Competitive antagonism of glycine at the N-methyl-D-aspartate (NMDA) receptor

Phosphodiesterase as a Target for Cognition Enhancement in Schizophrenia

Schizophrenia is a severe mental disorder that affects more than 1% of the population worldwide. Dopamine system dysfunction and alterations in glutamatergic neurotransmission are strongly implicated in the aetiology of schizophrenia. To date, antipsychotic drugs are the only available treatment for the symptoms of schizophrenia. These medications, which act as D2-receptor antagonist, adequately address the positive symptoms of the disease, but they fail to improve the negative symptoms and cognitive impairment. In schizophrenia, cognitive impairment is a core feature of the disorder. Therefore, the treatment of cognitive impairment and the other symptoms related to schizophrenia remains a significant unmet medical need. Currently, phosphodiesterases (PDEs) are considered the best drug target for the treatment of schizophrenia since many PDE subfamilies are abundant in the brain regions that are relevant to cognition. Thus, this review aims to illustrate the mechanism of PDEs in treating the symptoms of schizophrenia and summarises the encouraging results of PDE inhibitors as anti-schizophrenic drugs in preclinical and clinical studies.

Psychostimulant Drugs and Neuroplasticity

Drugs of abuse induce plastic changes in the brain that seem to underlie addictive phenomena. These plastic changes can be structural (morphological) or synaptic (biochemical), and most of them take place in the mesolimbic and mesostriatal circuits. Several addiction-related changes in brain circuits (hypofrontality, sensitization, tolerance) as well as the outcome of treatment have been visualized in addicts to psychostimulants using neuroimaging techniques. Repeated exposure to psychostimulants induces morphological changes such as increase in the number of dendritic spines, changes in the morphology of dendritic spines, and altered cellular coupling through new gap junctions. Repeated exposure to psychostimulants also induces various synaptic adaptations, many of them related to sensitization and neuroplastic processes, that include up- or down-regulation of D1, D2 and D3 dopamine receptors, changes in subunits of G proteins, increased adenylyl cyclase activity, cyclic AMP and protein kinase A in the nucleus accumbens, increased tyrosine hydroxylase enzyme activity, increased calmodulin and activated CaMKII in the ventral tegmental area, and increased deltaFosB, c-Fos and AP-1 binding proteins. Most of these changes are transient, suggesting that more lasting plastic brain adaptations should take place. In this context, protein synthesis inhibitors block the development of sensitization to cocaine, indicating that rearrangement of neural networks must develop for the long-lasting plasticity required for addiction to occur. Self-administration studies indicate the importance of glutamate neurotransmission in neuroplastic changes underlying transition from use to abuse. Finally, plastic changes in the addicted brain are enhanced and aggravated by neuroinflammation and neurotrophic disbalance after repeated psychostimulants.

Role for D-serine within the ventral tegmental area in the development of cocaine's sensitization

Repeated exposure to cocaine results in motor sensitization that, in the ventral tegmental area (VTA), is associated to enhanced glutamate release, which in turn leads to enhanced calcium levels in dopaminergic neurons. Calcium influx activates calcium-calmodulin-dependent protein kinases such as CaMKII. D-Serine could participate on these effects, and the objective was to discern the role of VTA D-serine after a sensitizing regimen of cocaine (10 mg/kg daily), and to discern consequent expression changes in CaMKII and its activated form. For this purpose, D-serine, sodium benzoate (inhibitor of D-amino acid oxidase, the degradating enzyme of D-serine), and 7-chlorokynurenate (inhibitor of the glycine site of NMDA receptors) were injected into the VTA (in either the induction or expression phase of sensitization), and activation state of CaMKII was assessed through blotting. The findings indicated that intra-VTA administration of D-serine (5 mM) and sodium benzoate (100 and 200 microg/microl) during the induction phase (not expression) reliably augmented the expression of behavioral sensitization to cocaine, providing evidence that D-serine in the VTA participates in the initiation of motor sensitization to this psychostimulant drug. Intra-VTA infusions of D-serine, sodium benzoate and 7-chlorokynurenate did not elicit a motor effect of their own. Confirming the important role of NMDA receptors and their activation at the glycine site, the employment of 7-chlorokynurenate (2 and 5 microg/microl) led to blocking of the development of sensitization to cocaine. CaMKII within the VTA was found to participate in D-serine's effects because this kinase, that is activated after repeated cocaine, was further activated after co-treatment with D-serine or sodium benzoate. Besides CaMKII activity was otherwise reduced by 7-chlorokynurenate.

CoMFA, synthesis, and pharmacological evaluation of (E)-3-(2-carboxy-2-arylvinyl)-4,6-dichloro-1H-indole-2-carboxylic acids: 3-[2-(3-aminophenyl)-2-carboxyvinyl]-4,6-dichloro-1H-indole-2-carboxylic acid, a potent selective glycine-site NMDA receptor antagonist

(E)-3-(2-Carboxy-2-phenylvinyl)-4,6-dichloro-1H-indole-2-carboxylic acid, 1, is a potent and selective antagonist of the glycine site of the N-methyl-d-aspartate (NMDA) receptor. Using 3D comparative molecular field analysis (CoMFA) to guide the synthetic effort, a series of aryl diacid analogues of 1 were synthesized to optimize in vivo potency, duration of action, and binding activity. It was found that the incorporation of a substituted aromatic with an electron withdrawing group or a heterocyclic group at the 2-position of the 3-propenyl moiety of 1 gave compounds with better affinity and potency in the murine stroke model. Ultimately this led to the discovery of 3-[2-(3-aminophenyl)-2-carboxyvinyl]-4,6-dichloro-1H-indole-2-carboxylic acid, 19, as a new potent selective glycine-site NMDA receptor antagonist.

N-methyl-D-aspartate antagonists for stroke and head trauma

The N-methyl-D-aspartate (NMDA) receptor is a ligand-gated ion channel which is widely distributed in the central nervous system (CNS), and which mediates most of the fast excitatory neuronal transmission in the CNS. As with other ligand-gated ion channels, the NMDA receptor is a macromolecular complex which possesses a number of intricate regulatory sites within and around a central ion channel. The key regulatory components for which prototypic antagonists have been developed are the competitive NMDA antagonist binding site, the non-competitive NMDA antagonist binding site within the ion channel, and the NMDA receptor-associated glycine antagonist site. The binding domains for each of these binding sites possess discrete and non-overlapping SAR with regard to the chemical series developed to date. The potential utility of NMDA antagonists in the treatment of stroke and traumatic brain injury was investigated soon after the synthesis of the first bioavailable NMDA antagonists. Efficacy in preclinical models was demonstrated with both competitive and non-competitive NMDA antagonists. However, preclinical testing also revealed potentially clinically-limiting side-effects which included phencyclidine (PCP)-like actions indicative of possible psychotomimetic activity, cerebral vacuolisation of limbic cortical neurones, low therapeutic indices relative to incapacitating motor side-effects and, in the case of non-competitive antagonists, hypertension. These limitations have led to the design of clinical trials that should define the therapeutic index for this type of compound in humans. Currently, the first competitive antagonist to enter clinical trials, selfotel, is on hold, while D-CPPene is still in development. The non-competitive antagonist, aptiganel, is currently in Phase III clinical trials and its therapeutic efficacy and index should be defined in 1997 and 1998. The well-defined limitations of competitive and non-competitive NMDA antagonists have been a key impetus in the investigation of alternative approaches to modulating the NMDA receptor complex. In the case of glycine site antagonists, these compounds have been shown in preclinical studies to be devoid of PCP-like actions and the neuronal vacuolisation associated with the competitive and non-competitive NMDA antagonists. This has induced the development of a number of chemical series with at least three compounds currently in Phase I and II clinical trials. These include ACEA 1021, GV150526A and ZD9379. Clinical efficacies and therapeutic indices of these compounds should be defined in 1998 and 1999. An alternative approach using a partial agonist of the glycine site (1-aminocyclopropane-carboxylic acid, ACPC) has been halted in Phase I. Another approach which has led to the development of NMDA receptor antagonists, selective for the NMDA receptor subunits 1A/2B (NR1A/2B subtype), was the discovery in early studies of the neuroprotective actions of ifenprodil. Structural analogues include eliprodil, CP-101,606 and lubeluzole. In the cases of eliprodil and lubeluzole, these compounds have demonstrated neuroprotection in preclinical models, but they possess the extremely dangerous side-effect of increasing cardiac repolarisation time (i.e., increased QTc interval). The therapeutic index for these compounds is low. This has led to the termination of eliprodil's development and has limited the current dosing strategy with lubeluzole. It has not been disclosed if CP-101,606 possesses this dose-limiting side-effect. In summary, strategies for drug design and development based on our knowledge of the NMDA receptor complex have led to the development of a new generation of compounds for the treatment of stroke and traumatic brain injury, which remain to be evaluated in the clinic. The success of this approach will be defined in the next two to three years.

Impaired visual memory in rats reared in isolation is reversed by D-cycloserine in the adult rat

Previous studies have shown that environmental factors can influence cholinergic and glutamatergic activity in the developing brain, and that the variations in neurochemistry are accompanied by behavioral changes in later life. Rats reared in isolated, social, or enriched environments were tested with a visual discrimination task in adulthood. The results show that saline-treated rats reared in isolation exhibited impaired retention of the discrimination task compared to rats raised in social or enriched environments. However, systemic administration of the NMDA receptor agonist, D-cycloserine (3 mg/kg), restored normal memory function in cognitively impoverished rats. Acquisition of the task was not affected by the rearing conditions. D-Cycloserine is considered to be an efficient cognitive enhancer probably able to compensate for assumed loss of NMDA receptors during isolated rearing.

Chronic high-dose glycine nutrition: effects on rat brain cell morphology

BACKGROUND

Facilitation of N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission via administration of glycine site agonists of the NMDA receptor (e.g., glycine, D-serine), and glycine transport inhibitors may represent an innovative pharmacologic strategy in schizophrenia; however, given the potential involvement of NMDA receptors in the neurotoxicity of excitatory amino acids, possible neurotoxic effects of glycinergic compounds need to be explored. Furthermore, studying brain adaptations to chronic administration of glycine site agonists may provide insights into the therapeutic mechanisms of these drugs.

METHODS

Adult rats were randomized to one of three nutritional regimens (no glycine supplementation, 1 g/kg/day, or 5 g/kg/day glycine supplementation) and to one of three treatment durations (1, 3, or 5 months). Serum glycine and serine levels at sacrifice and brain sections were examined using histologic markers of neurodegeneration (cresyl violet and silver impregnation staining) and immunohistochemical staining of glial fibrillary acidic protein, microtubule-associated protein, and neurofilament 200. To explore additional neural adaptations to high-dose glycine treatment, immunostaining was also performed for class B, N-type Ca(2+) channels.

RESULTS

Serum glycine levels increased dose dependently during glycine nutrition, whereas serine levels were not changed. In hippocampal dentate gyrus, the percentage of hypertrophied astrocytes transiently increased at 1 month. At 3 and 5 months of glycine treatment, the density of class B, N-type Ca(2+) channels was reduced in parietal cortex and hippocampus. No evidence of neuronal or glial cell excitotoxic damage or degeneration was registered at either of the treatment intervals studied.

CONCLUSIONS

These findings demonstrate for the first time that in vivo administration of high-dose glycine may induce brain morphological changes without causing neurotoxic effects. A reduction in density of class B, N-type Ca(2+) channels in specific brain regions may represent one general adaptation to long-term, high-dose glycine treatment.

Modulation of NMDA receptor function by ketamine and magnesium: Part I

N-methyl-D-aspartate (NMDA) receptors are important components of pain processing. Ketamine and Mg2+ block NMDA receptors and might therefore be useful analgesics, and combinations of Mg2+ and ketamine provide more effective analgesia. We investigated their interactions at NMDA receptors. Xenopus oocytes, expressing NR1/NR2A or NR1/NR2B glutamate receptors, were studied. The effects of Mg2+, racemic ketamine and its isomers, and the combination of Mg2+ and S(+)-ketamine on NMDA signaling were determined. Mg2+ and ketamine alone inhibited NMDA receptors noncompetitively (half-maximal inhibitory effect concentration: Mg2+ 4.2 +/- 1.2 x 10(-)(4) M at NR1/NR2A and 6.3 +/- 2.4 x 10(-)(4) M at NR1/NR2B; racemic ketamine 13.6 +/- 8.5 x 10(-)(6) M at NR1/NR2A and 17.6 +/- 7.2 x 10(-)(6) M at NR1/NR2B; S(+)-ketamine 4.1 +/- 2.5 x 10(-)(6) at NR1/NR2A and 3.0 +/- 0.3 at NR1/NR2B; R(-)-ketamine 24.4 +/- 4.1 x 10(-)(6) M at NR1/NR2A and 26.0 +/- 2.4 x 10(-)(6) M at NR1/NR2B). The combined application of Mg2+ and ketamine decreased the half-maximal inhibitory effect concentration >90% at both receptors. Isobolographic analysis demonstrated super-additive interactions. Ketamine and Mg2+ inhibit responses of recombinantly expressed NR1/NR2A and NR1/NR2B glutamate receptors, and combinations of the compounds act in a super-additive manner. These findings may explain, in part, why combinations of ketamine and Mg2+ are more effective analgesics than either compound alone.

IMPLICATIONS

Ketamine and Mg2+ inhibit functioning of recombinantly expressed NR1/NR2A and NR1/NR2B glutamate receptors, and combinations of the compounds act in a super-additive manner. These findings may explain, in part, why combinations of ketamine and Mg2+ are more effective analgesics than either compound alone.

Kynurenic acid antagonists and kynurenine pathway inhibitors

The kynurenine pathway accounts for the metabolism of around 80% of non-protein tryptophan metabolism. It includes both an agonist (quinolinic acid) at NMDA receptors and an antagonist (kynurenic acid). Since their discovery, there has been a major development of kynurenic acid analogues as neuroprotectants for the treatment of stroke and neurodegenerative disease. Several prodrugs of kynurenic acid or its analogues that can be hydrolysed within the CNS are also available. More recently, the pathway itself has proved to be a valuable drug target, affected by agents which reduce the synthesis of quinolinic acid and increase the formation of kynurenic acid. The change in the balance of these, away from the excitotoxin and towards the neuroprotectant, has anticonvulsant and neuroprotective properties.

Inhibition of excitotoxic neuronal death by methanol extract of Acori graminei rhizoma in cultured rat cortical neurons

Acori graminei rhizoma (AGR) are reported to exhibit a number of pharmacological actions in the central nervous system. The effects of the methanol extract of AGR on excitotoxic neuronal death were evaluated in the present study using cultured rat cortical neurons. Based on the phase-contrast microscopic examinations of cultures and lactate dehydrogenase activities measured in the culture media, the glutamate-induced excitotoxicity was significantly inhibited by the extract. The inhibitory action of the extract was more potent and selective for the N-methyl-D-aspartate (NMDA) receptor-mediated toxicity. The AGR extract competed with [3H]MDL 105,519 for the specific binding to the glycine site of the NMDA receptor with the IC(50) value of 164.7 microg/ml. Modulation of the NMDA receptor activity by the extract was determined using [3H]MK-801 binding studies. The reduction of the binding in the presence of the extract indicated the receptor inactivation by AGR. These results demonstrated that the methanol extract of AGR exhibited protective action against excitotoxic neuronal death, and that the neuroprotective action was primarily due to the blockade of NMDA receptor function by the interaction with the glycine binding site of the receptor.

Pre-training blocks the improving effect of tetrahydroaminoacridine and D-cycloserine on spatial navigation performance in aged rats

We investigated the effect of pre-training on the improvement of spatial navigation performance provided by a cholinesterase inhibitor, tetrahydroaminoacridine (3 mg/kg, i.p.), and a positive modulator of NMDA receptor, D-cycloserine (10 mg/kg, i.p.), or their combination in aged rats. Pre-training consisted of spatial or non-spatial conditions and took place in either the same or a separate room. We found that any kind of pre-training was able to eliminate the enhancing effect of tetrahydroaminoacridine and D-cycloserine on spatial navigation. However, none of these pre-training conditions was able to block the age-related deficit in spatial navigation. These results indicate that tetrahydroaminoacridine and D-cycloserine, separately or in combination, do not themselves alleviate the age-related spatial memory deficit, but may enhance procedural aspects of water maze learning in aged rats.

3-(2-Carbamoylvinyl)-4,5-dimethylpyrrole-2-carboxylic acids as ligands at the NMDA glycine-binding site: a study on the 2-carbamoylvinyl chain modification

Twenty 4,5-dimethylpyrrole-2-carboxylic acids (5a-t) with different 2-carbamoylvinyl chains in position 3 were prepared to further investigate the relationships between structure and in vitro affinity for the strychnine-insensitive glycine-binding site. None of these compounds was superior to (E)-3-(N-phenyl-2-carbamoylvinyl)-4,5-dimethylpyrrole-2-carb oxylic acid III (pKi = 6.70), which was taken as a reference standard, but overall the results obtained indicate that the N-phenyl-2-carbamoylvinyl substituent of III may be replaced with the N-(1-adamantyl)-2-carbamoylvinyl group as in 5h (pKi = 6.20) without considerable loss of affinity. This finding adds to previous knowledge.

Adverse psychological impact, glutamatergic dysfunction, and risk factors for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cell loss and pathological changes in neuronal transmission. In particular, malfunction in glutamatergic activity may be associated with the impairment of memory seen in Alzheimer patients. Both hypoactivation and hyperactivation of glutamatergic systems seem to cause impeded cognitive processing in animals. Rats subjected to rearing in isolation display reduced levels of glutamate in temporal regions accompanied by impaired learning and memory. Similar cognitive deficits are also seen in animals exposed to behavioral stress. Stress appears to have deleterious effects on cognition caused by glutamate neurotoxicity leading to attenuated synaptic activity. It is suggested that stress may represent a potential risk factor for AD. The known risk factors for AD (age, heredity, head trauma, low education, depression) may all be related to glutamatergic dysfunction. Some difficulties with pharmacological approaches based on glutamatergic agonists are discussed. It is suggested that optimal glutamate-mediated neurotransmission throughout life may prevent the occurrence of mental decline associated with AD.

Determination of the excitatory potencies of fluoroquinolones in the central nervous system by an in vitro model

Fluoroquinolones have been reported to induce central nervous system side effects, including seizures and psychiatric events. Although relatively rare in patients up to now, the proconvulsant activity depends on the chemical structure and might be a critical endpoint of some new representatives of this valuable class of antimicrobials. The electrophysiological determination of field potentials in the CA1 region of the rat hippocampus slice allowed an assessment of the excitatory potential of fluoroquinolones and might be predictive for their neurotoxic potency in vivo. An optimization of this method and its extension to other fluoroquinolones resulted in a defined rank order. Well-known already-marketed quinolones as well as some fluoroquinolones under evaluation and development were used. The dose range tested was between 0.5 and 4 mumol/liter, which was comparable to the therapeutic concentration in the brain. All tested compounds increased the population spike amplitude in a concentration-dependent manner, and the resulting excitatory potency was highly dependent on the chemical structure, with compounds ranging from least to most excitatory as follows: ofloxacin, ciprofloxacin, nalidixic acid, moxifloxacin, BAY x 8843, [corrected] fleroxacin, lomefloxacin, enoxacin, clinafloxacin (much more excitatory than enoxacin), tosufloxacin, trovafloxacin, BAY 15-7828, and BAY x 9181 (much more excitatory than BAY 15-7828). The proposed hippocampus slice model not only is suitable for giving valuable alerts as to convulsive potential during candidate selection but also enables mechanistic investigations. These investigations pointed to the N-methyl-D-aspartate receptor as the probable target of the fluoroquinolone effects.

(E)-3-(2-(N-phenylcarbamoyl)vinyl)pyrrole-2-carboxylic acid derivatives. A novel class of glycine site antagonists

The synthesis and preliminary biological evaluation of novel (E)-3-(2-(N-phenylcarbamoyl)-vinyl)pyrrole-2-carboxylic acids bearing alkyl, acyl, alkoxy, phenyl, and halo substituents at the 4- and 5-positions of the pyrrole ring are reported. These compounds were studied for their in vitro affinity at the strychnine-insensitive glycine-binding site of the N-methyl-D-aspartate (NMDA) receptor complex. In the [3H]glycine binding assay (E)-4,5-dibromo-3-(2-(N-phenylcarbamoyl)vinyl)pyrrole-2-carboxylic acid 6w (pKi = 7.95 +/- 0.01) and the 4-bromo-5-methyl 6j (pKi = 7.24 +/- 0.01) and 4,5-dimethyl 6g (pKi = 6.70 +/- 0.03) analogues were the most active compounds of the series. Qualitative structure-activity analysis points to a negative correlation between bulk of the C-4 and C-5 substituents and affinity which is enhanced by halo-substituents. QSAR analysis by the Hansch descriptors F, R, pi, and MR, on a subset of compounds with pKi > or = 4, indicates that electron-withdrawing groups at C-4 and C-5 enhance the affinity. Bulk and lipophilicity are also relevant for the substituents at these positions. 6g was found to be a full antagonist (alpha = 0; enhancement of the [3H]TCP binding). The in vivo potency of 6g, 6j, and 6w was evaluated by the inhibition of NMDA-induced convulsions in mice by both the i.v. and po routes; 6w was the most active compound (ED50 = 3 x 10(-3) (0.8-10) g/kg, i.v. and 30 x 10(-3) (4.5-61) g/kg, p.o.). The results of this study indicate that the 3,4-disubstitutedpyrrole-2-carboxylate represents a novel template for the design of new glycine antagonists.

Tetrahydroaminoacridine and D-cycloserine stimulate acquisition of water maze spatial navigation in aged rats

We investigated the effect of tetrahydroaminoacridine, a cholinesterase inhibitor and D-cycloserine (a partial glycine-B agonist of the NMDA receptor complex) on the defect of water maze spatial navigation in rats induced by aging. Tetrahydroaminoacridine (3 mg/kg, i.p.) or D-cycloserine (10 mg/kg, i.p.) enhanced acquisition of the water maze task. A combination of subthreshold doses of tetrahydroaminoacridine (1 mg/kg) and D-cycloserine (3 mg/kg) improved water maze acquisition, but a combination of lower subthreshold doses (tetrahydroaminoacridine 0.3 mg/kg + D-cycloserine 1 mg/kg) was ineffective. Consolidation in water maze test was not improved by tetrahydroaminoacridine (3 mg/kg) and/or D-cycloserine (10 mg/kg). The results suggest that tetrahydroaminoacridine and D-cycloserine synergistically enhance acquisition of spatial navigation in aged rats.

Glycine increases arterial pressure and augments NMDA-induced pressor responses in the dorsomedial and ventrolateral medulla of cats

The present study is designed to determine and characterize two neurobiological events. Firstly, we investigated whether increases of systemic arterial pressure (SAP) and sympathetic vertebral nerve activity (VNA) produced by microinjection of glycine (Gly) in the dorsomedial (DM) or rostral ventrolateral medulla (RVLM) are mediated by pressor neurons in DM or RVLM. Secondly, we assessed whether simultaneous microinjections of Gly and N-methyl-D-aspartate (NMDA) in DM or RVLM potentiate the NMDA-pressor effects. Changes in SAP and VNA were recorded in 33 cats under alpha-chloralose and urethane anesthesia. Microinjection of sodium glutamate (Glu, 0.25 M, 30 nl) or Gly (1.0 M, 30 nl) into the DM or RVLM increased SAP and VNA in similar magnitude. Latencies of changes in SAP and VNA induced by Gly, however, were longer (3 s) than those induced by Glu. Prior microinjection of the following antagonists blocked the Gly-induced pressor responses: 2-amino-5-phosphonopentanoate (AP-5, 25 mM, 30 nl), a specific NMDA receptor antagonist; or glutamate diethyl ester (GDEE, 0.5 M, 30 nl), a quisqualate receptor antagonist; or kynurenic acid (KYN, 10 mM, 30 nl), a broad spectrum competitive Glu antagonist. Prior treatment with strychnine (3 mM, 30 nl), a specific Gly antagonist, also blocked the Gly-induced pressor responses. Since Gly is believed to be an inhibitory neurotransmitter, these data suggest that Gly may produce pressor actions via an inhibition on specific inhibitory neurons synapsing with the pressor neurons. NMDA (0.1 M, 30 nl) and Gly (1.0 M, 30 nl) microinjected simultaneously in DM or RVLM produced a greater pressor action than NMDA alone. This potentiation was blocked by KYN, another known antagonist for such potentiation, but was only partially blocked by strychnine.

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.

Infusion of D-cycloserine into temporal-hippocampal areas and restoration of mnemonic function in rats with disrupted glutamatergic temporal systems

Partial transections of the fiber connections between the temporal cortex and the lateral entorhinal cortex at a site of the white matter corresponding to the perirhinal cortex result in impaired visual memory accompanied by reduced concentrations of glutamate in both the temporal cortex and lateral entorhinal cortex. Intraperitoneal administration of the glycinergic receptor agonist D-cycloserine produces complete restoration of memory function, as measured by a brightness discrimination task in rats with temporal cortex/lateral entorhinal cortex transections. The purpose of the present study was to identify in which brain structures the compensatory activity might take place. The results show that infusion of cycloserine into either the temporal cortex or lateral entorhinal cortex fully ameliorated the impairment of temporal cortex/lateral entorhinal cortex lesions, whereas infusion into the hippocampal region caused only a mild improvement of the retention performance. Infusion of cycloserine into the frontal cortex or saline into the temporal cortex or lateral entorhinal cortex had no ameliorating effects on the memory dysfunction of rats bearing temporal cortex/lateral entorhinal cortex transections. It is concluded that the temporal cortex, lateral entorhinal cortex and perirhinal cortex are highly critical in forming visual memory.

Modulatory treatment of NMDA receptors in neonatal rats affects cognitive behavior in adult age

The glutamatergic NMDA receptor is probably involved in establishing functional connections during development, and interference may promote or impair cognitive functions in adult age. In the present study, rat pups received one daily injection of the NMDA receptor partial agonist D-cycloserine in various concentrations (3, 10, 50 mg/kg), the NMDA receptor antagonist (+/-)-HA-966 (30 mg/kg), or saline throughout postnatal days 10-20 (Experiment 1). In Experiment 2, effects of the (+)-enantiomer of HA-966 were similarly examined. The rats were tested in a novelty task in adult age (postnatal days 98-112). The results from Experiment 1 show that injections of D-cycloserine in the concentration of 10 mg/kg or (+/-)-HA-966 caused a slight increase in locomotor activity only. The results from Experiment 2 show that (+)-HA-966-treated rats displayed reduced preference for novelty, a slight reduction in exploratory activity and locomotor behavior, and increased rate of grooming. These results suggest that neonatal treatment with (+)-HA-966 can impair cognitive behavior in adult life. It was not possible to record any effects on cognitive function after neonatal administration of the glutamatergic agonist D-cycloserine.

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.

Nicotine and D-cycloserine enhance acquisition of water maze spatial navigation in aged rats

We investigated the effect of nicotine, a nicotinic agonist, and D-cycloserine (DCS, a partial glycine-B agonist of the N-methyl-D-aspartate (NMDA) receptor complex) on aging-induced defects of water maze (WM) spatial navigation in rats. Nicotine (0.3 mg kg-1, s.c.) or DCS (10 mg kg-1, i.p.) enhanced acquisition of the WM task. A combination of subthreshold doses of nicotine (0.1 mg kg-1) and DCS (3 mg kg-1) improved WM acquisition. A subthreshold dose of a competitive NMDA antagonist, CPP (1 mg kg-1), blocked the effect of nicotine (0.3 mg kg-1) on WM acquisition. A nicotine antagonist, mecamylamine (10 mg kg-1), impaired WM acquisition and had no effect on retention, but did not block the effect of DCS 10 mg kg-1. The results suggest that nicotine and DCS synergistically enhance spatial navigation in aged rats.

Volatile anesthetics and glutamate activation of N-methyl-D-aspartate receptors

Several studies have indicated important functional interactions between volatile anesthetics and the N-methyl-D-aspartate (NMDA) class of glutamate receptors. In the present study, we examined the effects of diethyl ether, chloroform, methoxyflurane, halothane, enflurane, and isoflurane on (1) glutamate activation of the NMDA receptor complex, including glycine reversal of anesthetic action, as revealed by [3H]-(5R, 10S)-(+)methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine, dizocilpine (MK-801) binding to the cation channel, and (2) [3H]cis-4-( phosphonomethyl)piperidine-2-carboxylic acid (CGS 19755) binding to the glutamate recognition site of the NMDA receptor In agreement with previous studies, glutamate increased the binding of 1 nM [3H]MK-801, measured after a 1-hr incubation at 37 degrees, by up to several hundred fold. This stimulation was blocked by glutamate antagonists and potentiated by glycine with an EC50 of approximately 0.03 muM. Glycine also had a direct stimulatory effect on [3H]MK-801 binding at much higher concentrations ( > or = 10 muM). All of the anesthetics examined depressed glutamate stimulation of [3H]MK-801 binding in a concentration-dependent manner with the following order of potency: halothane > or = enflurane > methoxyflurane > chloroform > diethyl ether. This inhibition of [3H]MK-801 binding was observed at concentrations that are routinely attained in the cerebrospinal fluid during surgical anesthesia. Moreover, the inhibition was reversed rapidly following removal of the anesthetics from the assay medium. Inclusion of glycine in the incubation medium markedly attenuated anesthetic-induced inhibition of glutamate-sensitive [3H]MK-801 binding with an EC50 of between 0.1 and 1 muM. Thus, this reversal by glycine correlated with its potentiating as opposed to its direct stimulatory, effect on NMDA receptors. Anesthetic inhibition of [3H]MK-801 binding could not be overcome by raising the glutamate concentration (i.e. the interaction did not appear to be competitive with respect to glutamate) unless glycine was present. Binding of [3H]CGS 19755 to the glutamate recognition site was also inhibited by each of the anesthetics examined. However, with the exception of chloroform, all of the anesthetics were more potent inhibitors of glutamate-stimulated [3H]MK-801 binding than they were of [3H]CGS 19755 binding. [3H]CGS 19755 binding saturation curves in the presence of halothane and enflurane indicated a decrease in the density of [3H]-CGS 19755 binding sites with no change in binding affinity (i.e. the inhibition did not appear to be competitive). These findings support the idea that anesthetic drugs disrupt NMDA receptor transmission through multiple allosteric effects on the receptor-channel activation mechanisms and the glutamate binding site.

R-(+)-HA-966, an antagonist for the glycine/NMDA receptor, prevents locomotor sensitization to repeated cocaine exposures

Repeated administration of cocaine results in a reverse tolerance or sensitization to the locomotor stimulant properties of cocaine. In this study, we examined the effect of an antagonist for the strychinine-insensitive glycine receptor of the NMDA receptor complex, R-(+)-HA-966, on the development of locomotor sensitization to cocaine. Co-administration of R-(+)-HA-966 with repeated cocaine prevented locomotor sensitization to a subsequent challenge dose of cocaine. However, R-(+)-HA-966 (15 mg/kg i.p.) did not attenuate locomotor activation to an acute dose of cocaine (15 mg/kg). These results indicate that the glycine/NMDA receptor is involved in locomotor sensitization to repeated cocaine administration but not in the locomotor activation to the acute stimulant effects of cocaine administration.

Characterization of sodium-independent beta-alanine binding to cerebral cortical membranes from 7-day-old and adult mice

The sodium-independent binding of beta-alanine to cerebral cortical membranes from adult (3- and 12-month-old) and developing (7-day-old) mice was characterized for the first time. The binding was saturable in each age group, consisting of only one component. The affinity for beta-alanine was highest and the number of available binding sites greatest in young animals. The binding was not affected by strychnine, but inhibited by beta-alanine itself, glycine, L-alanine and L-serine, the IC50 values being lower in immature mice. Glycine was shown to be a competitive inhibitor. The binding was also inhibited, albeit only in adults, by N-methyl-D-aspartate receptor antagonists acting at the glycine modulatory site and by some GABAergic substances. It is concluded that even though beta-alanine may possess binding sites of its own, particularly in the immature cerebral cortex, beta-alanine could at least partly bind to strychnine-insensitive glycine sites in the N-methyl-D-aspartate receptor complex.

Glycine inhibition of glutamate evoked-release of norepinephrine in the hypothalamus is strychnine-insensitive

The effects of glycine (GLY), D-serine (D-SER), kynurenic acid (KYN), and 7-Cl kynurenic acid (7-Cl KYN), on the evoked release of endogenous norepinephrine (NE) by L-glutamate (L-GLU) in rat hypothalamic slices was investigated. KYN (500 and 100 microM) and 7-Cl KYN (10 microM) blocked evoked release of NE by L-GLU (1 mM). The inhibitory effects of 100 microM KYN on evoked release of NE by L-GLU were reversed by 10 microM and 100 microM D-SER and, but not 10 or 100 microM GLY. When KYN was not present in the superfusate, superfusion with either 10 or 100 microM GLY alone simultaneously with 1 mM L-GLU blocked the evoked release of NE produced by L-GLU. The addition of strychnine (3 or 100 microM) to the superfusate did not block the inhibitory effects of 10 microM GLY on L-GLU-evoked NE release. These findings suggest that a novel strychnine-resistant GLY receptor is present in the rat hypothalamus.

Excitatory amino acid-induced excitation of dopamine-containing neurons in the rat substantia nigra: modulation by kynurenic acid

Kynurenic acid (KYNA), an endogenous antagonist of ionotropic excitatory amino acid (EAA) receptors, was tested for its ability to modulate N-methyl-D-aspartate (NMDA)- and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced excitation of dopamine (DA)-containing neurons in the zona compacta of the rat substantia nigra (SNc). Experiments were conducted using extracellular recording techniques in conjunction with an in vitro brain slice preparation. Bath application of NMDA (1-20 microM) or AMPA (0.5-10 microM) produced a concentration-dependent increase in the firing rate of SNc DA neurons but had no effect on firing pattern. The highest concentration of both agonists produced a rapid and reversible cessation of activity that was attributed to acute induction of depolarization block. Addition of glycine (GLY) (up to 100 microM) to the bathing solution had no effect on either basal firing rate or the increase in activity produced by NMDA. KYNA (10 microM-1 mM) antagonized the excitatory effects of both NMDA (15 microM) and AMPA (3 microM) in a concentration-dependent fashion (IC50:102 microM and 64 microM, respectively) without affecting basal firing rate. Perfusion of tissue slices with a modified Ringer's solution containing low Mg2+ (0.12 mM) increased NMDA-induced excitation but did not affect the antagonist properties of KYNA. D-serine (100 microM) reversed the ability of KYNA to block the excitatory effects of NMDA, suggesting that KYNA attenuates NMDA-induced excitation of SNc DA neurons via blockade of the GLY allosteric site on the NMDA receptor. The ability of KYNA to modulate the excitatory effects of both NMDA and non-NMDA agonists implies that endogenous KYNA may play a physiological role in regulating DA cell excitability.

A putative cytoprotective receptor in the kidney: relation to the neuronal strychnine-sensitive glycine receptor

The neutral amino acid glycine has been demonstrated to prevent cell death in numerous cell types exposed to a variety of toxic insults. Recently, the central nervous system (CNS) glycine antagonist strychnine was demonstrated to bind specifically to the plasma membrane of renal proximal tubules (RPT) and mimic glycine cytoprotection. Further, it has been demonstrated in RPT that glycine and strychnine block chloride influx in the late stages of cell injury. The aim of this study was to determine if the RPT cytoprotective site is related to neuronal glycine receptors. Only antagonists to the CNS strychnine-sensitive glycine receptor (strychnine, brucine), and not antagonists to the glycine modulatory site of the NMDA receptor (DCQX, 7-CKA, HA-966) or the GABAA receptor (bicuculline methiodide, picrotoxin), prevented mitochondrial inhibitor (antimycin A)-induced RPT cell death. Using immunoblot analysis, proteins corresponding to the 58 kDa beta-subunit of the strychnine-sensitive glycine receptor and the associated protein gephyrin were identified in rabbit kidney cortical membrane fractions and RPT. No protein corresponding to the 48 kDa alpha-subunit was identified. Thus, glycine and strychnine may exert their cytoprotective effects via a putative plasma membrane receptor that is related to the strychnine-sensitive glycine receptor found in the CNS.

Strychnine-insensitive glycine binding to cerebral cortical membranes in developing and ageing mice

The strychnine-insensitive binding of [3H]glycine was characterized in purified cerebral cortical membranes from mice aged from 7 days to 22 months. The binding was saturable, exhibiting only one component during the whole life-span studied. The binding constant KD did not change during development and ageing, whereas the maximal binding capacity Bmax, calculated per protein content, increased up to the age of two weeks and then again in ageing animals (18- and 22-month-olds). The binding was similarly inhibited by the antagonists 7-chlorokynurenate, 3-amino-1-hydroxypyrrolidin-2-one (HA-966) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in 7-day-, 3-month- and 12-month-old mice. The inhibition caused by glycine, L-serine and beta-alanine also remained unaltered during the whole life-span. beta-Alanine was a noncompetitive inhibitor. The alterations in the maximal binding capacities during development and ageing could be of importance in the regulation of NMDA receptors, which have been implicated in synaptic potentiation, developmental processes and various pathological conditions.

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.

Animal models of Alzheimer's disease: glutamatergic denervation as an alternative approach to cholinergic denervation

Alzheimer's disease (AD) is a neurodegenerative disorder that severely reduces lifespan. In this article, a new, glutamatergic denervation model of AD is presented as a supplement to the well known cholinergic one, because these models are trying to mimic different aspects of the pathology in AD. Impaired memory and disorientation are prominent features in the symptomatology of AD. In searching for neurochemical systems associated with the initial cognitive disorders of AD, a reorientation from cholinergic to glutamatergic systems is suggested. Results from recent behavioral studies of damage to the temporal and entorhinal cortices in rats imply that these structures are strongly involved in mnemonic function. Findings from Alzheimer brains and laboratory animals indicate that major losses of glutamatergic receptors may underly the cognitive impairment seen in AD patients. A growing body of evidence appears to support a glutamatergic hypothesis of AD. Possible pharmacological approaches are suggested.

Ethanol inhibits NMDA receptor-mediated excitotoxicity in rat primary neuronal cultures

Excessive or prolonged stimulation of N-methyl-D-aspartate (NMDA) receptors appears to play an important role in many neurodegenerative processes in brain through a process known as excitotoxicity. This study examined the effects of ethanol on NMDA receptor-mediated excitotoxicity in primary neuronal cultures obtained from embryonic rat whole brain. Neurotoxicity was quantitated by measuring the amount of lactate dehydrogenase released into the media during a 20-hr time period following NMDA washout. Exposure of 12- to 14-day-old cultures to NMDA in Mg(2+)-free HEPES buffer (pH 7.4) for a 25-min period resulted in a concentration-dependent toxicity (EC50 = 54 microM). Time-course experiments showed that exposure to NMDA for as little as 5 min was excitotoxic and reached a plateau after a 20-min exposure period. Preincubation of the cultures with ethanol (25 to 200 mM) resulted in a concentration-dependent inhibition of NMDA-mediated toxicity with approximately 38% inhibition produced by 25 mM ethanol and essentially complete inhibition at 200 mM ethanol (IC50 = 60 mM). Increasing the glycine concentration to 100 microM did not potentiate NMDA neurotoxicity or antagonize the neuroprotective effect of ethanol. NMDA-Mediated excitotoxicity was reduced by approximately 50% by the glycine antagonist 7-chlorokynurenate (50 microM). Ethanol (50 mM) reduced NMDA neurotoxicity similar to 7-chlorokynurenate, and the two together produced greater inhibition than either alone. These results show that intoxicating concentrations of ethanol can potently inhibit NMDA receptor-mediated excitotoxicity and may have important implications in terms of ethanols interactions with brain trauma, ischemia, and other neuropathologies associated with NMDA receptor-mediated neurotoxicity.

Measurement of glutamate, aspartate and glycine and its potential precursors in human brain using high-performance liquid chromatography by pre-column derivatization with diethylaminoazobenzene sulphonyl chloride

This paper describes a high-performance liquid chromatographic technique, with dimethylaminoazobenzene sulphonyl chloride derivatization, for the measurement of glutamate, aspartate and glycine and its potential precursors in human brain tissue. The derivatization procedure is simple, sensitive and highly reproducible. The derivatized amino acids are stable and can be analysed by reversed-phase chromatography with visible detection at an absorption wavelength of 436 nm. A preliminary application to the determination of the concentrations of several amino acids in several regions of the human brain is described.

Mechanism of glycine protection in hypoxic injury: analogies with glycine receptor

Addition of glycine to the recirculating perfusate of isolated perfused rat kidneys protects against hypoxic injury to the medullary thick ascending limb and slows functional deterioration in the course of perfusion. This effect is dependent on dose; the earliest significant protection is seen at 0.25 mM, and the protective effects increase as glycine concentration is increased to 2 mM, the highest level tested. Two specific agonists of the strychnine-insensitive (NMDA) glycine receptor in neural membranes, 1-aminocyclopropane carboxylic acid (ACC) and d-serine, also exerted a cytoprotective effect at a concentration of 2 mM. On the other hand, 1-serine and taurine, ineffective agonists of the NMDA-glycine receptor but effective agonists of the strychnine-sensitive glycine receptor, had no protective effect in this system. Two antagonists to glycine at its binding site on the N-methyl-D-Aspartate (NMDA) receptor, 7-chlorokynurenic acid (2 mM) and indole-2-carboxylic acid (12.5 mM), did not reverse the cytoprotective action of 0.25 mM glycine. The data are consistent with a ligand-acceptor type of interaction to account for cytoprotection. The configuration of the glycine acceptor may resemble, but is not identical with, that of certain glycine receptors in the nervous system.

In vitro and in vivo characterization of the NMDA receptor-linked strychnine-insensitive glycine site

Modulation of the NMDA receptor by the strychnine-insensitive glycine site was studied both in vitro and in vivo. In vitro the glycinergic stimulation of [3H]MK801 binding was measured in three different rat forebrain membrane preparations. An increased association rate of [3H]MK801 in the presence of glycine was observed. The binding of the radioligand was also enhanced by D-serine, whereas L-serine was less potent. The concentration-effect curves were shifted to the right by the glycine antagonist 7-chlorokynurenic acid (7CKA). In vivo modulation of the N-methyl-D-aspartate (NMDA) receptor was studied using NMDA induced convulsions in 7 day old rats. The NMDA effect was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten,5,10-imine maleate (MK801) and D-(-)-2-amino-5-phosphono-pentanoic acid (AP5). The effect of a submaximal dose of NMDA was dose-dependently potentiated by 1-10 mg/kg D-serine, whereas higher doses of L-serine were needed to obtain a similar effect. 7CKA did not affect NMDA-induced convulsions but reduced the D-serine potentiation of NMDA responses. This study illustrates the ability of the strychnine-insensitive glycine site to modulate the NMDA receptor function both in vitro and in vivo.

Stimulation of noradrenaline release in human cerebral cortex mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors

1. Human brain cortical slices from patients undergoing neurosurgery for treatment of epilepsy resistant to antiepileptic drugs were used to identify and characterize N-methyl-D-aspartate (NMDA) and non-NMDA receptors mediating stimulation of noradrenaline release. The slices preincubated with [3H]-noradrenaline were superfused with Krebs-Henseleit solution with or without Mg2+ (1.2 mmol l-1) and were stimulated by 2-min exposure to NMDA, kainic acid or (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). 2. In slices superfused without Mg2+, NMDA induced a concentration-dependent tritium overflow. 3. The NMDA-evoked tritium overflow was almost abolished by tetrodotoxin (TTX), Mg2+ or by omission of Ca2+ from the superfusion fluid. 2-Amino-5-phosphonopentanoic acid (AP5; a competitive NMDA receptor antagonist) or dizocilpine (formerly MK-801; an antagonist at the phencyclidine receptor within the NMDA-gated ion channel) inhibited the NMDA-evoked tritium overflow. The stimulatory effect of NMDA was not significantly enhanced by glycine added to the superfusion fluid but was reduced by 7-chlorokynurenic acid (an antagonist at the glycine site coupled to the NMDA receptor). 4. In slices superfused with solution containing Mg2+, kainic acid or AMPA induced a concentration-dependent tritium overflow which was susceptible to blockade by TTX. 5. The kainic acid-evoked tritium overflow was not affected by DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP37849; a competitive NMDA receptor antagonist), but was inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; an antagonist at glutamate receptors of the non-NMDA type). 6. The AMPA-evoked tritium overflow was also inhibited by CNQX.2ń

Enhancement of NMDA-evoked neuronal activity by glycine in the rat spinal cord in vivo

The effects of iontophoretically administered N-methyl-D-aspartate (NMDA), glycine and strychnine on nociceptive dorsal horn neurons of the rat spinal cord were studied to test the hypothesis that their responses to NMDA are influenced in vivo by glycine acting at a strychnine-insensitive site. Experiments were carried out on 44 dorsal horn neurons responsive to microiontophoretic application of NMDA and peripheral stimulation. Glycine alone either enhanced or inhibited NMDA responses depending upon its dose (151% and 68% of control, respectively). Strychnine alone increased the NMDA-induced neural firing (129%), suggesting the presence of endogenous glycine. When glycine was co-ejected with strychnine, NMDA responses were further elevated (171%) revealing the activation of strychnine-insensitive binding sites. These data provide evidence that glycine can potentiate NMDA responses of nociceptive dorsal horn neurons in vivo and thus that glycine sites on NMDA receptors on these neurons are not saturated.

Presynaptic site of action underlying the ethanol-induced inhibition of norepinephrine release evoked by stimulation of N-methyl-d-aspartate (NMDA) receptors in rat cerebral cortex

Rat brain cortex synaptosomes pre-incubated with [3H]norepinephrine were used (1) to provide evidence that part of the NMDA receptors mediating stimulation of norepinephrine (NE) release are located on the noradrenergic varicosities themselves, (2) to characterize these receptors and (3) to examine whether ethanol specifically inhibits the NMDA-evoked NE release via a presynaptic site of action. In synaptosomes superfused with Mg(2+)-free Krebs-Henseleit solution, NMDA (2-min exposure) stimulated tritium overflow in a concentration- and glycine-dependent manner. The stimulatory effect of NMDA was not altered by tetrodotoxin but was abolished by omission of Ca2+ from the superfusion fluid and was considerably reduced in the presence of 1.2 mM Mg2+. DL-(E)-2-Amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849; a competitive NMDA receptor antagonist) produced a parallel shift of the concentration-response curve for NMDA to the right, whereas dizocilpine (MK-801; an antagonist at the phencyclidine, PCP, recognition site of the NMDA-gated ion channel) reduced the maximum effect of NMDA. Ethanol inhibited the NMDA-evoked tritium overflow in a concentration-dependent manner. In contrast, in synaptosomes superfused with Ca(2+)-free Krebs-Henseleit solution containing 15 mM K+ throughout, ethanol did not affect the tritium overflow evoked by 2 min introduction of 75 microM Ca2+ into the superfusion fluid. This Ca(2+)-evoked overflow was also not altered by tetrodotoxin and dizocilpine, but was inhibited by the inorganic Ca2+ channel antagonist Cd2+.(ABSTRACT TRUNCATED AT 250 WORDS)

New antiepileptic drugs: from serendipity to rational discovery

Antiepileptic drug discovery has made enormous progress from the serendipity and screening processes of earlier days to the rational drug development of today. The modern era of research began with the recognition that enhancement of inhibitory processes in the brain might favorably influence the propensity for seizures, gamma-aminobutyric acid (GABA) being the main inhibitory transmitter. Work in this field led to the development of vigabatrin, which inhibits the enzyme responsible for the degradation of GABA. More recently, research has focused on the therapeutic potential of blocking excitatory amino acids--in particular glutamate. Of the three receptors for glutamate, the N-methyl-D-aspartate (NMDA) receptor is considered the one of most interest in epilepsy, and research on a series of competitive NMDA receptor antagonists--especially those that are orally active--is in the forefront of antiepileptic drug development today. A further alternative for diminishing neuronal excitability is to modulate sodium, potassium, or calcium channels. The latter are especially implicated in absence seizures.