Potential neuroprotection mechanisms in PD: focus on dopamine agonist pramipexole

Three paradoxes related to the mode of action of pramipexole: The path from D2/D3 dopamine receptor stimulation to modification of dopamine-modulated functions

Pramipexole, a D2/D3 dopamine receptor agonist, is used to treat the motor symptoms of Parkinson's disease, caused by degeneration of the dopaminergic nigrostriatal pathway. There are three paradoxes associated with its mode of action. Firstly, stimulation of D2/D3 receptors leads to neuronal inhibition, although pramipexole does not inhibit but promotes some dopamine-modulated functions, such as locomotion and reinforcement. Secondly, another dopamine-modulated function, arousal, is not promoted but inhibited by pramipexole, leading to sedation. Thirdly, pramipexole-evoked sedation is associated with an increase in pupil diameter, although sedation is expected to cause pupil constriction. To resolve these paradoxes, the path from stimulation of D2/D3 receptors to the modification of dopamine-modulated functions has been tracked. The functions considered are modulated by midbrain dopaminergic nuclei: locomotion - substantia nigra pars compacta (SNc), reinforcement/motivation - ventral tegmental area (VTA), sympathetic activity (as reflected in pupil function) - VTA; arousal - ventral periaqueductal grey (vPAG), with contributions from VTA and SNc. The application of genetics-based molecular techniques (optogenetics and chemogenetics) has enabled tracing the chains of neurones from the dopaminergic nuclei to their final targets executing the functions. The functional neuronal circuits linked to the D2/D3 receptors in the dorsal and ventral striata, stimulated by inputs from SNc and VTA, respectively, may explain how neuronal inhibition induced by pramipexole is translated into the promotion of locomotion, reinforcement/motivation and sympathetic activity. As the vPAG may increase arousal mainly by stimulating cortical D1 dopamine receptors, pramipexole would stimulate only presynaptic D2/D3 receptors on vPAG neurones, curtailing their activity and leading to sedation.

P2B001 (Extended Release Pramipexole and Rasagiline): A New Treatment Option in Development for Parkinson's Disease

Despite levodopa’s superior efficacy in reducing the motor symptoms of Parkinson’s disease (PD), its risk to induce motor complications requires consideration of the pros and cons of initiating treatment with levodopa-sparing strategies. The current drive toward early levodopa monotherapy is primarily driven by safety and tolerability concerns with dopamine agonists and only mild efficacy of other available approaches. Recently, P2B001, a novel once-daily combination of low-dose, extended-release formulations of pramipexole and rasagiline (0.6 mg and 0.75 mg respectively), has entered clinical development. In this drug evaluation, we review the preclinical and current clinical data for P2B001 and its components. The P2B001 combination has the potential to provide greater efficacy than either pramipexole or rasagiline alone and a better tolerability profile compared to higher dosage dopamine agonist monotherapy, while maintaining the advantage of lower motor complication risk than levodopa.

Parkinson’s disease is the fastest growing neurologic disorder across the globe. Once diagnosed, it is now generally agreed that there is no clinical rationale to postpone symptomatic treatment in people who develop Parkinson’s-related disability. There are three main treatment options available for use in early Parkinson’s disease: levodopa, dopamine agonists and monoamine oxidase type B (MAO-B) inhibitors. Of these, there is a current push toward using levodopa as the main first-line therapy. This is primarily because of the significant safety and tolerability concerns with dopamine agonists and only mild efficacy of MAO-B inhibitors. Recently, P2B001, a novel drug formulation combining once-daily, extended-release, low dosages of the dopamine agonist pramipexole and the MAO-B inhibitor rasagiline (0.6 mg and 0.75 mg respectively), has entered clinical development. In this article, the authors review the preclinical and current clinical data on P2B001 and its components. The P2B001 combination has the potential to provide greater efficacy than either pramipexole or rasagiline alone and a better tolerability profile compared to higher dosage dopamine agonist monotherapy, while maintaining the advantage of lower motor complication risk than levodopa.

Pramipexole attenuates neuronal injury in Parkinson's disease by targeting miR-96 to activate BNIP3-mediated mitophagy

BACKGROUND

Parkinson's disease is a common neurodegenerative problem. Pramipexole (PPX) plays protective role in Parkinson's disease. Nevertheless, the mechanism of PPX in Parkinson's disease-like neuronal injury is largely uncertain.

METHODS

1-methyl-4-phenylpyridinium (MPP⁺)-stimulated neuronal cells and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mice were used as the model of Parkinson's disease. MPP⁺-induced neuronal injury was assessed via cell viability, lactic dehydrogenase (LDH) release and apoptosis. microRNA-96 (miR-96) and BCL2/adenovirus E1B 19 kDa interacting protein 3 (BNIP3) abundances were examined by quantitative reverse transcription polymerase chain reaction (qRT-PCR) or Western blotting. Mitophagy was tested by Western blotting and immunofluorescence staining. MPTP-induced neuronal injury in mice was investigated via behavioral tests and TUNEL.

RESULTS

PPX alleviated MPP⁺-induced neuronal injury via increasing cell viability and decreasing LDH release and apoptosis. PPX reversed MPP⁺-induced miR-96 expression and inhibition of mitophagy. miR-96 overexpression or BNIP3 interference weakened the suppressive role of PPX in MPP⁺-induced neuronal injury. miR-96 targeted BNIP3 to inhibit PTEN-induced putative kinase 1 (PINK1)/Parkin signals-mediated mitophagy. miR-96 overexpression promoted MPP⁺-induced neuronal injury via decreasing BNIP3. PPX weakened MPTP-induced neuronal injury in mice via regulating miR-96/BNIP3-mediated mitophagy.

CONCLUSION

PPX mitigated neuronal injury in MPP⁺-induced cells and MPTP-induced mice by activating BNIP3-mediated mitophagy via directly decreasing miR-96.

Risk for Parkinson's Disease in Patients with Behçet's Disease: A Nationwide Population-Based Dynamic Cohort Study in Korea

BACKGROUND

Although several studies have reported a correlation between Behçet's disease (BD) and Parkinson's disease, this association has not yet been clarified.

OBJECTIVE

To determine the risk of Parkinson's disease in patients with BD.

METHODS

BD patients (n = 17,179; mean age, 51.12±8.09 years; male, 34.95%) without known previous diagnosis of Parkinson's disease were selected from the Korean National Health Insurance Database between 2010 and 2013. An age- and gender-matched control population of individuals without BD was randomly sampled at a ratio of 3:1 (n = 34,575). Both cohorts were followed for development of Parkinson's disease until 2015.

RESULTS

Among a total of 11,525 BD patients, 50 (0.43%) developed Parkinson's disease, whereas only 51 among the 34,575 individuals in the non-BD group developed Parkinson's (0.15%, P < 0.0001). The incidence of Parkinson's disease was significantly higher in BD patients than in controls. In addition, during the follow-up period, patients with BD were more likely to be diagnosed with Parkinson's disease than those in the non-BD group.

CONCLUSIONS

BD patients had a higher risk of Parkinson's disease compared to non-BD controls. Physicians should carefully monitor patients with BD for potential development of Parkinson's disease.

Nicotine prevents alpha-synuclein accumulation in mouse and human iPSC-derived dopaminergic neurons through activation of the dopamine D3- acetylcholine nicotinic receptor heteromer

We recently found that in mouse dopaminergic neurons, the heteromer formed by the dopamine D3 receptor (D3R) and the β2 subunit of acetylcholine nicotinic receptor (nAChR) exerts neurotrophic effects when activated by nicotine, leading to neurons with enlarged cell bodies and increased dendrite arborization. Beside this action, we now show that nicotine, by activating the D3R-nAChR heteromer, protects dopaminergic neurons against neuronal injury. In primary cultures of mouse dopaminergic neurons, in fact, the ability of nicotine to inhibit both the pathological accumulation of alpha-synuclein induced by glucose deprivation and the consequent morphological defects were strongly prevented by disrupting the D3R-nAChR heteromer with specific interfering TAT-peptides; the relevance of the phosphoinositide 3-kinase (PI3K) intracellular signaling in mediating nicotine prevention of alpha-synuclein aggregation has been also demonstrated. Moreover, the ability of nicotine in restoring the ubiquitin-proteasome system has been found as a mechanism contributing to the neuroprotective properties of nicotine. By using the proximity ligation assay, we have shown that the D3R-nAChR heteromer is also expressed in human dopaminergic neurons derived from induced pluripotent stem cells. In this human cell model, nicotine exerts neuroprotective effects specifically acting through the D3R-nAChR complex thus indicating that this heteromer is a relevant molecular effector involved in the protection of human dopaminergic neurons.

The effect of pramipexole extended release on the levodopa equivalent daily dose in Lebanese Parkinson diseased patients

Objective

The objective of this study is to compute the potential benefit of Pramipexole ER on total levodopa equivalent dose (LED) and Unified Parkinson Disease Rating Score (UPDRS-III) compared to mono- or combined therapy of pramipexole IR and/or carbidopa/levodopa.

Methods

This is a retrospective observational study conducted in a specialized PD (Parkinson Disease) and movement disease center in Lebanon between January and December 2017.

Results

A total of 176 patient's record was reviewed. Pramipexole ER showed a significant difference on the mean changes in LED and UPDRS-III score. After 13 weeks of initiating Pramipexole ER, the mean decrease in LED was -49.42 mg for all patients (p < 0.001, CI 95% [35.28-63.55]) and the mean decrease in UPDRS-III score for all patients was -6 points (P< 0.001).According to the subgroup analysis, patients aged 65 years and below, the change in mean total LED from baseline (350.80 mg) was a decrease of 63.19 mg with a p<0.001, CI 95% [42.07-84.31]. In patients aged more than 65 years and shifted to mono or combined pramipexole ER therapy, the change in mean total LED from baseline (559.25 mg) was a decrease of 34.67 mg with a p<0.001 CI 95% [16.16-53.18]. In addition the results showed that in patients having an UPDRS-III score of less than or equal to 33, the change in mean total LED from baseline (436.73 mg) was a decrease of 56.76 mg (p<0.001; CI 95% [41.32-72.20]). However, in patients having an UPDRS-III score of more than 33 the change in mean total LED from baseline (545.06 mg) was a decrease of 2.96 mg with a p value < 0.844 CI 95% [27.32-33.15].

Conclusions

This study demonstrated the efficacy of Pramipexole ER on decreasing the total levodopa equivalent dose (LED).The role of health care professionals is to maintain the patient on the lowest effective levodopa equivalent daily dose and optimize the treatment therapy, thus decreasing the side effects that might arise from overdosing of antiparkinsonian drugs.

Analysis of pramipexole dose-response relationships in Parkinson's disease

BACKGROUND

Pramipexole (PPX), a non-ergot dopamine receptor agonist, is a first-line treatment for Parkinson's disease (PD). A critical dose level above which a better benefit-to-harm ratio exists has not been examined.

METHODS

Chinese PD patients (n=464) were retrospectively analyzed by PPX maintenance dose, PD stage, combined levodopa dose, and baseline tremor contribution. The sum score of Baseline Activities of Daily Living (part II) and Motor Examination (III) of the Unified Parkinson's Disease Rating Scale (UPDRS II+III) was used as a covariate for final score adjustment.

RESULTS

Sustained-release (SR) and immediate-release (IR) PPX showed similar efficacy based on score changes at 18 weeks, with comparable tolerability. Approximately two-third of patients received PPX at ≥1.5 mg/d, and one fourth of patients had ≥20% tremor contribution to UPDRS II+III. After treatment, patients receiving PPX ≥1.5 mg/d showed better improvement in UPDRS II+III scores (P=0.0025), with similar trends with the IR and SR formulations. Patients with ≥20% tremor contribution showed better improvement in UPDRS II+III scores (P=0.0017). No differences were seen based on PD stage or combined levodopa dose. The overall proportions of adverse events (AEs) were similar. More patients discontinued because of intolerable side effects, and more investigator-defined drug-related AEs were recorded in the <1.5 mg/d subgroup.

CONCLUSION

UPDRS II+III improvement was better with PPX ≥1.5 than with <1.5 mg/d in Chinese PD patients after 18 weeks of treatment, with similar trends seen with IR and SR formulations. The frequency of AEs in PPX ≥1.5 and <1.5 mg/d subgroups was similar.

Medicinal plant Combretum leprosum mart ameliorates motor, biochemical and molecular alterations in a Parkinson's disease model induced by MPTP

ETHNOPHARMACOLOGICAL RELEVANCE

Combretum leprosum is a popular medicinal plant distributed in north and northeastern regions of Brazil. Many different parts of this plant are used in traditional medicine to treat several inflammatory diseases. Parkinson's disease (PD) is a disorder associated with inflammatory toxic factors and the treatments available provide merely a delay of the neurodegeneration.

AIM OF THE STUDY

We investigated the potential neuroprotective properties of the C. leprosum ethanolic extract (C.l.EE) in a murine model of PD using the toxin 1-methyl-4 phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP).

MATERIALS AND METHODS

The mice were split into four groups: V/S (vehicle/saline), E/S (extract/saline), V/M (vehicle/MPTP) and E/M (extract/ MPTP). Mice received MPTP (30mg/kg, i.p.) or vehicle (10ml/kg, i.p.) once a day for 5 consecutive days and vehicle (10ml/kg) or C.l.EE (100mg/kg) orally by intra-gastric gavage (i.g.) during a 14-d period, starting 3 days before the first MPTP injection. All groups were assessed for behavioural impairments (amphetamine-induced locomotor activity and muscle strength), dopamine content in striatum using high performance liquid chromatography (HPLC), tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene expressions using qPCR.

RESULTS

Animals were injected with d-amphetamine (2mg/kg) and the activity was recorded. Amphetamine-induced hyperlocomotion was observed in all groups; however animals treated with MPTP showed exacerbated hyperlocomotion (approximately 3 fold increase compared to control groups). By contrast, mice treated with MPTP that received C.l.EE exhibited attenuation of the hyperlocomotion and did not differ from control groups. Muscle strength test pointed that C.l.EE strongly avoided muscular deficits caused by MPTP (approximately 2 fold increase compared to V/M group). Dopamine and its metabolites were measured in the striatum. The V/M group presented a dopamine reduction of 80%. On the other hand, the E/M group exhibited an increase in dopamine and its metabolites levels (approximately 3 fold increase compared to V/M group). Tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene expressions were significantly reduced in the V/M group (60%). Conversely, C.l.EE treatment was able to increase the mRNA levels of those genes in the E/M group (approximately 2 fold for TH and DAT).

CONCLUSIONS

These data show, for the first time, that C. leprosum ethanolic extract prevented motor and molecular changes induced by MPTP, and partially reverted dopamine deficit. Thus, our results demonstrate that C.l.EE has potential for the treatment and prevention of PD.

Effects of ceftriaxone on ethanol, nicotine or sucrose intake by alcohol-preferring (P) rats and its association with GLT-1 expression

Increased glutamatergic neurotransmission appears to mediate the reinforcing properties of drugs of abuse, including ethanol (EtOH). We have shown that administration of ceftriaxone (CEF), a β-lactam antibiotic, reduced EtOH intake and increased glutamate transporter 1 (GLT-1) expression in mesocorticolimbic regions of male and female alcohol-preferring (P) rats. In the present study, we tested whether CEF administration would reduce nicotine (NIC) and/or EtOH intake by adult female P rats. P rats were randomly assigned to 4 groups: (a) 5% sucrose (SUC) or 10% SUC [SUC], (b) 5% SUC+0.07mg/ml NIC and 10% SUC+0.14mg/ml NIC [NIC-SUC], 15% EtOH and 30% EtOH [EtOH] and (d) 15% EtOH+0.07mg/ml NIC and 30% EtOH+0.14mg/ml NIC [NIC-EtOH]. After achieving stable intakes (4weeks), the rats were administered 7 consecutive, daily i.p. injections of either saline or 200mg/kg CEF. The effects of CEF on intake were significant but differed across the reinforcers; such that ml/kg/day SUC was reduced by ∼30%, mg/kg/day NIC was reduced by ∼70% in the NIC-SUC group and ∼40% in the EtOH-NIC group, whereas g/kg/day EtOH was reduced by ∼40% in both the EtOH and EtOH-NIC group. The effects of CEF on GLT-1 expression were also studied. We found that CEF significantly increased GLT-1 expression in the prefrontal cortex and the nucleus accumbens of the NIC and NIC-EtOH rats as compared to NIC and NIC-EtOH saline-treated rats. These findings provide further support for GLT-1-associated mechanisms in EtOH and/or NIC abuse. The present results along with previous reports of CEF's efficacy in reducing cocaine self-administration in rats suggest that modulation of GLT-1 expression and/or activity is an important pharmacological target for treating polysubstance abuse and dependence.

Prolonged treatment with pramipexole promotes physical interaction of striatal dopamine D3 autoreceptors with dopamine transporters to reduce dopamine uptake

The dopamine (DA) transporter (DAT), a membrane glycoprotein expressed in dopaminergic neurons, clears DA from extracellular space and is regulated by diverse presynaptic proteins like protein kinases, α-synuclein, D2 and D3 autoreceptors. DAT dysfunction is implicated in Parkinson's disease and depression, which are therapeutically treated by dopaminergic D2/D3 receptor (D2/D3R) agonists. It is, then, important to improve our understanding of interactions between D3R and DAT. We show that prolonged administration of pramipexole (0.1mg/kg/day, 6 to 21 days), a preferential D3R agonist, leads to a decrease in DA uptake in mouse striatum that reflects a reduction in DAT affinity for DA in the absence of any change in DAT density or subcellular distribution. The effect of pramipexole was absent in mice with genetically-deleted D3R (D3R(-/-)), yet unaffected in mice genetically deprived of D2R (D2R(-/-)). Pramipexole treatment induced a physical interaction between D3R and DAT, as assessed by co-immunoprecipitation and in situ proximity ligation assay. Furthermore, it promoted the formation of DAT dimers and DAT association with both D2R and α-synuclein, effects that were abolished in D3R(-/-) mice, yet unaffected in D2R(-/-) mice, indicating dependence upon D3R. Collectively, these data suggest that prolonged treatment with dopaminergic D3 agonists provokes a reduction in DA reuptake by dopaminergic neurons related to a hitherto-unsuspected modification of the DAT interactome. These observations provide novel insights into the long-term antiparkinson, antidepressant and additional clinical actions of pramipexole and other D3R agonists.

Mavoglurant as a treatment for Parkinson's disease

INTRODUCTION

A major unresolved issue in the Parkinson's disease (PD) treatment is the development of l-DOPA-induced dyskinesias (LIDs) as a side effect of chronic L-DOPA administration. Currently, LIDs are managed in part by reducing the L-DOPA dose or by the administration of amantadine. However, this treatment is only partially effective. A potential strategy, currently under investigation, is the coadministration of metabotropic glutamate receptor 5 (mGluR5) negative allosteric modulators (NAMs) and L-DOPA; a treatment that results in the improvement of dyskinesia symptoms and that permits reductions in l-DOPA dosage frequency.

AREAS COVERED

The authors examine the role of mGluR5 in the pathophysiology of PD and the potential use of mGluR5 NAM as an adjuvant therapy together with a primary treatment with L-DOPA. Specifically, the authors look at the mavoglurant therapy and the evidence presented through preclinical and clinical trials.

EXPERT OPINION

Interaction between mGluR5 NAM and L-DOPA is an area of interest in PD research as concomitant treatment results in the improvement of LID symptoms in humans, thus enhancing the patient's quality of life. However, few months ago, Novartis decided to discontinue clinical trials of mavoglurant for the treatment of LID, due to the lack of efficacy demonstrated in trials NCT01385592 and NCT01491529, although no safety concerns were involved in this decision. Nevertheless, the potential application of mGluR5 antagonists as neuroprotective agents must be considered and further studies are warranted to better investigate their potential.

Assessing the roles of stimulants/stimulant-like drugs and dopamine-agonists in the treatment of bipolar depression

Bipolar depression is considered the most difficult-to-treat phase of bipolar disorder, in relation to its pervasiveness and efficacy and/or tolerability limitations of available treatments. Indeed, most mood stabilizers and atypical antipsychotics are not as effective in ameliorating depressive compared with manic symptoms, and entail substantial tolerability limitations. However, the use of antidepressants is highly controversial, as their efficacy appears less robust in bipolar compared with unipolar depression. In addition, antidepressants, in spite of generally having adequate somatic tolerability, in BD may be associated with a higher risk of manic/hypomanic switch, suicidality and rapid cycling. Among alternative pharmacological strategies, compounds with stimulant and pro-dopaminergic effects, such as methylphenidate, modafinil, armodafinil and pramipexole, have showed potential antidepressant activity, even though their use in clinical practice has been limited by the paucity of controlled evidence. This article seeks to review available evidence about the use of the aforementioned compounds in the treatment of bipolar depression. Findings from reviewed studies suggested that pro-dopaminergic compounds, such as pramipexole and stimulants/stimulant-like agents, deserve consideration as adjunctive therapies in bipolar depressed patients, at least in some subgroups of patients. Nevertheless, caution regarding their use is recommended as further clinical trials with larger samples and longer follow-up periods are necessary to clarify the roles of these medications in bipolar depression.

The role of inflammatory and oxidative stress mechanisms in the pathogenesis of Parkinson's disease: focus on astrocytes

Neuroinflammation plays a key role in the pathogenesis of Parkinson's disease (PD). Epidemiologic, animal, human, and therapeutic studies support the role of oxidative stress and inflammatory cascade in initiation and progression of PD. In Parkinson's disease pathophysiology, activated glia affects neuronal injury and death through production of neurotoxic factors like glutamate, S100B, tumor necrosis factor alpha (TNF-α), prostaglandins, and reactive oxygen and nitrogen species. As disease progresses, inflammatory secretions engage neighboring cells, including astrocytes and endothelial cells, resulting in a vicious cycle of autocrine and paracrine amplification of inflammation leading to neurodegeneration. The exact mechanism of these inflammatory mediators in the disease progression is still poorly understood. In this review, we highlight and discuss the mechanisms of oxidative stress and inflammatory mediators by which they contribute to the disease progression. Particularly, we focus on the altered role of astroglial cells that presumably initiate and execute dopaminergic neurodegeneration in PD. In conclusion, we focus on the molecular mechanism of neurodegeneration, which contributes to the basic understanding of the role of neuroinflammation in PD pathophysiology.

The neurobiology of depression and antidepressant action

We present a comprehensive overview of the neurobiology of unipolar major depression and antidepressant drug action, integrating data from affective neuroscience, neuro- and psychopharmacology, neuroendocrinology, neuroanatomy, and molecular biology. We suggest that the problem of depression comprises three sub-problems: first episodes in people with low vulnerability ('simple' depressions), which are strongly stress-dependent; an increase in vulnerability and autonomy from stress that develops over episodes of depression (kindling); and factors that confer vulnerability to a first episode (a depressive diathesis). We describe key processes in the onset of a 'simple' depression and show that kindling and depressive diatheses reproduce many of the neurobiological features of depression. We also review the neurobiological mechanisms of antidepressant drug action, and show that resistance to antidepressant treatment is associated with genetic and other factors that are largely similar to those implicated in vulnerability to depression. We discuss the implications of these conclusions for the understanding and treatment of depression, and make some strategic recommendations for future research.

Amyotrophic lateral sclerosis and the clinical potential of dexpramipexole

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder that leads to progressive weakness from loss of motor neurons and death on average in less than 3 years after symptom onset. No clear causes have been found and just one medication, riluzole, extends survival. Researchers have identified some of the cellular processes that occur after disease onset, including mitochondrial dysfunction, protein aggregation, oxidative stress, excitotoxicity, inflammation, and apoptosis. Mitochondrial disease may be a primary event in neurodegeneration or occur secondary to other cellular processes, and may itself contribute to oxidative stress, excitotoxicity, and apoptosis. Clinical trials currently aim to slow disease progression by testing drugs that impact one or more of these pathways. While every agent tested in the 18 years after the approval of riluzole has been ineffective, basic and clinical research methods in ALS have become dramatically more sophisticated. Dexpramipexole (RPPX), the R(+) enantiomer of pramiprexole, which is approved for symptomatic treatment of Parkinson disease, carries perhaps the currently largest body of pre-and early clinical data that support testing in ALS. The neuroprotective properties of RPPX in various models of neurodegeneration, including the ALS murine model, may be produced through protective actions on mitochondria. Early phase trials in human ALS suggest that the drug can be taken safely by patients in doses that provide neuroprotection in preclinical models. A Phase III trial to test the efficacy of RPPX in ALS is underway.

Stereochemical and neuroanatomical selectivity of pramipexole effects on sensorimotor gating in rats

BACKGROUND

In rats, prepulse inhibition (PPI) of acoustic startle is disrupted by systemic administration of dopaminergic agonists, such as the dopamine D3 receptor (D3R)-preferential agonist pramipexole (PPX). PPX has D3R-active (S) and -inactive (R) stereoisomers. Here, we tested the neuroanatomical and stereochemical selectivity of PPX effects on PPI.

METHODS

(S)-PRA or (R)-PRA (0, 0.47, 1.42, 4.73 μmol/kg) was injected sc 15 min prior to PPI testing in adult male Sprague Dawley rats. In separate rats, (S)-PPX (0, 3, 10 μg/0.5μl/side, ic) was infused into the nucleus accumbens (NAc), caudodorsal striatum (CS), or olfactory tubercle/Islands of Calleja (ICj) 15 min prior to PPI testing. D3R expression in these brain regions was assessed using quantitative rt-PCR. The PPI-disruptive effects of systemic (S)-PPX were also tested after pretreatment with the D3R-selective antagonist, U99194 (10mg/kg).

RESULTS

Systemic administration of PPX stereoisomers demonstrated a dose-dependent effect of (S)-PPX on PPI, while (R)-PPX had no effect on PPI. PPX decreased PPI when infused into the NAc and ICj, but not the CS. Quantitative rt-PCR revealed D3R expression in ICj>NAc>CS. The PPI-disruptive effects of PPX were prevented by U99194.

CONCLUSION

The PPI-reducing effects of PPX are stereospecific for the D3R-active (S)-isomer, neuroanatomically preferential for the D3R-rich ventral vs. D3R poor caudodorsal striatum, and prevented by pharmacologic D3R blockade. These findings are consistent with the conclusion that PPX disrupts PPI via stimulation of mesolimbic D3Rs.

Pramipexole and its extended release formulation for Parkinson's disease

Pramipexole has been a widely used dopamine agonist for the last decade. Recently an extended release formulation of pramipexole has been introduced as both monotherapy for patients with early Parkinson’s disease as well as for patients with more advanced disease, as an adjunct to L-DOPA. Along with the enhanced patient compliance seen with once a day dosing, there are other potential advantages of extended release preparations of dopamine agonists. Patients initiated on pramipexole have a lower incidence of developing motor fluctuations including dyskinesia than those initiated on L-DOPA. Pramipexole requires a prolonged dose titration compared to L-DOPA, and generally does not have the efficacy of L-DOPA. The extended release form of pramipexole shows comparable mean and peak serum levels with once a day dosing as seen with three times a day dosing of the immediate release preparation. The extended release preparation has been studied in randomized multicenter clinical trial against both placebo and the immediate release preparation in the setting of early Parkinson’s disease as monotherapy and in more advanced patients with motor fluctuations on L-DOPA. In both settings the extended release preparation was superior to placebo and comparable to the immediate release form in efficacy with a similar side effect profile including nausea, sleepiness, leg edema, dyskinesias, hallucinations and impulse control disorders.

Role of pramipexole in the management of Parkinson's disease

The non-ergot dopamine agonist pramipexole is currently indicated for the treatment of the signs and symptoms of idiopathic Parkinson's disease and for the treatment of moderate-to-severe primary restless legs syndrome. A new extended-release formulation of pramipexole has now also been launched in Europe and the US to improve ease of use, compliance and provide a more continuous therapeutic effect over 24 hours. Before initiating any treatment, the benefit-risk ratio to the individual patient must be considered. For pramipexole in the treatment of Parkinson's disease, this means taking into account the available evidence regarding its symptomatic efficacy, effect on delaying long-term levodopa-related motor complications, beneficial effect on non-motor symptoms such as depression, and its safety and tolerability profile. Studies have shown that pramipexole is effective as monotherapy in early Parkinson's disease and as adjunctive therapy in advanced disease. Trials further suggest that the benefits of pramipexole may extend beyond the relief of motor symptoms (akinesia, rigidity and tremor at rest) to the amelioration of depressive symptoms in Parkinson's disease. Pramipexole is generally well tolerated; however, compared with levodopa treatment, pramipexole is associated with a higher rate of some dopaminergic adverse effects.

Neuroprotection of pramipexole in UPS impairment induced animal model of Parkinson's disease

Pramipexole (PPX), a dopamine (DA) receptor D3 preferring agonist, has been used as monotherapy or adjunct therapy to treat Parkinson's disease (PD) for many years. Several in vitro and in vivo studies in neurotoxin-induced DA neuron injury models have reported that PPX may possess neuroprotective properties. The present study is to evaluate the neuroprotection of PPX in a sustained DA neuron degeneration model of PD induced by ubiquitin-proteasome system (UPS) impairment. Adult C57BL/6 mice were treated with PPX (low dose 0.1 mg/kg or high dose 0.5 mg/kg, i.p, twice a day) started 7 days before, and continued after microinjection of proteasome inhibitor lactacystin in the medial forebrain bundle for a total 4 weeks. Animal behavior observation, and pathological and biochemical assays were conducted to determine the neuroprotective effects of PPX. We report here that PPX treatment significantly improves rotarod performance, attenuates DA neuron loss and striatal DA reduction, and alleviates proteasomal inhibition and microglial activation in the substantia nigra of lactacystin-lesioned mice. PPX can increase the levels of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor and induce an activation of autophagy. Furthermore, pretreatment with D3 receptor antagonist U99194 can significantly block the PPX-mediated neuroprotection. These results suggest that multiple molecular pathways may be attributed to the neuroprotective effects of PPX in the UPS impairment model of PD.

Pramipexole extended release: in Parkinson's disease

Pramipexole extended release (ER) is a non-ergolinic dopamine receptor agonist available for use as a once-daily oral treatment for the signs and symptoms of early and advanced idiopathic Parkinson's disease. Once-daily pramipexole ER and three times-daily pramipexole immediate release (IR) have similar exposure over 24 hours. The ER formulation is associated with fewer fluctuations in plasma pramipexole concentrations over this period. Pramipexole ER improved the symptoms of Parkinson's disease in three well designed trials in adults with early or advanced disease, as measured by changes from baseline in the sum of the Unified Parkinson's Disease Rating Scale (UPDRS) parts II and III subtotal scores. In a 9-week study, the majority of patients with early Parkinson's disease who were receiving stable pramipexole IR treatment were successfully switched to pramipexole ER. Relative to placebo at week 18, pramipexole ER 0.375-4.5 mg (of the salt) once daily significantly decreased the sum of the UPDRS parts II and III subtotal scores from baseline in two trials in patients with early or advanced Parkinson's disease, and also reduced the percentage of off-time during waking hours in patients with advanced disease. The efficacy of pramipexole ER was maintained after 33 weeks of treatment in the trials in patients with early or advanced Parkinson's disease. Pramipexole ER was generally well tolerated in patients with Parkinson's disease, with the rate of adverse events being generally similar to that with pramipexole IR.