Alcohol preference influences the subthalamic nucleus control on motivation for alcohol in rats

Subthalamic high-frequency deep brain stimulation reduces addiction-like alcohol use and the possible negative influence of a peer presence

RATIONALE

The immediate social context significantly influences alcohol consumption in humans. Recent studies have revealed that peer presence could modulate drugs use in rats. The most efficient condition to reduce cocaine intake is the presence of a stranger peer, naive to drugs. Deep brain stimulation (DBS) of the Subthalamic Nucleus (STN), which was shown to have beneficial effects on addiction to cocaine or alcohol, also modulates the protective influence of peer's presence on cocaine use.

OBJECTIVES

This study aimed to: 1) explore how the presence of an alcohol-naive stranger peer affects recreational and escalated alcohol intake, and 2) assess the involvement of STN on alcohol use and in the modulation induced by the presence of an alcohol-naïve stranger peer.

METHODS

Rats with STN DBS and control animals self-administered 10% (v/v) ethanol in presence, or absence, of an alcohol-naive stranger peer, before and after escalation of ethanol intake (observed after intermittent alcohol (20% (v/v) ethanol) access).

RESULTS

Neither STN DBS nor the presence of an alcohol-naive stranger peer modulated significantly recreational alcohol intake. After the escalation procedure, STN DBS reduced ethanol consumption. The presence of an alcohol-naive stranger peer increased consumption only in low drinkers, which effect was suppressed by STN DBS.

CONCLUSIONS

These results highlight the influence of a peer's presence on escalated alcohol intake, and confirm the role of STN in addiction-like alcohol intake and in the social influence on drug consumption.

Role of deep brain stimulation (DBS) in addiction disorders

Background

Addiction disorders pose significant challenges to public health, necessitating innovative treatments. This assesses deep brain stimulation (DBS) as a potential intervention for addiction disorders.

Methods

A literature review was carried out with a focus on the role of DBS in addiction disorders and its future implications in neurosurgical research.

Results

The online literature shows that DBS precisely modulates certain brain regions to restore addiction-related neural circuits and promote behavioral control.

Conclusion

Preclinical evidence demonstrates DBS's potential to rebalance neural circuits associated with addiction, and early clinical trials provide encouraging outcomes in enhancing addiction-related outcomes. Ethical considerations, long-term safety, and personalized patient selection require further investigation.

A role for the subthalamic nucleus in aversive learning

The subthalamic nucleus (STN) is critical for behavioral control; its dysregulation consequently correlated with neurological and neuropsychiatric disorders, including Parkinson's disease. Deep brain stimulation (DBS) targeting the STN successfully alleviates parkinsonian motor symptoms. However, low mood and depression are affective side effects. STN is adjoined with para-STN, associated with appetitive and aversive behavior. DBS aimed at STN might unintentionally modulate para-STN, causing aversion. Alternatively, the STN mediates aversion. To investigate causality between STN and aversion, affective behavior is addressed using optogenetics in mice. Selective promoters allow dissociation of STN (e.g., Pitx2) vs. para-STN (Tac1). Acute photostimulation results in aversion via both STN and para-STN. However, only STN stimulation-paired cues cause conditioned avoidance and only STN stimulation interrupts on-going sugar self-administration. Electrophysiological recordings identify post-synaptic responses in pallidal neurons, and selective photostimulation of STN terminals in the ventral pallidum replicates STN-induced aversion. Identifying STN as a source of aversive learning contributes neurobiological underpinnings to emotional affect.

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.

Chronic Alcoholism and HHV-6 Infection Synergistically Promote Neuroinflammatory Microglial Phenotypes in the Substantia Nigra of the Adult Human Brain

Both chronic alcoholism and human herpesvirus-6 (HHV-6) infection have been identified as promoters of neuroinflammation and known to cause movement-related disorders. Substantia Nigra (SN), the dopaminergic neuron-rich region of the basal ganglia, is involved in regulating motor function and the reward system. Hence, we hypothesize the presence of possible synergism between alcoholism and HHV-6 infection in the SN region and report a comprehensive quantification and characterization of microglial functions and morphology in postmortem brain tissue from 44 healthy, age-matched alcoholics and chronic alcoholics. A decrease in the perivascular CD68+ microglia in alcoholics was noted in both the gray and white matter. Additionally, the CD68+/Iba1− microglial subpopulation was found to be the dominant type in the controls. Conversely, in alcoholics, dystrophic changes in microglia were seen with a significant increase in Iba1 expression and perivascular to diffuse migration. An increase in CD11b expression was noted in alcoholics, with the Iba1+/CD11b− subtype promoting inflammation. All the controls were found to be negative for HHV-6 whilst the alcoholics demonstrated HHV-6 positivity in both gray and white matter. Amongst HHV-6 positive alcoholics, all the above-mentioned changes were found to be heightened when compared with HHV-6 negative alcoholics, thereby highlighting the compounding relationship between alcoholism and HHV-6 infection that promotes microglia-mediated neuroinflammation.

New pharmacological and neuromodulation approaches for impulsive-compulsive behaviors in Parkinson's disease

INTRODUCTION

A significant proportion of patients with Parkinson's disease (PD) display a set of impulsive-compulsive behaviors at some point during the course of illness. These behaviors range from the so-called behavioral addictions to dopamine dysregulation syndrome, punding and hoarding disorders. These behaviors have been consistently linked to the use of dopaminergic medications used to treat PD motor symptoms (dopamine agonists, levodopa, and other agents) and less consistently to neuromodulation techniques such as deep brain stimulation (DBS). Since there are still no approved treatments for these conditions, their pharmacological management is still a big challenge for clinicians.

METHODS

We conducted an extensive review of current pharmacological and neuromodulation literature for the management of impulsive-compulsive disorders in PD patients.

RESULTS

Pharmacological treatment approaches for impulsive-compulsive behaviors and DDS in PD patients include reduction of levodopa (LD), reduction/cessation of dopamine agonist (DA), and initiation of infusion therapies (apomorphine infusion and duodopa). Also, atomoxetine, a noradrenergic agent approved for the treatment of attention deficit hyperactivity disorder, showed some interesting preliminary results but there is still a lack of controlled longitudinal studies. Finally, while DBS effects on impulsive-compulsive disorders are still controversial, non-invasive techniques (such as transcranial magnetic stimulation and transcranial direct current stimulation) could have a potential positive effect but, again, there is still a lack of controlled trials.

CONCLUSION

Managing impulsivity and compulsivity in PD patients is still a non-evidence-based challenge for clinicians. Controlled trials on promising approaches such as atomoxetine and non-invasive neuromodulation techniques are needed.

Subthalamic low-frequency oscillations predict vulnerability to cocaine addiction

Identifying vulnerable individuals before they transition to a compulsive pattern of drug seeking and taking is a key challenge in addiction to develop efficient prevention strategies. Oscillatory activity within the subthalamic nucleus (STN) has been associated with compulsive-related disorders. To study compulsive cocaine-seeking behavior, a core component of drug addiction, we have used a rat model in which cocaine seeking despite a foot-shock contingency only emerges in some vulnerable individuals having escalated their cocaine intake. We show that abnormal oscillatory activity within the alpha/theta and low-beta bands during the escalation of cocaine intake phase predicts the subsequent emergence of compulsive-like seeking behavior. In fact, mimicking STN pathological activity in noncompulsive rats during cocaine escalation turns them into compulsive ones. We also find that 30 Hz, but not 130 Hz, STN deep brain stimulation (DBS) reduces pathological cocaine seeking in compulsive individuals. Our results identify an early electrical signature of future compulsive-like cocaine-seeking behavior and further advocates the use of frequency-dependent STN DBS for the treatment of addiction.

Advanced Therapies for the Management of Dopamine Dysregulation Syndrome in Parkinson's Disease

Background

Dopamine Dysregulation Syndrome (DDS) is an adverse non-motor complication of dopamine replacement therapy in Parkinson's disease. The current literature on this syndrome is limited, and it remains underdiagnosed and challenging to manage.

Objective

To assess the role of advanced therapies in the management of DDS.

Methods

We performed a retrospective chart review and identified patients who fit the inclusion criteria for DDS. They were classified according to risk factors that have been identified in the literature, motor and complication scores, intervention (medical or surgical) and outcome. Multivariate analyses were performed to analyze these characteristics.

Results

Twenty-seven patients were identified (23 males, mean age of onset: 49 ± 8.8 years). Average levodopa equivalent daily dose was 1916.7 ± 804 mg and a history of impulse control disorders, psychiatric illness, and substance abuse was present in 89%, 70% and 3.7% of the patients, respectively. Overall 81.5% of patients had symptom resolution at follow up, on average 4.8 ± 3.5 years after management, with medication only (7/9), levodopa-carbidopa intestinal gel (1/3), deep brain stimulation of subthalamic nucleus (10/13), or globus pallidus pars interna (2/2). Reduction of medications occurred with deep brain stimulation of subthalamic nucleus (P = 0.01) but was associated with a relapse in two patients.

Conclusion

Although the small sample size of some subgroups limits our ability to draw meaningful conclusions, our results did not suggest superiority of a single treatment option. Advanced therapies including deep brain stimulation can be considered in patients with DDS refractory to conservative measures, but outcome is variable and relapse is possible.

BEHAVIORAL AND NEUROBIOLOGICAL MECHANISMS OF PAVLOVIAN AND INSTRUMENTAL EXTINCTION LEARNING

This article reviews the behavioral neuroscience of extinction, the phenomenon in which a behavior that has been acquired through Pavlovian or instrumental (operant) learning decreases in strength when the outcome that reinforced it is removed. Behavioral research indicates that neither Pavlovian nor operant extinction depends substantially on erasure of the original learning but instead depends on new inhibitory learning that is primarily expressed in the context in which it is learned, as exemplified by the renewal effect. Although the nature of the inhibition may differ in Pavlovian and operant extinction, in either case the decline in responding may depend on both generalization decrement and the correction of prediction error. At the neural level, Pavlovian extinction requires a tripartite neural circuit involving the amygdala, prefrontal cortex, and hippocampus. Synaptic plasticity in the amygdala is essential for extinction learning, and prefrontal cortical inhibition of amygdala neurons encoding fear memories is involved in extinction retrieval. Hippocampal-prefrontal circuits mediate fear relapse phenomena, including renewal. Instrumental extinction involves distinct ensembles in corticostriatal, striatopallidal, and striatohypothalamic circuits as well as their thalamic returns for inhibitory (extinction) and excitatory (renewal and other relapse phenomena) control over operant responding. The field has made significant progress in recent decades, although a fully integrated biobehavioral understanding still awaits.

Anticipatory human subthalamic area beta-band power responses to dissociable tastes correlate with weight gain

The availability of enticing sweet, fatty tastes is prevalent in the modern diet and contribute to overeating and obesity. In animal models, the subthalamic area plays a role in mediating appetitive and consummatory feeding behaviors, however, its role in human feeding is unknown. We used intraoperative, subthalamic field potential recordings while participants (n = 5) engaged in a task designed to provoke responses of taste anticipation and receipt. Decreased subthalamic beta-band (15-30 Hz) power responses were observed for both sweet-fat and neutral tastes. Anticipatory responses to taste-neutral cues started with an immediate decrease in beta-band power from baseline followed by an early beta-band rebound above baseline. On the contrary, anticipatory responses to sweet-fat were characterized by a greater and sustained decrease in beta-band power. These activity patterns were topographically specific to the subthalamic nucleus and substantia nigra. Further, a neural network trained on this beta-band power signal accurately predicted (AUC ≥ 74%) single trials corresponding to either taste. Finally, the magnitude of the beta-band rebound for a neutral taste was associated with increased body mass index after starting deep brain stimulation therapy. We provide preliminary evidence of discriminatory taste encoding within the subthalamic area associated with control mechanisms that mediate appetitive and consummatory behaviors.

Deep Brain Stimulation of the Subthalamic Nucleus Modulates Reward-Related Behavior: A Systematic Review

Deep brain stimulation of the subthalamic nucleus (STN-DBS) is an effective treatment for the motor symptoms of movement disorders including Parkinson's Disease (PD). Despite its therapeutic benefits, STN-DBS has been associated with adverse effects on mood and cognition. Specifically, apathy, which is defined as a loss of motivation, has been reported to emerge or to worsen following STN-DBS. However, it is often challenging to disentangle the effects of STN-DBS per se from concurrent reduction of dopamine replacement therapy, from underlying PD pathology or from disease progression. To this end, pre-clinical models allow for the dissociation of each of these factors, and to establish neural substrates underlying the emergence of motivational symptoms following STN-DBS. Here, we performed a systematic analysis of rodent studies assessing the effects of STN-DBS on reward seeking, reward motivation and reward consumption across a variety of behavioral paradigms. We find that STN-DBS decreases reward seeking in the majority of experiments, and we outline how design of the behavioral task and DBS parameters can influence experimental outcomes. While an early hypothesis posited that DBS acts as a "functional lesion," an analysis of lesions and inhibition of the STN revealed no consistent pattern on reward-related behavior. Thus, we discuss alternative mechanisms that could contribute to the amotivational effects of STN-DBS. We also argue that optogenetic-assisted circuit dissection could yield important insight into the effects of the STN on motivated behavior in health and disease. Understanding the mechanisms underlying the effects of STN-DBS on motivated behavior-will be critical for optimizing the clinical application of STN-DBS.

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.

Medications, Deep Brain Stimulation, and Other Factors Influencing Impulse Control Disorders in Parkinson's Disease

Impulse control disorders (ICDs) in Parkinson's disease (PD) have a high cumulative incidence and negatively impact quality of life. ICDs are influenced by a complex interaction of multiple factors. Although it is now well-recognized that dopaminergic treatments and especially dopamine agonists underpin many ICDs, medications alone are not the sole cause. Susceptibility to ICD is increased in the setting of PD. While causality can be challenging to ascertain, a wide range of modifiable and non-modifiable risk factors have been linked to ICDs. Common characteristics of PD patients with ICDs have been consistently identified across many studies; for example, males with an early age of PD onset and dopamine agonist use have a higher risk of ICD. However, not all cases of ICDs in PD can be directly attributable to dopamine, and studies have concluded that additional factors such as genetics, smoking, and/or depression may be more predictive. Beyond dopamine, other ICD associations have been described but remain difficult to explain, including deep brain stimulation surgery, especially in the setting of a reduction in dopaminergic medication use. In this review, we will summarize the demographic, genetic, behavioral, and clinical contributions potentially influencing ICD onset in PD. These associations may inspire future preventative or therapeutic strategies.

The pathophysiological mechanisms of motivational deficits in Parkinson's disease

Parkinson's disease (PD) is a progressive degenerative disorder that leads to disabling motor symptoms and a wide variety of neuropsychiatric symptoms. Apathy is the most common psychiatric disorder in the early stages of untreated PD and can be defined as a hypodopaminergic syndrome, which also includes anxiety and depression. Apathy is also considered the core feature of the parkinsonian triad (apathy, anxiety and depression) of behavioural non-motor signs, including a motivational deficit. Moreover, apathy is recognised as a distinct chronic neuropsychiatric behavioural disorder based on specific diagnostic criteria. Given the prevalence of apathy in approximately 40% of the general Parkinson's disease population, this appears to be a contributing factor to dementia in PD; also, apathy symptoms are factors that potentially contribute to morbidity, leading to a major impairment of health-related quality of life, thus stressing the importance of understanding the pathophysiology of this disease. Several studies have clearly established a prominent role for DA-mediated signals in PD apathy. However, synergistic interaction between dopaminergic impairment resulting from the neurodegenerative process and deep brain stimulation of the subthalamic nucleus may cause or exacerbate apathy. Furthermore, serotoninergic mechanism signalling is also likely to be of importance in this pathophysiology.

Subthalamic Nucleus Deep Brain Stimulation: Basic Concepts and Novel Perspectives

Over the last decades, extensive basic and clinical knowledge has been acquired on the use of subthalamic nucleus (STN) deep brain stimulation (DBS) for Parkinson's disease (PD). It is now clear that mechanisms involved in the effects of this therapy are far more complex than previously anticipated. At frequencies commonly used in clinical practice, neural elements may be excited or inhibited and novel dynamic states of equilibrium are reached. Electrode contacts used for chronic DBS in PD are placed near the dorsal border of the nucleus, a highly cellular region. DBS may thus exert its effects by modulating these cells, hyperdirect projections from motor cortical areas, afferent and efferent fibers to the motor STN. Advancements in neuroimaging techniques may allow us to identify these structures optimizing surgical targeting. In this review, we provide an update on mechanisms and the neural elements modulated by STN DBS.

Targeting the subthalamic nucleus in a preclinical model of alcohol use disorder

BACKGROUND

The subthalamic nucleus (STN) has only recently been considered to have a role in reward processing. In rats, inactivation of the STN by lesion or high-frequency stimulation (HFS) decreases motivation for cocaine but increases motivation for sucrose. For ethanol, the effect of STN lesion depends on the individual's baseline intake; decreasing motivation for ethanol in rats with lower ethanol intake, while increasing motivation for ethanol in rats with higher-but still limited-ethanol intake. However, the involvement of the STN in behaviour more closely resembling some aspects of alcohol use disorder has not been assessed. This study aimed to determine the effect of STN lesions on the escalation of ethanol intake, subsequent increases in the motivation to "work" for ethanol and the choice of ethanol over a non-drug alternative.

RESULTS

We found that STN lesion prevented increases in ethanol intake observed during intermittent ethanol access and after a long period of ethanol privation. STN lesion also decreased the motivation to work for ethanol after escalated intake. Surprisingly, STN lesion increased the choice of alcohol over saccharin. This was associated with a blunting of the hedonic responses to the taste of the reinforcement alternatives.

CONCLUSION

These results evidence the involvement of the STN in different ethanol-motivated behaviours and therefore position the STN as an interesting target for the treatment of alcohol use disorders.

Inhibiting subthalamic nucleus decreases cocaine demand and relapse: therapeutic potential

Preclinical evidence indicates that inactivation of subthalamic nucleus (STN) may be effective for treating cocaine addiction, and therapies that target STN, e.g. deep brain stimulation, are available indicating that this may have clinical promise. Here, we assessed the therapeutic potential of STN inactivation using a translationally relevant economic approach that quantitatively describes drug-taking behavior, and tested these results with drug-seeking tasks. Economic demand for cocaine was assessed in rats (n = 11) using a within-session threshold procedure in which cocaine price (responses/mg cocaine) was sequentially increased throughout the session. Cocaine demand was assessed in this manner immediately after bilateral microinfusions into STN of either vehicle (artificial cerebrospinal fluid) or the GABAA receptor agonist muscimol. A separate group of animals (n = 8) was tested for changes in cocaine seeking either during extinction or in response to cocaine-associated cues. Muscimol-induced inhibition of STN significantly attenuated cocaine consumption at high prices, drug seeking during extinction and cued reinstatement of cocaine seeking. In contrast, STN inhibition did not reduce cocaine consumption at low prices or locomotor activity. Thus, STN inactivation reduced economic demand for cocaine and multiple measures of drug seeking during extinction. In view of the association between economic demand and addiction severity in both rat and human, these results indicate that STN inactivation has substantial clinical potential for treatment of cocaine addiction.

Ventral Pallidum Output Pathways in Context-Induced Reinstatement of Alcohol Seeking

Ventral pallidum (VP) is a well-established locus for the reinforcing effects of drugs of abuse and reinstatement of drug seeking. However, VP neurons are at the origin of multiple output pathways, with strong projections to ventral tegmental area (VTA), subthalamic nucleus (STN), lateral hypothalamus, among others, and the roles of these VP output pathways in reinstatement of drug seeking remain poorly understood. Here we addressed these issues using a combination of neuroanatomical tracing and chemogenetic approaches. First, using dual-retrograde tracing, we show that VP neurons projecting to either VTA or STN are recruited during context-induced reinstatement of extinguished alcohol seeking in rats. Then, using chemogenetics, we show modulation of context-induced reinstatement and reacquisition of alcohol seeking via designer receptors exclusively activated by designer drugs excitation or inhibition of the VP. To determine the causal roles of VP → VTA and VP → STN pathways in context-induced reinstatement and reacquisition we used a chemogenetic disconnection approach and show that silencing either the VP → VTA or VP → STN pathways is sufficient to reduce both reinstatement and reacquisition of alcohol seeking. Moreover, these disconnections also each reduced responding and motivation during a progressive ratio test but had no effect on locomotor activity. Together, these results show that multiple ventral pallidal output pathways contribute to relapse to alcohol seeking.

SIGNIFICANCE STATEMENT

Ventral pallidum (VP) serves important roles in reward and motivation and is a critical node in the neural circuitry for reinstatement of drug seeking. Despite being a common locus for different forms of reinstatement, fundamental aspects of neural circuitry for these VP contributions to reinstatement of drug seeking remain unknown. Here we used a combination of neuroanatomical tracing and chemogenetic approaches to map the VP output pathways for context-induced reinstatement and reacquisition of alcohol seeking. We show that VP output pathways to the subthalamic nucleus and also to the ventral tegmental area are necessary for these forms of reinstatement.

How contexts promote and prevent relapse to drug seeking

The contexts where drugs are self-administered play an important role in regulating persistent drug taking and in relapse to such taking after periods of abstinence. Here, we review the behavioral and brain mechanisms enabling contexts to promote and prevent relapse to drug seeking. We review the key brain structures, their neuropharmacology and their connectivity. We discuss the similarities and differences between the mechanisms for context-induced reinstatement of drug seeking vs. other forms of relapse to drug seeking in animal models and we highlight the numerous deficits in our understanding. We emphasize that current understanding, although significant, defies explanations in terms of models at the level of brain structures and their connectivity. Rather, we show that there is significant functional compartmentalization and segregation within these structures during reinstatement and extinction of drug seeking that parallels their anatomical segregation into circuits and channels. A key challenge is to recognize this complexity, understand how these circuits and channels are organized, as well as understand how different modes of activity of ensembles of neurons within them promote abstinence or relapse to drug seeking.

The neurocircuitry involved in oxytocin modulation of methamphetamine addiction

The role of oxytocin in attenuating the abuse of licit and illicit drugs, including the psychostimulant methamphetamine, has been examined with increased ferocity in recent years. This is largely driven by the potential application of oxytocin as a pharmacotherapy. However, the neural mechanisms by which oxytocin modulates methamphetamine abuse are not well understood. Recent research identified an important role for the accumbens core and subthalamic nucleus in this process, which likely involves an interaction with dopamine, glutamate, GABA, and vasopressin. In addition to providing an overview of methamphetamine, the endogenous oxytocin system, and the effects of exogenous oxytocin on drug abuse, we propose a neural circuit through which exogenous oxytocin modulates methamphetamine abuse, focusing on its interaction with neurochemicals within the accumbens core and subthalamic nucleus. A growing understanding of exogenous oxytocin effects at a neurochemical and neurobiological level will assist in its evaluation as a pharmacotherapy for drug addiction.

The Subthalamic Nucleus, Limbic Function, and Impulse Control

It has been well documented that deep brain stimulation (DBS) of the subthalamic nucleus (STN) to address some of the disabling motor symptoms of Parkinson's disease (PD) can evoke unintended effects, especially on non-motor behavior. This observation has catalyzed more than a decade of research concentrated on establishing trends and identifying potential mechanisms for these non-motor effects. While many issues remain unresolved, the collective result of many research studies and clinical observations has been a general recognition of the role of the STN in mediating limbic function. In particular, the STN has been implicated in impulse control and the related construct of valence processing. A better understanding of STN involvement in these phenomena could have important implications for treating impulse control disorders (ICDs). ICDs affect up to 40% of PD patients on dopamine agonist therapy and approximately 15% of PD patients overall. ICDs have been reported to be associated with STN DBS. In this paper we will focus on impulse control and review pre-clinical, clinical, behavioral, imaging, and electrophysiological studies pertaining to the limbic function of the STN.

Brain activation induced by voluntary alcohol and saccharin drinking in rats assessed with manganese-enhanced magnetic resonance imaging

The neuroanatomical and neurochemical basis of alcohol reward has been studied extensively, but global alterations of neural activity in reward circuits during chronic alcohol use remain poorly described. Here, we measured brain activity changes produced by long-term voluntary alcohol drinking in the alcohol-preferring AA (Alko alcohol) rats using manganese-enhanced magnetic resonance imaging (MEMRI). MEMRI is based on the ability of paramagnetic manganese ions to accumulate in excitable neurons and thereby enhance the T1-weighted signal in activated brain areas. Following 6 weeks of voluntary alcohol drinking, AA rats were allowed to drink alcohol for an additional week, during which they were administered manganese chloride (MnCl2 ) with subcutaneous osmotic minipumps before MEMRI. A second group with an identical alcohol drinking history received MnCl2 during the abstinence week following alcohol drinking. For comparing alcohol with a natural reinforcer, MEMRI was also performed in saccharin-drinking rats. A water-drinking group receiving MnCl2 served as a control. We found that alcohol drinking increased brain activity extensively in cortical and subcortical areas, including the mesocorticolimbic and nigrostriatal dopamine pathways and their afferents. Remarkably similar activation maps were seen after saccharin ingestion. Particularly in the prelimbic cortex, ventral hippocampus and subthalamic nucleus, activation persisted into early abstinence. These data show that voluntary alcohol recruits an extensive network that includes the ascending dopamine systems and their afferent connections, and that this network is largely shared with saccharin reward. The regions displaying persistent alterations after alcohol drinking could participate in brain networks underlying alcohol seeking and relapse.

The Involvement of Oxytocin in the Subthalamic Nucleus on Relapse to Methamphetamine-Seeking Behaviour

The psychostimulant methamphetamine (METH) is an addictive drug of abuse. The neuropeptide oxytocin has been shown to modulate METH-related reward and METH-seeking behaviour. Recent findings implicated the subthalamic nucleus (STh) as a key brain region in oxytocin modulation of METH-induced reward. However, it is unclear if oxytocin acts in this region to attenuate relapse to METH-seeking behaviour, and if this action is through the oxytocin receptor. We aimed to determine whether oxytocin pretreatment administered into the STh would reduce reinstatement to METH use in rats experienced at METH self-administration, and if this could be reversed by the co-administration of the oxytocin receptor antagonist desGly-NH2,d(CH2)5[D-Tyr2,Thr4]OVT. Male Sprague Dawley rats underwent surgery to implant an intravenous jugular vein catheter and bilateral microinjection cannulae into the STh under isoflourane anaesthesia. Rats were then trained to self-administer intravenous METH (0.1 mg/kg/infusion) by lever press during 2-hour sessions under a fixed ratio 1 schedule for 20 days. Following extinction of lever press activity, the effect of microinjecting saline, oxytocin (0.2 pmol, 0.6 pmol, 1.8 pmol, 3.6 pmol) or co-administration of oxytocin (3.6 pmol) and desGly-NH₂,d(CH₂)₅[D-Tyr²,Thr⁴]OVT (3 nmol) into the STh (200 nl/side) was examined on METH-primed reinstatement (1 mg/kg; i.p.). We found that local administration of the highest oxytocin dose (3.6 pmol) into the STh decreased METH-induced reinstatement and desGly-NH2,d(CH2)5[D-Tyr2,Thr4]OVT had a non-specific effect on lever press activity. These findings highlight that oxytocin modulation of the STh is an important modulator of relapse to METH abuse.

The role of the subthalamic nucleus in cognition

Because the complex functions of the basal ganglia have been increasingly studied over the past several decades, the understanding of the role of the subthalamic nucleus (STN) in motor and cognitive functions has evolved. The traditional role in motor function ascribed to the STN, based on its involvement in the cortico-striato-thalamo-cortical motor loops, the pathologic STN activity seen in Parkinson's disease, and the benefits in motor symptoms following STN lesions and deep brain stimulation, has been revised to include wider cognitive functions. The increased attention focused on such nonmotor functions housed within the STN partially arose from the observed cognitive and affective side effects seen with STN deep brain stimulation. The multiple modalities of research have corroborated these findings and have provided converging evidence that the STN is critically involved in cognitive processes. In particular, numerous experiments have demonstrated the involvement of the STN in high-conflict decisions. The different STN functions appear to be related to activity in anatomically distinct subregions, with the ventral STN contributing to high-conflict decision-making through its role in the hyperdirect pathway involving the prefrontal cortex.

Different populations of subthalamic neurons encode cocaine vs. sucrose reward and predict future error

The search for treatment of cocaine addiction raises the challenge to find a way to diminish motivation for the drug without decreasing it for natural rewards. Subthalamic nucleus (STN) inactivation decreases motivation for cocaine while increasing motivation for food, suggesting that STN can dissociate different rewards. Here, we investigated how rat STN neurons respond to cues predicting cocaine or sucrose and to reward delivery while rats are performing a discriminative stimuli task. We show that different neuronal populations of STN neurons encode cocaine and sucrose. In addition, we show that STN activity at the cue onset predicts future error. When changing the reward predicted unexpectedly, STN neurons show capacities of adaptation, suggesting a role in reward-prediction error. Furthermore, some STN neurons show a response to executive error (i.e., "oops neurons") that is specific to the missed reward. These results position the STN as a nexus where natural rewards and drugs of abuse are coded differentially and can influence the performance. Therefore, STN can be viewed as a structure where action could be taken for the treatment of cocaine addiction.

Cortico-subthalamic connection predicts individual differences in value-driven choice bias

It has been suggested that a connection between the STN and value-sensitive areas of the prefrontal cortex might mediate value-based actions in perceptual decision making. In this study, we first seek to quantify a structural connection between the STN and a cortical region that was associated with mechanisms underlying bias in choice behavior (vmPFC). Next, we tested whether individual differences in the probabilistic tract-strength of this connection were predictive for individual differences in the magnitude of bias in a perceptual decision-making task. Probabilistic tractography was used to measure the tract-strength between the STN and the vmPFC. Bias was quantified using an accumulation-to-bound model where a shift in the starting point of the accumulation of sensory evidence causes faster and more choices for an alternative that is more likely or more valuable. Results show that vmPFC is structurally connected with the STN and that the strength of this connection is predictive for choice bias towards an alternative that is more valuable, but not for choice bias towards an alternative that is more likely. These findings confirm the involvement of the cortico-subthalamic circuit in mechanisms underlying value-based actions in perceptual decision making.

Pathological gambling in Parkinson's disease: subthalamic oscillations during economics decisions

Pathological gambling develops in up to 8% of patients with Parkinson's disease. Although the pathophysiology of gambling remains unclear, several findings argue for a dysfunction in the basal ganglia circuits. To clarify the role of the subthalamic nucleus in pathological gambling, we studied its activity during economics decisions. We analyzed local field potentials recorded from deep brain stimulation electrodes in the subthalamic nucleus while parkinsonian patients with (n = 8) and without (n = 9) pathological gambling engaged in an economics decision-making task comprising conflictual trials (involving possible risk-taking) and non conflictual trials. In all parkinsonian patients, subthalamic low frequencies (2-12 Hz) increased during economics decisions. Whereas, in patients without gambling, low-frequency oscillations exhibited a similar pattern during conflictual and non conflictual stimuli, in those with gambling, low-frequency activity increased significantly more during conflictual than during non conflictual stimuli. The specific low-frequency oscillatory pattern recorded in patients with Parkinson's disease who gamble could reflect a subthalamic dysfunction that makes their decisional threshold highly sensitive to risky options. When parkinsonian patients process stimuli related to an economics task, low-frequency subthalamic activity increases. This task-related change suggests that the cognitive-affective system that drives economics decisional processes includes the subthalamic nucleus. The specific subthalamic neuronal activity during conflictual decisions in patients with pathological gambling supports the idea that the subthalamic nucleus is involved in behavioral strategies and in the pathophysiology of gambling.

Oxytocin modulates dopamine-mediated reward in the rat subthalamic nucleus

The subthalamic nucleus (STh) is increasingly recognized as an important region involved in the motivation for drug reward. It is not yet known if dopamine, the neurotransmitter primarily responsible for reward signaling, is also involved in mediating reward-related activity in the STh. The neuropeptide oxytocin acts within the STh to reduce the rewarding effects of the psychostimulant methamphetamine, through a proposed interaction with dopamine. However, the mechanisms of this interaction are unclear. The current study aimed to determine whether (i) dopamine microinjected into the STh would result in a significant place preference following a single-trial conditioning session, (ii) co-administered dopamine receptor antagonist would block the formation of a conditioned place preference (CPP) for dopamine, (iii) co-administered oxytocin would prevent CPP for dopamine and (iv) whether the selective oxytocin antagonist desGly-NH(2),d(CH(2))(5)[D-Tyr(2),Thr(4)]OVT, when co-administered with oxytocin and dopamine, would reverse the effects of oxytocin and result in a CPP for dopamine. Results showed that male Sprague Dawley rats i) formed a preference for the context paired with dopamine (100 nmol/side) administration into the STh, which was prevented by co-administration of ii) the mixed dopamine receptor antagonist fluphenazine (10 nmol/side) or iii) oxytocin (0.6 pmol/side), [corrected] with the oxytocin effect on dopamine CPP reversed by the co-administration of the oxytocin receptor antagonist (3 nmol/side). These data suggest that dopamine neurotransmission in the STh produces rewarding effects that can be reduced by activation of local oxytocin receptors.

Resiliency in adolescents at high risk for substance abuse: flexible adaptation via subthalamic nucleus and linkage to drinking and drug use in early adulthood

INTRODUCTION

The personality trait resiliency is the ability to flexibly adapt impulse control relative to contextual demand. Low resiliency has been linked to later alcohol/drug problems. The underlying psychological and neural mechanisms are unknown, but neurocomputational models suggested relations between resiliency and working memory. Cortical-striatal connectivity has been proposed to underlie adaptive switches between cautious and risky behaviors.

METHODS

Working memory was probed in sixty-seven 18- to 22-year-olds from a larger community study of alcoholism, using the n-back task during functional magnetic resonance imaging. Functional connectivity between task-related regions was investigated with psychophysiological interaction analysis. Resiliency was measured in early teen years and related to early adulthood measures of drinking/drug use, task activation, and connectivity. Relationships with risk factors, including family history, age of drinking onset, and number of alcohol problems, were also investigated.

RESULTS

Higher resiliency was related to lower levels of substance use, fewer alcohol problems, and better working memory performance. Whole-brain regression revealed resiliency negatively correlated with activation of subthalamic nucleus (STN) and pallidum during the n-back. High and Low resiliency quartile groups (n = 17 each) differed in coupling strength between STN and median cingulate cortex, a region of reduced activation during working memory. The high resiliency group had later onset of drinking, fewer alcohol problems, had used fewer illicit drugs, and were less likely to smoke cigarettes than their low resiliency counterparts.

CONCLUSIONS

These findings suggest that resiliency in early adolescence may protect against alcohol problems and drug use, although the direction of this effect is currently unknown. This protective factor may relate to executive functioning as supported by the finding of a neural link shared between resiliency and working memory in basal ganglia structures. The STN, a key basal ganglia structure, may adaptively link flexible impulse control with cognitive processing, potentially modulating substance use outcomes.

Neuromodulation in psychiatric disorders

Psychiatric disorders are worldwide a common cause of severe and long-term disability and socioeconomic burden. The management of patients with psychiatric disorders consists of drug therapy and/or psychotherapy. However, in some patients, these treatment modalities do not produce sufficient therapeutic effects or induce intolerable side effects. For these patients, neuromodulation has been suggested as a potential treatment modality. Neuromodulation includes deep brain stimulation, vagal nerve stimulation, and transcranial magnetic and electrical stimulation. The rationale for neuromodulation is derived from the research identifying neurobiologically localized substrates for refractory psychiatric symptoms. Here, we review the clinical data on neuromodulation in the major psychiatric disorders. Relevant data from animal models will also be discussed to explain the neurobiological basis of the therapy.

Oxytocin directly administered into the nucleus accumbens core or subthalamic nucleus attenuates methamphetamine-induced conditioned place preference

Accumulating evidence indicates that the neuropeptide oxytocin (OXY) may modulate reward-related behavioural responses to methamphetamine (METH) administration. Limited research has examined the effect of OXY on METH-induced conditioned place preference (CPP) and little is known about the neural mechanisms involved. A Fos immunohistochemistry study recently demonstrated that peripheral OXY administration reduced METH-induced Fos expression within the nucleus accumbens (NAc) core and subthalamic nucleus (STh) in rats. The current study aimed to (i) investigate the effect of systemically administered OXY on METH-induced CPP, (ii) determine the effectiveness of a single-trial CPP procedure with METH, in order to (iii) evaluate whether pretreatment with OXY injected directly into the NAc core or the STh attenuates METH-induced CPP. Results showed that male Sprague Dawley rats learned to associate unique compartmental cues with METH (1 mg/kg, i.p.) such that they spent more time in the METH-paired compartment and less time in the saline-paired compartment. Pretreatment with systemic OXY (0.6 mg, i.p.), or OXY (0.6 ng, i.c.) microinjected into the NAc core or the STh prior to METH administration attenuated the formation of a CPP to METH. This provides further evidence that OXY acts within either the NAc core or the STh to reduce the rewarding effects of METH administration.

Frontal cortex-like functions of the subthalamic nucleus

The subthalamic nucleus (STN) has been considered a motor structure for a long time. Over the last 20 years, anatomical and behavioral data have highlighted the position of the STN within a prefrontal-associative and a limbic loops, suggesting that the STN should play a critical role in frontal functions such as attention, inhibitory control (including impulsive action, compulsivity, impulsive choice), and motivation. Here we will review the work highlighting these functions of the STN.

Frontal cortex-like functions of the subthalamic nucleus

The subthalamic nucleus (STN) has been considered a motor structure for a long time. Over the last 20 years, anatomical and behavioral data have highlighted the position of the STN within a prefrontal-associative and a limbic loops, suggesting that the STN should play a critical role in frontal functions such as attention, inhibitory control (including impulsive action, compulsivity, impulsive choice), and motivation. Here we will review the work highlighting these functions of the STN.

Effect of prenatal stress on alcohol preference and sensitivity to chronic alcohol exposure in male rats

RATIONALE

In rats, prenatal restraint stress (PRS) induces persistent behavioral and neurobiological alterations leading to a greater consumption of psychostimulants during adulthood. However, little is known about alcohol vulnerability in this animal model.

OBJECTIVES

We examined in adolescent and adult male Sprague Dawley rats the long-lasting impact of PRS exposure on alcohol consumption.

METHODS

PRS rats were subjected to a prenatal stress (three daily 45-min sessions of restraint stress to the mothers during the last 10 days of pregnancy). Alcohol preference was assessed in a two-bottle choice paradigm (alcohol 2.5%, 5%, or 10% versus water), in both naïve adolescent rats and adult rats previously exposed to a chronic alcohol treatment. Behavioral indices associated with incentive motivation for alcohol were investigated. Finally, plasma levels of transaminases (marker of hepatic damages) and ΔFosB levels in the nucleus accumbens (a potential molecular switch for addiction) were evaluated following the chronic alcohol exposure.

RESULTS

Alcohol preference was not affected by PRS. Contrary to our expectations, stressed and unstressed rats did not display signs of compulsive alcohol consumption. The consequences of the alcohol exposure on locomotor reactivity and on transaminase levels were more prominent in PRS group. Similarly, PRS potentiated alcohol-induced ΔFosB levels in the nucleus accumbens.

CONCLUSION

Our data suggest that negative events occurring in utero do not modulate alcohol preference in male rats but potentiate chronic alcohol-induced molecular neuroadaptation in the brain reward circuitry. Further studies are needed to determine whether the exacerbated ΔFosB upregulation in PRS rats could be extended to other reinforcing stimuli.

The Varied Uses of Conditioned Place Preference in Behavioral Neuroscience Research: An Investigation of Alcohol Administration in Model Organisms

Place conditioning procedures have been used to study human addiction to alcohol for the past several years. This experimental resource has been utilized successfully due to the fact that investigators can carefully manipulate the experimental design in order to explore specific hypotheses. Only three choices exist regarding animal response to place conditioning: aversion, preference, or no change. This review provides an in-depth analysis of five variables commonly adjusted or changed in place conditioning experiments with ethanol. These include: apparatus design, administration methods, choice of model organism, age of model organism, and model paradigms. It is suggested that the two-chamber design, the intragastric administration, the mouse model, the adolescent age group, and the pre-exposure to stress paradigm are the best current options available in place conditioning experiments with ethanol. The basis for evaluation used throughout this review is that investigators should adjust the variables employed in place conditioning experiments in a manner that most accurately represents and models complex human addiction to alcohol.

Systemically administered oxytocin decreases methamphetamine activation of the subthalamic nucleus and accumbens core and stimulates oxytocinergic neurons in the hypothalamus

Recent preclinical evidence indicates that the neuropeptide oxytocin may have potential in the treatment of drug dependence and drug withdrawal. Oxytocin reduces methamphetamine self-administration, conditioned place preference and hyperactivity in rodents. However, it is unclear how oxytocin acts in the brain to produce such effects. The present study examined how patterns of neural activation produced by methamphetamine were modified by co-administered oxytocin. Male Sprague-Dawley rats were pretreated with either 2 mg/kg oxytocin (IP) or saline and then injected with either 2 mg/kg methamphetamine (IP) or saline. After injection, locomotor activity was measured for 80 minutes prior to perfusion. As in previous studies, co-administered oxytocin significantly reduced methamphetamine-induced behaviors. Strikingly, oxytocin significantly reduced methamphetamine-induced Fos expression in two regions of the basal ganglia: the subthalamic nucleus and the nucleus accumbens core. The subthalamic nucleus is of particular interest given emerging evidence for this structure in compulsive, addiction-relevant behaviors. When administered alone, oxytocin increased Fos expression in several regions, most notably in the oxytocin-synthesizing neurons of the supraoptic nucleus and paraventricular nucleus of the hypothalamus. This provides new evidence for central actions of peripheral oxytocin and suggests a self-stimulation effect of exogenous oxytocin on its own hypothalamic circuitry. Overall, these results give further insight into the way in which oxytocin might moderate compulsive behaviors and demonstrate the capacity of peripherally administered oxytocin to induce widespread central effects.

Reducing the desire for cocaine with subthalamic nucleus deep brain stimulation

Deep brain stimulation (DBS) is a reversible technique that is currently used for the treatment of Parkinson disease and may be suitable for the treatment of psychiatric disorders. Whether DBS inactivates the target structure is still a matter of debate. Here, from findings obtained in rats, we propose DBS of the subthalamic nucleus (STN) as a possible treatment for cocaine addiction to be further tested in human studies. We show that STN DBS reversibly reduces the motivation to work for an i.v. injection of cocaine, and it increases motivation to work for sucrose pellets. These opposite effects may result from STN DBS effect on the positive affective properties of these rewards. Indeed, we further show that STN DBS reduces the preference for a place previously associated with the rewarding properties of cocaine, and it increases the preference for a place associated with food. Because these findings are consistent with those observed after STN lesions [Baunez C, Dias C, Cador M, Amalric M (2005) Nat Neurosci 8:484-489], they suggest that STN DBS mimics an inactivation of the STN on motivational processes. Furthermore, given that one of the major challenges for cocaine addiction is to find a treatment that reduces the craving for the drug without diminishing the motivation for naturally rewarding activities, our findings validate STN as a good target and DBS as the appropriate technique for a promising therapeutic strategy in the treatment of cocaine addiction.

Beyond the Reward Pathway: Coding Reward Magnitude and Error in the Rat Subthalamic Nucleus

It was recently shown that subthalamic nucleus (STN) lesions affect motivation for food, cocaine, and alcohol, differentially, according to either the nature of the reward or the preference for it. The STN may thus code a reward according to its value. Here, we investigated how the firing of subthalamic neurons is modulated during expectation of a predicted reward between two possibilities (4 or 32% sucrose solution). The firing pattern of neurons responding to predictive cues and to reward delivery indicates that STN neurons can be divided into subpopulations responding specifically to one reward and less or giving no response to the other. In addition, some neurons (“oops” neurons) specifically encode errors as they respond only during error trials. These results reveal that the STN plays a critical role in ascertaining the value of the reward and seems to encode that value differently depending on the magnitude of the reward. These data highlight the importance of the STN in the reward circuitry of the brain.

Neurobiological substrates of cue-elicited craving and anhedonia in recently abstinent opioid-dependent males

AIM

Drug-related stimuli may induce craving in addicted patients, prompting drug-seeking behaviour. In addition, studies have shown addicted patients to be less sensitive to pleasant, but non-drug-related, stimuli; a condition generally referred to as anhedonia. The neural correlates of cue-induced craving and anhedonia in opioid-dependent patients are, however, not well understood. We studied brain activation patterns following visual presentation of neutral, pleasant and heroin-related cues.

METHODS

Detoxified opioid-dependent males (n=12) and healthy male control subjects (n=17) underwent functional magnetic resonance imaging (fMRI) while subjects viewed neutral, pleasant and heroin-related images. In addition, subjective cue-elicited craving (OCDUS and DDQ) and anhedonia (SHAPS) were measured.

RESULTS

Opioid-dependent subjects, but not control subjects, showed significant increases in activation in hippocampal region and subcortical limbic structures in response to heroin-related stimuli with a significant group x stimulus interaction effect for the subthalamic nucleus (STN). Control subjects, but not opioid-dependent subjects, showed significant increases in activation of anterior frontal areas and basal ganglia while viewing pleasant images with a significant group x stimulus interaction effect for bilateral anterior prefrontal cortex. Regression analyses showed a positive association between cue-elicited craving and ventral tegmental area (VTA) activation in response to heroin-related stimuli in heroin-dependent patients. In addition, a negative correlation was found between self-reported anhedonia and medial prefrontal regions in both groups.

CONCLUSIONS

Our findings suggest that the VTA is prominently involved in cue-induced opioid craving for heroin stimuli, in addition to mesolimbic and mesocortical pathways as identified in previous research. The present study also provides further evidence for the involvement of the STN in reward processing. Finally, our data support the presence of reduced brain activation in heroin-dependent patients in response to pleasant (non-drug-related) stimuli.