Effect of intermittent washout periods on progressive lesioning of the nigrostriatal pathway with 1-methyl-2-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

Abstract

We have previously reported that a progressively increased dose of MPTP over the course of 4 weeks induces the gradual impairment of the nigrostriatal dopamine (DA) pathway and several behaviors [Goldberg et al. (in press) Neuroscience]. To our knowledge, this is the first report of specific behavioral deficits correlated with discrete thresholds of DA loss in this pathway. In that study, MPTP was administered 5 d/wk, with behavioral and tissue analysis being carried out 3 days following the final injection at each dose. However, in order to better represent long-term progressive neurodegeneration the present study introduced a washout period of 10 days between each increased dose of MPTP. This implementation also controlled for any transient de-activation of tyrosine hydroxylase (TH), the enzyme that catalyzes synthesis of DA, caused by MPTP-induced oxidative stress which has been suggested following acute administration of the toxin [Smeyne and Jackson-Lewis (2005) Brian Res Mol Brain Res 134:57-66]. Additionally, by the end of the previous study, there was an ultimate decrease of 62% in the mean number of TH-labeled neurons/section in the substantia nigra pars compacta (SNpc) and a 74% decrease in caudate putamen (CPu) TH optical density with continuous MPTP. In the present study, we find that the washout periods lead to a final 79% decrease in the mean number of TH-labeled SNpc neurons/section, and a similar 74% decrease in CPu TH following the 32 mg/kg MPTP dose. Additionally, a dose-dependent decrease was observed in the mean number of SNpc TH-ir neurons/section in the current study which was not seen in the continuous MPTP protocol. These results suggest that a washout period following each increased MPTP dose allows for observation of continued cell death that might occur during the week following MPTP administration, and for therapeutic interventions to be applied at any of several stages during progressive neurodegeneration.