The risk is in the air: Intranasal administration of MPTP to rats reproducing clinical features of Parkinson's disease

Many studies have shown that deficits in olfactory and cognitive functions precede the classical motor symptoms seen in Parkinson's disease (PD) and that olfactory testing may contribute to the early diagnosis of this disorder. Although the primary cause of PD is still unknown, epidemiological studies have revealed that its incidence is increased in consequence of exposure to certain environmental toxins. In the present study, we demonstrated that rats treated with intranasal infusion of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) at low concentrations (0.1 mg/nostril) suffered progressive impairments in olfactory, cognitive and motor functions that were assessed with the olfactory discrimination, Morris water maze and open field tests, respectively. Moreover, intranasal administration of MPTP reduced the expression of the enzyme tyrosine hydroxylase in the olfactory bulb and substantia nigra of rats, resulting in a significant reduction of dopamine concentration in the olfactory bulb, prefrontal cortex and striatum, but not in the hippocampus. These results reinforce the notion that the olfactory system represents a particularly sensitive route for the transport of neurotoxins into the central nervous system that may be related to the etiology of PD. In addition, the time course of the olfactory, cognitive and motor impairments verified in rats treated intranasally with MPTP, which appears to be correlated with different stages of the human PD, suggest that the MPTP intranasal model in rats may provide new insights into the underlying mechanisms of PD pathogenesis.