In vitro and in vivo acetylcholine release from rat striatum as a functional paradigm of signal transduction via a D-2 dopamine receptor

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.

Strength of cholinergic tone dictates the polarity of dopamine D2 receptor modulation of striatal cholinergic interneuron excitability in DYT1 dystonia

Balance between cholinergic and dopaminergic signaling is central to striatal control of movement and cognition. In dystonia, a common disorder of movement, anticholinergic therapy is often beneficial. This observation suggests there is a pathological increase in cholinergic tone, yet direct confirmation is lacking. In DYT1, an early-onset genetic form of dystonia caused by a mutation in the protein torsinA (TorA), the suspected heightened cholinergic tone is commonly attributed to faulty dopamine D2 receptor (D2R) signaling where D2R agonists cause excitation of striatal cholinergic interneurons (ChIs), rather than the normal inhibition of firing observed in wild-type animals, an effect known as "paradoxical excitation". Here, we provide for the first time direct measurement of elevated striatal extracellular acetylcholine (ACh) in a knock-in mouse model of human DYT1 dystonia (TorA^∆E/+ mice), confirming a striatal hypercholinergic state. We hypothesized that this elevated extracellular ACh might cause chronic over-activation of muscarinic acetylcholine receptors (mAChRs) and disrupt normal D2R function due to their shared coupling to G_i/o-proteins. We tested this concept in vitro first using a broad-spectrum mAChR antagonist, and then using a M2/M4 mAChR selective antagonist to specifically target mAChRs expressed by ChIs. Remarkably, we found that mAChR inhibition reverses the D2R-mediated paradoxical excitation of ChIs recorded in slices from TorA^∆E/+ mice to a typical inhibitory response. Furthermore, we recapitulated the paradoxical D2R excitation of ChIs in striatal slices from wild-type mice within minutes by simply increasing cholinergic tone through pharmacological inhibition of acetylcholinesterase (AChE) or by prolonged agonist activation of mAChRs. Collectively, these results show that enhanced mAChR tone itself is sufficient to rapidly reverse the polarity of D2R regulation of ChI excitability, correcting the previous notion that the D2R mediated paradoxical ChI excitation causes the hypercholinergic state in dystonia. Further, using a combination of genetic and pharmacological approaches, we found evidence that this switch in D2R polarity results from a change in coupling from the preferred G_i/o pathway to non-canonical β-arrestin signaling. These results highlight the need to fully understand how the mutation in TorA leads to pathologically heightened extracellular ACh. Furthermore the discovery of this novel ACh-dopamine interaction and the participation of β-arrestin in regulation of cholinergic interneurons is likely important for other basal ganglia disorders characterized by perturbation of ACh-dopamine balance, including Parkinson and Huntington diseases, l-DOPA-induced dyskinesia and schizophrenia.

Muscarinic mechanisms of antipsychotic atypicality

The interactions of the atypical antipsychotic drugs (APD) clozapine, olanzapine, risperidone, quetiapine and ziprasidone with muscarinic receptors were reviewed. Only clozapine and olanzapine have marked affinity for muscarinic receptors in radioligand binding studies; however, the affinity of these compounds is considerably lower than classical muscarinic antagonists. Although functional assays in cell lines transfected with muscarinic receptors suggest that olanzapine and clozapine have weak partial agonist activity at muscarinic receptors, particularly M4 receptors, studies in vitro and in vivo indicate that the compounds function as antagonists. In animal studies and in humans, clozapine has pronounced antimuscarinic effects whereas olanzapine has weak antimuscarinic effects. However, olanzapine significantly occupies central muscarinic receptors in humans. Overall, the role of muscarinic receptors in the antipsychotic effects of clozapine and olanzapine is controversial and complex.

Cholinergic lesions of mouse striatum induced by AF64A alter D2 dopaminergic behavior and reduce D2 dopamine receptors and D2 dopamine receptor mRNA

To determine whether dopamine receptors are expressed in acetylcholine-containing neurons intrinsic to the striatum, and to study further the interactions between the dopaminergic and cholinergic systems, the irreversibly acting cholinergic neurotoxin, ethylcholine mustard aziridinium ion (AF64A), was injected unilaterally into the mouse corpus striatum, and rotational behavior induced by dopamine agonists and certain molecular events associated with this lesion were determined 7 days after lesioning. Brains were analyzed for D2 dopamine receptors by autoradiography, using [3H](-)sulpiride as a ligand, and for D2 dopamine receptor mRNA and glutamic acid decarboxylase mRNA by Northern blot analysis, using selective radiolabelled oligonucleotide probes. Choline uptake sites were determined by binding assays using [3H]hemicholinium-3, a selective choline reuptake blocker, as a ligand. Mice with intrastriatal injections of AF64A showed ipsilateral rotational responses to the non-selective dopamine agonist apomorphine and to the D2 dopamine agonists, pergolide and quinpirole, but not to the D1 dopamine agonist SKF 38393. This was associated with a significant reduction in D2 dopamine receptors in the ipsilateral striatum and a significant decrease in the amount of D2 dopamine receptor mRNA. That AF64A produced a relatively selective cholinergic deficit was supported by the evidence showing that AF64A lesions significantly reduced [3H]hemicholinium-3 binding sites but did not alter glutamic acid decarboxylase (GAD) mRNA. Further, hemicholinium-3, prevented the AF64A-induced changes in rotational behavior. These results suggest that striatal cholinergic interneurons contain D2 dopamine receptors and express the D2 dopamine receptor gene, and that these interneurons are involved in dopamine-mediated rotational behavior.