Neuroscience Research 74 (2012) 248–255 Contents lists available at SciVerse ScienceDirect Neuroscience Research jo u rn al hom epa g e: www.elsevier.com/locate/neures Opto-fMRI analysis for exploring the neuronal connectivity of the hippocampal formation in rats Yoshifumi Abe a , Masaki Sekino b,c,e , Yasushi Terazono d , Hiroyuki Ohsaki b,c , Yugo Fukazawa f , Seiichiro Sakai g , Hiromu Yawo g , Tatsuhiro Hisatsune a,b,∗ a Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan b Bioimaging Center, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan c Department of Advanced Energy, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan d Department of Complexity Sciences and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan e Department of Electrical Engineering and Information Systems, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan f Department of Anatomy and Molecular Cell Biology, Nagoya University Graduate School of Medicine, 65 Turumai, Syouwa, Nagoya 466-8550, Japan g Department of Developmental Biology and Neuroscience, Tohoku University Graduate School of Life Sciences, 2-1-1Katahira, Aoba-ku, Sendai 980-8577, Japan a r t i c l e i n f o Article history: Received 24 April 2012 Received in revised form 21 August 2012 Accepted 28 August 2012 Available online 5 September 2012 Keywords: Optogenetics Channelrhodopsin-2 fMRI Hippocampus Transgenic rat Neuronal network a b s t r a c t In recent years, optical stimulation of neurons that bear a light-gated cation channel, “Optogenetics”, has opened a new avenue for exploring neuronal connectivity of the nervous system. In this study, we applied a technique, “Opto-fMRI”, which combined optogenetics with blood oxygenation level-dependent (BOLD) functional MRI (fMRI), for examining the neuronal connectivity of the hippocampal formation in rats. Although the hippocampal formation is very important for memory formation and retrieval, there is little information on its neuronal connectivity, especially on its longitudinal axis of connection. For this purpose, we utilized a transgenic rat strain, expressing the light-gated cation channel channelrhodopsin- 2 (ChR2) under the regulation of the Thy1.2 promoter which permits the expression of the integrated gene in neurons. After optical stimulation targeting the dentate gyrus of the transgenic rat, we detected BOLD response of not only the dentate gyrus (DG) but also at the CA3 area. In addition, we detected the longitudinal-axis activation of the hippocampus after optical stimulation. Our study suggests that Opto- fMRI could be a tool for exploring the neuronal connectivity of the hippocampal formation, to understand the neural basis of memory formation and retrieval. © 2012 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved. 1. Introduction Optical stimulation of neurons expressing a light-gated cation channel, channelrhodopsin-2 (ChR2), is a new technique to explore the neural system (Boyden et al., 2005; Ishizuka et al., 2006). Optical stimulation enabled us to control the neural activity at a high temporal and spatial level. Also, functional magnetic reso- nance imaging (fMRI) based on blood oxygenation level-dependent (BOLD) response is a powerful tool to noninvasively investigate whole-brain activity (Ogawa et al., 1990). Furthermore, a technique which combined optogenetics with fMRI was developed and called optogenetics-fMRI (opto-fMRI). This technique enables measure- ment of altered neuronal activity and networks with a high spatial resolution across an entire structure or even the whole brain, by using optical stimulation of the region of interest. In recent studies, ∗ Corresponding author at: Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan. Tel.: +81 4 7136 3632; fax: +81 4 7136 3632. E-mail address: hisatsune@k.u-tokyo.ac.jp (T. Hisatsune). Lee et al. have demonstrated the potential of opto-fMRI to study the function of the motor cortex and the thalamus (Lee et al., 2010). Subsequently, the researchers have also demonstrated a BOLD response after optical stimulation in the primary somatosen- sory cortex (Desai et al., 2011; Kahn et al., 2011). In this study, we focused on the hippocampal formation. Hip- pocampal networks are very important for memory formation, retrieval, neurogenesis, and cognitive function. The hippocampus has been extensively studied in many levels of neuroscience. It is anatomically subdivided into three areas called CA1, CA2, and CA3 and, together with the dentate gyrus (DG), subiculum (Sub), and the entorhinal cortex (EC). The granule cells of the DG project to pyramidal cell’s dendrites in CA3 via the mossy fiber system and the axons of CA3 pyramidal cells run in the stratum radiatum of the CA1 region via the Schaffer collateral system. We understood the structural hippocampus, but did not comprehend the function of hippocampal circuit. Furthermore, the longitudinal-axis func- tional activation of the hippocampus and the functional relation between the hippocampus and other regions are not yet thoroughly understood. Therefore, a direct stimulation at the hippocampal for- mation using optogenetical stimulation and an analysis with fMRI 0168-0102/$ – see front matter © 2012 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved. http://dx.doi.org/10.1016/j.neures.2012.08.007