Investigation of the effects of P2 purinoceptor ligands on the micturition reflex in female urethane-anaesthetized rats

In vivo paracrine effects of ATP-induced urothelial acetylcholine in the rat urinary bladder

Mechanical stretch of the urothelium induces the release of ATP that activates bladder afferent nerves. In the rat urinary bladder, ATP is also a contractile co-transmitter in the parasympathetic innervation. In isolated preparations, ATP evokes a urothelial release of acetylcholine that substantially contributes to ATP-evoked contractile responses. Currently we aimed to further examine the interactions of ATP and acetylcholine in the rat urinary bladder in two in vivo models. In the whole bladder preparation, atropine reduced ATP-evoked responses by about 50% in intact but denervated bladders, while atropine had no effect after denudation of the urothelium. In a split bladder preparation, reflex-evoked responses of the contralateral half were studied by applying stimuli (agonists or stretch) to the ipsilateral half. Topical administration of ATP and methacholine as well as of stretch induced contralateral reflex-evoked contractions. While topical administration of atropine ipsilaterally reduced the ATP- and stretch-induced contralateral contractions by 27 and 39%, respectively, the P2X purinoceptor antagonist PPADS reduced them by 74 and 84%. In contrary, the muscarinic M2-(M4)-selective receptor antagonist methoctramine increased the responses by 38% (ATP) and 75% (stretch). Pirenzepine (M1-selective antagonist) had no effect on the reflex. In vitro, in the absence of the reflex, methoctramine did not affect the ATP-induced responses. It is concluded that urothelial ATP potently induces the micturition reflex and stimulates urothelial release of acetylcholine. Acetylcholine subsequently acts on afferents and on the detrusor muscle. While muscarinic M2 and/or M4 receptors in the sensory innervation exert inhibitory modulation, muscarinic M3 receptors cause excitation.

Extracellular adenosine 5'-triphosphate concentrations changes in rat spinal cord associated with the activation of urinary bladder afferents. A microdialysis study

Objective

To determine adenosine 5’-triphosphate levels in the interstice of spinal cord L₆-S₁ segment, under basal conditions or during mechanical and chemical activation of urinary bladder afferents.

Methods

A microdialysis probe was transversally implanted in the dorsal half of spinal cord L₆-S₁ segment in female rats. Microdialysate was collected at 15 minutes intervals during 135 minutes, in anesthetized animals. Adenosine 5’-triphosphate concentrations were determined with a bioluminescent assay. In one group of animals (n=7) microdialysate samples were obtained with an empty bladder during a 10-minutes bladder distension to 20 or 40cmH₂O with either saline, saline with acetic acid or saline with capsaicin. In another group of animals (n=6) bladder distention was performed and the microdialysis solution contained the ectonucleotidase inhibitor ARL 67156.

Results

Basal extracellular adenosine triphosphate levels were 110.9±35.34fmol/15 minutes, (mean±SEM, n=13), and bladder distention was associated with a significant increase in adenosine 5’-triphosphate levels which was not observed after bladder distention with saline solution containing capsaicin (10µM). Microdialysis with solution containing ARL 67156 (1mM) was associated with significantly higher extracellular adenosine 5’-triphosphate levels and no further increase in adenosine 5’-triphosphate was observed during bladder distension.

Conclusion

Adenosine 5’-triphosphate was present in the interstice of L₆-S₁ spinal cord segments, was degraded by ectonucleotidase, and its concentration increased following the activation of bladder mechanosensitive but not of the chemosensitive afferents fibers. Adenosine 5’-triphosphate may originate either from the central endings of bladder mechanosensitive primary afferent neurons, or most likely from intrinsic spinal neurons, or glial cells and its release appears to be modulated by capsaicin activated bladder primary afferent or by adenosine 5’-triphosphate itself.

Contractile effects and receptor analysis of adenosine-receptors in human detrusor muscle from stable and neuropathic bladders

To measure the relative transcription of adenosine receptor subtypes and the contractile effects of adenosine and selective receptor-subtype ligands on detrusor smooth muscle from patients with neuropathic overactive (NDO) and stable bladders and also from guinea-pigs. Contractile function was measured at 37°C in vitro from detrusor smooth muscle strips. Contractions were elicited by superfusate agonists or by electrical field stimulation. Adenosine-receptor (A1, A2A, A2B, A3) transcription was measured by RT-PCR. Adenosine attenuated nerve-mediated responses with equivalent efficacy in human and guinea-pig tissue (pIC50 3.65–3.86); the action was more effective at low (1–8 Hz) compared to high (20–40 Hz) stimulation frequencies in human NDO and guinea-pig tissue. With guinea-pig detrusor the action of adenosine was mirrored by the A1/A2-agonist N-ethylcarboxamidoadenosine (NECA), partly abolished in turn by the A2B-selectve antagonist alloxazine, as well as the A1-selective agonist N6- cyclopentyladenosine (CPA). With detrusor from stable human bladders the effects of NECA and CPA were much smaller than that of adenosine. Adenosine also attenuated carbachol contractures, but mirrored by NECA (in turn blocked by alloxazine) only in guinea-pig tissue. Adenosine receptor subtype transcription was measured in human detrusor and was similar in both groups, except reduced A2A levels in overactive bladder. Suppression of the carbachol contracture in human detrusor is independent of A-receptor activation, in contrast to an A2B-dependent action with guinea-pig tissue. Adenosine also reduced nerve-mediated contractions, by an A1- dependent action suppressing ATP neurotransmitter action.

Medicinal chemistry of adenosine, P2Y and P2X receptors

Pharmacological tool compounds are now available to define action at the adenosine (ARs), P2Y and P2X receptors. We present a selection of the most commonly used agents to study purines in the nervous system. Some of these compounds, including A1 and A3 AR agonists, P2Y1R and P2Y12R antagonists, and P2X3, P2X4 and P2X7 antagonists, are potentially of clinical use in treatment of disorders of the nervous system, such as chronic pain, neurodegeneration and brain injury. Agonists of the A2AAR and P2Y2R are already used clinically, P2Y12R antagonists are widely used antithrombotics and an antagonist of the A2AAR is approved in Japan for treating Parkinson's disease. The selectivity defined for some of the previously introduced compounds has been revised with updated pharmacological characterization, for example, various AR agonists and antagonists were deemed A1AR or A3AR selective based on human data, but species differences indicated a reduction in selectivity ratios in other species. Also, many of the P2R ligands still lack bioavailability due to charged groups or hydrolytic (either enzymatic or chemical) instability. X-ray crystallographic structures of AR and P2YRs have shifted the mode of ligand discovery to structure-based approaches rather than previous empirical approaches. The X-ray structures can be utilized either for in silico screening of chemically diverse libraries for the discovery of novel ligands or for enhancement of the properties of known ligands by chemical modification. Although X-ray structures of the zebrafish P2X4R have been reported, there is scant structural information about ligand recognition in these trimeric ion channels. In summary, there are definitive, selective agonists and antagonists for all of the ARs and some of the P2YRs; while the pharmacochemistry of P2XRs is still in nascent stages. The therapeutic potential of selectively modulating these receptors is continuing to gain interest in such fields as cancer, inflammation, pain, diabetes, ischemic protection and many other conditions. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Activation of P2Y6 receptors increases the voiding frequency in anaesthetized rats by releasing ATP from the bladder urothelium

BACKGROUND AND PURPOSE

Despite the abundant expression of the UDP-sensitive P2Y6 receptor in urothelial cells and sub-urothelial myofibroblasts its role in the control of bladder function is not well understood.

EXPERIMENTAL APPROACH

We compared the effects of UDP and of the selective P2Y6 receptor agonist, PSB0474, on bladder urodynamics in anaesthetized rats; the voided fluid was tested for ATP bioluminescence. The isolated urinary bladder was used for in vitro myographic recordings and [(3) H]-ACh overflow experiments.

KEY RESULTS

Instillation of UDP or PSB0474 into the bladder increased the voiding frequency (VF) without affecting the amplitude (A) and the duration (Δt) of bladder contractions; an effect blocked by the P2Y6 receptor antagonist, MRS2578. Effects mediated by urothelial P2Y6 receptors required extrinsic neuronal circuitry as they were not detected in the isolated bladder. UDP-induced bladder hyperactvity was also prevented by blocking P2X3 and P2Y1 receptors, respectively, with A317491 and MRS2179 applied i.v.. UDP decreased [(3) H]-ACh release from stimulated bladder strips with urothelium, but not in its absence. Inhibitory effects of UDP were converted into facilitation by the P2Y1 receptor antagonist, MRS2179. The P2Y6 receptor agonist increased threefold ATP levels in the voided fluid.

CONCLUSIONS AND IMPLICATIONS

Activation of P2Y6 receptors increased the voiding frequency indirectly by releasing ATP from the urothelium and activation of P2X3 receptors on sub-urothelial nerve afferents. Bladder hyperactivity may be partly reversed following ATP hydrolysis to ADP by E-NTPDases, thereby decreasing ACh release from cholinergic nerves expressing P2Y1 receptors.

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

Purinergic inhibitory regulation of murine detrusor muscles mediated by PDGFRα+ interstitial cells

Purines induce transient contraction and prolonged relaxation of detrusor muscles. Transient contraction could be due to activation of inward currents in smooth muscle cells, but the mechanism of purinergic relaxation has not been determined. We recently reported a new class of interstitial cells in detrusor muscles and showed that these cells could be identified with antibodies against platelet-derived growth factor receptor-α (PDGFRα(+) cells). The current density of small conductance Ca(2+)-activated K(+) (SK) channels in these cells is far higher (∼100 times) than in smooth muscle cells. Thus, we examined purinergic receptor (P2Y) mediated SK channel activation as a mechanism for purinergic relaxation. P2Y receptors (mainly P2ry1 gene) were highly expressed in PDGFRα(+) cells. Under voltage clamp conditions, ATP activated large outward currents in PDGFRα(+) cells that were inhibited by blockers of SK channels. ATP also induced significant hyperpolarization under current clamp conditions. A P2Y1 agonist, MRS2365, mimicked the effects of ATP, and a P2Y1 antagonist, MRS2500, inhibited ATP-activated SK currents. Responses to ATP were largely abolished in PDGFRα(+) cells of P2ry1(-/-) mice, and no response was elicited by MRS2365 in these cells. A P2X receptor agonist had no effect on PDGFRα(+) cells but, like ATP, activated transient inward currents in smooth muscle cells (SMCs). A P2Y1 antagonist decreased nerve-evoked relaxation. These data suggest that purines activate SK currents via mainly P2Y1 receptors in PDGFRα(+) cells. Our findings provide an explanation for purinergic relaxation in detrusor muscles and show that there are no discrete inhibitory nerve fibres. A dual receptive field for purines provides the basis for inhibitory neural regulation of excitability.

Extracellular UDP enhances P2X-mediated bladder smooth muscle contractility via P2Y(6) activation of the phospholipase C/inositol trisphosphate pathway

Bladder dysfunction characterized by abnormal bladder smooth muscle (BSM) contractions is pivotal to the disease process in overactive bladder, urge incontinence, and spinal cord injury. Purinergic signaling comprises one key pathway in modulating BSM contractility, but molecular mechanisms remain unclear. Here we demonstrate, using myography, that activation of P2Y6 by either UDP or a specific agonist (MRS 2693) induced a sustained increase in BSM tone (up to 2 mN) in a concentration-dependent manner. Notably, activation of P2Y6 enhanced ATP-mediated BSM contractile force by up to 45%, indicating synergistic interactions between P2X and P2Y signaling. P2Y6-activated responses were abolished by phospholipase C (PLC) and inositol trisphosphate (IP3) receptor antagonists U73122 and xestospongin C, demonstrating involvement of the PLC/IP3 signal pathway. Mice null for Entpd1, an ectonucleotidase on BSM, demonstrated increased force generation on P2Y6 activation (150%). Thus, in vivo perturbations to purinergic signaling resulted in altered P2Y6 activity and bladder contractility. We conclude that UDP, acting on P2Y6, regulates BSM tone and in doing so selectively maximizes P2X1-mediated contraction forces. This novel neurotransmitter pathway may play an important role in urinary voiding disorders characterized by abnormal bladder motility.

The impact of commercially available purinergic ligands on purinergic signalling research

Due to the extremely wide-spread expression of purinergic receptors, purinergic signalling has been implicated in numerous physiological and pathophysiological areas. To better understand the involvement of purinergic receptors in such areas, the researcher's requirement for diverse and varied purinergic receptor ligands has greatly increased. This has generated increased commercial opportunities for life science suppliers, and ultimately, has led to a rapid expansion in the number of commercially available purinergic receptor ligands. The wide-spread availability of ligands to researchers has greatly benefited the scientific community, nurturing the rapid and continued expansion of the purinergic signalling field.

ATP and P2X purinoceptors in urinary tract disorders

The pharmacological concept of specifically targeting purinoceptors (receptors for ATP and related nucleotides) has emerged over the last two decades in the quest for novel, differentiated therapeutics. Investigations from many laboratories have established a prominent role for ATP in the functional regulation of most tissue and organ systems, including the urinary tract, under normal and pathophysiological conditions. In the particular case of the urinary tract, ATP signaling via P2X1 receptors participates in the efferent control of detrusor smooth muscle excitability, and this function may be heightened in disease and aging. Perhaps of greater interest, ATP also appears to be involved in bladder sensation, operating via activation of P2X3-containing receptors on sensory afferent neurones, both on peripheral terminals within the urinary tract tissues (e.g., ureters, bladder) and on central synapses in the dorsal horn of the spinal cord. Such findings are based on results from classical pharmacological and localization studies in nonhuman and human tissues, gene knockout mice, and studies using recently identified pharmacological antagonists - some of which have progressed as candidate drug molecules. Based on recent advances in this field, it is apparent that the development of selective antagonists for these receptors will occur that could lead to therapies offering better relief of storage, voiding, and sensory symptoms for patients, while minimizing the systemic side effects that curb the clinical effectiveness of current urologic medicines.

Endogenous purinergic control of bladder activity via presynaptic P2X3 and P2X2/3 receptors in the spinal cord

P2X(3) and P2X(2/3) receptors are localized on sensory afferents both peripherally and centrally and have been implicated in various sensory functions. However, the physiological role of these receptors expressed presynaptically in the spinal cord in regulating sensory transmission remains to be elucidated. Here, a novel selective P2X(3) and P2X(2/3) antagonist, AF-792 [5-(5-ethynyl-2-isopropyl-4-methoxy-phenoxy)-pyrimidine-2,4-diamine, previously known as RO-5], in addition to less selective purinoceptor ligands, was applied intrathecally in vivo. Cystometry recordings were made to assess changes in the micturition reflex contractions after drug treatments. We found that AF-792 inhibited micturition reflex activity significantly (300 nmol; from baseline contraction intervals of 1.18 +/- 0.07 to 9.33 +/- 2.50 min). Furthermore, inhibition of P2X(3) and P2X(2/3) receptors in the spinal cord significantly attenuated spinal activation of extracellular-signal regulated kinases induced by acute peripheral stimulation of the bladder with 1% acetic acid by 46.4 +/- 12.0% on average. Hence, the data suggest that afferent signals originating from the bladder are regulated by spinal P2X(3) and P2X(2/3) receptors and establish directly an endogenous central presynaptic purinergic mechanism to regulate visceral sensory transmission. Identification of this spinal purinergic control in visceral activities may help the development of P2X(3) and P2X(2/3) antagonist to treat urological dysfunction, such as overactive bladder, and possibly other debilitating sensory disorders, including chronic pain states.

Assessment and characterization of purinergic contractions and relaxations in the rat urinary bladder

The aim of the present study was to assess the purinoceptor functional responses of the urinary bladder by using isolated rat urinary bladder strip preparations. ATP elicited a transient bladder contraction followed by a sustained relaxation and ADP, UDP and UTP generated predominantly potent relaxations (relaxatory potencies: ADP = ATP > UDP = UTP). The ATP contractions were desensitized with the P2X(1/3) purinoceptor agonist/desensitizer alpha,beta-meATP and reduced by the P2 purinoceptor antagonist PPADS but unaffected by the P2 purinoceptor antagonist suramin. Electrical field stimulation (1-60 Hz) evoked frequency-dependent bladder contractions that were decreased by incubation with alpha,beta-meATP but not further decreased by PPADS. Suramin antagonized relaxations generated by UDP but not those by ADP, ATP or UTP. PPADS antagonized and tended to antagonize UTP and UDP relaxations, respectively, but did neither affect ADP nor ATP relaxations. ADP relaxations were insensitive to the P2Y(1) purinoceptor antagonist MRS 2179 and the ATP-sensitive potassium channel antagonist glibenclamide. The ATP relaxations were inhibited by the P1 purinoceptor antagonist 8-p-sulfophenyltheophylline but unaffected by the A2A adenosine receptor antagonist 8-(3-chlorostyryl)caffeine and glibenclamide. Adenosine evoked relaxations that were antagonized by the A2B adenosine receptor antagonist PSB 1115. Thus, in the rat urinary bladder purinergic contractions are elicited predominantly by stimulation of the P2X(1) purinoceptors, while UDP/UTP-sensitive P2Y purinoceptor(s) and P1 purinoceptors of the A2B adenosine receptor subtype are involved in bladder relaxation.

Purinergic mechanosensory transduction and visceral pain

In this review, evidence is presented to support the hypothesis that mechanosensory transduction occurs in tubes and sacs and can initiate visceral pain. Experimental evidence for this mechanism in urinary bladder, ureter, gut, lung, uterus, tooth-pulp and tongue is reviewed. Potential therapeutic strategies are considered for the treatment of visceral pain in such conditions as renal colic, interstitial cystitis and inflammatory bowel disease by agents that interfere with mechanosensory transduction in the organs considered, including P2X₃ and P2X_2/3 receptor antagonists that are orally bioavailable and stable in vivo and agents that inhibit or enhance ATP release and breakdown.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

Bladder dysfunction in multiple sclerosis

Multiple sclerosis (MS) is a relatively common disease of young adults. Patients with MS can have a wide range of symptoms and may develop significant disability. The cause of MS is unknown, but immunological mechanisms are important. In MS, the pathological features include prominent demyelination and inflammation, but there is also evidence of neurodegeneration. Bladder symptoms are common in MS. The bladder is under neural control, and bladder disturbance is usually attributed to demyelination or loss of axons from the neural pathways, particularly those in the spinal cord, that control the bladder. However, as with other symptoms in MS, the presence of bladder disturbance does not always correlate well with MRI lesions. We speculate that other possible causes of bladder dysfunction in MS might include the effects of circulating toxic factors. Urgency of micturition is prominent in MS, and better understanding of the receptors involved in bladder sensation suggests possible treatment strategies through inhibiting these receptors.

Emerging drugs for treatment of overactive bladder and detrusor overactivity

BACKGROUND

Overactive bladder (OAB) signifies the presence of urinary urgency and can have major effects on quality of life and social functioning. Standard antimuscarinic drugs have good initial response rates but substantial adverse effects and long-term compliance problems.

OBJECTIVES

To review the complexities of the mechanisms underlying OAB and the current drugs available for treating its symptoms.

METHODS

The literature was reviewed to define current therapies and drugs in clinical trials. Articles were identified by means of a computerised PubMed and Cochrane Library search (using the following keywords: overactive bladder, detrusor overactivity, urgency and bladder), supported by a search of the PharmaProjects database.

CONCLUSIONS

New drug classes, such as beta-3 adrenergic agonists, may work by reducing contractility or excitability of bladder muscle. Moderation of afferent activity may allow improved OAB symptoms, with lower risk of affecting voiding function. Agents acting on the CNS could influence OAB favourably, but target selection and adverse effects are an issue. The recognition of the functional contribution of the urothelium and the diversity of nerve transmitters has sparked interest in both peripheral and central modulation of OAB pathophysiology.

Potential therapeutic targets for ATP-gated P2X receptor ion channels

P2X receptors make up a novel family of ligand-gated ion channels that are activated by binding of extracellular ATP. These receptors can form a number of homomeric and heteromeric ion channels, which are widely distributed throughout the human body. They are thought to play an important role in many cellular processes, including synaptic transmission and thrombocyte aggregation. These ion channels are also involved in the pathology of several disease states, including chronic inflammation and neuropathic pain, and thus are the potential targets for drug development. The recent discovery of potent and highly selective antagonists for P2X(7) receptors, through the use of high-throughput screening, has helped to further understand the P2X receptor pharmacology and provided new evidence that P2X(7) receptors play a specific role in chronic pain states. In this review, we discuss how the P2X family of ion channels has distinguished itself as a potential new drug target. We are optimistic that safe and effective candidate drugs will be suitable for progression into clinical development.

Expression and function of rat urothelial P2Y receptors

The control and regulation of the lower urinary tract are partly mediated by purinergic signaling. This study investigated the distribution and function of P2Y receptors in the rat urinary bladder. Application of P2Y agonists to rat urothelial cells evoked increases in intracellular calcium; the rank order of agonist potency (pEC(50) +/- SE) was ATP (5.10 +/- 0.07) > UTP (4.91 +/- 0.14) > UTPgammaS (4.61 +/- 0.16) = ATPgammaS (4.70 +/- 0.05) > 2-methylthio adenosine 5'-diphosphate = 5'-(N-ethylcarboxamido)adenosine = ADP (<3.5). The rank order potency for these agonists indicates that urothelial cells functionally express P2Y(2)/P2Y(4) receptors, with a relative lack of contribution from other P2Y or adenosine receptors. Real-time PCR, Western blotting, and immunocytochemistry confirmed the expression of P2Y(2) and to a lesser extent P2Y(4) in the urothelium. Immunocytochemical studies revealed expression of P2Y(2) staining in all layers of the urothelium, with relative absence of P2Y(4). P2Y(2) staining was also present in suburothelial nerve bundles and underlying detrusor smooth muscle. Addition of UTP and UTPgammaS was found to evoke ATP release from cultured rat urothelial cells. These findings indicate that cultured rat urothelial cells functionally express P2Y(2)/P2Y(4) receptors. Activation of these receptors could have a role in autocrine and paracrine signaling throughout the urothelium. This could lead to the release of bioactive mediators such as additional ATP, nitric oxide, and acetylcholine, which can modulate the micturition reflex by acting on suburothelial myofibroblasts and/or pelvic afferent fibers.

Connexins 37 and 40 transduce purinergic signals mediating renal autoregulation

Our previous data indicated that various subtypes of connexin (Cx) were expressed in the juxtaglomerular apparatus. Experiments were performed to characterize the effects on renal autoregulation of specific mimetic peptides that inhibit these Cx subtypes in Wistar-Kyoto rats. Intrarenal infusion of (Cx37,43)GAP27 increased autoregulatory index of renal plasma flow (0.06 +/- 0.05 to 0.47 +/- 0.06, n = 6, P < 0.05) and glomerular filtration rate (GFR; 0.01 +/- 0.07 to 0.49 +/- 0.07, P < 0.05). The additional administration of 8-cyclopentyl- 1,3-dipropylxanthine (CPX) produced a further elevation of autoregulatory index of RPF (0.86 +/- 0.07, P < 0.05) and GFR (0.88 +/- 0.09, P < 0.05), compared with (Cx37,43)GAP27 alone. However, the addition of pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid (PPADS) to (Cx37,43)GAP27 did not. Combined treatment with CPX and PPADS markedly worsened autoregulatory index of RPF (0.04 +/- 0.10 to 0.81 +/- 0.06, n = 6 P < 0.01) and GFR (0.05 +/- 0.08 to 0.79 +/- 0.05, P < 0.01). (Cx40)GAP27 induced similar changes to (Cx37,43)GAP27. Renal autoregulation was preserved in the presence of (Cx43)GAP26. Our results indicate that the inhibition of gap junction impaired renal autoregulation. Furthermore, the present data provide evidence that both adenosine and purinergic receptors contribute to glomerular autoregulation. Finally, our findings suggest that gap junctions, at least in part, transduce purinergic signals mediating renal autoregulation.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Purinoceptors as therapeutic targets for lower urinary tract dysfunction

Lower urinary tract symptoms (LUTS) are present in many common urological syndromes. However, their current suboptimal management by muscarinic and alpha(1)-adrenoceptor antagonists leaves a significant opportunity for the discovery and development of superior medicines. As potential targets for such therapeutics, purinoceptors have emerged over the last two decades from investigations that have established a prominent role for ATP in the regulation of urinary bladder function under normal and pathophysiological conditions. In particular, evidence suggests that ATP signaling via P2X(1) receptors participates in the efferent control of detrusor smooth muscle excitability, and that this function may be heightened in disease and aging. ATP also appears to be involved in bladder sensation, via activation of P2X(3) and P2X(2/3) receptors on sensory afferent neurons, both within the bladder itself and possibly at central synapses. Such findings are based on results from classical pharmacological and localization studies in non-human and human tissues, knockout mice, and studies using recently identified pharmacological antagonists--some of which possess attributes that offer the potential for optimization into candidate drug molecules. Based on recent advances in this field, it is clearly possible that the development of selective antagonists for these receptors will occur that could lead to therapies offering better relief of sensory and motor symptoms for patients, while minimizing the systemic side effects that limit current medicines.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Detrusor overactivity induced by intravesical application of adenosine 5'-triphosphate under different delivery conditions in rats

OBJECTIVES

To investigate the effects of intravesical application of adenosine 5'-triphosphate (ATP) on bladder activity to elucidate the role of urothelial barrier function and ecto-ATPase activity in the ATP-mediated mechanism inducing detrusor overactivity.

METHODS

Continuous cystometry by an intravesical catheter inserted from the bladder dome was performed in conscious female rats.

RESULTS

ATP solutions adjusted to pH 6.0 did not elicit significant detrusor overactivity at a concentration of 60 mM. However, in bladders pretreated with protamine sulfate (10 mg/mL) to increase urothelial permeability, ATP solution (pH 6.0) induced detrusor overactivity by decreasing the intercontraction intervals. These irritant effects of ATP after protamine treatment were antagonized by P2X receptor antagonists, such as pyridoxal-5-phosphate-6-azophenyl-2',4'-disulfonic acid (70 micromol/kg) and 2',3'-O-(2,4,6, trinitrophenyl) ATP (30 micromol/kg). These were also suppressed in rats pretreated with systemic capsaicin (125 mg/kg subcutaneously). Alpha,beta-methylene ATP (5 mM, pH 6.0) or ATP (60 mM, pH6) after intravesical infusion of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (5 mM, pH 6.0), an ecto-ATPase inhibitor, induced detrusor overactivity without protamine pretreatment, but the reduction in intercontraction intervals was smaller compared with that with ATP after protamine treatment.

CONCLUSIONS

Low permeability of bladder epithelium and ecto-ATPase activity can prevent ATP activation of subepithelial P2X receptors to induce bladder overactivity. Thus, enhanced penetration of endogenous ATP owing to urothelial damage may contribute to urinary frequency and bladder pain in hypersensitive bladder disorders such as interstitial cystitis.

Antagonism of ATP responses at P2X receptor subtypes by the pH indicator dye, Phenol red

1 Many types of culture media contain a pH-sensitive dye. One commonly occurring dye, Phenol red sodium (Na(+)) salt, was tested for blocking activity at rat P2X(1-4) receptors (P2X(1-4)Rs) expressed in Xenopus oocytes. 2 Phenol red Na(+)-salt antagonised adenosine 5'-triphosphate (ATP) responses at P2X(1)R (IC(50), 3 microM) and, at higher concentrations, also blocked P2X(2)R and P2X(3)R. Phenol red Na(+)-salt, purified of lipophilic contaminants, blocked P2X(1)R and P2X(3)R by acting as an insurmountable antagonist. 3 Two lipophilic extracts of Phenol red antagonised ATP responses at P2XRs. Extract A was a potent antagonist at P2X(1)R (IC(50), 1.4 microM), whereas extract B was a potent antagonist at P2X(3)R (IC(50), 4.1 microM). A bisphenolic compound (RS151030) found in these extracts was a potent antagonist at P2X(1)R (IC(50), 0.3 microM) and at P2X(3)R (IC(50), 2.4 microM). 4 Phenolphthalein base was a potent irreversible antagonist at P2X(1)R (IC(50), 1 microM), whereas Phenolphthalein K(+)-salt was 25-fold less potent here. 5 Phenolphthalein base was a reversible antagonist of ATP responses at rat P2X(4)R (IC(50), 26 microM), whereas Phenolphthalein K(+)-salt was inactive. 6 Dimethyl sulphoxide (DMSO), used to dissolve lipophilic extracts, showed pharmacological activity by itself at rat P2X(1)R and P2X(4)R. 7 Thus, Phenol red and related compounds are antagonists at rat P2X(1)R, but are also active at other rat P2XRs. Phenolphthalein base is a newly identified, low potency antagonist of ATP responses at P2X(4)R. Culture media containing these red dyes should be used cautiously in future pharmacological studies of P2XRs. Also, wherever possible, the solvent DMSO should be used with caution.

Discovery and synthesis of a novel and selective drug-like P2X1 antagonist

Although there is extensive literature to indicate that many different types of P2 purinoceptors are present in the lower urinary tract, the physiological role of these receptors in micturition is still uncertain. In part, this uncertainty has been caused by a lack of P2 subtype selective ligands. In this paper we report the discovery, gram scale synthesis, and binding results for 1, the first potent, drug-like, selective P2X(1) receptor antagonist described. Compound 1 was shown to be more than 30-fold selective over other purinergic receptor subtypes.