A characterization of anxiolytic-like actions induced by the novel NMDA/glycine site antagonist, L-701,324

 The effects of the NMDA/glycine site antagonist, 7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(1H)-quinolone (L-701,324), and the benzodiazepine receptor agonist, diazepam, were examined in the elevated plus-maze and in the Vogel’s conflict test. Oral administration of L-701,324 caused a dose-dependent increase (2.5 and 5.0 mg/kg, −30 min) in the percent time spent in the open arms with no change in the total number of arm entries or in the percent entries into the open arms of the plus-maze. The same doses of L-701,324 increased punished responding in the Vogel’s conflict test in a dose-dependent fashion, with no influence on unpunished drinking behavior. The anxiolytic-like effects of L-701,324 were obtained at doses which by themselves had no influence on the locomotor activity of the animals. Diazepam (2 mg/kg, IP, −30 min) was slightly more effective than L-701,324 in the plus-maze situation, whereas the increase in punished drinking in the Vogel’s test was of the same magnitude for both compounds. Our present results suggest that inhibition of NMDA receptor activity via a blockade of the NMDA/glycine-sensitive site at the NMDA receptor is accompanied by a reduction of anxiety-like behavior in both non-conditioned and conditioned conflict behavior situations. Received: 13 April 1997/Final version: 22 July 1997     

Effects of L-701,324, a high-affinity antagonist at the N-methyl-D-aspartate (NMDA) receptor glycine site, on the rat electroencephalogram

L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy) phenyl-2-(1H)-quinolone) is a novel, orally active antagonist at the N-methyl-D-aspartate (NMDA) receptor glycine site. As NMDA receptor antagonism is generally associated with anaesthetic effects, we have examined the electroencephalographic alterations produced by doses of L-701,324 that effectively reduce NMDA-evoked responses in vivo. Microdialysis probes incorporating an electrode were implanted in the striatum of rats and perfused with artificial cerebrospinal fluid (ACSF). Under light halothane anaesthesia, 12 consecutive depolarizations were elicited by switching to ACSF containing 200 μM NMDA for 2 or 3 min, every 20 min. NMDA-evoked depolarizations and EEG were recorded with the microdialysis electrode. L-701,324 (5 or 10 mg kg^-1 i.v.) or vehicle were administered 5 min after the 3rd NMDA stimulus. L-701,324 dose-dependently inhibited NMDA-evoked depolarizations, with 10 mg kg^-1 reducing these responses by 50 % for at least 3 h. The average amplitude of the EEG in the window 0.25-6 Hz (low frequencies) and 6-21 Hz (high frequencies) did not change in the control group. At the higher dose of 10 mg kg^-1 L-701,324 transiently increased the amplitude of low frequencies by around 20 %. In contrast, both 5 and 10 mg kg^-1 significantly reduced the high frequencies to around 70 % of control, and this action was sustained with the higher dose. Analysis of the relative EEG power spectra confirmed a small, but persistent shift from high to low EEG frequencies. Our results suggest that L-701,324 slightly strengthened halothane anaesthesia at doses inhibiting effectively NMDA receptor function. Accordingly, the resulting anticonvulsant and neuroprotective actions of L-701,324 may not be associated with marked anaesthesia-like side-effects. Received: 27 December 1996 / Accepted: 14 February 1997     

The glycine/NMDA receptor antagonist,L-701,324 reverses isolation-induced deficits in prepulse inhibition in the rat

Previous studies have demonstrated that the glycine/NMDA receptor antagonist, L-701,324 (7-chloro-4-hydroxy-3(3-phenoxy)phenyl-2(H)quinolone) blocks the activation of mesolimbic dopamine systems induced following psychostimulant administration in the rat (Bristow et al. 1994). In the present study, pretreatment with L-701,324 also reversed the deficit in prepulse inhibition (PPI) observed in rats reared in social isolation after weaning. Given that PPI is also attenuated in schizophrenic patients and that isolation rearing induces both neurochemical and behavioural abnormalities suggestive of a physiologically induced state of dopaminergic hyperactivity, these results suggest that blockade of the glycine/NMDA receptor may offer a new strategy for the development of novel antipsychotic agents.     

Neuroprotective effects of RPR 104632, a novel antagonist at the glycine site of the NMDA receptor, in vitro

The NMDA antagonist and neuroprotective effects of RPR 104632 (2H-1,2,4-benzothiadiazine-1-dioxide-3-carboxylic acid), a new benzothiadiazine derivative, with affinity for the glycine site of the NMDA receptor-channel complex are described. RPR 104632 antagonized the binding of [³H]5,7-dichlorokynurenic acid to the rat cerebral cortex, with a K_i of 4.9 nM. This effect was stereospecific, since the (-)-isomer was 500-fold more potent than the (+)-isomer. The potent affinity of RPR 104632 for the glycine site was confirmed by the observation that RPR 104632 inhibited [³H]N-(1-(2-thienyl)cyclohexyl]-3,4-piperidine ([³H]TCP) binding in the presence of N-methyl- -aspartate (NMDA) (IC₅₀ = 55 nM), whereas it had no effect on the competitive NMDA site or on the dissociative anaesthetic site. RPR 104632 inhibited the NMDA-evoked increase in guanosine 3′,5′-cyclic monophosphate (cGMP) levels of neonatal rat cerebellar slices (IC₅₀ = 890 nM) in a non-competitive manner and markedly reduced NMDA-induced neurotoxicity in rat hippocampal slices and in cortical primary cell cultures. These results suggest that RPR 104632 is a high-affinity specific antagonist of the glycine site coupled to the NMDA receptor channel with potent neuroprotective properties in vitro.     

Pharmacology of ACEA-1416: a potent systemically active NMDA receptor glycine site antagonist

Excitatory amino acid receptor antagonists show potential for the treatment of ischemic stroke and head trauma. In search of novel antagonists, a series of alkyl- and alkoxyl-substituted 1,4-dihydro-2,3-quinoxalinediones were synthesized and assayed for inhibition of glutamate receptors. We report on the pharmacological characterization of one such compound, 7-chloro-6-methyl-5-nitro-1,4-dihydro-2,3-quinoxalinedione (ACEA-1416). Electrophysiological assays showed that ACEA-1416 is a potent antagonist of rat brain NMDA receptors expressed in Xenopus oocytes, and NMDA receptors expressed by cultured rat cortical neurons. Antagonism is via competitive inhibition at glycine co-agonist sites (K_b = 7.9 nM in oocytes, K_b = 11 nM in neurons). ACEA-1416 also antagonizes AMPA receptors, though potency is considerably lower (K_b = 3.5 μM in oocytes, K_b = 1.6 μM in neurons). Oocyte assays indicated that ACEA-1416 is weak or inactive as an antagonist at NMDA receptor glutamate binding sites (K_b > 5.9 μM) and metabotropic glutamate receptors (K_b > 57 μM). Many NMDA receptor glycine site antagonists show poor penetration of the blood-brain barrier. Systemic bioavailability of ACEA-1416 was assessed by measuring the ability of the compound to protect against electroshock-induced seizures in mice. Protective effects of ACEA-1416 had rapid onset following i.v. administration. Peak efficacy was at 2 min and the biological half-time of protection was 60 min. The ED₅₀ measured at peak efficacy was 1.5 mg/kg. Our results show that ACEA-1416 is a high potency systemically active NMDA receptor glycine site antagonist and a moderate potency AMPA receptor antagonist. Separate studies indicate that ACEA-1416 is efficacious as a neuroprotectant in a rat model of focal cerebral ischemia. Taken together, our results suggest that ACEA-1416 has potential for clinical development as a neuroprotectant.     

MTEP, a new selective antagonist of the metabotropic glutamate receptor subtype 5 (mGluR5), produces antiparkinsonian-like effects in rats

The aim of the present study was to examine a potential antiparkinsonian-like action of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), a new non-competitive antagonist of mGluR5, in the rat models. This compound has affinity for mGluR5 in a nanomolar concentration range and seems to be superior to the earlier known antagonists in terms of its specificity and bioavailability. Catalepsy and muscle rigidity induced by haloperidol administered at doses of 0.5 and 1 mg/kg were regarded as models of parkinsonian akinesia and muscle rigidity, respectively. MTEP at doses between 0.5 and 3 mg/kg i.p. decreased the haloperidol-induced muscle rigidity measured as an increased muscle resistance of the rat's hind leg in response to passive extension and flexion at the ankle joint. The strongest and the longest effect was observed after the dose of 1 mg/kg. MTEP (0.5-3 mg/kg i.p.) also reduced the haloperidol-induced increase in electromyographic (EMG) activity recorded in the gastrocnemius and tibialis anterior muscles. MTEP (3 and 5 mg/kg i.p.) inhibited the catalepsy induced by haloperidol. The present study confirms earlier suggestions that the antagonists of mGluR5 may possess antiparkinsonian properties. However, selective mGluR5 antagonists may be more effective in inhibiting parkinsonian muscle rigidity than parkinsonian akinesia.     

Synthesis and biological evaluation of pro-drugs of GW196771, a potent glycine antagonist acting at the NMDA receptor

GW196771 is a potent antagonist of the modulatory glycine site of the N-methyl-D-aspartate (NMDA) receptor exhibiting outstanding in vivo profile in different animal models of chronic pain. With the aim to maximize the drug delivery to the target organs a suitable "pro-drug approach" was attempted; in this regards two conjugates of GW196771 with nutrients actively transported into the brain, namely adenosine and glucose, were prepared and investigated. These compounds, were evaluated in vitro in terms of their stability in rat plasma and in vivo on rats. Although an improvement was observed in terms of brain penetration of the esters vs. the parent compound, the amount of the latter did not increase significantly, probably due to some degradation events in the brain, different from the expected ester hydrolysis, resulting in a reduced availability of GW196771.     

Combined stimulation of the glycine and polyamine sites of the NMDA receptor attenuates NMDA blockade-induced learning deficits of rats in a 14-unit T-maze

 The present study examined the effects of multi-site activation of the glycine and polyamine sites of the NMDA receptor on memory formation in rats learning a 14-unit T-maze task. The competitive NMDA receptor antagonist, (±)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP, 9 mg/kg), was used to impair learning. The objectives were two-fold: (1) to investigate the effects of independent stimulation of the strychnine-insensitive glycine site or the polyamine site; (2) to investigate the effects of simultaneous activation of these two sites. Male, Fischer-344 rats were pretrained to a criterion of 13 out of 15 shock avoidances in a straight runway, and 24 h later were trained in a 14-unit T-maze that also required shock avoidance. Prior to maze training, rats received intraperitoneal (IP) injections of saline, saline plus CPP, CPP plus the glycine agonist, D-cycloserine (DCS, 30 or 40 mg/kg), CPP plus the polyamine agonist, spermine (SPM, 2.5 or 5 mg/kg), or CPP plus a combination of DCS (7.5 mg/kg) and SPM (0.625 mg/kg). Individual administration of either DCS or SPM attenuated the CPP-induced maze learning impairment in a dose-dependent manner. However, the combined treatment with both DCS and SPM completely reversed the learning deficit at doses five-fold less than either drug given alone. These findings provide additional evidence that the glycine and polyamine modulatory sites of the NMDA receptor are involved in memory formation. Furthermore, the potent synergistic effect resulting from combined activation of the glycine and polyamine sites would suggest a stronger interaction between these two sites than previously considered, and might provide new therapeutic approaches for enhancing glutamatergic function. Received: 9 May 1997/Final version: 13 August 1997     

The NMDA receptor glycine modulatory site: a therapeutic target for improving cognition and reducing negative symptoms in schizophrenia

Numerous clinical studies demonstrate that subanaesthetic doses of dissociative anaesthetics, which are non-competitive antagonists at the NMDA receptor, replicate in normal subjects the cognitive impairments, negative symptoms and brain functional abnormalities of schizophrenia. Post-mortem and genetic studies have identified several abnormalities associated with schizophrenia that would interfere with the activation of the glycine modulatory site on the NMDA receptor. Placebo controlled clinical trials with agents that directly or indirectly activate the glycine modulatory site consistently reduce negative symptoms and frequently improve cognition in patients with chronic schizophrenia, who are receiving concurrent typical antipsychotics. Thus, there is convincing evidence that the glycine modulatory site on the NMDA receptor is a valid therapeutic target for improving cognition and associated negative symptoms in schizophrenia.     

Synthesis of 7,8-dichloro-6-Nitro-1H-1,5-benzodiazepine-2,4-(3H, 5H)-dione as a potential NMDA receptor glycine site antagonist

An efficient procedure for the preparation of 7,8-dichloro-6-nitro-1H-1,5-benzodiazepine-2,4-(3H, 5H)-dione(7) as a potential lead compound for the NMDA receptor glycine binding site antagonist, starting from readily available 4,5-dichloro-2-nitroaniline(8), is described. The key step in the synthesis involves the cyclization of malonic ester amide 10 to compound 11.     

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

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

Actions of adenosine A2A receptor antagonist KW-6002 on drug-induced catalepsy and hypokinesia caused by reserpine or MPTP

Rationale: Current treatment of Parkinson’s disease (PD) is based on dopamine replacement therapy, but this leads to long term complications, including dyskinesia. Adenosine A_2A receptors are particularly abundant in the striatum and would be a target for an alternative approach to the treatment of PD. Objectives: The purpose of this study is to examine the efficacy and potency of the novel selective adenosine A_2A receptor antagonist (E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methyl-3,7-dhydro-1H-purine-2,6-dione (KW-6002) in ameliorating the motor deficits in various mouse models of Parkinson’s disease. Methods: We evaluated the efficacy and potency of KW-6002 and other reference compounds in the selective adenosine A_2A receptor agonist 2-[p-(2-carboxyethyl)phenethylamino]-5’-N-ethylcarboxamidoadenosine (CGS 21680)-, haloperidol- or reserpine-induced catalepsy models. The effect of KW-6002 on reserpine or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride(MPTP)-induced hypolocomotion was also examined. Results: The ED₅₀s of KW-6002 in the reversal of CGS21680-induced and reserpine-induced catalepsy were 0.05 mg/kg, PO and 0.26 mg/kg, PO, respectively. Compared to the ED₅₀ of other adenosine antagonists and dopamine agonist drugs, KW-6002 is over 10 times as potent in these models. KW-6002 also ameliorated the hypolocomotion (minimum effective dose; 0.16 mg/kg) induced by nigral dopaminergic dysfunction with MPTP or reserpine treatment. Combined administrations of subthreshold doses of KW-6002 and l-dopa (50 mg/kg, PO) exerted prominent effects on haloperidol-induced and reserpine-induced catalepsy, suggesting that there may be a synergism between the adenosine A_2A receptor antagonist KW-6002 and dopaminergic agents. Conclusions: To our knowledge, KW-6002 is the most potent and orally active adenosine A_2A receptor antagonist in experimental models of Parkinson’s disease, and may offer a new therapeutic approach to the treatment of Parkinson’s disease. Received: 22 March 1999 / Final version: 25 May 1999     

GV196771A, an NMDA receptor/glycine site antagonist, attenuates mechanical allodynia in neuropathic rats and reduces tolerance induced by morphine in mice

The effects of the N-methyl- -aspartate (NMDA) receptor/glycine site antagonist, GV196771A (E-4,6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-ylidenemethyl)-1H-indole-2-carboxylic acid sodium salt), on mechanical allodynia and on tolerance to the antinociceptive effects induced by morphine were evaluated. Its antiallodynic properties were studied in a model of chronic constriction injury applied to rat sciatic nerve. GV196771A (0.3–10 mg/kg, p.o.) dose-dependently inhibited established mechanical allodynia when tested 14 or 21 days after nerve ligation. In the formalin test in mice, GV196771A (10 or 20 mg/kg, p.o.), administered for 8 days together with morphine 10 mg/kg, i.p. inhibited morphine tolerance development in both early and late phases of the test. This finding reinforces the key role of the NMDA receptors in the plastic event, such as allodynia, which develops in some conditions of painful neuropathy. Moreover, the capability to strongly reduce morphine-induced tolerance suggests that GV196771A could be an alternative agent for the treatment of difficult pain states not only when given alone, but also in combination, in order to prolong the analgesic effects of the opiates.     

Inhibition of cortical spreading depression by L-701,324, a novel antagonist at the glycine site of the N-methyl-D-aspartate receptor complex.

1. Spreading depression (SD) is a propagating transient suppression of electrical activity, associated with cellular depolarization, which probably underlies the migraine aura and may contribute to neuronal damage in focal ischaemia. The purpose of this study was to examine whether L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2-(1H)-quinolone), a high affinity antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex, inhibits the initiation and propagation of K(+)-induced SD in the rat cerebral cortex in vivo. 2. Microdialysis probes incorporating a recording electrode were implanted in the cerebral cortex of anaesthetized rats and perfused with artificial cerebrospinal fluid (ACSF). Five episodes of repetitive SD were elicited by switching to a medium containing 130 mM K+ for 20 min, each separated by 40 min of recovery (i.e. perfusion with normal ACSF). The brief negative shifts of the extracellular direct current (d.c.) potential, characteristic of SD elicitation, were recorded with the microdialysis electrode and a reference electrode placed under the scalp. Propagation of SD was examined using glass capillary electrodes inserted about 3 mm posterior to the microdialysis electrode. L-701,324 (5 or 10 mg kg-1) or its vehicle were administered i.v. 10 min after the end of the second K(+)-stimulus. The effects of L-701,324 were compared to those of dizocilpine (MK-801; 1 mg kg-1 i.v.), a NMDA-channel blocker known to potently block SD elicitation. 3. Potassium-induced SD initiation was inhibited by 10 mg kg-1 (but not by 5 mg kg-1) of L-701,324. Thirty minutes after administration of 10 mg kg-1 L-701,324, the cumulative area of SD peaks elicited during 20 min was 15.3 +/- 2.1 mV min, versus 23.2 +/- 1.1 mV min in animals which received only the drug vehicle (P < 0.02; n = 6). The delay between application of 130 mM K+ and occurrence of the first SD was also significantly increased. It was approximately doubled in animals treated with 10 mg kg-1 of L-701,324. 4. SD propagation was more sensitive than SD elicitation to L-701,324, as both 5 and 10 mg kg-1 produced an effective inhibition. Even at the lower dose of 5 mg kg-1, L-701,324 completely blocked the propagation of SD elicited 30 min after drug administration. This differential sensitivity of SD elicitation and propagation is not specific to L-701,324 since it was previously observed with other drugs. At doses effective against SD, L-701,324 did not produce any marked alterations of the electroencephalogram. 5. L-701,324 (10 mg kg-1) and MK-801 (1 mg kg-1) had identical effects on the d.c. potential when administered during the recovery which followed the second K+ stimulus. Both drugs produced a positive shift of around 4.5 mV within 10 min of i.v. drug administration, indicating rapid drug penetration into the CNS. Paradoxically, L-701,324 (10 mg kg-1) was markedly less effective than MK-801 (1 mg kg-1) in blocking SD, since this dose of MK-801 was sufficient virtually to abolish SD initiation and completely block its propagation. The higher potency of MK-801 against SD may reflect its use-dependency, i.e. binding of MK-801 and channel blockade are enhanced when the NMDA-receptor ionophore is open. 6. Taken together, these data demonstrate that L-701,324 has an inhibitory effect on both SD initiation and propagation. This action may be beneficial in focal ischaemia, and possibly also against migraine, especially as this drug was shown to be active when administered orally.     

Role of morphine maintenance dose in the development of tolerance and its attenuation by an NMDA receptor antagonist

Rationale: Current research shows that N-methyl-d-aspartate (NMDA) receptor antagonists attenuate the development of morphine tolerance in rodent antinociceptive assays. Objective: The purpose of this study was to determine the role of morphine maintenance dose in the attenuation of morphine tolerance by the competitive NMDA receptor antagonist, LY235959. Methods: A rat warm-water tail-withdrawal procedure was used to measure the antinociceptive effects of morphine and LY235959. In this procedure, the distal 8 cm of each rat’s tail is immersed in 40° (non-noxious) and 55°C (noxious) water, and the latency to remove the tail is recorded. Results: Morphine (0.3–10 mg/kg, SC) produced dose-dependent increases in tail-withdrawal latencies from the 55°C water. Following determination of the morphine dose-effect curves, rats were administered chronically one of three doses of morphine (10, 20, or 40 mg/kg) either alone or in combination with LY235959 (1.0, 3.0, or 5.6 mg/kg, SC) twice daily for 7 days. Chronic administration of 10, 20, and 40 mg/kg morphine produced rightward shifts in the morphine dose-effect curves of approximately 3-, 6-, and 12-fold, respectively. When LY235959 (1.0–5.6 mg/kg) was co-administered with 10 mg/kg morphine, the development of morphine tolerance was attenuated in a dose-dependent manner, with complete prevention observed following 3.0 mg/kg LY235959. LY235959 (1.0, 3.0 mg/kg) also attenuated the development of tolerance to 20 and 40 mg/kg morphine; however, tolerance was not completely prevented. Administering 3.0 mg/kg LY235959 along with 20 and 40 mg/kg morphine was functionally equivalent to treating rats with half the amount of morphine. Conclusion: These data suggest that the maintenance dose of morphine, and thus the magnitude of tolerance, can determine the effectiveness of an NMDA receptor antagonist to attenuate morphine tolerance. Received: 5 April 1999 / Final version: 23 July 1999     

Effects of the NMDA receptor channel blockers memantine and MRZ 2/579 on morphine withdrawal-facilitated aggression in mice

Rationale: Opioid withdrawal is known to facilitate aggressive behavior in laboratory rodents. Aggression develops as the somatic signs disappear and thus may reflect protracted withdrawal-related behavioral alterations. Antagonists acting at the NMDA receptor are known to attenuate the expression of morphine withdrawal syndrome in laboratory animals. Objective: The present study aimed to evaluate the effects of low-affinity NMDA receptor channel blockers (memantine and MRZ 2/579) on aggression facilitated by morphine withdrawal in mice. Methods: Significant increases in aggressive behavior were observed 48 h after repeated morphine administration (8 days, b.i.d., 10–80 mg/kg, SC) was discontinued. Separate groups of mice were treated intraperitoneally with vehicles or different doses of memantine (1, 3, 10 or 30 mg/kg) or MRZ 2/579 (1, 3 or 10 mg/kg) 48 h after the last morphine injection. Results: Both compounds dose-dependently reduced the expression of aggressive behavior while having no significant effect upon the intensity of non-aggressive social contacts. Memantine significantly diminished the occurrence of all recorded components of aggressive behavior (attacks/bites, threats, tail rattling) while MRZ 2/579 affected mainly the appetitive events of aggressive bursts (threats, tail rattling). For both compounds, anti-aggressive effects occurred at dose levels that did not produce motor impairment in the Rotarod test. Conclusions: Taken together with the evidence on the lack of selective anti-aggressive effects of these drugs in morphine-naive mice, attenuation of the aggression observed in the present study may be due to specific interaction with morphine withdrawal-triggered processes. Received: 2 May 1999 / Final version: 17 November 1999     

Contribution of the serotonin 5-HT1A receptor agonism of 8-OH-DPAT and EMD 128130 to the regulation of haloperidol-induced muscle rigidity in rats

The aim of the present study was to find out whether (±)-8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a prototypical 5-HT_1A agonist, and (R)-(−)-2-[5-(4-fluorophenyl)-3-pyridylmethylaminomethyl]-chromane HCl (EMD 128130), a compound with serotonin 5-HT_1A-agonist and dopamine D₂-like antagonist properties, are able to attenuate the haloperidol-induced (1 mg/kg) muscle rigidity in rats. Muscle tone was examined using a combined mechano- and electromyographic (EMG) method that simultaneously measured the mechanical muscle resistance (MMG) of the rat’s hind foot to passive movements in the ankle joint, and the EMG activity of two antagonist muscles. Both 8-OH-DPAT (0.125–0.5 mg/kg i.p.) and EMD 128130 (1–10 mg/kg i.p.) dose-dependently decreased the haloperidol-enhanced MMG to passive movements, as well as the tonic and the long-latency reflex EMG activities.

Provided these results can be extrapolated to humans, the efficacy of EMD 128130 in relieving the haloperidol-induced muscle rigidity supports the concept that novel antipsychotics with 5-HT_1A agonist and dopamine D₂ antagonist activities should have a favourable extrapyramidal side-effect profile.     

Effects of RPR 118723, a novel antagonist at the glycine site of the NMDA receptor, in vitro

RPR 118723 ((8-chloro-5-methyl-2,3-dioxo-1,4-dihydro-5H-indeno[1, 2-b]pyrazin-5-yl) acetic acid) was previously reported to exhibit potent affinity for the glycine site of the N-methyl-D-aspartate (NMDA) receptor-channel complex in the nanomolar range (K(i)=3.1+/-0. 8 nM). We now report on the effects of RPR 118723 in two functional tests reflecting the interaction between the glycine site and the NMDA receptor. First, RPR 118723 potently inhibited [3H]N-[1-(2-thienyl)cyclohexyl]-3,4-piperidine ([3H]TCP) binding in the presence of NMDA (IC(50)=3.5+/-0.4 nM). Second, RPR 118723 antagonized the NMDA-induced increase in [3H]dopamine release in mouse striatal slices (IC(50)=8.0+/-1.1 nM). In both experimental models, an excess of glycine reversed the effect of RPR 118723. These results show that RPR 118723 interferes functionally in the nanomolar range with the glycine site coupled to the NMDA receptor in vitro. The blockade of the glycine site with RPR 118723 may be useful for the therapy of the disorders linked to excessive NMDA stimulation.     

L-687,414, a low efficacy NMDA receptor glycine site partial agonist in vitro,does not prevent hippocampal LTP in vivo at plasma levels known to be neuroprotective

1. N-methyl-D-aspartic acid (NMDA) receptors are known to play a key role in the induction phase of long-term potentiation (LTP) at certain hippocampal synapses and to represent some component of spatial learning in animals. The ability of NMDA receptor antagonists (or gene knockout) to impair LTP has led to the suggestion that the therapeutic use of such antagonists may impair cognitive function in humans. The present study compares the effects on LTP of NMDA receptor ion channel block by MK-801 and glycine-site antagonism by 3R(+)cis-4-methyl-pyrrollid-2-one (L-687,414).
2. In vitro experiments using rat cortical slices revealed L-687,414 to be ∼3.6 fold more potent than its parent analogue, R(+)HA-966 at antagonizing NMDA-evoked population depolarizations (apparent K_bs: 15 μM and 55 μM, respectively).
3. Whole-cell voltage-clamp experiments using rat cultured cortical neurones revealed L-687,414 to shift to the right the concentration-response relationship for NMDA-evoked inward current responses (pK_b=6.2±0.12). L-687,414 affinity for the glycine site on the NMDA receptor complex was also determined from concentration-inhibition curves, pKi=6.1±0.09. In the latter experiments, L-687,414 and R(+)HA-966 were unable to completely abolish inward current responses suggesting each compound to be a low efficacy partial agonist (estimated intrinsic activity=∼10 and 20% of glycine, respectively).
4. L-687,414 and MK-801 were compared for their effects on NMDA receptor-dependent LTP in the dentate gyrus of anaethestized rats following high frequency stimulation of the medial perforant path (mPP) afferents. Control rats, administered saline (0.4 ml kg⁻¹ followed by 0.0298 ml min⁻¹), showed a robust augmentation of the population e.p.s.p. risetime (LTP) recorded in the dentate hilus following tetanic stimulation of the mPP. LTP was effectively abolished in a separate group of rats treated with an MK-801 dosing regimen (0.12 mg kg⁻¹ i.v. followed by 1.8 μg kg⁻¹ h⁻¹) known to produce maximal neuroprotection in a rat stroke model but, by contrast, remained largely intact in a third group of animals given a similarly neuroprotective L-687,414 treatment (28 mg kg⁻¹ i.v. followed by 28 mg kg⁻¹ h⁻¹).
5. These experiments suggest that a low level of intrinsic activity at the glycine site may be sufficient to support NMDA receptor-dependent LTP but in circumstances where there is likely to be an excessive NMDA receptor activation the agonism associated with a low efficacy partial agonist, such as L-687,414, is dominated by the antagonist properties. Thus, an NMDA receptor partial agonist profile may offer a therapeutic advantage over neutral antagonists by permitting an acceptable level of `normal'' synaptic transmission whilst curtailing excessive receptor activation.

     

Ethanol and N-methyl-D-aspartate receptor complex interactions: a detailed drug discrimination study in the rat

The discriminative stimulus properties of compounds that interact with the NMDA receptor complex were investigated in rats trained to discriminate ethanol from saline. Male Wistar rats were trained in a two-lever operant drug discrimination paradigm to make differential responses [fixed ratio 10 (FR10)] for food after ethanol (1 g/kg IP; 12% v/v ethanol solution) and saline vehicle injections. Drug effects were assessed by means of generalization and antagonism tests. In the generalization tests, the noncompetitive NMDA antagonists acting at the ion channel dizocilpine, memantine, phencyclidine (PCP) and the sigma₁ receptor agonists (+)-pentazocine and (+)-N-allyl-normetazocine (NANM) dose-dependently generalized for ethanol, whereas the α-amino-3-hydroxy- 5-methyl-4-isoxazole-propionate (AMPA) antagonist GYKI 52466, the glycine antagonists L-701,324 and MRZ 2/502, the polyamine site antagonist arcaine and the polyamine site ligand spermidine, did not. Our results show that the noncompetitive NMDA antagonists fully substitute dose-dependently for ethanol in a drug-discrimination task. The ethanol-like discriminative stimulus effects of PCP, pentazocine and NANM, which are also sigma receptor ligands, are likely to be attributed to their activity at NMDA receptors. We therefore assume that some of the acute effects of ethanol are mediated via NMDA receptor antagonism at the PCP binding site. Received: 28 January 1997/Final version: 10 June 1997