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

Nicotinic acetylcholine receptor subtype expression, function, and pharmacology: Therapeutic potential of α-conotoxins

The pentameric nicotinic acetylcholine receptors (nAChRs) are typically classed as muscle- or neuronal-type, however, the latter has also been reported in non-neuronal cells. Given their broad distribution, nAChRs mediate numerous physiological and pathological processes including synaptic transmission, presynaptic modulation of transmitter release, neuropathic pain, inflammation, and cancer. There are 17 different nAChR subunits and combinations of these subunits produce subtypes with diverse pharmacological properties. The expression and role of some nAChR subtypes have been extensively deciphered with the aid of knock-out models. Many nAChR subtypes expressed in heterologous systems are selectively targeted by the disulfide-rich α-conotoxins. α-Conotoxins are small peptides isolated from the venom of cone snails, and a number of them have potential pharmaceutical value.

Striatal synaptic adaptations in Parkinson's disease

The striatum is densely innervated by mesencephalic dopaminergic neurons that modulate acquisition and vigor of goal-directed actions and habits. This innervation is progressively lost in Parkinson's disease (PD), contributing to the defining movement deficits of the disease. Although boosting dopaminergic signaling with levodopa early in the course of the disease alleviates these deficits, later this strategy leads to the emergence of debilitating dyskinesia. Here, recent advances in our understanding of how striatal cells and circuits adapt to this progressive de-innervation and to levodopa therapy are discussed. First, we discuss how dopamine (DA) depletion triggers cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity but also lead to disruption of the synaptic architecture sculpted by experience. Second, we discuss the roles played by cholinergic and nitric oxide-releasing interneurons in these adaptations. Third, we examine recent work in freely moving mice suggesting that alterations in the spatiotemporal dynamics of striatal ensembles contributes to PD movement deficits. Lastly, we discuss recently published evidence from a progressive model of PD suggesting that contrary to the classical model, striatal pathway imbalance is necessary but not sufficient to produce frank parkinsonism.

Cysteine [2,4] Disulfide Bond as a New Modifiable Site of α-Conotoxin TxIB

α-Conotoxin TxIB, a selective antagonist of α6/α3β2β3 nicotinic acetylcholine receptor, could be a potential therapeutic agent for addiction and Parkinson's disease. As a peptide with a complex pharmacophoric conformation, it is important and difficult to find a modifiable site which can be modified effectively and efficiently without activity loss. In this study, three xylene scaffolds were individually reacted with one pair of the cysteine residues ([1,3] or [2,4]), and iodine oxidation was used to form a disulfide bond between the other pair. Overall, six analogs were synthesized with moderate isolated yields from 55% to 65%, which is four times higher than the traditional two-step oxidation with orthogonal protection on cysteines. The cysteine [2,4] modified analogs, with higher stability in human serum than native TxIB, showed obvious inhibitory effect and selectivity on α6/α3β2β3 nicotinic acetylcholine receptors (nAChRs), which was 100 times more than the cysteine [1,3] modified ones. This result demonstrated that the cysteine [2,4] disulfide bond is a new modifiable site of TxIB, and further modification can be a simple and feasible strategy for the exploitation and utilization of α-Conotoxin TxIB in drug discovery.

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.

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.

Trihexyphenidyl rescues the deficit in dopamine neurotransmission in a mouse model of DYT1 dystonia

Trihexyphenidyl, a nonselective muscarinic receptor antagonist, is the small molecule drug of choice for the treatment of DYT1 dystonia, but it is poorly tolerated due to significant side effects. A better understanding of the mechanism of action of trihexyphenidyl is needed for the development of improved treatments. Because DTY1 dystonia is associated with both abnormal cholinergic neurotransmission and abnormal dopamine regulation, we tested the hypothesis that trihexyphenidyl normalizes striatal dopamine release in a mouse model of DYT1 dystonia using ex vivo fast scan cyclic voltammetry and in vivo microdialysis. Trihexyphenidyl increased striatal dopamine release and efflux as assessed by ex vivo voltammetry and in vivo microdialysis respectively. In contrast, ʟ-DOPA, which is not usually effective for the treatment of DYT1 dystonia, did not increase dopamine release in either Dyt1 or control mice. Trihexyphenidyl was less effective at enhancing dopamine release in Dyt1 mice relative to controls ex vivo (mean increase WT: 65% vs Dyt1: 35%). Trihexyphenidyl required nicotinic receptors but not glutamate receptors to increase dopamine release. Dyt1 mice were more sensitive to the dopamine release decreasing effects of nicotinic acetylcholine receptor antagonism (IC₅₀: WT = 29.46 nM, Dyt1 = 12.26 nM) and less sensitive to acetylcholinesterase inhibitors suggesting that nicotinic acetylcholine receptor neurotransmission is altered in Dyt1 mice, that nicotinic receptors indirectly mediate the differential effects of trihexyphenidyl in Dyt1 mice, and that nicotinic receptors may be suitable therapeutic targets for DYT1 dystonia.

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.

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

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.

Deletion of lynx1 reduces the function of α6* nicotinic receptors

The α6 nicotinic acetylcholine receptor (nAChR) subunit is an attractive drug target for treating nicotine addiction because it is present at limited sites in the brain including the reward pathway. Lynx1 modulates several nAChR subtypes; lynx1-nAChR interaction sites could possibly provide drug targets. We found that dopaminergic cells from the substantia nigra pars compacta (SNc) express lynx1 mRNA transcripts and, as assessed by co-immunoprecipitation, α6 receptors form stable complexes with lynx1 protein, although co-transfection with lynx1 did not affect nicotine-induced currents from cell lines transfected with α6 and β2. To test whether lynx1 is important for the function of α6 nAChRs in vivo, we bred transgenic mice carrying a hypersensitive mutation in the α6 nAChR subunit (α6L9′S) with lynx1 knockout mice, providing a selective probe of the effects of lynx1 on α6* nAChRs. Lynx1 removal reduced the α6 component of nicotine-mediated rubidium efflux and dopamine (DA) release from synaptosomal preparations with no effect on numbers of α6β2 binding sites, indicating that lynx1 is functionally important for α6* nAChR activity. No effects of lynx1 removal were detected on nicotine-induced currents in slices from SNc, suggesting that lynx1 affects presynaptic α6* nAChR function more than somatic function. In the absence of agonist, lynx1 removal did not alter DA release in dorsal striatum as measured by fast scan cyclic voltammetry. Lynx1 removal affected some behaviors, including a novel-environment assay and nicotine-stimulated locomotion. Trends in 24-hour home-cage behavior were also suggestive of an effect of lynx1 removal. Conditioned place preference for nicotine was not affected by lynx1 removal. The results show that some functional and behavioral aspects of α6-nAChRs are modulated by lynx1.

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.

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.

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.

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.