Animal models of Parkinson's disease in rodents induced by toxins: an update.

The development of animal models of Parkinson's disease is of great importance in order to test substitutive or neuroprotective strategies for Parkinson's disease. Such models should reproduce the main characteristics of the disease, such as a selective lesion of dopaminergic neurons that evolves over time and the presence of neuronal inclusions known as Lewy bodies. Optimally, such models should also reproduce the lesion of non-dopaminergic neurons observed in a great majority of patients with Parkinson's disease. From a behavioral point of view, a parkinsonian syndrome should be observed, ideally with akinesia, rigidity and rest tremor. These symptoms should be alleviated by dopamine replacement therapy, which may in turn lead to side effects such as dyskinesia. In this review, we analyze the main characteristics of experimental models of Parkinson's disease induced by neurotoxic compounds such as 6-hydroxydopamine, MPTP and rotenone. We show that, whereas MPTP and 6-hydroxydopamine induce a selective loss of catecholaminergic neurons that in most cases evolves over a short period of time, rotenone infusion by osmotic pumps can induce a chronically progressive degeneration of dopaminergic neurons and also of non-dopaminergic neurons in both the basal ganglia and the brainstem.

The role of glial reaction and inflammation in Parkinson's disease.

The glial reaction is generally considered to be a consequence of neuronal death in neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, and Parkinson's disease. In Parkinson's disease, postmortem examination reveals a loss of dopaminergic neurons in the substantia nigra associated with a massive astrogliosis and the presence of activated microglial cells. Recent evidence suggests that the disease may progress even when the initial cause of neuronal degeneration has disappeared, suggesting that toxic substances released by the glial cells may be involved in the propagation and perpetuation of neuronal degeneration. Glial cells can release deleterious compounds such as proinflammatory cytokines (TNF-alpha, Il-1beta, IFN-gamma), which may act by stimulating nitric oxide production in glial cells, or which may exert a more direct deleterious effect on dopaminergic neurons by activating receptors that contain intracytoplasmic death domains involved in apoptosis. In line with this possibility, an activation of proteases such as caspase-3 and caspase-8, which are known effectors of apoptosis, has been reported in Parkinson's disease. Yet, caspase inhibitors or invalidation of TNF-alpha receptors does not protect dopaminergic neurons against degeneration in experimental models of the disease, suggesting that manipulation of a single signaling pathway may not be sufficient to protect dopaminergic neurons. In contrast, the antiinflammatory drugs pioglitazone, a PPAR-gamma agonist, and the tetracycline derivative minocycline have been shown to reduce glial activation and protect the substantia nigra in an animal model of the disease. Inhibition of the glial reaction and the inflammatory processes may thus represent a therapeutic target to reduce neuronal degeneration in Parkinson's disease.

Protective action of the peroxisome proliferator-activated receptor-gamma agonist pioglitazone in a mouse model of Parkinson's disease.

We examined the effect of pioglitazone, a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist of the thiazolidinedione class, on dopaminergic nerve cell death and glial activation in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. The acute intoxication of C57BL/6 mice with MPTP led to nigrostriatal injury, as determined by tyrosine hydroxylase (TH) immunocytochemistry, and HPLC detection of striatal dopamine and metabolites. Damage to the nigrostriatal dopamine system was accompanied by a transient activation of microglia, as determined by macrophage antigen-1 (Mac-1) and inducible nitric oxide synthase (iNOS) immunoreactivity, and a prolonged astrocytic response. Orally administered pioglitazone (approximately 20 mg/kg/day) attenuated the MPTP-induced glial activation and prevented the dopaminergic cell loss in the substantia nigra pars compacta (SNpc). In contrast, there was little reduction of MPTP-induced dopamine depletion, with no detectable effect on loss of TH immunoreactivity and glial response in the striatum of pioglitazone-treated animals. Low levels of PPARgamma expression were detected in the ventral mesencephalon and striatum, and were unaffected by MPTP or pioglitazone treatment. Since pioglitazone affects primarily the SNpc in our model, different PPARgamma-independent mechanisms may regulate glial activation in the dopaminergic terminals compared with the dopaminergic cell bodies after acute MPTP intoxication.

Transport of monoamine transmitters by the organic cation transporter type 2, OCT2.

The recently cloned apical renal transport system for organic cations (OCT2) exists in dopamine-rich tissues such as kidney and some brain areas (Gründemann, D., Babin-Ebell, J., Martel, F., Ording, N., Schmidt, A., and Schömig, E. (1997) J. Biol. Chem. 272, 10408-10413). The study at hand was performed to answer the question of whether OCT2 accepts dopamine and other monoamine transmitters as substrate. 293 cells were stably transfected with the OCT2r cDNA resulting in the 293OCT2r cell line. Expression of OCT2r in 293 cells induces specific transport of tritiated dopamine, noradrenaline, adrenaline, and 5-hydroxytryptamine (5-HT). Initial rates of specific 3H-dopamine, 3H-noradrenaline, 3H-adrenaline, and 3H-5-HT transport were saturable, the Km values being 2.1, 4.4, 1.9, and 3.6 mmol/liter. The corresponding Vmax values were 3.9, 1.0, 0. 59, and 2.5 nmol min-1.mg of protein-1, respectively. 1, 1'-diisopropyl-2,4'-cyanine (disprocynium24), a known inhibitor of OCT2 with a potent eukaliuric diuretic activity, inhibited 3H-dopamine uptake into 293OCT2r cells with an Ki of 5.1 (2.6, 9.9) nmol/liter. In situ hybridization reveals that, within the kidney, the OCT2r mRNA is restricted to the outer medulla and deep portions of the medullary rays indicating selective expression in the S3 segment of the proximal tubule. These findings open the possibility that OCT2r plays a role in renal dopamine handling.

Catecholamine transport by the organic cation transporter type 1 (OCT1).

1. Liver and kidney extract adrenaline and noradrenaline from the circulation by a mechanism which does not seem to be one of the classical catecholamine transporters. The hypothesis that OCT1 is involved the organic cation transporter type 1 which exists in rat kidney and liver-was tested. 2. Based on human embryonic kidney cells (293), we constructed a cell line which stably expresses OCT1r (293OCT1r cells). Transfection with OCT1 resulted in a transport activity not only for prototypical known substrates of OCT1 such as 3H-1-methyl-4-phenylpyridinium and 14C-tetraethylammonium but also for the catecholamines 3H-adrenaline, 3H-noradrenaline (3H-NA) and 3H-dopamine (3H-DA), the indoleamine 3H-5-hydroxytryptamine (3H-5HT) as well as the indirect sympathomimetic 14C-tyramine. 3. For 3H-DA, 3H-5HT and 3H-NA, at non-saturating concentrations, the rate constants for inwardly directed substrate flux (kin) were 6.9+/-0.8, 3.1+/-0.2, and 1.2+/-0.1 microl min(-1) mg protein(-1). In wild type cells (293WT) the corresponding kin's were considerably lower, being 0.94+/-0.40, 0.47+/-0.08 and 0.23+/-0.05 microl min(-1) mg protein ' (n=12). The indirectly determined half-saturating concentrations of DA, 5HT, and NA were 1.1 (95% c.i.: 0.8, 1.4), 0.65 (0.49, 0.86), and 2.8 (2.1, 3.7) mmol l(-1) (n=3). 4. Specific 3H-DA uptake in 293OCT1r cells was resistant to cocaine (1 micromoll(-1)), 3H-5HT uptake was resistant to citalopram (300 nmol l(-1)) and 3H-NA uptake was resistant to desipramine (100 nmoll(-1)), corticosterone (1 micromol l(-1)), and reserpine (10 nmoll(-1)) which rules out the involvement of classical transporters for biogenic amines. 5. The findings demonstrate that OCTI efficiently transports catecholamines and other biogenic amines and support the hypothesis that OCT1 is responsible for hepatic and renal inactivation of circulating catecholamines.