Subthalamic nucleus lesion regularizes firing patterns in globus pallidus and substantia nigra pars reticulata neurons in rats

Present and future of subthalamotomy in the management of Parkinson´s disease: a systematic review

Introduction: The subthalamic nucleus (STN) is known to be involved in the pathophysiology of Parkinson´s disease and by reducing its abnormal activity, normal output of basal ganglia can be restored along with improvement in PD cardinal motor features. Deep brain stimulation of the STN is currently the main surgical procedure for PD with motor complications, but lesioning can be an alternative.Areas covered: Here, the authors systematically review the current evidence regarding subthalamotomy both with radiofrequency and, more recently, with focused ultrasound (FUS) for the treatment of PD.Expert opinion: Unilateral subthalamotomy for the treatment of PD motor features can be considered a viable option in asymmetric patients, particularly with FUS which allows a minimally invasive safe and effective ablation of the STN. Risk of inducing dyskinesia (i.e., hemichorea/ballism) may be strikingly reduced when lesions enlarge dorsally to impinge on pallidothalamic fibers.

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.

Phosphoinositol metabolism affects AMP kinase-dependent K-ATP currents in rat substantia nigra dopamine neurons

We reported recently that ligand-gated ATP-sensitive K⁺ (K-ATP) current is potentiated by AMP-activated protein kinase (AMPK) in rat substantia nigra compacta (SNC) dopamine neurons. Because phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) regulates K-ATP current, we explored the hypothesis that changes in PI(4,5)P2 modify the ability of AMPK to augment K-ATP current. To influence PI(4,5)P2 levels, we superfused brain slices with phospholipase C (PLC) activators and inhibitors while recording whole-cell currents in SNC dopamine neurons. Diazoxide, superfused for 5 min every 20 min, evoked K-ATP currents that, on average, increased from 38 pA at first application to 122 pA at the fourth application, a 220% increase. This enhancement of diazoxide-induced current was AMPK dependent because K-ATP current remained at baseline when slices were superfused with either the AMPK inhibitor dorsomorphin or the upstream kinase inhibitor STO-609. The PLC inhibitor U73122 significantly increased diazoxide current over control values, and this increase was blocked by dorsomorphin. Enhancement of diazoxide-induced current was also completely prevented by the PLC activator m-3M3FBS. Agonists at 5-HT_2C and group I metabotropic glutamate receptors, both of which activate PLC, also prevented augmentation of diazoxide-induced current. Finally, inhibition of spike discharges by diazoxide was significantly antagonized by m-3M3FBS. These results suggest that PLC activity significantly influences the inhibitory effect of K-ATP channels by altering PI(4,5)P2 content. Results also suggest that modification of K-ATP current by PLC requires AMPK activity.

Subthalamotomy in the treatment of Parkinson's disease: clinical aspects and mechanisms of action

Parkinson's disease (PD) is a neurodegenerative condition that can be pharmacologically treated with levodopa. However, important motor and nonmotor symptoms appear with its long-term use. The subthalamic nucleus (STN) is known to be involved in the pathophysiology of PD and to contribute to levodopa-induced complications. Surgery is considered in patients who have advanced PD that is refractory to pharmacotherapy and who display disabling dyskinesia. Deep brain stimulation of the STN is currently the main surgical procedure for PD, but lesioning is still performed. This review covers the clinical aspects and complications of subthalamotomy as one of the lesion-based options for PD patients with levodopa-induced dyskinesias. Moreover, the authors discuss the possible effects of subthalamic lesioning.

Presynaptic serotonergic gating of the subthalamonigral glutamatergic projection

The GABAergic projection neurons in the substantia nigra pars reticulata (SNr) are key basal ganglia output neurons. The activity of these neurons is critically influenced by the glutamatergic projection from the subthalamic nucleus (STN). The SNr also receives an intense serotonin (5-HT) innervation, raising the possibility that 5-HT may regulate the STN→SNr glutamatergic transmission and the consequent STN-triggered spike firing in SNr neurons. Here we show that 5-HT reduced STN stimulation-evoked long-lasting polysynaptic complex EPSCs in SNr GABA neurons. This inhibitory 5-HT effect was mimicked by the 5-HT1B receptor agonist CP93129 and blocked by the 5-HT1B antagonist NAS-181. 5-HT1A receptor ligands were ineffective. Additionally, 5-HT and CP93129 reduced the frequency but not the amplitude of miniature EPSCs, suggesting a reduced vesicular release. 5-HT and CP93129 also decreased the amplitude but increased the paired pulse ratio of the monosynaptic EPSCs in SNr GABA neurons, indicating a presynaptic 5-HT1B receptor-mediated inhibition of glutamate release. Furthermore, 5-HT and CP93129 inhibited STN-triggered burst firing in SNr GABA neurons, and CP93129's inhibitory effect was strongest when puffed to STN→SNr axon terminals in SNr, indicating a primary role of the 5-HT1B receptors in these axon terminals. Finally, the 5-HT1B receptor antagonist NAS-181 increased the STN-triggered complex EPSCs and burst firing in SNr GABA neurons, demonstrating the effects of endogenous 5-HT. These results suggest that nigral 5-HT, via presynaptic 5-HT1B receptor activation, gates the excitatory STN→SNr projection, reduces burst firing in SNr GABA neurons, and thus may play a critical role in movement control.

Intrinsic and integrative properties of substantia nigra pars reticulata neurons

The GABA projection neurons of the substantia nigra pars reticulata (SNr) are output neurons for the basal ganglia and thus critical for movement control. Their most striking neurophysiological feature is sustained, spontaneous high frequency spike firing. A fundamental question is: what are the key ion channels supporting the remarkable firing capability in these neurons? Recent studies indicate that these neurons express tonically active type 3 transient receptor potential (TRPC3) channels that conduct a Na-dependent inward current even at hyperpolarized membrane potentials. When the membrane potential reaches -60 mV, a voltage-gated persistent sodium current (I(NaP)) starts to activate, further depolarizing the membrane potential. At or slightly below -50 mV, the large transient voltage-activated sodium current (I(NaT)) starts to activate and eventually triggers the rapid rising phase of action potentials. SNr GABA neurons have a higher density of I(NaT), contributing to the faster rise and larger amplitude of action potentials, compared with the slow-spiking dopamine neurons. I(NaT) also recovers from inactivation more quickly in SNr GABA neurons than in nigral dopamine neurons. In SNr GABA neurons, the rising phase of the action potential triggers the activation of high-threshold, inactivation-resistant Kv3-like channels that can rapidly repolarize the membrane. These intrinsic ion channels provide SNr GABA neurons with the ability to fire spontaneous and sustained high frequency spikes. Additionally, robust GABA inputs from direct pathway medium spiny neurons in the striatum and GABA neurons in the globus pallidus may inhibit and silence SNr GABA neurons, whereas glutamate synaptic input from the subthalamic nucleus may induce burst firing in SNr GABA neurons. Thus, afferent GABA and glutamate synaptic inputs sculpt the tonic high frequency firing of SNr GABA neurons and the consequent inhibition of their targets into an integrated motor control signal that is further fine-tuned by neuromodulators including dopamine, serotonin, endocannabinoids, and H₂O₂.

The Subthalamic Nucleus becomes a Generator of Bursts in the Dopamine-Depleted State. Its High Frequency Stimulation Dramatically Weakens Transmission to the Globus Pallidus

Excessive burst firing in the dopamine-depleted basal ganglia correlates with severe motor symptoms of Parkinson's disease that are attenuated by high frequency electrical stimulation of the subthalamic nucleus (STN). Here we test the hypothesis that pathological bursts in dopamine-deprived basal ganglia are generated within the STN and transmitted to globus pallidus neurons. To answer this question we recorded excitatory synaptic currents and potentials from subthalamic and pallidal neurons in the basal ganglia slice (BGS) from dopamine-depleted mice while continuously blocking GABA_A receptors. In control mice, a single electrical stimulus delivered to the internal capsule or the rostral pole of the STN evoked a short duration, small amplitude, monosynaptic EPSC in subthalamic neurons. In contrast, in the dopamine-depleted BGS, this monosynaptic EPSC was amplified and followed by a burst of polysynaptic EPSCs that eventually reverberated three to seven times, providing a long lasting response that gave rise to bursts of EPSCs and spikes in GP neurons. Repetitive (10–120 Hz) stimulation delivered to the STN in the dopamine-depleted BGS attenuated STN-evoked bursts of EPSCs in pallidal neurons after several minutes of stimulation but only high frequency (90–120 Hz) stimulation replaced them with small amplitude EPSCs at 20 Hz. We propose that the polysynaptic pathway within the STN amplifies subthalamic responses to incoming excitation in the dopamine-depleted basal ganglia, thereby transforming the STN into a burst generator and entraining pallidal neurons in pathogenic bursting activities. High frequency stimulation of the STN prevents the transmission of this pathological activity to globus pallidus and imposes a new glutamatergic synaptic noise on pallidal neurons.

Subthalamic nucleus evokes similar long lasting glutamatergic excitations in pallidal, entopeduncular and nigral neurons in the basal ganglia slice

The subthalamic nucleus (STN) modulates the activity of globus pallidus (GP), entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr) neurons via its direct glutamatergic projections. To investigate the mechanism by which STN affects activity in these structures and whether STN induced activity is comparable among STN target neurons, we performed patch clamp recordings in a tilted, parasagittal, basal ganglia slice (BGS) that preserves these functional connections. We report that single, brief stimulation of the STN generates a brief monosynaptic AMPA-mediated excitatory postsynaptic current (EPSC) in GP, EP and SNr neurons. A higher intensity, supra-threshold activation evokes a compound EPSC consisting of an early monosynaptic component followed by a slow inward NMDA-mediated current with an overlying barrage of AMPA-mediated EPSCs. These late EPSCs were polysynaptic and gave rise to bursts of spikes that lasted several hundreds of milliseconds. They were eliminated by surgical removal of the STN from the BGS slice, indicating that the STN is required for their generation. Reconstruction of biocytin-filled STN neurons revealed that a third of STN neurons project intra-STN axon collaterals that may underlie polysynaptic activity. We propose that activation of the STN yields comparable long lasting excitations in its target neurons by means of a polysynaptic network.

Complex EPSCs evoked in substantia nigra reticulata neurons are disrupted by repetitive stimulation of the subthalamic nucleus

Although substantia nigra reticulata (SNR) neurons fire bursts of action potentials during normal movement, excessive burst firing correlates with symptoms of Parkinson's disease. A major excitatory output from the subthalamic nucleus (STN) to the SNR is thought to provide the synaptic impetus for burst firing in SNR neurons. Using patch pipettes to record from SNR neurons in rat brain slices, we found that a single electrical stimulus delivered to the STN evokes a burst of action potentials. Under voltage-clamp conditions, STN stimulation evokes a complex EPSC that is comprised of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a long-lasting inward current. Using varied stimulation frequencies, we found that the initial EPSC was significantly reduced or abolished after 2 s of 50-100 Hz STN stimulation. However, only 4 s of 1 Hz stimulation was required to abolish the late component of the complex EPSC. We suggest that differential effects of repetitive STN stimulation on early and late components of complex EPSCs may help explain the frequency-dependent effects of deep brain stimulation of the STN that is used in the treatment of Parkinson's disease.

Subthalamic stimulation evokes complex EPSCs in the rat substantia nigra pars reticulata in vitro

The subthalamic nucleus (STN) plays an important role in movement control by exerting its excitatory influence on the substantia nigra pars reticulata (SNR), a major output structure of the basal ganglia. Moreover, excessive burst firing of SNR neurons seen in Parkinson's disease has been attributed to excessive transmission in the subthalamonigral pathway. Using the 'blind' whole-cell patch clamp recording technique in rat brain slices, we found that focal electrical stimulation of the STN evoked complex, long-duration excitatory postsynaptic currents (EPSCs) in SNR neurons. Complex EPSCs lasted 200-500 ms and consisted of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a slow inward shift in holding current. Focal stimulation of regions outside the STN failed to evoke complex EPSCs. The late component of complex EPSCs was markedly reduced by ionotropic glutamate receptor antagonists (2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitro-quinoxalone) and by a GABAA receptor agonist (isoguvacine) when these agents were applied directly to the STN using a fast-flow microapplicator. Moreover, the complex EPSC was greatly enhanced by bath application of the GABAA receptor antagonists picrotoxin or bicuculline. These data suggest that recurrent glutamate synapses in the STN generate polysynaptic, complex EPSCs that are under tonic inhibition by GABA. Because complex EPSCs are expected to generate bursts of action potentials in SNR neurons, we suggest that complex EPSCs may contribute to the pathological burst firing that is associated with the symptoms of Parkinson's disease.

Models of the subthalamic nucleus. The importance of intranuclear connectivity

A coherent set of models is presented that provide novel and testable predictions about the functional role of the subthalamic nucleus (STN) in the basal ganglia. The STN is emerging as an important target for novel therapeutic strategies for the alleviation of Parkinsonian type symptoms [Lancet 345 (1995) 91; Science 249 (1990) 1436]. Computational and mathematical models based on the properties of the STN and its interactions are reviewed. These models focus on core anatomical and physiological data that span many levels. By assessing models of anatomy, dynamic network models, and a detailed model of a recent pharmacological experiment, we can expose the primary modes of STN function and highlight their underlying properties. We show that the presence of functional interactions between STN projection neurons is critical in defining its behaviour and how it interacts with other basal ganglia nuclei. Pulses or switch-like activity patterns emerge in the models as a consequence of these local interactions. Furthermore, the models demonstrate that this behaviour can break down under abnormal conditions resulting in low frequency bursting oscillations. Such oscillations may play a role in symptoms of Parkinson's disease.

High frequency stimulation of the subthalamic nucleus eliminates pathological thalamic rhythmicity in a computational model

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) or the internal segment of the globus pallidus (GPi) has recently been recognized as an important form of intervention for alleviating motor symptoms associated with Parkinson's disease, but the mechanism underlying its effectiveness remains unknown. Using a computational model, this paper considers the hypothesis that DBS works by replacing pathologically rhythmic basal ganglia output with tonic, high frequency firing. In our simulations of parkinsonian conditions, rhythmic inhibition from GPi to the thalamus compromises the ability of thalamocortical relay (TC) cells to respond to depolarizing inputs, such as sensorimotor signals. High frequency stimulation of STN regularizes GPi firing, and this restores TC responsiveness, despite the increased frequency and amplitude of GPi inhibition to thalamus that result. We provide a mathematical phase plane analysis of the mechanisms that determine TC relay capabilities in normal, parkinsonian, and DBS states in a reduced model. This analysis highlights the differences in deinactivation of the low-threshold calcium T -current that we observe in TC cells in these different conditions. Alternative scenarios involving convergence of thalamic signals in the cortex are also discussed, and predictions associated with these results, including the occurrence of rhythmic rebound bursts in certain TC cells in parkinsonian states and their drastic reduction by DBS, are stated. These results demonstrate how DBS could work by increasing firing rates of target cells, rather than shutting them down.

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.

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.

Deep brain stimulation in epilepsy

Since the pioneering studies of Cooper et al. to influence epilepsy by cerebellar stimulation, numerous attempts have been made to reduce seizure frequency by stimulation of deep brain structures. Evidence from experimental animal studies suggests the existence of a nigral control of the epilepsy system. It is hypothesized that the dorsal midbrain anticonvulsant zone in the superior colliculi is under inhibitory control of efferents from the substantia nigra pars reticulata. Inhibition of the subthalamic nucleus (STN) could release the inhibitory effect of the substantia nigra pars reticulata on the dorsal midbrain anticonvulsant zone and thus activate the latter, raising the seizure threshold. Modulation of the seizure threshold by stimulation of deep brain structures-in particular, of the STN-is a promising future treatment option for patients with pharmacologically intractable epilepsy. Experimental studies supporting the existence of the nigral control of epilepsy system and preliminary results of STN stimulation in animals and humans are reviewed, and alternative mechanisms of seizure suppression by STN stimulation are discussed.

Time-course of changes in firing rates and firing patterns of subthalamic nucleus neuronal activity after 6-OHDA-induced dopamine depletion in rats

The subthalamic nucleus (STN) plays a key role in motor control. Disorganization of its neuronal activity is implicated in the manifestation of parkinsonian motor symptoms. The aim of the present work was to study the time-course of changes in the firing activity of STN neurons in a rat model of parkinsonism. Electrophysiological recordings were done in normal rats and four groups of rats at different time points after 6-hydroxydopamine (6-OHDA) microinjection into the pars compacta of substantia nigra (SNc). Results showed a significant decrease in firing rate during the first and second weeks post lesion (5.53+/-0.56 and 7.66+/-0.73 spikes/s, respectively) compared to normal rats (11.13+/-0.59 spikes/s). From the 3rd week after 6-OHDA injection the firing rates returned toward baseline, with an average of 9.71+/-0.51 spikes/s during the 3rd week and 11.13+/-0.71 spikes/s during the 4th week. With regard to firing pattern, the majority of STN cells (90%) discharged regularly or slightly irregularly in normal animals. Only 4% exhibited burst activity and 6% had mixed firing patterns. After SNc-lesion, the percentage of cells exhibiting burst and mixed patterns increased progressively from 35% during the first week to 56% at week 4 post-lesion. In sum, these experiments revealed that the firing rate of STN neurons was altered only transiently following nigral lesions, whereas a progressive and stable change in the firing pattern was observed up to 4 weeks post lesion, suggesting that the persistence of bursts firing more closely relates to the motor pathologies of this rat model of parkinsonism.

Synaptic localization of ionotropic glutamate receptors in the rat substantia nigra

Glutamatergic neurotransmission in the substantia nigra pars compacta and pars reticulata is mediated through N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxaline propionic acid/kainate (AMPA) type receptors as well as other glutamate receptors and is critical for basal ganglia functioning. A major glutamatergic input to the substantia nigra originates in the subthalamic nucleus, and the long-lasting stimulation of the dopaminergic cells of the substantia nigra pars compacta by the subthalamic neurons has been implicated in the pathophysiology of Parkinson's disease. The objectives of the present study were to determine the subcellular and subsynaptic localization of subunits of the N-methyl-D-aspartate and AMPA receptors in the substantia nigra, and also to determine whether co-localization of N-methyl-D-aspartate and AMPA receptor subunits occur at individual synapses. To achieve this, pre-embedding and post-embedding immunocytochemistry was applied to sections of substantia nigra using antibodies that recognize the NR1 and NR2A/B subunits of the N-methyl-D-aspartate receptor, and GluR2/3 subunits of the AMPA receptor. In both regions of the substantia nigra, immunolabelling for each of the subunits was observed in numerous perikarya and proximal dendrites. At the subcellular level, silver-intensified immunogold particles localizing N-methyl-D-aspartate and AMPA receptor subunits were most commonly present within dendrites where they were associated with a variety of intracellular organelles and with the internal surface of the plasma membrane. Post-embedding immunogold labelling revealed immunoparticles labelling for NR1, NR2A/B and GluR2/3 to be enriched at asymmetric synaptic specializations, although a large proportion of asymmetric synapses were immunonegative. Double immunolabelling revealed, in addition to single-labelled synapses, the co-localization of subunits of the N-methyl-D-aspartate receptor and subunits of the AMPA receptor at individual asymmetric synapses. Similarly, double immunolabelling also revealed the co-localization of the NRl and NR2A/B subunits of the N-methyl-D-aspartate receptor at individual asymmetric synapses. Labelling for NR1 and GluR2/3 was, on average, relatively evenly distributed across the width of the synapse with a gradual reduction towards the periphery when analysed in single sections. In summary, the present results demonstrate that AMPA and N-methyl-D-aspartate receptors are selectively localized at a subpopulation of asymmetric synapses in the substantia nigra pars compacta and reticulata and that the two receptor types, at least partially co-localize at individual synapses. It is concluded that glutamatergic transmission in the substantia nigra pars compacta and pars reticulata occurs primarily at asymmetric synapses and, at least in part, is mediated by both N-methyl-D-aspartate and AMPA receptors.

Lesion of the subthalamic nucleus or globus pallidus does not cause chaotic firing patterns in basal ganglia neurons in rats

The basal ganglia appears to play an important role in behavioral selection. One model (Berns and Sejnowski's) of basal ganglia function argues that the subthalamic nucleus plays a critical role in this selection process and predicts that the subthalamic nucleus prevents the basal ganglia and its re-entrant circuits with the thalamus and cerebral cortex from developing chaotic oscillations. We tested this prediction by generating three-dimensional sequential interval state space plots of the spike trains from 684 globus pallidus, substantia nigra pars reticulata and subthalamic neurons recorded in intact, subthalamic lesioned and globus pallidus lesioned rats, neurons which had previously been analyzed with more standard statistical methods. Only 1 neuron (a globus pallidus neuron in a subthalamic lesioned rat) of the 684 showed a chaotic attractor. In no case did subthalamic nucleus lesion induce a chaotic firing pattern elsewhere in the basal ganglia.

Excitatory cortical inputs to pallidal neurons via the subthalamic nucleus in the monkey

How the motor-related cortical areas modulate the activity of the output nuclei of the basal ganglia is an important issue for understanding the mechanisms of motor control by the basal ganglia. In the present study, by using awake monkeys, the polysynaptic effects of electrical stimulation in the forelimb regions of the primary motor and primary somatosensory cortices on the activity of globus pallidus (GP) neurons, especially mediated by the subthalamic nucleus (STN), have been characterized. Cortical stimulation induced an early, short-latency excitation followed by an inhibition and a late excitation in neurons of both the external and internal segments of the GP. It also induced an early, short-latency excitation followed by a late excitation and an inhibition in STN neurons. The early excitation in STN neurons preceded that in GP neurons. Blockade of STN neuronal activity by muscimol (GABA(A) receptor agonist) injection resulted in abolishment of both the early and late excitations evoked in GP neurons by cortical stimulation. At the same time, the spontaneous discharge rate of GP neurons decreased, pauses between the groups of spikes of GP neurons became prominent, and the firing pattern became regular. Injection of (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) [N-methyl-D-aspartate (NMDA) receptor antagonist], but not 1,2,3, 4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium [NBQX (non-NMDA receptor antagonist)], into the STN attenuated the early and late excitations in GP neurons, suggesting that cortico-subthalamic transmission is mediated mainly by NMDA receptors. Interference with the pallido-subthalamic transmission by bicuculline (GABA(A) receptor antagonist) injection into the STN made the inhibition distinct without affecting the early excitation. The present results indicate that the cortico-subthalamo-pallidal pathway conveys powerful excitatory effects from the motor-related cortical areas to the GP with shorter conduction time than the effects conveyed through the striatum.

Relationship of activity in the subthalamic nucleus-globus pallidus network to cortical electroencephalogram

One of the functions of the excitatory subthalamic nucleus (STN) is to relay cortical activity to other basal ganglia structures. The response of the STN to cortical input is shaped by inhibition from the reciprocally connected globus pallidus (GP). To examine the activity in the STN-GP network in relation to cortical activity, we recorded single and multiple unit activity in STN and/or GP together with cortical electroencephalogram in anesthetized rats during various states of cortical activation. During cortical slow-wave activity (SWA), STN and GP neurons fired bursts of action potentials at frequencies that were similar to those of coincident slow ( approximately 1 Hz) and spindle (7-14 Hz) cortical oscillations. Spontaneous or sensory-driven global activation was associated with a reduction of SWA and a shift in STN-GP activity from burst- to tonic- or irregular-firing. Rhythmic activity in STN and GP neurons was lost when the cortex was inactivated by spreading depression and did not resume until SWA had recovered. Although rhythmic STN-GP activity was correlated with SWA, the phase relationships of activities of neurons within the STN and GP and between the nuclei were variable. Even when neurons displayed synchronous bursting activity, correlations on the millisecond time scale, which might indicate shared synaptic input, were not observed. These data indicate that (1) STN and GP activity is intimately related to cortical activity and hence the sleep-wake cycle; (2) rhythmic oscillatory activity in the STN-GP network in disease states may be driven by the cortex; and (3) activity of the STN-GP network is regulated in space in a complex manner.

High-frequency stimulation of the subthalamic nucleus suppresses experimental resting tremor in the monkey

The effect of high-frequency stimulation of the subthalamic nucleus on parkinsonian-like resting tremor was investigated in two monkeys (Macaca fascicularis). Unilateral tremor of the arm and leg was induced by electrical coagulation of the brainstem area including the substantia nigra and the red nucleus. The tremor was only seen at rest condition with a very stable frequency of 4.46+/-0.59 Hz (mean+/-S.D.). Apomorphine (0.10-0.4 mg/kg, s.c.) completely blocked the tremor, suggesting that it was a dopaminergic-dependent symptom just like the parkinsonian tremor. When the stimulating frequency varied from 20 to 1000 Hz, both mono- and bipolar stimulation (square pulses, 0-5 mA, 0.06 ms) of the subthalamic nucleus suppressed resting tremor in a frequency-dependent manner but monopolar stimulation was more effective. These effects remained stable for more than two years. The present results suggest that the subthalamic nucleus is involved in the control and mechanism of resting tremor and that the high-frequency stimulation of the subthalamic nucleus can be used as an alternative therapy in parkinsonian patients with akinesia, rigidity and resting tremor.

Selective innervation of neostriatal interneurons by a subclass of neuron in the globus pallidus of the rat

A subpopulation of neurons in the globus pallidus projects to the neostriatum, which is the major recipient of afferent information to the basal ganglia. Given the moderate nature of this projection, we hypothesized that the pallidostriatal projection might exert indirect but powerful control over principal neuron activity by targeting interneurons, which comprise only a small percentage of neostriatal neurons. This was tested by the juxtacellular labeling and recording of pallidal neurons in combination with immunolabeling of postsynaptic neurons. In addition to innervating the subthalamic nucleus and output nuclei, 6 of 23 labeled pallidal neurons projected to the neostriatum. Both the firing characteristics and the extent of the axonal arborization in the neostriatum were variable. However, light and electron microscopic analysis of five pallidostriatal neurons revealed that each neuron selectively innervated neostriatal interneurons. A large proportion of the boutons of an individual axon (19-66%) made contact with parvalbumin-immunoreactive interneurons. An individual parvalbumin-immunoreactive neuron (n = 27) was apposed on average by 6.7 boutons (SD = 6.1) from a single pallidal axon (n = 2). Individual pallidostriatal boutons typically possessed more than one symmetrical synaptic specialization. In addition, 3-32% of boutons of axons from four of five pallidal neurons contacted nitric oxide synthase-immunoreactive neurons. Descending collaterals of pallidostriatal neurons were also found to make synaptic contact with dopaminergic and GABAergic neurons of the substantia nigra. These data imply that during periods of cortical activation, individual pallidal neurons may influence the activity of GABAergic interneurons of the neostriatum (which are involved in feed-forward inhibition and synchronization of principle neuron activity) while simultaneously patterning neuronal activity in basal ganglia downstream of the neostriatum.

A massively connected subthalamic nucleus leads to the generation of widespread pulses

A composite model of the subthalamic nucleus is developed from physiological and anatomical considerations. First, study of a geometric model of the anatomical arrangements of projection neurons within the nucleus indicates that they form a massively connected network. Second, given the excitatory nature of these neurons, their threshold and peak firing rates, a simple model of neuron responses reveals that large regions of this highly interconnected nucleus can respond to excitatory input in the form of a wide-spread uniform pulse. Such widespread pulses of activity may act as a braking signal that resets the major basal ganglia output nuclei.

Targeting the subthalamic nucleus in the treatment of Parkinson's disease

As more is learnt about the functional implications of basal ganglia connectivity, the role of the subthalamic nucleus as a target site for stereotactic procedures in the amelioration of the symptoms of Parkinson's disease is becoming clearer. A comparison of various neurosurgical procedures in the disease is discussed in relation to current thinking about circuitry. Experimental investigations involving lesioning or stimulation of the subthalamic nucleus in nonhuman primate models and in clinical studies of Parkinson's disease are compared. Neurosurgical procedures that lesion structures bilaterally are more likely to induce side effects than is deep-brain stimulation, which has the added advantage of reversibility and which is more amenable to titration in relation to medication and dosage. A small but growing number of parkinsonian patients have received subthalamic stimulation either unilaterally or bilaterally. Stimulation of the subthalamic nucleus ameliorates tremor, rigidity and hypokinesia, as opposed to thalamic stimulation which is probably best reserved for tremor-dominant patients. Such procedures also do not involve the same complex technical and ethical issues that are associated with foetal mesencephalic grafting. Although subthalamic stimulation shows great promise, it has not been developed to the point where it can be used as more than an experimental treatment. Further experimental research is required before the new strategies can be used on a larger scale.

Behavioural effects of subthalamic nucleus lesions in the hemiparkinsonian marmoset (Callithrix jacchus)

Recent studies in non-human primates support a role for the subthalamic nucleus in the expression of parkinsonian symptomatology, and it has been proposed that subthalamic lesions may provide a surgical treatment for the symptoms of Parkinson's disease in humans. We have applied a broad range of behavioural tests to characterize the effects of lesions of the subthalamic nucleus on parkinsonian symptoms in the unilateral 6-hydroxydopamine (6-OHDA) lesioned marmoset (Callithrix jacchus). Thirteen marmosets were trained on a battery of behavioural tasks that were conducted at regular intervals before and after surgery. All received unilateral 6-OHDA lesions to the medial forebrain bundle. Seven animals were then given an additional N-methyl-D-aspartate lesion of the ipsilateral subthalamic nucleus, whereas the remaining six animals received a variety of control or sham lesions to the nucleus. The 6-OHDA lesions induced a strong ipsilateral bias in head position; mild-moderate ipsilateral rotation spontaneously and after injection of saline or amphetamine; and contralateral rotation after injection of apomorphine. Hemineglect was evident as delayed initiation of reaches on the contralateral side on the staircase reaching task. Additional subthalamic lesions significantly reversed the bias in head position from ipsilateral to contralateral and decreased neglect as evidenced by improved latencies to initiate reaching on the contralateral side at the staircase. However, deficits in skilled movements persisted in the subthalamic nucleus lesion group in that they did not complete the staircase task any faster than the control group and remained impaired on another task which required reaching into tubes. These behavioural effects demonstrate that excitotoxic lesioning of the subthalamic nucleus can ameliorate some, but not all, parkinsonian-like deficits in the unilateral 6-OHDA lesioned marmoset.

A computational model of how the basal ganglia produce sequences

We propose a systems-level computational model of the basal ganglia based closely on known anatomy and physiology. First, we assume that the thalamic targets, which relay ascending information to cortical action and planning areas, are tonically inhibited by the basal ganglia. Second, we assume that the output stage of the basal ganglia, the internal segment of the globus pallidus (Gpi), selects a single action from several competing actions via lateral interactions. Third, we propose that a form of local working memory exists in the form of reciprocal connections between the external globus pallidus (Gpe) and the subthalamic nucleus (STN). As a test of the model, the system was trained to learn a sequence of states that required the context of previous actions. The striatum, which was assumed to represent a conjunction of cortical states, directly selected the action in the GP during training. The STN-to-GP connection strengths were modified by an associative learning rule and came to encode the sequence after 20 to 40 iterations through the sequence. Subsequently, the system automatically reproduced the sequence when cued to the first action. The behavior of the model was found to be sensitive to the ratio of the striatal-nigral learning rate to the STN-GP learning rate. Additionally, the degree of striatal inhibition of the globus pallidus had a significant influence on both learning and the ability to select an action. Low learning rates, which would be hypothesized to reflect low levels of dopamine, as in Parkinson's disease, led to slow acquisition of contextual information. However, this could be partially offset by modeling a lesion of the globus pallidus that resulted in an increase in the gain of the STN units. The parameter sensitivity of the model is discussed within the framework of existing behavioral and lesion data.

Substantia nigra pars reticulata single unit activity in normal and 60HDA-lesioned rats: effects of intrastriatal apomorphine and subthalamic lesions

The spontaneous activity and the response to intrastriatal application of apomorphine of substantia nigra pars reticulata (SNpr) single units was studied in four experimental groups of rats: (1) normal rats; (2) subthalamic nucleus (STN) lesioned rats; (3) rats bearing a 6-hydroxydopamine (60HDA) lesion; and (4) 60HDA-lesioned animals with an additional STN lesion. Thirty-eight percent of units from 60HDA-lesioned rats showed a bursting pattern of spontaneous activity, which was never found in normal rats. STN lesions had no effect on the spontaneous activity of SNpr units from normal rats, but reduced the percentage of burst units in 60HDA-lesioned animals. Intrastriatal apomorphine produced responses in 62% of SNpr units from normal rats and 85% of units from 60HDA-lesioned animals (P < 0.05). In addition, the modifications in the firing rate and in the coefficient of variation of the interspike intervals induced by intrastriatal apomorphine were significantly greater for the units isolated from 60HDA-lesioned rats. In particular, it was noted that all the burst units responded to apomorphine, showing the highest changes in firing rate and coefficient of variation. However, intrastriatal apomorphine did not always turn the activity of burst units into a more physiological pattern. STN lesions reduced the percentage of units responding to intrastriatal apomorphine in normal rats. In 60HDA-lesioned rats, STN lesions reduced the number of responsive units, and their change in mean firing rate and coefficient of variation. Our results show that the STN participates in the genesis of the bursting pattern of activity of SNpr units in 60HDA-lesioned rats, and that STN lesions can partially revert the abnormal spontaneous and apomorphine-induced responses of SNpr units in these animals.

Subthalamic ablation reverses changes in basal ganglia oxidative metabolism and motor response to apomorphine induced by nigrostriatal lesion in rats

In Parkinson's disease, the functional architecture of the basal ganglia nuclei undergoes profound alterations, one of the most important of which is overactivity of the basal ganglia output nuclei. This phenomenon seems to be intimately related to pathological overactivity of the subthalamic nucleus, which directly modulates the basal ganglia output through its glutamatergic projections. In this study, we investigated the effects of unilateral subthalamic nucleus lesions on the activities of succinate dehydrogenase and cytochrome oxidase, two markers of neuronal activity, in rats with prior unilateral lesions of the nigrostriatal tract. We also explored the effect of subthalamic nucleus lesions on the rotational response to systemic apomorphine. Rats with unilateral lesions of the nigrostriatal tract showed ipsilateral increases in enzyme activity in the basal ganglia output nuclei, entopeduncular nucleus and substantia nigra pars reticulata. Selective subthalamic nucleus destruction completely reversed this phenomenon. In addition, subthalamic nucleus lesions abolished the rotational response to apomorphine. These results confirm that overactivity of the subthalamic nucleus plays a pivotal role in the functional alterations of basal ganglia associated with Parkinson's disease. They also shed further light on the neural mechanisms through which manipulations of subthalamic activity can ameliorate Parkinson's disease symptoms.

A role for the subthalamic nucleus in 5-HT2C-induced oral dyskinesia

The 5-hydroxytryptamine2C serotonin receptor is broadly distributed in brain, however, its functional role is unknown. Peripheral administration of drugs acting at the 5-hydroxytryptamine2C receptor induces abnormal oral dyskinesias, hyperkinetic motor disorders that often result from dysfunction of the basal ganglia. The subthalamic nucleus, a brain region anatomically and functionally related to the basal ganglia, has been implicated in oral dyskinesia. The subthalamic nucleus contains messenger RNA encoding 5-hydroxytryptamine2C receptors, suggesting its potential role in 5-hydroxytryptamine2C-mediated oral dyskinesia. Both systemic administration and local unilateral infusion of the 5-hydroxytryptamine2C/1B agonist, 1-(m-chlorophenyl)piperazine into the subthalamic nucleus increased orofacial movements. Oral movements following subthalamic infusion of 1-(m-chlorophenyl)piperazine were blocked by systemic administration of the 5-hydroxytryptamine2C/2A antagonists mianserin, ketanserin and mesulergine but were not altered by systemic pretreatment with either the 5-hydroxytryptamine1A/2A and dopamine antagonist spiperone or the 5-hydroxytryptamine1A/1B antagonist pindolol. Co-infusion of mesulergine with 1-(m-chlorophenyl)piperazine into the subthalamic nucleus blocked 1-(m-chlorophenyl)piperazine-stimulated oral movements. Oral bouts following systemically administered 1-(m-chlorophenyl)piperazine were markedly reduced following bilateral subthalamic infusion of either mesulergine or the selective 5-hydroxytryptamine2C antagonist SDZ SER 082. The findings indicate that stimulating 5-hydroxytryptamine2C receptors in the subthalamic nucleus elicits orofacial dyskinesia in the rat. These data are novel in providing a behavioral model for central 5-hydroxytryptamine2C receptor stimulation attributed to a specific anatomical location, and suggest that antagonists at the 5-hydroxytryptamine2C receptor could be useful in treating hyperkinetic motor disorders.

Turning behavior in rats with unilateral lesions of the subthalamic nucleus: synergism between D1 and D2 receptors

Rats with unilateral kainic acid lesion of the subthalamic nucleus showed a dose dependent rotational response to the lesioned side (ipsilateral) after systemic administration of the non-selective dopaminergic agonist apomorphine. Both D2 and D1 selective antagonists ((-)sulpiride and SCH23390) inhibited the response to apomorphine in these rats. Selective D2 and D1 agonists (quinpirole and SKF38393) were unable to induce turning behavior. However, an ipsilateral circling response was obtained after the simultaneous application of both agonists. The interaction mechanism between dopaminergic receptor subtypes seems to be similar to that of other normosensitive models of turning previously studied (Barone et al., 1986; Robertson and Robertson, 1986; Arnt and Perregard, 1987; Asim et al., 1990; Pazo et al., 1993). It is proposed that the ipsilateral turning response to dopaminergic agonists in rats with subthalamic nucleus lesion results from an impaired behavioral expression of the action of dopaminergic agonists on one side, leading the rats to turn away from the intact hemisphere.

Glutamate decarboxylase messenger RNA in rat pallidum: comparison of the effects of haloperidol, clozapine and combined haloperidol-scopolamine treatments

We have investigated the effects of neuroleptic treatments which do, or do not, induce catalepsy on the level of expression of glutamate decarboxylase, the rate limiting enzyme in GABA synthesis, in efferent neurons of the pallidum in adult rats. Different regimens of haloperidol (1 mg/kg s.c., three, seven or 14 days; 2 mg/kg, s.c., 10 days) induced catalepsy in a majority of rats and increased glutamate decarboxylase messenger RNA levels in the globus pallidus (external pallidum) in those rats exhibiting catalepsy. Levels of glutamate decarboxylase messenger RNA were also increased in the entopeduncular nucleus (internal pallidum), but only after 14 days of treatment with haloperidol. The atypical antipsychotic clozapine (seven days, 20 mg/kg, s.c.), which did not induce catalepsy, slightly decreased glutamate decarboxylase messenger RNA levels in the globus pallidus. When co-administered with haloperidol (seven days, 1 mg/kg s.c.), the muscarinic antagonist scopolamine (1 mg/kg, s.c.) completely blocked both haloperidol-induced catalepsy and increases in glutamate decarboxylase messenger RNA levels in the globus pallidus. In contrast, scopolamine was not able to block increased glutamate decarboxylase and enkephalin messenger RNA expression induced by haloperidol in the striatum. These results reveal a good correlation between increases in glutamate decarboxylase messenger RNA levels in the globus pallidus and catalepsy after these drug treatments and suggest that anticholinergic blockade of the behavioral and molecular effects of neuroleptics may involve non-striatal mechanisms.

A role for non-NMDA excitatory amino acid receptors in regulating the basal activity of rat globus pallidus neurons and their activation by the subthalamic nucleus

We have investigated the hypothesis that excitatory amino acid (EAA) receptors in the globus pallidus (GP) play a significant role in maintaining the firing rates of GP neurons under basal conditions and following activation of the subthalamic nucleus (STN). Drugs were infused directly into the GP and/or STN while the extracellular single unit activity of Type II GP neurons was recorded in ketamine-anesthetized rats. Local infusions of the EAA agonists NMDA (30-300 pmol/200 nl) or AMPA (0.1-1 pmol/200 nl) elicited increases in the firing rate of GP neurons in a dose-dependent fashion. Infusion of the GABAA receptor antagonist bicuculline methiodide (1-10 pmol/100 nl) into the STN also elicited dose-related increases in the firing rate of GP neurons. Intrapallidal infusion of the non-NMDA (AMPA/kainate) receptor antagonist NBQX (0.1-1.0 nmol) reduced the basal firing rate of GP neurons by 40%. In contrast, the NMDA antagonist MK-801 (0.01-0.1 nmol) produced no significant effect on basal firing rate. Intrapallidal infusion of the non-selective EAA receptor antagonist kynurenic acid or NBQX reversed or blocked the increase in firing rate of GP neurons following bicuculline-induced activation of the STN. Similar treatment with MK-801, however, had no significant effect on this response. These results indicate that tonic stimulation of non-NMDA receptors plays an important role in maintaining the basal activity of GP neurons and in mediating the effects of increased excitatory input from subthalamic afferent neurons.

Subthalamic nucleus and globus pallidus lesions alter activity in nigrothalamic neurons in rats

Lesions of the subthalamic nucleus or the globus pallidus altered the response of substantia nigra pars reticulata neurons (antidromically identified as projecting to the thalamus) to electrical stimulation of the frontal agranular cortex. In intact animals, cortical stimulation evokes three independent responses (excitation, inhibition, excitation) that may occur singly or in various combinations. The independence of the various responses, especially the temporally coincident excitatory and inhibitory responses, suggests that the net inhibitory and excitatory pathways carrying these signals from the cortex may converge to varying degrees on individual nigrothalamic neurons. Subthalamic lesions increased total response duration (from 28.4 to 39.7 ms), increased the duration of inhibition (from 18 to 30 ms), decreased the occurrence of excitatory responses, and decreased the intensity of the second excitation (from 1.1 to 0.6 spikes/s). Lesion of the globus pallidus also increased total response duration (up to 38 ms), but by increasing the duration of the second excitation (from 15.1 up to 23.8 ms). The intensity of the second excitation (from 1.1 to 1.5 spikes/stimulus) and the number of cells showing the first and second excitations also increased. The incidence, but not the duration, of the inhibition increased. The mean firing rate increased after subthalamic nucleus lesion (34.2 spikes/s) as compared to intact (27.0) or globus pallidus lesion (25.6). These changes may reflect changes in the relative contribution of the five different pathways transmitting information from the cortex to the substantia nigra. In all cases the cortico-striato-nigral pathway is largely intact.(ABSTRACT TRUNCATED AT 250 WORDS)