Whole-Brain Functional Magnetic Resonance Imaging Mapping of Acute Nociceptive Responses Induced by Formalin in Rats Using Atlas Registration-Based Event-Related Analysis Yen-Yu I. Shih, 1,2 You-Yin Chen, 3 Chiao-Chi V. Chen, 2 Jyh-Cheng Chen, 4,5 Chen Chang, 2 * and Fu-Shan Jaw * Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan, Republic of China Department of Electrical and Control Engineering, National Chiao-Tung University, Hsinchu, Taiwan, Republic of China Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei, Taiwan, Republic of China Department of Education and Research, Taipei City Hospital, Taipei, Taiwan, Republic of China Nociceptive neuronal activation in subcortical regions has not been well investigated in functional magnetic resonance imaging (fMRI) studies. The present report aimed to use the blood oxygenation level-dependent (BOLD) fMRI technique to map nociceptive responses in both subcortical and cortical regions by employing a refined data processing method, the atlas registration- based event-related (ARBER) analysis technique. During fMRI acquisition, 5% formalin (50 ll) was injected into the left hindpaw to induce nociception. ARBER was then used to normalize the data among rats, and images were analyzed using automatic selection of the atlas-based region of interest. It was found that forma- lin-induced nociceptive processing increased BOLD sig- nals in both cortical and subcortical regions. The corti- cal activation was distributed over the cingulate, motor, somatosensory, insular, and visual cortices, and the subcortical activation involved the caudate putamen, hippocampus, periaqueductal gray, superior colliculus, thalamus, and hypothalamus. With the aid of ARBER, the present study revealed a detailed activation pattern that possibly indicated the recruitment of various parts of the nociceptive system. The results also demon- strated the utilization of ARBER in establishing an fMRI- based whole-brain nociceptive map. The formalin induced nociceptive images may serve as a template of central nociceptive responses, which can facilitate the future use of fMRI in evaluation of new drugs and pre- clinical therapies for pain. V V C 2008 Wiley-Liss, Inc. Key words: fMRI; pain; formalin; rat; atlas registration Assessing nociception in small animals using imag- ing techniques has drastically increased in the last decade. Blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) technique provides noninvasive, in vivo measurement of cerebral hemody- namics. Thus, it is a valuable approach in rapidly map- ping central nociceptive responses of small animals. Sev- eral pain paradigms have been used to image brain acti- vation, such as electrical stimulation-induced pain (Lowe et al., 2007) or chemical-induced nociception with, e.g., formalin (Tuor et al., 2000; Shah et al.,, 2005), capsaicin (Malisza and Docherty, 2001; Moylan Governo et al., 2006), and zymosan (Hess et al., 2007). Intense electrical stimulation tends to excite the Ab,Ad, and C fibers col- lectively (Chang and Shyu, 2001) and generates less spe- cific activation patterns (Tuor et al., 2000). In contrast, formalin is a more specific nociceptive stimulus, insofar as it activates mainly C fibers (Tjolsen et al., 1992). Tuor et al. (2002) reported that formalin induced BOLD responses occurring primarily in the frontal corti- cal areas of a-chloralose-anesthetized rats. However, formalin-evoked BOLD responses in subcortical regions, Contract grant sponsor: National Science Council, Taiwan, Republic of China; Contract grant number: NSC-94-2213-E-002-001; Contract grant number: NSC95-3112-B-001-009; Contract grant number: NSC95-3112-B-001-004. *Correspondence to: Fu-Shan Jaw, Institute of Biomedical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan, Republic of China. E-mail: jaw@ntu.edu.tw or Chen Chang, Institute of Biomedical Sciences, Academia Sinica, No. 128, Sec. 2, Aca- demia Road, Taipei, Taiwan, Republic of China. E-mail: bmcchen@ibms.sinica.edu.tw Received 26 August 2007; Revised 27 November 2007; Accepted 29 November 2007 Published online 21 February 2008 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jnr.21638 Journal of Neuroscience Research 86:1801–1811 (2008) ' 2008 Wiley-Liss, Inc.