A new minimally-invasive method for microinjection into the mouse spinal dorsal horn

Exploring the role of spinal astrocytes in the onset of hyperalgesic priming signals in acid-induced chronic muscle pain

Hyperalgesic priming, a form of pain plasticity initiated by initial injury, leads to heightened sensitivity to subsequent noxious stimuli, contributing to chronic pain development in animals. While astrocytes play active roles in modulating synaptic transmission in various pain models, their specific involvement in hyperalgesic priming remains elusive. Here, we show that spinal astrocytes are essential for hyperalgesic priming formation in a mouse model of acid-induced muscle pain. We observed spinal astrocyte activation 4 h after initial acid injection, and inhibition of this activation prevented chronic pain development upon subsequent acid injection. Chemogenetic activation of spinal astrocytes mimicked the first acid-induced hyperalgesic priming. We also demonstrated that spinal phosphorylated extracellular regulated kinase (pERK)-positive neurons were mainly vesicular glutamate transporter-2 positive (Vglut2+) neurons after the first acid injection, and inhibition of spinal pERK prevented astrocyte activation. Furthermore, pharmacological inhibition of astrocytic glutamate transporters glutamate transporter-1 and glutamate-aspartate transporter abolished the hyperalgesic priming. Collectively, our results suggest that pERK activation in Vglut2+ neurons activate astrocytes through astrocytic glutamate transporters. This process eventually establishes hyperalgesic priming through spinal D-serine. We conclude that spinal astrocytes play a crucial role in the transition from acute to chronic pain.

Tiam1-mediated maladaptive plasticity underlying morphine tolerance and hyperalgesia

Opioid pain medications, such as morphine, remain the mainstay for treating severe and chronic pain. Prolonged morphine use, however, triggers analgesic tolerance and hyperalgesia (OIH), which can last for a long period after morphine withdrawal. How morphine induces these detrimental side effects remains unclear. Here, we show that morphine tolerance and OIH are mediated by Tiam1-coordinated synaptic structural and functional plasticity in the spinal nociceptive network. Tiam1 is a Rac1 GTPase guanine nucleotide exchange factor that promotes excitatory synaptogenesis by modulating actin cytoskeletal dynamics. We found that prolonged morphine treatment activated Tiam1 in the spinal dorsal horn and Tiam1 ablation from spinal neurons eliminated morphine antinociceptive tolerance and OIH. At the same time, the pharmacological blockade of Tiam1-Rac1 signalling prevented the development and reserved the established tolerance and OIH. Prolonged morphine treatment increased dendritic spine density and synaptic NMDA receptor activity in spinal dorsal horn neurons, both of which required Tiam1. Furthermore, co-administration of the Tiam1 signalling inhibitor NSC23766 was sufficient to abrogate morphine tolerance in chronic pain management. These findings identify Tiam1-mediated maladaptive plasticity in the spinal nociceptive network as an underlying cause for the development and maintenance of morphine tolerance and OIH and provide a promising therapeutic target to reduce tolerance and prolong morphine use in chronic pain management.

Inner and outer penetrating spinal cord injuries lead to distinct overground walking in mice

Spinal cord injury (SCI) is a devastating mechanical trauma. Although locomotion of model animals that mimic contusion SCI was actively examined, locomotion after penetrating SCI caused by sharp objects was not extensively studied. Severity of walking difficulty after partial transection of the spinal cord including penetrating SCI likely depends on the regions affected. Therefore, we compared beam walking and overground walking between mice after penetrating SCI at inner spinal cord region and mice with the injury at the outer region. Mice with the both penetrating SCIs did not display changes in beam walking. When appearance and movements of hindlimbs during overground walking was rated using Basso Mouse Scale for locomotion (BMS), however, mice with inner penetrating SCI showed low score shortly after the SCI. However, the score became high at later time points, as seen in contusion SCI mice. By contrast, BMS score did not decrease shortly after the outer penetrating SCI. However, the score became low 3 weeks after the SCI. As quantitative values during overground walking, movement duration in an open field were shorter at 1 day after the two penetrating SCIs. However, slower moving speed and fewer number of movement at 1 day were specific to mice with inner and outer penetrating SCIs, respectively. Moreover, BMS score was correlated with walking distance in open field only in mice with inner penetrating SCI. Thus, inner and outer penetrating SCI cause difficulty in overground walking with different severity and progress.

SIRT1 mediates the excitability of spinal CaMKIIα-positive neurons and participates in neuropathic pain by controlling Nav1.3

AIMS

Neuropathic pain is a common chronic pain disorder, which is largely attributed to spinal central sensitization. Calcium/calmodulin-dependent protein kinase II alpha (CaMKIIα) activation in the spinal dorsal horn (SDH) is a major contributor to spinal sensitization. However, the exact way that CaMKIIα-positive (CaMKIIα+) neurons in the SDH induce neuropathic pain is still unclear. This study aimed to explore the role of spinal CaMKIIα+ neurons in neuropathic pain caused by chronic constriction injury (CCI) and investigate the potential epigenetic mechanisms involved in CaMKIIα+ neuron activation.

METHODS

CCI-induced neuropathic pain mice model, Sirt1loxP/loxP mice, and chemogenetic virus were used to investigate whether the activation of spinal CaMKIIα+ neurons is involved in neuropathic pain and its involved mechanism. Transcriptome sequence, western blotting, qRT-PCR, and immunofluorescence analysis were performed to assay the expression of related molecules and activation of neurons. Co-immunoprecipitation was used to observe the binding relationship of protein. Chromatin immunoprecipitation (ChIP)-PCR was applied to analyze the acetylation of histone H3 in the Scn3a promoter region.

RESULTS

The expression of sodium channel Nav1.3 was increased and the expression of SIRT1 was decreased in the spinal CaMKIIα+ neurons of CCI mice. CaMKIIα neurons became overactive after CCI, and inhibiting their activation relieved CCI-induced pain. Overexpression of SIRT1 reversed the increase of Nav1.3 and alleviated pain, while knockdown of SIRT1 or overexpression of Nav1.3 promoted CaMKIIα+ neuron activation and induced pain. By knocking down spinal SIRT1, the acetylation of histone H3 in the Scn3a (encoding Nav1.3) promoter region was increased, leading to an increased expression of Nav1.3.

CONCLUSION

The findings suggest that an aberrant reduction of spinal SIRT1 after nerve injury epigenetically increases Nav1.3, subsequently activating CaMKIIα+ neurons and causing neuropathic pain.

Sigma-1 Receptor Inhibition Reduces Mechanical Allodynia and Modulate Neuroinflammation in Chronic Neuropathic Pain

Neuropathic pain is one of the most debilitating forms of chronic pain, resulting from an injury or disease of the somatosensory nervous system, which induces abnormal painful sensations including allodynia and hyperalgesia. Available treatments are limited by severe side-effects and reduced efficacy in the chronic phase of the disease. Sigma-1 receptor (σ1R) has been identified as a chaperone protein, which modulate opioid receptors activities and the functioning of several ion channels, exerting a role in pain transmission. As such, it represents a druggable target to treat neuropathic pain. This study aims at investigating the therapeutic potential of the novel compound (+)-2R/S-LP2, a σ1R antagonist, in reducing painful behaviour and modulating the neuroinflammatory environment. We showed that repeated administration of the compound significantly inhibited mechanical allodynia in neuropathic rats, increasing the withdrawal threshold as compared to CCI-vehicle rats. Moreover, we found that (+)-2R/S-LP2-mediated effects resolve the neuroinflammatory microenvironment by reducing central gliosis and pro-inflammatory cytokines expression levels. This effect was coupled with a significant reduction of connexin 43 (Cx43) expression levels and gap junctions/hemichannels mediated microglia-to-astrocyte communication. These results suggest that inhibition of σ1R significantly attenuates neuropathic pain chronicization, thus representing a viable effective strategy.

Laminar-selective spinal astrocyte population capable of converting tactile information into nociceptive in rats

We previously identified a spinal astrocyte population that expresses hairy and enhancer of split 5 (Hes5) and is selectively present in superficial laminae in mice. However, it was unclear whether such astrocyte heterogeneity is commonly observed across species. Using adeno-associated viral (AAV) vectors incorporating a rat Hes5 promotor (AAV-Hes5P), we found that AAV-Hes5P-captured astrocytes were selectively located in the superficial laminae in rats. Furthermore, activation of AAV-Hes5P+ astrocytes elicited allodynia-like behavior and increased c-FOS+ cells in the superficial laminae. Thus, laminar-selective Hes5+ astrocytes are conserved beyond species and have the capability to convert tactile information to nociceptive.

Optogenetic inhibition of spinal inhibitory neurons facilitates mechanical responses of spinal wide dynamic range neurons and causes mechanical hypersensitivity

Inhibitory interneurons in the spinal dorsal horn (DH) play a major role in regulating innocuous and noxious information. Reduction in inhibitory synaptic transmission is thought to contribute to the development of touch-evoked pain (allodynia), a common symptom of neuropathic pain. However, it is not fully understood how inhibitory neurons in the DH regulate sensory responses in surrounding neurons and modulate sensory transmission. In this study, we established a novel experimental method to analyze temporal activity of DH neurons during the optogenetically induced disinhibition state by combining extracellular recording and optogenetics. We investigated how specific and temporally restricted dysfunction of DH inhibitory neurons affected spinal neuronal activities evoked by cutaneous mechanical stimulation. In behavioral experiments, the specific and temporally restricted spinal optogenetic suppression of DH inhibitory neurons induced mechanical hypersensitivity. Furthermore, this manipulation enhanced the mechanical responses of wide dynamic range (WDR) neurons, which are important for pain transmission, in response to brush and von Frey stimulation but not in response to nociceptive pinch stimulation. In addition, we examined whether a neuropathic pain medication, mirogabalin, suppressed these optogenetically induced abnormal pain responses. We found that mirogabalin treatment attenuated the abnormal firing responses of WDR neurons and mechanical hypersensitivity. These results suggest that temporally restricted and specific reduction of spinal inhibitory neuronal activity facilitates the mechanical responses of WDR neurons, resulting in neuropathic-like mechanical allodynia which can be suppressed by mirogabalin. Our optogenetic methods could be useful for developing novel therapeutics for neuropathic pain.

Characterization of synaptic structural plasticity in mouse spinal dorsal horn neurons

Here, we present a pipeline for the characterization of synaptic structural plasticity in mouse spinal dorsal horn (SDH) neurons. We describe steps for the intra-SDH microinjection of the EGFP virus to sparsely label L4 SDH neurons without laminectomy, wide dynamic range neuron imaging, dendritic spine morphometric analysis, and F-actin to G-actin ratio measurement. This protocol can be applied to investigate the synaptic structural plasticity mechanisms in the SDH as well as in the brain. For complete details on the use and execution of this protocol, please refer to Li et al. (2023).1.

Human OPRM1 and murine Oprm1 promoter driven viral constructs for genetic access to μ-opioidergic cell types

With concurrent global epidemics of chronic pain and opioid use disorders, there is a critical need to identify, target and manipulate specific cell populations expressing the mu-opioid receptor (MOR). However, available tools and transgenic models for gaining long-term genetic access to MOR+ neural cell types and circuits involved in modulating pain, analgesia and addiction across species are limited. To address this, we developed a catalog of MOR promoter (MORp) based constructs packaged into adeno-associated viral vectors that drive transgene expression in MOR+ cells. MORp constructs designed from promoter regions upstream of the mouse Oprm1 gene (mMORp) were validated for transduction efficiency and selectivity in endogenous MOR+ neurons in the brain, spinal cord, and periphery of mice, with additional studies revealing robust expression in rats, shrews, and human induced pluripotent stem cell (iPSC)-derived nociceptors. The use of mMORp for in vivo fiber photometry, behavioral chemogenetics, and intersectional genetic strategies is also demonstrated. Lastly, a human designed MORp (hMORp) efficiently transduced macaque cortical OPRM1+ cells. Together, our MORp toolkit provides researchers cell type specific genetic access to target and functionally manipulate mu-opioidergic neurons across a range of vertebrate species and translational models for pain, addiction, and neuropsychiatric disorders.

Tiam1 coordinates synaptic structural and functional plasticity underpinning the pathophysiology of neuropathic pain

Neuropathic pain is a common, debilitating chronic pain condition caused by damage or a disease affecting the somatosensory nervous system. Understanding the pathophysiological mechanisms underlying neuropathic pain is critical for developing new therapeutic strategies to treat chronic pain effectively. Tiam1 is a Rac1 guanine nucleotide exchange factor (GEF) that promotes dendritic and synaptic growth during hippocampal development by inducing actin cytoskeletal remodeling. Here, using multiple neuropathic pain animal models, we show that Tiam1 coordinates synaptic structural and functional plasticity in the spinal dorsal horn via actin cytoskeleton reorganization and synaptic NMDAR stabilization and that these actions are essential for the initiation, transition, and maintenance of neuropathic pain. Furthermore, an antisense oligonucleotides (ASO) targeting spinal Tiam1 persistently alleviate neuropathic pain sensitivity. Our findings suggest that Tiam1-coordinated synaptic functional and structural plasticity underlies the pathophysiology of neuropathic pain and that intervention of Tiam1-mediated maladaptive synaptic plasticity has long-lasting consequences in neuropathic pain management.

A novel co-target of ACY1 governing plasma membrane translocation of SphK1 contributes to inflammatory and neuropathic pain

Previous studies validate that inhibiting sodium channel 1.8 (Nav1.8) effectively relieves inflammatory and neuropathic pain. However, Nav1.8 blockers have cardiac side effects in addition to analgesic effects. Here, we constructed a spinal differential protein expression profile using Nav1.8 knockout mice to screen common downstream proteins of Nav1.8 in inflammatory and neuropathic pain. We found that aminoacylase 1 (ACY1) expression was increased in wild-type mice compared to Nav1.8 knockout mice in both pain models. Moreover, spinal ACY1 overexpression induced mechanical allodynia in naive mice, while ACY1 suppression alleviated inflammatory and neuropathic pain. Further, ACY1 could interact with sphingosine kinase 1 and promote its membrane translocation, resulting in sphingosine-1-phosphate upregulation and the activation of glutamatergic neurons and astrocytes. In conclusion, ACY1 acts as a common downstream effector protein of Nav1.8 in inflammatory and neuropathic pain and could be a new and precise therapeutic target for chronic pain.

A novel convex acoustic lens-attached ultrasound drug delivery system and its testing in a murine melanoma subcutaneous modelo

Target-specific drug release is indispensable to improve chemotherapeutic efficacy as it enhances drug uptake and penetration into tumors. Sono-responsive drug-loaded nano-/micro-particles are a promising solution for achieving target specificity by exposing them to ultrasound near tumors. However, the complicated synthetic processes and limited ultrasound (US) exposure conditions, such as limited control of ultrasound focal depth and acoustic power, prevent the practical application of this approach in clinical practice. Here, we propose a convex acoustic lens-attached US (CALUS) as a simple, economic, and efficient alternative of focused US for drug delivery system (DDS) application. The CALUS was characterized both numerically and experimentally using a hydrophone. In vitro, microbubbles (MBs) inside microfluidic channels were destroyed using the CALUS with various acoustic parameters (acoustic pressure [P], pulse repetition frequency [PRF], and duty cycle) and flow velocity. In vivo, tumor inhibition was evaluated using melanoma-bearing mice by characterizing tumor growth rate, animal weight, and intratumoral drug concentration with/without CALUS DDS. US beams were measured to be efficiently converged by CALUS, which was consistent with our simulation results. The acoustic parameters were optimized through the CALUS-induced MB destruction test (P = 2.34 MPa, PRF = 100 kHz, and duty cycle = 9%); this optimal parameter combination successfully induced MB destruction inside the microfluidic channel with an average flow velocity of up to 9.6 cm/s. The CALUS also enhanced the therapeutic effects of an antitumor drug (doxorubicin) in vivo in a murine melanoma model. The combination of the doxorubicin and the CALUS inhibited tumor growth by ∼55% more than doxorubicin alone, clearly indicating synergistic antitumor efficacy. Our tumor growth inhibition performance was better than other methods based on drug carriers, even without a time-consuming and complicated chemical synthesis process. This result suggests that our novel, simple, economic, and efficient target-specific DDS may offer a transition from preclinical research to clinical trials and a potential treatment approach for patient-centered healthcare.

Histone deacetylase 5-induced deficiency of signal transducer and activator of transcription-3 acetylation contributes to spinal astrocytes degeneration in painful diabetic neuropathy

Diabetes patients with painful diabetic neuropathy (PDN) show severe spinal atrophy, suggesting pathological changes of the spinal cord contributes to central sensitization. However, the cellular changes and underlying molecular mechanisms within the diabetic spinal cord are less clear. By using a rat model of type 1 diabetes (T1D), we noted an extensive and irreversible spinal astrocyte degeneration at an early stage of T1D, which is highly associated with the chronification of PDN. Molecularly, acetylation of astrocytic signal transducer and activator of transcription-3 (STAT3) that is essential for maintaining the homeostatic astrocytes population was significantly impaired in the T1D model, resulting in a dramatic loss of spinal astrocytes and consequently promoting pain hypersensitivity. Mechanistically, class IIa histone deacetylase, HDAC5 were aberrantly activated in spinal astrocytes of diabetic rats, which promoted STAT3 deacetylation by direct protein-protein interactions, leading to the PDN phenotypes. Restoration of STAT3 signaling or inhibition of HDAC5 rescued astrocyte deficiency and attenuated PDN in the T1D model. Our work identifies the inhibitory axis of HDAC5-STAT3 induced astrocyte deficiency as a key mechanism underlying the pathogenesis of the diabetic spinal cord that paves the way for potential therapy development for PDN.

GPR55 contributes to neutrophil recruitment and mechanical pain induction after spinal cord compression in mice

Pain transmission and processing in the nervous system are modulated by various biologically active substances, including lysophospholipids, through direct and indirect actions on the somatosensory pathway. Lysophosphatidylglucoside (LysoPtdGlc) was recently identified as a structurally unique lysophospholipid that exerts biological actions via the G protein-coupled receptor GPR55. Here, we demonstrated that GPR55-knockout (KO) mice show impaired induction of mechanical pain hypersensitivity in a model of spinal cord compression (SCC) without the same change in the models of peripheral tissue inflammation and peripheral nerve injury. Among these models, only SCC recruited peripheral inflammatory cells (neutrophils, monocytes/macrophages, and CD3⁺ T-cells) in the spinal dorsal horn (SDH), and GPR55-KO blunted these recruitments. Neutrophils were the first cells recruited to the SDH, and their depletion suppressed the induction of SCC-induced mechanical hypersensitivity and inflammatory responses in compressed SDH. Furthermore, we found that PtdGlc was present in the SDH and that intrathecal administration of an inhibitor of secretory phospholipase A₂ (an enzyme required for producing LysoPtdGlc from PtdGlc) reduced neutrophil recruitment to compressed SDH and suppressed pain induction. Finally, by screening compounds from a chemical library, we identified auranofin as a clinically used drug with an inhibitory effect on mouse and human GPR55. Systemically administered auranofin to mice with SCC effectively suppressed spinal neutrophil infiltration and pain hypersensitivity. These results suggest that GPR55 signaling contributes to the induction of inflammatory responses and chronic pain after SCC via the recruitment of neutrophils and may provide a new target for reducing pain induction after spinal cord compression, such as spinal canal stenosis.

Voltage-gated calcium channel subunit α2δ-1 in spinal dorsal horn neurons contributes to aberrant excitatory synaptic transmission and mechanical hypersensitivity after peripheral nerve injury

Neuropathic pain, an intractable pain symptom that occurs after nerve damage, is caused by the aberrant excitability of spinal dorsal horn (SDH) neurons. Gabapentinoids, the most commonly used drugs for neuropathic pain, inhibit spinal calcium-mediated neurotransmitter release by binding to α₂δ-1, a subunit of voltage-gated calcium channels, and alleviate neuropathic pain. However, the exact contribution of α₂δ-1 expressed in SDH neurons to the altered synaptic transmission and mechanical hypersensitivity following nerve injury is not fully understood. In this study, we investigated which types of SDH neurons express α₂δ-1 and how α₂δ-1 in SDH neurons contributes to the mechanical hypersensitivity and altered spinal synaptic transmission after nerve injury. Using in situ hybridization technique, we found that Cacna2d1, mRNA coding α₂δ-1, was mainly colocalized with Slc17a6, an excitatory neuronal marker, but not with Slc32a1, an inhibitory neuronal marker in the SDH. To investigate the role of α₂δ-1 in SDH neurons, we used clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 system and showed that SDH neuron-specific ablation of Cacna2d1 alleviated mechanical hypersensitivity following nerve injury. We further found that excitatory post-synaptic responses evoked by electrical stimulation applied to the SDH were significantly enhanced after nerve injury, and that these enhanced responses were significantly decreased by application of mirogabalin, a potent α₂δ-1 inhibitor, and by SDH neuron-specific ablation of Cacna2d1. These results suggest that α₂δ-1 expressed in SDH excitatory neurons facilitates spinal nociceptive synaptic transmission and contributes to the development of mechanical hypersensitivity after nerve injury.

Astrocytic connexin 43 deletion ameliorates SNI-induced neuropathic pain by reducing microglia activation

Neuropathic pain (NP) is a chronic disease caused by damage to the peripheral or central nervous system. Connexin 43 (Cx43), the primary connexin expressed by astrocytes, has been reported to be significantly increased in NP. However, the roles and mechanisms of Cx43 in the development and maintenance of NP remain largely unknown, while microglia activation has been commonly regarded as a key factor of NP. In the present study, we found that Cx43 deletion significantly ameliorated spared nerve injury (SNI)-induced NP and suppressed SNI induced c-Fos expression in the spinal cord. Notably, Cx43 deletion led to much less SNI-induced microglia activation in the spinal cord. These results suggest that astrocyte Cx43 may play a significant role in regulating microglial activation and NP.

Identification of Spinal Inhibitory Interneurons Required for Attenuating Effect of Duloxetine on Neuropathic Allodynia-like Signs in Rats

Neuropathic pain is a chronic pain condition that occurs after nerve damage; allodynia, which refers to pain caused by generally innocuous stimuli, is a hallmark symptom. Although allodynia is often resistant to analgesics, the antidepressant duloxetine has been used as an effective therapeutic option. Duloxetine increases spinal noradrenaline (NA) levels by inhibiting its transporter at NAergic terminals in the spinal dorsal horn (SDH), which has been proposed to contribute to its pain-relieving effect. However, the mechanism through which duloxetine suppresses neuropathic allodynia remains unclear. Here, we identified an SDH inhibitory interneuron subset (captured by adeno-associated viral (AAV) vectors incorporating a rat neuropeptide Y promoter; AAV-NpyP⁺ neurons) that is mostly depolarized by NA. Furthermore, this excitatory effect was suppressed by pharmacological blockade or genetic knockdown of α_1B-adrenoceptors (ARs) in AAV-NpyP⁺ SDH neurons. We found that duloxetine suppressed Aβ fiber-mediated allodynia-like behavioral responses after nerve injury and that this effect was not observed in AAV-NpyP⁺ SDH neuron-selective α_1B-AR-knockdown. These results indicate that α_1B-AR and AAV-NpyP⁺ neurons are critical targets for spinal NA and are necessary for the therapeutic effect of duloxetine on neuropathic pain, which can support the development of novel analgesics.

Hevin/Sparcl1 drives pathological pain through spinal cord astrocyte and NMDA receptor signaling

High endothelial venule protein/SPARC-like 1 (hevin/Sparcl1) is an astrocyte-secreted protein that regulates synapse formation in the brain. Here we show that astrocytic hevin signaling plays a critical role in maintaining chronic pain. Compared with WT mice, hevin-null mice exhibited normal mechanical and heat sensitivity but reduced inflammatory pain. Interestingly, hevin-null mice have faster recovery than WT mice from neuropathic pain after nerve injury. Intrathecal injection of WT hevin was sufficient to induce persistent mechanical allodynia in naive mice. In hevin-null mice with nerve injury, adeno-associated-virus-mediated (AAV-mediated) re-expression of hevin in glial fibrillary acidic protein-expressing (GFAP-expressing) spinal cord astrocytes could reinstate neuropathic pain. Mechanistically, hevin is crucial for spinal cord NMDA receptor (NMDAR) signaling. Hevin-potentiated N-Methyl-D-aspartic acid (NMDA) currents are mediated by GluN2B-containing NMDARs. Furthermore, intrathecal injection of a neutralizing Ab against hevin alleviated acute and persistent inflammatory pain, postoperative pain, and neuropathic pain. Secreted hevin that was detected in mouse cerebrospinal fluid (CSF) and nerve injury significantly increased CSF hevin abundance. Finally, neurosurgery caused rapid and substantial increases in SPARCL1/HEVIN levels in human CSF. Collectively, our findings support a critical role of hevin and astrocytes in the maintenance of chronic pain. Neutralizing of secreted hevin with monoclonal Ab may provide a new therapeutic strategy for treating acute and chronic pain and NMDAR-medicated neurodegeneration.

Somatotopy of Mouse Spinothalamic Innervation and the Localization of a Noxious Stimulus Requires Deleted in Colorectal Carcinoma Expression by Phox2a Neurons

Anterolateral system (AS) neurons transmit pain signals from the spinal cord to the brain. Their morphology, anatomy, and physiological properties have been extensively characterized and suggest that specific AS neurons and their brain targets are concerned with the discriminatory aspects of noxious stimuli, such as their location or intensity, and their motivational/emotive dimension. Among the recently unraveled molecular markers of AS neurons is the developmentally expressed transcription factor Phox2a, providing us with the opportunity to selectively disrupt the embryonic wiring of AS neurons to gain insights into the logic of their adult function. As mice with a spinal-cord-specific loss of the netrin-1 receptor deleted in colorectal carcinoma (DCC) have increased AS neuron innervation of ipsilateral brain targets and defective noxious stimulus localization or topognosis, we generated mice of either sex carrying a deletion of Dcc in Phox2a neurons. Such DccPhox2a mice displayed impaired topognosis along the rostrocaudal axis but with little effect on left-right discrimination and normal aversive responses. Anatomical tracing experiments in DccPhox2a mice revealed defective targeting of cervical and lumbar AS axons within the thalamus. Furthermore, genetic labeling of AS axons revealed their expression of DCC on their arrival in the brain, at a time when many of their target neurons are being born and express Ntn1 Our experiments suggest a postcommissural crossing function for netrin-1:DCC signaling during the formation of somatotopically ordered maps and are consistent with a discriminatory function of some of the Phox2a AS neurons.SIGNIFICANCE STATEMENT How nociceptive (pain) signals are relayed from the body to the brain remains an important question relevant to our understanding of the basic physiology of pain perception. Previous studies have demonstrated that the AS is a main effector of this function. It is composed of AS neurons located in the spinal cord that receive signals from nociceptive sensory neurons that detect noxious stimuli. In this study, we generate a genetic miswiring of mouse AS neurons that results in a decreased ability to perceive the location of a painful stimulus. The precise nature of this defect sheds light on the function of different kinds of AS neurons and how pain information may be organized.

Selective Involvement of a Subset of Spinal Dorsal Horn Neurons Operated by a Prodynorphin Promoter in Aβ Fiber-Mediated Neuropathic Allodynia-Like Behavioral Responses in Rats

Mechanical allodynia (pain produced by innocuous stimuli such as touch) is the main symptom of neuropathic pain. Its underlying mechanism remains to be elucidated, but peripheral nerve injury (PNI)-induced malfunction of neuronal circuits in the central nervous system, including the spinal dorsal horn (SDH), is thought to be involved in touch-pain conversion. Here, we found that intra-SDH injection of adeno-associated viral vectors including a prodynorphin promoter (AAV-PdynP) captured a subset of neurons that were mainly located in the superficial laminae, including lamina I, and exhibited mostly inhibitory characteristics. Using transgenic rats that enable optogenetic stimulation of touch-sensing Aβ fibers, we found that the light-evoked paw withdrawal behavior and aversive responses after PNI were attenuated by selective ablation of AAV-PdynP-captured SDH neurons. Notably, the ablation had no effect on withdrawal behavior from von Frey filaments. Furthermore, Aβ fiber stimulation did not excite AAV-PdynP⁺ SDH neurons under normal conditions, but after PNI, this induced excitation, possibly due to enhanced Aβ fiber-evoked excitatory synaptic inputs and elevated resting membrane potentials of these neurons. Moreover, the chemogenetic silencing of AAV-PdynP⁺ neurons of PNI rats attenuated the Aβ fiber-evoked paw withdrawal behavior and c-FOS expression in superficial SDH neurons. Our findings suggest that PNI renders AAV-PdynP-captured neurons excitable to Aβ fiber stimulation, which selectively contributes to the conversion of Aβ fiber-mediated touch signal to nociceptive. Thus, reducing the excitability of AAV-PdynP-captured neurons may be a new option for the treatment of neuropathic allodynia.

Microglia-mediated degradation of perineuronal nets promotes pain

Activation of microglia in the spinal cord dorsal horn following peripheral nerve injury contributes to the development of pain hypersensitivity. How activated microglia selectively enhance the activity of spinal nociceptive circuits is not well understood. We discovered that following peripheral nerve injury, microglia degrade extracellular matrix structures, perineuronal nets (PNNs), in lamina I of the spinal cord dorsal horn. Lamina I PNNs selectively enwrap spinoparabrachial projection neurons, which integrate nociceptive information in the spinal cord and convey it to supraspinal brain regions to induce pain sensation. Degradation of PNNs by microglia enhances the activity of projection neurons and induces pain-related behaviors. Thus, nerve injury-induced degradation of PNNs is a mechanism by which microglia selectively augment the output of spinal nociceptive circuits and cause pain hypersensitivity.

Neuronal pentraxin 2 is required for facilitating excitatory synaptic inputs onto spinal neurons involved in pruriceptive transmission in a model of chronic itch

An excitatory neuron subset in the spinal dorsal horn (SDH) that expresses gastrin-releasing peptide receptors (GRPR) is critical for pruriceptive transmission. Here, we show that glutamatergic excitatory inputs onto GRPR+ neurons are facilitated in mouse models of chronic itch. In these models, neuronal pentraxin 2 (NPTX2), an activity-dependent immediate early gene product, is upregulated in the dorsal root ganglion (DRG) neurons. Electron microscopy reveals that NPTX2 is present at presynaptic terminals connected onto postsynaptic GRPR+ neurons. NPTX2-knockout prevents the facilitation of synaptic inputs to GRPR+ neurons, and repetitive scratching behavior. DRG-specific NPTX2 expression rescues the impaired behavioral phenotype in NPTX2-knockout mice. Moreover, ectopic expression of a dominant-negative form of NPTX2 in DRG neurons reduces chronic itch-like behavior in mice. Our findings indicate that the upregulation of NPTX2 expression in DRG neurons contributes to the facilitation of glutamatergic inputs onto GRPR+ neurons under chronic itch-like conditions, providing a potential therapeutic target.

Carrier strategies boost the application of CRISPR/Cas system in gene therapy

Emerging clustered regularly interspaced short palindromic repeat/associated protein (CRISPR/Cas) genome editing technology shows great potential in gene therapy. However, proteins and nucleic acids suffer from enzymatic degradation in the physiological environment and low permeability into cells. Exploiting carriers to protect the CRISPR system from degradation, enhance its targeting of specific tissues and cells, and reduce its immunogenicity is essential to stimulate its clinical applications. Here, the authors review the state-of-the-art CRISPR delivery systems and their applications, and describe strategies to improve the safety and efficacy of CRISPR mediated genome editing, categorized by three types of cargo formats, that is, Cas: single-guide RNA ribonucleoprotein, Cas mRNA and single-guide RNA, and Cas plasmid expressing CRISPR/Cas systems. The authors hope this review will help develop safe and efficient nanomaterial-based carriers for CRISPR tools.

A spinal microglia population involved in remitting and relapsing neuropathic pain

Neuropathic pain is often caused by injury and diseases that affect the somatosensory system. Although pain development has been well studied, pain recovery mechanisms remain largely unknown. Here, we found that CD11c-expressing spinal microglia appear after the development of behavioral pain hypersensitivity following nerve injury. Nerve-injured mice with spinal CD11c⁺ microglial depletion failed to recover spontaneously from this hypersensitivity. CD11c⁺ microglia expressed insulin-like growth factor-1 (IGF1), and interference with IGF1 signaling recapitulated the impairment in pain recovery. In pain-recovered mice, the depletion of CD11c⁺ microglia or the interruption of IGF1 signaling resulted in a relapse in pain hypersensitivity. Our findings reveal a mechanism for the remission and recurrence of neuropathic pain, providing potential targets for therapeutic strategies.

A distinct hypothalamus-to-β cell circuit modulates insulin secretion

The central nervous system has long been thought to regulate insulin secretion, an essential process in the maintenance of blood glucose levels. However, the anatomical and functional connections between the brain and insulin-producing pancreatic β cells remain undefined. Here, we describe a functional transneuronal circuit connecting the hypothalamus to β cells in mice. This circuit originates from a subpopulation of oxytocin neurons in the paraventricular hypothalamic nucleus (PVN^OXT), and it reaches the islets of the endocrine pancreas via the sympathetic autonomic branch to innervate β cells. Stimulation of PVN^OXT neurons rapidly suppresses insulin secretion and causes hyperglycemia. Conversely, silencing of these neurons elevates insulin levels by dysregulating neuronal signaling and secretory pathways in β cells and induces hypoglycemia. PVN^OXT neuronal activity is triggered by glucoprivation. Our findings reveal that a subset of PVN^OXT neurons form functional multisynaptic circuits with β cells in mice to regulate insulin secretion, and their function is necessary for the β cell response to hypoglycemia.

Microglial deactivation by adeno-associated virus expressing small-hairpin GCH1 has protective effects against neuropathic pain development in a spinothalamic tract-lesion model

AIMS

Neuropathic pain after spinal cord injury is one of the most difficult clinical problems after the loss of mobility, and pharmacological or neuromodulation therapy showed limited efficacy. In this study, we examine the possibility of pain modulation by a recombinant adeno-associated virus (rAAV) encoding small-hairpin RNA against GCH1 (rAAV-shGCH1) in a spinal cord injury model in which neuropathic pain was induced by a spinothalamic tract (STT) lesion.

METHODS

Micro-electric lesioning was used to damage the left STT in rats (n = 32), and either rAAV-shGCH1 (n = 19) or rAAV control (n = 6) was injected into the dorsal horn of the rats at the same time. On postoperative days 3, 7, and 14, we evaluated neuropathic pain using a behavioral test and microglial activation by immunohistochemical staining.

RESULTS

A pain modulation effect of shGCH1 was observed from postoperative days 3 to 14. The mechanical withdrawal threshold was 13.0 ± 0.95 in the shGCH1 group, 4.3 ± 1.37 in the control group, and 3.49 ± 0.85 in sham on postoperative day 3 (p < 0.0001) and continued to postoperative day 14 (shGCH1 vs. control: 11.4 ± 1.1 vs. 2.05 ± 0.60, p < 0.001 and shGCH1 vs. sham: 11.4 ± 1.1 vs. 1.43 ± 0.54, p < 0.001). Immunohistochemical staining of the spinal cord dorsal horn showed deactivation of microglia in the shGCH1 group without any change of delayed pattern of astrocyte activation as in STT model.

CONCLUSIONS

Neuropathic pain after spinal cord injury can be modulated bilaterally by deactivating microglial activation after a unilateral injection of rAAV-shGCH1 into the dorsal horn of a STT lesion spinal cord pain model. This new attempt would be another therapeutic approach for NP after SCI, which once happens; there is no clear curative options still now.

TTI-101: A competitive inhibitor of STAT3 that spares oxidative phosphorylation and reverses mechanical allodynia in mouse models of neuropathic pain

Signal Transducer and Activator of Transcription (STAT) 3 emerged rapidly as a high-value target for treatment of cancer. However, small-molecule STAT3 inhibitors have been slow to enter the clinic due, in part, to serious adverse events (SAE), including lactic acidosis and peripheral neuropathy, which have been attributed to inhibition of STAT3's mitochondrial function. Our group developed TTI-101, a competitive inhibitor of STAT3 that targets the receptor pY705-peptide binding site within the Src homology 2 (SH2) domain to block its recruitment and activation. TTI-101 has shown target engagement, no toxicity, and evidence of clinical benefit in a Phase I study in patients with solid tumors. Here we report that TTI-101 did not affect mitochondrial function, nor did it cause STAT3 aggregation, chemically modify STAT3 or cause neuropathic pain. Instead, TTI-101 unexpectedly suppressed neuropathic pain induced by chemotherapy or in a spared nerve injury model. Thus, in addition to its direct anti-tumor effect, TTI-101 may be of benefit when administered to cancer patients at risk of developing chemotherapy-induced peripheral neuropathy (CIPN).

Secreted PLA2-III is a possible therapeutic target to treat neuropathic pain

Lysophosphatidic acid (LPA) plays a critical role in developing and maintaining chronic pain in various animal models. Previous studies have reported that cytosolic and calcium-independent phospholipase A2 (PLA2) is involved in the LPA receptor-mediated amplification of LPA production in the spinal dorsal horn (SDH) after nerve injury, while the involvement of secreted PLA2 (sPLA2) remains unclear. The present study revealed that only sPLA2 -III among 11 species of PLA2 showed a significant upregulation of gene expression in the SDH. Intraspinal injection of adeno-associated virus-miRNA targeting sPLA2-III prevented hyperalgesia and unique hypoalgesia in mice treated with partial sciatic nerve ligation. In addition, intrathecal treatment with antisense oligodeoxynucleotide or siRNA targeting sPLA2-III significantly reversed the established thermal hyperalgesia. In the high-throughput screening of sPLA2-III inhibitors from the chemical library, we identified two hit compounds. Through in vitro characterization of PLA2 inhibitor profiles and in vivo assessment of the anti-hyperalgesic effects of known PLA2 inhibitors as well as hit compounds, sPLA2-III was found to be a novel therapeutic target molecule for the treatment of Neuropathic pain.

A subset of spinal dorsal horn interneurons crucial for gating touch-evoked pain-like behavior

A cardinal, intractable symptom of neuropathic pain is mechanical allodynia, pain caused by innocuous stimuli via low-threshold mechanoreceptors such as Aβ fibers. However, the mechanism by which Aβ fiber-derived signals are converted to pain remains incompletely understood. Here we identify a subset of inhibitory interneurons in the spinal dorsal horn (SDH) operated by adeno-associated viral vectors incorporating a neuropeptide Y promoter (AAV-NpyP⁺) and show that specific ablation or silencing of AAV-NpyP⁺ SDH interneurons converted touch-sensing Aβ fiber-derived signals to morphine-resistant pain-like behavioral responses. AAV-NpyP⁺ neurons received excitatory inputs from Aβ fibers and transmitted inhibitory GABA signals to lamina I neurons projecting to the brain. In a model of neuropathic pain developed by peripheral nerve injury, AAV-NpyP⁺ neurons exhibited deeper resting membrane potentials, and their excitation by Aβ fibers was impaired. Conversely, chemogenetic activation of AAV-NpyP⁺ neurons in nerve-injured rats reversed Aβ fiber-derived neuropathic pain-like behavior that was shown to be morphine-resistant and reduced pathological neuronal activation of superficial SDH including lamina I. These findings suggest that identified inhibitory SDH interneurons that act as a critical brake on conversion of touch-sensing Aβ fiber signals into pain-like behavioral responses. Thus, enhancing activity of these neurons may offer a novel strategy for treating neuropathic allodynia.

Astrocytic STAT3 activation and chronic itch require IP3R1/TRPC-dependent Ca2+ signals in mice

BACKGROUND

Chronic itch is a debilitating symptom of inflammatory skin diseases, but the underlying mechanism is poorly understood. We have recently demonstrated that astrocytes in the spinal dorsal horn become reactive in models of atopic and contact dermatitis via activation of the transcription factor signal transducer and activator of transcription 3 (STAT3) and critically contribute to chronic itch. In general, STAT3 is transiently activated; however, STAT3 activation in reactive astrocytes of chronic itch model mice persistently occurs via an unknown mechanism.

OBJECTIVE

We aimed to determine the mechanisms of persistent activation of astrocytic STAT3 in chronic itch conditions.

METHODS

To determine the factors that are required for persistent activation of astrocytic STAT3, Western blotting and calcium imaging with cultured astrocytes or spinal cord slices were performed. Thereafter, chronic itch model mice were used for genetic and behavioral experiments to confirm the role of the factors determined to mediate persistent STAT3 activation from in vitro and ex vivo experiments in chronic itch.

RESULTS

IP3 receptor type 1 (IP3R1) knockdown in astrocytes suppressed IL-6-induced persistent STAT3 activation and expression of lipocalin-2 (LCN2), an astrocytic STAT3-dependent inflammatory factor that is required for chronic itch. IP3R1-dependent astrocytic Ca2+ responses involved Ca2+ influx through the cation channel transient receptor potential canonical (TRPC), which was required for persistent STAT3 activation evoked by IL-6. IL-6 expression was upregulated in dorsal root ganglion neurons in a mouse model of chronic itch. Dorsal root ganglion neuron-specific IL-6 knockdown, spinal astrocyte-specific IP3R1 knockdown, and pharmacologic spinal TRPC inhibition attenuated LCN2 expression and chronic itch.

CONCLUSION

Our findings suggest that IP3R1/TRPC channel-mediated Ca2+ signals elicited by IL-6 in astrocytes are necessary for persistent STAT3 activation, LCN2 expression, and chronic itch, and they may also provide new targets for therapeutic intervention.

Netrin-1 receptor DCC is required for the contralateral topography of lamina I anterolateral system neurons

Anterolateral system (AS) neurons relay nociceptive information from the spinal cord to the brain, protecting the body from harm by evoking a variety of behaviours and autonomic responses. The developmental programs that guide the connectivity of AS neurons remain poorly understood. Spinofugal axons cross the spinal midline in response to Netrin-1 signalling through its receptor deleted in colorectal carcinoma (DCC); however, the relevance of this canonical pathway to AS neuron development has only been demonstrated recently. Here, we disrupted Netrin-1:DCC signalling developmentally in AS neurons and assessed the consequences on the path finding of the different classes of spinofugal neurons. Many lamina I AS neurons normally innervate the lateral parabrachial nucleus and periaqueductal gray on the contralateral side. The loss of DCC in the developing spinal cord resulted in increased frequency of ipsilateral projection of spinoparabrachial and spinoperiaqueductal gray neurons. Given that contralateral spinofugal projections are largely associated with somatotopic representation of the body, changes in the laterality of AS spinofugal projections may contribute to reduced precision in pain localization observed in mice and humans carrying Dcc mutations.

Intrinsic braking role of descending locus coeruleus noradrenergic neurons in acute and chronic itch in mice

Itch is defined as an unpleasant sensation that provokes a desire to scratch. Our understanding of neuronal circuits for itch information transmission and processing in the spinal dorsal horn (SDH) has progressively advanced following the identification of SDH neuron subsets that are crucial for scratching behavior in models of itch. However, little is known about the control of acute and chronic itch by descending signals from the brain to the SDH. In this study, using genetic approaches that enable cell-type and circuit-specific functional manipulation, we reveal an intrinsic potential of locus coeruleus (LC)-noradrenergic (NAergic) neurons that project to the SDH to control acute and chronic itch. Activation and silencing of SDH-projecting LC-NAergic neurons reduced and enhanced scratching behavior, respectively, in models of histamine-dependent and -independent acute itch. Furthermore, in a model of chronic itch associated with contact dermatitis, repetitive scratching behavior was suppressed by the activation of the descending LC-NAergic pathway and by knocking out NA transporters specific to descending LC-NAergic neurons using a CRISPR-Cas9 system. Moreover, patch-clamp recording using spinal slices showed that noradrenaline facilitated inhibitory synaptic inputs onto gastrin-releasing peptide receptor-expressing SDH neurons, a neuronal subset known to be essential for itch transmission. Our findings suggest that descending LC-NAergic signaling intrinsically controls acute and chronic itch and provide potential therapeutic strategies for the treatment of acute and chronic itch.

Mechanical pain of the lower extremity after compression of the upper spinal cord involves signal transducer and activator of transcription 3-dependent reactive astrocytes and interleukin-6

Chronic pain is one of the main symptoms of spinal disorders such as spinal canal stenosis. A major cause of this pain is related to compression of the spinal cord, and chronic pain can develop at the level of the compressed spinal segment. However, in many patients chronic pain arises in an area that does not correspond to the compressed segment, and the underlying mechanism involved remains unknown. This was investigated in the present study using a mouse model of spinal cord compression in which mechanical pain of the hindpaws develops after compression of the first lumbar segment (L1) of the spinal cord. Compression induced the activation of astrocytes in the L1 spinal dorsal horn (SDH)-but not the L4 SDH that corresponds to the hindpaws-and activated signal transducer and activator of transcription 3 (STAT3). Suppressing reactive astrocytes by expressing a dominant negative form of STAT3 (dnSTAT3) in the compressed SDH prevented mechanical pain. Expression of interleukin (IL)-6 was also upregulated in the compressed SDH, and it was inhibited by astrocytic expression of dnSTAT3. Intrathecal administration of a neutralizing anti-IL-6 antibody reversed the compression-induced mechanical pain. These results suggest that astrocytic STAT3 and IL-6 in the compressed SDH are involved in remote mechanical pain observed in the lower extremity, and may provide a target for treating chronic pain associated with spinal cord compression such as spinal canal stenosis.

Role of reactive astrocytes in the spinal dorsal horn under chronic itch conditions

Astrocytes are the most abundant glial cells in the central nervous system (CNS), including the spinal cord. Neuronal damage induces astrocytes to become reactive and contribute to various CNS pathologies. Recent studies have demonstrated that astrocytes in the spinal dorsal horn (SDH) become reactive in a transcription factor signal transducer and activator of transcription 3-dependent manner without neuronal damage under chronic itch conditions, causing release of the factor lipocalin-2, leading to induction of sensitization of gastrin releasing peptide-induced chemical itch signaling in the SDH. In this review, we describe recent advances in our understanding of SDH neuronal pathways for itch transmission, the mechanisms of SDH astrocytic activation and its contribution to abnormal itch processing and discuss the role of reactive astrocytes in the SDH in abnormal sensory processing under chronic itch conditions.

Insights Into Spinal Dorsal Horn Circuit Function and Dysfunction Using Optical Approaches

Somatosensation encompasses a variety of essential modalities including touch, pressure, proprioception, temperature, pain, and itch. These peripheral sensations are crucial for all types of behaviors, ranging from social interaction to danger avoidance. Somatosensory information is transmitted from primary afferent fibers in the periphery into the central nervous system via the dorsal horn of the spinal cord. The dorsal horn functions as an intermediary processing center for this information, comprising a complex network of excitatory and inhibitory interneurons as well as projection neurons that transmit the processed somatosensory information from the spinal cord to the brain. It is now known that there can be dysfunction within this spinal cord circuitry in pathological pain conditions and that these perturbations contribute to the development and maintenance of pathological pain. However, the complex and heterogeneous network of the spinal dorsal horn has hampered efforts to further elucidate its role in somatosensory processing. Emerging optical techniques promise to illuminate the underlying organization and function of the dorsal horn and provide insights into the role of spinal cord sensory processing in shaping the behavioral response to somatosensory input that we ultimately observe. This review article will focus on recent advances in optogenetics and fluorescence imaging techniques in the spinal cord, encompassing findings from both in vivo and in vitro preparations. We will also discuss the current limitations and difficulties of employing these techniques to interrogate the spinal cord and current practices and approaches to overcome these challenges.

Microglia-Mediated Regulation of Neuropathic Pain: Molecular and Cellular Mechanisms

Pain is a defense system that responds rapidly to harmful internal and external stimuli through the somatosensory neuronal pathway. However, damage to the nervous system through cancer, diabetes, infection, autoimmune disease, chemotherapy or trauma often leads to neuropathic pain, a debilitating chronic pain condition. Neuropathic pain is not simply a temporal continuum of acute nociceptive signals from the periphery, but rather due to pathologically altered functions in the nervous system, which shift the net neuronal excitatory balance toward excitation. Although alterations were long thought to be a result of changes in neurons, but an increasing body of evidence over the past decades indicates the necessity and sufficiency of microglia, the tissue-resident macrophages of the spinal cord and brain, for nerve injury-induced malfunction of the nervous system. In this review article, I describe our current understanding of the molecular and cellular mechanisms underlying the role of microglia in the pathogenesis of neuropathic pain and discuss the therapeutic potential of microglia from recent advances in the development of new drugs targeting microglia.

Emerging technologies to study glial cells

Development, physiological functions, and pathologies of the brain depend on tight interactions between neurons and different types of glial cells, such as astrocytes, microglia, oligodendrocytes, and oligodendrocyte precursor cells. Assessing the relative contribution of different glial cell types is required for the full understanding of brain function and dysfunction. Over the recent years, several technological breakthroughs were achieved, allowing "glio-scientists" to address new challenging biological questions. These technical developments make it possible to study the roles of specific cell types with medium or high-content workflows and perform fine analysis of their mutual interactions in a preserved environment. This review illustrates the potency of several cutting-edge experimental approaches (advanced cell cultures, induced pluripotent stem cell (iPSC)-derived human glial cells, viral vectors, in situ glia imaging, opto- and chemogenetic approaches, and high-content molecular analysis) to unravel the role of glial cells in specific brain functions or diseases. It also illustrates the translation of some techniques to the clinics, to monitor glial cells in patients, through specific brain imaging methods. The advantages, pitfalls, and future developments are discussed for each technique, and selected examples are provided to illustrate how specific "gliobiological" questions can now be tackled.

Alterations of voluntary behavior in the course of disease progress and pharmacotherapy in mice with collagen-induced arthritis

Background

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic synovitis and bone destruction at the joints, causing pain and motor disturbance. Despite the better control of inflammation and joint deformity afforded by modern disease-modifying anti-rheumatic drugs, many patients with RA remain dissatisfied with their treatment, primarily because of sensory-emotional distress. Pre-clinical tests that can evaluate not only the symptoms of arthritis but also the associated pain as sensory-emotional experience are urgently needed.

Methods

Here, we introduce two types of novel methods for evaluation of voluntary behavior in a commonly used model of RA (collagen-induced arthritis; CIA) in male mice. First, spontaneous motor activity was assessed with a running wheel placed in home cages and the number of rotations was continuously recorded in a 12:12-h light environment. Second, temperature preference was assessed by measuring the time spent in either of the floor plates with augmenting (25 to 49 °C) or fixed temperature (25 °C). We also evaluated the effects of tofacitinib on CIA-associated changes in voluntary wheel running and temperature preference.

Results

We detected a significant decrease in voluntary wheel running, a significant shift in the distribution of movement in the dark phase, and a significant increase in the time spent in warmer environments than the room temperature in the mice with CIA. These alterations in voluntary behavior have never been described with conventional methods. We also revealed tofacitinib-resistant significant changes in the voluntary behavior and choice of temperature despite significant mitigation of the symptoms of arthritis.

Conclusions

We described for the first time significant alterations of the voluntary behavior of the mice with CIA during the clinical periods, indicating that the overall physical/motivational states and its circadian variation, as well as the specific preference to a certain environmental temperature, are modified in the mice with CIA, as observed in human patients. Some of these did not parallel with the conventional arthritis scores, particularly during the pharmacotherapy suggesting that mice with CIA show not only the peripheral symptoms but also the central consequences. The use of these approaches would also help clarify the biological mechanisms underlying physician-patient discordance in the assessment of RA.

Imaging spinal cord activity in behaving animals

The spinal cord is the primary neurological link between the brain and peripheral organs. How important it is in everyday life is apparent in patients with spinal cord injury or motoneuron disease, who have dramatically reduced musculoskeletal control or capacity to sense their environment. Despite its crucial role in sensory and motor processing little is known about the cellular and molecular signaling events that underlie spinal cord function under naturalistic conditions. While genetic, electrophysiological, pharmacological, and circuit tracing studies have revealed important roles for different molecularly defined neurons, these approaches insufficiently describe the moment-to-moment neuronal and non-neuronal activity patterns that underlie sensory-guided motor behaviors in health and disease. The recent development of imaging methods for real-time interrogation of cellular activity in the spinal cord of behaving mice has removed longstanding technical obstacles to spinal cord research and allowed new insight into how different cell types encode sensory information from mechanoreceptors and nociceptors in the skin. Here, we review the current state-of-the-art in interrogating cellular and microcircuit function in the spinal cord of behaving mammals and discuss current opportunities and technological challenges.

Surgical preparations, labeling strategies, and optical techniques for cell-resolved, in vivo imaging in the mouse spinal cord

In vivo optical imaging has enabled detailed studies of cellular dynamics in the brain of rodents in both healthy and diseased states. Such studies were made possible by three advances: surgical preparations that give optical access to the brain; strategies for in vivo labeling of cells with structural and functional fluorescent indicators; and optical imaging techniques that are relatively insensitive to light scattering by tissue. In vivo imaging in the rodent spinal cord has lagged behind than that in the brain, largely due to the anatomy around the spinal cord that complicates the surgical preparation, and to the strong optical scattering of the dorsal white matter that limits the ability to image deep into the spinal cord. Here, we review recent advances in surgical methods, labeling strategies, and optical tools that have enabled in vivo, high-resolution imaging of the dynamic behaviors of cells in the spinal cord in mice. Surgical preparations that enable long-term optical access and robust stabilization of the spinal cord are now available. Labeling strategies that have been used in the spinal cord tend to follow those that have been used in the brain, and some recent advances in genetically-encoded labeling strategies remain to be capitalized on. The optical imaging methods used to date, including two photon excited fluorescence microscopy, are largely limited to imaging the superficial layers of the spinal cord by the optical scattering of the white matter. Finally, we show preliminary data that points to the use of higher-order nonlinear optical processes, such as three photon excited fluorescence, as a means to image deeper into the mouse spinal cord.

Cav3.2 T-type calcium channels shape electrical firing in mouse Lamina II neurons

The T-type calcium channel, Cav3.2, is necessary for acute pain perception, as well as mechanical and cold allodynia in mice. Being found throughout sensory pathways, from excitatory primary afferent neurons up to pain matrix structures, it is a promising target for analgesics. In our study, Cav3.2 was detected in ~60% of the lamina II (LII) neurons of the spinal cord, a site for integration of sensory processing. It was co-expressed with Tlx3 and Pax2, markers of excitatory and inhibitory interneurons, as well as nNOS, calretinin, calbindin, PKCγ and not parvalbumin. Non-selective T-type channel blockers slowed the inhibitory but not the excitatory transmission in LII neurons. Furthermore, T-type channel blockers modified the intrinsic properties of LII neurons, abolishing low-threshold activated currents, rebound depolarizations, and blunting excitability. The recording of Cav3.2-positive LII neurons, after intraspinal injection of AAV-DJ-Cav3.2-mcherry, showed that their intrinsic properties resembled those of the global population. However, Cav3.2 ablation in the dorsal horn of Cav3.2^GFP-Flox KI mice after intraspinal injection of AAV-DJ-Cav3.2-Cre-IRES-mcherry, had drastic effects. Indeed, it (1) blunted the likelihood of transient firing patterns; (2) blunted the likelihood and the amplitude of rebound depolarizations, (3) eliminated action potential pairing, and (4) remodeled the kinetics of the action potentials. In contrast, the properties of Cav3.2-positive neurons were only marginally modified in Cav3.1 knockout mice. Overall, in addition to their previously established roles in the superficial spinal cord and in primary afferent neurons, Cav3.2 channel appear to be necessary for specific, significant and multiple controls of LII neuron excitability.

NALCN channels enhance the intrinsic excitability of spinal projection neurons

Spinal projection neurons convey nociceptive signals to multiple brain regions including the parabrachial (PB) nucleus, which contributes to the emotional valence of pain perception. Despite the clear importance of projection neurons to pain processing, our understanding of the factors that shape their intrinsic membrane excitability remains limited. Here, we investigate a potential role for the Na leak channel NALCN in regulating the activity of spino-PB neurons in the developing rodent. Pharmacological reduction of NALCN current (INALCN), or the genetic deletion of NALCN channels, significantly reduced the intrinsic excitability of lamina I spino-PB neurons. In addition, substance P (SP) activated INALCN in ascending projection neurons through downstream Src kinase signaling, and the knockout of NALCN prevented SP-evoked action potential discharge in this neuronal population. These results identify, for the first time, NALCN as a strong regulator of neuronal activity within central pain circuits and also elucidate an additional ionic mechanism by which SP can modulate spinal nociceptive processing. Collectively, these findings indicate that the level of NALCN conductance within spino-PB neurons tightly governs ascending nociceptive transmission to the brain and thereby potentially influences pain perception.

Targeted intraspinal injections to assess therapies in rodent models of neurological disorders

Despite decades of research, pharmacological therapies for spinal cord motor pathologies are limited. Alternatives using macromolecular, viral, or cell-based therapies show early promise. However, introducing these substances into the spinal cord, past the blood-brain barrier, without causing injury is challenging. We describe a technique for intraspinal injection targeting the lumbar ventral horn in rodents. This technique preserves motor performance and has a proven track record of translation into phase 1 and 2 clinical trials in amyotrophic lateral sclerosis (ALS) patients. The procedure, in brief, involves exposure of the thoracolumbar spine and dissection of paraspinous muscles over the target vertebrae. Following laminectomy, the spine is affixed to a stereotactic frame, permitting precise and reproducible injection throughout the lumbar spine. We have used this protocol to inject various stem cell types, primarily human spinal stem cells (HSSCs); however, the injection is adaptable to any candidate therapeutic cell, virus, or macromolecule product. In addition to a detailed procedure, we provide stereotactic coordinates that assist in targeting of the lumbar spine and instructional videos. The protocol takes ~2 h per animal.

Regulating nociceptive transmission by VGluT2-expressing spinal dorsal horn neurons

Vesicular glutamate transporter-2 (VGluT2) mediates the uptake of glutamate into synaptic vesicles in neurons. Spinal cord dorsal horn interneurons are highly heterogeneous and molecularly diverse. The functional significance of VGluT2-expressing dorsal horn neurons in physiological and pathological pain conditions has not been explicitly demonstrated. Designer receptors exclusively activated by designer drugs (DREADDs) are a powerful chemogenetic tool to reversibly control neuronal excitability and behavior. Here, we used transgenic mice with Cre recombinase expression driven by the VGluT2 promoter, combined with the chemogenetic approach, to determine the contribution of VGluT2-expressing dorsal horn neurons to nociceptive regulation. Adeno-associated viral vectors expressing double-floxed Cre-dependent Gαq-coupled human M3 muscarinic receptor DREADD (hM3D)-mCherry or Gαi-coupled κ-opioid receptor DREADD (KORD)-IRES-mCitrine were microinjected into the superficial spinal dorsal horn of VGluT2-Cre mice. Immunofluorescence labeling showed that VGluT2 was predominantly expressed in lamina II excitatory interneurons. Activation of excitatory hM3D in VGluT2-expressing neurons with clozapine N-oxide caused a profound increase in neuronal firing and synaptic glutamate release. Conversely, activation of inhibitory KORD in VGluT2-expressing neurons with salvinorin B markedly inhibited neuronal activity and synaptic glutamate release. In addition, chemogenetic stimulation of VGluT2-expressing neurons increased mechanical and thermal sensitivities in naive mice, whereas chemogenetic silencing of VGluT2-expressing neurons reversed pain hypersensitivity induced by tissue inflammation and peripheral nerve injury. These findings indicate that VGluT2-expressing excitatory neurons play a crucial role in mediating nociceptive transmission in the spinal dorsal horn. Targeting glutamatergic dorsal horn neurons with inhibitory DREADDs may be a new strategy for treating inflammatory pain and neuropathic pain.

AP-1 and the injury response of the GFAP gene

Increased GFAP gene expression is a common feature of CNS injury, resulting in its use as a reporter to investigate mechanisms producing gliosis. AP-1 transcription factors are among those proposed to participate in mediating the reactive response. Prior studies found a consensus AP-1 binding site in the GFAP promoter to be essential for activity of reporter constructs transfected into cultured cells, but to have little to no effect on basal transgene expression in mice. Since cultured astrocytes display some properties of reactive astrocytes, these findings suggested that AP-1 transcription factors are critical for the upregulation of GFAP in injury, but not for its resting level of expression. We have examined this possibility by comparing the injury response in mice of lacZ transgenes driven by human GFAP promoters that contain the wild-type AP-1 binding site to those in which the site is mutated. An intact AP-1 site was found critical for a GFAP promoter response to the three different injury models used: physical trauma produced by cryoinjury, seizures produced by kainic acid, and chronic gliosis produced in an Alexander disease model. An unexpected additional finding was that the responses of the lacZ transgenes driven by the wild-type promoters were substantially less than that of the endogenous mouse GFAP gene. This suggests that the GFAP gene has previously unrecognized injury-responsive elements that reside further upstream of the transcription start site than the 2.2 kb present in the GFAP promoter segments used here.