**4. Estrogen and Parkinson's disease**

#### **4.1 Introduction**

Parkinson's disease is the second most common neurodegenerative movement disorder. It is mainly characterized by the slow and gradual emergence of motor disorders such as tremor, rigidity, bradykinesia, and postural instability (Lang, 2007). Parkinson's disease is less prevalent in women than in men by an approximate 2:3 ratio and evidence suggests that estrogen influences the onset and severity of disease-associated symptoms (Currie, 2004; Shulman, 2006). Women with Parkinson's disease tend to have an earlier menopause, are more likely to have undergone hysterectomy, and used estrogen therapy less frequently than control subjects (Benedetti et al., 2001). Ragonese et al. (2004) suggested that factors reducing estrogen contribute to the development of Parkinson's disease (Ragonese et al., 2004). This was recently supported by the Observational Study of the Women's Health Initiative (WHI-OS) that employed 83,482 women. The study showed association between the number of women with longer fertile lifespan and a reduced risk of Parkinson's disease (Saunders-Pullman et al., 2009). In another human study, women with Parkinson's disease were less likely to have used postmenopausal estrogen therapy (Currie et al., 2004), suggesting that estrogen produces a beneficial effect on Parkinson's disease.

#### **4.2 Dopamine neurotransmission**

Dopamine is a neurotransmitter that has multiple functions in the brain such as cognition, reward, mood, and voluntary movement. The substantia nigra is a brain area that governs these functions. So far, this neurotransmitter has been the major player in Parkinson's disease such that dopamine synthesizing neurons are progressively depleted in the substantia nigra of Parkinson's patients (Emborg, 2004). Aberrant dopamine transmission is implicated in Parkinson's disease, particularly because the symptoms are ameliorated by a drug which increases dopamine signaling. Dopamine is actively eliminated from the extracellular space by astrocytes and neurons through dopamine transporters. Afterwards, dopamine is either recycled into vesicles or metabolized. In previous studies, estrogen increased the availability of dopamine by inhibiting uptake and by decreasing the affinity of the transporter for dopamine (Disshon et al., 1998). Estrogen also increases the synthesis of dopamine in the substantia nigra and the release of dopamine from axon terminals. In rodents and in neuronal cell culture studies, estrogen protected dopaminergic neurons from injury (B. Liu & Dluzen, 2006; Arvin et al., 2000). Given this, the beneficial effect of estrogen on Parkinson's disease may be mediated through estrogen's action on dopamine.

Studies have further identified how estrogen acts on the dopamine system. Estrogen modulates the development of dopaminergic neurons and neurotransmission (Bourque,

that inflammatory mechanisms significantly contribute to the pathogenesis of Alzheimer's

Collectively, animal studies on Alzheimer's disease have shown beneficial effects of estrogen through inhibiting the synthesis of amyloid β, facilitating its metabolisms, modulating protein kinases, and inhibiting inflammatory pathways. Human studies on the effects of estrogen on Alzheimer's disease have resulted in both positive and negative effects. It is unclear what causes the inconsistent results. Nevertheless, it seems clear that estrogen influences Alzheimer's disease pathology, if not etiology. How to identify and adjust factors

Parkinson's disease is the second most common neurodegenerative movement disorder. It is mainly characterized by the slow and gradual emergence of motor disorders such as tremor, rigidity, bradykinesia, and postural instability (Lang, 2007). Parkinson's disease is less prevalent in women than in men by an approximate 2:3 ratio and evidence suggests that estrogen influences the onset and severity of disease-associated symptoms (Currie, 2004; Shulman, 2006). Women with Parkinson's disease tend to have an earlier menopause, are more likely to have undergone hysterectomy, and used estrogen therapy less frequently than control subjects (Benedetti et al., 2001). Ragonese et al. (2004) suggested that factors reducing estrogen contribute to the development of Parkinson's disease (Ragonese et al., 2004). This was recently supported by the Observational Study of the Women's Health Initiative (WHI-OS) that employed 83,482 women. The study showed association between the number of women with longer fertile lifespan and a reduced risk of Parkinson's disease (Saunders-Pullman et al., 2009). In another human study, women with Parkinson's disease were less likely to have used postmenopausal estrogen therapy (Currie et al., 2004),

Dopamine is a neurotransmitter that has multiple functions in the brain such as cognition, reward, mood, and voluntary movement. The substantia nigra is a brain area that governs these functions. So far, this neurotransmitter has been the major player in Parkinson's disease such that dopamine synthesizing neurons are progressively depleted in the substantia nigra of Parkinson's patients (Emborg, 2004). Aberrant dopamine transmission is implicated in Parkinson's disease, particularly because the symptoms are ameliorated by a drug which increases dopamine signaling. Dopamine is actively eliminated from the extracellular space by astrocytes and neurons through dopamine transporters. Afterwards, dopamine is either recycled into vesicles or metabolized. In previous studies, estrogen increased the availability of dopamine by inhibiting uptake and by decreasing the affinity of the transporter for dopamine (Disshon et al., 1998). Estrogen also increases the synthesis of dopamine in the substantia nigra and the release of dopamine from axon terminals. In rodents and in neuronal cell culture studies, estrogen protected dopaminergic neurons from injury (B. Liu & Dluzen, 2006; Arvin et al., 2000). Given this, the beneficial effect of estrogen

suggesting that estrogen produces a beneficial effect on Parkinson's disease.

on Parkinson's disease may be mediated through estrogen's action on dopamine.

Studies have further identified how estrogen acts on the dopamine system. Estrogen modulates the development of dopaminergic neurons and neurotransmission (Bourque,

disease and support the use of estrogen in the fight against Alzheimer's disease.

underlying the discrepancies seems to be an essential task.

**4. Estrogen and Parkinson's disease** 

**4.2 Dopamine neurotransmission** 

**4.1 Introduction** 

2009) by promoting neurite plasticity (Beyer et al., 2000). These effects are either mediated through a direct action on dopaminergic neurons or interactions with local astroglia (Ivanova et al., 2001, 2002). Alternatively, estrogen may act on genetic levels to modulate dopamine. For instance, estrogen regulates dopamine gene expression by activating transcriptional factors (DonCarlos et al., 2009). Estrogen also exerts non-genomic membrane effects, interaction with neurotransmitter receptors, and ionic channel regulation (Garcia-Segura et al., 2009). These studies suggest that estrogen protects against Parkinson's disease through genomic and non-genomic effects on the dopamine system.

Dopamine transporters mediate the uptake of dopamine from synapses to presynaptic vesicles, thereby restoring depleted vesicular dopamine levels (Jourdain et al., 2005). Estrogen stimulated dopamine uptake by nerve cells through neuronal dopamine transporter (D'Astous et al., 2004). On the other hand, estrogen decreased astroglial dopamine uptake, increasing the available levels of synaptic dopamine. This allowed more synaptic dopamine to be taken up by neurons. These studies suggest a few important points: first, not only dopamine neurons but also nigrostriatal astroglia contribute to the metabolic processes of dopamine (Karakaya et al., 2007); second, astroglia are implicated in estrogen-transmitted neuroprotection during dopamine neuro-degeneration (Morale et al, 2006), and finally, as the complementary action of estrogen on neurons, astrocyte and microglia may represent a potential pharmacological target for Parkinson's disease management (Vegeto et al., 2008).

#### **4.3 Oxidative stress**

In the process of dopamine being catalyzed by monoamine oxidase, a large amount of reactive oxygen species is produced, resulting in cell death (Hastings et al., 1996; Luo et al., 1998). In addition, dopamine aldehyde generated in the oxidative deamination reaction is 1000-fold more toxic than dopamine (Burke, 2003). Dopamine neurons in Parkinson's disease become vulnerable to oxidative stress (Dexter et al., 1989; Sian et al., 1994) perhaps due to lower levels of glutathione (endogenous antioxidant) than other cell types.

The brain has a predominant defense mechanism against superoxide radicals through antioxidant enzymes such as superoxide dismutase. Studies have demonstrated that superoxide dismutase is implicated in dopamine and Parkinson's disease. Mutant mice that over-expressed or lacked superoxide dismutase were more resistant to (Przedborski et al., 1992) or vulnerable to (Andreassen et al., 2001; J. Zhang et al., 2000) dopamine neurotoxin than wild type mice, respectively. The expression of superoxide dismutase was upregulated in the substantia nigra following the dopamine neurotoxin insult, yet the loss of dopaminergic neurons still occurred (Tripanichkul et al., 2007). These results suggest that there is an attempt to combat the oxidative stress in nigral neurons but not sufficient to spare neurons. The implication of superoxide dismutase in the antioxidant effect of estrogen has been shown in a study done by Tripanichkul et al. (2007). In that study, estrogen treatment increased the expression of superoxide dismutase in the substantia nigra of animals that were treated with the dopamine neurotoxin. This study suggests that estrogen up-regulates superoxide dismutase in critical brain areas, thereby exerting protection against dopamine neurotoxin or Parkinson's disease.

#### **4.4 Neuroinflammation**

Neuroinflammation and microglial activation are often seen in Parkinson's disease (McGeer et al., 1988; Hunot et al., 2003) and anti-inflammatory drugs reduce the risk of this disease

Estrogen and Brain Protection 149

Chronic ethanol consumption and ethanol withdrawal both generate oxidative free radicals and subsequent lipid peroxidation (Nordmann et al., 1990; Montoliu et al., 1994). Lipid peroxidation reflects the interaction between oxygen and the polyunsaturated fatty acids of membrane lipids, generating deteriorating breakdown products. Since the brain consists of a high content of unsaturated membrane lipids, it is a preferred target of both reactive oxygen species and ethanol (Hernandez-Munoz et al., 2000). Ethanol withdrawal-induced oxidative stress was associated with an increase in glutamatergic neurotransmission (Rossetti & Carboni, 1995), the upregulation of calcium channels, and the accumulation of intracellular calcium (Rewal et al., 2005). The functional consequence of prooxidant ethanol withdrawal is shown in several animal and human studies. For instance, enhanced reactive oxygen species concurred with ethanol withdrawal-induced seizure activity in rats (Vallett et al., 1997). The cerebrospinal fluid of patients who underwent ethanol withdrawal showed higher concentrations of excitatory neurotransmitters and oxidative markers (Marotta et al., 1997; Tsai et al., 1998) than control subjects. Higher levels of lipid peroxide and lower levels of superoxide dismutase (antioxidant enzyme) activity were also seen in those patients (Tsai et al., 1998). These studies indicate that the redox imbalance has a causative relationship

If ethanol withdrawal is a prooxidant stimulus, estrogen treatment should be able to mitigate the stress through its antioxidant property. Our recent findings essentially confirmed the hypothesis using the in vivo and in vitro model of ethanol withdrawal. Estrogen treatment mitigated reactive oxygen species generation, lipid peroxidation, and protein oxidation (Jung et al. 2004, 2006). Estrogen protection against the prooxidant effect of ethanol withdrawal may involve glutamate transmission because glutamate-induced oxidative stress is attenuated by estrogen (Behl & Manthey, 2000) and the quinol derived from estrogen (Prokai et al., 2003). It is also possible that estrogen elevates the levels of endogenous antioxidants, such as glutathione, so that a favorable redox potential for an antioxidant environment is created (Prokai et al., 2003). Since oxidative molecules are generated mainly from mitochondria, these studies suggest that the antioxidant protection of estrogen against ethanol withdrawal is linked to the mitoprotective activity of estrogen.

Indeed, the mitoprotective effects of estrogen are interactive with the antioxidant effect by virtue of the fact that mitochondria are the major source and target of oxidative free radicals. The mitoprotective effect of estrogen has been extended to the ethanol withdrawal model in our recent study in which ethanol withdrawal provokes the oxidation of mitochondrial proteins in rats, in a manner mitigated by estrogen. Since cellular energy ATP is mainly generated in mitochondria, it is not surprising that estrogen protects against mitochondrial respiratory deficit during ethanol withdrawal (Jung et al., 2011). Presumably, estrogen plays a role in alleviating the oxidative burden in mitochondria, thus increasing mitochondrial

P38 is referred to as a stress-activated protein kinase because it is often activated in response to a variety of stress. A transient, moderate activation of P38 normally occurs in association

respiration efficiency (J.Q. Chen & Yager 2004; Jung et al., 2011).

**5.2 Oxidative stress** 

with ethanol withdrawal insults.

**5.3 Mitochondria** 

**5.4 Signaling pathways** 

(H. Chen et al., 2003; Wahner et al., 2007). A positive correlation was found between antecedent brain injuries, such as trauma or exposure to infectious agents and the development of Parkinson's disease (B. Liu et al., 2003). This correlation implies that the brain inflammatory response to these noxious events, and specifically microglial activation, plays a critical role in Parkinson's disease. In support of this view, researchers have detected pro-inflammatory molecules (e.g. TNF-α) and excessive reactive oxygen species in the nervous system of Parkinson's disease patients (Hunot et al., 1996; Knott et al., 2000). The inflammatory molecules seem to amplify neuroinflammation as well as neuro-toxicity, ultimately leading to a slow and irreversible destruction of dopaminergic neurons. Using estrogen receptor-null mice, several studies have demonstrated that estrogen receptor-α is involved in the anti-inflammatory activity of estrogen (Dubal et al., 2001; Vegeto et al., 2003). Although estrogen receptor-β is expressed widely in brain, it does not seem to mediate the protective effect of estrogen. Or the effects of estrogen receptors on inflammation depend on the brain area (Harris et al., 2003). Whether or which receptor mediates estrogen's protection against inflammatory response still remains unclear.

Collectively, the protective effects of estrogen on Parkinson's disease appear to involve dopaminergic neuroprotection, anti-oxidant activities, anti-inflammatory activities, and estrogen receptors. Considering that Parkinson's disease is more prevalent in male than female patients, how these effects of estrogen can be implemented to clinical usages is an open question. At the very least, estrogen can be used as an interventional tool for a new mechanistic insight into this neurodegenerative disease.
