Therapeutic potential of excitatory amino acid antagonists: channel blockers and 2,3-benzodiazepines

Remacemide: current status and clinical applications

Remacemide (RMC) is a non-competitive, low-affinity N-methyl-D-aspartate (NMDA) receptor antagonist that does not cause the behavioural and neuropathological side effects seen with other NMDA receptor antagonists. RMC and its active metabolite, AR-R 12495 AR, which has moderate affinity for the NMDA receptor, also interact with voltage-dependent neuronal sodium channels. Both agents show efficacy in a variety of animal models of epilepsy, parkinsonism and cerebral ischaemia. There is no evidence for teratogenicity or genotoxicity. RMC delays the absorption of L-dopa and elevates the concentrations of drugs metabolised by the hepatic cytochrome P450 3A4 isoform. RMC and AR-R 12495 AR have moderate protein binding and linear pharmacokinetics. Controlled studies show evidence of efficacy in treating epilepsy and Parkinson's disease. Post-surgical outcomes in RMC-treated patients at risk for intra-operative cerebral ischaemia are also encouraging. Adverse effects are related to the gastrointestinal and central nervous systems. RMC is a promising drug with numerous potential applications for both acute or chronic conditions associated with glutamate-mediated neurotoxicity.

The low-affinity, use-dependent NMDA receptor antagonist AR-R 15896AR. An update of progress in stroke

Use-dependent N-methyl-D-aspartate (NMDA) receptor antagonists protect neurons from the lethal consequences of excessive stimulation by excitatory amino acids. Clinical development of high-affinity compounds such as MK801 have been limited due to untoward side effects. Toward this end, the lower-affinity use-dependent NMDA antagonists have greater margins of safety and have advanced to clinical trials for stroke, epilepsy, head trauma and chronic neurodegenerative disorders. AR-R 15896AR is currently in Phase II trials for stroke and has been repeatedly demonstrated to afford neuroprotection in a variety of in vivo and in vitro models associated with ischemia/excitotoxic conditions.

Effects of GYKI 52466 and some 2,3-benzodiazepine derivatives on hippocampal in vitro basal neuronal excitability and 4-aminopyridine epileptic activity

In order to determine whether the anticonvulsant effect of 2, 3-benzodiazepines is also displayed in a model of in vitro epilepsy, such as the "epileptiform" hippocampal slice, we studied the effects of 2,3-benzodiazepine 1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxe-5H 2,3-benzodiazepine hydrochloride (GYKI 52466) and some new 2,3-benzodiazepine derivatives on CA1 basal neuronal excitability and on CA1 epileptiform burst activity produced by 4-aminopyridine in rat hippocampal slices. The results showed that GYKI 52466 affected basal neuronal excitability as evidenced by its influence on the magnitude of the CA1 orthodromic-evoked field potentials. 2,3-Benzodiazepines showed their antiepileptic effect also in an in vitro model of experimental epilepsy. The effects of the new 2,3-benzodiazepine derivatives suggest that the methylenedioxidation in positions 7 and 8 of the 2,3-benzodiazepine ring is the main structural modification for the antiepileptic effect of 2,3-benzodiazepines to take place.

The anticonvulsant SGB-017 (ADCI) blocks voltage-gated sodium channels in rat and human neurons: comparison with carbamazepine

PURPOSE

SGB-017 (ADCI) is a novel anticonvulsant that blocks both voltage-activated sodium channels and N-methyl-D-aspartate (NMDA)-receptor-gated channels. Results by Rogawski et al. suggested that SGB-017 produces its anticonvulsant action primarily by inhibition of NMDA-receptor channels. However, SGB-017 is effective in several animal models of epilepsy that are unresponsive to NMDA antagonists. These results indicate that block of NMDA-receptor channels is not the only mechanism contributing to its anticonvulsant activity. Thus the effects of SGB-017 on neuronal sodium channels were investigated.

METHODS

Whole cell voltage-clamp techniques were used to record sodium currents in freshly dissociated rat superior cervical ganglion (SCG) and hippocampal neurons and cultured human NT2 neurons. The effects of SGB-017 on the amplitude of sodium currents, elicited by a depolarizing pulse to 0 mV from different holding potentials, were measured and compared with those of carbamazepine (CBZ).

RESULTS

SGB-017 inhibited sodium currents in rat SCG and hippocampal neurons with a similar potency to CBZ. Like CBZ, the inhibition of sodium channels by SGB-017 was voltage dependent. Its median inhibitory concentration (IC50) for inhibition of sodium channels at depolarized holding potentials is similar to that for its inhibition of NMDA receptor channels. In human hNT2 neurons, SGB-017 was more potent than CBZ at inhibiting sodium currents.

CONCLUSIONS

SGB-017 produces its anticonvulsant activity by blocking both sodium- and NMDA-receptor channels in a voltage- and use-dependent manner. The combination of these two mechanisms of action makes SGB-017 an effective AED in several different animal models of epilepsy.

The contributions of GluR2 to allosteric modulation of AMPA receptors

Native AMPA receptor complexes in the CNS are composed of hetero-oligomers of the GluR1-4 subunits, and generally contain the GluR2 subunit. To determine the contributions of GluR2 to pharmacological properties of receptor complexes, the effect of hetero-oligomerization with GluR2 on allosteric modulation of recombinant AMPA receptors was studied. The study of homo-oligomeric GluR2 was facilitated with a site-directed mutant of the pore, GluR2(R607Q), which allowed robust currents from this normally low-conducting subunit. The efficacy of the allosteric modulators was tested on homo-oligomeric GluR1-4, and then compared with hetero-oligomeric GluR1/GluR2, GluR3/GluR2 and GluR4/GluR2. Two selective allosteric modulators were tested, a positive modulator, cyclothiazide, and a negative modulator, LY300164. The results show that the pharmacological properties of homo-oligomeric GluR2 are not significantly different from those of GluR1, GluR3 or GluR4. The apparent affinity of cyclothiazide is not significantly changed upon hetero-oligomerization. However, the extent of potentiation of kainate responses by cyclothiazide is significantly decreased upon hetero-oligomerization. Hetero-oligomerization increases the apparent affinity of LY300164, a (-) isomer of the 2,3-benzodiazepine LY293606. These data indicate that although GluR2 has a dominant effect on the permeation properties, this subunit does not have a similarly dominant effect on pharmacological properties of native receptors. However, the state of hetero-oligomerization can alter the pharmacological properties of AMPA receptors.

Differential effects of NMDA-receptor antagonists on long-term potentiation and hypoxic/hypoglycaemic excitotoxicity in hippocampal slices

Whole-cell patch clamp recording from cultured hippocampal neurones was used to investigate the NMDA antagonistic effects of the glycineB antagonist 5,7-DCKA and the competitive antagonist CGP 37849. Extracellular field potential recording from area CA1 of hippocampal slices was used to investigate their effects on the induction of LTP and hypoxia/hypoglycaemia-induced suppression of fEPSPs. Additionally, memantine and (+)MK-801 were tested in the later model. 5,7-DCKA inhibited NMDA-induced plateau currents (IC50=0.24+/-0.02 microM) with around nine times higher potency than against peak (IC50=2.14+/-0.17 microM). In contrast, CGP 37849 slowed the onset of NMDA-induced currents considerably and antagonized currents at the time point when the peak component occurred in control responses (IC50=0.18+/-0.01 microM) with around seven times higher potency than against plateau (IC50=1.26+/-0.19 microM). Both 5,7-DCKA and CGP 37849 inhibited the induction of LTP (IC50s=2.53+/-0.13 and 0.37+/-0.04 microM respectively) with potencies close to those inhibiting peak currents in patch clamp studies. 5,7-DCKA and CGP 37849 also blocked the hypoxia/hypoglycaemia-induced suppression of fEPSPs but CGP 37849 (EC50=4.3+/-0.33 microM) was far less potent than against the induction of LTP whilst 5,7-DCKA (EC50=1.47+/-0.04 microM) had similar potency in these two models. Memantine and (+)MK-801 also blocked hypoxia/hypoglycaemia-induced suppression of fEPSPs with EC50s of 14.1+/-0.52 and 0.53+/-0.02 microM respectively. Whereas memantine blocked this effect with similar potency as we previously reported for LTP, (+)MK-801 was four time less potent in this model. The calculated relative therapeutic indices (IC50 LTP over EC50 hypoxia/hypoglycaemia) for 5,7-DCKA, CGP 37849, memantine and (+)MK-801 were 1.72, 0.09, 0.82 and 0.24 respectively. These results show that even in a severe model of hypoxia/hypoglycaemia, glycineB site antagonists and moderate affinity channel blockers exhibit a better therapeutic index than competitive antagonists and high affinity channel blockers. It is likely that in milder forms of pathology the observed differences in therapeutic indices remain the same but the absolute values are expected to be higher.

Specific antagonists of NMDA receptors prevent osteoclast sealing zone formation required for bone resorption

N-Methyl-d-aspartate (NMDA) glutamate receptors, widely distributed in the nervous system, have recently been identified in bone. They are expressed and are functional in osteoclasts. In the present work, we have studied the effects of specific antagonists of NMDA receptors on osteoclast activation and bone resorption. Using an in vitro assay of bone resorption, we showed that several antagonists of NMDA receptors binding to different sites of the receptor inhibit bone resorption. Osteoclast activation requires adhesion to the bone surface, cytoskeletal reorganization and survival. We demonstrated by autoradiography that the specific NMDA receptor channel blocker, MK 801, binds to osteoclasts. This antagonist had no effect on osteoclast attachment to bone and did not induce osteoclast apoptosis. In contrast, MK 801 rapidly decreased the percentage of osteoclasts with actin ring structures that are associated with actively resorbing osteoclasts. These results suggest that NMDA receptors expressed by osteoclasts may be involved in adhesion-induced formation of the sealing zone required for bone resorption.

Effects of N-methyl-D-aspartate receptor antagonists on reinstatement of cocaine-seeking behavior by priming injections of cocaine or exposures to cocaine-associated cues in rats

The reinstatement of extinguished cocaine self-administration behavior was studied in rats pretreated with N-methyl-D-aspartate receptor antagonists. Rats were trained to self-administer intravenous cocaine (0.32 mg/kg/infusion) during five consecutive daily sessions that were followed by five consecutive daily extinction sessions, during which cocaine was unavailable and cocaine-associated cues (sound and light) were absent. Neither the competitive N-methyl-D-aspartate receptor antagonist D-CPPene (0.3-3 mg/kg) nor the low-affinity N-methyl-D-aspartate receptor channel blocker memantine (1-10 mg/kg) reinstated extinguished responding. Priming injections of intravenous cocaine (Experiment 1), and exposures to cocaine-associated stimuli (buzzer and light; Experiment 2) engendered responding on the reinforced lever in excess of that on the non-reinforced lever. In Experiment 1, administration of D-CPPene or memantine prior to the priming injection of cocaine eliminated the difference between reinforced-lever and non-reinforced-lever response rates. For both D-CPPene and memantine, however, this effect was largely due to increased responding upon the non-reinforced lever rather than to decreased reinforced-lever responding. In Experiment 2, D-CPPene, but not memantine, abolished in a dose-dependent manner the selective increase in reinforced-lever over non-reinforced-lever responding that was induced by exposures to cocaine-related stimuli. This effect of D-CPPene was not due to increased non-reinforced-lever responding. These data help define the boundaries within which N-methyl-D-aspartate receptor antagonists can prevent reinstatement of cocaine-seeking behavior (e.g. type of antagonist used and reinstatement procedure).

Pre-incisional epidural ketamine, morphine and bupivacaine combined with epidural and general anaesthesia provides pre-emptive analgesia for upper abdominal surgery

BACKGROUND

Previous studies have shown that N-methyl-D-asparate (NMDA) receptor antagonists provide a pre-emptive analgesic effect in humans. This study investigated the benefits of pre-emptive analgesia for upper abdominal surgery, using pre-incisional epidural ketamine + morphine + bupivacaine (K+M+B) treatment for achieving postoperative pain relief.

METHODS

Sixty ASA 1-2 patients scheduled for upper abdominal surgery were allocated to three groups in a randomized, single-blinded study. Patients in the control group (I) received general anaesthesia followed by an infusion of normal saline. Group II and III patients received general anaesthesia with a continuous epidural infusion of 2% lidocaine. Thirty minutes after the incision in groups I and II, an epidural pain control regimen was administered using ketamine (10 mg) and morphine (1 mg) in 10 ml of 0.085% bupivacaine (K+M+B). Group III patients also received K+M+B, but it was administered 10 min after the 2% lidocaine injection and 30 min before skin incision. All patients received an epidural pain control regimen (q12 h) for 3 days after their first injection. Patient-controlled analgesia (PCA) with morphine was used to control subsequent postoperative pain. During the 3-day period following surgery, duration to PCA trigger (h), morphine consumption (mg), pain intensity at rest and when coughing/moving, and analgesic-related adverse effects were recorded. The VAS scale (0-10) was used to assess pain intensity.

RESULTS

Median times to first PCA trigger were 1.2 (0.5-2.0) h, 3.0 (0.7-4.2) h, and 4.0 (2.5-7.5) h for groups I, II, and III, respectively. Both the incident and resting pain scores were consistently lower for group III patients than groups I and II. The number of PCA triggers (all attempts/successful triggers) during the day following surgery were 14.0 (3-30)/8.0 (3-24) times, 10.0 (3-23)/6.0 (2-20) times, and 7.0 (3-12)/4.5 (1-10) times for groups I, II, and III. Total morphine consumption for the 3-day observation period was 12.5 (3-42) mg, 10.5 (2-29) mg, and 6.0 (1-20) for groups I, II, and III, respectively.

CONCLUSION

Pre-incisional epidural K+M+B treatment combined with continuous epidural anaesthesia and general anaesthesia provides an ideal pre-emptive analgesic therapy, exhibiting better postoperative pain relief than general anaesthesia and post-incisional K+M+B treatment.

NMDA-receptor antagonists in neuropathic pain: experimental methods to clinical trials

Recent clinical data suggest that chronic pain due to nerve or soft tissue injury may result in the sensitization of the central nervous system, mediated in part by the excitatory amino acids, glutamate and aspartate. Only a handful of N-methyl-D-aspartate antagonists are clinically available. These include ketamine, dextromethorphan, memantine, and amantadine, as well as three clinically used opioids (methadone, dextropropoxyphene, and ketobemidone). This review summarizes the single-dose efficacy of the first two compounds in the treatment of experimental and neuropathic pain. In all examples presented here, NMDA-receptor antagonists with affinity at the phencyclidine site have been shown to modulate pain and hyperalgesia but are limited by dose-limiting side effects. Thus, provided their therapeutic ratio is favorable, NMDA-receptor antagonists may be effective in the treatment of some types of chronic pain.

Effects of AMPA/kainate glutamate receptor antagonists on cocaine-induced convulsions and lethality in mice

Prior studies demonstrate that NMDA receptor antagonists attenuate cocaine-induced convulsions and lethality. Since glutamate is the primary neurotransmitter for NMDA receptors, pharmacological interventions to lower glutamatergic activity through non-NMDA ionotropic receptor-mediated mechanisms were evaluated for their ability to prevent the convulsive and lethal effects of cocaine. Pre-treatment of male, Swiss Webster mice with the alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA)/kainate receptor antagonists 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX; 10-80 mg/kg, i.p.) or 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine hydrochloride (GYKI 52466; 10-20 mg/kg, i.p.) failed to significantly attenuate cocaine-induced convulsions or lethality. Although ineffective when administered alone, NBQX enhanced the protective effects of 5-nitro-6,7-dichloro-1, 4-dihydro-2,3-quinoxalinedione (ACEA-1021), an NMDA/glycine site antagonist, when administered in combination. The mixed NMDA/non-NMDA receptor competitive antagonist 5-chloro-7-trifluoromethyl-1,2,3,4-tetrahydroquinoxaline-2,3-dione (ACEA-1011) also protected against the convulsive effects of cocaine. The data suggest that AMPA/kainate receptors indirectly influence the pathophysiological changes that occur after a cocaine overdose through modulation of NMDA receptors.

Synthesis and anticonvulsant activity of novel and potent 2,3-benzodiazepine AMPA/kainate receptor antagonists

We have previously shown that 1-aryl-3,5-dihydro-7, 8-methylenedioxy-4H-2,3-benzodiazepin-4-ones (3) possess marked anticonvulsant properties and antagonize seizures induced by 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) in analogy to the structurally related 1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxy-5H-2,3-benzodiazepine (1, GYKI 52466), a well-known noncompetitive AMPA/kainate receptor antagonist. We now report the synthesis of 3-(N-alkylcarbamoyl)-1-aryl-3,5-dihydro-7, 8-methylenedioxy-4H-2,3-benzodiazepin-4-ones (4a-h) and 1-aryl-3, 5-dihydro-7,8-methylenedioxy-4H-2,3-benzodiazepine-4-thiones (5a-c). The activity of all compounds, intraperitoneally (ip) injected, was evaluated against audiogenic seizures in DBA/2 mice and against seizures induced by maximal electroshock (MES) and pentylenetetrazole (PTZ) in Swiss mice. Some of the new compounds 4 and 5 showed remarkable anticonvulsant activity, and their toxicity, as evidenced by the rotarod test, is lower than that of 1. The time course of anticonvulsant activity of derivatives 4b and 5b,c was studied and compared to that of 1 and 3b,c. Compounds 4a,b and 5a-c antagonize seizures induced by AMPA and kainate (KA) and their anticonvulsant activity is reversed by pretreatment with aniracetam. Using the patch-clamp technique, the capability of derivatives 3c, 4b, and 5c to antagonize KA-evoked currents in primary cultures of granule neurons was tested and compared with that of the parent compounds 1 and 1-(4-aminophenyl)-3, 4-dihydro-4-methyl-3-methylcarbamoyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine (2, GYKI 53655).

N-methyl-D-aspartate antagonists and drug discrimination

Excitatory amino acids (EAA), such as glutamate, are thought to be involved in various disorders (e.g., ischemic brain damage, epilepsy, Parkinson's disease), and EAA antagonists have been suggested as potential treatments for these disorders. Phencyclidine (PCP), with produces psychotomimetic effects in humans, has antagonist properties at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors that have been suggested to underlie some of its actions. This suggestion, and concern about possible psychotomimetic activity, has stimulated research aimed at examining to what extent the behavioral profile of other NMDA antagonists resembles that of PCP. Drug discrimination (DD) is prominent among the procedures used to carry out such comparisons. The results of clinical studies with NMDA antagonists provide feedback about the predictive validity of the DD procedures used to characterize their preclinical behavioral profile. Further, DD is used also to examine the ability of compounds to attenuate the discriminative stimulus (DS) effects of PCP-type drugs, and results of such studies have been suggested to provide evidence of antipsychotic potential. Finally, although many instances of intermediate responding in DD can be explained by low efficacy at the receptors that mediate the DS effects of the training drug, certain outcomes produced by PCP-type drugs do not offer valid measures of efficacy, and require more detailed behavioral analyzes.

Memantine restores long term potentiation impaired by tonic N-methyl-D-aspartate (NMDA) receptor activation following reduction of Mg2+ in hippocampal slices

This study compared the ability of memantine and (+)MK-801 to counteract deficits in the induction of long term potentiation (LTP) following reduction of Mg2+ in hippocampal slices--a model of increased synaptic noise due to tonic N-methyl-D-aspartate (NMDA) receptor activation. Decreasing Mg2+ from 1 mM to 10 microM for 60 min enhanced baseline field excitatory post-synaptic potential (fEPSP) slopes (87.2 +/- 10.6% above control) and impaired LTP (-4.1 +/- 9.8% compared to pre-tetanic levels). Long pre-incubations with memantine (1 microM), a concentration achieved in the CSF of dementia patients, almost fully restored the induction of LTP (to 43.4 +/- 8.4%) without changing the enhancement of baseline fEPSP slopes (84.1 +/- 11.6%). Memantine (10 microM) fully restored the induction of LTP (61.5 +/- 5.3%) and also decreased the enhancement of baseline fEPSP slopes (30.1 +/- 4.9%). In contrast, although (+)MK-801 (0.01, 0.1 and 1 microM) caused a concentration-dependent reduction in the low Mg2+ -induced enhancement of baseline fEPSP slopes, it was not able to restore the induction of LTP (3.0 +/- 9.8%, 16.3 +/- 5.7% and 4.8 +/- 6.7% respectively). These data indicate that memantine could produce symptomatological improvement in learning under conditions of tonic NMDA receptor activation such as those occurring in chronic neurodegenerative diseases whereas (+)MK-801 is likely to have only negative effects.

The preoperative administration of intravenous dextromethorphan reduces postoperative morphine consumption

We evaluated the effect of dextromethorphan on postoperative pain management. Sixty ASA physical status I-III female patients undergoing major abdominal surgery underwent standardized general anesthesia. Thirty patients received an i.v. infusion of dextromethorphan 5 mg/kg before anesthetic induction (Pre group), whereas the remaining 30 patients received the same volume of isotonic sodium chloride solution, followed by a postoperative i.v. infusion of dextromethorphan 5 mg/kg (Post group). Patients in the Pre group received the same volume of isotonic sodium chloride solution postoperatively. All patients were then treated with patient-controlled i.v. analgesia, which administered a 0.6-mg bolus of morphine on demand (maximal 4 h dose 20 mg). The mean visual analog pain score during cough or movement and at rest were similar in the two groups in the first 3 days postoperatively. However, Post group patients consumed more morphine than Pre group patients during the first 2 days (P < 0.01). The sedation scores, patient satisfaction, and the incidence of morphine-related side effects were similar between the two groups. We conclude that the preoperative administration of dextromethorphan 5 mg/kg reduces postoperative morphine consumption compared with postoperative administration.

IMPLICATIONS

In this double-blinded study, we found that the preoperative administration of i.v. dextromethorphan 5 mg/kg, compared with postoperative administration, reduces postoperative morphine consumption, which may provide clinical evidence of preemptive or preventive analgesic effects of dextromethorphan.

Use and efficacy of low-dose ketamine in the management of acute postoperative pain: a review of current techniques and outcomes

Ketamine hydrochloride is a well known general anesthetic and short acting analgesic in use for almost 3 decades. The role of the NMDA receptor in the processing of nociceptive input has led naturally to renewed clinical interest in N-methyl-D-aspartate (NMDA) receptor antagonists such as ketamine. This paper reviews the use and efficacy of low-dose ketamine in the management of acute postoperative pain. The literature was obtained from a computer search of the MEDLINE database from 1966 through December 1998. Studies were included for review if they were randomized, prospective, controlled, double-blind and reported pain scores. We evaluate the clinical literature and discuss the efficacy of low-dose ketamine in the management of acute postoperative pain when administered alone or in conjunction with other agents via the oral, intramuscular, subcutaneous, intravenous and intraspinal routes. Low-dose ketamine is defined as a bolus dose of less than 2 mg/g when given intramuscularly or less than 1 mg/kg when administered via the intravenous or epidural route. For continuous i.v. administration low-dose ketamine is defined as a rate of < or =20 microg/kg per min. We conclude that ketamine may provide clinicians with a tool to improve postoperative pain management and to reduce opioid related adverse effects. The evidence suggests that low-dose ketamine may play an important role in postoperative pain management when used as an adjunct to local anesthetics, opioids, or other analgesic agents. Further research is required in the following areas: (a) dose-finding studies for ketamine as an adjunct to opioids and local anesthetics (b) efficacy and optimal route of administration (c) the role of S(+)-ketamine; (d) the influence of ketamine on long-term outcome such as chronic pain (e) long-term physical and chemical stability of mixtures containing ketamine (f) spinal toxicity of ketamine and (g) effects of low-dose ketamine on cognitive and memory functioning after surgery.

Anticonvulsant activity and plasma level of 2,3-benzodiazepin-4-ones (CFMs) in genetically epilepsy-prone rats

Anticonvulsant properties of some 2,3-benzodiazepine derivatives acting as alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) antagonists have been examined in vivo in the genetically epilepsy-prone rats using an audiogenic seizures assay. 2,3-Benzodiazepin-4-ones (CFMs) are nonselective AMPA antagonists that have been found to be potent anticonvulsant compound is in acute models of epilepsy. Because very little is known about their actions in a chronic model of epilepsy, and no correlations exist between anticonvulsant potency and plasma levels of these derivatives, we planned to investigate such a relationship. Maximal anticonvulsant protection occurred 15-60 min after the IP administration of GYKI 52466, 30-90 min after CFM-2, and 45-120 min after CFM-3. In addition, maximal anticonvulsant effect was observed 60-120 min after the IP administration of CFM-4 and at 90 min after CFM-5. The therapeutic index revealed that GYKI 52466 was slightly more toxic than CFM-2 and CFM-3. The time course of plasma levels of rats treated showed that peak plasma concentration was observed 45 min after IP administration of CFM-2 and CFM-3 and 75 min after CFM-4 and CFM-5. Following IP administration of CFM-3 two curves were detected, one is referred to the injected compound, and the other to its demethylated metabolite, which corresponds to CFM-2. Also. for the nitroderivative CFM-4 two curves were detected: one of an injected compound and the second due to its reduced metabolite (CFM-2). Finally, three different metabolites were detected in rat plasma after IP administration of CFM-5. The present study demonstrated that CFMs showed a significant protection against auditory stimulation during the period of peak plasma concentrations, suggesting a marked inhibition of those brain structures involved in the initiation and/or spreading of the audiogenic seizures.

Low-affinity NMDA receptor channel blockers inhibit acquisition of intravenous morphine self-administration in naive mice

Experimental evidence suggests that NMDA receptor antagonists modulate behavioral effects of morphine in models assessing abuse potential of drugs. The present study sought to evaluate the ability of NMDA receptor channel blockers to affect the acquisition of morphine i.v. self-administration in drug- and experimentally naive mice. DBA/2 mice were allowed to self-administer morphine (0.125-4.0 mg/ml) or saline during the 30-min test. Each nose-poke of the active mouse resulted in a 1.6-microl infusion to both the active mouse and the passive (yoked control) mouse. In vehicle-treated mice, differences between operant activity of active and passive mice were most obvious when active mice were allowed to self-inject morphine at the concentration of 0.5 mg/ml (the optimum concentration). Pretreatment with MRZ 2/579 (1-amino-1,3,3,5,5-pentamethyl-cyclohexan hydrochloride; 1, 3.2 and 10 mg/kg) shifted the optimum concentration to 0.75 mg/ml. Memantine (1-amino-3,5-dimethyladamantane hydrochloride; 0.3, 1, 3.2 and 10 mg/kg) suppressed both the morphine intake and the difference in nose-poke activity of active vs. passive mice across all tested concentrations of morphine. Dizocilpine ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate; 0.1 mg/kg) was ineffective. Taken together with earlier reports, the present results suggest that low-affinity NMDA receptor channel blockers--in contrast to dizocilpine--attenuate the rewarding potential of morphine.

Synthesis and anticonvulsant activity of 3-aryl-5H-2,3-benzodiazephine AMPA antagonists

A novel series of 3-aryl-5H-2,3-benzodiazepines with N-3 aromatic substituents has been synthesized. Good in vivo anticonvulsant activity of the new compounds has been demonstrated employing the maximal electroshock seizure test in mice. Evaluation of a subset of the compounds in the cortical wedge assay confirmed the new structures to be AMPA antagonists.

Active NMDA glutamate receptors are expressed by mammalian osteoclasts

1. The N-methyl-D-aspartate (NMDA) glutamate receptor, widely distributed in the mammalian nervous system, has recently been identified in bone. In this study, we have investigated whether NMDA receptors expressed by osteoclasts have an electrophysiological activity. 2. Using the patch clamp technique two agonists of the NMDA receptor, L-glutamate (Glu) and NMDA, were shown to activate whole-cell currents recorded in isolated rabbit osteoclasts. 3. The current-voltage (I-V ) relationships of the currents induced by Glu (IGlu) and NMDA (INMDA) were studied using Mg2+-free solutions. The agonist-induced currents had a linear I-V relationship with a reversal potential near 0 mV, as expected for a voltage independent and non-selective cationic current. 4. IGlu and INMDA were sensitive to specific blockers of NMDA subtype glutamate receptors, such as magnesium ions, (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten -5,10-imine (MK-801) and 1-(1,2-diphenylethyl) piperidine (DEP). The block of IGlu and INMDA by these specific antagonists was voltage dependent, strong for negative potentials (inward current) and absent for positive potentials (outward current). 5. These results demonstrate that NMDA receptors are functional in rabbit osteoclasts, and that their electrophysiological and pharmacological properties in these cells are similar to those documented for neuronal cells. Active NMDA receptors expressed by osteoclasts may represent a new target for regulating bone resorption.

The spinal antinociceptive effects of a novel competitive AMPA receptor antagonist, YM872, on thermal or formalin-induced pain in rats

Alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonists have spinally mediated analgesic effects on acute nociception; however, their current formulations are not water-soluble and have toxic side effects. A new competitive AMPA antagonist, YM872 (2,3-dioxo-7-[1H-imidazol-1-yl]-6-nitro-1,2,3,4-tetrahydro-1-quinoxal inyl acetic acid) is water-soluble and may have fewer side effects. The purpose of this study was to investigate the analgesic effects of YM872 on both acute thermal and irritant-induced pain. Sprague-Dawley rats were implanted with chronic lumbar intrathecal catheters and were tested for their tail withdrawal response by the tail flick test and for their paw flinches by formalin injection after the intrathecal administration of YM872. The tail flick latency increased dose-dependently with a 50% effective dose (ED50) value of 1.0 microg. The number of flinches in both Phase 1 and Phase 2 of the formalin test decreased with increasing dose of YM872. ED50 values were 0.24 microg in Phase 1 and 0.21 microg in Phase 2. YM872 10 and 30 microg induced motor disturbance and flaccidity. In rats, the intrathecal administration of YM872 had analgesic effects on both acute thermal and formalin-induced nociceptions. Transient motor disturbance and flaccidity occurred only with large doses. YM872 may have potential in the clinical management of both acute and chronic pain.

IMPLICATIONS

A novel alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist, YM872, may have an analgesic effect on both acute and chronic pain when administered intrathecally.

Memantine is a clinically well tolerated N-methyl-D-aspartate (NMDA) receptor antagonist--a review of preclinical data

N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in numerous CNS disorders ranging from acute neurodegeneration (e.g. stroke and trauma), chronic neurodegeneration (e.g. Parkinson's disease, Alzheimer's disease, Huntington's disease, ALS) to symptomatic treatment (e.g. epilepsy, Parkinson's disease, drug dependence, depression, anxiety and chronic pain). However, many NMDA receptor antagonists also produce highly undesirable side effects at doses within their putative therapeutic range. This has unfortunately led to the conclusion that NMDA receptor antagonism is not a valid therapeutic approach. However, memantine is clearly an uncompetitive NMDA receptor antagonist at therapeutic concentrations achieved in the treatment of dementia and is essentially devoid of such side effects at doses within the therapeutic range. This has been attributed to memantine's moderate potency and associated rapid, strongly voltage-dependent blocking kinetics. The aim of this review is to summarise preclinical data on memantine supporting its mechanism of action and promising profile in animal models of chronic neurodegenerative diseases. The ultimate purpose is to provide evidence that it is indeed possible to develop clinically well tolerated NMDA receptor antagonists, a fact reflected in the recent interest of several pharmaceutical companies in developing compounds with similar properties to memantine.

Changes in somatosensory evoked potentials following forebrain ischemia in the gerbils: effects of felbamate

Somatosensory evoked potentials (SEPs) as well as change following transient cerebral ischemia in the gerbil were characterized in this study. SEPs were measured in each gerbil before ischemia (day -1), during ischemia, 10 min, 2, 4, 8, 24, 48 h and 8 days after recirculation. During bilateral carotid occlusion, SEP amplitude was dramatically reduced and central conduction time was significantly increased. During recirculation these values showed an improvement when compared to ischemic but not to control values. Moreover at 8 days of recirculation they were still statistically different from control values. Felbamate administration at the dose of 150 mg kg(-1), immediately after recirculation was shown to ameliorate neurophysiological recovery following cerebral ischemia.

Synthesis and anticonvulsant activity of new 2,3-benzodiazepines as AMPA receptor antagonists

Novel 1-aryl-3,5-dihydro-7,8-methylenedioxy-4H-2,3-benzodiazepine-4-ones (12a-j) were prepared and their anticonvulsant effects were evaluated by using various models of experimental epilepsy. The seizures were evoked both by means of auditory stimulation in DBA/2 mice and by pentylenetetrazole or maximal electroshock in Swiss mice. Some of these compounds possess marked anticonvulsant properties in all tests employed. Compounds 12 antagonise seizures induced by AMPA in analogy to the structurally-related 1-(4'-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3- benzodiazepine (1) (GYKI 52466), a well-known non-competitive AMPA-receptor antagonist. On the other hand, these novel 2,3-benzodiazepines exhibit anticonvulsant properties that are not affected by flumazenil, but are reversed by aniracetam. In addition, when compared to model compound 1, compounds 12 show a longer-lasting anticonvulsant activity and a lower toxicity. A structure-activity relationship study carried out on compounds 12 as well as analogous 7,8-dimethoxy derivatives 2 offers an approach for designing more potent agents.

Stereoselective neuroprotection by novel 2,3-benzodiazepine non-competitive AMPA antagonist against non-NMDA receptor-mediated excitotoxicity in primary rat hippocampal cultures

Glutamate excitotoxicity has been implicated in a variety of acute and chronic neurodegenerative diseases but early phase clinical trials with competitive antagonists at both N-methyl-D-aspartate (NMDA)-receptors and alpha-amino-3-hydroxy-5-methyl-isoxazolepropionate (AMPA) receptors have been disappointing. A family of atypical 2,3 benzodiazepines, exemplified by GYKI 52466, have been described recently which function as non-competitive AMPA-receptor antagonists. We have investigated the neuroprotective efficacy of LY303070 and LY300164, two analogs of GYKI-52466, in an embryonic rat hippocampal culture model of non-NMDA receptor-mediated excitotoxicity using kainic acid (KA) as an agonist at the AMPA/KA receptor. Overnight treatment with 500 microM KA resulted in prominent neuronal excitotoxicity as assessed by lactate dehydrogenase efflux. LY300164 and LY303070 attenuated KA-excitotoxicity in a dose-dependent manner with IC50s of 4 and 2 microM, respectively. In contrast, their stereoisomers, LY300165 and LY303071 showed no neuroprotection at concentrations up to 25 microM. In addition, AMPA-mediated excitotoxicity in cyclothiazide pre-treated cultures was also completely blocked by LY303070. Finally, neuroprotection by this class of 2,3 benzodiazepines was not influenced by antagonism of the classical benzodiazepine receptor. LY303070 and LY300164 represent novel non-competitive AMPA-receptor antagonists which may offer unique advantages in the clinic over competitive AMPA-receptor antagonists.

The role of excitotoxicity in neurodegenerative disease: implications for therapy

Glutamic acid is the principal excitatory neurotransmitter in the mammalian central nervous system. Glutamic acid binds to a variety of excitatory amino acid receptors, which are ligand-gated ion channels. It is activation of these receptors that leads to depolarisation and neuronal excitation. In normal synaptic functioning, activation of excitatory amino acid receptors is transitory. However, if, for any reason, receptor activation becomes excessive or prolonged, the target neurones become damaged and eventually die. This process of neuronal death is called excitotoxicity and appears to involve sustained elevations of intracellular calcium levels. Impairment of neuronal energy metabolism may sensitise neurones to excitotoxic cell death. The principle of excitotoxicity has been well-established experimentally, both in in vitro systems and in vivo, following administration of excitatory amino acids into the nervous system. A role for excitotoxicity in the aetiology or progression of several human neurodegenerative diseases has been proposed, which has stimulated much research recently. This has led to the hope that compounds that interfere with glutamatergic neurotransmission may be of clinical benefit in treating such diseases. However, except in the case of a few very rare conditions, direct evidence for a pathogenic role for excitotoxicity in neurological disease is missing. Much attention has been directed at obtaining evidence for a role for excitotoxicity in the neurological sequelae of stroke, and there now seems to be little doubt that such a process is indeed a determining factor in the extent of the lesions observed. Several clinical trials have evaluated the potential of antiglutamate drugs to improve outcome following acute ischaemic stroke, but to date, the results of these have been disappointing. In amyotrophic lateral sclerosis, neurolathyrism, and human immunodeficiency virus dementia complex, several lines of circumstantial evidence suggest that excitotoxicity may contribute to the pathogenic process. An antiglutamate drug, riluzole, recently has been shown to provide some therapeutic benefit in the treatment of amyotrophic lateral sclerosis. Parkinson's disease and Huntington's disease are examples of neurodegenerative diseases where mitochondrial dysfunction may sensitise specific populations of neurones to excitotoxicity from synaptic glutamic acid. The first clinical trials aimed at providing neuroprotection with antiglutamate drugs are currently in progress for these two diseases.

Neurotoxicity of dextrorphan

BACKGROUND

The noncompetitive NMDA antagonists phencyclidine (PCP) and dizocilpine (MK-801) have been considered for use as neuroprotective therapeutic agents, although both produce injury in neurons of cingulate and retrosplenial cortices in rodents. The low-affinity, noncompetitive NMDA antagonist dextrorphan has been considered for use as a neuroprotective therapeutic drug. The aim of the present work was to evaluate the neurotoxicity of dextrorphan.

METHODS

Sprague-Dawley male rats were used and injected with either saline or dextrorphan (30 mg/kg i.p.). The animals were sacrificed 30 min later, and the brain was examined for histopathological changes.

RESULTS

After systemic administration of the drug, hyperchromatic and shrunken nuclei with chromatin condensation and disruption were observed. Also, granular and vacuolated cytoplasm was apparent in pyramidal neurons in the retrosplenial (posterior cingulate) cortex. Status spongiosus (spongy degeneration) of the neuropil was also detected.

CONCLUSIONS

Morphological changes are similar to those described previously, which are induced by high-affinity, noncompetitive NMDA antagonists, such as MK-801.

Chronic interleukin-6 alters NMDA receptor-mediated membrane responses and enhances neurotoxicity in developing CNS neurons

Recent studies show that the cytokine interleukin-6 (IL-6) is expressed at elevated levels in the CNS in several disease states and contributes to the neuropathological process. The mechanisms through which IL-6 exerts its CNS effects are primarily unknown. We have investigated the pathophysiological effects of IL-6 on developing CNS neurons using a culture model system and a chronic treatment paradigm. Here, we show, using current- and voltage-clamp recordings, that chronic IL-6 treatment of developing cerebellar granule neurons increases the membrane and current response to NMDA and that these effects are the primary mechanism through which IL-6 produces an enhanced calcium signal to NMDA. We also show that calcium influx through voltage-sensitive calcium channels contributes to the enhanced calcium signal to NMDA in the IL-6-treated neurons in a developmentally regulated manner and that the membrane depolarization to NMDA is more sensitive to the NMDA receptor antagonist ifenprodil in the IL-6-treated neurons compared with control neurons at a late developmental stage, consistent with a larger proportion of NMDA receptors containing the NMDAR2B subunit in the IL-6-treated neurons. Additional studies show that IL-6 treatment reduces the number of granule neurons in culture and enhances neurotoxicity involving NMDA receptors. These results support a pathological role for IL-6 in the CNS and indicate that NMDA receptor-mediated functions are likely to play a critical role in neuropathological changes observed in CNS diseases associated with elevated CNS levels of IL-6.

Allosteric modulators of the AMPA receptor: novel 6-substituted dihydrophthalazines

Novel analogs of the allosteric AMPA receptor modulator SYM 2206 have been prepared. Structure/activity correlations of these novel analogs and other dihydrophthalazines (DHPs) reveal the important contribution of the heteroatom-based aryl substituents in this class of noncompetitive inhibitors. One of the analogs (6, SYM 2189) is equipotent with the early series, but with reduced sedation.

Antihyperalgesic effect of the N-methyl-D-aspartate receptor antagonist dextromethorphan in the oral surgery model

Peripheral neuronal barrage from tissue injury produces central nervous system hyperexcitability through the activation of N-methyl-D-aspartate (NMDA) receptor sites by excitatory amino acids and neuropeptides. This study evaluated if attenuation of NMDA receptor activation with dextromethorphan (DM) suppresses the postoperative development of hyperalgesia. Seventy-five patients undergoing oral surgery in a parallel-group, double-blind study randomly received either a placebo or the maximally tolerated dose of DM administered orally prior to and continuing for 48 hours following surgery. Pain as measured by category, visual analog, and verbal descriptor scales was not significantly different between groups during the first 6 hours following surgery. However, pain at 48 hours was decreased in the DM group as measured by scales for pain intensity and unpleasantness. Subjects in the DM group also self-administered fewer acetaminophen tablets for unrelieved pain over 24 to 48 hours postoperatively. The results suggest that DM at maximally tolerated doses does not produce an analgesic effect in the immediate postoperative period but reduces pain at 48 hours. This may be related to antagonism of NMDA receptors necessary for the expression of hyperalgesia associated with noxious afferent input postoperatively.

Neuroprotective interaction effects of NMDA and AMPA receptor antagonists in an in vitro model of cerebral ischemia

An in vitro model of ischemia was developed and characterized using the acute rat hippocampal slice preparation. Neuroprotective concentrations of several competitive and noncompetitive glutamate subtype-selective antagonists (CGS-19755, MK-801, YM90K and GYKI-52466) were initially determined in anoxia-enhanced agonist-induced excitotoxicity experiments. Concentrations which proved to be effective in these studies were subsequently tested for their effectiveness against an ischemic episode. Ischemia was defined as a 30-min exposure to aglycemic media ending in 5 min of concurrent anoxia, a protocol which was arrived at by empirically determining the effect of various hypoglycemic and anoxic insults on the ability of hippocampal slices to retain their electrophysiological viability. Exposure to such an ischemic episode resulted in a loss of viability by most slices, an effect which was strongly dependent on extracellular calcium. AMPA antagonists applied alone produced no neuroprotective effect in the present model of in vitro ischemia, while NMDA antagonists applied alone had a modest neuroprotective effect. In contrast, the coapplication of 10 microM MK-801 and 300 microM GYKI-52466, noncompetitive NMDA and AMPA receptor antagonists, respectively, resulted in almost complete neuroprotection. This protection was comparable to that obtained by withholding extracellular calcium, indicating that the toxic effects of glutamate receptor overstimulation can be accounted for solely by calcium influx. The effect of this combination treatment on the survival rate of hippocampal slices was synergistic, that is greater than the sum of the effects of the individual compounds. The results indicate that neuroprotection against acute ischemic insults may require a combination therapy approach.

A new pyrrolyl-quinoxalinedione series of non-NMDA glutamate receptor antagonists: pharmacological characterization and comparison with NBQX and valproate in the kindling model of epilepsy

Antagonists at the ionotropic non-NMDA [AMPA (amino-methyl proprionic acid)/kainate] type of glutamate receptors have been suggested to possess several advantages compared to NMDA (N-methyl-D-aspartate) receptor antagonists, particularly in terms of risk/benefit ratio, but the non-NMDA receptor antagonists available so far have not fulfilled this promise. From a large series of pyrrolyl-quinoxalinedione derivatives, we selected six new competitive non-NMDA receptor antagonists. The basis of selection was high potency and selectivity for AMPA and/or kainate receptors, high in vivo potency after systemic administration, and an acceptable ratio between neuroprotective or anticonvulsant effects and adverse effects. Pharmacological characteristics of these novel compounds are described in this study with special emphasis on their effects in the kindling model of temporal lobe epilepsy, the most common type of epilepsy in humans. In most experiments, NBQX and the major antiepileptic drug valproate were used for comparison with the novel compounds. The novel non-NMDA receptor antagonists markedly differed in their AMPA and kainate receptor affinities from NBQX. Thus, while NBQX essentially did not bind to kainate receptors at relevant concentrations, several of the novel compounds exhibited affinity to rat brain kainate receptors or recombinant kainate receptor subtypes in addition to AMPA receptors. One compound, LU 97175, bound to native high affinity kainate receptors and rat GluR5-GluR7 subunits, i.e. low affinity kainate binding sites, with much higher affinities than to AMPA receptors. All compounds potently blocked AMPA-induced cell death in vitro and, except LU 97175, AMPA-induced convulsions in vivo. In the kindling model, compounds with a high affinity for GluR7 (LU 97175) or compounds (LU 115455, LU 136541) which potently bind to AMPA receptors and low affinity kainate receptor subunits were potent anticonvulsants in the kindling model, whereas the AMPA receptor-selective LU 112313 was the least selective compound in this model, indicating that non-NMDA antagonists acting at both AMPA and kainate receptors are more effective in this model than AMPA receptor-selective drugs. Three of the novel compounds, i.e. LU 97175, LU 115455 and LU 136541, exerted potent anticonvulsant effects without inducing motor impairment in the rotarod test. This combination of actions is thought to be a prerequisite for selective anticonvulsant drug action.

Amino-alkyl-cyclohexanes are novel uncompetitive NMDA receptor antagonists with strong voltage-dependency and fast blocking kinetics: in vitro and in vivo characterization

The present study characterized the in vitro NMDA receptor antagonistic properties of novel amino-alkyl-cyclohexane derivatives and compared these effects with their ability to block excitotoxicity in vitro and MES-induced convulsions in vivo. The 36 amino-alkyl-cyclohexanes tested displaced [3H]-(+)-MK-801 binding to rat cortical membranes with K(i)s between 1.5 and 143 microM. Current responses of cultured hippocampal neurones to NMDA were antagonized by the same compounds with a wide range of potencies (IC50s of 1.3-245 microM, at -70 mV) in a use- and strongly voltage-dependent manner (delta 0.55-0.87). The offset kinetics of NMDA receptor blockade was correlated with equilibrium affinity (Corr Coeff. 0.87 P < 0.0001). As an example, MRZ 2/579 (1-amino-1,3,3,5,5-pentamethyl-cyclohexane HCl) had similar blocking kinetics to those previously reported for memantine (K(on) 10.67 +/- 0.09 x 10(4) M(-1) s(-1), K(off) 0.199 +/- 0.02 s(-1), K(d) = K(off)/K(on) = 1.87 microM c.f. IC50 of 1.29 microM). Most amino-alkyl-cyclohexanes were protective against glutamate toxicity in cultured cortical neurones (e.g. MRZ 2/579 IC50 2.16 +/- 0.03 microM). Potencies in the three in vitro assays showed a relatively strong cross correlation (all corr. coeffs. > 0.72, P < 0.0001). MRZ 2/579 was also effective in protecting hippocampal slices against 7 min. hypoxia/hypoglycaemia-induced reduction of fEPSP amplitude in CA1 with an EC50 of 7.01 +/- 0.24 microM. MRZ 2/579 showed no selectivity between NMDA receptor subtypes expressed in Xenopus oocytes but was somewhat more potent than in patch clamp experiments-IC50s of 0.49 +/- 0.11, 0.56 +/- 0.01 microM, 0.42 +/- 0.04 and 0.49 +/- 0.06 microM on NR1a/2A /2B, /2C and 2/D, respectively. In contrast, memantine and amantadine were both 3-fold more potent at NR1a/2C and NR1a/2D than NR1a/2A receptors. All Merz amino-alkyl-cyclohexane derivatives inhibited MES-induced convulsions in mice with ED50s ranging from 3.6 to 130 mg/kg i.p. The in vivo and in vitro potencies correlated indicating similar access of most compounds to the CNS. MRZ 2/579 administered at 10 mg/kg resulted in peak plasma concentrations of 5.3 and 1.4 microM following i.v. and p.o. administration respectively, which then declined with a half life of around 170-210 min. Analysis of A.U.C. concentrations indicates a p.o./i.v. bioavailability ratio for MRZ 2/579 of 60%. MRZ 2/579 injected i.p. at a dose of 5 mg/kg resulted in peak brain extracellular fluid (ECF) concentrations of 0.78 microM (brain microdialysates). Of the compounds tested MRZ 2/579, 2/615, 2/632, 2/633, 2/639 and 2/640 had affinities, kinetics and voltage-dependency most similar to those of memantine and had good therapeutic indices against MES-induced convulsions. We predict that these amino-alkyl-cyclohexanes, which all had methyl substitutions at R1, R2, and R5, at least one methyl or ethyl at R3 or R4 and a charged amino-containing substitution at R6, could be useful therapeutics in a wide range of CNS disorders proposed to involve disturbances of glutamatergic transmission.

Neuroprotective activities of carvedilol and a hydroxylated derivative: role of membrane biophysical interactions

Carvedilol is a vasodilating beta-blocker and antioxidant approved for treatment of mild to moderate hypertension, angina, and congestive heart failure. SB 211475 (4-[2-hydroxyl-3-[[2-(2-methoxyphenoxy)ethyl]amino]propoxyl]-9H-++ +carbazol-3-ol), a hydroxylated carvedilol analogue, is an even more potent antioxidant in several assay systems. Carvedilol also has neuroprotective capacity with modulatory actions at N-methyl-D-aspartate (NMDA) receptors and Na+ channels. In the present study, we demonstrated that in cultured rat cerebellar neurons, SB 211475 has 28-fold greater antioxidant activity than carvedilol, but is 2- to 6-fold less potent, respectively, at inhibiting neurotoxic activities at Na+ channels and at NMDA receptor channels. To determine a biophysical rationale for these differential activities, small angle x-ray scattering data were obtained from model lipid and brain membrane bilayers containing either carvedilol, SB 211475, or dihydropyridine calcium channel blockers. Electron density profiles revealed that the location of SB 211475 was restricted to the glycerol backbone/hydrocarbon interface and significantly reduced membrane width by 5%, whereas the time-averaged location for carvedilol and flunarizine also extended to the hydrated surface of the bilayer. Comparison of carvedilol with several dihydropyridines showed a correlation between high ClogP values (lipophilicity), Na+ channel inhibitory potency, and bilayer localization. The antioxidant activity of SB 211475 could be explained by restricted intercalation into the glycerol phosphate/hydrocarbon interface, creating an increase in volume associated with the phospholipid acyl chains, which would then become resistant to lipid peroxidation. Differential channel modulation may also be explained by these membrane structural results, which indicate that carvedilol and the less spatially restricted dihydropyridine molecules are more likely to inhibit transmembrane receptor channels.

Anticonvulsant action of a NMDA receptor antagonist CGP 40116 varies only quantitatively during ontogeny in rats

Anticonvulsant action of CGP 40116, a competitive antagonist of N-methyl-D-aspartate type of excitatory amino acid receptors, was studied in rats during development (7, 12, 18, 25 and 90 days old). Two types of motor seizures were elicited by a subcutaneous injection of pentylenetetrazol. Pretreatment with CGP 40116 did not influence minimal, predominantly clonic seizures in any age group. Generalized tonic-clonic seizures were at first modified--their tonic phase was restricted to forelimbs, then selectively suppressed--and with increasing dosage the clonic phase was blocked too. This effect exhibited only minor quantitative changes during development.

Electrophysiological effects of felbamate

Felbamate is a broad spectrum antiepileptic drug recently introduced into clinical practice for controlling seizures in patients affected by Lennox-Gastaut epilepsy, complex partial seizures or otherwise intractable epilepsies. However, the cellular mechanisms by which the drug exerts its anticonvulsant actions are not fully understood. The aim of the present article is to outline the possible mechanisms of action of felbamate as suggested by findings obtained with electrophysiological approaches.

Open channel block and alteration of N-methyl-D-aspartic acid receptor gating by an analog of phencyclidine

We investigated inhibition of the N-methyl-D-aspartic acid (NMDA) receptor-channel complex by N-ethyl-1,4,9, 9alpha-tetrahydro-4alphaR-cis-4alphaH-fluoren-++ +4alpha-amine (NEFA), a structural analog of phencyclidine (PCP). Using the whole-cell recording technique, we demonstrated that NEFA inhibits NMDA responses with an IC50 of 0.51 microM at -66 mV. We determined that NEFA binds to the open channel, and subsequently the channel can close and trap the blocker. Once the channel has closed, NEFA is unable to dissociate until the channel reopens. Single-channel recordings revealed that NEFA reduces the mean open time of single NMDA-activated channels in a concentration-dependent manner with a forward blocking rate (k+) of 39.9 microM-1 s-1. A computational model of antagonism by NEFA was developed and constrained using kinetic measurements of single-channel data. By multiple criteria, only models in which blocker binding in the channel causes a change in receptor operation adequately fit or predicted whole-cell data. By comparing model predictions and experimental measurements of NEFA action at a high NMDA concentration, we determined that NEFA affects receptor operation through an influence on channel gating. We conclude that inhibition of NMDA receptors by PCP-like blockers involves a modification of channel gating as well as block of current flow through the open channel.

Evidence for overshadowing by components of the heterogeneous discriminative stimulus effects of ethanol

The present study used a drug discrimination paradigm to characterize the contribution of separate receptor systems to the stimulus effects of different training doses of ethanol. In a two-lever drug discrimination paradigm two groups of adult male Long-Evans rats (n = 8 per group) were trained to discriminate either 1.0 g/kg ethanol from water or 2.0 g/kg ethanol from water, administered intragastrically (i.g.), 30 min prior to the start of daily sessions in which responding was maintained under a fixed ratio 20 schedule of food presentation. Following training, cumulative dosing substitution tests were conducted with the GABAA positive modulator pentobarbital (1-17 mg/kg, i.p.), the uncompetitive NMDA antagonist dizocilpine (0.01-0.3 mg/kg, i.p.) and the 5-HT1B/2C agonist m-trifluoromethylphenylpiperazine (TFMPP 0.17-1.7 mg/kg, i.p.). Next, the rats initially trained at 1.0 g/kg ethanol were retrained to discriminate 2.0 g/kg ethanol from water, and the rats initially trained at 1.0 g/kg were retrained to discriminate 2.0 g/kg ethanol from water. Both groups were then re-tested with the same ligands. Regardless of training history, animals currently discriminating 1.0 g/kg were more sensitive to the ethanol-like effects of TFMPP and pentobarbital compared to rats discriminating 2.0 g/kg ethanol. However, no difference in sensitivity to the ethanol-like effects of dizocilpine based on ethanol training dose was detected. These results support the view that ethanol is a heterogeneous discriminative stimulus comprised of GABAA, NMDA and 5-HT1B/2C receptor-mediated activity. Furthermore, changes in sensitivity to GABAA and 5-HT ligands as a function of training dose could be indicative of overshadowing by other components of ethanol's heterogeneous cue. Finally, it appears that the current profile of ethanol's heterogeneous stimulus effects, rather than an interaction with ethanol training history, determines the substitution pattern of specific receptor ligands.

Interactions of GYKI 52466 and NBQX with cyclothiazide at AMPA receptors: experiments with outside-out patches and EPSCs in hippocampal neurones

In outside-out patches from cultured hippocampal neurones, glutamate (1 mM) applied for 1 ms evoked currents which rose rapidly (tau(on) 451 +/- 31 micros) to a peak and then deactivated with slower kinetics (1.95 +/- 0.13 ms). Offset time constants were significantly slower with longer application durations (tau(off) 3.10 +/- 0.19, 3.82 +/- 0.25, 4.80 +/- 0.65 and 7.56 +/- 0.65 ms with 10, 20, 100 and 500 ms applications respectively). Desensitization was complete within 100 ms with a similar rate for all application durations (4.74 +/- 0.34 ms with 100 ms applications). GYKI 52466 reduced inward peak currents with an IC50 of 11.7 +/- 0.6 microM and had similar potency on steady-state currents to longer glutamate applications. GYKI 52466 had no significant effect on desensitization or deactivation time constants but caused a modest and significant prolongation of onset kinetics at higher concentrations. Cyclothiazide (100 microM) potentiated steady-state currents 25-fold at 100 ms and caused a modest but significant slowing in onset kinetics (601 +/- 49 micros with 1 ms applications) but a more pronounced prolongation of deactivation time constants (5.55 +/- 0.66 ms with 1 ms applications). In 50% of neuronal patches cyclothiazide completely eliminated desensitization. In those patches with residual desensitization, the rate was not significantly different to control (5.36 +/- 0.43 ms with 100 ms applications). Following 100 ms applications of glutamate, GYKI 52466 had IC50s of 11.7 +/- 1.1 microM and 75.1 +/- 7.0 microM in the absence and presence of cyclothiazide (100 microM) respectively. Onset kinetics were slowed from 400 +/- 20 micros to 490 +/- 30 micros by cyclothiazide (100 microM) and then further prolonged by GYKI 52466 (100 microM) to a double exponential function (tau(on1) 1.12 +/- 0.13 ms and tau(on2) 171.5 +/- 36.5 ms). GYKI 52466 did not re-introduce desensitization but concentration-dependently weakened cyclothiazide's prolongation of deactivation time constants (1 ms applications: 5.01 +/- 0.71, 4.47 +/- 0.80 and 2.28 +/- 0.64 ms with GYKI 52466 30, 100 and 300 microM respectively). NBQX reduced peak current responses with an IC50 of 28.2 +/- 1.3 nM. Paradoxically, steady-state currents with 500 ms applications of glutamate were potentiated from 3.3 +/- 1.2 pA to 29.4 +/- 6.4 pA by NBQX (1 nM). Higher concentrations of NBQX then antagonized this potentiated response. The potency of NBQX in antagonizing steady-state currents to 500 ms applications of glutamate (IC50 120.9 +/- 30.2 nM) was 2-fold less than following 100 ms applications (IC50 67.7 +/- 2.6 nM). NBQX had no effect on rapid onset, desensitization or deactivation time constants. However, a slow relaxation of inhibition was seen with longer applications. NBQX was 2-5-fold less potent against inward currents in the presence of cyclothiazide (100 microM) depending on the application duration but had no effect on the rapid onset, desensitization or deactivation time constants. The same relaxation of inhibition was seen as with NBQX alone. NBQX (1 microM) reduced AMPA receptor-mediated EPSC amplitude to 7 +/- 1% of control with no effect on kinetics. Cyclothiazide (330 microM) caused a 2.8-fold prolongation of the decay time constant (control 26.6 +/- 2.2 ms, cyclothiazide 74.2 +/- 7.6 ms, n = 9). Additional application of NBQX (1 microM) partly reversed this prolongation to 1.9 fold (47.7 +/- 2.5 ms, n = 5). These results support previous findings that cyclothiazide also allosterically influences AMPA receptor agonist/antagonist recognition sites. There were no interactions between NBQX and cyclothiazide on desensitization or deactivation time constants of glutamate-induced currents but clear interactions on EPSC deactivation kinetics. This raises the possibility that the interactions of NBQX, GYKI 52466 and cyclothiazide on AMPA-receptor-mediated EPSC kinetics observed are due to modulation of glutamate-release at presynaptic AMPA receptors.

Pattern and pharmacology of propagating epileptiform activity in mouse cerebral cortex

Multiple extracellular recording electrodes were used to study the intra- and interhemispheric spread of stimulus-evoked epileptiform responses in adult mouse neocortical slices. Bath application of 20 microM bicuculline methiodide induced epileptiform activity that propagated at approximately 0.08 m/s over several millimeters in rostro-caudal and medio-lateral direction within the ipsilateral hemisphere and across the corpus callosum to the contralateral hemisphere. A vertical incision from layer II to subcortical regions did not prevent the spread to remote cortical regions, indicating that layer I plays a major role in the lateral propagation of epileptiform activity. The intra- and interhemispheric spread was not influenced by application of an N-methyl-d-aspartate (NMDA) receptor antagonist, but blocked by an antagonist acting at the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptor. The potential role of potassium channel activation in controlling the generation or spread of epileptiform activity was tested by applying the potassium channel opener cromakalim and the serotonin type 1A (5-HT1A) receptor agonist (+/-)-8-hydroxydipropylaminotetralin (8-OH-DPAT) to the disinhibited slices. Whereas cromakalim reduced the neuronal excitability and blocked all epileptiform responses, 8-OH-DAPT did not affect the activity pattern. Our results suggest that propagating epileptiform activity in disinhibited neocortical structures is predominantly mediated by activation of AMPA receptors and controllable by activation of a voltage-dependent potassium current.

3,5-Dihydro-4H-2,3-benzodiazepine-4-thiones: a new class of AMPA receptor antagonists

Synthesis and evaluation of anticonvulsant activity of a series of 2,3-benzodiazepin-4-ones (2) chemically related to 1-(4'-aminophenyl)-4-methyl-7,8-(methylenedioxy)-5H-2,3-benzodiazepine (1, GYKI 52466) have been reported in our recent publications. Compounds 2 manifested marked anticonvulsant properties acting as 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptor antagonists. In an attempt to better define the structure-activity relationships (SAR) and to obtain more potent and selective anticonvulsant agents, 1-aryl-3,5-dihydro-4H-2, 3-benzodiazepine-4-thiones 3 were synthesized from the corresponding isosteres 2. The evaluation is reported of their anticonvulsant effects, both in the audiogenic seizures test with DBA/2 mice and against the maximal electroshock- and pentylenetetrazole-induced seizures in Swiss mice. New derivatives 3 showed higher potency, less toxicity and longer-lasting anticonvulsant action than those of the parent compounds 2 in all tests employed. Analogous to derivatives 2, new compounds 3 do not affect the benzodiazepine receptor (BZR) while they do antagonize AMPA-induced seizures; their anticonvulsant activity is reversed by pretreatment with aniracetam but not with flumazenil, thus suggesting a clear involvement of AMPA receptors. Electrophysiological data indicate a noncompetitive blocking mechanism at the AMPA receptor sites for 3i, the most active of the series and over 5-fold more potent than 1.

Inhibition of reinforcing effects of morphine and naloxone-precipitated opioid withdrawal by novel glycine site and uncompetitive NMDA receptor antagonists

The glycine site (MRZ 2/570 and L-701,324), and uncompetitive (MRZ 2/579) NMDA receptor antagonists inhibited morphine-produced behaviors related to drug-abuse. The expression of morphine dependence was blocked by pretreatment with all three compounds (3-7.5 mg/kg); the effects of glycine/NMDA antagonists were not dose-dependent. Mice which were morphine-free for 3 days still displayed a significant severity of the withdrawal syndrome when challenged again with naloxone. This extinction of a residual morphine dependence was markedly diminished by treatment with similar doses of NMDA receptor antagonists at the test following the wash-out period. The rewarding impact of morphine was investigated in rats using the place preference (CPP) paradigm. All NMDA receptor antagonists (2.5-10 mg/kg) inhibited both the acquisition and expression of morphine-induced CPP. Once established, morphine-induced CPP was observed until 2 weeks after conditioning. NMDA receptor antagonists given for 3 days after the end of conditioning did not influence the extinction of morphine-induced CPP. Microdialysis studies revealed that the behaviorally effective doses of MRZ 2/579 resulted in a brain concentration close to its in vitro potency as an NMDA receptor antagonist. These data suggest that novel glycine site and uncompetitive NMDA receptor antagonists may have therapeutic potential in the treatment of opioid abuse.

4H-1,2,4-Pyridothiadiazine 1,1-dioxides and 2,3-dihydro-4H-1,2, 4-pyridothiadiazine 1,1-dioxides chemically related to diazoxide and cyclothiazide as powerful positive allosteric modulators of (R/S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid receptors: design, synthesis, pharmacology, and structure-activity relationships

A series of 4H-1,2,4-pyridothiadiazine 1,1-dioxides and 2, 3-dihydro-4H-1,2,4-pyridothiadiazine 1,1-dioxides bearing various alkyl and aryl substituents on the 2-, 3-, and 4-positions was synthesized and tested as possible positive allosteric modulators of the (R/S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors. Many compounds were found to be more potent than the reference compounds diazoxide and aniracetam as potentiators of the AMPA current in rat cortex mRNA-injected Xenopus oocytes. The most active compound, 4-ethyl-2,3-dihydro-4H-pyrido[3,2-e]-1,2, 4-thiadiazine 1,1-dioxide (31b), revealed an in vitro activity on Xenopus oocytes not far from that of cyclothiazide, the most potent allosteric modulator of AMPA receptors reported to date. Moreover, 31b, but not cyclothiazide, was found to potentiate the duration and the amplitude of the excitatory postsynaptic field potentials induced by electric stimulation in rat hippocampal slices. Such an effect could indicate, for 31b, but not for cyclothiazide, a possible interaction with postsynaptic AMPA receptor binding sites located on hippocampal CA1 neurons. Structure-activity relationships indicated that the structural requirements responsible for a biological activity on AMPA receptors are different from those responsible for an inhibitory activity on the insulin releasing process (putative ATP-sensitive K+-channel openers). For instance, 31b and other related dihydropyridothiadiazines were found to be ineffective as inhibitors of insulin release from rat pancreatic B-cells, in contrast to diazoxide and known pyridothiadiazines reported as ATP-sensitive K+-channel openers. Conversely, the pyridothiadiazines active on B-cells were found to be ineffective as potentiators of the AMPA currents in Xenopus oocytes. Thus, 31b appeared to be more specific than diazoxide as an AMPA receptor modulator. This compound may be considered as a new pharmacological tool, different from diazoxide and cyclothiazide, for studying AMPA receptors. Moreover, 31b can also constitute a new therapeutic agent for the treatment of cognitive disorders.