Bradykinin (BK) is well known as a potent mediator of pain and hyperalgesia. Using a highly sensitive nociception test, we found that intraplantar (i.pl.) injection of BK produced nociceptive hyper-responses in partial sciatic nerve-injured mice, compared with the control sham-operated animals. By use of selective agonists and antagonists, we revealed that BK nociception in sham-operated mice was mediated through B2 receptor, whereas that in injured mice was mediated through B1 receptor. When we examined the activation of extracellular signal-regulated protein kinase (ERK) in dorsal root ganglion (DRG) neurons upon i.pl. injection of BK, phosphorylated ERK was mainly observed in unmyelinated neurons in sham-operated mice, and in case of nerve-injured mice, ERK was mainly activated in myelinated neurons and satellite cells. The B1 receptor agonist, [Lys-des-Arg 9 ]-BK also produced nociceptive response and activated ERK only in nerve-injured mice. BK or B1 agonist-induced activation of ERK in DRG neurons of nerveinjured mice was completely blocked by pretreatment with antisense oligodeoxynucleotide (AS-ODN) for B1 receptor. We found that in sham-operated mice mainly B2 receptors were expressed in unmyelinated DRG neurons with a very little presence of B1 receptor. After nerve injury, B2 receptor expression drastically decreased, whereas B1 receptors were newly expressed mainly in myelinated DRG neurons and satellite cells. Finally, BK nociception in sham-operated mice was blocked by AS-ODN for B2 receptors and that in injured mice by AS-ODN for B1 receptors. Altogether, these findings confirm a switching of receptor and fiber subtype for BK nociception after peripheral nerve injury, which might contribute to the pathobiology of neuropathic pain.Traumatic nerve injury is often associated with local inflammation of the injured nerve and release of proinflammatory mediators including bradykinin, prostaglandins, tumor necrosis factor-␣, interleukin-1, etc. It has been proposed that the local release of such proinflammatory mediators might play a critical role in the development and maintenance of neuropathic pain (Bennett 1999;Cui et al., 2000;Schafers et al., 2003). The nonapeptide bradykinin is one of the most potent among the proinflammatory mediators and is produced from plasma globulin kininogens by action of specific enzyme kallikreins (Bhoola et al., 1992). It participates in processes such as tissue permeability, vascular dilation, smooth muscle contraction, and sensory nerve-ending stimulation (Regoli and Barabe 1980;Bhoola et al., 1992). Since the kallikreins are normally present in their inactive forms, only a small amount of bradykinin is present in the plasma and tissues under normal physiological conditions (Bhoola et al., 1992). After tissue injury, kallikreins are activated, and bradykinin is released from damaged tissue and mediates pain and hyperalgesia (Bhoola et al., 1992;Calixto et al., 2000).Upon release, bradykinin exerts its physiological effects through activation of two pharmacologi...