Neurotoxic effects of excitatory amino acids in the mouse spinal cord: quisqualate and kainate but not N-methyl-D-aspartate induce permanent neural damage

Intermediate gray matter interneurons in the lumbar spinal cord play a critical and necessary role in coordinated locomotion

Locomotion is a complex task involving excitatory and inhibitory circuitry in spinal gray matter. While genetic knockouts examine the function of individual spinal interneuron (SpIN) subtypes, the phenotype of combined SpIN loss remains to be explored. We modified a kainic acid lesion to damage intermediate gray matter (laminae V-VIII) in the lumbar spinal enlargement (spinal L2-L4) in female rats. A thorough, tailored behavioral evaluation revealed deficits in gross hindlimb function, skilled walking, coordination, balance and gait two weeks post-injury. Using a Random Forest algorithm, we combined these behavioral assessments into a highly predictive binary classification system that strongly correlated with structural deficits in the rostro-caudal axis. Machine-learning quantification confirmed interneuronal damage to laminae V-VIII in spinal L2-L4 correlates with hindlimb dysfunction. White matter alterations and lower motoneuron loss were not observed with this KA lesion. Animals did not regain lost sensorimotor function three months after injury, indicating that natural recovery mechanisms of the spinal cord cannot compensate for loss of laminae V-VIII neurons. As gray matter damage accounts for neurological/walking dysfunction in instances of spinal cord injury affecting the cervical or lumbar enlargement, this research lays the groundwork for new neuroregenerative therapies to replace these lost neuronal pools vital to sensorimotor function.

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

Reactivity of microglia and astrocytes after an excitotoxic injury induced by kainic acid in the rat spinal cord

After injury of the nervous system glial cells react according to the stimuli by modifying their morphology and function. Glia activation was reported in different kainic acid (KA)-induced neurodegeneration models. Here, we describe glial morphometric changes occurring in an excitotoxic KA-induced cervical spinal cord injury model. Concomitant degenerative and apoptotic processes are also reported. Male rats injected at the spinal cord C5 segment either with KA or saline were euthanized at post-injection (PI) days 1, 2, 3 or 7. Anti-IBA-1 and anti-GFAP antibodies were used to identify microglia and activated astrocytes, respectively, and to morphometrically characterized them. Fluoro-Jade B staining and TUNEL reaction were used to determine neuronal and glial degeneration and apoptosis. KA-injected group showed a significant increase in microglia number at the ipsilateral side by PI day 3. Different microglia reactive phenotypes were observed. Reactive microglia was still present by PI day 7. Astrocytes in KA-injected group showed a biphasic increase in number at PI days 1 and 3. Degenerative and apoptotic events were only observed in KA-injected animals, increasing mainly by PI day 1. Understanding the compromise of glia in different neurodegenerative processes may help to define possible common or specific therapeutic approaches directed towards neurorestorative strategies.

Pathogenic Mechanisms in Sporadic Amyotrophic Lateral Sclerosis

In recognition of the 100th anniversary of Charcot’s death we have reviewed possible pathogenic mechanisms in amyotrophic lateral sclerosis (ALS). Advances in the last 5 years in molecular biology and genetics have identified mutations in the cytosolic dismutase (SODI) gene in some patients with familial ALS raising the possibility that oxidative stress may be involved in the pathogenesis. An excitotoxic pathogenesis has been implicated based on elevated plasma and CSF levels of amino acids and altered contents of amino acids in the nervous system of ALS patients and changes in the number of excitatory amino acid receptors. ALS sera containing antibodies to L-type calcium channels and the development of immune mediated lower and upper and lower motor neuron models have revitalized research efforts focusing on an immune basis for ALS. Other pathogenic mechanisms which have been the subject of recent research include elemental toxicity, apoptosis and programmed cell death and possibly a deficiency or abnormality in growth factors. Pathogenic processes for ALS must account for an increasing incidence of ALS, male preponderance, and the selective vulnerability of the corticomotoneuronal system.

Comparative Study on the Effects of Ceftriaxone and Monocytes on Recovery after Spinal Cord Injury in Rat

PURPOSE

Comparison between the efficacy of ceftriaxone and monocytes on improvement of neuron protection and functional recovery after spinal cord injury (SCI) in rat.

METHODS

Rats were randomly divided into three groups of ten. Spinal cord injury was performed on rats under general anesthesia using the weight dropping method. Ceftriaxone was injected intraperitoneally 200 mg/kg/day for seven days after SCI. Monocytes were injected 2 × 105 cells 4 days after SCI. Hind limb motor function was assessed using the Basso, Beattie and Bresnahan (BBB) scale. Corticospinal tract (CST) axons were traced by injection of biotin dextran amine (BDA) into the sensorimotor cortex.

RESULTS

There were statistically significant differences in BBB scores in ceftriaxone in comparison to both monocytes receiving and control groups. On the other hand there were statistically significant differences in axon counting in both ceftriaxone and monocytes receiving groups in comparison to control group.

CONCLUSION

Our findings suggest that ceftriaxone improves functional recovery more effective than monocytes in rats after SCI. These results are from an experimental model and validation is required for further investigation.

Comparison of the spinal neuropathic pain induced by intraspinal injection of N-methyl-d-aspartate and quisquate in rats

OBJECTIVE

Excitatory amino acids play important roles in the development of secondary pathology following spinal cord injury (SCI). This study was designed to evaluate morphological changes in the dorsal horn of the spinal cord and assess profiles of pain behaviors following intraspinal injection of N-methyl-D-aspartate (NMDA) or quisqualate (QUIS) in rats.

METHODS

Forty male Sprague-Dawley rats were randomized into three groups : a sham, and two experimental groups receiving injections of 125 mM NMDA or QUIS into their spinal dorsal horn. Following injection, hypersensitivity to cold and mechanical stimuli, and excessive grooming behaviors were assessed serially for four weeks. At the end of survival periods, morphological changes in the spinal cord were evaluated.

RESULTS

Cold allodynia was developed in both the NMDA and QUIS groups, which was significantly higher in the QUIS group than in the NMDA group. The mechanical threshold for the ipsilateral hind paw in both QUIS and NMDA groups was significantly lower than that in the control group. The number of groomers was significantly higher in the NMDA group than in the QUIS group. The size of the neck region of the spinal dorsal horn, but not the superficial layer, was significantly smaller in the NMDA and QUIS groups than in the control group.

CONCLUSION

Intraspinal injection of NMDA or QUIS can be used as an excitotoxic model of SCI for further research on spinal neuropathic pain.

Excitotoxic motoneuron degeneration induced by glutamate receptor agonists and mitochondrial toxins in organotypic cultures of chick embryo spinal cord

Glutamate receptor-mediated excitotoxicity and mitochondrial dysfunction appear to play an important role in motoneuron (MN) degeneration in amyotrophic lateral sclerosis (ALS). In the present study we used an organotypic slice culture of chick embryo spinal cord to explore the responsiveness of mature MNs to different excitotoxic stimuli and mitrochondrial inhibition. We found that, in this system, MNs are highly vulnerable to excitotoxins such as glutamate, N-methyl-D-aspartate (NMDA), and kainate (KA), and that the neuroprotective drug riluzole rescues MNs from KA-mediated excitotoxic death. MNs are also sensitive to chronic mitochondrial inhibition induced by malonate and 3-nitropropionic acid (3-NP) in a dose-dependent manner. MN degeneration induced by treatment with mitochondrial toxins displays structural changes similar to those seen following excitotoxicity and can be prevented by applying either the antiexcitotoxic drug 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) or riluzole. Excitotoxicity results in an increased frequency of normal spontaneous Ca2+ oscillations in MNs, which is followed by a sustained deregulation of intracellular Ca2+. Tolerance to excitotoxic MN death resulting from chronic exposure to excitotoxins correlates with a reduced excitotoxin-induced increase in intracellular Ca2+ and increased thapsigargin-sensitive Ca2+ stores.

Excitotoxicity and amyotrophic lateral sclerosis

Since its description by Charcot more than 130 years ago, the pathogenesis of selective motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains unsolved. Over the years, many pathogenic mechanisms have been proposed. Amongst others these include: oxidative stress, excitotoxicity, aggregate formation, inflammation, growth factor deficiency and neurofilament disorganization. This multitude of contributing factors indicates that ALS is a complex disease and also suggests that ALS is a multifactorial disorder. Excitotoxicity is not the newest and most spectacular hypothesis in the ALS field, but it is undoubtedly one of the most robust pathogenic mechanisms supported by an impressive amount of evidence. Moreover, the therapeutic efficacy of riluzole, the only drug proven to slow disease progression in ALS, is most likely related to its anti-excitotoxic properties. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS and of the possible mechanisms leading to motor neuron death. We will also summarize the intrinsic properties of motor neurons that render these cells particularly vulnerable to excitotoxicity and could explain the selective vulnerability of motor neurons in ALS. All this information could help to develop new and better therapeutic strategies that could protect motor neurons from excitotoxicity.

The role of excitotoxicity in the pathogenesis of amyotrophic lateral sclerosis

Unfortunately and despite all efforts, amyotrophic lateral sclerosis (ALS) remains an incurable neurodegenerative disorder characterized by the progressive and selective death of motor neurons. The cause of this process is mostly unknown, but evidence is available that excitotoxicity plays an important role. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS. The most important one is that the only drug proven to slow the disease process in humans, riluzole, has anti-excitotoxic properties. Moreover, consumption of excitotoxins can give rise to selective motor neuron death, indicating that motor neurons are extremely sensitive to excessive stimulation of glutamate receptors. We will summarize the intrinsic properties of motor neurons that could render these cells particularly sensitive to excitotoxicity. Most of these characteristics relate to the way motor neurons handle Ca(2+), as they combine two exceptional characteristics: a low Ca(2+)-buffering capacity and a high number of Ca(2+)-permeable AMPA receptors. These properties most likely are essential to perform their normal function, but under pathological conditions they could become responsible for the selective death of motor neurons. In order to achieve this worst-case scenario, additional factors/mechanisms could be required. In 1 to 2% of the ALS patients, mutations in the SOD1 gene could shift the balance from normal motor neuron excitation to excitotoxicity by decreasing glutamate uptake in the surrounding astrocytes and/or by interfering with mitochondrial function. We will discuss point by point these different pathogenic mechanisms that could give rise to classical and/or slow excitotoxicity leading to selective motor neuron death.

Widespread loss of neuronal populations in the spinal ventral horn in sporadic motor neuron disease. A morphometric study

The cytopathology and loss of neurons was studied in 7670 neurons from the ventral horn of the third lumbar segment of the spinal cord of six sporadic motor neuron disease (MND) patients compared with 7568 neurons in seven age matched control subjects. A modified Tomlinson et al. [Tomlinson BE, Irving D, Rebeiz JJ. Total numbers of limb motor neurones in the human lumbosacral cord and an analysis of the accuracy of various sampling procedures. J Neurol Sci 1973;20:313-27] sampling procedure was used for neuronal counts. The ventral horn was divided in quadrants. Neuronal populations were also classified by the maximum cell diameter through the nucleolus. There was widespread loss of neurons in all quadrants of the ventral horn in MND. Size distribution histograms showed similar neuron loss across all populations of neurons. The dorsomedial quadrant contains almost exclusively interneurons and the ventrolateral quadrant mostly motor neurons. The cytopathology of neurons in the dorsomedial quadrant and of large motorneurons in the ventrolateral quadrant MND was similar. In the dorsomedial quadrant, neuron loss (56.7%) was similar to the loss of large motor neurons in the ventrolateral quadrant (64.4%). The loss of presumed motor neurons and interneurons increased with increased disease duration. There was no evidence that loss of presumed interneurons occurred prior, or subsequent, to loss of motor neurons. We conclude that, in sporadic MND, all neuronal populations in the ventral horn are affected and that interneurons are involved to a similar extent and in parallel with motor neurons, as reported in the G86R transgenic mouse model of familial MND. The increasing evidence of loss of neurons other than motor neurons in MND suggests the need for revising the concept of selective motor neuron vulnerability.

AMPA receptor activation, but not the accumulation of endogenous extracellular glutamate, induces paralysis and motor neuron death in rat spinal cord in vivo

The mechanisms of motor neuron (MN) degeneration in amyotrophic lateral sclerosis (ALS) are unknown, but glutamate-mediated excitotoxicity may be involved. To examine directly this idea in vivo, we have used microdialysis in the rat lumbar spinal cord and showed that four- to fivefold increases in the concentration of endogenous extracellular glutamate during at least 1 h, by perfusion with the glutamate transport inhibitor L-2,4-trans-pyrrolidine-dicarboxylate, elicited no motor alterations or MN damage. Stimulation of glutamate release with 4-aminopyridine induced transitory ipsilateral hindlimb muscular twitches but no MN damage. In contrast, perfusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) did not modify glutamate levels but produced intense muscular spasms, followed by ipsilateral permanent hindlimb paralysis and a remarkable loss of MNs. These effects of AMPA were prevented by co-perfusion with the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline. Perfusion with NMDA or kainate produced no motor effects or MN damage. Thus, the elevation of endogenous extracellular glutamate in vivo due to blockade of its transport is innocuous for spinal MNs. Because this resistance is observed under the same experimental conditions in which MNs are highly vulnerable to AMPA, these results indicate that excitotoxicity due to this mechanism might not be an important factor in the pathogenesis of ALS.

Temporal and spatial distribution of activated caspase-3 after subdural kainic acid infusions in rat spinal cord

The molecular events initiating apoptosis following traumatic spinal cord injury (SCI) remain poorly understood. Soon after injury, the spinal cord is exposed to numerous secondary insults, including elevated levels of glutamate, that contribute to cell dysfunction and death. In the present study, we attempted to mimic the actions of glutamate by subdural infusion of the selective glutamate receptor agonist, kainic acid, into the uninjured rat spinal cord. Immunohistochemical colocalization studies revealed that activated caspase-3 was present in ventral horn motor neurons at 24 hours, but not 4 hours or 96 hours, following kainic acid treatment. However, at no time point examined was there evidence of significant neuronal loss. Kainic acid resulted in caspase-3 activation in several glial cell populations at all time points examined, with the most pronounced effect occurring at 24 hours following infusion. In particular, caspase-3 activation was observed in a significant number of oligodendroglia in the dorsal and ventral funiculi, and there was a pronounced loss of oligodendroglia at 96 hours following treatment. The results of these experiments indicate a role for glutamate as a mediator of oligodendroglial apoptosis in traumatic SCI. In addition, understanding the apoptotic signaling events activated by glutamate will be important for developing therapies targeting this cell death process.

Vocalization responses after intrathecal administration of ionotropic glutamate receptor agonists in rats

Inotropic glutamate receptors in the spinal cord (N-methyl-D-aspartic acid [NMDA], alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA], and kainate receptors) seem to play a key role in acute pain transmission and the neuronal plasticity in chronic pain states. Vocalization responses produced by activation of these receptors on the pain pathways can be quantified semiautomatically and thus could be used as a research tool. We studied vocalization responses induced by intrathecal administration of various agonists acting at the glutamate receptors in normal rats and in the presence of peripheral inflammation and a chronic constriction injury model of neuropathic pain. The nonselective endogenous agonist, glutamate, and the NMDA receptor glycine site agonist D-serine did not produce vocalization, whereas selective agonists acting at AMPA, NMDA, and kainate receptors produced dose-related vocalization responses. The vocalization response evoked by the administration of AMPA was significantly increased in the neuropathic pain model. In conclusion, spinal administration of ionotropic glutamate receptor agonists produce short-lasting, dose-related vocalization responses that can be used as a basic research and screening tool for analgesic studies. However, peripheral inflammation or nerve injury did not substantially alter vocalization responses overall, possibly indicating that the vocalization test is not a good tool for studying the role of excitatory amino acids in these pathological pain conditions.

IMPLICATIONS

Vocalization responses evoked by spinal administration of ionotropic glutamate receptor agonists can be used for experimental analgesic studies. However, pathological pain models did not substantially alter vocalization responses, possibly indicating that this test is not suitable for studying the role of spinal excitatory amino acids in central sensitization.

NBQX treatment improves mitochondrial function and reduces oxidative events after spinal cord injury

The purpose of this study was to examine the effects of inhibiting ionotropic glutamate receptor subtypes on measures of oxidative stress events at acute times following traumatic spinal cord injury (SCI). Rats received a moderate contusion injury and 15 min later were treated with one of two doses of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzol[f]quinoxaline-7-sulfonamide disodium (NBQX), MK-801, or the appropriate vehicle. At 4 h following injury, spinal cords were removed and a crude synaptosomal preparation obtained to examine mitochondrial function using the MTT assay, as well as measures of reactive oxygen species (ROS), lipid peroxidation, and glutamate and glucose uptake. We report here that intraspinal treatment with either 15 or 30 nmol of NBQX improves mitochondrial function and reduces the levels of ROS and lipid peroxidation products. In contrast, MK-801, given intravenously at doses of 1.0 or 5.0 mg/kg, was without effect on these same measures. Neither drug treatment had an effect on glutamate or glucose uptake, both of which are reduced at acute times following SCI. Previous studies have documented that drugs acting on non-N-methyl-D-aspartate (NMDA) receptors exhibit greater efficacy compared to NMDA receptor antagonists on recovery of function and tissue sparing following traumatic spinal cord injury. The results of this study provide a potential mechanism by which blockade of the non-NMDA ionotropic receptors exhibit positive effects following traumatic SCI.

Calcineurin-mediated BAD dephosphorylation activates the caspase-3 apoptotic cascade in traumatic spinal cord injury

We report here that activation of the caspase-3 apoptotic cascade in spinal cord injury is regulated, in part, by calcineurin-mediated BAD dephosphorylation. BAD, a proapoptotic member of the bcl-2 gene family, is rapidly dephosphorylated after injury, dissociates from 14-3-3 in the cytosol, and translocates to the mitochondria of neurons where it binds to Bcl-x(L). Pretreatment of animals with FK506, a potent inhibitor of calcineurin activity, or MK801, an NMDA glutamate receptor antagonist, blocked BAD dephosphorylation and abolished activation of the caspase-3 apoptotic cascade. These findings extend previous in vitro observations and are the first to implicate the involvement of glutamate-mediated calcineurin activation and BAD dephosphorylation as upstream, premitochondrial signaling events leading to caspase-3 activation in traumatic spinal cord injury.

Glutamate release and neuronal injury after intrathecal injection of local anesthetics

High concentrations of local anesthetics are neurotoxic, but the mechanism for this neurotoxicity is obscure. Here, we report increased concentrations of glutamate in the cerebrospinal fluid after intrathecal injections of high concentrations of tetracaine (a local anesthetic). The peak concentrations of glutamate after administration of 1%, 2%, and 4% tetracaine were 4-fold, 6-fold, and 10-fold higher than baseline values, respectively. Animals in the 1% group were all neurologically normal one week after tetracaine injection. In the group receiving 4%, no animal was able to hop and vacuolation of the white matter and/or central chromatolysis of the motor neurons were observed. Because high concentrations of glutamate are known to be neurotoxic, our results may provide some insight into the mechanisms for neurotoxicity of intrathecal local anesthetics.

Superoxide dismutase activity and the effects of NBQX and CPP on lipid peroxidation in experimental spinal cord injury

The endogenous activity of the neuroprotective enzyme superoxide dismutase (SOD) and the amount of lipid peroxidation in the early phase of experimental spinal cord injury, together with the effects of N-methyl-D-aspartate (NMDA) antagonist CPP and non-NMDA antagonist NBQX on lipid peroxidation were evaluated. The clip compression model was used for the production of a standardized spinal cord trauma. SOD activity and malondialdehyde (MDA) levels--as an indicator of lipid peroxidation--were determined in the injured segment of the spinal cord 30 and 60 min after injury. SOD activity did not change in this period, whereas MDA levels at 30 and 60 min after trauma were significantly elevated. Intrathecal administration of CPP or NBQX 15 min after injury produced statistically significant reductions in MDA elevation 60 min after injury. NBQX was found to be more effective than CPP. These results demonstrated that intrathecal local application of excitatory amino acid receptor antagonists can protect the spinal cord from secondary damage caused by the generation of lipid peroxides in experimental spinal cord injury.

Dual localization of neuropeptide FF receptors in the rat dorsal horn

Although neuropeptide FF (NPFF) is generally considered an anti-opioid, its intrathecal administration produces analgesia. In the present study, the stable analog 1DMe ([D.Tyr(1), (NMe)Phe(3)]neuropeptide FF) was used in quantitative autoradiographic experiments in combination with surgical and chemical lesions to precisely localize NPFF receptors in the rat spinal cord. Ligation of lumbar dorsal spinal roots revealed the presence of NPFF receptors in dorsal root fibers and it induced a significant accumulation of [(125)I]1DMe-specific binding on the side peripheral to the ligature, demonstrating that a population of NPFF receptors is synthesized in dorsal root ganglia and migrates anterogradely towards primary afferent nerve endings. Complete mid-thoracic spinal cord transection failed to modify the [(125)I]1DMe labeling density in the dorsal horn, indicating that NPFF receptors are not located on the descending fiber terminals. In contrast, unilateral microinjections of kainic acid into the dorsal horn dramatically reduced [(125)I]1DMe-specific binding in the superficial layers, revealing localization of a population of NPFF receptors on the spinal intrinsic neurons. NPFF receptor binding was not modified during the development of spinal opioid tolerance. The pre- and postsynaptic localization of spinal NPFF receptors provide further support for heterogeneity in the pain modulation by NPFF and related agonists.

Parenterally administered kainic acid induces a persistent hyperalgesia in the mouse and rat

Nociceptive primary afferent C-fibers express a subset of glutamate receptors that are sensitive to kainic acid. Thus, we tested the possibility that activation of these receptors alters nociception. Intraperitoneal (i.p.) injection of kainic acid induced a persistent thermal hyperalgesia, when tested using the hot plate (mice) and tail flick (mice and rats) assays, and mechanical hyperalgesia when tested using von Frey monofilaments (rats), but had no effect on acetic acid-induced chemical nociception (mice). When administered i. p., 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid HBr/kainate (AMPA/KA) antagonist, completely blocked hyperalgesia. When injected intrathecally (i.t.), kainic acid itself failed to induce hyperalgesia and AMPA/KA antagonists given i.t. also failed to attenuate the hyperalgesic effect of kainic acid administered i.p. , indicating that the spinal cord is not the primary site of action. Kainic acid injected subcutaneously in the back of mice decreased response latencies in the hot plate and tail flick assays, indicating that hyperalgesia is achieved by a variety of parenteral routes of injection. Histological evaluation of rat spinal cord and dorsal root ganglia revealed no neurodegenerative changes 24 h after kainic acid. Together these data suggest that a persistent hyperalgesia results from the transient activation of AMPA/KA receptors that are located outside the spinal cord, perhaps on the distal projections of primary afferent fibers.

Comparing deficits following excitotoxic and contusion injuries in the thoracic and lumbar spinal cord of the adult rat

The majority of human spinal cord injuries involve gray matter loss from the cervical or lumbar enlargements. However, the deficits that arise from gray matter damage are largely masked by the severe deficits due to associated white matter damage. We have developed a model to examine gray matter-specific deficits and therapeutic strategies that uses intraspinal injections of the excitotoxin kainic acid into the T9 and L2 regions of the spinal cord. The resulting deficits have been compared to those from standard contusion injuries at the same levels. Injuries were assessed histologically and functional deficits were determined using the Basso, Beattie, and Bresnahan (BBB) 21-point open field locomotor scale and transcranial magnetic motor evoked potentials (tcMMEPs). Kainic acid injections into T9 resulted in substantial gray matter damage; however, BBB scores and tcMMEP response latencies were not different from those of controls. In contrast, kainic acid injections into L2 resulted in paraplegia with BBB scores similar to those following contusion injuries at either T9 or L2, without affecting tcMMEP response latencies. These observations demonstrate that gray matter loss can result in significant functional deficits, including paraplegia, in the absence of a disruption of major descending pathways.

Excitotoxic death of a subset of embryonic rat motor neurons in vitro

We have used cultures of purified embryonic rat spinal cord motor neurons to study the neurotoxic effects of prolonged ionotropic glutamate receptor activation. NMDA and non-NMDA glutamate receptor agonists kill a maximum of 40% of the motor neurons in a concentration- and time-dependent manner, which can be blocked by receptor subtype-specific antagonists. Subunit-specific antibodies stain all of the motor neurons with approximately the same intensity and for the same repertoire of subunits, suggesting that the survival of the nonvulnerable population is unlikely to be due to the lack of glutamate receptor expression. Extracellular Ca2+ is required for excitotoxicity, and the route of entry initiated by activation of non-NMDA, but not NMDA, receptors is L-type Ca2+ channels. Ca2+ imaging of motor neurons after application of specific glutamate receptor agonists reveals a sustained rise in intracellular Ca2+ that is present to a similar degree in most motor neurons, and can be blocked by appropriate receptor/channel antagonists. Although the lethal effects of glutamate receptor agonists are seen in only a subset of cultured motor neurons, the basis of this selectivity is unlikely to be simply the glutamate receptor phenotype or the level/pattern of rise in agonist-evoked intracellular Ca2+.

AMPA receptor-mediated slow neuronal death in the rat spinal cord induced by long-term blockade of glutamate transporters with THA

Excitotoxicity secondary to the loss of glutamate transporters (GluT) has been proposed as a possible pathogenetic mechanism for neuronal degeneration in amyotrophic lateral sclerosis. We therefore investigated whether prolonged in vivo pharmacologic inhibition of GluT would result in neuronal damage in the rat. DL-Threo-beta-hydroxyaspartate (THA), a potent GluT inhibitor, and glutamate were continuously infused into the rat spinal subarachnoid space by using a mini-osmotic pump. Animals that received both THA and glutamate, but not those received either singly, displayed tail paralysis with or without hind-limb paralysis and urinary incontinence after the third postoperative day. Pathologically, symptomatic animals exhibited neuronal loss with a variable extent of gliosis preferentially involving the dorsal horn of the lumbosacral cord. In the rostral spinal segments adjacent to those regions of intense pathologic changes, small neurons in the dorsal horn were selectively destroyed, a pattern similar to the late-onset neuronal damage induced by continuous intrathecal administration of 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) [R. Nakamura et al., Brain Res. 654 (1994) 279-285]. These behavioral and pathologic changes were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), suggesting that pharmacologic blockade of GluT causes selective neuronal damage in vivo by AMPA receptor activation.

Concentration-dependent changes in motor behavior produced by continuous intrathecal infusion of excitatory amino acids in the rat spinal cord

A growing body of evidence has suggested that glutamate receptors mediate selective degeneration of neurons in the central nervous system during the development of neurodegenerative diseases. Glutamate receptors are divided into N-methyl-D-aspartate (NMDA)-type and non-NMDA-type. Neurotoxicity mediated by the latter has attracted much interest as a possible causative mechanism underlying amyotrophic lateral sclerosis (ALS). As the clinical course of ALS is chronic and progressive, investigation of chronic effects of non-NMDA receptor agonists on neuronal function would be useful for evaluating the role of glutamate receptor-mediated neurotoxicity in ALS. However, chronic non-NMDA receptor-mediated neurotoxicity has been investigated less thoroughly than acute neurotoxicity. We infused an aqueous solution of R,S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) intrathecally and continuously by an osmotic minipump in rats. This method of continuous infusion enabled us to keep the drug concentration relatively constant in the cerebrospinal fluid surrounding the spinal cord. The present method of AMPA administration is more suitable for investigating ALS pathogenesis than acute injections, in view of the gradual progression of the disease and the selectivity of lesions produced.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

NMDA and non-NMDA receptor antagonists protect against excitotoxic injury in the rat spinal cord

The neuroprotective properties of the N-methyl-D-aspartate (NMDA) antagonist dizocilpine (MK-801) and the non-NMDA antagonists 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f]quinoxaline (NBQX) and alpha-methyl-4-carboxyphenylglycine (MCPG) were evaluated against neuronal injury produced by the intraspinal injection of NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). Forty-nine animals were divided into eight groups in order to evaluate the effects of different drug combinations: (a) NMDA; (b) NMDA + MCPG; (c) NMDA + NBQX; (d) NMDA + MK-801; (e) AMPA; (f) AMPA + MCPG; (g) AMPA + MK-801; and (h) AMPA + NBQX. Drugs were microinjected into spinal segments T12-L3 through a micropipette attached to a Hamilton microliter syringe. Spinal cords were evaluated after a survival period of 48 h at which time NMDA and AMPA were found to produce morphological changes over the concentration ranges of 125-500 mM and 75-500 microM, respectively. Neuronal loss following injections of NMDA + MK-801 or AMPA + NBQX was significantly less than that following injections of NMDA or AMPA alone. By contrast, neuronal loss following co-injections of NMDA or AMPA with inappropriate antagonists, i.e., NMDA + NBQX/MCPG or AMPA + MCPG/MK-801, was not significantly different from that produced by NMDA or AMPA. The results suggest that elevations in spinal levels of glutamate followed by prolonged activation of NMDA and AMPA receptor subtypes initiate an excitotoxic cascade resulting in neuronal injury. Blockade of NMDA and AMPA effects by MK-801 and NBQX respectively confirms the well documented neuroprotective effects of these drugs and lends support to the potential importance of NMDA and especially AMPA receptor antagonists as therapeutic agents in the treatment of acute spinal cord injury.

Detrimental effects of exogenous glutamate on spinal cord neurons during brief ischemia in vivo

BACKGROUND

Paraplegia remains a serious complication of thoracoabdominal aortic operations. However, despite growing in vitro evidence, it has been difficult to demonstrate glutamate neurotoxicity in vivo because of the reuptake activity that occurs. We hypothesized that glutamate can be toxic to the spinal cord under metabolic stress.

METHODS

Infrarenal aortic isolation was performed in New Zealand white rabbits. Group A animals (n = 7) then received a segmental infusion of glutamate (50 mmol/L) for 5 minutes. Group B animals (n = 7) received saline as a negative control. Group C animals (n = 6) were pretreated with a segmental infusion of 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (4 mg/kg), a competitive alpha-amino-3-hydroxy-5-methylisoazole-4-propionic acid/kainate antagonist, followed by the segmental infusion of glutamate (30 mmol/L) for 4 minutes. Group D animals (n = 6) received the vehicle agents only, followed by the same glutamate infusion (30 mmol/L) as in group C as a control for group C. Neurologic status was assessed at 12, 24, and 48 hours after operation and scored using the Tarlov system.

RESULTS

Group A animals exhibited paraplegia or paraparesis with marked neuronal necrosis. Group B animals recovered fully. Group C animals had better neurologic function than group D animals (p = 0.0039).

CONCLUSIONS

Exogenous glutamate can have detrimental effects on spinal cord neurons during a brief period of ischemia. This model may be useful for the purpose of assaying a glutamate receptor antagonist in vivo.

Alterations in apolipoprotein E expression during aging and neurodegeneration

Apolipoprotein E (apoE) is a 34 kDa protein that plays an important role in cholesterol transport, uptake and redistribution. Within the nervous system, apoE might be involved in maintaining synaptic integrity after injury and during aging. ApoE might help maintain the integrity of the synaptodendritic complex by several different mechanisms. Among them, recent studies have suggested that apoE: (1) stabilizes the neuronal cytoskeleton; (2) plays an important role in transporting esterified cholesterol to neurons undergoing reinnervation where it is taken up by the low density lipoprotein receptor-related protein pathway and used as a precursor for the synthesis of new synaptic terminals; (3) regulates interactions between neurons and the extracellular matrix (e.g. laminin); and (4) regulates levels of intracellular calcium. The main objective of the manuscript is to review the current progress in understanding the functions of apoE in the nervous system and how malfunctioning of this molecule might result in neurodegenerative disorders such as Alzheimer's disease.

Acute neurotoxicity of L-glutamate induced by impairment of the glutamate uptake system

We examined the effect of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) on the neurotoxicity of L-glutamate in organotypic cultures of rat spinal cord. Eighteen-day-old cultures were incubated with 500 microM L-glutamate, 1 mM PDC, or both. After 72 hours, the tissues were stained for acetylcholinesterase (AChE), and the ventral horn AChE-positive neurons (VHANs) analyzed using morphometry. Neither L-glutamate nor PDC affected AChE staining, but in combination they produced markedly reduced AChE staining in the dorsal horn and a significant decrease in the number of VHANs (especially the smaller VHANs) as compared with the control. Moreover, treatment with 200 microM PDC for 2 weeks preferentially affected the smaller VHANs. The neurotoxicity of L-glutamate plus PDC was blocked by the N-methyl-D-aspartate (NMDA) antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). Results suggest that glutamate uptake system has an important protective function in the aggravation of acute neuronal damage.

Normal and abnormal calcium homeostasis in neurons: a basis for the pathophysiology of traumatic and ischemic central nervous system injury

Clinical recovery after central nervous system (CNS) trauma or ischemia may be limited by a neural injury process that is triggered and perpetuated at the cellular level, rather than by a lesion amenable to surgical repair. It is widely thought that one such process, a fundamental pathological mechanism initiated by CNS injury, is a disruption of cellular Ca2+ homeostasis. Because of the critical role of Ca2+ ions in regulating innumerable cellular functions, this major homeostatic disturbance is thought to trigger neuronal and axonal degeneration and produce clinical disability. We review those aspects of normal and pathological Ca2+ homeostasis in neurons that relate to neurodegeneration and to the application of neuroprotective strategies for the treatment of CNS injury. In particular, we examine the contribution of Ca(2+)-permeable ionic channels, Ca2+ pumps, intracellular Ca2+ stores, intracellular Ca2+ buffering systems, and the roles of secondary, Ca(2+)-dependent processes in neurodegeneration. A number of hypotheses linking Ca2+ ions and Ca2+ permeable channels to neurotoxicity are discussed with an emphasis on strategies for lessening Ca(2+)-related damage. A number of these strategies may have a future role in the treatment of traumatic and ischemic CNS injury.

Selective degeneration of inhibitory interneurons in the rat spinal cord induced by intrathecal infusion of acromelic acid

We investigated the characteristics of the neurotoxicity mediated by non-N-methyl-D-aspartic acid (NMDA)-type glutamate receptors in the spinal cord by infusing rats intrathecally with three specific agonists (acromelic acid A (ACRO), kainic acid and 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)). When ACRO was infused intrathecally continuously for 2 h, the rats developed dose-dependent flaccid paraplegia during the infusion, followed by pure motor, rigid-spastic, long-lasting paraparesis (ED50: 220 pmol/h). The paraparetic rats showed selective degeneration of interneurons in the spinal cord with about 50% loss of their contents of glycine and aspartic and glutamic acids. The alpha-motoneurons in the ventral horns were largely free from permanent damage. These changes were selectively ameliorated by concomitant administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist at non-NMDA receptors. Kainate induced long-lasting paraplegia with neuronal damage at doses about 40-times higher than those of ACRO that caused paraplegia. When examined 30 days after kainate infusion, the neurotransmitter markers had decreased markedly in the lumbar cords of the rats with long-lasting paraplegia. Intrathecal infusion of large doses of AMPA caused long-lasting flaccid paraplegia, which were accompanied by extensive necrosis in the caudal spinal cord. It seems that ACRO exerts its unique pharmacological actions by activating a subclass of non-NMDA receptors distinct from those activated by kainate and AMPA, and may become a useful tool for investigating the biological roles of glutamate receptors.

Acute and late neurotoxicity in the rat spinal cord in vivo induced by glutamate receptor agonists

We investigated glutamate receptor-mediated neurotoxicity in vivo by means of infusing three specific agonists for non-NMDA receptors (acromelic acid A (ACRO), kainic acid and 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)) into the adult rat spinal subarachnoid space. ACRO induced long-lasting pure motor, rigid-spastic paraparesis in a dose-dependent manner (EC50: 220 pmol/h) that was accompanied by selective degeneration of spinal interneurons; leaving large anterior horn cells intact. Kainate and AMPA induced paraplegia but with relatively non-selective neuronal damage when given in doses more than 40-fold larger than those required for ACRO. When AMPA (> 100 nmol/h) was infused continuously using a mini-osmotic pump for more than 2 days, rats displayed progressive changes in motor behavior due to extensive damage in the caudal spinal cord where small neurons in the dorsal horns were the most vulnerable. Co-administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) specifically prevented neurotoxicity, suggesting a non-NMDA receptor-mediated mechanism. These results indicate that the non-NMDA receptor is heterogeneous, mediating neuronal damage with different selectivity. It is also suggested that chronic activation of glutamate receptors is capable of inducing slowly progressive neuronal death, which suggests relevance to the pathogenesis of ALS.

An experimental model combining microdialysis with electrophysiology, histology, and neurochemistry for studying excitotoxicity in spinal cord injury. Effect of NMDA and kainate

We used an experimental model that we previously developed to characterize the damage caused by the agonists of glutamate receptors, N-methyl-D-aspartate (NMDA) and kainate, in the spinal cord in vivo, thereby testing further the utility of this model. Microdialysis was used to administer the toxins and to sample the release of other substances in response to these agents. The blockage of electrical conduction was monitored by recording the amplitudes of evoked potentials during administration of the damaging substances, and damage was assessed by postmortem histological examination. The released amino acids in microdialysates were measured by HPLC. Administration of 5 mM NMDA + 5 mM kainate into the gray matter blocked most postsynaptic responses and caused the release of amino acids. Administration of 10 mM NMDA and 10 mM kainate significantly destroyed cell bodies near the fiber. The advantage of this model is that histological, neurochemical, and electrophysiological parameters were obtained in the same experiment.

Late-onset selective neuronal damage in the rat spinal cord induced by continuous intrathecal administration of AMPA

Neurotoxicity mediated by 1-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) was investigated by infusing this agent continuously for 7 days intrathecally to adult rats using a mini-osmotic pump. Behavioral changes were apparent only after the second postoperative day, when the rats displayed hindlimb palsy or incontinence of urine. The behavioral deficits became progressively severe and the rats usually displayed both hindlimb paraplegia and incontinence of urine by the 7th postoperative day. These progressive behavioral deficits were induced in a dose-dependent manner in the rats that received AMPA at a dose of > 100 pmol/h (100 microM at 1 microliter/h, 17 nmol in total dose). The severity of behavioral deficits was in parallel with that of neuropathological changes in the lumbosacral cords. In spinal segments rostrally adjacent to those with severe pathological changes, only the neurons in the dorsal horns (Rexed's laminae II-IV) were destroyed with intense gliosis. These changes were not induced by infusing AMPA for 1 day. The concomitant administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist for non-N-methyl-D-aspartate (NMDA) receptors, with AMPA, but not that of 2-amino-5-phosphonovalerate (APV), an antagonist for NMDA receptor, prevented induction of the behavioral and neuropathological changes. The findings of the present study suggest that this late-onset, selective neurotoxicity is mediated by AMPA-type glutamate receptors.

The role of intracellular free calcium in motor neuron disease

The intracellular calcium (Ca2+) concentrations of motoneurons can be altered by the influx of Ca2+ into the cell by the opening of voltage-dependent Ca2+ channels and ligand-gated channels linked to Ca2+ influx, especially by the N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. Intracellular Ca2+ concentration is also affected by the release of Ca2+ buffered in mitochondria and endoplasmic reticulum. Evidence that motoneurons may be selectively vulnerable to Ca(2+)-induced cell death include the following observations: (i) the presence of excitatory amino acid receptors on the cell membranes of motoneurons, some of which would permit Ca2+ influx (e.g. NMDA receptors); (ii) the availability of the presynaptic terminal for antibody-mediated effects leading to changes in cell permeability and Ca2+ influx; and (iii) the limited amounts of intracellular Ca(2+)-binding proteins such as calbindin D28K and parvalbumin in motoneurons. Elevation of intracellular free Ca2+ may also be a common event in a number of independent mechanisms leading to motoneuron death in motor neuron disease.

Quisqualic acid-induced seizures during development: a behavioral and EEG study

Quisqualic acid (QA) is an excitatory amino acid analogue that binds to the glutamate ionotropic receptor subclass AMPA (alpha-amino-3 hydroxy-5 methyl-4 isoxazol propionic acid) and metabotropic receptor phospholipase C. To study its epileptogenic properties, we administered QA through an intraventricular cannula to 23-, 41-, and 60-day-old rats with recording electrodes implanted in amygdala, hippocampus, and neocortex. The frequency power spectra of the recorded EEG was computed by fast fourier transform (FFT), and coherence between anatomic sites was computed. Seizures occurred in all animals receiving QA. The behavioral manifestations of the seizures varied as a function of age, with younger rats demonstrating rigidity and immobility followed by circling activity and intermittent forelimb clonus and 60-day-old animals exhibiting severe, wild running followed by generalized clonus. Ictal electrical discharges occurred in all animals. Neocortical ictal discharges occurred more prominently in the younger animals, and amygdala ictal discharges were more prominent in the older animals. Marked increases in spectral power occurred during the seizures in all anatomic structures and at all frequencies. Our results demonstrate that the clinical manifestations of QA seizures vary during development; results of the neurophysiologic studies suggested that neocortex may play an important role in genesis of QA seizures in immature brain.

A quantitative autoradiographic study of [3H]kainate binding sites in the normal human spinal cord, brainstem and motor cortex

The quantitative autoradiographic distribution of the kainate subtype of non-NMDA receptor in the normal human motor cortex, brainstem and spinal cord has been investigated using [3H]kainate. In the motor cortex specific [3H]kainate binding was present in all cortical laminae with the highest density in laminae and II and the upper part of III and lower densities in the middle and deep laminae. In the premotor cortex a band of high density was found in laminae V and VI as well as in the superficial laminae. In the normal brainstem kainate receptors had a heterogeneous distribution. Brainstem motor nuclei which tend to be affected in motor neuron disease (MND) had very low densities of binding sites, whereas the oculomotor nucleus had a higher density. Specific [3H]kainate binding was found throughout the spinal grey matter, the greatest density being found in the substantia gelatinosa and much lower densities in the rest of the grey matter including the ventral horns. Excitotoxicity at non-NMDA receptors has been implicated in the pathogenesis of MND. This study shows that the motor neuron groups vulnerable in MND express a low density of [3H]kainate binding sites and suggests that the density of kainate receptors does not account for selective vulnerability in this disorder.

Excitotoxicity and neurological disorders: involvement of membrane phospholipids

Excitatory amino acids and their receptors play an important role in membrane phospholipid metabolism. Persistent stimulation of excitatory amino acid receptors by glutamate may be involved in neurodegenerative diseases and brain and spinal cord trauma. The molecular mechanism of neurodegeneration induced by excitatory amino acids is, however, not known. Excitotoxin-induced calcium entry causes the stimulation of phospholipases and lipases. These enzymes act on neural membrane phospholipids and their stimulation results in accumulation of free fatty acids, diacylglycerols, eicosanoids, and lipid peroxides in neurodegenerative diseases and brain and spinal cord trauma. Other enzymes, such as protein kinase C and calcium-dependent proteases, may also contribute to the neuronal injury. Excitotoxin-induced alterations in membrane phospholipid metabolism in neurodegenerative diseases and neural trauma can be studied in animal and cell culture models. These models can be used to study the molecular mechanisms of the neurodegenerative processes and to screen the efficacy of therapeutic drugs.

Differential responses of rat cerebral somatostatinergic and cholinergic cells to glutamate agonists

Reductions in cortical somatostatin (SRIH) and choline acetyl-transferase (ChAT) are major biochemical deficits in Alzheimer disease (AD). SRIH and ChAT were measured in fetal rat cerebral neurons after exposure to the glutamate agonists N-methyl-D-aspartate (NMDA), kainate (KA), and quisqualate (Q). NMDA (96 h incubation) stimulated SRIH release and content in a dose-dependent manner with a Bmax of 10(-5)M and EC50 of 2-3 x 10(-6)M. KA showed a small stimulation in SRIH levels at 10(-5)M, but produced marked inhibition at 10(-4)M. Q decreased both intracellular and secreted SRIH. KA (51-76% of basal) and Q (27-56% of basal) but not NMDA (91-114% of basal) also inhibited the incorporation of [35S]methionine into proteins. In similar experiments 10(-4)M Q (23 +/- 9% of basal) and KA (20 +/- 3% of basal) but not NMDA (80 +/- 16% of basal) reduced ChAT levels in hypothalamic/septal cultures. These inhibitory actions on ChAT activity by KA and Q were reversed by gamma-glutamyltaurine (GT) but not by 2-amino-5-phosphonopentanoic acid (AP5). Chronic NMDA exposure partially inhibited muscarinic acetylcholine receptor (mAChR) mediated inositol phospholipid (PI) turnover, whereas it was abolished after KA and Q pretreatment. These findings suggest that in cerebral cell cultures, NMDA has a stimulatory action on somatostatinergic neurons and non-NMDA receptor agonism could play an important role in EAA-mediated neural damage.

Neuronal degeneration and spinal cavitation following intraspinal injections of quisqualic acid in the rat

Microinjections of quisqualic acid were made in the spinal cord to evaluate the excitotoxic effects of this excitatory amino acid agonist on spinal neurons in the rat. Animals were divided into four groups based on post injection survival times of 7-49 days. Injections ranging from 0.3 to 2.0 microL of 8.3, 83, and 125 mM quisqualic acid or normal saline were made in the lower thoracic and upper lumbar spinal cord. At all survival times evaluated unilateral injections of quisqualic acid produced unilateral or bilateral cell death and a prominent inflammatory reaction. In 23/25 animals spinal cavities were also observed. Spinal cord segments at or near quisqualate injection sites contained darkly stained, hypertrophied neuronal profiles, and increased staining for glial fibrillary acidic factor. Immunostaining for glial fibrillary acidic factor was especially intense in areas of neuronal degeneration and in border areas of spinal cavities. The results of this study suggest that the intraspinal injection of quisqualic acid may be an effective method to study the mechanisms of excitatory amino acid neurotoxicity, and the pathogenesis of spinal cavitation following neuronal injury.

Novel pharmacologic therapies in the treatment of experimental traumatic brain injury: a review

Delayed or secondary neuronal damage following traumatic injury to the central nervous system (CNS) may result from pathologic changes in the brain's endogenous neurochemical systems. Although the precise mechanisms mediating secondary damage are poorly understood, posttraumatic neurochemical changes may include overactivation of neurotransmitter release or re-uptake, changes in presynaptic or postsynaptic receptor binding, or the pathologic release or synthesis of endogenous "autodestructive" factors. The identification and characterization of these factors and the timing of the neurochemical cascade after CNS injury provides a window of opportunity for treatment with pharmacologic agents that modify synthesis, release, receptor binding, or physiologic activity with subsequent attenuation of neuronal damage and improvement in outcome. Over the past decade, a number of studies have suggested that modification of postinjury events through pharmacologic intervention can promote functional recovery in both a variety of animal models and clinical CNS injury. This article summarizes recent work suggesting that pharmacologic manipulation of endogenous systems by such diverse pharmacologic agents as anticholinergics, excitatory amino acid antagonists, endogenous opioid antagonists, catecholamines, serotonin antagonists, modulators of arachidonic acid, antioxidants and free radical scavengers, steroid and lipid peroxidation inhibitors, platelet activating factor antagonists, anion exchange inhibitors, magnesium, gangliosides, and calcium channel antagonists may improve functional outcome after brain injury.

Effect of kynurenate on functional deficits resulting from traumatic spinal cord injury

The potential role of excitatory amino acid (EAA) receptors in spinal cord trauma was examined in a standardized rat model of contusive injury. EAA antagonists were administered in a split-dose protocol with half given 5 min prior to and the remainder 15 min after contusion produced at the T8 vertebral level. Hindlimb function was assessed using a battery of tests of reflex and more complex behaviors at 1 day after injury and weekly thereafter through 4-8 weeks. Functional deficits were compared for groups administered intravenous MK 801 (1 mg/kg), dextromethorphan (10 mg/kg) and kynurenate (300 mg/kg) or the vehicle, saline, alone. In addition, possible effect of the drugs themselves on hindlimb function were assessed in uninjured controls. None of the drugs produced more than transient effect on uninjured rats. In contused rats, only kynurenate produced significant reductions in functional deficits as compared to saline controls. Significant improvement of hindlimb function was also observed when the thoracic cord was locally perfused with kynurenate via intrathecal cannulas and when kynurenate was directly infused into the contusive injury site by stereotaxic microinjection. Using the latter route of administration, a dose-dependent effect of kynurenate (100, 200 and 400 nmol) on the ability of contused rats to use their hindlimbs in locomotion was demonstrated. The highest dose also resulted in a significant reduction in overall functional deficits from 1 week through 1 month and at 2 months after injury. Our results support the hypothesis that EAA receptors at or near the injury site are involved in producing a proportion of the overall functional deficits stemming from traumatic injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that local non-NMDA receptors contribute to functional deficits in contusive spinal cord injury

To investigate the role of non-N-methyl-D-aspartate (non-NMDA) types of excitatory amino acid (EAA) receptors in traumatic spinal cord injury, we administered 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline (NBQX), a potent and specific antagonist of non-NMDA receptors, to rats with a standardized contusive spinal cord injury. Focal infusion of NBQX into the injury site significantly reduced long-term hindlimb functional deficits as well as decreasing the time required for the rats to establish a reflex bladder. The results suggest that non-NMDA receptors at or near the injury site are involved in producing a portion of the functional deficits that result from contusive spinal cord injury.

Decreased glutamate transport by the brain and spinal cord in amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a chronic degenerative neurologic disorder characterized by the death of motor neurons in the cerebral cortex and spinal cord. Recent studies have suggested that the metabolism of glutamate, a potentially neurotoxic amino acid, is abnormal in patients with ALS. We hypothesized that the high-affinity glutamate transporter is the site of the defect.

METHODS

We measured high-affinity, sodium-dependent glutamate transport in synaptosomes from neural tissue obtained from 13 patients with ALS, 17 patients with no neurologic disease, and 27 patients with other neuro-degenerative diseases (Alzheimer's disease in 15 patients and Huntington's disease in 12 patients). The groups were comparable with respect to age and the interval between death and autopsy. Synaptosomes were prepared from spinal cord, motor cortex, sensory cortex, visual cortex, striatum, and hippocampus. We also measured sodium-dependent transport of gamma-aminobutyric acid and phenylalanine in the synaptosomal preparations.

RESULTS

In patients with ALS, there was a marked decrease in the maximal velocity of transport for high-affinity glutamate uptake in synaptosomes from spinal cord (-59 percent, P less than 0.001), motor cortex (-70 percent, P less than 0.001), and somatosensory cortex (-39 percent, P less than 0.05), but not in those from visual cortex, striatum, or hippocampus. The affinity of the transporter for glutamate was not altered. No abnormalities in glutamate transport were found in synaptosomes from patients with other chronic neurodegenerative disorders. The transport of gamma-aminobutyric acid and phenylalanine was normal in patients with ALS.

CONCLUSIONS

ALS is associated with a defect in high-affinity glutamate transport that has disease, region, and chemical specificity. Defects in the clearance of extracellular glutamate because of a faulty transporter could lead to neurotoxic levels of extracellular glutamate and thus be pathogenic in ALS.

Acromelic acid, a novel kainate analogue, induces long-lasting paraparesis with selective degeneration of interneurons in the rat spinal cord

A single systemic administration of acromelic acid, a novel kainate analogue (kainoid), induces a series of characteristic behavioral changes in association with selective damage of interneurons in the caudal spinal cord in adult rats. When an effective dose of acromelic acid (5 mg/kg) was systemically administered, forced extension of hindlimbs with or without cramps appeared in all rats. In the course of the intensified hindlimb extension, 10 of 16 rats suffered from generalized convulsive seizures during which 6 rats died without apparent neuropathological change. Of 4 surviving rats that experienced seizures, two developed long-lasting spastic paraparesis which remained unchanged for at least 3 months, whereas the other two were normal in behavior on the days following the administration. In lower doses (less than 4 mg/kg), the rats transiently displayed forced extension of hindlimbs, and in a higher dose (5.5 mg/kg), all rats died during an attack of severe generalized convulsion. Neuropathological changes were observed only in the rats with persistent paraparesis, in which neuron damage was identified selectively in small interneurons in the lumbosacral cord. The morphological change of the degenerated spinal interneurons resembles that of degenerated hippocampal CA1 pyramidal cells seen after systemic administration of kainate. Large motoneurons, spinal roots, and white matter of the spinal cord were well preserved. Unlike the case of systemic administration of kainate, other structures in the central and peripheral nervous system and muscles were morphologically intact except the hippocampal CA4 and the stratum moleculare-lacnosum in which there were reactive astrocytes. The regional difference between kainate-induced and acromelate-induced neuron damage suggests that systemically administered acromelic acid, a kainoid, induces selective neuron damage through activating a particular kainate receptor subtype. The clinicopathological feature of the paraparetic rats resembles closely that of stiffman syndrome, a progressive human neurological disorder with selective loss of interneurons in the spinal cord.