MPTP toxicity was discovered after inadvertent self-administration by drug abusers

These people had ingested a compound produced during illicit synthesis of a narcotic related to meperidine. The users rapidly developed a movement disorder that closely resembles Parkinson's disease, including low concentrations of HVA in the CSF. Similarly, MPTP induces most of the biochemical, pathological and clinical features akin to Parkinson's disease in nonhuman primates.

The mechanisms implicated in MPTP toxicity [19] are depicted in Figure 45-6. MPTP, which is lipid-soluble, readily penetrates the blood—brain barrier and enters the brain cells. Because it is amphiphilic, it is captured into acidic organelles, mostly lysosomes, of astrocytes. MPTP itself does not appear to be toxic, but its oxidized product, 1-methyl-4-phenylpyridinium (MPP⁺), is toxic. Astrocytes and serotonergic neurons contain MAO-B, which converts MPTP to MPP⁺. The toxic oxidation product reaches the extracellular fluid and then is transported by the DA transporter into DA nerve terminals. Inhibition of either MAO-B or the DA transporter protects against MPTP-generated MPP⁺ toxicity.

Figure 45-6

Schematic representation of the mechanisms involved in toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). BBB, blood—brain barrier; MPDP⁺, 1-methyl-4-phenyl-2,3-dihydropyridinium; MPTP, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; (more...)

Although the precise mechanism(s) underlying the mode of MPP⁺ toxicity is unknown, it has been suggested that the toxicity of MPP⁺ is dependent on a mitochondrial concentrating mechanism via selective uptake. Energy-driven mitochondrial uptake of MPP⁺ results in sufficiently high concentrations of the toxin to interfere with mitochondrial respiration. The site at which MPP⁺ acts, complex I, appears to be at or near the region where several other agents, such as rotenone, act to block mitochondrial oxidation. Blockade of mitochondrial respiration has two cytotoxic consequences. First, it impairs ATP formation, resulting in the inhibition of energy-dependent processes such as ion transport. A recent study suggests that inhibition of complex I is not solely involved in eliciting cell death. Indeed, disruption of calcium ion homeostasis plays a vital role in MPP⁺ toxicity. This results in an elevation of intracellular Ca²⁺, leading to the activation of Ca²⁺-dependent enzymes, for example, protein kinase and calpains I and II, which disturbs the normal cell function, resulting in cellular damage (see Chaps. 23 and 34). Second, MPP⁺ appears to support the occurrence of oxidative stress. This notion is demonstrated by the generation of reactive oxygen radicals and free iron. MPP⁺ and mitochondrial NADH dehydrogenase have been suggested to yield toxic hydroxyl radicals derived from hydrogen peroxide. In monkeys, MPP⁺ has been shown to release toxic iron (II) (Fe²⁺), which in turn may react with hydrogen peroxide via the Fenton reaction and, thus, yield hydroxyl radicals (^•OH) (see Chap. 34). Hydroxyl and other free radical species and Fe²⁺ have been strongly implicated in the pathogenesis of Parkinson's disease. More importantly, MPTP mimics another fundamental nigral biochemical change in Parkinson's disease, that is, a reduction of glutathione content, as observed in rodents.

However, there are some fundamental changes found in Parkinson's disease that are not induced by MPTP. Consequently, this has raised some doubts about the involvement of some MPTP-like toxins in Parkinson's disease. Indeed, in nonhuman primates and rodents, there has been no evidence for the occurrence of the pathological hallmark of Parkinson's disease, namely, Lewy bodies. In some older, MPTP-treated primates, eosinophilic inclusions were observed in the substantia nigra and locus ceruleus; however, the identity of these features remains largely unresolved. Resting tremor, which is one of the most prominent clinical features of Parkinson's disease, is seldom observed in MPTP-induced parkinsonism. The neurodegeneration of the nigral neurons and the manifestation of Parkinson's disease clinical symptoms are progressive. In contrast, the development of Parkinson's disease-like symptoms subsequent to MPTP exposure is acute. In Parkinson's disease, the cholinergic cells of the substantia innominata exhibit evidence for neuronal loss and Lewy body pathology, whereas MPTP does not elicit these changes. Some of these diversities may be based on species differences; however, they do seem to suggest a divergence in the mechanism(s) evoking cell death in Parkinson's disease and MPTP.

Nevertheless, MPTP remains the “best” model for investigating Parkinson's disease to date, although many other dopaminergic neurotoxins have evolved, which are being actively employed. In addition, the MPTP scenario had a great impact on the quest to unravel the putative pathogenesis underlying the disease. It furnished a possible mechanism(s) by which the dopaminergic neurons in Parkinson's disease may degenerate. Additionally, it triggered the search for some endogenous or exogenous neurotoxin which may be involved in eliciting the nigral cell death characteristic of the disease. Some of these neurotoxins include 6-hydroxydopamine (6-OH-DA), iron and methamphetamine.

MPTP provides clues to the pathogenesis of Parkinson's disease

The selective vulnerability of nigrostriatal DA neurons to MPTP toxicity and the resemblance of the resulting clinical syndrome to Parkinson's disease refocused attention on determining the etiological factors that contribute to the development of Parkinson's disease. Three separate, but not necessarily exclusive, hypotheses have been explored [20].

The first hypothesis suggests that there are one or more toxic substances acquired from the environment or produced in the brain, at least for some vulnerable persons. A genetic component may ultimately determine the predisposition of those individuals to the particular toxin, although the familial coincidence of the disease is low.

The second hypothesis suggests that oxidative stress may play a pivotal role in dopaminergic cell death (Table 45-5). However, it remains debatable as to whether oxidative stress represents a cause or a consequence of the disease. Oxidative stress is a condition in which reactive oxygen-derived free radical species comprise the chief factor leading to cell degeneration (see Chap. 34). The catabolism of DA itself, via both enzymatic deamination and auto-oxidation, is reputed to generate toxic superoxide and hydroxyl radicals, which may in turn trigger a self-amplifying cell-destruction cycle. The postmortem evidence for the occurrence of oxidative stress is vast and highly supportive of its involvement in the pathogenesis of Parkinson's disease (Table 45-5). The first documentation of oxidative stress was provided by the nigral depletion of the antioxidant glutathione in Parkinson's disease. Other antioxidants, such as catalase, were also found to be depleted in the substantia nigra in Parkinson's disease. This reflects a general reduced state of cellular defenses in the disease, perhaps due to “consumption” of antioxidant molecules by active, free radical-generating processes. The mitochondrial dysfunction in Parkinson's disease, as reflected by the decrease of respiratory complex I activity, may generate superoxide radicals, which may elicit cell destruction and exacerbate the complex I defect. Also, the shift in the Fe²⁺: Fe³⁺ ratio from 2:1 in the normal substantia nigra to 1:2 in that of Parkinson's disease may generate hydroxyl radicals from hydrogen peroxide through the iron-dependent Fenton reaction. Thus, the elevation of mitochondrial Mn²⁺-dependent superoxide dismutase activity found in Parkinson's disease perhaps represents a compensatory mechanism to curtail the effects of excess superoxide radical formation. These free radical species may ultimately induce cell death via disruption of normal cellular Ca²⁺ homeostasis (Chap. 34).

Table 45-5

Evidence From Postmortem Findings For the Occurrence of Oxidative Stress in Parkinson's Disease.

Intracerebral administration of the neurotoxin 6-OHDA provides an acute animal model for Parkinson's disease. This is a particularly important model because this toxin also induces oxidative stress and, thus, allows the possibility of investigating biochemical parameters affected by this cytotoxic process. Moreover, the biochemical alterations that 6-OHDA induces are very similar to those reported in Parkinson's disease and, thus, support the occurrence of oxidative stress in the latter. The neurotoxicity of 6-OHDA is believed to be related to production of hydrogen peroxide-derived hydroxyl radicals, which probably induce destruction of nigral neurons. In addition, 6-OHDA initiates the release of iron from ferritin, which may account for its ability to generate hydroxyl radicals via the Fenton reaction. It is supposed that 6-OHDA is more effective than MPP⁺ at inhibiting mitochondrial complex I activity and may also generate other toxic free radicals, such as superoxide radicals. Consequently, the reductions in the activity of superoxide dismutase and in glutathione content in the striatum may represent compensatory actions against a 6-OHDA-elicited toxic mechanism(s).

The third hypothesis suggests a putative association between oxidative stress and other free radical-generating processes, such as excitatory and immune pathways (see Chap. 34). Immune-related mechanisms may also be conducive to selective regional cell death in Parkinson's disease. Indeed, microgliosis is most marked in the ventral tier of the SNc of Parkinson's disease, an area blighted by the degenerative processes of the disease. It has been shown that reactive microglia can mediate secondary cell destruction by releasing cytotoxic species, such as hydroxyl radicals, superoxide radicals, NO and glutamate. Furthermore, the fact of elevated concentrations of interleukin-6 in the CSF of de novo parkinsonian patients confirms the occurrence of immunologically mediated processes in the disorder.

Although the cause(s) of Parkinson's disease has not yet been elucidated, reactive oxygen species appear to play a salient role in the degenerative process. Therefore, neuroprotection represents one of the strategies evolved to combat some of these active degenerative processes.

Neuroprotective strategies have some benefits in Parkinson's disease

Neuroprotective drugs that have demonstrated some benefit in experimental animal models of Parkinson's disease and have fulfilled the safety requirements were subsequently assessed in parkinsonian patients. Deprenyl and α-tocopherol were the first two compounds to be clinically evaluated as potential candidates for neuroprotective treatment in Parkinson's disease [21]. Deprenyl effects a triad of cellular protective mechanism(s). These include neuroprotection, neurorescue and neurorestoration. Its neuroprotective effects are exerted by inhibiting the degradation of DA or other MPTP-like neurotoxins and, thus, the production of potential cytotoxic metabolites, including hydrogen peroxide, via MAO-mediated deamination, and DA quinones, via auto-oxidation. In vivo experiments for oxidative stress have clearly shown protective actions of the drug in nigrostriatal neurons of animals treated with haloperidol or reserpine. In addition, deprenyl affords neuroprotection against other dopaminergic toxins, including 6-OHDA and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine. Its neuroprotective ability may also be related to the attenuation of the neurotoxic actions of the endogenous toxins β-carbolines and tetrahydroisoquinolines.

The neurorescuing actions of deprenyl are clearly demonstrated by the protection it affords to dopaminergic neurons after the administration of MPTP at a time when most of the latter is metabolized to toxic MPP⁺. It has been suggested that the mode of neurorescue in this case may involve an interaction of deprenyl or its metabolites with some cellular antiapoptotic factors. Deprenyl also has been reported to promote the production of neurotrophic factors; this may explain its role as a neurorestorative agent. This particular feature may be of importance in the preservation of the nigral neurons in a progressive and degenerative disorder such as Parkinson's disease. Although not proven, these attributes may explain its described ability to increase life expectancy in parkinsonian patients [22].

There has been some implication that the MAO-A inhibitor moclobemide (p-chloro-N-[2morpholinoethyl]benzamide) may also provide cellular protection by inhibition of MAO-A-derived hydrogen peroxide generation. α-Tocopherol treatment may exert some neuroprotective effect, probably by scavenging free radicals and, thus, inhibiting cytotoxic processes such as lipid peroxidation. However, no clinical benefit was demonstrated by oral ingestion of 2 g per day of α-tocopherol in the DATATOP clinical study [23]. The absence of protective clinical effect from ingestion may be due to insufficient brain concentrations of α-tocopherol. Little is known about vitamin transport into the brain (see Chap. 33).

Findings from in vitro and in vivo studies suggest that some DA agonists may afford neuroprotection by scavenging reactive free radical species, although this attribute has not yet been clinically proven. Pergolide is currently being evaluated for potential neuroprotective activity in Parkinson's disease (Fig. 45-5). Other DA agonists also believed to afford some neuronal protection by virtue of their ability to scavenge free radicals include ropinirole, bromocriptine and pramipexole. Positron emission tomography may be used to assess the neuroprotective actions of ropinirole, which is advocated in early Parkinson's disease (see Chap. 54).

Amantadine possibly provides some protective activity in Parkinson's disease. These cellular protective effects are believed to be based on the ability of amantadine to block the excitatory amino acid NMDA receptor. If present in intact human brain, such an action may confer an ability to increase life expectancy in the parkinsonian patient.

Multiple synergistic pathways have been suggested to play crucial roles in the cell death cascade. Therefore, it would be an effective therapeutic strategy to employ a combination of compounds to affect multiple “target sites” in the cell-destruction pathway(s). Drugs such as budipine, an NMDA receptor antagonist; the dihydropyridine type of Ca²⁺ channel antagonists; N-nitro-l-arginine, an NO inhibitor; and iron chelators represent candidates for Parkinson's disease treatment. Perhaps the most effective mode of cell-protective therapy would be a combination of these compounds as opposed to monotherapy. However, for these drugs to implement their neuroprotective actions effectively, they would have to be administered in the early stages of the disease. This therapeutic requirement highlights the need to elucidate a marker for asymptomatic, or early, Parkinson's disease. Furthermore, this contention is emphasized by the fact that it is difficult to demonstrate the neuroprotective benefit of these drugs in clinical trials involving advanced disease since they do not produce instant clinical improvement or restoration of neurons.

Both experimental and clinical studies suggest that neurotrophic factors may play significant roles in the survival, growth and differentiation of dopaminergic neurons. These observations provide support for potential roles of these factors, to some degree, in neuroprotection and regeneration of the denervated neuronal network in Parkinson's disease. It appears that the trophic factors exert their beneficial effect by supporting differentiation of the neuronal phenotype rather than survival, but a reduction in their availability or in the receptors for neurotrophic factors may exert serious consequences on both cellular function and survival. However, a deficit in neurotrophic factors is not known to cause Parkinson's disease or any other neurological disease. Neurotrophic factors, such as nerve growth factor, brain-derived neurotrophic factors or glial-derived neurotrophic factors, do not effectively cross the blood—brain barrier and, therefore, need to be administered either directly into the ventricles or into the striatum. This may prove to be cumbersome and a hindrance to the use of these factors in the long-term management of the disease.