Dependence of anticonvulsant drug action on central monoamines

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

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

Paradoxical effect of noradrenaline-mediated neurotransmission in the antinociceptive phenomenon that accompanies tonic-clonic seizures: role of locus coeruleus neurons and α(2)- and β-noradrenergic receptors

The postictal state is generally followed by antinociception. It is known that connections between the dorsal raphe nucleus, the periaqueductal gray matter, and the locus coeruleus, an important noradrenergic brainstem nucleus, are involved in the descending control of ascending nociceptive pathways. The aim of the present study was to determine whether noradrenergic mechanisms in the locus coeruleus are involved in postictal antinociception. Yohimbine (an α(2)-receptor antagonist) or propranolol (a β-receptor antagonist) was microinjected unilaterally into the locus coeruleus, followed by intraperitoneal administration of pentylenetetrazole (PTZ), a noncompetitive antagonist that blocks GABA-mediated Cl(-) influx. Although the administration of both yohimbine and propranolol to the locus coeruleus/subcoeruleus area resulted in a significant decrease in tonic or tonic-clonic seizure-induced antinociception, the effect of yohimbine restricted to the locus coeruleus was more distinct compared with that of propranolol, possibly because of the presynaptic localization of α(2)-noradrenergic receptors in locus coeruleus neurons. These effects were related to the modulation of noradrenergic activity in the locus coeruleus. Interestingly, microinjections of noradrenaline into the locus coeruleus also decrease the postictal antinociception. The present results suggest that the mechanism underlying postictal antinociception involves both α(2)- and β-noradrenergic receptors in the locus coeruleus, although the action of noradrenaline on these receptors causes a paradoxical effect, depending on the nature of the local neurotransmission.

Cerebrospinal fluid and serum levels of dopa, catechols, and monoamine metabolites in patients with epilepsy

We measured CSF and serum concentrations of monoamines and monoamine metabolites in normal control subjects and in patients with partial epilepsy between and less than 2 h after complex partial seizures (CPS) or secondarily generalized tonic-clonic seizures (SGTCs). After SGTCs, concentrations of norepinephrine in CSF were significantly higher (p less than 0.05) than interictal concentrations, concentrations after PSs, and concentrations in control subjects. Serum epinephrine levels also were significantly higher after SGTCs than interictal and control subjects' levels. CSF HVA levels were significantly higher after PSs than interictal or control subjects' levels. CSF concentrations of norepinephrine and its intraneuronal metabolite, dihydroxyphenylglycol, were highly correlated, both interictally and following SGTCs, whereas correlations between serum and CSF levels of these catechols generally were not statistically significant. The results indicate that seizures are associated with release of catecholamines in the central nervous system.

Influence of MK-801 on the anticonvulsant activity of antiepileptics

MK-801 (a potent non-competitive antagonist of N-methyl-D-aspartic acid-mediated events) in subcutaneous doses of 0.1 and 0.2 mg/kg increased the threshold for electroconvulsions and in doses of 0.0031 and 0.0125 mg/kg enhanced the protective activity of valproate against maximal electroshock-induced convulsions in mice. Valproate-induced side-effects (evaluated by means of dark-avoidance acquisition and retention testing and the chimney test) at its ED50 against maximal electroshock (i.e. 268 mg/kg) were pronounced whereas they were absent in the case of a combined treatment with MK-801 (0.0125 mg/kg) and valproate (91 mg/kg). This treatment provided 50% protection against maximal electroshock-induced seizures. Moreover, MK-801 (0.0125 and 0.05 mg/kg) potentiated the anticonvulsant action of phenobarbital, reducing phenobarbital-induced motor impairment totally at 0.05 mg/kg, but did not influence the protection offered by carbamazepine and diphenylhydantoin at 0.05 mg/kg. The N-methyl-D-aspartic acid antagonist did not affect the total plasma levels of either valproate or phenobarbital (as measured by immunofluorescence), so a pharmacokinetic interaction, in terms of total plasma levels at least, is unlikely to be involved in the observed effects. The finding that the combined treatment of MK-801 with valproate or phenobarbital, apart from the distinct potentiation of their anticonvulsant activities, is devoid of side-effects should be carefully considered.

Monoamine turnover in the brain of mice during development of tolerance to the anticonvulsant effect of clonazepam

Mice were treated for 14 days with clonazepam, 0.5 mg/kg i.p. twice daily, during which time partial tolerance to the anticonvulsant effect against pentetrazole developed. The development of tolerance was paralleled by a reduced turnover of noradrenaline in the whole brain, and of dopamine in the midbrain. The turnover of 5-HT was increased during the first week of treatment, but decreased thereafter. These changes in monoamine turnover, which are thought to be GABA-mediated, are consistent with an increased seizure susceptibility, and may contribute to the development of tolerance to the anticonvulsant effect of benzodiazepines.

A possible role for spinal noradrenaline in the mechanisms of 6-hydroxydopamine against pentylenetetrazol induced convulsions in rats

The threshold of the generalized clonic convulsions induced by intravenous infusion of pentylenetetrazol (PTZ) was significantly increased by the intraperitoneal administration of noradrenaline (NA) neurotoxin, 6-hydroxydopamine, which produced no changes in the levels of catecholamines in discrete areas of rat brain, but the effect was accompanied by spinal depletion of NA. Moreover, the anticonvulsant effects of phenobarbitone (PB) and diphenylhydantoin (DPH) against PTZ convulsions were also significantly increased in the animals pretreated with 6-OHDA. These results suggest that the observed elevation of PTZ convulsive threshold and the potentiation of anticonvulsant activity of PB and DPH in 6-OHDA treated rats were possibly mediated through spinal cord depletion of NA.

Convulsive thresholds and severity and the anticonvulsant effect of phenobarbital and phenytoin in adult rats administered 6-hydroxydopamine or 5,7-dihydroxytryptamine during postnatal development

Rats were administered intracisternal 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT) within the first three postnatal days, at several ages centered on the third postnatal week or on postnatal day 180. When the rats were 210-days-old, maximal electroshock convulsive thresholds and responses and the anticonvulsant effect of phenobarbital and phenytoin were determined. All 5,7-DHT treatments resulted in an approximate 21% decrease in the tonic convulsive threshold and increased the incidence of tonic hindlimb extension (HLE). Only the 5,7-DHT treatment at 180 days was associated with a more severe HLE response (shortened onset and prolonged duration). All neonatal 6-OHDA treatments were associated with no change in the tonic threshold, but increased the incidence and severity of HLE. The latter effect depended on the postnatal age of 6-OHDA-treatment: treatment at postnatal days 14 and 15 resulted in the greatest increase in severity (52% decrease in onset and 48% increase in duration). The 6-OHDA treatment to 180-day-old rats increased the incidence and duration of HLE but had no influence on the tonic threshold or onset of extension. The effectiveness of both phenobarbital and phenytoin to block HLE was variably decreased by all neurotoxin treatments. The results suggest that interference with the postnatal maturation of monoaminergic influences on seizure processes can have a long-lasting influence on the ability of the brain to limit the generation and spread of seizure activity and on the effectiveness of anticonvulsant drugs.

Stimulation of serotonin synthesis in rat brain after antiepilepsirine, an antiepileptic piperine derivative

Piperine and two of its derivatives, antiepilepsirine (AE or 3,4-methylendioxycynnamoylpiperine) and compound 7448 (N-isopropyl 3 (4 chloro-phenyl) propenoylamide) are very effective in stimulating serotonin (5HT) synthesis. AE raises the ratio of free-to-bound tryptophan (TP) in plasma and induces a long-lasting increase of this aminoacid in brain. At the same time in striatum and limbic area it causes a lasting increase in 5 hydroxyindolacetic acid (5HIAA) a 5HT metabolite and to a lesser extent, an increase in the levels of the monoamine itself. Together with this action on 5HT metabolism we found that AE caused release of 3H-5HT from an in vitro synaptosomal preparation. It thus appears that piperine and its derivatives AE and compound 7148 affect the central serotonergic system.

Effects of some antiepileptic drugs in pentetrazol-induced convulsions in mice lesioned with kainic acid

Mice were injected with intracerebroventricular (i.c.v.) kainic acid (KA; 0.1 micrograms per animal) and the pentetrazol test was carried out on the fifth day after the administration of the amino acid. The following antiepileptic drugs were tested for anticonvulsant activity in mice lesioned with KA: diazepam (0.4 mg/kg), phenobarbital (12.5 and 25 mg/kg), trimethadione (200 and 400 mg/kg), depakine (200 and 400 mg/kg), carbamazepine (10 and 20 mg/kg), lefadol (bromophenylsuccinimide; 20 mg/kg), and acetazolamide (320 mg/kg). All drugs were given intraperitoneally, except for carbamazepine, which was also given orally in doses of 100 and 200 mg/kg. Pentetrazol was administered subcutaneously in a dose of 110 mg/kg, and the animals were subsequently observed for the occurrence of clonic and tonic convulsions within 30 min. The protective effects of diazepam and phenobarbital were significantly reduced in the KA-lesioned animals, while the actions of the remaining anticonvulsants were unaltered. Moreover, a substantial loss of pyramidal cells in the CA 3 field of the hippocampus was noted after i.c.v. injection of KA. It may therefore be concluded that the mechanism of the action of diazepam and phenobarbital are partially dependent on the intact functions of the hippocampal formation.

Handling-induced seizures and rotational behavior in th Mongolian gerbil

The aim of this report was to examine the relationship between sensory-induced seizures, cerebral laterality (as measured by rotation) and nigrostriatal asymmetry in Mongolian gerbils. Seizure resistant gerbils made proportionally more spontaneous turns to be preferred direction than sensitive animals. Three prototypical antiepileptic drugs strongly elicited rotational behavior (carbamazepine (10-20 mg/kg), diazepam (16 mg/kg) and pentobarbital (40 mg/kg)) and two others (phenobarbital 20-40 mg/kg) and ethosuximide (500 mg/kg)) also appeared to potentiate rotation; only diphenylhydantoin and trimethadione were effective. Two dopaminergic agonists, amphetamine (4 mg/kg) and apomorphine (16 mg/kg) enhanced rotation at anticonvulsant doses while the dopaminergic antagonist haloperidol reduced rotational behavior at a dose (1 mg/kg) which exacerbate seizure severity. Finally, surgical induction of nigrostriatal asymmetry by means of unilateral electrolytic striatal lesions reduced seizure severity; sham and bilateral striatal lesions had no significant effects on seizures. These results suggest that seizure activity and rotational behavior are inversely related and, furthermore, that the link between these two behaviors may be the asymmetry between nigrostriatal dopaminergic systems.

Effect of dopaminergic and GABA-ergic drugs given alone or in combination on the anticonvulsant action of phenobarbital and diphenylhydantoin in the electroshock test in mice

In the electroshock test--taking hind-limb tonic extension as the end point--apomorphine (10 mg/kg) exerted no effect on the anticonvulsant action of phenobarbital (PB; 20 mg/kg) or diphenylhydantoin (DPH; 8 mg/kg); amantadine (25 and 100 mg/kg) decreased that of DPH, while L-DOPA (500 mg/kg) and d,l-amphetamine (10 mg/kg) potentiated the action of both anticonvulsants. Fluphenazine (4 mg/kg) had no influence on the effects of the two anticonvulsants, but haloperidol lessened that of DPH. All GABA-ergic stimulants used, i.e., gamma-hydroxybutyric acid (GHBA/250 mg/kg), baclofen (2.5 and 10 mg/kg) and aminooxoacetic acid (AOAA; 15 and 20 mg/kg) potentiated the action of PB; the action of DHP was unaffected by these drugs except for AOAA (20 mg/kg). The combined treatment with dopaminergic and GABA-ergic stimulants, being ineffective in terms of anticonvulsant activity, resulted in a marked potentiation of the action of the anticonvulsants tested in this study. The most distinct potentiation was noted in the case of PB, baclofen (1 mg/kg), and amantadine (25 mg/kg).

Inhibition of the action of anticonvulsants by lithium treatment

The present study demonstrates that the effectiveness of anticonvulsants significantly decreases following lithium treatment (3 doses of 50 mg/kg of LiCl given every 12 hr). The anticonvulsant action of phenytoin, phenobarbital and of 3 carbonic anhydrase inhibitors: methazolamide, acetazolamide, ethoxzolamide, was assessed in rats subjected to maximal electroshock. In a chronic study on tolerance development to the anticonvulsant action of acetazolamide, lithium treatment (10 mg/kg per day) has been shown to inhibit gradually the action of acetazolamide. These results are consistent with the hypothesis that norepinephrine and dopamine are involved in the action of anticonvulsants.

Evidence against serotonin involvement in the tonic component of electrically induced convulsions and in carbamazepine anticonvulsant activity

Intraventricular injection of 5,7-dihydroxytryptamine, selective destruction of descending serotoninergic neurons by 5,6-dihydroxytryptamine or electrolytic and chemical lesions of the nucleus raphe dorsalis did not affect the electroconvulsive threshold in rats. No effect was observed after the systemic administration of drugs known to increase central serotonin transmission, such as quipazine, m-chlorophenylpiperazine, and moderate doses of d-fenfluramine, whereas p-chlorophenylalanine, an inhibitor of serotonin synthesis, decreased seizure susceptibility. The anticonvulsant activity of carbamazepine was not modified in animals with the same experimental lesions. The results, in relation to the high selectivity of the experimental procedures employed to deplete brain and spinal cord serotonin, do not bear out any involvement of serotonin in the tonic component of electrically induced convulsions or in the action of carbamazepine.

Anticonvulsant action of cannabis in the rat: role of brain monoamines

The role of brain monoamines in the anticonvulsant action of Cannabis indica resin (CI), against maximal electroshock-induced seizures in albino rats, was investigated by using pharmacologic agents that influence brain monoamine activity. Delta-9-tetrahydrocannabinol content of cannabis resin was estimated to be 17%. The anticonvulsant action of CI (200 mg/kg, i.p.) was significantly inhibited after pretreatment with drugs that reduce brain serotonin activity but not by drugs that reduce brain catecholamine activity. Similarly, the anticonvulsant action of a subanticonvulsant dose (50 mg/kg, i.p.) of CI was potentiated by serotonin precursors but not by catecholamine precursors. Potentiation of the anticonvulsant action of CI by nialamide or by imipramine was inhibited after pretreatment with 5,6-dihydroxytryptamine. The results suggest that the anticonvulsant action of CI in the rat is serotonin- and not catecholamine-mediated.

Prostaglandin E1-induced potentiation of the anticonvulsant action of phenobarbitone in the rat. Role of brain monoamines

Prostaglandin E1 (PGE1) significantly potentiated the anticonvulsant action of a sub-effective (EDO) dose of phenobarbitone, against maximal electroshock-induced seizures in the rat. PGE1-induced potentiation of phenobarbitone was significantly inhibited after pretreatment with drugs which are known to reduce brain serotonin activity, but was unaffected by drugs known to decrease brain catecholamine activity. Prostaglandin F2 alpha produced a moderate though statistically insignificant inhibition of PGE1 effect. The results suggest that PGE1-induced potentiation of phenobarbitone is not a direct effect but an indirect one, mediated through an increase in brain serotonin activity.

Brain monoamines in seizure mechanism (review)

A majority of experimental studies have strongly suggested that catecholamine and/or 5-HT play an important role for regulating the seizure susceptibility. As mentioned previously, however, the relative significance of individual monoamine has not yet been fully clarified. Since it is well known that the interaction between catecholaminergic and serotonergic neuronal activity is quite complex, a causal relationship between each monoamine and seizure susceptibility cannot be easily established if results are obtained from the whole brain study on pharmacologic manipulation of experimental seizures. The extensive study, including the monoamine turnover rates and concentrations in specific brain regions, may help to delineate such a correlation in the future. Further, when an object of study is concerned in excitability of the central nervous system, it should be necessary to elucidate the mutual relationship of monoamines to the other putative neurotransmitters (e.g., acetylcholine, amino acid). Does there exist an abnormal metabolism of monoamines in the brain of epileptic patients? If so, how is it related to the elaboration or maintenance of epileptic seizures? Unfortunately, we have no sufficeint information on the monoamine metabolism of epileptic patients.

Decreased anticonvulsant activity of carbamazepine in 6-hydroxydopamine-treated rats

An intraventricular injection of 6-hydroxydopamine, which produced a marked decrease of catecholamines in the forebrain, significantly lowered the electroconvulsive threshold in rats. The anticonvulsant effect of carbamazepine was also significantly reduced in the animals treated with 6-hydroxydopamine. The results are consistent with the hypothesis that brain catecholamines may play an important role in seizure susceptibility as well as in the anticonvulsant activity of carbamazepine in rats.

Effect of convulsant and anticonvulsant agents on level and metabolism of gamma-aminobutyric acid in mouse brain

1. The effect of the convulsant agents pentetrazole, picrotoxin, bicuculline, strychnine and isoniazid on the central level of gamma-aminobutyric acid (GABA) and the activity of the enzymes glutamate decarboxylase (GAD) and GABA-alpha-oxoglutarate aminotransferase (GABA-T) from mice brain was studied in vivo and vitro. In vivo, convulsant doses of picrotoxin and isoniazid lowered the level of GABA and the activity of GAD, whereas strychnine and bicuculline had no such effect. Pentetrazole inhibited GAD, but did not alter the GABA content. In vitro, all convulsants, except bicuculline, inhibited the activity of GAD; however, the concentrations of strychnine were far beyond the range that is reached in vivo by convulsant doses. Only isoniazid inhibited the activity of GABA-T in vivo as well as in vitro. 2. Phenobarbital, ethosuximide and trimethadione were about equally active in preventing convulsions induced by strychnine and picrotoxin, whereas diazepam was 9 times, and sodium valproate 3.5 times more active against convulsions elicited by picrotoxin. Phenytoin up to 100 mg/kg was ineffective against all chemoconvulsants. 3. Diazepam, sodium valproate, ethosuximide and trimethadione antagonized the inhibition of GAD and the decrease in GABA concentrations caused by isoniazid. Phenobarbital and phenytoin prevented the decrease of GABA but did not reverse the inhibition of GAD. 4. The results suggest a role played by the transmitter pool of GABA in the convulsant action of chemoconvulsants and in the anticonvulsant effect of antiepileptics clinically used in petit mal epilepsy.

Folate and monoamine metabolism in epilepsy

In 27 drug-treated epileptics there was a significant fall in serum, red cell and CSF folate levels compared with 15 untreated epileptics and 22 neurological controls. The 3 folate parameters were positively correlated with each other and negatively correlated with serum phenobarbitone, diphenylhydantoin and primidone. There was also a significant elevation of CSF 5-hydroxyindoleacetic acid (5HIAA) in the drug-treated epileptics; but this was not seen until "therapeutic" serum levels of phenobarbitone and diphenylhydantoin had been achieved and was most marked in clinically intoxicated patients. Similar trends were observed in CSF homovanillic acid (HVA). CSF 5HIAA and HVA were positively correlated with each other, especially in the drug-treated patients, in whom both amine metabolites were also negatively correlated with CSF folate. A possible relationship between folate and monoamine metabolism is discussed with particular reference to the antiepileptic and toxic effects of phenobarbitone, diphenylhydantoin and primidone.