Background and objective: Previous studies have reported immunoreactive opioid nerve fibers in the detrusor and lower urinary tract sphincters. However, there is a paucity of in vivo studies demonstrating the direct effect of endogenous opioids in these structures. In the present study, we investigated the contractile actions of intra-arterially administered exogenous Dynorphin-A, Met-enkephalin, Leu-enkephalin, morphine, and the opioid antagonist naltrexone on the female rat intrinsic urethral sphincter in vivo.
Methods: Intraurethral pressure was recorded by a catheter placed at the maximum pressure zone of the intrinsic urethral sphincter in anesthetized female Sprague-Dawley rats. The effects of different opioids were studied and expressed as means and as percentages of pressure change (cmH(2)O) of the baseline intraurethral pressure.
Results: Dynorphin-A, Met-enkephalin, and Leu-enkephalin evoked rapid, long-lasting contractile effects on the female rat urethra. The greatest intraurethral pressure increase was evoked by Dynorphin-A (89.2 +/- 15.3%). For Met-enkephalin, intraurethral pressure increased by 70.2 +/- 21.8% and for Leu-enkephalin, the pressure increase was 60.6 +/- 20%. Morphine, however, evoked inconsistent intraurethral pressure changes, increasing the urethral pressure in three subjects and lowering the pressure in the remaining six subjects. The opioid antagonist naltrexone reduced the intraurethral pressure by a mean of -19.0 +/- 5.8%.
Conclusion: Results of the present study suggest that endogenous opioids by their contractile action on the intrinsic urethral sphincter may play a role in the control of continence in rats, additional to cholinergic and noradrenergic pathways.
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