Markus Schmutz

Epilepsy, Hyperalgesia, Impaired Memory, and Loss of Pre- and Postsynaptic GABA B Responses in Mice Lacking GABA B(1)

GABA(B) (gamma-aminobutyric acid type B) receptors are important for keeping neuronal excitability under control. Cloned GABA(B) receptors do not show the expected pharmacological diversity of native receptors and it is unknown whether they contribute to pre- as well as postsynaptic functions. Here, we demonstrate that Balb/c mice lacking the GABA(B(1)) subunit are viable, exhibit spontaneous seizures, hyperalgesia, hyperlocomotor activity, and memory impairment. Upon GABA(B) agonist application, null mutant mice show neither the typical muscle relaxation, hypothermia, or delta EEG waves. These behavioral findings are paralleled by a loss of all biochemical and electrophysiological GABA(B) responses in null mutant mice. This demonstrates that GABA(B(1)) is an essential component of pre- and postsynaptic GABA(B) receptors and casts doubt on the existence of proposed receptor subtypes.

Redistribution of GABAB(1) Protein and Atypical GABAB Responses in GABAB(2)-Deficient Mice

GABAB receptors mediate slow synaptic inhibition in the nervous system. In transfected cells, functional GABAB receptors are usually only observed after coexpression of GABAB(1) and GABAB(2) subunits, which established the concept of heteromerization for G-protein-coupled receptors. In the heteromeric receptor, GABAB(1) is responsible for binding of GABA, whereas GABAB(2) is necessary for surface trafficking and G-protein coupling. Consistent with these in vitro observations, the GABAB(1) subunit is also essential for all GABAB signaling in vivo. Mice lacking the GABAB(1) subunit do not exhibit detectable electrophysiological, biochemical, or behavioral responses to GABAB agonists. However, GABAB(1) exhibits a broader cellular expression pattern than GABAB(2), suggesting that GABAB(1) could be functional in the absence of GABAB(2). We now generated GABAB(2)-deficient mice to analyze whether GABAB(1) has the potential to signal without GABAB(2) in neurons. We show that GABAB(2)-/- mice suffer from spontaneous seizures, hyperalgesia, hyperlocomotor activity, and severe memory impairment, analogous to GABAB(1)-/- mice. This clearly demonstrates that the lack of heteromeric GABAB(1,2) receptors underlies these phenotypes. To our surprise and in contrast to GABAB(1)-/- mice, we still detect atypical electrophysiological GABAB responses in hippocampal slices of GABAB(2)-/- mice. Furthermore, in the absence of GABAB(2), the GABAB(1) protein relocates from distal neuronal sites to the soma and proximal dendrites. Our data suggest that association of GABAB(2) with GABAB(1) is essential for receptor localization in distal processes but is not absolutely necessary for signaling. It is therefore possible that functional GABAB receptors exist in neurons that naturally lack GABAB(2) subunits.

Specific γ‐hydroxybutyrate‐binding sites but loss of pharmacological effects of γ‐hydroxybutyrate in GABAB(1)‐deficient mice

γ‐Hydroxybutyrate (GHB), a metabolite of γ‐aminobutyric acid (GABA), is proposed to function as a neurotransmitter or neuromodulator. γ‐Hydroxybutyrate and its prodrug, γ‐butyrolactone (GBL), recently received increased public attention as they emerged as popular drugs of abuse. The actions of GHB/GBL are believed to be mediated by GABAB and/or specific GHB receptors, the latter corresponding to high‐affinity [3H]GHB‐binding sites coupled to G‐proteins. To investigate the contribution of GABAB receptors to GHB actions we studied the effects of GHB in GABAB(1)−/− mice, which lack functional GABAB receptors. Autoradiography reveals a similar spatial distribution of [3H]GHB‐binding sites in brains of GABAB(1)−/− and wild‐type mice. The maximal number of binding sites and the KD values for the putative GHB antagonist [3H]6,7,8,9‐tetrahydro‐5‐hydroxy‐5H‐benzocyclohept‐6‐ylidene acetic acid (NCS‐382) appear unchanged in GABAB(1)−/− compared with wild‐type mice, demonstrating that GHB‐ are distinct from GABAB‐binding sites. In the presence of the GABAB receptor positive modulator 2,6‐di‐tert‐butyl‐4‐(3‐hydroxy‐2,2‐dimethyl‐propyl)‐phenol GHB induced functional GTPγ[35S] responses in brain membrane preparations from wild‐type but not GABAB(1)−/− mice. The GTPγ[35S] responses in wild‐type mice were blocked by the GABAB antagonist [3‐[[1‐(S)‐(3,4dichlorophenyl)ethyl]amino]‐2‐(S)‐hydroxy‐propyl]‐cyclohexylmethyl phosphinic acid hydrochloride (CGP54626) but not by NCS‐382. Altogether, these findings suggest that the GHB‐induced GTPγ[35S] responses are mediated by GABAB receptors. Following GHB or GBL application, GABAB(1)−/− mice showed neither the hypolocomotion, hypothermia, increase in striatal dopamine synthesis nor electroencephalogram delta‐wave induction seen in wild‐type mice. It, therefore, appears that all studied GHB effects are GABAB receptor dependent. The molecular nature and the signalling properties of the specific [3H]GHB‐binding sites remain elusive.

Endogenous Serine Protease Inhibitor Modulates Epileptic Activity and Hippocampal Long-Term Potentiation

Protease nexin-1 (PN-1), a member of the serpin superfamily, controls the activity of extracellular serine proteases and is expressed in the brain. Mutant mice overexpressing PN-1 in brain under the control of the Thy-1 promoter (Thy 1/PN-1) or lacking PN-1 (PN-1−/−) were found to develop epileptic activity in vivo and in vitro. Theta burst-induced long-term potentiation (LTP) and NMDA receptor-mediated synaptic transmission in the CA1 field of hippocampal slices were augmented in Thy 1/PN-1 mice and reduced in PN-1−/− mice. Compensatory changes in GABA-mediated inhibition in Thy 1/PN-1 mice suggest that altered brain PN-1 levels lead to an imbalance between excitatory and inhibitory synaptic transmission.

The competitive NMDA receptor antagonists CGP 37849 and CGP 39551 are potent, orally-active anticonvulsants in rodents

SummaryAnticonvulsant properties of CGP 37849 and CGP 39551, two novel phosphono-amino acids which are competitive NMDA receptor antagonists, were examined in rodents. At optimal pretreatment times CGP 37849 suppressed electroshock-induced seizures in mice and rats with ED50s ranging from 8 to 22 mg/kg after oral administration, and 0.4 to 2.4 mg/kg after i. v. and i. p. injection. Relative to CGP 37849, CGP 39551 was more potent after p. o. (ED50 3.7–8.1 mg/kg), and less potent after i.v. or i.p. treatment (ED50 2.7–8.7 mg/kg). Following oral treatment, the duration of action of CGP 37849 was about 8 h, while CGP 39551 still showed good activity after 24 h (ED50 8.7 mg/kg, mouse; 21 mg/kg, rat). Both compounds were anticonvulsant at doses below those at which overt behavioural side effects were apparent. CGP 39551 delayed the development of kindling in rats at doses of 10 mg/kg p. o. and above, and showed weak anticonvulsant activity against pentylenetetrazolevoked seizures. CGP 37849 and CGP 39551 are the first competitive NMDA antagonists to show oral anticonvulsant properties in a therapeutically-useful dose-range, and hence are interesting candidates for novel antiepileptic therapy in man.

γ-Vinyl GABA: comparison of neurochemical and anticonvulsant effects in mice

Biochemical and pharmacological effects ofγ-vinyl GABA (Vigabatrin®, GVG), an irreversible enzyme-activated inhibitor of 4-aminobutyrate: 2-oxoglutarate aminotransferase (EC 2.6.1.19; GABA-T), were measured in mice. This anticonvulsant produced a time- and dose-dependent elevation of the GABA, phenylalanine and lysine contents of cortical tissue and simultaneously decreased glutamate, aspartate and alanine levels. In addition, GVG caused a biphasic change in glutamine concentrations (a decline 1–4 hours after administration, followed 20 hours later by an increase). Moreover, we found a new, as yet unidentified amino acid in the brain eluting with the same retention time as α-aminoadipic acid from an HPLC cation-exchange column. The level of this novel chemical entity was greatly increased by GVG 20 hours after injection of the drug. At all tested intervals between 1 and 60 hours after injection, GVG was ineffective against maximal electroshock. The GABA-T inhibitor dose-dependently protected mice against isoniazid-induced seizures, simultaneously causing an increase in brain GABA concentrations. However, this apparent correlation applied only until 4 hours after treatment. To better define the anticonvulsant profile of GVG, groups of mice were treated, 1, 2, 4, and 24 hours prior to challenge with convulsant doses of strychnine, pentetrazole (PTZ), and picrotoxin, and brain amino acid levels, including brain concentrations of GVG, were measured. In all instances, the time dependency of the anticonvulsant effects of GVG and of increases in brain GABA levels differed. Amino acid concentrations in animals treated only with GVG were similar to those in animals given GVG and a chemical convulsant. GVG showed no selectivity for seizures produced by impairment of GABA-ergic neurotransmission. Although GVG is an effective GABA-T inhibitor, it apparently affects several other pyridoxal-phosphate-dependent cerebral enzymes and/or interacts with other neurotransmitter systems as well.

Oxcarbazepine: Preclinical Anticonvulsant Profile and Putative Mechanisms of Action

Summary: Oxcarbazepine (OCBZ, Trileptal) and its main human monohydroxy metabolite (MHD) protected mice and rats against generalized tonic‐clonic seizures induced by electroshock with ED50 values between 13.5 and 20.5 mg/kg p.o. No tolerance toward this anticonvulsant effect was observed when rats were treated with OCBZ or MHD daily for 4 weeks. The therapeutic indices were 4 (OCBZ) and >6 (MHD) for sedation (observation test, mice and rats) and 8 (MHD) or 10 (OCBZ) for motor impairment (rotorod test, mice). Both compounds were less potent in suppressing chemically induced seizures and did not significantly influence rat kindling development. At doses of 50 mg/kg p.o. and 20 mg/kg i.m. and higher, OCBZ and, to a lesser extent, MHD protected Rhesus monkeys from aluminum‐induced chronically recurring partial seizures. In vitro, OCBZ and MHD suppressed sustained high‐frequency repetitive firing of sodium‐dependent action potentials in mouse neurons in cell culture with equal potency (medium effective concentration 5 × 10‐8M/L). This effect is probably due in part to a direct effect on sodium channels. Patch‐clamp studies on rat dorsal root ganglia cells revealed that up to a concentration of 3 × 10‐4M, MHD did not significantly interact with L‐type calcium currents, whereas OCBZ diminished them by about 30% at the concentration of 3 × 10‐4M. In biochemical investigations, no brain neurotransmitter or modulator receptor site responsible for the anticonvulsant mechanism of action of OCBZ and MHD was identified. MHD and both of its enantiomers were of equal anticonvulsant profile and potency in rodent screening tests, with ED50 values ranging from 13 to 34 and 32 to 46 mg/kg p.o. in the electroshock and pentylenetetrazol test in mice, respectively. In addition, all three compounds showed a very similar profile of unwanted side effects. In vitro, they inhibited penicillin‐induced epileptic‐like discharges in the CA3 area of rat hippocampal slices with equal potency and efficacy at concentrations of 100–500 nm. This effect was attenuated when the potassium‐channel blocker 4‐aminopyridine was added to the bath fluid, thus indicating that potassium channels may also contribute to the antiepileptic activity of OCBZ.

Anticonvulsant properties of the methanolic extract of Cyperus articulatus (Cyperaceae)

The methanolic extract of rhizomes of Cyperus articulatus, a plant used in traditional medicine in Africa and Latin America for many diseases, possesses anticonvulsant activity in mice. This extract protected mice against maximal electroshock (MES)- and pentylenetetrazol (PTZ)-induced seizures. It also delayed the onset of seizures induced by isonicotinic acid hydrazide and strongly antagonized N-methyl-D-aspartate-induced turning behavior. The ED(50) for protection against seizures was 306 (154-541) mg/kg intraperitoneally (i.p.) for the PTZ test and 1005 (797-1200) mg/kg i.p. for the MES test. The ED(50) of methanolic extract against N-methyl-D-aspartate-induced turning behavior was 875 (623-1123) mg/kg i.p. C. articulatus L. methanolic extract protected 54% of mice from seizures induced by strychnine at the dose of 1000 mg/kg i.p. but had no or a moderate effect only against picrotoxin- or bicuculline-induced seizures. With these effects, the rhizome of C. articulatus L. possesses anticonvulsant properties in animals that might explain its use as a traditional medicine for epilepsy in Africa.

Effect of carbamazepine, oxcarbazepine and lamotrigine on the increase in extracellular glutamate elicited by veratridine in rat cortex and striatum

Lamotrigine, carbamazepine and oxcarbazepine inhibit veratrine-induced neurotransmitter release from rat brain slices in concentrations corresponding to those reached in plasma or brain in experimental animals or humans after anticonvulsant doses, presumably due to their sodium channel blocking properties. Microdialysis measurements of extracellular glutamate and aspartate were carried out in conscious rats in order to investigate whether corresponding effects occur in vivo. Veratridine (10 μM) was applied via the perfusion medium to the cortex and the corpus striatum in the presence of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (1 mM in perfusion medium). Maximally effective anticonvulsant doses of carbamazepine (30 mg/kg), oxycarbazepine ( 60 mg/kg) and lamotrigine (15 mg/kg) were given orally.The uptake inhibitor increased extracellular glutamate and aspartate about 2-fold in striatum and about 7-fold and 3-fold, respectively, in cortex. Veratridine caused a further 2–3-fold increase in extracellular glutamate in striatum and cortex, respectively, but its effect on extracellular aspartate was less marked in both areas. None of the anticonvulsant compounds affected the veratridine-induced increases in extracellular glutamate or aspartate in the striatum which were, however, markedly inhibited by tetrodotoxin (1 μM) and thus are sensitive to sodium channel blockade. In the cortex, the same drugs at the same doses did cause about 50% inhibition of the veratridine-induced increase in extracellular glutamate. Carbamazepine and to a lesser extent lamotrigine, but not oxcarbazepine, also inhibited the veratridine-induced increase in extracellular aspartate in the cortex.Although our results might seem to support the view that inhibition of glutamate and aspartate release is responsible for the anticonvulsant effects of lamotrigine, carbamazepine and oxcarbazepine, two complementary findings argue against this interpretation. First, as previously shown, inhibition of electrically induced release of glutamate requires 5 to 7 times higher concentrations of these compounds than release elicited by veratrine. Second, the present study indicates that doses totally suppressing convulsions caused no inhibition in the striatum and at best a 50% inhibition in the brain cortex. From this we conclude that the doses used here, although to some extent effective against veratridine, did not suppress the release of GLU and ASP elicited by the normal ongoing electrical activity of the glutamatergic and aspartatergic neurons and that the mechanism of the suppression of convulsions must be sought elsewhere.

Inhibition or enhancement of kindling evolution by antiepileptics

The influence of antiepileptics on the evolution of rat amygdaloid kindling was studied. Under placebo conditions clonic convulsions and a spike-wave EEG pattern developed. Diazepam, clonazepam, clobazam and phenobarbital were most effective in suppressing the evolution of kindling; the effects of valproate sodium, ethosuximide and acetazolamide were somewhat less pronounced in this respect. Carbamazepine, oxcarbazepine and phenytoin, on the other hand, enhanced kindling development, i.e. the increase in duration of after-discharge was faster than in the placebo group. The results indicate that under the above experimental conditions drugs with no anti-absence component can be distinguished from those with an anti-absence component. The mechanism of action underlying the observed effects is not yet known; the hypothesis that under special conditions protective inhibitory neuronal activity can develop to absence type seizures is proposed.