Experimental parkinsonism in primates

Neural Stem Cells Implanted into MPTP-Treated Monkeys Increase the Size of Endogenous Tyrosine Hydroxylase-Positive Cells Found in the Striatum: A Return to Control Measures

Neural stem cells (NSC) have been shown to migrate towards damaged areas, produce trophic factors, and replace lost cells in ways that might be therapeutic for Parkinson's disease (PD). However, there is very little information on the effects of NSC on endogenous cell populations. In the current study, effects of implanted human NSC (hNSC) on endogenous tyrosine hydroxylase-positive cells (TH+ cells) after treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were explored in nonhuman primates. After MPTP damage and in PD, the primate brain is characterized by decreased numbers of dopamine neurons in the substantia nigra (SN) and an increase in neurons expressing TH in the caudate nucleus. To determine how implanted NSC might affect these cell populations, 11 St. Kitts African green monkeys were treated with the selective dopaminergic neurotoxin, MPTP. Human NSC were implanted into the left and right caudate nucleus and the right SN of eight of the MPTP-treated monkeys. At either 4 or 7 months after NSC implants, the brains were removed and the size and number of TH+ cells in the target areas were assessed. The results were compared to data obtained from normal untreated control monkeys and to the three unimplanted MPTP-treated monkeys. The majority of hNSC were found bilaterally along the nigrostriatal pathway and in the substantia nigra, while relatively few were found in the caudate. In the presence of NSC, the number and size of caudate TH+ cells returned to non-MPTP-treated control levels. MPTP-induced and hNSC-induced changes in the putamen were less apparent. We conclude that after MPTP treatment in the primate, hNSC prevent the MPTP-induced upregulation of TH+ cells in the caudate and putamen, indicating that hNSC may be beneficial to maintaining a normal striatal environment.

A potential compensatory role for endogenous striatal tyrosine hydroxylase-positive neurons in a nonhuman primate model of Parkinson's disease

The possibility of enhancing endogenous brain repair following neurological disorders, such as Parkinson's disease (PD), is of considerable recent interest. One such mechanism may exist in the striatum as an upregulated population of tyrosine hydroxylase (TH)-immunoreactive neurons that appear after 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine (MPTP) lesions in nonhuman primates as well as in humans with PD. An intriguing possibility is that these endogenous neurons reflect a compensatory mechanism to mitigate the loss of striatal DA due to progressive destruction of the nigrostriatal pathway. The possibility of enhancing the number and function of this population is attractive; however, it is crucial to gain further information about these cells in order to comprehend more fully their possible therapeutic potential. The current research was designed to investigate the fate of this endogenous population in African green monkeys rendered parkinsonian by MPTP lesions. Specifically, we assessed changes in the numbers of striatal neurons expressing TH at differing stages of the toxin-induced behavioral disability and discovered a close relationship with symptom severity and striatal DA neuron numbers. Increased numbers of striatal TH-positive neurons were associated with MPTP treatment that produced parkinsonian symptoms compared to numbers of these neurons in MPTP-treated asymptomatic animals and untreated controls. Expression of striatal DA neurons peaked at the manifestation of symptoms in mild/moderate animals and remained stable in animals that were severely parkinsonian. Furthermore, in severely debilitated animals that improved after fetal dopaminergic grafts, we discovered a return to control levels of the endogenous population. Taken together, our results further support the concept that this population of DA neurons responds to variations in striatal DA tone and may serve as a compensatory mechanism to restore striatal DA levels in the context of significant depletion. Artificially manipulating this endogenous population could prove beneficial for PD treatment, especially for individuals in early disease stages.

Changes in the neuronal activity in the pedunculopontine nucleus in chronic MPTP-treated primates: an in situ hybridization study of cytochrome oxidase subunit I, choline acetyl transferase and substance P mRNA expression

The pedunculopontine nucleus is a mesencephalic nucleus that has widespread and reciprocal connections with the basal ganglia. It has been implicated in the physiopathology of akinesia, rigidity, gait failure and sleep disorders associated with Parkinson's disease. In this study, in situ hybridization was used to examine the changes in neuronal metabolic activity (measuring cytochrome oxidase subunit I) and in the level of acetylcholine and Substance P synthesis in the pedunculopontine nucleus of monkeys chronically treated with MPTP. Significant reductions were observed in cytochrome oxidase subunit I (p = 0.001), choline acetyl transferase (p = 0.003) and substance P (p = 0.006) mRNA expression in parkinsonian animals compared with controls, indicating that pedunculopontine cholinergic neurons activity decreases with parkinsonism.

Lewy body disease: thalamic cholinergic activity related to dementia and parkinsonism

Within the spectrum of Lewy body disease cognitive impairment occurs in PD with dementia (PDD) and dementia with Lewy bodies (DLB). Although neocortical cholinergic deficits are associated with cognitive impairments in PDD and DLB, no neurochemical study has been published describing the thalamic cholinergic activity whereas the thalamus plays a major role in modulating cortical activity. Choline acetyltransferase (ChAT) activity was analyzed in reticular (Re), mediodorsal (MD) and centromedian (CM) thalamic nuclei in series of nine controls, five DLB with parkinsonism (DLB + P), five DLB without parkinsonism (DLB - P), six PD without dementia and 14 PDD cases. Significant reductions in ChAT were apparent in PDD as follows: in Re and MD nuclei compared with controls; in MD and CM nuclei compared with DLB + P; and in MD compared with PD. Increased ChAT activity was found in CM nuclei in DLB + P compared with DLB - P. These findings show that significant thalamic presynaptic cholinergic deficits occur only in cases of combined cortical and subcortical neurodegeneration in which dementia developed after prolonged parkinsonism.

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

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

The pedunculopontine tegmental nucleus and experimental parkinsonism

In this article, I review a series of my experiments on the role of the pedunculopontine tegmental nucleus (Ppn) in voluntary movements. I sought to elucidate the functional role of the Ppn in the control of voluntary movements using macaque monkeys. I propose a functional model of basal ganglia circuitry in which I emphasize the role of the Ppn and discuss the underlying mechanisms of parkinsonism.

Unilateral pallidotomy in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated common marmosets exhibiting levodopa-induced dyskinesia

Pallidotomy paradoxically reduces the intensity of levodopa-induced dyskinesia without worsening motor symptoms. The reasons for this are not clear and no experimental study has investigated this phenomenon. The objective of this investigation was to evaluate the effects of unilateral pallidotomy on locomotor activity, motor disability and levodopa-induced dyskinesia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated levodopa-primed common marmosets. Animals were primed to exhibit dyskinesia by daily administration of levodopa until stable dyskinesia was evoked by each dose. Locomotor activity, motor disability and dyskinesia were assessed weekly at baseline and following an acute levodopa challenge. Prior to pallidotomies, two distinct groups of animals emerged: poor responders to levodopa with mild dyskinesia (Group 1) and those exhibiting a marked increase in motor activity and pronounced dyskinesia (Group 2). Electrolytic lesions were placed in the left internal segment of the globus pallidus. Pallidotomy had no effect on basal or levodopa-induced motor activity in either group but significantly improved basal motor disability in Group 2. Following pallidotomy, the ability of levodopa to reduce motor disability was significantly increased in both groups. Pallidotomy improved dyskinesia in both Groups 1 and 2 but it was more effective in reducing dystonia compared with chorea. The effect of pallidotomy on dyskinesia in Group 2 was transient, with the intensity of involuntary movements reverting to presurgery levels 4 weeks later. This study shows that in levodopa-primed, parkinsonian marmosets, placement of discrete globus pallidus lesions can ameliorate levodopa-induced dyskinesia but not akinesia. This model allows the evaluation of pallidotomy-induced biochemical changes in dyskinetic primates.