Synaptic plasticity of 5-hydroxytryptamine-immunoreactive terminals in the phrenic nucleus following spinal cord injury: A quantitative electron microscopic analysis

Serotonergic innervation of respiratory motor nuclei after cervical spinal injury: Impact of intermittent hypoxia

Although cervical spinal cord injury (cSCI) disrupts bulbo-spinal serotonergic projections, partial recovery of spinal serotonergic innervation below the injury site is observed after incomplete cSCI. Since serotonin contributes to functional recovery post-injury, treatments to restore or accelerate serotonergic reinnervation are of considerable interest. Intermittent hypoxia (IH) was reported to increase serotonin innervation near respiratory motor neurons in spinal intact rats, and to improve function after cSCI. Here, we tested the hypotheses that spontaneous serotonergic reinnervation of key respiratory (phrenic and intercostal) motor nuclei: 1) is partially restored 12 weeks post C2 hemisection (C2Hx); 2) is enhanced by IH; and 3) results from sprouting of spared crossed-spinal serotonergic projections below the site of injury. Serotonin was assessed via immunofluorescence in male Sprague Dawley rats with and without C2Hx (12 wks post-injury); individual groups were exposed to 28 days of: 1) normoxia; 2) daily acute IH (dAIH28: 10, 5 min 10.5% O2 episodes per day; 5 min normoxic intervals); 3) mild chronic IH (IH28-5/5: 5 min 10.5% O2 episodes; 5 min intervals; 8 h/day); or 4) moderate chronic IH (IH28-2/2: 2 min 10.5% O2 episodes; 2 min intervals; 8 h/day), simulating IH experienced during moderate sleep apnea. After C2Hx, the number of ipsilateral serotonergic structures was decreased in both motor nuclei, regardless of IH protocol. However, serotonergic structures were larger after C2Hx in both motor nuclei, and total serotonin immunolabeling area was increased in the phrenic motor nucleus but reduced in the intercostal motor nucleus. Both chronic IH protocols increased serotonin structure size and total area in the phrenic motor nuclei of uninjured rats, but had no detectable effects after C2Hx. Although the functional implications of fewer but larger serotonergic structures are unclear, we confirm that serotonergic reinnervation is substantial following injury, but IH does not affect the extent of reinnervation.

High-frequency epidural stimulation across the respiratory cycle evokes phrenic short-term potentiation after incomplete cervical spinal cord injury

C2 spinal hemilesion (C2Hx) paralyzes the ipsilateral diaphragm, but recovery is possible through activation of "crossed spinal" synaptic inputs to ipsilateral phrenic motoneurons. We tested the hypothesis that high-frequency epidural stimulation (HF-ES) would potentiate ipsilateral phrenic output after subacute and chronic C2Hx. HF-ES (300 Hz) was applied to the ventrolateral C4 or T2 spinal cord ipsilateral to C2Hx in anesthetized and mechanically ventilated adult rats. Stimulus duration was 60 s, and currents ranged from 100 to 1,000 µA. Bilateral phrenic nerve activity and ipsilateral hypoglossal (XII) nerve activity were recorded before and after HF-ES. Higher T2 stimulus currents potentiated ipsilateral phasic inspiratory activity at both 2 and 12 wk post-C2Hx, whereas higher stimulus currents delivered at C4 potentiated ipsilateral phasic phrenic activity only at 12 wk (P = 0.028). Meanwhile, tonic output in the ipsilateral phrenic nerve reached 500% of baseline values at the high currents with no difference between 2 and 12 wk. HF-ES did not trigger inspiratory burst-frequency changes. Similar responses occurred following T2 HF-ES. Increases in contralateral phrenic and XII nerve output were induced by C4 and T2 HF-ES at higher currents, but the relative magnitude of these changes was small compared with the ipsilateral phrenic response. We conclude that following incomplete cervical spinal cord injury, HF-ES of the ventrolateral midcervical or thoracic spinal cord can potentiate efferent phrenic motor output with little impact on inspiratory burst frequency. However, the substantial increases in tonic output indicate that the uninterrupted 60-s stimulation paradigm used is unlikely to be useful for respiratory muscle activation after spinal injury.NEW & NOTEWORTHY Previous studies reported that high-frequency epidural stimulation (HF-ES) activates the diaphragm following acute spinal transection. This study examined HF-ES and phrenic motor output following subacute and chronic incomplete cervical spinal cord injury. Short-term potentiation of phrenic bursting following HF-ES illustrates the potential for spinal stimulation to induce respiratory neuroplasticity. Increased tonic phrenic output indicates that alternatives to the continuous stimulation paradigm used in this study will be required for respiratory muscle activation after spinal cord injury.

Enhanced recovery of breathing capacity from combined adenosine 2A receptor inhibition and daily acute intermittent hypoxia after chronic cervical spinal injury

Daily acute intermittent hypoxia (dAIH) improves breathing capacity after C2 spinal hemisection (C2HS) in rats. Since C2HS disrupts spinal serotonergic innervation below the injury, adenosine-dependent mechanisms underlie dAIH-induced functional recovery 2weeks post-injury. We hypothesized that dAIH-induced functional recovery converts from an adenosine-dependent to a serotonin-dependent, adenosine-constrained mechanism with chronic injury. Eight weeks post-C2HS, rats began dAIH (10, 5-min episodes, 10.5% O2; 5-min intervals; 7days) followed by AIH 3× per week (3×wAIH) for 8 additional weeks with/without systemic A2A receptor inhibition (KW6002) on each AIH exposure day. Tidal volume (VT) and bilateral diaphragm (Dia) and T2 external intercostal motor activity were assessed in unanesthetized rats breathing air and during maximum chemoreflex stimulation (MCS: 7% CO2, 10.5% O2). Nine weeks post-C2HS, dAIH increased VT versus time controls (p<0.05), an effect enhanced by KW6002 (p<0.05). dAIH increased bilateral Dia activity (p<0.05), and KW6002 enhanced this effect in contralateral (p<0.05) and ipsilateral Dia activity (p<0.001), but not T2 inspiratory activity. Functional benefits of combined AIH plus systemic A2A receptor inhibition were maintained for 4weeks. Thus, in rats with chronic injuries: 1) dAIH improves VT and bilateral diaphragm activity; 2) VT recovery is enhanced by A2A receptor inhibition; and 3) functional recovery with A2A receptor inhibition and AIH "reminders" last 4weeks. Combined dAIH and A2A receptor inhibition may be a simple, safe, and effective strategy to accelerate/enhance functional recovery of breathing capacity in patients with respiratory impairment from chronic spinal injury.

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.

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.

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.

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.

Cough following low thoracic hemisection in the cat

A function of the abdominal expiratory muscles is the generation of cough, a critical respiratory defense mechanism that is often disrupted following spinal cord injury. We assessed the effects of a lateral T9/10 hemisection on cough production at 4, 13 and 21 weeks post-injury in cats receiving extensive locomotor training. The magnitudes of esophageal pressure as well as of bilateral rectus abdominis electromyogram activity during cough were not significantly different from pre-injury values at all time points evaluated. The results show that despite considerable interruption of the descending pre-motor drive from the brainstem to the expiratory motoneuron pools, the cough motor system shows a significant function by 4 weeks following incomplete thoracic injury.

Localization of serotoninergic neurons that participate in regulating diaphragm activity in the cat

Although a considerable body of literature indicates that serotoninergic neurons affect diaphragm activity both through direct inputs to phrenic motoneurons and multisynaptic connections involving the brainstem respiratory groups, the locations of the serotoninergic neurons that modulate breathing have not been well defined. The present study identified these neurons in cats by combining the transneuronal retrograde transport of rabies virus from the diaphragm with the immunohistochemical detection of the N-terminal region of tryptophan hydroxylase-2 (TPH2), the brain-specific isoform of the enzyme responsible for the initial and rate-limiting step in serotonin synthesis. TPH2-immunopositive neurons were present in the midline raphe nuclei, formed a column in the ventrolateral medulla near the lateral reticular nucleus, and were spread across the dorsal portion of the pons just below the fourth ventricle. In most animals, only a small fraction of neurons (typically <20%) labeled for TPH2 in each of the medullary raphe nuclei and the medullary ventrolateral column were infected with rabies virus. However, the percentage of medullary neurons dual-labeled for both rabies and TPH2 was much higher in animals with very advanced infections where virus had spread transneuronally through many synapses. Furthermore, in all cases, TPH2-immunopositive neurons that were infected by rabies virus were significantly less prevalent in the pons than the medulla. These findings suggest that although serotoninergic neurons with direct influences on diaphragm activity are widely scattered in the brainstem, the majority of these neurons are located in the medulla. Many non-serotoninergic neurons in the raphe nuclei were also infected with rabies virus, indicating that midline cells utilizing multiple neurotransmitters participate in the control of breathing.

Glutamate receptor plasticity and activity-regulated cytoskeletal associated protein regulation in the phrenic motor nucleus may mediate spontaneous recovery of the hemidiaphragm following chronic cervical spinal cord injury

High cervical spinal cord hemisection results in paralysis of the ipsilateral hemidiaphragm; however, functional recovery of the paralyzed hemidiaphragm can occur spontaneously. The mechanisms mediating this recovery are unknown. In chronic, experimental contusive spinal cord injury, an upregulation of the NMDA receptor 2A subunit and a downregulation of the AMPA receptor GluR2 subunit have been correlated with improved hind limb motor recovery. Therefore, we hypothesized that NR2A is upregulated, whereas GluR2 is down-regulated following chronic C2 hemisection to initiate synaptic strengthening in respiratory motor pathways. Since NMDA receptor activation can lead to the delivery of AMPA receptor subunits to the post-synaptic membrane, we also hypothesized that there would be an upregulation of the GluR1 AMPA receptor subunit and that activity-regulated cytoskeletal associated protein may mediate the post-synaptic membrane delivery. Female rats were hemisected at C2 and allowed to recover for different time points following hemisection. At these time points, protein levels of NR2A, GluR1, and GluR2 subunits were assessed via Western blot analysis. Western blot analysis revealed that there were increases in NR2A subunit at six and twelve weeks post C2 hemisection. At six, twelve, and sixteen weeks post hemisection, the GluR1 subunit was increased over controls, whereas the GluR2 subunit decreased sixteen weeks post hemisection. Immunocytochemical data qualitatively supported these findings. Results also indicated that activity-regulated cytoskeletal associated protein may be associated with the above changes. These findings suggest a role of NR2A, GluR1, and GluR2 in mediating chronic spontaneous functional recovery of the paralyzed hemidiaphragm following cervical spinal cord hemisection.

Effect of spinal cord injury on the neural regulation of respiratory function

Injury at any level of the spinal cord can impair respiratory motor function. Indeed, complications associated with respiratory function are the number one cause of mortality in humans following spinal cord injury (SCI) at any level of the cord. This review is aimed at describing the effect of SCI on respiratory function while highlighting the recent advances made by basic science research regarding the neural regulation of respiratory function following injury. Models of SCI that include upper cervical hemisection and contusion injury have been utilized to examine the underlying neural mechanisms of respiratory control following injury. The approaches used to induce motor recovery in the respiratory system are similar to other studies that examine recovery of locomotor function after SCI. These include strategies to initiate regeneration of damaged axons, to reinnervate paralyzed muscles with peripheral nerve grafts, to use spared neural pathways to induce motor function, and finally, to initiate mechanisms of neural plasticity within the spinal cord to increase motoneuron firing. The ultimate goals of this research are to restore motor function to previously paralyzed respiratory muscles and improve ventilation in patients with SCI.

Effect of spinal cord injury on the respiratory system: basic research and current clinical treatment options

Spinal cord injury (SCI) often leads to an impairment of the respiratory system. The more rostral the level of injury, the more likely the injury will affect ventilation. In fact, respiratory insufficiency is the number one cause of mortality and morbidity after SCI. This review highlights the progress that has been made in basic and clinical research, while noting the gaps in our knowledge. Basic research has focused on a hemisection injury model to examine methods aimed at improving respiratory function after SCI, but contusion injury models have also been used. Increasing synaptic plasticity, strengthening spared axonal pathways, and the disinhibition of phrenic motor neurons all result in the activation of a latent respiratory motor pathway that restores function to a previously paralyzed hemidiaphragm in animal models. Human clinical studies have revealed that respiratory function is negatively impacted by SCI. Respiratory muscle training regimens may improve inspiratory function after SCI, but more thorough and carefully designed studies are needed to adequately address this issue. Phrenic nerve and diaphragm pacing are options available to wean patients from standard mechanical ventilation. The techniques aimed at improving respiratory function in humans with SCI have both pros and cons, but having more options available to the clinician allows for more individualized treatment, resulting in better patient care. Despite significant progress in both basic and clinical research, there is still a significant gap in our understanding of the effect of SCI on the respiratory system.

Recovery of phrenic activity and ventilation after cervical spinal hemisection in rats

We tested two hypotheses: 1) that the spontaneous enhancement of phrenic motor output below a C2 spinal hemisection (C2HS) is associated with plasticity in ventrolateral spinal inputs to phrenic motoneurons; and 2) that phrenic motor recovery in anesthetized rats after C2HS correlates with increased capacity to generate inspiratory volume during hypercapnia in unanesthetized rats. At 2 and 4 wk post-C2HS, ipsilateral phrenic nerve activity was recorded in anesthetized, paralyzed, vagotomized, and ventilated rats. Electrical stimulation of the ventrolateral funiculus contralateral to C2HS was used to activate crossed spinal synaptic pathway phrenic motoneurons. Inspiratory phrenic burst amplitudes ipsilateral to C2HS were larger in the 4- vs. 2-wk groups ( P < 0.05); however, no differences in spinally evoked compound phrenic action potentials could be detected. In unanesthetized rats, inspiratory volume and frequency were quantified using barometric plethysmography at inspired CO2 fractions between 0.0 and 0.07 (inspired O2 fraction 0.21, balance N2) before and 2, 3, and 5 wk post-C2HS. Inspiratory volume was diminished, and frequency enhanced, at 0.0 inspired CO2 fraction ( P < 0.05) 2-wk post-C2HS; further changes were not observed in the 3- and 5-wk groups. Inspiratory frequency during hypercapnia was unaffected by C2HS. Hypercapnic inspiratory volumes were similarly attenuated at all time points post-C2HS ( P < 0.05), thereby decreasing hypercapnic minute ventilation ( P < 0.05). Thus increases in ipsilateral phrenic activity during 4 wk post-C2HS have little impact on the capacity to generate inspiratory volume in unanesthetized rats. Enhanced crossed phrenic activity post-C2HS may reflect plasticity associated with spinal axons not activated by our ventrolateral spinal stimulation.

The education and re-education of the spinal cord

In normal life, activity-dependent plasticity occurs in the spinal cord as well as in the brain. Like CNS plasticity elsewhere, this spinal cord plasticity can occur at many neuronal and synaptic sites and by a variety of mechanisms. Spinal cord plasticity is prominent in postnatal development and contributes to acquisition of standard behaviors such as locomotion and rapid withdrawal from pain. Later on in life, spinal cord plasticity contributes to acquisition and maintenance of specialized motor skills, and to compensation for the peripheral and central changes associated with aging, disease, and trauma. Mastery of even the simplest behaviors is accompanied by complex spinal and supraspinal plasticity. This complexity is necessary, to preserve the full roster of behaviors, and is also inevitable, due to the ubiquity of activity-dependent plasticity in the CNS. Careful investigation of spinal cord plasticity is essential for understanding motor skills; and, because of the relative simplicity and accessibility of the spinal cord, is a logical and convenient starting point for exploring skill acquisition. Appropriate induction and guidance of activity-dependent plasticity in the spinal cord is likely to be a key part of the realization of effective new rehabilitation methods for spinal cord injuries, cerebral palsy, and other chronic motor disorders.

Cervical spinal cord injury upregulates ventral spinal 5-HT2A receptors

Following chronic C2 spinal hemisection (C2HS), crossed spinal pathways to phrenic motoneurons exhibit a slow, spontaneous increase in efficacy by a serotonin (5-HT)-dependent mechanism associated with 5-HT2A receptor activation. Further, the spontaneous appearance of cross-phrenic activity following C2HS is accelerated and enhanced by exposure to chronic intermittent hypoxia (CIH). We hypothesized that chronic C2HS would increase 5-HT and 5-HT2A receptor expression in ventral cervical spinal segments containing phrenic motoneurons. In addition, we hypothesized that CIH exposure would further increase 5-HT and 5-HT2A receptor density in this region. Control, sham-operated, and C2HS Sprague-Dawley rats were studied following normoxia or CIH (11% O2-air; 5-min intervals; nights 7-14 post-surgery). At 2 weeks post-surgery, ventral spinal gray matter extending from C4 and C5 was isolated ipsilateral and contralateral to C2HS. Neither C2HS nor CIH altered 5-HT concentration measured with an ELISA on either side of the spinal cord. However, 5-HT2A receptor expression assessed with immunoblots increased in ipsilateral gray matter following C2HS, an effect independent of CIH. Immunocytochemistry revealed increased 5-HT2A receptor expression on identified phrenic motoneurons (p<0.05), as well as in the surrounding gray matter. Contralateral to injury, 5-HT2A receptor expression was elevated in CIH, but not normoxic C2HS rats (p<0.05). Our data are consistent with the hypothesis that spontaneous increase in 5-HT2A receptor expression on or near phrenic motoneurons contributes to strengthened crossed-spinal synaptic pathways to phrenic motoneurons following C2HS.

Spinal shock revisited: a four-phase model

Spinal shock has been of interest to clinicians for over two centuries. Advances in our understanding of both the neurophysiology of the spinal cord and neuroplasticity following spinal cord injury have provided us with additional insight into the phenomena of spinal shock. In this review, we provide a historical background followed by a description of a novel four-phase model for understanding and describing spinal shock. Clinical implications of the model are discussed as well.

Invited review: Neural network plasticity in respiratory control

Respiratory network plasticity is a modification in respiratory control that persists longer than the stimuli that evoke it or that changes the behavior produced by the network. Different durations and patterns of hypoxia can induce different types of respiratory memories. Lateral pontine neurons are required for decreases in respiratory frequency that follow brief hypoxia. Changes in synchrony and firing rates of ventrolateral and midline medullary neurons may contribute to the long-term facilitation of breathing after brief intermittent hypoxia. Long-term changes in central respiratory motor control may occur after spinal cord injury, and the brain stem network implicated in the production of the respiratory rhythm could be reconfigured to produce the cough motor pattern. Preliminary analysis suggests that elements of brain stem respiratory neural networks respond differently to hypoxia and hypercapnia and interact with areas involved in cardiovascular control. Plasticity or alterations in these networks may contribute to the chronic upregulation of sympathetic nerve activity and hypertension in sleep apnea syndrome and may also be involved in sudden infant death syndrome.

The crossed phrenic phenomenon: a model for plasticity in the respiratory pathways following spinal cord injury

Hemisection of the cervical spinal cord rostral to the level of the phrenic nucleus interrupts descending bulbospinal respiratory pathways, which results in a paralysis of the ipsilateral hemidiaphragm. In several mammalian species, functional recovery of the paretic hemidiaphragm can be achieved by transecting the contralateral phrenic nerve. The recovery of the paralyzed hemidiaphragm has been termed the "crossed phrenic phenomenon." The physiological basis for the crossed phrenic phenomenon is as follows: asphyxia induced by spinal hemisection and contralateral phrenicotomy increases central respiratory drive, which activates a latent crossed respiratory pathway. The uninjured, initially latent pathway mediates the hemidiaphragm recovery by descending into the spinal cord contralateral to the hemisection and then crossing the midline of the spinal cord before terminating on phrenic motoneurons ipsilateral and caudal to the hemisection. The purpose of this study is to review work conducted on the crossed phrenic phenomenon and to review closely related studies focusing particularly on the plasticity associated with the response. Because the review deals with recovery of respiratory muscles paralyzed by spinal cord injury, the clinical relevance of the reviewed studies is highlighted.

Neuroplasticity in respiratory motor control

Although recent evidence demonstrates considerable neuroplasticity in the respiratory control system, a comprehensive conceptual framework is lacking. Our goals in this review are to define plasticity (and related neural properties) as it pertains to respiratory control and to discuss potential sites, mechanisms, and known categories of respiratory plasticity. Respiratory plasticity is defined as a persistent change in the neural control system based on prior experience. Plasticity may involve structural and/or functional alterations (most commonly both) and can arise from multiple cellular/synaptic mechanisms at different sites in the respiratory control system. Respiratory neuroplasticity is critically dependent on the establishment of necessary preconditions, the stimulus paradigm, the balance between opposing modulatory systems, age, gender, and genetics. Respiratory plasticity can be induced by hypoxia, hypercapnia, exercise, injury, stress, and pharmacological interventions or conditioning and occurs during development as well as in adults. Developmental plasticity is induced by experiences (e.g., altered respiratory gases) during sensitive developmental periods, thereby altering mature respiratory control. The same experience later in life has little or no effect. In adults, neuromodulation plays a prominent role in several forms of respiratory plasticity. For example, serotonergic modulation is thought to initiate and/or maintain respiratory plasticity following intermittent hypoxia, repeated hypercapnic exercise, spinal sensory denervation, spinal cord injury, and at least some conditioned reflexes. Considerable work is necessary before we fully appreciate the biological significance of respiratory plasticity, its underlying cellular/molecular and network mechanisms, and the potential to harness respiratory plasticity as a therapeutic tool.

Theophylline-induced respiratory recovery following cervical spinal cord hemisection is augmented by serotonin 2 receptor stimulation

Cervical spinal cord hemisection leads to a disruption of bulbospinal innervation of phrenic motoneurons resulting in paralysis of the ipsilateral hemidiaphragm. We have previously demonstrated separate therapeutic roles for theophylline, and more recently serotonin (5-HT) as modulators to phrenic nerve motor recovery; mechanisms that likely occur via adenosine A1 and 5-HT2 receptors, respectively. The present study was designed to specifically determine if concurrent stimulation of 5-HT2 receptors may enhance motor recovery induced by theophylline alone. Adult female rats (250-350 g; n=7 per group) received a left cervical (C2) hemisection that resulted in paralysis of the ipsilateral hemidiaphragm. Twenty-four hours later rats were given systemic theophylline (15 mg/kg, i.v.), resulting in burst recovery in the ipsilateral phrenic nerve. Theophylline-induced recovery was enhanced with the 5-HT2A/2C receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI; 1.0 mg/kg). DOI-evoked augmentation of theophylline-induced recovery was attenuated following subsequent injection of the 5-HT2 receptor antagonist, ketanserin (2.0 mg/kg). In a separate group, rats were pretreated with ketanserin, which did not prevent subsequent theophylline-induced respiratory recovery. However, pretreatment with ketanserin did prevent DOI-induced augmentation of the theophylline-evoked phrenic nerve burst recovery. Lastly, using immunocytochemistry and in situ hybridization, we showed for the first time a positive co-localization of adenosine A1 receptor mRNA and immunoreactivity with phrenic motoneurons of the cervical ventral horns. Taken together, the results of the present study suggest that theophylline may induce motor recovery likely at adenosine A1 receptors located at the level of the spinal cord, and the concurrent stimulation of converging 5-HT2 receptors may augment the response.

Serotonin(2) receptors mediate respiratory recovery after cervical spinal cord hemisection in adult rats

The aim of the present study was to specifically investigate the involvement of serotonin [5-hydroxytryptamine (5-HT(2))] receptors in 5-HT-mediated respiratory recovery after cervical hemisection. Experiments were conducted on C(2) spinal cord-hemisected, anesthetized (chloral hydrate, 400 mg/kg ip), vagotomized, pancuronium- paralyzed, and artificially ventilated female Sprague-Dawley rats in which CO(2) levels were monitored and maintained. Twenty-four hours after spinal hemisection, the ipsilateral phrenic nerve displayed no respiratory-related activity indicative of a functionally complete hemisection. Intravenous administration of the 5-HT(2A/2C)-receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) induced respiratory-related activity in the phrenic nerve ipsilateral to hemisection under conditions in which CO(2) was maintained at constant levels and augmented the activity induced under conditions of hypercapnia. The effects of DOI were found to be dose dependent, and the recovery of activity could be maintained for up to 2 h after a single injection. DOI-induced recovery was attenuated by the 5-HT(2)-receptor antagonist ketanserin but not with the 5-HT(2C)-receptor antagonist RS-102221, suggesting that 5-HT(2A) and not necessarily 5-HT(2C) receptors may be involved in the induction of respiratory recovery after cervical spinal cord injury.

Distribution of Serotonin 2A and 2C Receptor mRNA Expression in the Cervical Ventral Horn and Phrenic Motoneurons Following Spinal Cord Hemisection

Cervical spinal cord injury leads to a disruption of bulbospinal innervation from medullary respiratory centers to phrenic motoneurons. Animal models utilizing cervical hemisection result in inhibition of ipsilateral phrenic nerve activity, leading to paralysis of the hemidiaphragm. We have previously demonstrated a role for serotonin (5-HT) as one potential modulator of respiratory recovery following cervical hemisection, a mechanism that likely occurs via 5-HT2A and/or 5-HT2C receptors. The present study was designed to specifically examine if 5-HT2A and/or 5-HT2C receptors are colocalized with phrenic motoneurons in both intact and spinal-hemisected rats. Adult female rats (250-350 g; n = 6 per group) received a left cervical (C2) hemisection and were injected with the fluorescent retrograde neuronal tracer Fluorogold into the left hemidiaphragm. Twenty-four hours later, animals were killed and spinal cords processed for in situ hybridization and immunohistochemistry. Using (35)S-labeled cRNA probes, cervical spinal cords were probed for 5-HT2A and 5-HT2C receptor mRNA expression and double-labeled using an antibody to Fluorogold to detect phrenic motoneurons. Expression of both 5-HT2A and 5-HT2C receptor mRNA was detected in motoneurons of the cervical ventral horn. Despite positive expression of both 5-HT2A and 5-HT2C receptor mRNA-hybridization signal over phrenic motoneurons, only 5-HT2A silver grains achieved a signal-to-noise ratio representative of colocalization. 5-HT2A mRNA levels in identified phrenic motoneurons were not significantly altered following cervical hemisection compared to sham-operated controls. Selective colocalization of 5-HT2A receptor mRNA with phrenic motoneurons may have implications for recently observed 5-HT2A receptor-mediated regulation of respiratory activity and/or recovery in both intact and injury-compromised states.

Do synaptic rearrangements underlie compensatory reflex enhancement in spinal motoneurons after partial cell loss?

In adult cats, avulsion of a spinal ventral root induces retrograde cell death among the corresponding motoneurons and, also, enhanced monosynaptic reflexes ipsilaterally in the adjacent uninjured spinal cord segments. The present study investigates possible mechanisms behind this reflex potentiation. At 1-12 weeks after unilateral L7 ventral root avulsion, the L7 dorsal root ganglia were bilaterally injected with choleragenoid-HRP to light microscopically quantify the amount of HRP-labeled terminals in the motor nuclei of the lesioned L7 segment and adjacent intact L6+S1 segments. In addition, motoneuron synaptology and individual HRP-labeled boutons were analyzed electron microscopically. In the L7 segment, the loss of motoneurons at 12 weeks after ventral root avulsion was accompanied by a marked loss of HRP-labeled boutons in the corresponding ventral horn. In the L6/S1 segments, the monosynaptic reflex enhancement found ipsilaterally at 12 weeks postoperatively (mean 212%) was not accompanied by an increased HRP-labeling in the ventral horn (mean 109%), indicating that no sprouting or enlargement of the monosynaptic boutons had occurred. Ultrastructurally, the values for apposition length, total active site length, cross-sectional area, and mitochondrial density of the labeled boutons were also similar between the two sides. However, ipsilaterally the L6/S1 motoneurons exhibited an increased membrane covering by presumably excitatory boutons. The present results indicate that after partial cell death in a motoneuron pool the remaining motoneurons may undergo compensatory synaptic rearrangements leading to increased excitability and enhanced reflexes.

5-Hydroxytryptophan-induced respiratory recovery after cervical spinal cord hemisection in rats

The present study investigates the role of serotonin in respiratory recovery after spinal cord injury. Experiments were conducted on C(2) spinal cord hemisected, anesthetized, vagotomized, paralyzed, and artificially ventilated rats in which end-tidal CO(2) was monitored and maintained. Before drug administration, the phrenic nerve ipsilateral to hemisection showed no respiratory-related activity due to the disruption of the descending bulbospinal respiratory pathways by spinal cord hemisection. 5-Hydroxytryptophan (5-HTP), a serotonin precursor, was administrated intravenously. 5-HTP induced time- and dose-dependent increases in respiratory recovery in the phrenic nerve ipsilateral to hemisection. Although the 5-HTP-induced recovery was initially accompanied by an increase in activity in the contralateral phrenic nerve, suggesting an increase in descending respiratory drive, the recovery persisted well after activity in the contralateral nerve returned to predrug levels. 5-HTP-induced effects were reversed by a serotonin receptor antagonist, methysergide. Because experiments were conducted on animals subjected to C(2) spinal cord hemisection, the recovery was most likely mediated by the activation of a latent respiratory pathway spared by the spinal cord injury. The results suggest that serotonin is an important neuromodulator in the unmasking of the latent respiratory pathway after spinal cord injury. In addition, the results also suggest that the maintenance of 5-HTP-induced respiratory recovery may not require a continuous enhancement of central respiratory drive.

Plasticity of cardiorespiratory neural processing: classification and computational functions

Neural plasticity, or malleability of neuronal structure and function, is an important attribute of the mammalian forebrain and is generally thought to be a kernel of biological intelligence. In this review, we examine some reported manifestations of neural plasticity in the cardiorespiratory system and classify them into four functional categories, integral; differential; memory; and statistical-type plasticity. At the cellular and systems level the myriad forms of cardiorespiratory plasticity display emergent and self-organization properties, use- and disuse-dependent and pairing-specific properties, short-term and long-term potentiation or depression, as well as redundancy in series or parallel structures, convergent pathways or backup and fail-safe surrogate pathways. At the behavioral level, the cardiorespiratory system demonstrates the capability of associative and nonassociative learning, classical and operant conditioning as well as short-term and long-term memory. The remarkable similarity and consistency of the various types of plasticity exhibited at all levels of organization suggest that neural plasticity is integral to cardiorespiratory control and may subserve important physiological functions.

Effects of serotonin inhibition on neuronal and astrocyte plasticity in the phrenic nucleus 4 h following C2 spinal cord hemisection

C2 spinal cord hemisection results in synaptic and astroglial changes in the phrenic nucleus which have been associated with the recovery of the ipsilateral hemidiaphragm during expression of the crossed phrenic phenomenon. As part of our ongoing analysis of the neurotransmitters involved, the present study investigated the effects of systemic administration of para-chlorophenylalanine (p-CPA), a serotonin (5-HT) synthesis inhibitor, on plasticity in the rat phrenic nucleus 4 h following C2 hemisection. Hemisected control rats demonstrated typical morphological changes in the ipsilateral phrenic nucleus including: (1) an increased number and length of synaptic active zones and (2) an increased number and length of dendrodendritic membrane appositions. p-CPA treatment 3 days prior to hemisection reduced 5-HT levels and resulted in an attenuation of these changes in the ipsilateral phrenic nucleus 4 h following hemisection compared to hemisected controls. In addition, p-CPA treatment attenuated injury-induced alterations in immunohistochemical staining of glial fibrillary acidic protein (GFAP), although Western blot analysis demonstrated that overall levels of GFAP did not differ significantly between groups. The results suggest that inhibition of 5-HT synthesis by p-CPA attenuates hemisection-induced plasticity in the phrenic nucleus 4 h following an ipsilateral C2 hemisection.

Effects of serotonin on crossed phrenic nerve activity in cervical spinal cord hemisected rats

The present study investigates the effect of 5-hydroxytryptophan (5-HTP), a serotonin precursor, on crossed phrenic nerve activity (CPNA) in rats subjected to a left C2 spinal cord hemisection. Electrophysiological experiments were conducted on anesthetized, vagotomized, paralyzed, and artificially ventilated rats to assess phrenic nerve activity. The left phrenic nerve lost rhythmic activity due to the disruption of the bulbospinal respiratory pathways following spinal cord hemisection. Activity was induced in the left phrenic nerve (CPNA) by temporary asphyxia. 5-HTP administration increased CPNA during asphyxia in the left phrenic nerve in a dose-dependent fashion. Specifically, in a group of eight animals, application of 5-HTP at 0.5, 1.0, and 2.0 mg/kg significantly increased CPNA by 102.2+/-18.5%, 200.8+/-58.1%, and 615.0+/-356.9% compared with predrug control values, respectively. 5-HTP-induced increases in CPNA were reversed by methysergide (2-6 mg/kg, i.v.), a serotonin receptor antagonist. The results suggest that serotonin is involved in the modulation of crossed phrenic nerve activity following spinal cord injury.

Serotonergic neuronal sprouting as a potential mechanism of recovery in multiple sclerosis

Experimental allergic encephalomyelitis (EAE) is widely considered as an animal model of multiple sclerosis (MS). Damage to the bulbospinal serotonergic (5-HT) neurons occurs in the early paralytic stages of EAE in rats with the severity of neurologic signs corresponding to spinal serotonergic depletion. Neurologic recovery of EAE rats is associated with reestablishment of spinal 5-HT transmission possibly through sprouting of undamaged axons and nerve terminals. Damage to the bulbospinal serotonergic fibers also occurs in patients with MS (as reflected by reduced lumbar CSF 5-HIAA levels) and may contribute to several manifestations of the disease including autonomic dysregulation, sensory symptoms (i.e., paresthesias, pain) and motor symptoms (weakness, spasticity, clonus). Spinal serotonergic neuronal sprouting with regeneration of 5-HT nerve terminals may also occur in the early stages of MS and may be associated with spontaneous remission of MS symptoms following an acute relapse. Sprouting of serotonergic neurons may also explain the disparity in MS between the extent of demyelinating plaques and clinical signs of the disease. The chronic course of MS may be associated with progressive axonal degenerative changes with reduction of serotonergic nerve terminals and loss of their sprouting capability. It is proposed that the beneficial effects of treatment with AC pulsed electromagnetic fields on the symptoms and course of the disease in patients with chronic progressive MS may be related in part to renewed sprouting of serotonergic neurons.