Recent Advances in the Discovery of Nicotinic Acetylcholine Receptor Allosteric Modulators

Positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent agonists, allosteric activating PAMs and neutral or silent allosteric modulators are compounds capable of modulating the nicotinic receptor by interacting at allosteric modulatory sites distinct from the orthosteric sites. This survey is focused on the compounds that have been shown or have been designed to interact with nicotinic receptors as allosteric modulators of different subtypes, mainly α7 and α4β2. Minimal chemical changes can cause a different pharmacological profile, which can then lead to the design of selective modulators. Experimental evidence supports the use of allosteric modulators as therapeutic tools for neurological and non-neurological conditions.

Ropanicant (SUVN-911), an α4β2 nicotinic acetylcholine receptor antagonist intended for the treatment of depressive disorders: pharmacological, behavioral, and neurochemical characterization

RATIONALE

Ropanicant (SUVN-911) (3-(6-Chloropyridine-3-yloxymethyl)-2-azabicyclo (3.1.0) hexane hydrochloride) is a novel α4β2 nicotinic acetylcholine receptor (nAChR) antagonist being developed for the treatment of depressive disorders.

OBJECTIVES

Pharmacological and neurochemical characterization of Ropanicant to support a potential molecule for the treatment of depressive disorders.

METHODS

Ropanicant was assessed for antidepressant-like activity using the rat forced swimming test (FST) and differential reinforcement of low rate -72 s (DRL-72 s). Alleviation of anhedonia was assessed in chronic mild stress model using sucrose preference test. To understand the mechanism of action, serotonin levels, ionized calcium-binding adaptor molecule 1 (Iba1), and brain-derived neurotrophic factor (BDNF) were determined. The onset of antidepressant-like activity was determined using the reduction in submissive behavior assay. The effects on cognition and sexual functions were assessed using the object recognition task and sexual dysfunction assay respectively. Interaction of Ropanicant, TC-5214, and methyllycaconitine (MLA) with citalopram was investigated individually in mice FST.

RESULTS

Ropanicant exhibited antidepressant like properties in the FST and DRL-72 s. A significant reduction in anhedonia was observed in the sucrose preference test. Oral administration of Ropanicant produced a significant increase in serotonin and BDNF levels, with a reduction in the Iba1 activity. The onset of antidepressant like effect with Ropanicant was within a week of treatment, and was devoid of cognitive dulling and sexual dysfunction. While Ropanicant potentiated the effect of citalopram in FST, such an effect was not observed with MLA or TC-5214.

CONCLUSIONS

Preclinical studies with Ropanicant support the likelihood of its therapeutic utility in the treatment of depressive disorders.

An electrophysiological characterization of naturally occurring tobacco alkaloids and their action on human α4β2 and α7 nicotinic acetylcholine receptors

Nicotinic acetylcholine receptor (nAChR) subtype-selective pharmacological profiles of tobacco alkaloids are essential for understanding the physiological effects of tobacco products. In this study, automated electrophysiology was used to functionally characterize the effects of distinct groups of tobacco alkaloids on human α4β2 and α7 nAChRs. We found that, in tobacco alkaloids, pyridine as a hydrogen bond acceptor and a basic nitrogen atom at a distance of 4-7 Å are pharmacophoric elements necessary for molecular recognition by α4β2 and α7 nAChRs with various degrees of selectivity, potency, and efficacy. While four alkaloids-nicotine, nornicotine, anabasine and R-anatabine-potently activated α4β2, they were also weak agonists of α7 nAChRs. Nicotine was the most potent agonist of α4β2, while anabasine elicited the highest activation of α7. None of the tobacco alkaloids enhanced nAChR activity elicited by the endogenous ligand acetylcholine; therefore, none was considered to be a positive allosteric modulator (PAM) of either α4β2 or α7 nAChRs. In contrast, we identified tobacco alkaloids, such as the tryptophan metabolite 6-hydroxykynurenic acid, that decreased the activity of both α4β2 and α7 nAChRs. Our study identified a class of alkaloids with positive and negative effects against human α4β2 and α7 nAChRs. It also revealed human α4β2 to be the principal receptor for sensing the most abundant alkaloids in tobacco leaves.

Synthesis of Novel Nicotinic Ligands with Multimodal Action: Targeting Acetylcholine α4β2, Dopamine and Serotonin Transporters

Nicotinic acetylcholine receptors (nAChRs), serotonin transporters (SERT) and dopamine transporters (DAT) represent targets for the development of novel nicotinic derivatives acting as multiligands associated with different health conditions, such as depressive, anxiety and addiction disorders. In the present work, a series of functionalized esters structurally related to acetylcholine and nicotine were synthesized and pharmacologically assayed with respect to these targets. The synthesized compounds were studied in radioligand binding assays at α4β2 nAChR, h-SERT and h-DAT. SERT experiments showed not radioligand [³H]-paroxetine displacement, but rather an increase in the radioligand binding percentage at the central binding site was observed. Compound 20 showed K_i values of 1.008 ± 0.230 μM for h-DAT and 0.031 ± 0.006 μM for α4β2 nAChR, and [³H]-paroxetine binding of 191.50% in h-SERT displacement studies, being the only compound displaying triple affinity. Compound 21 displayed K_i values of 0.113 ± 0.037 μM for α4β2 nAChR and 0.075 ± 0.009 μM for h-DAT acting as a dual ligand. Molecular docking studies on homology models of α4β2 nAChR, h-DAT and h-SERT suggested potential interactions among the compounds and agonist binding site at the α4/β2 subunit interfaces of α4β2 nAChR, central binding site of h-DAT and allosteric modulator effect in h-SERT.

Bidirectional dopamine modulation of excitatory and inhibitory synaptic inputs to subthalamic neuron subsets containing α4β2 or α7 nAChRs

The subthalamic nucleus (STN) possesses microcircuits distinguished by subtypes of nicotinic acetylcholine receptors (nAChRs). Although dysfunction of the STN is well-known in Parkinson's disease, there is still little information about whether dopamine differentially modulates excitatory and inhibitory synaptic inputs to STN neurons expressing different nAChR subtypes. To address this issue, we performed brain slice patch-clamp recordings on STN neurons, while we pharmacologically manipulated dopaminergic inputs. In STN neuron subsets containing either α4β2 or α7 nAChRs, D₁ and D₂ receptors respectively enhanced and inhibited spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) and firing rates. The elevation of dopamine levels resulted in diverse regulations of synaptic transmission in these two neuron subsets, and interestingly, the dopamine regulation of sIPSCs significantly correlated with that of sEPSCs. Surprisingly, depletion of dopamine either by reserpine treatment or by unilateral 6-OHDA lesion of nigrostriatal dopaminergic neurons did not alter synaptic inputs to STN neurons, but STN neurons in the 6-OHDA-lesioned side exhibited hyperactivity. In summary, dopamine regulated both GABAergic and glutamatergic synaptic inputs to STN neuron subsets containing either α4β2 or α7 nAChRs, forming a balancing machinery to control neuronal activity. In parkinsonian mice, postsynaptic mechanisms may exist and contribute to the hyperactivity of STN neurons.

Allosteric modulation of α4β2* nicotinic acetylcholine receptors: Desformylflustrabromine potentiates antiallodynic response of nicotine in a mouse model of neuropathic pain

BACKGROUND

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels. The α4β2 subtype of nAChRs plays an important role in the mediation of pain and several nicotine-evoked responses. Agonists and partial agonists of α4β2 nAChRs show efficacy in animal pain models. In addition, the antinociceptive properties of nicotine, a non-selective nAChR agonist with a high affinity for α4β2 nAChRs, is well-known. There is a growing body of evidence pointing to allosteric modulation of nAChRs as an alternative treatment strategy in experimental pain. Desformylflustrabromine (dFBr) is a positive allosteric modulator (PAM) at α4β2 nAChRs that enhances agonist responses without activating receptors. We hypothesized that dFBr may enhance nicotine-induced antinociception.

METHODS

The present study investigated whether dFBr could attenuate mouse chronic constriction injury (CCI)-induced neuropathic pain by increasing endogenous cholinergic tone or potentiating the nicotine-evoked antiallodynic response.

RESULTS

We found that subcutaneous administration of dFBr failed to reduce pain behaviour on its own. However, the combination of dFBr with nicotine significantly reversed neuropathic pain behaviour dose- and time-dependently without motor impairment. Our data revealed that this effect was mediated by the α4β2 nAChRs by using competitive α4β2 antagonist dihydro-β-erythroidine. In addition, dFBr failed to potentiate the antiallodynic effect of morphine, which shows the effect of dFBr is unique to α4β2 nAChRs.

CONCLUSIONS

The present results suggest that allosteric modulation of α4β2 nAChR may provide new strategies in chronic neuropathic pain.

SIGNIFICANCE

α4β2 nAChRs are involved in pain modulation. dFBr, a PAM at α4β2 nAChRs, potentiates the nicotine response dose-dependently in neuropathic pain. Thus, the present results suggest that allosteric modulation of α4β2* nAChR may provide new strategies in chronic neuropathic pain.

[18 F]Nifene test-retest reproducibility in first-in-human imaging of α4β2* nicotinic acetylcholine receptors

The aim of this study was to examine the suitability of [¹⁸ F]nifene, a novel α4β2* nicotinic acetylcholine receptor (nAChR) radiotracer, for in vivo brain imaging in a first-in-human study.

METHODS

Eight healthy subjects (4 M,4 F;21-69,44 ± 21 yrs) underwent a [¹⁸ F]nifene positron emission tomography scan (200 ± 3.7 MBq), and seven underwent a second scan within 58 ± 31 days. Regional estimates of DVR were measured using the multilinear reference tissue model (MRTM2) with the corpus callosum as reference region. DVR reproducibility was evaluated with test-retest variability (TRV) and intraclass correlation coefficient (ICC).

RESULTS

The DVR ranged from 1.3 to 2.5 across brain regions with a TRV of 0-7%, and did not demonstrate a systematic difference between test and retest. The ICCs ranged from 0.2 to 0.9. DVR estimates were stable after 40 min.

CONCLUSION

The binding profile and tracer kinetics of [¹⁸ F]nifene make it a promising α4β2* nAChR radiotracer for scientific research in humans, with reliable DVR test-retest reproducibility.

The Role of Nicotine in Schizophrenia

Schizophrenia is associated with by severe disruptions in thought, cognition, emotion, and behavior. Patients show a marked increase in rates of smoking and nicotine dependence relative to nonaffected individuals, a finding commonly ascribed to the potential ameliorative effects of nicotine on symptom severity and cognitive impairment. Indeed, many studies have demonstrated improvement in patients following the administration of nicotine. Such findings have led to an increased emphasis on the development of therapeutic agents to target the nicotinic system as well as increasing the impetus to understand the genetic basis for nicotinic dysfunction in schizophrenia. The goal of this review article is to provide a critical summary of evidence for the role of the nicotinic system in schizophrenia. The first part will review the role of nicotine in normalization of primary dysfunctions and endophenotypical changes found in schizophrenia. The second part will provide a summary of genetic evidence linking polymorphisms in nicotinic receptor genes to smoking and schizophrenia. The third part will summarize attempts to treat schizophrenia using agents specifically targeting nicotinic and nicotinic receptor subtypes. Although currently available antipsychotic treatments are generally able to manage some aspects of schizophrenia (e.g., positive symptoms) they fail to address several other critically effected aspects of the disease. As such, the search for novel mechanisms to treat this disease is necessary.

In vivo PET imaging of the α4β2 nicotinic acetylcholine receptor as a marker for brain inflammation after cerebral ischemia

PET imaging of nicotinic acetylcholine receptors (nAChRs) could become an effective tool for the diagnosis and therapy evaluation of neurologic diseases. Despite this, the role of nAChRs α4β2 receptors after brain diseases such as cerebral ischemia and its involvement in inflammatory reaction is still largely unknown. To investigate this, we performed in parallel in vivo magnetic resonance imaging (MRI) and positron emission tomography (PET) with 2[(18)F]-fluoro-A85380 and [(11)C]PK11195 at 1, 3, 7, 14, 21, and 28 d after middle cerebral artery occlusion (MCAO) in rats. In the ischemic territory, PET with 2[(18)F]-fluoro-A85380 and [(11)C]PK11195 showed a progressive binding increase from days 3-7, followed by a progressive decrease from days 14-28 after cerebral ischemia onset. Ex vivo immunohistochemistry for the nicotinic α4β2 receptor and the mitochondrial translocator protein (18 kDa) (TSPO) confirmed the PET findings and demonstrated the overexpression of α4β2 receptors in both microglia/macrophages and astrocytes from days 7-28 after experimental ischemic stroke. Likewise, the role played by α4β2 receptors on neuroinflammation was supported by the increase of [(11)C]PK11195 binding in ischemic rats treated with the α4β2 antagonist dihydro-β-erythroidine hydrobromide (DHBE) at day 7 after MCAO. Finally, both functional and behavioral testing showed major impaired outcome at day 1 after ischemia onset, followed by a recovery of the sensorimotor function and dexterity from days 21-28 after experimental stroke. Together, these results suggest that the nicotinic α4β2 receptor could have a key role in the inflammatory reaction underlying cerebral ischemia in rats.

Decreasing nicotinic receptor activity and the spatial learning impairment caused by the NMDA glutamate antagonist dizocilpine in rats

Nicotinic systems have been shown by a variety of studies to be involved in cognitive function. Nicotinic receptors have an inherent property to become desensitized after activation. The relative role of nicotinic receptor activation vs. net receptor inactivation by desensitization in the cognitive effects of nicotinic drugs remains to be fully understood. In these studies, we tested the effects of the α7 nicotinic receptor antagonist methyllycaconitine (MLA), the α4β2 nicotinic receptor antagonist dihydro-β-erythroidine (DHβE), the nonspecific nicotinic channel blocker mecamylamine and the α4β2 nicotinic receptor desensitizing agent sazetidine-A on learning in a repeated acquisition test. Adult female Sprague-Dawley rats were trained on a repeated acquisition learning procedure in an 8-arm radial maze. MLA (1-4mg/kg), DHβE (1-4mg/kg), mecamylamine (0.125-0.5mg/kg) or sazetidine-A (1 and 3mg/kg) were administered in four different studies either alone or together with the NMDA glutamate antagonist dizocilpine (0.05 and 0.10mg/kg). MLA significantly counteracted the learning impairment caused by dizocilpine. The overall choice accuracy impairment caused by dizocilpine was significantly attenuated by co-administration of DHβE. Low doses of the non-specific nicotinic antagonist mecamylamine also reduced dizocilpine-induced repeated acquisition impairment. Sazetidine-A reversed the accuracy impairment caused by dizocilpine. These studies provide evidence that a net decrease in nicotinic receptor activity can improve learning by attenuating learning impairment induced by NMDA glutamate blockade. This adds to evidence in cognitive tests that nicotinic antagonists can improve cognitive function. Further research characterizing the efficacy and mechanisms underlying nicotinic antagonist and desensitization induced cognitive improvement is warranted.

Internal dose assessment of (-)-18F-flubatine, comparing animal model datasets of mice and piglets with first-in-human results

(-)-(18)F-flubatine is a promising tracer for neuroimaging of nicotinic acetylcholine receptors (nAChRs), subtype α4β2, using PET. Radiation doses after intravenous administration of the tracer in mice and piglets were assessed to determine the organ doses (ODs) and the effective dose (ED) to humans. The results were compared with subsequent clinical investigations in human volunteers.

METHODS

Twenty-seven female CD1 mice (weight ± SD, 28.2 ± 2.1 g) received intravenous injection of 0.75 ± 0.33 MBq of (-)-(18)F-flubatine. Up to 240 min after injection, 3 animals per time point were sacrificed and the organs harvested, weighed, and counted in a γ counter to determine mass and activity, respectively. Furthermore, whole-body PET scans of 5 female piglets (age ± SD, 44 ± 3 d; weight ± SD, 13.7 ± 1.7 kg) and 3 humans (2 men and 1 woman; age ± SD, 59.6 ± 3.9 y; weight ± SD, 74.3 ± 3.1 kg) were obtained up to 236 min (piglets) and 355 min (humans) after injection of 186.6 ± 7.4 and 353.7 ± 10.2 MBq of (-)-(18)F-flubatine, respectively, using a PET/CT scanner. The CT was used for delineation of the organs. Exponential curves were fitted to the time-activity-data, and time and mass scales were adapted to the human anatomy. The ODs were calculated using OLINDA/EXM (version 1.0); EDs were calculated with the tissue-weighting factors of ICRP103.

RESULTS

After the injection of (-)-(18)F-flubatine, there were no adverse or clinically detectable pharmacologic effects in any of the subjects. The highest activities after injection were found in the kidneys, urinary bladder, and liver. The urinary bladder receives the highest OD in all investigated species, followed by the kidneys and the liver for animals and humans, respectively. On the basis of mouse, piglet, and human kinetic data, the projected human ED of (-)-(18)F-flubatine was estimated to be 12.5 μSv/MBq in mice, 14.7 ± 0.7 μSv/MBq in piglets, and 23.4 ± 0.4 μSv/MBq in humans.

CONCLUSION

As has been demonstrated for other PET radiotracers, preclinical (i.e., animal-derived) dosimetry underestimates the ED to humans, in the current case of (-)-(18)F-flubatine by 34%-44%.

Identification and pharmacological characterization of 3,6-diazabicyclo[3.1.1]heptane-3-carboxamides as novel ligands for the α4β2 and α6/α3β2β3 nicotinic acetylcholine receptors (nAChRs)

We have synthesized a novel series of compounds, 3,6-diazabicyclo[3.1.1]heptane-3-carboxamides, targeting both the α4β2 and α6/α3β2β3 nAChRs. Members of the obtained chemical library are partial or full agonists at both the high sensitivity (α4)2(β2)3 and α6/α3β2β3 nAChRs. 3-(Cyclopropylcarbonyl)-3,6-diazabicyclo[3.1.1]heptane (TC-8831 or compound 7 herein) demonstrated a safe in vitro pharmacological profile and the potential for reducing or preventing L-dopa-induced dyskinesias (LID) in several in vivo animal models [1-4]. In vivo metabolism studies in rat and in vitro metabolism studies in liver microsomes from human, rat, dog and monkey showed TC-8831 to be relatively stable. In vivo pharmacokinetic analysis in the rat confirmed brain penetration, with an average brain:plasma ratio of approximately 0.3 across time points from 0.5 to 4 h. Docking into homology models predicted alternative binding modes for TC-8831 and highlighted the importance of the cationic center, hydrogen-bond acceptor, and hydrophobic aliphatic features in promoting binding affinity to both nAChRs. Pharmacophore elucidation confirmed the importance of these key interactions. QSAR modeling suggested that binding affinity is primarily driven by ligand shape, relative positive charge distribution onto the molecular surface, and molecular flexibility. Of the two subtypes, ligand binding to α6β2β3 appears to be more sensitive to bulkiness and flexibility.

Functional expression of α7-nicotinic acetylcholine receptors by muscle afferent neurons

The exercise pressor reflex (EPR) is generated by group III and IV muscle afferents during exercise to increase cardiovascular function. Muscle contraction is triggered by ACh, which is metabolized into choline that could serve as a signal of exercise-induced activity. We demonstrate that ACh can induce current in muscle afferents neurons isolated from male Sprague-Dawley rats. The nicotinic ACh receptors (nAChRs) appear to be expressed by some group III-IV neurons since capsaicin (TRPV1) and/or ATP (P2X) induced current in 56% of ACh-responsive neurons. α7- And α4β2-nAChRs have been shown to be expressed in sensory neurons. An α7-nAChR antibody stained 83% of muscle afferent neurons. Functional expression was demonstrated by using the specific α7-nAChR blockers α-conotoxin ImI (IMI) and methyllycaconitine (MLA). MLA inhibited ACh responses in 100% of muscle afferent neurons, whereas IMI inhibited ACh responses in 54% of neurons. Dihydro-β-erythroidine, an α4β2-nAChR blocker, inhibited ACh responses in 50% of muscle afferent neurons, but recovery from block was not observed. Choline, an α7-nAChR agonist, elicited a response in 60% of ACh-responsive neurons. Finally, we demonstrated the expression of α7-nAChR by peripherin labeled (group IV) afferent fibers within gastrocnemius muscles. Some of these α7-nAChR-positive fibers were also positive for P2X3 receptors. Thus choline could serve as an activator of the EPR by opening α7-nAChR expressed by group IV (and possible group III) afferents. nAChRs could become pharmacological targets for suppressing the excessive EPR activation in patients with peripheral vascular disease.

Selective potentiation of (α4)3(β2)2 nicotinic acetylcholine receptors augments amplitudes of prefrontal acetylcholine- and nicotine-evoked glutamatergic transients in rats

Prefrontal glutamate release evoked through activation of α4β2* nicotinic acetylcholine receptors (nAChRs) situated on thalamic glutamatergic afferents mediates cue detection processes and thus contributes to attentional performance. However, little is known about the respective contributions of the high sensitivity and low sensitivity (LS) stoichiometries of the α4β2 nAChR, (α4)2(β2)3 and (α4)3(β2)2, to these processes. In the present study we employed glutamate-sensitive microelectrodes and the (α4)3(β2)2-selective positive allosteric modulator (PAM) NS9283 to investigate the importance of the LS α4β2 nAChR for glutamate release in the rat medial prefrontal cortex (mPFC). Firstly, the signaling evoked by physiologically relevant ACh concentrations through the (α4)3(β2)2 nAChR in HEK293 cells was potentiated by NS9283, consistent with the classification of NS9283 as a PAM. In urethane-anesthetized rats, intra-prefrontal pressure ejections of NS9283 evoked glutamatergic transients. Importantly, this glutamate release was attenuated by removal of cholinergic projections to the recording area. This finding indicates that the effects of NS9283 depend on endogenous ACh, again consistent with effects of a PAM. We then conducted microdialysis to demonstrate the presence of extracellular ACh in urethane-anesthetized control rats. While detectable, those levels were significantly lower than in awake rats. Finally, the amplitudes of glutamatergic transients evoked by local pressure ejections of a low concentration of nicotine were significantly augmented following systemic administration of NS9283 (3.0mg/kg). In conclusion, our results indicate that a LS α4β2 nAChR PAM such as NS9283 may enhance the cholinergic modulation of glutamatergic neurotransmission in the cortex, thereby perhaps alleviating the attentional impairments common to a range of brain disorders.

Evaluation of [18F]Nifene biodistribution and dosimetry based on whole-body PET imaging of mice

INTRODUCTION

[(18)F]Nifene is a novel radiotracer specific to the nicotinic acetylcholine α4β2 receptor class. In preparation for using this tracer in humans we have performed whole-body PET studies in mice to evaluate the in vivo biodistribution and dosimetry of [(18)F]Nifene.

METHODS

Seven BALB/c mice (3 males, 4 females) received IV tail injections of [(18)F]Nifene and were scanned for 2 h in an Inveon dedicated PET scanner. Each animal also received a high resolution CT scan using an Inveon CT. The CT images were used to draw volume of interest (VOI) on the following organs: brain, large intestine, small intestine, stomach, heart, kidneys, liver, lungs, pancreas, bone, spleen, testes, thymus, uterus and urinary bladder. All organ time activity curves had the decay correction reversed and were normalized to the injected activity. The area under the normalized curves was then used to compute the residence times in each organ. The absorbed doses in mouse organs were computed using the RAdiation Dose Assessment Resource (RADAR) animal models for dose assessment. The residence times in mouse organs were converted to human values using scale factors based on differences between organ and body weights. OLINDA 1.1 software was used to compute the absorbed human doses in multiple organs for both female and male phantoms.

RESULTS

The highest mouse residence times were found in urinary bladder, liver, bone, small intestine and kidneys. The largest doses in mice were found in urinary bladder and kidneys for both females and males. The elimination of radiotracer was primarily via kidney and urinary bladder with the urinary bladder being the limiting organ. The projected human effective doses were 1.51E-02 mSv/MBq for the adult male phantom and 1.65E-02 mSv/MBq for the adult female model phantom.

CONCLUSION

This study indicates that the whole-body mouse imaging can be used as a preclinical tool for initial estimation of the absorbed doses of [(18)F]Nifene in humans.

Varenicline decreases alcohol consumption in heavy-drinking smokers

RATIONALE

Emerging evidence suggests that the α4β2 form of the nicotinic acetylcholine receptor (nAChR) modulates the rewarding effects of alcohol. The nAChR α4β2 subunit partial agonist varenicline (Chantix™), which is approved by the Food and Drug Administration for smoking cessation, also decreases ethanol consumption in rodents (Steensland et al., Proc Natl Acad Sci U S A 104:12518-12523, 2007) and in human laboratory and open-label studies (Fucito et al., Psychopharmacology (Berl) 215:655-663, 2011; McKee et al., Biol Psychiatry 66:185-190 2009).

OBJECTIVES

We present a randomized, double-blind, 16-week study in heavy-drinking smokers (n = 64 randomized to treatment) who were seeking treatment for their smoking. The study was designed to determine the effects of varenicline on alcohol craving and consumption. Outcome measures included number of alcoholic drinks per week, cigarettes per week, amount of alcohol craving per week, cumulative cigarettes and alcoholic drinks consumed during the treatment period, number of abstinent days, and weekly percentage of positive ethyl glucuronide and cotinine screens.

RESULTS

Varenicline significantly decreases alcohol consumption (χ (2) = 35.32, p < 0.0001) in smokers. Although varenicline has previously been associated with suicidality and depression, side effects were low in this study and declined over time in the varenicline treatment group.

CONCLUSIONS

Varenicline can produce a sustained decrease in alcohol consumption in individuals who also smoke. Further studies are warranted to assess varenicline efficacy in treatment-seeking alcohol abusers who do not smoke and to ascertain the relationship between varenicline effects on smoking and drinking.

Safety and efficacy of ABT-089 in pediatric attention-deficit/hyperactivity disorder: results from two randomized placebo-controlled clinical trials

OBJECTIVE

To assess the safety and efficacy of ABT-089, a novel α(4)β(2) neuronal nicotinic receptor partial agonist, vs. placebo in children with attention-deficit/hyperactivity disorder (ADHD).

METHOD

Two multicenter, randomized, double-blind, placebo-controlled, parallel-group studies of children 6 through 12 years of age were conducted. Study 1 (n = 274) assessed six treatment groups over 8 weeks: 4 once-daily (QD) ABT-089 doses (0.085-0.700 mg/kg), QD atomoxetine, and placebo. Study 2 (n = 119) assessed three treatment groups over 6 weeks: 2 QD ABT-089 doses (0.7 mg/kg, 1.4 mg/kg) and placebo. The primary efficacy variable was the investigator-administered Attention-Deficit/Hyperactivity Disorder Rating Scale-IV: Home Version (ADHD-RS-IV [HV]) Total Score. Safety was assessed by adverse event (AE) monitoring, laboratory tests, vital signs, physical examinations, and electrocardiogram measures.

RESULTS

There was no statistically significant difference between ABT-089 and placebo in mean change from baseline to final evaluation of ADHD-RS-IV (HV) Total Score or other outcome measures at any dose in either study. In Study 1, atomoxetine showed statistically significant improvement for the primary and most secondary endpoints. ABT-089 was generally safe and well tolerated, with no statistically significant difference between any ABT-089 dose and placebo in the overall incidence of any specific AE, and no clinically significant changes in other safety measures.

CONCLUSIONS

ABT-089 did not show efficacy on the primary efficacy variable, the ADHD-RS-IV (HV) Total Score, or other measures of ADHD symptomatology in children with ADHD, and had a safety profile similar to placebo. These results contrast with published reports of efficacy of nicotinic modulators in adults with ADHD.

Assembly of alpha4beta2 nicotinic acetylcholine receptors assessed with functional fluorescently labeled subunits: effects of localization, trafficking, and nicotine-induced upregulation in clonal mammalian cells and in cultured midbrain neurons

Fura-2 recording of Ca2+ influx was used to show that incubation in 1 microm nicotine (2-6 d) upregulates several pharmacological components of acetylcholine (ACh) responses in ventral midbrain cultures, including a MLA-resistant, DHbetaE-sensitive component that presumably corresponds to alpha4beta2 receptors. To study changes in alpha4beta2 receptor levels and assembly during this upregulation, we incorporated yellow and cyan fluorescent proteins (YFPs and CFPs) into the alpha4 or beta2 M3-M4 intracellular loops, and these subunits were coexpressed in human embryonic kidney (HEK) 293T cells and cultured ventral midbrain neurons. The fluorescent receptors resembled wild-type receptors in maximal responses to ACh, dose-response relations, ACh-induced Ca2+ influx, and somatic and dendritic distribution. Transfected midbrain neurons that were exposed to nicotine (1 d) displayed greater levels of fluorescent alpha4 and beta2 nicotinic ACh receptor (nAChR) subunits. As expected from the hetero-multimeric nature of alpha4beta2 receptors, coexpression of the alpha4-YFP and beta2-CFP subunits resulted in robust fluorescence resonance energy transfer (FRET), with a FRET efficiency of 22%. In midbrain neurons, dendritic alpha4beta2 nAChRs displayed greater FRET than receptors inside the soma, and in HEK293T cells, a similar increase was noted for receptors that were translocated to the surface during PKC stimulation. When cultured transfected midbrain neurons were incubated in 1 microm nicotine, there was increased FRET in the cell body, denoting increased assembly of alpha4beta2 receptors. Thus, changes in alpha4beta2 receptor assembly play a role in the regulation of alpha4beta2 levels and responses in both clonal cell lines and midbrain neurons, and the regulation may result from Ca2+-stimulated pathways.

Chronic exposure to nicotine upregulates the human (alpha)4((beta)2 nicotinic acetylcholine receptor function

Widely expressed in the brain, the alpha4beta2 nicotinic acetylcholine receptor (nAChR) is proposed to play a major role in the mechanisms that lead to and maintain nicotine addiction. Using the patch-clamp technique and pharmacological protocols, we examined the consequences of long-term exposure to 0.1-10 micrometer nicotine in K-177 cells expressing the major human brain alpha4beta2 receptor. The acetylcholine dose-response curves are biphasic and revealed both a high- and a low-affinity component with apparent EC(50) values of 1.6 and 62 micrometer. Ratios of receptors in the high- and low-affinity components are 25 and 75%, respectively. Chronic exposure to nicotine or nicotinic antagonists [dihydro-beta-erytroidine (DHbetaE) or methyllycaconitine (MLA)] increases the fraction of high-affinity receptors up to 70%. Upregulated acetylcholine-evoked currents increase by twofold or more and are less sensitive to desensitization. Functional upregulation is independent of protein synthesis as shown by the lack of effect of 20 micrometer cycloheximide. Single-channel currents recorded with 100 nm acetylcholine show predominantly high conductances (38.8 and 43.4 pS), whereas additional smaller conductances (16.7 and 23.5 pS) were observed with 30 micrometer acetylcholine. In addition, long-term exposure to dihydro-beta-erytroidine increases up to three times the frequency of channel openings. These data indicate, in contrast to previous studies, that human alpha4beta2 nAChRs are functionally upregulated by chronic nicotine exposure.

Human alpha4beta2 neuronal nicotinic acetylcholine receptor in HEK 293 cells: A patch-clamp study

The cloning and expression of genes encoding for the human neuronal nicotinic acetylcholine receptors (nAChRs) has opened new possibilities for investigating their physiological and pharmacological properties. Cells (HEK 293) stably transfected with two of the major brain subunits, alpha4 and beta2, were characterized electrophysiologically using the patch-clamp technique. Fast application of the natural ligand ACh can evoke currents up to 3500 pA, with an apparent affinity (EC50) of 3 microM and a Hill coefficient of 1.2. The rank order of potency of four nAChR ligands to activate human alpha4beta2 receptors is (-)-nicotine > ACh > (-)-cytisine > ABT-418. At saturating concentrations, the efficacy of these ligands is ABT-418 >> (-)-nicotine > ACh >> (-)-cytisine > GTS-21 (previously named DMXB). Coapplication of 1 microM ACh with known nAChR inhibitors such as dihydro-beta-erythroidine and methyllycaconitine reversibly reduces the current evoked by the agonist with respective IC50 values of 80 nM and 1.5 microM. The current-voltage relationship of human alpha4beta2 displays a strong rectification at positive potentials. Experiments of ionic substitutions suggest that human alpha4beta2 nAChRs are permeable to sodium and potassium ions. In the "outside-out" configuration, ACh evokes unitary currents (main conductance 46 pS) characterized by a very fast rundown. Potentiation of the ACh-evoked currents is observed when the extracellular calcium concentration is increased from 0.2 to 2 mM. In contrast, however, a reduction of the evoked currents is observed when calcium concentration is elevated above 2 mM.