Effect of Tadalafil on Seizure Threshold and Activity of Antiepileptic Drugs in Three Acute Seizure Tests in Mice

Tadalafil, a selective phosphodiesterase type 5 inhibitor, is a long-acting oral agent for the treatment of erectile dysfunction of multiple etiologies. Although generalized tonic-clonic seizures were reported in a healthy man after taking tadalafil, the influence of tadalafil on seizure susceptibility has not been studied so far. Therefore, the aim of the present study was to investigate the effect of tadalafil on seizure threshold as well as on the activity of some first- and second-generation antiepileptic drugs in three acute seizure tests in mice. The obtained results showed that tadalafil, at the highest dose tested (20 mg/kg), significantly decreased the threshold for the first myoclonic twitch in the intravenous pentylenetetrazole (i.v. PTZ) seizure test. It did not affect the threshold for generalized clonic seizure and forelimb tonus in the i.v. PTZ, for tonic hindlimb extension in the maximal electroshock seizure threshold test, and for psychomotor seizure in the 6-Hz-induced seizure threshold test. Tadalafil did not alter the anticonvulsant activity of any of the studied antiepileptic drugs in electrically induced seizure tests. Interestingly, tadalafil potentiated the anticonvulsant activity of clonazepam and decreased the anticonvulsant activity of oxcarbazepine in the i.v. PTZ test. These interactions were pharmacodynamic in nature, as tadalafil did not alter clonazepam and oxcarbazepine concentrations both in serum and brain tissue. Furthermore, neither tadalafil alone nor its combinations with the studied antiepileptic drugs produced any significant impairment of motor coordination (assessed in the chimney test), muscular strength (investigated in the grip-strength test), and long-term memory (assessed in the passive avoidance task). In conclusion, tadalafil may increase the risk of myoclonic seizure and decrease the anticonvulsant efficacy of oxcarbazepine. Further studies are warranted to evaluate the safety of tadalafil usage in patients with epilepsy.     

Ralitoline: a reevaluation of anticonvulsant profile and determination of "active" plasma concentrations in comparison with prototype antiepileptic drugs in mice

Ralitoline (RLT) is a new thiazolidinone derivative with potent anticonvulsant activity in different seizure models. During Phase I studies, RLT was well tolerated in human volunteers and showed linear pharmacokinetics in the dose range tested (up to 150 mg). Since RLT will soon be entering clinical Phase II studies, we were interested in obtaining predictive data for effective plasma concentrations in patients. For this purpose, the anticonvulsant potency of RLT was determined in four seizure models in mice, and plasma levels were measured at time of peak drug effect. The four models were the threshold for maximal (tonic extension) electroshock seizures (MES), the threshold for clonic seizures determined by i.v. infusion of pentylenetetrazol (PTZ), the traditional MES test with supramaximal (50 mA) stimulation, and generalized clonic seizures induced by s.c. administration of PTZ. Furthermore, median minimal "neurotoxic" doses (TD50s) were determined by the rotorod and chimney test for calculation of protective indices. All data obtained for RLT were compared with data obtained with standard antiepileptic drugs: phenobarbital, phenytoin, valproate, and diazepam. The onset of anticonvulsant action after i.p. injection of RLT was very rapid, and the peak drug effect was already obtained after 2 min. In the MES models, RLT was the most potent compound. "Active" plasma levels ranged from approximately 300 ng/ml in the MES threshold test to approximately 1,300 ng/ml in the MES test. RLT was also capable of increasing the PTZ threshold, whereas, possibly because of its short duration of action in mice, it was not very active in the s.c. PTZ seizure test.(ABSTRACT TRUNCATED AT 250 WORDS)     

Parenteral magnesium sulfate fails to control electroshock and pentylenetetrazol seizures in mice

Magnesium sulfate has been used as an anticonvulsant in the treatment of eclampsia, but efficacy of magnesium in other types of seizure disorders is poorly documented. We examined the effects of magnesium sulfate (MgSO4) on seizures produced in mice by maximal electroshock (MES) and pentylenetetrazol (PTZ), MgSO4 injection (6.7 mEq/kg i.p.) caused weakness in all animals. With suprathreshold electroshock, 10/10 controls and 11/12 treated animals had seizures with tonic hind limb extension (P = NS). Electroshock threshold was unaltered by magnesium treatment (n = 48; P = 0.47). PTZ induced clonic seizures in 12/12 controls and 5/14 treated animals (P less than 0.05). This difference was likely due to muscular weakness because frequency of EEG spikes was the same in PTZ and PTZ + MgSO4 groups. Mean serum magnesium levels were 2.3 +/- 0.3 mEq/l in animals not given MgSO4; 10.9 +/- 1.4 mEq/l and 12.8 +/- 2.2 mEq/l in treated animals with and without seizures (P = NS). We conclude that magnesium sulfate had no significant anticonvulsant activity in mouse MES and PTZ models for epilepsy. The relevance of these findings to the possible efficacy of magnesium sulfate in eclamptic seizures and other types of epilepsy remains to be determined.     

Involvement of the nitric oxide pathway in the anticonvulsant effect of tramadol on pentylenetetrazole-induced seizures in mice

In the present study, the effects of tramadol on pentylenetetrazole (PTZ)-induced seizures and involvement of nitric oxide (NO) were assessed in mice. To determine the threshold for clonic seizures, PTZ was administered intravenously. Tramadol was administered intraperitoneally (0.5-50mg/kg) 30 minutes prior to induction of seizures. The effects of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 0.5, 1, 5, and 10mg/kg), the nitric oxide precursor L-arginine (10, 30, and 60 mg/kg), and the nonspecific opioid receptor antagonist naloxone (0.1, 0.5, 1, and 5mg/kg) on the anticonvulsant effect of tramadol were investigated. Administration of tramadol (1mg/kg) increased the threshold for seizures induced with PTZ in a monophasic, dose-independent, and time-dependent manner. Acute administration of L-NAME (5 and 10mg/kg) inhibited the anticonvulsant effect of tramadol (1mg/kg), whereas L-arginine, in the noneffective dose range (30 and 60 mg/kg), potentiated the seizure threshold when co-administered with a subeffective dose of tramadol (0.5mg/kg). Naloxone partially and dose-independently antagonized the anticonvulsant effect of tramadol (1mg/kg). These results indicate that the anticonvulsant effect of tramadol is mediated by the nitric oxide pathway and also by classic opioid receptors.     

Timed pentylenetetrazol infusion test: a comparative analysis with s.c.PTZ and MES models of anticonvulsant screening in mice.

The intravenous pentylenetetrazol (i.v.PTZ) seizure test provides threshold dose for induction of seizures in individual animals. In the present study, the i.v. and s.c.PTZ seizure models in mice were compared for seizure pattern, intra- and interanimal variability. Anticonvulsant activities of several antiepileptic drugs (AEDs) at non-ataxic dose levels were evaluated in the PTZ and maximal electroshock (MES) seizure tests. In the i.v.PTZ test, at 0.5 ml/min rate of administration, the mean threshold PTZ doses for induction of clonus and tonic extensor were 44.17 and 99.59 mg/kg, respectively. The intra- and interanimal variabilities in the seizure response were low in the i.v.PTZ as compared to the s.c.PTZ model. Phenobarbital sodium, ethosuximide, sodium valproate, diazepam, tiagabine, oxcarbazepine and zonisamide enhanced threshold or onset latency for clonus in the i.v. and s.c.PTZ tests, respectively. Levetiracetam and pregabalin were active in the i.v.PTZ test, but had no effect in the s.c.PTZ test. Ability of AEDs to protect from tonic extensor was compared in the MES and i.v.PTZ tests. For this effect, phenobarbital sodium, phenytoin, carbamazepine, sodium valproate, gabapentin, oxcarbazepine, zonisamide and pregabalin were effective in the i.v.PTZ and MES tests. Ethosuximide, diazepam and levetiracetam were effective in the i.v.PTZ test, but not the MES test. On the contrary, lamotrigine and topiramate were active in the MES, but not the i.v.PTZ test. These results indicate that it is advantageous to use i.v.PTZ test as an acute seizure model for screening of antiepileptic drugs. This model can identify molecules with varied mechanism of action and broad clinical utility in the treatment of epilepsy.     

Avian brainstem neurogenesis is stimulated during cochlear hair cell regeneration

Cyclosporin A (CsA) is known to decrease nitric oxide (NO) release in the nervous system. The present study was aimed at investigating the effects of acute administration of CsA on pentylenetetrazole (PTZ)-induced seizure threshold and latency and probable modulation of these effects by NO synthesis substrate L-arginine, and NO synthesis inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) or aminoguanidine. Moreover, the effect of CsA per se or concomitant with L-arginine on the development of PTZ-induced kindling was assessed. CsA (0.05, 1, 5, 10 and 20 mg/kg, s.c.) dose-dependently increased PTZ-induced clonic seizure threshold and the latency for onset of myoclonic jerks, clonic seizures and clonic-tonic generalized seizures following PTZ administration. L-NAME (10 and 30 mg/kg, i.p.) but not aminoguanidine (50 and 100 mg/kg, i.p.) potentiated the anticonvulsant effects of CsA (1 and 10 mg/kg). L-arginine (60, 100 and 200 mg/kg, i.p.) inhibited the anticonvulsant effects of CsA (20 mg/kg) in a dose-related manner. The inhibitory effect of L-arginine on CsA-induced alterations of seizure threshold and latency was blocked by L-NAME but not with aminoguanidine. CsA (20 mg/kg) significantly inhibited the development of PTZ kindling and decreased the seizure intensity as tested by a challenge dose of PTZ. Pretreatment with L-arginine (60 mg/kg) reversed the inhibitory effects of CsA on kindling development. It was concluded that CsA exerts some anticonvulsant properties that may be due to its inhibition of nitric oxide synthesis.     

Striking differences in proconvulsant-induced alterations of seizure threshold in two rat models

During drug development, seizure threshold tests are widely used to identify potential proconvulsant activity of investigational drugs. The most commonly used tests in this respect are the timed intravenous pentylenetetrazole (PTZ) infusion seizure test and the maximal electroshock seizure threshold (MEST) test in mice or rats. To our knowledge, no study is available in which proconvulsant drug activities in these models are directly compared, which prompted us to perform such experiments in male Wistar rats. Five drugs with reported proconvulsant activity were tested in the two models: d-amphetamine, chlorpromazine, caffeine, theophylline, and tramadol. Furthermore, the anticonvulsant drug phenobarbital was included in the experiments. While phenobarbital exerted anticonvulsant activity in both models, the five proconvulsant drugs markedly differed in their effects. In the dose range tested, d-amphetamine significantly lowered the PTZ seizure threshold but increased the MEST, caffeine and theophylline did not alter the PTZ seizure threshold but decreased the MEST, and tramadol reduced the PTZ threshold but increased the MEST. These marked differences between seizure threshold tests are most likely a consequence of the mechanisms underlying seizure induction in these tests. Our data indicate that using only one seizure threshold model during preclinical drug development may pose the risk that potential proconvulsant activity of an investigational drug is overseen. However, the label "proconvulsant" may be misleading if such activity only occurs at doses high above the therapeutic range, but the drug is not proconvulsant or even exerts anticonvulsant effects at lower, therapeutically relevant doses.     

The involvement of endogenous opioids and nitricoxidergic pathway in the anticonvulsant effects of foot-shock stress in mice

The involvement of endogenous opioids and nitric oxide (NO) in the anticonvulsant effects of stress against pentylenetetrazole (PTZ)- or electroconvulsive shock-induced seizures was assessed in mice. The prolonged and intermittent foot-shock stress, which induced opioid-mediated analgesia, had significant protective effects against both seizure types which was reversible by naloxone (0.3, 1 or 2 mg/kg), while brief and continuous foot-shock did not alter the seizure susceptibility. Pre-treatment with non-specific nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 1, 2, 5, 10 or 30 mg/kg), but not with specific inducible NOS (iNOS) inhibitor, aminoguanidine (50 or 100 mg/kg), blocked the stress-induced anticonvulsant effects. The lower doses of naloxone (0.3 mg/kg) and L-NAME (2 mg/kg) showed additive effects in blocking the stress-induced anticonvulsant properties. L-arginine at a per se non-effective dose of 20 mg/kg potentiated the stress-induced anticonvulsant properties, an effect which was inhibited by L-NAME but not by aminoguanidine. Furthermore, a low dose of morphine (0.5 mg/kg) showed potentiation with stress in increasing PTZ seizure threshold. This potentiation was reversed by either naloxone or L-NAME at low doses but not by aminoguanidine. Taken together, these results show that NO synthesis, through constitutive but not iNOS, is involved in opioid-dependent stress-induced anticonvulsant effects against electrical and PTZ-induced convulsions.     

The role of nitric oxide in anticonvulsant and proconvulsant effects of morphine in mice

Acute subcutaneous administration of lower doses of morphine (0.5, 1 and 3 mg/kg) increase the threshold of seizures induced by pentylenetetrazole (PTZ) in mice, whereas higher doses of morphine (15, 30 and 60 mg/kg) have proconvulsant effects. The effect of systemic administration of nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-nitro-L-arginine (L-NNA) and nitric oxide synthase (NOS) L-arginine on biphasic effect of morphine was investigated. Acute administration of both L-NAME (1, 3 and 10 mg/kg) and L-NNA (1 and 10 mg/kg) as well as chronic pretreatment with L-NAME (1 and 10 mg/kg, 4 days) dose-dependently inhibited both the anticonvulsant and proconvulsant effects of morphine (1 and 30 mg/kg, respectively). The inhibition was complete for anticonvulsant effect while partial for proconvulsant effect. L-arginine at doses that did not affect seizure threshold per se (acute, 30 and 60 mg/kg; chronic, 60 mg/kg) potentiated both anticonvulsant and proconvulsant properties of less potent doses of morphine (0.5 and 15 mg/kg, respectively). The L-arginine induced potentiation of both phases of morphine effect was blocked by L-NAME (0.5-30 mg/kg). Moreover, low and per se non-effective doses of naloxone (0.1 mg/kg) and L-NAME (0.3, 0.5 or 1 mg/kg) showed additive effects in inhibiting both phases of morphine effects. These results support the involvement of L-arginine/nitric oxide pathway in the modulation of seizure threshold by morphine.     

Neurosteroid Withdrawal Model of Perimenstrual Catamenial Epilepsy

PURPOSE: Perimenstrual catamenial epilepsy, the increase in seizure frequency that some women with epilepsy experience near the time of menstruation, may in part be related to withdrawal of the progesterone metabolite allopregnanolone, an endogenous anticonvulsant neurosteroid that is a potent positive allosteric gamma-aminobutyric acidA (GABA(A)) receptor modulator. The objective of this study was to develop an animal model of perimenstrual catamenial epilepsy for use in evaluating drug-treatment strategies. METHODS: A state of prolonged high serum progesterone (pseudopregnancy) was induced in 26-day-old female rats by sequential injection of pregnant mares' serum gonadotropin and human chorionic gonadotropin. Neurosteroid withdrawal was induced by treatment with finasteride (100 mg/kg, i.p.), a 5alpha-reductase inhibitor that blocks the conversion of progesterone to allopregnanolone. Plasma progesterone and allopregnanolone levels were measured by gas chromatography/electron capture negative chemical ionization mass spectrometry. Seizure susceptibility was evaluated with the convulsant pentylenetetrazol (PTZ). RESULTS: Plasma allopregnanolone levels were markedly increased during pseudopregnancy (peak level, 55.1 vs. control diestrous level, 9.3 ng/mL) and were reduced by 86% 24 h after finasteride treatment (6.4 ng/mL). Progesterone levels were unaffected by finasteride. After finasteride-induced withdrawal, rats showed increased susceptibility to PTZ seizures. There was a significant increase in the number of animals exhibiting clonic seizures when challenged with subcutaneous PTZ (60 mg/kg) compared with control pseudopregnant animals not undergoing withdrawal and nonpseudopregnant diestrous females. The CD50 (50% convulsant dose) was 46 mg/kg, compared with 73 mg/kg in nonwithdrawn pseudopregnant animals and 60 mg/kg in diestrous controls. The threshold doses for induction of various seizure signs, measured by constant intravenous infusion of PTZ, were reduced by 30-35% in neurosteroid-withdrawing animals compared with control diestrous females. No change in threshold was observed in pseudopregnant rats treated from days 7 to 11 with finasteride, demonstrating that high levels of progesterone alone do not alter seizure reactivity. CONCLUSIONS: Neurosteroid withdrawal in pseudopregnant rats results in enhanced seizure susceptibility, providing an animal model of perimenstrual catamenial epilepsy that can be used for the evaluation of new therapeutic approaches.     

Repeated anticonvulsant testing: contingent tolerance to diazepam and clobazam in kindled rats.

The acute anticonvulsant efficacy of diazepam (1.5 mg/kg, i.p.) was evaluated by repeated test injection in kindled rats subcutaneously implanted with diazepam-filled or empty silastic tubes for 3 weeks. Tolerance developed to acute test injections in both diazepam- and sham-implanted rats. Tolerance developed to a lesser extent in another group of diazepam-implanted rats which did not receive acute intermittent anticonvulsant tests. The hypothesis that contingent tolerance had developed to the anticonvulsant actions of benzodiazepines (diazepam, 1.5 mg/kg, i.p. and clobazam, 10 mg/kg, i.p.) in kindled rats given acute intermittent injections was investigated using a 'before-after' design. Significant contingent tolerance developed in rats which received intermittent benzodiazepine treatment before, but not after, amygdala stimulation. Tolerance developed to different extents depending on the seizure measure evaluated (forelimb clonus duration, amygdala afterdischarge duration, motor seizure latency and duration, and seizure stage). Contingent tolerance to both benzodiazepines developed at a similar rate. The findings suggest that contingent tolerance may contribute a sizeable component to the overall functional benzodiazepine tolerance measured in long-term anticonvulsant drug studies in kindled rats. Several questions regarding contingent tolerance phenomena are posed and the implications of these findings for studies using repeated anticonvulsant testing are discussed.     

The interaction between morphine and propranolol in chemical and electrical seizure models of mice

Objectives: Morphine and propranolol have different effects on seizure. Several studies have shown interaction between adrenergic and opioid systems in different models. In this study, interaction between morphine and propranolol in different seizure models was examined in mice.

Methods: In this study, three seizure models, including intravenous pentylenetetrazole (PTZ), intraperitoneal PTZ and electroshock, were examined in mice. Animals were injected with different doses of morphine or propranolol in the 60th and 45th min, before seizure induction, respectively.

Results: Acute administration of propranolol or lower doses of morphine induced an anticonvulsant effect in intravenous PTZ, intraperitoneal PTZ and electroshock-induced seizure models; on the contrary, higher doses of morphine exert proconvulsant effects in all three models. Also additive anticonvulsant effect of propranolol and lower doses of morphine was observed in all examined models. The additive anti-seizure effect of propranolol and lower doses of morphine was blocked by naltrexone in intraperitoneal PTZ model. Moreover, the anticonvulsant effect of propranolol was inhibited by naltrexone in intraperitoneal PTZ seizure model of mice. Propranolol restrained the proconvulsant effects in higher doses of morphine in clonic seizures of intravenous and intraperitoneal PTZ models.

Discussion: In conclusion, we believe that this is the first study that has indicated the interaction of propranolol and lower doses of morphine in the anticonvulsant effects in three seizure models of intravenous PTZ, intraperitoneal PTZ and electroshock. The involvement of μ-opioid receptor in this interaction was also demonstrated. Simultaneously, we showed the interaction between propranolol and higher doses of morphine in proconvulsant effects.     

Anatomically dependent anticonvulsant properties of temporally-coded electrical stimulation

In the PTZ animal model of epilepsy, electrical stimulation applied to the amygdaloid complex may result in either pro-convulsive or anticonvulsant effect, depending on the temporal pattern used (i.e. periodic-PS and non-periodic-NPS electrical stimulation). Our hypothesis is that the anatomical target is a determinant factor for the differential effect of temporally-coded patterns on seizure outcome. The threshold dose of PTZ to elicit forelimb clonus and generalized tonic-clonic seizure behavior was measured. The effect of amygdaloid complex PS on forelimb clonus threshold showed a pro-convulsive effect while NPS was anticonvulsant. NPS also significantly increased generalized tonic-clonic threshold; while PS, although at lower threshold levels, did not present statistical significance. Thalamus stimulation did not affect forelimb clonus threshold and showed similar anticonvulsant profiles for both PS and NPS on generalized tonic-clonic threshold. In summary, the anatomical target is a determinant factor on whether temporally-coded ES differentially modulates seizure outcome.     

Selectivity of the protective effects of dihydropyridine calcium channel antagonists against the ethanol withdrawal syndrome

Four dihydropyridine calcium channel antagonists were compared for their ability to protect against the hyperexcitability produced in mice by withdrawal from chronic ethanol treatment and to protect against seizures due to bicuculline or pentylenetetrazol. Comparison was also made of their effects on locomotor activity, body temperature and motor co-ordination, and with the corresponding effects of the benzodiazepine, diazepam. Nitrendipine, nimodipine, nicardipine (at 50 and 10 mg/kg) and isradipine (at 10 and 4 mg/kg) decreased the withdrawal hyperexcitability, but showed no anticonvulsant action against either bicuculline or pentylenetetrazol. Diazepam (1.5 and 4 mg/kg) both protected against the withdrawal signs and decreased seizure incidence after bicuculline and pentylenetetrazol, although the latter effects were of shorter duration than those on the withdrawal signs. The four dihydropyridines decreased spontaneous locomotor activity, an effect which lasted up to 6 h. Only isradipine and diazepam had any ataxic actions at the doses tested. All the dihydropyridines had hypothermic actions, considerably shorter in duration than effects on withdrawal hyperexcitability, with little evidence of dose dependence, except for nicardipine, which had a larger, dose-related, hypothermic action. Of the four compounds, isradipine was more potent in terms of dose, but not any more selective for effectiveness against the withdrawal signs, than the other three dihydropyridines, and nicardipine was slightly less effective in protecting against the withdrawal signs. The results indicate that the anticonvulsant effects of the dihydropyridines were selective for ethanol withdrawal hyperexcitability, whereas diazepam showed no such selectivity.     

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)     

Anticonvulsant effect of flutamide on seizures induced by pentylenetetrazole: involvement of benzodiazepine receptors

PURPOSE: There is some structural similarity between the androgen receptor antagonist, flutamide (Flut) and benzodiazepines (BZDs). We evaluated the possible anticonvulsant effect and interaction of Flut with BZD receptors in common seizure models. METHODS: (a) Different groups of mice each were pretreated i.p. with Flut, and after 0.5 h, they received chemoconvulsants [pentylenetetrazole (PTZ), bicuculline, aminophylline, strychnine or kainic acid]. Latency and incidence of a clonic seizure were recorded. (b) Mice were pretreated i.p. with Flut, and after 0.5 h, transauricular electroshock was applied. Occurrence of a tonic seizure was observed. (c) Amygdala-kindled rats were pretreated i.p. with Flut, and 0.5, 1, or 2 h later, they were stimulated at afterdischarge threshold. Then the seizure parameters (afterdischarge duration, seizure severity, and stage 5 duration) were recorded. (d) The effect of Flut on clonic seizure threshold was determined by i.v. infusion of bicuculline or PTZ to different groups of Flut-receiving mice. To determine the possible interaction of Flut with BZD receptors, the flumazenil (FMZ)+Flut effect on clonic seizure threshold was compared with the effect of Flut. (e) Neurotoxicity of Flut was evaluated by rotarod test at 30 min after administration. RESULTS: Flut produced a dose-dependent anticonvulsant effect against PTZ-induced seizures [median effective dose (ED50), 67.0 mg/kg]. Moreover, Flut elevated the clonic seizure threshold induced by bicuculline or PTZ. FMZ reversed the effect of Flut on the threshold of PTZ seizures. A median toxic dose (TD50) value of 124.8 mg/kg was obtained for Flut. CONCLUSIONS: Flut both blocks PTZ-induced clonic seizures and elevates the threshold of PTZ or bicuculline-induced clonic seizures, through interaction with BZD receptors.     

Mechanisms of Tolerance to the Anticonvulsant Effects of Acetazolamide in Mice: Relation to the Activity and Amount of Carbonic Anhydrase in Brain

The mechanism by which tolerance develops to the anticonvulsant effects of acetazolamide (AZM) was investigated in Swiss-Webster mice. The effects of single and six daily doses of 40 mg or 200 mg/kg AZM on electroshock seizure threshold (EST), maximal electroshock (MES) seizure pattern, and on the activity and total amount of carbonic anhydrase II (CAII) in various subcellular fractions (cytosol, microsomes, and myelin) of cerebral cortex, cerebellum, and brainstem were assessed. The activity of CAII was measured by microassay, and the total amount was measured by immunoassay methods developed in this laboratory. From the activity (units per microgram of protein) and total amount (nanograms per microgram protein) data, the specific activity (units per nanogram CAII) of the enzyme was calculated. With multiple doses, tolerance developed to both elevation of the EST and modification of the MES pattern noted with single doses of AZM. Accompanying the development of tolerance to the anticonvulsant effects of AZM was an increase in both the activity and specific activity of CAII in the various subcellular fractions and areas of the brain. The effects were dose dependent. Tolerance to the EST-elevating effects of AZM correlated with increases in the activity, total amount, and specific activity of CAII in the myelin fraction of the cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of nitric oxide in the anticonvulsive effect of progesterone

Described here is an investigation of the potential interaction of the nitric oxide signaling pathway with the anticonvulsant effects of progesterone. In ovariectomized Swiss mice, the threshold for seizures induced by intravenous infusion of pentylenetetrazole was determined after treatment with progesterone (25, 50, or 75 mg/kg, given subcutaneously 6h before seizure testing) or vehicle. Progesterone induced significant anticonvulsive activity at moderate (50 mg/kg) and high (75 mg/kg) doses. This effect of progesterone was abolished by the NO precursor compound L-arginine (200 mg/kg). Moreover, when subeffective doses of progesterone (25 mg/kg) and the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (10 mg/kg) were injected, a strong anticonvulsant effect was observed. These findings suggest a potential role for NO signaling as an anticonvulsant target in females.     

Anticonvulsant efficacy of the low-affinity partial benzodiazepine receptor agonist ELB 138 in a dog seizure model and in epileptic dogs with spontaneously recurrent seizures

PURPOSE: Ataxia, sedation, amnesia, ethanol and barbiturate potentiation, loss of efficacy (tolerance), development of dependence, and the potential for drug abuse limit the clinical use of benzodiazepines (BZDs) for long-term treatment of epilepsy or anxiety. BZD ligands that are in current use act as full allosteric modulators of gamma-aminobutyric acid (GABA)-gated chloride channels and, on long-term administration, trigger a functional uncoupling between the GABAA and BZD recognition sites. Partial allosteric modulators, which have a low intrinsic activity at the BZD recognition site of the GABAA receptor, might eventually overcome the limitations of full agonists such as diazepam (DZP). METHODS: In the present study, the new low-affinity partial BZD-receptor agonist ELB 138 [former name AWD 131-138; 1-(4-chlorophenyl)-4-morpholino-imidazolin-2-one] was evaluated in a dog seizure model and in epileptic dogs with spontaneously recurrent seizures. RESULTS: ELB 138 was shown to increase potently the pentylenetetrazole (PTZ) seizure threshold in dogs. Prolonged oral administration with twice-daily dosing of ELB 138 with either 5 or 40 mg/kg over a 5-week period was not associated with loss of anticonvulsant efficacy in the PTZ dog model. To study whether physical dependence developed during long-term treatment, the BZD antagonist flumazenil was injected after 5 weeks of treatment with ELB 138. Compared with prolonged treatment with DZP, only relatively mild abstinence symptoms were precipitated in dogs treated with ELB 138, particularly at the lower dosage (5 mg/kg, b.i.d.). In a prospective trial in dogs with newly diagnosed epilepsy, ELB 138 markedly reduced seizure frequency and severity without significant difference to standard treatments (phenobarbital or primidone) but was much better tolerated than the standard drugs. In dogs with chronic epilepsy, most dogs exhibited a reduction in seizure frequency and severity during add-on treatment with ELB 138. CONCLUSIONS: The data demonstrate that the partial BZD receptor agonist ELB 138 exerts significant anticonvulsant efficacy without tolerance in a dog seizure model as well as in epileptic dogs with spontaneously recurrent seizures. These data thus substantiate that partial agonism at the BZD site of GABAA receptors offers advantages versus full agonism and constitutes a valuable approach for treatment of seizures.     

Evidence of the antiepileptic potential of amiloride with neuropharmacological benefits in rodent models of epilepsy and behavior

Sodium-hydrogen exchangers (NHEs) in the brain play a key role in regulating neuronal pH and, hence, modulate bioelectric and seizure activity. In this study, we investigated the anticonvulsant effect of amiloride (a NHE inhibitor) on increasing current electroshock (ICES) and pentylenetetrazole (PTZ)-induced seizures in mice. Further, the effect of amiloride on mood, memory, grip strength, and rotarod performance was also evaluated. The forced swimming test (FST) and spontaneous alternation behavior (SAB) models were employed to assess the effects on mood and memory, respectively. Amiloride produced a dose-dependent increase in seizure threshold in both rodent models of epilepsy. It was observed that amiloride reduced behavioral depression in the FST in mice. In addition, it resulted in memory improvement in the SAB model. Amiloride did not affect grip strength and rotarod performance, suggesting it is devoid of behavioral impairment. The results indicate the potential antiseizure activity of amiloride along with additional neurological advantages.