Therapies for dopaminergic-induced dyskinesias in parkinson disease

Comparison of the D₁ dopamine full agonists, dihydrexidine and doxanthrine, in the 6-OHDA rat model of Parkinson's disease

RATIONALE

Preclinical evidence indicates that D₁ dopamine receptor full agonists have potential as therapeutic agents for a variety of neurological conditions. Dihydrexidine (DHX) was the first high potency selective D₁ dopamine receptor full agonist and has been studied as a possible treatment for Parkinson's disease (PD). Recently, we discovered doxanthrine (DOX), an oxygen bioisostere of DHX that has even greater selectivity for the D₁ dopamine receptor.

OBJECTIVES

Using the unilateral 6-hydroxydopamine-lesioned rat model of PD, DOX and DHX were compared at several doses (0.625, 1.25, 2.5, or 5.0 mg/kg) for their ability to elicit contralateral rotation by either intraperitoneal injection or oral gavage.

RESULTS

After intraperitoneal administration, both DOX and DHX showed robust contralateral rotation at doses of 2.5 and 5.0 mg/kg compared to vehicle. In addition, after intraperitoneal administration at doses of 2.5 and 5.0 mg/kg, DHX had a significantly longer duration of action than DOX (p < 0.05). Areas under the curves (AUC) for DOX and DHX were not significantly different, however, indicating that DOX and DHX have similar potency after intraperitoneal administration. By contrast, after oral administration, 2.5 and 5.0 mg/kg of DOX produced significant contralateral rotations (p < 0.05), whereas DHX showed no significant activity after oral administration of any dose.

CONCLUSION

These results demonstrate that although DHX and DOX have similar activity after intraperitoneal administration, DOX demonstrated greater activity after oral administration compared to DHX. Despite its catechol functionality, DOX may possess sufficient oral availability for development as a human therapeutic agent.

Domain-dependent effects of DAT inhibition in the rat dorsal striatum

The rat dorsal striatum exhibits domain-dependent kinetics of dopamine release and clearance. The present report describes the domain-dependent actions of nomifensine (20 mg/kg i.p.), a competitive dopamine uptake inhibitor, on evoked dopamine responses recorded by voltammetry during electrical stimulation of the medial forebrain bundle. In slow domains, nomifensine increases the initial rate of evoked overflow, increases response overshoot, does not affect the slope of the linear segment of the dopamine clearance profile, and slows the non-linear segment of the clearance profile. In fast domains, nomifensine does not affect the initial rate of overflow, increases the end-of-stimulus overshoot, and decreases the slope of the linear segment of the dopamine clearance profile. Collectively, these findings do not concur with existing models of evoked dopamine release that describe the effect of nomifensine as an increase in the effective KM of dopamine uptake. These findings suggest that dopamine clearance after evoked release is affected by both dopamine uptake and a restricted extracellular diffusion process.

Ghrelin prevents levodopa-induced inhibition of gastric emptying and increases circulating levodopa in fasted rats

BACKGROUND

Levodopa (L-dopa) is the most commonly used treatment for alleviating symptoms of Parkinson's disease. However, L-dopa delays gastric emptying, which dampens its absorption. We investigated whether ghrelin prevents L-dopa action on gastric emptying and enhances circulating L-dopa in rats.

METHODS

Gastric emptying of non-nutrient methylcellulose/phenol red viscous solution was determined in fasted rats treated with orogastric or intraperitoneal (i.p.) L-dopa, or intravenous (i.v.) ghrelin 10 min before orogastric L-dopa. Plasma L-dopa and dopamine levels were determined by high pressure liquid chromatography. Plasma acyl ghrelin levels were assessed by radioimmunoassay. Fos expression in the brain was immunostained after i.v. ghrelin (30 μg kg(-1)) 10 min before i.p. L-dopa.

KEY RESULTS

Levodopa (5 and 15 mg kg(-1)) decreased significantly gastric emptying by 32% and 62%, respectively, when administered orally, and by 91% and 83% when injected i.p. Ghrelin (30 or 100 μg kg(-1), i.v.) completely prevented L-dopa's (15 mg kg(-1), orogastrically) inhibitory action on gastric emptying and enhanced plasma L-dopa and dopamine levels compared with vehicle 15 min after orogastric L-dopa. Levodopa (5 mg kg(-1)) did not modify plasma acyl ghrelin levels at 30 min, 1, and 2 h after i.v. injection. Levodopa (15 mg kg(-1), i.p.) induced Fos in brain autonomic centers, which was not modified by i.v. ghrelin.

CONCLUSIONS & INFERENCES

Ghrelin counteracts L-dopa-induced delayed gastric emptying but not Fos induction in the brain and enhances circulating L-dopa levels. Potential therapeutic benefits of ghrelin agonists in Parkinson's disease patients treated with L-dopa remain to be investigated.

Rapid and reliable quantitation of amino acids and myo-inositol in mouse brain by high performance liquid chromatography and tandem mass spectrometry

Amino acids and myo-inositol have long been proposed as putative biomarkers for neurodegenerative diseases. Accurate measures and stability have precluded their selective use. To this end, a sensitive liquid chromatography tandem mass spectrometry (LC-MS/MS) method based on multiple reaction monitoring was developed to simultaneously quantify glutamine, glutamate, γ-aminobutyric acid (GABA), aspartic acid, N-acetyl aspartic acid, taurine, choline, creatine, phosphocholine and myo-inositol in mouse brain by methanol extractions. Chromatography was performed using a hydrophilic interaction chromatography silica column within in a total run time of 15 min. The validated method is selective, sensitive, accurate, and precise. The method has a limit of quantification ranging from 2.5 to 20 ng/ml for a range of analytes and a dynamic range from 2.5-20 to 500-4000 ng/ml. This LC-MS/MS method was validated for biomarker discovery in models of human neurological disorders.

New pharmacological avenues for the treatment of L-DOPA-induced dyskinesias in Parkinson's disease: targeting glutamate and adenosine receptors

INTRODUCTION

Parkinson's disease (PD) therapy is still centered on the use of L-3,4-dihydroxyphenylalanine (L-DOPA), which is hampered by numerous side effects, including abnormal involuntary movements known as L-DOPA-induced dyskinesias (LIDs). LIDs are the result of pre- and postsynaptic changes at the corticostriatal level, induced by chronic and pulsatile stimulation of striatal dopaminergic receptors. These changes impact on synaptic plasticity and involve also selected, nondopaminergic receptors expressed by striatal projection neurons.

AREAS COVERED

Among nondopaminergic receptors, glutamate receptors - NMDA and mGluR5 subtypes in particular - and adenosine A(2A) receptors are those most likely involved in LIDs. The aim of the present review is to summarize results of studies undertaken with specific antagonists of these receptors, first conducted in animal models of LIDs, which in selected cases have been translated into clinical trials.

EXPERT OPINION

Selected antagonists of glutamate and adenosine receptors have been proposed as anti-dyskinetic agents. Promising results have been obtained in preclinical investigations and in initial clinical trials, but long-term safety, tolerability and efficacy studies in patients are still required. The current development of novel antagonists, including tools able to act on receptor mosaics, may provide innovative tools for LIDs management in the next future.

Strategies for Parkinson’s disease care: prevention and management of motor fluctuations

SUMMARY Parkinson’s disease (PD) is characterized clinically by the hallmark motor signs of bradykinesia, rest tremor and rigidity. Current pharmacological management goals include control of motor symptoms as well as prevention and management of motor complications including motor fluctuations and dyskinesias. While the use of levodopa revolutionized the pharmacological management of PD, multiple other agents and strategies have emerged with many demonstrable, albeit sometimes controversial, advantages to a ‘levodopa’ only approach. Despite these developments, the progressive nature of PD requires vigilance and creativity from clinicians as both motor and nonmotor complications grow in number and severity over time.