Effects of intravesical capsaicin and resiniferatoxin on distension-induced bladder contraction in conscious rats with and without chronic spinal cord injury

Purpose: To compare conscious, normal rats and rats with chronic spinal cord injury (CSI) in terms of the rhythmic bladder contractions (RBCs) induced by intravesical infusion of saline, and to determine how these contractions are influenced by intravesical capsaicin and resiniferatoxin, Materials and Methods: Female Sprague-Dawley rats, normal or with spinal transection at the level of Th-8-Th-9, were investigated cystometrically under isovolumetric conditions before and after intravesical administration of capsaicin or resiniferatoxin. Results: Spinal transection induced a significant increase in bladder weight. In both control and CSI animals, intravesical saline instillation induced reproducible RBCs that could be blocked by hexamethonium. Four weeks after the transection, the CSI animals had a significantly larger threshold volume than the controls, even after correction for bladder weight. The mean amplitude and duration of the RBCs did not differ between the two groups, but the frequency was significantly lower in CSI animals. Both capsaicin (0.1 and 1 mM) and resiniferatoxin (1 and 10 mu M), instilled intravesically, were found to inhibit; RBCs in both normal and CSI rats. There were no qualitative differences in the response to the drugs between the two groups. However, resiniferatoxin was approximately 100 times more potent than capsaicin. Conclusion: Capsaicin and resiniferatoxin inhibited RBCs in both normal and CSI rats, suggesting that activity in sensory fibers (C and A delta), which are sensitive to the action of these drugs, is initiated by bladder filling in both types of rat.