Abstracts

g-hydroxybutyric (GHB) aciduria is an inherited deficiency of succinic semialdehyde dehydrogenase (SSADH) and is characterized by seizures and mental retardation. The SSADH deficient mouse (SSADH^-/-) mirrors the clinical condition with markedly elevated levels of both GABA and GHB in urine and homogenates of liver and brain. Further, the SSADH^-/- phenotype is characterized by absence seizures which transition to lethal status epilepticus during the third postnatal week of life (Pharmacology, Biochemistry and Behaviour 2004: 79: pp 547-553). During the course of searching for rescue strategies that would abort or delay the onset of the status epilepticus in the mutant animals we investigated carbenoxolone, a gap junction blocker, because of its reported efficacy in other seizure models. In vivo experiments, carbenoxolone (45 mg/Kg, b.i.d) or saline was administered to SSADH^-/- mice and the onset of status epilepticus was determined in drug and control animals. In vitro experiments, [³H]GHB with specific activity of 60 Ci/mmol, [³H]CGP54626A with specific activity of 40 Ci/mmol and [³H]NCS-382 with specific activity of 20 Ci/mM were used to test whether carbenoxolone would compete for GHB and/or GABA[sub]B[/sub] receptor using membrane binding techniques on whole brain obtained from SSADH^-/- and SSADH^+/+ mice. The binding studies were also done with glycyrrhetinic acid, a structurally related gap junction blocker, to dissect out the mechanism of anticonvulsant effects of carbenoxolone. Carbenoxolone significantly delayed the onset of status epilepticus and prolonged the life span of SSADH^-/- mice (75% survival at P25) compared to mutant animals that received saline alone (5% survival at P25). As well, 0.5mM carbenoxolone replaced 82.5% [plusmn] 1.8% binding of H3-GHB in mouse brain membrane preparations (n=6, p[lt]0.01). Further study showed that carbenoxolone at the level from 1 [mu]M to 0.5 mM significantly inhibited the binding of H3-NCS and H3-CGP54626 (specific, high affinity ligands to the GHB and GABA[sub]B[/sub] receptors) in mouse brain membrane preparations. In contrast, glycyrrhetinic acid has no effect on the binding of H3-GHB in mouse brain membrane preparations. Carbenoxolone may represent a novel treatment to prevent the evolution of absence seizures to generalized convulsive seizures and may act in this regard by both GHB- and GABA[sub]B[/sub] receptor-mediated mechanisms. (Supported by the Canadian Institutes for Health Research and the Bloorview Children[apos]s Hospital Foundation.)