ABSTRACT
1Dept of Internal Medicine and Therapeutics and 3Dept of Biochemistry, IRCCS San Matteo, University of Pavia; 2Laboratory of Functional Neurochemistry, Interdepartmental Research Center for Parkinson's Disease (CRIMP), IRCCS Neurological Institute C. Mondino, Pavia, Italy.
Background and aim: We recently demonstrated the presence of hepatic-mitochondrial dysfunctions in parkinsonian rats bearing a 6-hydroxydopamine (6-OHDA) induced lesion of the nigrostriatal tracts. This well-known animal model of Parkinson’s disease (PD) is characterized by the prompt damage of striatal dopaminergic terminals followed by the progressive retrograde cell loss of nigral neurons. Recent data have supported the use of this model also to study PD-related non-motor symptoms. We have previously shown that hepatocytes express mGlu5 metabotropic glutamate receptors and that administration of 2-methyl-6-(phenylethynyl)pyridine (MPEP), a well characterized receptor antagonist, protects hepatocytes against hypoxic damage (Storto et al., 2000). Similarly, chronic treatment with MPEP, significantly reduced the motor symptoms in 6-OHDA-lesioned rats supporting that mGluR5 modulation may be a viable strategy to counteract PD motor symptoms (Ambrosi et al., 2010). In this study we evaluated whether the chronic administration of MPEP can protect against decreased energy capacity observed in hepatic mitochondria isolated from 6-OHDA-lesioned rats.
Materials and methods: Male Sprague Dawley rats received a unilateral intrastriatal injection of 6-OHDA (20μg/3μL) or vehicle, and were sacrificed 4 or 8 weeks after the neurotoxic insult. A four-week, daily systemic with MPEP (1.5 mg/kg, i.p.) was started (1) immediately or (2) 4 weeks after the neurotoxin injection. At the end of the treatment period animals were sacrificed by decapitation and both brain and liver were immediately removed. The toxin-induced lesion was evaluated by western blot analysis and immunohistochemical staining for the enzyme tyrosine hydroxylase (TH) and the dopamine transporter (DAT), two dopaminergic-specific markers. Intact hepatic mitochondria were isolated immediately using differential centrifugation. Mitochondrial ATP content and production were measured by the luciferin-luciferase method; rhodamine 123 was used for evaluation of mitochondrial membrane potential. Serum levels of hepatic enzyme release were also evaluated.
Results: Stable loss of dopaminergic terminals in the striatum and neurons in the substantia nigra, 80 and 60% loss vs intact hemisphere, was observed 4 and 8 weeks after 6-OHDA injection. Liver mitochondria obtained from animals sacrificed at these time points exhibited a decrease in ATP content and ATP production compared with the respective vehicle treated rats. Similarly, the measurement of membrane potential showed a statistically significant decrease 8 weeks after infusion of the toxin, respect to mitochondria obtained from control animal. The administration of MPEP, simultaneously to 6-OHDA injection, prevented the decrease in ATP content and production in hepatic mitochondria. On the contrary, delayed MPEP treatment, started 4 weeks after 6-OHDA injection, had no significant effects. No changes in membrane potential were observed after MPEP treatment compared with to the respective 6-OHDA control groups. The serum levels of hepatic enzymes after MPEP treatment did not differ from those observed in sham operated rats; interestingly ALT levels significantly decreased in rats MPEP-treated for 4 weeks as compared with sham group.
Conclusions: Chronic treatment with MPEP administration, starting immediately after 6-OHDA-injection, can counteract the lesion-dependent decrease in ATP content and production in isolated hepatic mitochondria. These results demonstrate that MPEP has no evident side effect on hepatic function and that the selective mGlu5 antagonist can restore altered mitochondrial function in 6-OHDA-lesioned rats. Taken together our results further indicate that MPEP might represent a new chance in the control of the Parkinson's disease.
Storto et al.(2000). Hepatology. 31, 649-55.
Ambrosi et al. (2010). Brain Res Bull. 82, 29-38.