Evaluation of spontaneous oro–facial neuropathic pain in animal model using high resolution MRI

Introduction: In our clinical situation, pain is evaluated by the subjective complaints reported by patients because an objective measurement is still unrealizable. As an alter native, we usually evaluate pain–related behaviour to analyze animal pain. However, the phenomenon of analyzing pain–related behaviour is limited because some amounts of stimulations are required to be provided first. At present, the opinion that pain–related behaviour is not a response to pain but a mere reflex still cannot be completely disproved. Moreover, as observed with chronic pain patients, pain triggered by some kind of stimuli is less important than pain experienced continuous ly in a given period from our group’s clinical data. This means that the common pain study using the general way of evaluation cannot contribute to new analgesics’ discovery which patients are eager to await. The goal of this study is to evaluate rodents’ spontaneous pain objectively without any stimulation by high resolutio n MRI. Methods: All surgical and experimental protocols were approved by the Institutional Animal Care and Use Committee of Osaka university graduate school of medicine. 31 male Wister rats were anesthetized with sodium pentobarbital (60 mg/kg i.p.) and sevoflurane (2 – 3%). An infra–orbital nerve (ION) loose ligation model was made by the procedure that two nylon (5–0) ligatures (2 mm apart) were loosely tied around the ION. Sham–operation was performed only by cutting the skin on the face and suturing in the same way as the procedure of ION loose ligation model. The scanning schedule was set 3 weeks after surgery and 3 days before the scanning day, 5 mg/kg manganese was injected intraperitoneally for three days. MRI measurements were performed with an 11.7 T MR scanner (AVANCE 500WB, Bruker BioSpin MRI GmbH, Ettlingen, Germany), under isoflurane anesthesia (2 – 2.5%), T1 weighted MR–images were acquired with a spin echo sequence. All the images were standardized and divided into 52 areas automatically and uptakes of manganese were analyzed. Tukey–Kramer multiple comparison test and correlation analyses were performed using JMP12.0.0 software.