The effects of purine nucleotides on transmission in vesical parasympathetic ganglia of the cat

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.

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.

A possible role of the cholinergic and purinergic receptor interaction in the regulation of the rat urinary bladder function

The contractile activation of the upper (dome) and lower (base) parts of the urinary bladder show some differences. Cellular mechanisms that might be responsible for cholinergic effects blocking non-adrenergic non-cholinergic contractions in the base of the rat urinary bladder were investigated. Smooth muscle cells were thus freshly isolated or cultured both from the dome and the base of the rat urinary bladder and the contribution from cholinergic and purinergic pathways to their Ca(2+) homeostasis was examined. The expression of nicotinic acetylcholine (nAChR) and P2X2 purinergic receptors on the cultured cells and on tissue sections was investigated. The ATP-evoked Ca(2+) transients in rat smooth muscle cells did not show any desensitization. However, when ATP was administered together with carbamylcholine (CCh), the latter essentially prevented ATP from evoking Ca(2+) transients in smooth muscle cells from the base (suppression to 12 ± 2.5% of control, n = 57; p < 0.01), but not from the dome (99 ± 5% of control, n = 52; p > 0.05) of the rat urinary bladder. While atropine was unable to modify (6 ± 3% of control, n = 14; p < 0.05), α-bungarotoxin (118 ± 12% of control, n = 20; p > 0.05) blocked the inhibitory effects of CCh. Additionally, α7 subunits of nAChR and P2X2 purinergic receptors were identified using immunocytochemistry, immunohistochemistry, and Western blot in cultured urinary bladder smooth muscle cells, in urinary bladder sections, and in urinary bladder muscle strips, respectively, suggesting that the activation of nAChR modifies the action of ATP.

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.

Expression of P2X and P2Y receptors in the intramural parasympathetic ganglia of the cat urinary bladder

The distribution and function of P2X and P2Y receptor subtypes were investigated on intact or cultured intramural ganglia of the cat urinary bladder by immunocytochemistry and calcium-imaging techniques, respectively. Neurons were labeled by all seven P2X receptor subtype antibodies and antibodies for P2Y2, P2Y4, P2Y6, and P2Y12 receptor subtypes with a staining intensity of immunoreactivity in the following order: P2X3=P2Y2=P2Y4=P2Y6=P2Y12>P2X1=P2X2=P2X4>P2X5=P2X6=P2X7. P2Y1 receptor antibodies labeled glial cells, but not neurons. P2X3 and P2Y4 polyclonal antibodies labeled ∼95 and 40% of neurons, respectively. Double staining showed that 100, 48.8, and 97.4% of P2X3 receptor-positive neurons coexpressed choline acetyl transferase (ChAT), nitric oxide synthase (NOS), and neurofilament 200 (NF200), respectively, whereas 100, 59.2, and 97.6% of P2Y4 receptor-positive neurons coexpressed ChAT, NOS, and NF200, respectively. Application of ATP, α,β-methylene ATP, and uridine triphosphate elevated intracellular Ca2+ concentration in a subpopulation of dissociated cultured cat intramural ganglia neurons, demonstrating the presence of functional P2Y4 and P2X3 receptors. This study indicates that P2X and P2Y receptor subtypes are expressed by cholinergic parasympathetic neurons innervating the urinary bladder. The neurons were also stained for NF200, usually regarded as a marker for large sensory neurons. These novel histochemical properties of cholinergic neurons in the cat bladder suggest that the parasympathetic pathways to the cat bladder may be modulated by complex purinergic synaptic mechanisms.

Elementary purinergic Ca2+ transients evoked by nerve stimulation in rat urinary bladder smooth muscle

The translation of nerve transmission to Ca2+ signals in urinary bladder smooth muscle (UBSM) is incompletely understood. Thus, we sought to characterize Ca2+ signals in strips of UBSM loaded with the Ca2+-sensitive fluorescent dye, fluo-4, using laser scanning confocal microscopy. Two types of Ca2+ signals occurred spontaneously and could be evoked with field stimulation: large, rapid, global Ca2+ transients termed 'global Ca2+ flashes', and much smaller, localized Ca2+ transients. Global Ca2+ flashes were inhibited by the L-type voltage-dependent Ca2+ channel (VDCC) inhibitor, diltiazem and with P2X receptor blockade. Simultaneous intracellular recordings and Ca2+ measurements indicated that these events are caused by Ca2+ influx through VDCCs during action potentials. Small, local Ca2+ transients occurred spontaneously, and their frequency could be elevated with field stimulation. Atropine, an inhibitor of muscarinic receptors, did not affect these local Ca2+ transients. However, the desensitizing P2X receptor agonist alpha,beta-methylene ATP, and the purinergic antagonist, suramin, effectively inhibited the local Ca2+ transients. The frequency of these 'purinergic Ca2+ transients' was increased about 7-fold by a 10 s stimulus train (1 Hz). The amplitude, duration at one-half amplitude and the spatial spread of the evoked purinergic Ca2+ transients were F/F(o) = 2.4 +/- 0.13, 111.7 +/- 9.3 ms and 14.0 +/- 1.0 microm2, respectively. Tetrodotoxin inhibited evoked purinergic Ca2+ transients, indicating that they were dependent on nerve fibre activation. Purinergic Ca2+ transients were not dependent on VDCC activity. Neither 2-APB, an inhibitor of inositol 1,4,5-triphosphate (Ins(1,4,5)P3) (IP3)-induced Ca2+ release, nor ryanodine inhibited the purinergic Ca2+ transients. We have identified two novel Ca2+ signals in rat UBSM. Large, rapid, global Ca2+ flashes that represent Ca2+ influx through VDCCs during action potentials, and local, purinergic Ca2+ transients that represent Ca2+ entry through P2X receptors. Our results indicate that purinergic Ca2+ transients evoked by release of ATP from nerve varicosities are elementary signals in the process of nerve-smooth muscle communication.

Alterations in P2X and P2Y purinergic receptor expression in urinary bladder from normal cats and cats with interstitial cystitis

Purinergic mechanisms appear to be involved in motor as well as sensory functions in the urinary bladder. ATP released from efferent nerves excites bladder smooth muscle, whereas ATP released from urothelial cells can activate afferent nerves and urothelial cells. In the present study, we used immunohistochemical techniques to examine the distribution of purinoceptors in the urothelium, smooth muscle, and nerves of the normal cat urinary bladder as well as possible changes in the expression of these receptors in cats with a chronic painful bladder condition termed feline interstitial cystitis (FIC) in which ATP release from the urothelium is increased. In normal cats, a range of P2X (P2X1, P2X2, P2X3, P2X4, P2X5, P2X6, and P2X7) and P2Y (P2Y1, P2Y2, and P2Y4) receptor subtypes was expressed throughout the bladder urothelium. In FIC cats, there is a marked reduction in P2X1 and loss of P2Y2 receptor staining. Both P2X3 and P2Y4 are present in nerves in normal cat bladder, and no obvious differences in staining were detected in FIC. Smooth muscle in the normal bladder did not exhibit P2Y receptor staining but did exhibit P2X (P2X2, P2X1) staining. In the FIC bladder smooth muscle, there was a significant reduction in P2X1 expression. These findings raise the possibility that purinergic mechanisms in the urothelium and bladder smooth muscle are altered in FIC cats. Because the urothelial cells appear to have a sensory function in the bladder, it is possible that the plasticity in urothelial purinergic receptors is linked with the painful bladder symptoms in IC.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Differing effects of N(G)-monomethyl L-arginine and 7-nitroindazole on detrusor activity

AIMS

Previous studies reported that nitric oxide (NO) synthase (NOS) inhibition decreases micturition volume threshold (MVT), the volume required to produce a centrally mediated micturition contraction, and that NO can be released from urothelium by means of certain stimuli. With elucidation of multiple isoforms of NOS, studies were performed to determine whether inhibition of specific isoforms of NOS altered MVT in different ways.

METHODS

In naive, anesthetized cats, the urinary bladder was exposed by means of a midline abdominal incision and cannulated through a slit in the internal urethra approximately 4-5 cm distal to the neck of the bladder. The left renal artery and left radial vein were cannulated for the intra-arterial and intravenous administration of drugs, respectively. All nerves were left intact. A control MVT was determined by slowly infusing saline into the bladder at a rate of 0.018 mL/kg per minute. Varying doses of L-NMMA (N(G)-monomethyl-L-arginine) or 7-NI (7-nitro indazole) were administered and the MVT was again determined.

RESULTS

Inhibition of endothelial NOS (eNOS), by L-NMMA, or neuronal NOS (nNOS), by 7-NI, produces varying effects on certain detrusor activities and that inhibition of different isoforms of NOS produces qualitatively different effects. L-NMMA significantly decreases MVT (up to 60% decrease), whereas 7-NI significantly increases MVT (over 300% increase). L-NMMA increases frequency and onset of small bladder contractions, whereas 7-NI produces opposite effects.

CONCLUSIONS

The results suggest that detrusor relaxation and contractility may be modulated by NO levels and that NO released from the urothelium may be a mediator of detrusor relaxation during the storage phase of micturition.

P2X receptors in peripheral neurons

P2X receptors are a family of ligand-gated ion channels, activated by extracellular ATP. The seven subunits cloned (P2X1-7) can assemble to form homomeric and heteromeric receptors. Peripheral neurons of neural crest origin (e.g. those in dorsal root, trigeminal, sympathetic and enteric ganglia) and placodal origin (e.g. those in nodose and petrosal ganglia) express mRNAs for multiple P2X subunits. In this review, we summarize the molecular biological, electrophysiological and immunohistochemical evidence for P2X receptor subunits in sensory, sympathetic, parasympathetic, pelvic and myenteric neurons and adrenomedullary chromaffin cells. We consider the pharmacological properties of these native P2X receptors and their physiological roles. The responses of peripheral neurons to ATP show considerable heterogeneity between cells in the same ganglia, between ganglia and between species. Nevertheless, these responses can all be accounted for by the presence of P2X2 and P2X3 subunits, giving rise to varying proportions of homomeric and heteromeric receptors. While dorsal root ganglion neurons express predominantly P2X3 and rat sympathetic neurons express mainly P2X2 receptors, nodose and guinea-pig sympathetic neurons express mixed populations of P2X2 and heteromeric P2X2/3 receptors. P2X receptors are important for synaptic transmission in enteric ganglia, although their roles in sympathetic and parasympathetic ganglia are less clear. Their presence on sensory neurons is essential for some processes including detection of filling of the urinary bladder. The regulation of P2X receptor expression in development and in pathological conditions, along with the interactions between purinergic and other signalling systems, may reveal further physiological roles for P2X receptors in autonomic and sensory ganglia.

Pharmacology of the lower urinary tract

The functions of the lower urinary tract, to store and periodically release urine, are dependent on the activity of smooth and striated muscles in the urinary bladder, urethra, and external urethral sphincter. This activity is in turn controlled by neural circuits in the brain, spinal cord, and peripheral ganglia. Various neurotransmitters, including acetylcholine, norepinephrine, dopamine, serotonin, excitatory and inhibitory amino acids, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in the neural regulation of the lower urinary tract. Injuries or diseases of the nervous system, as well as drugs and disorders of the peripheral organs, can produce voiding dysfunctions such as urinary frequency, urgency, and incontinence or inefficient voiding and urinary retention. This chapter will review recent advances in our understanding of the pathophysiology of voiding disorders and the targets for drug therapy.

Neurophysiology of micturition and continence in women

Micturition and continence involve the coordination of complex neural events between the central and peripheral nervous systems. An understanding of these events provides a foundation for the treatment of voiding disorders in women such as stress urinary incontinence, urge incontinence and interstitial cystitis. The purpose of this paper is to comprehensively review the neuroanatomy, neurophysiology and neuropharmacology of micturition and continence. However, a brief section discussing clinical correlations will follow each of these topics to help integrate the basic science with clinical observations.

The effect of NG-monomethyl-L-arginine on bladder function

Recent studies have demonstrated the presence of nitric oxide synthase (NO synthase) in lower urinary tract tissues, however, its role in the detrusor is unclear. The current study was designed to determine if NO synthase inhibition alters detrusor activities, including micturition volume threshold, and inhibition of pelvic nerve-evoked contractions by various stimuli. In naive, anesthetized adult cats, inhibition of pelvic nerve-evoked bladder contractions, induced by hypogastric nerve stimulation or the intraarterial administration of NA, ATP, adenosine, beta,gamma-methylene ATP and 2-methylthio ATP, was measured before and after inhibition of NO synthase. The micturition volume threshold was also measured before and after NO synthase inhibition. L-NMMA decreased the micturition volume threshold by 38% (2 mg intravesical administration) or 80% (4 mg/kg i.a.). The magnitude of the micturition contractions was modestly increased. These results, and information in the literature, indicate that NO may play a role in the collection phase of the bladder cycle and any alteration of nitric oxide availability could induce or allow development of various bladder malfunctions, such as small bladder diseases, like interstitial cystitis.

Purinergic cation channels in neurons of rabbit vesical parasympathetic ganglia

Membrane current was recorded from neurons in rabbit vesical parasympathetic ganglia, utilizing single electrode voltage clamp techniques. ATP (0.1-1 mM) caused an inward current (IATP) associated with an increased conductance at a holding potential of -50 mV. ADP (0.1-1 mM) and 5'-O-3-thiotriphosphate (0.1-0.6 mM) but not AMP (0.3-2 mM) and adenosine (0.1-2 mM) mimicked the actions of ATP. The IATP reversed its polarity at -12.1 +/- 1.4 mV. The amplitude of the IATP was depressed in low sodium solutions and in nominally calcium-free solutions but not in low chloride solutions. Suramin (10-100 microM) and reactive blue 2 (10-100 microM), P2-antagonists, reversibly depressed the IATP. In contrast, hexamethonium (100 microM) did not affect the IATP. These data suggest that ATP activates cation channels through P2X receptor subtypes in parasympathetic neurons.

Neurophysiology of micturition and continence

This article reviews the neuroanatomy, neurophysiology, and neuropharmacology involved in micturition and continence. Knowledge of these topics helps the clinician diagnose and treat voiding disorders that are caused by disease, trauma, drugs, and aging.

Pharmacological studies to examine the source of ATP released by pelvic nerve stimulation in the feline lower urinary tract

1. Pretreatment with 6-OHDA or bilateral sectioning of the hypogastric nerves decreased the noradrenaline content of lower urinary tract tissue. 2. Pelvic nerve stimulation-induced contractions were not significantly changed by either 6-OHDA pretreatment or bilateral sectioning of the hypogastric nerves. 3. Hypogastric nerve stimulation-induced contractions were significantly attenuated by the pretreatments. 4. Neither ATP- nor acetylcholine-induced contractions were significantly altered by either 6-OHDA or bilateral sectioning of the hypogastric nerves. 5. The data indicate that ATP, released by pelvic nerve stimulation, which induces a bladder contraction, is released from parasympathetic, cholinergic nerve fibres in the pelvic nerve trunks and not sympathetic, nor adrenergic nerve fibres also present in the pelvic nerve trunks.

Characterization and autoradiographic localization of [3H] alpha,beta-methylene ATP binding sites in cat urinary bladder

1. The characteristics and distribution of [3H] alpha,beta-methylene ATP ([3H] alpha,beta-MeATP), a radioligand for P2X-purinoceptors, binding sites in cat urinary bladder detrusor were examined. 2. Saturation analysis revealed that, in cat bladder membrane preparations, only one population of binding sites with high affinity (Kd = 1.8 nM) was present, in contrast to other species where both high-and low-affinity binding sites are present. Another feature is that the density of the binding sites in the cat bladder (Bmax = 21.2 pmol/mg protein) is considerably higher (about 2-fold) than the high-affinity binding component in the rat bladder membrane preparations. 3. Displacement experiments with unlabelled purinoceptor ligands indicate that [3H] alpha,beta-MeATP mainly binds to P2X-purinoceptors. The order of binding displacement activity was: alpha,beta-methylene ATP, beta,gamma-methylene ATP > 2-methylthioATP > ATP > suramin and L-beta,gamma-methylene ATP > > adenosine. 4. Autoradiographic study demonstrated dense specific binding sites of [3H] alpha,beta-MeATP on detrusor smooth muscle of cat bladder. 5. The results of this study are consistent with pharmacological studies for the existence of P2X-purinoceptors in cat bladder.

Current state of purinoceptor research

It is hoped that this summary of the history and current status of purinoceptors will convince readers that receptors for purines are now established alongside other well-known extracellular messenger systems. These receptors are primitive, widespread and serve many different systems. Receptors of adenosine (P1-purinoceptors) are clearly different from receptors of ATP (P2-purinoceptors). As for other major transmitters such as acetylcholine, GABA, glutamate and 5-HT, receptors of two major families are activated by ATP, one (the P2X-purinoceptor family) mediates fast responses via ligand-gated ion channels, while the other (the P2Y-purinoceptor family) mediates slower responses via G-proteins (see Table 3). Subclasses of these two families have been suggested on the basis of recent molecular biology studies and the development of new selective agonists and antagonists (Abbracchio and Burnstock, 1994). It would indeed be helpful if the work on purinoceptors could be extended to studies of their chemical structure employing crystallography.

Purinergic and cholinergic components of bladder contractility and flow

The role of ATP as a neurotransmitter/neuromodulator in the urinary tract has been the subject of much study, particularly whether ATP has a functional role in producing urine flow. Recent studies suggested significant species variation, specifically a variation between cat and other species. This study was performed to determine the in vivo response of cat urinary bladder to pelvic nerve stimulation (PNS) and to the exogenous administration of cholinergic and purinergic agents. In anesthetized cats, bladder contractions and fluid expulsion was measured in response to PNS and to the exogenous administration of cholinergic and purinergic agents. Fluid was instilled into the bladder and any fluid expelled by bladder contractions induced by PNS or exogenous agents was collected in a beaker. The volume was measured in a graduated cylinder and recorded. PNS, carbachol and APPCP produced sustained contractions with significant expulsion of fluid. ATP, ACh and hypogastric nerve stimulation did not produce any significant expulsion of fluid. Atropine, a cholinergic antagonist, inhibited PNS contractions and fluid expulsion with no effect on purinergic actions. There was a significant relationship between the magnitude of the contraction, duration of the contractions and volume of fluid expelled. The data and information from other studies, strongly suggests a functional role for ATP as a cotransmitter in the lower urinary tract different from ACh's role. ATP stimulation of a specific purinergic receptor plays a role in initiation of bladder contractions and perhaps in the initiation of urine flow from the bladder. ACh's role is functionally different and appears to be more involved in maintenance of contractile activity and flow.

Subclasses of purinoceptors in feline bladder

Purines have been shown to inhibit and excite feline detrusor smooth muscle through P1 and P2 receptor activation. Several recent studies have demonstrated differences in agonist potency orders for subclasses of purinoceptors, including P2Y and nucleotide, or P2U receptors. The current studies were performed to determine the presence of such receptor subtypes in feline detrusor smooth muscle. Cats were surgically prepared for monitoring detrusor smooth muscle contractions as increases in intravesical pressure. Contractions were induced by pelvic nerves stimulation (PNS), ATP, and ATP analogs, such as beta, gamma-methylene ATP (APPCP), 5' adenylimido diphosphate (AMP-PNP) and 2-methylthio ATP (2-MeSATP), ATP gamma S, UTP, CTP and GTP. These agents all produced contractions and had an agonist potency order of AMP-PNP = APPCP > ATP gamma S = 2-MeSATP >> ATP > UTP = CTP = GTP. The agonist potency order for inhibition of PNS nerve-evoked bladder contractions was APPCP = AMP-PNP = ATP gamma S > 2-MeSATP = ATP > UTP = CTP = GTP. Reactive Blue 2 and Coomassie's Brilliant Blue G, two putative P2Y receptor antagonists, antagonized purine-induced actions. This antagonism and the agonist potency orders suggest the possible presence of novel receptors in detrusor smooth muscle and/or the presence of multiple receptors in detrusor smooth muscle.