The effect of tramadol on sneeze-induced urethral continence reflex through μ-opioid receptors in the spinal cord in rats.
We examined the efficacy of tramadol on the urethral reflex during sneezing, as well as the role of µ-opioid receptors in the spinal cord, in rats. Forty-one female Sprague-Dawley rats were used. The rats were divided into normal female rats and rats with vaginal distension (VD), which mimics stress urinary incontinence (SUI) in humans. Under urethane anesthesia, the sneeze-induced amplitude of urethral responses (AUR) and baseline pressure (BP) were examined after intravenous injection of tramadol using a microtransducer-tipped catheter in both rat groups. The effect of intrathecal cyprodime, a selective µ-opioid receptor antagonist, following intravenous tramadol injection was examined in normal rats. The tilt leak point pressure (tilt LPP) after intravenous tramadol injection was also evaluated in both groups. In normal rats, tramadol enhanced the AUR and BP by 33.2% and 19.5%, respectively. Tramadol also increased BP by 13.9% in rats with VD, but it did not change AUR. Intrathecal cyprodime alone did not change AUR, but it decreased BP. However, tramadol-provoked increments in AUR were blocked by intrathecal cyprodime, while BP was recovered to the level that it was before administration of cyprodime. Tramadol was associated with a significant elevation in tilt LPP: 24.8% and 19.5% in normal and VD rats, respectively. These findings suggest that tramadol effectively enhances the AUR at the spinal level and BP peripherally. Therefore, stimulation of the spinal µ-opioid receptors may be useful for the treatment of SUI.