Engraftment of serotonergic precursors enhances locomotor function and attenuates chronic central pain behavior following spinal hemisection injury in the rat

Hydralazine plays an immunomodulation role of pro-regeneration in a mouse model of spinal cord injury

Spinal cord injury (SCI) results in severe motor and sensory dysfunction with no effective therapy. Spinal cord debris (sp) from injured spinal cord evokes secondary SCI continuously. We and other researchers have previously clarified that it is mainly bone marrow derived macrophages (BMDMs) infiltrating in the lesion epicenter to clear sp, rather than local microglia. Unfortunately, the pro-inflammatory phenotype of these infiltrating BMDMs is predominant which impairs wound healing. Hydralazine, as a potent vasodilator and scavenger of acrolein, has protective effects in many diseases. Hydralazine is also confirmed to promote motor function and hypersensitivity in SCI rats through scavenging acrolein. However, few studies have explored the effects of hydralazine on immunomodulation, as well as spontaneous pain and emotional response, the important syndromes in clinical patients. It remains unclear whether hydralazine affects infiltrating BMDMs after SCI. In this study, we targeted BMDMs to explore the influence of hydralazine on immune cells in a mouse model of SCI, and also investigated the contribution of polarized BMDMs to hydralazine-induced neurological function recovery after SCI in male mice. The adult male mice underwent T10 spinal cord compression. The results showed that in addition to improving motor function and hypersensitivity, hydralazine relieved SCI-induced spontaneous pain and emotional response, which is a newly discovered function of hydralazine. Hydralazine inhibited the recruitments of pro-inflammatory BMDMs and educated infiltrated BMDMs to a more reparative phenotype involving in multiple biological processes associated with SCI pathology, including immune/inflammation response, neurogenesis, lipid metabolism, oxidative stress, fibrosis formation, and angiogenesis, etc. As an overall effect, hydralazine-treated BMDMs loaden with sp partially rescued neurological function after SCI. It is concluded that hydralazine plays an immunomodulation role of educating pro-inflammatory BMDMs to a more reparative phenotype; and hydralazine-educated BMDMs contribute to hydralazine-induced improvement of neurological function in SCI mice, which provides support for drug and cell treatment options for SCI therapy.

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

The Effects of a Ketogenic Diet on Sensorimotor Function in a Thoracolumbar Mouse Spinal Cord Injury Model

Spinal cord injury and peripheral nerve injuries are traumatic events that greatly impact quality of life. One factor that is being explored throughout patient care is the idea of diet and the role it has on patient outcomes. But the effects of diet following neurotrauma need to be carefully explored in animal models to ensure that they have beneficial effects. The ketogenic diet provides sufficient daily caloric requirements while being potentially neuroprotective and analgesic. In this study, animals were fed a high-fat, low-carbohydrate diet that led to a high concentration of blood ketone that was sustained for as long as the animals were on the diet. Mice fed a ketogenic diet had significantly lower levels of tyrosine and tryptophan, but the levels of other monoamines within the spinal cord remained similar to those of control mice. Mice were fed a standard or ketogenic diet for 7 d before and 28 d following the injury. Our results show that mice hemisected over the T10–T11 vertebrae showed no beneficial effects of being on a ketogenic diet over a 28 d recovery period. Similarly, ligation of the common peroneal and tibial nerve showed no differences between mice fed normal or ketogenic diets. Tests included von Frey, open field, and ladder-rung crossing. We add to existing literature showing protective effects of the ketogenic diet in forelimb injuries by focusing on neurotrauma in the hindlimbs. The results suggest that ketogenic diets need to be assessed based on the type and location of neurotrauma.

Regional Hyperexcitability and Chronic Neuropathic Pain Following Spinal Cord Injury

Spinal cord injury (SCI) causes maladaptive changes to nociceptive synaptic circuits within the injured spinal cord. Changes also occur at remote regions including the brain stem, limbic system, cortex, and dorsal root ganglia. These maladaptive nociceptive synaptic circuits frequently cause neuronal hyperexcitability in the entire nervous system and enhance nociceptive transmission, resulting in chronic central neuropathic pain following SCI. The underlying mechanism of chronic neuropathic pain depends on the neuroanatomical structures and electrochemical communication between pre- and postsynaptic neuronal membranes, and propagation of synaptic transmission in the ascending pain pathways. In the nervous system, neurons are the only cell type that transmits nociceptive signals from peripheral receptors to supraspinal systems due to their neuroanatomical and electrophysiological properties. However, the entire range of nociceptive signaling is not mediated by any single neuron. Current literature describes regional studies of electrophysiological or neurochemical mechanisms for enhanced nociceptive transmission post-SCI, but few studies report the electrophysiological, neurochemical, and neuroanatomical changes across the entire nervous system following a regional SCI. We, along with others, have continuously described the enhanced nociceptive transmission in the spinal dorsal horn, brain stem, thalamus, and cortex in SCI-induced chronic central neuropathic pain condition, respectively. Thus, this review summarizes the current understanding of SCI-induced neuronal hyperexcitability and maladaptive nociceptive transmission in the entire nervous system that contributes to chronic central neuropathic pain.

Grafting Embryonic Raphe Neurons Reestablishes Serotonergic Regulation of Sympathetic Activity to Improve Cardiovascular Function after Spinal Cord Injury

Cardiovascular dysfunction often occurs after high-level spinal cord injury. Disrupting supraspinal vasomotor pathways affects basal hemodynamics and contributes to the development of autonomic dysreflexia (AD). Transplantation of early-stage neurons to the injured cord may reconstruct the descending projections to enhance cardiovascular performance. To determine the specific role of reestablishing serotonergic regulation of hemodynamics, we implanted serotonergic (5-HT⁺) neuron-enriched embryonic raphe nucleus-derived neural stem cells/progenitors (RN-NSCs) into a complete spinal cord transection lesion site in adult female rats. Grafting embryonic spinal cord-derived NSCs or injury alone served as 2 controls. Ten weeks after injury/grafting, histological analysis revealed well-survived grafts and partial integration with host tissues in the lesion site. Numerous graft-derived serotonergic axons topographically projected to the caudal autonomic regions. Neuronal tracing showed that host supraspinal vasomotor pathways regenerated into the graft, and 5-HT⁺ neurons within graft and host brainstem neurons were transsynaptically labeled by injecting pseudorabies virus (PRV-614) into the kidney, indicating reconnected serotonergic circuits regulating autonomic activity. Using an implanted telemeter to record cardiovascular parameters, grafting RN-NSCs restored resting mean arterial pressure to normal levels and remarkably alleviated naturally occurring and colorectal distension-induced AD. Subsequent pharmacological blockade of 5-HT_2A receptors with ketanserin in RN-NSC-grafted rats reduced resting mean arterial pressure and increased heart rate in all but 2 controls. Furthermore, spinal cord retransection below RN-NSC grafts partially eliminated the recovery in AD. Collectively, these data indicate that RN-NSCs grafted into a spinal cord injury site relay supraspinal control of serotonergic regulation for sympathetic activity to improve cardiovascular function.SIGNIFICANCE STATEMENT Disruption of supraspinal vasomotor pathways results in cardiovascular dysfunction following high-level spinal cord injury. To reestablish the descending regulation of autonomic function, we transplanted serotonergic neuron enriched embryonic raphe nucleus-derived neural stem cells/progenitors into the lesion site of completely transected rat spinal cord. Consequently, grafted raphe nucleus-derived neural stem cells/progenitors acted as a neuronal relay to reconnect supraspinal center and spinal sympathetic neurons below the injury. The reconstituted serotonergic regulation of sympathetic activity led to the improvement of hemodynamic parameters and mitigated autonomic dysreflexia. Based on morphological and physiological results, this study validates the effectiveness of transplanting early-stage serotonergic neurons into the spinal cord for cardiovascular functional recovery after spinal cord injury.

Serotonergic mechanisms in spinal cord injury

Spinal cord injury (SCI) is a tragic event causing irreversible losses of sensory, motor, and autonomic functions, that may also be associated with chronic neuropathic pain. Serotonin (5-HT) neurotransmission in the spinal cord is critical for modulating sensory, motor, and autonomic functions. Following SCI, 5-HT axons caudal to the lesion site degenerate, and the degree of axonal degeneration positively correlates with lesion severity. Rostral to the lesion, 5-HT axons sprout, irrespective of the severity of the injury. Unlike callosal fibers and cholinergic projections, 5-HT axons are more resistant to an inhibitory milieu and undergo active sprouting and regeneration after central nervous system (CNS) traumatism. Numerous studies suggest that a chronic increase in serotonergic neurotransmission promotes 5-HT axon sprouting in the intact CNS. Moreover, recent studies in invertebrates suggest that 5-HT has a pro-regenerative role in injured axons. Here we present a brief description of 5-HT discovery, 5-HT innervation of the CNS, and physiological functions of 5-HT in the spinal cord, including its role in controlling bladder function. We then present a comprehensive overview of changes in serotonergic axons after CNS damage, and discuss their plasticity upon altered 5-HT neurotransmitter levels. Subsequently, we provide an in-depth review of therapeutic approaches targeting 5-HT neurotransmission, as well as other pre-clinical strategies to promote an increase in re-growth of 5-HT axons, and their functional consequences in SCI animal models. Finally, we highlight recent findings signifying the direct role of 5-HT in axon regeneration and suggest strategies to further promote robust long-distance re-growth of 5-HT axons across the lesion site and eventually achieve functional recovery following SCI.

Longitudinal Magnetic Resonance Imaging Analysis and Histological Characterization after Spinal Cord Injury in Two Mouse Strains with Different Functional Recovery: Gliosis as a Key Factor

Spinal cord injuries (SCI) are disastrous neuropathologies causing permanent disabilities. The availability of different strains of mice is valuable for studying the pathophysiological mechanisms involved in SCI. However, strain differences have a profound effect on spontaneous functional recovery after SCI. CX3CR1^+/eGFP and Aldh1l1-EGFP mice that express green fluorescent protein in microglia/monocytes and astrocytes, respectively, are particularly useful to study glial reactivity. Whereas CX3CR1^+/eGFP mice have C57BL/6 background, Aldh1l1-EGFP are in Swiss Webster background. We first assessed spontaneous functional recovery in CX3CR1^+/eGFP and Aldh1l1-EGFP mice over 6 weeks after lateral spinal cord hemisection. Second, we carried out a longitudinal follow-up of lesion evolution using in vivo T2-weighted magnetic resonance imaging (MRI). Finally, we performed in-depth analysis of the spinal cord tissue using ex vivo T2-weighted MRI as well as detailed histology. We demonstrate that CX3CR1^+/eGFP mice have improved functional recovery and reduced anxiety after SCI compared with Aldh1l1-EGFP mice. We also found a strong correlation between in vivo MRI, ex vivo MRI, and histological analyses of the injured spinal cord in both strain of mice. All three modalities revealed no difference in lesion extension and volume between the two strains of mice. Importantly, histopathological analysis identified decreased gliosis and increased serotonergic axons in CX3CR1^+/eGFP compared with Aldh1l1-EGFP mice following SCI. These results thus suggest that the strain-dependent improved functional recovery after SCI may be linked with reduced gliosis and increased serotonergic innervation.

Metaplasticity within the spinal cord: Evidence brain-derived neurotrophic factor (BDNF), tumor necrosis factor (TNF), and alterations in GABA function (ionic plasticity) modulate pain and the capacity to learn

Evidence is reviewed that behavioral training and neural injury can engage metaplastic processes that regulate adaptive potential. This issue is explored within a model system that examines how training affects the capacity to learn within the lower (lumbosacral) spinal cord. Response-contingent (controllable) stimulation applied caudal to a spinal transection induces a behavioral modification indicative of learning. This behavioral change is not observed in animals that receive stimulation in an uncontrollable manner. Exposure to uncontrollable stimulation also engages a process that disables spinal learning for 24-48 h. Controllable stimulation has the opposite effect; it engages a process that enables learning and prevents/reverses the learning deficit induced by uncontrollable stimulation. These observations suggest that a learning episode can impact the capacity to learn in future situations, providing an example of behavioral metaplasticity. The protective/restorative effect of controllable stimulation has been linked to an up-regulation of brain-derived neurotrophic factor (BDNF). The disruption of learning has been linked to the sensitization of pain (nociceptive) circuits, which is enabled by a reduction in GABA-dependent inhibition. After spinal cord injury (SCI), the co-transporter (KCC2) that regulates the outward flow of Cl^- is down-regulated. This causes the intracellular concentration of Cl^- to increase, reducing (and potentially reversing) the inward flow of Cl^- through the GABA-A receptor. The shift in GABA function (ionic plasticity) increases neural excitability caudal to injury and sets the stage for nociceptive sensitization. The injury-induced shift in KCC2 is related to the loss of descending serotonergic (5HT) fibers that regulate plasticity within the spinal cord dorsal horn through the 5HT-1A receptor. Evidence is presented that these alterations in spinal plasticity impact pain in a brain-dependent task (place conditioning). The findings suggest that ionic plasticity can affect learning potential, shifting a neural circuit from dampened/hard-wired to excitable/plastic.

The Lesioned Spinal Cord Is a "New" Spinal Cord: Evidence from Functional Changes after Spinal Injury in Lamprey

Finding a treatment for spinal cord injury (SCI) focuses on reconnecting the spinal cord by promoting regeneration across the lesion site. However, while regeneration is necessary for recovery, on its own it may not be sufficient. This presumably reflects the requirement for regenerated inputs to interact appropriately with the spinal cord, making sub-lesion network properties an additional influence on recovery. This review summarizes work we have done in the lamprey, a model system for SCI research. We have compared locomotor behavior (swimming) and the properties of descending inputs, locomotor networks, and sensory inputs in unlesioned animals and animals that have received complete spinal cord lesions. In the majority (∼90%) of animals swimming parameters after lesioning recovered to match those in unlesioned animals. Synaptic inputs from individual regenerated axons also matched the properties in unlesioned animals, although this was associated with changes in release parameters. This suggests against any compensation at these synapses for the reduced descending drive that will occur given that regeneration is always incomplete. Compensation instead seems to occur through diverse changes in cellular and synaptic properties in locomotor networks and proprioceptive systems below, but also above, the lesion site. Recovery of locomotor performance is thus not simply the reconnection of the two sides of the spinal cord, but reflects a distributed and varied range of spinal cord changes. While locomotor network changes are insufficient on their own for recovery, they may facilitate locomotor outputs by compensating for the reduction in descending drive. Potentiated sensory feedback may in turn be a necessary adaptation that monitors and adjusts the output from the “new” locomotor network. Rather than a single aspect, changes in different components of the motor system and their interactions may be needed after SCI. If these are general features, and where comparisons with mammalian systems can be made effects seem to be conserved, improving functional recovery in higher vertebrates will require interventions that generate the optimal spinal cord conditions conducive to recovery. The analyses needed to identify these conditions are difficult in the mammalian spinal cord, but lower vertebrate systems should help to identify the principles of the optimal spinal cord response to injury.

When Pain Hurts: Nociceptive Stimulation Induces a State of Maladaptive Plasticity and Impairs Recovery after Spinal Cord Injury

Spinal cord injury (SCI) is often accompanied by other tissue damage (polytrauma) that provides a source of pain (nociceptive) input. Recent findings are reviewed that show SCI places the caudal tissue in a vulnerable state that exaggerates the effects nociceptive stimuli and promotes the development of nociceptive sensitization. Stimulation that is both unpredictable and uncontrollable induces a form of maladaptive plasticity that enhances nociceptive sensitization and impairs spinally mediated learning. In contrast, relational learning induces a form of adaptive plasticity that counters these adverse effects. SCI sets the stage for nociceptive sensitization by disrupting serotonergic (5HT) fibers that quell overexcitation. The loss of 5HT can enhance neural excitability by reducing membrane-bound K+-Cl- cotransporter 2, a cotransporter that regulates the outward flow of Cl-. This increases the intracellular concentration of Cl-, which reduces the hyperpolarizing (inhibitory) effect of gamma-aminobutyric acid. Uncontrollable noxious stimulation also undermines the recovery of locomotor function, and increases behavioral signs of chronic pain, after a contusion injury. Nociceptive stimulation has a greater effect if experienced soon after SCI. This adverse effect has been linked to a downregulation in brain-derived neurotrophic factor and an upregulation in the cytokine, tumor necrosis factor. Noxious input enhances tissue loss at the site of injury by increasing the extent of hemorrhage and apoptotic/pyroptotic cell death. Intrathecal lidocaine blocks nociception-induced hemorrhage, cellular indices of cell death, and its adverse effect on behavioral recovery. Clinical implications are discussed.

Intrathecal Transplantation of Embryonic Stem Cell-Derived Spinal GABAergic Neural Precursor Cells Attenuates Neuropathic Pain in a Spinal Cord Injury Rat Model

Neuropathic pain following spinal cord injury (SCI) is a devastating disease characterized by spontaneous pain such as hyperalgesia and allodynia. In this study, we investigated the therapeutic potential of ESC-derived spinal GABAergic neurons to treat neuropathic pain in a SCI rat model. Mouse embryonic stem cell–derived neural precursor cells (mESC-NPCs) were cultured in media supplemented with sonic hedgehog (SHH) and retinoic acid (RA) and efficiently differentiated into GABAergic neurons. Interestingly, low doses of SHH and RA induced MGE-like progenitors, which expressed low levels of DARPP32 and Nkx2.1 and high levels of Irx3 and Pax6. These cells subsequently generated the majority of the DARPP32- GABAergic neurons after in vitro differentiation. The spinal mESC-NPCs were intrathecally transplanted into the lesion area of the spinal cord around T10–T11 at 21 days after SCI. The engrafted spinal GABAergic neurons remarkably increased both the paw withdrawal threshold (PWT) below the level of the lesion and the vocalization threshold (VT) to the level of the lesion (T12, T11, and T10 vertebrae), which indicates attenuation of chronic neuropathic pain by the spinal GABAergic neurons. The transplanted cells were positive for GABA antibody staining in the injured region, and cells migrated to the injured spinal site and survived for more than 7 weeks in L4–L5. The mESC-NPC-derived spinal GABAergic neurons dramatically attenuated the chronic neuropathic pain following SCI, suggesting that the spinal GABAergic mESC-NPCs cultured with low doses of SHH and RA could be alternative cell sources for treatment of SCI neuropathic pain by stem cell-based therapies.

Thoracic Hemisection in Rats Results in Initial Recovery Followed by a Late Decrement in Locomotor Movements, with Changes in Coordination Correlated with Serotonergic Innervation of the Ventral Horn

Lateral thoracic hemisection of the rodent spinal cord is a popular model of spinal cord injury, in which the effects of various treatments, designed to encourage locomotor recovery, are tested. Nevertheless, there are still inconsistencies in the literature concerning the details of spontaneous locomotor recovery after such lesions, and there is a lack of data concerning the quality of locomotion over a long time span after the lesion. In this study, we aimed to address some of these issues. In our experiments, locomotor recovery was assessed using EMG and CatWalk recordings and analysis. Our results showed that after hemisection there was paralysis in both hindlimbs, followed by a substantial recovery of locomotor movements, but even at the peak of recovery, which occurred about 4 weeks after the lesion, some deficits of locomotion remained present. The parameters that were abnormal included abduction, interlimb coordination and speed of locomotion. Locomotor performance was stable for several weeks, but about 3-4 months after hemisection secondary locomotor impairment was observed with changes in parameters, such as speed of locomotion, interlimb coordination, base of hindlimb support, hindlimb abduction and relative foot print distance. Histological analysis of serotonergic innervation at the lumbar ventral horn below hemisection revealed a limited restoration of serotonergic fibers on the ipsilateral side of the spinal cord, while on the contralateral side of the spinal cord it returned to normal. In addition, the length of these fibers on both sides of the spinal cord correlated with inter- and intralimb coordination. In contrast to data reported in the literature, our results show there is not full locomotor recovery after spinal cord hemisection. Secondary deterioration of certain locomotor functions occurs with time in hemisected rats, and locomotor recovery appears partly associated with reinnervation of spinal circuitry by serotonergic fibers.

Changes in functional properties and 5-HT modulation above and below a spinal transection in lamprey

In addition to the disruption of neural function below spinal cord injuries (SCI), there also can be changes in neuronal properties above and below the lesion site. The relevance of these changes is generally unclear, but they must be understood if we are to provide rational interventions. Pharmacological approaches to improving locomotor function have been studied extensively, but it is still unclear what constitutes an optimal approach. Here, we have used the lamprey to compare the modulatory effects of 5-HT and lesion-induced changes in cellular and synaptic properties in unlesioned and lesioned animals. While analyses typically focus on the sub-lesion spinal cord, we have also examined effects above the lesion to see if there are changes here that could potentially contribute to the functional recovery. Cellular and synaptic properties differed in unlesioned and lesioned spinal cords and above and below the lesion site. The cellular and synaptic modulatory effects of 5-HT also differed in lesioned and unlesioned animals, again in region-specific ways above and below the lesion site. A role for 5-HT in promoting recovery was suggested by the potential for improvement in locomotor activity when 5-HT was applied to poorly recovered animals, and by the consistent failure of animals to recover when they were incubated in PCPA to deplete 5-HT. However, PCPA did not affect swimming in animals that had already recovered, suggesting a difference in 5-HT effects after lesioning. These results show changes in 5-HT modulation and cellular and synaptic properties after recovery from a spinal cord transection. Importantly, effects are not confined to the sub-lesion spinal cord but also occur above the lesion site. This suggests that the changes may not simply reflect compensatory responses to the loss of descending inputs, but reflect the need for co-ordinated changes above and below the lesion site. The changes in modulatory effects should be considered in pharmacological approaches to functional recovery, as assumptions based on effects in the unlesioned spinal cord may not be justified.

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.

Serotonergic transmission after spinal cord injury

Changes in descending serotonergic innervation of spinal neural activity have been implicated in symptoms of paralysis, spasticity, sensory disturbances and pain following spinal cord injury (SCI). Serotonergic neurons possess an enhanced ability to regenerate or sprout after many types of injury, including SCI. Current research suggests that serotonine (5-HT) release within the ventral horn of the spinal cord plays a critical role in motor function, and activation of 5-HT receptors mediates locomotor control. 5-HT originating from the brain stem inhibits sensory afferent transmission and associated spinal reflexes; by abolishing 5-HT innervation SCI leads to a disinhibition of sensory transmission. 5-HT denervation supersensitivity is one of the key mechanisms underlying the increased motoneuron excitability that occurs after SCI, and this hyperexcitability has been demonstrated to underlie the pathogenesis of spasticity after SCI. Moreover, emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. There are functional relevant connections between the descending serotonergic system from the rostral ventromedial medulla in the brainstem, the 5-HT receptors in the spinal dorsal horn, and the descending pain facilitation after tissue and nerve injury. This narrative review focussed on the most important studies that have investigated the above-mentioned effects of impaired 5-HT-transmission in humans after SCI. We also briefly discussed the promising therapeutical approaches with serotonergic drugs, monoclonal antibodies and intraspinal cell transplantation.

Neutralization of inhibitory molecule NG2 improves synaptic transmission, retrograde transport, and locomotor function after spinal cord injury in adult rats

NG2 belongs to the family of chondroitin sulfate proteoglycans that are upregulated after spinal cord injury (SCI) and are major inhibitory factors restricting the growth of fibers after SCI. Neutralization of NG2's inhibitory effect on axon growth by anti-NG2 monoclonal antibodies (NG2-Ab) has been reported. In addition, recent studies show that exogenous NG2 induces a block of axonal conduction. In this study, we demonstrate that acute intraspinal injections of NG2-Ab prevented an acute block of conduction by NG2. Chronic intrathecal infusion of NG2-Ab improved the following deficits induced by chronic midthoracic lateral hemisection (HX) injury: (1) synaptic transmission to lumbar motoneurons, (2) retrograde transport of fluororuby anatomical tracer from L5 to L1, and (3) locomotor function assessed by automated CatWalk gait analysis. We collected data in an attempt to understand the cellular and molecular mechanisms underlying the NG2-Ab-induced improvement of synaptic transmission in HX-injured spinal cord. These data showed the following: (1) that chronic NG2-Ab infusion improved conduction and axonal excitability in chronically HX-injured rats, (2) that antibody treatment increased the density of serotonergic axons with ventral regions of spinal segments L1-L5, (3) and that NG2-positive processes contact nodes of Ranvier within the nodal gap at the location of nodal Na(+) channels, which are known to be critical for propagation of action potentials along axons. Together, these results demonstrate that treatment with NG2-Ab partially improves both synaptic and anatomical plasticity in damaged spinal cord and promotes functional recovery after HX SCI. Neutralizing antibodies against NG2 may be an excellent way to promote axonal conduction after SCI.

Review of the history and current status of cell-transplant approaches for the management of neuropathic pain

Treatment of sensory neuropathies, whether inherited or caused by trauma, the progress of diabetes, or other disease states, are among the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord would be the logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the transplant of cells or a cell line to treat human disease. The history of the research and development of useful cell-transplant-based approaches offers an understanding of the advantages and problems associated with these technologies, but as an adjuvant or replacement for current pharmacological treatments, cell therapy is a likely near future clinical tool for improved health care.

Potential for Cell-Transplant Therapy with Human Neuronal Precursors to Treat Neuropathic Pain in Models of PNS and CNS Injury: Comparison of hNT2.17 and hNT2.19 Cell Lines

Effective treatment of sensory neuropathies in peripheral neuropathies and spinal cord injury (SCI) is one of the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord is a logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the potential of transplant of cells to treat chronic pain. Cell lines derived from the human neuronal NT2 cell line parentage, the hNT2.17 and hNT2.19 lines, which synthesize and release the neurotransmitters gamma-aminobutyric acid (GABA) and serotonin (5HT), respectively, have been used to evaluate the potential of cell-based release of antinociceptive agents near the lumbar dorsal (horn) spinal sensory cell centers to relieve neuropathic pain after PNS (partial nerve and diabetes-related injury) and CNS (spinal cord injury) damage in rat models. Both cell lines transplants potently and permanently reverse behavioral hypersensitivity without inducing tumors or other complications after grafting. Functioning as cellular minipumps for antinociception, human neuronal precursors, like these NT2-derived cell lines, would likely provide a useful adjuvant or replacement for current pharmacological treatments for neuropathic pain.

Chondroitinase ABC combined with neurotrophin NT-3 secretion and NR2D expression promotes axonal plasticity and functional recovery in rats with lateral hemisection of the spinal cord

Elevating spinal levels of neurotrophin NT-3 (NT3) while increasing expression of the NR2D subunit of the NMDA receptor using a HSV viral construct promotes formation of novel multisynaptic projections from lateral white matter (LWM) axons to motoneurons in neonates. However, this treatment is ineffective after postnatal day 10. Because chondroitinase ABC (ChABC) treatment restores plasticity in the adult CNS, we have added ChABC to this treatment and applied the combination to adult rats receiving a left lateral hemisection (Hx) at T8. All hemisected animals initially dragged the ipsilateral hindpaw and displayed abnormal gait. Rats treated with ChABC or NT3/HSV-NR2D recovered partial hindlimb locomotor function, but animals receiving combined therapy displayed the most improved body stability and interlimb coordination [Basso-Beattie-Bresnahan (BBB) locomotor scale and gait analysis]. Electrical stimulation of the left LWM at T6 did not evoke any synaptic response in ipsilateral L5 motoneurons of control hemisected animals, indicating interruption of the white matter. Only animals with the full combination treatment recovered consistent multisynaptic responses in these motoneurons indicating formation of a detour pathway around the Hx. These physiological findings were supported by the observation of increased branching of both cut and intact LWM axons into the gray matter near the injury. ChABC-treated animals displayed more sprouting than control animals and those receiving NT3/HSV-NR2D; animals receiving the combination of all three treatments showed the most sprouting. Our results indicate that therapies aimed at increasing plasticity, promoting axon growth and modulating synaptic function have synergistic effects and promote better functional recovery than if applied individually.

Subarachnoid Transplant of the Human Neuronal hNT2.19 Serotonergic Cell Line Attenuates Behavioral Hypersensitivity without Affecting Motor Dysfunction after Severe Contusive Spinal Cord Injury

Transplant of cells which make biologic agents that can modulate the sensory and motor responses after spinal cord injury (SCI) would be useful to treat pain and paralysis. To address this need for clinically useful human cells, a unique neuronal cell line that synthesizes and secretes/releases the neurotransmitter serotonin (5HT) was isolated. Hind paw tactile allodynia and thermal hyperalgesia induced by severe contusive SCI were potently reversed after lumbar subarachnoid transplant of differentiated cells, but had no effect on open field motor scores, stride length, foot rotation, base of support, or gridwalk footfall errors associated with the SCI. The sensory effects appeared 1 week after transplant and did not diminish during the 8-week course of the experiment when grafts were placed 2 weeks after SCI. Many grafted cells were still present and synthesizing 5HT at the end of the study. These data suggest that the human neuronal serotonergic hNT2.19 cells can be used as a biologic minipump for receiving SCI-related neuropathic pain, but likely requires intraspinal grafts for motor recovery.

Cellular and molecular insights into neuropathy-induced pain hypersensitivity for mechanism-based treatment approaches

Neuropathic pain is currently being treated by a range of therapeutic interventions that above all act to lower neuronal activity in the somatosensory system (e.g. using local anesthetics, calcium channel blockers, and opioids). The present review highlights novel and often still largely experimental treatment approaches based on insights into pathological mechanisms, which impact on the spinal nociceptive network, thereby opening the 'gate' to higher brain centers involved in the perception of pain. Cellular and molecular mechanisms such as ectopia, sensitization of nociceptors, phenotypic switching, structural plasticity, disinhibition, and neuroinflammation are discussed in relation to their involvement in pain hypersensitivity following either peripheral neuropathies or spinal cord injury. A mechanism-based treatment approach may prove to be successful in effective treatment of neuropathic pain, but requires more detailed insights into the persistence of cellular and molecular pain mechanisms which renders neuropathic pain unremitting. Subsequently, identification of the therapeutic window-of-opportunities for each specific intervention in the particular peripheral and/or central neuropathy is essential for successful clinical trials. Most of the cellular and molecular pain mechanisms described in the present review suggest pharmacological interference for neuropathic pain management. However, also more invasive treatment approaches belong to current and/or future options such as neuromodulatory interventions (including spinal cord stimulation) and cell or gene therapies, respectively.

The animal model of spinal cord injury as an experimental pain model

Pain, which remains largely unsolved, is one of the most crucial problems for spinal cord injury patients. Due to sensory problems, as well as motor dysfunctions, spinal cord injury research has proven to be complex and difficult. Furthermore, many types of pain are associated with spinal cord injury, such as neuropathic, visceral, and musculoskeletal pain. Many animal models of spinal cord injury exist to emulate clinical situations, which could help to determine common mechanisms of pathology. However, results can be easily misunderstood and falsely interpreted. Therefore, it is important to fully understand the symptoms of human spinal cord injury, as well as the various spinal cord injury models and the possible pathologies. The present paper summarizes results from animal models of spinal cord injury, as well as the most effective use of these models.

Validity of acute and chronic tactile sensory testing after spinal cord injury in rats

Spinal cord injury (SCI) impairs sensory systems causing allodynia. Measuring the development of allodynia in rodent models of SCI is challenging due to spinal shock and marked motor impairments. Assessment of SCI-induced allodynia is not standardized across labs, making interpretation of results difficult. Therefore, we validated sensory threshold assessment after SCI and developed a novel assessment of allodynia prior to motor recovery in a rat SCI model. One hundred fifty-six Sprague-Dawley rats received T8 laminectomy or mild to moderate SCI using the OSU SCI device (0.3 mm to 1.3 mm cord displacement). To determine tactile thresholds, von Frey hairs (VFH) were applied in Up-Down or ascending order to the dorsal or plantar hindpaw. The most efficient and valid procedures that maintain high sensitivity and specificity were identified. Ten Up-Down VFH applications yielded stable thresholds; reducing the risk of threshold decay and unnecessary exposure to painful stimuli. Importantly, distraction of SCI-rats with food revealed differential decay of thresholds than when distraction is not provided. The new test uses dorsal VFH stimulation and is independent of trunk or hindlimb control. Acute dorsal VFH thresholds collected before recovery of hindlimb weight support accurately predicted plantar VFH thresholds measured at late timepoints (chi(2)=8.479; p<0.05). Thus, standardized testing early after SCI using the dorsal VFH test or later using 10 stimuli in the Up-Down test produces valid measures of tactile sensation across many SCI severities. Early detection of allodynia in experimental SCI will allow identification of mechanisms responsible for pain development and determine targets for therapeutic interventions.

Decrease in the descending inhibitory 5-HT system in rats with spinal nerve ligation

The descending serotonergic (5-HT) system is shown to be plastically altered under pathological conditions such as inflammation or peripheral nerve lesion. Although much evidence indicates that the potentiation of descending facilitatory 5-HT pathways may contribute to the development of chronic pain, the inhibition of descending inhibitory 5-HT system may be functionally more important to the development of central sensitization and neuropathic pain. In the present study, we observed that the inhibitory effects of 5-HT and its receptor agonists including 1A, 1B, 3, 4, and probably 2C receptor agonists, on the C-fiber responses of dorsal horn wide dynamic range (WDR) neurons in the spinal cord decreased significantly following spinal nerve ligation (SNL). Furthermore, we found that the antagonistic effects of 5-HT 1B, 2C, 3, and 4 receptor antagonists on the 5-HT-induced inhibition of C-fiber responses of WDR neurons were also attenuated after SNL. In consistent with these observations, we also found an obvious decrease in the content of 5-HT and 5-HIAA, and a marked increase in the turnover rate of 5-HT (5-HIAA/5-HT) in the ipsilateral dorsal half of the lumbar spinal cord after SNL. These data indicate that a loss or decrease in the descending inhibitory 5-HT system upon the spinal processing of nociceptive information appears to occur following spinal nerve injury, and this kind of decrease in the descending inhibitory 5-HT system is proposed to be involved in the development of central sensitization and ultimately to the nerve injury-induced neuropathic pain.

Blockade of the 5-HT3 receptor for days causes sustained relief from mechanical allodynia following spinal cord injury

Chronic neuropathic pain is a frequent, serious outcome of spinal cord injury (SCI) that is highly refractory to treatment. Serotonin can contribute to neuropathic pain after SCI, as suggested by our previous observation that transient blockade of the 5-HT(3) receptor by intrathecal injections of the antagonist ondansetron reduces mechanical allodynia after SCI in rats. The current study determined whether intrathecal or intravenous infusion of ondansetron for 3 or 7 days, respectively, could cause sustained blockade of mechanical allodynia at and below the level of a twelfth thoracic clip compression injury in rats. Intrathecal 3-day infusion of ondansetron (2.0 microg/hr), targeted to the cord rostral to the SCI and commencing at 28 days after SCI, decreased at-level mechanical allodynia by 40% and below-level allodynia by 60% compared with saline-treated rats (controls). This reduction was sustained throughout drug delivery and for 1 day afterward. During the next 3 days, allodynia gradually returned toward the values of saline-treated rats. An initial experiment showed that bolus intravenous injections of ondansetron (20-100 microg) at 28 days after SCI decreased both at- and below-level allodynia for 90-120 min. Intravenous 7-day infusions (20 microg/hr), commencing at 28 days after SCI, significantly decreased at-level allodynia by 48% and below-level allodynia by 51% compared with controls. This reduction of allodynia lasted throughout the infusion and for 1-3 days afterward while pain responses gradually approached those of controls. These findings suggest a potential role of 5-HT(3) receptor antagonism in the relief of neuropathic pain after SCI in humans.

Recovery of control of posture and locomotion after a spinal cord injury: solutions staring us in the face

Over the past 20 years, tremendous advances have been made in the field of spinal cord injury research. Yet, consumed with individual pieces of the puzzle, we have failed as a community to grasp the magnitude of the sum of our findings. Our current knowledge should allow us to improve the lives of patients suffering from spinal cord injury. Advances in multiple areas have provided tools for pursuing effective combination of strategies for recovering stepping and standing after a severe spinal cord injury. Muscle physiology research has provided insight into how to maintain functional muscle properties after a spinal cord injury. Understanding the role of the spinal networks in processing sensory information that is important for the generation of motor functions has focused research on developing treatments that sharpen the sensitivity of the locomotor circuitry and that carefully manage the presentation of proprioceptive and cutaneous stimuli to favor recovery. Pharmacological facilitation or inhibition of neurotransmitter systems, spinal cord stimulation, and rehabilitative motor training, which all function by modulating the physiological state of the spinal circuitry, have emerged as promising approaches. Early technological developments, such as robotic training systems and high-density electrode arrays for stimulating the spinal cord, can significantly enhance the precision and minimize the invasiveness of treatment after an injury. Strategies that seek out the complementary effects of combination treatments and that efficiently integrate relevant technical advances in bioengineering represent an untapped potential and are likely to have an immediate impact. Herein, we review key findings in each of these areas of research and present a unified vision for moving forward. Much work remains, but we already have the capability, and more importantly, the responsibility, to help spinal cord injury patients now.

L1 cell adhesion molecule is essential for the maintenance of hyperalgesia after spinal cord injury

Spinal cord injury (SCI) results in a loss of normal motor and sensory function, leading to severe disability and reduced quality of life. A large proportion of individuals with SCI also suffer from neuropathic pain symptoms. The causes of abnormal pain sensations are not well understood, but can include aberrant sprouting and reorganization of injured or spared sensory afferent fibers. L1 is a cell adhesion molecule that contributes to axonal outgrowth, guidance and fasciculation in development as well as synapse formation and plasticity throughout life. In the present study, we used L1 knockout (KO) mice to determine whether this adhesion molecule contributes to sensory dysfunction after SCI. Both wild-type (WT) and KO mice developed heat hyperalgesia following contusion injury, but the KO mice recovered normal response latencies beginning at 4 weeks post-injury. Histological analyses confirmed increased sprouting of sensory fibers containing calcitonin-gene related peptide (CGRP) in the deep dorsal horn of the lumbar spinal cord and increased numbers of interneurons expressing protein kinase C gamma (PKCgamma) in WT mice 6 weeks after injury. In contrast, L1 KO mice had less CGRP(+) fiber sprouting, but even greater numbers of PKCgamma(+) interneurons at the 6 week time point. These data demonstrate that L1 plays a role in maintenance of thermal hyperalgesia after SCI in mice, and implicate CGRP(+) fiber sprouting and the upregulation of PKCgamma expression as potential contributors to this response.

The combination of human neuronal serotonergic cell implants and environmental enrichment after contusive SCI improves motor recovery over each individual strategy

A human neuronal cell line, hNT2.19, which secretes serotonin (5-HT) after differentiation, was used as a transplant source to improve motor dysfunction following severe contusive spinal cord injury (SCI). Also, environmental enrichment (EE) was added to the interspinal transplant treatment paradigm. Motor testing was performed weekly before and following SCI, with and without EE and/or cell transplant conditions. Motor recovery was maximal when both cell transplant and EE were used. Individual treatment paradigms also significantly improved foot rotation and reduced footfall errors but not stride length or base of support dysfunction. This recovery of motor function after SCI suggests that the combinatory use of serotonergic hNT2.19 cell grafts plus EE is a meaningful strategy to modestly improve motor dysfunction that accompanies contusive SCI.

Locomotor dysfunction and pain: the scylla and charybdis of fiber sprouting after spinal cord injury

Injury to the spinal cord (SCI) can produce a constellation of problems including chronic pain, autonomic dysreflexia, and motor dysfunction. Neuroplasticity in the form of fiber sprouting or the lack thereof is an important phenomenon that can contribute to the deleterious effects of SCI. Aberrant sprouting of primary afferent fibers and synaptogenesis within incorrect dorsal horn laminae leads to the development and maintenance of chronic pain as well as autonomic dysreflexia. At the same time, interruption of connections between supraspinal motor control centers and spinal cord output cells, due to lack of successful regenerative sprouting of injured descending fiber tracts, contributes to motor deficits. Similarities in the molecular control of axonal growth of motor and sensory fibers have made the development of cogent therapies difficult. In this study, we discuss recent findings related to the degradation of inhibitory barriers and promotion of sprouting of motor fibers as a strategy for the restoration of motor function and note that this may induce primary afferent fiber sprouting that can contribute to chronic pain. We highlight the importance of careful attentiveness to off-target molecular- and circuit-level modulation of nociceptive processing while moving forward with the development of therapies that will restore motor function after SCI.

Spinal GABAergic Transplants Attenuate Mechanical Allodynia in a Rat Model of Neuropathic Pain

Injury to the spinal cord or peripheral nerves can lead to the development of allodynia due to the loss of inhibitory tone involved in spinal sensory function. The potential of intraspinal transplants of GABAergic cells to restore inhibitory tone and thus decrease pain behaviors in a rat model of neuropathic pain was investigated. Allodynia of the left hind paw was induced in rats by unilateral L5- 6 spinal nerve root ligation. Mechanical sensitivity was assessed using von Frey filaments. Postinjury, transgenic fetal green fluorescent protein mouse GABAergic cells or human neural precursor cells (HNPCs) expanded in suspension bioreactors and differentiated into a GABAergic phenotype were transplanted into the spinal cord. Control rats received undifferentiated HNPCs or cell suspension medium only. Animals that received either fetal mouse GABAergic cell or differentiated GABAergic HNPC intraspinal transplants demonstrated a significant increase in paw withdrawal thresholds at 1 week post-transplantation that was sustained for 6 weeks. Transplanted fetal mouse GABAergic cells demonstrated immunoreactivity for glutamic acid decarboxylase and GABA that colocalized with green fluorescent protein. Intraspinally transplanted differentiated GABAergic HNPCs demonstrated immunoreactivity for GABA and beta-III tubulin. In contrast, intraspinal transplantation of undifferentiated HNPCs, which predominantly differentiated into astrocytes, or cell suspension medium did not affect any behavioral recovery. Intraspinally transplanted GABAergic cells can reduce allodynia in a rat model of neuropathic pain. In addition, HNPCs expanded in a standardized fashion in suspension bioreactors and differentiated into a GABAergic phenotype may be an alternative to fetal cells for cell-based therapies to treat chronic pain syndromes.

Serotonin concentrations in the lumbosacral spinal cord of the adult rat following microinjection or dorsal surface application

Application of neuroactive substances, including monoamines, is common in studies examining the spinal mechanisms of sensation and behavior. However, affected regions and time courses of transmitter activity are uncertain. We measured the spatial and temporal distribution of serotonin [5-hydroxytryptamine (5-HT)] in the lumbosacral spinal cord of halothane-anesthetized adult rats, following its intraspinal microinjection or surface application. Carbon fiber microelectrodes (CFMEs) were positioned at various locations in the spinal cord and oxidation currents corresponding to extracellular 5-HT were measured by fast cyclic voltammetry. Intraspinal microinjection of 5-HT (100 microM, 1-3 microl) produced responses that were most pronounced at CFMEs positioned <or=800 microm from the drug micropipette: 5-HT concentration was significantly higher (1.43 vs. <0.28% of initial concentration) and response latency was shorter (67.1 vs. 598.2 s) compared with more distantly positioned CFMEs. Treatment with the selective 5-HT reuptake inhibitor clomipramine only slightly affected the spread of microinjected 5-HT. Surface application over several segments led to a transient rise in concentration that was usually apparent within 30 s and was dramatically attenuated with increasing depth: 0.25% of initial concentration (1 mM) within 400 microm of the dorsal surface and <0.001% between 1,170 and 2,000 microm. This initial response to superfusion was sometimes followed by a gradual increase to a new concentration plateau. In sum, compared with bath application, microinjection can deliver about tenfold higher transmitter concentrations, but to much more restricted areas of the spinal cord.

Deviations in gait pattern in experimental models of hindlimb paresis shown by a novel pressure mapping system

Injuries to the central and peripheral nervous system result in varying degrees of paresis and as such alter gait. We developed novel quantitative measures to assess compensatory patterns of gait in experimental models of unilateral and bilateral hindlimb paresis. We hypothesized that hindlimb paresis results in unique alterations in the gait cycle that reflect the symmetry of the initial lesion. To test this hypothesis, adult, male Sprague-Dawley rats were subjected to a unilateral sciatic nerve crush injury or a moderately severe spinal cord contusion injury at T8. Kinematic and timing parameters were captured simultaneously in all four limbs and alterations in gait were then compared to relevant sham controls. Gait analysis consisted of walking trials through a gait tunnel positioned over a Tekscan pressure sensor grid. After sciatic nerve injury, animals unweighted the injured limb by shifting their center of mass toward the contralateral forelimb and hindlimb. These changes in weight-bearing occurred simultaneously with an increase in stance time on the contralateral limbs. As might be expected spinal cord injured animals unweighted their hindlimbs, as shown by reduced hindlimb contact force and contact pressure. These adjustments coincided with a shortening of forelimb stance time and stride length. These findings show both alterations and compensatory changes in gait that reflect the symmetry of the initial injury.

Predifferentiated embryonic stem cells prevent chronic pain behaviors and restore sensory function following spinal cord injury in mice

Embryonic stem (ES) cells have been investigated in repair of the CNS following neuronal injury and disease; however, the efficacy of these cells in treatment of postinjury pain is far from clear. In this study, we evaluated the therapeutic potential of predifferentiated mouse ES cells to restore sensory deficits following spinal cord injury (SCI) in mice. The pain model used unilateral intraspinal injection of quisqualic acid (QUIS) into the dorsal horn between vertebral levels T13 and L1. Seven days later, 60,000 predifferentiated ES cells or media were transplanted into the site of the lesion. Histological analysis at 7, 14, and 60 days post-transplantation revealed that animals receiving ES cell transplants suffered significantly less tissue damage than animals receiving media alone. Transplanted cells provided immediate effects on both spontaneous and evoked pain behaviors. Treatment with ES cells resulted in 0% (n = 28) excessive grooming behavior versus 60% (18 of 30) in media-treated animals. In the acetone test (to assess thermal allodynia), mice recovered to preinjury levels by 12 days after ES cell transplant, whereas control animals injected with media after SCI did not show any improvement up to 60 days. Similarly, the von Frey test (to assess mechanical allodynia) and the formalin test (to assess nociceptive hyperalgesia) showed that transplantation of predifferentiated ES cells significantly reduced these pain behaviors following injury. Here we show that predifferentiated ES cells act in a neuroprotective manner and provide antinociceptive and therapeutic effects following excitotoxic SCI.

Recovery of function following grafting of human bone marrow-derived stromal cells into the injured spinal cord

This study evaluates functional recovery after transplanting human bone marrow-derived stromal cells (BMSCs) into contusion models of spinal cord injury (SCI). The authors used a high-throughput process to expand BMSCs and characterized them by flow cytometry, ELISA, and gene expression. They found that BMSCs secrete neurotrophic factors and cytokines with therapeutic potential for cell survival and axon growth. In adult immune-suppressed rats, mild, moderate, or severe contusions were generated using the MASCIS impactor. One week following injury, 0.5 to 1 x 106 BMSCs were injected into the lesioned spinal cord; control animals received vehicle injection. Biweekly behavioral tests included the Basso, Beattie, and Bresnahan Locomotor Rating Scale (BBB), exploratory rearing, grid walking, and thermal sensitivity. Animals receiving moderate contusions followed by BMSC grafts showed significant behavioral recovery in BBB and rearing tests when compared to controls. Animals receiving BMSC grafts after mild or severe contusion showed trends toward improved recovery. Immunocytochemistry identified numerous axons passing through the injury in animals with BMSC grafts but few in controls. BMSCS were detected at 2 weeks after transplantation; however, at 11 weeks very few grafted cells remained. The authors conclude that BMSCs show potential for repairing SCI. However, the use of carefully characterized BMSCs improved transplantation protocols ensuring BMSC, survival, and systematic motor and sensory behavioral testing to identify robust recovery is imperative for further improvement.

Cell and Molecular Approaches to the Attenuation of Pain after Spinal Cord Injury

Recent experimental research to treat spinal cord injury (SCI) pain has greatly increased our understanding of how such chronic pain might be modulated in the human population. Neuropathic pain is caused by the structural and biochemical changes associated with the peripheral and central nervous system damage associated with nervous system trauma, often leading to an imbalance in endogenous excitatory and inhibitory spinal systems that modulate sensory processing. But current pharmacological therapies are often ineffective over time for the greater number of patients. Although there are a variety of useful surgical and pharmacologic interventions (including electric stimulation, implantable mechanical pumps and a myriad of drugs for pain relief) cell and molecular technologies are a new frontier in pain medicine. These other potential therapeutic agents of pain are based on current and developing treatment strategies elucidated from recent research, especially concerning central spinal sensitization, and the spinal mechanisms that are thought to be the origin and ongoing cause of chronic pain, even when the injury is peripheral in location. Newly developing translational strategies such as molecular agents, viral-mediated gene transfer or cellular transplants to treat chronic pain are being evaluated in a variety of peripheral and central injury models. They seek to address both the causes of neuropathic pain, to interfere with its development and maintenance over time, and give the injured person with pain an improved quality-of-life that allows them to better deal with the larger tasks of daily life and the strenuous rehabilitation that might also improve motor function after SCI.

Spinal synaptic enhancement with acute intermittent hypoxia improves respiratory function after chronic cervical spinal cord injury

Respiratory insufficiency is the leading cause of death after high-cervical spinal cord injuries (SCIs). Although respiratory motor recovery can occur with time after injury, the magnitude of spontaneous recovery is limited. We hypothesized that partial respiratory motor recovery after chronic cervical SCI could be strengthened using a known stimulus for spinal synaptic enhancement, intermittent hypoxia. Phrenic motor output was recorded before and after intermittent hypoxia from anesthetized, vagotomized, and pump-ventilated control and C2 spinally hemisected rats at 2, 4, and 8 weeks after injury. Weak spontaneous phrenic motor recovery was present in all C2-injured rats via crossed spinal synaptic pathways that convey bulbospinal inspiratory premotor drive to phrenic motoneurons on the side of injury. Intermittent hypoxia augmented crossed spinal synaptic pathways [phrenic long-term facilitation; pLTF] for up to 60 min after hypoxia at 8 weeks, but not 2 weeks, after injury. Ketanserin, a serotonin 2A receptor antagonist, administered before intermittent hypoxia at 8 weeks after injury prevented pLTF. Serotonergic innervation near phrenic motoneurons was assessed after injury. The limited magnitude of pLTF at 2 weeks was associated with an injury-induced reduction in serotonin-containing nerve terminals in the vicinity of phrenic motoneurons ipsilateral to C2 hemisection. Thereafter, pLTF magnitude progressively increased with the recovery of serotonergic innervation in the phrenic motor nucleus. Intermittent hypoxia (or pLTF) has intriguing possibilities as a therapeutic tool, because its greatest efficacy may be in patients with chronic SCI, a time when most patients have already achieved maximal spontaneous functional recovery.

Spinal cord injury triggers sensitization of wide dynamic range dorsal horn neurons in segments rostral to the injury

A spinal cord injury (SCI) was produced in adult rats by complete spinal cord transection at L6-S1. Neuropathic pain behaviors similar to the chronic central pain (CCP) syndrome in human, such as thermal hyperalgesia, mechanical allodynia and autotomy, were present in these rats after spinal cord injury. Meanwhile, wide dynamic range (WDR) neurons recorded in the spinal dorsal horn rostral to the lesion responded as high frequency of spontaneous activities, long duration of after-discharges to noxious electrical stimuli and an augmented wind-up to 0.5 Hz stimuli. By using bupivacaine powder, a sodium channel blocker, at the locus of transection immediate after nerve injury, the chronic pain behaviors were prevented; the hyperexcitability of WDR neurons was also substantially reduced. It is suggested that spinal cord transection induces the CCP syndromes, which may be evoked and maintained by the hyperexcitability in WDR neurons rostrally. Reducing the neuronal activity at the site of lesion following injury may prevent the development of CCP after SCI.

Human fetal neural stem cells grafted into contusion-injured rat spinal cords improve behavior

Grafted human neural stem cells (hNSCs) may help to alleviate functional deficits resulting from spinal cord injury by bridging gaps, replacing lost neurons or oligodendrocytes, and providing neurotrophic factors. Previously, we showed that primed hNSCs differentiated into cholinergic neurons in an intact spinal cord. In this study, we tested the fate of hNSCs transplanted into a spinal cord T10 contusion injury model. When grafted into injured spinal cords of adult male rats on either the same day or 3 or 9 days after a moderate contusion injury, both primed and unprimed hNSCs survived for 3 months postengraftment only in animals that received grafts at 9 days postinjury. Histological analyses revealed that primed hNSCs tended to survive better and differentiated at higher rates into neurons and oligodendrocytes than did unprimed counterparts. Furthermore, only primed cells gave rise to cholinergic neurons. Animals receiving primed hNSC grafts on the ninth day postcontusion improved trunk stability, as determined by rearing activity measurements 3 months after grafting. This study indicates that human neural stem cell fate determination in vivo is influenced by the predifferentiation stage of stem cells prior to grafting. Furthermore, stem cell-mediated facilitation of functional improvement depends on the timing of transplantation after injury, the grafting sites, and the survival of newly differentiated neurons and oligodendrocytes.

Combined Treatment with Neurotrophin-3 and LSD Facilitates Behavioral Recovery from Double-Hemisection Spinal Injury in Neonatal Rats

We explored functional recovery in two spinal cord injury models following a novel combination treatment (NT-3 + LSD). One group of rats received a staggered double hemisection (DH) at postnatal day 2 (P2) of the left hemicord at T11 and the right hemicord at T12. Another group received complete transection (CT) at T11 on P2. A third group was sham operated. Each of these groups was also treated with the drug combination. Drugs were administered intrathecally above the lesion during surgery, and again s.c. at P4, P6, P8, and P10. Intracellular recording in an in vitro spinal cord preparation at P10-P12 in DH rats revealed weak polysynaptic connections to lumbar motoneurons through the injury region, but only in those receiving NT-3 + LSD; NT-3 or LSD alone had no effect. In behavioral experiments, the frequency of rearing in an open field and hindlimb kicks during swimming was assessed every 3-4 days from P9 to P58. Both CT and DH injury severely impaired rearing and hindlimb kicking during swimming. DH rats treated with NT-3 + LSD showed significantly more kicks during swimming than untreated DH or CT rats and treated CT rats beginning as early as P9 and lasting through the duration of testing. Rearing behavior was also improved by treatment but beginning only in the 3rd postnatal week, the time at which it normally develops. Rearing frequency reached sham control levels by P40. Our results suggest this combination treatment may be a promising new strategy for facilitating recovery from moderate spinal cord injury.

Axon growth and recovery of function supported by human bone marrow stromal cells in the injured spinal cord exhibit donor variations

Bone marrow stromal cells (MSC) are non-hematopoietic support cells that can be easily derived from bone marrow aspirates. Human MSC are clinically attractive because they can be expanded to large numbers in culture and reintroduced into patients as autografts or allografts. We grafted human MSC derived from aspirates of four different donors into a subtotal cervical hemisection in adult female rats and found that cells integrated well into the injury site, with little migration away from the graft. Immunocytochemical analysis demonstrated robust axonal growth through the grafts of animals treated with MSC, suggesting that MSC support axonal growth after spinal cord injury (SCI). However, the amount of axon growth through the graft site varied considerably between groups of animals treated with different MSC lots, suggesting that efficacy may be donor-dependent. Similarly, a battery of behavioral tests showed partial recovery in some treatment groups but not others. Using ELISA, we found variations in secretion patterns of selected growth factors and cytokines between different MSC lots. In a dorsal root ganglion explant culture system, we tested efficacy of conditioned medium from three donors and found that average axon lengths increased for all groups compared to control. These results suggest that human MSC produce factors important for mediating axon outgrowth and recovery after SCI but that MSC lots from different donors vary considerably. To qualify MSC lots for future clinical application, such notable differences in donor or lot-lot efficacy highlight the need for establishing adequate characterization, including the development of relevant efficacy assays.

Serotonergic fiber sprouting to external anal sphincter motoneurons after spinal cord contusion

The present study analyzed the anatomical plasticity of serotonergic immunoreactive projections to external anal sphincter (EAS) motoneurons, and the behavioral plasticity of EAS reflexes, penile erection, and locomotion in rats with spinal contusion injury (SCI) or complete spinal cord transection (TX). Electromyographic activity of the EAS, penile erection latency, and BBB locomotor score exhibited parallel recovery over the 6-week recovery period after contusion SCI. This pattern of recovery was not observed in TX animals. While locomotor scores demonstrated a small increase after TX, erectile and anorectal function remained at abnormal levels established immediately after injury. Serotonergic immunofluorescent (5-HT-IF) staining at the lesion site identified a small number of fibers spared after SCI that may provide a substrate for functional recovery. Pixel density measurements of 5-HT-IF in the vicinity of retrogradely labeled EAS and unlabeled pudendal motoneurons necessary for penile erection provide indirect evidence of serotonergic sprouting that parallels the observed functional recovery in animals with SCI. No 5-HT-IF was detected caudal to the injury site in TX animals. These studies indicate: (1) lumbosacral eliminative and reproductive reflexes provide a valid means of studying the mechanisms of post-SCI plasticity; (2) the similar recovery curves suggest similar return of descending control, perhaps through sprouting of descending serotonergic fibers; (3) the observed deficits after TX likely represent the permanent removal of descending inhibition and reflect reorganization of segmental circuitry.

Uncontrollable stimulation undermines recovery after spinal cord injury

Prior studies have shown that neurons within the spinal cord are sensitive to response-outcome relations, a form of instrumental learning. Spinally transected rats that receive shock to one hind leg learn to maintain the leg in a flexed position that minimizes net shock exposure (controllable shock). Prior exposure to uncontrollable stimulation (intermittent shock) inhibits this spinally mediated learning. Here it is shown that uncontrollable stimulation undermines the recovery of function after a spinal contusion injury. Rats received a moderate injury (12.5 mm drop) and recovery was monitored for 6 weeks. In Experiment 1, rats received varying amounts of intermittent tailshock 1-2 days after injury. Just 6 min of intermittent shock impaired locomotor recovery. In Experiment 2, rats were shocked 1, 4, or 14 days after injury. Delaying the application of shock exposure reduced its negative effect on recovery. In Experiment 3, rats received controllable or uncontrollable shock 24 and 48 h after injury. Only uncontrollable shock disrupted recovery of locomotor function. Uncontrollably shocked rats also exhibited higher vocalization thresholds to aversive stimuli (heat and shock) applied below the injury. Across the three experiments, exposure to uncontrollable shock, (1) delayed the recovery of bladder function; (2) led to greater mortality and spasticity; and (3) increased tissue loss (white and gray matter) in the region of the injury. The results indicate that uncontrollable stimulation impairs recovery after spinal cord injury and suggest that reducing sources of uncontrolled afferent input (e.g., from peripheral tissue injury) could benefit patient recovery.

Cell therapy for neuropathic pain in spinal cord injuries

Cell therapy to treat neuropathic pain after spinal cord injury (SCI) is in its infancy. However, the development of cellular strategies that would replace or be used as an adjunct to existing pharmacological treatments for neuropathic pain have progressed tremendously over the past 20 years. The earliest cell therapy studies for pain relief tested adrenal chromaffin cells from rat or bovine sources, placed in the subarachnoid space, near the spinal cord pain- processing pathways. These grafts functioned as cellular minipumps, secreting a cocktail of antinociceptive agents around the spinal cord for peripheral nerve injury, inflammatory or arthritic pain. These initial animal, and later clinical, studies suggested that the spinal intrathecal space was a safe and accessible location for the placement of cell grafts. However, one major problem was the lack of a homogeneous, expandable cell source to supply the antinociceptive agents. Cell lines that can be reversibly immortalised are the next phase for the development of a practical, homogenous cell source. These technologies have been modelled with a variety of murine cell lines, derived from embryonic adrenal medulla or CNS brainstem, in which cells are transplanted, which downregulate their proliferative, oncogenic phenotype either before or after transplant. An alternative approach for existing human cell lines is the use of neural or adrenal precursors, in which the antinociceptive properties are induced by in vitro treatment with molecules that move the cells to an irreversible neural or chromaffin, and non-oncogenic, phenotype. Although such human cell lines are at an early stage of investigation, their clinical antinociceptive potential is significant given the daunting problem of difficult-to-treat neuropathic SCI pain.

The 5-HT3 receptor facilitates at-level mechanical allodynia following spinal cord injury

Spinal cord injury (SCI) results in the development of mechanical allodynia immediately rostral to the lesion site, within the dermatome border of normal sensation and sensory loss (at-level mechanical allodynia). We propose that an observed threefold increase in serotonergic fibre immunoreactivity within spinal segments corresponding to these allodynic dermatomes facilitates the maintenance of chronic neuropathic pain via activation of the 5-HT(3) receptor (5-HT(3)-R). Serotonin (5-HT), the non-selective 5-HT(1)/5-HT(2) receptor antagonist, methysergide, the 5-HT(3)-R agonist, m-chlorophenylbiguanide (m-CPBG) or the 5-HT(3)-R antagonist, ondansetron were intrathecally administered five weeks following SCI in rats. Ondansetron produced a robust, long-term reduction of at-level mechanical allodynia, while m-CPBG exacerbated allodynia. Exogenous 5-HT transiently reduced at-level mechanical allodynia. This effect was opposed by methysergide, which enhanced mechanical allodynia. Co-administration of 5-HT and ondansetron produced a short-lasting partial summation of effects, further decreasing mechanical allodynia while co-administration of methysergide attenuated the anti-allodynic effect of ondansetron. Depletion of spinal 5-HT via 5,7-dihydroxytryptamine (5,7-DHT) resulted in decreased at-level mechanical allodynia. The reduction of allodynia by ondansetron was lost following 5,7-DHT administration, suggesting that reduced allodynia following intrathecal ondansetron is via blockade of 5-HT-induced excitation of the 5-HT(3)-R. These results suggest that increased 5-HT fibre density immediately rostral to the SCI lesion site could have transient effects to reduce mechanical allodynia via actions at 5-HT(1) and/or 5-HT(2) receptors. However, the more long-lasting effects of this enhanced serotonergic input may facilitate chronic, at-level allodynia via the 5-HT(3)-R.

Delayed transplantation of fibroblasts genetically modified to secrete BDNF and NT-3 into a spinal cord injury site is associated with limited recovery of function

Delivery of neurotrophic factors in acute models of spinal cord injury in adult rats can rescue axotomized neurons, promote axonal growth, and partially restore function. The extent to which repair and recovery of function can be achieved after chronic injury has received less attention. In the companion paper we show that transplanting fibroblasts genetically modified to produce neurotrophic factors into chronic (6-week) hemisection injuries results in sprouting, partial neuroprotection, but only limited regeneration. Here we describe functional consequences of this treatment using a series of behavioral tests. Adult rats received a complete unilateral C3/C4 hemisection and recovery from the injury was assessed over 5 weeks. At 6 weeks postoperative, the experimental group received grafts of a combination of fibroblasts modified to secrete BDNF or NT-3. The operated control groups received grafts of either gelfoam or gelfoam with fibroblasts expressing GFP into the lesion site. Behavioral recovery in the three groups was assessed over the next 10 weeks. Severe deficits with no recovery in any of the groups were observed in several tests (BBB, limb preference, narrow beam, horizontal rope test) that measure primarily motor function. Recovery was observed in the grid test, a measure of sensorimotor function, and the von Frey test, a measure of response to mechanical stimulation, but there were no differences between the operated control or experimental groups. Both groups also showed recovery from heat-induced hyperalgesia, with the experimental group exhibiting greater recovery than the operated control groups. In this test, delivery of neurotrophic factors from transplanted fibroblasts does not worsen responses to nociceptive stimuli and in fact appears to reduce hypersensitivity. Our data also demonstrate that additional damage to the spinal cord upon placement of a graft further compromises behavioral recovery for locomotor and postural function. Additional therapeutic interventions will be necessary to provide greater levels of recovery after chronic injuries.

Neurotrophins in spinal cord nociceptive pathways

Neurotrophins are a well-known family of growth factors for the central and peripheral nervous systems. In the course of the last years, several lines of evidence converged to indicate that some members of the family, particularly NGF and BDNF, also participate in structural and functional plasticity of nociceptive pathways within the dorsal root ganglia and spinal cord. A subpopulation of small-sized dorsal root ganglion neurons is sensitive to NGF and responds to peripheral NGF stimulation with upregulation of BDNF synthesis and increased anterograde transport to the dorsal horn. In the latter, release of BDNF appears to modulate or even mediate nociceptive sensory inputs and pain hypersensitivity. We summarize here the status of the art on the role of neurotrophins in nociceptive pathways, with special emphasis on short-term synaptic and intracellular events that are mediated by this novel class of neuromessengers in the dorsal horn. Under this perspective we review the findings obtained through an array of techniques in naïve and transgenic animals that provide insight into the modulatory mechanisms of BDNF at central synapses. We also report on the results obtained after immunocytochemistry, in situ hybridization, and monitoring intracellular calcium levels by confocal microscopy, that led to hypothesize that also NGF might have a direct central effect in pain modulation. Although it is unclear whether or not NGF may be released at dorsal horn endings of certain nociceptors in vivo, we believe that these findings offer a clue for further studies aiming to elucidate the putative central effects of NGF and other neurotrophins in nociceptive pathways.