Pharmacological antagonism of kainate receptor rescues dysfunction and loss of dopamine neurons in a mouse model of human parkin-induced toxicity

Chronic hyperactivation of midbrain dopamine neurons causes preferential dopamine neuron degeneration

Parkinson's disease (PD) is characterized by the death of substantia nigra (SNc) dopamine (DA) neurons, but the pathophysiological mechanisms that precede and drive their death remain unknown. The activity of DA neurons is likely altered in PD, but we understand little about if or how chronic changes in activity may contribute to degeneration. To address this question, we developed a chemogenetic (DREADD) mouse model to chronically increase DA neuron activity, and confirmed this increase using ex vivo electrophysiology. Chronic hyperactivation of DA neurons resulted in prolonged increases in locomotor activity during the light cycle and decreases during the dark cycle, consistent with chronic changes in DA release and circadian disturbances. We also observed early, preferential degeneration of SNc projections, recapitulating the PD hallmarks of selective vulnerability of SNc axons and the comparative resilience of ventral tegmental area axons. This was followed by eventual loss of midbrain DA neurons. Continuous DREADD activation resulted in a sustained increase in baseline calcium levels, supporting an important role for increased calcium in the neurodegeneration process. Finally, spatial transcriptomics from DREADD mice examining midbrain DA neurons and striatal targets, and cross-validation with human patient samples, provided insights into potential mechanisms of hyperactivity-induced toxicity and PD. Our results thus reveal the preferential vulnerability of SNc DA neurons to increased neural activity, and support a potential role for increased neural activity in driving degeneration in PD.

Role of dopamine neuron activity in Parkinson's disease pathophysiology

Neural activity is finely tuned to produce normal behaviors, and disruptions in activity likely occur early in the course of many neurodegenerative diseases. However, how neural activity is altered, and how these changes influence neurodegeneration is poorly understood. Here, we focus on evidence that the activity of dopamine neurons is altered in Parkinson's disease (PD), either as a compensatory response to degeneration or as a result of circuit dynamics or pathologic proteins, based on available human data and studies in animal models of PD. We then discuss how this abnormal activity may augment other neurotoxic phenomena in PD, including mitochondrial deficits, protein aggregation and spread, dopamine toxicity, and excitotoxicity. A more complete picture of how activity is altered and the resulting effects on dopaminergic neuron health and function may inform future therapeutic interventions to target and protect dopamine neurons from degeneration.

Parkinson's disease, epilepsy, and amyotrophic lateral sclerosis-emerging role of AMPA and kainate subtypes of ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) mediate the majority of excitatory neurotransmission and are implicated in various neurological disorders. In this review, we discuss the role of the two fastest iGluRs subtypes, namely, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors, in the pathogenesis and treatment of Parkinson's disease, epilepsy, and amyotrophic lateral sclerosis. Although both AMPA and kainate receptors represent promising therapeutic targets for the treatment of these diseases, many of their antagonists show adverse side effects. Further studies of factors affecting the selective subunit expression and trafficking of AMPA and kainate receptors, and a reasonable approach to their regulation by the recently identified novel compounds remain promising directions for pharmacological research.

Integrative multilevel exploration of the mechanism by which Er-Zhi-Wan alleviates the Parkinson's disease (PD)-like phenotype in the MPTP-induced PD mouse model

The neuroprotective effects of Er-Zhi-Wan (EZW), a well-known traditional Chinese formulation, in MPTP-induced Parkinson's disease (PD) models are poorly understood and require evaluation. A model of PD induced by MPTP was used to evaluate the neuroprotective effects of EZW in mice. The underlying pharmacological mechanisms of EZW for the prevention and treatment of PD were then explored using a combination of multilevel databases, network pharmacology, biological experiments, and LCMS/MS. In vivo data showed that pretreatment with EZW can be neuroprotective against MPTP-induced motor dysfunction and can effectively rescue dopaminergic neurons from MPTP-induced degeneration in mice. Furthermore, data from combined multilevel databases and network pharmacology analysis strategies suggested that the neuroprotective activity of EZW in the treatment of PD is mediated by a complicated multicomponent, multitarget network. Genes such as Grm2, Grm5, Drd2, and Grik2 were identified as important therapeutic targets. Subsequent experimental validation showed that EZW can broadly regulate the mRNA levels of these receptor genes as well as BDNF, and consequently increase the phosphorylation levels of CREB to stimulate CREB signaling. These targets and signaling systems may be responsible for the reversal of neuronal death by EZW after MPTP exposure. The LC-MS/MS results also identified a wide range of chemical components of EZW, including at least 53 precise compounds, further demonstrating the complexity of the network in which EZW exerts its neuroprotective activity. Our work provides evidence for the mechanism of EZW in MPTP-PD models and supports the neuroprotective function of EZW in neurodegenerative diseases.

(S)-2-Mercaptohistidine: A First Selective Orthosteric GluK3 Antagonist

The development of tool compounds for the ionotropic glutamate receptors (iGluRs) remains an important research objective, as these are essential for the study and understanding of the roles of these receptors in health and disease. Herein, we report on the pharmacological characterization of (S)-2-hydroxyhistidine (2a) and (S)-2-mercaptohistidine (2b) as mediators of glutamatergic neurotransmission. While 2a displayed negligible binding affinity or activity at all glutamate receptors and transporters investigated, 2b displayed selectivity for homomeric GluK3 with binding affinities in the low micromolar range (K_i = 6.42 ± 0.74 μM). The iGluR subtype selectivity ratio for 2b was calculated at ∼30-fold for GluK1/GluK3, GluA3/GluK3, and GluA4/GluK3 and >100-fold for GluK2/GluK3, GluA1/GluK3, and GluA2/GluK3. Unexpectedly, functional characterization of 2b revealed that the compound is an antagonist (K_b = 7.6 μM) at homomeric GluK3 receptors while exhibiting only weak agonist activity at GluA2 (EC₅₀ = 3.25 ± 0.55 mM). The functional properties of 2b were explored further in electrophysiological recordings of mouse hippocampal neurons.

Pathophysiological Features of Nigral Dopaminergic Neurons in Animal Models of Parkinson's Disease

The degeneration of nigral dopaminergic neurons is considered the hallmark of Parkinson’s disease (PD), and it is triggered by different factors, including mitochondrial dysfunction, Lewy body accumulation, neuroinflammation, excitotoxicity and metal accumulation. Despite the extensive literature devoted to unravelling the signalling pathways involved in neuronal degeneration, little is known about the functional impairments occurring in these cells during illness progression. Of course, it is not possible to obtain direct information on the properties of the dopaminergic cells in patients. However, several data are available in the literature reporting changes in the function of these cells in PD animal models. In the present manuscript, we focus on dopaminergic neuron functional properties and summarize shared or peculiar features of neuronal dysfunction in different PD animal models at different stages of the disease in an attempt to design a picture of the functional modifications occurring in nigral dopaminergic neurons during disease progression preceding their eventual death.

A Critical Review of Zebrafish Models of Parkinson's Disease

A wide variety of human diseases have been modelled in zebrafish, including various types of cancer, cardiovascular diseases and neurodegenerative diseases like Alzheimer’s and Parkinson’s. Recent reviews have summarized the currently available zebrafish models of Parkinson’s Disease, which include gene-based, chemically induced and chemogenetic ablation models. The present review updates the literature, critically evaluates each of the available models of Parkinson’s Disease in zebrafish and compares them with similar models in invertebrates and mammals to determine their advantages and disadvantages. We examine gene-based models, including ones linked to Early-Onset Parkinson’s Disease: PARKIN, PINK1, DJ-1, and SNCA; but we also examine LRRK2, which is linked to Late-Onset Parkinson’s Disease. We evaluate chemically induced models like MPTP, 6-OHDA, rotenone and paraquat, as well as chemogenetic ablation models like metronidazole-nitroreductase. The article also reviews the unique advantages of zebrafish, including the abundance of behavioural assays available to researchers and the efficiency of high-throughput screens. This offers a rare opportunity for assessing the potential therapeutic efficacy of pharmacological interventions. Zebrafish also are very amenable to genetic manipulation using a wide variety of techniques, which can be combined with an array of advanced microscopic imaging methods to enable in vivo visualization of cells and tissue. Taken together, these factors place zebrafish on the forefront of research as a versatile model for investigating disease states. The end goal of this review is to determine the benefits of using zebrafish in comparison to utilising other animals and to consider the limitations of zebrafish for investigating human disease.

Restoration and targeting of aberrant neurotransmitters in Parkinson's disease therapeutics

Neurotransmitters are considered as a fundamental regulator in the process of neuronal growth, differentiation and survival. Parkinson's Disease (PD) occurs due to extensive damage of dopamine-producing neurons; this causes dopamine deficits in the midbrain, followed by the alternation of various other neurotransmitters (glutamate, GABA, serotonin, etc.). It has been observed that fluctuation of neurotransmission in the basal ganglia exhibits a great impact on the pathophysiology of PD. Dopamine replacement therapy, such as the use of L-DOPA, can increase the dopamine level, but it majorly ameliorates the motor symptoms and is also associated with long-term complications (for e.g., LID). While the non-dopaminergic system can efficiently target non-motor symptoms, for instance, the noradrenergic system regulates the synthesis of BDNF via the MAPK pathway, which is important in learning and memory. Herein, we briefly discuss the role of different neurotransmitters, implementation of neurotransmitter receptors in PD. We also illustrate the recent advances of neurotransmitter-based drugs, which are currently under in vivo and clinical studies. Reinstating normal neurotransmitter levels has been believed to be advantageous in the treatment of PD. Thus, there is an increasing demand for drugs that can specifically target the neurotransmission system and reinstate the normal levels of neurotransmitters, which might prevent or delay neurodegeneration in PD.

Cell-attached and Whole-cell Patch-clamp Recordings of Dopamine Neurons in the Substantia Nigra Pars Compacta of Mouse Brain Slices

The Substantia Nigra pars compacta (SNc) is a midbrain dopaminergic nucleus that plays a key role in modulating motor and cognitive functions. It is crucially involved in several disorders, particularly Parkinson's disease, which is characterized by a progressive loss of SNc dopaminergic cells. Electrophysiological studies on SNc neurons are of paramount importance to understand the role of dopaminergic transmission in health and disease. Here, we provide an extensive protocol to prepare SNc-containing mouse brain slices and record the electrical activity of dopaminergic cells. We describe all the necessary steps, including mouse transcardiac perfusion, brain extraction, slice cutting, and patch-clamp recordings.

Animal Models of Autosomal Recessive Parkinsonism

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder. The neuropathological hallmark of the disease is the loss of dopamine neurons of the substantia nigra pars compacta. The clinical manifestations of PD are bradykinesia, rigidity, resting tremors and postural instability. PD patients often display non-motor symptoms such as depression, anxiety, weakness, sleep disturbances and cognitive disorders. Although, in 90% of cases, PD has a sporadic onset of unknown etiology, highly penetrant rare genetic mutations in many genes have been linked with typical familial PD. Understanding the mechanisms behind the DA neuron death in these Mendelian forms may help to illuminate the pathogenesis of DA neuron degeneration in the more common forms of PD. A key step in the identification of the molecular pathways underlying DA neuron death, and in the development of therapeutic strategies, is the creation and characterization of animal models that faithfully recapitulate the human disease. In this review, we outline the current status of PD modeling using mouse, rat and non-mammalian models, focusing on animal models for autosomal recessive PD.

Early Dysfunction of Substantia Nigra Dopamine Neurons in the ParkinQ311X Mouse

Mutations in the PARK2 gene encoding the protein parkin cause autosomal recessive juvenile parkinsonism (ARJP), a neurodegenerative disease characterized by early dysfunction and loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). No therapy is currently available to prevent or slow down the neurodegeneration in ARJP patients. Preclinical models are key to clarifying the early events that lead to neurodegeneration and reveal the potential of novel neuroprotective strategies. ParkinQ311X is a transgenic mouse model expressing in DA neurons a mutant parkin variant found in ARJP patients. This model was previously reported to show the neuropathological hallmark of the disease, i.e., the progressive loss of DA neurons. However, the early dysfunctions that precede neurodegeneration have never been investigated. Here, we analyzed SNc DA neurons in parkinQ311X mice and found early features of mitochondrial dysfunction, extensive cytoplasmic vacuolization, and dysregulation of spontaneous in vivo firing activity. These data suggest that the parkinQ311X mouse recapitulates key features of ARJP and provides a useful tool for studying the neurodegenerative mechanisms underlying the human disease and for screening potential neuroprotective drugs.