Spinal cord injury-induced neurogenic bowel: A role for host-microbiome interactions in bowel pain and dysfunction.

Background and aims: Spinal cord injury (SCI) affects roughly 300,000 Americans with 17,000 new cases added annually. In addition to paralysis, 60% of people with SCI develop neurogenic bowel (NB), a syndrome characterized by slow colonic transit, constipation, and chronic abdominal pain. The knowledge gap surrounding NB mechanisms after SCI means that interventions are primarily symptom-focused and largely ineffective. The goal of the present studies was to identify mechanism(s) that initiate and maintain NB after SCI as a critical first step in the development of evidence-based, novel therapeutic treatment options. Methods: Following spinal contusion injury at T9, we observed alterations in bowel structure and function reflecting key clinical features of NB. We then leveraged tissue-specific whole transcriptome analyses (RNAseq) and fecal 16S rRNA amplicon sequencing in combination with histological, molecular, and functional (Ca2+ imaging) approaches to identify potential mechanism(s) underlying the generation of the NB phenotype. Results: In agreement with prior reports focused on SCI-induced changes in the skin, we observed a rapid and persistent increase in expression of calcitonin gene-related peptide (CGRP) expression in the colon. This is suggestive of a neurogenic inflammation-like process engaged by antidromic activity of below-level primary afferents following SCI. CGRP has been shown to disrupt colon homeostasis and negatively affect peristalsis and colon function. As predicted, contusion SCI resulted in increased colonic transit time, expansion of lymphatic nodules, colonic structural and genomic damage, and disruption of the inner, sterile intestinal mucus layer corresponding to increased CGRP expression in the colon. Gut microbiome colonization significantly shifted over 28 days leading to the increase in Anaeroplasma, a pathogenic, gram-negative microbe. Moreover, colon specific vagal afferents and enteric neurons were hyperresponsive after SCI to different agonists including fecal supernatants. Conclusions: Our data suggest that SCI results in overexpression of colonic CGRP which could alter colon structure and function. Neurogenic inflammatory-like processes and gut microbiome dysbiosis can also sensitize vagal afferents, providing a mechanism for visceral pain despite the loss of normal sensation post-SCI. These data may shed light on novel therapeutic interventions targeting this process to prevent NB development in patients.

Pharmacological and non-pharmacological therapeutic interventions for the treatment of spinal cord injury-induced pain.

Spinal cord injury (SCI) is a complex neurophysiological disorder, which can result in many long-term complications including changes in mobility, bowel and bladder function, cardiovascular function, and metabolism. In addition, most individuals with SCI experience some form of chronic pain, with one-third of these individuals rating their pain as severe and unrelenting. SCI-induced chronic pain is considered to be "high impact" and broadly affects a number of outcome measures, including daily activity, physical and cognitive function, mood, sleep, and overall quality of life. The majority of SCI pain patients suffer from pain that emanates from regions located below the level of injury. This pain is often rated as the most severe and the underlying mechanisms involve injury-induced plasticity along the entire neuraxis and within the peripheral nervous system. Unfortunately, current therapies for SCI-induced chronic pain lack universal efficacy. Pharmacological treatments, such as opioids, anticonvulsants, and antidepressants, have been shown to have limited success in promoting pain relief. In addition, these treatments are accompanied by many adverse events and safety issues that compound existing functional deficits in the spinally injured, such as gastrointestinal motility and respiration. Non-pharmacological treatments are safer alternatives that can be specifically tailored to the individual and used in tandem with pharmacological therapies if needed. This review describes existing non-pharmacological therapies that have been used to treat SCI-induced pain in both preclinical models and clinical populations. These include physical (i.e., exercise, acupuncture, and hyper- or hypothermia treatments), psychological (i.e., meditation and cognitive behavioral therapy), and dietary interventions (i.e., ketogenic and anti-inflammatory diet). Findings on the effectiveness of these interventions in reducing SCI-induced pain and improving quality of life are discussed. Overall, although studies suggest non-pharmacological treatments could be beneficial in reducing SCI-induced chronic pain, further research is needed. Additionally, because chronic pain, including SCI pain, is complex and has both emotional and physiological components, treatment should be multidisciplinary in nature and ideally tailored specifically to the patient.

Comprehensive phenotyping of cutaneous afferents reveals early-onset alterations in nociceptor response properties, release of CGRP, and hindpaw edema following spinal cord injury.

Spinal cord injury (SCI) is a complex syndrome that has profound effects on patient well-being, including the development of medically-resistant chronic pain. The mechanisms underlying SCI pain have been the subject of thorough investigation but remain poorly understood. While the majority of the research has focused on changes occurring within and surrounding the site of injury in the spinal cord, there is now a consensus that alterations within the peripheral nervous system, namely sensitization of nociceptors, contribute to the development and maintenance of chronic SCI pain. Using an ex vivo skin/nerve/DRG/spinal cord preparation to characterize afferent response properties following SCI, we found that SCI increased mechanical and thermal responding, as well as the incidence of spontaneous activity (SA) and afterdischarge (AD), in below-level C-fiber nociceptors 24 hr following injury relative to naïve controls. Interestingly, the distribution of nociceptors that exhibit SA and AD are not identical, and the development of SA was observed more frequently in nociceptors with low heat thresholds, while AD was found more frequently in nociceptors with high heat thresholds. We also found that SCI resulted in hindpaw edema and elevated cutaneous calcitonin gene-related peptide (CGRP) concentration that were not observed in naïve mice. These results suggest that SCI causes a rapidly developing nociceptor sensitization and peripheral inflammation that may contribute to the early emergence and persistence of chronic SCI pain.

Identifying genetic determinants of inflammatory pain in mice using a large-scale gene-targeted screen.

ABSTRACT: Identifying the genetic determinants of pain is a scientific imperative given the magnitude of the global health burden that pain causes. Here, we report a genetic screen for nociception, performed under the auspices of the International Mouse Phenotyping Consortium. A biased set of 110 single-gene knockout mouse strains was screened for 1 or more nociception and hypersensitivity assays, including chemical nociception (formalin) and mechanical and thermal nociception (von Frey filaments and Hargreaves tests, respectively), with or without an inflammatory agent (complete Freund's adjuvant). We identified 13 single-gene knockout strains with altered nocifensive behavior in 1 or more assays. All these novel mouse models are openly available to the scientific community to study gene function. Two of the 13 genes (Gria1 and Htr3a) have been previously reported with nociception-related phenotypes in genetically engineered mouse strains and represent useful benchmarking standards. One of the 13 genes (Cnrip1) is known from human studies to play a role in pain modulation and the knockout mouse reported herein can be used to explore this function further. The remaining 10 genes (Abhd13, Alg6, BC048562, Cgnl1, Cp, Mmp16, Oxa1l, Tecpr2, Trim14, and Trim2) reveal novel pathways involved in nociception and may provide new knowledge to better understand genetic mechanisms of inflammatory pain and to serve as models for therapeutic target validation and drug development.

MrgprdCre lineage neurons mediate optogenetic allodynia through an emergent polysynaptic circuit.

ABSTRACT: Most cutaneous C fibers, including both peptidergic and nonpeptidergic subtypes, are presumed to be nociceptors and respond to noxious input in a graded manner. However, mechanically sensitive, nonpeptidergic C fibers also respond to mechanical input in the innocuous range, so the degree to which they contribute to nociception remains unclear. To address this gap, we investigated the function of nonpeptidergic afferents using the MrgprdCre allele. In real-time place aversion studies, we found that low-frequency optogenetic activation of MrgrpdCre lineage neurons was not aversive in naive mice but became aversive after spared nerve injury (SNI). To address the underlying mechanisms of this allodynia, we recorded responses from lamina I spinoparabrachial (SPB) neurons using the semi-intact ex vivo preparation. After SNI, innocuous brushing of the skin gave rise to abnormal activity in lamina I SPB neurons, consisting of an increase in the proportion of recorded neurons that responded with excitatory postsynaptic potentials or action potentials. This increase was likely due, at least in part, to an increase in the proportion of lamina I SPB neurons that received input on optogenetic activation of MrgprdCre lineage neurons. Intriguingly, in SPB neurons, there was a significant increase in the excitatory postsynaptic current latency from MrgprdCre lineage input after SNI, consistent with the possibility that the greater activation post-SNI could be due to the recruitment of a new polysynaptic circuit. Together, our findings suggest that MrgprdCre lineage neurons can provide mechanical input to the dorsal horn that is nonnoxious before injury but becomes noxious afterwards because of the engagement of a previously silent polysynaptic circuit in the dorsal horn.

A Role for Global DNA Methylation Level and IL2 Expression in the Transition From Acute to Chronic Low Back Pain.

Objectives: The transition from acute low back pain (aLBP) to chronic LBP (cLBP) results from a variety of factors, including epigenetic modifications of DNA. The aim of this study was to (1) compare global DNA (gDNA) methylation and histone acetylation at LBP onset between the aLBP and cLBP participants, (2) compare mRNA expression of genes with known roles in the transduction, maintenance, and/or modulation of pain between the aLBP and cLBP participants, (3) compare somatosensory function and pain ratings in our participants, and (4) determine if the aforementioned measurements were associated. Methods: A total of 220 participants were recruited for this prospective observational study following recent onset of an episode of LBP. We retained 45 individuals whose gDNA was of sufficient quality for analysis. The final sample included 14 participants whose pain resolved within 6 weeks of onset (aLBP),15 participants that reported pain for 6 months (cLBP), and 16 healthy controls. Participants were subjected to quantitative sensory testing (QST), blood was drawn via venipuncture, gDNA isolated, and global DNA methylation and histone acetylation, as well as mRNA expression of 84 candidate genes, were measured. Results: Individuals that develop cLBP display multimodal somatosensory hypersensitivity relative to aLBP participants. cLBP participants also had significantly lower global DNA methylation, which was negatively correlated with interleukin-2 (IL2) mRNA expression. Discussion: cLBP is characterized by somatosensory hypersensitivity, lower global DNA methylation, and higher IL2 expression level compared to those whose pain will resolve quickly (aLBP). These results suggest potential diagnostic and therapeutic relevance for global DNA methylation and IL2 expression in the pathology underlying the transition from acute to chronic LBP.

Contribution of COMT and BDNF Genotype and Expression to the Risk of Transition From Acute to Chronic Low Back Pain.

OBJECTIVES: A number of factors, including heritability and the environment, contribute to risk of transition from acute low back pain to chronic low back pain (CLBP). The aim of this study was to (1) compare somatosensory function and pain ratings at low back pain (LBP) onset between the acute low back pain and CLBP conditions and (2) evaluate associations between BDNF and COMT polymorphisms and expression levels at LBP onset to acute and chronic pain burden and risk for transition to the chronic pain state. METHODS: In this longitudinal study, 220 participants were enrolled following recent onset of LBP and data were collected until the LBP resolved or until the end of the study at 6 months. Forty-two participants' pain resolved before 6 weeks from onset and 42 participants continued to have pain at 6 months. Patient-reported pain burden, somatosensory function (quantitative sensory testing), and blood samples were collected at each study visit. RESULTS: CLBP is associated with greater pain burden and somatosensory hypersensitivity at the time of LBP onset. COMT rs4680 genotype (GG) was associated with acute cold pain sensitivity and with the risk for transition to CLBP while COMT expression was independently associated with risk for transition. DISCUSSION: CLBP was characterized by higher reported pain burden and augmented hypersensitivity at LBP onset. COMT expression and genotype were associated with acute pain burden and likelihood of transition to CLBP.

TIMP-1 Attenuates the Development of Inflammatory Pain Through MMP-Dependent and Receptor-Mediated Cell Signaling Mechanisms.

Unresolved inflammation is a significant predictor for developing chronic pain, and targeting the mechanisms underlying inflammation offers opportunities for therapeutic intervention. During inflammation, matrix metalloproteinase (MMP) activity contributes to tissue remodeling and inflammatory signaling, and is regulated by tissue inhibitors of metalloproteinases (TIMPs). TIMP-1 and -2 have known roles in pain, but only in the context of MMP inhibition. However, TIMP-1 also has receptor-mediated cell signaling functions that are not well understood. Here, we examined how TIMP-1-dependent cell signaling impacts inflammatory hypersensitivity and ongoing pain. We found that hindpaw injection of complete Freund's adjuvant (CFA) increased cutaneous TIMP-1 expression that peaked prior to development of mechanical hypersensitivity, suggesting that TIMP-1 inhibits the development of inflammatory hypersensitivity. To examine this possibility, we injected TIMP-1 knockout (T1KO) mice with CFA and found that T1KO mice exhibited rapid onset thermal and mechanical hypersensitivity at the site of inflammation that was absent or attenuated in WT controls. We also found that T1KO mice exhibited hypersensitivity in adjacent tissues innervated by different sets of afferents, as well as skin contralateral to the site of inflammation. Replacement of recombinant murine (rm)TIMP-1 alleviated hypersensitivity when administered at the site and time of inflammation. Administration of either the MMP inhibiting N-terminal or the cell signaling C-terminal domains recapitulated the antinociceptive effect of full-length rmTIMP-1, suggesting that rmTIMP-1inhibits hypersensitivity through MMP inhibition and receptor-mediated cell signaling. We also found that hypersensitivity was not due to genotype-specific differences in MMP-9 activity or expression, nor to differences in cytokine expression. Administration of rmTIMP-1 prevented mechanical hypersensitivity and ongoing pain in WT mice, collectively suggesting a novel role for TIMP-1 in the attenuation of inflammatory pain.

Injectable nanocomposite analgesic delivery system for musculoskeletal pain management.

Musculoskeletal pain is a major health issue which results from surgical procedures (i.e. total knee and/or hip replacements and rotator cuff repairs), as well as from non-surgical conditions (i.e. sympathetically-mediated pain syndrome and occipital neuralgia). Local anesthetics, opioids or corticosteroids are currently used for the pain management of musculoskeletal conditions. Even though local anesthetics are highly preferred, the need for multiple administration presents significant disadvantages. Development of unique delivery systems that can deliver local anesthetics at the injection site for prolonged time could significantly enhance the therapeutic efficacy and patient comfort. The goal of the present study is to evaluate the efficacy of an injectable local anesthetic nanocomposite carrier to provide sustained analgesic effect. The nanocomposite carrier was developed by encapsulating ropivacaine, a local anesthetic, in lipid nanocapsules (LNC-Rop), and incorporating the nanocapsules in enzymatically crosslinked glycol chitosan (0.3GC) hydrogels. Cryo Scanning Electron Microscopic (Cryo SEM) images showed the ability to distribute the LNCs within the hydrogel without adversely affecting their morphology. The study demonstrated the feasibility to achieve sustained release of lipophilic molecules from the nanocomposite carrier in vitro and in vivo. A rat chronic constriction injury (CCI) pain model was used to evaluate the efficacy of the nanocomposite carrier using thermal paw withdrawal latency (TWL). The nanocomposite carriers loaded with ropivacaine and dexamethasone showed significant improvement in pain response compared to the control groups for at least 7 days. The study demonstrated the clinical potential of these nanocomposite carriers for post-operative and neuropathic pain. STATEMENT OF SIGNIFICANCE: Acute or chronic pain associated with musculoskeletal conditions is considered a major health issue, with healthcare costs totaling several billion dollars. The opioid crisis presents a pressing clinical need to develop alternative and effective approaches to treat musculoskeletal pain. The goal of this study was to develop a long-acting injectable anesthetic formulation which can sustain a local anesthetic effect for a prolonged time. This in turn could increase the quality of life and rehabilitation outcome of patients, and decrease opioid consumption. The developed injectable nanocomposite demonstrated the feasibility to achieve prolonged pain relief in a rat chronic constriction injury (CCI) model.

Neurobiological Consequences of Early Painful Experience: Basic Science Findings and Implications for Evidence-Based Practice.

As healthcare teams have worked to improve infant survival rates, the management of painful events experienced by these hospitalized neonates has increased and yet pain management remains highly variable between healthcare institutions. At the same time, emerging evidence suggests that these early painful experiences may alter the trajectory of development for pain-processing pathways both peripherally and centrally. This concise review highlights findings from both the basic and clinical science literature supporting the hypothesis that early painful experiences can have long-lasting negative effects on biological, psychological, and socioemotional functions. Implications for pain management in neonates and considerations for evidence-based practice change are discussed.

Variations in COMT and NTRK2 Influence Symptom Burden in Women Undergoing Breast Cancer Treatment.

Women with breast cancer frequently report distressing symptoms during and after treatment that can significantly erode quality of life (QOL). Symptom burden among women with breast cancer is of complex etiology and is likely influenced by disease, treatment, and environmental factors as well as individual genetic differences. The purpose of the present study was to examine the relationships between genetic polymorphisms within Neurotrophic tyrosine kinase receptor 1 (NTRK1), Neurotrophic tyrosine kinase receptor 2 (NTRK2), and catechol-O-methyltransferase ( COMT) and patient symptom burden of QOL, pain, fatigue, anxiety, depression, and sleep disturbance before, during, and after treatment for breast cancer in a subset of participants ( N = 51) in a randomized clinical trial of a novel symptom-management modality for women with breast cancer undergoing chemotherapy. Patients were recruited at the time of initial breast cancer diagnosis and completed all survey measures at the time of recruitment, after the initiation of treatment (surgery and/or chemotherapy), and then following treatment conclusion. Multiple linear regression analyses revealed significant associations between NTRK2 and COMT single nucleotide polymorphism (SNP) genotype and symptom burden. Two COMT variants were associated with the specific symptoms of anxiety and QOL measures prior to the initiation of chemotherapy as well as pain interference and severity during and after treatment. Genotype at the NTRK2 SNP rs1212171 was associated with both sleep disturbance and fatigue. These findings, while exploratory, indicate that the genotypes of NTRK2 and COMT may contribute to relative risk for symptom burden during and shortly after the period of chemotherapy in women with early stage breast cancer.

Fixed spaced stimulation restores adaptive plasticity within the spinal cord: Identifying the eliciting conditions.

Prior work has shown that neurons within the spinal cord are sensitive to temporal relations and that stimulus regularity impacts nociceptive processing and adaptive plasticity. Application of brief (80ms) shocks (180-900) in a variable manner induces a form of maladaptive plasticity that inhibits spinally-mediated learning and enhances nociceptive reactivity. In contrast, an extended exposure (720-900) to stimuli given at regular (fixed spaced) intervals has a restorative effect that counters nociceptive sensitization and enables learning. The present paper explores the stimulus parameters under which this therapeutic effect of fixed spaced stimulation emerges. Spinally transected rats received variably spaced stimulation (180 shocks) to the sciatic nerve at an intensity (40-V) that recruits pain (C) fibers, producing a form of maladaptive plasticity that impairs spinal learning. As previously shown, exposure to 720 fixed spaced shocks had a therapeutic effect that restored adaptive learning. This therapeutic effect was most robust at a lower shock intensity (20V) and was equally strong irrespective of pulse duration (20-80ms). A restorative effect was observed when stimuli were given at a frequency between 0.5 and 5Hz, but not at a higher (50Hz) or lower (0.05Hz) rate. The results are consistent with prior work implicating neural systems related to the central pattern generator that drives stepping behavior. Clinical implications are discussed.

Cutaneous neurturin overexpression alters mechanical, thermal, and cold responsiveness in physiologically identified primary afferents.

Neurotrophic factors play an important role in the regulation of functional properties of sensory neurons under normal and pathological conditions. The GDNF family member neurturin is one such factor that has been linked to modulating responsiveness to peripheral stimuli. Neurturin binds to the GFRα2 receptor, a receptor found primarily in isolectin B4-expressing polymodal cutaneous nociceptors. Previous work has shown that knockout of GFRα2 alters heat, but not mechanical, responses in dissociated sensory neurons and reduces pain-related behaviors during the second phase of the formalin test. Research has also shown that overexpression of neurturin in basal keratinocytes increases behavioral responsiveness to mechanical stimulation and innocuous cooling of the skin without affecting noxious heat responses. Here we directly examined the impact of neurturin overexpression on cutaneous afferent function. We compared physiological responses of individual sensory neurons to mechanical and thermal stimulation of the skin, using an ex vivo skin-nerve-dorsal root ganglion-spinal cord preparation produced from neurturin-overexpressing (NRTN/OE) mice and wild-type littermate controls. We found that neurturin overexpression increases responsiveness to innocuous mechanical stimuli in A-fiber nociceptors, alters thermal responses in the polymodal subpopulation of C-fiber sensory neurons, and changes the relative numbers of mechanically sensitive but thermally insensitive C-fiber afferents. These results demonstrate the potential roles of different functional groups of sensory neurons in the behavioral changes observed in mice overexpressing cutaneous neurturin and highlight the importance of neurturin in regulating cutaneous afferent response properties.NEW & NOTEWORTHY GDNF family neurotrophic factors regulate the development and function of primary sensory neurons. Of these, neurturin has been shown to modulate mechanical and cooling sensitivity behaviorally. Here we show that overexpression of neurturin in basal keratinocytes regulates mechanical responsiveness in A-fiber primary sensory neurons while increasing the overall numbers of cold-sensing units. Results demonstrate a crucial role for cutaneous neurturin in modulating responsiveness to peripheral stimuli at the level of the primary afferent.

Semi-intact ex vivo approach to investigate spinal somatosensory circuits.

The somatosensory input that gives rise to the perceptions of pain, itch, cold and heat are initially integrated in the superficial dorsal horn of the spinal cord. Here, we describe a new approach to investigate these neural circuits in mouse. This semi-intact somatosensory preparation enables recording from spinal output neurons, while precisely controlling somatosensory input, and simultaneously manipulating specific populations of spinal interneurons. Our findings suggest that spinal interneurons show distinct temporal and spatial tuning properties. We also show that modality selectivity - mechanical, heat and cold - can be assessed in both retrogradely labeled spinoparabrachial projection neurons and genetically labeled spinal interneurons. Finally, we demonstrate that interneuron connectivity can be determined via optogenetic activation of specific interneuron subtypes. This new approach may facilitate key conceptual advances in our understanding of the spinal somatosensory circuits in health and disease.

Deletion of the murine ATP/UTP receptor P2Y2 alters mechanical and thermal response properties in polymodal cutaneous afferents.

P2Y2 is a member of the P2Y family of G protein-coupled nucleotide receptors that is widely co-expressed with TRPV1 in peripheral sensory neurons of the dorsal root ganglia. To characterize P2Y2 function in cutaneous afferents, intracellular recordings from mouse sensory neurons were made using an ex vivo preparation in which hindlimb skin, saphenous nerve, dorsal root ganglia and spinal cord are dissected intact. The peripheral response properties of individual cutaneous C-fibers were analyzed using digitally controlled mechanical and thermal stimuli in male P2Y2(+/+) and P2Y2(-/-) mice. Selected sensory neurons were labeled with Neurobiotin and further characterized by immunohistochemistry. In wildtype preparations, C-fibers responding to both mechanical and thermal stimuli (CMH or CMHC) preferentially bound the lectin marker IB4 and were always immunonegative for TRPV1. Conversely, cells that fired robustly to noxious heat, but were insensitive to mechanical stimuli, were TRPV1-positive and IB4-negative. P2Y2 gene deletion resulted in reduced firing by TRPV1-negative CMH fibers to a range of heat stimuli. However, we also identified an atypical population of IB4-negative, TRPV1-positive CMH fibers. Compared to wildtype CMH fibers, these TRPV1-positive neurons exhibited lower firing rates in response to mechanical stimulation, but had increased firing to noxious heat (43-51°C). Collectively, these results demonstrate that P2Y2 contributes to response properties of cutaneous afferents, as P2Y2 deletion reduces responsiveness of conventional unmyelinated polymodal afferents to heat and appears to result in the acquisition of mechanical responsiveness in a subset of TRPV1-expressing afferents.

Managing Chronic Pain in Special Populations with Emphasis on Pediatric, Geriatric, and Drug Abuser Populations.

In the adult population chronic pain can lead to loss of productivity and earning potential, and decreased quality of life. There are distinct groups with increased vulnerability for the emergence of chronic pain. These groups may be defined by developmental status and/or life circumstances. Within the pediatric, geriatric, and drug abuser populations, chronic pain represents a significant health issue. This article focuses on known anatomic, physiologic, and genetic mechanisms underlying chronic pain in these populations, and highlights the need for a multimodal approach from multiple health care professionals for management of chronic pain in those with the most risk.

Learning about time within the spinal cord: evidence that spinal neurons can abstract and store an index of regularity.

Prior studies have shown that intermittent noxious stimulation has divergent effects on spinal cord plasticity depending upon whether it occurs in a regular (fixed time, FT) or irregular (variable time, VT) manner: In spinally transected animals, VT stimulation to the tail or hind leg impaired spinal learning whereas an extended exposure to FT stimulation had a restorative/protective effect. These observations imply that lower level systems are sensitive to temporal relations. Using spinally transected rats, it is shown that the restorative effect of FT stimulation emerges after 540 shocks; fewer shocks generate a learning impairment. The transformative effect of FT stimulation is related to the number of shocks administered, not the duration of exposure. Administration of 360 FT shocks induces a learning deficit that lasts 24 h. If a second bout of FT stimulation is given a day after the first, it restores the capacity to learn. This savings effect implies that the initial training episode had a lasting (memory-like) effect. Two bouts of shock have a transformative effect when applied at different locations or at difference frequencies, implying spinal systems abstract and store an index of regularity (rather than a specific interval). Implications of the results for step training and rehabilitation after injury are discussed.

Keratinocytes can modulate and directly initiate nociceptive responses.

How thermal, mechanical and chemical stimuli applied to the skin are transduced into signals transmitted by peripheral neurons to the CNS is an area of intense study. Several studies indicate that transduction mechanisms are intrinsic to cutaneous neurons and that epidermal keratinocytes only modulate this transduction. Using mice expressing channelrhodopsin (ChR2) in keratinocytes we show that blue light activation of the epidermis alone can produce action potentials (APs) in multiple types of cutaneous sensory neurons including SA1, A-HTMR, CM, CH, CMC, CMH and CMHC fiber types. In loss of function studies, yellow light stimulation of keratinocytes that express halorhodopsin reduced AP generation in response to naturalistic stimuli. These findings support the idea that intrinsic sensory transduction mechanisms in epidermal keratinocytes can directly elicit AP firing in nociceptive as well as tactile sensory afferents and suggest a significantly expanded role for the epidermis in sensory processing.

Metaplasticity and behavior: how training and inflammation affect plastic potential within the spinal cord and recovery after injury.

Research has shown that spinal circuits have the capacity to adapt in response to training, nociceptive stimulation and peripheral inflammation. These changes in neural function are mediated by physiological and neurochemical systems analogous to those that support plasticity within the hippocampus (e.g., long-term potentiation and the NMDA receptor). As observed in the hippocampus, engaging spinal circuits can have a lasting impact on plastic potential, enabling or inhibiting the capacity to learn. These effects are related to the concept of metaplasticity. Behavioral paradigms are described that induce metaplastic effects within the spinal cord. Uncontrollable/unpredictable stimulation, and peripheral inflammation, induce a form of maladaptive plasticity that inhibits spinal learning. Conversely, exposure to controllable or predictable stimulation engages a form of adaptive plasticity that counters these maladaptive effects and enables learning. Adaptive plasticity is tied to an up-regulation of brain derived neurotrophic factor (BDNF). Maladaptive plasticity is linked to processes that involve kappa opioids, the metabotropic glutamate (mGlu) receptor, glia, and the cytokine tumor necrosis factor (TNF). Uncontrollable nociceptive stimulation also impairs recovery after a spinal contusion injury and fosters the development of pain (allodynia). These adverse effects are related to an up-regulation of TNF and a down-regulation of BDNF and its receptor (TrkB). In the absence of injury, brain systems quell the sensitization of spinal circuits through descending serotonergic fibers and the serotonin 1A (5HT 1A) receptor. This protective effect is blocked by surgical anesthesia. Disconnected from the brain, intracellular Cl(-) concentrations increase (due to a down-regulation of the cotransporter KCC2), which causes GABA to have an excitatory effect. It is suggested that BDNF has a restorative effect because it up-regulates KCC2 and re-establishes GABA-mediated inhibition.