Dopaminergic innervation of the basal ganglia in the squirrel monkey as revealed by tyrosine hydroxylase immunohistochemistry

Mesial temporal dopamine: From biology to behaviour

While colloquially recognized for its role in pleasure, reward, and affect, dopamine is also necessary for proficient action control. Many motor studies focus on dopaminergic transmission along the nigrostriatal pathway, using Parkinson's disease as a model of a dorsal striatal lesion. Less attention to the mesolimbic pathway and its role in motor control has led to an important question related to the limbic-motor network. Indeed, secondary targets of the mesolimbic pathway include the hippocampus and amygdala, and these are linked to the motor cortex through the substantia nigra and thalamus. The modulatory impact of dopamine in the hippocampus and amygdala in humans is a focus of current investigations. This review explores dopaminergic activity in the mesial temporal lobe by summarizing dopaminergic networks and transmission in these regions and examining their role in behaviour and disease.

Architecture of the subthalamic nucleus

The subthalamic nucleus (STN) is a major neuromodulation target for the alleviation of neurological and neuropsychiatric symptoms using deep brain stimulation (DBS). STN-DBS is today applied as treatment in Parkinson´s disease, dystonia, essential tremor, and obsessive-compulsive disorder (OCD). STN-DBS also shows promise as a treatment for refractory Tourette syndrome. However, the internal organization of the STN has remained elusive and challenges researchers and clinicians: How can this small brain structure engage in the multitude of functions that renders it a key hub for therapeutic intervention of a variety of brain disorders ranging from motor to affective to cognitive? Based on recent gene expression studies of the STN, a comprehensive view of the anatomical and cellular organization, including revelations of spatio-molecular heterogeneity, is now possible to outline. In this review, we focus attention to the neurobiological architecture of the STN with specific emphasis on molecular patterns discovered within this complex brain area. Studies from human, non-human primate, and rodent brains now reveal anatomically defined distribution of specific molecular markers. Together their spatial patterns indicate a heterogeneous molecular architecture within the STN. Considering the translational capacity of targeting the STN in severe brain disorders, the addition of molecular profiling of the STN will allow for advancement in precision of clinical STN-based interventions.

Anatomical characterisation of three different psychosurgical targets in the subthalamic area: from the basal ganglia to the limbic system

Effective neural stimulation for the treatment of severe psychiatric disorders needs accurate characterisation of surgical targets. This is especially true for the medial subthalamic region (MSR) which contains three targets: the anteromedial STN for obsessive compulsive disorder (OCD), the medial forebrain bundle (MFB) for depression and OCD, and the "Sano triangle" for pathological aggressiveness. Blocks containing the subthalamic area were obtained from two human brains. After obtaining 11.7-Tesla MRI, blocks were cut in regular sections for immunohistochemistry. Fluorescent in situ hybridisation was performed on the macaque MSR. Electron microscopic observation for synaptic specialisation was performed on human and macaque subthalamic fresh samples. Images of human brain sections were reconstructed in a cryoblock which was registered on the MRI and histological slices were then registered. The STN contains glutamatergic and fewer GABAergic neurons and has no strict boundary with the adjacent MSR. The anteromedial STN has abundant dopaminergic and serotoninergic innervation with very sparse dopaminergic neurons. The MFB is composed of dense anterior dopaminergic and posterior serotoninergic fibres, and fewer cholinergic and glutamatergic fibres. Medially, the Sano triangle presumably contains orexinergic terminals from the hypothalamus, and neurons with strong nuclear oestrogen receptor-alpha staining with a decreased anteroposterior and mediolateral gradient of staining. These findings provide new insight regarding MSR cells and their fibre specialisation, forming a transition zone between the basal ganglia and the limbic systems. Our 3D reconstruction enabled us to visualize the main histological features of the three targets which should enable better targeting and understanding of neuromodulatory stimulation results in severe psychiatric conditions.

Increased Free Water in the Putamen in Idiopathic REM Sleep Behavior Disorder

BACKGROUND

It has been suggested that the loss of nigrostriatal dopaminergic axon terminals occurs before the loss of dopaminergic neurons in the substantia nigra (SN) in Parkinson's disease (PD). This study aimed to use free-water imaging to evaluate microstructural changes in the dorsoposterior putamen (DPP) of idiopathic rapid eye movement (REM) sleep behavior disorder (iRBD) patients, which is considered a prodromal stage of synucleinopathies.

METHODS

Free water values in the DPP, dorsoanterior putamen (DAP), and posterior SN were compared between the healthy controls (n = 48), iRBD (n = 43) and PD (n = 47) patients. In iRBD patients, the relationships between baseline and longitudinal free water values and clinical manifestations or dopamine transporter (DAT) striatal binding ratio (SBR) were analyzed.

RESULTS

Free water values were significantly higher in the DPP and posterior substantia nigra (pSN), but not in the DAP, in the iRBD and PD groups than in controls. In iRBD patients, free water values in the DPP were progressively increased and correlated with the progression of clinical manifestations and the striatal DAT SBR. Baseline free water in the DPP was negatively correlated with striatal DAT SBR and hyposmia and positively correlated with motor deficits.

CONCLUSIONS

This study demonstrates that free water values in the DPP are increased cross-sectionally and longitudinally and associated with clinical manifestations and the function of the dopaminergic system in the prodromal stage of synucleinopathies. Our findings indicate that free-water imaging of the DPP has the potential to be a valid marker of early diagnosis and progression of synucleinopathies. © 2023 International Parkinson and Movement Disorder Society.

The Network Systems Underlying Emotions: The Rational Foundation of Deep Brain Stimulation Psychosurgery

Science and philosophy have tried to understand the origin of emotions for centuries. However, only in the last 150 years have we started to try to understand them in a neuroscientific scope. Emotions include physiological changes involving different systems, such as the endocrine or the musculoskeletal, but they also cause a conscious experience of those changes that are embedded in memory. In addition to the cortico-striato-thalamo-cortical circuit, which is the most important of the basal ganglia, the limbic system and prefrontal circuit are primarily involved in the process of emotion perceptions, thoughts, and memories. The purpose of this review is to describe the anatomy and physiology of the different brain structures involved in circuits that underlie emotions and behaviour, underlying the symptoms of certain psychiatric pathologies. These circuits are targeted during deep brain stimulation (DBS) and knowledge of them is mandatory to understand the clinical-physiological implications for the treatment. We summarize the main outcomes of DBS treatment in several psychiatric illness such as obsessive compulsive disorder, refractory depression, erethism and other conditions, aiming to understand the rationale for selecting these neural systems as targets for DBS.

Neurobiological and Pharmacological Perspectives of D3 Receptors in Parkinson’s Disease

The discovery of the D3 receptor (D3R) subtypes of dopamine (DA) has generated an understandable increase in interest in the field of neurological diseases, especially Parkinson’s disease (PD). Indeed, although DA replacement therapy with l-DOPA has provided an effective treatment for patients with PD, it is responsible for invalidating abnormal involuntary movements, known as L-DOPA-induced dyskinesia, which constitutes a serious limitation of the use of this therapy. Of particular interest is the finding that chronic l-DOPA treatment can trigger the expression of D1R–D3R heteromeric interactions in the dorsal striatum. The D3R is expressed in various tissues of the central nervous system, including the striatum. Compelling research has focused on striatal D3Rs in the context of PD and motor side effects, including dyskinesia, occurring with DA replacement therapy. Therefore, this review will briefly describe the basal ganglia (BG) and the DA transmission within these brain regions, before going into more detail with regard to the role of D3Rs in PD and their participation in the current treatments. Numerous studies have also highlighted specific interactions between D1Rs and D3Rs that could promote dyskinesia. Finally, this review will also address the possibility that D3Rs located outside of the BG may mediate some of the effects of DA replacement therapy.

Mapping reward mechanisms by intracerebral self-stimulation in the rhesus monkey (Macaca mulatta)

The objective of the study was to identify brain structures that mediate reward as evidenced by positive reinforcing effects of stimuli on behavior. Testing by intracerebral self-stimulation enabled monkeys to inform whether activation of ~2900 sites in 74 structures of 4 sensorimotor pathways and 4 modulatory loop pathways was positive, negative or neutral. Stimulation was rewarding at 30% of sites, negative at 17%, neutral at 52%. Virtually all (99%) structures yielded some positive or negative sites, suggesting a ubiquitous distribution of pathways transmitting valence information. Mapping of sites to structures with dense versus sparse dopaminergic (DA) or noradrenergic (NA) innervation showed that stimulation of DA-pathways was rewarding or neutral. Stimulation of NA-pathways was not rewarding. Stimulation of association areas was generally rewarding; stimulation of purely sensory or motor structures was generally negative. Reward related more to structures' sensorimotor function than to density of DA-innervation. Stimulation of basal ganglia loop pathways was rewarding except in lateral globus pallidus, an inhibitory structure in the negative feedback loop; stimulation of the cerebellar loop was rewarding in anterior vermis and the spinocerebellar pathway; and stimulation of the hippocampal CA1 loop was rewarding. While most positive sites were in the DA reward system, numerous sites in sparsely DA-innervated posterior cingulate and parietal cortices may represent a separate reward system. DA-density represents concentrations of plastic synapses that mediate acquisition of new synaptic connections. DA-sparse areas may represent innate, genetically programmed reward-associated pathways. Implications of findings in regard to response habituation and addiction are discussed.

Therapeutic potential of intranasal photobiomodulation therapy for neurological and neuropsychiatric disorders: a narrative review

The application of photobiomodulation therapy (PBMT) for neuronal stimulation is studied in different animal models and in humans, and has shown to improve cerebral metabolic activity and blood flow, and provide neuroprotection via anti-inflammatory and antioxidant pathways. Recently, intranasal PBMT (i-PBMT) has become an attractive and potential method for the treatment of brain conditions. Herein, we provide a summary of different intranasal light delivery approaches including a nostril-based portable method and implanted deep-nasal methods for the effective systemic or direct irradiation of the brain. Nostril-based i-PBMT devices are available, using either lasers or light emitting diodes (LEDs), and can be applied either alone or in combination to transcranial devices (the latter applied directly to the scalp) to treat a wide range of brain conditions such as mild cognitive impairment, Alzheimer's disease, Parkinson's disease, cerebrovascular diseases, depression and anxiety as well as insomnia. Evidence shows that nostril-based i-PBMT improves blood rheology and cerebral blood flow, so that, without needing to puncture blood vessels, i-PBMT may have equivalent results to a peripheral intravenous laser irradiation procedure. Up to now, no studies were conducted to implant PBMT light sources deep within the nose in a clinical setting, but simulation studies suggest that deep-nasal PBMT via cribriform plate and sphenoid sinus might be an effective method to deliver light to the ventromedial part of the prefrontal and orbitofrontal cortex. Home-based i-PBMT, using inexpensive LED applicators, has potential as a novel approach for neurorehabilitation; comparative studies also testing sham, and transcranial PBMT are warranted.

The squirrel monkey model in clinical neuroscience

Clinical neuroscience research relying on animal models brought valuable translational insights into the function and pathologies of the human brain. The anatomical, physiological, and behavioural similarities between humans and mammals have prompted researchers to study cerebral mechanisms at different levels to develop and test new treatments. The vast majority of biomedical research uses rodent models, which are easily manipulable and have a broadly resembling organisation to the human nervous system but cannot satisfactorily mimic some disorders. For these disorders, macaque monkeys have been used as they have a more comparable central nervous system. Still, this research has been hampered by limitations, including high costs and reduced samples. This review argues that a squirrel monkey model might bridge the gap by complementing translational research from rodents, macaque, and humans. With the advent of promising new methods such as ultrasound imaging, tool miniaturisation, and a shift towards open science, the squirrel monkey model represents a window of opportunity that will potentially fuel new translational discoveries in the diagnosis and treatment of brain pathologies.

A theoretical framework for the site-specific and frequency-dependent neuronal effects of deep brain stimulation

BACKGROUND

Deep brain stimulation is an established therapy for several neurological disorders; however, its effects on neuronal activity vary across brain regions and depend on stimulation settings. Understanding these variable responses can aid in the development of physiologically-informed stimulation paradigms in existing or prospective indications.

OBJECTIVE

Provide experimental and computational insights into the brain-region-specific and frequency-dependent effects of extracellular stimulation on neuronal activity.

METHODS

In patients with movement disorders, single-neuron recordings were acquired from the subthalamic nucleus, substantia nigra pars reticulata, ventral intermediate nucleus, or reticular thalamus during microstimulation across various frequencies (1-100 Hz) to assess single-pulse and frequency-response functions. Moreover, a biophysically-realistic computational framework was developed which generated postsynaptic responses under the assumption that electrical stimuli simultaneously activated all convergent presynaptic inputs to stimulation target neurons. The framework took into consideration the relative distributions of excitatory/inhibitory afferent inputs to model site-specific responses, which were in turn embedded within a model of short-term synaptic plasticity to account for stimulation frequency-dependence.

RESULTS

We demonstrated microstimulation-evoked excitatory neuronal responses in thalamic structures (which have predominantly excitatory inputs) and inhibitory responses in basal ganglia structures (predominantly inhibitory inputs); however, higher stimulation frequencies led to a loss of site-specificity and convergence towards neuronal suppression. The model confirmed that site-specific responses could be simulated by accounting for local neuroanatomical/microcircuit properties, while suppression of neuronal activity during high-frequency stimulation was mediated by short-term synaptic depression.

CONCLUSIONS

Brain-region-specific and frequency-dependant neuronal responses could be simulated by considering neuroanatomical (local microcircuitry) and neurophysiological (short-term plasticity) properties.

Current Directions in Deep Brain Stimulation for Parkinson's Disease-Directing Current to Maximize Clinical Benefit

Several single-center studies and one large multicenter clinical trial demonstrated that directional deep brain stimulation (DBS) could optimize the volume of tissue activated (VTA) based on the individual placement of the lead in relation to the target. The ability to generate axially asymmetric fields of stimulation translates into a broader therapeutic window (TW) compared to conventional DBS. However, changing the shape and surface of stimulating electrodes (directional segmented vs. conventional ring-shaped) also demands a revision of the programming strategies employed for DBS programming. Model-based approaches have been used to predict the shape of the VTA, which can be visualized on standardized neuroimaging atlases or individual magnetic resonance imaging. While potentially useful for optimizing clinical care, these systems remain limited by factors such as patient-specific anatomical variability, postsurgical lead migrations, and inability to account for individual contact impedances and orientation of the systems of fibers surrounding the electrode. Alternative programming tools based on the functional assessment of stimulation-induced clinical benefits and side effects allow one to collect and analyze data from each electrode of the DBS system and provide an action plan of ranked alternatives for therapeutic settings based on the selection of optimal directional contacts. Overall, an increasing amount of data supports the use of directional DBS. It is conceivable that the use of directionality may reduce the need for complex programming paradigms such as bipolar configurations, frequency or pulse width modulation, or interleaving. At a minimum, stimulation through directional electrodes can be considered as another tool to improve the benefit/side effect ratio. At a maximum, directionality may become the preferred way to program because of its larger TW and lower energy consumption.

Anatomy and Connectivity of the Subthalamic Nucleus in Humans and Non-human Primates

The Subthalamic Nucleus (STh) is an oval-shaped diencephalic structure located ventrally to the thalamus, playing a fundamental role in the circuitry of the basal ganglia. In addition to being involved in the pathophysiology of several neurodegenerative disorders, such as Huntington’s and Parkinson’s disease, the STh is one of the target nuclei for deep brain stimulation. However, most of the anatomical evidence available derives from non-human primate studies. In this review, we will present the topographical and morphological organization of the nucleus and its connections to structurally and functionally related regions of the basal ganglia circuitry. We will also highlight the importance of additional research in humans focused on validating STh connectivity, cytoarchitectural organization, and its functional subdivision.

Differential calcium channel-mediated dopaminergic modulation in the subthalamonigral synapse

Dopamine (DA) modulates basal ganglia (BG) activity for initiation and execution of goal-directed movements and habits. While most studies are aimed to striatal function, the cellular and molecular mechanisms underlying dopaminergic regulation in other nuclei of the BG are not well understood. Therefore, we set to analyze the dopaminergic modulation occurring in subthalamo-nigral synapse, in both pars compacta (SNc) and pars reticulata (SNr) neurons, because these synapses are important for the integration of information previously processed in striatum and globus pallidus. In this study, electrophysiological and pharmacological evidence of dopaminergic modulation on glutamate release through calcium channels is presented. Using paired pulse ratio (PPR) measurements and selective blockers of these ionic channels, together with agonists and antagonists of DA D₂ -like receptors, we found that blockade of the Ca_V 3 family occludes the presynaptic inhibition produced by the activation of DA receptors pharmacologically profiled as D₃ -type in the STh-SNc synapses. On the contrast, the blockade of Ca_V 2 channels, but not Ca_V 3, occlude with the effect of the D₃ agonist, PD 128907, in the STh-SNr synapse. The functional role of this differential distribution of calcium channels that modulate the release of glutamate in the SN implies a fine adjustment of firing for both classes of neurons. Dopaminergic neurons of the SNc establish a DA tone within the SN based on the excitatory/inhibitory inputs; such tone may contribute to processing information from subthalamic nucleus and could also be involved in pathological DA depletion that drives hyperexcitation of SNr neurons.

Evidence for incentive salience sensitization as a pathway to alcohol use disorder

The incentive salience sensitization (ISS) theory of addiction holds that addictive behavior stems from the ability of drugs to progressively sensitize the brain circuitry that mediates attribution of incentive salience (IS) to reward-predictive cues and its behavioral manifestations. In this article, we establish the plausibility of ISS as an etiological pathway to alcohol use disorder (AUD). We provide a comprehensive and critical review of evidence for: (1) the ability of alcohol to sensitize the brain circuitry of IS attribution and expression; and (2) attribution of IS to alcohol-predictive cues and its sensitization in humans and non-human animals. We point out gaps in the literature and how these might be addressed. We also highlight how individuals with different alcohol subjective response phenotypes may differ in susceptibility to ISS as a pathway to AUD. Finally, we discuss important implications of this neuropsychological mechanism in AUD for psychological and pharmacological interventions attempting to attenuate alcohol craving and cue reactivity.

Dopamine signals related to appetitive and aversive events in paradigms that manipulate reward and avoidability

Using environmental cues to acquire good and avoid harmful things is critical for survival. Rewarding and aversive outcomes both drive behavior through reinforcement learning and sometimes occur together in the environment, but it remains unclear how these signals are encoded within the brain and if signals for positive and negative reinforcement are encoded similarly. Recent studies demonstrate that the dopaminergic system and interconnected brain regions process both positive and negative reinforcement necessary for approach and avoidance behaviors, respectively. Here, we review these data with a special focus on behavioral paradigms that manipulate both expected reward and the avoidability of aversive events to reveal neural correlates related to value, prediction error encoding, motivation, and salience.

State transitions in the substantia nigra reticulata predict the onset of motor deficits in models of progressive dopamine depletion in mice

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose cardinal motor symptoms are attributed to dysfunction of basal ganglia circuits under conditions of low dopamine. Despite well-established physiological criteria to define basal ganglia dysfunction, correlations between individual parameters and motor symptoms are often weak, challenging their predictive validity and causal contributions to behavior. One limitation is that basal ganglia pathophysiology is studied only at end-stages of depletion, leaving an impoverished understanding of when deficits emerge and how they evolve over the course of depletion. In this study, we use toxin- and neurodegeneration-induced mouse models of dopamine depletion to establish the physiological trajectory by which the substantia nigra reticulata (SNr) transitions from the healthy to the diseased state. We find that physiological progression in the SNr proceeds in discrete state transitions that are highly stereotyped across models and correlate well with the prodromal and symptomatic stages of behavior.

Netrin-1/DCC-mediated PLCγ1 activation is required for axon guidance and brain structure development

Coordinated expression of guidance molecules and their signal transduction are critical for correct brain wiring. Previous studies have shown that phospholipase C gamma1 (PLCγ1), a signal transducer of receptor tyrosine kinases, plays a specific role in the regulation of neuronal cell morphology and motility in vitro However, several questions remain regarding the extracellular stimulus that triggers PLCγ1 signaling and the exact role PLCγ1 plays in nervous system development. Here, we demonstrate that PLCγ1 mediates axonal guidance through a netrin-1/deleted in colorectal cancer (DCC) complex. Netrin-1/DCC activates PLCγ1 through Src kinase to induce actin cytoskeleton rearrangement. Neuronal progenitor-specific knockout of Plcg1 in mice causes axon guidance defects in the dorsal part of the mesencephalon during embryogenesis. Adult Plcg1-deficient mice exhibit structural alterations in the corpus callosum, substantia innominata, and olfactory tubercle. These results suggest that PLCγ1 plays an important role in the correct development of white matter structure by mediating netrin-1/DCC signaling.

Effects of feedback delay on learning from positive and negative feedback in patients with Parkinson's disease off medication

Phasic dopamine (DA) signals conveyed from the substantia nigra to the striatum and the prefrontal cortex crucially affect learning from feedback, with DA bursts facilitating learning from positive feedback and DA dips facilitating learning from negative feedback. Consequently, diminished nigro-striatal dopamine levels as in unmedicated patients suffering from Parkinson's Disease (PD) have been shown to lead to a negative learning bias. Recent studies suggested a diminished striatal contribution to feedback processing when the outcome of an action is temporally delayed. This study investigated whether the bias towards negative feedback learning induced by a lack of DA in PD patients OFF medication is modulated by feedback delay. To this end, PD patients OFF medication and healthy controls completed a probabilistic selection task, in which feedback was given immediately (after 800 ms) or delayed (after 6800 ms). PD patients were impaired in immediate but not delayed feedback learning. However, differences in the preference for positive/negative learning between patients and controls were seen for both learning from immediate and delayed feedback, with evidence of stronger negative learning in patients than controls. A Bayesian analysis of the data supports the conclusion that feedback timing did not affect the learning bias in the patients. These results hint at reduced, but still relevant nigro-striatal contribution to feedback learning, when feedback is delayed.

Antipsychotic Drug Responsiveness and Dopamine Receptor Signaling; Old Players and New Prospects

Antipsychotic drugs targeting dopamine neurotransmission are still the principal mean of therapeutic intervention for schizophrenia. However, about one third of people do not respond to dopaminergic antipsychotics. Genome wide association studies (GWAS), have shown that multiple genetic factors play a role in schizophrenia pathophysiology. Most of these schizophrenia risk variants are not related to dopamine or antipsychotic drugs mechanism of action. Genetic factors have also been implicated in defining response to antipsychotic medication. In contrast to disease risk, variation of genes coding for molecular targets of antipsychotics have been associated with treatment response. Among genes implicated, those involved in dopamine signaling mediated by D2-class dopamine receptor, including DRD2 itself and its molecular effectors, have been implicated as key genetic predictors of response to treatments. Studies have also reported that genetic variation in genes coding for proteins that cross-talk with DRD2 at the molecular level, such as AKT1, GSK3B, Beta-catenin, and PPP2R2B are associated with response to antipsychotics. In this review we discuss the relative contribution to antipsychotic drug responsiveness of candidate genes and GWAS identified genes encoding proteins involved in dopamine responses. We also suggest that in addition of these older players, a deeper investigation of new GWAS identified schizophrenia risk genes such as FXR1 can provide new prospects that are not clearly engaged in dopamine function while being targeted by dopamine-associated signaling molecules. Overall, further examination of genes proximally or distally related to signaling mechanisms engaged by medications and associated with disease risk and/or treatment responsiveness may uncover an interface between genes involved in disease causation with those affecting disease remediation. Such a nexus would provide realistic targets for therapy and further the development of genetically personalized approaches for schizophrenia.

Compensatory mechanisms in Parkinson's disease: Circuits adaptations and role in disease modification

The motor features of Parkinson's disease (PD) are well known to manifest only when striatal dopaminergic deficit reaches 60-70%. Thus, PD has a long pre-symptomatic and pre-motor evolution during which compensatory mechanisms take place to delay the clinical onset of disabling manifestations. Classic compensatory mechanisms have been attributed to changes and adjustments in the nigro-striatal system, such as increased neuronal activity in the substantia nigra pars compacta and enhanced dopamine synthesis and release in the striatum. However, it is not so clear currently that such changes occur early enough to account for the pre-symptomatic period. Other possible mechanisms relate to changes in basal ganglia and motor cortical circuits including the cerebellum. However, data from early PD patients are difficult to obtain as most studies have been carried out once the diagnosis and treatments have been established. Likewise, putative compensatory mechanisms taking place throughout disease evolution are nearly impossible to distinguish by themselves. Here, we review the evidence for the role of the best known and other possible compensatory mechanisms in PD. We also discuss the possibility that, although beneficial in practical terms, compensation could also play a deleterious role in disease progression.

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.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

A Dynamic Circuit Hypothesis for the Pathogenesis of Blepharospasm

Blepharospasm (sometimes called “benign essential blepharospasm,” BEB) is one of the most common focal dystonias. It involves involuntary eyelid spasms, eye closure, and increased blinking. Despite the success of botulinum toxin injections and, in some cases, pharmacologic or surgical interventions, BEB treatments are not completely efficacious and only symptomatic. We could develop principled strategies for preventing and reversing the disease if we knew the pathogenesis of primary BEB. The objective of this study was to develop a conceptual framework and dynamic circuit hypothesis for the pathogenesis of BEB. The framework extends our overarching theory for the multifactorial pathogenesis of focal dystonias (Peterson et al., 2010) to incorporate a two-hit rodent model specifically of BEB (Schicatano et al., 1997). We incorporate in the framework three features critical to cranial motor control: (1) the joint influence of motor cortical regions and direct descending projections from one of the basal ganglia output nuclei, the substantia nigra pars reticulata, on brainstem motor nuclei, (2) nested loops composed of the trigeminal blink reflex arc and the long sensorimotor loop from trigeminal nucleus through thalamus to somatosensory cortex back through basal ganglia to the same brainstem nuclei modulating the reflex arc, and (3) abnormalities in the basal ganglia dopamine system that provide a sensorimotor learning substrate which, when combined with patterns of increased blinking, leads to abnormal sensorimotor mappings manifest as BEB. The framework explains experimental data on the trigeminal reflex blink excitability (TRBE) from Schicatano et al. and makes predictions that can be tested in new experimental animal models based on emerging genetics in dystonia, including the recently characterized striatal-specific D1R dopamine transduction alterations caused by the GNAL mutation. More broadly, the model will provide a guide for future efforts to mechanistically link multiple factors in the pathogenesis of BEB and facilitate simulations of how exogenous manipulations of the pathogenic factors could ultimately be used to prevent and reverse the disorder.

Challenges and opportunities for brainstem neuroimaging with ultrahigh field MRI

The human brainstem plays a central role in connecting the cerebrum, the cerebellum and the spinal cord to one another, hosting relay nuclei for afferent and efferent signaling, and providing source nuclei for several neuromodulatory systems that impact central nervous system function. While the investigation of the brainstem with functional or structural magnetic resonance imaging has been hampered for years due to this brain structure's physiological and anatomical characteristics, the field has seen significant advances in recent years thanks to the broader adoption of ultrahigh-field (UHF) MRI scanning. In the present review, we focus on the advantages offered by UHF in the context of brainstem imaging, as well as the challenges posed by the investigation of this complex brain structure in terms of data acquisition and analysis. We also illustrate how UHF MRI can shed new light on the neuroanatomy and neurophysiology underlying different brainstem-based circuitries, such as the central autonomic network and neurotransmitter/neuromodulator systems, discuss existing and foreseeable clinical applications to better understand diseases such as chronic pain and Parkinson's disease, and explore promising future directions for further improvements in brainstem imaging using UHF MRI techniques.

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.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Chemical anatomy of pallidal afferents in primates

Neurons of the globus pallidus receive massive inputs from the striatum and the subthalamic nucleus, but their activity, as well as those of their striatal and subthalamic inputs, are modulated by brainstem afferents. These include serotonin (5-HT) projections from the dorsal raphe nucleus, cholinergic (ACh) inputs from the pedunculopontine tegmental nucleus, and dopamine (DA) afferents from the substantia nigra pars compacta. This review summarizes our recent findings on the distribution, quantitative and ultrastructural aspects of pallidal 5-HT, ACh and DA innervations. These results have led to the elaboration of a new model of the pallidal neuron based on a precise knowledge of the hierarchy and chemical features of the various synaptic inputs. The dense 5-HT, ACh and DA innervations disclosed in the associative and limbic pallidal territories suggest that these brainstem inputs contribute principally to the planification of motor behaviors and the regulation of attention and mood. Although 5-HT, ACh and DA inputs were found to modulate pallidal neurons and their afferents mainly through asynaptic (volume) transmission, genuine synaptic contacts occur between these chemospecific axon varicosities and pallidal dendrites, revealing that these brainstem projections have a direct access to pallidal neurons, in addition to their indirect input through the striatum and subthalamic nucleus. Altogether, these findings reveal that the brainstem 5-HT, ACh and DA pallidal afferents act in concert with the more robust GABAergic inhibitory striatopallidal and glutamatergic excitatory subthalamopallidal inputs. We hypothesize that a fragile equilibrium between forebrain and brainstem pallidal afferents plays a key role in the functional organization of the primate basal ganglia, in both health and disease.

Morphological changes of glutamatergic synapses in animal models of Parkinson's disease

The striatum and the subthalamic nucleus (STN) are the main entry doors for extrinsic inputs to reach the basal ganglia (BG) circuitry. The cerebral cortex, thalamus and brainstem are the key sources of glutamatergic inputs to these nuclei. There is anatomical, functional and neurochemical evidence that glutamatergic neurotransmission is altered in the striatum and STN of animal models of Parkinson’s disease (PD) and that these changes may contribute to aberrant network neuronal activity in the BG-thalamocortical circuitry. Postmortem studies of animal models and PD patients have revealed significant pathology of glutamatergic synapses, dendritic spines and microcircuits in the striatum of parkinsonians. More recent findings have also demonstrated a significant breakdown of the glutamatergic corticosubthalamic system in parkinsonian monkeys. In this review, we will discuss evidence for synaptic glutamatergic dysfunction and pathology of cortical and thalamic inputs to the striatum and STN in models of PD. The potential functional implication of these alterations on synaptic integration, processing and transmission of extrinsic information through the BG circuits will be considered. Finally, the significance of these pathological changes in the pathophysiology of motor and non-motor symptoms in PD will be examined.

Morphological evidence for dopamine interactions with pallidal neurons in primates

The external (GPe) and internal (GPi) segments of the primate globus pallidus receive dopamine (DA) axonal projections arising mainly from the substantia nigra pars compacta and this innervation is here described based on tyrosine hydroxylase (TH) immunohistochemical observations gathered in the squirrel monkey (Saimiri sciureus). At the light microscopic level, unbiased stereological quantification of TH positive (+) axon varicosities reveals a similar density of innervation in the GPe (0.19 ± 0.02 × 10⁶ axon varicosities/mm³ of tissue) and GPi (0.17 ± 0.01 × 10⁶), but regional variations occur in the anteroposterior and dorsoventral axes in both GPe and GPi and along the mediolateral plane in the GPe. Estimation of the neuronal population in the GPe (3.47 ± 0.15 × 10³ neurons/mm³) and GPi (2.69 ± 0.18 × 10³) yields a mean ratio of, respectively, 28 ± 3 and 68 ± 15 TH+ axon varicosities/pallidal neuron. At the electron microscopic level, TH+ axon varicosities in the GPe appear significantly smaller than those in the GPi and very few TH+ axon varicosities are engaged in synaptic contacts in the GPe (17 ± 3%) and the GPi (15 ± 4%) compared to their unlabeled counterparts (77 ± 6 and 50 ± 12%, respectively). Genuine synaptic contacts made by TH+ axon varicosities in the GPe and GPi are of the symmetrical and asymmetrical type. Such synaptic contacts together with the presence of numerous synaptic vesicles in all TH+ axon varicosities observed in the GPe and GPi support the functionality of the DA pallidal innervation. By virtue of its predominantly volumic mode of action, DA appears to exert a key modulatory effect upon pallidal neurons in concert with the more direct GABAergic inhibitory and glutamatergic excitatory actions of the striatum and subthalamic nucleus. We argue that the DA pallidal innervation plays a major role in the functional organization of the primate basal ganglia under both normal and pathological conditions.

Striatal cholinergic dysfunction as a unifying theme in the pathophysiology of dystonia

Dystonia is a movement disorder of both genetic and non-genetic causes, which typically results in twisted posturing due to abnormal muscle contraction. Evidence from dystonia patients and animal models of dystonia indicate a crucial role for the striatal cholinergic system in the pathophysiology of dystonia. In this review, we focus on striatal circuitry and the centrality of the acetylcholine system in the function of the basal ganglia in the control of voluntary movement and ultimately clinical manifestation of movement disorders. We consider the impact of cholinergic interneurons (ChIs) on dopamine-acetylcholine interactions and examine new evidence for impairment of ChIs in dysfunction of the motor systems producing dystonic movements, particularly in animal models. We have observed paradoxical excitation of ChIs in the presence of dopamine D2 receptor agonists and impairment of striatal synaptic plasticity in a mouse model of DYT1 dystonia, which are improved by administration of recently developed M1 receptor antagonists. These findings have been confirmed across multiple animal models of DYT1 dystonia and may represent a common endophenotype by which to investigate dystonia induced by other types of genetic and non-genetic causes and to investigate the potential effectiveness of pharmacotherapeutics and other strategies to improve dystonia.

Calbindin content and differential vulnerability of midbrain efferent dopaminergic neurons in macaques

Calbindin (CB) is a calcium binding protein reported to protect dopaminergic neurons from degeneration. Although a direct link between CB content and differential vulnerability of dopaminergic neurons has long been accepted, factors other than CB have also been suggested, particularly those related to the dopamine transporter. Indeed, several studies have reported that CB levels are not causally related to the differential vulnerability of dopaminergic neurons against neurotoxins. Here we have used dual stains for tyrosine hydroxylase (TH) and CB in 3 control and 3 MPTP-treated monkeys to visualize dopaminergic neurons in the ventral tegmental area (VTA) and in the dorsal and ventral tiers of the substantia nigra pars compacta (SNcd and SNcv) co-expressing TH and CB. In control animals, the highest percentages of co-localization were found in VTA (58.2%), followed by neurons located in the SNcd (34.7%). As expected, SNcv neurons lacked CB expression. In MPTP-treated animals, the percentage of CB-ir/TH-ir neurons in the VTA was similar to control monkeys (62.1%), whereas most of the few surviving neurons in the SNcd were CB-ir/TH-ir (88.6%). Next, we have elucidated the presence of CB within identified nigrostriatal and nigroextrastriatal midbrain dopaminergic projection neurons. For this purpose, two control monkeys received one injection of Fluoro-Gold into the caudate nucleus and one injection of cholera toxin (CTB) into the postcommissural putamen, whereas two more monkeys were injected with CTB into the internal division of the globus pallidus (GPi). As expected, all the nigrocaudate- and nigroputamen-projecting neurons were TH-ir, although surprisingly, all of these nigrostriatal-projecting neurons were negative for CB. Furthermore, all the nigropallidal-projecting neurons co-expressed both TH and CB. In summary, although CB-ir dopaminergic neurons seem to be less prone to MPTP-induced degeneration, our data clearly demonstrated that these neurons are not giving rise to nigrostriatal projections and indeed CB-ir/TH-ir neurons only originate nigroextrastriatal projections.

Cerebellum in levodopa-induced dyskinesias: the unusual suspect in the motor network

The exact mechanisms that generate levodopa-induced dyskinesias (LID) during chronic levodopa therapy for Parkinson’s disease (PD) are not yet fully established. The most widely accepted theories incriminate the non-physiological synthesis, release and reuptake of dopamine generated by exogenously administered levodopa in the striatum, and the aberrant plasticity in the cortico-striatal loops. However, normal motor performance requires the correct recruitment of motor maps. This depends on a high level of synergy within the primary motor cortex (M1) as well as between M1 and other cortical and subcortical areas, for which dopamine is necessary. The plastic mechanisms within M1, which are crucial for the maintenance of this synergy, are disrupted both during “OFF” and dyskinetic states in PD. When tested without levodopa, dyskinetic patients show loss of treatment benefits on long-term potentiation and long-term depression-like plasticity of the intracortical circuits. When tested with the regular pulsatile levodopa doses, they show further impairment of the M1 plasticity, such as inability to depotentiate an already facilitated synapse and paradoxical facilitation in response to afferent input aimed at synaptic inhibition. Dyskinetic patients have also severe impairment of the associative, sensorimotor plasticity of M1 attributed to deficient cerebellar modulation of sensory afferents to M1. Here, we review the anatomical and functional studies, including the recently described bidirectional connections between the cerebellum and the basal ganglia that support a key role of the cerebellum in the generation of LID. This model stipulates that aberrant neuronal synchrony in PD with LID may propagate from the subthalamic nucleus to the cerebellum and “lock” the cerebellar cortex in a hyperactive state. This could affect critical cerebellar functions such as the dynamic and discrete modulation of M1 plasticity and the matching of motor commands with sensory information from the environment during motor performance. We propose that in dyskinesias, M1 neurons have lost the ability to depotentiate an activated synapse when exposed to acute pulsatile, non-physiological, dopaminergic surges and become abnormally receptive to unfiltered, aberrant, and non-salient afferent inputs from the environment. The motor program selection in response to such non-salient and behaviorally irrelevant afferent inputs would be abnormal and involuntary. The motor responses are worsened by the lack of normal subcortico–cortical inputs from cerebellum and basal ganglia, because of the aberrant plasticity at their own synapses. Artificial cerebellar stimulation might help re-establish the cerebellar and basal ganglia control over the non-salient inputs to the motor areas during synaptic dopaminergic surges.

Involvement of dopamine loss in extrastriatal basal ganglia nuclei in the pathophysiology of Parkinson's disease

Parkinson's disease (PD) is a neurological disorder characterized by the manifestation of motor symptoms, such as akinesia, muscle rigidity and tremor at rest. These symptoms are classically attributed to the degeneration of dopamine neurons in the pars compacta of substantia nigra (SNc), which results in a marked dopamine depletion in the striatum. It is well established that dopamine neurons in the SNc innervate not only the striatum, which is the main target, but also other basal ganglia nuclei including the two segments of globus pallidus and the subthalamic nucleus (STN). The role of dopamine and its depletion in the striatum is well known, however, the role of dopamine depletion in the pallidal complex and the STN in the genesis of their abnormal neuronal activity and in parkinsonian motor deficits is still not clearly determined. Based on recent experimental data from animal models of Parkinson's disease in rodents and non-human primates and also from parkinsonian patients, this review summarizes current knowledge on the role of dopamine in the modulation of basal ganglia neuronal activity and also the role of dopamine depletion in these nuclei in the pathophysiology of Parkinson's disease.

Localization and function of dopamine receptors in the subthalamic nucleus of normal and parkinsonian monkeys

The subthalamic nucleus (STN) receives a dopaminergic innervation from the substantia nigra pars compacta, but the role of this projection remains poorly understood, particularly in primates. To address this issue, we used immuno-electron microscopy to localize D1, D2, and D5 dopamine receptors in the STN of rhesus macaques and studied the electrophysiological effects of activating D1-like or D2-like receptors in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys. Labeling of D1 and D2 receptors was primarily found presynaptically, on preterminal axons and putative glutamatergic and GABAergic terminals, while D5 receptors were more significantly expressed postsynaptically, on dendritic shafts of STN neurons. The electrical spiking activity of STN neurons, recorded with standard extracellular recording methods, was studied before, during, and after intra-STN administration of the dopamine D1-like receptor agonist SKF82958, the D2-like receptor agonist quinpirole, or artificial cerebrospinal fluid (control injections). In normal animals, administration of SKF82958 significantly reduced the spontaneous firing but increased the rate of intraburst firing and the proportion of pause-burst sequences of firing. Quinpirole only increased the proportion of such pause-burst sequences in STN neurons of normal monkeys. In MPTP-treated monkeys, the D1-like receptor agonist also reduced the firing rate and increased the proportion of pause-burst sequences, while the D2-like receptor agonist did not change any of the chosen descriptors of the firing pattern of STN neurons. Our data suggest that dopamine receptor activation can directly modulate the electrical activity of STN neurons by pre- and postsynaptic mechanisms in both normal and parkinsonian states, predominantly via activation of D1 receptors.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

Transgenic mouse lines subdivide external segment of the globus pallidus (GPe) neurons and reveal distinct GPe output pathways

Cell-type diversity in the brain enables the assembly of complex neural circuits, whose organization and patterns of activity give rise to brain function. However, the identification of distinct neuronal populations within a given brain region is often complicated by a lack of objective criteria to distinguish one neuronal population from another. In the external segment of the globus pallidus (GPe), neuronal populations have been defined using molecular, anatomical, and electrophysiological criteria, but these classification schemes are often not generalizable across preparations and lack consistency even within the same preparation. Here, we present a novel use of existing transgenic mouse lines, Lim homeobox 6 (Lhx6)-Cre and parvalbumin (PV)-Cre, to define genetically distinct cell populations in the GPe that differ molecularly, anatomically, and electrophysiologically. Lhx6-GPe neurons, which do not express PV, are concentrated in the medial portion of the GPe. They have lower spontaneous firing rates, narrower dynamic ranges, and make stronger projections to the striatum and substantia nigra pars compacta compared with PV-GPe neurons. In contrast, PV-GPe neurons are more concentrated in the lateral portions of the GPe. They have narrower action potentials, deeper afterhyperpolarizations, and make stronger projections to the subthalamic nucleus and parafascicular nucleus of the thalamus. These electrophysiological and anatomical differences suggest that Lhx6-GPe and PV-GPe neurons participate in different circuits with the potential to contribute to different aspects of motor function and dysfunction in disease.

Long-term treatment with l-DOPA and an mGlu5 receptor antagonist prevents changes in brain basal ganglia dopamine receptors, their associated signaling proteins and neuropeptides in parkinsonian monkeys

Brain glutamate overactivity is well documented in Parkinson's disease (PD) and antiglutamatergic drugs decrease L-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesias (LID); the implication of dopamine neurotransmission is not documented in this anti-LID activity. Therefore, we evaluated changes of dopamine receptors, their associated signaling proteins and neuropeptides mRNA, in normal control monkeys, in saline-treated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in L-DOPA-treated MPTP monkeys, without or with an adjunct treatment to reduce the development of LID: 2-methyl-6-(phenylethynyl)pyridine (MPEP), the prototypal metabotropic glutamate 5 (mGlu5) receptor antagonist. All de novo treatments were administered for 1 month and the animals were sacrificed thereafter. MPTP monkeys treated with l-DOPA + MPEP developed significantly less LID than MPTP monkeys treated with l-DOPA alone. [(3)H]SCH-23390 specific binding to D1 receptors of all MPTP monkeys was decreased as compared to controls in the basal ganglia and no difference was observed between all MPTP groups, while striatal D1 receptor mRNA levels remained unchanged. [(3)H]raclopride specific binding to striatal D2 receptors and mRNA levels of D2 receptors were increased in MPTP monkeys compared to controls; l-DOPA treatment reduced this binding in MPTP monkeys while it remained elevated with the l-DOPA + MPEP treatment. Striatal [(3)H]raclopride specific binding correlated positively with D2 receptor mRNA levels of all MPTP-lesioned monkeys. Striatal preproenkephalin/preprodynorphin mRNA levels and phosphorylated ERK1/2 and Akt/GSK3β levels increased only in L-DOPA-treated MPTP monkeys as compared to controls, saline treated-MPTP and l-DOPA + MPEP treated MPTP monkeys. Hence, reduction of development of LID with MPEP was associated with changes in D2 receptors, their associated signaling proteins and neuropeptides.

Independent circuits in the basal ganglia for the evaluation and selection of actions

The basal ganglia are critical for selecting actions and evaluating their outcome. Although the circuitry for selection is well understood, how these nuclei evaluate the outcome of actions is unknown. Here, we show in lamprey that a separate evaluation circuit, which regulates the habenula-projecting globus pallidus (GPh) neurons, exists within the basal ganglia. The GPh neurons are glutamatergic and can drive the activity of the lateral habenula, which, in turn, provides an indirect inhibitory influence on midbrain dopamine neurons. We show that GPh neurons receive inhibitory input from the striosomal compartment of the striatum. The striosomal input can reduce the excitatory drive to the lateral habenula and, consequently, decrease the inhibition onto the dopaminergic system. Dopaminergic neurons, in turn, provide feedback that inhibits the GPh. In addition, GPh neurons receive direct projections from the pallium (cortex in mammals), which can increase the GPh activity to drive the lateral habenula to increase the inhibition of the neuromodulatory systems. This circuitry, thus, differs markedly from the "direct" and "indirect" pathways that regulate the pallidal (e.g., globus pallidus) output nuclei involved in the control of motion. Our results show that a distinct reward-evaluation circuit exists within the basal ganglia, in parallel to the direct and indirect pathways, which select actions. Our results suggest that these circuits are part of the fundamental blueprint that all vertebrates use to select actions and evaluate their outcome.

Molecular Pathogenesis and Pathophysiology of Parkinson’s Disease: New Targets for New Therapies

Parkinson’s disease (PD) is a complex chronic neurodegenerative disease of unknown etiology. A conceptual framework for all chronic diseases involves a series of channels or pathways (aging, genetic, environment, oxidative stress, mitochondrial damage, protein aggregation, etc.) and their interactions. Those channels with specificities may explain the ‘developmental’ program that through transcriptional reprogramming results in stressed dopamine neurons that eventually become dysfunctional or die, giving rise to the clinical manifestations of PD. In Chapter 2 we review the molecular mechanisms of those channels that may be implicated in the pathogenesis of PD and the pathophysiology of the disease based on the anatomo‐physiological complexity of the basal ganglia. This illustrates that understanding the molecular mechanisms of a disease may not be enough, or we have to reach an adequate system level to understand the disease process. Finally, we suggest that common therapies used for the treatment of other chronic diseases may be useful for the treatment (or help to advance the understanding) of PD, as well as new targets for new therapies that may be useful in the prevention of, or to stop the progression of, PD and other synucleinopathies.

Differential striatal spine pathology in Parkinson's disease and cocaine addiction: a key role of dopamine?

In the striatum, the dendritic tree of the two main populations of projection neurons, called "medium spiny neurons (MSNs)", are covered with spines that receive glutamatergic inputs from the cerebral cortex and thalamus. In Parkinson's disease (PD), striatal MSNs undergo an important loss of dendritic spines, whereas aberrant overgrowth of striatal spines occurs following chronic cocaine exposure. This review examines the possibility that opposite dopamine dysregulation is one of the key factors that underlies these structural changes. In PD, nigrostriatal dopamine degeneration results in a significant loss of dendritic spines in the dorsal striatum, while rodents chronically exposed to cocaine and other psychostimulants, display an increase in the density of "thin and immature" spines in the nucleus accumbens (NAc). In rodent models of PD, there is evidence that D2 dopamine receptor-containing MSNs are preferentially affected, while D1-positive cells are the main targets of increased spine density in models of addiction. However, such specificity remains to be established in primates. Although the link between the extent of striatal spine changes and the behavioral deficits associated with these disorders remains controversial, there is unequivocal evidence that glutamatergic synaptic transmission is significantly altered in both diseased conditions. Recent studies have suggested that opposite calcium-mediated regulation of the transcription factor myocyte enhancer factor 2 (MEF2) function induces these structural defects. In conclusion, there is strong evidence that dopamine is a major, but not the sole, regulator of striatal spine pathology in PD and addiction to psychostimulants. Further studies of the role of glutamate and other genes associated with spine plasticity in mediating these effects are warranted.

Acetylcholine-based entropy in response selection: a model of how striatal interneurons modulate exploration, exploitation, and response variability in decision-making

The basal ganglia play a fundamental role in decision-making. Their contribution is typically modeled within a reinforcement learning framework, with the basal ganglia learning to select the options associated with highest value and their dopamine inputs conveying performance feedback. This basic framework, however, does not account for the role of cholinergic interneurons in the striatum, and does not easily explain certain dynamic aspects of decision-making and skill acquisition like the generation of exploratory actions. This paper describes basal ganglia acetylcholine-based entropy (BABE), a model of the acetylcholine system in the striatum that provides a unified explanation for these phenomena. According to this model, cholinergic interneurons in the striatum control the level of variability in behavior by modulating the number of possible responses that are considered by the basal ganglia, as well as the level of competition between them. This mechanism provides a natural way to account for the role of basal ganglia in generating behavioral variability during the acquisition of certain cognitive skills, as well as for modulating exploration and exploitation in decision-making. Compared to a typical reinforcement learning model, BABE showed a greater modulation of response variability in the face of changes in the reward contingences, allowing for faster learning (and re-learning) of option values. Finally, the paper discusses the possible applications of the model to other domains.

The corticostriatal and corticosubthalamic pathways: two entries, one target. So what?

The basal ganglia receive cortical inputs through two main stations - the striatum and the subthalamic nucleus (STN). The information flowing along the corticostriatal system is transmitted to the basal ganglia circuitry via the "direct and indirect" striatofugal pathways, while information that flows through the STN is transmitted along the so-called "hyperdirect" pathway. The functional significance of this dual entry system is not clear. Although the corticostriatal system has been thoroughly characterized anatomically and electrophysiologically, such is not the case for the corticosubthalamic system. In order to provide further insights into the intricacy of this complex anatomical organization, this review examines and compares the anatomical and functional organization of the corticostriatal and corticosubthalamic systems, and highlights some key issues that must be addressed to better understand the mechanisms by which these two neural systems may interact to regulate basal ganglia functions and dysfunctions.

The Basal Ganglia as a Substrate for the Multiple Actions of Amphetamines

Amphetamines are psychostimulant drugs with high abuse potential. Acute and chronic doses of amphetamines affect dopamine (DA) neurotransmission in the basal ganglia. The basal ganglia are a group of subcortical nuclei that are anatomically positioned to integrate cognitive, motor and sensorimotor inputs from the cortex. Amphetamines can differentially alter the functioning of specific BG circuits to produce neurochemical changes that affect cognition, movement, and drug seeking behavior through their effects on DA neurotransmission. This review focuses on how alterations in dopaminergic neurotransmission within distinct basal ganglia pathways can modify their functional output to predict and explain the acute and long term behavioral consequences of amphetamine exposure.