Non-dopaminergic drug treatment of Parkinson's disease

N-Propargyl Caffeate Amide (PACA) Potentiates Nerve Growth Factor (NGF)-Induced Neurite Outgrowth and Attenuates 6-Hydroxydopamine (6-OHDA)-Induced Toxicity by Activating the Nrf2/HO-1 Pathway

Insufficient production of neurotrophic factors is implicated in the pathogenesis of various neurodegenerative disorders. The aim of the present study was to evaluate the potential of N-propargyl caffeate amide (PACA) to enhance nerve growth factor (NGF)-induced neurite outgrowth and the underlying mechanisms. We discovered that PACA not only potentiated NGF-induced neurite outgrowth but also attenuated 6-hydroxydopamine (6-OHDA) neurotoxicity in dopaminergic PC12 cells and primary rat midbrain neurons. To identify the PACA-binding proteins, we introduced a biotin tag to the covalent PACA-protein adducts via "click chemistry" alkyne-azido cycloaddition. As a result, kelch-like ECH-associated protein 1 (Keap1) was isolated as the predominant protein from PACA treated PC12 cells. We demonstrated that the formation of PACA-Keap1 conjugates induced the nuclear translocation of transcription factor Nrf2 and the expression of antioxidant heme oxygenase-1 (HO-1). Importantly, specific HO-1 inhibitor SnPP diminished the neuroprotective and neuritogenic activities of PACA. Moreover, PACA attenuated 6-OHDA-induced production of neurotoxic reactive oxygen species and reactive nitrogen species. PACA also preserved mitochondrial membrane integrity and enhanced the cellular resistance against 6-OHDA neurotoxicity. These results suggest that PACA may exhibit neuroprotective and neuritogenic activities via activating the Nrf2/HO-1 antioxidant pathway.

Noradrenaline-induced release of newly-synthesized accumbal dopamine: differential role of alpha- and beta-adrenoceptors

Previous studies have shown that intra-accumbens infusion of isoproterenol (ISO), a beta-adrenoceptor-agonist, and phenylephrine (PE), an alpha-adrenoceptor-agonist, increase the release of accumbal dopamine (DA). In the present study we analyzed whether the ISO-induced release of DA is sensitive to pretreatment with the DA synthesis inhibitor alpha-methyl-para-tyrosine (AMPT). Earlier studies have shown that the PE-induced release of DA is derived from DA pools that are resistant to AMPT. In addition to PE, the alpha-adrenoceptor-antagonist phentolamine (PA) was also found to increase accumbal DA release. Therefore, we investigated whether similar to the DA-increasing effect of PE, the DA increase induced by PA is resistant to AMPT. Pretreatment with AMPT prevented the ISO-induced increase of accumbal DA. The accumbal DA increase after PA was not reduced by the DA synthesis inhibitor, independently of the amount of DA released. These results show that mesolimbic beta-, but not alpha-adrenoceptors, control the release of accumbal newly-synthesized DA pools. The DA-increasing effects of PE have previously been ascribed to stimulation of presynaptic receptors located on noradrenergic terminals, whereas the DA-increasing effects of PA and ISO have been ascribed to an action of these drugs at postsynaptic receptors on dopaminergic terminals. The fact that AMPT did not affect the accumbal DA response to PE and PA, whereas it did prevent the accumbal DA increase to ISO, supports our previously reported hypothesis that the noradrenergic neurons of the nucleus accumbens containing presynaptic alpha-adrenoceptors impinge upon the dopaminergic terminals in the nucleus accumbens containing postsynaptic adrenoceptors of the alpha but not of the beta type. The putative therapeutic effects of noradrenergic agents in the treatment of DA-related disorders are shortly discussed.

Reduced plasma homocysteine levels in levodopa/entacapone treated Parkinson patients

Hyperhomocysteinemia is not only a major risk factor for atherothrombotic disease, but is also strongly associated with an increased risk of dementia and cognitive impairment, both of which are common in the course of Parkinson's disease (PD). Previous work has found that levodopa increases plasma homocysteine concentrations. Animal studies have indicated that the catechol-O-methyltransferase (COMT) inhibitors can prevent levodopa-induced elevation of homocysteine concentrations by reducing the O-methylation of levodopa. The objective of our study was to assess the impact of entacapone, a COMT inhibitor, on plasma levels of homocysteine, serum folate, and vitamin B12 in levodopa-treated PD patients. Nineteen PD patients receiving only levodopa and 21 PD patients on a combination of levodopa and entacapone participated in the cross-sectional study. The control group consisted of 17 subjects on dopamine agonists. The mean plasma homocysteine concentration in the subjects on only levodopa was higher than that in the subjects on a combination of levodopa and entacapone (P=0.001) or in the control group (P=0.0001). Concentrations of serum vitamin B12 and serum folate were on average normal in all groups, but levodopa-treated subjects (with or without entacapone therapy) were more prone to have hypovitaminosis B12 (45%) than controls on dopamine agonists (6%). We suggest that the COMT inhibition may play a promising role in successfully controlling levodopa-induced hyperhomocysteinemia and in reducing the risk of pathologies probably linked to it. These preliminary findings and postulated hypotheses must now be confirmed in prospective studies.

Neurorestoration in Parkinson's disease by cell replacement and endogenous regeneration

Parkinson's disease (PD) is characterised by a continuous and selective loss of dopaminergic neurons in the substantia nigra pars compacta with a subsequent reduction of the neurotransmitter dopamine. Thus, the prospect of replacing the missing or damaged dopaminergic cells is very attractive. Possible regenerative therapies include transplanting developing neural tissue or neural stem cells into the degenerated host brain and inducing proliferation of endogenous stem cells by pharmacological manipulations. Neural stem cells, with the capacity to self renew and produce the major cell types of the brain, exist in the developing and adult CNS. These cells can be generated and expanded in vitro while retaining the potential to differentiate into nervous tissue. However, one major problem is the control of growth and differentiation of these cells. This review discusses new data on stem cell technology in cell replacement strategies in PD as well as endogenous dopaminergic regeneration.

Prevalence, etiology, and treatment of depression in Parkinson's disease

Parkinson's disease (PD) is primarily a disease of elderly individuals with a peak age at onset of 55 to 66 years. It is characterized by bradykinesia, rigidity, tremor, and postural instability; and affects approximately 1 million individuals in the US and is the second most common neurodegenerative disease next to Alzheimer's disease. The motor symptoms of PD are the focus of pharmacotherapy, yet the nonmotor symptoms (e.g., dementia, psychosis, anxiety, insomnia, autonomic dysfunction, and mood disturbances) can be the most disturbing, disabling, and misunderstood aspects of the disease. Depressive symptoms occur in approximately half of PD patients and are a significant cause of functional impairment for PD patients. There is accumulating evidence suggesting that depression in PD is secondary to the underlying neuroanatomical degeneration, rather than simply a reaction to the psychosocial stress and disability. The incidence of depression is correlated with changes in central serotonergic function and neurodegeneration of specific cortical and subcortical pathways. Understanding comorbid depression in PD may therefore add to the understanding of the neuroanatomical basis of melancholia.

Dopaminergic substitution in Parkinson's disease

Motor complications associated with long-term levodopa application, which follow the so-called honeymoon period of well-tolerated levodopa administration and are looked upon as one clinical marker for progression of Parkinson's disease (PD), initiated a long and controversial debate on the putative neurotoxicity of levodopa. Since dopamine agonists (DA) delay onset of motor complications, they support the neuroprotective treatment strategy in PD. Efficacy and tolerability of DA differs in particular due to their affinity to various dopamine receptor subtypes. The accumulating evidence for levodopa-associated homocysteinaemia, which represents a risk factor for increased incidence of vascular disease in PD, supports the strategy of initial DA application and the use of levodopa as an add-on compound in as low a dose as possible in young PD patients.

Decrease of methionine and S-adenosylmethionine and increase of homocysteine in treated patients with Parkinson's disease

Levodopa is administered with dopa decarboxylase inhibitors (DDI) to prevent its peripheral degradation. This increases conversion of levodopa to 3-O-methyldopa (3-OMD) by catechol-O-methyltransferase (COMT). S-adenosylmethionine (SAM), which is synthesized from adenosine triphosphate and methionine (MET), serves as methyl donor for this O-metabolisation of levodopa with resulting conversion of SAM to total homocysteine (tHcy) via S-adenosylhomocysteine (SAH). Previous studies showed augmented plasma levels of tHcy in long-term levodopa/DDI-treated patients with Parkinson's disease (PP). Objective of this study was to compare MET, SAM, levodopa, 3-OMD, tHcy and SAH in plasma of 20 levodopa/DDI treated PP and corresponding controls. A significant decrease of MET respectively SAM and an increase of tHcy appeared in PP. SAH with its short half-life did not differ. Levodopa/DDI long-term treatment contributes to altered levels of substrates of the O-methylation cycle in PP.