Novel dual 'small' vesicle model of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

Perivascular Adipose Tissue: the Sixth Man of the Cardiovascular System

Perivascular adipose tissue (PVAT) refers to the local aggregate of adipose tissue surrounding the vascular tree, exhibiting phenotypes from white to brown and beige adipocytes. Although PVAT has long been regarded as simply a structural unit providing mechanical support to vasculature, it is now gaining reputation as an integral endocrine/paracrine component, in addition to the well-established modulator endothelium, in regulating vascular tone. Since the discovery of anti-contractile effect of PVAT in 1991, the use of multiple rodent models of reduced amounts of PVAT has revealed its regulatory role in vascular remodeling and cardiovascular implications, including atherosclerosis. PVAT does not only release PVAT-derived relaxing factors (PVRFs) to activate multiple subsets of endothelial and vascular smooth muscle potassium channels and anti-inflammatory signals in the vasculature, but it does also provide an interface for neuron-adipocyte interactions in the vascular wall to regulate arterial vascular tone. In this review, we outline our current understanding towards PVAT and attempt to provide hints about future studies that can sharpen the therapeutic potential of PVAT against cardiovascular diseases and their complications.

VNUT and VMAT2 segregate within sympathetic varicosities and localize near preferred Cav2 isoforms in the rat tail artery

ATP and norepinephrine (NE) are coreleased from peripheral sympathetic nerve terminals. Whether they are stored in the same vesicles has been debated for decades. Preferential dependence of NE or ATP release on Ca²⁺ influx through specific voltage-gated Ca²⁺ channel (Cav2) isoforms suggests that NE and ATP are stored in separate vesicle pools, but simultaneous imaging of NE and ATP containing vesicles within single varicosities has not been reported. We conducted an immunohistochemical study of vesicular monoamine transporter 2 (VMAT2/SLC18A2) and vesicular nucleotide translocase (VNUT/SLC17A9) as markers of vesicles containing NE and ATP in sympathetic nerves of the rat tail artery. A large fraction of varicosities exhibited neighboring, rather than overlapping, VNUT and VMAT2 fluorescent puncta. VMAT2, but not VNUT, colocalized with synaptotagmin 1. Cav2.1, Cav2.2, and Cav2.3 are expressed in nerves in the tunica adventitia. VMAT2 preferentially localized adjacent to Cav2.2 and Cav2.3 rather than Cav2.1. VNUT preferentially localized adjacent to Cav2.3 > Cav2.2 >> Cav2.1. With the use of wire myography, inhibition of field-stimulated vasoconstriction with the Cav2.3 blocker SNX-482 (0.25 µM) mimicked the effects of the P2X inhibitor suramin (100 µM) rather than the α-adrenergic inhibitor phentolamine (10 µM). Variable sensitivity to SNX-482 and suramin between animals closely correlated with Cav2.3 staining. We concluded that a majority of ATP and NE stores localize to separate vesicle pools that use different synaptotagmin isoforms and that localize near different Cav2 isoforms to mediate vesicle release. Cav2.3 appears to play a previously unrecognized role in mediating ATP release in the rat tail artery. NEW & NOTEWORTHY Immunofluorescence imaging of vesicular nucleotide translocase and vesicular monoamine transporter 2 in rat tail arteries revealed that ATP and norepinephrine, classical cotransmitters, localize to well-segregated vesicle pools. Furthermore, vesicular nucleotide translocase and vesicular monoamine transporter 2 exhibit preferential localization with specific Cav2 isoforms. These novel observations address long-standing debates regarding the mechanism(s) of sympathetic neurotransmitter corelease.

Purinergic modulation of glutamate transmission: An expanding role in stress-linked neuropathology

Chronic stress has been extensively linked to disturbances in glutamatergic signalling. Emerging from this field of research is a considerable number of studies identifying the ability of purines at the pre-, post-, and peri-synaptic levels to tune glutamatergic neurotransmission. While the evidence describing purinergic control of glutamate has continued to grow, there has been relatively little attention given to how chronic stress modulates purinergic functions. The available research on this topic has demonstrated that chronic stress can not only disturb purinergic receptors involved in the regulation of glutamate neurotransmission, but also perturb glial-dependent purinergic signalling. This review will provide a detailed examining of the complex literature relating to glutamatergic-purinergic interactions with a focus on both neuronal and glial contributions. Once these detailed interactions have been described and contextualised, we will integrate recent findings from the field of stress research.

Imaging sympathetic neurogenic Ca2+ signaling in blood vessels

We review the information that has been provided by optical imaging experiments directed at understanding the role and effects of sympathetic nerve activity (SNA) in the functioning of blood vessels. Earlier studies utilized electric field stimulation of nerve terminals (EFS) in isolated arteries and vascular tissues (ex vivo) to elicit SNA, but more recently, imaging studies have been conducted in vivo, enabling the study of SNA in truly physiological conditions. Ex vivo: In vascular smooth muscle cells (VSMC) of isolated arteries, the three sympathetic neurotransmitters, norepinephrine (NE), ATP and neuropeptide Y (NPY), elicit or modulate distinct patterns of Ca²⁺ signaling, as revealed by confocal imaging of exogenous fluorescent Ca²⁺ indicators. Purinergic junctional Ca²⁺ transients (jCaTs) arise from Ca²⁺ influx during excitatory junction potentials (eJPs), and are associated with the initial neurogenic contraction. Adrenergic Ca²⁺ waves and oscillations cause contraction while SNA-induced endothelial Ca²⁺ 'pulsars' cause relaxation. In vivo: optical biosensor mice, which express genetically encoded Ca²⁺ indicators (GECI's) specifically in smooth muscle, combined with non-invasive imaging techniques has enabled imaging SNA-induced Ca²⁺ signaling and arterial diameter in vivo. SNA induces Ca²⁺ oscillations in intact arteries. [Ca²⁺] of arterial smooth muscle cells increased in hypertension, in association with increased SNA. High resolution imaging has revealed local sympathetic, neurogenic Ca²⁺ signaling within smooth muscle and endothelial cells of the vasculature. The ongoing development of in vivo imaging together with an expanding availability of different biosensor animals promises to enable the further assessment of SNA and its effects in the vasculature of living animals.

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

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

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.

Vas deferens neuro-effector junction: from kymographic tracings to structural biology principles

The vas deferens is a simple bioassay widely used to study the physiology of sympathetic neurotransmission and the pharmacodynamics of adrenergic drugs. The role of ATP as a sympathetic co-transmitter has gained increasing attention and furthered our understanding of its role in sympathetic reflexes. In addition, new information has emerged on the mechanisms underlying the storage and release of ATP. Both noradrenaline and ATP concur to elicit the tissue smooth muscle contractions following sympathetic reflexes or electrical field stimulation of the sympathetic nerve terminals. ATP and adenosine (its metabolic byproduct) are powerful presynaptic regulators of co-transmitter actions. In addition, neuropeptide Y, the third member of the sympathetic triad, is an endogenous modulator. The peptide plus ATP and/or adenosine play a significant role as sympathetic modulators of transmitter's release. This review focuses on the physiological principles that govern sympathetic co-transmitter activity, with special interest in defining the motor role of ATP. In addition, we intended to review the recent structural biology findings related to the topology of the P2X1R based on the crystallized P2X4 receptor from Danio rerio, or the crystallized adenosine A2A receptor as a member of the G protein coupled family of receptors as prototype neuro modulators. This review also covers structural elements of ectonucleotidases, since some members are found in the vas deferens neuro-effector junction. The allosteric principles that apply to purinoceptors are also reviewed highlighting concepts derived from receptor theory at the light of the current available structural elements. Finally, we discuss clinical applications of these concepts.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Neuronal and extraneuronal release of ATP and NAD(+) in smooth muscle

Adenosine 5'-triphosphate (ATP) and nicotinamide adenine dinucleotide (NAD(+) ) are key intracellular constituents involved in energy transfer and redox homeostasis in the cell. ATP is also released in the extracellular space and in the past half century it has been assumed to be the purinergic neurotransmitter in many systems including smooth muscle. In some smooth muscles (i.e., the human urinary bladder detrusor muscle), ATP does appear to be primarily released from nerves upon action potential firings, but in other smooth muscles (i.e., the human large intestine), ATP does not mimic the endogenous purine neurotransmitter. It was recently found that NAD(+) , another ubiquitous intracellular adenine nucleotide, also follows a regulated release in neurosecretory cells, vascular and visceral smooth muscles, and the brain. In some cases, NAD(+) fulfills presynaptic and postsynaptic criteria for a neurotransmitter better than ATP. Therefore, the purine hypothesis of neural regulation in smooth muscle is in need of reevaluation. This article will briefly review the current understanding of neuronal and extraneuronal release of purines in smooth muscle with emphasis on the roles of extracellular ATP and NAD(+) and, further, will discuss more recent information about the likely involvement of multiple purines in smooth muscle neurotransmission.

Perspective on the role of P2X-purinoceptor activation in human vas deferens contractility

The contractile actions of α,β-methylene ATP (α,β-meATP) and ATP and the effects of K(+) channel blockers in longitudinal and circular muscles of human vas deferens were investigated with a view to clarifying the functional importance of P2X(1)-purinoceptor activation and K(+) channels in modulating contractility of the tissues. The results provide an experiment-based perspective for resolving differing reports on purinergic activation of the tissues and uncertain roles of large-conductance Ca(2+)-activated K(+) (BK(Ca)) and voltage-gated delayed rectifier K(+) (K(V)) channels. α,β-Methylene ATP (3-100 μm) evoked suramin-sensitive contractions of longitudinal muscle but rarely of circular muscle. ATP (0.1-3 mm) less reliably activated only longitudinal muscle contractions. These were enhanced by ARL 67156 (100 μm), but a different ectonucleotidase inhibitor, POM 1, was ineffective. Both muscle types were unresponsive to ADP-βS (100 μm), a P2Y-purinoceptor agonist. Longitudinal muscle contractions in response to α,β-meATP were enhanced by FPL 64176 (1 μm), an L-type Ca(2+) agonist, TEA (1 mm), a non-specific K(+) channel blocker, 4-aminopyridine (0.3 mm), a selective blocker of K(V) channels, and iberiotoxin (0.1 μm), a selective blocker of BK(Ca) channels. Quiescent circular muscles responded to α,β-meATP reliably in the presence of FPL 64176 or iberiotoxin. Apamin (0.1 μm), a selective blocker of small conductance Ca(2+)-activated K(+) (SK(Ca)) channels had no effect in both muscle types. Y-27632 (1-10 μm) reduced longitudinal muscle contractions in response to α,β-meATP, suggesting involvement of Rho-kinase-dependent contractile mechanisms. The results indicate that P2X(1)-purinoceptor stimulation elicits excitatory effects that: (a) lead to longitudinal muscle contraction and secondary activation of 4-aminopyridine-sensitive (K(V)) and iberiotoxin-sensitive (BK(Ca)) K(+) channels; and (b) are subcontractile in circular muscle due to ancillary activation of BK(Ca) channels. The novel finding of differential action by P2X(1)-purinoceptor agonists in the muscle types has functional implication in terms of the purinergic contribution to overall contractile function of human vas deferens. The modulatory effects of K(V) and BK(Ca) channels following P2X(1)-purinoceptor activation may be pivotal in providing the crucial physiological mechanism that ensures temporal co-ordination of longitudinal and circular muscle contractility.

Sympathetic nerves and the endothelium influence the vasoconstrictor effect of low concentrations of ouabain in pressurized small arteries

We hypothesized that in salt-dependent forms of hypertension, endogenous ouabain acts on arterial smooth muscle to cause enhanced vasoconstriction. Here, we tested for the involvement of the arterial endothelium and perivascular sympathetic nerve terminals in ouabain-induced vasoconstriction. Segments of rat mesenteric or renal interlobar arteries were pressurized to 70 mmHg at 37 degrees C and exposed to ouabain (10(-11)-10(-7) M). Removal of the endothelium enhanced ouabain-induced vasoconstriction by as much as twofold (at an ouabain concentration of 10(-9) M). A component of the ouabain-induced vasoconstriction is due to the enhanced spontaneous release of norepinephrine (NE) from nerve terminals in the arterial wall. The alpha(1)-adrenoceptor blocker prazosin (10(-6) M) decreased ouabain-induced vasoconstrictions by as much as 50%. However, neither the contraction induced by sympathetic nerve activity (SNA) nor the NE release evoked by SNA (measured directly by carbon fiber amperometry) was increased by ouabain (<10(-7) M). Nevertheless, the converse case was true: after brief bursts of SNA, vasoconstrictor responses to ouabain were transiently increased (1.75-fold). This effect may be mediated by neuropeptide Y and Y(1) receptors on smooth muscle. In arteries lacking the endothelium and exposed to prazosin, ouabain (10(-11) M and greater) caused vasoconstriction, indicating a direct effect of very "low" concentrations of ouabain on arterial smooth muscle. In conclusion, in intact arteries, the endothelium opposes ouabain (10(-11)-10(-7)M)-induced vasoconstriction, which is caused by both enhanced spontaneous NE release and direct effects on smooth muscle. Ouabain (<10(-7)M) does not enhance SNA-mediated contractions, but SNA enhances ouabain-induced contractions. The effects of endogenous ouabain may be accentuated in forms of hypertension that involve sympathetic nerve hyperactivity and/or endothelial dysfunction.

Sympathetic neurogenic Ca2+ signalling in rat arteries: ATP, noradrenaline and neuropeptide Y

The sympathetic nervous system (SNS) plays an essential role in the control of total peripheral vascular resistance by controlling the contraction of small arteries. The SNS also exerts long-term trophic influences in health and disease; SNS hyperactivity accompanies most forms of human essential hypertension, obesity and heart failure. At their junctions with smooth muscle cells, the peri-arterial sympathetic nerves release ATP, noradrenaline (NA) and neuropeptide Y (NPY) onto smooth muscle cells. Confocal Ca(2+) imaging studies reveal that ATP and NA each produce unique types of postjunctional Ca(2+) signals and consequent smooth muscle cell contractions. Neurally released ATP activates postjunctional P2X(1) receptors to produce local, non-propagating Ca(2+) transients, termed 'junctional Ca(2+) transients', or 'jCaTs'. Neurally released NA binds to alpha(1)-adrenoceptors and can activate Ca(2+) waves or more uniform global changes in [Ca(2+)]. Neurally released NPY does not appear to produce Ca(2+) transients directly, but significantly modulates NA-induced Ca(2+) signalling. The neural release of ATP and NA, as judged by postjunctional Ca(2+) signals, electrical recording of excitatory junction potentials and carbon fibre amperometry to measure NA, varies markedly with the pattern of nerve activity. This probably reflects both pre- and postjunctional mechanisms, which are not yet fully understood. These phenomena, together with different temporal patterns of sympathetic nerve activity in different regional circulations, are probably an important mechanistic basis of the important selective regulation of regional vascular resistance and blood flow by the sympathetic nervous system.

Neuroeffector Ca2+ transients for the direct measurement of purine release and indirect measurement of cotransmitters in rodents

Determining whether ATP and noradrenaline are released from the same vesicle at mature autonomic neuroeffector junctions is challenging because of the difficulty of simultaneously detecting the packeted release of these neurotransmitters. Contraction, overflow and electrophysiology experiments all show that both ATP and noradrenaline are released following field stimulation (although the ratio might vary) from autonomic nerves in tissues including the vas deferens, rat tail artery and mesenteric artery. The occurrence of purinergic neuroeffector Ca(2+) transients (NCTs) has been used to detect the packeted release of the neurotransmitter ATP acting on postjunctional P2X receptors to cause Ca(2+) influx. Neuroeffector Ca(2+) transients can also be used to detect the local effects of noradrenaline through its alpha(2)-adrenoceptor-mediated prejunctional autoinhibitory effects on nerve terminal Ca(2+) concentration and the probability of exocytosis (measured by counting NCTs). Evidence is presented that exocytosis from sympathetic varicosities does not occur in a manner independent of the history of that varicosity, but rather that the release of a packet of ATP transiently suppresses (or predicts the transient suppression of) subsequent release. This could arise by autoinhibition (by the prejunctional action of noradrenaline or purines) or due to a transient shortage of vesicles readily available for release. In summary, two high-resolution approaches are proposed to measure the intermittent release of packets of neurotransmitter: (1) local transient suppression of nerve terminal Ca(2+) transients; and (2) the local and transient inhibition of NCTs to infer local autoinhibition, hence transmitter release. Such approaches may allow the packeted corelease of ATP and noradrenaline to be investigated without the need to measure both neurotransmitters directly.

In vitro continuous amperometry with a diamond microelectrode coupled with video microscopy for simultaneously monitoring endogenous norepinephrine and its effect on the contractile response of a rat mesenteric artery

Continuous amperometry with a diamond microelectrode and video microscopy were used to record (in vitro) endogenous norepinephrine release simultaneously with the evoked contractile response of a mesenteric artery from a healthy Sprague Dawley rat. Norepinephrine (NE) is a vasoconstricting neurotransmitter released from sympathetic nerves that innervate the smooth muscle cell layers surrounding arteries and veins. Using these two techniques along with several drugs, the NE released at sympathetic neuroeffector junctions nearby the microelectrode was measured as an oxidation current. Key to the amperometric measurement was the use of a diamond microelectrode because of the response sensitivity, reproducibility, and stability it provided. NE release was elicited by electrical stimulation at frequencies between 1 and 60 Hz, with a maximum response seen at 20 Hz. Confirmation that the oxidation current was, in fact, associated with endogenous NE came from the results of several drugs. Tetrodotoxin (TTX, 0.3 microM), a voltage-dependent sodium channel antagonist that blocks nerve conduction, abolished both the oxidation current and the arterial constriction. The alpha(2)-adrenergic autoreceptor antagonist, yohimbine (1.0 microM), caused an increase in the oxidation current and the corresponding constriction. The addition of cocaine (10 microM), an antagonist that inhibits neuronal NE reuptake, caused both the oxidation current and the contractile response to increase. These results, combined with the fact that the hydrodynamic voltammetric E(1/2) for endogenous NE was identical to that for a standard solution, confirmed that the oxidation current was due to NE and that this compound caused, at least in part, the contractile response. The results demonstrate that continuous amperometric monitoring of NE with a diamond microelectrode and video imaging of vascular tone allow real time local measurement of the temporal relationship between nerve-stimulated NE release and arterial constriction.

Sympathetically evoked Ca2+ signaling in arterial smooth muscle

The sympathetic nervous system plays an essential role in the control of total peripheral vascular resistance and blood flow, by controlling the contraction of small arteries. Perivascular sympathetic nerves release ATP, norepinephrine (NE) and neuropeptide Y. This review summarizes our knowledge of the intracellular Ca2+ signals that are activated by ATP and NE, acting respectively on P2X1 and alpha1-adrenoceptors in arterial smooth muscle. Each neurotransmitter produces a unique type of post-synaptic Ca2+ signal and associated contraction. The neural release of ATP and NE is thought to vary markedly with the pattern of nerve activity, probably reflecting both pre- and post-synaptic mechanisms. Finally, we show that Ca2+ signaling during neurogenic contractions activated by trains of sympathetic nerve fiber action potentials are in fact significantly different from that elicited by simple bath application of exogenous neurotransmitters to isolated arteries (a common experimental technique), and end by identifying important questions remaining in our understanding of sympathetic neurotransmission and the physiological regulation of contraction of small arteries.

P2X1 receptors mediate sympathetic postjunctional Ca2+ transients in mesenteric small arteries

Brief, spatially localized Ca(2+) transients occur in the smooth muscle adjacent to perivascular nerves of small arteries during neurogenic contractions. We named these "junctional Ca(2+) transients" (jCaTs) and postulated that they arose from Ca(2+) entering smooth muscle cells through P2X(1) receptors activated by neurally released ATP. Nevertheless, the lack of potent, subtype-selective P2X-receptor antagonists made determining the exact molecular identity of the channels difficult. Here we used small, pressurized mesenteric arteries from P2X(1)-receptor-deficient mice (KO) to test the hypothesis that jCaTs arise from Ca(2+) entering the smooth muscle cell via P2X(1) receptors. In wild-type (WT) arteries, confocal microscopy of fluo-4 fluorescence during electrical field stimulation (EFS) of perivascular sympathetic nerves revealed jCaTs in the smooth muscle cells adjacent to the perivascular nerves, similar to those reported previously in rat arteries, and alpha-latrotoxin (2.5 nM) markedly increased the frequency of "spontaneous" jCaTs. In the KO arteries, however, neither EFS nor alpha-latrotoxin elicited any jCaTs. A potent P2X-receptor agonist, alpha,beta-methylene ATP (10.0 microM), elicited strong contractions and increased intracellular Ca(2+) concentration in WT arteries but elicited neither in KO arteries. A biphasic vasoconstriction in response to EFS was observed in WT arteries. In KO arteries, however, the initial rapid, transient component of the biphasic vasoconstriction was absent. The data support the hypothesis that jCaTs represent Ca(2+) that enters the smooth muscle cells through P2X(1) receptors activated by neurally released ATP and that this Ca(2+) is involved in the initial rapid component of the sympathetic neurogenic contraction.

Vesicular release of ATP at central synapses

Adenosine triphosphate (ATP) acts as a fast excitatory transmitter in several regions of the central nervous system (CNS) including the medial habenula, dorsal horn, locus coeruleus, hippocampus, and somatosensory cortex. Postsynaptic actions of ATP are mediated through an extended family of P2X receptors, widely expressed throughout the CNS. ATP is released via several pathways, including exocytosis from presynaptic terminals and diffusion through large transmembrane pores (e.g., hemichannels, P2X(7) receptors, or volume-sensitive chloride channels) expressed in astroglial membranes. In presynaptic terminals, ATP is accumulated and stored in the synaptic vesicles. In different presynaptic terminals, these vesicles may contain ATP only or ATP and another neurotransmitter [e.g., gamma-amino-butyric acid (GABA) or glutamate]; in the latter case, two transmitters can be coreleased. Here, we discuss the mechanisms of vesicular release of ATP in the CNS and present our own data, which indicate that in central neuronal terminals, ATP is primarily stored and released from distinct pool of vesicles; the release of ATP is not synchronized either with GABA or with glutamate.

Selective release of ATP from sympathetic nerves of rat vas deferens by the toxin TsTX-I from Brazilian scorpion Tityus serrulatus

1. The effects of the main component of the Tityus serrulatus scorpion venom, toxin TsTX-I, were studied on the contractility and release of neurotransmitters in the rat vas deferens. Since TsTX-I is known to act on sodium channels, we used veratridine, another sodium channel agent, for comparison. 2. Toxin TsTX-I induced concentration-dependent contractions with an EC(50) value of 47.8+/-0.1 nM and a maximum effect of 84.4+/-10.4% of that for BaCl(2). 3. Contractions by TsTX-I were abolished by denervation or tetrodotoxin (0.1 microM), showing that the toxin effects depend on the integrity of sympathetic nerve terminals. 4. To check for the presence of a noradrenergic component, experiments were conducted after removal of adrenergic stores in nerve terminals by reserpinization (10 mg kg(-1), 24 h prior to experiments) or blockade of alpha(1) adrenoceptors by prazosin (30 microM), showing that these procedures did not modify the response to TsTX-I, and therefore that adrenoceptors were not involved in contractions. 5. To check for the presence of a purinergic component, experiments were carried out after blockade of P(2X) receptors by suramin (0.1 mM) or desensitization by alpha,beta-methylene-ATP (30 microM). These agents greatly abolished the contractile response to TsTX-I (about 83% by desensitization and 96% by suramin), showing the involvement of purinergic receptors. 6. The release of noradrenaline and purinergic agents (ATP, ADP, AMP and adenosine) was detected by HPLC. Together, the total release of purines in the presence of TsTX-I was about 42 times higher than in the control group. In contrast, TsTX-I did not modify the overflow of noradrenaline, showing that the release was selective for purines. 7. The release of purinergic agents was reduced by the N-type calcium channel blocker omega-conotoxin GVIA (1 microM) and by the P/Q-type blocker omega-conotoxin MVIIC (1 microM), showing that the effects of TsTX-I are calcium-dependent. 8. The results show that TsTX-I produced a selective release of purines from postganglionic sympathetic nerves in the rat vas deferens.

Real-time measurements of noradrenaline release in periphery and central nervous system

Noradrenaline (NA) plays important hormonal and neurotransmitter roles in the periphery and central nervous system, respectively. The cells that produce and release NA, namely, adrenal chromaffin cells (ACCs), sympathetic postganglionic neurones and central neurones, show both commonalities as well as profound differences in morphology, physiological function and characteristics of NA secretion. In order to address disorders which have been associated with the dysregulation of NA release, such as essential hypertension, a better understanding of the molecular mechanisms governing and modulating NA release in neurones is urgently required. Due to profound technical challenges, the molecular basis of NA release has been investigated much more thoroughly in ACCs than in neurones. This review discusses suitable approaches for detecting NA secretion in periphery as well as brain tissues. Membrane capacitance and high-resolution electrochemical measurements have proven particularly useful when combined with fluorescence microscopy. ACCs and peripheral and central NAergic neurones are compared regarding their vesicle morphologies, as well as possible locations of release sites, and the trajectory of secreted NA. Further, current views on the properties of single vesicle release events, including proposed release probabilities in these cell types, are presented.

Ectonucleoside triphosphate diphosphohydrolase 1/CD39, localized in neurons of human and porcine heart, modulates ATP-induced norepinephrine exocytosis

Using a guinea pig heart synaptosomal preparation, we previously observed that norepinephrine (NE) exocytosis was attenuated by a blockade of P2X purinoceptors, potentiated by inhibition of ectonucleoside triphosphate diphosphohydrolase-1 (E-NTPDase1)/CD39, and reduced by soluble CD39, a recombinant form of human E-NTPDase1/CD39. This suggests that norepinephrine and ATP are coreleased upon depolarization of cardiac sympathetic nerve endings and that ATP enhances norepinephrine exocytosis by an action modulated by E-NTPDase1/CD39 activity. Whether E-NTPDase1/CD39 is localized to cardiac neurons and modulates norepinephrine exocytosis in intact heart tissue remained untested. We report that E-NTPDase1/CD39 is selectively localized in human and porcine cardiac neurons and that depolarization of porcine heart tissue elicits omega-conotoxin-inhibitable release of both norepinephrine and ATP. Inhibition of E-NTPDase1/CD39 with ARL67156 markedly potentiated ATP release, demonstrating that E-NTPDase1/CD39 is a major determinant of ATP availability at sympathetic nerve terminals. Notably, inhibition of E-NTPDase1/CD39 enhanced both ATP and NE exocytosis, whereas administration of soluble CD39 reduced both ATP and NE exocytosis. The strong correlation between ATP and norepinephrine release was abolished in the presence of the purinergic P2X receptor (P2XR) antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). We conclude that released ATP governs norepinephrine exocytosis by activating presynaptic P2XR and that this action is controlled by neuronal E-NTPDase1/CD39. Clinically, excessive norepinephrine release is a major cause of arrhythmic and coronary vascular dysfunction during myocardial ischemia. By curtailing NE release, in addition to its effects as an antithrombotic agent, soluble CD39 may constitute a novel therapeutic approach to ischemic complications in the myocardium.

A quantitative description of the diffusion of noradrenaline in the media of blood vessels following its release from sympathetic varicosities

A quantitative model is provided which describes how noradrenaline (NAd), released from varicosities at the adventitial surface of an artery, either diffuses into the media of the vessel to reach the intimal surface, diffuses into the volume of solution surrounding the artery, or is removed by the uptake 1 process in the varicosities. These predictions are then compared with experimental evaluations of the extent of changes in NAd to be found at the adventitial and intimal surfaces of the rat-tail artery, during and after trains of impulses, as determined using amperometry. In the model of the blood vessel there is a sequential decrease in the diffusion constant of NAd from the surrounding solution, to the adventitia, to the media, to the endothelium, to rise again in the lumen of the vessel; there is also an uptake 1 NAd pump in the varicosities described by Michaelis-Menten kinetics. This model is shown to provide a quantitative account of the spatial and temporal changes in NAd observed following trains of impulses at different frequencies of stimulation (5-40 Hz) for different periods of times (10-40 s). Changes in the spatio-temporal distribution of NAd observed following block of the uptake 1 NAd pump were also successfully predicted by the model. It is concluded that, within the context of the model, there is no need to evoke special mechanisms of buffering at the sympathetic varicosities, nor distinctions on the basis that only secreting varicosities utilize the uptake 1 mechanism, in order to describe the dynamics of NAd distribution in arteries during nerve activity.

Differential sensitivity to calcium and osmotic pressure of fast and slow ATP currents at sympathetic varicosities in mouse vas deferens

Secretion of noradrenaline from large dense-core vesicles in chromaffin cells involves both rapid and slow components of exocytosis which are differentially sensitive to changes in external calcium, osmotic pressure and interruption of the interacting SNARE proteins. Electrical signs of secretion of ATP from sympathetic nerve terminals of mouse vas deferens, the excitatory junctional currents (EJCs), also indicate both rapid and slow mechanisms of exocytosis, which might also show such differential sensitivity. We report here that the large and fast EJCs are highly sensitive to changes in extracellular calcium ions whereas the small and slow EJCs are not. Furthermore, the frequency of fast EJCs is accelerated by hypotonic solutions whereas the slow EJCs are accelerated by hypertonic solution. Fast EJCs, but not slow EJCs, are blocked by peptide fragments of alpha-SNAP and syntaxin whereas slow EJCs are not. These observations point to two classes of exocytosis from sympathetic nerve terminals that parallel those of exocytosis from chromaffin cells.

ATP as a cotransmitter in sympathetic nerves and its inactivation by releasable enzymes

ATP and norepinephrine (NE) are cotransmitters released from many postganglionic sympathetic nerves. In this article, we review the evidence for ATP and NE cotransmission in the rodent vas deferens with special attention to the mechanisms involved in removing the cotransmitters from the neuroeffector junction. Although the clearance of NE is well understood (e.g., the primary mechanism being reuptake into the nerves), the clearance of ATP is just beginning to be explained. The general belief has been that ATP is metabolized by cell-fixed ecto-nucleotidases. It now seems, however, that when ATP is released from nerves as a transmitter there is a concomitant release of nucleotidases that rapidly degrade ATP sequentially to ADP, AMP, and adenosine, thereby terminating the action of ATP. In the guinea pig vas deferens, there appear to be at least two enzymes, one that converts ATP to ADP and ADP to AMP (an ATPDase) and a second enzyme that converts AMP to adenosine (an AMPase). An important feature of this process is that the transmitter-metabolizing nucleotidases are released into the synaptic space as opposed to being fixed to cell membranes. A preliminary characterization of these enzymes suggests that the releasable ATPDase exhibits some similarities to known ectonucleoside triphosphate/diphosphohydrolases, whereas the releasable AMPase exhibits some similarities to ecto-5'-nucleotidases.

Intermittent ATP release from nerve terminals elicits focal smooth muscle Ca2+ transients in mouse vas deferens

A confocal Ca2+ imaging technique has been used to detect ATP release from individual sympathetic varicosities on the same nerve terminal branch. Varicose nerve terminals and smooth muscle cells in mouse vas deferens were loaded with the Ca2+ indicator Oregon Green 488 BAPTA-1. Field (nerve) stimulation evoked discrete, focal increases in [Ca2+] in smooth muscle cells adjacent to identified varicosities. These focal increases in [Ca2+] have been termed 'neuroeffector Ca2+ transients' (NCTs). NCTs were abolished by alpha,beta-methylene ATP (1 microM), but not by nifedipine (1 microM) or prazosin (100 nM), suggesting that NCTs are generated by Ca2+ influx through P2X receptors without a detectable contribution from L-type Ca2+ channels or alpha(1)-adrenoceptor-mediated pathways. Action potential-evoked ATP release was highly intermittent (mean probability 0.019 +/- 0.002; range 0.001-0.10) at 1 Hz stimulation, even though there was no failure of action potential propagation in the nerve terminals. Twenty-eight per cent of varicosities failed to release transmitter following more than 500 stimuli. Spontaneous ATP release was very infrequent (0.0014 Hz). No Ca2+ transient attributable to noradrenaline release was detected even in response to 5 Hz stimulation. There was evidence of local noradrenaline release as the alpha(2)-adrenoceptor antagonist yohimbine increased the probability of occurrence of NCTs by 55 +/- 21 % during trains of stimuli at 1 Hz. Frequency-dependent facilitation preferentially occurred at low probability release sites. The monitoring of NCTs now allows transmitter release to be detected simultaneously from each functional varicosity on an identified nerve terminal branch on an impulse-to-impulse basis.