Michael P. Hill

Enhanced levels of endogenous cannabinoids in the globus pallidus are associated with a reduction in movement in an animal model of Parkinson’s disease

In recent years, cannabinoid receptors and their endogenous ligands (endocannabinoids) have been identified within the brain. The high density of CB1 cannabinoid receptors within the basal ganglia suggests a potential role for endocannabinoids in the control of voluntary movement and in basal ganglia‐related movement disorders such as Parkinsons disease. However, whether endocannabinoids play a role in regulating motor behavior in health and disease is unknown. Here we report the presence in two regions of the basal ganglia, the globus pallidus and substantia nigra, of the endocannabinoids 2‐arachidonoylglycerol (2AG) and anand‐amide. The levels of the latter compound are —threefold higher than those previously reported in any other brain region. In the reserpine‐treated rat, an animal model of Parkinsons disease, suppression of locomotion is accompanied by a sevenfold increase in the levels of the 2AG in the globus pallidus, but not in the other five brain regions analyzed. Stimulation of locomotion in the reserpine‐treated rat by either of the two selective agonists of D2 and D1 dopamine receptors, quinpirole and R‐(±)‐3‐allyl‐6‐chloro‐7,8‐dihydroxy‐1‐phenyl‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepine hydrobromide (Cl‐APB), respectively, results in the reduction of both anandamide and 2AG levels in the globus pallidus. Finally, full restoration of locomotion in the reserpine‐treated rat is obtained by coadministration of quinpirole and the selective antagonist of the cannabinoid CB1 receptor subtype, SR141716A. These findings indicate a link between endocannabinoid signaling in the globus pallidus and symptoms of Parkinsons disease in the reserpine‐treated rat, and suggest that modulation of the endocannabinoid signaling system might prove useful in treating this or other basal ganglia‐related movement disorders.—Di Marzo, V., Hill, M. P., Bisogno, T., Crossman, A. R., Brotchie, J. M. Enhanced levels of endogenous cannabinoids in the globus pallidus are associated with a reduction in movement in an animal model of Parkinsons disease. FASEB J. 14, 1432–1438 (2000)

Antiparkinsonian actions of ifenprodil in the MPTP-lesioned marmoset model of Parkinson's disease.

Dopamine-replacement strategies form the basis of most symptomatic treatments for Parkinsons disease. However, since long-term dopamine-replacement therapies are characterized by many side effects, most notably dyskinesia, the concept of a nondopaminergic therapy for Parkinsons disease has attracted great interest. To date, it has proved difficult to devise a nondopaminergic therapy with efficacy comparable to that of dopamine replacement. In animal models of Parkinsons disease, loss of striatal dopamine leads to enhanced excitation of striatal NR2B-containing NMDA receptors. This is responsible, in part at least, for generating parkinsonian symptoms. Here we demonstrate that, in the MPTP-lesioned marmoset, monotherapy with the NR2B-selective NMDA receptor antagonist, ifenprodil, administered de novo, has antiparkinsonian effects equivalent to those of l-DOPA (administered as its methyl ester form). In MPTP-lesioned marmosets, median mobility scores, following vehicle-treatment were 12.5/h (range 6-21), compared to 61/h (range 26-121) in normal, non-MPTP-lesioned animals. Following ifenprodil (10 mg/kg) treatment in MPTP-lesioned marmosets, the median mobility score was 66/h (range 34-93), and following l-DOPA (10 mg/kg i.p.) treatment 89/h (range 82-92). The data support the proposal that NR2B-selective NMDA receptor antagonists have potential as a nondopaminergic monotherapy for the treatment of parkinsonian symptoms when given de novo.

Metabotropic glutamate receptor agonists inhibit endogenous glutamate release from rat striatal synaptosomes

A striatal synaptosomal preparation was used to assess the action of metabotropic glutamate receptor (mGlu receptor) agonists on 4-aminopyridine (2 mM)-stimulated endogenous glutamate release. 4-Aminopyridine alone increased basal glutamate release by 6.89 +/- 0.74 nmol/mg. The mGlu receptor agonists L-2-amino-4-phosphonobutyric acid (L-AP4) (IC50 approximately 0.2 microM) and (1S,3S)-1-amino-cyclopentane-1,3-dicarboxylic acid (IC50 approximately 0.2 microM) inhibited 4-aminopyridine-stimulated release. The putative mGlu receptor antagonist (S)-alpha-methyl-L-AP4, which itself inhibited 4-aminopyridine-stimulated release (IC50 approximately 10 microM), did not inhibit the effects of the two agonists.

Novel antiepileptic drug levetiracetam decreases dyskinesia elicited by L-dopa and ropinirole in the MPTP-lesioned marmoset

Long‐term dopamine replacement therapy of Parkinsons disease leads to the occurrence of dyskinesias. Altered firing patterns of neurons of the internal globus pallidus, involving a pathological synchronization/desynchronization process, may contribute significantly to the genesis of dyskinesia. Levetiracetam, an antiepileptic drug that counteracts neuronal (hyper)synchronization in animal models of epilepsy, was assessed in the MPTP–lesioned marmoset model of Parkinsons disease, after coadministration with (1) levodopa (L‐dopa) or (2) ropinirole/L‐dopa combination. Oral administration of levetiracetam (13–60 mg/kg) in combination with either L‐dopa (12 mg/kg) alone or L‐dopa (8 mg/kg)/ropinirole (1.25 mg/kg) treatments was associated with significantly less dyskinesia, in comparison to L‐dopa monotherapy during the first hour after administration. Thus, new nondopaminergic treatment strategies targeting normalization of abnormal firing patterns in basal ganglia structures may prove useful as an adjunct to reduce dyskinesia induced by dopamine replacement therapy without affecting its antiparkinsonian action.

Modulation of glutamate release by a κ-opioid receptor agonist in rodent and primate striatum

Abstract The influence of the κ-opioid receptor agonist, enadoline, on endogenous glutamate release was investigated in rat and marmoset striatal synaptosomes. Enadoline decreased 4-aminopyridine (2 mM)-stimulated glutamate release (rat: IC 50 ∼ 8.7 μM, marmoset: IC 50 ∼ 2.9 μM). The effect of enadoline was reversed by nor-binaltorphimine (5 μM). These data indicate that, in the striatum of the rat and marmoset, κ-opioid receptor agonists can modulate glutamate release. These findings may have implications for the treatment of Parkinsons disease.

Synergistic antidyskinetic effects of topiramate and amantadine in animal models of Parkinson's disease

L‐Dopa‐induced dyskinesia in patients with Parkinsons disease can be alleviated by amantadine, an antagonist at N‐methyl‐D‐aspartate glutamate receptors. The antiepileptic drug topiramate, which blocks α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptors, has also been shown to reduce dyskinesia. The purpose of this study was to examine the behavioral pharmacology of topiramate alone and in combination with amantadine in animal models of PD and L‐dopa‐induced dyskinesia. The effects of topiramate (5–20 mg/kg) and amantadine (5–20 mg/kg) on abnormal involuntary movements (the rat homologue of dyskinesia) and Rotarod performance were assessed alone and in combination in the 6‐hydroxydopamine‐lesioned rat following chronic L‐dopa treatment. Dyskinesia, parkinsonian disability, and “on‐time” were assessed in the MPTP‐lesioned nonhuman primate following administration of topiramate (5–20 mg/kg) and amantadine (0.1–1.0 mg/kg) alone and in combination. Topiramate and amantadine dose‐dependently reduced dyskinesia in the 6‐hydroxydopamine‐lesioned rat, whereas topiramate reduced Rotarod performance; there was no effect on parkinsonian disability in the MPTP‐lesioned nonhuman primate, in which both drugs reduced dyskinesia. Topiramate and amantadine exhibited differential antidyskinetic effects on dyskinesia elicited by the dopamine D1 receptor agonist SKF 38393 (2 mg/kg). Subthreshold doses of both drugs in combination had a synergistic effect on dyskinesia in the 6‐hydroxydopamine‐lesioned rat, with no worsening of motor performance; this effect was confirmed in the MPTP‐lesioned nonhuman primate, with a selective reduction in “bad on‐time.” These data confirm the antidyskinetic potential of topiramate and suggest that combination with low‐dose amantadine may allow better reduction of dyskinesia with no adverse motor effects.

Rotigotine Polyoxazoline Conjugate SER-214 Provides Robust and Sustained Antiparkinsonian Benefit

Currently available dopaminergic drugs such as levodopa and dopamine (DA) receptor agonists impart considerable improvement in Parkinsons disease (PD) motor symptoms but often lead to significant motor complications including “wearing‐off” and dyskinesia. Such complications are believed to stem from the pulsatile nature of dopaminergic stimulation with these agents. Continuous dopaminergic drug delivery using polyoxazoline (POZ) polymer conjugation may improve motor symptoms, while avoiding development of side effects. The purposes of the current study are to characterize the in vitro and in vivo pharmacokinetics of POZ conjugation of a U.S. Food and Drug Administration (FDA)‐approved DA agonist, rotigotine, and to evaluate their effects in an established rat model of PD. After determination of release profiles of several POZ‐conjugated constructs (“fast”: SER‐212; “moderate”: SER‐213; and “slow”: SER‐214) using in vitro hydrolysis, normal male Sprague‐Dawley rats were used for determination of the pharmacokinetic profile of both acute and chronic exposure. Finally, a separate group of rats was rendered hemiparkinsonian using intracranial 6‐hydroxydopamine (6‐OHDA) infusions, treated acutely with POZ‐rotigotine, and assessed for rotational behavior and antiparkinsonian benefit using the cylinder test. POZ‐rotigotine formulations SER‐213 and SER‐214 led to substantial pharmacokinetic improvement compared to unconjugated rotigotine. In addition, SER‐214 led to antiparkinsonian effects in DA‐lesioned rats that persisted up to 5 days posttreatment. Repeated weekly dose administration of SER‐214 to normal rats for up to 12 weeks demonstrated highly reproducible pharmacokinetic profiles. The continuous dopaminergic stimulation profile afforded by SER‐214 could represent a significant advance in the treatment of PD, with potential to be a viable, once‐per‐week therapy for PD patients.

Levetiracetam Interferes With the L-dopa priming process in MPTP-lesioned drug-naive marmosets.

ObjectiveLevetiracetam (LEV; Keppra, UCB Pharma) has been shown to reduce established l-3,4 dihydroxyphenylalanine (l-dopa)-induced dyskinesia. This study investigated whether LEV can modify induction of dyskinesia by l-dopa or the process of priming. MethodsDrug-naive MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) -lesioned marmosets were treated for 21 days with l-dopa/LEV or l-dopa alone. Subsequently, the animals were left untreated for 1 week and then both groups were challenged with a single dose of l-dopa alone on day 29. Behavior was assessed by automated activity counts and by post hoc analysis of videotapes using validated rating scales. ResultsLEV had no significant effect on the appearance of dyskinesia when administered de novo in combination with l-dopa. However, after a week of drug holiday, the 2 groups exhibited a different response to an acute l-dopa challenge. Thus, animals previously treated with l-dopa alone exhibited a similar level of dyskinesia to that seen on day 21 of the repeated treatment phase of the study. However, animals previously treated with l-dopa/LEV demonstrated significantly reduced dyskinesia compared with day 21 of the repeated treatment phase of the study. ConclusionsLEV does not modify the onset of dyskinesia following de novo treatment with l-dopa. However, concomitant treatment with l-dopa/LEV reduces the level of dyskinesia induced by l-dopa following a drug holiday. Thus, prior treatment with LEV appears to modify the mechanisms responsible for the maintenance of l-dopa-induced dyskinesia.

Antiparkinsonian effects of the novel D3/D2 dopamine receptor agonist, S32504, in MPTP-lesioned marmosets: Mediation by D2, not D3, dopamine receptors.

L‐dopa remains the most common treatment for Parkinsons disease. However, there is considerable interest in D3/D2 receptor agonists such as the novel agent S32504, since they exert antiparkinsonian properties in the absence of dyskinesia. An important question concerns the roles of D2 vs. D3 receptors, an issue we addressed with the 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)–lesioned nonhuman primate model of Parkinsons disease. In L‐dopa–primed animals, S32504 (0.16–2.5 mg/kg p.o.) dose‐dependently enhanced locomotor activity. This action was abolished by the D2 antagonist, L741,626 (2.5 mg/kg), but potentiated by the D3 antagonist, S33084 (0.63 mg/kg). Both antagonists were inactive alone. In drug‐naive animals, a maximally effective dose of S32504 (2.5 mg/kg p.o.) displayed pronounced antiparkinsonian properties from the third day of administration, and its actions were expressed rapidly and durably. Thus, on day 33, antiparkinsonian properties of S32504 were apparent within 5 minutes and present for > 4 hours. Moreover, they were associated with neither wearing off nor significant dyskinesia. In conclusion, the novel D3/D2 agonist S32504 may offer advantages over L‐dopa in the treatment of newly diagnosed parkinsonian patients. Its actions are expressed primarily by activation of D2, not D3, receptors.

Use of catechol-O-methyltransferase inhibition to minimize L-3,4-dihydroxyphenylalanine-induced dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaque

L‐3,4‐dihydroxyphenylalanine (L‐DOPA)‐induced dyskinesia is a complication of dopaminergic treatment in Parkinsons disease. Lowering the L‐DOPA dose reduces dyskinesia but also reduces the antiparkinsonian benefit. A therapy that could enhance the antiparkinsonian action of low‐dose L‐DOPA (LDl) without exacerbating dyskinesia would thus be of considerable therapeutic benefit. This study assessed whether catechol‐O‐methyltransferase (COMT) inhibition, as an add‐on to LDl, might be a means to achieve this goal. Cynomolgus macaques were administered 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine. Dyskinesia was established by chronic treatment with L‐DOPA. Two doses of L‐DOPA were identified – high‐dose L‐DOPA (LDh), which provided good antiparkinsonian benefit but was compromised by disabling dyskinesia, and LDl, which was sub‐threshold for providing significant antiparkinsonian benefit, without dyskinesia. LDh and LDl were administered in acute challenges in combination with vehicle and, for LDl, with the COMT inhibitor entacapone (5, 15 and 45 mg/kg). The duration of antiparkinsonian benefit (ON‐time), parkinsonism and dyskinesia were determined. The ON‐time after LDh was ∼170 min and the ON‐time after LDl alone (∼98 min) was not significantly different to vehicle (∼37 min). In combination with LDl, entacapone significantly increased the ON‐time (5, 15 and 45 mg/kg being ∼123, ∼148 and ∼180 min, respectively). The ON‐time after LDl/entacapone 45 mg/kg was not different to that after LDh. However, whereas the percentage ON‐time that was compromised by disabling dyskinesia was ∼56% with LDh, it was only ∼31% with LDl/entacapone 45 mg/kg. In addition to the well‐recognized action of COMT inhibition to reduce wearing‐OFF, the data presented suggest that COMT inhibition in combination with low doses of L‐DOPA has potential as a strategy to alleviate dyskinesia.