Lance A. Smith

Combined Use of the Adenosine A2A Antagonist KW-6002 with l-DOPA or with Selective D1 or D2 Dopamine Agonists Increases Antiparkinsonian Activity but Not Dyskinesia in MPTP-Treated Monkeys

The novel selective adenosine A(2A) receptor antagonist KW-6002 improves motor disability in MPTP-treated parkinsonian marmosets without provoking dyskinesia. In this study we have investigated whether KW-6002 in combination with l-DOPA or selective D1 or D2 dopamine receptor agonists enhances antiparkinsonian activity in MPTP-treated common marmosets. Combination of KW-6002 with the selective dopamine D2 receptor agonist quinpirole or the D1 receptor agonist SKF80723 produced an additive improvement in motor disability. Coadministration of KW-6002 with a low dose of L-DOPA also produced an additive improvement in motor disability, and increased locomotor activity. The ability of KW-6002 to enhance antiparkinsonian activity was more marked with L-DOPA and quinpirole than with the D1 agonist. However, despite producing an enhanced antiparkinsonian response KW-6002 did not exacerbate L-DOPA-induced dyskinesia in MPTP-treated common marmosets previously primed to exhibit dyskinesia by prior exposure to L-DOPA. Selective adenosine A(2A) receptor antagonists, such as KW-6002, may be one means of reducing the dosage of L-DOPA used in treating Parkinsons disease and are potentially a novel approach to treating the illness both as monotherapy and in combination with dopaminergic drugs.

Multiple small doses of levodopa plus entacapone produce continuous dopaminergic stimulation and reduce dyskinesia induction in MPTP-treated drug-naïve primates

Long‐acting dopamine agonist drugs induce a lower incidence of dyskinesia in MPTP‐treated primates and patients with Parkinsons disease compared to pulsatile treatment with levodopa, supporting the concept of continuous dopaminergic stimulation as a means of dyskinesia avoidance. We examined the effects of L‐dopa administered with or without the COMT inhibitor entacapone on dyskinesia induction in previously untreated MPTP‐treated common marmosets. Administration of L‐dopa (12.5 mg/kg p.o.) plus carbidopa twice daily produced fluctuating improvement in motor behavior coupled with dyskinesia. Coadministration with entacapone produced similar patterns of motor improvement and dyskinesia that were not different from that produced by L‐dopa alone. Treatment with L‐dopa (6.25 mg/kg p.o.) plus carbidopa four times daily reversed motor disability and induced dyskinesia in a manner that was not different from the twice‐daily treatment regimens. However, coadministration with entacapone produced more continuous improvement in locomotor activity with less dyskinesia than animals treated with L‐dopa four times daily alone. These data support the notion that pulsatile stimulation contributes to the development of dyskinesia and suggests that more frequent dosing of L‐dopa plus entacapone may be a useful treatment strategy for patients in the early stages of Parkinsons disease.

Antiparkinsonian activity of Ro 25-6981, a NR2B subunit specific NMDA receptor antagonist, in animal models of Parkinson's disease

N-methyl-D-aspartate (NMDA) receptor antagonists have antiakinetic and antidyskinetic effects in animals models of Parkinsons disease (PD). However, non-selective inhibition of NMDA receptors throughout the central nervous system may result in undesired effects such as ataxia and psychosis. We therefore studied Ro 25-6981, an activity-dependent antagonist of NMDA receptors containing the NR2B subunit which are predominantly expressed in the striatum. Ro 25-6981 induced contraversive rotations in 6-hydroxydopamine (6-OHDA)-lesioned rats without stimulating locomotion in normal rats and reversed parkinsonian symptoms in 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine (MPTP)-treated common marmosets. Due to the small number of marmosets, there were no significant differences between Ro 25-6981 and vehicle though there was a significant trend toward differences, as shown by the Page test. Furthermore, Ro 25-6981 potentiated the action of levodopa in both species and attenuated the maximal levodopa response in 6-OHDA-lesioned rats chronically treated with levodopa without reducing the overall response. Ro 25-6981 also potentiated the action of the dopamine receptor agonists apomorphine, A68930 and quinpirole in 6-OHDA-lesioned rats. The present observations suggest a therapeutic potential of NR2B-selective NMDA receptor antagonists in the management of PD.

Effect of pulsatile administration of levodopa on Dyskinesia induction in drug-naive MPTP-treated common marmosets: Effect of dose, frequency of administration, and brain exposure

Levodopa (L‐dopa) consistently primes basal ganglia for the appearance of dyskinesia in parkinsonian patients and 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine hydrochloride (MPTP) ‐treated primates. This finding may reflect its relatively short duration of effects resulting in pulsatile stimulation of postsynaptic dopamine receptors in the striatum. We have compared the relationship between L‐dopa dose and frequency of administration on dyskinesia initiation in drug‐naïve, MPTP‐treated common marmosets. We have also studied the effect of increased brain exposure to pulsatile administration by combining a low‐dose of L‐dopa with the peripheral catechol‐O‐methyltransferase inhibitor (COMT‐I), entacapone. Pulsatile administration of a low (dose range, 5.0–7.5 mg/kg p.o.) or a high (12.5 mg/kg) dose of L‐dopa plus carbidopa b.i.d. produced a dose‐related reversal of motor deficits. Repeated administration of low and high doses of L‐dopa for 26 days to drug‐naïve, MPTP‐treated animals also caused a dose‐related induction of peak‐dose dyskinesia. Repeated administration of high‐dose L‐dopa b.i.d. compared to once daily caused a frequency‐related improvement of motor symptoms, resulting in a more rapid and initially more intense appearance of peak‐dose dyskinesia. Administration of low‐dose L‐dopa b.i.d. for 26 days in combination with entacapone enhanced the increase in locomotor activity and reversal of disability produced by L‐dopa alone, but with no obvious change in duration of L‐dopas effect. However, combining entacapone with L‐dopa resulted in the more rapid appearance of dyskinesia, which was initially more severe than occurred with L‐dopa alone. Importantly, increasing pulsatile exposure of brain to L‐dopa by preventing its peripheral breakdown also increases dyskinesia induction.

A modified MPTP treatment regime produces reproducible partial nigrostriatal lesions in common marmosets

Standard MPTP treatment regimens in primates result in > 85% destruction of nigral dopaminergic neurons and the onset of marked motor deficits that respond to known symptomatic treatments for Parkinsons disease (PD). The extent of nigral degeneration reflects the late stages of PD rather than events occurring at its onset. We report on a modified MPTP treatment regimen that causes nigral dopaminergic degeneration in common marmosets equivalent to that occurring at the time of initiation of motor symptoms in man. Subcutaneous administration of MPTP 1 mg/kg for 3 consecutive days caused a reproducible 60% loss of nigral tyrosine hydroxylase (TH)‐positive cells, which occurred mainly in the calbindin‐D28k‐poor nigrosomes with a similar loss of TH‐immunoreactivity (TH‐ir) in the caudate nucleus and the putamen. The animals showed obvious motor abnormalities with reduced bursts of activity and the onset of motor disability. However, the loss of striatal terminals did not reflect early PD because a greater loss of TH‐ir occurred in the caudate nucleus than in the putamen and a marked reduction in TH‐ir occurred in striatal patches compared to the matrix. Examination of striatal fibres following a partial MPTP lesion showed a conspicuous increase in the number and the diameter of large branching fibres in the putaminal and to some extent caudatal matrix, pointing to a possible compensatory sprouting of dopaminergic terminals. In addition, these partially lesioned animals did not respond to acute treatment with L‐DOPA. This primate partial lesions model may be useful for examining potential neuroprotective or neurorestorative agents for PD.

Dopamine, but not norepinephrine or serotonin, reuptake inhibition reverses motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primates

Monoamine reuptake inhibitors that do not discriminate between the transporters for dopamine (DA), norepinephrine (NE), or 5-hydroxytryptamine (5-HT, serotonin) can reverse locomotor deficits and motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets. DA reuptake inhibition is presumed to be primarily responsible, but the role played by inhibition of NE and 5-HT reuptake is unknown. We now evaluate the efficacy of a range of monoamine reuptake inhibitors either alone or in combination in MPTP-treated common marmosets to determine the actions required for effective antiparkinsonian activity. Monoamine reuptake inhibitors not discriminating between the DA, NE, and 5-HT transporters [1-[1-(3,4-dichlororphenyl)cyclobutyl]-2-(3-diaminethylaminopropylthio)ethanone monocitrate (BTS 74 398) and nomifensine] reversed locomotor deficits and motor disability in MPTP-treated marmosets but bupropion was without effect. The selective DA reuptake inhibitor 1-(2-(bis-(4-fluorophenyl)-methoxy)ethyl)-4-(3-phenylpropyl) piperazine) dihydrochloride (GBR 12909) also reversed these motor deficits. The relative efficacy of the compounds (BTS 74 398 > GBR 12909 > nomifensine ≫ bupropion) paralleled their potency in inhibiting DA uptake in vitro and in vivo. In contrast, the selective NE reuptake inhibitor nisoxetine and the 5-HT reuptake inhibitor sertraline administered alone failed to improve motor function and tended to worsen the deficits. Coadministration of nisoxetine attenuated the improvement in motor deficits produced by GBR 12909. Coadministration of sertraline also abolished the reversal of motor deficits produced by GBR 12909. Coadministration of both sertraline and nisoxetine similarly abolished the improvement of motor deficits produced by GBR 12909. Molecules possessing potent DA reuptake inhibitory activity may be useful in the treatment of the motor symptoms of Parkinsons disease. In contrast, there seems to be no role for NE or 5-HT reuptake inhibitors, and they may impair antiparkinsonian activity mediated through dopaminergic mechanisms.

Differential anti-parkinsonian effects of benzazepine D1 dopamine agonists with varying efficacies in the MPTP-treated common marmoset

In common marmosets systemically treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), the behavioural effects of benzazepine D1 dopamine (DA) agonists with full/supramaximal (SKF 80723 and SKF 82958), partial (SKF 38393, SKF 75670 and SKF 83565) and no efficacies (SKF 83959) in stimulating adenylate cyclase (AC) activity were investigated. The benzazepine derivatives, with the exception of SKF 82958 (8 fold D1 DA receptor selectivity), demonstrated high D1 DA receptor affinity and selectivity (approximately 100 fold or more) in rat striatal homogenates. Administration of MPTP in marmosets induced locomotor hypoactivity, rigidity and motor disability. SKF 38393 (7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and SKF 75670 (3-CH3 analogue) further reduced locomotor activity (by −70 to −80%) and increased motor disability (by +22 to +67%) in these animals. SKF 83565 (6-Cl, 3-CH3, 3′-Cl analogue) and SKF 82958 (6-Cl, 3-C3H5 analogue) had only a slight effect on locomotor activity but decreased motor disability at high doses (−46 to −60%). In contrast, SKF 83959 (6-Cl, 3-CH3, 3′-CH3 analogue) and SKF 80723 (6-Br analogue) produced pronounced increases in locomotion (6–10 fold) and a reversal in motor disability (by −64 to −77%). Oral activity, consisting largely of abnormal, ‘dyskinetic’ tongue protrusions and vacuous chews, was increased in animals treated with SKF 38393, SKF 83565, SKF 82958 and more especially with SKF 80723 and SKF 83959. Grooming was increased with SKF 82958 and more especially with SKF 80723 and SKF 83959. In contrast, quinpirole (D2 DA agonist), reversed the MPTP-induced motor deficits in the marmoset, with no effect on grooming and oral activity. The present findings further demonstrate the antiparkinsonian actions of some D1 DA agonists in MPTP-treated primates. However, in general the behavioural effects of benzazepines failed to correlate with either their D1 DA receptor affinity/selectivity or their efficacy in stimulating adenylate cyclase (AC) activity. These observations further implicate a behavioural role for D1 DA receptors uncoupled to AC and/or a role for extrastriatal D1 DA receptors in mediating the behavioural response to D1 DA agonists.

Switching from levodopa to the long-acting dopamine D2/D3 agonist piribedil reduces the expression of dyskinesia while maintaining effective motor activity in MPTP-treated primates.

Background: The control of motor complications following dopaminergic medication in late-stage Parkinson disease remains problematic. Objective: We now investigate the potential of oral administration of the long-acting dopamine D2/D3 agonist piribedil to decrease the expression of dyskinesia induced by prior exposure to levodopa in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP)-treated primates. Methods: MPTP-treated common marmosets were treated with equieffective doses of levodopa (10.0-12.5 mg/kg PO, twice daily) or piribedil (3.0-4.0 mg/kg PO, once daily) for 30 days and then switched to the alternative treatment for a further 35 days. Results: Levodopa administration markedly improved motor function, but dyskinesia rapidly appeared and intensified as treatment progressed. Administration of piribedil produced a similar reversal of MPTP-induced motor deficits but with comparatively mild dyskinesia. On switching from levodopa to piribedil, the intensity of dyskinesia decreased without altering the improvement in motor deficits. However, on switching from piribedil to levodopa, the rapid increase in dyskinesia despite the improvement in motor function being maintained suggests that piribedil also primes for but does not markedly express dyskinesia. Conclusion: The study confirms the low dyskinesia expression resulting from piribedil treatment compared with an equieffective dose of levodopa. Importantly, the results show that switching from levodopa to piribedil rapidly results in a sustained decrease in dyskinesia intensity.

Osteopontin expression in substantia nigra in MPTP-treated primates and in Parkinson's disease

Parkinsons disease (PD) is characterised by the loss of dopaminergic neurones in the substantia nigra (SN) but the pathogenic mechanism remains unknown. Cell death involves oxidative stress and inflammatory mechanisms, and these may be altered by the actions of the glycosylated phosphoprotein osteopontin (OPN). OPN is present in the rat SN, but its presence in human and non-human primate brain has not been extensively studied. Both OPN mRNA and protein were present in the normal marmoset SN, and OPN protein was localised to nigral neurones although these were not dopaminergic cells and it was not present in glial cells. In contrast, OPN protein was found in dopaminergic neurones in the normal human SN but again not in glial cells with some accumulation in the extracellular matrix. Following MPTP treatment of common marmosets, OPN protein expression was decreased, although its mRNA levels were unchanged and it was not present in either activated microglia or astrocytes. In the SN in PD, OPN protein expression was decreased in the remaining dopaminergic neurones and it was present in activated microglia but not in astrocytes. This was not specific to PD as OPN protein expression was also decreased in the SN in multiple system atrophy and progressive supranuclear palsy with an identical localisation of the protein. The presence of OPN in the normal human and non-human primate SN coupled to its decreased expression following nigral cell degeneration suggests that it may play an important role in dopaminergic neurone survival.

Repeated administration of piribedil induces less dyskinesia than L-dopa in MPTP-treated common marmosets: A behavioural and biochemical investigation

Piribedil ([1‐(3,4‐methylenedioxybenzyl)‐4‐(2‐pyrimidinyl)piperazine]; S 4200) is a dopamine agonist with equal affinity for D2/D3 dopamine receptors effective in treating Parkinsons disease as monotherapy or as an adjunct to levodopa (L‐dopa). However, its ability to prime basal ganglia for the appearance of dyskinesia is unknown. We now report on the ability of repeated administration of piribedil to induce dyskinesia in drug naïve 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) ‐lesioned common marmosets compared with L‐dopa and its actions on the direct and indirect striatal outflow pathways. Administration of piribedil (4.0–5.0 mg/kg orally) or L‐dopa (12.5 mg/kg orally plus carbidopa 12.5 mg/kg orally twice daily) produced equivalent increases in locomotor activity and reversal of motor deficits over a 28‐day study period. Administration of L‐dopa resulted in the progressive development of marked dyskinesia over the period of study. In contrast, administration of piribedil produced a significantly lower degree and intensity of dyskinesia. Surprisingly, piribedil caused an increase in vigilance and alertness compared to L‐dopa, which may relate to the recently discovered α2‐noradrenergic antagonist properties of piribedil. The behavioural differences between piribedil and L‐dopa are reflected in the biochemical changes associated with the direct striatal output pathway. Administration of L‐dopa or piribedil did not reverse the MPTP‐induced up‐regulation of preproenkephalin A mRNA in rostral or caudal areas of the putamen or caudate nucleus. In contrast, administration of either piribedil or L‐dopa reversed the downregulation of preprotachykinin mRNA induced by MPTP in rostral and caudal striatum. L‐dopa, but not Piribedil, reversed the decrease in preproenkephalin B mRNA produced by MPTP treatment.