recently identified ER membrane complex (EMC). Our data suggest this complex is required for protein folding and is connected to ER-associated degradation. We demonstrated that inactivation of additional EMC members in C. elegans also impaired AChR synthesis and induced the unfolded protein response. These results suggest that the EMC is a component of the ER folding machinery. AChRs might provide a valuable proxy to decipher the function of the EMC further.

Contrasting properties of α7-selective orthosteric and allosteric agonists examined on native nicotinic acetylcholine receptors

Subtype-selective ligands are important tools for the pharmacological characterisation of neurotransmitter receptors. This is particularly the case for nicotinic acetylcholine receptors (nAChRs), given the heterogeneity of their subunit composition. In addition to agonists and antagonists that interact with the extracellular orthosteric nAChR binding site, a series of nAChR allosteric modulators have been identified that interact with a distinct transmembrane site. Here we report studies conducted with three pharmacologically distinct nicotinic ligands, an orthosteric agonist (compound B), a positive allosteric modulator (TQS) and an allosteric agonist (4BP-TQS). The primary focus of the work described in this study is to examine the suitability of these compounds for the characterisation of native neuronal receptors (both rat and human). However, initial experiments were conducted on recombinant nAChRs demonstrating the selectivity of these three compounds for α7 nAChRs. In patch-clamp recordings on rat primary hippocampal neurons we found that all these compounds displayed pharmacological properties that mimicked closely those observed on recombinant α7 nAChRs. However, it was not possible to detect functional responses with compound B, an orthosteric agonist, using a fluorescent intracellular calcium assay on either rat hippocampal neurons or with human induced pluripotent stem cell-derived neurons (iCell neurons). This is, presumably, due to the rapid desensitisation of α7 nAChR that is induced by orthosteric agonists. In contrast, clear agonist-evoked responses were observed in fluorescence-based assays with the non-desensitising allosteric agonist 4BP-TQS and also when compound B was co-applied with the non-desensitising positive allosteric modulator TQS. In summary, we have demonstrated the suitability of subtype-selective orthosteric and allosteric ligands for the pharmacological identification and characterisation of native nAChRs and the usefulness of ligands that minimise receptor desensitisation for the characterisation of α7 nAChRs in fluorescence-based assays.

Point-to-point ligand-receptor interactions across the subunit interface modulate the induction and stabilization of conformational states of alpha7 nAChR by benzylidene anabaseines

The homomeric α7 nicotinic acetylcholine receptor is a well-studied therapeutic target, though its characteristically rapid desensitization complicates the development of drugs with specific agonist effects. Moreover, some experimental compounds such as GTS-21 (2,4diMeOBA), a derivative of the α7-selective partial agonist benzylidene anabaseine (BA), produce a prolonged residual desensitization (RD) in which the receptor remains non-activatable long after the drug has been removed from extracellular solution. In contrast, the desensitization caused by GTS-21's dihydroxy metabolite (2,4diOHBA) is relatively short-lived. RD is hypothetically due to stable binding of the ligand to the receptor in its desensitized state. We can attribute the reduction in RD to a single BA hydroxyl group on the 4' benzylidene position. Computational prediction derived from homology modeling showed the serine36 (S36) residue of α7 as a reasonable candidate for point-to-point interaction between BA compounds and the receptor. Through evaluating the activity of BA and simple derivatives on wild-type and mutant α7 receptors, it was observed that the drug-receptor pairs which were capable of hydrogen bonding at residue 36 exhibited significantly less stable desensitization. Further experiments involving the type II positive allosteric modulator (PAM) PNU-120596 showed that the various BA compounds' preference to induce either a PAM-sensitive (D(s)) or PAM-insensitive (D(i)) desensitized state is concentration dependent and suggested that both states are destabilized by S36 H-bonding. These results indicate that the fine-tuning of agonists for specific interaction with S36 can facilitate the development of therapeutics with targeted effects on ion channel desensitization properties and conformational state stability.

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 antinociceptive effects of nicotinic receptors α7-positive allosteric modulators in murine acute and tonic pain models

The α7 nicotinic acetylcholine receptor (nAChR) subtype is abundantly expressed in the central nervous system and in the periphery. Recent evidence suggests that α7 nAChR subtypes, which can be activated by an endogenous cholinergic tone, comprising acetylcholine and the α7 nAChR agonist choline, play an important role in subchronic pain and inflammation. This study's objective was to test whether α7 nAChR positive allosteric modulators (PAMs) produce antinociception in in vivo mouse models of acute and persistent pain. Testing type I [N-(5-chloro-2-hydroxyphenyl)-N'-[2-chloro-5-(trifluoromethyl)phenyl] (NS1738)] and type II [1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl) (PNU-120596)] α7 nAChR PAMs in acute and persistent pain, we found that, although neither reduced acute thermal pain, only PNU-120596 dose-dependently attenuated paw-licking behavior in the formalin test. The long-acting effect of PNU-120596 in this test was in discordance with its pharmacokinetic profile in mice, which suggests the involvement of postreceptor signaling mechanisms. Our results with selective mitogen-activated protein kinase kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene monoethanolate (U0126) argues for an important role of extracellular signal-regulated kinase-1/2 pathways activation in PNU-120596's antinociceptive effects. The α7 antagonist MLA, administered intrathecally, reversed PNU-120596's effects, confirming PNU-120596's action, in part, through central α7 nAChRs. Importantly, tolerance to PNU-120596 was not developed after subchronic treatment of the drug. Surprisingly, PNU-120596's antinociceptive effects were blocked by NS1738. Our results indicate that type II α7 nAChR PAM PNU-120596, but not type I α7 nAChR PAM NS1738, shows significant antinociception effects in persistent pain models in mice.

Neuronal nicotinic acetylcholine receptors: common molecular substrates of nicotine and alcohol dependence

Alcohol and nicotine are often co-abused. As many as 80-95% of alcoholics are also smokers, suggesting that ethanol and nicotine, the primary addictive component of tobacco smoke, may functionally interact in the central nervous system and/or share a common mechanism of action. While nicotine initiates dependence by binding to and activating neuronal nicotinic acetylcholine receptors (nAChRs), ligand-gated cation channels normally activated by endogenous acetylcholine (ACh), ethanol is much less specific with the ability to modulate multiple gene products including those encoding voltage-gated ion channels, and excitatory/inhibitory neurotransmitter receptors. However, emerging data indicate that ethanol interacts with nAChRs, both directly and indirectly, in the mesocorticolimbic dopaminergic (DAergic) reward circuitry to affect brain reward systems. Like nicotine, ethanol activates DAergic neurons of the ventral tegmental area (VTA) which project to the nucleus accumbens (NAc). Blockade of VTA nAChRs reduces ethanol-mediated activation of DAergic neurons, NAc DA release, consumption, and operant responding for ethanol in rodents. Thus, ethanol may increase ACh release into the VTA driving activation of DAergic neurons through nAChRs. In addition, ethanol potentiates distinct nAChR subtype responses to ACh and nicotine in vitro and in DAergic neurons. The smoking cessation therapeutic and nAChR partial agonist, varenicline, reduces alcohol consumption in heavy drinking smokers and rodent models of alcohol consumption. Finally, single nucleotide polymorphisms in nAChR subunit genes are associated with alcohol dependence phenotypes and smoking behaviors in human populations. Together, results from pre-clinical, clinical, and genetic studies indicate that nAChRs may have an inherent role in the abusive properties of ethanol, as well as in nicotine and alcohol co-dependence.

Short- and long-term consequences of nicotine exposure during adolescence for prefrontal cortex neuronal network function

More than 70% of adolescents report to have smoked a cigarette at least once. At the adolescent stage the brain has not completed its maturation. The prefrontal cortex (PFC), the brain area responsible for executive functions and attention performance, is one of the last brain areas to mature and is still developing during adolescence. Smoking during adolescence increases the risk of developing psychiatric disorders and cognitive impairment in later life. In addition, adolescent smokers suffer from attention deficits, which aggravate with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and that underlie the lasting effects on cognitive function. Here we provide an overview of these recent findings.

Activation of α7 nicotinic receptors by orthosteric and allosteric agonists: influence on single-channel kinetics and conductance

Nicotinic acetylcholine receptors (nAChRs) are oligomeric transmembrane proteins in which five subunits coassemble to form a central ion channel pore. Conventional agonists, such as acetylcholine (ACh), bind to an orthosteric site, located at subunit interfaces in the extracellular domain. More recently, it has been demonstrated that nAChRs can also be activated by ligands binding to an allosteric transmembrane site. In the case of α7 nAChRs, ACh causes rapid activation and almost complete desensitization. In contrast, allosteric agonists such as 4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c] quin oline-8-sulfonamide (4BP-TQS) activate α7 nAChRs more slowly and cause only low levels of apparent desensitization. In the present study, single-channel patch-clamp recording has been used to investigate differences in the mechanism of activation of α7 nAChRs by ACh and 4BP-TQS. The most striking difference between activation by ACh and 4BP-TQS is in single-channel kinetics. In comparison with activation by ACh, single-channel open times and burst lengths are substantially longer (~160-800-fold, respectively), and shut times are shorter (~8-fold) when activated by 4BP-TQS. In addition, coapplication of ACh and 4BP-TQS results in a further increase in single-channel burst lengths. Mean burst lengths seen when the two agonists are coapplied (3099 ± 754 ms) are ~2.5-fold longer than with 4BP-TQS alone and ∼370-fold longer than with ACh alone. Intriguingly, the main single-channel conductance of α7 nAChRs, was significantly larger when activated by 4BP-TQS (100.3 ± 2.4 pS) than when activated by ACh (90.0 ± 2.7 pS), providing evidence that activation by allosteric and orthosteric agonists results in different α7 nAChRs open-channel conformations.

The ly-6 protein, lynx1, is an endogenous inhibitor of nicotinic signaling in airway epithelium

Our laboratory has previously reported that bronchial epithelial cells (BEC) express a regulatory cascade of classic neurotransmitters and receptors that communicate in an almost neuronal-like manner to achieve physiological regulation. In this paper we show that the similarity between neurotransmitter signaling in neurons and BEC extends to the level of transmitter receptor allosteric modulators. Lynx1 is a member of the ly-6/three-finger superfamily of proteins, many of which modulate receptor signaling activity. Lynx1 specifically has been shown to modulate nicotinic acetylcholine receptor (nAChR) function in neurons by altering receptor sensitivity and desensitization. We now report that lynx1 forms a complex with α7 nAChR in BEC and serves to negatively regulate α7 downstream signaling events. Treatment of primary cultures of BEC with nicotine increased levels of nAChR subunits and that increase was potentiated by lynx1 knockdown. Lynx1 knockdown also potentiated the nicotine-induced increase in GABA(A) receptors (GABA(A)R) and MUC5AC mRNA expression, and that effect was blocked by α7 antagonists and α7 knockdown. In parallel with the increases in nAChR, GABA(A)R, and mucin mRNA levels, lynx1 knockdown also increased levels of p-Src. Consistent with this, inhibition of Src signaling blocked the ability of the lynx1 knockdown to increase basal and nicotine-stimulated GABA(A)R and mucin mRNA expression. Thus lynx1 appears to act as a negative modulator of α7 nAChR-induced events by inhibiting Src activation. This suggests that lynx1 agonists or mimetics are a potentially important therapeutic target to develop new therapies for smoking-related diseases characterized by increased mucin expression.

Synthesis of selective agonists for the α7 nicotinic acetylcholine receptor with in situ click-chemistry on acetylcholine-binding protein templates

The acetylcholine-binding proteins (AChBPs), which serve as structural surrogates for the extracellular domain of nicotinic acetylcholine receptors (nAChRs), were used as reaction templates for in situ click-chemistry reactions to generate a congeneric series of triazoles from azide and alkyne building blocks. The catalysis of in situ azide-alkyne cycloaddition reactions at a dynamic subunit interface facilitated the synthesis of potentially selective compounds for nAChRs. We investigated compound sets generated in situ with soluble AChBP templates through pharmacological characterization with α7 and α4β2 nAChRs and 5-hydroxytryptamine type 3A receptors. Analysis of activity differences between the triazole 1,5-syn- and 1,4-anti-isomers showed a preference for the 1,4-anti-triazole regioisomers among nAChRs. To improve nAChR subtype selectivity, the highest-potency building block for α7 nAChRs, i.e., 3α-azido-N-methylammonium tropane, was used for additional in situ reactions with a mutated Aplysia californica AChBP that was made to resemble the ligand-binding domain of the α7 nAChR. Fourteen of 50 possible triazole products were identified, and their corresponding tertiary analogs were synthesized. Pharmacological assays revealed that the mutated binding protein template provided enhanced selectivity of ligands through in situ reactions. Discrete trends in pharmacological profiles were evident, with most compounds emerging as α7 nAChR agonists and α4β2 nAChR antagonists. Triazoles bearing quaternary tropanes and aromatic groups were most potent for α7 nAChRs. Pharmacological characterization of the in situ reaction products established that click-chemistry synthesis with surrogate receptor templates offered novel extensions of fragment-based drug design that were applicable to multisubunit ion channels.

Effects of glaucoma on Chrna6 expression in the retina

PURPOSE

Recent advances in technology now provide tools capable of tracking genome-wide expression changes occurring in progressive pathological processes. The present experiments were carried out to determine if acetylcholine receptor α 6 subunit (Chrna6) is a reliable retinal ganglion cell (RGC) marker in adult mouse eyes and if Chrna6 expression can be used to track progressive loss of RGCs, such as is observed in the DBA/2J glaucoma model.

METHODS

Data sets derived from the BXD strains were used to extract gene expression signatures for RGCs. Pooled retinas from DBA/2J or C57BL/6J cases at 1-3 months, 12 months, and 16-17 months were prepared for gene-array and RT-PCR analysis. Globes were fixed in paraformaldehyde and sectioned for immunofluorescence with antibodies against Chrna6.

RESULTS

Chrna6 has a cellular expression signature for RGCs with high correlation to Thy1 (r = 0.65), a recognized RGC marker. Immunofluorescence experiments confirm that in the young and adult mouse retina, Chrna6 is preferentially expressed by RGCs. We further show that C3H/HeJ retinas, which lack photoreceptors, also express Chrna6 in the RGC layer. Gene expression array analyses, confirmed by RT-PCR, show progressive loss of Chrna6 expression in retinas of the DBA/2J glaucomatous mouse retinas.

CONCLUSIONS

Quantitative trait locus analysis provides support for Chrna6 as a RGC marker. Chrna6 expression decreases with death of RGCs in glaucomatous DBA/2J mice and after optic nerve crush injury, further supporting Chrna6 as a reliable RGC marker. High expression of RGC Chrna6 in the absence of photoreceptors is suggestive that Chrna6 expression by RGCs is independent of photoreceptor-derived stimuli.

Nicotine-mediated improvement in L-dopa-induced dyskinesias in MPTP-lesioned monkeys is dependent on dopamine nerve terminal function

L-dopa-induced dyskinesias (LIDs) are abnormal involuntary movements that develop with long term L-dopa therapy for Parkinson's disease. Studies show that nicotine administration reduced LIDs in several parkinsonian animal models. The present work was done to understand the factors that regulate the nicotine-mediated reduction in LIDs in MPTP-lesioned nonhuman primates. To approach this, we used two groups of monkeys, one with mild-moderate and the other with more severe parkinsonism rendered dyskinetic using L-dopa. In mild-moderately parkinsonian monkeys, nicotine pretreatment (300 μg/ml via drinking water) prevented the development of LIDs by ~75%. This improvement was maintained when the nicotine dose was lowered to 50 μg/ml but was lost with nicotine removal. Nicotine re-exposure again decreased LIDs. By contrast, nicotine treatment did not reduce LIDs in monkeys with more severe parkinsonism. We next determined how nicotine's ability to reduce LIDs correlated with lesion-induced changes in the striatal dopamine transporter and (3)H-dopamine release in these two groups of monkeys. The striatal dopamine transporter was reduced to 54% and 28% of control in mild-moderately and more severely parkinsonian monkeys, respectively. However, basal, K(+), α4β2* and α6β2* nAChR-evoked (3)H-dopamine release were near control levels in striatum of mild-moderately parkinsonian monkeys. By contrast, these same release measures were reduced to a significantly greater extent in striatum of more severely parkinsonian monkeys. Thus, nicotine best improves LIDs in lesioned monkeys in which striatal dopamine transmission is still relatively intact. These data suggest that nicotine treatment would most effectively reduce LIDs in patients with mild to moderate Parkinson's disease.

Molecular basis for the differential sensitivity of rat and human α9α10 nAChRs to α-conotoxin RgIA

The α9α10 nicotinic acetylcholine receptor (nAChR) may be a potential target in pathophysiology of the auditory system, chronic pain, and breast and lung cancers. Alpha-conotoxins, from the predatory marine snail Conus, are potent nicotinic antagonists, some of which are selective for the α9α10 nAChR. Here, we report a two order of magnitude species difference in the potency of α-conotoxin RgIA for the rat versus human α9α10 nAChR. We investigated the molecular mechanism of this difference. Heterologous expression of the rat α9 with the human α10 subunit in Xenopus oocytes resulted in a receptor that was blocked by RgIA with potency similar to that of the rat α9α10 nAChR. Conversely, expression of the human α9 with that of the rat α10 subunit resulted in a receptor that was blocked by RgIA with potency approaching that of the human α9α10 receptor. Systematic substitution of residues found in the human α9 subunit into the homologous position in the rat α9 subunit revealed that a single point mutation, Thr56 to Ile56, primarily accounts for this species difference. Remarkably, although the α9 nAChR subunit has previously been reported to provide the principal (+) binding face for binding of RgIA, Thr56 is located in the (-) complementary binding face.

Nicotine exposure during adolescence alters the rules for prefrontal cortical synaptic plasticity during adulthood

The majority of adolescents report to have smoked a cigarette at least once. Adolescence is a critical period of brain development during which maturation of areas involved in cognitive functioning, such as the medial prefrontal cortex (mPFC), is still ongoing. Tobacco smoking during this age may compromise the normal course of prefrontal development and lead to cognitive impairments in later life. In addition, adolescent smokers suffer from attention deficits, which progress with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and underlie the lasting effects on cognitive function. In particular, the expression and function of metabotropic glutamate receptors (mGluRs) are changed and this has an impact on short- and long-term plasticity of glutamatergic synapses in the PFC and ultimately on the attention performance. Here, we review and discuss these recent findings.

Variations in binding among several agonists at two stoichiometries of the neuronal, α4β2 nicotinic receptor

Drug-receptor binding interactions of four agonists, ACh, nicotine, and the smoking cessation compounds varenicline (Chantix) and cytisine (Tabex), have been evaluated at both the 2:3 and 3:2 stoichiometries of the α4β2 nicotinic acetylcholine receptor (nAChR). Previous studies have established that unnatural amino acid mutagenesis can probe three key binding interactions at the nAChR: a cation-π interaction, and two hydrogen-bonding interactions to the protein backbone of the receptor. We find that all drugs make a cation-π interaction to TrpB of the receptor. All drugs except ACh, which lacks an N(+)H group, make a hydrogen bond to a backbone carbonyl, and ACh and nicotine behave similarly in acting as a hydrogen-bond acceptor. However, varenicline is not a hydrogen-bond acceptor to the backbone NH that interacts strongly with the other three compounds considered. In addition, we see interesting variations in hydrogen bonding interactions with cytisine that provide a rationalization for the stoichiometry selectivity seen with this compound.

Nicotine as a potential neuroprotective agent for Parkinson's disease

Converging research efforts suggest that nicotine and other drugs that act at nicotinic acetylcholine receptors (nAChRs) may be beneficial in the management of Parkinson's disease. This idea initially stemmed from the results of epidemiological studies that demonstrated that smoking is associated with a decreased incidence of Parkinson's disease. The subsequent finding that nicotine administration protected against nigrostriatal damage in parkinsonian animal models led to the idea that nicotine in tobacco products may contribute to this apparent protective action. Nicotine most likely exerts its effects by interacting at nAChRs. Accumulating research indicates that multiple subtypes containing nAChRs, including α4β2, α6β2, and/or α7, may be involved. Stimulation of nAChRs initially activates various intracellular transduction pathways primarily via alterations in calcium signaling. Consequent adaptations in immune responsiveness and trophic factors may ultimately mediate nicotine's ability to reduce/halt the neuronal damage that arises in Parkinson's disease. In addition to a potential neuroprotective action, nicotine also has antidepressant properties and improves attention/cognition. Altogether, these findings suggest that nicotine and nAChR drugs represent promising therapeutic agents for the management of Parkinson's disease.

Potential state-selective hydrogen bond formation can modulate activation and desensitization of the α7 nicotinic acetylcholine receptor

A series of arylidene anabaseines were synthesized to probe the functional impact of hydrogen bonding on human α7 nicotinic acetylcholine receptor (nAChR) activation and desensitization. The aryl groups were either hydrogen bond acceptors (furans), donors (pyrroles), or neither (thiophenes). These compounds were tested against a series of point mutants of the ligand-binding domain residue Gln-57, a residue hypothesized to be proximate to the aryl group of the bound agonist and a putative hydrogen bonding partner. Q57K, Q57D, Q57E, and Q57L were chosen to remove the dual hydrogen bonding donor/acceptor ability of Gln-57 and replace it with hydrogen bond donating, hydrogen bond accepting, or nonhydrogen bonding ability. Activation of the receptor was compromised with hydrogen bonding mismatches, for example, pairing a pyrrole with Q57K or Q57L, or a furan anabaseine with Q57D or Q57E. Ligand co-applications with the positive allosteric modulator PNU-120596 produced significantly enhanced currents whose degree of enhancement was greater for 2-furans or -pyrroles than for their 3-substituted isomers, whereas the nonhydrogen bonding thiophenes failed to show this correlation. Interestingly, the PNU-120596 agonist co-application data revealed that for wild-type α7 nAChR, the 3-furan desensitized state was relatively stabilized compared with that of 2-furan, a reversal of the relationship observed with respect to the barrier for entry into the desensitized state. These data highlight the importance of hydrogen bonding on the receptor-ligand state, and suggest that it may be possible to fine-tune features of agonists that mediate state selection in the nAChR.

A series of α7 nicotinic acetylcholine receptor allosteric modulators with close chemical similarity but diverse pharmacological properties

Acetylcholine activates nicotinic acetylcholine receptors (nAChRs) by binding to an extracellular site located at the interface of two adjacent subunits. In contrast, recent studies have provided evidence that positive allosteric modulators (PAMs) such as TQS (4-(naphthalen-2-yl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) and allosteric agonists such as 4BP-TQS (4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) interact at an intrasubunit transmembrane site. Here, we describe the synthesis and pharmacological characterization of a series of chemically related allosteric modulators of the α7 nAChR. Minimal changes in the chemical structure of these compounds have been found to exert profound effects on their pharmacological properties. For example, compounds containing a bromine atom at either the ortho or meta position on the phenyl ring, such as 2BP-TQS (4-(2-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) and 3BP-TQS (4-(3-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide), rather than at the para position (4BP-TQS), display no allosteric agonist activity but retain PAM activity on α7 nAChRs, demonstrating the importance of the location of the halogen atom on pharmacological properties. Replacement of the bromine atom in 4BP-TQS with either a chlorine [4CP-TQS (4-(4-chloroophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] or an iodine atom [4IP-TQS (4-(4-iodoophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] results in compounds that have pharmacological properties characteristic of allosteric agonists but display differences in activation rates, in inactivation rates, and in levels of desensitization. In contrast, replacement of the bromine atom in 4BP-TQS with a fluorine atom [4FP-TQS (4-(4-fluorophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] generated a compound that lacks allosteric agonist activity but acts a potentiator of responses to acetylcholine. In addition, 4FP-TQS was found to act as an antagonist of responses evoked by allosteric agonists such as 4BP-TQS. These findings provide evidence of the pharmacological diversity of compounds interacting with the allosteric transmembrane site on α7 nAChRs.

Prefrontal neuromodulation by nicotinic receptors for cognitive processes

RATIONALE

The prefrontal cortex (PFC) mediates executive functions, a set of control processes that optimize performance on cognitive tasks. It enables appropriate decision-making and mediates adapted behaviors, all processes impaired in psychiatric or degenerative disorders. Key players of normal functioning of the PFC are neurotransmitter (NT) systems arising from subcortical nuclei and targeting PFC subareas and, also, neuronal nicotinic acetylcholine receptors (nAChRs). These ion channels, located on multiple cell compartments in all brain areas, mediate direct cholinergic transmission and modulate the release of NTs that cross onto PFC neurons or interneurons.

OBJECTIVE

We compiled current knowledge concerning the role of nAChRs in NT release, focusing on the PFC. We point out plausible mechanisms of interaction among PFC circuits implicated in executive functions and emphasized the role of β2-containing nAChRs, the high-affinity receptors for acetylcholine (ACh). These receptors are more directly implicated in behavioral flexibility either when located on PFC neurons or in the monoaminergic or cholinergic systems targeting the PFC.

RESULTS

We shed light on potentially crucial roles played by nAChRs in complex interactions between local and afferent NTs. We show how they could act on cognition via PFC networks.

CONCLUSIONS

nAChRs are crucial for decision-making, during integration of emotional and motivational features, both mediated by different NT pathways in the PFC. We review the knowledge recently gained on cognitive functions in mice and our current understanding of PFC NT modulation. The combination of these data is expected to provide new hypotheses concerning the role of AChRs in cognitive processes.

Nicotine reduces antipsychotic-induced orofacial dyskinesia in rats

Antipsychotics are an important class of drugs for the management of schizophrenia and other psychotic disorders. They act by blocking dopamine receptors; however, because these receptors are present throughout the brain, prolonged antipsychotic use also leads to serious side effects. These include tardive dyskinesia, repetitive abnormal involuntary movements of the face and limbs for which there is little treatment. In this study, we investigated whether nicotine administration could reduce tardive dyskinesia because nicotine attenuates other drug-induced abnormal movements. We used a well established model of tardive dyskinesia in which rats injected with the commonly used antipsychotic haloperidol develop vacuous chewing movements (VCMs) that resemble human orofacial dyskinesias. Rats were first administered nicotine (minipump; 2 mg/kg per day). Two weeks later, they were given haloperidol (1 mg/kg s.c.) once daily. Nicotine treatment reduced haloperidol-induced VCMs by ∼20% after 5 weeks, with a significant ∼60% decline after 13 weeks. There was no worsening of haloperidol-induced catalepsy. To understand the molecular basis for this improvement, we measured the striatal dopamine transporter and nicotinic acetylcholine receptors (nAChRs). Both haloperidol and nicotine treatment decreased the transporter and α6β2* nAChRs (the asterisk indicates the possible presence of other nicotinic subunits in the receptor complex) when given alone, with no further decline with combined drug treatment. By contrast, nicotine alone increased, while haloperidol reduced α4β2* nAChRs in both vehicle and haloperidol-treated rats. These data suggest that molecular mechanisms other than those directly linked to the transporter and nAChRs underlie the nicotine-mediated improvement in haloperidol-induced VCMs in rats. The present results are the first to suggest that nicotine may be useful for improving the tardive dyskinesia associated with antipsychotic use.

Changes in temperature have opposing effects on current amplitude in α7 and α4β2 nicotinic acetylcholine receptors

We have examined the effect of temperature on the electrophysiological properties of three neuronal nicotinic acetylcholine receptor (NACHR) subtypes: the rapidly desensitizing homomeric α7 nAChR, the more slowly desensitizing heteromeric α4β2 nAChR and on α7 nAChRs containing a transmembrane mutation (L247T) that results in dramatically reduced desensitization. In all cases, the functional properties of receptors expressed in Xenopus oocytes at room temperature (RT; 21°C) were compared to those recorded at either physiological temperature (37°C) or at lower temperature (4°C). Alterations in temperature had dramatically differing effects on the amplitude of whole-cell responses detected with these three nAChR subtypes. Compared to responses at RT, the amplitude of agonist-evoked responses with α4β2 nAChRs was increased at high temperature (125±9%, n = 6, P<0.01) and reduced at low temperature (47±5%, n = 6, P<0.01), whereas the amplitude of α7 responses was reduced at high temperature (27±7%, n = 11, P<0.001) and increased at low temperatures (224±16%, n = 10, P<0.001). In contrast to the effects of temperature on α4β2 and wild type α7 nAChRs, the amplitude of α7 nAChRs containing the L247T mutation was unaffected by changes in temperature. In addition, changes in temperature had little or no effect on current amplitude when α7 nAChRs were activated by the largely non-desensitizing allosteric agonist 4BP-TQS. Despite these differing effects of temperature on the amplitude of agonist-evoked responses in different nAChRs, changes in temperature had a consistent effect on the rate of receptor desensitization on all subtypes examined. In all cases, higher temperature resulted in increased rates of desensitization. Thus, it appears that the differing effects of temperature on the amplitudes of whole-cell responses cannot be explained by temperature-induced changes in receptor desensitization rates.

Nicotinic receptors in the dorsal and ventral hippocampus differentially modulate contextual fear conditioning

Nicotine administration alters various forms of hippocampus-dependent learning and memory. Increasing work has found that the dorsal and ventral hippocampus differentially contribute to multiple behaviors. Thus, the present study examined whether the effects of nicotine in the dorsal and ventral hippocampus have distinct influences on contextual fear learning in male C57BL/6J mice. Direct infusion of nicotine into the dorsal hippocampus resulted in an enhancement of contextual fear learning, whereas nicotine infused into the ventral hippocampus resulted in deficits. Nicotine infusions into the ventral hippocampus did not alter hippocampus-independent cued fear conditioning or time spent in the open arm of the elevated plus maze, a measure of anxiety, suggesting that the effects are due to alterations in contextual learning and not other general processes. Finally, results from using direct infusions of MLA, a low-affinity α7 nicotinic acetylcholine receptor (nAChR) antagonist, in conjunction with systemic nicotine, provide evidence that α7-nAChRs in the ventral hippocampus mediate the detrimental effect of ventral hippocampal nicotine on contextual fear learning. These results suggest that with systemic nicotine administration, competition exists between the dorsal and ventral hippocampus for behavioral control over contextual learning.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Investigation of the molecular mechanism of the α7 nicotinic acetylcholine receptor positive allosteric modulator PNU-120596 provides evidence for two distinct desensitized states

Although α7 nicotinic acetylcholine receptors are considered potentially important therapeutic targets, the development of selective agonists has been stymied by the α7 receptor's intrinsically low probability of opening (P(open)) and the concern that an agonist-based therapeutic approach would disrupt endogenous cholinergic function. Development of α7 positive allosteric modulators (PAMs) holds promise of avoiding both issues. N-(5-Chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea (PNU-120596) is one of the most effective α7 PAMs, with a mechanism associated, at least in part, with the destabilization of desensitized states. We studied the mechanism of PNU-120596 potentiation of α7 receptors expressed in Xenopus laevis oocytes and outside-out patches from BOSC 23 cells. We identify two forms of α7 desensitization: one is destabilized by PNU-120596 (D(s)), and the other is induced by strong episodes of activation and is stable in the presence of the PAM (D(i)). Our characterization of prolonged bursts of single-channel currents that occur with PNU-120596 provide a remarkable contrast to the behavior of the channels in the absence of the PAM. Individual channels that avoid the D(i) state show a 100,000-fold increase in P(open) compared with receptors in the nonpotentiated state. In the presence of PNU-120596, balance between D(s) and D(i) is dynamically regulated by both agonist and PAM binding, with maximal ion channel activity at intermediate levels of binding to both classes of sites. In the presence of high agonist concentrations, competitive antagonists may have the effect of shifting the balance in favor of D(s) and increasing ion channel currents.

Desensitization of nicotinic acetylcholine receptors in central nervous system neurons of the stick insect (Carausius morosus) by imidacloprid and sulfoximine insecticides

Imidacloprid, sulfoxaflor and two experimental sulfoximine insecticides caused generally depressive symptoms in stick insects, characterized by stillness and weakness, while also variably inducing postural changes such as persistent ovipositor opening, leg flexion or extension and abdomen bending that could indicate excitation of certain neural circuits. We examined the same compounds on nicotinic acetylcholine receptors in stick insect neurons, which have previously been shown to desensitize in the presence of ACh. Brief U-tube application of 10(-4) M solutions of insecticides for 1 s evoked currents that were much smaller than ACh-evoked currents, and depressed subsequent ACh-evoked currents for several minutes, indicating that the compounds are low-efficacy partial agonists that potently desensitize the receptors. Much lower concentrations of insecticides applied in the bath for longer periods did not activate currents, but inhibited ACh-evoked currents via desensitization of the receptors. Previously described fast- and slowly-desensitizing nACh currents, I(ACh1) and I(ACh2) respectively, were each found to consist of two components with differing sensitivities to the insecticides. Imidacloprid applied in the bath desensitized high-sensitivity components, I(ACh1H) and I(ACh2H) with IC(50)s of 0.18 and 0.13 pM, respectively. It desensitized the low-sensitivity slowly desensitizing component, I(ACh2L), with an IC(50) of 2.6 nM, while a component of the fast-desensitizing current, I(ACh1L), was least sensitive, with an IC(50) of 81 nM I(ACh1L) appeared to be insensitive to the three sulfoximines tested, whereas all three sulfoximines potently desensitized I(ACh1H) and both slowly desensitizing components, with IC(50)s between 2 and 7 nM. We conclude that selective desensitization of certain nAChR subtypes can account for the insecticidal actions of imidacloprid and sulfoximines in stick insects.

Allosteric modulators of the α4β2 subtype of neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are ligand-gated ion conducting transmembrane channels from the Cys-loop receptor super-family. The α4β2 subtype is the predominant heteromeric subtype of nicotinic receptors found in the brain. Allosteric modulators for α4β2 receptors interact at a site other than the orthosteric site where acetylcholine binds. Many compounds which act as allosteric modulators of the α4β2 receptors have been identified, with both positive and negative effects. Such allosteric modulators either increase or decrease the response induced by agonist on the α4β2 receptors. Here we discuss the concept of allosterism as it pertains to the α4β2 receptors and summarize the important features of allosteric modulators for this nicotinic receptor subtype.

Positive and negative effects of alcohol and nicotine and their interactions: a mechanistic review

Nicotine and alcohol are two of the most commonly abused legal substances. Heavy use of one drug can often lead to, or is predictive of, heavy use of the other drug in adolescents and adults. Heavy drinking and smoking alone are of significant health hazard. The combination of the two, however, can result in synergistic adverse effects particularly in incidences of various cancers (e.g., esophagus). Although detrimental consequences of smoking are well established, nicotine by itself might possess positive and even therapeutic potential. Similarly, alcohol at low or moderated doses may confer beneficial health effects. These opposing findings have generated considerable interest in how these drugs act. Here we will briefly review the negative impact of drinking-smoking co-morbidity followed by factors that appear to contribute to the high rate of co-use of alcohol and nicotine. Our main focus will be on what research is telling us about the central actions and interactions of these drugs, and what has been elucidated about the mechanisms of their positive and negative effects. We will conclude by making suggestions for future research in this area.

Decreased α4β2 nicotinic receptor number in the absence of mRNA changes suggests post-transcriptional regulation in the spontaneously hypertensive rat model of ADHD

The spontaneously hypertensive rat (SHR) is widely used as a model of attention-deficit/hyperactivity disorder (ADHD). Deficits in central nicotinic receptors (nAChRs) have been previously observed in SHRs, which is interesting since epidemiological studies have identified an association between smoking and ADHD symptoms in humans. Here, we examine whether nAChR deficits in SHRs compared with Wistar Kyoto rat (WKY) controls are nAChR subtype-specific and whether these deficits correlate with changes at the level of mRNA transcription in specific brain regions. Levels of binding sites (B(max) ) and dissociation constants (K(d)) for nAChRs were determined from saturation curves of high-affinity [³H]epibatidine- and [³H] Methyllycaconitine (MLA) binding to membranes from cortex, striatum, hippocampus and cerebellum. In additional brain regions, nAChRs were examined by autoradiography with [¹²⁵I]A-85380 and [¹²⁵I]α-bungarotoxin. Levels of mRNA encoding nAChR subunits were measured using quantitative real-time PCR (qPCR). We showed that the number of α4β2 nAChR binding sites is lower globally in the SHR brain compared with WKY in the absence of significant differences in mRNA levels, with the exception of lower α4 mRNA in cerebellum of SHR compared with WKY. Furthermore, nAChR deficits were subtype- specific because no strain difference was found in α7 nAChR binding or α7 mRNA levels. Our results suggest that the lower α4β2 nAChR number in SHR compared with WKY may be a consequence of dysfunctional post-transcriptional regulation of nAChRs.

Effects of chronic nicotine on heteromeric neuronal nicotinic receptors in rat primary cultured neurons

Nicotine increases the number of neuronal nicotinic acetylcholine receptors (nAChRs) in brain. This study investigated the effects of chronic nicotine treatment on nAChRs expressed in primary cultured neurons. In particular, we studied the chronic effects of nicotine exposure on the total density, surface expression and turnover rate of heteromeric nAChRs. The receptor density was measured by [¹²⁵I]epibatidine ([¹²⁵I]EB) binding. Untreated and nicotine-treated neurons were compared from several regions of embryonic (E19) rat brain. Twelve days of treatment with 10 μM nicotine produced a twofold up-regulation of nAChRs. Biotinylation and whole-cell binding studies indicated that up-regulation resulted from an increase in the number of cell surface receptors as well as intracellular receptors. nAChR subunit composition in cortical and hippocampal neurons was assessed by immunoprecipitation with subunit-selective antibodies. These neurons contain predominantly α4, β2 and α5 subunits, but α2, α3, α6 and β4 subunits were also detected. Chronic nicotine exposure yielded a twofold increase in the β2-containing receptors and a smaller up-regulation in the α4-containing nAChRs. To explore the mechanisms of up-regulation we investigated the effects of nicotine on the receptor turnover rate. We found that the turnover rate of surface receptors was > 2 weeks and chronic nicotine exposure had no effect on this rate.

Nicotine reduces L-DOPA-induced dyskinesias by acting at beta2* nicotinic receptors

L-DOPA-induced dyskinesias or abnormal involuntary movements (AIMs) are a debilitating adverse complication associated with prolonged L-DOPA administration for Parkinson's disease. Few treatments are currently available for dyskinesias. Our recent data showed that nicotine reduced L-DOPA-induced AIMs in parkinsonian animal models. An important question is the nicotinic acetylcholine receptor (nAChR) subtypes through which nicotine exerts this beneficial effect, because such knowledge would allow for the development of drugs that target the relevant receptor population(s). To address this, we used β2 nAChR subunit knockout [β2(-/-)] mice because β2-containing nAChRs are key regulators of nigrostriatal dopaminergic function. All of the mice were lesioned by intracranial injection of 6-hydroxydopamine into the right medial forebrain bundle. Lesioning resulted in a similar degree of nigrostriatal damage and parkinsonism in β2(-/-) and wild-type mice. All of the mice then were injected with L-DOPA (3 mg/kg) plus benserazide (15 mg/kg) once daily for 4 weeks until AIMs were fully developed. L-DOPA-induced AIMs were approximately 40% less in the β2(-/-) mice compared with the wild-type mice. It is interesting to note that nicotine (300 μg/ml in drinking water) reduced L-DOPA-induced AIMs by 40% in wild-type mice but had no effect in β2(-/-) mice with partial nigrostriatal damage. The nicotine-mediated decline in AIMs was much less pronounced in wild-type mice with near-complete degeneration, suggesting that presynaptic nAChRs on dopaminergic terminals have a major influence. These data demonstrate an essential role for β2* nAChRs in the antidyskinetic effect of nicotine and suggest that drugs targeting these subtypes may be useful for the management of L-DOPA-induced dyskinesias in Parkinson's disease.

Glutamate and nicotinic receptor interactions in working memory: importance for the cognitive impairment of schizophrenia

This article reaches across disciplines to correlate results in molecular, cellular, behavioral, and clinical research to develop a more complete picture of how working memory (WM) functions. It identifies a new idea that deserves further investigation. NMDA glutamate receptors (NMDAR) are critical for memory function. NMDAR inhibition effectively reproduces principal manifestations of schizophrenia (SP), such as WM impairment and GABAergic deficit (mainly reduction of glutamic acid decarboxylase 67 (GAD67) and parvalbumin (PV) content). Nicotine and selective α7 nicotinic acetylcholine receptor (nAChR) agonists reduce WM impairments in patients with SP and reverse WM deficits in animals treated with NMDAR antagonists. The mechanism of this effect is unknown. Importantly, WM recovery occurs even before restoration of NMDAR blockade-induced molecular alterations, including reduced GAD67 in interneurons. Our insight into the cognitive-enhancing effect of α7 nAChR agonists, particularly in the animal models of SP, combines reviews of recent findings on glutamate and nicotinic receptor expression in the neuronal circuits involved in WM, the properties of these receptors, their implication in WM regulation, generation of rhythmic neuronal activity, resulting in a proposed hypothesis for further investigations. We suggest that (1) cortical/hippocampal interneurons, particularly PV positive, play a crucial role in WM and that impairment of these cells in SP could be behind the WM deficit; (2) activation of α7 nAChRs could restore calcium signaling and intrinsic properties of these interneurons, and associated with these events, computational capacity, gamma rhythmic activity, and WM would also be restored.

Competitive binding at a nicotinic receptor transmembrane site of two α7-selective positive allosteric modulators with differing effects on agonist-evoked desensitization

Positive allosteric modulators (PAMs) of nicotinic acetylcholine receptors (nAChRs) have attracted considerable interest as a novel area of therapeutic drug discovery. Two types of α7-selective PAMs have been identified (type I and type II). Whilst both potentiate peak agonist-induced responses, they have different effects on the rate of agonist-induced receptor desensitization. Type I PAMs have little or no effect on the rapid rate of desensitization that is characteristic of α7 nAChRs, whereas type II PAMs cause dramatic slowing of receptor desensitization. Previously, we have obtained evidence indicating that PNU-120596, a type II PAM, causes potentiation by interacting with an allosteric transmembrane site. In contrast, other studies have demonstrated the importance of the ‘M2–M3 segment’ in modulating the effects of the type I PAM NS1738 and have led to the proposal that NS1738 may interact with the extracellular N-terminal domain. Here, our aim has been to compare the mechanism of allosteric potentiation of α7 nAChRs by NS1738 and PNU-120596. Functional characterization of a series of mutated α7 nAChRs indicates that mutation of amino acids within a proposed intrasubunit transmembrane cavity have a broadly similar effect on these two PAMs. In addition, we have employed a functional assay designed to examine the ability of ligands to act competitively at either the orthosteric or allosteric binding site of α7 nAChRs. These data, together with computer docking simulations, lead us to conclude that both the type I PAM NS1738 and the type II PAM PNU-120596 bind competitively at a mutually exclusive intrasubunit transmembrane site.

Role of α6 nicotinic receptors in CNS dopaminergic function: relevance to addiction and neurological disorders

Although a relative newcomer to the nicotinic acetylcholine receptor (nAChR) family, substantial evidence suggests that α6 containing nAChRs play a key role in CNS function. This subtype is unique in its relatively restricted localization to the visual system and catecholaminergic pathways. These latter include the mesolimbic and nigrostriatal dopaminergic systems, which may account for the involvement of α6 containing nAChRs in the rewarding properties of nicotine and in movement. Here, we review the literature on the role of α6 containing nAChRs with a focus on the striatum and nucleus accumbens. This includes molecular, electrophysiological and behavioral studies in control and lesioned animal models, as well as in different genetic models. Converging evidence suggest that the major α6 containing nAChRs subtypes in the nigrostriatal and mesolimbic dopamine system are the α6β2β3 and α6α4β2β3 nAChR populations. They appear to have a dominant role in regulating dopamine release, with consequent effects on nAChR-modulated dopaminergic functions such as reinforcement and motor behavior. Altogether these data suggest that drugs directed to α6 containing nAChRs may be of benefit for the treatment of addiction and for neurological disorders with locomotor deficits such as Parkinson's disease.

An autoradiographic survey of mouse brain nicotinic acetylcholine receptors defined by null mutants

Nine nicotinic receptor subunits are expressed in the central nervous system indicating that a variety of nicotinic acetylcholine receptors (nAChR) may be assembled. A useful method with which to identify putative nAChR is radioligand binding. In the current study the binding of [(125)I]α-bungarotoxin, [(125)I]α-conotoxinMII, 5[(125)I]-3-((2S)-azetidinylmethoxy)pyridine (A-85380), and [(125)I]epibatidine has been measured autoradiographically to provide data on many nAChR binding sites. Each binding site was evaluated semi-quantitatively for samples prepared from wild-type and α2, α4, α6, α7, β2, β4, α5 and β3 null mutant mice. Deletion of the α7 subunit completely and selectively eliminated [(125)I]α-bungarotoxin binding. The binding of [(125)I]α-conotoxinMII was eliminated in most brain regions by deletion of either the α6 or β2 subunit and is reduced by deletion of either the α4 or β3 subunit. The binding of 5[(125)I]A-85380 was completely eliminated by deletion of the β2 subunit and significantly reduced by deletion of the α4 subunit. Most, but not all, α4-independent sites require expression of the α6 subunit. The effect of gene deletion on total [(125)I]epibatidine binding was very similar to that on [(125)I]A-85380 binding. [(125)I]Epibatidine also labels β4* nAChR, which was readily apparent for incubations conducted in the presence of 100nM cytisine. The effects of α3 gene deletion could not be evaluated, but persistence of residual sites implies the expression of α3* nAChR. Taken together these results confirm and extend previously published evaluations of the effect of nAChR gene deletion and help to define the nAChR subtypes measurable by ligand binding.

A rapid agonist application system for fast activation of ligand-gated ion channels

The synaptic delay between neurotransmitter release across the synaptic cleft and activation of neurotransmitter gated ion channels is less than a ms. Nicotinic acetylcholine receptors (nAChRs), like many other classes of ligand-gated ion channels, are comprised of different protein subunits forming a variety of receptors with different activation and desensitization kinetics and pharmacological sensitivities. To measure and fully characterize ligand-gated ion channel currents accurately, one must apply agonists in a fraction of a ms and repeatedly at various concentrations without any prior desensitization of the receptors. In this paper, we describe an economical, easy to assemble and operate rapid drug application system. The drug applicator system consists of a parallel array of three pinch valves, which allow either agonist or wash solution into a theta tube. Solution exchanges of 0.16 ms can be achieved. In transfected cells, ACh elicited α4β2 nicotinic currents with mean rise times of 55±13 ms. We recorded α7 nAChRs, which desensitize very rapidly, and obtained very fast rise times of 19±2 ms. With this novel drug applicator, agonists can be applied repeatedly without any loss of current. Hence, complete dose-response relations can be obtained for even α7 nAChRs, which are very sensitive to desensitization caused by agonist exposure on a ms time scale. The drug application system can also be extended to the study of ligand-gated ion channels in brain slices. The theta tube valve-driven drug applicator system can be applied to study other ligand-gated ion channels including glutamate and GABA receptors.

Insect nicotinic acetylcholine receptor agonists as flea adulticides in small animals

Fleas are significant ectoparasites of small animals. They can be a severe irritant to animals and serve as a vector for a number of infectious diseases. In this article, we discuss the pharmacological characteristics of four insect nicotinic acetylcholine receptor (nAChR) agonists used as flea adulticides in dogs and cats, which include three neonicotinoids (imidacloprid, nitenpyram, and dinotefuran) and a macrocyclic lactone (spinosad). Insect nAChR agonists are one of the most important classes of insecticides, which are used to control sucking insects on both plants and animals. These novel compounds provide a new approach for practitioners to safely and effectively eliminate adult fleas.

The effects of galantamine on nicotine withdrawal-induced deficits in contextual fear conditioning in C57BL/6 mice

Current smoking cessation aids are relatively ineffective at maintaining abstinence during withdrawal. Nicotine withdrawal is associated with a variety of symptoms including cognitive deficits and targeting these deficits may be a useful strategy for maintaining abstinence. Galantamine is an acetylcholinesterase inhibitor and allosteric modulator of nicotinic acetylcholine receptors (nAChRs) with cognitive enhancing effects that may alleviate cognitive deficits associated with nicotine withdrawal. The effects of galantamine on nicotine withdrawal-induced deficits in contextual fear conditioning in C57BL/6 mice were examined. An initial acute dose-response experiment revealed that 0.5 and 1mg/kg galantamine had no effect on fear conditioning. To determine if galantamine would reverse nicotine withdrawal-related deficits in contextual fear conditioning, mice were implanted with osmotic mini-pumps that delivered chronic saline or 6.3mg/kg/d nicotine for 12 days and then pumps were removed. Training and testing of fear conditioning occurred 24 and 48 h later, respectively. Nicotine withdrawal disrupted contextual fear conditioning, which was reversed with 1 but not 0.5mg/kg galantamine. Across all conditions in both studies 2mg/kg galantamine led to high levels of freezing that were likely due to nonspecific effects. The ability of galantamine to reverse nicotine withdrawal-deficits in contextual conditioning is likely mediated through enhanced levels of acetylcholine via inhibition of acetylcholinesterase, potentiation of hippocampal α4β2* nAChRs, or both. The present study suggests that acetylcholinesterase inhibitors and/or drugs that act as allosteric modulators of nAChRs might be targets for smoking cessation aids because they may alleviate withdrawal symptoms such as cognitive deficits that can lead to relapse.

A fungal metabolite asperparaline a strongly and selectively blocks insect nicotinic acetylcholine receptors: the first report on the mode of action

Asperparalines produced by Aspergillus japonicus JV-23 induce paralysis in silkworm (Bombyx mori) larvae, but the target underlying insect toxicity remains unknown. In the present study, we have investigated the actions of asperparaline A on ligand-gated ion channels expressed in cultured larval brain neurons of the silkworm using patch-clamp electrophysiology. Bath-application of asperparaline A (10 µM) had no effect on the membrane current, but when delivered for 1 min prior to co-application with 10 µM acetylcholine (ACh), it blocked completely the ACh-induced current that was sensitive to mecamylamine, a nicotinic acetylcholine receptor (nAChR)-selective antaogonist. In contrast, 10 µM asperparaline A was ineffective on the γ-aminobutyric acid- and L-glutamate-induced responses of the Bombyx larval neurons. The fungal alkaloid showed no-use dependency in blocking the ACh-induced response with distinct affinity for the peak and slowly-desensitizing current amplitudes of the response to 10 µM ACh in terms of IC₅₀ values of 20.2 and 39.6 nM, respectively. Asperparaline A (100 nM) reduced the maximum neuron response to ACh with a minimal shift in EC₅₀, suggesting that the alkaloid is non-competitive with ACh. In contrast to showing marked blocking action on the insect nAChRs, it exhibited only a weak blocking action on chicken α3β4, α4β2 and α7 nAChRs expressed in Xenopus laevis oocytes, suggesting a high selectivity for insect over certain vertebrate nAChRs.

Agonist activation of alpha7 nicotinic acetylcholine receptors via an allosteric transmembrane site

Conventional nicotinic acetylcholine receptor (nAChR) agonists, such as acetylcholine, act at an extracellular "orthosteric" binding site located at the interface between two adjacent subunits. Here, we present evidence of potent activation of α7 nAChRs via an allosteric transmembrane site. Previous studies have identified a series of nAChR-positive allosteric modulators (PAMs) that lack agonist activity but are able to potentiate responses to orthosteric agonists, such as acetylcholine. It has been shown, for example, that TQS acts as a conventional α7 nAChR PAM. In contrast, we have found that a compound with close chemical similarity to TQS (4BP-TQS) is a potent allosteric agonist of α7 nAChRs. Whereas the α7 nAChR antagonist metyllycaconitine acts competitively with conventional nicotinic agonists, metyllycaconitine is a noncompetitive antagonist of 4BP-TQS. Mutation of an amino acid (M253L), located in a transmembrane cavity that has been proposed as being the binding site for PAMs, completely blocks agonist activation by 4BP-TQS. In contrast, this mutation had no significant effect on agonist activation by acetylcholine. Conversely, mutation of an amino acid located within the known orthosteric binding site (W148F) has a profound effect on agonist potency of acetylcholine (resulting in a shift of ∼200-fold in the acetylcholine dose-response curve), but had little effect on the agonist dose-response curve for 4BP-TQS. Computer docking studies with an α7 homology model provides evidence that both TQS and 4BP-TQS bind within an intrasubunit transmembrane cavity. Taken together, these findings provide evidence that agonist activation of nAChRs can occur via an allosteric transmembrane site.

Nicotinic receptor agonists decrease L-dopa-induced dyskinesias most effectively in partially lesioned parkinsonian rats

L-dopa therapy for Parkinson's disease leads to dyskinesias or abnormal involuntary movement (AIMs) for which there are few treatment options. Our previous data showed that nicotine administration reduced L-dopa-induced AIMs in parkinsonian monkeys and rats. To further understand how nicotine mediates its antidyskinetic action, we investigated the effect of nicotinic receptor (nAChR) agonists in unilateral 6-OHDA-lesioned rats with varying striatal damage. We first tested the drugs in L-dopa-treated rats with a near-complete striatal dopamine lesion (>99%), the standard rodent dyskinesia model. Varenicline, an agonist that interacts with multiple nAChRs, did not significantly reduce L-dopa-induced AIMs, while 5-iodo-A-85380 (A-85380), which acts selectively at α4β2* and α6β2* subtypes, reduced AIMs by 20%. By contrast, both varenicline and A-85380 reduced L-dopa-induced AIMs by 40-50% in rats with a partial striatal dopamine lesion. Neither drug worsened the antiparkinsonian action of L-dopa. The results show that selective nicotinic agonists reduce dyskinesias, and that they are optimally effective in animals with partial striatal dopamine damage. These findings suggest that presynaptic dopamine terminal α4β2* and α6β2* nAChRs are critical for nicotine's antidyskinetic action. The current data have important implications for the use of nicotinic receptor-directed drugs for L-dopa-induced dyskinesias, a debilitating motor complication of dopamine replacement therapy for Parkinson's disease.

Differential sensitivity of Ctenocephalides felis and Drosophila melanogaster nicotinic acetylcholine receptor α1 and α2 subunits in recombinant hybrid receptors to nicotinoids and neonicotinoid insecticides

Nicotinic acetylcholine receptors (nAChRs) are the binding sites for nicotinoid drugs, such as nicotine and epibatidine, and are the molecular targets of the selectively insecticidal neonicotinoids. In this study we report the full length cDNA cloning of the three Ctenocephalides (C.) felis (cat flea) nAChR α subunits Cfα1, Cfα2, and Cfα3. When expressed in Xenopus oocytes as hybrid receptors with the Gallus gallus (chicken) β2 (Ggβ2) subunit, these cat flea α subunits formed acetylcholine-responsive ion channels. Acetylcholine-evoked currents of Cfα2/Ggβ2 were resistant to α-bungarotoxin, while those of Cfα1/Ggβ2 were sensitive to this snake toxin. The pharmacological profiles of Cfα1/Ggβ2, Cfα2/Ggβ2 and the chicken neuronal receptor Ggα4/Ggβ2 for acetylcholine, two nicotinoids and 6 insecticidal neonicotinoids were determined and compared. Particularly remarkable was the finding that Cfα1/Ggβ2 was far more sensitive to acetylcholine, nicotine and neonicotinoid agonists than either Cfα2/Ggβ2 or Ggα4/Ggβ2: for the anti flea neonicotinoid market compound imidacloprid the respective EC₅₀s were 0.02 μM, 1.31 μM and 10 μM. These results were confirmed for another insect species, Drosophila melanogaster, where the pharmacological profile of the Dmα1 and Dmα2 subunits as hybrid receptors with Ggβ2 in Xenopus oocyte expressions resulted in a similar sensitivity pattern as those identified for the C. felis orthologs. Our results show that at least in a Ggβ2 hybrid receptor setting, insect α1 subunits confer higher sensitivity to neonicotinoids than α2 subunits, which may contribute in vivo to the insect-selective action of this pesticide class.

Brain nicotinic receptors as emerging targets for drug addiction: neurobiology to translational research

Drug addiction, a chronic relapsing disorder, is a serious public health problem around the world. A growing body of preclinical and clinical evidence suggests that mammalian brain nicotinic acetylcholine receptors (nAChRs), the heterogeneous family of ion channels, play a pivotal role in drug addiction, including nicotine and alcohol dependence. As a result, there is an increasing interest in developing nAChR-based therapies for the treatment of addictive disorders. The current review summarizes the important preclinical and clinical data, demonstrating the ability of nAChR ligands to modulate nicotine and alcohol-induced biobehavioral and neurochemical changes in laboratory animals and humans. Recent studies suggest that partial agonists and antagonists at nAChRs have therapeutic potential for the management of nicotine and alcohol dependence. The complexity of nAChRs and their regulation for the development of nAChR-based drug candidates as novel pharmacotherapy for other addictive disorders will also be discussed. Taken together, this review will provide new insights into nAChR-based compounds and offer innovative translational strategies for combating drug addictive disorders.

Tethered agonist analogs as site-specific probes for domains of the human α7 nicotinic acetylcholine receptor that differentially regulate activation and desensitization

Homomeric α7 nicotinic acetylcholine receptors represent an important and complex pharmaceutical target. They can be activated by structurally diverse agonists and are highly likely to enter and remain in desensitized states at rates determined by the structures of the agonists. To identify structural elements regulating this function, we introduced reactive cysteines into the α7 ligand-binding domain allowing us to bind sulfhydryl-reactive (SH) agonist analogs or control reagents onto specific positions in the ligand binding domain. We identified four α7 mutants (S36C, L38C, W55C, and L119C) in which the tethering of the SH reagents blocked further acetylcholine-evoked activation of the receptor. However, after selective reaction with SH agonist analogs, the type II allosteric modulator N-(5-chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl-3-isoxazolyl)-urea (PNU-120596) could reactivate L119C and W55C mutants and receptors with a reduced or modified C-loop. Modified S36C and L38C mutants were insensitive to reactivation by PNU-120596, whether they were reacted with agonist analogs or alternative SH reagents. Molecular modeling showed that in the W55C and L119C mutants, the ammonium pharmacophore of the agonist analog methanethiosulfonate-ethyltrimethylammonium would be in a similar but nonidentical position underneath the C-loop. The orientation assumed by the ligand tethered to 119C was approximately 3-fold more sensitive to PNU-120596 than the alternative pose at 55C. Our results support the hypothesis that a single ligand can bind within the receptor in different ways and, depending on the specific binding pose, may variously promote activation or desensitization, or, alternatively, function as a competitive antagonist. This insight may provide a new approach for drug development.

The auto-antigen repertoire in myasthenia gravis

Myasthenia Gravis (MG) is an antibody-mediated autoimmune disorder affecting the postsynaptic membrane of the neuromuscular junction (NMJ). MG is characterized by an impaired signal transmission between the motor neuron and the skeletal muscle cell, caused by auto-antibodies directed against NMJ proteins. The auto-antibodies target the nicotinic acetylcholine receptor (nAChR) in about 90% of MG patients. In approximately 5% of MG patients, the muscle specific kinase (MuSK) is the auto-antigen. In the remaining 5% of MG patients, however, antibodies against the nAChR or MuSK are not detectable (idiopathic MG, iMG). Although only the anti-nAChR and anti-MuSK auto-antibodies have been demonstrated to be pathogenic, several other antibodies recognizing self-antigens can also be found in MG patients. Various auto-antibodies associated with thymic abnormalities have been reported, as well as many non-MG-specific auto-antibodies. However, their contribution to the cause, pathology and severity of the disease is still poorly understood. Here, we comprehensively review the reported auto-antibodies in MG patients and discuss their role in the pathology of this autoimmune disease.