Neuroscience Letters 384 (2005) 87–92 Ischemic tolerance induction in organotypic hippocampal slices: Role for the GABA A receptor? Christian Lange-Asschenfeldt a,b , Ami P. Raval a , Miguel A. P´ erez-Pinz´ on a,∗ a Cerebral Vascular Disease Research Center, Department of Neurology and Neuroscience, University of Miami School of Medicine, Miami, FL 33101, USA b Department of Neurology, University of Ulm, Ulm, Germany Received 23 January 2005; received in revised form 30 March 2005; accepted 16 April 2005 Abstract Ischemic preconditioning (IPC) refers to sublethal ischemic insults rendering brain tissue tolerant against subsequent ischemic insults. We investigated the role of the GABA A receptor (GABA A R) upon IPC induction. Rat organotypic hippocampal slices were subjected to IPC by 15 min of oxygen–glucose deprivation (OGD) followed by 40 min of OGD 48 h later, resulting in robust cell death reduction as assessed by the propidium iodide fluorescence method (‘late’ or ‘second window’ IPC). Superfusion with the GABA A R antagonist bicuculline during IPC ameliorated propidium iodide uptake at a high but not at low doses indicating that GABA A R activation may be assigned a limited role in neuroprotection. In previous studies, we found that increased neuronal excitability can promote IPC neuroprotection. We, therefore, tested the hypothesis that blockade of inhibitory GABAergic transmission conferred ischemic tolerance. However, temporary administration of bicuculline 48 h prior to ischemic challenge was not neuroprotective. In another approach, we tested whether preconditioning with the GABA A R agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) mediated ischemic tolerance and found no significant neuroprotection. The results are discussed in light of the intrinsic excitatory-inhibitory balance of glutamate and GABA. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Ischemia; Preconditioning; Organotypic; GABA A receptor; Bicuculline; THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) During the past decade, both exogenous pharmacological and intrinsic cellular mechanisms of neuroprotection against brain ischemia have been studied extensively. A reasonable approach in discovering tools with potentially protective capabilities may result from observations of nature’s very own adaptive responses. Ischemic preconditioning (IPC) is a prototype of such an endogenous defense mechanism. It is now well established that not only brief sublethal ischemic episodes, but also subthreshold stresses of almost any kind may induce robust and sustained neuroprotection (for review, see [6]). Growing evidence suggests a wide variety of factors and pathways involved in achieving this state of tolerance. For example, activation of adenosine receptors, the ATP-sensitive potassium channel (K + ATP ) [15,18], glutamate receptors [7] ∗ Corresponding author. Tel.: +1 305 243 7698; fax: +1 305 243 5830. E-mail address: perezpinzon@miami.edu (M.A. P´ erez-Pinz´ on). and induction of stress proteins [10] have been well char- acterized. However, the specific mechanisms remain largely undefined. Among the multiple subcellular alterations associated with IPC other groups have recently found enhanced -aminobutyric acid (GABA) release during ischemia after IPC treatment [8]. Moreover, upregulation of the GABA receptor after IPC treatment has been reported [20]. As the main inhibitory central receptor in vertebrates and the physi- ological counterpart of the excitotoxic N-methyl-d-aspartate (NMDA) receptor system and given its potential clinical accessibility, the GABA receptor seems an interesting target for neuroprotection. Thus, an abundance of literature deals with the putative neuroprotective potential of GABAergic compounds (review: [19]). Many in vitro and in vivo data demonstrate efficacy in various neuropathological models, however, studies tend to be equivocal and clinical trials have so far been disappointing. Moreover, the functional 0304-3940/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2005.04.053