Gamma hydroxybutyrate in the monkey. II. Effect of chronic oral anticonvulsant drugs.

Gamma hydroxybutyrate (GHB) was administered intravenously to monkeys that had been pretreated orally for 2 weeks with various anticonvulsant drugs or with L-DOPA at different dosage levels. Continuous electroencephalographic (EEG) monitoring was performed during and after GHB administration. Bloood was assayed for GHB and for the anticonvulsant drug the animal was receiving. The EEG and behavioral changes produced by GHB were improved by ethosuximide and phenobarbital, made worse by phenytoin, and unchanged by L-DOPA.     

Gamma hydroxybutyrate in the monkey. IV. Dopaminergic mechanisms.

The electrical seizure activity and trancelike state induced in the rhesus monkey by gamma-hydroxybutyrate (GHB) were abolished by dextroamphetamine. Dextroamphetamine blockade of this neurophysiologic effect was overcome with chlorpromazine, a dopamine receptor blocker. These results suggest that the electroencephalographic (EEG) and behavioral effects of GHB are related to effects on dopaminergic systems. Such a relationship, if substantiated by further studies, might indicate that anticonvulsant drugs used to treat petit mal epilepsy have a dopaminergic mode of action.     

Gamma hydroxybutyrate in the monkey. I. Electroencephalographic, behavioral, and pharmacokinetic studies.

Gamma hydroxybutyrate (GHB) was administered to adult and prepubescent rhesus monkeys intravenously in varying dosages while an electroencephalogram (EEG) was recorded from scalp electrodes and the body core temperature was monitored. Blood and cerebrospinal fluid samples were assayed for gamma hydroxybutyrate. GHB produced a trancelike stupor in all the monkeys, associated with marked EEG changes and hypothermia. There was a striking age specificity in that prepubescent rhesus monkeys responded to a lower threshold dosage, had a higher incidence of myoclonic jerking, and showed characteristic EEG changes not seen in the adult animals. The EEG-behavioral changes paralleled the hypothermia. There was good correlation between the serum levels of GHB and the EEG-behavioral effects. These studies suggest that the GHB-treated monkey may have utility as a petit mal seizure model.     

Effect of anticonvulsant drugs on kainic acid-induced epileptiform activity

Five antiepileptic drugs were tested for their ability to block limbic seizures induced by systemic injection of kainic acid and to suppress kainic acid-induced epileptiform discharges in incubated hippocampal slices. Phenytoin, phenobarbital, ethosuximide, and valproic acid inhibited epileptiform discharges in hippocampal slices at concentrations approximating their respective clinically effective anticon-vulsant blood concentration in humans, and diazepam had a similar action at significantly higher concentrations. At these concentrations none of the drugs blocked evoked orthodromic responses of monosynaptic excitatory connections in the hippocampal slices. In contrast, none of the drugs, at therapeutic doses, prevented kainic acid-induced seizure discharges in the hippocampus, in situ. Phenobarbital and diazepam were effective at higher concentrations. These data demonstrate that antiepileptic drugs do not have identical effects on seizure discharges in one type of brain tissue in situ and in vitro even when both are elicited by the same convulsant agent. The results also indicate that limbic seizures induced by kainic acid in vivo, like many cases of complex partial seizures in humans, are highly resistant to conventional anticonvulsant drug therapy.     

Effect of anticonvulsant drugs on glutamate neurotoxicity in cortical cell culture

Pretreatment with anticonvulsants partially protects animals against the brain damage induced by intraparenchymal injection of kainate, an analogue of the neurotransmitter glutamate. In murine cortical cell culture, high concentrations of phenobarbital, diazepam, phenytoin, or GABA itself did not prevent glutamate-induced neuronal loss. Addition of a glutamate receptor antagonist (gamma-D-glutamyl glycine) did reduce glutamate neurotoxicity. The in vivo protective effect of anticonvulsant drugs against the toxicity of excitatory amino acids must be indirect.     

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. III. Pentylenetetrazole seizure models

Although seizure models using systemic administration of the chemoconvulsant pentylenetetrazol (PTZ) for induction of generalized clonic seizures in rodents are widely employed to identify potential anticonvulsants, the important role of diverse technical, biological and pharmacological factors in interpretation of results obtained with these models is often not recognized. The aim of this study was to delineate factors other than sex, age, diet, climate, and circadian rhythms, which are generally known. For this purpose, experiments with 8 clinically established antiepileptic drugs were undertaken in the following PTZ models: (1) the threshold for different types of PTZ seizures, i.e., initial myoclonic twitch, generalized clonus with loss of righting reflexes, and tonic backward extension of forelimbs (forelimb tonus), in mice; (2) the traditional PTZ seizure test with s.c. injection of the CD97 for generalized clonic seizures in mice; and (3) the s.c. PTZ seizure test in rats. In rats, in addition to evaluating drug effects on generalized clonic seizures, a ranking system was used to determine drug effects on other seizure types. When drugs were dissolved in vehicles which themselves did not exert effects on seizure susceptibility, the most important factors which influenced drug potencies were: (1) bishaped dose-response curves, i.e., a decline in anticonvulsant dose-response at high doses of some drugs, leading to misinterpretations of drug efficacy if only a single high drug dosage is tested; (2) effects of route of PTZ administration (i.v. infusion vs. s.c. injection) on estimation of anticonvulsant potency; (3) species differences in drug metabolism; (4) differences in drug potencies calculated on the basis of administered doses compared to potency calculations based on 'active' drug concentrations in plasma; (5) qualitative and quantitative species differences in drug actions; (6) endpoints used for PTZ tests; (7) misleading predictions from PTZ seizure models. Analysis of anticonvulsant drug actions indicated that myoclonic or clonic seizures induced by i.v. or s.c. PTZ might be suitable for predicting efficacy against myoclonic petit mal seizures in humans, but certainly not to predict efficacy against absence seizures. Tonic seizures induced by PTZ were blocked by drugs, such as ethosuximide, which exert no effect on tonic seizures in humans. In order to reduce the variability among estimates of anticonvulsant activity in PTZ seizure models, the various factors delineated in this study should be rigidly controlled in experimental situations involving assay of anticonvulsant agents.     

Psychiatric implications of anticonvulsant drugs

The spectrum of contemporary antiepileptics are categorically reviewed with reference to adverse behavioral sequelae. These iatrogenic disturbances may have peripheral (eg. heptatic dysfunction, blood dyscrasia, or lupus-like syndromes) or central (eg. altered neurotransmission) manifestations. Whether an idiosyncratic reaction, toxic circumstance, or untoward drug interaction, there is scant understanding of the predisposing factors or recognition of the neuropsychiatric influence of anticonvulsant therapy.     

A primate model for testing anticonvulsant drugs.

Senegalese baboons (Papio papio), with a natural syndrome of photosensitive epilepsy, consistently show generalized myoclonic jerks if stimulated stroboscopically at hourly intervals, two to eight hours after the intravenous administration of allylglycine, 200 mg/kg. This provides a model for testing the acute antiepileptic effects of established or new drugs. The relationship between concentration of drug, antiepileptic action, and acute neurological toxic effects can be studied. Pnehobarbital (15 mg/kg) and diazepam (0;5 to 1.5 mg/kg) were highly effective in the absence of signs of toxic reaction (plasma levels: phenobarbital sodium, 0.7 to 1.7 mg/100 ml; diazepam, greater than 0.5 mug/ml). After administration of carbamazepine (30 to 40 mg/kg) and diphenylhydantoin sodium (40 to 50 mg/kg), antiepileptic action was seen, but was accompanied by severe toxic signs (nystagmus and ataxia). Sulthiame (20 to 125 mg/kg) and ethosuximide (50 to 100 mg/kg) had little antiepileptic activity and no acute toxic effects. This primate model may aid the identification of new drugs that are active against grand mal seizures and status epilepticus.     

Effects of anticonvulsant drugs on 4-aminopyridine-induced seizures in mice.

The K+ channel blocker 4-aminopyridine (4-AP) causes epileptiform activity in in vitro preparations and is a potent convulsant in animals and man. In mice, 4-AP produces behavioral activation, clonic limb movements and wild running, followed by tonic hindlimb extension and death (ED97, 13.3 mg/kg, s.c.). We evaluated the ability of a series of anticonvulsant drugs to protect against 4-AP-induced seizures using lethality as the endpoint. Drugs with a phenytoin-like profile of activity were protective with ED50 values (all in mg/kg, i.p.) of 34.4 for phenytoin, 18.6 for carbamazepine, 26.9 for felbamate, and 41.5 for zonisamide. Phenobarbital and valproate also protected against 4-AP-induced seizures and lethality (ED50s, 30.6 and 301, respectively). In contrast the NMDA antagonists (+/-)-CPP and (+)-MK-801 were inactive as were the GABA enhancers diazepam, vigabatrin and tiagabine; the antiabsence drug ethosuximide; and the L-type Ca2+ channel blocker nimodipine. We conclude that drugs like phenytoin which block seizure spread are effective antagonists of seizures induced by K+ channel blockade. Drugs with specific actions on other cellular targets may be weak or inactive, presumably because they are unable to attenuate the spread of intense (non-NMDA receptor mediated) excitation evoked by 4-AP.     

In vivo interaction of anticonvulsant drugs

The phenytoin plasma levels were measured in 45 epileptic patients whose only treatment was phenytoin. The plasma of 20 other patients receiving both phenytoin and phenobarbital was also tested for concentration of these two drugs and 18 patients treated with phenytoin, phenobarbital and primidone were investigated in the same way. The results were used to calculate the plasma levels of phenytoin in relation to dosage and to measure the effect of the simultaneous use of phenobarbital on the phenytoin plasma levels and of primidone together with phenobarbital on phenytoin concentration. The results led to the following conclusions: The population of epileptic patients can be divided into 2 groups. In the first group the patients reach equilibrium at the relatively high phenytoin plasma level for a given dose of phenytoin, and in the second group the phenytoin plasma level tends to be significantly lower for parallel dosages. Both groups, in their behavior, obey mathematically an exponential graph specific for each group. Phenobarbital tends to lower the plasma phenytoin level when the two drugs are used simultaneously. It is also possible, by the graphs produced, to calculate the expected phenytoin plasma levels when using the drugs together. Primidone and phenobarbital together decrease the phenytoin level much more than expected from the effect of phenobarbital alone.