Estrogen and SERM neuroprotection in animal models of Parkinson's disease.

A higher prevalence and incidence of Parkinson disease (PD) is observed in men and beneficial motor effects of estrogens are observed in parkinsonian women. Lesion of the dopamine (DA) nigrostriatal pathway in animals with 1-methyl 4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) provides a model of PD and this is based on its use in humans as side-product of a drug abuse. Presently treatment of PD is mainly symptomatic. The MPTP mouse is used to study the neuroprotective roles of estrogenic drugs on the DA system. Estrogens, but not androgens, are active neuroprotectants as well as progesterone and dehydroepiandrosterone. An estrogen receptor agonist PPT and the selective estrogen receptor modulator raloxifene are also neuroprotective. Striatal DA neurons of estrogen receptor alpha knockout mice are more susceptible to MPTP toxicity than wild-type mice and neuroprotection by estradiol is associated with the activation of the PI3-K pathway involving Akt, GSK3beta, Bcl2 and BAD.

Regulation of striatal preproenkephalin mRNA levels in MPTP-lesioned mice treated with estradiol.

We reported previously the protective effect of 17beta-estradiol (17beta-E(2)) on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopamine (DA) depletion. This protection was stereospecific, because 17beta-E(2) showed activity but 17alpha-estradiol (17alpha-E(2)) did not. The mechanisms by which estradiol exerts its beneficial effects, however, remain unknown. We investigated a possible implication of enkephalins (ENK) in neuroprotective activity of 17beta-E(2). Protection against MPTP-induced DA depletion was obtained with 17beta-E(2) but not 17alpha-E(2). MPTP lesion increased striatal preproenkephalin (PPE) mRNA levels and they remained elevated in 17alpha-E(2)-treated MPTP mice whereas 17beta-E(2) treatment decreased these levels to control values. This is the first report of estradiol modulation of striatal PPE mRNA in mice. Negative and significant correlations between DA levels, vesicular monoamine transporter (VMAT(2)) density, and PPE mRNA were observed in the striatum of lesioned animals. This effect of 17beta-E(2) on PPE mRNA after a lesion could be one of many mechanisms by which this steroid exerts its neuroprotective activity.

The degeneration of dopamine neurons in Parkinson's disease: insights from embryology and evolution of the mesostriatocortical system.

Parkinson's disease (PD) is, to a large extent, specific to the human species. Most symptoms are the consequence of the preferential degeneration of the dopamine-synthesizing cells of the mesostriatal-mesocortical neuronal pathway. Reasons for that can be traced back to the evolutionary mechanisms that shaped the dopamine neurons in humans. In vertebrates, dopamine-containing neurons and nuclei do not exhibit homogenous phenotypes. In this respect, mesencephalic dopamine neurons of the substantia nigra and ventral tegmental area are characterized by a molecular combination (tyrosine hydroxylase, aromatic amino acid decarboxylase, monoamine oxidase, vesicular monoamine transporter, dopamine transporter--to name a few), which is not found in other dopamine-containing neurons of the vertebrate brain. In addition, the size of these mesencephalic DA nuclei is tremendously expanded in humans as compared to other vertebrates. Differentiation of the mesencephalic neurons during development depends on genetic mechanisms, which also differ from those of other dopamine nuclei. In contrast, pathophysiological approaches to PD have highlighted the role of ubiquitously expressed molecules such as a-synuclein, parkin, and microtubule-associated proteins. We propose that the peculiar phenotype of the dopamine mesencephalic neurons, which has been selected during vertebrate evolution and reshaped in the human lineage, has also rendered these neurons particularly prone to oxidative stress, and thus, to the fairly specific neurodegeneration of PD. Numerous evidence has been accumulated to demonstrate that perturbed regulation of DAT-dependent dopamine uptake, DAT-dependent accumulation of toxins, dysregulation of TH activity as well as high sensitivity of DA mesencephalic neurons to oxidants are key components of the neurodegeneration process of PD. This view points to the contribution of nonspecific mechanisms (alpha-synuclein aggregation) in a highly specific cellular environment (the dopamine mesencephalic neurons) and provides a robust framework to develop novel and rational therapeutic schemes in PD.

Evolution and cell biology of dopamine receptors in vertebrates.

Dopamine, one of main modulatory neurotransmitters of the nervous system acts on target cells through two classes of G protein-coupled receptors, D1 and D2. The two dopamine receptor classes display different structures, interact with different regulatory partners (including heterotrimeric G proteins) and, accordingly, have independent evolutionary origins. In vertebrates, each of these receptor classes comprises several subtypes, generated by two steps of gene duplications, early in vertebrate evolution. In the D1 receptor class, the D1A, D1B, D1C and D1D subtypes, and in the D2 class, the D2, D3 and D4 receptor subtypes have been conserved in most vertebrate groups. This conservation has been driven by the acquisition, by each receptor subtype, of a small number of specific properties, which were selected for adaptive purpose in vertebrates. Among these properties, affinity for dopamine, the natural ligand, intrinsic receptor activity, and agonist-induced desensitization clearly distinguish the receptor subtypes. In addition, each dopamine receptor subtype is addressed to a specific location within neuronal networks, although detailed information is lacking for several receptor subtypes. Receptors localization at diverse subcellular places in neurons may also differ from one subtype to another, resulting in different ways of regulating cell signalisation. One challenge for future research on dopamine and its receptors would be to identify the nature of the protein partners and the molecular mechanisms involved in localizing receptors to the neuronal plasma membrane. In this respect, the evolutionary approach we have undertaken suggests that, due to gene duplications, a reasonable degree of freedom exists in the tight organisation of dopamine receptors in neurons. This "evolvability" of dopamine systems has been instrumental to adapt the vertebrate species to nearly all the possible environments.

Dehydroepiandrosterone (DHEA) such as 17beta-estradiol prevents MPTP-induced dopamine depletion in mice.

Previous work from our laboratory has shown prevention of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced striatal dopamine (DA) depletion in mice by 17beta-estradiol, progesterone, and raloxifene. Dehydroepiandrosterone (DHEA), a neurosteroid, was shown to have neuroprotective activities in various paradigms of neuronal death but its effect in vivo in mice on MPTP toxicity has not been reported. We investigated the effects of 17beta-estradiol (2 microg/day) and DHEA (3 mg/day) for 5 days before and after an acute treatment of four MPTP (10 mg/kg) injections in male C57Bl/6 mice. Striatal DA concentrations and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured by HPLC. MPTP mice that received 17beta-estradiol or DHEA had striatal DA, DOPAC, and HVA concentrations comparable to intact animals and higher than striatal DA, DOPAC, and HVA levels in saline-MPTP-treated mice. MPTP treatment led to an increase of striatal DA turnover (assessed with the HVA/DA ratio); DHEA and 17beta-estradiol prevented this increase. 17beta-Estradiol did not affect striatal DA and metabolites concentrations in intact mice in this paradigm. Furthermore, in the substantia nigra DHEA and 17beta-estradiol prevented the MPTP-induced dopamine transporter and tyrosine hydroxylase mRNA decreases measured by in situ hybridization. Therefore, DHEA such as 17beta-estradiol is active in preventing the catecholamine-depleting effect of MPTP and our results suggest that this involves neuroprotection of DA neurons.

Evaluation of the protective effect of oestradiol against toxicity induced by 6-hydroxydopamine and 1-methyl-4-phenylpyridinium ion (Mpp+) towards dopaminergic mesencephalic neurones in primary culture.

Recent findings suggest that gonadal steroid hormones are neuroprotective and may provide clinical benefits in delaying the development of Parkinson's disease. In this report we investigated the ability of oestradiol to protect mesencephalic dopaminergic neurones cultured in serum-free or serum-supplemented medium from toxicity induced by 6-hydroxydopamine or 1-methyl-4-phenylpyridinium ion (MPP+). The efficiency of both toxins and oestradiol was evaluated by tyrosine hydroxylase (TH) immunocytochemistry, [3H]dopamine ([3H]DA) uptake, length of dopaminergic processes and lactate dehydrogenase (LDH) release measurement. In cultures grown in serum-supplemented medium, a 2-h pre-treatment with high concentrations (10-100 microM) of 17beta-oestradiol or 17alpha-oestradiol, the stereoisomer with weak oestrogenic activity, protected both dopaminergic and non-dopaminergic neurones from toxicity induced by 6-hydroxydopamine (6-OHDA; 40 or 100 microM) and by the high MPP+ concentrations (50 microM) necessary to obtain significant neuronal death under those culture conditions. At these concentrations, MPP+ was no longer selective for dopaminergic neurones but affected all cells present in the culture. In contrast, the hormonal treatments did not protect against selective degeneration of dopaminergic neurones induced by lower MPP+ concentrations (below 10 microM), related to inhibition of complex I of respiratory chain. In cultures grown in serum-free medium, oestradiol concentrations higher than 1 microM induced neuronal degeneration and no protection against 6-OHDA or MPP+ toxicity was observed at lower concentrations of the steroid. The neuroprotective effects of 17alpha- or 17beta-oestradiol evidenced in this model might be due to the antioxidant properties of these compounds. However, other non-genomic effects of the steroids cannot be excluded.