The parkinsonian toxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP): a technical review of its utility and safety (2001)

Parkinson's disease (PD) is a common disabling neurodegenerative disorder the cardinal clinical features of which include tremor, rigidity and slowness of movement (Fahn and Przedborski 2000). These symptoms are attributed mainly to a profound reduction of dopamine in the striatum due to a dramatic loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) (Fahn and Przedborski 2000). Thus far, both the cause and the mechanisms of PD remain unknown. Over the years, investigators have used experimental models of PD produced by several compounds such as reserpine, 6-hydroxydopamine, methamphetamine, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to provide insights into the mechanisms responsible for the demise of dopaminergic neurons in PD. To this end, MPTP has emerged unquestionably as a popular tool for inducing a model of PD in a variety of animal species including monkeys, rodents, cats, and pigs (Kopin and Markey 1988). The sensitivity to MPTP and therefore its ability to induce parkinsonism closely follows the phylogenetic tree where the species most closely related to humans are the most vulnerable to this neurotoxin. Due to the significant neurotoxicity of MPTP, it is important that researchers appreciate the potential hazards of this toxin. Given this, the purpose of this review is to inform the researcher of the hazardous nature of MPTP and to provide guidance for its safe handling and use. MPTP is a by-product of the chemical synthesis of a meperidine analog with potent heroin-like effects. MPTP can induce a parkinsonian syndrome in humans almost indistinguishable from PD (Langston and Irwin 1986). Recognition of MPTP as a neurotoxin occured early in 1982, when several young drug addicts mysteriously developed a profound parkinsonian syndrome after the intravenous use of street preparations of meperidine analogs which, unknown to anyone, were contaminated with MPTP (Langston et al. 1983). In humans and non-human primates, depending on the regimen used, MPTP can produce an irreversible and severe parkinsonian syndrome that replicates almost all of the features of PD, including tremor, rigidity, slowness of movement, postural instability, and even freezing; in non-human primates, a resting tremor characteristic of PD has only been demonstrated convincingly in the African green monkey (Tetrud et al. 1986). The responses, as well as the complications, to traditional antiparkinsonian therapies are virtually identical to those seen in PD. It is believed that in PD the neurodegenerative process occurs over several years, while the most active phase of neurodegeneration is completed within a few days following MPTP administration (Langston 1987; Jackson-Lewis et al. 1995). However, recent data suggest that, following the main phase of neuronal death, MPTP-induced neurodegeneration may continue to progress ‘silently’ over several decades, at least in humans intoxicated with MPTP (Vingerhoets et al. 1994; Langston et al. 1999). Except for four cases (Davis et al. 1979; Langston et al. 1999), no human pathological material has been available for studies and thus, the comparison between PD and the MPTP model is largely limited to primates (Forno et al. 1993). Neuropathological data show that MPTP administration causes damage to the nigrostriatal dopaminergic pathway identical to that seen in PD (Agid et al. 1987), yet there is a resemblance that goes beyond the loss of SNpc dopaminergic neurons. Like PD, MPTP causes greater loss of dopaminergic neurons in SNpc than in ventral tegmental area (Seniuk et al. 1990; Muthane et al. 1994) and, at least in monkeys treated with low doses of MPTP but not in humans, greater degeneration of dopaminergic nerve terminals in the putamen than in the caudate nucleus (Moratalla et al. 1992; Snow et al. 2000). However, two typical neuropathologic features of PD have, until now, been lacking in the MPTP model. First, except for SNpc, pigmented nuclei such as locus coeruleus have been spared, according to most published reports. Second, the eosinophilic intraneuronal inclusions, called Lewy bodies, so characteristic of PD, thus far, have not been convincingly observed in MPTP-induced parkinsonism (Forno et al. 1993), although, in MPTP-injected monkeys, intraneuronal inclusions reminiscent of Lewy bodies have been described (Forno et al. 1986). To date, the most frequently used animals for MPTP studies are monkeys, mice and rats. The administration of MPTP through a number of different routes using different dosing regimens has led to the development of several distinct models, each characterized by some unique behavioral and/or biochemical features. The manner in which these models were developed is based on the concept of delivering MPTP in a fashion that creates the most severe and stable form of SNpc damage with the least number of undesirable consequences such as acute death, dehydration and malnutrition. Although MPTP can be given by a number of different routes, including gavage and stereotaxic injection into the brain, the most common, reliable, and reproducible lesion is provided by its systemic administration (i.e. subcutaneous, intravenous, intraperitoneal or intramuscular). The most commonly used regimens in monkeys are the multiple intraperitoneal or intramuscular injections and the intracarotid infusion of MPTP (Petzinger and Langston 1998). The former is easy to perform and produces a bilateral parkinsonian syndrome. However, often the monkey exhibits a generalized parkinsonian syndrome so severe that chronic administration of levodopa is required to enable the animal to eat and drink adequately (Petzinger and Langston 1998). On the other hand, the unilateral intracarotid infusion is technically more difficult, but causes symptoms mainly on one side (Bankiewicz et al. 1986; Przedborski et al. 1991), which enables the monkey to maintain normal nutrition and hydration without the use levodopa. For many years monkeys were mainly, if not exclusively, treated with harsh regimens of MPTP to produce an acute and severe dopaminergic neurodegeneration (Petzinger and Langston 1998). More recently, several investigators have treated monkeys with low doses of MPTP (e.g. 0.05 mg/kg 2–3-times per week) for a prolonged period of time (i.e. weeks to months) in an attempt to better model the slow neurodegenerative process of PD (Schneider and Roeltgen 1993; Bezard et al. 1997; Schneider et al. 1999). While both the acute and the chronic MPTP-monkey models are appropriate for the testing of experimental therapies aimed at alleviating PD symptoms, it is the chronic model that is, presumably, the most suitable for the testing of neuroprotective strategies. In addition to monkeys, many other mammalian species are also susceptible to MPTP (Kopin and Markey 1988; Heikkila et al. 1989; Przedborski et al. 2000). Mice have become the most commonly used species for both technical and financial reasons. However, several problems need to be emphasized. First, mice are much less sensitive to MPTP than monkeys; thus, much higher doses are required to produce significant SNpc damage in this animal species, presenting a far greater hazardous situation. Second, in contrast to the situation in monkeys, mice treated with MPTP do not develop parkinsonism. Third, the magnitude of nigrostriatal damage depends on the dose and dosing schedule (Sonsalla and Heikkila 1986). The use of MPTP in rats presents an interesting situation (Kopin and Markey 1988). For instance, rats injected with mg/kg doses of MPTP comparable to those used in mice do not exhibit any significant dopaminergic neurodegeneration (Giovanni et al. 1994a; Giovanni et al. 1994b). Conversely, rats injected with much higher doses of MPTP do exhibit significant dopaminergic neurodegeneration (Giovanni et al. 1994a; Giovanni et al. 1994b) although, at these high doses, rats have to be pretreated with guanethidine to prevent dramatic peripheral catecholamine release and extensive mortality (Giovanni et al. 1994a). These findings indicate that rats are relatively insensitive to MPTP, but regardless of this drawback, rats continue to be used often in MPTP studies (Storey et al. 1992; Giovanni et al. 1994a; Giovanni et al. 1994b; Staal and Sonsalla 2000; Staal et al. 2000). In rats, the systemic administration of MPTP is rarely used and the vast majority of studies involve the stereotaxic infusion of MPTP's toxic metabolite, 1-methyl-4-phenylpyridinium (MPP+) (Storey et al. 1992; Giovanni et al. 1994a; Giovanni et al. 1994b; Staal and Sonsalla 2000; Staal et al. 2000). Several factors influence the reproducibility of the lesion in monkeys, rats, and mice. However, to our knowledge, the extensive and systematic assessment of these factors has only been done in mice, and can be found in the following references (Heikkila et al. 1989; Giovanni et al. 1991; Giovanni et al. 1994a; Giovanni et al. 1994b; Miller et al. 1998; Hamre et al. 1999; Staal and Sonsalla 2000), the highlights of which can be summarized as follows: different strains of mice (and even within a given strain obtained from different vendors) can exhibit strikingly distinct sensitivity to MPTP. This differential sensitivity acts in an autosomal dominant fashion (Hamre et al. 1999). Gender, age, and body weight are also factors that modulate MPTP sensitivity as well as reproducibility of the lesion, in that female mice are less sensitive and exhibit more variability in the extent of damage than males, as do mice younger than 8 weeks and lighter than 25 g. From our experience, optimal reproducibility in MPTP neurotoxicity is obtained using male C57 BL/6 mice 8–10 weeks of age and 25–30 g in weight. Also of importance is that, following MPTP administration, some mice will die within the first 48 h postinjection; note that C57 BL/6 mice from different vendors exhibit dramatically different magnitudes of acute lethality, ranging from 5% to 90%. This common issue is unlikely related to a toxic effect in the central nervous system but rather toxicity to the peripheral nervous and cardiovascular systems. Although, to our knowledge, this possibility has never been formally studied, we believe that, following acute MPTP administration, mice develop fatal alterations in and MPTP causes a in body which not only can modulate the extent of dopaminergic damage et al. but can also to acute can be by the body of the injected mice using a not use a which can mice by as there is no of the MPTP has a and 1993; Przedborski et al. 2000). It is and and the a after MPTP of the are in the et al. in the brain, MPTP is to by the in is to the active MPTP metabolite, which is into the where it is by the dopamine and is within dopaminergic where it its toxic effects. The of these different in MPTP-induced neurotoxicity and the that is the are demonstrated by the following with such as MPTP to and dopaminergic toxicity (Heikkila et al. Markey et al. with dopamine (e.g. into dopaminergic neurons and also dopaminergic toxicity et al. at least in and with dopaminergic toxicity in mice (Giovanni et al. where MPTP and its toxic metabolite, both and of the body of the injected animal following MPTP administration is to the of any of for the safe use of MPTP. MPTP administration to both mice and monkeys, only the of the the that the animals and/or including and are contaminated with MPTP and its et al. 1988). Conversely, no of is found of the or on the et al. 1988). two days of the injected dose of MPTP and its is from the and of which in mice and in monkeys is MPTP, while the is due to MPTP such as it that the of MPTP occurs mainly the first while mainly MPTP are to days et al. 1988). is no in mice or in monkeys that MPTP and its are after days MPTP Although high of MPTP are found in the the main of MPTP is the et al. 1986). MPTP in will be and not and be well by the animal less than of the injected dose of MPTP is as MPTP, which from the animals MPTP or from from contaminated et al. 1988). after an injection of MPTP to mice, most of the is in the and the while all other of et al. 1986). of the species from different and body such as and of MPTP after but by h all of the to et al. et al. 1986). From the it that the potential of to MPTP are through with the the animal and its is from due to or of MPTP. Although as be the period of of MPTP is from the of injection to the time that MPTP or its are no found in the of treated as a we the period of from days to days to MPTP injection and animal it is to the of the and a only investigators and/or are in handling hazardous and are with MPTP and and MPTP, and the animals the period (i.e. days any these and not be into on it is that all of the MPTP including and in a or area within the animal and not in a For when using MPTP, researchers are required to the described the of the MPTP the injection and days as required to animals is It is important to that in to MPTP one that to even of MPTP over many years of the and/or may have This is one more a of be for any in MPTP MPTP including of be in a or animal from MPTP and its can be from and animal animals be to the for days to any MPTP to for MPTP injection alterations may lesion The or animal have a a with a a and be For animals mice, it also be with an animal to all of the and a be of or of any except that is and including the and animal be with that are on the side and on the The of the or area in the animal for animals be with MPTP be on the of the or animal The be at all and the animal of the use of MPTP and its also be that this is to by the responsible This or area be with all of the for the MPTP It also a as hazardous a with a hazardous for animal for animals and MPTP, with in a and an for to MPTP. It is that the material data for MPTP, which is by the be in the in the or there be no need to the injection be all MPTP, including the days The is far more important when mice than monkeys as mice higher doses of MPTP. The of a with an and of a and material that is and from This be easy to into and to after one For of with and and an can be with that is to the is for and a with that is by the of and for that is to the using the can also be The is and be with with after each be of in the hazardous and the the can be obtained from a The of and in any where MPTP is to be used be for guidance in for use with MPTP. For animals such as mice, and are as of without with is to both and of other animal be on the animal in the to and the period injections be in the in the For animals such as monkeys, be The of the and the be with MPTP can be from several do not use MPTP as the but only as the or other MPTP and handling be to the or area within the animal the use of and handling of MPTP and never MPTP or of MPTP of the MPTP may be in of or in of MPTP be until used and at in a within a This be in a with a This be to a in the or investigators in the handling of MPTP perform the of will handling problems that from the of It is that a to MPTP be in the to MPTP the area with with to the of MPTP To the of MPTP the described by et al. is a safe a (e.g. with a the and an of MPTP in and with to and all other with of all in a hazardous if a given a dose of less than or of MPTP, it is not to the and the but to the of to the or It be that this MPTP has to be used in one and the MPTP in at to the used MPTP a of to the of MPTP into the the as found that MPTP at its as MPTP stable to However, one has a for MPTP other such as will and that even without the of and of MPTP on its be injected only with of MPTP by through a or by the in or not MPTP as this will the and may to from a number of different injection regimens have been used to produce the MPTP These are based on a number of including experimental of lesion, and species mice, which greater of MPTP to produce can be injected or or multiple and with a of the regimen used, it is that all MPTP injections to mice be in a from which MPTP is have a or be with to potential and and to on the frequently the of and at the of the injection This will prevent any of the and the possibility of of On the of or on the the all animals are and a the injection MPTP be to the animal be to to animal will this For the in the and when the animal so that any will into the and not on the mice, when to which can significant of MPTP et al. 1988). the is not so as to cause of the injected MPTP from the animals such as monkeys be in for It is not to animals in a injection for or and with a with do not or from the with and the with in a to be of as the of the injection the MPTP be with an of as described The potential for to MPTP and its is from contaminated and following MPTP injection and the period that MPTP or its are to be in the of treated when handling and it is important that the be contaminated be with and be into a and for using also be with in the and of as after and with for with and with may be to central The material that the be with to for and of as hazardous For animal that the and with to for and in the used with This to be done on a and with with and with The described that MPTP-injected animals remain in the for days and only after the days In the of prolonged MPTP (i.e. weeks to while the or area will remain the period the days for mice, only a following the described and, for monkeys, it is to monkeys to other and to the as described in the or area be with be and of as hazardous animal can be days MPTP injection and the or area has been by the responsible and/or of to MPTP or may when animals are for to days following MPTP this mice be in the and the appropriate by the researcher and are with which be if with body is the of for animals in MPTP be to prevent and and be are on an with and on including the used for be in for with and with be with and and the which may MPTP and et al. be following for animal For the of a a and will be in the and can be into a or can be as per the of the be with For the of be to the of the and a This will any and prevent of the The will be as the the be with with and with one may that is used for following MPTP However, we have that to and after for more than h at not MPTP at Conversely, a 5% in MPTP almost However, is such a it can produce hazardous with several compounds and be with to as have also found that is as as in MPTP, yet more to use as it not cause with and not to is available as a It can be to the with and at a in which to the for we found that the of on of in is in that after at there is no any MPTP The of MPTP by the as in is not a the not to an but to a biochemical In we found that of with different of ranging from to on MPTP. our for MPTP is of in In contrast to on MPTP, 5% even after an This is not as is stable and even after to harsh chemical and doses of (i.e. 25 mg/kg to mice produce damage to the but to the nervous system et al. This is with our that the intraperitoneal or injection of different doses of and to mice to show any of in the striatum or to produce any damage to the dopaminergic of the the injection of into the striatum produce dopaminergic neurotoxicity (Giovanni et al. 1994b). These data indicate that the hazards of involve peripheral such as the and only if high the or the is far less hazardous than its and thus the is the of MPTP. effect of on MPTP. of in at for different of time with of in the time of an of the injected into an system and MPTP were as described et al. investigators in the handling of MPTP to any the of MPTP in the and the of the the is as and will any In a the the researcher the and with to for and these in hazardous In the that and have not all of the MPTP with to prevent MPTP from and as hazardous The area is with with several with with and with these in hazardous as area and inform the and that an MPTP has occured and were used to that To MPTP with a with all and into a hazardous the area with with several with with and with these in hazardous inform the and that a MPTP has occured and were to and the become contaminated with MPTP, and to a of any potential MPTP is and the to of contaminated or these are to and can and can be using by and all with with is to other and to of that may have become To date, there has been no in the of the of a researcher to MPTP while MPTP of a a fatal to of MPTP its synthesis has been and the only human to MPTP (Langston and 1983). However, the safe of MPTP be to potential from acute to the as a prevent the of MPTP to its toxic metabolite, For of animals with a potent irreversible MPTP-induced neurotoxicity et al. and et al. 1988). On the other hand, except for a 1993), there is no that by or by other following to MPTP any However, in of to MPTP, in an attempt to the of any MPTP to it is that be far as we there is no regimen for to MPTP. the is to prevent the of MPTP by as and as we suggest an dose of (e.g. four be at Although it may be to continue (e.g. a for some it is unknown this is is for the of from the or the development of acute parkinsonian symptoms from the MPTP In following the administration of a dose of be in (i.e. and in active to any MPTP be available for use and be in a at all in the or area for if it is that are to a of MPTP a of a of to (e.g. days and the This may be for a first the or if there is an of with MPTP. This be done only after To date, MPTP the experimental model of PD. To this end, it is used in animal species and in mice. However, even as a MPTP is an hazardous which can be and/or of its demonstrated toxicity to humans, the use of MPTP researchers is a Over the years, a better of the of this its and its body has investigators to develop and for the safe use of this These include for MPTP and for its injection into potential from animal and and and in of the that we have to the most common and related to MPTP this review all of the safe use of this hazardous there can be no for common and in the use of compounds such as MPTP. It is that this review has the by in the and, in with our recent of MPTP, will to the and safe use of the MPTP animal model of PD. The of this review are by the of and the of the Parkinson's the the the the and The are to Markey and for on the review and to Sonsalla and for