Spinal neuronal pathology associated with continuous intrathecal infusion ofN-methyl-d-aspartate in the rat

Summary Continuous intrathecal infusion ofN-methyl-d-aspartate (NMDA) at the level of the lumbar enlargement of the spinal cord in middle-aged rats produced dose-dependent toxicity of spinal cord neuronal systems. Toxicity was enhanced by coadministration of glycine, but was significantly reduced when NMDA was coadministered with the competitive inhibitordl-2-amino-5-phosphovaleric acid or the noncompetitive inhibitor MgSO₄. The toxic effects of NMDA were manifest most dramatically and at the lowest concentrations in the neuropil, while neuronal loss was obvious at higher concentrations. The distribution and intensity of reactive astrocytosis was consistent with the known regional and subcellular distribution of NMDA receptors in the spinal cord of rats. The increase in ribosomes and rough endoplasmic reticulum observed in anterior horn cells suggested an increase of cell metabolism reflecting either a nonspecific response to injury or a specific increase in cell metabolism secondary to sustained activation of NMDA receptors. The present studies implicate excitatory amino acid receptors of the NMDA type in producing toxicity to selected neuronal populations of the spinal cord. This model provides a system for studies of the protective effects and rescue of neuronal populations susceptible to the toxic effects of excitatory amino acids.Supported by the ALS Society of Canada     

Effects ofl-cysteine-sulphinate andl-aspartate, mixed excitatory amino acid agonists, on the membrane potential of cat caudate neurons

Responses evoked byl-cysteine-sulphinate (l-CSA) andl-aspartate (l-Asp) were recorded with intracellular electrodes from caudate neurons in halothane anesthetized cats.l-CSA andl-Asp were applied microiontophoretically to caudate cells and their effects on membrane and action potentials, as well as on cortically evoked synaptic potentials were evaluated.l-CSA andl-Asp induced depolarizations accompanied by regular firing resembling kainate (KA)- or quisqualate (QUIS)-induced excitation patterns (type 1) in 82% and 72% of the recorded neurons, respectively, and a mixed pattern consisting of aN-methyl-d-aspartate (NMDA)-like excitation (type 2) followed by a regular type 1 pattern in the remaining cells. In about a quarter of the cells the effects ofl-CSA andl-Asp, but not those of KA or QUIS, were partially antagonized by 2-amino-7-phosphonoheptanoate (AP-7), a specific NMDA receptor antagonist. Kynurenate, a broad spectrum excitatory amino acid antagonist, blocked responses elicited by eitherl-CSA or QUIS. The actions ofl-CSA andl-Asp on the firing pattern and membrane potential of cat caudate neurons in situ provide evidence in favor of their mixed agonist nature with respect to NMDA and non-NMDA excitatory amino acid receptors.     

17Beta-estradiol blocks NMDA-induced increases in regional cerebral O(2) consumption

We tested the hypothesis that 17beta-estradiol would reduce the cerebral O(2) consumption response to stimulation of N-methyl-D-aspartate (NMDA) receptors. We determined NMDA receptor density in 10 ovariectomized Wistar female rats equally divided into a control group and 17beta-estradiol (500 microg/21 days) treated group. An autoradiographic assay using 125I-MK-801, an NMDA antagonist, was used to measure specific binding to NMDA receptors. Another 14 ovariectomized rats were separated into 17beta-estradiol and control groups to determine cerebral blood flow (14C-iodoantipyrine) and O(2) consumption (microspectrophotometry). 17Beta-estradiol caused a 20% decrease in specific binding to cortical NMDA receptors. After topical cortical stimulation with 10(-3)M and 10(-4)M NMDA, blood flow increased significantly in control from 73+/-5 in the saline treated cortex to 110+/-8 ml/min/100 g with 10(-3)M NMDA. In contrast, there was no significant change in blood flow in the 17beta-estradiol treated animals. Cerebral O(2) extraction increased significantly in the 10(-3)M NMDA treated cortex in both groups. Cerebral O(2) consumption in the control group significantly increased by 53%, from 3.7+/-0.2 to 5.7+/-0.5 with 10(-4)M NMDA and 72% to 6.4+/-2.4 ml O(2)/min/100 g with 10(-3)M NMDA. The 17beta-estradiol group demonstrated no significant difference between the saline treated and NMDA treated cortex. Thus, 17beta-estradiol blocked the effects of NMDA on cerebral O(2) consumption and this was associated with a slightly decreased number of NMDA receptors.     

Calcineurin inhibitors, FK506 and cyclosporin A, suppress the NMDA receptor-mediated potentials and LTP, but not depotentiation in the rat hippocampus

The effects of FK506, a Ca²⁺/calmodulin-dependent phosphatase 2B (calcineurin) inhibitor, on the NMDA receptor-mediated potentials and synaptic plasticity were investigated in the CA1 region of the rat hippocampus. Bath application of FK506 (50 μM) produced a 45% inhibition on the NMDA receptor-mediated potentials. FK506 also inhibited the induction of long-term potentiation (LTP), but had no effect on the depotentiation in the CA1 hippocampus. Cyclosporin A (100 μM), another calcineurin inhibitor, mimicked the effects of FK506 on the NMDA responses and synaptic plasticity. These results suggest that FK506 inhibits the activity of NMDA receptors via the involvement of calcineurin. The differential effects of FK506 on LTP and depotentiation may attribute to the partial inhibition on the activity of NMDA receptors and the subsequent attenuation of intracellular Ca²⁺ increase.     

Neuroprotection against NMDA excitotoxicity by group I metabotropic glutamate receptors is associated with reduction of NMDA stimulated currents

The neurotransmitter glutamate can have both excitotoxic and protective effects on neurons. The excitotoxic effects have been intensively studied, whereas the protective effects, including the involvement of metabotropic glutamate receptors (mGluRs), remain unclear. In the present study, we tested the protective effects of the group-I-mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) on organotypic hippocampal slice cultures exposed to excitotoxic concentrations of N-methyl-D-aspartate (NMDA). Effects of DHPG on electrophysiological responses induced by NMDA receptor activation were also recorded. Experiments were performed on organotypic hippocampal slice cultures derived from 7-day-old rats, with cellular uptake of propidium iodide as a marker for neuronal cell death. Slice cultures pretreated with DHPG (10 or 100 microM) for 2 h prior to exposure to 50 microM NMDA for 30 min displayed reduced propidium iodide uptake, compared to cultures exposed to NMDA only. The neuroprotective effect was confirmed by Hoechst 33342 staining, where the appearance of pycnotic nuclei after NMDA treatment was prevented by the DHPG pretreatment. Using caspase-3 activity to monitor the presence of apoptosis, failed to demonstrate this type of cell death in CA1 after NMDA application. The protective effect of DHPG was abolished by the mGluR1 selective antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385; 5 or 10 microM), whereas the mGluR5-selective antagonist 2-methyl-6-phenylethynylpyridine (MPEP; 1 microM) had no effect. Voltage-clamping of CA1 pyramidal cells in cultures treated with 10 microM DHPG for 2 h showed a significant depression of NMDA-induced inward currents compared to untreated controls. We conclude that neuroprotection induced by activation of group-I-mGluRs involve mGluR1 and is associated with decreased NMDA-stimulated currents.     

NMDA antagonists attenuate hypertension induced by carotid clamping in the rostral ventrolateral medulla of rats

The purpose of these experiments were to study the interactions of N-methyl-D-aspartate (NMDA) with baroreceptor reflexes induced by transient carotid clamping. Adult male Sprague-Dawley rats were anesthetized with urethane. Bilateral common carotid artery occlusion resulted in a reversible and reproducible hypertension in the vagotomized animals. This hypertensive reaction was blocked by intraventricular injection of NMDA antagonists, such as 2-amino-7-phosphono-heptaneoate (AP-7) and phencyclidine (PCP). We also found that blood pressure-sensitive neurons of the rostral ventrolateral medulla (RVLM) could be classified into two groups, on the basis of their responses to norepinephrine given intravenously. Using pressure microejection and single unit recording, we observed that clamping of the common carotids resulted in excitation of type I neurons. This evoked excitation, similar to that induced by NMDA, was blocked by locally applied AP-7. However, the carotid occlusion-induced responses of type II neurons were not blocked by AP-7. In conclusion, the present data suggest that NMDA receptors are involved in hypertensive responses during carotid occlusion, perhaps involving a site in the rostral ventrolateral medulla.     

Enhancement of NMDA receptor-mediated neurotoxicity in the hippocampal slice by depolarization and ischemia

Evidence from animal stroke models suggests that the proximate cause of neuronal degeneration after ischemia is massive release of glutamate and activation of NMDA receptors. However, in the physiologic presence of oxygen and glucose in the rat hippocampal slice preparation, the neurotoxicity of glutamate, as measured by inhibition of protein synthesis, requires high concentrations and is not prevented by glutamate receptor antagonists. Thus, the NMDA receptor-mediated neurotoxic effects of extracellular glutamate accumulation during ischemia might depend on additional factors, such as neuronal depolarization. In the experiments reported here, slices were exposed to glutamate in a medium intended to mimic the ionic conditions found during ischemia, high potassium (128 mM) and low sodium (26 mM). This depolarizing medium itself inhibited protein synthesis in a manner which was partially mediated by NMDA receptor activation, since it was significantly reversed by the noncompetitive NMDA antagonist, MK-801. Furthermore, the effect of glutamate under depolarizing conditions was also significantly decreased by MK-801, suggesting that glutamate was acting at NMDA receptors. Thus, depolarization appears to enhance the sensitivity of neurons to toxic NMDA receptor activation by glutamate. Under conditions that mimic ischemia, hypoxia plus hypoglycemia, a similar protective effect of NMDA receptor antagonists was observed. Depolarization and ischemia both appeared to attenuate the neurotoxicity of non-NMDA receptor agonists. It appears that under conditions of normal glucose and oxygen, high concentrations of bath applied glutamate inhibit protein synthesis at sites other than the NMDA receptor. However, when the Na+ gradient is decreased, as occurs during ischemia, glutamate's NMDA effects predominate. These findings suggest that ionic shifts may play a central role in permitting NMDA receptor-mediated ischemic neuronal damage.     

NMDA, kainate, and AMPA depolarize nondopamine neurons in the rat ventral tegmentum

The possible existence of N-methyl-d-aspartate (NMDA) and non-NMDA receptors on electrophysiologically identified nondopamine neurones in the ventral tegmental area (VTA) was tested in rat midbrain slice preparations. NMDA, kainate (KA), and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) depolarized the membrane potential of nondopamine neurons in a dose-dependent manner. The NMDA effect was blocked by the selective NMDA receptor antagonist, CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylate), but not by the non-NMDA receptor antagonist, NBOX [2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline]. In contrast, the effects of KA and AMPA were antagonized by NBOX, but not by CGS 19755. The rank order potency of the three agonists was AMPA > KA > NMDA, with thresholds of 0.1, 0.3, and 3 μM, respectively. These results provide clear electrophysiological evidence that nondopamine neurons in the ventral tegmental area possess both NMDA and non-NMDA receptors.     

NS-257, a novel competitive AMPA receptor antagonist, interacts with kainate and NMDA receptors

In this study, we examined the effects of a novel, water-soluble, putative competitive AMPA receptor antagonist, 1,2,3,6,7, 8-hexahydro-3-(hydroxyimino)-N,N,7-trimethyl-2-oxobenzo[2,1- b:3, 4-c']dipyrrole-5-sulfonamide (NS-257) on AMPA, kainate and NMDA receptors using the two-electrode voltage-clamp technique in Xenopus oocytes. All glutamate receptor subtypes were inhibited by NS-257 in a voltage-independent way. When kainate was applied to oocytes injected with total mouse brain mRNA, mainly AMPA receptors were activated. The antagonistic effects of NS-257 on these kainate-induced currents were concentration-dependent and competitive. In the same way, NS-257 blocked kainate-induced currents recorded from oocytes expressing homomeric GluR-1 receptors. In our experiments higher concentrations (>1 microM) of NS-257 also produced inhibitory effects on kainate and to a lesser extent on NMDA receptor function as indicated by recordings from GluR-6 or NR-1b/2A cRNA injected oocytes. While NMDA receptor function was inhibited in a competitive fashion, kainate responses recorded from homomeric GluR-6 receptors were blocked in a mixed competitive-noncompetitive manner. This mixed antagonistic action of NS-257 might have been caused by preincubating oocytes with concanavalin A, which blocks desensitization of kainate receptors. Although NS-257 appeared to be a less potent AMPA receptor antagonist then other known antagonists like NBQX, its main advantage over all other reported compounds so far is its higher aqueous solubility which still represents the major weakness of the other AMPA receptor antagonists, especially for clinical use.     

Ethanol-induced death of postnatal hippocampal neurons

Fetal alcohol exposure causes severe neuropsychiatric problems, but mechanisms of the ethanol-associated changes in central nervous system development are unclear. In vivo, ethanol's interaction with N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid type A (GABA(A)) receptors may cause increased apoptosis in the immature forebrain. We examined whether ethanol affects survival of neonatal hippocampal neurons in primary cultures. A 6-day ethanol exposure killed hippocampal neurons with an LD50 of approximately 25 mM. Elevated extracellular potassium or insulin-related growth factor 1 inhibited cell loss. Although potentiation of GABA(A) receptors or complete block of NMDA receptors also kills hippocampal neurons, pharmacological studies suggest that ethanol's interaction with GABA(A) and NMDA receptors is not sufficient to explain ethanol's effects on neuronal survival. Ca(2+) influx in response to depolarization was depressed >50% by chronic ethanol treatment. We suggest that chronic ethanol may promote neuronal loss through a mechanism affecting Ca(2+) influx in addition to effects on postsynaptic GABA and glutamate receptors.     

Stimulation of N-methyl-d-aspartate receptors induces apoptosis in rat brain

To evaluate the contribution of apoptotic mechanisms to excitotoxin-induced neurodegeneration as well as to characterize the glutamate receptor subtypes involved, biochemical and morphological effects of intrastriatally administered NMDA receptor agonist N-methyl-d-aspartate (NMDA) or quinolinic acid (QA) were studied. Receptor autoradiography showed that NMDA (75–300 nmol) caused a loss of 18–68% of striatal D₁ dopamine (DA) and 10–43% of NMDA receptors 7 days after drug administration. Treatment with QA (60–240 nmol) also led to a loss of 60–73% of D₁ DA and 37–44% of NMDA receptors in the ipsilateral striatum. Agarose gel electrophoresis revealed that both NMDA and QA induced internucleosomal DNA fragmentation in the striatum 12 to 48 h after drug administration. NMDA- and QA-induced internucleosomal DNA fragmentation was attenuated by the protein synthesis inhibitor cycloheximide in a dose-dependent manner. Terminal transferase-mediated deoxyuridine triphosphate (d-UTP)-digoxigenin nick end labeling (TUNEL)-positive nuclei were found in the ipsilateral striatum in response to NMDA or QA treatment. In addition, many fragmented nuclei were observed in the NMDA or QA-treated striatum and propidium iodide staining showed profound nuclear condensation in the NMDA or QA-treated striatum. NMDA- and QA-induced internucleosomal DNA fragmentation and TUNEL-positive nuclei as well as nuclear condensation were abolished by the NMDA receptor antagonist MK-801, but not by the AMPA/KA receptor antagonist NBQX. MK-801, but not NBQX, also prevented NMDA or QA-induced striatal cell death. These results suggest that apoptotic mechanisms are involved in excitotoxin-induced striatal cell death. The initiation of an apoptotic cascade by NMDA or QA appears to be mediated by stimulation of NMDA but not AMPA/KA receptors.     

Involvement of vasoactive intestinal polypeptide in NMDA-induced phase delay of firing activity rhythm in the suprachiasmatic nucleus in vitro

Glutamate has been reported to be involved in the transmission of photic information from the retina to the suprachiasmatic nucleus (SCN). Therefore, we investigated whether the application of N-methyl-d-aspartate (NMDA), a glutamate receptor agonist could, reset the circadian rhythm of SCN firing activity in vitro. Treatment with NMDA for 1 h between projected zeitgeber time (ZT) 13–14 produced a phase delay in a concentration-dependent manner. The NMDA-induced phase delay was antagonized by an NMDA-receptor antagonist, MK-801 (100 μM). The retinohypothalamic tract has been reported to make terminals on neurons possessing vasoactive intestinal polypeptide (VIP). Therefore, we investigated the effects of NMDA on VIP release from the SCN and on VIP immunoreactivity in the SCN. Application of NMDA for 15 min between ZT 13–15 increased release of VIP from the SCN. In contrast to release, the content of VIP in the SCN tissue was reduced by application of NMDA. Immunohistochemical analysis revealed that application of NMDA for 4 h or 1 h reduced VIP immunoreactivity in the SCN. To investigate the possibility that VIP released by NMDA could reset SCN neuronal activity, we examined the effects of VIP on the SCN neuronal activity rhythm. Cotreatment with VIP (1 μM) and gastrin-releasing peptide (1 μM) for 1 h between ZT 13–14 caused a phase-delay of SCN activity rhythm. These findings suggest that activation of NMDA receptors during early subjective night causes a phase delay of the SCN neuronal activity via facilitation of VIP release in this nucleus.     

N-Methyl-D-aspartate microinjected into the suprachiasmatic nucleus mimics the phase-shifting effects of light in the diurnal Nile grass rat (Arvicanthis niloticus)

Mammals exhibit circadian rhythms in behavior generated by the suprachiasmatic nucleus (SCN). Exposure to light synchronizes the circadian clock to the environmental light:dark cycle through the release of glutamate into the SCN. In nocturnal animals such as Syrian hamsters, direct application of NMDA to the SCN results in phase shifts similar to those produced by exposure to light. This study was designed to determine if light phase shifts the circadian pacemaker of diurnal Nile grass rats (Arvicanthis niloticus) housed in constant darkness by acting on NMDA-type glutamate receptors in the suprachiasmatic nucleus (SCN). N-Methyl-D-aspartate (NMDA; 0, 1, 10, 50, and 100 mM) was administered through guide cannulae aimed at the SCN at circadian times when light induces phase shifts. Maximal phase delays were attained with 50 mM NMDA, and maximal phase advances were seen after 100 mM NMDA. A phase-response curve (PRC) for NMDA, determined by administering NMDA at each hour over the circadian cycle, resembled the PRC to light in this species. These data support the hypothesis that NMDA-type glutamate receptors play a critical role in mediating the phase shifting effects of light in diurnal, as well as nocturnal, animals. In addition, these data suggest that diurnal grass rats may be less sensitive to the phase shifting properties of NMDA than nocturnal rodents.     

Metabotropic glutamate receptor activation selectively limits excitotoxic damage in the intact neostriatum

The present study was designed to compare the protective consequences of activation of metabotropic glutamate receptors (mGluRs) onN-methyl-d-aspartate (NMDA)- and kainic acid (KA)-induced excitotoxicity in vivo. Pretreatment with the mGluR agonist 1SR,3RS-1-aminocyclo-pentane-1,3-dicarboxylic acid (tACPD) limited the anatomical and behavioral consequences of the intrastriatal administration of the NMDA agonist quinolinic acid (QA). In contrast, pretreatment with tACPD did not alter the effects of intrastriatal injection of KA.     

NMDA Receptors in glia.

The amino acid L-Glutamate acts as the most ubiquitous mediator of excitatory synaptic transmission in the central nervous system. Glutamatergic transmission is central for diverse brain functions, being particularly important for learning, memory, and cognition. In brain pathology, excessive release of glutamate triggers excitotoxic neural cell death through necrotic or apoptotic pathways. Glutamate effects are mediated by several classes of glutamate receptors, expressed in virtually all cells of neural origin. Specifically important for both physiological information processing and cell damage are glutamate receptors of NMDA (N-methyl-D-aspartate) type, which, for a long time, were considered to be expressed exclusively in neurons. Recent studies have found functional NMDA receptors in brain macroglia, in astrocytes, and oligodendrocytes. Glial and neuronal NMDA receptors are functionally and structurally different; the glial receptors are weakly (if at all) sensitive to the extracellular magnesium block, which may indicate a predominant expression of the NR3 receptor subunit. In the cortex, astroglial NMDA receptors are activated upon physiological synaptic transmission. The physiological relevance of NMDA receptors in the white matter remains unknown; their activation upon ischemia triggers Ca(2+)-dependent damage of oligodendrocytes and myelin. The discovery of glial NMDA receptors further indicates the complex nature of intercellular signaling mechanisms in the brain, which involve all types of neural cells, connected through diverse types of chemical and electrical synapses.     

A novel interaction between dynorphin(1–13) and an N-methyl-d-aspartate site

Dynorphin injected intrathecally in the rat results in a neurotoxicity behaviorally expressed as an irreversible loss of the thermally evoked tail-flick reflex. The excitatory amino acid antagonists DL-2-amino-5-phosphonovalerate (APV) and gamma-D-glutamylglycine (DGG) blocked the loss of the tail-flick reflex. The order of potency (APV greater than DGG) suggests that the N-methyl-D-aspartate (NMDA) subclass of excitatory amino acid receptors participate in the neurotoxicity. Additionally, intrathecal injection of APV results in a reversible loss of the tail-flick reflex, whereas with DGG doses which block the tail-flick reflex also result in hindlimb paralysis. These data suggest that neurotransmission in the tail-flick reflex pathway is, in part, mediated by NMDA receptors. From these and previous findings it was concluded that dynorphin neurotoxicity results from enhanced, excitotoxic, transmission across these synapses utilizing NMDA receptors.     

Dynorphin A (1–13) Neurotoxicity in Vitro: Opioid and Non-Opioid Mechanisms in Mouse Spinal Cord Neurons

Dynorphin A is an endogenous opioid peptide that preferentially activates κ-opioid receptors and is antinociceptive at physiological concentrations. Levels of dynorphin A and a major metabolite, dynorphin A (1–13), increase significantly following spinal cord trauma and reportedly contribute to neurodegeneration associated with secondary injury. Interestingly, both κ-opioid and N-methyl- -aspartate (NMDA) receptor antagonists can modulate dynorphin toxicity, suggesting that dynorphin is acting (directly or indirectly) through κ-opioid and/or NMDA receptor types. Despite these findings, few studies have systematically explored dynorphin toxicity at the cellular level in defined populations of neurons coexpressing κ-opioid and NMDA receptors. To address this question, we isolated populations of neurons enriched in both κ-opioid and NMDA receptors from embryonic mouse spinal cord and examined the effects of dynorphin A (1–13) on intracellular calcium concentration ([Ca²⁺]_i) and neuronal survival in vitro. Time-lapse photography was used to repeatedly follow the same neurons before and during experimental treatments. At micromolar concentrations, dynorphin A (1–13) elevated [Ca²⁺]_i and caused a significant loss of neurons. The excitotoxic effects were prevented by MK-801 (Dizocilpine) (10 μM), 2-amino-5-phosphopentanoic acid (100 μM), or 7-chlorokynurenic acid (100 μM)—suggesting that dynorphin A (1–13) was acting (directly or indirectly) through NMDA receptors. In contrast, cotreatment with (−)-naloxone (3 μM), or the more selective κ-opioid receptor antagonist nor-binaltorphimine (3 μM), exacerbated dynorphin A (1–13)-induced neuronal loss; however, cell losses were not enhanced by the inactive stereoisomer (+)-naloxone (3 μM). Neuronal losses were not seen with exposure to the opioid antagonists alone (10 μM). Thus, opioid receptor blockade significantly increased toxicity, but only in the presence of excitotoxic levels of dynorphin. This provided indirect evidence that dynorphin also stimulates κ-opioid receptors and suggests that κ receptor activation may be moderately neuroprotective in the presence of an excitotoxic insult. Our findings suggest that dynorphin A (1–13) can have paradoxical effects on neuronal viability through both opioid and non-opioid (glutamatergic) receptor-mediated actions. Therefore, dynorphin A potentially modulates secondary neurodegeneration in the spinal cord through complex interactions involving multiple receptors and signaling pathways.     

Reduced density of NMDA receptors and increased sensitivity to dizocilpine-induced learning impairment in aged rats

About 20 min prior to training in a shock-motivated 14-unit T-maze, young (3-4 months) and aged (24-25 months) male Fischer-344 rats were given s.c. injections of either saline or dizocilpine (MK-801, 0.02 or 0.04 mg/kg), a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. The aged rats showed a dose-dependent impairment in maze performance. Deficiencies were manifested as increases in errors, in runtime from start to goal, and in the number and duration of shocks received. In contrast, young rats exhibited no detrimental effects of dizocilpine on maze performance. Analysis of [3H]glutamate binding in these rats revealed a marked age-related decline in NMDA receptor binding in hippocampus. A significant correlation was observed between errors in the maze and hippocampal [3H]-glutamate binding, but the correlation was positive, i.e., rats that made the most errors had the highest level of NMDA receptor binding. Thus, compared to young rats, aged rats were more sensitive to the behavioral effects of NMDA receptor antagonism and they showed a hippocampal loss of [3H]glutamate in binding, which may be related to the increased sensitivity to dizocilpine. The positive correlation between poor maze performance and NMDA receptor binding suggests that the behaviors assessed involve complex interactions between NMDA receptors and other neuronal systems in the hippocampus.     

Differential blockade of early and late components of acoustic startle following intrathecal infusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) ord,l-2-amino-5-phosphonovaleric acid (AP-5)

The present study investigated the individual contributions of spinal cord N-methyl-d-aspartate (NMDA) and non-NMDA receptors to the acoustic startle reflex in rats. The first experiment measured whole body acoustic startle before and after intrathecal infusion of various doses of either the NMDA receptor antagonist,d,l-2-amino-5-phosphonovaleric acid (AP-5), or the non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Both compounds depressed startle in a dose-dependent fashion with similar potencies. A second experiment measured startle electromyographically (EMG) in the quadriceps femoris muscle complex in the hindlimbs during auditory stimulation to characterize the effects of these two compounds on the early ( 8ms) or late ( 15ms) EMG components of the startle response. CNQX preferentially blocked the early EMG component of startle, whereas AP-5 preferentially blocked the late component. These results suggest that the acoustic startle reflex involves an early EMG component mediated by spinal non-NMDA receptors, and a late EMG component mediated by spinal NMDA receptors.