Anticonvulsant and adverse effects of MK-801, LY 235959, and GYKI 52466 in combination with Ca2+ channel inhibitors in mice

Isobolographic analysis of the mechanisms of action of anticonvulsants from a combination effect

The nature of the pharmacodynamic interactions of drugs is influenced by the drugs׳ mechanisms of action. It has been hypothesized that drugs with different mechanisms are likely to interact synergistically, whereas those with similar mechanisms seem to produce additive interactions. In this review, we describe an extensive investigation of the published literature on drug combinations of anticonvulsants, the nature of the interaction of which has been evaluated by type I and II isobolographic analyses and the subthreshold method. The molecular targets of antiepileptic drugs (AEDs) include Na(+) and Ca(2+) channels, GABA type-A receptor, and glutamate receptors such as NMDA and AMPA/kainate receptors. The results of this review indicate that the nature of interactions evaluated by type I isobolographic analyses but not by the two other methods seems to be consistent with the above hypothesis. Type I isobolographic analyses may be used not only for evaluating drug combinations but also for predicting the targets of new drugs.

Effects of rhynchophylline on GluN1 and GluN2B expressions in primary cultured hippocampal neurons

N-methyl-d-aspartate (NMDA) receptor subunits GluN1 and GluN2B in hippocampal neurons play key roles in anxiety. Our previous studies show that rhynchophylline, an active component of the Uncaria species, down-regulates GluN2B expression in the hippocampal CA1 area of amphetamine-induced rat. The effects of rhynchophylline on expressions of GluN1 and GluN2B in primary hippocampal neurons in neonatal rats in vitro were investigated. Neonatal hippocampal neurons were cultured with neurobasal-A medium. After incubation for 6h or 48 h with rhynchophylline (non-competitive NMDAR antagonist) and MK-801 (non-competitive NMDAR antagonist with anxiolytic effect, as the control drug) from day 6, neuron toxicity, mRNA and protein expressions of GluN1 and GluN2B were analyzed. GluN1 is mainly distributed on neuronal axons and dendritic trunks, cytoplasm and cell membrane near axons and dendrites. GluN2B is mainly distributed on the membrane, dendrites, and axon membranes. GluN1 and GluN2B are codistributed on dendritic trunks and dendritic spines. After 48 h incubation, a lower concentration of rhynchophylline (lower than 400 μmol/L) and MK-801 (lower than 200 μmol/L) have no toxicity on neonatal hippocampal neurons. Rhynchophylline up-regulated GluN1 mRNA expression at 6h and mRNA and protein expressions at 48h, but down-regulated GluN2B mRNA and protein expressions at 48 h. However, GluN1 and GluN2B mRNA expressions were down-regulated at 6h, and mRNA and protein expressions were both up-regulated by MK-801 at 48h. These findings show that rhynchophylline reciprocally regulates GluN1 and GluN2B expressions in hippocampal neurons, indicating a potential anxiolytic property for rhynchophylline.

Flunarizine: a possible adjuvant medication against soman poisoning?

Organophosphate (OP) nerve agents are amongst the most toxic chemicals. One of them, soman, can induce severe epileptic seizures and brain damage for which therapy is incomplete. The present study shows that pretreatment with flunarizine (Flu), a voltage-dependent calcium channel blocker, when used alone, does not produce any beneficial effect against the convulsions, neuropathology and lethality induced by soman. Flu was also tested in combination with atropine sulfate and diazepam. In this case, although only some results reach statistical significance, an encouraging general trend toward an improvement of the anticonvulsant, neuroprotective and antilethal capacities of this classical anti-OP two-drug regimen is constantly observed. In the light of these findings, it seems premature to definitely reject (or recommend) Flu as a possible adjuvant medication against soman poisoning. Further studies are required to determine its real potential interest.

Pre- rather than co-application of vigabatrin increases the efficacy of tiagabine in hippocampal slices

PURPOSE

The antiepileptic drug vigabatrin (VGB) increases intracellular availability of the inhibitory transmitter gamma-aminobutyric acid (GABA) by inhibition of GABA-transaminase. A blockade of the GABA uptake is the main mechanism of action of tiagabine (TGB). Based on this, the two antiepileptic drugs (AEDs) can be speculated to act synergistically so that their combined antiepileptic efficacy is supraadditive.

METHODS

To test this, experiments were performed on hippocampal slices of guinea-pigs. As an epilepsy model, epileptiform field potentials (EFPs) were induced by omission of Mg2+ from the bath solution and recorded in stratum pyramidale of the CA3 region. VGB (7.5 microM) and TGB (0.75 microM) were added to the superfusate.

RESULTS

VGB, given alone, failed to decrease the repetition rate of EFPs. Similarly, TGB applied alone only transiently led to a nonsignificant reduction of the EFP frequency. Combining VGB and TGB, their suppressive efficacy increased, yielding a significant reduction of EFP frequency, which, however, again did not persist. Pretreatment of the preparations with VGB for 2 h, followed by additional application of TGB, or TGB alone, drastically and persistently potentiated the effects.

CONCLUSIONS

These results demonstrate that VGB and TGB show favorable pharmacodynamic interactions, provided VGB is allowed to block intracellular GABA degradation before GABA uptake block by TGB.

Amlodipine enhances the activity of antiepileptic drugs against pentylenetetrazole-induced seizures

Amlodipine (AML), which belongs to the 1,4-dihydropyridine calcium channel antagonists, possesses pharmacological and pharmacokinetic profile that distinguishes it from other agents of this class. Pentylenetetrazole (PTZ)-induced clonic and tonic convulsions in mice were significantly reduced by administration of AML at 10 mg/kg. At this dose AML remained without influence upon the plasma level of PTZ. The ED50 value of AML against clonic seizures induced by PTZ was 5.4 mg/kg. This calcium channel antagonist (at 2.5 mg/kg) combined with ethosuximide (ETX), valproate magnesium (VPA) or phenobarbital (PB) significantly reduced their ED50 values against clonic phase of PTZ-induced seizures. AML administered alone or in combination with antiepileptic drugs (AEDs) worsened the motor performance of mice in the chimney test. However, these treatments remained without significant influence on the retention time in the passive avoidance test. Plasma levels of antiepileptics remained unchanged in the presence of AML. The results indicate that AML does not seem a good candidate for a combination therapy in epileptic patients because of its adverse potential.

Nicardipine improves the antidepressant action of ECT but does not improve cognition

INTRODUCTION

Cognitive impairment, the most important adverse effect of electroconvulsive therapy (ECT), may involve elevated intracellular calcium ion signaling. Animal research suggests that calcium channel-blocking agents, which attenuate excessive intracellular calcium activity, may reduce cognitive dysfunction caused by ECT.

METHOD

The lipid-soluble calcium channel-blocking drug nicardipine or matching placebo were randomly assigned to 26 patients with major depressive disorder receiving ECT. A rater blind to the experimental condition administered the Hamilton Depression Rating Scale, the Montgomery-Asberg Depression Rating Scale, the Beck Depression Inventory, the Mini-Mental State Examination and a comprehensive battery of neuropsychological tests prior to ECT, at the completion of ECT, and 6 months after ECT completion.

RESULTS

Compared with patients receiving placebo, patients taking nicardipine had significantly lower scores on the Hamilton and Montgomery-Asberg but not the Beck Depression rating scale scores at the completion of ECT. There were no differences between placebo and nicardipine groups in depression scores 6 months after ECT. Cognitive function declined over the course of ECT and improved over the next 6 months in both groups, but changes were statistically significant for only two subtests on the neuropsychological battery. Changes in Mini-Mental State Examination scores were small and were not significant at any point. There were no significant differences between nicardipine and placebo treated groups in any assessment of cognition.

DISCUSSION

Standard approaches to ECT in younger patients without preexisting neurological impairment do not produce cognitive side effects of sufficient severity for calcium channel-blocking agents to reduce these side effects demonstrably. Studies of treatments for cognitive impairment should be conducted in patients with risk factors for more severe cognitive impairment such as geriatric patients or patients with a history of interictal delirium during previous treatment with ECT. A possible effect of nicardipine in enhancing the antidepressant action of ECT requires further investigation in a study designed to test this action.

Felbamate demonstrates low propensity for interaction with methylxanthines and Ca2+ channel modulators against experimental seizures in mice

The aim of this study was to determine the interaction potential of the new antiepileptic drug felbamate (2-phenyl-1,3-propanediol dicarbamate) with three Ca2+ channel blockers (nicardipine, nifedipine, and flunarizine), one Ca2+ channel activator (Bay K 8644; 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridi ne carboxylic acid), and two methylxanthines (caffeine and aminophylline (theophylline2 . ethylenediamine)) which are all known to markedly change protective effects of conventional antiepileptic drugs. To do so, the maximal electroshock seizure test in mice (an experimental model predicting drug efficacy in the treatment of human generalized tonic-clonic seizures) was employed to (1) quantify changes in the protective efficacy and potency of felbamate produced by adjunct drugs and (2) assess the ability of aminophylline and caffeine to affect protective efficacy afforded by a submaximal protective dose of felbamate against maximal electroshock-induced seizures. Doses of adjunct drugs were selected based on their effects on the threshold for electroconvulsions and on appropriate literature. Nicardipine (10-30 mg/kg), nifedipine (5-20 mg/kg), flunarizine (2.5-10 mg/kg), Bay K 8644 (2.5-5 mg/kg), and aminophylline (50-75 mg/kg) did not change the protective efficacy and potency of felbamate against maximal electroshock-induced tonic convulsions. Aminophylline in the dose of 100 mg/kg, however, diminished the protective potency of felbamate as evidenced by a statistically significant increase in the protective ED50 value of felbamate (a dose, in mg/kg, predicted to protect 50% of mice against convulsive stimulus) from 79.6 to 118 mg/kg; P < 0.05). Aminophylline and caffeine only at high doses (100 and 161.7 mg/kg, respectively) significantly diminished the protective efficacy of felbamate (110 mg/kg) from 96% to 27% and 40% (P < 0.05), respectively. In conclusion, felbamate shows low interaction potential with Ca2+ channel modulators and methylxanthines. Such low interaction potential clearly differentiates felbamate from conventional antiepileptic drugs where protective effects are readily altered by the compounds tested in the present study.