Spinal seizures and excitatory amino acid-mediated synaptic transmission

Right-sided vagus nerve stimulation inhibits induced spinal cord seizures

We have previously shown that left-sided vagus nerve stimulation results in cessation of induced spinal cord seizures. To test our hypothesis that right-sided vagus nerve stimulation will also abort seizure activity, we have initiated seizures in the spinal cord and then performed right-sided vagus nerve stimulation in an animal model. Four pigs were anesthetized and placed in the lateral position and a small laminectomy performed in the lumbar region. Topical penicillin, a known epileptogenic drug to the cerebral cortex and spinal cord, was next applied to the dorsal surface of the exposed cord. With the exception of the control animal, once seizure activity was discernible via motor convulsion or increased electrical activity, the right vagus nerve previously isolated in the neck was stimulated. Following multiple stimulations of the vagus nerve and with seizure activity confirmed, the cord was transected in the midthoracic region and vagus nerve stimulation performed. Right-sided vagus nerve stimulation resulted in cessation of spinal cord seizure activity in all animals. Transection of the spinal cord superior to the site of seizure induction resulted in the ineffectiveness of vagus nerve stimulation in causing cessation of seizure activity in all study animals. As with left-sided vagus nerve stimulation, right-sided vagus nerve stimulation results in cessation of induced spinal cord seizures. Additionally, the effects of right-sided vagus nerve stimulation on induced spinal cord seizures involve descending spinal pathways. These data may aid in the development of alternative mechanisms for electrical stimulation for patients with medically intractable seizures and add to our knowledge regarding the mechanism for seizure cessation following peripheral nerve stimulation.

Activity of common anticonvulsant drugs on spinal seizure-induced by sudden cooling

Although some of the clinical signs associated with epilepsy have their origin in supraspinal structures, it is the spinal cord in the end, which is responsible for generating the typical pattern of tonic-clonic contractions associated with a convulsion. Indeed, the spinal cord isolated from influence of the brain is capable of convulsive and paroxysmal activity that exhibits the same stereotyped motor pattern seen in the intact animal. This motor pattern can be reproduced experimentally by cooling the isolated spinal cord of amphibians. The isolated spinal cord-hindleg preparation of toad was used. Convulsive activity was induced by placing the isolated spinal cord into a Ringer's bath kept at 7 degrees C. The characteristic phases of the convulsion and their intensity were assessed by recording tonic-clonic contractions of hindleg muscles. Two main endpoints were used to assess the anticonvulsive activity of the drugs tested: first, their ability to block only the tonic hind-limb extension (THE) and second, their ability to block all tonic-clonic activity. The ED50 values and its 95% confidence interval estimated for abolition of THE for each drugs was (mg/kg): carbamazepine 8.6 (6.8-10.8), phenytoin 13 (7.1-23.6), diazepam 0.007 (0.004-0.01), MK-801 3.4 (2.0-5.7), valproate 120 (40-400), phenobarbital 17.1 (12.2-23.9), pentobarbital 10 (6-16.4), mephenesin 2-5 and acetazolamide >500. The ability of some of these drugs to inhibit this kind of seizure activity at doses within therapeutic range suggests a potential use of this isolated preparation as a model in the study and testing of new anticonvulsive drugs.

Vagus nerve stimulation for induced spinal cord seizures: insights into seizure cessation

OBJECT

Vagus nerve stimulation is known to decrease the frequency, duration, and intensity of some types of intracranial seizures in both humans and animals. Although many theories abound concerning the mechanism for this action, the true cause remains speculative. To potentially elucidate a pathway in which vagus nerve stimulation aborts seizure activity, seizures were initiated not in the cerebral cortex but in the spinal cord and then vagus nerve stimulation was performed.

METHODS

Ten pigs were anesthetized and placed in the lateral position, and a small laminectomy was performed in the lumbar region. Topical penicillin, a known epileptogenic drug to the cerebral cortex and spinal cord, was applied to the dorsal surface of the exposed cord. With the exception of two animals that were used as controls, once seizure activity was discernible via motor convulsion or increased electrical activity the left vagus nerve, which had been previously isolated in the neck, was stimulated. Following multiple stimulations of the vagus nerve and with seizure activity confirmed, the cord was transected in the midthoracic region and vagus nerve stimulation was performed. Vagus nerve stimulation resulted in cessation of spinal cord seizure activity in all (87.5%) but one experimented animal. Transection of the spinal cord superior to the site of seizure induction resulted in the ineffectiveness of vagus nerve stimulation to cause cessation of seizure activity in all study animals.

CONCLUSIONS

The effects of vagus nerve stimulation on induced spinal cord seizures involve descending spinal pathways. The authors believe that this experiment is the first to demonstrate that spinal cord neuronal hyperactivity can be suppressed by stimulation of a cranial nerve. These data may aid in the development of alternative mechanisms for electrical stimulation in patients with medically intractable seizures. Further studies are now necessary to isolate which specific tracts, nuclei, and neurotransmitters are involved in this process.

The WAG/Rij rat model for nonconvulsive absence epilepsy: involvement of nonNMDA receptors

The involvement of AMPA and kainate receptors in nonconvulsive epilepsy was studied by intracerebroventricular injections of AMPA, GDEE, kainic acid and kynurenic acid in WAG/Rij rats. The WAG/Rij rat strain is recognized as an animal model for human absence epilepsy. EEG registrations showed that AMPA (0.1 pmol/5 microliters; 1 pmol/5 microliters; 10 pmol/5 microliters) dose-dependently increased the nonconvulsive absence epilepsy while GDEE (0.2 mumol/5 microliters; 1 mumol/5 microliters; 5 umol/5 microliters) caused a dose-dependent decrease. All effects of GDEE could be blocked by an inactive AMPA dosage. Kainic acid (0.01 nmol/5 microliters; 0.1 nmol/5 microliters; 0.15 nmol/5 microliters) had no effects on the nonconvulsive epilepsy but induced convulsions in the two highest dosages. Kynurenic acid (50 nmol/5 microliters; 100 nmol/5 microliters; 500 nmol/5 microliters) decreased dose-dependently the incidence of nonconvulsive epilepsy. The effect of kynurenic acid could be blocked by a nonconvulsive dosage of kainic acid. These results show that the AMPA and kainate receptor appear to be involved in nonconvulsive epilepsy. Furthermore, blockage of these two receptor subtypes led to an antiepileptic effect without inducing behavioural alterations. Therefore, selective AMPA and kainate receptor antagonists might be potent anti-epileptics.

Activation of alpha-adrenoceptors indirectly facilitates sodium pumping in frog motoneurons

The effects of clonidine on Na+ pumping in motoneurons of the isolated frog spinal cord was investigated using sucrose gap recordings from ventral roots. Na+ pump activity, induced in motoneurons either by tetanizing the dorsal root or by rapidly exposing the cord to normal medium following 30 min in K(+)-free Ringer's solution (K(+)-activated hyperpolarizations), was increased by application of clonidine (100 microM). These actions of clonidine were blocked by the preferential alpha 2-adrenergic antagonist yohimbine, but not by alpha 1-adrenergic antagonist prazosin or the beta-blocker propranolol. Clonidine's effects on Na+ pumping appeared to be indirect (presumably via interneurons) because its effects on K(+)-activated hyperpolarizations were reduced by tetrodotoxin (TTX) or high concentrations of Mg2+. This indirect mechanism involved activation of non-NMDA excitatory amino acid receptors. Thus, in the presence of clonidine, CNQX, but not APH, limited the ability of clonidine to enhance K(+)-activated hyperpolarizations. The AMPA receptor may play a role in the process, K(+)-activated hyperpolarizations were augmented by the presence of AMPA; NMDA had no effect. The present results are consistent with the idea that activation of alpha 2-adrenoceptors produces the following: the release of excitatory amino acids from interneurons; the activation of non-NMDA receptors on motoneurons; increased Na+ influx and loading and increased Na+ pump activity.

N-methyl-D-aspartate receptor antagonists and channel blockers have different effects upon a spinal seizure model in mice

Spinal seizures in mice induced by handling following pretreatment with a subconvulsive dose of strychnine could be blocked by competitive N-methyl-D-aspartate (NMDA) receptor antagonists (D-, L-, DL-CPPene (CPPene = (E)-4-(3-phophonoprop-2-enyl)-piperazine-2-carboxylic acid), D-AP5 (D-2-amino-5-phophonovalerate)) and compounds acting at receptor-coupled modulatory sites (R-HA 966, ifenprodil). NMDA cation channel antagonists (MK-801, phencyclidine) however, resulted in ataxia, tremor and loss of righting. There are differences between NMDA antagonists acting via the receptor and the cation channel in this model of spinal seizure.

Cell damage associated with changing the medium of mesencephalic cultures in serum-free medium is mediated via N-methyl-D-aspartate receptors

Dopaminergic neurons from embryonic rat mesencephalon were grown in simple serum-free media. The cells develop over a period of several weeks in vitro, particularly between day 14 and day 23. Removing the culture medium and replacing it with fresh medium during this interval caused severe damage to the cultures; this damage is mediated by excitatory amino acids acting through glutamate receptors. Damage could be completely prevented by antagonists of the N-methyl-D-aspartate subtype of glutamate receptor. As expected, medium that contains glutamate (i.e., Ham's F-12 medium) caused damage; however, medium that contains no glutamate or aspartate (i.e., Dulbecco's modified Eagle medium) also caused severe damage, and most of the damage was dependent on the presence of glutamine in the medium. The presence of the antibiotics penicillin and streptomycin greatly enhanced damage caused by medium change.

Involvement of NMDA receptors in non-convulsive epilepsy in WAG/Rij rats

The involvement of the NMDA receptor in spontaneous non-convulsive epilepsy was studied by intracerebroventricular injections of APH and NMDA in WAG/Rij rats. The WAG/Rij rat strain is recognized as an animal model for human absence epilepsy. EEG registrations showed that APH (5 nmol/5 microliters; 25 nmol/5 microliters; 50 nmol/5 microliters) causes a dose-dependent decrease in the number and mean duration of the spike-wave discharges, while NMDA (50 pmol/5 microliters; 500 pmol/5 microliters; 5 nmol/5 microliters) induces a dose-dependent increase in the number. The effects of NMDA (5 nmol/5 microliters) can be blocked completely by APH (50 nmol/5 microliters). These results suggest an involvement of the NMDA receptor in experimental non-convulsive epilepsy, observed in the WAG/Rij model.

Inhibition of the development of electrical kindling of the prepyriform cortex by daily focal injections of excitatory amino acid antagonists

The effects of daily focal injections of excitatory amino acid antagonists into the prepyriform cortex on the development of electrically kindled seizures at this site were studied. The selective 'NMDA receptor' antagonists 2-amino-7-phosphonoheptanoic acid (AP7) and 3-[+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) significantly inhibited the development of the electrically evoked afterdischarge over a 10 day period and prevented the development of the motor seizure responses. The 'kainate and quisqualate receptor' antagonist gamma-D-glutamylaminomethyl sulphonic acid (GAMS) showed less potent but still significant inhibitory actions on these responses. When drug treatment ceased, kindling progressed in all animals at a rate similar to that of the control (buffer-treated) animals. These results suggest a critical role for NMDA receptors in the primary neuronal events initiating the epileptiform activity in this animal model of epilepsy.

Retardation of amygdala kindling by antagonism of NMD-aspartate and muscarinic cholinergic receptors: evidence for the summation of excitatory mechanisms in kindling

DL-2-amino-5-phosphonovaleric acid (APV), a specific antagonist of N-methyl-D-aspartate (NMDA) receptors, was administered alone and in combination with scopolamine (SCO), an antagonist of muscarinic cholinergic receptors, to different groups of rats undergoing electrical kindling of the amygdala. Both groups were significantly retarded in their rate of kindling during 15 drug sessions compared to controls, and the group receiving APV and SCO kindled significantly slower than the group receiving APV alone. These results indicate that both NMDA and muscarinic cholinergic receptors are involved in kindling of the amygdala, and implicate a mechanism involving the summation of excitatory neurotransmission in kindling of the amygdala.

Seizure-like discharges induced by lowering [Mg2+]o in the human epileptogenic neocortex maintained in vitro

Seizure-like discharges were observed in slices of human epileptogenic neocortex maintained in vitro when [Mg2+]o was lowered near to zero. This type of epileptiform activity: (1) could occur spontaneously or following extracellular focal stimuli; (2) resembled the electrographic pattern associated with tonic-clonic seizures; (3) was accompanied by increases in [K+]o (maximally 6.2 mM from a baseline of 3.25 mM) and decreases in [Ca2+]o (maximally 0.23 mM from a baseline of 1.8 mM). Application of the selective antagonist of N-methyl-D-aspartate (NMDA) receptors, DL-2-amino-5-phosphonovalerate, suppressed in a reversible manner both spontaneous and stimulus-induced seizure-like discharges, suggesting that NMDA-activated conductances are important for the genesis of prolonged epileptiform discharges generated by human epileptogenic neocortical slices.

Involvement of N-methyl-D-aspartate receptors in epileptiform bursting in the rat hippocampal slice

The effects of the N-methyl-D-aspartate (NMDA) receptor antagonist, D-2-amino-5-phosphonovaleric acid (D-APV), and other excitatory amino acid antagonists, were studied on CA1 pyramidal neurones treated with picrotoxin or bicuculline to reduce synaptic inhibition mediated by gamma-aminobutyric acid (GABA). Under these conditions epileptiform burst firing is readily produced by orthodromic stimulation of the pyramidal cell population. D-APV reduced the plateau amplitude and duration of the depolarization underlying evoked and spontaneous bursts without affecting membrane potential, input resistance or the ability of the cell to fire a Ca2+ spike or a short train of Na+ spikes. A late component of the subthreshold excitatory post-synaptic potential (e.p.s.p.) was voltage dependent, being reduced in amplitude on membrane hyperpolarization. D-APV selectively removed this component of the e.p.s.p. in disinhibited slices. In contrast, in the absence of GABA antagonists, D-APV had no noticeable effect on the e.p.s.p. as studied with field potential recordings. The concentration-response relationship of the inhibitory effect of D-APV and L-APV on population spike bursts was studied. The action of APV was highly stereoselective; the EC50 of D-APV was approximately 700 nM, whereas a similar inhibition required 540 microM-L-APV. A number of other excitatory amino acid antagonists were tested at a fixed concentration (100 microM). Among them, the quisqualate antagonist gamma-D-glutamylaminomethyl sulphonic acid was ineffective against epileptiform bursts. In the low nanomolar concentration range both D- and L-APV potentiated bursting. These results suggest that in the absence of GABAergic inhibition, a significant component of the slow depolarization underlying burst firing is voltage dependent, synaptic in origin and mediated by NMDA receptors. We propose that, under normal (non-epileptic) physiological conditions, the balance between synaptic inhibition mediated by GABA receptors and synaptic excitation mediated by NMDA receptors may modulate the excitability of pyramidal cell dendrites.

Ketamine selectively suppresses synchronized afterdischarges in immature hippocampus

The role of excitatory amino acid neurotransmission in epileptogenesis was investigated in the developing hippocampus. Bath application of ketamine blocked penicillin-induced, synchronized afterdischarges in immature rat CA3 hippocampal neurons. Ketamine also decreased the duration of the preceding intracellularly recorded depolarization shift but had no measurable effect on the resting membrane potential or input impedance of pyramidal cells. Concentrations of ketamine that blocked afterdischarge generation dramatically depressed intracellular depolarizations produced by iontophoretic application of N-methyl-D-aspartate (NMDA) but not quisqualate. The effects of the NMDA antagonist 2-amino-7-phosphonoheptanoic acid on epileptiform discharges were identical to those of ketamine. These results suggest that an endogenous excitatory amino acid acting on an NMDA receptor plays a key role in the pronounced capacity of immature hippocampus for seizures.

Neuronal sensitivity to GABA and glutamate in generalized epilepsy with spike and wave discharges

In awake but painlessly immobilized cats the extracellular activity of the same cortical neurons was recorded before and for 2 to 5 h after the injection of penicillin G (350,000 IU/kg, i.m.) during the development of generalized epilepsy with bilaterally synchronous spike and wave discharges. Possible changes in their sensitivity to microiontophoretically applied glutamate and GABA during this period were searched for using computer-generated periejection histograms at intervals of about 30 min. In contrast to reported studies in other models of epilepsy, glutamate excited and GABA depressed virtually all neurons tested during fully developed spike and wave epilepsy. Spike height was not apparently affected either by the amino acids or by the development of epilepsy. Comparison of relative thresholds for the above effects on rhythmical neuronal activity associated with spike and wave discharge versus effects on random neuronal activity during the interburst periods, supported the idea that spikes and waves result from strong excitatory and inhibitory synaptic drives of the neurons. In all neurons until the appearance of spike and wave discharges, changes in the effect of amino acids, if observed, were small and statistically nonsignificant. This suggests that the hyperexcitability of cortical neurons which reportedly leads to the appearance of spike and wave discharges depends on mechanisms other than an increase in sensitivity to glutamate or a desensitization to GABA. Sometimes the sensitivity to GABA decreased later in this experimental model when the very frequent appearance of spike and wave discharges eventually led to EEG tonic-clonic seizures.

A selective N-methyl-D-aspartate antagonist depresses epileptiform activity in rat hippocampal slices

The sensitivity of convulsant-induced epileptiform activity in the hippocampus to the selective N-methyl-D-aspartate (NMDA) antagonist D-2-amino-5-phosphonovalerate (D-APV) was examined using in vitro electrophysiological techniques. This compound reduced the number and size of the synaptically evoked population spikes recorded in the CA1 region in the presence of the convulsants, pentylenetetrazol, bicuculline or folate. Intracellular recordings in the presence of bicuculline showed that D-APV reduced the late component of the excitatory postsynaptic potential and the number of action potentials evoked synaptically. A mechanism is suggested to explain how NMDA receptors, which are known not to be involved in normal synaptic transmission in hippocampal slices, can contribute to epileptiform activity.