Nicotine reduces L-DOPA-induced dyskinesias by acting at beta2* nicotinic receptors

Levodopa-Induced Dyskinesias in Parkinson's Disease: An Overview on Pathophysiology, Clinical Manifestations, Therapy Management Strategies and Future Directions

Since its first introduction, levodopa has become the cornerstone for the treatment of Parkinson's disease and remains the leading therapeutic choice for motor control therapy so far. Unfortunately, the subsequent appearance of abnormal involuntary movements, known as dyskinesias, is a frequent drawback. Despite the deep knowledge of this complication, in terms of clinical phenomenology and the temporal relationship during a levodopa regimen, less is clear about the pathophysiological mechanisms underpinning it. As the disease progresses, specific oscillatory activities of both motor cortical and basal ganglia neurons and variation in levodopa metabolism, in terms of the dopamine receptor stimulation pattern and turnover rate, underlie dyskinesia onset. This review aims to provide a global overview on levodopa-induced dyskinesias, focusing on pathophysiology, clinical manifestations, therapy management strategies and future directions.

Cholinergic Receptor Modulation as a Target for Preventing Dementia in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative condition characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) in the midbrain resulting in progressive impairment in cognitive and motor abilities. The physiological and molecular mechanisms triggering dopaminergic neuronal loss are not entirely defined. PD occurrence is associated with various genetic and environmental factors causing inflammation and mitochondrial dysfunction in the brain, leading to oxidative stress, proteinopathy, and reduced viability of dopaminergic neurons. Oxidative stress affects the conformation and function of ions, proteins, and lipids, provoking mitochondrial DNA (mtDNA) mutation and dysfunction. The disruption of protein homeostasis induces the aggregation of alpha-synuclein (α-SYN) and parkin and a deficit in proteasome degradation. Also, oxidative stress affects dopamine release by activating ATP-sensitive potassium channels. The cholinergic system is essential in modulating the striatal cells regulating cognitive and motor functions. Several muscarinic acetylcholine receptors (mAChR) and nicotinic acetylcholine receptors (nAChRs) are expressed in the striatum. The nAChRs signaling reduces neuroinflammation and facilitates neuronal survival, neurotransmitter release, and synaptic plasticity. Since there is a deficit in the nAChRs in PD, inhibiting nAChRs loss in the striatum may help prevent dopaminergic neurons loss in the striatum and its pathological consequences. The nAChRs can also stimulate other brain cells supporting cognitive and motor functions. This review discusses the cholinergic system as a therapeutic target of cotinine to prevent cognitive symptoms and transition to dementia in PD.

Roles of the Functional Interaction between Brain Cholinergic and Dopaminergic Systems in the Pathogenesis and Treatment of Schizophrenia and Parkinson's Disease

Most physiologic processes in the brain and related diseases involve more than one neurotransmitter system. Thus, elucidation of the interaction between different neurotransmitter systems could allow for better therapeutic approaches to the treatments of related diseases. Dopaminergic (DAergic) and cholinergic neurotransmitter system regulate various brain functions that include cognition, movement, emotion, etc. This review focuses on the interaction between the brain DAergic and cholinergic systems with respect to the pathogenesis and treatment of schizophrenia and Parkinson’s disease (PD). We first discussed the selection of motor plans at the level of basal ganglia, the major DAergic and cholinergic pathways in the brain, and the receptor subtypes involved in the interaction between the two signaling systems. Next, the roles of each signaling system were discussed in the context of the negative symptoms of schizophrenia, with a focus on the α7 nicotinic cholinergic receptor and the dopamine D₁ receptor in the prefrontal cortex. In addition, the roles of the nicotinic and dopamine receptors were discussed in the context of regulation of striatal cholinergic interneurons, which play crucial roles in the degeneration of nigrostriatal DAergic neurons and the development of L-DOPA-induced dyskinesia in PD patients. Finally, we discussed the general mechanisms of nicotine-induced protection of DAergic neurons.

The Rodent Models of Dyskinesia and Their Behavioral Assessment

Dyskinesia, a major motor complication resulting from dopamine replacement treatment, manifests as involuntary hyperkinetic or dystonic movements. This condition poses a challenge to the treatment of Parkinson's disease. So far, several behavioral models based on rodent with dyskinesia have been established. These models have provided an important platform for evaluating the curative effect of drugs at the preclinical research level over the past two decades. However, there are differences in the modeling and behavioral testing procedures among various laboratories that adversely affect the rat and mouse models as credible experimental tools in this field. This article systematically reviews the history, the pros and cons, and the controversies surrounding rodent models of dyskinesia as well as their behavioral assessment protocols. A summary of factors that influence the behavioral assessment in the rodent dyskinesia models is also presented, including the degree of dopamine denervation, stereotaxic lesion sites, drug regimen, monitoring styles, priming effect, and individual and strain differences. Besides, recent breakthroughs like the genetic mouse models and the bilateral intoxication models for dyskinesia are also discussed.

Comparison of Mono-dopaminergic and Multi-target Pharmacotherapies in Primary Parkinson Syndrome and Assessment Tools to Evaluate Motor and Non-motor Symptoms

Background:

Primary Parkinson syndrome is mostly treated by dopaminergic drugs, while the progression of the disease is not altered. Some non-dopaminergic are available, which are administered only after the Parkinsonian symptoms get worse.

Objective:

The objective of this review is to give basic results in order to compare a dopaminergic and non-dopaminergic pharmacotherapy in Parkinson’s disease and to control whether the add-on pharmacotherapy with non-dopaminergic drugs can inhibit the progression of the disease.

Methods:

In primary Parkinson syndrome, the altered activity of classical neurotransmitters and neuropeptides in the extrapyramidal system is summarized and up-dated. Anatomical studies on neural networks in the basal ganglia are mentioned. The direct, motor facilitatory pathway (D1 dopaminergic neurons) from the substantia nigra to the thalamus, via the internal globus pallidus, and the indirect, motor inhibitory pathway via D2 dopaminergic neurons have been considered. These established anatomical pathways have been brought in line with the neural interactions derived from neurotransmitter balances or imbalances. Besides, preclinical and clinical studies of effective non-dopaminergic anti-Parkinsonian drugs are reviewed.

Results:

It can be hypothesized that glutamatergic neurons enhance dopamine deficiency in the substantia nigra and putamen through an increased presynaptic inhibition mediated by NMDA receptors. In the putamen, 5-HT2A serotonergic neurons counteract D2 dopaminergic neurons and A2A adenosine neurons antagonize D2 dopaminergic neurons by activating glutamatergic neurons, which presynaptically inhibit via subtype 5 of metabotropic glutamatergic receptors, D2 dopaminergic neurons. In the extrapyramidal system, an up-dated neural network, which harmonizes established anatomical pathways with derived neural interactions, is presented. In Parkinson’s disease, a question should be answered, whether a combination of dopaminergic and non-dopaminergic drugs can promote an increased motor and non-motor functioning.

Conclusion:

A mono-target pharmacotherapy (using only dopaminergic drugs) and a multi-target pharmacotherapy (i.e. by combining dopaminergic and non-dopaminergic drugs) are compared. The alternate administration of dopaminergic and non-dopaminergic anti-Parkinsonian drugs, administered at different times during the day, must be tested in order to inhibit the progression of the disease. Assessment tools can be used to evaluate motor and cognitive functions. Moreover, imaging examination techniques can be also applied to control the course of the disease.

Receptor Ligands as Helping Hands to L-DOPA in the Treatment of Parkinson's Disease

Levodopa (LD) is the most effective drug in the treatment of Parkinson’s disease (PD). However, although it represents the “gold standard” of PD therapy, LD can cause side effects, including gastrointestinal and cardiovascular symptoms as well as transient elevated liver enzyme levels. Moreover, LD therapy leads to LD-induced dyskinesia (LID), a disabling motor complication that represents a major challenge for the clinical neurologist. Due to the many limitations associated with LD therapeutic use, other dopaminergic and non-dopaminergic drugs are being developed to optimize the treatment response. This review focuses on recent investigations about non-dopaminergic central nervous system (CNS) receptor ligands that have been identified to have therapeutic potential for the treatment of motor and non-motor symptoms of PD. In a different way, such agents may contribute to extending LD response and/or ameliorate LD-induced side effects.

Potential Therapeutic Application for Nicotinic Receptor Drugs in Movement Disorders

Emerging studies indicate that striatal cholinergic interneurons play an important role in synaptic plasticity and motor control under normal physiological conditions, while their disruption may lead to movement disorders. Here we discuss the involvement of the cholinergic system in motor dysfunction, with a focus on the role of the nicotinic cholinergic system in Parkinson's disease and drug-induced dyskinesias. Evidence for a role for the striatal nicotinic cholinergic system stems from studies showing that administration of nicotine or nicotinic receptor drugs protects against nigrostriatal degeneration and decreases L-dopa-induced dyskinesias. In addition, nicotinic receptor drugs may ameliorate tardive dyskinesia, Tourette's syndrome and ataxia, although further study is required to understand their full potential in the treatment of these disorders. A role for the striatal muscarinic cholinergic system in movement disorders stems from studies showing that muscarinic receptor drugs acutely improve Parkinson's disease motor symptoms, and may reduce dyskinesias and dystonia. Selective stimulation or lesioning of striatal cholinergic interneurons suggests they are primary players in this regulation, although multiple central nervous systems appear to be involved.

IMPLICATIONS

Accumulating data from preclinical studies and clinical trials suggest that drugs targeting CNS cholinergic systems may be useful for symptomatic treatment of movement disorders. Nicotinic cholinergic drugs, including nicotine and selective nAChR receptor agonists, reduce L-dopa-induced dyskinesias, as well as antipsychotic-induced tardive dyskinesia, and may be useful in Tourette's syndrome and ataxia. Subtype selective muscarinic cholinergic drugs may also provide effective therapies for Parkinson's disease, dyskinesias and dystonia. Continued studies/trials will help address this important issue.

Dyskinesia and brain-derived neurotrophic factor levels after long-term levodopa and nicotinic receptor agonist treatments in female mice with near-total unilateral dopaminergic denervation

BACKGROUND

The treatment of Parkinson's disease is often complicated by levodopa-induced dyskinesia (LID). Nicotinic acetylcholine receptor agonists can alleviate LID in animal models but may be less effective in conditions of severe dopaminergic denervation. While the mechanisms of LID remain incompletely understood, elevated corticostriatal levels of the brain-derived neurotrophic factor (BDNF) have been suggested to play a role. Here, female mice with near-total unilateral 6-hydroxydopamine-induced nigrostriatal lesions were chronically treated with levodopa, and the effects of the α7 nicotinic receptor partial agonist AZD0328 and nicotine on LID were assessed. At the end of the experiment, BDNF protein levels in the prefrontal cortex and striatum were measured.

RESULTS

Five-day treatments with three escalating doses of AZD0328 and a 10-week treatment with nicotine failed to alleviate LID. BDNF levels in the lesioned striatum correlated positively with LID severity, but no evidence was found for a levodopa-induced elevation of corticostriatal BDNF in the lesioned hemisphere. The nicotine treatment decreased BDNF levels in the prefrontal cortex but had no effect on striatal BDNF.

CONCLUSIONS

The findings suggest that treatment of LID with nicotinic agonists may lose its effectiveness as the disease progresses, represent further evidence for a role for BDNF in LID, and expand previous knowledge on the effects of long-term nicotine treatment on BDNF.

Genetic Knockdown of mGluR5 in Striatal D1R-Containing Neurons Attenuates L-DOPA-Induced Dyskinesia in Aphakia Mice

L-DOPA is the main pharmacological therapy for Parkinson's disease. However, long-term exposure to L-DOPA induces involuntary movements termed dyskinesia. Clinical trials show that dyskinesia is attenuated by metabotropic glutamate receptor type 5 (mGluR5) antagonists. Further, the onset of dyskinesia is delayed by nicotine and mGluR5 expression is lower in smokers than in non-smokers. However, the mechanisms by which mGluR5 modulates dyskinesia and how mGluR5 and nicotine interact have not been established. To address these issues, we studied the role of mGluR5 in D1R-containing neurons in dyskinesia and examined whether nicotine reduces dyskinesia via mGluR5. In the aphakia mouse model of Parkinson's disease, we selectively knocked down mGluR5 in D1R-containing neurons (aphakia-mGluR5^KD-D1). We found that genetic downregulation of mGluR5 decreased dyskinesia in aphakia mice. Although chronic nicotine increased the therapeutic effect of L-DOPA in both aphakia and aphakia-mGluR5^KD-D1 mice, it caused a robust reduction in dyskinesia only in aphakia, and not in aphakia-mGluR5^KD-D1 mice. Downregulating mGluR5 or nicotine treatment after L-DOPA decreased ERK and histone 3 activation, and FosB expression. Combining nicotine and mGluR5 knockdown did not have an added antidyskinetic effect, indicating that the effect of nicotine might be mediated by downregulation of mGluR5 expression. Treatment of aphakia-mGluR5^KD-D1 mice with a negative allosteric modulator did not further modify dyskinesia, suggesting that mGluR5 in non-D1R-containing neurons does not play a role in its development. In conclusion, this work suggests that mGluR5 antagonists reduce dyskinesia by mainly affecting D1R-containing neurons and that the effect of nicotine on dyskinetic signs in aphakia mice is likely via mGluR5.

Cholinergic control of striatal neurons to modulate L-dopa-induced dyskinesias

L-dopa induced dyskinesias (LIDs) are a disabling motor complication of L-dopa therapy for Parkinson's disease (PD) management. Treatment options remain limited and the underlying network mechanisms remain unclear due to a complex pathophysiology. What is well-known, however, is that aberrant striatal signaling plays a key role in LIDs development. Here, we discuss the specific contribution of striatal cholinergic interneurons (ChIs) and GABAergic medium spiny projection neurons (MSNs) with a particular focus on how cholinergic signaling may integrate multiple striatal systems to modulate LIDs expression. Enhanced ChI transmission, altered MSN activity and the associated abnormal downstream signaling responses that arise with nigrostriatal damage are well known to contribute to LIDs development. In fact, enhancing M4 muscarinic receptor activity, a receptor favorably expressed on D1 dopamine receptor-expressing MSNs dampens their activity to attenuate LIDs. Likewise, ChI activation via thalamostriatal neurons is shown to interrupt cortical signaling to enhance D2 dopamine receptor-expressing MSN activity via M1 muscarinic receptors, which may interrupt ongoing motor activity. Notably, numerous preclinical studies also show that reducing nicotinic cholinergic receptor activity decreases LIDs. Taken together, these studies indicate the importance of cholinergic control of striatal neuronal activity and point to muscarinic and nicotinic receptors as significant pharmacological targets for alleviating LIDs in PD patients.

Attenuated dopaminergic neurodegeneration and motor dysfunction in hemiparkinsonian mice lacking the α5 nicotinic acetylcholine receptor subunit

Preclinical studies suggest the involvement of various subtypes of nicotinic acetylcholine receptors in the pathophysiology of Parkinson's disease, a neurodegenerative disorder characterized by the death of dopaminergic neurons in the substantia nigra pars compacta (SNC). We studied for the first time the effects of α5 nicotinic receptor subunit gene deletion on motor behavior and neurodegeneration in mouse models of Parkinson's disease and levodopa-induced dyskinesia. Unilateral dopaminergic lesions were induced in wild-type and α5-KO mice by 6-hydroxydopamine injections into the striatum or the medial forebrain bundle. Subsequently, rotational behavior induced by dopaminergic drugs was measured. A subset of animals received chronic treatments with levodopa and nicotine to assess levodopa-induced dyskinesia and antidyskinetic effects by nicotine. SNC lesion extent was assessed with tyrosine hydroxylase immunohistochemistry and stereological cell counting. Effects of α5 gene deletion on the dopaminergic system were investigated by measuring ex vivo striatal dopamine transporter function and protein expression, dopamine and metabolite tissue concentrations and dopamine receptor mRNA expression. Hemiparkinsonian α5-KO mice exhibited attenuated rotational behavior after amphetamine injection and attenuated levodopa-induced dyskinesia. In the intrastriatal lesion model, dopaminergic cell loss in the medial cluster of the SNC was less severe in α5-KO mice. Decreased striatal dopamine uptake in α5-KO animals suggested reduced dopamine transporter function as a mechanism of attenuated neurotoxicity. Nicotine reduced dyskinesia severity in wild-type but not α5-KO mice. The attenuated dopaminergic neurodegeneration and motor dysfunction observed in hemiparkinsonian α5-KO mice suggests potential for α5 subunit-containing nicotinic receptors as a novel target in the treatment of Parkinson's disease.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

The striatal cholinergic system in L-dopa-induced dyskinesias

Cholinergic signaling plays a key role in regulating striatal function. The principal source of acetylcholine in the striatum is the cholinergic interneurons which, although low in number, densely arborize to modulate striatal neurotransmission. This modulation occurs via strategically positioned nicotinic and muscarinic acetylcholine receptors that influence striatal dopamine, GABA and other neurotransmitter release. Cholinergic interneurons integrate multiple striatal synaptic inputs and outputs to regulate motor activity under normal physiological conditions. Consequently, an imbalance between these systems is associated with basal ganglia disorders. Here, we provide an overview of how striatal cholinergic interneurons modulate striatal activity under normal and pathological conditions. Numerous studies show that nigrostriatal damage such as that occurs with Parkinson's disease affects cholinergic receptor-mediated striatal activity. This altered cholinergic signaling is an important contributor to Parkinson's disease as well as to the dyskinesias that develop with L-dopa therapy, the gold standard for treatment. Indeed, multiple preclinical studies show that cholinergic receptor drugs may be beneficial for the treatment of L-dopa-induced dyskinesias. In this review, we discuss the evidence indicating that therapeutic modulation of the cholinergic system, particularly targeting of nicotinic cholinergic receptors, may offer a novel approach to manage this debilitating side effect of dopamine replacement therapy for Parkinson's disease.

Animal models of l-dopa-induced dyskinesia in Parkinson's disease

Understanding the biological mechanisms of l-dopa-induced motor complications is dependent on our ability to investigate these phenomena in animal models of Parkinson's disease. The most common motor complications consist in wearing-off fluctuations and abnormal involuntary movements appearing when plasma levels of l-dopa are high, commonly referred to as peak-dose l-dopa-induced dyskinesia. Parkinsonian models exhibiting these features have been well-characterized in both rodent and nonhuman primate species. The first animal models of peak-dose l-dopa-induced dyskinesia were produced in monkeys lesioned with N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and treated chronically with l-dopa to elicit choreic movements and dystonic postures. Seminal studies were performed in these models using both metabolic mapping and electrophysiological techniques, providing fundamental pathophysiological insights that have stood the test of time. A decade later, it was shown possible to reproduce peak-dose l-dopa-induced dyskinesia in rats and mice rendered parkinsonian with nigrostriatal 6-hydroxydopamine lesions. When treated with l-dopa, these animals exhibit abnormal involuntary movements having both hyperkinetic and dystonic components. These models have enabled molecular- and cellular-level investigations into the mechanisms of l-dopa-induced dyskinesia. A flourishing literature using genetically engineered mice is now unraveling the role of specific genes and neural circuits in the development of l-dopa-induced motor complications. Both non-human primate and rodent models of peak-dose l-dopa-induced dyskinesia have excellent construct validity and provide valuable tools for discovering therapeutic targets and evaluating potential treatments. © 2018 International Parkinson and Movement Disorder Society.

Effects of antidyskinetic nicotine treatment on dopamine release in dorsal and ventral striatum

The treatment of Parkinson's disease is often complicated by levodopa-induced dyskinesia (LID), and antidyskinetic treatment options are currently sparse. Nicotinic acetylcholine receptors have been suggested as potential targets for treatment of LID, as nicotinic agonists have been reported to alleviate LID in animal models. We aimed at the first independent replication of an antidyskinetic effect by nicotine using a mouse model of LID, and at investigation of its mechanisms by studying the release of [³H]dopamine from synaptosomes prepared from the dorsal and ventral striatum. Chronic nicotine treatment in drinking water inhibited the development of LID in mice lesioned unilaterally with 6-hydroxydopamine and treated chronically with levodopa and benserazide. The antidyskinetic nicotine treatment had no effect on [³H]dopamine release mediated by α4β2* nicotinic receptors, but decreased α6β2*-mediated [³H]dopamine release in the lesioned dorsal striatum and the ventral striatum. In addition, nicotine treatment restored [³H]dopamine release in the lesioned ventral striatum to intact levels. The results support a role for nicotinic receptors as drug targets for treatment of LID, and suggest that striatal presynaptic α6β2* receptors are important mediators of nicotine's antidyskinetic effect.

PET Molecular Imaging Research of Levodopa-Induced Dyskinesias in Parkinson's Disease

Purpose of Review

To review the current status of positron emission tomography (PET) molecular imaging research of levodopa-induced dyskinesias (LIDs) in Parkinson’s disease (PD).

Recent Findings

Recent PET studies have provided robust evidence that LIDs in PD are associated with elevated and fluctuating striatal dopamine synaptic levels, which is a consequence of the imbalance between dopaminergic and serotonergic terminals, with the latter playing a key role in mishandling presynaptic dopamine release. Long-term exposure to levodopa is no longer believed to solely induce LIDs, as studies have highlighted that PD patients who go on to develop LIDs exhibit elevated putaminal dopamine release before the initiation of levodopa treatment, suggesting the involvement of other mechanisms, including altered neuronal firing and abnormal levels of phosphodiesterase 10A.

Summary

Dopaminergic, serotonergic, glutamatergic, adenosinergic and opioid systems and phosphodiesterase 10A levels have been shown to be implicated in the development of LIDs in PD. However, no system may be considered sufficient on its own for the development of LIDs, and the mechanisms underlying LIDs in PD may have a multisystem origin. In line with this notion, future studies should use multimodal PET molecular imaging in the same individuals to shed further light on the different mechanisms underlying the development of LIDs in PD.

Biomarkers for Parkinson's Disease: Recent Advancement

As a multi-factorial degenerative disease, Parkinson's disease (PD) leads to tremor, gait rigidity, and hypokinesia, thus hampering normal living. As this disease is usually detected in the later stages when neurons have degenerated completely, cure is on hold, ultimately leading to death due to the lack of early diagnostic techniques. Thus, biomarkers are required to detect the disease in the early stages when prevention is possible. Various biomarkers providing early diagnosis of the disease include those of imaging, cerebrospinal fluid, oxidative stress, neuroprotection, and inflammation. Also, biomarkers, alone or in combination, are used in the diagnosis and evolution of PD. This review encompasses various biomarkers available for PD and discusses recent advances in their development.

Striatal D1 medium spiny neuron activation induces dyskinesias in parkinsonian mice

BACKGROUND

Dyskinesias are a disabling motor complication that arises with prolonged l-dopa treatment. Studies using D1 receptor drugs and genetically modified mice suggest that medium spiny neurons expressing D1 receptors play a primary role in l-dopa-induced dyskinesias. However, the specific role of these neurons in dyskinesias is not fully understood.

METHODS

We used optogenetics, which allows for precise modulation of select neurons in vivo, to investigate whether striatal D1-expressing medium spiny neuron activity regulates abnormal involuntary movements or dyskinesia in parkinsonian mice. D1-cre mice unilaterally lesioned with 6-hydroxydopamine received striatal injections of cre-dependent channelrhodopsin2 virus or control virus. After stable virus expression, the effect of optical stimulation on dyskinesia was tested in l-dopa-naïve and l-dopa-primed mice.

RESULTS

Single-pulse and burst-optical stimulation of D1-expressing medium spiny neurons induced dyskinesias in l-dopa-naïve channelrhodopsin2 mice. In stably dyskinetic mice, l-dopa injection induced dyskinesia to a similar or somewhat greater extent than optical stimulation. Combined l-dopa administration and stimulation resulted in an additive increase in dyskinesias, indicating that other mechanisms also contribute. Molecular studies indicate that changes in extracellular signal-regulated kinase phosphorylation in D1-expressing medium spiny neurons are involved. Optical stimulation did not ameliorate parkinsonism in l-dopa-naïve mice. However, it improved parkinsonism in l-dopa-primed mice to a similar extent as l-dopa administration. None of the stimulation paradigms enhanced dyskinesia or modified parkinsonism in l-dopa-naïve or l-dopa-primed control virus mice.

CONCLUSION

The data provide direct evidence that striatal D1-expressing medium spiny neuron stimulation is sufficient to induce dyskinesias and contributes to the regulation of motor control. © 2017 International Parkinson and Movement Disorder Society.

Striatal cholinergic interneurons and D2 receptor-expressing GABAergic medium spiny neurons regulate tardive dyskinesia

Tardive dyskinesia (TD) is a drug-induced movement disorder that arises with antipsychotics. These drugs are the mainstay of treatment for schizophrenia and bipolar disorder, and are also prescribed for major depression, autism, attention deficit hyperactivity, obsessive compulsive and post-traumatic stress disorder. There is thus a need for therapies to reduce TD. The present studies and our previous work show that nicotine administration decreases haloperidol-induced vacuous chewing movements (VCMs) in rodent TD models, suggesting a role for the nicotinic cholinergic system. Extensive studies also show that D2 dopamine receptors are critical to TD. However, the precise involvement of striatal cholinergic interneurons and D2 medium spiny neurons (MSNs) in TD is uncertain. To elucidate their role, we used optogenetics with a focus on the striatum because of its close links to TD. Optical stimulation of striatal cholinergic interneurons using cholineacetyltransferase (ChAT)-Cre mice expressing channelrhodopsin2-eYFP decreased haloperidol-induced VCMs (~50%), with no effect in control-eYFP mice. Activation of striatal D2 MSNs using Adora2a-Cre mice expressing channelrhodopsin2-eYFP also diminished antipsychotic-induced VCMs, with no change in control-eYFP mice. In both ChAT-Cre and Adora2a-Cre mice, stimulation or mecamylamine alone similarly decreased VCMs with no further decline with combined treatment, suggesting nAChRs are involved. Striatal D2 MSN activation in haloperidol-treated Adora2a-Cre mice increased c-Fos⁺ D2 MSNs and decreased c-Fos⁺ non-D2 MSNs, suggesting a role for c-Fos. These studies provide the first evidence that optogenetic stimulation of striatal cholinergic interneurons and GABAergic MSNs modulates VCMs, and thus possibly TD. Moreover, they suggest nicotinic receptor drugs may reduce antipsychotic-induced TD.

Potential Role of Caffeine in the Treatment of Parkinson's Disease

Parkinson’s disease [PD] is the second most common neurodegenerative disorder after Alzheimer’s disease, affecting 1% of the population over the age of 55. The underlying neuropathology seen in PD is characterised by progressive loss of dopaminergic neurons in the substantia nigra pars compacta with the presence of Lewy bodies. The Lewy bodies are composed of aggregates of α-synuclein. The motor manifestations of PD include a resting tremor, bradykinesia, and muscle rigidity. Currently there is no cure for PD and motor symptoms are treated with a number of drugs including levodopa [L-dopa]. These drugs do not delay progression of the disease and often provide only temporary relief. Their use is often accompanied by severe adverse effects. Emerging evidence from both in vivo and in vitro studies suggests that caffeine may reduce parkinsonian motor symptoms by antagonising the adenosine A_2A receptor, which is predominately expressed in the basal ganglia. It is hypothesised that caffeine may increase the excitatory activity in local areas by inhibiting the astrocytic inflammatory processes but evidence remains inconclusive. In addition, the co-administration of caffeine with currently available PD drugs helps to reduce drug tolerance, suggesting that caffeine may be used as an adjuvant in treating PD. In conclusion, caffeine may have a wide range of therapeutic effects which are yet to be explored, and therefore warrants further investigation in randomized clinical trials.

Optogenetic activation of striatal cholinergic interneurons regulates L-dopa-induced dyskinesias

L-dopa-induced dyskinesias (LIDs) are a serious complication of L-dopa therapy for Parkinson's disease. Emerging evidence indicates that the nicotinic cholinergic system plays a role in LIDs, although the pathways and mechanisms are poorly understood. Here we used optogenetics to investigate the role of striatal cholinergic interneurons in LIDs. Mice expressing cre-recombinase under the control of the choline acetyltransferase promoter (ChAT-Cre) were lesioned by unilateral injection of 6-hydroxydopamine. AAV5-ChR2-eYFP or AAV5-control-eYFP was injected into the dorsolateral striatum, and optical fibers implanted. After stable virus expression, mice were treated with L-dopa. They were then subjected to various stimulation protocols for 2h and LIDs rated. Continuous stimulation with a short duration optical pulse (1-5ms) enhanced LIDs. This effect was blocked by the general muscarinic acetylcholine receptor (mAChR) antagonist atropine indicating it was mAChR-mediated. By contrast, continuous stimulation with a longer duration optical pulse (20ms to 1s) reduced LIDs to a similar extent as nicotine treatment (~50%). The general nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine blocked the decline in LIDs with longer optical pulses showing it was nAChR-mediated. None of the stimulation regimens altered LIDs in control-eYFP mice. Lesion-induced motor impairment was not affected by optical stimulation indicating that cholinergic transmission selectively regulates LIDs. Longer pulse stimulation increased the number of c-Fos expressing ChAT neurons, suggesting that changes in this immediate early gene may be involved. These results demonstrate that striatal cholinergic interneurons play a critical role in LIDs and support the idea that nicotine treatment reduces LIDs via nAChR desensitization.

α4 nicotinic acetylcholine receptor modulated by galantamine on nigrostriatal terminals regulates dopamine receptor-mediated rotational behavior

Galantamine, an acetylcholine esterase (AChE) inhibitor used to treat dementia symptoms, also acts as an allosteric potentiating ligand (APL) at nicotinic acetylcholine receptors (nAChRs). This study was designed to evaluate the allosteric effect of galantamine on nAChR regulation of nigrostrial dopaminergic neuronal function in the hemiparkinsonian rat model established by unilateral nigral 6-hydroxydopamine (6-OHDA) injection. Methamphetamine, a dopamine releaser, induced ipsilateral rotation, whereas dopamine agonists apomorphine (a non-selective dopamine receptor agonist), SKF38393 (a selective dopamine D1 receptor agonist), and quinpirole (a selective dopamine D2 receptor agonist) induced contralateral rotation. When 6-OHDA-injected rats were co-treated with nomifensine, a dopamine transporter inhibitor, a more pronounced and a remarkable effect of nicotine and galantamine was observed. Under these conditions, the combination of nomifensine with nicotine or galantamine induced the ipsilateral rotation similar to the methamphetamine-induced rotational behavior, indicating that nicotine and galantamine also induce dopamine release from striatal terminals. Both nicotine- and galantamine-induced rotations were significantly blocked by flupenthixol (an antagonist of both D1 and D2 dopamine receptors) and mecamylamine (an antagonist of nAChRs), suggesting that galantamine modulation of nAChRs on striatal dopaminergic terminals regulates dopamine receptor-mediated movement. Immunohistochemical staining showed that α4 nAChRs were highly expressed on striatal dopaminergic terminals, while no α7 nAChRs were detected. Pretreatment with the α4 nAChR antagonist dihydroxy-β-erythroidine significantly inhibited nicotine- and galantamine-induced rotational behaviors, whereas pretreatment with the α7 nAChR antagonist methyllycaconitine was ineffective. Moreover, the α4 nAChR agonist ABT-418 induced ipsilateral rotation, while the α7 nAChR agonist PNU282987 had no significant effect on rotational behavior. These results suggest that galantamine can enhance striatal dopamine release through allosteric modulation of α4 nAChRs on nigrostriatal dopaminergic terminals.

α7 nicotinic receptor agonists reduce levodopa-induced dyskinesias with severe nigrostriatal damage

BACKGROUND

ABT-126 is a novel, safe, and well-tolerated α7 nicotinic receptor agonist in a Phase 2 Alzheimer's disease study. We tested the antidyskinetic effect of ABT-126 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated squirrel monkeys with moderate and more severe nigrostriatal damage.

METHODS

Monkeys (n = 21, set 1) were lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine 1-2×. When parkinsonian, they were gavaged with levodopa (10 mg/kg)/carbidopa (2.5 mg/kg) twice daily and dyskinesias rated. They were then given nicotine in drinking water (n = 5), or treated with vehicle (n = 6) or ABT-126 (n = 10) twice daily orally 30 min before levodopa. Set 1 was then re-lesioned 1 to 2 times for a total of 3 to 4 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injections. The antidyskinetic effect of ABT-126, nicotine, and the β2* nicotinic receptor agonist ABT-894 was re-assessed. Another group of monkeys (n = 23, set 2) were lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine only 1× to 2×. They were treated with levodopa/carbidopa, administered the α7 agonist ABT-107 (n = 6), ABT-894 (n = 6), nicotine (n = 5), or vehicle (n = 6) and dyskinesias evaluated. All monkeys were euthanized and the dopamine transporter measured.

RESULTS

With moderate nigrostriatal damage (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine 1×-2×), ABT-126 dose-dependently decreased dyskinesias (∼60%), with similar results seen with ABT-894 (∼60%) or nicotine (∼60%). With more severe damage (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine 3-4×), ABT-126 and nicotine reduced dyskinesias, but ABT-894 did not. The dopamine transporter was 41% and 8.9% of control, with moderate and severe nigrostriatal damage, respectively. No drug modified parkinsonism.

CONCLUSION

The novel α7 nicotinic receptor drug ABT-126 reduced dyskinesias in monkeys with both moderate and severe nigrostriatal damage. ABT-126 may be useful to reduce dyskinesias in both early- and later-stage Parkinson's disease.

Preclinical Evidence for a Role of the Nicotinic Cholinergic System in Parkinson's Disease

One of the primary deficits in Parkinson's disease (PD) is the loss of dopaminergic neurons in the substantia nigra pars compacta which leads to striatal dopaminergic deficits that underlie the motor symptoms associated with the disease. A plethora of animal models have been developed over the years to uncover the molecular alterations that lead to PD development. These models have provided valuable information on neurotransmitter pathways and mechanisms involved. One such a system is the nicotinic cholinergic system. Numerous studies show that nigrostriatal damage affects nicotinic receptor-mediated dopaminergic signaling; therefore therapeutic modulation of the nicotinic cholinergic system may offer a novel approach to manage PD. In fact, there is evidence showing that nicotinic receptor drugs may be useful as neuroprotective agents to prevent Parkinson's disease progression. Additional preclinical studies also show that nicotinic receptor drugs may be beneficial for the treatment of L-dopa induced dyskinesias. Here, we review preclinical findings supporting the idea that nicotinic receptors are valuable therapeutic targets for PD.

Alpha7 nicotinic receptors as therapeutic targets for Parkinson's disease

Accumulating evidence suggests that CNS α7 nicotinic acetylcholine receptors (nAChRs) are important targets for the development of therapeutic approaches for Parkinson's disease. This progressive neurodegenerative disorder is characterized by debilitating motor deficits, as well as autonomic problems, cognitive declines, changes in affect and sleep disturbances. Currently l-dopa is the gold standard treatment for Parkinson's disease motor problems, particularly in the early disease stages. However, it does not improve the other symptoms, nor does it reduce the inevitable disease progression. Novel therapeutic strategies for Parkinson's disease are therefore critical. Extensive pre-clinical work using a wide variety of experimental models shows that nicotine and nAChR agonists protect against damage to nigrostriatal and other neuronal cells. This observation suggests that nicotine and/or nAChR agonists may be useful as disease modifying agents. Additionally, studies in several parkinsonian animal models including nonhuman primates show that nicotine reduces l-dopa-induced dyskinesias, a side effect of l-dopa therapy that may be as incapacitating as Parkinson's disease itself. Work with subtype selective nAChR agonists indicate that α7 nAChRs are involved in mediating both the neuroprotective and antidyskinetic effects, thus offering a targeted strategy with optimal beneficial effects and minimal adverse responses. Here, we review studies demonstrating a role for α7 nAChRs in protection against neurodegenerative effects and for the reduction of l-dopa-induced dyskinesias. Altogether, this work suggests that α7 nAChRs may be useful targets for reducing Parkinson's disease progression and for the management of the dyskinesias that arise with l-dopa therapy.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Analysis of gait in rats with olivocerebellar lesions and ability of the nicotinic acetylcholine receptor agonist varenicline to attenuate impairments

Studies have demonstrated that administration of the neuronal nicotinic receptor agonist varenicline to rats with olivocerebellar lesions attenuates balance deficits on a rotorod and balance beam, but the effects of this drug on gait deficits have not been investigated. To accomplish this, male Sprague-Dawley rats were trained to walk on a motorized treadmill at 25 and 35 cm/s and baseline performance determined; both temporal and spatial gait parameters were analyzed. A principal component analysis (PCA) was used to identify the key components of gait, and the cumulative gait index (CGI) was calculated, representing deviations from prototypical gait patterns. Subsequently, animals either remained as non-lesioned controls or received injections of 3-acetylpyridine (3-AP)/nicotinamide to destroy the climbing fibers innervating Purkinje cells. The gait of the non-lesioned group was assessed weekly to monitor changes in the normal population, while the gait of the lesioned group was assessed 1 week following 3-AP administration, and weekly following the daily administration of saline or varenicline (0.3, 1.0, or 3.0mg free base/kg) for 2 weeks. Non-lesioned animals exhibited a 60-70% increased CGI over time due to increases in temporal gait measures, whereas lesioned animals exhibited a nearly 3-fold increased CGI as a consequence of increases in spatial measures. Following 2 weeks of treatment with the highest dose of varenicline (3.0mg free base/kg), the swing duration of lesioned animals normalized, and stride duration, stride length and step angle in this population did not differ from the non-lesioned population. Thus, varenicline enabled animals to compensate for their impairments and rectify the timing of the gait cycle.

Molecular imaging of levodopa-induced dyskinesias

Levodopa-induced dyskinesias (LIDs) occur in the majority of patients with Parkinson's disease (PD) following years of levodopa treatment. The pathophysiology underlying LIDs in PD is poorly understood, and current treatments generate only minor benefits for the patients. Studies with positron emission tomography (PET) molecular imaging have demonstrated that in advanced PD patients, levodopa administration induces sharp increases in striatal dopamine levels, which correlate with LIDs severity. Fluctuations in striatal dopamine levels could be the result of the attenuated buffering ability in the dopaminergically denervated striatum. Lines of evidence from PET studies indicate that serotonergic terminals could also be responsible for the development of LIDs in PD by aberrantly processing exogenous levodopa and by releasing dopamine in a dysregulated manner from the serotonergic terminals. Additionally, other downstream mechanisms involving glutamatergic, cannabinoid, opioid, cholinergic, adenosinergic, and noradrenergic systems may contribute in the development of LIDs. In this article, we review the findings from preclinical, clinical, and molecular imaging studies, which have contributed to our understanding the pathophysiology of LIDs in PD.

Nicotinic receptor subtype-selective circuit patterns in the subthalamic nucleus

The glutamatergic subthalamic nucleus (STN) exerts control over motor output through nuclei of the basal ganglia. High-frequency electrical stimuli in the STN effectively alleviate motor symptoms in movement disorders, and cholinergic stimulation boosts this effect. To gain knowledge about the mechanisms of cholinergic modulation in the STN, we studied cellular and circuit aspects of nicotinic acetylcholine receptors (nAChRs) in mouse STN. We discovered two largely divergent microcircuits in the STN; these are regulated in part by either α4β2 or α7 nAChRs. STN neurons containing α4β2 nAChRs (α4β2 neurons) received more glutamatergic inputs, and preferentially innervated GABAergic neurons in the substantia nigra pars reticulata. In contrast, STN neurons containing α7 nAChRs (α7 neurons) received more GABAergic inputs, and preferentially innervated dopaminergic neurons in the substantia nigra pars compacta. Interestingly, local electrical stimuli excited a majority (79%) of α4β2 neurons but exerted strong inhibition in 58% of α7 neurons, indicating an additional diversity of STN neurons: responses to electrical stimulation. Chronic exposure to nicotine selectively affects α4β2 nAChRs in STN: this treatment increased the number of α4β2 neurons, upregulated α4-containing nAChR number and sensitivity, and enhanced the basal firing rate of α4β2 neurons both ex vivo and in vivo. Thus, chronic nicotine enhances the function of the microcircuit involving α4β2 nAChRs. This indicates chronic exposure to nicotinic agonist as a potential pharmacological intervention to alter selectively the balance between these two microcircuits, and may provide a means to inhibit substantia nigra dopaminergic neurons.

Evidence for a role for α6(∗) nAChRs in l-dopa-induced dyskinesias using Parkinsonian α6(∗) nAChR gain-of-function mice

l-Dopa-induced dyskinesias (LIDs) are a serious side effect of dopamine replacement therapy for Parkinson's disease. The mechanisms that underlie LIDs are currently unclear. However, preclinical studies indicate that nicotinic acetylcholine receptors (nAChRs) play a role, suggesting that drugs targeting these receptors may be of therapeutic benefit. To further understand the involvement of α6β2(∗) nAChRs in LIDs, we used gain-of-function α6(∗) nAChR (α6L9S) mice that exhibit a 20-fold enhanced sensitivity to nAChR agonists. Wildtype (WT) and α6L9S mice were lesioned by unilateral injection of 6-hydroxydopamine (6-OHDA, 3μg/ml) into the medial forebrain bundle. Three to 4wk later, they were administered l-dopa (3mg/kg) plus benserazide (15mg/kg) until stably dyskinetic. l-dopa-induced abnormal involuntary movements (AIMs) were similar in α6L9S and WT mice. WT mice were then given nicotine in the drinking water in gradually increasing doses to a final 300μg/ml, which resulted in a 40% decline AIMs. By contrast, there was no decrease in AIMs in α6L9S mice at a maximally tolerated nicotine dose of 20μg/ml. However, the nAChR antagonist mecamylamine (1mg/kg ip 30min before l-dopa) reduced l-dopa-induced AIMs in both α6L9S and WT mice. Thus, both a nAChR agonist and antagonist decreased AIMs in WT mice, but only the antagonist was effective in α6L9S mice. Since nicotine appears to reduce LIDs via desensitization, hypersensitive α6β2(∗) nAChRs may desensitize less readily. The present data show that α6β2(∗) nAChRs are key regulators of LIDs, and may be useful therapeutic targets for their management in Parkinson's disease.

Nicotinic acetylcholine receptor density in cognitively intact subjects at an early stage of Parkinson's disease

We investigated in vivo brain nicotinic acetylcholine receptor (nAChR) distribution in cognitively intact subjects with Parkinson’s disease (PD) at an early stage of the disease. Fourteen patients and 13 healthy subjects were imaged with single photon emission computed tomography and the radiotracer 5-[¹²³I]iodo-3-[2(S)-2-azetidinylmethoxy]pyridine ([¹²³I]5IA). Patients were selected according to several criteria, including short duration of motor signs (<7 years) and normal scores at an extensive neuropsychological evaluation. In PD patients, nAChR density was significantly higher in the putamen, the insular cortex and the supplementary motor area and lower in the caudate nucleus, the orbitofrontal cortex, and the middle temporal gyrus. Disease duration positively correlated with nAChR density in the putamen ipsilateral (ρ = 0.56, p < 0.05) but not contralateral (ρ = 0.49, p = 0.07) to the clinically most affected hemibody. We observed, for the first time in vivo, higher nAChR density in brain regions of the motor and limbic basal ganglia circuits of subjects with PD. Our findings support the notion of an up-regulated cholinergic activity at the striatal and possibly cortical level in cognitively intact PD patients at an early stage of disease.

The α7 nicotinic receptor agonist ABT-107 decreases L-Dopa-induced dyskinesias in parkinsonian monkeys

Previous studies in Parkinsonian rats and monkeys have shown that β2-selective nicotinic acetylcholine receptor (nAChR) agonists reduce l-Dopa-induced dyskinesias (LIDs), a serious complication of l-Dopa therapy for Parkinson's disease. Since rodent studies also suggested an involvement of α7 nAChRs in LIDs, we tested the effect of the potent, selective α7 agonist ABT-107 [5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole]. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-lesioned monkeys were gavaged with l-Dopa/carbidopa (10 and 2.5 mg/kg, respectively) twice daily, which resulted in stable LIDs. A dose-response study (0.03-1.0 mg/kg) showed that oral ABT-107 decreased LIDs by 40-60%. LIDs returned to control levels only after a 6-week ABT-107 washout, suggesting that long-term molecular changes were involved. Subsequent readministration of ABT-107 decreased LIDs by 50-60%, indicating that tolerance did not develop. ABT-107 had no effect on Parkinsonism or cognitive performance. We next tested ABT-107 together with the β2 agonist ABT-894 [(3-(5,6-dichloro-pyridin-3-yl)-1(S),5 (S)-3,6-diazabicyclo[3.2.0]heptane], previously shown to reduce LIDs in Parkinsonian monkeys. In one study, the monkeys were first given oral ABT-894 (0.01 mg/kg), which maximally decreased LIDs by 50-60%; they were then also treated with 0.1 mg/kg ABT-107, a dose that maximally reduced LIDs. The effect of combined treatment on LIDs was similar to that with either drug alone. Comparable results were observed in a group of monkeys first treated with ABT-107 and then also given ABT-894. Thus, α7 and β2 nAChR-selective drugs may function via a final common mechanism to reduce LIDs. The present results suggest that drugs targeting either α7 or β2 nAChRs may be useful as antidyskinetic agents in Parkinson's disease.

AQW051, a novel and selective nicotinic acetylcholine receptor α7 partial agonist, reduces l-Dopa-induced dyskinesias and extends the duration of l-Dopa effects in parkinsonian monkeys

Nicotinic acetylcholine receptor (nAChR)-mediated signaling has been implicated in levodopa (l-Dopa)-induced dyskinesias (LID). This study investigated the novel selective α7 nAChR partial agonist (R)-3-(6-ρ-Tolyl-pyridin-3-yloxy)-1-aza-bicyclo(2.2.2)octane (AQW051) for its antidyskinetic activity in l-Dopa-treated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned cynomolgus monkeys. Six MPTP monkeys were repeatedly treated with l-Dopa to develop reproducible dyskinesias. AQW051 (2, 8, and 15 mg/kg) administered 1 h before l-Dopa treatment did not affect their parkinsonian scores or locomotor activity, but did significantly extend the duration of the l-Dopa antiparkinsonian response, by 30 min at the highest AQW051 dose (15 mg/kg). Dyskinesias were significantly reduced for the total period of l-Dopa effect following treatment with 15 mg/kg; achieving a reduction of 60% in median values. Significant reductions in 1 h peak dyskinesia scores and maximal dyskinesias were also observed with AQW051 (15 mg/kg). To understand the exposure-effect relationship and guide dose selection in clinical trials, plasma concentration-time data for the 15 mg/kg AQW051 dose were collected from three of the MPTP monkeys in a separate pharmacokinetic experiment. No abnormal behavioral or physiological effects were reported following AQW051 treatment. Our results show that AQW051 at a high dose can reduce LID without compromising the benefits of l-Dopa and extend the duration of the l-Dopa antiparkinsonian response in MPTP monkeys. This supports the clinical testing of α7 nAChR agonists to modulate LID and extend the duration of the therapeutic effect of l-Dopa.

Role for the nicotinic cholinergic system in movement disorders; therapeutic implications

A large body of evidence using experimental animal models shows that the nicotinic cholinergic system is involved in the control of movement under physiological conditions. This work raised the question whether dysregulation of this system may contribute to motor dysfunction and whether drugs targeting nicotinic acetylcholine receptors (nAChRs) may be of therapeutic benefit in movement disorders. Accumulating preclinical studies now show that drugs acting at nAChRs improve drug-induced dyskinesias. The general nAChR agonist nicotine, as well as several nAChR agonists (varenicline, ABT-089 and ABT-894), reduces l-dopa-induced abnormal involuntary movements or dyskinesias up to 60% in parkinsonian nonhuman primates and rodents. These dyskinesias are potentially debilitating abnormal involuntary movements that arise as a complication of l-dopa therapy for Parkinson's disease. In addition, nicotine and varenicline decrease antipsychotic-induced abnormal involuntary movements in rodent models of tardive dyskinesia. Antipsychotic-induced dyskinesias frequently arise as a side effect of chronic drug treatment for schizophrenia, psychosis and other psychiatric disorders. Preclinical and clinical studies also show that the nAChR agonist varenicline improves balance and coordination in various ataxias. Lastly, nicotine has been reported to attenuate the dyskinetic symptoms of Tourette's disorder. Several nAChR subtypes appear to be involved in these beneficial effects of nicotine and nAChR drugs including α4β2*, α6β2* and α7 nAChRs (the asterisk indicates the possible presence of other subunits in the receptor). Overall, the above findings, coupled with nicotine's neuroprotective effects, suggest that nAChR drugs have potential for future drug development for movement disorders.

ABT-089 and ABT-894 reduce levodopa-induced dyskinesias in a monkey model of Parkinson's disease

Levodopa-induced dyskinesias (LIDs) are a serious complication of levodopa therapy for Parkinson's disease for which there is little treatment. Accumulating evidence shows that nicotinic acetylcholine receptor (nAChR) drugs decrease LIDs in parkinsonian animals. Here, we examined the effect of two β2 nAChR agonists, ABT-089 and ABT-894, that previously were approved for phase 2 clinical trials for other indications. Two sets of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys were administered levodopa/carbidopa (10 mg/kg and 2.5 mg/kg, respectively) twice daily 5 days a week until they were stably dyskinetic. Each set had a vehicle-treated group, an nAChR agonist-treated group, and a nicotine-treated group as a positive control. Set A monkeys had previously received other nAChR drugs (nAChR drug-primed), whereas Set B monkeys were initially nAChR drug-naive. Both sets were administered the partial agonist ABT-089 (range, 0.01-1.0 mg/kg) orally 5 days a week twice daily 30 minutes before levodopa with each dose given for 1 to 5 weeks. ABT-089 decreased LIDs by 30% to 50% compared with vehicle-treated monkeys. Nicotine reduced LIDs by 70% in a parallel group. After 4 weeks of washout, the effect of the full agonist ABT-894 (range, 0.0001-0.10 mg/kg) was assessed on LIDs in Set A and Set B. ABT-894 reduced LIDs by 70%, similar to nicotine. Both drugs acted equally well at α4β2* and α6β2* nAChRs; however, ABT-089 was 30 to 60 times less potent than ABT-894. Tolerance did not develop for the time periods tested (range, 3-4 months). The nAChR drugs did not worsen parkinsonism or cognitive ability. Emesis, a common problem with nAChR drugs, was not observed. ABT-894 and ABT-089 appear to be good candidate nAChR drugs for the management of LIDs in Parkinson's disease.

Nicotine-encapsulated poly(lactic-co-glycolic) acid nanoparticles improve neuroprotective efficacy against MPTP-induced parkinsonism

For some instances of Parkinson disease (PD), current evidence in the literature is consistent with reactive oxygen species being involved in the etiology of the disease. The management of PD is still challenging owing to its ambiguous etiology and lack of permanent cure. Because nicotine offers neuroprotection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism, the neuroprotective efficacy of nicotine-encapsulated poly(lactic-co-glycolic) acid (PLGA) nanoparticles and the underlying mechanism of improved efficacy, if any, over bulk nicotine were assessed in this study. The selected indicators of oxidative stress, dopaminergic neurodegeneration and apoptosis, were measured in both in vitro and rodent models of parkinsonism in the presence or absence of "nanotized" or bulk nicotine. The levels of dopamine and its metabolites were measured in the striatum, nicotine and its metabolite in the nigrostriatal tissues while the immunoreactivities of tyrosine hydroxylase (TH), metallothionein-III (MT-III), inducible nitric oxide synthase (iNOS) and microglial activation were checked in the substantia nigra of controls and treated mice. GSTA4-4, heme oxygenase (HO)-1, tumor suppressor protein 53 (p53), caspase-3, lipid peroxidation (LPO), and nitrite levels were measured in the nigrostriatal tissues. Nicotine-encapsulated PLGA nanoparticles improved the endurance of TH-immunoreactive neurons and the number of fiber outgrowths and increased the mRNA expression of TH, neuronal cell adhesion molecule, and growth-associated protein-43 over bulk against 1-methyl-4-phenyl pyridinium ion-induced degeneration in the in vitro model. MPTP reduced TH immunoreactivity and levels of dopamine and its metabolites and increased microglial activation, expression of GSTA4-4, iNOS, MT-III, HO-1, p53, and caspase-3, and levels of nitrite and LPO. Whereas both bulk nicotine and nicotine-encapsulated PLGA nanoparticles modulated the changes toward controls, the modulation was more pronounced in nicotine-encapsulated PLGA nanoparticle-treated parkinsonian mice. The levels of nicotine and cotinine were elevated in nicotine-encapsulated PLGA nanoparticle-treated PD mouse brain compared with bulk. The results obtained from this study demonstrate that nanotization of nicotine improves neuroprotective efficacy by enhancing its bioavailability and subsequent modulation in the indicators of oxidative stress and apoptosis.

Nicotinic receptor agonists reduce L-DOPA-induced dyskinesias in a monkey model of Parkinson's disease

Abnormal involuntary movements or dyskinesias are a serious complication of long-term l-DOPA treatment of Parkinson's disease, for which there are few treatment options. Accumulating preclinical data show that nicotine decreases l-DOPA-induced dyskinesias (LIDs), suggesting that it may be a useful antidyskinetic therapy for Parkinson's disease. Here, we investigated whether nicotinic acetylcholine receptor (nAChR) agonists reduced LIDs in nonhuman primates. We first tested the nonselective nAChR agonist 1, 6,7,8,9-tetrahydro-6,10-methano-6H-pyrazino[2,3-h][3]benzazepine (varenicline), which offers the advantage that it is approved by the U.S. Food and Drug Administration for use in humans. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys (n = 23) were first administered l-DOPA/carbidopa (10/2.5 mg/kg) twice daily 5 days/week until stably dyskinetic. Oral varenicline (0.03-0.10 mg/kg) decreased LIDs ∼50% compared with vehicle-treated monkeys, whereas nicotine treatment (300 µg/ml in drinking water) reduced LIDs by 70% in a parallel group of animals. We next tested the selective α4β2*/α6β2* nAChR agonist TC-8831 [3-cyclopropylcarbonyl-3,6-diazabicyclo[3.1.1]heptane] on LIDs in the same set of monkeys after a 10-week washout. We also tested TC-8831 in another set of MPTP-lesioned monkeys (n = 16) that were nAChR drug-naïve. Oral TC-8831 (0.03-0.3 mg/kg) reduced LIDs in both sets by 30-50%. After a washout period, repeat TC-8831 dosing led to a greater decline in LIDs (60%) in both sets of monkeys that was similar to the effect of nicotine. Tolerance to any nAChR drug did not develop over the course of the study (3-4 months). NAChR drug treatment did not worsen parkinsonism or cognitive ability. These data suggest that nAChR agonists may be useful for the management of dyskinesias in l-DOPA-treated Parkinson's disease patients.

Nicotine reduces established levodopa-induced dyskinesias in a monkey model of Parkinson's disease

Although 3,4-dihydroxyphenylalanine (levodopa) is the gold-standard treatment for Parkinson's disease, it can lead to disabling dyskinesias. Previous work demonstrated that nicotine reduces levodopa-induced dyskinesias (LIDs) in several parkinsonian animal models. The goal of this study was to determine whether the duration of nicotine administration affects its ability to reduce LIDs in levodopa-primed and levodopa-naíve monkeys and also to test whether tolerance develops to the beneficial effects of nicotine. Monkeys were injected with MPTP (1.9-2.0 mg/kg subcutaneously) over 3 to 5 months until parkinsonism developed. Nicotine (300 μg/mL) was administered in drinking water (over 4-6 months) to levodopa-primed or levodopa-naíve monkeys, with levodopa/carbidopa (10/2.5 mg/kg) gavaged twice daily. One set of MPTP-lesioned monkeys (n = 23) was first gavaged with levodopa and subsequently received nicotine 4 weeks later, when dyskinesias plateaued, or 8 weeks later, when dyskinesias were established. A 60% to 70% decrease in LIDs was observed after several weeks of nicotine treatment in both groups. A second set of monkeys (n = 26) received nicotine 8 or 2 weeks before levodopa. In the 8-week nicotine pretreatment group, there was an immediate reduction in LIDs, which plateaued at 60% to 70%. In the 2-week nicotine pretreatment group, there were initial small decreases in LIDs, which plateaued at 60% to 70% several weeks later. Thus, nicotine pretreatment and nicotine post-treatment were similarly efficacious in reducing LIDs. The beneficial effect of nicotine persisted throughout the study (17-23 weeks). Nicotine did not worsen parkinsonism. These data suggest that nicotine treatment has potential as a successful antidyskinetic therapy for patients with Parkinson's disease.

Multiple CNS nicotinic receptors mediate L-dopa-induced dyskinesias: studies with parkinsonian nicotinic receptor knockout mice

Accumulating evidence supports the idea that drugs acting at nicotinic acetylcholine receptors (nAChRs) may be beneficial for Parkinson's disease, a neurodegenerative movement disorder characterized by a loss of nigrostriatal dopaminergic neurons. Nicotine administration to parkinsonian animals protects against nigrostriatal damage. In addition, nicotine and nAChR drugs improve L-dopa-induced dyskinesias, a debilitating side effect of L-dopa therapy which remains the gold-standard treatment for Parkinson's disease. Nicotine exerts its antidyskinetic effect by interacting with multiple nAChRs. One approach to identify the subtypes specifically involved in L-dopa-induced dyskinesias is through the use of nAChR subunit null mutant mice. Previous work with β2 and α6 nAChR knockout mice has shown that α6β2* nAChRs were necessary for the development/maintenance of L-dopa-induced abnormal involuntary movements (AIMs). The present results in parkinsonian α4 nAChR knockout mice indicate that α4β2* nAChRs also play an essential role since nicotine did not reduce L-dopa-induced AIMs in such mice. Combined analyses of the data from α4 and α6 knockout mice suggest that the α6α4β2β3 subtype may be critical. In contrast to the studies with α4 and α6 knockout mice, nicotine treatment did reduce L-dopa-induced AIMs in parkinsonian α7 nAChR knockout mice. However, α7 nAChR subunit deletion alone increased baseline AIMs, suggesting that α7 receptors exert an inhibitory influence on L-dopa-induced AIMs. In conclusion, α6β2*, α4β2* and α7 nAChRs all modulate L-dopa-induced AIMs, although their mode of regulation varies. Thus drugs targeting one or multiple nAChRs may be optimal for reducing L-dopa-induced dyskinesias in Parkinson's disease.

TC-8831, a nicotinic acetylcholine receptor agonist, reduces L-DOPA-induced dyskinesia in the MPTP macaque

Long-term L-DOPA treatment for Parkinson's disease (PD) is limited by motor complications, particularly L-DOPA-induced dyskinesia (LID). A therapy with the ability to ameliorate LID without reducing anti-parkinsonian benefit would be of great value. We assessed the ability of TC-8831, an agonist at nicotinic acetylcholine receptors (nAChR) containing α6β2/α4β2 subunit combinations, to provide such benefits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine- (MPTP) lesioned macaques with established LID. Animals were treated orally for consecutive 14-day periods with twice-daily vehicle (weeks 1-2) or TC-8831 (0.03, 0.1 or 0.3 mg/kg, weeks 3-8). L-DOPA was also administered, once-daily, (weeks 1-12, median-dose 30 mg/kg, p.o.). For the following two-weeks (weeks 9-10), TC-8831 was washed out, while once-daily L-DOPA treatment was maintained. The effects of once-daily amantadine (3 mg/kg, p.o.) were then assessed over weeks 11-12. LID, parkinsonism, duration and quality of ON-time were assessed weekly by a neurologist blinded to treatment. TC-8831 reduced the duration of 'bad' ON-time (ON-time with disabling dyskinesia) by up to 62% and decreased LID severity (median score 18 cf. 34 (vehicle), 0.1 mg/kg, 1-3 h period). TC-8831 also significantly reduced choreiform and dystonic dyskinesia (median scores 6 and 31 cf. 19 and 31 respectively (vehicle), both 0.03 mg/kg, 1-3 h). At no time did TC-8831 treatment result in a reduction in anti-parkinsonian benefit of L-DOPA. By comparison, amantadine also significantly reduced dyskinesia and decreased 'bad' ON-time (up to 61%) but at the expense of total ON-time (reduced by up to 23%). TC-8831 displayed robust anti-dyskinetic actions and improved the quality of ON-time evoked by L-DOPA without any reduction in anti-parkinsonian benefit.

Neuronal nicotinic receptor agonists improve gait and balance in olivocerebellar ataxia

Clinical studies have reported that the nicotinic receptor agonist varenicline improves balance and coordination in patients with several types of ataxia, but confirmation in an animal model has not been demonstrated. This study investigated whether varenicline and nicotine could attenuate the ataxia induced in rats following destruction of the olivocerebellar pathway by the neurotoxin 3-acetylpyridine (3-AP). The administration of 3-AP (70 mg/kg followed by 300 mg niacinamide/kg; i.p.) led to an 85% loss of inferior olivary neurons within one week without evidence of recovery, and was accompanied by a 72% decrease in rotorod activity, a 3-fold increase in the time to traverse a stationary beam, a 19% decrease in velocity and 31% decrease in distance moved in the open field, and alterations in gait parameters, with a 19% increase in hindpaw stride width. The daily administration of nicotine (0.33 mg free base/kg) for one week improved rotorod performance by 50% and normalized the increased hindpaw stride width, effects that were prevented by the daily preadministration of the nicotinic antagonist mecamylamine (0.8 mg free base/kg). Varenicline (1 and 3 mg free base/kg daily) also improved rotorod performance by approximately 50% following one week of administration, and although it did not alter the time to traverse the beam, it did improve the ability to maintain balance on the beam. Neither varenicline nor nicotine, at doses that improved balance, affected impaired locomotor activity in the open field. Results provide evidence that nicotinic agonists are of benefit for alleviating some of the behavioral deficits in olivocerebellar ataxia and warrant further studies to elucidate the specific mechanism(s) involved.

α4β2 Nicotinic receptors play a role in the nAChR-mediated decline in L-dopa-induced dyskinesias in parkinsonian rats

L-Dopa-induced dyskinesias are a serious long-term side effect of dopamine replacement therapy for Parkinson's disease for which there are few treatment options. Our previous studies showed that nicotine decreased l-dopa-induced abnormal involuntary movements (AIMs). Subsequent work with knockout mice demonstrated that α6β2* nicotinic receptors (nAChRs) play a key role. The present experiments were done to determine if α4β2* nAChRs are also involved in l-dopa-induced dyskinesias. To approach this, we took advantage of the finding that α6β2* nAChRs are predominantly present on striatal dopaminergic nerve terminals, while a significant population of α4β2* nAChRs are located on other neurons. Thus, a severe dopaminergic lesion would cause a major loss in α6β2*, but not α4β2* nAChRs. Experiments were therefore done in which rats were unilaterally lesioned with 6-hydroxydopamine, at a dose that led to severe nigrostriatal damage. The dopamine transporter, a dopamine nerve terminal marker, was decreased by >99%. This lesion also decreased striatal α6β2* nAChRs by 97%, while α4β2* nAChRs were reduced by only 12% compared to control. A series of β2* nAChR compounds, including TC-2696, TI-10165, TC-8831, TC-10600 and sazetidine reduced l-dopa-induced AIMs in these rats by 23-32%. TC-2696, TI-10165, TC-8831 were also tested for parkinsonism, with no effect on this behavior. Tolerance did not develop with up to 3 months of treatment. Since α4α5β2 nAChRs are also predominantly on striatal dopamine terminals, these data suggest that drugs targeting α4β2 nAChRs may reduce l-dopa-induced dyskinesias in late stage Parkinson's disease.

The nicotine-mediated decline in l-dopa-induced dyskinesias is associated with a decrease in striatal dopamine release

l-dopa-induced dyskinesias (LIDs) are a side effect of Parkinson's disease therapy that is thought to arise, at least in part, because of excessive dopaminergic activity. Thus, drugs that regulate dopaminergic tone may provide an approach to manage LIDs. Our previous studies showed that nicotine treatment reduced LIDs in Parkinsonian animal models. This study investigates whether nicotine may exert its beneficial effects by modulating pre-synaptic dopaminergic function. Rats were unilaterally lesioned by injection of 6-hydroxydopamine (6-OHDA) (2 × 3 ug per site) into the medial forebrain bundle to yield moderate Parkinsonism. They were then implanted with minipumps containing vehicle or nicotine (2.0 mg/kg/d) and rendered dyskinetic with l-dopa (8 mg/kg plus 15 mg/kg benserazide). Lesioning alone decreased the striatal dopamine transporter, nicotinic receptor (nAChR) levels, and nAChR-mediated (3)H-dopamine release, consistent with previous results. Nicotine administration reduced l-dopa-induced abnormal involuntary movements throughout the course of the study (4 months). Nicotine treatment led to declines in the striatal dopamine transporter, α6β2* nAChRs and various components of α6β2* and α4β2* nAChR-mediated release. l-dopa treatment had no effect. These data suggest that nicotine may improve LIDs in Parkinsonian animal models by dampening striatal dopaminergic activity.

Nicotine-mediated improvement in L-dopa-induced dyskinesias in MPTP-lesioned monkeys is dependent on dopamine nerve terminal function

L-dopa-induced dyskinesias (LIDs) are abnormal involuntary movements that develop with long term L-dopa therapy for Parkinson's disease. Studies show that nicotine administration reduced LIDs in several parkinsonian animal models. The present work was done to understand the factors that regulate the nicotine-mediated reduction in LIDs in MPTP-lesioned nonhuman primates. To approach this, we used two groups of monkeys, one with mild-moderate and the other with more severe parkinsonism rendered dyskinetic using L-dopa. In mild-moderately parkinsonian monkeys, nicotine pretreatment (300 μg/ml via drinking water) prevented the development of LIDs by ~75%. This improvement was maintained when the nicotine dose was lowered to 50 μg/ml but was lost with nicotine removal. Nicotine re-exposure again decreased LIDs. By contrast, nicotine treatment did not reduce LIDs in monkeys with more severe parkinsonism. We next determined how nicotine's ability to reduce LIDs correlated with lesion-induced changes in the striatal dopamine transporter and (3)H-dopamine release in these two groups of monkeys. The striatal dopamine transporter was reduced to 54% and 28% of control in mild-moderately and more severely parkinsonian monkeys, respectively. However, basal, K(+), α4β2* and α6β2* nAChR-evoked (3)H-dopamine release were near control levels in striatum of mild-moderately parkinsonian monkeys. By contrast, these same release measures were reduced to a significantly greater extent in striatum of more severely parkinsonian monkeys. Thus, nicotine best improves LIDs in lesioned monkeys in which striatal dopamine transmission is still relatively intact. These data suggest that nicotine treatment would most effectively reduce LIDs in patients with mild to moderate Parkinson's disease.

Nicotine as a potential neuroprotective agent for Parkinson's disease

Converging research efforts suggest that nicotine and other drugs that act at nicotinic acetylcholine receptors (nAChRs) may be beneficial in the management of Parkinson's disease. This idea initially stemmed from the results of epidemiological studies that demonstrated that smoking is associated with a decreased incidence of Parkinson's disease. The subsequent finding that nicotine administration protected against nigrostriatal damage in parkinsonian animal models led to the idea that nicotine in tobacco products may contribute to this apparent protective action. Nicotine most likely exerts its effects by interacting at nAChRs. Accumulating research indicates that multiple subtypes containing nAChRs, including α4β2, α6β2, and/or α7, may be involved. Stimulation of nAChRs initially activates various intracellular transduction pathways primarily via alterations in calcium signaling. Consequent adaptations in immune responsiveness and trophic factors may ultimately mediate nicotine's ability to reduce/halt the neuronal damage that arises in Parkinson's disease. In addition to a potential neuroprotective action, nicotine also has antidepressant properties and improves attention/cognition. Altogether, these findings suggest that nicotine and nAChR drugs represent promising therapeutic agents for the management of Parkinson's disease.