Effect of catecholaminergic drugs on quinolinate- and kynurenine-induced seizures in mice

Anticonvulsant profile of flunarizine and relation to Na(+) channel blocking effects

The present study will summarize our findings concerning the anticonvulsant properties of the Ca2+ channel blocker flunarizine in a variety of experimental models of epilepsy. Flunarizine exhibits anticonvulsant effects against tonic seizures induced by electroshock or various chemoconvulsants in mice, however, did not protect against pentylenetetrazol-induced clonic seizures. In the MES test, the efficacy of clinically established antiepileptics was increased by co-medication. In the rotarod test, a minimal "neurotoxic" dose (TD50) of 18.0 mg/kg intraperitoneally was determined. In models of complex partial seizures like the hippocampal stimulation and the amygdala kindling in rats, flunarizine showed only a moderate activity. Thus, it can be suggested that the anticonvulsant potency of flunarizine in various screening tests is lower than that of standard antiepileptics such as carbamazepine and phenytoin. Concerning the possible mode of action, whole-cell patch-clamp experiments with cultured neonatal rat cardiomyocytes showed that flunarizine depressed the fast inward Na+ current in a concentration- and frequency-dependent manner well comparable with the action of phenytoin. It is concluded that the use-dependent inhibition of voltage-dependent Na+ channels may essentially contribute to the anticonvulsant activity of flunarizine in models for generalized tonic-clonic seizures. The clinical efficacy as add-on therapy is critically discussed in view of the present data.

Anticonvulsant profile and mechanism of action of propranolol and its two enantiomers

The anticonvulsant properties of the ss-adrenoceptor antagonist propranolol and its two enantiomers were examined in various screening tests in order to characterize the anticonvulsant profile as well as the possible molecular mechanism of action. These compounds dose-dependently raised the threshold for tonic electroshock seizures in mice and were effective in the traditional maximal electroshock test (ED (50)s 15- 20 mg kg (-1)i.p.). In combination with clinically used antiepileptics, the anticonvulsant effectiveness of the latter was significantly increased. In the pentylenetetrazol (85 mg kg (-1)s.c.) seizure threshold test, ( +/-)- and ( +)-propranolol were not effective in preventing clonic seizures. In unrestrained rats with chronically implanted electrodes in the dorsal hippocampus, propranolol and its ( +)-enantiomer equieffectively reduced the duration of electrically-evoked hippocampal afterdischarges (10 and 20 mg kg (-1)i.p.) and raised the focal stimulation threshold (20 mg kg (-1)i.p.). In amygdala-kindled rats, both drugs ( >or= 10 mg kg (-1)i.p.) reduced the seizure severity from stage 5 (generalized clonic-tonic) to stage 3 (unilateral forelimb) seizures. Furthermore, whole-cell patch-clamp experiments showed that ( +)- as well as ( -)-propranolol ( 10(-6)to 10(-4)M) depressed the fast inward sodium current in a concentration- and use-dependent manner in cultured rat cardiomyocytes and inhibited picrotoxin-induced burst firing activity of mouse spinal cord neurones in culture. In conclusion, propranolol and its two enantiomers have anticonvulsant effects in models for generalized tonic-clonic and complex partial seizures which may be accounted for by the sodium channel blocking and not by the ss-adrenoceptor blocking activity.

Norepinephrine-deficient mice have increased susceptibility to seizure-inducing stimuli

Several lines of evidence suggest that norepinephrine (NE) can modulate seizure activity. However, the experimental methods used in the past cannot exclude the possible role of other neurotransmitters coreleased with NE from noradrenergic terminals. We have assessed the seizure susceptibility of genetically engineered mice that lack NE. Seizure susceptibility was determined in the dopamine beta-hydroxylase null mutant (Dbh -/-) mouse using four different convulsant stimuli: 2,2,2-trifluroethyl ether (flurothyl), pentylenetetrazol (PTZ), kainic acid, and high-decibel sound. Dbh -/- mice demonstrated enhanced susceptibility (i.e., lower threshold) compared with littermate heterozygous (Dbh +/-) controls to flurothyl, PTZ, kainic acid, and audiogenic seizures and enhanced sensitivity (i.e., seizure severity and mortality) to flurothyl, PTZ, and kainic acid. c-Fos mRNA expression in the cortex, hippocampus (CA1 and CA3), and amygdala was increased in Dbh -/- mice in association with flurothyl-induced seizures. Enhanced seizure susceptibility to flurothyl and increased seizure-induced c-fos mRNA expression were reversed by pretreatment with L-threo-3, 4-dihydroxyphenylserine, which partially restores the NE content in Dbh -/- mice. These genetically engineered mice confirm unambiguously the potent effects of the noradrenergic system in modulating epileptogenicity and illustrate the unique opportunity offered by Dbh -/- mice for elucidating the pathways through which NE can regulate seizure activity.

Anticonvulsant activity of melatonin against seizures induced by quinolinate, kainate, glutamate, NMDA, and pentylenetetrazole in mice

Melatonin was tested in an ongoing attempt to find the endogenous antagonists of quinolinic acid, an endogenous convulsant. Among a great number of metabolites that have been tried before, only a few were found (cerulein and quinaldic acid in mice and kynurenic acid in rats). In SHR (bred from Swiss) male mice, intracerebroventricular (i.c.v.) pretreatment with melatonin (1.25-10.0 microg) attenuated (in the descending order of potency) the convulsant effect of i.c.v. administered kainate, quinolinate, glutamate, N-methyl-D-aspartate, and pentylenetetrazole. Melatonin was ineffective against i.p. administered pentylenetetrazole. Systemically (intraperitoneal, i.p.) administered melatonin (12.5-100.0 mg/kg) attenuated the convulsant effect of quinolinate, while the action of other convulsants used remained unaltered. It is suggested that melatonin could be tried against grand mal seizures in epileptic patients.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Serotonin-GABA treatment is hypothesized for self-injury in Lesch-Nyhan syndrome

The self-injurious movements of Lesch-Nyhan patients are considered extremely severe and currently intractable. Lesch-Nyhan syndrome is a genetic disorder of purine metabolism resulting in several abnormalities, one of which is elevated levels of xanthine. The author reasons that elevated plasma xanthine sets off a chain of events that produce excessive endogenous convulsants and diminished endogenous anticonvulsants. Treatment is proposed for self-injury in Lesch-Nyhan that entails reducing production of two endogenous convulsants (kynurenine and quinolinic acid) and enhancing two endogenous anticonvulsants (serotonin and gamma aminobutyric acid).