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NEUROPHARMACOLOGY

The Mechanism of Carbamazepine Aggravation of Absence Seizures

Departments of Medicine (L.L., T.Z., C.W., T.J.O.), Surgery (T.J.O.), and Neurology (T.J.O.), University of Melbourne, Royal Melbourne Hospital, Melbourne, Victoria, Australia; Beijing Friendship Hospital, Affiliate of Capital University of Medical Sciences, Beijing, China (L.L.); Department of Physiology and Pharmacology, University of New South Wales, Sydney, Australia (M.J.M.); and Howard Florey Institute of Physiology and Medicine, Melbourne, Victoria, Australia (A.L.C., C.A.R., S.P.)

Carbamazepine (CBZ) aggravates many generalized seizures types, particularly absence seizures, but the mechanisms underlying this are poorly understood. GABA signaling within the reticular nucleus (Rt) and the ventrobasal complex (VB) of the thalamus is critical to the neurophysiology of absence seizures. The hypothesis that CBZ aggravates absence seizures by acting at the VB thalamus via a GABA_A receptor-mediated mechanism was investigated in a genetic rat model, generalized absence epilepsy rats from Strasbourg (GAERS). Seizure activity was quantified by a 90-min electroencephalogram recording postdrug injection. Intracerebroventricular injections of CBZ (15 µg in 4 µl) resulted in seizure aggravation versus vehicle treatment, with a mean increase in seizure time of 40%. This indicates that CBZ acts directly, rather than via a metabolite, on the brain to aggravate seizures. Seizure aggravation also occurred following bilateral microinjection of CBZ (0.75 µg in 0.2 µl) into the VB (53%) but not following injection into the Rt (–9%). However, seizure aggravation was blocked when the GABA_A receptor antagonist, bicuculline (BIC, 0.04 µg in 0.2 µl), was coinjected with CBZ into the VB. Injection of BIC alone (versus vehicle) into the VB also blocked seizure aggravation following systemic administration of CBZ (15 mg/kg i.p.). In vitro studies in Xenopus oocytes expressing recombinant GABA_A receptors demonstrated that CBZ produced a dose-dependent potentiation of the GABA current at a physiological relevant concentration range (1–100 µM). These data demonstrate that CBZ acts at the VB thalamus to aggravate absence seizures in GAERS and that activation of GABA_A receptors is critical to this effect.

Address correspondence to: Dr. Terence J. O'Brien, Department of Medicine, University of Melbourne, Royal Melbourne Hospital, Royal Parade, Parkville 3050, Victoria, Australia. E-mail: obrientj{at}unimelb.edu.au

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