MPTP
MPTP (1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine) is a neurotoxin that causes permanent symptoms of Parkinson's disease by killing certain neurons in the substantia nigra of the brain. It is used to study the disease in monkeys.
While MPTP itself does not have opioid effects, it is related to MPPP, a synthetic opioid drug with effects similar to those of heroin and morphine. MPTP can be accidentally produced during the illicit manufacture of MPPP, and that is how its Parkinson-inducing effects were first discovered.

# Toxicity
Injection of MPTP causes rapid onset of Parkinsonism, hence users of MPPP contaminated with MPTP will develop these symptoms.
MPTP itself is not toxic, and as a lipophilic compound can cross the blood-brain barrier. Once inside the brain, MPTP is metabolized into the toxic cation 1-methyl-4-phenylpyridinium (MPP+) by the enzyme MAO-B of glial cells. MPP+ primarily kills dopamine-producing neurons in a part of the brain called the substantia nigra. MPP+ interferes with complex I of the electron transport chain, a component of mitochondrial metabolism, which leads to cell death and causes the buildup of free radicals, toxic molecules that contribute further to cell destruction.
MPTP has quite selective abilities to cause neuronal death in dopaminergic cells, apparently through a high-affinity uptake process in nerve terminals normally used to reuptake dopamine after it has been released into the synaptic cleft. The dopamine transporter moves MPP+ inside the cell.
The resulting gross depletion of dopaminergic neurons has severe implications on cortical control of complex movements. The direction of complex movement is based from the substantia nigra to the putamen and caudate nucleus which then relay signals to the rest of the brain. This pathway is controlled via dopamine-using neurons, which MPTP selectively destroys, resulting over time in parkinsonism.
MPTP causes parkinsonism in primates including humans. Rodents are much less susceptible. Rats are almost immune to the adverse effects of MPTP. Mice suffer from cell death in the substantia nigra (to differing degree according to the strain of mice used) but do not show parkinsonian symptoms. It is believed that the lower levels of MAO B in the rodent brain's capillaries may be responsible for this.

# Discovery in users of illicit drugs
The neurotoxicity of MPTP was hinted at in 1976 after Barry Kidston, a 23-year-old chemistry graduate student in Maryland, synthesized MPPP incorrectly and injected the result. It was contaminated with MPTP, and within three days he began exhibiting symptoms of Parkinson's disease. The National Institute of Mental Health found traces of MPTP and other meperidine analogues in his lab. They tested the substances on rats, but due to rodents' tolerance for this type of neurotoxin nothing was observed. Kidston's parkinsonism was successfully treated with Levo-dopa but he died 18 months later from a cocaine overdose. Upon autopsy, destruction of dopamine-neurons in the substantia nigra was discovered.
In 1982, seven people in Santa Clara County, California were diagnosed with Parkinsonism after using MPPP contaminated with MPTP. The neurologist J. William Langston in collaboration with NIH tracked down MPTP as the cause, researched its effects on primates, and was eventually able to successfully treat motor symptoms of three of the seven patients with neural grafts of fetal stem cells from aborted human fetuses in collaboration with neuroscientists from Lund University Hospital in Sweden.  This experience was documented in a book he authored, The Case of the Frozen Addicts (ISBN 0-679-42465-2), about his quest for a cure, which was later featured in two NOVA productions by PBS.

# Contribution of MPTP to research into Parkinson's disease
Langston et al.(1984) 
found that injections of MPTP in squirrel monkeys resulted in parkinsonism, symptoms of which were subsequently reduced by Levo-dopa, a precursor for the neurotransmitter dopamine, currently the drug-of-choice in treatment of Parkinson's. The symptoms and brain structures of MPTP-induced Parkinson's are fairly indistinguishable to the point that MPTP may be used to simulate the disease in order to study Parkinson's physiology and possible treatments within the laboratory. Mouse studies have shown that susceptibility to MPTP increases with age.
Knowledge of MPTP and its use in reliably recreating Parkinson's disease in experimental models has inspired scientists to investigate the possibilities of surgically replacing neuron loss through fetal tissue implants, subthalamic electrical stimulation and stem cell research, all of which have demonstrated initial, provisional successes.
It has been postulated that Parkinson's disease may be caused by minute amounts of MPP+ like compounds from ingestion or exogenously through repeated exposure and that these substances are too minute to be detected significantly by epidemiological studies.
In 2000 another animal model for Parkinson's Disease was found. It was shown that the pesticide and insecticide rotenone causes parkinsonism in rats by killing dopaminergic neurons in the substantia nigra. Like MPP+, rotenone also interferes with complex I of the electron transport chain.

# Synthesis and uses
MPTP was first synthesized as an analgesic in 1947 by Ziering et al.. It can be formed by mixing formaldehyde, methylamine and alpha-methylstyrene. 
It was tested as a treatment for various conditions, but the tests were halted when Parkinson-like symptoms were noticed in monkeys. In one test of the substance, two of six human subjects died.
MPTP is used in industry as a chemical intermediate; the chloride of the toxic metabolite MPP+ was turned into the herbicide Cyperquat.