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Aim This study examined the role of glycinergic transmission in nociceptive and non-nociceptive bladder reflexes and in inhibition of these reflexes by pudendal nerve stimulation (PNS). Methods Cystometrograms (CMGs) were performed in α-chloralose anesthetized cats by intravesical infusion of saline or 0.25% acetic acid (AA) to trigger, respectively, non-nociceptive or nociceptive bladder reflexes. PNS at 2 or 4 times threshold (T) intensity for inducing anal twitch was used to inhibit the bladder reflexes. Strychnine (a glycine receptor antagonist) was administered in cumulative doses (0.001–0.3 mg/kg, i.v.) at 60–120 min intervals. Results Strychnine at 0.001–0.3 mg/kg significantly (P

Elevated dopamine levels are believed to contribute to the rewarding sensation of ethanol (EtOH), and previous research has shown that strychnine-sensitive glycine receptors in the nucleus accumbens (nAc) are involved in regulating dopamine release and in mediating the reinforcing effects of EtOH. Furthermore, the osmoregulator taurine, which is released from astrocytes treated with EtOH, can act as an endogenous ligand for the glycine receptor, and increase extracellular dopamine levels. The aim of this study was to address if EtOH-induced swelling of astrocytes could contribute to elevated dopamine levels by increasing the extracellular concentration of taurine. Cell swelling was estimated by optical sectioning of fluorescently labeled astrocytes in primary cultures from rat, and showed that EtOH (25-150 mM) increased astrocyte cell volumes in a concentration- and ion-dependent manner. The EtOH-induced cell swelling was inhibited in cultures treated with the Na(+)/K(+)/2Cl(-) cotransporter blocker furosemide (1 mM), Na(+)/K(+)-ATPase inhibitor ouabain (0.1 mM), potassium channel inhibitor BaCl(2) (50 microM) and in cultures containing low extracellular sodium concentration (3 mM). In vivo microdialysis performed in the nAc of awake and freely moving rats showed that local treatment with EtOH enhanced the concentrations of dopamine and taurine in the microdialysate, while glycine and beta-alanine levels were not significantly modulated. EtOH-induced dopamine release was antagonized by local treatment with the glycine receptor antagonist strychnine (20 microM) or furosemide (100 microM or 1 mM). Furosemide also prevented EtOH-induced taurine release in the nAc. In conclusion, our data suggest that extracellular concentrations of dopamine and taurine are interconnected and that swelling of astrocytes contributes to the acute rewarding sensation of EtOH.

This study investigated the role of electrical and chemical synapses in sustaining 4-aminopyridine (4-AP)-evoked network activity recorded extracellularly from substantia gelatinosa (SG) of young rat spinal cord in vitro. Superfusion of 4-AP (50 μm) induced two types of activity, the first was observed as large amplitude field population spiking activity and the second manifested within the inter-spike interval as low amplitude rhythmic oscillations in the 4–12 Hz frequency range (mean peak of 8.0 ± 0.1 Hz). The AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 μm) abolished field population spiking and disrupted 4–12 Hz rhythmic oscillatory activity whereas the NMDA receptor antagonist d-AP5 (50 μm) had no significant effect on either activity component. The glycine receptor antagonist strychnine (4 μm) and the GABAA receptor antagonist bicuculline (10 μm) diminished and abolished, respectively, field population spiking and both antagonists reduced the power of 4–12 Hz oscillations. The non-specific gap junction blockers carbenoxolone (100 μm) and octanol (1 mm) attenuated both types of 4-AP-induced activity. By comparison, the neuronal-specific gap junction uncouplers quinine (250 μm) and mefloquine (500 nm) both disrupted 4–12 Hz oscillations but only quinine reduced the frequency of field population spiking. These data demonstrate the existence of 4-AP-sensitive neuronal networks within SG that can generate rhythmic activity, are differentially modulated by excitatory and inhibitory ionotropic neurotransmission and are at least partly reliant on neuronal and/or glial-mediated electrical connectivity. The physiological significance of these putative intrinsic SG networks and the implications in the context of processing of nociceptive inputs are discussed.

Identical male twins with non-ketotic hyperglycinaemia did not exhibit any development progress while on strychnine treatment. Since other therapeutic attempts with single agents previously had failed in most instances, we tried a combination of various treatment modalities. The results indicated that the administration of glycine receptor antagonist (strychnine) together with a low-protein diet (1.5 gm/kg/day), augmentation of 1-C units (N(5)-formlytetrahydrofolate), and removal of glycine from extracellular fluid (sodium salicylate and sodium benzoate) yielded some clinical improvement and positive biochemical changes in out patients.

[3H]Strychnine binding to rat pons + medulla membranes was used as a measure of glycine receptors or glycine receptor-coupled chloride channels in vitro. A series of compounds structurally related to 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), which previously were shown to antagonize glycine responses in cat spinal cord, inhibited [3H]strychnine binding in micromolar concentrations. The most potent of these glycine antagonists, 5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol (iso-THAZ), was also the most potent inhibitor of [3H]strychnine binding, with a Ki of 1,400 nM. The Ki value for strychnine was 7.0 nM, whereas the Ki value for the mixed gamma-aminobutyric acid (GABA)/glycine antagonist 3 alpha-hydroxy-16-imino-5 beta-17-aza-androstan-11-one (RU 5135) was only 4.6 nM. Sodium chloride (1,000 mM) enhanced the affinity of strychnine, brucine, isostrychnine, and the nonselective GABA antagonist pitrazepin for [3H]strychnine binding sites, whereas the affinities of glycine, beta-alanine, and taurine were reduced. These sodium chloride shifts, however, were not predictive of antagonist or agonist properties, since the sodium chloride shift for the glycine antagonist iso-THAZ and of the other THIP-related antagonists were similar to those of the glycine-like agonists. The various sodium chloride shifts show that different groups of ligands bind to glycine receptor sites in different ways.

Thromboembolic stroke is a severe, disabling disease characterized by an abrupt reduction of cerebral blood flow, which leads to deprivation of oxygen and nutrients to neuronal tissue, followed by permanent brain damage. Evidence has been accumulated to implicate excitotoxicity in the pathogenesis of ischemic brain injury. Overstimulation of excitatory amino acid receptors becomes deleterious for neuronal cell survival. Glutamate antagonists can ameliorate the ischemic injury by any of several mechanisms. Because blockade of the glycine site of the N-methyl-D-aspartate (NMDA) receptor seems to offer a better side-effect profile, glycine antagonists are attractive targets for blocking excitotoxicity following stroke. GV150526 is a selective and potent glycine antagonist at the NMDA receptor complex. It binds to the glycine site with both high affinity and high selectivity in in vitro binding studies. In vivo studies have shown that GV150526 significantly reduces infarct volume in the middle cerebral artery occlusion model of stroke. This effect remained statistically significant, even if treatment was delayed for as long as 6 h post-occlusion. GV150526 showed no evidence of adverse effects usually associated with NMDA receptor blockers, such as neuronal vacuolization in standard assays or cognitive impairment in behavioral tests. GV150526 had no significant treatment-related respiratory or cardiovascular effects or effects on behavior, body temperature, or blood pressure in mice or rats. Pharmacokinetic studies indicated that GV150526 has low clearance and volume of distribution in both the rat and the dog. Preclinical toxicology studies have shown that the compound is well tolerated in both species. Phase I/II studies were undertaken to assess the safety, tolerability, and pharmacokinetics of GV150526 in healthy volunteers and acute stroke patients, and from these a dose was selected to be studied in Phase III clinical trials. These efficacy studies have now completed recruitment and data reconciliation is ongoing. GV150526 has the potential to be an effective therapy for acute ischemic stroke.

Background: Glutamatergic neurotransmission has been implicated in drug-environment conditioning, but little is known about the role of glutamate in alcohol seeking maintained by alcohol-associated cues. Therefore, we examined the effects of ionotropic glutamate receptor antagonists on cue-induced ethanol-seeking behavior in the extinction/reinstatement model. Methods: Rats were trained to orally self-administer ethanol (10% w/v) and a nonrewarding (80 μM) quinine solution on randomly alternating days. Ethanol and quinine availability were signaled by olfactory discriminative stimuli (S + /S - ). In addition, ethanol delivery was accompanied by a light stimulus (CS + ) and quinine delivery by an auditory stimulus (CS - ). Thereafter, rats were subjected to extinction training during which responding had no programmed consequences. Reinstatement of responding was tested under three conditions: in the presence of the S - /CS - , S + /CS + , and S + /CS + together with a small (0.2 ml) response-contingent oral ethanol dose at the beginning of the reinstatement session (S + /CS + /priming). We examined the effects of the noncompetitive NMDA receptor antagonist MK-801 (0, 0.05, 0.15 mg/kg intraperitoneally), the competitive NMDA antagonist CGP39551 (0, 5, 10 mg/kg intraperitoneally), the NMDA/glycine receptor antagonist L-701,324 (0, 2, 4 mg/kg intraperitoneally), the AMPA/kainate receptor antagonist CNQX (0, 0.5, 1.5 mg/kg intraperitoneally), and the opioid receptor antagonist naltrexone (0, 0.3, 1 mg/kg subcutaneously) on ethanol seeking under the S + /CS + /priming condition. Results: Presentation of the S + /CS + stimulus condition reinstated extinguished responding, whereas presentation of the S - /CS - condition did not. Response-contingent ethanol priming enhanced reinstatement further. Under these reinstatement conditions, L-701,324, CNQX, and naltrexone inhibited ethanol-seeking behavior significantly. In contrast, MK-801 and CGP39551 failed to affect reinstated responding. Conclusions: These results show that glutamate antagonism suppresses ethanol-seeking behavior induced by ethanol-paired stimuli. Furthermore, the data suggest that ionotropic glutamate receptors may have differential roles in mediation of this behavior.

The roles played by GABAA and glycine receptors in inspiratory-expiratory motor co-ordination and in tonic inhibitory regulation of expiratory motor activity were studied using brainstem-spinal cord (-rib) preparations from neonatal rats. Inspiratory activity was recorded from the C4 ventral root. Expiratory activity in internal intercostal muscle, internal oblique muscle or T13 ventral root was evoked by a decrease in perfusate pH from 7.4 to 7.1 (i.e. from normal to low pH conditions) and was limited to the first part of the expiratory phase. Under low pH conditions, bath application of 10 microM bicuculline, a GABAA receptor antagonist, caused the inspiratory burst to overlap the expiratory burst in 2/7 preparations. Overlapping of the expiratory burst with the inspiratory burst was observed in 7/7 preparations made under 10 microM bicuculline. Furthermore, such preparations exhibited expiratory bursts under bicuculline-containing normal pH conditions. Local application of 10 microM bicuculline to the brainstem under normal pH conditions evoked expiratory bursts, some of which overlapped the inspiratory bursts. Picrotoxin, another antagonist of the GABAA receptor, had similar effects. Under normal pH conditions, application of strychnine (0.2- 2.0 microM; a glycine receptor antagonist) to the brainstem did not evoke expiratory bursts. On subsequent application of strychnine-containing low pH solution, expiratory bursts were evoked and some (0.5 microM) or all (2.0 microM) of these overlapped the inspiratory burst. Simultaneous application of picrotoxin and strychnine to the brainstem evoked expiratory bursts that overlapped the inspiratory bursts and a subsequent decrease in perfusate pH to 7.1 increased the frequency of the respiratory rhythm. It was a characteristic finding that the duration of the expiratory burst exceeded that of the inspiratory burst under control low pH conditions. This remained true during concurrent blockade of GABAA and glycine receptors. The results suggest that in the in vitro preparation from neonatal rats: (1) GABAA and glycine receptors within the brainstem play important roles in the co-ordination between inspiratory and expiratory motor activity, (2) tonic inhibition via GABAA receptors, but not glycine receptors, plays a role in the regulation of expiratory motor activity and (3) inspiratory and expiratory burst termination is independent of both GABAA and glycine receptors.

Systemic administration of the anxiogenic benzodiazepine inverse agonist FG 7142 has been shown to increase selectively dopamine utilization in the medial prefrontal cortex and the shell, but not core, subregion of the nucleus accumbens. In the present study, we examined the functional interaction between benzodiazepine and N-methyl-d-aspartate receptor influences on dopamine utilization in these areas. Male Sprague-Dawley rats were pretreated with the glycine receptor antagonist (+)-HA 966 (15 mg/kg, i.p.) or saline 15 min before FG 7142 (20 mg/kg, i.p.) or vehicle administration. Subjects were killed 30 min later and assayed for tissue concentrations of dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid in the core and shell subdivisions of the nucleus accumbens and the medial prefrontal cortex. (+)-HA 966 administration blocked FG 7142-induced increased dopamine utilization in both the medial prefrontal cortex and the shell subdivision of the nucleus accumbens. Results are discussed in terms of N-methyl-d-aspartate receptor influences on the response of mesoaccumbal dopamine neurons to stress.

1. Guanidinoethyl sulphonate (GES) is an analogue of taurine and an inhibitor of taurine transport. Interactions of GES with GABA(A) and glycine receptors are studied by whole cell recording and fast drug application in isolated striatal neurons of the mouse. 2. We confirm that GES is a weak agonist at GABA(A) receptors, and is able to antagonize GABA-evoked responses. GES did not gate GlyR. 3. GES antagonized glycine responses in a concentration-dependent and surmountable manner. Glycine dose-response curves were shifted to the right by GES (0.5 mM), yielding EC(50)s and Hill coefficients of 62 micro M and 2.5 in control, 154 micro M and 1.3 in the presence of GES. 4. GlyR-mediated taurine responses were competitively antagonized by GES. Taurine dose-response curves, in contrast to the glycine dose-response curves were shifted by GES to the right in a parallel manner. 5. The GlyR-block by GES was not voltage-dependent. 6. In contrast to our findings in the mouse, in rat striatal neurons which lack expression of the alpha3 GlyR subunit, GES shifted the glycine dose-response curve to the right in a parallel way without affecting the maximal response. Subtype-specificity of the GES action at GlyR must await further investigation in artificial expression systems. 7. We conclude that GES is a competitive antagonist at GlyR. The antagonistic action of GES at inhibitory ionotropic receptors can explain its epileptogenic action. Care must be taken with the interpretation of data on GES evoked taurine release.

A series of different substituted benzimidazole derivatives has been synthesized and evaluated for the ability to displace [3H]MDL-105,519 to rat cortical membranes. Two benzimidazole-2-carboxylic acids 9 b and 9 c, in this substitution pattern not yet described as glycine antagonists, showed IC50 values of 0.89 microM (9 b) and 38.0 microM (9 c). Replacement of the carboxylate function in 2-position by a sulfonic acid moiety appreciably increased solubility, but decreased the affinity giving evidence for the strong need of the carboxylate group within the ligand. Further structure-activity studies using benzimidazole-2-one derivatives with an acetic acid moiety adjacent to a ring nitrogen revealed new insights into the importance of amide functionalities within the heterocycle for the affinity of antagonist glycine-site ligands.

A novel series of 3-carbamoylethynyl-2-carboxyindoles, antagonists acting at the strychnine-insensitive glycine binding site associated with the NMDA receptor, has been synthesised. This new versatile approach involves the introduction of a 2-chloroethenyl moiety in position C-3 with subsequent derivatisation of the terminal carboxyl group, followed by an unusual elimination of HCl to afford the ethynyl functionality. This novel series of glycine antagonists was evaluated in terms of in vitro affinity at the glycine binding site and the most potent compound was tested in vivo in the NMDA-induced convulsions model in mice.

PMBA is a novel antagonist of strychnine-sensitive glycine receptors in the rat spinal cord, however, its mode of action is unknown. The actions of PMBA on rat glycine receptor α1 and α2 homomers in Xenopus oocytes were studied under two-electrode voltage-clamp. Co-application of PMBA and glycine to both α1 and α2 homomers yielded inward currents which decayed to a steady-state. Responses rose slowly to the same steady-state amplitude following a 2 min pre-incubation in PMBA. Strychnine, but not picrotoxinin, showed similar antagonism to PMBA. The potency of PMBA was independent of membrane potential between −100 and 0 mV. When tested against EC50 concentrations of glycine, PMBA was almost equally potent on α1 (IC50, 406±41 nM: Hill coefficient, 1.5±0.2) and α2 (IC50, 539±56 nM; Hill coefficient, 1.4±0.2) homomers. PMBA (1–10 μM) and strychnine (200 nM) reduced the potency of glycine and the amplitude of the maximal agonist response of α1 and α2 homomers. In 10 μM PMBA, two distinct classes of glycine response were observed on α2, only a single class of responses were observed on α1. There are similarities in PMBA and strychnine antagonism, although these compounds are structurally distinct. The possibility that PMBA interacts at two binding sites which differ in α1 and α2 subunits is discussed. PMBA may provide a lead structure for novel antagonists with which to investigate structural differences in glycine receptor at α1 and α2 subunits. British Journal of Pharmacology (1999) 126, 1230–1236; doi:10.1038/sj.bjp.0702402

The effects of several glycine antagonists at the strychnine-insensitive site on N-methyl-D-aspartate (NMDA) receptors in an acute in vitro electrophysiological model of ischemia in the rat hippocampus were studied. Hippocampal slices were subjected to 9 min of hypoxia and hypoglycemia (ischemia). The noncompetitive NMDA antagonist MK-801 produced significant protection of the slices, whereas the competitive NMDA antagonist D-CPP did not. Administration of the strychnine-insensitive glycine-modulatory site antagonists (ACEA 1021, GV150,526A, and L701,324) up to 10 μM did not result in significant recovery of function. Other animal models have suggested that glycine antagonists are beneficial 6–48 hours after ischemic insult; this model assayed functional recovery for only 1 hour after the insult. Therefore, we conclude that this acute in vitro electrophysiologic model may not be useful in detecting the potential neuroprotective properties of glycine antagonists. Drug Dev. Res. 46:134–138, 1999. © 1999 Wiley-Liss, Inc.

Development of neuronal circuits generating locomotor activity was studied using an isolated lumbar spinal cord preparation from fetal and neonatal rats. Bath application of N-methyl-d-aspartate (NMDA) or 5-HT evoked patterned motor activity resembling that seen during normal fictive locomotion on embryonic day (E) 20.5. Glycine-mediated inhibition was essential to the formation of this coordinated motor activity. In preparations from fetuses at the earlier stages (E14.5-E16.5), we observed spontaneous motoneuronal activity and chemically induced rhythmic bursts, which were synchronized on the two sides in the corresponding ventral roots. The spontaneous activity was not blocked by kynurenate, the glutamate receptor blocker, although it was completely abolished by strychnine, the glycine receptor antagonist. A brief application of glycine evoked excitatory responses resembling the spontaneous bursts in both time course and amplitude. It is concluded that glycine functions transiently as excitatory transmitters at these stages. These results suggest that functional change in glycine-induced responses during development plays an important role in differentiation of the neuronal circuits generating locomotion.

ACEA 1021 (6,7-dichloro-5-nitro-1,4-dihydroquinoxaline-2,3-dione, Licostinel) is a potent antagonist for the glycine site of the NMDA receptor. With the purpose of evaluating the drug's biodistribution in vivo as well as its potential as a PET tracer for the glycine binding sites, ACEA 1021 was labelled in the heterocyclic ring with carbon-11 in a five-step synthesis. The radiolabelling precursor, derived from [11C]cyanide, was diethyl [1-11C]oxalate. Yields of its cyclization with the deactivated nitrated diamine, 4,5-dichloro-3-nitro-1,2-phenylenediamine, were too low to be reliable for the planned in vivo studies. Instead, diethyl [1-11C]oxalate was reacted with 4,5-dichloro-1,2- phenylenediamine to give [2-11C]6,7-dichloro-1,4-dihydroquinoxaline-2,3-dione (DCQX). Interference from the excess diamine during the subsequent nitration reaction was reduced by two methods. After formation of [2-11C]DCQX, unlabelled diethyl oxalate was added and allowed to cyclize before adding the nitrating agent, giving a carrier-added product suitable for use in pharmacokinetic studies. For the non-carrier-added tracer studies, the diamine was condensed with acetic acid before adding fuming HNO3/concentrated H2SO4. Both procedures gave high conversions of [2-11C]DCQX to [11C]ACEA 1021, which was subsequently isolated by semi-preparative liquid chromatography. The total synthesis time was 70–80 min. The conversions according to radio-analytical LC were 25–30% and isolated yields for the five-step synthesis were≈5–10% (decay-corrected, based on [11C]CN− at end of trapping). The specific activity of the no-carrier-added product was 15–20 GBq/μmol at end-of-synthesis. © 1998 John Wiley & Sons, Ltd.

1. Inherited defects in human glycine receptors give rise to hyperekplexia (startle disease). We expressed human glycine receptors in Xenopus oocytes, in order to examine the pharmacological and single-channel properties of receptors that contain a mutation, alpha1(K276E), associated with an atypical form of hyperekplexia. 2. Equilibrium concentration-response curves showed that recombinant human alpha1(K276E)beta receptors had a 29-fold lower glycine sensitivity than wild-type alpha1beta receptors, and a greatly reduced Hill coefficient. The maximum response to glycine also appeared much reduced, whereas the equilibrium constant for the glycine receptor antagonist strychnine was unchanged. 3. Both wild-type and mutant channels opened to multiple conductance levels with similar main conductance levels (33 pS) and weighted mean conductances (41.5 versus 49.8 pS, respectively). 4. Channel openings were shorter for the alpha1(K276E)beta mutant than for the wild-type alpha1beta, with mean overall apparent open times of 0.82 and 6.85 ms, respectively. 5. The main effect of the alpha1(K276E) mutation is to impair the opening of the channel rather than the binding of glycine. This is shown by the results of fitting glycine dose-response curves with particular postulated mechanisms, the shorter open times of mutant channels, the properties of single-channel bursts, and the lack of an effect of the mutation on the strychnine-binding site.

1. Total RNA isolated from embryonic chick paravertebral sympathetic ganglia was used in a reverse transcription-polymerase chain reaction (RT-PCR) assay with a pair of degenerate oligonucleotide primers deduced from conserved regions of mammalian glycine receptor alpha-subunits. Three classes of cDNA were identified which encode portions of the chicken homologues of the mammalian glycine receptor alpha 1, alpha 2 and alpha 3 subunits. 2. The presence of functional glycine receptors was investigated in the whole-cell configuration of the patch-clamp technique in neurons dissociated from the ganglia and kept in culture for 7-8 days. In cells voltage clamped to -70 mV, glycine consistently induced inward currents in a concentration-dependent manner and elicited half-maximal peak current amplitudes at 43 microM. 3. The steady-state current-voltage relation for glycine-induced currents was linear between +80 and -60 mV, but showed outward rectification at more hyperpolarized potentials. Reversal potentials of these currents shifted with changes in intracellular chloride concentrations and matched the calculated Nernst potentials for chloride. 4. beta-Alanine and taurine were significantly less potent than glycine in triggering inward currents, with half-maximal responses at 79 and 86 microM, respectively. At maximally active concentrations, beta-alanine-evoked currents were identical in amplitude to those induced by glycine. Taurine-evoked currents, in contrast, never reached the same amplitude as glycine-induced currents. 5. The classical glycine receptor antagonist strychnine reversibly reduced glycine-induced currents, with half-maximal inhibition occurring at 62 nM. Two more recently characterized glycine receptor antagonists, isonipecotic acid (half-maximal inhibition at 2 mM) and 7-trifluoromethyl-4-hydroxyquinoline-3-carboxylic acid (half-maximal inhibition at 67 microM), also blocked glycine-evoked currents in a reversible manner. The chloride channel blocker picrotoxin reduced glycine-evoked currents, with half-maximal effects at 348 microM. Inhibition by the glycine receptor channel blocker cyanotriphenylborate was half-maximal at 4 microM. 6. Apart from evoking inward currents, glycine occasionally triggered short (< 100 ms) spike-like currents which were abolished by hexamethonium and thus reflected synaptic release of endogenous acetylcholine. In addition, glycine caused Ca(2+)-dependent and tetrodotoxin-sensitive tritium overflow from neurons previously labelled with [3H]noradrenaline. This stimulatory action of glycine was reduced in the presence of strychnine and after treatment with the chloride uptake inhibitor furosemide (frusemide). 7. In 65% of neurons loaded with the Ca2+ indicator fura-2 acetoxymethyl ester, glycine increased the ratio of the fluorescence signal obtained with excitation wavelengths of 340 and 380 nm, respectively, which indicates a rise in intracellular Ca2+ concentration. 8. The results show that sympathetic neurons contain transcripts for different glycine receptor alpha-subunits and carry functional heteromeric glycine receptors which depolarize the majority of neurons to trigger transmitter release.

Based on our molecular modeling investigations of the glycinergic receptor, we expanded our studies to similarly investigate the GABAergic receptor. New data suggest there may exist a slightly different agonistic mechanism for the molecules described herein as compared to glycine. The origin of this is undoubtedly the fact that, while glycine has a positive and two negative binding sites, it is significantly shorter than GABA and the other GABA agonists. Clearly, discovery of more glycine agonists is needed to further clarify this point. Moreover, we find a remarkedly different antagonistic mechanism exists for this phylogenetically newer inhibitory system in the central ncrvous system (CNS) than recently reported for strychnine and eight weaker glycine antagonists. We used GABA and six agonists (muscimol, dihydromuscimol, TRIP, isoguvacine, trans-3-aminocyclopentane-1-carboxylic acid, piperidine-4-sulfonic acid) and five antagonists (bicuculline-Nl5-rnethobromide, R5135, pitrazepin, iso-THAZ and securinine) to derive our conclusions. We found that each of the agonists have three clearly defined atoms that can serve as attachment points at the GABAA receptor site. One of the three attachment atoms includes a carbonyl or carboxylate oxygen. The role of the carbonyl or carboxylate atom is very important. First, we theorize that a rapid two-point attachment occurs (one from the positive end and one from one of the other two negative atoms on the ligand) at the recognition site in the receptor where GABA or a GABAergic agonistt binds. The positive end of the agonist perhaps associates through hydrogen bonding to a β-carboxyl group in one of the aspartate molecules in the poly. peptide. The negative attachment points perhaps bind through hydrogen bonding to arginine molecules in this polypeptide. The second negative site in the agonist immediately triggers a conformational change by pulling together the aforementioned groups by electrostatic attraction, and hence opening the chloride channel. We propose the carbonyl oxygen is partly responsible for triggering the opening by formation of a double hydrogen bond to arginine. We postulate that this attraction is the first step inducing the conformational change. In the case of the GABA antagonists investigated, a fourth attachment site was not found. In fact only two sites have been identified similar to the group II glycine antagonists. Our data support a hypothesis for GABAergic antagonist activity which suggests that the antagonist simply binds to the recognition site and blocks the neurotransmitter, GABA, from entering this site thereby preventing the opening of the chloride channel; it just stays closed. This mechanism is different from the mechanism proposed for the large number of Group I glycine antagonists (Aprison et al.: J Neurosci Res 41:259-269, 1995). © 1995 Wiley-Liss, Inc.

We used molecular modeling techniques to examine six reported antagonists of glycine with varying K; values against strychnine. We found the data suggest two groups operating with different mechanisms. In group 1 (strychnine, brucine, Pitrazepin, and bicuculline methobromide) the antagonist contains two or three sites that can electrostatically bind to the three comparable groups of opposite charge in the recognition site where the natural neurotransmitter binds, thus opening the chloride channel. In addition, when in this position, the antagonist is able to also block the now opened chloride channel with a different portion of its structure. In many cases, this involves an interaction between a carbonyl group, on the antagonist and the guanidinium group of arginine which is part of the polypeptide segment of the outer mouth of the chloride channel (Grenningloh et al., Nature 330:2526, 1987). In group 2 (R5135 and 1,5-diphenyl-3,7-diazaadamantan-9-ol) the antagonist contains charged sites but when one of these molecules attaches to the recognition site, the chloride channel is not opened. In addition, R5135 contains a carbonyl group which attaches to arginine as pointed out in the text, whereas 1,5-diphenyl-3,7-diazaadamantan-9-ol contains a phenyl group that can block the channel. © 1995 Wiley-Liss, Inc

Using molecular modeling techniques, we studied nine glycine antagonists in order to try to identify the molecular descriptors that characterize strychnine as a strong antagonist and N, N-dimethyl-muscimol, iso-THIA, THIA, N-methyl-THIP, iso-THAZ, THAZ, iso-THPO, and iso-THAO (see Experimental for chemical names) as weak glycine antagonists. We confirm that all nine compounds have the three-atom regions (two negative and one positive) that we have postulated are necessary to permit such compounds to attach to the recognition site in the glycinergic synapse. Furthermore, in the case of antagonists we have postulated the presence of a fourth atom that can attach to the top of the chloride ion channel. Each of the nine antagonists has such a fourth negative atom and the latter property gives each of these compounds their antagonistic characteristic. Further, only in the case of strychnine is there evidence that at its positively charged end does the positive charge extend to cover a region that could bind through electrostatic domains to a tertiary carboxyl group in an amino acid like aspartate. Published molecular biological data show that such an amino acid is present in the portion of the polypeptides identified in the glycine receptor. The bidentate binding is superior to the single site attachment that is present in the other eight weak glycine antagonists. In addition, the two negative atom sites in each antagonist are also in a position to participate in electrostatic binding through bidentate involvement with the positively charged guanidinium group of arginine. The latter amino acid also has been identified in the portion of the polypeptide chain at the glycine receptor. Finally, our molecular data predict that after strychnine, the eight weak glycine antagonists listed above are in order of decreasing potency, i.e., N,N-dimethyl-muscimol is the best of the weak antagonists and iso-THAO should be the weakest. © 1995 Wiley-Liss, Inc.

1. The possible roles of glycine and gamma-aminobutyric acid (GABA) as inhibitory transmitters in the spinal dorsal horn were studied by intracellular recordings from substantia gelatinosa (SG) neurones in transverse slices of the adult rat spinal cord which retained an attached dorsal root. 2. Stimulation of primary afferent A delta fibres evoked an initial excitatory postsynaptic potential (fast EPSP) followed by a short and/or long inhibitory postsynaptic potential (short and long IPSP). The short IPSP, observed in twenty-nine SG neurones (37%) which received inhibitory inputs, had a mean latency of 3.6 ms and a half-decay time of 11 ms, while the long IPSP had a mean latency of 3.7 ms and a half-decay time of 42 ms and was observed in thirty-seven SG neurones (47%). The remaining twelve neurones (16%) exhibited both short and long IPSPs. Both IPSPs reversed polarity at a membrane potential of -70 +/- 4 mV. The short IPSP was reversibly blocked by the glycine receptor antagonist strychnine (0.5-2 microM), while the long IPSP was reversibly blocked by the GABAA receptor antagonist bicuculline (10-20 microM). 3. In the majority of SG neurones, the short and long IPSPs appeared to be disynaptic and were blocked by the non-N-methyl-D-aspartic acid (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5-10 microM). Both IPSPs were less sensitive (depressed by less than 30%) to the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 50-100 microM). 4. In ten SG neurones (13%), bath-applied glutamate (0.5-2 mM) increased the amplitude and frequency of IPSPs, which had a similar time course to that of the short IPSP evoked by afferent A delta fibres. The glutamate-induced short IPSPs were blocked by tetrodotoxin (0.5 microM) or strychnine (0.5-1 microM). In twelve neurones (16%), glutamate hyperpolarized the membrane or increased the amplitude and frequency of IPSPs that had a similar time course to that of the A delta fibre-evoked long IPSPs. The glutamate-induced membrane hyperpolarization and long IPSPs decreased in amplitude with membrane hyperpolarization and reversed polarity at -70 +/- 6 mV. These hyperpolarizing responses were blocked by tetrodotoxin (0.5 microM) or bicuculline (10 microM). 5. These observations suggest that primary afferent A delta fibres activate glycinergic and/or GABAergic interneurones primarily through the non-NMDA receptor subclass and result in inhibition of nearby SG neurones in the dorsal horn of the spinal cord.(ABSTRACT TRUNCATED AT 400 WORDS)

Pyrido[2,3-b]pyrazines and pyrido[2,3-b]purazines-N-oxides have been synthesized and evaluated for in vitro/in vivo antagonistic activity at the glycine site on the NMDA receptor

Two novel antagonists of the glycine-NMDA receptor have been synthesized and tested for their ability to displace [ 3 H]glycine from the specific binding site in rat brain cortical membranes. The 3-substituted pyrazino[1,2,3-de]quinoxaline-2,5,6-triones 1a-b contain all the essential pharmacophoric descriptors of a glycine receptor antagonist. A model is proposed for the binding of these compounds that includes some of the interactions postulated for the potent glycine-NMDA receptor antagonist L-689560

Two approaches leading to the enantiomerically pure tricyclic quinoxalinedione class of NMDA-glycine antagonists using enzymatic resolutions are described. An intermediate, racemic methyl 1,2,3,4-tetrahydroquinoline-2-carboxylate 3 , was resolved to ( S )- 3 in 97% ee and 47% yield (E=67) using α-chymotrypsin. In an improved method, hydrolysis of another intermediate, racemic methyl 1,2,3,4-tetrahydroquinoline-2-acetate 4 , with Novozym ® 435 provided the desired ( S )- 4 in high enantioselectivity and yield (93% ee, 50%, E=94).

N-Substituted 5-aminomethylquinoxalinediones containing carboxy or phosphonic acids yield potent and selective AMPA and/or NMDA (glycine-binding site) antagonists. Phosphonic acid derivatives are particularly water-soluble and display potent anticonvulsant effects in the electroshock-induced convulsion assay in mice.

A novel series of 3-carbamoylethynyl-2-carboxyindoles, antagonists acting at the strychnine-insensitive glycine binding site associated with the NMDA receptor, has been synthesised. This new versatile approach involves the introduction of a 2-chloroethenyl moiety in position C-3 with subsequent derivatisation of the terminal carboxyl group, followed by an unusual elimination of HCl to afford the ethynyl functionality. This novel series of glycine antagonists was evaluated in terms of in vitro affinity at the glycine binding site and the most potent compound was tested in vivo in the NMDA-induced convulsions model in mice.

A series of different substituted benzimidazole derivatives has been synthesized and evaluated for the ability to displace [3H]MDL-105,519 to rat cortical membranes. Two benzimidazole-2-carboxylic acids 9 b and 9 c, in this substitution pattern not yet described as glycine antagonists, showed IC50 values of 0.89 microM (9 b) and 38.0 microM (9 c). Replacement of the carboxylate function in 2-position by a sulfonic acid moiety appreciably increased solubility, but decreased the affinity giving evidence for the strong need of the carboxylate group within the ligand. Further structure-activity studies using benzimidazole-2-one derivatives with an acetic acid moiety adjacent to a ring nitrogen revealed new insights into the importance of amide functionalities within the heterocycle for the affinity of antagonist glycine-site ligands.

HA-966 (1-hydroxy-3-aminopyrrolidone-2) is an antagonist at the glycine allosteric site of the N-methyl-D-aspartate receptor ionophore complex. Unlike presently known glycine antagonists, HA-966 is chiral. We report stereoselectivity for the (R)-enantiomer at the glycine antagonist site. In [3H]glycine binding, the (R)-enantiomer has an IC50 of 4.1 +/- 0.6 microM. The racemic mixture has an IC50 of 11.2 +/- 0.5 microM, whereas (S)-HA-966 has an IC50 greater than 900 microM. In glycine-stimulated [3H]1-[1-(2- thienyl)cyclohexyl]piperidine binding, the (R)-enantiomer inhibits with an IC50 of 121 +/- 61 microM, whereas the racemic mixture has an IC50 of 216 +/- 113 microM and (S)-HA-966 is inactive. The inhibition by (R)-HA-966 can be prevented by the addition of glycine. (R)-HA-966 and racemic HA-966, but not (S)-HA-966, also prevent N-methyl-D-aspartate cytotoxicity in cortical cultures. The (R)-enantiomer and, less potently, the (S)-enantiomer inhibit N-methyl-D-aspartate-evoked [3H]norepinephrine release from rat hippocampal slices (IC50 values of about 0.3 mM and 1.6 mM, respectively), but only the inhibition by (R)-HA-966 is reversed by added glycine. In glutamate-evoked contractions of the guinea pig ileum, (R)-HA-966 causes a glycine-reversible inhibition (IC50 of about 150 microM), whereas (S)-HA-966 is much less potent (IC50 of greater than 1 mM). These results demonstrate stereoselectivity of the glycine antagonist site of the N-methyl-D-aspartate receptor complex in a variety of tissues and assays. The stereoselectivity also confirms the specificity of N-methyl-D-aspartate receptors in glutamate-evoked contractions of the guinea pig ileum, and supports their similarity to central N-methyl-D-aspartate receptors.

In synaptic plasma membranes from rat forebrain, the potencies of glycine recognition site agonists and antagonists for modulating [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding and for displacing strychnine-insensitive [3H]glycine binding are altered in the presence of N-methyl-D-aspartate (NMDA) recognition site ligands. The NMDA competitive antagonist, cis-4-phosphonomethyl-2-piperidine carboxylate (CGS 19755), reduces [3H]glycine binding, and the reduction can be fully reversed by the NMDA recognition site agonist, L-glutamate. Scatchard analysis of [3H]glycine binding shows that in the presence of CGS 19755 there is no change in Bmax (8.81 vs. 8.79 pmol/mg of protein), but rather a decrease in the affinity of glycine (KD of 0.202 microM vs. 0.129 microM). Similar decreases in affinity are observed for the glycine site agonists, D-serine and 1-aminocyclopropane-1-carboxylate, in the presence of CGS 19755. In contrast, the affinity of glycine antagonists, 1-hydroxy-3-amino-2-pyrrolidone and 1-aminocyclobutane-1-carboxylate, at this [3H]glycine recognition site increases in the presence of CGS 19755. The functional consequence of this change in affinity was addressed using the modulation of [3H]TCP binding. In the presence of L-glutamate, the potency of glycine agonists for the stimulation of [3H]TCP binding increases, whereas the potency of glycine antagonists decreases. These data are consistent with NMDA recognition site ligands, through their interactions at the NMDA recognition site, modulating activity at the associated glycine recognition site.

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.

To identify novel classes of glycine antagonists, compounds potentially useful as neuroprotective after stroke, novel substituted tetrahydroquinoline derivatives, satisfying the key pharmacophoric requirements for glycine antagonists, were designed. To explore the SAR of these compounds an efficient synthetic route was set up exploiting the outstanding reactivity of suitable N-aryl imine derivatives. In particular an allylmetalation reaction or the addition of bis(trimethyldisilyl)ketene acetals allowed the preparation of versatile intermediates which were smoothly transformed into the desired bicycle tetrahydroquinoline derivatives by a Heck-type cyclization reaction.Key words: glycine antagonists, neuroprotection, tetrahydroquinolines, synthesis, allylmetalation.

Polyclonal antibodies have been raised in rabbits against the glycine receptor antagonist strychnine, coupled through a 2-amino substituent to the antigenic protein key-hole limpet haemocyanin. Strychnine binding of the predominantly immunoglobulin G (IgG) class of antibodies was measured by incubation with [3H]strychnine, followed by adsorption of IgG onto Staphylococcus aureus cells and filtration through glass-fibre filters under vacuum. Only strychnine and structurally related alkaloids or derivatives were able to inhibit [3H]strychnine binding to the IgG. A significant rank correlation was found between the potencies of these compounds to inhibit [3H]strychnine binding to the antibodies and to the glycine receptor in mouse spinal cord membranes. In contrast, preincubation of strychnine antibodies with a variety of ligands at other neurotransmitter, drug, or hormone receptors in the CNS (at 10(-4) M) failed to inhibit binding significantly. The failure of glycine to inhibit strychnine antibody binding is consistent with previous suggestions that the recognition sites for this amino acid on the CNS receptor may be conformationally distinct from those for the antagonist alkaloid. Strychnine antibodies may now help in the identification and purification of possible endogenous ligands at this alkaloid binding site in the CNS.

In this study we characterized the distribution of glycine receptor immunoreactivity in the spinal cord of the rat by using monoclonal antisera directed against the purified glycine receptor. There was dense, punctate glycine receptor immunoreactive staining in all regions of the gray matter ventral to the substantia gelatinosa. The densest staining was found in laminae III and IV of the dorsal horn. There were also distinct, tributarylike bands of punctate staining that extended well into the white matter of the lateral and ventral funiculi. The only consistent cell body staining was found in small neurons of the ventral horn. The labelled neurons were distributed among larger, unlabelled motoneurons. In general, the pattern of glycine receptor immunoreactivity was similar at all levels of the spinal cord and was comparable to that seen with binding of a tritiated glycine receptor antagonist, strychnine, to sections of rat spinal cord (Zarbin et al.: J. Neurosci. 1:532-547, '81). Two important exceptions, however, were observed. In contrast to the high levels of strychnine binding reported in the substantia gelatinosa, we found almost no glycine receptor immunoreactivity in laminae I and II of the superficial dorsal horn of the spinal cord or of the trigeminal nucleus caudalis. There was also a notable absence of antibody staining in the intermediolateral cell column of the thoracic cord. The presence of dense glycine receptor immunoreactivity in the region of lamina V and its absence in the superficial dorsal horn are discussed in terms of a possible differential glycinergic control of nociceptive neurons of laminae I and V.

The bicyclic 5-isoxazolol zwitterions 4,5,6,7-tetrahydroisoxazolo[4,3-c] pyridin-3-ol (3, iso-THPO), 5,6,7,8-tetrahydro-4H-isoxazolo [4,3-c]azepin-3-ol (12, iso-THAO), and 5,6,7,8-tetrahydro-4H-isoxazolo [3,4-c]azepin-3-ol (13, iso-THIA), which are structurally related to the glycine antagonist 5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol (iso-THAZ), have been synthesized and tested biologically. All of these compounds were glycine antagonists approximately equipotent with iso-THAZ during microelectrophoretic ejection near cat spinal neurons. In contrast to iso-THAZ, which also interacts with 4-aminobutyric acid (GABA) receptors in rat brains, neither 12 nor 13 show any significant affinities for GABA binding or uptake mechanisms in vitro. The glycine antagonist 3 was, however, shown also to be a moderately potent inhibitor of GABA uptake. The structure of 12 was established by an X-ray analysis. The bond lengths of the 5-isoxazolol anionic moiety of 12 are in agreement with a pronounced delocalization of the negative charge of this compound.