Spinal GABAergic disinhibition allows microglial activation mediating the development of nociplastic pain in male mice.

Previously we developed a murine model in which postinjury stimulation of an injured area triggers a transition to a nociplastic pain state manifesting as persistent mechanical hypersensitivity outside of the previously injured area. This hypersensitivity was maintained by sex-specific mechanisms; specifically, activated spinal microglia maintained the hypersensitivity only in males. Here we investigated whether spinal microglia drive the transition from acute injury-induced pain to nociplastic pain in males, and if so, how they are activated by normally innocuous stimulation after peripheral injury. Using intraplantar capsaicin injection as an acute peripheral injury and vibration of the injured paw as postinjury stimulation, we found that inhibition of spinal microglia prevents the vibration-induced transition to a nociplastic pain state. The transition was mediated by the ATP-P2X4 pathway, but not BDNF-TrkB signaling. Intrathecally injected GABA receptor agonists after intraplantar capsaicin injection prevented the vibration-induced transition to a nociplastic pain state. Conversely, in the absence of intraplantar capsaicin injection, intrathecally injected GABA receptor antagonists allowed the vibration stimulation of a normal paw to trigger the transition to a spinal microglia-mediated nociplastic pain state only in males. At the spinal level, TNF-α, IL-1ß, and IL-6, but not prostaglandins, contributed to the maintenance of the nociplastic pain state in males. These results demonstrate that in males, the transition from acute injury-induced pain to nociplastic pain is driven by spinal microglia causing neuroinflammation and that peripheral injury-induced spinal GABAergic disinhibition is pivotal for normally innocuous stimulation to activate spinal microglia.

Piezo2-peripheral baroreceptor channel expressed in select neurons of the mouse brain: a putative mechanism for synchronizing neural networks by transducing intracranial pressure pulses.

Here we use immunohistochemistry to examine the expression of Piezo2 in neurons of the mouse dorsal root ganglia and brain. Whereas Piezo2 is expressed in the large majority (≥ 90%) of dorsal root ganglia neurons, Piezo2 expression is restricted to select neuron types in specific brain regions, including neocortical and hippocampal pyramidal neurons, cerebellar Purkinje cells and mitral cells of the olfactory bulb. Given the well-established role of Piezo2 as a low-threshold pressure sensor (i.e., ≤5 mmHg) in peripheral mechanosensation, including the regulation of breathing and blood pressure, its expression in central neurons has interesting implications. In particular, we hypothesize that Piezo2 provides neurons with an intrinsic resonance that promotes their entrainment by the normal intracranial pressure pulses (~5 mmHg) associated with breathing and cardiac cycles. The pressure-induced change in neural activity need only be very subtle to increase, for example, the robustness of respiration-entrained oscillations reported previously in widely distributed neuronal networks in both rodent and human brains. This idea of a "global brain rhythm" first arose from the effect of nasal airflow in activating mechanosensitive olfactory sensory neurons, which then synaptically entrain mitral cells within the olfactory bulb and through their projections, neural networks in other brain regions, including the hippocampus and neocortex. Our proposed, non-synaptic, intrinsic mechanism, where Piezo2 tracks the highly predictable and "metronome-like" intracranial pressure pulses-to date generally considered epiphenomena-would have the advantage that a physical force rapidly transmitted throughout the brain also contributes to this synchronization.

Peripheral and central oxidative stress in chemotherapy-induced neuropathic pain.

Chemotherapy-induced peripheral neuropathy (CIPN) is an adverse side effect of many anti-cancer chemotherapeutic treatments. CIPN often causes neuropathic pain in extremities, and oxidative stress has been shown to be a major contributing factor to this pain. In this study, we determined the site of oxidative stress associated with pain (specifically, mechanical hypersensitivity) in cisplatin- and paclitaxel-treated mouse models of CIPN and investigated the neurophysiological mechanisms accounting for the pain. C57BL/6N mice that received either cisplatin or paclitaxel (2 mg/kg, once daily on four alternate days) developed mechanical hypersensitivity to von Frey filament stimulations of their hindpaws. Cisplatin-induced mechanical hypersensitivity was inhibited by silencing of Transient Receptor Potential channels V1 (TRPV1)- or TRPA1-expressing afferents, whereas paclitaxel-induced mechanical hypersensitivity was attenuated by silencing of Aß fibers. Although systemic delivery of phenyl N-tert-butylnitrone, a reactive oxygen species scavenger, alleviated mechanical hypersensitivity in both cisplatin- and paclitaxel-treated mice, intraplantar phenyl N-tert-butylnitrone was effective only in cisplatin-treated mice, and intrathecal phenyl N-tert-butylnitrone, only in paclitaxel-treated mice. In a reactive oxygen species-dependent manner, the mechanosensitivity of Aδ/C fiber endings in the hindpaw skin was increased in cisplatin-treated mice, and the excitatory synaptic strength in the spinal dorsal horn was potentiated in paclitaxel-treated mice. Collectively, these results suggest that cisplatin-induced mechanical hypersensitivity is attributed to peripheral oxidative stress sensitizing mechanical nociceptors, whereas paclitaxel-induced mechanical hypersensitivity is due to central (spinal) oxidative stress maintaining central sensitization that abnormally produces pain in response to Aß fiber inputs.

5-HT2C Receptor Knockdown in the Amygdala Inhibits Neuropathic-Pain-Related Plasticity and Behaviors.

Neuroplasticity in the amygdala drives pain-related behaviors. The central nucleus (CeA) serves major amygdala output functions and can generate emotional-affective behaviors and modulate nocifensive responses. The CeA receives excitatory and inhibitory inputs from the basolateral nucleus (BLA) and serotonin receptor subtype 5-HT2CR in the BLA, but not CeA, has been implicated anxiogenic behaviors and anxiety disorders. Here, we tested the hypothesis that 5-HT2CR in the BLA plays a critical role in CeA plasticity and neuropathic pain behaviors in the rat spinal nerve ligation (SNL) model. Local 5-HT2CR knockdown in the BLA with stereotaxic injection of 5-HT2CR shRNA AAV vector decreased vocalizations and anxiety- and depression-like behaviors and increased sensory thresholds of SNL rats, but had no effect in sham controls. Extracellular single-unit recordings of CeA neurons in anesthetized rats showed that 5-HT2CR knockdown blocked the increase in neuronal activity (increased responsiveness, irregular spike firing, and increased burst activity) in SNL rats. At the synaptic level, 5-HT2CR knockdown blocked the increase in excitatory transmission from BLA to CeA recorded in brain slices from SNL rats using whole-cell patch-clamp conditions. Inhibitory transmission was decreased by 5-HT2CR knockdown in control and SNL conditions to a similar degree. The findings can be explained by immunohistochemical data showing increased expression of 5-HT2CR in non-GABAergic BLA cells in SNL rats. The results suggest that increased 5-HT2CR in the BLA contributes to neuropathic-pain-related amygdala plasticity by driving synaptic excitation of CeA neurons. As a rescue strategy, 5-HT2CR knockdown in the BLA inhibits neuropathic-pain-related behaviors.SIGNIFICANCE STATEMENT Neuroplasticity in the amygdala has emerged as an important pain mechanism. This study identifies a novel target and rescue strategy to control abnormally enhanced amygdala activity in an animal model of neuropathic pain. Specifically, an integrative approach of gene transfer, systems and brain slice electrophysiology, behavior, and immunohistochemistry was used to advance the novel concept that serotonin receptor subtype 5-HT2C contributes critically to the imbalance between excitatory and inhibitory drive of amygdala output neurons. Local viral vector-mediated 5-HT2CR knockdown in the amygdala normalizes the imbalance, decreases neuronal activity, and inhibits neuropathic-pain-related behaviors. The study provides valuable insight into serotonin receptor (dys)function in a limbic brain area.

Mitochondrial superoxide increases excitatory synaptic strength in spinal dorsal horn neurons of neuropathic mice.

Reactive oxygen species has been suggested as a key player in neuropathic pain, causing central sensitization by changing synaptic strengths in spinal dorsal horn neurons. However, it remains unclear as to what type of reactive oxygen species changes what aspect of synaptic strengths for central sensitization in neuropathic pain conditions. In this study, we investigated whether mitochondrial superoxide affects both excitatory and inhibitory synaptic strengths in spinal dorsal horn neurons after peripheral nerve injury. Upregulation of mitochondrial superoxide level by knockout of superoxide dismutase-2 exacerbated neuropathic mechanical hypersensitivity caused by L5 spinal nerve ligation, whereas downregulation of mitochondrial superoxide level by overexpression of superoxide dismutase-2 alleviated the hypersensitivity. In spinal nerve ligation condition, the frequency of miniature excitatory postsynaptic currents increased, while that of miniature inhibitory postsynaptic currents decreased in spinal dorsal horn neurons. Superoxide dismutase-2-knockout augmented, whereas superoxide dismutase-2-overexpression prevented, the spinal nerve ligation-increased miniature excitatory postsynaptic currents frequency. However, superoxide dismutase-2-knockout had no effect on the spinal nerve ligation-decreased miniature inhibitory postsynaptic current frequency, and superoxide dismutase-2-overexpression unexpectedly decreased miniature inhibitory postsynaptic current frequency in the normal condition. When applied to the spinal cord slice during in vitro recordings, mitoTEMPO, a specific scavenger of mitochondrial superoxide, reduced the spinal nerve ligation-increased miniature excitatory postsynaptic currents frequency but failed to normalize the spinal nerve ligation-decreased miniature inhibitory postsynaptic current frequency. These results suggest that in spinal dorsal horn neurons, high levels of mitochondrial superoxide increase excitatory synaptic strength after peripheral nerve injury and contribute to neuropathic mechanical hypersensitivity. However, mitochondrial superoxide does not seem to be involved in the decreased inhibitory synaptic strength in this neuropathic pain condition.

Differential involvement of reactive oxygen species in a mouse model of capsaicin-induced secondary mechanical hyperalgesia and allodynia.

Abstract: Intradermally injected capsaicin induces secondary mechanical hyperalgesia and allodynia outside the primary (i.e., capsaicininjected) site. This secondary mechanical hypersensitivity is attributed to central sensitization in which reactive oxygen species (ROS) play a key role. We examined whether ROS would be differentially involved in secondary mechanical hyperalgesia and allodynia using a mouse intraplantar capsaicin injection model. In mice, capsaicin-induced secondary mechanical hyperalgesia outlasted its allodynia counterpart. Unlike the hyperalgesia, the allodynia was temporarily abolished by an anesthetic given at the capsaicin-injected site. The ROS scavenger phenyl-N-tert-butylnitrone slowed the development of both secondary mechanical hyperalgesia and allodynia when administered before intraplantar capsaicin injection, whereas it inhibited only the allodynia when administered after capsaicin had already induced secondary mechanical hyperalgesia and allodynia. Intrathecal injection of the ROS donor KO2 induced both mechanical hyperalgesia and allodynia with the former outlasting the latter. Metformin, an activator of redox-sensitive adenosine monophosphate-activated protein kinase, selectively inhibited capsaicin-induced secondary mechanical allodynia and intrathecal KO2-induced mechanical allodynia. These results suggest that ROS is required for rapid activation of central sensitization mechanisms for both secondary mechanical hyperalgesia and allodynia after intraplantar capsaicin injection. Once activated, the mechanism for the hyperalgesia is longlasting without being critically dependent on ongoing afferent activities arising from the capsaicin-injected site and the continuous presence of ROS. On the contrary, the ongoing afferent activities, ROS presence and adenosine monophosphate-activated protein kinase inhibition are indispensable for the maintenance mechanism for capsaicin-induced secondary mechanical allodynia.

Gonadal hormone-dependent nociceptor sensitization maintains nociplastic pain state in female mice.

ABSTRACT: Nociplastic pain conditions develop predominantly in women. We recently established a murine nociplastic pain model by applying postinjury thermal (40°C) stimulation to an injured (capsaicin-injected) area, triggering a transition to a nociplastic pain state manifesting as persistent mechanical hypersensitivity outside of the previously injured area. The nociplastic pain state was centrally maintained by spinal microglia in males but peripherally by ongoing afferent activity at the previously injured area in females. Here, we investigated whether gonadal hormones are critical for the development of this peripherally maintained nociplastic pain state in females. Although the transition to a nociplastic pain state still occurred in ovariectomized females, the pain state was maintained neither by ongoing afferent activity at the previously injured area nor by spinal microglia. Estradiol reconstitution a week before the injury plus postinjury stimulation, but not after the transition had already occurred, restored the development of peripherally maintained nociplastic mechanical hypersensitivity in ovariectomized females. G protein-coupled estrogen receptor antagonism during the transition phase mimicked ovariectomy in gonad-intact females, whereas the receptor antagonism after the transition gradually alleviated the nociplastic mechanical hypersensitivity. At the previously injured area, afferents responsive to allyl isothiocyanate (AITC), a TRPA1 agonist, contributed to the maintenance of nociplastic mechanical hypersensitivity in gonad-intact females. In ex vivo skin-nerve preparations, only AITC-responsive afferents from the nociplastic pain model in gonad-intact females showed ongoing activities greater than control. These results suggest that gonadal hormones are critical for peripherally maintained nociplastic pain state in females by sensitizing AITC-responsive afferents to be persistently active.

Intraoperative Spinal Cord Stimulation Mitigates Central Sensitization After Spine Surgery in Mice.

STUDY DESIGN: Double-blinded, prospective laboratory animal study. OBJECTIVE: To examine whether intraoperative spinal cord stimulation (SCS) inhibits the development of spine surgery-induced hypersensitivity. SUMMARY OF BACKGROUND DATA: Managing postoperative pain after spine surgery is challenging, and as many as 40% of patients may develop failed back surgery syndrome. Although SCS has been shown to effectively reduce chronic pain symptoms, it is unknown whether intraoperative SCS can mitigate the development of central sensitization that causes postoperative pain hypersensitivity and potentially leads to failed back surgery syndrome after spine surgery. MATERIALS AND METHODS: Mice were randomly stratified into three experimental groups: (1) sham surgery, (2) laminectomy alone, and (3) laminectomy plus SCS. Secondary mechanical hypersensitivity was measured in hind paws using von Frey assay one day before and at predetermined times after surgery. In addition, we also performed a conflict avoidance test to capture the affective-motivational domain of pain at selected time points postlaminectomy. RESULTS: Mice that underwent unilateral T13 laminectomy developed mechanical hypersensitivity in both hind paws. Intraoperative SCS applied to the exposed side of the dorsal spinal cord significantly inhibited the development of hind paw mechanical hypersensitivity on the SCS-applied side. Sham surgery did not produce any obvious secondary mechanical hypersensitivity in the hind paws. CONCLUSIONS: These results demonstrate that spine surgery for unilateral laminectomy induces central sensitization that results in postoperative pain hypersensitivity. Intraoperative SCS after laminectomy may be able to mitigate the development of this hypersensitivity in appropriately selected cases.

Mitochondrial Ca(2+) uptake is essential for synaptic plasticity in pain.

The increase of cytosolic free Ca(2+) ([Ca(2+)](c)) due to NMDA receptor activation is a key step for spinal cord synaptic plasticity by altering cellular signal transduction pathways. We focus on this plasticity as a cause of persistent pain. To provide a mechanism for these classic findings, we report that [Ca(2+)](c) does not trigger synaptic plasticity directly but must first enter into mitochondria. Interfering with mitochondrial Ca(2+) uptake during a [Ca(2+)](c) increase blocks induction of behavioral hyperalgesia and accompanying downstream cell signaling, with reduction of spinal long-term potentiation (LTP). Furthermore, reducing the accompanying mitochondrial superoxide levels lessens hyperalgesia and LTP induction. These results indicate that [Ca(2+)](c) requires downstream mitochondrial Ca(2+) uptake with consequent production of reactive oxygen species (ROS) for synaptic plasticity underlying chronic pain. These results suggest modifying mitochondrial Ca(2+) uptake and thus ROS as a type of chronic pain therapy that should also have broader biologic significance.

Regulation of Wnt signaling by nociceptive input in animal models.

BACKGROUND: Central sensitization-associated synaptic plasticity in the spinal cord dorsal horn (SCDH) critically contributes to the development of chronic pain, but understanding of the underlying molecular pathways is still incomplete. Emerging evidence suggests that Wnt signaling plays a crucial role in regulation of synaptic plasticity. Little is known about the potential function of the Wnt signaling cascades in chronic pain development. RESULTS: Fluorescent immunostaining results indicate that ß-catenin, an essential protein in the canonical Wnt signaling pathway, is expressed in the superficial layers of the mouse SCDH with enrichment at synapses in lamina II. In addition, Wnt3a, a prototypic Wnt ligand that activates the canonical pathway, is also enriched in the superficial layers. Immunoblotting analysis indicates that both Wnt3a a ß-catenin are up-regulated in the SCDH of various mouse pain models created by hind-paw injection of capsaicin, intrathecal (i.t.) injection of HIV-gp120 protein or spinal nerve ligation (SNL). Furthermore, Wnt5a, a prototypic Wnt ligand for non-canonical pathways, and its receptor Ror2 are also up-regulated in the SCDH of these models. CONCLUSION: Our results suggest that Wnt signaling pathways are regulated by nociceptive input. The activation of Wnt signaling may regulate the expression of spinal central sensitization during the development of acute and chronic pain.

Postinjury stimulation triggers a transition to nociplastic pain in mice.

ABSTRACT: Acute injury-induced pain can transition to chronic nociplastic pain, which predominantly affects women. To facilitate studies on the underlying mechanisms of nociplastic pain, we developed a mouse model in which postinjury thermal stimulation (intermittent 40°C water immersion for 10 minutes at 2 hours postcapsaicin) prolongs capsaicin (ie, experimental injury)-induced transient mechanical hypersensitivity outside of the injury area. Although capsaicin injection alone induced mechanical and thermal hypersensitivity that resolved in ∼7 days (slower recovery in females), the postinjury stimulation prolonged capsaicin-induced mechanical, but not thermal, hypersensitivity up to 3 weeks in both sexes. When postinjury stimulation was given at a lower intensity (30°C) or at later time points (40°C at 1-3 days postcapsaicin), chronification of mechanical hypersensitivity occurred only in females. Similar chronification could be induced by a different postinjury stimulation modality (vibration of paw) or with a different injury model (plantar incision). Notably, the 40°C postinjury stimulation did not prolong capsaicin-induced inflammation in the hind paw, indicating that the prolonged mechanical hypersensitivity in these mice arises without clear evidence of ongoing injury, reflecting nociplastic pain. Although morphine and gabapentin effectively alleviated this persistent mechanical hypersensitivity in both sexes, sexually dimorphic mechanisms mediated the hypersensitivity. Specifically, ongoing afferent activity at the previously capsaicin-injected area was critical in females, whereas activated spinal microglia were crucial in males. These results demonstrate that postinjury stimulation of the injured area can trigger the transition from transient pain to nociplastic pain more readily in females, and sex-dependent mechanisms maintain the nociplastic pain state.

Effect of deuteration on the single dose pharmacokinetic properties and postoperative analgesic activity of methadone.

Although methadone is effective in the management of acute pain, the complexity of its absorption-distribution-metabolism-excretion profile limits its use as an opioid of choice for perioperative analgesia. Because deuteration is known to improve the pharmacokinetic, pharmacodynamic and toxicological properties of some drugs, here we characterized the single dose pharmacokinetic properties and post-operative analgesic efficacy of d9-methadone. The pharmacokinetic profiles of d9-methadone and methadone administered intravenously to CD-1 male mice revealed that deuteration leads to a 5.7- and 4.4-fold increase in the area under the time-concentration curve and maximum concentration in plasma, respectively, as well as reduction in clearance (0.9 ± 0.3 L/h/kg vs 4.7 ± 0.8 L/h/kg). The lower brain-to-plasma ratio of d9-methadone compared to that of methadone (0.35 ± 0.12 vs 2.05 ± 0.62) suggested that deuteration decreases the transfer of the drug across the blood-brain barrier. The estimated LD50 value for a single intravenous dose of d9-methadone was 2.1-fold higher than that for methadone. Moreover, d9-methadone outperformed methadone in the efficacy against postoperative pain by primarily activating peripheral opioid receptors. Collectively, these data suggest that the replacement of three hydrogen atoms in three methyl groups of methadone altered its pharmacokinetic properties, improved safety, and enhanced its analgesic efficacy.

Neuron Type-Dependent Synaptic Activity in the Spinal Dorsal Horn of Opioid-Induced Hyperalgesia Mouse Model.

Opioids are widely used for pain relief; however, chronic opioid use causes a paradoxical state of enhanced pain sensitivity, termed "Opioid-induced hyperalgesia (OIH)." Despite the clinical importance of OIH, the detailed mechanism by which it enhances pain sensitivity remains unclear. In this study, we tested whether repeated morphine induces a neuronal circuit polarization in the mouse spinal dorsal horn (SDH). Transgenic mice expressing GFP to neurokinin 1 receptor-expressing neurons (sNK1Rn) and GABAergic interneurons (sGABAn) that received morphine [20 mg/kg, once daily for four consecutive days (i.p.)] developed mechanical hypersensitivity. Repeated morphine altered synaptic strengths in the SDH as a specific cell-type but not in a gender-dependent manner. In sNK1Rn and non-tonic firing neurons, repeated morphine treatment significantly increased frequency of spontaneous excitatory postsynaptic current (sEPSC) and evoked EPSC (eEPSC). In addition, repeated morphine treatment significantly decreased evoked inhibitory postsynaptic current (eIPSC) in sNK1Rn. Conversely, in sGABAn and tonic firing neurons, repeated morphine treatment significantly decreased sEPSC frequency and eEPSC, but had no change of eIPSC in sGABAn. Interestingly, repeated morphine treatment significantly decreased neuronal rheobase of sNK1Rn but had no effect on sGABAn. These findings suggest that spinal neuronal circuit polarization maybe the mechanism of OIH and identify a potential therapeutic mechanism to prevent or treat opioid-induced pain.

Persistent pain is dependent on spinal mitochondrial antioxidant levels.

Reactive oxygen species (ROS) scavengers have been shown to relieve persistent pain; however, the mechanism is not clearly understood. Superoxide produced from mitochondrial oxidative phosphorylation is considered the major source of ROS in neurons during excitation where mitochondrial superoxide levels are normally controlled by superoxide dismutase (SOD-2). The present study hypothesizes that capsaicin-induced secondary hyperalgesia is a consequence of superoxide build-up in spinal dorsal horn neurons and SOD-2 is a major determinant. To test this hypothesis, the spinal levels of SOD-2 activity, inactivated SOD-2 proteins, and mitochondrial superoxide were measured and correlated to the levels of capsaicin-induced secondary hyperalgesia in mice with and without SOD-2 manipulations. The data suggest that superoxide accumulation is a culprit in the abnormal sensory processing in the spinal cord in capsaicin-induced secondary hyperalgesia. Our studies also support the notion that SOD-2 nitration is a critical mechanism that maintains elevated superoxide levels in the spinal cord after capsaicin treatment. Finally, our findings suggest a therapeutic potential for the manipulation of spinal SOD-2 activity in pain conditions.

PIEZO1 Is Selectively Expressed in Small Diameter Mouse DRG Neurons Distinct From Neurons Strongly Expressing TRPV1.

Using a high resolution in situ hybridization technique we have measured PIEZO1, PIEZO2, and TRPV1 transcripts in mouse dorsal root ganglion (DRG) neurons. Consistent with previous studies, PIEZO2 transcripts were highly expressed in DRG neurons of all sizes, including most notably the largest diameter neurons implicated in mediating touch and proprioception. In contrast, PIEZO1 transcripts were selectively expressed in smaller DRG neurons, which are implicated in mediating nociception. Moreover, the small neurons expressing PIEZO1 were mostly distinct from those neurons that strongly expressed TRPV1, one of the channels implicated in heat-nociception. Interestingly, while PIEZO1- and TRPV1- expressing neurons form essentially non-overlapping populations, PIEZO2 showed co-expression in both populations. Using an in vivo functional test for the selective expression, we found that Yoda1, a PIEZO1-specific agonist, induced a mechanical hyperalgesia that displayed a significantly prolonged time course compared with that induced by capsaicin, a TRPV1-specific agonist. Taken together, our results indicate that PIEZO1 should be considered a potential candidate in forming the long sought channel mediating mechano-nociception.

A surgical ankle sprain pain model in the rat: effects of morphine and indomethacin.

Ankle sprain is a frequent injury in humans that results in pain, swelling and difficulty in walking on the affected side. Currently a suitable animal model resembling human ankle sprain is lacking. Here, we describe an animal ankle sprain model induced by ankle ligament injury (ALI) in rats. Cutting combinations of the lateral ankle ligament complex produced pain, edema and difficulty of weight bearing, thereby mimicking severe (grade III) ankle sprain in humans. Analgesic compounds, morphine and indomethacin, significantly reversed the reduced weight bearing, thus indicating that reduction of weight bearing is partially due to pain. The ALI model is a new ankle sprain model that may be useful for the study of ankle sprain pain mechanisms and treatments, as well as for the screening of new analgesic drugs.

Phenyl N-t-butylnitrone, a reactive oxygen species scavenger, reduces zymosan-induced visceral pain in rats.

To examine a possible involvement of reactive oxygen species (ROS) in visceral pain, the levels of ROS in the colon and the effect of a ROS scavenger phenyl N-t-butylnitrone (PBN) on pain were examined in zymosan-induced colitis rats. Zymosan was instilled into the colon of adult rats. The electromyograms (EMGs) of abdominal muscle contractions in response to colorectal distension (CRD) were recorded as an indicator of visceral pain. After zymosan treatment, the rats showed enhanced EMG and elevated levels of H2O2 in the colon. PBN treatment (intraperitoneal, intrathecal or intracolonic) significantly reduced the enhanced EMGs induced by zymosan. The results suggest that elevated ROS in the spinal cord and the colon are involved in visceral pain.

An Energy-Efficient Wirelessly Powered Millimeter-Scale Neurostimulator Implant Based on Systematic Codesign of an Inductive Loop Antenna and a Custom Rectifier.

In this work, a switched-capacitor-based stimulator circuit that enables efficient energy harvesting for neurostimulation applications is presented, followed by the discussion on the optimization of the inductive coupling front-end through a codesign approach. The stimulator salvages input energy and stores it in a storage capacitor, and, when the voltage reaches a threshold, releases the energy as an output stimulus. The dynamics of the circuit are automatically enabled by a positive feedback, eliminating any stimulation control circuit blocks. The IC is fabricated in a 180 nm CMOS process and achieves a quiescent current consumption of 1.8 µA. The inductive coupling front-end is optimized as a loaded resonator, in which the input impedance of the custom rectifier directly loads the inductive loop antenna. The loaded quality factor and the rectifier's efficiency determine the reception sensitivity of the stimulator, while a balance should be achieved for the robustness of the system against dielectric medium variations by increasing the reception bandwidth. The finalized stimulator adopts a 4.9 mm × 4.9 mm inductive loop antenna and achieves an overall assembly dimension of 5 mm × 7.5 mm. Operating at the resonant frequency of 198 MHz, the stimulator works at a 14 cm distance from the transmitter in the air. An animal experiment was performed, in which a fully implanted stimulator excited the sciatic nerve of a rat that consequently triggered the movement of the limb.

Functional motoneurons develop from human neural stem cell transplants in adult rats.

We have shown previously that primed human fetal neural stem cells, after transplantation into rat spinal cords, differentiated into cholinergic motoneurons that sent axons to contact medial gastrocnemius myocytes. Here we demonstrate that (i) axons from the transplanted cells are cholinergic and myelinated, (ii) putative synapses form on transplanted somata and dendrites in the ventral horn, (iii) human fetal neural stem cells transplantation led to normal electromyograms from medial gastrocnemius muscles, and (iv) the gait of transplanted animals was much improved. Accumulatively, our data indicate that some transplanted human fetal neural stem cells in adult motoneuron-deficient ventral horns differentiate into relatively normal motoneurons that are integrated into spinal and peripheral circuitry. These findings are steps towards the long-term goal of providing stem cell transplants for motoneuron loss.

Reactive oxygen species affect spinal cell type-specific synaptic plasticity in a model of neuropathic pain.

Spinal synaptic plasticity is believed to drive central sensitization that underlies the persistent nature of neuropathic pain. Our recent data showed that synaptic plasticity in the dorsal horn is cell type specific: intense afferent stimulation produced long-term potentiation (LTP) in excitatory spinothalamic tract neurons (STTn), whereas it produced long-term depression (LTD) in inhibitory GABAergic interneurons (GABAn). In addition, reactive oxygen species (ROS) were shown to be involved in LTP in STTn (STTn-LTP) and in LTD in GABAn (GABAn-LTD). This study examined the roles of 2 biologically important ROS--superoxide [·O2] and hydroxyl radicals [·OH]--in neuropathic mechanical hyperalgesia and cell type-specific spinal synaptic plasticity. The [·O2] donor induced stronger mechanical hyperalgesia than the [·OH] donor in naive mice. The [·O2] scavenger showed greater antihyperalgesic effect than [·OH] scavengers in the spinal nerve ligation (SNL) mouse model of neuropathic pain. In addition, the [·O2] donor induced both STTn-LTP and GABAn-LTD, but the [·OH] donor induced only GABAn-LTD. On the other hand, the [·O2] scavenger inhibited STTn-LTP and GABAn-LTD induction in naive mice and alleviated SNL-induced potentiation in STTn and depression in GABAn. The [·OH] scavenger, however, inhibited depression in GABAn but did not interfere with potentiation in STTn. These results indicate that mechanical hyperalgesia in SNL mice is the result of the combination of STTn-LTP and GABAn-LTD. Behavioral outcomes compliment electrophysiological results which suggest that [·O2] mediates both STTn-LTP and GABAn-LTD, whereas [·OH] is involved primarily in GABAn-LTD.