Editorial Central & Peripheral Nervous Systems: Dopamine D1receptor agonists and dopamine reuptake blockers: new treatment stratagems for Parkinson's disease

The monoamine reuptake inhibitor BTS 74 398 fails to evoke established dyskinesia but does not synergise with levodopa in MPTP-treated primates

Long-term treatment of Parkinson's disease (PD) with levodopa (L-dopa) induces dyskinesia that, once established, is provoked by each dose of L-dopa or a dopamine (DA) agonist. In contrast, monoamine reuptake inhibitors may reverse motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated primates without provoking established involuntary movements. We now examine whether the potent monoamine reuptake blocker BTS 74 398 induces established dyskinesia in MPTP-treated common marmosets primed previously with L-dopa and whether co-administration of BTS 74 398 with L-dopa potentiates motor behaviour and dyskinesia induced by acute L-dopa treatment. Administration of BTS 74 398 (2.5, 5.0, or 10.0 mg/kg, p.o.) in MPTP-treated common marmosets increased locomotor activity and reduced motor disability in a dose-related manner but did not provoke involuntary movements. BTS 74 398 (2.5, 5.0, or 10.0 mg/kg p.o.) co-administered with a threshold dose of L-dopa (2.5 mg/kg p.o.) did not evoke a motor response or induce dyskinesia. Similarly, concomitant administration of BTS 74 398 (5.0 mg/kg p.o.) with a submaximal L-dopa dose (12.5 mg/kg p.o.) did not potentiate the motor response produced by L-dopa alone and there was no alteration in the dyskinesia provoked by L-dopa challenge. BTS 74 398 reverses motor abnormalities in MPTP-treated marmosets without evoking established dyskinesia but no additive improvement occurs when administered in combination with L-dopa. The lack of synergy with L-dopa may suggest different sites of drug action.

Dopamine reuptake inhibition and failure to evoke dyskinesia in MPTP-treated primates

Nonspecific monoamine reuptake inhibitors reverse motor abnormalities in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated marmosets without evoking established dyskinesia. However, it is not known whether dopamine reuptake inhibition alone explains these actions or whether noradrenaline and/or serotonin reuptake blockade also contributes. L-DOPA (12.5 mg/kg, p.o.) rapidly reversed the baseline locomotor deficits and motor disabilities, but evoked dyskinesia (especially limb chorea) in MPTP-treated common marmosets primed to exhibit involuntary movements. In contrast, the selective dopamine reuptake inhibitor 1-(2-(bis-(4-fluorophenyl)-methoxy)ethyl)-4-(3-phenylpropyl) piperazine dihydrochloride (GBR 12909) reversed motor deficits in a dose-dependent manner but, unlike L-DOPA, did not evoke established dyskinesia in these animals. Therefore, inhibition of dopamine reuptake does not evoke established dyskinesia in MPTP-treated primates.

The monoamine reuptake blocker brasofensine reverses akinesia without dyskinesia in MPTP-treated and levodopa-primed common marmosets

The common marmoset develops motor deficits after MPTP treatment and exhibits dyskinesia after chronic levodopa (L-dopa) dosing and subsequent re-challenge with L-dopa and other dopaminergic agents. We report on the actions of the potent monoamine reuptake blocker brasofensine on motor disability, locomotor activity, and dyskinesia in the 1-methyl-4-1, 2,3,6-tetrahydropyridine (MPTP) -treated marmoset model of Parkinson's disease. Oral administration of brasofensine (0.25, 0.5, 1.0, or 2.5 mg/kg) to MPTP-treated marmosets produced a long-lasting, dose-dependent increase in locomotor activity and reduction in disability scores. In addition, coadministration of the lowest dose of brasofensine (0.25 mg/kg orally) with a threshold oral dose of L-dopa (2.5 mg/kg) caused a marked increase in locomotor activity, greater than that produced by either drug alone. In other MPTP-treated marmosets previously primed to exhibit dyskinesia by repeated L-dopa dosing, brasofensine effectively reversed akinesia with a naturalistic and prolonged motor response without the appearance of dyskinesia or stereotypy. This finding contrasts with the severe dyskinesia, stereotypy, and hyperkinesis produced by equivalent doses of L-dopa. The ability of brasofensine to produce a prolonged and naturalistic antiparkinsonian response without eliciting dyskinesia after previous L-dopa priming may relate to actions on D(1) receptor-linked pathways. These findings suggest that monoamine reuptake blockade may be of value in the treatment of Parkinson's disease, both early in the disease course and when L-dopa-induced dyskinesias complicate treatment.

De novo administration of ropinirole and bromocriptine induces less dyskinesia than L-dopa in the MPTP-treated marmoset

In contrast to levodopa (L-dopa), de novo administration of the D2-like receptor agonist bromocriptine to patients with Parkinson's disease (PD) or to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated subhuman primates is not associated with the onset of significant dyskinesia. We now compare the ability of the novel D2-like selective dopamine agonist ropinirole with that of bromocriptine and L-dopa to induce dyskinesia in MPTP-treated common marmosets. MPTP-treated common marmosets were treated with placebo, L-dopa plus carbidopa, ropinirole, or bromocriptine daily for 30 days (n = 4 per group) in doses that were titrated to similarly increase locomotion and improve motor disability. L-dopa rapidly induced dyskinesia of moderate to severe intensity, whereas ropinirole and bromocriptine produced mild dyskinesia over the course of the study that was significantly less severe than in the L-dopa-treated group (p < 0.05). However, in a separate group of marmosets previously primed with L-dopa to exhibit dyskinesia, ropinirole administration elicited severe dyskinesias comparable with that of L-dopa in a dose-dependent fashion. Ropinirole, in common with bromocriptine, has a lesser tendency than L-dopa to produce dyskinesia while similarly improving motor performance in drug-naive MPTP-treated marmosets. However, in common with other dopamine agonists, ropinirole will elicit comparable dyskinesia once L-dopa priming has occurred. These results predict a similar response to ropinirole and other long-acting dopamine agonists in L-dopa-naive patients with PD and emphasize the importance of avoiding initial dyskinesia induction through early use of dopamine agonist drugs.