Role of pharmacological factors in the evaluation of anticonvulsant drugs

Root extract of Anacyclus pyrethrum ameliorates seizures, seizure-induced oxidative stress and cognitive impairment in experimental animals

In Ayurveda, Anacyclus pyrethrum has been used as a brain tonic. The present study evaluates the effect of hydroalcoholic extract of A. pyrethrum (HEAP) root against seizures, seizure-induced oxidative stress and cognitive impairment in experimental models of seizures. Male Wistar rats were used in the study. HEAP was administered in doses of 50, 100, 250, 500 in pentylenetetrazole (PTZ) model and 250, 500 and 1000 mg/kg in maximal electroshock (MES) model. Myoclonic jerk latency and generalized tonic clonic seizures (GTCS) were noted in PTZ whereas occurrence of tonic hind limb extension (THLE) was observed in MES seizures. Cognitive deficit was assessed using elevated plus maze and passive avoidance tests. Whole brain reduced glutathione, malondialdehyde levels and cholinesterase activity were measured. HEAP showed 50, 66.7, 83.3 and 100% protection at 50,100, 250 and 500 mg/kg, respectively against GTCS in PTZ induced seizures. In MES induced seizures, HEAP produced 16.7, 33.3 and 50% protection against THLE at 250, 500 and 1000 mg/kg, respectively. HEAP administration significantly prevented seizure induced oxidative stress and cognitive impairment in a dose-dependent manner. HEAP also normalized the decrease in cholinesterase activity caused by seizures. Thus, HEAP showed protective effect against seizures, seizure-induced oxidative stress and cognitive impairment in rats.

Sex differences in sensitivity to seizures elicited by pentylenetetrazol in mice

Sex differences in sensitivity to seizures elicited by intraperitoneally injected pentylenetetrazol (PTZ) were studied in 240 (120 males and 120 females) adult Swiss mice. Animals were separated into four groups according to the dose that was injected: 40, 50, 60 and 70 mg/kg. Seizure severity was expressed by the following scoring scale: (0) no abnormal behavior; (1) myoclonus; (2) running bouncing (RB) clonus; (3) tonic hind limb extension (THE). The analyses of the dose-response curves indicated that females were more susceptible than males when the 50- and 60-mg/kg doses were used. Specifically, females often displayed RB clonus, while males frequently displayed only myoclonus or no abnormal behavior. No significant sex differences were demonstrated when either the 40- or the 70-mg/kg doses were used. These data indicate that, for a specific range of doses, sex differences in seizure susceptibility can be clearly demonstrated with the use of intraperitoneally injected PTZ. In this sense, this method could be used as a tool to investigate the role played by sexual hormones in regulating the sensitivity of the gamma-aminobutiric acid (GABA(A)) receptor complex (GRC).

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. VI. Seasonal influences on maximal electroshock and pentylenetetrazol seizure thresholds

There is strong evidence for circadian rhythmicity in certain seizure types, whereas only a few studies have addressed the possibility of seasonal rhythms in convulsive activity. In the present experiments, seizure thresholds to different types of seizures were determined twice per month over a period of 13 months in mice under controlled environmental conditions, i.e., constant photoperiod, temperature, humidity, and food. Each group of animals was used for only one experiment, and the age of the mice used per month was the same throughout the study. Furthermore, all experiments were done at the same time in the morning to avoid circadian variation. Thresholds for the following seizure types were determined: (1) tonic hindlimb seizures induced by electrical (transauricular) stimulation; (2) myoclonic seizures induced by intravenous (i.v.) infusion of pentylenetetrazol (PTZ); (3) generalized clonic seizures in response to i.v. PTZ; and (4) tonic forelimb seizures induced by PTZ. A significant seasonality was determined for myoclonic and clonic PTZ seizure thresholds with highest thresholds between February and April and lowest thresholds between July and September. No clear seasonality was seen for chemically or electrically induced tonic seizures. Determination of plasma corticosterone did not disclose any seasonal rhythm in adrenal corticosteroid production that resembled the circannual variation in myoclonic and clonic seizure thresholds. In conclusion, our experiments suggest the existence of seasonal rhythms in PTZ seizure thresholds in laboratory animals despite standardized environmental conditions. A possible explanation for the findings may be the known seasonal alteration of the geomagnetic field which, by its effect on the pineal production of melatonin, may act as a seasonal synchronizor ("zeitgeber") in animals in the absence of other synchronizing forces, such as seasonal changes in photoperiod and ambient temperature.

The role of technical, biological, and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. V. Lack of seasonal influences on amygdala kindling in rats

Previous studies have suggested that seizure models may be affected by seasonal rhythms, even under controlled environmental conditions. In the present experiments in rats with chronically implanted electrodes in the basolateral amygdala, kindling was initiated at different seasons of the year over a period of 3 years. In a total of 109 animals, the following parameters were determined: the threshold for induction of amygdaloid afterdischarges prior to kindling (pre-kindling ADT), the number of daily amygdaloid stimulations to fully kindled (stage 5) seizures, and the post-kindling ADT. No seasonal influences were found with respect to rate of kindling development and post-kindling ADT. In contrast, pre-kindling ADTs appeared to be higher in spring than in other seasons, which, however, could not be reproduced in another spring. Thus, the data do not indicate that seasonal rhythms affect the kindling model of epilepsy.

Lack of changes in seizure susceptibility during the estrous cycle in kindled rats

The threshold and pattern of focal and generalized seizures in fully kindled rats during the estrous cycle were investigated. Two groups of rats were studied; one was kindled from stimulation of the basolateral amygdala, the other from stimulation of the anterior portion of the posterior piriform cortex. Determinations of the threshold for focal afterdischarges were either carried out in the morning (between 8 and 9 a.m.) or in the afternoon (between 2 and 3 p.m.). In all experiments, stable and reproducible afterdischarge thresholds were obtained during the different stages of the estrous cycle. The only significant alteration was a reduced seizure duration during metestrus and/or the first day of diestrus in amygdala kindled rats. The data indicate that the natural changes in sex hormone levels during the estrous cycle in rats do not affect seizure susceptibility, at least in the kindling model of epilepsy.

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. IV. Protective indices

Calculation of protective or therapeutic indices is widely used in primary and secondary screening for drugs with selective anticonvulsant activity. The protective index is the median minimal 'neurotoxic' dose, TD50, divided by median effective dose, ED50. TD50s are usually determined by tests, such as the rotarod test or the chimney test, for quantification of 'minimal neurological deficit', such as motor impairment, while median effective doses are commonly determined in the maximal electroshock seizure (MES) test or the s.c. pentylenetetrazol (PTZ) seizure test in mice or rats. For antiepileptic drug development, it has been proposed previously that only compounds with an estimated protective index of at least 5 should proceed to further evaluation. However, various technical, biological and pharmacological factors can influence anticonvulsant or 'neurotoxic' potencies and thereby protective indices. In order to reevaluate the value of protective indexes in the prediction of drugs with selective anticonvulsant action, protective indices were determined for various clinically used antiepileptic drugs in standardized seizure tests, i.e. MES and s.c. PTZ tests in mice and rats, as well as in seizure threshold tests. For most drugs, similar TD50s were determined in the rotarod and chimney test. When protective indices were calculated for the different seizure models, only few drugs reached an index of 5 (some not even reaching an index of 2) in the traditional MES or s.c. PTZ tests in mice and rats. In contrast, using anticonvulsant doses determined by seizure threshold tests, the 5 primary drugs against generalized tonic-clonic seizures, i.e., carbamazepine, phenytoin, phenobarbital, primidone and valproate, had indices of more than 5 in the MES threshold model, while drugs with efficacy against absence and myoclonic seizures, i.e., valproate, ethosuximide and the benzodiazepines, had protective indices of at least 5 in the i.v. PTZ seizure threshold model. The data substantiate that valuable information can be obtained by estimation of protective indices. However, in order to minimize the possibility that an interesting new anticonvulsant compound is overlooked during primary or secondary screening, a protective index of 2 should be considered sufficient in case of traditional MES or s.c. PTZ models with fixed seizure stimulus. Alternatively, seizure threshold models could be used for calculation of protective indices in order to avoid underestimation of anticonvulsant selectivity of test compounds.

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.

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. II. Maximal electroshock seizure models

Although seizure models using electrical stimulation for the induction of generalized tonic-clonic seizures in rodents are widely employed to identify potential anticonvulsants, the important role of various technical, biological and pharmacological factors in the 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 electroshock seizure models: (1) the maximal (tonic extensor) electroshock seizure threshold (MEST) in mice; (2) the traditional maximal electroshock seizure (MES) test with supra-threshold stimulation in mice; and (3) the MES test with suprathreshold stimulation in rats. When drugs were dissolved in vehicles which did not themselves exert effects on seizure susceptibility, the most important factors which influenced drug potencies were (1) marked differences between drugs and species in terms of peak drug effect, duration of action and the formation of active metabolites; (2) differences in drug potencies calculated on the basis of administered doses compared to potency calculations based on active drug concentrations; (3) the equipment used for seizure induction; (4) marked effects of current strength on results obtained in electroshock seizure models; (5) site of application of the electrical stimulus (transcorneal vs. transauricular). In order to reduce the variability among estimates of anticonvulsant activity, the various factors delineated in this study should be rigidly controlled in experimental situations involving assay of anticonvulsant agents.

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. I. The influence of administration vehicles

Although animal models, such as electroshock seizures, pentylenetetrazol (PTZ)-induced seizures and the rotorod test, are widely employed in the search for and evaluation of new anticonvulsant drugs, the important role of diverse technical, biological and pharmacological factors in the interpretation of results obtained with these models is often not recognized. In order to delineate factors other than strain, sex, age, diet, climate, and circadian rhythms, which are generally known, a series of studies was undertaken. In the experiments described here, the influence of administration vehicles and drug formulations on bioavailability, potency and time course of anticonvulsant drugs was studied in mice. Two standard anticonvulsant drugs, primidone and carbamazepine, with poor aqueous solubility were used for these experiments, because water insolubility is a common problem in the laboratory evaluation of anticonvulsant agents. Since vehicles, especially organic solvents or detergents, may exert effects of their own, sensitive electroshock and PTZ seizure threshold tests were used for the assessment of vehicle-related actions. Of various aqueous or lipophilic vehicles tested, only glycofurol increased seizure thresholds, when concentrations exceeding 10% were administered. However, even at a concentration of 30%, the solubilizer did not exert measurable effects in the maximal electroshock seizure (MES) test in mice, but markedly potentiated the effect of primidone. In contrast, polyethylene glycol 400 (PEG 400) up to a concentration of 30% did not affect electrical or chemical seizure thresholds nor did it alter the pharmacological potency of primidone. When primidone or carbamazepine were administered as a suspension in a Tween/water vehicle, their anticonvulsant effects were considerably lower compared to injections of the same doses as a solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Flurothyl seizure thresholds in mice treated neonatally with a single injection of monosodium glutamate (MSG): evaluation of experimental parameters in flurothyl seizure testing

Monosodium glutamate (MSG) administration to neonatal rodents produces convulsions and results in numerous biochemical and behavioral deficits. These studies were undertaken to determine if neonatal administration of MSG produced permanent alterations in seizure susceptibility, since previous investigations were inconclusive. A flurothyl ether seizure screening technique was used to evaluate seizure susceptibility in adult mice that received neonatal injections of MSG (4 mg/g and 1 mg/g). MSG treatment resulted in significant reductions in whole brain weight but did not alter seizure threshold. A naloxone (5 mg/kg) challenge was also ineffective in altering the seizure thresholds of either control of MSG-treated mice. Flurothyl ether produced hypothermia which was correlated with the duration of flurothyl exposure; however, the relationship of hypothermia to seizure induction was unclear. Flurothyl seizure testing proved to be a rapid and reliable technique with which to evaluate seizure susceptibility.

Kinetics of drug action in disease states III: Effect of pregnancy on the relationship between phenytoin concentration and antiseizure activity in rats

The purposes of this investigation were to determine the effect of pregnancy on the susceptibility of female rats to experimentally induced seizures and on the relationship between serum phenytoin concentration and antiseizure activity. Pregnant rats (on the 18th day of gestation) were more susceptible than nonpregnant female rats to seizures produced by maximal electroshock or by a body-weight-based dose of pentylenetetrazol. There was no apparent difference between pregnant (20th day of gestation) and nonpregnant rats in the relationship between seizure protection (percent of animals protected) and the serum concentration of total (free plus protein-bound) phenytoin. The relationship between concentration and effect was essentially the same 20 min after an injection of phenytoin and 2 h after the start of a constant-rate infusion preceded by a loading dose of the drug. Since the protein binding of phenytoin is appreciably decreased in late pregnancy, the serum concentration of free phenytoin required for seizure protection tended to be higher in pregnant than in nonpregnant rats. This may be due to the increased susceptibility of pregnant rats to seizure stimuli.

Hippocampal kindling: corticosterone modulation of induced seizures

The effect of adrenalectomy (ADX) and corticosterone replacement was studied on seizures induced by hippocampal kindling. A complex series of changes occurred in after-discharge (AD) and behavioural depression (BD) during the immediate hours after ADX, culminating at day 1 in markedly decreased AD and BD, which returned to normal over the next several days. These changes were normalized after replacement of the ADX group with low doses of corticosterone. It is concluded that the expression and maintenance of hippocampal kindled seizures is under short-term control of corticosterone.

Hormonal influences on seizure kindling: the effects of post-stimulation ACTH or cortisone injections

Repeated application of brain stimulation can lead to a progressively augmenting electrical and behavioral response-- a phenomenon termed seizure kindling. In this experiment, stimulation was delivered once per day, and was followed by peripheral (intraperitoneal) administration of ACTH or cortisone. An intermediate or a high dose of either hormone (0.3 IU or 3.0 IU of ACTH/animal, 10 mg or 25 mg cortisone/animal) delayed the completion of kindling if administered shortly after each kindling stimulation. Lower doses (0.03 IU of ACTH or 2 mg of cortisone) had no significant effects. The high dose of ACTH or cortisone was no longer effective if administration was delayed more than 4 h after stimulation. Peripherally administered ACTH and cortisone can influence processes initiated by the brain stimulation which presumably underlie the augmentation of response to successive stimulations. This time-limited action is analogous to the effects of these hormones on memory consolidation.

Anticonvulsant activity of cyclopentano amino acids

The hypothesis that certain amino acid analogues possessing a five-membered ring structure or amino acid analogues that can be viewed as fragments derived from such a ring would have anticonvulsant activity was proposed and tested. The compounds 1-aminocyclopentane carboxylic acid, 1-amino-3-methylcyclopentane carboxylic acid, 3-aminotetrahydrothiophene carboxylic acid, and alpha-aminoisobutyric acid were found to protect rats against seizures in the maximal electroshock test but offered no protection against metrazol- (pentylenetetrazol) induced seizures in mice. The structural feature of this class of anticonvulsants that allows for hydrophobic interactions at the receptor site is considered to be a major physical factor necessary in promoting the activity of this class of anticonvulsants.

Influences of pituitary-adrenal hormones on kindling

The kindling process represents a progressively augmenting electrical and behavioral response to brain stimulation, that has been proposed as a model for the development of epileptogenesis. The first experiment examined the effects of hypophysectomy on the kindling process. The effects of hypophysectomy were found to be dependent on the number of days between hypophysectomy and the beginning of kindling. The kindling rate was slowed, compared to intact controls, when hypophysectomy was performed less than 2 weeks prior to the first kindling stimulation. However, if more than 4 weeks and passed since hypophysectomy, the kindling rate was faster than that of intact controls. The second experiment tested the influence of systemic administration of cortisone (10 mg/animal), upon kindling of control and hypophysectomized animals. Cortisone was found to reverse the kindling deficit of hypophysectomized animals at less than 2 weeks and to slow kindling of intact controls, but to have no effect on hypophysectomized animals at more than 4 weeks (at this dose). These results are interpreted as indicating that pituitary-adrenal hormones have profound influences on the kindling process, but it is unclear whether the influence is upon brain excitability or more directly upon modification of synaptic function.

Identification of urinary catechol and methylated catechol metabolites of phenytoin in humans, monkeys, and dogs by GLC and GLC-mass spectrometry

A catechol metabolite, 5-(3,4-dihydroxyphenyl)-5-phenylhydantoin, and a methylated catechol metabolite, 5-(3-methoxy-4-hydroxyphenyl)-5-phenylhydantoin, were identified as urinary metabolites in humans, monkeys, and dogs following the administration of phenytoin. These metabolites were separated from each other and from other known metabolites of phenytoin as n-butyl derivatives by GLC and positively identified by combined GLC-mass spectrometry.

Ontogenetic studies of seizure patterns and seizure activities induced by cortical focus

Ontogenetic studies of epileptogenic process were carried out in albino rats ranging in age from birth to 45 days. Experimental epilepsy was produced by two different procedures and the results were compared with each other. Tungstic acid gel was applied to the motor area of the left side of the cortex, and the following results were obtained. The latency of the seizure appearance was long during 10 days after birth, became progressively short thereafter and reached the minimum in about 20 days of age, and gradually returned to the adult level again by 45 days of age. No abvvious seizure was exhibited until five days of age. Seizure patterns developed from tonic or twitch-like jerky convulsion (10 days old) to rhythmic or clonic type of seizure (13 days old), and the seizure patterns similar to those in the adult rat were observed in about 20 days of age. Cortical seizure activity was initially observed in about 10-day-old rats; single high amplitude slow wave appeared and small spikes became superimposed on it in the course of maturation. Atypical spike and wave complexes were observed after 20 days of age. Electrical stimulation was applied to the left cortical motor area by constant current stimulator, and the following seizure patterns were observed: No obvious seizure could be elicited in newborn rat, whereas from three days of age, tonic seizure of the whole body, and from seven days old twitch-like convulsion of extremities were observed. In ages from 10 to 20 days, seizure induced by electrical stimulation was mainly tonic in pattern; extension of forelimbs and flexion of hindlimbs in most cases were observed before 13 days old, but both fore-and hindlimbs were extended therafter. Tonic-clonic seizure patterns were exhibited after 20 days of age. From these results, it was considered that tonic convulsions and high voltage slow cortical seizure activites were produced from the activites of the local cortical neuronal connections, and rhythmic and/or clonic seizure patterns and spike and wave seizure activities were elicited from the more complex, i.e. cortico-subcortical neuronal circuits. Possible contributing factors for the determination of seizure susceptibility in immautre rats were also discussed.

Effects of acute cerebellectomy on maximal electroshock seizures and anticonvulsant efficacy of diazepam in the rat

Rats were cerebellectomized 72-96 hr prior to evaluation (1) during maximum electroshock seizures and (2) for their capacity to respond to pentylenetetrazol-induced clonic seizures. Cerebellectomized rats failed to exhibit tonic hindlimb extension, an endpoint characteristic of maximal electroshock seizures. The dose of pentylenetetrazol required to produce clonic seizures or death was not different in cerebellectomized and sham-operated controls. The anticonvulsant efficacy of diazepam, when assessed as a pentylenetetrazol antagonist, was not influenced by removal of the cerebellum. These data indicate that whereas cerebellar influences may suppress seizure activity which is largely focal, seizures of more diffuse origin are not markedly influenced by cerebellar activity. It is, therefore, essential that the role of the cerebellum in suppressing seizures be characterized for each kind of experimentally induced seizure process.

Bioavailability of phenytoin. A comparison of two preparations

Plasma phenytoin levels were measured in 60 patients under steady-state conditions for a period of six weeks. During the trial, the preparation of phenytoin was changed from Phenytoin BP (Regent) to Epanutin Infatabs. A significant increase in plasma phenytoin levels following the change of tablet was matched by a decrease in the number of seizures.