Cervical spinal cord injury upregulates ventral spinal 5-HT2A receptors

Prolonged intermittent hypoxia differentially regulates phrenic motor neuron serotonin receptor expression in rats following chronic cervical spinal cord injury

Low-dose (< 2 h/day), acute intermittent hypoxia (AIH) elicits multiple forms of serotonin-dependent phrenic motor plasticity and is emerging as a promising therapeutic strategy to restore respiratory and non-respiratory motor function after spinal cord injury (SCI). In contrast, high-dose (> 8 h/day), chronic intermittent hypoxia (CIH) undermines some forms of serotonin-dependent phrenic motor plasticity and elicits pathology. CIH is a hallmark of sleep disordered breathing, which is highly prevalent in individuals with cervical SCI. Interestingly, AIH and CIH preconditioning differentially impact phrenic motor plasticity. Although mechanisms of AIH-induced plasticity in the phrenic motor system are well-described in naïve rats, we know little concerning how these mechanisms are affected by chronic SCI or intermittent hypoxia preconditioning. Thus, in a rat model of chronic, incomplete cervical SCI (lateral spinal hemisection at C2 (C2Hx), we assessed serotonin type 2A, 2B and 7 receptor expression in and near phrenic motor neurons and compared: 1) intact vs. chronically injured rats; and 2) the impact of preconditioning with varied "doses" of intermittent hypoxia (IH). While there were no effects of chronic injury or intermittent hypoxia alone, CIH affected multiple receptors in rats with chronic C2Hx. Specifically, CIH preconditioning (8 h/day; 28 days) increased serotonin 2A and 7 receptor expression exclusively in rats with chronic C2Hx. Understanding the complex, context-specific interactions between chronic SCI and CIH and how this ultimately impacts phrenic motor plasticity is important as we leverage AIH-induced motor plasticity to restore breathing and other non-respiratory motor functions in people with chronic SCI.

Subclinical respiratory dysfunction and impaired ventilatory adaptation in degenerative cervical myelopathy

Degenerative cervical myelopathy (DCM) is a debilitating neurological condition characterized by chronic compression of the cervical spinal cord leading to impaired upper and lower limb function. Despite damage to areas of the cervical spinal cord that house the respiratory network, respiratory dysfunction is not a common symptom of DCM. However, DCM may be associated with respiratory dysfunction, and this can affect the ventilatory response to respiratory challenges during emergence from anesthesia, exercise, or pulmonary disease. Surgical spinal cord decompression, which is the primary treatment for DCM, leads to improved sensorimotor function in DCM; yet its impact on respiratory function is unknown. Here, using a clinically relevant model of DCM, we evaluate respiratory function during disease progression and assess adaptive ventilation to hypercapnic challenge before and after surgical intervention. We show that despite significant and progressive forelimb and locomotor deficits, there was no significant decline in eupneic ventilation from the early to late phases of spinal cord compression. Additionally, for the first time, we demonstrate that despite normal ventilation under resting conditions, DCM impairs acute adaptive ventilatory ability in response to hypercapnia. Remarkably, akin to DCM patients, surgical decompression treatment improved sensorimotor function in a subset of mice. In contrast, none of the mice that underwent surgical decompression recovered their ability to respond to hypercapnic ventilatory challenge. These findings underscore the impact of chronic spinal cord compression on respiratory function, highlighting the challenges associated with ventilatory response to respiratory challenges in individuals with DCM. This research highlights the impact of cervical spinal cord compression on respiratory dysfunction in DCM, as well as the persistence of adaptive ventilatory dysfunction after surgical spinal cord decompression. These results indicate the need for additional interventions to enhance recovery of respiratory function after surgery for DCM.

The Development of Hindlimb Postural Asymmetry Induced by Focal Traumatic Brain Injury Is Not Related to Serotonin 2A/C Receptor Expression in the Spinal Cord

Brain injury and stroke are leading causes of adult disability. Motor deficits are common problems, and their underlying pathological mechanisms remain poorly understood. The serotoninergic system is implicated in both functional recovery from and the occurrence of spasticity after injuries to the central nervous system. This study, which was conducted on rats, investigated the development of limb postural changes and their relationship to the expression of serotonin (5-HT) 2A and 2C receptors in the spinal cord in the 4 weeks after focal traumatic brain injury (TBI) to the right hindlimb sensorimotor cortex. The limb motor deficits were assessed by measuring gait pattern changes during walking and hindlimb postural asymmetry at different time intervals (3−28 days) after surgery. The expressions of the 5-HT2A and 2C receptors in the lumbar spinal cord were investigated using immunohistochemistry. The results showed that all the rats with TBI, independently of the duration of the interval, displayed postural asymmetry with flexion on the contralateral (left) side (>2 mm), while the sham-operated rats showed no apparent postural asymmetry. The TBI rats also had longer stride lengths during walking in both their hindlimbs and their forelimbs compared with the sham rats. For both the TBI and the sham rats, the hind-paw placement angles were larger on the contralateral side in some of the groups. Compared to the sham-operated rats, the 5-HT2A and 2C receptor expression did not significantly change on either side of the lumbar spinal cords of the TBI rats in any of the groups. These results suggest that focal TBI can induce motor deficits lasting a relatively long time, and that these deficits are not related to the expression of the 5-HT2A and 2C receptors in the spinal cord.

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.

Rostral-caudal effect of cervical magnetic stimulation on the diaphragm motor evoked potential following cervical spinal cord contusion in the rat

The present study was designed to investigate the rostro-caudal effect of spinal magnetic stimulation on diaphragmatic motor-evoked potentials following cervical spinal cord injury. The diaphragm electromyogram was recorded in rats that received a laminectomy or a left mid-cervical contusion at the acute (1 day), subchronic (2 weeks), or chronic (8 weeks) injured stages. The center of a figure-eight coil was placed at 30 mm lateral to bregma on the left side, and the effect of magnetic stimulation was evaluated by stimulating the rostral, middle, and caudal cervical regions in spontaneously breathing rats. The results demonstrated that cervical magnetic stimulation induced intensity-dependent motor-evoked potentials in the bilateral diaphragm in both uninjured and contused rats; however, the left diaphragm exhibited a higher amplitude and earlier onset than the right diaphragm. Moreover, the intensity-response curve was shifted upward in the rostral-to-caudal direction of magnetic stimulation, suggesting that caudal cervical magnetic stimulation produced more robust diaphragmatic motor-evoked potentials compared to rostral cervical magnetic stimulation. Interestingly, the diaphragmatic motor-evoked potentials were similar between uninjured and contused rats during cervical magnetic stimulation despite weaker inspiratory diaphragmatic activity in contused rats. Additionally, in contused animals but not uninjured animals, diaphragmatic motor-evoked potential amplitude were greater at the chronic stage than during earlier injured stages. These results demonstrated that cervical magnetic stimulation can excite the residual phrenic motor circuit to activate the diaphragm in the presence of a significant lesion in the cervical spinal cord. These findings indicate that this non-invasive approach is effective for modulating diaphragmatic excitability following cervical spinal cord injury.

Phrenic motor neuron survival below cervical spinal cord hemisection

Cervical spinal cord injury (cSCI) severs bulbospinal projections to respiratory motor neurons, paralyzing respiratory muscles below the injury. C2 spinal hemisection (C2Hx) is a model of cSCI often used to study spontaneous and induced plasticity and breathing recovery post-injury. One key assumption is that C2Hx dennervates motor neurons below the injury, but does not affect their survival. However, a recent study reported substantial bilateral motor neuron death caudal to C2Hx. Since phrenic motor neuron (PMN) death following C2Hx would have profound implications for therapeutic strategies designed to target spared neural circuits, we tested the hypothesis that C2Hx minimally impacts PMN survival. Using improved retrograde tracing methods, we observed no loss of PMNs at 2- or 8-weeks post-C2Hx. We also observed no injury-related differences in ChAT or NeuN immunolabeling within labelled PMNs. Although we found no evidence of PMN loss following C2Hx, we cannot rule out neuronal loss in other motor pools. These findings address an essential prerequisite for studies that utilize C2Hx as a model to explore strategies for inducing plasticity and/or regeneration within the phrenic motor system, as they provide important insights into the viability of phrenic motor neurons as therapeutic targets after high cervical injury.

Diaphragm Motor-Evoked Potential Induced by Cervical Magnetic Stimulation following Cervical Spinal Cord Contusion in the Rat

Cervical spinal injury is typically associated with respiratory impairments due to damage to bulbospinal respiratory pathways and phrenic motoneurons. Magnetic stimulation is a non-invasive approach for the evaluation and modulation of the nervous system. The present study was designed to examine whether cervical magnetic stimulation can be applied to evaluate diaphragmatic motor outputs in a pre-clinical rat model of cervical spinal injury. The bilateral diaphragm was monitored in anesthetized rats using electromyogram at the acute, subchronic, and chronic stages following left mid-cervical contusion. The center of a figure-of-eight coil was placed 20 mm caudal to bregma to stimulate the cervical spinal cord. The results demonstrated that a single magnetic stimulation can evoke significant motor-evoked potentials in the diaphragms of uninjured animals when the animal's head was placed 30 mm right or left from the center of the coil. The spontaneous bursting of the diaphragm was significantly attenuated by contusion injury at all-time-points post-injury. However, the threshold of the diaphragmatic motor-evoked potential was reduced, and the amplitude of the diaphragmatic motor-evoked potential was enhanced in response to cervical magnetic stimulation at the acute injury stage. Moreover, the motor-evoked potentials of the bilateral diaphragm in animals with contusions were generally larger when the coil was placed at the left spinal cord at the subchronic and chronic injury stages. These results suggested that cervical magnetic stimulation can be used to examine the excitability of phrenic motor outputs post-injury, and magnetic stimulation applied more laterally may be more effective for triggering diaphragmatic motor-evoked potentials.

Electrical epidural stimulation of the cervical spinal cord: implications for spinal respiratory neuroplasticity after spinal cord injury

Traumatic cervical spinal cord injury (cSCI) can lead to damage of bulbospinal pathways to the respiratory motor nuclei and consequent life-threatening respiratory insufficiency due to respiratory muscle paralysis/paresis. Reports of electrical epidural stimulation (EES) of the lumbosacral spinal cord to enable locomotor function after SCI are encouraging, with some evidence of facilitating neural plasticity. Here, we detail the development and success of EES in recovering locomotor function, with consideration of stimulation parameters and safety measures to develop effective EES protocols. EES is just beginning to be applied in other motor, sensory, and autonomic systems; however, there has only been moderate success in preclinical studies aimed at improving breathing function after cSCI. Thus, we explore the rationale for applying EES to the cervical spinal cord, targeting the phrenic motor nucleus for the restoration of breathing. We also suggest cellular/molecular mechanisms by which EES may induce respiratory plasticity, including a brief examination of sex-related differences in these mechanisms. Finally, we suggest that more attention be paid to the effects of specific electrical parameters that have been used in the development of EES protocols and how that can impact the safety and efficacy for those receiving this therapy. Ultimately, we aim to inform readers about the potential benefits of EES in the phrenic motor system and encourage future studies in this area.

Spinal Cord Injury Alters Spinal Shox2 Interneurons by Enhancing Excitatory Synaptic Input and Serotonergic Modulation While Maintaining Intrinsic Properties in Mouse

Neural circuitry generating locomotor rhythm and pattern is located in the spinal cord. Most spinal cord injuries (SCIs) occur above the level of spinal locomotor neurons; therefore, these circuits are a target for improving motor function after SCI. Despite being relatively intact below the injury, locomotor circuitry undergoes substantial plasticity with the loss of descending control. Information regarding cell type-specific plasticity within locomotor circuits is limited. Shox2 interneurons (INs) have been linked to locomotor rhythm generation and patterning, making them a potential therapeutic target for the restoration of locomotion after SCI. The goal of the present study was to identify SCI-induced plasticity at the level of Shox2 INs in a complete thoracic transection model in adult male and female mice. Whole-cell patch-clamp recordings of Shox2 INs revealed minimal changes in intrinsic excitability properties after SCI. However, afferent stimulation resulted in mixed excitatory and inhibitory input to Shox2 INs in uninjured mice which became predominantly excitatory after SCI. Shox2 INs were differentially modulated by serotonin (5-HT) in a concentration-dependent manner in uninjured conditions but following SCI, 5-HT predominantly depolarized Shox2 INs. 5-HT7 receptors mediated excitatory effects on Shox2 INs from both uninjured and SCI mice, but activation of 5-HT2B/2C receptors enhanced excitability of Shox2 INs only after SCI. Overall, SCI alters sensory afferent input pathways to Shox2 INs and 5-HT modulation of Shox2 INs to enhance excitatory responses. Our findings provide relevant information regarding the locomotor circuitry response to SCI that could benefit strategies to improve locomotion after SCI.SIGNIFICANCE STATEMENT Current therapies to gain locomotor control after spinal cord injury (SCI) target spinal locomotor circuitry. Improvements in therapeutic strategies will require a better understanding of the SCI-induced plasticity within specific locomotor elements and their controllers, including sensory afferents and serotonergic modulation. Here, we demonstrate that excitability and intrinsic properties of Shox2 interneurons, which contribute to the generation of the locomotor rhythm and pattering, remain intact after SCI. However, SCI induces plasticity in both sensory afferent pathways and serotonergic modulation, enhancing the activation and excitation of Shox2 interneurons. Our findings will impact future strategies looking to harness these changes with the ultimate goal of restoring functional locomotion after SCI.

Respiratory axon regeneration in the chronically injured spinal cord

Promoting the combination of robust regeneration of damaged axons and synaptic reconnection of these growing axon populations with appropriate neuronal targets represents a major therapeutic goal following spinal cord injury (SCI). A key impediment to achieving this important aim includes an intrinsic inability of neurons to extend axons in adult CNS, particularly in the context of the chronically-injured spinal cord. We tested whether an inhibitory peptide directed against phosphatase and tensin homolog (PTEN: a central inhibitor of neuron-intrinsic axon growth potential) could restore inspiratory diaphragm function by reconnecting critical respiratory neural circuitry in a rat model of chronic cervical level 2 (C2) hemisection SCI. We found that systemic delivery of PTEN antagonist peptide 4 (PAP4) starting at 8 weeks after C2 hemisection promoted substantial, long-distance regeneration of injured bulbospinal rostral Ventral Respiratory Group (rVRG) axons into and through the lesion and back toward phrenic motor neurons (PhMNs) located in intact caudal C3-C5 spinal cord. Despite this robust rVRG axon regeneration, PAP4 stimulated only minimal recovery of diaphragm function. Furthermore, re-lesion through the hemisection site completely removed PAP4-induced functional improvement, demonstrating that axon regeneration through the lesion was responsible for this partial functional recovery. Interestingly, there was minimal formation of putative excitatory monosynaptic connections between regrowing rVRG axons and PhMN targets, suggesting that (1) limited rVRG-PhMN synaptic reconnectivity was responsible at least in part for the lack of a significant functional effect, (2) chronically-injured spinal cord presents an obstacle to achieving synaptogenesis between regenerating axons and post-synaptic targets, and (3) addressing this challenge is a potentially-powerful strategy to enhance therapeutic efficacy in the chronic SCI setting. In conclusion, our study demonstrates a non-invasive and transient pharmacological approach in chronic SCI to repair the critically-important neural circuitry controlling diaphragmatic respiratory function, but also sheds light on obstacles to circuit plasticity presented by the chronically-injured spinal cord.

Prelude to the special issue on novel neurocircuit, cellular and molecular targets for developing functional rehabilitation therapies of neurotrauma

The poor endogenous recovery capacity and other impediments to reinstating sensorimotor or autonomic function after adult neurotrauma have perplexed modern neuroscientists, bioengineers, and physicians for over a century. However, despite limited improvement in options to mitigate acute pathophysiological sequalae, the past 20 years have witnessed marked progresses in developing efficacious rehabilitation strategies for chronic spinal cord and brain injuries. The achievement is mainly attributable to research advancements in elucidating neuroplastic mechanisms for the potential to enhance clinical prognosis. Innovative cross-disciplinary studies have established novel therapeutic targets, theoretical frameworks, and regiments to attain treatment efficacy. This Special Issue contained eight papers that described experimental and human data along with literature reviews regarding the essential roles of the conventionally undervalued factors in neural repair: systemic inflammation, neural-respiratory inflammasome axis, modulation of glutamatergic and monoaminergic neurotransmission, neurogenesis, nerve transfer, recovery neurobiology components, and the spinal cord learning, respiration and central pattern generator neurocircuits. The focus of this work was on how to induce functional recovery from manipulating these underpinnings through their interactions with secondary injury events, peripheral and supraspinal inputs, neuromusculoskeletal network, and interventions (i.e., activity training, pharmacological adjuncts, electrical stimulation, and multimodal neuromechanical, brain-computer interface [BCI] and robotic assistance [RA] devices). The evidence suggested that if key neurocircuits are therapeutically reactivated, rebuilt, and/or modulated under proper sensory feedback, neurological function (e.g., cognition, respiration, limb movement, locomotion, etc.) will likely be reanimated after neurotrauma. The efficacy can be optimized by individualizing multimodal rehabilitation treatments via BCI/RA-integrated drug administration and neuromechanical protheses.

Ampakines stimulate phrenic motor output after cervical spinal cord injury

Activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors increases phrenic motor output. Ampakines are a class of drugs that are positive allosteric modulators of AMPA receptors. We hypothesized that 1) ampakines can stimulate phrenic activity after incomplete cervical spinal cord injury (SCI), and 2) pairing ampakines with brief hypoxia could enable sustained facilitation of phrenic bursting. Phrenic activity was recorded ipsilateral (IL) and contralateral (CL) to C2 spinal cord hemisection (C2Hx) in anesthetized adult rats. Two weeks after C2Hx, ampakine CX717 (15 mg/kg, i.v.) increased IL (61 ± 46% baseline, BL) and CL burst amplitude (47 ± 26%BL) in 8 of 8 rats. After 90 min, IL and CL bursting remained above baseline (BL) in 7 of 8 rats. Pairing ampakine with a single bout of acute hypoxia (5-min, arterial partial pressure of O₂ ~ 50 mmHg) had a variable impact on phrenic bursting, with some rats showing a large facilitation that exceeded the response of the ampakine alone group. At 8 weeks post-C2Hx, 7 of 8 rats increased IL (115 ± 117%BL) and CL burst amplitude (45 ± 27%BL) after ampakine. The IL burst amplitude remained above BL for 90-min in 7 of 8 rats; CL bursting remained elevated in 6 of 8 rats. The sustained impact of ampakine at 8 weeks was not enhanced by hypoxia exposure. Intravenous vehicle (10% 2-Hydroxypropyl-β-cyclodextrin) did not increase phrenic bursting at either time point. We conclude that ampakines effectively stimulate neural drive to the diaphragm after cervical SCI. Pairing ampakines with a single hypoxic exposure did not consistently enhance phrenic motor facilitation.

Spinal cord injury and diaphragm neuromotor control

Introduction: Neuromotor control of diaphragm muscle and the recovery of diaphragm activity following spinal cord injury have been narrowly focused on ventilation. By contrast, the understanding of neuromotor control for non-ventilatory expulsive/straining maneuvers (including coughing, defecation, and parturition) is relatively impoverished. This variety of behaviors are achieved via the recruitment of the diverse array of motor units that comprise the diaphragm muscle.Areas covered: The neuromotor control of ventilatory and non-ventilatory behaviors in health and in the context of spinal cord injury is explored. Particular attention is played to the neuroplasticity of phrenic motor neurons in various models of cervical spinal cord injury.Expert opinion: There is a remarkable paucity in our understanding of neuromotor control of maneuvers in spinal cord injury patients. Dysfunction of these expulsive/straining maneuvers reduces patient quality of life and contributes to severe morbidity and mortality. As spinal cord injury patient life expectancies continue to climb steadily, a nexus of spinal cord injury and age-associated comorbidities are likely to occur. While current research remains concerned only with the minutiae of ventilation, the major functional deficits of this clinical cohort will persist intractably. We posit some future research directions to avoid this scenario.

5-HT7 Receptor Inhibition Transiently Improves Respiratory Function Following Daily Acute Intermittent Hypercapnic-Hypoxia in Rats With Chronic Midcervical Spinal Cord Contusion

Background. Intermittent hypoxia can induce respiratory neuroplasticity to enhance respiratory motor outputs following hypoxic treatment. This type of respiratory neuroplasticity is primarily mediated by the activation of Gq-protein-coupled 5-HT2 receptors and constrained by Gs-protein-coupled 5-HT7 receptors. Objective. The present study hypothesized that the blockade of 5-HT7 receptors can potentiate the effect of intermittent hypercapnic-hypoxia on respiratory function after cervical spinal cord contusion injury. Methods. The ventilatory behaviors of unanesthetized rats with midcervical spinal cord contusions were measured before, during, and after daily acute intermittent hypercapnic-hypoxia (10 episodes of 5 minutes of hypoxia [10% O2, 4% CO2, 86% N2] with 5 minutes of normoxia intervals for 5 days) at 8 weeks postinjury. On a daily basis, 5 minutes before intermittent hypercapnic-hypoxia, rats received either a 5-HT7 receptor antagonist (SB269970, 4 mg/kg, intraperitoneal) or a vehicle (dimethyl sulfoxide). Results. Treatment with intermittent hypercapnic-hypoxia induced a similar increase in tidal volume between rats that received SB269970 and those that received dimethyl sulfoxide within 60 minutes post-hypoxia on the first day. However, after 2 to 3 days of daily acute intermittent hypercapnic-hypoxia, the baseline tidal volumes of rats treated with SB269970 increased significantly. Conclusions. These results suggest that inhibiting the 5-HT7 receptor can transiently improve daily intermittent hypercapnic-hypoxia–induced tidal volume increase in midcervical spinal contused animals. Therefore, combining pharmacological treatment with rehabilitative intermittent hypercapnic-hypoxia training may be an effective strategy for synergistically enhancing respiratory neuroplasticity to improve respiratory function following chronic cervical spinal cord injury.

Intraspinal Grafting of Serotonergic Neurons Modifies Expression of Genes Important for Functional Recovery in Paraplegic Rats

Serotonin (5-hydroxytryptamine; 5-HT) plays an important role in control of locomotion, partly through direct effects on motoneurons. Spinal cord complete transection (SCI) results in changes in 5-HT receptors on motoneurons that influence functional recovery. Activation of 5-HT_2A and 5-HT₇ receptors improves locomotor hindlimb movements in paraplegic rats. Here, we analyzed the mRNA of 5-HT_2A and 5-HT₇ receptors (encoded by Htr2a and Htr7 genes, resp.) in motoneurons innervating tibialis anterior (TA) and gastrocnemius lateralis (GM) hindlimb muscles and the tail extensor caudae medialis (ECM) muscle in intact as well as spinal rats. Moreover, the effect of intraspinal grafting of serotonergic neurons on Htr2a and Htr7 gene expression was examined to test the possibility that the graft origin 5-HT innervation in the spinal cord of paraplegic rats could reverse changes in gene expression induced by SCI. Our results indicate that SCI at the thoracic level leads to changes in Htr2a and Htr7 gene expression, whereas transplantation of embryonic serotonergic neurons modifies these changes in motoneurons innervating hindlimb muscles but not those innervating tail muscles. This suggests that the upregulation of genes critical for locomotor recovery, resulting in limb motoneuron plasticity, might account for the improved locomotion in grafted animals.

Spontaneous respiratory plasticity following unilateral high cervical spinal cord injury in behaving rats

Unilateral cervical C2 hemisection (C2Hx) is a classic model of spinal cord injury (SCI) for studying respiratory dysfunction and plasticity. However, most previous studies were performed under anesthesia, which significantly alters respiratory network. Therefore, the goal of this work was to assess spontaneous diaphragm recovery post-C2Hx in awake, freely behaving animals. Adult rats were chronically implanted with diaphragm EMG electrodes and recorded during 8 weeks post-C2Hx. Our results reveal that ipsilateral diaphragm activity partially recovers within days post-injury and reaches pre-injury amplitude in a few weeks. However, the full extent of spontaneous ipsilateral recovery is significantly attenuated by anesthesia (ketamine/xylazine, isoflurane, and urethane). This suggests that the observed recovery may be attributed in part to activation of NMDA receptors which are suppressed by anesthesia. Despite spontaneous recovery in awake animals, ipsilateral hemidiaphragm dysfunction still persists: i) Inspiratory bursts during basal (slow) breathing exhibit an altered pattern, ii) the amplitude of sighs - or augmented breaths - is significantly decreased, and iii) the injured hemidiaphragm exhibits spontaneous events of hyperexcitation. The results from this study offer an under-appreciated insight into spontaneous diaphragm activity and recovery following high cervical spinal cord injury in awake animals.

Contribution of 5-HT2A receptors on diaphragmatic recovery after chronic cervical spinal cord injury

Unilateral C2 spinal cord hemisection (C2Hx) interrupts bulbospinal respiratory pathways innervating ipsilateral phrenic motoneurons, resulting in cessation of ipsilateral diaphragm motor output. Plasticity within the spinal neural circuitry controlling the diaphragm can induce partial recovery of phrenic bursting which correlates with the time-dependent return of spinal serotonin (5-HT) immunoreactivity in the vicinity of phrenic motoneurons. The 5-HT_2A receptor subtype is present on phrenic motoneurons and its expression is up-regulated after cervical spinal cord injury; however the functional role of these receptors following injury has not been clearly defined. The present study evaluated the functional role of 5-HT_2A receptors by testing the hypothesis that pharmacologic blockade would attenuate diaphragm activity in rats with chronic cervical spinal cord injury. Bilateral diaphragm electromyography (EMG) was performed in vagal-intact and spontaneously breathing rats before and after intravenous administration of the 5-HT_2A receptor antagonist Ketanserin (1mg/kg). Intravenous ketanserin significantly attenuated ipsilateral diaphragm EMG activity in C2Hx animals but had no impact on diaphragm output in uninjured animals. We conclude that 5-HT_2A receptor activation contributes to the recovery of ipsilateral phrenic motor output after chronic cervical spinal cord injury.

Anatomical Recruitment of Spinal V2a Interneurons into Phrenic Motor Circuitry after High Cervical Spinal Cord Injury

More than half of all spinal cord injuries (SCIs) occur at the cervical level, often resulting in impaired respiration. Despite this devastating outcome, there is substantial evidence for endogenous neuroplasticity after cervical SCI. Spinal interneurons are widely recognized as being an essential anatomical component of this plasticity by contributing to novel neuronal pathways that can result in functional improvement. The identity of spinal interneurons involved with respiratory plasticity post-SCI, however, has remained largely unknown. Using a transgenic Chx10-eGFP mouse line (Strain 011391-UCD), the present study is the first to demonstrate the recruitment of excitatory interneurons into injured phrenic circuitry after a high cervical SCI. Diaphragm electromyography and anatomical analysis were used to confirm lesion-induced functional deficits and document extent of the lesion, respectively. Transneuronal tracing with pseudorabies virus (PRV) was used to identify interneurons within the phrenic circuitry. There was a robust increase in the number of PRV-labeled V2a interneurons ipsilateral to the C2 hemisection, demonstrating that significant numbers of these excitatory spinal interneurons were anatomically recruited into the phrenic motor pathway two weeks after injury, a time known to correspond with functional phrenic plasticity. Understanding this anatomical spinal plasticity and the neural substrates associated with functional compensation or recovery post-SCI in a controlled, experimental setting may help shed light onto possible cellular therapeutic candidates that can be targeted to enhance spontaneous recovery.

Impact of glutamatergic and serotonergic neurotransmission on diaphragm muscle activity after cervical spinal hemisection

Incomplete cervical spinal cord hemisection at C2 (SH) disrupts descending excitatory drive to phrenic motoneurons, paralyzing the ipsilateral diaphragm muscle. Spontaneous recovery over time is associated with increased phrenic motoneuron expression of glutamatergic N-methyl-d-aspartate (NMDA) and serotonergic 5-HT2A receptors. We hypothesized that NMDA and 5-HT2A receptor-mediated neurotransmission play a role in ipsilateral diaphragm muscle activity post-SH. Adult male Sprague-Dawley rats were implanted with bilateral diaphragm EMG electrodes for chronic EMG recordings up to 28 days post-SH (SH 28D). The extent of recovery was calculated by peak root-mean-square (RMS) EMG amplitude. In all animals, absence of ipsilateral activity was verified at 3 days post-SH. Diaphragm EMG activity was also recorded during exposure to hypoxia-hypercapnia (10% O₂-5% CO₂). In SH animals displaying recovery of ipsilateral diaphragm EMG activity at SH 28D, cervical spinal cord segments containing the phrenic motor nucleus (C3-C5) were surgically exposed and either the NMDA receptor antagonist d-2-amino-5-phosphonovalerate (d-AP5; 100 mM, 30 μl) or 5-HT2A receptor antagonist ketanserin (40 mM, 30 μl) was instilled intrathecally. Following d-AP5, diaphragm EMG amplitude was reduced ipsilaterally, during both eupnea (42% of pre-d-AP5 value; P = 0.007) and hypoxia-hypercapnia (31% of pre-d-AP5 value; P = 0.015), with no effect on contralateral EMG activity or in uninjured controls. Treatment with ketanserin did not change ipsilateral or contralateral RMS EMG amplitude in SH animals displaying recovery at SH 28D. Our results suggest that spinal glutamatergic NMDA receptor-mediated neurotransmission plays an important role in ipsilateral diaphragm muscle activity after cervical spinal cord injury.NEW & NOTEWORTHY Spontaneous recovery following C2 spinal hemisection (SH) is associated with increased phrenic motoneuron expression of glutamatergic and serotonergic receptors. In this study, we show that pharmacological inhibition of glutamatergic N-methyl-d-aspartate (NMDA) receptors blunts ipsilateral diaphragm activity post-SH. In contrast, pharmacological inhibition of serotonergic 5-HT2A receptors does not change diaphragm EMG activity post-SH. Our results suggest that NMDA receptor-mediated glutamatergic neurotransmission plays an important role in enhancing rhythmic respiratory-related diaphragm activity after spinal cord injury.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

Neuroprotective and Neurorestorative Processes after Spinal Cord Injury: The Case of the Bulbospinal Respiratory Neurons

High cervical spinal cord injuries interrupt the bulbospinal respiratory pathways projecting to the cervical phrenic motoneurons resulting in important respiratory defects. In the case of a lateralized injury that maintains the respiratory drive on the opposite side, a partial recovery of the ipsilateral respiratory function occurs spontaneously over time, as observed in animal models. The rodent respiratory system is therefore a relevant model to investigate the neuroplastic and neuroprotective mechanisms that will trigger such phrenic motoneurons reactivation by supraspinal pathways. Since part of this recovery is dependent on the damaged side of the spinal cord, the present review highlights our current understanding of the anatomical neuroplasticity processes that are developed by the surviving damaged bulbospinal neurons, notably axonal sprouting and rerouting. Such anatomical neuroplasticity relies also on coordinated molecular mechanisms at the level of the axotomized bulbospinal neurons that will promote both neuroprotection and axon growth.

Intermittent hypoxia promotes recovery of respiratory motor function in spinal cord-injured mice depleted of serotonin in the central nervous system

We examined the effect of repeated daily exposure to intermittent hypoxia (IH) on the recovery of respiratory and limb motor function in mice genetically depleted of central nervous system serotonin. Electroencephalography, diaphragm activity, ventilation, core body temperature, and limb mobility were measured in spontaneously breathing wild-type (Tph2+/+) and tryptophan hydroxylase 2 knockout (Tph2−/−) mice. Following a C2 hemisection, the mice were exposed daily to IH (i.e., twelve 4-min episodes of 10% oxygen interspersed with 4-min normoxic periods followed by a 90-min end-recovery period) or normoxia (i.e., sham protocol, 21% oxygen) for 10 consecutive days. Diaphragm activity recovered to prehemisection levels in the Tph2+/+ and Tph2−/− mice following exposure to IH but not normoxia [Tph2+/+ 1.3 ± 0.2 (SE) vs. 0.3 ± 0.2; Tph2−/− 1.06 ± 0.1 vs. 0.3 ± 0.1, standardized to prehemisection values, P < 0.01]. Likewise, recovery of tidal volume and breathing frequency was evident, although breathing frequency values did not return to prehemisection levels within the time frame of the protocol. Partial recovery of limb motor function was also evident 2 wk after spinal cord hemisection. However, recovery was not dependent on IH or the presence of serotonin in the central nervous system. We conclude that IH promotes recovery of respiratory function but not basic motor tasks. Moreover, we conclude that spontaneous or treatment-induced recovery of respiratory and motor limb function is not dependent on serotonin in the central nervous system in a mouse model of spinal cord injury.

Serotonergic activation of locomotor behavior and posture in one-day old rats

The purpose of this study was to determine what dose of quipazine, a serotonergic agonist, facilitates air-stepping and induces postural control and patterns of locomotion in newborn rats. Subjects in both experiments were 1-day-old rat pups. In Experiment 1, pups were restrained and tested for air-stepping in a 35-min test session. Immediately following a 5-min baseline, pups were treated with quipazine (1.0, 3.0, or 10.0 mg/kg) or saline (vehicle control), administered intraperitoneally in a 50 μL injection. Bilateral alternating stepping occurred most frequently following treatment with 10.0 mg/kg quipazine, however the percentage of alternating steps, interlimb phase, and step period were very similar between the 3.0 and 10.0 mg/kg doses. For interlimb phase, the forelimbs and hindlimbs maintained a near perfect anti-phase pattern of coordination, with step period averaging about 1s. In Experiment 2, pups were treated with 3.0 or 10.0 mg/kg quipazine or saline, and then were placed on a surface (open field, unrestrained). Both doses of quipazine resulted in developmentally advanced postural control and locomotor patterns, including head elevation, postural stances, pivoting, crawling, and a few instances of quadrupedal walking. The 3.0 mg/kg dose of quipazine was the most effective at evoking sustained locomotion. Between the 2 experiments, behavior exhibited by the rat pup varied based on testing environment, emphasizing the role that environment and sensory cues exert over motor behavior. Overall, quipazine administered at a dose of 3.0 mg/kg was highly effective at promoting alternating limb coordination and inducing locomotor activity in both testing environments.

TrkB gene therapy by adeno-associated virus enhances recovery after cervical spinal cord injury

Unilateral cervical spinal cord hemisection at C2 (C2SH) interrupts descending bulbospinal inputs to phrenic motoneurons, paralyzing the diaphragm muscle. Recovery after C2SH is enhanced by brain derived neurotrophic factor (BDNF) signaling via the tropomyosin-related kinase subtype B (TrkB) receptor in phrenic motoneurons. The role for gene therapy using adeno-associated virus (AAV)-mediated delivery of TrkB to phrenic motoneurons is not known. The present study determined the therapeutic efficacy of intrapleural delivery of AAV7 encoding for full-length TrkB (AAV-TrkB) to phrenic motoneurons 3 days post-C2SH. Diaphragm EMG was recorded chronically in male rats (n=26) up to 21 days post-C2SH. Absent ipsilateral diaphragm EMG activity was verified 3 days post-C2SH. A greater proportion of animals displayed recovery of ipsilateral diaphragm EMG activity during eupnea by 14 and 21 days post-SH after AAV-TrkB (10/15) compared to AAV-GFP treatment (2/11; p=0.031). Diaphragm EMG amplitude increased over time post-C2SH (p<0.001), and by 14 days post-C2SH, AAV-TrkB treated animals displaying recovery achieved 48% of the pre-injury values compared to 27% in AAV-GFP treated animals. Phrenic motoneuron mRNA expression of glutamatergic AMPA and NMDA receptors revealed a significant, positive correlation (r(2)=0.82), with increased motoneuron NMDA expression evident in animals treated with AAV-TrkB and that displayed recovery after C2SH. Overall, gene therapy using intrapleural delivery of AAV-TrkB to phrenic motoneurons is sufficient to promote recovery of diaphragm activity, adding a novel potential intervention that can be administered after upper cervical spinal cord injury to improve impaired respiratory function.

Effects of serotonergic agents on respiratory recovery after cervical spinal injury

Unilateral cervical spinal cord hemisection (i.e., C2Hx) usually interrupts the bulbospinal respiratory pathways and results in respiratory impairment. It has been demonstrated that activation of the serotonin system can promote locomotor recovery after spinal cord injury. The present study was designed to investigate whether serotonergic activation can improve respiratory function during the chronic injury state. Bilateral diaphragm electromyogram and tidal volume were measured in anesthetized and spontaneously breathing adult rats at 8 wk post-C2Hx or C2laminectomy. A bolus intravenous injection of a serotonin precursor [5-hydroxytryptophan (5-HTP), 10 mg/kg], a serotonin reuptake inhibitor (fluoxetine, 10 mg/kg), or a potent agonist for serotonin 2A receptors (TCB-2, 0.05 mg/kg) was used to activate the serotonergic system. Present results demonstrated that 5-HTP and TCB-2, but not fluoxetine, significantly increased the inspiratory activity of the diaphragm electromyogram ipsilateral to the lesion for at least 30 min in C2Hx animals, but not in animals that received sham surgery. However, the tidal volume was not increased after administration of 5-HTP or TCB-2, indicating that the enhancement of ipsilateral diaphragm activity is not associated with improvement of the tidal volume. These results suggest that exogenous activation of the serotonergic system can specifically enhance the ipsilateral diaphragmatic motor outputs, but this approach may not be sufficient to improve respiratory functional recovery following chronic cervical spinal injury.

Functional recovery after cervical spinal cord injury: Role of neurotrophin and glutamatergic signaling in phrenic motoneurons

Cervical spinal cord injury (SCI) interrupts descending neural drive to phrenic motoneurons causing diaphragm muscle (DIAm) paralysis. Recent studies using a well-established model of SCI, unilateral spinal hemisection of the C2 segment of the cervical spinal cord (SH), provide novel information regarding the molecular and cellular mechanisms of functional recovery after SCI. Over time post-SH, gradual recovery of rhythmic ipsilateral DIAm activity occurs. Recovery of ipsilateral DIAm electromyogram (EMG) activity following SH is enhanced by increasing brain-derived neurotrophic factor (BDNF) in the region of the phrenic motoneuron pool. Delivery of exogenous BDNF either via intrathecal infusion or via mesenchymal stem cells engineered to release BDNF similarly enhance recovery. Conversely, recovery after SH is blunted by quenching endogenous BDNF with the fusion-protein TrkB-Fc in the region of the phrenic motoneuron pool or by selective inhibition of TrkB kinase activity using a chemical-genetic approach in TrkB(F616A) mice. Furthermore, the importance of BDNF signaling via TrkB receptors at phrenic motoneurons is highlighted by the blunting of recovery by siRNA-mediated downregulation of TrkB receptor expression in phrenic motoneurons and by the enhancement of recovery evident following virally-induced increases in TrkB expression specifically in phrenic motoneurons. BDNF/TrkB signaling regulates synaptic plasticity in various neuronal systems, including glutamatergic pathways. Glutamatergic neurotransmission constitutes the main inspiratory-related, excitatory drive to motoneurons, and following SH, spontaneous neuroplasticity is associated with increased expression of ionotropic N-methyl-d-aspartate (NMDA) receptors in phrenic motoneurons. Evidence for the role of BDNF/TrkB and glutamatergic signaling in recovery of DIAm activity following cervical SCI is reviewed.

Phrenic long-term facilitation requires PKCθ activity within phrenic motor neurons

Acute intermittent hypoxia (AIH) induces a form of spinal motor plasticity known as phrenic long-term facilitation (pLTF); pLTF is a prolonged increase in phrenic motor output after AIH has ended. In anesthetized rats, we demonstrate that pLTF requires activity of the novel PKC isoform, PKCθ, and that the relevant PKCθ is within phrenic motor neurons. Whereas spinal PKCθ inhibitors block pLTF, inhibitors targeting other PKC isoforms do not. PKCθ is highly expressed in phrenic motor neurons, and PKCθ knockdown with intrapleural siRNAs abolishes pLTF. Intrapleural siRNAs targeting PKCζ, an atypical PKC isoform expressed in phrenic motor neurons that underlies a distinct form of phrenic motor plasticity, does not affect pLTF. Thus, PKCθ plays a critical role in spinal AIH-induced respiratory motor plasticity, and the relevant PKCθ is localized within phrenic motor neurons. Intrapleural siRNA delivery has considerable potential as a therapeutic tool to selectively manipulate plasticity in vital respiratory motor neurons.

Spinal interneurons and forelimb plasticity after incomplete cervical spinal cord injury in adult rats

Cervical spinal cord injury (cSCI) disrupts bulbospinal projections to motoneurons controlling the upper limbs, resulting in significant functional impairments. Ongoing clinical and experimental research has revealed several lines of evidence for functional neuroplasticity and recovery of upper extremity function after SCI. The underlying neural substrates, however, have not been thoroughly characterized. The goals of the present study were to map the intraspinal motor circuitry associated with a defined upper extremity muscle, and evaluate chronic changes in the distribution of this circuit following incomplete cSCI. Injured animals received a high cervical (C2) lateral hemisection (Hx), which compromises supraspinal input to ipsilateral spinal motoneurons controlling the upper extremities (forelimb) in the adult rat. A battery of behavioral tests was used to characterize the time course and extent of forelimb motor recovery over a 16 week period post-injury. A retrograde transneuronal tracer - pseudorabies virus - was used to define the motor and pre-motor circuitry controlling the extensor carpi radialis longus (ECRL) muscle in spinal intact and injured animals. In the spinal intact rat, labeling was observed unilaterally within the ECRL motoneuron pool and within spinal interneurons bilaterally distributed within the dorsal horn and intermediate gray matter. No changes in labeling were observed 16 weeks post-injury, despite a moderate degree of recovery of forelimb motor function. These results suggest that recovery of the forelimb function assessed following C2Hx injury does not involve recruitment of new interneurons into the ipsilateral ECRL motor pathway. However, the functional significance of these existing interneurons to motor recovery requires further exploration.

Daily acute intermittent hypoxia elicits functional recovery of diaphragm and inspiratory intercostal muscle activity after acute cervical spinal injury

A major cause of mortality after spinal cord injury is respiratory failure. In normal rats, acute intermittent hypoxia (AIH) induces respiratory motor plasticity, expressed as diaphragm (Dia) and second external intercostal (T2 EIC) long-term facilitation (LTF). Dia (not T2 EIC) LTF is enhanced by systemic adenosine 2A (A2A) receptor inhibition in normal rats. We investigated the respective contributions of Dia and T2 EIC to daily AIH-induced functional recovery of breathing capacity with/without A2A receptor antagonist (KW6002, i.p.) following C2 hemisection (C2HS). Rats received daily AIH (dAIH: 10, 5-min episodes, 10.5% O2; 5-min normoxic intervals; 7 successive days beginning 7days post-C2HS) or daily normoxia (dNx) with/without KW6002, followed by weekly (reminder) presentations for 8weeks. Ventilation and EMGs from bilateral diaphragm and T2 EIC muscles were measured with room air breathing (21% O2) and maximum chemoreceptor stimulation (

MCS

7% CO2, 10.5% O2). dAIH increased tidal volume (VT) in C2HS rats breathing room air (dAIH+vehicle: 0.47±0.02, dNx+vehicle: 0.40±0.01ml/100g; p<0.05) and MCS (dAIH+vehicle: 0.83±0.01, dNx+vehicle: 0.73±0.01ml/100g; p<0.001); KW6002 had no significant effect. dAIH enhanced contralateral (uninjured) diaphragm EMG activity, an effect attenuated by KW6002, during room air breathing and MCS (p<0.05). Although dAIH enhanced contralateral T2 EIC EMG activity during room air breathing, KW6002 had no effect. dAIH had no statistically significant effects on diaphragm or T2 EIC EMG activity ipsilateral to injury. Thus, two weeks post-C2HS: 1) dAIH enhances breathing capacity by effects on contralateral diaphragm and T2 EIC activity; and 2) dAIH-induced recovery is A2A dependent in diaphragm, but not T2 EIC. Daily AIH may be a useful in promoting functional recovery of breathing capacity after cervical spinal injury, but A2A receptor antagonists (e.g. caffeine) may undermine its effectiveness shortly after injury.

The role of the serotonergic system in locomotor recovery after spinal cord injury

Serotonin (5-HT), a monoamine neurotransmitter synthesized in various populations of brainstem neurons, plays an important role in modulating the activity of spinal networks involved in vertebrate locomotion. Following spinal cord injury (SCI) there is a disruption of descending serotonergic projections to spinal motor areas, which results in a subsequent depletion in 5-HT, the dysregulation of 5-HT transporters as well as the elevated expression, super-sensitivity and/or constitutive auto-activation of specific 5-HT receptors. These changes in the serotonergic system can produce varying degrees of locomotor dysfunction through to paralysis. To date, various approaches targeting the different components of the serotonergic system have been employed to restore limb coordination and improve locomotor function in experimental models of SCI. These strategies have included pharmacological modulation of serotonergic receptors, through the administration of specific 5-HT receptor agonists, or by elevating the 5-HT precursor 5-hydroxytryptophan, which produces a global activation of all classes of 5-HT receptors. Stimulation of these receptors leads to the activation of the locomotor central pattern generator (CPG) below the site of injury to facilitate or improve the quality and frequency of movements, particularly when used in concert with the activation of other monoaminergic systems or coupled with electrical stimulation. Another approach has been to employ cell therapeutics to replace the loss of descending serotonergic input to the CPG, either through transplanted fetal brainstem 5-HT neurons at the site of injury that can supply 5-HT to below the level of the lesion or by other cell types to provide a substrate at the injury site for encouraging serotonergic axon regrowth across the lesion to the caudal spinal cord for restoring locomotion.

Chronic Intermittent Hypoxia Blunts the Expression of Ventilatory Long Term Facilitation in Sleeping Rats

We have previously reported that chronic intermittent hypoxia (CIH), a central feature of human sleep-disordered breathing, causes respiratory instability in sleeping rats (Edge D, Bradford A, O'halloran KD. Adv Exp Med Biol 758:359-363, 2012). Long term facilitation (LTF) of respiratory motor outputs following exposure to episodic, but not sustained, hypoxia has been described. We hypothesized that CIH would enhance ventilatory LTF during sleep. We examined the effects of 3 and 7 days of CIH exposure on the expression of ventilatory LTF in sleeping rats. Adult male Wistar rats were exposed to 20 cycles of normoxia and hypoxia (5 % O(2) at nadir; SaO(2) ~ 80 %) per hour, 8 h per day for 3 or 7 consecutive days (CIH, N = 7 per group). Corresponding sham groups (N = 7 per group) were subjected to alternating cycles of air under identical experimental conditions in parallel. Following gas exposures, breathing during sleep was assessed in unrestrained, unanaesthetized animals using the technique of whole-body plethysmography. Rats were exposed to room air (baseline) and then to an acute IH (AIH) protocol consisting of alternating periods of normoxia (7 min) and hypoxia (FiO(2) 0.1, 5 min) for 10 cycles. Breathing was monitored during the AIH exposure and for 1 h in normoxia following AIH exposure. Baseline ventilation was elevated after 3 but not 7 days of CIH exposure. The hypoxic ventilatory response was equivalent in sham and CIH animals after 3 days but ventilatory responses to repeated hypoxic challenges were significantly blunted following 7 days of CIH. Minute ventilation was significantly elevated following AIH exposure compared to baseline in sham but not in CIH exposed animals. LTF, determined as the % increase in minute ventilation from baseline following AIH exposure, was significantly blunted in CIH exposed rats. In summary, CIH leads to impaired ventilatory responsiveness to AIH. Moreover, CIH blunts ventilatory LTF. The physiological significance of ventilatory LTF is context-dependent but it is reasonable to consider that it can potentially destabilize respiratory control, in view of the potential for LTF to give rise to hypocapnia. CIH-induced blunting of LTF may represent a compensatory mechanism subserving respiratory homeostasis. Our results suggest that CIH-induced increase in apnoea index (Edge D, Bradford A, O'halloran KD. Adv Exp Med Biol 758:359-363, 2012) is not related to enhanced ventilatory LTF. We conclude that the mature adult respiratory system exhibits plasticity and metaplasticity with potential consequences for the control of respiratory homeostasis. Our results may have implications for human sleep apnoea.

Intermittent hypoxia: a low-risk research tool with therapeutic value in humans

Intermittent hypoxia has generally been perceived as a high-risk stimulus, particularly in the field of sleep medicine, because it is thought to initiate detrimental cardiovascular, respiratory, cognitive, and metabolic outcomes. In contrast, the link between intermittent hypoxia and beneficial outcomes has received less attention, perhaps because it is not universally understood that outcome measures following exposure to intermittent hypoxia may be linked to the administered dose. The present review is designed to emphasize the less recognized beneficial outcomes associated with intermittent hypoxia. The review will consider the role intermittent hypoxia has in cardiovascular and autonomic adaptations, respiratory motor plasticity, and cognitive function. Each section will highlight the literature that contributed to the belief that intermittent hypoxia leads primarily to detrimental outcomes. The second segment of each section will consider the possible risks associated with experimentally rather than naturally induced intermittent hypoxia. Finally, the body of literature indicating that intermittent hypoxia initiates primarily beneficial outcomes will be considered. The overarching theme of the review is that the use of intermittent hypoxia in research investigations, coupled with reasonable safeguards, should be encouraged because of the potential benefits linked to the administration of a variety of low-risk intermittent hypoxia protocols.

Reprint of "Drawing breath without the command of effectors: the control of respiration following spinal cord injury"

The maintenance of blood gas and pH homeostasis is essential to life. As such breathing, and the mechanisms which control ventilation, must be tightly regulated yet highly plastic and dynamic. However, injury to the spinal cord prevents the medullary areas which control respiration from connecting to respiratory effectors and feedback mechanisms below the level of the lesion. This trauma typically leads to severe and permanent functional deficits in the respiratory motor system. However, endogenous mechanisms of plasticity occur following spinal cord injury to facilitate respiration and help recover pulmonary ventilation. These mechanisms include the activation of spared or latent pathways, endogenous sprouting or synaptogenesis, and the possible formation of new respiratory control centres. Acting in combination, these processes provide a means to facilitate respiratory support following spinal cord trauma. However, they are by no means sufficient to return pulmonary function to pre-injury levels. A major challenge in the study of spinal cord injury is to understand and enhance the systems of endogenous plasticity which arise to facilitate respiration to mediate effective treatments for pulmonary dysfunction.

Recovery of the pulmonary chemoreflex and functional role of bronchopulmonary C-fibers following chronic cervical spinal cord injury

Persistent impairment of pulmonary defense reflexes is a critical factor contributing to pulmonary complications in patients with spinal cord injuries. The pulmonary chemoreflex evoked by activation of bronchopulmonary C-fibers has been reported to be abolished in animals with acute cervical hemisection (C2Hx). The present study examined whether the pulmonary chemoreflex can recover during the chronic injury phase and investigated the role of bronchopulmonary C-fibers on the altered breathing pattern after C2Hx. In the first protocol, bronchopulmonary C-fibers were excited by intrajugular capsaicin administration in uninjured and complete C2Hx animals 8 wk postsurgery. Capsaicin evoked pulmonary chemoreflexes in both groups, but the reflex intensity was significantly weaker in C2Hx animals. To examine whether spared spinal white matter tissue contributes to pulmonary chemoreflex recovery, the reflex was evaluated in animals with different extents of lateral injury. Linear regression analyses revealed that tidal volume significantly correlated with the extent of spared tissue; however, capsaicin-induced apnea was not related to injury severity when the ipsilateral-to-contralateral white matter ratio was <50%. In the second protocol, the influence of background bronchopulmonary C-fiber activity on respiration was investigated by blocking C-fiber conduction via perivagal capsaicin treatment. The rapid shallow breathing of C2Hx animals persisted after perivagal capsaicin treatment despite attenuation of pulmonary chemoreflexes. These results indicate that the pulmonary chemoreflex can recover to some extent following spinal injury, but remains attenuated even when there is moderate spinal tissue sparing, and that altered breathing pattern of C2Hx animals cannot be attributed to endogenous activation of bronchopulmonary C-fibers.

Traumatic injury induces changes in the expression of the serotonin 1A receptor in the spinal cord of lampreys

After spinal cord injury (SCI) in mammals, the loss of serotonin coming from the brainstem reduces the excitability of motor neurons and leads to a compensatory overexpression of serotonin receptors. Despite the key role of the serotonin receptor 1a in the control of locomotion, little attention has been put in the study of this receptor after SCI. In contrast to mammals, lampreys recover locomotion after a complete SCI, so, studies in this specie could help to understand events that lead to recovery of function. Here, we showed that in lampreys there is an acute increase in the expression of the serotonin 1A receptor transcript (5-ht1a) after SCI and a few weeks later expression levels go back to normal rostrally and caudally to the lesion. Overexpression of the 5-ht1a in rostral levels after SCI has not been reported in mammals, suggesting that this could be part of the plastic events that lead to the recovery of function in lampreys. The analysis of changes in 5-ht1a expression by zones (periventricular region and horizontally extended grey matter) showed that they followed the same pattern of changes detected in the spinal cord as a whole, with the exception of the caudal periventricular layer, where no significant differences were observed between control and experimental animals at any time post lesion. This suggests that different molecular signals act on the periventricular cells of the rostral and caudal regions to injury site and thus affecting their response to the injury in terms of expression of the 5-ht1a.

Quantitative expression profiling in mouse spinal cord reveals changing relationships among channel and receptor mRNA levels across postnatal maturation

Neural networks ultimately arrive at functional output via interaction of the excitability of individual neurons and their synaptic interactions. We investigated the relationships between voltage-gated ion channel and neurotransmitter receptor mRNA levels in mouse spinal cord at four different postnatal time points (P5, P11, P17, and adult) and three different adult cord levels (cervical, thoracic, and lumbosacral) using quantitative reverse-transcription polymerase chain reaction (qRT-PCR). Our analysis and data visualization are novel in that we chose a focal group of voltage-gated channel subunits and transmitter receptor subunits, performed absolute quantitation of mRNA copy number for each gene from a sample, and used multiple correlation analyses and correlation matrices to detect patterns in correlated mRNA levels across all genes of interest. These correlation profiles suggest that postnatal maturation of the spinal cord includes changes among channel and receptor subunits that proceed from widespread co-regulation to more refined and distinct functional relationships.

Phrenic motoneuron discharge patterns following chronic cervical spinal cord injury

Cervical spinal cord injury (SCI) dramatically disrupts synaptic inputs and triggers biochemical, as well as morphological, plasticity in relation to the phrenic motor neuron (PhMN) pool. Accordingly, our primary purpose was to determine if chronic SCI induces fundamental changes in the recruitment profile and discharge patterns of PhMNs. Individual PhMN action potentials were recorded from the phrenic nerve ipsilateral to lateral cervical (C2) hemisection injury (C2Hx) in anesthetized adult male rats at 2, 4 or 8 wks post-injury and in uninjured controls. PhMNs were phenotypically classified as early (Early-I) or late inspiratory (Late-I), or silent according to discharge patterns. Following C2Hx, the distribution of PhMNs was dominated by Late-I and silent cells. Late-I burst parameters (e.g., spikes per breath, burst frequency and duration) were initially reduced but returned towards control values by 8wks post-injury. In addition, a unique PhMN burst pattern emerged after C2Hx in which Early-I cells burst tonically during hypocapnic inspiratory apnea. We also quantified the impact of gradual reductions in end-tidal CO2 partial pressure (PETCO2) on bilateral phrenic nerve activity. Compared to control rats, as PETCO2 declined, the C2Hx animals had greater inspiratory frequencies (breaths∗min(-1)) and more substantial decreases in ipsilateral phrenic burst amplitude. We conclude that the primary physiological impact of C2Hx on ipsilateral PhMN burst patterns is a persistent delay in burst onset, transient reductions in burst frequency, and the emergence of tonic burst patterns. The inspiratory frequency data suggest that plasticity in brainstem networks is likely to play an important role in phrenic motor output after cervical SCI.

The role of the crossed phrenic pathway after cervical contusion injury and a new model to evaluate therapeutic interventions

More than 50% of all spinal cord injury (SCI) cases are at the cervical level and usually result in the impaired ability to breathe. This is caused by damage to descending bulbospinal inspiratory tracts and the phrenic motor neurons which innervate the diaphragm. Most investigations have utilized a lateral C2 hemisection model of cervical SCI to study the resulting respiratory motor deficits and potential therapies. However, recent studies have emerged which incorporate experimental contusion injuries at the cervical level of the spinal cord to more closely reflect the type of trauma encountered in humans. Nonetheless, a common deficit observed in these contused animals is the inability to increase diaphragm motor activity in the face of respiratory challenge. In this report we tested the hypothesis that, following cervical contusion, all remaining tracts to the phrenic nucleus are active, including the crossed phrenic pathway (CPP). Additionally, we investigated the potential function these spared tracts might possess after injury. We find that, following a lateral C3/4 contusion injury, not all remaining pathways are actively exciting downstream phrenic motor neurons. However, removing some of these pathways through contralateral hemisection results in a cessation of all activity ipsilateral to the contusion. This suggests an important modulatory role for these pathways. Additionally, we conclude that this dual injury, hemi-contusion and post contra-hemisection, is a more effective and relevant model of cervical SCI as it results in a more direct compromise of diaphragmatic motor activity. This model can thus be used to test potential therapies with greater accuracy and clinical relevance than cervical contusion models currently allow.

Targeted delivery of TrkB receptor to phrenic motoneurons enhances functional recovery of rhythmic phrenic activity after cervical spinal hemisection

Progressive recovery of rhythmic phrenic activity occurs over time after a spinal cord hemisection involving unilateral transection of anterolateral funiculi at C₂ (SH). Brain-derived neurotrophic factor (BDNF) acting through its full-length tropomyosin related kinase receptor subtype B (TrkB.FL) contributes to neuroplasticity after spinal cord injury, but the specific cellular substrates remain unclear. We hypothesized that selectively targeting increased TrkB.FL expression to phrenic motoneurons would be sufficient to enhance recovery of rhythmic phrenic activity after SH. Several adeno-associated virus (AAV) serotypes expressing GFP were screened to determine specificity for phrenic motoneuron transduction via intrapleural injection in adult rats. GFP expression was present in the cervical spinal cord 3 weeks after treatment with AAV serotypes 7, 8, and 9, but not with AAV2, 6, or rhesus-10. Overall, AAV7 produced the most consistent GFP expression in phrenic motoneurons. SH was performed 3 weeks after intrapleural injection of AAV7 expressing human TrkB.FL-FLAG or saline. Delivery of TrkB.FL-FLAG to phrenic motoneurons was confirmed by FLAG protein expression in the phrenic motor nucleus and human TrkB.FL mRNA expression in microdissected phrenic motoneurons. In all SH rats, absence of ipsilateral diaphragm EMG activity was confirmed at 3 days post-SH, verifying complete interruption of ipsilateral descending drive to phrenic motoneurons. At 14 days post-SH, all AAV7-TrkB.FL treated rats (n = 11) displayed recovery of ipsilateral diaphragm EMG activity compared to 3 out of 8 untreated SH rats (p<0.01). During eupnea, AAV7-TrkB.FL treated rats exhibited 73±7% of pre-SH root mean squared EMG vs. only 31±11% in untreated SH rats displaying recovery (p<0.01). This study provides direct evidence that increased TrkB.FL expression in phrenic motoneurons is sufficient to enhance recovery of ipsilateral rhythmic phrenic activity after SH, indicating that selectively targeting gene expression in spared motoneurons below the level of spinal cord injury may promote functional recovery.

The time course of serotonin 2C receptor expression after spinal transection of rats: an immunohistochemical study

In the spinal cord serotonin (5-HT) systems modulate the spinal network via various 5-HT receptors. Serotonin 2A receptor and serotonin 2C receptor (5-HT2A and 2C receptors) are likely the most important 5-HT receptors for enhancing the motoneuron excitability by facilitating the persistent inward current (PIC), and thus play an important role for the pathogenesis of spasticity after spinal cord injury. In conjunction with our 5-HT2A receptor study, using a same sacral spinal transection rat model we have in this study examined 5-HT2C receptor immunoreactivity (5-HT2CR-IR) changes at seven different time intervals after spinal injury. We found that 5-HT2CR-IR was widely distributed in different regions of the spinal gray matter and was predominantly located in the neuronal somata and their dendrites although it seemed also present in axonal fibers in the superficial dorsal horn. 5-HT2CR-IR in different regions of the spinal gray matter was seen to be increased at 14days after transection (with an average ∼1.3-fold higher than in sham-operated group) but did not reach a significant level until at 21days (∼1.4-fold). The increase sustained thereafter and a plateau level was reached at 45days (∼1.7-fold higher), a value similar as that at 60days. When 5-HT2CR-IR analysis was confined to the ventral horn motoneuron somata (including a proportion of proximal dendrites) a significant increase was not detected until 45days post-operation. 5-HT2CR upregulation in the spinal gray matter is confirmed with Western blot in the rats 60days post-operation. The time course of 5-HT2CR upregulation in the spinal gray matter and motoneurons was positively correlated with the development of tail spasticity (clinical scores). This indicates that 5-HT2CR is probably an important factor underlying this pathophysiological development by increasing the excitability of both motoneurons and interneurons.

Repetitive acute intermittent hypoxia increases expression of proteins associated with plasticity in the phrenic motor nucleus

Acute intermittent hypoxia (AIH) initiates plasticity in respiratory motor control, including phrenic long term facilitation (pLTF). Since pLTF is enhanced by preconditioning with repetitive exposure to AIH (rAIH), we hypothesized that a rAIH protocol consisting of 3 AIH exposures per week for 10 weeks (3×wAIH; AIH: 10, 5-min episodes of 10.5% O(2); 5-min normoxic intervals) would enhance expression of molecules that play key roles in pLTF within the phrenic motor nucleus. Immunohistochemical analyses revealed that 3×wAIH for 10 weeks increased serotonin terminal density in the C4 phrenic motor nucleus and serotonin 2A (5-HT(2A)) receptor expression in presumptive phrenic motor neurons. Immunoreactive brain derived neurotrophic factor (BDNF) and its high affinity receptor (TrkB) also increased following 3×wAIH. 3×wAIH also increased expression of another hypoxia-sensitive growth factor known to elicit phrenic motor facilitation, vascular endothelial growth factor (VEGF), and its receptor (VEGFR-2). Kinases "downstream" from TrkB and VEGFR-2 were up-regulated in or near presumptive phrenic motor neurons, including phosphorylated extracellular-signal regulated kinase (p-ERK) and protein kinase B (p-AKT). Thus, 3×wAIH up-regulates neurochemicals known to be associated with phrenic motor plasticity. Since 3×wAIH upregulates pro-plasticity molecules without evidence for CNS pathology, it may be a useful therapeutic tool in treating disorders that cause respiratory insufficiency, such as spinal injury or motor neuron disease.

Phrenic motoneuron expression of serotonergic and glutamatergic receptors following upper cervical spinal cord injury

Following cervical spinal cord injury at C(2) (SH hemisection model) there is progressive recovery of phrenic activity. Neuroplasticity in the postsynaptic expression of neurotransmitter receptors may contribute to functional recovery. Phrenic motoneurons express multiple serotonergic (5-HTR) and glutamatergic (GluR) receptors, but the timing and possible role of these different neurotransmitter receptor subtypes in the neuroplasticity following SH are not clear. The current study was designed to test the hypothesis that there is an increased expression of serotonergic and glutamatergic neurotransmitter receptors within phrenic motoneurons after SH. In adult male rats, phrenic motoneurons were labeled retrogradely by intrapleural injection of Alexa 488-conjugated cholera toxin B. In thin (10μm) frozen sections of the spinal cord, fluorescently-labeled phrenic motoneurons were visualized for laser capture microdissection (LCM). Using quantitative real-time RT-PCR in LCM samples, the time course of changes in 5-HTR and GluR mRNA expression was determined in phrenic motoneurons up to 21 days post-SH. Expression of 5-HTR subtypes 1b, 2a and 2c and GluR subtypes AMPA, NMDA, mGluR1 and mGluR5 was evident in phrenic motoneurons from control and SH rats. Phrenic motoneuron expression of 5-HTR2a increased ~8-fold (relative to control) at 14 days post-SH, whereas NMDA expression increased ~16-fold by 21-days post-SH. There were no other significant changes in receptor expression at any time post-SH. This is the first study to systematically document changes in motoneuron expression of multiple neurotransmitter receptors involved in regulation of motoneuron excitability. By providing information on the neuroplasticity of receptors expressed in a motoneuron pool that is inactivated by a higher-level spinal cord injury, appropriate pharmacological targets can be identified to alter motoneuron excitability.

Treatments to restore respiratory function after spinal cord injury and their implications for regeneration, plasticity and adaptation

Spinal cord injury (SCI) often leads to impaired breathing. In most cases, such severe respiratory complications lead to morbidity and death. However, in the last few years there has been extensive work examining ways to restore this vital function after experimental spinal cord injury. In addition to finding strategies to rescue breathing activity, many of these experiments have also yielded a great deal of information about the innate plasticity and capacity for adaptation in the respiratory system and its associated circuitry in the spinal cord. This review article will highlight experimental SCI resulting in compromised breathing, the various methods of restoring function after such injury, and some recent findings from our own laboratory. Additionally, it will discuss findings about motor and CNS respiratory plasticity and adaptation with potential clinical and translational implications.

Functional regeneration of respiratory pathways after spinal cord injury

Spinal cord injuries (SCI) often occur at the cervical level above the phrenic motor pools, which innervate the diaphragm. Unfortunately, the untoward effects of impaired breathing are a leading cause of SCI-related death, underscoring the importance of developing strategies to restore respiratory activity. Here we show that after cervical SCI, there is upregulation of the perineuronal net (PNN) associated chondroitin sulfate proteoglycans (CSPGs) around phrenic motor neurons. Digestion of these potently inhibitory extracellular matrix molecules with Chondroitinase ABC (ChABC) can, by itself, promote plasticity of spared tracts and restore limited activity to the paralyzed diaphragm. However, when combined with application of a peripheral nerve autograft, ChABC treatment results in lengthy regeneration of serotonergic axons and other bulbospinal fibers with remarkable recovery of diaphragm function. Following recovery and initial transection of the bridge, there occurs an unusual, overall increased tonic diaphragmatic EMG activity, suggesting considerable remodeling of spinal cord circuitry after regeneration. This is followed by complete elimination of the restored activity proving that regeneration is critical for the return of function. Overall, these experiments present a way to profoundly restore function of a single muscle following debilitating CNS trauma, through both plasticity of spared tracts and regeneration of essential pathways.

Contribution of 5-HT to locomotion - the paradox of Pet-1(-/-) mice

Serotonin (5-HT) plays a critical role in locomotor pattern generation by modulating the rhythm and the coordinations. Pet-1, a transcription factor selectively expressed in the raphe nuclei, controls the differentiation of 5-HT neurons. Surprisingly, inactivation of Pet-1 (Pet-1(-/-) mice) that causes a 70% reduction in the number of 5-HT-positive neurons in the raphe does not impair locomotion in adult mice. The goal of the present study was to investigate the operation of the locomotor central pattern generator (CPG) in neonatal Pet-1(-/-) mice. We first confirmed, by means of immunohistochemistry, that there is a marked reduction of 5-HT innervation in the lumbar spinal cord of Pet-1(-/-) mice. Fictive locomotion was induced in the in vitro neonatal mouse spinal cord preparation by bath application of N-methyl-d,l-Aspartate (NMA) alone or together with dopamine and 5-HT. A locomotor pattern characterized by left-right and flexor-extensor alternations was observed in both conditions. Increasing the concentration of 5-HT from 0.5 to 5 μm impaired the pattern in Pet-1(-/-) mice. We tested the role of endogenous 5-HT in the NMA-induced fictive locomotion. Application of 5-HT(2) or 5-HT(7) receptor antagonists affected the NMA-induced fictive locomotion in both heterozygous and homozygous mice although the effects were weaker in the latter strain. This may be, at least partly, explained by the reduced expression of 5-HT(2A) R as observed by means of immunohistochemistry. These results suggest that compensatory mechanisms take place in Pet-1(-/-) mice that make locomotion less dependent upon 5-HT.

The time course of serotonin 2A receptor expression after spinal transection of rats: an immunohistochemical study

Hyperexcitability of motoneurons is one of the key mechanism that has been demonstrated to underlie the pathogenesis of spasticity after spinal injury. Serotonin (5-HT) denervation supersensitivity is one of the mechanisms underlying this increased motoneuron excitability. In this study, to examine whether the supersensitivity is caused by 5-HT receptor upregulation we investigated changes in levels of 5-HT2A receptor immunoreactivity (5-HT2AR-IR) following a spinal transection in the sacral spinal cord of rats at seven different time points post injury: 2, 8, 16 h, and 1, 2, 7 and 28 days, respectively. 5-HT2AR-IR density was analyzed in motoneurons (regions containing their somata and dendrites) in the spinal segments below the lesion. The results showed no significant changes in 5-HT2AR-IR in the motoneurons up to 16 h following the transection. After 1-day, however the levels of 5-HT2AR-IR increased in the motoneurons and their dendrites, with the density level being 3.4-fold higher in spinalized rats than in sham-operated rats. The upregulation increased progressively until a maximal level was reached at 28 days post-injury. We also investigated 5-HT and 5-HT transporter expressions at five different post injury time points: 1, 2, 7, 21 and 60 days and they showed concurrent down-regulation changes after 2 days. These results suggest that the upregulation of 5-HT2ARs may at least partly underlie the development of 5-HT denervation supersensitivity in spinal motoneurons following spinal injury and thereby implicates their involvement in the pathogenesis of the subsequent development of spasticity.

Spinal plasticity following intermittent hypoxia: implications for spinal injury

Plasticity is a fundamental property of the neural system controlling breathing. One frequently studied model of respiratory plasticity is long-term facilitation of phrenic motor output (pLTF) following acute intermittent hypoxia (AIH). pLTF arises from spinal plasticity, increasing respiratory motor output through a mechanism that requires new synthesis of brain-derived neurotrophic factor, activation of its high-affinity receptor, tropomyosin-related kinase B, and extracellular-related kinase mitogen-activated protein kinase signaling in or near phrenic motor neurons. Because intermittent hypoxia induces spinal plasticity, we are exploring the potential to harness repetitive AIH as a means of inducing functional recovery in conditions causing respiratory insufficiency, such as cervical spinal injury. Because repetitive AIH induces phenotypic plasticity in respiratory motor neurons, it may restore respiratory motor function in patients with incomplete spinal injury.