Cerebral metabolism of parkinsonian primates 21 days after MPTP

The trajectory of disturbed resting-state cerebral function in Parkinson's disease at different Hoehn and Yahr stages

OBJECTIVE

We aim to investigate the disturbance of neural network associated with the different clinical stages of Parkinson's disease (PD).

METHOD

We recruited 80 patients at different H&Y stages of PD (28 at H&Y stage I, 28 at H&Y stage II, 24 at H&Y stage III) and 30 normal controls. All participants underwent resting-state fMRI scans on a 3-T MR system. The amplitude of low-frequency fluctuation (ALFF) of blood oxygen level-dependent signals was used to characterize regional cerebral function. Functional integration across the brain regions was evaluated by a seed voxel correlation approach.

RESULTS

PD patients had decreased regional activities in left occipital and lingual regions; these regions show decreased functional connection pattern with temporal regions, which is deteriorating as H&Y stage ascending. In addition, PD patients, especially those at stage II, exhibit increased regional activity in the posterior regions of default mode network (DMN), increased anticorrelation between posterior cingulate cortex (PCC) and cortical regions outside DMN, and higher temporal coherence within DMN. Those indicate more highly functioned DMN in PD patients at stage II.

CONCLUSIONS

Our study demonstrated the trajectories of resting-state cerebral function disturbance in PD patients at different H&Y stages. Impairment in functional integration of occipital-temporal cortex might be a promising measurement to evaluate and potentially track functional substrates of disease evolution of PD.

Pathological basal ganglia activity in movement disorders

Our understanding of the pathophysiology of movement disorders and associated changes in basal ganglia activities has significantly changed during the last few decades. This process began with the development of detailed anatomical models of the basal ganglia, followed by studies of basal ganglia activity patterns in animal models of common movement disorders and electrophysiological recordings in movement disorder patients undergoing functional neurosurgical procedures. These investigations first resulted in an appreciation of global activity changes in the basal ganglia in parkinsonism and other disorders, and later in the detailed description of pathological basal ganglia activity patterns, specifically burst patterns and oscillatory synchronous discharge of basal ganglia neurons. In this review, we critically summarize our current knowledge of the pathological discharge patterns of basal ganglia neurons in Parkinson's disease, dystonia, and dyskinesias.

Primary motor cortex of the parkinsonian monkey: differential effects on the spontaneous activity of pyramidal tract-type neurons

Dysfunction of primary motor cortex (M1) is thought to contribute to the pathophysiology of parkinsonism. What specific aspects of M1 function are abnormal remains uncertain, however. Moreover, few models consider the possibility that distinct cortical neuron subtypes may be affected differently. Those questions were addressed by studying the resting activity of intratelencephalic-type corticostriatal neurons (CSNs) and distant-projecting lamina 5b pyramidal-tract type neurons (PTNs) in the macaque M1 before and after the induction of parkinsonism by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Contrary to previous reports, the general population of M1 neurons (i.e., PTNs, CSNs, and unidentified neurons) showed reduced baseline firing rates following MPTP, attributable largely to a marked decrease in PTN firing rates. CSN firing rates were unmodified. Although burstiness and firing patterns remained constant in M1 neurons as a whole and CSNs in particular, PTNs became more bursty post-MPTP and less likely to fire in a regular-spiking pattern. Rhythmic spiking (found in PTNs predominantly) occurred at beta frequencies (14-32 Hz) more frequently following MPTP. These results indicate that MPTP intoxication induced distinct modifications in the activity of different M1 neuronal subtypes. The particular susceptibility of PTNs suggests that PTN dysfunction may be an important contributor to the pathophysiology of parkinsonian motor signs.

The biochemical and cellular basis for nutraceutical strategies to attenuate neurodegeneration in Parkinson's disease

Future therapeutic intervention that could effectively decelerate the rate of degeneration within the substantia nigra pars compacta (SNc) could add years of mobility and reduce morbidity associated with Parkinson’s disease (PD). Neurodegenerative decline associated with PD is distinguished by extensive damage to SNc dopaminergic (DAergic) neurons and decay of the striatal tract. While genetic mutations or environmental toxins can precipitate pathology, progressive degenerative succession involves a gradual decline in DA neurotransmission/synaptic uptake, impaired oxidative glucose consumption, a rise in striatal lactate and chronic inflammation. Nutraceuticals play a fundamental role in energy metabolism and signaling transduction pathways that control neurotransmission and inflammation. However, the use of nutritional supplements to slow the progression of PD has met with considerable challenge and has thus far proven unsuccessful. This review re-examines precipitating factors and insults involved in PD and how nutraceuticals can affect each of these biological targets. Discussed are disease dynamics (Sections 1 and 2) and natural substances, vitamins and minerals that could impact disease processes (Section 3). Topics include nutritional influences on α-synuclein aggregation, ubiquitin proteasome function, mTOR signaling/lysosomal-autophagy, energy failure, faulty catecholamine trafficking, DA oxidation, synthesis of toxic DA-quinones, o-semiquinones, benzothiazolines, hyperhomocyseinemia, methylation, inflammation and irreversible oxidation of neuromelanin. In summary, it is clear that future research will be required to consider the multi-faceted nature of this disease and re-examine how and why the use of nutritional multi-vitamin-mineral and plant-based combinations could be used to slow the progression of PD, if possible.

Variable toxicological response to the loss of OXPHOS through 1-methyl-4-phenylpyridinium-induced mitochondrial damage and anoxia in diverse neural immortal cell lines

Immortal cell lines are used to investigate various aspects of neurodegeneration. These cells display high glycolytic turnover rate and produce an abundant amounts of lactate. Our previous studies indicate that these cells survive the loss of mitochondrial oxidative phosphorylation (OXPHOS) with ample glucose supply. In the current study, we investigate if cell type (w/variation in basal metabolic rate (MR)), can alter glucose utilization patterns which in turn may affect LC(50) for the mitochondrial toxin 1-methyl-4-phenylpyridinium (MPP(+)) in various cell lines. The data obtained indicate that cell lines MRs examined were generally consistent with the average of species adult body weight where mouse N-2A > rat-PC-12 > human SH-SY5Y. A higher MR was associated with accelerated utilization of glucose and earlier cell death with MPP(+): LC(50) mouse = 294 µM, rat = 695 µM, and human = 5.25 mM at 24 h. Cell death appears to be a function of the velocity by which glucose disappears, leading to the failure of glycolysis and subsequent halt of energy production. Similar effects were also observed at higher plating densities where the demand for glucose is amplified. A time-lapse study of MPP(+) toxicity (0-36 h) in N-2A cells indicates that an anaerobic shift occurs as early as 2 h (evidenced by a rise in lactate), followed by a descent in glucose concentrations at 4 h and exhaustion of glucose supplies at 22 h which was associated with the first detectable sign of cell death. It was also noted that MPP(+) toxicity was not associated with the generation of reactive oxygen species (O (2) (-) , H(2)0(2), and NO(2)) and was not attenuated by adding catalase or superoxide dismutase to the media. On the other hand, MPP(+) toxicity was reversed by providing additional supply of glucose, pyruvate ± mitochondrial monocarboxylate transporter blocker (α-cyano-4-HCA), or pyruvate ± pyruvate dehydrogenase inhibitor (octanoyl-CoA), suggesting that the exclusive anaerobic survival compensates for the loss of OXPHOS by MPP(+). To examine if neuroblastoma were capable of surviving the deprivation of O(2) for 24 h, a range of hypoxia to anoxia was established with various concentrations of dithionite. The data suggest that cell lines examined continue to thrive when incubated with high-glucose media (25 mM). In summary, vulnerability of immortal neuroblastoma cell lines to MPP(+) toxicity is dependent upon glucose concentrations within the media and cell MR, which indirectly dominates the velocity of glucose use and its end point disappearance, leading to cell death by ergogenic failure.

Subcortical elevation of metabolism in Parkinson's disease--a critical reappraisal in the context of global mean normalization

In a recent issue of NeuroImage, we presented evidence that biased global mean (GM) normalization of brain PET data can generate the appearance of subcortical foci with relative hypermetabolism in patients with Parkinson's disease (PD), and other degenerative disorders. In a commentary to our article, Ma and colleagues presented a study seeking to establish that a pattern of widespread hypermetabolism, known as the Parkinson's disease related pattern (PDRP) is a genuine metabolic feature of PD. In the present paper, we respond to the arguments presented by Ma et al., and we provide a critical reappraisal of the evidence for the existence of the PDRP. To this end, we present new analyses of PET data sets, which demonstrate that very similar patterns of relative subcortical increases are seen in PD, Alzheimer's disease, hepatic encephalopathy, healthy aging, and simulation data. Furthermore, longitudinal studies of PD previously reported relative hypermetabolism in very small anatomical structures such as the subthalamic nucleus. We now demonstrate how focal hypermetabolism attributed to small nuclei can similarly arise as a consequence of GM normalization. Finally, we give a comprehensive summary of the entire deoxyglucose autoradiography literature on acquired parkinsonism in experimental animals. Based on this evidence, we conclude that (1) there is no quantitative evidence for widespread subcortical hypermetabolism in PD, (2) very similar patterns of subcortical hyperactivity are evident in various other brain disorders whenever GM normalization is utilized, and (3) the PDRP is not evident in animal models of PD. In the absence of quantitative evidence for the PDRP, our alternative interpretation of normalization bias seems the more parsimonious explanation for the reports of relative hypermetabolism in PD.

Altered neuronal activity relationships between the pedunculopontine nucleus and motor cortex in a rodent model of Parkinson's disease

The pedunculopontine nucleus (PPN) is a new deep brain stimulation (DBS) target for Parkinson's disease (PD), but little is known about PPN firing pattern alterations in PD. The anesthetized rat is a useful model for investigating the effects of dopamine loss on the transmission of oscillatory cortical activity through basal ganglia structures. After dopamine loss, synchronous oscillatory activity emerges in the subthalamic nucleus and substantia nigra pars reticulata in phase with cortical slow oscillations. To investigate the impact of dopamine cell lesion-induced changes in basal ganglia output on activity in the PPN, this study examines PPN spike timing with reference to motor cortex (MCx) local field potential (LFP) activity in urethane- or ketamine-anesthetized rats. Seven to ten days after unilateral 6-hydroxydopamine lesion of the medial forebrain bundle, spectral power in PPN spike trains and coherence between PPN spiking and PPN LFP activity increased in the approximately 1 Hz range in urethane-anesthetized rats. PPN spike timing also changed from firing predominantly in phase with MCx slow oscillations in the intact urethane-anesthetized rat to firing predominantly antiphase to MCx oscillations in the hemi-parkinsonian rat. These changes were not observed in the ketamine-anesthetized preparation. These observations suggest that dopamine loss alters PPN spike timing by increasing inhibitory oscillatory input to the PPN from basal ganglia output nuclei, a phenomenon that may be relevant to motor dysfunction and PPN DBS efficacy in PD patients.

Lesions in monkey globus pallidus externus exacerbate parkinsonian symptoms

To further define the role of the external segment of the globus pallidus (GPe) in the development of parkinsonian motor signs, two rhesus monkeys were made parkinsonian with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Behavioral assessments of bradykinesia and akinesia as well as single neuron recordings in the internal segment of the globus pallidus (GPi) were performed in both monkeys before and after ablating the sensorimotor portion of GPe. The effects of apomorphine on behavior and neuronal activity were also assessed in the parkinsonian monkeys before and after GPe ablation. We found that lesions in GPe exacerbated parkinsonian symptoms, altered neuronal activity in GPi, and reduced the therapeutic effects of apomorphine. These results support the hypothesis that GPe can influence GPi neuronal activity and is directly involved in parkinsonism. In addition, these data suggest that the inclusion of GPe in pallidotomy lesions for the treatment of Parkinson's disease can block the beneficial effects of antiparkinsonian medications and should be avoided.

Cognitive- and motor-related regions in Parkinson's disease: FDOPA and FDG PET studies

OBJECTIVE

Using 6-[(18)F]fluoro-L-dopa (FDOPA) and [(18)F]fluorodeoxyglucoce (FDG) positron emission tomography (PET), multiple regression analyses were performed to determine the specific brain regions that are related to cognitive and motor symptoms in nondemented patients with Parkinson's disease.

METHODS

Spatially normalized images of FDOPA influx rate constant (Ki) values and relative regional cerebral metabolic rates for glucose (rrCMRglc) were created. Raven's Coloured Progressive Matrices (RCPM) scores and the Unified Parkinson's Disease Rating Scale (UPDRS) motor scores were used to determine the patients' cognitive and motor functions, respectively. Multiple correlation analyses between the FDOPA and FDG images and the cognitive and motor scores were performed for each voxel.

RESULTS

RCPM score was significantly positively correlated with the FDOPA Ki in the left hippocampus and with the rrCMRglc in the left middle frontal gyrus and right retrosplenial cortex. Motor function was significantly positively correlated with the FDOPA Ki in the bilateral striatum and with the rrCMRglc in association areas and primary visual cortex. The level of motor function was significantly inversely correlated with the FDOPA Ki in the anterior cingulate gyrus and with the rrCMRglc in bilateral primary motor cortex and right putamen.

CONCLUSIONS

Changes of striatal FDOPA uptake and rrCMRglc in the primary motor cortex likely represent dysfunction in the motor system involving the corticobasal ganglia-thalamocortical loop. Change of FDOPA uptake in the anterior cingulate gyrus may be related to up-regulation of dopamine synthesis in surviving dopamine neurons. The regions where correlation with cognitive function was observed belong to a cognitive frontoparietal-hippocampal network.

Enhanced synchrony among primary motor cortex neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine primate model of Parkinson's disease

Primary motor cortex (MI) neurons discharge vigorously during voluntary movement. A cardinal symptom of Parkinson's disease (PD) is poverty of movement (akinesia). Current models of PD thus hypothesize that increased inhibitory pallidal output reduces firing rates in frontal cortex, including MI, resulting in akinesia and muscle rigidity. We recorded the simultaneous spontaneous discharge of several neurons in the arm-related area of MI of two monkeys and in the globus pallidus (GP) of one of the two. Accelerometers were fastened to the forelimbs to detect movement, and surface electromyograms were recorded from the contralateral arm of one monkey. The recordings were conducted before and after systemic treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rendering the animals severely akinetic and rigid with little or no tremor. The mean spontaneous MI rates during periods of immobility (four to five spikes/sec) did not change after MPTP; however, in this parkinsonian state, MI neurons discharged in long bursts (sometimes >2 sec long). These bursts were synchronized across many cells but failed to elicit detectable movement, indicating that even robust synchronous MI discharge need not result in movement. These synchronized population bursts were absent from the GP and were on a larger timescale than oscillatory synchrony found in the GP of tremulous MPTP primates, suggesting that MI parkinsonian synchrony arises independently of basal ganglia dynamics. After MPTP, MI neurons responded more vigorously and with less specificity to passive limb movement. Abnormal MI firing patterns and synchronization, rather than reduced firing rates, may underlie PD akinesia and persistent muscle rigidity.

Enhancement of sensorimotor behavioral recovery in hemiparkinsonian rats with intrastriatal, intranigral, and intrasubthalamic nucleus dopaminergic transplants

One of the critical variables that influences the efficacy of clinical neural transplantation for Parkinson's disease (PD) is optimal graft placement. The current transplantation paradigm that focuses on ectopic placement of fetal grafts in the striatum (ST) fails to reconstruct the basal ganglia circuitry or normalize neuronal activity in important basal ganglia structures, such as the substantia nigra (SN) and the subthalamic nucleus (STN). The aim of this study was to investigate a multitarget neural transplantation strategy for PD by assessing whether simultaneous dopaminergic transplants in the ST, SN, and STN induce functional recovery in hemiparkinsonian rats. Forty-six female Wistar rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway were randomly divided into eight groups and received lesions only or injections of 900,000 embryonic rat ventral mesencephalic cells in the (1) ST, (2) SN, (3) STN, (4) ST and SN, (5) ST, SN, and STN, (6) ST and STN, or (7) SN and STN. The number of cells transplanted was equally divided among grafting sites. Animals with two grafts received 450,000 cells in each structure, and animals with three grafts received 300,000 cells per structure. Recovery was assessed by amphetamine-induced rotations and the stepping tests. Graft survival was assessed using tyrosine hydroxylase immunohistochemistry. At 8 weeks after transplantation, simultaneous dopaminergic transplants in the ST, SN, and STN induced significant improvement in rotational behavior and stepping test scores. Intrastriatal transplants were associated with significant recovery of rotational asymmetry, whereas SN and STN transplants were associated with improved forelimb function scores. These results suggest that restoration of dopaminergic activity to multiple basal ganglia targets, such as the ST and SN, or the ST and STN, promotes a more complete functional recovery of complex sensorimotor behaviors. A multitarget transplant strategy aimed at optimizing dopaminergic reinnervation of the basal ganglia may be crucial in improving clinical outcomes in PD patients.

Tremor is associated with PET measures of nigrostriatal dopamine function in MPTP-lesioned monkeys

Unilateral intracarotid artery (ICA) MPTP infusion, along with sequential systemic doses of MPTP, produces near complete degeneration of the nigrostriatal pathway on the side of infusion (ipsilateral) and variable levels of damage in the contralateral hemisphere accompanied by varying levels of parkinsonism (overlesioned hemiparkinsonian model). Positron emission tomography and the dopamine (DA) metabolism tracer [(18)F]6-fluoro-l-m-tyrosine (FMT) were used to evaluate the relationship between DA metabolism and clinical features of parkinsonism in 14 overlesioned hemiparkinsonian monkeys. Monkeys were rated on a parkinsonian scale that included ratings of bradykinesia, fine motor skills (FMS), and rest tremor. Because the monkeys tended to show more severe clinical signs on the side of the body contralateral to ICA MPTP infusion, we calculated asymmetry scores for each of the clinical features as well as for FMT uptake (K(i)) in the caudate and putamen. Tremor asymmetry was associated with FMT uptake asymmetry in the putamen. No such relationship was observed for FMS or bradykinesia. The overall severity of tremor (mild, moderate/severe) was associated with FMT uptake in the caudate and putamen. Postmortem biochemical analysis for a subset of monkeys showed that the monkeys with moderate/severe tremor had significantly lower DA levels in both caudate and putamen than those with mild tremor. In addition, K(i) values were significantly correlated with DA levels in both caudate and putamen. These findings support the idea that nigrostriatal degeneration contributes to rest tremor.

Metabolic effects of nigrostriatal denervation in basal ganglia

In the past, functional changes in the circuitry of the basal ganglia that occur in Parkinson's disease were primarily analyzed with electrophysiological and 2-deoxyglucose measurements. The increased activity of the subthalamic nucleus (STN) observed has been attributed to a reduction in inhibition mediated by the external segment of the globus pallidus (GPe), secondary to the loss of dopaminergic-neuron influence on D2-receptor-bearing striato-pallidal neurons. More recently, in situ hybridization studies of cytochrome oxidase subunit I have confirmed the overactivity of the STN in the parkinsonian state. In addition, this technique has provided evidence that the change in STN activity is owing not only to decreased inhibition from the GPe but to hyperactivity of excitatory inputs from the parafascicular nucleus of the thalamus and the pedunculopontine nucleus in the brainstem.

Ablative surgery and deep brain stimulation for Parkinson's disease

Surgical options for Parkinson's disease (PD) are rapidly expanding and include ablative procedures, deep brain stimulation, and cell transplantation. The target nuclei for ablative surgery and deep brain stimulation are the motor thalamus, the globus pallidus, and the subthalamic nucleus. Multiple factors have led to the resurgence of interest in the surgical treatment of PD: 1) recognition that long-term medical therapy for PD is often unsatisfactory, with patients eventually suffering from drug-induced dyskinesias, motor fluctuations, and variable responses to medication; 2) greater understanding of the pathophysiology of PD, providing a better scientific rationale for some previously developed procedures and suggesting new targets; and 3) use of improved techniques, such as computed tomography- and magnetic resonance imaging-guided stereotaxy and single-unit microelectrode recording, making surgical intervention in the basal ganglia more precise. We review the present status of ablative surgery and deep brain stimulation for PD, including theoretical aspects, surgical techniques, and clinical results.

Experimental models of Parkinson's disease: from the static to the dynamic

The experimental models of Parkinson's disease (PD) available today can be divided into two categories according to the mode of action of the compound used: transient pharmacological impairment of dopaminergic transmission along the nigrostriatal pathway or selective destruction by a neurotoxic agent of the dopaminergic neurons of the substantia nigra pars compacta. The present article looks at the relative merits of each model, the clinical symptoms and neuronal impairment it induces, and the contribution it could make to the development of a truly dynamic model. It is becoming more and more clear that there is an urgent need for a chronic model integrating all the clinical features of PD including resting tremor, and reproducing the gradual but continuous nigral degeneration observed in the human pathology. Discrepancies have been reported several times between results obtained in classic animal models and those described in PD, and it would seem probable that such contradictions can be ascribed to the fact that animal models do not, as yet, reproduce the continuous evolution of the human disease. Dynamic experimental models which come closer to the progressive neurodegeneration and gradual intensification of motor disability so characteristic of human PD will enable us to investigate crucial aspects of the disease, such as compensatory mechanisms and dyskinesia.

Morphofunctional studies of the interactions of the glutamatergic, cholinergic, and dopaminergic systems in the neostriatum

Morphological, electrophysiological, and behavioral experiments were used to study the interactions of the glutamatergic, cholinergic, and dopaminergic systems in the neostriatum of white rats with unilateral lesions of the mesostriatal dopaminergic system induced by 6-hydroxydopamine. The neostriatum was shown to contain both synaptic and interneuronal nonsynaptic interactions between these neurochemical systems. It is suggested that glutamate, which is present in excess in conditions of prolonged dopamine deficiency, has toxic effects on corticoneostriatal synaptic connections.

Intrastriatal mesencephalic grafts affect neuronal activity in basal ganglia nuclei and their target structures in a rat model of Parkinson's disease

Nigrostriatal dopamine (DA) lesions lead to changes of neuronal activity in basal ganglia nuclei such as the globus pallidus (GP, the rodent homolog of lateral globus pallidus), entopeduncular nucleus (EP, the rodent homolog of medial globus pallidus), substantia nigra pars reticulata (SNR), and subthalamic nucleus (STN). We investigated in rats whether embryonic mesencephalic DA neurons grafted in the striatum may affect the lesion-induced alterations of neuronal activity in these structures. Regional neuronal activity was determined by use of quantitative cytochrome oxidase histochemistry. It was also examined in lesioned rats whether the grafts may regulate the expression of c-Fos after systemic administration of apomorphine in the basal ganglia nuclei as well as their target structures, including the ventromedial thalamic nucleus (VM), superior colliculus (SC), and pedunculopontine nucleus (PPN). Lesioned rats exhibited an increased activity of CO in the GP, EP, SNR, and STN ipsilateral to the lesion. Intrastriatal nigral grafts reversed the increases in the CO activity in the EP and SNR, whereas the grafts failed to affect the enzyme activity in the GP or STN. Apomorphine induced an increased expression of c-Fos in the GP, STN, VM, SC, and PPN on the lesioned side. The enhanced expression of this protein in all the structures except for the STN was attenuated by nigral grafts. The present results indicate that intrastriatal DA neuron grafts can normalize the lesion-induced changes of neuronal activity in the output nuclei of the basal ganglia as well as their target structures.

Re-evaluation of the functional anatomy of the basal ganglia in normal and Parkinsonian states

In the late 1980s, a functional and anatomical model of basal ganglia organization was proposed in order to explain the clinical syndrome of Parkinson's disease. According to this model, the pathological overactivity observed in the subthalamic nucleus and the output station of the basal ganglia plays a crucial role in the pathophysiology of the motor signs of Parkinson's disease. The hyperactivity of subthalamic neurons in Parkinsonism is viewed as a direct consequence of a pathological hypoactivity of the external segment of the pallidum. This article reviews recent data from different experimental approaches that challenge the established model of basal ganglia organization by reinterpreting the functional interaction between the external segment of the pallidum and the subthalamic nucleus in both the normal and pathological state. Indeed, recent neurobiochemical studies have rather unexpectedly shown that the GABAergic and metabolic activities of the external pallidum are not decreased in human and non-human primates with Parkinsonism. This absence of any decrease in activity might be explained by the functionally antagonistic influences of the striatal and subthalamic afferences within the external pallidum, as suggested by several anatomical studies. In addition, there are clues from electrophysiological studies to suggest that the hyperactivity found in the subthalamic neurons in Parkinsonism may not depend solely on the level of activity in the external pallidum. In such a framework, the hyperactivity of the subthalamic neurons would have to be explained, at least in part, by other sources of excitation or disinhibition. However, any explanation for the origin of the subthalamic overactivity in Parkinsonism remains speculative.

Short and long-term changes in cerebral [14C]-2-deoxyglucose uptake in the MPTP-treated marmoset: relationship to locomotor activity

The "short-term" (0.7 +/- 0.1 months post-MPTP) and "long-term" effects (36.7 +/- 4.4 months) of MPTP treatment on motor behaviour and [14C]-2DG uptake were investigated in the common marmoset. The subcutaneous administration of MPTP greatly reduced locomotor activity (-94% with respect to controls) and induced motor disability in the "short-term" MPTP-treated marmoset group. In the "long-term" MPTP group, MPTP treatment did not significantly affect locomotor activity (-27% with respect to controls) and there was partial recovery of motor disability. In the "short-term" MPTP group, there were increases in [14C]-2DG uptake in the GPl (+31 to +37%), SNc (+34 to +42%), VTA (+35%), LC (+23%), PPN (+19%) and in the VA (+19%), VL (+20%) and AM (+17%) thalamic nuclei. [14C]-2DG uptake was decreased in the STN (-15%). In the "long-term" MPTP group, [14C]-2DG uptake was increased in the GPl (+18%), SNc (+27%), VTA (+25%), PPN (+19%), ventral caudate nucleus (+18 to +23%), NAc (+22%), F.Ctx (+18%) and in the VA (+34%), VL (+28%), AV (+33%) and AM (+24%) thalamic nuclei. [14C]-2DG uptake was unchanged in the STN. The increase in metabolic activity of the surviving DA neurones and/or the reactive gliosis may account for the initial increase in [14C]-2DG uptake in the SNc and VTA. On the other hand, in the "long-term" MPTP-treated animals the increase in [14C]-2DG uptake in the SNc (though less than in the "short-term" MPTP group), ventral caudate and NAc may reflect the regenerative changes in the dopaminergic system in these areas. Despite the behavioural recovery, [14C]-2DG uptake remained elevated in the target areas for medial pallidal output (the thalamic nuclei and PPN). However, the attenuation of the changes in [14C]-2DG uptake in the GPl and STN of "long-term" MPTP-treated marmosets suggest that the striato-GPl and GPl-STN outputs closely reflect motor function in this primate model of Parkinson's disease.

Cortical glucose metabolism in Parkinson's disease without dementia

There have been several reports of decreased regional cerebral metabolic rates for glucose (rCMRglc) in Parkinson's disease (PD), although others find no differences between PD patients and controls. Differences in the cognitive status of the PD patients may account for some of these inconsistencies. We report the results of a PET study using 18F-fluorodeoxyglucose (FDG) to measure rCMRglc in eight nondemented PD patients, six of whom were receiving dopaminergic medications, and eight age-matched control subjects. We scanned one tomographic level through the temporal lobes that included both temporal neocortex and mesial temporal cortex, and one tomographic level through the basal ganglia that included frontal and parietal cortex. Previously determined rate constants and an operational equation were used to determine rCMRglc. On average, rCMRglc values were 23% below control values for all regions studied, with the greatest differences in posterior brain regions (visual association cortex, primary visual cortex, and parietal cortex) and thalamus. These results indicate that PD patients may show neocortical hypometabolism, especially in posterior brain regions, in the absence of any demonstrable cognitive deficits.

Differential effect of chronic dopaminergic treatment on dopamine D1 and D2 receptors in the monkey brain in MPTP-induced parkinsonism

Dopamine D1 and D2 receptors located within the striatum (caudate nucleus and putamen) were studied autoradiographically, using [3H]SCH 23390 and [3H]sulpiride respectively, in (i) seven monkeys rendered parkinsonian by the systemic administration of MPTP, four of which were chronically exposed to anti-parkinsonian drugs (levodopa or apomorphine), (ii) two hemi-parkinsonian monkeys (induced by intra-carotid infusion of MPTP), one of which received chronic exposure to apomorphine, and (iii) three control monkeys which received neither MPTP nor dopaminergic drugs. Anti-parkinsonian drug exposure resulted in a reversal of symptoms and was accompanied by the development of limb dyskinesias. In parkinsonian monkeys not chronically exposed to drugs. [3H]SCH 23390 binding was slightly but not significantly elevated above control values, whilst in the same animals [3H]sulpiride binding was significantly increased above that found in the control group. Rostrally [3H]SCH 23390 binding was similar in the control and drug-exposed parkinsonian groups but more caudally there was a small consistent, although not significant, increase in [3H]SCH 23390 binding in the drug-exposed animals as compared to the parkinsonian monkeys not exposed to drugs. In contrast at all rostro-caudal levels [3H]sulpiride binding in the drug-exposed parkinsonian group was lower than the corresponding values from the non-drug exposed animals. [3H]SCH 23390 binding showed no major side-to-side difference in the hemi-parkinsonian animal which was not exposed to levodopa/apomorphine, whilst in the hemi-parkinsonian monkey which received apomorphine there was again an increase in binding on the MPTP-treated side of the brain. In both drug- and non-drug exposed hemi-parkinsonian animals there was a greater density of [3H]sulpiride binding in the parkinsonian side of the brain; the general level of binding in the drug-exposed monkey was less than that seen in the other animal. These results would support the idea that in MPTP-induced parkinsonism, dopaminergic denervation results in a greater change in the D2 receptors, but furthermore would indicate a differential effect of levodopa/apomorphine exposure on the D1 and D2 receptor populations. Drug exposure apparently encourages the reversal of the MPTP-induced increase in the D2 receptor binding, whilst the D1 receptor binding appears to proliferate in response to these drugs. These results may have important implications in relation to the development of dyskinesias, subsequent to the chronic use of some anti-parkinsonian drug treatments.

Parkinson's disease-like effects of S-: Effects of l-Dopa

The major symptoms of Parkinson's disease (PD) are due to degeneration of the nigrostriatal pathway and depletion of dopamine (DA). Tyrosine hydroxylase (TH), norepinephrine (NE), serotonin (5-HT), and melanin pigments are also decreased and acetylcholinergic activity increased. Biochemically, increased methylation can cause the depletion of DA, NE, 5-HT, and melanin pigments and also an increase of acetylcholine; thus, increased methylation can present a biochemical picture that resembles the biochemical changes that occur in PD. During the therapy of PD with L-dopa, it is well known that L-dopa reacts avidly with S-adenosyl-L-methionine (SAM), the biologic methyl donor, to produce 3-O-methyl-dopa. Correspondingly, L-dopa has been shown to deplete the concentration of SAM, and SAM has been found to induce PD-like motor impairments in rodents; therefore, an excess of SAM-dependent methylation may be associated with Parkinsonism. To further study the effects of methylation, SAM was injected into the lateral ventricle of rats. SAM caused tremors, rigidity, abnormal posture, and dose-related hypokinesia. Doses of 9.38, 50, and 400 nM/rat caused 61.9, 73.4, and 94.8% reduction, respectively, of motor activity. A 200-mg/kg IP dose of L-dopa, given before 50 nM SAM, blocked the SAM-induced hypokinesia. SAM also caused a decrease in TH immunoreactivity, apparent degeneration of TH-containing fibers, loss of neurons, and the accumulation of phagocytic cells in the substantia nigra. These results showed that excess SAM in the brain, probably due to its ability to increase methylation, can induce symptoms that resemble some of the changes that occur in PD.

Subthalamic nucleus cell firing in the 6-OHDA-treated rat: basal activity and response to haloperidol

Single unit recordings from neurons of the subthalamic nucleus were made in control and 6-hydroxydopamine (6-OHDA)-treated rats anesthetized with chloral hydrate. Subthalamic nucleus cells in this preparation exhibited a wide range of firing rates and three different firing patterns. These patterns were defined as 'burst', 'normal', and 'mixed' based on comparisons of their interspike interval histograms. Four to 6 weeks after 6-OHDA treatment there was no change in the basal firing rates of subthalamic nucleus cells, but there was a significant shift in firing pattern, with a smaller proportion of cells exhibiting the 'normal' firing pattern. The response of subthalamic nucleus neurons to acute administration of haloperidol was also altered in 6-OHDA-treated rats tested 4-6 weeks post-lesion, with a significantly greater proportion of cells responding to doses of haloperidol as low as 0.2 mg/kg (i.v.) with increases in firing rate of 20% or more. These results suggest that the subthalamic nucleus is probably not involved in the increases in basal levels of dopamine cell activity observed previously in the 6-OHDA-treated rat, but may play a role in the acute induction of depolarization block of dopamine cell firing in response to haloperidol administration in this model.

Changes in brain catecholamines and dopamine uptake sites at different stages of MPTP parkinsonism in monkeys

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been shown to produce parkinsonism in primates. We have studied the changes in brain catecholamines and the distribution of desipramine insensitive mazindol binding sites in MPTP parkinsonian primates at different levels of parkinsonism. Thirty-seven monkeys (Macaca fascicularis) were utilized in this study. Twelve naive animals received no treatment and served as controls. Twenty-five animals were rendered parkinsonian with serial injections of MPTP. All animals were given scored neurologic examinations throughout the study. Their movement was quantitated in an activity box. The animals were sacrificed 30-360 days after their last MPTP injection. The clinical exam of the MPTP parkinsonian monkeys demonstrated mildly to severely affected animals. There was an exponential decrease in brain catecholamine levels with increased clinical parkinsonism. The MPTP parkinsonian animals showed the greatest decrease (67-99.8%) in tissue dopamine levels in the caudate nucleus. The putamen followed closely in severity (48-99.8%) and the nucleus accumbens was much less affected (0-40%). The percent reduction of norepinephrine in the anterior pole of the frontal cortex (0-48%) was similar in degree to the decreased dopamine levels in the nucleus accumbens. Mazindol binding was decreased 30-98% in the caudate nucleus, 20-97% in the putamen, 0-26% in the nucleus accumbens, 80-96% in the substantia nigra pars compacta and 49-94% in the ventral tegmental area. In the striatum, the decreased mazindol binding was more pronounced laterally and posteriorly. In each animal, there was good correlation between tissue dopamine levels and the number of mazindol binding sites.

Cerebral metabolic effects of monoamine oxidase inhibition in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine acutely treated monkeys

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces dopaminergic cell death in the substantia nigra pars compacta (SNpc) and clinical parkinsonism in humans and experimental animals. Pretreatment with monoamine oxidase inhibitors prevents this cell death and associated parkinsonism by blocking the oxidation of MPTP to a toxic intermediate. The 2-deoxyglucose method was used to study the acute effects of MPTP in the monkey brain and the effects of monoamine oxidase inhibition on local cerebral glucose utilization in both normal and MPTP-treated monkeys. MPTP administration alone caused a major increase in glucose utilization in the SNpc and smaller increases in some subnuclei within the ventral tegmental area in which eventual dopaminergic cell loss also occurs. Pretreatment with pargyline abolished these metabolic increases, a finding suggesting both that the oxidized product of MPTP generates the metabolic increases and that the increased glucose consumption may contribute to cell toxicity. On the other hand, in most cortical, thalamic, striatal, brainstem, and cerebellar areas MPTP alone caused reductions in glucose utilization, and pargyline failed to prevent these effects. Pargyline alone depressed metabolism in the locus coeruleus and a few other monoaminergic structures.

The role of striatopallidal neurones utilizing gamma-aminobutyric acid in the pathophysiology of MPTP-induced parkinsonism in the primate: evidence from [3H]flunitrazepam autoradiography

The GABA/benzodiazepine receptor complex in the basal ganglia of primates treated with the neurotoxin n-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been studied by semi-quantitative autoradiography with [3H]flunitrazepam ([3H]FNZ). Systemic treatment with MPTP produced a stable and lasting parkinsonian condition, with pronounced bradykinesia, akinesia and tremor. In the lateral segment of the globus pallidus (GPL) there was a significant reduction of [3H]FNZ binding compared with non-treated animals. There were no significant changes in the [3H]FNZ binding in the caudate nucleus, putamen and medial globus pallidus (GPM). This suggests that MPTP-treatment increases GABA release within the GPL exclusively. In view of the available evidence suggesting increased striatal output, and reduced unit activity within the GPL of the MPTP-treated primate, it seems likely that the striatal GABAergic output to the GPL is overactive in this model of Parkinson's disease. Furthermore, as there is no evidence for a change in GABA function within the GPM using this measure, the striatal neurones which innervate the GPM may be differentially affected by loss of dopamine innervation. In line with structural evidence and extrastriatal dopamine receptor distribution this suggests that the two striatopallidal systems are functionally heterogeneous. A hemi-parkinsonian primate model has also been used in this study. This model was produced by injection of MPTP directly into one carotid artery. The substantia nigra pars compacta (SNc) was destroyed on the injected side alone, and consequently the appearance of parkinsonian symptoms was confined to the contralateral side. [3H]FNZ binding in the GPL appears to be bilaterally reduced in this model, suggesting an interaction between the treated and non-treated side of the brain. In addition there is increased binding in the putamen and GPM with respect to the non-treated side of the brain. The increased [3H]FNZ binding in the GPM of the unilateral model may be due to the greater disruption of the nigropallidal and/or nigrostiatal dopamine neurones relative to the systemic model. The former would have the effect of uncoupling D1 dopamine receptors located on the terminals of striatal efferents from nigropallidal dopamine input, and as D1 dopamine receptors are implicated in the presynaptic control of GABA release from the terminals of striatal efferents, this would consequently reduce the level of GABA release in the GPM. The latter possibility would suggest that striatopallidal neurones projecting to GPM are more resistant to the effects of dopaminergic denervation than those projecting to GPL.

Primate models of movement disorders of basal ganglia origin

Movement disorders associated with basal ganglia dysfunction comprise a spectrum of abnormalities that range from the hypokinetic disorders (of which Parkinson's disease is the best-known example) at one extreme to the hyperkinetic disorders (exemplified by Huntington's disease and hemiballismus) at the other. Both extremes of this movement disorder spectrum can be accounted for by postulating specific disturbances within the basal ganglia-thalamocortical 'motor' circuit. In this paper, Mahlon DeLong describes the changes in neuronal activity in the motor circuit in animal models of hypo- and hyperkinetic disorders.