Trace amine-associated receptor 1 and drug abuse

Abstract

Trace amine-associated receptor 1 (TAAR1) is the best characterized receptor selectively activated by trace amines. It is broadly expressed in the monoaminergic system in the brain including ventral tegmental area (VTA), nucleus accumbens (NAc), dorsal raphe (DR) and substantial nigra (SN). Extensive studies have suggested that TAAR1 plays an important role in the modulation of monoaminergic system, especially dopamine (DA) transmission which may underlie the mechanisms by which TAAR1 interventions affect drug abuse-like behaviors. TAAR1 activation inhibits the rewarding and reinforcing effects of drugs from different classes including psychostimulants, opioid and alcohol as well as drug-induced increase in DA accumulation. The mechanisms of TAAR1’s function in mediating drug abuse-like behaviors are not clear. However, it is hypothesized that TAAR1 interaction with DA transporter (DAT) and dopamine D2 receptor (D2) and the subsequent modulation of cellular cascades may contribute to the effects of TAAR1 in regulating drug abuse. Further studies are needed to investigate the role of TAAR1 in other drugs of abuse-related behaviors and its safety and efficacy for prolonged medications. Together, TAAR1 inhibits drug-induced DA transmission and drug abuse-related behaviors. Therefore, TAAR1 may be a promising therapeutic target for the treatment of drug addiction.

1. Introduction

The research and interest in trace amines, which include β-phenylethylamine (PEA), p-tyramine (TYR), tryptamine (TRP), and p-octopamine (OCT), dates back to almost 50 years ago (Boulton, 1974, 1980; Sandler et al., 1980). Trace amines (TA) represent a group of endogenous amines which express at levels that are at least 100-fold lower than those of classic monoamine transmitters such as dopamine (DA), serotonin and norepinephrine (NE) (Berry, 2004; Grandy, 2007). Although trace amines share the most obvious structural similarity with the well-established classic amines (Figure 1) and are implicated to exert physiological effects in the brain (Berry, 2007; Burchett & Hicks, 2006), their functions remained largely unknown for decades due to the lack of specific receptors (Miller, 2011). Fortunately, in 2001, a new family of G-protein coupled receptors (GPCR), termed Trace-Amine Receptors (TAs), were cloned by two independent groups (Borowsky et al., 2001; Bunzow et al., 2001) where two members of this family (TA₁ and TA₂) were reported to be selectively activated by trace amines.

Figure 1.

Structures of representative trace amines and classic monoamine transmitters.

Four years later, a new TA receptor nomenclature was suggested to be changed to Trace Amine-Associated Receptor (TAAR) since only TA₁ and TA₂ can be activated by trace amines (Lindemann & Hoener, 2005). Thus, TA₁ was renamed as TAAR1 (Lindemann & Hoener, 2005). However, it was later suggested by the International Union of Pharmacology (IUPHAR) that the name of receptor remained TA₁ in 2009 (Maguire et al., 2009). While the IUPHAR’s suggestion is based on the tradition that receptor be named for their endogenous ligands (Gainetdinov, Hoener, & Berry, 2018), it presents an uncomfortable condition in this field since almost all recent studies refer to this receptor as TAAR1 (Lam, Espinoza, Gerasimov, Gainetdinov, & Salahpour, 2015). Therefore, we will also be using the TAAR1 in this review for simplicity and consistency.

2. The implication of TAAR1 in the brain

Because TAAR4 (previous TA₂) is actually a pseudogene in humans (I. S. Liu, Kusumi, Ulpian, Tallerico, & Seeman, 1998; Maguire et al., 2009), most of the subsequent research interest focused on TAAR1. The human, rat and mouse TAAR1 coupled to a Gα_s class G-protein and increased cAMP accumulation (Borowsky et al., 2001; Bunzow et al., 2001; Hartmann, Sherriff, & Kent, 1995). While all TAARs (except for TAAR1) are expressed in the olfactory epithelium and shown to potentially function as a group of olfactory receptors (Dewan, Pacifico, Zhan, Rinberg, & Bozza, 2013; Liberles & Buck, 2006; J. Zhang, Pacifico, Cawley, Feinstein, & Bozza, 2013), TAAR1 mRNA was reported in several brain regions including amygdala, hippocampus, dorsal root ganglia and cerebellum (Borowsky et al., 2001). The expression of TAAR1 was also found in spinal cord and periphery where its role in regulating immune functions was overlooked, mainly because the TAAR1 presence was further shown in several key monoaminergic brain regions including ventral tegmental area (VTA), locus coeruleus (LC), dorsal raphe (DR), substantial nigra (SN), hypothalamus, striatum and frontal cortex (Di Cara et al., 2011; Lindemann et al., 2008; Revel et al., 2011; Xie et al., 2007). In addition, the gene for TAAR1 in humans is mapped to chromosome 6q23.2, a putative susceptibility locus for schizophrenia and mood disorders (Lindemann et al., 2005; Lindemann & Hoener, 2005). These studies strongly suggest that TAAR1 is involved in the modulation of monoaminergic system and therefore participate in the regulation of neuropsychiatric disorders like schizophrenia, depression, Parkinson’s disease, bipolar disorder and drug addiction.

The potential role of TAAR1 in modulation monoaminergic transmission was further suggested by studies of genetic animal models. From 2007 to 2011, three separate groups generated TAAR1 knockout animals (Di Cara et al., 2011; Lindemann et al., 2008; Wolinsky et al., 2007). It is shown that TAAR1-KO mice have no differences in sensor and motor functions like motor coordination and spontaneous locomotor activity, however, they demonstrate an impairment of sensorimotor gating function, which might be a phenotype related to schizophrenia (Wolinsky et al., 2007). They also have increased sensitivity to the behavioral and neurochemical responses to amphetamine and higher proportion of high affinity D2 receptors in the striatum (Lindemann et al., 2008; Wolinsky et al., 2007). Moreover, knockout of TAAR1 induces a remarkable increase in the spontaneous firing rate of VTA dopaminergic neurons and DR serotonergic neurons which might be due to TAAR1’s role in tonic activation of inwardly rectifying potassium ion channels in these neurons in order to reduce basal firing rate (Bradaia et al., 2009; Lindemann et al., 2008; Revel et al., 2011). In addition to TAAR1-KO animals, a transgenic TAAR1 overexpression model (TAAR1-OE) was generated in 2012 (Revel, Meyer, et al., 2012). Mice overexpressing TAAR1 show hyposensitivity to amphetamine which is the opposite of TAAR1-KO animals (Revel, Meyer, et al., 2012). However, they also show the similar augmented spontaneous firing rate in VTA dopaminergic neurons and DR serotonergic neurons as TAAR1-KO mice (Revel, Meyer, et al., 2012). This finding is difficult to interpret since in these animals, TAAR1 was overexpressed in the brain regions which do not normally express TAAR1 (Lam et al., 2015). Nevertheless, these studies further supported the role of TAAR1 in mediating dopaminergic, glutamatergic and serotonergic system and led to the excitement of exploring TAAR1 in different neuropsychiatric disorders.

Considering the fact that endogenous ligands of TAAR1 have effects through targets other than TAAR1 itself (Burchett & Hicks, 2006; Ledonne et al., 2010; H. N. Panas et al., 2010), however, the lack of highly selective ligands challenged the examinations whether modulations of TAAR1 could be potential therapeutic strategies to improve these neuropsychiatric disorders. Fortunately, the development of highly potent and selective full (RO5166017, RO5263390) (Revel et al., 2011; Revel et al., 2013) and partial (RO5203648, RO5263397) (Revel, Moreau, et al., 2012; Revel et al., 2013) agonists and first antagonist (EPPTB) (Bradaia et al., 2009) contributed substantially to the identification of TAAR1 biological functions (Figure 2). Among all the neuropsychiatric disorders that TAAR1 is involved in, drug addiction is best characterized. In this review, we will focus on the role of TAAR1 in regulating drug addiction and its underlying mechanisms. We will be discussing the effects of TAAR1 modulation in addiction to different drug classes, including psychostimulants (amphetamines/cocaine/nicotine), opioid (morphine) and alcohol with the studies of selective and characterized TAAR1 ligands and transgenetic animal models (Table 1).

Figure 2.

Structures of synthetic TAAR1 ligands (full agonists: RO5256390, RO5166017; antagonist: EPPTB; partial agonists: RO5203648, RO5263397, RO5273012)

Table 1.

Modulations of TAAR1 activity on addiction-related behaviors of drugs

Drug	TAAR1 Modulation	Species	Behavior	Effect	Reference
Amphetamine	TAAR1 knockout	mouse	locomotor activity	enhanced	(Wolinsky et al., 2007)
	TAAR1 overexpression	mouse	locomotor activity	reduced	(Revel, Meyer, et al., 2012)
	TAAR1 partial agonist RO5073012	mouse	locomotor activity	enhanced
	TAAR1 knockout	mouse	locomotor activity	enhanced	(Achat-Mendes et al., 2012)
	TAAR1 knockout	mouse	locomotor activity	enhanced	(Sukhanov et al., 2016)
		mouse	priming-induced reinstatement of amphetamine-induced CPP	enhanced
METH	TAAR1 knockout	mouse	locomotor activity	enhanced	(Achat-Mendes et al., 2012)
	TAAR1 knockout	mouse	METH-induced CPP	enhanced
	Non-functional allele of Taar1	mouse	METH intake	enhanced	(Harkness et al., 2015)
		mouse	METH-induced CTA and hypothermia	reduced
	TAAR1 polymorphism	human	METH craving	enhanced	(Loftis et al., 2019)
	TAAR1 partial agonist RO5263397	rat	expression of METH induced-behavioral sensitization	reduced	(Jing et al., 2014)
			METH self-administration	reduced
			cue- and drug-induced reinstatement of METH-seeking	reduced
	TAAR1 partial agonist RO5203648	rat	METH-stimulated hyperactivity	reduced	(Cotter et al., 2015)
			development of METH sensitization	reduced
			METH self-administration	reduced
	TAAR1 partial agonist RO5263397	rat	break-point for METH	reduced	(Pei et al., 2017)
			drug-primed reinstatement of METH-seeking	reduced
	TAAR1 partial agonist RO5263397	rat	METH-induced premature response	reduced	(Xue et al., 2018)
Cocaine	TAAR1 partial agonist RO5203648	rat and mouse	cocaine-induced hyperlocomotion	reduced	(Revel, Moreau, et al., 2012)
		rat	cocaine taking	reduced
	TAAR1 partial agonist RO5203648 and full agonist RO5256390	rat	cocaine-seeking	reduced	(Pei et al., 2014)
	TAAR1 partial agonist RO5203648	rat	drug-primed reinstatement of cocaine-seeking	reduced
	TAAR1 partial agonist RO5263397	rat	expression of cocaine-induced behavioral sensitization	reduced	(Thorn, Zhang, et al., 2014)
			development of cocaine-induced behavioral sensitization	no effect
			cocaine CPP	reduced
			cue- and drug-induced reinstatement	reduced
			elasticity of cocaine demand curve	reduced
	TAAR1 partial agonist RO5203648 and full agonist RO5256390	rat	cocaine dose-response curve	reduced	(Pei et al., 2015)
	TAAR1 full agonist RO5256390	rat	ICSS threshold	reduced
	TAAR1 partial agonist RO5263397	rat	induction of CPP induced behavioral sensitization	reduced	(Thorn, Zhang, et al., 2014)
	TAAR1 full agonist RO5166017	rat	expression of cocaine CPP	reduced	(J. F. Liu et al., 2016)
	TAAR1 full agonist RO5166017 (intra-VTA, PrL)	rat	cue- and drug-induced reinstatement	reduced	(J. F. Liu et al., 2017)
	TAAR1 full agonist RO5166017 (intra-NAc core)	rat	cue-induced reinstatement	reduced
	TAAR1 full agonist RO5166017 (intra-NAc shell)	rat	drug-induced reinstatement	reduced
	TAAR1 full agonist RO5256390 and partial agonist RO5263397	mice	cocaine-induced hyperactivity	reduced	(Revel et al., 2013)
	TAAR1 full agonist RO5166017	mice	cocaine-induced hyperactivity	reduced	(Revel et al., 2011)
	TAAR1 partial agonist RO5263397	rat	cocaine intake, tolerance, binge intake	reduced	(J. Liu et al., 2020)
			cue- and drug-induced reinstatement	reduced
			yohimbine-induced reinstatement	reduced
Nicotine	TAAR1 partial agonist RO5263397	rat	Nicotine-induced hyperlocomotion	reduced	(Sukhanov et al., 2018)
			development and expression of nicotine sensitization	reduced
	TAAR1 knockout	mice	nicotine-induced hyperlocomotion	no effect
	TAAR1 partial agonist RO5263397	rat	expression and development of nicotine sensitization	reduced	(J. F. Liu et al., 2018)
			discriminative effects of nicotine	reduced
			nicotine demand curve	reduced
			nicotine-induced hypothermia	No effect
	TAAR1 full agonist RO5166017 (systemic and intra-NAc)	rat	reinstatement of nicotine-seeking	reduced
	TAAR1 knockout	rat	Nicotine-seeking	enhanced
Morphine	TAAR1 knockout	mouse	morphine-induced CPP	no effects	(Achat-Mendes et al., 2012)
	TAAR1 partial agonist RO5263397	mouse	expression of morphine-induced sensitization	reduced	(J. Liu et al., 2021)
		rat	Morphine intake, progressive ratio test	reduced
		rat	cue- and drug-induced reinstatement of morphine-seeking	reduced
		mouse/rat	morphine CPP	no effect
		mouse	Naltrexone-induced jumping and CPA	no effect
		mouse	hot water tail immersion test	no effect
		rat	CFA-induced inflammatory test	no effect
Alcohol	TAAR1 knockout	mouse	preference for and consumption of ethanol	enhanced	(Lynch et al., 2013)
			Sedative-like effects of acute ethanol	enhanced
			locomotor activity induced by acute ethanol	reduced
	TAAR1 partial agonist RO5263397	mouse	expression and development of ethanol-induced sensitization	reduced	(Wu et al., 2020)
	TAAR1 knockout	mouse	ethanol-induced sensitization	no effect

TAAR1, trace amine-associated receptor 1; METH, methamphetamine; CPP, conditioned place preference; CTA, conditioned taste aversion; ICSS, intracranial self-stimulation; CPA, conditioned place aversion

3. Neural Mechanisms of TAAR1 in drug addiction

3.1. Dopaminergic system

The inhibitory effect of TAAR1 was thought to be at least partially through regulations of dopaminergic system (Figure 3). It is reported that methamphetamine (METH) interacts with TAAR1 and subsequently inhibits DA uptake, enhance DA efflux and induces DAT internalization, and these effects are dependent on TAAR1 (Xie & Miller, 2009). For example, METH-induced inhibition of DA uptake was observed in TAAR1 and DAT co-transfected cells and WT mouse and monkey striatal synaptosomes but not in DAT-only transfected cells or in striatal synaptosomes of TAAR1-KO mice (Xie & Miller, 2009). TAAR1 activation was enhanced by co-expression of monoamine transporters and this effect could be blocked by monoamine transporter antagonists (Xie & Miller, 2007; Xie et al., 2007). Furthermore, DA activation of TAAR1 induced C-FOS-luciferase expression only in the presence of DAT (Xie et al., 2007).

Figure 3.

Schematic representation of the functional interaction of TAAR1 with dopaminergic system. It is hypothesized that in the pre-synaptic membrane, DATs might serve as conduits for the entry of dopamine and amphetamines into the membrane which then induces activation of intracellular TAAR1. This TAAR1 activation promotes PKC-mediated internalization of DAT, which results in increased dopamine efflux. TAAR1 activation might also trigger trafficking of TAAR1 into the plasma membrane and potentiate D2-mediated inhibition of dopamine transmission. In the post-synaptic membrane, TAAR1 activation signals through cAMP/PKA and ERK/CREB which can be inhibited by the activation of TAAR1/D2 heterodimers. TAAR1/D2 heterodimers activation might also reverse the inhibition of β-arrestin 2/AKT/GSK3β pathway induced by D2 activation. TAAR1 can also signals through Ca²⁺/PKC/NFAT pathway. TAAR1, trace amine-associated receptor 1; D2, dopamine D2 receptors; PKA, protein kinase A; PKC, protein kinase C; ERK1/2, extracellular signal-regulated protein kinases 1 and 2; AKT, protein kinase B; GSK3β, glycogen synthase kinase 3β; CREB, cAMP response element binding protein; NFAT, nuclear factor of activated T-cells.

Recent study also showed that amphetamines mediates the trafficking of both DA and glutamate transporters by increasing the activation of small GTPase RhoA and of protein kinase A (Underhill, Colt, & Amara, 2020; Underhill et al., 2014; Wheeler et al., 2015) and this regulation is dependent on the direct activation of TAAR1 by its coupling to two G proteins G₁₃ and Gs α-subunits (Underhill et al., 2021). Moreover, although intracellular entry through DAT may not be prerequisite for METH-induced activation of TAAR1 signaling, DAT serves as an important avenue for the access to the cytoplasm. Therefore, octopamine, which is a potent agonist of TAAR1 and a poor substrate for DAT, did not activate RhoA in the cells (Underhill et al., 2021). However, Yue et al showed that RO5203648 blocked cocaine-induced DA overflow in the nucleus accumbens without altering DA half-life, suggesting that partial activation of TAAR1 by RO5203648 attenuated DA overflow by mechanisms other interaction with DAT (Pei et al., 2014). Consistently, RO5166017 did not affect the half-life of DA in the NAc and striatum (Leo et al., 2014) and decreased the hyperlocomotion in DAT-KO mice (Revel et al., 2011), suggesting that the inhibitory effect of TAAR1 in drug addiction might be independent of DAT.

Bradaia et al. found that antagonism of TAAR1 by acute application of EPPTB increased the potency of DA at D2 auto-receptor in dopaminergic neurons (Bradaia et al., 2009). Espinoa et al. further explored the interaction between TAAR1 and D2 using a bioluminescence resonance energy transfer (BRET) biosensor for cAMP (Espinoza et al., 2011). They discovered that TAAR1 agonist PEA induced increase of cAMP was enhanced by D2 receptor antagonists (haloperidol, raclopride, and amisulpride) (Espinoza et al., 2011). Furthermore, TAAR1 and D2 receptor were able to form heterodimers when co-expressed in HEK 293 cells, and this effect was disrupted by D2 receptor antagonist haloperidol (Espinoza et al., 2011). However, haloperidol induced c-Fos activation in the striatum and catalepsy were significantly attenuated in TAAR1-KO mice (Espinoza et al., 2011). In addition, RO5256390 blockade of cocaine-induced DA release in slices of NAc was blocked by the antagonism of D2 receptor (Asif-Malik, Hoener, & Canales, 2017). Taken together, these data suggest that the interactions of TAAR1 and D2 receptor might underlie the functions of TAAR1 in modulations of monoaminergic system.

3.2. Cellular signaling

Consistent studies have suggested that the TAAR1 expression is probably intracellular. It was challenging to study TAAR1 initially because of the lack of expression at the plasma membrane in vitro as observed in microscopy images (Miller, 2011). Thus, many earlier studies of TAAR1 pharmacology required modifications to the receptor so as to increase its membrane expression (Bunzow et al., 2001; Lindemann & Hoener, 2005; Reese, Bunzow, Arttamangkul, Sonders, & Grandy, 2007; Wainscott et al., 2007). Moreover, it is shown that rhesus monkey TAAR1 interacts with monoamine transporters and thus provide an access by which TAAR1 ligands can enter the cytoplasm and bind to TAAR1. A more recent study suggested that trace amines action is dependent on intracellular uptake by pentamidine-sensitive Na(+)-independent membrane transporters but not monoamine transporters (Gozal et al., 2014). Furthermore, since traces amine have delayed onset in locomotor-like activity (LLA) in rat neonatal spinal cord compared with serotonin (Gozal et al., 2014), it supports the hypothesis that TAAR1 locates in intracellular compartments and its activation requires intracellular transportation.

TAAR1 couples to Gαs protein and increases the accumulation of cAMP upon activation. Moreover, it is speculated that TAAR1 may also signal directly or indirectly through β-arrestin 2/Akt/GSK3 pathway. The β-arrestin 2 knock-out mice showed similar hypoactivity in response to amphetamine as TAAR1-OE mice (Revel, Meyer, et al., 2012). However, in the striatum of TAAR1-KO mice, an overexpression of D2 receptors was observed, paralleled with a selective inhibition of AKT/GSK3 signaling pathway (Espinoza, Ghisi, et al., 2015). While the interaction of TAAR1 with D2 receptor reduced the β-arrestin 2 recruitment to D2 receptor, cAMP signaling of TAAR1 was reduced in the presence of D2 receptor (Harmeier et al., 2015). These findings suggest that β-arrestin 2/Akt/GSK3 signaling may be an important pathway for TAAR1 function and the activation of TAAR1/D2 receptor heterodimers negatively regulates this signaling. In addition, TAAR1 activation by 3-methoxytyramine (3-MT) significantly increased the phosphorylation of ERK and CREB in mouse striatum while in TAAR1-KO mice, the responses caused by 3-MT was significantly attenuated (Sotnikova et al., 2010). In contrast, upon the interaction with D2 receptor, activation of TAAR1 leads to a reduction in phosphorylation of ERK and CREB (Harmeier et al., 2015), suggesting the involvement of ERK/CREB signaling in TAAR1’s functions. Moreover, TAAR1 activation was shown to increase the phosphorylation of PKC and PKA in HEK293 transfected cells (M. W. Panas et al., 2012).

3.3. Neuroanatomic sites of TAAR1 actions

TAAR1 is expressed in a wide range of brain regions especially in the dopaminergic system, however, studies using systemic manipulations of TAAR1 cannot answer the question that which brain regions are crucial for TAAR1’s function. By using local microinfusions into different brain regions including PFC, NAc, VTA, Amygdala and SN, our group identified that TAAR1 in specific brain regions of reward system plays distinct role in drug addiction (J. F. Liu, Siemian, Seaman, Zhang, & Li, 2017). For example, microinfusion of RO5166017 into the VTA and prelimbic area of medial PFC attenuated both cue- and drug-induced reinstatement of cocaine-seeking. Moreover, RO5266017 within the NAc core and shell were key to cue- and drug-induced seeking behavior, respectively (J. F. Liu et al., 2017). However, RO5166017 in brain regions like SN, Amygdala and infralimbic PFC may not participate in cocaine relapse-like behaviors (J. F. Liu et al., 2017).

4. TAAR1 and Psychostimulants

4.1. Amphetamines

When first cloned in 2001, TAAR1 was shown to be activated potently by amphetamine and its analogues like METH, para-hydroxyamphetamine (POHA) and 3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) (Borowsky et al., 2001; Bunzow et al., 2001). As we know that trace amines are TAAR1 agonists, they show high level of structural and physiological similarities with amphetamine. Therefore, β-PEA has been described as endogenous “amphetamine” (Borison, Mosnaim, & Sabelli, 1975; Janssen, Leysen, Megens, & Awouters, 1999). It was further characterized by Grandy lab and others that both S-(+)- and R-(−) isomers of METH were full agonists at mouse and human-rat chimera TAAR1 while were partial agonists at rat TAAR1 (Lewin, Miller, & Gilmour, 2011; Reese et al., 2007), and TAAR1 had a stereoselective binding site for compounds in the amphetamine class in vitro (Simmler, Buchy, Chaboz, Hoener, & Liechti, 2016).

The findings from in vitro studies suggest that TAAR1 may play a role in the effects of psychostimulants in vivo. In fact, enhanced behavioral and neurochemical responses of amphetamine were observed in mice lacking the TAAR1 gene (TAAR1-KO) compared with their wide-type (WT) littermates (Wolinsky et al., 2007). On the contrary, TAAR1-OE mice showed hyposensitivity to the psychostimulant effects of amphetamine (Revel, Meyer, et al., 2012). These studies suggest that TAAR1 may be implicated in the rewarding effects of psychostimulants. By measuring locomotor activity and conditioned place preference (CPP), it was shown that d-amphetamine and METH produced higher levels of locomotion in TAAR1-KO mice (Achat-Mendes, Lynch, Sullivan, Vallender, & Miller, 2012; Sukhanov et al., 2016). Moreover, TAAR1-KO mice acquired METH-induced CPP earlier and retained CPP longer than WT mice (Achat-Mendes et al., 2012). This finding was later supported by the study which showed that TAAR1-KO mice consumed more METH and demonstrated less sensitivity to METH-induced hypothermia and conditioned taste aversion (CTA) (Harkness, Shi, Janowsky, & Phillips, 2015; Miner, Elmore, Baumann, Phillips, & Janowsky, 2017), implicating an inhibitory role of TAAR1 in the effects of amphetamines. Most recently, it is shown that TAAR1 gene variants contribute to the increased METH intake (Reed et al., 2017; Reed et al., 2021; Stafford et al., 2019) which is consistent with a report that the V288V polymorphism in TAAR1 gene increases drug craving in individuals with METH dependence (Loftis et al., 2019).

With the utilization of TAAR1 selective agonists, the role of TAAR1 in amphetamines stimulant effects was further clarified. In 2014, Jing et al first examined the TAAR1 partial agonist RO5263397 in addiction-related behaviors of METH (Jing, Zhang, & Li, 2014). They found that RO5263397 dose-dependently reduced the expression METH-induced behavioral sensitization. Using the self-administration model, a gold standard animal model for drug addiction research, they further reported that RO5263397 decreased the cue- and drug-induced reinstatement of METH-seeking behavior (Jing et al., 2014). Meanwhile, RO5263397 also reduced the break-point for METH self-administration, a behavioral model of motivation (Pei, Asif-Malik, Hoener, & Canales, 2017). Similarly, another partial agonist RO5203648 showed consistent inhibitory effects demonstrated by the decreased METH-induced hyperactivity, sensitization and self-administration (Cotter et al., 2015). Importantly, both agonists showed no effect on operant responding maintained by sucrose (Cotter et al., 2015; Jing et al., 2014) and were not self-administered when tested as a METH’s substitute, indicating no abuse potential (Cotter et al., 2015; Pei et al., 2017). Moreover, neurochemical data showed that RO5203648 and RO5263397 inhibited METH-induced increase of extracellular DA levels in the NAc and in the slices of the NAc, respectively (Cotter et al., 2015; Pei et al., 2017), which might contribute to its inhibitory effects in METH addiction-related behaviors. In regards to METH-induced impulsivity, an important trait related to drug addiction, RO5263397 was also able to attenuate acute but not chronic METH-induced premature responses (Xue, Siemian, Johnson, Zhang, & Li, 2018). The increased impulsivity caused by discontinuation of METH treatment was reduced by RO5263397 as well (Xue et al., 2018). Consistently, while TAAR1-KO mice showed perseverative and impulsive phenotype, activation of TAAR1 by RO5166017 and RO5203648 reduced premature impulsive responses observed in the fixed-interval conditioning schedule in WT mice (Espinoza, Lignani, et al., 2015).

4.2. Cocaine

Cocaine is an DA modulator as it is known to increase DA accumulation in the NAc resulted from blockade of DA reuptake through interaction with DA transporter (DAT) (Greco & Garris, 2003). While cocaine is not an agonist of TAAR1, recent studies support a critical role of TAAR1 in mediating DA transmission. For example, TAAR1-KO mice showed increased spontaneous firing rate of dopaminergic neurons in the VTA and higher expression of high-affinity D2 receptors in the striatum (Lindemann et al., 2008). Furthermore, similar to TAAR1 endogenous agonist p-tyramine, the pharmacologically selective full agonist RO5166017 inhibited the firing frequency of dopaminergic neurons in brain slice of WT but not TAAR1-KO mice, which can be blocked by TAAR1 selective antagonist EPPTB (Revel et al., 2011). TAAR1 agonists RO5166017, RO5256390, RO5263397, and RO5203648 were able to attenuate cocaine-induced hyperlocomotion and drug-taking behaviors (Revel et al., 2011; Revel, Moreau, et al., 2012; Revel et al., 2013). The inhibitory role of TAAR1 in the regulation of dopaminergic system strongly suggest that TAAR1 could be a potential therapeutic target for the treatment of cocaine addiction.

In light of this, more studies have tested the effect of TAAR1 activation in abuse-related effects of cocaine. Canales lab examined the effects partial and full activation of TAAR1 on cocaine-taking, cocaine-induced lowering in intracranial self-stimulation (ICSS) and relapse-like behaviors (Pei et al., 2014; Pei, Mortas, Hoener, & Canales, 2015). They found that both RO5256390 and RO5203648 suppressed the reinforcing and rewarding effects of cocaine as evidenced by a downward shift in the cocaine dose-response curve and the prevention of cocaine-induced changes in ICSS (Pei et al., 2015). The inhibitory role of TAAR1 was further supported by the findings that RO5256390 and RO5203648 attenuated cocaine-seeking both in withdrawal and extinction-reinstatement paradigms (Pei et al., 2014). Meanwhile, our lab showed consistent findings. RO5263397 and RO5166017 blocked the induction and expression of cocaine-induced behavioral sensitization and CPP and suppressed the dose-effect response of cocaine, respectively (J. F. Liu, Thorn, Zhang, & Li, 2016; Thorn, Jing, et al., 2014; Thorn, Zhang, Zhang, & Li, 2014). Moreover, RO5263397 decreased the cue- and drug-induced reinstatement of cocaine-seeking while suppressed the motivation of cocaine in behavioral economic analysis (Thorn, Jing, et al., 2014). Recently, our group further characterized the role of TAAR1 in compulsive cocaine use using an extended access self-administration model (J. Liu et al., 2020). Here, RO5263397 reduced cocaine intake in this model without developing tolerance and decreased cue- and stress-induced relapse-like behaviors after prolonged abstinence (J. Liu et al., 2020), further strengthening the importance of TAAR1 in cocaine addiction.

4.3. Nicotine

Nicotine produces stimulant effects by binding to the nicotinic acetylcholine receptors within the mesocorticolimbic system to modulate DA transmission (Subramaniyan & Dani, 2015). In fact, nicotine increases the firing rate and phasic bursts in the dopaminergic neurons in the VTA and its projecting areas, resulting in enhanced DA release (Stoker & Markou, 2013; L. Zhang, Doyon, Clark, Phillips, & Dani, 2009). As TAAR1 mediates central dopaminergic system, recent studies have begun to illustrate whether TAAR1 played a role in nicotine addiction-like behavior. It was shown that RO5263397 attenuated the acute reinforcing effect of nicotine as evidenced by decreased nicotine-induced hyperactivity (Sukhanov, Dorofeikova, Dolgorukova, Dorotenko, & Gainetdinov, 2018). Moreover, RO5263397 prevented the development of nicotine-induced sensitization and reduced the hyperactivity in nicotine-sensitized rats (Sukhanov et al., 2018). A more comprehensive study from our lab further examined the ability of TAAR1 in mediating nicotine addiction-like behavior (J. F. Liu et al., 2018). Consistent with previous study, we showed that TAAR1 activation by RO5263397 reduced the expression and development of nicotine-induced sensitization (J. F. Liu et al., 2018). TAAR1 activation attenuated the discriminative effects of nicotine, nicotine intake and shifted the demand curve for nicotine downward (J. F. Liu et al., 2018). For relapse-like behaviors, we found that TAAR1 activation suppressed the cue- and drug-induced reinstatement of nicotine-seeking behavior while knockout of TAAR1 further enhanced this effect (J. F. Liu et al., 2018). Moreover, TAAR1 activation reduced nicotine-induced c-Fos expression and DA release in the NAc, which might contribute to the inhibitory effects of TAAR1 in nicotine addiction (J. F. Liu et al., 2018). Meanwhile, local activation of TAAR1 in the NAc reduced the nicotine-seeking behavior, suggesting that the NAc is important for TAAR1’s effects in the reinstatement of nicotine-seeking (J. F. Liu et al., 2018).

While nicotine has a modest reinforcing effect compared with other psychostimulants (Mansvelder, Keath, & McGehee, 2002; Nestler, 2005), the pronounced withdrawal symptoms after drug discontinuation contribute substantially to the development of nicotine addiction (Epping-Jordan, Watkins, Koob, & Markou, 1998; Hughes, Higgins, & Bickel, 1994; Koob & Bloom, 1988). However, the involvement of TAAR1 in the negative reinforcement of drug addiction has not been studied. Using the short-access (ShA, 1h) and long-access (LA, 21h) self-administration models, we have found that rats in the LA group showed more robust withdrawal symptoms than those from ShA group, evidenced by increased anxiety-like behavior, hyperalgesia and augmented withdrawal signs induced by drug abstinence (R. Wu et al., 2021). Importantly, RO5263397 was able to attenuate those withdrawal-related effects in rats, suggesting a role in mediating withdrawal-related effects of nicotine (R. Wu et al., 2021). However, the neural mechanisms through which TAAR1 mediates nicotine’s withdrawal effects remain unclear, which is currently under investigation.

5. TAAR1 and Opioids

5.1. Morphine

Studies of the role of TAAR1 in morphine addiction are limited. It was previously showed by Achat-Mendes et al. that both TAAR1-KO and WT mice developed the same preference for morphine in CPP paradigm (Achat-Mendes et al., 2012). Moreover, there was no difference between TAAR1-KO and WT mice in the extinction and reinstatement of morphine-induced CPP (Achat-Mendes et al., 2012), suggesting that endogenous TAAR1 is not important for morphine-induced CPP. However, although TAAR1-KO animals had similar infusions in nicotine self-administration as their WT counterparts, TAAR1 agonist was able to reduce the nicotine intake in WT rats (J. F. Liu et al., 2018). Thus, the result that TAAR1-KO mice had similar morphine CPP may not rule out the possibility that TAAR1 is involved in other morphine addiction-like behaviors.

This hypothesis was recently tested by our group using a battery of behavioral assays including morphine-induced behavioral sensitization, morphine self-administration, morphine-induced CPP and two different pain models (J. Liu et al., 2021). In this study, RO5263397 decreased the expression of morphine-induced behavioral sensitization in WT but not TAAR1-KO mice (J. Liu et al., 2021). RO5263397 in the doses that did not affect food-maintained response shifted the dose-effect curve of morphine self-administration downward and reduced the breakpoint in a progressive ratio test (J. Liu et al., 2021). RO5263397 also attenuated the relapse-like behaviors of morphine as it decreased the cue- and drug-induced reinstatement of morphine-seeking behavior (J. Liu et al., 2021). Importantly, RO5263397 alone did not affect the locomotor activity and reinstate morphine-seeking behavior (J. Liu et al., 2021). Interestingly, RO5263397 had no effect on morphine-induced CPP both in mice and rats (J. Liu et al., 2021), which is consistent with previous study. This finding suggests that TAAR1 may not participate in the mechanisms underlying morphine CPP (J. Liu et al., 2021). Nonetheless, these studies suggest that TAAR1 inhibits the reinforcing and rewarding effects of morphine.

Similar to nicotine addiction, morphine addiction is also marked with a robust withdrawal effect after drug discontinuation. However, RO5263397 did not alter naltrexone-induced jumping behaviors and the expression of naltrexone-precipitated conditioned place aversion (CPA) in morphine-dependent mice (J. Liu et al., 2021), suggesting that activation of TAAR1 by RO5263397 did not affect the negative effects of morphine in mice. Considering that TAAR1 attenuated the withdrawal effects of nicotine but not morphine, it seems that the mechanisms underlying negative reinforcement may differ from different drug classes. As morphine is a powerful analgesic, it is worth testing whether TAAR1 activation could affect the analgesic effect of morphine. Using two different pain models (acute thermal pain and chronic inflammatory pain), RO5263397 was shown not to affect tail withdrawal in the hot water tail immersion test and withdrawal threshold in the Complete Freund’s adjuvant (CFA)-induced inflammation test (J. Liu et al., 2021). Taken together, activation of TAAR1 did not change the antinociceptive effect of morphine, which suggests that TAAR1 activation might be an efficient therapeutic strategy for patients with comorbidity of drug addiction and pain.

6. TAAR1 and Alcohol

It was shown that TAAR1-KO mice displayed significantly higher preference and intake for ethanol in two-bottle choice (TBC) test compared with WT mice (Lynch et al., 2013). However, this effect was not observed in TBC with sucrose, suggesting the specificity to the drug (Lynch et al., 2013). TAAR1-KO mice also had enhanced sedative-like effects of ethanol shown by severer motor impairment in loss of righting reflex (LORR) test (Lynch et al., 2013). Moreover, they also showed less ethanol induced locomotor stimulating effects, demonstrating less reinforcing effect of ethanol in the lack of TAAR1 (Lynch et al., 2013). Meanwhile, TAAR1-KO mice had similar blood ethanol level with WT mice, suggesting that these effects were not due to the differences in clearance rates between these two groups (Lynch et al., 2013). A more recent study tested the effect of TAAR1 activation in ethanol-induced behavioral sensitization (Wu et al., 2020). RO5263397 significantly decreased the expression and development of ethanol-induced behavioral sensitization both in female and male WT mice (Wu et al., 2020). Furthermore, while TAAR1-KO mice developed comparable levels of sensitization, RO5263397 had no effect in this behavior in TAAR1-KO mice (Wu et al., 2020). These results suggest that TAAR1 is involved in addiction-like behaviors and TAAR1 agonist may be useful to treat alcohol-related disorders.

7. Current challenges

7.1. TAAR1 and Other drugs of abuse

So far, we have discussed the essential roles of TAAR1 in modulations of addiction-like behaviors of several drugs of abuse including amphetamines, cocaine, nicotine, morphine and alcohol. It is clear that TAAR1 activation negatively mediates the rewarding and reinforcing effects of most of the studied drugs of abuse, of which its role in modulation of dopaminergic system contributes substantially to the effects observed. Pharmacological studies also suggest that highly potent and selective TAAR1 agonists might be useful for the treatment of drug addiction.

However, although the accumulating evidence form the past several decades provide us with much knowledge about the function of TAAR1 and its role in drug addiction, there are still blanks to be filled in in the near future. For example, there are limited studies focused on the role of TAAR1 in opioids except for the only two studies of morphine (Achat-Mendes et al., 2012; J. Liu et al., 2021). We have demonstrated that TAAR1 activation inhibits the reinforcing effects of morphine while sparing its analgesic effects (J. Liu et al., 2021). Thus, it will be interesting to test if this effect can be generalized to other opioids with higher potency, like fentanyl or remifentanil. The results from these studies could greatly help to curb the current opioid epidemic. Moreover, few studies have examined the effect of TAAR1 activation using ethanol self-administration or CPP model which could otherwise be useful to measure the effects of TAAR1 in mediating the rewarding effects, motivation and relapse-like behaviors of ethanol. Such studies could add valuable information for the potential of TAAR1 agonists to treat alcohol addiction.

7.2. TAAR1 agonists and antagonists

The studies of TAAR1 agonists suggest that these compounds showed good pharmacokinetic properties and efficacy in regulating addiction of many drugs of abuse. Intriguingly, TAAR1 partial agonists RO5263397 and RO5203648 increased the firing rate of dopaminergic neurons in the brain slices (Revel, Moreau, et al., 2012; Revel et al., 2013), the effect similar to what was seen in the TAAR1 antagonist EPPTB (Bradaia et al., 2009). However, one of the pharmacological properties of partial agonists is that they can act as an agonist or antagonist dependent on endogenous intrinsic activity of the receptor (Lam et al., 2015). Moreover, it is suggested that TAAR1 is tonically active or constitutively activated to downregulate DA neuron firing frequency (Lindemann et al., 2008). Therefore, it is possible that TAAR1 partial agonists have “inhibiting effects” in vitro. Nevertheless, the above in vivo behavioral studies showed that the partial agonists act in a similar fashion as TAAR1 full agonists like RO5166017 and RO5256390 (J. Liu et al., 2020; Revel et al., 2011; Revel et al., 2013). Based on these data, TAAR1 agonists may serve as promising therapeutic targets for the treatment of drug addiction. As the treatment of drug addiction requires repeated and long-term medications, it is important to determine whether repeated TAAR1 activation could cause tolerance or toxicity. These preclinical studies will provide crucial data to support future clinical developments.

Thus far, EPPTB is the only TAAR1 antagonist that has been identified. It is a highly potent mouse TAAR1-selective antagonist that prevents the reduction of firing rate of dopaminergic neurons induced by TYR in vitro (Bradaia et al., 2009). EPPTB could also increase the firing rate of DA neurons in the brain slices from WT but not TAAR1-KO mice (Bradaia et al., 2009), ruling out off-target effects. However, EPPTB is mostly inactive at rat and human TAAR1s (Berry, Gainetdinov, Hoener, & Shahid, 2017) and even in mice, the pharmacokinetic properties prevent the in vivo use of EPPTB (Bradaia et al., 2009). Therefore, the development of new TAAR1 antagonists with more favorable pharmacological profiles could boost the progress in elucidating TAAR1’s function. Moreover, TAAR1 antagonists may also have therapeutic potentials for the treatment of hypodopaminergic diseases, such as Parkinson’s disease (Alvarsson et al., 2015).

7.3. Clinical studies of TAAR1 agonists

With the promising preclinical data of the role of TAAR1 in mediating neuropsychiatric disorders, several clinical trials for TAAR1 agonists have been initiated for the treatment of mental disorders. RG7351, a partial agonist on TAAR1, is currently under phase I clinical trial by Hoffmann–La Roche for the treatment of depression. In a 4-week randomized, controlled clinical trial, SEP-363856, a compound that act on TAAR1 and 5-HT1A other than dopamine D2 receptors, significantly alleviated the severe psychotic symptoms in patients with schizophrenia as evidenced by remarkable reduction in the Positive and Negative Symptom scale (PNSS) (Koblan et al., 2020). Certainly, longer and larger trials are needed to confirm the effects of SEP-363856 in schizophrenia, but it represents an important step forward to develop novel treatments for mental disorders. Although there remains a long way ahead, it might be promising to test their efficacy for the treatment of drug addiction as well in the future.

8. Conclusions

TAAR1 negatively regulates addiction to different drug classes including psychostimulants, opioid and alcohol. The extensive inhibitory effect of TAAR1 agonists on drug addiction suggests that TAAR1 agonists might serve as potential and effective pharmacological therapeutics for the treatment of drug addiction. The mechanism underlying TAAR1’s function in drug addiction mainly involves modulation of dopaminergic system. Emerging evidence suggests that the interaction of TAAR1 with DAT and D2 receptors contribute substantially to the regulation of DA transmission. Together with the subsequent downstream cellular signaling including β-arrestin 2/Akt/GSK3 pathway, ERK/CREB pathway and PKA/PKC activation, TAAR1 induced-neurobiological alterations underlie the overall behavioral output. However, more studies are needed to further understand the role of TAAR1 in addiction to other drugs of abuse and examine the potency and efficacy of novel pharmacotherapy for drug addiction based on TAAR1.

Abbreviations

TAAR1

Trace amine-associated receptor 1

VTA

ventral tegmental area

NAc

nucleus accumbens

DR

dorsal raphe

SN

substantial nigra

DA

dopamine

DAT

dopamine transporter

PEA

β-phenylethylamine

TYR

p-tyramine

TRP

tryptamine

OCT

p-octopamine

NE

norepinephrine

GPCR

G-protein coupled receptors

TAs

Trace-Amine Receptors

IUPHAR

International Union of Pharmacology

LC

locus coeruleus

TAAR1-KO

TAAR1 knockout

TAAR1-OE

TAAR1 overexpression

METH

methamphetamine

BRET

bioluminescence resonance energy transfer

LLA

locomotor-like activity

3-MT

3-methoxytyramine

POHA

para-hydroxyamphetamine

MDMA, ecstasy

3,4-Methylenedioxymethamphetamine

WT

wide-type

CPP

conditioned place preference

CTA

conditioned taste aversion

ICSS

intracranial self-stimulation

ShA

short-access

LA

long-access

CPA

conditioned place aversion

CFA

Complete Freund’s adjuvant

TBC

two-bottle choice

LORR

loss of righting reflex

PNSS

Positive and Negative Symptom scale