Apomorphine and dopamine D(1) receptor agonists increase the firing rates of subthalamic nucleus neurons

Age- and sex-related oculomotor manifestation of dopamine deficiency in Segawa disease

OBJECTIVE

To investigate the oculomotor manifestations of Segawa disease (SD), considered to represent mild dopamine deficiency and discuss their pathophysiological basis.

METHODS

We recorded visually- (VGS) and memory-guided saccade (MGS) tasks in 31 SD patients and 153 age-matched control subjects to study how basal ganglia (BG) dysfunction in SD evolves with age for male and female subjects.

RESULTS

SD patients were impaired in initiating MGS, showing longer latencies with occasional failure. Patients showed impaired ability to suppress reflexive saccades; saccades to cues presented in MGS were more frequent and showed a shorter latency than in control subjects. These findings were more prominent in male patients, particularly between 13 and 25 years. Additionally, male patients showed larger delay in MGS latency in trials preceded by saccades to cue than those unpreceded.

CONCLUSIONS

The findings can be explained by a dysfunction of the BG-direct pathway impinging on superior colliculus (SC) due to dopamine deficiency. The disturbed inhibitory control of saccades may be explained by increased SC responsivity to visual stimuli.

SIGNIFICANCE

Oculomotor abnormalities in SD can be explained by dysfunction of the BG inhibitory pathways reaching SC, with a delayed maturation in male SD patients, consistent with previous pathological/physiological studies.

Nine Hereditary Movement Disorders First Described in Asia: Their History and Evolution

Clinical case studies and reporting are important to the discovery of new disorders and the advancement of medical sciences. Both clinicians and basic scientists play equally important roles leading to treatment discoveries for both cures and symptoms. In the field of movement disorders, exceptional observation of patients from clinicians is imperative, not just for phenomenology but also for the variable occurrences of these disorders, along with other signs and symptoms, throughout the day and the disease course. The Movement Disorders in Asia Task Force (TF) was formed to help enhance and promote collaboration and research on movement disorders within the region. As a start, the TF has reviewed the original studies of the movement disorders that were preliminarily described in the region. These include nine disorders that were first described in Asia: Segawa disease, PARK-Parkin, X-linked dystonia-parkinsonism, dentatorubral-pallidoluysian atrophy, Woodhouse-Sakati syndrome, benign adult familial myoclonic epilepsy, Kufor-Rakeb disease, tremulous dystonia associated with mutation of the calmodulin-binding transcription activator 2 gene, and paroxysmal kinesigenic dyskinesia. We hope that the information provided will honor the original researchers and help us learn and understand how earlier neurologists and basic scientists together discovered new disorders and made advances in the field, which impact us all to this day.

Optimal attention tuning in a neuro-computational model of the visual cortex-basal ganglia-prefrontal cortex loop

Visual attention is widely considered a vital factor in the perception and analysis of a visual scene. Several studies explored the effects and mechanisms of top-down attention, but the mechanisms that determine the attentional signal are less explored. By developing a neuro-computational model of visual attention including the visual cortex-basal ganglia loop, we demonstrate how attentional alignment can evolve based on dopaminergic reward during a visual search task. Unlike most previous modeling studies of feature-based attention, we do not implement a manually predefined attention template. Dopamine-modulated covariance learning enable the basal ganglia to learn rewarded associations between the visual input and the attentional gain represented in the PFC of the model. Hence, the model shows human-like performance on a visual search task by optimally tuning the attention signal. In particular, similar as in humans, this reward-based tuning in the model leads to an attentional template that is not centered on the target feature, but a relevant feature deviating away from the target due to the presence of highly similar distractors. Further analyses of the model shows, attention is mainly guided by the signal-to-noise ratio between target and distractors.

Screening of Parkinsonian subtle fine-motor impairment from touchscreen typing via deep learning

Fine-motor impairment (FMI) is progressively expressed in early Parkinson’s Disease (PD) patients and is now known to be evident in the immediate prodromal stage of the condition. The clinical techniques for detecting FMI may not be robust enough and here, we show that the subtle FMI of early PD patients can be effectively estimated from the analysis of natural smartphone touchscreen typing via deep learning networks, trained in stages of initialization and fine-tuning. In a validation dataset of 36,000 typing sessions from 39 subjects (17 healthy/22 PD patients with medically validated UPDRS Part III single-item scores), the proposed approach achieved values of area under the receiver operating characteristic curve (AUC) of 0.89 (95% confidence interval: 0.80–0.96) with sensitivity/specificity: 0.90/0.83. The derived estimations result in statistically significant (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$p<0.05$$\end{document}p<0.05) correlation of 0.66/0.73/0.58 with the clinical standard UPDRS Part III items 22/23/31, respectively. Further validation analysis on 9 de novo PD patients vs. 17 healthy controls classification resulted in AUC of 0.97 (0.93–1.00) with 0.93/0.90. For 253 remote study participants, with self-reported health status providing 252.000 typing sessions via a touchscreen typing data acquisition mobile app (iPrognosis), the proposed approach predicted 0.79 AUC (0.66–0.91) with 0.76/0.71. Remote and unobtrusive screening of subtle FMI via natural smartphone usage, may assist in consolidating early and accurate diagnosis of PD.

On the Role of Cortex-Basal Ganglia Interactions for Category Learning: A Neurocomputational Approach

In addition to the prefrontal cortex (PFC), the basal ganglia (BG) have been increasingly often reported to play a fundamental role in category learning, but the circuit mechanisms mediating their interaction remain to be explored. We developed a novel neurocomputational model of category learning that particularly addresses the BG-PFC interplay. We propose that the BG bias PFC activity by removing the inhibition of cortico-thalamo-cortical loop and thereby provide a teaching signal to guide the acquisition of category representations in the corticocortical associations to the PFC. Our model replicates key behavioral and physiological data of macaque monkey learning a prototype distortion task from Antzoulatos and Miller (2011) Our simulations allowed us to gain a deeper insight into the observed drop of category selectivity in striatal neurons seen in the experimental data and in the model. The simulation results and a new analysis of the experimental data based on the model's predictions show that the drop in category selectivity of the striatum emerges as the variability of responses in the striatum rises when confronting the BG with an increasingly larger number of stimuli to be classified. The neurocomputational model therefore provides new testable insights of systems-level brain circuits involved in category learning that may also be generalized to better understand other cortico-BG-cortical loops.SIGNIFICANCE STATEMENT Inspired by the idea that basal ganglia (BG) teach the prefrontal cortex (PFC) to acquire category representations, we developed a novel neurocomputational model and tested it on a task that was recently applied in monkey experiments. As an advantage over previous models of category learning, our model allows to compare simulation data with single-cell recordings in PFC and BG. We not only derived model predictions, but already verified a prediction to explain the observed drop in striatal category selectivity. When testing our model with a simple, real-world face categorization task, we observed that the fast striatal learning with a performance of 85% correct responses can teach the slower PFC learning to push the model performance up to almost 100%.

Revealing a novel nociceptive network that links the subthalamic nucleus to pain processing

Pain is a prevalent symptom of Parkinson's disease, and is effectively treated by deep brain stimulation of the subthalamic nucleus (STN). However, the link between pain and the STN remains unclear. In the present work, using in vivo electrophysiology in rats, we report that STN neurons exhibit complex tonic and phasic responses to noxious stimuli. We also show that nociception is altered following lesions of the STN, and characterize the role of the superior colliculus and the parabrachial nucleus in the transmission of nociceptive information to the STN, physiologically from both structures and anatomically in the case of the parabrachial nucleus. We show that STN nociceptive responses are abnormal in a rat model of PD, suggesting their dependence on the integrity of the nigrostriatal dopaminergic system. The STN-linked nociceptive network that we reveal is likely to be of considerable clinical importance in neurological diseases involving a dysfunction of the basal ganglia.

Neurotransmission systems in Parkinson’s disease

Parkinson’s disease (PD) is histologically characterized by the accumulation of α-synuclein particles, known as Lewy bodies. The second most common neurodegenerative disorder, PD is widely known because of the typical motor manifestations of active tremor, rigidity, and postural instability, while several prodromal non-motor symptoms including REM sleep behavior disorders, depression, autonomic disturbances, and cognitive decline are being more extensively recognized. Motor symptoms most commonly arise from synucleinopathy of nigrostriatal pathway. Glutamatergic, γ-aminobutyric acid (GABA)ergic, cholinergic, serotoninergic, and endocannabinoid neurotransmission systems are not spared from the global cerebral neurodegenerative assault. Wide intrabasal and extrabasal of the basal ganglia provide enough justification to evaluate network circuits disturbance of these neurotransmission systems in PD. In this comprehensive review, English literature in PubMed, Science direct, EMBASE, and Web of Science databases were perused. Characteristics of dopaminergic and non-dopaminergic systems, disturbance of these neurotransmitter systems in the pathophysiology of PD, and their treatment applications are discussed.

Neurotransmitters and Novelty: A Systematic Review

Our brains are highly responsive to novelty. However, how novelty is processed in the brain, and what neurotransmitter systems play a role therein, remains elusive. Here, we systematically review studies on human participants that have looked at the neuromodulatory basis of novelty detection and processing. While theoretical models and studies on nonhuman animals have pointed to a role of the dopaminergic, cholinergic, noradrenergic and serotonergic systems, the human literature has focused almost exclusively on the first two. Dopamine was found to affect electrophysiological responses to novelty early in time after stimulus presentation, but evidence on its effects on later processing was found to be contradictory: While neuropharmacological studies mostly yielded null effects, gene studies did point to an important role for dopamine. Acetylcholine seems to dampen novelty signals in the medial temporal lobe, but boost them in frontal cortex. Findings on 5-HT (serotonin) were found to be mostly contradictory. Two large gaps were identified in the literature. First, few studies have looked at neuromodulatory influences on behavioral effects of novelty. Second, no study has looked at the involvement of the noradrenergic system in novelty processing.

Pre-movement gating of somatosensory evoked potentials in Segawa disease

INTRODUCTION

Segawa disease (SD), an autosomal dominant dopa-responsive dystonia with marked diurnal fluctuation, can be clinically classified into the postural dystonia type (SD-P) and action dystonia type (SD-A). Compared to SD-A, SD-P has an earlier onset and is characterized by postural dystonia. In SD-A, along with postural dystonia, dystonic movements appear in late childhood. To evaluate the differences between these two types of SD, we studied the gating of SEPs, which is useful to investigate sensory-motor integration and might be one of the methods to detect the thalamo-cortical involvement.

METHODS

Fourteen patients with SD (11-63 years) and 18 age-matched normal subjects (11-51 years) were studied. Among the 14 patients with SD, 8 patients had SD-P and 6 had SD-A. Using median nerve stimulation at the wrist, the amplitude of the frontal N30 (FrN30) was compared between pre-movement and rest conditions.

RESULTS

We found that the amplitude of the contralateral FrN30 was attenuated before movement in normal controls and in the majority of both SD types. On the other hand, the pre-movement-rest amplitude ratio in patients with SD-A was significantly larger than in patients with SD-P (P=0.0025). No significant differences were observed in the pre-movement-rest ratio between SD-P and normal subjects.

CONCLUSION

The preservation or impairment of pre-movement gating shown here suggests a physiological difference between the two types of SD. More specifically, sensorimotor integration of the basal ganglia-thalamo-cortical circuits may be intact in SD-P, but are affected in SD-A. We discuss the different pathophysiology seen in the different phenotype of SD based on the different developmental involvement in the basal ganglia.

Abnormal neuronal activity in Tourette syndrome and its modulation using deep brain stimulation

Tourette syndrome (TS) is a common childhood-onset disorder characterized by motor and vocal tics that are typically accompanied by a multitude of comorbid symptoms. Pharmacological treatment options are limited, which has led to the exploration of deep brain stimulation (DBS) as a possible treatment for severe cases. Multiple lines of evidence have linked TS with abnormalities in the motor and limbic cortico-basal ganglia (CBG) pathways. Neurophysiological data have only recently started to slowly accumulate from multiple sources: noninvasive imaging and electrophysiological techniques, invasive electrophysiological recordings in TS patients undergoing DBS implantation surgery, and animal models of the disorder. These converging sources point to system-level physiological changes throughout the CBG pathway, including both general altered baseline neuronal activity patterns and specific tic-related activity. DBS has been applied to different regions along the motor and limbic pathways, primarily to the globus pallidus internus, thalamic nuclei, and nucleus accumbens. In line with the findings that also draw on the more abundant application of DBS to Parkinson's disease, this stimulation is assumed to result in changes in the neuronal firing patterns and the passage of information through the stimulated nuclei. We present an overview of recent experimental findings on abnormal neuronal activity associated with TS and the changes in this activity following DBS. These findings are then discussed in the context of current models of CBG function in the normal state, during TS, and finally in the wider context of DBS in CBG-related disorders.

Dopa-responsive dystonia

Clinical characteristics and pahophysiologies of dopa-responsive dystonia are discussed by reviewing autosomal-dominant GTP cyclohydrolase-I deficiency (AD GCHI D), recessive deficiencies of enzymes of pteridine metabolism, and recessive tyrosine hydroxylase (TH). Pteridine and TH metabolism involve TH activities in the terminals of the nigrostriatal dopamine neuron which show high in early childhood and decrease exponentially with age, attaining stational low levels by the early 20s. In these disorders, TH in the terminals follows this course with low levels and develops particular symptoms with functional maturation of the downstream structures of the basal ganglia; postural dystonia through the direct pathway and descending output matured earlier in early childhood and parkinsonism in TH deficiency in teens through the D2 indirect pathway ascending output matured later. In action-type AD GCHI D, deficiency of TH in the terminal on the subthalamic nucleus develops action dystonia through the descending output in childhood, focal and segmental dystonia and parkinsonism in adolescence and adulthood through the ascending pathway maturing later. Dysfunction of dopamine in the terminals does not cause degenerative changes or higher cortical dysfunction. In recessive disorders, hypofunction of serotonin and noradrenaline induces hypofunction of the dopamine in the perikaryon and shows cortical dysfunction.

Extrastriatal dopaminergic circuits of the Basal Ganglia

The basal ganglia are comprised of the striatum, the external and internal segment of the globus pallidus (GPe and GPi, respectively), the subthalamic nucleus (STN), and the substantia nigra pars compacta and reticulata (SNc and SNr, respectively). Dopamine has long been identified as an important modulator of basal ganglia function in the striatum, and disturbances of striatal dopaminergic transmission have been implicated in diseases such as Parkinson's disease (PD), addiction and attention deficit hyperactivity disorder. However, recent evidence suggests that dopamine may also modulate basal ganglia function at sites outside of the striatum, and that changes in dopaminergic transmission at these sites may contribute to the symptoms of PD and other neuropsychiatric disorders. This review summarizes the current knowledge of the anatomy, functional effects and behavioral consequences of the dopaminergic innervation to the GPe, GPi, STN, and SNr. Further insights into the dopaminergic modulation of basal ganglia function at extrastriatal sites may provide us with opportunities to develop new and more specific strategies for treating disorders of basal ganglia dysfunction.

Apomorphine reduces subthalamic neuronal entropy in parkinsonian patients

Dopamine depletion in Parkinson's disease (PD) alters the neuronal activity in basal ganglia circuits. Characterizing these changes in network activity is an important step in understanding the disease and how therapies mitigate symptoms. Non-linear analysis methods can complement the traditional description of neuronal firing characteristics. Here we examine the entropy of subthalamic neurons in PD patients undergoing stereotactic surgery for deep brain stimulation (DBS). The activity of 8 neurons was recorded prior to, during, and following systemic administration of the dopamine agonist apomorphine at clinically effective doses. Apomorphine induced a decrease in entropy measured in the inter-spike intervals of sub-thalamic neurons in 6 of the 8 neurons. This is the first report that anti-parkinsonian drugs affect non-linear features of neuronal firing in the basal ganglia of parkinsonian patients.

A model for altered neural network dynamics related to prehension movements in Parkinson disease

In this paper, we present a neural network model of the interactions between cortex and the basal ganglia during prehensile movements. Computational neuroscience methods are used to explore the hypothesis that the altered kinematic patterns observed in Parkinson's disease patients performing prehensile movements is mainly due to an altered neuronal activity located in the networks of cholinergic (ACh) interneurons of the striatum. These striatal cells, under a strong influence of the dopaminergic system, significantly contribute to the neural processing within the striatum and in the cortico-basal ganglia loops. In order to test this hypothesis, a large-scale model of neural interactions in the basal ganglia has been integrated with previous models accounting for the cortical organization of goal directed reaching and grasping movements in normal and perturbed conditions. We carry out a discussion of the model hypothesis validation by providing a control engineering analysis and by comparing results of real experiments with our simulation results in conditions resembling these original experiments.

Functional organization of the basal ganglia: therapeutic implications for Parkinson's disease

The basal ganglia (BG) are a highly organized network, where different parts are activated for specific functions and circumstances. The BG are involved in movement control, as well as associative learning, planning, working memory, and emotion. We concentrate on the "motor circuit" because it is the best understood anatomically and physiologically, and because Parkinson's disease is mainly thought to be a movement disorder. Normal function of the BG requires fine tuning of neuronal excitability within each nucleus to determine the exact degree of movement facilitation or inhibition at any given moment. This is mediated by the complex organization of the striatum, where the excitability of medium spiny neurons is controlled by several pre- and postsynaptic mechanisms as well as interneuron activity, and secured by several recurrent or internal BG circuits. The motor circuit of the BG has two entry points, the striatum and the subthalamic nucleus (STN), and an output, the globus pallidus pars interna (GPi), which connects to the cortex via the motor thalamus. Neuronal afferents coding for a given movement or task project to the BG by two different systems: (1) Direct disynaptic projections to the GPi via the striatum and STN. (2) Indirect trisynaptic projections to the GPi via the globus pallidus pars externa (GPe). Corticostriatal afferents primarily act to inhibit medium spiny neurons in the "indirect circuit" and facilitate neurons in the "direct circuit." The GPe is in a pivotal position to regulate the motor output of the BG. Dopamine finely tunes striatal input as well as neuronal striatal activity, and modulates GPe, GPi, and STN activity. Dopaminergic depletion in Parkinson's disease disrupts the corticostriatal balance leading to increased activity the indirect circuit and reduced activity in the direct circuit. The precise chain of events leading to increased STN activity is not completely understood, but impaired dopaminergic regulation of the GPe, GPi, and STN may be involved. The parkinsonian state is characterized by disruption of the internal balance of the BG leading to hyperactivity in the two main entry points of the network (striatum and STN) and excessive inhibitory output from the GPi. Replacement therapy with standard levodopa creates a further imbalance, producing an abnormal pattern of neuronal discharge and synchronization of neuronal firing that sustain the "off" and "on with dyskinesia" states. The effect of levodopa is robust but short-lasting and converts the parkinsonian BG into a highly unstable system, where pharmacological and compensatory effects act in opposing directions. This creates a scenario that substantially departs from the normal physiological state of the BG.

NMDA-mediated release of glutamate and GABA in the subthalamic nucleus is mediated by dopamine: an in vivo microdialysis study in rats

The present study investigated the effects of N-methyl-D-aspartic acid.H2O (NMDA) on the dopamine, glutamate and GABA release in the subthalamic nucleus (STN) by using in vivo microdialysis in rats. NMDA (100 micromol/L) perfused through the microdialysis probe evoked an increase in extracellular dopamine in the STN of the intact rat of about 170%. This coincided with significant increases in both extracellular glutamate (350%) and GABA (250%). The effect of NMDA perfusion on neurotransmitter release at the level of the STN was completely abolished by co-perfusion of the selective NMDA-receptor antagonist MK-801 (10 micromol/L), whereas subthalamic perfusion of MK-801 alone had no effect on extracellular neurotransmitter concentrations. Furthermore, NMDA induced increases in glutamate were abolished by both SCH23390 (8 micromol/L), a selective D1 antagonist, and remoxipride (4 micromol/L), a selective D2 antagonist. The NMDA induced increase in GABA was abolished by remoxipride but not by SCH23390. Perfusion of the STN with SCH23390 or remoxipride alone had no effect on extracellular neurotransmitter concentrations. The observed effects in intact animals depend on the nigral dopaminergic innervation, as dopamine denervation, by means of 6-hydroxydopamine lesioning of the substantia nigra, clearly abolished the effects of NMDA on neurotransmitter release at the level of the STN. Our work points to a complex interaction between dopamine, glutamate and GABA with a crucial role for dopamine at the level of the STN.

Control of the Subthalamic Innervation of the Rat Globus Pallidus by D2/3 and D4 Dopamine Receptors

The effects of activating dopaminergic D2/3 and D4 receptors during activation of the subthalamic projection to the globus pallidus (GP) were explored in rat brain slices using the whole cell patch-clamp technique. Byocitin labeling and both orthodromic and antidromic activation demonstrated the integrity of some subthalamopallidal connections in in vitro parasagittal brain slices. Excitatory postsynaptic currents (EPSCs) that could be blocked by CNQX and AP5 were evoked onto pallidal neurons by local field stimulation of the subthalamopallidal pathway in the presence of bicuculline. Bath application of dopamine and quinpirole, a dopaminergic D2-class receptor agonist, reduced evoked EPSCs by about 35%. This effect was only partially blocked by sulpiride, a D2/3 receptor antagonist. The sulpiride-sensitive reduction of the subthalamopallidal EPSC was associated with an increase in the paired-pulse ratio (PPR) and a reduction in the frequency but not the mean amplitude of spontaneous EPSCs (sEPSCs), indicative of a presynaptic site of action, which was confirmed by variance–mean analysis. The sulpiride-resistant EPSC reduction was mimicked by PD 168,077 and blocked by L-745,870, selective D4 receptor agonist and antagonist, respectively, suggesting the involvement of D4 receptors. The reduction of EPSCs produced by PD 168,077 was not accompanied by changes in PPR or the frequency of sEPSCs; however, it was accompanied by a reduction in mean sEPSC amplitude, indicative of a postsynaptic site of action. These results show that dopamine modulates subthalamopallidal excitation by presynaptic D2/3 and postsynaptic D4 receptors. The importance of this modulation is discussed.

Effects of nitric oxide-active drugs on the discharge of subthalamic neurons: microiontophoretic evidence in the rat

The presence of nitric oxide (NO) synthase and of soluble guanylyl cyclase, the main NO-activated metabolic pathway, has been demonstrated in many cells of the subthalamic nucleus. In this study, the effects induced on the firing of 96 subthalamic neurons by microiontophoretically administering drugs modifying NO neurotransmission were explored in anaesthetized rats. Recorded neurons were classified into regularly and irregularly discharging on the basis of their firing pattern. Nomega-nitro-L-arginine methyl ester (L-NAME; a NO synthase inhibitor), 3-morpholino-sydnonimin-hydrocloride (SIN-1; a NO donor), S-nitroso-glutathione (SNOG; another NO donor) and 8-Br-cGMP (a cell-permeable analogue of cGMP, the main second-messenger of NO neurotransmission) were iontophoretically applied while performing single-unit extracellular recordings. The activity of most neurons was influenced in a statistically significant way: in particular, both current-related inhibitory L-NAME-induced effects (20/39 tested cells) and excitatory effects of SIN-1 (25/41 tested neurons), SNOG (19/32 tested cells) and 8-Br-cGMP (13/19 tested neurons) were demonstrated. Neither statistically significant differences between the responses of regularly and irregularly discharging cells, nor specific topographical clustering of responding neurons, were demonstrated. Neurons administered drugs oppositely modulating the NO neurotransmission often displayed responses to only one treatment. We hypothesize that NO neurotransmission could exert a modulatory influence upon subthalamic neurons, with a prevalent excitatory effect. However, in the light of the presence of some responses of opposite sign to the same drug displayed by different subthalamic neurons, more complex effects of NO neurotransmission could be suggested, probably due to interactions with other classical neurotransmitter systems.

Lesions in monkey globus pallidus externus exacerbate parkinsonian symptoms

To further define the role of the external segment of the globus pallidus (GPe) in the development of parkinsonian motor signs, two rhesus monkeys were made parkinsonian with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Behavioral assessments of bradykinesia and akinesia as well as single neuron recordings in the internal segment of the globus pallidus (GPi) were performed in both monkeys before and after ablating the sensorimotor portion of GPe. The effects of apomorphine on behavior and neuronal activity were also assessed in the parkinsonian monkeys before and after GPe ablation. We found that lesions in GPe exacerbated parkinsonian symptoms, altered neuronal activity in GPi, and reduced the therapeutic effects of apomorphine. These results support the hypothesis that GPe can influence GPi neuronal activity and is directly involved in parkinsonism. In addition, these data suggest that the inclusion of GPe in pallidotomy lesions for the treatment of Parkinson's disease can block the beneficial effects of antiparkinsonian medications and should be avoided.

L-DOPA-induced dyskinesia in adult rats with a unilateral 6-OHDA lesion of dopamine neurons is paralleled by increased c-fos gene expression in the subthalamic nucleus

Levodopa (L-DOPA), the metabolic precursor of dopamine, is widely used as a pharmacological agent for the symptomatic treatment of Parkinson's disease. However, long-term L-DOPA use results in abnormal involuntary movements such as dyskinesias. There is evidence that abnormal cell signaling in the basal ganglia is involved in L-DOPA-induced dyskinesia. The subthalamic nucleus (STN) plays a key role in the circuitry of the basal ganglia and in the pathophysiology of Parkinson's disease. However, the contribution of the STN to L-DOPA-induced dyskinesias remains unclear. The objective of this work was to study the effects of acute or chronic systemic administration of L-DOPA to adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of dopamine neurons on c-fos expression in the STN and test the hypothesis that these effects correlate with L-DOPA-induced dyskinesias. c-fos mRNA expression was measured in the STN by in situ hybridization histochemistry at the single cell level. Our results confirm earlier evidence that the chronic administration of L-DOPA to rats with a unilateral 6-OHDA lesion increases c-fos expression in the STN. We also report that c-fos expression can be increased following an acute injection of L-DOPA to 6-OHDA-lesioned rats but not following a chronic injection of L-DOPA to sham-operated, unlesioned rats. Finally, we provide evidence that the occurrence and severity of dyskinesia is correlated with c-fos mRNA levels in the ipsilateral STN. These results suggest that altered cell signaling in the STN is involved in some of the behavioral effects induced by systemic L-DOPA administration.

Subthalamic neurons coordinate basal ganglia function through differential neural pathways

The subthalamic nucleus (STN) is a key component of basal ganglia circuitry that mediates a variety of motor functions. The STN neurons send glutamatergic projections to the output structures of basal ganglia, including the substantia nigra pars reticulata (SNr) and the entopeduncular nucleus, and also innervate the globus pallidus (GP). However, the mechanism by which the STN regulates motor functions in the neural circuitry is not fully understood. Here we performed conditional ablation of the STN neurons by using immunotoxin-mediated cell targeting. We then analyzed dopamine (DA)-mediated motor behavior and firing activity of the SNr and GP neurons. Ablation of the STN neurons increased spontaneous movement and reduced hyperactivity in response to DA stimulation. Ablation of these neurons modulated the pattern and rate of spontaneous firing of the SNr neurons, although it did not substantially affect spontaneous firing of the GP neurons. The ablation attenuated DA-induced suppression of the firing rate of the SNr neurons and inhibited DA-induced elevation of the rate of the GP neurons. In addition, pharmacological blockade of GP activation in response to DA stimulation inhibited the suppression of SNr activity and the resultant motor activation. These results suggest that the STN neurons suppress spontaneous behavior through their direct projection to the output neurons and that, in response to DA, they contribute to expression of behavior by acting on the output neurons mainly through the GP-mediated pathways. We conclude that the STN coordinates motor behavior through differential neural pathways depending on the state of DA transmission.

The discharge of subthalamic neurons is modulated by inhibiting the nitric oxide synthase in the rat

The effects induced on the discharge of subthalamic spontaneously active neurons by inhibiting the enzyme nitric oxide synthase was studied in two groups of urethane-anesthetized rats. In the first group of animals (n = 10), the activity of subthalamic single units was recorded before and after the systemic administration of 7-nitro-indazole (7-NI, 50 mg/kg i.p.), a selective inhibitor of neuronal nitric oxide synthase. In the second group of rats (n = 15), Nomega-nitro-L-arginine methyl ester (L-NAME), another inhibitor of nitric oxide synthase, was iontophoretically administered while performing single unit extracellular recordings. The activity of most tested spontaneously discharging neurons (8/10) was influenced by 7-NI administration, which always caused a statistically significant decrease in the firing rate of the responsive cells. In contrast, the iontophoretic administration of L-NAME, although influencing many cells (24/32), did not have univocal effects: in fact, 18 cells were inhibited while 6 neurons were excited in a statistically significant manner. We hypothesize that nitric oxide neurotransmission could exert a tonic modulatory influence upon spontaneously discharging subthalamic neurons, with a prevalent excitatory effect.

Silent plateau potentials, rhythmic bursts, and pacemaker firing: three patterns of activity that coexist in quadristable subthalamic neurons

Subthalamic neurons display uncommon intrinsic behaviors that are likely to contribute to the motor and cognitive functions of the basal ganglia and to many of its disorders. Here, we report silent plateau potentials in these cells. These plateau responses start with a transient burst of action potentials that quickly diminish in amplitude because of spike inactivation and current shunt. The resulting interruption of spiking reveals a stable depolarization (up state) that clamps the cell membrane potential near -40 mV for several seconds. These plateau potentials coexist in single subthalamic neurons with more familiar patterns of burst and pacemaker firing. Within a narrow range of baseline membrane potentials (-67 to -60 mV), depolarization abruptly switches single cells from bistable to rhythmic bursts or tonic firing modes, thus selecting entirely distinct algorithms for integrating cortical and pallidal synaptic inputs.

Dopamine receptors set the pattern of activity generated in subthalamic neurons

Information processing in the brain requires adequate background neuronal activity. As Parkinson's disease progresses, patients typically become akinetic; the death of dopaminergic neurons leads to a dopamine-depleted state, which disrupts information processing related to movement in a brain area called the basal ganglia. Using agonists of dopamine receptors in the D1 and D2 families on rat brain slices, we show that dopamine receptors in these two families govern the firing pattern of neurons in the subthalamic nucleus, a crucial part of the basal ganglia. We propose a conceptual frame, based on specific properties of dopamine receptors, to account for the dominance of different background firing patterns in normal and dopamine-depleted states.

MK801 and amantadine exert different effects on subthalamic neuronal activity in a rodent model of Parkinson's disease

Efforts to develop adjuvant therapies for the treatment of Parkinson's disease (PD) have led to interest in drugs that could mimic the therapeutic effects of lesion or deep brain stimulation of the subthalamic nucleus (STN). Extracellular single unit recordings were conducted to determine whether noncompetitive NMDA receptor blockade, suggested to have potential as an adjuvant treatment in PD, attenuates rate increases and firing pattern changes observed in the STN in a rodent model of PD. Systemic administration of the noncompetitive NMDA antagonist MK801 to rats with unilateral dopamine cell lesions did not significantly alter burstiness or interspike interval coefficient of variation, although mean firing rate decreased by a modest 20% with 50% of neurons showing decreases in rate >15% and spike train power in the 3-8-Hz (theta) range was reduced. MK801, combined with the D1 dopamine agonist SKF 38393 in intact rats or administered alone in lesioned rats, also significantly reduced incidence of multisecond (2-60 s) periodic oscillatory activity. Amantadine, a drug currently used as an adjuvant agent in PD whose beneficial effects are commonly attributed to its noncompetitive NMDA antagonist properties, had effects that contrasted with those of MK801. In both intact and lesioned animals, amantadine significantly increased STN firing rates and total spike train power in the 8-50-Hz range and did not alter spike power in the 3-8-Hz range or multisecond oscillatory activity. These observations show that an effective noncompetitive NMDA antagonist such as MK801 induces modest change in STN activity in 6-hydroxydopamine (6-OHDA)-lesioned rats, with the most notable effect on multisecond periodicities in firing rate and theta frequency total spike power. Amantadine's effects differed from MK801's, raising questions about its primary mechanism of action and the role in PD pharmacotherapy of the STN rate increases induced by this drug.

Electrophysiological alterations in subthalamic neurons after unilateral dopamine depletion in the rat

The present study examined the effects of unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) on electrophysiological properties of subthalamic neurons (STN) in adult rats. Most neurons displayed regular spontaneous tonic firing patterns in both control and lesioned animals; however, the percentage of neurons with spontaneous burst firing at hyperpolarized membrane potentials was increased significantly in lesioned animals compared with controls (45% vs. 14% respectively). In the presence of bicuculline, a gamma-aminobutyric acid type A (GABAA) receptor antagonist, electrical stimulation of the internal capsule produced monosynaptic excitatory postsynaptic potentials (EPSPs) in almost all recorded neurons. DA (50 microM) increased the amplitude and/or duration of the EPSPs in neurons from both groups, whereas the DA D1 receptor agonist SKF 81297 (10 microM) produced a significant increase in amplitude and/or duration of EPSPs in neurons from the lesioned group only. This latter increase was blocked by pretreatment with the DA D1 antagonist SCH 23390 (10 microM). These data suggest that unilateral degeneration of DA neurons in the SNc changes firing properties and enhances electrophysiological responsiveness of STN neurons to activation of DA D1 receptors.

Subthalamic nucleus lesions alter basal and dopamine agonist stimulated electrophysiological output from the rat basal ganglia

The subthalamic nucleus (STN) is an important link in the "indirect" striatal efferent pathway. To assess its role on basal ganglia output via the substantia nigra pars reticulata (SNr), we monitored the single unit activities of SNr neurons in chloral hydrate-anesthetized rats 5-8 days after bilateral kainic acid lesions (0.75 microg/0.3 microl/side) of the STN. Consistent with loss of an excitatory input, the average basal firing rate of SNr neurons was significantly reduced in STN-lesioned animals. Moreover, the lesions modified the responses of SNr neurons to individual and concurrent stimulation of striatal D1 and D2 receptors. Bilateral striatal infusions of the D1/D2 agonist apomorphine (10 microg/microl/side) into the ventral-lateral striatum (VLS) were previously shown to cause significant increases in SNr cell firing (to 133% of baseline) in normal rats. However, in STN-lesioned rats, identical infusions caused no overall change in SNr activity (mean, 103% of basal rates). Conversely, selective stimulation of striatal D2 receptors by bilateral co-infusion of the D2 agonist quinpirole and the D1 antagonist SCH 23390 that previously caused little change in SNr firing in normal rats significantly inhibited their firing in STN-lesioned rats. Finally, the modest excitatory responses of SNr neurons to selective stimulation of striatal D1 receptors by co-infusions of SKF 82958 with the D2 antagonist YM09151-2 were not altered by lesions of the STN. These results implicate the STN as a mediator of excitatory response of SNr neurons to D2, and mixed D1/D2, dopamine receptor agonists in normal rats, and challenge conventional views on the role of the STN and the "indirect" pathway in regulating dopamine-stimulated output from the SNr.

Subthalamic high frequency stimulation induced rotations are differentially mediated by D1 and D2 receptors

High frequency stimulation (HFS) of the subthalamic nucleus (STN) has clinically emerged as a promising approach in the treatment of Parkinson's disease, epilepsy, dystonia as well as compulsive and possibly other mood disorders. The underlying mechanisms are incompletely understood, but are definitely related to high frequency and likely to involve the dopamine (DA)-system. To further test this hypothesis the present study investigated the modulation of STN-HFS-induced circling by systemic and intracerebral injection of drugs acting on DA receptors in naive freely moving rats. Within this experimental setup, unilateral STN-HFS alone induced intensity-dependent circling. Systemic injections of selective D1- (SCH-23390) and D2-((-)-sulpiride) antagonists as well as the mixed D1 and D2 agonist apomorphine dose-dependently reduced STN-HFS-induced rotational behavior. Intracerebral microinjections of (-)-sulpiride but not SCH-23390 decreased circling when injected intrastriatally and increased the number of rotations when injected intranigrally (pars reticulata (SNr)). These data reveal that STN-HFS-induced contralateral circling is differentially modulated by D1 and D2 receptors. While D2 receptor-mediated effects involve the dorso-/ventrolateral striatum and the SNr, D1 receptors probably exert their actions via brain areas outside the striatum and SNr. These findings suggest the nigrostriatal DA-system to be specifically involved in the mediation of STN-HFS-induced motor effects.

Calcium-dependent subthreshold oscillations determine bursting activity induced by N-methyl-D-aspartate in rat subthalamic neurons in vitro

We used whole-cell patch recordings in current clamp to investigate the ionic dependence of burst firing induced by N-methyl-d-aspartate (NMDA) in neurons of the subthalamic nucleus (STN) in slices of rat brain. NMDA (20 microm) converted single-spike firing to burst firing in 87% of STN neurons tested. NMDA-induced bursting was blocked by AP5 (50 microm), and was not mimicked by the non-NMDA receptor agonist AMPA (0.6 microm). Tetrodotoxin (1 microm) converted bursts to oscillations of membrane potential, which were most robust when oscillations ranged between -50 and -70 mV. The NMDA bursts were blocked by an elevated extracellular concentration of Mg(2+), but superfusate containing no added Mg(2+) either reduced or increased burst firing, depending upon the amount of intracellular current injection. Block of K(+) conductances by apamin and tetraethylammonium prolonged burst duration, but iberiotoxin had no effect. NMDA-induced burst firing and membrane oscillations were completely blocked by superfusate containing no added Ca(2+), and they were significantly reduced when patch pipettes contained BAPTA. Selective antagonists for T-type (mibefradil, 10 microm), L-type (nifedipine, 3 microm), and N-type (omega-conotoxin GVIA, 1 micro m) Ca(2+) channels had no effect on NMDA burst firing. Superfusate containing a low concentration of Na(+) (20 mm) completely abolished NMDA-induced burst firing. Flufenamic acid (10 microm), which blocks current mediated by Ca(2+)-activated nonselective cation channels (I(CAN)), reversibly abolished NMDA-depended bursting. These results are consistent with the hypothesis that NMDA-induced burst firing in STN neurons requires activation of either an I(CAN) or a Na(+)-Ca(2+) exchanger.

Dopamine receptor supersensitivity in rat subthalamus after 6-hydroxydopamine lesions

The subthalamic nucleus (STN) receives direct dopaminergic innervation from the substantia nigra pars compacta, but the importance of this input in the pathophysiology of parkinsonism remains to be determined. We used whole-cell patch-clamp recordings in brain slices to study presynaptic dopaminergic modulation of synaptic inputs to the STN in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. Here, we report that dopamine was more potent for inhibiting GABA IPSCs and glutamate EPSCs in the STN ipsilateral to the lesion, and was less potent for suppressing IPSCs and EPSCs in the STN contralateral to the lesion, compared with the effects of dopamine in control STN. Dopamine reduced IPSCs with an IC50 value of 20.9 +/- 3.6 microM in control STN, whereas IC50 values were 0.83 +/- 0.15 and 55.1 +/- 11.1 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Dopamine also inhibited EPSCs with an IC50 value of 12.8 +/- 2.8 microM in control STN, whereas IC50 values were 4.5 +/- 0.9 and 41.6 +/- 9.8 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Results with paired stimuli to evoke EPSCs and IPSCs suggest that endogenous dopamine acts presynaptically to inhibit transmitter release in the STN. These results show that chronic dopamine denervation significantly alters the regulation of synaptic input to the STN. Our results also suggest that the STN may be an important target for levodopa therapy in Parkinson's disease.

Autosomal dominant guanosine triphosphate cyclohydrolase I deficiency (Segawa disease)

Autosomal dominant guanosine triphosphate cyclohydrolase I (GCH-I) deficiency (Segawa disease) is a dopa-responsive dystonia caused by mutation of the GCH-I gene located on 14q22.1-q22.2. Neurohistochemical examination revealed a decrease of the tyrosine hydroxylase protein as well as its activity in the striatum and decrease of dopamine content, particularly in its ventral portion rich in D1 receptors (striatal direct pathways). Neuroimaging, clinical neurophysiological, and biochemical studies showed preservation of the structure and function of the terminal of the nigrostriatal DA neuron. Clinical neurophysiological studies showed no progressive decrement of DA activities. As the enzymatic activity of pteridine metabolism is highest in the early developmental course, it may modulate dopamine receptors maturing early in the developmental course. Its product, tetrahydrobiopterin, has higher affinity to tyrosine hydroxylase among hydroxylases. Thus, partial deficiency of tetrahydrobiopterin caused by heterozygous mutation of the GCH-I gene decreases dopamine activity rather selectively. This affects the DA receptors that mature early and demonstrates characteristic symptoms age-dependently along with the developmental decrement of the tyrosine hydroxylase activities at the terminals and the maturational processes of the projecting neurons of the basal ganglia. A difference in the ratio of mutant/wild-type GCH-I mRNA that depends on the locus of mutation may explain intrafamilial and interfamilial variation of phenotype.

Noradrenaline increases the firing rate of a subpopulation of rat subthalamic neurones through the activation of α1-adrenoceptors

In the rat subthalamic nucleus, which plays a critical role in the control of motor behaviour, specific binding of [3H]-prazosin was detected by radioligand binding to homogenates and by autoradiography in slices. [3H]-Prazosin binding to homogenates (Bmax 71 +/- 5 fmol/mg protein; Kd 0.27 +/- 0.05 nM) was competed for by alpha1-antagonists. In subthalamic nucleus slices and in the presence of 10 mM LiCl, noradrenaline (100 microM) produced a modest, but consistent, stimulation of [3H]-inositol phosphate accumulation (146 +/- 6% of basal), reversed by the alpha1-antagonist prazosin (1 microM). Extracellular single-unit recordings in slices showed that in a subpopulation (61 out of 94 cells) of rat subthalamic neurones with regular, single-spike firing pattern, noradrenaline induced a concentration-dependent increase in the firing rate (EC50 2.5 +/- 0.2 microM, maximum effect 272 +/- 33% of basal). The action of noradrenaline was mimicked by the selective alpha1-agonist phenylephrine but not by selective alpha2- or beta-agonists, and was blocked by the alpha1-antagonist prazosin but not by alpha2- or beta-antagonists. The excitatory effect of noradrenaline was not prevented by perfusion with low Ca2+/high Mg2+ solution. In four out of 11 neurones perfusion with 3 microM noradrenaline resulted in a shift from bursting to regular firing. Taken together, our results indicate that rat subthalamic neurones express alpha1-adrenoceptors responsible for noradrenaline-induced stimulation of the firing rate of a subpopulation of neurones. By modulating the spontaneous activity of STN neurones, noradrenergic pathways might have a significant role in regulating basal ganglia function and thus motor activity.

Excitation by dopamine of rat subthalamic nucleus neurones in vitro-a direct action with unconventional pharmacology

Recent anatomical and physiological studies have pointed to a functional innervation of the subthalamic nucleus by dopamine. This nucleus has a pivotal role in basal ganglia function and voluntary movement control and the possibility that dopamine, and dopaminergic medication used in Parkinson's disease, might directly influence its activity is of considerable interest. We have evaluated electrophysiologically the action and pharmacology of dopamine on single subthalamic neurones in rat brain slices. Dopamine increased firing rate to up to a mean of 60% in 98% of the 261 neurones tested when examined using extracellular single-unit recording. This excitation was unaffected by the GABA antagonist picrotoxin, and the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, and persisted in a low Ca(2+)/raised Mg(2+) solution, indicative of a direct action, independent of synaptic transmission. Of the 33 cells examined using whole patch-clamp recording, only 13 showed measurable increases in firing rate and/or depolarisations in response to dopamine. Dopamine-responsive cells displayed significantly greater access resistance, suggesting that an unidentified cytoplamic constituent, removed by whole-cell dialysis, was required for the response. Using extracellular recording, the D2-like dopamine receptor agonists quinpirole and bromocryptine, but not the D1-like receptor agonist 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol, also consistently caused an excitation. This was mimicked by the catecholamine releaser amphetamine in 60% of cells tested. However, the dopamine excitation was not significantly reduced either by the D1-like receptor antagonist 7-chloro8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine or the D2-like receptor antagonists (-)-sulpiride, eticlopride and (+)-butaclamol, and the quinpirole excitation was also unaffected by (-)-sulpiride. In contrast, (-)-sulpiride, eticlopride and (+)-butaclamol all abolished the D2-like receptor-mediated inhibition by dopamine of substantia nigra pars compacta neurones. The alpha-adrenoceptor antagonist phentolamine was a weak antagonist of dopamine excitations, but not of those caused by quinpirole. Dopamine excitations also showed weak sensitivity to the 5-HT(2) antagonist ritanserin, but were unaffected by the alpha(1)-adrenoceptor antagonist prazocin and the beta-adrenoceptor antagonist propranolol. The pharmacology of this dopamine excitation is inconsistent with an action on any known catecholamine receptor. However, the effect of amphetamine indicates that an unidentified monamine--possibly dopamine--can be released within the subthalamic nucleus to cause an excitation. The anomalies of its pharmacological characterisation do not strongly support a physiologically relevant direct action of dopamine in the rat subthalamic nucleus.

Correlated multisecond oscillations in firing rate in the basal ganglia: modulation by dopamine and the subthalamic nucleus

Previous studies from this laboratory have shown that many neurons in the basal ganglia have multisecond (<0.5 Hz) periodicities in firing rate in awake rats. The frequency and regularity of these oscillations are significantly increased by systemically injected dopamine (DA) agonists. Because oscillatory activity should have greater functional impact if shared by many neurons, the level of correlation of multisecond oscillations was assessed by recording pairs of neurons in the globus pallidus and substantia nigra pars reticulata in the same hemisphere, or pairs of globus pallidus neurons in opposite hemispheres in awake, immobilized rats. Cross-correlation (90-180 s lags) and spectral analysis were used to characterize correlated oscillations. Thirty-eight percent of pairs recorded in baseline (n=50) demonstrated correlated multisecond oscillations. Phase relationships were near 0 or 180 degrees. DA agonist injection significantly increased the incidence of correlation (intra- and interhemispheric) to 94% (n=17). After DA agonist injection, phase relationships of globus pallidus/substantia nigra neuron pairs were exclusively concentrated near 180 degrees, and phases of interhemispheric pairs of globus pallidus neurons were concentrated near 0 degrees. After subthalamic nucleus lesion (n=8), the incidence of correlated multisecond oscillations (or of multisecond oscillations per se) was not changed, although the consistent phase relationship between the globus pallidus and substantia nigra pars reticulata was disrupted. Subthalamic lesion also blocked apomorphine-induced decreases in oscillatory period and increases in oscillation amplitude, and significantly attenuated apomorphine-induced changes in mean firing rate. The data demonstrate that multisecond oscillations in the basal ganglia can be correlated between nuclei, and that DA receptor activation increases the level of correlation and organizes internuclear phase relationships at these multisecond time scales. While the subthalamic nucleus is not necessary for generating or transmitting these slow oscillations, it is involved in DA agonist-induced modulation of mean firing rate, oscillatory period, and internuclear phase relationship. These data further support a role for DA in modulating coherent oscillatory activity in the basal ganglia, and for the subthalamic nucleus in shaping the effects of DA receptor stimulation on basal ganglia output.

Cocaine-induced psychomotor activity is associated with its ability to induce c-fos mRNA expression in the subthalamic nucleus: effects of dose and repeated treatment

Factors that modulate the psychomotor activating effects of amphetamine and cocaine, such as environmental novelty and dose, also regulate the ability of these drugs to induce c-fos mRNA expression in the subthalamic nucleus (STN). We hypothesized therefore that engagement of the STN may be important for stimulant-induced psychomotor activation. To further test this hypothesis we examined whether repeated treatment with cocaine, which enhances its psychomotor activating effects (i.e. produces behavioural sensitization), also enhances its ability to induce c-fos expression in the STN. In addition, given that STN activity is thought to be influenced by preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen, we also examined whether repeated cocaine treatment alters c-fos expression in ENK+ cells. We report that: (i) cocaine pretreatment enhances the ability of a cocaine challenge to induce c-fos mRNA expression in the STN, and this effect is most robust at challenge doses where behavioural sensitization is observed; (ii) the ability of cocaine to induce c-fos in the STN is independent of the ability of cocaine to engage ENK+ cells. These results support the idea that the STN is involved in stimulant-induced psychomotor activation and sensitization, but suggest that stimulant-induced engagement of the STN is not dependent on ENK+ cells in the caudate-putamen. These findings may have implications concerning the neurobiological mechanisms underlying the behavioural effects of psychostimulant drugs.

Amphetamine-induced c-fos mRNA expression in the caudate-putamen and subthalamic nucleus: interactions between dose, environment, and neuronal phenotype

When administered in a novel environment relatively low doses of amphetamine induce c-fos mRNA in the subthalamic nucleus (STN) and in preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen (CPu). When administered at home, however, low doses of amphetamine do not produce these effects. Environmental novelty also facilitates the behavioral effects of acute and repeated amphetamine, but this is dose-dependent. The purpose of the present experiment therefore was to determine if the effect of context on amphetamine-induced c-fos expression is also dose-dependent. It was found that: (i) No dose of amphetamine tested (1-10 mg/kg) induced c-fos in many ENK+ cells when given at home. (ii) When given in a novel environment low to moderate doses of amphetamine (1-5 mg/kg) induced c-fos in substantial numbers of ENK+ cells, but the highest dose examined (10 mg/kg) did not. (iii) Environmental novelty enhanced the ability of low to moderate doses of amphetamine to induce c-fos in the STN, but the highest dose of amphetamine induced robust c-fos mRNA expression in the STN regardless of context. The results do not support the idea that engaging ENK+ cells, at least as indicated by c-fos mRNA expression, is critical to produce robust behavioral sensitization, but do suggest a possible role for the STN. Furthermore, the results highlight the importance of drug-environment interactions on the neurobiological effects of drugs, and have implications for thinking about the circuits by which context modulates the acute and long-lasting consequences of amphetamine treatment.

D5 (not D1) dopamine receptors potentiate burst-firing in neurons of the subthalamic nucleus by modulating an L-type calcium conductance

Dopamine is a crucial factor in basal ganglia functioning. In current models of basal ganglia, dopamine is postulated to act on striatal neurons. However, it may also act on the subthalamic nucleus (STN), a key nucleus in the basal ganglia circuit. The data presented here were obtained in brain slices using whole-cell patch clamp. They reveal that D5 dopamine receptors strengthen electrical activity in the subset of subthalamic neurons endowed with burst-firing capacity, resulting in longer discharges of spontaneous or evoked bursts. To distinguish between D1 and D5 subtypes, the action of agonists in the D1/D5 receptor family was first investigated on rat subthalamic neurons. Single-cell reverse transcription-PCR profiling showed that burst-competent neurons only expressed D5 receptors. Accordingly, receptors localized in postsynaptic membranes within the STN were labeled by a D5-specific antibody. Second, agonists in the D1/D5 family were tested in mouse brain slices. It was found that these agonists were active in D1 receptor knock-out mice in a similar way to wild-type mice or rats. This proved that D5 rather than D1 receptors were involved. Pharmacological tools (dihydropyridines, omega-conotoxins, and calciseptine) were used to identify the target of D5 receptors as an L-type channel. This was reached via G-protein and protein kinase A. The action of dopamine on D5 receptors therefore shapes neuronal activity. It contributes to normal information processing in basal ganglia outside striatum. This finding may be useful in drug therapy for various disorders involving changes in STN activity, such as Parkinson's disease and related disorders.

The switch of subthalamic neurons from an irregular to a bursting pattern does not solely depend on their GABAergic inputs in the anesthetic-free rat

The subthalamic nucleus (STN) powerfully controls basal ganglia outputs and has been implicated in movement disorders observed in Parkinson's disease because of its pathological mixed burst firing mode and hyperactivity. A recent study suggested that reciprocally connected glutamatergic STN and GABAergic globus pallidus (GP) neurons act in vitro as a generator of bursting activity in basal ganglia. In vivo, we reported that GP neurons increased their firing rate in wakefulness (W) compared with slow-wave sleep (SWS) without any change in their random pattern. In contrast, STN neurons exhibited similar firing rates in W and SWS, with an irregular pattern in W and a bursty one in SWS. Thus, the pallidal GABAergic tone might control the STN pattern. This hypothesis was tested by mimicking such variations with microiontophoresis of GABA receptor ligands. GABA agonists specifically decreased the STN firing rate but did not affect its firing pattern. GABA(A) (but not GABA(B)) antagonists strongly enhanced the STN mean discharge rate during all vigilance states up to three to five times its basal activity. However, such applications did not change the typical W random pattern. When applied during SWS, GABA(A) antagonists strongly reinforced the spontaneous bursty pattern into a particularly marked one with instantaneous frequencies reaching 500-600 Hz. SWS-W transitions occurring during ongoing antagonist iontophoresis invariably disrupted the bursty pattern into a random one. Thus GABA(A) receptors play a critical, but not exclusive, role in regulating the excitatory STN influence on basal ganglia outputs.

The monoamine reuptake blocker brasofensine reverses akinesia without dyskinesia in MPTP-treated and levodopa-primed common marmosets

The common marmoset develops motor deficits after MPTP treatment and exhibits dyskinesia after chronic levodopa (L-dopa) dosing and subsequent re-challenge with L-dopa and other dopaminergic agents. We report on the actions of the potent monoamine reuptake blocker brasofensine on motor disability, locomotor activity, and dyskinesia in the 1-methyl-4-1, 2,3,6-tetrahydropyridine (MPTP) -treated marmoset model of Parkinson's disease. Oral administration of brasofensine (0.25, 0.5, 1.0, or 2.5 mg/kg) to MPTP-treated marmosets produced a long-lasting, dose-dependent increase in locomotor activity and reduction in disability scores. In addition, coadministration of the lowest dose of brasofensine (0.25 mg/kg orally) with a threshold oral dose of L-dopa (2.5 mg/kg) caused a marked increase in locomotor activity, greater than that produced by either drug alone. In other MPTP-treated marmosets previously primed to exhibit dyskinesia by repeated L-dopa dosing, brasofensine effectively reversed akinesia with a naturalistic and prolonged motor response without the appearance of dyskinesia or stereotypy. This finding contrasts with the severe dyskinesia, stereotypy, and hyperkinesis produced by equivalent doses of L-dopa. The ability of brasofensine to produce a prolonged and naturalistic antiparkinsonian response without eliciting dyskinesia after previous L-dopa priming may relate to actions on D(1) receptor-linked pathways. These findings suggest that monoamine reuptake blockade may be of value in the treatment of Parkinson's disease, both early in the disease course and when L-dopa-induced dyskinesias complicate treatment.

Excitatory effects of dopamine on subthalamic nucleus neurons: in vitro study of rats pretreated with 6-hydroxydopamine and levodopa

Increased output from the subthalamic nucleus (STN) following chronic dopamine depletion has been linked to the rigidity and tremor seen in Parkinson's disease (PD). We used extracellular microelectrode recordings from rat brain slices to investigate effects of dopamine on STN neurons. In brain slices prepared from rats that received unilateral 6-hydroxydopamine (6-OHDA) treatment, the spontaneous firing rate of STN neurons was reduced by 63%, and the firing pattern was more irregular, compared to STN neurons from normal rats. However, treatment with levodopa (50 mg/kg, i.p., daily) for 4 weeks normalized the firing rate and pattern of STN neurons in the 6-OHDA-treated rats. Dopamine (3-300 microM), added to the superfusate, significantly increased the firing rates of STN neurons in a concentration-dependent fashion, and also produced a more regular firing pattern in 6-OHDA-lesioned tissue. This excitatory effect of dopamine was mimicked by a D2 receptor agonist (quinpirole), and was reduced by the D2 antagonists haloperidol, clozapine and sulpiride. Antagonists of the D1 receptor (SCH-23390) and ionotropic glutamatergic receptors (CNQX and AP5) could not block the effect of dopamine on firing rate. These results suggest that dopamine exerts a direct excitatory influence on STN neurons via the activation of D2-like receptors.

Nigrostriatal lesion and dopamine agonists affect firing patterns of rodent entopeduncular nucleus neurons

Altered activity of the entopeduncular nucleus, the rodent homologue of the globus pallidus internal segment in primates, is thought to mediate behavioral consequences of midbrain dopamine depletion in rodents. Few studies, however, have examined dopaminergic modulation of spiking activity in this nucleus. This study characterizes changes in entopeduncular neuronal activity after nigrostriatal dopaminergic lesion and the effects of systemic treatment with selective D(1) (SKF 38393) and D(2) (quinpirole) agonists in lesioned rats. Extracellular single-unit recordings were performed in awake immobilized rats, either in neurologically intact animals (n = 42) or in animals that had received unilateral 6-hydroxydopamine infusion into the medial forebrain bundle several weeks previously (n = 35). Nigrostriatal lesion altered baseline activity of entopeduncular neurons in several ways. Interspike interval distributions had significantly decreased modes and significantly increased coefficient of variation, skewness and kurtosis; yet interspike interval mean (the inverse of firing rate) was not affected. Also, spectral analysis of autocorrelograms indicated that lesion significantly reduced the incidence of regular-spiking neurons and increased the incidence of neurons with 4-18 Hz oscillations. Dopamine agonist treatment reversed some lesion-induced effects: quinpirole reversed changes in interspike interval distribution mode and coefficient of variation, while combined quinpirole and SKF 38393 blocked the appearance of 4-18 Hz oscillations. However, no agonist treatment normalized all aspects of entopeduncular activity. Additionally, inhibition of firing rates by D(1) or combined D(1)/D(2) receptor activation indicated that dopamine agonists affected the overall level of entopeduncular activity in a manner similar to that found in the substantia nigra pars reticulata and globus pallidus internal segment after dopamine neuron lesion. These data demonstrate that lesion of the nigrostriatal tract leads to modifications of several aspects of firing pattern in the rodent entopeduncular nucleus and so expand on similar findings in the rodent substantia nigra pars reticulata and in the globus pallidus internal segment in humans and nonhuman primates. The results support the view that dysfunction in the basal ganglia after midbrain dopamine neuron loss relates more consistently to abnormal activity patterns than to net changes in firing rate in the basal ganglia output nuclei, while overall decreases in firing rate in these structures may play a more important role in adverse motor reactions to dopamine agonist treatments.

Pharmacological identification of inward current evoked by dopamine in rat subthalamic neurons in vitro

Dopaminergic mechanisms in the subthalamic nucleus (STN) are implicated in the pathophysiology of Parkinson's disease. Here, electrophysiological responses of STN neurons to dopamine (DA) were investigated by using whole-cell patch-clamp recordings in the rat brain slice preparation. Under current-clamp, DA depolarized membrane potential and increased the frequency of spontaneous action potentials of STN neurons. Under voltage-clamp, DA (3-300 microM) produced a reversible concentration-dependent inward current (I(DA); 6-40 pA) with an EC(50) of 13 microM. This DA-induced current had a negative slope conductance which reversed at -102 mV. It was partially reduced by barium and by superfusion with an elevated concentration of extracellular K(+). Moreover, TTX and glutamate receptor antagonists (CNQX and AP5) did not significantly affect the DA responses, indicating that I(DA) is not dependent upon afferent synaptic activity in the STN. Quinpirole, a D(2) receptor agonist, mimicked the DA action more effectively than did the D(1) agonist SKF-38393. The D(2) antagonist sulpiride, but not the D(1) antagonist SCH-23390, blocked responses induced by DA. Intracellular application of G-protein inhibitor GDP-beta-S also suppressed I(DA). GTP-gamma-S, added to the pipette solution, evoked a sustained inward shift in the absence of DA. These results suggest that DA increases the activity of STN neurons via activation of G-protein-coupled D(2)-like receptors which reduce a K(+) conductance.

Amphetamine-induced Fos expression is evident in gamma-aminobutyric acid neurons in the globus pallidus and entopeduncular nucleus in rats treated with intrastriatal c-fos antisense oligodeoxynucleotides

Double immunostaining for Fos and gamma-aminobutyric acid (GABA) was used in a previously established animal model of striatal dysfunction to examine whether GABA-immunoreactive neurons in the globus pallidus (GP) and entopeduncular nucleus (EP) are activated to express Fos immunoreactivity by intraperitoneal injection of amphetamine. Striatal efferent activity was suppressed by intrastriatal infusions of antisense oligodeoxynucleotide targeted to the messenger RNA of the immediate early gene, c-fos. This suppression produced robust rotational behavior and expression of Fos in the ipsilateral GP and EP following amphetamine challenge. The expression of Fos in the ipsilateral GP and EP following amphetamine challenge is not observed in naïve or control antisense-treated animals. Quantitative analysis revealed that a majority of the amphetamine-activated (Fos-immunoreactive) neurons in the GP and EP express GABA. The present results suggest that inhibitory GABAergic projection neurons within these two nuclei are regulated by inhibitory striatal output and suggests that decreased inhibitory striatal output may contribute to the motor dysfunction observed in patients with Huntington's disease.