[3H]D-2-amino-5-phosphonopentanoate as a ligand for N-methyl-D-aspartate receptors in the mammalian central nervous system

Prolonged synaptic currents increase relay neuron firing at the developing retinogeniculate synapse

The retinogeniculate synapse, the connection between retinal ganglion cells (RGC) and thalamic relay neurons, undergoes robust changes in connectivity over development. This process of synapse elimination and strengthening of remaining inputs is thought to require synapse specificity. Here we show that glutamate spillover and asynchronous release are prominent features of retinogeniculate synaptic transmission during this period. The immature excitatory postsynaptic currents exhibit a slow decay time course that is sensitive to low-affinity glutamate receptor antagonists and extracellular calcium concentrations, consistent with glutamate spillover. Furthermore, we uncover and characterize a novel, purely spillover-mediated AMPA receptor current from immature relay neurons. The isolation of this current strongly supports the presence of spillover between boutons of different RGCs. In addition, fluorescence measurements of presynaptic calcium transients suggest that prolonged residual calcium contributes to both glutamate spillover and asynchronous release. These data indicate that, during development, far more RGCs contribute to relay neuron firing than would be expected based on predictions from anatomy alone.

Relative roles of different mechanisms of depression at the mouse endbulb of Held

Several mechanisms can underlie short-term synaptic depression, including vesicle depletion, receptor desensitization, and changes in presynaptic release probability. To determine which mechanisms affect depression under physiological conditions, we studied the synapse formed by auditory nerve fibers onto bushy cells in the anteroventral cochlear nucleus (the "endbulb of Held") using voltage-clamp recordings of brain slices from P15-P21 mice near physiological temperatures. Depression of both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) excitatory postsynaptic currents (EPSCs) showed two phases of recovery. The fast component of depression for the AMPA EPSC was eliminated by cyclothiazide and aniracetam, suggesting it results from desensitization. The fast component of depression for the NMDA EPSC was reduced by the low-affinity antagonist l-AP5, suggesting it results from saturation. The remaining depression in AMPA and NMDA components is identical and therefore presynaptic in origin. It is likely to result from presynaptic vesicle depletion. Recovery from depression after trains of activity was slowed by the application of EGTA-AM, suggesting that the endbulb has a residual-calcium-dependent form of recovery. We developed a model that incorporates depletion, desensitization, and calcium-dependent recovery. This model replicated experimental findings over a range of experimental conditions. The model further indicated that desensitization plays only a minor role during prolonged activity, in large part because presynaptic release is so depleted. Thus depletion appears to be the dominant mechanism of depression at the endbulb during normal activity. Furthermore, calcium-dependent recovery at the endbulb is critical to prevent complete rundown during high activity and to preserve the reliability of information transmission.

Pattern-specific synaptic mechanisms in a multifunctional network. II. Intrinsic modulation by metabotropic glutamate receptors

The in vitro respiratory network contained in the transverse brain stem slice of mice simultaneously generates fast (approximately 15 min(-1)) and slow ( approximately 0.5 min(-1)) rhythmic activities corresponding to fictive eupnea ("normal" breathing) and fictive sighs. We show that these two activity patterns are differentially controlled through the modulatory actions of metabotropic glutamate receptors (mGluRs). Sighs were selectively inhibited by agonists of the group III mGluRs according to a pharmacological profile most consistent with activation of mGluR8. Sighs were also blocked by the supposedly inactive L-isomer of the widely used N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanoic acid (L-AP5, 5 microM), an effect that was abolished in the presence of group III mGluR antagonists. Excitatory postsynaptic potentials (EPSPs) were recorded in pre-Bötzinger Complex neurons after stimulation of the contralateral ventral respiratory group (VRG); evoked EPSP amplitude was variably reduced after bath application of the group III agonist L-serine-O-phosphate (L-SOP), with an average reduction of 15%. Therefore although group III mGluRs do play a role in regulating synapse strength, this seems to be only a minor factor in the regulation of synapses made by midline-crossing axons. Intrinsic modulation of the respiratory central pattern generator by mGluRs appears to be an essential component of the multifunctionality that characterizes this network.

Hindbrain administration of NMDA receptor antagonist AP-5 increases food intake in the rat

Hindbrain administration of MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) channel blocker, increases meal size, suggesting NMDA receptors in this location participate in control of food intake. However, dizocilpine (MK-801) reportedly antagonizes some non-NMDA ion channels. Therefore, to further assess hindbrain NMDA receptor participation in food intake control, we measured deprivation-induced intakes of 15% sucrose solution or rat chow after intraperitoneal injection of either saline vehicle or D(-)-2-amino-5-phosphonopentanoic acid (AP5), a competitive NMDA receptor antagonist, to the fourth ventricular, or nucleus of the solitary tract (NTS). Intraperitoneal injection of AP5 (0.05, 0.1, 1.0, 3.0, and 5.0 mg/kg) did not alter 30-min sucrose intake at any dose (10.7 +/- 0.4 ml, saline control) (11.0 +/- 0.8, 11.2 +/- 1.0, 11.2 +/- 1.0, 13.1 +/- 2.2, and 11.0 +/- 1.9 ml, AP5 doses, respectively). Fourth ventricular administration of both 0.2 mug (16.7 +/- 0.6 ml) and 0.4 mug (14.9 +/- 0.5 ml) but not 0.1 and 0.6 mug of AP5 significantly increased 60-min sucrose intake compared with saline (11.2 +/- 0.4 ml). Twenty-four hour chow intake also was increased compared with saline (AP5: 31.5 +/- 0.1 g vs. saline: 27.1 +/- 0.6 g). Furthermore, rats did not increase intake of 0.2% saccharin after fourth ventricular AP5 administration (AP5: 9.8 +/- 0.7 ml, vs. saline: 10.5 +/- 0.5 ml). Finally, NTS AP5 (20 ng/30 nl) significantly increased 30- (AP5: 17.2 +/- 0.7 ml vs. saline: 14.6 +/- 1.7 ml), and 60-min (AP5: 19.4 +/- 0.6 ml vs. saline: 15.5 +/- 1.4 ml) sucrose intake, as well as 24-h chow intake (AP5: 31.6 +/- 0.3 g vs. saline: 26.1 +/- 1.2 g). These results support the hypothesis that hindbrain NMDA receptors participate in control of food intake and suggest that this participation also may contribute to control of body weight over a 24-h period.

Direct binding properties of conantokins to native N-methyl-d-aspartate receptors

Conantokin-G (con-G) is a small, gamma-carboxyglutamic acid (Gla)-containing peptide that functions neurophysiologically by inhibiting the N-methyl-d-aspartate receptor (NMDAR). In the current study, the receptor binding properties of an alanine-rich, Gla-deficient con-G variant, Ala-con-G, were assessed following tracer radioiodination with 125I. Direct binding experiments with [125I]Ala-con-G yielded a single site defined by a Kd value of 516 +/- 120 nm. Displacement of [125I]Ala-con-G binding by Ala-con-G resulted in 100% displacement with an IC50 value of 564 +/- 33 nm, while heterologous displacement by con-G[S16Y], con-G, con-T, and con-R[1-17] yielded IC50 values in the range of 15-45 microm. No displacement was observed with d-gamma-con-G or con-G[L5A], analogs that are inactive at NMDARs. Specific [125I]Ala-con-G binding was displaced by NMDA and 2-amino-5-phosphopentanoic acid in a dose-dependent manner, suggesting an interaction at the glutamate binding site. The direct binding of [125I]Ala-con-G to adult rat brain sections revealed an anatomical distribution of binding sites in all regions known to contain the NR2B subunit of the NMDAR. These results constitute the only known demonstration of the direct binding of a radiolabeled conantokin to the NMDARs present in rat brain membrane preparations and rat brain sections, and suggest that radiolabeled Ala-con-G, and similar conantokin derivatives, may find utility as probes of NMDARs in a variety of systems.

Enkephalin Expression in Spinal Cord Neurons is Modulated by Drugs Related to Classical and Peptidergic Transmitters

The effects of various neurotransmitter agonists and antagonists on the synthesis and release of methionine enkephalin (mENK) in neuronal cultures of mouse spinal cord and dorsal root ganglia have been measured. Blockade of electrical activity with tetrodotoxin between days 9 and 13 in culture caused a > 95% decrease in the number of mENK-immunoreactive neurons. This effect was also seen upon the blockade of glycine and beta-adrenergic receptors with strychnine and propranolol, respectively, and stimulation of GABA receptors with muscimol. Stimulation of beta-adrenergic receptors with isoproterenol, or blockade of glutamate and GABA receptors with 2-aminophosphonovalerate and strychnine, respectively, had a qualitatively opposite action on both the number of mENK-immunoreactive neurons and enkephalin peptide levels measured by radioimmunoassay. Application of substance P also enhanced the mENK cell number. These data suggest that, at least in the spinal cord, characteristics other than the average level of impulse activity in the afferent input may be critical to the regulation of expression of mENK.

Contributions of receptor desensitization and saturation to plasticity at the retinogeniculate synapse

The retinogeniculate synapse conveys visual information from the retina to thalamic relay neurons. Here, we examine the mechanisms of short-term plasticity that can influence transmission at this connection in mouse brain slices. Our studies show that synaptic strength is modified by physiological activity patterns due to marked depression at high frequencies. Postsynaptic mechanisms of plasticity make prominent contributions to this synaptic depression. During trains of retinal input stimulation, receptor desensitization attenuates the AMPA EPSC while the NMDA EPSC saturates. This differential plasticity may help explain the distinct roles of these receptors in shaping the relay neuron response to visual stimulation with the AMPA component being important for transient responses, while sustained high frequency responses rely more on the NMDA component.

The synthetic enantiomer of pregnenolone sulfate is very active on memory in rats and mice, even more so than its physiological neurosteroid counterpart: distinct mechanisms?

The demonstration that the neurosteroid pregnenolone sulfate (PREGS) is active on memory function at both the physiological and pharmacological levels led to us examining in detail the effects of the steroid on spatial working memory by using a two-trial recognition task in a Y-maze, a paradigm based on the natural drive in rodents to explore a novel environment. Dose-response studies in young male adult Sprague-Dawley rats and Swiss mice, after the postacquisition intracerebroventricular injection of steroid, showed an U-inverted curve for memory performance and indicated a greater responsiveness in rats compared with mice. Remarkably, the synthetic (-) enantiomer of PREGS not only also displayed promnesiant activity, but its potency was 10 times higher than that of the natural steroid. Intracerebroventricular coadministration experiments with DL-2-amino-5-phosphonovaleric acid, a competitive selective antagonist of the N-methyl-D-aspartate receptor, abolished the memory-enhancing effect of PREGS, but not that of the PREGS enantiomer, evoking enantiomeric selectivity at the N-methyl-d-aspartate receptor and/or different mechanisms for the promnestic function of the two enantiomers.

Effects of sevoflurane on dopamine, glutamate and aspartate release in an in vitro model of cerebral ischaemia

Release of excitatory amino acids and dopamine plays a central role in neuronal damage after cerebral ischaemia. In the present study, we used an in vitro model of ischaemia to investigate the effects of sevoflurane on dopamine, glutamate and aspartate efflux from rat corticostriatal slices. Slices were superfused with artificial cerebrospinal fluid at 34 degrees C and episodes of 'ischaemia' were mimicked by removal of oxygen and reduction in glucose concentration from 4 to 2 mmol litre(-1) for < or = 30 min. Dopamine efflux was monitored in situ by voltammetry while glutamate and aspartate concentrations in samples of the superfusate were measured by HPLC with fluorescence detection. Neurotransmitter outflow from slices was measured in the absence or presence of sevoflurane (4%). After induction of ischaemia in control slices, there was a mean (SEM) delay of 166 (7) s (n = 5) before sudden efflux of dopamine which reached a maximum extracellular concentration of 77.0 (15.2) micromol litre(-1). Sevoflurane (4%) reduced the rate of dopamine efflux during ischaemia (6.90 (1.5) and 4.73 (1.76) micromol litre(-1) s(-1) in controls and sevoflurane-treated slices, respectively; P<0.05), without affecting its onset or magnitude. Excitatory amino acid efflux was much slower. lschaemia-induced glutamate efflux had not reached maximum after 30 min of ischaemia. Basal (pre-ischaemic) glutamate and aspartate efflux per slice was 94.8 (24.8) and 69.3 (31.5) nmol litre(-1) superfusate (n = 4) and was not significantly reduced by 4% sevoflurane. lschaemia greatly increased glutamate and aspartate efflux (to a maximum of 919 (244)% and 974 (489)% of control, respectively). However, ischaemia-induced efflux of both glutamate and aspartate was significantly reduced by 4% sevoflurane (P < 0.001 for glutamate, P < 0.01 for aspartate). In summary, sevoflurane may owe part of its reported neuroprotective effect to a reduction of ischaemia-induced efflux of excitatory amino acids and, to a lesser extent, dopamine.

Spontaneous openings of NMDA receptor channels in cultured rat hippocampal neurons

Spontaneous and N-methyl-D-aspartate (NMDA)-evoked single-channel currents were studied in outside-out patches isolated from cultured rat hippocampal neurons. Both spontaneous and NMDA-evoked single-channel currents reversed at potentials close to 0 mV and exhibited multiple amplitude levels of similar amplitude. Both spontaneous and NMDA-evoked single-channel currents were inhibited by Mg2+ in a voltage-dependent manner and by 7-chlorokynurenic acid. The activity of spontaneous single-channel currents was reduced by the competitive NMDA receptor antagonists, but by one to three orders of magnitude less than expected assuming that the spontaneous activity is due to an ambient NMDA receptor agonist present in the extracellular solution. Our results suggest that, similar to other ligand-gated ion channels, NMDA receptor channels have a dual mode of activation--spontaneous and agonist induced.

Infusion of NMDA antagonists into the nucleus reticularis pontis oralis inhibits the maximal electroshock seizure response

The nucleus reticularis pontis oralis (RPO) is necessary for the expression of tonic hindlimb extension (THE) in maximal electroshock (MES) seizures of rats. Previous work in this laboratory has demonstrated that both systemic administration and focal RPO microinfusion of D-cycloserine inhibits THE. The purpose of the present study was to characterize specific components of the NMDA receptor/ionophore complex that regulate the anticonvulsant activity mediated by the RPO. Bilateral RPO microinfusion of the competitive NMDA antagonists (-)AP7 and D-CPP as well as the uncompetitive antagonist dizocilpine ((+)MK-801) inhibited THE in a dose-related fashion. Bilateral RPO microinfusion of NMDA did not affect the THE response to MES but did induce convulsions resembling audiogenic seizures in genetically epilepsy prone rats. Bilateral RPO microinfusion of the strychnine-insensitive glycine site partial agonist D-cycloserine and the antagonist 5,7-dichlorokynurenic acid inhibited THE. The strychnine-insensitive glycine partial agonists (+)HA-966 and ACPC, as well as the agonists glycine and D-serine, did not significantly affect the THE response. Strychnine microinfusions in the RPO had no effect on THE. The results support a hypothesis that the RPO is a site of anticonvulsant drug action in MES and indicate that either competitive or uncompetitive NMDA antagonist action regulates the anticonvulsant activity mediated by the RPO. The role of the strychnine-insensitive glycine site in the regulation of the anticonvulsant activity medicated by the RPO is uncertain.

Excitatory amino acid-induced cytotoxicity in primary cultures of mouse cerebellar granule cells correlates with elevated, sustained c-fos proto-oncogene expression

An elevated, sustained expression of c-fos mRNA was found in primary cultures of mouse cerebellar granule cells following exposure to toxic concentrations of the excitatory amino acids, L-glutamate, L-homocysteate, S-sulpho-L-cysteine and N-methyl-D-aspartate (NMDA), using leakage of lactate dehydrogenase (LDH) as an indicator of cytotoxicity. In contrast, when used at non-toxic concentrations these compounds induced a rapid and transient increase in c-fos mRNA levels. Both LDH release and elevated, sustained c-fos mRNA induction were blocked (in the case of L-homocysteate) or reduced (in the case of L-glutamate and S-sulpho-L-cysteine) by the selective NMDA receptor antagonist (DL(+/-)-2-amino- 5-phosphonopentanoic acid) whereas 6-cyano-7-nitroquinoxaline-2,3-dione (a selective antagonist at non-NMDA ionotropic receptors) had no effect. These data suggest a role for altered c-fos mRNA expression in excitotoxic mechanisms.

Oxygen radical-induced neurotoxicity in spinal cord neuron cultures

The neurotoxic effects of oxygen radicals on spinal cord neuron cultures derived from fetal mouse have been studied. Oxygen radicals, superoxide radical and hydrogen peroxide, were generated by adding xanthine oxidase and hypoxanthine in the culture medium. Exposure of neurons to this oxygen radical-generating system resulted in a significant cell death and decrease of choline acetyltransferase (ChAT) activity in a time-dependent manner in spinal cord neuron cultures. The decrease in cell viability and ChAT enzyme activity induced by the oxygen radicals was blocked by scavengers such as superoxide dismutase (SOD), catalase, and tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), a metal chelator. Antagonists of the N-methyl-D-aspartate (NMDA) receptor, including MK801 (a noncompetitive NMDA antagonist), D-2-amino-5-phosphonovaleric acid (APV) (a competitive NMDA antagonist), and 7-chlorokynurenic acid (an antagonist at the glycine site associated with the NMDA receptor), similarly blocked oxygen radical-induced decrease in cell viability and ChAT activity in spinal cord neuron cultures. These results indicate that both oxygen radicals and excitotoxic amino acids were involved in the oxidant-initiated neurotoxicity of spinal cord neurons.

Traumatic brain injury-induced excitotoxicity assessed in a controlled cortical impact model

Using a controlled cortical impact model of traumatic brain injury (TBI) coupled with tissue microdialysis, interstitial concentrations of aspartate and glutamate (together with serine and glutamine) were assessed in rat frontal cortex. Histological analysis indicated that the severity of injury following severe TBI (depth of deformation = 3.5 mm) was approximately twice that occurring following moderate TBI (depth of deformation = 1.5 mm). Both groups demonstrated significant postinjury maximal increases in excitatory amino acid (EAA) concentration, which were proportional to the severity of injury. The mean +/- SEM fold increase in dialysate concentrations of aspartate was 38 +/- 13 (n = 5) for moderate TBI and 74 +/- 12 (n = 5) for severe TBI. Fold increases in glutamate concentrations were 81 +/- 26 and 144 +/- 23 for moderate and severe TBI, respectively. Although these increases normalized within 20-30 min following moderate TBI, concentrations of aspartate and glutamate took > 60 min to normalize after severe TBI. Changes in levels of nontransmitter amino acids were much smaller. Fold increases for serine concentrations were 4.6 +/- 0.6 and 7.6 +/- 1.7 in moderate and severe TBI, respectively; glutamine concentrations had similar small fold increases (2.6 +/- 0.2 and 4.1 +/- 0.6, respectively). Calculation of interstitial concentrations following severe TBI indicated that aspartate and glutamate maximally increased to 123 +/- 20 and 414 +/- 66 microM, respectively. To determine the extent to which such tissue concentrations of EAAs could contribute to the injury seen in TBI, the EAA receptor agonists N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid were slowly injected into rat cortex. Remarkably similar histological injuries were produced by this procedure, supporting the notion that TBI is an excitotoxic injury.

Selective reduction of N-methyl-D-aspartate-evoked responses by 1,3-di(2-tolyl)guanidine in mouse and rat cultured hippocampal pyramidal neurones

1. The effects of 1,3-di(2-tolyl)guanidine (DTG) were examined on the responses of cultured hippocampal neurones to the excitatory amino acid analogues N-methyl-D-aspartate (NMDA), kainate, quisqualate and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). 2. In rat hippocampal neurones loaded with the Ca(2+)-sensitive dye Fura-2, DTG (10-100 microM) produced a concentration-dependent depression of the NMDA-evoked rises in intracellular free calcium ([Ca2+]i), an effect that was not modified by changes in the extracellular glycine concentration. DTG (at 50 and 100 microM) also attenuated, although to a lesser extent, the rises in [Ca2+]i evoked by naturally-derived quisqualate. In contrast, 50 and 100 microM DTG did not depress responses evoked by kainate, AMPA and synthetic, glutamate-free (+)-quisqualate although on occasions DTG enhanced kainate- and AMPA-evoked rises in [Ca2+]i. 3. DTG attenuated NMDA-evoked currents recorded from mouse hippocampal neurones under whole-cell voltage-clamp with an IC50 (mean +/- s.e. mean) of 37 +/- 5 microM at a holding potential of -60 mV. The DTG block of NMDA-evoked responses was not competitive in nature and was not dependent on the extracellular glycine or spermine concentration. The block did, however, exhibit both voltage-, and use-, dependency. The steady-state current evoked by naturally-derived quisqualate was also attenuated by DTG whereas those evoked by kainate and AMPA were not. 4. We conclude that DTG, applied at micromolar concentrations, is a selective NMDA antagonist in cultured hippocampal neurones, the block exhibiting both Mg(2+)- and phencyclidine-like characteristics. Given the nanomolar affinity of DTG for sigma binding sites it is unlikely that the antagonism observed here is mediated by sigma-receptors, but the data emphasize the potential danger of ascribing the functional consequences of DTG administration solely to sigma receptor-mediated events.

Do NMDA receptors have a critical function in visual cortical plasticity?

The theoretical framework, by which we understand the function of NMDA receptors, is derived, in large part, from work conducted on the hippocampal slice preparation, where NMDA receptors are crucial for a form of synaptic plasticity known as long-term potentiation (LTP). Establishing their role in plasticity mechanisms in the neocortex is proving to be far more difficult than originally envisaged, in part due to the fact that the operation of NMDA receptors is different in the intact animal than in vitro. For example, NMDA receptors are activated at low levels of sensory input in intact animals but only by high levels of input in slice preparations. Recent results suggest that a re-evaluation of the role of NMDA receptors in neocortical plasticity is required. Here we discuss some of the issues and introduce four criteria by which any factor supposedly involved in plasticity can be judged. NMDA receptors fulfill more of these criteria than any of the other factors so far investigated in the visual cortex, but maybe this is because they have been studied more intensively.

Cortically evoked excitatory synaptic transmission in the cat red nucleus is antagonised by D-AP5 but not by D-AP7

Extracellular recordings were made from magnocellular red nucleus neurons (mRN) in alpha-chloralose (50 mg/kg, iv.) anaesthetised cats. Iontophoretically applied N-methyl-D-aspartate (NMDA) excited the neuronal firing which was antagonised by 4 selective NMDA receptor antagonists: 2-amino-5-phosphonopentanoate (AP5), 2-amino-7-phosphonoheptanoate (AP7), RS-4-(phosphonomethyl) piperazine-2-carboxylic acid (PMPC) and R-4-(3-phosphonopropyl) piperazine-2-carboxylic acid (CPP), whereas AMPA responses were uneffected. Monosynaptic excitatory responses were produced by stimulation of the sensorimotor cortex. These responses were reduced and often abolished by AP5 and PMPC but not by AP7 or CPP. It is postulated that two NMDA receptor subtypes exist on mRN neurones.

The time course of glutamate in the synaptic cleft

The peak concentration and rate of clearance of neurotransmitter from the synaptic cleft are important determinants of synaptic function, yet the neurotransmitter concentration time course is unknown at synapses in the brain. The time course of free glutamate in the cleft was estimated by kinetic analysis of the displacement of a rapidly dissociating competitive antagonist from N-methyl-D-aspartate (NMDA) receptors during synaptic transmission. Glutamate peaked at 1.1 millimolar and decayed with a time constant of 1.2 milliseconds at cultured hippocampal synapses. This time course implies that transmitter saturates postsynaptic NMDA receptors. However, glutamate dissociates much more rapidly from alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Thus, the time course of free glutamate predicts that dissociation contributes to the decay of the AMPA receptor-mediated postsynaptic current.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of N-methyl-d,l-aspartate on Beta-endorphin and prolactin secretion in rams exposed to long or short days

Abstract In a previous study we demonstrated that the injection of the excitatory amino-acid N-methyl-D, L-aspartate (NMDA) stimulates an acute increase in the peripheral blood concentrations of luteinizing hormone in Soay rams, and this response varies with the photoperiodically-induced reproductive cycle. To extend these observations, we have now measured the changes in the blood concentrations of beta-endorphin and prolactin in the same animals to establish whether NMDA stimulates the secretion of other pituitary hormones. Groups of adult Soay rams were exposed to alternating 16-weekly periods of long and short days to induce a long-term cycle in the endogenous secretion of beta-endorphin (maximum under short days) and prolactin (maximum under long days). NMDA injected intravenously caused a dose-dependent increase in the blood plasma concentrations of beta-endorphin (over the range 1 to 20 mg/kg NMDA). The initial increase in beta-endorphin occurred within 2 to 4 min with peak levels after 20 to 100 min. The magnitude of the beta-endorphin response was greatest following exposure to long days when the endogenous secretion of beta-endorphin was low, while the duration of the response was greatest following exposure to short days when the endogenous secretion of beta-endorphin was high. NMDA injected intravenously also caused a dose-dependent increase in the blood plasma concentrations of prolactin but this only occurred following exposure to long days when the endogenous secretion of prolactin was high. At this time the initial increase in prolactin concentrations occurred within 2 to 4 min after the injection of NMDA as for beta-endorphin but the values continued to increase for 2 to 4 h. In a separate experiment, it was shown that pretreatment of the rams with dexamethasone (synthetic glucocorticoid, 133.4 mug/kg iv) blocked the beta-endorphin response to NMDA but had no effect on the prolactin response. This indicates that NMDA stimulates the secretion of beta-endorphin from the corticotrophs probably acting centrally to induce the release of corticotrophin-releasing hormone and/or arginine vasopressin, while NMDA acts through separate mechanisms to affect the secretion of prolactin. The overall results show that NMDA can be used as a probe to investigate the neuroendocrine control of beta-endorphin and prolactin in addition to luteinizing hormone as described previously. The multiple responses are likely to represent the effects of NMDA acting on the hypothalamus to induce the acute release of peptides and other hormones into the pituitary portal blood system to affect the corticotrophs, lactotrophs and gonadotrophs. The variation in responsiveness to NMDA related to the photoperiodic cycle may reflect changes in the synthesis and storage of the hypothalamic hormones and the influence of other neural systems. Alternatively, the changes in the secretion of endogenous excitatory amino-acids and NMDA receptors may constitute part of the neuroendocrine mechanism relaying the effects of photoperiod.

The pharmacological specificity of N-methyl-D-aspartate receptors in rat cerebral cortex: correspondence between radioligand binding and electrophysiological measurements

1. The pharmacological specificity of N-methyl-D-aspartate (NMDA) receptors has been investigated in the rat cerebral cortex by use of radioligand binding and electrophysiological techniques. 2. A comparison was made between a functional assay (NMDA-induced depolarizations in a rat cortical slice preparation) and NMDA-sensitive L-[3H]-glutamate binding in the same brain region and species, to provide accurate affinity values for agonists and antagonists at the NMDA recognition site. 3. In a preparation of crude postsynaptic densities (PSD) from rat cortex, L-[3H]-glutamate bound with high affinity to an NMDA-sensitive population of sites with KD (geometric mean (-s.e.mean. + s.e. mean) = 120 (114, 126) nM, Bmax (mean +/- s.e.mean) = 11.4 +/- 0.8 pmol mg-1 protein and Hill coefficient (mean +/- s.e.mean) = 1.2 +/- 0.17 (n = 3). 4. There was a good agreement between the relative affinities in radioligand binding and electrophysiological assays for the receptor agonists NMDA, N-methyl-L-aspartate, quinolinate and trans-2,3-piperidine dicarboxylate, which are poor substrates of acidic amino acid transport systems. However, agonists which are good substrates for high affinity uptake systems (L- and D-glutamate, L- and D-aspartate, D-aspartate-beta-hydroxamate and L-glutamate-gamma-hydroxamate) were much weaker in the electrophysiological experiments. 5 Schild analysis of the antagonism of NMDA responses in the rat cortical slice by DL-3(2- carboxypiperazin-4-yl)propyl-1-phosphonate, D- and DL-2-amino-5-phosphonovalerate, D- and DL-2- amino- 7-phosphonoheptanoate, D-beta-aspartylaminomethylphosphonate, D-gamma-glutamylglycine and D-Ofaminoadipate (D-AA) indicated a competitive interaction with respective pA2 values of 6.17, 5.62, 5.24, 5.28, 5.20, 5.00, 4.43 and 3.97. 6 In the radioligand binding experiments the same antagonists inhibited only the NMDA-sensitive component of L-[3H]-glutamate binding. IC50 values showed a good correlation with the pA2 values (correlation coefficient = 0.96), with the exception of D-AA which was more potent than anticipated in the binding experiments (IC50 = 9.8 microM).7 These results confirm that NMDA-sensitive L-[3H]-glutamate binding sites represent the NMDA recognition site of the NMDA receptor and provide affinity values for both agonists and antagonists in the rat cerebral cortex, agreeing well with previous estimates in this and other tissues.

Antagonism of AP-5-induced sniffing stereotypy links umespirone to atypical antipsychotics

Blockade of glutamatergic transmission in the striatum (using the NMDA-antagonist DL-2-amino-5-phosphonovaleric acid AP-5) was recently shown to induce stereotyped sniffing in rats. Comparable stereotyped behaviour is well known to be elicited by stimulation of dopamine activity, which since long was the basis for experimental models to check for possible antipsychotic activity of new compounds. However, whereas dopamine-induced stereotypies are antagonized only by classical neuroleptics, stereotypies induced by blockade of glutamatergic transmission are antagonized by classical as well as by atypical antipsychotics. Umespirone, a novel psychotropic which has been reported to exhibit behavioural effects predictive for antipsychotic as well as anxiolytic potential was evaluated for antagonistic effects against AP-5-induced behaviour. The profile of umespirone was compared with the profile of a non-benzodiazepine anxiolytic buspirone as well as with previously published data of neuroleptics. Umespirone like clozapine specifically antagonized AP-5-induced sniffing, i.e. did not impair spontaneous sniffing but reversed AP-5-induced excessive sniffing. In contrast, buspirone impaired spontaneous and AP-5-induced sniffing to about the same extend. These results are in accordance with the glutamate hypothesis of schizophrenia and again give evidence that umespirone should have antipsychotic potential and a very low liability to exhibit unspecific sedative action.

Competitive antagonists and partial agonists at the glycine modulatory site of the mouse N-methyl-D-aspartate receptor

1. Kynurenate (Kyn), 7-chlorokynurenate (7-Cl-Kyn), 3-amino-1-hydroxypyrrolid-2-one (HA-966) and D-cycloserine are known to bind to the glycine site that modulates the N-methyl-D-aspartate (NMDA) response of vertebrate central neurones. The effects of these compounds were investigated with patch-clamp and fast-perfusion techniques on mouse cortical neurones in primary culture in an effort to establish whether they act as antagonists, partial agonists and/or inverse agonists of glycine. A fast drug application method allowed the study of both steady-state and transient responses. 2. The analysis of steady-state responses indicates that the main effects of Kyn and 7-Cl-Kyn are those expected from competitive antagonists of glycine, with a dissociation constant of 15 microM for Kyn, and of 0.3 microM for 7-Cl-Kyn. Concentration jumps indicate that at all concentrations of glycine, and in particular in the absence of added glycine, the blockade by Kyn and 7-Cl-Kyn develops at a rate which is close to the rate of dissociation of glycine from its binding site and is independent of antagonist concentration. 3. The main effects of D-cycloserine and of HA-966 are those of partial agonists of high and low efficacy, respectively. In the absence of added glycine, D-cycloserine always produced a potentiation, while HA-966 produced either a potentiation or an inhibition. This can be explained by assuming the presence of a variable level of contaminating glycine. With both D-cycloserine and HA-966, concentration jumps produced biphasic relaxations in which the onset rate of the slow component was, here again, close to the rate of dissociation of glycine from its binding site. 4. These results can be interpreted by assuming that (1) Kyn and 7-Cl-Kyn are competitive antagonists of glycine, (2) HA-966 and D-cycloserine are partial agonists, (3) in the absence of added glycine some glycine is present in the extracellular solution and (4) the response in the total absence of glycine is very small or negligible.

N-Methyl-DL-Aspartate Receptor Antagonism by D-2-Amino-5-Phosphonovaleric Acid Delays Onset of Puberty in the Female Rat

Abstract To investigate the possible role of N-methyl-DL-aspartate (NMDA) receptor activation in the initiation of puberty, we examined the effects of the selective competitive antagonist 2-amino-5-phosphonovaleric acid (AP5) on the timing of vaginal opening. Paired and weight-matched litter mates of immature female rats were implanted with osmotic minipumps for the intracerebroventricular infusion of DL- or D-AP5 or artificial cerebrospinal fluid from 27 to 30 days of age for 14 days. Each animal was weighed and examined daily for vaginal opening as the indicator of first oestrous. Infusion of 20 or 40 mM DL-AP5 beginning on Day 30 failed to delay vaginal opening. Administration 50mM of the single enantiomer D-AP5 beginning on Day 27 significantly delayed the age of vaginal opening to 40.6+/-1.1 (mean +/- SEM) days compared to the cerebrospinal fluid-infused controls (36.5 +/- 0.6 days). Blockade of NMDA receptors in the D-AP5-treated animals was confirmed on Day 32 by the suppression of luteinizing hormone response to intravenous NMDA (20 mg/kg) while the response to exogenous luteinizing hormone-releasing hormone (50 ng/kg) remained intact. AP5-treated animals had a slower rate of growth (3.1 +/- 0.2 g/day) compared to controls (4.2 +/- 0.2 g/day). However, a similar degree of growth retardation produced by a 75% restricted diet in untreated juvenile animals did not delay vaginal opening. This suggests that the slower growth rate in the D-AP5-treated animals could not account for the delayed onset of puberty. In conclusion, these data suggest that blockade of central NMDA receptors inhibits excitatory mechanisms which may be important in the control of pubertal onset in the female rat.

In vivo evidence for an inhibitory glutamatergic control of serotonin release in the cat caudate nucleus: involvement of GABA neurons

The local effect of L-glutamic acid (5 x 10(-5) M) on the release of [3H]serotonin continuously synthesized from [3H]tryptophan was examined in the caudate nucleus of 'encéphale isolé' unanaesthetized cats implanted with push-pull cannula. L-Glutamic acid (5 x 10(-5) M) decreased [3H]serotonin release from nerve terminals of the dorsalis raphe-striatal serotonergic neurons. The effect was antagonized by 2-amino-6-trifluoromethoxybenzothiazole (PK 26124) (10(-6) M), an antagonist of glutamatergic transmission. This effect was mimicked by N-methoxy-D-aspartic acid NMDA (5 x 10(-5) M) and prevented by DL-2-phosphono-valeric acid (APV) (5 x 10(-6) M), indicating that L-glutamic acid decreased serotonin release via a N-methoxy-D-aspartate type receptor. The superfusion of serotonergic nerve terminals in the caudate nucleus with tetrodotoxin prevented the inhibitory L-glutamic acid-induced effect on serotonin release. Furthermore, L-glutamic acid-induced inhibition of [3H]serotonin release was antagonized by bicuculline (5 x 10(-5) M). These data suggest that the glutamatergic receptors involved were not located directly on serotonin nerve terminals. The inhibitory control exerted by L-glutamic acid on serotonergic transmission could involve gamma-aminobutyric acid interneurons. Since no reduction of spontaneous [3H]serotonin release was observed in the presence of bicuculline, GABAergic neurons appeared to exert a phasic influence on serotonin release. Indirect inhibitory presynaptic control on serotonin release mediated by corticostriatal glutamatergic fibers is discussed in light of previous findings.

L-homocysteate stimulates [3H]MK-801 binding to the phencyclidine recognition site and is thus an agonist for the N-methyl-D-aspartate-operated cation channel

Rat brain synaptosomal membranes that are depleted of endogenous excitatory amino acids cannot bind [(+)-5-methyl-10, 11-dihydro-5H-dibenzo(a,d]cyclohept-5,10-imine maleate] ([3H]MK-801). However, they do so upon the restoration of excitatory amino acid agonists such as L-glutamate. [3H]MK-801 provides a molecular probe which is specific for a binding site located within the ionophore of the N-methyl-D-aspartate-type excitatory amino acid receptor, [3H]MK-801 does not bind to non-N-methyl-D-aspartate excitatory amino acid receptors. Exploiting [3H]MK-801 binding as a quantitative measure of agonist activity with respect to ability of inducing the open channel conformation, the present study demonstrates that L-homocysteate is an agonist almost equivalent to L-glutamate in terms of efficacy (maximal N-methyl-D-aspartate response) as well as potency (EC50). The effect of L-homocysteate was dose-dependent, stereospecific (L-homocysteate greater than DL-homocysteate greater than D-homocysteate), suppressible by the N-methyl-D-aspartate-selective competitive antagonist (+/-)-3(2-carboxy-piperazine-4-yl)propyl-l-phosphonate, and potentiated by the N-methyl-D-aspartate-selective "allosteric" modulator glycine. The demonstrated inactivity of L-homocysteine (and virtually all naturally occurring, non-acidic amino acids) implies that the omega-sulphonic acid moiety is an acceptable substitute for the omega carboxyl group for activating the N-methyl-D-aspartate receptor. While the potency of L-homocysteate at N-methyl-D-aspartate receptors was by a factor of only 1.6 smaller than that of L-glutamate, the affinity of L-homocysteate for kainate-type excitatory amino acid receptors was approximately four-fold lower than that of L-glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

A study in rat brain cortex synaptic vesicles of endogenous ligands for N-methyl-D-aspartate receptors

The presence of endogenous ligands for the N-methyl-D-aspartate receptor was looked for in highly purified rat brain cortex synaptic vesicles, the contents of which were extracted and fractionated by gel filtration on Sephadex G-10, or by three different high-voltage electrophoresis procedures. The presence of endogenous ligands was detected by their ability to compete with 50 nM L-[3H]glutamate for binding to whole rat brain N-methyl-D-aspartate receptors. The receptor preparations used were those present in purified postsynaptic densities, in which the quisqualate receptors were blocked by 10 microM quisqualate. Synaptic vesicles had a high content of N-methyl-D-aspartate receptor ligands, which on fractionation always coincided with glutamate or aspartate. A variable and very small amount of a highly acidic endogenous ligand was also found. The latter substance did not coincide in the electrophoresis with homocysteic, cysteic, quinolinic, cysteine sulphinic or homocysteine sulphinic acids, or with N-acetyl-aspartyl-glutamic acid, S-sulphocysteine or sulphoserine. We also found that a single centrifugation, in 0.25 M sucrose, 25 mM Tris-citrate, pH 7.1, of purified synaptic vesicles, at 135,000 gmax for 45 min, led to a 51% loss of endogenous glutamate, but did not change their aspartate content. Thus, in uncentrifuged vesicles the glutamate/aspartate ratio was 9.4, while in centrifuged ones the ratio was 3.9 ATP markedly enhanced L-[3H]glutamate uptake into synaptic vesicles, but did not change the binding of L-[3H]aspartate. Differences in labelled aspartate and glutamate efflux from the vesicles were also found.(ABSTRACT TRUNCATED AT 250 WORDS)

Allosteric modulation of N-methyl-D-aspartate receptors

In this review we have attempted to describe the basis for current models of the NMDA receptor, and justify the need for the various binding sites that have been proposed. The NMDA receptor is clearly a complex molecule with a number of modulatory sites, any of which may have great functional significance. From the data presented above it is apparent that the NMDA recognition site is closely coupled with the glycine site, and can also be regulated by Zn2+. The glycine site is reciprocally coupled to the NMDA site, and may also be coupled to a divalent-cation site outside the channel. However, the glycine site is insensitive to Zn2+. The Zn2+ site is probably not inside the channel to any degree, but can profoundly affect the ability of NMDA site ligands to operate the channel. However, the determination of reciprocal effects at the Zn2+ site await the development of a suitably potent and selective ligand for this site. Several lines of evidence suggest that the phencyclidine and channel-blocking Mg2+ site are located within the NMDA-operated ion channel. Glutamate, glycine, and Zn2+ alter the binding of ligands to these sites. However, this is most likely to be due to alteration of access of the ligands to their sites rather than a direct allosteric coupling. It does appear that phencyclidine site drugs and Mg2+ bind to separate sites within the channel, and that these separate sites are allosterically coupled. This complex series of interactions, many of which are mediated by endogenous agents, may allow very fine control over the expression of NMDA receptor-mediated synaptic transmission. In addition to these ligand-produced modulatory effects, there may also be covalent modification of the channel by receptor phosphorylation. Furthermore, the voltage sensitivity of some of the effects allows control of NMDA receptor-mediated signaling by alteration of the membrane potential in the postsynaptic cell, which can be achieved in a wide variety of ways. The level of sophistication possible in adjusting the responsiveness of this receptor seems entirely appropriate given its central involvement in a wide variety of fundamental neurobiological events, and underscores the deleterious pathological sequelae of the system tilting out of balance. At the same time, the wide array of possible therapeutic targets raises hopes that it may soon be possible to treat effectively some severely debilitating and currently untreatable diseases.

Neurotoxicity at the N-methyl-D-aspartate receptor in energy-compromised neurons. An hypothesis for cell death in aging and disease

Our results demonstrated that the neurotoxicity of glutamate and closely related agonists was mediated by the NMDA receptor in rat cerebellar granule cells. Evidence was presented to support our hypothesis that the pivotal event in the transition of these EAAs from neurotransmitters to neurotoxins is relief of the voltage-dependent Mg++ block of the NMDA channel due to changes in membrane potential which can be caused by depletion of highly phosphorylated nucleotides or by other depolarizing stimuli. Persistent stimulation of NMDA receptors whose channels are unblocked by Mg++ can permit excessive influx of Na+ and Ca++ and neuronal death can follow by a mechanism not yet understood. Glutamate is not toxic at kainate receptors although they are present on these cells. These findings underline the potential importance of perturbations in energy metabolism in a variety of neurodegenerative disorders and in the normal process of aging which share the common feature of the loss of neurons.

Glutamate-induced 45Ca2+ uptake into immature cerebral cortex neurons shows a distinct pharmacological profile

Glutamate-induced 45Ca2+ uptake was studied in cerebral cortex neurons cultured for 4 days, i.e., at a developmental stage where the neurons are sensitive to the mixed agonist glutamate but not to the actions of N-methyl-D-aspartate or other excitatory amino acids. Using this experimental approach, allowing the investigation of effects elicited only by glutamate, it was demonstrated that the glutamate-stimulated Ca2+ influx could be completely antagonized by MK-801, phencyclidine, and cyclazocine in the nanomolar range, and by 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate and D-2-amino-5-phosphonopentanoate (APV) in the low micromolar range. However, the glutamate response was unaffected by variations in the Mg2+ concentration in the exposure media. In addition, the two quinoxalinediones 6-cyano-7-nitroquinoxaline-2,3-dione and 6,7-dinitroquinoxaline-2,3-dione were equipotent with APV in blocking the glutamate-stimulated Ca2+ uptake. PK 26124 blocked the response in the high micromolar concentration range. Ketamine and gamma-glutamylaminomethylsulfonate were essentially without effect at concentrations up to 10 microM and 300 microM, respectively. These results may suggest the existence of a glutamate receptor with a pharmacological profile not compatible with the existent classification of glutamate receptor subtypes.

Excitatory amino acid neurotoxicity at the N-methyl-D-aspartate receptor in cultured neurons: pharmacological characterization

L-Glutamate neurotoxicity at the N-methyl-D-aspartate (NMDA) receptor was characterized in cultured cerebellar granule cells. When deprived of glucose for 40 min, these cells were killed by 20-60 microM L-glutamate. However, the neurons were resistant to glutamate at concentrations as high as 5 mM when glucose and Mg2+ were present throughout. Both competitive and non-competitive NMDA receptor antagonists completely blocked neurotoxicity due to glutamate and other NMDA receptor agonists. CPP [+/-)-3-(2-carboxypiperazin-4-yl)-prophyl-1-phosphonic acid) was the most effective competitive antagonist with full protection at 100 microM while MK-801 [+/-)-10,11-dihydro-5-methyl-5H-dibenzo[a,d]-cyclohepten-5,10-imin e) was the most effective non-competitive antagonist with full protection at 20 nM. Other antagonists with higher selectivity for other subtypes of glutamate receptors were ineffective. We conclude that glutamate toxicity in energy-deprived cerebellar granule cells is mediated by NMDA receptors. Results are discussed in terms of an hypothesis offering an explanation for the transition of glutamate from neurotransmitter to neurotoxin which emphasizes the responsiveness of the receptor to agonists rather than focusing on the presence of high concentrations of agonist.

AMPA, kainic acid, and N-methyl-D-aspartic acid stimulate locomotor activity after injection into the substantia innominata/lateral preoptic area

The substantia innominata/lateral preoptic area (SI/LPO) is a subpallidal region which has been shown to regulate the hypermotility produced by drugs acting in the nucleus accumbens. Evidence has been presented that the SI/LPO contains glutamatergic nerve terminals and receptors for excitatory amino acids. The purpose of this study was to determine the effects of the activation of excitatory amino acid receptors in the SI/LPO on locomotor activity following the direct injection of excitatory amino acids into this brain site. It was found that the bilateral injection of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), kainic acid, and N-methyl-D-aspartic acid into the SI/LPO produced marked dose-dependent stimulations of locomotor activity which resembled the effects of these agents after their injection into the nucleus accumbens. The effect, however, was bell-shaped in that at high doses, the locomotor activity values decreased from their peak values. The coinjection of gamma-glutamylaminomethylsulfonate (GAMS) with AMPA into the SI/LPO was found to inhibit the hypermotility response to AMPA at doses that were unable to produce a significant inhibition of the hypermotility responses to kainic acid or N-methyl-D-aspartic acid. The injection of 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the SI/LPO inhibited the hypermotility responses to AMPA or kainic acid while having no significant inhibitory effect on N-methyl-D-aspartic acid stimulated locomotor activity. The injection of D-alpha-aminoadipic acid into the SI/LPO produced a significant inhibition of the hypermotility response produced by N-methyl-D-aspartic acid at a dose that did not produce a significant inhibition of the hypermotility response produced by AMPA.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of excitatory amino acid receptors on sustained ganglion cells in the cat retina

Iontophoretic effects of N-methyl-D-aspartate, quisqualate and kainate and a variety of excitatory amino acid receptor antagonists, on retinal ganglion cells, were studied in optically intact eyes of barbiturate anesthetized cats. All three agonists raised the spontaneous firing of both ON- and OFF-sustained retinal ganglion cells, with the potency order of kainate much greater than quisqualate greater than N-methyl-D-aspartate. However, the excitatory amino acid analogues readily saturated the receptors and reduced the visually driven firing of cells with high spontaneous firing, but mimicked an increase in endogenous excitatory amino acid release and raised the visually induced response in cells with low spontaneous firing. The quinoxaline compound, 6-cyano-2,3 dihydroxy-7-nitroquinoxaline and 6-7-dinitroquinoxaline-2,3-dione, blocked the visually driven firing and kainate- and quisqualate-induced excitation, whilst 3[+)-2-carboxypiperazin-4-yl)propyl-1-phosphonate, antagonized the N-methyl-D-aspartate-induced excitation, but failed to block visually driven firing of the retinal ganglion cells. The broadband excitatory amino acid receptor antagonists, such as kynurenate, were also effective in antagonizing the visually driven response and also blocked the N-methyl-D-aspartate- as well as kainate- and quisqualate-induced responses. These results suggest that the receptors at the bipolar/ganglion cell synapse are of the non-N-methyl-D-aspartate type, but that N-methyl-D-aspartate receptors are also present on ganglion cells although their physiological role is unclear.

Subchronic administration of MK-801 in the rat decreases cortical binding of [3H]D-AP5, suggesting down-regulation of the cortical N-methyl-D-aspartate receptors

The effects of the subchronic administration of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801) (0.5 mg/kg twice daily, 7 days) on N-methyl-D-aspartate, phencyclidine and sigma binding sites, behaviour and catecholamine turnover were investigated in the rat. Overt behaviours induced by MK-801 on day 7 were significantly altered relative to day 1 with subchronically treated rats not showing head weaving, goss ataxia or loss of hindlimb control: locomotion and sniffing were largely unaffected. The mean intensities of behaviour were 1.8 and 5.4 for days 7 and 1, respectively. Behavioural tolerance was accompanied by a significant reduction in the density of cortical N-methyl-D-aspartate receptors as measured by [3H]D-2-amino-5-phosphonopentanoic acid binding, while affinity was unchanged: the density of binding sites was 3.52 and 1.88 pmol/mg protein for saline- and MK-801-treated rats, respectively. The N-methyl-D-aspartate ion channel as measured by the binding of [3H]N-(1-[2-thienyl]cyclohexyl)piperidine was not affected by the schedule of MK-801. Additionally, changes were not observed to N-methyl-D-aspartate- or glycine-stimulated [3H]N-(1-[2-thienyl]cyclohexyl)piperidine binding or to sigma binding. Catecholamine turnover was unaltered in the nucleus accumbens septi after the schedule of MK-801. Our results demonstrate that the subchronic administration of MK-801 produces behavioural tolerance and down-regulation of N-methyl-D-aspartate binding sites and suggest differential regulation of the domains of the N-methyl-D-aspartate receptor-ionophore complex.

Two classes of N-methyl-D-aspartate recognition sites: differential distribution and differential regulation by glycine

The N-methyl-D-aspartate (NMDA) receptor, a subtype of excitatory amino acid receptor, mediates synaptic responses in many regions of the central nervous system. This receptor plays a critical role in the mechanisms of both synaptic plasticity and excitotoxicity. Although these receptors were generally thought to be a single homogeneous receptor population, we report observations indicating that two anatomically distinct forms of the NMDA-receptor complex exist. (i) The distribution of NMDA receptors, as labeled by the NMDA agonist L-[3H]glutamate, differs from that obtained with the radiolabeled antagonist 3H-labeled 3-[(+/-)2-carboxypiperazine-4-yl]propyl-1-phosphonic acid [( 3H]CPP). Relative to L-[3H]glutamate, [3H]CPP binding is low in the striatum and septum and high in the thalamus and inner cerebral cortex. (ii) NMDA antagonists are relatively more potent than agonists at displacing L-[3H]glutamate binding in the thalamus and cerebral cortex; agonists are relatively more potent in the striatum and cerebellum. (iii) Glycine, which potentiates NMDA-receptor responses to glutamate, causes a greater percentage increase in L-[3H]glutamate binding to NMDA receptors in the thalamus and cerebral cortex than in the striatum, septum, and cerebellum. Radiolabeled NMDA-antagonist binding, in contrast, is inhibited by glycine. Thus, as observed for gamma-aminobutyric acid type A receptors, NMDA receptors have an agonist-preferring binding-site population and an antagonist-preferring binding site population. These may represent two distinct receptors and/or two interconverting forms. It could be of significant clinical importance if these two sites differ in their response to NMDA.

Structure/activity relations of N-methyl-D-aspartate receptor ligands as studied by their inhibition of [3H]D-2-amino-5-phosphonopentanoic acid binding in rat brain membranes

Structure/activity relations of agonists and antagonists for the N-methyl-D-aspartate receptor have been investigated by measuring the ability of a large range of substances to inhibit binding of [3H]2-amino-5-phosphonopentanoate to rat brain membranes. A major difference between optimum structures for agonist and antagonist activity lay in the differential effectiveness of sulphonic and phosphonic acid groups as the omega-acidic terminal in these two types of compound. The sulphonic acid moiety was an effective omega-acidic terminal in short chain agonists, but not in longer chain antagonists, while the phosphonic acid group was the most effective omega-acidic terminal in longer chain antagonists, but was only very weakly active in short chain agonists. It is proposed that the binding site of the omega-acidic terminal of antagonists is different from that for the omega-acidic group of agonists. Other structural features conducive to effective interaction of ligands with the receptor are discussed.