Glutamatergic neurotransmission is modulated by two different types of receptors: ionotropic glutamate (iGlu) receptors and metabotropic glutamate (mGlu) receptors. iGlu receptors are located postsynaptically and modulate fast glutamatergic neurotransmission. Nicotine self-administration resulted in changes in iGlu and mGlu receptor levels, which likely contributed to the cue-induced reinstatement of nicotine-seeking behavior (Gipson et al. 2013; Liechti et al. 2007). Moreover, pharmacologically targeting iGlu receptors with the NMDA receptor antagonists ifenprodil and acamprosate attenuated the cue-induced reinstatement of nicotine-seeking behavior (Gipson et al. 2013; Pechnick et al. 2011). The development of novel pharmacotherapies for the treatment of drug dependence, however, has focused primarily on mGlu receptors because of the side effects of iGlu receptor antagonists in humans (for review, see Gass and Olive 2008). Metabotropic glutamate receptors have attracted much interest in recent years as targets for novel therapeutics in the treatment of nicotine dependence (Markou 2007). Compared with iGlu receptors, the activity of mGlu receptors is more slow acting and modulatory, presumably resulting in a reduced side effect profile. During nicotine withdrawal, presynaptic mGlu2/3 receptors were downregulated in the VTA and NAc, and mGlu2/3 receptor activation in these brain areas induced by the agonist LY379268 attenuated the cue-induced reinstatement of nicotine seeking (Liechti et al. 2007). N-acetylcysteine, a compound that has been suggested to increase the glutamatergic tone of presynaptic mGlu2/3 receptors (Kupchik et al. 2012), similarly attenuated nicotine reinstatement elicited by environmental cues (Ramirez-Nino et al. 2013). Furthermore, the blockade of postsynaptic mGlu5 receptors (Bespalov et al. 2005) and mGlu1 receptors (see Fig. 1c adapted from Dravolina et al. 2007) decreased cue-induced reinstatement. Specifically, nicotine seeking was attenuated by administration of the mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine hydrochloride (MPEP; Bespalov et al. 2005) or mGlu1 receptor antagonist 3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM; Dravolina et al. 2007). In parallel to cue-induced reinstatement, EMQMCM also decreased the nicotine-induced reinstatement of nicotine-seeking behavior in rats (see Fig. 1d adapted from Dravolina et al. 2007). These results of experimental studies in animals on the role of mGlu receptors in nicotine reinstatement demonstrated that pharmacologically targeting glutamatergic neurotransmission effectively attenuates both cue- and nicotine-induced reinstatement and may attenuate relapse to tobacco smoking in humans. In fact, these experimental animal studies resulted in a Phase I clinical trial by Novartis that assessed the efficacy and safety of the mGlu5 receptor antagonist AFQ056 as a treatment option for voluntary smoking cessation. This clinical trial has been completed, but the results of the study have not yet been published (Clinicaltrials.gov 2007).

As described in Sects. 2.1 and 2.3, dopaminergic neurotransmission, which is mediated by G-protein-coupled dopamine receptors, in the mesolimbic circuit and striatum is critically involved in drug dependence (see chapter entitled The Role of Mesoaccumbens Dopamine in Nicotine Dependence; this volume). The cue-induced reinstatement of nicotine-seeking behavior can be attenuated by pharmacological compounds that decrease dopaminergic tone, including antagonists of dopamine D₁ and D₂ receptors (Liu et al. 2010), D₃ receptors (Khaled et al. 2010), and D₄ receptors (see Fig. 1e adapted from Yan et al. 2013). Consistent with these findings, a reduction of dopaminergic tone with the α-type peroxisome proliferator-activated receptor (PPAR-α) agonist clofibrate decreased cue-induced reinstatement in squirrel monkeys (Panlilio et al. 2012). These studies suggest that the pharmacological inhibition of dopaminergic neurotransmission consistently attenuates the cue-induced reinstatement of nicotine-seeking behavior. Furthermore, nicotine-induced reinstatement is similarly attenuated by dopamine D₃ and D₄ receptor agonists (Andreoli et al. 2003; Yan et al. 2013, see Fig. 1f adapted from Yan et al. 2013) and PPAR-α agonists (Mascia et al. 2011; Panlilio et al. 2012). The inhibition of dopaminergic neurotransmission, therefore, appears to be an interesting possibility in the identification for novel smoking cessation medication targets.

GABA is the main inhibitory transmitter in the central nervous system. Inhibitory GABAergic activity attenuates dopaminergic mesocorticolimbic neurotransmission through GABA interneurons located in the VTA, medium spiny GABA neurons in the NAc, and GABAergic projections to the VTA from the NAc, ventral pallidum, and pedunculopontine tegmental nucleus (Klitenick et al. 1992). Inhibitory GABA receptors, therefore, would have to be activated by full agonists or positive allosteric modulators to decrease excitatory neurotransmission in the VTA which, as discussed above, generally attenuates the reinstatement of nicotine seeking. GABAergic neurotransmission is regulated through ionotropic GABA_A and GABA_C receptors and metabotropic GABA_B receptors (Bormann 1986). Of these various GABA receptor subtypes, G-protein-coupled GABA_B receptors are primarily of interest in the treatment of nicotine dependence (Li et al. 2014; Vlachou and Markou 2010). GABA_B receptor activation induced by the GABA_B receptor agonist CPG44532 attenuated the cue-induced reinstatement of nicotine seeking in rats (see Fig. 1g adapted from Paterson et al. 2005). Similar to the GABA_B agonist, the GABA_B receptor positive allosteric modulator BHF177 also decreased cue-induced reinstatement in rats (Vlachou et al. 2011). The effects of GABA_B agonists on the nicotine-induced reinstatement of nicotine seeking have been less extensively explored. One study reported that baclofen decreased reinstatement induced by nicotine priming (see Fig. 1h adapted from Fattore et al. 2009). Furthermore, the GABA_B receptor agonist baclofen was suggested to potentially facilitate smoking cessation in humans (Cousins et al. 2001). These studies indicate that GABA_B receptors may be a promising target in the treatment of smoking cessation. Moreover, it has been proposed that GABA_B positive allosteric modulators may be particularly effective in the treatment of nicotine dependence because of their modulatory actions at GABA_B receptors that may result in an improved side effect profile and decreased development of tolerance to these compounds compared with GABA_B full receptor agonists (Guery et al. 2007; Vlachou et al. 2011).

Of the two endocannabinoid receptors cloned to date, CB₁ receptors are of primary interest in the treatment of dependence on drugs of abuse (Howlett et al. 2004) because these receptors are found on glutamatergic and GABAergic inputs to dopaminergic neurons (Gardner 2005). In contrast, CB₂ receptors are primarily localized on immune cells in both the central and peripheral nervous systems (Howlett 2002). As expected, the reinstatement of nicotine-seeking behavior was unaffected by the CB₂ receptor antagonist AM630 or CB₂ receptor agonist AM1241 (Gamaleddin et al. 2012b). CB₁ receptors located on presynaptic glutamatergic neurons in the VTA are hypothesized to decrease the inhibitory control that GABAergic neurons exert on dopaminergic neurons (Schlicker and Kathmann 2001). Consequently, CB₁ receptor activation would result in the increased firing activity of VTA dopamine neurons (French 1997; French et al. 1997) and increased dopamine release in the NAc (Gardner and Vorel 1998; Tanda et al. 1997), suggesting therapeutic potential for CB₁ receptor antagonism in attenuating the reinstatement of nicotine seeking. Indeed, antagonism of the CB₁ receptor consistently attenuated the cue-induced reinstatement of nicotine-seeking behavior, demonstrated by the administration of rimonabant (Diergaarde et al. 2008; Forget et al. 2009, see Fig. 1i adapted from Forget et al. 2009), SR141716 (Cohen et al. 2005; de Vries et al. 2005), and AM404 (Gamaleddin et al. 2013) in rats. The CB_1/2 receptor agonist WIN 55,212-2 facilitated the cue-induced reinstatement of nicotine-seeking behavior (Gamaleddin et al. 2012a), presumably by activating CB₁ receptors. Furthermore, antagonism at CB₁ receptors attenuated nicotine-induced reinstatement in rats, demonstrated by the administration of rimonabant (see Fig. 1j adapted from Forget et al. 2009), AM251 (Shoaib 2008), and AM404 (Gamaleddin et al. 2013). Additionally, reinstatement induced by the combination of both cue presentation and nicotine priming was attenuated by administration of the CB₁ receptor antagonist AM251 (Shoaib 2008).

After various clinical trials assessed the efficacy of rimonabant as a smoking cessation medication, it was approved for this purpose in various European countries in 2006. Inopportunely, treatment of smoking cessation with rimonabant was halted in 2007 after reports of severe side effects that included anxiety and depression (Moreira and Crippa 2009). The development of pharmacological compounds that target the endocannabinoid system in smoking cessation is therefore currently directed toward developing compounds that indirectly target endocannabinoid neurotransmission, including anandamide transport inhibitors and fatty acid amid hydrolase (FAAH). Anandamide is one of the endogenous ligands that act at cannabinoid receptors (Giang and Cravatt 1997) and eliminated by reuptake into cells by anandamide transporters and subsequent hydrolysis by FAAH (Beltramo et al. 1997; Cravatt et al. 1996). The enhancement of endocannabinoid signaling by inhibiting the reuptake or hydrolysis of anandamide attenuated both cue- and nicotine-induced reinstatement of nicotine seeking (Forget et al. 2009; Gamaleddin et al. 2011). Notably, inhibiting the reuptake or hydrolysis of anandamide opposes the effects of CB₁ receptor antagonists. That is, CB₁ receptor antagonists attenuate endocannabinoid signaling, while FAAH inhibitors and anandamide transport inhibitors enhance endocannabinoid signaling. The same direction of effect (i.e., decrease) on the reinstatement of nicotine seeking by these seemingly opposing mechanisms may be due to the action of CB₁ receptor antagonists on neurocircuits that express endocannabinoid ligands other than anandamide (Scherma et al. 2008). Interestingly, compounds that target FAAH may exert dual actions on the reinstatement of nicotine seeking because FAAH also breaks down fatty acid amides that can activate PPAR-α (Fegley et al. 2005). As discussed above, the activation of PPAR-α decreases the cue-induced reinstatement of nicotine seeking, presumably by decreasing dopaminergic neurotransmission, further emphasizing the potential of FAAH-inhibiting compounds in treating the reinstatement of nicotine seeking.

Whereas the aforementioned neurotransmitter systems have been the most extensively explored as targets for pharmacotherapy in attenuating the reinstatement of nicotine seeking, several other neurotransmitter systems have been suggested in the development of novel smoking cessation aids. Serotonergic receptors, for example, modulate dopaminergic neurotransmission and were suggested as potential targets in smoking cessation medications (for review, see Fletcher et al. 2008). The attenuation of serotonergic neurotransmission by the 5-HT_2C receptor antagonists Ro60-0175 and locaserin decreased both nicotine- and cue-induced reinstatement (Fletcher et al. 2012; Higgins et al. 2012). Similarly, the modulation of noradrenergic neurotransmission was shown to effectively reduce both cue- and nicotine-induced reinstatement with the noradrenergic α1 receptor antagonist prazosin (Forget et al. 2010b) and β-blocker propranolol, supporting the involvement of noradrenergic neurotransmission in cue-induced reinstatement (Chiamulera et al. 2010). Finally, the T-type calcium channel antagonist TTA-A2 also attenuated both cue- and nicotine-induced reinstatement (Uslaner et al. 2010), potentially by modulating glutamatergic or dopaminergic neurotransmission (Uslaner et al. 2012). Tricyclic antidepressants, which decrease both serotonergic and noradrenergic neurotransmission, have been suggested to facilitate smoking cessation in humans (Edwards et al. 1989; Hall et al. 1998; Prochazka et al. 1998). However, the severe side effects of tricyclic antidepressants, including cardiovascular effects and the severity of overdose symptoms (Biggs et al. 1977; Roose et al. 1991), make these compounds unfavorable as smoking cessation aids. The reversible monoamine oxidase-A (MAO-A) inhibitor moclobemide, which reduces both serotonergic and noradrenergic neurotransmission similarly to tricyclic antidepressants but with a more favorable side effect profile (Stabl et al. 1989), attenuated smoking cessation in a study group of heavy smokers (Berlin et al. 1995). These results suggest that MAO-A inhibitors may be preferred over tricyclic antidepressants as smoking cessation aids. However, bupropion remains the main antidepressant used as a smoking cessation medication (Tables 1 and 2).