Noradrenergic modulation of subthalamic nucleus activity: behavioral and electrophysiological evidence in intact and 6-hydroxydopamine-lesioned rats

The subthalamic nucleus (STN) plays a key role in the pathophysiology of Parkinson's disease. The modulation of the STN by norepinephrine, however, is unknown. The present study aims at characterizing the effects of systemic administration of noradrenergic agents on locomotor activity and on in vivo extracellularly recorded STN neuronal activity in intact and 6-hydroxydopamine (6-OHDA)-lesioned rats. Using selective agonists and antagonists of alpha1 and alpha2 adrenergic receptors (ARs), we show that STN neurons have functional alpha1- and alpha2-AR controlling STN firing with an impact on locomotor activity. We further demonstrate that those systemic effects are supported, at least in part, by a direct modulation of STN neuronal activity, using patch-clamp recordings of STN neurons in brain slices. These findings support the premise that hypokinesia is associated with an increased STN neuronal activity, and that improvements of parkinsonian motor abnormalities are associated with a decrease in STN activity. Our data challenge assumptions about the role of alpha1-AR and alpha2-AR in the regulation of STN neurons in both intact and 6-OHDA-lesioned rats and further ground the rationale for using alpha2-AR noradrenergic antagonists in Parkinson's disease, albeit via an unexpected mechanism.

Dopamine lesion-induced changes in subthalamic nucleus activity are not associated with alterations in firing rate or pattern in layer V neurons of the anterior cingulate cortex in anesthetized rats

Dysfunctional activity in the subthalamic nucleus (STN) is thought to underlie movement deficits of patients with Parkinson's disease. Alterations in STN firing patterns are also evident in the anesthetized rat model of Parkinson's disease, where studies show that loss of striatal dopamine and concomitant changes in the indirect pathway are associated with bursty and oscillatory firing patterns in STN output. However, the extent to which alterations in cortical activity contribute to changes in STN activity is unclear. As pyramidal neurons in the cingulate cortex project directly to the STN, cingulate output was assessed after dopamine lesion by simultaneously recording single-unit and local field potential (LFP) activities in STN and anterior cingulate cortex in control, dopamine-lesioned and non-lesioned hemispheres of urethane-anesthetized rats. Correlated oscillations were observed in cross-correlograms of spike trains from STN and cingulate layer V neurons with broad waveforms indicative of pyramidal neurons. One-2 weeks after dopamine cell lesion, firing rate, incidence of bursty and 0.3-2.5 Hz oscillatory activity of neurons and LFP power in the STN all increased significantly. In contrast, firing rate, incidence of bursty and 0.3-2.5 Hz oscillatory activity of cingulate layer V putative pyramidal neurons and power in cingulate LFPs did not differ significantly between dopamine-lesioned, non-lesioned or control hemispheres, despite significant loss of dopamine in the lesioned cingulate cortex. Data show that alterations in STN activity in the dopamine-lesioned hemisphere are not associated with alterations in neuronal activity in layer V of the anterior cingulate cortex in anesthetized rats.

Differential dynamic of action on cortical and subcortical structures of anesthetic agents during induction of anesthesia

BACKGROUND

Dynamic action of anesthetic agents was compared at cortical and subcortical levels during induction of anesthesia. Unconsciousness involved the cortical brain but suppression of movement in response to noxious stimuli was mediated through subcortical structures.

METHODS

Twenty-five patients with Parkinson disease, previously implanted with a deep-brain stimulation electrode, were enrolled during the implantation of the definitive pulse generator. During induction of anesthesia with propofol (n = 13) or sevoflurane (n = 12) alone, cortical (EEG) and subcortical (ESCoG) electrogenesis were obtained, respectively, from a frontal montage (F3-C3) and through the deep-brain electrode (p0-p3). In EEG and ESCoG spectral analysis, spectral edge (90%) frequency, median power frequency, and nonlinear analysis dimensional activation calculations were determined.

RESULTS

Sevoflurane and propofol decreased EEG and ESCoG activity in a dose-related fashion. EEG values decreased dramatically at loss of consciousness, whereas there was little change in ESCoG values. Quantitative parameters derived from EEG but not from ESCoG were able to predict consciousness versus unconsciousness. Conversely, quantitative parameters derived from ESCoG but not from EEG were able to predict movement in response to laryngoscopy.

CONCLUSION

These data suggest that in humans, unconsciousness mainly involves the cortical brain, but that suppression of movement in response to noxious stimuli is mediated through the effect of anesthetic agents on subcortical structures.

Survival of midbrain dopaminergic cells after lesion or deep brain stimulation of the subthalamic nucleus in MPTP-treated monkeys

We have examined dopaminergic cell survival after alteration of the subthalamic nucleus (STN) in methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys. The STN was lesioned with kainic acid (B series) or underwent deep brain stimulation (DBS) at high frequency (C series). In another series, MPTP-treated and non-MPTP-treated monkeys had no STN alteration (intact animals; A series). Animals were treated with MPTP either after (B1, C1) or before (B2, C2) STN alteration. We also explored the long-term ( approximately 7 months) effect of DBS in non-MPTP-treated monkeys (D series). Brains were aldehyde-fixed and processed for routine Nissl staining and tyrosine hydroxylase immunocytochemistry. Our results showed that there were significantly more (20-24%) dopaminergic cells in the substantia nigra pars compacta (SNc) of the MPTP-treated monkeys that had STN alteration, either with kainic acid lesion or DBS, compared to the non-MPTP-treated monkeys (intact animals). We suggest that this saving or neuroprotection was due to a reduction in glutamate excitotoxicity, as a result of the loss or reduction of the STN input to the SNc. Our results also showed that SNc cell number in the B1 and C1 series were very similar to those in the B2 and C2 series. In the cases that had long-term DBS of the STN (D series), there was no adverse impact on SNc cell number. In summary, these results indicated that STN alteration offered neuroprotection to dopaminergic cells that would normally die as part of the disease process.

High-frequency stimulation of the subthalamic nucleus increases glutamate in the subthalamic nucleus of rats as demonstrated by in vivo enzyme-linked glutamate sensor

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective therapy for Parkinson's disease; however, the mechanism whereby DBS ameliorates the symptoms of Parkinson's disease remains an area of intense research. In the present study, we investigated the hypothesis that the neurotransmitter glutamate is released within the STN during high-frequency stimulation (HFS) of the STN. Direct measurements of extracellular glutamate concentration in the STN were made using a dual enzyme-based electrochemical sensor. The studies were carried out in ketamine/xylazine anesthetized rats placed in a Kopf stereotaxic head frame. Various electrical stimulations (100-micros cathodic pulses; 100-3000 microA; 10- to 1000-Hz frequency; 5-s to 60-min stimulus durations) using bipolar stimulating electrodes were delivered to the STN. Stimulation of the STN elevated the concentration of glutamate in the STN. The concentration of glutamate rose quickly during HFS, remained elevated for the duration of stimulation, and descended slowly towards baseline upon cessation of stimulation. Elevation of the extracellular concentration of glutamate in the STN may be an important mechanism whereby DBS in the STN improves the symptoms of Parkinson's disease. Furthermore, our data argue against the hypothesis that DBS works primarily by electrotonic inhibition of the stimulated structure.

Effects of unilateral subthalamic and pallidal deep brain stimulation on fine motor functions in Parkinson's disease

Deep brain stimulation (DBS) is an effective treatment for selected patients with disabling Parkinson's disease (PD). The two main targets are the subthalamic nucleus (STN) and the globus pallidus internus (GPi), although it has not been established whether stimulation at one target is superior to the other. This prospective randomized study assessed the effects of unilateral DBS of the STN versus GPi on fine motor skills in 33 patients with advanced PD. Stimulation of either the STN (18 subjects) or GPi (15 subjects) in the off medication state significantly improved movement time and dexterity, but had little or no effect on reaction time. Overall, the extent of improvement did not differ between the two targets. The degree of improvement in movement time, but not dexterity, was correlated with the extent of preoperative medication responsiveness. Our findings suggest that DBS of the STN or GPi results in a similar improvement in hand movements at short-term follow-up. Preoperative medication responsiveness predicts improvement in some but not other motor tasks.

Localization and expression of group I metabotropic glutamate receptors in the mouse striatum, globus pallidus, and subthalamic nucleus: regulatory effects of MPTP treatment and constitutive Homer deletion

Group I metabotropic glutamate receptors (mGluRs), mGluR1 and mGluR5, regulate activity in the globus pallidus (GP) and subthalamic nucleus (STN). To test whether the localization of group I mGluRs is altered in parkinsonism, we used immunoelectron microscopy to analyze the subcellular and subsynaptic distribution of mGluR1a and mGluR5 in GP and STN of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice. Homer1 and Homer2 knock-out mice were used to assess the role of Homer in MPTP-induced redistribution of group I mGluRs. We also examined the effects of MPTP on the expression levels of group I mGluRs and Homer proteins in GP and striatum. MPTP treatment significantly reduced the expression levels of H1a and mGluR1a in striatum but not in GP. Although light microscopy did not reveal noticeable effects of MPTP treatment on the distribution of group I mGluRs and Homer proteins in GP and STN, specific changes in the ultrastructural localization of mGluR1a were found in MPTP-treated normal and Homer knock-out mice. An increase in the expression of presynaptic axonal and terminal mGluR1a labeling and an increased level of mGluR1a immunoreactivity in the postsynaptic specialization of putative GABAergic synapses were among the most significant effects induced by dopamine depletion. However, neither of these changes was found for mGluR5, which, in contrast, displayed complex regulatory alterations in its subsynaptic distribution in response to Homer deletion and MPTP lesion. Thus, nigrostriatal dopaminergic lesion and Homer deletion lead to changes in the trafficking of group I mGluRs in vivo that are specific to receptor subtypes and brain areas.

[Neuroprotective effect of dopamine agonists]

Neuroprotection in Parkinson's disease is defined as halting or slowing of the progression of neurodegeneration. Neuroprotective properties of dopamine agonists may be mediated via several mechanisms, (levodopa-sparing effect, decreased dopamine turnover, antioxidant activity or inhibition of the subthalamic nucleus). Many preclinical studies demonstrate that bromocriptine, pergolide, pramipexole, ropinirole and other dopamine agonists provide significant protection against neuronal loss in experimental models. Only a few studies have investigated the potential neuroprotective effect of dopamine agonists at the clinical level. Several recent trials have used neuroimaging biomarkers to assess the rate of dopamine neuron loss in dopamine agonist-treated versus levodopa-treated patients using fluoro-dopa PET or beta-CIT SPECT. Ropinirole slowed the decline of putaminal dopamine storage capacity and pramipexole reduced decline in striatal beta-CIT uptake to a similar extent, compared to levodopa. It is not possible to exclude, however, some pharmacologic or pharmacodynamic effects of drugs on imaging markers in these studies, as well as symptomatic effects of dopamine agonist treatment in clinical outcomes.

5,7-dihydroxytryptamine lesion of the dorsal raphe nucleus alters neuronal activity of the subthalamic nucleus in normal and 6-hydroxydopamine-lesioned rats

The subthalamic nucleus receives serotonergic projections from the dorsal raphe nucleus. However, the role of serotonergic innervation in the activity of subthalamic neurons in vivo is unknown. The aim of the present work is to study the changes in the firing of subthalamic neurons in rats with 5,7-dihydroxytryptamine lesions of the dorsal raphe nucleus and rats with combined 5,7-dihydroxytryptamine lesions in the dorsal raphe nucleus and 6-hydroxydopamine lesions in the substantia nigra pars compacta by using single-unit extracellular recordings. In rats with 5,7-dihydroxytryptamine lesions of the dorsal raphe nucleus, the firing rate of subthalamic neurons increased significantly compared with normal rats and the firing pattern changed significantly towards a more bursting firing in the majority of the neurons observed. In rats with combined dorsal raphe nucleus and substantia nigra pars compacta lesions, the firing rate and firing pattern of subthalamic neurons did not show a significant difference compared to rats with lesions of the substantia nigra pars compacta. However, dorsal raphe nucleus and substantia nigra pars compacta lesions combined increased significantly the percentage of subthalamic neurons with burst-firing pattern compared to normal rats, while having no effect on their firing rate. These results show that the serotonergic efferent projections of the dorsal raphe nucleus significantly influence on the activity of subthalamic neurons and that the loss of dopaminergic projection by substantia nigra pars compacta lesion decreases the effect of the lesions of the dorsal raphe nucleus on subthalamic nucleus neuronal activity, suggesting that the role of the dorsal raphe nucleus may be exerted by the dorsal raphe nucleus-substantia nigra pars compacta-subthalamic nucleus pathway.

Interaction of novel positive allosteric modulators of metabotropic glutamate receptor 5 with the negative allosteric antagonist site is required for potentiation of receptor responses

Exciting advances have been made in the discovery of selective positive allosteric modulators of the metabotropic glutamate receptor (mGluR) mGluR5. These compounds may provide a novel approach that could be useful in the treatment of certain central nervous system disorders. However, because of their low potencies, previously described mGluR5 potentiators are not useful for functional studies in native preparations. In addition, binding sites at which these compounds act have not been identified. It has been suggested that two allosteric potentiators, 3,3'-difluorobenzaldazine and 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB), act by binding to the same allosteric site as the negative allosteric modulators of mGluR5 such as 2-methyl-6-(phenylethynyl)pyridine (MPEP). However, another mGluR5 potentiator, N-{4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)m-ethyl]phenyl}-2-hydroxybenzamide, does not bind to this site, bringing this hypothesis into question. We have synthesized a series of CDPPB analogs and report that these compounds bind to the MPEP site with affinities that are closely related to their potencies as mGluR5 potentiators. Furthermore, allosteric potentiation is antagonized by a neutral ligand at the MPEP site and reduced by a mutation of mGluR5 that eliminates MPEP binding. Together, these data suggest that interaction with the MPEP site is important for allosteric potentiation of mGluR5 by CDPPB and related compounds. In addition, whole-cell patch-clamp studies in midbrain slices reveal that a highly potent analog of CDPPB, 4-nitro-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (VU-29), selectively potentiates mGluR5 but not mGluR1-mediated responses in midbrain neurons, whereas a previously identified allosteric potentiator of mGluR1 has the opposite effect.

Zolpidem modulates GABA(A) receptor function in subthalamic nucleus

The subthalamic nucleus occupies a position in the indirect pathway of basal ganglia circuit, which plays an important role in the movement regulation. Zolpidem is an imidazopyridine agonist with a high affinity on the benzodiazepine site of GABA(A) receptors containing alpha 1 subunit. Recently, zolpidem has been reported to be useful in treating subgroups of parkinsonian patients. A high density of zolpidem binding sites has been shown in rat subthalamic nucleus. To further investigate the modulation of zolpidem on GABA(A) receptor-mediated inhibitory synaptic current in subthalamic nucleus, whole-cell patch clamp recordings were used in the present study. Zolpidem at 100nM significantly prolonged the decay time and rise time of miniature inhibitory postsynaptic currents, with no effect on the amplitude and frequency. The benzodiazepine antagonist flumazenil could completely block the potentiation induced by zolpidem, confirming the specificity on the benzodiazepine site. At a high concentration of 1 microM, zolpidem significantly increased the decay time, rise time, amplitude and frequency of miniature inhibitory postsynaptic currents. In the behaving rats, unilateral microinjection of zolpidem into subthalamic nucleus induced a significant contralateral rotation. The present findings on the effect of zolpidem in subthalamic nucleus provide a rationale for further investigations into its potential in the treatment of Parkinson's disease.

Dopaminergic therapy promotes lateralized motor activity in the subthalamic area in Parkinson's disease

Treatment of patients with Parkinson's disease with levodopa has profound effects on both movement and the pattern of movement-related reactivity in the subthalamic nucleus (STN), as reflected in the local field potential (LFP). The most striking change is the promotion of reactivity in the gamma frequency band, but it remains unclear whether the latter is itself a pathological feature, possibly associated with levodopa induced dyskinesias, or is primarily physiological. Gamma band reactivity in the cerebral cortex of humans without Parkinson's disease occurs contralateral to movement, so we posited that lateralization of subcortical gamma reactivity should occur following levodopa if the latter restores a more physiological pattern in patients with Parkinson's disease. Accordingly, we studied movement-related changes in STN LFP activity in 11 Parkinson's disease patients (age 59 +/- 2.7 years, three females) while they performed ipsi- and contralateral self-paced joystick movements ON and OFF levodopa. A bilaterally symmetrical gamma band power increase occurred around movement onset in the OFF state. Following levodopa this feature became significantly more pronounced in the subthalamic region contralateral to movement. The physiological nature of this asymmetric pattern of gamma reactivity was confirmed in the STN of two tremor patients without Parkinson's disease. Although levodopa treatment in the Parkinson's disease patients did not lead to lateralization of power suppression at lower frequencies (8-30 Hz), it did increase the degree of power suppression. These findings suggest that dopaminergic therapy restores a more physiological pattern of reactivity in the STN of patients with Parkinson's disease.

Repetitive Activation of Glutamatergic Inputs Evokes a Long-Lasting Excitation in Rat Globus Pallidus Neurons In Vitro

External globus pallidus (GPe) neurons express abundant metabotropic glutamate receptor 1 (mGluR1) in their somata and dendrites and receive glutamatergic inputs mainly from the subthalamic nucleus. We investigated whether synaptically released glutamate could activate mGluR1s using whole cell and cell-attached recordings in rat brain slice preparations. Repetitive internal capsule stimulation evoked EPSPs followed by a slow depolarizing response (sDEPO) lasting 10–20 s. Bath application of both GABAA and GABAB receptor antagonists increased the amplitude of sDEPOs. A mixture of AMPA/kainate and N-methyl-d-aspartate receptor antagonists did not alter sDEPOs. The induction of sDEPOs was only partially mediated by mGluR1 because mGluR1 antagonists reduced but failed to completely block the responses. Voltage-clamp recordings revealed that slow inward currents sensitive to mGluR1 antagonist were larger at −60 than at −100 mV, whereas the currents insensitive to mGluR1 antagonist were larger at −100 than at −60 mV. In cell-attached recordings, repetitive internal capsule stimulation evoked long-lasting excitations in GPe neurons, which were also partially suppressed by mGluR1 antagonists. Application of a glutamate uptake inhibitor or an mGluR1 agonist significantly increased the spontaneous firing rate but decreased the excitations to repetitive stimulation. These results suggest that synaptically released glutamate can activate mGluR1, contributing to the induction of long-lasting excitation in GPe neurons and that background mGluR1 activation suppresses the slow mGluR1 responses. Thus mGluR1 may play important roles in the control of GPe neuronal activity.

Subthalamic nucleus neurones in slices from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mice show irregular, dopamine-reversible firing pattern changes, but without synchronous activity

The loss of dopamine in idiopathic or animal models of Parkinson's disease induces synchronized low-frequency oscillatory burst-firing in subthalamic nucleus neurones. We sought to establish whether these firing patterns observed in vivo were preserved in slices taken from dopamine-depleted animals, thus establishing a role for the isolated subthalamic-globus pallidus complex in generating the pathological activity. Mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) showed significant reductions of over 90% in levels of dopamine as measured in striatum by high pressure liquid chromatography. Likewise, significant reductions in tyrosine hydroxylase immunostaining within the striatum (>90%) and tyrosine hydroxylase positive cell numbers (65%) in substantia nigra were observed. Compared with slices from intact mice, neurones in slices from MPTP-lesioned mice fired significantly more slowly (mean rate of 4.2 Hz, cf. 7.2 Hz in control) and more irregularly (mean coefficient of variation of inter-spike interval of 94.4%, cf. 37.9% in control). Application of ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2-amino-5-phosphonopentanoic acid (AP5) and the GABA(A) receptor antagonist picrotoxin caused no change in firing pattern. Bath application of dopamine significantly increased cell firing rate and regularized the pattern of activity in cells from slices from both MPTP-treated and control animals. Although the absolute change was more modest in control slices, the maximum dopamine effect in the two groups was comparable. Indeed, when taking into account the basal firing rate, no differences in the sensitivity to dopamine were observed between these two cohorts. Furthermore, pairs of subthalamic nucleus cells showed no correlated activity in slices from either control (21 pairs) or MPTP-treated animals (20 pairs). These results indicate that the isolated but interconnected subthalamic-globus pallidus network is not itself sufficient to generate the aberrant firing patterns in dopamine-depleted animals. More likely, inputs from other regions, such as the cortex, are needed to generate pathological oscillatory activity.

Gender-related differences in the human subthalamic area: a local field potential study

The objective of this study was to investigate the possible existence of gender-related neurophysiological differences in the oscillatory activity of the human subthalamic area. To this end, we recorded local field potentials (LFPs) after neurosurgical procedures for deep brain stimulation (DBS) in 24 patients (12 males and 12 females) with Parkinson's disease. LFP recordings at rest before levodopa medication (19 nuclei from 11 female patients and 16 nuclei from ten male patients) showed significantly higher power in the alpha/low-beta band (8-12 Hz, P<0.01; 13-20 Hz, P=0.03) in females than in males. After levodopa medication (ten nuclei from six female patients and 11 nuclei from seven male patients), the power in the high-gamma band (60-90 Hz) and of the 300 Hz rhythm was significantly higher in females than in males (high-gamma, P=0.007; 300 Hz, P=0.002). These findings show that functional gender-related differences in the central nervous system involve the human subthalamic area (STN) and its response to levodopa in Parkinson's disease. Gender-related neurophysiological differences may be important for understanding gender-specific features of neurodegenerative disorders and should be considered when interpreting LFP data from the human basal ganglia.

Chronic high-frequency stimulation of the subthalamic nucleus and L-DOPA treatment in experimental parkinsonism: effects on motor behaviour and striatal glutamate transmission

Hyperactivity of striatal glutamatergic synaptic transmission in response to dopamine depletion plays a major role in the pathogenesis of parkinsonian motor symptoms. In the present study we investigated the impact, on this hyperactivity, of chronic dyskinesiogenic L-DOPA treatment, combined or not with high-frequency stimulation (HFS) of the subthalamic nucleus (STN). In vitro patch-clamp recordings were performed from striatal spiny neurons of hemiparkinsonian rats (intranigral 6-OHDA injection). Here we show that dyskinesiogenic L-DOPA treatment exacerbated striatal glutamatergic hyperactivity induced by 6-OHDA lesion. Chronic 5-day STN HFS had the opposite effect, reducing striatal glutamatergic transmission in both parkinsonian and dyskinetic animals. Consistently, chronic HFS stimulation could progressively ameliorate motor parkinsonian signs (akinesia) but, conversely, did not improve L-DOPA-induced dyskinesia (LID). Thus, the effects of L-DOPA and HFS on corticostriatal transmission seem to be dissociated. These data show for the first time that dyskinesiogenic L-DOPA treatment and chronic STN HFS with antiakinetic effects induce opposite plastic rearrangements in the striatum. The interaction between these two treatments provides further evidence that striatal glutamatergic hyperactivity is a pathophysiological correlate of akinesia rather than LID.

Unilateral lesion of the subthalamic nucleus enhances cortical fos expression associated with focally evoked seizures in the rat

Reducing subthalamic nucleus (STN) activity has been proposed as an anti-epileptic procedure. Here we show that, on the contrary, a unilateral lesion of the STN causes slight (nonsignificant) increases in the severity of limbic seizures evoked by bicuculline infusion into the piriform cortex, associated with marked Fos expression throughout the cerebral cortex. Abolishing the STN control over the basal ganglia output may therefore play a facilitatory role on cortical activation associated with limbic seizures.

Repeated l-DOPA treatment increases c-fos and BDNF mRNAs in the subthalamic nucleus in the 6-OHDA rat model of Parkinson's disease

The subthalamic nucleus and the striatum are input regions of the basal ganglia. This study used the unilateral 6-OHDA rat model of Parkinson's disease to examine effects of l-DOPA on the expression of c-fos and BDNF mRNAs in these nuclei. Dopamine depletion per se did not affect c-fos or BDNF. Both a single and repeated injections of l-DOPA induced c-fos, but not BDNF, in the dopamine-depleted striatum. However, repeated l-DOPA treatment increased c-fos and BDNF in the dopamine-depleted subthalamic nucleus. These molecular adaptations may reflect changes in neuronal plasticity that underlie some therapeutic actions and/or side effects of l-DOPA in Parkinson's disease.

Experimental manipulations of the subthalamic nucleus fail to suppress tonic seizures in the electroshock model of epilepsy

Recently, it has been shown that the subthalamic nucleus (STN) has anticonvulsant effects on epileptic seizures originating from the forebrain. The aim of the present study was to determine whether the anticonvulsant properties of the STN extend to the suppression of tonic seizures originating from the brainstem elicited by electroshock in rats. Three different procedures were used to manipulate activity in the STN and in each case the duration of tonic hindlimb extension elicited by electroshock was used as a measure of seizure-severity. Under general anesthesia, two groups of rats received chronic implants of either bilateral stainless steel guide cannulae or bilateral bipolar stimulating electrodes stereotaxically implanted and aimed at the STN. After 3 days of recovery, each rat in the first group was tested with electroshock on three consecutive days after having received 220 nl bilateral microinjections into the STN of either 200 or 400 pmol of muscimol (a GABA agonist) dissolved in saline or the same volume of normal saline. In the second group the electroshock test was conducted, again on three consecutive days, immediately following high frequency electrical stimulation (HFS) of the STN at 130 or 260 Hz or a no current control condition. In the third group, rats were tested with electroshock before and after bilateral excitotoxic lesions of the STN with either kainic or ibotenic acids. None of these manipulations produced significant suppression of the tonic hind limb extension elicited by electroshock compared with the relevant control conditions. This suggests that, within the limitations of the current procedures, the anticonvulsant properties of the STN appear to be ineffective against tonic seizures originating in the brainstem.

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 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.

Blockade of mGluR glutamate receptors in the subthalamic nucleus ameliorates motor asymmetry in an animal model of Parkinson's disease

It has been suggested that Group I metabotropic glutamate receptor antagonists could have potential therapeutic value in the treatment of Parkinson's disease. There is evidence that when given systemically, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a metabotropic glutamate receptor type 5 (mGluR5) antagonist, produces anti-parkinsonian effects in animal models, but the site of action has not been directly established. In the present study, we examined whether the subthalamic nucleus (STN) and its output structures may mediate such an effect using a unilateral rat model of Parkinson's disease. A battery of simple behavioral tests, reliably sensitive to dopamine depletion, was applied consecutively: (i) prior to surgery; (ii) 3 weeks following a unilateral 6-hydroxydopamine lesion of the substantia nigra pars compacta; (iii) at 1 h, 24 h and 4 days following a microinjection of MPEP, via an indwelling cannula, into the STN, entopeduncular nucleus (EP) or substantia nigra zona reticulata. Unilaterally dopamine-depleted animals typically had severe motor and sensorimotor asymmetries 3 weeks following surgery. Microinjection of 25 nmol MPEP into the STN of these animals significantly attenuated these asymmetries relative to vehicle. Further microinjections of lower doses (5 and 10 nmol) revealed a dose-response effect. Microinjection of MPEP into either the EP or substantia nigra zona reticulata was without effect. These data suggest that MPEP may act at the level of the STN to reduce glutamatergic overactivity and thereby induce anti-parkinsonian effects.

Prolonged blockade of NMDA or mGluR5 glutamate receptors reduces nigrostriatal degeneration while inducing selective metabolic changes in the basal ganglia circuitry in a rodent model of Parkinson's disease

We compared the neuroprotective and metabolic effects of chronic treatment with ionotropic or metabotropic glutamate receptor antagonists, in rats bearing a unilateral nigrostriatal lesion induced by 6-hydroxydopamine (6-OHDA). The ionotropic, N-methyl-D-aspartate receptor antagonist MK-801 increased cell survival in the substantia nigra pars compacta (SNc) and corrected the metabolic hyperactivity (increased cytochrome oxidase activity) of the ipsilateral substantia nigra pars reticulata (SNr) associated with the lesion, but showed no effects on the 6-OHDA-induced hyperactivity of the subthalamic nucleus (STN). Significant-although less pronounced-protection of SNc neurons was also observed following treatment with the metabotropic glutamate receptor (mGluR5) antagonist 2-methyl-6-(phenylehtynyl)-pyridine (MPEP). As opposed to MK-801, MPEP abolished the STN metabolic hyperactivity associated with the nigrostriatal lesion, without affecting SNr activity. Specific modulation of STN hyperactivity obtained with mGluR5 blockade may, therefore, open interesting perspectives for the use of this class of compounds in the treatment of Parkinson's disease.

Balance of monosynaptic excitatory and disynaptic inhibitory responses of the globus pallidus induced after stimulation of the subthalamic nucleus in the monkey

The subthalamic nucleus (STN) plays a pivotal role in controlling the activity of both the external and internal segments of the globus pallidus (GPe and GPi, respectively). Both nuclei receive monosynaptic excitatory and disynaptic GPe-mediated inhibitory inputs from the STN. Thus, we investigated the balance of these antagonistic inputs that may determine the overall response of pallidum to STN activation in monkeys. Single stimulation of the STN evoked a short-latency excitation followed by a weak inhibition in GPe neurons and a short-latency, very short-duration excitation followed by a strong inhibition in GPi neurons. Burst high-frequency stimulation (BHFS) (10 stimuli with 100 Hz) of the STN (STN-BHFS) evoked powerful excitatory responses in GPe neurons. Local injection of a mixture of 1, 2, 3, 4-tetrahydro-6-nitro-2, 3-dioxobenzo[f]quinoxaline-7-sulfonamide (NBQX; AMPA/kainate receptor blocker) and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; NMDA receptor blocker) greatly diminished or abolished excitatory responses to the STN stimulation. In contrast to the GPe, STN-BHFS evoked a predominantly inhibitory response in GPi neurons. The inhibition could be blocked either by a local application of the GABAA receptor antagonist gabazine or by an injection of an NBQX/CPP/gabazine mixture into the GPe. STN-BHFS induced weak excitatory or inhibitory responses in a small number of phasically active putamen neurons. These data suggest that with single stimulation and during STN-BHFS, the STN-GPe excitatory response dominates over the STN-GPe-GPe recurrent inhibition in the GPe, whereas the STN-GPe-GPi inhibitory response dominates over the STN-GPi excitatory response in the GPi.

Enhancement of excitatory synaptic integration by GABAergic inhibition in the subthalamic nucleus

The activity patterns of subthalamic nucleus (STN) neurons, which are intimately related to normal movement and abnormal movement in Parkinson's disease (PD), are sculpted by feedback GABAergic inhibition from the reciprocally connected globus pallidus (GP). To understand the principles underlying the integration of GABAergic inputs, we used gramicidin-based patch-clamp recording of STN neurons in rat brain slices. Voltage-dependent Na+ (Nav) channels actively truncated synthetic IPSPs and were required for autonomous activity. In contrast, hyperpolarization-activated cyclic nucleotide-gated and class 3 voltage-dependent Ca2+ channels contributed minimally to the integration of single or low-frequency trains of IPSPs and autonomous activity. Interestingly, IPSPs modified action potentials (APs) in a manner that suggested IPSPs enhanced postsynaptic Nav channel availability. This possibility was confirmed in acutely isolated STN neurons using current-clamp recordings containing IPSPs as voltage-clamp waveforms. Tetrodotoxin-sensitive subthreshold and spike-associated Na+ currents declined during autonomous spiking but were indeed transiently boosted after IPSPs. A functional consequence of inhibition-dependent augmentation of postsynaptic excitability was that EPSP-AP coupling was dramatically improved when IPSPs preceded EPSPs. Because STN neuronal activity exhibits coherence with cortical beta-oscillations in PD, we tested how rhythmic sequences of cortical EPSPs were integrated in the absence and presence of feedback inhibition. STN neuronal activity was consistently entrained by EPSPs only in the presence of feedback inhibition. These observations suggest that feedback inhibition from the GP is critical for the emergence of coherent beta-oscillations between the cortex and STN in PD.