Cocaine Dysregulates Dynorphin Modulation of Inhibitory Neurotransmission in the Ventral Pallidum in a Cell-Type-Specific Manner

Mu opioid receptors expressed in striatal D2 medium spiny neurons have divergent contributions to cocaine and morphine reward

While our understanding of the neurobiological mechanisms underlying cocaine and opiate reward has historically been dopamine-focused, evidence from genetic and pharmacological approaches indicates that µ-opioid receptors (MORs) in the striatum are important contributors. Within the striatum, MORs are expressed in both dopamine D1-receptor and D2-receptor expressing GABAergic medium spiny neurons (MSNs), as well as in interneurons and various afferents. Thus, it remains unclear how these distinct MOR populations regulate drug reward. To address this, we generated mice with a targeted deletion of MORs from dopamine D2 receptor-expressing MSNs (D2-MORKO) and tested the locomotor and conditioned rewarding effects of cocaine and morphine. D2-MORKO mice showed blunted acquisition of cocaine place preference and suppressed expression of preference when tested in the presence of cocaine. Conversely, the acute and sensitized locomotor responses to cocaine and morphine, as well as morphine conditioned place preference, were normal in D2-MORKOs. This indicates MORs expressed in D2-MSNs facilitate cocaine reward. Further, these data suggest these MORs play divergent roles in cocaine and morphine reward.

Refining the circuits of drug addiction: The ventral pallidum

The ventral pallidum is a prominent structure within the basal ganglia, regulating reward and motivational processes. Positioned at the interface between motor and limbic structures, its function is crucial to the development and maintenance of substance use disorders. Chronic drug use induces neuroplastic events in this structure, leading to long-term changes in VP neuronal activity and synaptic communication. Moreover, different neuronal populations within the VP drive drug-seeking behavior in opposite directions. This review explores the role of the VP as a hub for reward, motivation, and aversion, establishing it as an important contributor to the pathophysiology of substance use disorders.

Ventral pallidum GABA and glutamate neurons drive approach and avoidance through distinct modulation of VTA cell types

The ventral pallidum (VP) contains GABA and glutamate neurons projecting to ventral tegmental area (VTA) whose stimulation drives approach and avoidance, respectively. Yet little is known about the mechanisms by which VP cell types shape VTA activity and drive behavior. Here, we found that both VP GABA and glutamate neurons were activated during approach to reward or by delivery of an aversive stimulus. Stimulation of VP GABA neurons inhibited VTA GABA, but activated dopamine and glutamate neurons. Remarkably, stimulation-evoked activation was behavior-contingent such that VTA recruitment was inhibited when evoked by the subject's own action. Conversely, VP glutamate neurons activated VTA GABA, as well as dopamine and glutamate neurons, despite driving aversion. However, VP glutamate neurons evoked dopamine in aversion-associated ventromedial nucleus accumbens (NAc), but reduced dopamine release in reward-associated dorsomedial NAc. These findings show how heterogeneous VP projections to VTA can be engaged to shape approach and avoidance behaviors.

Multimodal Interrogation of Ventral Pallidum Projections Reveals Projection-Specific Signatures and Effects on Cocaine Reward

The ventral pallidum (VP) is a central hub in the reward circuitry with diverse projections that have different behavioral roles attributed mostly to the connectivity with the downstream target. However, different VP projections may represent, as in the striatum, separate neuronal populations that differ in more than just connectivity. In this study, we performed in mice of both sexes a multimodal dissection of four major projections of the VP-to the lateral hypothalamus (VP→LH), ventral tegmental area (VP→VTA), lateral habenula (VP→LHb), and mediodorsal thalamus (VP→MDT)-with physiological, anatomical, genetic, and behavioral tools. We also tested for physiological differences between VP neurons receiving input from nucleus accumbens medium spiny neurons (MSNs) that express either the D1 (D1-MSNs) or the D2 (D2-MSNs) dopamine receptor. We show that each VP projection (1) when inhibited during a cocaine conditioned place preference (CPP) test affects performance differently, (2) receives a different pattern of inputs using rabies retrograde labeling, (3) shows differentially expressed genes using RNA sequencing, and (4) has projection-specific characteristics in excitability and synaptic input characteristics using whole-cell patch clamp. VP→LH and VP→VTA projections have different effects on CPP and show low overlap in circuit tracing experiments, as VP→VTA neurons receive more striatal input, while VP→LH neurons receive more olfactory input. Additionally, VP→VTA neurons are less excitable, while VP→LH neurons are more excitable than the average VP neuron, a difference driven mainly by D2-MSN-responding neurons. Thus, VP→VTA and VP→LH neurons may represent largely distinct populations of VP neurons.

A female-specific role for trigeminal dynorphin in orofacial pain comorbidity

ABSTRACT

Migraine is commonly reported in patients with temporomandibular disorders (TMDs), but little is known about the mechanisms underlying the comorbid condition. Here, we prepared a mouse model to investigate this comorbidity, in which masseter muscle tendon ligation (MMTL) was performed to induce a myogenic TMD, and the pre-existing TMD enabled a subthreshold dose of nitroglycerin (NTG) to produce migraine-like pain in mice. RNA sequencing followed by real-time quantitative polymerase chain reaction confirmation showed that MMTL plus NTG treatment increased prodynorphin ( Pdyn ) mRNA expression in the spinal trigeminal nucleus caudalis (Sp5C) of female mice but not in male mice. Chemogenetic inhibition of Pdyn -expressing neurons or microinjection of antidynorphin antiserum in the Sp5C alleviated MMTL-induced masseter hypersensitivity and diminished the MMTL-enabled migraine-like pain in female mice but not in male mice. Moreover, chemogenetic activation of Pdyn -expressing neurons or microinjection of dynorphin A (1-17) peptide in the Sp5C enabled a subthreshold dose of NTG to induce migraine-like pain in female mice but not in male mice. Taken together, our results suggest that trigeminal dynorphin has a female-specific role in the modulation of comorbid TMDs and migraine.

Pallidal circuits drive addiction behavior

Understanding the neural mechanisms that control addiction processes, including drug-seeking and relapse, is key to finding new targets for substance use disorder (SUD) pharmacotherapies and circuit-based therapies. Addictive drugs alter activity in distinct neural circuits that can lead to SUD symptoms, including compulsive drug craving and taking. This includes the pallidum, a region in the basal ganglia that acts as an integrator of associative, sensorimotor, and limbic information to shape motor responses, promote reward-learning, and regulate habit formation. Here, we review key findings that demonstrate the sub-regional and circuit-specific functions of the pallidum that drive addiction-related behaviors in rodents. We also highlight newly discovered mechanisms within distinct cell types and circuits of the pallidum that drive drug-seeking. Overall, this review serves to emphasize the importance of the pallidum in addiction processes and underscore the need for studying circuit-specific mechanisms in SUD research.

Ventral pallidum GABA and glutamate neurons drive approach and avoidance through distinct modulation of VTA cell types

The ventral pallidum (VP) contains GABA and glutamate (Glut) neurons projecting to ventral tegmental area (VTA) whose stimulation drives approach and avoidance, respectively. Yet little is known about the cell-type-specific mechanisms by which VP projections to VTA drive behavior. Here, we found that both VP GABA and Glut neurons were activated during approach to reward or delivery of an aversive stimulus. Stimulation of VP GABA neurons inhibited VTA GABA, but activated dopamine (DA) and glutamate neurons. Remarkably, this cell-type-specific recruitment was behavior-contingent such that VTA recruitment was inhibited when evoked by the subject's own action. Conversely, VP Glut neurons activated VTA GABA, as well as DA and Glut neurons, despite driving aversion. However, VP Glut neurons evoked DA in reward-associated ventromedial nucleus accumbens (NAc), but reduced DA in aversion-associated dorsomedial NAc. These findings show how heterogeneous VP cell types can engage VTA cell types to shape approach and avoidance behaviors.

Ventral pallidal regulation of motivated behaviors and reinforcement

The interconnected nuclei of the ventral basal ganglia have long been identified as key regulators of motivated behavior, and dysfunction of this circuit is strongly implicated in mood and substance use disorders. The ventral pallidum (VP) is a central node of the ventral basal ganglia, and recent studies have revealed complex VP cellular heterogeneity and cell- and circuit-specific regulation of reward, aversion, motivation, and drug-seeking behaviors. Although the VP is canonically considered a relay and output structure for this circuit, emerging data indicate that the VP is a central hub in an extensive network for reward processing and the regulation of motivation that extends beyond classically defined basal ganglia borders. VP neurons respond temporally faster and show more advanced reward coding and prediction error processing than neurons in the upstream nucleus accumbens, and regulate the activity of the ventral mesencephalon dopamine system. This review will summarize recent findings in the literature and provide an update on the complex cellular heterogeneity and cell- and circuit-specific regulation of motivated behaviors and reinforcement by the VP with a specific focus on mood and substance use disorders. In addition, we will discuss mechanisms by which stress and drug exposure alter the functioning of the VP and produce susceptibility to neuropsychiatric disorders. Lastly, we will outline unanswered questions and identify future directions for studies necessary to further clarify the central role of VP neurons in the regulation of motivated behaviors. Significance: Research in the last decade has revealed a complex cell- and circuit-specific role for the VP in reward processing and the regulation of motivated behaviors. Novel insights obtained using cell- and circuit-specific interrogation strategies have led to a major shift in our understanding of this region. Here, we provide a comprehensive review of the VP in which we integrate novel findings with the existing literature and highlight the emerging role of the VP as a linchpin of the neural systems that regulate motivation, reward, and aversion. In addition, we discuss the dysfunction of the VP in animal models of neuropsychiatric disorders.

Ventral pallidum GABA neurons bidirectionally control opioid relapse across rat behavioral models

Opioid addiction is a chronic, relapsing disorder. Whether addicted individuals are forced to abstain or they decide themselves to quit using drugs, relapse rates are high-especially upon encountering contexts and stimuli associated with prior opioid use. Rodents similarly show context- and cue-induced reinstatement of drug seeking following abstinence, and intriguingly, the neural circuits underlying these relapse-like behaviors differ when abstinence is involuntarily imposed, responding is extinguished, or animals decide themselves to cease taking drug. Here, we employ two complementary rat behavioral models of relapse-like behavior for the highly reinforcing opioid drug remifentanil, and asked whether GABAergic neurons in the ventral pallidum (VPGABA) control opioid seeking under these behavioral conditions. Specifically, we asked how chemogenetically stimulating VPGABA neurons with clozapine-N-oxide (CNO) influences the ability of contextual or discrete remifentanil-paired cues to reinstate drug seeking following either voluntary abstinence (punishment-induced; GroupPunish), or extinction training (GroupExt). In GroupPunish rats, we also chemogenetically inhibited VPGABA neurons, and examined spontaneous VP activity (Fos) during cued reinstatement. In both GroupPunish and GroupExt rats, stimulating Gq-signaling in VPGABA neurons augmented remifentanil reinstatement in a cue- and context-dependent manner. Conversely, engaging inhibitory Gi-signaling in VPGABA neurons in GroupPunish suppressed cue-induced reinstatement, and cue-triggered seeking was correlated with Fos expression in rostral, but not caudal VP. Neither stimulating nor inhibiting VPGABA neurons influenced unpunished remifentanil self-administration. We conclude that VPGABA neurons bidirectionally control opioid seeking regardless of the specific relapse model employed, highlighting their fundamental role in opioid relapse-like behavior across behavioral models, and potentially across species.

Cocaine induces input and cell-type-specific synaptic plasticity in ventral pallidum-projecting nucleus accumbens medium spiny neurons

Cocaine use and abstinence induce long-term synaptic alterations in the excitatory input to nucleus accumbens (NAc) medium spiny neurons (MSNs). The NAc regulates reward-related behaviors through two parallel projections to the ventral pallidum (VP)-originating in D1 or D2-expressing MSNs (D1-MSNs_→VP; D2-MSNs_→VP). The activity of these projections depends on their excitatory synaptic inputs, but it is not known whether and how abstinence from cocaine affects the excitatory transmission to D1-MSNs_→VP and D2-MSNs_→VP. Here we examined different forms of cocaine-induced synaptic plasticity in the inputs from the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) to NAc D1-MSNs_→VP and putative D2-MSNs_→VP (pD2-MSNs_→VP) in the core and shell subcompartments of the NAc. We used the whole-cell patch-clamp technique to record excitatory postsynaptic currents from D1-tdTomato mice injected with ChR2 in either the BLA or the mPFC and retrograde tracer (RetroBeads) in the VP. We found that cocaine conditioned place preference (CPP) followed by abstinence potentiated the excitatory input from the BLA and mPFC to both D1-MSNs_→VP and pD2-MSNs_→VP. Interestingly, while the strengthening of the inputs to D1-MSNs_→VP was of postsynaptic origin and manifested as increased AMPA to NMDA ratio, in pD2-MSNs_→VP plasticity was predominantly presynaptic and was detected as changes in the paired-pulse ratio and coefficient of variation. Lastly, some of the changes were sex-specific. Overall our data show that abstinence from cocaine changes the excitatory inputs to both D1-MSNs_→VP and pD2-MSNs_→VP but with different mechanisms. This may help understand how circuits converging into the VP change after cocaine exposure.

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Traumatic Stress-Induced Vulnerability to Addiction: Critical Role of the Dynorphin/Kappa Opioid Receptor System

Substance use disorders (SUD) may emerge from an individual’s attempt to limit negative affective states and symptoms linked to stress. Indeed, SUD is highly comorbid with chronic stress, traumatic stress, or post-traumatic stress disorder (PTSD), and treatments approved for each pathology individually often failed to have a therapeutic efficiency in such comorbid patients. The kappa-opioid receptor (KOR) and its endogenous ligand dynorphin (DYN), seem to play a key role in the occurrence of this comorbidity. The DYN/KOR function is increased either in traumatic stress or during drug use, dependence acquisition and DYN is released during stress. The behavioural effects of stress related to the DYN/KOR system include anxiety, dissociative and depressive symptoms, as well as increased conditioned fear response. Furthermore, the DYN/KOR system is implicated in negative reinforcement after the euphoric effects of a drug of abuse ends. During chronic drug consumption DYN/KOR functions increase and facilitate tolerance and dependence. The drug-seeking behaviour induced by KOR activation can be retrieved either during the development of an addictive behaviour, or during relapse after withdrawal. DYN is known to be one of the most powerful negative modulators of dopamine signalling, notably in brain structures implicated in both reward and fear circuitries. KOR are also acting as inhibitory heteroreceptors on serotonin neurons. Moreover, the DYN/KOR system cross-regulate with corticotropin-releasing factor in the brain. The sexual dimorphism of the DYN/KOR system could be the cause of the gender differences observed in patients with SUD or/and traumatic stress-related pathologies. This review underlies experimental and clinical results emphasizing the DYN/KOR system as common mechanisms shared by SUD or/and traumatic stress-related pathologies, and suggests KOR antagonist as a new pharmacological strategy to treat this comorbidity.

Endogenous opiates and behavior: 2020

This paper is the forty-third consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2020 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

The role of the nucleus accumbens and ventral pallidum in feeding and obesity

Obesity is a growing global epidemic that stems from the increasing availability of highly-palatable foods and the consequent enhanced calorie consumption. Extensive research has shown that brain regions that are central to reward seeking modulate feeding and evidence linking obesity to pathology in such regions have recently started to accumulate. In this review we focus on the contribution of two major interconnected structures central to reward processing, the nucleus accumbens and the ventral pallidum, to obesity. We first review the known literature linking these structures to feeding behavior, then discuss recent advances connecting pathology in the nucleus accumbens and ventral pallidum to obesity, and finally examine the similarities and differences between drug addiction and obesity in the context of these two structures. The understanding of how pathology in brain regions involved in reward seeking and consumption may drive obesity and how mechanistically similar obesity and addiction are, is only now starting to be revealed. We hope that future research will advance knowledge in the field and open new avenues to studying and treating obesity.

Ventral pallidum cellular and pathway specificity in drug seeking

The ventral pallidum (VP) is central to the reinforcing effects across a variety of drugs and relapse to drug seeking. Emerging studies from animal models of reinstatement reveal a complex neurobiology of the VP that contributes to different aspects of relapse to drug seeking. This review builds on classical understanding of the VP as part of the final common pathway of relapse but also discusses the properties of the VP as an independent structure. These include VP neural anatomical subregions, cellular heterogeneity, circuitry, neurotransmitters and peptides. Collectively, this review provides a current understanding of the VP from molecular to circuit level architecture that contributes to both the appetitive and aversive symptoms of drug addiction. We show the complex neurobiology of the VP in drug seeking, emphasizing its critical role in addiction, and review strategic approaches that target the VP to reduce relapse rates.

Metaplasticity in the Ventral Pallidum as a Potential Marker for the Propensity to Gain Weight in Chronic High-Calorie Diet

A major driver of obesity is the increasing palatability of processed foods. Although reward circuits promote the consumption of palatable food, their involvement in obesity remains unclear. The ventral pallidum (VP) is a key hub in the reward system that encodes the hedonic aspects of palatable food consumption and participates in various proposed feeding circuits. However, there is still no evidence for its involvement in developing diet-induced obesity. Here we examine, using male C57BL6/J mice and patch-clamp electrophysiology, how chronic high-fat high-sugar (HFHS) diet changes the physiology of the VP and whether mice that gain the most weight differ in their VP physiology from others. We found that 10-12 weeks of HFHS diet hyperpolarized and decreased the firing rate of VP neurons without a major change in synaptic inhibitory input. Within the HFHS group, the top 33% weight gainers (WGs) had a more hyperpolarized VP with longer latency to fire action potentials on depolarization compared with bottom 33% of weight gainers (i.e., non-weight gainers). WGs also showed synaptic potentiation of inhibitory inputs both at the millisecond and minute ranges. Moreover, we found that the tendency to potentiate the inhibitory inputs to the VP might exist in overeating mice even before exposure to HFHS, thus making it a potential property of being an overeater. These data point to the VP as a critical player in obesity and suggest that hyperpolarized membrane potential of, and potentiated inhibitory inputs to, VP neurons may play a significant role in promoting the overeating of palatable food.SIGNIFICANCE STATEMENT In modern world, where highly palatable food is readily available, overeating is often driven by motivational, rather than metabolic, needs. It is thus conceivable that reward circuits differ between obese and normal-weight individuals. But is such difference, if it exists, innate or does it develop with overeating? Here we reveal synaptic properties in the ventral pallidum, a central hub of reward circuits, that differ between mice that gain the most and the least weight when given unlimited access to highly palatable food. We show that these synaptic differences also exist without exposure to palatable food, potentially making them innate properties that render some more susceptible than others to overeat. Thus, the propensity to overeat may have a strong innate component embedded in reward circuits.

The ventral pallidum and relapse in alcohol seeking

Alcohol-use disorders are chronically relapsing conditions characterized by cycles of use, abstinence and relapse. The ventral pallidum (VP) is a key node in the neural circuits controlling relapse to alcohol seeking and a key target of pharmacotherapies for relapse prevention. There has been a significant increase in our understanding of the molecular, anatomical, pharmacological and functional properties of the ventral pallidum, laying foundations for a new understanding of its role in relapse to alcohol seeking and motivation. Here we review these advances, placing special emphasis on how advances in understanding in the cellular and circuit architectures of ventral pallidum contributes to the relapse to alcohol seeking. We show how this knowledge improves mechanistic understanding of current relapse prevention pharmacotherapies, how it may be used to tailor these against different forms of relapse and how it may help provide insights into the mental health problems frequently co-morbid with alcohol-use disorders.