Metaphit-induced audiogenic seizures in mice: I. Pharmacologic characterization

Moderate body hypothermia alleviates behavioral and EEG manifestations of audiogenic seizures in metaphit-treated rats

We investigated the effects of hypothermia on the incidence and EEG signs of audiogenic seizures in rats treated with metaphit (1-[1(3isothiocyanatophenyl)-cyclohexyl] piperidine), an experimental model of generalized reflex epilepsy. After i.p. injection with metaphit (10 mg/kg) Wistar rats were exposed to audiogenic stimulation at hourly intervals during the time course of the experiment. After intermittent use of an ice pack 8 h after the metaphit treatment, when seizure was fully developed, the body temperature was reduced to 30 +/- 0.5 degrees C in one half of the rats, and maintained at 37 +/- 0.5 degrees C in the other half. Saline-injected rats served as a control group. In the hypothermia group, the incidence of audiogenic seizures induced by metaphit was completely suppressed during the 3 consecutive testing times, while no signs of epileptiform activity were noted in EEG tracings. The termination of hypothermic treatment resulted in the recovery of seizure susceptibility, and during audiogenic stimulation, bursts of spiking activity were recorded in the EEGs of metaphit-treated rats. These findings indicate that moderate body hypothermia is an effective anticonvulsant treatment for audiogenic seizures in metaphit-treated rats.

Delta-Sleep-Inducing Peptide Potentiates Anticonvulsive Activity of Valproate against Metaphit-Provoked Audiogenic Seizure in Rats

The effect of delta-sleep-inducing peptide (DSIP) on the anticonvulsive activity of a nonprotective valproate (VPA) dose in a metaphit model of generalized, reflex audiogenic seizures in adult Wistar rats was studied. The animals that received metaphit (10 mg/kg) were exposed to audiogenic stimulation (100 +/- 3 dB, 60 s) at hourly intervals. Metaphit-treated rats displaying seizures in 8 previous tests were i.p. injected with VPA (50 mg/kg) or DSIP (1.0 mg/kg) or their combination. Latency to seizure was behaviorally assessed. The EEGs and power spectra were recorded and analyzed. Neurotoxicity was evaluated by the chimney test. DSIP or VPA alone expressed no significant effect on the latency duration, but their combination significantly prolonged latency to seizure during 6 h after injection, while inducing no significant motor impairment. Neither the applied drugs nor their combination abolished metaphit-provoked EEG epileptiform activity. The results show that DSIP potentiated anticonvulsive effects of a nonprotective VPA dose in a metaphit model of audiogenic seizures without influencing its neurotoxicity.

Ontogenetic study of metaphit-induced audiogenic seizures in rats

Ontogenetic differences in susceptibility to metaphit (1-(1-(3-isothiocyanatophenyl)cyclohexyl)-piperidine)-induced audiogenic seizures were examined in young, developing (ages: 12, 18, and 25 days) and adult (90 days old) Wistar albino rats. Metaphit was injected in a dose of 10 mg/kg i.p. and animals were subjected to intense audio stimulation (100 +/- 3 dB, 60 s) at hourly intervals after administration. Audiogenic seizures (AGS) were scored according to a four point descriptive rating scale (0-3). AGS were elicited in all age groups; they were induced for 12, 15, 15, and 30 h in 12-, 18-, 25-day-old, and adult rats, respectively. Younger animals reached a peak incidence and severity of seizures before adult rats. Twenty-five-day-old rats showed greatest incidence and severity of seizures, and shortest latency. Twelve-day-old animals had longest latencies. Besides audiogenic seizures, we observed convulsions induced by metaphit only in the form of running episodes, forelimb clonus, clonic convulsions, and rearing. Results suggest that young rats develop metaphit-induced sound seizures more rapidly, but that adults have longer period of seizure susceptibility. Different susceptibility to seizures is probably due to changes in excitatory and inhibitory pathways, while maturation of blood-brain barrier is less probable, since metaphit has a lipophilic nature.

N-Methyl-D-aspartic acid- and metaphit-induced audiogenic seizures in rat model of seizures

The effects of NMDA (N-methyl-D-aspartic acid) on metaphit (1-[1(3-isothiocyanatophenyl)-cyclohexyl]piperidine)-induced audiogenic seizures in adult male Wistar albino rats were studied with the aim of developing a suitable animal model of seizures. The animals were divided into four experimental groups: 1, saline control; 2, metaphit-injected; 3, metaphit + NMDA administered and 4, NMDA-treated. Upon the treatment, the rats were exposed to sound stimulation (100 +/- 3 dB, for 60 s) at hourly intervals and the incidence and severity (running, clonus and tonus) of seizures were analysed. In group 3, only the animals which did not exhibit any metaphit-induced audiogenic seizures over 8 h were given a subconvulsive NMDA dose after the eighth audiogenic testing. For EEG recordings, three gold-plated screws were implanted into the rat skull. In most animals, metaphit led to EEG abnormalities and elicited epileptiform activity recorded as spikes, polyspikes and spike-wave complexes. Maximum incidence and severity of metaphit-induced convulsions occurred 8 h after injection (incidence 9/12), abating gradually until disappearing 30 h later. NMDA alone provoked no seizure response but the initial signs characterized by isolated spike activity evolving into sporadic slow-wave complexes, thus representing a proconvulsive brain state, were observed. This compound led to stereotyped behaviour seen as asymmetric posture, loss of righting reflex and tonic hind limb extension lasting for 60-90 min. It also potentiated metaphit-induced audiogenic seizures. Potentiation of metaphit-related audiogenic seizures by NMDA was recorded in three out of 17 rats that had never displayed seizures in eight previous testings, with a maximum incidence of eight out of 17 animals, 13-14 h after metaphit administration and seizures lasted for 10 h.

Magnesium deficiency-dependent audiogenic seizures (MDDASs) in adult mice: a nutritional model for discriminatory screening of anticonvulsant drugs and original assessment of neuroprotection properties

A great many animal models for audiogenic seizures have been described. The extent to which these models may provide insight into neuroscience fields such as abnormal locomotor behavior (wild running), seizures and anticonvulsants, and neuroinsults and neuroprotectors is examined here by our study of magnesium deficiency-dependent audiogenic seizures (MDDASs) in adult mice. MDDASs were induced in all of the eight tested adult murine strains and are presented as a sequence of four successive components (latency, wild running, convulsion, and recovery phase periods). Compared with several classic seizure tests, the nutritional MDDAS model responded to low doses of prototype antiepileptic drugs (AEDs), including phenytoin (PHT), carbamazepine (CBZ), phenobarbital (PB), valproic acid (VPA), ethosuximide (ESM), and diazepam (DZP). Modulation by AEDs of the four components of MDDAS indicated that this seizure test was discriminatory, distinguishing between phenytoinergic (PHT, CBZ), GABAergic (PB, VPA, DZP), and ethosuximide (ESM) compounds. Suitability of the MDDAS test for evaluation of neuroprotective compounds was also examined: it showed partial (melatonin) and complete (WEB2170, an anti-PAF agent) reduction of recovery phase by non-anticonvulsant doses of test compounds. These neuroprotective responses were compared with neuroprotective potentials determined in a model of neonatal cerebral injury induced by focal injection of ibotenate (a glutamate analog). WEB2170 and melatonin reduced the size of lesions in white matter, but only WEB2170 protected cortical plate against ibotenate-induced lesions. In addition to the original neuroprotective behavior of WEB2170, studies on the neuroprotectors also supported GABAergic anticonvulsant activity of melatonin in the MDDAS test.

APH, an N-methyl-D-aspartate receptor antagonist, blocks the metaphit-induced audiogenic seizures in rats

The effect of the competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, (+/-)2-amino-7-phosphonoheptanoic acid (APH) on electrocorticographic (ECoG) activity and behavior was studied in the model of epilepsy induced by systemic application of metaphit (1-(1-(3-isothiocyanatophenyl)-cyclohexyl)-piperidine). Male Wistar rats were injected with metaphit intraperitoneally (10 mg/kg, i.p.), and exposed to intense audio stimulation (electric bell generating 100 +/- 3 dB at animal level for 60 s) 1 h after administration and at 1-h intervals thereafter. ECoG tracings showed appearance of paroxysmal activity in form of spikes, spike-wave complexes and ECoG seizures. Audiogenic seizures consisted of wild running followed by clonic and tonic convulsions. Each behavioral seizure response had a characteristic ECoG correlate. The incidence and severity of seizures increased with time, reaching a peak 8-12 h after metaphit administration, and then gradually decreased until 31 h, when no animal responded to sound stimulation. APH was injected intracerebroventricularly (0.005, 0.01, 0.02, 0.03 and 0.05 mumol icv in 5 microL of sterile saline) after the 8th hour of audiogenic testing (AGS). APH inhibited seizures in a dose-dependent manner. The minimum dose which blocked seizures in all animals was 0.03 mumol. However, ECoG signs of seizure susceptibility were not suppressed by APH. After varying periods of time, behavioral seizures reappeared. It seems that APH blocks epileptiform propagation, but has less influence on the epileptogenic activity caused by metaphit.

Acute effects of MK801 on kainic acid-induced seizures in neonatal rats

Kainic acid (KA) causes behavioral and electrographic status epilepticus (SE) in rats of all ages. In adult rats, the noncompetitive N-methyl-D-aspartate (NMDA) channel blocker MK801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine ) is anticonvulsant against KA-induced seizures: it reduces their severity and protects against neuronal damage, although it may worsen electrographic seizures. Here we examined the effects of MK801 on KA seizures in the immature brain. Neonatal rats (P11-P12) were pretreated with MK801 (0.01, 0.1, 0.5 or 1.0 mg/kg, i.p.) or saline twenty minutes prior to KA (2 mg/kg, i.p.). Clinical seizure behavior was monitored for > 6 hrs, and in some rats the EEG was monitored with an intrahippocampal or intracortical electrode. MK801 caused immobility alternating with hyperactivity, ataxia, scratching and sometimes alternate limb cycling, which correlated with the appearance of spikes on the EEG. Compared to KA alone or KA preceded by 0.01 mg/kg MK801, the higher doses of MK801 (0.1, 0.5 and 1.0 mg/kg) significantly lowered the latency to electrographic seizures (P < 0.001), ictal scratching (P < 0.0001), and status epilepticus (P < 0.0001). MK801 pretreatment did not lower significantly the death rate due to KA seizures. No histologic damage was seen after MK801, KA or both agents together. These results suggest that MK801 exacerbates KA-induced seizures in the neonatal brain, and may even cause ictal behavioral and electrographic manifestations by itself. The findings point to an age-dependency of NMDA antagonist action, and suggest caution in considering the use of NMDA antagonists in neonates.

Ethanol withdrawal induces increased firing in inferior colliculus neurons associated with audiogenic seizure susceptibility

Ethanol withdrawal (ETX) in ethanol-dependent rats results in susceptibility to seizures, including generalized tonic-clonic audiogenic seizures (AGS). The inferior colliculus (IC) is strongly implicated in AGS initiation during ETX, but IC neuronal mechanisms subserving AGS are unclear. The present study examined IC (central nucleus) single neuronal firing during repeated (4 day) intragastric ethanol administration and during ETX. This involved microwire electrodes implanted chronically into freely moving rats and acoustic stimulation in intensities up to 105 dB SPL. During initial ethanol administration the animals were stuporous, and IC spontaneous neuronal firing and acoustically evoked firing at high stimulus intensities were significantly reduced. This firing reduction is consistent with the action of ethanol to enhance gamma-aminobutyric acid (GABA)-mediated inhibition, which is prominent in IC neurons at high stimulus intensities. During ETX the animals were agitated, and spontaneous IC neuronal firing and acoustically evoked firing at all stimulus intensities were significantly increased during the period of AGS susceptibility. Previous studies indicate that IC neuronal responses are tightly regulated by GABA and glutamate. The IC firing increases during ETX in the present study may involve the down-regulation of GABAA receptors and supersensitivity of glutamate receptors reported to occur during ETX. Previous studies also indicate that focal blockade of GABAA receptors or activation of glutamate receptors produces AGS susceptibility in normal rats. Therefore, the IC neuronal firing increases observed in the present study may play a critical role in initiation of AGS during ethanol withdrawal.

Seizures during ethanol withdrawal are blocked by focal microinjection of excitant amino acid antagonists into the inferior colliculus and pontine reticular formation

Physical dependence on ethanol can result in seizure susceptibility during ethanol withdrawal. In rats, generalized tonic-clonic seizures are precipitated by auditory stimulation during the ethanol withdrawal syndrome. Excitant amino acids (EAAs) are implicated as neurotransmitters in the inferior colliculus and the brain stem reticular formation, which play important roles in the neuronal network for genetic models of audiogenic seizures (AGSs). Ethanol blocks the actions of EAAs in various brain regions, including the inferior colliculus. In this study, dependence was produced by intragastric administration of ethanol for 4 days. During ethanol withdrawal, AGSs were blocked by systemic administration of competitive or noncompetitive NMDA antagonists 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or dizocilpine (MK-801). Focal microinjections of NMDA or non-NMDA antagonists into the inferior colliculus or the pontine reticular formation also inhibited AGSs. MK-801 was the most potent anticonvulsant systemically. When injected into the inferior colliculus, CPP had a more potent anticonvulsant effect than either MK-801 or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. The inferior colliculus was more sensitive than the pontine reticular formation to the anticonvulsant effects of both competitive NMDA and non-NMDA antagonists. The results of the present support the idea that continued ethanol administration may lead to development of supersensitivity to the action of EAAs in inferior colliculus and pontine reticular formation neurons. This may be a critical mechanism subserving AGS susceptibility during ethanol withdrawal.

Metaphit amplifies long-term potentiation (LTP) in the mouse hippocampus

The influence of metaphit, a phencyclidine derivative, on the amplitude of Long-Term Potentiation (LTP) in the mouse hippocampus was investigated. Mice (C57BL/6) of both sexes were injected with metaphit (80 mg/kg) and hippocampal slices were prepared at 3, 24, 48 hrs and 6 days following injection. The extracellular evoked potentials were recorded from the pyramidal cell layer following Schaffer collateral stimulation. The threshold value, defined as the minimum strength of the stimuli to evoke a 0.1 mV potential, was about 5 fold greater in metaphit slices than in control slices 3 hr following injection, and then declined to the control value within 6 days. The magnitude of LTP was also amplified by metaphit in a time-dependent fashion. The effect was visible three hours after injection, reached its maximum at 48 hr and then declined to a level slightly higher than control at 6 days following injection. These results demonstrate that metaphit, a compound known to induce audiogenic seizures, is able to modify synaptic plasticity in the hippocampus. Presented results are also in agreement with our previous data which demonstrated an interaction between the mechanisms of LTP and audiogenic seizure.

Facilitation of amygdala kindling development and kindled seizures by metaphit

The effect of metaphit (a phencyclidine analogue with an acylating isothiocyanate) on kindling development and kindled seizures from amygdala was investigated in rats pretreated once with metaphit. Administration of a single dose of metaphit (10 or 20 mg/kg intraperitoneally i.p.) 4 h before the first electrical stimulation of the amygdala did not in itself induce seizures, but greatly facilitated development of behavioral seizures during kindling. This effect persisted throughout the whole process of electrical amygdala kindling without further dosing. In contrast, metaphit only transiently and modestly increased the growth of afterdischarge (AD) duration. In kindled rats, pretreatment with a single dose of metaphit (20 mg/kg) 8 h before the test stimulation reduced the threshold current required to elicit a stage 5 seizure and shortened the latency for bilateral forelimb clonus (BFC) without changing AD duration or BFC duration. The facilitation of kindling development and kindled seizures may be due to an excessive excitatory transmission by metaphit in the limbic seizure circuitry.

Metaphit-induced audiogenic seizures in mice: II. Studies on N-methyl-D-aspartic acid, GABA, and sodium channel receptors and on the disposition of metaphit in the brain

We previously demonstrated that metaphit (a phencyclidine analogue with an acylating isothiocyanate group) induces occurrence of audiogenic seizures in mice exposed to audio stimulation 24 h after metaphit administration. We have studied various receptor systems associated with excitatory and inhibitory networks: sites for competitive and noncompetitive antagonists of the N-methyl D-aspartic acid (NMDA) receptor complex, for [3H]muscimol on the gamma-aminobutyric acid (GABA) receptor complex, and for [3H]batrachotoxinin A20-alpha-benzoate on the voltage-dependent sodium channel. Mice were examined for neurochemical changes at 24 h after pretreatment with metaphit, when susceptibility to audiogenic seizures is greatest. Ex vivo receptor binding studies detected no changes; in vivo labeling of the phencyclidine site in the NMDA receptor complex was reduced by 20% in cortical and midbrain regions. A separate group of experiments was aimed at measuring brain levels of metaphit. One minute after retroorbital administration of [3H]metaphit at a dose sufficient to produce susceptibility to audiogenic seizures 24 h later, the brain level of [3H]metaphit (determined by high-performance liquid chromatography, HPLC) was 49 pmol/mg tissue; at 1, 4, and 24 h, the level was 12, 6, and 1.4 pmol/mg tissue or microM if metaphit was evenly distributed throughout the brain. Although the observed metaphit concentrations during the first 4 h are high enough to acylate receptors, no firm evidence for acylation was found for most of the examined receptors. Finally, the time course of the brain level of metaphit showing a continuous decrease is entirely different from that of development of the seizure susceptibility, which peaks at 18-24 h.