NMDA receptor-independent epileptiform activity induced by magnesium-free solution in rat amygdala neurons is blocked by CNQX

Short-term neuronal effects of fumonisin B1 on neuronal activity in rodents

Fumonisin B1 (FB1) is a mycotoxin produced by microscopic fungi (mostly Fusarium species), which may infect our major crops. The toxin inhibits the development of these plants and may also have harmful effects on animals and humans consuming the infected crops. FB1 inhibits sphingolipid biosynthesis which leads to altered membrane characteristics and consequently, altered cellular functions. There are some indications that the toxin has inhibitory effects on neuronal activity in case of repeated consumption, presumably due to sphingolipid depletion. However, according to new literature data, FB1 may have acute excitatory neural effects, too, via different mechanisms of action. Therefore, in the present study, we addressed the neuronal network effects of FB1 following acute treatment, using different electrophysiological techniques in vitro and in vivo. Acute treatments with FB1 (10-100 μM) were carried out on brain slices, tissue cultures and live animals. After direct treatment of samples, electrically evoked or spontaneous field potentials were examined in the hippocampus and the neocortex of rat brain slices and in hippocampal cell cultures. In the hippocampus, a short-term increase in the excitability of neuronal networks and individual cells was observed in response to FB1 treatment. In some cases, the initially enhanced excitation was reversed presumably due to overactivation of neuronal networks. Normal spontaneous activity was found to be stimulated in hippocampal cell cultures. Seizure susceptibility was not affected in the neocortex of brain slices. For the verification of the results caused by direct treatment, effects of systemic administration of FB1 (7.5 mg/kg, i.p.) were also examined. Evoked field potentials recorded in vivo from the somatosensory cortex and cell activation measured by the c-fos technique in hippocampus and somatosensory cortex were analyzed. However, the hippocampal and cortical stimulatory effect detected in vitro could not be demonstrated by these in vivo assays. Altogether, the toxin enhanced the basic excitability of neurons and neuronal networks after direct treatment but there were no effects on the given brain areas after systemic treatment in vivo. Based on the observed in vitro FB1 effects and the lack of data on the penetration of FB1 across the blood-brain barrier, we assume that in vivo consequences of FB1 administration can be more prominent in case of perturbed blood-brain barrier functions.

Transient removal of extracellular Mg(2+) elicits persistent suppression of LTP at hippocampal CA1 synapses via PKC activation

Previous work has shown that seizure-like activity can disrupt the induction of long-term potentiation (LTP). However, how seizure-like event disrupts the LTP induction remains unknown. To understand the cellular and molecular mechanisms underlying this process better, a set of studies was implemented in area CA1 of rat hippocampal slices using extracellular recording methods. We showed here that prior transient seizure-like activity generated by perfused slices with Mg(2+)-free artificial cerebrospinal fluid (ACSF) exhibited a persistent suppression of LTP induction. This effect lasted between 2 and 3 h after normal ACSF replacement and was specifically inhibited by N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphovaleric acid (D-APV) and L-type voltage-operated Ca(2+) channel (VOCC) blocker nimodipine, but not by non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In addition, this suppressive effect was specifically blocked by the selective protein kinase C (PKC) inhibitor NPC-15437. However, neither Ca(2+)/calmodulin-dependent protein kinase II inhibitor KN-62 nor cAMP-dependent protein kinase inhibitor Rp-adenosine 3', 5'-cyclic monophosphothioate (Rp-cAMPS) affected this suppressive effect. This persistent suppression of LTP was not secondary to the long-lasting changes in NMDA receptor activation, because the isolated NMDA receptor-mediated responses did not show a long-term enhancement in response to a 30-min Mg(2+)-free ACSF application. Additionally, in prior Mg(2+)-free ACSF-treated slices, the entire frequency-response curve of LTP and long-term depression (LTD) is shifted systematically to favor LTD. These results suggest that the increase of Ca(2+) influx through NMDA channels and L-type VOCCs in turn triggering a PKC-dependent signaling cascade is a possible cellular basis underlying this seizure-like activity-induced inhibition of LTP.

Hyperexcitability of amygdala neurons of senescence-accelerated mouse revealed by electrical and optical recordings in an in vitro slice preparation

In the amygdala (AMG) slices obtained from both the young (4-7 months old) and aged (17-20 months old) groups of Senescence-Accelerated Mouse (SAM) P10, spontaneous bursts were recorded in the medial, central and basolateral nuclei. The spontaneous bursts were also observed in the slices from the young group of SAMR1, whereas the mean frequency was significantly lower than that from the young group of SAMP10. The spontaneous burst was barely detectable in slices from the aged group of SAMR1 during perfusing with the standard solution, while bicuculline methiodide (10 microM), a GABAA receptor antagonist, or Mg2+-free solution induced a similar bursting activity observed in the young group. The burst response was also evoked in the medial, central and basolateral AMG following stimulation of the stria terminalis (ST). Both spontaneous and evoked bursts were completely suppressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 4 microM), an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist, but not by (+)-5-methyl-10, 11-dihydro-5H-dibenzo-[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801, 30 microM), an N-methyl-d-aspartate receptor antagonist. The hyperexcitability of the AMG neurons was further substantiated by optical recordings. Following stimulation of the ST, the optical signals reflected postsynaptic responses spread into the medial and central AMG areas at 2-5 ms and faded out at 20-30 ms after stimulation. The intensity of the optical signal recorded in the slice from the young SAMP10 was significantly higher than that from SAMR1 or ddY mice. These observations indicate that bursts mediated by AMPA/kainate receptors were transiently generated in the AMG of SAMR1 at the young age, while the bursts with higher frequency were continuously generated in the AMG of SAMP10. The chronic neuronal hyperactivity in the AMG may be partially responsible for the age-related deterioration of memory and learning abilities observed in SAMP10.

Age dependence of NMDA receptor involvement in epileptiform activity in rat hippocampal slices

The pattern of epileptiform activity recorded from a number of in vitro seizure models is age dependent: ictal discharges are observed in immature brain slices while interictal bursts are seen in adult brain slices. This study evaluated the involvement of the N-methyl-D-aspartate (NMDA) receptor in the age-dependency of epileptiform activity recorded in area CA1 of hippocampal slices in Mg(2+)-free medium. Incubation in Mg(2+)-free medium induced ictal activity in 84% of hippocampal slices from immature rats (postnatal 10-15 days). In contrast, adult slices responded with interictal bursting, while ictal activity was rare (9%). Bath application of the NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (DL-APV, 20 microM) converted ictal activity to interictal activity in the hippocampal slices from immature rats. In adult slices, bath application of NMDA (10-20 microM) in Mg(2+)-free medium induced ictal-like discharges. Perfusion with NMDA (20 microM) in a medium containing 1.5 mM Mg2+ induced ictal activity in immature slices while it evoked only interictal bursts in adult slices. These results suggest that differences in NMDA receptor function may be involved in the age-dependency of epileptiform activity induced by Mg(2+)-free medium. Enhanced NMDA receptor-mediated activity may partially underlie increased seizure susceptibility in the immature brain.

NBQX, a selective antagonist of the AMPA receptor, affects neither field potentials nor long-term potentiation in vivo

NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline), a selective antagonist of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptors, at an anticonvulsant dose of 40 mg/kg (i.p.), was shown in vivo to have no effects on either the field potentials in the piriform cortex or those in the dentate gyrus of the hippocampus. The same dose of NBQX also exerted no significant effects on long-term potentiation in the hippocampus. These results suggest that the mechanism underlying the anticonvulsant action of NBQX in vivo does not involve a suppressive action on physiological synaptic transmission. The possible clinical usefulness of AMPA receptor antagonists as antiepileptic drugs is suggested.

In vivo assessment of prevention of white-noise-induced seizure in magnesium-deficient rats by N-methyl-D-aspartate receptor blockers

The behavioral changes associated with seizures induced by auditory stimulation in magnesium (Mg)-deficient rats originate in deep brain structures and secondarily project to neocortex. In the present study, we examined the roles of N-methyl-D-aspartate (NMDA) receptors in this seizure model. The intraperitoneal administration of the competitive NMDA receptor blocker DL-2-amino-7-phosphonoheptanoic acid (36 and 72 mg/kg) and the non-competitive NMDA receptor blocker MK-801 (1.35 and 2.7 mg/kg), completely prevented the induction of seizure and bradyarrhythmia or sudden death resulting from seizure. Therefore, the white-noise-induced seizures in Mg-deficient rats are linked to increased neuronal excitability via the NMDA receptor.

Antiepileptogenic and anticonvulsant effects of NBQX, a selective AMPA receptor antagonist, in the rat kindling model of epilepsy

To investigate the role of non-NMDA receptors in epileptic seizures, we examined the antiepileptogenic and anticonvulsant effects of NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline), a potent and selective AMPA receptor antagonist, in the rat kindling model. Systemic administration of 10-40 mg/kg NBQX significantly and dose dependently suppressed previously kindled seizures from the amygdala (AM), assessed in terms of the motor seizure stage and afterdischarge (AD) duration. The maximal effects were observed at 0.5-1 h after drug injection. When the intensity of electrical stimulation was increased to twice the generalized seizure-triggering threshold (GST), the anticonvulsant effects of NBQX on AM-kindled seizures were not reversed, suggesting that the effects were not due to non-specific elevation of the GST. In contrast to AM-kindled seizures, 20-40 mg/kg NBQX significantly suppressed only the motor seizure stage without reducing the AD duration of previously hippocampal-kindled seizures. Daily administration of 15 or 30 mg/kg NBQX prior to each electrical stimulation of the AM markedly and significantly suppressed the development of kindling. During drug sessions, the growth of the AD duration was blocked almost completely, while the waveform of ADs became more complex. These results indicate that NBQX has potent antiepileptogenic and anticonvulsant actions on kindling, at least from the AM and that non-NMDA receptors have an important role in seizure propagation.

Roles of the NMDA and quisqualate/kainate receptors in the induction and expression of kindled bursts in rat hippocampal slices

We used combinations of NMDA and quisqualate/kainate (Q/K) receptor antagonists and low Mg2+ (0.1 mM) solutions to study the respective roles of these receptors during in vitro kindling of interictal bursts in the CA3 area of rat hippocampal slices. Intracellular and extracellular recordings in CA3 showed that Q/K receptors were not necessary for the induction of kindling once the Mg2+ block of NMDA was alleviated, but that the expression of bursts kindled via NMDA-driven mechanisms was Q/K-dependent.

CNQX increases spontaneous inhibitory input to CA3 pyramidal neurones in neonatal rat hippocampal slices

Whole-cell recordings were made from immature CA3 pyramidal neurones in the rat hippocampal slice. The addition of the glutamate receptor antagonist, CNQX, caused a robust increase in the frequency of spontaneous inhibitory post-synaptic currents (IPSC) concomitant with the expected reduction of excitatory drive to these neurones. This effect of CNQX was not shared by structurally related quinoxalinediones or kynurenic acid, which are also antagonists of non-NMDA glutamate receptors. This effect of CNQX was abolished by tetrodotoxin suggesting that an increase in interneurone spiking was responsible for the IPSCs. Recordings from stratum radiatum interneurones of CA3 confirmed this suggestion, since some interneurones were directly depolarized by CNQX. The excitation by CNQX of a small population of stratum radiatum interneurones of CA3 complicates interpretation of experiments designed to assess the consequences of blocking excitatory transmission with this drug.

Ethanol inhibits epileptiform activity and NMDA receptor-mediated synaptic transmission in rat amygdaloid slices

The effect of ethanol on the epileptiform activity induced by Mg(++)-free solution was studied in rat amygdalar slices using intracellular recording techniques. The spontaneous and evoked epileptiform discharges consisting of an initial burst followed by afterdischarges were observed 20-30 min after switching to Mg(++)-free medium. Superfusion with ethanol (20-100 mM) reversibly reduced the duration of spontaneous and evoked bursting discharges in a concentration-dependent manner. Synaptic response mediated by N-methyl-D-aspartate (NMDA) receptor activation was isolated by application of a solution containing the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and either in Mg(++)-free solution or in the presence of 50 microM bicuculline. Application of ethanol reversibly suppressed the duration of NMDA receptor-mediated synaptic response. These results suggest that intoxicating concentrations of ethanol possess anticonvulsant activity through blocking the NMDA receptor-mediated synaptic excitation. In addition, the observed effect of ethanol on NMDA receptor-mediated synaptic response could be relevant to the cognitive and behavioral manifestations seen in some alcoholics.

Bursting discharges in disinhibited amygdala slices: the role of excitatory amino acid receptors

The involvement of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the epileptiform activity, induced by bicuculline, was studied in slices of amygdala in the rat, using intracellular recording techniques. Stimulation of the ventral endopyriform nucleus evoked an excitatory postsynaptic potential (EPSP). After exposure to bicuculline (20 microM), the same stimulus evoked burst firing. Occasionally, spontaneous bursts similar in waveform to synaptically triggered bursts also occurred in disinhibited slices. Superfusion of DL-2-amino-5-phosphonovalerate (DL-APV, 50 microM) or 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acids (CPP, 10 microM), rapidly blocked the late component of the paroxysmal depolarizing shift. The spontaneous and evoked bursts were never completely abolished in the presence of DL-APV or CPP. These results suggest that NMDA receptors may contribute to but are not required for the generation of these bursts. In contrast, application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) largely abolished the bursts, indicating that activation of non-NMDA receptors is of primary importance in this model of epilepsy.