Effect of the selective N-methyl-D-aspartate receptor agonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid on [3H]flunitrazepam binding

6 Hz Active Anticonvulsant Fluorinated N-Benzamide Enaminones and Their Inhibitory Neuronal Activity

A small library of novel fluorinated N-benzamide enaminones were synthesized and evaluated in a battery of acute preclinical seizure models. Three compounds (GSA 62, TTA 35, and WWB 67) were found to have good anticonvulsant activity in the 6-Hz 'psychomotor' 44-mA rodent model. The focus of this study was to elucidate the active analogs' mode of action on seizure-related molecular targets. Electrophysiology studies were employed to evaluate the compounds' ability to inhibit neuronal activity in central olfactory neurons, mitral cells, and sensory-like ND7/23 cells, which express an assortment of voltage and ligand-gated ion channels. We did not find any significant effects of the three compounds on action potential generation in mitral cells. The treatment of ND7/23 cells with 50 µM of GSA 62, TTA 35, and WWB 67 generated a significant reduction in the amplitude of whole-cell sodium currents. Similar treatment of ND7/23 cells with these compounds had no effect on T-type calcium currents, indicating that fluorinated N-benzamide enaminone analogs may have a selective effect on voltage-gated sodium channels, but not calcium channels.

Synthesis and anticonvulsant screening of 1,2,4-triazole derivatives

BACKGROUND

Currently available antiepileptic drugs offer limited symptomatic treatment and fail to cure more than 30% of the epileptic seizures. (Arylalkyl)azoles are a class of anticonvulsants including nafimidone and loreclezole. Here, we report the design and synthesis of new (arylalkyl)azoles in N-[1-(4-chlorophenyl)-2-(1H-1,2,4-triazol-1-yl)ethylidene]hydroxylamine ester structure, their anticonvulsant screening and in silico prediction studies of their pharmacokinetic properties.

METHODS

The title compounds were synthesized according to the Steglich esterification of N-[1-(4-chlorophenyl)-2-(1H-1,2,4-triazol-1-yl)ethylidene]hydroxylamine with various carboxylic acids. Anticonvulsant identification and quantification tests were performed in mice by the Epilepsy Therapy Screening Program (ETSP) of the National Institutes of Health (NIH) using 6Hz psychomotor, maximal electroshock (MES), and rotorod tests. Their physicochemical and pharmacokinetic properties were calculated using QikProp.

RESULTS

Most of the compounds showed protection against 6Hz- and/or MES-induced seizures. 4a, 4b, and 4g were active at 100mg/kg, 4g was active in both tests without neurotoxicity. According to the QikProp calculations the title compounds were druglike and had some favourable properties such as high membrane permeability and oral absorptivity.

CONCLUSION

Anticonvulsant screening of a set N-[1-(4-chlorophenyl)-2-(1H-1,2,4-triazol-1-yl)ethylidene]hydroxylamine esters yielded some active derivatives in 6Hz and MES test. Especially, 4g emerged as a promising compound with activity at 100mg/kg and no toxicity. The compounds were predicted to be drug like and have good pharmacokinetic properties except hERG inhibition, which needs to be addressed in further optimization studies.

Anti-exploratory effect of N-methyl-D-aspartate in elevated plus-maze. Involvement of NMDA and CCK receptors

N-Methyl-D-aspartate (NMDA, 20 mg/kg) produced a clear decrease in mouse exploration in open parts of an elevated plus-maze. Paradoxically, 40 mg/kg NMDA did not modify the behavior of the mice in the plus-maze. NMDA at a dose of 80 mg/kg again depressed the exploratory activity of mice, but this effect was accompanied with tremor and compulsive tail biting. The 'anti-exploratory' dose of NMDA (20 mg/kg) increased, whereas the 'tremorigenic' dose (80 mg/kg) significantly decreased the number of cholecystokinin (CCK) binding sites in the mouse cerebral cortex. The competitive NMDA antagonist (+/-)-CPP (2.5-5 mg/kg) and the non-competitive antagonist MK-801 (0.25 mg/kg) antagonized the anti-exploratory effect of NMDA (20 mg/kg). The tricyclic antidepressant imipramine (5 mg/kg, but not 1 or 10 mg/kg) also attenuated the inhibition of exploratory activity induced by NMDA. Of three CCK receptor antagonists tested, the unselective CCK antagonist proglumide (1 mg/kg, but not 0.1 and 10 mg/kg) significantly opposed the anti-exploratory action of NMDA. The selective CCK antagonists L-365,260 (1 microgram/kg) and devazepide (1 microgram/kg) were evidently weaker antagonists of NMDA. Furthermore, 10 micrograms/kg of L-365,260, a CCK-B receptor antagonist, and 1 mg/kg of devazepide, a CCK-A receptor antagonist, even tended to augment the effect of NMDA in the plus-maze. The results of the present study seem to give some support to the notion that not only NMDA receptors, but also CCK-ergic mechanisms are involved in the modulation of anti-exploratory action of NMDA in the elevated plus-maze.

3-((+-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and phencyclidine produce a deficit of passive avoidance retention in rats

3-((+-)-2-Carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), phencyclidine (PCP) and diazepam were evaluated for their ability to produce a deficit for a single trial step-through passive avoidance response in rats. Pretraining administration with CPP at doses ranging from 2.0 to 10.0 mg/kg s.c. significantly decreased retention latencies 24 h after passive avoidance training. Similar effects were found with PCP at doses ranging from 0.5 to 1.7 mg/kg s.c. and diazepam at doses between 5.0-18.0 mg/kg s.c. Pretraining administration with the benzodiazepine antagonist, RO15-1788 at doses between 0.1-15 mg/kg s.c., did not alter retention latencies. Co-administration of RO15-1788 (0.01-15.0 mg/kg s.c.) with CPP (6.0 mg/kg s.c.) or PCP (1.0 mg/kg s.c.) failed to block decreases in latencies. However, when RO15-1788 was co-administered with diazepam (9.0 mg/kg s.c.) a dose-related antagonism of diazepam's effects were found. These results suggest that the behavioral actions of CPP and PCP on passive avoidance retention are not mediated via the benzodiazepine receptor complex.

Drug discrimination models in anxiety and depression

Drug discrimination is a technique for investigating the stimulus properties of centrally active drugs. Although many studies have employed animals to investigate the stimulus properties of substances used clinically for the treatment of anxiety and depression, it would be a mistake to consider the internal discriminative stimuli as being related specifically to the anxiolytic or antidepressant properties of these drugs. Rather drug cues are better considered as relating to the pharmacological action of classes of compounds. Thus, benzodiazepine cues generalize to other compounds acting at benzodiazepine receptors, but not to substances (anxiolytic or otherwise) acting at 5-HT1A receptors. Similarly, antidepressants with different pharmacological properties, for example the tricyclic imipramine, or the phenylaminoketone buproprion produce distinct, unrelated discriminative stimuli. For this reason, the limits of drug discrimination techniques for investigating novel anxiolytic or antidepressant drugs should be clearly recognized. Attempts to identify an anxiogenic discriminative stimulus using pentylenetetrazole have also been misguided. In this technique it has proven difficult to separate unequivocally the pharmacological proconvulsant effects of the drug from the psychological construct anxiety. Nevertheless, drug discrimination remains a valuable technique for investigating pharmacological interactions in animals and man.

Effects of 5-HT1A receptor agonists and NMDA receptor antagonists in the social interaction test and the elevated plus maze

The effects of several 5-HT1A agonists and excitatory amino acid antagonists were compared to the standard benzodiazepines, diazepam and chlordiazepoxide (CDP) in two assays predictive of anxiolytic activity, the social interaction and elevated plus maze procedures. Indicative of anxiolytic effects the 5-HT1A agonists, buspirone, gepirone and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) all significantly increased social interaction time and open arm exploration time in the social interaction and elevated plus maze procedures, respectively. Likewise, anxiolytic activity in these assays were also produced by the competitive N-methyl-D-aspartate (NMDA) antagonists, 2-amino-5-phosphonovaleric acid (AP-5), 2-amino-7-phosphonoheptanoic acid (AP-7), 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and the non-competitive NMDA antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) while NMDA produced anxiogenic effects. Furthermore, the anxiolytic effects of these agents were of equal magnitude to the benzodiazepines. These two classes of compounds were differentiated in the yohimbine-induced seizure assay, with the NMDA antagonists dose dependently antagonizing seizures similar to the benzodiazepines while the 5-HT1A agonists were inactive. These results suggest that the 5-HT1A agonists and the NMDA antagonists may be potential non-classical anxiolytic agents with different mechanisms of action.

NMDA antagonists block restraint-induced increase in extracellular DOPAC in rat nucleus accumbens

The effects of the N-methyl-D-aspartate (NMDA) receptor antagonists CPP, TCP, PK 26124 and ifenprodil, and of the minor tranquillizer diazepam on stress-induced changes of dopamine metabolism in the nucleus accumbens were investigated in the rat. Dopamine metabolism was assessed by measuring the extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) by means of in vivo differential pulse voltammetry with electrochemically pretreated carbon fiber electrodes. Physical immobilization of the rats for 4 min caused a marked and long-lasting increase in extracellular DOPAC levels in the nucleus accumbens. A similar, though shorter-lasting, augmentation of extracellular DOPAC was observed in the nucleus accumbens after systemic administration of the anxiogenic agent methyl-beta-carboline-3-carboxylate (beta-CCM) (10 mg/kg s.c.). Pretreatment with CPP (1 mg/kg i.p.), TCP (3 mg/kg i.p.), PK 26124 (3 mg/kg i.p.), ifenprodil (3 mg/kg i.p.) or diazepam (2 mg/kg i.p.) totally antagonized the immobilization-induced increase in extracellular DOPAC in the nucleus accumbens. Diazepam and the benzodiazepine (omega 1-2) receptor antagonist flumazenil (30 mg/kg i.p.), but not ifenprodil, also antagonized the beta-CCM-induced activation of dopamine metabolism in the nucleus accumbens. Finally, systemic administration of haloperidol (25 micrograms/kg i.p.) increased the extracellular concentrations of DOPAC in the nucleus accumbens, but pretreatment with ifenprodil (3 mg/kg i.p.) did not modify this response. These data indicate that NMDA receptor antagonists prevent the activation of dopamine metabolism in the nucleus accumbens caused by immobilization stress but not by beta-CCM-induced anxiogenic stimulation. These results suggest that NMDA receptor antagonists may possess an anxiolytic-like action in the rodent, which is exerted via neuroanatomical circuits distinct from those acted upon by diazepam.

The mechanism of action and pharmacological specificity of the anticonvulsant NMDA antagonist MK-801: a voltage clamp study on neuronal cells in culture

1. Some possible molecular mechanisms of action of the anxiolytic, anticonvulsant and neuroprotective agent MK-801 have been examined in 'whole-cell' voltage clamp recordings performed on rat hippocampal and cortical neurones, bovine adrenomedullary chromaffin cells and N1E-115 neuroblastoma cells maintained in cell culture. 2. Transmembrane currents recorded from rat hippocampal and cortical neurones in response to locally applied N-methyl-D-aspartate (NMDA) were antagonized by MK-801 (0.1-3.0 microM). Blockade was use-dependent, and little influenced by transmembrane potential. MK-801 (3 microM) had no effect on currents evoked by kainate (100 microM). 3. The antagonism of NMDA-induced currents by MK-801 was only slowly and incompletely reversed when the cell membrane potential was clamped at -60 mV during washout. Prolonged applications of NMDA at +40, but not -60 mV during washout, markedly accelerated recovery from block. 4. In contrast to MK-801, ketamine (10 microM) blocked NMDA-induced currents in a voltage-dependent manner. Blockade increased with membrane hyperpolarization and was completely reversible upon washout. 5. MK-801 (1-10 microM) produced a voltage- and concentration-dependent block of membrane currents elicited by ionophoretically applied acetylcholine (ACh) recorded from bovine chromaffin cells. The block was readily reversible upon washout. 6. gamma-Aminobutyric acidA (GABAA) receptor-mediated chloride currents of chromaffin cells were unaffected by MK-801 (1-100 microM). In contrast, such currents were potentiated by diazepam (1 microM). MK-801 (100 microM) had no effect on currents evoked by GABA on hippocampal neurones. 7. MK-801 (10 microM) had little effect on membrane currents recorded from N1E-115 neuroblastoma cells in response to ionophoretically applied 5-hydroxytryptamine (5-HT). Such currents were antagonized by the 5-HT3 receptor antagonist GR 38032F (1 nM) and also by MK-801 at high concentration (100 microM). 8. Voltage-activated, tetrodotoxin-sensitive, sodium currents of chromaffin cells were unaffected by 10 microM MK-801. However, at a relatively high concentration (100 microM), MK-801 reduced the amplitude of such currents to approximately 77% of control. 9. The relevance of the present results to the central actions of MK-801 is discussed.

Endogenous and network bursts induced by N-methyl-D-aspartate and magnesium free medium in the CA3 region of the hippocampal slice

The epileptogenic properties of N-methyl-D-aspartate and magnesium-free medium were investigated in the CA3 region of the hippocampal slice preparation in the rat. Bath application of N-methyl-D-aspartate (5-10 microM) or magnesium-free medium induced both spontaneous and stimulus-evoked bursts. Both endogenous and network bursts were generated, the former always preceding the latter. The paroxysmal depolarizing shift underlying the network bursts generated by N-methyl-D-aspartate and magnesium-free medium resembled a giant excitatory postsynaptic potential with a reversal potential near 0 mV and a synaptic input in the apical dendrites above the mossy fibre zone. In the presence of N-methyl-D-aspartate or magnesium-free medium, population bursts were synchronized by activating single CA3 neurons. N-methyl-D-aspartate receptor antagonists prevented the development of N-methyl-D-aspartate-induced spontaneous and stimulus-evoked bursts. However, the only N-methyl-D-aspartate receptor antagonist effective in preventing such bursts in magnesium-free medium was DL-3-[(+/-)-2-carboxypiperazin-4-yl-]-propyl-1-phosphonic acid. Endogenous bursting in the CA3 region has not been observed with other convulsants and thus may reflect the novel voltage dependence of the N-methyl-D-aspartate receptor gated ionic channel. N-methyl-D-aspartate receptors may also partially contribute to the excitatory interaction between CA3 neurons and thereby account for the synchronization of the population observed when activating single CA3 neurons.

The NMDA antagonists, CPP and CGS 19755, lack affinity for central benzodiazepine receptors

CPP (3-(2-carboxypiperazin-4-yl-propyl-1-phosphonic acid), a rigid analog of AP7 (2-amino-7-phosphonoheptanoate), previously shown to be a selective antagonist of the NMDA (N-methyl-D-aspartate) receptor (IC50 = 209 nM) has been reported to be exceptionally active (IC50 = 430 pM) at benzodiazepine binding sites. Re-examination of CPP, and the rigid AP5 analog, CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylic acid; 0.001-10,000 nM), showed that, as previously reported, neither compound affected the binding of [3H]flunitrazepam. These compounds are thus selective NMDA receptor antagonists.

Epileptiform bursts elicited in CA3 hippocampal neurons by a variety of convulsants are not blocked by N-methyl-D-aspartate antagonists

Intracellular and extracellular recordings from CA3 hippocampal neurons in vitro were used to study the ability of several NMDA (N-methyl-D-aspartate) receptor antagonists to suppress epileptiform bursts induced by NMDA and convulsants not thought to act at NMDA receptors. The antagonists, APV (D-2-amino-5-phosphonovalerate), AP-7 (D,L-2-amino-7-phosphonohepatanoate) and CPP (D,L-3[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid), blocked the spontaneous and evoked bursts induced by NMDA. CPP, but not APV or AP-7, prevented the development of bursts induced by Mg-free medium. The NMDA antagonists failed to block bursting induced by kainate, 7 mM K+, mast cell degranulating peptide, anoxia or spontaneous bursting. In some cases the NMDA antagonists induced spontaneous bursts or enhanced burst frequency, a proconvulsant effect. It is concluded that activation of NMDA receptors is sufficient but not necessary for burst generation in the CA3 region.