Nicotine reduces levodopa-induced dyskinesias in lesioned monkeys

The dual role of striatal interneurons: circuit modulation and trophic support for the basal ganglia

ABSTRACT

Striatal interneurons play a key role in modulating striatal-dependent behaviors, including motor activity and reward and emotional processing. Interneurons not only provide modulation to the basal ganglia circuitry under homeostasis but are also involved in changes to plasticity and adaptation during disease conditions such as Parkinson's or Huntington's disease. This review aims to summarize recent findings regarding the role of striatal cholinergic and GABAergic interneurons in providing circuit modulation to the basal ganglia in both homeostatic and disease conditions. In addition to direct circuit modulation, striatal interneurons have also been shown to provide trophic support to maintain neuron populations in adulthood. We discuss this interesting and novel role of striatal interneurons, with a focus on the maintenance of adult dopaminergic neurons from interneuron-derived sonic-hedgehog.

Striosomes and Matrisomes: Scaffolds for Dynamic Coupling of Volition and Action

Striosomes form neurochemically specialized compartments of the striatum embedded in a large matrix made up of modules called matrisomes. Striosome-matrix architecture is multiplexed with the canonical direct-indirect organization of the striatum. Striosomal functions remain to be fully clarified, but key information is emerging. First, striosomes powerfully innervate nigral dopamine-containing neurons and can completely shut down their activity, with a following rebound excitation. Second, striosomes receive limbic and cognition-related corticostriatal afferents and are dynamically modulated in relation to value-based actions. Third, striosomes are spatially interspersed among matrisomes and interneurons and are influenced by local and global neuromodulatory and oscillatory activities. Fourth, striosomes tune engagement and the motivation to perform reinforcement learning, to manifest stereotypical behaviors, and to navigate valence conflicts and valence discriminations. We suggest that, at an algorithmic level, striosomes could serve as distributed scaffolds to provide formats of the striatal computations generated through development and refined through learning. We propose that striosomes affect subjective states. By transforming corticothalamic and other inputs to the functional formats of the striatum, they could implement state transitions in nigro-striato-nigral circuits to affect bodily and cognitive actions according to internal motives whose functions are compromised in neuropsychiatric conditions.

Mucuna pruriens Seed Aqueous Extract Improved Neuroprotective and Acetylcholinesterase Inhibitory Effects Compared with Synthetic L-Dopa

L-dopa, a dopaminergic agonist, is the gold standard for the treatment of Parkinson’s disease. However, due to the long-term toxicity and adverse effects of using L-dopa as the first-line therapy for Parkinson’s disease, a search for alternative medications is an important current challenge. Traditional Ayurvedic medicine has suggested the use of Mucuna pruriens Linn. (Fabaceae) as an anti-Parkinson’s agent. The present study aimed to quantify the amount of L-dopa in M. pruriens seed extract by HPLC analysis. The cytotoxicity and neuroprotective properties of M. pruriens aqueous extract were investigated by two in vitro models including the serum deprivation method and co-administration of hydrogen peroxide assay. The results showed the significant neuroprotective activities of M. pruriens seed extracts at a concentration of 10 ng/mL. In addition, the effects of L-dopa and M. pruriens seed extract on in vitro acetylcholinesterase activities were studied. M. pruriens seed extract demonstrated acetylcholinesterase inhibitory activity, while synthetic L-dopa enhanced the activity of the enzyme. It can be concluded that the administration of M. pruriens seed might be effective in protecting the brain against neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases. M. prurience seed extract containing L-dopa has shown less acetylcholinesterase activity stimulation compared with L-dopa, suggesting that the extract might have a superior benefit for use in the treatment of Parkinson’s disease.

Pathophysiological Mechanisms and Experimental Pharmacotherapy for L-Dopa-Induced Dyskinesia

L-dopa-induced dyskinesia (LID) is the most frequent motor complication associated with chronic L-dopa treatment in Parkinson’s disease (PD). Recent advances in the understanding of the pathophysiological mechanisms underlying LID suggest that abnormalities in multiple neurotransmitter systems, in addition to dopaminergic nigrostriatal denervation and altered dopamine release and reuptake dynamics at the synaptic level, are involved in LID development. Increased knowledge of neurobiological LID substrates has led to the development of several drug candidates to alleviate this motor complication. However, with the exception of amantadine, none of the pharmacological therapies tested in humans have demonstrated clinically relevant beneficial effects. Therefore, LID management is still one of the most challenging problems in the treatment of PD patients. In this review, we first describe the known pathophysiological mechanisms of LID. We then provide an updated report of experimental pharmacotherapies tested in clinical trials of PD patients and drugs currently under study to alleviate LID. Finally, we discuss available pharmacological LID treatment approaches and offer our opinion of possible issues to be clarified and future therapeutic strategies.

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.

Targeting the cholinergic system in Parkinson's disease

Motor control in the striatum is an orchestra played by various neuronal populations. Loss of harmony due to dopamine deficiency is considered the primary pathological cause of the symptoms of Parkinson’s disease (PD). Recent progress in experimental approaches has enabled us to examine the striatal circuitry in a much more comprehensive manner, not only reshaping our understanding of striatal functions in movement regulation but also leading to new opportunities for the development of therapeutic strategies for treating PD. In addition to dopaminergic innervation, giant aspiny cholinergic interneurons (ChIs) within the striatum have long been recognized as a critical node for balancing dopamine signaling and regulating movement. With the roles of ChIs in motor control further uncovered and more specific manipulations available, striatal ChIs and their corresponding receptors are emerging as new promising therapeutic targets for PD. This review summarizes recent progress in functional studies of striatal circuitry and discusses the translational implications of these new findings for the treatment of PD.

Characterization of AN6001, a positive allosteric modulator of α6β2-containing nicotinic acetylcholine receptors

α6β2-Containing nicotinic acetylcholine receptors (α6β2* nAChRs) are predominantly expressed in midbrain dopaminergic neurons, including substantia nigra pars compacta (SNc) neurons and their projections to striatal regions, where they regulate dopamine release and nigrostriatal activity. It is well established that nAChR agonists exert protection against dopaminergic neurotoxicity in cellular assays and parkinsonian animal models. Historically, drug development in the nAChR field has been mostly focused on development of selective agonists and positive allosteric modulators (PAMs) for the predominant neuronal nAChRs, α7 and α4β2. Here, we report the discovery and characterization of AN6001, a novel selective α6β2* nAChR PAM. AN6001 mediated increases in both nicotine potency and efficacy at the human α6/α3β2β3^V9'S nAChR in HEK293 cells, and it positively modulated ACh-evoked currents through both α6/α3β2β3^V9'S and a concatenated β3-α6-β2-α6-β2 receptor in Xenopus oocytes, displaying EC₅₀ values of 0.58 µM and 0.40 µM, respectively. In contrast, the compound did not display significant modulatory activity at α4β2, α3β4, α7 and muscle nAChRs. AN6001 also increased agonist-induced dopamine release from striatal synaptosomes and augmented agonist-induced global cellular responses and inward currents in dopaminergic neurons in SNc slices (measured by Ca²⁺ imaging and patch clamp recordings, respectively). Finally, AN6001 potentiated the neuroprotective effect of nicotine at MPP⁺-treated primary dopaminergic neurons. Overall, our studies demonstrate the existence of allosteric sites on α6β2* nAChRs and that positive modulation of native α6β2* receptors strengthens DA signaling. Hence, AN6001 represents an important tool for studies of α6β2* nAChRs and furthermore underlines the therapeutic potential in these receptors in Parkinson's disease.

Nicotine Bitartrate Reduces Falls and Freezing of Gait in Parkinson Disease: A Reanalysis

Objective: Determine if NC001, an oral formulation of nicotine that reduces levodopa-induced dyskinesias (LIDs) in MPTP-Parkinson monkeys, could reduce falls, freezing of gait (FOG), and LIDs in Parkinson disease (PD) patients.

Methods: Previously collected data from a study analyzing the effects of NC001 on LIDs in PD patients were reanalyzed. Because indirect-acting cholinergic drugs are sometimes helpful in reducing falls, we hypothesized that NC001, a direct-acting cholinergic agonist, could reduce falls in PD. The original 12-center, double-blind, randomized trial enrolled 65 PD patients. NC001 or placebo was administered 4 times per day for 10 weeks, beginning at 4 mg/day and escalating to 24 mg/day. Assessments included the Unified Dyskinesia Rating Scale (UDysRS) and Parts II-III of the original Unified Parkinson's Disease Rating Scale (UPDRS).

Results: Randomization (1:1) resulted in 35 patients on NC001 and 30 on placebo at baseline. Thirty and 27 patients, respectively, had data available for an intent-to-treat analysis. NC001 was safe and well-tolerated. After 10 weeks, NC001 patients (14/30) had a significant reduction in falls vs. placebo patients (3/27) (p = 0.0041) as assessed by UPDRS Part II. NC001 patients (12/30) also had significantly reduced FOG vs. placebo patients (4/27) (p = 0.0043). NC001 patients, compared with placebo patients, had a significant improvement (p = 0.01) in UDysRS ambulation subtest (40% vs. 3%, respectively). Although NC001 patients had a greater reduction in dyskinesias on the UDysRS than placebo patients (30% vs. 19%, respectively), this was not significant (p = 0.09).

Conclusions: NC001 significantly improved two refractory symptoms of PD, falls and FOG. The reduction in falls and FOG is attributed to selective stimulation of nicotinic receptors.

Clinical Trial Registration: Conducted under IND 105, 268, serial number 0000. ClinicalTrials.gov identifier NCT00957918.

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.

The MPTP Story

The identification of MPTP, a relatively simple compound which causes selective degeneration of the substantia nigra after systemic administration, has had an a significant impact on the understanding and treatment of Parkinson’s disease (PD) over the last 30 years. This article is prefaced by the intriguing “medical detective story” that lead to the discovery of the biological effects of MPTP in humans. The steps that lead to the unraveling its mechanism of action and their impact on research into pathways underlying nigrostriatal degeneration are reviewed. The impact of the animal models that have been developed utilizing MPTP is also described with a focus on the translational implications of MPTP-related research. These include use of MAO-B inhibitors aimed at neuroprotection in PD and the importance of a stable primate model for PD which was utilized to better understand the circuitry of the basal ganglia, and the identification of the subthalamic nucleus as a target for deep brain stimulation. Finally, the results of a broad range of epidemiologic studies aimed as assessing the impact of environmental factors in PD that have been inspired by MPTP are summarized, including the discovery of other neurotoxicants (rotenone and paraquat) with parkinsonogenic effects. Overall, this article attempts to describe how the discovery of this nigral neurotoxicant began, where it is currently, and what the future may hold.

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.

Moist smokeless tobacco (Snus) use and risk of Parkinson's disease

Background

Cigarette smoking is associated with a lower risk of Parkinson's disease. It is unclear what constituent of tobacco smoke may lower the risk. Use of Swedish moist smokeless tobacco (snus) can serve as a model to disentangle what constituent of tobacco smoke may lower the risk. The aim of this study was to determine whether snus use was associated with a lower risk of Parkinson's disease.

Methods

Individual participant data were collected from seven prospective cohort studies, including 348 601 men. We used survival analysis with multivariable Cox regression to estimate study-specific relative risk of Parkinson's disease due to snus use, and random-effects models to pool estimates in a meta-analysis. The primary analyses were restricted to never-smokers to eliminate the potential confounding effect of tobacco smoking.

Results

During a mean follow-up time of 16.1 years, 1199 incident Parkinson's disease cases were identified. Among men who never smoked, ever-snus users had about 60% lower Parkinson's disease risk compared with never-snus users [pooled hazard ratio (HR) 0.41, 95% confidence interval (CI) 0.28-0.61]. The inverse association between snus use and Parkinson's disease risk was more pronounced in current (pooled HR 0.38, 95% CI 0.23-0.63), moderate-heavy amount (pooled HR 0.41, 95% CI 0.19-0.90) and long-term snus users (pooled HR 0.44, 95% CI 0.24-0.83).

Conclusions

Non-smoking men who used snus had a substantially lower risk of Parkinson's disease. Results also indicated an inverse dose-response relationship between snus use and Parkinson's disease risk. Our findings suggest that nicotine or other components of tobacco leaves may influence the development 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.

Emerging preclinical pharmacological targets for Parkinson's disease

Parkinson's disease (PD) is a progressive neurological condition caused by the degeneration of dopaminergic neurons in the basal ganglia. It is the most prevalent form of Parkinsonism, categorized by cardinal features such as bradykinesia, rigidity, tremors, and postural instability. Due to the multicentric pathology of PD involving inflammation, oxidative stress, excitotoxicity, apoptosis, and protein aggregation, it has become difficult to pin-point a single therapeutic target and evaluate its potential application. Currently available drugs for treating PD provide only symptomatic relief and do not decrease or avert disease progression resulting in poor patient satisfaction and compliance. Significant amount of understanding concerning the pathophysiology of PD has offered a range of potential targets for PD. Several emerging targets including AAV-hAADC gene therapy, phosphodiesterase-4, potassium channels, myeloperoxidase, acetylcholinesterase, MAO-B, dopamine, A2A, mGlu5, and 5-HT-1A/1B receptors are in different stages of clinical development. Additionally, alternative interventions such as deep brain stimulation, thalamotomy, transcranial magnetic stimulation, and gamma knife surgery, are also being developed for patients with advanced PD. As much as these therapeutic targets hold potential to delay the onset and reverse the disease, more targets and alternative interventions need to be examined in different stages of PD. In this review, we discuss various emerging preclinical pharmacological targets that may serve as a new promising neuroprotective strategy that could actually help alleviate PD and its symptoms.

Prior nicotine self-administration attenuates subsequent dopaminergic deficits of methamphetamine in rats: role of nicotinic acetylcholine receptors

Preclinical studies have demonstrated that oral nicotine exposure attenuates long-term dopaminergic damage induced by toxins, including repeated, high doses of methamphetamine. It is suggested that alterations in nicotinic acetylcholine receptor (nAChR) expression, including α4β2* and α6β2* subtypes, likely contribute to this protection. The current study extended these findings by investigating whether nicotine self-administration in male, Sprague-Dawley rats (a) attenuates short-term dopaminergic damage induced by methamphetamine and (b) causes alterations in levels of α4β2* and α6β2* nAChR subtypes. The findings indicate that nicotine self-administration (0.032 mg/kg/infusion for 14 days) per se did not alter α4β2* and α6β2* nAChR expression or dopamine transporter (DAT) expression and function. Interestingly, prior nicotine self-administration attenuated methamphetamine-induced decreases in DAT function when assessed 24 h, but not 1 h, after methamphetamine treatment (4×7.5 mg/kg/injection). The ability of nicotine to attenuate the effects of methamphetamine on DAT function corresponded with increases in α4β2*, but not α6β2*, nAChR binding density. Understanding the role of nAChRs in methamphetamine-induced damage has the potential to elucidate mechanisms underlying the etiology of disorders involving dopaminergic dysfunction, as well as to highlight potential new therapeutic strategies for prevention or reduction of dopaminergic neurodegeneration.

Patient-driven N-of-1 in Parkinson's Disease. Lessons Learned from a Placebo-controlled Study of the Effect of Nicotine on Dyskinesia

BACKGROUND

New insights and knowledge in biomedical science often come from observation and experimentation. Methods traditionally used include self-experimentation, case reports, randomised controlled trials, and N-of-1 studies. Technological advances have lead to an increasing number of individuals and patients engaging in self-tracking. We use the term patient-driven N-of-1 for self-tracking performed with the explicit intention to disseminate the results by academic publishing.

OBJECTIVES

The aim of the study was to: 1) explore the potential role for patient-driven N-of-1 studies as a tool for improving self-management in Parkinson's disease (PD) using the example of managing levodopa-induced dyskinesia (LID) with nicotine, and 2) based on this example; identify some specific challenges of patient-driven N-of-1 studies.

METHODS

We used a placebo controlled patient-driven N-of-1 study with nicotine administered via e-cigarette to treat LID. The first author initiated and conducted the experiment on herself and noted her observations. The evaluations of the potential of N-of-1 for improving self-management of PD as well as the effects of nicotine on dyskinesia were based on the perception of the subject. During the planning and undertaking of the experiment, notes were made to identify challenges specific to patient-driven N-of-1 studies.

RESULTS

The subject was able to distinguish a decrease of her LID from nicotine but no effect from placebo. The main challenges of patient-driven N-of-1 studies were identified to be associated with planning of the study, recruiting a suitable research team, making sure the data collection is optimal, analysis of data, and publication of results.

CONCLUSIONS

Our study indicates that nicotine administered via e-cigarette may have an effect on levodopa-induced dyskinesia in individual patients with PD. The main contribution is however highlighting the work done by patients on a daily basis for understanding their conditions and conducting self-tracking experiments. More work is needed to further develop methods around patient-driven N-of-1 studies for PD.

Evaluation of AZD1446 as a Therapeutic in DYT1 Dystonia

DYT1 dystonia is an early-onset, hyperkinetic movement disorder caused by a deletion in the gene TOR1A, which encodes the protein torsinA. Several lines of evidence show that in animal models of DTY1 dystonia, there is impaired basal dopamine (DA) release and enhanced acetylcholine tone. Clinically, anticholinergic drugs are the most effective pharmacological treatment for DYT1 dystonia, but the currently used agents are non-selective muscarinic antagonists and associated with side effects. We used a DYT1 ∆GAG knock-in mouse model (DYT1 KI) to investigate whether nicotine and/or a non-desensitizing nicotinic agonist, AZD1446, would increase DA output in DYT1 dystonia. Using in vivo microdialysis, we found that DYT1 KI mice showed significantly increased DA output and greater sensitivity to nicotine compared to wild type (WT) littermate controls. In contrast, neither systemic injection (0.25–0.75 mg/kg) or intrastriatal infusion (30 μM–1 mM) of AZD1446 had a significant effect on DA efflux in WT or DYT1 KI mice. In vitro, we found that AZD1446 had no effect on the membrane properties of striatal spiny projection neurons (SPNs) and did not alter the spontaneous firing of ChI interneurons in either WT or DYT1 KI mice. We did observe that the firing frequency of dopaminergic neurons was significantly increased by AZD1446 (10 μM), an effect blocked by dihydro-beta-erythroidine (DHβE 3 μM), but the effect was similar in WT and DYT1 KI mice. Our results support the view that DYT1 models are associated with abnormal striatal cholinergic transmission, and that the DYT1 KI animals have enhanced sensitivity to nicotine. We found little effect of AZD1446 in this model, suggesting that other approaches to nicotinic modulation should be explored.

Investigational drugs in Phase I and Phase II for Levodopa-induced dyskinesias

INTRODUCTION

Prolonged treatment of Parkinson's disease (PD) with levodopa (L-DOPA) results in motor complications, including motor fluctuations and involuntary movements known as L-DOPA induced dyskinesias (LIDs). LIDs represent an additional cause of disability for PD patients and a major challenge for the clinical neurologist. Preclinical research has provided invaluable insights into the molecular and neural substrates of LIDs, identifying a number of potential targets for new anti-dyskinetic strategies. Areas covered: This review article is centered on drugs currently in Phase I and II clinical trials for LIDs and their relative pharmacological targets, which include glutamate, acetylcholine, serotonin, adrenergic receptors and additional targets of potential therapeutic interest. Expert opinion: LIDs are sustained by complex molecular and neurobiological mechanisms that are difficult to disentangle or target, unless one or more prevalent mechanisms are identified. In this context, the role of the serotonergic system and mGluR5 glutamate receptors seem to stand out. Interesting results have been obtained, for example, with partial 5-HT1A/5-HT1B receptor agonist eltoprazine and mGluR5 negative allosteric modulator dipraglurant. Confirmation of these results through large-scale, Phase III clinical trials will be needed, to obtain new pharmacological tools that may be used to optimize the treatment of PD patients with motor complications.

Cholinergic/glutamatergic co-transmission in striatal cholinergic interneurons: new mechanisms regulating striatal computation

It is well established that neurons secrete neuropeptides and ATP with classical neurotransmitters; however, certain neuronal populations are also capable of releasing two classical neurotransmitters by a process named co-transmission. Although there has been progress in our understanding of the molecular mechanism underlying co-transmission, the individual regulation of neurotransmitter secretion and the functional significance of this neuronal 'bilingualism' is still unknown. Striatal cholinergic interneurons (CINs) have been shown to secrete glutamate (Glu) in addition to acetylcholine (ACh) and are recognized for their role in the regulation of striatal circuits and behavior. Our review highlights the recent research into identifying mechanisms that regulate the secretion and function of Glu and ACh released by CINs and the roles these neurons play in regulating dopamine secretion and striatal activity. In particular, we focus on how the transporters for ACh (VAChT) and Glu (VGLUT3) influence the storage of neurotransmitters in CINs. We further discuss how these individual neurotransmitters regulate striatal computation and distinct aspects of behavior that are regulated by the striatum. We suggest that understanding the distinct and complementary functional roles of these two neurotransmitters may prove beneficial in the development of therapies for Parkinson's disease and addiction. Overall, understanding how Glu and ACh secreted by CINs impacts striatal activity may provide insight into how different populations of 'bilingual' neurons are able to develop sophisticated regulation of their targets by interacting with multiple receptors but also by regulating each other's vesicular storage. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

A Placebo-Controlled Trial of AQW051 in Patients With Moderate to Severe Levodopa-Induced Dyskinesia

BACKGROUND

This phase 2 randomized, double-blind, placebo-controlled study evaluated the efficacy and safety of the nicotinic acetylcholine receptor α7 agonist AQW051 in patients with Parkinson's disease and levodopa-induced dyskinesia.

METHODS

Patients with idiopathic Parkinson's disease and moderate to severe levodopa-induced dyskinesia were randomized to AQW051 10 mg (n = 24), AQW051 50 mg (n = 24), or placebo (n = 23) once daily for 28 days. Coprimary end points were change in Modified Abnormal Involuntary Movement Scale and Unified Parkinson's Disease Rating Scale part III scores. Secondary outcomes included pharmacokinetics.

RESULTS

In total, 67 patients completed the study. AQW051-treated patients experienced no significant improvements in Modified Abnormal Involuntary Movement Scale or Unified Parkinson's Disease Rating Scale part III scores by day 28. AQW051 was well tolerated; the most common adverse events were dyskinesia, fatigue, nausea, and falls.

CONCLUSIONS

AQW051 did not significantly reduce dyskinesia or parkinsonian severity. © 2016 International Parkinson and Movement Disorder Society.

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.

Deficits in cholinergic neurotransmission and their clinical correlates in Parkinson's disease

In view of its ability to explain the most frequent motor symptoms of Parkinson’s Disease (PD), degeneration of dopaminergic neurons has been considered one of the disease’s main pathophysiological features. Several studies have shown that neurodegeneration also affects noradrenergic, serotoninergic, cholinergic and other monoaminergic neuronal populations. In this work, the characteristics of cholinergic deficits in PD and their clinical correlates are reviewed. Important neurophysiological processes at the root of several motor and cognitive functions remit to cholinergic neurotransmission at the synaptic, pathway, and circuital levels. The bulk of evidence highlights the link between cholinergic alterations and PD motor symptoms, gait dysfunction, levodopa-induced dyskinesias, cognitive deterioration, psychosis, sleep abnormalities, autonomic dysfunction, and altered olfactory function. The pathophysiology of these symptoms is related to alteration of the cholinergic tone in the striatum and/or to degeneration of cholinergic nuclei, most importantly the nucleus basalis magnocellularis and the pedunculopontine nucleus. Several results suggest the clinical usefulness of antimuscarinic drugs for treating PD motor symptoms and of inhibitors of the enzyme acetylcholinesterase for the treatment of dementia. Data also suggest that these inhibitors and pedunculopontine nucleus deep-brain stimulation might also be effective in preventing falls. Finally, several drugs acting on nicotinic receptors have proved efficacious for treating levodopa-induced dyskinesias and cognitive impairment and as neuroprotective agents in PD animal models. Results in human patients are still lacking.

Drugs of abuse and Parkinson's disease

The term "drug of abuse" is highly contextual. What constitutes a drug of abuse for one population of patients does not for another. It is therefore important to examine the needs of the patient population to properly assess the status of drugs of abuse. The focus of this article is on the bidirectional relationship between patients and drug abuse. In this paper we will introduce the dopaminergic systems of the brain in Parkinson's and the influence of antiparkinsonian drugs upon them before discussing this synergy of condition and medication as fertile ground for drug abuse. We will then examine the relationship between drugs of abuse and Parkinson's, both beneficial and deleterious. In summary we will draw the different strands together and speculate on the future merit of current drugs of abuse as treatments for Parkinson's disease.

Plant-Derived Natural Products for Parkinson's Disease Therapy

Plant-derived natural products have made their own niche in the treatment of neurological diseases since time immemorial. Parkinson's disease (PD), the second most prevalent neurodegenerative disorder, has no cure and the treatment available currently is symptomatic. This chapter thoughtfully and objectively assesses the scientific basis that supports the increasing use of these plant-derived natural products for the treatment of this chronic and progressive disorder. Proper considerations are made on the chemical nature, sources, preclinical tests and their validity, and mechanisms of behavioural or biochemical recovery observed following treatment with various plants derived natural products relevant to PD therapy. The scientific basis underlying the neuroprotective effect of 6 Ayurvedic herbs/formulations, 12 Chinese medicinal herbs/formulations, 33 other plants, and 5 plant-derived molecules have been judiciously examined emphasizing behavioral, cellular, or biochemical aspects of neuroprotection observed in the cellular or animal models of the disease. The molecular mechanisms triggered by these natural products to promote cell survivability and to reduce the risk of cellular degeneration have also been brought to light in this study. The study helped to reveal certain limitations in the scenario: lack of preclinical studies in all cases barring two; heavy dependence on in vitro test systems; singular animal or cellular model to establish any therapeutic potential of drugs. This strongly warrants further studies so as to reproduce and confirm these reported effects. However, the current literature offers scientific credence to traditionally used plant-derived natural products for the treatment of PD.

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.

New treatments for the motor symptoms of Parkinson's disease

Levodopa remains the most potent drug to treat motor symptoms in Parkinson's disease (PD); however, motor fluctuations and levodopa-induced dyskinesia that occur with long-term use restrict some of its therapeutic value. Despite these limitations, the medical treatment of PD strives for continuous relief of symptoms using different strategies throughout the course of the illness: increasing the half-life of levodopa, using 'levodopa-sparing agents' and adding non-dopaminergic drugs. New options to 'improve' delivery of levodopa are under investigation, including long-acting levodopa, nasal inhalation and continuous subcutaneous or intrajejunal administration of levodopa. Long-acting dopamine agonists were recently developed and are undergoing further comparative studies to investigate potential superiority over the immediate-release formulations. Non-dopaminergic drugs acting on adenosine receptors, cholinergic, adrenergic, serotoninergic and glutamatergic pathways are newly developed and many are being evaluated in Phase II and Phase III trials. This article focuses on promising novel therapeutic approaches for the management of PD motor symptoms and motor complications. We will provide an update since 2011 on new formulations of current drugs, new drugs with promising results in Phase II and Phase III clinical trials, old drugs with new possibilities and some new potential strategies that are currently in Phase I and II of development (study start date may precede 2011 but are included as study is still ongoing or full data have not yet been published). Negative Phase II and Phase III clinical trials published since 2011 will also be briefly mentioned.

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.

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.

Lynx1 supports neuronal health in the mouse dorsal striatum during aging: an ultrastructural investigation

Nicotinic acetylcholine receptors have been shown to participate in neuroprotection in the aging brain. Lynx protein modulators dampen the activity of the cholinergic system through direct interaction with nicotinic receptors. Although lynx1 null mutant mice exhibit augmented learning and plasticity, they also exhibit macroscopic vacuolation in the dorsal striatum as they age, detectable at the optical microscope level. Despite the relevance of the lynx1 gene to brain function, little is known about the cellular ultrastructure of these age-related changes. In this study, we assessed degeneration in the dorsal striatum in 1-, 3-, 7-, and 13-month-old mice, using optical and transmission electron microscopy. We observed a loss of nerve fibers, a breakdown in nerve fiber bundles, and a loss of neuronal nuclei in the 13-month-old lynx1 null striatum. At higher magnification, these nerve fibers displayed intracellular vacuoles and disordered myelin sheaths. Few or none of these morphological alterations were present in younger lynx1 null mutant mice or in heterozygous lynx1 null mutant mice at any age. These data indicate that neuronal health can be maintained by titrating lynx1 dosage and that the lynx1 gene may participate in a trade-off between neuroprotection and augmented learning.

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.