Practical Considerations for the Use of DREADD and Other Chemogenetic Receptors to Regulate Neuronal Activity in the Mammalian Brain

Targeting Spinal Interneurons for Respiratory Recovery After Spinal Cord Injury

Spinal interneurons (SpINs) are pivotal to the function of neural circuits, orchestrating motor, sensory, and autonomic functions in the healthy, intact central nervous system. These interneurons (INs) are heterogeneous, with diverse types contributing to various neural systems, including those that control respiratory function. Research in the last few decades has highlighted the complex involvement of SpINs in modulating motor control. SpINs also partake in motor plasticity by aiding in adapting and rewiring neural circuits in response to injury or disease. This plasticity is crucial in the context of spinal cord injury (SCI), where damage often leads to severe and long-term breathing deficits. Such deficits are a leading cause of morbidity and mortality in individuals with SCI, emphasizing the need for effective interventions. This review will focus on SpIN circuits involved in the modulation of breathing and explore current and emerging approaches that leverage SpINs as therapeutic targets to promote respiratory recovery following SCI.

Perturbing cortical networks: in vivo electrophysiological consequences of pan-neuronal chemogenetic manipulations using deschloroclozapine

Introduction

Chemogenetic techniques, specifically the use of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), have become invaluable tools in neuroscience research. Yet, the understanding of how Gq- and Gicoupled DREADDs alter local field potential (LFP) oscillations in vivo remains incomplete.

Methods

This study investigates the in vivo electrophysiological effects of DREADD actuation by deschloroclozapine, on spontaneous firing rate and LFP oscillations recorded from the anterior cingulate cortex in lightly anesthetized male rats.

Results

Unexpectedly, in response to the administration of deschloroclozapine, we observed inhibitory effects with pan-neuronal hM3D(Gq) stimulation, and excitatory effects with pan-neuronal hM4D(Gi) stimulation in a significant portion of neurons. These results emphasize the need to account for indirect perturbation effects at the local neuronal network level in vivo, particularly when not all neurons express the chemogenetic receptors uniformly. In the current study, for instance, the majority of cells that were transduced with both hM3D(Gq) and hM4D(Gi) were GABAergic. Moreover, we found that panneuronal cortical chemogenetic modulation can profoundly alter oscillatory neuronal activity, presenting a potential research tool or therapeutic strategy in several neuropsychiatric models and diseases.

Discussion

These findings help to optimize the use of chemogenetic techniques in neuroscience research and open new possibilities for novel therapeutic strategies.

Neuronal Activity Promotes Glioma Progression by Inducing Proneural-to-Mesenchymal Transition in Glioma Stem Cells

Neuronal activity can drive progression of high-grade glioma by mediating mitogen production and neuron-glioma synaptic communications. Glioma stem cells (GSC) also play a significant role in progression, therapy resistance, and recurrence in glioma, which implicates potential cross-talk between neuronal activity and GSC biology. Here, we manipulated neuronal activity using chemogenetics in vitro and in vivo to study how it influences GSCs. Neuronal activity supported glioblastoma (GBM) progression and radioresistance through exosome-induced proneural-to-mesenchymal transition (PMT) of GSCs. Molecularly, neuronal activation led to elevated miR-184-3p in neuron-derived exosomes that were taken up by GSCs and reduced the mRNA N6-methyladenosine (m6A) levels by inhibiting RBM15 expression. RBM15 deficiency decreased m6A modification of DLG3 mRNA and subsequently induced GSC PMT by activating the STAT3 pathway. Loss of miR-184-3p in cortical neurons reduced GSC xenograft growth, even when neurons were activated. Levetiracetam, an antiepileptic drug, reduced the neuronal production of miR-184-3p-enriched exosomes, inhibited GSC PMT, and increased radiosensitivity of tumors to prolong survival in xenograft mouse models. Together, these findings indicate that exosomes derived from active neurons promote GBM progression and radioresistance by inducing PMT of GSCs.

SIGNIFICANCE

Active neurons secrete exosomes enriched with miR-184-3p that promote glioblastoma progression and radioresistance by driving the proneural-to-mesenchymal transition in glioma stem cells, which can be reversed by antiseizure medication levetiracetam.

Comparison of three DREADD agonists acting on Gq-DREADDs in the ventral tegmental area to alter locomotor activity in tyrosine hydroxylase:Cre male and female rats

RATIONALE

Despite the increasingly pervasive use of chemogenetic tools in preclinical neuroscience research, the in vivo pharmacology of DREADD agonists remains poorly understood. The pharmacological effects of any ligand acting at receptors, engineered or endogenous, are influenced by numerous factors including potency, time course, and receptor selectivity. Thus, rigorous comparison of the potency and time course of available DREADD ligands may provide an empirical foundation for ligand selection.

OBJECTIVES

Compare the behavioral pharmacology of three different DREADD ligands clozapine-N-oxide (CNO), compound 21 (C21), and deschloroclozapine (DCZ) in a locomotor activity assay in tyrosine hydroxylase:cre recombinase (TH:Cre) male and female rats.

METHODS

Locomotor activity in nine adult TH:Cre Sprague-Dawley rats (5 female, 4 male) was monitored for two hours following administration of d-amphetamine (vehicle, 0.1-3.2 mg/kg, IP), DCZ (vehicle, 0.32-320 µg/kg, IP), CNO (vehicle, 0.32-10 mg/kg), and C21 (vehicle, 0.1-3.2 mg/kg, IP). Behavioral sessions were conducted twice per week prior to and starting three weeks after bilateral intra-VTA hM3Dq DREADD virus injection.

RESULTS

d-Amphetamine significantly increased locomotor activity pre- and post-DREADD virus injection. DCZ, CNO, and C21 did not alter locomotor activity pre-DREADD virus injection. There was no significant effect of DCZ, CNO, and C21 on locomotor activity post-DREADD virus injection; however, large individual differences in both behavioral response and receptor expression were observed.

CONCLUSIONS

Large individual variability was observed in both DREADD agonist behavioral effects and receptor expression. These results suggest further basic research would facilitate the utility of these chemogenetic tools for behavioral neuroscience research.

Parvalbumin Role in Epilepsy and Psychiatric Comorbidities: From Mechanism to Intervention

Parvalbumin is a calcium-binding protein present in inhibitory interneurons that play an essential role in regulating many physiological processes, such as intracellular signaling and synaptic transmission. Changes in parvalbumin expression are deeply related to epilepsy, which is considered one of the most disabling neuropathologies. Epilepsy is a complex multi-factor group of disorders characterized by periods of hypersynchronous activity and hyperexcitability within brain networks. In this scenario, inhibitory neurotransmission dysfunction in modulating excitatory transmission related to the loss of subsets of parvalbumin-expressing inhibitory interneuron may have a prominent role in disrupted excitability. Some studies also reported that parvalbumin-positive interneurons altered function might contribute to psychiatric comorbidities associated with epilepsy, such as depression, anxiety, and psychosis. Understanding the epileptogenic process and comorbidities associated with epilepsy have significantly advanced through preclinical and clinical investigation. In this review, evidence from parvalbumin altered function in epilepsy and associated psychiatric comorbidities were explored with a translational perspective. Some advances in potential therapeutic interventions are highlighted, from current antiepileptic and neuroprotective drugs to cutting edge modulation of parvalbumin subpopulations using optogenetics, designer receptors exclusively activated by designer drugs (DREADD) techniques, transcranial magnetic stimulation, genome engineering, and cell grafting. Creating new perspectives on mechanisms and therapeutic strategies is valuable for understanding the pathophysiology of epilepsy and its psychiatric comorbidities and improving efficiency in clinical intervention.

Ultrasound Mediated Cellular Deflection Results in Cellular Depolarization

Ultrasound has been used to manipulate cells in both humans and animal models. While intramembrane cavitation and lipid clustering have been suggested as likely mechanisms, they lack experimental evidence. Here, high-speed digital holographic microscopy (kiloHertz order) is used to visualize the cellular membrane dynamics. It is shown that neuronal and fibroblast membranes deflect about 150 nm upon ultrasound stimulation. Next, a biomechanical model that predicts changes in membrane voltage after ultrasound exposure is developed. Finally, the model predictions are validated using whole-cell patch clamp electrophysiology on primary neurons. Collectively, it is shown that ultrasound stimulation directly defects the neuronal membrane leading to a change in membrane voltage and subsequent depolarization. The model is consistent with existing data and provides a mechanism for both ultrasound-evoked neurostimulation and sonogenetic control.

Chemogenetics: Beyond Lesions and Electrodes

The field of chemogenetics has rapidly expanded over the last decade, and engineered receptors are currently utilized in the lab to better understand molecular interactions in the nervous system. We propose that chemogenetic receptors can be used for far more than investigational purposes. The potential benefit of adding chemogenetic neuromodulation to the current neurosurgical toolkit is substantial. There are several conditions currently treated surgically, electrically, and pharmacologically in clinic, and this review highlights how chemogenetic neuromodulation could improve patient outcomes over current neurosurgical techniques. We aim to emphasize the need to take these techniques from bench to bedside.

Neural circuits and activity dynamics underlying sex-specific effects of chronic social isolation stress

Exposure to prolonged stress in critical developmental periods induces heightened vulnerability to psychiatric disorders, which may have sex-specific consequences. Here we investigate the neuronal circuits mediating behavioral changes in mice after chronic adolescent social isolation stress. Escalated aggression is exhibited in stressed males, while social withdrawal is shown in stressed females. In vivo multichannel recordings of free-moving animals indicate that pyramidal neurons in prefrontal cortex (PFC) from stressed males exhibit the significantly decreased spike activity during aggressive attacks, while PFC pyramidal neurons from stressed females show a blunted increase of discharge rates during sociability tests. Chemogenetic and electrophysiological evidence shows that PFC hypofunctioning and BLA principal neuron hyperactivity contribute to the elevated aggression in stressed males, while PFC hypofunctioning and VTA dopamine neuron hypoactivity contribute to the diminished sociability in stressed females. These results establish a framework for understanding the circuit and physiological mechanisms underlying sex-specific divergent effects of stress.

Compound 21, a two-edged sword with both DREADD-selective and off-target outcomes in rats

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) represent a technical revolution in integrative neuroscience. However, the first used ligands exhibited dose-dependent selectivity for their molecular target, leading to potential unspecific effects. Compound 21 (C21) was recently proposed as an alternative, but in vivo characterization of its properties is not sufficient yet. Here, we evaluated its potency to selectively modulate the activity of nigral dopaminergic (DA) neurons through the canonical DREADD receptor hM4Di using TH-Cre rats. In males, 1 mg.kg-1 of C21 strongly increased nigral neurons activity in control animals, indicative of a significant off-target effect. Reducing the dose to 0.5 mg.kg-1 circumvented this unspecific effect, while activated the inhibitory DREADDs and selectively reduced nigral neurons firing. In females, 0.5 mg.kg-1 of C21 induced a transient and residual off-target effect that may mitigated the inhibitory DREADDs-mediated effect. This study raises up the necessity to test selectivity and efficacy of chosen ligands for each new experimental condition.

Targeted activation of spinal respiratory neural circuits

Spinal interneurons which discharge in phase with the respiratory cycle have been repeatedly described over the last 50 years. These spinal respiratory interneurons are part of a complex propriospinal network that is synaptically coupled with respiratory motoneurons. This article summarizes current knowledge regarding spinal respiratory interneurons and emphasizes chemical, electrical and physiological methods for activating spinal respiratory neural circuits. Collectively, the work reviewed here shows that activating spinal interneurons can have a powerful impact on spinal respiratory motor output, and can even drive rhythmic bursting in respiratory motoneuron pools under certain conditions. We propose that the primary functions of spinal respiratory neurons include 1) shaping the respiratory pattern into the final efferent motor output from the spinal respiratory nerves; 2) coordinating respiratory muscle activation across the spinal neuraxis; 3) coordinating postural, locomotor and respiratory movements, and 4) enabling plasticity of respiratory motor output in health and disease.

The Reuniens Nucleus of the Thalamus Has an Essential Role in Coordinating Slow-Wave Activity between Neocortex and Hippocampus

Sleep is a period of profound neural synchrony throughout the brain, a phenomenon involved in various physiological functions. The coordination between neocortex and hippocampus, in particular, appears to be critical for episodic memory, and, indeed, enhanced synchrony in this circuit is a hallmark of slow-wave sleep. However, it is unclear how this coordination is mediated. To this end, we examined the role of the thalamic nucleus reuniens (RE), a midline body with reciprocal connections to both prefrontal and hippocampal cortices. Using a combination of electrophysiological, optogenetic, and chemogenetic techniques in the urethane-anesthetized rat (a model of forebrain sleep activity), we directly assessed the role of the RE in mediating slow oscillatory synchrony. Using unit recording techniques, we confirmed that RE neurons showed slow rhythmic activity patterns during deactivated forebrain states that were coupled to ongoing slow oscillations. Optogenetic activation of RE neurons or their projection fibers in the cingulum bundle caused an evoked potential in hippocampus that was maximal at the level of stratum lacunosum-moleculare of CA1. A similar but longer-latency response could be evoked by stimulation of the medial prefrontal cortex that was then abolished by chemogenetic inhibition of the RE. Inactivation of the RE also severely reduced the coherence of the slow oscillation across cortical and hippocampal sites, suggesting that its activity is necessary to couple slow-wave activity across these regions. These results indicate an essential role of the RE in coordinating neocortico-hippocampal slow oscillatory activity, which may be fundamental for slow-wave sleep-related episodic memory consolidation.