Spotlight on pain: optogenetic approaches for interrogating somatosensory circuits

Neural circuit-selective, multiplexed pharmacological targeting of prefrontal cortex-projecting locus coeruleus neurons drives antinociception

Selective manipulation of neural circuits using optogenetics and chemogenetics holds great translational potential but requires genetic access to neurons. Here, we demonstrate a general framework for identifying genetic tool-independent, pharmacological strategies for neural circuit-selective modulation. We developed an economically accessible calcium imaging-based approach for large-scale pharmacological scans of endogenous receptor-mediated neural activity. As a testbed for this approach, we used the mouse locus coeruleus due to the combination of its widespread, modular efferent neural circuitry and its wide variety of endogenously expressed GPCRs. Using machine learning-based action potential deconvolution and retrograde tracing, we identified an agonist cocktail that selectively inhibits medial prefrontal cortex-projecting locus coeruleus neurons. In vivo, this cocktail produces synergistic antinociception, consistent with selective pharmacological blunting of this neural circuit. This framework has broad utility for selective targeting of other neural circuits under different physiological and pathological states, facilitating non-genetic translational applications arising from cell type-selective discoveries.

Reproducible and fully automated testing of nocifensive behavior in mice

Pain in rodents is often inferred from their withdrawal from noxious stimulation. Threshold stimulus intensity or response latency is used to quantify pain sensitivity. This usually involves applying stimuli by hand and measuring responses by eye, which limits reproducibility and throughput. We describe a device that standardizes and automates pain testing by providing computer-controlled aiming, stimulation, and response measurement. Optogenetic and thermal stimuli are applied using blue and infrared light, respectively. Precise mechanical stimulation is also demonstrated. Reflectance of red light is used to measure paw withdrawal with millisecond precision. We show that consistent stimulus delivery is crucial for resolving stimulus-dependent variations in withdrawal and for testing with sustained stimuli. Moreover, substage video reveals "spontaneous" behaviors for consideration alongside withdrawal metrics to better assess the pain experience. The entire process was automated using machine learning. RAMalgo (reproducible automated multimodal algometry) improves the standardization, comprehensiveness, and throughput of preclinical pain testing.

A photoswitchable GPCR-based opsin for presynaptic inhibition

Optical manipulations of genetically defined cell types have generated significant insights into the dynamics of neural circuits. While optogenetic activation has been relatively straightforward, rapid and reversible synaptic inhibition has proven more elusive. Here, we leveraged the natural ability of inhibitory presynaptic GPCRs to suppress synaptic transmission and characterize parapinopsin (PPO) as a GPCR-based opsin for terminal inhibition. PPO is a photoswitchable opsin that couples to Gi/o signaling cascades and is rapidly activated by pulsed blue light, switched off with amber light, and effective for repeated, prolonged, and reversible inhibition. PPO rapidly and reversibly inhibits glutamate, GABA, and dopamine release at presynaptic terminals. Furthermore, PPO alters reward behaviors in a time-locked and reversible manner in vivo. These results demonstrate that PPO fills a significant gap in the neuroscience toolkit for rapid and reversible synaptic inhibition and has broad utility for spatiotemporal control of inhibitory GPCR signaling cascades.

Surgical implantation of wireless, battery-free optoelectronic epidural implants for optogenetic manipulation of spinal cord circuits in mice

The use of optogenetics to regulate neuronal activity has revolutionized the study of the neural circuitry underlying a number of complex behaviors in rodents. Advances have been particularly evident in the study of brain circuitry and related behaviors, while advances in the study of spinal circuitry have been less striking because of technical hurdles. We have developed and characterized a wireless and fully implantable optoelectronic device that enables optical manipulation of spinal cord circuitry in mice via a microscale light-emitting diode (µLED) placed in the epidural space (NeuroLux spinal optogenetic device). This protocol describes how to surgically implant the device into the epidural space and then analyze light-induced behavior upon µLED activation. We detail optimized optical parameters for in vivo stimulation and demonstrate typical behavioral effects of optogenetic activation of nociceptive spinal afferents using this device. This fully wireless spinal µLED system provides considerable versatility for behavioral assays compared with optogenetic approaches that require tethering of animals, and superior temporal and spatial resolution when compared with other methods used for circuit manipulation such as chemogenetics. The detailed surgical approach and improved functionality of these spinal optoelectronic devices substantially expand the utility of this approach for the study of spinal circuitry and behaviors related to mechanical and thermal sensation, pruriception and nociception. The surgical implantation procedure takes ~1 h. The time required for the study of behaviors that are modulated by the light-activated circuit is variable and will depend upon the nature of the study.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Anisotropic light scattering from myelinated axons in the spinal cord

Optogenetics has become an integral tool for studying and dissecting the neural circuitries of the brain using optical control. Recently, it has also begun to be used in the investigation of the spinal cord and peripheral nervous system. However, information on these regions' optical properties is sparse. Moreover, there is a lack of data on the dependence of light propagation with respect to neural tissue organization and orientation. This information is important for effective simulations and optogenetic planning, particularly in the spinal cord where the myelinated axons are highly organized. To this end, we report experimental measurements for the scattering coefficient, validated with three different methods in both the longitudinal and radial directions of multiple mammalian spinal cords. In our analysis, we find that there is indeed a directional dependence of photon propagation when interacting with organized myelinated axons. Specifically, light propagating perpendicular to myelinated axons in the white matter of the spinal cord produced a measured reduced scattering coefficient ( μ s ' ) of 3.52 ± 0.1    mm - 1 , and light that was propagated along the myelinated axons in the white matter produced a measured μ s ' of 1.57 ± 0.03    mm - 1 , across the various species considered. This 50% decrease in scattering power along the myelinated axons is observed with three different measurement strategies (integrating spheres, observed transmittance, and punch-through method). Furthermore, this directional dependence in scattering power and overall light attenuation did not occur in the gray matter regions where the myelin organization is nearly random. The acquired information will be integral in preparing future light-transport simulations and in overall optogenetic planning in both the spinal cord and the brain.

Optogenetic Inhibition of CGRPα Sensory Neurons Reveals Their Distinct Roles in Neuropathic and Incisional Pain

Cutaneous somatosensory neurons convey innocuous and noxious mechanical, thermal, and chemical stimuli from peripheral tissues to the CNS. Among these are nociceptive neurons that express calcitonin gene-related peptide-α (CGRPα). The role of peripheral CGRPα neurons (CANs) in acute and injury-induced pain has been studied using diphtheria toxin ablation, but their functional roles remain controversial. Because ablation permanently deletes a neuronal population, compensatory changes may ensue that mask the physiological or pathophysiological roles of CANs, particularly for injuries that occur after ablation. Therefore, we sought to define the role of intact CANs in vivo under baseline and injury conditions by using noninvasive transient optogenetic inhibition. We assessed pain behavior longitudinally from acute to chronic time points. We generated adult male and female mice that selectively express the outward rectifying proton pump archaerhodopsin-3 (Arch) in CANs, and inhibited their peripheral cutaneous terminals in models of neuropathic (spared nerve injury) and inflammatory (skin-muscle incision) pain using transdermal light activation of Arch. After nerve injury, brief activation of Arch reversed the chronic mechanical, cold, and heat hypersensitivity, alleviated the spontaneous pain, and reversed the sensitized mechanical currents in primary afferent somata. In contrast, Arch inhibition of CANs did not alter incision-induced hypersensitivity. Instead, incision-induced mechanical and heat hypersensitivity was alleviated by peripheral blockade of CGRPα peptide-receptor signaling. These results reveal that CANs have distinct roles in the time course of pain during neuropathic and incisional injuries and suggest that targeting peripheral CANs or CGRPα peptide-receptor signaling could selectively treat neuropathic or postoperative pain, respectively.SIGNIFICANCE STATEMENT The contribution of sensory afferent CGRPα neurons (CANs) to neuropathic and inflammatory pain is controversial. Here, we left CANs intact during neuropathic and perioperative incision injury by using transient transdermal optogenetic inhibition of CANs. We found that peripheral CANs are required for neuropathic mechanical, cold, and heat hypersensitivity, spontaneous pain, and sensitization of mechanical currents in afferent somata. However, they are dispensable for incisional pain transmission. In contrast, peripheral pharmacological inhibition of CGRPα peptide-receptor signaling alleviated the incisional mechanical and heat hypersensitivity, but had no effect on neuropathic pain. These results show that CANs have distinct roles in neuropathic and incisional pain and suggest that their targeting via novel peripheral treatments may selectively alleviate neuropathic versus incisional pain.

Optogenetic recruitment of spinal reflex pathways from large-diameter primary afferents in non-transgenic rats transduced with AAV9/Channelrhodopsin 2

Key points

We demonstrated optical activation of primary somatosensory afferents with high selectivity to fast‐conducting fibres by means of adeno‐associated virus 9 (AAV9)‐mediated gene transduction in dorsal root ganglion (DRG) neurons.

AVV9 expressing green fluorescent protein showed high selectivity and transduction efficiency for fast‐conducting, large‐sized DRG neurons.

Compared with conventional electrical stimulation, optically elicited volleys in primary afferents had higher sensitivity with stimulus amplitude, but lower sensitivity with stimulus frequency.

Optically elicited dorsal root volleys activated postsynaptic neurons in the segmental spinal pathway.

This proposed technique will help establish the causal relationships between somatosensory afferent inputs and neural responses in the CNS as well as behavioural outcomes in higher mammals where transgenic animals are not available.

Abstract

Previously, fundamental structures and their mode of action in the spinal reflex circuit were determined by confirming their input–output relationship using electrophysiological techniques. In those experiments, the electrical stimulation of afferent fibres was used as a core element to identify different types of reflex pathways; however, a major disadvantage of this technique is its non‐selectivity. In this study, we investigated the selective activation of large‐diameter afferents by optogenetics combined with a virus vector transduction technique (injection via the sciatic nerve) in non‐transgenic male Jcl:Wistar rats. We found that green fluorescent protein gene transduction of rat dorsal root ganglion (DRG) neurons with a preference for medium‐to‐large‐sized cells was achieved using the adeno‐associated virus 9 (AAV9) vector compared with the AAV6 vector (P = 0.021). Furthermore, the optical stimulation of Channelrhodopsin 2 (ChR2)‐expressing DRG neurons (transduced by AAV9) produced compound action potentials in afferent nerves originating from fast‐conducting nerve fibres. We also confirmed that physiological responses to different stimulus amplitudes were comparable between optogenetic and electrophysiological activation. However, compared with electrically elicited responses, the optically elicited responses had lower sensitivity with stimulus frequency. Finally, we showed that afferent volleys evoked by optical stimulation were sufficient to activate postsynaptic neurons in the spinal reflex arc. These results provide new ways for understanding the role of sensory afferent input to the central nervous system regarding behavioural control, especially when genetically manipulated animals are not available, such as higher mammals including non‐human primates.

We demonstrated optical activation of primary somatosensory afferents with high selectivity to fast‐conducting fibres by means of adeno‐associated virus 9 (AAV9)‐mediated gene transduction in dorsal root ganglion (DRG) neurons.

AVV9 expressing green fluorescent protein showed high selectivity and transduction efficiency for fast‐conducting, large‐sized DRG neurons.

Compared with conventional electrical stimulation, optically elicited volleys in primary afferents had higher sensitivity with stimulus amplitude, but lower sensitivity with stimulus frequency.

Optically elicited dorsal root volleys activated postsynaptic neurons in the segmental spinal pathway.

This proposed technique will help establish the causal relationships between somatosensory afferent inputs and neural responses in the CNS as well as behavioural outcomes in higher mammals where transgenic animals are not available.

From Mechanism to Cure: Renewing the Goal to Eliminate the Disease of Pain

Objective

Persistent pain causes untold misery worldwide and is a leading cause of disability. Despite its astonishing prevalence, pain is undertreated, at least in part because existing therapeutics are ineffective or cause intolerable side effects. In this review, we cover new findings about the neurobiology of pain and argue that all but the most transient forms of pain needed to avoid tissue damage should be approached as a disease where a cure can be the goal of all treatment plans, even if attaining this goal is not yet always possible.

Design

We reviewed the literature to highlight recent advances in the area of the neurobiology of pain.

Results

We discuss barriers that are currently hindering the achievement of this goal, as well as the development of new therapeutic strategies. We also discuss innovations in the field that are creating new opportunities to treat and even reverse persistent pain, some of which are in late-phase clinical trials.

Conclusion

We conclude that the confluence of new basic science discoveries and development of new technologies are creating a path toward pain therapeutics that should offer significant hope of a cure for patients and practitioners alike. Classification of Evidence. Our review points to new areas of inquiry for the pain field to advance the goal of developing new therapeutics to treat chronic pain.

Closed-loop functional optogenetic stimulation

Optogenetics has been used to orchestrate temporal- and tissue-specific control of neural tissues and offers a wealth of unique advantages for neuromuscular control. Here, we establish a closed-loop functional optogenetic stimulation (CL-FOS) system to control ankle joint position in murine models. Using the measurement of either joint angle or fascicle length as a feedback signal, we compare the controllability of CL-FOS to closed-loop functional electrical stimulation (CL-FES) and demonstrate significantly greater accuracy, lower rise times and lower overshoot percentages. We demonstrate orderly recruitment of motor units and reduced fatigue when performing cyclical movements with CL-FOS compared with CL-FES. We develop and investigate a 3-phase, photo-kinetic model to elucidate the underlying mechanisms for temporal variations in optogenetically activated neuromusculature during closed-loop control experiments. Methods and insights from this study lay the groundwork for the development of closed-loop optogenetic neuromuscular stimulation therapies and devices for peripheral limb control.

Inhibitory Gi/O-coupled receptors in somatosensory neurons: Potential therapeutic targets for novel analgesics

Primary sensory neurons in the dorsal root ganglia and trigeminal ganglia are responsible for sensing mechanical and thermal stimuli, as well as detecting tissue damage. These neurons express ion channels that respond to thermal, mechanical, or chemical cues, conduct action potentials, and mediate transmitter release. These neurons also express a large number of G-protein coupled receptors, which are major transducers for extracellular signaling molecules, and their activation usually modulates the primary transduction pathways. Receptors that couple to phospholipase C via heterotrimeric G_q/11 proteins and those that activate adenylate cyclase via G_s are considered excitatory; they positively regulate somatosensory transduction and they play roles in inflammatory sensitization and pain, and in some cases also in inducing itch. On the other hand, receptors that couple to G_i/o proteins, such as opioid or GABA_B receptors, are generally inhibitory. Their activation counteracts the effect of G_s-stimulation by inhibiting adenylate cyclase, as well as exerts effects on ion channels, usually resulting in decreased excitability. This review will summarize knowledge on G_i-coupled receptors in sensory neurons, focusing on their roles in ion channel regulation and discuss their potential as targets for analgesic and antipruritic medications.

Ensembles of change-point detectors: implications for real-time BMI applications

Brain-machine interfaces (BMIs) have been widely used to study basic and translational neuroscience questions. In real-time closed-loop neuroscience experiments, many practical issues arise, such as trial-by-trial variability, and spike sorting noise or multi-unit activity. In this paper, we propose a new framework for change-point detection based on ensembles of independent detectors in the context of BMI application for detecting acute pain signals. Motivated from ensemble learning, our proposed "ensembles of change-point detectors" (ECPDs) integrate multiple decisions from independent detectors, which may be derived based on data recorded from different trials, data recorded from different brain regions, data of different modalities, or models derived from different learning methods. By integrating multiple sources of information, the ECPDs aim to improve detection accuracy (in terms of true positive and true negative rates) and achieve an optimal trade-off of sensitivity and specificity. We validate our method using computer simulations and experimental recordings from freely behaving rats. Our results have shown superior and robust performance of ECPDS in detecting the onset of acute pain signals based on neuronal population spike activity (or combined with local field potentials) recorded from single or multiple brain regions.

Differential Regulation of Bladder Pain and Voiding Function by Sensory Afferent Populations Revealed by Selective Optogenetic Activation

Bladder-innervating primary sensory neurons mediate reflex-driven bladder function under normal conditions, and contribute to debilitating bladder pain and/or overactivity in pathological states. The goal of this study was to examine the respective roles of defined subtypes of afferent neurons in bladder sensation and function in vivo via direct optogenetic activation. To accomplish this goal, we generated transgenic lines that express a Channelrhodopsin-2-eYFP fusion protein (ChR2-eYFP) in two distinct populations of sensory neurons: TRPV1-lineage neurons (Trpv1^Cre;Ai32, the majority of nociceptors) and Na_v1.8⁺ neurons (Scn10a^Cre;Ai32, nociceptors and some mechanosensitive fibers). In spinal cord, eYFP+ fibers in Trpv1^Cre;Ai32 mice were observed predominantly in dorsal horn (DH) laminae I-II, while in Scn10a^Cre;Ai32 mice they extended throughout the DH, including a dense projection to lamina X. Fiber density correlated with number of retrogradely-labeled eYFP+ dorsal root ganglion neurons (82.2% Scn10a^Cre;Ai32 vs. 62% Trpv1^Cre;Ai32) and degree of DH excitatory synaptic transmission. Photostimulation of peripheral afferent terminals significantly increased visceromotor responses to noxious bladder distension (30–50 mmHg) in both transgenic lines, and to non-noxious distension (20 mmHg) in Scn10a^Cre;Ai32 mice. Depolarization of ChR2+ afferents in Scn10a^Cre;Ai32 mice produced low- and high-amplitude bladder contractions respectively in 53% and 27% of stimulation trials, and frequency of high-amplitude contractions increased to 60% after engagement of low threshold (LT) mechanoreceptors by bladder filling. In Trpv1^Cre;Ai32 mice, low-amplitude contractions occurred in 27% of trials before bladder filling, which was pre-requisite for light-evoked high-amplitude contractions (observed in 53.3% of trials). Potential explanations for these observations include physiological differences in the thresholds of stimulated fibers and their connectivity to spinal circuits.

Usefulness of knockout mice to clarify the role of the opioid system in chronic pain

Several lines of knockout mice deficient in the genes encoding each component of the endogenous opioid system have been used for decades to clarify the specific role of the different opioid receptors and peptide precursors in many physiopathological conditions. The use of these genetically modified mice has improved our knowledge of the specific involvement of each endogenous opioid component in nociceptive transmission during acute and chronic pain conditions. The present review summarizes the recent advances obtained using these genetic tools in understanding the role of the opioid system in the pathophysiological mechanisms underlying chronic pain. Behavioural data obtained in these chronic pain models are discussed considering the peculiarities of the behavioural phenotype of each line of knockout mice. These studies have identified the crucial role of specific components of the opioid system in different manifestations of chronic pain and have also opened new possible therapeutic approaches, such as the development of opioid compounds simultaneously targeting several opioid receptors. However, several questions still remain open and require further experimental effort to be clarified. The novel genetic tools now available to manipulate specific neuronal populations and precise genome editing in mice will facilitate in a near future the elucidation of the role of each component of the endogenous opioid system in chronic pain.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Miniaturized, Battery-Free Optofluidic Systems with Potential for Wireless Pharmacology and Optogenetics

Combination of optogenetics and pharmacology represents a unique approach to dissect neural circuitry with high specificity and versatility. However, conventional tools available to perform these experiments, such as optical fibers and metal cannula, are limited due to their tethered operation and lack of biomechanical compatibility. To address these issues, a miniaturized, battery-free, soft optofluidic system that can provide wireless drug delivery and optical stimulation for spatiotemporal control of the targeted neural circuit in freely behaving animals is reported. The device integrates microscale inorganic light-emitting diodes and microfluidic drug delivery systems with a tiny stretchable multichannel radiofrequency antenna, which not only eliminates the need for bulky batteries but also offers fully wireless, independent control of light and fluid delivery. This design enables a miniature (125 mm³ ), lightweight (220 mg), soft, and flexible platform, thus facilitating seamless implantation and operation in the body without causing disturbance of naturalistic behavior. The proof-of-principle experiments and analytical studies validate the feasibility and reliability of the fully implantable optofluidic systems for use in freely moving animals, demonstrating its potential for wireless in vivo pharmacology and optogenetics.

Real-time particle filtering and smoothing algorithms for detecting abrupt changes in neural ensemble spike activity

Sequential change-point detection from time series data is a common problem in many neuroscience applications, such as seizure detection, anomaly detection, and pain detection. In our previous work (Chen Z, Zhang Q, Tong AP, Manders TR, Wang J. J Neural Eng 14: 036023, 2017), we developed a latent state-space model, known as the Poisson linear dynamical system, for detecting abrupt changes in neuronal ensemble spike activity. In online brain-machine interface (BMI) applications, a recursive filtering algorithm is used to track the changes in the latent variable. However, previous methods have been restricted to Gaussian dynamical noise and have used Gaussian approximation for the Poisson likelihood. To improve the detection speed, we introduce non-Gaussian dynamical noise for modeling a stochastic jump process in the latent state space. To efficiently estimate the state posterior that accommodates non-Gaussian noise and non-Gaussian likelihood, we propose particle filtering and smoothing algorithms for the change-point detection problem. To speed up the computation, we implement the proposed particle filtering algorithms using advanced graphics processing unit computing technology. We validate our algorithms, using both computer simulations and experimental data for acute pain detection. Finally, we discuss several important practical issues in the context of real-time closed-loop BMI applications. NEW & NOTEWORTHY Sequential change-point detection is an important problem in closed-loop neuroscience experiments. This study proposes novel sequential Monte Carlo methods to quickly detect the onset and offset of a stochastic jump process that drives the population spike activity. This new approach is robust with respect to spike sorting noise and varying levels of signal-to-noise ratio. The GPU implementation of the computational algorithm allows for parallel processing in real time.

In Vivo Interrogation of Spinal Mechanosensory Circuits

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

Fully implantable, battery-free wireless optoelectronic devices for spinal optogenetics

The advent of optogenetic tools has allowed unprecedented insights into the organization of neuronal networks. Although recently developed technologies have enabled implementation of optogenetics for studies of brain function in freely moving, untethered animals, wireless powering and device durability pose challenges in studies of spinal cord circuits where dynamic, multidimensional motions against hard and soft surrounding tissues can lead to device degradation. We demonstrate here a fully implantable optoelectronic device powered by near-field wireless communication technology, with a thin and flexible open architecture that provides excellent mechanical durability, robust sealing against biofluid penetration and fidelity in wireless activation, thereby allowing for long-term optical stimulation of the spinal cord without constraint on the natural behaviors of the animals. The system consists of a double-layer, rectangular-shaped magnetic coil antenna connected to a microscale inorganic light-emitting diode (μ-ILED) on a thin, flexible probe that can be implanted just above the dura of the mouse spinal cord for effective stimulation of light-sensitive proteins expressed in neurons in the dorsal horn. Wireless optogenetic activation of TRPV1-ChR2 afferents with spinal μ-ILEDs causes nocifensive behaviors and robust real-time place aversion with sustained operation in animals over periods of several weeks to months. The relatively low-cost electronics required for control of the systems, together with the biocompatibility and robust operation of these devices will allow broad application of optogenetics in future studies of spinal circuits, as well as various peripheral targets, in awake, freely moving and untethered animals, where existing approaches have limited utility.

Optical inactivation of the anterior cingulate cortex modulate descending pain pathway in a rat model of trigeminal neuropathic pain created via chronic constriction injury of the infraorbital nerve

Purpose

The anterior cingulate cortex (ACC) plays a critical role in the initiation, development, and maintenance of neuropathic pain. Recently, the effects of optical stimulation on pain have been investigated, but the therapeutic effects of optical stimulation on trigeminal neuralgia (TN) have not been clearly shown. Here, we investigated the effects of optical inhibition of the ACC on TN lesions to determine whether the alleviation of pain affects behavior performance and thalamic neuron signaling.

Materials and methods

TN lesions were established in animals by generating a chronic constriction injury of the infraorbital nerve, and the animals received injections of AAV-hSyn-eNpHR3.0-EYFP or a vehicle (phosphate-buffered saline [PBS]) in the ACC. The optical fiber was fixed into the ipsilateral ACC after the injection of adeno-associated virus plasmids or vehicle. Behavioral testing, consisting of responses to an air puff and cold allodynia, was performed, and thalamic neuronal activity was monitored following optical stimulation in vivo. Optical stimulation experiments were executed in three steps: during pre-light-off, stimulation-light-on, and post-light-off states. The role of the optical modulation of the ACC in response to pain was shown using a combination of optical stimulation and electrophysiological recordings in vivo.

Results

Mechanical thresholds and facial cold allodynia scores were significantly improved in the TN lesion group during optical stimulation compared to those in the control group. Thalamic neuronal activity, consisting of the firing rate (spikes/s) and burst rate (bursts/s), was also decreased during optical stimulation.

Conclusion

Reciprocal optical inhibition of the ACC can alleviate pain-associated behavior and decrease abnormal thalamic sensory neuron activity in the trigeminal neuropathic rat model. The descending pain pathway can modulate thalamic neurons from the ACC following optical stimulation.

Will optogenetics be used to treat chronic pain patients?

Chronic pain affects a third of the population and current treatments produce limited relief and severe side effects. An alternative strategy to decrease pain would be to directly modulate somatosensory pathways using optogenetics. Optogenetics involves the use of genetically encoded and optically active proteins, namely opsins, to control neuronal circuits. In preclinical animal models, optical silencing of peripheral nociceptors has been shown to alleviate both inflammatory and neuropathic pain. An opsin-based gene therapy to treat chronic pain patients is not ready yet, but encouraging advances have been made in optical and viral technology. In view of the increasing burden of chronic pain in our aging society, innovative analgesic approaches based on optogenetics are definitely worth exploring.

Reticular Formation and Pain: The Past and the Future

The involvement of the reticular formation (RF) in the transmission and modulation of nociceptive information has been extensively studied. The brainstem RF contains several areas which are targeted by spinal cord afferents conveying nociceptive input. The arrival of nociceptive input to the RF may trigger alert reactions which generate a protective/defense reaction to pain. RF neurons located at the medulla oblongata and targeted by ascending nociceptive information are also involved in the control of vital functions that can be affected by pain, namely cardiovascular control. The RF contains centers that belong to the pain modulatory system, namely areas involved in bidirectional balance (decrease or enhancement) of pain responses. It is currently accepted that the imbalance of pain modulation towards pain facilitation accounts for chronic pain. The medullary RF has the peculiarity of harboring areas involved in bidirectional pain control namely by the existence of specific neuronal populations involved in antinociceptive or pronociceptive behavioral responses, namely at the rostroventromedial medulla (RVM) and the caudal ventrolateral medulla (VLM). Furthermore the dorsal reticular nucleus (also known as subnucleus reticularis dorsalis; DRt) may enhance nociceptive responses, through a reverberative circuit established with spinal lamina I neurons and inhibit wide-dynamic range (WDR) neurons of the deep dorsal horn. The components of the triad RVM-VLM-DRt are reciprocally connected and represent a key gateway for top-down pain modulation. The RVM-VLM-DRt triad also represents the neurobiological substrate for the emotional and cognitive modulation of pain, through pathways that involve the periaqueductal gray (PAG)-RVM connection. Collectively, we propose that the RVM-VLM-DRt triad represents a key component of the “dynamic pain connectome” with special features to provide integrated and rapid responses in situations which are life-threatening and involve pain. The new available techniques in neurobiological studies both in animal and human studies are producing new and fascinating data which allow to understand the complex role of the RF in pain modulation and its integration with several body functions and also how the RF accounts for chronic pain.

Deciphering neuronal population codes for acute thermal pain

OBJECTIVE

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Current pain research mostly focuses on molecular and synaptic changes at the spinal and peripheral levels. However, a complete understanding of pain mechanisms requires the physiological study of the neocortex. Our goal is to apply a neural decoding approach to read out the onset of acute thermal pain signals, which can be used for brain-machine interface.

APPROACH

We used micro wire arrays to record ensemble neuronal activities from the primary somatosensory cortex (S1) and anterior cingulate cortex (ACC) in freely behaving rats. We further investigated neural codes for acute thermal pain at both single-cell and population levels. To detect the onset of acute thermal pain signals, we developed a novel latent state-space framework to decipher the sorted or unsorted S1 and ACC ensemble spike activities, which reveal information about the onset of pain signals.

MAIN RESULTS

The state space analysis allows us to uncover a latent state process that drives the observed ensemble spike activity, and to further detect the 'neuronal threshold' for acute thermal pain on a single-trial basis. Our method achieved good detection performance in sensitivity and specificity. In addition, our results suggested that an optimal strategy for detecting the onset of acute thermal pain signals may be based on combined evidence from S1 and ACC population codes.

SIGNIFICANCE

Our study is the first to detect the onset of acute pain signals based on neuronal ensemble spike activity. It is important from a mechanistic viewpoint as it relates to the significance of S1 and ACC activities in the regulation of the acute pain onset.

Surgical extraction of human dorsal root ganglia from organ donors and preparation of primary sensory neuron cultures

Primary cultures of rodent sensory neurons are widely used to investigate the cellular and molecular mechanisms involved in pain, itch, nerve injury and regeneration. However, translation of these preclinical findings may be greatly improved by direct validation in human tissues. We have developed an approach to extract and culture human sensory neurons in collaboration with a local organ procurement organization (OPO). Here we describe the surgical procedure for extraction of human dorsal root ganglia (hDRG) and the necessary modifications to existing culture techniques to prepare viable adult human sensory neurons for functional studies. Dissociated sensory neurons can be maintained in culture for >10 d, and they are amenable to electrophysiological recording, calcium imaging and viral gene transfer. The entire process of extraction and culturing can be completed in <7 h, and it can be performed by trained graduate students. This approach can be applied at any institution with access to organ donors consenting to tissue donation for research, and is an invaluable resource for improving translational research.