Bladder wall tension and mechanoreceptor discharge

Reflex voiding in rat occurs at consistent bladder volume regardless of pressure or infusion rate

AIMS

The central nervous system (CNS) regulates lower urinary tract reflexes using information from sensory afferents; however, the mechanisms of this process are not well known. Pressure and volume were measured at the onset of the guarding and micturition reflexes across a range of infusion rates to provide insight into what the CNS is gauging to activate reflexes.

METHODS

Female Sprague Dawley rats were anesthetized with urethane for open outlet cystometry. A set of 10 infusion rates (ranging 0.92-65.5 mL/h) were pseudo-randomly distributed across 30 single-fill cystometrograms. Bladder pressure and external urethral sphincter electromyography were used for the determination of the onset of the micturition and guarding reflexes, respectively. The bladder volume at the onset of both reflexes was estimated from the total infusion rate during a single fill.

RESULTS

In response to many single-fill cystometrograms, there was an increased volume the bladder could store without a significant increase in pressure. Volume was adjusted for this effect for the analysis of how pressure and volume varied with infusion rate at the onset of the micturition and guarding reflexes. In 25 rats, the micturition reflex was evoked at similar volumes across all infusion rates, whereas the pressure at micturition reflex onset increased with increasing infusion rates. In 11 rats, the guarding reflex was evoked at similar pressures across infusion rates, but the volume decreased with increasing infusion rates.

CONCLUSIONS

These results suggest that the CNS is interpreting volume from the bladder to activate the micturition reflex and pressure from the bladder to activate the guarding reflex.

Responsiveness of lumbosacral superficial dorsal horn neurons during the voiding reflex and functional loss of spinal urethral-responsive neurons in streptozotocin-induced diabetic rats

AIMS

Sensory information from the lower urinary tract (LUT) is conveyed to the spinal cord to trigger and co-ordinate micturition. However, it is not fully understood how spinal dorsal horn neurons are excited during the voiding reflex. In this study, we developed an in vivo technique allowing recording of superficial dorsal horn (SDH) neurons concurrent with intravesical pressure (IVP) during the micturition cycle in both normal and diabetic rats.

METHODS

Lumbosacral dorsal horn neuronal activity and IVP were recorded from urethane-anesthetized naive and streptozotocin (STZ)-induced diabetic rats. Saline was continuously perfused into the urinary bladder through a cannula to induce micturition.

RESULTS

We classified SDH neurons into bladder- and urethral-responsive neurons, based on their responsiveness during the voiding reflex. Bladder-responsive SDH neurons responded to the rapid increase in IVP at the start of voiding. In contrast, urethral-responsive SDH neuronal firing increased at the peak IVP and their firing lasted during the voiding phase (the high-frequency oscillations). Urethral-responsive SDH neurons were more sensitive to capsaicin, received C afferent fiber inputs, and were rarely detected in STZ-diabetes rats. Administration of a cyclohexenoic long-chain fatty alcohol (TAC-302), which is reported to promote neurite outgrowth of peripheral nerves in STZ-diabetic rats, prevented the functional loss of spinal urethral response.

CONCLUSIONS

Sensory information from the bladder and urethra is conveyed separately to different groups of SDH neurons. Functional loss of spinal urethral sensory information through unmyelinated C afferent fibers may contribute to diabetic bladder dysfunction.

An innovative, non-invasive sensation meter allows for a more comprehensive understanding of bladder sensation events: A prospective study in participants with normal bladder function

AIMS

There is currently no standardized method of characterizing changes in bladder sensation during bladder filling outside of the urodynamics laboratory. The purpose of this investigation was to characterize real-time bladder sensation events using a sensation meter during oral hydration in individuals with normal bladder function.

METHODS

Participants enrolled in an accelerated hydration study drank 2 L Gatorade-G2® and utilized a sensation meter to record real-time bladder sensation (0-100%), verbal sensory thresholds, and sensation descriptors of "tense," "pressure," "tingling," "painful," and "other" for two consecutive fill-void cycles.

RESULTS

Data from 21 participants (12 females/9 males) were obtained and demonstrated an average of 8-9 sensation events (significant changes in sensation) per fill with no differences in the total number of sensation events and volume between sensation events (fill 1 vs fill 2). An increased number of sensation events occurred at higher capacity quartiles. Event descriptors of "pressure" and "tingling" were the most commonly chosen descriptors in both fills.

CONCLUSIONS

The innovative sensation meter includes the sensation event descriptors of "tense," "tingling," "pressure," and "painful," to enable a more comprehensive understanding of bladder sensation as well as real-time identification, quantification, and characterization of sensation events. The study demonstrates 8-9 events per fill, acceleration of sensation during filling, and unique sensation event descriptor patterns. This technology may be helpful in the identification of novel sensation patterns associated with overactive bladder (OAB) and aging.

Hysteretic behavior of bladder afferent neurons in response to changes in bladder pressure

Background

Mechanosensitive afferents innervating the bladder increase their firing rate as the bladder fills and pressure rises. However, the relationship between afferent firing rates and intravesical pressure is not a simple linear one. Firing rate responses to pressure can differ depending on prior activity, demonstrating hysteresis in the system. Though this hysteresis has been commented on in published literature, it has not been quantified.

Results

Sixty-six bladder afferents recorded from sacral dorsal root ganglia in five alpha-chloralose anesthetized felines were identified based on their characteristic responses to pressure (correlation coefficient ≥ 0.2) during saline infusion (2 ml/min). For saline infusion trials, we calculated a maximum hysteresis ratio between the firing rate difference at each pressure and the overall firing rate range (or Hmax) of 0.86 ± 0.09 (mean ± standard deviation) and mean hysteresis ratio (or Hmean) of 0.52 ± 0.13 (n = 46 afferents). For isovolumetric trials in two experiments (n = 33 afferents) Hmax was 0.72 ± 0.14 and Hmean was 0.40 ± 0.14.

Conclusions

A comprehensive state model that integrates these hysteresis parameters to determine the bladder state may improve upon existing neuroprostheses for bladder control.

Anatomy and physiology of the lower urinary tract

Functions of the lower urinary tract to store and periodically eliminate urine are regulated by a complex neural control system in the brain, spinal cord, and peripheral autonomic ganglia that coordinates the activity of smooth and striated muscles of the bladder and urethral outlet. Neural control of micturition is organized as a hierarchic system in which spinal storage mechanisms are in turn regulated by circuitry in the rostral brainstem that initiates reflex voiding. Input from the forebrain triggers voluntary voiding by modulating the brainstem circuitry. Many neural circuits controlling the lower urinary tract exhibit switch-like patterns of activity that turn on and off in an all-or-none manner. The major component of the micturition switching circuit is a spinobulbospinal parasympathetic reflex pathway that has essential connections in the periaqueductal gray and pontine micturition center. A computer model of this circuit that mimics the switching functions of the bladder and urethra at the onset of micturition is described. Micturition occurs involuntarily during the early postnatal period, after which it is regulated voluntarily. Diseases or injuries of the nervous system in adults cause re-emergence of involuntary micturition, leading to urinary incontinence. The mechanisms underlying these pathologic changes are discussed.

Neural control of the lower urinary tract

This article summarizes anatomical, neurophysiological, pharmacological, and brain imaging studies in humans and animals that have provided insights into the neural circuitry and neurotransmitter mechanisms controlling the lower urinary tract. The functions of the lower urinary tract to store and periodically eliminate urine are regulated by a complex neural control system in the brain, spinal cord, and peripheral autonomic ganglia that coordinates the activity of smooth and striated muscles of the bladder and urethral outlet. The neural control of micturition is organized as a hierarchical system in which spinal storage mechanisms are in turn regulated by circuitry in the rostral brain stem that initiates reflex voiding. Input from the forebrain triggers voluntary voiding by modulating the brain stem circuitry. Many neural circuits controlling the lower urinary tract exhibit switch-like patterns of activity that turn on and off in an all-or-none manner. The major component of the micturition switching circuit is a spinobulbospinal parasympathetic reflex pathway that has essential connections in the periaqueductal gray and pontine micturition center. A computer model of this circuit that mimics the switching functions of the bladder and urethra at the onset of micturition is described. Micturition occurs involuntarily in infants and young children until the age of 3 to 5 years, after which it is regulated voluntarily. Diseases or injuries of the nervous system in adults can cause the re-emergence of involuntary micturition, leading to urinary incontinence. Neuroplasticity underlying these developmental and pathological changes in voiding function is discussed.

Afferent nerve regulation of bladder function in health and disease

The afferent innervation of the urinary bladder consists primarily of small myelinated (Adelta) and unmyelinated (C-fiber) axons that respond to chemical and mechanical stimuli. Immunochemical studies indicate that bladder afferent neurons synthesize several putative neurotransmitters, including neuropeptides, glutamic acid, aspartic acid, and nitric oxide. The afferent neurons also express various types of receptors and ion channels, including transient receptor potential channels, purinergic, muscarinic, endothelin, neurotrophic factor, and estrogen receptors. Patch-clamp recordings in dissociated bladder afferent neurons and recordings of bladder afferent nerve activity have revealed that activation of many of these receptors enhances neuronal excitability. Afferent nerves can respond to chemicals present in urine as well as chemicals released in the bladder wall from nerves, smooth muscle, inflammatory cells, and epithelial cells lining the bladder lumen. Pathological conditions alter the chemical and electrical properties of bladder afferent pathways, leading to urinary urgency, increased voiding frequency, nocturia, urinary incontinence, and pain. Neurotrophic factors have been implicated in the pathophysiological mechanisms underlying the sensitization of bladder afferent nerves. Neurotoxins such as capsaicin, resiniferatoxin, and botulinum neurotoxin that target sensory nerves are useful in treating disorders of the lower urinary tract.

Does the cystometric filling rate affect the afferent bladder response pattern? A study on single fibre pelvic nerve afferents in the rat urinary bladder

AIMS

To study the effect of physiological and supraphysiological filling rates on the response pattern of single fibre pelvic nerve afferents of the rat urinary bladder.

MATERIALS AND METHODS

A total of 37 single afferent bladder units were identified electrophysiologically and their mechanosensitive properties studied. Afferent activity of 13 units was studied at filling rates of 40 and 200 microl/min. Afferent activity of the other units was studied at 200 and 400 microl/min.

RESULTS

At the physiological filling rate of 40 microl/min two clearly different types of response pattern were noted. However, at higher supraphysiological filling rates, all units exhibited nearly the same pattern. The difference in mechanosensitive properties of the units that showed a change in response pattern at the supraphysiological filling rate, was characterized by a decrease in pressure at which afferent firing rate peaked. For all units it was found that an increase in filling rate induced an increase in the activation pressure threshold for afferent units, whereas the afferent firing rate at all pressures decreased.

CONCLUSIONS

In rats supraphysiological filling rates delay afferent activation, lower afferent firing activity and even change the characteristics of some afferents completely. These data may elucidate some of the differences between ambulatory and conventional urodynamics observed in man.

Afferent bladder nerve activity in the rat: a mechanism for starting and stopping voiding contractions

The objective of this work was to study the relation between afferent bladder nerve activity and bladder mechanics and the mechanisms that initiate and terminate bladder contractions. Bladder nerve activity, pressure and volume were recorded during the micturition cycle in the rat. The highest correlation was found between afferent nerve activity and stress (pressure x volume). Afferent nerve activity depended linearly on stress within 6%, and both slope and offset were independent of the bladder-filling rate. The levels of afferent bladder nerve activity at the onset and cessation of efferent firing to the bladder were highly reproducible with coefficients of variation of <or=17%. We propose a model in which afferent activity is proportional to bladder wall stress, and bladder contraction is initiated when afferent activity exceeds a threshold due to an increasing pressure and volume. The contraction continues until afferent activity drops below a threshold again as a result of a decreasing volume.