Evidence for coexistence of ATP and nitric oxide in non-adrenergic, non-cholinergic (NANC) inhibitory neurones in the rat ileum, colon and anococcygeus muscle

Neurochemical profile of the myenteric plexus in the rat colorectal region

The enteric nervous system, a major subdivision of the autonomic nervous system, is known for its neurochemical heterogeneity and complexity. The myenteric plexus, one of its two principal components, primarily controls peristalsis and its dysfunction may lead to a number of gastrointestinal motility disorders. The myenteric neurons have been described to use a wide variety of neurotransmitters although no evidence has been reported for the existence of adrenergic neurons in the hindgut. This study aims at elucidating the chemical coding of neurons in the myenteric plexus of the rat colon and anorectal region with particular emphasis on cholinergic and the so-called nonadrenergic, noncholinergic (NANC) transmitter systems. The immunostaining for choline acetyltransferase revealed an intense staining of the myenteric ganglia with clear delineation of their neuronal cell bodies and without local distributional differences in the colonic region. The myenteric ATPergic structures were mostly limited to fiber bundles surrounding unstained myenteric neurons and penetrating the two muscle layers. We also observed an abundance of intensely stained varicose substance P-immunopositive fibers, ensheathing the immunonegative myenteric neuronal cell bodies in a basket-like manner. Applying NADPH-diaphorase histochemistry and nitric oxide synthase immunohistochemistry, we were able to demonstrate numerous nitrergic somata of myenteric neurons with Dogiel Type I morphology. Apart from the observed nitrergic distributional patterns, no distinct variations were found in the staining intensity or distribution of myenteric structures in the colon and anorectal area. Our results suggest that myenteric neurons in the distal intestinal portion utilize a broad spectrum of enteric transmitters, including classical and NANC transmitters.

The role of enteric inhibitory neurons in intestinal motility

The enteric nervous system controls much of the mixing and propulsion of nutrients along the digestive tract. Enteric neural circuits involve intrinsic sensory neurons, interneurons and motor neurons. While the role of the excitatory motor neurons is well established, the role of the enteric inhibitory motor neurons (IMNs) is less clear. The discovery of inhibitory transmission in the intestine in the 1960's in the laboratory of Geoff Burnstock triggered the search for the unknown neurotransmitter. It has since emerged that most neurons including the IMNs contain and may utilise more than one transmitter substances; for IMNs these include ATP, the neuropeptide VIP/PACAP and nitric oxide. This review distinguishes the enteric neural pathways underlying the 'standing reflexes' from the pathways operating physiologically during propulsive and non-propulsive movements. Morphological evidence in small laboratory animals indicates that the IMNs are located in the myenteric plexus and project aborally to the circular muscle, where they act by relaxing the muscle. There is ongoing 'tonic' activity of these IMNs to keep the intestinal muscle relaxed. Accommodatory responses to content further activate enteric pathways that involve the IMNs as the final neural element. IMNs are activated by mechanical and chemical stimulation induced by luminal contents, which activate intrinsic sensory enteric neurons and the polarised interneuronal ascending excitatory and descending inhibitory reflex pathways. The latter relaxes the muscle ahead of the advancing bolus, thus facilitating propulsion.

In Memoriam Geoffrey Burnstock: Creator of Purinergic Signaling

Geoff Burnstock (1929–2020) discovered purinergic signaling in a fastidious research that started in early 1960 and culminated in a concept of purinergic nerves in 1972. Subsequently, Geoff developed the concept of purinergic transmission and demonstrated ATP storage, release, and degradation in the context of cotransmission, which was another fundamental concept developed by him. Purinergic transmission contributes to the most fundamental physiological functions such as sensory transduction, regulation of heart rate, smooth muscle contraction, bile secretion, endocrine regulation, immune responses, as well as to various pathophysiological conditions, including inflammation, cancer, neuropathic pain, diabetes, and kidney failure.

Introduction to Purinergic Signaling

Purinergic signaling was proposed in 1972, after it was demonstrated that adenosine 5'-triphosphate (ATP) was a transmitter in nonadrenergic, noncholinergic inhibitory nerves supplying the guinea-pig taenia coli. Later, ATP was identified as an excitatory cotransmitter in sympathetic and parasympathetic nerves, and it is now apparent that ATP acts as a cotransmitter in most, if not all, nerves in both the peripheral nervous system and central nervous system (CNS). ATP acts as a short-term signaling molecule in neurotransmission, neuromodulation, and neurosecretion. It also has potent, long-term (trophic) roles in cell proliferation, differentiation, and death in development and regeneration. Receptors to purines and pyrimidines have been cloned and characterized: P1 adenosine receptors (with four subtypes), P2X ionotropic nucleotide receptors (seven subtypes) and P2Y metabotropic nucleotide receptors (eight subtypes). ATP is released from different cell types by mechanical deformation, and after release, it is rapidly broken down by ectonucleotidases. Purinergic receptors were expressed early in evolution and are widely distributed on many different nonneuronal cell types as well as neurons. Purinergic signaling is involved in embryonic development and in the activities of stem cells. There is a growing understanding about the pathophysiology of purinergic signaling and there are therapeutic developments for a variety of diseases, including stroke and thrombosis, osteoporosis, pain, chronic cough, kidney failure, bladder incontinence, cystic fibrosis, dry eye, cancer, and disorders of the CNS, including Alzheimer's, Parkinson's. and Huntington's disease, multiple sclerosis, epilepsy, migraine, and neuropsychiatric and mood disorders.

Effects of exendin-4 on colonic motility in rats and its underlying mechanism

BACKGROUND

Glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) agonists modulate gastrointestinal motility; however, the effects of GLP-1R agonists on colonic motility are still controversial, and the molecular mechanism is unclear. Exendin-4 shares 53% homology with GLP-1 and is a full agonist of GLP-1R. In this study, our aims were to explore the role and mechanism of exendin-4 in isolated rat colonic tissues and cells.

METHODS

An organ bath system was used to examine the spontaneous contractions of smooth muscle strips. The whole-cell patch-clamp technique was used to investigate the currents of L-type voltage-dependent calcium channels and large conductance Ca²⁺ -activated K⁺ (BK_Ca ) channels in smooth muscle cells.

KEY RESULTS

Exendin-4 decreased both the amplitude and frequency of spontaneous contractions of smooth muscle strips in a concentration-dependent manner. The inhibitory effect was completely blocked by exendin-4(9-39), a GLP-1R antagonist. Moreover, this effect was partially abolished by tetrodotoxin (TTX), a blocker of neuronal voltage-dependent Na⁺ channels, Nω-Nitro-l-arginine (L-NNA), a nitric oxide synthase (NOS) inhibitor, apamin, an inhibitor of small-conductance Ca²⁺ -activated K⁺ (SK) channels. Whole-cell patch-clamp recordings revealed that exendin-4 inhibited the peak current of L-type calcium channels in colonic smooth muscle cells, but did not change the shape of the current-voltage (I-V) curves. The steady-state activation and steady-state inactivation of L-type calcium channels were not affected. Likewise, BK_Ca currents were significantly inhibited by exendin-4.

CONCLUSIONS

Exendin-4 indirectly inhibits colonic muscle activity via a nitrergic and a purinergic neural pathway through NO and ATP release and inhibits L-type voltage-dependent calcium channels and BK_Ca channels in smooth muscle cells.

General Principles of Neuronal Co-transmission: Insights From Multiple Model Systems

It is now accepted that neurons contain and release multiple transmitter substances. However, we still have only limited insight into the regulation and functional effects of this co-transmission. Given that there are 200 or more neurotransmitters, the chemical complexity of the nervous system is daunting. This is made more-so by the fact that their interacting effects can generate diverse non-linear and novel consequences. The relatively poor history of pharmacological approaches likely reflects the fact that manipulating a transmitter system will not necessarily mimic its roles within the normal chemical environment of the nervous system (e.g., when it acts in parallel with co-transmitters). In this article, co-transmission is discussed in a range of systems [from invertebrate and lower vertebrate models, up to the mammalian peripheral and central nervous system (CNS)] to highlight approaches used, degree of understanding, and open questions and future directions. Finally, we offer some outlines of what we consider to be the general principles of co-transmission, as well as what we think are the most pressing general aspects that need to be addressed to move forward in our understanding of co-transmission.

Inhibitory action of oxytocin on spontaneous contraction of rat distal colon by nitrergic mechanism: involvement of cyclic GMP and apamin-sensitive K+ channels

AIM

The mechanisms underlying the inhibitory effects of oxytocin (OT) on colon tone are not totally understood. We explore the mechanisms of OT on spontaneous contractility in rat distal colon and identify the mediators involved in this action.

METHODS

In rat distal colon strips, mechanical activity was analysed and the production of nitric oxide (NO) in tissue loaded with the fluorochrome DAF-FM was visualized by confocal microscopy. OT receptor (OTR) expression was determined by Western blotting and immunofluorescence.

RESULTS

In rat distal colon, OT produced a concentration-dependent reduction in the spontaneous contraction, which was abolished by the OTR antagonist atosiban, the neural blocker tetrodotoxin and the inhibitor of neuronal nitric oxide synthase (nNOS) NPLA. The inhibitory effects of OT were not affected by propranolol, atropine, the nicotinic cholinoceptor blocker hexamethonium, the vasoactive intestinal peptide receptor antagonist VIPHyb, the P₂ purinoceptor antagonist PPADS, the adenosine A1 receptors antagonist DPCPX and the prostacyclin receptor antagonist Ro1138452. The soluble guanylyl cyclase (sGC) inhibitor ODQ and the small conductance Ca²⁺ -activated K⁺ (_Ca K⁺ ) channels blocker apamin significantly reduced the relaxation induced by OT, nicotine, sodium nitroprusside and the sGC activator BAY 41-2272. The neural release of NO elicited by OT was prevented by NPLA, tetrodotoxin and atosiban. The presence of the OTR and its co-localization with nNOS was detected by immunohistochemistry and Western blotting experiments.

CONCLUSION

These results demonstrate the NO release from enteric neurones induced by activation of OTR mediates distal colon relaxation. sGC and small conductance _Ca K⁺ channels are involved in this relaxation.

Increase of transcription factor EB (TFEB) and lysosomes in rat DRG neurons and their transportation to the central nerve terminal in dorsal horn after nerve injury

In the spinal dorsal horn (DH), nerve injury activates microglia and induces neuropathic pain. Several studies clarified an involvement of adenosine triphosphate (ATP) in the microglial activation. However, the origin of ATP together with the release mechanism is unclear. Recent in vitro study revealed that an ATP marker, quinacrine, in lysosomes was released from neurite terminal of dorsal root ganglion (DRG) neurons to extracellular space via lysosomal exocytosis. Here, we demonstrate a possibility that the lysosomal ingredient including ATP released from DRG neurons by lysosomal-exocytosis is an additional source of the glial activation in DH after nerve injury. After rat L5 spinal nerve ligation (SNL), mRNA for transcription factor EB (TFEB), a transcription factor controlling lysosomal activation and exocytosis, was induced in the DRG. Simultaneously both lysosomal protein, LAMP1- and vesicular nuclear transporter (VNUT)-positive vesicles were increased in L5 DRG neurons and ipsilateral DH. The quinacrine staining in DH was increased and co-localized with LAMP1 immunoreactivity after nerve injury. In DH, LAMP1-positive vesicles were also co-localized with a peripheral nerve marker, Isolectin B4 (IB4) lectin. Injection of the adenovirus encoding mCherry-LAMP1 into DRG showed that mCherry-positive lysosomes are transported to the central nerve terminal in DH. These findings suggest that activation of lysosome synthesis including ATP packaging in DRG, the central transportation of the lysosome, and subsequent its exocytosis from the central nerve terminal of DRG neurons in response to nerve injury could be a partial mechanism for activation of microglia in DH. This lysosome-mediated microglia activation mechanism may provide another clue to control nociception and pain.

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.

Purinergic signalling: pathophysiology and therapeutic potential

The article begins with a review of the main conceptual steps involved in the development of our understanding of purinergic signalling, including non-adrenergic, non-cholinergic (NANC) neurotransmission; identification of ATP as a NANC transmitter; purinergic cotransmission; recognition of two families of purinoceptors [P1 (adenosine) and P2 (ATP/ADP)]; and, later, cloning and characterisation of P1 (G protein-coupled), P2X (ion channel) and P2Y (G protein-coupled) receptor subtypes. Further studies have established the involvement of ATP in synaptic neurotransmission in both ganglia and in the central nervous system; long-term (trophic) purinergic signalling in cell proliferation, differentiation and death occurring in development and regeneration; and short-term purinergic signalling in neurotransmission, neuromodulation and secretion. ATP is released from most cell types in response to gentle mechanical stimulation and is rapidly degraded to adenosine by ecto-nucleotidases. This review then focuses on the pathophysiology of purinergic signalling in a wide variety of systems, including urinogenital, cardiovascular, airway, musculoskeletal and gastrointestinal. Consideration is also given to the involvement of purinoceptors in pain, cancer and diseases of the central nervous system. Purinergic therapeutic approaches for the treatment of some of these diseases are discussed.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Progress in understanding of inhibitory purinergic neuromuscular transmission in the gut

Recent studies with genetic deletion of P2Y1 receptor (P2Y1-/-) have clinched its role in enteric purinergic inhibitory neurotransmission and suggested that β-NAD may be the purinergic inhibitory neurotransmitter in the colon. In this issue of the Journal, Gil and colleagues extend their earlier observations to the cecum and gastric antrum, showing that P2Y1 receptor mediated purinergic inhibition may be a general phenomenon in the gut. However, the authors made an unexpected observation in contrast with their earlier findings in the colon that neither the selective P2Y1 receptor antagonist MRS2500, nor P2Y1 receptor deletion, blocked the hyperpolarizing action of β-NAD in the cecum. These observations suggest that β-NAD may be the purinergic inhibitory neurotransmitter in the colon, but not in the cecum. This group had previously reported that the selective P2Y1 receptor antagonist MRS 2179 suppressed the hyperpolarizing action of ATP or ADP. Further studies are now needed to determine whether the hyperpolarizing actions of ATP and ADP are suppressed by the more potent P2Y1 antagonist MRS2500, and in P2Y1-/- mutants to test the intriguing possibility that different purines serve as purinergic inhibitory neurotransmitters in the colon and cecum and perhaps in different parts of the gut. Studies in P2Y1-/- mice will resolve other issues in purinergic neurotransmission including cellular localization of the β-NAD or ATP-activated P2Y1 receptors on either smooth muscle cells or PDGFRα+ fibroblast-like cells, relationship of purinergic to nitrergic neurotransmission and understanding the physiological and clinical importance of purinergic transmission in gastrointestinal motility and its disorders.

A novel method using an acedan-based Zn(DPA) probe to monitor ATP localization in an in vivo system

In in vitro and in vivo systems, understanding localization and the functional role of ATP is essential, but effective methods to monitor ATP in cells and tissues are limited. Although quinacrine dihydrochloride is a well-known fluorescent dye used to detect ATP, it is limited in its use because it shows non-specific nuclear staining both in vitro and in vivo. A commercial luciferin-luciferase bioluminescence assay has also been used to detect ATP, but it can not be easily used in vivo. Thus, to effectively monitor ATP in vivo, we employed a novel two-photon ATP fluorescent probe, acedan-based Zn(DPA). Using the acedan-based Zn(DPA) probe, we show that this probe produces high quality images of ATP in lung, spleen, liver and spinal cord tissues.

Discovery of purinergic signalling, the initial resistance and current explosion of interest

There has been a remarkable growth of papers published about purinergic signalling via ATP since 1972. I am most grateful to the wonderful PhD students and postdoctoral fellows who have worked with me over the years to pursue the purinergic hypothesis despite early opposition and to the many outstanding scientists around the world who are currently extending the story. Recently, therapeutic approaches to pathological disorders include the development of selective P1 and P2 receptor subtype agonists and antagonists, as well as of inhibitors of extracellular ATP breakdown and of ATP transport enhancers and inhibitors. Medicinal chemists are starting to develop small molecule purinergic drugs that are orally bioavailable and stable in vivo.

Role of myosin Va in purinergic vesicular neurotransmission in the gut

We examined the hypothesis that myosin Va, by transporting purinergic vesicles to the varicosity membrane for exocytosis, plays a key role in purinergic vesicular neurotransmission. Studies were performed in wild-type (WT) and myosin Va-deficient dilute, brown, nonagouti (DBA) mice. Intracellular microelectrode recordings were made in mouse antral muscle strips. Purinergic inhibitory junction potential (pIJP) was recorded under nonadrenergic noncholinergic conditions after masking the nitrergic junction potentials. DBA mice showed reduced pIJP but normal hyperpolarizing response to P2Y1 receptor agonist MRS-2365. To investigate the mechanism of reduced purinergic transmission in DBA mice, studies were performed in isolated varicosities obtained from homogenates of whole gut tissues by ultracentrifugation and sucrose cushion purification. Purinergic varicosities were identified in tissue sections and in isolated varicosities by immunostaining for the vesicular ATP transporter, the solute carrier protein SLC17A9. The varicosities were similar in WT and DBA mice. Myosin Va was markedly reduced in DBA varicosities compared with the WT varicosities. Proximity ligation assay showed that myosin Va was closely associated with SLC17A9. Vesicular exoendocytosis was examined by FM1-43 staining of varicosities, which showed that exoendocytosis after KCl stimulation was impaired in DBA varicosities compared with WT varicosities. These studies show that SLC17A9 identifies ATP-containing purinergic varicosities. Myosin Va associates with SLC17A9-stained vesicles and possibly transports them to varicosity membrane for exocytosis. In myosin Va-deficient mice, purinergic inhibitory neurotransmission is impaired.

Investigation of neurogenic excitatory and inhibitory motor responses and their control by 5-HT(4) receptors in circular smooth muscle of pig descending colon

The aim of this study was to investigate whether the pig colon descendens might be a good model for the responses mediated via the different locations of human colonic 5-HT(4) receptors. The intrinsic excitatory and inhibitory motor neurotransmission in pig colon descendens was therefore first characterized. In circular smooth muscle strips, electrical field stimulation (EFS) at basal tone induced only in the combined presence of the NO synthase inhibitor N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME) and the SK channel blocker apamin voltage-dependent on-contractions. These on-contractions were largely reduced by the neuronal conductance blocker tetrodotoxin (TTX) and by the muscarinic receptor antagonist atropine, illustrating activation of cholinergic neurons. The 5-HT(4) receptor agonist prucalopride facilitated submaximal EFS-evoked cholinergic contractions and this effect was prevented by the 5-HT(4) receptor antagonist GR113808, supporting the presence of facilitating 5-HT(4) receptors on the cholinergic nerve endings innervating circular muscle in pig colon descendens. Relaxations were induced by EFS in strips pre-contracted with substance P in the presence of atropine. The responses at lower stimulation voltages were abolished by TTX. L-NAME or apamin alone did not influence or only moderately reduced the relaxations, but L-NAME plus apamin abolished the relaxations at lower stimulation voltages, suggesting that NO and ATP act as inhibitory neurotransmitters in a redundant way. Prucalopride did not influence the EFS-induced relaxations at lower stimulation voltage, nor did it per se relax contracted circular muscle strips. No evidence for relaxing 5-HT(4) receptors, either on inhibitory neurons or on the muscle cells was thus obtained in pig colon descendens circular muscle.

Inhibitory effect of schisandrin on spontaneous contraction of isolated rat colon

This study examined the effect of schisandrin, one of the major lignans isolated from Schisandra chinensis, on spontaneous contraction in rat colon and its possible mechanisms. Schisandrin produced a concentration-dependent inhibition (EC₅₀=1.66 μM) on the colonic spontaneous contraction. The relaxant effect of schisandrin could be abolished by the neuronal Na+ channel blocker tetrodotoxin (1 μM) but not affected by propranolol (1 μM), phentolamine (1 μM), atropine (1 μM) or nicotine desensitization, suggesting possible involvement of non-adrenergic non-cholinergic (NANC) transmitters released from enteric nerves. N(ω)-nitro-l-arginine methyl ester (100-300 μM), a nitric oxide synthase inhibitor, attenuated the schisandrin response. The role of nitric oxide (NO) was confirmed by an increase in colonic NO production after schisandrin incubation, and the inhibition on the schisandrin responses by soluble guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,3-α]-quinoxalin-1-one (1-30 μM). Non-nitrergic NANC components may also be involved in the action of schisandrin, as suggested by the significant inhibition of apamin on the schisandrin-induced responses. Pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt hydrate (100 μM), a selective P2 purinoceptor antagonist, markedly attenuated the responses to schisandrin. In contrast, neither 8-cyclopentyl-1,3-dipropylxanthine, an antagonist for adenosine A₁ receptors, nor chymotrypsin, a serine endopeptidase, affected the responses. All available results have demonstrated that schisandrin produced NANC relaxation on the rat colon, with the involvement of NO and acting via cGMP-dependent pathways. ATP, but not adenosine and VIP, likely plays a role in the non-nitrergic, apamin-sensitive component of the response.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Purinergic signalling in the nervous system: an overview

Purinergic receptors, represented by several families, are arguably the most abundant receptors in living organisms and appeared early in evolution. After slow acceptance, purinergic signalling in both peripheral and central nervous systems is a rapidly expanding field. Here, we emphasize purinergic co-transmission, mechanisms of release and breakdown of ATP, ion channel and G-protein-coupled-receptor subtypes for purines and pyrimidines, the role of purines and pyrimidines in neuron-glial communication and interactions of this system with other transmitter systems. We also highlight recent data involving purinergic signalling in pathological conditions, including pain, trauma, ischaemia, epilepsy, migraine, psychiatric disorders and drug addiction, which we expect will lead to the development of therapeutic strategies for these disorders with novel mechanisms of action.

Sub-micromolar increase in [Ca(2+)](i) triggers delayed exocytosis of ATP in cultured astrocytes

Astrocytes release a variety of transmitter molecules, which mediate communication between glial cells in the brain and modulate synaptic transmission. ATP is a major glia-derived transmitter, but the mechanisms and kinetics of ATP release from astrocytes remain largely unknown. Here, we combined epifluorescence and total internal reflection fluorescence microscopy to monitor individual quinacrine-loaded ATP-containing vesicles undergoing exocytosis in cultured astrocytes. In resting cells, vesicles exhibited three-dimensional motility, spontaneous docking and release at low rate. Extracellular ATP application induced a Ca(2+)-dependent increase in the rate of exocytosis, which persisted for several minutes. Using UV flash photolysis of caged Ca(2+), the threshold [Ca(2+)](i) for ATP exocytosis was found to be approximately 350 nM. Subthreshold [Ca(2+)](i) transients predominantly induced vesicle docking at plasma membrane without subsequent release. ATP exocytosis triggered either by purinergic stimulation or by Ca(2+) uncaging occurred after a substantial delay ranging from tens to hundreds of seconds, with only approximately 4% of release occurring during the first 30 s. The time course of the cargo release from vesicles had two peaks centered on <or=10 s and 60 s. These results demonstrate that: (1) [Ca(2+)](i) elevations in cultured astrocytes trigger docking and release of ATP-containing vesicles; (2) vesicle docking and release have different Ca(2+) thresholds; (3) ATP exocytosis is delayed by several minutes and highly asynchronous; (4) two populations of ATP-containing vesicles with distinct (fast and slow) time course of cargo release exist in cultured astrocytes.

Do ATP and NO interact in the CNS?

Enzymatically derived NO and extracellular ATP are receiving greater attention due to their role as messengers in the CNS during different physiological and pathological processes. Ionotropic (P2XR) and metabotropic (P2YR) purinergic receptors mediate ATP effects and are present throughout the body. Particularly P2XR are crucial for brain plasticity mechanisms, and are involved in the pathogenesis of different CNS illnesses. NO does not have a specific receptor and its actions are directly dependent on the production on demand by different nitric oxide synthase isoforms. NO synthesizing enzymes are present virtually in all tissues, and NO influences multifarious physiological and pathological functions. Interestingly, various are the tissue and organs modulated by both ATP and NO, such as the immune, brain and vascular systems. Moreover, direct interactions between purinergic and nitrergic mechanisms outside the CNS are well documented, with several studies also indicating that ATP and NO do participate to the same CNS functions. In the past few years, further experimental evidence supported the physiological and pathological relevance of ATP and NO direct interactions in the CNS. The aim of the present review is to provide an account of the available information on the interplay between purinergic and nitrergic systems, focussing on the CNS. The already established relevance of ATP and NO in different pathological processes would predict that the knowledge of ATP/NO cross-talk mechanisms would support pharmacological approaches toward the development of novel ATP/NO combined pharmacological agents.

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.

P2 purinoceptors account for the non-nitrergic NANC relaxation in the rat ileum

The transmitters involved in the non-nitrergic component of the non-adrenergic, non-cholinergic (NANC) inhibitory response of the rat small intestinal longitudinal muscle to electrical field stimulation of its nerves is a matter of controversy. The present study is the first one to utilise a combination of a nitric oxide synthase inhibitor and a P(2) purinoceptor antagonist for studying this response. We found that the P(2) purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 5x10(-5) M) abolished the non-nitrergic NANC relaxation to electrical field stimulation (10 Hz). PPADS alone provided a significant, moderate inhibitory action. PPADS specifically inhibited relaxations due to exogenous adenosine 5'-triphosphate (ATP) or alpha,beta-methylene ATP. The guanylate cyclase blocker 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10(-6) M) did not add to the inhibitory action of N(G)-nitro-L-arginine on field stimulation-induced relaxation. ODQ abolished the relaxant effect of the nitric oxide donors nitroglycerin or sodium nitroprusside. These data indicate that: (1) nitric oxide and ATP fully account for the field stimulation-induced relaxation in the rat ileal strip under the experimental conditions of this study, and (2) no ODQ-sensitive guanylate cyclase-mediated mechanism is involved in the non-nitrergic component of the NANC relaxation.

Gastrointestinal function regulation by nitrergic efferent nerves

Gastrointestinal (GI) smooth muscle responses to stimulation of the nonadrenergic noncholinergic inhibitory nerves have been suggested to be mediated by polypeptides, ATP, or another unidentified neurotransmitter. The discovery of nitric-oxide (NO) synthase inhibitors greatly contributed to our understanding of mechanisms involved in these responses, leading to the novel hypothesis that NO, an inorganic, gaseous molecule, acts as an inhibitory neurotransmitter. The nerves whose transmitter function depends on the NO release are called "nitrergic", and such nerves are recognized to play major roles in the control of smooth muscle tone and motility and of fluid secretion in the GI tract. Endothelium-derived relaxing factor, discovered by Furchgott and Zawadzki, has been identified to be NO that is biosynthesized from l-arginine by the constitutive NO synthase in endothelial cells and neurons. NO as a mediator or transmitter activates soluble guanylyl cyclase and produces cyclic GMP in smooth muscle cells, resulting in relaxation of the vasculature. On the other hand, NO-induced GI smooth muscle relaxation is mediated, not only by cyclic GMP directly or indirectly via hyperpolarization, but also by cyclic GMP-independent mechanisms. Numerous cotransmitters and cross talk of autonomic efferent nerves make the neural control of GI functions complicated. However, the findingsrelated to the nitrergic innervation may provide us a new way of understanding GI tract physiology and pathophysiology and might result in the development of new therapies of GI diseases. This review article covers the discovery of nitrergic nerves, their functional roles, and pathological implications in the GI tract.

The distribution of P2X5 purinergic receptors in the enteric nervous system of mouse

The distribution of the P2X(5) purinoceptor in the enteric nervous system of the mouse was studied by immunohistochemistry. P2X(5) receptor immunoreactivity was widely distributed in myenteric and submucosal plexuses throughout the gastrointestinal tract. In myenteric plexuses, immunoreactivity for the P2X(5) receptor was observed in nerve fibres that enveloped ganglion cell bodies, and possibly on glial cell processes. P2X(5) receptor immunoreactivity was colocalised with vasoactive intestinal peptide and surrounded ganglion cells that contained calretinin, calbindin or nitric oxide synthase. In the submucous plexus, P2X(5) receptor immunoreactivity occurred throughout the cytoplasm and on the surface membranes of the nerve cells. Double-labelling studies showed that 22%, 9%, 6% and 68% of P2X(5) receptor-immunoreactive neurones were also immunoreactive for calretinin, calbindin, nitric oxide synthase and vasoactive intestinal peptide, respectively. Thus, the P2X(5) receptor subunit is expressed in specific functional groups of neurones. P2X(2) and P2X(3) receptors were also present in the mouse enteric plexuses but no immunoreactivity for P2X(1), P2X(4) or P2X(6) receptors was found.

NANC inhibitory neurotransmission in mouse isolated stomach: involvement of nitric oxide, ATP and vasoactive intestinal polypeptide

1. The neurotransmitters involved in NANC relaxation and their possible interactions were investigated in mouse isolated stomach, recording the motor responses as changes of endoluminal pressure from whole organ. 2. Field stimulation produced tetrodotoxin-sensitive, frequency-dependent, biphasic responses: rapid transient relaxation followed by a delayed inhibitory component. 3. The inhibitor of the synthesis of nitric oxide (NO), l-NAME, abolished the rapid relaxation and significantly reduced the slow relaxation. Apamin, blocker of Ca2+-dependent K+ channels, or ADPbetaS, which desensitises P2y purinoceptors, reduced the slow relaxation to 2-8 Hz, without affecting that to 16-32 Hz or the fast relaxation. alpha-Chymotrypsin or vasoactive intestinal polypeptide 6-28 (VIP6-28), antagonist of VIP receptors, failed to affect the fast component or the delayed relaxation to 2-4 Hz, but antagonised the slow component to 8-32 Hz. 4. Relaxation to sodium nitroprusside was not affected by l-NAME, apamin or ADPbetaS, but was reduced by alpha-chymotrypsin or VIP6-28. Relaxation to VIP was abolished by alpha-chymotrypsin, antagonised by VIP6-28, but was not affected by l-NAME, apamin or ADPbetaS. Relaxation to ATP was abolished by apamin, antagonised by ADPbetaS, but was not affected by l-NAME or alpha-chymotrypsin. 5. The present results suggest that NO is responsible for the rapid relaxation and partly for the slow relaxation. ATP is involved in the slow relaxation evoked by low frequencies of stimulation. VIP is responsible for the slow relaxation evoked by high frequencies of stimulation. The different neurotransmitters appear to work in parallel, although NO could serve also as a neuromodulator that facilitates release of VIP.

Nitrergic prejunctional inhibition of purinergic neuromuscular transmission in the hamster proximal colon

Neurogenic ATP and nitric oxide (NO) may play important roles in the physiological control of gastrointestinal motility. However, the interplay between purinergic and nitrergic neurons in mediating the inhibitory neurotransmission remains uncertain. This study investigated whether neurogenic NO modulates the purinergic transmission to circular smooth muscles of the hamster proximal colon. Electrical activity was recorded from circular muscle cells of the hamster proximal colon by using the microelectrode technique. Intramural nerve stimulation with a single pulse evoked a fast purinergic inhibitory junction potential (IJP) followed by a slow nitrergic IJP. The purinergic component of the second IJP evoked by paired stimulus pulses at pulse intervals between 1 and 3 s became smaller than that of the first IJP. This purinergic IJP depression could be observed at pulse intervals <3 s, but not at longer ones, and failed to occur in the presence of NO synthase inhibitor. Exogenous NO (0.3-1 microM), at which no hyperpolarization is produced, inhibited purinergic IJPs, without altering the nitrergic IJP and exogenously applied ATP-induced hyperpolarization. In the presence of both purinoceptor antagonist and nitric oxide synthase (NOS) inhibitor, intramural nerve stimulation with 5 pulses at 20 Hz evoked vasoactive intestinal peptide (VIP)-associated IJPs, suggesting that VIP component may be masked in the IJPs of the hamster proximal colon. Our results suggest that neurogenic NO may modulate the purinergic transmission to circular smooth muscles of the hamster proximal colon via a prejunctional mechanism. In addition, VIP may be involved in the neurotransmitter in the hamster proximal colon.

Involvement of a purinergic pathway in the sympathetic regulation of motility in rat ileum

We investigated extrinsic neuronal regulation of intestinal motility. The mesenteric nerve stimulation (MNS; duration 0.5 ms, 10 Hz for 30 s) evoked relaxation in the longitudinal muscle direction of the isolated rat ileum. The MNS-induced relaxation was abolished by guanethidine (2 microM) or propranolol (10 microM), but was not affected by prazosin (10 microM), rauwolscine (10 microM), hexamethonium (100 microM) or capsaicin (1 microM). Exposure to a high concentration (100 microM) of ATP (ATP-desensitization) or ADP (ADP-desensitization) reduced the MNS-induced relaxation to 44.7% or 32.5% of the control (P<0.01), respectively. P2 purinoceptor antagonists [suramin (100 microM) and reactive blue-2 (RB-2, 50 microM)] or small conductance Ca(2+)-activated K(+) channel blocker, apamin (0.5 microM), significantly decreased the relaxation to 54.4% and 25.6% or 19.4% of the control (P<0.01), respectively, whereas selective P2Y(1) purinoceptor antagonist MRS2179 (10 microM) failed to affect the relaxation. Furthermore, exogenous ATP (1 microM) or ADP (1 microM) elicited relaxation in the rat ileum, which was almost abolished by reactive blue-2 (50 microM, 9.1% of control remained, P<0.05). In contrast, relaxation induced by noradrenalin (10 microM) was not antagonized by ATP-desensitization, apamin (0.5 microM) or reactive blue-2 (50 microM). From the present results, we conclude that noradrenergic sympathetic nerves might regulate intestinal motility mediated through a purinergic inhibitory neuronal pathway in the rat small intestine.

Alterations of intestinal motor responsiveness in a model of mild mesenteric ischemia/reperfusion in rats

In this study we investigate the changes in intestinal motor responsiveness after mild mesenteric ischemia/reperfusion in anaesthetized rats. Motor responsiveness to pharmacological/electrical stimulation was studied in isolated ileum excised from sham-operated rats or animals which underwent occlusion of superior mesenteric artery (1 h) plus interruption of collateral blood flow and reperfusion for 0, 24, 72 h. Only 24 h reperfusion resulted in a significant suppression in acetylcholine induced contractile response and in indomethacin induced relaxation. In the presence of adrenergic and cholinergic blockade a greater relaxant response to field stimulation (trains 10 s every min, 120 mA, 1 ms and 10 Hz) was unmasked in all groups except 24 h reperfused rats. Such effect was sensitive to N(G)-Nitro-L-arginine methyl ester (NOS unselective inhibitor) and the proteolytic enzyme alpha-chymotrypsin but resistant to aminoguanidine (iNOS selective inhibitor). In conclusion, in this rat model, intestinal mild ischemia/24 h reperfusion induces reversible changes in enteric motility attributable to a decrease in eicosanoids, nitric oxide and neuropeptides availability.

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

Nitrergic and purinergic regulation of the rat pylorus

The role of nitric oxide (NO) and ATP in the regulation of nonadrenergic, noncholinergic (NANC) inhibitory transmission in the pylorus remains unclear. In the presence of atropine and guanethidine, electric field stimulation induced NANC relaxations in a frequency-dependent manner (1-20 Hz) in the rat pylorus. NANC relaxations were significantly inhibited by N(G)-nitro-L-arginine methyl ester (L-NAME; 10(-4) M). P(2X) purinoceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 3 x 10(-5) M) and P(2Y) purinoceptor antagonist reactive blue 2 (2 x 10(-5) M) had no effect on NANC relaxations. However, the combined administration of L-NAME and PPADS, but not reactive blue 2, evoked greater inhibitory effects on NANC relaxation than that evoked by L-NAME alone. alpha-Chymotrypsin and vasoactive intestinal polypeptide antagonist did not affect NANC relaxations. ATP (10(-5)-10(-3) M) and P(2X) purinoceptor agonist alpha, beta-methyleneadenosine 5'-triphosphate (10(-7)-10(-5) M), but not P(2Y) purinoceptor agonist 2-methylthioadenosine 5'-triphosphate (10(-7)-10(-5) M), induced muscle relaxations in a dose-dependent manner, and relaxations were significantly reduced by PPADS and unaffected by TTX. These studies suggest that NO and ATP act in concert to mediate NANC relaxation of the rat pylorus. ATP-induced relaxation appears to be mediated by P(2X) purinoceptors located on smooth muscle cells.

Effect of exogenous ATP on canine jejunal smooth muscle

Intracellular recordings were made from the circular smooth muscle cells of the canine jejunum to study the effect of exogenous ATP and to compare the ATP response to the nonadrenergic, noncholinergic (NANC) inhibitory junction potential (IJP) evoked by electrical field stimulation (EFS). Under NANC conditions, exogenous ATP evoked a transient hyperpolarization (6.5 +/- 0.6 mV) and EFS evoked a NANC IJP (17 +/- 0.4 mV). Omega-conotoxin GVIA (100 nM) and a low-Ca(2+), high-Mg(2+) solution abolished the NANC IJP but had no effect on the ATP-evoked hyperpolarization. The ATP-evoked hyperpolarization and the NANC IJP were abolished by apamin (1 microM) and N(G)-nitro-L-arginine (100 microM). Oxyhemoglobin (5 microM) partially (38.8 +/- 5.5%) reduced the amplitude of the NANC IJP but had no effect on the ATP-evoked hyperpolarization. Neither the NANC IJP nor the ATP-evoked hyperpolarization was affected by P2 receptor antagonists or agonists, including suramin, reactive blue 2, 1-(N, O-bis-[5-isoquinolinesulfonyl]-N-methyl-L-tyrosyl)-4-phenylpiperazine , pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, alpha, beta-methylene ATP, 2-methylthioadenosine 5'-triphosphate tetrasodium salt, and adenosine 5'-O-2-thiodiphosphate. The data suggest that ATP evoked an apamin-sensitive hyperpolarization in circular smooth muscle cells of the canine jejunum via local production of NO in a postsynaptic target cell.

Inhibitory neural pathway regulating gastric emptying in rats

The relaxation of the pylorus is one of the most important factors for promoting gastric emptying. However, the role of inhibitory neurotransmitters in the regulation of pyloric relaxation and gastric emptying remains unclear. In this study, we investigated the effects of NO biosynthesis inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), and calcium dependent potassium channel blocker, apamin, on vagal stimulation-induced pyloric relaxation and gastric emptying in rats. Sodium nitroprusside (SNP), adenosine 5'-triphosphate (ATP), vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) caused pyloric relaxations in a dose dependent manner in vivo. Apamin (120 microg/kg) significantly reduced ATP and PACAP-induced pyloric relaxations without affecting SNP- or VIP-induced relaxations. Vagal stimulation (10 V, 1 ms, 1-20 Hz)-induced pyloric relaxation was significantly inhibited by L-NAME (10 mg/kg). The combined administration of L-NAME and apamin almost completely abolished vagal stimulation-induced pyloric relaxation. L-NAME and apamin significantly increased spontaneous contractions in the antrum, pylorus and duodenum. Increased motility index by L-NAME and apamin was significantly higher in the pylorus and duodenum, compared to that of antrum. L-NAME and apamin significantly delayed liquid gastric emptying. These results suggest that besides NO, probably ATP and PACAP, act as inhibitory neurotransmitters in the rat pylorus and regulate gastric emptying.

Pattern of distribution and co-localization of NOS and ATP in the myenteric plexus of human fetal stomach and intestine

The pattern of distribution and co-localization of nitric oxide synthase (NOS) and quinacrine fluorescence (indicative of vesicular adenosine 5'-triphosphate, ATP), and co-localization of NADPH-diaphorase (NADPH-d) activity and NOS-immunoreactivity in the myenteric plexus of pre-term human fetal (6-17 weeks of gestation) stomach and small intestine was examined using immunohistochemical and histochemical techniques. In all stages of gestation investigated, NOS-immunoreactive and NADPH-d-reactive myenteric neurons and nerve fibres were seen in the fetal intestine and stomach. However, in fetuses of 6-10 weeks of gestation, only 15% of the NADPH-d-positive myenteric neurons were NOS-immunoreactive, whereas a 100% co-localization was found in samples of 12-17 weeks of gestation. Quinacrine fluorescent myenteric neurons and nerve fibres were found only in the fetal intestine of 12-17 weeks of gestation, of which 25% of the NADPH-d-positive myenteric neurons in these samples were quinacrine fluorescent. These findings demonstrate the presence and co-localization of markers for nitric oxide (NO)- and ATP-utilizing myenteric neurons and nerve fibres in the early stages of gestation, suggesting possible co-transmitter and/or trophic roles of ATP and NO in the process of development and maturity of human myenteric neurons. In addition, the fact that only a small percentage of NADPH-d-reactive myenteric neurons express NOS immunoreactivity at 6-10 weeks of gestation confirms that NADPH-d-reactivity does not always represent NOS activity.

Diabetes does not alter the activity and localisation of nitric oxide synthase in the rat anococcygeus muscle

Functional studies have revealed diabetes specifically impairs smooth muscle reactivity to nitric oxide in the rat anococcygeus muscle. The present study was conducted to examine whether concurrent prejunctional defects in nitrergic neurotransmission exist in anococcygeus muscles from diabetic rats. Nitric oxide synthase (NOS) activity was assessed by the conversion of 3H-L-arginine to 3H-L-citrulline in homogenates of anococcygeus muscles obtained from 8-week diabetic rats and control rats. NOS activity measured in all tissue samples was dependent on the presence of calcium (2 mM), NADPH (1 mM), tetrahydrobiopterin (100 microM) and flavin adenine dinucleotide (10 microM); however, removal of calmodulin (50 U/ml) did not reduce L-citrulline production. Both N(G)-nitro-L-arginine (100 microM) and N(G)-nitro-L-arginine methyl ester (100 microM) produced significant inhibition of enzyme activity. NOS activity measured in tissue samples from diabetic rats (369.6 +/- 75.9 fmol L-citrulline/mg protein) did not significantly differ from that measured in samples from control rats (423.9 +/- 110.6 fmol L-citrulline/mg protein). However, NOS activity measured after removal of the cofactor tetrahydrobiopterin, was significantly greater in samples from control rats than that from the diabetic group. NOS-immunoreactive and NADPH-diaphorase reactive nerve terminals were found to be sparsely distributed throughout longitudinal sections or whole mounts of anococcygeus muscles from both control and diabetic rats. Quantification of NADPH-diaphorase positive fibres intersecting transects of whole tissue mounts, revealed no significant difference in fibre number between the treatment groups. All NOS-immunoreactive fibres also showed vasoactive-intestinal-polypeptide immunoreactivity. In conclusion, the findings together provide no evidence to indicate that diabetes can induce prejunctional changes in NOS activity or localisation, concurrent with the reported postjunctional impairment in smooth muscle reactivity to nitric oxide, in the rat anococcygeus muscle.

Neural modulation of the cyclic electrical and mechanical activity in the rat colonic circular muscle: putative role of ATP and NO

1. The rat colonic circular muscle displays cyclic episodes of myenteric potential oscillations (MPOs), each of them associated with a spontaneous contraction. Nifedipine 1 microM abolished both MPOs and their associated contractions. TTX (1 microM) increased the amplitude and frequency of spontaneous contractions. 2. Electrical field stimulation (EFS) induced a non-adrenergic non-cholinergic (NANC) inhibitory junction potential (IJP), with two phases: an initial fast hyperpolarization (characterized by IJP amplitude) and a sustained hyperpolarization (characterized by IJP duration). 3. Sodium nitroprusside (10 microM) hyperpolarized and abolished spontaneous contractions even in presence of TTX or 1 microM apamin. ATP (100 microM) also hyperpolarized and abolished spontaneous contractions but its effects were decreased by TTX and abolished by apamin. 4. Suramin (100 microM) or apamin reduced the amplitude of the IJPs, but did not affect their duration. Incubation with L-NOARG (1 mM) reduced the duration but not the amplitude of the IJPs. In presence of L-NOARG plus suramin or L-NOARG plus apamin, both duration and amplitude of the IJPs were reduced but a residual IJP could still be recorded. 5. We conclude that the mechanical and electrical cyclic activity of the rat colonic circular muscle is modulated but not originated by the enteric nervous system and involves L-type calcium channel activity. EFS induces release of NANC inhibitory neurotransmitters which hyperpolarize and relax smooth muscle cells. Both ATP and NO are involved in IJP generation: ATP is responsible for the first phase of the IJPs involving activation of apamin-sensitive potassium channels, whereas NO initiates the second phase which is independent of the activation of such channels.

NANC relaxation of the circular smooth muscle of the oesophagus of the Agama lizard involves the L-arginine-nitric oxide synthase pathway

On carbachol (CCh; 10-30 microM) pre-contracted circular muscle strips of the Agama lizard oesophagus, electrical field stimulation evoked frequency-dependent relaxations in the presence of guanethidine (1 microM) and indomethacin (1 microM). These non-adrenergic inhibitory responses were concentration-dependently inhibited by the nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) within a concentration range of 30-300 microM but not D-NAME (up to 300 microM), although a component remained at 4-16 Hz even with 300 microM L-NAME. The inhibition by L-NAME (300 microM) was completely prevented when L-arginine (L-Arg; 15 mM) but not D-Arg (up to 15 mM) was applied simultaneously with L-NAME (300 microM). Increasing the L-NAME concentration to 1 mM had no additional inhibitory effect. Sodium nitroprusside (SNP) concentration-dependently relaxed pre-contracted oesophageal strips, L-NAME (up to 300 microM) had no effect. Neither adenosine 5'-triphosphate (up to 0.1 mM) nor vasoactive intestinal polypeptide (up to 0.1 microM) caused the pre-contracted oesophagus to relax. This study has shown that the NANC inhibitory response of the Agama lizard oesophagus circular muscle largely involves the L-Arg-NOS pathway as seen by the effect of L-NAME, L-Arg and SNP. The identity of the L-NAME-resistant component(s) and the lack of effect of tetrodotoxin (up to 3 microM) and omega-conotoxin GVIA (up to 0.1 microM) in relation to the nature of the inhibitory response are discussed.

Distribution of myenteric NO neurons along the guinea-pig esophagus

Intrinsic nitrergic (NO) neurons of the guinea-pig esophagus were histologically studied to elucidate the physiological significance of the myenteric plexus located in the esophageal striated muscle and smooth muscle of the lower esophageal sphincter. Double staining for PGP 9.5 immunohistochemistry and NADPH-diaphorase histochemistry, which depicts whole neuronal elements and nitrergic NO neurons, respectively, revealed that the plexus had different network patterns along the entire course of the esophagus, and that NADPH-diaphorase positive neurons made up on average 69% of the total number of myenteric neurons. Motor endplates of the esophageal striated muscles that were stained by acetylcholinesterase histochemistry, were often observed in association with NADPH-diaphorase positive varicose fibers that were traced to the myenteric ganglia, though their direct continuity with the neuronal cell bodies could not be ascertained. We conclude that the myenteric NADPH-diaphorase positive neurons in the guinea-pig esophagus contribute to the innervation of the striated muscles as well as the smooth muscles of the lower esophageal sphincter.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

The distribution of nitric oxide synthase-, adenosine deaminase- and neuropeptide Y-immunoreactivity through the entire rat nucleus tractus solitarius: Effect of unilateral nodose ganglionectomy

The present study has employed immunocytochemistry on free-floating sections of adult rat medulla oblongata to characterise the distribution of nitric oxide synthase- (NOS), adenosine deaminase- (ADA) and neuropeptide Y- (NPY) immunoreactivity (IR) throughout the entire rostro-caudal axis of the nucleus tractus solitarius (NTS). In addition, unilateral nodose ganglionectomy was performed in a group of rats to determine whether any observed immunoreactivity was associated with central vagal afferent terminals. NOS-IR was found throughout the entire NTS, in cells, and both varicose and non-varicose fibres. Furthermore, unilateral nodose ganglionectomy resulted in a clear reduction in NOS-IR (visualised with diaminobenzidine) in a highly restricted portion of the ipsilateral medial NTS. Similarly, ADA- and NPY-containing cells, fibres and terminals were also found throughout the adult rat NTS. However, following unilateral nodose ganglionectomy, there was no apparent reduction in either ADA-IR or NPY-IR on the denervated side of the NTS. These data indicate a role for nitric oxide, purines and neuropeptide Y as neuromodulators within the rat NTS, although only nitric oxide appears to be primarily associated with vagal afferent input. Adenosine deaminase and neuropeptide Y-containing neurons appear to be predominantly postsynaptic to vagal input, although their possible association with vagal afferents cannot be completely excluded.