P2Y purinoceptors inhibit exocytosis in adrenal chromaffin cells via modulation of voltage-operated calcium channels

Autocrine activation of P2X7 receptors mediates catecholamine secretion in chromaffin cells

BACKGROUND AND PURPOSE

ATP is highly accumulated in secretory vesicles and secreted upon exocytosis from neurons and endocrine cells. In adrenal chromaffin granules, intraluminal ATP reaches concentrations over 100 mM. However, how these large amounts of ATP contribute to exocytosis has not been investigated.

EXPERIMENTAL APPROACH

Exocytotic events in bovine and mouse adrenal chromaffin cells were measured with single cell amperometry. Cytosolic Ca2+ measurements were carried out in Fluo-4 loaded cells. Submembrane Ca2+ was examined in PC12 cells transfected with a membrane-tethered Ca2+ indicator Lck-GCaMP3. ATP release was measured using the luciferin/luciferase assay. Knockdown of P2X7 receptors was induced with short interfering RNA (siRNA). Direct Ca2+ influx through this receptor was measured using a P2X7 receptor-GCamp6 construct.

KEY RESULTS

ATP induced exocytosis in chromaffin cells, whereas the ectonucleotidase apyrase reduced the release events induced by the nicotinic agonist dimethylphenylpiperazinium (DMPP), high KCl, or ionomycin. The purinergic agonist BzATP also promoted a secretory response that was dependent on extracellular Ca2+. A740003, a P2X7 receptor antagonist, abolished secretory responses of these secretagogues. Exocytosis was also diminished in chromaffin cells when P2X7 receptors were silenced using siRNAs and in cells of P2X7 receptor knockout mice. In PC12 cells, DMPP induced ATP release, triggering Ca2+ influx through P2X7 receptors. Furthermore, BzATP, DMPP, and KCl allowed the formation of submembrane Ca2+ microdomains inhibited by A740003.

CONCLUSION AND IMPLICATIONS

Autocrine activation of P2X7 receptors constitutes a crucial feedback system that amplifies the secretion of catecholamines in chromaffin cells by favouring submembrane Ca2+ microdomains.

María Teresa Miras Portugal: a pioneer in the study of purinoceptors in chromaffin cells

María Teresa Miras Portugal devoted most of her scientific life to the study of purinergic signalling. In an important part of her work, she used a model system: the chromaffin cells of the adrenal medulla. It was in these cells that she identified diadenosine polyphosphates, from which she proceeded to the study of adrenomedullary purinome: nucleotide synthesis and degradation, adenosine transport, nucleotide uptake into chromaffin granules, exocytotic release of nucleotides and autocrine regulation of chromaffin cell function via purinoceptors. This short review will focus on the current state of knowledge of the purinoceptors of adrenal chromaffin cells, a subject to which María Teresa made seminal contributions and which she continued to study until the end of her scientific life.

ADP-mediated Modulation of Intracellular Calcium Responses in Chromaffin Cells: The Role of Ectonucleoside Triphosphate Diphosphohydrolase 2 on Rat Adrenal Medulla Function

The present study investigated the localization and the adenosine 5'-triphosphate (ATP)-degrading function of the plasma membrane-bound ecto-nucleotidase, ectonucleoside triphosphate diphosphohydrolase 2 (NTPDase2), in the rat adrenal medulla. The effect of ATP degradation product, adenosine 5'-diphosphate (ADP), on carbachol (CCh)-induced intracellular Ca2+ ([Ca2+]i) responses in adrenal chromaffin cells was examined using calcium imaging. NTPDase2-immunoreactive cells were distributed between chromaffin cells. NTPDase2-immunoreactive cells were immunoreactive for glial fibrillary acidic protein and S100B, suggesting that they were sustentacular cells. NTPDase2-immunoreactive cells surrounded chromaffin cells immunoreactive for vesicular nucleotide transporter and P2Y12 ADP-selective purinoceptors. In ATP bioluminescence assays using adrenal medullary slices, ATP was rapidly degraded and its degradation was attenuated by the NTPDase inhibitors sodium polyoxotungstate (POM-1) and 6-N, N-diethyl-d-β,γ-dibromomethylene ATP (ARL67156). ADP inhibited CCh-induced [Ca2+]i increases of chromaffin cells in adrenal medullary slices. The inhibition of CCh-induced [Ca2+]i increases by ADP was blocked by the P2Y12 purinoceptor antagonist AZD1283. CCh-induced [Ca2+]i increases were also inhibited by the P2Y1, P2Y12, and P2Y13 purinoceptor agonist 2-methylthioadenosine diphosphate trisodium (2MeSADP), in combination with the P2Y1 purinoceptor antagonist MRS2179. These results suggest that sustentacular cells express NTPDase2 to degrade ATP released from adrenal chromaffin cells, and ADP modulates the excitability of chromaffin cells via P2Y12 purinoceptors to regulate catecholamine release during preganglionic sympathetic stimuli. (J Histochem Cytochem 72: 41-60, 2024).

Membrane Capacitance Measurements of Stimulus-Evoked Exocytosis in Adrenal Chromaffin Cells

Research using membrane capacitance (C_m) measurements in adrenal chromaffin cells has transformed our understanding of the molecular mechanisms controlling regulated exocytosis. This is in part due to the exquisite temporal resolution of the technique, and the possibility of combining quantification of exo-/endocytosis at the whole-cell level, with the ability to simultaneously monitor and control the calcium signals triggering vesicle fusion. In this regard, experiments performed with C_m measurements complement amperometry experiments that give a measure of secreted transmitter and the behavior of the fusion pore, and fluorescent microscopy studies used to monitor vesicle and protein dynamics in imaged regions of the cell. In this chapter, we provide a detailed account of the methodology used to perform whole-cell patch clamp measurements of C_m in combination with voltage-clamp recordings of voltage-gated calcium channels to quantify stimulus-secretion coupling in chromaffin cells. Stimulus protocols developed for investigation of functionally distinct releasable vesicle pools are also described.

Adrenal G Protein-Coupled Receptors and the Failing Heart: A Long-distance, Yet Intimate Affair

ABSTRACT

Systolic heart failure (HF) is a chronic clinical syndrome characterized by the reduction in cardiac function and still remains the disease with the highest mortality worldwide. Despite considerable advances in pharmacological treatment, HF represents a severe clinical and social burden. Chronic human HF is characterized by several important neurohormonal perturbations, emanating from both the autonomic nervous system and the adrenal glands. Circulating catecholamines (norepinephrine and epinephrine) and aldosterone elevations are among the salient alterations that confer significant hormonal burden on the already compromised function of the failing heart. This is why sympatholytic treatments (such as β-blockers) and renin-angiotensin system inhibitors or mineralocorticoid receptor antagonists, which block the effects of angiotensin II (AngII) and aldosterone on the failing heart, are part of the mainstay HF pharmacotherapy presently. The adrenal gland plays an important role in the modulation of cardiac neurohormonal stress because it is the source of almost all aldosterone, of all epinephrine, and of a significant amount of norepinephrine reaching the failing myocardium from the blood circulation. Synthesis and release of these hormones in the adrenals is tightly regulated by adrenal G protein-coupled receptors (GPCRs), such as adrenergic receptors and AngII receptors. In this review, we discuss important aspects of adrenal GPCR signaling and regulation, as they pertain to modulation of cardiac function in the context of chronic HF, by focusing on the 2 best studied adrenal GPCR types in that context, adrenergic receptors and AngII receptors (AT 1 Rs). Particular emphasis is given to findings from the past decade and a half that highlight the emerging roles of the GPCR-kinases and the β-arrestins in the adrenals, 2 protein families that regulate the signaling and functioning of GPCRs in all tissues, including the myocardium and the adrenal gland.

Glucagon-like peptide-1 receptor controls exocytosis in chromaffin cells by increasing full-fusion events

Agonists for glucagon-like-peptide-1 receptor (GLP-1R) are currently used for the treatment of type 2 diabetes and obesity. Their benefits have been centered on pancreas and hypothalamus, but their roles in other organ systems are not well understood. We studied the action of GLP-1R on secretions of adrenal medulla. Exendin-4, a synthetic analog of GLP-1, increases the synthesis and the release of catecholamines (CAs) by increasing cyclic AMP (cAMP) production, without apparent participation of cAMP-regulated guanine nucleotide exchange factor (Epac). Exendin-4, when incubated for 24 h, increases CA synthesis by promoting the activation of tyrosine hydroxylase. Short incubation (20 min) increases the quantum size of exocytotic events by switching exocytosis from partial to full fusion. Our results give a strong support to the role of GLP-1 in the fine control of exocytosis.

Serotonin and Serotonin Transporters in the Adrenal Medulla: A Potential Hub for Modulation of the Sympathetic Stress Response

Serotonin (5-HT) is an important neurotransmitter in the central nervous system where it modulates circuits involved in mood, cognition, movement, arousal, and autonomic function. The 5-HT transporter (SERT; SLC6A4) is a key regulator of 5-HT signaling, and genetic variations in SERT are associated with various disorders including depression, anxiety, and autism. This review focuses on the role of SERT in the sympathetic nervous system. Autonomic/sympathetic dysfunction is evident in patients with depression, anxiety, and other diseases linked to serotonergic signaling. Experimentally, loss of SERT function (SERT knockout mice or chronic pharmacological block) has been reported to augment the sympathetic stress response. Alterations to serotonergic signaling in the CNS and thus central drive to the peripheral sympathetic nervous system are presumed to underlie this augmentation. Although less widely recognized, SERT is robustly expressed in chromaffin cells of the adrenal medulla, the neuroendocrine arm of the sympathetic nervous system. Adrenal chromaffin cells do not synthesize 5-HT but accumulate small amounts by SERT-mediated uptake. Recent evidence demonstrated that 5-HT1A receptors inhibit catecholamine secretion from adrenal chromaffin cells via an atypical mechanism that does not involve modulation of cellular excitability or voltage-gated Ca2+ channels. This raises the possibility that the adrenal medulla is a previously unrecognized peripheral hub for serotonergic control of the sympathetic stress response. As a framework for future investigation, a model is proposed in which stress-evoked adrenal catecholamine secretion is fine-tuned by SERT-modulated autocrine 5-HT signaling.

An interplay between the serotonin transporter (SERT) and 5-HT receptors controls stimulus-secretion coupling in sympathoadrenal chromaffin cells

Adrenal chromaffin cells (ACCs), the neuroendocrine arm of the sympathetic nervous system, secrete catecholamines to mediate the physiological response to stress. Although ACCs do not synthesize 5-HT, they express the serotonin transporter (SERT). Genetic variations in SERT are linked to several CNS disorders but the role(s) of SERT/5-HT in ACCs has remained unclear. Adrenal glands from wild-type mice contained 5-HT at ≈ 750 fold lower abundance than adrenaline, and in SERT(-/-) mice this was reduced by ≈80% with no change in catecholamines. Carbon fibre amperometry showed that SERT modulated the ability of 5-HT1A receptors to inhibit exocytosis. 5-HT reduced the number of amperometric spikes (vesicular fusion events) evoked by KCl in SERT(-/-) cells and wild-type cells treated with escitalopram, a SERT antagonist. The 5-HT1A receptor antagonist WAY100635 blocked the inhibition by 5-HT which was mimicked by the 5-HT1A agonist 8-OH-DPAT but not the 5-HT1B agonist CP93129. There was no effect on voltage-gated Ca(2+) channels, K(+) channels, or intracellular [Ca(2+)] handling, showing the 5-HT receptors recruit an atypical inhibitory mechanism. Spike charge and kinetics were not altered by 5-HT receptors but were reduced in SERT(-/-) cells compared to wild-type cells. Our data reveal a novel role for SERT and suggest that adrenal chromaffin cells might be a previously unrecognized hub for serotonergic control of the sympathetic stress response.

GPCRs of adrenal chromaffin cells & catecholamines: The plot thickens

The circulating catecholamines (CAs) epinephrine (Epi) and norepinephrine (NE) derive from two major sources in the whole organism: the sympathetic nerve endings, which release NE on effector organs, and the chromaffin cells of the adrenal medulla, which are cells that synthesize, store and release Epi (mainly) and NE. All of the Epi in the body and a significant amount of circulating NE derive from the adrenal medulla. The secretion of CAs from adrenal chromaffin cells is regulated in a complex way by a variety of membrane receptors, the vast majority of which are G protein-coupled receptors (GPCRs), including adrenergic receptors (ARs), which act as "presynaptic autoreceptors" in this regard. There is a plethora of CA-secretagogue signals acting on these receptors but some of them, most notably the α2ARs, inhibit CA secretion. Over the past few years, however, a few new proteins present in chromaffin cells have been uncovered to participate in CA secretion regulation. Most prominent among these are GRK2 and β-arrestin1, which are known to interact with GPCRs regulating receptor signaling and function. The present review will discuss the molecular and signaling mechanisms by which adrenal chromaffin cell-residing GPCRs and their regulatory proteins modulate CA synthesis and secretion. Particular emphasis will be given to the newly discovered roles of GRK2 and β-arrestins in these processes and particular points of focus for future research will be highlighted, as well.

ATPergic signalling during seizures and epilepsy

Much progress has been made over the last few decades in the identification of new anti-epileptic drugs (AEDs). However, 30% of epilepsy patients suffer poor seizure control. This underscores the need to identify alternative druggable neurotransmitter systems and drugs with novel mechanisms of action. An emerging concept is that seizure generation involves a complex interplay between neurons and glial cells at the tripartite synapse and neuroinflammation has been proposed as one of the main drivers of epileptogenesis. The ATP-gated purinergic receptor family is expressed throughout the brain and is functional on neurons and glial cells. ATP is released in high amounts into the extracellular space after increased neuronal activity and during chronic inflammation and cell death to act as a neuro- and gliotransmitter. Emerging work shows pharmacological targeting of ATP-gated purinergic P2 receptors can potently modulate seizure generation, inflammatory processes and seizure-induced brain damage. To date, work showing the functional contribution of P2 receptors has been mainly performed in animal models of acute seizures, in particular, by targeting the ionotropic P2X7 receptor subtype. Other ionotropic P2X and metabotropic P2Y receptor family members have also been implicated in pathological processes following seizures such as the P2X4 receptor and the P2Y12 receptor. However, during epilepsy, the characterization of P2 receptors was mostly restricted to the study of expressional changes of the different receptor subtypes. This review summarizes the work to date on ATP-mediated signalling during seizures and the functional impact of targeting the ATP-gated purinergic receptors on seizures and seizure-induced pathology. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Regulation of neuronal ion channels via P2Y receptors

Within the last 15 years, at least 8 different G protein-coupled P2Y receptors have been characterized. These mediate slow metabotropic effects of nucleotides in neurons as well as non-neural cells, as opposed to the fast ionotropic effects which are mediated by P2X receptors. One class of effector systems regulated by various G protein-coupled receptors are voltage-gated and ligand-gated ion channels. This review summarizes the current knowledge about the modulation of such neuronal ion channels via P2Y receptors. The regulated proteins include voltage-gated Ca²⁺ and K⁺ channels, as well as N-methyl-d-aspartate, vanilloid, and P2X receptors, and the regulating entities include most of the known P2Y receptor subtypes. The functional consequences of the modulation of ion channels by nucleotides acting at pre- or postsynaptic P2Y receptors are changes in the strength of synaptic transmission. Accordingly, ATP and related nucleotides may act not only as fast transmitters (via P2X receptors) in the nervous system, but also as neuromodulators (via P2Y receptors). Hence, nucleotides are as universal transmitters as, for instance, acetylcholine, glutamate, or γ-aminobutyric acid.

Inhibition of Ca2+ channels and adrenal catecholamine release by G protein coupled receptors

Catecholamines and other transmitters released from adrenal chromaffin cells play central roles in the "fight-or-flight" response and exert profound effects on cardiovascular, endocrine, immune, and nervous system function. As such, precise regulation of chromaffin cell exocytosis is key to maintaining normal physiological function and appropriate responsiveness to acute stress. Chromaffin cells express a number of different G protein coupled receptors (GPCRs) that sense the local environment and orchestrate this precise control of transmitter release. The primary trigger for catecholamine release is Ca2+ entry through voltage-gated Ca2+ channels, so it makes sense that these channels are subject to complex regulation by GPCRs. In particular G protein βγ heterodimers (Gbc) bind to and inhibit Ca2+ channels. Here I review the mechanisms by which GPCRs inhibit Ca2+ channels in chromaffin cells and how this might be altered by cellular context. This is related to the potent autocrine inhibition of Ca2+ entry and transmitter release seen in chromaffin cells. Recent data that implicate an additional inhibitory target of Gβγ on the exocytotic machinery and how this might fine tune neuroendocrine secretion are also discussed.

Regulation of calcium channels and exocytosis in mouse adrenal chromaffin cells by prostaglandin EP3 receptors

Prostaglandin (PG) E(2) controls numerous physiological functions through a family of cognate G protein-coupled receptors (EP1-EP4). Targeting specific EP receptors might be therapeutically useful and reduce side effects associated with nonsteroidal anti-inflammatory drugs and selective cyclooxygenase-2 inhibitors that block prostanoid synthesis. Systemic immune challenge and inflammatory cytokines have been shown to increase expression of the synthetic enzymes for PGE(2) in the adrenal gland. Catecholamines and other hormones, released from adrenal chromaffin cells in response to Ca(2+) influx through voltage-gated Ca(2+) channels, play central roles in homeostatic function and the coordinated stress response. However, long-term elevation of circulating catecholamines contributes to the pathogenesis of hypertension and heart failure. Here, we investigated the EP receptor(s) and cellular mechanisms by which PGE(2) might modulate chromaffin cell function. PGE(2) did not alter resting intracellular [Ca(2+)] or the peak amplitude of nicotinic acetylcholine receptor currents, but it did inhibit Ca(V)2 voltage-gated Ca(2+) channel currents (I(Ca)). This inhibition was voltage-dependent and mediated by pertussis toxin-sensitive G proteins, consistent with a direct Gβγ subunit-mediated mechanism common to other G(i/o)-coupled receptors. mRNA for all four EP receptors was detected, but using selective pharmacological tools and EP receptor knockout mice, we demonstrated that EP3 receptors mediate the inhibition of I(Ca). Finally, changes in membrane capacitance showed that Ca(2+)-dependent exocytosis was reduced in parallel with I(Ca). To our knowledge, this is the first study of EP receptor signaling in mouse chromaffin cells and identifies a molecular mechanism for paracrine regulation of neuroendocrine function by PGE(2).

Modulation by endogenously released ATP and opioids of chromaffin cell calcium channels in mouse adrenal slices

Modulation of high-threshold voltage-dependent calcium channels by neurotransmitters has been the subject of numerous studies in cultures of neurons and chromaffin cells. However, no studies on such modulation exist in chromaffin cells in their natural environment, the intact adrenal medullary tissue. Here we performed such a study in voltage-clamped chromaffin cells of freshly prepared mouse adrenal slices under the whole cell configuration of the patch-clamp technique. The subcomponents of the whole cell inward Ca(2+) current (I(Ca)) accounted for 49% for L-, 28% for N-, and 36% for P/Q-type channels. T-type Ca(2+) channels or residual R-type Ca(2+) currents were not seen. However, under the perforated-patch configuration, 20% of I(Ca) accounted for a toxin-resistant R-type Ca(2+) current. Exogenously applied ATP and methionine-enkephalin (Met-enk) inhibited I(Ca) by 33%. Stop-flow and Ca(2+) replacement by Ba(2+), which favored the release of endogenous ATP and opioids, also inhibited I(Ca), with no changes in activation or inactivation kinetics. This inhibition was partially voltage independent and insensitive to prepulse facilitation. Furthermore, in about half of the cells, suramin and naloxone augmented I(Ca) in the absence of exogenous application of ATP/Met-enk. No additional modulation of I(Ca) was obtained after bath application of exogenous ATP and opioids to these already inhibited cells. Augmentation of I(Ca) was also seen upon intracellular dialysis of guanosine 5'-[β-thio]diphosphate (GDPβS), indicating the existence in the intact slice of a tonic inhibition of I(Ca) in resting conditions. These results suggest that in the intact adrenal tissue a tonic inhibition of I(Ca) exists, mediated by purinergic and opiate receptors.

Down-modulation of Ca2+ channels by endogenously released ATP and opioids: from the isolated chromaffin cell to the slice of adrenal medullae

Modifications in Ca(2+) influx may lead to profound changes in the cell activity associated with Ca(2+)-dependent processes, from muscle contraction and neurotransmitter release to calcium-mediated cell death. Therefore, calcium entry into the cell requires fine regulation. In this context, understanding of the modulation of voltage-dependent Ca(2+) channels seems to be critical. The modulatory process results in the enhancement or decrement of calcium influx that may regulate the local and global cytosolic Ca(2+) concentrations. Here, we summarize the well-established data on this matter described in isolated chromaffin cells by our laboratory and others, and the new results we have obtained in a more physiological preparation: freshly isolated slices of mouse adrenal medullae.

G protein betagamma subunits modulate the number and nature of exocytotic fusion events in adrenal chromaffin cells independent of calcium entry

G-protein-coupled receptors (GPCR) play important roles in controlling neurotransmitter and hormone release. Inhibition of voltage-gated Ca(2+) channels (Ca(2+) channels) by G protein betagamma subunits (Gbetagamma) is one prominent mechanism, but there is evidence for additional effects distinct from those on calcium entry. However, relatively few studies have investigated the Ca(2+)-channel-independent effects of Gbetagamma on transmitter release, so the impact of this mechanism remains unclear. We used carbon fiber amperometry to analyze catecholamine release from individual vesicles in bovine adrenal chromaffin cells, a widely used neurosecretory model. To bypass the effects of Gbetagamma on Ca(2+) entry, we stimulated secretion using ionomycin (a Ca(2+) ionophore) or direct intracellular application of Ca(2+) through a patch pipette. Activation of endogenous GPCR or transient transfection with exogenous Gbetagamma significantly reduced the number of amperometric spikes (the number of vesicular fusion events). The charge ("quantal size") and amplitude of the amperometric spikes were also significantly reduced by GPCR/Gbetagamma. We conclude that independent from effects on calcium entry, Gbetagamma can regulate both the number of vesicles that undergo exocytosis and the amount of catecholamine released per fusion event. We discuss possible mechanisms by which Gbetagamma might exert these novel effects including interaction with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

N- and P/Q-type Ca2+ channels in adrenal chromaffin cells

Ca2+ is the most ubiquitous second messenger found in all cells. Alterations in [Ca2+]i contribute to a wide variety of cellular responses including neurotransmitter release, muscle contraction, synaptogenesis and gene expression. Voltage-dependent Ca2+ channels, found in all excitable cells (Hille 1992), mediate the entry of Ca2+ into cells following depolarization. Ca2+ channels are composed of a large pore-forming subunit, called the alpha1 subunit, and several accessory subunits. Ten different alpha1 subunit genes have been identified and classified into three families, Ca(v1-3) (Dunlap et al. 1995, Catterall 2000). Each alpha1 gene produces a unique Ca2+ channel. Although chromaffin cells express several different types of Ca2+ channels, this review will focus on the Cav(2.1) and Cav(2.2) channels, also known as P/Q- and N-type respectively (Nowycky et al. 1985, Llinas et al. 1989b, Wheeler et al. 1994). These channels exhibit physiological and pharmacological properties similar to their neuronal counterparts. N-, P/Q and to a lesser extent R-type Ca2+ channels are known to regulate neurotransmitter release (Hirning et al. 1988, Horne & Kemp 1991, Uchitel et al. 1992, Luebke et al. 1993, Takahashi & Momiyama 1993, Turner et al. 1993, Regehr & Mintz 1994, Wheeler et al. 1994, Wu & Saggau 1994, Waterman 1996, Wright & Angus 1996, Reid et al. 1997). N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they colocalize with synaptic vesicles. Similarly, these channels play a role in neurotransmitter release in chromaffin cells (Garcia et al. 2006). N- and P/Q-type channels are subject to many forms of regulation (Ikeda & Dunlap 1999). This review pays particular attention to the regulation of N- and P/Q-type channels by heterotrimeric G-proteins, interaction with SNARE proteins, and channel inactivation in the context of stimulus-secretion coupling in adrenal chromaffin cells.

Presynaptic adenosine and P2Y receptors

Adenine-based purines, such as adenosine and ATP, are ubiquitous molecules that, in addition to their roles in metabolism, act as modulators of neurotransmitter release through activation of presynaptic P1 purinoceptors or adenosine receptors (activated by adenosine) and P2 receptors (activated by nucleotides). Of the latter, the P2Y receptors are G protein-coupled, whereas the P2X receptors are ligand-gated ion channels and not covered in this review.

Modulation of neurotransmitter release by P2X and P2Y receptors in the rat spinal cord

In this study, the P2 receptor-mediated modulation of [3H]glutamate and [3H]noradrenaline release were examined in rat spinal cord slices. Adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), and 2-methylthioadenosine 5'-diphosphate (2-MeSADP) decreased the electrical stimulation-evoked [3H]glutamate efflux with the following order of potency: ADP>2-MeSADP>ATP. The effect of ATP was antagonized by suramin (300microM), the P2Y12,13 receptor antagonist 2-methylthioadenosine 5'-monophosphate (2-MeSAMP, 10microM), and partly by 4-[[4-Formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid (PPADS, 30microM) and the P2Y1 receptor antagonist 2'-deoxy-N6-methyladenosine 3',5'-diphosphate (MRS 2179, 10muM). ATP, ADP and 2-MeSADP also decreased evoked [3H]noradrenaline outflow; the order of agonist potency was ADP> or =2-MeSADP>ATP. The effect of ATP was reversed by 2-MeSAMP (10microM), and partly by MRS 2179 (10microM). By contrast, 2-methylthioadenosine-5'-triphosphate (2-MeSATP, 10-300microM) increased resting and electrically evoked [3H]glutamate and [3H]noradrenaline efflux, and this effect was prevented by the P2X1 receptor selective antagonist 4,4',4'',4'''-[carbonylbis[imino-5,1,3-benzenetriyl bis (carbonyl-imino)]] tetrakis (benzene-1,3-disulfonic acid) octasodium salt (NF449, 100nM). Reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that mRNAs encoding P2Y12 and P2Y13 receptors are expressed in the brainstem, whereas P2Y13 but not P2Y12 receptor mRNA is present in the dorsal root ganglion and spinal cord. P2Y1 receptor expression in the spinal cord is also demonstrated at the protein level. In conclusion, inhibitory P2Y and facilitatory P2X1-like receptors, involved in the regulation of glutamate (P2Y13 and/or P2Y1) and noradrenaline (P2Y13 and/or P2Y1, P2Y12) release have been identified, which provide novel target sites for analgesics acting at the spinal cord level.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

Selective stimulation of catecholamine release from bovine adrenal chromaffin cells by an ionotropic purinergic receptor sensitive to 2-methylthio ATP

Background

2-Methylthioadenosine 5'-triphosphate (2-MeSATP), formerly regarded as a specific P2Y (metabotropic) purinergic receptor agonist, stimulates Ca²⁺ influx and evokes catecholamine release from adrenal chromaffin cells. These cells express P2Y and P2X (ionotropic) purinoceptors, with the latter providing an important Ca²⁺ influx pathway. Using single cell calcium imaging techniques, we have determined whether 2-MeSATP might be a specific P2X receptor agonist in bovine chromaffin cells and assessed the relative role of P2X and P2Y receptors on catecholamine secretion from these cells.

Results

ATP raised the [Ca²⁺]_i in ~50% of the cells. Removing extracellular Ca²⁺ suppressed the [Ca²⁺]_i-raising ability of 2-MeSATP, observed in ~40% of the ATP-sensitive cells. This indicates that 2-MeSATP behaves as a specific ionotropic purinoceptor agonist in bovine chromaffin cells. The 2-MeSATP-induced [Ca²⁺]_i-rises were suppressed by PPADS. UTP raised the [Ca²⁺]_i in ~40% of the ATP-sensitive cells, indicating that these expressed Ca²⁺-mobilizing P2Y receptors. UTP-sensitive receptors may not be the only P2Y receptors present, as suggested by the observation that ~20% of the ATP-sensitive pool did not respond to either 2-MeSATP or UTP. The average sizes of the ATP- and 2-MeSATP-evoked [Ca²⁺]_i responses were identical in UTP-insensitive cells. 2-MeSATP stimulated Ca²⁺ influx and evoked catecholamine release, whereas UTP elicited Ca²⁺ release from intracellular stores but did not evoke secretion. 2-MeSATP-induced secretion was strongly inhibited by Cd²⁺ and suppressed by extracellular Ca²⁺ or Na⁺ removal. TTX inhibited 2-MeSATP-evoked secretion by ~20%.

Conclusion

2-MeSATP is a specific P2X purinoceptor agonist and a potent secretagogue in bovine chromaffin cells. Activation of 2-MeSATP-sensitive receptors stimulates Ca²⁺ influx mainly via voltage-sensitive Ca²⁺ channels. For the most part, these are activated by the depolarization brought about by Na⁺ influx across P2X receptor pores.

Functional distribution of Ca2+-coupled P2 purinergic receptors among adrenergic and noradrenergic bovine adrenal chromaffin cells

BACKGROUND

Adrenal chromaffin cells mediate acute responses to stress through the release of epinephrine. Chromaffin cell function is regulated by several receptors, present both in adrenergic (AD) and noradrenergic (NA) cells. Extracellular ATP exerts excitatory and inhibitory actions on chromaffin cells via ionotropic (P2X) and metabotropic (P2Y) receptors. We have taken advantage of the actions of the purinergic agonists ATP and UTP on cytosolic free Ca2+ concentration ([Ca2+]i) to determine whether P2X and P2Y receptors might be asymmetrically distributed among AD and NA chromaffin cells.

RESULTS

The [Ca2+]i and the [Na+]i were recorded from immunolabeled bovine chromaffin cells by single-cell fluorescence imaging. Among the ATP-sensitive cells ~40% did not yield [Ca2+]i responses to ATP in the absence of extracellular Ca2+ (Ca2+o), indicating that they expressed P2X receptors and did not express Ca2+- mobilizing P2Y receptors; the remainder expressed Ca2+-mobilizing P2Y receptors. Relative to AD-cells approximately twice as many NA-cells expressed P2X receptors while not expressing Ca2+- mobilizing P2Y receptors, as indicated by the proportion of cells lacking [Ca2+]i responses and exhibiting [Na+]i responses to ATP in the absence and presence of Ca2+o, respectively. The density of P2X receptors in NA-cells appeared to be 30-50% larger, as suggested by comparing the average size of the [Na+]i and [Ca2+]i responses to ATP. Conversely, approximately twice as many AD-cells expressed Ca2+-mobilizing P2Y receptors, and they appeared to exhibit a higher (~20%) receptor density. UTP raised the [Ca2+]i in a fraction of the cells and did not raise the [Na+]i in any of the cells tested, confirming its specificity as a P2Y agonist. The cell density of UTP-sensitive P2Y receptors did not appear to vary among AD- and NA-cells.

CONCLUSION

Although neither of the major purinoceptor types can be ascribed to a particular cell phenotype, P2X and Ca2+-mobilizing P2Y receptors are preferentially located to noradrenergic and adrenergic chromaffin cells, respectively. ATP might, in addition to an UTP-sensitive P2Y receptor, activate an UTP-insensitive P2Y receptor subtype. A model for a short-loop feedback interaction is presented whereby locally released ATP acts upon P2Y receptors in adrenergic cells, inhibiting Ca2+ influx and contributing to terminate evoked epinephrine secretion.

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.

Resolution of fusion pore formation in a cell-attached patch

Exocytosis of neurotransmitter is initiated by formation of a fusion pore, a narrow channel connecting the vesicle lumen to the extracellular space. Opening of the fusion pore can produce a flow of capacitative current, which was used to resolve the exocytosis of mast cell giant secretory granules by Breckenridge and Almers [Breckenridge LJ, Almers W. Currents through the fusion pore that forms during exocytosis of a secretory vesicle. Nature 1987;328:814-7]. We present an extension of this method to resolve fusion pore formation initiating exocytosis of single vesicles in bovine chromaffin cells. Cell-attached patch recordings revealed a capacitative current that was evoked by an increase of intracellular calcium (Ca(2+)) from application of a Ca(2+) ionophore or by the opening of a co-localized Ca(2+) channel. Calculated values for fusion pore conductance and vesicular membrane potential were in accord with previous estimates. Finally, we show that a single opening of a co-localized calcium channel evoked fusion pore formation, with delay times between channel opening and exocytosis agreeing with other methods. This method can be applied to resolve exocytosis regardless of what the vesicle contains or where exocytosis occurs, giving the possibility of using this method to resolve the release of synaptic vesicles.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Functions of neuronal P2Y receptors

Within the last 15 years, at least eight different G protein-coupled nucleotide receptors, i.e., P2Y receptors, have been characterized by molecular means. While ionotropic P2X receptors are mainly involved in fast synaptic neurotransmission, P2Y receptors rather mediate slower neuromodulatory effects. This P2Y receptor-dependent neuromodulation relies on changes in synaptic transmission via either pre- or postsynaptic sites of action. At both sites, the regulation of voltage-gated or transmitter-gated ion channels via G protein-linked signaling cascades has been identified as the predominant underlying mechanisms. In addition, neuronal P2Y receptors have been found to be involved in neurotoxic and neurotrophic effects of extracellular adenosine 5-triphosphate. This review provides an overview of the most prominent actions mediated by neuronal P2Y receptors and describes the signaling cascades involved.

Internalization and desensitization of a green fluorescent protein-tagged P2Y nucleotide receptor are differently controlled by inhibition of calmodulin-dependent protein kinase II

De- and re-sensitization and trafficking of P2Y nucleotide receptors modulate physiological responses of these receptors. Here, we used the rat brain P2Y1 receptor tagged with green fluorescent protein (P2Y1-GFP receptor) expressed in HEK293 human embryonic kidney cells. Ca2+ release was used as a functional test to investigate ATP-induced receptor de- and re-sensitization. By confocal laser scanning microscopy (CLSM), endocytosis of P2Y1-GFP receptor was visualized in live cells. Stimulation of the cells with ATP induced complete receptor endocytosis within 30 min and appearance of the P2Y1 receptor in small vesicles. Removal of the agonist resulted in reappearance of the receptor after 60 min on the plasma membrane. Exposure of the cells to KN-62 and KN-93, inhibitors of the calmodulin dependent protein kinase II (CaMKII), prevented receptor internalization upon stimulation with ATP. However, the receptor which was still present on the plasma membrane was desensitized, seen by decreased Ca2+ response. The decreased Ca2+ response after 30-min exposure to ATP can be attributed to desensitization and is not as a result of depletion of internal stores, as the cells exposed to ATP for 30 min exhibited a normal Ca2+ response upon stimulation with thrombin. However, okadaic acid, an inhibitor of protein phosphatase 2A (PP2A), did not affect ATP-induced P2Y1 receptor endocytosis, but delayed the reappearance of the P2Y1 receptor on the plasma membrane after ATP withdrawal. Consistently, in okadaic acid-treated cells the ATP-induced Ca2+ response observed after the 30-min exposure to ATP recovered only partially. Thus, CaMKII seems to be involved in P2Y1 receptor internalization, but not desensitization, whereas protein phosphatase 2A might play a role in recycling of the receptor back to the plasma membrane.

Enhancement of Asynchronous and Train-Evoked Exocytosis in Bovine Adrenal Chromaffin Cells Infected With a Replication Deficient Adenovirus

Bovine adrenal chromaffin cells share many characteristics with neurons and are often used as a simple model system to study ion channels and neurotransmitter release. We infected bovine adrenal chromaffin cells with a replication deficient adenovirus that induces expression of the common reporters β-galactosidase and Green Fluorescent Protein via a bicistronic sequence. In perforated-patch recordings performed 48-h postinfection, peak calcium currents were reduced 32%, primarily due to loss of ω-conotoxin-GVIA-sensitive current. In contrast, sodium currents were increased 17%. Exocytosis, detected as an increase in membrane capacitance immediately after a single step depolarization, was reduced in proportion to the decrease in calcium influx. However, capacitance continued to increase for seconds after the depolarization. The amplitude of this poststimulus drift, or asynchronous exocytosis, was approximately three times that which occurred in a small fraction of control cells. Exocytosis evoked by repetitive stimulation with a train of brief depolarizations was increased 50%. Intracellular calcium levels measured during and after stimulation were lower, not higher, in adenovirus-infected cells. Electroporated cells showed reduced calcium currents but no enhancement of exocytosis. Cells infected with UV-irradiated virus showed reduced calcium currents and enhancement of exocytosis, but the changes were smaller than those caused by intact virus. Our results are consistent with the idea that adenovirus capsid and adenoviral DNA contribute to a Ca2+influx- and [Ca2+]i-independent enhancement of exocytosis in bovine chromaffin cells.

Changes in P2Y 2 receptor localization on adrenaline‐ and noradrenaline‐containing chromaffin cells in the rat adrenal gland during development and aging

Using immunohistochemistry, the occurrence and age-related changes of the P2Y2 receptor was investigated in the adrenal gland of rat at different ages, ranging from embryonic day E16 to 22 months. Immunoreactivity for the P2Y2 receptor was present in chromaffin cells and nerve fibres at all ages examined. Double labeling with the antibody against phenyl ethanolamine-N-methyltransferase, which marks adrenaline-producing chromaffin cells, revealed that only a few of the P2Y2-immunoreactive cells were adrenaline producing at embryonic day E16, the vast majority being noradrenaline-containing cells. However, immunoreactivity for adrenaline-containing cells in the P2Y2 receptor-labeled chromaffin cells increased with increasing age and at 1 week post-natal almost all chromaffin cells were positive for both P2Y2 and phenyl ethanolamine-N-methyltransferase, while noradrenaline-containing cells were minimal. At 2 weeks, there was a dramatic drop in P2Y2-immunoreactive chromaffin cells and this was maintained in adult rats, noradrenaline-containing cells dominating. In the aging rat adrenals, P2Y2 receptor-immunoreactivity was localized in subpopulations of both adrenaline and noradrenaline-producing cells. Intrinsic neurones were also visible that were positively labeled with the P2Y2 receptor antibody in the adrenals of both adult and aging rats. P2Y2-immunoreactive nerve fibres formed a plexus around the adrenal cortical cells of zona glomerulosa in the post-natal, but not in adult or aging rats. In conclusion, this study suggests that ATP, acting through P2Y2 receptors, may influence the phenotypic expression of chromaffin cells during the development and aging of the rat adrenal gland. However, during early development, when the chromaffin cells are actively dividing and during aging, when the adrenal medullary cells are known to show hyperplastic lesions, ATP acting through P2Y2 receptors may be involved in other physiological activities, such as proliferation and/or differentiation of the chromaffin cells associated with their adrenaline or noradrenaline phenotype.

Activation of GPCRs modulates quantal size in chromaffin cells through G(betagamma) and PKC

Exocytosis proceeds by either full fusion or 'kiss-and-run' between vesicle and plasma membrane. Switching between these two modes permits the cell to regulate the kinetics and amount of secretion. Here we show that ATP receptor activation reduces secretion downstream from cytosolic Ca2+ elevation in rat adrenal chromaffin cells. This reduction is mediated by activation of a pertussis toxin-sensitive G(i/o) protein, leading to activation of G(betagamma) subunits, which promote the 'kiss-and-run' mode by reducing the total open time of the fusion pore during a vesicle fusion event. Furthermore, parallel activation of the muscarinic acetylcholine receptor removes the inhibitory effects of ATP on secretion. This is mediated by a G(q) pathway through protein kinase C activation. The inhibitory effects of ATP and its reversal by protein kinase C activation are also shared by opioids and somatostatin. Thus, a variety of G protein pathways exist to modulate Ca2+-evoked secretion at specific steps in fusion pore formation.

An electron microscopy study of changes in dense core vesicles of PC12 cells following pulse depolarization

Depolarization of PC12 cells by brief pulses of high K+ or ATP produces electrophysiological responses consistent with the quantal release of ATP. Quantitative electron microscopy was used to validate whether the same protocol changed dense core vesicles containing endogenous ATP. The total vesicle number fell by 54% after high K+ and by 21% after ATP. Perimembrane vesicles were significantly depleted by high K+, yet unchanged by ATP, suggesting differential contribution by vesicle pools to distinct stimuli during the release process. Vesicle changes were consistent with vesicular release mechanisms for the liberation of discrete packets of endogenous ATP. These data thus support the use of clustered PC12 cells as a model to study the process of ATP release.

Identification of the P2Y(12) receptor in nucleotide inhibition of exocytosis from bovine chromaffin cells

Nucleotides are released from bovine chromaffin cells and take part in a feedback loop to inhibit further exocytosis. To identify the nucleotide receptors involved, we measured the effects of a range of exogenous nucleotides and related antagonists on voltage-operated calcium currents (I(Ca)), intracellular calcium concentration ([Ca(2+)](i)), and membrane capacitance changes. In comparative parallel studies, we also cloned the bovine P2Y(12) receptor from chromaffin cells and determined its properties by coexpression in Xenopus laevis oocytes with inward-rectifier potassium channels made up of Kir3.1 and Kir3.4. In both systems, the agonist order of potency was essentially identical (2-methylthio-ATP approximately 2-methylthio-ADP >> ATP approximately ADP > UDP). alphabeta-Methylene-ATP and adenosine were inactive. UTP inhibited I(Ca) in chromaffin cells (pEC(50) = 4.89 +/- 0.11) but was essentially inactive at the cloned P2Y(12) receptor. The relatively nonselective P2 antagonist pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid blocked nucleotide responses in both chromaffin cells and X. laevis oocytes, whereas the P2Y(12)- and P2Y(13)-selective antagonist N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene ATP (ARC69931MX) blocked responses to ATP in both chromaffin cells and X. laevis oocytes but not to UTP in chromaffin cells. These results identify the P2Y(12) purine receptor as a key component of the nucleotide inhibitory pathway and also demonstrate the involvement of a UTP-sensitive G(i/o) -coupled pyrimidine receptor.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

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

Adenine nucleotides inhibit recombinant N-type calcium channels via G protein-coupled mechanisms in HEK 293 cells; involvement of the P2Y13 receptor-type

1. N-type Ca(2+) channel modulation by an endogenous P2Y receptor was investigated by the whole-cell patch-clamp method in HEK 293 cells transfected with the functional rabbit N-type calcium channel. 2. The current responses (I(Ca(N))) to depolarizing voltage steps were depressed by ATP in a concentration-dependent manner. Inclusion of either guanosine 5'-O-(3-thiodiphosphate) or pertussis toxin into the pipette solution as well as a strongly depolarizing prepulse abolished the inhibitory action of ATP. 3. In order to identify the P2Y receptor subtype responsible for this effect, several preferential agonists and antagonists were studied. Whereas the concentration-response curves of ADP and adenosine 5'-O-(2-thiodiphosphate) indicated a higher potency of these agonists than that of ATP, alpha,beta-methylene ATP, UTP and UDP were considerably less active. The effect of ATP was abolished by the P2Y receptor antagonists suramin and N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, 2'deoxy-N(6)-methyladenosine-3',5'-diphosphate or 2-methylthio AMP. 4. Using reverse transcription and polymerase chain reaction, mRNA for the P2Y(1), P2Y(4), P2Y(6), P2Y(11) and P2Y(13) receptor subtypes, but not the P2Y(2), and P2Y(12) subtypes, was detected in HEK 293 cells. 5. Immunocytochemistry confirmed the presence of P2Y(1), and to a minor extent that of P2Y(4), but not of P2Y(2) receptors. 6. Hence, it is tempting to speculate that P2Y(13) receptors may inhibit N-type Ca(2+) channels via the betagamma subunits of the activated G(i) protein.

ATP modulates noradrenaline release by activation of inhibitory P2Y receptors and facilitatory P2X receptors in the rat vas deferens

The role of ATP on the modulation of noradrenaline release elicited by electrical stimulation (100 pulses/8 Hz) was studied in the prostatic portion of rat vas deferens preincubated with [3H]noradrenaline. In the presence of P1 antagonists, the nucleotides 2-methylthioadenosine-5'-triphosphate (2-MeSATP), 2-methylthioadenosine 5'-diphosphate (2-MeSADP), ADP, and ATP decreased electrically evoked tritium overflow up to 44%, with the following order of potency: 2-MeSATP > 2-MeSADP > ADP > or = ATP. The P2Y antagonists reactive blue 2 (RB2) and 2-methylthioadenosine 5'-monophosphate (2-MeSAMP) increased, whereas the P2X antagonist pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) (PPNDS) decreased evoked tritium overflow. The inhibitory effect of 2-MeSATP was antagonized by RB2 (10 microM) and by 2-MeSAMP (10 microM) but not by the selective P2Y1 receptor antagonist 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS 2179; 10 microM). When, besides P1 receptors, inhibitory P2Y receptors were blocked with RB2, alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-meATP), beta,gamma-imidoadenosine 5'-triphosphate (beta,gamma-imidoATP), beta,gamma-methyleneadenosine 5'-triphosphate (beta,gamma-meATP), 2-MeSATP, and ATP enhanced tritium overflow up to 140%, with the following order of potency: alpha,beta-meATP > 2-MeSATP = ATP = beta,gamma-meATP > or = beta,gamma-imidoATP. The facilitatory effects of alpha,beta-MeATP and beta,gamma-imidoATP were prevented by PPNDS. Under the same conditions, apyrase attenuated, whereas the ectonucleotidase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethylene 5'-triphosphate enhanced tritium overflow, an effect that was prevented by PPNDS. In the prostatic portion of the rat vas deferens, endogenous ATP exerts a dual and opposite modulation of noradrenaline release: an inhibition through activation of P2Y receptors with a pharmacological profile similar to that of the P2Y12 and P2Y13 receptors and a facilitation through activation of P2X receptors with a pharmacological profile similar to that of P2X1 and P2X3, or PX2/P2X3 receptors.

Increased vesicle recycling in response to osmotic cell swelling. Cause and consequence of hypotonicity-provoked ATP release

Osmotic swelling of Intestine 407 cells leads to an immediate increase in cell surface membrane area as determined using the fluorescent membrane dye FM 1-43. In addition, as measured by tetramethylrhodamine isothiocyanate (TRITC)-dextran uptake, a robust (>100-fold) increase in the rate of endocytosis was observed, starting after a discrete lag time of 2-3 min and lasting for approximately 10-15 min. The hypotonicity-induced increase in membrane surface area, like the cell swelling-induced release of ATP (Van der Wijk, T., De Jonge, H. R., and Tilly, B. C. (1999) Biochem. J. 343, 579-586), was diminished after 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester loading or cytochalasin B treatment. Uptake of TRITC-dextrans, however, was not affected. Treatment of the cells with the vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptor-specific protease Clostridium botulinum toxin F not only nearly eliminated the hypotonicity-induced increase in membrane surface area but also strongly diminished the release of ATP, indicating the involvement of regulated exocytosis. Both the ATP hydrolase apyrase and the MEK inhibitor PD098059 diminished the osmotic swelling-induced increase in membrane surface area as well as the subsequent uptake of TRITC-dextrans. Taken together, the results indicate that extracellular ATP is required for the hypotonicity-induced vesicle recycling and suggest that a positive feedback loop, involving purinergic activation of the Erk-1/2 pathway, may contribute to the release of ATP from hypo-osmotically stimulated cells.

Adenosine triphosphate inhibits cytokine release from lipopolysaccharide-activated microglia via P2y receptors

Microglial proliferation and activation have been reported to occur after several central nervous system injuries. In this study, we tested the effects of adenosine triphosphate (ATP) on cultured microglia obtained from the spinal cord of rat embryos. The amounts of tumor necrosis factor alpha (TNF-alpha), interleukin 1beta and interleukin 6 released from the microglia, which were stimulated by lipopolysaccharide (LPS; 100 ng/ml), were inhibited by the simultaneous addition of ATP in a dose dependent manner (10-300 microM). We examined the effect of several endogenous purines (ATP, ADP, CTP, UDP, UTP) and P(2)y receptor agonists (ADPbetaS and 2-methylthio-ATP) on LPS-induced TNF-alpha release. The rank order of inhibitory potency of endogenous purines on TNF-alpha release was: ATP>ADP>>UTP>UDP>CTP. The latter three were much less potent than the former two. The addition of 10 microM 2-methylthio-ATP showed a potency similar to 100 microM ATP. The addition of ADPbetaS, however, showed less effect. These endogenous purines and selective ATP receptor agonists showed a similar inhibitory effect in their rank order on LPS-induced interleukin 6 release. We demonstrate that ATP inhibits cytokine release from LPS-activated microglia via metabotropic receptors. The application of P(2)y receptor agonists might be considered as a pharmacological treatment of several pathological conditions of the spinal cord, including toxic immunoreactions.

Distinct potentiation of L-type currents and secretion by cAMP in rat chromaffin cells

We have investigated the potentiating action of cAMP on L-currents of rat chromaffin cells and the corresponding increase of Ca(2+)-evoked secretory responses with the aim of separating the action of cAMP on Ca(2+) entry through L-channels and the downstream effects of cAMP/protein kinase A (PKA) on exocytosis. In omega-toxin-treated rat chromaffin cells, exposure to the permeable cAMP analog 8-(4-chlorophenylthio)-adenosine 3',5'-monophosphate (pCPT-cAMP; 1 mM, 30 min) caused a moderate increase of Ca(2+) charge carried through L-channels (19% in 10 mM Ca(2+) at +10 mV) and a drastic potentiation of secretion ( approximately 100%), measured as membrane capacitance increments (deltaC). The apparent Ca(2+) dependency of exocytosis increased with pCPT-cAMP and was accompanied by 83% enhancement of the readily releasable pool of vesicles with no significant change of the probability of release, as evaluated with paired-pulse stimulation protocols. pCPT-cAMP effects could be mimicked by stimulation of beta(1)-adrenoreceptors and reversed by the PKA inhibitor H89, suggesting strict PKA dependence. For short pulses to +10 mV (100 ms), potentiation of exocytosis by pCPT-cAMP was proportional to the quantity of charge entering the cell and occurred independently of whether L, N, or P/Q channels were blocked, suggesting that cAMP acts as a constant amplification factor for secretion regardless of the channel type carrying Ca(2+). Analysis of statistical variations among depolarization-induced capacitance increments indicates that pCPT-cAMP acts downstream of Ca(2+) entry by almost doubling the mean size of unitary exocytic events, most likely as a consequence of an increased granule-to-granule rather than a granule-to-membrane fusion.

Opposite action of beta1- and beta2-adrenergic receptors on Ca(V)1 L-channel current in rat adrenal chromaffin cells

Voltage-gated Ca(2+) channels of chromaffin cells are modulated by locally released neurotransmitters through autoreceptor-activated G-proteins. Clear evidence exists in favor of a Ca(2+) channel gating inhibition mediated by purinergic, opioidergic, and alpha-adrenergic autoreceptors. Few and contradictory data suggest also a role of beta-adrenergic autoreceptors (beta-ARs), the action of which, however, remains obscure. Here, using patch-perforated recordings, we show that rat chromaffin cells respond to the beta-AR agonist isoprenaline (ISO) by either upmodulating or downmodulating the amplitude of Ca(2+) currents through two distinct modulatory pathways. ISO (1 microm) could cause either fast inhibition (approximately 25%) or slow potentiation (approximately 25%), or a combination of the two actions. Both effects were completely prevented by propranolol. Slow potentiation was more evident in cells pretreated with pertussis toxin (PTX) or when beta(1)-ARs were selectively stimulated with ISO + ICI118,551. Potentiation was absent when the beta(2)-AR-selective agonist zinterol (1 microm), the protein kinase A (PKA) inhibitor H89, or nifedipine was applied, suggesting that potentiation is associated with a PKA-mediated phosphorylation of L-channels (approximately 40% L-current increase) through beta(1)-ARs. The ISO-induced inhibition was fast and reversible, preserved in cell treated with H89, and mimicked by zinterol. The action of zinterol was mostly on L-channels (38% inhibition). Zinterol action preserved the channel activation kinetics, the voltage-dependence of the I-V characteristic, and was removed by PTX, suggesting that beta(2)AR-mediated channel inhibition was mainly voltage independent and coupled to G(i)/G(o)-proteins. Sequential application of zinterol and ISO mimicked the dual action (inhibition/potentiation) of ISO alone. The two kinetically and pharmacologically distinct beta-ARs signaling uncover alternative pathways, which may serve the autocrine control of Ca(2+)-dependent exocytosis and other related functions of rat chromaffin cells.

P2Y-receptors mediating an inhibition of the evoked entry of calcium through N-type calcium channels at neuronal processes

In the search for P2-receptors modulating the stimulation-evoked entry of calcium at processes of PC12 cells differentiated in the presence of nerve growth factor and neurotrophin-3, electrically evoked increases in free calcium were assessed by fura-2 microfluorimetry. Omission of calcium and addition of cadmium (100 microM) or the N-type calcium channel blocker omega-conotoxin GVIA (0.5 microM) abolished or markedly reduced the evoked responses. The P2Y-receptor agonists 2-methylthio adenosine 5'-diphosphate (2-methylthio-ADP), ADP, and adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS) inhibited the electrically evoked entry of calcium without any changes in basal calcium concentrations. 2-Methylthio-ADP was the most potent agonist. Adenosine, P(1),P(4)-di(adenosine-5')-tetraphosphate (Ap4A), UDP, and UTP (30 microM each) had no effect. The effect of ADPbetaS (30 microM) was abolished by the P2-antagonists reactive blue 2 (3 microM), suramin (100 microM), 2-methylthio-AMP (10 microM), p-chloromercuriphenyl sulfonic acid (1 microM), and AR-C 69931MX [N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene adenosine 5'-triphosphate] (300 nM). In contrast, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (10 microM), the selective P2Y1-receptor antagonist MRS 2179 (N(6)-methyl-2'-deoxyadenosine 3',5'-bisphosphate; 10 microM), as well as the adenosine A(1)-receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine; 100 nM), caused no change. Pretreatment with pertussis toxin abolished the effect of ADPbetaS. Reverse transcriptase-polymerase chain reaction revealed the presence of mRNA for P2Y12-receptors in nondifferentiated and differentiated PC12 cells. The results indicate that processes of differentiated PC12 cells possess P2Y12-receptors coupling to pertussis toxin-sensitive G-proteins and mediating an inhibition of the stimulation-evoked entry of calcium through omega-conotoxin GVIA-sensitive calcium channels. This suggests a role of P2Y12-receptors in neuromodulation in addition to their involvement in platelet aggregation.

Neurotensin modulates the amplitude and frequency of voltage-activated Ca2+ currents in frog pituitary melanotrophs: implication of the inositol triphosphate/protein kinase C pathway

Many excitatory neurotransmitters and neuropeptides regulate the activity of neuronal and endocrine cells by modulating voltage-operated Ca2+ channels. Paradoxically, however, excitatory neuromediators that provoke mobilization of intracellular calcium from inositol trisphosphate (IP3)-sensitive stores usually inhibit voltage-gated Ca2+ currents. We have recently demonstrated that neurotensin (NT) stimulates the electrical and secretory activities of frog pituitary melanotrophs, and increases intracellular calcium concentration in these cells. In the present study, we have investigated the effects of NT on Ca2+ currents in cultured frog melanotrophs by using the perforated patch-clamp technique. Frog neurotensin (f NT) reduced the amplitude and facilitated the inactivation of both L- and N-type Ca2+ currents. Application of the membrane-permeant Ca2+ chelator BAPTA-AM, the sarcoendoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, or the IP3 receptor antagonist 2-APB suppressed the reduction of Ca2+ currents induced by f NT. Incubation of melanotrophs with the diacylglycerol analogue PMA, which causes desensitization of protein kinase C (PKC), or with the PKC inhibitors chelerythrine and calphostin C, reduced the inhibitory effect of f NT. The NT-induced action potential waveforms, applied as voltage-clamp commands, decreased the amplitude of Ca2+ currents, and enhanced Ca2+ influx by increasing the Ca2+ spike frequency. Altogether, these data indicate that the inhibitory effect of f NT on Ca2+ currents results from activation of the IP3/PKC pathway. The observation that NT controls Ca2+ signalling through both amplitude and frequency modulations of Ca2+ currents suggests that NT might induce spacial and temporal changes of intracellular Ca2+ concentration leading to stimulation of exocytosis.

Histamine promotes excitability in bovine adrenal chromaffin cells by inhibiting an M-current

The current study has investigated the electrophysiological responses evoked by histamine in bovine adrenal chromaffin cells using perforated-patch techniques. Histamine caused a transient hyperpolarization followed by a sustained depolarization of 7.2 +/- 1.4 mV associated with an increase in spontaneous action potential frequency. The hyperpolarization was abolished after depleting intracellular Ca(2+) stores with thapsigargin (100 nM), and was reduced by 40 % with apamin (100 nM). Membrane resistance increased by about 60 % during the histamine-induced depolarization suggesting inhibition of a K(+) channel. An inward current relaxation, typical of an M-current, was observed in response to negative voltage steps from a holding potential of -30 mV. This current reversed at -81.6 +/- 1.8 mV and was abolished by the M-channel inhibitor linopirdine (100 microM). During application of histamine, the amplitude of M-currents recorded at a time corresponding with the sustained depolarization was reduced by 40 %. No inward current rectification was observed in the range -150 to -70 mV, and glibenclamide (10 microM) had no effect on either resting membrane potential or the response to histamine. The results show that an M-current is present in bovine chromaffin cells and that this current is inhibited during sustained application of histamine, resulting in membrane depolarization and increased discharge of action potentials. These results demonstrate for the first time a possible mechanism coupling histamine receptors to activation of voltage-operated Ca(2+) channels in these cells.

Voltage-independent inhibition of P/Q-type Ca2+ channels in adrenal chromaffin cells via a neuronal Ca2+ sensor-1-dependent pathway involves Src family tyrosine kinase

In common with many neurons, adrenal chromaffin cells possess distinct voltage-dependent and voltage-independent pathways for Ca(2+) channel regulation. In this study, the voltage-independent pathway was revealed by addition of naloxone and suramin to remove tonic blockade of Ca(2+) currents via opioid and purinergic receptors due to autocrine feedback inhibition. This pathway requires the Ca(2+)-binding protein neuronal calcium sensor-1 (NCS-1). The voltage-dependent pathway was pertussis toxin-sensitive, whereas the voltage-independent pathway was largely pertussis toxin-insensitive. Characterization of the voltage-independent inhibition of Ca(2+) currents revealed that it did not involve protein kinase C-dependent signaling pathways but did require the activity of a Src family tyrosine kinase. Two structurally distinct Src kinase inhibitors, 4-amino-5-(4-methylphenyl)7-(t-butyl)pyrazolo[3,4-d] pyrimidine (PP1) and a Src inhibitory peptide, increased the Ca(2+) currents, and no further increase in Ca(2+) currents was elicited by addition of naloxone and suramin. In addition, the Src-like kinase appeared to act in the same pathway as NCS-1. In contrast, addition of PP1 did not prevent a voltage-dependent facilitation elicited by a strong pre-pulse depolarization indicating that this pathway was independent of Src kinase activity. PPI no longer increased Ca(2+) currents after addition of the P/Q-type channel blocker omega-agatoxin TK. The alpha(1A) subunit of P/Q-type Ca(2+) channels was immunoprecipitated from chromaffin cell extracts and found to be phosphorylated in a PP1-sensitive manner by endogenous kinases in the immunoprecipitate. A high molecular mass (around 220 kDa) form of the alpha(1A) subunit was detected by anti-phosphotyrosine, suggesting a possible target for Src family kinase action. These data demonstrate a voltage-independent mechanism for autocrine inhibition of P/Q-type Ca(2+) channel currents in chromaffin cells that requires Src family kinase activity and suggests that this may be a widely distributed pathway for Ca(2+) channel regulation.

[(35)S]GTPgammaS autoradiography reveals a wide distribution of G(i/o)-linked ADP receptors in the nervous system: close similarities with the platelet P2Y(ADP) receptor

No G(i)-linked P2Y receptors have been cloned to date but the presence of such receptors is thought to be restricted to platelets and certain clonal cell lines. Using the functional approach of [(35)S]guanosine 5'-[gamma-thio]-triphosphate autoradiography, we uncovered the widespread presence of such receptors in the CNS. Under conditions in which the prominent signal due to tonic adenosine receptor activity is masked, ADP and ATP stimulated G-protein activity in multiple grey and white matter regions. Localization in the grey matter suggests inhibitory auto-/heteroreceptor function. In the white matter, activated G proteins appeared as 'hot spots' (presumed oligodendrocyte progenitors) with scattered distribution along the main fibre tracts. Responses to ATP were diminished under conditions that inhibited degradation, suggesting that prior conversion to ADP explained agonist action. Uracil nucleotides were ineffective but 2-methylthio-ADP activated G proteins approximately 500-fold more potently than ADP, although both were similarly degraded. Throughout the brain, ADP-dependent G-protein activity was reversed by 2-hexylthio-AdoOC(O)Asp(2), a non-phosphate ATP analogue, whereas selective P2Y(1) receptor antagonists proved ineffective. A similar receptor was also disclosed from the adrenal medulla. These data witness a hitherto unrecognized abundance of G(i/o)-linked ADP receptors in the nervous system. Biochemical and pharmacological behaviour suggests striking similarities to the elusive platelet P2Y(ADP) receptor.

Molecular physiology of P2X receptors and ATP signalling at synapses

ATP is found in every cell, where it is a major source of energy. But in the nervous system, ATP also has additional actions, which include its role in fast synaptic transmission and modulation. Here I discuss the 'fast' actions of ATP at synapses, the properties of the receptors that are activated by ATP and the physiology of ATP signalling, with emphasis on its role in pain processing.