Langston "et al."(1984) found that injections of MPTP in squirrel monkeys resulted in Parkinsonism, symptoms of which were subsequently reduced by levodopa, the drug-of-choice in the treatment of Parkinson's disease along with carbidopa and entacapone. The symptoms and brain structures of MPTP-induced Parkinson's disease are fairly indistinguishable to the point that MPTP may be used to simulate the disease in order to study Parkinson's disease physiology and possible treatments within the laboratory. Mouse studies have shown that susceptibility to MPTP increases with age.