Entopeduncular Nucleus Projections to the Lateral Habenula Contribute to Cocaine Avoidance.

The aversive properties associated with drugs of abuse influence both the development of addiction and relapse. Cocaine produces strong aversive effects after rewarding effects wear off, accompanied by increased firing in the lateral habenula (LHb) that contributes to downstream activation of the rostromedial tegmental nucleus (RMTg). However, the sources of this LHb activation are unknown, as the LHb receives many excitatory inputs whose contributions to cocaine aversion remain uncharacterized. Using cFos activation and in vivo electrophysiology in male rats, we demonstrated that the rostral entopeduncular nucleus (rEPN) was the most responsive region to cocaine among LHb afferents examined and that single cocaine infusions induced biphasic responses in rEPN neurons, with inhibition during cocaine's initial rewarding phase transitioning to excitation during cocaine's delayed aversive phase. Furthermore, rEPN lesions reduced cocaine-induced cFos activation by 2-fold in the LHb and by a smaller proportion in the RMTg, while inactivation of the rEPN or the rEPN-LHb pathway attenuated cocaine avoidance behaviors measured by an operant runway task and by conditioned place aversion (CPA). These data show an essential but not exclusive role of rEPN and its projections to the LHb in processing the aversive effects of cocaine, which could serve as a novel target for addiction vulnerability.SIGNIFICANCE STATEMENT Cocaine produces well-known rewarding effects but also strong aversive effects that influence addiction propensity, but whose mechanisms are poorly understood. We had previously reported that the lateral habenula (LHb) is activated by cocaine and contributes to cocaine's aversive effects, and the current findings show that the rostral entopeduncular nucleus (rEPN) is a major contributor to this LHb activation and to conditioned avoidance of cocaine. These findings show a critical, though not exclusive, rEPN role in cocaine's aversive effects, and shed light on the development of addiction.

Globus pallidus, but not entopeduncular nucleus, 6-OHDA-induced lesion attenuates L-Dopa-induced dyskinesia in the rat model of Parkinson's disease.

Although extrastriatal dopaminergic (DAergic) systems are being recognized as contributors to Parkinson's disease (PD) pathophysiology, the role of extrastriatal DA depletion in L-Dopa-induced dyskinesia (LID) is still unknown. In view of the physiologic actions of DA on pallidal neuronal activity and the effects on motor behavior of local injection of DA drugs, the loss of the external (GPe, GP in rodents) and internal (GPi, entopeduncular nucleus (EP) in rodents) pallidal DAergic innervation might differentially contribute to LID. A role of pallidal serotonergic (SER) terminals in LID has been highlighted, however, the effect of DAergic innervation is unknown. We investigated the role of DAergic pallidal depletion on LID. Rats were distributed in groups which were concomitantly lesioned with 6-OHDA or vehicle (sham) in the GP, or EP, and in the medial forebrain bundle (MFB) as follows: a) MFB-sham+GP-sham, b) MFB-sham+GP-lesion, c) MFB-lesion+GP-sham, d) MFB-lesion+GP-lesion, e) MFB-sham+EP-sham, f) MFB-sham+EP-lesion, g) MFB-lesion+EP-sham, and h) MFB-lesion+EP-lesion. Four weeks later, animals were treated with L-Dopa (6 mg/kg) twice daily for 22 days.. Immunohistochemical studies were performed in order to investigate the changes in pallidal SER and serotonin transporter (SERT) levels. GP, but not EP, DAergic denervation attenuated LID in rats with a concomitant MFB lesion (p < 0.01). No differences were found in GP SERT expression between groups of animals developing or not LID. These results provide evidence of the relevance of GP DAergic innervation in LID. The conversion of levodopa to DA in GP serotonergic nerve fibers appears not to be the major mechanism underlying LID.

Botulinum toxin A injection into the entopeduncular nucleus improves dynamic locomotory parameters in hemiparkinsonian rats.

Parkinson's disease is associated with hyperactivity of the subthalamic nucleus (STN), contributing to motor and gait disturbances. Although deep brain stimulation of the STN alleviates certain motor dysfunction, its specific effect on gait abnormalities remains controversial. This study investigated the long-term changes in locomotion following direct infusions of botulinum toxin-A into the globus pallidus internal segment (GPi) to suppress the flow of information from the STN to the GPi in a hemiparkinsonian rat model. Static and dynamic gait parameters were quantified using a CatWalk apparatus. Interestingly, botulinum toxin-A at 0.5 ng significantly reduced only the dynamic gait parameters of hemiparkinsonian rats at 1 week and 1 month post-infusion, while static gait parameters did not change. This study offers new insights into the complexity of basal ganglia in locomotor control and shows the potential of central infusion of botulinum toxin-A as a novel intervention in the study of experimental hemiparkinson's disease.

Acute L-DOPA administration reverses changes in firing pattern and low frequency oscillatory activity in the entopeduncular nucleus from long term L-DOPA treated 6-OHDA-lesioned rats.

The pathophysiology of Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID) is associated with aberrant neuronal activity and abnormal high levels of oscillatory activity and synchronization in several basal ganglia nuclei and the cortex. Previously, we have shown that the firing activity of neurons in the substantia nigra pars reticulata (SNr) is relevant in dyskinesia and may be driven by subthalamic nucleus (STN) hyperactivity. Conversely, low frequency oscillatory activity and synchronization in these structures seem to be more important in PD because they are not influenced by prolonged L-DOPA administration. The aim of the present study was to assess (through single-unit extracellular recording techniques under urethane anaesthesia) the neuronal activity of the entopeduncular nucleus (EPN) and its relationship with LID and STN hyperactivity, together with the oscillatory activity and synchronization between these nuclei and the cerebral cortex in 6-OHDA-lesioned rats that received long term L-DOPA treatment (or not). Twenty-four hours after the last L-DOPA injection the firing activity of EPN neurons in long term L-DOPA treated 6-OHDA-lesioned rats was more irregular and bursting compared to sham rats, being those alterations partially reversed by the acute challenge of L-DOPA. No correlation between EPN neurons firing activity and abnormal involuntary movements score was found. However, there was a significant correlation between the firing activity parameters of EPN and STN neurons recorded from long term L-DOPA treated 6-OHDA-lesioned rats. Low frequency oscillatory activity and synchronization both within the EPN and with the cerebral cortex were enhanced in 6-OHDA-lesioned animals. These changes were reversed by the acute L-DOPA challenge only in long term L-DOPA treated 6-OHDA-lesioned rats. Altogether, these results obtained from long term L-DOPA treated 6-OHDA-lesioned rats suggest (1) a likely relationship between STN and EPN firing patterns and spiking phases induced by changes after prolonged L-DOPA administration and (2) that the effect of L-DOPA on the firing pattern, low frequency oscillatory activity and synchronization in the EPN may have a relevant role in LID.

The anterior and posterior pedunculopontine tegmental nucleus are involved in behavior and neuronal activity of the cuneiform and entopeduncular nuclei.

Loss of cholinergic neurons in the mesencephalic locomotor region, comprising the pedunculopontine nucleus (PPN) and the cuneiform nucleus (CnF), is related to gait disturbances in late stage Parkinson's disease (PD). We investigate the effect of anterior or posterior cholinergic lesions of the PPN on gait-related motor behavior, and on neuronal network activity of the PPN area and basal ganglia (BG) motor loop in rats. Anterior PPN lesions, posterior PPN lesions or sham lesions were induced by stereotaxic microinjection of the cholinergic toxin AF64-A or vehicle in male Sprague-Dawley rats. First, locomotor activity (open field), postural disturbances (Rotarod) and gait asymmetry (treadmill test) were assessed. Thereafter, single-unit and oscillatory activities were measured in the non-lesioned area of the PPN, the CnF and the entopeduncular nucleus (EPN), the BG output region, with microelectrodes under urethane anesthesia. Additionally, ECoG was recorded in the motor cortex. Injection of AF64-A into the anterior and posterior PPN decreased cholinergic cell counts as compared to naive controls (P<0.001) but also destroyed non-cholinergic cells. Only anterior PPN lesions decreased the front limb swing time of gait in the treadmill test, while not affecting other gait-related parameters tested. Main electrophysiological findings were that anterior PPN lesions increased the firing activity in the CnF (P<0.001). Further, lesions of either PPN region decreased the coherence of alpha (8-12 Hz) band between CnF and motor cortex (MCx), and increased the beta (12-30 Hz) oscillatory synchronization between EPN and the MCx. Lesions of the PPN in rats had complex effects on oscillatory neuronal activity of the CnF and the BG network, which may contribute to the understanding of the pathophysiology of gait disturbance in PD.

Coherence of neuronal firing of the entopeduncular nucleus with motor cortex oscillatory activity in the 6-OHDA rat model of Parkinson's disease with levodopa-induced dyskinesias.

The pathophysiological mechanisms leading to dyskinesias in Parkinson's disease (PD) after long-term treatment with levodopa remain unclear. This study investigates the neuronal firing characteristics of the entopeduncular nucleus (EPN), the rat equivalent of the human globus pallidus internus and output nucleus of the basal ganglia, and its coherence with the motor cortex (MCx) field potentials in the unilateral 6-OHDA rat model of PD with and without levodopa-induced dyskinesias (LID). 6-hydroxydopamine-lesioned hemiparkinsonian (HP) rats, 6-OHDA-lesioned HP rats with LID (HP-LID) rats, and naïve controls were used for recording of single-unit activity under urethane (1.4 g/kg, i.p) anesthesia in the EPN "on" and "off" levodopa. Over the MCx, the electrocorticogram output was recorded. Analysis of single-unit activity in the EPN showed enhanced firing rates, burst activity, and irregularity compared to naïve controls, which did not differ between drug-naïve HP and HP-LID rats. Analysis of EPN spike coherence and phase-locked ratio with MCx field potentials showed a shift of low (12-19 Hz) and high (19-30 Hz) beta oscillatory activity between HP and HP-LID groups. EPN theta phase-locked ratio was only enhanced in HP-LID compared to HP rats. Overall, levodopa injection had no stronger effect in HP-LID rats than in HP rats. Altered coherence and changes in the phase lock ratio of spike and local field potentials in the beta range may play a role for the development of LID.

Neuronal Entropy-Rate Feature of Entopeduncular Nucleus in Rat Model of Parkinson's Disease.

The function of the nigro-striatal pathway on neuronal entropy in the basal ganglia (BG) output nucleus, i.e. the entopeduncular nucleus (EPN) was investigated in the unilaterally 6-hyroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease (PD). In both control subjects and subjects with 6-OHDA lesion of dopamine (DA) the nigro-striatal pathway, a histological hallmark for parkinsonism, neuronal entropy in EPN was maximal in neurons with firing rates ranging between 15 and 25 Hz. In 6-OHDA lesioned rats, neuronal entropy in the EPN was specifically higher in neurons with firing rates above 25 Hz. Our data establishes that the nigro-striatal pathway controls neuronal entropy in motor circuitry and that the parkinsonian condition is associated with abnormal relationship between firing rate and neuronal entropy in BG output nuclei. The neuronal firing rates and entropy relationship provide putative relevant electrophysiological information to investigate the sensory-motor processing in normal condition and conditions such as movement disorders.

Long-term increase in coherence between the basal ganglia and motor cortex after asphyxial cardiac arrest and resuscitation in developing rats.

BACKGROUND: The basal ganglia are vulnerable to injury during cardiac arrest. Movement disorders are a common morbidity in survivors. Yet, neuronal motor network changes post-arrest remain poorly understood. METHODS: We compared function of the motor network in adult rats that, during postnatal week 3, underwent 9.5 min of asphyxial cardiac arrest (n = 9) or sham intervention (n = 8). Six months after injury, we simultaneously recorded local field potentials (LFP) from the primary motor cortex (MCx) and single neuron firing and LFP from the rat entopeduncular nucleus (EPN), which corresponds to the primate globus pallidus pars interna. Data were analyzed for firing rates, power, and coherence between MCx and EPN spike and LFP activity. RESULTS: Cardiac arrest survivors display chronic motor deficits. EPN firing rate is lower in cardiac arrest survivors (19.5 ± 2.4 Hz) compared with controls (27.4 ± 2.7 Hz; P < 0.05). Cardiac arrest survivors also demonstrate greater coherence between EPN single neurons and MCx LFP (3-100 Hz; P < 0.001). CONCLUSIONS: This increased coherence indicates abnormal synchrony in the neuronal motor network after cardiac arrest. Increased motor network synchrony is thought to be antikinetic in primary movement disorders. Characterization of motor network synchrony after cardiac arrest may help guide management of post-hypoxic movement disorders.

l-dopa-induced dyskinesias in unilateral 6-hydroxydopamine-lesioned rats are not modified by excitotoxic lesion of the entopeduncular nucleus and substantia nigra pars reticulata.

l-Dopa-induced dyskinesias (LIDs) are a troublesome complication in Parkinson's disease after long-term therapy and a major reason for surgical treatment. LIDs are effectively eliminated by surgery. We aimed to reproduce such effect in the 6-hydroxydopamine (6-OHDA)-lesioned rat model. Single or combined lesions with quinolinic acid were caused in the entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr) on 6-hydroxydopamine (6-OHDA)-lesioned rats treated for 3 weeks with l-Dopa (6 mg/kg plus 15 mg/kg benserazide, i.p.). l-Dopa administration was continued for a further week following the lesion and abnormal involuntary movements (AIMs) scored at the end of treatment. Neither the individual lesions of the EP and SNr nor the combined lesions had any antidyskinetic effect nor decreased the total number of rotations. These results suggest that excitotoxic lesions of neurons bodies of the output nuclei of the basal ganglia, which destroy cell bodies and spare fibers of passage, do not induce a beneficial reduction of dyskinesias in contrast to thermolytic lesions in humans (which provokes a complete tissue destruction), thus supporting the possibility that other nuclei or systems might be involved in the antidyskinetic effect of pallidotomy.

Effects of repeated deep brain stimulation on depressive- and anxiety-like behavior in rats: comparing entopeduncular and subthalamic nuclei.

BACKGROUND: Deep brain stimulation (DBS) of the subthalamic nucleus (STN) or internal globus pallidus (GPi) has been routinely used for the treatment of some movement disorders. However, DBS may be associated with adverse psychiatric effects, such as depression, anxiety and impulsivity. OBJECTIVE: To compare DBS applied to the entopeduncular nucleus (EPN; the rodent homolog of the GPi) and STN in terms of their effects on depressive- and anxiety-like behavior in rats. METHODS: DBS was applied for 21 days (4 h a day) to either the STN or EPN. Rats then underwent behavioral testing on learned helplessness and elevated plus maze tasks before being sacrificed for brain analyses of zif268, BDNF and trkB mRNA as well as BDNF protein levels. RESULTS: Repeated DBS of the STN, but not of the EPN, led to impaired performance in the learned helplessness task, suggesting that STN-DBS induces or potentiates depressive-like behavior. There was no effect of DBS on elevated plus maze or on open field behavior. Repeated STN-DBS, but not EPN-DBS, led to decreased levels of BDNF and trkB mRNA in hippocampus. Acute stimulation of the STN or EPN resulted in similar changes in zif268 levels in several brain areas, except for the raphe where decreases were seen only after STB-DBS. CONCLUSIONS: Together these results indicate that the effects of STN- and EPN-DBS differ in behavioral and neurochemical respects. Results further suggest that the EPN may be a preferable target for clinical DBS when psychiatric side effects are considered insofar as it may be associated with a lower incidence of depressive-like behavior than the STN.

Activity modulation of the globus pallidus and the nucleus entopeduncularis affects compulsive checking in rats.

Deep brain stimulation at high frequencies (HFS) is currently studied in the treatment of therapy-refractory obsessive-compulsive disorder (OCD). The diversity of targeted brain areas and the discrepancy in demonstrating beneficial effects, highlight the need for better mapping of brain regions in which HFS may yield anti-compulsive effects. This goal may be achieved by investigating the effects of HFS in appropriate animal models of OCD. The present study tested the effect of bilateral HFS or pharmacological inactivation (as induced by intracerebral administration of the GABA-agonist muscimol) of both the Globus pallidus (GP; rodent equivalent to human GP externus) and the Nucleus entopeduncularis (EP; rodent equivalent to human GP internus) on checking behaviour in the quinpirole rat model of OCD. We demonstrate that HFS of the GP does not and HFS of the EP only partially reduces OCD-like behaviour in rats. In contrast, pharmacological inactivation of both GP and EP significantly reduces OCD-like behaviour in the model. These data contrast previously derived data on the effectiveness of HFS of the subthalamic nucleus, nucleus accumbens, GP and EP in the same and other rat models of OCD. We conclude that (i) although GP and EP play an important role in the pathophysiology of OCD, these areas may not represent first choice target structures for HFS, (ii) the effectiveness of HFS may depend on different subtypes of OCD, represented in different animal models, and (iii) differential net mechanisms may subserve the effectiveness of HFS and pharmacological inactivation.

Effects of pharmacological entopeduncular manipulations on idiopathic dystonia in the dt(sz) mutant hamster.

The pathophysiology of idiopathic dystonias is still unknown, but it is regarded as a basal ganglia disorder. Previous experiments in the dt(sz) hamster, a model of primary paroxysmal dystonia, demonstrated reduced discharge rates and an abnormal pattern within the entopeduncular nucleus (EPN), a basal ganglia output structure. To clarify if this is based on abnormal gamma-aminobutyric acid(GABA)ergic or glutamatergic input, microinjections into the EPN were done in mutant hamsters in the present study. The GABA(A) receptor antagonists pentylenetetrazole and bicuculline exerted moderate antidystonic effects, while previous systemic administrations worsened dystonia in the dt(sz) mutant. GABA-potentiating drugs, i.e., the GABA(A) receptor agonist muscimol and the GABA transporter inhibitor 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene)amino]oxy]ethyl]-3-pyridinecarboxy-lic acid (NNC-711), which are known to improve dystonia after systemic treatment in mutant hamsters, did not exert significant effects after EPN injections, but NNC-711 tended to increase the severity at the highest dose (2.5 ng bilateral). The NMDA receptor antagonist D(-)-2-amino-5-phosphopentanoic acid (AP-5) retarded the onset of a dystonic attack. However, this effect was not dose dependent and the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzol(f)quinoxaline (NBQX) alone or in combination with AP-5 and NNC-711, also failed to show any effects on dystonia. The present data do not provide clear evidence for an enhanced striatal GABAergic input or a reduced glutamatergic activation of the EPN via the subthalamic nucleus, i.e., more pronounced antidystonic effects of GABA(A) receptor antagonists and stronger prodystonic effects of GABAmimetics and glutamate receptor antagonists were expected. Nevertheless, previously found changes in entopeduncular activity probably play a critical pathophysiological role in dystonic hamsters.

Deep brain stimulation changes basal ganglia output nuclei firing pattern in the dystonic hamster.

Dystonia is a heterogeneous syndrome of movement disorders characterized by involuntary muscle contractions leading to abnormal movements and postures. While medical treatment is often ineffective, deep brain stimulation (DBS) of the internal pallidum improves dystonia. Here, we studied the impact of DBS in the entopeduncular nucleus (EP), the rodent equivalent of the human globus pallidus internus, on basal ganglia output in the dt(sz)-hamster, a well-characterized model of dystonia by extracellular recordings. Previous work has shown that EP-DBS improves dystonic symptoms in dt(sz)-hamsters. We report that EP-DBS changes firing pattern in the EP, most neurons switching to a less regular firing pattern during DBS. In contrast, EP-DBS did not change the average firing rate of EP neurons. EP neurons display multiphasic responses to each stimulation impulse, likely underlying the disruption of their firing rhythm. Finally, neurons in the substantia nigra pars reticulata display similar responses to EP-DBS, supporting the idea that EP-DBS affects basal ganglia output activity through the activation of common afferent fibers.

High frequency stimulation of the entopeduncular nucleus sets the cortico-basal ganglia network to a new functional state in the dystonic hamster.

High frequency stimulation (HFS) of the internal pallidum is effective for the treatment of dystonia. Only few studies have investigated the effects of stimulation on the activity of the cortex-basal ganglia network. We here assess within this network the effect of entopeduncular nucleus (EP) HFS on the expression of c-Fos and cytochrome oxidase subunit I (COI) in the dt(sz)-hamster, a well-characterized model of paroxysmal dystonia. In dt(sz)-hamsters, we identified abnormal activity in motor cortex, basal ganglia and thalamus. These structures have already been linked to the pathophysiology of human dystonia. EP-HFS (i) increased striatal c-Fos expression in controls and dystonic hamsters and (ii) reduced thalamic c-Fos expression in dt(sz)-hamsters. EP-HFS had no effect on COI expression. The present results suggest that EP-HFS induces a new network activity state which may improve information processing and finally reduces the severity of dystonic attacks in dt(sz)-hamsters.

Deficient sensorimotor gating induced by selective breeding in rats is improved by entopeduncular nucleus lesions.

Deficient prepulse inhibition (PPI) of startle reflects disturbed sensorimotor gating found in certain neuropsychiatric disorders, such as Tourette's syndrome, ADHD, Huntington's and schizophrenia. We here tested, whether lesions of the entopeduncular nucleus (EPN) would improve a PPI-deficit induced by selective breeding. Rats with breeding induced high and low expression of PPI were stereotaxically microinjected with ibotenate (0.2 microg in 0.3 microl phosphate buffered saline) or vehicle into the EPN and two weeks later tested for PPI of the acoustic startle response (ASR) and motor activity. Lesions of the EPN counteracted the breeding-induced PPI-deficit and reduced ASR in the PPI low group without affecting their motor activity. In the PPI high group EPN lesions did not affect PPI, ASR, and motor activity. This work indicates an important role of the EPN in the modulation of sensorimotor gating. Additionally, PPI low rats may provide a non-pharmacological model that can be used to develop new therapeutic strategies for neuropsychiatric disorders.

Glutamatergic pallidothalamic projections and their implications in the pathophysiology of Parkinson's disease.

GABAergic projections emitted from the entopeduncular nucleus (ENT) and the substantia nigra pars reticulata (SNr) innervate different thalamic nuclei and they are known to be hyperactive after dopaminergic depletion. Here we show that isoform 2 of the vesicular glutamate transporter (VGLUT2) is expressed by neurons in the ENT nucleus but not in the SNr. Indeed, dual in situ hybridization demonstrated that the ENT nucleus contains two different subpopulations of projection neurons, one single-expressing GAD65/67 mRNAs and another one that co-expresses either of the GAD isoforms together with VGLUT2 mRNA. Unilateral dopaminergic depletion induced marked changes in pallidothalamic-projecting neuron gene expression, resulting in increased expression of GAD65/67 mRNAs together with a clear down-regulation of VGLUT2 mRNA expression. Our results indicate that the increased thalamic inhibition typical of dopamine depletion might be explained by a synergistic effect of increased GABA outflow coupled to decreased glutamate levels, both neurotransmitters coming from ENT neurons.

Modulation of torsinA expression in the globus pallidus internus is associated with levodopa-induced dyskinesia in hemiparkinsonian rats.

TorsinA is the causative protein of DYT1 dystonia, a major representative of hyperkinetic movement disorders. In this study, the distribution of torsinA was investigated in the basal ganglia of hemiparkinsonian rats with or without levodopa-induced dyskinesia (LID). Two months after 6-hydroxydopamine (OHDA) treatment, Wistar-albino rats were subjected to intermittent intraperitoneal injection of levodopa/benserazid (LID-group, n=5) or vehicle (control, n=5) for 21 days. Immunohistochemical analysis disclosed that in the caudal portion of the entopeduncular nucleus (EP), homologous to the globus pallidus internus (GPi) in primates, on the parkinsonian side, there was a significant decrease of torsinA-immunopositive neurons in rats with LID, but not in those without LID. However, Nissl-staining showed no loss of GPi neurons in rats with LID. In both groups, there was no significant difference between ipsi- and contralateral sides with respect to the density of torsinA-positive neuronal cells in the striatum, globus pallidus externus, and subthalamic nucleus. Ours are the first data to demonstrate the specific modulation of torsinA expression in the basal ganglia of the hyperkinesia model, suggesting that GPi neurons containing torsinA possess pathologic plasticity for LID.

Effect of subthalamic nucleus or entopeduncular nucleus lesion on levodopa-induced neurochemical changes within the basal ganglia and on levodopa-induced motor alterations in 6-hydroxydopamine-lesioned rats.

Inactivation of the subthalamic nucleus (STN) or the internal segment of the pallidum (GPi)/entopeduncular nucleus (EP) by deep brain stimulation or lesioning alleviates clinical manifestations of Parkinson's disease (PD) as well as reducing the side-effects of levodopa treatment. However, the effects of STN or entopeduncular nucleus (EP) lesion on levodopa-related motor fluctuations and on neurochemical changes induced by levodopa remain largely unknown. The effects of such lesions on levodopa-induced motor alterations were studied in 6-hydroxydopamine (6-OHDA)-lesioned rats and were assessed neurochemically by analyzing the functional activity of the basal ganglia nuclei, using the expression levels of the mRNAs coding for glutamic acid decarboxylase and cytochrome oxidase as molecular markers of neuronal activity. At the striatal level, preproenkephalin (PPE) mRNA levels were analyzed. We found in 6-OHDA-lesioned rats that a unilateral STN or EP lesion ipsilateral to the 6-OHDA lesion had no effect on either the shortening in the duration of the levodopa-induced rotational response or the levodopa-induced biochemical changes in the basal ganglia nuclei. In contrast, overexpression of PPE mRNA due to levodopa treatment was reversed by the STN or EP lesion. Our study thus shows that lesion of the EP or STN may counteract some of the neurochemical changes induced by levodopa treatment within the striatum.

Altered discharge pattern of basal ganglia output neurons in an animal model of idiopathic dystonia.

A decreased activity of basal ganglia output neurons is thought to underlie idiopathic dystonias and other hyperkinetic movement disorders. We found recently a reduced spontaneous discharge rate of entopeduncular neurons (internal globus pallidus in primates) in dt(sz) hamsters, an unique model for idiopathic paroxysmal dystonia in which stress-inducible attacks show an age-dependent severity. Otherwise, it has been suggested that an altered discharge pattern may be more important for the occurrence of dystonia than a reduced discharge rate. Based on qualitative and computerized quantitative evaluations of interspike interval histograms and spike trains of extracellularly recorded single neurons, we investigated the spontaneous discharge pattern of GABAergic entopeduncular and nigral neurons in dt(sz) hamsters at different ages. The discharge pattern of entopeduncular neurons was highly irregular and showed an altered burst-like firing in dt(sz) hamsters at the age of the most marked expression of dystonia when compared with age-matched nondystonic controls. In line with a recently reported normalization of discharge rates after age-dependent disappearance of dystonia, we found an almost complete normalization of the discharge pattern of entopeduncular neurons after remission of dystonia in dt(sz) hamsters. Investigations of GABAergic nigral neurons, reported recently to have the same spontaneous discharge rates in dystonic and nondystonic hamsters, did not show an altered firing pattern in dt(sz) hamsters. The present data clearly indicate the fundamental importance of an altered discharge pattern of entopeduncular neurons for the expression of paroxysmal dystonia, and probably also for other dyskinesias, and may explain the improvements obtained by pallidotomy in dystonic patients despite an obviously reduced pallidal output.

Effects of intralaminar thalamic nuclei lesion on glutamic acid decarboxylase (GAD65 and GAD67) and cytochrome oxidase subunit I mRNA expression in the basal ganglia of the rat.

This study investigated the influence of thalamic inputs on neuronal metabolic activity in the rat basal ganglia. By means of in situ hybridization histochemistry, we examined the consequences of ibotenate-induced unilateral lesion of intralaminar thalamic nuclei on mRNA expression of cytochrome oxidase subunit-I (CoI) in the striatum and the subthalamic nucleus (STN) and of the two isoforms of glutamate decarboxylase (GAD65 and GAD67) in the striatum, globus pallidus (GP), entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr). In the striatum, GAD67 mRNA expression decreased selectively in the rostral part of the structure at 5 and 12 days postlesion (approximately -30%), whereas, GAD65 mRNA levels was downregulated only in the caudal striatum at 12 days (-29%). In both the striatum and STN, CoI mRNA expression decreased ipsilaterally at 5 and bilaterally at 12 days. In GP, GAD67 and GAD65 mRNA expression decreased ipsilaterally at 5 (-20% and -26%) and 12 days (-23% and -36%). In EP, selective bilateral decreases in GAD67 mRNA expression were found at 5 and 12 days (-50% and -40%). Conversely, in SNr, only GAD65 mRNA expression was reduced bilaterally at both time points. These data show that the thalamus exerts a widespread excitatory influence on the basal ganglia network that cannot be accounted for solely by its known direct connections. Given the recent data showing that intralaminar thalamic nuclei are a major nondopaminergic site of neurodegeneration in Parkinson's disease, these results may have a critical bearing on understanding the cellular basis of basal ganglia dysfunction in parkinsonism.