Selective electrical silencing of mammalian neurons in vitro by the use of invertebrate ligand-gated chloride channels

How can ethology inform the neuroscience of fear, aggression and dominance?

The study of behaviour is dominated by two approaches. On the one hand, ethologists aim to understand how behaviour promotes adaptation to natural contexts. On the other, neuroscientists aim to understand the molecular, cellular, circuit and psychological origins of behaviour. These two complementary approaches must be combined to arrive at a full understanding of behaviour in its natural setting. However, methodological limitations have restricted most neuroscientific research to the study of how discrete sensory stimuli elicit simple behavioural responses under controlled laboratory conditions that are only distantly related to those encountered in real life. Fortunately, the recent advent of neural monitoring and manipulation tools adapted for use in freely behaving animals has enabled neuroscientists to incorporate naturalistic behaviours into their studies and to begin to consider ethological questions. Here, we examine the promises and pitfalls of this trend by describing how investigations of rodent fear, aggression and dominance behaviours are changing to take advantage of an ethological appreciation of behaviour. We lay out current impediments to this approach and propose a framework for the evolution of the field that will allow us to take maximal advantage of an ethological approach to neuroscience and to increase its relevance for understanding human behaviour.

Chemogenetic Tools and their Use in Studies of Neuropsychiatric Disorders

Chemogenetics is a newly developed set of tools that allow for selective manipulation of cell activity. They consist of a receptor mutated irresponsive to endogenous ligands and a synthetic ligand that does not interact with the wild-type receptors. Many different types of these receptors and their respective ligands for inhibiting or excitating neuronal subpopulations were designed in the past few decades. It has been mainly the G-protein coupled receptors (GPCRs) selectively responding to clozapine-N-oxide (CNO), namely Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), that have been employed in research. Chemogenetics offers great possibilities since the activity of the receptors is reversible, inducible on demand by the ligand, and non-invasive. Also, specific groups or types of neurons can be selectively manipulated thanks to the delivery by viral vectors. The effect of the chemogenetic receptors on neurons lasts longer, and even chronic activation can be achieved. That can be useful for behavioral testing. The great advantage of chemogenetic tools is especially apparent in research on brain diseases since they can manipulate whole neuronal circuits and connections between different brain areas. Many psychiatric or other brain diseases revolve around the dysfunction of specific brain networks. Therefore, chemogenetics presents a powerful tool for investigating the underlying mechanisms causing the disease and revealing the link between the circuit dysfunction and the behavioral or cognitive symptoms observed in patients. It could also contribute to the development of more effective treatments.

GluCl.Cre ON enables selective inhibition of molecularly defined pain circuits

ABSTRACT

Insight into nociceptive circuits will ultimately build our understanding of pain processing and aid the development of analgesic strategies. Neural circuit analysis has been advanced greatly by the development of optogenetic and chemogenetic tools, which have allowed function to be ascribed to discrete neuronal populations. Neurons of the dorsal root ganglion, which include nociceptors, have proved challenging targets for chemogenetic manipulation given specific confounds with commonly used DREADD technology. We have developed a cre/lox dependant version of the engineered glutamate-gated chloride channel (GluCl) to restrict and direct its expression to molecularly defined neuronal populations. We have generated GluCl.Cre ON that selectively renders neurons expressing cre-recombinase susceptible to agonist-induced silencing. We have functionally validated our tool in multiple systems in vitro, and subsequently generated viral vectors and tested its applicability in vivo. Using Nav1.8 Cre mice to restrict AAV-GluCl.Cre ON to nociceptors, we demonstrate effective silencing of electrical activity in vivo and concomitant hyposensitivity to noxious thermal and noxious mechanical pain, whereas light touch and motor function remained intact. We also demonstrated that our strategy can effectively silence inflammatory-like pain in a chemical pain model. Collectively, we have generated a novel tool that can be used to selectively silence defined neuronal circuits in vitro and in vivo. We believe that this addition to the chemogenetic tool box will facilitate further understanding of pain circuits and guide future therapeutic development.

Optogenetic control of neural activity: The biophysics of microbial rhodopsins in neuroscience

Optogenetics, the use of microbial rhodopsins to make the electrical activity of targeted neurons controllable by light, has swept through neuroscience, enabling thousands of scientists to study how specific neuron types contribute to behaviors and pathologies, and how they might serve as novel therapeutic targets. By activating a set of neurons, one can probe what functions they can initiate or sustain, and by silencing a set of neurons, one can probe the functions they are necessary for. We here review the biophysics of these molecules, asking why they became so useful in neuroscience for the study of brain circuitry. We review the history of the field, including early thinking, early experiments, applications of optogenetics, pre-optogenetics targeted neural control tools, and the history of discovering and characterizing microbial rhodopsins. We then review the biophysical attributes of rhodopsins that make them so useful to neuroscience - their classes and structure, their photocycles, their photocurrent magnitudes and kinetics, their action spectra, and their ion selectivity. Our hope is to convey to the reader how specific biophysical properties of these molecules made them especially useful to neuroscientists for a difficult problem - the control of high-speed electrical activity, with great precision and ease, in the brain.

Chemogenetic Silencing of NaV1.8-Positive Sensory Neurons Reverses Chronic Neuropathic and Bone Cancer Pain in FLEx PSAM4-GlyR Mice

Drive from peripheral neurons is essential in almost all pain states, but pharmacological silencing of these neurons to effect analgesia has proved problematic. Reversible gene therapy using long-lived chemogenetic approaches is an appealing option. We used the genetically activated chloride channel PSAM4-GlyR to examine pain pathways in mice. Using recombinant AAV9-based delivery to sensory neurons, we found a reversal of acute pain behavior and diminished neuronal activity using in vitro and in vivo GCaMP imaging on activation of PSAM4-GlyR with varenicline. A significant reduction in inflammatory heat hyperalgesia and oxaliplatin-induced cold allodynia was also observed. Importantly, there was no impairment of motor coordination, but innocuous von Frey sensation was inhibited. We generated a transgenic mouse that expresses a CAG-driven FLExed PSAM4-GlyR downstream of the Rosa26 locus that requires Cre recombinase to enable the expression of PSAM4-GlyR and tdTomato. We used NaV1.8 Cre to examine the role of predominantly nociceptive NaV1.8+ neurons in cancer-induced bone pain (CIBP) and neuropathic pain caused by chronic constriction injury (CCI). Varenicline activation of PSAM4-GlyR in NaV1.8-positive neurons reversed CCI-driven mechanical, thermal, and cold sensitivity. Additionally, varenicline treatment of mice with CIBP expressing PSAM4-GlyR in NaV1.8+ sensory neurons reversed cancer pain as assessed by weight-bearing. Moreover, when these mice were subjected to acute pain assays, an elevation in withdrawal thresholds to noxious mechanical and thermal stimuli was detected, but innocuous mechanical sensations remained unaffected. These studies confirm the utility of PSAM4-GlyR chemogenetic silencing in chronic pain states for mechanistic analysis and potential future therapeutic use.

Local modulation by presynaptic receptors controls neuronal communication and behaviour

Central nervous system neurons communicate via fast synaptic transmission mediated by ligand-gated ion channel (LGIC) receptors and slower neuromodulation mediated by G protein-coupled receptors (GPCRs). These receptors influence many neuronal functions, including presynaptic neurotransmitter release. Presynaptic LGIC and GPCR activation by locally released neurotransmitters influences neuronal communication in ways that modify effects of somatic action potentials. Although much is known about presynaptic receptors and their mechanisms of action, less is known about when and where these receptor actions alter release, especially in vivo. This Review focuses on emerging evidence for important local presynaptic receptor actions and ideas for future studies in this area.

Chemogenetics of cell surface receptors: beyond genetic and pharmacological approaches

Cell surface receptors transmit extracellular information into cells. Spatiotemporal regulation of receptor signaling is crucial for cellular functions, and dysregulation of signaling causes various diseases. Thus, it is highly desired to control receptor functions with high spatial and/or temporal resolution. Conventionally, genetic engineering or chemical ligands have been used to control receptor functions in cells. As the alternative, chemogenetics has been proposed, in which target proteins are genetically engineered to interact with a designed chemical partner with high selectivity. The engineered receptor dissects the function of one receptor member among a highly homologous receptor family in a cell-specific manner. Notably, some chemogenetic strategies have been used to reveal the receptor signaling of target cells in living animals. In this review, we summarize the developing chemogenetic methods of transmembrane receptors for cell-specific regulation of receptor signaling. We also discuss the prospects of chemogenetics for clinical applications.

Melding Synthetic Molecules and Genetically Encoded Proteins to Forge New Tools for Neuroscience

Unraveling the complexity of the brain requires sophisticated methods to probe and perturb neurobiological processes with high spatiotemporal control. The field of chemical biology has produced general strategies to combine the molecular specificity of small-molecule tools with the cellular specificity of genetically encoded reagents. Here, we survey the application, refinement, and extension of these hybrid small-molecule:protein methods to problems in neuroscience, which yields powerful reagents to precisely measure and manipulate neural systems. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Molecular Tools for Targeted Control of Nerve Cell Electrical Activity. Part II

In modern life sciences, the issue of a specific, exogenously directed manipulation of a cell's biochemistry is a highly topical one. In the case of electrically excitable cells, the aim of the manipulation is to control the cells' electrical activity, with the result being either excitation with subsequent generation of an action potential or inhibition and suppression of the excitatory currents. The techniques of electrical activity stimulation are of particular significance in tackling the most challenging basic problem: figuring out how the nervous system of higher multicellular organisms functions. At this juncture, when neuroscience is gradually abandoning the reductionist approach in favor of the direct investigation of complex neuronal systems, minimally invasive methods for brain tissue stimulation are becoming the basic element in the toolbox of those involved in the field. In this review, we describe three approaches that are based on the delivery of exogenous, genetically encoded molecules sensitive to external stimuli into the nervous tissue. These approaches include optogenetics (overviewed in Part I), as well as chemogenetics and thermogenetics (described here, in Part II), which is significantly different not only in the nature of the stimuli and structure of the appropriate effector proteins, but also in the details of experimental applications. The latter circumstance is an indication that these are rather complementary than competing techniques.

Multiplexing viral approaches to the study of the neuronal circuits

Neural circuits are composed of multitudes of elaborately interconnected cell types. Understanding neural circuit function requires not only cell-specific knowledge of connectivity, but the ability to record and manipulate distinct cell types independently. Recent advances in viral vectors promise the requisite specificity to perform true "circuit-breaking" experiments. However, such new avenues of multiplexed, cell-specific investigation raise new technical issues: one must ensure that both the viral vectors and their transgene payloads do not overlap with each other in both an anatomical and a functional sense. This review describes benefits and issues regarding the use of viral vectors to analyse the function of neural circuits and provides a resource for the design and implementation of such multiplexing experiments.

Turning a Drug Target into a Drug Candidate: A New Paradigm for Neurological Drug Discovery?

The conventional paradigm for developing new treatments for disease mainly involves either the discovery of new drug targets, or finding new, improved drugs for old targets. However, an ion channel found only in invertebrates offers the potential of a completely new paradigm in which an established drug target can be re-engineered to serve as a new candidate therapeutic agent. The L-glutamate-gated chloride channels (GluCls) of invertebrates are absent from vertebrate genomes, offering the opportunity to introduce this exogenous, inhibitory, L-glutamate receptor into vertebrate neuronal circuits either as a tool with which to study neural networks, or a candidate therapy. Epileptic seizures can involve L-glutamate-induced hyper-excitation and toxicity. Variant GluCls, with their inhibitory responses to L-glutamate, when engineered into human neurons, might counter the excitotoxic effects of excess L-glutamate. In reviewing recent studies on model organisms, it appears that this approach might offer a new paradigm for the development of candidate therapeutics for epilepsy.

Optogenetic and chemogenetic therapies for epilepsy

Drug-resistant epilepsy remains a significant health-care burden. The most effective treatment is surgery, but this is suitable for very few patients because of the unacceptable consequences of removing brain tissue. In contrast, gene therapy can regulate neuronal excitability in the epileptic focus whilst preserving function. Optogenetics and chemogenetics have the advantage that they are titratable therapies. Optogenetics uses light to control the excitability of specific neuronal populations. Optogenetics can be used in a closed-loop paradigm in which the light source is activated only when seizures are detected. However, expression of foreign proteins raises concerns about immunogenicity. Chemogenetics relies on the modification of an endogenous receptor or the production of a modified chimeric receptor that responds to an exogenous ligand. The main chemogenetic approach applied to epilepsy is to use designer receptors exclusively activated by designer drugs (DREADDs), which have been mainly modified muscarinic receptors or kappa-opioid receptors. Genetically modified human muscarinic receptor DREADDs are activated not by acetylcholine but by specific drugs such as clozapine-n-oxide or olanzepine. The dose of the drugs can be titrated in order to suppress seizures without adverse effects. Lastly, there is a chemogenetic approach that is activated by an endogenous ligand, glutamate. This takes advantage of invertebrate glutamate receptors that are chloride permeable. These bind glutamate released during seizure activity, and the resultant chloride current inhibits neuronal activity. The exogenous ligand, ivermectin, can also be given to reduce neuronal activity either chronically or as a rescue medication. The translation of this technology is hampered by the expression of a foreign protein. This article is part of the special issue entitled 'New Epilepsy Therapies for the 21st Century - From Antiseizure Drugs to Prevention, Modification and Cure of Epilepsy'.

A Crucial Role for Side-Chain Conformation in the Versatile Charge Selectivity of Cys-Loop Receptors

Whether synaptic transmission is excitatory or inhibitory depends, to a large extent, on whether the ion channels that open upon binding the released neurotransmitter conduct cations or anions. The mechanistic basis of the opposite charge selectivities of Cys-loop receptors has only recently begun to emerge. It is now clear that ionized side chains-whether pore-facing or buried-in the first α-helical turn of the second transmembrane segments underlie this phenomenon and that the electrostatics of backbone atoms are not critically involved. Moreover, on the basis of electrophysiological observations, it has recently been suggested that not only the sign of charged side chains but also their conformation are crucial determinants of cation-anion selectivity. To challenge these ideas with the chemical and structural rigor that electrophysiological observations naturally lack, we performed molecular dynamics, Brownian dynamics, and electrostatics calculations of ion permeation. To this end, we used structural models of the open-channel conformation of the α1 glutamate-gated Cl- channel and the α1 glycine receptor. Our results provided full support to the notion that the conformation of charged sides chains matters for charge selectivity. Indeed, whereas some rotamers of the buried arginines at position 0' conferred high selectivity for anions, others supported the permeation of cations and anions at similar rates or even allowed the faster permeation of cations. Furthermore, we found that modeling glutamates at position -1' of the anion-selective α1 glycine receptor open-state structure-instead of the five native alanines-switches charge selectivity also in a conformation-dependent manner, with some glutamate rotamers being much more effective at conferring selectivity for cations than others. Regarding pore size, we found that the mere expansion of the pore has only a minimal impact on cation-anion selectivity. Overall, these results bring to light the previously unappreciated impact of side-chain conformation on charge selectivity in Cys-loop receptors.

Designer receptor technology for the treatment of epilepsy

Epilepsy remains refractory to medical treatment in ~30% of patients despite decades of new drug development. Neurosurgery to remove or disconnect the seizure focus is often curative but frequently contraindicated by risks of irreversible impairment to brain function. Novel therapies are therefore required that better balance seizure suppression against the risks of side effects. Among experimental gene therapies, chemogenetics has the major advantage that the action on the epileptogenic zone can be modulated on demand. Two broad approaches are to use a designer G-protein-coupled receptor or a modified ligand gated ion channel, targeted to specific neurons in the epileptogenic zone using viral vectors and cell-type selective promoters. The receptor can be activated on demand by either an exogenous compound or by pathological levels of extracellular glutamate that occur in epileptogenic tissue. We review the principal designer receptor technologies and their modes of action. We compare the drawbacks and benefits of each designer receptor with particular focus on the drug activators and the potential for clinical translation in epilepsy.

Ultrapotent chemogenetics for research and potential clinical applications

Chemogenetics enables noninvasive chemical control over cell populations in behaving animals. However, existing small-molecule agonists show insufficient potency or selectivity. There is also a need for chemogenetic systems compatible with both research and human therapeutic applications. We developed a new ion channel-based platform for cell activation and silencing that is controlled by low doses of the smoking cessation drug varenicline. We then synthesized subnanomolar-potency agonists, called uPSEMs, with high selectivity for the chemogenetic receptors. uPSEMs and their receptors were characterized in brains of mice and a rhesus monkey by in vivo electrophysiology, calcium imaging, positron emission tomography, behavioral efficacy testing, and receptor counterscreening. This platform of receptors and selective ultrapotent agonists enables potential research and clinical applications of chemogenetics.

A Photoactivatable Botulinum Neurotoxin for Inducible Control of Neurotransmission

Regulated secretion is critical for diverse biological processes ranging from immune and endocrine signaling to synaptic transmission. Botulinum and tetanus neurotoxins, which specifically proteolyze vesicle fusion proteins involved in regulated secretion, have been widely used as experimental tools to block these processes. Genetic expression of these toxins in the nervous system has been a powerful approach for disrupting neurotransmitter release within defined circuitry, but their current utility in the brain and elsewhere remains limited by lack of spatial and temporal control. Here we engineered botulinum neurotoxin B so that it can be activated with blue light. We demonstrate the utility of this approach for inducibly disrupting excitatory neurotransmission, providing a first-in-class optogenetic tool for persistent, light-triggered synaptic inhibition. In addition to blocking neurotransmitter release, this approach will have broad utility for conditionally disrupting regulated secretion of diverse bioactive molecules, including neuropeptides, neuromodulators, hormones, and immune molecules. VIDEO ABSTRACT.

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

Chemogenetics is the process of genetically expressing a macromolecule receptor capable of modulating the activity of the cell in response to selective chemical ligand. This chapter will cover the chemogenetic technologies that are available to date, focusing on the commonly available engineered or otherwise modified ligand-gated ion channels and G-protein-coupled receptors in the context of neuromodulation. First, we will give a brief overview of each chemogenetic approach as well as in vitro/in vivo applications, then we will list their strengths and weaknesses. Finally, we will provide tips for ligand application in each case.Each technology has specific limitations that make them more or less suitable for different applications in neuroscience although we will focus mainly on the most commonly used and versatile family named designer receptors exclusively activated by designer drugs or DREADDs. We here describe the most common cases where these can be implemented and provide tips on how and where these technologies can be applied in the field of neuroscience.

Chemogenetic Tools for Causal Cellular and Neuronal Biology

Chemogenetic technologies enable selective pharmacological control of specific cell populations. An increasing number of approaches have been developed that modulate different signaling pathways. Selective pharmacological control over G protein-coupled receptor signaling, ion channel conductances, protein association, protein stability, and small molecule targeting allows modulation of cellular processes in distinct cell types. Here, we review these chemogenetic technologies and instances of their applications in complex tissues in vivo and ex vivo.

Transient Magnetothermal Neuronal Silencing Using the Chloride Channel Anoctamin 1 (TMEM16A)

Determining the role and necessity of specific neurons in a network calls for precisely timed, reversible removal of these neurons from the circuit via remotely triggered transient silencing. Previously, we have shown that alternating magnetic field mediated heating of magnetic nanoparticles, bound to neurons, expressing temperature-sensitive cation channels TRPV1 remotely activates these neurons, evoking behavioral responses in mice. Here, we demonstrate how to apply magnetic nanoparticle heating to silence target neurons. Rat hippocampal neuronal cultures were transfected to express the temperature gated chloride channel, anoctamin 1 (TMEM16A). Spontaneous firing was suppressed within seconds of alternating magnetic field application to anoctamin 1 (TMEM16A) channel expressing, magnetic nanoparticle decorated neurons. Five seconds of magnetic field application leads to 12 s of silencing, with a latency of 2 s and an average suppression ratio of more than 80%. Immediately following the silencing period spontaneous activity resumed. The method provides a promising avenue for tether free, remote, transient neuronal silencing in vivo for both scientific and therapeutic applications.

Elucidating Gene-by-Environment Interactions Associated with Differential Susceptibility to Chemical Exposure

BACKGROUND

Modern societies are exposed to vast numbers of potentially hazardous chemicals. Despite demonstrated linkages between chemical exposure and severe health effects, there are limited, often conflicting, data on how adverse health effects of exposure differ across individuals.

OBJECTIVES

We tested the hypothesis that population variability in response to certain chemicals could elucidate a role for gene-environment interactions (GxE) in differential susceptibility.

METHODS

High-throughput screening (HTS) data on thousands of chemicals in genetically heterogeneous zebrafish were leveraged to identify a candidate chemical (Abamectin) with response patterns indicative of population susceptibility differences. We tested the prediction by generating genome-wide sequence data for 276 individual zebrafish displaying susceptible (Affected) vs. resistant (Unaffected) phenotypes following identical chemical exposure.

RESULTS

We found GxE associated with differential susceptibility in the sox7 promoter region and then confirmed gene expression differences between phenotypic response classes.

CONCLUSIONS

The results for Abamectin in zebrafish demonstrate that GxE associated with naturally occurring, population genetic variation play a significant role in mediating individual response to chemical exposure. https://doi.org/10.1289/EHP2662.

The antihelminthic moxidectin enhances tonic GABA currents in rodent hippocampal pyramidal neurons

Macrocyclic lactones (MLs) are commonly used treatments for parasitic worm and insect infections in humans, livestock, and companion animals. MLs target the invertebrate glutamate-activated chloride channel that is not present in vertebrates. MLs are not entirely inert in vertebrates, though; they have been reported to have activity in heterologous expression systems consisting of ligand-gated ion channels that are present in the mammalian central nervous system (CNS). However, these compounds are typically not able to reach significant concentrations in the CNS because of the activity of the blood-brain barrier P-glycoprotein extrusion system. Despite this, these compounds are able to reach low levels in the CNS that may be useful in the design of novel "designer" ligand-receptor systems that can be used to directly investigate neuronal control of behavior in mammals and have potential for use in treating human neurological diseases. To determine whether MLs might affect neurons in intact brains, we investigated the activity of the ML moxidectin (MOX) at native GABA receptors. Specifically, we recorded tonic and phasic miniature inhibitory postsynaptic currents (mIPSCs) in ex vivo brain slices. Our data show that MOX potentiated tonic GABA currents in a dose-dependent manner but had no concomitant effects on phasic GABA currents (i.e., MOX had no effect on the amplitude, frequency, or decay kinetics of mIPSCs). These studies indicate that behavioral experiments that implement a ML-based novel ligand-receptor system should take care to control for potential effects of the ML on native tonic GABA receptors. NEW & NOTEWORTHY We have identified a novel mechanism of action in the mammalian central nervous system for the antihelminthic moxidectin, commonly prescribed to animals worldwide and currently being evaluated for use in humans. Specifically, moxidectin applied to rodent brain slices selectively enhanced the tonic GABA conductance of hippocampal pyramidal neurons.

Using an engineered glutamate-gated chloride channel to silence sensory neurons and treat neuropathic pain at the source

See Basbaum (doi:10.1093/brain/awx227) for a scientific commentary on this article. Peripheral neuropathic pain arises as a consequence of injury to sensory neurons; the development of ectopic activity in these neurons is thought to be critical for the induction and maintenance of such pain. Local anaesthetics and anti-epileptic drugs can suppress hyperexcitability; however, these drugs are complicated by unwanted effects on motor, central nervous system and cardiac function, and alternative more selective treatments to suppress hyperexcitability are therefore required. Here we show that a glutamate-gated chloride channel modified to be activated by low doses of ivermectin (but not glutamate) is highly effective in silencing sensory neurons and reversing neuropathic pain-related hypersensitivity. Activation of the glutamate-gated chloride channel expressed in either rodent or human induced pluripotent stem cell-derived sensory neurons in vitro potently inhibited their response to both electrical and algogenic stimuli. We have shown that silencing is achieved both at nerve terminals and the soma and is independent of membrane hyperpolarization and instead likely mediated by lowering of the membrane resistance. Using intrathecal adeno-associated virus serotype 9-based delivery, the glutamate-gated chloride channel was successfully targeted to mouse sensory neurons in vivo, resulting in high level and long-lasting expression of the channel selectively in sensory neurons. This enabled reproducible and reversible modulation of thermal and mechanical pain thresholds in vivo; analgesia was observed for 3 days after a single systemic dose of ivermectin. We did not observe any motor or proprioceptive deficits and noted no reduction in cutaneous afferent innervation or upregulation of the injury marker ATF3 following prolonged glutamate-gated chloride channel expression. Established mechanical and cold pain-related hypersensitivity generated by the spared nerve injury model of neuropathic pain was reversed by ivermectin treatment. The efficacy of ivermectin in ameliorating behavioural hypersensitivity was mirrored at the cellular level by a cessation of ectopic activity in sensory neurons. These findings demonstrate the importance of aberrant afferent input in the maintenance of neuropathic pain and the potential for targeted chemogenetic silencing as a new treatment modality in neuropathic pain.

Silencing Neurons: Tools, Applications, and Experimental Constraints

Reversible silencing of neuronal activity is a powerful approach for isolating the roles of specific neuronal populations in circuit dynamics and behavior. In contrast with neuronal excitation, for which the majority of studies have used a limited number of optogenetic and chemogenetic tools, the number of genetically encoded tools used for inhibition of neuronal activity has vastly expanded. Silencing strategies vary widely in their mechanism of action and in their spatial and temporal scales. Although such manipulations are commonly applied, the design and interpretation of neuronal silencing experiments present unique challenges, both technically and conceptually. Here, we review the most commonly used tools for silencing neuronal activity and provide an in-depth analysis of their mechanism of action and utility for particular experimental applications. We further discuss the considerations that need to be given to experimental design, analysis, and interpretation of collected data. Finally, we discuss future directions for the development of new silencing approaches in neuroscience.

Mutations on M3 helix of Plutella xylostella glutamate-gated chloride channel confer unequal resistance to abamectin by two different mechanisms

Abamectin is one of the most widely used avermectins for agricultural pests control, but the emergence of resistance around the world is proving a major threat to its sustained application. Abamectin acts by directly activating glutamate-gated chloride channels (GluCls) and modulating other Cys-loop ion channels. To date, three mutations occurring in the transmembrane domain of arthropod GluCls are associated with target-site resistance to abamectin: A309V in Plutella xylostella GluCl (PxGluCl), G323D in Tetranychus urticae GluCl1 (TuGluCl1) and G326E in TuGluCl3. To compare the effects of these mutations in a single system, A309V/I/G and G315E (corresponding to G323 in TuGluCl1 and G326 in TuGluCl3) substitutions were introduced individually into the PxGluCl channel. Functional analysis using Xenopus oocytes showed that the A309V and G315E mutations reduced the sensitivity to abamectin by 4.8- and 493-fold, respectively. In contrast, the substitutions A309I/G show no significant effects on the response to abamectin. Interestingly, the A309I substitution increased the channel sensitivity to glutamate by one order of magnitude (∼12-fold). Analysis of PxGluCl homology models indicates that the G315E mutation interferes with abamectin binding through a steric hindrance mechanism. In contrast, the structural consequences of the A309 mutations are not so clear and an allosteric modification of the binding site is the most likely mechanism. Overall the results show that both A309V and G315E mutations may contribute to target-site resistance to abamectin and may be important for the future prediction and monitoring of abamectin resistance in P. xylostella and other arthropod pests.

Mutational Analysis at Intersubunit Interfaces of an Anionic Glutamate Receptor Reveals a Key Interaction Important for Channel Gating by Ivermectin

The broad-spectrum anthelmintic drug ivermectin (IVM) activates and stabilizes an open-channel conformation of invertebrate chloride-selective glutamate receptors (GluClRs), thereby causing a continuous inflow of chloride ions and sustained membrane hyperpolarization. These effects suppress nervous impulses and vital physiological processes in parasitic nematodes. The GluClRs are pentamers. Homopentameric receptors assembled from the Caenorhabditis elegans (C. elegans) GluClα (GLC-1) subunit can inherently respond to IVM but not to glutamate (the neurotransmitter). In contrast, heteromeric GluClα/β (GLC-1/GLC-2) assemblies respond to both ligands, independently of each other. Glutamate and IVM bind at the interface between adjacent subunits, far away from each other; glutamate in the extracellular ligand-binding domain, and IVM in the ion-channel pore periphery. To understand the importance of putative intersubunit contacts located outside the glutamate and IVM binding sites, we introduced mutations at intersubunit interfaces, between these two binding-site types. Then, we determined the effect of these mutations on the activation of the heteromeric mutant receptors by glutamate and IVM. Amongst these mutations, we characterized an α-subunit point mutation located close to the putative IVM-binding pocket, in the extracellular end of the first transmembrane helix (M1). This mutation (αF276A) moderately reduced the sensitivity of the heteromeric GluClαF276A/βWT receptor to glutamate, and slightly decreased the receptor subunits’ cooperativity in response to glutamate. In contrast, the αF276A mutation drastically reduced the sensitivity of the receptor to IVM and significantly increased the receptor subunits’ cooperativity in response to IVM. We suggest that this mutation reduces the efficacy of channel gating, and impairs the integrity of the IVM-binding pocket, likely by disrupting important interactions between the tip of M1 and the M2-M3 loop of an adjacent subunit. We hypothesize that this physical contact between M1 and the M2-M3 loop tunes the relative orientation of the ion-channel transmembrane helices M1, M2 and M3 to optimize pore opening. Interestingly, pre-exposure of the GluClαF276A/βWT mutant receptor to subthreshold IVM concentration recovered the receptor sensitivity to glutamate. We infer that IVM likely retained its positive modulation activity by constraining the transmembrane helices in a preopen orientation sensitive to glutamate, with no need for the aforementioned disrupted interactions between M1 and the M2-M3 loop.

Trapping of ivermectin by a pentameric ligand-gated ion channel upon open-to-closed isomerization

Ivermectin (IVM) is a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. By activating invertebrate pentameric glutamate-gated chloride channels (GluCl receptors; GluClRs), IVM induces sustained chloride influx and long-lasting membrane hyperpolarization that inhibit neural excitation in nematodes. Although IVM activates the C. elegans heteromeric GluClα/β receptor, it cannot activate a homomeric receptor composed of the C. elegans GluClβ subunits. To understand this incapability, we generated a homopentameric α7-GluClβ chimeric receptor that consists of an extracellular ligand-binding domain of an α7 nicotinic acetylcholine receptor known to be potentiated by IVM, and a chloride-selective channel domain assembled from GluClβ subunits. Application of IVM prior to acetylcholine inhibited the responses of the chimeric α7-GluClβR. Adding IVM to activated α7-GluClβRs, considerably accelerated the decline of ACh-elicited currents and stabilized the receptors in a non-conducting state. Determination of IVM association and dissociation rate constants and recovery experiments suggest that, following initial IVM binding to open α7-GluClβRs, the drug induces a conformational change and locks the ion channel in a closed state for a long duration. We further found that IVM also inhibits the activation by glutamate of a homomeric receptor assembled from the C. elegans full-length GluClβ subunits.

Ivermectin-Activated, Cation-Permeable Glycine Receptors for the Chemogenetic Control of Neuronal Excitation

The ability to control neuronal activation is rapidly advancing our understanding of brain function and is widely viewed as having eventual therapeutic application. Although several highly effective optogenetic, optochemical genetic, and chemogenetic techniques have been developed for this purpose, new approaches may provide better solutions for addressing particular questions and would increase the number of neuronal populations that can be controlled independently. An early chemogenetic neuronal silencing method employed a glutamate receptor Cl^- channel engineered for activation by 1-3 nM ivermectin. This construct has been validated in vivo. Here, we sought to develop cation-permeable ivermectin-gated receptors that were either maximally Ca²⁺-permeable so as to induce neuro-excitotoxic cell death or minimally Ca²⁺-permeable so as to depolarize neurons with minimal excitotoxic risk. Our constructs were based on the human α1 glycine receptor Cl^- channel due to its high conductance, human origin, high ivermectin sensitivity (once mutated), and because pore mutations that render it permeable to Na⁺ alone or Na⁺ plus Ca²⁺ are well characterized. We developed a Ca²⁺-impermeable excitatory receptor by introducing the F207A/P-2'Δ/A-1'E/T13'V/A288G mutations and a Ca²⁺-permeable excitatory receptor by introducing the F207A/A-1'E/A288G mutations. The latter receptor efficiently induces cell death and strongly depolarizes neurons at nanomolar ivermectin concentrations.

Cell-Specific Targeting of Genetically Encoded Tools for Neuroscience

Genetically encoded tools for visualizing and manipulating neurons in vivo have led to significant advances in neuroscience, in large part because of the ability to target expression to specific cell populations of interest. Current methods enable targeting based on marker gene expression, development, anatomical projection pattern, synaptic connectivity, and recent activity as well as combinations of these factors. Here, we review these methods, focusing on issues of practical implementation as well as areas for future improvement.

Systems Biology of Circadian Rhythms: An Outlook

The circadian system in higher organisms temporally orchestrates rhythmic changes in a vast number of genes and gene products in different organs. Complex interactions between these components, both within and among cells, ultimately lead to rhythmic behavior and physiology. Identifying the plethora of circadian targets and mapping their interactions with one another is therefore essential to comprehend the molecular mechanisms of circadian regulation. The emergence of new technology for unbiased identification of biomolecules and for mapping interactions at the genome-wide scale is offering powerful tools to decipher the regulatory networks underpinning circadian rhythms. In this review, the authors discuss the potential application of these genome-wide approaches in the study of circadian rhythms.

Functional characterization of neurotransmitter activation and modulation in a nematode model ligand-gated ion channel

The superfamily of pentameric ligand-gated ion channels includes neurotransmitter receptors that mediate fast synaptic transmission in vertebrates, and are targets for drugs including alcohols, anesthetics, benzodiazepines, and anticonvulsants. However, the mechanisms of ion channel opening, gating, and modulation in these receptors leave many open questions, despite their pharmacological importance. Subtle conformational changes in both the extracellular and transmembrane domains are likely to influence channel opening, but have been difficult to characterize given the limited structural data available for human membrane proteins. Recent crystal structures of a modified Caenorhabditis elegans glutamate-gated chloride channel (GluCl) in multiple states offer an appealing model system for structure-function studies. However, the pharmacology of the crystallographic GluCl construct is not well established. To establish the functional relevance of this system, we used two-electrode voltage-clamp electrophysiology in Xenopus oocytes to characterize activation of crystallographic and native-like GluCl constructs by L-glutamate and ivermectin. We also tested modulation by ethanol and other anesthetic agents, and used site-directed mutagenesis to explore the role of a region of Loop F which was implicated in ligand gating by molecular dynamics simulations. Our findings indicate that the crystallographic construct functionally models concentration-dependent agonism and allosteric modulation of pharmacologically relevant receptors. Specific substitutions at residue Leu174 in loop F altered direct L-glutamate activation, consistent with computational evidence for this region's role in ligand binding. These insights demonstrate conservation of activation and modulation properties in this receptor family, and establish a framework for GluCl as a model system, including new possibilities for drug discovery. In this study, we elucidate the validity of a modified glutamate-gated chloride channel (GluClcryst ) as a structurally accessible model for GABAA receptors. In contrast to native-like controls, GluClcryst exhibits classical activation by its neurotransmitter ligand L-glutamate. The modified channel is also sensitive to allosteric modulators associated with human GABAA receptors, and to site-directed mutations predicted to alter channel opening.

Viral vector-based tools advance knowledge of basal ganglia anatomy and physiology

Viral vectors were originally developed to deliver genes into host cells for therapeutic potential. However, viral vector use in neuroscience research has increased because they enhance interpretation of the anatomy and physiology of brain circuits compared with conventional tract tracing or electrical stimulation techniques. Viral vectors enable neuronal or glial subpopulations to be labeled or stimulated, which can be spatially restricted to a single target nucleus or pathway. Here we review the use of viral vectors to examine the structure and function of motor and limbic basal ganglia (BG) networks in normal and pathological states. We outline the use of viral vectors, particularly lentivirus and adeno-associated virus, in circuit tracing, optogenetic stimulation, and designer drug stimulation experiments. Key studies that have used viral vectors to trace and image pathways and connectivity at gross or ultrastructural levels are reviewed. We explain how optogenetic stimulation and designer drugs used to modulate a distinct pathway and neuronal subpopulation have enhanced our mechanistic understanding of BG function in health and pathophysiology in disease. Finally, we outline how viral vector technology may be applied to neurological and psychiatric conditions to offer new treatments with enhanced outcomes for patients.

Subunit stoichiometry and arrangement in a heteromeric glutamate-gated chloride channel

The invertebrate glutamate-gated chloride-selective receptors (GluClRs) are ion channels serving as targets for ivermectin (IVM), a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. The native GluClR is a heteropentamer consisting of α and β subunit types, with yet unknown subunit stoichiometry and arrangement. Based on the recent crystal structure of a homomeric GluClαR, we introduced mutations at the intersubunit interfaces where Glu (the neurotransmitter) binds. By electrophysiological characterization of these mutants, we found heteromeric assemblies with two equivalent Glu-binding sites at β/α intersubunit interfaces, where the GluClβ and GluClα subunits, respectively, contribute the "principal" and "complementary" components of the putative Glu-binding pockets. We identified a mutation in the IVM-binding site (far away from the Glu-binding sites), which significantly increased the sensitivity of the heteromeric mutant receptor to both Glu and IVM, and improved the receptor subunits' cooperativity. We further characterized this heteromeric GluClR mutant as a receptor having a third Glu-binding site at an α/α intersubunit interface. Altogether, our data unveil heteromeric GluClR assemblies having three α and two β subunits arranged in a counterclockwise β-α-β-α-α fashion, as viewed from the extracellular side, with either two or three Glu-binding site interfaces.

Cell type-specific pharmacology of NMDA receptors using masked MK801

N-Methyl-D-aspartate receptors (NMDA-Rs) are ion channels that are important for synaptic plasticity, which is involved in learning and drug addiction. We show enzymatic targeting of an NMDA-R antagonist, MK801, to a molecularly defined neuronal population with the cell-type-selectivity of genetic methods and the temporal control of pharmacology. We find that NMDA-Rs on dopamine neurons are necessary for cocaine-induced synaptic potentiation, demonstrating that cell type-specific pharmacology can be used to dissect signaling pathways within complex brain circuits.

DOI:http://dx.doi.org/10.7554/eLife.10206.001

Learning is critical to survival for humans and other animals. The learning process is regulated by receptors on the surface of brain cells called N-Methyl-D-aspartate receptors (or NMDA receptors for short). These receptors help to strengthen signals between brain cells, which allows a new concept or action to be learned. However, it has been difficult to pin down how the role of NMDA receptors selectively in specific types of brain cells. While drugs can be used to quickly block NMDA receptors throughout the brain, it is hard to target drugs to a specific cell type. Also, genetic engineering can be used to selectively knock out NMDA receptors in certain types of brain cells, but these techniques are too slow, and can take weeks or even a lifetime to work.

Now, Yang et al. have developed a clever way to combine an NMDA-blocking drug and genetic engineering to study NMDA receptors' responses to cocaine in specific brain cells. This approach involved first creating an inactive form of an NMDA-blocking drug that can only becomes active when it is processed by an enzyme that is normally produced in pigs' livers. Next, living mouse brain cells, including some that were engineered to express the pig enzyme, were exposed to the drug in the laboratory. The drug blocked the NMDA receptors on brain cells that expressed the enzyme, but not the receptors on nearby brain cells that lacked the enzyme. This occurred even though all the cells produced NMDA receptors and all were exposed to the drug.

NMDA receptors have been known to play an important role in cocaine addiction for more than 20 years. Drugs like cocaine can co-opt the normally healthy learning process involving NMDA receptors and lead to a maladaptive form of learning that is commonly called addiction. Cocaine strengthens signals between brain cells causing the behaviors associated with using cocaine to become deeply ingrained and difficult to change. Yang et al. used cell type-specific targeting of a drug that blocks NMDA receptors to observe what happened in cocaine-exposed brain cells with, or without, working NMDA receptors.

As expected, the experiments showed that cocaine didn't strengthen brain signals in cells without working NMDA receptors. Specifically, the experiments showed that NMDA receptors on a type of brain cell that release a pleasure-inducing chemical called dopamine are necessary for cocaine–induced synaptic plasticity. The combination technique developed by Yang et al. will likely be used by other scientists to further study the role of NMDA receptors in specific brain cells during addiction and normal brain activity.

DOI:http://dx.doi.org/10.7554/eLife.10206.002

Changing channels in pain and epilepsy: Exploiting ion channel gene therapy for disorders of neuronal hyperexcitability

Chronic pain and epilepsy together affect hundreds of millions of people worldwide. While traditional pharmacotherapy provides essential relief to the majority of patients, a large proportion remains resistant, and surgical intervention is only possible for a select few. As both disorders are characterised by neuronal hyperexcitability, manipulating the expression of the most direct modulators of excitability - ion channels - represents an attractive common treatment strategy. A number of viral gene therapy approaches have been explored to achieve this. These range from the up- or down-regulation of channels that control excitability endogenously, to the delivery of exogenous channels that permit manipulation of excitability via optical or chemical means. In this review we highlight the key experimental successes of each approach and discuss the challenges facing their clinical translation.

Acute inhibition of a cortical motor area impairs vocal control in singing zebra finches

Genetically targeted approaches that permit acute and reversible manipulation of neuronal circuit activity have enabled an unprecedented understanding of how discrete neuronal circuits control animal behavior. Zebra finch singing behavior has emerged as an excellent model for studying neuronal circuit mechanisms underlying the generation and learning of behavioral motor sequences. We employed a newly developed, reversible, neuronal silencing system in zebra finches to test the hypothesis that ensembles of neurons in the robust nucleus of the arcopallium (RA) control the acoustic structure of specific song parts, but not the timing nor the order of song elements. Subunits of an ivermectin-gated chloride channel were expressed in a subset of RA neurons, and ligand administration consistently suppressed neuronal excitability. Suppression of activity in a group of RA neurons caused the birds to sing songs with degraded elements, although the order of song elements was unaffected. Furthermore some syllables disappeared in the middle or at the end of song motifs. Thus, our data suggest that generation of specific song parts is controlled by a subset of RA neurons, whereas elements order coordination and timing of whole songs are controlled by a higher premotor area.

Allosteric modulation of ligand gated ion channels by ivermectin

Ivermectin acts as a positive allosteric regulator of several ligand-gated channels including the glutamate-gated chloride channel (GluCl), gamma aminobutyric acid type-A receptor, glycine receptor, neuronal alpha7-nicotinic receptor and purinergic P2X4 receptor. In most of the ivermectin-sensitive channels, the effects of ivermectin include the potentiation of agonist-induced currents at low concentrations and channel opening at higher concentrations. Based on mutagenesis, electrophysiological recordings and functional analysis of chimeras between ivermectin-sensitive and ivermectin-insensitive receptors, it has been concluded that ivermectin acts by insertion between transmembrane helices. The three-dimensional structure of C. elegans GluCl complexed with ivermectin has revealed the details of the ivermectin-binding site, however, no generic motif of amino acids could accurately predict ivermectin binding site for other ligand gated channels. Here, we will review what is currently known about ivermectin binding and modulation of Cys-loop receptor family of ligand-gated ion channels and what are the critical structural determinants underlying potentiation of the P2X4 receptor channel.

Dissecting inhibitory brain circuits with genetically-targeted technologies

The evolution of genetically targeted tools has begun to allow us to dissect anatomically and functionally heterogeneous interneurons, and to probe circuit function from synapses to behavior. Over the last decade, these tools have been used widely to visualize neurons in a cell type-specific manner, and engage them to activate and inactivate with exquisite precision. In this process, we have expanded our understanding of interneuron diversity, their functional connectivity, and how selective inhibitory circuits contribute to behavior. Here we discuss the relative assets of genetically encoded fluorescent proteins (FPs), viral tracing methods, optogenetics, chemical genetics, and biosensors in the study of inhibitory interneurons and their respective circuits.

Beyond the bolus: transgenic tools for investigating the neurophysiology of learning and memory

Understanding the neural mechanisms underlying learning and memory in the entorhinal-hippocampal circuit is a central challenge of systems neuroscience. For more than 40 years, electrophysiological recordings in awake, behaving animals have been used to relate the receptive fields of neurons in this circuit to learning and memory. However, the vast majority of such studies are purely observational, as electrical, surgical, and pharmacological circuit manipulations are both challenging and relatively coarse, being unable to distinguish between specific classes of neurons. Recent advances in molecular genetic tools can overcome many of these limitations, enabling unprecedented control over neural activity in behaving animals. Expression of pharmaco- or optogenetic transgenes in cell-type-specific "driver" lines provides unparalleled anatomical and cell-type specificity, especially when delivered by viral complementation. Pharmacogenetic transgenes are specially designed neurotransmitter receptors exclusively activated by otherwise inactive synthetic ligands and have kinetics similar to traditional pharmacology. Optogenetic transgenes use light to control the membrane potential, and thereby operate at the millisecond timescale. Thus, activation of pharmacogenetic transgenes in specific neuronal cell types while recording from other parts of the circuit allows investigation of the role of those neurons in the steady state, whereas optogenetic transgenes allow one to determine the immediate network response.

The duplicated α7 subunits assemble and form functional nicotinic receptors with the full-length α7

The α7 nicotinic acetylcholine receptor gene (CHRNA7) is linked to schizophrenia. A partial duplication of CHRNA7 (CHRFAM7A) is found in humans on 15q13-14. Exon 6 of CHRFAM7A harbors a 2-bp deletion polymorphism, CHRFAM7AΔ2bp, which is also associated with schizophrenia. To understand the effects of the duplicated subunits on α7 receptors, we fused α7, dupα7, and dupΔα7 subunits with various fluorescent proteins. The duplicated subunits co-localized with full-length α7 subunits in mouse neuroblastoma cells (Neuro2a) as well as rat hippocampal neurons. We investigated the interaction between the duplicated subunits and full-length α7 by measuring Förster resonance energy transfer using donor recovery after photobleaching and fluorescence lifetime imaging microscopy. The results revealed that the duplicated proteins co-assemble with α7. In electrophysiological studies, Leu at the 9'-position in the M2 membrane-spanning segment was replaced with Cys in dupα7 or dupΔα7, and constructs were co-transfected with full-length α7 in Neuro2a cells. Exposure to ethylammonium methanethiosulfonate inhibited acetylcholine-induced currents, showing that the assembled functional nicotinic acetylcholine receptors (nAChRs) included the duplicated subunit. Incorporation of dupα7 and dupΔα7 subunits modestly changes the sensitivity of receptors to choline and varenicline. Thus, the duplicated proteins are assembled and transported to the cell membrane together with full-length α7 subunits and alter the function of the nAChRs. The characterization of dupα7 and dupΔα7 as well as their influence on α7 nAChRs may help explain the pathophysiology of schizophrenia and may suggest therapeutic strategies.

Chemogenetic tools to interrogate brain functions

Elucidating the roles of neuronal cell types for physiology and behavior is essential for understanding brain functions. Perturbation of neuron electrical activity can be used to probe the causal relationship between neuronal cell types and behavior. New genetically encoded neuron perturbation tools have been developed for remotely controlling neuron function using small molecules that activate engineered receptors that can be targeted to cell types using genetic methods. Here we describe recent progress for approaches using genetically engineered receptors that selectively interact with small molecules. Called "chemogenetics," receptors with diverse cellular functions have been developed that facilitate the selective pharmacological control over a diverse range of cell-signaling processes, including electrical activity, for molecularly defined cell types. These tools have revealed remarkably specific behavioral physiological influences for molecularly defined cell types that are often intermingled with populations having different or even opposite functions.

Defining functional gene-circuit interfaces in the mouse nervous system

Complexity in the nervous system is established by developmental genetic programs, maintained by differential genetic profiles and sculpted by experiential and environmental influence over gene expression. Determining how specific genes define neuronal phenotypes, shape circuit connectivity and regulate circuit function is essential for understanding how the brain processes information, directs behavior and adapts to changing environments. Mouse genetics has contributed greatly to current percepts of gene-circuit interfaces in behavior, but considerable work remains. Large-scale initiatives to map gene expression and connectivity in the brain, together with advanced techniques in molecular genetics, now allow detailed exploration of the genetic basis of nervous system function at the level of specific circuit connections. In this review, we highlight several key advances for defining the function of specific genes within a neural network.

Transient and specific inactivation of Drosophila neurons in vivo using a native ligand-gated ion channel

A key tool in neuroscience is the ability to transiently inactivate specific neurons on timescales of milliseconds to minutes. In Drosophila, there are two available techniques for accomplishing this (shibire(ts) and halorhodopsin [1-3]), but both have shortcomings [4-9]. Here we describe a complementary technique using a native histamine-gated chloride channel (Ort). Ort is the receptor at the first synapse in the visual system. It forms large-conductance homomeric channels that desensitize only modestly in response to ligand [10]. Many regions of the CNS are devoid of histaminergic neurons [11, 12], raising the possibility that Ort could be used to artificially inactivate specific neurons in these regions. To test this idea, we performed in vivo whole-cell recordings from antennal lobe neurons misexpressing Ort. In these neurons, histamine produced a rapid and reversible drop in input resistance, clamping the membrane potential below spike threshold and virtually abolishing spontaneous and odor-evoked activity. Every neuron type in this brain region could be inactivated in this manner. Neurons that did not misexpress Ort showed negligible responses to histamine. Ort also performed favorably in comparison to the available alternative effector transgenes. Thus, Ort misexpression is a useful tool for probing functional connectivity among Drosophila neurons.

An engineered glutamate-gated chloride (GluCl) channel for sensitive, consistent neuronal silencing by ivermectin

A modified invertebrate glutamate-gated Cl(-) channel (GluCl αβ) was previously employed to allow pharmacologically induced silencing of electrical activity in CNS neurons upon exposure to the anthelmintic drug ivermectin (IVM). Usefulness of the previous receptor was limited by 1) the high concentration of IVM necessary to elicit a consistent silencing phenotype, raising concern about potential side effects, and 2) the variable extent of neuronal spike suppression, due to variations in the co-expression levels of the fluorescent protein-tagged α and β subunits. To address these issues, mutant receptors generated via rational protein engineering strategies were examined for improvement. Introduction of a gain-of-function mutation (L9'F) in the second transmembrane domain of the α subunit appears to facilitate β subunit incorporation and substantially increase heteromeric GluCl αβ sensitivity to IVM. Removal of an arginine-based endoplasmic reticulum retention motif (RSR mutated to AAA) from the intracellular loop of the β subunit further promotes heteromeric expression at the plasma membrane possibly by preventing endoplasmic reticulum-associated degradation of the β subunit rather than simply reducing endoplasmic reticulum retention. A monomeric XFP (mXFP) mutation that prevents fluorescent protein dimerization complements the mutant channel effects. Expression of the newly engineered GluCl opt α-mXFP L9'F + opt β-mXFP Y182F RSR_AAA receptor in dissociated neuronal cultures markedly increases conductance and reduces variability in spike suppression at 1 nm IVM. This receptor, named "GluClv2.0," is an improved tool for IVM-induced silencing.

Alzheimer's disease risk factor lymphocyte-specific protein tyrosine kinase regulates long-term synaptic strengthening, spatial learning and memory

The lymphocyte-specific protein tyrosine kinase (Lck), which belongs to the Src kinase-family, is expressed in neurons of the hippocampus, a structure critical for learning and memory. Recent evidence demonstrated a significant downregulation of Lck in Alzheimer's disease. Lck has additionally been proposed to be a risk factor for Alzheimer's disease, thus suggesting the involvement of Lck in memory function. The neuronal role of Lck, however, and its involvement in learning and memory remain largely unexplored. Here, in vitro electrophysiology, confocal microscopy, and molecular, pharmacological, genetic and biochemical techniques were combined with in vivo behavioral approaches to examine the role of Lck in the mouse hippocampus. Specific pharmacological inhibition and genetic silencing indicated the involvement of Lck in the regulation of neuritic outgrowth. In the functional pre-established synaptic networks that were examined electrophysiologically, specific Lck-inhibition also selectively impaired the long-term hippocampal synaptic plasticity without affecting spontaneous excitatory synaptic transmission or short-term synaptic potentiation. The selective inhibition of Lck also significantly altered hippocampus-dependent spatial learning and memory in vivo. These data provide the basis for the functional characterization of brain Lck, describing the importance of Lck as a critical regulator of both neuronal morphology and in vivo long-term memory.

Heterologous expression of a glial Kir channel (KCNJ10) in a neuroblastoma spinal cord (NSC-34) cell line

Heterologous expression of Kir channels offers a tool to modulate excitability of neurons which provide insight into Kir channel functions in general. Inwardly-rectifying K+ channels (Kir channels) are potential candidate proteins to hyperpolarize neuronal cell membranes. However, heterologous expression of inwardly-rectifying K+ channels has previously proven to be difficult. This was mainly due to a high toxicity of the respective Kir channel expression. We investigated the putative role of a predominantly glial-expressed, weakly rectifying Kir channel (Kir4.1 channel subunit; KCNJ10) in modulating electrophysiological properties of a motoneuron-like cell culture (NSC-34). Transfection procedures using an EGFP-tagged Kir4.1 protein in this study proved to have no toxic effects on NSC-34 cells. Using whole cell-voltage clamp, a substantial increase of inward rectifying K+ currents as well as hyperpolarization of the cell membrane was observed in Kir4.1-transfected cells. Na+ inward currents, observed in NSC-34 controls, were absent in Kir4.1/EGFP motoneuronal cells. The Kir4.1-transfection did not influence the NaV1.6 sodium channel expression. This study demonstrates the general feasibility of a heterologous expression of a weakly inward-rectifying K+ channel (Kir4.1 subunit) and shows that in vitro overexpression of Kir4.1 shifts electrophysiological properties of neuronal cells to a more glial-like phenotype and may therefore be a candidate tool to dampen excitability of neurons in experimental paradigms.

Optogenetic and potassium channel gene therapy in a rodent model of focal neocortical epilepsy

Neocortical epilepsy is frequently drug-resistant. Surgery to remove the epileptogenic zone is only feasible in a minority of cases, leaving many patients without an effective treatment. We report the potential efficacy of gene therapy in focal neocortical epilepsy using a rodent model in which epilepsy is induced by tetanus toxin injection in the motor cortex. By applying several complementary methods that use continuous wireless electroencephalographic monitoring to quantify epileptic activity, we observed increases in high frequency activity and in the occurrence of epileptiform events. Pyramidal neurons in the epileptic focus showed enhanced intrinsic excitability consistent with seizure generation. Optogenetic inhibition of a subset of principal neurons transduced with halorhodopsin targeted to the epileptic focus by lentiviral delivery was sufficient to attenuate electroencephalographic seizures. Local lentiviral overexpression of the potassium channel Kv1.1 reduced the intrinsic excitability of transduced pyramidal neurons. Coinjection of this Kv1.1 lentivirus with tetanus toxin fully prevented the occurrence of electroencephalographic seizures. Finally, administration of the Kv1.1 lentivirus to an established epileptic focus progressively suppressed epileptic activity over several weeks without detectable behavioral side effects. Thus, gene therapy in a rodent model can be used to suppress seizures acutely, prevent their occurrence after an epileptogenic stimulus, and successfully treat established focal epilepsy.

Coordinated optimization of visual cortical maps (II) numerical studies

In the juvenile brain, the synaptic architecture of the visual cortex remains in a state of flux for months after the natural onset of vision and the initial emergence of feature selectivity in visual cortical neurons. It is an attractive hypothesis that visual cortical architecture is shaped during this extended period of juvenile plasticity by the coordinated optimization of multiple visual cortical maps such as orientation preference (OP), ocular dominance (OD), spatial frequency, or direction preference. In part (I) of this study we introduced a class of analytically tractable coordinated optimization models and solved representative examples, in which a spatially complex organization of the OP map is induced by interactions between the maps. We found that these solutions near symmetry breaking threshold predict a highly ordered map layout. Here we examine the time course of the convergence towards attractor states and optima of these models. In particular, we determine the timescales on which map optimization takes place and how these timescales can be compared to those of visual cortical development and plasticity. We also assess whether our models exhibit biologically more realistic, spatially irregular solutions at a finite distance from threshold, when the spatial periodicities of the two maps are detuned and when considering more than 2 feature dimensions. We show that, although maps typically undergo substantial rearrangement, no other solutions than pinwheel crystals and stripes dominate in the emerging layouts. Pinwheel crystallization takes place on a rather short timescale and can also occur for detuned wavelengths of different maps. Our numerical results thus support the view that neither minimal energy states nor intermediate transient states of our coordinated optimization models successfully explain the architecture of the visual cortex. We discuss several alternative scenarios that may improve the agreement between model solutions and biological observations.

Neurons in the visual cortex of carnivores, primates and their close relatives form spatial representations or maps of multiple stimulus features. In part (I) of this study we theoretically predicted maps that are optima of a variety of optimization principles. When analyzing the joint optimization of two interacting maps we showed that for different optimization principles the resulting optima show a stereotyped, spatially perfectly periodic layout. Experimental maps, however, are much more irregular. In particular, in case of orientation columns it was found that different species show apparently species invariant statistics of point defects, so-called pinwheels. In this paper, we numerically investigate whether the spatial features of the stereotyped optima described in part (I) are expressed on biologically relevant timescales and whether other, spatially irregular, long-living states emerge that better reproduce the experimentally observed statistical properties of orientation maps. Moreover, we explore whether the coordinated optimization of more than two maps can lead to spatially irregular optima.