Activation and inhibition of the micturition reflex by penile afferents in the cat

Abstract

Coordination of the urinary bladder and the external urethral sphincter is controlled by descending projections from the pons and is also subject to modulation by segmental afferents. We quantified the effects on the micturition reflex of sensory inputs from genital afferents traveling in the penile component of the somatic pudendal nerve by electrical stimulation of the dorsal nerve of the penis (DNP) in alpha-chloralose anesthetized male cats. Depending on the frequency of stimulation (range, 1-40 Hz), activation of penile afferents either inhibited contractions of the bladder and promoted urine storage or activated the bladder and produced micturition. Stimulation of the DNP at 5-10 Hz inhibited distension-evoked contractions and increased the maximum bladder capacity before incontinence. Conversely, stimulation at 33 and 40 Hz augmented distension-evoked contractions. When the bladder was filled above a threshold volume (70% of the volume necessary for distension-evoked contractions), stimulation at 20-40 Hz activated de novo the micturition reflex and elicited detrusor contractions that increased voiding efficiency compared with distension-evoked voiding. Electrical stimulation of the DNP with a cuff electrode or percutaneous wire electrode produced similar results. The ability to evoke detrusor contractions by activation of the DNP was preserved following acute spinal cord transection. These results demonstrate a clear role of genital afferents in modulating the micturition reflex and suggest the DNP as a potential target for functional restoration of bladder control using electrical stimulation.

Figure 1. Anatomy of the dorsal nerve of the penis (DNP, a) of the cat

Dorsolateral view of the penile body from the ventral side of the cat. A branch of the DNP was observed leaving the body of the penis and innervating the skin of the prepuce and perineum (b). Another branch of the DNP coursed along the lateral body of the penis to the glans and gave off nerves towards the urethra (c). A third, more superficial branch traveled along the dorsal aspect of the body of the penis to the glans penis and prepuce (d).

FIGURE 2. Frequency dependent bladder responses to direct electrical stimulation of the DNP

(A) Direct stimulation of the DNP is shown evoking destrusor contractions at bladder volumes between the STV and DTV in 2 different cats. Contractions were generated within 5 seconds of the onset of high frequency stimulation (33 and 40 Hz in the upper pressure trace, 20–40 Hz in the lower pressure trace) and ended with the termination of stimulation or shortly thereafter. Stimulation at low frequencies (≤10 Hz) did not elicit contractions. The black bars indicate the duration of stimulation, which consisted of 20 or 30 second trains at the frequency and amplitude above each bar. (B) Percent of trials in which direct stimulation of the DNP elicited detrusor contractions at different stimulus frequencies. The ability to elicit detrusor contraction by stimulation of the DNP was dependent on stimulation frequency (p<0.001, Kruskal-Wallis test, n=453 trials across 7 cats). Stimulation at 33 and 40 Hz consistently evoked detrusor contractions when stimulation was applied at appropriate bladder volumes and stimulus amplitude. Stimulation at 20 Hz evoked detrusor contractions in 4 of the 7 cats represented in the figure. Stimulation at 20, 33, and 40 Hz elicited contractions in a significantly greater percentage of trials than stimulation at 2–10 Hz (*p<0.01, Bonferroni inequalities). Bladder volumes were above STVs and below DTVs. Stimulus amplitudes ranged from 150μA-600μA (all amplitudes were 2–4x the threshold to elicit a bladder response). The number above each bar is the number of trials.

FIGURE 3. Direct stimulation of the DNP inhibited or augmented distension evoked detrusor contractions dependent on the stimulation frequency

(A) Intravesical pressure in 2 cats shows inhibition and augmentation of distension evoked detrusor contractions by stimulation of the DNP. Stimulation at 10 Hz (and 20Hz in the lower trace) inhibited distension evoked contractions, while stimulation at 33 and 40 Hz augmented distension evoked contractions. Bladder volumes were above the DTVs. The black bars indicate the duration of the DNP stimulation, which consisted of 20 or 30 second trains at the frequency and amplitude above each bar. (B) A more detailed view of the detrusor inhibition (left panel) and augmentation (right panel) from the lower trace in (A). Stimulation at 10 and 20 Hz after the onset of distension evoked contractions caused the intravesical pressure to rapidly return to baseline. Stimulation at 33 Hz during a distension evoked contraction caused a rapid rise (after a brief decrease) in the intravesical pressure. (C) Percent of trials in which direct stimulation inhibited or augmented distension evoked detrusor contractions at different stimulus frequencies. Stimulation at 5–10 Hz inhibited detrusor contractions in a greater percentage of trials than stimulation at 2, 20, 33, and 40 Hz (*p<0.01, Bonferroni inequalities, n=7 cats). Stimulation at 33 and 40 Hz augmented contractions in a greater percentage of trials than stimulation at 2–20 Hz (**p<0.05, Bonferroni inequalities, n=7 cats). Bladder volumes were above the DTV and stimulus amplitudes ranged from 160μA-800μA (all amplitudes were 2–4x threshold). Numbers above each bar are the number of trials at each frequency.

FIGURE 4. Effect of direct stimulation of the DNP on distension evoked detrusor contractions

(A) The pressure-time produce (PTP) was computed as the area under the intravesical pressure trace (minus baseline pressure) for 20 seconds after the onset of a distension evoked contraction. The intravesical pressure trace shows the PTP (shaded area) for 10 Hz stimulation, a distension evoked contraction with no stimulation, and 33 Hz stimulation. (B) The normalized pressure-time products (PTPs) of the first 20 seconds of distension evoked detrusor contractions with or without stimulation are shown (mean ± standard deviation). The relative PTP of the detrusor contraction was dependent on the stimulation frequency (p<0.001, Kruskal-Wallis test, n=112 trials across 6 cats). Inhibitory stimulation (5–10 Hz) within the first 10 seconds of a distension evoked contraction reduced the PTP of distension evoked detrusor contractions significantly, while excitatory stimulation (20–40 Hz) increased detrusor PTP (*p<0.01, Bonferroni inequalities).

FIGURE 5. Periurethral (PU) and external anal sphincter (EAS) electromyograms evoked by direct electrical stimulation of the DNP

Stimulation was delivered at either 1 or 20 Hz (10 second trains, 100μs pulses, 200μA) with the bladder empty. Reflex responses were elicited following each pulse during 1 Hz stimulation, but the response disappeared after the first 2–4 pulses during 20 Hz stimulation.

FIGURE 6. Direct stimulation of the DNP improved continence and voiding

Cystometrograms are shown for a single cat with no stimulation, inhibitory stimulation, and excitatory stimulation. The bladder was filled at 1ml/min. (A) Intravesical pressure during bladder filling in the absence of stimulation (upper plot) and the volume voided by the distension evoked contractions (lower plot). Continence was lost at 15.3ml (indicated by the dashed vertical line in the upper plot) and the total volume voided was 8.2ml. (B) Intravesical pressure during bladder filling with continuous 10 Hz stimulation of the DNP starting at 8ml infused volume. A sustained detrusor contraction occurred at 23.4ml (indicated by the dashed vertical line). (D) Intravesical pressure during bladder filling in the absence of stimulation but with stimulation evoked bladder voiding. Continence was lost at 14.2ml (indicated by the dashed vertical line in the pressure trace), and the total volume voided was 11.0ml. Stimulation trains are indicated by the black bars above the intravesical pressure traces.

FIGURE 7. The contractile detrusor response to direct electrical stimulation of the DNP was present following acute SCI

The intravesical pressure responses to 20 second trains of stimulation at 33 Hz, 200μA before SCT (A) and 15 hours after SCT at T10 (B) in one cat. (C) The percent of trials that elicited detrusor contractions at different frequencies shows that detrusor contraction was dependent on stimulation frequency (p<0.0001, Kruskal-Wallis test, n=75 trials across 2 cats). Stimulation at 20, 33, and 40 Hz elicited contractions significantly more frequently than stimulation at 2, 5, and 10 Hz (*p<0.05, Bonferroni inequalities). Stimulation at 2, 5, or 10 Hz did not elicit any detrusor contractions following SCT.