Absence Of N-methyl-D-aspartate Research Articles

Nanomolar concentrations of anabolic–androgenic steroids amplify excitotoxic neuronal death in mixed mouse cortical cultures

The use of anabolic–androgenic steroids (AASs) in the world of sport has raised a major concern for the serious, sometimes life-threatening, side effects associated with these drugs. Most of the CNS effects are of psychiatric origin, and whether or not AASs are toxic to neurons is yet unknown. We compared the effect of testosterone with that of the AASs, 19-nortestosterone (nandrolone), stanozolol, and gestrinone, on excitotoxic neuronal death induced by N-methyl- d-aspartate (NMDA) in primary cultures of mouse cortical cells. In the most relevant experiments, steroids were applied to the cultures once daily during the 4 days preceding the NMDA pulse. Under these conditions, testosterone amplified excitotoxic neuronal death only at very high concentrations (10 μM), whereas it was protective at concentrations of 10 nM and inactive at intermediate concentrations. Low concentrations of testosterone became neurotoxic in the presence of the aromatase inhibitors, i.e. anastrozole and aminoglutethimide, suggesting that the intrinsic toxicity of testosterone was counterbalanced by its aromatization into 17β-estradiol. As opposed to testosterone, nortestosterone, stanozolol and gestrinone amplified NMDA toxicity at nanomolar concentrations; their action was insensitive to aromatase inhibitors, but was abrogated by the androgen receptor antagonist, flutamide. None of the AASs were toxic in the absence of NMDA. These data suggest that AASs increase neuronal vulnerability to an excitotoxic insult and may therefore facilitate neuronal death associated with acute or chronic CNS disorders.

Evaluating the NMDA-Glutamate Receptor as a Site of Action for Toluene, In Vivo

Acute exposure to toluene and other volatile organic solvents results in neurotoxicity characterized by nervous system depression, cognitive and motor impairment, and alterations in visual function. In vitro, toluene disrupts the function of N-methyl-D-aspartate (NMDA)-glutamate receptors, indicating that effects on NMDA receptor function may contribute to toluene neurotoxicity. NMDA-glutamate receptors are widely present in the visual system and contribute to pattern-elicited visual-evoked potentials (VEPs) in rodents, a measure that is altered by toluene exposure. The present study tested the hypothesis that effects on NMDA receptors contribute to toluene-induced alterations in pattern-elicited VEPs. Prior to examining the effects of NMDA receptor agonists and antagonists on toluene-exposed animals, a dose-range study was conducted to determine the optimal dose for NMDA (agonist) and MK801 (antagonist). Dose levels of 2.5 mg/kg NMDA and 0.1 mg/kg MK801 were selected from these initial studies. In the second study, Long-Evans rats were exposed to toluene by inhalation, and VEPs were measured during toluene exposure in the presence or absence of NMDA or MK801. Pattern-elicited VEPs were collected by exposing rats to a sinusoidal pattern modulated at a temporal frequency of 4.55 Hz. Following collection of baseline VEPs, rats were injected with either saline, NMDA (2.5 mg/kg, ip), or MK801 (0.1 mg/kg, ip) and 10 min later were exposed to air or toluene (2000 ppm). VEP amplitudes were calculated for 1x (F1) and 2x stimulus frequency (F2). The F2 amplitude was reduced by approximately 60, 60, and 50% in the toluene-exposed groups (TOL): SALINE/TOL (n = 11), NMDA/TOL (2.5 mg/kg; n = 13), and NMDA/TOL (10 mg/kg, n = 11), respectively. Thus, NMDA (2.5 and 10 mg/kg) did not significantly affect toluene-mediated F2 amplitude effects. Administration of 0.1 mg/kg MK801 prior to toluene exposure blocked the F2 amplitude decreases caused by toluene (n = 9). However, when 0.1 mg/kg MK801 was administered 20 min after the onset of toluene exposure, toluene-mediated F2 amplitude decreases persisted despite the challenge by MK801. These data support the hypothesis that acute actions of toluene on pattern-elicited VEPs involve NMDA receptors.

Cellular mechanisms underlying the rhythmic bursts induced by NMDA microiontophoresis at the apical dendrites of CA1 pyramidal neurons.

This article reports the cellular mechanisms underlying a form of intracellular "theta-like" (theta-like) rhythm evoked in vitro by microiontophoresis of N-methyl-D-aspartate (NMDA) at the apical dendrites of CA1 pyramidal neurons. Rhythmic membrane potential (Vm) oscillations and action potential (AP) bursts (approximately 6 Hz; approximately 20 mV; approximately 2-5 APs) were evoked in all cells. The response lasted approximately 2 s, and the initial oscillations were usually small (< 20 mV) and below AP threshold. Rhythmic bursts were never evoked by imposed depolarization in the absence of NMDA. Block of Na+ conductance with tetrodotoxin (TTX) (1.5 microM), of non-NMDA receptors with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (20 microM) and of synaptic inhibition by bicuculline (50 microM) and picrotoxin (50 microM) did not prevent NMDA oscillation. Inhibition of the voltage dependence of the NMDA conductance in Mg2+-free Ringer's solution blocked oscillations. Preventing Ca2+ influx with Ca2+-free and Co2+ (2-mM) solutions and block of the slow Ca2+-dependent afterhyperpolarization (sAHP) by carbamilcholine (5 microM), isoproterenol (10 microM), and intracellular BAPTA blocked NMDA oscillations. Inhibition of L-type Ca2+ conductance with nifedipine (30 microM) reduced oscillation amplitude. Block of tetraethylammonium (TEA) (10 mM) and 4AP (10 mM)-sensitive K+ conductance increased the duration and amplitude, but not the frequency, of oscillations. In conclusion, theta-like bursts relied on the voltage dependence of the NMDA conductance and on high-threshold Ca2+ spikes to initiate and boost the depolarizing phase of oscillations. The repolarization is initiated by TEA-sensitive K+ conductance and is controlled by the sAHP. These results suggest a role of interactions between NMDA conductance and intrinsic membrane properties in generating the CA1 theta-rhythm.

Synaptically evoked membrane potential oscillations induced by substance P in lamprey motor neurons.

Short-lasting application (10 min) of tachykinin neuropeptides evokes long-lasting (>24 h) modulation of N-methyl-D-aspartate (NMDA)-evoked locomotor network activity in the lamprey spinal cord. In this study, the net effects of the tachykinin substance P on the isolated spinal cord have been examined by recording from motor neurons in the absence of NMDA and ongoing network activity. Brief bath application of substance P (30 s to 2 min) induced irregular membrane potential oscillations in motor neurons. These oscillations consisted of depolarizing and hyperpolarizing phases and were associated with phasic ventral-root activity. The oscillations were blocked by the tachykinin antagonist spantide II. They were also blocked by tetrodotoxin (TTX), suggesting that they were not dependent on intrinsic membrane properties of the motor neurons but were synaptically mediated. Substance P could also have a direct effect, however, because a membrane potential depolarization persisted in the presence of TTX. Protein kinase agonists and antagonists were used to investigate the intracellular pathways through which substance P acted. The oscillations were blocked by the selective protein kinase C (PKC) antagonist chelerythrine. However, the TTX-resistant membrane potential depolarization was not significantly affected by blocking PKC. The protein kinase A and G antagonist H8 did not affect either the oscillations or the direct TTX-resistant membrane potential depolarization. The glutamate receptor antagonist kynurenic acid abolished the substance-P-evoked oscillations, suggesting that they were dependent on glutamate release. The oscillations were abolished or reduced by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione but were only reduced by the NMDA receptor antagonist D-AP5. The oscillations were thus mediated by glutamatergic inputs with a greater dependence on non-NMDA receptors. Blocking glycinergic inputs with strychnine resulted in large depolarizing plateaus and bursts of spikes. The glutamatergic and glycinergic inputs underlying the oscillations are apparently evoked through direct and indirect excitatory effects on inhibitory and excitatory premotor interneurons. Substance P thus has a distributed excitatory effect in the spinal cord. While it can activate premotor networks, this activation alone is not able to evoke a coordinated behaviorally relevant motor output.

Effects of volatile anesthetics on N-methyl-D-aspartate excitotoxicity in primary rat neuronal-glial cultures.

Volatile anesthetics are known to ameliorate experimental ischemic brain injury. A possible mechanism is inhibition of excitotoxic cascades induced by excessive glutamatergic stimulation. This study examined interactions between volatile anesthetics and excitotoxic stress. Primary cortical neuronal-glial cultures were exposed to N-methyl-D-aspartate (NMDA) or glutamate and isoflurane (0.1-3.3 mM), sevoflurane (0.1-2.9 mM), halothane (0.1-2.9 mM), or 10 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801). Lactate dehydrogenase release was measured 24 h later. In other cultures, effects of volatile anesthetics on Ca++ uptake and mitochondrial membrane potential were determined in the presence or absence of NMDA (0-200 microM). Volatile anesthetics reduced excitotoxin induced lactate dehydrogenase release by up to 52% in a dose-dependent manner. At higher concentrations, this protection was reversed. When corrected for olive oil solubility, the three anesthetics offered equivalent protection. MK-801 provided near-complete protection. Ca++ uptake was proportionally reduced with increasing concentrations of anesthetic but did not account for reversal of protection at higher anesthetic concentrations. Given equivalent NMDA-induced Ca++ loads, cells treated with volatile anesthetic had greater lactate dehydrogenase release than those left untreated. At protective concentrations, volatile anesthetics partially inhibited NMDA-induced mitochondrial membrane depolarization. At higher concentrations, volatile anesthetics alone were sufficient to induce mitochondrial depolarization. Volatile anesthetics offer similar protection against excitotoxicity, but this protection is substantially less than that provided by selective NMDA receptor antagonism. Peak effects of NMDA receptor antagonism were observed at volatile anesthetic concentrations substantially greater than those used clinically.

Effects of inhibitory neurotransmitters on the mudpuppy (Necturus maculatus) locomotor pattern in vitro.

Effects of inhibitory neurotransmitters on the locomotor rhythm and pattern generation were investigated using an in vitro preparation isolated from the mudpuppy (Necturus maculatus). The preparation consisted of the first five segments of the spinal cord and the right forelimb attached by the brachial nerves. During N-methyl-d-aspartate (NMDA)-induced locomotion, the rhythmic motor output (EMG) was recorded unilaterally from elbow flexor and extensor muscles. While neither glycine nor gamma-aminobutyric acid (GABA)-related substances induced locomotion in the absence of NMDA, they modulated NMDA-induced locomotion. Bath application of glycine and GABA suppressed the rhythmic motor pattern induced by NMDA. Addition of glycine receptor antagonist strychnine or GABA(A) receptor antagonist bicuculline disrupted the phase relationship between antagonistic motor pools during ongoing locomotion, thereby changing the normal alternating pattern into synchronous EMG bursts. Both the GABA(A) receptor agonist muscimol and GABA(B) receptor agonist baclofen mimicked the effects of GABA as they either slowed down or stopped locomotion. Nipecotic acid, a GABA uptake blocker, had a similar effect. This suggested that an endogenous release of GABA modulated the locomotor rhythm. The endogenous release was antagonized by the GABA(A) and GABA(B) receptor antagonists bicuculline and CGP-35348, respectively. Immunocytochemistry revealed that glycine and GABA-positive neurons and fibers were present in mudpuppy spinal cord. Although the GABAergic neurons were more numerous than glycinergic neurons, both cell types contributed processes directed towards the white matter and occasionally towards the ependymal lining of the central canal. Our results suggest that inhibitory neurotransmitters exert powerful actions upon the neuronal network governing forelimb locomotion in the mudpuppy. The effects we observed may be mediated by a network of segmentally distributed glycinergic and GABAergic spinal neurons.

Dopaminergic modulation of early signs of excitotoxicity in visualized rat neostriatal neurons

Cell swelling induced by activation of excitatory amino acid receptors is presumably the first step in a toxic cascade that may ultimately lead to cell death. Previously we showed that bath application of N-methyl-D-aspartate (NMDA) or kainate (KA) produces swelling of neostriatal cells. The present experiments examined modulation of NMDA and KA-induced cell swelling by dopamine (DA) and its receptor agonists. Nomarski optics and infra-red videomicroscopy were utilized to visualize neostriatal medium-sized neurons in thick slices from rat pups (12-18 postnatal days). Increase in somatic cross-sectional area served as the indicator of swelling induced by bath application of glutamate receptor agonists. NMDA induced cell swelling in a dose-dependent manner. Activation of DA receptors in the absence of NMDA did not produce swelling. DA and the D1 receptor agonist SKF 38393, increased the magnitude of swelling produced by NMDA. This effect was reduced in the presence of the D1 receptor antagonist, SCH 23390. In contrast, activation of D2 receptors by quinpirole decreased the magnitude of NMDA-induced cell swelling. DA slightly attenuated cell swelling induced by activation of KA receptors. Quinpirole produced a significant concentration-dependent reduction in KA-induced swelling while SKF38393 increased KA-induced swelling, but only at a low concentration of KA. Together, these results provide additional support for the hypothesis that the direction of DA modulation depends on the glutamate receptor subtype, as well as the DA receptor subtype activated. One possible consequence of these observations is that endogenous DA may be an important contributing factor in the mechanisms of cell death in Huntington's disease.

Evidence for functional native NMDA receptors activated by glycine or D-serine alone in the absence of glutamatergic coagonist.

In this study we have examined the effects of N-methyl-D-aspartate (NMDA) receptor activation on the release of cholecystokinin and somatostatin from rat neocortical nerve endings. The release of cholecystokinin-like immunoreactivity (CCK-LI) and of somatostatin-like immunoreactivity (SRIF-LI) elicited by 12 mM K+ from superfused synaptosomes, but not the spontaneous release, was increased by NMDA in a concentration-dependent manner. The effects of NMDA could be prevented by antagonists selective for the glutamate recognition site, the receptor channel and the glycine site of the NMDA receptor. In the absence of NMDA, glycine increased on its own and in a concentration-dependent manner the depolarization-evoked release of both CCK-LI and SRIF-LI. This effect of glycine was strychnine-insensitive and could be mimicked by D-serine, a stereoselective agonist at the NMDA receptor glycine site. Antagonists selective for the glycine site or for the NMDA receptor channel prevented the effects of glycine/D-serine; these effects were, however, insensitive to blockade of the glutamate recognition site of the NMDA receptor, suggesting that glutamate released from synaptosomes or present as contaminant was not involved. The neuropeptide release elicited by D-serine was strongly inhibited by ifenprodil (0.3 microM) and by Zn2+ ions (50 nM), selective ligands at the NR2B and NR2A subunits of NMDA receptors, respectively. It is concluded that nerve terminals of CCK- and SRIF-releasing neurons possess non-conventional NMDA receptors whose channels can be operated by glycine or D-serine without apparent activation of the glutamatergic coagonist site. These receptors may display the triple subunit combination NR1/NR2A/NR2B.

Voltage Oscillations in Xenopus Spinal Cord Neurons: Developmental Onset and Dependence on Co‐activation of NMDA and 5HT Receptors

The development of intrinsic, N-methyl-D-aspartate (NMDA) receptor-mediated voltage oscillations and their dependence on co-activation of 5-hydroxytryptamine (5HT) receptors was explored in motor neurons of late embryonic and early larval Xenopus laevis. Under tetrodotoxin, 100 microM NMDA elicited a membrane depolarization of around 20 mV, but did not lead to voltage oscillations. However, following the addition of 2-5 microM 5HT, oscillations were observed in 12% of embryonic and 70% of larval motor neurons. The voltage oscillations depended upon co-activation of NMDA and 5HT receptors since they were curtailed by selectively blocking NMDA receptors with D-2-amino-5-phosphonovaleric acid (APV) or by excluding Mg2+ from the experimental saline. 5HT applied in the absence of NMDA also failed to elicit oscillations. Oscillations could be induced by the non-selective 5HT1alpha receptor agonist, 5-carboxamidotryptamine (5CT) and both 5HT- and 5CT-induced oscillations were abolished by pindobind-5HT1, a selective 5HT1alpha receptor antagonist. To test whether 5HT enables voltage oscillations by modulating the voltage-dependent block of NMDA channels by Mg2+, membrane conductance was monitored under tetrodotoxin. Although 5HT caused membrane hyperpolarization of 4-8 mV, there was little detectable change in conductance. NMDA application caused an approximate 20 mV depolarization and an 'apparent' decrease in conductance, presumably due to the conductance pulse bringing the membrane into a voltage region where Mg2+ blocks the NMDA ionophore. 5HT further decreased conductance, which we propose is due to its enhancement of the voltage-dependent Mg2+ block. When the membrane potential was depolarized by approximately 20 mV via depolarizing current injection (to mimic the NMDA-induced depolarization), 5HT increased rather than decreased membrane conductance. Furthermore, 5HT did not affect the increase in membrane conductance following NMDA applications in zero Mg2+ saline. The results suggest that intrinsic, NMDA receptor-mediated voltage oscillations develop in a brief period after hatching, and that they depend upon the co-activation of 5HT and NMDA receptors. The enabling function of 5HT may involve the facilitation of the voltage-dependent block of the NMDA ionophore by Mg2+ through activation of receptors with 5HT1alpha-like pharmacology.

The neurosteroid pregnenolone sulphate enhances NMDA-induced phasic firing of vasopressin neurones in the rat supraoptic nucleus

The effect of the neurosteroid pregnenolone sulphate (PS) on N-methyl- d-aspartate (NMDA)-induced phasic firing of supraoptic vasopressin (VP) neurones was studied in rat hypothalamic slices in vitro. In VP neurones which were induced to fire phasically by continuous perifusion with NMDA (9–30 μM), addition of 100 μM PS to the incubation medium significantly increased overall spike frequency, with a rise in both proportion of time active and intraburst firing rate. A similar effect was seen during picrotoxin block of GABAergic transmission. No significant change in NMDA-induced phasic firing was observed with 100 μM dehydroepiandrosterone sulphate. VP neurones became silent in the absence of NMDA, and under these conditions PS had no effect. In conclusion, PS increases NMDA-induced phasic firing in VP neurones, providing a mechanism whereby this neurosteroid may participate in the regulation of VP secretion.

Facilitation of N-methyl-D-aspartate-evoked acetylcholine release by hydroxyl radical scavengers.

Effects of hydroxyl radical scavengers such as dimethylthiourea (DMTU), uric acid and mannitol on acetylcholine (ACh) release evoked by N-methyl-D-aspartate (NMDA) were examined in cerebral cortical neurones. NMDA increased dose-dependently ACh release. This increase in the release was significantly suppressed by MK-801, N omega-nitro-L-arginine and haemoglobin, indicating that NMDA evokes ACh release via the formation of NO subsequent to NMDA receptor activation. DMTU dose-dependently enhanced the NMDA-induced ACh release. Uric acid (100 microM) and mannitol (1 mM) facilitated significantly the NMDA-induced ACh release, although these scavengers themselves showed no effects on ACh release in the absence of NMDA. These results indicate that the removal of hydroxyl radical facilitates the NO-induced ACh release.

Polyamine potentiation and inhibition of NMDA-mediated increases of intracellular free Ca 2+ in cultured chick cortical neurons

Polyamine potentiation and inhibition of N- methyl- d-aspartate (NMDA) receptor-mediated CA 2+ changes was studied in cultured chick cortical neurons. Spermidine and spermine potentiated the effect of saturating concentrations of NMDA and glycine. No effect of spermidine or spermine was observed in the absence of NMDA or in the presence of either kainate or quisqualate. Similarly, antagonism of the NMDA receptor complex with dizocilpine (an open channel blocker), or with competitive antagonists to the NMDA or glycine binding sites greatly attenuated or completely abolished the combined effects of polyamines plus NMDA and glycine. N-Acetylspermine and N-acetylspermidine, in the presence or absence of NMDA and glycine, were without effect. These data strongly suggest that spermidine and spermine are potent and selective agonists at the polyamine binding site. Putrescine and diethylenetriamine were ineffective as antagonists of NMDA-mediated intracellular free Ca 2+ increases in the presence or absence of added spermine or spermidine. Arcaine and 1,10-diaminodecane, however, antagonized NMDA-mediated intracellular free Ca 2+ increases in the presence and absence of spermine and spermidine, and therefore appear to act either as inverse agonists at the polyamine binding site or as open channel blockers of the NMDA receptor.

Equilibrium and kinetic study of glycine action on the N-methyl-D-aspartate receptor in cultured mouse brain neurons.

1. The characteristics of the activation of the N-methyl-D-aspartate (NMDA) response by glycine were studied using whole-cell and outside-out patch clamp recording techniques. 2. Glycine concentration-response (C-R) curves were measured in the presence of 10 microM-NMDA and fitted with the Hill equation modified to account for the response to NMDA observed in the absence of added glycine. The mean value of the apparent dissociation constant (KD) was 150 nM, and the mean value of the Hill coefficient (nH) was 1.1. When the KD was corrected for the concentration of contaminating glycine in nominally glycine-free solutions, estimated assuming that there is no response in the absence of glycine, the value was 130 nM. 3. The question of how many glycine binding sites there are on each NMDA receptor-channel complex was addressed by examining the curvature at the foot of the glycine C-R curve. An equation that allowed estimation of both the concentration of contaminating glycine and of the value of nH was fitted to glycine C-R data up to 50 nM. The mean value of nH was found to be 1.0, consistent with the idea that there is one glycine binding site. 4. The kinetics of the interaction of glycine with the NMDA receptor were measured by fitting single exponential curves to the current relaxation following a jump in glycine concentration in the presence of 10 microM-NMDA. The plot of the inverse of the relaxation time constant as a function of glycine concentration after the concentration jump was linear. The association rate constant was estimated from these data as 1.2 x 10(7) M-1 s-1 and the dissociation rate as 1.0 s-1. 5. Experiments were devised to allow the evaluation of the KD and dissociation rates of glycine in the absence of NMDA. They led to a value for KD of 80 nM, slightly but significantly lower than the value of 150 nM estimated in the presence of 10 microM-NMDA. The glycine dissociation rate in the absence of NMDA was found to be 0.7 s-1, not significantly different from that measured in the presence of 10 microM-NMDA. 6. The results are consistent with a model of the NMDA receptor with a single glycine binding site. The characteristics of glycine binding are similar in the absence and the presence of 10 microM-NMDA, although NMDA binding may cause a small increase in the glycine KD.(ABSTRACT TRUNCATED AT 250 WORDS)

Formula omitted] stimulation of the survival of rat cerebellar granule cells in culture is not dependent upon increased c- fos expression and is not mimicked by protein kinase C activation

The role of c- fos expression and protein kinase C in the survival of primary cultures of rat cerebellar granule cells was investigated. Results from immunocytochemistry and immunoblotting suggest that increased c- fos expression is not essential for the survival of cells grown in low K + media in the presence of N- methyl- d-aspartate (NMDA) at the critical time point when sensitivity to survival requirements develops. In addition the phorbol ester, phorbol 12-myristate 13-acetate failed to bring about survival of cells cultured in low K + media in the absence of NMDA when given chronically, suggesting that protein kinase C activation alone is not sufficient to maintain granule cell survival in culture.

Mode of blockade by MK-801 of N-methyl- d-aspartate-induced increase in intracellular Ca 2+ in cultured mouse hippocampal neurons

Microfluorometry with fura-2 was applied to study the action of the anticonvulsant (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) on N-methyl- d-aspartate (NMDA)-induced increase in intracellular Ca 2+ concentration ([Ca 2+] 1) in cultured mouse hippocampal neurons. MK-801 caused a potent and long-lasting blockade of the NMDA-activated [Ca 2+] i elevation in a selective manner, not affecting the [Ca 2+] i rise induced by quisqualate or kainate. Blockade and recovery from the blockade by MK-801 showed use dependency; the degree of blockade was dependent on the presence of NMDA. The use-dependent onset of antagonism was, however, highly sensitive to the bath temperature. MK-801 applied in the absence of NMDA had no effect on the response to subsequent application of NMDA at 22°C, whereas it reduced the subsequent response to NMDA significantly at 37°C. MK-801 interacted with the receptor-ion channel complex even when Mg 2+, which is considered to block the open channel, had already blocked the NMDA-induced [Ca 2+] i. The recovery from blockade by MK-801 was not accelerated by the application of 10 mM Mg 2+ for 5 min. These results suggest that MK-801 can gain access to its binding site in the absence of NMDA at physiological temperature, and that this binding site is distinct from that for Mg 2+.

N-methyl-D-aspartate-evoked calcium uptake by kitten visual cortex maintained in vitro

As a functional measure of NMDA receptor effectiveness in kitten striate cortex, the uptake of 45Ca by visual cortical slices was measured after 2 minute bath applications of N-methyl-D-aspartate (NMDA). Significant Ca uptake occurred in response to 12.5-100 microM NMDA in slices prepared from visual cortex of normal animals aged 28-48 days. Basal uptake (in the absence of NMDA) was increased and evoked uptake was decreased in visual cortical slices prepared from age-matched dark-reared animals. Four days of binocular deprivation in otherwise normally reared animals had no effect on basal uptake, but significantly lowered NMDA-evoked Ca uptake at agonist concentrations greater than 25 microM. These data suggest that even brief manipulations of sensory experience are sufficient to alter visual cortical calcium regulation.

Argiotoxin-636 blocks effects of N-methyl- D-aspartate on lateral line of Xenopus laevis at concentrations which do not alter spontaneous or evoked neural activity

Activity of primary afferent neurons in acousticolateralis organs can be modulated by excitatory amino acids (EAA) and their antagonists, and EAA are among the better candidates for the transmitter(s) between hair cells and afferent neurons. Argiotoxin-636 (ATX) is a spider venom toxin that is a selective antagonist of the N-methyl- D-aspartate (NMDA) subtype of excitatory amino acid receptors in vertebrates. In the present study, the effects of NMDA alone and in combination with ATX on resting firing rate (spontaneous activity) and mechanically stimulated activity of lateral line afferent neurons of post-metamorphic Xenopus laevis were compared. Perfusion of NMDA (100 or 200 μM) typically produced a biphasic effect on spontaneous activity consisting of a transient increase followed by a decrease in firing rate. Mechanical stimulation applied after the initial response to NMDA produced an increase in firing activity that was not significantly different from the increase in firing activity evoked in the absence of NMDA. ATX at 1–2 μM did not alter either spontaneous or evoked activity, but blocked in a reversible manner both the excitatory and inhibitory actions of NMDA on spontaneous activity. These results suggest the presence of an NMDA receptor in the lateral line but do not support the hypothesis that the generation of action potentials by the afferent transmitter is solely dependent on activation of postsynaptic NMDA receptor-ion channels.

Formula omitted] promotes the survival of cerebellar granule cells: Pharmacological characterization

The survival of cerebellar granule cells in culture is promoted by chronic exposure to N- methyl- d-aspartate (NMDA). The effect is due to the stimulation of ‘conventional’ NMDA receptor-ionophore complex: it is concentration dependent, voltage dependent and blocked by the selective antagonists d-2-amino-5-phosphonovalerate, d-2-amino-7-phosphonoheptanoate, dextromethorphan and (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-iminemaleate (MK 801). The most potent antagonist tested was MK-801. In contrast, non-selective antagonists, including kynurenate, were much less effective. Further, the trophic effect of NMDA is not reproduced by ibotenate or quinolinate at the concentration range tested. It could also be shown that glutamate released into the culture medium is responsible for limited cell survival in the absence of NMDA.

The effect of the neuronal excitant N-methyl- d-aspartate on the metabolism of mouse brain amino acids labelled from [ 14C]bicarbonate and l-[U- 14C]aspartate

Summary [ 14 C]Bicarbonate and l -[U- 14 C]aspartate were injected intraventricularly into the brains of nembutalized mice, and the cerebral metabolism of each of these substrates compared with that occurring when the injection solution also contained the neuronal excitant, N -methyl- d -aspartate (NMDA). Over a 7-min period, during which the distribution of radioactivity was measured at 1-min intervals, NMDA caused an increased rate of loss of radioactivity newly incorporated into endogenous aspartate from [ 14 C]bicarbonate. A commensurate increase in the labelling of either glutamate or glutamine was not detected. These results are taken to indicate heterogeneity of the systems responsible for the metabolism of the injected bicarbonate. The major system would appear to involve the predominant part of the glutamate and glutamine labelled from this precursor and to be unaffected by the neuronal excitation effected by NMDA. This system may involve a substantial glial component. The minor system may be localized intraneuronally, and be associated with that particular aspartate pool, the turnover rate of which was shown previously, in [U- 14 C]glucose studies, to be greatly increased by the action of NMDA. Slight changes observed in the metabolism of l -[U- 14 C]aspartate in the presence and absence of NMDA, studied over a single 5-min period, could be interpreted in a similar way, but the differences were not statistically significant in this case.