Mechanisms of reflex bladder activation by pudendal afferents

Abstract

Activation of pudendal afferents can evoke bladder contraction or relaxation dependent on the frequency of stimulation, but the mechanisms of reflex bladder excitation evoked by pudendal afferent stimulation are unknown. The objective of this study was to determine the contributions of sympathetic and parasympathetic mechanisms to bladder contractions evoked by stimulation of the dorsal nerve of the penis (DNP) in α-chloralose anesthetized adult male cats. Bladder contractions were evoked by DNP stimulation only above a bladder volume threshold equal to 73 ± 12% of the distension-evoked reflex contraction volume threshold. Bilateral hypogastric nerve transection (to eliminate sympathetic innervation of the bladder) or administration of propranolol (a β-adrenergic antagonist) decreased the stimulation-evoked and distension-evoked volume thresholds by -25% to -39%. Neither hypogastric nerve transection nor propranolol affected contraction magnitude, and robust bladder contractions were still evoked by stimulation at volume thresholds below the distension-evoked volume threshold. As well, inhibition of distention-evoked reflex bladder contractions by 10 Hz stimulation of the DNP was preserved following bilateral hypogastric nerve transection. Administration of phentolamine (an α-adrenergic antagonist) increased stimulation-evoked and distension-evoked volume thresholds by 18%, but again, robust contractions were still evoked by stimulation at volumes below the distension-evoked threshold. These results indicate that sympathetic mechanisms contribute to establishing the volume dependence of reflex contractions but are not critical to the excitatory pudendal to bladder reflex. A strong correlation between the magnitude of stimulation-evoked bladder contractions and bladder volume supports that convergence of pelvic afferents and pudendal afferents is responsible for bladder excitation evoked by pudendal afferents. Further, abolition of stimulation-evoked bladder contractions following administration of hexamethonium bromide confirmed that contractions were generated by pelvic efferent activation via the pelvic ganglion. These findings indicate that pudendal afferent stimulation evokes bladder contractions through convergence with pelvic afferents to increase pelvic efferent activity.

Fig. 1.

Volume thresholds for stimulation-evoked and distension-evoked reflex bladder contractions were determined under control and sympathetic block conditions. A: control filling trial consisting of a 1-ml bolus of saline injected into the bladder every minute with 20 s of 33 Hz stimulation of pudendal afferents in the dorsal nerve of the penis (DNP) delivered between boluses. The stimulation-evoked contraction volume threshold (S) was 17 ml and the distension-evoked threshold (D) was 21 ml. B: a filling trial after bilateral hypogastric nerve transection. The stimulation-evoked contraction volume thresholds decreased to 10 ml and the distension-evoked threshold decreased to 13 ml. C: an example bladder pressure trace at the stimulation-evoked volume threshold. D: an example bladder pressure trace at the distension-evoked volume threshold. A–D: black bars under the pressure trace indicate DNP stimulation at 33 Hz and 2× the intensity threshold for evoking a reflex response in the external anal sphincter.

Fig. 2.

Effect of propranolol administration on bladder contractions evoked by DNP stimulation or distension. A: example bladder responses evoked by 33 Hz DNP stimulation before and after propranolol administration (1 mg/kg). The contractions were evoked during bladder filling trials at bladder volumes 1 ml less than the distension-evoked threshold volume. B: normalized volume thresholds before and after propranolol administration were significantly different. (P < 10⁻⁴, ANOVA, n = 6 cats). *Significant difference between stimulation-evoked and distension-evoked volume thresholds for control trials or for propranolol trials (P < 0.05, post hoc Bonferroni comparisons). †Significant difference between control and propranolol stimulation volume thresholds or for distension volume thresholds (P < 0.05, post hoc Bonferroni comparisons). C: normalized mean contraction magnitudes evoked by stimulation before and after propranolol were not different (P = 0.15, t-test, n = 6 cats). Also, distension-evoked contraction magnitudes increased slightly following propranolol, but the difference compared with control distension-evoked contractions was not significant (P = 0.28).

Fig. 3.

Effect of phentolamine administration on bladder contractions evoked by DNP stimulation or distension. A: contractions evoked at bladder volumes 1 ml less than the distension-evoked threshold volumes by 33 Hz DNP stimulation before and after phentolamine (2 mg/kg). B: normalized stimulation-evoked and distension-evoked reflex bladder contraction volume thresholds before and after phentolamine were significantly different (P < 10⁻⁵, ANOVA, n = 6 cats). *Significant difference between stimulation-evoked and distension-evoked volume thresholds for control trials or for phentolamine trials (P < 0.05, post hoc Bonferroni comparisons). †Significant difference between control and phentolamine stimulation volume thresholds or for distension volume thresholds (P < 0.05, post hoc Bonferroni comparisons). C: normalized mean contraction magnitudes evoked by 33 Hz DNP stimulation before and after phentolamine (*P < 0.02, t-test, n = 6 cats). Distension-evoked contractions decreased after phentolamine, but the difference in relative contraction magnitudes was not significant (P = 0.34).

Fig. 4.

Effect of bilateral hypogastric nerve transection and subsequent adrenergic antagonist administration on bladder contractions evoked by DNP stimulation or distension. A: contractions evoked by 33 Hz DNP stimulation at a volume 1 cc less than the distension-evoked threshold volumes before and after bilateral hypogastric nerve transection. B: normalized volume thresholds before and after hypogastric nerve transection and after subsequent propranolol and phentolamine administration. Volume thresholds were significantly different (P < 10⁻¹², ANOVA, n = 36 trials). *Stimulation-evoked volume thresholds were significantly less than distension-evoked thresholds for the treatment group (P < 0.05, post hoc Bonferroni comparisons). †Significant difference between treatment and control volume thresholds for stimulation thresholds or for distension thresholds (P < 0.05, post hoc Bonferroni comparisons). C: normalized mean contraction amplitudes evoked by DNP stimulation before and after hypogastric nerve transection and subsequent drug administration were not significantly different (P = 0.35, t-test, n = 36 contractions). Normalized distension-evoked contraction magnitudes also were not significantly altered by hypogastric nerve transection or subsequent drug administration (P = 0.96).

Fig. 5.

Hexamethonium bromide abolished the bladder contractions evoked by DNP stimulation. DNP stimulation [black bars, 33 Hz at (A) 300 μA or (B) 200 μA] evoked bladder contractions following (A) hypogastric nerve transection and (B) 6 h after acute spinal cord transection at the T10 vertebral level, but administration of hexamethonium bromide (1 mg/kg) abolished the stimulation-evoked contractions. The stimulation-evoked bladder contractions returned ∼14 min (A) and 20 min (B) later.

Fig. 6.

The magnitude of bladder contractions evoked by DNP stimulation were strongly correlated with bladder volume before (A) and after (B) bilateral hypogastric nerve transection. The magnitudes of DNP stimulation-evoked bladder contractions were determined at 1-cc volume intervals between the stimulation-evoked contraction volume threshold and suprathreshold volumes. Contraction magnitudes were normalized by scaling between 0 (minimum value) and 1 (maximum value). A, left: normalized mean contraction pressures as a function of normalized bladder volume; right: regression lines (linear or quadratic) for the relationship between bladder volume and mean contraction magnitude for each cat. A significant correlation was found between mean contraction magnitude and bladder volume for all 3 cats investigated (P < 0.0001, Cat 1: r = 0.8518, n = 3; Cat 2: r = 0.8480, n = 7; Cat 3: r = 0.8708, n = 4). B: after bilateral hypogastric nerve transection. Left: normalized maximum contraction magnitudes as a function of normalized bladder volume. Right: regression lines (linear or quadratic) for the relationship between bladder volume and maximum contraction magnitude for each cat. A significant correlation was found between maximum contraction magnitude and bladder volume for all 3 cats investigated (P < 0.0001, Cat 1: r = 0.8893, n = 3; Cat 2: r = 0.8687, n = 7; Cat 3: r = 0.8688, n = 4).

Fig. 7.

Inhibition of distension-evoked reflex bladder contractions by 10 Hz DNP stimulation before and after bilateral hypogastric nerve transection. A: bladder inhibition evoked by 10 Hz DNP stimulation under control conditions. B: after hypogastric transection, 10 Hz DNP stimulation still evoked bladder inhibition. A–B: black bars indicate DNP stimulation. C: bladder pressures during the last 10 s of 10 Hz DNP stimulation were compared with the baseline bladder pressure before the distension-evoked contractions. Bladder pressure relative to baseline was not significantly different for DNP stimulation-evoked inhibition before and after hypogastric nerve transection (P = 0.74, 2-tailed, unpaired t-test, n = 20 trials across 3 cats).