Aroyl(aminoacyl)pyrroles, a new class of anticonvulsant agents

2-Aroyl-4-(omega-aminoacyl)- (4) and 4-aroyl-2-(omega-aminoacyl)pyrroles (9) represent a new, structurally novel class of anticonvulsant agents. Compounds of type 4 were prepared by Friedel-Crafts acylation of a 2-aroylpyrrole with an omega-chloroacyl chloride followed by displacement of the chloro group by a primary or secondary amine. Compounds of type 9 were prepared by Friedel-Crafts aroylation of a 2-(omega-chloroacyl)pyrrole followed by displacement by an amine. These compounds were active in the mouse and rat maximal electroshock tests but not in the mouse metrazole test. The lead compound, RWJ-37868, 2-(4-chlorobenzoyl)-4-(1-piperidinyl-acetyl)-1,3,5-trimethylpyrrole++ + (4d), showed potency and therapeutic index comparable to those of phenytoin and carbamazepine and greater than those of sodium valproate. This compound blocked bicuculline induced seizures, did not elevate seizure threshold following iv infusion of metrazole, and blocked influx of Ca2+ ions into cerebellar granule cells induced by K+ or veratridine.

Analysis of anticonvulsant and neurotoxic responses to combination therapy with carbamazepine, felbamate and phenytoin by response-surface modeling

Various combinations of carbamazepine (CAS 298-46-4), felbamate (CAS 25451-15-4), and phenytoin (CAS 57-41-0) were evaluated in mice (i.p.) for anticonvulsant activity (maximal electroshock seizure test) and minimal neurotoxicity (rotarod test). The results obtained from these studies were analyzed using response surface methodologies (RSM). The outcomes of these analyses in regard to anticonvulsant activity suggest that, under these experimental study conditions, at 0.5 h post treatment there is a significant carbamazepine/phenytoin synergism even though none of the drugs has a significant dose-response by that time when given alone, and that at 1.0 h post treatment, the combination dose-response is additive. Thus, there appears to be an important dose/time relationship. In regard to the neurotoxic response, the results suggest a significant carbamazepine/phenytoin synergism at 0.25 h post treatment and an additive neurotoxic effect due to the combination of felbamate/carbamazepine/phenytoin at 0.5, 1.0 and 2.0 h post exposure.

Anticonvulsant activity of the gamma-aminobutyric acid uptake inhibitor N-4,4-diphenyl-3-butenyl-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol

The N-4,4-diphenyl-3-butenyl derivative of the glial selective gamma-aminobutyric acid (GABA) uptake inhibitor 4,5,6,7-tetrahydroisoxazolo [4,5-c]pyridin-3-ol (N-DPB-THPO), was tested for its ability to block sound-induced seizures in the audiogenic seizure-susceptible Frings mouse model of epilepsy. Following intracerebroventricular (i.c.v.) administration, N-DPB-THPO blocked tonic hindlimb extension in a dose- and time-dependent manner. At the doses tested no gross behavioral effects were noted.

Characterization of the anticonvulsant properties of F-721

The anticonvulsant properties of F-721 (3-diethylamino-2,2-dimethylpropyl-5-[p-trifluoromethylphenyl]-2-f uroate hydrochloride) were investigated in a battery of in vivo and in vitro anticonvulsant model systems. After intraperitoneal (ip) administration in mice, F-721 was effective in nontoxic doses against maximal electroshock (MES), subcutaneous picrotoxin clonic, intracerebroventricular (icv) N-methyl-D-aspartate (NMDA) tonic, icv NMDA clonic and icv quisqualic acid tonic seizures (ED50s: 11.1, 28.4, 1.76, 3.4, and 4.4 mg/kg, respectively). F-721 exhibited only partial activity against clonic seizures induced in the subcutaneous Metrazol and subcutaneous bicuculline test in mice and was inactive in this species against tonic seizures induced in the subcutaneous strychnine test. F-721 was effective against MES seizures following oral administration to mice (ED50: 31.3 mg/kg) and only partially effective by this route against clonic seizures induced by subcutaneous Metrazol. In rats, F-721 was a potent anticonvulsant in the maximal electroshock model following oral administration (ED50: 9.9 mg/kg). F-721 was also effective against corneal-kindled and amygdaloid-kindled seizures in rats. F-721 suppressed stage 5 seizures in corneal-kindled rats with an ED50 of 15 mg/kg, ip. In addition, it also decreased the afterdischarge duration and behavioral seizure stage in amygdaloid-kindled rats at doses that did not cause sedation or ataxia. At 40 mg/kg, F-721 reduced afterdischarge duration by 83.2% and reduced the seizure severity score to 1.7. The ED50 for 50% reduction of afterdischarge duration was 16.3 mg/kg, ip. In cultured mouse spinal cord neurons, F-721 suppressed sustained repetitive firing in response to a depolarizing current with a median inhibitory concentration (IC50) of 1.9 microM. F-721 had no effect on adenosine uptake, gamma-aminobutyric acid or NMDA receptor binding. Comparative data from previous studies with clinically established antiepileptic agents reveal that F-721's profile of activity most closely resembles that of phenytoin and carbamazepine. However, F-721 was notably more efficacious in suppressing amygdaloid-kindled seizures in rats and was a more potent antagonist of icv NMDA clonic seizures. Our studies indicate that F-721 is a potent, orally active anticonvulsant with a favorable margin of safety. The profile of anticonvulsant activity of F-721 suggests potential utility in the management of generalized tonic-clonic, simple and complex partial seizures.

A neuropharmacological evaluation of felbamate as a novel anticonvulsant

Felbamate (2-phenyl-1,3-propanediol dicarbamate, FBM) was subjected to a series of carefully selected in vivo and in vitro tests to provide additional insight into mechanism of action, margin of safety, and clinical potential. FBM was effective against intracerebroventricular (i.c.v.) N-methyl-D-aspartate (NMDA)-induced clonus and i.c.v. NMDA- and quisqualic acid (quis)-induced forelimb tonic extension in mice and ineffective against i.c.v. quis-induced clonus in mice. FBM was also effective in preventing the expression of Stage 5 kindled seizures in corneal-kindled rats. The calculated protective indices (rotorod median toxic dose divided by anticonvulsant median effective dose) ranged from 28 to 146 for those tests in which FBM displayed activity. With the in vitro tests, FBM did not significantly displace [3H]MK-801 from its binding site. In contrast, FBM was effective in blocking sustained repetitive firing in mouse spinal cord neurons grown in tissue culture (median inhibitory concentration 67 micrograms/ml). This effect on repetitive firing suggests indirectly that FBM modulates sodium channel conductance. The results, when compared to similar data for phenytoin, carbamazepine, valproate, and ethosuximide, support the concept that FBM is a relatively nontoxic agent with a unique profile of anticonvulsant action, a broad margin of safety, and a clinical potential that includes at least generalized tonic-clonic and complex partial seizures.

Anticonvulsant profiles of the potent and orally active GABA uptake inhibitors SK&F 89976-A and SK&F 100330-A and four prototype antiepileptic drugs in mice and rats

The anticonvulsant profiles of two potent and orally active gamma-aminobutyric acid (GABA) uptake inhibitors, 1-(4,4-diphenyl-3-butenyl)-3-piperidine-carboxylic acid hydrochloride (SK&F 89976-A) and 1-(4,4-diphenyl-3-butenyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (SK&F 100330-A), were determined with a battery of well-standardized tests in mice and rats and compared with the profiles of phenytoin (PHT), carbamazepine (CBZ), valproate (VPA) and clonazepam (CZP) when subjected to the same tests. ED50 values were calculated and compared with TD50 values for minimal motor impairment to provide protective indexes (PI = TD50/ED50). The anticonvulsant profiles of SK&F 89976-A and SK&F 100330-A were similar and suggest that these compounds raise the threshold for seizure initiation rather than inhibit seizure spread. Like intraperitoneal (i.p.) PHT, CBZ, VPA, and CZP, SK&F 89976-A and SK&F 100330-A inhibited seizures in corneally kindled rats. The profiles of SK&F 89976-A and SK&F 100330-A were most similar to that of CZP and virtually opposite to that of PHT. Intraperitoneal SK&F 100330-A provided complete protection against pentylenetetrazol-induced seizures [subcutaneous (s.c.) PTZ] in mice but was ineffective against seizures induced by maximal electroshock (MES) at doses slightly greater than its TD50. SK&F 100330-A provided complete protection against picrotoxin-induced seizures (s.c. Pic) and against both clonus and forelimb tonic extension induced by NMDA N-methyl-D-aspartate [intracerebral ventricular (i.c.v.)-NMDA] in mice; however, SK&F 100330-A was ineffective against seizures induced by bicuculline (s.c. Bic) and strychnine (s.c. Strych) at doses slightly greater than its TD50. SK&F 89976-A was similar but provided partial protection against NMDA-induced clonus.(ABSTRACT TRUNCATED AT 250 WORDS)

AHR-12245: a potential anti-absence drug

AHR-12245, 2-(4-chlorophenyl)-3H-imadazo[4,5-b]pyridine-3-acetamid, ethosuximide, Na valproate, phenytoin, and clonazepam were evaluated in mice and rats with a battery of well-standardized anticonvulsant test procedures. The results obtained indicate that the anticonvulsant profile of AHR-12245 is similar to that for ethosuximide and clonazepam. AHR-12245 is effective in nontoxic intraperitoneal doses in mice by the maximal electroshock seizure (MES), pentylenetatrazol (s.c. PTZ), bicuculline, and picrotoxin tests but ineffective against strychnine-induced seizures; it is effective after nontoxic oral doses in both mice and rats by the s.c. PTZ test and ineffective by the MES test. The candidate antiepileptic substance was also ineffective against seizures induced in amygdala and corneally kindled rats. The PIs for AHR-12245 by the s.c. PTZ test were 4.5 to 12 times higher than those for the prototype agents, except that for clonazepam when administered orally in mice. The in vitro studies indicate that AHR-12245 is a weak inhibitor of benzodiazepine (BDZ) receptor binding but does inhibit adenosine uptake. These results indicate that AHR-12245 is a relatively nontoxic agent with a profile of anticonvulsant action which suggests it should be useful in generalized absence seizures.

Preclinical profile of stereoisomers of the anticonvulsant remacemide in mice

Stereoisomers of remacemide (racemate form) were compared for anticonvulsant efficacy and safety in mice. In the maximal electroshock seizure (MES) test for oral efficacy, the (-) stereoisomer, FPL 14145, was more potent than the racemate or the (+) stereoisomer, FPL 14144. Respective ED50 values (expressed as mg/kg) were: remacemide, 58; FPL 14145, 45; FPL 14144, 79. In 2 of 3 tests for neural impairment, FPL 14145 yielded significantly better therapeutic indices (toxic dose 50/ED50) than the racemate. The margin of safety (estimated median lethal dose ED50) was more favorable for FPL 14144: remacemide, 15.1; FPL 14144, 18.9; FPL 14145, 15.7. The duration of protection against MES indicated the stereoisomers were longer acting than the racemate. After intravenous administration the order of potency against MES was similar: FPL 14145 greater than remacemide greater than FPL 14144. Following daily administration of the oral ED98 for 4 days, with a dose response curve run on day 5, the MES ED50 values for all compounds were increased. The test indicates tolerance. In the pentylenetetrazol infusion test the racemate and FPL 14144 demonstrated more proconvulsant properties than FPL 14145. Intraperitoneal administration of 50 mg/kg or more produced changes in behavior with all compounds. At higher doses the racemate and FPL 14145 elicited more severe symptoms with death at 200 mg/kg.

Comparison of midazolam and diazepam by the intramuscular route for the control of seizures in a mouse model of status epilepticus

A model system is described in which sustained clonic seizures are produced by a combination of phenytoin (PHT) and pentylenetetrazol (PTZ) in the mouse, the former agent preventing the terminal tonic spasms produced by the latter. In this system, midazolam (MDL), a water-soluble benzodiazepine, was compared with diazepam (DZP), a sparingly soluble agent which is widely used to treat status epilepticus (SE) in humans. Both agents were administered intramuscularly (i.m.) in approximately equieffective doses in animals exhibiting clonic seizure activity. MDL proved to be about twice as potent as DZP. Whereas control animals convulsed for a period of approximately 90 min, those treated with DZP 0.2 and 0.4 mg/kg convulsed for 7.8 and 3.9 min, respectively; mice receiving MDL 0.1 and 0.2 mg/kg convulsed for 1.9 and 1.4 min, respectively. MDL arrested seizures substantially more rapidly than diazepam (p less than 0.05). These data suggest that MDL has sufficiently rapid anticonvulsant action to merit evaluation for control of SE in humans when a rapidly absorbed antiepileptic drug (AED) is needed and intravenous (i.v.) administration is not feasible.

Pharmacodynamics and pharmacokinetics of AHR-11748, a new antiepileptic agent, in rodents

AHR-11748, the desmethyl metabolite of fluzinamide (an effective antiepileptic), was active in preventing maximal seizures induced in mice or rats by electroshock and threshold seizures induced in mice by Metrazol, bicuculline, and picrotoxin. The compound showed a profile of anticonvulsant activity similar to those of phenobarbital and valproic acid and different from those of phenytoin and ethosuximide. ED50s were less than those of valproic acid, but greater than those of phenobarbital. Analysis of plasma and whole brain homogenates of mice indicated that AHR-11748 has an apparent terminal half-life (t1/2, beta) of 1.0 h. The brain:plasma ratio of AHR-11748 was 3.4:1 from 0.5 h to 6 h.

Hypnotic action of pentobarbital in mice: a possible mechanism

The effect of GABA receptor agonists (THIP and baclofen) on the hypnotic activity (loss of righting reflex and sleep time) of pentobarbital in mice was investigated. Combinations of either THIP-pentobarbital or baclofen-pentobarbital interacted synergistically by increasing hypnotic activity. The GABAa receptor antagonist, bicuculline, decreased the hypnotic effect of THIP-pentobarbital combinations but not that of baclofen-pentobarbital combinations. These results suggest that the hypnotic activity of pentobarbital involves GABAa receptor function.

The effect of chronic felbamate administration on anticonvulsant activity and hepatic drug-metabolizing enzymes in mice and rats

The possibility of tolerance development from chronic administration of felbamate (FBM) was investigated in mice and rats. Chronic administration (15 days) of FBM (150 mg/kg i.p.) in mice had no significant effect on either intravenous pentylenetetrazol (PTZ) seizure threshold or hexobarbital sleep time; however, hexobarbital sleep time was significantly increased after a single dose. Chronic administration (5-7 days) of FBM (48 or 95 mg/kg orally) in rats also had no significant effect on either maximal electroshock seizure activity or hexobarbital sleep time. Chronic administration of FBM at 238 mg/kg slightly decreased anti-subcutaneous PTZ activity in chronically treated rats (one of eight protected) as compared with those receiving only a single dose (three of eight protected), but there was no significant change in hexobarbital sleep time. Chronic treatment of rats for 7 days with 48 mg/kg had no significant effect on any hepatic parameters. However, 95 or 238 mg/kg of FBM significantly increased p-nitroanisole O-demethylase activity. It is concluded that the increased hexobarbital sleep time induced by an acute dose of FBM reflects the CNS-depressant effect of the substance. The increased p-nitroanisole O-demethylase activity observed after chronic administration may be indicative of some liver microsomal induction. Overall, FBM in doses ranging from 48 to 238 mg/kg appears to have minimal potential for tolerance development.

Correlation of the hypnotic potency of benzodiazepines with inhibition of voltage-dependent calcium uptake into mouse brain synaptosomes

Nine benzodiazepines were tested for their ability to inhibit 45Ca2+ uptake into mouse whole brain synaptosomes and for hypnotic activity as indicated by their ability to produce loss of the righting reflex. Eight of the benzodiazepines significantly inhibited fast-phase voltage-dependent 45Ca2+ uptake and five exhibited hypnotic activity. There was a direct correlation between the hypnotic potency of these five benzodiazepines and their ability to inhibit 45Ca2+ uptake. There does not appear to be a correlation between the anticonvulsant potency of the benzodiazepines and their potency for inhibiting 45Ca2+ uptake. These results support previous findings with other sedative/hypnotic drugs and suggest that inhibition of presynaptic calcium uptake may be linked with hypnotic but not anticonvulsant actions of benzodiazepines.

Effects of gamma-aminobutyric acid (GABA) receptor agonists on the neurotoxicity and anticonvulsant activity of barbiturates in mice

The effects of gamma-aminobutyric acid (GABA) receptor agonists [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol THIP]; [progabide and baclofen] on the minimal neurotoxicity and anticonvulsant activity of pentobarbital and phenobarbital in mice were investigated. When either progabide, THIP or baclofen were administered with pentobarbital, the components of this combination interacted additively by the rotorod test. Combinations of pentobarbital and progabide or phenobarbital and progabide interacted additively when subjected to the pentylenetetrazol (PTZ) minimal threshold seizure (clonic) test. THIP, even at toxic doses, did not alter the anti-PTZ activity of either pentobarbital or phenobarbital. In contrast, baclofen at toxic doses potentiated the anti-PTZ activity of pentobarbital and phenobarbital. Combinations of progabide and pentobarbital or progabide and phenobarbital interacted additively by the maximal electroshock seizure (MES) test. THIP, even when given in toxic doses, had no effect on the anti-MES activity of pentobarbital and phenobarbital. However, baclofen at nontoxic doses potentiated the anti-MES activity of phenobarbital but not that of pentobarbital. These results suggest that 1) in vitro interactions between barbiturates and GABAa receptor agonists may not be the same in vivo, 2) GABAa receptors may play a minor role in the minimal neurotoxicity and anticonvulsant activity of barbiturates and 3) inhibition of glutamate-induced excitation by baclofen may be an important action in potentiating the anti-MES activity of phenobarbital.

Effect of ethosuximide alone and in combination with gamma-aminobutyric acid receptor agonists on brain gamma-aminobutyric acid concentration, anticonvulsant activity and neurotoxicity in mice

The acute administration of an anticonvulsant dose of ethosuximide (150 mg/kg) had no effect on brain gamma-aminobutyric acid (GABA) concentration, whereas a toxic dose (400 mg/kg) increased significantly the concentration of brain GABA (1.23 +/- 0.05 vs. 1.92 +/- 0.14 mumol/g of wet tissue). The administration of 500 mg/kg/day of ethosuximide for 1, 2, 4, 6, 8 and 11 days induced neurotoxicity in 100, 100, 67, 0, 0 and 0% of animals, respectively, and increased brain GABA concentration 46, 38, 25, 14, 9 and 0%, respectively. These results imply that the tolerance that develops in response to the chronic administration of toxic doses of ethosuximide correlates well with the concentration of brain GABA. 4,5,6,7-Tetrahydroisoxazolo [5,4-c]pyridin-3-ol even in toxic doses had no effect on the anticonvulsant activity of ethosuximide. Combination studies with ethosuximide and progabide demonstrated that the antipentylenetetrazol activity of the individual components interacts additively. Likewise, combinations of either ethosuximide and 4,5,6-tetrahydroisoxazolo [5,4-c]pyridin-3-ol or ethosuximide and progabide showed an additive effect by the rotorod test. These results indicate that the antipentylenetetrazol activity of ethosuximide is unrelated to GABA function and that the increase in brain GABA concentration induced by toxic doses of ethosuximide contributes to its neurotoxicity.

Involvement of a GABAergic mechanism in the pharmacologic action of phenytoin

In vivo interactions between phenytoin (PHT) and baclofen (a GABAb receptor agonist) or PHT and progabide (a GABAa receptor agonist) were investigated by the rotorod minimal neurotoxicity test and maximal electroshock seizure (MES) test. The combination of PHT and baclofen produced an additive effect by the rotorod test, whereas the combination of PHT and progabide elicited a supra-additive (synergistic) effect. The median minimal neurotoxic dose of baclofen augmented the anti-MES activity of PHT. The combination of PHT and progabide induced a supra-additive effect by the MES test. These results imply that GABAa receptors are involved in both the minimal neurotoxicity and anti-MES activity of PHT.

Benzodiazepine inhibition of [3H]flunitrazepam binding and caffeine-induced seizures in mice

The median inhibitory and anticonvulsant potencies of seven benzodiazepine (BDZ) agonists and one BDZ antagonist (Ro15-1788) were established by two tests: inhibition of [3H]flunitrazepam receptor binding and prevention of caffeine-induced seizures in mice. The effect of Ro15-1788 on the anticonvulsant potency of diazepam against caffeine-induced seizures was also investigated. The [3H]flunitrazepam receptor binding inhibitory potencies (IC50s) of the BDZ agonists correlate well with their anticonvulsant potencies (ED50s) against caffeine-induced seizures (r = 0.831; P greater than 0.01 and less than 0.05). Ro15-1788 and clonazepam are the most potent inhibitors (IC50s: 1.72 and 1.75 nM, respectively), but differ markedly in their ability to obtund caffeine-induced seizures (ED50s: 43.2 and 0.226 mg/kg, respectively). Although both Ro15-1788 and diazepam are effective against caffeine-induced seizures, when used in combination Ro15-1788 antagonizes the anti-caffeine effect of diazepam. These data indicate that Ro15-1788 is a BDZ partial agonist with low efficacy as well as a potent antagonist.

Comparative anticonvulsant activity and neurotoxicity of felbamate and four prototype antiepileptic drugs in mice and rats

Felbamate (2-phenyl-1,3-propanediol dicarbamate), phenytoin, phenobarbital, ethosuximide, and valproate were evaluated in mice and rats with a battery of well-standardized anticonvulsant test procedures. The results obtained indicate that felbamate exhibits a wider range of experimental anticonvulsant activity than either phenytoin or ethosuximide and a somewhat more restricted range than either phenobarbital or valproate. Felbamate is effective in nontoxic intraperitoneal doses in mice by the maximal electroshock seizure (MES), pentylenetetrazol (s.c. PTZ), and picrotoxin (s.c. Pic) tests but ineffective against bicuculline- and strychnine-induced seizures; it is effective after nontoxic oral doses in both mice and rats by the MES and s.c. PTZ tests. When compared on the basis of protective indices (PI = TD50/ED50) calculated from the intraperitoneal data in mice, the PIs for felbamate were from 1.05 to 2.37 times higher than those of the prototype antiepileptics. Overall, except for the s.c. PTZ test in mice and rats after oral administration, the PIs were equal to or higher than those of the prototype agents. The PIs for the s.c. PTZ test in mice and rats after oral administration were within the range of the prototype agents. These data indicate that felbamate is a relatively nontoxic agent with a unique profile of anticonvulsant action.

An analysis of the in vivo interactions between chemical convulsants and anticonvulsants

The interactions between pentylenetetrazol (PTZ), picrotoxin (PIC), or bicuculline (BIC) and diazepam, phenobarbital, or valproate were subjected to Schild plot analysis. Log dose-probit response curves for minimal clonic seizures were determined for three chemical convulsants in the absence and in the presence of various concentrations of three anticonvulsants. The calculated median convulsant doses were subjected to Schild plot analysis and the pA2 values determined. A comparison of the pA2 values for the various convulsant/anticonvulsant combinations suggested the following conclusions: (i) the sequence of events leading to minimal clonic seizures evoked by PTZ or PIC involves a common receptor, (ii) BIC acts through a different receptor, and (iii) Schild plot analysis of the antagonism between convulsant and anticonvulsant is in agreement with their antagonism in vitro studies. Thus, Schild plot analysis can be useful in the evaluation of anticonvulsant activity in vivo and may offer some insight into the potential clinical usefulness of anticonvulsant substances.

Effect of picrotoxinin on benzodiazepine receptor binding

The effect of picrotoxinin on [3H]flunitrazepam binding to benzodiazepine receptors was investigated. In mouse forebrain membranes, picrotoxinin inhibited basal, GABA- and pentobarbital-stimulated [3H]flunitrazepam binding; this inhibitory activity was temperature- and chloride ion-dependent. Scatchard analysis of the data indicates that picrotoxinin decreases the number of binding sites without altering binding affinity. In cerebellar membranes, picrotoxinin did not alter [3H]flunitrazepam receptor binding.

gamma-Aminobutyric acid: temperature dependence in benzodiazepine receptor binding

The effects of muscimol and/or incubation temperature on the inhibition of [3H]flunitrazepam receptor binding by benzodiazepine receptor ligands were investigated. At 0 degree C muscimol decreased the Ki values for some ligands as displacers of [3H]flunitrazepam binding to brain-specific sites while increasing or having no effect on the Ki values for other ligands. The Ki values for some ligands are higher at 37 degrees C than at 0 degree C but are reduced by muscimol at both 0 degrees and 37 degrees C. In contrast, the ligands whose Ki values are increased by muscimol either decreased or did not alter the Ki values at 37 degrees C as compared to those at 0 degree C. Incubation of membranes at 37 degrees C for 30 min accelerated gamma-aminobutyric acid (GABA) release by 221% over that at 0 degree C. These results indicate that changes in incubation temperature alter benzodiazepine receptor affinity for ligands via GABA.

Antiepileptic drugs: detection, quantification, and evaluation

The anticonvulsant potential of chemical substances can be identified with test procedures that act at various biological levels, ranging from subcellular elements to the normal or modified intact animal. All of these procedures modify either some minimal overt threshold electrochemical or neurochemical event or a suprathreshold manifestation such as seizure spread. This suggests that laboratory tests for the detection, quantification, and evaluation of antiepileptic drugs should be designed to identify substances that elevate seizure threshold and/or prevent seizure spread. The s.c. Metrazol (pentylenetetrazol) seizure threshold test and the supramaximal electroshock seizure test are commonly used to achieve this objective. Additional chemoshock tests may be used to delineate further the mechanisms of anticonvulsant action. Numerous variables, such as experimental animals, electroshock apparatus, parameters of electrical and chemical stimulus, and routes of drug administration, must be controlled to ensure accurate, reliable, and reproducible results. The in vivo procedures described are reliable and reproducible, and predict clinical utility of the drugs tested. New models for testing anticonvulsant activity are evaluated against clinically effective antiepileptic drugs originally identified by these same procedures.

Benzodiazepine receptors are not involved in the neurotoxicity and anti-electroshock activity of phenytoin in mice

The present study was undertaken to investigate whether benzodiazepine receptors are involved in either the anticonvulsant activity of phenytoin against supramaximal seizures or maximal threshold seizures induced by electroshock or the neurotoxicity of phenytoin. Ro 15-1788, a benzodiazepine receptor antagonist, reversed the anticonvulsant activity of clonazepam against both supramaximal and maximal threshold seizures induced by electroshock but not that of phenytoin. Moreover, Ro 15-1788 did not decrease the neurotoxicity of phenytoin. These results suggest that benzodiazepine receptors are not involved in either the anticonvulsant action of phenytoin against seizures induced by electroshock or the neurotoxicity of phenytoin.

gamma-Aminobutyric acid modulation of benzodiazepine receptor binding in vitro does not predict the pharmacologic activity of all benzodiazepine receptor ligands

gamma-Aminobutyric acid (GABA) modulation of triazolam and nicotinamide binding to benzodiazepine (BDZ) receptors in vitro was compared with the neurotoxicity and anticonvulsant activity of these two drugs in vivo. GABA had no significant effect on the inhibitory potency of triazolam in [3H]flunitrazepam receptor binding, whereas GABA decreased the inhibitory potency of nicotinamide. When administered to mice, both triazolam and nicotinamide exhibited neurotoxicity by the rotorod test and anticonvulsant activity by the pentylenetetrazol seizure threshold test. This suggests that GABA modulation of the receptor binding of a BDZ ligand in vitro is not a reliable predictor of the pharmacologic activity of the ligand.

Effect of stimulus intensity on the profile of anticonvulsant activity of phenytoin, ethosuximide and valproate

The relative ability of phenytoin, ethosuximide and valproate to prevent minimal (clonic) threshold, maximal (tonic extension of the hindlimbs) threshold and supramaximal (tonic extension of the hindlimbs) seizures elicited by electrical and chemical (Metrazol, bicuculline, and picrotoxin) stimulation was determined. Ethosuximide and valproate were effective against minimal (clonic) threshold seizures, whereas phenytoin was ineffective and even activated them. All three anticonvulsants were effective against maximal (tonic extension of the hindlimbs) threshold seizures. Phenytoin and valproate, but not ethosuximide, were effective against supramaximal (tonic extension of the hindlimbs) seizures. The results suggest that the specificity of experimental models of epilepsy used in the evaluation of potential antiepileptic drugs is primarily due to the intensity rather than the nature of the stimulus used or the kind of seizure component evoked. Models based only on maximal (tonic extension of the hindlimbs) threshold seizures may identify anticonvulsant activity, but do not differentiate between substances that prevent seizure spread and those that increase seizure threshold.

Effects of SKF525A, phenobarbital, fasting, and carnitine on the anticonvulsant activity and neurotoxicity of valproate in mice

The anticonvulsant potency of valproate (VPA), as measured by the maximal electroshock seizure model in mice, was improved by pretreatment with the microsomal inhibitor SKF525A and by fasting, but was reduced by chronic phenobarbital pretreatment. Carnitine did not significantly alter the anticonvulsant properties of VPA. These findings suggest that the efficacy of VPA can be improved by factors that alter its metabolic pattern.

Anticonvulsant activity and neurotoxicity of piperonyl butoxide in mice

Piperonyl butoxide, a microsomal monooxygenase inhibitor, administered intraperitoneally to mice exerts peak anti-maximal electroshock activity and peak neurotoxicity at 5 and 7h, respectively. The median neurotoxic dose is 1,690 mg/kg. In the maximal electroshock seizure test, the median effective dose (ED50) is 457 mg/kg and the protective index (PI) is 3.69. In the subcutaneous pentylenetetrazol test, the ED50 is 443 mg/kg and the PI is 3.81. Piperonyl butoxide prevents seizure spread and elevates seizure threshold. Its PI compares favorably with PIs of clinically useful anticonvulsants.

Hypnotic action of benzodiazepines: a possible mechanism

The objective of this investigation was to determine whether the effects of muscimol on benzodiazepine receptor binding relate to the hypnotic activity of nine benzodiazepines (clonazepam, triazolam, diazepam, flurazepam, nitrazepam, oxazepam, temazepam, clobazam, and chlordiazepoxide) and CL 218,872. There was no correlation between the basal receptor binding affinities of the drugs tested and their hypnotic potencies, whereas the benzodiazepine receptor agonists whose receptor bindings are strongly modulated by muscimol possess potent hypnotic activity. These results indicate that benzodiazepine receptors that couple to GABA receptors are involved in the hypnotic activity of the benzodiazepines.

Benzodiazepine inhibition of flunitrazepam receptor binding, adenosine uptake, and pentylenetetrazol-induced seizures in mice

The potencies of seven benzodiazepines (BDZs) were established by three tests: inhibition of [3H]flunitrazepam receptor binding, inhibition of [3H]adenosine uptake, and prevention of pentylenetetrazol-induced seizures in mice. There is a high correlation between the potency for inhibition of [3H]flunitrazepam receptor binding and the antipentylenetetrazol potencies of benzodiazepines (r = 0.941; p less than 0.01). The antipentylenetetrazol potencies of benzodiazepines correlate well with their ability to inhibit [3H]adenosine uptake (r = 0.860; p less than 0.05 and greater than 0.01). However, there is no significant correlation between the potency for the inhibition of [3H]flunitrazepam receptor binding and the potency for inhibition of [3H]adenosine uptake (r = 0.751; p greater than 0.05). In addition, there is a marked difference in BDZ potency as measured by these two tests in vitro. The ratios of the Ki values (Ki2/Ki1) range from 98 for BDZ I to 64615 for clonazepam. The data presented indicate that antipentylenetetrazol activity of benzodiazepines results from an interaction between BDZ and nanomolar affinity BDZ receptors.

Correlations among minimal neurotoxicity, anticonvulsant activity, and displacing potencies in [3H]flunitrazepam binding of benzodiazepines

Five clinically available 1,4-benzodiazepines (BDZs) (chlordiazepoxide, diazepam, oxazepam, nitrazepam, and clonazepam) and four investigational 1,4-BDZs (BDZ I, BDZ II, BDZ III, and BDZ IV) were tested in vivo for minimal neurotoxicity (TD50) and for ability to obtund seizures (ED50) induced by a battery of five well-standardized procedures (maximal electroshock, strychnine, pentylenetetrazol, bicuculline, and picrotoxin). In addition, these BDZs were also tested in vitro as inhibitors of [3H]flunitrazepam binding to BDZ receptors. The results with each of the six in vivo tests were compared with those with the in vitro receptor binding test, and the correlation coefficients (r) were calculated. There was a high correlation between the inhibitor constants (Ki) derived from BDZ binding studies and the TD50 values (r = 0.882) and the pentylenetetrazol ED50 values (r = 0.946). There was also good correlation between ED50 values of BDZs effective by the bicuculline and picrotoxin tests and their Ki values in the BDZ receptor binding studies (r = 0.868 and 0.892, respectively). However, BDZ I, BDZ II, and BDZ IV had Ki values of 1.830, 0.075, and 0.015 microM, respectively, but BDZ I was ineffective in nontoxic doses by the bicuculline and picrotoxin tests, BDZ II by the picrotoxin test, and BDZ IV by the bicuculline test. In contrast, there was no correlation between the BDZs' anticonvulsant potency, determined by either the maximal electroshock or strychnine test, and their inhibitory potency on [3H]flunitrazepam binding to receptor sites. The possible methodological and neurochemical bases for these differences are discussed. These studies indicate that BDZ binding studies can be used to screen BDZs for anticonvulsant activity (selectively for antipentylenetetrazol activity and less selectively for antibicuculline and antipicrotoxin activity). More importantly, they complement the conventional in vivo anticonvulsant threshold tests. Therefore, BDZ binding studies can be used in concert with conventional in vivo procedures for the pharmacological differentiation of candidate antiepileptic BDZs.

Pentylenetetrazol may discriminate between different types of benzodiazepine receptors

The effect of various concentrations of pentylenetetrazol (PTZ) on [3H]flunitrazepam binding to benzodiazepine receptors was investigated by Hofstee and Hill plot analyses. These analyses indicate the presence of two PTZ binding sites in forebrain, whereas a single PTZ binding site is present in cerebellum. The relative proportions of the two PTZ binding sites in forebrain are close to those of benzodiazepine Type II and Type I receptors, respectively. These results suggest that PTZ may actually discriminate between different types of benzodiazepine receptors.

Comparative anticonvulsant activity and neurotoxicity of clobazam, diazepam, phenobarbital, and valproate in mice and rats

The 1.5-benzodiazepine (clobazam), the 1,4-benzodiazepine (diazepam), and two nonbenzodiazepine antiepileptic drugs (phenobarbital and valproate) were evaluated in mice and rats with a battery of well-standardized anticonvulsant test procedures. The results obtained indicate that clobazam and valproate exhibit a wider range of experimental anticonvulsant activity than either diazepam or phenobarbital. Except for clobazam by the maximal electroshock seizure (MES) test in rats, clobazam and valproate are effective in nontoxic doses against MES and all four chemically induced seizures (Metrazol, bicuculline, picrotoxin, and strychnine). Clobazam is effective by the MES test in rats only in doses that exceed the median minimal toxic dose. Phenobarbital is effective against all of the above tests, but minimal toxic doses must be employed to prevent strychnine seizures. Diazepam, on the other hand, is effective in nontoxic doses against seizures induced by Metrazol, bicuculline, and picrotoxin, but protects animals from maximal electroshock and strychnine seizures only when given in toxic doses. When compared on the basis of protective indices (PI = TD50/ED50) calculated from intraperitoneal data, the PIs for clobazam were 1.6 to 13 times higher than those for diazepam. Overall, except for the MES test in rats, the PIs for clobazam were from 1.5 to 44 times higher than those for any of the other three substances. With respect to the MES test in rats, the PI for clobazam was 10.8 times higher than that for diazepam; however, the PIs for phenobarbital and valproate were 3.5 and 4.4 times higher, respectively, than that for clobazam. These data suggest that the spectrum of anticonvulsant activity for the 1,5-benzodiazepine (clobazam) is superior to that for the 1,4-benzodiazepine (diazepam). Also, the broad experimental profile of anticonvulsant activity of clobazam agrees well with its reported broad clinical efficacy.

Altered neurochemical parameters in the brains of mice (Frings) susceptible to audiogenic seizures

In this study, several biochemical aspects of serotonergic neurons were compared in mice susceptible to audiogenic seizures (Frings) and in mice not susceptible to audiogenic seizures (CF 1). Tryptophan hydroxylase (TPH) activity was significantly lower in brains of Frings mice compared to CF1 mice; however, brain levels of 5-HT were similar in both strains. The significantly lower TPH activity in Frings mice was due to the altered kinetic characteristics of TPH activity in this strain. TPH from Frings mice had a significantly lower apparent maximal velocity and a significantly higher affinity for the substrate, as indicated by a lower apparent Km for tryptophan. The uptake of tryptophan and accumulation of 5-HT (following pargyline) were similar in Frings and CF 1 mice. The levels of 5-HT in selected brain regions of Frings and CF 1 mice were also similar, but NE levels were higher in the cerebral cortex and DA levels were higher in the neostriatum of Frings mice. It is unclear at this point what role, if any, 5-HT plays in susceptibility to audiogenic seizures in brains of Frings mice.

Drug contamination of mortars and pestles

Evidence is presented suggesting that potent water-insoluble antipentylenetetrazol agents triturated in porcelain mortars and pestles are not removed from this mixing device by the usual laboratory washing procedure. Moreover, amounts sufficient to contaminate the next substance triturated in this vessel can be demonstrated by the subcutaneous pentylenetetrazol seizure threshold test. The data show that a rigorous washing routine must be followed to achieve a "clean" mortar and pestle. Attention is also directed to the importance of using disposable hypodermic syringes, test tubes, etc., whenever possible and of designing an internal control test to determine when implements that must be reused are "clean."

Nonspecific effect of bis(2-ethylhexyl) phthalate on hexobarbital sleep time

The data presented suggest that the intravenous or intraperitoneal administration of bis(2-ethylhexyl) phthalate prolongs hexobarbital sleep time in mice and rats by enlarging the lipophilic pool, a phenomenon that can be replicated by pretreatment of the animals with olive oil. These results call attention to the importance of considering the physical characteristics of a substance when evaluating its effect on a pharmacologically active agent.