Shared Behavioral and Neurocircuitry Disruptions in Drug Addiction, Obesity and Binge Eating Disorder: Focus on Group I mGluRs in the Mesolimbic Dopamine Pathway

Abstract

Accumulated data from clinical and preclinical studies suggest that in drug addiction and states of overeating, such as obesity and binge eating disorder (BED), there is an imbalance in circuits that are critical for motivation, reward saliency, executive function, and self-control. Central to these pathologies and the extensive topic of this review are the aberrations in dopamine (DA) and glutamate (Glu) within the mesolimbic pathway. Group I metabotropic glutamate receptors are highly expressed in the mesolimbic pathways and are poised in key positions to modulate disruptions in synaptic plasticity and neurotransmitter release observed in drug addiction, obesity and BED. The use of allosteric modulators of group I metabotropic glutamate receptors (mGluRs) have been studied in drug addiction, as they offer several advantages over traditional orthosteric agents. However, they have yet to be studied in obesity or BED. With the substantial overlap between the neurocircuitry involved in drug addiction and eating disorders, group I mGluRs may also provide novel targets for obesity and BED.

Graphical Abstract

1.1. Introduction

Numerous reviews have highlighted the common changes in brain circuits involved in drug addiction and obesity, which involves a dysregulation of corticostriatal and reward-related circuitry ^(1–8). The loss of control and compulsive pattern of food intake, commonly seen in binge eating disorder (BED) is reminiscent of drug intake patterns seen in substance use disorder (SUD) ^(2,9). In particular, responses to processed foods high in sugar, fat, and salt can lead to an escalation of intake and pronounced cravings – a syndrome that has come to be known as food addiction ^(10–17). These parallels have led to the current hypothesis that addictive-like process may contribute to excess food consumption. The first questionnaire to assess food addiction was the Yale Food Addiction Scale (YFAS) ^(18–21), which examines seven core symptoms of addiction as applied to highly palatable food. The YFAS criterion has been used to explore the prevalence of food addiction in obese subjects from the general population as well as clinical populations with and without BED ^(10,21–24). According to the DSM-5, there is a phenomenological overlap between Substance-related and Addictive Disorders and Feeding and Eating Disorders ^(25–29), in that impulse control plays a prominent role in the criteria for these disorders ^(30,31). While the concept of food addiction continues to gain attention, the YFAS is currently the only validated measure to operationalize addictive-like eating behavior ^(18,19), therefore, exploring obesity and BED in an addiction-like context in preclinical models may help to establish a better understanding of these disorders. It is important to acknowledge that the concept of food addiction is not a universally accepted definition, a topic which we briefly discuss below (for detailed reviews see ^(32–34)).

One of the major issues centering around the concept of food addiction is that food addiction is regarded as phenotypic model that is based on similarities between overeating and the DSM-5 criteria for substance addiction ^(32,33,35). The YFAS was developed with the aim of identifying and quantifying a specific clinical phenotypic entry. Namely, the YFAS cannot identify the transition from general consumption to abuse and the scoring system represents a continuous severity measure. The YFAS is an important research tool; however, it is does not necessarily follow that the syndrome it captures is food addiction, but rather another type of eating disorder. An additional issue with the concept of food addiction is the translatability of preclinical models (as discussed in detail in ^(33,34)). It is important to keep in mind that while preclinical models are not an exact correlate to clinical populations, these models provide us with a detailed understanding of the neurocircuitry involved. While the concept of food addiction is not universally accepted, we use this term as it encompasses the neural and phenotypic overlap with substance addiction.

It is not surprising that there is an overlap in the neuronal mechanisms and circuits implicated in the loss of control and overconsumption of food intake seen in obesity and BED ^(4–8) as well as in the compulsive intake of drugs seen in SUD⁽³⁶⁾. In both drug and food addiction, the salience value of the reward becomes abnormally enhanced relative to other rewards ^(37–39). This model is consistent with the hypothesis that both natural and drug rewards have powerful reinforcing effects that are mediated, in part, by abrupt dopamine (DA) increases in the brain reward system ^(8,40–42). Changes in DA following chronic drug use or food consumption are also reported in DA pathways involved in the modulation of habit formation, motivation, and executive functions ^{(8,40,43–49)}. Additionally, a wealth of preclinical studies has demonstrated that many of these DA responses are mediated by crosstalk with the glutamate (Glu) system ^(49–53). This review will focus on changes to the neurocircuitry underlying motivated behavior as well as DA and Glu abnormalities reported in drug addiction, obesity and BED. It is important to note that disruptions are observed in other neurotransmitter systems and circuits, however, that is beyond the scope of this review ^(30,53–61).

1.2. The Mesolimbic Dopamine Pathway

The ventral tegmental area (VTA) is comprised of a group of neurons located around the midline of the midbrain floor ⁽⁶²⁾ and contains mainly neurons that produce dopamine (DA). Studies using immunohistochemistry and in situ hybridization in the VTA have revealed that 55–65% of neurons express the enzyme tyrosine hydroxylase (TH, critical for DA synthesis), 30% are positive for glutamic acid decarboxylase mRNA (GAD, necessary for GABA synthesis), and a small percentage (2–3%) express vesicular glutamate transporter 2 (vGluT2, required for reuptake and packaging of glutamate) ^(63–66). Recently it has been demonstrated that subpopulations of VTA neurons contain and release different neurotransmitters, termed multiplexed neurotransmission, from distinct compartments within a single axon, a single terminal, or the same vesicle ^(67–70); subsets of VTA neurons are capable of co-releasing (i) DA and Glu^(71,72), (ii) Glu and GABA⁽⁷³⁾ or (iii) DA and GABA⁽⁷⁴⁾. It has been suggested that multiplexed neurotransmission conveys distinct messages depending upon neurotransmitter content and time scale of neurotransmitter function to influence pre- and postsynaptic changes that result in observable changes in behavior ⁽⁷⁵⁾. Interestingly, synapses that are capable of multiplexed neurotransmission have been speculated to produce aberrant signaling in various states ⁽⁷⁵⁾, including drugs of abuse and neurodegenerative diseases. In general, VTA neurons are thought to play distinct roles in positive and negative reinforcement, decision making, working memory, incentive salience, and aversion ^(76–83). This behavioral heterogeneity is thought to reflect the diverse phenotypic characteristics of VTA neurons and the brain structures with which they are connected ⁽⁸⁴⁾.

The DA neurons in the VTA exhibit two distinct modes of spike firing: tonic activity (1–8 Hz) or a transient high-frequency phasic mode (>15 Hz) ^(85–87), with the phasic bursts resulting in larger increases in synaptic DA at projection regions than tonic activity in this same population. Tonic firing refers to spontaneously occurring activity and is driven by intrinsic properties of DA neurons. The control of tonic firing of VTA DA neurons is regulated by GABAerigic input from the bed nucleus of the stria terminals (BNST) and the ventral pallidum (VP) as well as local GABAergic inhibition from interneurons (^81,88,89). In contrast to tonic DA neuron population activity, phasic firing is dependent on glutamatergic excitatory synaptic drive onto VTA DA neurons from a number of areas, such as pedunculo pontine tegmentum (PPTg), the subthalamic nucleus (STN), and the laterodorsal tegmentum (LDTg) ^(90,91) as well as cholinergic signaling from the LDTg ⁽⁹²⁾ and medial habenular (MHb) subregion ⁽⁹³⁾ Within the VTA, acetylcholine and agonists of nicotinic and muscarinic receptors increase activity of DA and GABA cells, promote burst firing and a phasic efflux in terminal regions ^(92,94–96).

Projections from VTA DA neurons to the nucleus accumbens (NAc) and associated forebrain regions comprise the mesolimbic DA pathway, which has long been implicated in reward and reinforcement processes that respond to natural stimuli and numerous drugs of abuse ^(40,97–100). In the NAc, DA interacts with membrane receptors belonging to a family of seven transmembrane domain G-protein-coupled receptors (GPCR). A general subdivision into two groups has been made based on their structural, neuropeptide content and coupling properties ⁽¹⁰¹⁾. D₁-like receptors stimulate heterotrimetric G proteins, Gα_s and Gα_olf, which are positively coupled to adenylyl cyclase (AC), leading to the production of cyclic adenosine monophosphate (cAMP) and the activation of protein kinase A (PKA). By contrast, D₂-like receptors activate Gα_i and Gα_o proteins, which inhibit AC and limit PKA activation. DA receptors can also signal independently of cAMP/PKA to modulate intracellular calcium (Ca²⁺) levels and regulate ligand- and voltage-gated ion channels ^(101,102). In addition to their effects on G-protein regulated pathways, D₁ and D₂ can alter membrane trafficking of Ca²⁺ channels as well as N-methyl-D-aspartate (NMDA) and GABA_A receptors through direct protein-protein interactions or following the activation of intracellular tyrosine kinases ⁽¹⁰³⁾. DA signaling through both D₁- and D₂-like receptors have been reported the play an important role in response to both natural- and drug-reward ^{(40,104–107)}.

The NAc is comprised of multiple neuronal subtypes including four different types of interneurons (i.e., cholinergic interneurons and GABAergic interneurons, which express parvalbumin, calretinin, or nitric oxide synthase/neuropeptide Y/somatostatin) ^(108,109). However, the majority of striatal neurons (~95%) are medium spiny neurons (MSNs) ⁽¹¹⁰⁾, which are the primary projection neurons of the striatum. The MSNs consist of two subtypes, D₁-type DA containing MSNs (D₁-MSNs) that are characterized by the expression of the neuropeptides dynorphin (DYN) and substance P (SP) and D₂-type DA containing MSNs(D₂-MSNs) that express the neuropeptide enkephalin (ENK). It should also be noted that there is a small population of neurons in the NAc that coexpress both D₁ and D₂, though this is largely restricted to the NAc shell ⁽¹¹¹⁾. In contrast to the canonical concept of the basal ganglia, pathways of the NAc are not coded by MSN cell type ^(112,113). D₁- and D₂-MSNs both project to the ventral pallidum (VP) and D₁-MSNs also project to ventral mesencephalon structures, such as the VTA. D₁- and D₂-MSNs have distinct roles in modulating reward and motivation, which will be discussed in further detail below.

1.3. Expression of Group I Metabotropic Glutamate Receptors in the Mesolimbic Pathway

To date, eight different metabotropic glutamate (mGlu) receptor subtypes have been cloned and characterized, and each appears to have a diverse neuroanatomical distribution as well as unique pharmacological and intracellular signaling properties ⁽¹¹⁴⁾. mGlu receptors are typically subcategorized into Group I receptors (mGlu₁ and mGlu₅), which are coupled to various classes of G-proteins such as G_q/11. It is important to note that mGlu₅ is pleiotropically coupled and is able to active multiple signaling pathways. While mGlu₅ is predominantly coupled to G_q and mobilizes Ca²⁺, mGlu₅ also couples to iCa²⁺-independent signaling pathways, such as extracellular signal-regulated kinases 1 and 2 (ERK 1/2) and cyclic adenosine monophosphate (cAMP) and can interact and modulate the activity of other GPCRs and ion channels ⁽¹¹⁵⁾ Group I mGlu receptors are densely expressed in striatal MSNs and are found in DA neurons of the ventral midbrain ⁽¹¹⁶⁾. In contrast, Group II (mGlu₂ and mGlu₃) and Group III (mGlu₄, mGlu₆, mGlu₇, and mGlu₈) receptors are coupled to G_i/o protein signaling mechanisms and upon activation inhibit adenylyl cyclase activity. For the purpose of this review, we will focus on group I mGlu receptors, however, detailed information regarding mGlu receptors in addiction has been reviewed elsewhere ^{(114,117–120)}.

Group I mGlu receptors possess four typical intracellular domains: three loops and a C terminus (CT) and are coupled to phospholipase Cβ1 (PLCβ1) via G_αq proteins ^(114,121). Upon activation, these receptors trigger Ca²⁺ release and PKC signaling pathways. It is important to note that depending on the cell type or neuronal population, group I mGlu receptors can activate a range of downstream effectors ^(122–126). Phosphorylation of these receptors are mediated by protein kinases and are essential elements in long-term remodeling of excitatory synaptic transmission ⁽¹²⁶⁾.

Group I mGlu receptors positioned at excitatory synapses and are known to facilitate or induce both long-term depression (LTD) and long-term potentiation (LTP) of synaptic strength ^(127,128). These receptors are also able to trigger plasticity of non-synaptic conductance that lead to enhanced neuronal excitability ⁽¹²⁹⁾. Within the NAc, long-term synaptic plasticity is the likely cellular correlate modulating DA signaling that underlies the learned behavioral response or the addictive component of both natural and drug reward ⁽¹³⁰⁾. Some forms of long-term synaptic plasticity in the mesolimbic pathway are facilitated by retrograde signaling via endocannabinoids (eCBs) ⁽¹²⁸⁾. This eCB-dependent plasticity is most often mediated by activation of postsynaptic group I mGlu receptors, resulting in eCB synthesis and activation of eCB receptors, such as the cannabinoid receptor 1 (CB₁). It has previously been reported that pharmacological activation of group I mGlu receptors in the NAc by the agonist (S)-3,5-dihydroxyphenylglycine (DHPG) promotes mobilization of eCBs and subsequent reduction in Glu release through activation of CB₁ leading to a presynaptic form of long-term depression (LTD) ⁽¹³¹⁾. In the DA neurons of the VTA, under normal physiological states group I mGlu receptors do not drive LTD, even when strongly stimulated ⁽⁵¹⁾. However, after a single exposure to a drug, group I mGlu-mediated plasticity can be unmasked. The plasticity observed has been shown to be dependent on exchanging receptors with distinct subunit compositions. For example, it has been reported that following drug exposure a fraction of a-amino-3-hydrpxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors become GluR2-lacking and excitatory transmission is therefore Ca²⁺ permeable ^(132,133). Like many other brain regions, in the mesolimbic pathway a common cellular mechanism for mGlu-LTD is the reliance on rapid protein synthesis, such as activity-regulated cytoskeletal associated protein (Arc) and microtubule associate protein 1b (MAP1b), which is beyond the scope of this review ^(126,134).

1.4. Mesolimbic Dopamine in Drug and Food Reward: Homeostatic and Dysregulated States

Given the importance of the VTA in drug-related behaviors, the synaptic adaptations in VTA DA neurons have been extensively studied and reviewed elsewhere ^{(130,135–137)}. Numerous studies from a variety of laboratories have consistently demonstrated an increase in excitatory synaptic strength onto VTA DA neurons after in vivo exposure to drugs of abuse ^{(135,138,139)}. Many of these studies have examined the effect of drugs of abuse on the ratio of the AMPA to NMDA receptor current (AMPA/NMDA) in VTA neurons ⁽¹⁴⁰⁾, which allow one to compare the excitatory synaptic strength between different groups of animals (i.e., drug treated vs. drug naive). In vivo exposure to drugs of abuse increases the AMPA/NMDA ratio, which is mediated by insertion of Ca²⁺-permeable AMPA (CP-AMPA) receptors and removal of NMDA receptors in VTA DA neurons ⁽¹⁴¹⁾. The activity of VTA DA neurons is also important for feeding behavior ⁽⁴⁰,^142–144) as well as necessary for the formation of cue-reward associations ^(145–147) and effort- related food seeking ^(148,149). Previous studies have shown that these DA neurons have an important role in signaling the value of food-predictive cues that drive motivated behavior and encode properties related to feeding, such as reward identity and meal size ⁽¹⁵⁰⁾. Based on the shared neurocircuitry between drug addiction, obesity and BED, one could hypothesize that the AMPA/NMDA ratio in VTA DA neurons in obesity and BED could be altered; however, this still needs to be investigated.

A single exposure to a drug of abuse can impair NAc mGlu-LTD via the reduction of mGlu₅ and dysfunction of CB₁ ^{(51,128,137,151,152)}, thus leading to a remodeling of excitatory synaptic transmission and synaptic plasticity. It has been hypothesized that the abolishment of NAc mGlu-LTD following drug exposure may represent a mechanism to counteract the decrease in Glu neurotransmission ^(51,137), however, the underlying signaling mechanism remains unclear. In the context of food addiction mGlu-LTD has been much less reported on. This is particularly interesting given the parallels and overlap in natural and drug-reward neurocircuits and the anatomical location of group I mGlu receptors. Obesity is associated with changes in plasticity of the mesolimbic system ⁽¹⁵³⁾, therefore it would be interesting to investigate the role of group I mGlu receptors in this circuit. Based on the drug-addiction literature, one could hypothesize that mGlu-LTD would be impaired in preclinical models of obesity and BED.

Nearly, all addictive drugs display an ability to increase DA release within the ventral striatum ^(41,45), which is thought to underlie their rewarding effects. The rewarding and conditioning effects of drugs and food seem to be predominantly driven by transient and pronounced increases in DA by direct effects on the terminals ⁽⁶⁰⁾ that result in high DA concentrations that are necessary to stimulate low affinity D₁ ⁽¹⁵⁴⁾. Depletion of DA in the NAc induced by local 6-hydroxydopamine (6-OHDA) injections severely attenuates the rewarding effects of psychostimulants, as assessed by instrumental responses ^(155,156) or conditioned place preference (CPP) ⁽¹⁵⁷⁾. Interestingly, these dopaminergic responses might also play a role in the rewarding effects of foods and contribute to excessive consumption and obesity. Certain foods, particularly those rich in sugars and fat, are rewarding and may promote over-eating because like drugs, they increase NAc DA release ^(42,158). Chronic drug abuse induces dopaminergic stimulation that results in impaired inhibitory control, compulsive drug intake, and enhanced emotional reactivity to drugs ^{(8,41,59,135)}. Similarly, repeated exposure to high fat and sugar content foods results in compulsive food consumption, poor control of food intake, and food stimulus conditioning ^(3,31,159). Recent findings from human imaging studies support the idea that mechanisms of abnormal eating behaviors, including those observed in obese subjects, may have similarities to those underlying addiction to drugs of abuse. A recent study by Peleg- Raibstein et al. (2016) found that maternal over nutrition can lead to enhanced sensitivity to drugs of abuse and palatable food ⁽¹⁶⁰⁾, suggesting a common underlying neural mechanism.

The VP serves a critical role in reward and motivation and is frequently referred to as the limbic final common pathway because of its’ extensive connections to many regions involved in processing sensory information, reward and movement ^(161–163). It has been proposed to mediate various aspects of reward and translate motivational signals into actions. In addition to reward signals from the NAc, the VP also receives input from forebrain limbic structures and gustatory inputs ^(164,165). Signals from NAc to the VP are carried by MSNs that release GABA and opioid neurotransmitters ⁽¹⁶⁶⁾, and VP neurons express μ-opioid and GABA_A receptors ^(166–168), which form a gradient within the VP that mediate hedonic-liking reactions. Direct manipulations of the VP cause changes in the hedonic impact of sweet tastes ^{(162,169–171)} and reward value of other incentives, such as alcohol ⁽¹⁷²⁾ or cocaine ⁽¹⁷³⁾. It is important to note that changes to the mesolimbic DA system do not change hedonic values of natural or drug reward but are important for motivational aspects of reward learning. Rather, the hedonic aspects of reward learning are mediated by hedonic hotspots in the NAc shell and VP ⁽¹⁷⁴⁾. Within the NAc shell, hedonic reactions are housed rostrally and are mediated by subtypes of opioid receptors within this region. Similar to the NAc, the VP contains a hedonic hotspot. Stimulation of opioid receptors located in the posterior VP hotspot causes increases in the number of liking reactions elicited by sucrose ⁽¹⁶²⁾, whereas lesions of the VP cause liking reactions to be lost and replaced by aversive reactions ⁽¹⁷⁵⁾. Hedonic signals in the VP are encoded by neural firing rates ⁽¹⁷⁶⁾ and VP neurons respond to cues that predict the delivery of reward ^{(162,176,177)}. In addition to opioid signals, orexin signals in the posterior VP also can enhance hedonic impact of sucrose. It has been suggested that both the VP anatomical map and neuronal representations of hedonic liking (i.e., firing rates) are important for the normal hedonic impact of rewards ^(162,176). Pathological distortions of hedonic coding may cause hedonic dysfunctions that are implicated in eating disorders, drug addiction and other affective disorders.

Beyond natural rewards, the VP also has a well-established role in the reinforcing effects of drugs of abuse and reinstatement of drug seeking. Inhibition of GABAergic MSNs projecting to the VP and the resulting suppression of GABA release is common to various drugs of abuse and could be important for their reinforcing properties. Several drugs of abuse have been shown to suppress extracellular levels of GABA or inhibit GABAergic evoked inhibitory postsynaptic potentials (eIPSCs) in the VP ^(178–181). Recent work suggests that the VP is at a strategic anatomical position to integrate D₁- and D₂-MSN activity and relay this information to key motivational circuits ^{(161,162,182,183)}. Findings from Creed et al. (2016) suggests that the VP may be a site of convergence for drug-evoked synaptic changes that contribute to both positive and negative symptoms following cocaine experience ⁽¹⁸⁴⁾. In addition to changes in synaptic plasticity, administration of psychostimulants has been shown to increase DA release within the VP ⁽¹⁸⁵⁾. The neural activity in the VP has also been reported to encode the incentive value of both food and drug reward, and it has been suggested that maladaptive behavior in VP neural processes can lead to overeating and addiction ⁽¹⁸²⁾.

1.5. Impulsivity Impairments

Impulsivity has been defined as a failure to resist an impulse, despite potentially harmful consequences to oneself or others ^(186,187). Additional definitions of impulsivity include a deficient tolerance for delay of gratification and the inability to inhibit or delay voluntary behavior ⁽¹⁸⁸⁾. Human neuroimaging studies in both healthy controls with no prior report of mental illness and patient populations have provided a functional neuroanatomical link between impulsive phenotypes and the processing of appetitive and aversive stimuli in the mesolimbic reward system ^(189–193). Activation of the NAc has primarily been observed during DA-dependent reward tasks, with a dual role in signaling both reward prediction and prediction errors ⁽¹⁹⁴⁾. Importantly, converging evidence suggests that impulsivity modulates NAc reward responses differentially in healthy controls compared to patient populations. Most neuroimaging studies that have linked striatal reward processing to impulsivity suggest that the response of the NAc to reward is positively correlated with self-reported impulsivity ⁽¹⁸⁸⁾. In contrast to these findings, higher rates of impulsivity observed in both food- and drug-addiction are accompanied by reduced NAc activation during reward anticipation and feedback processing ^(195,196).

Inhibitory response control is often linked to interconnecting regions of the frontal lobe and basal ganglia ⁽¹⁹⁷⁾. The umbrella term of impulsivity can be further broken down into “waiting” and “stopping” impulsivity, both of which are dependent on different brain structures. “Stopping” impulsivity is dependent on motor areas, the orbitofrontal cortex as well as interactions with the dorsal striatum ^(198,199). Whereas “waiting” impulsivity, also known as premature or anticipatory responding, depends on top-down prefrontal cortical interactions with the hippocampus, amygdala and NAc ^{(188,200,201)}. Dopaminergic mechanisms and DA receptor subtypes play an important role in “waiting” impulsivity ⁽²⁰²⁾. Given that reductions in striatal D₂ availability has been found in individuals with SUDs, obesity and BED ^{(5,43,203,204)}, discussed in more detail below, these findings are of clinical interest because these findings suggest a reduction in activity in ventral striatal regions.

An example of a test of “waiting” impulsivity that has high translational value is delay discounting. Delay discounting is defined by the choice for a small, immediate reinforcer over a larger, delayed reinforcer; that is, delay discounting is the decline in the present value of a reward to its receipt. The neurocircuitry involved in delay discounting can be mapped onto multiple neural regions that are important for the evaluation of reward, cognitive control and prospection ^(205,206). Clinically, impulsive individuals may not only be more likely to suffer from SUD but also with food addiction ⁽⁶⁾. Impulsivity has been consistently linked to the development and expression of both substance abuse and BED ^(207,208). Furthermore, impairments in delayed discounting have been positively correlated with family history of drug use disorders, suggesting that deficits in impulsivity may represent a potential risk factor for development of SUDs. Similar to what is observed with drugs of abuse, individuals with BED have impairments in delayed discounting ⁽²⁰⁹⁾, such that patients will choose immediate over larger, delayed rewards. It has been suggested that in both food and drug addiction the increase in impulsivity that is observed is due to underlying changes in the DA system; specifically, there are changes in DA in brain regions that are important for cue-encoding ⁽²¹⁰⁾. In addition to these clinical results, data from preclinical studies have shown that deficits in delayed discounting can predict higher self-administration rates, escalation of drug intake, increased drug-seeking during abstinence, and greater vulnerability to cue-induced reinstatement ^(211–215). In the context of natural reinforcers, deficits in delayed discounting are associated with greater escalation of sucrose-seeking behavior and reinstatement after extinction ⁽²¹⁶⁾. However, the relationship between impulsivity and BED is not well-established; therefore additional studies are needed to examine this relationship.

In addition to the changes reported in DAergic systems, group I mGlu receptors have also been shown to modulate impulsive choice. Studies using positive allosteric modulators (PAMs) of mGlu₅ have repeatedly shown that activation of mGlu₅ decreases impulsive choice ⁽²¹⁷⁾ and can attenuate disruptive effects of NMDA antagonism ⁽²¹⁸⁾. In contrast to mGlu₅, literature focusing on impulsive choice following blockade of mGlu₁ has yielded mixed results. Earlier studies have reported that administration of an mGlu₁ negative allosteric modulator (NAM) reduces impulsive choice in rodents ⁽²¹⁹⁾. However, a recent study conducted by Yates and colleges (2017) found that antagonism of mGlu₁ increases impulsive choice ⁽²²⁰⁾. The inconsistency in these findings may be dependent on the task used or lack of selective compounds. It is evident that mGlu₁ is an important mediator of impulse-control; however, future studies utilizing highly selective compounds are needed to investigate the role of mGlu₁ in “waiting” impulsivity. mGlu₁ is highly expressed in both NAc and VP ⁽¹¹⁶⁾, however, it is unknown whether mGlu₁ preferentially influences D₁- or D₂-MSN activity. Therefore, future studies should investigate the modulatory role of mGlu₁ on D₁ and D₂-MSNs.

1.6. Motivational Impairments: Obesity and BED

The NAc is a terminal field of the mesolimbic dopaminergic system involved in hedonic and motivational aspects of feeding, as well as a location in which endogenous opioids act to both modulate DA release and affect hedonic processes associated with food evaluation, consumption and reward processes ^(143,221). NAc DA is equally important for regulating behavioral activation, energy expenditure and enabling organisms to overcome work-related response costs ^(99,222). Dysregulated DA signaling is associated with enhanced motivation to procure drugs ^(45,46,59), a hallmark of SUD. The effect of NAc DA on food-reinforced behavior interacts powerfully with the response requirements of instrumental tasks. Research with concurrent choice tasks involving distinct food reinforcers that can be obtained by instrumental behaviors with different work requirements has shown that rodents with accumbens DA depletions or DA receptor antagonism reallocate their instrumental behavior away from food- reinforced tasks that have high response requirements (e.g., ratio requirements or vigorous activities such as climbing) and instead select a less-effortful option ^(99,222,223).

One way to assess the reinforcing efficacy of natural and drug rewards is the use of a progressive ratio (PR) schedule of reinforcement. Under PR schedules, the response requirement increases with each subsequent reinforcer delivery. Reinforcing efficacy is operationally defined by PR performance as the breakpoint, i.e. the highest ratio the animal completes, the total responses made, or the reinforcers earned. PR schedules promote high levels of responding while limiting the number of reinforcers earned. Rodents fed a chronic high fat diet (HFD) have reduced instrumental performance and a lower break point when trained on interval, but not ratio, schedules of reinforcement ⁽²²⁴⁾. The impact of reinforcement schedules on instrumental performance has long been recognized and random ratio schedules tend to maintain higher rates of behavior than interval schedules, even when reinforcement rate is controlled. Whereas interval schedules maintain a weaker relationship between the rate of lever pressing and rate of reinforcement, a difference which may be exacerbated by HFD exposure explaining decreased instrumental performance and motivation ^(224–226). It is important to note that the size of the incremental value of the PR schedule can affect performance in rodents maintained on HFD as well as type of reinforcer. Interestingly, there are discrepancies in PR performance between animals maintained on intermittent and daily schedules of HFD, however, the mechanism underlying these differences are not clear.

In addition to DA being an instrumental requirement for obtaining access to motivational stimuli, considerable research indicates that physical activities can have intrinsic motivational or reinforcing properties. In preclinical studies, one of the most commonly studied voluntary physical activities is wheel running. Wheel running can be used as the motivational stimulus as an explicit reinforcer in operant-conditioning procedures ^(227–229). Interestingly, if a running wheel is present in a complex environment that offers other alternatives animals will spend a considerable amount of time engaged in running activity. Interestingly, running wheel activity can be attenuated following systemic or intra-accumbens infusion of the D₂ antagonist, haloperidol ^(230,231), at doses which do not suppress locomotor activity. These findings are particularly interesting in light of human imaging data showing reduced activation and DA turnover in the NAc in obese ⁽²³²⁾ and BED ⁽¹⁵⁸⁾ patients. Recent clinical data suggests that an increase in physical activity leads to a reduction in binge episodes ^(233,234). Future studies should therefore investigate effort allocation in a concurrent choice assay of sedentary and physical activity in animals on intermittent access to HFD.

1.6. Polymorphisms in D₂ Receptor and the DA Transporter

A seminal paper played an important role suggesting that there is a parallel between obesity and drugs of abuse, namely alterations in D₂ ⁽²³⁵⁾. D₂ is located both pre-synaptically as autoreceptors as well as post-synaptically ⁽²³⁶⁾. D₂ is widely expressed in the striatum, VTA and prefrontal cortex (PFC), areas involved in the primary reinforcing effects of natural and drug rewards. Imaging studies show a reduction in D₂ and DA release in the striatum in SUD patients compared to healthy controls ⁽⁴⁶⁾. Interestingly, this decrease is seen across SUDs independent of the substance used. Moreover, obese individuals and SUD patients show reduced expression of D₂ in striatal areas ⁽⁴⁰⁾ and imaging studies have demonstrated that similar brain areas are activated by food and drug-related cues ^(45,46).

The influence of D₂ on addiction-related behaviors has been a curiosity, in part, because receptor availability has been linked to response to both natural and drug rewards ⁽⁴⁰⁾. Radioligand binding studies have shown that individuals with a wide variety of SUDs, including but not limited to alcohol, cocaine, and opioids, have significant reductions in D₂ availability in both dorsal and ventral striatum that persists months after withdrawal ^(46,237). Reduced striatal D₂ availability has also been found in morbidly obese individuals and is correlated with higher body weights. Furthermore, obese subjects following gastric bypass surgery had increased D₂ availability which was proportional to weight loss ^{(203,238,239)}. In non-addicted individuals, baseline measures of D₂ have been shown to predict the subjective responses to drugs of abuse ^(45,240). For example, individuals describing the drug use as pleasant had lower levels of D₂ compared with individuals that described the experience as unpleasant⁽⁴⁵⁾. Together these findings suggest a possible role for D₂ in the inhibitory control of compulsive behaviors. Therefore, it is conceivable that in both obesity and drug addiction D₂ may play a greater role in regulating vulnerability to addictive-like behaviors, such as compulsive food intake, in obese individuals.

The midbrain DA system plays a crucial role in regulating reward-related behaviors. Decreased somatodendritic D₂ availability is implicated in novelty seeking and impulsivity in humans and rodents ^(241,242). These character traits have been associated with clinical studies of drug addiction and obesity and further supported by preclinical studies. For instance, rats that exhibit enhanced cocaine self-administration show sub-sensitivity of D₂ somatodendritic autoreceptors ⁽²⁴³⁾. Likewise, mice lacking the D₂ autoreceptor display elevated DA release ⁽²⁴⁴⁾ and are hypersensitive to the psychomotor effect of cocaine ⁽²⁴⁵⁾. Interestingly, a recent study by de Jong et al. (2015) compared food and drug seeking in VTA D₂ knockdown mice ⁽²⁴⁶⁾. The researchers found that decreased VTA D₂ expression markedly increased motivation for both food and drug reward under a progressive ratio schedule of reinforcement, but acquisition or maintenance of behavior were not affected, suggesting VTA D₂ downregulation renders animals more motivated to work for a reward. This is consistent with the well-established notion that mesolimbic DA mediates incentive motivation and willingness to work for rewards, especially when the effort requirement is high ⁽²²³⁾. It has been noted that obese patients that underwent gastric bypass surgery have reduced progressive ratio performance for refined sugars and fat compared to unoperated obese patients ^(247,248), however, whether this difference in performance is due to increased availability of VTA D₂ is unknown.

Allelic variants of the Taq1A polymorphism, located 10 kb downstream from the coding region of the D₂ gene, have been investigated for possible associations with substance abuse, including abuse of alcohol, cocaine, nicotine, and opioids ^(203,249), as well as food-related addictions, including obesity and BED ^(250,251). There are three allelic variants of the Taq1A polymorphism: A1/A1, A1/A2 and A2/A2. Based on evidence from imaging and postmortem studies, individuals with one or two copies of the A1 allele contain approximately 40% D₂ compared to those without the allele and lower mean glucose metabolism rate in dopaminergic brain regions ^(252,253). Association analysis studies have illustrated TaqA1 polymorphisms have a positive association with drugs of abuse, however, whether this polymorphism increases risk for drug dependence remains to be elucidated. Interestingly, the link between TaqA1 and feeding-related behavior seems to be well documented. Studies have shown food reinforcement and energy intake was greater in obese individuals that carried the TaqA1 allele compared to non-carriers. Obese individuals comorbid with BED were characterized by stronger activation of their striatum when compared to obese but non-binging counterparts, a difference that was associated with carriers of TaqIA ^(251,254). Interestingly, an imaging study conducted by Stice and colleagues found weaker activation of striatal regions in response to food intake in obese relative to lean individuals and this abnormal signal strongly predicted future weight gain ⁽²⁵⁵⁾. The Taq1A allele has been implicated in both obesity and substance use disorders and is thought to increase reward sensitivity in the striatum by elevated DA activity levels.

Polymorphisms involving the variable number of tandem repeats (VNTR) have been described in the 3’ untranslated region of the SLC6A3 gene coding for the DA transporter (DAT) has been reported in both obesity ^(256,257) and drug addiction ^(258–260). DAT is important for maintaining dopaminergic tone via sequestering DA back into the presynaptic neuron. In vitro studies suggest that the SLC6A3 VNTR polymorphism influences gene expression and DAT availability. Several groups have reported shorter alleles of VNRT are associated with substance dependence ^(258,261) as well as obesity and binge eating disorder ⁽²⁶²⁾. These data suggesting that food and drug addiction may share a genetic causative mechanism associated with the dopaminergic system.

1.7. Shared Glutamate Alterations: Group I mGus

In addition to the shared overlap with the dopaminergic system, both food and drug addiction share similarities in the Glu system as well ^(140,263). Glu has been found to play a role in the regulation of food intake, drug seeking behavior, and modifying binge eating ^{(140,263–267)}. Heightened glutamatergic intervention and pre- and post-synaptic receptor distribution is an equally important regulator as dopamine in addiction. In fact, the mesocorticolimbic DA system is intricately connected with glutamatergic structures or afferents and acts as a high pass filter onto incoming glutamatergic synapses ^(52,268). Glutamate interacts with both ionotropic (iGluRs) and mGlu receptors to regulate a variety of cellular activities within the basal ganglia and both receptor families have been implicated in drug addiction, obesity and BED ^{(140,269–271)}.

Direct and indirect modulation of synaptic plasticity by addictive drugs has received much attention, because there is an emerging consensus that SUD ultimately is a disease of goal-directed learning ^(58,272). In brief, this model posits that drugs promote the learning of drug-related behaviors with such efficiency that they become compulsive. Excessive levels of DA in response to the exposure to an addictive drug would be permissive for a pathological form of synaptic plasticity of glutamatergic transmission. With normal rewards, the learning signal becomes quiescent once the behavior is predictive of the outcome, and addictive drugs override this mechanism. It will be important to test whether obesity is related to the development of habit-like consummatory behavior resulting from plasticity in dorsal striatum in the same way that drug addiction may be related to striatal remodeling and the emergence of habit-like drug seeking behaviors ^{(47,273–275)}.

On a cellular level several studies demonstrate that addictive drugs evoke long-term alterations of synaptic transmission in the mesolimbic dopaminergic system ^(137,276). Interestingly, although there are several shared similarities between drug addiction and obesity, there are only a handful of studies that have tested whether BED changes excitatory and inhibitory synapses in the mesolimbic pathway. A recent study conducted by Brown and colleagues (2017) found that obesity-prone rodents had potentiated NAc core Glu synapses as measured by NMDA decay currents and these synapses were unable to undergo long-term depression (LTD). These results are highly consistent with what is observed in animal models of drug addiction. The findings generated by Brown et al. (2017) provide the first evidence for addiction-like synaptic impairments in the NAc core of diet-induced obese rats ⁽²⁷⁷⁾, but whether the mechanisms leading to these impairments are similar to those implicated in drug addiction is not known and therefore should be explored.

There is a myriad of studies suggesting a role for mGlu₅ in the formation of addictive-like behavior ^{(53,278–280)} and recent work implicates mGlu₅ in central reward pathways. ⁽²⁸¹⁾. Data from a number of behavioral studies using mice with targeted deletions as well as pharmacological inhibition of mGlu₅ via antagonists or negative allosteric modulators (NAMs) ^(282–287), suggest that this receptor plays a critical role in behavioral responses to psychostimulants as well as other addictive substances. It is important to note however that the findings with mGlu₅-deficient mice on drug or alcohol self-administration are contradictory and may be attributed to strain differences. For example, Chiamulera (2001) reported that mGlu₅ deficient mice fail to acquire cocaine self-administration despite showing increased extracellular DA levels in the NAc ⁽²⁸¹⁾. These findings are in stark contrast to recent studies from studies utilizing a line of mGlu₅-deficient mice utilized by Lawrence and colleagues demonstrated that while mGlu₅-deficient mice acquire self-administration, but the loss of mGlu₅ results in an impairment in the ability to extinguish drug related behaviors ⁽²⁸⁸⁾. Furthermore, the impairment in spatial learning is thought to be due to an impairment in hippocampal LTP ⁽²⁸⁹⁾. Interestingly, few studies have investigated the potential utility of mGlu₅ antagonists for the treatment of BED ^(269,290). Bisaga and coworkers (2008) demonstrated that the mGlu₅ antagonist MPEP decreased candy consumption in a baboon model of BED ⁽²⁶⁹⁾. Moreover, mice lacking mGlu₅ maintain lower body weight compared to wildtype controls ⁽²⁹¹⁾. These findings are not surprising as mGlu₅ is well positioned to regulate and fine-tune neuronal excitability and synaptic transmission. Several transmitter systems interact with mGlu₅-containing cellular signaling pathways, including dopaminergic, cannabinoid, and serotonergic systems, and they have the potential to modify reward behavior through interactions at the level of the mesolimbic DA system. It has been suggested that reduced drug and food intake observed after antagonizing mGlu₅ may be related to a reduction in the rewarding value of reinforcing stimuli ⁽²⁶⁹⁾, however, future studies are needed to investigate the role of mGlu₅ in the formation of addictive-like behaviors. Additionally, it would be interesting in clinical subpopulations to correlate mGlu₅ polymorphisms to predisposition of addictive-like behaviors. Data from several behavioral studies suggest a role for mGlu1 in the expression of reward seeking behavior for drugs of abuse, such as reduced self-administration and CPP, however, the contribution of mGlu is less understood than mGlu₅. Previous studies have shown that activation of mGlu₁ via administration of a PAM causes synaptic depression in the NAc and suppresses cue-induced craving, which is mediated via reduction of CP-AMPA receptors ⁽²⁹²⁾. Similarly, the upregulation of NAc CP-AMPARs seen following cue-induced drug cravings, is also observed in obesity-susceptible rats ⁽²⁹³⁾. Enhancing mGlu activity by a PAM reverses cocaine-induced plasticity in DA neurons ^(294,295) and MSNs of the NAc. Although direct comparisons to drugs of abuse are more difficult to make, it would be interesting to explore the ability of mGlu modulators to suppress cue-induced cravings and CP-AMPARs in rodent models of BED and plasticity changes observed in obesity. Additionally, mGlu within the NAc regulates drug intake ⁽²⁹⁶⁾, to date no studies have investigated the role of mGlu on food-maintained responding. A recent report by Yohn et al. (2018) found that activation of mGlu₁ does not reduce progressive ratio (PR) responding for liquid reward ⁽²⁹⁷⁾ but the role of mGlu₁ in chronic pathological conditions within the mesolimibic pathway has not been investigated. Therefore, future studies should examine the role of mGlu₁ in the development and maintenance of food-related responding. This basic understanding of mGlu₁ in instrumental conditioning will be influential to our understanding of alterations in mGlu₁ in eating disorders, such as obesity and BED, which can be regarded as disorders of motivation and decisionmaking.

1.8. Shared Glutamate Alterations: Group II and III mGlus

While a detailed discussion about group II and III mGlu in the neuropathology of drug addiction, obesity and BED is beyond the scope of this review (see ^{(120,298,299)}for detailed review), it is important to briefly note the overlap alterations in other mGlu families as they relate to the mesolimbic pathway. Both group II (mGlu_2/3) and III mGlu receptors (mGlu_4/6/7/8) are expressed in the mesolimbic DA pathway. It is important to note alterations in group II and III mGlus are not limited to addiction. Malfunction of both group II and III mGlus have been linked to the pathogenesis of many other neurological diseases, such as schizophrenia, anxiety, depression, and Parkinson’s disease.

NAc DA release is bi-directionally controlled by group II mGlu receptors. For example, intra-accumbens infusions of agonists or antagonists decrease or increase basal DA levels, respectively ^{(50,300–302)}. Group II mGlu receptor regulation of DA release depends on activation of voltage-dependent Ca²⁺ channels ⁽⁵⁰⁾; however, it is important to note that it is unclear whether this is mediated by mGlu_2/3 on DA terminals or whether mGlu_2/3 regulates Glu terminals on MSNs which in turn project to dopaminergic cells in the VTA and other regions within the motivational circuit ^(303,304) are mediated by mGlu_2/3. Therefore, it is not surprising that repeated exposure to drugs of abuse alters Group II mGlu receptor function. mGlu_2/3 exert inhibitory efforts on excitatory transmission in the VTA and NAc, an effect which is enhanced after early withdrawal from chronic morphine ⁽³⁰⁵⁾. Alterations in mGlu_2/3 function have also been observed following chronic exposure to drugs of abuse, such as decreased or uncoupling from Gi subunits ⁽²⁹⁹⁾. In addition to deficits in receptor density and signaling, mGlu_2/3 plasticity is also impaired following exposure to drugs of abuse and has been suggested to be related to impairments in behavioral flexibility observed following drug use ⁽²⁹⁹⁾. Interestingly, the role of group II mGluRs in obesity and BED has not yet been explored, therefore, future studies are needed to investigate the role of mGlu_2/3 preclinical models in the mesolimbic circuit.

Of particular interest to both drug addiction and obesity may be the role of mGlu₃ selective compounds to regulate microglia alterations. Microglial cells are the most abundant immune cells and are relatively quiescent under baseline conditions; however, when exposed to injury signals, microglia undergo rapid reaction characterized by morphological and functional changes. Activated microglia are thought to contribute to addiction-related plasticity changes in several ways ⁽³⁰⁶⁾, including activation of proinflammatory cytokines, synaptic remodeling, and neurotoxicity. In obesity models, exposure to HFD causes hypothalamic microglial to undergo morphological and function changes (see ⁽³⁰⁷⁾ for detailed review) and is linked to obesity- associated cognitive decline ⁽³⁰⁸⁾. Approaches aimed at dampening inflammatory activity in preclinical models have proven to be beneficial to correct both drug and obesity phenotypes, thus future investigations using selective mGlu₃ agents are needed.

Similar to the effects observed with group II mGlu receptors, evidence suggests that Group III mGlu receptors (mGlu_4/7/8) regulate behavioral sensitivity to psychostimulants, such as amphetamine and cocaine, and bi-directional regulate DA release in the ventral striatum (see ⁽¹²⁰⁾ for review). The inhibition of DA release observed following activation of group III mGlu receptors has been attributed to either a heteroreceptor located on DA terminals or an inhibition of Glu release by autoreceptors. However, the lack of selective ligands for many of the individual group III mGlu receptor subtypes has made it difficult to fully establish the roles of these receptors. Furthermore, the roles of group III mGlus have not yet been investigated in the context of obesity and BED.

1.9. Novel Approaches: Allosteric Modulators

While the use of traditional orthosteric site mGlu agonists and antagonists have been suggested for the treatment of BED and obesity, these compounds have several limitations, which include increased risk of adverse effects and potential for greater tolerance with chronic dosing ^(309,310). The approach of targeting allosteric binding sites, which are topographically distinct from the orthosteric site and less conserved across receptor subtypes, have been developed and hold several advantages over traditional compounds. Namely, allosteric modulators possess high subtype selectivity and can either activate the receptor by themselves or modulate receptor activation. Allosteric activators can include allosteric agonists, which act at a site removed from the orthosteric site to directly activate the receptor in the absence of the endogenous ligand ^{(309,311,312)}. Furthermore, allosteric modulators can also be neutral, positive or negative, which do not activate the receptor directly but modulate activation of the receptor by the endogenous orthosteric agonist. A key advantage of positive allosteric modulators (PAMs) is that they can maintain the temporal and spatial organization of physiological receptor activation and impose a “ceiling” on the magnitude of allosteric effect. Together, these properties may reduce the side effect potential relative to orthosteric agonists, which stimulate a given receptor independently of its physiological state ^{(309,313–315)}. Multiple studies suggest that mGlu₅ receptors are critical regulators of learning and memory and are important for reinforcer-associated conditioned stimuli ⁽³¹⁶⁾, which may explain why mGlu₅ NAMs attenuate drug self-administration for substances, such as cocaine ^(317,318), ethanol ^(283,319) and nicotine ^{(285,286,318,320)}. Furthermore, it has been found that mGlu₅ expression in D₁-MSNs are important for modulation of cocaine reinforcement and learning ⁽³²¹⁾. Additionally, mGlu₅ PAMs have been suggested to be beneficial in alleviating the cognitive deficits associated with chronic drug abuse and animal models of addiction have shown that these compounds facilitate extinction^(322,323). Taken together these findings suggest that mGlu₅ is positioned to regulate the neurocircuitry involved in addictive-like behaviors ^(53,278,324). Clinical studies with mGlu₅ allosteric compounds should be conducted in accordance with the National Institute of Mental Health (NIMH) Research Domain Criteria (RDoC) should consider cutting across disorders characterized by action-to-habit devolution ^(325,326), these include substance abuse disorders but BED, bulimia nervosa, and pathological gambling.

Interestingly, recent studies reveal that mGlu5 allosteric modulators have the potential to confer stimulus bias to mGlu₅ signaling. The term biased agonism describes the phenomenon in which an agonist can preferentially activates specific signaling pathways that are coupled to a given receptor ^{(115,327,328)}. Allosteric modulators can confer bias by selectively modulating specific responses to the natural agonist, such as glutamate. to different signaling pathways. mGlu₅ is pleiotropically coupled, thus it is conceivable that activation by diverse ligands may prompt unique receptor conformations. It has previously been reported that some mGlu₅ PAMs can bias signaling toward increased ERK 1/2 phosphorylation relative to Ca²⁺ mobilization ⁽³²⁸⁾. Furthermore, mGlu₅ PAMs have been identified that selectively potentiate mGlu₅-mediated Ca²⁺ mobilization without modulating coupling of the receptor to potentiation of NMDA receptor currents⁽³²⁹⁾. At present, the impact of stimulus bias on responses to mGlu₅ modulators in models of addiction, obesity, and BED have not been evaluated. However, it is possible that selective potentiation or inhibition of specific signaling pathways could confer advantages for specific therapeutic outcomes, which could encourage development of mGlu₅ PAMs that have been optimized for biased modulation.

Compared with the extensive literature that has been reviewed on the role of mGlu₅ in mediating drug reward, reinforcement, and relapse, few studies have been published on the specific role of mGlu₁, due to the non-selectivity of earlier compounds. Studies have shown that blockade of mGlu₁ reduces alcohol self-administration ^(296,330) and reinstatement of nicotineseeking behavior ⁽³³¹⁾. However, additional studies are needed to provide a comprehensive understanding of the neural circuitry in which these receptors regulate various aspects of obesity and BED. The role of mGlu₁ in the pathology of addictive-like behaviors can be further explored due to the recent development of highly selective allosteric modulators, such as VU6004909, a novel mGlu₁ PAM ⁽³³²⁾, which will serve as a tool to probe the neurocircuitry underlying addiction. The high expression of mGlu₁ in the VTA and the ability of mGlu₁ to influence DA neuron firing patterns ⁽³³³⁾, presents an interesting series of questions that can be explored through utilization of mGlu₁ PAMs. Moreover, mGlu₁ has been found to regulate dorsal striatal DA release through mobilization of an eCB ⁽²⁹⁷⁾. This is of particular interest based on findings by Oleson and colleagues (2012) which found eCBs shape NAc encoding of cue- motivation ⁽³³⁴⁾, suggesting that mGlu₁ may be positioned to modulate DA through local release of an eCB. However, future studies are needed to investigate the role of mGlu₁ on accumbens D₁- and D₂-MSNs.

2.0. Conclusions

A key feature of drug addiction is compulsive use despite adverse consequences, a feature that also occurs in BED and obesity. Neuroimaging techniques are starting to reveal significant overlap in the brain circuitry underlying addiction and disorders of dysfunction over rewarding behaviors (such as BED and obesity). Several imaging studies have documented brain activation abnormalities that implicate DA-modulated pathways, such as the mesolimbic system, as well as alterations in Glu and DA transmission and receptor expression within this circuit. It has been suggested that decreased sensitivity of reward circuits could result in a decreased interest for environmental stimuli, possibly predisposing subjects to seek drug stimulation as a means to temporarily activate these reward circuits. Neuroimaging studies have also revealed an interaction of increased incentive response to drug cues, propensity for habit formation, poor self-control, and heightened negative emotionality in response to drugs of abuse and overeating ⁽³³⁵⁾. Overall, imaging studies provide evidence for the existence of shared neural mechanisms associated with obesity and different forms of addiction. As discussed above, the disruptions in the mesolimbic pathway are also characterized by maladaptive decision-making and motivated behavior. It is critical to note these maladaptive motivational states are also characterized by deficits in top-down control, a component which is strongly correlated to the mesocortical pathway (for detailed review see ⁽⁴⁾). Additionally, it is important to keep in mind that several neurotransmitters, hormones and peptides are involved in food intake have been implicated in the rewarding effects of food and drugs of abuse and the overlap in the DA system is best characterized.

A key research tool to gain further insight into overlapping pathophysiological mechanisms in drug addiction, obesity and BED is preclinical models. While it is important to recognize that no animal model can recapitulate all aspects of the clinical disease, these experimental systems serve as valuable tools for examining component processes and pathophysiologic/therapeutic mechanisms with a level of precision that cannot be achieved in clinical research. These preclinical models, aim to mimic disturbances hypothesized to contribute to the etiology of disease states. Genetics and epigenetic changes in the DA system have been extensively studied in drug addiction; however, relatively few studies have investigated these changes in obesity or BED. Future studies should investigate whether genetic changes observed in drug addiction are similar to those seen in states of overeating. In line with this notion, research on cellular and molecular changes in group I mGlu receptor-related proteins, second messengers, and receptors are currently being investigated in addiction, obesity and BED. This is particularly interesting in light of recent findings showing increased mGlu₁ signaling in the dorsal striatum facilitates hypermotivation for reward ⁽³³⁶⁾. It would be interesting if elevated mGlu₁ signaling is seen during reinstatement for drugs of abuse as well as in states of overeating.

A wealth of preclinical studies demonstrate that cue-triggered motivational responses are mediated by brain mesolimbic circuits, particularly DA and Glu transmission. Enhanced cue triggered motivation and accompanying increases in NAc responsivity are thought to underlie drug addiction and NAc CP-AMPARs mediate enhanced cue-trigged cocaine- seeking (see ⁽¹²⁾ for review). Excitingly, recent findings suggest that obesity-prone animals exhibit a similar enhancement in NAc responsivity and cue-triggered ⁽²⁹³⁾ cues as that seen in drug addiction. It should be mentioned, however, drug-cues are more powerful triggers of reinforcer-seeking behavior than food cues, so it may not an exact one to one correlation. With this in mind, future studies are need to confirm these similarities and differences in cue-induced reinstatement and responding. Additionally, preclinical models of motivational dysfunction, such as impulsivity impairments, cost/benefit decision making, and work output in response to drug or natural rewards, have high constructive and predictive validity to clinical populations and therefore should be utilized to test novel therapeutic agents. Specifically, the development of highly selective compounds will provide researchers with a tool to probe group I mGlu receptor subtypes in the mesolimbic pathway in relation to both drugs of abuse and natural rewards. These pharmacological tools in conjunction with techniques, such as optogenetics, fiber photometry, and in vivo electrophysiology, will pave the way to novel avenues of research.

Figure 1: Shared Basal State Mesolimbic Circuitry.

Drug addiction, obesity, and binge eating disorder (BED) have a significant degree of overlap in the mesolimbic dopamine (DA) pathway. (A) Both natural and drug rewards exert their mechanisms of effect in the ventral tegmental area (VTA) to augment activity of GABAergic and DA neurons. The VTA receives GABAergic input from D₁-medium spiny neurons (MSNs), which have iGlu and group I metabotropic glutamate (mGlu) receptors located on the terminal. The VTA also receives Glu inputs from the prefrontal cortex (PFC) and pedunculopontine tegmental nucleus (PPTg) as well as cholinergic input from the pedunculopontine nucleus (PPn) and mesopontine laterodorsal tegmental nucleus (LDTn). (B) MSNs within the nucleus accumbens (NAc) receive DA input from the VTA, Glu input from the PFC, thalamus, amgydala, and hippocampus (HPC), as well as GABA input from the ventral pallidum (VP). MSNs also receive cholinergic modulation via NAc cholinergic interneurons. (C) GABAergic neurons in the VP receive input from both D₁- and D₂-MSNs, DA input from the VTA, and cholinergic modulation from cholinergic interneurons.

Figure 2: Proposed Site of Action of Allosteric Modulators of Group I Metabotropic Glutamate Receptors.

Administration of group I allosteric modulators is hypothesized to attenuate deficits in inward current, protein kinase-dependent responses and dopamine (DA) release within the ventral tegmental area (VTA); restore deficits in synaptic plasticity (i.e., longterm depression [LTD] and potentiation [LTP]) from cortical regions; restore augmentations in dendritic spine density on medium spiny neurons (MSNs), MSN excitability, and synaptic plasticity between the nucleus accumbens (NAc) and the ventral pallidum (VP).