Excitatory aminoacids and epileptic seizures in immature brain

Data on convulsant and anticonvulsant action of drugs influencing excitatory amino acid receptors in developing rats are reviewed. Agonists of NMDA type of receptors NMDA and homocysteic acid, elicited an age-related seizure pattern--flexion, emprosthotonic seizures--in the first three postnatal weeks of rats. Generalized clonic-tonic seizures appeared only after a longer latency. Kainic acid administration resulted in epileptic automatisms and later in minimal, clonic seizures followed by generalized tonic-clonic seizures. A decrease of sensitivity to convulsant action with age is a general rule for all agonists tested. Different anticonvulsant action of NMDA and nonNMDA antagonists was demonstrated in a model of generalized tonic-clonic seizures induced by pentetrazol, whereas their action against epileptic afterdischarges elicited by electrical stimulation of cerebral cortex was similar. Again, higher efficacy in younger animals was a rule. As far as metabotropic glutamate receptors are concerned, agonists of groups II and III were shown to protect against convulsant action of homocysteic acid in immature rats and an antagonist of group I receptors MPEP suppressed the tonic phase of generalized tonic-clonic seizures induced by pentetrazol more efficiently in younger than in more mature rat pups. Unfortunately, a higher sensitivity to the action of antagonists of ionotropic glutamate receptors was demonstrated also for unwanted side effects (motor functions were compromized). In contrast, glutamate metabotropic receptor antagonist MPEP did not exhibit any serious side effects in rat pups.     

Medical management of organophosphate-induced seizures

Recent studies concerning management of soman-induced seizures are reviewed. While drugs classically used against epilepsy in hospital appear ineffective against soman, muscarinic receptor blockers are shown to be able to prevent or stop seizures within the first 5 min after their onset. Benzodiazepine could also be considered as an emergency treatment useful during the first 10 min of seizure. Comparatively NMDA antagonists appear to be able to terminate soman-induced seizures even if the treatment is delayed after 40 min of epileptic activity. Drugs with both antimuscarinic and anti-NMDA properties may represent the most adequate pharmacological treatment to treat soman intoxication. However, the results obtained until now with these drugs must be completed in relation with their possible efficacy after i.m. administration. Propositions for future studies are reviewed.     

Naloxone and the kindling of seizures.

In view of evidence indicating that endogenous opiate-like peptides have epileptiform effects, we examined the effect of the opiate antagonist naloxone on the kindling of seizures produced by repeated electrical stimulation of the amygdala or the caudate nucleus in rats. Naloxone had no effect on the threshold for local after-discharge in the two areas and failed to retard the rate of kindling of clinical seizures. These results suggest that an interaction of opiate-like peptides with central opiate receptors does not play any critical role in the kindling of seizures.     

Experimental febrile seizures are precipitated by a hyperthermia-induced respiratory alkalosis

Febrile seizures are frequent during early childhood, and prolonged (complex) febrile seizures are associated with an increased susceptibility to temporal lobe epilepsy. The pathophysiological consequences of febrile seizures have been extensively studied in rat pups exposed to hyperthermia. The mechanisms that trigger these seizures are unknown, however. A rise in brain pH is known to enhance neuronal excitability. Here we show that hyperthermia causes respiratory alkalosis in the immature brain, with a threshold of 0.2-0.3 pH units for seizure induction. Suppressing alkalosis with 5% ambient CO2 abolished seizures within 20 s. CO2 also prevented two long-term effects of hyperthermic seizures in the hippocampus: the upregulation of the I(h) current and the upregulation of CB1 receptor expression. The effects of hyperthermia were closely mimicked by intraperitoneal injection of bicarbonate. Our work indicates a mechanism for triggering hyperthermic seizures and suggests new strategies in the research and therapy of fever-related epileptic syndromes.     

Serotonin mediation of the protective effect of clonidine against pentylenetetrazol-induced seizures in rats

An intraperitoneal injection of 0.5 mg/kg clonidine significantly increased the latency to the first convulsion and reduced tonic seizures and mortality caused by pentylenetetrazol (PTZ), 90 mg/kg, administered subcutaneously to rats. 1 mg/kg clonidine produced similar effects except that tonic seizures were not significantly affected. No effect was observed with 0.01 or 0.1 mg/kg clonidine. Metergoline (1 mg/kg) and methysergide (10 mg/kg), administered intraperitoneally, completely prevented the effect of 0.5 mg/kg clonidine on PTZ-induced seizures. An intraperitoneal injection of 5 mg/kg of d-fenfluramine, a releaser of 5HT from nerve terminals, significantly reduced tonic seizures and completely blocked mortality caused by PTZ but did not significantly modify the latency to the first convulsion. The results suggest that serotonin plays an important role in the protective effect of 0.5 mg/kg clonidine against PTZ-induced seizures. Possible reasons for the different effects of clonidine on different experimental seizures are discussed.     

Pharmacological studies on mechanisms of aminophylline-induced seizures in rats

In the present study, the possible role of free radicals in aminophylline-induced seizures was evaluated in albino rats. Aminophylline (theophylline in ethylene diamine; 50 - 300 mg/kg) induced convulsions in rats in a dose-dependent manner, and both incidence of seizure and mortality were maximum at 300 mg/kg. Conventional anti-epileptics, diphenylhydantoin and dizocilpine, as well as adenosine agonists were ineffective in antagonizing these seizures. On the other hand, phosphodiesterase inhibitors, pentoxyphylline and rolipram, showed insignificant seizurogenic effects. Pretreatment with antioxidants (ascorbic acid, alpha-tocopherol, and melatonin) showed differential attenuating effects on aminophylline seizures and lethality. Further, prior administration of 1-buthionine sulfoxamine (BSO, glutathione depletor) and triethyltetramine (TETA, superoxide dismutase inhibitor), precipitated seizures and enhanced lethality in response to subthreshold doses of aminophylline. The present results suggested of the possible involvement of oxidative stress during aminophylline-induced seizures.     

The effect of naloxone administration on pregnancy-associated seizures

Pregnant mice are more susceptible to flurothyl-induced seizures than are non-pregnant controls. The possibility that the well-known increase in beta-endorphin concentration which accompanies pregnancy was involved in this effect was examined by testing whether naloxone administration could block the increased seizure susceptibility. Pregnant female, control female and male C3H mice were treated with 5-50 mg/kg naloxone 5 min before flurothyl seizure testing. Naloxone markedly increased clonic seizure susceptibility in all three groups at a dose of 50 mg/kg, but had little effect at lower doses. In contrast, naloxone had differential effects on myoclonic seizures in pregnant and control female mice, being anticonvulsant in the controls, but proconvulsant in the pregnant mice. A role for endogenous opiates is unlikely in mediating clonic seizures in pregnant mice, but may be involved in myoclonic seizures.     

Vasopressin and the kindling of seizures

In an attempt to confirm a previous report that two intracerebroventricular injections of arginine-vasopressin (AVP) can kindle convulsive seizures, we injected AVP up to 14 times. Although behavioral and electrographic effects were observed, seizures failed to develop with any reliability. These results call into question the phenomenon of AVP kindling.     

Effects of iloprost, prostaglandin E1 (PGE1) and prostacyclin (PGI2) on chemically and electrically induced seizures in mice

The effects of iloprost (ZK 36,374), a new chemically stable analogue of prostacyclin (PGI2), on strychnine-, pentylene-tetrazol-, and maximal electroshock-induced seizures were studied in mice. The time from the beginning of the injection of the convulsant or inducing electroshock to the stage of persistent seizures was determined, and lack of tonic hindlimb extension was regarded as inhibition of convulsions. In doses of 8 micrograms--16 micrograms kg-1 iloprost already exhibited an anticonvulsant action by markedly reducing the incidence of seizures and mortality following strychnine, pentylenetetrazol or maximal electroshock. The onset of tonic seizures was also reduced by iloprost. PGE1 and PGI2 were generally effective in 7 to 13 times higher doses than iloprost. It is suggested that the anticonvulsant activity of iloprost, PGE1 and PGI2 might involve a common basic mechanism. Due to its efficacy, iloprost is a useful tool in the investigation of the anticonvulsant action of prostaglandins.     

Ethosuximide affects both pentylenetetrazole- and kainate-induced clonic seizures but differentiates between tonic-clonic seizures

Young (25-day-old) and adult (90-day-old) rats pretreated with ethosuximide (62.5 or 125 mg/kg i.p.) were injected with either s.c. pentylenetetrazole (100 mg/kg) or i.p. kainate (10 or 14 mg/kg). The incidences and latencies of minor (clonic) and major (tonic-clonic) seizures were registered. Ethosuximide (125 mg/kg) completely blocked clonic seizures induced by the lower dose of kainate, and slightly suppressed or delayed those induced by the higher dose of kainate or pentylenetetrazole in both age groups. The effect of ethosuximide on major kainate-induced seizures (elicited in young rats only) was insignificant (ethosuximide only partially decreased the incidence of major seizures), whereas ethosuximide abolished major pentylenetetrazole-induced seizures in both age groups. Ethosuximide also failed to affect the latencies of kainate-induced automatisms (e.g., scratching, wet dog shakes). Similarities between kainate- and pentylenetetrazole-induced clonic seizures, as well as a similar action of ethosuximide on both, suggest a common generator for the pattern of clonic seizures.     

Suppression of generalized seizures activity by intrathalamic 2-chloroadenosine application

In the present study, we investigated the effects of micro-injecting 2-chloroadenosine (2-CADO; an adenosine receptor agonist) into the thalamus alone and with theophylline (a nonspecific adenosine receptor antagonist) pretreatment on pentylenetetrazol (PTZ)-induced tonic-clonic seizures in male Wistar albino rats. Following intrathalamic 2-CADO injection alone or theophylline pretreatment, 50 mg kg(-1) PTZ was given ip after 1 and 24 hrs. The duration of epileptic seizure activity was recorded by cortical electroencephalogram (EEG), and seizure severity was behaviorally scored. Intrathalamic 2-CADO administration induced significant decreases in both seizure duration and seizure severity scores at 1 and 24 hrs, but the effects were more abundant on the seizures induced after 24 hrs. On the other hand, pretreatment with theophylline prevented the inhibitor effect of 2-CADO on seizure activity and increased both seizure duration and seizure scores. Present results suggest that the activation of adenosine receptors in the thalamus may represent another anticonvulsant/modulatory site of adenosine action during the course of the PTZ-induced generalized tonic-clonic seizures and provide additional data for the involvement of the adenosinergic system in the generalized seizures model.     

Repeated inhibitory effects of NPY on hippocampal CA3 seizures and wet dog shakes

Intracerebroventricular injection of NPY inhibits epileptiform seizures and seizure-related "wet dog shakes" (WDS) following electrical stimulation of the dentate gyrus or subiculum. This study examined the effects of NPY on seizures and WDS elicited in hippocampal CA3. Like in the other hippocampal regions, NPY significantly inhibited both seizures and accompanying WDS consistent with in vitro data. The identification of an additional antiepileptic hippocampal target for NPY could prove therapeutically relevant considering that the hippocampal formation is a frequent seizure focus in human epilepsy. The effects of NPY were found to persist on seven repeated NPY injection days. Thus tolerance to the anti-seizure effects of NPY does not appear to develop rapidly. Tolerance being a problem with several current antiepileptic drugs, this further strengthens the concept of NPY receptors as a potential future antiepileptic target.     

Anticonvulsive efficacy of flumazenil in the electroinduced seizures in the rabbit.

The anticonvulsive efficacy of flumazenil, a specific antagonist of the ligands of the benzodiazepine receptor, was studied in the experimental seizures induced by electrical stimulation of corpus Amygdaloideum and Cornu Ammonis dorsale of the hippocampus in rabbits. In the amygdaloid seizure model flumazenil raised the threshold and/or reduced the afterdischarge duration. Results observed in the seizures induced by stimulation of hippocampus were less consistent. Possible explanations are discussed.     

Epileptic seizures induce structural and functional alterations on brain tissue membranes

Epilepsy is characterized by disruption of balance between cerebral excitation and inhibition, leading to recurrent and unprovoked convulsions. Studies are still underway to understand mechanisms lying epileptic seizures with the aim of improving treatment strategies. In this context, the research on brain tissue membranes gains importance for generation of epileptic activities. In order to provide additional information for this field, we have investigated the effects of pentylenetetrazol-induced and audiogenetically susceptible epileptic seizures on structure, content and function of rat brain membrane components using Fourier transform infrared (FT-IR) spectroscopy.     

The effect of taurine on kindled seizures in the rat

Recently, it has been reported that taurine, an amino acid with anticonvulsant properties, does not suppress experimental seizures generated by the "kindling" technique. This finding seems somewhat paradoxical since taurine antagonizes other sorts of experimental convulsion and since kindled seizures are easily suppressed by other anticonvulsant drugs. Further tests were therefore conducted during which taurine's anticonvulsant effects were assessed: (1) when kindling stimulation was dropped to near-threshold levels; (2) when cortical as well as limbic kindled foci were stimulated; (3) when developing as well as fully kindled seizures were involved; and (4) when taurine was introduced directly into the ventricles of the brain. Even in these tests which were specifically designed to favour the appearance of anticonvulsant effects, no taurine antagonism of kindled seizures was found.     

Erythropoietin prevents the increase in blood-brain barrier permeability during pentylentetrazol induced seizures

Recombinant human erythropoietin (r-Hu EPO) has been shown to exert neuroprotection in ischemic, excitotoxicity, trauma, convulsions and neurodegenerative disorders. Blood-brain barrier (BBB) leakage plays a role in the pathogenesis of many pathological states of the brain including neurodegenerative disorders. This study aimed to investigate the effects of r-Hu EPO on BBB integrity in pentylentetrazol (PTZ) induced seizures in rats. Seizures were observed and evaluated regard to latency and intensity for an hour. Macroscopical and spectrophotometrical measurement of Evans Blue (EB) leakage were observed for BBB integrity. r-Hu EPO was given intraperitoneally 24 h prior to seizure induction. Total seizure duration of 720+/-50 s after single PTZ administration (80 mg/kg i.p.) was declined to 190+/-40 s in r-Hu EPO pretreatment. A typical BBB breakdown pattern (i.e. staining in cerebellum, cerebral cortex, midbrain, hippocampus, thalamus and corpus striatum) was observed in rat brains with PTZ induced seizures; whereas, EPO pretreatment confined BBB leakage to cerebellum and cortical areas, and lessened the intensity of tonic-clonic seizures observed in PTZ seizures. The protective effect of r-Hu EPO on BBB permeability in seizures is a new and original finding. The protective action of r-Hu EPO in seizures and some of CNS pathologies warrant further investigations.     

Lovastatin exacerbates atypical absence seizures with only minimal effects on brain sterols

AY-9944 (AY) exacerbates chronic recurrent seizures in rats that are analogous to atypical absence epilepsy in humans. The mechanism by which AY affects the slow spike-and-wave discharges associated with these seizures is not known, but is thought to involve inhibition of cholesterol synthesis. We tested the hypothesis that seizures seen with AY are due to significant reduction in brain cholesterol and/or elevated brain 7-dehydrocholesterol by assessing whether three other cholesterol synthesis inhibitors mimic AY seizures in rats. Effects of AY on brain sterols and spike-and-wave discharge duration were compared with those of two other late-stage cholesterol inhibitors [BM 15.766 (BM) and U18666A (UA)] and to an HMG-CoA reductase (early-stage cholesterol) inhibitor, lovastatin. With BM or UA, prolongation of seizure duration and brain sterol changes was similar to that caused by AY. AY effects on both brain sterols and seizure duration were dose-related. Lovastatin, with or without concurrent AY, mimicked AY seizures but reduced brain cholesterol by <10% and did not significantly change brain 7-dehydrocholesterol. Either lovastatin has a different mechanism of action than these late-stage cholesterol inhibitors or the brain sterol changes are not directly responsible for seizures in this model.     

Effects of lipopolysaccharide on blood-brain barrier permeability during pentylenetetrazole-induced epileptic seizures in rats

We investigated the effects of lipopolysachharide (LPS) on functional and structural properties of the blood-brain barrier (BBB) during pentylenetetrazole (PTZ)-induced epileptic seizures in rats. Arterial blood pressure was significantly elevated during epileptic seizures irrespective of LPS pretreatment. Plasma levels of interleukin (IL)-1, interleukin (IL)-6, nitric oxide (NO) and malondialdehyde (MDA) increased while catalase concentrations decreased in animals treated with LPS, PTZ and LPS plus PTZ. The significantly increased BBB permeability to Evans blue (EB) dye in the cerebral cortex, diencephalon and cerebellum regions of rats by PTZ-induced seizures was markedly reduced upon LPS pretreatment. Immunoreactivity for tight junction proteins, zonula occludens-1 and occludin, did not change in brain vessels of animals treated with PTZ and LPS plus PTZ. Glial fibrillary acidic protein immunoreactivity was increased in LPS, but not in PTZ and LPS plus PTZ. These results indicate that LPS pretreatment reduces the passage of EB dye bound to albumin into the brain, at least partly, by increasing plasma NO and IL-6 levels during PTZ-induced epileptic seizures. We suggest that LPS may provide protective effects on the BBB integrity during epileptic seizures through transcellular pathway, since the paracellular route remained unaffected by LPS and LPS plus PTZ.     

Role of the brain-stem reticular formation in tonic-clonic seizures: lesion and pharmacological studies

Bilateral lesions of the pontine tegmentum involving the superior cerebellar peduncles and the nucleus reticularis pontis oralis have been shown to attenuate the tonic components of maximal seizures induced by electroshock, sound stimulation (audiogenic), or pentylenetetrazol, although having no effect on clonus in three separate seizure models. The pontine tegmental lesion also abolishes the clonus of minimal audiogenic seizures that have a motor pattern different from that of other clonic models, and are believed to originate in the brain stem. The preponderant suppression of tonus by the pontine tegmental lesion as well as the inhibition of clonus in audiogenic seizures is strikingly similar to the effects of phenytoin in these same seizure models. The findings presented are consistent with the hypothesis that the pontine reticular formation (RF) plays a key role in the generation and/or expression of tonic convulsions. Additional findings are presented that suggest that serotonin may attenuate the tonic components of maximal electroshock seizures by an action on the brain stem. Thus, it seems likely that pontine tegmental lesions as well as antiepileptic drugs and neurotransmitters with preferential effects on tonic seizures act on a common neural substrate that appears to include the brain-stem RF.     

Effect of calcium channel blockers on experimentally induced seizures in rats

Chemically different classes of calcium channel blockers were examined in rats for their effects on behavior, tolerability and protection against maximal electroshock seizures (MES) and pentylenetetrazol (PTZ) induced seizures. In MES test at doses (mg/kg, ip) that were devoid of side effects, felodipine, 50, afforded 100% protection, while nimodipine, 5; pimozide, 10; and thioridazine, 25, showed 50 to 66% protection. Nifedipine, 10, and diltiazem, 50, showed 30 and 66% protection respectively, but were associated with side effects. Verapamil and loperamide were ineffective against MES and PTZ induced seizures. Nimodipine, 1 mg/kg, ip, was the most potent agent and produced 100% protection against PTZ. Equieffective doses were pimozide, 25, felodipine, 50, and thioridazine, 50. The rest of the calcium channel blockers showed marginal to moderate activity against chemoshock. The data obtained suggest that some calcium channel blockers possess anticonvulsant activity and may be considered as adjuvant therapeutic agents in epileptics refractory to conventional antiepileptic medication.