Indole-2-carboxylic acid: a competitive antagonist of potentiation by glycine at the NMDA receptor

Pharmacological characterization and differential expression of NMDA receptor subunits in the chicken vestibular system during development

Previous studies demonstrated that in vitro preparations of the isolated vestibular system of diverse animal species still exhibit stable resting electrical activity and mechanically evoked synaptic transmission between hair cells and primary afferent endings. However, there are no reports related to their neurodevelopment. Therefore, this research aimed to examine whether NMDA receptors mediate these electrical signals in an isolated preparation of the chicken vestibular system at three developmental stages, E15, E18, and E21. We found that the spontaneous and mechanically evoked discharges from primary afferents of the posterior semicircular canal were modulated by agonists NMDA and glycine, but not by the agonist d-serine applied near the synapses. Moreover, the individually applied by bath perfusion of three NMDA receptor antagonists (MK-801, ifenprodil, and 2-naphthoic acid) or high Mg²⁺ decreased the resting discharge rate, the NMDA response, and the discharge rate of mechanically evoked activity from these primary afferents. Furthermore, we found that the vestibular ganglion shows a stage-dependent increase in the expression of NMDA receptor subunits GluN1, GluN2 (A-C), and GluN3 (A-B), being greater at E21, except for GluN2D, which was inversely related to the developmental stage. However, in the crista ampullaris, the expression pattern remained constant throughout development. This could suggest the possible existence of presynaptic NMDA receptors. Our results highlight that although the NMDA receptors are functionally active at the early embryonic stages of the vestibular system, NMDA and glycine reach their mature functionality to increase NMDA responses close to hatching (E21).

Pathogenesis and Host Immune Response in Leprosy

Leprosy is an ancient insidious disease caused by Mycobacterium leprae, where the skin and peripheral nerves undergo chronic granulomatous infections, leading to sensory and motor impairment with characteristic deformities. Susceptibility to leprosy and its disease state are determined by the manifestation of innate immune resistance mediated by cells of monocyte lineage. Due to insufficient innate resistance, granulomatous infection is established, influencing the specific cellular immunity. The clinical presentation of leprosy ranges between two stable polar forms (tuberculoid to lepromatous) and three unstable borderline forms. The tuberculoid form involves Th1 response, characterized by a well demarcated granuloma, infiltrated by CD4⁺ T lymphocytes, containing epitheloid and multinucleated giant cells. In the lepromatous leprosy, there is no characteristic granuloma but only unstructured accumulation of ineffective macrophages containing engulfed pathogens. Th1 response, characterised by IFN-γ and IL-2 production, activates macrophages in order to kill intracellular pathogens. Conversely, a Th2 response, characterized by the production of IL-4, IL-5 and IL-10, helps in antibody production and consequently downregulates the cell-mediated immunity induced by the Th1 response. M. lepare has a long generation time and its inability to grow in culture under laboratory conditions makes its study challenging. The nine-banded armadillo still remains the best clinical and immunological model to study host-pathogen interaction in leprosy. In this chapter, we present cellular morphology and the genomic uniqueness of M. leprae, and how the pathogen shows tropism for Schwann cells, macrophages and dendritic cells.

Circadian Modulation of Neurons and Astrocytes Controls Synaptic Plasticity in Hippocampal Area CA1

Most animal species operate according to a 24-h period set by the suprachiasmatic nucleus (SCN) of the hypothalamus. The rhythmic activity of the SCN modulates hippocampal-dependent memory, but the molecular and cellular mechanisms that account for this effect remain largely unknown. Here, we identify cell-type-specific structural and functional changes that occur with circadian rhythmicity in neurons and astrocytes in hippocampal area CA1. Pyramidal neurons change the surface expression of NMDA receptors. Astrocytes change their proximity to synapses. Together, these phenomena alter glutamate clearance, receptor activation, and integration of temporally clustered excitatory synaptic inputs, ultimately shaping hippocampal-dependent learning in vivo. We identify corticosterone as a key contributor to changes in synaptic strength. These findings highlight important mechanisms through which neurons and astrocytes modify the molecular composition and structure of the synaptic environment, contribute to the local storage of information in the hippocampus, and alter the temporal dynamics of cognitive processing.

Immunocytochemical localization of a putative strychnine-sensitive glycine receptor in Hydra vulgaris

Previous biochemical studies have identified strychnine-sensitive glycine receptors in membrane preparations of Hydra vulgaris (Cnidaria: Hydrozoa). Electrophysiological and behavioral evidence has shown that these receptors play a role in modulating pacemaker activity and feeding behavior. Here, we present our genomic analysis that revealed hydra proteins having strong homology with the strychnine-binding region of the human receptor protein, GlyRα1. We further present immunocytochemical evidence for the specific labeling of cell and tissue preparations of hydra by a commercially available polyclonal anti-GlyRα1 antibody, selected through our genomic analysis. Tissue pieces and cell macerates from the upper and lower thirds of the body and ablated tentacles were double-labeled with this antibody and with an antibody specific for α-tubulin, to identify the glycine receptors and microtubules, respectively. Extensive receptor labeling was evident on the membranes, cell bodies and myonemes of endodermal and ectodermal epithelial cells, cell bodies and neurites of nerve cells, cnidocytes and interstitial cells. Labeling of the membranes of epithelial cells frequently corresponded to conspicuous varicosities (presumptive presynaptic sites) in the associated nerve net. Our findings support the idea that glycine receptors form an integral part of the nerve and effector systems that control hydra behavior.

Radial symmetry in a chimeric glutamate receptor pore

Ionotropic glutamate receptors comprise two conformationally different A/C and B/D subunit pairs. Closed channels exhibit fourfold radial symmetry in the transmembrane domain (TMD) but transition to twofold dimer-of-dimers symmetry for extracellular ligand binding and N-terminal domains. Here, to evaluate symmetry in open pores we analysed interaction between the Q/R editing site near the pore loop apex and the transmembrane M3 helix of kainate receptor subunit GluK2. Chimeric subunits that combined the GluK2 TMD with extracellular segments from NMDA receptors, which are obligate heteromers, yielded channels made up of A/C and B/D subunit pairs with distinct substitutions along M3 and/or Q/R site editing status, in an otherwise identical homotetrameric TMD. Our results indicate that Q/R site interaction with M3 occurs within individual subunits and is essentially the same for both A/C and B/D subunit conformations, suggesting that fourfold pore symmetry persists in the open state.

Molecular electrostatic potentials in aromatic substituted 4-hydroxyquino-2-lones: glycine/NMDA receptor antagonists

Hydroxyquinolone derivatives have proven to be useful for inhibition at the glycine binding site of N-methyl-D-aspartate (NMDA) receptor. In this work the electronic structure, molecular electrostatic potential (MESP) and vibrational characteristics of a set of C(3) substituted 4-hydroxyquino-2-lone (HQ) derivatives, which act as Glycine/NMDA receptor antagonists, have been investigated using the density functional calculations. In the optimized structures a substituent at the C(3) site of HQ tends to adopt a helical structure. MESP investigations reveal that the ligands showing better inhibition activity should possess electron-rich regions extending over the substituent and carbonyl group of HQ. A correlation of inhibitory activity to the molecular electrostatic potential topography at the carbonyl oxygen as well as to the molecular electron density topography turns out to be a significant output of the investigation.

In vitro and in vivo pharmacological profile of AS057278, a selective d-amino acid oxidase inhibitor with potential anti-psychotic properties

Non-competitive N-methyl-d-aspartate (NMDA) blockers induce schizophrenic-like behavior in healthy volunteers and exacerbate symptomatology in schizophrenic patients. Hence, a compound able to enhance NMDA neurotransmission by increasing levels of d-serine, an endogenous full agonist at the glycine site of the NMDA receptors, could have anti-psychotic activity. One way to increase d-serine levels is the inhibition of d-amino acid oxidase (DAAO), the enzyme responsible for d-serine oxidation. Indeed AS057278, a potent in vitro (IC(50)=0.91 microM) and ex vivo (ED(50)=2.2-3.95 microM) DAAO inhibitor, was able to increase d-serine fraction in rat cortex and midbrain (10 mg/kg i.v.). AS057278 was able to normalize phencyclidine (PCP)-induced prepulse inhibition after acute (80 mg/kg) and chronic (20 mg/kg b.i.d.) oral administration in mice. Finally, AS057278 after oral chronic treatment (10 mg/kg b.i.d.) was able to normalize PCP-induced hyperlocomotion. These results suggest that AS057278 has the potential to anti-psychotic action toward both cognitive and positive symptoms of schizophrenia.

Pacemaker activity in hydra is modulated by glycine receptor ligands

In the mammalian central nervous system, the neurotransmitter, glycine, acts both on an inhibitory, strychnine-sensitive receptor (GlyR) and an excitatory, strychnine-insensitive site at the NMDA receptor. Here we present electrophysiological evidence that the strychnine-sensitive glycine agonists, glycine and taurine, and the antagonist, strychnine, affect the endodermal rhythmic potential (RP) system and that the ectodermal contraction burst (CB) pacemaker system is modulated by glycine and strychnine in hydra. The RP and CB pacemaker systems are responsible for the respective elongation and contraction of hydra's body column. Activity of the CB system, quantified by the rate of contraction bursts (CBs), the number of pulses per contraction burst (P/CB), and the duration of bursts, was decreased by glycine. Glycine, coadministered with the strychnine-insensitive glycine site blocker, indole-2-carboxylic acid (I2CA), decreased RPs but not CBs or P/CB. The effect was mimicked by taurine. Strychnine increased the duration of RP production, and decreased CB duration. The effect of glycine with I2CA was counteracted by strychnine. The results support the idea that a vertebrate-like GlyR may be involved in modulating activity of the endodermal RP system and suggest that a glycine site on an NMDA receptor may be involved in the CB system.

Glutamate but not glycine agonist affinity for NMDA receptors is influenced by small cations

NMDA receptor currents desensitize in an agonist-dependent manner when either the glutamate or glycine agonist is subsaturating. This may result from a conformational change in the NMDA receptor protein that lowers glutamate and glycine binding site affinity induced by co-agonist binding, channel opening, or ion permeation. We have used whole-cell voltage clamp of cultured hippocampal neurons with agonist paired-pulse protocols to demonstrate that glutamate and glycine dissociate 7.9- and 6.8-fold slower in the absence of their respective co-agonists than when their co-agonists are present. Paired-pulse and desensitization protocols were used to show that co-agonist binding and channel opening are sufficient to cause a reduction in glycine affinity, but extracellular sodium or magnesium binding was required in addition to conformational changes leading to channel opening to reduce glutamate binding-site affinity. Use of cesium or potassium as the major extracellular cation prevented the reduction of glutamate affinity. In addition, the use of choline-, sodium-, or cesium-based intracellular solutions did not alter desensitization characteristics, indicating that the site responsible for reduction of glutamate affinity is not in the intracellular domain. The fact that the reduction of glutamate affinity is dependent on certain small extracellular cations whereas the reduction of glycine affinity is insensitive to such cations indicates that conformational changes induced by the binding of glutamate are not completely paralleled by the conformational changes induced by glycine. Although glutamate and glycine are essential co-agonists, these data suggest that they have differential roles in the process of NMDA receptor activation.

Vilsmeier formylation of 2-carboxyindoles and preparation of O-benzylhydroxyureas on solid phase

The Vilsmeier formylation has been introduced for the solid-phase functionalization of five different 2-carboxyindoles. The aldehyde functionality has been utilized in the preparation of O-benzylhydroxyureas.

Desensitization of NMDA receptor channels is modulated by glutamate agonists

Two distinct forms of desensitization have been characterized for N-methyl-D-aspartate (NMDA) receptors. One form results from a weakening of agonist affinity when channels are activated whereas the other form of desensitization results when channels enter a long-lived nonconducting state. A weakening of glycine affinity upon NMDA receptor activation has been reported. Cyclic reaction schemes for NMDA receptor activation require that a concomitant affinity shift should be observed for glutamate agonists. In this study, measurements of peak and steady-state NMDA receptor currents yielded EC50 values for glutamate that differed by 1.9-fold, but no differences were found for another agonist, L-cysteine-S-sulfate (LCSS). Simulations show that shifts in EC50 values may be masked by significant degrees of desensitization resulting from channels entering a long-lived nonconducting state. Simulations also show that a decrease in the degree of desensitization with increasing agonist concentration is a good indicator for the existence of desensitization resulting from a weakening of agonist affinity. Both glutamate and LCSS exhibited this trend. An affinity difference of three- to eightfold between high-and low-affinity agonist-binding states was estimated from fitting of dose-response data with models containing both types of desensitization. This indicates that activation of NMDA receptors causes a reduction in both glutamate and glycine affinities.

Biochemical, cellular, and molecular mechanisms in the evolution of secondary damage after severe traumatic brain injury in infants and children: Lessons learned from the bedside

OBJECTIVE: To present a state-of-the-art review of mechanisms of secondary injury in the evolution of damage after severe traumatic brain injury in infants and children. DATA SOURCES: We reviewed 152 peer-reviewed publications, 15 abstracts and proceedings, and other material relevant to the study of biochemical, cellular, and molecular mechanisms of damage in traumatic brain injury. Clinical studies of severe traumatic brain injury in infants and children were the focus, but reports in experimental models in immature animals were also considered. Results from both clinical studies in adults and models of traumatic brain injury in adult animals were presented for comparison. DATA SYNTHESIS: Categories of mechanisms defined were those associated with ischemia, excitotoxicity, energy failure, and resultant cell death cascades; secondary cerebral swelling; axonal injury; and inflammation and regeneration. CONCLUSIONS: A constellation of mediators of secondary damage, endogenous neuroprotection, repair, and regeneration are set into motion in the brain after severe traumatic injury. The quantitative contribution of each mediator to outcome, the interplay between these mediators, and the integration of these mechanistic findings with novel imaging methods, bedside physiology, outcome assessment, and therapeutic intervention remain an important target for future research.

Synthesis of 4,6-dichloro-3-[(1-N-arylaminocarbonyl)-hydrazono]-1,3-dihydro-indole-2 -one as a potential NMDA receptor glycine site antagonist

A synthetic procedure for the preparation of indole-2,3-dione derivatives 6 as a potential NMDA receptor glycine site antagonist with improved pharmacological profile compared with 2-carboxyindole derivative 5, starting from readily available 3,5-dichloroaniline (7), is described.

Synthesis and pharmacological properties of novel glycine antagonists

The NMDA receptor is an ionotropic receptor complex widely distributed in the central nervous system and its activation, particularly in hypoxic conditions such as stroke, traumatic head injury and hypoglycemia, results in a massive influx of calcium ions into the post-synaptic neurones, leading to cell death through the activation of several neurotoxic cascades. The NMDA receptor is a unique ionotropic receptor complex because its activation requires the simultaneous binding of glutamate and glycine and selective antagonists at the glycine binding site are endowed with a better side-effect profile than competitive NMDA antagonists. Then, considerable efforts have been devoted to find potent and selective ligands, resulting in the identification of several classes of glycine antagonists. The research at Glaxo Wellcome has been aimed at the identification of novel in vivo active glycine antagonists, and led to the synthesis and pharmacological characterization of a number of novel, potent and systemically active compounds belonging to different chemical classes.

The non-NMDA glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline, but not NMDA antagonists, block the intrastriatal neurotoxic effect of MPP+

Altered glutamatergic neurotransmission appears to be central to the pathophysiology of Parkinson's disease; consequently, considerable effort has been made to elucidate neuroprotective mechanisms against such toxicity. In the present study, the possible neuroprotective effect of glutamate receptor antagonists against MPP+ neurotoxicity on dopaminergic terminals of rat striatum was investigated. Different doses of glutamate receptor antagonists were coinfused with 1.5 microg of MPP+ into the striatum; kynurenic acid, a nonselective antagonist of glutamate receptors (30 and 60 nmol), partially protected dopaminergic terminal degeneration in terms of rescue of dopamine levels and tyrosine hydroxylase immunohistochemistry. Dizocilpine, a channel blocker of the NMDA receptor (1, 4, and 8 nmol), and 7-chlorokynurenic acid, a selective antagonist at the glycine site of the NMDA receptor (1 and 10 nmol), failed to protect dopaminergic terminals from MPP+ toxicity. However, 6-cyano-7-nitroquinoxaline-2,3-dione (0.5 and 1 nmol) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (1 nmol), two AMPA-kainate receptor antagonists, protected against MPP toxicity. Our findings suggest that the toxic effects of MPP+ on dopaminergic terminals are not mediated through a direct interaction with the NMDA subtype of glutamate receptor, but with the AMPA-kainate subtype.

Modulation of a calcium-activated chloride current by Maitotoxin

The effect of Maitotoxin (MTX) on the calcium-activated chloride current (ICl-Ca) from Xenopus oocytes was studied, applying the two-electrode voltage clamp technique. MTX increased the current amplitude at all the voltages explored and reduced the time to reach the maximum current level (time to peak). At low toxin concentrations (15 pM), both effects were fully reversible. Activation of ICl-Ca by MTX was secondary to the increment in the intracellular Ca2+ concentration induced by this toxin, since incubation of the oocytes with the cell-permeant Ca2+ chelator BAPTA-AM, greatly reduced the effect of MTX on ICl-Ca. Furthermore, external chloride ions removal also diminished the MTX effect on the current, strongly suggesting that the main current activated by MTX is ICl-Ca. Subsequent applications of a fixed toxin concentration after toxin washout resulted in enhanced ICl-Ca, suggesting that the toxin effect potentiates.

Channel properties of NMDA receptors on magnocellular neuroendocrine cells cultured from the rat supraoptic nucleus

Application of N-methyl-D-aspartate (NMDA) to the supraoptic nucleus of the hypothalamus (SON) generates clustered firing that may be important in hormone release. However, synaptically evoked EPSPs recorded from SON neurons exhibit varying contributions from NMDA receptors. We used the high resolution of single-channel recording to examine the receptor and ion channel properties of NMDA receptors expressed by SON neurons in 'punch' culture. Biocytin introduced into individual neurons during patch clamp recording revealed large (32.1+/-3.3 micron), oblong somas and bipolar extensions typical of magnocellular neuroendocrine cells (MNCs). Rapid application of NMDA (100-300 microM) in the presence of 10 microM glycine to outside-out macropatches resulted in openings with an average conductance of 46. 9 pS and reversal potential of +3.9 mV. Increasing glycine from 0.03 to 30 microM increased the apparent frequency, duration and occurrence of overlapping NMDA-elicited openings. NMDA responses were inhibited by Mg2+ in a voltage-dependent manner and by the NMDA-site antagonist, D-(-)-2-amino-5-phosphonovaleric acid (D-APV). Application of saturating NMDA or glycine alone with the glycine-site antagonist, 5,7-dichlorokynurenate (DCK) or with D-APV, respectively, did not result in agonist-induced openings. NR1 immunoreactivity was observed in large neurons (>25 micron) with MNC-like morphology. These single-channel and immunocytochemical data confirm the presence of functional NR1-containing NMDA receptors in MNCs.

Enhanced sensitivity of medullary depressor neurons to N-methyl-D-aspartate-glycine site antagonists in the spontaneously hypertensive rat

1. The effects of the specific N-methyl-D-aspartate (NMDA)-glycine site antagonist 5-fluoro indole-2-carboxylic acid (FICA) and NMDA, microinjected into the vasodepressor caudal ventrolateral medulla, were compared in spontaneously hypertensive rats (SHR) and in Wistar-Kyoto (WKY) rats. 2. 5-Fluoro indole-2-carboxylic acid elicited a significant pressor response (+20.0 +/- 4.9 mmHg) in SHR, but no change was found in the basal blood pressure of WKY rats. 3. The depressor response due to NMDA microinjection was significantly larger in SHR (-48.0 +/- 4 mmHg) than in WKY rats (-23.0 +/- 1.9 mmHg). 4. Pre-injection of FICA attenuated the depressor effects of NMDA significantly, this blockade being significantly more pronounced in SHR (37.0 +/- 2.7 mmHg) than in WKY rats (12.0 +/- 1.2 mmHg). 5. The enhanced responses to FICA may reflect the lower levels of the endogenous NMDA-glycine antagonist kynurenic acid in SHR compared with WKY rats.

2,3-Dihydro-6,7-dichloro-pyrido[2,3-b]pyrazine-8-oxide as selective glycine antagonist with in vivo activity

2,3-Dihydro-6,7-dichloro-pyrido[2,3-b]pyrazine-8-oxide was synthesized and evaluated for in vitro/in vivo antagonistic activity at the strychnine insensitive glycine binding site on the NMDA receptor revealing it to be a useful tool to evaluate the effectiveness of glycine antagonists in vivo.

Chronic administration of NMDA glycine partial agonists induces tolerance in the Porsolt swim test

The Porsolt swim test (PST) was used to assess behavioral effects following acute or chronic treatment with two N-methyl-D-aspartate (NMDA) glycine partial agonists, 1-aminocyclopropanecarboxylic acid (ACPC), and D-cycloserine (DCS). Consistent with previous findings in mice, single intravenous doses of ACPC in rats produced a significant, dose-dependent reduction in immobility in the PST compared to saline. Single dose DCS also elicited significant dose-dependent reductions in PST immobility times. Single-dose ACPC or DCS (200 mg/kg) reduced immobility (p < 0.05) by 26 or 30%, respectively, compared to saline. However, multiple dosing with either ACPC or DCS (6 daily doses, 200 mg/kg) produced an apparent behavioral adaptation, as the immobility data were indistinguishable from chronic saline administration. Moreover, pretreatment with a 5-day course of ACPC or DCS promoted the development of a behavioral cross-tolerance following a sixth dose of DCS or ACPC, respectively. The development of a behavioral tolerance in the PST following chronic therapy of these drugs appears to be a general feature of glycine partial agonists. In toto, these findings support the hypothesis that chronic administration of NMDA glycine partial agonists produces a behavioral tolerance putatively through an adaptation of the NMDA receptor complex.

Prolonged pre-exposure to 1-aminocyclopropanecarboxylic acid protects against subsequent glutamate toxicity in vitro

Sustained 20 h pre-exposure to 1 mM 1-aminocyclopropanecarboxylic acid (ACPC, which was removed 30 min before addition of 25 microM glutamate) significantly reduced the subsequent neurotoxicity of glutamate in cultured forebrain and cerebellar neurons. The magnitude of neuronal protection was further enhanced if the neurons pretreated with ACPC were re-exposed to ACPC during glutamate challenge. These results closely resemble earlier findings with cultured spinal cord neurons and indicate that these primary cell culture preparations might be suitable for the assessment of the mechanism(s) underlying chronic ACPC-induced modification of the NMDA receptor complex.

Melatonin blocks the induction of long-term potentiation in an N-methyl-D-aspartate independent manner

Perfusion of 100 microM melatonin had no effect on low frequency synaptic transmission, but prevented the induction of tetanically induced long-term potentiation (LTP) when recorded in the dendritic region of the CA1 in rat hippocampal slices. Perfusion of 100 microM melatonin in this preparation had no effect on the multiple population spikes recorded in Mg2+-free medium, and, in grease-gap recordings from the CA1-subiculum slice, 100 microM melatonin had no effect on depolarisations evoked by N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). This suggests that melatonin has the ability to prevent the formation of LTP, and that this effect is not mediated by blockade of NMDA receptors.

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

Glycine causes increased excitability and neurotoxicity by activation of NMDA receptors in the hippocampus

Glycine is an inhibitory neurotransmitter in the spinal cord and also acts as a permissive cofactor required for activation of the N-methyl-D-aspartate (NMDA) receptor. We have found that high concentrations of glycine (10 mM) cause marked hyperexcitability and neurotoxicity in organotypic hippocampal slice cultures. The hyperexcitability, measured using intracellular recording in CA1 pyramidal neurons was completely blocked by the NMDA receptor antagonist MK-801 (10 microM), but not by the AMPA receptor antagonist DNQX (100 microM). The neurotoxicity caused by glycine occurred in all regions of hippocampal cultures but was most marked in area CA1. There was significant CA1 neuronal damage in cultures exposed to 10 mM glycine for 30 min or longer (P < 0.01) or those exposed to 4 mM glycine for 24 h compared to control cultures (P < 0.01). The NMDA antagonists MK-801 (10 microM) and APV (100 microM) significantly reduced glycine-induced neuronal damage in all hippocampal subfields (P < 0.01). The AMPA antagonists CNQX, DNQX, and NBQX (100 microM) had no effect on glycine-induced neuronal damage. High concentrations of glycine therefore appear to enhance the excitability of hippocampal slices in an NMDA receptor-dependent manner. The neurotoxic actions of glycine are also blocked by NMDA receptor antagonists.

Picolinic acid and indole-2-carboxylic acid: two types of glycinergic compounds modulate motor function differentially

1. A putative agonist for the strychnine-sensitive glycine receptor picolinic acid was tested for its anticonvulsant activities in mice and muscle-relaxant activities in rats and compared with indole-2-carboxylic acid (I2CA), an antagonist for the strychnine-insensitive glycine receptor. Their effects on segmental reflexes in the cat spinal cord were examined to elucidate their sites of action. 2. Picolinic acid (200 and 400 mg/kg IP) delayed the onsets of strychnine- but not pentylenetetrazole-induced seizures. It delayed the onsets of bicuculline-induced seizures only at the higher dose. I2CA (200 and 400 mg/kg IP) delayed the onsets of these 3 kinds of seizures. Both compounds reduced muscle tone in rat decerebrate rigidity at a dose of 100 mg/kg IV. 3. Picolinate methylester, a picolinate derivative with higher lipophilicity, depressed spinal reflexes in both intact and spinalized cats at cumulative doses of 25 to 200 mg/kg IV. I2CA (50 mg/kg IV) inhibited spinal reflexes only in intact preparations. 4. These results suggest that the anticonvulsant and muscle-relaxant activities of picolinic acid (PA) are due to inhibition of spinal neurons, but that I2CA selectively affects supraspinal structures.

Substituted indole-2-carboxylates as in vivo potent antagonists acting as the strychnine-insensitive glycine binding site

A series of indole-2-carboxylates bearing suitable chains at the C-3 position of the indole nucleus was synthesized and evaluated in terms of in vitro affinity using [3H]glycine binding assay and in vivo potency by inhibition of convulsions induced by N-methyl-D-aspartate (NMDA) in mice. 3-[2-[(Phenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxyl ic acid (8) was an antagonist at the strychnine-insensitive glycine binding site (noncompetitive inhibition of the binding of [3H]TCP, pA2 = 8.1) displaying nanomolar affinity for the glycine binding site (pKi = 8.5), coupled with high glutamate receptor selectivity (> 1000-fold relative to the affinity at the NMDA, AMPA, and kainate binding sites). This indole derivative inhibited convulsions induced by NMDA in mice, when administered by both iv and po routes (ED50 = 0.06 and 6 mg/kg, respectively). The effect of the substituents on the terminal phenyl ring of the C-3 side chain was investigated. QSAR analysis suggested that the pKi value decreases with lipophilicity and steric bulk of substituents and increases with the electron donor resonance effect of the groups present in the para position of the terminal phenyl ring. According to these results the terminal phenyl ring of the C-3 side chain should lie in a nonhydrophobic pocket of limited size, refining the proposed pharmacophore model of the glycine binding site associated with the NMDA receptor.

Familial and developmental abnormalities of front lobe function and neurochemistry in schizophrenia

structural abnormalities of the cerebral cortex in schizophrenia have been revealed by magnetic resonance imaging, although it is not clear whether these abnormalities are diffuse or local. We predicted that changes in cortical structure would result in abnormalities in biochemical markers for the glutamate system in post-mortem brain, and that the pattern of neurochemical abnormalities would be a clue to the distribution and extent of pathology. A number of studies have now reported increases in biochemical and other markers of glutamatergic cell bodies and terminals in the frontal cortex in schizophrenia. These findings are consistent with the presence of an abnormally abundant glutamatergic innervation, which may be due to an arrest in the normal developmental process of synaptic elimination. In the anterior temporal cortex and hippocampus there is evidence of an asymmetric loss of glutamate terminals, and of reduced GABA function, which may be secondary to the glutamatergic deficit. Glutamate cell body markers are spared in the temporal lobe; we argue that the loss of glutamate uptake sites may reflect the loss of an extrinsic glutamatergic innervation of the polar temporal cortex which arises from the frontal cortex. These fronto-temporal projections may be vulnerable because they arise from a cytoarchitecture which has not been stabilized by remodelling during early post-natal life. There have been several therapeutic studies of drugs with actions on brain glutamate systems. Based on the glutamate deficiency theories, one approach has been to enhance glutamatergic function using agonists of the N-methyl-D-aspartate-linked glycine site. However, there are no clear therapeutic effects, and some studies report aggravation of positive symptoms. This might be expected if, as part of our post-mortem studies suggested, there is excess glutamatergic innervation in some brain regions in schizophrenia. There is neuropsychological evidence that frontal abnormalities in schizophrenia may be genetically determined. We found that first degree relatives of schizophrenic patients were selectively impaired in tests of frontal lobe function, whereas both frontal and temporal function is impaired in patients We conclude that the genetic predisposition to schizophrenia involves impaired frontal lobe function. Psychotic symptoms develop only when a second process results in a loss of fronto-temporal projections and leads to temporal lobe dysfunction.

Acute or prolonged exposure to 1-aminocyclopropanecarboxylic acid protects spinal neurons against NMDA toxicity

1-Aminocyclopropanecarboxylic acid (ACPC) is a high affinity partial agonist for the glycine binding site within the NMDA receptor complex. Chronic treatment with ACPC in vivo appears to reversibly desensitize the NMDA receptor complex, prompting suggestions that it might provide an effective means of ameliorating degenerative mechanisms mediated through this ligand-gated ion channel. In the present experiments, cultured rat spinal cord neurons were used to further examine the effects of acute and sustained ACPC exposures on N-methyl-D-aspartate (NMDA)-induced neurotoxicity. Cell damage was quantitatively assessed using a tetrazolium salt colorimetric assay. With coincubation, 1 mM ACPC significantly reduced the neuronal cell damage caused by 30 min exposure to 25 or 50 microM concentrations of NMDA, but, in contrast to other competitive and non-competitive NMDA receptor antagonists (D-(-)-2-amino-5-phosphonovaleric acid (APV), dizocilpine maleate (MK-801) and 7-chlorokynurenic acid (7-CK)), it failed to alter the cell injury induced by 100 microM NMDA. The protective effect of ACPC was competitively abolished by coaddition of glycine, verifying that it was mediated through glycine binding sites. Sustained 20 h exposure to 1 mM ACPC (which was removed 30 min before addition of 25 microM NMDA) also caused cells to be significantly less responsive to the neurotoxic effects of NMDA. Pre-exposure to ACPC for shorter intervals ( < 1 h) failed to alter subsequent NMDA toxicity. Acute or sustained exposures to ACPC alone did not affect cell viability. These results support earlier indications that: (1) ACPC provides an effective means of antagonizing excitotoxic phenomena, and (2) sustained exposure to ACPC desensitizes the NMDA receptor complex.

Single-channel evidence for glycine and NMDA requirement in NMDA receptor activation

N-Methyl-D-aspartate (NMDA) receptor dose-response relationships that are based on macroscopic currents suggest that NMDA and a different agonist molecule, glycine, must together activate the channel. Since single-channel recordings have a much higher resolution than whole-cell currents, they provide a highly sensitive test for the absolute requirement of NMDA channel opening for glycine. Rapid application of 10-300 microM NMDA to outside-out patches from cultured cortical neurons evoked substantial single-channel activity in the absence of added glycine. However, in the presence of a high affinity and highly selective glycine-site antagonist, 5,7-dichlorokynurenate (DCK), NMDA failed to evoke any openings on its own. Channel openings could not be produced by saturating concentrations of NMDA (up to 1 mM) but were evoked when glycine was added to the test solution. Glycine alone (up to 100 microM) was similarly ineffective in the continuous presence of D(-)-2-amino-5-phosphonovaleric acid (D-APV), an NMDA-site antagonist. Reversal of antagonist blockade by the appropriate ligand (glycine or NMDA) and the normal appearance and duration of channel openings evoked in the presence of either antagonist ruled out open channel block. These single-channel data confirm the hypothesis that both NMDA and glycine are coagonists of the NMDA receptor. Furthermore, the coagonist requirement increases the potential targets for therapeutic drugs aimed at blocking the pathologies resulting from overactivation of NMDA receptors.

Neurochemical mechanisms in brain injury and treatment: a review

This article reviews cellular energy transformation processes and neurochemical events that take place at the time of brain injury and shortly thereafter emphasizing hypoxia-ischemia, cerebrovascular accident, and traumatic brain injury. New interpretations of established concepts, such as diffuse axonal injury, are discussed; specific events, such as free radical production, excess production of excitatory amino acids, and disruption of calcium homeostasis, are reviewed. Neurochemically-based interventions are also presented: calcium channel blockers, excitatory amino acid antagonists, free radical scavengers, and hypothermia treatment. Concluding remarks focus on the role of clinical neuropsychologists in validation of treatment interventions.

Effect of 1-aminocyclopropanecarboxylic acid on N-methyl-D-aspartate-stimulated [3H]-noradrenaline release in rat hippocampal synaptosomes

1. The effect of 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex that exhibits neuroprotective, anxiolytic and antidepressant-like actions, was investigated in a functional assay for presynaptic NMDA receptors. 2. NMDA (100 microM) produced a 36% increase of tritium efflux above basal efflux in rat hippocampal synaptosomes preincubated with [3H]-noradrenaline ([3H]-NA), reflecting a release of tritiated noradrenaline. This effect was prevented by 10 microM 7-chlorokynurenic acid, an antagonist of the glycine site of the NMDA receptor. 3. Glycine enhanced the effect of NMDA with Emax and EC50 values of 84 +/- 11% and 1.82 +/- 0.04 microM, respectively. ACPC potentiated the effect of NMDA on tritium overflow with a lower EC50 (43 +/- 6 nM) and a lower maximal effect (Emax = 40 +/- 9%) than glycine. Furthermore, ACPC (0.1 microM) shifted the EC50 of glycine from 1.82 microM to > or = 3 mM. 4. These results show that ACPC can reduce the potentiation by glycine of NMDA-evoked [3H]-NA release and hence, may act as an antagonist at the glycine site of presynaptic hippocampal NMDA receptors when the concentration of glycine is high.

Benzimidazole-type glycine antagonists: the role of the ring nitrogen atoms

Several derivatives of 1H-benzimidazole-2-carboxylic acid (BICA, 2a) were tested in vitro in comparison to 1H-indole-2-carboxylic acid (ICA, 1e) for their ability to displace [3H]glycine from rat hippocampal membranes. Compound 2a was 8 times more potent than 1e (Ki 5.3 microM, as compared to 42 microM). However, introduction of a carboxymethyl group or a corresponding ester at position 3 had no positive effect on the potency of 2a, while this type of structural modification increased the potency of 1e significantly. Nevertheless, 1-carboxymethyl-BICA (2b) displaced [3H]glycine with similar potency as the corresponding 3-carboxymethyl-ICA 1c, indicating that also a nitrogen atom lacking a hydrogen atom can be engaged in glycine receptor interaction. N-Methylation strongly reduced the potencies of both BICA and ICA derivatives.

The effect of N-acetylaspartate on the intracellular free calcium concentration in NTera2-neurons

We have studied the effect of relatively high concentrations of extracellular N-acetylaspartate (NAA) on the intracellular free calcium concentration [Ca2+]i in NTera2-neurons. While low concentrations of extracellular NAA (0.1, 1 mM) had no effect on the [Ca2+]i, high concentrations of extracellular NAA (3, 10 mM) elicited sharp and statistically significant elevations of [Ca2+]i. Different classes of antagonists of the N-methyl-D-aspartate (NMDA) receptor abolished the NAA induced elevations of the [Ca2+]i, indicating the involvement of the NMDA receptor in NAA-induced elevations of [Ca2+]i.

Temporal response and effects of excitatory amino acid antagonism on microtubule-associated protein 2 immunoreactivity following experimental brain injury in rats

Alterations in microtubule-associated protein 2 (MAP2) immunoreactivity following lateral fluid-percussion (FP) brain injury were investigated in rats with survival times ranging between 10 min and 7 days. MAP2 immunoreactivity was profoundly diminished in the cortex and hippocampus ipsilateral to the site of injury by 10 min and remained diminished up to 7 days after injury. Nissl staining and silver impregnation histochemistry demonstrated a correlation between the loss of MAP2 and neuronal degeneration. The effect of excitatory amino acid receptor antagonism on MAP2 immunoreactivity was evaluated by administering kynurenate or buffer 15 min after FP injury. Administration of kynurenate significantly attenuated the loss of MAP2 observed in the cortex two weeks after injury when compared to buffer treated control animals (P < 0.02). We conclude that significant and prolonged cytoskeletal changes occur following lateral FP brain injury, and that these alterations can be attenuated by blocking excitatory amino acid receptors.

Glycine site NMDA receptor antagonists provide protection against ischemia-induced neuronal damage in hippocampal slice cultures

Ischemia-induced neuronal injury can be reduced by glutamate antagonists acting at the N-methyl-D-aspartate (NMDA) receptor. 7-Chlorokynurenic acid and the recently synthesized compound Acea 1021 block NMDA receptors by acting at the strychnine-insensitive glycine site. The anti-ischemic properties of these compounds were tested by evaluating their ability to reduce CA1 neuronal damage in hippocampal slice cultures deprived of oxygen and glucose. Acea 1021 and 7-chlorokynurenic acid significantly reduced CA1 injury produced by oxygen and glucose deprivation in a dose-dependent manner. The neuroprotective effect of these compounds was reversed by the addition of glycine. The phencyclidine site NMDA antagonist MK-801 also provided significant protection to CA1 neurons against the same insult, and this protection was not affected by the addition of glycine. These results indicate that Acea 1021 and 7-chlorokynurenic acid can provide protection to CA1 neurons against ischemia-induced injury by a glycine-sensitive mechanism.

Histological and ultrastructural changes in the rat brain following systemic administration of picolinic acid

Picolinic acid was administered intraperitoneally in a dose of 30, 60, or 100 mmol, once every 24 h for 8 days. Histologically, under normal conditions as well as when picolinic acid was administered in a dose of 30 mmol the brain formations exhibited characteristic features. When picolinic acid was administered in a dose of 60 mmol or 100 mmol, the alterations were profound and developed selectively in hippocampus, being much less intense in the substantia nigra and striatum. In such cases, injuries of neuronal cell bodies were accompanied by symptoms of spongiosis. Within the hippocampus, the neuronal cell body injury was selectively restricted to the hilar and CA3 regions of stratum pyramidale. Tissue spongiosis was more intense at the granular layer, particularly within the hilus and in the mossy fiber area at CA3. Histochemically, a variable intensity of the reaction of succinic and alpha-glycerophosphate dehydrogenases was demonstrated. A decrease in their activities was observed in areas where the neuronal cell body injuries and spongiosis took place. No changes in the Ca-ATP-ase activity in brain formation after picolinic acid treatment were observed. Ultrastructurally, the changes within substantia nigra were manifested by neuronal cell bodies of the dark type and dendritic degenerations. Also less damaged neuronal cell bodies were seen. They were swollen, depleted of polyribosomes with dilated elements of RER and altered mitochondria. Some of the dendritic profiles were swollen with lucent cytoplasm. Most of the boutons in synaptic contact zones were unchanged. Most presynaptic terminals which were in junction with dark dendrites were swollen with or without crystal-like aggregates of synaptic vesicles.

Kynurenate is neuroprotective following experimental brain injury in the rat

Pharmacologic inhibition of excitatory amino acid neurotransmission improves physiologic, metabolic, and neurobehavioral outcome following experimental brain trauma. However, no studies to date have demonstrated pharmacologically-induced attenuation of histopathological changes associated with experimental brain injury models. The present study examined the effects of kynurenate, an NMDA and non-NMDA receptor antagonist, on neuronal survival in the hippocampus after lateral fluid-percussion brain injury in the rat. Animals (n = 10/treatment) randomly received an intravenous injection of either kynurenate (300 mg/kg) or buffer (equal volume) 15 min following fluid-percussion brain injury of moderate severity. Two weeks after injury, animals were sacrificed and neuronal cell loss in the hippocampus was examined with Nissl staining. Selective loss of neurons in the CA3 region of the hippocampus, which has previously been characterized in this model of brain injury, was found to be significantly attenuated following kynurenate treatment (P < 0.05). These data suggest that pharmacologic compounds which are known to have beneficial effects on neurobehavioral and physiological outcome following brain injury may also significantly attenuate post-traumatic neuronal cell loss. Our results also support other recent data that pharmacological intervention with an excitatory amino acid receptor antagonist may be of therapeutic value in the treatment of brain injury.

1-Aminocyclopropanecarboxylic acid protects against dynorphin A-induced spinal injury

Lumbar subarachnoid injection of dynorphin A causes an ischemia-induced neuronal degeneration and persistent hindlimb paralysis. The protective effects of a variety of competitive and non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists indicate that activation of the NMDA receptor complex is essential for dynorphin A-induced spinal cord injury. 1-Aminocyclopropanecarboxylic acid (ACPC) is a high affinity, partial agonist at strychnine-insensitive glycine receptors associated with the NMDA receptor complex. Pretreatment of rats with ACPC (100 and 200 mg/kg, i.p., 30 min prior to dynorphin A) significantly eliminated the persistent hindlimb motor deficits and neuropathological changes produced by 20 nmol of this peptide. The neuroprotective effects of ACPC (100 mg/kg, i.p.) were abolished by parenteral administration of glycine (800 mg/kg, 30 min prior to ACPC), consistent with other in vivo and in vitro studies indicating that the pharmacological actions of ACPC are effected through strychnine-insensitive glycine receptors. When given instead as six daily injections (200 mg/kg, i.p.) followed by an injection-free day, ACPC also significantly improved neurological recovery following dynorphin-A injection. These results support earlier indications that: (1) activation of the NMDA receptor complex plays a critical role in mediating dynorphin A-induced rat spinal cord injury; (2) ACPC provides an effective means of antagonizing excitotoxic phenomena; and (3) chronic administration of ACPC can elicit a persistent change in the NMDA receptor complex.

Anticonvulsant activity of antagonists for the NMDA-associated glycine binding site

Coupled to the N-methyl-D-aspartate (NMDA) receptor-channel complex is a strychnine-insensitive binding site for glycine. Pharmacological antagonism of glycine binding at this site can produce anticonvulsant activity. Derivatives of the glycine antagonists kynurenic acid and 2-carboxy-indole were synthesized and evaluated for anticonvulsant effects. Compounds were tested in mice against seizures induced by electroshock and pentylenetetrazole, and in the rotorod assay for neurological deficit. The derivatives were also assayed for binding at the NMDA-associated glycine site. The most potent anticonvulsant was ethyl 4-methylamino-5,7-dichloro-2-quinoline carboxylate. This compound provided protection against maximal electroshock (MES) induced seizures at a dose level including 5-fluoro-2-indole carboxylic acid and the diethyl ester of 2,6-pyridine dicarboxylic acid.

Antagonists of excitatory amino acids and endogenous opioid peptides in the treatment of experimental central nervous system injury

Trauma to the central nervous system can lead to primary injuries occurring at the time of impact as well as secondary or delayed injury processes that can result from cellular hypoxia, oligemia/ischemia, edema and swelling, and intracranial hypertension that are manifested over a period of hours to weeks after the initial event. Although the mechanisms underlying delayed tissue injury are poorly understood, they appear to be associated with endogenous neurochemical changes resulting from traumatic nervous system injury. These neurochemical changes may include excessive neurotransmitter release, deregulation of ion homeostasis, and the synthesis, release, or activation of various "autodestructive" neurochemical factors. Experimental studies over the past decade indicate that these alterations mediate important components of the neurochemical cascade leading to central nervous system injury. Furthermore, pharmacologic manipulations of these neurochemical changes have been reported to attenuate secondary central nervous system damage, ameliorate neuronal death, and promote functional recovery after central nervous system injury. This article focuses on the role of excitatory amino acid neurotransmitters, endogenous opioid peptides, and magnesium in the pathophysiology of central nervous system injury and on the therapeutic manipulation of these systems to improve functional outcome after central nervous system injury.

Glutamate and glycine decrease the affinity of [3H]MK-801 binding in the presence of Mg2+

The NMDA receptor is coupled to a cation-selective ion channel, which has been implicated in important brain functions such as long-term potentiation and burst firing, and in neuronal death associated with stroke and epilepsy. We have investigated the binding properties of [3H]MK-801, which binds selectively to the open state of the NMDA channel, at physiological concentrations of Mg2+ in membrane preparations of the rat cerebral cortex. Glutamate and glycine were found to enhance [3H]MK-801 binding at low concentrations and inhibit [3H]MK-801 binding at high concentrations. The inhibition of [3H]MK-801 binding was due to an enhancement of the dissociation rate constant and was reversed by competitive glutamate and glycine antagonists. These findings could be explained by a glutamate- and glycine-induced decrease in the affinity of [3H]MK-801 binding sites within activated NMDA channels, in the presence of Mg2+. This decrease in [3H]MK-801 affinity may correspond to a decreased affinity of the site where Mg2+ causes a voltage-dependent block of the NMDA channel.

Multiple effects of spermine on N-methyl-D-aspartic acid receptor responses of rat cultured hippocampal neurones

1. The modulation by polyamines of responses to N-methyl-D-aspartic acid (NMDA) was studied using a rapid perfusion system and whole-cell voltage-clamp recording from rat hippocampal neurons in dissociated culture. 2. Concentration jump responses to 100 microM NMDA in the presence of 10 microM glycine revealed potentiation by 3 mM spermine at a membrane potential of +60 mV, but depression at -120 mV; the degree of potentiation at +60 mV was variable from cell to cell while marked depression at -120 mV was observed in all cells. The depression of responses to NMDA by spermine was highly voltage dependent (z delta = 1.17) with an apparent equilibrium dissociation constant for block at 0 mV of 27 mM. 3. Analysis of spermine dose-potentiation curves for responses recorded at +60 mV in the presence of 10 microM glycine revealed a half-maximal effect at 125 microM. Under the same conditions, but at -60 mV, analysis of spermine-evoked depression was performed for cells with less than 5% potentiation at +60 mV, and revealed half-maximal inhibition at 344 microM. 4. Dose-response analysis for the glycine-sensitive activation of NMDA receptors at +60 mV revealed a 3.5-fold increase in apparent affinity for glycine in the presence of 1 mM spermine. This increase in affinity for glycine was accompanied by a 3.3-fold decrease in the rate of development of glycine-sensitive desensitization, and a 2.4-fold decrease in the rate of dissociation of glycine from NMDA receptors, while the rate constant for dissociation of NMDA was not reduced. 5. In the presence of non-saturating concentrations of glycine, spermine-induced potentiation at +60 mV developed with two exponential components: a slow glycine-sensitive component, the amplitude and time constant of which decreased with increasing glycine concentration (30 nM glycine, amplitude = 80.2 +/- 5.1%, tau = 780 +/- 79 ms; 3 microM glycine, amplitude = 22.6 +/- 7.1%, tau = 45 +/- 13 ms), and a faster component (tau < 20 ms at all concentrations of glycine), the amplitude of which varied from cell to cell, and which became larger with increase in concentration of glycine. When responses to the application of spermine were measured in the presence 10 microM L-alanine instead of 100 nM glycine, the slow component of potentiation was absent.(ABSTRACT TRUNCATED AT 400 WORDS)

Desensitization of the NMDA receptor complex by glycinergic ligands in cerebellar granule cell cultures

Glutamate neurotoxicity was examined in cultured cerebellar granule neurons following both prolonged (20-24 h) and brief (45 min) exposure to compounds acting at strychnine-insensitive glycine receptors. Glutamate neurotoxicity was reduced in a concentration-dependent fashion by brief exposure to the glycine partial agonists 1-aminocyclopropanecarboxylic acid (ACPC) and (+-)-3-amino-1-hydroxy-2-pyrrolidone (HA-966) and the competitive antagonist, 7-chlorokynurenic acid (7-CK) with a rank order efficacy: 7-CK > HA-966 > ACPC. Neither D-cycloserine (D-CS) nor glycine affected neurotoxicity produced by maximum glutamate concentrations, while glycine but not D-CS augmented the effects of submaximum glutamate concentrations. Prolonged exposure of cultures to either full (glycine) or partial agonists (ACPC, D-CS, HA-966) abolished the neuroprotective effects of ACPC and significantly diminished the neuroprotective effects of HA-966. In contrast, the neuroprotective effects of 7-CK were only marginally reduced by prolonged exposure to glycinergic ligands, while the neuroprotection afforded by compounds acting at other loci on the NMDA receptor complex (e.g. 2-amino-5-phosphonopentanoate (APV) and dizocilpine (MK-801)) were unaltered. These effects may represent homologous desensitization of the NMDA receptor complex at its strychnine-insensitive glycine receptor induced by prolonged exposure to glycinergic agonists and partial agonists. Nonetheless, levels of the NMDA receptor subunit zeta 1 mRNA were unaffected by prolonged exposure to ACPC, indicating the apparent desensitization could involve a post-translational modification of the NMDA receptor complex.