M.A. Sambrook

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.

Sites of the neurotoxic action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the macaque monkey include the ventral tegmental area and the locus coeruleus

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces a profound parkinsonian state when systemically administered in monkeys and man. Previous studies have shown MPTP to be toxic to only the dopamine (DA) cells in the substantia nigra pars compacta and not to other catecholamine (CA)-containing cells. The data presented here suggest that MPTP also has a specific neurotoxic effect on the DA-containing cells of the ventral tegmental area and the noradrenaline-containing cells of the locus coeruleus in macaque monkeys with a moderate-to-severe parkinsonian syndrome. The results suggest that MPTP-induced parkinsonism in the monkey more closely replicates the neurochemical changes seen in idiopathic Parkinsons disease than previously thought.

Regional brain uptake of 2-deoxyglucose in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)—induced parkinsonism in the macaque monkey

This preliminary report describes application of the 2-deoxyglucose (2DG) autoradiographic technique to study regional changes in brain metabolism in experimental parkinsonism, induced in the monkey by administration of the neurotoxin MPTP. In one monkey, rendered severely parkinsonian by MPTP, there was a marked increase in the uptake of 2DG in the globus pallidus (both medial and lateral segments) and in the ventral anterior and ventral lateral thalamic nuclei, in comparison to non-parkinsonian animals. Increased uptake of 2DG in the globus pallidus may reflect increased activity of striatopallidal synapses secondary to loss of nigrostriatal dopaminergic neurones. The findings are in sharp contrast to our observations on regional brain metabolism in experimental choreiform dyskinesia in the monkey.

Subthalamic nucleotomy alleviates parkinsonism in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-exposed primate.

Research into the neural mechanisms underlying the symptoms of parkinsonism utilizing the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-exposed primate model have shown that the subthalamic nucleus (STN) occupies a central role. As a logical development of this theory, we have studied the effects of thermocoagulative lesions of the STN in the primate model. Such lesions can cause remarkable symptom reversal in the experimental primate model.

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.

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.

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.

Autoradiographic studies in animal models of hemi-parkinsonism reveal dopamine D2 but not D1 receptor supersensitivity. II. Unilateral intra-carotid infusion of MPTP in the monkey (Macaca fascicularis)

The selective dopaminergic antagonist ligands [3H]SCH 23390 and [3H]sulpiride were used to reveal autoradiographically dopamine D1 and D2 receptors, respectively, in brain sections from monkeys which had received unilateral intracarotid infusions of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), causing loss of dopamine-containing neurones of the substantia nigra pars compacta. The monkeys developed hemi-parkinsonian symptoms (tremor, bradykinesia) in limbs contralateral to the side of the toxin infusion. Administration of apomorphine (0.05-0.25 mg/kg) caused contralateral rotational behaviour, and reversal of the parkinsonian symptoms. Loss of forebrain dopaminergic terminals was assessed autoradiographically using [3H]mazindol to label dopamine uptake sites. A reduction in these sites of 97% (mean brain value) in the caudate nucleus, and 91% in the putamen, as compared with binding values from untreated control monkeys, was accompanied by a significant increase in the binding of [3H]sulpiride (D2) in these structures. In contrast, in the same animals there was no similar increase in [3H]SCH 23390 binding to D1 receptors in the denervated areas. These results suggest that in the parkinsonian brain, where the dopaminergic innervation of the caudate nucleus and putamen has been lost, D2 receptors may be more susceptible than D1 receptors to changes, revealed here as an increase in [3H]sulpiride binding sites.

Effect of the NMDA antagonist MK-801 on MPTP-induced parkinsonism in the monkey

Current evidence suggests that the motor symptoms of parkinsonism are due to abnormal overactivity of the medial segment of the globus pallidus, brought about by overactivity of the subthalamic nucleus, from which it receives an excitatory amino acid-mediated projection. The possibility exits, therefore, that excitatory amino acid antagonists might have an anti-parkinson effect by normalising medial pallidal activity. The NMDA antagonist MK-801 was administered i.m. to a single cynomolgus monkey with parkinsonism induced by the neurotoxin MPTP. In fact, MK-801 exacerbated the symptoms of parkinsonism. When administered after a therapeutic dose of L-DOPA it antagonised the anti-parkinson action of L-DOPA. The results suggest that any potential anti-parkinson action of excitatory amino acid antagonists will depend upon an action at non-NMDA sites. The administration of the selective neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to produce a primate model of Parkinsons disease is well-documented (Burns, Markey, Phillips & Chiuch, 1984; Crossman, 1987; Langston, Forno, Rebert & Irwin, 1984). Intravenous injection of MPTP, titrated judiciously over a period of several weeks, can produce a stable manifestation of the motor disability seen in the idiopathic disease of man, with a remarkable correspondence of both symptoms and pathology. Additionally, primates rendered parkinsonian by MPTP respond well to L-DOPA treatment. As in human Parkinsons disease, long-term L-DOPA therapy of MPTP-induced parkinsonism tends to be complicated by the emergence of choreiform movements and dystonic postures (Boyce, Clarke, Luquin, Peggs, Robertson, Mitchell, Sambrook & Crossman, 1989; Clarke, Sambrook, Mitchell & Crossman, 1987).(ABSTRACT TRUNCATED AT 250 WORDS)

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.