Current drug therapy for Parkinson's disease. A review

Prospective treatment of Parkinson's disease by a siRNA-LDH nanoconjugate

In the world, among the neurodegenerative diseases, Parkinson's is the second most common disease. Although several medications are available in the market, this disease still remains incurable and only the symptoms are controlled to a certain extent with severe side effects. For these reasons we decided to search for a novel therapeutic measure. The objective of this publication was to find a therapeutic procedure to cure this devastating disease. In this study, a biocompatible, easily permeable, cationic nanoparticle-layered double hydroxide was synthesized. Within the layers of these nanoparticles we intercalated α synuclein siRNA, which helps to silence the α synuclein gene. After the intercalation, which was optimized at a 1 : 40 ratio of siRNA : (LDH), we studied its stability in blood by a RNase protection test and serum protection assay. Both proved that LDH was an excellent nanocarrier that can protect intercalated molecules within its layers. After that, several cellular studies were performed by FACS to evaluate its biocompatibility after intercalation and cellular internalization. Results of the biocompatibility studies found it to be nontoxic and in the cellular internalization study, 51.55% of cells were taken into the nanoconjugate and confocal microscopy supported the data from FACS. Lastly, ELISA was performed to discover protein levels in the control, overexpressed, and treated groups of the SH-SY5Y cell line. These results verified that this nanoconjugate is a protective treatment procedure for Parkinson's disease.

Levodopa-induced response fluctuations in patients with Parkinson's disease: strategies for management

Fluctuations in response to levodopa in patients in the advanced stages of idiopathic Parkinson's disease occur frequently and are a difficult problem to treat. Patients who are treated with levodopa have an additional 10% risk of experiencing response fluctuations with each year of treatment: 50% of patients have this problem after 5 years of receiving levodopa therapy and almost 100% of patients after 10 years. The mechanisms by which response fluctuations occur are only partially understood and can be divided into three main types: (i) presynaptic neuronal degeneration leading to a lack of buffering of released levodopa, which is mainly related to wearing-off phenomena; (ii) postsynaptic changes in dopamine receptor sensitivity and number, partially caused by the presynaptic changes, which are clinically related to at-random response fluctuations; and (iii) pharmacokinetic and pharmacodynamic influences of exogenously administered dopaminergic agents. Several oral and parenteral treatment strategies are recommended to manage response fluctuations, such as optimisation of dopamine receptor agonist therapy in combination with a reduction of the levodopa load; use of slow-release levodopa formulations; use of catechol-O-methyltransferase inhibitors; an increase of levodopa dose frequency; use of high-dose amantadine; and intermittent or continuous use of apomorphine and/or levodopa. Continuous stimulation of dopamine receptors with dopaminergic agents is one of the crucial basic steps in the treatment of patients at an advanced stage of Parkinson's disease, and the preferential use of dopamine receptor agonists has proven to be successful in the prevention and treatment of response fluctuations.

Microdialysis study of bromocriptine and its metabolites in rat pituitary and striatum

Bromocriptine, a D2 receptor agonist, was administered intravenously (1mg/kg) to anesthetized rats. Microdialysis probes were implanted in the pituitary and the striatum, known sites of D2 agonist action. Bromocriptine and its metabolites were monitored in plasma and tissue dialysates for 4 h. Drug analyses were performed using two different enzyme immunoassays specific for untransformed bromocriptine or a pool of parent drug plus hydroxylated metabolites. The metabolites/parent drug ratio for areas under the curve was 5.5 in plasma and 1 in the pituitary. No metabolites could be detected in the striatum. Bromocriptine penetration was at least 10-fold greater in the pituitary than in the striatum. The kinetics of bromocriptine in the pituitary and striatum did not parallel those in plasma, indicating that the prolonged action of bromocriptine reported by other authors may be due to slow dissociation from receptors.

Scopolamine prevents tolerance to the effects of caffeine on rotational behavior in 6-hydroxydopamine-denervated rats

Continuous administration of caffeine has been shown to induce tolerance to its psychostimulant effects. In this study, using unilateral 6-hydroxydopamine nigrostriatal denervated rats, we tested the hypothesis that the muscarinic receptor antagonist, scopolamine, would prevent the tolerance to caffeine-induced contralateral rotational behavior. For that purpose we administered either caffeine (40 mg/kg) plus saline or scopolamine (5, 10 and 20 mg/kg) plus saline, as well as caffeine in combination with the various doses of scopolamine for 7 consecutive days, and measured ipsilateral and contralateral rotational behavior. The results showed that acute injections of scopolamine plus saline produced similar levels of both ipsilateral and contralateral turning, while caffeine produced more contralateral than ipsilateral turning. Tolerance to caffeine-induced contralateral turning was observed as of the second administration, while scopolamine plus saline injections did not produce significant changes in rotational behavior with repeated treatment. Scopolamine co-administered with caffeine significantly attenuated the increased contralateral turning produced by acute injections of caffeine plus saline, but significantly prevented the tolerance effects with repeated administration. These findings strongly suggest that muscarinic cholinergic processes may be involved in tolerance to caffeine-induced contralateral turning. The results are interpreted in terms of the possible interactions between dopamine, adenosine and acetylcholine neurotransmitter systems within the basal ganglia circuitry involved in motor behavior.

GlycineB antagonists and partial agonists in rodent models of Parkinson's disease--comparison with uncompetitive N-methyl-D-aspartate receptor antagonist

Antiparkinsonian-like activity of glutamate receptor antagonists (mostly of N-methyl-D-aspartate (NMDA) receptors) has been demonstrated in animals and for uncompetitive agents, also in humans. In the present study we investigated the potential antiparkinsonian-like activity of compounds acting at the glycine site of the NMDA receptor complex in three animal models of Parkinson's disease and compared them with the new uncompetitive NMDA receptor antagonist MRZ 2/579. Haloperidol-induced catalepsy was inhibited by the Merz glycine site antagonists MRZ 2/570, MRZ 2/571 and MRZ 2/576 but not by another antagonist L-701,324 or the glycine site partial agonists ACPC and D-CS. None of the tested glycine site antagonists or partial agonists increased locomotor activity or potentiated L-DOPA responses in reserpine and alpha-MT treated rats. In rats with a unilateral 6-OHDA medial forebrain bundle lesion neither glycine site antagonists nor partial agonists affected rotations on their own or enhanced the contralateral rotations induced by L-DOPA. In contrast, the uncompetitive NMDA receptor antagonist MRZ 2/579 was active in all antiparkinsonian tests used in this study. Based on the present data the therapeutic potential of the glycine site antagonists and partial agonists tested for the treatment of Parkinson's disease is rather doubtful. Uncompetitive NMDA receptor antagonists seem to possess a better profile as antiparkinsonian agents.

Nicotinic acetylcholine receptors in health and disease

Nicotinic acetylcholine receptors (AChRs) are a family of acetylcholine-gated cation channels that form the predominant excitatory neurotransmitter receptors on muscles and nerves in the peripheral nervous system. AChRs are also expressed on neurons in lower amounts throughout the central nervous system. AChRs are even being reported on unexpected cell types such as keratinocytes. Structures of these AChRs are being determined with increasing precision, but functions of some orphan subunits are just beginning to be established. Functional roles for postsynaptic AChRs in muscle are well known, but in neurons the post-, peri-, extra-, and presynaptic roles of AChRs are just being revealed. Pathogenic roles of AChRs are being discovered in many diseases involving mechanisms ranging from mutations, to autoimmune responses, to the unknown; involving cell types ranging from muscles, to neurons, to keratinocytes; and involving signs and symptoms ranging from muscle weakness to epilepsy, to neurodegenerative disease, to psychiatric disease, to nicotine addiction. Awareness of AChR involvement in some of these diseases has provoked new interests in development of therapeutic agonists for specific AChR subtypes and the use of expressed cloned AChR subunits as possible immunotherapeutic agents. Highlights of recent developments in these areas will be briefly reviewed.

Pharmacokinetic-pharmacodynamic interaction between the COMT inhibitor tolcapone and single-dose levodopa

1. Single oral doses of the catechol-O-methyltransferase (COMT) inhibitor tolcapone (10-800 mg) or placebo were administered simultaneously with a dose of levodopa/benserazide 100/25 mg to seven sequential groups of six healthy male subjects in a two-way crossover study. 2. Plasma concentrations of tolcapone, its metabolite 3-O-methyltolcapone, levodopa and 3-O-methyldopa (3-OMD) were determined in conjunction with COMT activity in erythrocytes. 3. The drug combination was well tolerated at all dose levels and there were no signs indicative of an increase in dopaminergic stimulation. 4. Tolcapone caused a rapid and reversible inhibition of COMT activity in erythrocytes in parallel with a dose-dependent decrease in the formation of 3-OMD. Tolcapone increased the area under the concentration-time curve and elimination half-life of levodopa. The maximum effects were obtained at a dose of about 200 mg when both parameters increased approximately twofold. The drug had no influence on the maximum concentration of levodopa. 5. Tolcapone was rapidly absorbed and eliminated with, on average, a tmax of 1.5 h and a t1/2 of 2.3 h. The drug showed dose-proportional pharmacokinetics, in contrast to 3-O-methyltolcapone whose formation was relatively decreased at higher doses. 6. Plasma concentrations of tolcapone correlated with inhibition of COMT activity in erythrocytes and suppression of 3-OMD levels, but not with changes in levodopa pharmacokinetics.

Effect of coadministration of glutamate receptor antagonists and dopaminergic agonists on locomotion in monoamine-depleted rats

Combinations of dopaminergic agonists with glutamate receptor antagonists have been suggested to be a possible alternative treatment of Parkinson's disease. To gain further insights into this possibility, the antagonist of the competitive AMPA-type glutamate receptor NBQX and the ion-channel blocker of the NMDA glutamate receptor (+)-MK-801 in combination with the dopamine D1 receptor agonists: SKF 38393, SKF 82958 and dihydrexidine; the dopamine D2 receptor agonist bromocriptine and the dopamine-precursor L-DOPA were tested in rats pretreated with reserpine and alpha-methyl-p-tyrosine. MK-801 on its own induced locomotor behaviour and potentiated the antiakinetic effects of dihydrexidine and L-DOPA but not of the other dopamine agonists tested. NBQX neither on its own nor coadministered with the dopamine agonists tested had an antiakinetic effect. These results indicate that agents, blocking the ion-channel of the NMDA receptor, might be useful adjuvants to some but not all dopaminomimetics in therapy of Parkinson's disease. The same does not seem to be true for the AMPA-antagonist NBQX.

Role of D1 receptor mechanisms in the potentiation of motor responses to L-dopa and apomorphine by MK 801 in the reserpine-treated mouse

In 24 h reserpine-treated akinetic mice, locomotion was induced by the D1-selective agonist SKF 38393 (30 mg/kg IP), or by the mixed D1/D2 agonists L-dopa (150 mg/kg IP, plus benserazide 100 mg/kg IP) and apomorphine (0.5 mg/kg SC). The non-competitive NMDA receptor antagonist MK 801 (0.01-1.6 mg/kg IP) did not induce motor activity by itself, but potentiated the motor responses to L-dopa and apomorphine at roughly 10-fold lower doses than those which facilitated D1 responding. These data cast doubt on the notion that glutamate antagonists enhance the antiparkinsonian efficacy of mixed D1/D2 agonists solely through a D1 receptor mechanism.

Memantine, amantadine, and L-deprenyl potentiate the action of L-dopa in monoamine-depleted rats

Some treatments used for Parkinson's disease attenuate locomotor depression in rats treated with reserpine and alpha-methyl-p-tyrosine. In the present study memantine (2.5, 5.0 mg/kg), amantadine (10, 20 mg/kg) (both uncompetitive NMDA antagonists), and L-deprenyl (1.0, 5.0 mg/kg; MAO-B inhibitor) were tested for possible synergistic interactions with the dopamine agonists: bromocriptine (2.5, 5.0 mg/kg) and L-dopa (50, 100 mg/kg, +benserazide, 100 mg/kg). At higher doses, memantine (10 mg/kg), amantadine (40 mg/kg), bromocriptine (5 and 10 mg/kg) and L-dopa (100, 200 mg/kg) but not L-deprenyl (up to 10 mg/kg) produced a pronounced increase in locomotor activity when given alone. The combination of memantine, amantadine and L-deprenyl with bromocriptine did not result in synergism of action and, at best, an additive effect was seen. On the other hand the combination of these agents with L-dopa produced a pronounced synergistic effect. Hence, the clinical observation that coadministration of L-dopa with either memantine or amantadine results in enhancement of their action is also reflected in an animal model of Parkinson's disease. Such a combination therapy should allow the use of lower doses of both drugs which may reduce the occurrence of side effects and may also be predicted to have additional benefits related to the neuroprotective properties of memantine, amantadine, and L-deprenyl.

Oxidative stress: free radical production in neural degeneration

It is not yet established whether oxidative stress is a major cause of cell death or simply a consequence of an unknown pathogenetic factor. Concerning chronic diseases, as Parkinson's and Alzheimer's disease are assumed to be, it is possible that a gradual impairment of cellular defense mechanisms leads to cell damage because of toxic substances being increasingly formed during normal cellular metabolism. This point of view brings into consideration the possibility that, besides exogenous factors, the pathogenetic process of neurodegeration is triggered by endogenous mechanisms, either by an endogenous toxin or by inherited metabolic disorders, which become progressively more evident with aging. In the following review, we focus on the oxidative stress theory of neurodegeneration, on excitotoxin-induced cell damage and on impairment of mitochondrial function as three major noxae being the most likely causes of cell death either independently or in connection with each other. First, having discussed clinical, pathophysiological, pathological and biochemical features of movement and cognitive disorders, we discuss the common features of these biochemical theories of neurodegeneration separately. Second, we attempt to evaluate possible biochemical links between them and third, we discuss experimental findings that confirm or rule out the involvement of any of these theories in neurodegeneration. Finally, we report some therapeutic strategies evolved from each of these theories.