The Journal of Neurophysiology Vol. 86 No. 6 December 2001, pp. 2727-2735
Copyright ©2001 by the American Physiological Society

B2 Receptor-Mediated Enhanced Bradykinin Sensitivity of Rat Cutaneous C-Fiber Nociceptors During Persistent Inflammation

 ¹Department of Neural Regulation, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan; and  ²Department of Biochemistry and Molecular Genetics, University of Colorado Medical School, Denver, Colorado 80262

Banik, Ratan Kumar, Yasuko Kozaki, Jun Sato, Lajos Gera, and Kazue Mizumura. B2 Receptor-Mediated Enhanced Bradykinin Sensitivity of Rat Cutaneous C-Fiber Nociceptors During Persistent Inflammation. J. Neurophysiol. 86: 2727-2735, 2001. Bradykinin (BK), which has potent algesic and sensitizing effect on nociceptors, is of current interest in understanding the mechanisms of chronic pain. BK response is mediated by B2 receptor in normal conditions; however, findings that B1 receptor blockade alleviated hyperalgesia in inflammation have been highlighting the role of B1 receptor in pathological conditions. It has not yet been clear whether nociceptor activities are modified by B1 receptor agonists or antagonists during inflammation. In addition, previous studies reported the change in BK sensitivity of nociceptors during short-lasting inflammation, and data in persistent inflammation are lacking. Therefore we investigated whether an experimentally induced persistent inflammatory state modulates the BK sensitivity of nociceptors and which receptor subtype plays a more important role in this condition. Complete Freund's adjuvant was injected into the rat-tail and after 2-3 wk, persistent inflammation developed, which was prominent in the ankle joint. Using an in vitro skin-saphenous nerve preparation, single-fiber recordings were made from mechano-heat sensitive C-fiber nociceptors innervating rat hairy hindpaw skin, and their responses were compared with those obtained from C-fibers tested similarly in normal animals. BK at 10⁸ M excited none of the 10 C-fibers in normal animals while it excited 5 of 11 (45%) C-fibers of inflamed animals, and at 10⁶ M BK excited all of the 11 inflamed C-fibers (or 94% of 36 tested C-fibers) but only 4 of 10 (or 45% of 58 tested C-fibers) in normal animals. Thus the concentration-response curves based on the incidence of BK induced excitation, and the total number of impulses evoked in response to BK were significantly shifted to the left. Moreover, an increased percentage of the inflamed C-fibers responded to 10⁶ M BK with bursting or high-frequency discharges. Thirty-percent of inflamed C-fibers had spontaneous activity, and these fibers showed comparatively less tachyphylaxis to consecutive second and third 10⁶ M BK stimulation. A B2 receptor antagonist (D-Arg-[Hyp³, Thi^5,8,D-phe⁷]-BK) completely eliminated BK responses in inflamed rats, while B1 receptor antagonists (B 9958 and Des-Arg⁹-[Leu⁸]-BK) had no effect. Selective B1 receptor agonist (Des-Arg¹⁰-Kallidin) excited 46% (n = 13) of inflamed C-fibers at 10⁵ M concentration, which is 1,000 times higher than that of BK needed to excite the same percentage of inflamed C-fibers. We conclude that in chronically inflamed tissue, sensitivity of C-fiber nociceptors to BK, which is B2 receptor mediated, is strongly increased and that B1 receptor may not be important to a persistent inflammatory state, at least at the primary afferent level.