Optogenetics at the presynapse

In Situ Synthesis and Assembly of Functional Materials and Devices in Living Systems

ConspectusIntegrating functional materials and devices with living systems enables novel methods for recording, manipulating, or augmenting organisms not accessible by traditional chemical, optical, or genetic approaches. (The term "device" refers to the fundamental components of complex electronic systems, such as transistors, capacitors, conductors, and electrodes.) Typically, these advanced materials and devices are synthesized, either through chemical or physical reactions, outside the biological systems (ex situ) before they are integrated. This is due in part to the more limited repertoire of biocompatible chemical transformations available for assembling functional materials in vivo. Given that most of the assembled bulk materials are impermeable to cell membranes and cannot go through the blood-brain barrier (BBB), the external synthesis poses challenges when trying to interface these materials and devices with cells precisely and in a timely manner and at the micro- and nanoscale─a crucial requirement for modulating cellular functions. In contrast to presynthesis in a separate location, in situ assembly, wherein small molecules or building blocks are directly assembled into functional materials within a biological system at the desired site of action, has offered a potential solution for spatiotemporal and genetic control of material synthesis and assembly.In this Account, we highlight recent advances in spatially and temporally targeted functional material synthesis and assembly in living cells, tissues and animals and provide perspective on how they may enable novel probing, modulation, or augmentation of fundamental biology. We discuss several strategies, starting from the traditional nontargeted methods to targeted assembly of functional materials and devices based on the endogenous markers of the biological system. We then focus on genetically targeted assembly of functional materials, which employs enzymatic catalysis centers expressed in living systems to assemble functional materials in specific molecular-defined cell types. We introduce the recent efforts of our group to modulate membrane capacitance and neuron excitability using in situ synthesized electrically functional polymers in a genetically targetable manner. These advances demonstrate the promise of in situ synthesis and assembly of functional materials and devices, including the optogenetic polymerization developed by our lab, to interface with cells in a cellular- or subcellular-specific manner by incorporating genetic and/or optical control over material assembly. Finally, we discuss remaining challenges, areas for improvement, potential applications to other biological systems, and novel methods for the in situ synthesis of functional materials that could be elevated by incorporating genetic or material design strategies. As researchers expand the toolkit of biocompatible in situ functional material synthetic techniques, we anticipate that these advancements could potentially offer valuable tools for exploring biological systems and developing therapeutic solutions.

A light-controlled phospholipase C for imaging of lipid dynamics and controlling neural plasticity

Phospholipase C (PLC) is a key enzyme that regulates physiological processes via lipid and calcium signaling. Despite advances in protein engineering, no tools are available for direct PLC control. Here, we developed a novel optogenetic tool, light-controlled PLCβ (opto-PLCβ). Opto-PLCβ uses a light-induced dimer module, which directs an engineered PLC to the plasma membrane in a light-dependent manner. Our design includes an autoinhibitory capacity, ensuring stringent control over PLC activity. Opto-PLCβ triggers reversible calcium responses and lipid dynamics in a restricted region, allowing precise spatiotemporal control of PLC signaling. Using our system, we discovered that phospholipase D-mediated phosphatidic acid contributes to diacylglycerol clearance on the plasma membrane. Moreover, we extended its applicability in vivo, demonstrating that opto-PLCβ can enhance amygdala synaptic plasticity and associative fear learning in mice. Thus, opto-PLCβ offers precise spatiotemporal control, enabling comprehensive investigation of PLC-mediated signaling pathways, lipid dynamics, and their physiological consequences in vivo.

Natural phasic inhibition of dopamine neurons signals cognitive rigidity

When animals unexpectedly fail, their dopamine neurons undergo phasic inhibition that canonically drives extinction learning-a cognitive-flexibility mechanism for discarding outdated strategies. However, the existing evidence equates natural and artificial phasic inhibition, despite their spatiotemporal differences. Addressing this gap, we targeted a GABAA-receptor antagonist precisely to dopamine neurons, yielding three unexpected findings. First, this intervention blocked natural phasic inhibition selectively, leaving tonic activity unaffected. Second, blocking natural phasic inhibition accelerated extinction learning-opposite to canonical mechanisms. Third, our approach selectively benefitted perseverative mice, restoring rapid extinction without affecting new reward learning. Our findings reveal that extinction learning is rapid by default and slowed by natural phasic inhibition-challenging foundational learning theories, while delineating a synaptic mechanism and therapeutic target for cognitive rigidity.

A bistable inhibitory optoGPCR for multiplexed optogenetic control of neural circuits

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein-coupled receptor pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable G-protein-coupled receptor that can suppress synaptic transmission in mammalian neurons with high temporal precision in vivo. PdCO has useful biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

DART.2: bidirectional synaptic pharmacology with thousandfold cellular specificity

Precision pharmacology aims to manipulate specific cellular interactions within complex tissues. In this pursuit, we introduce DART.2 (drug acutely restricted by tethering), a second-generation cell-specific pharmacology technology. The core advance is optimized cellular specificity-up to 3,000-fold in 15 min-enabling the targeted delivery of even epileptogenic drugs without off-target effects. Additionally, we introduce brain-wide dosing methods as an alternative to local cannulation and tracer reagents for brain-wide dose quantification. We describe four pharmaceuticals-two that antagonize excitatory and inhibitory postsynaptic receptors, and two that allosterically potentiate these receptors. Their versatility is showcased across multiple mouse-brain regions, including cerebellum, striatum, visual cortex and retina. Finally, in the ventral tegmental area, we find that blocking inhibitory inputs to dopamine neurons accelerates locomotion, contrasting with previous optogenetic and pharmacological findings. Beyond enabling the bidirectional perturbation of chemical synapses, these reagents offer intersectional precision-between genetically defined postsynaptic cells and neurotransmitter-defined presynaptic partners.

Postoperatively Noninvasive Optogenetic Stimulation via Upconversion Nanoparticles Enhancing Sciatic Nerve Repair

The efficacy of electrical stimulation facilitating peripheral nerve regeneration is evidenced extensively, while the associated secondary damage resulting from repeated electrode invasion and indiscriminate stimulation is inevitable. Here, we present an optogenetics strategy that utilizes upconversion nanoparticles (UCNPs) to convert deeply penetrating near-infrared excitation into blue emission, which activates an adeno-associated virus-encoding ChR2 photoresponsive ion channel on cell membranes. The induced Ca2+ flux, similar to the ion flux in the electrical stimulation approach, efficiently regulates viability and proliferation, secretion of nerve growth factor, and neural function of RSC96 cells. Furthermore, deep near-infrared excitation is harnessed to stimulate autologous Schwann cells in situ via a UCNP-composited scaffold, which enhances nerve sprouting and myelination, consequently promoting functional recovery, electrophysiological restoration, and reinnervation of damaged nerves. This developed postoperatively noninvasive optogenetics strategy presents a novel, minimally traumatic, and enduring therapeutic stimulus to effectively promote peripheral nerve repair.

Kalium channelrhodopsins effectively inhibit neurons

The analysis of neural circuits has been revolutionized by optogenetic methods. Light-gated chloride-conducting anion channelrhodopsins (ACRs)-recently emerged as powerful neuron inhibitors. For cells or sub-neuronal compartments with high intracellular chloride concentrations, however, a chloride conductance can have instead an activating effect. The recently discovered light-gated, potassium-conducting, kalium channelrhodopsins (KCRs) might serve as an alternative in these situations, with potentially broad application. As yet, KCRs have not been shown to confer potent inhibitory effects in small genetically tractable animals. Here, we evaluated the utility of KCRs to suppress behavior and inhibit neural activity in Drosophila, Caenorhabditis elegans, and zebrafish. In direct comparisons with ACR1, a KCR1 variant with enhanced plasma-membrane trafficking displayed comparable potency, but with improved properties that include reduced toxicity and superior efficacy in putative high-chloride cells. This comparative analysis of behavioral inhibition between chloride- and potassium-selective silencing tools establishes KCRs as next-generation optogenetic inhibitors for in vivo circuit analysis in behaving animals.

All-optical presynaptic plasticity induction by photoactivated adenylyl cyclase targeted to axon terminals

Intracellular signaling plays essential roles in various cell types. In the central nervous system, signaling cascades are strictly regulated in a spatiotemporally specific manner to govern brain function; for example, presynaptic cyclic adenosine monophosphate (cAMP) can enhance the probability of neurotransmitter release. In the last decade, channelrhodopsin-2 has been engineered for subcellular targeting using localization tags, but optogenetic tools for intracellular signaling are not well developed. Therefore, we engineered a selective presynaptic fusion tag for photoactivated adenylyl cyclase (bPAC-Syn1a) and found its high localization at presynaptic terminals. Furthermore, an all-optical electrophysiological method revealed rapid and robust short-term potentiation by bPAC-Syn1a at brain stem-amygdala synapses in acute brain slices. Additionally, bPAC-Syn1a modulated mouse immobility behavior. These results indicate that bPAC-Syn1a can manipulate presynaptic cAMP signaling in vitro and in vivo. The all-optical manipulation technique developed in this study can help further elucidate the dynamic regulation of various cellular functions.

Intracellular microbial rhodopsin-based optogenetics to control metabolism and cell signaling

Microbial rhodopsin (MRs) ion channels and pumps have become invaluable optogenetic tools for neuroscience as well as biomedical applications. Recently, MR-optogenetics expanded towards subcellular organelles opening principally new opportunities in optogenetic control of intracellular metabolism and signaling via precise manipulations of organelle ion gradients using light. This new optogenetic field expands the opportunities for basic and medical studies of cancer, cardiovascular, and metabolic disorders, providing more detailed and accurate control of cell physiology. This review summarizes recent advances in studies of the cellular metabolic processes and signaling mediated by optogenetic tools targeting mitochondria, endoplasmic reticulum (ER), lysosomes, and synaptic vesicles. Finally, we discuss perspectives of such an optogenetic approach in both fundamental and applied research.

Beyond a Transmission Cable-New Technologies to Reveal the Richness in Axonal Electrophysiology

The axon is a neuronal structure capable of processing, encoding, and transmitting information. This assessment contrasts with a limiting, but deeply rooted, perspective where the axon functions solely as a transmission cable of somatodendritic activity, sending signals in the form of stereotypical action potentials. This perspective arose, at least partially, because of the technical difficulties in probing axons: their extreme length-to-diameter ratio and intricate growth paths preclude the study of their dynamics through traditional techniques. Recent findings are challenging this view and revealing a much larger repertoire of axonal computations. Axons display complex signaling processes and structure-function relationships, which can be modulated via diverse activity-dependent mechanisms. Additionally, axons can exhibit patterns of activity that are dramatically different from those of their corresponding soma. Not surprisingly, many of these recent discoveries have been driven by novel technology developments, which allow for in vitro axon electrophysiology with unprecedented spatiotemporal resolution and signal-to-noise ratio. In this review, we outline the state-of-the-art in vitro toolset for axonal electrophysiology and summarize the recent discoveries in axon function it has enabled. We also review the increasing repertoire of microtechnologies for controlling axon guidance which, in combination with the available cutting-edge electrophysiology and imaging approaches, have the potential for more controlled and high-throughput in vitro studies. We anticipate that a larger adoption of these new technologies by the neuroscience community will drive a new era of experimental opportunities in the study of axon physiology and consequently, neuronal function.

Orexin neurons play contrasting roles in itch and pain neural processing via projecting to the periaqueductal gray

Pain and itch are recognized as antagonistically regulated sensations; pain suppresses itch, whilst pain inhibition enhances itch. The neural mechanisms at the central nervous system (CNS) underlying these pain-itch interactions still need to be explored. Here, we revealed the contrasting role of orexin-producing neurons (ORX neurons) in the lateral hypothalamus (LH), which suppresses pain while enhancing itch neural processing, by applying optogenetics to the acute pruritus and pain model. We also revealed that the circuit of ORX neurons from LH to periaqueductal gray regions served in the contrasting modulation of itch and pain processing using optogenetic terminal inhibition techniques. Additionally, by using an atopic dermatitis model, we confirmed the involvement of ORX neurons in regulating chronic itch processing, which could lead to a novel therapeutic target for persistent pruritus in clinical settings. Our findings provide new insight into the mechanism of antagonistic regulation between pain and itch in the CNS.

Gamma oscillation plasticity is mediated via parvalbumin interneurons

Understanding the plasticity of neuronal networks is an emerging field of (patho-) physiological research, yet the underlying cellular mechanisms remain poorly understood. Gamma oscillations (30 to 80 hertz), a biomarker of cognitive performance, require and potentiate glutamatergic transmission onto parvalbumin-positive interneurons (PVIs), suggesting an interface for cell-to-network plasticity. In ex vivo local field potential recordings, we demonstrate long-term potentiation of hippocampal gamma power. Gamma potentiation obeys established rules of PVI plasticity, requiring calcium-permeable AMPA receptors (CP-AMPARs) and metabotropic glutamate receptors (mGluRs). A microcircuit computational model of CA3 gamma oscillations predicts CP-AMPAR plasticity onto PVIs critically outperforms pyramidal cell plasticity in increasing gamma power and completely accounts for gamma potentiation. We reaffirm this ex vivo in three PVI-targeting animal models, demonstrating that gamma potentiation requires PVI-specific signaling via a Gq/PKC pathway comprising mGluR5 and a Gi-sensitive, PKA-dependent pathway. Gamma activity-dependent, metabotropically mediated CP-AMPAR plasticity on PVIs may serve as a guiding principle in understanding network plasticity in health and disease.

Chemo- and optogenetic activation of hypothalamic Foxb1-expressing neurons and their terminal endings in the rostral-dorsolateral PAG leads to tachypnea, bradycardia, and immobility

Foxb1 -expressing neurons occur in the dorsal premammillary nucleus (PMd) and further rostrally in the parvafox nucleus, a longitudinal cluster of neurons in the lateral hypothalamus of rodents. The descending projection of these Foxb1+ neurons end in the dorsolateral part of the periaqueductal gray (dlPAG). The functional role of the Foxb1+ neuronal subpopulation in the PMd and the parvafox nucleus remains elusive. In this study, the activity of the Foxb1+ neurons and of their terminal endings in the dlPAG in mice was selectively altered by employing chemo- and optogenetic tools. Our results show that in whole-body barometric plethysmography, hM3Dq-mediated, global Foxb1+ neuron excitation activates respiration. Time-resolved optogenetic gain-of-function manipulation of the terminal endings of Foxb1+ neurons in the rostral third of the dlPAG leads to abrupt immobility and bradycardia. Chemogenetic activation of Foxb1+ cell bodies and ChR2-mediated excitation of their axonal endings in the dlPAG led to a phenotypical presentation congruent with a 'freezing-like' situation during innate defensive behavior.

Optogenetics 2.0: challenges and solutions towards a quantitative probing of neural circuits

To exploit the full potential of optogenetics, we need to titrate and tailor optogenetic methods to emulate naturalistic circuit function. For that, the following prerequisites need to be met: first, we need to target opsin expression not only to genetically defined neurons per se, but to specifically target a functional node. Second, we need to assess the scope of optogenetic modulation, i.e. the fraction of optogenetically modulated neurons. Third, we need to integrate optogenetic control in a closed loop setting. Fourth, we need to further safe and stable gene expression and light delivery to bring optogenetics to the clinics. Here, we review these concepts for the human and rodent brain.

Corticotropin-releasing hormone neurons control trigeminal neuralgia-induced anxiodepression via a hippocampus-to-prefrontal circuit

Anxiety and depression are frequently observed in patients suffering from trigeminal neuralgia (TN), but neural circuits and mechanisms underlying this association are poorly understood. Here, we identified a dedicated neural circuit from the ventral hippocampus (vHPC) to the medial prefrontal cortex (mPFC) that mediates TN-related anxiodepression. We found that TN caused an increase in excitatory synaptic transmission from vHPCCaMK2A neurons to mPFC inhibitory neurons marked by the expression of corticotropin-releasing hormone (CRH). Activation of CRH+ neurons subsequently led to feed-forward inhibition of layer V pyramidal neurons in the mPFC via activation of the CRH receptor 1 (CRHR1). Inhibition of the vHPCCaMK2A-mPFCCRH circuit ameliorated TN-induced anxiodepression, whereas activating this pathway sufficiently produced anxiodepressive-like behaviors. Thus, our studies identified a neural pathway driving pain-related anxiodepression and a molecular target for treating pain-related psychiatric disorders.

Optogenetic Activation of Peripheral Somatosensory Neurons in Transgenic Mice as a Neuropathic Pain Model for Assessing the Therapeutic Efficacy of Analgesics

Optogenetics is a novel biotechnology widely used to precisely manipulate a specific peripheral sensory neuron or neural circuit. However, the use of optogenetics to assess the therapeutic efficacy of analgesics is elusive. In this study, we generated a transgenic mouse stain in which all primary somatosensory neurons can be optogenetically activated to mimic neuronal hyperactivation in the neuropathic pain state for the assessment of analgesic effects of drugs. A transgenic mouse was generated using the advillin-Cre line mated with the Ai32 strain, in which channelrhodopsin-2 fused to enhanced yellow fluorescence protein (ChR2-EYFP) was conditionally expressed in all types of primary somatosensory neurons (advillincre/ChR2+/+). Immunofluorescence and transdermal photostimulation on the hindpaws were used to verify the transgenic mice. Optical stimulation to evoke pain-like paw withdrawal latency was used to assess the analgesic effects of a series of drugs. Injury- and pain-related molecular biomarkers were investigated with immunohistofluorescence. We found that the expression of ChR2-EYFP was observed in many primary afferents of paw skin and sciatic nerves and in primary sensory neurons and laminae I and II of the spinal dorsal horns in advillincre/ChR2+/+ mice. Transdermal blue light stimulation of the transgenic mouse hindpaw evoked nocifensive paw withdrawal behavior. Treatment with gabapentin, some channel blockers, and local anesthetics, but not opioids or COX-1/2 inhibitors, prolonged the paw withdrawal latency in the transgenic mice. The analgesic effect of gabapentin was also verified by the decreased expression of injury- and pain-related molecular biomarkers. These optogenetic mice provide a promising model for assessing the therapeutic efficacy of analgesics in neuropathic pain.

Metal Modulation: An Effortless Tactic for Refining Photoredox Catalysis in Living Cells

The remarkable impact of photoredox catalytic chemistries has sparked a wave of innovation, opening doors to novel biotechnologies in the realm of catalytic antitumor therapy. Yet, the quest for novel photoredox catalysts (PCs) suitable for living systems, or the enhancement of catalytic efficacy in existing biocompatible PC systems, persists as a formidable challenge. Within this context, we introduce a readily applicable metal modulation strategy that significantly augments photoredox catalysis within living cells, exemplified by a set of metalloporphyrin complexes termed M-TCPPs (M = Zn, Mn, Ni, Co, Cu). Among these complexes, Zn-TCPP emerges as an exceptional catalyst, displaying remarkable photocatalytic activity in the oxidation of nicotinamide adenine dinucleotide (NADH), nicotinamide adenine dinucleotide phosphate (NADPH), and specific amino acids. Notably, comprehensive investigations reveal that Zn-TCPP's superior catalytic prowess primarily arises from the establishment of an efficient oxidative cycle for PC, in contrast to previously reported PCs engaged in reductive cycles. Moreover, theoretical calculations illuminate that amplified intersystem crossing rates and geometry alterations in Zn-TCPP contribute to its heightened photocatalytic performance. In vitro studies demonstrated that Zn-TCPP exhibits therapeutic potential and is found to be effective for photocatalytic antitumor therapy in both glioblastoma G98T cells and 3D multicellular spheroids. This study underscores the transformative role of "metal modulation" in advancing high-performance PCs for catalytic antitumor therapy, marking a significant stride toward the realization of this innovative therapeutic approach.

Optical Approaches for Investigating Neuromodulation and G Protein-Coupled Receptor Signaling

Despite the fact that roughly 40% of all US Food and Drug Administration (FDA)-approved pharmacological therapeutics target G protein-coupled receptors (GPCRs), there remains a gap in our understanding of the physiologic and functional role of these receptors at the systems level. Although heterologous expression systems and in vitro assays have revealed a tremendous amount about GPCR signaling cascades, how these cascades interact across cell types, tissues, and organ systems remains obscure. Classic behavioral pharmacology experiments lack both the temporal and spatial resolution to resolve these long-standing issues. Over the past half century, there has been a concerted effort toward the development of optical tools for understanding GPCR signaling. From initial ligand uncaging approaches to more recent development of optogenetic techniques, these strategies have allowed researchers to probe longstanding questions in GPCR pharmacology both in vivo and in vitro. These tools have been employed across biologic systems and have allowed for interrogation of everything from specific intramolecular events to pharmacology at the systems level in a spatiotemporally specific manner. In this review, we present a historical perspective on the motivation behind and development of a variety of optical toolkits that have been generated to probe GPCR signaling. Here we highlight how these tools have been used in vivo to uncover the functional role of distinct populations of GPCRs and their signaling cascades at a systems level. SIGNIFICANCE STATEMENT: G protein-coupled receptors (GPCRs) remain one of the most targeted classes of proteins for pharmaceutical intervention, yet we still have a limited understanding of how their unique signaling cascades effect physiology and behavior at the systems level. In this review, we discuss a vast array of optical techniques that have been devised to probe GPCR signaling both in vitro and in vivo.

Photoactivated Protein Degrader for Optical Control of Synaptic Function

Hundreds of proteins determine the function of synapses, and synapses define the neuronal circuits that subserve myriad brain, cognitive, and behavioral functions. It is thus necessary to precisely manipulate specific proteins at specific sub-cellular locations and times to elucidate the roles of particular proteins and synapses in brain function. We developed PHOtochemically TArgeting Chimeras (PHOTACs) as a strategy to optically degrade specific proteins with high spatial and temporal precision. PHOTACs are small molecules that, upon wavelength-selective illumination, catalyze ubiquitylation and degradation of target proteins through endogenous proteasomes. Here, we describe the design and chemical properties of a PHOTAC that targets Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIα), which is abundant and crucial for the baseline synaptic function of excitatory neurons. We validate the PHOTAC strategy, showing that the CaMKIIα-PHOTAC is effective in mouse brain tissue. Light activation of CaMKIIα-PHOTAC removed CaMKIIα from regions of the mouse hippocampus only within 25 μm of the illuminated brain surface. The optically controlled degradation decreases synaptic function within minutes of light activation, measured by the light-initiated attenuation of evoked field excitatory postsynaptic potential (fEPSP) responses to physiological stimulation. The PHOTACs methodology should be broadly applicable to other key proteins implicated in synaptic function, especially for evaluating their precise roles in the maintenance of long-term potentiation and memory within subcellular dendritic domains.

Illuminating the Brain: Advances and Perspectives in Optoelectronics for Neural Activity Monitoring and Modulation

Optogenetic modulation of brain neural activity that combines optical and electrical modes in a unitary neural system has recently gained robust momentum. Controlling illumination spatial coverage, designing light-activated modulators, and developing wireless light delivery and data transmission are crucial for maximizing the use of optical neuromodulation. To this end, biocompatible electrodes with enhanced optoelectrical performance, device integration for multiplexed addressing, wireless transmission, and multimodal operation in soft systems have been developed. This review provides an outlook for uniformly illuminating large brain areas while spatiotemporally imaging the neural responses upon optoelectrical stimulation with little artifacts. Representative concepts and important breakthroughs, such as head-mounted illumination, multiple implanted optical fibers, and micro-light-delivery devices, are discussed. Examples of techniques that incorporate electrophysiological monitoring and optoelectrical stimulation are presented. Challenges and perspectives are posed for further research efforts toward high-density optoelectrical neural interface modulation, with the potential for nonpharmacological neurological disease treatments and wireless optoelectrical stimulation.

A Comprehensive Review of Emerging Trends and Innovative Therapies in Epilepsy Management

Epilepsy is a complex neurological disorder affecting millions worldwide, with a substantial number of patients facing drug-resistant epilepsy. This comprehensive review explores innovative therapies for epilepsy management, focusing on their principles, clinical evidence, and potential applications. Traditional antiseizure medications (ASMs) form the cornerstone of epilepsy treatment, but their limitations necessitate alternative approaches. The review delves into cutting-edge therapies such as responsive neurostimulation (RNS), vagus nerve stimulation (VNS), and deep brain stimulation (DBS), highlighting their mechanisms of action and promising clinical outcomes. Additionally, the potential of gene therapies and optogenetics in epilepsy research is discussed, revealing groundbreaking findings that shed light on seizure mechanisms. Insights into cannabidiol (CBD) and the ketogenic diet as adjunctive therapies further broaden the spectrum of epilepsy management. Challenges in achieving seizure control with traditional therapies, including treatment resistance and individual variability, are addressed. The importance of staying updated with emerging trends in epilepsy management is emphasized, along with the hope for improved therapeutic options. Future research directions, such as combining therapies, AI applications, and non-invasive optogenetics, hold promise for personalized and effective epilepsy treatment. As the field advances, collaboration among researchers of natural and synthetic biochemistry, clinicians from different streams and various forms of medicine, and patients will drive progress toward better seizure control and a higher quality of life for individuals living with epilepsy.

Excitation and Inhibition Delays within a Feedforward Inhibitory Pathway Modulate Cerebellar Purkinje Cell Output in Mice

The cerebellar cortex computes sensorimotor information from many brain areas through a feedforward inhibitory (FFI) microcircuit between the input stage, the granule cell (GC) layer, and the output stage, the Purkinje cells (PCs). Although in other brain areas FFI underlies a precise excitation versus inhibition temporal correlation, recent findings in the cerebellum highlighted more complex behaviors at GC-molecular layer interneuron (MLI)-PC pathway. To dissect the temporal organization of this cerebellar FFI pathway, we combined ex vivo patch-clamp recordings of PCs in male mice with a viral-based strategy to express Channelrhodopsin2 in a subset of mossy fibers (MFs), the major excitatory inputs to GCs. We show that although light-mediated MF activation elicited pairs of excitatory and inhibitory postsynaptic currents in PCs, excitation (E) from GCs and inhibition (I) from MLIs reached PCs with a wide range of different temporal delays. However, when GCs were directly stimulated, a low variability in E/I delays was observed. Our results demonstrate that in many recordings MF stimulation recruited different groups of GCs that trigger E and/or I, and expanded PC temporal synaptic integration. Finally, using a computational model of the FFI pathway, we showed that this temporal expansion could strongly influence how PCs integrate GC inputs. Our findings show that specific E/I delays may help PCs encoding specific MF inputs.SIGNIFICANCE STATEMENT Sensorimotor information is conveyed to the cerebellar cortex by mossy fibers. Mossy fiber inputs activate granule cells that excite molecular interneurons and Purkinje cells, the sole output of the cerebellar cortex, leading to a sequence of synaptic excitation and inhibition in Purkinje cells, thus defining a feedforward inhibitory pathway. Using electrophysiological recordings, optogenetic stimulation, and mathematical modeling, we demonstrated that different groups of granule cells can elicit synaptic excitation and inhibition with various latencies onto Purkinje cells. This temporal variability controls how granule cells influence Purkinje cell discharge and may support temporal coding in the cerebellar cortex.

Quantitative analysis of the optogenetic excitability of CA1 neurons

Introduction

Optogenetics has emerged as a promising technique for modulating neuronal activity and holds potential for the treatment of neurological disorders such as temporal lobe epilepsy (TLE). However, clinical translation still faces many challenges. This in-silico study aims to enhance the understanding of optogenetic excitability in CA1 cells and to identify strategies for improving stimulation protocols.

Methods

Employing state-of-the-art computational models coupled with Monte Carlo simulated light propagation, the optogenetic excitability of four CA1 cells, two pyramidal and two interneurons, expressing ChR2(H134R) is investigated.

Results and discussion

The results demonstrate that confining the opsin to specific neuronal membrane compartments significantly improves excitability. An improvement is also achieved by focusing the light beam on the most excitable cell region. Moreover, the perpendicular orientation of the optical fiber relative to the somato-dendritic axis yields superior results. Inter-cell variability is observed, highlighting the importance of considering neuron degeneracy when designing optogenetic tools. Opsin confinement to the basal dendrites of the pyramidal cells renders the neuron the most excitable. A global sensitivity analysis identified opsin location and expression level as having the greatest impact on simulation outcomes. The error reduction of simulation outcome due to coupling of neuron modeling with light propagation is shown. The results promote spatial confinement and increased opsin expression levels as important improvement strategies. On the other hand, uncertainties in these parameters limit precise determination of the irradiance thresholds. This study provides valuable insights on optogenetic excitability of CA1 cells useful for the development of improved optogenetic stimulation protocols for, for instance, TLE treatment.

A bistable inhibitory OptoGPCR for multiplexed optogenetic control of neural circuits

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein coupled receptor (GPCRs) pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision, or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable GPCR that can suppress synaptic transmission in mammalian neurons with high temporal precision in-vivo. PdCO has superior biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

A self-inactivating invertebrate opsin optically drives biased signaling toward Gβγ-dependent ion channel modulation

Animal opsins, light-sensitive G protein-coupled receptors, have been used for optogenetic tools to control G protein-dependent signaling pathways. Upon G protein activation, the Gα and Gβγ subunits drive different intracellular signaling pathways, leading to complex cellular responses. For some purposes, Gα- and Gβγ-dependent signaling needs to be separately modulated, but these responses are simultaneously evoked due to the 1:1 stoichiometry of Gα and Gβγ Nevertheless, we show temporal activation of G protein using a self-inactivating invertebrate opsin, Platynereis c-opsin1, drives biased signaling for Gβγ-dependent GIRK channel activation in a light-dependent manner by utilizing the kinetic difference between Gβγ-dependent and Gα-dependent responses. The opsin-induced transient Gi/o activation preferentially causes activation of the kinetically fast Gβγ-dependent GIRK channels rather than slower Gi/oα-dependent adenylyl cyclase inhibition. Although similar Gβγ-biased signaling properties were observed in a self-inactivating vertebrate visual pigment, Platynereis c-opsin1 requires fewer retinal molecules to evoke cellular responses. Furthermore, the Gβγ-biased signaling properties of Platynereis c-opsin1 are enhanced by genetically fusing with RGS8 protein, which accelerates G protein inactivation. The self-inactivating invertebrate opsin and its RGS8-fusion protein can function as optical control tools biased for Gβγ-dependent ion channel modulation.

Advances in the Electrophysiological Recordings of Long-Term Potentiation

Understanding neuronal firing patterns and long-term potentiation (LTP) induction in studying learning, memory, and neurological diseases is critical. However, recently, despite the rapid advancement in neuroscience, we are still constrained by the experimental design, detection tools for exploring the mechanisms and pathways involved in LTP induction, and detection ability of neuronal action potentiation signals. This review will reiterate LTP-related electrophysiological recordings in the mammalian brain for nearly 50 years and explain how excitatory and inhibitory neural LTP results have been detected and described by field- and single-cell potentials, respectively. Furthermore, we focus on describing the classic model of LTP of inhibition and discuss the inhibitory neuron activity when excitatory neurons are activated to induce LTP. Finally, we propose recording excitatory and inhibitory neurons under the same experimental conditions by combining various electrophysiological technologies and novel design suggestions for future research. We discussed different types of synaptic plasticity, and the potential of astrocytes to induce LTP also deserves to be explored in the future.

A Photoactivated Protein Degrader for Optical Control of Synaptic Function

Hundreds of proteins determine the function of synapses, and synapses define the neuronal circuits that subserve myriad brain, cognitive, and behavioral functions. It is thus necessary to precisely manipulate specific proteins at specific sub-cellular locations and times to elucidate the roles of particular proteins and synapses in brain function. We developed PHOtochemically TArgeting Chimeras (PHOTACs) as a strategy to optically degrade specific proteins with high spatial and temporal precision. PHOTACs are small molecules that, upon wavelength-selective illumination, catalyze ubiquitylation and degradation of target proteins through endogenous proteasomes. Here we describe the design and chemical properties of a PHOTAC that targets Ca 2+ /calmodulin-dependent protein kinase II alpha (CaMKIIα), which is abundant and crucial for baseline synaptic function of excitatory neurons. We validate the PHOTAC strategy, showing that the CaMKIIα-PHOTAC is effective in mouse brain tissue. Light activation of CaMKIIα-PHOTAC removed CaMKIIα from regions of the mouse hippocampus only within 25 μm of the illuminated brain surface. The optically-controlled degradation decreases synaptic function within minutes of light activation, measured by the light-initiated attenuation of evoked field excitatory postsynaptic potential (fEPSP) responses to physiological stimulation. The PHOTACs methodology should be broadly applicable to other key proteins implicated in synaptic function, especially for evaluating their precise roles in the maintenance of long-term potentiation and memory within subcellular dendritic domains.

Rapid and reversible optogenetic silencing of synaptic transmission by clustering of synaptic vesicles

Acutely silencing specific neurons informs about their functional roles in circuits and behavior. Existing optogenetic silencers include ion pumps, channels, metabotropic receptors, and tools that damage the neurotransmitter release machinery. While the former hyperpolarize the cell, alter ionic gradients or cellular biochemistry, the latter allow only slow recovery, requiring de novo synthesis. Thus, tools combining fast activation and reversibility are needed. Here, we use light-evoked homo-oligomerization of cryptochrome CRY2 to silence synaptic transmission, by clustering synaptic vesicles (SVs). We benchmark this tool, optoSynC, in Caenorhabditis elegans, zebrafish, and murine hippocampal neurons. optoSynC clusters SVs, observable by electron microscopy. Locomotion silencing occurs with tauon ~7.2 s and recovers with tauoff ~6.5 min after light-off. optoSynC can inhibit exocytosis for several hours, at very low light intensities, does not affect ion currents, biochemistry or synaptic proteins, and may further allow manipulating different SV pools and the transfer of SVs between them.

Green light analgesia in mice is mediated by visual activation of enkephalinergic neurons in the ventrolateral geniculate nucleus

Green light exposure has been shown to reduce pain in animal models. Here, we report a vision-associated enkephalinergic neural circuit responsible for green light-mediated analgesia. Full-field green light exposure at an intensity of 10 lux produced analgesic effects in healthy mice and in a model of arthrosis. Ablation of cone photoreceptors completely inhibited the analgesic effect, whereas rod ablation only partially reduced pain relief. The analgesic effect was not modulated by the ablation of intrinsically photosensitive retinal ganglion cells (ipRGCs), which are atypical photoreceptors that control various nonvisual effects of light. Inhibition of the retino-ventrolateral geniculate nucleus (vLGN) pathway completely abolished the analgesic effects. Activation of this pathway reduced nociceptive behavioral responses; such activation was blocked by the inhibition of proenkephalin (Penk)-positive neurons in the vLGN (vLGN^Penk). Moreover, green light analgesia was prevented by knockdown of Penk in the vLGN or by ablation of vLGN^Penk neurons. In addition, activation of the projections from vLGN^Penk neurons to the dorsal raphe nucleus (DRN) was sufficient to suppress nociceptive behaviors, whereas its inhibition abolished the green light analgesia. Our findings indicate that cone-dominated retinal inputs mediated green light analgesia through the vLGN^Penk-DRN pathway and suggest that this signaling pathway could be exploited for reducing pain.

High-resolution optogenetics in space and time

To understand the neural mechanisms of behavior, it is necessary to both monitor and perturb the activity of ensembles of neurons with high specificity. While neural ensemble recordings have been available for decades, progress in high-resolution manipulation techniques has lagged behind. Optogenetics has enabled the manipulation of genetically defined cell types in behaving animals, and recent developments, including multipoint nanofabricated light sources, provide spatiotemporal resolution on a par with that of physiological recordings. Here we review current advances in optogenetic methods for cellular-resolution stimulation and intervention, as well as their integration with real-time neural recordings for closed-loop experimentation. We discuss how these approaches open the door to new kinds of experiments aimed at dissecting the role of specific neural patterns and discrete cellular populations in orchestrating the activity of brain circuits that support behavior and cognition.