Optogenetic investigation of neural circuits underlying brain disease in animal models

Optogenetic inhibition of behavior with anion channelrhodopsins

Optogenetics uses light exposure to manipulate physiology in genetically modified organisms. Abundant tools for optogenetic excitation are available, but the limitations of current optogenetic inhibitors present an obstacle to demonstrating the necessity of neuronal circuits. Here we show that anion channelrhodopsins can be used to specifically and rapidly inhibit neural systems involved in Drosophila locomotion, wing expansion, memory retrieval and gustation, thus demonstrating their broad utility in the circuit analysis of behavior.

The epileptic amygdala: Toward the development of a neural prosthesis by temporally coded electrical stimulation

Many patients with epilepsy do not obtain proper control of their seizures through conventional treatment. We review aspects of the pathophysiology underlying epileptic phenomena, with a special interest in the role of the amygdala, stressing the importance of hypersynchronism in both ictogenesis and epileptogenesis. We then review experimental studies on electrical stimulation of mesiotemporal epileptogenic areas, the amygdala included, as a means to treat medically refractory epilepsy. Regular high-frequency stimulation (HFS) commonly has anticonvulsant effects and sparse antiepileptogenic properties. On the other hand, HFS is related to acute and long-term increases in excitability related to direct neuronal activation, long-term potentiation, and kindling, raising concerns regarding its safety and jeopardizing in-depth understanding of its mechanisms. In turn, the safer regular low-frequency stimulation (LFS) has a robust antiepileptogenic effect, but its pro- or anticonvulsant effect seems to vary at random among studies. As an alternative, studies by our group on the development and investigation of temporally unstructured electrical stimulation applied to the amygdala have shown that nonperiodic stimulation (NPS), which is a nonstandard form of LFS, is capable of suppressing both acute and chronic spontaneous seizures. We hypothesize two noncompetitive mechanisms for the therapeutic role of amygdala in NPS, 1) a direct desynchronization of epileptic circuitry in the forebrain and brainstem and 2) an indirect desynchronization/inhibition through nucleus accumbens activation. We conclude by reintroducing the idea that hypersynchronism, rather than hyperexcitability, may be the key for epileptic phenomena and epilepsy treatment.

Preparation and implementation of optofluidic neural probes for in vivo wireless pharmacology and optogenetics

This Protocol Extension describes the fabrication and technical procedures for implementing ultrathin, flexible optofluidic neural probe systems that provide targeted, wireless delivery of fluids and light into the brains of awake, freely behaving animals. As a Protocol Extension article, this article describes an adaptation of an existing Protocol that offers additional applications. This protocol serves as an extension of an existing Nature Protocol describing optoelectronic devices for studying intact neural systems. Here, we describe additional features of fabricating self-contained platforms that involve flexible microfluidic probes, pumping systems, microscale inorganic LEDs, wireless-control electronics, and power supplies. These small, flexible probes minimize tissue damage and inflammation, making long-term implantation possible. The capabilities include wireless pharmacological and optical intervention for dissecting neural circuitry during behavior. The fabrication can be completed in 1-2 weeks, and the devices can be used for 1-2 weeks of in vivo rodent experiments. To successfully carry out the protocol, researchers should have basic skill sets in photolithography and soft lithography, as well as experience with stereotaxic surgery and behavioral neuroscience practices. These fabrication processes and implementation protocols will increase access to wireless optofluidic neural probes for advanced in vivo pharmacology and optogenetics in freely moving rodents.This protocol is an extension to: Nat. Protoc. 8, 2413-2428 (2013); doi:10.1038/nprot.2013.158; published online 07 November 2013.

Chronic wide-field imaging of brain hemodynamics in behaving animals

Chronically monitoring cerebral activities in awake and freely moving status is very important in physiological and pathological studies. We present a novel standalone micro-imager for monitoring the cerebral blood flow (CBF) and total hemoglobin (HbT) activities in freely moving animals using the laser speckle contrast imaging (LSCI) and optical intrinsic signal (OIS) methods. A new cranial window method, using contact lens and wide field optics, is also proposed to achieve the chronic and wide-field imaging of rat's cerebral cortex. The hemodynamic activities of rats' cortex were measured for the first time without restriction of cables or fibers in awake and behaving animals. Chronic imaging showed the increase of CBF and HbT in motor cortex when the rats were climbing on the cage wall. Interestingly, the CBF activation of supplying vessel was smaller than that of parenchyma. Furthermore, after the climbing, CBF demonstrated fully return to the baseline while HbT showed a delayed recovery. The standalone micro-imager technology provides new possibilities of brain imaging in cognitive neuroscience studies.

Stress-related disorders, pituitary adenylate cyclase-activating peptide (PACAP)ergic system, and sex differences

Trauma-related disorders, such as posttraumatic stress disorder (PTSD) are remarkably common and debilitating, and are often characterized by dysregulated threat responses. Across numerous epidemiological studies, females have been found to have an approximately twofold increased risk for PTSD and other stress-related disorders. Understanding the biological mechanisms of this differential risk is of critical importance. Recent data suggest that the pituitary adenylate cyclase-activating polypeptide (PACAP) pathway is a critical regulator of the stress response across species. Moreover, increasing evidence suggests that this pathway is regulated by both stress and estrogen modulation and may provide an important window into understanding mechanisms of sex differences in the stress response. We have recently shown that PACAP and its receptor (PAC1R) are critical mediators of abnormal processes after psychological trauma. Notably, in heavily traumatized human subjects, there appears to be a robust sex-specific association of PACAP blood levels and PAC1R gene variants with fear physiology, PTSD diagnosis, and symptoms, specifically in females. The sex-specific association occurs within a single-nucleotide polymorphism (rs2267735) that resides in a putative estrogen response element involved in PAC1R gene regulation. Complementing these human data, the PAC1R messenger RNA is induced with fear conditioning or estrogen replacement in rodent models. These data suggest that perturbations in the PACAP-PAC1R pathway are regulated by estrogen and are involved in abnormal fear responses underlying PTSD.

Microbial Proteins as Novel Industrial Biotechnology Hosts to Treat Epilepsy

Epilepsy is characterized by the hyperexcitability of various neuronal circuits that results due to the imbalance between glutamate-mediated excitation of voltage-gated cation channels and γ-amino butyric acid (GABA)-mediated inhibition of anion channels leading to aberrant, sporadic oscillations or fluctuations in neuronal electrical activity. Epilepsy with a risk of mortality and around 65 million sufferers of all ages all over the world is limited therapeutically with high rates of adverse reactions, lack of complete seizure control, and over 30% patients with refractory epilepsy. The only alternative to medicines is to identify and surgically remove the seizure foci in the brain or to abort the seizures just as they begin using an implanted cerebral electrode. However, these alternatives are unable to precisely aim aberrant neuronal circuits while leaving others unaltered. Epilepsy animal models also constitute the identical constraint. Thus, a better target-specific approach is needed to study and treat epilepsy. Unicellular green algae Chlamydomonas reinhardtii expresses a channelrhodopsin-2 (ChR2) sodium ion channel protein that controls the phototaxis movement of algae in response to blue light. Similarly, archaeon Natronomonas pharaonis (NpHR) expresses a monovalent Cl^- channel protein halorhodopsin that responds to yellow light. These features of ChR2 and NpHR proteins can be used in optogenetic techniques to manipulate the bi-directional firing pattern of neuronal circuits in an attempt to better understand the pathophysiology of epileptic seizures as well as to discover novel potential drugs to treat epilepsy.

Efficacy of nonselective optogenetic control of the medial septum over hippocampal oscillations: the influence of speed and implications for cognitive enhancement

Optogenetics holds great promise for both the dissection of neural circuits and the evaluation of theories centered on the temporal organizing properties of oscillations that underpin cognition. To date, no studies have examined the efficacy of optogenetic stimulation for altering hippocampal oscillations in freely moving wild-type rats, or how these alterations would affect performance on behavioral tasks. Here, we used an AAV virus to express ChR2 in the medial septum (MS) of wild-type rats, and optically stimulated septal neurons at 6 Hz and 30 Hz. We measured the corresponding effects of these stimulations on the oscillations of the MS and hippocampal subfields CA1 and CA3 in three different contexts: (1) With minimal movement while the rats sat in a confined chamber; (2) Explored a novel open field; and (3) Learned and performed a T-maze behavioral task. While control yellow light stimulation did not affect oscillations, 6-Hz blue light septal stimulations altered hippocampal theta oscillations in a manner that depended on the animal's mobility and speed. While the 30 Hz blue light septal stimulations only altered theta frequency in CA1 while the rat had limited mobility, it robustly increased the amplitude of hippocampal signals at 30 Hz in both regions in all three recording contexts. We found that animals were more likely to make a correct choice during Day 1 of T-maze training during both MS stimulation protocols than during control stimulation, and that improved performance was independent of theta frequency alterations.

Optogenetic activation of serotonergic terminals facilitates GABAergic inhibitory input to orexin/hypocretin neurons

Orexin/hypocretin neurons play a crucial role in the regulation of sleep/wakefulness, primarily in the maintenance of wakefulness. These neurons innervate wide areas of the brain and receive diverse synaptic inputs including those from serotonergic (5-HT) neurons in the raphe nucleus. Previously we showed that pharmacological application of 5-HT directly inhibited orexin neurons via 5-HT1A receptors. However, it was still unclear how 5-HT neurons regulated orexin neurons since 5-HT neurons contain not only 5-HT but also other neurotransmitters. To reveal this, we generated new triple transgenic mice in which orexin neurons express enhanced green fluorescent protein (EGFP) and 5-HT neurons express channelrhodopsin2 (ChR2). Immunohistochemical studies show that nerve endings of ChR2-expressing 5-HT neurons are in close apposition to EGFP-expressing orexin neurons in the lateral hypothalamic area. Using these mice, we could optogenetically activate 5-HT nerve terminals and record postsynaptic effects from orexin neurons. Activation of nerve terminals of 5-HT neurons directly inhibited orexin neurons via the 5HT1A receptor, and also indirectly inhibited orexin neurons by facilitating GABAergic inhibitory inputs without affecting glutamatergic inputs. Increased GABAergic inhibitory inputs in orexin neurons were confirmed by the pharmacological application of 5-HT. These results suggest that orexin neurons are inhibited by 5-HT neurons, primarily via 5-HT, in both direct and indirect manners.

Cell- and region-specific expression of depression-related protein p11 (S100a10) in the brain

P11 (S100a10), a member of the S100 family of proteins, has widespread distribution in the vertebrate body, including in the brain, where it has a key role in membrane trafficking, vesicle secretion, and endocytosis. Recently, our laboratory has shown that a constitutive knockout of p11 (p11-KO) in mice results in a depressive-like phenotype. Furthermore, p11 has been implicated in major depressive disorder (MDD) and in the actions of antidepressants. Since depression affects multiple brain regions, and the role of p11 has only been determined in a few of these areas, a detailed analysis of p11 expression in the brain is warranted. Here we demonstrate that, although widespread in the brain, p11 expression is restricted to distinct regions, and specific neuronal and nonneuronal cell types. Furthermore, we provide comprehensive mapping of p11 expression using in situ hybridization, immunocytochemistry, and whole-tissue volume imaging. Overall, expression spans multiple brain regions, structures, and cell types, suggesting a complex role of p11 in depression. J. Comp. Neurol. 525:955-975, 2017. © 2016 Wiley Periodicals, Inc.

Optogenetic Control of Mouse Outer Hair Cells

Normal hearing in mammals depends on sound amplification by outer hair cells (OHCs) presumably by their somatic motility and force production. However, the role of OHC force production in cochlear amplification and frequency tuning are not yet fully understood. Currently, available OHC manipulation techniques for physiological or clinical studies are limited by their invasive nature, lack of precision, and poor temporal-spatial resolution. To overcome these limitations, we explored an optogenetic approach based on channelrhodopsin 2 (ChR-2), a direct light-activated nonselective cation channel originally discovered in Chlamydomonas reinhardtii. Three approaches were compared: 1) adeno-associated virus-mediated in utero transfer of the ChR-2 gene into the developing murine otocyst, 2) expression of ChR-2(H134R) in an auditory cell line (HEI-OC1), and 3) expression of ChR-2 in the OHCs of a mouse line carrying a ChR-2 conditional allele. Whole cell recording showed that blue light (470 nm) elicited the typical nonselective cation current of ChR-2 with reversal potential around zero in both mouse OHCs and HEI-OC1 cells and generated depolarization in both cell types. In addition, pulsed light stimulation (10 Hz) elicited a 1:1 repetitive depolarization and ChR-2 currents in mouse OHCs and HEI-OC1 cells, respectively. The time constant of depolarization in OHCs, 1.45 ms, is 10 times faster than HEI-OC1 cells, which allowed light stimulation up to rates of 10/s to elicit corresponding membrane potential changes. Our study demonstrates that ChR-2 can successfully be expressed in mouse OHCs and HEI-OC1 cells and that these present a typical light-sensitive current and depolarization. However, the amount of ChR-2 current induced in our in vivo experiments was insufficient to result in measurable cochlear effects.

Optogenetic and Chemogenetic Approaches for Studying Astrocytes and Gliotransmitters

The brain consists of heterogeneous populations of neuronal and non-neuronal cells. The revelation of their connections and interactions is fundamental to understanding normal brain functions as well as abnormal changes in pathological conditions. Optogenetics and chemogenetics have been developed to allow functional manipulations both in vitro and in vivo to examine causal relationships between cellular changes and functional outcomes. These techniques are based on genetically encoded effector molecules that respond exclusively to exogenous stimuli, such as a certain wavelength of light or a synthetic ligand. Activation of effector molecules provokes diverse intracellular changes, such as an influx or efflux of ions, depolarization or hyperpolarization of membranes, and activation of intracellular signaling cascades. Optogenetics and chemogenetics have been applied mainly to the study of neuronal circuits, but their use in studying non-neuronal cells has been gradually increasing. Here we introduce recent studies that have employed optogenetics and chemogenetics to reveal the function of astrocytes and gliotransmitters.

Synapse engineering: A new level of brain modulation

Brain modulation is a powerful approach to study brain function in vivo. Tremendous progress had been made by controlling brain activity with different brain modulation tools. Synapse is the more fundamental functional unit of brain. In theory, synapse engineering could modulate brain function more precisely. However this had not been possible until recently. Our review provides a brief introduction of various brain modulation methods, and elaborates on a recently developed synapse-engineering tool. This technique allows modulation of specific synapses in vivo for the first time and has been used to clarify the causal role of synaptic plasticity in learning and memory. We also discuss its potentials for further development.

Probing pain pathways with light

We have witnessed an accelerated growth of photonics technologies in recent years to enable not only monitoring the activity of specific neurons, while animals are performing certain types of behavior, but also testing whether specific cells, circuits, and regions are sufficient or necessary for initiating, maintaining, or altering this or that behavior. Compared to other sensory systems, however, such as the visual or olfactory system, photonics applications in pain research are only beginning to emerge. One reason pain studies have lagged behind is that many of the techniques originally developed cannot be directly implemented to study key relay sites within pain pathways, such as the skin, dorsal root ganglia, spinal cord, and brainstem. This is due, in part, to difficulties in accessing these structures with light. Here we review a number of recent advances in design and delivery of light-sensitive molecular probes (sensors and actuators) into pain relay circuits to help decipher their structural and functional organization. We then discuss several challenges that have hampered hardware access to specific structures including light scattering, tissue movement and geometries. We review a number of strategies to circumvent these challenges, by delivering light into, and collecting it from the different key sites to unravel how nociceptive signals are encoded at each level of the neuraxis. We conclude with an outlook on novel imaging modalities for label-free chemical detection and opportunities for multimodal interrogation in vivo. While many challenges remain, these advances offer unprecedented opportunities to bridge cellular approaches with context-relevant behavioral testing, an essential step toward improving translation of basic research findings into clinical applications.

Optogenetic stimulation of the cochlea-A review of mechanisms, measurements, and first models

This review evaluates the potential of optogenetic methods for the stimulation of the auditory nerve and assesses the feasability of optogenetic cochlear implants (CIs). It provides an overview of all critical steps like opsin targeting strategies, how opsins work, how their function can be modeled and included in neuronal models and the properties of light sources available for optical stimulation. From these foundations, quantitative estimates for the number of independent stimulation channels and the temporal precision of optogenetic stimulation of the auditory nerve are derived and compared with state-of-the-art electrical CIs. We conclude that optogenetic CIs have the potential to increase the number of independent stimulation channels by up to one order of magnitude to about 100, but only if light sources are able to deliver confined illumination patterns independently and parallelly. Already now, opsin variants like ChETA and Chronos enable driving of the auditory nerve up to rates of 200 spikes/s, close to the physiological value of their maximum sustained firing rate. Apart from requiring 10 times more energy than electrical stimulation, optical CIs still face major hurdles concerning the safety of gene transfection and optrode array implantation, for example, before becoming an option to replace electrical CIs.

Excessive Sensory Stimulation during Development Alters Neural Plasticity and Vulnerability to Cocaine in Mice

Early life experiences affect the formation of neuronal networks, which can have a profound impact on brain function and behavior later in life. Previous work has shown that mice exposed to excessive sensory stimulation during development are hyperactive and novelty seeking, and display impaired cognition compared with controls. In this study, we addressed the issue of whether excessive sensory stimulation during development could alter behaviors related to addiction and underlying circuitry in CD-1 mice. We found that the reinforcing properties of cocaine were significantly enhanced in mice exposed to excessive sensory stimulation. Moreover, although these mice displayed hyperactivity that became more pronounced over time, they showed impaired persistence of cocaine-induced locomotor sensitization. These behavioral effects were associated with alterations in glutamatergic transmission in the nucleus accumbens and amygdala. Together, these findings suggest that excessive sensory stimulation in early life significantly alters drug reward and the neural circuits that regulate addiction and attention deficit hyperactivity. These observations highlight the consequences of early life experiences and may have important implications for children growing up in today's complex technological environment.

Adeno-associated virus and lentivirus vectors: a refined toolkit for the central nervous system

The last two decades have witnessed the increasing instrumentalization of viruses, which have progressively evolved into highly potent gene transfer vehicles for a wide spectrum of applications. In the context of the central nervous system (CNS), their unique gene delivery features and targeting specificities have been exploited not only to improve our understanding of basic neurobiology, but also to investigate diseases or deliver therapeutic candidates. As a result, we have started moving away from the opportunistic use of recombinant vectors that are derived from naturally existing viruses toward the rational engineering of tailored lentivirus (LV) and adeno-associated virus (AAV) vectors for specific use in the CNS.

Preserved dopaminergic homeostasis and dopamine-related behaviour in hemizygous TH-Cre mice

Cre-driver mouse lines have been extensively used as genetic tools to target and manipulate genetically defined neuronal populations by expression of Cre recombinase under selected gene promoters. This approach has greatly advanced neuroscience but interpretations are hampered by the fact that most Cre-driver lines have not been thoroughly characterized. Thus, a phenotypic characterization is of major importance to reveal potential aberrant phenotypes prior to implementation and usage to selectively inactivate or induce transgene expression. Here, we present a biochemical and behavioural assessment of the dopaminergic system in hemizygous tyrosine hydroxylase (TH)-Cre mice in comparison to wild-type (WT) controls. Our data show that TH-Cre mice display preserved dopaminergic homeostasis with unaltered levels of TH and dopamine as well as unaffected dopamine turnover in striatum. TH-Cre mice also show preserved dopamine transporter expression and function supporting sustained dopaminergic transmission. In addition, TH-Cre mice demonstrate normal responses in basic behavioural paradigms related to dopaminergic signalling including locomotor activity, reward preference and anxiolytic behaviour. Our results suggest that TH-Cre mice represent a valid tool to study the dopamine system, though careful characterization must always be performed to prevent false interpretations following Cre-dependent transgene expression and manipulation of selected neuronal pathways.

Optical control of endogenous receptors and cellular excitability using targeted covalent photoswitches

Light-regulated drugs allow remotely photoswitching biological activity and enable plausible therapies based on small molecules. However, only freely diffusible photochromic ligands have been shown to work directly in endogenous receptors and methods for covalent attachment depend on genetic manipulation. Here we introduce a chemical strategy to covalently conjugate and photoswitch the activity of endogenous proteins and demonstrate its application to the kainate receptor channel GluK1. The approach is based on photoswitchable ligands containing a short-lived, highly reactive anchoring group that is targeted at the protein of interest by ligand affinity. These targeted covalent photoswitches (TCPs) constitute a new class of light-regulated drugs and act as prosthetic molecules that photocontrol the activity of GluK1-expressing neurons, and restore photoresponses in degenerated retina. The modularity of TCPs enables the application to different ligands and opens the way to new therapeutic opportunities.

Biological activity can be photoswitched by light-regulated drugs, but so far only diffusible ligands have been shown to work on endogenous receptors. Here the authors develop targeted covalent photoswitches that couple to a protein target by ligand affinity, and demonstrate photocontrol of GluK1-expressing neurons.

Effects of D1 receptor knockout on fear and reward learning

Dopamine signaling is involved in a variety of neurobiological processes that contribute to learning and memory. D1-like dopamine receptors (including D1 and D5 receptors) are thought to be involved in memory and reward processes, but pharmacological approaches have been limited in their ability to distinguish between D1 and D5 receptors. Here, we examine the effects of a specific knockout of D1 receptors in associative learning tasks involving aversive (shock) or appetitive (cocaine) unconditioned stimuli. We find that D1 knockout mice show similar levels of cued and contextual fear conditioning to WT controls following conditioning protocols involving one, two, or four shocks. D1 knockout mice show increased generalization of fear conditioning and extinction across contexts, revealed as increased freezing to a novel context following conditioning and decreased freezing to an extinguished cue during a contextual renewal test. Further, D1 knockout mice show mild enhancements in extinction following an injection of SKF81297, a D1/D5 receptor agonist, suggesting a role for D5 receptors in extinction enhancements induced by nonspecific pharmacological agonists. Finally, although D1 knockout mice show decreased locomotion induced by cocaine, they are able to form a cocaine-induced conditioned place preference. We discuss these findings in terms of the role of dopamine D1 receptors in general learning and memory processes.

The antiepileptic and ictogenic effects of optogenetic neurostimulation of PV-expressing interneurons

Parvalbumin (PV)-expressing interneurons exert powerful inhibitory effects on the normal cortical network; thus optogenetic activation of PV interneurons may also possess antiepileptic properties. To investigate this possibility we expressed channelrhodopsin 2 in PV interneurons by locally injecting the Cre-dependent viral vector AAV2/1-EF1a-DIO-ChETA-EYFP into the S1 barrel cortex of PV-Cre mice. Approximately 3-4 wk later recurrent electrographic seizures were evoked by local application of the chemoconvulsant 4-aminopyridine (4-AP); the ECoG and unit activity were monitored with extracellular silicone electrodes; and PV interneurons were activated optogenetically during the ictal and interictal phases. Five- to ten-second optogenetic activation of PV interneurons applied during electrographic seizures (ictal phase) terminated 33.7% of electrographic seizures compared with only 6% during sham stimulation, and the average electrographic seizure duration shortened by 38.7 ± 34.2% compared with sham stimulation. In contrast, interictal optogenetic activation of PV interneurons showed powerful and robust ictogenic effects. Approximately 60% of interictal optogenetic stimuli resulted in electrographic seizure initiation. Single-unit recordings revealed that presumptive PV-expressing interneurons markedly increased their firing during optogenetic stimulation, while many presumptive excitatory pyramidal neurons showed a biphasic response, with initial suppression of firing during the optogenetic pulse followed by a synchronized rebound increase in firing at the end of the laser pulse. Our findings indicated that ictal activation of PV-expressing interneurons possesses antiepileptic properties probably due to suppression of firing in pyramidal neurons during the laser pulse. However, in addition interictal activation of PV-expressing interneurons possesses powerful ictogenic properties, probably due to synchronized postinhibition rebound firing of pyramidal neurons.

Chemogenetic and Optogenetic Activation of Gαs Signaling in the Basolateral Amygdala Induces Acute and Social Anxiety-Like States

Anxiety disorders are debilitating psychiatric illnesses with detrimental effects on human health. These heightened states of arousal are often in the absence of obvious threatening cues and are difficult to treat owing to a lack of understanding of the neural circuitry and cellular machinery mediating these conditions. Activation of noradrenergic circuitry in the basolateral amygdala is thought to have a role in stress, fear, and anxiety, and the specific cell and receptor types responsible is an active area of investigation. Here we take advantage of two novel cellular approaches to dissect the contributions of G-protein signaling in acute and social anxiety-like states. We used a chemogenetic approach utilizing the Gαs DREADD (rM3Ds) receptor and show that selective activation of generic Gαs signaling is sufficient to induce acute and social anxiety-like behavioral states in mice. Second, we use a recently characterized chimeric receptor composed of rhodopsin and the β2-adrenergic receptor (Opto-β2AR) with in vivo optogenetic techniques to selectively activate Gαs β-adrenergic signaling exclusively within excitatory neurons of the basolateral amygdala. We found that optogenetic induction of β-adrenergic signaling in the basolateral amygdala is sufficient to induce acute and social anxiety-like behavior. These findings support the conclusion that activation of Gαs signaling in the basolateral amygdala has a role in anxiety. These data also suggest that acute and social anxiety-like states may be mediated through signaling pathways identical to β-adrenergic receptors, thus providing support that inhibition of this system may be an effective anxiolytic therapy.

Optogenetic manipulation of neural circuits in awake marmosets

Optogenetics has revolutionized the study of functional neuronal circuitry (Boyden ES, Zhang F, Bamberg E, Nagel G, Deisseroth K. Nat Neurosci 8: 1263-1268, 2005; Deisseroth K. Nat Methods 8: 26-29, 2011). Although these techniques have been most successfully implemented in rodent models, they have the potential to be similarly impactful in studies of nonhuman primate brains. Common marmosets (Callithrix jacchus) have recently emerged as a candidate primate model for gene editing, providing a potentially powerful model for studies of neural circuitry and disease in primates. The application of viral transduction methods in marmosets for identifying and manipulating neuronal circuitry is a crucial step in developing this species for neuroscience research. In the present study we developed a novel, chronic method to successfully induce rapid photostimulation in individual cortical neurons transduced by adeno-associated virus to express channelrhodopsin (ChR2) in awake marmosets. We found that large proportions of neurons could be effectively photoactivated following viral transduction and that this procedure could be repeated for several months. These data suggest that techniques for viral transduction and optical manipulation of neuronal populations are suitable for marmosets and can be combined with existing behavioral preparations in the species to elucidate the functional neural circuitry underlying perceptual and cognitive processes.

A soft, transparent, freely accessible cranial window for chronic imaging and electrophysiology

Chronic in vivo imaging and electrophysiology are important for better understanding of neural functions and circuits. We introduce the new cranial window using soft, penetrable, elastic, and transparent, silicone-based polydimethylsiloxane (PDMS) as a substitute for the skull and dura in both rats and mice. The PDMS can be readily tailored to any size and shape to cover large brain area. Clear and healthy cortical vasculatures were observed up to 15 weeks post-implantation. Real-time hemodynamic responses were successfully monitored during sensory stimulation. Furthermore, the PDMS window allowed for easy insertion of microelectrodes and micropipettes into the cortical tissue for electrophysiological recording and chemical injection at any location without causing any fluid leakage. Longitudinal two-photon microscopic imaging of Cx3Cr1^{+/− GFP} transgenic mice was comparable with imaging via a conventional glass-type cranial window, even immediately following direct intracortical injection. This cranial window will facilitate direct probing and mapping for long-term brain studies.

From Molecular Circuit Dysfunction to Disease: Case Studies in Epilepsy, Traumatic Brain Injury, and Alzheimer's Disease

Complex circuitry with feed-forward and feed-back systems regulate neuronal activity throughout the brain. Cell biological, electrical, and neurotransmitter systems enable neural networks to process and drive the entire spectrum of cognitive, behavioral, and motor functions. Simultaneous orchestration of distinct cells and interconnected neural circuits relies on hundreds, if not thousands, of unique molecular interactions. Even single molecule dysfunctions can be disrupting to neural circuit activity, leading to neurological pathology. Here, we sample our current understanding of how molecular aberrations lead to disruptions in networks using three neurological pathologies as exemplars: epilepsy, traumatic brain injury (TBI), and Alzheimer's disease (AD). Epilepsy provides a window into how total destabilization of network balance can occur. TBI is an abrupt physical disruption that manifests in both acute and chronic neurological deficits. Last, in AD progressive cell loss leads to devastating cognitive consequences. Interestingly, all three of these neurological diseases are interrelated. The goal of this review, therefore, is to identify molecular changes that may lead to network dysfunction, elaborate on how altered network activity and circuit structure can contribute to neurological disease, and suggest common threads that may lie at the heart of molecular circuit dysfunction.

Architectural Representation of Valence in the Limbic System

In order to thrive, animals must be able to recognize aversive and appetitive stimuli within the environment and subsequently initiate appropriate behavioral responses. This assignment of positive or negative valence to a stimulus is a key feature of emotional processing, the neural substrates of which have been a topic of study for several decades. Until recently, the result of this work has been the identification of specific brain regions, such as the basolateral amygdala (BLA) and nucleus accumbens (NAc), as important to valence encoding. The advent of modern tools in neuroscience has allowed further dissection of these regions to identify specific populations of neurons signaling the valence of environmental stimuli. In this review, we focus upon recent work examining the mechanisms of valence encoding, and provide a model for the systematic investigation of valence within anatomically-, genetically-, and functionally defined populations of neurons.

Contemporary approaches to neural circuit manipulation and mapping: focus on reward and addiction

Tying complex psychological processes to precisely defined neural circuits is a major goal of systems and behavioural neuroscience. This is critical for understanding adaptive behaviour, and also how neural systems are altered in states of psychopathology, such as addiction. Efforts to relate psychological processes relevant to addiction to activity within defined neural circuits have been complicated by neural heterogeneity. Recent advances in technology allow for manipulation and mapping of genetically and anatomically defined neurons, which when used in concert with sophisticated behavioural models, have the potential to provide great insight into neural circuit bases of behaviour. Here we discuss contemporary approaches for understanding reward and addiction, with a focus on midbrain dopamine and cortico-striato-pallidal circuits.

Animal models of monogenic migraine

Migraine is a highly prevalent and disabling neurological disorder with a strong genetic component. Rare monogenic forms of migraine, or syndromes in which migraine frequently occurs, help scientists to unravel pathogenetic mechanisms of migraine and its comorbidities. Transgenic mouse models for rare monogenic mutations causing familial hemiplegic migraine (FHM), cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), and familial advanced sleep-phase syndrome (FASPS), have been created. Here, we review the current state of research using these mutant mice. We also discuss how currently available experimental approaches, including epigenetic studies, biomolecular analysis and optogenetic technologies, can be used for characterization of migraine genes to further unravel the functional and molecular pathways involved in migraine.

Neurobiological Insights from mGlu Receptor Allosteric Modulation

Allosteric modulation of metabotropic glutamate (mGlu) receptors offers a promising pharmacological approach to normalize neural circuit dysfunction associated with various psychiatric and neurological disorders. As mGlu receptor allosteric modulators progress through discovery and clinical development, both technical advances and novel tool compounds are providing opportunities to better understand mGlu receptor pharmacology and neurobiology. Recent advances in structural biology are elucidating the structural determinants of mGlu receptor-negative allosteric modulation and supplying the means to resolve active, allosteric modulator-bound mGlu receptors. The discovery and characterization of allosteric modulators with novel pharmacological profiles is uncovering the biological significance of their intrinsic agonist activity, biased mGlu receptor modulation, and novel mGlu receptor heterodimers. The development and exploitation of optogenetic and optopharmacological tools is permitting a refined spatial and temporal understanding of both mGlu receptor functions and their allosteric modulation in intact brain circuits. Together, these lines of research promise to provide a more refined understanding of mGlu receptors and their allosteric modulation that will inform the development of mGlu receptor allosteric modulators as neurotherapeutics in the years to come.

Electro-optical Neural Platform Integrated with Nanoplasmonic Inhibition Interface

Engineering of neural interfaces with nanomaterials for remote manipulation facilitates the development of platforms for the study and treatment of brain disorders, yet extending their capability to inhibiting the electrical activities of unmodified neurons has been difficult. Here we report the development of an electro-optical neural platform integrated with gold nanorods for simultaneous electrical excitation and readout, and photothermal inhibition of neural activities. A monolayer of gold nanorods was placed at the electrode-neuron interfaces of a microelectrode array for photothermal stimulation of neural activities. This nanoplasmonic interface interacted well with neurons and metal electrodes without affecting the biological and electrical properties. We demonstrated that spontaneous firing of neurons and their signal propagation along the neurites evoked by electrical stimulation were optically inhibited on this neural platform. We believe that our platform could be an alternative to the optogenetic approach and may ultimately be applied to prosthetic devices based on optical neuromodulation.

Marmosets: A Neuroscientific Model of Human Social Behavior

The common marmoset (Callithrix jacchus) has garnered interest recently as a powerful model for the future of neuroscience research. Much of this excitement has centered on the species' reproductive biology and compatibility with gene editing techniques, which together have provided a path for transgenic marmosets to contribute to the study of disease as well as basic brain mechanisms. In step with technical advances is the need to establish experimental paradigms that optimally tap into the marmosets' behavioral and cognitive capacities. While conditioned task performance of a marmoset can compare unfavorably with rhesus monkey performance on conventional testing paradigms, marmosets' social behavior and cognition are more similar to that of humans. For example, marmosets are among only a handful of primates that, like humans, routinely pair bond and care cooperatively for their young. They are also notably pro-social and exhibit social cognitive abilities, such as imitation, that are rare outside of the Apes. In this Primer, we describe key facets of marmoset natural social behavior and demonstrate that emerging behavioral paradigms are well suited to isolate components of marmoset cognition that are highly relevant to humans. These approaches generally embrace natural behavior, which has been rare in conventional primate testing, and thus allow for a new consideration of neural mechanisms underlying primate social cognition and signaling. We anticipate that through parallel technical and paradigmatic advances, marmosets will become an essential model of human social behavior, including its dysfunction in neuropsychiatric disorders.

Illuminating the Undergraduate Behavioral Neuroscience Laboratory: A Guide for the in vivo Application of Optogenetics in Mammalian Model Organisms

Optogenetics is a technology that is growing rapidly in neuroscience, establishing itself as a fundamental investigative tool. As this tool is increasingly utilized across the neuroscience community and is one of the primary research techniques being presented at neuroscience conferences and in journals, we believe that it is important that this technology is introduced into the undergraduate neuroscience research laboratory. While there has been a significant body of work concentrated to deploy optogenetics in invertebrate model organisms, little to no work has focused on brining this technology to mammalian model organisms in undergraduate neuroscience laboratories. The establishment of in vivo optogenetics could provide for high-impact independent research projects for upper-level undergraduate students. Here we review the considerations for establishing in vivo optogenetics with the use of rodents in an undergraduate laboratory setting and provide some cost-saving guidelines to assist in making optogenetic technologies financially accessible. We discuss opsin selection, cell-specific opsin expression strategies, species selection, experimental design, selection of light delivery systems, and the construction of implantable optical fibers for the application of in vivo optogenetics in rodents.

Optogenetic activation of axon guidance receptors controls direction of neurite outgrowth

Growth cones of extending axons navigate to correct targets by sensing a guidance cue gradient via membrane protein receptors. Although most signaling mechanisms have been clarified using an in vitro approach, it is still difficult to investigate the growth cone behavior in complicated extracellular environment of living animals due to the lack of tools. We develop a system for the light-dependent activation of a guidance receptor, Deleted in Colorectal Cancer (DCC), using Arabidopsis thaliana Cryptochrome 2, which oligomerizes upon blue-light absorption. Blue-light illumination transiently activates DCC via its oligomerization, which initiates downstream signaling in the illuminated subcellular region. The extending axons are attracted by illumination in cultured chick dorsal root ganglion neurons. Moreover, light-mediated navigation of the growth cones is achieved in living Caenorhabditis elegans. The photo-manipulation system is applicable to investigate the relationship between the growth cone behavior and its surrounding environment in living tissue.

Ex Vivo Optogenetic Dissection of Fear Circuits in Brain Slices

Optogenetic approaches are now widely used to study the function of neural populations and circuits by combining targeted expression of light-activated proteins and subsequent manipulation of neural activity by light. Channelrhodopsins (ChRs) are light-gated cation-channels and when fused to a fluorescent protein their expression allows for visualization and concurrent activation of specific cell types and their axonal projections in defined areas of the brain. Via stereotactic injection of viral vectors, ChR fusion proteins can be constitutively or conditionally expressed in specific cells of a defined brain region, and their axonal projections can subsequently be studied anatomically and functionally via ex vivo optogenetic activation in brain slices. This is of particular importance when aiming to understand synaptic properties of connections that could not be addressed with conventional electrical stimulation approaches, or in identifying novel afferent and efferent connectivity that was previously poorly understood. Here, a few examples illustrate how this technique can be applied to investigate these questions to elucidating fear-related circuits in the amygdala. The amygdala is a key region for acquisition and expression of fear, and storage of fear and emotional memories. Many lines of evidence suggest that the medial prefrontal cortex (mPFC) participates in different aspects of fear acquisition and extinction, but its precise connectivity with the amygdala is just starting to be understood. First, it is shown how ex vivo optogenetic activation can be used to study aspects of synaptic communication between mPFC afferents and target cells in the basolateral amygdala (BLA). Furthermore, it is illustrated how this ex vivo optogenetic approach can be applied to assess novel connectivity patterns using a group of GABAergic neurons in the amygdala, the paracapsular intercalated cell cluster (mpITC), as an example.

Optogenetic approaches to evaluate striatal function in animal models of Parkinson disease

Optogenetics refers to the ability to control cells that have been genetically modified to express light-sensitive ion channels. The introduction of optogenetic approaches has facilitated the dissection of neural circuits. Optogenetics allows for the precise stimulation and inhibition of specific sets of neurons and their projections with fine temporal specificity. These techniques are ideally suited to investigating neural circuitry underlying motor and cognitive dysfunction in animal models of human disease. Here, we focus on how optogenetics has been used over the last decade to probe striatal circuits that are involved in Parkinson disease, a neurodegenerative condition involving motor and cognitive abnormalities resulting from degeneration of midbrain dopaminergic neurons. The precise mechanisms underlying the striatal contribution to both cognitive and motor dysfunction in Parkinson disease are unknown. Although optogenetic approaches are somewhat removed from clinical use, insight from these studies can help identify novel therapeutic targets and may inspire new treatments for Parkinson disease. Elucidating how neuronal and behavioral functions are influenced and potentially rescued by optogenetic manipulation in animal models could prove to be translatable to humans. These insights can be used to guide future brain-stimulation approaches for motor and cognitive abnormalities in Parkinson disease and other neuropsychiatric diseases.

Excitatory/inhibitory imbalance in autism spectrum disorders: Implications for interventions and therapeutics

OBJECTIVES

Imbalance between excitation and inhibition and increased excitatory-inhibitory (E-I) ratio is a common mechanism in autism spectrum disorders (ASD) that is responsible for the learning and memory, cognitive, sensory, motor deficits, and seizures occurring in these disorders. ASD are very heterogeneous and better understanding of E-I imbalance in brain will lead to better diagnosis and treatments.

METHODS

We perform a critical literature review of the causes and presentations of E-I imbalance in ASD.

RESULTS

E-I imbalance in ASD is due primarily to abnormal glutamatergic and GABAergic neurotransmission in key brain regions such as neocortex, hippocampus, amygdala, and cerebellum. Other causes are due to dysfunction of neuropeptides (oxytocin), synaptic proteins (neuroligins), and immune system molecules (cytokines). At the neuropathological level E-I imbalance in ASD is presented as a "minicolumnopathy". E-I imbalance alters the manner by which the brain processes information and regulates behaviour. New developments for investigating E-I imbalance such as optogenetics and transcranial magnetic stimulation (TMS) are presented. Non-invasive brain stimulation methods such as TMS for treatment of the core symptoms of ASD are discussed.

CONCLUSIONS

Understanding E-I imbalance has important implications for developing better pharmacological and behavioural treatments for ASD, including TMS, new drugs, biomarkers and patient stratification.

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.

Optogenetic Approaches to Target Specific Neural Circuits in Post-stroke Recovery

Stroke is a leading cause of death and disability in the USA, yet treatment options are very limited. Functional recovery can occur after stroke and is attributed, in part, to rewiring of neural connections in areas adjacent to or remotely connected to the infarct. A better understanding of neural circuit rewiring is thus an important step toward developing future therapeutic strategies for stroke recovery. Because stroke disrupts functional connections in peri-infarct and remotely connected regions, it is important to investigate brain-wide network dynamics during post-stroke recovery. Optogenetics is a revolutionary neuroscience tool that uses bioengineered light-sensitive proteins to selectively activate or inhibit specific cell types and neural circuits within milliseconds, allowing greater specificity and temporal precision for dissecting neural circuit mechanisms in diseases. In this review, we discuss the current view of post-stroke remapping and recovery, including recent studies that use optogenetics to investigate neural circuit remapping after stroke, as well as optogenetic stimulation to enhance stroke recovery. Multimodal approaches employing optogenetics in conjunction with other readouts (e.g., in vivo neuroimaging techniques, behavior assays, and next-generation sequencing) will advance our understanding of neural circuit reorganization during post-stroke recovery, as well as provide important insights into which brain circuits to target when designing brain stimulation strategies for future clinical studies.

Appetite controlled by a cholecystokinin nucleus of the solitary tract to hypothalamus neurocircuit

The nucleus of the solitary tract (NTS) is a key gateway for meal-related signals entering the brain from the periphery. However, the chemical mediators crucial to this process have not been fully elucidated. We reveal that a subset of NTS neurons containing cholecystokinin (CCK^NTS) is responsive to nutritional state and that their activation reduces appetite and body weight in mice. Cell-specific anterograde tracing revealed that CCK^NTS neurons provide a distinctive innervation of the paraventricular nucleus of the hypothalamus (PVH), with fibers and varicosities in close apposition to a subset of melanocortin-4 receptor (MC4R^PVH) cells, which are also responsive to CCK. Optogenetic activation of CCK^NTS axon terminals within the PVH reveal the satiating function of CCK^NTS neurons to be mediated by a CCK^NTS→PVH pathway that also encodes positive valence. These data identify the functional significance of CCK^NTS neurons and reveal a sufficient and discrete NTS to hypothalamus circuit controlling appetite.

DOI:http://dx.doi.org/10.7554/eLife.12225.001

Obesity primarily results from eating more food than the body requires, the energy from which is then stored as fat. In recent years obesity has become increasingly common, with the resulting health problems presenting one of the major healthcare challenges of the twenty-first century. New ways to tackle the obesity epidemic are therefore required to improve human health on a global scale.

To regulate how much food is eaten, the gut sends chemical messengers to the brain about how much food has been consumed. These messengers activate particular cells in the brain that signal to other brain regions to trigger a decision about whether we’ve had enough food to eat. This raises a question: if we can artificially activate these cells, can we ‘trick’ the brain into thinking that food has been consumed?

A brain region called the nucleus of the solitary tract (NTS) is known to play a key role in receiving signals from the gut about meals. By studying mice, D’Agostino et al. found that cells in the NTS that make a brain hormone called cholecystokinin (CCK) are particularly activated by food.

Further experiments then used a technique called optogenetics to activate these cells in mice that had free access to different types of food. This activation significantly reduced how hungry the mice were, causing them to eat less food and lose weight. D’Agostino et al. also showed that CCK cells relay the signal about food intake to a brain region called the hypothalamus.

Overall, D’Agostino et al. have found a way to trick the brain into thinking that food has been eaten when it actually hasn’t, and for this reason mice eat less without feeling hungry and lose weight. The next step is to try and find a way to activate the CCK cells in obese humans who have health complications associated with excess body weight.

DOI:http://dx.doi.org/10.7554/eLife.12225.002

Micro- and Nanotechnologies for Optical Neural Interfaces

In last decade, the possibility to optically interface with the mammalian brain in vivo has allowed unprecedented investigation of functional connectivity of neural circuitry. Together with new genetic and molecular techniques to optically trigger and monitor neural activity, a new generation of optical neural interfaces is being developed, mainly thanks to the exploitation of both bottom-up and top-down nanofabrication approaches. This review highlights the role of nanotechnologies for optical neural interfaces, with particular emphasis on new devices and methodologies for optogenetic control of neural activity and unconventional methods for detection and triggering of action potentials using optically-active colloidal nanoparticles.

Cortical-Subcortical Interactions in Depression: From Animal Models to Human Psychopathology

Depression is a debilitating disorder causing significant societal and personal suffering. Improvements in identification of major depressive disorder (MDD) and its treatment are essential to reduce its toll. Recent developments in rodent models of MDD and neuroimaging of humans suffering from the disorder provide avenues through which gains can be made towards reducing its burden. In this review, new findings, integrating across rodent models and human imaging are highlighted that have yielded new insights towards a basic understanding of the disorder. In particular, this review focuses on cortical-subcortical interactions underlying the pathophysiology of MDD. In particular, evidence is accruing that dysfunction in prefrontal-subcortical circuits including the amygdala, ventral striatum (VS), hippocampus and dorsal raphe nucleus (DRN) are associated with MDD status.

A family of photoswitchable NMDA receptors

NMDA receptors, which regulate synaptic strength and are implicated in learning and memory, consist of several subtypes with distinct subunit compositions and functional properties. To enable spatiotemporally defined, rapid and reproducible manipulation of function of specific subtypes, we engineered a set of photoswitchable GluN subunits ('LiGluNs'). Photo-agonism of GluN2A or GluN2B elicits an excitatory drive to hippocampal neurons that can be shaped in time to mimic synaptic activation. Photo-agonism of GluN2A at single dendritic spines evokes spine-specific calcium elevation and expansion, the morphological correlate of LTP. Photo-antagonism of GluN2A alone, or in combination with photo-antagonism of GluN1a, reversibly blocks excitatory synaptic currents, prevents the induction of long-term potentiation and prevents spine expansion. In addition, photo-antagonism in vivo disrupts synaptic pruning of developing retino-tectal projections in larval zebrafish. By providing precise and rapidly reversible optical control of NMDA receptor subtypes, LiGluNs should help unravel the contribution of specific NMDA receptors to synaptic transmission, integration and plasticity.

DREADDS: Use and application in behavioral neuroscience

Technological advances over the last decade are changing the face of behavioral neuroscience research. Here we review recent work on the use of one such transformative tool in behavioral neuroscience research, chemogenetics (or Designer Receptors Exclusively Activated by Designer Drugs, DREADDS). As transformative technologies such as DREADDs are introduced, applied, and refined, their utility in addressing complex questions about behavior and cognition becomes clear and exciting. In the behavioral neuroscience field, remarkable new findings now regularly appear as a result of the ability to monitor and intervene in neural processes with high anatomical precision as animals behave in complex task environments. As these new tools are applied to behavioral questions, individualized procedures for their use find their way into diverse labs. Thus, "tips of the trade" become important for wide dissemination not only for laboratories that are using the tools but also for those who are interested in incorporating them into their own work. Our aim is to provide an up-to-date perspective on how the DREADD technique is being used for research on learning and memory, decision making, and goal-directed behavior, as well as to provide suggestions and considerations for current and future users based on our collective experience. (PsycINFO Database Record

The endocannabinoid system in guarding against fear, anxiety and stress

The endocannabinoid (eCB) system has emerged as a central integrator linking the perception of external and internal stimuli to distinct neurophysiological and behavioural outcomes (such as fear reaction, anxiety and stress-coping), thus allowing an organism to adapt to its changing environment. eCB signalling seems to determine the value of fear-evoking stimuli and to tune appropriate behavioural responses, which are essential for the organism's long-term viability, homeostasis and stress resilience; and dysregulation of eCB signalling can lead to psychiatric disorders. An understanding of the underlying neural cell populations and cellular processes enables the development of therapeutic strategies to mitigate behavioural maladaptation.

Blue Light Modulates Murine Microglial Gene Expression in the Absence of Optogenetic Protein Expression

Neural optogenetic applications over the past decade have steadily increased; however the effects of commonly used blue light paradigms on surrounding, non-optogenetic protein-expressing CNS cells are rarely considered, despite their simultaneous exposure. Here we report that blue light (450 nm) repetitively delivered in both long-duration boluses and rapid optogenetic bursts gene-specifically altered basal expression of inflammatory and neurotrophic genes in immortalized and primary murine wild type microglial cultures. In addition, blue light reduced pro-inflammatory gene expression in microglia activated with lipopolysaccharide. These results demonstrate previously unreported, off-target effects of blue light in cells not expressing optogenetic constructs. The unexpected gene modulatory effects of blue light on wild type CNS resident immune cells have novel and important implications for the neuro-optogenetic field. Further studies are needed to elucidate the molecular mechanisms and potential therapeutic utility of blue light modulation of the wild type CNS.

Activity-dependent FMRP requirements in development of the neural circuitry of learning and memory

The activity-dependent refinement of neural circuit connectivity during critical periods of brain development is essential for optimized behavioral performance. We hypothesize that this mechanism is defective in fragile X syndrome (FXS), the leading heritable cause of intellectual disability and autism spectrum disorders. Here, we use optogenetic tools in the Drosophila FXS disease model to test activity-dependent dendritogenesis in two extrinsic neurons of the mushroom body (MB) learning and memory brain center: (1) the input projection neuron (PN) innervating Kenyon cells (KCs) in the MB calyx microglomeruli and (2) the output MVP2 neuron innervated by KCs in the MB peduncle. Both input and output neuron classes exhibit distinctive activity-dependent critical period dendritic remodeling. MVP2 arbors expand in Drosophila mutants null for fragile X mental retardation 1 (dfmr1), as well as following channelrhodopsin-driven depolarization during critical period development, but are reduced by halorhodopsin-driven hyperpolarization. Optogenetic manipulation of PNs causes the opposite outcome--reduced dendritic arbors following channelrhodopsin depolarization and expanded arbors following halorhodopsin hyperpolarization during development. Importantly, activity-dependent dendritogenesis in both neuron classes absolutely requires dfmr1 during one developmental window. These results show that dfmr1 acts in a neuron type-specific activity-dependent manner for sculpting dendritic arbors during early-use, critical period development of learning and memory circuitry in the Drosophila brain.