GABA agonists fail to protect the retina from ischemia–reperfusion injury Sindri Traustason, Thor Eysteinsson, Bjarni A. Agnarsson, Einar Stefa ´ nsson * Department of Ophthalmology and Physiology and National University Hospital, Department of Ophthalmology and Pathology, University of Iceland, Reykjavı ´k, Iceland article info Article history: Received 27 November 2007 Accepted in revised form 8 July 2008 Available online 10 December 2008 Keywords: retina ischemia reperfusion hypothermia gamma-aminobutyric acid abstract The purpose of this study was to test the hypothesis that ischemia/reperfusion injury in the rat retina may be ameliorated by reducing retinal metabolism with either hypothermia or inhibitory GABA agonists. The intraocular pressure of each right eye in rats was raised to 130 mmHg for 60 min with the left eye serving as normal control. The rats were divided into four groups in terms of drug and hypo- thermia treatment: (1) Untreated ischemia, (2) Hypothermia, (3) Baclofen/midazolam and (4) Baclofen/ muscimol. Electroretinogram was recorded before ischemia and again after 10-day reperfusion. Histo- logical analysis with H&E staining and cell counts was performed. Untreated ischemia/reperfusion resulted in severely reduced ERG responses. The ERG b-wave was reduced from 423  144 mV to 130  91 mV (mean  SD, n ¼ 5). With hypothermia the ERG b-wave was reduced from 499  80 mV to 237  111 mV(n ¼ 4). With combinations of baclofen and midazolam the ERG b-wave was reduced from 432  96 mV to 104  67 mV(n ¼ 7). In baclofen/muscimol treated eyes the ERG b-wave went from 426  101 mV to 148  118 mV(n ¼ 6). The histological tissue damage was severe in untreated ischemia and the baclofen/midazolam and baclofen/muscimol groups, but less severe in the hypothermia group. The GABA agonists do not provide any protection in our ischemia/reperfusion model. Our results are consistent with earlier reports that hypothermia may be helpful in ischemic conditions in the retina. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Ischemia is thought to play a role in some of the most common blinding diseases, such as diabetic retinopathy, glaucoma, retinal vascular occlusions and retinopathy of prematurity, as well as in less common acute occlusions of the central retinal artery. In ischemic tissue the metabolic requirements exceed the capacity of the circulation and the tissue suffers from lack of nutrients, hypoxia and build-up of waste products. In general ischemia may be ameliorated by either increasing blood flow or lowering the metabolic activity of the ischemic tissue. Both methods are commonly used in the eye as well as any other organ suffering ischemic situations. In glaucoma, retinal blood flow may be increased by lowering ocular pressure (Costa et al., 2003) and laser photocoagulation of the retina in diabetic retinopathy reduces metabolic demand of the tissue by destroying some of the energy demanding photoreceptors (Stefansson, 2006). In other organs various methods are used to decrease metabolic activity in ischemia and reperfusion, the most common being hypothermia and drugs that lower metabolic rate. b-Blockers are used in this role for ischemic heart disease (Kopecky, 2006) and hypothermia is a recommended treatment for patients in cardiac arrest for example in open heart surgery, protecting both the heart and brain from reperfusion damage (Nolan et al., 2003). Similarly general anesthesia is used to reduce brain damage in cerebrovas- cular attacks and head trauma (Kawaguchi et al., 2005). GABA (g-aminobutyric acid) is a major inhibitory transmitter in the central nervous system and retinal sensory neurons. Three structurally and functionally different subtypes of postsynaptic GABA receptors have been described to date, designated as GABA A , GABA B and GABA C (Davies et al., 1996; Koulen et al., 1998a; Slaughter, 1995). The presence of these GABA receptor subtypes on retinal neurons in the retina has been well established. GABA A - receptors are predominantly found in the inner retina, on hori- zontal cells, bipolar cells, amacrine cells and ganglion cells. GABA B -receptors have been identified presynaptically on amacrine cells and horizontal cells and postsynaptically on amacrine cells and ganglion cells (Koulen et al., 1998b). The presence and hyperpolarizing effect of GABA A receptors has been further established using the patch-clamp method in rabbits (Varela et al., 2005) and mice (Feigenspan and Weiler, 2004). GABA C -receptors have primarily been located on bipolar cells and evidence for their existence in other retinal cells, such as horizontal cells and ganglion cells is lacking (Koulen et al., 1998a). The ionotropic GABA A and GABA C receptors are ligand-gated Cl  ion channels that directly induce influx of chloride ions, while the * Corresponding author. Tel.: þ354 543 7217. E-mail address: einarste@landspitali.is (E. Stefa ´ nsson). Contents lists available at ScienceDirect Experimental Eye Research journal homepage: www.elsevier.com/locate/yexer 0014-4835/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.exer.2008.07.019 Experimental Eye Research 88 (2009) 361–366