Coadministration of entacapone with levodopa attenuates the severity of dyskinesias in hemiparkinsonian rats.

Levodopa-induced dyskinesias (LIDs) have been associated with a sequence of events that includes pulsatile stimulation of dopamine receptors. The degree of nigrostriatal degeneration, the half-life of dopaminomimetic agents, and the dose of levodopa used to treat parkinsonian symptoms are factors directly correlated with the development of motor complications in Parkinson's disease patients. Long-acting agents producing continuous dopaminergic stimulation are less likely to prime for dyskinesia than short-acting drugs that produce pulsatile stimulation of dopamine receptors. Inhibition of the enzyme catechol-O-methyl transferase (COMT) by entacapone extends the half-life of levodopa and minimizes variability in plasma levodopa levels. The aim of the present study was to characterize the effect of the early administration of the COMT inhibitor entacapone in the recently described model of LIDs in rats with a nigrostriatal lesion induced by 6-hydroxydopamine (6-OHDA). Male Sprague-Dawley rats received a unilateral 6-OHDA administration in the nigrostriatal pathway. Animals were treated either with levodopa (6 mg/kg, twice at day, i.p.) plus entacapone (30 mg/kg per day, i.p.) or levodopa (6 mg/kg, twice at day, i.p.) plus vehicle for 22 consecutive days. Early administration of entacapone, in association with levodopa, induces a decrease in the severity of dyskinesia and delays their onset in hemiparkinsonian rats. All dyskinesia subtypes evaluated, such as axial, limb, and orofacial dyskinesias, have shown similar reductions. These results suggest that entacapone, by extending levodopa elimination half-life, might reduce its propensity to induce motor complications.     

Expression of dopamine and vesicular monoamine transporters and differential vulnerability of mesostriatal dopaminergic neurons

Numerous studies suggest that the dopamine transporter (DAT), responsible for dopamine reuptake, may act as a vulnerability factor in the pathogenesis of Parkinson's disease (PD) and the vesicular monoamine transporter (VMAT2), responsible for its vesicular storage, as a neuroprotective factor. However, the relevance of each on the differential vulnerability of midbrain DA cells remains unknown. Here we studied the relationship between the expression pattern (mRNA and protein) of both transporters and the differential vulnerability of midbrain DA cells in a model of PD (intracerebroventricular injection of 6-OHDA in rats) and in monkey and human midbrain. Our results revealed that the expression patterns for VMAT2 mRNA and protein and DAT mRNA are similar, with the highest levels in the rostromedial region of substantia nigra (SNrm), followed by the caudoventral region of SN (SNcv), the ventral tegmental area and pigmented parabrabraquial nucleus (VTA/PBP), and finally the linear and interfascicular nuclei (Li/IF). In contrast, the expression of DAT protein in rats, monkeys, and humans followed a caudoventrolateral-to-rostrodorsomedial decreasing gradient (SNcv > SNrm > VTA/PBP > Li/IF), matching the degeneration profile observed after intracerebroventricular injection of 6-OHDA and in PD. In addition, DAT blockade made all midbrain DA cells equally resistant to 6-OHDA. These data indicate that DAT protein levels, but not DAT mRNA levels, are closely related to the differential vulnerability of midbrain DA cells and that this relationship is unaffected by the relative levels of VMAT2. Furthermore, the difference between DAT mRNA and protein profiles suggests internuclear differences in its posttransductional regulation.     

The majority of newly generated cells in the adult mouse substantia nigra express low levels of Doublecortin,but their proliferation is unaffected by 6‐OHDA‐induced nigral lesion or Minocycline‐mediated inhibition of neuroinflammation

Parkinson's disease is characterized by a selective loss of dopaminergic neurons in the substantia nigra (SN). However, whether regenerative endogenous neurogenesis is taking place in the mammalian SN of parkinsonian and non‐parkinsonian brains remains of debate. Here, we tested whether proliferating cells in the SN and their neurogenic potential would be affected by anti‐inflammatory treatment under physiological conditions and in the 6‐hydroxy‐dopamine (6‐OHDA) Parkinson's disease mouse model. We report that the majority of newly generated nigral cells are positive for Doublecortin (Dcx), which is an often used marker for neural progenitor cells. Yet, Dcx expression levels in these cells were much lower than in neural progenitor cells of the subventricular zone and the dentate gyrus neural progenitor cells. Furthermore, these newly generated nigral cells are negative for neuronal lineage markers such as TuJ1 and NeuN. Therefore, their neuronal commitment is questionable. Instead, we found evidence for oligodendrogenesis and astrogliosis in the SN. Finally, neither short‐term nor long‐term inhibition of neuroinflammation by Minocycline‐ or 6‐OHDA‐induced lesion affected the numbers of newly generated cells in our disease paradigm. Our findings of adult generated Dcx⁺ cells in the SN add important data for understanding the cellular composition and consequently the regenerative capacity of the SN.     

p-Quinone mediates 6-hydroxydopamine-induced dopaminergic neuronal death and ferrous iron accelerates the conversion of p-quinone into melanin extracellularly

Parkinson's disease (PD) is characterized by the selective loss of dopaminergic neurons in the substantia nigra (SN). 6-Hydroxydopamine (6-OHDA), a dopaminergic neurotoxin, is detected in human brains and the urine of PD patients. Using SH-SY5Y, a human neuroblastoma cell line, we demonstrated that 6-OHDA toxicity was determined by the amount of p-quinone produced in 6-OHDA auto-oxidation rather than by reactive oxygen species (ROS). Glutathione (GSH), which conjugated with p-quinone, provided significant protection whereas catalase, which detoxified hydrogen peroxide and superoxide anions, failed to block cell death caused by 6-OHDA. Although iron accumulated in the SN of patients with PD can cause dopaminergic neuronal degeneration by enhancing oxidative stress, we found that extracellular ferrous iron promoted the formation of melanin and reduced the amount of p-quinone. The addition of ferrous iron to the culture medium inhibited caspase-3 activation and apoptotic nuclear morphologic changes and blocked 6-OHDA-induced cytotoxicity in SH-SY5Y cells and primary cultured mesencephalic dopaminergic neurons. These data suggested that generation of p-quinone played a pivotal role in 6-OHDA-induced toxicity and extracellular iron in contrast to intracellular iron was protective rather than harmful because it accelerated the conversion of p-quinone into melanin.     

Alteration of nociceptive integration in the spinal cord of a rat model of Parkinson's disease

Background : Pain is a major non motor symptom that contributes to impaired quality of life in PD. However, its mechanism is unknown. Objectives and Methods : We sought to identify the pain phenotypes and parallel changes in spinal integration of peripheral stimuli in a rat model of PD induced by lesions of SN dopamine neurons, using behavioral plantar and von Frey tests as well as electrophysiology of the dorsal horn. Results : We show that dopamine depletion by 6‐OHDA induced hypersensitivity to mechanical and thermal stimuli. These abnormal behaviors were paralleled by increased neuronal responses and hyperexcitability of wide dynamic range neurons of lamina V of the dorsal horn of the spinal cord in response to electrical stimulation of the sciatic nerve in the 6‐OHDA model as compared to sham rats. Conclusions : These results provide evidence for alteration of nociceptive integration in the spinal dorsal horn neurons in 6‐OHDA rats that can reflect changes in pain behavior. © 2018 International Parkinson and Movement Disorder Society     

Neuroprotective role of ERK1/2 and ERK5 in a dopaminergic cell line under basal conditions and in response to oxidative stress

Loss of motor function in Parkinson's disease is due in part to degeneration of dopamine (DA) neurons. Pharmacological evidence suggests that the mitogen-activated protein kinase signaling pathways involving extracellular signal-regulated kinases (ERKs) play important roles in neuroprotection of DA neurons. However, the relative roles of the several ERK isoforms in the viability of DA neurons have not yet been determined. In the present study, we investigated the contributions of ERK5, as well as ERK1/2, to MN9D cell survival under basal conditions and in response to 6-hydroxydopamine (6-OHDA). We observed that U0126, an inhibitor of ERK activation, decreased basal survival of these cells. To differentiate between ERK1/2 and ERK5, cells were transfected with a dominant negative form of either ERK5 or MEK1, the upstream activator of ERK1/2. Transfection of MN9D cells with either dominant negative construct mimicked U0126, reducing cell survival. Moreover, transfection of the cells in such a way as to increase ERK5 or ERK1/2 activity inhibited 6-OHDA-induced cell death, although this effect was significant only in the case of ERK1/2 activation. These studies suggest that activations of ERK5 and ERK1/2 both promote basal DA cell survival and that ERK1/2 also protects DA cells from oxidative stress. These are the first studies to demonstrate a role for ERK5 in DA neuronal survival and to investigate the relative roles of ERK1/2 and ERK5 in basal DA survival and neuroprotection from oxidative stress.     

Relationship between dopamine deficit and the expression of depressive behavior resulted from alteration of serotonin system

Depression frequently accompanies in Parkinson's disease (PD). Previous research suggested that dopamine (DA) and serotonin systems are closely linked with depression in PD. However, comprehensive studies about the relationship between these two neurotransmitter systems are limited. Therefore, the purpose of this study is to evaluate the effect of dopaminergic destruction on the serotonin system. The interconnection between motor and depression was also examined. Two PET scans were performed in the 6‐hydroxydopamine (6‐OHDA) lesioned and sham operated rats: [¹⁸F]FP‐CIT for DA transporters and [¹⁸F]Mefway for serotonin 1A (5‐HT_1A) receptors. Here, 6‐OHDA is a neurotoxin for dopaminergic neurons. Behavioral tests were used to evaluate the severity of symptoms: rotational number for motor impairment and immobility time, acquired from the forced swim test for depression. Region‐of‐interests were drawn in the striatum and cerebellum for the DA system and hippocampus and cerebellum for the 5‐HT system. The cerebellum was chosen as a reference region. Nondisplaceable binding potential in the striatum and hippocampus were compared between 6‐OHDA and sham groups. As a result, the degree of DA depletion was negatively correlated with rotational behavior (R² = 0.79, P = 0.003). In 6‐OHDA lesioned rats, binding values for 5‐HT_1A receptors was 22% lower than the sham operated group. This decrement of 5‐HT_1A receptor binding was also correlated with the severity of depression (R² = 0.81, P = 0.006). Taken together, this research demonstrated that the destruction of dopaminergic system causes the reduction of the serotonergic system resulting in the expression of depressive behavior. The degree of dopaminergic dysfunction was positively correlated with the impairment of the serotonin system. Severity of motor symptoms was also closely related to depressive behavior. Synapse 69:453–460, 2015. © 2015 Wiley Periodicals, Inc.     

Intrasubthalamic injection of 6-hydroxydopamine induces changes in the firing rate and pattern of subthalamic nucleus neurons in the rat

The subthalamic nucleus (STN) receives dopaminergic projections from the substantia nigra pars compacta (SNc). To investigate the role of direct and indirect dopaminergic influences on STN neurons, the spontaneous activity was studied in four groups of animals: normal rats, rats with intrasubthalamic or intranigral injection of 6-hydroxydopamine (6-OHDA), and sham STN injection rats by using extracellular recordings 4 weeks postsurgery. After intrasubthalamic injection of 6-OHDA, the mean firing rate significantly decreased (7.29 +/- 0.39 spikes/sec, P < 0.01 vs. 11.13 +/- 0.59 spikes/sec in normal or 11.26 +/- 0.57 spikes/sec in sham group), and the percentage of STN neurons discharging regularly decreased significantly (81%, P < 0.05 vs. 90% in normal group or P < 0.01 vs. 92% in sham group) and that of bursty cells increased (19%, P < 0.05 vs. 10%; in normal group or P < 0.01 vs. 8% in sham group). In the group of rats with SNc lesion, the firing rate of subthalamic neurons did not show a significant difference (11.61 +/- 0.81 spikes/sec) compared with normal group. However, the firing pattern was dramatically changed: 74% of cells exhibited bursty pattern and only 26% of cells discharged regularly or slightly irregularly. Immunohistochemical results showed that intrasubthalamic injection of 6-OHDA induced a marked degeneration of dopaminergic cells in the lateral part of the ipsilateral SNc, whereas 6-OHDA injection into the SNc induced a total in situ lesion of dopamine cells. These results suggest that the SNc exerts an excitatory influence on STN neurons and that the loss of this dopaminergic projection could, at least partially, account for the changes in the firing pattern of STN neurons in the 6-OHDA rat model of parkinsonism.     

Elevated potassium provides an ionic mechanism for deep brain stimulation in the hemiparkinsonian rat

The mechanism of high‐frequency stimulation used in deep brain stimulation (DBS) for Parkinson's disease (PD) has not been completely elucidated. Previously, high‐frequency stimulation of the rat entopeduncular nucleus, a basal ganglia output nucleus, elicited an increase in [K⁺]_e to 18 mm , in vitro. In this study, we assessed whether elevated K⁺ can elicit DBS‐like therapeutic effects in hemiparkinsonian rats by employing the limb‐use asymmetry test and the self‐adjusting stepping test. We then identified how these effects were meditated with in‐vivo and in‐vitro electrophysiology. Forelimb akinesia improved in hemiparkinsonian rats undergoing both tests after 20 mm KCl injection into the substantia nigra pars reticulata (SNr) or the subthalamic nucleus. In the SNr, neuronal spiking activity decreased from 38.2 ± 1.2 to 14.6 ± 1.6 Hz and attenuated SNr beta‐frequency (12–30 Hz) oscillations after K⁺ treatment. These oscillations are commonly associated with akinesia/bradykinesia in patients with PD and animal models of PD. Pressure ejection of 20 mm KCl onto SNr neurons in vitro caused a depolarisation block and sustained quiescence of SNr activity. In conclusion, our data showed that elevated K⁺ injection into the hemiparkinsonian rat SNr improved forelimb akinesia, which coincided with a decrease in SNr neuronal spiking activity and desynchronised activity in SNr beta frequency, and subsequently an overall increase in ventral medial thalamic neuronal activity. Moreover, these findings also suggest that elevated K⁺ may provide an ionic mechanism that can contribute to the therapeutic effects of DBS for the motor treatment of advanced PD.     

Preservation of a functional nigrostriatal dopamine pathway by GDNF in the intrastriatal 6-OHDA lesion model depends on the site of administration of the trophic factor

Here we studied whether glial cell line-derived neurotrophic factor (GDNF), given as a single bolus injection before an intrastriatal 6-hydroxydopamine (6-OHDA) lesion, can protect the nigrostriatal dopamine neurons against the toxin-induced damage and preserve normal motor functions in the lesioned animals. GDNF or vehicle was injected in the striatum (25 microg), substantia nigra (25 microg) or lateral ventricle (50 microg) 6 h before the 6-OHDA lesion (20 microg/3 microL). Motor function was evaluated by the stepping and drug-induced motor asymmetry tests. Lesioned animals given vehicle alone showed a clear ipsilateral-side bias in response to amphetamine (13 turns/min), a moderate contralateral-side bias to apomorphine (4.5 turns/min) and a moderate to severe stepping deficit on the contralateral forepaw (three to four steps, as compared with 11-13 steps on the unimpaired side). Injection of GDNF into the striatum had a significant protective effect both on nigrostriatal function (1-2 turns/min in the rotation tests and seven to eight steps in the stepping test), and the integrity of the nigrostriatal pathway, seen as a protection of both the cell bodies in the substantia nigra and the dopamine innervation in the striatum. Injection of GDNF in the nigra had a protective effect on the nigral cell bodies, but not the striatal innervation, and failed to provide any functional benefit. In contrast, intranigral GDNF had deleterious effects on both the striatal TH-positive fibre density and on drug-induced rotation tests. Intraventricular injection had no effect. We conclude that preservation of normal motor functions in the intrastriatal 6-OHDA lesion model requires protection of striatal terminal innervation, and that this can be achieved by intrastriatal, but not nigral or intraventricular, administration of GDNF.     

Angiotensin-converting enzyme inhibition reduces oxidative stress and protects dopaminergic neurons in a 6-hydroxydopamine rat model of Parkinsonism

It is now established that the brain possesses a local renin-angiotensin system and that angiotensin II exerts multiple actions in the nervous system, including regulation of striatal dopamine release. Furthermore, angiotensin activates NADPH-dependent oxidases, which are a major source of superoxide, and angiotensin-converting enzyme inhibitors, commonly used in the treatment of hypertension and chronic heart failure, have shown antioxidant properties in several tissues. Oxidative stress is a key contributor to the pathogenesis and progression of Parkinson's disease. In the present study, we treated rats with intraventricular injections of the dopaminergic neurotoxin 6-hydroxydopamine and subcutaneous injections of the angiotensin-converting enzyme inhibitor Captopril to study the possible neuroprotective effect of the latter on the dopaminergic system and on 6-hydroxydopamine-induced oxidative stress. Rats treated with Captopril and 6-hydroxydopamine showed significantly less reduction in the number of dopaminergic neurons (i.e., immunoreactive to tyrosine hydroxylase) in the substantia nigra and in the density of striatal dopaminergic terminals than 6-hydroxydopamine-lesioned rats not treated with Captopril. In addition, Captopril reduced the levels of major oxidative stress indicators (i.e., lipid peroxidation and protein oxidation) in the ventral midbrain and the striatum of 6-hydroxydopamine-lesioned rats. Our results suggest that angiotensin-converting enzyme inhibitors may be useful for treatment of Parkinson's disease and that further investigation should focus on the neuroprotective capacity of these compounds.     

Behavioral and electrophysiological effects of 5‐HT in globus pallidus of 6‐hydroxydopamine lesioned rats

Anatomical studies have shown that the globus pallidus receives abundant 5‐hydroxytryptamine (5‐HT) innervations from raphe nuclei. 5‐HT may occupy an important position in the modulation of motor function through its affect on the activity of globus pallidus. In the present study, intrapallidal microinjection of 5‐HT (0.1 mM) alone did not induce any motor behavior or postural asymmetry in the unilateral 6‐hydroxydopamine (6‐OHDA)‐lesioned rats. However, when infused concomitantly with a low dose of 3, 4‐dihydroxyphenylalanine (L‐DOPA, 3 mg/kg i.p.), which itself can induce modest contralateral rotational behavior, 5‐HT significantly potentiated the number of contralateral rotations. To elucidate the cellular mechanism, in vivo extracellular recordings were performed to examine the effects of 5‐HT on globus pallidus neurons. In normal rats, the predominant effect of micropressure ejection of 5‐HT on pallidal neurons was excitation. In 6‐OHDA‐lesioned rats, although 5‐HT increased the firing rate in most pallidal neurons, 5‐HT‐induced inhibitory effects was stronger than that on the unlesioned side as well as normal rats. Furthermore, 5‐HT_1B receptors are mainly involved in 5‐HT‐induced excitation while 5‐HT_1A receptors are involved in 5‐HT‐induced inhibition. The results suggest that 5‐HT may potentiate the antiparkinsonian effect of L‐DOPA through modulating the activity of globus pallidus. © 2009 Wiley‐Liss, Inc.     

Alterations in electrophysiological activity and dye coupling of striatal spiny and aspiny neurons in dopamine‐denervated rat striatum recorded in vivo

We recently reported that pharmacological manipulations of the dopamine system can produce more than a 4‐fold increase in dye coupling between dopaminoceptive neurons in the adult rat striatal complex. During in vivo intracellular recordings, striatal neurons in control rats and in rats that had been treated with 6‐hydroxydopamine were injected with either Lucifer yellow or Neurobiotin. Only rats that exhibited severe loss (i.e., larger than ∼95%) of striatal dopamine terminals displayed a significant increase in the incidence of dye coupling between neurons in adult striatum. Moreover, this increased coupling was present only between neurons of the same morphological cell class, i.e., among clusters of spiny neurons or between aspiny neurons. Combining intracellular labeling of spiny neurons with parvalbumin immunocytochemistry demonstrated that coupling did not occur between anatomically adjacent neurons that comprised immunocytochemically and morphologically distinct cell classes. Therefore, gap junction conductance as reflected by dye coupling appears to undergo upregulation as a consequence of compromises in nigrostriatal and mesolimbic dopamine transmission. Synapse 33:1–15, 1999. © 1999 Wiley‐Liss, Inc.     

Subthalamic nucleus lesion improves cell survival and functional recovery following dopaminergic cell transplantation in parkinsonian rats

Subthalamic nucleus (STN) modulation is currently the gold standard in the treatment of Parkinson's disease (PD) cases refractory to medication. Cell transplantation is a tissue‐restorative approach and is a promising strategy in the treatment of PD. One of the obstacles to overcome in cell therapy is the poor dopaminergic cell survival. Our experiment investigates the impact of a partial subthalamotomy prior to ventral mesencephalic (VM) embryonic cell transplantation on dopaminergic cell survival and functional outcome. Unilateral dopamine depletion was carried out in rats, via medial forebrain bundle (MFB) injection of 6‐hydroxydopamine, and half of the animals went on to receive unilateral excitotoxic lesions of the STN/Zone Incerta (ZI) causing partial lesion of these structures on the same side as the MFB lesion. All MFB‐lesioned animals, with or without the STN/ZI lesion, received striatal ipsilateral embryonic VM cell grafts. The data suggest that the STN/ZI lesion could boost the dopamine cell survival in the grafts by 2.6‐fold compared with the control grafted‐only group. Moreover, performance on the drug‐induced rotation and the spontaneous behavior tests were ameliorated on the STN/ZI‐lesioned group to a significantly greater extent than the grafted‐only group. These data suggest that the STN/ZI partial lesion optimized the striatal environment, promoting an improvement in cell survival. Further studies are needed to see whether the synergy between STN modulation via deep brain stimulation and cell therapy might have clinical applications in the management of PD.     

Dynamic of neurochemical alterations in striatum,hippocampus and cortex after the 6-OHDA mesostriatal lesion

Immediate neurochemical alterations produced by 6-OHDA could explain the general toxic pattern in the central nervous system. However, no evidences describe the effects of 6-OHDA on early changes of neurotransmitters in rats’ striatum, cortex and hippocampus. In our study, unilateral 6-OHDA injection into medial forebrain bundle (MFB) was used in rats, then five neurotransmitters were analyzed at 3, 6, 12, 24, 48 and 72 h, respectively. Results showed that 6-OHDA injection caused a sharp decline of striatal dopamine (DA) levels in the first 12 h followed by a further reduction between 12 and 48 h. However, striatal levels of homovanillic acid (HVA) were stable in the first 12 h and showed a marked reduction between 12 and 24 h. Striatal levels of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) decreased linearly for 72 h, whereas levels of norepinephrine (NE) showed a slight reduction in the first 48 h, and returned back to normal afterwards. Striatal HVA/DA ratio increased significantly in the first 12 h, but 5-HIAA/5-HT ratio showed a sharp increase between 12 and 72 h. Besides, neurochemical alterations were also found in hippocampus and cortex, and the correlations of neurotransmitters were analyzed. Our study indicated that NE system had little influence in the early phase of 6-OHDA injection, moreover, early neurochemical alterations were involved with striatum, hippocampus and cortex.     

Actions of the sodium channel inhibitor 202W92 on rat midbrain dopaminergic neurons

Excessive glutamatergic activity is implicated in Parkinson's disease (PD) and sodium channel blockade, resulting in inhibition of glutamate release, is a potential therapeutic approach to PD therapy. Beneficial effects of riluzole and lamotrigine have been reported in animal models of PD, but these compounds have relatively low potency as sodium channel inhibitors and also inhibit N and P/Q-type calcium channels. 202W92, a structural analog of lamotrigine, is a potent sodium channel inhibitor, with no effect on N, P/Q-type channels. Here we present the effects of 202W92 on single patch-clamped dopaminergic neurons. 202W92 (> or =10 microM) inhibited spontaneous action potential firing and reduced amplitude and frequency of evoked action potentials. It also inhibited the frequency of 4-aminopyridine (4-AP)- and electrically evoked excitatory postsynaptic currents (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs), with >80% inhibition at 10 microM (IC(50) 1.5 microM). EPSC and IPSC amplitudes were partially inhibited. 202W92 did not affect postsynaptic responses to locally applied glutamate and GABA, nor spontaneously occurring mini-IPSCs. These actions of 202W92 are compatible with sodium channel inhibition and depression of transmitter release.     

Thalamic innervation of the direct and indirect basal ganglia pathways in the rat: Ipsi- and contralateral projections

The present study describes the thalamic innervation coming from the rat parafascicular nucleus (PF) onto striatal and subthalamic efferent neurons projecting either to the globus pallidus (GP) or to the substantia nigra pars reticulata (SNr) by using a protocol for multiple neuroanatomical tracing. Both striatofugal neurons targeting the ipsilateral SNr (direct pathway) as well as striatal efferent neurons projecting to the ipsilateral GP (indirect pathway) were located within the terminal fields of the thalamostriatal afferents. In the subthalamic nucleus (STN), both neurons projecting to ipsilateral GP as well as neurons projecting to ipsilateral SNr also appear to receive thalamic afferents. Although the projections linking the caudal intralaminar nuclei with the ipsilateral striatum and STN are far more prominent, we also noticed that thalamic axons could gain access to the contralateral STN. Furthermore, a small number of STN neurons were seen to project to both the contralateral GP and PF nuclei. These ipsi- and contralateral projections enable the caudal intralaminar nuclei to modulate the activity of both the direct and the indirect pathway.