Recruitment of unmyelinated C-fibers mediates the bladder-inhibitory effects of tibial nerve stimulation in a continuous-fill anesthetized rat model

Mechanism of Action of Tibial Nerve Stimulation in the Treatment of Lower Urinary Tract Dysfunction

BACKGROUND AND OBJECTIVE

Tibial nerve stimulation (TNS) has long been used to effectively treat lower urinary tract dysfunction (LUTD). Although numerous studies have concentrated on TNS, its mechanism of action remains elusive. This review aimed to concentrate on the mechanism of action of TNS against LUTD.

MATERIALS AND METHODS

A literature search was performed in PubMed on October 31, 2022. In this study, we introduced the application of TNS for LUTD, summarized different methods used in exploring the mechanism of TNS, and discussed the next direction to investigate the mechanism of TNS.

RESULTS AND CONCLUSIONS

In this review, 97 studies, including clinical studies, animal experiments, and reviews, were used. TNS is an effective treatment for LUTD. The study of its mechanisms primarily concentrated on the central nervous system, tibial nerve pathway, receptors, and TNS frequency. More advanced equipment will be used in human experiments to investigate the central mechanism, and diverse animal experiments will be performed to explore the peripheral mechanism and parameters of TNS in the future.

Transdermal tibial nerve optogenetic stimulation targeting C-fibers

Purpose: To explore whether stimulation of C-fibers in tibial nerves can induce bladder inhibition by optogenetic transdermal illumination. Methods: Ten rats were injected with AAV2/6-hSyn-ChR2(H134R)-EYFP into the tibial nerves. Transurethral cystometry was performed 4 weeks after the virus injection. Illumination (473-nm blue light at 100 mW) was performed with the fiber positioned above the right hind paw near the ankle. The light transmission efficiency was examined with a laser power meter. The effects on cystometry were compared before and after illumination with the bladder infused with normal saline and acetic acid, respectively. Result: Upon transdermal delivery of 473-nm light at a peak power of 100 mW, the irradiance value of 0.653 mW/mm2 at the target region was detected, which is sufficient to activate opsins. The photothermal effect of 473-nm light is unremarkable. Acute inhibitory responses were not observed during stimulation regarding any of the bladder parameters; whereas, after laser illumination for 30 min, a statistically significant increase in bladder capacity with the bladder infused with normal saline (from 0.53 ± 0.04 mL to 0.72 ± 0.05 mL, p < 0.001) and acetic acid (from 0.25 ± 0.02 mL to 0.37 ± 0.04 mL, p < 0.001) was detected. A similar inhibitory response was observed with pulsed illumination at both 10Hz and 50Hz. However, illumination did not significantly influence base pressure, threshold pressure, or peak pressure. Conclusion: In this preliminary study, it can be inferred that the prolonged bladder inhibition is mediated by the stimulation of C-fibers in the tibial nerves, with no frequency-dependent characteristics. Although the 473-nm blue light has limited penetration efficacy, it is sufficient to modulate bladder functions through transdermal illumination on the superficial peripheral nervous system.

Prolonged inhibition of bladder function is evoked by low-amplitude electrical stimulation of the saphenous nerve in urethane-anesthetized rats

To better understand the effects of saphenous nerve (SN) stimulation on bladder function, we investigated the duration of electrical stimulation as a key variable in eliciting urodynamic changes. SN stimulation is a novel approach to electrically modulating bladder function. In previous animal studies, bladder-inhibitory responses were evoked by low-amplitude (25 μA) stimulus pulses applied in short-duration (10 min) trials and at frequencies between 10 and 20 Hz. Experiments were performed in urethane-anesthetized rats that were separated into three groups: intravesical saline infusion + SN stimulation (group A), intravesical 0.1% acetic acid infusion + SN stimulation (group B), and intravesical saline infusion + no SN stimulation (group C). Changes in bladder function- basal bladder pressure (P base ), contraction amplitude (ΔP), and inter-contraction interval (T ICI )-were measured in response to stimulation trials applied for different durations (10, 20, and 40 min). Trials were also repeated at frequencies of 10 and 20 Hz. In group A, longer-duration (40 min) stimulation trials applied at 10 Hz evoked overflow incontinence (OI) episodes that were characterized by significant changes in P base (122.7 ± 9.1%, p = 0.026), ΔP (-60.8 ± 12.8%, p = 0.044), and T ICI (-43.2 ± 13.0%, p = 0.031). Stimulation-evoked OI was observed in 5 of 8 animals and lasted for 56.5 ± 10.7 min. In contrast, no significant changes in bladder function were observed in either group B or group C. Our findings show that longer-duration trials consisting of electrical pulses applied at 10 Hz are important stimulation parameters that elicit inhibitory bladder responses in anesthetized rodents.

Inhibitory effects of a smartphone-controlled wearable transcutaneous tibial nerve stimulation device on bladder reflexes in anesthetized cats

OBJECTIVES

To explore the inhibitory effects of a novel, smartphone-controlled, and wearable tibial nerve stimulation device on nonnociceptive and nociceptive bladder reflexes in anesthetized cats and to compare the stimulus results of two current waveforms outputted by this new stimulator.

MATERIALS AND METHODS

A novel, intelligent tibial nerve stimulator was put on the ankles of 14 cats and controlled by a mobile application. Cystometrograms (CMGs) were performed repeatedly by infusing 0.9% normal saline (NS) and 0.5% acetic acid (AA) through a urethral catheter. Inhibitory effects were explored by measuring the bladder capacity (BC) in two areas: (1) on nonnociceptive bladder reflex (infused with NS) and on nociceptive bladder reflex (filled with AA to induce overactive bladder [OAB] model); and (2) under the stimulation of two different current waveforms (waveforms A and B).

RESULTS

In Group 1, the BC of AA-induced OAB (41.48 ± 8.40%) was significantly different compared with the capacity of a NS-infused bladder (104.89 ± 1.32%, p < 0.05). Both NS-filled (151.35 ± 5.71%, p < 0.05) and AA-instilled (71.41 ± 9.34%, p < 0.05) bladder volumes significantly increased after tibial nerve stimulation (TNS). In Group 2, the BC increased to 166.18 ± 15.17% (p = 0.026) and 127.64 ± 13.00% (p = 0.239), respectively, after TNS with waveforms A and B current.

CONCLUSIONS

Results revealed that this novel, smartphone-based, wearable, and wireless tibial nerve stimulation system could inhibit the micturition reflex on physiological condition, serving as a potential option for OAB treatment. In addition, the waveforms of stimulation current had an important influence on the effects of TNS.

Prolonged Inhibitory Effects of Repeated Tibial Nerve Stimulation on the Micturition Reflex in Decorticated Rats

OBJECTIVE

This study aimed to determine whether a short-term repeated stimulation of tibial nerve afferents induces a prolonged modulation effect on the micturition reflex in a decorticated rat model.

MATERIAL AND METHODS

Fifteen female Sprague-Dawley rats (250-350 g) were fully decorticated and paralyzed in the study. Tibial nerve stimulation (TNS) was delivered by inserting two pairs of needle electrodes close to the nerves at the level of the medial malleolus. Constant flow cystometries (0.07 mL/min) at approximately ten-minute intervals were performed, and the micturition threshold volume (MTV) was recorded and used as a dependent variable. After four to five stable recordings, the tibial nerves of both sides were stimulated continuously for five minutes at 10 Hz and at an intensity of three times the threshold for α-motor axons. Six same stimulations were applied repeatedly, with an interval of five minutes between each stimulation. Mean MTV was calculated on the basis of several cystometries in each half-hour period before, during, and after the six repeated TNS.

RESULTS

During the experiment, all the animals survived in good condition with relatively stable micturition reflexes, and a significant increase in MTV was detected after TNS. The strongest effect (mean = 178%) was observed during the first 30 minutes after six repeated stimulations. This obvious threshold increase remained for at least five hours.

CONCLUSIONS

A prolonged poststimulation modulatory effect on the micturition reflex was induced by short-term repeated TNS in decorticated rats. This study provides a theoretical explanation for the clinical benefit of TNS in patients with overactive bladder and suggests decorticated rats as a promising model for further investigation of the neurophysiological mechanisms underlying the bladder inhibitory response induced by TNS.

Gate Mechanism and Parameter Analysis of Anodal-First Waveforms for Improving Selectivity of C-Fiber Nerves

PURPOSE

Few investigations have been conducted on the selective stimulation of small-radius unmyelinated C nerves (C), which are critical to both the recovery of damaged nerves and pain suppression. The purpose of this study is to understand how an anodal pulse in an anodal-first stimulation could improve C-selectivity over myelinated nociceptive Aδ nerves (Aδ) and to further clarify the landscape of the solution space.

MATERIALS AND METHODS

An adapted Hodgkin-Huxley (HH) model and the McIntyre-Richardson-Grill (MRG) model were used for modeling C and Aδ, respectively, to analyze the underlying ion dynamics and the influence of relevant stimulation waveforms, including monopolar, polarity-symmetric, and asymmetric pulses.

RESULTS

The results showed that polarity asymmetric waveforms with preceding anodal stimulations benefit C-selectivity the most, underlain by the decrease in the potassium ion current of C.

CONCLUSION

The optimal parameters for C-selectivity have been identified in the low-frequency band, remarkably benefiting the design of selective stimulation waveforms for the recovery of damaged nerves and pain management.