Pharmacologic evaluation of pressor and visceromotor reflex responses to bladder distension

Several mechanisms that are involved in acute rat bladder nociception were examined. The nociceptive response was measured by analyzing both cardiovascular and visceromotor reflex responses to urinary bladder distension. The contributions of micro-opioid receptor, kappa-opioid receptor, sodium channels, muscarinic receptors, and cyclooxygenase, were explored with morphine, U50,488, mexiletine, oxybutynin, and naproxen, respectively.

Female Sprague-Dawley rats were acutely instrumented with jugular venous, carotid arterial, and bladder cannulas. Needle electrodes were placed directly into the abdominal musculature to measure myoelectrical activity subsequent to repeated phasic urinary bladder distension (60 mmHg for 20 sec in 3 min intervals) under 1% isoflurane. Drugs were administered by i.v. bolus injection 2 min prior to distension.

The analgesics morphine (ID50 0.69 mg/kg), U50,488 (1.34 mg/kg), and mexiletine (2.60 mg/kg) significantly inhibited the visceromotor reflex response to noxious urinary bladder distension. Oxybutynin also attenuated reflex responses to noxious urinary bladder distension to 41% of the maximal pressor response and 32% of the control visceromotor reflex response (3.01 and 5.05 mg/kg), respectively, indicating a role of muscarinic receptors in bladder nociception. Naproxen did not attenuate the pressor response, but moderately inhibited visceromotor reflex to 45% of control at 30 mg/kg (P < 0.05).

Current results using the rat urinary bladder distension model are consistent with previous research demonstrating a role of the analgesics (morphine, U50,488, and mexiletine) in the inhibition of visceral nociceptive transmission. The utility of the reflex responses to urinary bladder distension may provide a method useful to examine mechanisms which target the bladder sensory pathway.