J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.52.Suppl.22 on 1 June 1989. Downloaded from Journal ofNeurology, Neurosurgery, and Psychiatry. Special Supplement 1989:22-28 Clues to the mechanism underlying dopamine cell death in Parkinson's disease P JENNER From the Parkinson's Disease Society Research Centre, University Department ofNeurology, Institute of Psychiatry and King's College Hospital Medical School, London UK SUMMARY The primary pathological change in Parkinson's disease is the destruction of dopamine containing cells in the zona compacta of substantia nigra. The cause of nigral cell death and the underlying mechanism remains elusive. However, the discovery of the selective nigral neurotoxin MPTP and its ability to inhibit mitochondrial energy metabolism via its metabolite MPP+ and to generate superoxide radicals suggests processes by which nigral cell death might occur. Recent post- mortem evidence in brain tissue from patients dying with Parkinson's disease also suggests the occurrence of some on-going toxic mechanism. This may be a free radical process stimulated by an excess ofiron within subsantia nigra coupled to a generalised decrease in brain ferritin content. These data suggest altered iron handling occurs in Parkinson's disease which may lead to the generation of toxic oxygen species such as superoxide radicals. There is also evidence for an inhibition ofProtected by copyright. mitochondrial function in the substantia nigra in patients with Parkinson's disease. So there may be a close association between the actions of the synthetic neurotoxin MPTP and the underlying cause of idiopathic Parkinson's disease. There are widespread pathological and biochemical However, MPTP does not provide an exact model changes in the brain ofpatients dying with Parkinson's of idiopathic Parkinson's disease since in general disease.'2 The primary alteration appears to be a loss neurotoxic effects are limited to substantia nigra and of dopamine containing cells in zona compacta of its corresponding losses of caudate-putamen substantia nigra with a corresponding generalised loss dopamine content.78 Overall other neuronal systems of dopamine content throughout the forebrain. While do not appear to be involved and there is no the details of these changes are extensively documen- occurrence of Lewy bodies as a marker of the process ted3 there has been little evidence as to the underlying underlying idiopathic Parkinson's disease.'"'2 One cause of dopamine cell death or the mechanism by reason for the discrepancies between MPTP induced which dopamine cells degenerate. Many theories have Parkinsonism and the idiopathic disease may relate to been advanced (for example, involvement of viruses, the age ofthe animals studied. In general, young adult aberrant metabolism of dopamine, involvement of primates are used in such experiments but where older neuromelanin) but none of these proven. More recen- animals have been employed the pathology has been http://jnnp.bmj.com/ tly interest has centred on the manner in which more extensive, involving also the locus coeruleus and endogenous or environmental toxins may be involved Lewy body like inclusions have been observed.'"'5 as a cause of Parkinson's disease. This stems from the However, MPTP has provided the most appropriate discovery of the selective nigral toxin, I-methyl-4- animal model of Parkinson's disease so far devised. It phenyl-1,2,3,6-tetrahydropyrdine (MPTP) which was has found use as a test-bed in which to evaluate novel found to induce persistent Parkinsonism in man and therapies for Parkinson's disease'"'9 and also to other primate species by destroying dopamine con- evaluate long-term complications of levodopa treat- taining cells within substantia nigra.' ment of Parkinson's disease.20'2 Similarly, MPTP on October 1, 2021 by guest. The discovery of MPTP provided a major impetus treated primates provide an ideal test-bed in which to for research into the cause of Parkinson's disease. evaluate the usefulness of the implantation of foetal nigral dopamine containing cells as a "cure" for Parkinson's disease.2223 However, it may be one other Address for reprint requests: Dr P Jenner, Department ofNeurology, aspect of the actions of MPTP which is the most Institute of Psychiatry, De Crespigny Park, London SE5 8AF, UK. important, namely the mechanism by which MPTP Accepted March 1989 kills nigral dopamine containing cells. This may 22 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.52.Suppl.22 on 1 June 1989. Downloaded from Clues to the mechanism underlying dopamine cell death in Parkinson's disease 23 provide clues to the selective vulnerability of this cell free radicals. This was derived from the chemical group and to the pathological process underlying similarity between MPP+ and a paraquat, a known dopamine cell death in Parkinson's disease. redox cycler. Although MPP+ bears a striking chemical resemblance to paraquat, MPP+ is extremely Mechanism ofaction ofMPTP stable compared with paraquat and does not undergo Many important steps in the mechanism of action of single electron reduction to produce oxygen MPTP have been uncovered. Following the adminis- radicals.474 This is emphasised by the electrochemical tration ofMPTP to primates it was found that another potential of MPP+ of < 1-0 V which compared with substance, namely, l-methyl-4-phenyl-pyridinium paraquat makes it improbable that it will undergo species (MPP+), and not MPTP itself, accumulated bioreduction to stimulate toxic oxygen radical for- and persisted within the brain.2425 The reason for this mation.49 However, this conclusion presumes that it is became clear when it was discovered that MPTP was MPP+ alone which causes the neurotoxicity associated an unexpected substrate for MAO B2"2' So it appeared with MPTP administration. that MPP+ rather than MPTP might be the neurotoxic MPTP is converted to MPP+ in a two-stage reaction species. Indeed, MPP+ was actively accumulated by via the intermediate dihydropyridine derivative, dopaminergic neurons since it is a substrate for MPDP+."52 This sequence ofevents gives more scope dopamine reuptake mechanisms29 and this accumula- for the production of oxygen species. Indeed, there is tion may be enhanced by an association with recent evidence that oxygen radicals may be involved neuromelanin.? MPTP neurotoxicity could be preven- in the toxicity of MPTP in a manner previously ted by prior treatment of animals with MAO B unsuspected. Thus, in aerobic mitochondrial prepara- inhibitors, such as deprenyl, or dopamine reuptake tions MPTP stimulated an electron spin resonance blockers, such as nomifensine.3"32 Also, MPP+ was (ESR) signal compatible with free radical formation.53 more toxic than MPTP itself in destroying dopamine This signal was prevented by inclusion of superoxide containing cells. Thus, intranigral infusions of MPP+, dismutase suggesting the generation of superoxide Protected by copyright. but not MPTP, destroyed nigral neurones in rats.33 radicals. The metabolism of MPTP appeared involved Similarly, intracerebroventricular injections of MPP+ since the ESR signal was prevented by the inclusion of destroyed nigral cells in the brains of mice.? In cell the MAO B inhibitor deprenyl but not by the MAO A culture MPP+ is toxic to mesencephalic cells.35 36 So all inhibitor clorgyline. In the absence of mitochondria this data led to the conclusion that it was MPP+ and neither of the MPTP metabolites, MPP+ or MPDP+, not MPTP that was responsible for the toxic actions alone produced an ESR signal. However, together observed. they caused a spectrum which increased in intensity These data did not, however, explain the mechanism with time. These data suggest a redox reaction occurs by which MPTP (or MPP+) killed dopamine contain- between MPP+ and MPDP+ to produce toxic oxygen ing cells. This issue was clarified with the discovery radicals and that these may be involved in the ability of that MPP+ induces oxidative stress owing to its ability MPTP to destroy nigral containing cells. Whether an to inhibit the oxidation ofmitochondrial NAD-linked action of MPTP based on superoxide formation is substrates.373' More specifically MPP+ inhibits mito- compatible with inhibition of mitochondrial function chondrial energy metabolism at the level of complex remains to be determined. I3. However, millimolar concentrations ofMPP+ were So the MPTP story reveals possible mechanisms by required in vitro to produce this effect and it was which nigral dopamine containing cells may be des- thought unlikely that these would be achieved in vivo. troyed. But what is its relevance to Parkinson's http://jnnp.bmj.com/ But MPP+ is actively accumulated by mitochondria disease? It may be that there is an MPTP like toxin such that 50-100 times the external concentration can involved as a cause of Parkinson's disease although at be achieved.A"2 So at present it is believed that MPP+ present this appears unlikely. On the other hand, it interferes with mitochondrial energy metabolism may be that the mechanism by which MPTP kills somewhere between NADH dehydrogenase and co- dopamine cells by interference with mitochondrial enzyme Q 10.43 Exactly how and where this occurs function or by superoxide formation may reflect a remains unknown. Nevertheless, its consequences can selective vulnerability of dopamine cells in substantia be observed in terms of depletion of cellular ATP, a nigra which is also apparent in idiopathic Parkinson's on October 1, 2021 by guest. decrease in reduced glutathione content, and altera- disease. So, are there any similarities between the tions in cellular calcium content."44" actions of MPTP and biochemical changes occurring in brain in Parkinson's disease itself? Free radical generation as a mechanism of MPTP toxicity Evidencefor a neurotoxic process occurring in brain in One basic mechanism initially proposed to explain the Parkinson's disease neurotoxic actions of MPP+ was the production of One approach to detecting a cause or mechanism J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.52.Suppl.22 on 1 June 1989.