CHEB, a convulsant barbiturate, evokes calcium-dependent spontaneous glutamate release from rat cerebrocortical synaptosomes

Potential pleiotropic effects of Mpdz on vulnerability to seizures

We previously mapped quantitative trait loci (QTL) responsible for approximately 26% of the genetic variance in acute alcohol and barbiturate (i.e., pentobarbital) withdrawal convulsion liability to a < 1 cM (1.8 Mb) interval of mouse chromosome 4. To date, Mpdz, which encodes the multiple PSD95/DLG/ZO-1 (PDZ) domain protein (MPDZ), is the only gene within the interval shown to have allelic variants that differ in coding sequence and/or expression, making it a strong candidate gene for the QTL. Previous work indicates that Mpdz haplotypes in standard mouse strains encode distinct protein variants (MPDZ1-3), and that MPDZ status is genetically correlated with severity of withdrawal from alcohol and pentobarbital. Here, we report that MPDZ status cosegregates with withdrawal convulsion severity in lines of mice selectively bred for phenotypic differences in severity of acute withdrawal from alcohol [i.e., High Alcohol Withdrawal (HAW) and Low Alcohol Withdrawal (LAW) lines] or pentobarbital [High Pentobarbital Withdrawal (HPW) and Low Pentobarbital Withdrawal (LPW) lines]. These analyses confirm that MPDZ status is associated with severity of alcohol and pentobarbital withdrawal convulsions. Using a panel of standard inbred strains of mice, we assessed the association between MPDZ status with seizures induced by nine chemiconvulsants. Our results show that MPDZ status is genetically correlated with seizure sensitivity to pentylenetetrazol, kainate and other chemiconvulsants. Our results provide evidence that Mpdz may have pleiotropic effects on multiple seizure phenotypes, including seizures associated with withdrawal from two classes of central nervous system (CNS) depressants and sensitivity to specific chemiconvulsants that affect glutaminergic and GABAergic neurotransmission.

Differential interaction of anaesthetics and antiepileptic drugs with neuronal Na+ channels, Ca2+ channels, and GABA(A) receptors

BACKGROUND

Current theories favour multiple agent-specific neuronal actions for both general anaesthetics and antiepileptic drugs, but the pharmacological properties that distinguish them are poorly understood. We compared the interactions of representative agents from each class on their putative targets using well-characterized radioligand binding assays.

METHODS

Synaptosomes or membranes prepared from rat cerebral cortex were used to analyse drug effects on [(35)S]t-butyl bicyclophosphorothionate ([(35)S]TBPS) binding to the picrotoxinin site of GABA(A) receptors, [(3)H]batrachotoxinin A 20-alpha benzoate ([(3)H]BTX-B) binding to site 2 of voltage-gated Na(+) channels, (+)-[methyl-(3)H]isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-methoxycarboxyl-2,6-dimethyl-3-pyridinecarboxylate ([(3)H]PN200-110; isradipine) binding to L-type Ca(2+) channels, and [cyclohexyl-2,3-(3)H](N)glibenclamide ([(3)H]GB) binding to K(ATP) channels.

RESULTS

I.V. anaesthetics other than ketamine preferentially inhibited [(35)S]TBPS binding (etomidate approximately equal alphaxalone > propofol > thiopental > pentobarbital). Volatile anaesthetics inhibited both [(35)S]TBPS and [(3)H]BTX-B binding with comparable potencies (halothane approximately equal isoflurane approximately equal enflurane). Antiepileptic drugs preferentially antagonized either [(35)S]TBPS (diazepam > phenobarbital) or [(3)H]BTX-B (phenytoin > carbamazepine) binding. Local anaesthetics (lidocaine, tertracaine) selectively antagonized [(3)H]BTX-B binding. None of the drugs tested were potent antagonists of [(3)H]PN200-110 or [(3)H]GB binding.

CONCLUSIONS

Comparative radioligand binding assays identified distinct classes of general anaesthetic and antiepileptic drugs based on their relative specificities for a defined target set. I.V. anaesthetics interacted preferentially with GABA(A) receptors, while volatile anaesthetics were essentially equipotent at Na(+) channels and GABA(A) receptors. Antiepileptic drugs could be classified by preferential actions at either Na(+) channels or GABA(A) receptors. Anaesthetics and antiepileptic drugs have agent-specific effects on radioligand binding. Both general anaesthetics and antiepileptic drugs interact with Na(+) channels and GABA(A) receptors at therapeutic concentrations, in most cases with little selectivity.

Effects of anticonvulsants on veratridine- and KCl-evoked glutamate release from rat cortical synaptosomes

We compared the effects of three conventional (phenytoin, carbamazepine and phenobarbital) and three novel (BW 1003C87, lamotrigine and riluzole) anticonvulsants on evoked glutamate release from rat cortical synaptosomes. Glutamate release was evoked by either 20 microM veratridine (which requires both Na+ and Ca2+ channel activation) or 30 mM KCl (which requires Ca2+ channel, but not Na+ channel, activation) to assess the involvement of Na+ and/or Ca2+ channels in the presynaptic actions of these anticonvulsants. All six compounds inhibited veratridine-evoked glutamate release; BW 1003C87 (IC50 = 2.0 microM) was the most potent and phenobarbital (IC50 = 3.2 mM) was the least potent inhibitor. Only phenobarbital (IC50 = 6.4 mM), riluzole (IC50 > 50 microM) and phenytoin (IC50 > 800 microM) significantly inhibited KCl-evoked glutamate release. These results suggest that therapeutic concentrations of BW 1003C87, lamotrigine, phenytoin, carbamazepine and riluzole, but not phenobarbital, inhibit synaptic glutamate release by preferentially blocking presynaptic Na+ channels. Presynaptic Na+ channel blockade with inhibition of the release of glutamate, and possibly other transmitters, may contribute to their anticonvulsant and/or neuroprotective effects.