Glutamate receptor subunit expression after spinal cord injury in young rats

Simultaneous intrathecal injection of muscimol and endomorphin-1 alleviates neuropathic pain in rat model of spinal cord injury

INTRODUCTION

Due to side effects of medications used for chronic pain, combination therapy seems to be an appropriate solution for alleviation of chronic pain and reducing the side effects. The role of inhibitory GABA system is well proven in reducing neuropathic pain. Also, special attention has been focused on endogenous morphine (endomorphins) in reducing chronic pain originates from damage to the nervous system. The purpose of this study is to investigate the analgesic effect of simultaneous administration of GABA agonist and endomorphin-1 on neuropathic pain in rat model of spinal cord injury (SCI). The role of oxidative stress, NR1 subunits of NMDA receptors, and α₂ subunits of GABA receptors in the spinal cord has also been investigated.

METHODS

Spinal cord at level of T6-T8 was compressed. Three weeks after spinal cord injury, muscimol and endomorphin-1 were injected (intrathecally once a day for 7 days) individually or in combination. Mechanical and cold allodynia, thermal and mechanical hyperalgesia were evaluated before injection and 15 and 60 min after injection. At the end of behavioral experiments, histological and biochemical evaluations were done on prepared spinal cord samples.

RESULTS

Isobologram results showed that combination therapy significantly increased the pain threshold comparing to injection of endomorphin-1 (EM) or muscimol alone. Histological studies indicated the increased expression of α2 subunits of GABA receptors, and NR1 subunits of NMDA receptors in the spinal cord. The combination therapy also increased the glutathione (GSH) and superoxide dismutase (SOD) level and decreased the malondialdehyde (MDA) levels in the spinal cord.

CONCLUSION

Simultaneous administration of muscimol and endomorphine-1 could be a new candidate for alleviation of pain resulting from spinal cord injury.

EphB3 signaling propagates synaptic dysfunction in the traumatic injured brain

Traumatic brain injury (TBI), ranging from mild concussion to severe penetrating wounds, can involve brain regions that contain damaged or lost synapses in the absence of neuronal death. These affected regions significantly contribute to sensory, motor and/or cognitive deficits. Thus, studying the mechanisms responsible for synaptic instability and dysfunction is important for protecting the nervous system from the consequences of progressive TBI. Our controlled cortical impact (CCI) injury produces ~20% loss of synapses and mild changes in synaptic protein levels in the CA3-CA1 hippocampus without neuronal losses. These synaptic changes are associated with functional deficits, indicated by >50% loss in synaptic plasticity and impaired learning behavior. We show that the receptor tyrosine kinase EphB3 participates in CCI injury-induced synaptic damage, where EphB3(-/-) mice show preserved long-term potentiation and hippocampal-dependent learning behavior as compared with wild type (WT) injured mice. Improved synaptic function in the absence of EphB3 results from attenuation in CCI injury-induced synaptic losses and reduced d-serine levels compared with WT injured mice. Together, these findings suggest that EphB3 signaling plays a deleterious role in synaptic stability and plasticity after TBI.

Administration of antagomir-223 inhibits apoptosis, promotes angiogenesis and functional recovery in rats with spinal cord injury

MicroRNAs (miRNAs) are recently described as a class of short non-coding RNAs, which play important roles in post-transcriptional gene regulation and involved in many physiological and pathological processes. MicroRNA-223 (miR-223) has been showed highly elevated in the injured spinal cord. However, the potential role and underlying mechanisms of miR-223 in spinal cord injury (SCI) were incompletely understood. In the present study, we observed the persistent high levels of miR-223 in the injured spinal cord at different time points (1, 3, 7, and 14 days) after SCI. Besides, inhibiting miR-223 by intrathecally injection with antagomir-223 significantly improved recovery in hindlimb motor function and attenuated cell apoptosis in spinal cord-injured rats. Additionally, antagomir-223 treatment markedly decreased the pro-apoptotic protein levels, including Bax and cleaved caspase-3, up-regulated the anti-apoptotic Bcl-2 protein level, as well as the expression of GluR2. Moreover, inhibition of miR-223 promoted angiogenesis, as evidenced by the increased CD31 expression and microvascular density. Taken together, our results indicate that inhibition of miR-223 with antagomir-223 exerts protective role in functional recovery, angiogenesis, and anti-apoptosis during SCI. Thereby, miR-223 may be a promising target of therapy for SCI.

Kainate receptor RNA editing is markedly altered by acute spinal cord injury

We have previously observed changes in the RNA editing of AMPA receptors after acute spinal cord injury (SCI); this implies that post-transcriptional modifications are capable of affecting the physiological properties of glutamate receptor channels and related signal transduction in this neurodegenerative condition. Here, we report that the editing of the ionotropic KAR is markedly decreased at both GluK1 and GluK2 Q/R sites in the epicenter of the lesion and with distinct magnitude and kinetics also in the caudal and rostral portions of the injured cord. These effects are persistent, being observed as late as 30 days after lesioning. In addition, also the I/V and Y/C sites of GluK2 were severely affected after SCI. These findings add novel information to the relevance of editing of glutamate receptors following acute SCI, thus expanding the recently emerged role of post-transcriptional mechanisms under these experimental conditions.

Differential plasticity of the GABAergic and glycinergic synaptic transmission to rat lumbar motoneurons after spinal cord injury

Maturation of inhibitory postsynaptic transmission onto motoneurons in the rat occurs during the perinatal period, a time window during which pathways arising from the brainstem reach the lumbar enlargement of the spinal cord. There is a developmental switch in miniature IPSCs (mIPSCs) from predominantly long-duration GABAergic to short-duration glycinergic events. We investigated the effects of a complete neonatal [postnatal day 0 (P0)] spinal cord transection (SCT) on the expression of Glycine and GABA(A) receptor subunits (GlyR and GABA(A)R subunits) in lumbar motoneurons. In control rats, the density of GlyR increased from P1 to P7 to reach a plateau, whereas that of GABA(A)R subunits dropped during the same period. In P7 animals with neonatal SCT (SCT-P7), the GlyR densities were unchanged compared with controls of the same age, while the developmental downregulation of GABA(A)R was prevented. Whole-cell patch-clamp recordings of mIPSCs performed in lumbar motoneurons at P7 revealed that the decay time constant of miniature IPSCs and the proportion of GABAergic events significantly increased after SCT. After daily injections of the 5-HT(2)R agonist DOI, GABA(A)R immunolabeling on SCT-P7 motoneurons dropped down to values reported in control-P7, while GlyR labeling remained stable. A SCT made at P5 significantly upregulated the expression of GABA(A)R 1 week later with little, if any, influence on GlyR. We conclude that the plasticity of GlyR is independent of supraspinal influences whereas that of GABA(A)R is markedly influenced by descending pathways, in particular serotoninergic projections.

The AMPA receptor as a therapeutic target: current perspectives and emerging possibilities

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a subtype of the ionotropic glutamate receptors that plays a prominent role in neurotransmission and is widespread throughout the CNS. Because of this, its malfunction can result in a multitude of nervous system diseases. This review looks at compounds that are able to modulate AMPAR function by binding to one of several sites on the receptor that either downregulate its function (competitive, noncompetitive and uncompetitive antagonists) or upregulate its function (positive modulators). It will also give an account of the various diseases that have implicated AMPAR dysfunction and how specific types of AMPAR modulator may be beneficial in their treatment. The AMPAR remains an unexploited but important therapeutic target.

Pediatric spinal cord injury in infant piglets: description of a new large animal model and review of the literature

OBJECTIVE

To develop a new, clinically relevant large animal model of pediatric spinal cord injury (SCI) and compare the clinical and experimental features of pediatric SCI.

METHODS

Infant piglets (3-5 weeks old) underwent contusive SCI by controlled cortical impactor at T7. Severe complete SCI was induced in 6 piglets, defined as SCI with no spontaneous return of sensorimotor function. Eight piglets received incomplete SCI, which was followed by partial recovery. Somatosensory evoked potentials, magnetic resonance imaging, neurobehavioral function, and histopathology were measured during a 28-day survival period.

RESULTS

Mean SCI volume (defined as volume of necrotic tissue) was larger after complete compared with incomplete SCI (387 +/- 29 vs 77 +/- 38 mm3, respectively, P < 0.001). No functional recovery occurred after complete SCI. After incomplete SCI, piglets initially had an absence of lower extremity sensorimotor function, urinary and stool retention, and little to no rectal tone. Sensory responses recovered first (1-2 days after injury), followed by spontaneous voiding, lower extremity motor responses, regular bowel movements, and repetitive flexion-extension of the lower extremities when crawling. No piglet recovered spontaneous walking, although 4 of 8 animals with incomplete injuries were able to bear weight by 28 days. In vivo magnetic resonance imaging was performed safely, yielded high-resolution images of tissue injury, and correlated closely with injury volume seen on histopathology, which included intramedullary hemorrhage, cellular inflammation, necrosis, and apoptosis.

CONCLUSION

Piglets performed well as a reproducible model of traumatic pediatric SCI in a large animal with chronic survival and utilizing multiple outcome measures, including evoked potentials, magnetic resonance imaging, functional outcome scores, and histopathology.

Neuroprotective effects of bone morphogenetic protein 7 (BMP7) treatment after spinal cord injury

Bone morphogenetic protein 7 (BMP7) has been shown to ameliorate reduced dendritic growth induced by glutamate excitotoxicity in neuronal tissue cultures and/or provide an enhancement of functional recovery in central nervous system (CNS) injury. BMP7 expression is modulated by spinal cord injury (SCI), but the molecular mechanisms involved in neuroprotection have not been clearly defined. Here, we show that BMP7 treatment of rats subjected to mild cervical SCI significantly increased the pro-survival mitogen-activated protein kinase-38 (MAPK-38) pathway and levels of N-methyl-D-aspartate receptor 1 (NMDAR-1) resulting in a significant increase in neuronal sparing in the ventral horn of the spinal cord. Moreover, BMP7 was neuroprotective against glutamate-mediated excitotoxicity in cultured cortical neurons. These studies show that BMP7 administration may be used as a therapeutic strategy to reduce the damaging excitotoxic effects following SCI.

The potential role of phrenic nucleus glutamate receptor subunits in mediating spontaneous crossed phrenic activity in neonatal rat

Cervical spinal cord hemisection rostral to the phrenic nucleus leads to paralysis of the ipsilateral hemidiaphragm in adult rats. Respiratory function can be restored to the paralyzed hemidiaphragm by activating a latent respiratory motor pathway. The latent pathway is called the crossed phrenic pathway. In adult rats, the pathway can be activated by drug-induced upregulation of NMDA receptor NR2A subunit and AMPA receptor GluR1 subunit in the phrenic nucleus following hemisection. In neonatal rats, this pathway is not latent as shown by the spontaneous expression of activity in the ipsilateral hemidiaphragm following hemisection. We hypothesized that the NR2A and GluR1 subunits may be highly expressed naturally on phrenic motoneurons of neonatal rats and may play a potential role in mediating the spontaneous expression of activity in the ipsilateral hemidiaphragm after hemisection. To test this hypothesis, the protein levels of NR2A and GluR1 in different age rats were assessed via Western blot analysis immediately following C2 hemisection and EMG recording of crossed phrenic activity. The protein levels of NR2A and GluR1 were transiently high in postnatal day 2 (P2) rats and then was significantly reduced in P7 and P35 animals. An immunofluorescence study qualitatively supported these findings. The present results indicate that the developmental downregulation of the phrenic nucleus glutamate receptor subunits correlates with the conversion of the crossed phrenic pathway in older postnatal animals from an active state to a latent state.

A neurochemical signature of visual recovery after extrastriate cortical damage in the adult cat

In adult cats, damage to the extrastriate visual cortex on the banks of the lateral suprasylvian (LS) sulcus causes severe deficits in motion perception that can recover as a result of intensive direction discrimination training. The fact that recovery is restricted to trained visual field locations suggests that the neural circuitry of early visual cortical areas, with their tighter retinotopy, may play an important role in attaining perceptual improvements after damage to higher level visual cortex. The present study tests this hypothesis by comparing the manner in which excitatory and inhibitory components of the supragranular circuitry in an early visual cortical area (area 18) are affected by LS lesions and postlesion training. First, the proportion of LS-projecting pyramidal cells as well as calbindin- and parvalbumin-positive interneurons expressing each of the four AMPA receptor subunits was estimated in layers II and III of area 18 in intact animals. The degree to which LS lesions and visual retraining altered these expression patterns was then assessed. Both LS-projecting pyramidal cells and inhibitory interneurons exhibited long-term, differential reductions in the expression of glutamate receptor (GluR)1, -2, -2/3, and -4 following LS lesions. Intensive visual training post lesion restored normal AMPAR subunit expression in all three cell-types examined. Furthermore, for LS-projecting and calbindin-positive neurons, this restoration occurred only in portions of the ipsi-lesional area 18 representing trained visual field locations. This supports our hypothesis that stimulation of early visual cortical areas-in this case, area 18-by training is an important factor in restoring visual perception after permanent damage to LS cortex.

Spinal cord injury in neonates alters respiratory motor output via supraspinal mechanisms

Upper cervical spinal cord injury (SCI) alters respiratory output and results in a blunted respiratory response to pH/CO2. Many SCI studies have concentrated on respiratory changes in neural function caudal to injury; however few have examined whether neural plasticity occurs rostral to SCI. Golder et al. (2001a) showed that supraspinal changes occur to alter respiratory output after SCI. Furthermore, Brown et al. (2004) showed that neural receptors change rostral to a thoracic SCI. We hypothesized that SCI in neonates will alter supraspinal output, show a blunted response to pH and alter receptor protein levels in the medulla. On postnatal day 0/1, a C2 SCI surgery was performed. Two days later, neonates were anesthetized and brainstem-spinal cords removed. Respiratory-related activity was recorded using the in vitro brainstem-spinal cord preparation and the superfusate pH was changed (pH 7.2, 7.4 and 7.8). The respiratory-like frequency was significantly reduced in SCI rats indicating supraspinal plasticity. Increasing the pH decreased respiratory-like frequency and peak amplitude in injured and sham controls. Increasing the pH increased burst duration and area in sham controls, whereas in injured rats, the burst duration and area decreased. Western blot analysis demonstrated significant changes in glutamate receptor subunits (NR1, NR2B and GluR2), adenosine receptors (A1, A2A), glutamic acid decarboxylase (65) and neurokinin-1 receptors in medullary tissue ipsilateral and contralateral to injury. These data show that supraspinal plasticity in the respiratory system occurs after SCI in neonate rats. The mechanisms remain unknown, but may involve alterations in receptor proteins involved in neurotransmission.

Spinal cord transection enhances afferent-evoked inhibition in lamina II neurons and abolishes BDNF-induced facilitation of their sensory input

We previously reported that the pronociceptive neurotrophin brain-derived neurotrophic factor (BDNF) induces facilitation of C-fiber evoked EPSCs and NMDA currents in lamina II neurons of rats up to P40. Here, patch-clamp recording was used to study BDNF-induced modification of synaptic and NMDA-evoked responses in transverse spinal slices from lumbar (L2-L5) spinal cord of rats from P3 to P21 following complete spinal cord transection at P2. After transection injury at either T13/L1 or L6/S1, BDNF failed to facilitate synaptic AMPA-kainate currents or agonist-induced NMDA currents. The evoked synaptic currents were smaller in amplitude, often consisting of a biphasic (excitatory-inhibitory) response. The EPSC decayed more rapidly in neurons from transected cords than in those from uninjured cords. In transected cords most neurons responded to the GABA(A) receptor antagonist bicuculline with a significant increase in duration of the excitatory synaptic response. This could subsequently be blocked by D-APV, suggesting an NMDA-receptor mediated component. These findings suggest that following spinal cord transection, BDNF spinal actions are no longer predominantly pronociceptive. It is possible that a diminished availability of full-length TrkB and an enhanced expression of truncated TrkB receptors in the injured cord could play an important role in reducing the effect of BDNF following injury. These data are compared to those obtained after contusion, and it is concluded that the physiological changes after spinal injury differ according to nature of the injury.

BDNF-Induced Facilitation of Afferent-Evoked Responses in Lamina II Neurons Is Reduced After Neonatal Spinal Cord Contusion Injury

We previously reported that brain-derived neurotrophic factor (BDNF), a pronociceptive neurotransmitter, induces synaptic facilitation of excitatory postsynaptic current (EPSC) in lamina II neurons of neonatal rats up to P14 in a N-methyl-d-aspartate (NMDA) receptor-dependent manner. Here we used the patch-clamp technique to study synaptic and NMDA-evoked responses in transverse spinal slices in the lumbar enlargement as well as the ability of BDNF to modify these responses from 1 day to 6 wk after neonatal contusion. In older uninjured animals (>P14), BDNF continued to evoke synaptic facilitation although superfusion of NMDA (in TTX) induced inward current of significantly smaller amplitude than that observed in younger rats. After contusion injury, BDNF was unable to facilitate dorsal root-evoked EPSCs in lamina II neurons despite the finding that NMDA-evoked currents were only slightly smaller than those observed in age-matched uninjured animals. These findings suggest that although BDNF-induced facilitation of the AMPA/kainate receptor-mediated response to dorsal root stimulation is maintained in the mature dorsal horn from intact rats, BDNF may no longer elicit these pronociceptive actions after neonatal contusion injury. The lack of change in NMDA-evoked currents in contused cords suggests that diminished NMDA receptor function is not the major cause of the decline in BDNF action after contusion. It seems more likely that diminished trkB expression and enhanced expression of truncated trkB receptors in the contused cord play a significant role in determining the reduced effect of BDNF under these conditions.

Rapid functional recovery after spinal cord injury in young rats

Responses to traumatic injury in the immature spinal cord may be different from those in adults. We modified an adult model of weight-drop injury to characterize the histopathology and functional recovery after spinal cord injury (SCI) in rat pups at postnatal day 14-15. A 10-g weight was dropped from 2.5 or 5.0 cm at T8-T9. Hindlimb function was evaluated at 24 h and 1, 2, 3, and 4 weeks after injury using the Combined Behavioral Score that estimates overall hind limb sensorimotor function, and the BBB scale for open field locomotion. Histopathology was examined at 15 min, 24 h, and 4 weeks after SCI. The initial hemorrhagic lesion was similar to that seen in adults, but the time course of secondary loss of ventral horn motor neurons was extended. By 4 weeks, only a partial rim of white matter surrounding a central cavity was seen. The 5.0 cm injury group exhibited significantly less recovery of function at 4 weeks than the 2.5 cm group. In the latter, the degree of hindlimb deficit at 4 weeks was similar to that previously described for adults with 10 g x 2.5 cm SCI. However, pups in both injury groups exhibited a significantly faster rate of recovery than adults. Recovery was maximal by 1 week after SCI in pups as compared to 3-4 weeks in adults. The more rapid functional recovery observed in the pups suggests that this new model may be useful for studying mechanisms of functional plasticity after SCI.