Regulation of limbic information outflow by the subthalamic nucleus: excitatory amino acid projections to the ventral pallidum

Transformation of valence signaling in a striatopallidal circuit

The ways in which sensory stimuli acquire motivational valence through association with other stimuli is one of the simplest forms of learning. Though we have identified many brain nuclei that play various roles in reward processing, a significant gap remains in understanding how valence encoding transforms through the layers of sensory processing. To address this gap, we carried out a comparative investigation of the olfactory tubercle (OT), and the ventral pallidum (VP) - 2 connected nuclei of the basal ganglia which have both been implicated in reward processing. First, using anterograde and retrograde tracing, we show that both D1 and D2 neurons of the OT project primarily to the VP and minimally elsewhere. Using 2-photon calcium imaging, we then investigated how the identity of the odor and reward contingency of the odor are differently encoded by neurons in either structure during a classical conditioning paradigm. We find that VP neurons robustly encode reward contingency, but not identity, in low-dimensional space. In contrast, OT neurons primarily encode odor identity in high-dimensional space. Though D1 OT neurons showed larger response vectors to rewarded odors than other odors, we propose this is better interpreted as identity encoding with enhanced contrast rather than as valence encoding. Finally, using a novel conditioning paradigm that decouples reward contingency and licking vigor, we show that both features are encoded by non-overlapping VP neurons. These results provide a novel framework for the striatopallidal circuit in which a high-dimensional encoding of stimulus identity is collapsed onto a low-dimensional encoding of motivational valence.

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.

Activity of a direct VTA to ventral pallidum GABA pathway encodes unconditioned reward value and sustains motivation for reward

Dopamine signaling from the ventral tegmental area (VTA) plays critical roles in reward-related behaviors, but less is known about the functions of neighboring VTA GABAergic neurons. We show here that a primary target of VTA GABA projection neurons is the ventral pallidum (VP). Activity of VTA-to-VP-projecting GABA neurons correlates consistently with size and palatability of the reward and does not change following cue learning, providing a direct measure of reward value. Chemogenetic stimulation of this GABA projection increased activity of a subset of VP neurons that were active while mice were seeking reward. Optogenetic stimulation of this pathway improved performance in a cue-reward task and maintained motivation to work for reward over days. This VTA GABA projection provides information about reward value directly to the VP, likely distinct from the prediction error signal carried by VTA dopamine neurons.

Astrocytes in the ventral pallidum extinguish heroin seeking through GAT-3 upregulation and morphological plasticity at D1-MSN terminals

GABAergic projections from the nucleus accumbens core to the dorsolateral ventral pallidum are necessary for drug-conditioned cues to initiate relapse-like drug seeking. Astrocytes in the ventral pallidum are situated perisynaptically and regulate GABA transmission through expression of GABA uptake transporters, but whether they are involved in regulating drug seeking is unknown. To determine the contribution of ventral pallidal astrocytes to heroin seeking, we labeled astrocytes in male and female rats with a membrane-bound fluorescent tag and used confocal microscopy to quantify astroglial expression of the GABA transporter GAT-3 and astrocyte synaptic proximity after withdrawal from heroin self-administration and during 15 min of cued heroin seeking. We found that GAT-3 was upregulated in rats that had extinguished heroin seeking, but not in animals that were withdrawn from heroin without extinction training or in rats that extinguished sucrose seeking. When GAT-3 upregulation was reversed using a vivo-morpholino oligo, heroin seeking was restored in the extinguished context and extinction of cued heroin seeking was disrupted compared to control animals. Although astrocyte synaptic proximity was not altered overall after heroin withdrawal, examination of astrocyte proximity to accumbens D1- or D2-expressing afferents revealed a selective increase in astrocyte proximity with D1-expressing terminals during extinction of heroin self-administration. Experimentally-induced reduction of astrocyte synaptic proximity through knockdown of the astrocyte-selective actin-binding protein ezrin also markedly disrupted extinction of heroin seeking. Notably, GAT-3 or ezrin knockdown had no impact on context- or cue-induced seeking in sucrose-trained animals. These data show that astrocytes in the ventral pallidum undergo plasticity after extinction of heroin use that reduces seeking and highlight the importance of astrocyte-neuron interactions in shaping behaviors associated with opioid use disorder.

Neural correlates of product attachment to cosmetics

The neurobiological basis of brand and product attachment has received much attention in consumer neuroscience research, although it remains unclear. In this study, we conducted functional MRI experiments involving female users of famous luxury brand cosmetics as participants, based on the regions of interest involved in human attachment and object attachment. The results showed that the left ventral pallidum (VP), which is involved in positive reward, and the right posterior cingulate cortex (PCC), which is involved in self-concept, a key concept in object attachment, are the core regions in cosmetic attachment. Moreover, the performed psychophysiological interaction analyses showed that VP-temporoparietal junction connectivity positively correlated with activity in the dorsal raphe nucleus, and PCC-anterior hippocampus (aHC) connectivity positively correlated with subjective evaluation of attachment. The former suggests that object attachment is a human-like attachment and a stronger tendency of anthropomorphism is associated with stronger feelings of security. The latter suggests that the individual's concept of attachment as well as the relationships with the attached cosmetics are represented in the aHC, and the PCC-aHC associations produce subjective awareness of the attachment relationships. These associations between memory and reward systems have been shown to play critical roles in cosmetic attachment.

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.

The role of dopamine pharmacotherapy and addiction-like behaviors in Parkinson's disease

Addictions involve a spectrum of behaviors that encompass features of impulsivity and compulsivity, herein referred to as impulsive-compulsive spectrum disorders (ICSDs). The etiology of ICSDs likely involves a complex interplay among neurobiological, psychological and social risk factors. Neurobiological risk factors include the status of the neuroanatomical circuits that govern ICSDs. These circuits can be altered by disease, as well as exogenous influences such as centrally-acting pharmacologics. The 'poster child' for this scenario is Parkinson's disease (PD) medically managed by pharmacological treatments. PD is a progressive neurodegenerative disease that involves a gradual loss of dopaminergic neurons largely within nigrostriatal projections. Replacement therapy includes dopamine receptor agonists that directly activate postsynaptic dopamine receptors (bypassing the requirement for functioning presynaptic terminals). Some clinically useful dopamine agonists, e.g., pramipexole and ropinirole, exhibit high affinity for the D2/D3 receptor subtypes. These agonists provide excellent relief from PD motor symptoms, but some patients exhibit debilitating ICSD. Teasing out the neuropsychiatric contribution of PD-associated pathology from the drugs used to treat PD motor symptoms is challenging. In this review, we posit that modern clinical and preclinical research converge on the conclusion that dopamine replacement therapy can mediate addictions in PD and other neurological disorders. We provide five categories of evidences that align with this position: (i) ICSD prevalence is greater with D2/D3 receptor agonist therapy vs PD alone. (ii) Capacity of dopamine replacement therapy to produce addiction-like behaviors is independent of disease for which the therapy is being provided. (iii) ICSD-like behaviors are recapitulated in laboratory rats with and without PD-like pathology. (iv) Behavioral pathology co-varies with drug exposure. (v) ICSD Features of ICSDs are consistent with agonist pharmacology and neuroanatomical substrates of addictions. Considering the underpinnings of ICSDs in PD should not only help therapeutic decision-making in neurological disorders, but also apprise ICSDs in general.

GABAergic and glutamatergic effects on nigrostriatal and mesolimbic dopamine release in the rat

In this review, a series of experiments is presented, in which γ-amino butyric acid (GABA)ergic and glutamatergic effects on dopamine function in the rat nigrostriatal and mesolimbic system was systematically assessed after pharmacological challenge with GABAA receptor (R) and and N-methyl d-aspartate (NMDA)R agonists and antagonists. In these studies, [123I]iodobenzamide binding to the D2/3R was mesured in nucleus accumbens (NAC), caudateputamen (CP), substantia nigra/ventral tegmental area (SN/VTA), frontal (FC), motor (MC) and parietal cortex (PC) as well as anterior (aHIPP) and posterior hippocampus (pHIPP) with small animal SPECT in baseline and after injection of either the GABAAR agonist muscimol (1 mg/kg), the GABAAR antagonist bicuculline (1 mg/kg), the NMDAR agonist d-cycloserine (20 mg/kg) or the NMDAR antagonist amantadine (40 mg/kg). Muscimol reduced D2/3R binding in NAC, CP, SN/VTA, THAL and pHIPP, while, after amantadine, decreases were confined to NAC, CP and THAL. In contrast, d-cycloserine elevated D2/3R binding in NAC, SN/VTA, THAL, frontal cortex, motor cortex, PC, aHIPP and pHIPP, while, after bicuculline, increases were confined to CP and THAL. Taken together, similar actions on regional dopamine levels were exterted by the GABAAR agonist and the NMDAR antagonist on the one side and by the GABAAR antagonist and the NMDAR agonist on the other, with agonistic action, however, affecting more brain regions. Thereby, network analysis suggests different roles of GABAARs and NMDARs in the mediation of nigrostriatal, nigrothalamocortical and mesolimbocortical dopamine function.

Ventral pallidum encodes relative reward value earlier and more robustly than nucleus accumbens

The ventral striatopallidal system, a basal ganglia network thought to convert limbic information into behavioral action, includes the nucleus accumbens (NAc) and the ventral pallidum (VP), typically described as a major output of NAc. Here, to investigate how reward-related information is transformed across this circuit, we measure the activity of neurons in NAc and VP when rats receive two highly palatable but differentially preferred rewards, allowing us to track the reward-specific information contained within the neural activity of each region. In VP, we find a prominent preference-related signal that flexibly reports the relative value of reward outcomes across multiple conditions. This reward-specific firing in VP is present in a greater proportion of the population and arises sooner following reward delivery than in NAc. Our findings establish VP as a preeminent value signaler and challenge the existing model of information flow in the ventral basal ganglia.

The role of the human globus pallidus in Huntington's disease

Huntington's disease (HD) is characterized by pronounced pathology of the basal ganglia, with numerous studies documenting the pattern of striatal neurodegeneration in the human brain. However, a principle target of striatal outflow, the globus pallidus (GP), has received limited attention in comparison, despite being a core component of the basal ganglia. The external segment (GPe) is a major output of the dorsal striatum, connecting widely to other basal ganglia nuclei via the indirect motor pathway. The internal segment (GPi) is a final output station of both the direct and indirect motor pathways of the basal ganglia. The ventral pallidum (VP), in contrast, is a primary output of the limbic ventral striatum. Currently, there is a lack of consensus in the literature regarding the extent of GPe and GPi neurodegeneration in HD, with a conflict between pallidal neurons being preserved, and pallidal neurons being lost. In addition, no current evidence considers the fate of the VP in HD, despite it being a key structure involved in reward and motivation. Understanding the involvement of these structures in HD will help to determine their involvement in basal ganglia pathway dysfunction in the disease. A clear understanding of the impact of striatal projection loss on the main neurons that receive striatal input, the pallidal neurons, will aid in the understanding of disease pathogenesis. In addition, a clearer picture of pallidal involvement in HD may contribute to providing a morphological basis to the considerable variability in the types of motor, behavioral, and cognitive symptoms in HD. This review aims to highlight the importance of the globus pallidus, a critical component of the cortical-basal ganglia circuits, and its role in the pathogenesis of HD. This review also summarizes the current literature relating to human studies of the globus pallidus in HD.

High-frequency stimulation of the subthalamic nucleus modulates neuronal activity in the lateral habenula nucleus

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is often used to treat movement disability in advanced Parkinson's disease, but some patients experience debilitating psychiatric effects including depression. Interestingly, HFS of the STN modulates 5-HT neurons in the dorsal raphe nucleus (DRN) which are linked to depression, but the neural substrate of this effect is unknown. Here, we tested the effect of STN stimulation on neuronal activity in the lateral habenula nucleus (LHb), an important source of input to DRN 5-HT neurons and also a key controller of emotive behaviours. LHb neurons were monitored in anaesthetized rats using single-unit extracellular recording, and localization within the LHb was confirmed by juxtacellular labelling. HFS of the STN (130 Hz) evoked rapid changes in the firing rate of the majority of LHb neurons tested (38 of 68). Some LHb neurons (19/68) were activated by HFS, while others (19/68), distinguished by a higher basal firing rate, were inhibited. LHb neurons that project to the DRN were identified using antidromic activation and collision testing (n = 17 neurons). Some of these neurons (5/17) were also excited by HFS of the STN, and others (7/17) were inhibited although this was only a statistical trend. In summary, HFS of the STN modulated the firing of LHb neurons, including those projecting to the DRN. The data identify that the STN impacts on the LHb-DRN pathway. Moreover, this pathway may be part of the circuitry mediating the psychiatric effects of STN stimulation experienced by patients with Parkinson's disease.

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.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Distinct populations of neurons respond to emotional valence and arousal in the human subthalamic nucleus

Both animal studies and studies using deep brain stimulation in humans have demonstrated the involvement of the subthalamic nucleus (STN) in motivational and emotional processes; however, participation of this nucleus in processing human emotion has not been investigated directly at the single-neuron level. We analyzed the relationship between the neuronal firing from intraoperative microrecordings from the STN during affective picture presentation in patients with Parkinson's disease (PD) and the affective ratings of emotional valence and arousal performed subsequently. We observed that 17% of neurons responded to emotional valence and arousal of visual stimuli according to individual ratings. The activity of some neurons was related to emotional valence, whereas different neurons responded to arousal. In addition, 14% of neurons responded to visual stimuli. Our results suggest the existence of neurons involved in processing or transmission of visual and emotional information in the human STN, and provide evidence of separate processing of the affective dimensions of valence and arousal at the level of single neurons as well.

Cellular distribution of AMPA receptor subunits and mGlu5 following acute and repeated administration of morphine or methamphetamine

Ionotropic AMPA receptors (AMPAR) and metabotropic glutamate group I subtype 5 receptors (mGlu5) mediate neuronal and behavioral effects of abused drugs. mGlu5 stimulation increases expression of striatal-enriched tyrosine phosphatase isoform 61 (STEP61 ) which internalizes AMPARs. We determined the rat brain profile of these proteins using two different classes of abused drugs, opiates, and stimulants. STEP61 levels, and cellular distribution/expression of AMPAR subunits (GluA1, GluA2) and mGlu5, were evaluated via a protein cross-linking assay in medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and ventral pallidum (VP) harvested 1 day after acute, or fourteen days after repeated morphine (8 mg/kg) or methamphetamine (1 mg/kg) (treatments producing behavioral sensitization). Acute morphine decreased GluA1 and GluA2 surface expression in mPFC and GluA1 in NAc. Fourteen days after repeated morphine or methamphetamine, mGlu5 surface expression increased in VP. In mPFC, mGlu5 were unaltered; however, after methamphetamine, STEP61 levels decreased and GluA2 surface expression increased. Pre-treatment with a mGlu5-selective negative allosteric modulator, blocked methamphetamine-induced behavioral sensitization and changes in mPFC GluA2 and STEP61 . These data reveal (i) region-specific distinctions in glutamate receptor trafficking between acute and repeated treatments of morphine and methamphetamine, and (ii) that mGlu5 is necessary for methamphetamine-induced alterations in mPFC GluA2 and STEP61 .

Axonal branching patterns of ventral pallidal neurons in the rat

The ventral pallidum (VP) is a key component of the cortico-basal ganglia circuits that process motivational and emotional information, and also a crucial site for reward. Although the main targets of the two VP compartments, medial (VPm) and lateral (VPl) have already been established, the collateralization patterns of individual axons have not previously been investigated. Here we have fully traced eighty-four axons from VPm, VPl and the rostral extension of VP into the olfactory tubercle (VPr), using the anterograde tracer biotinylated dextran amine in the rat. Thirty to fifty percent of axons originating from VPm and VPr collateralized in the mediodorsal thalamic nucleus and lateral habenula, indicating a close association between the ventral basal ganglia-thalamo-cortical loop and the reward network at the single axon level. Additional collateralization of these axons in diverse components of the extended amygdala and corticopetal system supports a multisystem integration that may take place at the basal forebrain. Remarkably, we did not find evidence for a sharp segregation in the targets of axons arising from the two VP compartments, as VPl axons frequently collateralized in the caudal lateral hypothalamus and ventral tegmental area, the well-known targets of VPm, while VPm axons, in turn, also collateralized in typical VPl targets such as the subthalamic nucleus, substantia nigra pars compacta and reticulata, and retrorubral field. Nevertheless, VPl and VPm displayed collateralization patterns that paralleled those of dorsal pallidal components, confirming at the single axon level the parallel organization of functionally different basal ganglia loops.

Weight gain is associated with medial contact site of subthalamic stimulation in Parkinson's disease

The aim of our study was to assess changes in body-weight in relation to active electrode contact position in the subthalamic nucleus. Regular body weight measurements were done in 20 patients with advanced Parkinson's disease within a period of 18 months after implantation. T1-weighted (1.5T) magnetic resonance images were used to determine electrode position in the subthalamic nucleus and the Unified Parkinson's disease rating scale (UPDRS-III) was used for motor assessment. The distance of the contacts from the wall of the third ventricle in the mediolateral direction inversely correlated with weight gain (r = −0.55, p<0.01) and with neurostimulation-related motor condition expressed as the contralateral hemi-body UPDRS-III (r = −0.42, p<0.01). Patients with at least one contact within 9.3 mm of the wall experienced significantly greater weight gain (9.4±(SD)4.4 kg, N = 11) than those with both contacts located laterally (3.9±2.7 kg, N = 9) (p<0.001). The position of the active contact is critical not only for motor outcome but is also associated with weight gain, suggesting a regional effect of subthalamic stimulation on adjacent structures involved in the central regulation of energy balance, food intake or reward.

Emerging, reemerging, and forgotten brain areas of the reward circuit: Notes from the 2010 Motivational Neural Networks conference

On April 24-27, 2010, the Motivational Neuronal Networks meeting took place in Wrightsville Beach, North Carolina. The conference was devoted to "Emerging, re-emerging, and forgotten brain areas" of the reward circuit. A central feature of the conference was four scholarly discussions of cutting-edge topics related to the conference's theme. These discussions form the basis of the present review, which summarizes areas of consensus and controversy, and serves as a roadmap for the next several years of research.

Modulation of subthalamic alpha activity to emotional stimuli correlates with depressive symptoms in Parkinson's disease

BACKGROUND

Deep brain stimulation of the subthalamic nucleus is an effective treatment for patients with advanced Parkinson's disease. However, affective side effects following subthalamic deep brain stimulation have been reported. Here, we aim to elucidate the influence of affective state on emotional processing as indexed by local field potential activity and to identify neurophysiological markers in patients at risk of developing depressive symptoms during subthalamic deep brain stimulation.

METHODS

Subthalamic local field potentials were directly recorded via electrodes implanted for deep brain stimulation in 12 Parkinson's disease patients while viewing emotionally salient and neutral pictures. Parkinson's disease patients were assessed for depressive symptoms using the Beck depression inventory at the time of operation and 3 months after continuous subthalamic nucleus deep brain stimulation.

RESULTS

We found a significant event-related desynchronization in the local alpha frequency band (8-12 Hz) for emotionally arousing but not neutral pictures. The the event-related desynchronization (ERD) in the alpha frequency band was reduced for pleasant stimuli in patients with mild to moderate depressive symptoms compared with patients without depression. The alpha-ERD to unpleasant stimuli showed the opposite pattern. Consistently, the index of event-related alpha desynchronization (alpha ERD for pleasant stimuli minus alpha ERD for unpleasant stimuli) correlated with the Beck depression inventory at the time of the recordings and at 3 months after continuous deep brain stimulation. The alpha ERD to unpleasant pictures correlated significantly with the Beck depression inventory score at 3 months after chronic deep brain stimulation.

DISCUSSION

In conclusion, we found mood-congruent stimulus processing in the subthalamic nucleus of Parkinson's disease patients. Electrophysiological markers such as event-related desynchronization of subthalamic alpha activity reflect state-dependent emotional processing and may potentially be used to predict depressive mood disturbances in Parkinson's disease patients with chronic subthalamic deep brain stimulation at an early stage.

Striatal structure and function in mood disorders: a comprehensive review

OBJECTIVES

  A large and diverse literature has implicated abnormalities of striatal structure and function in both unipolar and bipolar disorder. Recent functional imaging studies have greatly expanded this body of research. The aim of this review is to provide a comprehensive and critical appraisal of the relevant literature.

METHODS

  A total of 331 relevant articles were reviewed to develop an integrated overview of striatal function in mood disorders.

RESULTS

  There is compelling evidence from multiple studies that functional abnormalities of the striatum and greater corticostriatal circuitry exist in at least some forms of affective illness. The literature does not yet provide data to determine whether these aberrations represent primary pathology or they contribute directly to symptom expression. Finally, there is considerable evidence that bipolar disorder may be associated with striatal hyperactivity and some suggestion that unipolar illness may be associated with hypoactivation.

CONCLUSIONS

  Additional research investigating striatal function in affective disorders will be critical to the development of comprehensive models of the neurobiology of these conditions.

Cortico-basal ganglia circuitry: a review of key research and implications for functional connectivity studies of mood and anxiety disorders

There is considerable evidence that dysfunction of the cortico-basal ganglia circuits may be associated with several mood and anxiety disorders. However, it is unclear whether circuit abnormalities contribute directly either to the neurobiology of these conditions or to the manifestation of symptoms. Understanding the role of these pathways in psychiatric illness has been limited by an incomplete characterization of normal function. In recent years, studies using animal models and human functional imaging have greatly expanded the literature describing normal cortico-basal ganglia circuit function. In this paper, recent key studies of circuit function using human and animal models are reviewed and integrated with findings from other studies conducted over the previous decades. The literature suggests several hypotheses of cortico-basal ganglia circuitry function in mood and anxiety disorders that warrant further exploration. Hypotheses are proposed herein based upon the cortico-basal ganglia mechanisms of: (1) feedforward and feedback control, (2) circuit integration and (3) emotional control. These are presented as models of circuit function, which may be particularly relevant to future investigations using neuroimaging and functional connectivity analyses.

The reward circuit: linking primate anatomy and human imaging

Although cells in many brain regions respond to reward, the cortical-basal ganglia circuit is at the heart of the reward system. The key structures in this network are the anterior cingulate cortex, the orbital prefrontal cortex, the ventral striatum, the ventral pallidum, and the midbrain dopamine neurons. In addition, other structures, including the dorsal prefrontal cortex, amygdala, hippocampus, thalamus, and lateral habenular nucleus, and specific brainstem structures such as the pedunculopontine nucleus, and the raphe nucleus, are key components in regulating the reward circuit. Connectivity between these areas forms a complex neural network that mediates different aspects of reward processing. Advances in neuroimaging techniques allow better spatial and temporal resolution. These studies now demonstrate that human functional and structural imaging results map increasingly close to primate anatomy.

Ventral pallidum roles in reward and motivation

In recent years the ventral pallidum has become a focus of great research interest as a mechanism of reward and incentive motivation. As a major output for limbic signals, the ventral pallidum was once associated primarily with motor functions rather than regarded as a reward structure in its own right. However, ample evidence now suggests that ventral pallidum function is a major mechanism of reward in the brain. We review data indicating that (1) an intact ventral pallidum is necessary for normal reward and motivation, (2) stimulated activation of ventral pallidum is sufficient to cause reward and motivation enhancements, and (3) activation patterns in ventral pallidum neurons specifically encode reward and motivation signals via phasic bursts of excitation to incentive and hedonic stimuli. We conclude that the ventral pallidum may serve as an important 'limbic final common pathway' for mesocorticolimbic processing of many rewards.

Fos expression following activation of the ventral pallidum in normal rats and in a model of Parkinson's Disease: implications for limbic system and basal ganglia interactions

The circuit-related consequences of activating the ventral pallidum (VP) are not well known, and lacking in particular is how these effects are altered in various neuropathological states. To help to address these paucities, this study investigated the brain regions affected by VP activation by quantifying neurons that stain for Fos-like immunoreactivity (ir). Fos-ir was assessed after intra-pallidal injections of the excitatory amino acid agonist, NMDA, or the GABA(A) antagonist, bicuculline in normal rats and in those rendered Parkinsonian-like by lesioning dopaminergic neurons with the neurotoxin, 6-OHDA. We hypothesized that activation of the VP will alter the activity state of brain regions associated with both the basal ganglia and limbic system, and that this influence would be modified in the Parkinsonian state. Blocking tonically activated GABA(A) receptors with bicuculline (50 ng/0.5 microl) elevated Fos-ir in the VP to 423% above the contralateral, vehicle-injected side. Likewise, intra-VP NMDA (0.23 microg or 0.45 microg/0.5 microl), dose-dependently increased the number of pallidal neurons expressing Fos-ir by 224 and 526%, respectively. At higher NMDA doses, the density of Fos-ir neurons was not elevated above control levels. This inverted U-shaped profile was mirrored by a VP output structure, the medial subthalamic nucleus (mSTN). The mSTN showed a 289% increase in Fos-ir neurons with intra-VP injections of 0.45 microg NMDA, and this response was halved following intra-VP injections of 0.9 microg NMDA. Of the 12 other brain regions measured, three showed VP NMDA-induced enhancements in Fos-ir: the frontal cortex, entopeduncular nucleus and substantia nigra pars reticulata, all regions associated with the basal ganglia. In a second study, we evaluated the NMDA activation profile in a rat model of Parkinson's Disease (PD) which was created by a unilateral injection of 6-OHDA into the rostral substantia nigra pars compacta. Comparisons of responses to intra-VP NMDA between the hemispheres ipsilateral and contralateral to the lesion revealed that Fos-ir cells in the pedunculopontine nucleus was reduced by 62%, whereas Fos-ir for the basolateral amygdala and STN was reduced by 32 and 42%, respectively. These findings support the concept that the VP can influence both the basal ganglia and the limbic system, and that that the nature of this influence is modified in an animal model of PD. As the VP regulates motivation and cognition, adaptations in this system may contribute to the mood and mnemonic disorders that can accompany PD.

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

In addition to its role in motor and attentional processes, the subthalamic nucleus (STN) has also been recently demonstrated to be involved in motivational function. Indeed, bilateral STN lesions modulate differentially the motivation for natural rewards and drugs of abuse, increasing motivation for food and decreasing motivation for cocaine in rats. Here, we show that in outbred rats, the STN can modulate the motivation for alcohol according to alcohol preference, without affecting alcohol intake. When performed on 'High-Drinker' rats, STN lesions enhanced the breaking point (BP) under a progressive ratio schedule of reinforcement and increased the time spent in the environment previously paired with alcohol access in the place preference paradigm. In contrast, when performed on 'Low-Drinker' rats, STN lesions decreased the BP and increased the time spent in the environment paired with water. These results show that STN lesions enhance the motivation for alcohol in rats showing a high alcohol preference, whereas they decrease it in rats showing a low preference for alcohol. These results suggest that the STN plays a complex role in the reward circuit, that is not limited to a

Neurokinin-1 receptors in cholinergic neurons of the rat ventral pallidum have a predominantly dendritic distribution that is affected by apomorphine when combined with startle-evoking auditory stimulation

Cholinergic neurons of the basal forebrain are implicated in startle reflex inhibition by a prior weak stimulus often referred to as prepulse inhibition (PPI) and used as an index of sensorimotor gating deficits in schizophrenia. Gating deficits can be produced in rodent models by acute systemic administration of apomorphine, a non-selective dopamine D1 and D2 receptor agonist that also affects trafficking of neurokinin-1 (NK(1)) receptors induced by startle evoking auditory stimulation (AS) in midbrain neurons. We used electron microscopic immunolabeling of NK(1) receptors and the vesicular acetylcholine transporter (VAchT) to test the hypothesis that the subcellular distributions of these receptors in cholinergic neurons of the rat ventral pallidum are subject to a similar regulation. In vehicle controls, NK(1) immunogold was often seen near cytoplasmic endomembranes in somata and large dendrites, but was more equally distributed in cytoplasmic and plasmalemmal compartments of medium dendrites, and principally located on the plasma membrane of small dendrites. These labeling patterns appeared to be largely independent of whether the NK(1) receptor was co-expressed with VAchT, however only the medium and small VAchT-labeled dendrites showed significant treatment-specific differences in NK(1) immunogold distributions. The NK(1) receptor immunogold particle density on the plasma membrane of medium cholinergic dendrites was significantly enhanced by combined apomorphine and AS, while neither alone affected either the plasmalemmal density or the equality of the plasmalemmal and cytoplasmic distributions of NK(1) receptors in these dendrites. Small cholinergic dendrites showed a significant AS-induced increase in both the plasmalemmal and cytoplasmic density of NK(1) gold particles, and an apomorphine-induced disruption of the preferential plasmalemmal targeting of the NK(1) receptors. These results provide ultrastructural evidence that NK(1) receptors in cholinergic neurons of the ventral pallidum have subcellular locations and plasticity conducive to active involvement in dopamine-dependent sensorimotor processing.

Inhibition of 5-HT neuron activity and induction of depressive-like behavior by high-frequency stimulation of the subthalamic nucleus

Bilateral, high-frequency stimulation (HFS) of the subthalamic nucleus (STN) is the surgical therapy of choice for movement disability in advanced Parkinson's disease (PD), but this procedure evokes debilitating psychiatric effects, including depressed mood, of unknown neural origin. Here, we report the unexpected finding that HFS of the STN inhibits midbrain 5-hydroxytryptamine (5-HT) neurons to evoke depression-related behavioral changes. We found that bilateral HFS of the STN consistently inhibited (40-50%) the firing rate of 5-HT neurons in the dorsal raphe nucleus of the rat, but not neighboring non-5-HT neurons. This effect was apparent at clinically relevant stimulation parameters (> or =100 Hz, > or =30 microA), was not elicited by HFS of either neighboring or remote structures to the STN, and was still present in rat models of PD. We also found that bilateral HFS of the STN evoked clear-cut, depressive-like behavior in a widely used experimental paradigm of depression (forced swim test), and this effect was also observed in a PD model. Importantly, the depressive-like behavior elicited by HFS of the STN was reversed by a selective 5-HT-enhancing antidepressant, thereby linking the behavioral change to decreased 5-HT neuronal activity. Overall, these findings link reduced 5-HT function to the psychiatric effects of HFS of the STN observed in PD patients and provide a rational basis for their clinical management. More generally, the powerful interaction between the STN and 5-HT system uncovered here offers insights into the high level of comorbidity of basal ganglia disease and mood disorder.

Nullifying drug-induced sensitization: behavioral and electrophysiological evaluations of dopaminergic and serotonergic ligands in methamphetamine-sensitized rats

Repeated exposure to methamphetamine produces a persistent enhancement of the acute motor effects of the drug, commonly referred to as behavioral sensitization. Behavioral sensitization involves monoaminergic projections to several forebrain nuclei. We recently revealed that the ventral pallidum (VP) may also be involved. In this study, we sought to establish if treatments with antagonists or partial agonists to monoaminergic receptors could "reverse" methamphetamine-induced behavioral and VP neuronal sensitization. Behavioral sensitization was obtained in rats with five once-daily s.c. injections of 2.5mg/kg methamphetamine, an effect that persisted for at least 60 days. After the development of sensitization, 15 once-daily treatments of mirtazapine (a 5-HT(2/3), alpha(2) and H(1) antagonist), SKF38393 (D(1) partial agonist) or SCH23390 (dopamine D(1) antagonist) nullified indices of motor sensitization as assessed by measuring the motoric response to an acute methamphetamine challenge 30 days after the fifth repeated methamphetamine treatment. VP neurons recorded in vivo from methamphetamine-sensitized rats at the 30-day withdrawal time also showed a robust downward shift in the excitatory responses observed to an acute i.v. methamphetamine challenge in non-sensitized rats. This decreased excitatory effect was reversed by mirtazapine, but not by other antagonists that were tested. These data suggest a potential therapeutic benefit for mirtazapine in the treatment of methamphetamine addiction, and point to a possible role for the VP in the sensitization process to methamphetamine.

Changes in accumbal and pallidal pCREB and deltaFosB in morphine-sensitized rats: correlations with receptor-evoked electrophysiological measures in the ventral pallidum

Activation of mu-opioid receptors in the ventral pallidum (VP) is important for the induction of behavioral sensitization to morphine in rats. The present study was designed to ascertain if neurons within the VP demonstrate sensitization at a time when morphine-induced behavioral sensitization occurred (ie 3 or 14 days after five once-daily injections of 10 mg/kg i.p. morphine) in rats. Western blotting was used to evaluate transcription factors altered by opiates, CREB and deltaFosB. CREB levels did not change in the VP, but there was a significant decrease in levels of its active, phosphorylated form (pCREB) at both 3- and 14-days withdrawal. DeltaFosB levels were elevated following a 3-day withdrawal, but returned to normal by 14 days. This profile also was obtained from nucleus accumbens tissue. In a separate group of similarly treated rats, in vivo electrophysiological recordings of VP neuronal responses to microiontophoretically applied ligands were carried out after 14-days withdrawal. The firing rate effects of local applications of morphine were diminished in rats withdrawn from i.p. morphine. Repeated i.p. morphine did not alter GABA-mediated suppression of firing, or the rate enhancing effects of the D1 dopamine receptor agonist SKF82958 or glutamate. However, VP neurons from rats withdrawn from repeated i.p. morphine showed a higher propensity to enter a state of depolarization inactivation to locally applied glutamate. Overall, these findings reveal that decreased pCREB in brain regions such as the VP accompanies persistent behavioral sensitization to morphine and that this biochemical alteration may influence the excitability of neurons in this brain region.

Expectation enhances autonomic responses to stimulation of the human subthalamic limbic region

Recent studies show that the placebo component of a treatment can be investigated by administering therapies either overtly or covertly, without the administration of any placebo. Here, we analyze the effects of open (i.e., expected) versus hidden (i.e., unexpected) stimulations of the human subthalamic region on autonomic responses in Parkinson patients. To do this, we mapped the whole subthalamic region, from the dorsal to the ventral part, and recorded both heart rate and sympathetic responses by using spectral analysis of heart rate variability. We found that open stimulations were more effective than hidden ones only in the ventral subthalamic region, whereas no difference between the two conditions was found in the dorsal aspect. By analyzing the stimulus-response curves in the dorsal, middle, and ventral subthalamic regions, we found that the autonomic response threshold was higher in the hidden than open condition for both heart rate and sympathetic responses only in the ventral part. As this ventralmost portion of the subthalamic region is involved in associative-limbic functions, these data suggest that expectation enhances autonomic responses only if these are elicited in the limbic system. These results extend previous findings on the open-hidden paradigm in deep brain stimulation [Benedetti, F., Colloca, L., Lanotte, M., Bergamasco, B., Torre, E., Lopiano, L., 2004a. Autonomic and emotional responses to open and hidden stimulations of the human subthalamic region. Brain Res. Bull. 63, 203-211.], and indicate that expectation plays a major role in the therapeutic outcome. In light of the interactions between the sympathetic adrenergic system and the immune system, the open-hidden difference in autonomic responses might be relevant to the understanding of how expectations might affect the immune system.

Ventral pallidal neurons code incentive motivation: amplification by mesolimbic sensitization and amphetamine

Neurons in ventral pallidum fire to reward and its predictive cues. We tested mesolimbic activation effects on neural reward coding. Rats learned that a Pavlovian conditioned stimulus (CS+1 tone) predicted a second conditioned stimulus (CS+2 feeder click) followed by an unconditioned stimulus (UCS sucrose reward). Some rats were sensitized to amphetamine after training. Electrophysiological activity of ventral pallidal neurons to stimuli was later recorded under the influence of vehicle or acute amphetamine injection. Both sensitization and acute amphetamine increased ventral pallidum firing at CS+2 (population code and rate code). There were no changes at CS+1 and minimal changes to UCS. With a new 'Profile Analysis', we show that mesolimbic activation by sensitization/amphetamine incrementally shifted neuronal firing profiles away from prediction signal coding (maximal at CS+1) and toward incentive coding (maximal at CS+2), without changing hedonic impact coding (maximal at UCS). This pattern suggests mesolimbic activation specifically amplifies a motivational transform of CS+ predictive information into incentive salience coded by ventral pallidal neurons. Our results support incentive-sensitization predictions and suggest why cues temporally proximal to drug presentation may precipitate cue-triggered relapse in human addicts.

The functional role of the subthalamic nucleus in cognitive and limbic circuits

Once it was believed that the subthalamic nucleus (STN) was no more than a relay station serving as a "gate" for ascending basal ganglia-thalamocortical circuits. Nowadays, the STN is considered to be one of the main regulators of motor function related to the basal ganglia. The role of the STN in the regulation of associative and limbic functions related to the basal ganglia has generally received little attention. In the present review, the functional role of the STN in the control of cortico-basal ganglia-thalamocortical associative and limbic circuits is discussed. In the past 20 years the concepts about the functional role of the STN have changed dramatically: from being an inhibitory nucleus to a potent excitatory nucleus, and from being involved in hyperkinesias to hypokinesias. However, it has been demonstrated only recently, mainly by reports on the behavioral (side-) effects of STN deep brain stimulation (DBS), which is a popular surgical technique in the treatment of patients suffering from advanced Parkinson Disease (PD), that the STN is clinically involved in associative and limbic functions. These findings were confirmed by results from animal studies. Experimental studies applying STN DBS or STN lesions to investigate the neuronal mechanisms involved in these procedures found profound effects on cognitive and motivational parameters. The anatomical, electrophysiological and behavioral data presented in this review point towards a potent regulatory function of the STN in the processing of associative and limbic information towards cortical and subcortical regions. In conclusion, it can be stated that the STN has anatomically a central position within the basal ganglia thalamocortical associative and limbic circuits and is functionally a potent regulator of these pathways.

The subthalamic nucleus exerts opposite control on cocaine and 'natural' rewards

A challenge in treating drug addicts is preventing their pathological motivation for the drug without impairing their general affective state toward natural reinforcers. Here we have shown that discrete lesions of the subthalamic nucleus greatly decreased the motivation of rats for cocaine while increasing it for food reward. The subthalamic nucleus, a key structure controlling basal ganglia outputs, is therefore able to oppositely modulate the effect of 'natural' rewards and drugs of abuse on behavior. Modulating the activity of the subthalamic nucleus might prove to be a new target for the treatment of cocaine addiction.

Cross-sensitization to morphine in cocaine-sensitized rats: behavioral assessments correlate with enhanced responding of ventral pallidal neurons to morphine and glutamate, with diminished effects of GABA

Common neurobiological substrates contribute to the progressively increased behavioral effects (i.e., sensitization) that occur with repeated intermittent treatments of cocaine and morphine. Consequently, repeated exposure to cocaine can augment responding to morphine (termed cross-sensitization). Drug-induced sensitization in rats may model aspects of the dysfunction in motivation that are imposed by addiction. The ventral pallidum (VP) is involved in motivated behaviors and its function is altered by acute administration of cocaine and morphine, but the effects of repeated drug exposure remain unknown. Targeting this paucity, the present study evaluated electrophysiological changes in the VP of rats exposed to five once-daily cocaine treatments (15 mg/kg i.p.). This regimen also induced behavioral-sensitization that was expressed 3 days later when the rats received either an acute injection of cocaine (15 mg/kg i.p.) or morphine (10 mg/kg i.p.). VP neurons recorded in vivo 3 days after the repeated cocaine treatment regimen demonstrated increased excitatory responding to microiontophoretic applications of morphine and glutamate. The maximal effect (E(max)) was increased without altering potency, suggesting a change in the functional efficacy of the respective receptor systems. This did not represent a potentiation in transmission in general, for the effects of GABA were diminished. The results provide the first evidence for cellular adaptation in the VP after a sensitizing drug treatment paradigm and reveal that cross-sensitization of drug-induced behaviors temporally correlates with changes in VP neuronal responding. These findings advance an emerging theme that alterations in the VP may contribute to the increased motivation for drug seeking that occurs in drug-withdrawn addicts.

Limbic pallidal adaptations following long-term cessation of dopaminergic transmission: lack of upregulation of dopamine receptor function

Neurons in the ventral pallidum (VP) exhibit robust responding to activation of dopamine (DA) receptors of the D1 class. To determine if the VP adapts to chronic cessation of DA transmission, the present studies examined D1 receptor-mediated responses in the VP recorded extracellularly in chloral-hydrate anesthetized rats following destruction of DA neurons with 6-hydroxydopamine (6-OHDA) or long-term treatment with the D1 antagonist SCH23390. Indices of basal spiking (i.e., spontaneous firing rate and pattern) recorded 10-21 days after unilateral 6-OHDA treatment did not differ from controls. Moreover, DA depletion did not alter the proportion of VP neurons whose rate was enhanced with i.v. injections of the D1 agonist SKF38393, and the functional efficacy (Emax) and potency (ED50) were similar to controls. There also was no change in the direction of responses, the Emax or the ED50 measure of sensitivity (ECur50) to iontophoretic application of DA or SKF38393 in VP neurons. Forty-eight hours after 21 once-daily treatments with SCH23390, the number of [3H]SCH23390-labeled D1 receptors was increased in the striatum, but unchanged in the VP, globus pallidus, or septum. Accordingly, there was no functional upregulation of VP responses to i.v. SKF38393. Indeed, the proportion of SKF38393-sensitive neurons was decreased after chronic SCH23390. Distinguishing the VP from other forebrain regions, these findings indicate that basal spiking is not altered in the VP following chronic DA depletion, and that no upregulation of VP DA receptor function occurs following either dopaminergic lesions or chronic antagonism of D1 receptors.

Autonomic and emotional responses to open and hidden stimulations of the human subthalamic region

We performed a microstimulation study of the subthalamic region of Parkinsonian patients who underwent bilateral electrode implantation in the subthalamic nuclei and whose heart rate and heart rate variability were recorded. The stimulation of the dorsalmost region, which includes the zona incerta and the dorsal pole of the subthalamic nucleus, produced autonomic responses that were constant over time. In fact, hidden stimulations (the patient is not aware of being stimulated) and open stimulations (the patient is aware of being stimulated) always induced the same responses. By contrast, the stimulation of the ventralmost region, which includes the ventral pole of the subthalamic nucleus and the substantia nigra pars reticulata, produced autonomic and emotional responses that were inconstant over time and varied according to the condition. In fact, different responses were elicited with hidden and open stimulations. These data suggest that the dorsal subthalamic nucleus and/or the zona incerta are involved in autonomic control, whereas the ventral subthalamic nucleus and/or the substantia nigra reticulata are involved in associative/limbic-related autonomic activity. The difference between the open and hidden stimulations in the ventral subthalamic region can explain previous studies in which open and hidden stimulations produced different therapeutic outcomes.

Short-term, D2 receptor blockade induces synaptic degeneration, reduces levels of tyrosine hydroxylase and brain-derived neurotrophic factor, and enhances D2-mediated firing in the ventral pallidum

Repeated treatments with neuroleptics are associated with biochemical and morphological alterations in forebrain neurons as well as an upregulation of D2-mediated changes in neuronal function. The present study evaluated the histological and physiological effects of three once-daily treatments with two chemically divergent neuroleptics, haloperidol (1 mg/kg i.p./day) and eticlopride (3 mg/kg i.p./day), measured in rats 24 h after the last injection. It was determined that this short-term antagonism of D2-like receptors induced fiber and terminal degeneration and significantly decreased tyrosine hydroxylase (TH) and brain-derived neurotrophic factor (BDNF) immunoreactivity in the ventral pallidum (VP), as determined by optical density measurements. While other forebrain regions demonstrated changes in TH and BDNF, the neurodegeneration profile was unique to the VP. This was accompanied by an enhancement in the efficacy of the D2 agonist quinpirole to increase spiking rate of VP neurons recorded in chloral hydrate-anesthetized rats. These data indicate that short-term treatments with D2 antagonists are sufficient to induce changes in the biochemical and morphological profiles uniquely within the VP. Moreover, the functional ramifications of these changes appear to include profound alterations in the way dopamine regulates neuronal activity in this region.

Alterations in responses of ventral pallidal neurons to excitatory amino acids after long-term dopamine depletion

The present study explored the possibility that excitatory amino acid (EAA) sensitivity within the ventral pallidum (VP) is altered by long-term removal of dopamine (DA). Electrophysiological experiments were conducted in chloral hydrate-anesthetized rats 21 to 28 days after they received unilateral substantia nigra injections of the dopaminergic toxin 6-hydroxydopamine (6-OHDA). VP neurons increased firing at low microiontophoretic ejection currents of the EAA agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA); however, high currents decreased action potential amplitude and rapidly caused cessation of neuronal firing. These responses likely reflected the induction of depolarization block for they were reversed by coiontophoresis of the hyperpolarizing transmitter gamma-aminobutyric acid (GABA) at ejection current levels that normally suppressed firing. The ability of NMDA and AMPA to induce such inactivation was greater in the VP of 6-OHDA-lesioned hemispheres, but unchanged in reserpinized rats, verifying that the alterations in responding to NMDA were the result of chronic, rather than acute, DA removal. The adaptations do not appear to be the consequence of a diminished GABAergic tone for the ability of bicuculline to increase firing (due to blocking a tonic GABAergic input) was not changed. However, low ejection currents of GABA that were insufficient to alter firing rate greatly attenuated the ability of NMDA to induce an apparent depolarization inactivation when coiontophoresed with NMDA onto VP neurons of the lesioned, but not the unlesioned, hemisphere. These studies show that chronic DA removal altered the EAA-induced amplitude-decreasing (i.e., the apparent depolarization inactivation) effects in VP neurons in the absence of a decrease in GABAergic tone.

Enhanced food-related motivation after bilateral lesions of the subthalamic nucleus

Although inactivation of the subthalamic nucleus (STN) has beneficial effects on motor symptoms of parkinsonism, little is known of possible actions on nonmotor symptoms of cognition or mood. Here, we used several forms of converging evidence to show that STN lesions can enhance behavioral motivation. Thus, bilateral fiber-sparing lesions of the STN in rats reduced the time required to eat a standard number of food reward pellets, without affecting food intake, and altered performance on a number of behavioral measures consistent with enhanced motivation for food. Thus, STN-lesioned rats showed greater levels of locomotor activity conditioned to food presentation, enhanced control over responding by food-related conditioned reinforcers, and a higher breaking point associated with elevated rate of lever press under a progressive ratio schedule of reinforcement. These results reveal a new functional role schedule for STN, possibly because of its involvement in ventral, as well as dorsal, striatal circuitry and are relevant to the therapeutic effects of STN stimulation in Parkinson's disease.

Differential effects of an NMDA and a non-NMDA receptor antagonist on medullary lateral tegmental field neurons

We microiontophoresed an N-methyl-D-aspartate (NMDA) and a non-NMDA receptor antagonist onto medullary lateral tegmental field (LTF) neurons, the naturally occurring discharges of which were correlated to the cardiac-related rhythm in sympathetic nerve discharge (SND) of dialurethane-anesthetized cats. Some of these neurons were classified as sympathoexcitatory, because their firing rate decreased during baroreceptor reflex activation. Microiontophoresis of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo-[f]quinoxaline-7-sulfonamide (NBQX), a non-NMDA receptor antagonist, reduced the mean firing rates of these neurons (51 +/- 8% of control, P < 0.001, n = 20) without affecting their relationship to cardiac-related SND, as indicated by the lack of significant changes in the ratio of peak to background counts in arterial pulse (AP)-triggered histograms of LTF neuronal activity and the AP-LTF coherence value at the frequency of the heartbeat. In contrast, microiontophoresis of D(-)-2-amino-5-phosphonopentanoic acid, an NMDA receptor antagonist, onto LTF neurons reduced the ratio of peak to background counts in AP-triggered histograms to 57 +/- 9% of control (P = 0.002, n = 16) and the AP-LTF coherence value to 25 +/- 10% of control (P = 0.001, n = 10). These data support the view that non-NMDA and NMDA receptors are involved in setting the basal level of activity of LTF sympathoexcitatory neurons and in synchronizing their discharges to the AP, respectively.

Mu and kappa opioid agonists modulate ventral tegmental area input to the ventral pallidum

The ventral pallidum (VP) is situated at the convergence of midbrain dopamine and accumbal opioid efferent projections. Using in vivo electrophysiological procedures in chloral hydrate-anaesthetized rats, we examined whether discrete application of mu- [D-Ala2,N-Me-Phe4,Gly-ol5 (DAMGO)] or kappa- (U50488) opioid receptor agonists could alter VP responses to electrical stimulation of ventral tegmental area. Rate suppressions occurred frequently following ventral tegmental area stimulation. Consistent with an involvement of dopamine in this effect, none of the 12 spontaneously active ventral pallidal neurons recorded in rats that had monoamines depleted by reserpine responded to electrical stimulation of ventral tegmental area. Moreover, in intact rats, the dopamine antagonist flupenthixol attenuated evoked suppression in 100% of the neurons tested; however, the GABAA antagonist bicuculline was able to slightly attenuate the response in 50% of the neurons tested. These observations concur with our previous studies in indicating that ventral tegmental area stimulation releases dopamine (and sometimes GABA) onto ventral pallidal neurons. Both DAMGO and U50488 decreased the inhibitory effects of ventral tegmental area stimulation. These effects on the endogenously released transmitter differed from those seen with exogenously applied dopamine, for DAMGO did not alter the efficacy or potency of microiontophoretically applied dopamine. Taken together, these observations suggest that the interaction between DAMGO and dopamine does not occur at a site that is immediately postsynaptic to the dopaminergic input within the VP, but rather that opioid modulation involves mechanisms governing presynaptically released dopamine. These modulatory processes would enable ventral pallidal opioids to gate the influence of ventral tegmental area dopamine transmission on limbic system outputs at the level of the VP.