6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) increases GABA A receptor-mediated spontaneous postsynaptic currents in the dentate granule cells of rat hippocampal slices Yoshinori Hashimoto, Hiroyoshi Miyakawa, Yoshihisa Kudo, Masashi Inoue * Laboratory of Cellular Neurobiology, Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan Received 3 December 2003; received in revised form 19 December 2003; accepted 22 December 2003 Abstract 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) is widely used as an antagonist on non-NMDA glutamate receptors. However, several studies have shown that CNQX increases the spontaneous inhibitory postsynaptic current frequency at hippocampal pyramidal neurons and cerebellar granule cells. Dentate granule cells are known to be another distinctive type of principal neurons in hippocampus, and receive dense synaptic input from hilar interneurons. Thus, we examined the effects of CNQX on the dentate granule cells and hilar interneurons with whole-cell recording. CNQX increased the frequency of GABAergic spontaneous postsynaptic currents (sPSCs) on the granule cells, and increased the resting potential and the action potential frequency of the hilar interneurons. These increases were not observed with other glutamate receptor antagonists. The increases in sPSC frequency may be caused by the depolarization and the action potentials of the interneurons. q 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Dentate gyrus; Hilus; Interneuron; GYKI52466; NS-102; Inhibitory postsynaptic current A commonly used a-amino-3-hydroxy-5-methyl-4-isoxa- zolepropionic acid (AMPA)/kainate receptor antagonist [1], 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), has been reported to increase GABAergic input of hippocampal pyramidal neurons and cerebellar granule cells [2,5,6] with a mechanism that is unrelated to glutamate receptor blocks. In these studies researchers observed that direct generation of firing in interneurons lead to increases in frequency of spontaneous inhibitory postsynaptic currents on postsyn- aptic targets. Granule cells in the dentate gyrus are known to be another distinctive type of principal neurons in hippocampal formation. The granule cells receive dense synaptic input from interneurons in dentate gyrus and hilus [8]. Thus, in the present study, we examined the effect of CNQX on the spontaneous activities of dentate granule cells and hilar interneurons with whole-cell recording. Hippocampal slices (400 mm) were prepared from the brain of 6–8-week-old male Wistar rats, which were decapitated after ether anesthesia. Slices were immersed in oxygenated artificial cerebrospinal fluid (ACSF) at room temperature (23 – 25 8C) containing (in mM)124 NaCl, 26 NaHCO 3 , 10 glucose, 2.5 KCl, 1.25 NaH 2 PO 4 , 2.5 CaCl 2 , and 1.5 MgCl 2 . The granule cells or the interneurons were chosen by visual identification with infrared differential interference constant (IR-DIC) videomicroscopy. Whole- cell recordings were amplified with an AXOCLAMP-2B or AXOPATCH-1D amplifier (Axon Instruments, Foster City, CA). All responses were collected using pClamp 8.0 software (Axon Instruments). Responses were sampled at 33.3 kHz and stored in a PC. For the current clamp recordings from interneurons, patch pipettes were filled with internal solutions containing (in mM) 120 K-gluconate, 20 KCl, 4.0 NaCl, 5.0 EGTA, 10 HEPES, 2.0 Mg 2 -ATP, and 10 biocytin (pH adjusted to 7.3 with KOH). For the voltage clamp recordings of granule cells, patch pipettes were filled with internal solutions containing (in mM) 140 CsCl, 4.0 NaCl, 5.0 EGTA, 10 HEPES, and 2.0 Mg 2 -ATP (pH adjusted to 7.3 with CsOH). Patch electrode resistance was 5–8MV. Drugs were diluted in standard aCFS to their final concentrations. All drugs were obtained from Sigma– Aldrich (Tokyo, Japan). 0304-3940/03/$ - see front matter q 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2003.12.083 Neuroscience Letters 358 (2004) 33–36 www.elsevier.com/locate/neulet * Corresponding author. Tel.: þ 81-426-76-8831; fax: þ81-426-76-8841. E-mail address: inou@ls.toyaku.ac.jp (M. Inoue).