Involvement of neuronal β₂ subunit-containing nicotinic acetylcholine receptors in nicotine reward and withdrawal: Implications for pharmacotherapies

SUMMARY

What is known and objective

Tobacco smoking remains a major health problem. Nicotine binds to nicotinic acetylcholine receptors (nAChRs), which can cause addiction and withdrawal symptoms upon cessation of nicotine administration. Pharmacotherapies for nicotine addiction target brain alterations that underlie withdrawal symptoms. This review will delineate the involvement of the β₂ subunit of neuronal nAChRs in nicotine reward and in generating withdrawal symptoms to better understand the efficacy of smoking cessation pharmacotherapies.

Comment

Chronic nicotine desensitizes and upregulates β₂ subunit-containing nAChRs, and the prolonged upregulation of receptors may underlie symptoms of withdrawal. Experimental research has demonstrated that the β₂ subunit of neuronal nAChRs is necessary for generating nicotine reward and withdrawal symptoms.

What is new and conclusion

Smoking cessation pharmacotherapies act on β₂ subunit-containing nAChRs to reduce nicotine reward and withdrawal symptom severity.

WHAT IS KNOWN AND OBJECTIVE

Introduction

Tobacco use is the leading cause of preventable death and disease in the United States, causing approximately 443 000 premature deaths each year.¹ The health impacts of tobacco use include increased risk of lung and oral cancers, coronary heart disease and stroke.² Nicotine addiction is expensive for individuals and the national economy, costing approximately $96.8 billion annually in productivity loss.³ Approximately 70% of smokers report desiring to quit, but only 42% of smokers actually attempt to quit smoking. Of those who attempt to quit, only 3–5% are successful after 6 months when the quit attempt is unaided by behavioural treatments or pharmacotherapies.⁴ Clearly, tobacco smoking remains a major health problem for the United States and other countries.

Nicotine is one of the principal psychoactive and addictive drugs in tobacco products. Similarly to other drugs of abuse, nicotine addiction is characterized by compulsive use, craving, tolerance from continued use and withdrawal upon cessation.⁵ The symptoms of nicotine withdrawal include dysphoria, irritability, frustration and difficulty concentrating.⁶ Smokers are able to relieve symptoms of withdrawal by relapsing.^7–9 These effects and the health consequences of tobacco smoking point to the importance for a better understanding of the neurobiology of nicotine addiction.¹⁰

Nicotine alters neurobiological processes by binding to neuronal nicotinic acetylcholine receptors (nAChRs). Similarly to other drugs of abuse, acute nicotine administration causes signalling changes in brain reward substrates, such as increasing the phasic release of mesolimbic dopamine (DA).^11–15 Chronic nicotine leads to desensitization and upregulation of neuronal nAChRs, including heteropentameric β₂ subunit-containing nAChRs.^16–23 During nicotine withdrawal, changes in both brain reward and neuronal nAChR functioning have been observed, which underlie withdrawal symptom development.^21,24–28 It is evident that the β₂ subunit of neuronal nAChRs is pivotal for the generation of reward from nicotine administration^29–35 as well as for the generation of cognitive and affective withdrawal symptoms.^36–41

Objective

This review will focus on how β₂ subunit-containing nAChRs underlie the symptoms of nicotine withdrawal, as well as how these receptors underlie the rewarding and reinforcing properties of nicotine. Moreover, this review will discuss the utility of pharmacotherapies in reducing withdrawal symptom severity and the rewarding properties of nicotine via acting on β₂ subunit-containing nAChRs. The review will incorporate findings from human and animal studies to elucidate the involvement of the β₂ subunit in nicotine reward and withdrawal.

COMMENT

Nicotine in the brain: effects of chronic use and withdrawal

Neuronal nicotinic acetylcholine receptors

Nicotine and endogenous acetylcholine (ACh) bind to pentameric nicotinic acetylcholine receptors (nAChRs), which are formed from a combination of α and β subunits, to gate the transmembrane flow of cations.^42,43 In the mammalian central nervous system, predominant nAChRs include low-affinity homomeric α₇ and high-affinity heteromeric α₄ β₂ nAChRs. The β₂ subunit can couple with non-α₄ subunits, including α₂, α₃ and α₆ subunits to form functionally distinct nAChRs, but these receptors are much less densely expressed in the central nervous system relative to α₄β₂* nAChRs (*indicating potential assemblage of β₂ subunits with non-α₄ subunits).⁴⁴ In addition to nAChRs forming from different subunit assemblages, stoichiometric study has shown that α₄β₂ nAChRs can form as either (α₄)₃(β₂)₂ or (α₄)₂(β₂)₃, the latter having a greater affinity for both nicotine and ACh.⁴⁵ The high-affinity α₄β₂* nAChRs are densely expressed in cortex, striatum and hippocampus⁴⁶ and are involved in addiction and learning processes.^{30,32,34,38,40,47}

Dopamine, reward and the β₂ subunit of neuronal nAChRs

Drugs of abuse can cause alterations in mesocorticolimbic neurotransmitter release, and these release patterns have been associated with the rewarding effects of drug administration.⁴⁸ The nucleus accumbens (NAcc) receives a basal (i.e. ‘tonic’) release of DA from presynaptic dopaminergic cell bodies in the ventral tegmental area (VTA).⁴⁹ Rewarding events, such as administration of drugs of abuse, elicit large and transient (i.e. ‘phasic’) bouts of DA release in the NAcc from VTA cell bodies.^14,15 In vivo studies have demonstrated that acute nicotine administration elicits phasic DA release in the NAcc and that this phasic activity continues to be elicited after repeated administrations.^{11–13,50–53} Whereas acute nicotine leads to enhanced phasic dopaminergic signalling in the NAcc, withdrawal from chronic nicotine leads to both reduced tonic DA release in the NAcc^28,54 and a concordant reduction in brain reward thresholds.⁵⁵ Accordingly, nicotine withdrawal leads to a state of reduced pleasure and reward, which is known as anhedonia.⁵⁶ These studies reveal that: (1) acute nicotine elicits phasic DA release in the NAcc even after repeated administrations, (2) nicotine withdrawal leads to a hypodopaminergic state which is associated with anhedonia, and (3) re-administration of nicotine may temporarily resolve this anhedonic state by enhancing dopaminergic signalling in the NAcc.⁵⁷

The dopaminergic activity changes observed in the NAcc following nicotine administration are mediated by presynaptic β₂ subunit-containing nAChRs in the VTA.^29,30 Genetically engineered β₂-knockout (β₂^−/−) mice have allowed for delineating the involvement of the β₂ subunit of neuronal nAChRs in nicotine-induced reward.^58,59 It has been reported that β₂^−/− mice show significantly reduced phasic DA release in the NAcc upon nicotine administration relative to wild-type control animals and that β₂^−/− mice show reduced self-administration for nicotine when allowed intravenous³³ or direct intra-VTA³¹ self-administration. Targeted lentiviral expression of the β₂ subunit in the VTA of β₂^−/− mice restored nicotine self-administration^31,33,34 and increased phasic DA release in the NAcc after nicotine administration.³¹ Additionally, mice acquire conditioned place preference (CPP) to nicotine,⁶⁰ as evidenced by animals spending significantly more time in a nicotine-paired context relative to a saline-paired context. It was demonstrated that β₂^−/− mice do not acquire nicotine CPP.^32,35 Separate lines of evidence have revealed that other nAChR subunits, such as α₅ and α₆ subunits, can co-assemble with β₂ subunit-containing nAChRs and are involved in the rewarding properties of nicotine and that genetic mutations in the genes coding for these nAChR subunits may confer vulnerability for nicotine addiction.^61,62 Ultimately, the β₂ subunit of neuronal nAChRs is necessary for behavioural and neurochemical properties of nicotine-induced reward.

Receptor upregulation and the β₂ subunit of neuronal nAChRs: Receptor desensitization precedes upregulation

Although several lines of evidence have shown that chronic nicotine can alter receptor function of many nAChRs,⁶³ including α₇ and α₃β₄ subtypes, this review will focus on how chronic nicotine alters receptor function of β₂ subunit-containing nAChRs. Several studies have shown that β₂ subunit-containing nAChRs desensitize from chronic nicotine exposure and that these receptors recover from their desensitized states after nicotine is cleared from the system.^{16–20,22,23} Moreover, evidence has found that chronic nicotine leads to a dose-dependent increase in number of β₂ subunit-containing nAChRs throughout widespread brain regions and that these receptors persist in an upregulated state after nicotine is cleared from the system.^{21,24–27,64–67} Concordantly, β₂^−/− mice failed to show receptor upregulation following chronic nicotine treatment.⁶⁸ To explain the dose-dependent upregulation effect of chronic nicotine on β₂ subunit-containing nAChRs, evidence has found that a positive relationship exists between the recovery time from desensitization and the magnitude of receptor upregulation. Specifically, mutant α₄β₂ nAChRs in Xenopus laevis oocytes that were slow to recover from their desensitized state upregulated in response to low-dose, periodic exposures to nicotine, whereas wild-type channels did not upregulate from low doses of nicotine.²⁰ This evidence supports the idea that there might be a cause-and-effect relationship between receptor desensitization and upregulation and that the magnitude of nicotine dependence may underlie the degree of receptor upregulation, but this relationship has not been fully uncovered. Combined, these lines of evidence suggest that β₂ subunit-containing nAChRs desensitize from chronic nicotine treatment, which is followed by persistent upregulation, and that these nAChRs return from desensitization upon nicotine clearing the system. Thus, when nicotine is cleared from the system, it might be the case that an increased number of functional β₂ subunit-containing nAChRs are available on cell membranes relative to prenicotine receptor levels. Accordingly, it might be the case that a sensitized nAChR system develops during withdrawal as a result of increased functional β₂ subunit-containing nAChRs.^27,67,69

The findings from animal research on nicotine-mediated nAChR function have shown to be translational to humans. An early autoradiographic study revealed that smokers had increased nAChR binding sites in multiple brain regions, including the hippocampus, relative to non-smokers.⁷⁰ It has since been shown that increases in nAChR binding sites are positively correlated with the magnitude of nicotine dependence⁷¹ and are specific to increased densities of high-affinity β₂ subunit-containing nAC-hRs.²¹ Furthermore, smokers who had quit at least 2 months before death were found to have normalized nAChR binding sites that were comparable to levels observed in non-smokers.⁷¹ Initial post-mortem studies provided the first evidence of nAChR upregulation from brains of smokers, and a more extensive understanding of nAChR changes has since been made from imaging the brains of live smokers.

Subtype-selective radioligands have been developed for positron emission topography (PET)^72–75 and single-photon emission computed tomography (SPECT),^76,77 which have allowed for in vivo imaging of nAChRs in smokers. One PET study, using the α₄β₂* nAChR radioligand 2-[¹⁸F]fluoro-3-(2(S)azetidinylmethoxy) pyridine (2-F-A-85380), showed that smoking 1–3 cigarettes saturates α₄β₂* binding sites in widespread brain areas for several hours in smokers.⁷⁸ Related studies using iodide 123-labelled 5-iodo-A-85380 ([¹²³I]5-IA; herein referred to as 5IA) SPECT imaging provided evidence that smokers have increased levels of α₄β₂* nAChRs in thalamic, striatal and cortical brain regions at 1 week but not 24 h into withdrawal relative to non-smokers.^26,79 Notably, it is likely the case that nicotine metabolites (e.g. cotinine) interfered with radioligand binding at the 24 h post-cessation imaging session, such that high-affinity nAChRs may, indeed, have been upregulated 24 h post-cessation (as evidenced by rodent studies^24,27,80). Moreover, α₄β₂* nAChR levels were found to have normalized between 6–12 weeks into abstinence,²⁶ which extends initial post-mortem findings.⁷¹ Concordantly, Mamede and colleagues⁸¹ used 5IA SPECT imaging to show that the binding potential of α₄β₂* nAChRs is decreased on average by 33.5% in widespread brain areas, including in frontal and temporal cortices, 4 h after smoking a cigarette relative to non-smokers, which is due to the presence and continued binding of nicotine to neuronal nAChRs. The binding potential of α₄β₂* nAChRs was significantly increased at 10 days post-cessation by 25.7% on average, providing additional evidence of upregulated receptors during withdrawal. These levels returned to non-smoker levels by 21 days post-cessation. Together, these results suggest that the upregulation of neuronal nAChRs from smoking is an underlying neurobiological correlate of the early symptoms of nicotine withdrawal, but that other circuits may underlie relapse after nAChR levels have normalized.

Nicotine withdrawal symptomatology

Nicotine withdrawal in smokers

An early study by Shiffman and Jarvik⁸² showed that physical and psychological withdrawal symptoms were present between 2 days and 14 days post-cessation in abstaining smokers. Hughes⁸³ showed that 2-day abstainers had increased anxiety, restlessness and difficulty concentrating relative to precessation assessments. Since these early studies, a better understanding of both the time course of withdrawal symptoms and the relationships between withdrawal symptoms, craving and relapse propensity has been reached.

Smokers experiencing nicotine withdrawal report an array of cognitive and affective symptoms as nicotine levels decline in the body.⁶ The cognitive symptoms of nicotine withdrawal include concentration difficulty, working memory problems and attentional impairment, and these symptoms have been observed between 0.5 h and 2 days post-cessation.^9,83–95 The affective symptoms of nicotine withdrawal include anxiety, irritability and restlessness, and these symptoms have been observed between 1.5 h and 2 days post-cessation.^{83,88,90,92,94} An early report observed that cognitive and affective symptoms in abstaining smokers were resolved by 30 days post-cessation,⁸³ but the majority of these studies did not assess withdrawal symptoms beyond 2 days post-cessation. Thus, the precise time course of cognitive and affective symptoms during nicotine withdrawal has not been entirely determined. As nicotine plasma levels decline post-cessation, craving for nicotine tends to increase,⁹⁴ and craving has been observed in conjunction with cognitive and affective withdrawal symptoms in many of these studies.^{82,83,88,91–93} Symptoms of withdrawal and craving for nicotine in smokers abstaining from tobacco products combine to encourage relapse and propagate addiction.

Relapse propensity has been shown to be mediated by the severity of nicotine withdrawal symptoms.^96,97 For example, it has recently been demonstrated that deficits in cognitive functioning during withdrawal are predictive of relapse.⁹⁸ In this study, working memory performance during abstinence predicted relapse throughout a 7-day withdrawal period. Smokers with lower performance tended to relapse more quickly and at greater frequency than smokers with higher performance. Taken together, nicotine withdrawal involves the generation of negative cognitive and affective symptoms, which in turn function to promote relapse, and relapse propensity is mediated by the severity of withdrawal symptoms.^99,100

Examining nicotine withdrawal in rodents: Cognitive symptoms and the β₂ subunit of neuronal nAChRs

Nicotine withdrawal is characterized by impaired cognitive functioning. Multiple studies have shown that rodents withdrawn from chronic nicotine have impaired learning.^{27,40,41,80,101,102} Importantly, impairment was observed in tasks that require hippocampus-dependent learning.^103–106 The β₂ subunit of neuronal nAChRs underlies learning impairment during nicotine withdrawal, as β₂^−/− mice exhibited no deficits in hippocampus-dependent learning tasks during nicotine withdrawal relative to wild-type control mice.^40,41 Additionally, nicotine withdrawal can be precipitated by the administration of the high-affinity nAChR antagonist dihydro-beta-erythroidine (DHβE),¹⁰⁷ which has high binding affinity to α₄β₂* nAChRs. Whereas chronic nicotine-treated wild-type control animals had impaired learning when DHβE was administered systemically or directly into the hippocampus, chronic nicotine-treated β₂^−/− mice did not have impaired learning when DHβE was administered.^38,40 Additionally, antagonizing α₄β₂* nAChRs increased impulsivity in wildtype mice, suggesting the involvement of β₂ subunit-containing nAChRs in causing impulsivity during nicotine withdrawal.^36,108 Combined, these studies support the role of β₂ subunit-containing nAChRs in generating cognitive impairment during nicotine withdrawal.

Upregulation of β₂ subunit-containing nAChRs underlies learning impairment during nicotine withdrawal. In support, the duration of α₄β₂* nAChR upregulation in the hippocampus during nicotine withdrawal paralleled the duration of impairments observed in hippocampus-dependent learning.²⁷ Chronic nicotine administration causes nAChRs to desensitize¹⁶ and upregulate.^60,61 As nicotine clears from the system during withdrawal, receptors return from being desensitized and are maintained in an upregulated state.^19,20,24,27 During this time, endogenous ACh is the only ligand available to bind to the re-sensitized and increased nAChRs. Combined, these receptor-level changes during nicotine withdrawal may produce a hypersensitive nAChR system that could lead to cognitive impairment.

Affective symptoms and the β₂ subunit of neuronal nAChRs

Nicotine withdrawal is also characterized by changes in affect, and the β₂ subunit of neuronal nAChRs is involved in the generation of affective symptoms during nicotine withdrawal. Rodent studies have found that nicotine withdrawal produces anhedonia, which is defined by the American Psychological Association as the reduced ability to obtain pleasure from rewarding stimuli.^109–111 Administration of the high-affinity nAChR antagonist DHβE produced anhedonia in chronic nicotine-treated rodents, which suggests the involvement of β₂ subunit-containing nAChRs in this negative affective state during nicotine withdrawal.³⁹ Moreover, the β₂ subunit of neuronal nAChRs is involved in anxiogenesis during nicotine withdrawal, as β₂^−/− mice did not show signs of increased anxiety during nicotine withdrawal relative to wild-type control mice.³⁷ Taken together, nicotine withdrawal produces cognitive and affective symptoms that promote relapse and involve changes in β₂ subunit-containing receptor functioning. Pharmacotherapies that target β₂ subunit-containing nAChRs may help alleviate these symptoms.

Pharmacotherapies

Nicotine replacement therapies

The easiest way for smokers to relieve nicotine withdrawal symptoms is to smoke. The rationale behind nicotine replacement therapies (NRTs) is to administer nicotine by means other than smoking. Several forms of NRT that have been approved by the Food and Drug Administration (FDA) are currently available, including gum, transdermal patch, lozenge and intranasal spray.^112,113 A recent meta-analysis of 117 placebo-controlled clinical trials found a 6-month abstinence rate pooled risk ratio for NRT treatments to be 1.60 (95% CI: 1.53 to 1.68) relative to placebo treatment¹¹⁴; the authors conclude that all forms of commercially available FDA-approved NRTs are effective aides to smoking cessation.

Although NRTs are largely considered effective, studies have shown mixed results in terms of NRTs’ ability to reduce craving and symptoms of nicotine withdrawal.^{7,9,115,116–118} For example, an early study by West et al.¹¹⁶ showed that nicotine gum dose dependently reduced certain affective symptoms (e.g. depressed mood) yet failed to reduce cognitive symptoms (e.g. concentration difficulty) in smokers going through overnight withdrawal. A later study found that administration of nicotine nasal spray effectively reduced both cognitive and affective withdrawal symptoms yet did not reduce craving in overnight abstainers and that these effects were sex and dose dependent.⁹ Additionally, one study found that performance on a working memory task was significantly improved by the administration of nicotine gum in smokers going through 12 h of abstinence.^7,94 One rodent study found that acute nicotine administration restored cognitive functioning in mice undergoing withdrawal from chronic nicotine.¹⁰² The discrepant findings across NRT clinical literature could be attributed in large part to the type and dosing regimen of treatment used between studies.¹¹⁹ The studies mentioned here suggest that NRTs can be effective in relieving certain withdrawal symptoms during short-term nicotine withdrawal.

Electronic cigarettes are widely available in the United States. Electronic cigarettes are not approved by the FDA as smoking cessation aides, and concerns have been raised regarding their potentially adverse health effects and abuse liability.^120–122 The intention behind NRTs is to assist in cessation, whereas electronic cigarettes, similarly to smokeless tobacco, are often used as replacements or supplements for tobacco cigarettes.¹²³ Additionally, clinical trials have provided mixed results for the efficacy of electronic cigarettes to reduce withdrawal symptom severity and aide in cessation, and many of these studies fail to report nicotine abstinence rates (as opposed to tobacco product abstinence rates).^124–127 Moreover, there have been questions raised regarding the impurities in the nicotine delivery device cartridge liquid.¹²⁸ Although electronic cigarettes have been discussed by some from a ‘harm reduction’ perspective,^129,130 there are adverse health effects, particularly on cardiovascular functioning, associated with nicotine administration.^10,131–134 Additionally, studies (reviewed in^135,136) suggest that activation of nAChRs is mechanistically involved in carcinogenesis and that nicotine may be one of the compounds involved in causing cancer. Thus, more studies are needed to determine the safety and efficacy of electronic cigarettes for smoking cessation.¹³⁷

Varenicline

Varenicline is a partial agonist of α₄β₂ nAChRs, a full agonist at the α₇ nAChR and has been approved by the FDA for smoking cessation.^138,139 Clinically, varenicline treatment resulted in 43.9% tobacco product abstinence at 12 weeks following a target quit date and 23.0% abstinence at 52 weeks¹⁴⁰ (see Table 1). For a review and meta-analysis of varenicline’s clinical efficacy, see Hays et al.¹⁴¹ and Cahill et al.¹⁴², respectively.

Table 1.

Placebo-controlled clinical trials examining tobacco product abstinence rates from varenicline and bupropion SR treatments

Drug	Treatment regimen	Continuous abstinence or point prevalence rate (%)				Final N	Report
		7 weeks	12 weeks	24 weeks	52 weeks
Varenicline vs. Bupropion SR vs. Placebo
Varenicline	10 mg BID for 12 weeks	–	44.0***,‡	29.5***,†	21.9***	352	Gonzales et al. 2006188
Bupropion SR	150 mg BID for 12 weeksa	–	29.5***	20.7***	161***	352
Placebo		–	17.7	10.5	84	344
Varenicline	10 mg BID for 12 weeks	–	43.9***,‡	29.7***,†	23.0***,†	344	Jorenby et al. 2006140
Bupropion SR	150 mg BID for 12 weeksa	–	29.8***	20.2**	14.6	342
Placebo		–	17.6	13.2	10.3	341
Varenicline	0.3 mg QD for 6 weeks	254*	16.7	9.5	7.9	126	Nides et al. 2006153
	1.0 mg QD for 6 weeks	31.0**	15.1	9.5	5.6	126
	10 mg BID for 6 weeks	40.8***	28.8**	20.8**	144**	125
Bupropion SR	150 mg BID for 7 weeks	28.6**	19.8*	10.3	6.3	126
Placebo		13.8	10.6	7.3	4.9	123
Varenicline vs. Placebo
Varenicline	0.5 mg BID for 12 weeks	36.3***	44.0***	–	18.5***,b	123	Oncken et al. 2006154
	10 mg BID for 12 weeks	39.8***	49.4***	–	22.4***,b	146
Placebo		10.9	11.6	–	3.9b	40
Varenicline	10 mg BID for 12 weeksa	–	50.3***	38.2**	–	165	Wang et al. 2009190
Placebo		–	31.6	25.0	–	168
Bupropion SR vs. Placebo
Bupropion SR	100 mg QD for 7 weeks	28.8*	242*	24.2	19.6	153	Hurt et al. 1997165 b,c
	150 mg QD for 7 weeks	38.6***	261**	27.5**	22.9*	153
	300 mg QD for 7 weeks	44.2***	29.5***	26.9*	23.1**	156
Placebo		19.0	144	15.7	124	153
Bupropion SR	150 mg BID for 9 weeksa	–	–	34.8***	30.3***	169	Jorenby et al. 1999166 b
Placebo		–	–	18.8	15.6	82
Bupropion SR	150 mg BID for 7 weeksa	46.0***	32.0e	25.0***	21.0***	527	Tønnesen et al. 2003189 d
Placebo		23.0	18.0	13.0	110	180

Adapted from^{140,153,154, 165,166, 188–190}.

^*P ≤ .05 compared to placebo.

^**P ≤ .01 compared to placebo.

^***P ≤ .001 compared to placebo.

^†P ≤ .01 compared to bupropion SR.

^‡P ≤ .001 compared to bupropion SR.

^aDose titration within first 7 days of treatment.

^bPoint prevalence rate only at 52 weeks for¹⁵⁴ and at all time points for^165,166.

^cTime points for continued abstinence rate (CAR) findings are: 6 weeks, 3 months, 6 months and 12 months, respectively.

^dTime points for CAR findings are: 7 weeks, 12 weeks, 26 weeks and 52 weeks, respectively.

^eStatistics not performed.

Varenicline has been shown to be effective at reducing both craving and the reward associated with smoking in abstaining smokers.^140,143,144 The ability of varenicline to reduce the reward associated with smoking could be due to its activity on the mesolimbic DA system. Specifically, microdialysis study has shown that varenicline significantly reduced the phasic release of DA in the NAcc upon an acute nicotine challenge injection relative to animals that received a nicotine challenge injection but received a saline pretreatment.¹³⁸ Although varenicline has the ability by itself to elicit phasic DA release in the NAcc via β₂ subunit-containing nAChRs,^138,145 animal studies have shown that varenicline did not by itself induce conditioned place preference.¹⁴⁶ Moreover, varenicline reduced the reward associated with nicotine¹⁴⁷ and reduced nicotine self-administration.¹⁴⁸ Combined, these studies suggest that varenicline is effective in reducing both craving and the reward associated with nicotine administration during short-term withdrawal but does not by itself carry abuse liability.

Varenicline reduces cognitive and certain affective symptoms during short-term nicotine withdrawal. An initial study in mice found that varenicline ameliorated impairment in hippocampus-dependent learning when animals were withdrawn from chronic nicotine.¹⁴⁹ Moreover, it was found that varenicline reduced the anhedonic state normally experienced by rodents during nicotine withdrawal.¹⁵⁰ However, one animal study showed that varenicline might not be effective in reducing anxiety during short-term nicotine withdrawal.¹⁵¹ Varenicline treatment reduced negative affect and improved performance in neurocognitive tests of sustained attention and working memory in smokers undergoing short-term withdrawal.^143,152 Taken together, varenicline has been found to be effective in reducing some cognitive and certain affective symptoms during short-term nicotine withdrawal.

Varenicline use has been associated with side effects that discourage treatment and consequently encourage relapse. The most prominent adverse effect associated with varenicline is nausea.¹⁴² At the 1.0 mg/kg BID dose of varenicline, one study reported a 52% incidence rate of nausea.¹⁵³ However, it has been observed that titrating dosage of varenicline over the first week of treatment is associated with a reduction in the incidence rate of nausea.¹⁵⁴ Mechanistically, the side effects of varenicline are thought to be due in part to effects on non- α₄β₂ nAChRs, such as peripheral α₃β₄* and central α₇ nAChRs.^155–157 Ahmed et al.¹⁵⁸ reviewed 25 case reports that detailed neuropsychiatric adverse events, including depressed mood, insomnia, suicidal ideation and aggression, as a result of varenicline treatment. Importantly, 68% of these cases were reported from patients with a history of mental illness. However, a recent analysis of eight randomized, placebo-controlled clinical trials found that varenicline treatment decreased neuropsychiatric adverse events in patients with no history of mental illness.¹⁵⁹ Thus, the probability of developing neuropsychiatric adverse reactions to varenicline is greater in patients with a history of mental illness, but there appears to be less concern of neuropsychiatric symptom development in treatment-seeking smokers without a history of mental illness.

Bupropion

Bupropion is a DA and norepinephrine (NE) reuptake inhibitor,^160,161 as well as a non-competitive nAChR antagonist with selectivity to β₂ subunit-containing nAChRs.¹⁶² Bupropion is an FDA-approved antidepressant with documented clinical efficacy,¹⁶³ and bupropion sustained-released (SR) has been approved for smoking cessation in the United States as of 1997.¹⁶⁴

An initial clinical trial found that a 7-week daily treatment regimen of bupropion SR in smokers dose dependently increased tobacco product abstinence rates at 3, 6 and 12 months after a target quit date compared to placebo-treated smokers.¹⁶⁵ A study using a 9-week treatment regimen showed that bupropion-treated smokers were, on average, 2.3 times more likely to abstain at 6 and 12 months relative to placebo-treated smokers.¹⁶⁶

Combination treatment with a nicotine patch showed that smokers co-treated with bupropion SR were three times more likely to continue abstinence at 12 months relative to placebo-treated smokers, but this difference was not significantly different from either bupropion SR or nicotine patch treatment alone.¹⁶⁶ Additional evidence has suggested that combination treatment of bupropion SR and participant-selected NRT is not additionally effective relative to either treatment alone at 6-month follow-up¹⁶⁷ (bupropion SR: 26.7% abstinent, NRT: 24.2% abstinent, combination therapy: 23.4% abstinent). Overall, Stapleton et al.¹⁶⁷ conclude that bupropion SR might be best considered a second-line treatment for individuals not responding to NRT or varenicline.

Bupropion has been shown to reduce the cognitive and affective symptoms during nicotine withdrawal.¹⁶⁸ Specifically, working memory ability was significantly improved in bupropion SR-treated vs. placebo-treated overnight abstainers. An earlier study found that high-dose bupropion SR significantly reduced self-reported difficulty concentrating and craving during withdrawal relative to placebo-treated smokers.¹⁶⁹ Predominant adverse reactions to bupropion SR were found at moderate and high doses and included dry mouth and insomnia.^165,166 Preclinical studies have found that bupropion treatment reduced anhedonia during nicotine withdrawal.^170,171 A microdialysis study extended initial findings by showing that bupropion treatment increased DA release in the NAcc.¹⁷² An additional study found that administration of bupropion to nicotine-withdrawn mice reduced hippocampus-dependent learning impairment.¹⁷³ An in vivo microdialysis study found that acute nicotine administration increased extracellular NE release in the hippocampus,¹⁷⁴ and it has since been suggested that hippocampus-dependent learning impairment during nicotine withdrawal may be due in part to alterations in NE signalling in the hippocampus, as drugs that function primarily to increase NE ameliorate learning impairment.^173,175 The increase in hippocampal NE is one possible mechanism by which bupropion might be therapeutic for treating nicotine addiction. A second mechanism by which bupropion could aid in cessation and maintain abstinence is by antagonizing β₂ subunit-containing nAChRs,¹⁶² which would function to reduce activity in a potentially hypersensitive nAChR system. It is also possible that the therapeutic action of bupropion involves combined activity on NE- and ACh-mediated neural circuits. Additionally, metabolites of bupropion, such as a hydroxybupropion, have been found to reduce affective and somatic symptoms in nicotine-withdrawn mice,¹⁷⁶ and human studies have further suggested a role of bupropion metabolites in its therapeutic efficacy.^177–179 Delineation of the specific mechanism by which bupropion aids in smoking cessation has not been fully determined, and identification of this mechanism could aid in the development of novel smoking cessation aides.

WHAT IS NEW AND CONCLUSION

New developments

Nicotine vaccine

A nicotine vaccination study was recently published and showed that smokers treated with the vaccine showed significantly reduced neuronal α₄β₂* nAChR occupancy rates compared to prevaccination receptor occupancy rates after intravenous administration of nicotine.¹⁸⁰ This study was supported by an initial finding in rats, showing that a 2-h pretreatment with a nicotine vaccine reduced the amount of nicotine found in the brain by 35% relative to placebo-treated control animals.¹⁸¹ The nicotine vaccine works by having the treated individual create antibodies that bind to nicotine upon smoking, which serves to reduce the amount of nicotine able to cross the blood–brain barrier. The study conducted by Esterlis et al.¹⁸⁰ also showed that the 20-week vaccination regimen led to a 40% reduction in the amount of cigarettes smoked and significantly reduced craving. The nicotine vaccine is promising in terms of reducing the amount of nicotine available to bind to neuronal nAChRs, which would likely reduce the reward associated with smoking and help to prolong abstinence. Adverse reactions to nicotine vaccines predominantly include injection site tenderness, general discomfort, myalgia and headache.^182,183 Progress is continuing to be made with nicotine vaccine research, but results from clinical trials thus far have provided mixed evidence that nicotine vaccines promote abstinence at greater rates than currently available pharmacotherapies.¹⁸⁴

Although the focus of this review is on therapeutics acting on β₂ subunit-containing nAChRs, it is noteworthy that other targets exist. For example, nortriptyline is a tricyclic antidepressant that primarily functions to increase extracellular levels of serotonin and has been shown to have long-term smoking cessation efficacy.¹⁸⁵ Additionally, a selective and potent antagonist at α₃β₄* nAChRs was found to dose dependently reduce nicotine self-administration in rats without affecting natural reward seeking.¹⁸⁶ Moreover, recent developments have shown that antagonists at high-affinity nAChRs containing the α₆ subunit mediate nicotine-evoked mesolimbic DA release and may be effective smoking cessation aides.¹⁸⁷ Although the most effective pharmacotherapies have been found to target β₂ subunit-containing nAChRs, other neuronal circuits that are affected by nicotine have been explored as therapeutic avenues for smoking cessation.

Concluding remarks

One pathway by which nicotine produces reward and addiction is through binding to neuronal β₂ subunit-containing nAChRs. Chronic nicotine desensitizes β₂ subunit-containing nAChRs, and β₂ subunit-containing nAChRs upregulate after chronic nicotine exposure. Upregulation of neuronal β₂ subunit-containing nAChRs persists during withdrawal and is observed in concordance nicotine withdrawal symptoms. Pharmacotherapies, including varenicline and NRTs, are agonists of α₄β₂* nAChRs, and these compounds reduce cognitive and affective symptoms of withdrawal, reduce the reward associated with smoking and reduce craving. Bupropion is a smoking cessation aide that functions similarly by reducing nAChR system activity but also works by blocking the reuptake of DA and NE.