Both agonists and antagonists of the strychnine-insensitive glycine site of N-methyl-D-aspartate receptors modulate polysynaptic excitations in slices of mouse olfactory cortex

Summary In this study, it is reported that bath application of D-serine and, to a lesser extent glycine, potentiated polysynaptic but not monosynaptic excitations evoked in slices of mouse olfactory cortex perfused with solution containing Mg²⁺ (1 mmol/l), picrotoxin and strychnine (both 25 μmol/l). Effects were largely confined to the longer latency components of the field potentials and occurred at amino acid concentrations of between 0.01 and 1 mmol/l. The effects of D-serine and glycine were antagonized by 7-chlorokynurenate and indole-2-carboxylate, antagonists of the glycine regulatory site of the N-methyl-D-aspartate (NMDA) receptor complex. D-Serine (glycine not tested) also potentiated, and 7-chlorokynurenate partially inhibited the longer latency components of the polysynaptic field potentials evoked in slices perfused in the absence of picrotoxin and strychnine. However, neither D-serine nor glycine potentiated responses evoked by the bath application of NMDA. It is concluded that under the present experimental conditions, the glycine regulatory sites of those NMDA receptor involved in the mediation of polysynaptic excitations in the mouse olfactory cortex are not saturated with endogenous glycine.     

In vivo effects of the NMDA receptor agonist tetrazolyl-glycine related to the function of small-diameter primary afferents

The NMDA receptor agonist tetrazolyl-glycine (TG; 100 μg kg^–1, i.v.) caused a depressor reflex in anaesthetized rats. The NMDA receptor antagonist MK-801 (300 μg kg^–1, i.v.) inhibited this depressor reflex, but not that induced by pentylenetetrazol (PTZ; 100 mg kg^–1, i.v.), indicating a selective effect of TG on NMDA receptors in vivo. Capsaicin pretreatment, which excludes the function of small-diameter primary afferent fibres, caused only a reduction of the TG-induced depressor reflex, suggesting a reduction of NMDA receptors. The absence of effects of TG and PTZ on the blood pressure in pithed rats excluded any peripheral vascular actions of TG and PTZ. The depressor reflex evoked by afferent nerve stimulation was also inhibited by MK-801 (300 μg kg^–1, i.v.), but not by the tachykinin antagonist L-742694 (10 mg kg^–1, i.v.), confirming the essential role of glutamate in the neurotransmission of signals at central terminals of small-diameter afferent neurons. Plasma protein extravasation in the rat hind paw, induced by neurotransmitters released at peripheral terminals of small diameter afferent neurons by antidromic nerve stimulation, was not influenced by MK-801, indicating that glutamate is either not released or has no effect there. It is concluded that the NMDA agonist TG is a valuable tool to study the functions of primary afferents in vivo. Received: 8 February 1999 / Accepted: 12 April 1999 / Published online: 21 June 1999     

Effect of D-serine on AQP4 knockout-induced changes in behavior alteration to cocaine

OBJECTIVE AQP4 is widely expressed in brain astrocytes,and its function is mainly to maintain the steady-state of brain water.D-serine as an important neurotransmitter secreted by glial cells is an endogenous co-agonist of NMDA receptor,and is essential for the induction of long-term potentiation.The present study is to explore whether D-serine is mediated the change of synaptic plasticity and cocaine-induced addictive behavior induced by AQP4 knockout.METHODS The effects of D-serine on NAc LTD and behavior of AQP4 knockout mice were investigated by electrophysiological recording and behavioral tests.RESULTS 10 μmol·L-1 D-serine significantly reversed NMDA receptor-dependent LTD in AQP4 KO mice NAc slices.D-serine concentration-dependently antagonized the effects of AQP4 knockout on cocaine addiction behavior.500 mg·kg-1 D-serine significantly increased cocaine-induced locomotor activity in AQP4 KO mice.D-serine(500 mg·kg-1) significantly restored cocaine-induced conditioned place preferences in AQP4 KO mice.CONCLUSION AQP4 knockout changed the behavior of cocaine addiction through affecting the release of D-serine in glial cells.     

Potent, stereoselective, and brain region selective modulation of second messengers in the rat brain by (+)LY354740, a novel group II metabotropic glutamate receptor agonist

LY354740 is a highly potent and selective agonist for recombinant Group II mGlu receptors (mGlu2 and mGlu3), which has anxiolytic and drug withdrawal alleviating properties when administered systemically in rats and mice. The modulation of second messengers by LY354740 in rat brain tissues was investigated to understand the cellular basis for the pharmacological and potential therapeutic actions of LY354740. LY354740 potently decreased forskolin-stimulated cAMP formation in slices of the adult rat hippocampus (EC₅₀=22±3 nM) in a stereoselective manner. LY354740 (at 1 μM) greatly (>90%) suppressed forskolin-stimulated cAMP in the cerebral cortex, hippocampus, and striatum, while producing only partial suppression (about 50%) in midbrain regions and olfactory bulb, and no significant cAMP alterations in the cerebellum and brainstem regions. Inhibition of forskolin-stimulated cAMP formation was antagonized by (+)-α-methyl-4-carboxyphenylglycine [(+)MCPG], a competitive mGlu receptor antagonist. LY354740 did not alter phosphoinositide hydrolysis in the rat hippocampus per se, but potentiated stimulation of phophoinositide hydrolysis by the Group I mGlu receptor selective agonist 3,5-dihydroxyphenylglycine (DHPG) or stimulation of cAMP formation by the adenosine receptor agonist 5’-N-ethylcarboxamideoadenosine (NECA). These data indicate that LY354740 is a highly potent, efficacious, and selective Group II mGlu receptor (mGlu 2/3) agonist in the rat brain. The potent, stereoselective, and brain region selective actions of LY354740 on mGlu receptor linked second messenger systems likely underlie the in vivo potency and stereoselectivity of this compound in animal models. Received: 25 February 1998 / Accepted: 18 April 1998     

Histamine H3 receptor antagonist clobenpropit enhaces GABA release to protect against NMDA -induced excitotoxicity in cultured cortical neurons through the cAMP/PKA pathway

Aim： Using the H3 receptor antagonist clobenpropit, the roles of histamine H3 receptor in NMDA -induced necrosis were investigated in rat cultured cortical neurons. Methods： Experiments were conducted using primary cortical neuron cultures. Neuron survival was measured after insulted by NMDA. GABA concentrations were determined by an HPLC - ECD system. Results： Clobenpropit reversed the neurotoxicity in a concentration -dependent manner, and showed a peak protection at a concentration of 10^-7 M. This protection was antagonized by the H3 receptor agonist （R） - α - methylhis- tamine, but not by the H1 receptor antagonist pyrilamine and H2 receptor antagonist cimetidine. In addition, the protection by clobenpropit was inhibited by the GABAA receptor antagonists picrotoxin and bicuculline. Further study demonstrated that the protection by clobenpropit was due to increased GABA release. The inducible GABA release was also inhibited by （R） -α- methylhistamine, but not by pyrilamine and eimetidine. In contrast, both the adenylyl eyclase inhibitor SQ - 22536 and the PKA inhibitor H -89 reversed the protection and GABA release by clobenpropit. In addition, clobenpropit also prevented [ Ca^2＋] i increase induced by NMDA.     

Effects of carnosine on NMDA -induced necrosis in NGF- differentiated PC12 cells

The purpose of this study was to assess the effects of carnosine on N - methyl - D - aspartate acid （NMDA）-induced necrosis in NGF - differentiated PC12 rat pheoehromocytoma cells. Within 30 min of 2mM NMDA insults, 40% cells died by necrosis. Carnosine reversed the neurotoxicity in a time-dependent and concentration-dependent manner. After 18 hours application of carnosine, it showed a peak protection at a concentration of 5 raM. This protection was antagonized by histamine H1 receptor antagonist pyrilamine,     

Hyperlocomotion and increased dopamine efflux in the rat nucleus accumbens evoked by electrical stimulation of the ventral subiculum: role of ionotropic glutamate and dopamine D1 receptors

Rationale and objectives: The role of glutamatergic afferents from the hippocampus in the modulation of dopamine (DA) efflux in the nucleus accumbens (NAcc) and concomitant increases in locomotor activity was examined following brief high-frequency electrical stimulation of the ventral subiculum (vSub). Reverse dialysis of ionotropic glutamate receptor (iGluR) antagonists into the NAcc identified the relative contributions of N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors in the modulation of DA efflux, whereas microinjection of these compounds or selective DA D₁ or D₂ receptor antagonists were used to analyze the roles of glutamatergic and DA receptors in the stimulation-induced hyperlocomotion. Methods and results: Electrical stimulation of the vSub at 20 Hz (10 s, 300 μA) induced a significant increase in (1) DA levels in the NAcc (≈30% from pre-stimulation DA levels) and (2) locomotor activity (≈400%). The evoked DA release was completely blocked by reverse dialysis of a selective non-NMDA antagonist DNQX (10 μM and 100 μM), whereas only a high dose of the NMDA antagonist AP-V (100 μM) was effective. The increased motor activity, however, was only slightly attenuated by reverse dialysis of these drugs. Bilateral intra-NAcc injection of DNQX (1 μg/0.5 μl) blocked the increased motor activity induced by vSub stimulation relative to saline treatment. In contrast, bilateral intra-NAcc injection of AP-V (1 μg/ 0.5 μl) alone caused a significant increase in locomotor activity. The increased motor activity induced by vSub stimulation appears to be mediated through the DA D₁ receptor, as systemic administration of the D₁ antagonist SCH 23390 (0.25 mg/kg and 1 mg/kg), but not the D₂ antagonist sulpiride (2 mg/kg and 10 mg/kg) blocked these effects. Conclusions: These data indicate an important role for hippocampal glutamatergic afferents in modulating the release of DA through iGluR on DA-receptive neurons in the NAcc and possibly on output neurons to the ventral tegmental area, which subsequently elicits a prolonged increase in locomotor behavior. The role of this circuit in mediating context-dependent behavioral sensitization to repeated administration of psychostimulants is discussed. Received: 13 October 1999 / Accepted: 21 December 1999     

Requirement of PSD-95 for dopamine D1 receptor modulating glutamate NR1a/NR2B receptor function

Aim： To elucidate the role of scaffold protein postsynaptic density （PSD）-95 in the dopamine D1 receptor （D~R）-modulated NRla/NR2B receptor response. Methods： The human embryonic kidney 293 ceils expressing D1R （tagged with the enhanced yellow fluorescent protein） and NRla/NR2B with or without co-expres- sion of PSD-95 were used in the experiments. The Ca^2＋ influx measured by imaging technique was employed to monitor N-methyl-D-aspartic acid receptors （NMDAR） function. Results： The application of dopamine （DA, 100 μmol/L） did not alter glutamate/glycine （Glu/Gly）-induced NMDAR-mediated Ca^2＋ influx in cells only expressing the D1R/NRla/NR2B receptor. However, DA increased Glu/Glyinduced Ca^2＋ influx in a concentration-dependent manner while the cells were co-expressed with PSD-95. D1R-stimulated Ca^2＋ influx was inhibited by a selective D1R antagonist SCH23390. Moreover, pre-incubation with either the protein kinase A （PKA） inhibitor H89, or the protein kinase C （PKC） inhibitor chelerythrine attenuated D1R-enhanced Ca^2＋ influx induced by the N-methyl-D-aspartic acid （NMDA） agonist. The results clearly indicate that D1R-modulated NRla/NR2B receptor function depends on PSD-95 and is subjected to the regulation of PKA and PKC. Conclusion： The present study provides the first evidence that PSD-95 is essential in D1R-regulated NRla/NR2B receptor function.     

Environmental animal models for sensorimotor gating deficiencies in schizophrenia: a review

Rationale: Functional imaging studies have revealed overactivity of the hippocampus in schizophrenic patients. Neuropathological data indicate that hyperactivity of excitatory hippocampal afferents and decreased hippocampal GABA transmission contribute to this overactivity. In rats, excitation of the ventral hippocampus, e.g. by NMDA, results in hyperactivity and disruption of sensorimotor gating measured as prepulse inhibition (PPI) of the acoustic startle response, behavioral effects related to psychotic symptoms in humans. Objective: The present study examined whether disinhibition of the ventral hippocampus by the GABA_A antagonist picrotoxin would result in similar psychosis-related behavioral disturbances (hyperactivity, decreased PPI) as NMDA stimulation. Methods and results: Wistar rats received bilateral infusions of subconvulsive doses of picrotoxin (100 or 150 ng/0.5 μl per side) into the ventral hippocampus and were then immediately tested for open field locomotor activity or startle reactivity and PPI. Only the higher dose induced hyperactivity and decreased PPI. Both doses decreased acoustic startle reactivity to a similar extent. The decreased PPI appeared not to result from decreased startle reactivity, but was associated with a diminished potency of the prepulses to inhibit the startle reaction to the startle pulse, indicating a sensorimotor gating deficit. All effects were temporary, i.e. disappeared when the rats were tested 24 h after infusion. Conclusions: Decreased GABAergic inhibition in the ventral hippocampus of rats yielded psychosis-related behavioral effects, very similar to those induced by NMDA stimulation. Thus, a concurrence of decreased GABAergic inhibition and increased afferent excitation in the hippocampus of schizophrenic patients might contribute to psychotic symptoms. Electronic Publication     

D-serine modulates the NMDA receptor/nitric oxide/cGMP pathway in the rat cerebellum during in vivo microdialysis

Several lines of investigation indicate that D-serine may be an endogenous ligand for the glycine site of N-methyl-D-aspartate (NMDA) receptors in some CNS regions. We here studied the in vivo effects of D-serine on the NMDA receptor/nitric oxide/cGMP pathway by monitoring extracellular cGMP in the cerebellum of freely-moving rats subjected to transcerebral microdialysis. Local application of NMDA (200, 500 μM) through the dialysis probe for 20 min evoked transient, concentration-dependent cGMP responses which peaked in the fraction of drug administration, the nucleotide levels returning to basal values after 40 min. The NMDA-induced elevation of the extracellular nucleotide was completely inhibited by the selective receptor channel blocker dizocilpine (MK-801) locally co-perfused at the concentration of 10 μM. The non-competitive antagonist had no effect on its own suggesting that endogenous glutamic acid does not tonically activate NMDA receptors. The effect of 200 μM NMDA was largely attenuated by 30 μM 7-chloro-kynurenic acid and completely abrogated when the concentration of the strychnine-insensitive glycine receptor antagonist was raised to 100 μM. D-serine (300 μM), perfused in the presence of 7-chloro-kynurenate (30 μM), was able to fully restore the NMDA (200 μM)-induced increase of cGMP extracellular levels. On the other hand, the D-amino acid directly potentiated in a concentration-dependent manner (0.3, 1 and 10 mM) the NMDA (200 μM)-evoked cGMP production whereas it was inactive on its own. These data show that in vivo the activation of the strychnine-insensitive glycine site is essential for the functioning of the NMDA receptor complex and can be activated by the selective agonist D-serine. They also confirm that cerebellar NMDA receptors do not have their glycine sites saturated. Received: 25 June 1996 / Accepted: 23 September 1996     

环噻嗪对大鼠海马谷氨酸受体/NO/cGMP通路的调节

通过活体微透析的方法研究了环噻嗪对大鼠海马谷氨酸受体/NO/cGMP通路的影响. 局部灌流α- 氨基羟甲基异噁唑丙酸(AMPA)受体脱敏阻断剂环噻嗪能引起细胞外cGMP水平的提高. 环噻嗪的这种作用能够被NO合酶抑制剂N-硝基-L-精氨酸(L-NNA)或选择性的可溶性鸟苷酸环化酶抑制剂1H-［1，2，4］噁二唑［4，3-a］喹喔啉-1-酮(ODQ)所阻断. 在环噻嗪灌流过程中，大鼠呈现明显的痉挛前的行为变化--湿狗样反应(WDS). 由环噻嗪引起的cGMP增加和WDS反应能够被N-甲基-D-天冬氨酸(NMDA)受体通道阻断剂甲基二苯并环庚烯亚胺(MK-801)或镁离子所阻断. AMPA受体拮抗剂6，7-二硝基喹喔啉-2，3-二酮(DNQX)和2，3-二羟基-6-硝基-7-氨磺酰基 苯并(f)- 喹喔啉(NBQX)可拮抗WDS反应，但不能阻断环噻嗪引起的cGMP反应. 这种结果表明：（1）在海马内与NO-cGMP通路有关的AMPA受体由于内源性谷氨酸的存在保持部分脱敏状态.（2）环噻嗪对AMPA受体脱敏的阻断作用可导致内源性NMDA受体的激活.     

Activation of supraspinal NMDA receptors by both D-serine alone or in combination with morphine leads to the potentiation of antinociception in tail-flick test of rats

Although there is a variety of information concerning the effects of the N-methyl-D-aspartate (NMDA) receptor on opioid-induced antinociception at the spinal level, little is known about the effects at the supraspinal level. To clarify the role of the NMDA receptor on the morphine-induced antinociception at the supraspinal level, we investigated the effects of the intracerebroventricular (i.c.v.) administration of D-serine, a selective agonist for the glycine site of the NMDA receptors, alone or in combination with morphine using the tail-flick test. The i.c.v. administration of D-serine, but not L-serine, alone produced a dose-dependent antinociception in the tail-flick response. D-Serine also dose-dependently potentiated the antinociceptive effect induced by the i.c.v. administration of morphine and the simultaneous administration produced an additive effect. The potentiation of the antinociception produced by both D-serine alone or in combination with morphine was dose-dependently attenuated by the i.c.v. administration of L-701,324, a selective antagonist for the glycine site of the NMDA receptors. In addition, the potentiation of the D-serine-induced antinociception was antagonized by the i.c.v. administration of naloxone, a nonselective opioid receptor antagonist. These observations, together with the fact that D-serine is an endogenous and selective co-agonist for the glycine site of the NMDA receptors, strongly suggested that the activation of the supraspinal NMDA receptors by D-serine leads to the potentiation of the antinociception in the tail-flick test and that endogenous D-serine could modulate the mu-opioid receptor mediated antinociception via the glycine site of the NMDA receptors at the supraspinal level.     

Effect of 7-chloro kynurenic acid on glycine modulation of the N-methyl-D-aspartate response in guinea-pig myenteric plexus

The glycine modulation of the N-methyl-D-aspartate (NMDA) response in guinea-pig myenteric plexus was investigated by using D-serine and 7-chloro kynurenic acid as a glycine agonist and antagonist, respectively. D-serine caused a concentration-dependent enhancement of the NMDA response, an effect which was competitively inhibited by 7-chloro kynurenic acid (pA2 = 6.0). In addition, 7-chloro kynurenic acid induced a concentration-dependent, non-competitive inhibition of the NMDA response per se, even in the absence of added D-serine. This inhibition was fully reversed by exogenous D-serine, suggesting that this effect was also due to the occupancy of the glycine site. These results emphasize the usefulness of the guinea-pig myenteric plexus for studying the function of the NMDA receptor complex.     

P物质增强大鼠鞘内注射D—丝氨酸诱发的热痛过敏

目的：研究脊髓伤害性信息传递中P物质（SP）与N－甲基－D－天冬氨酸（NMDA）受体甘氨酸位点激动剂D－丝氨酸（D-serine)之间的功能联系。方法：在浅麻大鼠,采用行为学方法,测定甩尾反射潜伏期（TFL）并结合鞘内给药途径观察药物作用。结果：鞘内注射D-serine 1000nmol后1．5分钟,TEL明显缩短;在注射D-serine 10nmol前6分钟鞘内施加SP 0.05nmol,明显增强D-serine 10nmol引起的TEL缩短效应;选择性NMDA受体甘氨酸位点拮抗剂7－氯犬尿酸1pmol及非选择性PKC抑制剂H－7 10μmol均可阻断这种增强作用。结论：SP可使D－丝氨酸诱发的热痛过敏明显加强,NMDA受体甘氨酸位点及胞内蛋白激酶系统参与了脊髓SP与NMDA受体的相互作用。     

Modulation of seizure susceptibility in the mouse by the strychnine-insensitive glycine recognition site of the NMDA receptor/ion channel complex.

1. In order to determine whether the strychnine-insensitive glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor/ion channel complex is fully activated in vivo, the ability of the selective glycine receptor agonist, D-serine, to modulate seizure susceptibility in the mouse has been examined. 2. D-Serine (10-200 micrograms per mouse, i.c.v.) dose-dependently increased the potency of NMDLA in inducing seizures in Swiss Webster mice by approximately 3 fold. L-Serine was without significant effect. 3. The potency of pentylenetetrazol in inducing seizures was also enhanced by D-, but not L-serine, although the magnitude of the shift (1.6 fold) was considerably less than for NMDLA. 4. Similar doses of D-serine were also able to block the anticonvulsant effect of the non-selective glycine receptor antagonist, kynurenic acid, against seizures induced by NMDLA, but were without effect on the anticonvulsant effect of the competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). 5. D-Serine completely antagonized the protective effect of the selective glycine receptor antagonist, 7-chlorokynurenic acid, against sound-induced seizures in DBA/2 mice, but was less effective in this model against the less selective antagonist, kynurenic acid. 6 The results indicate that in vivo, NMDA receptors are not maximally potentiated by endogenous glycine and suggest an important involvement of the glycine modulatory site on the NMDA receptor/ion channel complex in the pathophysiology of epilepsy.     

Activity of 5,7-dichlorokynurenic acid, a potent antagonist at the N-methyl-D-aspartate receptor-associated glycine binding site

5,7-Dichlorokynurenic acid (5,7-DCKA), one of the most potent excitatory amino acid receptor antagonists yet described, binds to a strychnine-insensitive glycine binding site located on the N-methyl-D-aspartate (NMDA) receptor complex (Ki = 79 nM versus [3H]glycine). 5,7-DCKA (10 microM) antagonized the ability of NMDA to stimulate the binding of the radiolabeled ion channel blocker N-[3H][1-(2-thienyl)cyclohexyl]-piperidine ([3]TCP). Glycine was able to overcome this effect and in the presence of 5,7-DCKA enhanced [3H]TCP binding to antagonist-free levels. 5,7-DCKA completely and noncompetitively antagonized several NMDA receptor-mediated biochemical and electrophysiological responses. Thus, micromolar concentrations of 5,7-DCKA inhibited NMDA-stimulated elevation of cytosolic calcium in cultured hippocampal neurons, cGMP accumulation in cerebellar slices, and norepinephrine release from hippocampal slices. The glycine antagonist could also block the action of synaptically released agonist, as shown by its ability to inhibit the increase in the magnitude of the population spike that follows tetanic stimulation of the hippocampus in vitro (long term potentiation). Inclusion of glycine or D-serine prevented all these effects of the antagonist. 5,7-DCKA was a potent anticonvulsant when administered intracerebroventricularly to mice. As in the in vitro experiments, the dose-response curve for the antagonist was shifted rightward in a parallel fashion when D-serine was coinjected. This spectrum of activity displayed by a compound acting at the glycine binding site suggests that the therapeutic utility of glycine antagonists will be similar to those proposed for other types of glutamate receptor antagonists.     

Glycine, glycinamide and D-serine act as positive modulators of signal transduction at the N-methyl-D-aspartate (NMDA) receptor in vivo: differential effects on mouse cerebellar cyclic guanosine monophosphate levels

Direct intracerebellar (icb) administration of glycine, glycinamide and D-serine produced time- and dose-dependent changes in mouse cerebellar cGMP levels, indicating a modulation of ongoing neuronal activity through the NMDA receptor complex. Intracerebroventricular administration of glycinamide also produced a time-dependent change in cGMP levels, indicating a central mechanism of action. The icb dose-response data indicated a unimolecular interaction for these compounds. D-serine-, glycine-, and glycinamide-mediated increases in cGMP levels were reversed by the competitive NMDA antagonist, CPP and the NMDA-associated glycine receptor antagonist, HA-966, indicating mediation via the NMDA receptor complex. Glycine and D-serine were less effective than glycinamide at increasing cerebellar cGMP levels. In contrast, L- and D-serinamide did not affect cGMP levels. These results indicate that glycine receptor is not saturated under physiological conditions and also suggest possible existence of multiple glycine pools.     

Modulation of the NMDA receptor by D-serine in the cortex and the spinal cord, in vitro

We present a comparative study of the modulation of the N-methyl-D-aspartate (NMDA) receptor at the strychnine-insensitive glycine site in the spinal cord and in the cortex. The excitatory effect of NMDA was potentiated by D-serine (a glycine mimetic) in the hemisected rat spinal cord. The non-competitive NMDA antagonists 7-chlorokynurenic acid (7-Cl KYNA; 10 microM) and 3-amino-1-hydroxypyrrolid-2-one (HA-966; 100 or 200 microM) antagonized the effect of NMDA in the spinal cord and cortical wedge preparation. The antagonism was reversed by the addition of D-serine. This effect was strychnine-insensitive and hence not related to the inhibitory glycine receptor known to be present in the spinal cord. Our results suggest strongly that glycine positively modulates the NMDA system not only at a supraspinal level but also at the spinal level. As the positive modulation of NMDA responses by D-serine was also seen in the presence of tetrodotoxin, we conclude that the NMDA/glycine complex is (also) located on motoneurones in addition to the known glycine-mediated inhibitory system.