Excitatory amino acid antagonists protect mice against seizures induced by bicuculline

PACAP/VIP in the prefrontal cortex mediates the rapid antidepressant effects of zhizichi decoction

ETHNOPHARMACOLOGICAL RELEVANCE

Zhizichi decoction (ZZCD) is a traditional Chinese medicine formula that consists of Gardenia jasminoides J.Ellis (GJ) and Semen Sojae Praeparatum. It is used to treat insomnia and emotion-related disorders, such as irritability. Previous studies have found that GJ has a rapid antidepressant effect. The study found that ZZCD is safer than GJ at the same dosage. Consequently, ZZCD is a superior drug with quicker antidepressant effects than GJ. The rapid antidepressant effects of ZZCD were examined in this study, along with the components that make up this effect. It was determined that the activation of prefrontal Pituitary Adenylate Cyclase Activating Polypeptide (PACAP)/Vasoactive Intestinal Polypeptide (VIP) is essential for ZZCD's rapid antidepressant effects.

AIM

This study identified and discussed the rapid antidepressant effects and biological mechanisms of ZZCD.

MATERIALS AND METHODS

The tail suspension test (TST) and the forced swimming test (FST) were used to screen the effective dosage of ZZCD (0.67 g/kg, 1 g/kg, 4 g/kg). The effective dosage of ZZCD (1 g/kg) was tested in the TST conducted on Institute of Cancer Research (ICR) mice that were treated with lipopolysaccharide (LPS) at a concentration of 0.1 mg/mL. To confirm the expression of c-Fos, PACAP, and VIP in the prefrontal cortex (PFC), immunohistochemistry tests were conducted on mice following intragastric injection of ZZCD. Chemical characterization analysis and HPLC quality control analysis were conducted using UHPLC-Q-Obitrap-HRMS and chromatographic analysis.

RESULTS

The results showed that an acute administration of ZZCD (1 g/kg) decreased the immobility time of Kunming (KM) mice in TST and FST. Depressive behaviors in TST-induced ICR mice treated with LPS (0.1 mg/mL) were reversed by ZZCD (1 g/kg). The results of immunohistochemical experiments showed that ZZCD (1 g/kg) activated neurons in the PFC and PACAP/VIP in the PFC. In this study, 22 substances in ZZCD were identified. Five primary distinctive fingerprint peaks-geniposide, genistin, genipin-1-β-D-gentiobioside, glycitin, and daidzin-were found among the ten common peaks.

CONCLUSION

ZZCD (1 g/kg) had significant rapid antidepressant effects. PACAP/VIP in the PFC was found to mediate the rapid antidepressant effects of ZZCD.

The effects of electrical hippocampal kindling of seizures on amino acids and kynurenic acid concentrations in brain structures

Our study demonstrated that the development of seizures during the electrically induced kindling of seizures is associated with significant changes in the concentration of kynurenic acid (KYNA) and its precursor, tryptophan (TRP). The primary finding of our study was an increase in KYNA levels and the KYNA/TRP ratio (a theoretical index of activity of the kynurenine pathway) in the amygdala and hippocampus of kindled animals. We also found decreases in the concentration of tryptophan in the hippocampus and prefrontal cortex. Changes in the concentration of KYNA and TRP in the amygdala were accompanied by a significant decrease in γ-Aminobutryic Acid (GABA) levels and an increase in the glutamate/GABA ratio. Moreover, we found a significant negative correlation between the local concentrations of KYNA and glutamate in the amygdala of kindled rats. However, there were no changes in the local concentrations of the following amino acids: glutamate, aspartate, glutamine, glycine, taurine and alanine. In conclusion, these new results suggest a modulatory influence of KYNA on the process of epileptogenesis, characterized by a negative relationship between the KYNA and glutamate systems in the amygdala.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Time course of changes in the concentration of kynurenic acid in the brain of pentylenetetrazol-kindled rats

The time response of changes in the brain concentration of kynurenic acid (KYNA) was examined in rats subjected to the pentylenetetrazol (PTZ)-induced kindling of seizures (n=32). The development of seizures was accompanied by a progressive decrease in KYNA concentration in the caudate putamen, entorhinal cortex, piriform cortex, amygdala and hippocampus. A single injection of PTZ (35 mg/kg i.p.--the dose used in the kindling experiment, n=7) caused a much less pronounced KYNA depletion, with different structures affected: the nucleus accumbens, piriform cortex and amygdala. The comparison of KYNA concentration in rats subjected to the kindling of seizures with that in animals given a single, proconvulsive, dose of PTZ (55 mg/kg, n=7) showed that the kindling itself, rather than the occurrence of a fit of seizures, was responsible for the depletion of KYNA in the hippocampus and caudate putamen. Another control experiment showed that neither single nor repeated saline injections caused significant changes in KYNA concentration. The data indicate that changes in the brain concentration of an endogenous inhibitory neurotransmitter, KYNA, undergo selective modulation in the course of a kindling of seizures. This suggests that the depletion of KYNA within the hippocampus may be directly related to the development of kindled seizures in this model of epilepsy.

Involvement of NMDA Receptors in Thiopental-Induced Loss of Righting Reflex, Antinociception and Anticonvulsion Effects in Mice

Potentiation of GABA(A) receptor-mediated inhibitory neurotransmission contributes to the anesthetic action of thiopental. However, the inhibiting action of general anesthetic on excitatory neurotransmission also purportedly underlies its effects. The aim of the study was to elucidate the role of glutamate receptors (NMDA and AMPA receptors) in thiopental-induced anesthesia. Intracerebroventricular (i.c.v.) NMDA (50 ng) significantly increased the induction time of loss of righting reflex and decreased sleep time induced by intraperitoneal injection (i.p.) of thiopental (50 mg/kg). Furthermore, NMDA at 50 ng i.c.v. increased the 50% effective dose values for thiopental to produce loss of righting reflex and immobility in response to noxious tail clamp by 25% and 21% (p < 0.05), respectively. However, intrathecal (IT) administration of NMDA or both of i.c.v. or IT administration of AMPA did not show such antagonizing effects on thiopental action at subconvulsive dose. Finally, thiopental (25 mg/kg i.p.) inhibited convulsions induced by NMDA (0.4 microg i.c.v.) or bicuculline (0.6 microg i.c.v.). However, i.p. muscimol (1 mg/kg) blocked the convulsions induced by bicuculline, but not those induced by NMDA at 3 mg/kg. Similarly, i.p. MK-801 (0.1 mg/kg) antagonized NMDA-induced convulsions, but not bicuculline-induced convulsions at 0.3 mg/kg. Therefore, we suggest that the effects of the selective GABA(A) and NMDA receptors on convulsive behavior are special to their sites of action, and that the inhibitory action of thiopental on NMDA receptors is possibly not mediated by secondary effects of its GABA(A) receptors agonism. These results above indicate the involvement of NMDA receptors in thiopental-induced anesthesia in mice.

Enhancement of brain kynurenic acid production by anticonvulsants—Novel mechanism of antiepileptic activity?

In this study, we describe the effect of antiepileptic drugs on the production of kynurenic acid in rat cortical slices, and on the activity of kynurenic acid biosynthetic enzymes, kynurenine aminotransferases (KATs I and II) in the brain tissue. Phenobarbital, felbamate, phenytoin and lamotrigine (all at 0.5-3.0 mM) enhanced kynurenic acid production in vitro, and stimulated the activity of KAT I. In contrast, vigabatrin, gabapentin and tiagabine inhibited kynurenic acid synthesis in cortical slices with IC(50) of 3.9 (2.8-7.9), 3.7 (2.5-5.4) and 7.5 (3.5-14.3) mM, respectively. Vigabatrin, gabapentin and tiagabine reduced also the activity of KAT I with IC(50) of 1.6 (1.1-2.4), 0.1 (0.01-0.15), 0.9 (0.7-1.2) mM, and the activity of KAT II with IC(50) values of 6.0 (4.8-7.5), 0.2 (0.1-0.3) and 2.0 (1.5-2.6) mM, respectively. In conclusion, the enhancement of kynurenic acid formation displayed by carbamazepine, phenytoin, phenobarbital, felbamate and lamotrigine seems to be a novel mechanism, synergistic with other actions of these drugs, and potentially valuable in terms of better control of epilepsy.

Endogenous protectant kynurenic acid in amyotrophic lateral sclerosis

OBJECTIVES

Excitotoxicity may play a role in neurodegeneration in amyotrophic lateral sclerosis (ALS). Kynurenic acid (KYNA), an endogenous antagonist of excitatory amino acid receptors, may inhibit excitotoxic lesions. The aim of this study was to determine the concentration of KYNA in ALS patients.

MATERIAL AND METHODS

KYNA was measured by high-performance liquid chromatography in the serum and cerebrospinal fluid (CSF) from ALS and control patients.

RESULTS

Our study revealed that CSF KYNA concentration was significantly higher in patients with bulbar onset of ALS compared to controls, and compared to patients with limb onset of the disease. CSF KYNA was also higher in patients with severe clinical status compared to controls. Serum KYNA was significantly lower in ALS patients with severe clinical status compared to controls, and compared to patients with mild clinical status. There were no significant differences in CSF and serum KYNA concentration between the whole ALS group of patients and controls. There was no difference in CSF KYNA concentration between males and females, and there was no correlation between KYNA concentration and age of patients, and duration of ALS.

CONCLUSIONS

An increased CSF KYNA concentration in patients with bulbar onset of ALS and in patients with severe clinical status may indicate neuroprotective role of KYNA against excitotoxicity. The difference of KYNA concentration in CSF of patients with bulbar and limb onset of ALS suggests that these two variants of motor neuron disease may have different etiopathogenetic mechanisms.

Intracerebroventricular injection of the antibiotic cefoselis produces convulsion in mice via inhibition of GABA receptors

A majority of beta-lactam antibiotics (e.g., cephalosporins and penicillins) have convulsive activity to a greater or lesser extent. (6R,7R)-3-[[3-Amino-2-(2-hydroxyethyl)-2H-pyrazol-1-ium-1-yl]methyl]-7-[(Z)-2-(2-aminothiazol-4-yl)-2-methoxyiminoacetylamino]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate monosulfate (cefoselis), a newly developed injectable beta-lactam antibiotic with activity against methicillin-resistant Staphylococcus aureus (MRSA), might induce convulsions if cerebral concentrations become highly elevated. In the present study, we examined whether or not cefoselis had convulsive activity after direct brain administration, and we attempted to clarify the pharmacological mechanism of action. When cefoselis was injected into the lateral ventricle of the mouse brain at doses higher than 20 microg/animal, it produced convulsions dose-dependently. Cefoselis (50 microg/animal)-induced convulsions were prevented by pretreatment with 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), diazepam and phenobarbital (ED(50) values (mg/kg) of 0.78, 1.59 and 33.0, respectively), but not by carbamazepine or phenytoin. When the effects of these anticonvulsants on the convulsions induced by intracerebral injection of bicuculline methiodide (BMI) or N-methyl-D-aspartate (NMDA) were investigated, the inhibitory profile of anticonvulsants on cefoselis-induced convulsions was similar to those induced by BMI (125 ng/animal) but differed markedly in their inhibitory activity on NMDA (100 ng/animal)-induced convulsions, which were not inhibited by diazepam. These results suggest that cefoselis may be convulsive at higher concentrations through a mechanism involving inhibition of gamma-aminobutyric acid (GABA)(A) receptors.

NMDA- but not kainate-mediated events reduce efficacy of some antiepileptic drugs against generalized tonic-clonic seizures in mice

PURPOSE

The aim of this study was to evaluate the efficacy of conventional antiepileptic drugs (AEDs) against the generalized tonic-clonic seizures in mice subjected to the subconvulsive doses of N-methyl-D-aspartate (NMDA) or kainate.

METHODS

Mice were given NMDA and kainate in the doses of 50.0 and 9.0 mg/kg i.p., respectively [i.e., equal to 75% of their CD16 values (convulsive dose in 16% of the animals studied)]. Subsequently the anticonvulsive potential of conventional AEDs against the maximal electroshock-induced seizures was estimated. Where necessary, the plasma levels of AEDs were assessed.

RESULTS

NMDA or kainate application did not affect the electroconvulsive threshold. NMDA, but not kainate, diminished the antiepileptic activity of diazepam (DZP) and carbamazepine (CBZ), increasing their 50% effective doses (ED50s) from 14.1 and 8.6 to 19.0 and 12.1 mg/kg i.p., respectively. Neither NMDA nor kainate affected the ED50 for valproate (VPA), phenobarbital (PB), or diphenylhydantoin (DPH) against electroconvulsions. NMDA-evoked effects were reversed with the use of the NMDA antagonist, D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) and were not accompanied by the alterations in the free plasma levels of AEDs.

CONCLUSIONS

The NMDA-mediated events, but not kainate-related ones, seem to be involved in the protective action of DZP and CBZ against maximal electroshock-induced seizures. Moreover, it might be concluded that when subthreshold activation of NMDA receptors adds to other epileptogenic factors, DZP and CBZ are less efficacious. Presented data indicate that in such situations, adding the NMDA receptor antagonist (at very low doses) to the AED may yield beneficial therapeutic effects.

Convulsant actions of the neurosteroid pregnenolone sulfate in mice

Pregnenolone sulfate (PS) is an endogenous neurosteroid known to antagonize GABA(A) receptor-mediated inhibitory responses and potentiate NMDA receptor-mediated excitatory responses in vitro. To assess the actions of the steroid as a modulator of seizure susceptibility in vivo, PS (30-300 nmol) was administered intracerebroventricularly in mice. At doses of 50 to 150 nmol, PS elicited seizures characterized by head jerks, rearing and falling, severe forelimb and hindlimb clonus, opisthotonos and explosive running. The seizures increased in severity and frequency with time and eventually progressed to status epilepticus, tonic hindlimb extension and death. The doses producing convulsions in 50% (CD(50)) and 97% (CD(97)) of animals were 92 and 205 nmol, respectively. A subconvulsant dose of PS (50 nmol) significantly increased the convulsant potencies of systemically administered pentylenetetrazol (30-50 mg/kg) and NMDA (50-100 mg/kg). Systemically administered PS at doses as high as 100 mg/kg failed to induce seizures or alter the convulsant potencies of pentylenetetrazol and NMDA. Protection against PS (205 nmol)-induced seizures and lethality was conferred by the GABA(A) receptor positive allosteric modulators clonazepam and allopregnanolone, and by the NMDA receptor antagonists dizocilpine and (R)-CPP. The overall pharmacological profile suggests that the convulsant actions of PS are mediated predominantly via its effects on GABA(A) receptors, and also possibly by effects on NMDA receptors.

LY 300164, a novel antagonist of AMPA/kainate receptors, potentiates the anticonvulsive activity of antiepileptic drugs

LY 300164 [7-acetyl-5-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxolo(4, 5H)-2,3-benzodiazepine], administered intraperitoneally up to 2 mg/kg, did not influence the threshold for electroconvulsions. In doses of 2.5-4 mg/kg, LY 300164 significantly raised the threshold. In subprotective doses against electroconvulsions, this excitatory amino acid receptor antagonist enhanced the protective activity of intraperitoneally given valproate, carbamazepine and diphenylhydantoin against maximal electroshock-induced convulsions in mice. The anticonvulsive action of phenobarbital was potentiated by LY 300164 only at 2 mg/kg. The non-N-methyl-D-aspartate receptor antagonist did not affect the plasma levels of the antiepileptic drugs, so a pharmacokinetic interaction is not probable. Combined treatment with LY 300164 (2 mg/kg) and the antiepileptics studied (providing 50% protection against maximal electroshock) did not impair the motor performance of mice, evaluated in the chimney test. Valproate, at its ED50 of 280 mg/kg against maximal electroshock, produced motor impairment. As shown in the passive avoidance task, combination of LY 300164 (2 mg/kg) with valproate or diphenylhydantoin resulted in impairment of long-term memory. Alone among the antiepileptics, valproate (280 mg/kg) and phenobarbital (28.5 mg/kg) disturbed long-term memory. The results suggest that blockade of glutamate-mediated events via non-NMDA receptors leads to enhancement of the anticonvulsive activity of conventional antiepileptics. Some combinations of LY 300164 with antiepileptic drugs were superior to these antiepileptics alone in terms of their lack of adverse effects.

Chlormethiazole anticonvulsive efficacy diminished by N-methyl-D-aspartate but not kainate in mice

The aim of this study was to evaluate the effect of N-methyl-D-aspartate (NMDA) and kainate used at nonconvulsive doses upon protective efficacy of chlormethiazole against maximal electroshock-induced seizures. NMDA (50 mg/kg, i.p.) reduced the anticonvulsant potency of chlormethiazole increasing its ED50 value from 126.9 to 155.0 mg/kg. The effect of NMDA was completely reversed by the competitive NMDA receptor antagonist D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) (0.06 mg/kg i.p.). Kainic acid (9 mg/kg i.p.) did not affect the anticonvulsive properties of chlormethiazole. Our results suggest that NMDA but not kainate receptor-mediated events participate in the anticonvulsant action of chlormethiazole.

Excitatory amino acid antagonists alleviate convulsive and toxic properties of lindane in mice

Pesticides acting at GABAA receptors may induce convulsions in man and animals, but the mechanisms responsible for their convulsant activity are not fully explained. The following excitatory amino acid antagonists were studied for their protective action in mice intoxicated with chlorinated hydrocarbon insecticide lindane (gamma-hexachlorocyclohexane): the competitive NMDA antagonist: 3-(2-carboxypiperazine-4-yl)propenyl-1- phosphonic acid (D-CPPene, 20 mg/kg), the non-competitive NMDA antagonist: dizocilpine (MK-801, 0.4 mg/kg), the glycine site antagonist of NMDA receptor: 2-phenyl-1,3-propane-diol dicarbamate (felbamate, 400 mg/kg) and the competitive AMPA antagonist: 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX, 100 mg/kg). Systemic administration of an antagonist prior to lindane resulted in a strong anticonvulsant effect. D-CPPene, MK-801 and NBQX produced a marked increase of CD50 values of lindane for clonic convulsions. All the antagonists protected animals against tonic convulsions. Toxicity of lindane was potently reduced, as assessed 2, 24 and 120 hr after administration of the pesticide. Our results demonstrate that excitatory amino acid antagonists reduce convulsant properties and toxicity of lindane, suggesting that excitatory amino acid neurotransmission may be involved in its central action.

Glutamate in neurologic diseases

Excitotoxicity has been implicated as a mechanism of neuronal death in acute and chronic neurologic diseases. Cerebral ischemia, head and spinal cord injury, and prolonged seizure activity are associated with excessive release of glutamate into the extracellular space and subsequent neurotoxicity. Accumulating evidence suggests that impairment of intracellular energy metabolism increases neuronal vulnerability to glutamate which, even when present at physiologic concentrations, can damage neurons. This mechanism of slow excitotoxicity may be involved in neuronal death in chronic neurodegenerative diseases such as the mitochondrial encephalomyopathies, Huntington's disease, spinocerebellar degeneration syndromes, and motor neuron diseases. If so, glutamate antagonists in combination with agents that selectively inhibit the multiple steps downstream of the excitotoxic cascade or help improve intracellular energy metabolism may slow the neurodegenerative process and offer a therapeutic approach to treat these disorders.

Effects of some excitatory amino acid antagonists and drugs enhancing gamma-aminobutyric acid neurotransmission on pefloxacin-induced seizures in DBA/2 mice

The behavioral and convulsant effects of pefloxacin (PEFLO), a quinolone derivative, were studied after intraperitoneal (i.p.) administration to Dilute Brown Agouti DBA/2J (DBA/2) mice, a strain genetically susceptible to sound-induced seizures. The anticonvulsant effects of some excitatory amino acid (EAA) antagonists acting at N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate (KA) receptors and of some compounds enhancing gamma-aminobutyric acid (GABA)-ergic transmission against seizures induced by PEFLO were also evaluated. The present study demonstrated that both groups of compounds administered i.p. or intracerebroventricularly were able to protect against seizures induced by PEFLO. However, ifenprodil and (+/-)-alpha-(chlorophenyl)-4-[(4-fluorophenyl)methyl]-1-piperidine-ethan ol (SL 82.0715), two compounds acting on the polyamine site of the NMDA receptor complex, were unable to provide any protection. The relationship between the different sites of action and the anticonvulsant activities of these derivatives were discussed. Although the main mechanisms of PEFLO-induced seizures cannot be easily determined, potential interactions with the receptors of EAA exist. In fact, antagonists of EAA, and in particular, those acting at NMDA receptors, were able to increase the threshold for the seizures or to prevent the seizures induced by PEFLO, while compounds acting at the polyamine site did not provide any protection. The AMPA-KA receptor antagonists were also able to exert anticonvulsant activity, but with minor potency in comparison to those of NMDA antagonists. In addition, the fact that compounds enhancing GABA-ergic neurotransmission were also able to protect the mice against seizures induced by PEFLO suggests an involvement of GABA system.

Differential effects of agents acting at various sites of the NMDA receptor complex in a place preference conditioning model

A conditioned place preference paradigm was used to assess the potential rewarding properties of the uncompetitive NMDA receptor antagonist, MK-801 (dizolcipine), the two competitive NMDA receptor antagonists, CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentonoic acid) and its (R)-enantiomer CGP 40116, as well as the partial agonist at strychnine-insensitive glycine receptors, ACPC (1-aminocyclopropanecarboxylic acid). MK-801 (0.3 mg/kg), CGP 37849 (1.25-10 mg/kg) and CGP 40116 (1.25-10 mg/kg), administered in association with either the initially non-preferred or initially preferred side of the two-arm chamber, caused a significant increase in the time spent on that side in a post-conditioning test. In contrast, ACPC did not support the conditioned place preference. Thus, the time spent on the drug-associated side following conditioning with ACPC (50-400 mg/kg) did not significantly differ from that measured in the pre-conditioning test, irrespective of whether it was associated with the initially non-preferred black side or the initially preferred white side. These results are consistent with both clinical and pre-clinical data demonstrating differences in psychopharmacological properties among compounds acting at the multiple, allosteric regulatory sites on the NMDA receptor complex. Moreover, these results indicate that the abuse potential of ACPC, which acts as a functional NMDA receptor antagonist, may be lower than that of either uncompetitive or competitive NMDA receptor antagonists.

Effects of glutamate application on the rhythm of low magnesium-induced epileptiform activity in hippocampal slices of guinea-pigs

The extracellular concentration of glutamate has previously been reported to increase to more than 10-fold the basal level during seizure activity. In the present study, we tested whether localized increases in extracellular glutamate concentration influence the rhythm of epileptiform discharges in the low-magnesium epilepsy model. In hippocampal slices of guinea-pigs, epileptiform activity was induced by omission of magnesium from the bath fluid. Glutamate and its subreceptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were ejected into different strata of the CA3 and CA1 regions using microiontophoretic and micropressure application. Glutamate, NMDA and AMPA applied to the CA3 region, but not to the CA1 region, induced a short-lasting increase in epileptiform discharge frequency, often followed by a transient reduction. The effect was most pronounced with application into the stratum lacunosum-moleculare of the CA3 region and could only be evoked in slices exceeding 400 microns in thickness. The effects on the rhythm of epileptiform discharges induced by NMDA and AMPA were blocked by their specific receptor antagonists. They were not influenced by application of GABAA and GABAB receptor antagonists. Changes in somatic membrane potential of CA3 pyramidal neurons did not correlate with changes in the rhythm of epileptiform discharges elicited in this region. The transient suppression of epileptiform discharges that followed the increase in discharge frequency was abolished by an adenosine A1 receptor antagonist. We propose that localized increases in extracellular glutamate concentration modify the rhythm of epileptiform discharges due to changes in neuronal network activity.

Interactions of excitatory amino acid antagonists with conventional antiepileptic drugs

Excitatory amino acid antagonists possess anticonvulsant properties in many experimental models of epilepsy and were shown to potentiate the protective activity of conventional antiepileptics against maximal electroshock-induced seizures in mice. Combined treatments of valproate with either D,L-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid or dizocilpine (NMDA antagonists), which provided a 50% protection against maximal electroshock, produced no side-effects, as measured in the chimney test (motor coordination) or passive avoidance task (long-term memory). Valproate alone at its ED50 against maximal electroshock, induced severe adverse effects. The NMDA antagonists, D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid, memantine, procyclidine, and trihexyphenidyl also potentiated the protective activity of conventional antiepileptics but these treatments were associated with considerable side-effects. The non-NMDA receptor antagonists, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline and 1-(amino-phenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine, also enhanced the anticonvulsive action of antiepileptic drugs against maximal electroshock, and these combinations generally resulted in no adverse effects. The potential clinical importance of some combinations of common antiepileptics with excitatory amino acid antagonists is postulated.

Change in neurotrophins and their receptor mRNAs in the rat forebrain after status epilepticus induced by pilocarpine

We studied the effects of status epilepticus (SE) induced by lithium chloride/pilocarpine treatment on gene expression of neurotrophins of the nerve growth factor (NGF) family and of their high-affinity receptors of the tyrosine protein kinase (trk) family in the forebrain. Using in situ hybridization (ISH), we demonstrated an early (3 h after treatment) increase in brain-derived neurotrophic factor (BDNF) and trkB mRNA expression in the dentate gyrus, amygdala, and piriform cortex, as well as widespread increases in the cerebral cortex. NGF mRNA, but not the mRNA of its receptor trkA, was increased in the dentate gyrus. In contrast, 12 h after treatment, neurotrophin-3 (NT-3) decreased, and its receptor trkC mRNA increased. There was no change in NT-4 mRNA levels. All changes were blocked by pretreatment with scopolamine, a muscarinic antagonist. The noncompetitive N-methyl-D-aspartate (NMDA) antagonist ketamine blocked NGF, BDNF, and trkB mRNA increases in the hippocampus and cerebral cortex, but not in the amygdala and piriform cortex. In contrast, ketamine did not affect NT-3 and trkC changes. These results provide a complete description of changes in mRNA levels of neurotrophins and their receptors in the forebrain after SE and supply additional data supporting the view that neurotrophin gene expression is related to abnormal neuronal activity.

Effects of glutamate receptor antagonists on posthypoxic myoclonus in rats

Male Sprague-Dawley rats developed posthypoxic myoclonus following 10-min cardiac arrest and resuscitation. In current studies, roles of N-methyl-D-aspartate (NMDA), non-NMDA (a-amino-3-hydroxy-5-methylisoxazole-4-propionate, AMPA, and kainate), and metabotropic glutamate receptors in the pathophysiology of posthypoxic myoclonus were investigated. Treatments with the competitive or noncompetitive NMDA receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid (D[-]-AP-5) (ED50: 12.5 mg/kg, i.p.) or MK-801 maleate (ED50: 0.034 mg/kg, i.p.), and competitive or noncompetitive non-NMDA (AMPA/kainate) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) (ED50: 9.25 nM/5 microliters, i.c.v.) or 1-(4-ami -nophenyl)-4-methyl-7,8-methylenedioxy -5H-2,3-benzodiazepine hydrochloride (GYKI 52466) (ED50: 0.67 mg/kg, IP), significantly decreased myoclonus episodes in rats. On the other hand, treatment with the metabotropic glutamate receptor antagonist, L(+)-2-amino-3-phosphonopropionic acid (L[+]-AP-3) (50 or 500 nM/5 microliters, i.c.v., exerted no significant effect on myoclonus scores in posthypoxic rats. These results indicate that activation of NMDA and non-NMDA receptors receptors may mediate posthypoxic myoclonus in rats, whereas, involvement of metabotropic glutamate receptors needs to be studied further.

The non-competitive AMPA/kainate receptor antagonist, GYKI 52466, potentiates the anticonvulsant activity of conventional antiepileptics

1-(4-Aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466), up to 5 mg/kg, did not influence the electroconvulsive threshold but potentiated the anticonvulsant activity of valproate, carbamazepine and diphenylhydantoin against maximal electroshock-induced convulsions in mice. No potentiation was observed in the case of phenobarbital. Moreover, this non-NMDA receptor antagonist did not influence the plasma levels of the antiepileptic drugs studied, so a pharmacokinetic interaction, in terms of total and free plasma levels, is not probable. The combined treatment of GYKI 52466 with either carbamazepine or diphenylhydantoin (providing a 50% protection against maximal electroshock) was devoid of significant side effects (motor and long-term memory impairment). Valproate applied at a dose equal to its ED50 caused serious worsening of motor coordination and long-term memory. It is noteworthy that the combined treatment of GYKI 52466 with valproate was superior to valproate alone, as regards adverse effects. The results suggest that concomitant administration of GYKI 52466 with some conventional antiepileptic drugs may offer a novel approach in the treatment of epilepsy.

Influence of combined treatment with NMDA and non-NMDA receptor antagonists on electroconvulsions in mice

alpha-Amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/kainate) receptor antagonists (at subthreshold doses against electroconvulsions), 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466 at maximally 5 mg/kg) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX at maximally 20 mg/kg) enhanced the protective effects of NMDA receptor antagonists, MK-801 (dizocilpine) or 2-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid (D-CPP-ene), against electroconvulsions. Similarly, MK-801 or D-CPP-ene reduced the ED50 values of both NBQX and GYKI 52466 against maximal electroshock. The adverse effects of D-CPP-ene, evaluated in the chimney and rotorod tests, were potentiated by both GYKI 52466 (2.5 mg/kg) and NBQX (10 mg/kg). Also, D-CPP-ene (0.1 mg/kg) worsened the motor performance of mice pretreated with GYKI 52466 in the rotorod test. Neither MK-801 (0.025 mg/kg) nor D-CPP-ene (0.1 mg/kg) affected the NBQX-induced impairment of motor coordination. Similarly, GYKI 52466 (2.5 mg/kg) or NBQX (10 mg/kg) did not influence the performance of mice treated with MK-801 (0.2 mg/kg). It may be concluded that the blockade of more than one subtype of glutamate receptors leads to a more pronounced anticonvulsive effect when compared with the effect of blockade of an individual receptor subtype. In some cases more efficient seizure protection was not associated with increased adverse effects.

Kynurenic acid and 5,7-dichlorokynurenic acids improve social and object recognition in male rats

The present study describes the effect of kynurenic (KYNA) and 5,7-dichlorokynurenic (DCKA) acids, acting as selective antagonists at the glycine site on the NMDA receptor complex, upon short-term memory of male rats. Oxiracetam (OXIR) or pramiracetam (PRAM) were used as reference compounds. In the social recognition test, adult animals were injected SC with a drug or vehicle immediately after the first exposure to a juvenile male, 21-24 days old, and reexposed to the same or a novel juvenile 120 min later. Time spent by adults in social investigation of juveniles was measured. Animals treated with KYNA or DCKA (0.3, 3 and 30 mg/kg in both cases) and OXIR (30 and 60 mg/kg) had significantly reduced investigation time when reexposed to the same juvenile as compared to controls. No reduction of investigation time was found in those drugged animals reexposed to a novel juvenile. The findings suggest that KYNA and DCKA improved retention of memory for olfactory stimuli in adult male rats. In the object recognition test, the duration of exploration of two identical objects during the sample trial and the familiar and a new object during the choice trial, performed 60 min later, was evaluated. Drugs or vehicles were administered SC 30 min prior to the sample trial. On choice one, animals treated with KYNA or DCKA (0.6 and 30 mg/kg in both cases) and PRAM (30 mg/kg) spent more time in exploring a new object than the familiar one as compared to controls. This suggests that the drugged animals were able to remember the familiar object.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of the NMDA-antagonist, MK 801, on benzodiazepine-opioid interactions at the spinal and supraspinal level in rats

1. Benzodiazepines potentiate morphine antinociception at the spinal level via GABAergic mechanisms. At the supraspinal level, the inhibitory effect of midazolam on morphine antinociception cannot be easily explained by GABAA receptor activation. Since excitatory amino acids play a role in central transmission, we investigated the effect of dizocilpine (MK 801) on this interaction in spinal cord and brain. 2. In rats with an intrathecal or intracerebroventricular catheter, the mechanisms of the antinociceptive effect of benzodiazepine-morphine combinations were tested during thermal nociceptive tests. 3. The principal findings of this study were that at the spinal level, midazolam potentiation of morphine antinociception can be antagonized by the NMDA antagonist, MK 801 (10 micrograms), as assessed by hot-plate and tail-flick tests. When drugs were administered supraspinally, midazolam inhibited morphine antinociception only in the hot-plate test, an effect also inhibited by MK 801. In the tail-flick assay, midazolam failed to influence the morphine response. 4. The NMDA antagonist significantly affected midazolam antinociception at the spinal level, but was not effective following i.c.v. administration of the drugs. MK 801 had no effect on morphine antinociception after i.t. and i.c.v. administration of the drugs. 5. The paradoxical effect of midazolam on morphine antinociception and its reversal by MK 801 might be due to modulation at various levels of the neuraxis and/or modulation of different pathways mediated via both GABAA and NMDA receptor mechanisms.

Seizures increase trkC mRNA expression in the dentate gyrus of rat hippocampus. Role of glutamate receptor activation

In this study we have shown, by in situ hybridization and RNase protection assay, a significant trkC mRNA increase confined to the dentate gyrus of hippocampus, both after seizures induced by intracerebroventricular injection of kainic acid and bicuculline. Moreover, after bicuculline treatment we observed an earlier increase of trkC mRNA level, which peaked after 3 h and returned back to normal levels by 12 h. In contrast, the kainic acid treatment produced a delayed increase of trkC mRNA, which initiated after 6 h, peaked at 12 h, and returned to normal levels at 24 h. This increase, which involves also trkC mRNA receptor with tyrosine kinase activity, was mediated by non-NMDA receptors and counteracted by GABA potentiating agent diazepam. Using embryonic neuronal cultures from cerebral hemispheres, including hippocampus, we found that glutamate receptor agonists, including glutamate, kainate, NMDA, and t-ACPD, increase trkC mRNA levels with the following rank order of efficacy: NMDA > t-ACPD > kainic acid > glutamate. In conclusion, our data show that trkC mRNA expression in granule cells of the hippocampus dentate gyrus is increased during seizure activity and that it is mediated by non-NMDA receptors.

Effects of some excitatory amino acid antagonists on imipenem-induced seizures in DBA/2 mice

The behavioural and convulsant effects of imipenem (Imi), a carbapenem derivative, were studied after intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) administration in DBA/2 mice, a strain genetically susceptible to sound-induced seizures. The anticonvulsant effects of some excitatory amino acid antagonists and muscimol (Msc), a GABAA agonist, against seizures induced by i.p. or i.c.v. administration of Imi were also evaluated. The present study demonstrated that the order of anticonvulsant activity in our epileptic model, after i.p. administration, was (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)-cyclohepten-5,10-imine maleate (MK-801) > (+/-)(E)-2-amino-4-methyl-5-phosphono-3-pentenoate ethyl ester (CGP 39551) > 3-((+/-)-2-carboxypiperazin-4-yl)propenyl-1-phosphonic acid (CPPene) > 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CCP) > 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)-quinoxaline (NBQX). Ifenprodil, a compound acting on the polyamine site of NMDA receptor complex was unable to protect against seizures induced by Imi, suggesting that the poliamine site did not exert a principal role in the genesis of seizures induced by Imi. In addition, the order of anticonvulsant potency in our epileptic model, after i.c.v. administration, was CPPene > MK-801 > Msc > (-)-2-amino-7-phosphonic acid (AP7) > gamma-D-glutamylaminomethylsulphonate (gamma-D-GAMS) > NBQX > kynurenic acid (KYNA) > 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). The relationship between the different site of action and the anticonvulsant activity of these derivatives was discussed. Although the main mechanism of Imi induced seizures cannot be easily determined, potential interactions with the receptors of the excitatory amino acid neurotransmitters exists. In fact, antagonists of excitatory amino acids are able to increase the threshold for the seizures or to prevent the seizures induced by Imi. In addition, Imi acts on the central nervous system by inhibition of GABA neurotransmission and Msc, a selective GABAA agonist, was able to protect against seizures induced by Imi.

The competitive NMDA antagonist, D-CPP-ene, potentiates the anticonvulsant activity of conventional antiepileptics against maximal electroshock-induced seizures in mice

D-CPP-ene[3-(2-carboxy-piperazine-4-yl)-1-propenyl-1-phosphonic acid; a competitive antagonist of N-methyl-D-aspartic acid] in a dose of 2 mg/kg (i.p.) significantly increased the threshold for electroconvulsions. When given in a dose half that affecting the electroconvulsive threshold, D-CPP-ene potentiated the anticonvulsant activity of carbamazepine, diazepam, diphenylhydantoin, phenobarbital and valproate against maximal electroshock (50 mA)-induced seizures in mice. However, this NMDA antagonist did not influence the plasma levels of the antiepileptic drugs studied, so a pharmacokinetic interaction, in terms of total plasma levels at least, is not probable. The chimney test and retention test in mice revealed that the combined treatment of D-CPP-ene at 1.0 mg/kg (i.p.) with either diazepam, diphenylhydantoin, phenobarbital or valproate (providing a 50% protection against maximal electroshock convulsions) resulted in motor impairment and caused impairment of long-term memory. On the other hand, a combination of D-CPP-ene and carbamazepine was devoid of adverse effects. It can be concluded that the potential utility of D-CPP-ene in combination with conventional antiepileptic drugs does not seem promising, except for carbamazepine.

Competitive NMDA receptor antagonists enhance the antielectroshock activity of various antiepileptics

CGP 37849 (1 mg/kg i.p.) enhanced the protective action of carbamazepine, diphenylhydantoin and phenobarbital against maximal electroshock-induced convulsions in mice. At 0.25 mg/kg CGP 37849 was inactive and at 0.5 mg/kg it potentiated the anticonvulsive activity of phenobarbital. CGP 39551 (5 mg/kg i.p.) reduced the ED50 values of diphenylhydantoin and phenobarbital, being without influence on carbamazepine. In the dose of 1.25 mg/kg, CGP 39551 potentiated the antielectroshock action of diphenylhydantoin and at 2.5 mg/kg that of phenobarbital. Neither NMDA receptor antagonist elevated the total plasma levels of antiepileptic drugs. Consequently, a pharmacokinetic interaction (in terms of total plasma levels at least) seems unlikely to be responsible for the observed potentiation of the antiepileptic drugs' activity. Combinations of CGP 37849 with either carbamazepine or phenobarbital resulted in a motor and memory impairment quantified by the chimney test and passive avoidance task, respectively. Moreover, combined treatment with phenobarbital and CGP 39551 caused a memory deficit. In contrast, diphenylhydantoin combined with either CGP 37849 or 39551 was devoid of adverse effects. It may be concluded that NMDA receptor blockade results in enhanced anticonvulsive action of common antiepileptics against maximal electroshock-induced seizures.

Competitive antagonists of NMDA receptors, CGP 37849 and CGP 39551, enhance the anticonvulsant activity of valproate against electroconvulsions in mice

Two novel N-methyl-D-aspartic acid (NMDA) competitive antagonists, CGP 37849 (1.25 and 2.5 mg/kg) and CGP 39551 (10 mg/kg), significantly raised the threshold for electroconvulsions in mice. CGP 37849 in doses of 0.125-1.0 mg/kg and CGP 39551 in doses of 0.625-5 mg/kg i.p. considerably potentiated the protective activity of magnesium valproate against maximal electroshock-induced convulsions. The anticonvulsant activity of sodium valproate was potentiated by CGP 37849 (1 mg/kg) to a similar degree, which suggests that magnesium is not involved in the observed interaction. Neither CGP agent influenced the plasma level of valproate, so a pharmacokinetic interaction, in terms of total plasma levels, is not probable. Furthermore, the performance of mice injected with magnesium valproate (91 mg/kg) and CGP 37849 (0.25 mg/kg), which provided 50% protection against maximal electroshock-induced convulsions, in the long-term memory test and chimney test did not differ significantly from that of the control animals. The combination of magnesium valproate and CGP 39551 had a neurotoxic potential comparable to that of valproate alone. The results suggest that a combined treatment of valproate and some competitive NMDA antagonists may be important from a clinical point of view.

Intracerebroventricular application of competitive and non-competitive NMDA antagonists induce similar effects upon rat hippocampal electroencephalogram and local cerebral glucose utilization

In this study we have used electrophysiological and metabolic markers to investigate the effects of competitive and non-competitive NMDA antagonists in rats after central or peripheral administration. The non-competitive antagonist, MK-801, induced dose-dependent suppression of rat hippocampal EEG energy both after intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) application. Similar effects were observed after i.p. and i.c.v. application of the competitive antagonist, DL-CPP-ene. Whereas the MK-801 was more potent after i.p. application, DL-CPP-ene was more potent after i.c.v. administration. Intracerebroventricular administration of MK-801 and DL-CPP-ene resulted in similar changes in the pattern of local cerebral glucose utilization in the olfactory tubercle and regions of the limbic system such as the anteroventral thalamus, hippocampus and entorhinal cortex. Intravenous (i.v.) administration of MK-801 induced increases in glucose metabolism similar to those observed after i.c.v. application. In contrast, i.v. administration of DL-CPP-ene induced only small decreases of glucose utilization in several regions of the central sensory system. Thus the blockade of glutamatergic (NMDA) transmission results in decreased hippocampal EEG activity which is paralleled by increased metabolic activity in this area. We conclude from EEG recordings and [14C]2-deoxyglucose uptake experiments that both non-competitive and competitive NMDA antagonists produce the same pattern of alterations after i.c.v. administration. Apparent differences in efficacy after peripheral administration may be largely due to differences in bioavailability.

Effect of compounds modulating amino acid neurotransmission on the development and control of bicuculline-induced epileptogenic spiking in the rat

Dose-related EEG spiking was induced and monitored in urethane-anaesthetised rats by cortical superfusion of bicuculline methiodide, through a cortical cup incorporating recording electrodes. The total integrated spike voltage, total number of spikes as well as the average size of the spikes were monitored. Extracellular recording showed that each individual EEG spike coincided with the sudden, synchronous firing of a group of superficial cortical cells (layer II-III). gamma-Aminobutyric acid reduced both the size and frequency of the spikes, whilst muscimol and clonazepam mainly reduced the size of the spikes. (+/-) Baclofen reduced the frequency of the spikes, with no effect on their size. The NMDA receptor antagonists, AP5 and AP7 reduced spiking by attenuating size, with no effect on frequency. The NMDA channel blocker MK801 also reduced the size of the spikes but increased their frequency at large concentrations; increasing magnesium in the artificial CSF, from 1 to 10 mM, had a similar effect. Compounds believed to preferentially block non-NMDA receptors, GAMS and CNQX, reduced activity by mainly reducing the frequency of spikes. It is concluded that activation of non-NMDA and GABAB receptors are important for controlling the initiation of bicuculline-induced spikes and NMDA and GABAA receptors, for the control of their subsequent development.

Differential effects of antiepileptic drugs and beta-carbolines on seizures induced by excitatory amino acids

Agonists acting at subtypes of glutamate receptors, N-methyl-D-aspartate, kainate and quisqualate, induce convulsions in rodents. Clonic seizures induced in mice by intracerebral administration of N-methyl-D-aspartate, kainate or quisqualate were used to study the anti- and proconvulsant potential of antiepileptic drugs and beta-carbolines. Systemic administration showed that the benzodiazepines clonazepam and midazolam blocked convulsions induced by kainate and had no effect on seizures triggered by N-methyl-D-aspartate and quisqualate. In contrast, diazepam blocked convulsions induced by either excitatory amino acid, as did valproate. The benzodiazepine receptor agonist beta-carboline ZK 93423 blocked convulsions induced by kainate but had no effect on seizures induced by N-methyl-D-aspartate or quisqualate. The antagonist beta-carboline ZK 93426 did not affect convulsions induced by excitatory amino acids, while the inverse agonists FG 7142 and ethyl-beta-carboline-3-carboxylate increased the sensitivity of mice to kainate. Phenobarbital and 2-chloroadenosine protected mice against seizures induced by quisqualate and kainate, while baclofen was active against convulsions produced by kainate. MK-801 selectively blocked convulsions induced by N-methyl-D-aspartate, and enhanced the susceptibility of mice to seizures triggered by kainate and quisqualate. Ethosuximide increased the susceptibility of mice to N-methyl-D-aspartate and had little or no effect on other types of seizures. Diphenylhydantoin enhanced the convulsant potential of quisqualate. Trimethadione and carbamazepine did not affect convulsions induced by N-methyl-D-aspartate, kainate or quisqualate. Intracerebral administration of midazolam protected mice against seizures induced by kainate. Ethosuximide increased the susceptibility of mice to N-methyl-D-aspartate, while diphenylhydantoin to quisqualate convulsions.(ABSTRACT TRUNCATED AT 250 WORDS)