Unilateral 6-hydroxydopamine Lesions Research Articles

Effects of pallidotomy on motor symptoms in an animal model of Parkinson's disease

The present study was designed to evaluate the motor effects of lesioning the internal globus pallidus in an animal model of Parkinson's disease. Fourty rats were divided into four groups (each of 10 rats) which received either unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle (mfb) plus sham surgery to the pallidum, sham surgery of mfb plus N-methyl- d-aspartate (NMDA) induced pallidal lesions, combined 6-OHDA mfb + NMDA pallidal lesions or sham surgery to both structures. Animals with 6-OHDA lesions developed significant ipsilateral biases in head position, body axis and circling after amphetamine challenge (all P < 0.05). Prominent contralateral deficits were present in sensorimotor response latency and contralateral circling was induced by apomorphine challenge (both P < 0.05). The addition of an NMDA pallidal lesion, improved the head position and body axis biases, as well as dopamine-agonist induced rotation and contralateral reaction time in a sensorimotor task (all P < 0.05). There was, however, a slight worsening of sensorimotor response on the ipsilateral side ( P < 0.05). Pallidal lesions in the absence of 6-OHDA lesions produced contralateral head position and body axis biases (both P < 0.05). These data indicate that pallidotomy improves some, but not all aspects of parkinsonian motor dysfunction in an animal model of Parkinson's disease (PD).

Blockade of mGluR5 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.

Multi-electrode recordings reveal enhanced beta-synchrony along the cortico-striatal axis in the rat model of parkinsonism

Recent studies in primates and patients suffering from Parkinson's disease emphasize an excessive synchronization of activity in the basal ganglia in the beta-frequency range, emerging as a network effect of dopamine depletion. The striatum, as the head end of the circuit linking cerebral cortex and basal ganglia, may be a crucial point for the emergence or/and maintenance of increased temporal coupling. To assess changes in coupling between neuronal assemblies in the cortex and the striatum following unilateral 6-hydroxydopamine lesions of midbrain dopamine neurons, we recorded unit activity and local field potentials (LFP) in lesioned and in healthy rats. Signals were acquired bilaterally from various cortical areas and the striatum of halothane-anesthetized rats. Dopamine depletion led to a significant increase of beta-band (10–20Hz) oscillatory activity (BO) in units as well as LFPs. Pronounced BO appeared in short bursts recurring with a frequency of 0.2–2Hz, accompanying irregular slow wave activity in the EEG. BO was not present during spontaneously occurring episodes of clockwork-like delta oscillations with lower amplitudes. In contrast to the global coherence of the slow waves, BO was topographically restricted to sensorimotor cortical areas, and was present neither in parietal or occipital cortices nor in the hippocampus. The oscillations were found on both hemispheres, but coherence analysis of LFPs revealed a coupling of BO only for cortico-cortical and cortico-striatal pairs of recording sites within the same hemisphere. In line with these observations, cortical unit activity was found to correlate at beta-frequencies only within, but not across the hemispheres. Interestingly, administration of the dopamine receptor agonist apomorphine suppressed BO. Taken together, our results suggest that BO emerging due to dopamine depletion prevail even during anesthesia, are restricted to sensorimotor areas and may lead to an involvement of the non-lesioned hemisphere.

The atypical dopamine D1 receptor agonist SKF 83959 induces striatal Fos expression in rats

The effects of dopamine D1 receptor agonists are often presumed to result from an activation of adenylyl cyclase, but dopamine D1 receptors may also be linked to other signal transduction cascades and the relative importance of these various pathways is currently unclear. SKF 83959 is an agonist at dopamine D1 receptors linked to phospholipase C, but has been reported to be an antagonist at receptors linked to adenylyl cyclase. The current report demonstrates that SKF 83959 induces pronounced, nonpatchy, expression of the immediate-early gene product Fos in the striatum of intact rats which can be converted to a patchy pattern by pretreatment with the dopamine D2-like receptor agonist quinpirole. In rats with unilateral 6-hydroxydopamine lesions SKF 83959 induces strong behavioral rotation and a greatly potentiated Fos response. All of the responses to SKF 83959, in both intact and dopamine-depleted animals, can be blocked by pretreatment with the dopamine D1 receptor antagonist SCH-23390. In intact subjects, SKF 83959 induced Fos expression less potently than the standard dopamine D1 receptor agonist SKF 82958, but the two drugs were approximately equipotent in deinnervated animals. These results demonstrate for the first time that possession of full efficacy at dopamine D1 receptors linked to adenylyl cyclase is not a necessary requirement for the induction of striatal Fos expression in intact animals and suggest that alternative signal transduction pathways may play a role in dopamine agonist induced Fos expression, especially in dopamine-depleted subjects.

Unilateral lesion of the pedunculopontine nucleus induces hyperactivity in the subthalamic nucleus and substantia nigra in the rat

Recent data suggest a role for the pedunculopontine nucleus (PPN) in the pathophysiology of Parkinson's disease. Although there is anatomical evidence that the PPN and the basal ganglia are reciprocally connected, the functional importance of these connections is poorly understood. Lesioning of the PPN was shown to induce akinesia in primates, whereas in the 6-hydroxydopamine rat model the PPN was found to be hyperactive. As both nigrostriatal dopamine depletion and lesioning of the PPN were shown to induce akinesia and parkinsonism, the present study was performed in order to investigate the changes in neuronal activity of the subthalamic nucleus (STN) and the substantia nigra pars reticulata (SNr) after unilateral ibotenic acid lesioning of the PPN and after unilateral 6-hydroxydopamine lesioning of the substantia nigra pars compacta (SNc). The firing rate of STN neurones significantly increased from 10.2 +/- 6.2 (mean +/- SD) to 14.6 +/- 11.7 spikes/s after lesion of the PPN and to 18.6 +/- 14.5 spikes/s after lesion of the SNc. The activity of the SNr significantly increased from 19.6 +/- 10.5 to 28.7 +/- 13.4 spikes/s after PPN lesioning and to 23.5 +/- 10.8 spikes/s after SNc lesioning. Furthermore, PPN lesion decreased the number of spontaneously firing dopaminergic SNc cells, while having no effect on their firing rate. The results of our study show that lesion of the PPN leads to hyperactivity of the STN and SNr, similar to the changes induced by lesion of the SNc. Moreover, the decreased activity of SNc cells observed after PPN lesion might be at the origin of activity changes in the STN and SNr.

Olfactory ensheathing glial co-grafts improve functional recovery in rats with 6-OHDA lesions

Olfactory ensheathing cells (OEC) transplanted to the site of a spinal cord injury can promote axonal sparing/regeneration and functional recovery. The purpose of this study was to investigate if OEC enhance the effects of grafted dopamine-neuron-rich ventral mesencephalic tissue (VM) in a rodent model of Parkinson's disease. We co-grafted VM with either OEC or astrocytes derived from the same olfactory bulbs as the OEC to rats with a unilateral 6-hydroxydopamine lesion of the nigrostriatal system. Co-grafting fetal VM with OEC, but not with astrocytes enhanced dopamine cell survival, striatal reinnervation and functional recovery of amphetamine- and apomorphine-induced rotational behaviour compared with grafting embryonic VM alone. Grafting OEC or astrocytes alone had no effects. Intriguingly, only in the presence of OEC co-grafts, did dopamine neurons extend strikingly long neurites that reached peripheral striatal compartments. Comparable results were observed in a co-culture system where OEC promoted dopamine cell survival and neurite elongation through a mechanism involving both releasable factors and direct contact. Cell type analysis of fetal VM grafts suggested that dopamine neurons of the substantia nigra rather than of the ventral tegmental area were increased in the presence of OEC co-grafts. We conclude that the addition of OEC enhances efficacy of grafted immature dopamine neurons in a rat Parkinson's disease model.

Alteration of striatal [ 11C]raclopride and 6-[ 18F]fluoro- l-3,4-dihydroxyphenylalanine uptake precedes development of methamphetamine-induced rotation following unilateral 6-hydroxydopamine lesions of medial forebrain bundle in rats

We studied the positron emission tomography (PET) tracer distributions of ligands for dopamine D 1 receptors ([ 11C]SCH23390) and D 2 receptors ([ 11C]raclopride) and of the dopamine precursor analog 6-[ 18F]fluoro- l-3,4-dihydroxyphenylalanine ([ 18F]FDOPA) in the brain after 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle in rats. The number of methamphetamine-induced rotation was higher at 14 days than at 3 days after the 6-OHDA lesions. The brains of 6-OHDA-treated rats were analyzed by tissue dissection following i.v. bolus of each tracer at 3 days (acute stage) or 3 weeks (chronic stage) postlesion. [ 11C]Raclopride, but not [ 11C]SCH23390, showed higher accumulation in the striatum on the lesion side than on the non-lesion (intact) side both at 3 days and 3 weeks postlesion. On the other hand, lower accumulation of [ 18F]FDOPA was observed in the striatum on the lesion side at 3 days postlesion and in both the striatum and cerebral cortex on the lesion side at 3 weeks postlesion. Our studies demonstrate that an increase in [ 11C]raclopride and a decrease in [ 18F]FDOPA uptake in the denervated striatum is evident even at 3 days after the 6-OHDA lesions when the methamphetamine-induced rotational behavior is not established.

The effects of cocaine on CART expression in the rat nucleus accumbens: A possible role for corticosterone

CART (Cocaine- and Amphetamine-Regulated Transcript) was initially described as an mRNA which had increased expression in the rat striatum following administration of acute cocaine or amphetamine but not saline. However, not all subsequent studies confirmed this. The present study aimed to repeat experiments with conflicting results and to reexamine and extend the original finding of acute regulation of nucleus accumbens CART mRNA by cocaine. Acute administration of cocaine failed to produce any change in levels of CART mRNA or peptide. Chronic administration of cocaine, as well as unilateral 6-hydroxydopamine lesions, also failed to alter CART mRNA levels in the accumbens. However, binge administration of cocaine, which also caused some seizures, did cause a significant increase in CART message. Given the involvement of corticosteroids with both stress and the effects of psychostimulants, we examined the possible effects of corticosteroids. We acutely administered ascending doses of corticosterone and found an increase in CART message. Similar effects were seen on CART peptides after acute corticosterone administration, and acute metyrapone administration was found to reduce CART peptide levels in the accumbens. This suggests that CART mRNA may be regulated by cocaine under certain conditions, such as binge administration, and this may at least partly involve corticosterone.

Studies of ATP-sensitive potassium channels on 6-hydroxydopamine and haloperidol rat models of Parkinson's disease: Implications for treating Parkinson's disease?

In the present study, we first investigated the effects of unilateral 6-hydroxydopamine (6-OHDA) lesioning of the substantia nigra pars compacta (SNc) on the expression of subunits of ATP-sensitive potassium channels (KATP channels) in the prefrontal cortex (PFC), striatum and hippocampus of adult rats by utilizing semiquantitative reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry techniques. The results show that Kir6.2 and SUR2 expression in the PFC, Kir6.1, Kir6.2 and SUR1 expression in the striatum, and Kir6.1 and Kir6.2 expression in the hippocampus of injured side increased significantly after unilateral 6-OHDA lesioning of the SNc in rats. Afterward, we studied the effects of iptakalim (Ipt), a novel ATP-sensitive potassium channel opener (KCO), on parkinsonian symptoms, which were induced by acute injection of haloperidol. The results indicate that intraperitoneal injection of Ipt (0.125 mg/kg, 0.25 mg/kg or 0.5 mg/kg) partially alleviated haloperidol-induced catalepsy and hypolocomotion. Even though the observed effects (0.5 mg/kg) are better than those of l-3,4-dihydroxyphenylalanine ( l-DOPA) (100 mg/kg), Ipt (0.25 mg/kg) failed to enhance the anti-parkinsonian actions of l-DOPA (100 mg/kg). Our results suggest that KATP channels might be involved in the pathogenesis of Parkinson's disease (PD) induced in an animal model and conceptually support the idea that KATP channels may be new therapeutic targets for PD.

Unilateral lesions of mesostriatal dopaminergic pathway alters the withdrawal response of the rat hindpaw to mechanical stimulation

To investigate the role mesostriatal dopamine system plays in pain processing, we examined the withdrawal response of rat hindpaws to mechanical stimulus at 1, 4, and 12 weeks after unilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal pathway. In all of the 6-OHDA rats examined, almost no tyrosine hydroxylase (TH) immunoreactivity was detected in the substantia nigra, ventral tegmental area, and striatum ipsilateral to 6-OHDA lesions. Alteration in the withdrawal response in this model animal was evaluated by comparing the latency of withdrawal reflex following the mechanical stimulus to the hindpaw. The latency of withdrawal response in the 6-OHDA rats was significantly reduced in the side ipsilateral to 6-OHDA lesions at all times observed, whereas that was not changed through the period observed in the contralateral side, indicating that dopamine depletion in the mesostriatal system has the influence on withdrawal response to the mechanical stimulus. These results show that the unilateral dopamine depletion causes hypersensitivity to the mechanical stimulus in the ipsilateral side, suggesting that, at least in part, dopamine in the mesostriatal system may be involved in sensory processing including pain sensation induced by mechanical stimulation.

Dopaminergic Reinnervation of the Globus Pallidus by Fetal Nigral Grafts in the Rodent Model of Parkinson's Disease

The current neural transplantation strategy for Parkinson's disease (PD) involves the dopaminergic reinnervation of the striatum (STR). Although up to 85% reinnervation of the STR has been attained by neural transplantation, functional recovery in animal models and transplanted patients is incomplete. This limitation may be due to an incomplete restoration of the dopaminergic input to other basal ganglia structures such as the external segment of the globus pallidus (GPe, homologue of the rodent GP), which normally receives dopaminergic input from the substantia nigra (SN). As part of our investigation into a multiple grafting strategy for PD, we have explored the effects of dopaminergic grafts in the GP of rodents with unilateral 6-hydroxydopamine (6-OHDA) lesions. In this experiment, lesioned rats received either 300,000 fetal ventral mesencephalic (FVM) cells or a sham injection into the GP. Functional assessment consisted of rotational behavior at 3 and 6 weeks posttransplantation. A fluorogold tracer study was conducted to rule out any behavioral improvement due to striatal outgrowth of the GP graft. Sections were stained for glial fibrillary acidic protein (GFAP) to assess the degree of trauma in the GP by the graft in comparison to the sham injection. Immunohistochemistry for tyrosine hydroxylase (TH) was performed after transplantation to assess graft survival. Animals with GP grafts demonstrated a significant improvement in rotational behavior at 3 and 6 weeks posttransplantation (p < 0.05) while sham control animals did not improve. All animals receiving FVM cells showed TH-immunoreactive grafts in the GP posttransplantation. TH-positive neurons in the GP showed no double labeling with an intrastriatal injection of fluorogold, indicating that behavioral improvement was not due to striatal innervation by the GP graft. These observations suggest that functional recovery was the result of dopaminergic reinnervation of the GP and that this nucleus may be a potential target for neural transplantation in clinical PD.

Postural changes after lesions of the substantia nigra pars reticulata in hemiparkinsonian monkeys

Current neurosurgical strategies target overactive brain regions including the subthalamic nucleus, globus pallidus and thalamus to control various symptoms of Parkinson's disease. Subthalamotomy improves akinesia and can induce postural deficits in both parkinsonian humans and animals, pallidotomy improves limb dyskinesia and more variably, distal bradykinesia whilst thalamotomy improves tremor. Because the SNr also becomes overactive in PD and there are few surgical studies in parkinsonian primates, we therefore evaluated the effects of lesioning the SNr in hemiparkinsonian marmosets to establish the effects on symptomatology. Nine monkeys received unilateral 6-hydroxydopamine (6-OHDA) lesions. Seven weeks later, four received kainic acid lesions of the SNr. Behavioural tests were performed prior to 6-OHDA surgery and then fortnightly for 14 weeks. Unilateral 6-OHDA lesions induced ipsilateral postural bias, ipsilateral rotation after amphetamine injection and bradykinesia. Whilst, SNr lesions significantly altered the direction of head position and amphetamine-induced rotation relative to 6-OHDA lesions, there was no improvement in 6-OHDA-induced reaching deficits or sensorimotor neglect. Unbiased quantitation of the nigral lesions showed on average 88% loss of dopaminergic neurons after 6-OHDA lesions and 77% loss of non-dopaminergic neurons after SNr lesions. Our results demonstrate that the SNr is important in body orientation changes in parkinsonism.

Platelet-Derived Growth Factor (PDGF-BB) and Brain-Derived Neurotrophic Factor (BDNF) induce striatal neurogenesis in adult rats with 6-hydroxydopamine lesions

The effects of i.c.v. infused platelet-derived growth factor and brain-derived neurotrophic factor on cell genesis, as assessed with bromodeoxyuridine (BrdU) incorporation, were studied in adult rats with unilateral 6-hydroxydopamine lesions. Both growth factors increased the numbers of newly formed cells in the striatum and substantia nigra to an equal extent following 10 days of treatment. At 3 weeks after termination of growth factor treatment, immunostaining of BrdU-labeled cells with the neuronal marker NeuN revealed a significant increase in newly generated neurons in the striatum. In correspondence, many doublecortin-labeled neuroblasts were also observed in the denervated striatum following growth factor treatment. Further evaluation suggested that a subset of these new neurons expresses the early marker for striatal neurons Pbx. However, no BrdU-positive cells were co-labeled with DARPP-32, a protein expressed by mature striatal projection neurons. Both in the striatum and in the substantia nigra there were no indications of any newly born cells differentiating into dopaminergic neurons following growth factor treatment, such that BrdU-labeled cells never co-expressed tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. In conclusion, our results suggest that administration of these growth factors is capable of recruiting new neurons into the striatum of hemiparkinsonian rats.

Comparative effects of acute or chronic administration of levodopa to 6-hydroxydopamine-lesioned rats on the expression of glutamic acid decarboxylase in the neostriatum and GABA A receptors subunits in the substantia nigra, pars reticulata

Current evidence suggests that behavioral sensitization to the chronic administration of levodopa ( l-DOPA) to dopamine-depleted animals involves a plasticity of GABA-mediated signaling in output regions of the basal ganglia. The purpose of this study was to compare in adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion the effects of an acute or chronic (for 3 or 7 days) injection of l-DOPA on mRNA levels encoding for glutamic acid decarboxylase (GAD65 and GAD67) in the striatum and GABA A receptor α1, β2 and γ2 subunits in the substantia nigra, pars reticulata (SNr), by in situ hybridization histochemistry. In addition, immunostaining levels for the α1 subunit were examined in the SNr. In agreement with previous studies, we found that l-DOPA administration increased GAD mRNA levels in the striatum of 6-OHDA-lesioned rats. However, the magnitude of this effect increased with the number of injections of l-DOPA. On the other hand, we found that 6-OHDA lesions resulted in increases in α1, β2 and γ2 mRNA levels in the ipsilateral SNr, which were normalized or decreased compared with the contralateral side by the acute or chronic administration of l-DOPA. In addition, α1 immunostaining in the SNr was significantly decreased in rats injected for 7 days but not for 3 days or acutely with l-DOPA. Our results demonstrate that a chronic administration of l-DOPA results in a progressive increase in GAD and decrease in GABA A receptor expression in the striatum and SNr, respectively. They provide further evidence that behavioral sensitization and dyskinesia induced by a chronic administration of l-DOPA in an experimental model of Parkinson’s disease is paralleled by a plasticity of GABA-mediated signaling in the SNr.

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

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

Role of NMDA and AMPA glutamate receptors in the induction and the expression of dopamine-mediated sensitization in 6-hydroxydopamine-lesioned rats

Rats with unilateral 6-hydroxydopamine (6-OHDA) lesions exhibit behavioral sensitization following repeated treatment with dopamine agonists, a phenomenon called "priming." Priming has two distinct phases: induction and expression. Priming induction using three injections with D1/D2 agonist apomorphine (0.5 mg/kg) or D1 agonist SKF38393 (10 mg/kg) allows priming expression, robust contralateral rotational behavior and striatal Fos expression, following a challenge with the D2 agonist quinpirole (0.25 mg/kg). We examined the roles of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors on dopamine agonist priming. Administration of the NMDA antagonist (+)5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK801) (0.5 mg/kg) blocked apomorphine-priming of quinpirole-mediated responses, while MK801 dose-dependently attenuated SKF38393-priming of quinpirole-mediated striatal Fos expression and had no effect on SKF38393-priming of quinpirole-mediated rotational behavior. In contrast, administration of the AMPA antagonist 2,3-dihydroxy-6-nitro-7sulfamoyl-benzo[f]quinoxaline (NBQX) (5 or 10 mg/kg) potentiated apomorphine- and SKF38393-priming of quinpirole-mediated striatal Fos expression, but had no effect on their priming of quinpirole-mediated rotational behavior. In SKF38393-primed 6-OHDA rats, administration of MK801 (0.5 mg/kg) blocked the expression of quinpirole-mediated responses, while administration of NBQX (10 mg/kg) or the noncompetitive AMPA antagonist 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-5-yl)-benzenamine dihydrochloride (GYKI52466) (5 or 15 mg/kg) had no effect. These results suggest that NMDA and AMPA glutamate receptors have differing roles in dopamine agonist priming-with NMDA receptors required for D1/D2 priming induction and D2-mediated priming expression, and AMPA receptors inhibiting priming induction of D2-mediated immediate early gene expression in the striatum, but not affecting priming induction of D2-mediated rotational behavior or the expression of D2-mediated responses.

Preserved ipsilateral-to-lesion motor map organization in the unilateral 6-OHDA-treated rat model of Parkinson's disease

The classic view of dopamine (DA) loss in Parkinson's disease is that it produces a functional deafferentation in striatal–cortical circuitry that, in turn, contributes to sensorimotor deficits. The present study examines this view in the rat by assessing how DA-depletion affects the intracortical microstimulation (ICMS) topographic representation of movement in the rostral and caudal motor areas of the motor cortex. The ICMS map is used as an index of motor cortex function because it has been shown to reflect motor function and experience. Groups of rats received no training or skilled reach training and were then given unilateral 6-hydroxydopamine (6-OHDA) or sham lesions of the nigrostriatal bundle to deplete nigrostriatal DA. Lesion success was confirmed by abnormalities in skilled reaching, by apomorphine-induced rotation, and by loss of DA neurons in the substantia nigra. The size and threshold of the motor map in naïve and skilled reach trained DA-depleted rats were preserved. In addition, there was an increase in distal limb representation in the caudal forelimb area (CFA) in the DA-depleted rats suggesting a possible plastic response to the behavioral effects of DA-depletion. The presence of preserved size and modified map organization in DA-depleted rats is discussed in relation to the hypothesis that preserved motor cortex functionality despite DA loss underlies the spared motor abilities of DA-depleted rats.

Sparing of behavior and basal extracellular dopamine after 6-hydroxydopamine lesions of the nigrostriatal pathway in rats exposed to a prelesion sensitizing regimen of amphetamine

Repeated administration of amphetamine leads to enduring augmentation of its behavioral-activating effects, enhanced dopamine (DA) release in striatal regions, and morphological changes in DA target neurons. Here we show that exposure to a 2-week escalating-dose regimen of amphetamine prevents behavioral asymmetries of forelimb use and spontaneous (drug-independent) turning behavior following unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal pathway made 7–14 days after termination of amphetamine treatment (Experiments 1–3). Exposure to three nonescalating injections of amphetamine 7 days before 6-OHDA lesions had no effect (Experiment 2). Prelesion amphetamine treatment led to normalization of basal extracellular levels of striatal DA as measured by microdialysis on days 11–14 and 25–28 after lesioning (Experiment 3). However, there were no significant differences between treatment groups in postmortem tissue levels of DA and its metabolites, indicating a dissociation between the DA depletion and the extracellular levels of DA as measured by microdialysis. Finally, rats exposed to the escalating amphetamine regimen had reduced lesion-induced loss of TH-IR cells in the ipsilateral DA cell body regions (Experiment 3). Thus, prelesion exposure to the escalating doses of amphetamine may render the cells resistant to the consequences of damage after subsequent 6-OHDA lesions, possibly by accelerating the development of compensatory changes in the DA neurons that typically accompany behavioral recovery. The potential role of amphetamine-induced endogenous neurotrophic factors in the behavioral sparing and normalization of basal extracellular DA levels observed after subsequent 6-OHDA lesions is discussed.

High-frequency stimulation of the subthalamic nucleus reverses limb-use asymmetry in rats with unilateral 6-hydroxydopamine lesions

Deep brain stimulation (DBS) is a widely used clinical treatment for Parkinson's disease (PD). A rodent model of DBS is a necessary tool for understanding the neural mechanisms of this method. Our previous study showed that high-frequency stimulation (HFS) of the subthalamic nucleus (STN) improved treadmill locomotion in rats with unilateral 6-hydroxydopamine (6-OHDA)-induced lesions of nigrostriatal dopamine (DA) neurons. The present study tested DBS effects on limb-use asymmetry (LUA) during vertical/lateral exploration in a cylindrical chamber in rats with similar unilateral nigrostriatal DA lesions. Limb-use asymmetry assessment has been used to detect functional capacity over a wide range of dopamine depletion. Before lesioning, rats exhibited regular rearing activity and used both forelimbs equally often to support weight during exploration of the walls of the cylinder. After unilateral nigrostriatal DA lesioning, rats displayed reduced rearing activity and predominant use of the ipsilateral (good) forelimb to touch the wall. HFS of the STN, but not of other nearby regions surrounding the STN, in the lesioned rats restored normal rearing activity and reversed the limb-use asymmetry caused by the unilateral DA depletion. This study is consistent with the possibility that there can be beneficial effects of STN-DBS on behavioral impairments in unilateral DA-depleted rats and may suggest an appropriate rodent model for DBS study.

Liposomal formulations of tacrolimus and rapamycin increase graft survival and fiber outgrowth of dopaminergic grafts.

The immunosuppressive drugs tacrolimus (TAC) and rapamycin (RAPA) have both been found to have neuroprotective effects on dopaminergic neurons. The purpose of the present study was to investigate whether liposomal formulations of these drugs administered directly into the brain improve cell survival and fiber outgrowth. Rats with unilateral 6-hydroxydopamine lesions were transplanted with 800,000 fetal rat ventral mesencephalic cells and randomly divided to one of four groups. Group 1 received a transplant containing cells only; group 2 received a cell suspension containing 0.68 microM liposomal RAPA (LRAPA); group 3 received a cell suspension containing 2.0 microM liposomal TAC (LTAC); and group 4 received a cell suspension containing a liposomal formulation of both 0.68 microM RAPA and 2.0 microM TAC (LRAPATAC). Rats were sacrificed after 6 weeks, and cell survival and fiber outgrowth were assessed using tyrosine hydroxylase (TH) immunohistochemistry. The animals receiving a cell suspension containing either LTAC or LRAPATAC were found to have significantly more surviving TH-immunoreactive (TH-ir) cells than the control group receiving cells only. The group receiving LTAC had significantly longer fibers, the group receiving LRAPA had significantly more fibers close to the graft, and the group receiving LRAPATAC had significantly more fibers at all distances. This study shows the feasibility of using liposomal formulations of neuroimmunophilins directly in the brain at the time of implantation to improve graft survival and fiber outgrowth. Furthermore, we have shown that the combination of LTAC and LRAPA has a synergistic effect. These compounds may play an important role in optimizing graft survival and host reinnervation in cell-mediated brain repair strategies for the treatment of neurological conditions.