Evaluation of structurally diverse neuronal nicotinic receptor ligands for selectivity at the alpha6( *) subtype

TC299423, a Novel Agonist for Nicotinic Acetylcholine Receptors

(E)-5-(Pyrimidin-5-yl)-1,2,3,4,7,8-hexahydroazocine (TC299423) is a novel agonist for nicotinic acetylcholine receptors (nAChRs). We examined its efficacy, affinity, and potency for α6β2^∗ (α6β2-containing), α4β2^∗, and α3β4^∗ nAChRs, using [¹²⁵I]-epibatidine binding, whole-cell patch-clamp recordings, synaptosomal ⁸⁶Rb⁺ efflux, [³H]-dopamine release, and [³H]-acetylcholine release. TC299423 displayed an EC₅₀ of 30–60 nM for α6β2^∗ nAChRs in patch-clamp recordings and [³H]-dopamine release assays. Its potency for α6β2^∗ in these assays was 2.5-fold greater than that for α4β2^∗, and much greater than that for α3β4^∗-mediated [³H]-acetylcholine release. We observed no major off-target binding on 70 diverse molecular targets. TC299423 was bioavailable after intraperitoneal or oral administration. Locomotor assays, measured with gain-of-function, mutant α6 (α6L9′S) nAChR mice, show that TC299423 elicits α6β2^∗ nAChR-mediated responses at low doses. Conditioned place preference assays show that low-dose TC299423 also produces significant reward in α6L9′S mice, and modest reward in WT mice, through a mechanism that probably involves α6(non-α4)β2^∗ nAChRs. However, TC299423 did not suppress nicotine self-administration in rats, indicating that it did not block nicotine reinforcement in the dosage range that was tested. In a hot-plate test, TC299423 evoked antinociceptive responses in mice similar to those of nicotine. TC299423 and nicotine similarly inhibited mouse marble burying as a measure of anxiolytic effects. Taken together, our data suggest that TC299423 will be a useful small-molecule agonist for future in vitro and in vivo studies of nAChR function and physiology.

In vitro and in vivo neuronal nicotinic receptor properties of (+)- and (-)-pyrido[3,4]homotropane [(+)- and (-)-PHT]: (+)-PHT is a potent and selective full agonist at α6β2 containing neuronal nicotinic acetylcholine receptors

Pyrido[3,4]homotropane (PHT) is a conformationally rigid, high affinity analogue of nicotine. (+)-PHT was previously shown to be 266 times more potent than (-)-PHT for inhibition of [(3)H]epibatidine binding to nAChRs but had no antinociceptive activity in mouse tail-flick or hot-plate tests and was not a nicotinic antagonist even when administered intrathecally. While (-)-PHT had no agonist activity, it was a potent, nicotinic antagonist in the test. Here, electrophysiological studies with rat nAChRs show (+)-PHT to be a low efficacy partial agonist selective for α4β2-nAChRs, relative to α3β4-nAChRs (15-fold) and α7-nAChRs (45-fold). (-)-PHT was an antagonist with selectivity for α3β4, relative to α4β2- (3-fold) and α7- (11-fold) nAChRs. In [(3)H]DA release studies in mice, (+)-PHT was 10-fold more potent than (-)-PHT at α4β2*-nAChRs and 30-fold more potent at α6β2*-nAChRs. Studies using α5KO mice suggested that much of the activity at α4β2*-nAChRs is mediated by the α4β2α5-nAChR subtype. In conditioned place preference studies, (-)-PHT was more potent than (+)-PHT in blocking nicotine reward. Off-target screens showed (+)- and (-)-PHT to be highly selective for nAChRs. The high potency, full agonism of (+)- and (-)-PHT at α6*-nAChR contrasts with the partial agonism observed for α4*-nAChR, making these ligands intriguing probes for learning more about the pharmacophores for various nAChRs.

Analysis of rare variations reveals roles of amino acid residues in the N-terminal extracellular domain of nicotinic acetylcholine receptor (nAChR) alpha6 subunit in the functional expression of human alpha6*-nAChRs

Background

Functional heterologous expression of naturally-expressed and apparently functional mammalian α6*-nicotinic acetylcholine receptors (nAChRs; where ‘*’ indicates presence of additional subunits) has been difficult. Here we wanted to investigate the role of N-terminal domain (NTD) residues of human (h) nAChR α6 subunit in the functional expression of hα6*-nAChRs. To this end, instead of adopting random mutagenesis as a tool, we used 15 NTD rare variations (i.e., Ser43Pro, Asn46Lys, Asp57Asn, Arg87Cys, Asp92Glu, Arg96His, Glu101Lys, Ala112Val, Ser156Arg, Asn171Lys, Ala184Asp, Asp199Tyr, Asn203Thr, Ile226Thr and Ser233Cys) in nAChR hα6 subunit to probe for their effect on the functional expression of hα6*-nAChRs.

Results

N-terminal α-helix (Asp⁵⁷); complementary face/inner β-fold (Arg⁸⁷ or Asp⁹²) and principal face/outer β-fold (Ser¹⁵⁶ or Asn¹⁷¹) residues in the hα6 subunit are crucial for functional expression of the hα6*-nAChRs as variations in these residues reduce or abrogate the function of hα6hβ2*-, hα6hβ4- and hα6hβ4hβ3-nAChRs. While variations at residues Ser⁴³ or Asn⁴⁶ (both in N-terminal α-helix) in hα6 subunit reduce hα6hβ2*-nAChRs function those at residues Arg⁹⁶ (β2-β3 loop), Asp¹⁹⁹ (loop F) or Ser²³³ (β10-strand) increase hα6hβ2*-nAChR function. Similarly substitution of NTD α-helix (Asn⁴⁶), loop F (Asp¹⁹⁹), loop A (Ala¹¹²), loop B (Ala¹⁸⁴), or loop C (Ile²²⁶) residues in hα6 subunit increase the function of hα6hβ4-nAChRs. All other variations in hα6 subunit do not affect the function of hα6hβ2*- and hα6hβ4*-nAChRs. Incorporation of nAChR hβ3 subunits always increase the function of wild-type or variant hα6hβ4-nAChRs except for those of hα6(D57N, S156R, R87C or N171K)hβ4-nAChRs. It appears Asp57Lys, Ser156Arg or Asn171Lys variations in hα6 subunit drive the hα6hβ4hβ3-nAChRs into a nonfunctional state as at spontaneously open hα6(D57N, S156R or N171K)hβ4hβ3^V9’S-nAChRs (V9’S; transmembrane II 9’ valine-to-serine mutation) agonists act as antagonists. Agonist sensitivity of hα6hβ4- and/or hα6hβ4hβ3-nAChRs is nominally increased due to Arg96His, Ala184Asp, Asp199Tyr or Ser233Cys variation in hα6 subunit.

Conclusions

Hence investigating functional consequences of natural variations in nAChR hα6 subunit we have discovered additional bases for cell surface functional expression of various subtypes of hα6*-nAChRs. Variations (Asp57Asn, Arg87Cys, Asp92Glu, Ser156Arg or Asn171Lys) in hα6 subunit that compromise hα6*-nAChR function are expected to contribute to individual differences in responses to smoked nicotine.

A novel α-conotoxin MII-sensitive nicotinic acetylcholine receptor modulates [(3) H]-GABA release in the superficial layers of the mouse superior colliculus

Mouse superficial superior colliculus (SuSC) contains dense GABAergic innervation and diverse nicotinic acetylcholine receptor subtypes. Pharmacological and genetic approaches were used to investigate the subunit compositions of nicotinic acetylcholine receptors (nAChR) expressed on mouse SuSC GABAergic terminals. [(125) I]-Epibatidine competition-binding studies revealed that the α3β2* and α6β2* nicotinic subtype-selective peptide α-conotoxin MII-blocked binding to 40 ± 5% of SuSC nAChRs. Acetylcholine-evoked [(3) H]-GABA release from SuSC crude synaptosomal preparations is calcium dependent, blocked by the voltage-sensitive calcium channel blocker, cadmium, and the nAChR antagonist mecamylamine, but is unaffected by muscarinic, glutamatergic, P2X and 5-HT3 receptor antagonists. Approximately 50% of nAChR-mediated SuSC [(3) H]-GABA release is inhibited by α-conotoxin MII. However, the highly α6β2*-subtype-selective α-conotoxin PIA did not affect [(3) H]-GABA release. Nicotinic subunit-null mutant mouse experiments revealed that ACh-stimulated SuSC [(3) H]-GABA release is entirely β2 subunit-dependent. α4 subunit deletion decreased total function by >90%, and eliminated α-conotoxin MII-resistant release. ACh-stimulated SuSC [(3) H]-GABA release was unaffected by β3, α5 or α6 nicotinic subunit deletions. Together, these data suggest that a significant proportion of mouse SuSC nicotinic agonist-evoked GABA-release is mediated by a novel, α-conotoxin MII-sensitive α3α4β2 nAChR. The remaining α-conotoxin MII-resistant, nAChR agonist-evoked SuSC GABA release appears to be mediated via α4β2* subtype nAChRs.

Stable expression and functional characterization of a human nicotinic acetylcholine receptor with α6β2 properties: discovery of selective antagonists

BACKGROUND AND PURPOSE

Despite growing evidence that inhibition of α6β2-containing (α6β2*) nicotinic acetylcholine receptors (nAChRs) may be beneficial for the therapy of tobacco addiction, the lack of good sources of α6β2*-nAChRs has delayed the discovery of α6β2-selective antagonists. Our aim was to generate a cell line stably expressing functional nAChRs with α6β2 properties, to enable pharmacological characterization and the identification of novel α6β2-selective antagonists.

EXPERIMENTAL APPROACH

Different combinations of the α6, β2, β3, chimeric α6/3 and mutant β3(V273S) subunits were transfected in human embryonic kidney cells and tested for activity in a fluorescent imaging plate reader assay. The pharmacology of rat immune-immobilized α6β2*-nAChRs was determined with ¹²⁵I-epibatidine binding.

KEY RESULTS

Functional channels were detected after co-transfection of α6/3, β2 and β3(V273S) subunits, while all other subunit combinations failed to produce agonist-induced responses. Stably expressed α6/3β2β3(V273S)-nAChR pharmacology was unique, and clearly distinct from α4β2-, α3β4-, α7- and α1β1δε-nAChRs. Antagonist potencies in inhibiting α6/3β2β3(V273S) -nAChRs was similar to their binding affinity for rat native α6β2*-nAChRs. Agonist affinities for α6β2*-nAChRs was higher than their potency in activating α6/3β2β3(V273S)-nAChRs, but their relative activities were equivalent. Focussed set screening at α6/3β2β3(V273S)-nAChRs, followed by cross-screening with the other nAChRs, led to the identification of novel α6β2-selective antagonists.

CONCLUSIONS AND IMPLICATIONS

We generated a mammalian cell line stably expressing nAChRs, with pharmacological properties similar to native α6β2*-nAChRs, and used it to identify novel non-peptide, low molecular weight, α6β2-selective antagonists. We also propose a pharmacophore model of α6β2 antagonists, which offers a starting point for the development of new smoking cessation agents.

Progress and challenges in the study of α6-containing nicotinic acetylcholine receptors

Recent progress has been made in the understanding of the anatomical distribution, composition, and physiological role of nicotinic acetylcholine receptors containing the α6 subunit. Extensive study by many researchers has indicated that a collection of α6-containing receptors representing a nicotinic sub-family is relevant in preclinical models of nicotine self-administration and locomotor activity. Due to a number of technical difficulties, the state of the art of in vitro model systems expressing α6-containing receptors has lagged behind the state of knowledge of native α6 nAChR subunit composition. Several techniques, such as the expression of chimeric and concatameric α6 subunit constructs in oocytes and mammalian cell lines have been employed to overcome these obstacles. There remains a need for other critical tools, such as selective small molecules and radioligands, to advance the field of research and to allow the discovery and development of potential therapeutics targeting α6-containing receptors for smoking cessation, Parkinson's disease and other disorders.

A novel series of [3.2.1] azabicyclic biaryl ethers as alpha3beta4 and alpha6/4beta4 nicotinic receptor agonists

We report the synthesis of a series of [3.2.1]azabicyclic biaryl ethers as selective agonists of alpha3- and alpha6-containing nicotinic receptors. In particular, compound 17a from this series is a potent alpha3beta4 and alpha6/4beta4 receptor agonist in terms of both binding and functional activity. Compound 17a also shows potent in vivo activity in CNS-mediated animal models that are sensitive to antipsychotic drugs. Compound 17a may thus be a useful tool for studying the role of alpha3beta4 and alpha6/4beta4 nicotinic receptors in CNS pharmacology.