Abstract
We studied the mechanisms by which activation of primary afferent nociceptors inhibits bradykinin-induced plasma extravasation in the rat. First, capsaicin, administered into the plantar surface of the hindpaw, dose-dependently inhibited bradykinin-induced plasma extravasation in the knee joint, a site distant from the noxious stimulus. The inhibitory effect of capsaicin was markedly attenuated after T12/L1 spinal transection combined with lumbar preganglionic sympathectomy, which interrupts ascending spinal tracts to rostral sites and to spinal sympathetic and sympathoadrenal outflow. Second, interruption of the sympathetics (cutting the L1-3 white rami) or surgical adrenal denervation significantly attenuated capsaicin-induced inhibition of bradykinin-induced plasma extravasation. Interruption of the sympathoadrenal pathway produced the largest attenuation. Lesioning of the hypothalamic-pituitary-adrenal axis did not affect the inhibitory action of capsaicin. Third, intra-articular perfusion with phentolamine (10−5 M, an α-adrenoceptor antagonist), propranolol (10−5 M, a β-adrenoceptor antagonist), and naloxone (10−5 M, an opioidergic receptor antagonist) each attenuated the inhibitory action of capsaicin. Propranolol and naloxone produced the largest attenuation. Blocking glucocorticoid receptors (RU-38,486, 30 mg/kg s.c.) did not affect the inhibitory action of intraplantar capsaicin. Fourth, the magnitude of the attenuation of capsaicin-induced inhibition of bradykinin-induced plasma extravasation after a combined treatment of surgical lumbar sympathetic decentralization with intra-articular phentolamine or surgical adrenal denervation with intra-articular propranolol or naloxone was similar to each of the surgical or pharmacological treatments of the same axis alone. These results support the suggestion that two neural/endocrine circuits, sympathoadrenal and sympathetic, account for most, if not all, of nociceptor activity-induced inhibition of bradykinin-induced plasma extravasation produced by capsaicin.
Footnotes
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Send reprint requests to: Frederick Miao, M.D, Ph.D, National Institutes of Health Pain Center, University of California at San Francisco, 521 Parnassus Ave., C-522/Box 0440, San Francisco, CA 94143-0440. E-mail: fjmiao{at}itsa.ucsf.edu
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↵1 This work was supported by grants from the Tobacco-Related Disease Research Program (8RT-0032) and National Institutes of Health Grant AR32634. Portions of the present work were presented at the XIIIth International Congress of Pharmacology (IUPHAR), Munich, Germany, 1998.
- Abbreviations:
- HPA
- hypothalamo-pituitary-adrenal
- BK
- bradykinin
- PE
- synovial plasma extravasation
- Received June 16, 1999.
- Accepted August 13, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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