On the sacred disease: the neurochemistry of epilepsy

Mechanisms of vagus nerve stimulation for the treatment of neurodevelopmental disorders: a focus on microglia and neuroinflammation

The vagus nerve (VN) is the primary parasympathetic nerve, providing two-way communication between the body and brain through a network of afferent and efferent fibers. Evidence suggests that altered VN signaling is linked to changes in the neuroimmune system, including microglia. Dysfunction of microglia, the resident innate immune cells of the brain, is associated with various neurodevelopmental disorders, including schizophrenia, attention deficit hyperactive disorder (ADHD), autism spectrum disorder (ASD), and epilepsy. While the mechanistic understanding linking the VN, microglia, and neurodevelopmental disorders remains incomplete, vagus nerve stimulation (VNS) may provide a better understanding of the VN's mechanisms and act as a possible treatment modality. In this review we examine the VN's important role in modulating the immune system through the inflammatory reflex, which involves the cholinergic anti-inflammatory pathway, which releases acetylcholine. Within the central nervous system (CNS), the direct release of acetylcholine can also be triggered by VNS. Homeostatic balance in the CNS is notably maintained by microglia. Microglia facilitate neurogenesis, oligodendrogenesis, and astrogenesis, and promote neuronal survival via trophic factor release. These cells also monitor the CNS microenvironment through a complex sensome, including groups of receptors and proteins enabling microglia to modify neuroimmune health and CNS neurochemistry. Given the limitations of pharmacological interventions for the treatment of neurodevelopmental disorders, this review seeks to explore the application of VNS as an intervention for neurodevelopmental conditions. Accordingly, we review the established mechanisms of VNS action, e.g., modulation of microglia and various neurotransmitter pathways, as well as emerging preclinical and clinical evidence supporting VNS's impact on symptoms associated with neurodevelopmental disorders, such as those related to CNS inflammation induced by infections. We also discuss the potential of adapting non-invasive VNS for the prevention and treatment of these conditions. Overall, this review is intended to increase the understanding of VN's potential for alleviating microglial dysfunction involved in schizophrenia, ADHD, ASD, and epilepsy. Additionally, we aim to reveal new concepts in the field of CNS inflammation and microglia, which could serve to understand the mechanisms of VNS in the development of new therapies for neurodevelopmental disorders.

An insulin based model to explain changes and interactions in human breath-holding

Until now oxygen was thought to be the leading factor of hypoxic conditions. Whereas now it appears that insulin is the key regulator of hypoxic conditions. Insulin seems to regulate the redox state of the organism and to determine the breakpoint of human breath-holding. This new hypoxia-insulin hypotheses might have major clinical relevance. Besides the clinical relevance, this hypothesis could explain, for the first time, why the training of the diaphragm, among other factors, results in an increase in breath-holding performance. Elite freedivers/apnea divers are able to reach static breath-holding times to over 6 min. Untrained persons exhibit an unpleasant feeling after more or less a minute. Breath-holding is stopped at the breakpoint. The partial oxygen pressure as well as the carbon dioxide pressure failed to directly influence the breakpoint in earlier studies. The factors that contribute to the breakpoint are still under debate. Under hypoxic conditions the organism needs more glucose, because it changes from the oxygen consuming pentose phosphate (36 ATP/glucose molecule) to the anaerobic glycolytic pathway (2ATP/glucose molecule). Hence insulin, as it promotes the absorption of glucose, is set in the center of interest regarding hypoxic conditions. This paper provides an insulin based model that could explain the changes and interactions in human breath-holding. The correlation between hypoxia and reactive oxygen species (ROS) and their influence on the sympathetic nerve system and hypoxia-inducible factor 1 alpha (HIF-1α) is dealt with. It reviews as well the direct interrelation of HIF-1α and insulin. The depression of insulin secretion through the vagus nerve activation via inspiration is discussed. Furthermore the paper describes the action of insulin on the carotid bodies and the diaphragm and therefore a possible role in respiration pattern. Freedivers that go over the breakpoint of breath-holding could exhibit seizures and thus the effect of insulin, blood glucose levels and corticosteroids in hippocampal seizures is highlighted.

Prostaglandin d2 induced potentiation of the anticonvulsant actions of phenobarbitone and phenytoin in rats. Role of serotonin

Prostaglandin D2 (PGD2) produced a dose-related potentiation of the anticonvulsant actions of sub-effective doses of phenobarbitone and phenytoin against maximal electroshock-induced seizures in rats. PDG2-induced potentiation of phenobarbitone and phenytoin was significantly attenuated following pretreatment with centrally administered 5,6-dihydroxytryptamine, a selective neurotoxin for serotonergic neurones, p-chlorophenylalanine, a specific inhibitor of serotonin biosynthesis, and methysergide, a serotonin receptor antagonist, indicating that the potentiation was serotonin-mediated.

Vagus nerve stimulation for epilepsy: A review of central mechanisms

In a previous paper, the anatomy and physiology of the vagus nerve was discussed in an attempt to explain which vagus nerve fibers and branches are affected by clinically relevant electrical stimulation. This companion paper presents some of vagus nerve stimulation's putative central nervous system mechanisms of action by summarizing known anatomical projections of vagal afferents and their effects on brain biogenic amine pathways and seizure expression.

Pharmacological characterisation of the electrically evoked release of monoamines from chicken brainin vitro

1. A study was conducted to develop an in vitro model for examining the basal and electrical-stimulation-induced release of [3H]monoamines from chicken hyperstriatal neurones in order to demonstrate the presence of presynaptic autoreceptors for the three main monoamine transmitters: noradrenaline, dopamine and 5-HT. 2. Two sets of experiments were carried out: the first was to evaluate the effect of calcium and tetrodotoxin (TTX, sodium channel conductance inhibitor) in order to demonstrate that evoked release of monoamines was a consequence of exocytotic processes; the second to investigate the effect of selective agonists and antagonists on neurotransmitter release. 3. Ross and Cobb broiler chickens of either sex (approximately 7 to 8 weeks old) were used. Slices of hyperstriatal tissue were preincubated with [3H]noradrenaline, [3H]dopamine or [3H]5-hydroxytryptamine (5-HT), washed, perfused and electrically stimulated at three time points (S1, S2 and S3) which released [3H]noradrenaline, [3H]dopamine and [3H]5-HT, as determined by scintillation spectrometry. 4. When calcium was removed from, or TTX added to, the superfusion medium prior to and including the second period of electrical stimulation (S2) the evoked releases of [3H]noradrenaline, [3H]dopamine and [3H]5-HT at S2 were abolished. 5. In the presence of the selective alpha2-adrenoceptor agonist UK 14304 during the S2 period, the S2/S1 ratio was lower than the control ratio due to a reduction in the stimulated release of [3H]noradrenaline. The selective alpha2-adrenoceptor antagonist RX 821002 blocked the UK 14304-induced reduction of evoked release and the S2/S1 ratio was similar to the control ratio. 6. The D2-like receptor agonist quinpirole reduced the S2/S1 ratio for [3H]dopamine release, an effect blocked by the antagonist AJ 76. The 5-HT1B receptor agonist CP 94253 during S2 reduced the S2/S1 ratio due to a reduction in evoked [3H]5-HT. This effect was blocked by the 5-HT1B receptor antagonist GR 55562. 7. The results demonstrate, for the first time, the functional presence of presynaptic alpha2-adrenoceptors, presynaptic 5-HT1B autoreceptors and presynaptic D2-like autoreceptors in broiler chicken hyperstriatal neurones in vitro.

Acetylcholine receptors and thresholds for convulsions from flurothyl and 1,2-dichlorohexafluorocyclobutane

There are acetylcholine receptors throughout the central nervous system, and they may mediate some forms and aspects of convulsive activity. Most high-affinity binding sites on nicotinic acetylcholine receptors for nicotine, cytisine, and epibatidine in the brain contain the beta2 subunit of the receptor. Transitional inhaled compounds (compounds less potent than predicted from their lipophilicity and the Meyer-Overton hypothesis) and nonimmobilizers (compounds that do not produce immobility despite a lipophilicity that suggests anesthetic qualities as predicted from the Meyer-Overton hypothesis) can produce convulsions. The nonimmobilizer flurothyl [di-(2,2,2,-trifluoroethyl)ether] blocks the action of gamma-aminobutyric acid on gamma-aminobutyric acid(A) receptors, whereas the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N, also called F6) does not. 2N can block the action of acetylcholine on nicotinic acetylcholine receptors. We examined the relative capacities of these compounds to cause convulsions in mice having and lacking the beta2 subunit of the acetylcholine receptor. The partial pressure causing convulsions in half the mice (the 50% effective concentration [EC(50)]) was the same as in control mice. For the knockout mice, the EC(50) for flurothyl was 0.00170 +/- 0.00030 atm (mean +/- SD), and for 2N, it was 0.0345 +/- 0.0041 atm. For the control mice, the respective values were 0.00172 +/- 0.00057 atm and 0.0341 +/- 0.0048 atm. The ratio of the 2N to flurothyl EC(50) values was 20.8 +/- 3.5 for the knockout mice and 21.7 +/- 7.0 for the control mice. These results do not support the notion that acetylcholine receptors are important mediators of the capacity of 2N or flurothyl to cause convulsions. However, we also found that both nonimmobilizers inhibit rat alpha4beta2 neuronal nicotinic acetylcholine receptors at EC(50) partial pressures (0.00091 atm and 0.062 atm for flurothyl and 2N, respectively) that approximate those that produce convulsions (0.0015 atm and 0.04 atm).

IMPLICATIONS

The results from the present study provide conflicting data concerning the notion that acetylcholine receptors mediate the capacity of nonimmobilizers to produce convulsions.

Effects of psychotropic drugs on seizure threshold

Psychotropic drugs, especially antidepressants and antipsychotics, may give rise to some concern in clinical practice because of their known ability to reduce seizure threshold and to provoke epileptic seizures. Although the phenomenon has been described with almost all the available compounds, neither its real magnitude nor the seizurogenic potential of individual drugs have been clearly established so far. In large investigations, seizure incidence rates have been reported to range from approximately 0.1 to approximately 1.5% in patients treated with therapeutic doses of most commonly used antidepressants and antipsychotics (incidence of the first unprovoked seizure in the general population is 0.07 to 0.09%). In patients who have taken an overdose, the seizure risk rises markedly, achieving values of approximately 4 to approximately 30%. This large variability, probably due to methodological differences among studies, makes data confusing and difficult to interpret. Agreement, however, converges on the following: seizures triggered by psychotropic drugs are a dose-dependent adverse effect; maprotiline and clomipramine among antidepressants and chlorpromazine and clozapine among antipsychotics that have a relatively high seizurogenic potential; phenelzine, tranylcypromine, fluoxetine, paroxetine, sertraline, venlafaxine and trazodone among antidepressants and fluphenazine, haloperidol, pimozide and risperidone among antipsychotics that exhibit a relatively low risk. Apart from drug-related factors, seizure precipitation during psychotropic drug medication is greatly influenced by the individual's inherited seizure threshold and, particularly, by the presence of seizurogenic conditions (such as history of epilepsy, brain damage, etc.). Pending identification of compounds with less or no effect on seizure threshold and formulation of definite therapeutic guidelines especially for patients at risk for seizures, the problem may be minimised through careful evaluation of the possible presence of seizurogenic conditions and simplification of the therapeutic scheme (low starting doses/slow dose escalation, maintenance of the minimal effective dose, avoidance of complex drug combinations, etc.). Although there is sufficient evidence that psychotropic drugs may lower seizure threshold, published literature data have also suggested that an appropriate psychotropic therapy may not only improve the mental state in patients with epilepsy, but also exert antiepileptic effects through a specific action. Further scientific research is warranted to clarify all aspects characterising the complex link between seizure threshold and psychotropic drugs.

Antidepressant drugs and seizure susceptibility: from in vitro data to clinical practice

The use of antidepressant drugs (ADs) in patients with epilepsy still raises uncertainties because of the widespread conviction that this class of drugs facilitates seizures. A detailed knowledge of this issue in its various aspects may help in optimal management of patients suffering concurrently from epilepsy and depression. This article reviews the available data in vitro in animals and humans concerning the known potential of various ADs to induce epileptic seizures. Emphasis has been placed on those variables that may generate confusion in interpreting the results of the various studies. Most ADs at therapeutic dosages exhibit in nonepileptic patients a seizure risk close to that reported for the first spontaneous seizure in the general population (i.e., <0.1%). In patients taking high AD doses, seizure incidence rises markedly and may reach values up to 40%. With a patient history of epilepsy and/or concomitant drugs that act on neuronal excitability, low or therapeutic AD doses may be sufficient to trigger seizures. Experimental data are in partial conflict with human data on the relative potential seizure risk of the various ADs. Therefore, a reliable scale for assigning a relative value to an individual AD or to single AD classes cannot be made. It appears fair to say that maprotiline and amoxapine exhibit the greatest seizure risk, whereas trazodone, fluoxetine, and fluvoxamine exhibit the least. Some ADs may also display antiepileptic effects, especially in low doses, in experimental models of epilepsy and in humans, but the mechanism of this action is largely unknown. The available data suggest that ADs may display both convulsant and anticonvulsant effects and that the most important factor in determining the direction of a given compound in terms of excitation/inhibition is drug dosage. It is probable that drugs that increase serotonergic transmission are less convulsant or, even, more anticonvulsant than others. Because of mutual pharmacokinetic interactions between antiepileptic drugs and ADs, with consequent marked changes in plasma concentrations, it remains to be established whether or not plasma AD levels that are effective against depression also facilitate seizures. Finally, exploring the mechanisms through which ADs modulate neuronal excitability might open new possibilities in antiepileptic drug development.

Dynorphin and epilepsy

Studies on dynorphin involvement in epilepsy are summarised in this review. Electrophysiological, biochemical and pharmacological data support the hypothesis that dynorphin is implicated in specific types of seizures. There is clear evidence that this is true for complex partial (limbic) seizures, i.e. those characteristic of temporal lobe epilepsy, because; (1) dynorphin is highly expressed in various parts of the limbic system, and particularly in the granule cells of the hippocampus; (2) dynorphin appears to be released in the hippocampus (and in other brain areas) during complex partial seizures; (3) released dynorphin inhibits excitatory neurotransmission at multiple synapses in the hippocampus via activation of kappa opioid receptors; (4) kappa opioid receptor agonists are highly effective against limbic seizures. Data on generalised tonic-clonic seizures are less straightforward. Dynorphin release appears to occur after ECS seizures and kappa agonists exert a clear anticonvulsant effect in this model. However, more uncertain biochemical data and lack of efficacy of kappa agonists in other generalised tonic-clonic seizure models argue that the involvement of dynorphin in this seizure type may not be paramount. Finally, an involvement of dynorphin in generalised absence seizures appears unlikely on the basis of available data. This may not be surprising, given the presumed origin of absence seizures in alterations of the thalamo-cortical circuit and the low representation of dynorphin in the thalamus. In conclusion, it may be suggested that dynorphin plays a role as an endogenous anticonvulsant in complex partial seizures and in some cases of tonic-clonic seizures, but most likely not in generalised absence. This pattern of effects may coincide with the antiseizure spectrum of selective kappa agonists.

Antidepressants and seizures: clinical anecdotes overshadow neuroscience

Pharmacological treatment of depression in persons with epilepsy has been an area of controversy because some drugs commonly are perceived specifically to induce or exacerbate seizures in patients with seizure disorders. This prevailing misconception is unjustified by scientific studies, yet it continues to prevent afflicted persons from receiving appropriate therapy. The scientific literature shows that tricyclic antidepressant drugs cause seizures in overdose in both animals and humans. In lower doses, these drugs have anticonvulsant activity in humans and animals. Thus, the antidepressant drugs are like several antiepileptic drugs that can both prevent and cause seizures. The anticonvulsant activity of antidepressant drugs has been studied extensively in animals and almost certainly stems from their capacity to block norepinephrine and/or serotonin reuptake. The pharmacodynamic action responsible for their convulsant effects has not been well studied but may be due to their local anesthetic, antihistaminic, or antimuscarinic activity. The newer, more selective monoamine uptake blockers have very low convulsant liability, and it is suggested that their anticonvulsant activity, which is well documented in animals, be investigated further in humans. If their effects in humans are analogous to those in animals, these drugs can be used safely in epileptic patients with depression, and it is possible that their anticonvulsant activity can be exploited for use in the treatment of epilepsy.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Early changes in prodynorphin mRNA and ir-dynorphin A levels after kindled seizures in the rat

Prodynorphin mRNA and immunoreactive dynorphin A (ir-dynorphin A) levels were measured in different brain areas at various time points after amygdala kindled seizures. In the hippocampus, striatum and hypothalamus, prodynorphin mRNA levels were not significantly changed in kindled rats (killed 1 week after the last stimulus-evoked seizure), but they were significantly increased 1 h after seizures. The relative increase was the highest in the hippocampus (approximately 3-fold). In the brainstem, midbrain and cerebral cortex no changes in prodynorphin mRNA were detected in kindled rats, 1 h or 1 week after a kindled seizure. ir-Dynorphin A levels were significantly reduced in the hippocampus and in the striatum of kindled rats, as well as 5 and 60 min after kindled seizures, but they were increased back to control levels after 120 min. In the hypothalamus, ir-dynorphin A levels were significantly increased 120 min after a kindled seizure. ir-Dynorphin A levels were also significantly reduced in the brainstem and in the frontal, parietal and temporal cortex 120 min, but not 5 or 60 min, after a kindled seizure. Taken together, these data support the hypothesis that the dynorphinergic system is activated after amygdala kindled seizures, with different kinetics in different brain areas.

Models of simple partial and absence seizures in freely moving rats: action of ketamine

The action of ketamine was studied in two models of seizures: a) bilateral neocortical discharges produced by topical application of pentylenetetrazol (model of simple partial seizures); and b) rhythmic spike-and-wave activity induced by systemic administration of pentylenetetrazol (model of absence seizures). Ketamine exerted biphasic effects. In the first model, the dose of 20 mg/kg ketamine significantly suppressed the ictal neocortical discharges (i.e., continuous spiking or ictal activity) accompanied by clonic motor seizures. However, at the dose of 40 mg/kg ketamine significantly accentuated the onset and increased the number of individual discharges (interictal spikes) in bilateral neocortical foci. In the model of rhythmic spike-and-wave activity, the lower dose of ketamine (20 mg/kg) decreased the number of rhythmic spike-and-wave episodes when compared to the higher dose (40 mg/kg) of ketamine, which increased the number of episodes. However, neither result differed significantly from control values. The present results suggest a dose-dependent action of ketamine: Lower doses (10 and 20 mg/kg in the rat) are able to suppress seizure activity, whereas a higher dose (40 mg/kg) potentiates the seizures. Moreover, the action of ketamine may be dependent upon the seizure model used. The study presents a new model of acute epileptic focus in freely moving rats.

Disordered dopamine neurotransmission in the striatum of rats undergoing pilocarpine-induced generalized seizures, as revealed by microdialysis

In this study rats were fitted with a concentric dialysis probe in one striatum and extracellular concentrations of dopamine and HVA measured by reverse phase high performance liquid chromatography. Injections of saline or the D1 agonist SKF 38393 (30 mg/kg) did not affect the releases of these compounds. On the other hand, the D2 agonist LY 171555 (0.5 mg/kg) inhibited the release of both dopamine and HVA, whilst amphetamine (1 mg/kg) increased the output of dopamine but not HVA. Treatment with 200 mg/kg pilocarpine caused minimal signs of epileptic activity and did not affect striatal dopamine neurotransmission. Concomitant administration of SKF 38393 (30 mg/kg) to this dose of pilocarpine greatly facilitated the incidence and severity of motor seizures, which were accompanied by an irregular pattern of dopamine release and a significant rise in HVA overflow. Similar results were obtained with rats made to convulse with 400 mg/mg pilocarpine, and to a lesser extent if these animals were first pretreated with a protective dose of LY 171555 (0.5 mg/kg). It is concluded that dopamine neurotransmission in the striatum is disrupted in rats undergoing a pilocarpine-induced motor seizure, and that the extent of this disruption increases as the seizure becomes more severe. An irregular release of dopamine could signify a loss of sensorimotor control by the striatum, which might conceivably contribute to the intractability of the seizure. An increase in the dialysate concentrations of metabolite and not dopamine, is consistent with a heightened glutamate-stimulated release of dopamine from a discrete striatal pool, caused by the seizure spreading through the cortex and activating the cortico-striatal system.

Ethosuximide suppresses seizures and lethality induced by picrotoxin in developing rats

The action of ethosuximide (125 or 250 mg/kg, IP) against picrotoxin-induced seizures (3-6 mg/kg, IP) was assessed in rats 12, 18, 25, and 90 days old. In 18-day-old and older controls, picrotoxin regularly elicited clonic seizures; tonic-clonic seizures were induced in all age categories with high consequent mortality. Only the higher dose of ethosuximide (250 mg/kg) increased the latency of clonic seizures in 18- and 25-day-old pups. Tonic-clonic seizures were delayed by ethosuximide in 12-, 18-, and 90-day-old rats. Picrotoxin-induced lethality was suppressed only in 18- and 90-day-old rats by the 250-mg/kg dose of ethosuximide. In contrast, ethosuximide pretreatment increased the incidence of clonic seizures in 12-day-old rats. The results suggest that only high doses of ethosuximide can suppress clonic seizures, and this action is not consistent. Tonic-clonic seizures probably have model-specific sensitivity to ethosuximide because in previous studies ethosuximide completely suppressed pentylenetetrazol-induced tonic-clonic seizures but had no effect on kainic acid-induced tonic-clonic seizures. The suppression of mortality rates is probably due to nonspecific effects of high doses of ethosuximide.

Reduced susceptibility to seizures in carbonic anhydrase II deficient mutant mice

The seizure susceptibility of carbonic anhydrase II (CA) deficient mice and their normal littermates was determined and compared. In flurothyl-induced seizures, CA deficient mice displayed longer latencies to the onset of both clonic and tonic-clonic seizures. In pentylenetetrazole-induced seizures mutant mice exhibited a lower incidence of clonic seizures than did their normal littermates. Acetazolamide (a CA blocker) was used for the pretreatment of normal mice to compare them to CA deficient littermates. The pretreated mice displayed a lower incidence of flurothyl-induced tonic-clonic seizures and of both types of pentylenetetrazole seizures. The attempts to elicit audiogenic seizure did not reveal any difference between normal and mutant littermates. However, when the mice were primed by a loud sound during the critical period and retested for audiogenic seizures again at age 1.5 months, the CA deficient mice displayed a significantly lower incidence of seizures. The similarity between the anticonvulsant action of CA deficiency and the anticonvulsant action of acetazolamide suggests an important role of CA in seizures. The exact mechanism of anticonvulsant action by CA inhibition, however, remains to be elucidated.

Action of antiepileptic drugs against kainic acid-induced seizures and automatisms during ontogenesis in rats

Kainic acid (KA 4-14 mg/kg) administered intraperitoneally (i.p.) produces automatisms (scratching until third postnatal week, "wet dog" shakes thereafter), and clonic and tonic-clonic seizures in rats aged 7, 12, 18, 25, and 90 days. Administration of carbamazepine (CBZ) i.p. (25 or 50 mg/kg), phenobarbital (PB 20-80 mg/kg), clonazepam (CZP 0.2 or 1 mg/kg), or valproate (VPA 200 mg/kg) influenced neither incidence nor latency of automatisms. Clonic seizures that are regularly observed after the third postnatal week in controls were either abolished or substantially suppressed by any of the aforementioned antiepileptic drugs (AEDs). Tonic-clonic seizures observed in the first 3 postnatal weeks were suppressed only by solvent [including propyleneglycol (PEG), ethanol, and water]; the effect of AEDs on tonic-clonic seizures was proconvulsant instead. The automatisms were most resistant to AED therapy. These results induce some doubts about the adequacy of the KA model for identifying AEDs effective against complex partial seizures, but forthcoming AEDs that suppress automatisms in the KA rat model might also be active against human complex partial seizures.

Neurochemical effects of vagus nerve stimulation in humans

An implanted stimulating device chronically stimulated the left cervical vagus nerve in epileptic patients. Cerebrospinal fluid concentrations of free and total gamma-aminobutyric acid, homovanillic acid, 5-hydroxyindoleacetic acid, aspartate, glutamate, asparagine, serine, glutamine, glycine, phosphoethanolamine, taurine, alanine, tyrosine, ethanolamine, valine, phenylalanine, isoleucine, vasoactive intestinal peptide, beta-endorphin, and somatostatin were measured before and after 2 months of chronic stimulation in six patients. Significant increases were seen in homovanillic acid and 5-hydroxyindoleacetic acid in three patients, and significant decreases in aspartate were seen in five patients. These changes were associated with a decrease in seizure frequency.

Differential effects of naloxone on postictal depression

The influence of naloxone on the duration of the postictal depression was determined in 2 seizure models in the adult rats: hippocampal afterdischarges and maximal electroshock. For testing the intensity of postictal depression 2 subsequent stimulations were used. The interstimulation intervals were 3, 5, 10 and 60 min. Using interstimulation intervals 3, 5 and 10 min there was marked depression of the afterdischarge duration. Wet dog shakes accompanying hippocampal afterdischarges were suppressed only in 3- and 5-min intervals. Naioxone (1 mg/kg i.p.) abolished the suppression of afterdischarges when 10-min interstimulation interval was used. In maximal electroshock seizures where the duration of tonic flexion and extension was determined, no postictal depression was registered nor were any effects of naloxone present. The results suggest a limited role of the mu opioid receptor system in the late phases of postictal depression following hippocampal stimulation and different effects of the mu opioid system in tonic flexion/extension and behavioral depression induced by maximal electroshock.

Bicuculline-induced neocortical epileptiform foci and the effects of 6-hydroxydopamine in developing rats

Catecholamines (dopamine and norepinephrine) are considered to be predominantly inhibitory neurotransmitters in the brain and their depletion produced by 6-hydroxydopamine may result in proconvulsant effects. In our experiments on rats aged 5, 7, 9, 12, 15, 18, 25 and 90 days under urethane anesthesia we demonstrated the development of neocortical epileptic focus evoked by topical application of bicuculline methiodide. In experimental groups aged 7, 12, 18, 25 and 90 days a chronic depletion of catecholamines was induced using pretreatment with 6-hydroxydopamine early postnatally. An epileptogenic focus was induced in all age groups; duration of a single discharge decreased with age in both control and experimental animals. The spread of activity from the primary focus to contralateral frontal cortex via callosal connections was as rapid as in controls. However, the transfer of discharge to occipital regions was delayed and the number of discharges decreased in experimental rats. Our study demonstrated a substantial role of catecholamines for synchronization of focal discharges in neocortex and a promoting role of catecholamines in association pathways within neocortex.

Analysis of the convulsant-potentiating effects of lithium in rats

Lithium pretreatment of rats has previously been shown to potentiate the convulsant effects of cholinomimetic drugs, such as pilocarpine. The first objective of this project was to determine if lithium also potentiates seizures induced by other classes of drugs. Lithium pretreatment of rats did not affect seizure activity induced by administration of N-methyl-D-aspartate, kainic acid, bicuculline, or pentylenetetrazole. This suggests that the proconvulsant effect of lithium is largely selective for cholinomimetics. A second series of experiments investigated possible mechanisms of the lithium potentiation of pilocarpine-induced seizures. The alpha 2-adrenergic receptor agonist clonidine suppressed seizure development, and the antagonist idazoxan enhanced the onset of seizures, suggesting that endogenous norepinephrine provides anticonvulsant properties. Administration of the norepinephrine depleter DSP-4 potentiated pilocarpine-induced seizures. These results suggest that the previously reported impairment of noradrenergic function by lithium may play a role in its potentiation of cholinomimetic-induced seizures.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. II. The effects of MK-801

MK-801 is a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist with anticonvulsant and neuroprotective properties. The action of MK-801 (0.05-10 mg/kg IP) was assessed against pentylenetetrazol-induced seizures (PTZ; 100 mg/kg SC; 30 min after MK-801) in rats aged 7, 12, 18, 25, and 90 days (N = 263). We observed pronounced ataxia and hypermobility after MK-801 pretreatment during the whole ontogenesis, and the animals exhibited head-weaving and body-rolls. After the combination of MK-801 and PTZ "wet dog shakes" were detected in 18-, 25-, and 90-day-old rats (never seen in controls receiving PTZ only). MK-801 only insignificantly modified the latencies of minimal (clonic) seizures in 18-day-old and older rats where this seizure type is regularly elicited. In 12-day-old rats an increased incidence of minimal seizures was detected. MK-801 nearly completely blocked or strongly delayed major (generalized tonic-clonic) seizures and attenuated the seizure severity during ontogenesis in a dose-dependent manner. Present results suggest the important role of NMDA receptors in the genesis of generalized tonic-clonic seizures whilst the role of NMDA receptors in minimal seizures appears to be negligible during the whole ontogenetic development.

Ketamine suppresses both bicuculline- and picrotoxin-induced generalized tonic-clonic seizures during ontogenesis

An anticonvulsant action of ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist (5-40 mg/kg IP), on the bicuculline-induced (3-8 mg/kg IP) or picrotoxin-induced seizures (3-6 mg/kg IP) was assessed in male Wistar rats aged 7, 12, 18, 25 and 90 days. Ketamine alone caused moderate ataxia which was more pronounced in younger animals. In combination with both aforementioned convulsants, ketamine exerted anticonvulsant effects against generalized tonic-clonic seizures in all developmental stages studied. This effect was more pronounced in bicuculline-treated animals. Moreover, ketamine also suppressed the lethality induced by both drugs during all the development. On the contrary, the action of ketamine on minimal (clonic) seizures was moderate or absent. Our results suggest an important role of ketamine-affected transmission in the generation of the generalized tonic-clonic seizure pattern; moreover, an action of high doses of ketamine on GABA-A receptors might be present.

The ontogeny of GABAergic enhancement of the gamma-hydroxybutyrate model of generalized absence seizures

The ontogeny of GABAergic enhancement of generalized absence seizures was examined in the gamma-hydroxybutyrate (GHB) model of generalized absence seizures. The GHB seizure was quantitated in developing and adult rats in the presence of varying doses of the GABAa agonist muscimol or intracerebroventricularly (i.c.v.) administered GABA. Both GABA and muscimol potentiated GHB-induced seizures in an age-dependent fashion. The adult dose of 1 mg/kg muscimol was extremely potent in rats less than 28 days of age and resulted in the death of all younger animals tested secondary to profound hypothermia. A dose of 0.1 mg/kg muscimol was associated with a significant prolongation of GHB seizure in rats less than 35 days of age, but had no effect on older animals. The response to GHB was also age dependent, with the greatest sensitivity occurring during the fourth and fifth week of life. The developmental sensitivity of the rat to GHB seizure correlated with enhancement of the seizure by muscimol and GABA, and both phenomena parallel the maturation of thalamocortical recruiting mechanisms thought to play a role in the pathogenesis of the bilaterally synchronous spike wave discharges that characterize generalized absence seizures.

The effects of dihydropyridine calcium channel modulators on pentylenetetrazole convulsions

The effects of low doses of dihydropyridine (DHP) calcium channel antagonists nimodipine, nifedipine, (-)-R-202-791, and amlodipine, the DHP calcium channel agonist BAY K 8644 were investigated on clonic convulsions to pentylenetetrazole (PTZ) in mice. Nimodipine (2-20 mg/kg) produced a dose-dependent increase in the onset time for convulsions, but did not decrease the number of mice convulsing. Nifedipine, amlodipine (10 mg/kg) and BAY K 8644 (2 mg/kg) also produced an increase in the onset time for convulsions. (-)-R-202-791 (10 mg/kg) was without effect on clonic convulsions to PTZ. BAY K 8644 increased the number of mice dying from tonic-extension convulsions to PTZ. Nimodipine did not affect convulsions elicited by strychnine. Thus, low doses of DHP calcium antagonists possess anticonvulsant properties which are structurally dependent, while DHP calcium channel activators may act to promote convulsions. These observations suggest and support previous evidence that DHP receptors are important modulatory sites for the convulsive state.

Substantia nigra regulates action of antiepileptic drugs

The cholinergic agonist pilocarpine triggers sustained limbic seizures in rodents. Pilocarpine seizures were blocked by systemic administration of benzodiazepines, barbiturates, valproate and trimethadione, while diphenylhydantoin did not affect, and ethosuximide increased the susceptibility of rats to such seizures. This pattern of action of antiepileptic drugs is characteristic for pilocarpine seizures and different from other rodent models of epilepsy. Although the anatomical substrates in the forebrain involved in the expression of anticonvulsant activity are unknown, the basal ganglia are believed to be essential for the motor expression of pilocarpine seizures. Bilateral microinjections into the substantia nigra, a major output station of the basal ganglia, of midazolam (ED50 38.5 nmol; range 29-52 nmol), phenobarbital (ED50 16 nmol; range 7-39 nmol) and trimethadione (ED50 30 nmol; range 16-56 nmol) protected rats against pilocarpine seizures (380 mg/kg i.p.) Diphenylhydantoin (up to 100 nmol) remained inactive, while ethosuximide (ED50 38 nmol; range 22-65.5 nmol) reduced the threshold for pilocarpine seizures, converting subconvulsant doses of pilocarpine (200 mg/kg i.p.) into convulsant ones. The profiles of action of antiepileptic drugs on pilocarpine seizures were similar following intranigral and systemic administration. These observations suggest that the substantia nigra may mediate some actions of antiepileptic drugs.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. I. The effects of 2-amino-7-phosphonoheptanoate

The anticonvulsant action of 2-amino-7-phosphonoheptanoate (AP7) was assessed during ontogenesis of the rat. Animals of five age groups (7, 12, 18, 25, and 90 days) were pretreated with AP7 i.p. in the doses from 15 to 60 mg/kg 30 min prior to pentamethylenetetrazol (PTZ; metrazol; 100 mg/kg s.c.). The incidence and latency of minimal seizures (pure clonic without the loss of righting ability) and of generalized tonic-clonic seizures (major) were evaluated and compared with the control groups. Minimal metrazol seizures were not regularly observed in controls between ages 7 and 12 days. An increased incidence was noticed in AP7-treated groups. In animals of 18 days of age and older the AP7-pretreatment did not influence incidence of minimal seizures; the latencies were significantly lengthened only in 18-day-old animals. Major seizures were significantly suppressed with the highest dose of AP7 (60 mg/kg) in all groups except 7-day-old rats. In 90-day-old rats all doses of AP7 were effective in the suppression of major seizures. The latencies of major seizures were increased in 7 and 18 days old rats. It appears that the blockade of NMDA receptor substantially influences the major seizures induced by PTZ, whereas minimal (clonic) seizures are affected weakly. This suggests an important role of NMDA receptor-mediated transmission in the genesis of generalized tonic-clonic seizure pattern.

Injections of noradrenergic and GABAergic agonists into the inferior colliculus: effects on audiogenic seizures in genetically epilepsy-prone rats

Genetically epilepsy-prone rats (GEPRs) which display tonic seizures (GEPR-9s) in response to acoustic stimulation were used in these studies. Other laboratories have shown that GEPR-9s have a reduced concentration of brain norepinephrine (NE). Previous reports have also indicated that audiogenic seizures (AGS) in these animals are inhibited by treatments that enhance noradrenergic (NA) neurotransmission. AGS in GEPRs are believed to be initiated in the inferior colliculus (IC) where GABA has been shown to exert inhibitory influences in GEPRs that display submaximal AGS. The present study examined whether the IC is a crucial site for NA suppression of tonic seizures by examining the effect of microinfusing NA agonists into the IC. The intracollicular effect of a GABA agonist, muscimol, on sound-induced tonic convulsions in GEPR-9s was also examined. Bilateral microinfusion of NE, phenylephrine, clonidine or isoproterenol failed to alter the AGS. In contrast, muscimol (30 or 60 ng/side) infused into the IC abolished the tonic and clonic components of the AGS in GEPR-9s. These findings suggest that enhancement of GABAergic neurotransmission in the IC markedly attenuates AGS in the GEPR, while augmentation of NA neurotransmission has little effect in this brain region.

Evidence for a chronic loss of inhibition in the hippocampus after kindling: biochemical studies

Brain tissue from kindled animals prepared 1 month after their last seizure was compared to tissue from matched surgical control animals. Quantitative film autoradiography was used to study muscimol binding in the CA1 region and 3 other brain areas (dentate gyrus, cerebral cortex, and thalamus). The Kd values so obtained were constant from region to region and comparable to those published by others. Bmax values varied; of the 4 regions studied CA1 had the lowest value of Bmax. There were no differences in either Kd or Bmax values in any region studied between kindled and surgical control rats. The release of GABA from nerve terminals was assessed with hippocampal tissue maintained in vitro and perfused with different solutions in which the concentrations of K+ and Ca2+ were varied. This allowed the examination of K+-induced depolarization release and the Ca2+ dependence of this process. K+-induced, Ca2+-dependent release of GABA from hippocampus derived from kindled animals was significantly less than that from hippocampus derived from controls. The biochemical studies reported here provide additional support for the hypothesis that there is a chronic decrease in GABA-mediated inhibition in the hippocampus associated with kindling. The data point to a dysfunction at the presynaptic level, within the GABAergic interneuron, but do not exclude changes at a level postsynaptic to the GABAergic interneuron not detected with the methods employed.

Anticonvulsant activity of MK-801 and nimodipine alone and in combination against pentylenetetrazole and strychnine

The effects of the N-methyl-D-aspartate receptor antagonist MK-801 and the dihydropyridine calcium channel antagonist nimodipine were assessed for their anticonvulsant activity alone and in combination against clonic convulsions to pentylenetetrazole (PTZ) and strychnine (STR) in mice. Nimodipine (2-20 mg/kg) and MK-801 (0.1 and 0.5 mg/kg) did not affect the number of mice displaying clonic convulsions to PTZ. However, nimodipine in a dose-dependent manner increased (100%) the latency to clonic convulsions and lethality (mortality from tonic extension convulsions and respiratory failure) following PTZ. In contrast, MK-801 did not increase the latency to PTZ convulsions, but prevented the lethal effects of PTZ. When combined, MK-801 and nimodipine produced a significant reduction in the number of animals (40-60%) displaying PTZ convulsions and a greater increase in the latency to PTZ convulsions than did nimodipine alone. In contrast, MK-801 decreased the onset time, and increased the severity of STR convulsions. A combination of MK-801 and nimodipine which afforded significant protection against PTZ convulsions did not affect STR convulsions. These findings suggest that MK-801 and nimodipine, while possessing significant anticonvulsant activity on their own, produce a potent anticonvulsant synergism against PTZ but not STR.

Cellular mechanisms of epilepsy and potential new treatment strategies

Over the last 15 years, neurobiologists have begun to unravel the cellular mechanisms that underlie epileptiform activity. Such investigations have two main objectives: (1) to develop new methods for treating, "curing," or preventing epilepsy: and (2) to learn more about the normal functioning of the human brain, at the cellular/molecular and the neurological/psychological levels by analyzing abnormal brain functioning. The electroencephalogram (EEG) spike is a marker for the hyperexcitable cortex and arises in or near an area with a high epileptogenic potential. The depolarizing shift (DS) that underlies the interictal discharge (ID) appears to be generated by a combination of excitatory synaptic currents and intrinsic voltage-dependent membrane currents. The hyperpolarization that follows the DS (post-DS HP) limits ID duration, determines ID frequency, and prevents ID deterioration into seizures. The disappearance of the post-DS HP in some models is related to the onset of seizures and the spread of epileptiform activity. During the transition to seizures, the usually self-limited ID spreads in time and anatomical space. Several processes may intervene in the pathophysiological dysfunction. These include enhancing GABA-mediated inhibition, dampening NMDA-mediated excitability, interfering with specific Ca2+ currents in central neurons, and perhaps stimulating "gating" pathways.

Effects of ketamine on metrazol-induced seizures during ontogenesis in rats

An anticonvulsant action of ketamine, a noncompetitive NMDA receptor antagonist (1-40 mg/kg IP), on the metrazol-induced seizures was assessed in male Wistar rats aged 7, 12, 18, 25 and 90 days. Ketamine alone caused ataxia even in the lowest dose used. As concerens its interaction with metrazol it exerted a clearcut anticonvulsant effect against generalized tonic-clonic seizures at all developmental stages. On the contrary, the effects on clonic (i.e., minimal) seizures were only moderate or absent. Higher efficacy of ketamine was observed in young animals. Our results suggest a role of excitatory amino acids in the generation of generalized tonic-clonic metrazol seizures, but their share on the induction of clonic (minimal) seizures seems to be very small.

Proline and proline derivatives as anticonvulsants

1. The anticonvulsant properties of L-proline, of proline derivatives (trans-4-hydroxy-L-proline, cis-4-hydroxy-D-proline, 3,4-dehydro-D,L-proline) and of D- and L-pipecolic acid were studied alone and in combination with vigabatrin (R/S-4-aminohex-5-enoic acid). 3-Mercaptopropionic acid and pentylenetetrazol-induced convulsions in mice were used as animal models of epilepsy. 2. Proline and proline derivatives are weak anticonvulsants if given alone in doses up to 10 mmol/kg, however, they are capable of potentiating the anticonvulsant effects of vigabatrin, in a manner similar to that reported previously for glycine, and some glycine derivatives. Among the compounds tested, trans-4-hydroxy-L-proline was the most potent anticonvulsant in combination with the indirect GABA agonist vigabatrin. 3. A potential explanation for the synergistic anticonvulsant effect of the combination of the GABA agonist and proline is the presumed role of proline as inhibitory neurotransmitter, and/or its glutamate antagonistic effects. 4. The current study points out the lack of basic knowledge on the neurochemistry and pharmacology of proline and hydroxyproline.

Review: cholinergic mechanisms and epileptogenesis. The seizures induced by pilocarpine: a novel experimental model of intractable epilepsy

High-dose treatment with pilocarpine hydrochloride, a cholinergic muscarinic agonist, induces seizures in rodents following systemic or intracerebral administration. Pilocarpine seizures are characterized by a sequential development of behavioral patterns and electrographic activity. Hypoactivity, tremor, scratching, head bobbing, and myoclonic movements of the limbs progress to recurrent myoclonic convulsions with rearing, salivation, and falling, and status epilepticus. The sustained convulsions induced by pilocarpine are followed by widespread damage to the forebrain. The amygdala, thalamus, olfactory cortex, hippocampus, neocortex, and substantia nigra are the most sensitive regions to epilepsy-related damage following convulsions produced by pilocarpine. Spontaneous seizures are observed in the long-term period following the administration of convulsant doses of pilocarpine. Developmental studies show age-dependent differences in the response of rats to pilocarpine. Seizures are first noted in 7-12 day-old rats, and the adult pattern of behavioral and electroencephalographic sequelae of pilocarpine is seen in 15-21-day-old rats. During the third week of life the rats show an increased susceptibility to the convulsant action of pilocarpine relative to older and younger animals. The developmental progress of the convulsive response to pilocarpine does not correlate with evolution of the brain damage. The adult pattern of the damage is seen after a delay of 1-2 weeks in comparison with the evolution of seizures and status epilepticus. The susceptibility to seizures induced by pilocarpine increases in rats aged over 4 months. The basal ganglia curtail the generation and spread of seizures induced by pilocarpine. The caudate putamen, the substantia nigra, and the entopeduncular nucleus govern the propagation of pilocarpine-induced seizures. The antiepileptic drugs diazepam, clonazepam, phenobarbital, valproate, and trimethadione protect against pilocarpine-induced convulsions, while diphenylhydantoin and carbamazepine are ineffective. Ethosuximide and acetazolamide increase the susceptibility to convulsant action of pilocarpine. Lithium, morphine, and aminophylline also increase the susceptibility of rats to pilocarpine seizures. The pilocarpine seizure model may be of value in designing new therapeutic approaches to epilepsy.

Effects of GABA-ergic agents on spontaneous non-convulsive epilepsy, EEG and behaviour, in the WAG/RIJ inbred strain of rats

The effects of GABAergic agents on non-convulsive epilepsy were studied by intracerebroventricular injections of muscimol and bicuculline in WAG/Rij rats. The WAG/Rij rat strain is recognized as an animal model for human absence epilepsy. EEG registrations and behavioural observations showed that muscimol dose-dependently increased the non-convulsive absence epilepsy. Besides this, it induced EEG spikes and body twitches. Bicuculline induced spikes and body twitches as well but decreased the non-convulsive epilepsy. All effects of muscimol can be blocked by bicuculline and vice versa, which suggests that the observed effects are genuine GABAA effects. These results implicate that non-convulsive epilepsy can be caused by a GABAergic hyperfunction.

Effects of chronic cortical seizures on GABA and benzodiazepine receptors within seizure pathways

The effects of chronic cortical seizures on the autoradiographic distributions of two markers of the gamma-aminobutyric acid (GABA)/benzodiazepine receptor--chloride ionophore complex within local and long circuits connected to the focus were examined. Rats were subjected to electrically triggered seizures of the forelimb--sensorimotor overlap zone either daily or once every other day. At the time of sacrifice the rats had received a mean of 39 +/- 3 stimulations and their seizure responses had grown in intensity and duration. [3H]Muscimol binding and [3H]flunitrazepam binding, at near saturating ligand concentrations were unchanged in the focus, mirror focus, dorsolateral caudate, globus pallidus, ventrolateral and ventrobasal thalamic nuclei, and the substantia nigra pars reticularis. These results indicate that the progressive increases in strength and duration of recurrent focal cortical seizures are not accompanied by changes in the density of either GABA receptors or benzodiazepine receptors within the focus or projection pathways.

The effects of chemically and electrically-induced convulsions on [3H]nitrendipine binding in mouse brain

The effects of chemically and electrically-evoked seizures on [3H]nitrendipine binding to voltage-dependent calcium channels in mouse brain were determined 30 and 60 min following the initiation of convulsions. While maximal electroconvulsive shock, pentylenetetrazol and strychnine exhibited either no or marginal effects, Ro 5-4864 produced a decrease (14%) in the Bmax of [3H]nitrendipine at 30 min but not 60 min. The convulsant dihydropyridine calcium channel activator, BAY K 8644, produced a significant increase in the Kd (31%) of [3H]nitrendipine at 30 min, and a significant increase in both the Bmax (21%) and Kd (28%) of [3H]nitrendipine 60 min following the initiation of convulsions. While maximal electroconvulsive shock, pentylenetetrazol and strychnine exhibited either no or marginal effects, Ro 5-4864 produced a decrease (14%) in the Bmax of [3H]nitrendipine at 30 min but not 60 min following the initiation of convulsions. These findings indicate that modulation of voltage-dependent calcium channels by certain convulsants may be important in the genesis of seizures or in post-ictal compensatory processes.

Alterations in neurotransmitter amino acids in hippocampal kindled seizures

Rats were rapidly kindled with electrical stimuli to the hippocampus, and the concentrations of GABA and related amino acids were measured in several brain regions, both in the baseline kindled state and during active seizures. In the baseline kindled state, a consistent pattern was found throughout the hippocampus where GABA levels were depressed and glutamate and glutamine levels were elevated. During seizures GABA rose slightly while glutamate and glutamine increased to twice control values. These changes were seen to various degrees in other brain areas. Turnover in the GABA-glutamate-glutamine cycle was measured in kindled rats experiencing seizures and compared to control animals. During seizures there was an increased turnover of the excitatory compounds glutamate and aspartate relative to GABA. The data indicate that kindling produces a change of presynaptic GABA metabolism which results in diminished inhibition.

Prostaglandin D2 inhibits pentylenetetrazole-induced convulsions in rats by a serotonin-mediated mechanism

Prostaglandins (PGs) of the E series are known to exert anticonvulsant action in experimental animals. Earlier studies from this laboratory have indicated that PGE1 inhibits pentylenetetrazole (PTZ)-induced convulsions in rats through a serotonin-mediated mechanism. PGD2, the major PG in the rodent brain, shares a number of central pharmacological actions of the PGEs, and like the latter it potentiates the anticonvulsant action of phenobarbitone and phenytoin in rats. The present study was undertaken to investigate the putative anticonvulsant action of PGD2 against PTZ-induced convulsions in rats and to evaluate the role of serotonin in the anticonvulsant action of PGD2. PGD2 (5, 10, and 20 micrograms, icv) produced a dose-related inhibition of PTZ-induced clonic convulsions in rats. The anticonvulsant action of PGD2 (20 micrograms, icv) was significantly attenuated following pretreatment of the rats with pharmacologic agents known to reduce central serotonergic activity, including 5,6-dihydroxytryptamine, a selective neurotoxin for serotonergic neurons, p-chlorophenylalanine, a specific inhibitor of serotonin biosynthesis, metergoline, a serotonin postsynaptic receptor antagonist, and quipazine, which is known to inhibit neuronal release of serotonin. These findings, in conjunction with an earlier study from this laboratory indicating that PGD2 augments rat brain serotonergic activity, suggest that the anticonvulsant activity of PGD2 against PTZ-induced convulsions in rats is mediated through a serotonergic mechanism.

Effect of dihydropyridines and diphenylalkylamines on pentylenetetrazol-induced seizures and cerebral blood flow in cats

Nimodipine, a dihydropyridine that interacts with a Ca++ channel-associated binding site, when delivered (30 to 150 micrograms/kg) intra-arterially (ia) to enflurane-anesthetized cats, produced a dose-dependent suppression of seizures evoked by pentylenetetrazol. A comparable suppression was produced by clonazepam (1 to 30 micrograms/kg, ia). Phenytoin was maximally effective only at nearly lethal doses (90 mg/kg, ia). Verapamil, a diphenylalkylamine that interacts with a separate Ca++ channel-associated site, at the maximum nonlethal dose (6 mg/kg, ia) resulted in a mild facilitation of seizure activity. The drug vehicle used in these studies (50% polyethylene glycol-400) had no effect when given alone. Regional cerebral blood flow (rCBF) as measured by the clearance of xenon-133 was markedly elevated immediately after the onset of seizure activity (89 +/- 3 to 168 +/- 4 ml/100 gm/min). Concurrent with their resolution of the seizure activity, both nimodipine and clonazepam reduced rCBF to near preseizure levels and preserved the rCBF response to hypercarbia which would otherwise have been abolished following prolonged seizure activity. Moreover, the effect of nimodipine on rCBF and seizures occurred without any prominent alterations in mean arterial blood pressure as compared to preseizure levels. These data support the proposition that a dihydropyridine Ca++ channel binding site may play a role in modulating paroxysmal neuronal activity, and suggest that this class of agents may reflect a novel group of antiepileptic drugs.

Only certain antiepileptic drugs prevent seizures induced by pilocarpine

Seizures produced in rats by systemically administered pilocarpine (PILO) provide a model for studying the generation and spread of convulsive activity in the forebrain. PILO, 380 mg/kg, induces a sequence of behavioral and electroencephalographic alterations indicative of motor limbic seizures and status epilepticus which is followed by widespread damage to the limbic forebrain resembling that occurring subsequent to prolonged intractable seizures in humans. The present study was undertaken to determine whether clinically utilized antiepileptic drugs share an ability to suppress seizures and brain damage elicited by PILO in rats. Clonazepam, ED50 0.35 mg/kg (0.25-0.49), phenobarbital, 23.4 mg/kg (18.5-29.6), and valproic acid, 286 mg/kg (202-405), prevented the buildup of limbic seizures and protected against seizure-related brain damage. Pretreatment with trimethadione, 179 mg/kg (116-277), resulted in a moderate protection against PILO-induced seizures, whereas carbamazepine, 10-50 mg/kg, and diphenylhydantoin, 10-200 mg/kg, blocked neither convulsions nor brain damage produced by the drug. Surprisingly, ethosuximide, 196 mg/kg (141-272), and acetazolamide, 505 mg/kg (332-766), both lowered the threshold for seizures induced by PILO and converted a non-convulsant dose of PILO, 200 mg/kg, into a convulsant one. These results indicate that only certain anticonvulsant drugs elevate the threshold for PILO-induced seizures and prevent the occurrence of epilepsy-related brain damage. The resistance of seizures produced by PILO in rats to antiepileptic drugs reaffirms the clinically obvious lack of effective treatments for limbic convulsions.

Calcium antagonist binding in cat brain tolerant to electroconvulsive shock

Cats subjected to daily (25-30 days) electroconvulsive shock (ECS) demonstrated an elevation of their electroconvulsive threshold or tolerance to ECS. [3H] Nitrendipine binding was measured to brain regions from non-tolerant (sham shocked) and ECS tolerant cats 24 hr following the last shock. ECS produced a significant increase (45%) in the density of [3H] nitrendipine binding sites in the cerebral cortex and a significant decrease (33%) in the apparent affinity of [3H] nitrendipine in the cerebellum. No changes in binding were observed in the hippocampus. The effects of ECS were also investigated in the rat, an animal not displaying tolerance to repeated ECS. [3H] Nitrendipine binding to rat brain was measured 10 min and 24 hr following one shock (acute) or ten shocks delivered transauricularly once daily (chronic). Twenty-four hours following chronic ECS, there was a significant increase (19%) and decrease (11%) in the density, but no change in the apparent affinity of [3H] nitrendipine binding sites in the cerebral cortex and hippocampus respectively. No significant change in [3H] nitrendipine binding was observed in rat cerebellum 24 hr following chronic ECS. There were no changes in [3H] nitrendipine binding in the cerebral cortex and hippocampus 10 min and 24 hr following acute ECS. These results indicate that ECS can alter [3H] nitrendipine binding to calcium channel linked dihydropyridine binding sites in the central nervous system. It is suggested that changes in [3H] nitrendipine binding in the cat cerebellum may be involved in the development of tolerance to ECS.

Micro-vacuolation in rat brains after long term administration of GABA-transaminase inhibitors. Comparison of effects of ethanolamine-O-sulphate and vigabatrin

Two "suicide" inhibitors of GABA-aminotransferase which are known to raise the concentration of GABA in vivo and to have anti-convulsant properties, have been compared for the extent to which they produce micro-vacuoles in the brains of rats. The compounds gamma-vinyl-GABA (Vigabatrin) and ethanolamine-O-sulphate were administered orally for six months to rats at doses that produced the same increase in brain GABA levels. Micro-vacuolation was found to be present in the brains of animals treated with either compound but to be more severe in those treated with Vigabatrin. A quantitative assessment using computerised image analysis revealed that both the number of vacuoles, and the area occupied by them, was twice as high in the Vigabatrin treated animals as in those treated with ethanolamine-O-sulphate. This quantitative difference could be seen to be due to the fact that in the Vigabatrin treated animals the vacuoles extended into the white matter tracts between the cerebellar folia whereas in those animals treated with ethanolamine-O-sulphate it was confined to the roof nucleus.

Suppression of penicillin-induced epileptiform activity by noxious stimulation: mediation by 5-hydroxytryptamine

Noxious stimulation has a well known suppressant effect on epileptiform activity in both laboratory animals and man. To study this phenomenon in an animal model, focal epileptiform activity was induced in anaesthetized rats by applying penicillin from one barrel of a micropipette while recording intracortical electrical activity from another barrel. Penicillin produced either focal epileptiform activity or focal penicillin spikes, depending somewhat on the rate of penicillin release. Focal epileptiform activity was suppressed by noxious stimulation, both somatic and olfactory, whereas non-noxious stimulation was ineffective in this regard. Focal penicillin spikes were only rarely suppressed by noxious stimulation. Reserpine blocked the suppressant effect of noxious stimulation as did p-chlorophenylalanine, a more selective depletor of 5-hydroxytryptamine. L-5-Hydroxytryptophan, a 5-hydroxytryptamine precursor, restored the suppressant effect of noxious stimulation blocked by reserpine and p-chlorophenylalanine. These results suggest that the suppression of SW by noxious stimulation is mediated by 5-hydroxytryptamine. Data from experiments employing pharmacological antagonists suggest the suppression of spike and wave activity by noxious stimulation not to be mediated by activation of dopamine receptors, alpha 2 or beta adrenoceptors, or muscarinic cholinoceptors. Prazosin, a selective alpha 1 adrenoceptor antagonist, did block the suppressant effect of noxious stimulation but only at a very high dose (2.4 mumols/kg). This likely reflects a known analgesic action of prazosin or weak binding to the 5-HT receptor. Methysergide, a 5-hydroxytryptamine antagonist, failed to antagonize the suppressant effect of noxious stimulation, however, many inhibitory actions of 5-hydroxytryptamine are not blocked by methysergide. It is concluded that suppression of focal epileptiform activity by noxious stimulation is mediated, at least in part, by 5-hydroxytryptamine.

Time course of the reduction of GABA terminals in a model of focal epilepsy: a glutamic acid decarboxylase immunocytochemical study

Immunocytochemical localization of glutamic acid decarboxylase (GAD), the synthesizing enzyme for the neurotransmitter gamma-aminobutyric acid (GABA), has been used to study the time course of the decrease in putative GABAergic synaptic terminals that occurs in an alumina gel-induced model of focal epilepsy. Monkeys were studied at progressive intervals following unilateral application of alumina gel to sensorimotor cerebral cortex, and were categorized into 3 different experimental groups depending upon their clinical status. These groups respectively exhibited: (1) no abnormal bioelectrical (EEG and ECoG) activity; (2) abnormal bioelectrical activity, but no clinical seizures; and (3) both abnormal bioelectrical activity and clinical seizures. Normal and sham-operated monkeys were also studied. The amounts of GAD-positive terminal-like structures were determined on control and experimental sides of motor cortex (layer V) of all specimens with an image analysis system. This quantitative study revealed that monkeys from the 3 experimental groups showed reductions of GAD-positive terminals on the experimental cortical side, with greater losses occurring at progressively longer times following alumina gel implants. Statistical tests showed that there were no significant cortical side differences for the normal and sham groups, but that cortical side variations were significantly different for each of the 3 experimental groups. Conventional electron microscopy of an early experimental stage revealed degenerating axon terminals in layer V of motor cortex, as well as phagocytosis of degenerating material and astrogliosis. Similar findings were obtained from a chronically epileptic specimen, except that degenerating terminals were observed less often and fibrous astrocytic scarring was more prevalent, especially surrounding the somata of pyramidal neurons. The main conclusion drawn from the results of this investigation is that significant decreases of GAD-positive terminals occur prior to the onset of clinical seizures, and this is consistent with a causal role for a loss of GABAergic innervation in the development of seizure activity in this primate model of focal epilepsy.

Endogenous anticonvulsant substances

Epileptic seizures will normally arrest abruptly and spontaneously, and the brain will remain refractory to further seizures for some time thereafter. This paper reviews the possible mechanisms underlying this seizure arrest and refractoriness. The data suggests that neuronal fatigue is not involved in either of these processes, whereas the role of ions and excitatory systems are unclear. Rather, seizure arrest and refractoriness may come about by the seizure-induced release and/or activity of multiple endogenous anticonvulsant substances. The spontaneous arrest of the seizure may involve the purine adenosine, in addition to other unknown mechanisms. Seizure refractoriness involves multiple systems, the most important of which, on the available evidence, are prostaglandins and opioid peptides and possibly benzodiazepine systems, although other neuropeptides and the purines may also be involved. The implications of these conclusions to anti-epileptic drug development and status epilepticus are discussed.

Abnormalities in the central cholinergic transmitter system of the genetically epilepsy-prone rat

Seizure-experienced Genetically Epilepsy-prone Rats (GEPRs) have increased acetylcholine content and choline acetyltransferase activity in the thalamus and striatum. These cholinergic differences are accompanied by a slight but statistically significant reduction in acetylcholinesterase activity in the midbrain. In addition, no abnormalities were found in the numbers of specific 3H-QNB binding sites in the striatum, hippocampus, inferior colliculi or cortex. Other work has shown no difference in muscarinic receptor function as measured by carbachol-stimulated inositol-1-phosphate formation. These data suggest a possible presynaptic defect in the striatal and thalamic cholinergic system which may play some role in the seizure-prone state of the GEPR. However, caution must be used in interpreting these cholinergic derangements since more recent findings show no differences in thalamic acetylcholine content in seizure-naive GEPRs. Thus, the original cholinergic abnormalities detected in the seizure-experienced GEPR may be an enduring response to seizure activity.

Neurochemical consequences of status epilepticus induced in rats by coadministration of lithium and pilocarpine

Status epilepticus was produced in rats by administering pilocarpine (30 mg/kg, s.c.) 16 h after treatment with LiCl (3 meq/kg, i.p.). After 35 min of status epilepticus, several parameters of cholinergic activity were measured. Seizures had no effect on the in vivo concentration of acetylcholine or choline in cerebellum, cortex, hippocampus, or striatum. Synaptosomal high-affinity choline transport was also not changed by seizures in hippocampus, cortex, or striatum. Cortical slices from seizing rats had elevated concentrations of acetylcholine and released acetylcholine at a greater rate than did controls, but these effects seemed to be due to a reduction in the postmortem hydrolysis of acetylcholine. Synaptosomal 45calcium uptake during 2 to 60 s of incubation was no different from control rates in tissue prepared from seizing rats. These results indicate that presynaptic cholinergic activity is not markedly altered by 35 min of continuous seizure activity induced by lithium and pilocarpine. In contrast, the in vivo concentration of cyclic guanosine 5'-monophosphate was elevated above control values in seizing rats by 57 to 170% in cerebellum, cortex, hippocampus, and striatum.