Bradykinin‐induced activation of nociceptors: receptor and mechanistic studies on the neonatal rat spinal cord‐tail preparation in vitro

Open Access
Publisher Website
Google Scholar
Abstract
1 The effects of bradykinin on nociceptors have been characterized on a preparation of the neonatal rat spinal cord with functionally connected tail maintained in vitro. Administration of bradykinin to the tail activated capsaicin-sensitive peripheral fibres and evoked a concentration-dependent (EC50 = 130 nm) depolarization recorded from a spinal ventral root (L3-L5). 2 The response to bradykinin was unaffected by the peptidase inhibitors, bestatin (0.4 mm), thiorphan (1 μm), phosphoramidon (1 μm) and MERGETPA (10 μm) or by the presence of calcium blocking agents, cadmium (200 μm) and nifedipine (10 μm). 3 Inhibition of cyclo-oxygenase with indomethacin (1–5 μm), aspirin (1–10 μm) and paracetamol (10–50 μm) consistently attenuated responses to bradykinin. 4 The effect of bradykinin was mimicked by the phorbol ester PDBu, an activator of protein kinase C. The response to bradykinin was attenuated following desensitization to PDBu but desensitization to bradykinin did not induce a cross-desensitization to PDBu. The protein kinase C inhibitor staurosporine (10–500 nm) consistently attenuated the effects of PDBu and bradykinin. 5 Bradykinin responses were reversibly enhanced by dibutyryl cyclic AMP (100 μm). However dibutyryl cyclic GMP (0.5 mm) and nitroprusside (10 μm) produced prolonged block of responsiveness to bradykinin. Prolonged superfusion with pertussis toxin did not affect responses to bradykinin. 6 The B1-receptor agonist des Arg9-bradykinin (10–100 μm) was ineffective alone or after prolonged exposure of the tail to lipopolysaccharide (100 ng ml−1) or epidermal growth factor (100 ng ml−1) to induce B1 receptors. The B1-receptor antagonist, des Arg9 Leu8-bradykinin (10 μm) did not attenuate the response to bradykinin. A number of bradykinin B2 antagonists selectively and reversibly attenuated the response to bradykinin. The rank order potency was Hoe 140 > LysLys [Hyp3,Thi5,8,d-Phe7]-bradykinin > d-Arg[Hyp3, Thi5,8, d-Phe7]-bradykinin = d-Arg[Hyp2,Thi5,8, d-Phe7]-bradykinin. 7 These data show that bradykinin produces concentration-dependent activation of peripheral nociceptors in the neonatal rat tail. The responses were unaffected by calcium channel block and were partially dependent on the production of prostanoids. Bradykinin-evoked responses were consistent with the activation of protein kinase C-dependent mechanisms. Cyclic GMP-dependent mechanisms may be involved in bradykinin-receptor desensitization whereas cyclic-AMP dependent mechanisms increase fibre excitability and facilitate bradykinin-induced responses. The effects of bradykinin were mediated by a B2 receptor.