The effects of neuromodulation in a novel obese-prone rat model of detrusor underactivity

Association of detrusor underactivity with aging and metabolic syndrome: suggestions from animal models

Detrusor underactivity, a condition in which the bladder muscle does not contract strongly or long enough to empty the bladder completely or within the normal time frame, is a common cause of lower urinary tract symptoms in older individuals of both sexes. Although aging is a known risk factor for detrusor underactivity, its pathophysiological mechanisms are not fully understood. Therefore, establishing animal models that closely mimic the pathophysiology of detrusor underactivity in humans is necessary to elucidate these mechanisms. Metabolic syndrome is a cluster of several risk factors, including obesity, hyperlipidemia, hyperglycemia, and hypertension, which are associated with the development of diabetes, cardiovascular disease, and lower urinary tract dysfunction in both sexes. Notably, bladder dysfunction resulting from detrusor underactivity is observed at an earlier age in animal models with diabetes mellitus than in those without. Recently, detrusor underactivity-like phenotypes have been observed at a relatively early age in animal models with metabolic syndrome, involving obesity, hyperlipidemia, and hypertension, compared with those without. Therefore, this review introduces the association of detrusor underactivity with aging and metabolic syndrome, as well as possible pathophysiological mechanisms for detrusor underactivity from reports of various animal models. Notably, metabolic syndrome may accelerate the onset of age-related detrusor underactivity, and further analysis of old animal models with metabolic syndrome may help elucidate the pathogenesis of detrusor underactivity in humans.

Pathways and parameters of sacral neuromodulation in rats

The stimulation paradigm for sacral neuromodulation has remained largely unchanged since its inception. We sought to determine, in rats, whether stimulation-induced increases in bladder capacity correlated with the proportion of sensory pudendal (PudS) neurons at each stimulated location (L6, S1). If supported, this finding could guide the choice of stimulation side (left/right) and level (S2, S3, S4) in humans. Unexpectedly, we observed that acute stimulation at clinically relevant (low) amplitudes [1-1.5 × motor threshold (Tm)], did not increase bladder capacity, regardless of stimulus location (L6 or S1). More importantly for the ability to test our hypothesis, there was little anatomic variation, and S1 infrequently contributed nerve fibers to the PudS nerve. During mapping studies we noticed that large increases in PudS nerve activation occurred at amplitudes exceeding 2Tm. Thus, additional cystometric studies were conducted, this time with stimulation of the L6-S1 trunk, to examine further the relationship between stimulation amplitude and cystometric parameters. Stimulation at 1Tm to 6Tm evoked increases in bladder capacity and decreases in voiding efficiency that mirrored those produced by PudS nerve stimulation. Many animal studies involving electrical stimulation of nerves of the lower urinary tract use stimulation amplitudes that exceed those used clinically (∼1Tm). Our results confirm that high amplitudes generate immediate changes in cystometric parameters; however, the relationship to low-amplitude chronic stimulation in humans remains unclear. Additional studies are needed to understand changes that occur with chronic stimulation, how these changes relate to therapeutic outcomes, and the contribution of specific nerve fibers to these changes.NEW & NOTEWORTHY Acute low-amplitude electrical stimulation of sacral nerve (sacral neuromodulation) did not increase bladder capacity in anesthetized CD, obese-prone, or obese-resistant rats. Increasing stimulation amplitude correlated with increases in bladder capacity and pudendal sensory nerve recruitment. It is unclear how the high-amplitude acute stimulation that is commonly used in animal experiments to generate immediate effects compares mechanistically to the chronic low-amplitude stimulation used clinically.

Effects of different diets used to induce obesity/metabolic syndrome on bladder function in rats

Preclinical and human studies on the relationship between obesity/metabolic syndrome (MetS) and lower urinary tract dysfunction (LUTD) are inconsistent. We compared the temporal effects of feeding four different diets used to induce obesity/MetS, including 60% fructose, 2% cholesterol +10% lard, 30% fructose + 20% lard, or 32.5% lard diet, up to 42 wk, on metabolic parameters and bladder function in male Sprague-Dawley rats. Rats fed a 30% fructose + 20% lard or 32.5% lard diet consumed less food (grams), but only the 32.5% lard diet group took in more calories. Feeding rats a 60% fructose or 30% fructose + 20% lard diet led to glucose intolerance and increased blood pressure. Higher body weight and increased cholesterol levels were observed in the rats maintained on a 2% cholesterol +10% lard diet, whereas exposure to a 32.5% lard diet affected most of the above parameters. Voiding behavior measurement showed that voiding frequency and the total voided volume were lower in the experimental diet groups except for the 30% fructose + 20% lard group. The mean voided volume was lower in the 30% fructose + 20% lard and 32.5% lard groups compared with the control group. Cystometric analysis revealed a decreased bladder capacity, mean voided volume, intermicturition interval, and compliance in the 32.5% lard diet group. In conclusion, experimental diets including 60% fructose, 30% fructose + 20% lard, or 2% cholesterol + 10% lard diet differently affected physiological and metabolic parameters and bladder function to a limited extent, while exposure to a 32.5% lard diet had a greater impact.

Dysfunctional voiding behavior and impaired muscle contractility in a rat model of detrusor underactivity

AIMS

Detrusor underactivity (DU) is an understudied health concern with inadequate clinical management. The pathophysiology of DU is unclear, and current therapies fail to improve symptoms. The current studies characterized voiding function and contractility of bladder and urethral tissues in a novel rat model of DU.

METHODS

Female obese prone (OP) and obese resistant (OR) rats were fed a 60 kcal% fat diet at 8 weeks old. A subset of rats (n = 4/strain) underwent uroflowmetry biweekly for 18 weeks in metabolic cages. At 40-56 weeks old, rats (n = 9-10/strain) underwent instrumented cystometry under urethane anesthesia. Following cystometry, bladder and urethral tissues (n = 8-9/strain) were harvested for in vitro assessments of contractility in response to carbachol, electric field stimulation, atropine, alpha, beta-methylene ATP, and caffeine.

RESULTS

OP rats exhibited increased urinary frequency (p = 0.0031), decreased voided volume (p = 0.0093), and urine flow rate (p = 0.0064) compared to OR rats during uroflowmetry. Bethanechol (10 mg/kg) did not alter uroflowmetry parameters. During cystometry, OP rats exhibited decreased bladder emptying efficiency (p < 0.0001), decreased pressure to generate a void (p < 0.0001), and increased EUS activity during filling (p = 0.0011). Bladder contractility was decreased in OP rats when exposed to carbachol (p < 0.0003) and ATP (p = 0.0004), whereas middle urethral contractility was increased when exposed to carbachol (p = 0.0014), EFS (p = 0.0289), and caffeine (p = 0.0031).

CONCLUSION

Impaired cholinergic and purinergic signaling in the bladder may contribute to poor voiding function in OP rats. In addition, increased urethral activity may engage a guarding reflex to augment continence and exacerbate incomplete emptying.

Sensitivity of urethral flow-evoked voiding reflexes decline with age in the rat: insights into age-related underactive bladder

The prevalence of underactive bladder (UAB) increases with age, suggesting a link between age-related processes and lower urinary tract (LUT) symptoms; however, the underlying mechanisms of age-related UAB are poorly understood. Understanding how aging affects LUT reflexes may help in the development of new treatments by identifying mechanistic targets. In this work, we studied the relationship between age and systems-level function of the LUT and tested the hypothesis that aging is related to weakening of reflexes that control voiding. Three groups of anesthetized female rats, young (4–7 mo old), mature (11–14 mo old), and old (18–24 mo old), were used to quantify the effect of aging on LUT reflexes. A double-lumen catheter enabled us to control the bladder volume and urethral flow rate independently, under quasi-isovolumetric bladder conditions. We systematically investigated the reflex bladder contractions evoked by combinations of rates of urethral infusion and bladder fill volumes as a function of age. Urethral infusion with the same flow rate evoked bladder contractions (via the augmenting reflex) in old animals less often than in younger animals. Furthermore, old animals needed more fluid in their bladders (relative to their bladder capacity) before urethra flow-evoked bladder contractions could be triggered at all, suggesting a delay in the switch of the LUT to “voiding mode.” Old rats also showed longer and weaker bladder contractions than young or mature rats. Taken together, this suggests there is an age-related functional weakening and loss of sensitivity in LUT reflexes, which may contribute to age-related UAB symptoms.

Sensory pudendal nerve stimulation increases bladder capacity through sympathetic mechanisms in cyclophosphamide-induced cystitis rats

AIMS

Interstitial cystitis and bladder pain syndrome is a prevalent health concern with inadequate treatments. Neuromodulation has emerged as a therapeutic option to treat patients refractory to standard care. The objective of this study was to determine the efficacy and mechanism(s) of sensory pudendal nerve stimulation on bladder function in cystitis rats.

METHODS

Female rats were administered saline (n = 8) or cyclophosphamide (CYP, 150 mg/kg IP, n = 16) and single-trial cystometry experiments were conducted under urethane anesthesia 48 h after injection. Electrical stimulation (0.02-0.22 mA, 10-20 Hz) was delivered to the sensory branch of the pudendal nerve and its effect on the bladder and external urethral sphincter were measured. Stimulation trials were also conducted following bilateral hypogastric nerve transection (HGNT) or pharmacological inhibition of beta-adrenergic receptors (propranolol, 1 mg/kg IV) to determine the mechanisms of bladder inhibition.

RESULTS

CYP-induced cystitis decreased bladder capacity (P = 0.0352) and bladder compliance (P = 0.024) by up to 38% of control. Electrical stimulation of the sensory pudendal nerve increased bladder capacity (P < 0.0001) in control and CYP rats by up to 51-52% of their respective baselines. HGNT did not influence bladder inhibition generated by sensory pudendal nerve stimulation in control rats, whereas HGNT and propranolol decreased the efficacy of electrical stimulation in CYP rats.

CONCLUSIONS

Sympathetic reflex activity mediates sensory pudendal nerve stimulation in CYP treated but not control rats. These studies demonstrate an alternative approach to neuromodulation in cystitis and establish mechanistic changes during stimulation that may enable the development of novel therapeutics.

Closed-loop stimulation of the pelvic nerve for optimal micturition

OBJECTIVE

Neural stimulation to restore bladder function has traditionally relied on open-loop approaches that used pre-set parameters, which do not adapt to suboptimal outcomes. The goal of this study was to examine the effectiveness of a novel closed-loop stimulation paradigm for improving micturition or bladder voiding.

APPROACH

We compared the voiding efficiency obtained with this closed-loop framework against open-loop stimulation paradigms in anesthetized rats. The bladder pressures that preceded voiding, and the minimum current amplitudes for stimulating the pelvic nerves to evoke bladder contractions, were first calibrated for each animal. An automated closed-loop system was used to initiate voiding upon bladder fullness, adapt the stimulation current by using real-time bladder pressure changes to classify voiding outcomes, and halt stimulation when the bladder had been emptied or when the safe stimulation limit was reached.

MAIN RESULTS

In vivo testing demonstrated that the closed-loop system achieved high voiding efficiency or VE (75.7%  ±  3.07%, mean  ±  standard error of the mean) and outperformed open-loop systems with either conserved number of stimulation epochs (63.2%  ±  4.90% VE) or conserved charge injected (32.0%  ±  1.70% VE). Post-hoc analyses suggest that the classification algorithm can be further improved with data from additional closed-loop experiments.

SIGNIFICANCE

This novel approach may be applied to an implantable device for treating underactive bladder (<60% VE), especially in cases where under- or over-stimulation of the nerve is a concern.

Detrusor Underactivity and the Underactive Bladder: A Systematic Review of Preclinical and Clinical Studies

CONTEXT

Detrusor underactivity (DUA) is a common but relatively under-researched bladder dysfunction. Underactive bladder (UAB) is the symptom-based correlate of DUA. Recently, there has been renewed interest in this topic.

OBJECTIVE

To systematically review and summarise the most recent literature and discuss this in the context of what is already known.

EVIDENCE ACQUISITION

A systematic review of the literature was performed in December 2017 using Medline and Scopus databases. Separate searches of each database used a complex search strategy including "free text" protocols. Search terms included "underactive bladder", "detrusor underactivity", "acontractile bladder", "detrusor failure", "detrusor areflexia", "atonic bladder", "chronic retention", and "impaired bladder contractility".

EVIDENCE SYNTHESIS

The initial search retrieved a total of 1690 studies; of these 44 were included in the final analyses.

CONCLUSIONS

Although there has been an expansion in the literature concerning all aspects of DUA and UAB, knowledge on its epidemiology and aetiopathogenesis is still lacking; there remains a need to develop accurate reproducible diagnostic criteria and effective treatments, in particular drug therapies.

PATIENT SUMMARY

Recently, there has been renewed interest in underactive bladder with expanding research in this area. The lack of simple, reproducible, noninvasive diagnostic criteria has precluded an accurate estimation of the magnitude of the problem. Recent studies have highlighted the potential role of impaired bladder blood supply in causing bladder underactivity.