Contribution of serotonin neurons to the functional recovery after spinal cord injury in rats

Translational PET Imaging of Spinal Cord Injury with the Serotonin Transporter Tracer [11C]AFM

PURPOSE

The descending raphespinal serotonin (5-HT) system contributes to neural activities required for locomotion. The presynaptic serotonin transporter (SERT) is a marker of 5-HT innervation. In this study, we explored the use of PET imaging with the SERT radioligand [¹¹C]AFM as a biomarker of 5-HT axon damage after spinal cord injury (SCI) in a rodent model and its translation to imaging SCI in humans.

PROCEDURES

PET imaging with [¹¹C]AFM was performed in healthy rats under baseline and citalopram blocking conditions and a mid-thoracic transection rat model of SCI. The lumbar-to-cervical activity (L/C) ratio was calculated for the healthy and SCI animals to assess SERT binding decrease after SCI. Finally, translation of [¹¹C]AFM PET was attempted to explore its potential to image SCI in humans.

RESULTS

Intense uptake in the brain and intact spinal cord was observed at 30-60 min post-injection of [¹¹C]AFM in healthy rats. About 65% of [¹¹C]AFM uptake in the spinal cord was blocked by citalopram. In the SCI rat model, the cervical uptake of [¹¹C]AFM was similar to that in healthy rats, but the lumbar uptake was dramatically reduced, resulting in about half the L/C ratio in SCI rats compared to healthy rats. In contrast, [¹¹C]AFM uptake in the human spinal cord showed no obvious decrease after treatment with citalopram. In the human subjects with SCI, decreases in [¹¹C]AFM uptake were also not obvious in the section of spinal cord caudal to the injury point.

CONCLUSION

[¹¹C]AFM PET imaging of SERT provides a useful preclinical method to non-invasively visualize the rodent spinal cord and detect SERT changes in SCI rodent models. However, there appears to be little detectable specific binding signal for [¹¹C]AFM in the human spinal cord. An SERT tracer with higher affinity and lower non-specific binding signal is needed to image the spinal cord in humans and to assess the axonal status in SCI patients.

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.

An injury-induced serotonergic neuron subpopulation contributes to axon regrowth and function restoration after spinal cord injury in zebrafish

Spinal cord injury (SCI) interrupts long-projecting descending spinal neurons and disrupts the spinal central pattern generator (CPG) that controls locomotion. The intrinsic mechanisms underlying re-wiring of spinal neural circuits and recovery of locomotion after SCI are unclear. Zebrafish shows axonal regeneration and functional recovery after SCI making it a robust model to study mechanisms of regeneration. Here, we use a two-cut SCI model to investigate whether recovery of locomotion can occur independently of supraspinal connections. Using this injury model, we show that injury induces the localization of a specialized group of intraspinal serotonergic neurons (ISNs), with distinctive molecular and cellular properties, at the injury site. This subpopulation of ISNs have hyperactive terminal varicosities constantly releasing serotonin activating 5-HT_1B receptors, resulting in axonal regrowth of spinal interneurons. Axon regrowth of excitatory interneurons is more pronounced compared to inhibitory interneurons. Knock-out of htr1b prevents axon regrowth of spinal excitatory interneurons, negatively affecting coordination of rostral-caudal body movements and restoration of locomotor function. On the other hand, treatment with 5-HT_1B receptor agonizts promotes functional recovery following SCI. In summary, our data show an intraspinal mechanism where a subpopulation of ISNs stimulates axonal regrowth resulting in improved recovery of locomotor functions following SCI in zebrafish.

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.

Activation of Neurogenesis in Multipotent Stem Cells Cultured In Vitro and in the Spinal Cord Tissue After Severe Injury by Inhibition of Glycogen Synthase Kinase-3

The inhibition of glycogen synthase kinase-3 (GSK-3) can induce neurogenesis, and the associated activation of Wnt/β-catenin signaling via GSK-3 inhibition may represent a means to promote motor function recovery following spinal cord injury (SCI) via increased astrocyte migration, reduced astrocyte apoptosis, and enhanced axonal growth. Herein, we assessed the effects of GSK-3 inhibition in vitro on the neurogenesis of ependymal stem/progenitor cells (epSPCs) resident in the mouse spinal cord and of human embryonic stem cell-derived neural progenitors (hESC-NPs) and human-induced pluripotent stem cell-derived neural progenitors (hiPSC-NPs) and in vivo on spinal cord tissue regeneration and motor activity after SCI. We report that the treatment of epSPCs and human pluripotent stem cell-derived neural progenitors (hPSC-NPs) with the GSK-3 inhibitor Ro3303544 activates β-catenin signaling and increases the expression of the bIII-tubulin neuronal marker; furthermore, the differentiation of Ro3303544-treated cells prompted an increase in the number of terminally differentiated neurons. Administration of a water-soluble, bioavailable form of this GSK-3 inhibitor (Ro3303544-Cl) in a severe SCI mouse model revealed the increased expression of bIII-tubulin in the injury epicenter. Treatment with Ro3303544-Cl increased survival of mature neuron types from the propriospinal tract (vGlut1, Parv) and raphe tract (5-HT), protein kinase C gamma-positive neurons, and GABAergic interneurons (GAD65/67) above the injury epicenter. Moreover, we observed higher numbers of newly born BrdU/DCX-positive neurons in Ro3303544-Cl-treated animal tissues, a reduced area delimited by astrocyte scar borders, and improved motor function. Based on this study, we believe that treating animals with epSPCs or hPSC-NPs in combination with Ro3303544-Cl deserves further investigation towards the development of a possible therapeutic strategy for SCI.

Rehabilitation following spinal cord injury: how animal models can help our understanding of exercise-induced neuroplasticity

Spinal cord injury is a devastating condition that is followed by long and often unsuccessful recovery after trauma. The state of the art approach to manage paralysis and concomitant impairments is rehabilitation, which is the only strategy that has proven to be effective and beneficial for the patients over the last decades. How rehabilitation influences the remodeling of spinal axonal connections in patients is important to understand, in order to better target these changes and define the optimal timing and onset of training. While clinically the answers to these questions remain difficult to obtain, rodent models of rehabilitation like bicycling, treadmill training, swimming, enriched environments or wheel running that mimic clinical rehabilitation can be helpful to reveal the axonal changes underlying motor recovery. This review will focus on the different animal models of spinal cord injury rehabilitation and the underlying changes in neuronal networks that are improved by exercise and rehabilitation.

Effects of Dietary Vitamin E Supplementation in Bladder Function and Spasticity during Spinal Cord Injury

Traumatic spinal cord injury (SCI) results in debilitating autonomic dysfunctions, paralysis and significant sensorimotor impairments. A key component of SCI is the generation of free radicals that contributes to the high levels of oxidative stress observed. This study investigates whether dietary supplementation with the antioxidant vitamin E (alpha-tocopherol) improves functional recovery after SCI. Female adult Sprague-Dawley rats were fed either with a normal diet or a dietary regiment supplemented with vitamin E (51 IU/g) for eight weeks. The rats were subsequently exposed either to a contusive SCI or sham operation, and evaluated using standard functional behavior analysis. We report that the rats that consumed the vitamin E-enriched diet showed an accelerated bladder recovery and significant improvements in locomotor function relative to controls, as determined by residual volumes and Basso, Beatie, and Bresnaham BBB scores, respectively. Interestingly, the prophylactic dietary intervention did not preserve neurons in the ventral horn of injured rats, but it significantly increased the numbers of oligodendrocytes. Vitamin E supplementation attenuated the depression of the H-reflex (a typical functional consequence of SCI) while increasing the levels of supraspinal serotonin immunoreactivity. Our findings support the potential complementary use of vitamin E to ameliorate sensory and autonomic dysfunctions associated with spinal cord injury, and identified promising new cellular and functional targets of its neuroprotective effects.

Mechanism of Restoration of Forelimb Motor Function after Cervical Spinal Cord Hemisection in Rats: Electrophysiological Verification

The objective of this study was to electrophysiologically assess the corticospinal tracts of adult rats and the recovery of motor function of their forelimbs after cervical cord hemisection. Of 39 adult rats used, compound muscle action potentials (CMAPs) of the forelimbs of 15 rats were evaluated, before they received left C5 segmental hemisection of the spinal cord, by stimulating the pyramid of the medulla oblongata on one side using an exciting microelectrode. All 15 rats exhibited contralateral electrical activity, but their CMAPs disappeared after hemisection. The remaining 24 rats received hemisection first, and CMAPs of 12 rats were assessed over time to study their recovery time. All of them exhibited electrical activity of the forelimbs in 4 weeks after surgery. The remaining 12 rats received additional right C2 segmental hemisection, and variation of CMAPs between before and after surgery was examined. The right side of the 12 rats that received the additional hemisection exhibited no electrical activity in response to the stimulation of the pyramids on both sides. These results suggest that changes in path between the resected and healthy sides, activation of the ventral corticospinal tracts, and propriospinal neurons were involved in the recovery of motor function after cervical cord injury.

Improving the regenerative potential of olfactory ensheathing cells by overexpressing prostacyclin synthetase and its application in spinal cord repair

Background

Olfactory ensheathing cells (OEC), specialized glia that ensheathe bundles of olfactory nerves, have been reported as a favorable substrate for axonal regeneration. Grafting OEC to injured spinal cord appears to facilitate axonal regeneration although the functional recovery is limited. In an attempt to improve the growth-promoting properties of OEC, we transduced prostacyclin synthase (PGIS) to OEC via adenoviral (Ad) gene transfer and examined the effect of OEC with enhanced prostacyclin synthesis in co-culture and in vivo. Prostacyclin is a vasodilator, platelet anti-aggregatory and cytoprotective agent.

Results

Cultured OEC expressed high level of cyclooxygneases, but not PGIS. Infection of AdPGIS to OEC could selectively augument prostacyclin synthesis. When cocultured with either OEC or AdPGIS-OEC, neuronal cells were resistant to OGD-induced damage. The resulted OEC were further transplanted to the transected cavity of thoracic spinal cord injured (SCI) rats. By 6 weeks post-surgery, significant functional recovery in hind limbs occurred in OEC or AdPGIS-OEC transplanted SCI rats compared with nontreated SCI rats. At 10–12 weeks postgraft, AdPGIS-OEC transplanted SCI rats showed significantly better motor restoration than OEC transplanted SCI rats. Futhermore, regenerating fiber tracts in the distal spinal cord stump were found in 40–60% of AdPGIS-OEC transplanted SCI rats.

Conclusions

Enhanced synthesis of prostacyclin in grafted OEC improved fiber tract regeneration and functional restoration in spinal cord injured rats. These results suggest an important potential of prostacyclin in stimulating OEC therapeutic properties that are relevant for neural transplant therapies.

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.

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

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

Exposure to acute intermittent hypoxia augments somatic motor function in humans with incomplete spinal cord injury

BACKGROUND

Neural plasticity may contribute to motor recovery following spinal cord injury (SCI). In rat models of SCI with respiratory impairment, acute intermittent hypoxia (AIH) strengthens synaptic inputs to phrenic motor neurons, thereby improving respiratory function by a mechanism known as respiratory long-term facilitation. Similar intermittent hypoxia-induced facilitation may be feasible in somatic motor pathways in humans.

OBJECTIVE

Using a randomized crossover design, the authors tested the hypothesis that AIH increases ankle strength in people with incomplete SCI.

METHODS

Ankle strength was measured in 13 individuals with chronic, incomplete SCI before and after AIH. Voluntary ankle strength was estimated using changes in maximum isometric ankle plantar flexion torque generation and plantar flexor electromyogram activity following 15 low oxygen exposures (Fio(2) = 0.09, 1-minute intervals). Results were compared with trials where subjects received sham exposure to room air.

RESULTS

AIH increased plantar flexion torque by 82 ± 33% (P < .003) immediately following AIH and was sustained above baseline for more than 90 minutes (P < .007). Increased ankle plantar flexor electromyogram activity (P = .01) correlated with increased torque (r(2) = .5; P < .001). No differences in plantar flexion strength or electromyogram activity were observed in sham experiments.

CONCLUSIONS

AIH elicits sustained increases in volitional somatic motor output in persons with chronic SCI. Thus, AIH has promise as a therapeutic tool to induce plasticity and enhance motor function in SCI patients.

Implantation of adult bone marrow-derived mesenchymal stem cells transfected with the neurotrophin-3 gene and pretreated with retinoic acid in completely transected spinal cord

Implantation of marrow-derived mesenchymal stem cells (MSCs) is the most promising therapeutic strategy for the treatment of spinal cord injury (SCI), especially because of their potential for clinical application, such as the avoidance of immunologic rejection, their strong secretory properties, and their plasticity for developing into neural cells. However, the recovery from SCI after MSC implantation is minimal due to their limited capacity for the reduction of cystic cavitation, for the axonal regeneration and their uncertain neural plasticity in the spinal cord. We previously pretreated MSCs with all-trans retinoic acid (RA) in vitro. Then we genetically modified them to overexpress neurotrophin-3 (NT-3) via a recombinant adenoviral vector (Adv). This combined treatment not only permitted more neuronal differentiation of MSCs, but stimulated more NT-3 secretion prior to grafting, according to our previous and present results. When these cells were implanted into the transected spinal cord of rats, the animals had some improvement (both functionally and structurally), including the recovery of hindlimb locomotor function, shown by the highest Basso, Beattie, and Bresnahan (BBB) scores, as well as dramatically reduced cavity volume, clear axonal regeneration and more neuronal survival. In contrast, simple MSC implantation is not a very effective therapy for spinal transection. However, the neuronal differentiation of MSCs after treatment with a combination of Adv-mediated NT-3 gene transfer and RA was only mildly improved in vivo.

The recovery of 5-HT transporter and 5-HT immunoreactivity in injured rat spinal cord

STUDY DESIGN

Experimental spinal cord injury.

OBJECTIVE

To determine the role of serotonin (5-HT) and 5-HT transporter in recovery from spinal cord injury.

METHOD

We examined 5-HT and 5-HT transporter of spinal cord immunohistologically and assessed locomotor recovery after extradural compression at the thoracic (T8) spinal cord in 21 rats. Eighteen rats had laminectomy and spinal cord injury, while the remaining three rats received laminectomy only. All rats were evaluated every other day for 4 weeks, using a 0-14 point scale open field test.

RESULTS

Extradural compression markedly reduced mean hindlimbs scores from 14 to 1.5 +/- 2.0 (mean +/- standard error of mean). The rats recovered apparently normal walking by 4 weeks. The animals were perfused with fixative 1-3 days, 1, 2 and 4 weeks (three rats in each) after a spinal cord injury. The 5-HT transporter immunohistological study revealed a marked reduction of 5-HT transporter-containing terminals by 1 day after injury. By 4 weeks after injury, 5-HT transporter immunoreactive terminals returned to the control level. The 5-HT immunohistological study revealed a reduction of 5-HT-containing terminals by 1 week after injury. By 4 weeks after injury, 5-HT immunoreactive fibers and terminals returned to the control level.

CONCLUSION

We estimated the recovery of 5-HT transporter and 5-HT neural elements in lumbosacral ventral horn by ranking 5-HT transporter and 5-HT staining intensity and counting 5-HT and 5-HT transporter terminals. The return of 5-HT transporter and 5-HT immunoreactivity of the lumbosacral ventral horn correlated with locomotor recovery, while 5-HT transporter showed closer relationship with locomotor recovery than 5-HT. The presence of 5-HT transporter indicates that the 5-HT fibers certainly function. This study shows that return of the function of 5-HT fibers predict the time course and extent of locomotory recovery after thoracic spinal cord injury.

Embryonic and adult stem cells promote raphespinal axon outgrowth and improve functional outcome following spinal hemisection in mice

Spinal cord injury (SCI) often results in permanent neurological deficits below the injury site. Serotonergic raphespinal projections promote functional recovery after SCI, but spontaneous regeneration of most severed axons is limited by the glial cyst and scar that form at the lesion site. Stem cell (SC) transplantation offers a promising approach for inducing regeneration through the damaged area. Here we compare the effects of transplantation of embryonic neural precursors (NPs) or adult mesenchymal SCs, both of which are potential candidates for SC therapy. The spinal cord was hemisected at the L2 neuromer in adult mice. Two weeks post-injury, we transplanted neural precursors or mesenchymal SCs into the cord, caudal to the hemisection. Injured mice without a graft served as controls. Mice were tested for functional recovery on a battery of motor tasks, then killed and analysed for survival of grafted cells, for effects of engraftment on the local cellular environment and for the sprouting of serotonergic axons. Both types of SCs survived and were integrated into the host tissue, but only the NPs expressed neuronal markers. All transplanted animals displayed an increased number of serotonin-positive fibres caudal to the hemisection, compared with untreated mice. And both cell types led to improved motor performance. These results point to a therapeutic potential for such cell grafting.

Intermittent hypoxia induces functional recovery following cervical spinal injury

Respiratory-related complications are the leading cause of death in spinal cord injury (SCI) patients. Few effective SCI treatments are available after therapeutic interventions are performed in the period shortly after injury (e.g. spine stabilization and prevention of further spinal damage). In this review we explore the capacity to harness endogenous spinal plasticity induced by intermittent hypoxia to optimize function of surviving (spared) neural pathways associated with breathing. Two primary questions are addressed: (1) does intermittent hypoxia induce plasticity in spinal synaptic pathways to respiratory motor neurons following experimental SCI? and (2) can this plasticity improve respiratory function? In normal rats, intermittent hypoxia induces serotonin-dependent plasticity in spinal pathways to respiratory motor neurons. Early experiments suggest that intermittent hypoxia also enhances respiratory motor output in experimental models of cervical SCI (cervical hemisection) and that the capacity to induce functional recovery is greater with longer durations post-injury. Available evidence suggests that intermittent hypoxia-induced spinal plasticity has considerable therapeutic potential to treat respiratory insufficiency following chronic cervical spinal injury.

Anti-Nogo-A antibody treatment enhances sprouting of corticospinal axons rostral to a unilateral cervical spinal cord lesion in adult macaque monkey

After injury, regrowth of axons in mammalian adult central nervous system is highly limited. However, in monkeys subjected to unilateral cervical lesion (C7-C8 level), neutralization of an important neurite outgrowth inhibitor, Nogo-A, stimulated axonal sprouting caudal to the lesion, accompanied by enhanced functional recovery of manual dexterity, compared with lesioned monkeys treated with a control antibody (Freund et al. [2006] Nat. Med. 12:790-792). The present study aimed at comparing the same two groups of monkeys for axonal sprouting rostral to the cervical lesion. The corticospinal tract was labeled by injecting the anterograde tracer biotinylated dextran amine into the contralesional motor cortex. The corticospinal axons were interrupted at the level of the lesion, accompanied by retrograde axonal degeneration (axon dieback), reflected by the presence of terminal retraction bulbs. The number of terminal retraction bulbs was lower in anti-Nogo-A antibody treated monkeys, and, when present, they were found closer to the lesion than in control-antibody treated monkeys. Compared with control antibody treated monkeys, the anti-Nogo-A antibody treated monkeys exhibited an increased cumulated axon arbor length and a higher number of axon arbors going in the medial direction from the white to the gray matter. Higher in the cervical cord (at C5 level), the anti-Nogo-A treatment enhanced the number of corticospinal fibers crossing the midline, suggesting axonal sprouting. Thus, the anti-Nogo-A antibody treatment enhanced axonal sprouting rostral to the cervical lesion; some of these fibers grew around the lesion and into the caudal spinal segments. These processes paralleled the observed improved functional recovery.

Erythropoietin-mediated preservation of the white matter in rat spinal cord injury

We investigated the effect of a single administration of recombinant human erythropoietin (rhEPO) on the preservation of the ventral white matter of rats at 4 weeks after contusive spinal cord injury (SCI), a time at which functional recovery is significantly improved in comparison to the controls [Gorio A, Necati Gokmen N, Erbayraktar S, Yilmaz O, Madaschi L, Cichetti C, Di Giulio AM, Enver Vardar E, Cerami A, Brines M (2002) Recombinant human erythropoietin counteracts secondary injury and markedly enhances neurological recovery from experimental spinal cord trauma. Proc Natl Acad Sci U S A 99:9450-9455; Gorio A, Madaschi L, Di Stefano B, Carelli S, Di Giulio AM, De Biasi S, Coleman T, Cerami A, Brines M (2005) Methylprednisolone neutralizes the beneficial effects of erythropoietin in experimental spinal cord injury. Proc Natl Acad Sci U S A 102:16379-16384]. Specifically, we examined, by morphological and cytochemical methods combined with light, confocal and electron microscopy, i) myelin preservation, ii) activation of adult oligodendrocyte progenitors (OPCs) identified for the expression of NG2 transmembrane proteoglycan, iii) changes in the amount of the chondroitin sulfate proteoglycans neurocan, versican and phosphacan and of their glycosaminoglycan component labeled with Wisteria floribunda lectin, and iv) ventral horn density of the serotonergic plexus as a marker of descending motor control axons. Injured rats received either saline or a single dose of rhEPO within 30 min after SCI. The results showed that the significant improvement of functional outcome observed in rhEPO-treated rats was associated with a better preservation of myelin in the ventral white matter. Moreover, the significant increase of both the number of NG2-positive OPCs and the labeling for Nogo-A, a marker of differentiated oligodendrocytes, suggested that rhEPO treatment could result in the generation of new myelinating oligodendrocytes. Sparing of fiber tracts in the ventral white matter was confirmed by the increased density of the serotonergic plexus around motor neurons. As for chondroitin sulfate proteoglycans, only phosphacan, increased in saline-treated rats, returned to normal levels in rhEPO group, probably reflecting a better maintenance of glial-axolemmal relationships along nerve fibers. In conclusion, this investigation expands previous studies supporting the pleiotropic neuroprotective effect of rhEPO on secondary degenerative response and its therapeutic potential for the treatment of SCI and confirms that the preservation of the ventral white matter, which contains descending motor pathways, may be critical for limiting functional deficit.

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

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

Synaptic pathways to phrenic motoneurons are enhanced by chronic intermittent hypoxia after cervical spinal cord injury

Spinal hemisection at C2 reveals caudal synaptic pathways that cross the spinal midline (crossed phrenic pathways) and can restore inspiratory activity in ipsilateral phrenic motoneurons. Intermittent hypoxia induces plasticity in the cervical spinal cord, resulting in enhanced inspiratory phrenic motor output. We hypothesized that chronic intermittent hypoxia (CIH) (alternating 11% O(2) and air; 5 min periods; 12 hr per night; 7 nights) would strengthen crossed phrenic pathways. Experiments were performed on anesthetized, vagotomized, paralyzed, ventilated, and spinally injured (C2 hemisection) rats that were exposed to either normoxia or CIH before acute injury (preconditioning) or after chronic injury (postconditioning). Spontaneous inspiratory bursts or compound action potentials evoked via stimulation of the ventrolateral funiculus (contralateral to injury) were recorded in both phrenic nerves. Spontaneous or evoked activity in crossed phrenic pathways were minimal or absent in all acutely injured rats regardless of preconditioning. In rats postconditioned with normoxia, crossed phrenic inspiratory bursts were observed occasionally during baseline conditions and always during chemoreceptor stimulation (hypoxia and hypercapnia). However, CIH postconditioned rats had substantially larger crossed phrenic inspiratory bursts during baseline, hypoxia, and hypercapnia (all p < 0.05 vs normoxic group). Short-latency (0.7 msec) evoked crossed phrenic potentials were also enhanced by CIH conditioning in chronically injured rats (p < 0.05). We conclude that CIH induced spinal cord plasticity-enhanced phrenic motor output. This plasticity required preconditions established by chronic spinal injury.

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.

Changes in serotonin, serotonin transporter expression and serotonin denervation supersensitivity: involvement in chronic central pain after spinal hemisection in the rat

Spinal cord injury (SCI) results in abnormal locomotor and pain syndromes in humans. In a rodent SCI model, T13 unilateral spinal hemisection results in bilateral mechanical allodynia and thermal hyperalgesia, partly by interruption of tonic descending serotonin (5-HT) inhibition. In the current study, we examined changes in density and distribution of 5-HT and 5-HT(T) in cervical (C8) and lumbar (L5) enlargements after T13 spinal hemisection and studied the effects of intrathecally delivered 5-HT (10, 21, and 63 microg), 5-HT antagonist methysergide (125 microg/kg), and 5-HT reuptake inhibitor fluvoxamine (75 microg/kg) on pain-related behaviors. Thirty-day-old male Sprague-Dawley rats were spinally hemisected and sacrificed at 3 (n = 20) and 28 (n = 20) days postsurgery for immunohistochemistry, Western blot, and ELISA analysis and compared against sham-operated animals (n = 10). At day 3, C8 5-HT levels were not significantly changed but at L5 there was a significant decrease in ipsilateral 5-HT in laminae I-II followed by incomplete recovery at 28 days postinjury. At both 3 and 28 days postinjury, C8 5-HT(T) levels were not significantly changed, but at L5 there was significant ipsilateral up-regulation of 5-HT(T) in laminae I-II. A second group of animals (n = 30) was hemisected and, starting at 28 days postinjury, behaviorally tested with intrathecal compounds. Increasing doses of 5-HT attenuated both fore- and hindlimb mechanical allodynia and thermal hyperalgesia, and effects of endogenous 5-HT were attenuated by methysergide and enhanced with fluvoxamine, all without locomotor alterations. Sham controls (n = 10) were unaffected. Thus, permanent changes occur in 5-HT and 5-HT(T) after SCI, denervation 5-HT supersensitivity develops, and modulation of 5-HT attenuates pain-related behaviors. Insight gained by these studies may aid in the understanding of dynamic 5-HT systems which will be useful in treating chronic central pain after SCI.

Transplantation of nasal olfactory tissue promotes partial recovery in paraplegic adult rats

Recent reports have highlighted the potential therapeutic role of olfactory ensheathing cells for repair of spinal cord injuries. Previously ensheathing cells collected from the olfactory bulbs within the skull were used. In humans a source of these cells for autologous therapy lies in the nasal mucosa where they accompany the axons of the olfactory neurons. The aim of the present study was to test the therapeutic potential of nasal olfactory ensheathing cells for spinal cord repair. Olfactory ensheathing cells cultured from the olfactory lamina propria or pieces of lamina propria from the olfactory mucosa were transplanted into the transected spinal cord. Three to ten weeks later these animals partially recovered movement of their hind limbs and joints which was abolished by a second spinal cord transection. Control rats, receiving collagen matrix, respiratory lamina propria or culture medium, did not recover hind limb movement. Recovery of movement was associated with recovery of spinal reflex circuitry, assessed using the rate-sensitive depression of the H-reflex from an interosseous muscle. Histological analysis of spinal cords grafted with olfactory tissue demonstrated nerve fibres passing through the transection site, serotonin-positive fibres in the spinal cord distal to the transection site, and retrograde labelling of brainstem raphe and gigantocellularis neurons from injections into the distal cord, indicating regeneration of descending pathways. Thus, olfactory lamina propria transplantation promoted partial restoration of function after relatively short recovery periods. This study is particularly significance because it suggests an accessible source of tissue for autologous grafting in human paraplegia.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

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.

Increased spinal monoamine concentrations after chronic thoracic dorsal rhizotomy in goats

In goats, bilateral thoracic dorsal rhizotomy (TDR) causes severe ventilatory failure during exercise, followed by progressive functional recovery. We investigated spinal neurochemical changes associated with TDR and/or functional recovery by measuring spinal concentrations of the monoamines serotonin (5-HT), norepinephrine, and dopamine via HPLC. Changes in 5-HT and calcitonin gene-related peptide were visualized with immunohistochemistry. Goat spinal cords were compared 4-15 mo after TDR from T(2) to T(12) (n = 7) with sham-operated (n = 4) or unoperated controls (n = 4). TDR increased the concentration of cervical 5-HT (C(5)-C(6); 122% change), caudal thoracic norepinephrine (T(7)-T(11); 53% change), and rostral thoracic dopamine (T(3)-T(6); 234% change). TDR increased 5-HT-immunoreactive terminal density (dorsal and ventral horns) and nearly eliminated calcitonin gene-related peptide immunoreactivity in the superficial laminae of the dorsal horn in rostral thoracic segments; both effects became less pronounced in caudal thoracic segments. Thus TDR elevates monoamine concentrations in discrete spinal regions, including possible compensatory changes in descending serotonergic inputs to spinal segments not directly affected by TDR (i.e., cervical) but associated with functionally related motor nuclei (i.e., phrenic nucleus).

Effect of aluminum on neurological recovery in rats following spinal cord injury

OBJECT

This investigation was undertaken to study the effect of aluminum on neurobehavioral, electrophysiological, structural, and biochemical changes in rats following spinal cord injury (SCI).

METHODS

Adult male Sprague-Dawley rats classified into different groups were given aluminum sulfate-dosed drinking water in the concentrations of 0%, 0.25%, 0.5% and 1%, respectively. After 30 days of aluminum treatment, the animals were subjected to spinal cord trauma. Laminectomy was performed at T7-8 in anesthetized rats, followed by placement of a compression plate (2.2 x 5 mm) loaded with a 35-g weight over the exposed spinal cord for 5 minutes. Control animals underwent the same surgical procedure, but the compression injury was not induced (sham). Postoperative neurological function was assessed using the inclined-plane test and by obtaining a modified Tarlov score and vocal/sensory score daily for 10 days. Electrophysiological changes were assessed using corticomotor evoked potentials, whereas pathological changes were assessed by light microscopy. The level of vitamin E in the spinal cord was measured as an index of antioxidant defense. The behavioral, biochemical, and histological analyses were performed in a blinded fashion.

CONCLUSIONS

Analysis of results obtained in the behavioral studies revealed that the compression of spinal cord produced transient paraparesis in which a maximum motor deficit occurred at Day 1 following SCI and resolved over a period of 10 days. Administration of aluminum significantly impaired the recovery following SCI. Analysis of the results of the biochemical, electrophysiological, and histopathological studies also confirmed the deleterious effects of aluminum on recovery from SCI in rats.

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.

Gait analysis of adult paraplegic rats after spinal cord repair

This study presents a novel detailed method of analysis of rat gait and uses this method to demonstrate recovery of forward locomotion patterns in adult rats made paraplegic by surgical spinal cord transection and subjected to a novel strategy for spinal cord repair. Six normal rats were compared to five animals in which the cord was transected at T8-T9, and a 5-mm segment of the spinal cord removed, and to seven animals in which, following spinal cord transection and removal of a spinal cord segment, multiple intercostal peripheral nerve bridges were implanted, rerouting pathways from white to gray matter in both directions. The implanted area was filled with fibrin glue containing acidic fibroblast growth factor. Details of the repair strategy have been published (H. Cheng, Y. Cao, and L. Olson, 1996, Science 273: 510-513). Gait analysis was carried out 3 and 4 months after surgery and once in the normal animals. Animals were allowed to walk across a runway with a transparent floor. Each test consisted of five trials, and each trial was videorecorded from underneath. Using frame-by-frame playback, individual footprints were then recorded regarding location and order of limb use, as well as step quality (degree of weight bearing, etc.). These data allowed measuring runway transit time, five different measures of step numbers, all possible temporal patterns of limb use, stride length, and base of support. Transected controls remained paralyzed in the hindlimbs with only occasional reflex hindlimb movements without weight bearing. Animals subjected to the full repair procedure were significantly faster than the controls, used their hindlimbs for 25-30% of the movements, and regained several of the specific limb recruitment patterns used by normal rats. Taken together, the gait analysis data demonstrate remarkable recovery of coordinated gait in the repaired animals, which was significantly better than controls for all relevant parameters, while at the same time clearly inferior to normal rats for most of the examined parameters. We conclude that normal rats use a multitude of interchangeable step sequence patterns, and that our spinal cord repair strategy leads to recovery of some of these patterns following complete spinal cord transection. These data suggest functionally relevant neuronal communication across the lesion.

Kinematic analysis of limb position during quadrupedal locomotion in rats

A test of locomotor behavior using the coordinates of ipsilateral limb positions of rats walking on a moving treadmill is described. Specific points on the forelimb and hindlimb were digitized from video records for 20-sec continuous sequences of locomotion, and step periods and step distances were calculated. The extent to which a given limb position would predict its own position--or the position of another limb--at different points in time was mathematically determined by autocorrelation and cross-correlation, respectively. Autocorrelation of position data was performed using a three-step window and the standard formula for correlating phasic data. A novel method of data preparation, which included normalization of the step data to eliminate variability introduced by differences in step period length, was used prior to cross-correlations of forelimb to hindlimb positions. Rats walking at 0.10, 0.15, and 0.25 m/sec had high limb autocorrelations, comparable forelimb/hindlimb phase relationships, and consistently high average cross-correlation coefficients. This analysis has resulted in the quantification of rat locomotor behavior in terms of the degree of limb movement rhythmicity and the strength of the forelimb/hindlimb coordination, and has provided baseline data for comparisons with spinal cord-injured rats that have retained or recovered alternating hindpaw movements.

The hypothalamus-pituitary-ovary and hypothalamus-pituitary-thyroid axes in spinal cord-injured women

Sixteen women with spinal cord injury (SCI) underwent studies of the hypothalamus-pituitary-ovary (HPO) and hypothalamus-pituitary-thyroid (HPT) axes with luteinizing hormone (LH) releasing hormone (LHRH) and thyrotropin (TSH) releasing hormone (TRH) stimulation tests during the early follicular phase. The mean interval from injury to participation in this study was 7.5 years (range, 1.5 to 13.1). All subjects were menstruating regularly. Five (35.7%) SCI subjects who were menstruating before injury had postinjury amenorrhea for 1 to 12 months, and the other nine (64.3%) SCI subjects had no interruption of menstruation after injury. Two SCI subjects whose injury occurred in preadolescence proceeded to menarche without any delay. The amount of menstrual flow was noted to be reduced in nine (64.3%) SCI subjects. Two and three SCI subjects had elevated follicle-stimulating hormone (FSH) and prolactin (PRL) levels, respectively. LH responses to LHRH were significantly higher in the SCI group (P < .001). Ten (62.6%) SCI subjects had enhanced LH responses to LHRH. The mean TSH, PRL, and FSH responses to TRH and LHRH of the SCI group were not significantly different from those of age-matched controls. However, five (31.2%), four (25.0%), and five (31.2%) SCI subjects had enhanced TSH, PRL, and FSH responses to TRH and LHRH, respectively. Six (37.5%) SCI subjects had a delayed FSH response to LHRH. In total, 13 (81.2%) SCI subjects had at least one axis abnormality. These findings are consistent with the hypothesis that changes of central neurotransmitters may occur after SCI.

Effect of muscle denervation on the expression of substance P in the ventral raphe-spinal pathway of the rat

The medullary raphe nuclei, wherein serotonin (5-HT) coexists with substance P (SP) and thyrotropin-releasing hormone (TRH), innervate lower motor neurons in the spinal cord ventral horn by means of the ventral raphe-spinal pathway. Destruction of the ventral raphe-spinal pathway is associated with deficient recovery of denervated muscle, indicating that it may exert a trophic effect upon lower motor neurons. To determine whether SP could be a trophic factor for lower motor neurons within the ventral raphe-spinal pathway, the effect of muscle denervation with botulinum toxin type A on SP-encoding beta-preprotachykinin mRNA in the rat medullary raphe was examined by in situ hybridization histochemistry. Silver grain density over hybridized medullary raphe neurons was increased by up to 11%, although the number of hybridized neurons did not change in denervated as compared to control rats. Increased SP gene expression in the medullary raphe in response to motor unit lesioning suggests that raphe-spinal SP may be trophic to lower motor neurons.

Fictive motor activities in adult chronic spinal rats transplanted with embryonic brainstem neurons

The present study was designed to examine the effects of an intraspinal transplantation of embryonic brainstem neurons on fictive motor patterns which can develop in hindlimb nerves of adult chronic spinal rats. Seventeen adult rats were spinalized at T8-9 level and, 8 days later, a suspension of embryonic cells obtained either from the raphe region (RR, n = 8) or from the locus coeruleus (LC, n = 9) was injected caudally (T12-13) to the cord transection. Eight control animals (control rats) were spinalized and injected with vehicle under the same conditions. One to three months later, the animals were decorticated and fictive motor patterns were recorded in representative hindlimb nerves. The data revealed that both control and grafted spinal rats could exhibit two distinctly different fictive motor patterns, one which could be associated with stepping and the other with hindlimb paw shaking. They further showed that following transplantation of embryonic RR or LC neurons the excitability of the spinal stepping generator was increased, whereas that of the spinal neural circuits which generate hindlimb paw shaking was not significantly affected. A histological analysis performed on the spinal cord segments below the transection revealed complete absence of serotonin and noradrenaline immunoreactivity in control spinal animals and, in both types of grafted rats, an extensive monoaminergic reinnervation with synaptic contacts between monoaminergic transplanted neurons and host interneurons and/or motoneurons. The possible mechanisms by which grafted monoaminergic neurons can influence the spinal motor networks are discussed.

Phrenic responses to contralateral spinal stimulation in rats: effects of old age or chronic spinal hemisection

Serotonin reveals ineffective spinal pathways from the C2-lateral funiculus to contralateral phrenic motoneurons in young adult rats with acute spinal hemisection. We tested the hypothesis that old age (1.5-2 years) or chronic hemisection (3-5 days) strengthens these pre-existing crossed spinal pathways. There were no consistent differences between young adult rats with acute hemisection versus young adult rats with chronic hemisection or old rat with acute hemisection except that one long-latency phrenic excitation could not be elicited in old rats. The results indicate that neither old age nor chronic hemisection strengthens crossed-spinal pathways, but that old age may selectively diminish spinal pathways involved in the neural control of breathing.

Serotonergic manipulations in experimental neoplastic spinal cord compression

The effects of differing strategies of serotonergic manipulation on vascular permeability, prostaglandin E2 (PGE2) synthesis, and the clinical course are evaluated in an experimental model of neoplastic spinal cord compression in rats. Serotonergic manipulations include in vivo inhibition of serotonin (5-HT) synthesis by p-chlorophenylalanine (p-CPA) and in vivo blockage of serotonin type 2 (5-HT2) receptors either by the selective antagonist ketanserin or by cyproheptadine. In paralyzed rats, the ratio of 5-hydroxyindole-3-acetic acid (5-HIAA) to 5-HT is significantly elevated in the compressed segments, suggesting that 5-HT utilization is increased. Treatment with p-CPA attenuates spinal 5-HT levels by 62.8% +/- 5.1% (mean +/- standard deviation) and reduces the elevated 5-HIAA:5-HT ratio to the normal value. The increased synthesis of PGE2 observed in the compressed cord is unaffected by p-CPA or ketanserin treatment but is markedly attenuated by cyproheptadine. Ketanserin reduces the 10-fold increase in spinal cord permeability observed in paralyzed rats in a clearly dose-related manner. If given at the first sign of neurological dysfunction, ketanserin delays the onset of paraplegia with the 1-mg/kg dose being clearly superior. Cyproheptadine and p-CPA also reduce the increased permeability and protract the course to paraplegia. A comparison of the effect of dexamethasone, indomethacin, cyproheptadine, p-CPA, and ketanserin reveals that they protract the disease course by 48%, 57%, 60%, 64%, and 78%, respectively. These data suggest that 5-HT2 receptors mediate some of the deleterious vascular consequences observed in the compressed spinal cord by a mechanism not coupled with PGE2 synthesis. A potential benefit of serotonergic manipulations for the acute treatment of neoplastic spinal cord compression is suggested.

Thienylphencyclidine protection for the spinal cord of adult rats against extension of lesions secondary to a photochemical injury

The purpose of this study was to evaluate treatment with the N-methyl-D-aspartate antagonist thienyl-phencyclidine (TCP) after spinal cord injury for its behavioral, electrophysiological, morphological, and immunohistochemical effects. Five minutes after a photochemical lesion was produced in rats at the T-8 level, the animals received TCP (1 mg/kg, intravenously) or TCP vehicle (saline). The animals were evaluated on Day 18 for neurological recovery by testing motor and sensory functions. The TCP-treated group showed less neurological impairment than the untreated group (p < 0.05 for inclined-plane stability and withdrawal reflex to extension). Somatosensory evoked potential testing was performed on Days 21 to 23 and the wave amplitude between the onset and P1 in the TCP-treated group was higher than in the untreated group (p < 0.05). Mean arterial blood pressure was not significantly modified after TCP injection. Morphometric studies of the lesion area in cross section revealed a significantly reduced spinal cord infarction in the TCP-treated group (p < 0.05). Immunohistochemical evaluation of the spinal cord in lumbar area showed an increased level of serotonin immunoreactivity in the dorsal horn of animals treated by TCP. These results demonstrate the efficacy of TCP in reducing secondary lesions after spinal cord injury in rats.

Muscle denervation increases thyrotropin-releasing hormone (TRH) biosynthesis in the rat medullary raphe

To determine whether thyrotropin-releasing hormone (TRH) could exert a trophic role in ventral horn motor neurons, we examined the effect of muscle denervation with botulinum toxin A on TRH mRNA in the rat medullary raphe by in situ hybridization histochemistry. Compared to controls, denervated rats showed a significant increase in the number and silver grain density of hybridized medullary raphe neurons. Increased proTRH gene expression in the medullary raphe in response to motor unit perturbation indicates that TRH may be trophic to lower motor neurons.

Effect of thyrotropin-releasing hormone on the neurologic impairment in rats with spinal cord injury: treatment starting 24 h and 7 days after injury

The effect of treatment with thyrotropin-releasing hormone (TRH) or naloxone on the neurologic impairment after spinal cord injury was studied in rats with the severest neurologic impairment (complete paraplegia, no withdrawal response upon tail pinching, and urinary incontinence) 24 h and 7 days after injury. Subcutaneous treatment with TRH (2.5, 10 and 40 mg/kg per day) once daily for 7 consecutive days starting 24 h or 7 days after injury improved the neurologic function in the rats with cord injury in a dose-related manner, with a minimum effective dose of less than 2.5 mg/kg per day in both cases. However, subcutaneous treatment with naloxone (40 mg/kg per day) once daily for 7 consecutive days starting 24 h after injury did not exert any beneficial effects on neurologic function. These results indicate that TRH but not naloxone treatment starting 24 h and as late as 7 days after injury is effective in rats with the severest neurologic impairment following spinal cord injury. Thus, it is suggested that the duration of the effectiveness of late treatment with TRH on the neurologic impairment in rats with spinal cord injury is more than 1 week, while the duration with naloxone is less than 24 h.