Optimization of Neuromodulation for Bladder Control in a Rat Cystitis Model.

OBJECTIVES: In a bladder overactivity model of cystitis induced by intravesical infusion of acetic acid (a.a.), several parameters of spinal nerve stimulation (SNS) were optimized using continuous infusion cystometry. The optimal stimulation was further characterized through measurements of urodynamic function using single-fill cystometry. MATERIALS AND METHODS: In anesthetized male rats, a cannula was placed into the bladder dome for saline or 0.3% a.a. infusion and intravesical pressure monitoring. For SNS, two teflon-coated stainless steel electrodes were placed bilaterally under each of the L6 spinal nerves, and current stimulation was controlled independently using two Grass stimulators. RESULTS: Stimulation of 1 Hz or 50 Hz at motor threshold (Tmot ) was ineffective for altering bladder activities, but 10-Hz SNS increased the infused volume (IV) in a stimulation intensity-dependent fashion (P < 0.01, mixed model repeated analysis). Pairwise comparisons of IV differences to each stimulation intensity show that IV during 1 × Tmot stimulation was significantly larger than 0 × Tmot (no stim, P = 0.001), while the IV during 2 × Tmot stimulation was significantly larger than other intensities tested (P < 0.01). The mean IV (±SEM) during 0 × Tmot (no stim), 0.5 × Tmot , 1 × Tmot , and 2 × Tmot were 0.23 ± 0.04 mL, 0.25 ± 0.03 mL, 0.26 ± 0.03 mL, and 0.40 ± 0.04 mL, respectively. In single-fill cystometry, 10-Hz SNS at 1 × Tmot and 2 × Tmot stimulation increased the IV, or voiding duration and threshold pressure. SNS did not produce significant effects on basal pressure and micturition pressure. CONCLUSIONS: SNS significantly attenuates hypersensitive micturition reflex; 10 Hz and high-intensity stimulation are mostly effective. Acute peripheral nerve activation increases the functional bladder capacity, which may be via mechanisms on the afferent arm of the bladder micturition reflex.

Preclinical assessment of potential interactions between botulinum toxin and neuromodulation for bladder micturition reflex.

BACKGROUND: While botulinum toxin A (BoNT-A) has become a more commonly used second-line treatment for patients with detrusor overactivity, it remains unknown whether the impacts of this therapy may persist to influence other therapies such as sacral neuromodulation. In this preclinical study we have evaluated urodynamic functions to intradetrusor injection of BoNT-A and the bladder inhibitory effects of spinal nerve stimulation (SNS) following BoNT-A treatment. METHODS: Female rats were anesthetized with 3 % isoflurane. BoNT-A (2 units, 0.2 ml) or saline were injected into the detrusor. Rats then were housed for 2 days to 1 month before neuromodulation study. Monopolar electrodes were placed under each of the L6 spinal nerve bilaterally under urethane anesthesia. A bladder cannula was inserted via the urethra for saline infusion and intravesical pressure recording. RESULTS: Intradetrusor injection of BoNT-A for 1-2 weeks or 1 month significantly increased bladder capacity compared with saline injection (p < 0.05, two-way ANOVA). Following BoNT-A, SNS attenuated the frequency of bladder contractions, either eliminating bladder contractions or reducing the contraction frequency during electrical stimulation. Inhibition of the contraction frequency by SNS following BoNT-A treated rats was not different from that measured following saline injection. CONCLUSIONS: BoNT-A increased the bladder capacity, but compensating for additional saline infusion to the enlarged urinary bladder in BoNT-A pretreated rats, the bladder contractions induced by bladder filling were attenuated by SNS. BoNT-A did not alter the ability of SNS to inhibit bladder contraction following intradetrusor injection of BoNT-A for 2 days, 1-2 weeks or 1 month. These results support further pre-clinical and clinical studies to evaluate potential interactions or combination therapy with neuromodulation and intradetrusor BoNT-A therapeutic approaches.

Differentiation and interaction of tibial versus spinal nerve stimulation for micturition control in the rat.

AIMS: To determine time course of the bladder inhibitory response to unilateral or bilateral stimulation of the tibial nerve (TN) and spinal nerve (SN) as well as the interaction of stimulation at these two sites. METHODS: In anesthetized female rats, a wire electrode was placed under either one or both of the TN or L6 SN. A cannula was placed into the bladder via the urethra. Saline infusion induced bladder rhythmic contraction (BRC). RESULTS: Compared to SN neuromodulation, TN neuromodulation is less efficacious. The first 5-min stimulation at three times motor threshold on the SN and TN decreased the BRC frequency to 9% and 69% of controls, respectively. In contrast to SN stimulation, bilateral TN neuromodulation is not more effective than unilateral and sustained TN stimulation results in an apparent desensitization of the bladder response. If a 15-min TN stimulation was applied, BRCs were shutdown only during the first 5 min of stimulation. If a 5-min stimulation, using sufficient current to abolish BRC, is repeated, at least 20 min between stimulations was required in order for the responses to the first and second stimulations to be equivalent. Finally, stimulation of the TN combined with SN never produced a significantly greater effect than TN or SN stimulation alone. CONCLUSIONS: Based on the current experiments, it would appear that SN neuromodulation of bladder activity is preferable to TN stimulation and there is no evidence to suggest that stimulation at both sites would offer a therapeutic advantage over spinal stimulation alone.

Neuromodulation attenuates bladder hyperactivity in a rat cystitis model.

BACKGROUND: We investigated the regulation of urinary bladder function by electrical stimulation of the L6 spinal nerve (SN) using cystometry in normal rats and in rats with cystitis induced by intravesical infusion of dilute acetic acid. METHODS: In anesthetized rats, a cannula was placed into the bladder dome for saline/acetic acid infusion and intravesical pressure monitoring. Threshold pressure (TP), basal pressure (BP) and inter-contraction interval (ICI) were measured from the bladder pressure recording and void volume (VV) was measured by weighing the voided fluid. RESULTS: Comparison of cystometrograms obtained with infusion of saline or acetic acid showed that acetic acid decreases TP, ICI and VV. These excitatory effects, characteristic of acetic acid induced bladder hyperactivity, were significantly reversed by bilateral SN stimulation (P <0.05, vs pre-stimulation, Student t-test). In saline perfused rats, one hour of bilateral SN stimulation at 10 Hz and at motor threshold (0.19 ± 0.01 milliamps) increased ICI (265%) and VV (217%). In rats perfused with acetic acid, the corresponding increases produced by SN stimulation were 350% for ICI and 383% for VV. The percentage increases in the acetic acid-treated group were not significantly higher than those in saline-treated group. CONCLUSIONS: Using continuous flow cystyometry, we find that SN stimulation can produce effects on micturition consistent with its effects on isovolumetric model, and consistent with the therapeutic effect observed with InterStim® therapy in overactive bladder patients. Although the effect of SN stimulation was slightly greater in bladder irritated over normal rats, the difference was not statistically significant.

Quantification of effectiveness of bilateral and unilateral neuromodulation in the rat bladder rhythmic contraction model.

BACKGROUND: Using the isovolumetric bladder rhythmic contraction (BRC) model in anesthetized rats, we have quantified the responsiveness to unilateral and bilateral stimulation of the L6 spinal nerve (SN) and characterized the relationship between stimulus intensity and inhibition of the bladder micturition reflex. METHODS: A wire electrode was placed under either one or both of the L6 SN roots. A cannula was placed into the bladder via the urethra and the urethra was ligated. Saline infusion induced BRC. RESULTS: At motor threshold (T mot) intensity, SN stimulation of both roots (10 Hz) for 10 min reduced bladder contraction frequency from 0.63 ± 0.04 to 0.17 ± 0.09 contractions per min (26 ± 14% of baseline control; n = 10, p < 0.05). However, the same intensity of unilateral stimulation (n = 15) or sequential stimulation of both SNs (e.g. 5 min per side alternatively for a total of 10 min or 20 min) was less efficacious. The greater sensitivity to bilateral stimulation is not dependent upon precise bilateral timing of the stimulation pulses. Bilateral stimulation also produced both acute and prolonged- inhibition on bladder contractions in a stimulation intensity dependent fashion. CONCLUSIONS: Using the bladder rhythmic contraction model, bilateral stimulation was more effective than unilateral stimulation of the SN. Clinical testing should be conducted to further compare efficacies of unilateral and bilateral stimulation. Bilateral stimulation may allow the use of lower stimulation intensities to achieve higher efficacy for neurostimulation therapies on urinary tract control.

Role of the endogenous opioid system in modulation of urinary bladder activity by spinal nerve stimulation.

The role of the endogenous opioid system in modulation of urinary bladder activity by spinal nerve (SN) stimulation was studied in anesthetized female rats, using the rat model of isovolumetric bladder contraction. SN stimulation at a fixed frequency of 10 Hz attenuated bladder contraction frequency; the magnitude of the inhibition was directly proportional to the current intensity. Neither the κ-opioid antagonist nor-binaltorphimine (2 mg/kg iv) nor the δ-opioid antagonist naltrindole (5 mg/kg iv) attenuated the bladder inhibitory response to SN stimulation. In contrast, the µ-opioid receptor antagonist naloxone (NLX; 0.03 mg/kg iv) blocked the inhibitory responses evoked by SN stimulation at therapeutic current intensities at ≤1 × motor threshold current (Tmot). An action at spinal and supraspinal centers was further confirmed by the ability of intrathecal or intracerebroventricular administration of NLX methiodide to attenuate the bladder inhibitory effects of 1 × Tmot SN stimulation. The magnitude of SN-mediated neuromodulation using therapeutically relevant stimulation intensity (Tmot) is equivalent to 0.16 mg/kg of systemically administered morphine, which produces 50% inhibition of bladder contraction frequency. These results suggest that the inhibitory effects of lower intensity SN stimulation may be mediated through the release of endogenous µ-opioid peptides. Additionally, these data suggest that neuromodulation may offer a mode of treating the symptoms of overactive bladder with efficacy equal to the opioid drugs but without their liability for abuse and dependence.

Comparison of neural targets for neuromodulation of bladder micturition reflex in the rat.

Spinal nerve (SN) stimulation inhibits the bladder rhythmic contraction (BRC) in anesthetized rats. This preparation was used to study the effects of electrical stimulation of the tibial nerve (TN) and the dorsal nerve of the clitoris (DNC) on BRC. Stimulation of the TN and DNC for 10 min produced a frequency- and intensity-dependent attenuation of the frequency of bladder contractions. As observed with the SN, 10-Hz stimulation of either TN or DNC produced the greatest degree of inhibition, with lower or higher frequencies being either less efficacious or inactive. In contrast to the prolonged inhibition produced by SN stimulation, both TN and DNC stimulation produced "short" lasting inhibition of bladder contractions and the maximal inhibition occurred during stimulation. TN stimulation was effective over only a narrow range of current intensities [3-4 × motor threshold current for inducing a toe twitch (T(mot))] and only at a frequency of 10 Hz. Stimulation of TN at 10 Hz, 3 × T(mot) inhibited BRC to 23% of control. Ten-hertz DNC stimulation at 2 × T(EAS), the threshold current for evoking a reflex anal sphincter contraction, decreased the frequency of contractions to 4% of control. Although compared with the respective threshold current the BRC response was more sensitive to DNC compared with TN stimulation, the absolute current required to reduce BRC using DNC stimulation appeared to be higher. Comparing the effects of TN and DNC stimulation to our previous results with SN stimulation, SN stimulation produces the largest duration and efficacy of bladder inhibition.

Neuromodulation in a rat model of the bladder micturition reflex.

A rat model of bladder reflex contraction (BRC) was used to determine the optimal frequency and intensity of spinal nerve (SN) stimulation to produce neuromodulation of bladder activity and to assess the therapeutic mechanisms of this neuromodulation. In anesthetized female rats (urethane 1.2 g/kg ip), a wire electrode was used to produce bilateral stimulation of the L6 SN. A cannula was placed into the bladder via the urethra, and the urethra was ligated to ensure an isovolumetric bladder. Saline infusion induced BRC. Electrical stimulation of the SN produced a frequency- and intensity-dependent attenuation of the frequency of bladder contractions. Ten-herz stimulation produced maximal inhibition; lower and higher stimulation frequency produced less attenuation of BRC. Attenuation of bladder contraction frequency was directly proportional to the current intensity. At 10 Hz, stimulation using motor threshold pulses (T(mot)) produced a delayed inhibition of the frequency of bladder contractions to 34 ± 11% of control. Maximal bladder inhibition appeared at 10 min poststimulation. High current intensity at 0.6 mA (∼6 * T(mot)) abolished bladder contraction during stimulation, and the inhibition was sustained for 10 min poststimulation (prolonged inhibition). Furthermore, in rats pretreated with capsaicin (125 mg/kg sc), stimulation produced a stronger inhibition of BRC. The inhibitory effects on bladder contraction may be mediated by both afferent and efferent mechanisms. Lower intensities of stimulation may activate large, fast-conducting fibers and actions through the afferent limb of the micturition reflex arc in SN neuromodulation. Higher intensities may additionally act through the efferent limb.