Chronic nigral neuromodulation aggravates behavioral deficits and synaptic changes in an α-synuclein based rat model for Parkinson's disease

⍺-Synuclein levels in Parkinson's disease - Cell types and forms that contribute to pathogenesis

Parkinson's disease (PD) has two main pathological hallmarks, the loss of nigral dopamine neurons and the proteinaceous aggregations of ⍺-synuclein (⍺Syn) in neuronal Lewy pathology. These two co-existing features suggest a causative association between ⍺Syn aggregation and the underpinning mechanism of neuronal degeneration in PD. Both increased levels and post-translational modifications of ⍺Syn can contribute to the formation of pathological aggregations of ⍺Syn in neurons. Recent studies have shown that the protein is also expressed by multiple types of non-neuronal cells in the brain and peripheral tissues, suggesting additional roles of the protein and potential diversity in non-neuronal pathogenic triggers. It is important to determine (1) the threshold levels triggering ⍺Syn to convert from a biological to a pathologic form in different brain cells in PD; (2) the dominant form of pathologic ⍺Syn and the associated post-translational modification of the protein in each cell type involved in PD; and (3) the cell type associated biological processes impacted by pathologic ⍺Syn in PD. This review integrates these aspects and speculates on potential pathological mechanisms and their impact on neuronal and non-neuronal ⍺Syn in the brains of patients with PD.

Endocannabinoid-mediated rescue of somatosensory cortex activity, plasticity and related behaviors following an early in life concussion

Due to the assumed plasticity of immature brain, early in life brain alterations are thought to lead to better recoveries in comparison to the mature brain. Despite clinical needs, how neuronal networks and associated behaviors are affected by early in life brain stresses, such as pediatric concussions, have been overlooked. Here we provide first evidence in mice that a single early in life concussion durably increases neuronal activity in the somatosensory cortex into adulthood, disrupting neuronal integration while the animal is performing sensory-related tasks. This represents a previously unappreciated clinically relevant mechanism for the impairment of sensory-related behavior performance. Furthermore, we demonstrate that pharmacological modulation of the endocannabinoid system a year post-concussion is well-suited to rescue neuronal activity and plasticity, and to normalize sensory-related behavioral performance, addressing the fundamental question of whether a treatment is still possible once post-concussive symptoms have developed, a time-window compatible with clinical treatment.

The AAV-α-Synuclein Model of Parkinson's Disease: An Update

Targeted delivery of α-synuclein using AAV vectors has over the two decades since its introduction developed into a versatile tool for modeling different aspects of synucleinopathy, mimicking those seen in Parkinson's disease and related Lewy body disorders. The viral vector approach to disease modeling is attractive in that the expression of α-synuclein, wild-type or mutated, can be confined to defined anatomical structures and targeted to selected cell populations using either cell-type specific promoter constructs or different natural or engineered AAV serotypes. AAV-α-synuclein was initially used to model progressive α-synuclein pathology in nigral dopamine neurons, and, like the standard 6-OHDA model, it has most commonly been applied unilaterally, using the non-injected side as a reference and control. In recent years, however, the AAV-α-synuclein model has become more widely used to induce Parkinson-like synuclein pathology in other relevant neuronal systems, such as the brainstem noradrenergic and serotonergic neurons, the vagal motor neurons, as well as in oligodendrocytes, the prime target relevant to the pathology seen in multiple system atrophy. The purpose of this review is to give an overview of the progress made in the use of the AAV-α-synuclein model over the last two decades and summarize the state-of-the art in the use of the AAV-α-synuclein model for disease modeling in rats and mice.

Repeated chemogenetic activation of dopaminergic neurons induces reversible changes in baseline and amphetamine-induced behaviors

RATIONALE

Repeated chemogenetic stimulation is often employed to study circuit function and behavior. Chronic or repeated agonist administration can result in homeostatic changes, but this has not been extensively studied with designer receptors exclusively activated by designer drugs (DREADDs).

OBJECTIVES

We sought to evaluate the impact of repeated DREADD activation of dopaminergic (DA) neurons on basal behavior, amphetamine response, and spike firing. We hypothesized that repeated DREADD activation would mimic compensatory effects that we observed with genetic manipulations of DA neurons.

METHODS

Excitatory hM3D(Gq) DREADDs were virally expressed in adult TH-Cre and WT mice. In a longitudinal design, clozapine N-oxide (CNO, 1.0 mg/kg) was administered repeatedly. We evaluated basal and CNO- or amphetamine (AMPH)-induced locomotion and stereotypy. DA neuronal activity was assessed using in vivo single-unit recordings.

RESULTS

Acute CNO administration increased locomotion, but basal locomotion decreased after repeated CNO exposure in TH-CrehM3Dq mice relative to littermate controls. Further, after repeated CNO administration, AMPH-induced hyperlocomotion and stereotypy were diminished in TH-CrehM3Dq mice relative to controls. Repeated CNO administration reduced DA neuronal firing in TH-CrehM3Dq mice relative to controls. A two-month CNO washout period rescued the decreases in basal locomotion and AMPH response.

CONCLUSIONS

We found that repeated DREADD activation of DA neurons evokes homeostatic changes that should be factored into the interpretation of chronic DREADD applications and their impact on circuit function and behavior. These effects are likely to also be seen in other neuronal systems and underscore the importance of studying neuroadaptive changes with chronic or repeated DREADD activation.

Advances in understanding the function of alpha-synuclein: implications for Parkinson's disease

The critical role of alpha-synuclein in Parkinson's disease represents a pivotal discovery. Some progress has been made over recent years in identifying disease-modifying therapies for Parkinson's disease that target alpha-synuclein. However, these treatments have not yet shown clear efficacy in slowing the progression of this disease. Several explanations exist for this issue. The pathogenesis of Parkinson's disease is complex and not yet fully clarified and the heterogeneity of the disease, with diverse genetic susceptibility and risk factors and different clinical courses, adds further complexity. Thus, a deep understanding of alpha-synuclein physiological and pathophysiological functions is crucial. In this review, we first describe the cellular and animal models developed over recent years to study the physiological and pathological roles of this protein, including transgenic techniques, use of viral vectors and intracerebral injections of alpha-synuclein fibrils. We then provide evidence that these tools are crucial for modelling Parkinson's disease pathogenesis, causing protein misfolding and aggregation, synaptic dysfunction, brain plasticity impairment and cell-to-cell spreading of alpha-synuclein species. In particular, we focus on the possibility of dissecting the pre- and postsynaptic effects of alpha-synuclein in both physiological and pathological conditions. Finally, we show how vulnerability of specific neuronal cell types may facilitate systemic dysfunctions leading to multiple network alterations. These functional alterations underlie diverse motor and non-motor manifestations of Parkinson's disease that occur before overt neurodegeneration. However, we now understand that therapeutic targeting of alpha-synuclein in Parkinson's disease patients requires caution, since this protein exerts important physiological synaptic functions. Moreover, the interactions of alpha-synuclein with other molecules may induce synergistic detrimental effects. Thus, targeting only alpha-synuclein might not be enough. Combined therapies should be considered in the future.

Neuronal hyperactivity-induced oxidant stress promotes in vivo α-synuclein brain spreading

Interneuronal transfer and brain spreading of pathogenic proteins are features of neurodegenerative diseases. Pathophysiological conditions and mechanisms affecting this spreading remain poorly understood. This study investigated the relationship between neuronal activity and interneuronal transfer of α-synuclein, a Parkinson-associated protein, and elucidated mechanisms underlying this relationship. In a mouse model of α-synuclein brain spreading, hyperactivity augmented and hypoactivity attenuated protein transfer. Important features of neuronal hyperactivity reported here were an exacerbation of oxidative and nitrative reactions, pronounced accumulation of nitrated α-synuclein, and increased protein aggregation. Data also pointed to mitochondria as key targets and likely sources of reactive oxygen and nitrogen species within hyperactive neurons. Rescue experiments designed to counteract the increased burden of reactive oxygen species reversed hyperactivity-induced α-synuclein nitration, aggregation, and interneuronal transfer, providing first evidence of a causal link between these pathological effects of neuronal stimulation and indicating a mechanistic role of oxidant stress in hyperactivity-induced α-synuclein spreading.

The DREADDful Hurdles and Opportunities of the Chronic Chemogenetic Toolbox

The chronic character of chemogenetics has been put forward as one of the assets of the technique, particularly in comparison to optogenetics. Yet, the vast majority of chemogenetic studies have focused on acute applications, while repeated, long-term neuromodulation has only been booming in the past few years. Unfortunately, together with the rising number of studies, various hurdles have also been uncovered, especially in relation to its chronic application. It becomes increasingly clear that chronic neuromodulation warrants caution and that the effects of acute neuromodulation cannot be extrapolated towards chronic experiments. Deciphering the underlying cellular and molecular causes of these discrepancies could truly unlock the chronic chemogenetic toolbox and possibly even pave the way for chemogenetics towards clinical application. Indeed, we are only scratching the surface of what is possible with chemogenetic research. For example, most investigations are concentrated on behavioral read-outs, whereas dissecting the underlying molecular signature after (chronic) neuromodulation could reveal novel insights in terms of basic neuroscience and deregulated neural circuits. In this review, we highlight the hurdles associated with the use of chemogenetic experiments, as well as the unexplored research questions for which chemogenetics offers the ideal research platform, with a particular focus on its long-term application.

From chronic stress and anxiety to neurodegeneration: Focus on neuromodulation of the axon initial segment

To adapt to the sustained demands of chronic stress, discrete brain circuits undergo structural and functional changes often resulting in anxiety disorders. In some individuals, anxiety disorders precede the development of motor symptoms of Parkinson's disease (PD) caused by degeneration of neurons in the substantia nigra (SN). Here, we present a circuit framework for probing a causal link between chronic stress, anxiety, and PD, which postulates a central role of abnormal neuromodulation of the SN's axon initial segment by brainstem inputs. It is grounded in findings demonstrating that the earliest PD pathologies occur in the stress-responsive, emotion regulation network of the brainstem, which provides the SN with dense aminergic and cholinergic innervation. SN's axon initial segment (AIS) has unique features that support the sustained and bidirectional propagation of activity in response to synaptic inputs. It is therefore, especially sensitive to circuit-mediated stress-induced imbalance of neuromodulation, and thus a plausible initiating site of neurodegeneration. This could explain why, although secondary to pathophysiologies in other brainstem nuclei, SN degeneration is the most extensive. Consequently, the cardinal symptom of PD, severe motor deficits, arise from degeneration of the nigrostriatal pathway rather than other brainstem nuclei. Understanding when and how circuit dysfunctions underlying anxiety can progress to neurodegeneration, raises the prospect of timed interventions for reversing, or at least impeding, the early pathophysiologies that lead to PD and possibly other neurodegenerative disorders.

Cortical hyperexcitability: Diagnostic and pathogenic biomarker of ALS

Cortical hyperexcitability is an early and intrinsic feature of both sporadic and familial forms of amyotrophic lateral sclerosis (ALS).. Importantly, cortical hyperexcitability appears to be associated with motor neuron degeneration, possibly via an anterograde glutamate-mediated excitotoxic process, thereby forming a pathogenic basis for ALS. The presence of cortical hyperexcitability in ALS patients may be readily determined by transcranial magnetic stimulation (TMS), a neurophysiological tool that provides a non-invasive and painless method for assessing cortical function. Utilising the threshold tracking TMS technique, cortical hyperexcitability has been established as a robust diagnostic biomarker that distinguished ALS from mimicking disorders at early stages of the disease process. The present review discusses the pathophysiological and diagnostic utility of cortical hyperexcitability in ALS.

Does chronic systemic injection of the DREADD agonists clozapine-N-oxide or Compound 21 change behavior relevant to locomotion, exploration, anxiety, and depression in male non-DREADD-expressing mice?

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are chemogenetic tools commonly-used to manipulate brain activity. The most widely-used synthetic DREADD ligand, clozapine-N-oxide (CNO), is back-metabolized to clozapine which can itself activate endogenous receptors. Studies in non-DREADD-expressing rodents suggest CNO or a DREADD agonist that lacks active metabolites, such as Compound 21 (C21), change rodent behavior (e.g. decrease locomotion), but chronic injection of CNO does not change locomotion. However, it is unknown if chronic CNO changes behaviors relevant to locomotion, exploration, anxiety, and depression, or if chronic C21 changes any aspect of mouse behavior. Here non-DREADD-expressing mice received i.p. Vehicle (Veh), CNO, or C21 (1 mg/kg) 5 days/week for 16 weeks and behaviors were assessed over time. Veh, CNO, and C21 mice had similar weight gain over the 16-week-experiment. During the 3rd injection week, CNO and C21 mice explored more than Veh mice in a novel context and had more open field center entries; however, groups were similar in other measures of locomotion and anxiety. During the 14th-16th injection weeks, Veh, CNO, and C21 mice had similar locomotion and anxiety-like behaviors. We interpret these data as showing chronic Veh, CNO, and C21 injections given to male non-DREADD-expressing mice largely lack behavioral effects. These data may be helpful for behavioral neuroscientists when study design requires repeated injection of these DREADD agonists.