Exploring the Possible Role of Cannabinoids in Managing Post-Cardiac Surgery Complications: A Narrative Review of Preclinical Evidence and a Call for Future Research Directions.

Open-heart surgery with cardiopulmonary bypass (CPB) often leads to complications including pain, systemic inflammation, and organ damage. Traditionally managed with opioids, these pain relief methods bring potential long-term risks, prompting the exploration of alternative treatments. The legalization of cannabis in various regions has reignited interest in cannabinoids, such as CBD, known for their anti-inflammatory, analgesic, and neuroprotective properties. Historical and ongoing research acknowledges the endocannabinoid system's crucial role in managing physiological processes, suggesting cannabinoids could offer therapeutic benefits in post-surgical recovery. Specifically, CBD has shown promise in managing pain, moderating immune responses, and mitigating ischemia/reperfusion injury, underscoring its potential in postoperative care. However, the translation of these findings into clinical practice faces challenges, highlighting the need for extensive research to establish effective, safe cannabinoid-based therapies for patients undergoing open-heart surgery. This narrative review advocates for a balanced approach, considering both the therapeutic potential of cannabinoids and the complexities of their integration into clinical settings.

Attenuation of Colitis-Induced Visceral Hypersensitivity and Pain by Selective Silencing of TRPV1-Expressing Fibers in Rat Colon.

BACKGROUND: Transient receptor potential vanilloid 1 (TRPV1) cation channels, expressed on nociceptors, are well established as key contributors to abdominal pain in inflammatory bowel disease (IBD). Previous attempts at blocking these channels have been riddled with side effects. Here, we propose a novel treatment strategy, utilizing the large pore of TRPV1 channels as a drug delivery system to selectively inhibit visceral nociceptors. METHODS: We induced colitis in rats using intrarectal dinitrobenzene sulfonic acid. Visceral hypersensitivity, spontaneous pain, and responsiveness of the hind paws to noxious heat stimuli were examined before and after the intrarectal application of membrane-impermeable sodium channel blocker (QX-314) alone or together with TRPV1 channel activators or blockers. RESULTS: Intrarectal co-application of QX-314 with TRPV1 channel activator capsaicin significantly inhibited colitis-induced gut hypersensitivity. Furthermore, in the model of colitis, but not in naïve rats, QX-314 alone was sufficient to reverse gut hypersensitivity. The blockade of TRPV1 channels prevented this effect of QX-314. Finally, applying QX-314 alone to the inflamed gut inhibited colitis-induced ongoing pain. CONCLUSIONS: Selective silencing of gut nociceptors by a membrane-impermeable sodium channel blocker entering via exogenously or endogenously activated TRPV1 channels diminishes IBD-induced gut hypersensitivity. The lack of effect on naïve rats suggests a selective analgesic effect in the inflamed gut. Our results suggest that in the colitis model, TRPV1 channels are tonically active. Furthermore, our results emphasize the role of TRPV1-expressing nociceptive fibers in colitis-induced pain. These findings provide proof of concept for using charged activity blockers for the blockade of IBD-associated abdominal pain.

Here, we show that the selective silencing of a specific subtype of nociceptive neurons innervating the gut mitigates colitis-induced visceral hypersensitivity and pain. Our results provide a basis for developing effective and selective treatments for inflammatory bowel disease pain.

Age-dependent decrease in inhibitory drive on the excitatory superficial spinal dorsal horn neurons.

The excitatory and inhibitory interneurons of superficial laminae I-II of the spinal dorsal horn (SDH) receive and process pain-related information from the primary afferents and transmit it to the brain via the projection neurons. Thus, the interaction between excitatory and inhibitory SDH interneurons is crucial in determining the output from the spinal cord network. Disruption of this interaction in pathological conditions leads to increased SDH output to the higher brain centers, which could underlie pathological pain. Here, we examined whether the changes in the intrinsic SDH connectivity also occur with age, possibly underlying age-related increase in pain sensitivity. Using Vgat;tdTomato transgenic mouse line, we compared the spontaneous inhibitory postsynaptic currents (sIPSCs) in inhibitory tdTomato+ and excitatory tdTomato- interneurons between adult (3-5 m.o.) and aged (12-13 m.o.) mice. We demonstrate that in adult mice, the amplitude and frequency of the sIPSCs on the excitatory interneurons were significantly higher than on inhibitory interneurons. These differences were annulled in aged mice. Further, we show that in aged mice, excitatory neurons receive less inhibition than in adult mice. This could lead to overall disinhibition of the SDH network, which might underlie increased pain perception among the aged population.

Structural plasticity of axon initial segment in spinal cord neurons underlies inflammatory pain.

ABSTRACT: Physiological or pathology-mediated changes in neuronal activity trigger structural plasticity of the action potential generation site-the axon initial segment (AIS). These changes affect intrinsic neuronal excitability, thus tuning neuronal and overall network output. Using behavioral, immunohistochemical, electrophysiological, and computational approaches, we characterized inflammation-related AIS plasticity in rat's superficial (lamina II) spinal cord dorsal horn (SDH) neurons and established how AIS plasticity regulates the activity of SDH neurons, thus contributing to pain hypersensitivity. We show that in naive conditions, AIS in SDH inhibitory neurons is located closer to the soma than in excitatory neurons. Shortly after inducing inflammation, when the inflammatory hyperalgesia is at its peak, AIS in inhibitory neurons is shifted distally away from the soma. The shift in AIS location is accompanied by the decrease in excitability of SDH inhibitory neurons. These AIS location and excitability changes are selective for inhibitory neurons and reversible. We show that AIS shift back close to the soma, and SDH inhibitory neurons' excitability increases to baseline levels following recovery from inflammatory hyperalgesia. The computational model of SDH inhibitory neurons predicts that the distal shift of AIS is sufficient to decrease the intrinsic excitability of these neurons. Our results provide evidence of inflammatory pain-mediated AIS plasticity in the central nervous system, which differentially affects the excitability of inhibitory SDH neurons and contributes to inflammatory hyperalgesia.

Encoding of inflammatory hyperalgesia in mouse spinal cord.

ABSTRACT: Inflammation modifies the input-output properties of peripheral nociceptive neurons such that the same stimulus produces enhanced nociceptive firing. This increased nociceptive output enters the superficial dorsal spinal cord (SDH), an intricate neuronal network composed largely of excitatory and inhibitory interneurons and a small percentage of projection neurons. The SDH network comprises the first central nervous system network integrating noxious information. Using in vivo calcium imaging and a computational approach, we characterized the responsiveness of the SDH network in mice to noxious stimuli in normal conditions and investigated the changes in SDH response patterns after acute burn injury-induced inflammation. We show that the application of noxious heat stimuli to the hind paw of naïve mice results in an overall increase in SDH network activity. Single-cell response analysis reveals that 70% of recorded neurons increase or suppress their activity, while ∼30% of neurons remain nonresponsive. After acute burn injury and the development of inflammatory hyperalgesia, application of the same noxious heat stimuli leads to the activation of previously nonresponding neurons and desuppression of suppressed neurons. We further demonstrate that an increase in afferent activity mimics the response of the SDH network to noxious heat stimuli under inflammatory conditions. Using a computational model of the SDH network, we predict that the changes in SDH network activity result in overall increased activity of excitatory neurons, amplifying the output from SDH to higher brain centers. We suggest that during acute local peripheral inflammation, the SDH network undergoes dynamic changes promoting hyperalgesia.

Nociception and pain in humans lacking a functional TRPV1 channel.

BACKGROUNDChronic pain is a debilitating illness with currently limited therapy, in part due to difficulties in translating treatments derived from animal models to patients. The transient receptor potential vanilloid 1 (TRPV1) channel is associated with noxious heat detection and inflammatory pain, and reports of adverse effects in human trials have hindered extensive efforts in the clinical development of TRPV1 antagonists as novel pain relievers.METHODSWe examined 2 affected individuals (A1 and A2) carrying a homozygous missense mutation in TRPV1, rendering the channel nonfunctional. Biochemical and functional assays were used to analyze the mutant channel. To identify possible phenotypes of the affected individuals, we performed psychophysical and medical examinations.RESULTSWe demonstrated that diverse TRPV1 activators, acting at different sites of the channel protein, were unable to open the cloned mutant channel. This finding was not a consequence of impairment in the expression, cellular trafficking, or assembly of protein subunits. The affected individuals were insensitive to application of capsaicin to the mouth and skin and did not demonstrate aversive behavior toward capsaicin. Furthermore, quantitative sensory testing of A1 revealed an elevated heat-pain threshold but also, surprisingly, an elevated cold-pain threshold and extensive neurogenic inflammatory, flare, and pain responses following application of the TRPA1 channel activator mustard oil.CONCLUSIONOur study provides direct evidence in humans for pain-related functional changes linked to TRPV1, which is a prime target in the development of pain relievers.FUNDINGSupported by the Israel Science Foundation (368/19); Teva's National Network of Excellence in Neuroscience grant (no. 0394886) and Teva's National Network of Excellence in Neuroscience postdoctoral fellowship.

mTORC2 mediates structural plasticity in distal nociceptive endings that contributes to pain hypersensitivity following inflammation.

The encoding of noxious stimuli into action potential firing is largely mediated by nociceptive free nerve endings. Tissue inflammation, by changing the intrinsic properties of the nociceptive endings, leads to nociceptive hyperexcitability and thus to the development of inflammatory pain. Here, we showed that tissue inflammation-induced activation of the mammalian target of rapamycin complex 2 (mTORC2) triggers changes in the architecture of nociceptive terminals and leads to inflammatory pain. Pharmacological activation of mTORC2 induced elongation and branching of nociceptor peripheral endings and caused long-lasting pain hypersensitivity. Conversely, nociceptor-specific deletion of the mTORC2 regulatory protein rapamycin-insensitive companion of mTOR (Rictor) prevented inflammation-induced elongation and branching of cutaneous nociceptive fibers and attenuated inflammatory pain hypersensitivity. Computational modeling demonstrated that mTORC2-mediated structural changes in the nociceptive terminal tree are sufficient to increase the excitability of nociceptors. Targeting mTORC2 using a single injection of antisense oligonucleotide against Rictor provided long-lasting alleviation of inflammatory pain hypersensitivity. Collectively, we showed that tissue inflammation-induced activation of mTORC2 causes structural plasticity of nociceptive free nerve endings in the epidermis and inflammatory hyperalgesia, representing a therapeutic target for inflammatory pain.

In vivo optical recordings of ion dynamics in mouse corneal primary nociceptive terminals.

This protocol aims to measure ion dynamics in nociceptive terminal endings in intact mice in vivo. We describe viral injection of GCaMP6s + RFP into trigeminal ganglia (TG) of mice, followed by calcium imaging of corneal nociceptive terminals that express GCaMP6s and RFP. This fast and high-resolution optical recording technique enables studying a nociceptive terminal's functional molecular network in physiological and pathological conditions. This platform can be applied to studying the physiology of terminals of other neurons. For complete details on the use and execution of this protocol, please refer to Goldstein et al. (2019).

Photopharmacological modulation of native CRAC channels using azoboronate photoswitches.

SignificanceCalcium release-activated calcium (CRAC) channels play key roles in the regulation of cellular signaling, transcription, and migration. Here, we describe the design, chemical synthesis, and characterization of photoswitchable channel inhibitors that can be switched on and off depending on the wavelength of light used. We use the compounds to induce light-dependent modulation of channel activity and downstream gene expression in human immune cells. We further expand the usage of the compounds to control seeding of cancer cells in target tissue and regulation of response to noxious stimuli in vivo in mice.

Optical Assessment of Nociceptive TRP Channel Function at the Peripheral Nerve Terminal.

Free nerve endings are key structures in sensory transduction of noxious stimuli. In spite of this, little is known about their functional organization. Transient receptor potential (TRP) channels have emerged as key molecular identities in the sensory transduction of pain-producing stimuli, yet the vast majority of our knowledge about sensory TRP channel function is limited to data obtained from in vitro models which do not necessarily reflect physiological conditions. In recent years, the development of novel optical methods such as genetically encoded calcium indicators and photo-modulation of ion channel activity by pharmacological tools has provided an invaluable opportunity to directly assess nociceptive TRP channel function at the nerve terminal.

The Input-Output Relation of Primary Nociceptive Neurons is Determined by the Morphology of the Peripheral Nociceptive Terminals.

The output from the peripheral terminals of primary nociceptive neurons, which detect and encode the information regarding noxious stimuli, is crucial in determining pain sensation. The nociceptive terminal endings are morphologically complex structures assembled from multiple branches of different geometry, which converge in a variety of forms to create the terminal tree. The output of a single terminal is defined by the properties of the transducer channels producing the generation potentials and voltage-gated channels, translating the generation potentials into action potential (AP) firing. However, in the majority of cases, noxious stimuli activate multiple terminals; thus, the output of the nociceptive neuron is defined by the integration and computation of the inputs of the individual terminals. Here, we used a computational model of nociceptive terminal tree to study how the architecture of the terminal tree affects the input-output relation of the primary nociceptive neurons. We show that the input-output properties of the nociceptive neurons depend on the length, the axial resistance (Ra), and location of individual terminals. Moreover, we show that activation of multiple terminals by a capsaicin-like current allows summation of the responses from individual terminals, thus leading to increased nociceptive output. Stimulation of the terminals in simulated models of inflammatory or neuropathic hyperexcitability led to a change in the temporal pattern of AP firing, emphasizing the role of temporal code in conveying key information about changes in nociceptive output in pathologic conditions, leading to pain hypersensitivity.SIGNIFICANCE STATEMENT Noxious stimuli are detected by terminal endings of primary nociceptive neurons, which are organized into morphologically complex terminal trees. The information from multiple terminals is integrated along the terminal tree, computing the neuronal output, which propagates toward the CNS, thus shaping the pain sensation. Here, we revealed that the structure of the nociceptive terminal tree determines the output of nociceptive neurons. We show that the integration of noxious information depends on the morphology of the terminal trees and how this integration and, consequently, the neuronal output change under pathologic conditions. Our findings help to predict how nociceptive neurons encode noxious stimuli and how this encoding changes in pathologic conditions, leading to pain.

Abnormal Reinnervation of Denervated Areas Following Nerve Injury Facilitates Neuropathic Pain.

An injury to peripheral nerves leads to skin denervation, which often is followed by increased pain sensitivity of the denervated areas and the development of neuropathic pain. Changes in innervation patterns during the reinnervation process of the denervated skin could contribute to the development of neuropathic pain. Here, we examined the changes in the innervation pattern during reinnervation and correlated them with the symptoms of neuropathic pain. Using a multispectral labeling technique-PainBow, which we developed, we characterized dorsal root ganglion (DRG) neurons innervating distinct areas of the rats' paw. We then used spared nerve injury, causing partial denervation of the paw, and examined the changes in innervation patterns of the denervated areas during the development of allodynia and hyperalgesia. We found that, differently from normal conditions, during the development of neuropathic pain, these areas were mainly innervated by large, non-nociceptive neurons. Moreover, we found that the development of neuropathic pain is correlated with an overall decrease in the number of DRG neurons innervating these areas. Importantly, treatment with ouabain facilitated reinnervation and alleviated neuropathic pain. Our results suggest that local changes in peripheral innervation following denervation contribute to neuropathic pain development. The reversal of these changes decreases neuropathic pain.

2-APB and CBD-Mediated Targeting of Charged Cytotoxic Compounds Into Tumor Cells Suggests the Involvement of TRPV2 Channels.

Targeted delivery of therapeutic compounds to particular cell types such that they only affect the target cells is of great clinical importance since it can minimize undesired side effects. For example, typical chemotherapeutic treatments used in the treatment of neoplastic disorders are cytotoxic not only to cancer cells but also to most normal cells when exposed to a critical concentration of the compound. As such, many chemotherapeutics exhibit severe side effects, often prohibiting their effective use in the treatment of cancer. Here, we describe a new means for facilitated delivery of a clinically used chemotherapy compound' doxorubicin, into hepatocellular carcinoma cell line (BNL1 ME). We demonstrate that these cells express a large pore, cation non-selective transient receptor potential (TRP) channel V2. We utilized this channel to shuttle doxorubicin into BNL1 ME cells. We show that co-application of either cannabidiol (CBD) or 2-APB, the activators of TRPV2 channels, together with doxorubicin leads to significantly higher accumulation of doxorubicin in BNL1 ME cells than in BNL1 ME cells that were exposed to doxorubicin alone. Moreover, we demonstrate that sub-effective doses of doxorubicin when co-applied with either 2-APB or CBD lead to a significant decrease in the number of living BNL1 ME cell and BNL1 ME cell colonies in comparison to application of doxorubicin alone. Finally, we demonstrate that the doxorubicin-mediated cell death is significantly more potent, requiring an order of magnitude lower dose, when co-applied with CBD than with 2-APB. We suggest that CBD may have a dual effect in promoting doxorubicin-mediated cell death by facilitating the entry of doxorubicin via TRPV2 channels and preventing its clearance from the cells by inhibiting P-glycoprotein ATPase transporter. Collectively, these results provide a foundation for the use of large pore cation-non selective channels as "natural" drug delivery systems for targeting specific cell types.

Platelet-derived growth factor activates nociceptive neurons by inhibiting M-current and contributes to inflammatory pain.

Endogenous inflammatory mediators contribute to the pathogenesis of pain by acting on nociceptors, specialized sensory neurons that detect noxious stimuli. Here, we describe a new factor mediating inflammatory pain. We show that platelet-derived growth factor (PDGF)-BB applied in vitro causes repetitive firing of dissociated nociceptor-like rat dorsal root ganglion neurons and decreased their threshold for action potential generation. Injection of PDGF-BB into the paw produced nocifensive behavior in rats and led to thermal and mechanical pain hypersensitivity. We further detailed the biophysical mechanisms of these PDGF-BB effects and show that PDGF receptor-induced inhibition of nociceptive M-current underlies PDGF-BB-mediated nociceptive hyperexcitability. Moreover, in vivo sequestration of PDGF or inhibition of the PDGF receptor attenuates acute formalin-induced inflammatory pain. Our discovery of a new pain-facilitating proinflammatory mediator, which by inhibiting M-current activates nociceptive neurons and thus contributes to inflammatory pain, improves our understanding of inflammatory pain pathophysiology and may have important clinical implications for pain treatment.

Location and Plasticity of the Sodium Spike Initiation Zone in Nociceptive Terminals In Vivo.

Nociceptive terminals possess the elements for detecting, transmitting, and modulating noxious signals, thus being pivotal for pain sensation. Despite this, a functional description of the transduction process by the terminals, in physiological conditions, has not been fully achieved. Here, we studied how nociceptive terminals in vivo convert noxious stimuli into propagating signals. By monitoring noxious-stimulus-induced Ca2+ dynamics from mouse corneal terminals, we found that initiation of Na+ channel (Nav)-dependent propagating signals takes place away from the terminal and that the starting point for Nav-mediated propagation depends on Nav functional availability. Acute treatment with the proinflammatory cytokines tumor necrosis factor α (TNF-α) and interleukin 1ß (IL-1ß) resulted in a shift of the location of Nav involvement toward the terminal, thus increasing nociceptive excitability. Moreover, a shift of Nav involvement toward the terminal occurs in corneal hyperalgesia resulting from acute photokeratitis. This dynamic change in the location of Nav-mediated propagation initiation could underlie pathological pain hypersensitivity.

TRPM2 Mediates Neutrophil Killing of Disseminated Tumor Cells.

Neutrophils play a critical role in cancer, with both protumor and antitumor neutrophil subpopulations reported. The antitumor neutrophil subpopulation has the capacity to kill tumor cells and limit metastatic spread, yet not all tumor cells are equally susceptible to neutrophil cytotoxicity. Because cells that evade neutrophils have greater chances of forming metastases, we explored the mechanism neutrophils use to kill tumor cells. Neutrophil cytotoxicity was previously shown to be mediated by secretion of H2O2 We report here that neutrophil cytotoxicity is Ca2+ dependent and is mediated by TRPM2, a ubiquitously expressed H2O2-dependent Ca2+ channel. Perturbing TRPM2 expression limited tumor cell proliferation, leading to attenuated tumor growth. Concomitantly, cells expressing reduced levels of TRPM2 were protected from neutrophil cytotoxicity and seeded more efficiently in the premetastatic lung.Significance: These findings identify the mechanism utilized by neutrophils to kill disseminated tumor cells and to limit metastatic spread. Cancer Res; 78(10); 2680-90. ©2018 AACR.

Ultrafast optical recording reveals distinct capsaicin-induced ion dynamics along single nociceptive neurite terminals in vitro.

Pain signals are detected by terminals of nociceptive peripheral fibers situated among the keratinocytes and epithelial cells. Despite being key structures for pain-related stimuli detection and transmission, little is known about the functional organization of terminals. This is mainly due to their minute size, rendering them largely inaccessible by conventional experimental approaches. Here, we report the implementation of an ultrafast optical recording approach for studying cultured neurite terminals, which are readily accessible for assay manipulations. Using this approach, we were able to study capsaicin-induced calcium and sodium dynamics in the nociceptive processes, at a near-action potential time resolution. The approach was sensitive enough to detect differences in latency, time-to-peak, and amplitude of capsaicin-induced ion transients along the terminal neurites. Using this approach, we found that capsaicin evokes distinctive calcium signals along the neurite. At the terminal, the signal was insensitive to voltage-gated sodium channel blockers, and showed slower kinetics and smaller signal amplitudes, compared with signals that were measured further up the neurite. These latter signals were mainly abolished by sodium channel blockers. We propose this ultrafast optical recording approach as a model for studying peripheral terminal signaling, forming a basis for studying pain mechanisms in normal and pathological states.

The Role of Kv7/M Potassium Channels in Controlling Ectopic Firing in Nociceptors.

Peripheral nociceptive neurons encode and convey injury-inducing stimuli toward the central nervous system. In normal conditions, tight control of nociceptive resting potential prevents their spontaneous activation. However, in many pathological conditions the control of membrane potential is disrupted, leading to ectopic, stimulus-unrelated firing of nociceptive neurons, which is correlated to spontaneous pain. We have investigated the role of KV7/M channels in stabilizing membrane potential and impeding spontaneous firing of nociceptive neurons. These channels generate low voltage-activating, noninactivating M-type K+ currents (M-current, IM ), which control neuronal excitability. Using perforated-patch recordings from cultured, rat nociceptor-like dorsal root ganglion neurons, we show that inhibition of M-current leads to depolarization of nociceptive neurons and generation of repetitive firing. To assess to what extent the M-current, acting at the nociceptive terminals, is able to stabilize terminals' membrane potential, thus preventing their ectopic activation, in normal and pathological conditions, we built a multi-compartment computational model of a pseudo-unipolar unmyelinated nociceptive neuron with a realistic terminal tree. The modeled terminal tree was based on the in vivo structure of nociceptive peripheral terminal, which we assessed by in vivo multiphoton imaging of GFP-expressing nociceptive neuronal terminals innervating mice hind paw. By modifying the conductance of the KV7/M channels at the modeled terminal tree (terminal gKV7/M) we have found that 40% of the terminal gKV7/M conductance is sufficient to prevent spontaneous firing, while ~75% of terminal gKV7/M is sufficient to inhibit stimulus induced activation of nociceptive neurons. Moreover, we showed that terminal M-current reduces susceptibility of nociceptive neurons to a small fluctuations of membrane potentials. Furthermore, we simulated how the interaction between terminal persistent sodium current and M-current affects the excitability of the neurons. We demonstrated that terminal M-current in nociceptive neurons impeded spontaneous firing even when terminal Na(V)1.9 channels conductance was substantially increased. On the other hand, when terminal gKV7/M was decreased, nociceptive neurons fire spontaneously after slight increase in terminal Na(V)1.9 conductance. Our results emphasize the pivotal role of M-current in stabilizing membrane potential and hereby in controlling nociceptive spontaneous firing, in normal and pathological conditions.

Teriparatide attenuates scarring around murine cranial bone allograft via modulation of angiogenesis.

Nearly all bone fractures in humans can deteriorate into a non-union fracture, often due to formation of fibrotic tissue. Cranial allogeneic bone grafts present a striking example: although seemingly attractive for craniofacial reconstructions, they often fail due to fibrosis at the host-graft junction, which physically prevents the desired bridging of bone between the host and graft and revitalization of the latter. In the present study we show that intermittent treatment with recombinant parathyroid hormone-analogue (teriparatide) modulates neovascularization feeding in the graft surroundings, consequently reducing fibrosis and scar tissue formation and facilitates osteogenesis. Longitudinal inspection of the vascular tree feeding the allograft has revealed that teriparatide induces formation of small-diameter vessels in the 1st week after surgery; by the 2nd week, abundant formation of small-diameter blood vessels was detected in untreated control animals, but far less in teriparatide-treated mice, although in total, more blood capillaries were detected in the animals that were given teriparatide. By that time point we observed expression of the profibrogenic mediator TGF-ß in untreated animals, but negligible expression in the teriparatide-treated mice. To evaluate the formation of scar tissue, we utilized a magnetization transfer contrast MRI protocol to differentiate osteoid tissue from scar tissue, based on the characterization of collagen fibers. Using this method we found that significantly more bone matrix was formed in animals given teriparatide than in control animals. Altogether, our findings show how teriparatide diminishes scarring, ultimately leading to superior bone graft integration.

KV 7/M channels as targets for lipopolysaccharide-induced inflammatory neuronal hyperexcitability.

KEY POINTS: Neuroinflammation associated with CNS insults leads to neuronal hyperexcitability, which may culminate in epileptiform discharges. Application of the endotoxin lipopolysaccharide (LPS) to brain tissue initiates a neuroinflammatory cascade, providing an experimental model to study the mechanisms of neuroinflammatory neuronal hyperexcitability. Here we show that LPS application to hippocampal slices markedly enhances the excitability of CA1 pyramidal cells by inhibiting a specific potassium current, the M-current, generated by KV 7/M channels, which controls the excitability of almost every neuron in the CNS. The LPS-induced M-current inhibition is triggered by sequential activation of microglia, astrocytes and pyramidal cells, mediated by metabotropic purinergic and glutamatergic transmission, leading to blockade of KV 7/M channels by calcium released from intracellular stores. The identification of the downstream molecular target of neuroinflammation, namely the KV 7/M channel, potentially has far reaching implications for the understanding and treatment of many acute and chronic brain disorders. ABSTRACT: Acute brain insults and many chronic brain diseases manifest an innate inflammatory response. The hallmark of this response is glia activation, which promotes repair of damaged tissue, but also induces structural and functional changes that may lead to an increase in neuronal excitability. We have investigated the mechanisms involved in the modulation of neuronal activity by acute inflammation. Initiating inflammatory responses in hippocampal tissue rapidly led to neuronal depolarization and repetitive firing even in the absence of active synaptic transmission. This action was mediated by a complex metabotropic purinergic and glutamatergic glia-to-neuron signalling cascade, leading to the blockade of neuronal KV 7/M channels by Ca2+ released from internal stores. These channels generate the low voltage-activating, non-inactivating M-type K+ current (M-current) that controls intrinsic neuronal excitability, and its inhibition was the predominant cause of the inflammation-induced hyperexcitability. Our discovery that the ubiquitous KV 7/M channels are the downstream target of the inflammation-induced cascade, has far reaching implications for the understanding and treatment of many acute and chronic brain disorders.