DREADDs (designer receptors exclusively activated by designer drugs): chemogenetic tools with therapeutic utility

In Vivo Interrogation of Spinal Mechanosensory Circuits

Spinal dorsal horn circuits receive, process, and transmit somatosensory information. To understand how specific components of these circuits contribute to behavior, it is critical to be able to directly modulate their activity in unanesthetized in vivo conditions. Here, we develop experimental tools that enable optogenetic control of spinal circuitry in freely moving mice using commonly available materials. We use these tools to examine mechanosensory processing in the spinal cord and observe that optogenetic activation of somatostatin-positive interneurons facilitates both mechanosensory and itch-related behavior, while reversible chemogenetic inhibition of these neurons suppresses mechanosensation. These results extend recent findings regarding the processing of mechanosensory information in the spinal cord and indicate the potential for activity-induced release of the somatostatin neuropeptide to affect processing of itch. The spinal implant approach we describe here is likely to enable a wide range of studies to elucidate spinal circuits underlying pain, touch, itch, and movement.

Activation of the Medial Prefrontal Cortex Reverses Cognitive and Respiratory Symptoms in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2; Amir et al., 1999), a transcriptional regulatory protein (Klose et al., 2005). Mouse models of RTT (Mecp2 mutants) exhibit excitatory hypoconnectivity in the medial prefrontal cortex (mPFC; Sceniak et al., 2015), a region critical for functions that are abnormal in RTT patients, ranging from learning and memory to regulation of visceral homeostasis (Riga et al., 2014). The present study was designed to test the hypothesis that increasing the activity of mPFC pyramidal neurons in heterozygous female Mecp2 mutants (Hets) would ameliorate RTT-like symptoms, including deficits in respiratory control and long-term retrieval of auditory conditioned fear. Selective activation of mPFC pyramidal neurons in adult animals was achieved by bilateral infection with an AAV8 vector expressing excitatory hm3D(Gq) DREADD (Designer Receptors Exclusively Activated by Designer Drugs) (Armbruster et al., 2007) under the control of the CamKIIa promoter. DREADD activation in Mecp2 Hets completely restored long-term retrieval of auditory conditioned fear, eliminated respiratory apneas, and reduced respiratory frequency variability to wild-type (Wt) levels. Reversal of respiratory symptoms following mPFC activation was associated with normalization of Fos protein levels, a marker of neuronal activity, in a subset of brainstem respiratory neurons. Thus, despite reduced levels of MeCP2 and severe neurological deficits, mPFC circuits in Het mice are sufficiently intact to generate normal behavioral output when pyramidal cell activity is increased. These findings highlight the contribution of mPFC hypofunction to the pathophysiology of RTT and raise the possibility that selective activation of cortical regions such as the mPFC could provide therapeutic benefit to RTT patients.

Studying brain-regulation of immunity with optogenetics and chemogenetics; A new experimental platform

The interactions between the brain and the immune system are bidirectional. Nevertheless, we have far greater understanding of how the immune system affects the brain than how the brain affects immunity. New technological developments such as optogenetics and chemogenetics (using DREADDs; Designer Receptors Exclusively Activated by Designer Drugs) can bridge this gap in our understanding, as they enable an unprecedented mechanistic and systemic analysis of the communication between the brain and the immune system. In this review, we discuss new experimental approaches for revealing neuronal circuits that can participate in regulation of immunity. In addition, we discuss methods, specifically optogenetics and chemogenetics, that enable targeted neuronal manipulation to reveal how different brain regions affect immunity. We describe how these techniques can be used as an experimental platform to address fundamental questions in psychoneuroimmunology and to understand how neuronal circuits associate with different psychological states can affect physiology.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Slow-wave sleep is controlled by a subset of nucleus accumbens core neurons in mice

Sleep control is ascribed to a two-process model, a widely accepted concept that posits homoeostatic drive and a circadian process as the major sleep-regulating factors. Cognitive and emotional factors also influence sleep–wake behaviour; however, the precise circuit mechanisms underlying their effects on sleep control are unknown. Previous studies suggest that adenosine has a role affecting behavioural arousal in the nucleus accumbens (NAc), a brain area critical for reinforcement and reward. Here, we show that chemogenetic or optogenetic activation of excitatory adenosine A_2A receptor-expressing indirect pathway neurons in the core region of the NAc strongly induces slow-wave sleep. Chemogenetic inhibition of the NAc indirect pathway neurons prevents the sleep induction, but does not affect the homoeostatic sleep rebound. In addition, motivational stimuli inhibit the activity of ventral pallidum-projecting NAc indirect pathway neurons and suppress sleep. Our findings reveal a prominent contribution of this indirect pathway to sleep control associated with motivation.

In addition to circadian and homoeostatic drives, motivational levels influence sleep−wake cycles. Here the authors demonstrate that adenosine receptor-expressing neurons in the nucleus accumbens core that project to the ventral pallidum are inhibited by motivational stimuli and are causally involved in the control of slow-wave sleep.

Disrupted Glutamatergic Transmission in Prefrontal Cortex Contributes to Behavioral Abnormality in an Animal Model of ADHD

Spontaneously hypertensive rats (SHR) are the most widely used animal model for the study of attention deficit hyperactivity disorder (ADHD). Here we sought to reveal the neuronal circuits and molecular basis of ADHD and its potential treatment using SHR. Combined electrophysiological, biochemical, pharmacological, chemicogenetic, and behavioral approaches were utilized. We found that AMPAR-mediated synaptic transmission in pyramidal neurons of prefrontal cortex (PFC) was diminished in SHR, which was correlated with the decreased surface expression of AMPAR subunits. Administration of methylphenidate (a psychostimulant drug used to treat ADHD), which blocks dopamine transporters and norepinephrine transporters, ameliorated the behavioral deficits of adolescent SHR and restored AMPAR-mediated synaptic function. Activation of PFC pyramidal neurons with a CaMKII-driven Gq-coupled designer receptor exclusively activated by designer drug also led to the elevation of AMPAR function and the normalization of ADHD-like behaviors in SHR. These results suggest that the disrupted function of AMPARs in PFC may underlie the behavioral deficits in adolescent SHR and enhancing PFC activity could be a treatment strategy for ADHD.

Orexin OX2 Receptor Antagonists as Sleep Aids

The discovery of the orexin system represents the single major progress in the sleep field of the last three to four decades. The two orexin peptides and their two receptors play a major role in arousal and sleep/wake cycles. Defects in the orexin system lead to narcolepsy with cataplexy in humans and dogs and can be experimentally reproduced in rodents. At least six orexin receptor antagonists have reached Phase II or Phase III clinical trials in insomnia, five of which are dual orexin receptor antagonists (DORAs) that target both OX1 and OX2 receptors (OX2Rs). All clinically tested DORAs induce and maintain sleep: suvorexant, recently registered in the USA and Japan for insomnia, represents the first hypnotic principle that acts in a completely different manner from the current standard medications. It is clear, however, that in the clinic, all DORAs promote sleep primarily by increasing rapid eye movement (REM) and are almost devoid of effects on slow-wave (SWS) sleep. At present, there is no consensus on whether the sole promotion of REM sleep has a negative impact in patients suffering from insomnia. However, sleep onset REM (SOREM), which has been documented with DORAs, is clearly an undesirable effect, especially for narcoleptic patients and also in fragile populations (e.g. elderly patients) where REM-associated loss of muscle tone may promote an elevated risk of falls. Debate thus remains as to the ideal orexin agent to achieve a balanced increase in REM and non-rapid eye movement (NREM) sleep. Here, we review the evidence that an OX2R antagonist should be at least equivalent, or perhaps superior, to a DORA for the treatment of insomnia. An OX2R antagonist may produce more balanced sleep than a DORA. Rodent sleep experiments show that the OX2R is the primary target of orexin receptor antagonists in sleep modulation. Furthermore, an OX2R antagonist should, in theory, have a lower narcoleptic/cataplexic potential. In the clinic, the situation remains equivocal, since OX2R antagonists are in early stages: MK-1064 has completed Phase I, and MIN202 is currently in clinical Phase II/III trials. However, data from insomnia patients have not yet been released. Promotional material suggests that balanced sleep is indeed induced by MIN-202, whereas in volunteers MK-1064 has been reported to act similarly to DORAs.

Genetics-based manipulation of adipose tissue sympathetic innervation

There is renewed interest in leveraging the thermogenic capacity of brown adipose tissue (BAT) and browning of white adipose tissue (WAT) to improve energy balance and prevent obesity. In addition to these effects on energy expenditure, both BAT and WAT secrete large numbers of hormones and cytokines that play important roles in maintaining metabolic health. Both BAT and WAT are densely innervated by the sympathetic nervous system (SNS) and this innervation is crucial for BAT thermogenesis and WAT browning, making it a potentially interesting target for manipulating energy balance and treatment of obesity and metabolic disease. Peripheral neuromodulation in the form of electrical manipulation of the SNS and parasympathetic nervous system (PSNS) has been used for the management of pain and many other conditions, but progress is hampered by lack of detailed knowledge of function-specific neurons and nerves innervating particular organs and tissues. Therefore, the goal of the National Institutes of Health (NIH) Common Fund project "Stimulating Peripheral Activity to Relieve Conditions (SPARC)" is to comprehensively map both anatomical and neurochemical aspects of the peripheral nervous system in animal model systems to ultimately guide optimal neuromodulation strategies in humans. Compared to electrical manipulation, neuron-specific opto- and chemogenetic manipulation, now being extensively used to decode the function of brain circuits, will further increase the functional specificity of peripheral neuromodulation.

Hippocampal sharp-wave ripples in awake mice are entrained by respiration

Several recent studies have shown that respiration modulates oscillatory neuronal activity in the neocortex and hippocampus on a cycle-by-cycle basis. It was suggested that this respiratory influence on neuronal activity affects cognitive functions, including memory. Sharp-wave ripples (SWRs) are high-frequency local field potential activity patterns characteristic for the hippocampus and implicated in memory consolidation and recall. Here we show that the timing of SWR events is modulated by the respiratory cycle, with a significantly increased probability of SWRs during the early expiration phase. This influence of respiration on SWR occurrence was eliminated when olfactory bulb activity was inhibited. Our findings represent a possible neuronal mechanism for a direct influence of the respiratory cycle on memory function.

Input-Timing-Dependent Plasticity in the Hippocampal CA2 Region and Its Potential Role in Social Memory

Input-timing-dependent plasticity (ITDP) is a circuit-based synaptic learning rule by which paired activation of entorhinal cortical (EC) and Schaffer collateral (SC) inputs to hippocampal CA1 pyramidal neurons (PNs) produces a long-term enhancement of SC excitation. We now find that paired stimulation of EC and SC inputs also induces ITDP of SC excitation of CA2 PNs. However, whereas CA1 ITDP results from long-term depression of feedforward inhibition (iLTD) as a result of activation of CB1 endocannabinoid receptors on cholecystokinin-expressing interneurons, CA2 ITDP results from iLTD through activation of δ-opioid receptors on parvalbumin-expressing interneurons. Furthermore, whereas CA1 ITDP has been previously linked to enhanced specificity of contextual memory, we find that CA2 ITDP is associated with enhanced social memory. Thus, ITDP may provide a general synaptic learning rule for distinct forms of hippocampal-dependent memory mediated by distinct hippocampal regions.

Advancing therapeutic applications of synthetic gene circuits

Synthetic biology aims to introduce new sense-and-respond capabilities into living cells, which would enable novel therapeutic strategies. The development of regulatory elements, molecular computing devices, and effector screening technologies has enabled researchers to design synthetic gene circuits in many organisms, including mammalian cells. Engineered gene networks, such as closed-loop circuits or Boolean logic gate circuits, can be used to program cells to perform specific functions with spatiotemporal control and restoration of homeostasis in response to the extracellular environment and intracellular signaling. In addition, genetically modified microbes can be designed as local delivery of therapeutic molecules. In this review, we will discuss recent advances in therapeutic applications of synthetic gene circuits, as well as challenges and future opportunities for biomedicine.

Role of Somatostatin-Positive Cortical Interneurons in the Generation of Sleep Slow Waves

During non-rapid eye-movement (NREM) sleep, cortical and thalamic neurons oscillate every second or so between ON periods, characterized by membrane depolarization and wake-like tonic firing, and OFF periods, characterized by membrane hyperpolarization and neuronal silence. Cortical slow waves, the hallmark of NREM sleep, reflect near-synchronous OFF periods in cortical neurons. However, the mechanisms triggering such OFF periods are unclear, as there is little evidence for somatic inhibition. We studied cortical inhibitory interneurons that express somatostatin (SOM), because ∼70% of them are Martinotti cells that target diffusely layer I and can block excitatory transmission presynaptically, at glutamatergic terminals, and postsynaptically, at apical dendrites, without inhibiting the soma. In freely moving male mice, we show that SOM+ cells can fire immediately before slow waves and their optogenetic stimulation during ON periods of NREM sleep triggers long OFF periods. Next, we show that chemogenetic activation of SOM+ cells increases slow-wave activity (SWA), slope of individual slow waves, and NREM sleep duration; whereas their chemogenetic inhibition decreases SWA and slow-wave incidence without changing time spent in NREM sleep. By contrast, activation of parvalbumin+ (PV+) cells, the most numerous population of cortical inhibitory neurons, greatly decreases SWA and cortical firing, triggers short OFF periods in NREM sleep, and increases NREM sleep duration. Thus SOM+ cells, but not PV+ cells, are involved in the generation of sleep slow waves. Whether Martinotti cells are solely responsible for this effect, or are complemented by other classes of inhibitory neurons, remains to be investigated.SIGNIFICANCE STATEMENT Cortical slow waves are a defining feature of non-rapid eye-movement (NREM) sleep and are thought to be important for many of its restorative benefits. Yet, the mechanism by which cortical neurons abruptly and synchronously cease firing, the neuronal basis of the slow wave, remains unknown. Using chemogenetic and optogenetic approaches, we provide the first evidence that links a specific class of inhibitory interneurons-somatostatin-positive cells-to the generation of slow waves during NREM sleep in freely moving mice.

Magnetothermal genetic deep brain stimulation of motor behaviors in awake, freely moving mice

Establishing how neurocircuit activation causes particular behaviors requires modulating the activity of specific neurons. Here, we demonstrate that magnetothermal genetic stimulation provides tetherless deep brain activation sufficient to evoke motor behavior in awake mice. The approach uses alternating magnetic fields to heat superparamagnetic nanoparticles on the neuronal membrane. Neurons, heat-sensitized by expressing TRPV1 are activated with magnetic field application. Magnetothermal genetic stimulation in the motor cortex evoked ambulation, deep brain stimulation in the striatum caused rotation around the body-axis, and stimulation near the ridge between ventral and dorsal striatum caused freezing-of-gait. The duration of the behavior correlated tightly with field application. This approach provides genetically and spatially targetable, repeatable and temporarily precise activation of deep-brain circuits without the need for surgical implantation of any device.

Optogenetics and pharmacogenetics: principles and applications

Remote and selective spatiotemporal control of the activity of neurons to regulate behavior and physiological functions has been a long-sought goal in system neuroscience. Identification and subsequent bioengineering of light-sensitive ion channels (e.g., channelrhodopsins, halorhodopsin, and archaerhodopsins) from the bacteria have made it possible to use light to artificially modulate neuronal activity, namely optogenetics. Recent advance in genetics has also allowed development of novel pharmacological tools to selectively and remotely control neuronal activity using engineered G protein-coupled receptors, which can be activated by otherwise inert drug-like small molecules such as the designer receptors exclusively activated by designer drug, a form of chemogenetics. The cutting-edge optogenetics and pharmacogenetics are powerful tools in neuroscience that allow selective and bidirectional modulation of the activity of defined populations of neurons with unprecedented specificity. These novel toolboxes are enabling significant advances in deciphering how the nervous system works and its influence on various physiological processes in health and disease. Here, we discuss the fundamental elements of optogenetics and chemogenetics approaches and some of the applications that yielded significant advances in various areas of neuroscience and beyond.

Applications of optogenetic and chemogenetic methods to seizure circuits: Where to go next?

Epilepsy is the quintessential circuit disorder, with seizure activity propagating through anatomically constrained pathways. These pathways, necessary for normal sensory, motor, and cognitive function, are hijacked during seizures. Understanding the network architecture at the level of both local microcircuits and distributed macrocircuits may provide new therapeutic avenues for the treatment of epilepsy. Over the past decade, optogenetic and chemogenetic tools have enabled previously impossible levels of functional circuit mapping in neuroscience. In this review, examples of the application of optogenetics and chemogenetics to epilepsy are raised, the comparative strengths and weaknesses of these approaches are discussed for both preclinical and translational applications, and recent applications of these approaches in other areas of neuroscience are highlighted. These points are raised in an effort to highlight the potential of these methods to address additional unanswered questions in epilepsy.

Engineered AAVs for efficient noninvasive gene delivery to the central and peripheral nervous systems

Adeno-associated viruses (AAVs) are commonly used for in vivo gene transfer. Nevertheless, AAVs that provide efficient transduction across specific organs or cell populations are needed. Here, we describe AAV-PHP.eB and AAV-PHP.S, capsids that efficiently transduce the central and peripheral nervous systems, respectively. In the adult mouse, intravenous administration of 1 × 1011 vector genomes (vg) of AAV-PHP.eB transduced 69% of cortical and 55% of striatal neurons, while 1 × 1012 vg of AAV-PHP.S transduced 82% of dorsal root ganglion neurons, as well as cardiac and enteric neurons. The efficiency of these vectors facilitates robust cotransduction and stochastic, multicolor labeling for individual cell morphology studies. To support such efforts, we provide methods for labeling a tunable fraction of cells without compromising color diversity. Furthermore, when used with cell-type-specific promoters and enhancers, these AAVs enable efficient and targetable genetic modification of cells throughout the nervous system of transgenic and non-transgenic animals.

Basal Ganglia Output Controls Active Avoidance Behavior

Engrained avoidance behavior is highly adaptive when it keeps away harmful events and can be highly maladaptive when individuals elude harmless situations in anxiety disorders, but the neural circuits that mediate avoidance are poorly understood. Using DREADDs and optogenetics in mice, we show that the output of the basal ganglia through the substantia nigra pars reticulata (SNr) controls active avoidance. SNr excitation blocks avoidance to a conditioned sensory stimulus while preserving the ability to escape the harmful event. Conversely, SNr inhibition facilitates avoidance to the conditioned stimulus and suffices to drive avoidance without any conditioned sensory stimulus. The results highlight a midbrain circuit that gates avoidance responses, which can be targeted to ameliorate maladaptive avoidance in psychiatric disorders.

SIGNIFICANCE STATEMENT

In many circumstances, subjects respond to fearful situations with avoidance. This is a useful coping strategy in situations in which there is impending danger. However, avoidance responses can also be maladaptive, as in anxiety disorders such as phobias (e.g., avoiding air transportation) and social anxiety (e.g., avoiding social situations). Despite the obvious clinical relevance, little is known about the neural circuits that mediate active avoidance. Using chemogenetics and optogenetics, we show that the output of the basal ganglia fully controls active avoidance behavior.

Metabolism and Distribution of Clozapine-N-oxide: Implications for Nonhuman Primate Chemogenetics

The use of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) in neuroscience has rapidly expanded in rodent studies but has lagged behind in nonhuman primate (NHP) experiments, slowing the development of this method for therapeutic use in humans. One reason for the slow adoption of DREADD technology in primates is that the pharmacokinetic properties and bioavailability of clozapine-n-oxide (CNO), the most commonly used ligand for human muscarinic (hM) DREADDs, are not fully described in primates. We report an extensive pharmacokinetic study using subcutaneous (SC) administration of CNO in five adult rhesus monkeys. CNO reached maximal plasma and cerebrospinal fluid (CSF) concentrations within 2 h after injection, with an observed dose-dependent increase in levels following a 3 and 10 mg/kg SC dose. Since CSF concentrations were below values predicted from unbound plasma concentrations, we investigated whether CNO was restricted from the CNS through active transport at the blood-brain barrier. In vitro assessment demonstrated that CNO is a substrate for P-glycoprotein (Pgp; efflux ratio, 20), thus providing a likely mechanism limiting CNO levels in the CNS. Furthermore, CNO is metabolized to the psychoactive compounds clozapine and n-desmethylclozapine in monkeys. The concentrations of clozapine detected in the CSF are sufficient to activate several types of receptor (including the hM-DREADDs). Our results suggest that CNO metabolism and distribution may interfere with reproducibility and interpretation of DREADD-related experiments in NHPs and calls for a re-evaluation of the use of CNO in DREADD-related experiments in NHPs along with the need to test alternative compounds.

Effects of chemogenetic excitation or inhibition of the ventrolateral periaqueductal gray on the acquisition and extinction of Pavlovian fear conditioning

The midbrain periaqueductal gray (PAG) has been implicated in the generation and transmission of a prediction error signal that instructs amygdala-based fear and extinction learning. However, the PAG also plays a key role in the expression of conditioned fear responses. The evidence for a role of the PAG in fear learning and extinction learning has been obtained almost exclusively using PAG-dependent fear responses. It is less clear whether the PAG regulates fear learning when other measures of learned fear are used. Here we combined a chemogenetic approach, permitting excitation or inhibition of neurons in the ventrolateral PAG (VLPAG), with conditioned suppression as the measure of learned fear to assess the role of VLPAG in the acquisition and extinction of fear learning. We show that chemogenetic excitation of VLPAG (with some encroachment on lateral PAG [LPAG]) impairs acquisition of fear and, conversely, chemogenetic inhibition impairs extinction of fear. These effects on fear and extinction learning were specific to the combination of DREADD expression and injection of CNO because they were observed relative to both eYFP controls injected with CNO as well as DREADD expressing controls injected with vehicle. Taken together, these results show that activity of L/VLPAG neurons regulates both the acquisition and extinction of Pavlovian fear learning.

Reprogramming cellular functions with engineered membrane proteins

Taking inspiration from Nature, synthetic biology utilizes and modifies biological components to expand the range of biological functions for engineering new practical devices and therapeutics. While early breakthroughs mainly concerned the design of gene circuits, recent efforts have focused on engineering signaling pathways to reprogram cellular functions. Since signal transduction across cell membranes initiates and controls intracellular signaling, membrane receptors have been targeted by diverse protein engineering approaches despite limited mechanistic understanding of their function. The modular architecture of several receptor families has enabled the empirical construction of chimeric receptors combining domains from distinct native receptors which have found successful immunotherapeutic applications. Meanwhile, progress in membrane protein structure determination, computational modeling and rational design promise to foster the engineering of a broader range of membrane receptor functions. Marrying empirical and rational membrane protein engineering approaches should enable the reprogramming of cells with widely diverse fine-tuned functions.

Activation of cortical somatostatin interneurons prevents the development of neuropathic pain

Neuropathic pain involves long-lasting modifications of pain pathways that result in abnormal cortical activity. How cortical circuits are altered and contribute to the intense sensation associated with allodynia is unclear. Here we report a persistent elevation of layer V pyramidal neuron activity in the somatosensory cortex of a mouse model of neuropathic pain. This enhanced pyramidal neuron activity was caused in part by increases of synaptic activity and NMDA-receptor-dependent calcium spikes in apical tuft dendrites. Furthermore, local inhibitory interneuron networks shifted their activity in favor of pyramidal neuron hyperactivity: somatostatin-expressing and parvalbumin-expressing inhibitory neurons reduced their activity, whereas vasoactive intestinal polypeptide–expressing interneurons increased their activity. Pharmacogenetic activation of somatostatin-expressing cells reduced pyramidal neuron hyperactivity and reversed mechanical allodynia. These findings reveal cortical circuit changes that arise during the development of neuropathic pain and identify the activation of specific cortical interneurons as therapeutic targets for chronic pain treatment.

Depolarized GABAergic Signaling in Subicular Microcircuits Mediates Generalized Seizure in Temporal Lobe Epilepsy

Secondary generalized seizure (sGS) is a major source of disability in temporal lobe epilepsy (TLE) with unclear cellular/circuit mechanisms. Here we found that clinical TLE patients with sGS showed reduced volume specifically in the subiculum compared with those without sGS. Further, using optogenetics and extracellular electrophysiological recording in mouse models, we found that photoactivation of subicular GABAergic neurons retarded sGS acquisition by inhibiting the firing of pyramidal neurons. Once sGS had been stably acquired, photoactivation of GABAergic neurons aggravated sGS expression via depolarized GABAergic signaling. Subicular parvalbumin, but not somatostatin subtype GABAergic, neurons were easily depolarized in sGS expression. Finally, photostimulation of subicular pyramidal neurons genetically targeted with proton pump Arch, rather than chloride pump NpHR3.0, alleviated sGS expression. These results demonstrated that depolarized GABAergic signaling in subicular microcircuit mediates sGS in TLE. This may be of therapeutic interest in understanding the pathological neuronal circuitry underlying sGS. VIDEO ABSTRACT.

Cholinergic Mesopontine Signals Govern Locomotion and Reward through Dissociable Midbrain Pathways

The mesopontine tegmentum, including the pedunculopontine and laterodorsal tegmental nuclei (PPN and LDT), provides major cholinergic inputs to midbrain and regulates locomotion and reward. To delineate the underlying projection-specific circuit mechanisms, we employed optogenetics to control mesopontine cholinergic neurons at somata and at divergent projections within distinct midbrain areas. Bidirectional manipulation of PPN cholinergic cell bodies exerted opposing effects on locomotor behavior and reinforcement learning. These motor and reward effects were separable via limiting photostimulation to PPN cholinergic terminals in the ventral substantia nigra pars compacta (vSNc) or to the ventral tegmental area (VTA), respectively. LDT cholinergic neurons also form connections with vSNc and VTA neurons; however, although photo-excitation of LDT cholinergic terminals in the VTA caused positive reinforcement, LDT-to-vSNc modulation did not alter locomotion or reward. Therefore, the selective targeting of projection-specific mesopontine cholinergic pathways may offer increased benefit in treating movement and addiction disorders.

Ketamine-induced apoptosis in the mouse cerebral cortex follows similar characteristic of physiological apoptosis and can be regulated by neuronal activity

The effects of general anesthetics on inducing neuronal apoptosis during early brain development are well-documented. However, since physiological apoptosis also occurs during this developmental window, it is important to determine whether anesthesia-induced apoptosis targets the same cell population as physiological apoptosis or different cell types altogether. To provide an adequate plane of surgery, ketamine was co-administered with dexmedetomidine. The apoptotic neurons in the mouse primary somatosensory cortex (S1) were quantitated by immunohistochemistry. To explore the effect of neural activity on ketamine-induced apoptosis, the approaches of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and an environmental enrichment (EE) were performed. Ketamine-induced apoptosis in S1 is most prominent at postnatal days 5 and 7 (P5 – P7), and becomes insignificant by P12. Physiological and ketamine-induced apoptosis follow similar developmental patterns, mostly comprised of layer V pyramidal neurons at P5 and shifting to mostly layer II to IV GABAergic neurons by P9. Changes in neuronal activity induced by the DREADD system bidirectionally regulated the pattern of ketamine-induced apoptosis, with reduced activity inducing increased apoptosis and shifting the lamination pattern to a more immature form. Importantly, rearing mice in an EE significantly reduced the magnitude of ketamine-induced apoptosis and shifted its developmental pattern to a more mature form. Together, these results demonstrate that lamination pattern and cell-type dependent vulnerability to ketamine-induced apoptosis follow the physiological apoptosis pattern and are age- and activity-dependent. Naturally elevating neuronal activity is a possible method for reducing the adverse effects of general anesthesia.

Dorsal Raphe Dopamine Neurons Modulate Arousal and Promote Wakefulness by Salient Stimuli

Ventral midbrain dopamine (DA) is unambiguously involved in motivation and behavioral arousal, yet the contributions of other DA populations to these processes are poorly understood. Here, we demonstrate that the dorsal raphe nucleus DA neurons are critical modulators of behavioral arousal and sleep-wake patterning. Using simultaneous fiber photometry and polysomnography, we observed time-delineated dorsal raphe nucleus dopaminergic (DRN^DA) activity upon exposure to arousal-evoking salient cues, irrespective of their hedonic valence. We also observed broader fluctuations of DRN^DA activity across sleep-wake cycles with highest activity during wakefulness. Both endogenous DRN^DA activity and optogenetically driven DRN^DA activity were associated with waking from sleep, with DA signal strength predictive of wake duration. Conversely, chemogenetic inhibition opposed wakefulness and promoted NREM sleep, even in the face of salient stimuli. Therefore, the DRN^DA population is a critical contributor to wake-promoting pathways and is capable of modulating sleep-wake states according to the outside environment, wherein the perception of salient stimuli prompts vigilance and arousal.

Dopamine signaling and myopia development: What are the key challenges

In the face of an "epidemic" increase in myopia over the last decades and myopia prevalence predicted to reach 2.5 billion people by the end of this decade, there is an urgent need to develop effective and safe therapeutic interventions to slow down this "myopia booming" and prevent myopia-related complications and vision loss. Dopamine (DA) is an important neurotransmitter in the retina and mediates diverse functions including retina development, visual signaling, and refractive development. Inspired by the convergence of epidemiological and animal studies in support of the inverse relationship between outdoor activity and risk of developing myopia and by the close biological relationship between light exposure and dopamine release/signaling, we felt it is timely and important to critically review the role of DA in myopia development. This review will revisit several key points of evidence for and against DA mediating light control of myopia: 1) the causal role of extracellular retinal DA levels, 2) the mechanism and action of dopamine D1 and D2 receptors and 3) the roles of cellular/circuit retinal pathways. We examine the experiments that show causation by altering DA, DA receptors and visual pathways using pharmacological, transgenic, or visual environment approaches. Furthermore, we critically evaluate the safety issues of a DA-based treatment strategy and some approaches to address these issues. The review identifies the key questions and challenges in translating basic knowledge on DA signaling and myopia from animal studies into effective pharmacological treatments for myopia in children.

Chemogenetic Inhibition of Pain Neurons in a Mouse Model of Osteoarthritis

OBJECTIVE

To determine the ability of drugs that activate inhibitory G protein-coupled receptors (GPCRs) expressed in peripheral voltage-gated sodium channel 1.8 (Na_V 1.8)-positive sensory neurons to control osteoarthritis (OA)-associated pain.

METHODS

We used designer receptors exclusively activated by a designer drug (DREADD) technology, which employs engineered GPCRs to activate or inhibit neurons upon binding the synthetic ligand clozapine N-oxide (CNO). Na_V 1.8-Pdi C57BL/6 mice were generated to express the inhibitory DREADD receptor Pdi in Na_V 1.8-expressing sensory neurons. Destabilization of the medial meniscus (DMM) surgery was performed in 10-week-old male mice. Four, 8, 12, or 16 weeks after surgery, knee hyperalgesia or hind paw mechanical allodynia was tested. Subsequently, CNO or vehicle was administered, and the effect on pain-related behaviors was measured by a blinded observer. Morphine was used as a control.

RESULTS

Immunohistochemistry and electrophysiology confirmed functional expression of the inhibitory DREADD receptor Pdi by Na_V 1.8-positive sensory neurons. Acute inhibition of Na_V 1.8-expressing neurons in mice treated with CNO reduced knee hyperalgesia 4 weeks after DMM surgery and reduced mechanical allodynia 8 weeks after DMM surgery. Inhibition had no effect on pain-related behaviors 12 and 16 weeks after DMM surgery. Morphine, a drug that activates GPCRs in the peripheral and central nervous systems, was still effective in the later stage of experimental OA.

CONCLUSION

Chemogenetic inhibition of Na_V 1.8-expressing neurons blocks knee hyperalgesia and mechanical allodynia in early experimental OA, but is no longer efficacious in the later stages. These data indicate that activation of inhibitory GPCRs located solely outside the central nervous system may be ineffective in treating chronic OA pain.

Activation of Ventral Tegmental Area 5-HT2C Receptors Reduces Incentive Motivation

Obesity is primarily due to food intake in excess of the body's energetic requirements, intake that is not only associated with hunger but also the incentive value of food. The 5-hydroxytryptamine 2C receptor (5-HT_2CR) is a target for the treatment of human obesity. Mechanistically, 5-HT_2CRs are positioned to influence both homeostatic feeding circuits within the hypothalamus and reward circuits within the ventral tegmental area (VTA). Here we investigated the role of 5-HT_2CRs in incentive motivation using a mathematical model of progressive ratio (PR) responding in mice. We found that the 5-HT_2CR agonist lorcaserin significantly reduced both ad libitum chow intake and PR responding for chocolate pellets and increased c-fos expression in VTA 5-HT_2CR expressing γ-aminobutyric acid (GABA) neurons, but not 5-HT_2CR expressing dopamine (DA) neurons. We next adopted a chemogenetic approach using a 5-HT_2CR^CRE line to clarify the function of subset of 5-HT_2C receptor expressing VTA neurons in the modulation of appetite and food-motivated behavior. Activation of VTA 5-HT_2C receptor expressing neurons significantly reduced ad libitum chow intake, operant responding for chocolate pellets, and the incentive value of food. In contrast, chemogenetic inhibition of VTA 5-HT_2C receptor expressing neurons had no effect on the feeding behavior. These results indicate that activation of the subpopulation of 5-HT_2CR neurons within the VTA is sufficient to significantly reduce homeostatic feeding and effort-based intake of palatable food, and that this subset has an inhibitory role in motivational processes. These findings are relevant to the treatment of obesity.

Distinct Synaptic Strengthening of the Striatal Direct and Indirect Pathways Drives Alcohol Consumption

BACKGROUND

Repeated exposure to addictive drugs or alcohol triggers glutamatergic and gamma-aminobutyric acidergic (GABAergic) plasticity in many neuronal populations. The dorsomedial striatum (DMS), a brain region critically involved in addiction, contains medium spiny neurons (MSNs) expressing dopamine D1 or D2 receptors, which form direct and indirect pathways, respectively. It is unclear how alcohol-evoked plasticity in the DMS contributes to alcohol consumption in a cell type-specific manner.

METHODS

Mice were trained to consume alcohol using an intermittent-access two-bottle-choice drinking procedure. Slice electrophysiology was used to measure glutamatergic and GABAergic strength in DMS D1- and D2-MSNs of alcohol-drinking mice and control mice. In vivo chemogenetic and pharmacologic approaches were employed to manipulate MSN activity, and their consequences on alcohol consumption were measured.

RESULTS

Repeated cycles of alcohol consumption and withdrawal in mice strengthened glutamatergic transmission in D1-MSNs and GABAergic transmission in D2-MSNs. In vivo chemogenetic excitation of D1-MSNs, mimicking glutamatergic strengthening, promoted alcohol consumption; the same effect was induced by D2-MSN inhibition, mimicking GABAergic strengthening. Importantly, suppression of GABAergic transmission via D2 receptor-glycogen synthase kinase-3β signaling dramatically reduced excessive alcohol consumption, as did selective inhibition of D1-MSNs or excitation of D2-MSNs.

CONCLUSIONS

Our results suggest that repeated cycles of excessive alcohol intake and withdrawal potentiate glutamatergic strength exclusively in D1-MSNs and GABAergic strength specifically in D2-MSNs of the DMS, which concurrently contribute to alcohol consumption. These results provide insight into the synaptic and cell type-specific mechanisms underlying alcohol addiction and identify targets for the development of new therapeutic approaches to alcohol abuse.

Viral strategies for targeting cortical circuits that control cocaine-taking and cocaine-seeking in rodents

Addiction to cocaine is a chronic disease characterized by persistent drug-taking and drug-seeking behaviors, and a high likelihood of relapse. The prefrontal cortex (PFC) has long been implicated in the development of cocaine addiction, and relapse. However, the PFC is a heterogeneous structure, and understanding the role of PFC subdivisions, cell types and afferent/efferent connections is critical for gaining a comprehensive picture of the contribution of the PFC in addiction-related behaviors. Here we provide an update on the role of the PFC in cocaine addiction from recent work that used viral-mediated optogenetic and chemogenetic tools to study the role of the PFC in drug-taking and drug-seeking behavior in rodents. Following overviews of rodent PFC neuroanatomy and of viral-mediated optogenetic and chemogenetic techniques, we review studies of manipulations within the PFC, followed by a review of work that utilized targeted manipulations to PFC inputs and outputs.

A novel approach to map induced activation of neuronal networks using chemogenetics and functional neuroimaging in rats: A proof-of-concept study on the mesocorticolimbic system

Linking neural circuit activation at whole-brain level to neuronal activity at cellular level remains one of the major challenges in neuroscience research. We set up a novel functional neuroimaging approach to map global effects of locally induced activation of specific midbrain projection neurons using chemogenetics (Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-technology) combined with pharmacological magnetic resonance imaging (phMRI) in the rat mesocorticolimbic system. Chemogenetic activation of DREADD-targeted mesolimbic or mesocortical pathways, i.e. projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) or medial prefrontal cortex (mPFC), respectively, induced significant blood oxygenation level-dependent (BOLD) responses in areas with DREADD expression, but also in remote defined neural circuitry without DREADD expression. The time-course of brain activation corresponded with the behavioral output measure, i.e. locomotor (hyper)activity, in the mesolimbic pathway-targeted group. Chemogenetic activation specifically increased neuronal activity, whereas functional connectivity assessed with resting state functional MRI (rs-fMRI) remained stable. Positive and negative BOLD responses distinctively reflected simultaneous ventral pallidum activation and substantia nigra pars reticulata deactivation, respectively, demonstrating the concept of mesocorticolimbic network activity with concurrent activation of the direct and indirect pathways following stimulation of specific midbrain projection neurons. The presented methodology provides straightforward and widely applicable opportunities to elucidate relationships between local neuronal activity and global network activity in a controllable manner, which will increase our understanding of the functioning and dysfunctioning of large-scale neuronal networks in health and disease.

New Tools to Study Astrocyte Ca2+ Signal Dynamics in Brain Networks In Vivo

Sensory information processing is a fundamental operation in the brain that is based on dynamic interactions between different neuronal populations. Astrocytes, a type of glial cells, have been proposed to represent active elements of brain microcircuits that, through dynamic interactions with neurons, provide a modulatory control of neuronal network activity. Specifically, astrocytes in different brain regions have been described to respond to neuronal signals with intracellular Ca²⁺ elevations that represent a key step in the functional recruitment of astrocytes to specific brain circuits. Accumulating evidence shows that Ca²⁺ elevations regulate the release of gliotransmitters that, in turn, modulate synaptic transmission and neuronal excitability. Recent studies also provided new insights into the spatial and temporal features of astrocytic Ca²⁺ elevations revealing a surprising complexity of Ca²⁺ signal dynamics in astrocytes. Here we discuss how recently developed experimental tools such as the genetically encoded Ca²⁺ indicators (GECI), optogenetics and chemogenetics can be applied to the study of astrocytic Ca²⁺ signals in the living brain.

Gs-DREADD Knock-In Mice for Tissue-Specific, Temporal Stimulation of Cyclic AMP Signaling

Hundreds of hormones and ligands stimulate cyclic AMP (cAMP) signaling in different tissues through the activation of G-protein-coupled receptors (GPCRs). Although the functions and individual effectors of cAMP signaling are well characterized in many tissues, pleiotropic effects of GPCR agonists limit investigations of physiological functions of cAMP signaling in individual cell types at different developmental stages in vivo. To facilitate studies of cAMP signaling in specific cell populations in vivo, we harnessed the power of DREADD (designer receptors exclusively activated by designer drugs) technology by creating ROSA26-based knock-in mice for the conditional expression of a Gs-coupled DREADD (rM3Ds-green fluorescent protein [GFP], or “GsD”). After Cre recombinase expression, GsD is activated temporally by the administration of the ligand clozapine N-oxide (CNO). In the same allele, we engineered a CREB-luciferase reporter transgene for noninvasive bioluminescence monitoring of CREB activity. After viral delivery of Cre recombinase to hepatocytes in vivo, GsD is expressed and allows CNO-dependent cAMP signaling and glycogen breakdown. The long-term expression of GsD in the liver results in constitutive CREB activity and hyperglycemia. ROSA26-Gs-DREADD mice can be used to study the physiological effects of cAMP signaling, acute or chronic, in liver or any tissue or cell type for which transgenic or viral Cre drivers are available.

Activation of the Hypoglossal to Tongue Musculature Motor Pathway by Remote Control

Reduced tongue muscle tone precipitates obstructive sleep apnea (OSA), and activation of the tongue musculature can lessen OSA. The hypoglossal motor nucleus (HMN) innervates the tongue muscles but there is no pharmacological agent currently able to selectively manipulate a channel (e.g., Kir2.4) that is highly restricted in its expression to cranial motor pools such as the HMN. To model the effect of manipulating such a restricted target, we introduced a “designer” receptor into the HMN and selectively modulated it with a “designer” drug. We used cre-dependent viral vectors (AAV8-hSyn-DIO-hM3Dq-mCherry) to transduce hypoglossal motoneurons of ChAT-Cre⁺ mice with hM3Dq (activating) receptors. We measured sleep and breathing in three conditions: (i) sham, (ii) after systemic administration of clozapine-N-oxide (CNO; 1 mg/kg) or (iii) vehicle. CNO activates hM3Dq receptors but is otherwise biologically inert. Systemic administration of CNO caused significant and sustained increases in tongue muscle activity in non-REM (261 ± 33% for 10 hrs) and REM sleep (217 ± 21% for 8 hrs), both P < 0.01 versus controls. Responses were specific and selective for the tongue with no effects on diaphragm or postural muscle activities, or sleep-wake states. These results support targeting a selective and restricted “druggable” target at the HMN (e.g., Kir2.4) to activate tongue motor activity during sleep.

Divergent Modulation of Nociception by Glutamatergic and GABAergic Neuronal Subpopulations in the Periaqueductal Gray

The ventrolateral periaqueductal gray (vlPAG) constitutes a major descending pain modulatory system and is a crucial site for opioid-induced analgesia. A number of previous studies have demonstrated that glutamate and GABA play critical opposing roles in nociceptive processing in the vlPAG. It has been suggested that glutamatergic neurotransmission exerts antinociceptive effects, whereas GABAergic neurotransmission exert pronociceptive effects on pain transmission, through descending pathways. The inability to exclusively manipulate subpopulations of neurons in the PAG has prevented direct testing of this hypothesis. Here, we demonstrate the different contributions of genetically defined glutamatergic and GABAergic vlPAG neurons in nociceptive processing by employing cell type-specific chemogenetic approaches in mice. Global chemogenetic manipulation of vlPAG neuronal activity suggests that vlPAG neural circuits exert tonic suppression of nociception, consistent with previous pharmacological and electrophysiological studies. However, selective modulation of GABAergic or glutamatergic neurons demonstrates an inverse regulation of nociceptive behaviors by these cell populations. Selective chemogenetic activation of glutamatergic neurons, or inhibition of GABAergic neurons, in vlPAG suppresses nociception. In contrast, inhibition of glutamatergic neurons, or activation of GABAergic neurons, in vlPAG facilitates nociception. Our findings provide direct experimental support for a model in which excitatory and inhibitory neurons in the PAG bidirectionally modulate nociception.

The Insula: An Underestimated Brain Area in Clinical Neuroscience, Psychiatry, and Neurology

Supported by recent human neuroimaging studies, the insula is re-emerging as an important brain area not only in the physiological understanding of the brain, but also in pathological contexts in clinical research. In this opinion article, we briefly introduce the anatomical and histological features of the human insula. We then summarize the physiological functions of the insula and underscore its pathological roles in psychiatric and neurological disorders that have long been underestimated. We finally propose possible strategies through which the role of the insula may be further understood for both basic and clinical neuroscience.

Paraventricular Thalamus Balances Danger and Reward

Foraging animals balance the need to seek food and energy against the accompanying dangers of injury and predation. To do so, they rely on learning systems encoding reward and danger. Whereas much is known about these separate learning systems, little is known about how they interact to shape and guide behavior. Here we show a key role for the rat paraventricular nucleus of the thalamus (PVT), a nucleus of the dorsal midline thalamus, in this interaction. First, we show behavioral competition between reward and danger: the opportunity to seek food reward negatively modulates expression of species-typical defensive behavior. Then, using a chemogenetic approach expressing the inhibitory hM4Di designer receptor exclusively activated by a designer drug in PVT neurons, we show that the PVT is central to this behavioral competition. Chemogenetic PVT silencing biases behavior toward either defense or reward depending on the experimental conditions, but does not consistently favor expression of one over the other. This bias could not be attributed to changes in fear memory retrieval, learned safety, or memory interference. Rather, our results demonstrate that the PVT is essential for balancing conflicting behavioral tendencies toward danger and reward, enabling adaptive responding under this basic selection pressure.SIGNIFICANCE STATEMENT Among the most basic survival problems faced by animals is balancing the need to seek food and energy against the accompanying dangers of injury and predation. Although much is known about the brain mechanisms that underpin learning about reward and danger, little is known about how these interact to solve basic survival problems. Here we show competition between defensive (to avoid predatory detection) and approach (to obtain food) behavior. We show that the paraventricular thalamus, a nucleus of the dorsal midline thalamus, is integral to this behavioral competition. The paraventricular thalamus balances the competing behavioral demands of danger and reward, enabling adaptive responding under this selection pressure.

Structure-guided SCHEMA recombination generates diverse chimeric channelrhodopsins

Integral membrane proteins (MPs) are key engineering targets due to their critical roles in regulating cell function. In engineering MPs, it can be extremely challenging to retain membrane localization capability while changing other desired properties. We have used structure-guided SCHEMA recombination to create a large set of functionally diverse chimeras from three sequence-diverse channelrhodopsins (ChRs). We chose 218 ChR chimeras from two SCHEMA libraries and assayed them for expression and plasma membrane localization in human embryonic kidney cells. The majority of the chimeras express, with 89% of the tested chimeras outperforming the lowest-expressing parent; 12% of the tested chimeras express at even higher levels than any of the parents. A significant fraction (23%) also localize to the membrane better than the lowest-performing parent ChR. Most (93%) of these well-localizing chimeras are also functional light-gated channels. Many chimeras have stronger light-activated inward currents than the three parents, and some have unique off-kinetics and spectral properties relative to the parents. An effective method for generating protein sequence and functional diversity, SCHEMA recombination can be used to gain insights into sequence-function relationships in MPs.

Synaptic Ensemble Underlying the Selection and Consolidation of Neuronal Circuits during Learning

Memories are crucial to the cognitive essence of who we are as human beings. Accumulating evidence has suggested that memories are stored as a subset of neurons that probably fire together in the same ensemble. Such formation of cell ensembles must meet contradictory requirements of being plastic and responsive during learning, but also stable in order to maintain the memory. Although synaptic potentiation is presumed to be the cellular substrate for this process, the link between the two remains correlational. With the application of the latest optogenetic tools, it has been possible to collect direct evidence of the contributions of synaptic potentiation in the formation and consolidation of cell ensemble in a learning task specific manner. In this review, we summarize the current view of the causative role of synaptic plasticity as the cellular mechanism underlying the encoding of memory and recalling of learned memories. In particular, we will be focusing on the latest optoprobe developed for the visualization of such “synaptic ensembles.” We further discuss how a new synaptic ensemble could contribute to the formation of cell ensembles during learning and memory. With the development and application of novel research tools in the future, studies on synaptic ensembles will pioneer new discoveries, eventually leading to a comprehensive understanding of how the brain works.

Neural Recording and Modulation Technologies

Within the mammalian nervous system, billions of neurons connected by quadrillions of synapses exchange electrical, chemical and mechanical signals. Disruptions to this network manifest as neurological or psychiatric conditions. Despite decades of neuroscience research, our ability to treat or even to understand these conditions is limited by the tools capable of probing the signalling complexity of the nervous system. Although orders of magnitude smaller and computationally faster than neurons, conventional substrate-bound electronics do not address the chemical and mechanical properties of neural tissue. This mismatch results in a foreign-body response and the encapsulation of devices by glial scars, suggesting that the design of an interface between the nervous system and a synthetic sensor requires additional materials innovation. Advances in genetic tools for manipulating neural activity have fuelled the demand for devices capable of simultaneous recording and controlling individual neurons at unprecedented scales. Recently, flexible organic electronics and bio- and nanomaterials have been developed for multifunctional and minimally invasive probes for long-term interaction with the nervous system. In this Review, we discuss the design lessons from the quarter-century-old field of neural engineering, highlight recent materials-driven progress in neural probes, and look at emergent directions inspired by the principles of neural transduction.

Resisting the Urge to Act: DREADDs Modifying Habits

Recently, Meyer and Bucci used chemogenetic technology - artificial excitatory and inhibitory receptors - to modulate neuronal activity in two connected brain regions in opposite directions simultaneously. This innovative manipulation revealed that the two regions studied, orbitofrontal cortex and nucleus accumbens, are not sequentially dependent during contextual decision-making.

Gene therapy approaches in the non-human primate model of Parkinson's disease

The field of gene therapy has recently witnessed a number of major conceptual changes. Besides the traditional thinking that comprises the use of viral vectors for the delivery of a given therapeutic gene, a number of original approaches have been recently envisaged, focused on using vectors carrying genes to further modify basal ganglia circuits of interest. It is expected that these approaches will ultimately induce a therapeutic potential being sustained by gene-induced changes in brain circuits. Among others, at present, it is technically feasible to use viral vectors to (1) achieve a controlled release of neurotrophic factors, (2) conduct either a transient or permanent silencing of any given basal ganglia circuit of interest, (3) perform an in vivo cellular reprogramming by promoting the conversion of resident cells into dopaminergic-like neurons, and (4) improving levodopa efficacy over time by targeting aromatic L-amino acid decarboxylase. Furthermore, extensive research efforts based on viral vectors are currently ongoing in an attempt to better replicate the dopaminergic neurodegeneration phenomena inherent to the progressive intraneuronal aggregation of alpha-synuclein. Finally, a number of incoming strategies will soon emerge over the horizon, these being sustained by the underlying goal of promoting alpha-synuclein clearance, such as, for instance, gene therapy initiatives based on increasing the activity of glucocerebrosidase. To provide adequate proof-of-concept on safety and efficacy and to push forward true translational initiatives based on these different types of gene therapies before entering into clinical trials, the use of non-human primate models undoubtedly plays an instrumental role.

Neural regulation of immunity: molecular mechanisms and clinical translation

Studies bridging neuroscience and immunology have identified neural pathways that regulate immunity and inflammation. Recent research using methodological advances in molecular genetics has improved our understanding of the neural control of immunity. Here we outline mechanistic insights, focusing on translational relevance and conceptual developments. We also summarize findings from recent clinical studies of bioelectronic neuromodulation in inflammatory and autoimmune diseases.

Expanding frontiers in weight-control research explored by young investigators

At the 93rd annual meeting of the Physiological Society of Japan, a symposium entitled "Expanding frontiers in weight-control research explored by young investigators" was organized. The latest research on weight control was presented by young up-and-coming investigators. The symposium consisted of the following presentations: Gastrointestinal brush cells, immunity, and energy homeostasis; Impact of a brown rice-derived bioactive product on feeding regulation and fuel metabolism; A novel G protein-coupled receptor-regulated neuronal signaling pathway triggers sustained orexigenic effects; and NMDA receptor co-agonist D-serine regulates food preference. These four talks presented at the symposium were summarized as a series of short reviews in this review.