Sensitization of pelvic afferent nerves in the in vitro rat urinary bladder-pelvic nerve preparation by purinergic agonists and cyclophosphamide pretreatment

Effects of stimulation of muscarinic receptors on bladder afferent nerves in the in vitro bladder-pelvic afferent nerve preparation of the rat

Effects of a muscarinic receptor agonist oxotremorine-M (oxo-M) on bladder afferent nerve (BAN) activity were studied in an in vitro bladder-pelvic nerve preparation. Distension of the bladder induced rhythmic bladder contractions that were accompanied by multiunit afferent firing. Intravesical administration of 25 and 50 μM oxo-M significantly increased afferent firing from 41 ± 2 spikes/s to 51 ± 4 spikes/s and 60.5 ± 5 spikes/s, respectively, but did not change the maximum amplitude of spontaneous bladder contractions. The afferent nerve firing induced by isotonic distension of the bladder (10-40 cmH(2)O) was increased 22-100% by intravesical administration of 50 μM oxo-M. Electrical stimulation on the surface of the bladder elicited action potentials (AP) in BAN. Oxo-M significantly decreased the voltage threshold by 40% (p<0.05) and increased by 157% (p<0.05) the area of the AP evoked at a submaximal stimulus intensity. These effects were blocked by intravesical injection of 5 μM atropine methyl nitrate (AMN). Intravesical administration of 5 μM AMN alone did not alter BAN firing or the amplitude of bladder contractions. The facilitatory effects induced by oxo-M on BAN activity were also suppressed (p<0.05) by intravesical administration of 2',3'-0-trinitrophenyl-ATP (TNP-ATP) (30 μM). In preparations pretreated with capsaicin (125 mg/kg, s.c.) the facilitatory effects of 50 μM oxo-M on BAN activity were absent. These results suggest that activation of muscarinic receptors facilitates mechano-sensitive, capsaicin-sensitive BAN activity in part by mechanisms involving purinergic receptors located near the luminal surface of the bladder and ATP release which presumably occurs in the urothelium.

Emerging Families of Ion Channels Involved in Urinary Bladder Nociception

The expression of multiple ion channels and receptors is essential for nociceptors to detect noxious stimuli of a thermal, mechanical or chemical nature. The peripheral sensory transduction systems of the urinary bladder include sensory nerve endings, urothelial cells and others whose location is suitable for transducing mechanical and chemical stimuli. There is an increasing body of evidence implicating the Deg/ENaC and TRP channel families in the control of bladder afferent excitability under physiological and pathological conditions. Pharmacological interventions targeting these ion channels may provide a new strategy for the treatment of pathological bladder sensation and pain.

The P2Y2 receptor sensitizes mouse bladder sensory neurons and facilitates purinergic currents

Sensitization of bladder afferents is an underlying contributor to the development and maintenance of painful bladder syndrome/interstitial cystitis. Extracellular purines and pyrimidines (e.g., ATP and UTP), released during bladder distension or from damaged cells after tissue insult, are thought to play an important role in bladder physiological and pathological states by actions at ionotropic P2X and metabotropic P2Y receptors. In the present study, we examined the ability of P2Y receptors to sensitize and modulate P2X-mediated responses in mouse bladder sensory neurons. UTP (a P2Y(2) and P2Y(4) agonist) increased excitability of bladder neurons by depolarizing resting membrane potential, increasing action potential firing, and facilitating responses to suprathreshold current injection as well as to P2X agonist application. These effects of UTP on bladder neuron excitability were blocked by the P2Y(2) receptor antagonist suramin. UTP also facilitated bladder neuron homomeric P2X(2) sustained currents and homomeric P2X(3) fast currents. The facilitatory effect of UTP on P2X(2) sustained currents was mediated by a G-protein-coupled P2Y(2) receptor/PKC pathway, whereas the effect of UTP on P2X(3) fast currents was G-protein independent. We also examined P2X and P2Y receptor expression in bladder neurons. P2Y(2) and P2Y(4) transcripts were detected in approximately 50 and approximately 20% of bladder neurons, respectively. Approximately 50% of P2X(2)- and P2X(3)-positive bladder neurons expressed P2Y(2) transcripts, whereas < or =25% of the same bladder neurons expressed P2Y(4) transcripts. These results support involvement of P2Y(2) receptors in bladder sensation, suggesting an important contribution to bladder neuron excitability and hypersensitivity.

Bladder afferent signaling: recent findings

PURPOSE

Much current research on lower urinary tract physiology focuses on afferent mechanisms. The main goals are to define and control the signaling pathways by which afferent information is generated and conveyed to the central nervous system. We summarize recent research on bladder afferent mechanisms.

MATERIALS AND METHODS

We systematically reviewed the literature by searching PubMed up to June 2009 with focus on the last 5 years.

RESULTS

At least 2 signaling pathways can be identified, including the urothelial and the myogenic pathway. The urothelial pathway is a functional unit consisting of the urothelium, interstitial cells and afferent nerves in the lamina propria. Signaling occurs via muscle-mucosal mechanoreceptors, mucosal mechanoreceptors and chemoreceptors. The myogenic pathway is activated via in-series mechanoreceptors responding to distention and via spontaneous contractile activity in units of myocytes generating afferent noise.

CONCLUSIONS

To control dysfunctional micturition we must know more about all components involved in normal micturition control, including how afferent information is handled by the central nervous system.

Differential purinergic signaling in bladder sensory neurons of naïve and bladder-inflamed mice

This study explored purinergic signaling in lumbosacral (LS) and thoracolumbar (TL) dorsal root ganglion neurons innervating the urinary bladder. In naïve mice, a greater proportion of LS (93%) than that of TL (77%) bladder neurons responded to purinergic agonists. Three types of purinergic currents were identified: 'sustained' (homomeric P2X2) currents were detected only in LS neurons, rapidly activating, 'slow' deactivating (heteromeric P2X2/3) currents predominated in both LS and TL neurons, and 'fast' activating/de-activating (homomeric P2X3) currents were detected only in TL neurons. Relative to TL bladder neurons, slow current density was greater in LS neurons, which also had a more negative action potential threshold and generated more action potentials in response to purinergic agonists (suggesting greater excitability of LS neurons). Single cell nested PCR documented P2X2 and P2X3 subunit expression in both TL and LS bladder neurons. Relative to saline treatment, bladder wall thickness and weight increased after cyclophosphamide (CYP) treatment. Both LS and TL neuron excitability increased (rheobase was decreased and responses to purinergic agonists increased) after CYP treatment. The proportion of sustained currents in LS bladder neurons increased fourfold after CYP bladder inflammation. Although proportions of slow and fast purinergic currents in TL neurons were unchanged by CYP treatment, the fast current density was greater than in saline-treated mice. These results in mouse, as previously described in rat, reveal differential purinergic signaling in TL and LS bladder neurons. The predominant currents and significant changes after inflammation, however, occur in different ganglia/sensory pathways in mouse and rat.

In vitro release of adenosine triphosphate from the urothelium of human bladders with detrusor overactivity, both neurogenic and idiopathic

BACKGROUND

There is increased evidence to suggest a role for nonadrenergic-noncholinergic neurotransmission in the pathogenesis of bladder dysfunction.

OBJECTIVE

In this set of experiments, we have assessed the contribution of the urothelium to purinergic activity by quantifying the amount of adenosine triphosphate (ATP) released from the urothelium of patients with idiopathic detrusor overactivity (IDO) and with neurogenic detrusor overactivity (NDO) and comparing these releases to those of controls.

DESIGN, SETTING, AND PARTICIPANTS

Bladder tissue with urodynamically and clinically proven NDO (n=8) and IDO (n=8) were included in this study. The carefully dissected urothelium was stimulated by mechanically stretching as well as electrically stimulating and the ATP; thus, release was quantified.

MEASUREMENTS

We used a Lucy Anthos 1 luminometre (Anthos Labtec Instruments GmBH, Wals, Austria) to perform the assay. The results were analysed using Stingray software (Dazdaq Ltd, Brighton, UK).

RESULTS AND LIMITATIONS

Both mechanical stretch and electric field stimulation (EFS) led to increased ATP release in both sets of tissues with overactivity compared to the controls; this rise was even more significant for the IDO urothelium (2416.7±479.8 pmol/g [p<0.005]) than for the NDO urothelium (133.1±22.4 pmol/g [p<0.01]); values for the controls were 77.6±16.2 pmol/g. ATP release following mechanical stretch was more sensitive to tetrodotoxin in bladders with NDO compared to those with IDO as well as to the controls, with ATP levels falling from 233.5±20.7 pmol/g to 107.2±11.6 pmol/g, expressed as percentage of basal levels (p<0.002). The experiments were performed in vitro, and the female patients were a mix of peri- and postmenopausal states.

CONCLUSIONS

These experiments suggested a significant rise in ATP release from the urothelium of bladders with NDO as well as those with IDO in comparison to controls. Most of the ATP released from bladders with NDO is primarily from neuronal sources.

Involvement of orexin-A on micturition reflex in normal and cyclophosphamide-induced cystitis bladder in rat

The purpose of the present study was to investigate the effect of orexin-A in the spinal cord on bladder function in normal rats and cyclophosphamide (CYP)-induced cystitis rat models. The effects of intrathecal (i.t.) injection of orexin-A (0.01, 0.1 and 1.0 nmol) on bladder function were examined during continuous infusion cystometrogram (CMG) in urethane anesthetized normal and CYP-induced cystitis rats. The effects of i.t. injection of selective orexin-1 receptor (OXR1) antagonist SB334867 (10 nmol) on orexin-A-induced bladder overactivity in normal rats and SB334867 (10 and 30 nmol) on changes in bladder function in normal and CYP-induced cystitis rats were investigated. The effects of intravenous (i.v.) injection of orexin-A (0.3 and 1.0 nmol) on micturition reflex were also investigated in normal rats. I.t. injection of orexin-A (0.1 and 1.0 nmol) significantly decreased the intercontraction intervals (ICI) in normal and CYP-induced cystitis rats. I.t. injection of SB334867 (10 nmol) significantly increased the ICI of orexin-A induced overactive bladder in normal rats and i.t. injection of SB334867 (30 nmol) also increased the ICI in normal rat bladder. However, in CYP-injected cystitis rat models, i.t. injection of SB334867 did not change the bladder function. I.v. injection of orexin-A failed to affect the bladder function in normal rats. Orexin mRNA levels in the lateral hypothalamus were significantly decreased in CYP-induced cystitis rats. These results indicate that orexin-A in the spinal cord activates micturition reflex via OXR1 in normal rats. In addition, OXR1 antagonist did not have any effect on micturition reflex in CYP-induced cystitis rats.

Visceral pain: the neurophysiological mechanism

The mechanism of visceral pain is still less understood compared with that of somatic pain. This is primarily due to the diverse nature of visceral pain compounded by multiple factors such as sexual dimorphism, psychological stress, genetic trait, and the nature of predisposed disease. Due to multiple contributing factors there is an enormous challenge to develop animal models that ideally mimic the exact disease condition. In spite of that, it is well recognized that visceral hypersensitivity can occur due to (1) sensitization of primary sensory afferents innervating the viscera, (2) hyperexcitability of spinal ascending neurons (central sensitization) receiving synaptic input from the viscera, and (3) dysregulation of descending pathways that modulate spinal nociceptive transmission. Depending on the type of stimulus condition, different neural pathways are involved in chronic pain. In early-life psychological stress such as maternal separation, chronic pain occurs later in life due to dysregulation of the hypothalamic-pituitary-adrenal axis and significant increase in corticotrophin releasing factor (CRF) secretion. In contrast, in early-life inflammatory conditions such as colitis and cystitis, there is dysregulation of the descending opioidergic system that results excessive pain perception (i.e., visceral hyperalgesia). Functional bowel disorders and chronic pelvic pain represent unexplained pain that is not associated with identifiable organic diseases. Often pain overlaps between two organs and approximately 35% of patients with chronic pelvic pain showed significant improvement when treated for functional bowel disorders. Animal studies have documented that two main components such as (1) dichotomy of primary afferent fibers innervating two pelvic organs and (2) common convergence of two afferent fibers onto a spinal dorsal horn are contributing factors for organ-to-organ pain overlap. With reports emerging about the varieties of peptide molecules involved in the pathological conditions of visceral pain, it is expected that better therapy will be achieved relatively soon to manage chronic visceral pain.

Signaling at purinergic P2X receptors

P2X receptors are membrane cation channels gated by extracellular ATP. Seven P2X receptor subunits (P2X(1-7)) are widely distributed in excitable and nonexcitable cells of vertebrates. They play key roles in inter alia afferent signaling (including pain), regulation of renal blood flow, vascular endothelium, and inflammatory responses. We summarize the evidence for these and other roles, emphasizing experimental work with selective receptor antagonists or with knockout mice. The receptors are trimeric membrane proteins: Studies of the biophysical properties of mutated subunits expressed in heterologous cells have indicated parts of the subunits involved in ATP binding, ion permeation (including calcium permeability), and membrane trafficking. We review our current understanding of the molecular properties of P2X receptors, including how this understanding is informed by the identification of distantly related P2X receptors in simple eukaryotes.

A refocus on the bladder as the originator of storage lower urinary tract symptoms: a systematic review of the latest literature

CONTEXT

The focus of clinical understanding and management of male storage lower urinary tract symptoms (LUTS) has shifted from the prostate to the bladder. This is mirrored by an increasing body of experimental evidence suggesting that the bladder is the central organ in the pathogenesis of LUTS.

OBJECTIVE

A systematic review of the literature available on pathophysiologic aspects of storage LUTS.

EVIDENCE ACQUISITION

Medline was searched for the period ending December 2008 for studies on human and animal tissue exploring possible functional and structural alterations underlying bladder dysfunction. Further studies were chosen on the basis of manual searches of reference lists and review papers.

EVIDENCE SYNTHESIS

Numerous recent publications on LUTS pathophysiology were identified. They were grouped into studies exploring abnormalities on urothelial/suburothelial, muscular, or central levels.

CONCLUSIONS

Studies revealed both structural and functional alterations in bladders from patients with LUTS symptoms or animals with experimentally induced bladder dysfunction. In particular, the urothelium and the suburothelial space, containing afferent nerve fibres and interstitial cells, have been found to form a functional unit that is essential in the process of bladder function. Various imbalances within this suburothelial complex have been identified as significant contributors to the generation of storage LUTS, along with potential abnormalities of central function.

Ion channel and receptor mechanisms of bladder afferent nerve sensitivity

Sensory nerves of the urinary bladder consist of small diameter A(delta) and C fibers running in the hypogastic and pelvic nerves. Neuroanatomical studies have revealed a complex neuronal network within the bladder wall. Electrophysiological recordings in vitro and in vivo have revealed several distinct classes of afferent fibers that may signal a wide range of bladder stimulations including physiological bladder filling, noxious distension, cold, chemical irritation and inflammation. The exact mechanisms that underline mechanosensory transduction in bladder afferent terminals remain ambiguous; however, a wide range of ion channels (e.g., TTX-resistant Na(+) channels, Kv channels and hyperpolarization-activated cyclic nucleotide-gated cation channels) and receptors (e.g., TRPV1, TRPM8, TRPA1, P2X(2/3), etc) have been identified at bladder afferent terminals and implicated in the generation and modulation of afferent signals. Experimental investigations have revealed that expression and/or function of these ion channels and receptors may be altered in animal models and patients with overactive and painful bladder disorders. Some of these ion channels and receptors may be potential therapeutic targets for bladder diseases.

Neurokinins inhibit low threshold inactivating K+ currents in capsaicin responsive DRG neurons

Neurokinins (NK) released from terminals of dorsal root ganglion (DRG) neurons may control firing of these neurons by an autofeedback mechanism. In this study we used patch clamp recording techniques to determine if NKs alter excitability of rat L4-S3 DRG neurons by modulating K(+) currents. In capsaicin (CAPS)-responsive phasic neurons substance P (SP) lowered action potential (AP) threshold and increased the number of APs elicited by depolarizing current pulses. SP and a selective NK(2) agonist, [betaAla(8)]-neurokinin A (4-10) also inhibited low threshold inactivating K(+) currents isolated by blocking non-inactivating currents with a combination of high TEA, (-) verapamil and nifedipine. Currents recorded under these conditions were heteropodatoxin-sensitive (Kv4 blocker) and alpha-dendrotoxin-insensitive (Kv1.1 and Kv1.2 blocker). SP and NKA elicited a >10 mV positive shift of the voltage dependence of activation of the low threshold currents. This effect was absent in CAPS-unresponsive neurons. The effect of SP or NKA on K(+) currents in CAPS-responsive phasic neurons was fully reversed by an NK(2) receptor antagonist (MEN10376) but only partially reversed by a PKC inhibitor (bisindolylmaleimide). An NK(1) selective agonist ([Sar(9), Met(11)]-substance P) or direct activation of PKC with phorbol 12,13-dibutyrate, did not change firing in CAPS-responsive neurons, but did inhibit various types of K(+) currents that activated over a wide range of voltages. These data suggest that the excitability of CAPS-responsive phasic afferent neurons is increased by activation of NK(2) receptors and that this is due in part to inhibition and a positive voltage shift in the activation of heteropodatoxin-sensitive Kv4 channels.

Purinergic contribution to small intestinal afferent hypersensitivity in a murine model of postinfectious bowel disease

Increased sensitivity of the afferent innervation of the gastrointestinal tract reportedly underlies symptoms of discomfort and pain in functional bowel disorders. The present investigation aimed to examine whether the purinergic P2X(2) and P2X(3) receptor subunits contribute to the mechanosensitivity of small intestinal afferents in normal mice and in a murine model of postinfectious gut dysfunction. Mesenteric afferent nerve activity was recorded in a mouse jejunum preparation maintained in vitro. As has been shown previously, ramp distension of the jejunal segment evoked biphasic afferent discharge, reflecting activation of low and high threshold fibres. The average pressure-afferent response curve in mice deficient in both P2X(2) and P2X(3) subunits (n = 14) was not significantly different from that of the wild-type control preparations (n = 13). Application of pyridoxal 5-phosphate 6-azophenyl-2 ,4-disulphonic acid (PPADS) (30 micromol L(-1)), a P2X and P2Y antagonist, or 2,4,6-trinitrophenol-adenosine 5'-triphosphate (10 micromol L(-1)), an antagonist selective for homomeric P2X(3) and heteromeric P2X(2/3) receptors, had no effect on the averaged pressure-afferent response curve in wild-type animals. In Trichinella spiralis-infected mice, the magnitude of mesenteric afferent responses to jejunal distension was greater at day 21 and day 56 postinfection compared with the sham control preparations demonstrating the development of afferent hypersensitivity. PPADS had no significant effect upon mechanically evoked afferent discharge rates in sham treated preparations (n = 5), but significantly inhibited afferent sensitivity to jejunal distension in preparations from mice at day 21 (n = 6) and day 56 (n = 7) postinfection. These results suggest that purinergic mechanisms play no role in mechanosensory transduction in the normal small intestine but contribute significantly to postinfectious mechano-hypersensitivity.

Mechanotransduction and chemosensitivity of two major classes of bladder afferents with endings in the vicinity to the urothelium

The guinea pig bladder is innervated by at least five distinct major classes of extrinsic sensory neurons. In this study, we have examined the mechanisms of mechanotransduction and chemosensitivity of two classes of bladder afferents that have their endings in the vicinity of the urothelium: stretch-sensitive muscle-mucosal mechanoreceptors and stretch-insensitive, mucosal high-responding afferents. The non-selective P2 purinoreceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid did not affect stretch- or stroking-induced firing of these afferents but significantly reduced the excitatory action of alpha,beta-methylene ATP. Blocking synaptic transmission in Ca(2+)-free solution did not affect stretch-evoked firing but slightly reduced stretch-induced tension responses. Stroking-induced firing of both classes of afferents was also not affected in Ca(2+)-free solution. Of blockers of mechano-gated channels, benzamil (100 microM), but not amiloride (100 microM), Gd(3+) (100 microM) or SKF 96365 (50 microM), inhibited stretch- and stroking-induced firing. Serotonin (100 microM) applied directly onto receptive fields predominantly activated muscle-mucosal afferents. Muscarine (100 microM) and substance P (100 microM) in 24% and 36% cases activated only mucosal high-responding units. Bradykinin (10 microM), but not prostaglandin E2 (10 microM), excites predominantly mucosal units. High (80 mM) K(+) solution activated both afferent classes, but responses of mucosal units were 4 times greater. In contrast to muscle-mucosal units, most mucosal high-responding units were activated by hot Krebs solution (45-46 degrees C), low pH (pH 4) and capsaicin (3 microm). TRPV1 antagonist, capsazepine (10 microM) was without effect on mechanotransduction by mucosal high-responding afferents. The results show that mechanotransduction of these two types of afferents are not dependant upon Ca(2+)-dependent exocytotic release of mediators, or ATP, and it is likely that benzamil-sensitive stretch-activated ion channels on their endings are involved in direct mechanotransduction. The chemosensitivity to agonists and noxious stimuli differs significantly between these two major classes of bladder afferents that reflects their different physiological and pathophysiological roles in the bladder.

Cannabinoid CB1 receptors are expressed in the mouse urinary bladder and their activation modulates afferent bladder activity

Pharmacological studies have indirectly shown the possible presence of cannabinoid receptors in the urinary bladder and their potential role in reducing bladder inflammatory pain. However, the localization of cannabinoid receptors in the urinary bladder remains unknown and there are no published data on the effects of cannabinoids on the sensory system of the bladder. The present study was performed to evaluate the expression of the cannabinoid CB(1) receptors in the mouse urinary bladder and to assess their co-localization with the purinergic P2X(3) receptor, a major player in the transduction of sensory events in the bladder. Also, the effect of intravesical administration of a cannabinoid agonist on the electrical activity of bladder afferent fibers was studied. The expression of mRNA coding for CB(1) receptor was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR). Immunofluorescence experiments were performed to study CB(1) and P2X(3) protein expression in the bladder. The electrical activity of bladder afferent fibers was recorded using an ex vivo bladder-nerve preparation. Mechanical stimulation of the bladder was performed by a controlled slow inflation with an external pump. A bolus of a cannabinoid agonist (AZ12646915) was administered intravesically prior to a second inflation. Afferent activity was measured before and after administration of the cannabinoid compound or its vehicle. The effects of CB(1) receptor antagonist (AM251) on the AZ12646915 response were also analyzed. Cannabinoid receptor CB(1) mRNA was detected in the urinary bladder of the mouse. The protein was found in the urothelium, as well as in nerve fibers. CB(1) and P2X(3) receptors were found to be co-expressed in urothelial cells and in some nerve fibers. In addition, intravesical administration of a cannabinoid receptor agonist reduced the mechanically-evoked activity of bladder afferents in the pelvic nerve. This effect was abolished by the previous administration of the CB(1) antagonist AM251. These data demonstrate the presence of cannabinoid CB(1) receptor mRNA and the protein in the mouse urinary bladder. CB(1) and P2X(3) protein co-localization supports the hypothesis of an interaction between the cannabinoid and the purinergic systems in the transduction of sensory information in the urinary bladder. Finally, the reduction of nerve activity induced by cannabinoid-receptor activation implicates CB(1) receptors in the peripheral modulation of bladder afferent information.

Spontaneous contractions evoke afferent nerve firing in mouse bladders with detrusor overactivity

PURPOSE

Afferent nerve firing has been linked to spontaneous bladder contractions in a number of lower urinary tract pathologies and it may lead to urgency and incontinence. Using optical mapping, single unit recording and tension measurements we investigated the correlation between afferent nerve firing and spontaneous bladder contractions in spinal cord transected mice.

MATERIALS AND METHODS

Bladder-nerve preparations (bladder sheets and the associated L6-S2 pelvic nerves) were dissected from normal and spinal cord transected mice showing overactivity on cystometry and opened along the ventral aspect from base to dome. Bladder sheets were mounted horizontally in a temperature regulated chamber to simultaneously record Ca(2+) transients across the mucosal surface, single unit afferent nerve firing and whole bladder tension.

RESULTS

Single unit afferent fibers were identified by probing their receptive fields. Fibers showed a graded response to von Frey stimulation and a frequency of afferent firing that increased as a function of the degree of stretch. Optical maps of Ca(2+) transients in control bladders demonstrated multiple initiation sites that resulted in high frequency, low amplitude spontaneous contractions. Alternatively in maps of the bladders of spinal cord transected mice Ca(2+) transients arose from 1 or 2 focal sites, resulting in low frequency, high amplitude contractions and concomitant afferent firing.

CONCLUSIONS

Large amplitude, spontaneous bladder contractions evoke afferent nerve activity, which may contribute to incontinence.

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.

New insights into the pharmacology of the bladder

PURPOSE OF REVIEW

Pharmacotherapy of a number of bladder disorders has traditionally focused on targeting the 'sensory' component or bladder nerves and the smooth muscle. This review aims to provide an insight into recent (experimental and clinical) developments in mechanisms of existing therapies as well as novel targets.

RECENT FINDINGS

Traditionally, sensory signaling in the urinary bladder has been attributed to activation of bladder afferents, but new findings have pointed to the urothelium and interstitial cells as key participants in the transduction of sensory events. Recent advances provide strong support for the development of subtype selective receptor agonists/antagonists, the modulation of signal transduction cascades and new and expanded uses for various neurotoxins.

SUMMARY

The development of therapeutic options for the treatment of a number of bladder disorders is complicated, and most treatments are associated with an increased incidence of side effects or lack of specificity. Recent studies suggest that selective targeting of receptors/ion channels or a disease-specific (i.e. phosphorylated) form of the receptor may represent a viable therapeutic target. Though the mechanisms regulating ion channel expression under pathological conditions are not fully known, an increased understanding of these pathways has important implications for drug development.