1. Department of Anesthesiology, Yokohama City University, 3–9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan
2. Pain Mechanism Research Group, 1–757 Asahimachi-dori, Chuo-ku, Niigata 951-8510, Japan
3. Department of Neuroanatomy, Yokohama City University, 3–9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan
4. Present Address: Division of Anesthesiology, Niigata University Graduate School of Medical and Dental Sciences, 1–757 Asahimachi-dori, Chuo-ku, Niigata 951-8510, Japan