Identification and properties of N-methyl-D-aspartate receptors in rat brain synaptic plasma membranes

Preoptic and hypothalamic regulation of multi-tiered, chronologically arranged female rat sexual behavior

As in many mammalian behaviors, sexual behavior exhibits structure. Each modular components of the structure, that are linked together over time, occur in probabilistic manner. Endocrine milieu, in particular sex hormones, define the probability to synchronize the behavior with the production of gametes. Developmental experience and environmental cues affect the hormonal milieu of the brain. This is especially true in female mammals, in which ova mature with certain intervals along with ovarian secretion of sex hormones. Estrogens secreted by mature ovarian follicles support both affiliative and executive components of female sexual behavior. In the absence of the ovarian steroids, females avoid males when possible, or antagonize and reject males when put together. Female sexual behavior is intimately linked with the estrous cycle in many species such that females are only receptive for a brief period at the estrus stage surrounding ovulation. Thus, in the rat, females strongly influence the outcome of mating encounter with a male. Affiliative or solicitatory behavior shown by females in estrus leads to the female adapting the lordosis posture, which is characterized by hindleg postural rigidity and lordotic dorsiflexion of the spine, in response to touch-pressure somatosensory stimuli on the skin of the flanks, rump-tail base, perineum region given by male partner. The posture facilitates intromission and consequently fertilization. Although dependence on estrogens is the most important feature of female rat sexual behavior, cervical probing combined with palpation of the hindquarter skin acts as a supranormal stimulus to elicit lordosis. Thus, lordosis behavior is a hub of multi-tiered, chronologically arranged set of behaviors and estrogen appear to alter excitability of neural network for lordosis.

Activation of NMDA receptors in brain endothelial cells increases transcellular permeability

Neurovascular coupling is a precise mechanism that induces increased blood flow to activated brain regions, thereby providing oxygen and glucose. In this study, we hypothesized that N-methyl-D-aspartate (NMDA) receptor signaling, the most well characterized neurotransmitter signaling system which regulates delivery of essential molecules through the blood–brain barrier (BBB). Upon application of NMDA in both in vitro and in vivo models, increased delivery of bioactive molecules that was mediated through modulation of molecules involved in molecular delivery, including clathrin and caveolin were observed. Also, NMDA activation induced structural changes in the BBB and increased transcellular permeability that showed regional heterogeneity in its responses. Moreover, NMDA receptor activation increased endosomal trafficking and facilitated inactivation of lysosomal pathways and consequently increased molecular delivery mediated by activation of calmodulin-dependent protein kinase II (CaMKII) and RhoA/protein kinase C (PKC). Subsequent in vivo experiments using mice specifically lacking NMDA receptor subunit 1 in endothelial cells showed decreased neuronal density in the brain cortex, suggesting that a deficiency in NMDA receptor signaling in brain endothelial cells induces neuronal losses. Together, these results highlight the importance of NMDA-receptor-mediated signaling in the regulation of BBB permeability that surprisingly also affected CD31 staining.

Memory Disorders Related to Hippocampal Function: The Interest of 5-HT4Rs Targeting

The hippocampus has long been considered as a key structure for memory processes. Multilevel alterations of hippocampal function have been identified as a common denominator of memory impairments in a number of psychiatric and neurodegenerative diseases. For many years, the glutamatergic and cholinergic systems have been the main targets of therapeutic treatments against these symptoms. However, the high rate of drug development failures has left memory impairments on the sideline of current therapeutic strategies. This underscores the urgent need to focus on new therapeutic targets for memory disorders, such as type 4 serotonin receptors (5-HT₄Rs). Ever since the discovery of their expression in the hippocampus, 5-HT₄Rs have gained growing interest for potential use in the treatment of learning and memory impairments. To date, much of the researched information gathered by scientists from both animal models and humans converge on pro-mnesic and anti-amnesic properties of 5-HT₄Rs activation, although the mechanisms at work require more work to be fully understood. This review addresses a fundamental, yet poorly understood set of evidence of the potential of 5-HT₄Rs to re-establish or limit hippocampal alterations related to neurological diseases. Most importantly, the potential of 5-HT₄Rs is translated by refining hypotheses regarding the benefits of their activation in memory disorders at the hippocampal level.

The 1980s: D-AP5, LTP and a Decade of NMDA Receptor Discoveries

In the 1960s and 70s, biochemical and pharmacological evidence was pointing toward glutamate as a synaptic transmitter at a number of distinct receptor classes, known as NMDA and non-NMDA receptors. The field, however, lacked a potent and highly selective antagonist to block these putative postsynaptic receptors. So, the discoveries in the early 1980s of d-AP5 as a selective NMDA receptor antagonist and of its ability to block synaptic events and plasticity were a major breakthrough leading to an explosion of knowledge about this receptor subtype. During the next 10 years, the role of NMDA receptors was established in synaptic transmission, long-term potentiation, learning and memory, epilepsy, pain, among others. Hints at pharmacological heterogeneity among NMDA receptors were followed by the cloning of separate subunits. The purpose of this review is to recognize the important contributions made in the 1980s by Graham L. Collingridge and other key scientists to the advances in our understanding of the functions of NMDA receptors throughout the central nervous system.

HPLC methods for determination of D-aspartate and N-methyl-D-aspartate

D-Amino acids are stereoisomers or optical isomers of naturally occurring L-amino acids and thus possess the same chemical structure, but may differ in their biological/physiological properties. Until a half century ago, D-amino acids had been considered to be unnatural substances found only in microorganisms. However, improvements in analytical instruments and methods have revealed that D-amino acids are present in invertebrates and vertebrates, including humans, and that they possess important physiological functions. D-Aspartate (D-Asp) and its methylated form N-methyl-D-aspartate (NMDA) possess neuroendocrine properties in many species. Several methods have been developed for determination of D- and L-enantiomers of amino acids by high performance liquid chromatography (HPLC). We report here improved HPLC methods for the specific determination of D-Asp and NMDA in biological tissues.

Effect of homocysteine on properties of neutrophils activated in vivo

We have found that neutrophils begin to express NMDA receptors on their membranes after in vivo activation. These receptors are the target for action of homocysteine (HC). After incubation of activated neutrophils with HC, the degranulation process is stimulated and generation of reactive oxygen species is increased. We conclude that expression of NMDA receptors on neutrophil membrane makes neutrophils sensitive to HC. Thus, hyperhomocysteinemia may induce additional stimulation of immune competent cells.

Selective expression of ligand-gated ion channels in L5 pyramidal cell axons

NMDA receptor (NMDAR)-dependent strengthening of neurotransmitter release has been widely observed, including in layer 5 (L5) pyramidal cells of the visual cortex, and is attributed to the axonal expression of NMDARs. However, we failed to detect NMDAR-mediated depolarizations or Ca(2+) entry in L5 pyramidal cell axons when focally stimulated with NMDAR agonists. This suggests that NMDARs are excluded from the axon. In contrast, local GABA(A) receptor activation alters axonal excitability, indicating that exclusion of ligand-gated ion channels from the axon is not absolute. Because NMDARs are restricted to the dendrite, NMDARs must signal to the axon by an indirect mechanism to alter release. Although subthreshold somatic depolarizations were found to spread electrotonically hundreds of micrometers through the axon, the resulting axonal potential was insufficient to open voltage-sensitive Ca(2+) channels. Therefore, if NMDAR-mediated facilitation of release is cell autonomous, it may depend on voltage signaling but apparently is independent of changes in basal Ca(2+). Alternatively, this facilitation may be even less direct, requiring a cascade of events that are merely triggered by NMDAR activation.

HPLC determination of acidic D-amino acids and their N-methyl derivatives in biological tissues

D-Aspartate (D-Asp) and N-methyl-D-aspartate (NMDA) occur in the neuroendocrine systems of vertebrates and invertebrates, where they play a role in hormone release and synthesis, neurotransmission, and memory and learning. N-methyl-d-glutamate (NMDG) has also been detected in marine bivalves. Several methods have been used to detect these amino acids, but they require pretreatment of tissue samples with o-phthaldialdehyde (OPA) to remove primary amino acids that interfere with the detection of NMDA and NMDG. We report here a one-step derivatization procedure with the chiral reagent N-alpha-(5-fluoro-2,4-dinitrophenyl)-(D or L)-valine amide, FDNP-Val-NH2, a close analog of Marfey's reagent but with better resolution and higher molar absorptivity. The diastereomers formed were separated by HPLC on an ODS-Hypersil column eluted with TFA/water-TFA/MeCN. UV absorption at 340 nm permitted detection levels as low as 5-10 pmol. D-Asp, NMDA and NMDG peaks were not obscured by other primary or secondary amino acids; hence pretreatment of tissues with OPA was not required. This method is highly reliable and fast (less than 40 min HPLC run). Using this method, we detected D-Asp, NMDA and NMDG in several biological tissues (octopus brain, optical lobe and bucchal mass; foot and mantle of the mollusk Scapharca broughtonii), confirming the results of other researchers.

Dendritic NMDA receptors activate axonal calcium channels

NMDA receptor (NMDAR) activation can alter synaptic strength by regulating transmitter release from a variety of neurons in the CNS. As NMDARs are permeable to Ca(2+) and monovalent cations, they could alter release directly by increasing presynaptic Ca(2+) or indirectly by axonal depolarization sufficient to activate voltage-sensitive Ca(2+) channels (VSCCs). Using two-photon microscopy to measure Ca(2+) excursions, we found that somatic depolarization or focal activation of dendritic NMDARs elicited small Ca(2+) transients in axon varicosities of cerebellar stellate cell interneurons. These axonal transients resulted from Ca(2+) entry through VSCCs that were opened by the electrotonic spread of the NMDAR-mediated depolarization elicited in the dendrites. In contrast, we were unable to detect direct activation of NMDARs on axons, indicating an exclusive somatodendritic expression of functional NMDARs. In cerebellar stellate cells, dendritic NMDAR activation masquerades as a presynaptic phenomenon and may influence Ca(2+) -dependent forms of presynaptic plasticity and release.

Influence of extremely low frequency magnetic fields on Ca2+ signaling and NMDA receptor functions in rat hippocampus

Extremely low frequency (ELF<300Hz) electromagnetic fields affect several neuronal activities including memory. Because ELF magnetic fields cause altered Ca(2+) homeostasis in neural tissues, we examined their influence on Ca(2+) signaling enzymes in hippocampus and related them with NMDA receptor functions. Hippocampal regions were obtained from brains of 21-day-old rats that were exposed for 90 days to 50Hz magnetic fields at 50 and 100 microT intensities. In comparison to controls, ELF exposure caused increased intracellular Ca(2+) levels concomitant with increased activities of Ca(2+)-dependent protein kinase C (PKC), cAMP-dependent protein kinase and calcineurin as well as decreased activity of Ca(2+)-calmodulin-dependent protein kinase in hippocampal regions. Simultaneous ligand-binding studies revealed decreased binding to N-methyl-D-aspartic acid (NMDA) receptors. The combined results suggest that perturbed neuronal functions caused by ELF exposure may involve altered Ca(2+) signaling events contributing to aberrant NMDA receptor activities.

Non-steroidal anti-inflammatory agents, tolmetin and sulindac, attenuate oxidative stress in rat brain homogenate and reduce quinolinic acid-induced neurodegeneration in rat hippocampal neurons

Alzheimer's disease (AD) is the most common form of neurodegenerative disease in the elderly. Anti-inflammatory agents have been shown to be beneficial in preventing neurodegenerative disorders such as AD. In this study we investigated the possible antioxidant and neuroprotective properties of two non-steroidal anti-inflammatory drugs (NSAIDS), tolmetin and sulindac, using quinolinic acid (QA)-induced neurotoxicity as a model. We used the thiobarbituric acid assay to measure the extent of lipid peroxidation and the nitroblue tetrazolium assay to measure the superoxide anion generated in rat brain homogenate. QA (1 mM) induced lipid peroxidation in rat brain homogenate was significantly curtailed by co-treatment of the homogenate with tolmetin and/or sulindac. Tolmetin and sulindac both reduced the generation of superoxide anions by the known neurotoxin, potassium cyanide (KCN). Intrahippocampal injections of QA induced neurotoxicity in rat hippocampus. N-Methyl-D-Aspartate (NMDA) receptor counts were conducted do give an indication of the amount protection offered by the NSAIDS. QA drastically reduced the number of NMDA binding sites by approximately 37%. This sharp decrease was considerably attenuated by the pre-treatment of the rats with tolmetin and sulindac (5 mg/kg/bd for five days). This study shows the antioxidant and neuroprotective properties of tolmetin and sulindac and hereby postulates that these drugs have important implications in the prevention or treatment of neurodegenerative diseases such as AD.

Chronic lithium chloride administration attenuates brain NMDA receptor-initiated signaling via arachidonic acid in unanesthetized rats

It has been proposed that lithium is effective in bipolar disorder (BD) by inhibiting glutamatergic neurotransmission, particularly via N-methyl-D-aspartate receptors (NMDARs). To test this hypothesis and to see if the neurotransmission could involve the NMDAR-mediated activation of phospholipase A2 (PLA2), to release arachidonic acid (AA) from membrane phospholipid, we administered subconvulsant doses of NMDA to unanesthetized rats fed a chronic control or LiCl diet. We used quantitative autoradiography following the intravenous injection of radiolabeled AA to measure regional brain incorporation coefficients k* for AA, which reflect receptor-mediated activation of PLA2. In control diet rats, NMDA (25 and 50 mg/kg i.p.) compared with i.p. saline increased k* significantly in 49 and 67 regions, respectively, of the 83 brain regions examined. The regions affected were those with reported NMDARs, including the neocortex, hippocampus, caudate-putamen, thalamus, substantia nigra, and nucleus accumbens. The increases could be blocked by pretreatment with the specific noncompetitive NMDA antagonist MK-801 ((5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) (0.3 mg/kg i.p.), as well by a 6-week LiCl diet sufficient to produce plasma and brain lithium concentrations known to be effective in BD. MK-801 alone reduced baseline values for k* in many brain regions. The results show that it is possible to image NMDA signaling via PLA2 activation and AA release in vivo, and that chronic lithium blocks this signaling, consistent with its suggested mechanism of action in BD.

Aciclovir protects against quinolinic-acid-induced oxidative neurotoxicity

AIDS-related encephalopathy, including AIDS dementia complex (ADC) and the opportunistic disease, herpes simplex encephalitis (HSE), are postulated to arise due to the release of neurotoxic products, such as quinolinic acid (QUIN), by activated microglial cells in the brain. QUIN causes a cascade of events to occur, which leads to the production of reactive oxygen species (ROS), these being ultimately responsible for oxidative neurotoxicity. The antiherpes antiviral aciclovir has been reported to protect against neuron loss in HSE, but the mechanism for this neuroprotection is unknown. Therefore, this study was conducted to investigate whether aciclovir has the ability to inhibit QUIN-induced lipid peroxidation in rat brain homogenates, after in-vitro and in-vivo exposure to QUIN and aciclovir. The thiobarbituric acid (TBA) assay was the method used to analyse lipid peroxidation. Rat brains were also examined histologically after in-vivo exposure to visually assess whether neuron loss was suppressed. The results show that aciclovir inhibits the QUIN-induced lipid peroxidation, in a dose-dependent manner. Furthermore, aciclovir reduced necrosis of hippocampal neurons and retained the characteristic morphology, integrity and arrangement of these cells. Thus, it appears that aciclovir has neuroprotective properties, which could possibly be useful in the treatment of AIDS-related encephalopathy.

Regulation of Extracellular Glutamate in the Prefrontal Cortex: Focus on the Cystine Glutamate Exchanger and Group I Metabotropic Glutamate Receptors

Microdialysis was used to determine the in vivo processes contributing to extracellular glutamate levels in the prefrontal cortex of rats. Reverse dialysis of a variety of compounds proved unable to decrease basal levels of extracellular glutamate, including Na+ and Ca2+ channel blockers, cystine/glutamate exchange (x(c)-) antagonists, and group I (mGluR1/5) and group II (mGluR2/3) metabotropic glutamate receptor (mGluR) agonists or antagonists. In contrast, extracellular glutamate was elevated by blocking Na+-dependent glutamate uptake (X(AG)-) with DL-threo-beta-benzyloxyaspartate (TBOA) and stimulating group I mGluRs with (R,S)-3,5-dihydroxy-phenylglycine (DHPG). The accumulation of extracellular glutamate produced by blocking X(AG)- was completely reversed by inhibiting system x(c)- with 4-carboxyphenylglycine (CPG), but not by Na+ and Ca2+ channel blockers. Because CPG also inhibits group I mGluRs, two additional group I antagonists were examined, LY367385 [(+)-2-methyl-4-carboxyphenylglycine] and (R,S)-1-aminoindan-1,5-dicarboxylic acid (AIDA). Whereas LY367385 also reduced TBOA-induced increases in extracellular glutamate, AIDA did not. In contrast, all three group I antagonists reversed the increase in extracellular glutamate elicited by stimulating mGluR1/5. In vitro evaluation revealed that similar to CPG, LY367385 inhibited x(c)- and that stimulating or inhibiting mGluR1/5 did not directly affect [3H]glutamate uptake via x(c)- or X(AG)-. These experiments reveal that although inhibiting x(c)- cannot reduce basal extracellular glutamate in the prefrontal cortex, the accumulation of extracellular glutamate after blockade of X(AG)- arises predominately from x(c)-. The accumulation of glutamate elicited by mGluR1/5 stimulation does not seem to result from modulating X(AG)-, x(c)-, or synaptic glutamate release.

Injury-induced alterations in N-methyl-D-aspartate receptor subunit composition contribute to prolonged 45calcium accumulation following lateral fluid percussion

Cells that survive traumatic brain injury are exposed to changes in their neurochemical environment. One of these changes is a prolonged (48 h) uptake of calcium which, by itself, is not lethal. The N-methyl-D-aspartate receptor (NMDAR) is responsible for the acute membrane flux of calcium following trauma; however, it is unclear if it is involved in a flux lasting 2 days. We proposed that traumatic brain injury induced a molecular change in the NMDAR by modifying the concentrations of its corresponding subunits (NR1 and NR2). Changing these subunits could result in a receptor being more sensitive to glutamate and prolong its opening, thereby exposing cells to a sustained flux of calcium. To test this hypothesis, adult rats were subjected to a lateral fluid percussion brain injury and the NR1, NR2A and NR2B subunits measured within different regions. Although little change was seen in NR1, both NR2 subunits decreased nearly 50% compared with controls, particularly within the ipsilateral cerebral cortex. This decrease was sustained for 4 days with levels returning to control values by 2 weeks. However, this decrease was not the same for both subunits, resulting in a decrease (over 30%) in the NR2A:NR2B ratio indicating that the NMDAR had temporarily become more sensitive to glutamate and would remain open longer once activated. Combining these regional and temporal findings with 45calcium autoradiographic studies revealed that the degree of change in the subunit ratio corresponded to the extent of calcium accumulation. Finally, utilizing a combination of NMDAR and NR2B-specific antagonists it was determined that as much at 85% of the long term NMDAR-mediated calcium flux occurs through receptors whose subunits favor the NR2B subunit. These data indicate that TBI induces molecular changes within the NMDAR, contributing to the cells' post-injury vulnerability to glutamatergic stimulation.

Functional interactions between somatodendritic dopamine release, glutamate receptors and brain-derived neurotrophic factor expression in mesencephalic structures of the brain

Dopaminergic nigrostriatal neurons may be considered as bipolar functional entities since they are endowed with the ability to synthesize, store and release the transmitter dopamine (DA) at the somatodendritic level in the substantia nigra (SN). Such dendritic DA release seems to be distinct from the transmitter release occurring at the axon terminal and seems to rely preferentially on volume transmission to exert its physiological effects. An increased glutamatergic (Gluergic) transmission into the SN facilitates such dendritic DA release via activation of NMDA-receptors (NMDA-Rs) and to a lesser extent through group II metabotropic glutamate receptors (mGluRs). In addition, nigral mGluRs functionally interact with NMDA-Rs in the SN, further modulating the NMDA-R-mediated increase of DA release from dendrites in the SN. In turn, dendritically released DA may exert, via D1 receptors, a tonic inhibitory control upon nigral glutamate (Glu). Furthermore, released DA, via D2/D3 autoreceptors, produces an autoinhibitory effect upon DA cell firing and its own release process. An increased Gluergic transmission into the SN may also induce, via activation of NMDA-Rs, an augmented expression of different brain-derived neurotrophic factor (BDNF) gene transcripts in this brain area. Pharmacological evidence suggests that non-NMDA-Rs could also participate in the regulation of BDNF gene expression in the SN. Glu-mediated changes of nigral BDNF expression could regulate, in turn, the expression of important transmitter-related proteins in the SN, such as different NMDA-R subunits, mGluRs and DA-D3 receptors. In conclusion, Glu-DA-BDNF interactions in the SN may play an important role in modulating the flow of neuronal information in this brain structure under normal conditions, as well as during adaptive and plastic responses associated with various neurological and psychiatric disorders.

Structure-activity analysis of a novel NR2C/NR2D-preferring NMDA receptor antagonist: 1-(phenanthrene-2-carbonyl) piperazine-2,3-dicarboxylic acid

(2S*,3R*)-1-(biphenyl-4-carbonyl)piperazine-2,3-dicarboxylic acid (PBPD) is a moderate affinity, competitive N-methyl-d-aspartate (NMDA) receptor antagonist with an atypical pattern of selectivity among NMDA receptor 2 subunit (NR2) subunits. We now describe the activity of several derivatives of PBPD tested at both rat brain NMDA receptors using l-[3H]-glutamate binding assays and at recombinant receptors expressed in Xenopus oocytes. Substituting various branched ring structures for the biphenyl group of PBPD reduced NMDA receptor activity. However, substituting linearly arranged ring structures - fluorenone or phenanthrene groups - retained or enhanced activity. Relative to PBPD, the phenanthrene derivative (2S*, 3R*)-1-(phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA) displayed a 30- to 78-fold increase in affinity for native NMDA receptors. At recombinant receptors, PPDA displayed a 16-fold (NR2B) to 94-fold (NR2C) increase in affinity over PBPD. Replacement of the biphenyl group of PBPD with a 9-oxofluorene ring system resulted in small changes in receptor affinity and subtype selectivity. 2'-Bromo substitution on the biphenyl group of PBPD reduced antagonist affinity 3- to 5-fold at NR2A-, NR2B- and NR2D-containing receptors, but had little effect on NR2C-containing receptors. In contrast, 4'-fluoro substitution of the biphenyl ring of PBPD selectively increased NR2A affinity. The aromatic rings of PBPD and PPDA increase antagonist affinity and appear to interact with a region of the NMDA receptor displaying subunit heterogeneity. PPDA is the most potent and selective NR2C/NR2D-preferring antagonist yet reported and thus may be useful in defining NR2C/NR2D function and developing related antagonists with improved NMDA receptor subtype selectivity. British Journal of Pharmacology (2004) 141, 508-516. doi:10.1038/sj.bjp.0705644

Susceptibility of hippocampal neurons to mechanically induced injury

Experimental models of traumatic cortical brain injury in rodents reveal that specific regions of the hippocampus (e.g., CA3 and hilar subfields) are severely injured despite their distance from the initial insult. Hippocampal neurons may be intrinsically more vulnerable to mechanical insult than cortical neurons due to increased NMDA receptor densities and lower energy capacities, as evidenced by increased susceptibility to ischemic insults. The selective vulnerability of hippocampal neurons was evaluated using an in vitro model of TBI in which either primary rat cortical or hippocampal neurons (E17) seeded onto silicone substrates were subjected to graded levels of mechanical stretch. Although cortical neurons exhibited significantly longer increases in stretch-induced membrane permeability, injury of hippocampal neurons resulted in larger increases in intracellular free calcium concentration [Ca(2+)](i) and cell death. [ATP](i) deficits due to stretch were apparent by 60 min after injury in cortical neurons but recovered by 24 h, whereas significant deficits in [ATP](i) were not observed in hippocampal neurons until 24 h after injury. MK801 pretreatment decreased the stretch-induced [Ca(2+)](i) transients in both hippocampal and cortical cultures, thereby negating the regional specificity. However, MK801 pretreatment did not improve hippocampal viability and paradoxically, significantly increased cell death among cortical neurons. As the hippocampus is the primary brain region responsible for the memory deficits and epileptic seizures associated with TBI, understanding why this region is selectively damaged could lead to the development of more accurate mechanical tolerances as well as effective pharmaceutical agents.

Acute treatment with MgSO4 attenuates long-term hippocampal tissue loss after brain trauma in the rat

Previous studies have shown that magnesium salts and the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, NPS 1506, attenuated short-term cognitive deficits and histopathological changes associated with traumatic brain injury (TBI). We evaluated the long-term effects of both therapies after brain trauma. Young adult rats were subjected to parasagittal fluid-percussion brain injury and received either MgSO(4) (125 micromol/400 g rat; n = 12) 15 min post-injury, NPS 1506 (1.15 mg/kg; n = 12) 15 min and 4 hr post-injury, or vehicle (n = 9) 15 min post-injury. Uninjured animals (sham) received vehicle (n = 10). Learning function in these animals was evaluated using a water maze paradigm 8 months after injury or sham treatment, and the brains were examined for cortical and hippocampal tissue loss. Compared to sham animals, injured vehicle-treated animals displayed a substantial learning dysfunction, indicated by an increased latency to find a hidden platform in the water maze (P < 0.001). No improvements in learning, however, were found for injured animals treated with NPS 1506 or MgSO(4). Injury induced >30% loss of tissue in the ipsilateral cortex in vehicle-treated animals that was not reduced in animals treated with either NPS 1506 or MgSO(4). Treatment with MgSO(4) significantly reduced progressive tissue loss in the hippocampus (P < 0.001). These findings are the first to demonstrate long-term neuroprotection of hippocampal tissue by an acute treatment in a TBI model. These data also show that the previously reported broad efficacy of MgSO(4) or NPS 1506 observed shortly after brain trauma could not be detected 8 months post-injury.

Role of group I metabotropic glutamate receptors and NMDA receptors in homocysteine-evoked acute neurodegeneration of cultured cerebellar granule neurones

Hyperhomocysteinemia is a risk factor in neurodegeneration. It has been suggested that apart from disturbances in methylation processes, the mechanisms of this effect may include excitotoxicity mediated by the N-methyl-D-aspartate (NMDA) receptors. In this study we demonstrate that apart from NMDA receptors, also group I metabotropic glutamate receptors participate in acute homocysteine (Hcy)-induced neurotoxicity in cultured rat cerebellar granule neurones. Primary neuronal cultures were incubated for 30 min in the Mg(2+)-free ionic medium containing homocysteine and other ligands, and neurodegenerative changes were assessed 24h later using propidium iodide staining. D,L-Homocysteine given alone appeared to be a weak neurotoxin, with EC(50) of 17.4mM, whereas EC(50) for L-glutamate was 0.17 mM. Addition of 50 microM glycine enhanced homocysteine neurotoxicity, and only that portion of neurotoxicity was abolished by 0.5 microM MK-801, an uncompetitive NMDA receptor antagonist. The net stimulation of 45Ca uptake by granule cells incubated in the presence of 25 mM D,L-homocysteine with 50 microM glycine was only 3% of the net uptake evoked by 1mM glutamate. Application of an antagonist of group I metabotropic glutamate receptors (mGluRs) LY367385 at 25 and 250 microM concentrations, induced a dose-dependent partial neuroprotection, whereas given together with MK-801 completely prevented neurotoxicity. In the absence of glycine, LY367385 and MK-801 given alone failed to induce neuroprotection, while applied together completely prevented homocysteine neurotoxicity. Agonist of group I mGluRs, 10 trans-azetidine-2,3-dicarboxylic acid (t-ADA) induced significant neurotoxicity. This study shows for the first time that acute homocysteine-induced neurotoxicity is mediated both by group I mGluRs and NMDA receptors, and is not accompanied by massive influx of extracellular Ca(2+) to neurones.

Occurrence and neuroendocrine role ofD-aspartic acid andN-methyl-D-aspartic acid inCiona intestinalis

Probes for the occurrence of endogenous D-aspartic acid (D-Asp) and N-methyl-D-aspartic acid (NMDA) in the neural complex and gonads of a protochordate, the ascidian Ciona intestinalis, have confirmed the presence of these two excitatory amino acids and their involvement in hormonal activity. A hormonal pathway similar to that which occurs in vertebrates has been discovered. In the cerebral ganglion D-Asp is synthesized from L-Asp by an aspartate racemase. Then, D-Asp is transferred through the blood stream into the neural gland where it gives rise to NMDA by means of an NMDA synthase. NMDA, in turn, passes from the neuronal gland into the gonads where it induces the synthesis and release of a gonadotropin-releasing hormone (GnRH). The GnRH in turn modulates the release and synthesis of testosterone and progesterone in the gonads, which are implicated in reproduction.

Homocysteine-evoked 45Ca release in the rabbit hippocampus is mediated by both NMDA and group I metabotropic glutamate receptors: in vivo microdialysis study

This in vivo microdialysis study compared the effects of NMDA and D,L-homocysteine (Hcy) administered via dialysis medium on 45Ca efflux from prelabeled rabbit hippocampus. Application of these agonists evoked dose-dependent, and sensitive to MK-801, opposite effects: NMDA decreased the 45Ca radioactivity in the dialysate, whereas Hcy induced the release of 45Ca. The latter effect was potentiated by glycine, inhibited by the antagonist of group I metabotropic glutamate receptors (mGluR) LY367385, and mimicked by t-ADA, an agonist of these receptors. Electron microscopic examination of pyramidal neurones in the CA1 sector of the hippocampus in the vicinity of the microdialysis probe after NMDA application demonstrated swelling of mitochondria, which was prevented by cyclosporin A. This study shows, for the first time, Hcy-induced activation of both group I mGluR and NMDA receptors, which may play a role in acute Hcy neurotoxicity. We present new applications of brain microdialysis in studies on excitotoxicity and neuroprotection.

A specific enzymatic high-performance liquid chromatography method to determine N-methyl-D-aspartic acid in biological tissues

Recently we demonstrated that N-methyl-D-aspartic acid (NMDA) is present as an endogenous compound in the nervous tissues and endocrine glands of the rat where it plays a role in the regulation of the luteinizing hormone, growth hormone, and prolactin (FASEB J. 14 (2000) 699; Endocrinology 141 (2000) 3861). Based on the prediction that NMDA could have future importance in neuroendocrinology, we have devised an improved method for the specific and routine determination of NMDA in biological tissue. This method is based on the detection by HPLC of methylamine (CH(3)NH(2)) which comes from the oxidation of NMDA by D-aspartate oxidase, an enzyme which specifically oxidizes NMDA, yielding CH(3)NH(2) as one of the oxidative products of the reaction. The sensitivity of the method permits the accurate determination of NMDA in the supernatant of a tissue homogenate at levels of about 5-10 picomol/assay. However, for those tissues in which the concentration of NMDA is less than 1nmol/g, the sample must be further purified by treatment with o-phthaldialdehyde in order to separate the NMDA from the other amino acids and amino compounds and then concentrated and analyzed by HPLC. Using this method we have conducted a comparative study in order to measure the amount of NMDA in neuroendocrine and other tissues of various animal phyla from mollusks to mammals.

NPS 1506 attenuates cognitive dysfunction and hippocampal neuron death following brain trauma in the rat

Although several noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists have been shown to be substantially efficacious in experimental models of brain trauma, side effects associated with this class of compounds have impeded clinical application. Therefore, new noncompetitive NMDA receptor antagonists have been developed, including NPS 1506, that appear to be nontoxic but retain efficacy. In the present study, we evaluated the efficacy of NPS 1506 in a model of parasagittal fluid percussion brain trauma in the anesthetized rat. Administration of 1 mg/kg NPS 1506 at both 10 min and 4 h posttrauma induced no changes in brain temperature, mean arterial pressure, pulse, or arterial blood gasses. At 1 week postinjury, animals treated with the same dosing regimen of NPS 1506 demonstrated a dramatic attenuation of memory dysfunction evaluated by a water maze task (P < 0.02) and had greatly reduced neuron death in the CA3 subfield of the hippocampus (P < 0.01). However, NPS 1506 treatment did not significantly affect the extent of cortical tissue loss following injury. Since memory dysfunction and hippocampal damage are common and potentially related consequences of brain trauma in humans, our results suggest that NPS 1506 treatment may have clinical utility.

Neurotoxic potential of three structural analogs of beta-N-oxalyl-alpha,beta-diaminopropanoic acid (beta-ODAP)

Lathyrism is a non-progressive motor neuron disease produced by consumption of the excitatory amino acid, 3-N-oxalyl-L-2,3-diaminopropanoic acid (beta-ODAP). To learn more about the mechanisms underlying Lathyrism three structural analogs of beta-ODAP were synthesized. Carboxymethyl-alpha,beta-diaminopropanoic acid (CMDAP) evoked inward currents which were antagonized by APV (30 microM), but not by CNQX (10 microM). N-acetyl-alpha,beta-diaminopropanoic acid (ADAP) evoked no detectable ionic currents but potentiated N-methyl-D-aspartate (NMDA)-activated currents. The potentiation of NMDA currents by ADAP was blocked by 7-chlorokynurenic acid. Carboxymethylcysteine (CMC) did not activate any detectable ionic currents. None of the three beta-ODAP analogs produced visible symptoms of toxicity in day old chicks when administered for 2-3 consecutive days. Ligand binding studies demonstrated that all the three compounds were effective to in displacing [3H]glutamate. The maximum inhibition was 92% for CMDAP, 61% for ADAP, 65% for CMC and 99% for beta-ODAP. These data indicate that analogs of beta-ODAP may interact with glutamate receptors without producing neurotoxicity.

Novel pharmacologic strategies in the treatment of experimental traumatic brain injury: 1998

The mechanisms underlying secondary or delayed cell death following traumatic brain injury are poorly understood. Recent evidence from experimental models suggests that widespread neuronal loss is progressive and continues in selectively vulnerable brain regions for months to years after the initial insult. The mechanisms underlying delayed cell death are believed to result, in part, from the release or activation of endogenous "autodestructive" pathways induced by the traumatic injury. The development of sophisticated neurochemical, histopathological and molecular techniques to study animal models of TBI have enabled researchers to begin to explore the cellular and genomic pathways that mediate cell damage and death. This new knowledge has stimulated the development of novel therapeutic agents designed to modify gene expression, synthesis, release, receptor or functional activity of these pathological factors with subsequent attenuation of cellular damage and improvement in behavioral function. This article represents a compendium of recent studies suggesting that modification of post-traumatic neurochemical and cellular events with targeted pharmacotherapy can promote functional recovery following traumatic injury to the central nervous system.

Receptor interactions of beta-N-oxalyl-L-alpha,beta-diaminopropionic acid, the Lathyrus sativus putative excitotoxin, with synaptic membranes

Direct evidence for the excitotoxicity of 3-N-oxalyl-L-alpha,beta-diaminopropionic acid (ODAP), the Lathyrus sativus neurotoxin has been studied by examining the binding of chemically synthesized [2,3 3H]ODAP ([3H]ODAP) to synaptic membranes. [3H]ODAP binding to membranes was mostly nonspecific, with only a very low specific binding (15-20% of the total binding) and was also not saturable. The low specific binding of [3H]ODAP remained unaltered under a variety of assay conditions. A low Bmax of 3.2 +/- 0.4 pmol/mg and Kd 0.2 +/- 0.08 microM could be discerned for the high affinity interactions under conditions wherein more than 80-90% of the binding was nonspecific. While ODAP could inhibit the binding of [3H]glutamate to chick synaptic membranes with a Ki of 10 +/- 0.9 microM, even L-DAP, a non neurotoxic amino acid was also equally effective in inhibiting the binding of [3H]glutamate. The very low specific binding of [3H]ODAP to synaptic membranes thus does not warrant considering its interactions at glutamate receptors as a significant event. The results thus suggest that the reported in vitro excitotoxic potential of ODAP may not reflect its true mechanism of neurotoxicity.

Substituted quinolines as inhibitors of L-glutamate transport into synaptic vesicles

This study investigated the structure-activity relationships and kinetic properties of a library of kynurenate analogues as inhibitors of 3H-L-glutamate transport into rat forebrain synaptic vesicles. The lack of inhibitory activity observed with the majority of the monocyclic pyridine derivatives suggested that the second aromatic ring of the quinoline-based compounds played a significant role in binding to the transporter. A total of two kynurenate derivatives, xanthurenate and 7-chloro-kynurenate, differing only in the carbocyclic ring substituents, were identified as potent competitive inhibitors, exhibiting Ki values of 0.19 and 0.59 mM, respectively. The Km value for L-glutamate was found to be 2.46 mM. Parallel experiments demonstrated that while none of the kynurenate analogues tested effectively inhibited the synaptosomal transport of 3H-D-aspartate, some cross-reactivity was observed with the EAA ionotropic receptors. Molecular modeling studies were carried out with the identified inhibitors and glutamate in an attempt to preliminarily define the pharmacophore of the vesicular transporter. It is hypothesized that the ability of the kynurenate analogues to bind to the transporter may be tied to the capacity of the quinoline carbocyclic ring to mimic the negative charge of the gamma-carboxylate of glutamate. A total of two low energy solution conformers of glutamate were identified that exhibited marked functional group overlap with the most potent inhibitor, xanthurenate. These results help to further refine the pharmacological specificity of the glutamate binding site on the vesicular transporter and identify a series of inhibitors with which to investigate transporter function.

Mannose-specific lectins modulate ligand binding to AMPA-type glutamate receptors

Binding of [3H]AMPA was increased above control levels in rat brain membranes that had been incubated with concanavalin A (Con A) or a lectin from Lens culinaris (LC), both of which bind mannose residues. This did not occur with any of six lectins with other specificities. The magnitude of the increased binding varied from 15% in cortex to 70% in hippocampus and decreased significantly between 3 weeks and 6 months of age. Succinylated Con A was without effect and neither Con A nor LC increased binding to solubilized AMPA receptors. Increases in binding were not obtained in membranes purified from HEK293 cell lines expressing homomeric AMPA receptors. This indicates that mannose specific lectins may enhance binding by cross-linking AMPA receptors to each other or to proteins that are specific to brain. Con A has been reported to reduce glutamate receptor desensitization with higher efficacy at kainate than at AMPA receptors; the increase in binding reported here appears to be unrelated to such effects because (1) it was not affected by drugs that block desensitization and (2) [3H]kainate binding was reduced rather than increased by Con A. These observations suggest that AMPA receptor kinetic properties not involving desensitization are influenced by extracellular interactions between the receptors and other transmembrane proteins.

Riluzole attenuates cortical lesion size, but not hippocampal neuronal loss, following traumatic brain injury in the rat

The neuroprotective effects of Riluzole, a compound with several mechanisms of action including the inhibition of sodium channel activity and glutamate release, were evaluated in a rat model of parasagittal fluid-percussion (FP) brain injury. Male Sprague-Dawley rats (350-400 g, n = 17) were anesthetized with sodium pentobarbital (60 mg/kg i.p.) and subjected to parasagittal FP brain injury of moderate severity (2.3-2.5 atm). Fifteen min following injury, animals randomly received an i.v. bolus of either Riluzole (8 mg/kg, n = 8) or vehicle (n = 9), followed by subcutaneous injections (identical dose) at 6 hr and 24 hr. Two weeks after injury and drug treatment, animals were sacrificed and a series of brain sections, stained with Hematoxylin and Eosin (H&E) or cresyl violet, were evaluated for quantitative cortical lesion volume and cell counts of hippocampal CA3 neurons, respectively, using a computerized image analysis system. Administration of Riluzole significantly reduced FP-induced tissue loss in the temporal/occipital cortices ipsilateral to the site of impact by 46%, compared to vehicle-treated, brain-injured animals (P = 0.01). In contrast, the selective neuronal loss observed in the CA3 region of the ipsilateral hippocampus was unaffected by Riluzole treatment. The present study demonstrates that Riluzole can attenuate cortical lesion size following brain trauma. These neuroprotective effects may be related to the synergy of the different mechanisms of action of Riluzole.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

Modulation by magnesium of N-methyl-D-aspartate receptors in developing human brain

AIM

To investigate age related alterations in glutamate N-methyl-D-aspartate (NMDA) receptor binding produced by the modulatory compounds glutamate, glycine, and magnesium (Mg2+) sulphate.

METHODS

The effects produced by glutamate plus glycine, and Mg2+ on the binding of [3H]MK-801, a ligand for the N-methyl-D-aspartate ion channel phencyclidine site, were measured in membrane preparations made from prefrontal cortex from human neonate (n = 5), infant (n = 6), and adult (n = 6) necropsy brains.

RESULTS

Neonatal brains had the least [3H]MK-801 binding, suggesting either a low density of NMDA receptors or a more restricted access of [3H]MK-801 to cation channel sites. Infant brains had the most [3H]MK-801 binding which was stimulated to a greater extent by L-glutamate (100 microM) and glycine (10 microM) than in neonatal and adult brains. MG2+ invariably inhibited [3H]MK-801 binding. However, the Mg2+ IC50 value was higher in neonatal brain (3.6 mM) than infant (1.4 mM) and adult (0.87 mM) brains.

CONCLUSION

Infant brain may have excess NMDA receptors which are hyper responsive to glutamate and glycine. The lower potency of Mg2+ to inhibit [3H]MK-801 binding in neonatal cortex may be because newborn babies have NMDA receptors without the normal complement of Mg2+ sites. The findings suggest that therapeutic NMDA receptor block in neonates requires higher concentrations of magnesium sulphate in brain tissue.

A concussive-like brain injury model in mice (II): selective neuronal loss in the cortex and hippocampus

A novel concussive-like brain injury (CLBI) model characterized by transient neurobehavioral depression, short duration of brain edema, and long-lasting memory deficits has been reported in our companion paper. This was achieved by dropping a 21-g weight from a height of 25 cm onto the head of a mouse. In the present study, we examined the histopathological changes in this model. Male ddY mice were subjected to either the trauma or sham injury. Gross pathological examination of the brain 1 h posttrauma did not demonstrate subdural, subarachnoid, intraventricular, periventricular, and intraparenchymatous hemorrhage, focal lesions or contusions. Microscopic examination 24 h posttrauma with Nissl staining (cresyl violet), however, revealed a selective bilateral neuronal cell loss in the cerebral cortex and hippocampus but not in the regions of the thalamus, cerebellum, and brain stem. The characteristics of neuronal cell loss in the cortex suggested that this pathology was related in part, to the head impact dynamics, since the cell loss was noted in the central portion of the supraventricular cerebral cortex (p < 0.001), the site of the weight impact, gradually decreasing peripheral to this site, and disappearing in the areas remote from this locus. In contrast, neuronal cell loss seen in the hippocampus did not suggest that this pathology was directly associated with the impact site. Neuronal cell loss was concentrated in the pyramidal cell layer of CA2 (p < 0.01) and CA3 (p < 0.01), and a lesser degree was noted in the subfields of CA3c (p < 0.05) and the hilar region (p < 0.05) but not in the subfields of CA1 and the dentate gyrus layers. The present study characterized the histopathological change seen in the CLBI model, demonstrating the selective neuronal cell loss following weight-drop concussion in mice.

Methylation of the NMDA receptor agonist L-trans-2,3-pyrrolidine-dicarboxylate: enhanced excitotoxic potency and selectivity

This study investigated the excitotoxic properties of a novel series of NMDA analogues in which a methyl group was introduced to the 5-position of the pyrrolidine ring of L-trans-2,3-PDC, a previously identified NMDA receptor agonist. While all of these compounds induced NMDA-receptor-mediated injury, methylation increased in vivo excitotoxic potency 1000-fold. Injections (1 mu 1) in rat dorsal hippocampus of cis- and trans-5-methyl-L-trans-2,3-PDC (0.1 nmol) induced 50-70% neuronal damage to areas CA1 and CA4, comparable to that induced by 100 nmol of L-trans-2,3-PDC. Further, cis- and trans-methylated analogues induced distinct patterns of hippocampal pathology consistent with differential excitotoxic vulnerability of neurons expressing NMDA receptors. Neuronal damage produced by the 5-methyl-L-trans-2,3-PDCs could be blocked by coadministration of MK-801 (3 mg/kg ip), but not NBQX (25 nmol). Biochemical and physiological assays confirmed the action of the analogues as NMDA agonists, but did not provide an explanation for differences in excitotoxic potency between the methylated and nonmethylated 2,3-PDCs. or example, the activity of the compounds as inhibitors of 3H-glutamate binding (IC50 values: 0.4, 1.4, and 1.2 microM for cis-5-methyl-,trans-5-methyl-, and L-trans-2,3-PDC, respectively), agonists at NR1A/NR2B receptors (EC50 values: 5, 49, and 16 microM for cis-5-methyl-,trans-5-methyl-, and L-trans-2,3-PDC, respectively), and in vitro excitotoxins in cortical cultures varied only two- to fivefold as a consequence of methylation. Potential roles of NMDA receptor subtypes and transport in these effects are discussed. As potent and selective NMDA excitotoxins, cis- and trans-5-methyl-L-trans-2,3-PDC will be of value studying excitotoxic mechanisms, MDA-receptor-mediated pathology, and NMDA receptor heterogeneity.

In vitro metal inhibition of N-methyl-D-aspartate specific glutamate receptor binding in neonatal and adult rat brain

The in vitro effect of methyl mercury (MM) and lead (Pb) on N-methyl-D-aspartate (NMDA)-specific glutamate receptor binding in neonatal (10 days old) and adult rat brain was investigated. The cerebral cortex was isolated from the neonatal and adult male Sprague-Dawley rats and the synaptic plasma membranes were prepared to study the NMDA-specific glutamate receptor binding by using (3H)-glutamic acid. The metal salts such as methyl mercury chloride and lead acetate were used to study the effect of MM and Pb. Both MM and Pb significantly inhibited the receptor binding in neonatal and adult rat brain in a concentration-dependent manner. MM (IC50:0.95 +/- 0.08 microM) was more potent in inhibiting the receptor binding than Pb (IC50:60 +/- 7 microM) in neonatal rat brain. A similar high potency was observed for MM than Pb in adult rat brain but the IC50 values are very high (70 +/- 6 microM and 300 +/- 24 microM respectively) indicating less effect compared to neonatal brain. The data suggest that NMDA-receptor binding was more sensitive to MM and Pb in neonatal brain than in adult. MM was more effective than Pb because of its more lipophilicity.

Riluzole, a novel neuroprotective agent, attenuates both neurologic motor and cognitive dysfunction following experimental brain injury in the rat

Several potential mechanisms are involved in mediating the pathophysiology of traumatic brain injury (TBI), including inflammatory processes and excitotoxicity. In the present study, we evaluated the ability of the use-dependent sodium channel inhibitor Riluzole to attenuate cognitive and neurologic motor deficits and reduce regional cerebral edema and histologic cell damage following lateral fluid-percussion (FP) brain injury in rats (n = 109). In study 1, 58 anesthetized male Sprague-Dawley rats (350-400 g) were subjected to FP brain injury of moderate severity (2.3-2.5 atm). Fifteen minutes following brain injury, animals randomly received an i.v. bolus of either Riluzole (4 mg/kg, n = 11), Riluzole (8 mg/kg, n = 11), or glycol vehicle (n = 20), followed by 6 h and 24 h s.c. injections (identical dose). Surgically prepared but uninjured animals received vehicle (n = 16) and served as controls. Animals were evaluated for cognitive deficits at 48 h postinjury and killed for assessment of regional brain edema. Administration of vehicle or Riluzole (4 mg/kg x 3) had no significant effect on memory or edema, whereas Riluzole (8 mg/kg x 3) significantly attenuated post-traumatic cognitive dysfunction (p < 0.05). In study 2, a second group of animals (n = 25) was injured, treated with Riluzole (8 mg/kg x 3 doses, n = 13) or vehicle (n = 12), and evaluated for neurologic motor function over 2 weeks. Animals treated with Riluzole demonstrated significantly improved motor scores beginning 1 week postinjury (p < 0.05). In study 3, brain-injured animals were treated with Riluzole (8 mg/kg x 3 doses, n = 10) or vehicle (n = 10), and posttraumatic lesion volume was assessed at 48 h postinjury using 2,3,5-triphenyltetrazolium chloride (TTC) staining. Treatment with Riluzole had no significant effect on posttraumatic lesion volume. The present study demonstrates that use-dependent sodium channel inhibitors, such as Riluzole, can attenuate both cognitive and neuromotor dysfunction associated with brain trauma.

Glutamate receptors in the frontal cortex of alcoholics

This study tests the hypothesis that glutamate receptors are altered in the brains of alcoholics as a result of chronic alcohol neurotoxicity. Release of the neurotransmitter glutamate after seizures or brain ischemia may damage postsynaptic neurons by increasing calcium flux through N-methyl-D-aspartate (NMDA) receptor-gated ion channels. Alcohol has two opposite effects on glutamate receptor ion channel complexes, depending upon the duration of exposure. Acute exposure to alcohol inhibits ion flow through these receptor-channel complexes, whereas chronic exposure up-regulates the number of these receptors and thereby increases ion flow. Acute withdrawal from alcohol results in hyperexcitability and seizures in the presence of up-regulated channels, thereby making postsynaptic neurons vulnerable to excitotoxic damage. We selected 13 histologically normal brains from alcoholics and 13 brains from controls from our brain bank that were matched for age, postmortem interval, and storage time. Maximal binding and affinities of glutamate receptor subtypes were determined by quantitative autoradiography in the superior frontal cortex, Brodmann area 8. The most alcohol-sensitive subtype, NMDA receptor-channel complexes, were modestly but consistently increased in alcoholics. This included agonist sites (NMDA-sensitive [3H]glutamate), and antagonist site ([3H]CGP-39653), and a [3H]MK-801 binding site in the channel interior, although the increase of the latter did not reach statistical significance. Age, autopsy delay, time in storage, liver diseases, thiamine deficiency, CNS medications, and various diseases causing acute and chronic hypoxia did not significantly affect receptor density or affinity. In contrast, the other two glutamate channel subtypes, AMPA and kainate receptors, were not significantly different in alcoholics compared with controls. In conclusion, chronic alcoholism moderately increases the density of the NMDA subtype of glutamate receptors in the frontal cortex. This up-regulation may represent a stage of alcohol-induced chronic neurotoxicity.

Glutamatergic regulation of [3H]acetylcholine release in striatal slices of normotensive and spontaneously hypertensive rats

It has been proposed that central cholinergic neurons may actively participate in blood pressure control and other cardiovascular regulations. The present study was performed to investigate the role of the glutamate receptors in the regulation of acetylcholine release in rat central nervous system in vitro. In the Mg2+-free condition, L-glutamate, an endogenous ligand for glutamate receptors, elicited [3H]acetylcholine release from striatal slices of Sprague-Dawley rats in a dose-related fashion. Glycine, an allosteric agonist for the N-methyl-D-aspartate type of glutamate receptor, significantly potentiated the increase in [3H]acetylcholine release evoked by L-glutamate. A non-competitive N-methyl-D-aspartate receptor antagonist, MK-801, blocked the L-glutamate-induced increase in [3H]acetylcholine release, although MK-801 had no effects on its own. In spontaneously hypertensive rats, the facilitatory effect of L-glutamate on [3H]acetylcholine release was significantly smaller than that in Wistar-Kyoto rats. Moreover, L-glutamate in combination with glycine increased the release of [3H]acetylcholine to a lesser extent in SHR than in WKY rats. These results show that L-glutamate increased acetylcholine release from rat striatum, which was highly dependent on the N-methyl-D-aspartate type of glutamate receptor. Furthermore, the lesser facilitation of acetylcholine release by L-glutamate in spontaneously hypertensive rats suggests that the excitatory amino acid may be, at least in part, involved in the regulation of central cholinergic nerve activity in hypertension.

L-trans-2,3-pyrrolidine dicarboxylate: characterization of a novel excitotoxin

This study investigated the in vitro and in vivo excitotoxic properties of a novel conformationally constrained analogue of L-glutamate, L-trans-2,3-pyrrolidine dicarboxylate (L-trans-2,3-PDC). When tested for excitotoxic activity in rat cortical cultures, L-trans-2,3-PDC mimicked the action of NMDA in both acute (30 min) and chronic (24 h) exposure paradigms. This neurotoxicity was attenuated by co-addition of MK-801 (10 microM). Microinjections of L-trans-2,3-PDC into the dorsal hippocampus of male rats also induced a selective pattern of pathology indicative of an NMDA receptor excitotoxin. In contrast to the equipotency observed in vitro, 100 nmol of L-trans-2,3-PDC were needed to produce cellular damage comparable to that induced by 25 nmol of NMDA. Consistent with an action at NMDA receptors, L-trans-2,3-PDC-induced damage could be significantly reduced by co-administration of MK-801 (3 mg/kg i.p.), but not by NBQX (25 nmol). In radioligand binding assays L-trans-2,3-PDC inhibited the binding of 3H-L-glutamate to NMDA receptors (IC50 1 microM), although it also exhibited some cross reactivity with KA and AMPA receptors. L-trans-2,3-PDC was also identified as a competitive inhibitor (Ki = 33 microM) of 3H-D-aspartate uptake into rat forebrain synaptosomes. In contrast to the action of a transported substrate, such as L-glutamate, L-trans-2,3-PDC did not exchange with 3H-D-aspartate that had been previously loaded into the synaptosomes.

A model of parasagittal controlled cortical impact in the mouse: cognitive and histopathologic effects

Controlled cortical impact (CCI), using a pneumatically driven impactor to produce traumatic brain injury, has been characterized previously in both the ferret and in the rat. In the present study, we applied this technique to establish and characterize the CCI model of brain injury in another species, the mouse, evaluating cognitive and histopathologic outcome. In anesthetized (sodium pentobarbital, 65 mg/kg) male C57BL mice, we performed sham treatment (no injury, n = 12) or CCI injury (n = 12) at a velocity of 5.7-6.2 m/sec and depth of 1 mm, using a 3-mm diameter rounded-tip impounder, positioned over the left parietotemporal cortex (parasagittal). At this level of injury, we observed highly significant deficits in memory retention of a Morris water maze task 2 days following injury (p < 0.001). Postmortem histopathologic analysis performed at 48 h following injury revealed substantial cortical tissue loss in the region of impact and selective hippocampal neuronal cell loss in the CA2, CA3, and CA3c regions, using Nissl staining. Analysis of degenerating neurons using modified Gallyas silver staining techniques demonstrated consistent ipsilateral injury of neurons in the cortex adjacent to the impact site and in the dentate gyrus of the ipsilateral hippocampus. Bilateral degeneration was observed at the gray matter-white matter interface along the corpus callosum. Glial fibrillary acidic protein (GFAP) immunohistochemistry revealed extensive reactive gliosis appearing diffusely through the bilateral cortices, hippocampi, and thalami at 48 h postinjury. Breakdown of the blood-brain barrier was demonstrated with antimouse IgG immunohistochemistry, revealing extravasation of endogenous IgG throughout the ipsilateral cortex, hippocampus, and thalamus. These results suggest that this new model of parasagittal CCI in the mouse mimics a number of well-established sequelae observed in previously characterized brain injury models using other rodent species. This mouse model may be a particularly useful experimental tool for comparing behavioral and histopathologic characteristics of traumatic brain injury in wild-type and genetically altered mice.

Regulation of N-methyl-D-aspartate receptor-mediated calcium transport and norepinephrine release in rat hippocampus synaptosomes by polyamines

The role of polyamines (PA) synthesis in NMDA receptor-mediated 45Ca2+ fluxes and norepinephrine release was studied in rat hippocampal synaptosomes. NMDA (50 microM) caused a sharp (> 2-fold) transient increase in PA synthesis regulating enzyme, ornithine decarboxylase (ODC) activity with concomitant elevation in PA levels in the order putrescine > spermidine > spermine. ODC inhibitor, alpha-difluoromethylornithine (DFMO), and NMDA antagonist, 2-amino-5-phosphonovaleric acid (D-AP5), both blocked increases in ODC activity and PA levels. Activation of NMDA receptors induced a sharp (3 to 4-fold) and quick (15 seconds) increase in 45Ca2+ uptake by synaptosomes within 15 seconds of exposure at 37 degrees C. The efflux of 45Ca2+ and 3H-norepinephrine (NE) release at 22 degrees C from pre-loaded synaptosomes was also significantly (2 to 4-fold) enhanced by NMDA within 15 seconds. These NMDA receptor-mediated effects on calcium fluxes and NE release were blocked by NMDA receptor-antagonists (DAP-5 and MK-801) and PA synthesis inhibitor, DFMO and the DFMO inhibition nullified by exogenous putrescine. These observations establish that ODC/PA cascade play an important role in transduction of excitatory amino acid mediated signals at NMDA receptors.

[3H]CGP 39653 binding to the agonist site of the N-methyl-D-aspartate receptor is modulated by Mg2+ and polyamines independently of the arcaine-sensitive polyamine site

This study investigated the binding of [3H]CGP 39653, a novel high-affinity antagonist of the N-methyl-D-aspartate (NMDA) recognition site of the NMDA receptor complex. [3H]CGP 39653 bound to the NMDA receptor in well washed rat brain membranes with an affinity of about 15 nM. Other NMDA site drugs inhibited [3H]CGP 39653 binding with the following order of potency: DL-(tetrazol-5-yl)glycine > glutamate > CGS 19755 > DL-2-amino-5-phosphonovalerate (DL-AP5) > NMDA. Glycine and 5,7-dichlorokynurenate partially inhibited binding. The polyamines spermine and spermidine increased [3H]CGP 39653 binding (EC50 values of 10 and 22 microM, respectively). This effect was mimicked by arcaine, 1,5-diethylaminopiperidine, diaminodecane, diethylenetriamine, and Mg2+. The increase in [3H]CGP 39653 was a result of an increased affinity of the binding site for the ligand with very little effect on binding site density. Spermine and Mg2+ also increased the affinity of the antagonists DL-AP5 and CGS 19755, but had only minor effects on the affinity of glutamate and NMDA. Arcaine did not reverse the enhancement of [3H]CGP 39653 binding by spermine, spermidine, or Mg2+. Channel-blocking dissociative anesthetics, including dizocilpine and ketamine, did not alter basal or Mg(2+)-stimulated [3H]CGP 39653 binding. Spermine did not alter either the enhancement of [3H]dizocilpine by glutamate or the inhibition of [3H]dizocilpine by DL-AP5 or CGS 19755. These studies show that polyamines and divalent cations selectively enhance the affinity of antagonists for the agonist binding site on the NMDA receptor complex. However, this effect is mediated by a site independent of the primary polyamine site defined using [3H]dizocilpine binding.

Further evidence for multiple forms of an N-methyl-D-aspartate recognition domain in rat brain using membrane binding techniques

Pretreatment with sulfhydryl-reactive agents, such as N-ethylmaleimide and p-chloromercuriphenylsulfonic acid, invariably resulted in marked inhibition of the binding of DL-(E)-2-amino-4-[3H]propyl-5-phosphono-3-pentenoic acid ([3H]CGP 39653), a competitive antagonist at an N-methyl-D-aspartate (NMDA)-sensitive subclass of central excitatory amino acid receptors, in brain synaptic membranes extensively washed and treated with Triton X-100, but did not significantly affect the binding of L-[3H]-glutamic acid ([3H]Glu), an endogenous agonist. The pretreatment was effective in reducing the binding of [3H]-CGP 39653 at equilibrium, without altering the initial association rate, and decreased the affinity for the ligand. Pretreatment with sulfhydryl-reactive agents also enhanced the potencies of NMDA agonists to displace [3H]-CGP 39653 binding and attenuated those of NMDA antagonists, but had little effect on the potencies of the agonists and antagonists to displace [3H]Glu binding. The binding of both [3H]CGP 39653 and [3H]Glu was similarly sensitive to pretreatment with four different proteases in Triton-treated membranes, whereas pretreatment with phospholipase A2 or C markedly inhibited [3H]CGP 39653 binding without altering [3H]Glu binding. Moreover, both phospholipases not only induced enhancement of the abilities of NMDA agonists to displace the binding of [3H]CGP 39653 and [3H]Glu, but also caused diminution of those of NMDA antagonists. These results suggest that both sulfhydryl-reactive agents and phospholipases may predominantly interfere with radiolabeling of the NMDA recognition domain in a state favorable to an antagonist by [3H]CGP 39653, with concomitant facilitation of that in an antagonist-preferring form by [3H]Glu. The possible presence of multiple forms of the NMDA recognition domain is further supported by these data.

Stimulatory effects of protein kinase C and calmodulin kinase II on N-methyl-D-aspartate receptor/channels in the postsynaptic density of rat brain

To clarify the regulatory mechanism of the N-methyl-D-aspartate (NMDA) receptor/channel by several protein kinases, we examined the effects of purified type II of protein kinase C (PKC-II), endogenous Ca2+/calmodulin-dependent protein kinase II (CaMK-II), and purified cyclic AMP-dependent protein kinase on NMDA receptor/channel activity in the postsynaptic density (PSD) of rat brain. Purified PKC-II and endogenous CaMK-II catalyzed the phosphorylation of 80-200-kDa proteins in the PSD and L-glutamate- (or NMDA)-induced increase of (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate ([3H]MK-801; open channel blocker for NMDA receptor/channel) binding activity was significantly enhanced. However, the pretreatment of PKC-II- and CaMK-II-catalyzed phosphorylation did not change the binding activity of L-[3H]glutamate, cis-4-[3H](phosphonomethyl)piperidine-2-carboxylate ([3H]CGS-19755; competitive NMDA receptor antagonist), [3H]glycine, alpha-[3H]-amino-3-hydroxy-5-methyl-isoxazole-4-propionate, or [3H]-kainate in the PSD. Pretreatment with PKC-II- and CaMK-II-catalyzed phosphorylation enhanced L-glutamate-induced increase of [3H]MK-801 binding additionally, although purified cyclic AMP-dependent protein kinase did not change L-glutamate-induced [3H]MK-801 binding. From these results, it is suggested that PKC-II and/or CaMK-II appears to induce the phosphorylation of the channel domain of the NMDA receptor/channel in the PSD and then cause an enhancement of Ca2+ influx through the channel.

Two different families of NMDA receptors in mammalian brain: physiological function and role in neuronal development and degeneration

The data from purification, ligand binding, reconstitution and immunochemical studies indicate that there is a group of small molecular size proteins (30 to 70 kDa) that form what appear to be NMDA receptor complexes. Based on cell biological studies of the expression and localization of one of the subunits of this complex, the glutamate-binding subunit, it appears that this putative NMDA receptor plays a key role in neuronal sensitivity to NMDA and in neuronal survival in early development. However, brain neurons quite clearly express another family of proteins which have all functional characteristics of an NMDA receptor plus a great degree of variability that can account for the varieties of NMDA receptors found in brain. This family of NMDA receptors, the NMDAR1 and NMDAR2, are not homologous to the small molecular size proteins of the previously described receptor complex that was isolated from synaptic membranes. If brain neurons are indeed expressing two very diverse families of proteins that function as glutamate/NMDA receptors, this must be an indication that either there is a very selective expression of one of these forms in specific neurons or neuronal compartments, or that one of these forms of the receptor plays an important role in unique functions of the cell, such as synaptic plasticity or neurodegeneration. As more information is gathered about the structure and function of these receptors, a better understanding of the expression and role of these families of receptor proteins in normal neuronal excitability will be achieved. This enhanced level of understanding about the function and activity of these receptors will be needed in order to identify more precisely the NMDA receptors most affected in pathological conditions such as hyperammonemia.

N-methyl-D-aspartate receptor-independent long-term potentiation in area CA1 of rat hippocampus: input-specific induction and preclusion in a non-tetanized pathway

We previously reported that an N-methyl-D-aspartate receptor-independent component of long-term potentiation with an apparent delayed onset can be induced in area CA1 of the hippocampus. Here we show that some but not all of this delay in onset can be accounted for by a transient heterosynaptic depression. We also show that N-methyl-D-aspartate receptor-independent long-term potentiation is induced only in the input pathway tetanized, and not in a second pathway. However, prior induction of N-methyl-D-aspartate receptor-independent long-term potentiation in one pathway precludes later induction in an independent pathway. Calcium entry through dihydropyridine-sensitive Ca2+ channels may be a critical step for induction of N-methyl-D-aspartate receptor-independent long-term potentiation in area CA1 [Grover L. M. and Teyler T.J. (1990) Nature 347, 477-479]. Since the distribution [Westenbroek R. E. et al. (1990) Nature 347, 281-284] of dihydropyridine-sensitive Ca2+ channels in CA1 neuron dendrites does not suggest a basis for input-specific induction of long-term potentiation, an additional process may confer the specificity we observed. Tetanic stimulation of afferents into area CA1 can elicit several processes: a transient heterosynaptic depression, and a transient homosynaptic potentiation, as well as N-methyl-D-aspartate receptor-dependent and -independent long-term potentiation.

N-methyl-D-aspartate receptors are transiently expressed in the developing spinal cord ventral horn

Quantitative receptor autoradiography was used to map the distribution of N-methyl-D-aspartate (NMDA) receptors in the developing rat spinal cord. Three different specific ligands, which label partially overlapping subpopulations of NMDA receptors, were used: an agonist (L-[3H]glutamate), a noncompetitive antagonist ([3H]MK-801), and a competitive antagonist ([3H]CGP-39653). In the adult, NMDA receptors labeled with all three ligands are restricted to the substantia gelatinosa in the spinal dorsal horn. In marked distinction, at postnatal day 7 NMDA receptors labeled with L-[3H]glutamate and [3H]MK-801 are present throughout the spinal gray matter. NMDA receptors in the neonatal spinal ventral horn have a higher affinity for L-[3H]glutamate than those in the adult substantia gelatinosa. Over the second and third postnatal weeks, NMDA receptors are lost from all areas of the spinal gray matter except for the substantia gelatinosa. Neonatal NMDA receptors identified with [3H]CGP-39653 are restricted to the substantia gelatinosa. These results show that the immature ventral horn contains a subpopulation of NMDA receptors and raise the possibility that motor neurons transiently express NMDA receptors in early postnatal life. Ventral horn NMDA receptors may be a component of the mechanisms by which the mature phenotype of motor neurons is acquired through activity-dependent processes. The loss of NMDA receptors over the course of development may play a role in limiting the period of motor neuron plasticity.