R.G. Robertson

Neural mechanisms underlying Parkinsonian symptoms based upon regional uptake of 2-deoxyglucose in monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

The 2-deoxyglucose metabolic mapping technique has been used to investigate the neural mechanisms which underlie the symptoms of Parkinsonism in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine primate model of Parkinsons disease. In six cynomolgus monkeys, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was either (a) administered intravenously to induce generalized Parkinsonism, or (b) infused into one carotid artery to induce unilateral Parkinsonism. Post-mortem examination revealed profound cell loss from the substantia nigra, pars compacta either bilaterally or unilaterally in the two groups, respectively. In addition, there was pathological involvement of the ventral tegmental area and locus coeruleus in animals receiving intravenous 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. 2-Deoxyglucose autoradiography revealed widespread changes in 2-deoxyglucose uptake in the brains of parkinsonian animals when compared to controls. Most of these changes were in basal ganglia and related structures and were qualitatively similar in the two groups of experimental animals. Prominent increases in 2-deoxyglucose uptake were observed in the lateral segment of the globus pallidus (24-27%), the ventral anterior and ventral lateral nuclei of the thalamus (14-22%) and the nucleus tegmenti pedunculopontinus of the caudal midbrain (17-69%). A profound decrease (17-26%) in 2-deoxyglucose uptake was observed in the subthalamic nucleus. We propose these data to indicate that in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism there is the following pattern of abnormal neuronal activity in basal ganglia circuitry: (i) increased activity in the projection from the putamen to the lateral segment of the globus pallidus; (ii) decreased activity in the projection from the putamen to the medial segment of the globus pallidus; (iii) decreased activity in the projection from the lateral segment of the globus pallidus to the subthalamic nucleus; (iv) increased activity in the projection from the subthalamic nucleus to the globus pallidus; and (v) increased activity in neurons of the medial segment of the globus pallidus projecting to the ventral anterior/ventral lateral thalamus and the pedunculopontine nucleus. These results are compared to the 2-deoxyglucose uptake findings in previous studies from this laboratory in hemiballism and hemichorea in the monkey. The central importance of the subthalamic nucleus in all three conditions is proposed, and supportive evidence for the excitatory nature of subthalamic efferent fibres is adduced.

Injection of excitatory amino acid antagonists into the medial pallidal segment of a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treated primate reverses motor symptoms of parkinsonism

Intracerebral injections of the broad spectrum excitatory amino acid antagonist kynurenic acid (50 ug) alleviated the symptoms of akinesia, tremor and rigidity in a severely parkinsonian monkey. Unilateral injection of kynurenic acid within the medial pallidal segment produced rotational behaviour away from the side of the injection, and the limbs on the contralateral side showed relief of the MPTP-induced parkinsonian symptoms. The subsequent bilateral injection of the excitatory amino acid antagonist allowed the monkey to move freely, unhindered by tremor or rigidity. In addition unilateral injections of the NMDA antagonist MK-801 (5, 25 and 50 ug) within the medial pallidum also produced dose-related rotational behaviour, with alleviation of parkinsonian symptoms in the contralateral limbs. Systemic administration of MK-801 (1 ng/kg - 1 ug/kg i.m.) was without effect.

Dyskinesia in the primate following injection of an excitatory amino acid antagonist into the medial segment of the globus pallidus

Injection of an excitatory amino acid antagonist, kynurenic acid, into the medial segment of the globus pallidus of the conscious monkey elicited dyskinesia of the contralateral limbs. In most respects the dyskinesia was indistinguishable from the disorder that is produced by ablation of the subthalamic nucleus, or injection of a GABA antagonist into the subthalamic nucleus. Injections of kynurenic acid into the lateral segment of the globus pallidus, by contrast, did not provoke dyskinesia. The effect of kynurenic acid is attributed to the blockade of neurotransmission from the subthalamic nucleus to the medial pallidal segment, and the results suggest that the neurotransmitter utilised by this pathway is an excitatory amino acid.

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 Parkinsons 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.

Further investigations into the pathophysiology of MPTP-induced parkinsonism in the primate: an intracerebral microdialysis study of γ-aminobutyric acid in the lateral segment of the globus pallidus

The technique of intracerebral microdialysis has been employed to examine the extracellular level of gamma-aminobutyric acid (GABA) within the lateral segment of the globus pallidus of two cynomolgus monkeys, before and after the induction of parkinsonism with N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Microdialysis probes were acutely implanted through indwelling cannulae positioned at several sites above the lentiform nuclei in the conscious primates, and the basal and potassium (100 mM) evoked amino acid levels were monitored for 3 h. These procedures were repeated at fresh sites within the lateral globus pallidus following the induction of parkinsonism with MPTP. The levels of a number of amino acids, including Asn, Ala, Gln, Ser, and Tau were unchanged following MPTP treatment, whereas both the basal and K(+)-evoked release of GABA was consistently increased. This observation represents further corroboration of the evidence for increased GABAergic input to the lateral globus pallidus in MPTP-induced parkinsonism.

Drug-induced dyskinesia in primates rendered hemiparkinsonian by intracarotid administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The right common carotid artery was surgically exposed under general anaesthesia in 6 cynomolgus monkeys and MPTP (0.5-2.2 mg/kg) directly infused. This produced a hemiparkinsonian syndrome in the contralateral limbs which responded to treatment with both levodopa and apomorphine. These drugs also precipitated dose-dependent contralateral rotation which reached a peak 2 weeks after MPTP infusion. A massive depletion of large, presumably dopaminergic cells was found from the ipsilateral substantia nigra pars compacta. Three animals receiving chronic therapy with apomorphine developed choreoathetoid movements of the limbs and the face contralateral to the infusion 2 weeks after the commencement of treatment. The severity of the dyskinesia gradually increased and after 4 weeks peak-dose hemiballistic movements were seen. Levodopa and the selective D-2 and D-1 dopamine agonists LY-171555 and SKF 38393 also reversed parkinsonian features and produced contralateral rotation and peak-dose dyskinesia. This unilateral model of parkinsonism in the primate will be of value in the elucidation of the mechanisms by which chronic levodopa or dopamine agonist therapy enhance involuntary movements in parkinsonism.

Basal Ganglia Mechanisms Mediating Experimental Dyskinesia in the Monkey

Disordered function of the basal ganglia may lead to a wide spectrum of motor abnormalities, depending upon the nature of the precipitating factor. Thus, for example, destruction of the subthalamic nucleus produces hemiballismus, degeneration of the neostriatum induces chorea, and interruption of dopaminergic nigostriatal transmission gives rise to parkinsonism. The origin of abnormal activity in other conditions, such as athetosis and dystonia, remains speculative but almost certainly involves the basal ganglia, most likely the neostriatum.

Advances in the Understanding of Neural Mechanisms in Movement Disorders

Relatively little is known about how various primary pathologies within the basal ganglia manifest themselves in the appearance of movement disorders. For example, both Parkinson’s disease and Huntington’s disease are characterised by dysfunction of the striatum. In the case of Parkinson’s disease, loss of the ascending dopamine systems results in disordered striatal activity, whereas in Huntington’s disease, parts of the striatum degenerate. It is implicitly assumed that the abnormal movements result from the disordered striatal activity acting on lower motor centres, presumably via the intermediary of the thalamus and its connections with the cortex. However, the pathophysiological processes by which the abnormal striatal output acts upon the intervening basal ganglia nuclei are yet to be defined. In an attempt to address these issues we have developed primate models of a spectrum of movement disorders and then attempted to both elucidate the neural circuitry which is responsible for mediating them and to define the underlying characteristic changes in neural activity in each of the affected basal ganglia nuclei.

The Role of the Internal Segment of the Globus Pallidus in Mediating Dyskinesias

Changes in neuronal activity within the internal segment of the globus pallidus (GPi), which subsequently influence thalamo-cortical pathways, are believed to reflect movement-related information processed within the basal ganglia. Previous work from this laboratory would support the view that in the hypokinetic movement disorder of Parkinson’s disease, the GPi is overactive, being driven by a disinhibited subthalamic nucleus (STN) mediating excitatory transmission. In contrast it is well documented that lesion or damage within the STN in man gives rise to choreic or ballistic dyskinesias (hyperkinesias), which, if STN output is reduced, would imply the presence of an underactivity of GPi neurones in the production of these movements. Therefore the current view is that an overall increase or decrease in GPi neuronal activity explains the two extremes of the movement spectrum. In this chapter we examine the sufficiency of this view by discussing more specifically the role of the GPi in mediating dyskinesias (chorea, ballism and dystonia), by drawing upon results from our own and other related studies.

New Parallels Between Parkinson’s Disease and MPTP-Induced Parkinsonism in the Monkey

Despite the very significant advances that have been made in the treatment of Parkinson’s disease with the introduction of levodopa therapy the management of the condition still presents major problems to the clinician. While some patients undoubtedly have a benign, relatively non-progressive type of the disorder, others develop response fluctuations, the ‘on-off’ phenomenon, peak-dose dyskinesias related to levodopa medication or become unresponsive to treatment. It has been suggested that these complications may be related to prolonged levodopa medication (Rajput et al., 1984 and Melamed, 1986) and the concept of delayed treatment and wider use of dopamine agonists has been proposed. Insight into their aetiology may come fron the introduction of PET scanning as a non-invasive method of studying cerebral metabolism and neurotransmitter receptor binding but a non-human primate model of Parkinson’s disease still remains an attractive and important development. Such a model has ironically presented itself in humans with the discovery of a parkinsonian syndrome in drug addicts who have received l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP) produced during the synthesis of “designer drugs”. MPTP has been administered to monkeys and found to produce a very realistic parkinsonian syndrome but so far it has not been confirmed that the histological changes are similar in appearance and distribution to those found in Parkinson’s disease. Furthermore, it is not known whether the problems of levodopa administration are also encountered in the animal model.