Pablo Rey

Mechanism of 6-hydroxydopamine neurotoxicity: the role of NADPH oxidase and microglial activation in 6-hydroxydopamine-induced degeneration of dopaminergic neurons

Cell death induced by 6‐hydroxydopamine (6‐OHDA) is thought to be caused by reactive oxygen species (ROS) derived from 6‐OHDA autooxidation and by a possible direct effect of 6‐OHDA on the mitochondrial respiratory chain. However, the process has not been totally clarified. In rat primary mesencephalic cultures, we observed a significant increase in dopaminergic (DA) cell loss 24 h after administration of 6‐OHDA (40 μmol/L) and a significant increase in NADPH subunit expression, microglial activation and superoxide anion/superoxide‐derived ROS in DA cells that were decreased by the NADPH inhibitor apocynin. Low doses of 6‐OHDA (10 μmol/L) did not induce a significant loss of DA cells or a significant increase in NADPH subunit expression, microglial activation or superoxide‐derived ROS. However, treatment with the NADPH complex activator angiotensin II caused a significant increase in all the latter. Forty‐eight hours after intrastriatal 6‐OHDA injection in rats, there was still no loss of DA neurons although there was an increase in NADPH subunit expression and NADPH oxidase activity. The results suggest that in addition to the autooxidation‐derived ROS and the inhibition of the mitochondrial respiratory chain, early microglial activation and NADPH oxidase‐derived ROS act synergistically with 6‐OHDA and constitute a relevant and early component of the 6‐OHDA‐induced cell death.

Brain angiotensin enhances dopaminergic cell death via microglial activation and NADPH-derived ROS

Angiotensin II (AII) plays a major role in the progression of inflammation and NADPH-derived oxidative stress (OS) in several tissues. The brain possesses a local angiotensin system, and OS and inflammation are key factors in the progression of Parkinsons disease. In rat mesencephalic cultures, AII increased 6-OHDA-induced dopaminergic (DA) cell death, generation of superoxide in DA neurons and microglial cells, the expression of NADPH-oxidase mRNA, and the number of reactive microglial cells. These effects were blocked by AII type-1 (AT1) antagonists, NADPH inhibitors, or elimination of glial cells. DA degeneration increased angiotensin converting enzyme activity and AII levels. In rats, 6-OHDA-induced dopaminergic cell loss and microglial activation were reduced by treatment with AT1 antagonists. The present data suggest that AII, via AT1 receptors, increases the dopaminergic degeneration process by amplifying the inflammatory response and intraneuronal levels of OS, and that glial cells play a major role in this process.

The inflammatory response in the MPTP model of Parkinson’s disease is mediated by brain angiotensin: relevance to progression of the disease

The neurotoxin MPTP reproduces most of the biochemical and pathological hallmarks of Parkinson’s disease. In addition to reactive oxygen species (ROS) generated as a consequence of mitochondrial complex I inhibition, microglial NADPH‐derived ROS play major roles in the toxicity of MPTP. However, the exact mechanism regulating this microglial response remains to be clarified. The peptide angiotensin II (AII), via type 1 receptors (AT1), is one of the most important inflammation and oxidative stress inducers, and produces ROS by activation of the NADPH‐oxidase complex. Brain possesses a local angiotensin system, which modulates striatal dopamine (DA) release. However, it is not known if AII plays a major role in microglia‐derived oxidative stress and DA degeneration. The present study indicates that in primary mesencephalic cultures, DA degeneration induced by the neurotoxin MPTP/MPP+ is amplified by AII and inhibited by AT1 receptor antagonists, and that protein kinase C, NADPH‐complex activation and microglial activation are involved in this effect. In mice, AT1 receptor antagonists inhibited both DA degeneration and early microglial and NADPH activation. The brain angiotensin system may play a key role in the self‐propelling mechanism of Parkinson’s disease and constitutes an unexplored target for neuroprotection, as previously reported for vascular diseases.

Angiotensin type-1-receptor antagonists reduce 6-hydroxydopamine toxicity for dopaminergic neurons

Angiotensin II activates (via type 1 receptors) NAD(P)H-dependent oxidases, which are a major source of superoxide, and is relevant in the pathogenesis of several cardiovascular diseases and certain degenerative changes associated with ageing. Given that there is a brain renin-angiotensin system and that oxidative stress is a key contributor to Parkinsons disease, we investigated the effects of angiotensin II and angiotensin type 1 (AT(1)) receptor antagonists in the 6-hydroxydopamine model of Parkinsons disease. Rats subjected to intraventricular injection of 6-hydroxydopamine showed bilateral reduction in the number of dopaminergic neurons and terminals. Injection of angiotensin alone did not induce any significant effect. However, angiotensin increased the toxic effect of 6-hydroxydopamine. Rats treated with the AT(1) receptor antagonist ZD 7155 and then 6-hydroxydopamine (with or without exogenous administration of angiotensin) showed a significant reduction in 6-hydroxydopamine-induced oxidative stress (lipid peroxidation and protein oxidation) and dopaminergic degeneration. Dopaminergic degeneration was also reduced by the NAD(P)H inhibitor apocynin. Angiotensin may play a pivotal role, via AT(1) receptors, in increasing the oxidative damage of dopaminergic cells, and treatment with AT(1) antagonists may reduce the progression of Parkinsons disease.

Angiotensin-converting enzyme inhibition reduces oxidative stress and protects dopaminergic neurons in a 6-hydroxydopamine rat model of Parkinsonism.

It is now established that the brain possesses a local renin‐angiotensin system and that angiotensin II exerts multiple actions in the nervous system, including regulation of striatal dopamine release. Furthermore, angiotensin activates NADPH‐dependent oxidases, which are a major source of superoxide, and angiotensin‐converting enzyme inhibitors, commonly used in the treatment of hypertension and chronic heart failure, have shown antioxidant properties in several tissues. Oxidative stress is a key contributor to the pathogenesis and progression of Parkinsons disease. In the present study, we treated rats with intraventricular injections of the dopaminergic neurotoxin 6‐hydroxydopamine and subcutaneous injections of the angiotensin‐converting enzyme inhibitor Captopril to study the possible neuroprotective effect of the latter on the dopaminergic system and on 6‐hydroxydopamine‐induced oxidative stress. Rats treated with Captopril and 6‐hydroxydopamine showed significantly less reduction in the number of dopaminergic neurons (i.e., immunoreactive to tyrosine hydroxylase) in the substantia nigra and in the density of striatal dopaminergic terminals than 6‐hydroxydopamine‐lesioned rats not treated with Captopril. In addition, Captopril reduced the levels of major oxidative stress indicators (i.e., lipid peroxidation and protein oxidation) in the ventral midbrain and the striatum of 6‐hydroxydopamine‐lesioned rats. Our results suggest that angiotensin‐converting enzyme inhibitors may be useful for treatment of Parkinsons disease and that further investigation should focus on the neuroprotective capacity of these compounds.

Reduction of dopaminergic degeneration and oxidative stress by inhibition of angiotensin converting enzyme in a MPTP model of parkinsonism

There is growing evidence indicating that oxidative stress is a key contributor to the pathogenesis and progression of Parkinsons disease. The brain, and particularly the basal ganglia, possesses a local rennin-angiotensin system. Angiotensin activates NAD(P)H-dependent oxidases, which are a major intracellular source of superoxide, and angiotensin converting enzyme inhibitors (ACEIs) have shown antioxidant properties. We treated mice with MPTP and the ACEI captopril to study the possible neuroprotective and antioxidant effects of the latter on the dopaminergic system. Pre-treatment with captopril induced a significant reduction in the MPTP-induced loss of dopaminergic neurons in the substantia nigra and a significant reduction in the loss of dopaminergic terminals in the striatum. Furthermore, captopril reduced the MPTP-induced increase in the levels of major oxidative stress indicators (i.e. lipid peroxidation and protein oxidation) in the ventral midbrain and the striatum. Captopril did not reduce striatal MPP(+) levels, MAO-B activity or dopamine transporter activity, which may reduce MPTP neurotoxicity. Our results suggest that angiotensin-converting enzyme inhibitors may be useful for treatment of Parkinsons disease, and that further investigation should focus on the neuroprotective capacity of these compounds.

Systemic administration of N-acetylcysteine protects dopaminergic neurons against 6-hydroxydopamine-induced degeneration.

The results of several in vitro studies have shown that cysteine prodrugs, particularly N‐acetylcysteine, are effective antioxidants that increase the survival of dopaminergic neurons. N‐acetylcysteine can be systemically administered to deliver cysteine to the brain and is of potential use for providing neuroprotection in the treatment of Parkinsons disease. However, it has also been reported that an excess of cysteine may induce neurotoxicity. In the present study, we injected adult rats intrastriatally with 2.5 μl of 6‐hydroxydopamine (7.5 μg) and N‐acetylcysteine (240 mM) or cysteine (240 mM) or intraventricularly with 6‐hydroxydopamine (200 μg) and subcutaneously with N‐acetylcysteine (10 and 100 mg/kg). We studied the effects of these compounds on both the nigrostriatal dopaminergic terminals and the surrounding striatal tissue. The tissue was stained with fluoro‐jade (a marker of neuronal degeneration) and processed by immunohistochemistry to detect tyrosine hydroxylase, neuronal and glial markers, and the stress protein heme‐oxygenase‐1. After intrastriatal injection, both cysteine and N‐acetylcysteine had clear neuroprotective effects on the striatal dopaminergic terminals, but also led to neuronal degeneration (as revealed by fluoro‐jade staining) and astroglial and microglial activation, as well as intense induction of heme‐oxygenase‐1 in astrocytes and microglial cells. Subcutaneous administration of N‐acetylcysteine also induced significant reduction of the dopaminergic lesion (about 30% reduction). However, we did not observe appreciable N‐acetylcysteine‐induced fluoro‐jade labeling in striatal neurons or any of the above‐mentioned changes in striatal glial cells. The results suggest that low doses of cysteine prodrugs may be useful neuroprotectors in the treatment of Parkinsons disease.

Glial overexpression of heme oxygenase-1: a histochemical marker for early stages of striatal damage.

The level of heme oxygenase-1 (HO-1) in the normal striatum is below the limit of immunodetection. However, HO-1 is overexpressed in both neural and non-neural cells in response to a wide range of lesions. We induced different types of lesions affecting the striatal cells or the main striatal afferent systems in rats to investigate if overexpression of HO-1 could be a useful histochemical marker of striatal damage. Thirty-six hours after intrastriatal or intraventricular injection of excitotoxins that affect striatal neurons (ibotenic acid) or of neurotoxins that affect striatal dopaminergic (6-hydroxydopamine) or serotonergic (5,7-dihydroxytriptamine) afferent terminals, or after surgical lesioning of cortico-striatal projections, there was intense induction of striatal HO-1 immunoreactivity (HO-1-ir). Double immunolabeling revealed that the HO-1-ir was located in glial cells. After intrastriatal injection of ibotenic acid, a central zone of neuronal degeneration contained numerous round and pseudopodic HO-1-ir cells, and was surrounded by a ring of HO-1-ir cells, most of which were immunoreactive for astroglial markers. Intraventricular injection of neurotoxins induced astroglial HO-1-ir cells which were more evenly distributed throughout the lesioned or denervated areas. HO-1-ir microglial cells were also observed in areas subjected to mechanical damage. The HO-1-ir was markedly lower or absent 1 week after lesion, and even more so 3 weeks after, although some HO-1-ir cells were still observed after intrastriatal injection of ibotenic acid or surgical corticostriatal deafferentation. The results indicate that determination of glial HO-1-ir is a useful histochemical marker for early stages of striatal damage.

N-Acetylcysteine enhances production of dopaminergic neurons from mesencephalic-derived precursor cells

Epidermal growth-factor-responsive rat mesencephalic precursor cells incubated in differentiation media produce only a small number of dopaminergic (DA) cells. Supplementation of the differentiation medium with N-acetylcysteine (NAC) induced a marked increase (∼400%) in the number of tyrosine hydroxylase (TH)-positive cells. Treatment with interleukin-1α also induced a significant increase (∼300%) in the number of TH-positive cells. However, we did not find additive effects between these drugs. The results suggest that NAC is effective in the production of DA cells from precursors, and that this may be related to enhancement of generation and/or survival of DA cells.