Characterization of pressor and visceromotor reflex responses to bladder distention in rats: Sources of variability and effect of analgesics

Purpose: We assessed the usefulness of cardiovascular and visceromotor responses to bladder distention as measures of acute visceral nociception in rats by determining the reliability of these responses. Materials and Methods: Halothane anesthetized male and female Sprague-Dawley rats were acutely instrumented with tracheal, jugular venous, carotid arterial. and bladder cannulas. Wires were inserted into the abdominal musculature to enable myoelectrical activity measurement. Anesthesia was decreased until flexion reflexes were present. Repeat phasic and graded bladder distention was administered, and arterial blood pressure and abdominal electromyography activity were continuously monitored. We determined the effects of gender, vaginal smear estrous cycle stage and drug treatment on the measured responses. Results: Bladder distention produced reliable presser and visceromotor (abdominal contractile) responses. There was great inter-animal variability in response vigor but good reproducibility was noted within individual animals. During slow bladder filling bladder contractions were not noted at this level of anesthesia. Sex differences included a more vigorous reflex response in females than in males, which was most vigorous in females in proestrus. Repeat bladder distention led to increasingly vigorous presser responses and the improved reliability of visceromotor responses. Intravenous morphine and lidocaine dose dependently inhibited the reflex responses. Conclusions: Presser and visceromotor responses to bladder distention in halothane anesthetized rats are reliable measures of acute bladder nociception that may prove useful for analgesic screening and in studies of hormonal effects on nociception.