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

Concerning neuromodulation as treatment of neurological and neuropsychiatric disorder: Insights gained from selective targeting of the subthalamic nucleus, para-subthalamic nucleus and zona incerta in rodents

Neuromodulation such as deep brain stimulation (DBS) is advancing as a clinical intervention in several neurological and neuropsychiatric disorders, including Parkinson's disease, dystonia, tremor, and obsessive-compulsive disorder (OCD) for which DBS is already applied to alleviate severely afflicted individuals of symptoms. Tourette syndrome and drug addiction are two additional disorders for which DBS is in trial or proposed as treatment. However, some major remaining obstacles prevent this intervention from reaching its full therapeutic potential. Side-effects have been reported, and not all DBS-treated individuals are relieved of their symptoms. One major target area for DBS electrodes is the subthalamic nucleus (STN) which plays important roles in motor, affective and associative functions, with impact on for example movement, motivation, impulsivity, compulsivity, as well as both reward and aversion. The multifunctionality of the STN is complex. Decoding the anatomical-functional organization of the STN could enhance strategic targeting in human patients. The STN is located in close proximity to zona incerta (ZI) and the para-subthalamic nucleus (pSTN). Together, the STN, pSTN and ZI form a highly heterogeneous and clinically important brain area. Rodent-based experimental studies, including opto- and chemogenetics as well as viral-genetic tract tracings, provide unique insight into complex neuronal circuitries and their impact on behavior with high spatial and temporal precision. This research field has advanced tremendously over the past few years. Here, we provide an inclusive review of current literature in the pre-clinical research fields centered around STN, pSTN and ZI in laboratory mice and rats; the three highly heterogeneous and enigmatic structures brought together in the context of relevance for treatment strategies. Specific emphasis is placed on methods of manipulation and behavioral impact.

Differential effects of dopaminergic drugs on spontaneous motor activity in the common marmoset following pretreatment with a bilateral brain infusion of 6-hydroxydopamine

The differential effects of dopaminergic drugs with different pharmacological profiles were investigated with respect to spontaneous motor activity in the common marmoset following pretreatment with a bilateral brain infusion of 6-hydroxydopamine (6-OHDA). Three marmosets received infusions of 6-OHDA (either 30 or 40 μg/side) into the bilateral dopamine-rich area running from the substantia nigra to the striatum. The motor activity of the 6-OHDA marmosets was compared with that of three intact marmosets. Following the administration of apomorphine (0.5 and 1 mg/kg, subcutaneously), the 6-OHDA group showed a tendency toward a brief increase in activity counts, suggesting denervation supersensitivity at the dopamine receptors. After the administration of methamphetamine (1 and 2 mg/kg, subcutaneously), the 6-OHDA group showed a significant decrease in activity counts, indicating limited dopamine release from the degenerated neurons. After the administration of l-3,4-dihydroxyphenylalanine (10 and 20 mg/kg, orally), the 6-OHDA group showed a significant increase in activity counts without hyperexcitation, consistent with the contribution of exogenous l-3,4-dihydroxyphenylalanine toward dopamine synthesis in the degenerated neurons. The present findings indicate that bilateral brain infusion of 6-OHDA in the marmoset may have preclinical utility as a primate model for investigating the behavioral properties of dopaminergic drugs in brains with dopaminergic neural deficits.

Behavioral phenotypes associated with MPTP induction of partial lesions in common marmosets (Callithrix jacchus)

Parkinson's disease is a chronic neurodegenerative disorder with the core motor features of resting tremor, bradykinesia, rigidity, and postural instability. Non-motor symptoms also occur, and include cognitive dysfunction, mood disorders, anosmia (loss of smell), and REM sleep disturbances. As the development of medications and other therapies for treatment of non-motor symptoms is ongoing, it is essential to have animal models that aid in understanding the neural changes underlying non-motor PD symptoms and serve as a testing ground for potential therapeutics. We investigated several non-motor symptoms in 10 adult male marmosets using the MPTP model, with both the full (n=5) and partial (n=5) MPTP dosing regimens. Baseline data in numerous domains were collected prior to dosing; assessments in these same domains occurred post-dosing for 12 weeks. Marmosets given the partial MPTP dose (designed to mimic the early stages of the disease) differed significantly from marmosets given the full MPTP dose in several ways, including behavior, olfactory discrimination, cognitive performance, and social responses. Importantly, while spontaneous recovery of PD motor symptoms has been previously reported in studies of MPTP monkeys and cats, we did not observe recovery of any non-motor symptoms. This suggests that the neurochemical mechanisms behind the non-motor symptoms of PD, which appear years before the onset of symptoms, are independent of the striatal dopaminergic transmission. We demonstrate the value of assessing a broad range of behavioral change to detect non-motor impairment, anosmia, and differences in socially appropriate responses, in the marmoset MPTP model of early PD.

Monkey-based research on human disease: the implications of genetic differences

Assertions that the use of monkeys to investigate human diseases is valid scientifically are frequently based on a reported 90-93% genetic similarity between the species. Critical analyses of the relevance of monkey studies to human biology, however, indicate that this genetic similarity does not result in sufficient physiological similarity for monkeys to constitute good models for research, and that monkey data do not translate well to progress in clinical practice for humans. Salient examples include the failure of new drugs in clinical trials, the highly different infectivity and pathology of SIV/HIV, and poor extrapolation of research on Alzheimer's disease, Parkinson's disease and stroke. The major molecular differences underlying these inter-species phenotypic disparities have been revealed by comparative genomics and molecular biology - there are key differences in all aspects of gene expression and protein function, from chromosome and chromatin structure to post-translational modification. The collective effects of these differences are striking, extensive and widespread, and they show that the superficial similarity between human and monkey genetic sequences is of little benefit for biomedical research. The extrapolation of biomedical data from monkeys to humans is therefore highly unreliable, and the use of monkeys must be considered of questionable value, particularly given the breadth and potential of alternative methods of enquiry that are currently available to scientists.

Modeling Parkinson's disease in the common marmoset (Callithrix jacchus): overview of models, methods, and animal care

The common marmoset (Callithrix jacchus) is a small-bodied, popular New World monkey and is used widely in reproductive biology, neuroscience, and drug development, due to its comparative ease of handling, high reproductive efficiency, and its unique behavioral characters. In this review, we discuss the marmoset models in Parkinson's disease (PD), which is a neurological movement disorder primarily resulting from a degeneration of dopaminergic neurons with clinical features of tremor, rigidity, postural instability, and akinesia. The most common PD models involve the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6-hydroxydopamine to study the pathogenesis and to evaluate novel therapies. Following the systemic or local administration of these neurotoxins, the marmosets with very severe Parkinson's symptoms are recommended to be placed in an intensive care unit with artificial feeding to increase survival rate. All procedures with MPTP should be conducted in a special room with enclosed cages under negative-pressure by trained researchers with personal protection. Behavioral tests are conducted to provide an external measure of the brain pathology. Along with several biomarkers, including α-synuclein and DJ-1, non-invasive neuroimaging techniques such as positron emission tomography and magnetic resonance imaging are used to evaluate the functional changes associated with PD. With the recent growing interest in potential and novel therapies such as stem cell and gene therapy for PD in Korea, the marmoset can be considered as a suitable non-human primate model in PD research to bridge the gap between rodent studies and clinical applications.

Monoamine reuptake inhibitors in Parkinson's disease

The motor manifestations of Parkinson's disease (PD) are secondary to a dopamine deficiency in the striatum. However, the degenerative process in PD is not limited to the dopaminergic system and also affects serotonergic and noradrenergic neurons. Because they can increase monoamine levels throughout the brain, monoamine reuptake inhibitors (MAUIs) represent potential therapeutic agents in PD. However, they are seldom used in clinical practice other than as antidepressants and wake-promoting agents. This review article summarises all of the available literature on use of 50 MAUIs in PD. The compounds are divided according to their relative potency for each of the monoamine transporters. Despite wide discrepancy in the methodology of the studies reviewed, the following conclusions can be drawn: (1) selective serotonin transporter (SERT), selective noradrenaline transporter (NET), and dual SERT/NET inhibitors are effective against PD depression; (2) selective dopamine transporter (DAT) and dual DAT/NET inhibitors exert an anti-Parkinsonian effect when administered as monotherapy but do not enhance the anti-Parkinsonian actions of L-3,4-dihydroxyphenylalanine (L-DOPA); (3) dual DAT/SERT inhibitors might enhance the anti-Parkinsonian actions of L-DOPA without worsening dyskinesia; (4) triple DAT/NET/SERT inhibitors might exert an anti-Parkinsonian action as monotherapy and might enhance the anti-Parkinsonian effects of L-DOPA, though at the expense of worsening dyskinesia.

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

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

Potentiation of response to low doses of levodopa in MPTP-injected monkeys by chemical unilateral subthalamotomy

Object

Subthalamotomy is a stereotactic surgery performed in patients with disabling dyskinesias due to Parkinson disease. The authors set out to model this human condition in MPTP monkeys and determine if subthalamotomy allowed a reduction of levodopa for similar benefit.

Methods

The authors performed unilateral subthalamotomy in 4 parkinsonian dyskinetic monkeys by stereotactic injection of ibotenic acid. An optimal dose, defined as the highest dose of levodopa improving parkinsonian motor symptoms while inducing low or no dyskinesias, was established in these animals. Each monkey was scored for the antiparkinsonian and dyskinetic effects of the optimal dose of levodopa, as well as suboptimal and dyskinesia-inducing doses (60% and 140% of the optimal dose, respectively), and these scores were compared with those obtained at baseline before and after subthalamotomy. Bradykinesia was assessed by a prehension task.

Results

Unilateral subthalamotomy had a positive effect on the antiparkinsonian response for all doses of levodopa as well as the baseline. There were no differences in the antiparkinsonian response between the suboptimal dose postsurgery and the optimal dose presurgery. Dyskinesias were increased at the suboptimal and the optimal doses. After surgery, the duration of response to levodopa increased between 20% and 25% in the suboptimal dose, whereas it remained unchanged with higher doses. Bradykinesia was significantly reduced after surgery only at the suboptimal dose.

Conclusions

Subthalamotomy potentiated the response to suboptimal doses of levodopa. Thus, levodopa can be reduced by 40% after surgery for similar beneficial antiparkinsonian response and less dyskinesia than with an optimal dose before surgery.

The 6-Hydroxydopamine model of parkinson’s disease

The neurotoxin 6-hydroxydopamine (6-OHDA) continues to constitute a valuable topical tool used chiefly in modeling Parkinson's disease in the rat. The classical method of intracerebral infusion of 6-OHDA involving a massive destruction of nigrostriatal dopaminergic neurons, is largely used to investigate motor and biochemical dysfunctions in Parkinson's disease. Subsequently, more subtle models of partial dopaminergic degeneration have been developed with the aim of revealing finer motor deficits. The present review will examine the main features of 6-OHDA models, namely the mechanisms of neurotoxin-induced neurodegeneration as well as several behavioural deficits and motor dysfunctions, including the priming model, modeled by this means. An overview of the most recent morphological and biochemical findings obtained with the 6-OHDA model will also be provided, particular attention being focused on the newly investigated intracellular mechanisms at the striatal level (e.g., A(2A) and NMDA receptors, PKA, CaMKII, ERK kinases, as well as immediate early genes, GAD67 and peptides). Thanks to studies performed in the 6-OHDA model, all these mechanisms have now been hypothesised to represent the site of pathological dysfunction at cellular level in Parkinson's disease.

Behavioural changes after bilateral subthalamic stimulation in advanced Parkinson disease: a systematic review

INTRODUCTION

The long-lasting beneficial effects of subthalamic nucleus (STN) deep brain stimulation (DBS) on motor function have now largely been acknowledged. Whereas behavioural changes have been demonstrated in certain case reports and small case series, some authors have not observed behavioural changes at all. The extent to which these changes occur has not yet been established. The aim of the present study was to systematically analyse behavioural changes of bilateral STN DBS.

MATERIALS AND METHODS

A structured Medline search was conducted using previously described methods. Studies were selected according to specific in- and exclusion criteria. Data on patients, surgical technique, outcome and complications were collected and pooled.

RESULTS

In total 1,398 patients who underwent bilateral STN DBS were included. The total cumulative follow-up period was 1,480 patient-years. Cognitive problems were seen in 41%, depression in 8%, and (hypo)mania in 4% of the patients. Anxiety disorders were observed in less than 2%, and personality changes, hypersexuality, apathy, anxiety, and aggressiveness were observed in less than 0.5% of the group studied. About half of the patients did not experience behavioural changes.

CONCLUSION

Caregivers should be aware of the extent of these behavioural changes and a risk/benefit evaluation should be performed for individual patients.

The subthalamic nucleus: From response selection to execution

The involvement of the subthalamic nucleus in physiological and pathological motor behaviour has now largely been established. Clinical observations in patients suffering from Parkinson disease treated with Deep Brain Stimulation of the STN show that these patients can suffer from postoperative changes in non-motor behaviour mainly involving alterations in cognitive functions. The involvement of the STN in cognition has initially been demonstrated by non-human studies investigating the effects of STN lesions and stimulations on cognitive parameters. In the present review, we discuss the findings of these preclinical studies on cognitive parameters and outline the anatomical and functional place of the STN in the basal ganglia cognitive circuit.

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).

The functional role of the subthalamic nucleus in cognitive and limbic circuits

Once it was believed that the subthalamic nucleus (STN) was no more than a relay station serving as a "gate" for ascending basal ganglia-thalamocortical circuits. Nowadays, the STN is considered to be one of the main regulators of motor function related to the basal ganglia. The role of the STN in the regulation of associative and limbic functions related to the basal ganglia has generally received little attention. In the present review, the functional role of the STN in the control of cortico-basal ganglia-thalamocortical associative and limbic circuits is discussed. In the past 20 years the concepts about the functional role of the STN have changed dramatically: from being an inhibitory nucleus to a potent excitatory nucleus, and from being involved in hyperkinesias to hypokinesias. However, it has been demonstrated only recently, mainly by reports on the behavioral (side-) effects of STN deep brain stimulation (DBS), which is a popular surgical technique in the treatment of patients suffering from advanced Parkinson Disease (PD), that the STN is clinically involved in associative and limbic functions. These findings were confirmed by results from animal studies. Experimental studies applying STN DBS or STN lesions to investigate the neuronal mechanisms involved in these procedures found profound effects on cognitive and motivational parameters. The anatomical, electrophysiological and behavioral data presented in this review point towards a potent regulatory function of the STN in the processing of associative and limbic information towards cortical and subcortical regions. In conclusion, it can be stated that the STN has anatomically a central position within the basal ganglia thalamocortical associative and limbic circuits and is functionally a potent regulator of these pathways.

Subthalamic nucleus influences spatial orientation in extrapersonal space

While the role of frontal and parietal cortex in spatial orientation has been studied extensively, the contribution of the basal ganglia and especially the subthalamic nucleus to spatial orientation remains less clear. Here we use subthalamic nucleus (STN) deep brain stimulation (DBS) in Parkinson's disease (PD) as a reversible model of functional lesioning to evaluate the influence of the STN in extra-personal space orientation. To this end, 12 PD patients were examined 1 year after implantation of DBS electrodes in the STN after overnight withdrawal of L-dopa. Patients were tested in a pseudo-randomized order while both stimulators, the right only, the left only, or no stimulator, were switched on. Patients performed line bisection and a reaction time task responding to stimuli of the middle, the left, and the right extra-personal space. A separate assessment of the right and left hand responding to visual stimuli in each hemispace made it possible to distinguish hemi-spatial and hemi-motor impairments. No asymmetries in space orientation were found when both stimulators were switched OFF, when both stimulators were switched ON, and when only the right stimulator was switched ON. When only the left subthalamic stimulation was switched ON, the reaction times of both hands to visual stimuli in the left extra-personal hemispace increased significantly and the line bisection test showed a significant orientation to the right. These results lead to the conclusion that the STN and its cortical projections influence the network involved in visuospatial orientation. These patterns of symptoms of neglect demonstrate the influence of the STN on the attentional system of the nondominant hemisphere.

Marmoset monkey models of Parkinson's disease: which model, when and why?

Parkinson's disease (PD) is a debilitating neurodegenerative disease, with clinical features of tremor, muscular rigidity and akinesia, occurring as a result of midbrain dopamine loss. The search for treatments has relied heavily on animal models of the disorder. The use of monkey models of PD plays a distinct role in the development and assessment of novel treatments. The common marmoset (Callithrix jacchus) is a popular New World monkey used in the search for new treatments. These monkeys are easy to handle and survive well in captivity. This review examines the advantages of using marmoset monkeys in PD research and examines the different models available with reference to their use in pre-clinical assessment for novel therapeutic treatments. The most common models involve the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6-hydroxydopamine (6-OHDA). Recently, selective cerebral transgenic over-expression of alpha-synuclein has also been attempted in marmosets as a potential model for PD. Each model has its advantages. The MPTP-based model in marmosets resembles the disease with regards to the neuroanatomy of neurotransmitter loss; the unilateral application of 6-OHDA allows for the assessment of more complex sensorimotor deficits due to the presence of an intact 'control' side; the over-expression of alpha-synuclein in the midbrain results in the slow onset of behavioural symptoms allowing for a pre-symptomatic time window. The appropriateness of each of these marmoset models for the assessment of treatments depends on several factors including the experimental aim of the study and whether emphasis is placed on the analysis of behavioural deficits.

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.

Behavioural effects of parafascicular thalamic lesions in an animal model of parkinsonism

We recently reported that the centromedian-parafascicular thalamic complex (CM-Pf) degenerates in Parkinson's disease and progressive supranuclear palsy. The contribution of such thalamic pathology to disease symptoms has not yet been established. The present study therefore investigated the behavioural impact of lesioning the corresponding thalamic region (termed Pf) on a range of behaviours present in rodents. There were four surgical groups: (1) sham medial forebrain bundle (mfb)+sham Pf, (2) 6-OHDA mfb lesion+sham Pf, (3) sham mfb+NMDA Pf lesion, (4) 6-OHDA+NMDA Pf lesions. Posture, sensory functions and apomorphine-induced rotational asymmetry were assessed before and after each surgery. Other assessments performed including a timed motivational task, grooming behaviours and piloerection. 6-OHDA lesions induced postural (ipsilateral curling and head position biases), sensorimotor (increased latency to respond to tactile stimulation of the contralateral side when eating or grooming) and rotational abnormalities (contralateral circling after apomorphine). The main effects of combined 6-OHDA+Pf lesions were improved performance in a motivational task (decreased latency to retrieve reward) but worsened piloerection, relative to animals with either 6-OHDA or Pf lesions alone. The thalamic zone common to all lesioned animals involved the posterior Pf. Our data suggests that the posterior CM-Pf may be involved in motivational responses and autonomic dysfunction in parkinsonian disorders.

Experimental therapeutics of Parkinson's disease

1. The loss of central dopamine, which characterises Parkinson's disease, led to the main pharmacological strategy for treatment, namely levodopa, a dopamine-replacement therapy. Several years after treatment, the majority of patients experience dose-limiting side-effects and loss of symptom control. There is a resurgence of interest in neurosurgery for treating the Parkinson's disease, particularly in new techniques targeting the subthalamic nucleus (STN), which is overactive in Parkinson's disease and contributes to symptom development. 2. We performed unilateral subthalamotomy (lesioning the subthalamic nucleus via the toxin N-methyl-d-aspartate) in marmosets and rats with experimentally induced parkinsonism (induced using the toxin 6-hydroxydopamine). A range of similar behaviours common to both rodents and primates were evaluated before and after each type of surgery. Post-mortem histology was used to confirm the lesions. We also provide details of a case with Parkinson's disease who underwent high-frequency bilateral stimulation of the STN and in whom we analysed the STN post-mortem. 3. Unilateral subthalamotomy improved akinesia in parkinsonian primates. However, both monkeys and rodents showed postural abnormalities. The patient who underwent bilateral high-frequency stimulation showed improvement of akinesia and other disease symptoms and no postural abnormalities. Post-mortem analysis did not demonstrate substantial damage of the STN as a result of the electrodes. 4. Although unilateral subthalamotomy improves some aspects of parkinsonism, it causes postural abnormalities in animal models of Parkinson's disease. Because bilateral high-frequency STN stimulation improves disease symptoms, is reversible and is not reported to induce postural side-effects, it may be a better surgical therapy for Parkinson's disease than lesioning the STN.

Dissociation of hemi-spatial and hemi-motor impairments in a unilateral primate model of Parkinson's disease

Monkeys with unilateral lesions of nigrostriatal dopamine projections were tested on a series of spatial tasks. One task, in which monkeys were required to use one or the other arm to retrieve food rewards from different positions, allowed separate assessment of the use of each arm in each hemi-space in order to distinguish hemi-spatial and hemi-motor impairments. The lesioned monkeys exhibited a persistent neglect of contralesional space when using either arm which could be dissociated from a motor impairment in the contralesional arm alone. Another task allowed free use of either arm across peri-personal space and demonstrated an ipsilesional bias in the monkeys' self-determined attention (orientation) to a task which they were trying to perform. It is argued that the tendency for monkeys with this lesion to rotate ipsilesionally is due to an ipsilesional deviation of the 'centre of interest' (determined by telencephalic circuitry) relative to 'straight ahead' (determined by brainstem circuitry). The dopamine projections may contribute to cortico-subcortical circuits which determine the spatial layout of mental representation, attention and intention. The results in this primate model of unilateral Parkinson's disease (PD) support the view that patients with left-sided Parkinsonian symptoms exhibit a unilateral deficit in spatial mental representation as well as their well-recognised motor symptoms. Patients with bilateral Parkinson's symptoms may exhibit bilateral deficits in mental representation.

Ablation of the subthalamic nucleus protects dopaminergic phenotype but not cell survival in a rat model of Parkinson's disease

Inhibition or ablation of the hyperactive subthalamic nucleus (STN) in Parkinson's disease (PD) does not only reverse motor deficits, silencing the glutamatergic output of the subthalamic nucleus, but has also been implicated to have neuroprotective effects on nigral neurons in animal models of Parkinson's disease. Ablation of the subthalamic nucleus has been shown to increase the number of tyrosinhydroxylase-immunopositive cells and partially restores behavioral deficits in animal models of Parkinson's disease. However, it is unclear whether subthalamic nucleus ablation indeed prevents cell death or whether the effect is due to the rescue of the dopaminergic (DA) phenotype of impaired cells by upregulating tyrosine hydroxylase (TH). We therefore investigated the potential neuroprotective effects of a preceding subthalamic nucleus lesion on 6-hydroxydopamine (6-OHDA)-induced nigral cell death and compared the retrograde tracer fluorogold (FG) as a marker of cell survival with tyrosinhydroxylase immunoreactivity as a marker of the dopaminergic phenotype. In the present study, we show that ablation of the subthalamic nucleus does not affect the number of fluorogold-labeled cells but increases the number of tyrosinhydroxylase-positive neurons in subthalamic nucleus-lesioned hemiparkinsonian animals and leads to partial behavioral recovery of the rats. We conclude that subthalamic nucleus ablation exerts neuroprotective properties on the dopaminergic nigrostriatal pathway against 6-hydroxydopamine toxicity in terms of rescuing the neurotransmitter phenotype in the remaining neurons rather than enhancing the total number of nigral cells.

Convulsive and postural effects of lesioning the mid-substantia nigra pars reticulata in naïve and 6-hydroxydopamine lesioned rats

The subthalamic nucleus is targeted for the treatment of Parkinson's disease. Unilateral lesions improve some aspects of parkinsonism but produce postural abnormalities in animal models but the exact pathways producing these effects remain to be defined. Using a battery of tests we evaluated the effects of lesioning one of the two major subthalamic targets, the substantia nigra pars reticulata in naïve and 6-OHDA lesioned rats. Lesions targeting the mid-substantia nigra pars reticulata resulted in acute tonic-clonic seizures and intense contralateral rotational asymmetry. During the first month after substantia nigra pars reticulata lesions there was normalisation of the ipsilateral head position bias induced by unilateral 6-OHDA lesions, significant contralateral body axis bias but no significant alteration of apomorphine induced rotation and sensorimotor neglect in 6-OHDA lesioned rats. Combined with our previous data, this suggests that subthalamic projections via the substantia nigra pars reticulata are important in seizures and postural behaviours. Therefore unilateral subthalamotomy probably induces postural deficits in hemiparkinsonian animals via projections involving the substantia nigra pars reticulata. This has implications for patients undergoing subthalamotomy for treatment of Parkinson's disease.

Relationships between various behavioural abnormalities and nigrostriatal dopamine depletion in the unilateral 6-OHDA-lesioned rat

While rotational asymmetry is used as a characteristic behavioural sign of striatal dopamine (DA) loss in unilateral animal models of Parkinson's disease (PD), there is relatively little analysis of how other common behavioural deficits relate to nigrostriatal DA depletion. We analysed the relationships between several deficits induced by unilateral 6-OHDA lesions and striatal neurochemistry, as well as neuronal loss in the dopaminergic substantia nigra (SN). Behaviour was evaluated from before until 6 weeks after surgery and abnormalities appeared in body axis, head position and sensorimotor performance as well as apomorphine-induced rotation. As expected, rotational behaviour correlated with striatal DA loss and not with other striatal neurotransmitters measured. Similar observations were found for sensorimotor deficits ('disengage task'). Both deficits were observed in rats with >70% loss of TH+ nigral neurons and >80% loss of striatal DA. Additional postural abnormalities appeared with mean losses of 87% of nigral DA neurons and 97% striatal DA, consistent with observations in patients with advanced PD. The data show that the repertoire of behavioural abnormalities manifested by hemiparkinsonian rats relate directly to the degree of nigrostriatal DA loss and, therefore, mimic features of PD. Analysis of such behaviours are relevant for chronic therapeutic studies targeting PD.

The striatum as a target for anti-rigor effects of an antagonist of mGluR1, but not an agonist of group II metabotropic glutamate receptors

The aim of the present study was to find out whether the metabotropic receptor 1 (mGluR1) and group II mGluRs, localized in the striatum, are involved in antiparkinsonian-like effects in rats. Haloperidol (1 mg/kg ip) induced parkinsonian-like muscle rigidity, measured as an increased resistance of a rat's hind foot to passive flexion and extension at the ankle joint. (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 0.5-15 microg/0.5 microl), a potent and selective mGluR1 antagonist, or (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC; 7.5-15 microg/0.5 microl), a selective group II agonist, was injected bilaterally into the striatum of haloperidol-treated animals. AIDA in doses of 7.5-15 microg/0.5 microl diminished the haloperidol-induced muscle rigidity. In contrast, 2R,4R-APDC injections were ineffective. The present results may suggest that the blockade of striatal mGluR1, but not the stimulation of group II mGluRs, may ameliorate parkinsonian muscle rigidity.

Comparison of the basal ganglia in rats, marmosets, macaques, baboons, and humans: volume and neuronal number for the output, internal relay, and striatal modulating nuclei

This study compares the basal ganglia of rats, marmosets, macaques, baboons, and humans. It uses established protocols to estimate the volume and number of neurons within the output nuclei (internal globus pallidus, IGP; and nondopaminergic substantia nigra, SNND), two internal relay and modulating nuclei (subthalamic nucleus, STh; and external globus pallidus, EGP), and a modulator of the striatum (dopaminergic substantia nigra, SND). Nuclear boundaries were defined by using immunohistochemistry for striatal afferents. Total numbers of Nissl-stained and parvalbumin-immunoreactive neurons were calculated by using the fractionator technique. Comparisons between species were standardized relative to brain mass (rats < marmosets < macaques < baboons < humans). The EGP consistently had more neurons relative to the IGP, STh, and SND, which had similar neuronal numbers within each species. The SNND had proportionally more neurons in rats than in primates (especially humans). The distribution of SND neurons varied substantially between rats and primates (very few ventrally located neurons in rats) with humans containing fewer SND neurons than other primates. The reduction in SND neurons in humans suggests less dopaminergic regulation of the basal ganglia system compared with other species. The consistency in the number of IGP neurons across all species, combined with the reduction in SNND neurons in humans, suggests a greater emphasis on output pathways through the IGP and that there are proportionally more STh and EGP neurons in humans.

Postmortem analysis of bilateral subthalamic electrode implants in Parkinson's disease

This is the second neuropathological report detailing bilateral electrodes targeting the subthalamic nucleus (STN) in idiopathic Parkinson's disease (PD). The patient presented with unilateral tremor-dominant parkinsonism. Bilateral STN stimulation was carried out 7 years later due to significant disease progression and severe motor fluctuations. The patient exhibited bilateral improvements in rigidity and bradykinesia both intraoperatively and postoperatively. The patient died 2 months later from aspiration pneumonia. Neuropathological examination confirmed both the diagnosis of PD and the electrode placements. The tip of the left electrode was located medially and posteriorly in the left STN and the tip of the right electrode entered the base of the thalamus/zona incerta immediately above the right STN. Tissue changes associated with the subthalamic electrode tracts included mild cell loss, astrogliosis, and some tissue vacuolation. Our postmortem analysis indicates little tissue damage associated with STN stimulation for PD.

Postural asymmetries following unilateral subthalomotomy for advanced Parkinson's disease

Two cases of postural asymmetries following unilateral stereotaxic subthalamotomy were observed with head and body tilting to the side contralateral to the STN lesion, which corrected itself completely or partially with levodopa treatment. After subsequent contralateral STN surgery, the postural asymmetry disappeared in both patients. Possible mechanism is discussed.

Parkinsonian motor deficits are reflected by proportional A9/A10 dopamine neuron degeneration in the rat

In a model of Parkinson's disease (PD), amphetamine, a dopamine (DA)-releasing drug, fails to induce ipsilateral drug rotations in a proportion of rats with complete unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle and DA neurons of the substantia nigra. To investigate this phenomenon, individual 6-OHDA lesions (measured by tyrosine hydroxylase immunohistochemistry) in the substantia nigra pars compacta (A9), ventral tegmental area (A10), and striatum were examined in conjunction with outcomes of four behavioral tests. The behavioral tests were skilled paw reaching, a head-turning test, and apomorphine (0.05 mg/kg) and amphetamine (4 mg/kg) drug-induced rotations. Four weeks postlesion, ipsilateral side bias measured by the head-turning test correlated strongly with extent of A9 DA neuronal lesion. Additional A10 neuronal DA lesions did not substantially improve the model fit, indicating that the head-turning bias was primarily A9 dependent. In contrast, total head-turning activity increased monotonically with lesions of A10 striatal DA fibers. Skilled paw-reaching accuracy decreased with increased lesion of both A9 and A10 DA neuronal systems. Associating amphetamine-induced rotations with extent of A9 DA lesion generated a second-order polynomial model, y = -11.1x + 0.20 x(2) + 208.7 (R(2) = 0.73), with an overall F ratio (df = 2,21) of 28.4 (P < 0.0001). This model predicts that an A9 DA lesion of about 50% is required to induce an ipsilateral turning bias, after which rotations increase with the degree of A9 DA neuronal lesion. No further change in rotational behavior was seen until an additional A10 DA lesion reached 60%, after which the rotational response decreased. This analysis provides tests that differentiate between A9 DA degeneration and combined A9/A10 lesions in animal models and in addition allows predictive testing of PD therapeutic intervention at a preclinical level.

Differential effects of unilateral striatal and nigrostriatal lesions on grip strength, skilled paw reaching and drug-induced rotation in the rat

Lateralised motor deficits associated with basal ganglia dysfunction were compared in separate groups of rats receiving unilateral 6-hydroxydopamine-induced lesions of the dopaminergic nigrostriatal pathway, quinolinic acid-induced lesions of the striatum, or sham control injections. Amphetamine induced ipsilateral rotation in both lesion groups, whereas a low ('supersensitive') dose of apomorphine induced rotation only in the nigrostriatal lesion group. Both lesions induced impairments in skilled paw reaching with the contralateral paw in the 'staircase' test; by contrast the striatal lesions also induced a marked impairment with the ipsilateral paw, which was unaffected by the nigrostriatal lesion. A previously reported increase in grip strength with the contralateral paw after nigrostriatal lesion was replicated, whereas striatal lesions induced only minor bilateral deficits in this test. The results are discussed in the context of the utility, reliability and validity of alternative tests of motor deficit in animal models of Parkinson's and Huntington's diseases.

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

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

Implication of the subthalamic nucleus in the pathophysiology and pathogenesis of Parkinson's disease

The subthalamic nucleus (STN) has been shown to play an important role in the control of movement and has been considered as a key structure in the functional organization of the basal ganglia. Several studies postulated that the STN plays a critical role in the pathophysiology of Parkinson's disease and that its inhibition or its lesioning can reverse the cardinal motor symptoms. Nevertheless, the beneficial effect was accompanied by dyskinetic abnormal movements. In order to avoid unpleasant and irreversible side effects we used high-frequency stimulation (HFS) of the STN instead of lesions. We have shown that parkinsonian motor symptoms, akinesia, rigidity, and tremor can be alleviated by HFS of the STN in the nonhuman primate model. Side effects were controllable and appeared only at intensities higher than that inducing the improvement of motor symptoms. In severe parkinsonian patients, bilateral STN-HFS greatly improved parkinsonian motor symptoms. Motor fluctuations were attenuated and patients became independent in most activities of daily living. It appears that STN-HFS mimics the effects of lesions by inhibiting its neuronal activity. In a rat model of parkinsonism, we studied the implication of the STN in the excitotoxicity of nigral dopamine cells. We showed that kainic acid lesioning of the STN can protect nigral dopaminergic cells against 6-hydroxydopamine-induced toxicity. The evidence reviewed in the present article clearly demonstrates that the STN is implicated in the pathophysiology and pathogenesis of Parkinson's disease.

Subthalamic nucleus lesions induce deficits as well as benefits in the hemiparkinsonian rat

Lesions of the subthalamic nucleus can restore some imbalances in motor output of the basal ganglia induced by nigrostriatal dopamine depletion, and have been proposed as a potential therapy for Parkinson's disease. Although there is substantial supporting evidence from experimental studies in both rats and primates, there is less information on the effects of subthalamic lesions alone. In order to characterize potential side effects, the present study evaluates the behavioural effects of unilateral excitotoxic lesions of the subthalamic nucleus in rats that have previously received either unilateral saline or 6-hydroxydopamine injections into the nigrostriatal bundle on the same side. The 6-hydroxydopamine lesions induced ipsilateral orientation asymmetries in head position and body axis bias, rotational asymmetries following injections of direct or indirect dopamine agonists, neglect of contralateral stimuli, and a reduction in the numbers of pellets retrieved with the contralateral paw in a skilled reaching task. Subsequent excitotoxic lesions of the subthalamic nucleus reduced (but did not abolish) rotational asymmetries, had no effects on the measures of neglect and skilled paw-reaching, and produced contralateral orientation biases in head turning and body axis curling. Rats that received subthalamic lesions alone exhibited de novo impairments comprising contralateral biases in the orientation tests. These results support a neuromodulatory role of the subthalamic nucleus in regulating motor outputs of the basal ganglia, and caution that there may be distinct side effects of the lesion by itself. Whereas some impairments attributable to dopamine depletion may be alleviated by subthalamic manipulations, other symptoms are not, or may even be aggravated.

Intranigral transplantation of fetal substantia nigra allograft in the hemiparkinsonian rhesus monkey

Current clinical protocols for fetal cell transplantation for Parkinson's disease (PD) have focused on restoring dopamine in the striatum. However, there are now a number of human transplant recipients who have had robust innervation of the striatum by dopaminergic grafts (documented by positron emission tomography or by autopsy), but only a partial improvement in parkinsonian motor signs. Thus, there is a need for improved transplant strategies. In animal models of PD, there is recent evidence that restoring dopamine in the substantia nigra, instead of or in addition to the striatum, may be important to correct abnormal motor behavior. This pilot study examined the morphological features and behavioral effects of fetal dopaminergic neuronal allografts placed into the substantia nigra of three 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated hemiparkinsonian rhesus monkeys. We show that grafts can survive in host substantia nigra. Characteristics of the graft-host interface were variable. In one animal, reinnervation of host substantia nigra was observed, and this animal showed behavioral improvement in a reach-and-retrieval task.

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

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