Striatal dopamine depletion, dopamine receptor stimulation, and GABA metabolism: implications for the therapy of Parkinson's disease

Dopamine-denervated rat striata exhibit increased synaptosomal gamma-aminobutyric acid (GABA) synthetic capacity (glutamate decarboxylase activity) without an alteration in GABA transport capacity. Stimulating denervated striatal dopamine receptors with apomorphine selectively increases in vivo steady-state striatal GABA turnover. The striatal response in animals to isolated loss of dopamine innervation is an increase in glutamate decarboxylase activity, not the decrease frequently found in the brains of patients with Parkinson's disease. Neurochemical expression of denervated striatal dopamine receptor stimulation may involve increased synaptic activity of striatal GABA neurons. For these reasons, enhancement of basal ganglia GABA function may improve responsiveness to dopamimetic therapies in patients with Parkinson's disease.     

Synergistic effects of dopamine and Zn2+ on the induction of PC12 cell death and dopamine depletion in the striatum: possible implication in the pathogenesis of Parkinson's disease

The mechanism that underlies the progressive degeneration of the dopaminergic neurons in Parkinson's disease (PD) is not clear. The Zn(2+) level in the substantia nigra of Parkinson's patients is increased. However, it is unknown whether Zn(2+) has a role in the degeneration of dopaminergic neurons. This study identifies an interaction between dopamine and Zn(2+) that induces cell death. When PC12 cells were pretreated with Zn(2+) before dopamine treatment, dopamine and Zn(2+) synergistically increased cell death, while Zn(2+) and H(2)O(2) had only additive effects on cell death. The synergistic effect appeared to be caused by increased apoptosis rather than necrosis. The synergistic effect was specific for Zn(2+). The synergistic effect was inhibited by thiol antioxidants but was not significantly affected by calcium channel blockers. There is a similar synergistic effect when dopamine and Zn(2+) were coinfused into the striatum, resulting in striatal dopamine content depletion in vivo. Thus, both dopamine oxidation and Zn(2+) are possibly linked to the degeneration of dopaminergic neurons.     

Overexpression of estrogen receptor beta alleviates the toxic effects of beta-amyloid protein on PC12 cells via non-hormonal ligands

After binding to the estrogen receptor, estrogen can alleviate the toxic effects of beta-amyloid protein, and thereby exert a therapeutic effect on Alzheimer’s disease patients. Estrogen can increase the incidence of breast carcinoma and endometrial cancer in post-menopausal women, so it is not suitable for clinical treatment of Alzheimer’s disease. There is recent evidence that the estrogen receptor can exert its neuroprotective effects without estrogen dependence. Real-time quantitative PCR and flow cytometry results showed that, compared with non-transfected PC12 cells, adenovirus-mediated estrogen receptor β gene-transfected PC12 cells exhibited lower expression of tumor necrosis factor α and interleukin 1β under stimulation with beta-amyloid protein and stronger protection from apoptosis. The Akt-specific inhibitor Abi-2 decreased the anti-inflammatory and anti-apoptotic effects of estrogen receptor β gene-transfection. These findings suggest that overexpression of estrogen receptor β can alleviate the toxic effect of beta-amyloid protein on PC12 cells, without estrogen dependence. The Akt pathway is one of the potential means for the anti-inflammatory and anti-apoptotic effects of the estrogen receptor.     

Targeting the noradrenergic system for anti-inflammatory and neuroprotective effects: implications for Parkinson's disease

Degeneration of the locus coeruleus noradrenergic system is thought to play a key role in the pathogenesis of Parkinson's disease(PD), whereas pharmacological approaches to increase noradrenaline bioavailability may provide neuroprotection. Noradrenaline inhibits microglial activation and suppresses pro-inflammatory mediator production(e.g., tumor necrosis factor-α, interleukin-1β inducible nitric oxide synthase activity), thus limiting the cytotoxicity of midbrain dopaminergic neurons in response to an inflammatory stimulus. Neighbouring astrocyte populations promote a neurotrophic environment in response to β_2-adrenoceptor(β_2-AR) stimulation via the production of growth factors(e.g., brain derived neurotrophic factor, cerebral dopamine neurotrophic factor glial cell derived neurotrophic factor which have shown promising neuroprotective and neuro-restorative effects in the nigrostriatal dopaminergic system. More recent findings have demonstrated a role for the β_2-AR in down-regulating expression levels of the human α-synuclein gene SNCA and relative α-synuclein protein abundance. Given that α-synuclein is a major protein constituent of Lewy body pathology, a hallmark neuropathological feature in Parkinson's disease, these findings could open up new avenues for pharmacological intervention strategies aimed at alleviating the burden of α-synucleinopathies in the Parkinsonian brain. In essence, the literature reviewed herein supports our hypothesis of a tripartite neuroprotective role for noradrenaline in combating PD-related neuropathology and motor dysfunction via(1) inhibiting nigral microglial activation pro-inflammatory mediator production,(2) promoting the synthesis of neurotrophic factors from midbrain astrocytes and(3) downregulating α-synuclein gene expression and protein abundance in a β_2-AR-dependent manner. Thus, taken together, either pharmacologically enhancing extra-synaptic noradrenaline bioavailability or targeting glial β_2-ARs directly makes itself as a promising treatment option aimed at slowing/halting PD progression.     

Hemin-induced apoptosis in PC12 and neuroblastoma cells: implications for local neuronal death associated with intracerebral hemorrhage

The exact pathogenesis of neuronal death following bleeding in brain parenchyma is still unknown. Hemoglobin (Hb) toxicity has been postulated to be one of the underlying mechanisms. The purpose of this study was to examine the possible contribution to neurotoxicity of each of the Hb compounds and to characterize the death pathway. Pheochromocytoma (PC12) and neuroblastoma (SH- SY5Y) cell lines were exposed to Hb, globin, hemin, protoporphyrin IX and iron for 1.5- 24 h. We found that Hb and hemin are highly toxic (LD(50) of 8 and 20 &mgr; mol/l, respectively) as compared to globin that was not toxic. In addition, protoporphyrin IX and iron, compounds of hemin, were less toxic than hemin itself (LD(50) of 962 and 2070 &mgr; mol/l respectively). We also demonstrated that non-specific protein digestion with proteinase-K, markedly increased Hb toxicity. Hemin-treated cells caused a typical apoptotic cell death pattern as indicated by DNA fragmentation, caspase activation and reduction in the mitochondrial membrane potential. Treatment with the antioxidant N-acetyl-L-cysteine or iron chelator, deferoxamine, diminished hemin-induced cell death, indicating a role of oxidative stress in this deleterious process. Thus, therapeutic strategies, based on antioxidant, iron chelator and anti-apoptotic agents may be effective in counteracting Hb neurotoxicity.     

Pathogenesis of Parkinson's disease: implications from familial Parkinson's disease]

The deposition of alpha-synuclein (aS), a product of pathogenic gene for dominantly inherited familial Parkinson's disease (PD; park1), as fibrillary aggregates like Lewy bodies (LB), is a hallmark lesion of a set of neurodegenerative disorders termed synucleinopathies, including sporadic PD and dementia with Lewy bodies (DLB). We found that aS is the major component of LBs and further identified a specific phosphorylation of Ser129 of insoluble aS by mass spectrometric analysis. The roles of DJ-1 and PINK-1, products of pathogenic genes for autosomal recessive forms of early-onset parkinsonism, have subsequently been examined. Overexpression of DJ-1 conferred cultured cells resistance to oxidative stress, suggesting an antioxidant function of DJ-1. We also confirmed the anti-PTEN function of DJ-1 that may promote cell survival, showing decreased phosphorylation of Akt through upregulation of PTEN activity upon siRNA knockdown for DJ-1. PINK-1, that had been identified as a gene upregulated by PTEN overexpression, turned out to be a protein kinase localized in mitochondria. Collectively, information derived from studies on pathogenic genes for familial PD will open up the way toward the clarification of the pathogenesis of PD, underscoring the roles of protein aggregation, proteolysis, oxidative stress and protein phosphorylation in PD.     

Pathological synaptic plasticity in the striatum: implications for Parkinson's disease

Repetitive stimulation of the corticostriatal pathway can cause either a long-lasting increase, or an enduring decrease in synaptic strength, respectively referred to as long-term potentiation (LTP), and long-term depression (LTD), both requiring a complex sequence of biochemical events. Once established, LTP can be reversed to control levels by a low-frequency stimulation (LFS) protocol, an active phenomenon defined "synaptic depotentiation", required to erase redundant information. In the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease (PD), striatal synaptic plasticity has been shown to be impaired, though chronic treatment with l-dopa was able to restore it. Interestingly, a consistent number of l-dopa-treated animals developed involuntary movements, resembling human dyskinesias. Strikingly, electrophysiological recordings from the dyskinetic group of rats demonstrated a selective impairment of synaptic depotentiation. This survey will provide an overview of plastic changes occurring at striatal synapses. The potential relevance of these findings in the control of motor function and in the pathogenesis both of Parkinson's disease and l-dopa-induced motor complications will be discussed.     

Synergistic effects of pesticides and metals on the fibrillation of alpha-synuclein: implications for Parkinson's disease

Aggregation of alpha-synuclein has been implicated in the formation of proteinaceous inclusions in the brain (Lewy bodies, Lewy neurites) that are characteristic of neurodegenerative diseases, such as Parkinson's disease (PD) and dementia with Lewy bodies (DLBs). The etiology of PD is unknown, but recent work has shown that except in rare cases, there appears to be no direct genetic basis. However, several studies have implicated environmental factors, especially pesticides and metals. Here we show that certain pesticides and metals induce a conformational change in alpha-synuclein and directly accelerate the rate of formation of alpha-synuclein fibrils in vitro. In addition, the simultaneous presence of metal and pesticide led to synergistic effects on the rate of fibrillation. We propose a model in which environmentalfactors in conjunction with genetic susceptibility may form the underlying molecular basis for idiopathic PD.     

Parkinson's disease: depletion of substantia nigra neurotensin receptors

Neurotensin receptor autoradiography reveals dense binding in normal human substantia nigra. In nigra from patients with Parkinson's disease, nigral receptor binding is only about one-third of control values. ‘Saturation’ analysis suggests that these changes result from receptor loss. These results support a neurotensin-domapine interaction in human nigro-striatal circuits.     

Striatal function in normal aging: implications for Parkinson's disease

Central to several current theories of the etiology of Parkinson's disease is the premise that the nigrostriatal dopaminergic system degenerates with normal aging. Much of the evidence for this assertion has come from postmortem neurochemical studies. We have used L-6-[18F] fluoro-Dopa and positron emission tomography in 26 healthy volunteers (age range, 27-76 years) to examine striatal and frontal cortical tracer uptake. Data have been analyzed by using a graphical approach to calculate an influx constant (Ki) for L-6-[18F]fluoro-Dopa uptake into the caudate, putamen, and medial frontal cortex of each subject. In the population studied, there was no decline in Ki with age for any of these structures. A series of physiological measurements made on the older subjects also showed few significant changes with age. The positron emission tomographic findings demonstrate preservation of nigrostriatal dopaminergic function in normal aging. The pathological process causing Parkinson's disease may operate closer to the time of presentation than has been suggested.     

Oestrogen receptors and signalling pathways: implications for neuroprotective effects of sex steroids in Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disorder with a higher incidence in the male population. In the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, 17β-oestradiol but not androgens were shown to protect dopamine (DA) neurones. We report that oestrogen receptors (ER)α and β distinctly contribute to neuroprotection against MPTP toxicity, as revealed by examining the membrane DA transporter (DAT), the vesicular monoamine transporter 2 (VMAT2) and tyrosine hyroxylase in ER wild-type (WT) and knockout (ERKO) C57Bl/6 male mice. Intact ERKOβ mice had lower levels of striatal DAT and VMAT2, whereas ERKOα mice were the most sensitive to MPTP toxicity compared to WT and ERKOβ mice and had the highest levels of plasma androgens. In both ERKO mice groups, treatment with 17β-oestradiol did not provide neuroprotection against MPTP, despite elevated plasma 17β-oestradiol levels. Next, the recently described membrane G protein-coupled oestrogen receptor (GPER1) was examined in female Macaca fascicularis monkeys and mice. GPER1 levels were increased in the caudate nucleus and the putamen of MPTP-monkeys and in the male mouse striatum lesioned with methamphetamine or MPTP. Moreover, neuroprotective mechanisms in response to oestrogens transmit via Akt/glycogen synthase kinase-3 (GSK3) signalling. The intact and lesioned striata of 17β-oestradiol treated monkeys, similar to that of mice, had increased levels of pAkt (Ser 473)/βIII-tubulin, pGSK3 (Ser 9)/βIII-tubulin and Akt/βIII-tubulin. Hence, ERα, ERβ and GPER1 activation by oestrogens is imperative in the modulation of ER signalling and serves as a basis for evaluating nigrostriatal neuroprotection.     

Differential effects of lysophospholipids on exocytosis in rat PC12 cells

Secretory phospholipase A₂ (sPLA₂) activity is present in the CNS and the sPLA₂-IIA isoform has been shown to induce exocytosis in cultured hippocampal neurons. However, little is known about possible contributions of various lysophospholipid species to exocytosis in neuroendocrine cells. This study was therefore carried out to examine the effects of several lysophospholipid species on exocytosis on rat pheochromocytoma-12 (PC12) cells. An increase in vesicle fusion, indicating exocytosis, was observed in PC12 cells after external infusion of lysophosphatidylinositol (LPI), but not lysophosphatidylcholine or lysophosphatidylserine by total internal reflection microscopy. Similarly, external infusion of LPI induced significant increases in capacitance, or number of spikes detected at amperometry, indicating exocytosis. Depletion of cholesterol by pre-incubation of cells with methyl beta cyclodextrin and depletion of Ca²⁺ by thapsigargin and incubation in zero external Ca²⁺ resulted in attenuation of LPI induced exocytosis, indicating that exocytosis was dependent on the integrity of lipid rafts and intracellular Ca²⁺. Moreover, LPI induced a rise in intracellular Ca²⁺ suggesting that this could be the trigger for exocytosis. It is postulated that LPI may be an active participant in sPLA₂-mediated exocytosis in the CNS.     

Stem cells may reshape the prospect of Parkinson's disease therapy

The concept of cell replacement to compensate for cell loss and restore functionality has entered several disease entities including neurodegenerative disorders. Recent clinical studies have shown that transplantation of fetal dopaminergic (DA) cells into the brain of Parkinson's disease (PD) patients can reduce disease-associated motor deficits. However, the use of fetal tissue is associated with practical and ethical problems including low efficiency, variability in the clinical outcome and controversy regarding the use of fetuses as donor. An alternative cell resource could be embryonic stem (ES) cells, which can be cultivated in unlimited amounts and which have the potential to differentiate into mature DA cells. Several differentiation protocols have been developed, and some progress has been made in understanding the mechanisms underlying DA specification in ES cell development, but the "holy grail" in this paradigm, which is the production of sufficient amounts of the "right" therapeutic DA cell, has not yet been accomplished. To achieve this goal, several criteria on the transplanted DA cells need to be fulfilled, mainly addressing cell survival, accurate integration in the brain circuitry, normal function, no tumor formation, and no immunogenicity. Here, we summarize the current state of ES cell-derived DA neurogenesis and discuss the aspects involved in generating an optimal cell source for cell replacement in PD.     

Controlled pilot study of the effects of neuromuscular therapy in patients with Parkinson's disease.

The objectives of this study is to examine the effects of neuromuscular therapy (NMT) on motor and nonmotor symptoms in Parkinson's disease (PD). Thirty-six subjects with PD were randomly assigned to NMT or music relaxation (MR, or active control). Subjects received treatment twice a week for 4 weeks. Testing was conducted at baseline, after final treatment, and 8 days after final treatment. Primary outcome measures were the Motor subscale of the United Parkinson Disease Rating Scale (UPDRS) and the Clinical Global Impression scale (CGI-Change). Secondary outcome measures included a PD-specific quality of life scale (PDQ-39), quantitative measures of motor function, and severity scales for anxiety and depression symptoms. NMT resulted in a significant and sustained improvement in the Motor subscale of the UPDRS (P < or = 0.0001), most notable in the tremor scores. Also improved 1 week after the last treatment were the CGI scores (P = 0.007) and the finger-tapping speed (P = 0.001). The MR active control group had a slight improvement in tremor but evidenced no other change in motor function. Both groups exhibited a modest improvement in quality of life immediately after the last treatment. This effect was sustained for 8 days only in the MR group. In the nonmotor domains, the MR group evidenced improvements in mood (P = 0.001) and anxiety (P = 0.002), whereas NMT had no effect on mood (P = 0.09), and its initial effect on anxiety (P = 0.0009) dissipated after 8 days (P = 0.40). Group differences for UPDRS motor score and patient CGI-Change were superior in the NMT compared to the MR group. There was no group difference in PDQ-39 scores or in nonmotor measures. The findings suggest that NMT can improve motor and selected nonmotor symptoms in PD and that this effect is more durable for the motor symptoms. The results of this pilot study warrant larger controlled studies to examine dose range, durability, and mechanisms of NMT in PD function.     

Nicotine and Parkinson's disease: Implications for therapy

Accumulating evidence suggests that nicotine, a drug that stimulates nicotinic acetylcholine receptors, may be of therapeutic value in Parkinson's disease. Beneficial effects may be several‐fold. One of these is a protective action against nigrostriatal damage. This possibility stems from the results of epidemiological studies that consistently demonstrate an inverse correlation between tobacco use and Parkinson's disease. This reduced incidence of Parkinson's disease has been attributed to the nicotine in tobacco products, at least in part, based on experimental work showing a protective effect of nicotine against toxic insults. Second, several studies suggest a symptomatic effect of nicotine in Parkinson's disease, although effects are small and somewhat variable. Third, recent data in nonhuman primates show that nicotine attenuates levodopa‐induced dyskinesias, a debilitating side effect that develops in the majority of patients on levodopa therapy. Collectively, these observations suggest that nicotine or CNS selective nicotinic receptor ligands hold promise for Parkinson's disease therapy to reduce disease progression, improve symptoms, and/or decrease levodopa‐induced dyskinesias. © 2007 Movement Disorder Society     

3,4-Dihydroxyphenylacetaldehyde potentiates the toxic effects of metabolic stress in PC12 cells

3,4-Dihydroxyphenylacetaldehyde (DOPAL) is a toxic metabolite formed by the oxidative deamination of dopamine. This aldehyde is mainly oxidized to 3,4-dihydroxyphenylacetic acid (DOPAC) by aldehyde dehydrogenase (ALDH), but is also partly reduced to 3, 4-dihydroxyphenylethanol (DOPET) by aldehyde or aldose reductase (ARs). In a previous study, we found that rotenone, a complex I inhibitor, induced a rapid accumulation of DOPAL and DOPET in the medium of cultured PC12 cells. Here, we examined the potential role of DOPAL in the toxicity induced by complex I inhibition in PC12 cells and compared the effects of rotenone on concentrations of DOPAL and DOPET to those of MPP(+). DOPAL and DOPET levels were increased by rotenone but decreased by MPP(+). Inhibition of ALDH by daidzein reduced the formation of DOPAC and increased the accumulation of DOPAL. Inhibition of ARs (with AL1576) diminished DOPET formation and elevated DOPAL concentrations. Combined inhibition of ALDH and ARs markedly elevated DOPAL concentrations while diminishing DOPET and DOPAC levels. The elevation of DOPAL levels induced by combined inhibition of ALDH and ARs had no effect on cell viability. However, combined inhibition of ALDH and ARs potentiated rotenone-induced toxicity. Both the potentiation of toxicity and the increase in DOPAL levels were blocked by inhibition of monoamine oxidase with clorgyline indicating that accumulation of DOPAL was responsible for the potentiated rotenone-induced toxicity following combined inhibition of ALDH and ARs. Since complex I dysfunction is reported to be involved in the pathogenesis of Parkinson's disease, DOPAL potentiation of the deleterious effects of complex I inhibition may contribute to the specific vulnerability of dopaminergic neurons to injury.     

Adenosine/dopamine interaction: implications for the treatment of Parkinson's disease

Evidence for a role of dopaminergic neurotransmission in the motor effects of adenosine antagonists, such as caffeine, is reviewed, based on the existence of specific antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the striatum. Both adenosine A(1) and adenosine A(2A) receptor antagonists induce motor activation in rodents. At least a certain degree of dopaminergic activity is required to obtain adenosine antagonist-induced motor activation, with adenosine A(1) antagonists being the most sensitive and non-selective adenosine antagonists the most resistant to striatal dopamine depletion. When considering long-term treatment with adenosine antagonists concomitant administration of dopamine agonists might be required in order to obtain strong motor effects (cross-sensitization) and to avoid the development of telerance.