Jonathan M. Brotchie

Pathophysiology of levodopa-induced dyskinesia: Potential for new therapies

Involuntary movements — or dyskinesias — are a debilitating complication of levodopa therapy for Parkinsons disease that is experienced by most patients. Despite the importance of this problem, little was known about the cause of dyskinesia until recently; however, this situation has changed significantly in the past few years. Our increased understanding of levodopa-induced dyskinesia is not only valuable for improving patient care, but also in providing us with new insights into the functional organization of the basal ganglia and motor systems.

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)

Cannabinoids reduce levodopa-induced dyskinesia in Parkinson’s disease: A pilot study

The lateral segment of the globus pallidus (GPl) is thought to be overactive in levodopa-induced dyskinesia in PD. Stimulation of cannabinoid receptors in the GPl reduces γ-aminobutyric acid (GABA) reuptake and enhances GABA transmission and may thus alleviate dyskinesia. In a randomized, double-blind, placebo-controlled, crossover trial (n = 7), the authors demonstrate that the cannabinoid receptor agonist nabilone significantly reduces levodopa-induced dyskinesia in PD.

Effect of repeated L-DOPA, bromocriptine, or lisuride administration on preproenkephalin-A and preproenkephalin-B mRNA levels in the striatum of the 6-hydroxydopamine-lesioned rat

Abnormal involuntary movements, or dyskinesias, plague current symptomatic approaches to the treatment of Parkinsons disease. The neural mechanisms underlying the generation of dyskinesia following repeated l-3,4-dihydroxyphenylalanine (L-DOPA) or dopamine agonist administration in Parkinsons disease remain unknown. However, de novo administration of bromocriptine or lisuride to either l-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned primates or patients can alleviate parkinsonian symptoms without the development of dyskinesia. In this study, we have investigated behavioral responses and alterations in the expression of opioid neuropeptide precursors preproenkephalin-A (PPE-A, encoding methionine- and leucine-enkephalin) and preproenkephalin-B (PPE-B), the precursor encoding dynorphins (dynorphin A1-17 and B1-13, leucine-enkephalin, and alpha-neoendorphin) in striatal output pathways of the 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinsons disease. Expression was assessed following repeated L-DOPA, bromocriptine, or lisuride administration. Given the functional organization of basal ganglia circuitry into anatomically discrete parallel circuits, we investigated alterations in peptide expression with reference to the detailed topography of the striatum. Following repeated L-DOPA administration (6.5 mg/kg, b.d., 21 days) in the 6-OHDA-lesioned rat a rotational response was observed. This became markedly enhanced with repeated treatment. We have previously characterized the pharmacology of this enhanced response and have suggested that it is a useful model for the elucidation of the cellular and molecular mechanisms underlying L-DOPA- and dopamine agonist-induced dyskinesia. In contrast to l-DOPA, de novo administration of bromocriptine (1 or 5 mg/kg, b.d., 21 days) or lisuride (0.01 or 0.1 mg/kg, b.d., 21 days) did not lead to an enhanced behavioral response. In vehicle-treated, 6-OHDA-lesioned animals, PPE-A expression was elevated rostrally and dorsally, while PPE-B expression was reduced in the striatum at all rostrocaudal levels. Repeated l-DOPA administration was accompanied by elevations in striatal PPE-B mRNA levels and a further elevation, above lesion-induced levels, in PPE-A expression. This further elevation was restricted to the dorsolateral striatum. However, following repeated bromocriptine or lisuride administration no increase in PPE-B expression was observed and the lesion-induced increase in PPE-A expression was normalized to prelesion levels. Increased PPE-A and PPE-B levels may, through decreasing GABA and glutamate release, respectively, in output nuclei of the basal ganglia, play a role in the development of L-DOPA- and dopamine-agonist induced dyskinesia in Parkinsons disease. These studies suggest that anti-parkinsonian treatments which are not associated with an elevation in PPE-B and/or normalize elevated PPE-A precursor expression, such as NMDA-receptor antagonists or long-acting dopamine D2 receptor agonists, e.g., cabergoline or ropinirole, may reduce dyskinesia in Parkinsons disease.

On the role of enkephalin cotransmission in the GABAergic striatal efferents to the globus pallidus

In the MPTP-treated primate model of Parkinsons disease, loss of dopaminergic afferents to the striatum leads to increased activity in striatal efferents to the external segment of the globus pallidus. This pathway utilizes both GABA and enkephalin as cotransmitters. Little is known regarding either the role of this cotransmission in the generation of parkinsonian symptoms or of the nature of any functional interaction between GABA and enkephalin. We have investigated the roles played by enkephalin and GABA in mediating parkinsonian symptoms by injection the GABAA antagonist bicuculline and the broad spectrum opioid antagonist naloxone directly into the globus pallidus in the reserpine-treated rat model of parkinsonism. Injections of bicuculline, but not naloxone, had marked antiparkinsonian effects. However, naloxone attenuated the antiparkinsonian effects of bicuculline. We interpret these findings as suggesting that increased GABAergic transmission in the globus pallidus is responsible for the generation of parkinsonian symptoms in the reserpine-treated rat. However, overactive enkephalinergic transmission is not responsible for the generation of symptoms and appears to act to reduce the effects of increased GABAergic transmission. In complementary studies in vitro, we have demonstrated a potential mechanism for this negative interaction. Met-enkephalin (3-10 microM) reduced depolarization-evoked release of GABA from terminals in slices prepared from rat globus pallidus (IC50, 0.38 microM). A better comprehension of the mechanisms by which enkephalin and other peptides modulate the action of amino acid transmitters in the basal ganglia is critical to the understanding of the neural processes underlying basal ganglia function and movement disorders.

Fipamezole (JP-1730) is a potent alpha2 adrenergic receptor antagonist that reduces levodopa-induced dyskinesia in the MPTP-lesioned primate model of Parkinson's disease.

Previous studies in the MPTP‐lesioned primate model of Parkinsons disease have demonstrated that α2 adrenergic receptor antagonists such as idazoxan, rauwolscine, and yohimbine can alleviate L‐dopa–induced dyskinesia and, in the case of idazoxan, enhance the duration of anti‐parkinsonian action of L‐dopa. Here we describe a novel α2 antagonist, fipamezole (JP‐1730), which has high affinity at human α2A (Ki, 9.2 nM), α2B (17 nM), and α2C (55 nM) receptors. In functional assays, the potent antagonist properties of JP‐1730 were demonstrated by its ability to reduce adrenaline‐induced 35S‐GTPγS binding with KB values of 8.4 nM, 16 nM, 4.7 nM at human α2A, α2B, and α2C receptors, respectively. Assessment of the ability of JP‐1730 to bind to a range of 30 other binding sites showed that JP‐1730 also had moderate affinity at histamine H1 and H3 receptors and the serotonin (5‐HT) transporter (IC50 100 nM to 1 μM). In the MPTP‐lesioned marmoset, JP‐1730 (10 mg/kg) significantly reduced L‐dopa–induced dyskinesia without compromising the anti‐parkinsonian action of L‐dopa. The duration of action of the combination of L‐dopa and JP‐1730 (10 mg/kg) was 66% greater than that of L‐dopa alone. These data suggest that JP‐1730 is a potent α2 adrenergic receptor antagonist with potential as an anti‐dyskinetic agent in the treatment of Parkinsons disease.

The α2-adrenergic receptor antagonist idazoxan reduces dyskinesia and enhances anti-parkinsonian actions of L-dopa in the MPTP-lesioned primate model of Parkinson's disease

Dopamine replacement therapy in patients with Parkinsons disease is plagued by the emergence of abnormal involuntary movements known as L‐dopa‐induced dyskinesias. It has been demonstrated that yohimbine can reduce L‐dopa‐induced dyskinesia in the MPTP‐lesioned primate model of Parkinsons disease. Yohimbine is, among other things, an alpha‐adrenergic receptor antagonist. In this study, we demonstrate that the selective and potent α2‐adrenergic receptor antagonist idazoxan reduces L‐dopa‐induced dyskinesia in the MPTP‐lesioned marmoset model of Parkinsons disease. The α2‐adrenergic receptor antagonists rauwolscine and yohimbine also reduce L‐dopa‐induced dyskinesia. Furthermore, we demonstrate that coadministration of idazoxan with L‐dopa can provide an anti‐parkinsonian action more than twice the length of that seen with L‐dopa alone. However, idazoxan as a monotherapy displayed no anti‐parkinsonian actions. We propose that idazoxan in combination with L‐dopa may provide a novel approach to the treatment of Parkinsons disease that will not only reduce the dyskinetic side effects, but extend the anti‐parkinsonian actions of L‐dopa. Idazoxan, as an adjunct to dopamine replacement, may prove useful in the treatment of parkinsonian patients at all stages of disease progression.

Activation of the cannabinoid receptor by Δ9-tetrahydrocannabinol reduces γ-aminobutyric acid uptake in the globus pallidus

The interaction between GABA (gamma-aminobutyric acid) and cannabinoids in the globus pallidus was investigated by evaluating the effects of delta 9-tetrahydrocannabinol on [3H]GABA uptake into slices of rat globus pallidus. delta 9-Tetrahydrocannabinol caused a concentration-dependent decrease in GABA uptake (51% decrease at 100 microM delta 9-tetrahydrocannabinol, IC50 = 18.95 microM). This effect was reversed in a concentration-dependent manner (IC50 = 11.9 microM) by the cannabinoid receptor antagonist SR 141716A (N-(piperidin-1-yl-)5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1 H-pyrazole-3-arboxiamidehydrochloride. SR 141716A alone did not affect GABA uptake. These results show that cannabinoid receptor activation reduces GABA uptake in the globus pallidus.

Paradoxical action of the cannabinoid WIN 55,212‐2 in stimulated and basal cyclic AMP accumulation in rat globus pallidus slices

The effects of the synthetic cannabinoid WIN 55,212‐2 on forskolin‐stimulated and basal adenosine 3′ : 5′‐cyclic monophosphate (cyclic AMP) accumulation in globus pallidus slices were investigated. WIN 55,212‐2 caused a concentration‐dependent decrease in forskolin‐stimulated cyclic AMP accumulation in globus pallidus slices (maximum inhibition 36% for 30 μm). This effect was blocked by the cannabinoid receptor antagonist SR 141716A (100 μm). WIN 55,212‐2 alone caused a concentration‐dependent increase in cyclic AMP levels in unstimulated slices (maximum increase 52.6% for 100 μm). This effect was also blocked by SR 141716A (100 μm). In either forskolin‐stimulated or unstimulated conditions SR 1417161A (100 μm) did not affect cyclic AMP levels.

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.