Micturition in naive and morphine-dependent rats

Voiding responses were recorded in conscious water-loaded rats. Morphine sulphate (5 mg kg-1) elevated the volume threshold for micturition (MV); the group mean MV of 16 rats after morphine was 40% larger than control. Micturition was nevertheless complete since no urine remained in the bladder afterwards. The implantation of 2 or 4 morphine-base pellets (150 or 300 mg morphine) elevated for 12 days the MV in water-loaded rats. On the 3rd to the 10th day following implantation the group mean was approximately twice that of untreated controls. After micturition was over no residual urine was found in the bladder. Within 3 days the rats became tolerant to the antinociceptive action of the morphine-base pellets but little apparent tolerance developed to their action on micturition. On the 1st day after the pellets were removed, the mean MV was reduced. When withdrawal was precipitated by the administration of naloxone the MV was often too small to measure. This component of a withdrawal syndrome could be elicited in the rats throughout the 12 days of morphine pellet implantation. The administration of 20 mg kg-1 morphine sulphate to anaesthetized rats did not decrease the contractions of the urinary bladder to repetitive stimulation of its motor nerves at 1 and 20 Hz.