Mapping of reinforcing and analgesic effects of the mu opioid agonist endomorphin-1 in the ventral midbrain of the rat

Ventral tegmental area glutamate neurons establish a mu-opioid receptor gated circuit to mesolimbic dopamine neurons and regulate opioid-seeking behavior

A two-neuron model of ventral tegmental area (VTA) opioid function classically involves VTA GABA neuron regulation of VTA dopamine neurons via a mu-opioid receptor dependent inhibitory circuit. However, this model predates the discovery of a third major type of neuron in the VTA: glutamatergic neurons. We found that about one-quarter of VTA neurons expressing the mu-opioid receptor are glutamate neurons without molecular markers of GABA co-release. Glutamate-Mu opioid receptor neurons are largely distributed in the anterior VTA. The majority of remaining VTA mu-opioid receptor neurons are GABAergic neurons that are mostly within the posterior VTA and do not express molecular markers of glutamate co-release. Optogenetic stimulation of VTA glutamate neurons resulted in excitatory currents recorded from VTA dopamine neurons that were reduced by presynaptic activation of the mu-opioid receptor ex vivo, establishing a local mu-opioid receptor dependent excitatory circuit from VTA glutamate neurons to VTA dopamine neurons. This VTA glutamate to VTA dopamine pathway regulated dopamine release to the nucleus accumbens through mu-opioid receptor activity in vivo. Behaviorally, VTA glutamate calcium-related neuronal activity increased following oral oxycodone consumption during self-administration and response-contingent oxycodone-associated cues during abstinent reinstatement of drug-seeking behavior. Further, chemogenetic inhibition of VTA glutamate neurons reduced abstinent oral oxycodone-seeking behavior in male but not female mice. These results establish 1) a three-neuron model of VTA opioid function involving a mu-opioid receptor gated VTA glutamate neuron pathway to VTA dopamine neurons that controls dopamine release within the nucleus accumbens, and 2) that VTA glutamate neurons participate in opioid-seeking behavior.

The rostromedial tegmental nucleus RMTg is not a critical site for ethanol-induced motor activation in rats

RATIONALE

Opioid drugs indirectly activate dopamine (DA) neurons in the ventral tegmental area (VTA) through a disinhibition mechanism mediated by mu opioid receptors (MORs) present both on the GABA projection neurons located in the medial tegmental nucleus/tail of the VTA (RMTg/tVTA) and on the VTA GABA interneurons. It is well demonstrated that ethanol, like opioid drugs, provokes VTA DA neuron disinhibition by interacting (through its secondary metabolite, salsolinol) with MORs present in VTA GABA interneurons, but it is not known whether ethanol could disinhibit VTA DA neurons through the MORs present in the RMTg/tVTA.

OBJECTIVES

The objective of the present study was to determine whether ethanol, directly microinjected into the tVTA/RMTg, is also able to induce VTA DA neurons disinhibition.

METHODS

Disinhibition of VTA DA neurons was indirectly assessed through the analysis of the motor activity of rats. Cannulae were placed into the tVTA/RMTg to perform microinjections of DAMGO (0.13 nmol), ethanol (150 or 300 nmol) or acetaldehyde (250 nmol) in animals pre-treated with either aCSF or the irreversible antagonist of MORs, beta-funaltrexamine (beta-FNA; 2.5 nmol). After injections, spontaneous activity was monitored for 30 min.

RESULTS

Neither ethanol nor acetaldehyde directly administered into the RMTg/tVTA were able to increase the locomotor activity of rats at doses that, in previous studies performed in the posterior VTA, were effective in increasing motor activities. However, microinjections of 0.13 nmol of DAMGO into the tVTA/RMTg significantly increased the locomotor activity of rats. These activating effects were reduced by local pre-treatment of rats with beta-FNA (2.5 nmol).

CONCLUSIONS

The tVTA/RMTg does not appear to be a key brain region for the disinhibiting action of ethanol on VTA DA neurons. The absence of dopamine in the tVTA/RMTg extracellular medium, the lack of local ethanol metabolism or both could explain the present results.

The effect of intraperitoneal and intra-RMTg infusions of CTAP on rats' social interaction

Social interaction is necessary for the development of individuals and society. Social interaction behaviors are rewarding. Similar to exogenous opioids, social interaction behaviors are able to induce rewarding effects that are regulated by the endogenous opioid system as well. As one type of opioid receptor, μ-opioid receptors (MORs), are densely expressed in the rostromedial tegmental nucleus (RMTg), which results in the RMTg being extremely sensitive to rewarding effects induced by exogenous and endogenous opioids. Here, we investigated how RMTg MORs played a role in rewarding effects induced by social interaction behaviors of male Wistar rats, using a conditioned place preference (CPP) model. Results showed that the CPP induced by social interaction behaviors was inhibited when the function of MORs was blocked via injecting CTAP (a selective MOR antagonist) intraperitoneally, and intra-RMTg injections of lower doses of CTAP affected the CPP in the same way. In addition, injecting CTAP intraperitoneally significantly inhibited the expression of pouncing behavior, while intra-RMTg injections of CTAP significantly inhibited the expression of all three types of social behaviors. These results suggest that RMTg MORs may be a crucial target and remain to be further explored in order to better understand the mechanism of the rewarding effects of social interaction behaviors.

Mu opioid receptor stimulation in the medial preoptic area or nucleus accumbens facilitates song and reward in flocking European starlings

It has been proposed that social cohesion in gregarious animals is reinforced both by a positive affective state induced by social interactions and by the prevention of a negative state that would be caused by social separation. Opioids that bind to mu opioid receptors (MORs) act in numerous brain regions to induce positive and to reduce negative affective states. Here we explored a potential role for MORs in affective states that may impact flocking behavior in mixed-sex flocks of nonbreeding European starlings, Sturnus vulgaris. Singing behavior, which is considered central to flock cohesion, and other social behaviors were quantified after infusions of the MOR agonist D-Ala2, N-Me-Phe4, glycinol5-ENK (DAMGO) into either the medial preoptic area (POM) or the nucleus accumbens (NAC), regions previously implicated in affective state and flock cohesion. We focused on beak wiping, a potential sign of stress or redirected aggression in this species, to provide insight into a presumed negative state. We also used conditioned place preference (CPP) tests to provide insight into the extent to which infusions of DAMGO into POM or NAC that stimulated song might be rewarding. We found that MOR stimulation in either POM or NAC dose-dependently promoted singing behavior, reduced beak wiping, and induced a CPP. Subtle differences in responses to MOR stimulation between NAC and POM also suggest potential functional differences in the roles of these two regions. Finally, because the location of NAC has only recently been identified in songbirds, we additionally performed a tract tracing study that confirmed the presence of dopaminergic projections from the ventral tegmental area to NAC, suggesting homology with mammalian NAC. These findings support the possibility that MORs in POM and NAC play a dual role in reinforcing social cohesion in flocks by facilitating positive and reducing negative affective states.

Fos response of the tail of the ventral tegmental area to food restriction entails a prediction error processing

The tail of the ventral tegmental area (tVTA) or rostromedial tegmental nucleus (RMTg) receives lateral habenula inputs and projects heavily to midbrain dopamine neurons. Midbrain dopamine and lateral habenula neurons participate in learning processes predicting the outcomes of actions, placing the tVTA in a critical location into prediction error pathways. tVTA GABA neurons show electrophysiological inhibition or activation after reward and aversive stimuli, respectively, and their predictive cues. tVTA molecular recruitment, however, is not elicited by all aversive stimuli. Indeed, precipitated opioid withdrawal, repeated footshocks or food restriction raise tVTA Fos expression, whereas various other unpleasant, stressful or painful stimuli does not elicit that molecular response. However, the basis of that difference remains unknown. In the present study, we tried to disentangle whether the tVTA c-Fos induction observed after food restriction was due to the aversive state of food restriction or to procedure-related reward prediction error. To this end, male Sprague-Dawley rats were food-restricted for 7-8 days. During this period, animals were handled and weighed every day before feeding. On the test day, rats underwent several behavioral procedures to explore the impact of food restriction and food-predictive cue exposure on tVTA c-Fos expression. We showed that food restriction per se was not able to recruit c-Fos in the tVTA. On the contrary, the food-predicting cues induced c-Fos locally in the absence of feeding, whereas the food-predicting cues followed by feeding evoked lower c-Fos expression. Overall, our results support the proposed involvement of the tVTA in reward prediction error.

The rostromedial tegmental (RMTg) "brake" on dopamine and behavior: A decade of progress but also much unfinished work

Between 2005 and 2009, several research groups identified a strikingly dense inhibitory input to midbrain dopamine neurons arising from a previously uncharted region posterior to the ventral tegmental area (VTA). This region is now denoted as either the rostromedial tegmental nucleus (RMTg) or the "tail of the VTA" (tVTA), and is recognized to express distinct genetic markers, encode negative "prediction errors" (inverse to dopamine neurons), and play critical roles in behavioral inhibition and punishment learning. RMTg neurons are also influenced by many categories of abused drugs, and may drive some aversive responses to such drugs, particularly cocaine and alcohol. However, despite much progress, many important questions remain about RMTg molecular/genetic properties, diversity of projection targets, and applications to addiction, depression, and other neuropsychiatric disorders. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Association of the OPRM1 A118G polymorphism and Pavlovian-to-instrumental transfer: Clinical relevance for alcohol dependence

BACKGROUND

Pavlovian-to-instrumental transfer (PIT) quantifies the extent to which a stimulus that has been associated with reward or punishment alters operant behaviour. In alcohol dependence (AD), the PIT effect serves as a paradigmatic model of cue-induced relapse. Preclinical studies have suggested a critical role of the opioid system in modulating Pavlovian-instrumental interactions. The A118G polymorphism of the OPRM1 gene affects opioid receptor availability and function. Furthermore, this polymorphism interacts with cue-induced approach behaviour and is a potential biomarker for pharmacological treatment response in AD. In this study, we tested whether the OPRM1 polymorphism is associated with the PIT effect and relapse in AD.

METHODS

Using a PIT task, we examined three independent samples: young healthy subjects (N = 161), detoxified alcohol-dependent patients (N = 186) and age-matched healthy controls (N = 105). We used data from a larger study designed to assess the role of learning mechanisms in the development and maintenance of AD. Subjects were genotyped for the A118G (rs1799971) polymorphism of the OPRM1 gene. Relapse was assessed after three months.

RESULTS

In all three samples, participants with the minor OPRM1 G-Allele (G+ carriers) showed increased expression of the PIT effect in the absence of learning differences. Relapse was not associated with the OPRM1 polymorphism. Instead, G+ carriers displaying increased PIT effects were particularly prone to relapse.

CONCLUSION

These results support a role for the opioid system in incentive salience motivation. Furthermore, they inform a mechanistic model of aberrant salience processing and are in line with the pharmacological potential of opioid receptor targets in the treatment of AD.

Progress in opioid reward research: From a canonical two-neuron hypothesis to two neural circuits

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the national opioid crisis in the USA. The neural mechanisms underlying opioid abuse and addiction are still not fully understood. This review discusses recent progress in basic research dissecting receptor mechanisms and circuitries underlying opioid reward and addiction. We first review the canonical GABA-dopamine neuron hypothesis that was upheld for half a century, followed by major findings challenging this hypothesis. We then focus on recent progress in research evaluating the role of the mesolimbic and nigrostriatal dopamine circuitries in opioid reward and relapse. Based on recent findings that activation of dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is equally rewarding and that GABA neurons in the rostromedial tegmental nucleus (RMTg) and the substantia nigra pars reticula (SNr) are rich in mu opioid receptors and directly synapse onto midbrain DA neurons, we proposed that the RTMg→VTA → ventrostriatal and SNr → SNc → dorsostriatal pathways may act as the two major neural substrates underlying opioid reward and abuse. Lastly, we discuss possible integrations of these two pathways during initial opioid use, development of opioid abuse and maintenance of compulsive opioid seeking.

Dissecting the Role of GABA Neurons in the VTA versus SNr in Opioid Reward

Opioid reward has traditionally been thought to be mediated by GABA-induced disinhibition of dopamine (DA) neurons in the VTA. However, direct behavioral evidence supporting this hypothesis is still lacking. In this study, we found that the μ opioid receptor (MOR) gene, Oprm1, is highly expressed in GABA neurons, with ∼50% of GABA neurons in the substantia nigra pars reticulata (SNr), ∼30% in the VTA, and ∼70% in the tail of the VTA (also called the rostromedial tegmental nucleus) in male rats. No Oprm1 mRNA was detected in midbrain DA neurons. We then found that optogenetic inhibition of VTA DA neurons reduced intravenous heroin self-administration, whereas activation of these neurons produced robust optical intracranial self-stimulation in DAT-Cre mice, supporting an important role of DA neurons in opioid reward. Unexpectedly, pharmacological blockade of MORs in the SNr was more effective than in the VTA in reducing heroin reward. Optogenetic activation of VTA GABA neurons caused place aversion and inhibited cocaine, but not heroin, self-administration, whereas optogenetic activation of SNr GABA neurons caused a robust increase in heroin self-administration with an extinction pattern, suggesting a compensatory response in drug intake due to reduced heroin reward. In addition, activation of SNr GABA neurons attenuated heroin-primed, but not cue-induced, reinstatement of drug-seeking behavior, whereas inhibition of SNr GABA neurons produced optical intracranial self-stimulation and place preference. Together, these findings suggest that MORs on GABA neurons in the SNr play more important roles in opioid reward and relapse than MORs on VTA GABA neurons.SIGNIFICANCE STATEMENT Opioid reward has long been believed to be mediated by inhibition of GABA interneurons in the VTA that subsequently leads to disinhibition of DA neurons. In this study, we found that more μ opioid receptors (MORs) are expressed in GABA neurons in the neighboring SNr than in the VTA, and that pharmacological blockade of MORs in the SNr is more effective in reducing heroin reward than blockade of MORs in the VTA. Furthermore, optogenetic activation of VTA GABA neurons inhibited cocaine, but not heroin, self-administration, whereas activation of SNr GABA neurons inhibited heroin reward and relapse. These findings suggest that opioid reward is more likely mediated by stimulation of MORs in GABA afferents from other brain regions than in VTA GABA neurons.

Dopamine and Addiction

Addiction is commonly identified with habitual nonmedical self-administration of drugs. It is usually defined by characteristics of intoxication or by characteristics of withdrawal symptoms. Such addictions can also be defined in terms of the brain mechanisms they activate; most addictive drugs cause elevations in extracellular levels of the neurotransmitter dopamine. Animals unable to synthesize or use dopamine lack the conditioned reflexes discussed by Pavlov or the appetitive behavior discussed by Craig; they have only unconditioned consummatory reflexes. Burst discharges (phasic firing) of dopamine-containing neurons are necessary to establish long-term memories associating predictive stimuli with rewards and punishers. Independent discharges of dopamine neurons (tonic or pacemaker firing) determine the motivation to respond to such cues. As a result of habitual intake of addictive drugs, dopamine receptors expressed in the brain are decreased, thereby reducing interest in activities not already stamped in by habitual rewards.

The rostromedial tegmental nucleus: a key modulator of pain and opioid analgesia

A recently defined structure, the rostromedial tegmental nucleus (RMTg; aka tail of the ventral tegmental area [VTA]), has been proposed as an inhibitory control center for dopaminergic activity of the VTA. This region is composed of GABAergic cells that send afferent projections to the ventral midbrain and synapse onto dopaminergic cells in the VTA and substantia nigra. These cells exhibit µ-opioid receptor immunoreactivity, and in vivo, ex vivo, and optogenetic/electrophysiological approaches demonstrate that morphine excites dopamine neurons by targeting receptors on GABAergic neurons localized in the RMTg. This suggests that the RMTg may be a key modulator of opioid effects and a major brake regulating VTA dopamine systems. However, no study has directly manipulated RMTg GABAergic neurons in vivo and assessed the effect on nociception or opioid analgesia. In this study, multiplexing of GABAergic neurons in the RMTg was achieved using stimulatory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and inhibitory kappa-opioid receptor DREADDs (KORD). Our data show that locally infused RMTg morphine or selective RMTg GABAergic neuron inhibition produces 87% of the maximal antinociceptive effect of systemic morphine, and RMTg GABAergic neurons modulate dopamine release in the nucleus accumbens. In addition, chemoactivation of VTA dopamine neurons significantly reduced pain behaviors both in resting and facilitated pain states and reduced by 75% the dose of systemic morphine required to produce maximal antinociception. These results provide compelling evidence that RMTg GABAergic neurons are involved in processing of nociceptive information and are important mediators of opioid analgesia.

Endogenous opioids facilitate intrinsically-rewarded birdsong

Many songbirds sing in non-reproductive contexts while in flocks. Singing in such gregarious contexts is critical for maintaining and learning songs; however, song is not directed towards other individuals and has no obvious, immediate social consequences. Studies using conditioned place preference (CPP) tests of reward indicate that song production in gregarious contexts correlates positively with a bird’s intrinsic reward state and with opioid markers in the medial preoptic nucleus (mPOA). However, the causal involvement of opioids in gregarious song is unknown. Here we report that the selective mu opioid receptor (MOR) agonist fentanyl dose-dependently facilitates gregarious song and reduces stress/anxiety-related behavior in male and female European starlings. Furthermore, infusion of siRNA targeting MORs specifically in mPOA both suppresses gregarious song and disrupts the positive association between affective state and singing behavior, as revealed using CPP tests of song-associated reward. Results strongly implicate opioids in gregarious song and suggest that endogenous opioids in the mPOA may facilitate song by influencing an individual’s intrinsic reward state.

Sex Differences and the Role of Estradiol in Mesolimbic Reward Circuits and Vulnerability to Cocaine and Opiate Addiction

Although both men and women become addicted to drugs of abuse, women transition to addiction faster, experience greater difficulties remaining abstinent, and relapse more often than men. In both humans and rodents, hormonal cycles are associated with females' faster progression to addiction. Higher concentrations and fluctuating levels of ovarian hormones in females modulate the mesolimbic reward system and influence reward-directed behavior. For example, in female rodents, estradiol (E2) influences dopamine activity within the mesolimbic reward system such that drug-directed behaviors that are normally rewarding and reinforcing become enhanced when circulating levels of E2 are high. Therefore, neuroendocrine interactions, in part, explain sex differences in behaviors motivated by drug reward. Here, we review sex differences in the physiology and function of the mesolimbic reward system in order to explore the notion that sex differences in response to drugs of abuse, specifically cocaine and opiates, are the result of molecular neuroadaptations that differentially develop depending upon the hormonal state of the animal. We also reconsider the notion that ovarian hormones, specifically estrogen/estradiol, sensitize target neurons thereby increasing responsivity when under the influence of either cocaine or opiates or in response to exposure to drug-associated cues. These adaptations may ultimately serve to guide the motivational behaviors that underlie the factors that cause women to be more vulnerable to cocaine and opiate addiction than men.

Dopamine D3 receptor-based medication development for the treatment of opioid use disorder: Rationale, progress, and challenges

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Opioid abuse and overdose have become a national crisis in the USA.

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Although several opioid-based pharmacotherapies are available, they are ineffective in long-term relapse prevention.

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National Institute on Drug Abuse has listed dopamine D3 receptor antagonists as high priority for anti-opioid medication development.

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The novel D3 receptor antagonists (VK4-116, VK4-40) are effective in reducing opioid reward and relapse as well as potentiate opioid analgesia.

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D3 receptor antagonists deserve further studies as new pharmacotherapies for pain and of opioid use disorder.

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the current national opioid crisis. Although several opioid-based pharmacotherapies are available (e.g., methadone, buprenorphine, naloxone), they show limited effectiveness in long-term relapse prevention. In response to the opioid crisis, the National Institute on Drug Abuse proposed a list of pharmacological targets of highest priority for medication development for the treatment of opioid use disorders (OUD). Among these are antagonists of dopamine D3 receptors (D3R). In this review, we first review recent progress in research of the dopamine hypothesis of opioid reward and abuse and then describe the rationale and recent development of D3R ligands for the treatment of OUD. Herein, an emphasis is placed on the effectiveness of newly developed D3R antagonists in the animal models of OUD. These new drug candidates may also potentiate the analgesic effects of clinically used opioids, making them attractive as adjunctive medications for pain management and treatment of OUD.

Alcohol withdrawal drives depressive behaviors by activating neurons in the rostromedial tegmental nucleus

Rostromedial tegmental nucleus (RMTg) GABA neurons exert a primary inhibitory drive onto midbrain dopamine neurons and are excited by a variety of aversive stimuli. There is, however, little evidence that the RMTg-ventral tegmental area (VTA)-nucleus accumbens shell (Acb) circuit plays a role in the aversive consequences of alcohol withdrawal. This study was performed in adult male Long-Evans rats at 48-h withdrawal from chronic alcohol drinking in the intermittent schedule. These rats displayed clear anhedonia and depression-like behaviors, as measured with the sucrose preference, and forced swimming tests. These aberrant behaviors were accompanied by a substantial increase in cFos expression in the VTA-projecting RMTg neurons, identified by a combination of immunohistochemistry and retrograde-tracing techniques. Pharmacological or chemogenetic inhibition of RMTg neurons mitigated the anhedonia and depression-like behaviors. Ex vivo electrophysiological data showed that chemogenetic inactivation of RMTg neurons reduced GABA release and accelerated spontaneous firings of VTA dopamine neurons. Finally, using a functional hemispheric disconnection procedure, we demonstrated that inhibition of unilateral RMTg, when combined with activation of D1 and D2 dopamine receptors in the contralateral (but not ipsilateral) Acb, mitigated the anhedonia and depression-like behaviors in alcohol-withdrawal rats. These data show that the integrity in the RMTg-VTA-Acb pathway in a single hemisphere is sufficient to elicit depression-like behavior during ethanol-withdrawal. Overall, the present results reveal that the RMTg-VTA-Acb pathway plays a crucial role in the depression-like behavior in animals undergoing alcohol withdrawal, further advocating the RMTg as a potential therapeutic target for alcoholism.

Why Do Birds Flock? A Role for Opioids in the Reinforcement of Gregarious Social Interactions

The formation of social groups provides safety and opportunities for individuals to develop and practice important social skills. However, joining a social group does not result in any form of obvious, immediate reinforcement (e.g., it does not result in immediate copulation or a food reward), and individuals often remain in social groups despite agonistic responses from conspecifics. Much is known about neural and endocrine mechanisms underlying the motivation to perform mate- or offspring-directed behaviors. In contrast, relatively little is known about mechanisms underlying affiliative behaviors outside of these primary reproductive contexts. Studies on flocking behavior in songbirds are beginning to fill this knowledge gap. Here we review behavioral evidence that supports the hypothesis that non-sexual affiliative, flocking behaviors are both (1) rewarded by positive social interactions with conspecifics, and (2) reinforced because affiliative contact reduces a negative affective state caused by social isolation. We provide evidence from studies in European starlings, Sturnus vulgaris, that mu opioid receptors in the medial preoptic nucleus (mPOA) play a central role in both reward and the reduction of a negative affective state induced by social interactions in flocks, and discuss potential roles for nonapeptide/opioid interactions and steroid hormones. Finally, we develop the case that non-sexual affiliative social behaviors may be modified by two complementary output pathways from mPOA, with a projection from mPOA to the periaqueductal gray integrating information during social interactions that reduces negative affect and a projection from mPOA to the ventral tegmental area integrating information leading to social approach and reward.

Opioid-galanin receptor heteromers mediate the dopaminergic effects of opioids

Identifying non-addictive opioid medications is a high priority in medical sciences, but μ-opioid receptors mediate both the analgesic and addictive effects of opioids. We found a significant pharmacodynamic difference between morphine and methadone that is determined entirely by heteromerization of μ-opioid receptors with galanin Gal1 receptors, rendering a profound decrease in the potency of methadone. This was explained by methadone's weaker proficiency to activate the dopaminergic system as compared to morphine and predicted a dissociation of therapeutic versus euphoric effects of methadone, which was corroborated by a significantly lower incidence of self-report of "high" in methadone-maintained patients. These results suggest that μ-opioid-Gal1 receptor heteromers mediate the dopaminergic effects of opioids that may lead to a lower addictive liability of opioids with selective low potency for the μ-opioid-Gal1 receptor heteromer, exemplified by methadone.

Generality and opponency of rostromedial tegmental (RMTg) roles in valence processing

The rostromedial tegmental nucleus (RMTg), a GABAergic afferent to midbrain dopamine (DA) neurons, has been hypothesized to be broadly activated by aversive stimuli. However, this encoding pattern has only been demonstrated for a limited number of stimuli, and the RMTg influence on ventral tegmental (VTA) responses to aversive stimuli is untested. Here, we found that RMTg neurons are broadly excited by aversive stimuli of different sensory modalities and inhibited by reward-related stimuli. These stimuli include visual, auditory, somatosensory and chemical aversive stimuli, as well as “opponent” motivational states induced by removal of sustained rewarding or aversive stimuli. These patterns are consistent with broad encoding of negative valence in a subset of RMTg neurons. We further found that valence-encoding RMTg neurons preferentially project to the DA-rich VTA versus other targets, and excitotoxic RMTg lesions greatly reduce aversive stimulus-induced inhibitions in VTA neurons, particularly putative DA neurons, while also impairing conditioned place aversion to multiple aversive stimuli. Together, our findings indicate a broad RMTg role in encoding aversion and driving VTA responses and behavior.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

Gene expression and neurochemical characterization of the rostromedial tegmental nucleus (RMTg) in rats and mice

The rostromedial tegmental nucleus (RMTg), also known as the tail of the ventral tegmental area (tVTA), is a GABAergic structure identified in 2009 that receives strong inputs from the lateral habenula and other sources, sends dense inhibitory projections to midbrain dopamine (DA) neurons, and plays increasingly recognized roles in aversive learning, addiction, and other motivated behaviors. In general, little is known about the genetic identity of these neurons. However, recent work has identified the transcription factor FoxP1 as enhanced in the mouse RMTg (Lahti et al. in Development 143(3):516-529, 2016). Hence, in the current study, we used RNA sequencing to identify genes significantly enhanced in the rat RMTg as compared to adjacent VTA, and then examined the detailed distribution of two genes in particular, prepronociceptin (Pnoc) and FoxP1. In rats and mice, both Pnoc and FoxP1 were expressed at high levels in the RMTg and colocalized strongly with previously established RMTg markers. FoxP1 was particularly selective for RMTg neurons, as it was absent in most adjacent brain regions. We used these gene expression patterns to refine the anatomic characterization of RMTg in rats, extend this characterization to mice, and show that optogenetic manipulation of RMTg in mice bidirectionally modulates real-time place preference. Hence, RMTg neurons in both rats and mice exhibit distinct genetic profiles that correlate with their distinct connectivity and function.

The tail of the ventral tegmental area in behavioral processes and in the effect of psychostimulants and drugs of abuse

The tail of the ventral tegmental area (tVTA) is a recently identified structure that exerts a major inhibitory drive onto midbrain dopamine (DA) neurons. Also referred to as the rostromedial tegmental nucleus (RMTg), the tVTA is a cluster of gamma-aminobutyric acid (GABA)ergic neurons that starts within the posterior end of the VTA, where it is restricted dorsolateral to the caudal part of the interpeduncular nucleus, and extends into the pons. First identified in the rat, the tVTA has been described in many species, including mice and monkeys, as a region exhibiting similar anatomical and behavioral properties; it receives strong excitatory inputs from the lateral habenula (LHb), conveys negative reward-related information, and inhibits midbrain DA neuron activity. As an important inhibitory afferent to midbrain DA neurons, the tVTA is also implicated in drug abuse and in the complex interplay between reward and aversion processes. The overarching goal of this review is to provide the current state of knowledge on the anatomy and connectivity of the tVTA and to discuss recent evidence implicating this structure in reward-related processes and in the effect of psychostimulants and drugs of abuse.

Opioid-induced rewards, locomotion, and dopamine activation: A proposed model for control by mesopontine and rostromedial tegmental neurons

Opioids, such as morphine or heroin, increase forebrain dopamine (DA) release and locomotion, and support the acquisition of conditioned place preference (CPP) or self-administration. The most sensitive sites for these opioid effects in rodents are in the ventral tegmental area (VTA) and rostromedial tegmental nucleus (RMTg). Opioid inhibition of GABA neurons in these sites is hypothesized to lead to arousing and rewarding effects through disinhibition of VTA DA neurons. We review findings that the laterodorsal tegmental (LDTg) and pedunculopontine tegmental (PPTg) nuclei, which each contain cholinergic, GABAergic, and glutamatergic cells, are important for these effects. LDTg and/or PPTg cholinergic inputs to VTA mediate opioid-induced locomotion and DA activation via VTA M5 muscarinic receptors. LDTg and/or PPTg cholinergic inputs to RMTg also modulate opioid-induced locomotion. Lesions or inhibition of LDTg or PPTg neurons reduce morphine-induced increases in forebrain DA release, acquisition of morphine CPP or self-administration. We propose a circuit model that links VTA and RMTg GABA with LDTg and PPTg neurons critical for DA-dependent opioid effects in drug-naïve rodents.

Learning From One's Mistakes: A Dual Role for the Rostromedial Tegmental Nucleus in the Encoding and Expression of Punished Reward Seeking

BACKGROUND

Psychiatric disorders such as addiction and mania are marked by persistent reward seeking despite highly negative or aversive outcomes, but the neural mechanisms underlying this aberrant decision making are unknown. The recently identified rostromedial tegmental nucleus (RMTg) encodes a wide variety of aversive stimuli and sends robust inhibitory projections to midbrain dopamine neurons, leading to the hypothesis that the RMTg provides a brake to reward signaling in response to aversive costs.

METHODS

To test the role of the RMTg in punished reward seeking, adult male Sprague Dawley rats were tested in several cost-benefit decision tasks after excitotoxic lesions of the RMTg or temporally specific optogenetic inhibition of RMTg efferents in the ventral tegmental area.

RESULTS

RMTg lesions drastically impaired the ability of foot shock to suppress operant responding for food. Optogenetic inhibition showed that this resistance to punishment was due in part to RMTg activity at the precise moment of shock delivery and was mediated by projections to the ventral tegmental area, which is consistent with an aversive "teaching signal" role for the RMTg during encoding of the aversive event. We observed a similar resistance to punishment when the RMTg was selectively inhibited immediately prior to the operant lever press, which is consistent with a second distinct role for the RMTg during action selection. These effects were not attributable to RMTg effects on learning rate, locomotion, shock sensitivity, or perseveration.

CONCLUSIONS

The RMTg has two strong and dissociable roles during both encoding and recall of aversive consequences of behavior.

Hormones and Neuropeptide Receptor Heteromers in the Ventral Tegmental Area. Targets for the Treatment of Loss of Control of Food Intake and Substance Use Disorders

Hormones and neuropeptides represent biological correlates of internal homeostatic signals detected and integrated in the hypothalamus, which establishes a robust functional connection with the ventral tegmental area (VTA). The hypothalamus-VTA connection determines the ability of these signals to influence central dopaminergic neurotransmission and, therefore, their ability to increase responsiveness to their reward-associated stimuli and to establish appropriate associative learning. The hypothalamus also provides the main source of the multiple neuropeptides that are released in the VTA. With volume transmission of neuropeptides and hormones, extrasynaptic receptors within the VTA provide a fine-tune mechanism, which depends on the ability of molecularly different G protein-coupled receptors (GPCRs) to form heteromers. GPCR heteromer is defined as a macromolecular complex composed of at least two different receptor units (protomers) with biochemical properties that are demonstrably different from those of its individual components. GPCR heteromers can provide unique allosteric properties to specific ligands, which provides new avenues for drug development. We have identified specific GPCR heteromers in the VTA that integrate orexin and CRF neurotransmission and opioid and galanin neurotransmission, which play a very significant role in the modulation of dopaminergic neuronal activity and which can constitute targets for the treatment of loss of control of food intake and substance use disorders.

Response of the Tail of the Ventral Tegmental Area to Aversive Stimuli

The GABAergic tail of the ventral tegmental area (tVTA), also named rostromedial tegmental nucleus (RMTg), exerts an inhibitory control on dopamine neurons of the VTA and substantia nigra. The tVTA has been implicated in avoidance behaviors, response to drugs of abuse, reward prediction error, and motor functions. Stimulation of the lateral habenula (LHb) inputs to the tVTA, or of the tVTA itself, induces avoidance behaviors, which suggests a role of the tVTA in processing aversive information. Our aim was to test the impact of aversive stimuli on the molecular recruitment of the tVTA, and the behavioral consequences of tVTA lesions. In rats, we assessed Fos response to lithium chloride (LiCl), β-carboline, naloxone, lipopolysaccharide (LPS), inflammatory pain, neuropathic pain, foot-shock, restraint stress, forced swimming, predator odor, and opiate withdrawal. We also determined the effect of tVTA bilateral ablation on physical signs of opiate withdrawal, and on LPS- and LiCl-induced conditioned taste aversion (CTA). Naloxone-precipitated opiate withdrawal induced Fos in μ-opioid receptor-positive (15%) and -negative (85%) tVTA cells, suggesting the presence of both direct and indirect mechanisms in tVTA recruitment during withdrawal. However, tVTA lesion did not impact physical signs of opiate withdrawal. Fos induction was also present with repeated, but not single, foot-shock delivery. However, such induction was mostly absent with other aversive stimuli. Moreover, tVTA ablation had no impact on CTA. Although stimulation of the tVTA favors avoidance behaviors, present findings suggest that this structure may be important to the response to some, but not all, aversive stimuli.

Muscarinic cholinergic receptor antagonists in the VTA and RMTg have opposite effects on morphine-induced locomotion in mice

The ventral tegmental area (VTA) and the rostromedial tegmental nucleus (RMTg) each contribute to opiate reward and each receive inputs from the laterodorsal tegmental and pedunculopontine tegmental nuclei, the two principle brainstem cholinergic cell groups. We compared the contributions of VTA or RMTg muscarinic cholinergic receptors to locomotion induced by morphine infusions into the same sites. VTA co-infusion of atropine completely blocked VTA morphine-induced locomotion providing additional support for the important role of VTA muscarinic cholinergic receptors in the stimulant effects of opiates. By contrast, RMTg co-infusion of atropine increased RMTg morphine-induced locomotion. Furthermore, RMTg co-infusion of the M3-selective antagonist 4-DAMP, but not the M4-selective antagonist Tropicamide, strongly increased RMTg morphine-induced locomotion. RMTg infusions of 4-DAMP, but not of Tropicamide, by themselves strongly increased drug-free locomotion. Muscarinic cholinergic receptors in the RMTg thus also contribute to the stimulant effects of morphine, but in a way opposite to those in VTA. We suggest that the net effect of endogenous cholinergic input to the RMTg on drug-free and on RMTg morphine-induced locomotion is inhibitory.

The Effects of Perinatal Oxycodone Exposure on Behavioral Outcome in a Rodent Model

Opiate addiction is now a major public health problem. Perinatal insults and exposure to opiates such as morphine in utero are well known to affect development of the hypothalamic-pituitary-adrenal axis of the offspring adversely and are associated with a higher risk of developing neurobehavioral problems. Oxycodone is now one of the most frequently abused pain killers during pregnancy; however, limited data are available regarding whether and how perinatal oxycodone exposure (POE) alters neurobehavioral outcomes of the offspring. We demonstrated that exposure to 0.5 mg/kg/day oxycodone in utero was associated with hyperactivity in adult rats in an open field. No significant effects of POE were detected on isolation-induced ultrasonic vocalizations in the early postnatal period or on learning and memory in the water maze in adult offspring. Our findings are consistent with hyperactivity problems identified in children exposed to opiates in utero.

Functional μ-Opioid-Galanin Receptor Heteromers in the Ventral Tegmental Area

The neuropeptide galanin has been shown to interact with the opioid system. More specifically, galanin counteracts the behavioral effects of the systemic administration of μ-opioid receptor (MOR) agonists. Yet the mechanism responsible for this galanin-opioid interaction has remained elusive. Using biophysical techniques in mammalian transfected cells, we found evidence for selective heteromerization of MOR and the galanin receptor subtype Gal1 (Gal1R). Also in transfected cells, a synthetic peptide selectively disrupted MOR-Gal1R heteromerization as well as specific interactions between MOR and Gal1R ligands: a negative cross talk, by which galanin counteracted MAPK activation induced by the endogenous MOR agonist endomorphin-1, and a cross-antagonism, by which a MOR antagonist counteracted MAPK activation induced by galanin. These specific interactions, which represented biochemical properties of the MOR-Gal1R heteromer, could then be identified in situ in slices of rat ventral tegmental area (VTA) with MAPK activation and two additional cell signaling pathways, AKT and CREB phosphorylation. Furthermore, in vivo microdialysis experiments showed that the disruptive peptide selectively counteracted the ability of galanin to block the dendritic dopamine release in the rat VTA induced by local infusion of endomorphin-1, demonstrating a key role of MOR-Gal1R heteromers localized in the VTA in the direct control of dopamine cell function and their ability to mediate antagonistic interactions between MOR and Gal1R ligands. The results also indicate that MOR-Gal1R heteromers should be viewed as targets for the treatment of opioid use disorders.

SIGNIFICANCE STATEMENT

The μ-opioid receptor (MOR) localized in the ventral tegmental area (VTA) plays a key role in the reinforcing and addictive properties of opioids. With parallel in vitro experiments in mammalian transfected cells and in situ and in vivo experiments in rat VTA, we demonstrate that a significant population of these MORs form functional heteromers with the galanin receptor subtype Gal1 (Gal1R), which modulate the activity of the VTA dopaminergic neurons. The MOR-Gal1R heteromer can explain previous results showing antagonistic galanin-opioid interactions and offers a new therapeutic target for the treatment of opioid use disorder.

Ablation of μ opioid receptor-expressing GABA neurons in rostromedial tegmental nucleus increases ethanol consumption and regulates ethanol-related behaviors

There has been increasing interest in the rostromedial tegmental nucleus (RMTg), given its potential regulatory role in many aversion-related behaviors. The RMTg contains mostly GABAergic neurons, sends a dense inhibitory projection to dopamine neurons in the midbrain, and is rich with μ-opioid receptors (MOR). Like most addictive drugs, ethanol has both aversive and rewarding properties. However, the cellular mechanisms underlying the effects of ethanol, particularly the aversive effect that limits its intake are not well understood. Recent studies have linked aversion with synaptic inhibition of dopamine neurons in the ventral tegmental area. To determine a potential role that the RMTg plays in the effect of ethanol, in this study, we employed a neurotoxin, dermorphin-saporin (DS), to lesion RMTg neurons prior to assessing ethanol-related behaviors. Rats were infused with DS bilaterally into the RMTg. This manipulation substantially increased the intake and preference for ethanol but not sucrose. It also reduced the number of neurons with MOR and glutamic acid decarboxylase 67 immunoreactivity within the RMTg. These changes did not occur after intra-RMTg infusion of blank saporin or vehicle. Importantly, intra-RMTg DS infusion significantly enhanced expression of conditioned place preference induced by ethanol (2 g/kg, i.p.), and slowed the extinction process. These results suggest that MOR-expressing GABAergic neurons in the RMTg contribute significantly to the regulation of ethanol consumption and related behaviors.

Regulator of G-Protein Signaling 7 Regulates Reward Behavior by Controlling Opioid Signaling in the Striatum

BACKGROUND

Morphine mediates its euphoric and analgesic effects by acting on the μ-opioid receptor (MOR). MOR belongs to the family of G-protein coupled receptors whose signaling efficiency is controlled by the regulator of G-protein signaling (RGS) proteins. Our understanding of the molecular diversity of RGS proteins that control MOR signaling, their circuit specific actions, and underlying cellular mechanisms is very limited.

METHODS

We used genetic approaches to ablate regulator of G-protein signaling 7 (RGS7) both globally and in specific neuronal populations. We used conditioned place preference and self-administration paradigms to examine reward-related behavior and a battery of tests to assess analgesia, tolerance, and physical dependence to morphine. Electrophysiology approaches were applied to investigate the impact of RGS7 on morphine-induced alterations in neuronal excitability and plasticity of glutamatergic synapses. At least three animals were used for each assessment.

RESULTS

Elimination of RGS7 enhanced reward, increased analgesia, delayed tolerance, and heightened withdrawal in response to morphine administration. RGS7 in striatal neurons was selectively responsible for determining the sensitivity of rewarding and reinforcing behaviors to morphine without affecting analgesia, tolerance, and withdrawal. In contrast, deletion of RGS7 in dopaminergic neurons did not influence morphine reward. RGS7 exerted its effects by controlling morphine-induced changes in excitability of medium spiny neurons in nucleus accumbens and gating the compositional plasticity of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate receptors.

CONCLUSIONS

This study identifies RGS7 as a novel regulator of MOR signaling by dissecting its circuit specific actions and pinpointing its role in regulating morphine reward by controlling the activity of nucleus accumbens neurons.

Role for the Rostromedial Tegmental Nucleus in Signaling the Aversive Properties of Alcohol

BACKGROUND

While the rewarding effects of alcohol contribute significantly to its addictive potential, it is becoming increasingly appreciated that alcohol's aversive properties also play an important role in the propensity to drink. Despite this, the neurobiological mechanism for alcohol's aversive actions is not well understood. The rostromedial tegmental nucleus (RMTg) was recently characterized for its involvement in aversive signaling and has been shown to encode the aversive properties of cocaine, yet its involvement in alcohol's aversive actions have not been elucidated.

METHODS

Adult male and female Long-Evans rats underwent conditioned taste aversion (CTA) procedures where exposure to a novel saccharin solution was paired with intraperitoneal administration of saline, lithium chloride (LiCl), or ethanol (EtOH). Control rats underwent the same paradigm except that drug and saccharin exposure were explicitly unpaired. Saccharin consumption was measured on test day in the absence of drug administration, and rats were sacrificed 90 to 105 minutes following access to saccharin. Brains were subsequently harvested and processed for cFos immunohistochemistry. The number of cFos-labeled neurons was counted in the RMTg and the lateral habenula (LHb)-a region that sends prominent glutamatergic input to the RMTg.

RESULTS

In rats that received paired drug and saccharin exposure, EtOH and LiCl induced significant CTA compared to saline to a similar degree in males and females. Both EtOH- and LiCl-induced CTA significantly enhanced cFos expression in the RMTg and LHb but not the hippocampus. Similar to behavioral measures, no significant effect of sex on CTA-induced cFos expression was observed. cFos expression in both the RMTg and LHb was significantly correlated with CTA magnitude with greater cFos being associated with more pronounced CTA. In addition, cFos expression in the RMTg was positively correlated with LHb cFos.

CONCLUSIONS

These data suggest that the RMTg and LHb are involved in the expression of CTA and are consistent with previous work implicating the RMTg in aversive signaling. Furthermore, increased cFos expression in the RMTg following EtOH-induced CTA suggests that this region plays a role in signaling alcohol's aversive properties.

Alcohol preference and consumption are controlled by the caudal linear nucleus in alcohol-preferring rats

The neuroanatomical and neurochemical basis of alcohol drinking has been extensively studied, but the neural circuitry mediating alcohol reinforcement has not been fully delineated. In the present experiments, we used both neuroimaging and pharmacological tools to identify neural systems associated with alcohol preference and high voluntary alcohol drinking in alcohol-preferring AA (Alko Alcohol) rats. First, we compared the basal brain activity of AA rats with that of heterogeneous Wistar rats with manganese-enhanced magnetic resonance imaging (MEMRI). Briefly, alcohol-naïve rats were implanted with subcutaneous osmotic minipumps delivering 120 mg/kg MnCl2 over a 7-day period, and were then imaged using a three-dimensional rapid acquisition-relaxation enhanced pulse sequence. MEMRI analysis revealed that the most conspicuous subcortical activation difference was located in the caudal linear nucleus of raphe (CLi), with AA rats displaying significantly lower T1 signal in this region compared to Wistar rats. However, following long-term alcohol drinking, CLi activity was increased in AA rats. In the second experiment, the CLi was targeted with pharmacological tools. AA rats trained to drink 10% alcohol during 2-h sessions were implanted with guide cannulas aimed at the CLi and were given injections of the GABAA receptor agonist muscimol into the CLi before drinking sessions. Muscimol dose-dependently increased alcohol drinking, and co-administration of the gamma aminobutyric acid (GABA)A antagonist bicuculline blocked muscimol's effect. These findings suggest that the mediocaudal region of the ventral tegmental area, particularly the CLi, is important for the propensity for high alcohol drinking and controls alcohol reward via GABAergic transmission.

Pharmacological Manipulation of the Rostromedial Tegmental Nucleus Changes Voluntary and Operant Ethanol Self-Administration in Rats

BACKGROUND

The aversive properties of ethanol (EtOH) that limit its intake are poorly understood. There is an increasing interest in the role of the rostromedial tegmental nucleus (RMTg), because it encodes aversion signals and inhibits motivated behaviors. It is also a major source of inhibitory GABAergic inputs to the midbrain dopamine neurons. Up to this time, the role of the RMTg in EtOH-drinking behaviors has not been well explored.

METHODS

Male Long-Evans rats were trained either to drink EtOH under the intermittent 2-bottle-choice protocol or to self-administer EtOH in operant chambers under fixed-ratio-3 schedules. Changes in drinking behaviors induced by the bilateral infusion into the RMTg of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), an agonist of AMPA-type glutamate receptors, or muscimol, an agonist of GABAA receptors, were measured.

RESULTS

Consumption and preference for EtOH, numbers of active lever pressing, and head entrance to the EtOH port were all significantly decreased upon activation of the RMTg by the infusion of AMPA, but were increased upon inhibition of the RMTg by the infusion of muscimol. By contrast, intra-RMTg infusion of these agents did not change sucrose consumption.

CONCLUSIONS

These data show for the first time that EtOH-drinking and EtOH-seeking behaviors of rats changed inversely with RMTg function, supporting the idea that the RMTg plays a crucial role in EtOH-drinking behaviors.

Sources of input to the rostromedial tegmental nucleus, ventral tegmental area, and lateral habenula compared: A study in rat

Profound inhibitory control exerted on midbrain dopaminergic neurons by the lateral habenula (LHb), which has mainly excitatory outputs, is mediated by the GABAergic rostromedial tegmental nucleus (RMTg), which strongly innervates dopaminergic neurons in the ventral midbrain. Early reports indicated that the afferent connections of the RMTg, excepting its very strong LHb inputs, do not differ appreciably from those of the ventral tegmental area (VTA). Presumably, however, the RMTg contributes more to behavioral synthesis than to simply invert the valence of the excitatory signal coming from the LHb. Therefore, the present study was done to directly compare the inputs to the RMTg and VTA and, in deference to its substantial involvement with this circuitry, the LHb was also included in the comparison. Data indicated that, while the afferents of the RMTg, VTA, and LHb do originate within the same large pool of central nervous system (CNS) structures, each is also related to structures that project more strongly to it than to the others. The VTA gets robust input from ventral striatopallidum and extended amygdala, whereas RMTg biased inputs arise in structures with a more direct impact on motor function, such as deep layers of the contralateral superior colliculus, deep cerebellar and several brainstem nuclei, and, via a relay in the LHb, the entopeduncular nucleus. Input from the ventral pallidal-lateral preoptic-lateral hypothalamus continuum is strong in the RMTg and VTA and dominant in the LHb. Axon collateralization was also investigated, providing additional insights into the organization of the circuitry of this important triad of structures.

Persistent Adaptations in Afferents to Ventral Tegmental Dopamine Neurons after Opiate Withdrawal

Protracted opiate withdrawal is accompanied by altered responsiveness of midbrain dopaminergic (DA) neurons, including a loss of DA cell response to morphine, and by behavioral alterations, including affective disorders. GABAergic neurons in the tail of the ventral tegmental area (tVTA), also called the rostromedial tegmental nucleus, are important for behavioral responses to opiates. We investigated the tVTA-VTA circuit in rats after chronic morphine exposure to determine whether tVTA neurons participate in the loss of opiate-induced disinhibition of VTA DA neurons observed during protracted withdrawal. In vivo recording revealed that VTA DA neurons, but not tVTA GABAergic neurons, are tolerant to morphine after 2 weeks of withdrawal. Optogenetic stimulation of tVTA neurons inhibited VTA DA neurons similarly in opiate-naive and long-term withdrawn rats. However, tVTA inactivation increased VTA DA activity in opiate-naive rats, but not in withdrawn rats, resembling the opiate tolerance effect in DA cells. Thus, although inhibitory control of DA neurons by tVTA is maintained during protracted withdrawal, the capacity for disinhibitory control is impaired. In addition, morphine withdrawal reduced both tVTA neural activity and tonic glutamatergic input to VTA DA neurons. We propose that these changes in glutamate and GABA inputs underlie the apparent tolerance of VTA DA neurons to opiates after chronic exposure. These alterations in the tVTA-VTA DA circuit could be an important factor in opiate tolerance and addiction. Moreover, the capacity of the tVTA to inhibit, but not disinhibit, DA cells after chronic opiate exposure may contribute to long-term negative affective states during withdrawal.

SIGNIFICANCE STATEMENT

Dopaminergic (DA) cells of the ventral tegmental area (VTA) are the origin of a brain reward system and are critically involved in drug abuse. Morphine has long been known to affect VTA DA cells via GABAergic interneurons. Recently, GABAergic neurons caudal to the VTA were discovered and named the tail of VTA (tVTA). Here, we show that tVTA GABA neurons lose their capacity to disinhibit, but not to inhibit, VTA DA cells after chronic opiate exposure. The failure of disinhibition was associated with a loss of glutamatergic input to DA neurons after chronic morphine. These findings reveal mechanisms by which the tVTA may play a key role in long-term negative affective states during opiate withdrawal.

Dual motor responses elicited by ethanol in the posterior VTA: Consequences of the blockade of μ-opioid receptors

A recent hypothesis, based on electrophysiological and behavioural findings, suggests that ethanol simultaneously exerts opposed effects on the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) through two parallel mechanisms, one promoting and the other reducing the GABA release onto VTA DA neurons. In this sense, the activating effects are mediated by salsolinol, a metabolite of ethanol, acting on the μ-opioid receptors (MORs) located in VTA GABA neurons. The inhibitory effects are, however, triggered by the non-metabolized fraction of ethanol which would cause the GABAA receptors-mediated inhibition of VTA DA neurons. Since both trends tend to offset each other, only the use of appropriate pharmacological tools allows analysis of this phenomenon in depth. Herein, we present new behavioural findings supporting this hypothesis. Motor activity was evaluated in rats after intra-VTA administration of ethanol 35 nmol, an apparently ineffective dose, 24 h after the irreversible blockade of MORs in the VTA with β-FNA. Our results showed that this pre-treatment turned the initially ineffective ethanol dose into a depressant one, confirming that the activating effect of ethanol can be selectively suppressed without affecting the depressant effects mediated by the non-biotransformed fraction of ethanol.

Supplemental morphine infusion into the posterior ventral tegmentum extends the satiating effects of self-administered intravenous heroin

Rats learn to self-administer intravenous heroin; well-trained animals lever-press at a slow and regular pace over a wide range of intravenous doses. The pauses between successive earned infusions are proportional to the dose of the previous injection and are thought to reflect periods of drug satiety. Rats will also self-administer opiates by microinjection directly into sites in the posterior regions of the ventral tegmentum. To determine if the pauses between self-administered intravenous injections are due to opiate actions in posterior ventral tegmentum, we delivered supplemental morphine directly into this region during intravenous self-administration sessions in well-trained rats. Reverse dialysis of morphine into the posterior ventral tegmentum increased the intervals between earned injections. The inter-response intervals were greatest for infusion into the most posterior ventral tegmental sites, sites in a region variously known as the tail of the ventral tegmental area or as the rostromedial tegmental nucleus. These sites at which morphine prolongs inter-response intervals, correspond to the sites at which opiates have been found most effective in reinforcing instrumental behavior.

Spatiotemporal control of opioid signaling and behavior

Optogenetics is now a widely accepted tool for spatiotemporal manipulation of neuronal activity. However, a majority of optogenetic approaches use binary on/off control schemes. Here, we extend the optogenetic toolset by developing a neuromodulatory approach using a rationale-based design to generate a Gi-coupled, optically sensitive, mu-opioid-like receptor, which we term opto-MOR. We demonstrate that opto-MOR engages canonical mu-opioid signaling through inhibition of adenylyl cyclase, activation of MAPK and G protein-gated inward rectifying potassium (GIRK) channels and internalizes with kinetics similar to that of the mu-opioid receptor. To assess in vivo utility, we expressed a Cre-dependent viral opto-MOR in RMTg/VTA GABAergic neurons, which led to a real-time place preference. In contrast, expression of opto-MOR in GABAergic neurons of the ventral pallidum hedonic cold spot led to real-time place aversion. This tool has generalizable application for spatiotemporal control of opioid signaling and, furthermore, can be used broadly for mimicking endogenous neuronal inhibition pathways.

Basal ganglia circuit loops, dopamine and motivation: A review and enquiry

Dopamine neurons located in the midbrain play a role in motivation that regulates approach behavior (approach motivation). In addition, activation and inactivation of dopamine neurons regulate mood and induce reward and aversion, respectively. Accumulating evidence suggests that such motivational role of dopamine neurons is not limited to those located in the ventral tegmental area, but also in the substantia nigra. The present paper reviews previous rodent work concerning dopamine's role in approach motivation and the connectivity of dopamine neurons, and proposes two working models: One concerns the relationship between extracellular dopamine concentration and approach motivation. High, moderate and low concentrations of extracellular dopamine induce euphoric, seeking and aversive states, respectively. The other concerns circuit loops involving the cerebral cortex, basal ganglia, thalamus, epithalamus, and midbrain through which dopaminergic activity alters approach motivation. These models should help to generate hypothesis-driven research and provide insights for understanding altered states associated with drugs of abuse and affective disorders.

Direct bidirectional μ-opioid control of midbrain dopamine neurons

The ventral tegmental area (VTA) is required for the rewarding and motivational actions of opioids and activation of dopamine neurons has been implicated in these effects. The canonical model posits that opioid activation of VTA dopamine neurons is indirect, through inhibition of GABAergic inputs. However, VTA dopamine neurons also express postsynaptic μ-opioid peptide (MOP) receptors. We report here that in Sprague Dawley rat, the MOP receptor-selective agonist DAMGO (0.5-3 μM) depolarized or increased the firing rate of 87 of 451 VTA neurons (including 22 of 110 dopamine neurons). This DAMGO excitation occurs in the presence of GABAA receptor blockade and its EC50 value is two orders of magnitude lower than for presynaptic inhibition of GABA release on to VTA neurons. Consistent with a postsynaptic channel opening, excitations were accompanied by a decrease in input resistance. Excitations were blocked by CdCl2 (100 μM, n = 5) and ω-agatoxin-IVA (100 nM, n = 3), nonselective and Cav2.1 Ca(2+) channel blockers, respectively. DAMGO also produced a postsynaptic inhibition in 233 of 451 VTA neurons, including 45 of 110 dopamine neurons. The mean reversal potential of the inhibitory current was -78 ± 7 mV and inhibitions were blocked by the K(+) channel blocker BaCl2 (100 μM, n = 7). Blockade of either excitation or inhibition unmasked the opposite effect, suggesting that MOP receptors activate concurrent postsynaptic excitatory and inhibitory processes in most VTA neurons. These results provide a novel direct mechanism for MOP receptor control of VTA dopamine neurons.

Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Control of the nigrostriatal dopamine neuron activity and motor function by the tail of the ventral tegmental area

Midbrain dopamine neurons are implicated in various psychiatric and neurological disorders. The GABAergic tail of the ventral tegmental area (tVTA), also named the rostromedial tegmental nucleus (RMTg), displays dense projections to the midbrain and exerts electrophysiological control over dopamine cells of the VTA. However, the influence of the tVTA on the nigrostriatal pathway, from the substantia nigra pars compacta (SNc) to the dorsal striatum, and on related functions remains to be addressed. The present study highlights the role played by the tVTA as a GABA brake for the nigrostriatal system, demonstrating a critical influence over motor functions. Using neuroanatomical approaches with tract tracing and electron microscopy, we reveal the presence of a tVTA-SNc-dorsal striatum pathway. Using in vivo electrophysiology, we prove that the tVTA is a major inhibitory control center for SNc dopamine cells. Using behavioral approaches, we demonstrate that the tVTA controls rotation behavior, motor coordination, and motor skill learning. The motor enhancements observed after ablation of the tVTA are in this regard comparable with the performance-enhancing properties of amphetamine, a drug used in doping. These findings demonstrate that the tVTA is a major GABA brake for nigral dopamine systems and nigrostriatal functions, and they raise important questions about how the tVTA is integrated within the basal ganglia circuitry. They also warrant further research on the tVTA's role in motor and dopamine-related pathological contexts such as Parkinson's disease.

The antero-posterior heterogeneity of the ventral tegmental area

The ventral tegmental area (VTA) is a brain region processing salient sensory and emotional information, controlling motivated behaviors, natural or drug-related reward, reward-related learning, mood, and participating in their associated psychopathologies. Mostly studied for its dopamine neurons, the VTA also includes functionally important GABA and glutamate cell populations. Behavioral evidence supports the presence of functional differences between the anterior VTA (aVTA) and the posterior VTA (pVTA), which is the topic of this review. This antero-posterior heterogeneity concerns locomotor activity, conditioned place preference and intracranial self-administration, and can be seen in response to ethanol, acetaldehyde, salsolinol, opioids including morphine, cholinergic agonists including nicotine, cocaine, cannabinoids and after local manipulation of GABA and serotonin receptors. It has also been observed after viral-mediated manipulation of GluR1, phospholipase Cγ (PLCγ) and cAMP response element binding protein (CREB) expression, with impact on reward and aversion-related responses, on anxiety and depression-related behaviors and on pain sensitivity. In this review, the substrates potentially underlying these aVTA/pVTA differences are discussed, including the VTA sub-nuclei and the heterogeneity in connectivity, cell types and molecular characteristics. We also review the role of the tail of the VTA (tVTA), or rostromedial tegmental nucleus (RMTg), which may also participate to the observed antero-posterior heterogeneity of the VTA. This region, partly located within the pVTA, is an inhibitory control center for dopamine activity. It controls VTA and substantia nigra dopamine cells, thus exerting a major influence on basal ganglia functions. This review highlights the need for a more comprehensive analysis of VTA heterogeneity.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Blockade of ventral midbrain NMDA receptors enhances brain stimulation reward: a preferential role for GluN2A subunits

Ventral midbrain (VM) neurons that project to limbic structures play a role in reward and incentive motivation. It has been suggested that a reward-related signal is transmitted when the firing rate of VM dopamine neurons shifts from a tonic to a phasic mode. Since glutamate is necessary for this transduction process, it is likely to play a role in reward signaling. This study was aimed at determining the effect of VM N-Methyl-D-Aspartate (NMDA) receptor blockade on reward induced by electrical brain stimulation. Experiments were performed on rats trained to self-administer an electrical stimulation in the medial posterior mesencephalon. Reward thresholds were measured with the curve-shift paradigm before and after bilateral VM injections of the following NMDA receptor antagonists: R-CPP, 3-(R-2-Carboxypiperazin-4-yl)-propyl-1 phosphonic acid, (0, 20.6, 41.2 and 82.5 pmol/0.5 μl/side), PPPA, (2R,4S)-4-(3-Phosphonopropyl)-2-piperidinecarboxylic acid, (0, 0.825 and 1.65 nmol/0.5 μl/side) orRo04-5595, 1-[2-(4-Chlorophenyl)ethyl]-1,2,3,4-tetrqahydro-6-methoxy-2-methyl-7-isoquinolinol hydrochloride (0, 0.825, 1.65 nmol/0.5 μl/side). R-CPP and PPPA produced a dose and time dependent decrease in reward threshold, an effect that was, at some doses and times after the injection, accompanied by an increase in maximum responses. These effects were not observed with Ro04-5595 over the range of doses tested. While previous studies suggest a role for glutamate in reward signaling, the present results show that VM glutamate exerts a tonic inhibition on the reward-relevant pathway. The selectivity of Ro04-5595 for NMDA receptors composed of GluN2B subunits and the higher affinity of R-CPP and PPPA for GluN2A suggest that the inhibition is mediated by receptors composed of GluN2A subunits.

Cocaine drives aversive conditioning via delayed activation of dopamine-responsive habenular and midbrain pathways

Many strong rewards, including abused drugs, also produce aversive effects that are poorly understood. For example, cocaine can produce aversive conditioning after its rewarding effects have dissipated, consistent with opponent process theory, but the neural mechanisms involved are not well known. Using electrophysiological recordings in awake rats, we found that some neurons in the lateral habenula (LHb), where activation produces aversive conditioning, exhibited biphasic responses to single doses of intravenous cocaine, with an initial inhibition followed by delayed excitation paralleling cocaine's shift from rewarding to aversive. Recordings in LHb slice preparations revealed similar cocaine-induced biphasic responses and further demonstrated that biphasic responses were mimicked by dopamine, that the inhibitory phase depended on dopamine D2-like receptors, and that the delayed excitation persisted after drug washout for prolonged durations consistent with findings in vivo. c-Fos experiments further showed that cocaine-activated LHb neurons preferentially projected to and activated neurons in the rostromedial tegmental nucleus (RMTg), a recently identified target of LHb axons that is activated by negative motivational stimuli and inhibits dopamine neurons. Finally, pharmacological excitation of the RMTg produced conditioned place aversion, whereas cocaine-induced avoidance behaviors in a runway operant paradigm were abolished by lesions of LHb efferents, lesions of the RMTg, or by optogenetic inactivation of the RMTg selectively during the period when LHb neurons are activated by cocaine. Together, these results indicate that LHb/RMTg pathways contribute critically to cocaine-induced avoidance behaviors, while also participating in reciprocally inhibitory interactions with dopamine neurons.