Noradrenaline release from rat sympathetic neurons evoked by P2-purinoceptor activation

Modulation of the heart's electrical properties by the anticonvulsant drug retigabine

Retigabine, currently used as antiepileptic drug, has a wide range of potential medical uses. Administration of the drug in patients can lead to QT interval prolongation in the electrocardiogram and to cardiac arrhythmias in rare cases. This suggests that the drug may perturb the electrical properties of the heart, and the underlying mechanisms were investigated here. Effects of retigabine on currents through human cardiac ion channels, heterologously expressed in tsA-201 cells, were studied in whole-cell patch-clamp experiments. In addition, the drug's impact on the cardiac action potential was tested. This was done using ventricular cardiomyocytes isolated from Langendorff-perfused guinea pig hearts and cardiomyocytes derived from human induced pluripotent stem cells. Further, to unravel potential indirect effects of retigabine on the heart which might involve the autonomic nervous system, membrane potential and noradrenaline release from sympathetic ganglionic neurons were measured in the absence and presence of the drug. Retigabine significantly inhibited currents through hKv11.1 potassium, hNav1.5 sodium, as well as hCav1.2 calcium channels, but only in supra-therapeutic concentrations. In a similar concentration range, the drug shortened the action potential in both guinea pig and human cardiomyocytes. Therapeutic concentrations of retigabine, on the other hand, were sufficient to inhibit the activity of sympathetic ganglionic neurons. We conclude that retigabine- induced QT interval prolongation, and the reported cases of cardiac arrhythmias after application of the drug in a typical daily dose range, cannot be explained by a direct modulatory effect on cardiac ion channels. They are rather mediated by indirect actions at the level of the autonomic nervous system.

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.

Excitatory P2-receptors at sympathetic axon terminals: role in temperature control of cutaneous blood flow

The mechanisms underlying the reduction in cutaneous blood flow in response to cooling are only partially understood. A study published in this issue of the British Journal of Pharmacology now provides evidence for the involvement of excitatory P2-receptors located at sympathetic axon terminals in the cooling-induced vasoconstriction in the skin. Cooling appears to cause the release of adenine nucleotides followed by the activation of excitatory presynaptic P2-receptors at noradrenergic axon terminals. Activation of these excitatory P2-receptors induces the release of noradrenaline, which subsequently causes constriction of blood vessels in the skin by action on smooth muscle alpha(1)- and alpha(2)-adrenoceptors. The commentary discusses the implication of the results and remaining questions.

P2X(2), P2X(2-2) and P2X(5) receptor subunit expression and function in rat thoracolumbar sympathetic neurons

The present study investigated the pharmacological properties of excitatory P2X receptors and P2X(2) and P2X(5) receptor subunit expression in rat-cultured thoracolumbar sympathetic neurons. In patch-clamp recordings, ATP (3-1000 microM; applied for 1 s) induced inward currents in a concentration-dependent manner. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS; 30 microM) counteracted the ATP response. In contrast to ATP, alpha,beta-meATP (30 microM; for 1 s) was virtually ineffective. Prolonged application of ATP (100 microM; 10 s) induced receptor desensitization in a significant proportion of sympathetic neurons in a manner typical for P2X(2-2) splice variant-mediated responses. Using single-cell RT-PCR, P2X(2), P2X(2-2) and P2X(5) mRNA expression was detectable in individual tyrosine hydroxylase-positive neurons; coexpression of both P2X(2) isoforms was not observed. Laser scanning microscopy revealed both P2X(2) and P2X(5) immunoreactivity in virtually every TH-positive neuron. P2X(2) immunoreactivity was largely distributed over the cell body, whereas P2X(5) immunoreactivity was most distinctly located close to the nucleus. In summary, the present study demonstrates the expression of P2X(2), P2X(2-2) and P2X(5) receptor subunits in rat thoracolumbar neurons. The functional data in conjunction with a preferential membranous localization of P2X(2)/P2X(2-2) compared with P2X(5) suggest that the excitatory P2X responses are mediated by P2X(2) and P2X(2-2) receptors. Apparently there exist two types of P2X(2) receptor-bearing sympathetic neurons: one major population expressing the unspliced isoform and another minor population expressing the P2X(2-2) splice variant.

Independent receptors for diadenosine pentaphosphate and ATP in rat midbrain single synaptic terminals

Diadenosine pentaphosphate (Ap5A) and adenosine 5'-triphosphate (ATP) stimulate a intrasynaptosomal calcium concentration [Ca(2+)](i) increase via specific purinergic receptors in rat midbrain synaptosomes, although nothing is known about their distribution in presynaptic terminals. A microfluorimetric technique to measure [Ca(2+)](i) increase using the dye FURA-2AM, has permitted study of the presence of dinucleotide and P2X receptors in independent isolated synaptic terminals. Our results demonstrate the existence of three populations of synaptosomes: one with dinucleotide receptors (12%), another with P2X receptors (20%) and a third with both (14%). It has been possible to demonstrate that the activation of these receptors occurs only in the presence of extracellular Ca(2+) and that it is also coupled with voltage-dependent Ca(2+) channels. Finally 54% of the synaptosomes that responded to K(+) did not present any calcium increase mediated by the nucleotides used. In summary, ATP and dinucleotides exhibit specific ionotropic receptors that can coexist or not on the same synaptic terminal.

UTP evokes noradrenaline release from rat sympathetic neurons by activation of protein kinase C

The pathway involved in UTP-evoked noradrenaline release was investigated in cultures of rat superior cervical ganglia. Northern blots revealed an age-related increase in levels of mRNA for P2Y6 receptors in cultures obtained at postnatal days 1 and 5, respectively, but no change in transcripts for P2Y1 and P2Y2. Likewise, UTP-evoked overflow of previously incorporated [(3)H]noradrenaline was six-fold higher in neurons obtained at postanatal day 5. Various protein kinase C inhibitors diminished UTP-, but not electrically, induced tritium overflow by > 70%, as did down-regulation of protein kinase C by 24 h exposure to phorbol ester. beta-Phorbol-12,13-dibutyrate and dioctanoylglycerol caused concentration-dependent increases in [(3)H] outflow of up to 6% of total radioactivity, and the secretagogue actions of these agents were reduced in the presence of protein kinase C inhibitors and in neurons pretreated with phorbol ester. Overflow evoked by dioctanoylglycerol was attenuated in the absence of extracellular Ca(2+) and in the presence of tetrodotoxin or Cd(2+). In addition to triggering tritium overflow, UTP reduced currents through muscarinic K(+) channels which, however, were not affected by phorbol esters. This action of UTP was not altered by protein kinase C inhibitors. These results indicate that P2Y6 receptors mediate UTP-evoked noradrenaline release from rat sympathetic neurons via activation of protein kinase C, but not inhibition of K(M) channels.

Effects of purinergic and adrenergic antagonists in a rat model of painful peripheral neuropathy

In previous studies, pain behaviors produced in the spinal nerve ligation rat model of neuropathic pain were partly reduced by surgical lumbar sympathectomy. However, systemic injection of phentolamine, an alpha-adrenoceptor blocker, was not effective in reducing pain behaviors, at least in the Sprague-Dawley strain of rats. This suggests that sympathectomy removes not only adrenoceptor function but also other factors that must contribute importantly to the generation of neuropathic pain behaviors. Since the purinergic substance adenosine 5'-triphosphate (ATP) is known to be co-released with norepinephrine (NE) from the sympathetic nerve terminals, we hypothesized that ATP might be involved in the sympathetic dependency of neuropathic pain. The present study tested this hypothesis by examining the effects of systemic injection of an adrenoceptor blocker (phentolamine), a purinoceptor blocker (suramin), and a combination of these two on behavioral signs of mechanical allodynia in the spinal nerve ligation model of neuropathic pain. The results of the present study showed two novel findings. First, the mechanical hypersensitivity (allodynia) resulting from the L5/6 spinal nerve ligation can be reduced either by sympathetic block accomplished by application of a local anesthetic or by surgical sympathectomy of the L2-L6 sympathetic ganglia. Second, suramin (at 100 mg/kg, i.p.) can reduce mechanical hypersensitivity in neuropathic rats when given in combination with 5 mg/kg of phentolamine. This effect was observed in a subset of neuropathic rats, and the drug responses were consistent in repeated treatments within the animal group. Neither phentolamine nor suramin changed the mechanical sensitivity of neuropathic rats when given alone. The data suggest that the purinergic substance ATP is co-released with NE from sympathetic nerve terminals and these two are together involved, at least in part, in the maintenance of the sympathetically dependent component of pain behaviors in some neuropathic rats.

P2 receptors in the central and peripheral nervous systems modulating sympathetic vasomotor tone

Arterial pressure depends on the level of activity of sympathetic vasoconstrictor outflow to blood vessels. This activity is generated in the central nervous system, and involves inputs from a variety of brain regions projecting to sympathetic preganglionic neurones. Of especial interest are a group of neurones in the rostral ventrolateral medulla (RVLM), as they have been demonstrated to have a fundamental role in reflex regulation of the cardiovascular system, and in generation of tonic drive to sympathetic outflow. Sympathetic outflow to blood vessels is additionally modulated at sympathetic ganglia, and at the peripheral terminals of sympathetic nerves. This review considers the role of P2 purine receptors in this neural pathway. Ionotropic P2X receptors are expressed in the RVLM, in sympathetic ganglia, and at the sympathetic neuromuscular junction, and mediate fast excitatory neurotransmission, indicating a general role for ATP as a regulator of sympathetic vasomotor tone. P2Y receptors couple to G proteins and mediate slower signalling to ATP; they have been reported to inhibit prejunctionally neurotransmission at the peripheral terminals of sympathetic nerves, but little is known about their possible role in the central nervous system and in sympathetic ganglia.

Noradrenaline release from cultured mouse postganglionic sympathetic neurons: autoreceptor-mediated modulation

The possible existence of alpha2-autoreceptors, P2-autoreceptors, and adenosine A1- or A2A-receptors was studied in cultured thoracolumbar postganglionic sympathetic neurons from mice. The cells were preincubated with [3H]noradrenaline and then superfused. The selective alpha2-adrenoceptor agonist UK 14,304 reduced the electrically evoked overflow of tritium. When the cultures were stimulated by trains of increasing pulse number, ranging from a single pulse to 72 pulses at 3 Hz, the concentration-inhibition curve of UK 14,304 was shifted progressively to the right and the maximal inhibition obtainable became progressively smaller. Six alpha-adrenoceptor antagonists shifted the concentration-inhibition curve of UK 14,304 in a parallel manner to the right. Neither ATP (3-300 microM), adenosine (0.01-100 microM), the selective A1-receptor agonist cyclopentyladenosine (1-1,000 nM), nor the selective A2A-receptor agonist CGS-21680 (1-10,000 nM) changed the basal or the electrically evoked overflow of tritium. It is concluded that the cultured neurons possess presynaptic, release-inhibiting alpha2-autoreceptors. As in intact tissues, the effectiveness of presynaptic alpha2-adrenergic inhibition depends on the "strength" of the releasing stimulus. The pK(D) values of the six antagonists against UK 14,304 indicate that the autoreceptors belong to the pharmacological alpha2D and hence the genetic alpha(2A/D) subtype of alpha2-adrenoceptor. Neither P2-autoreceptors nor receptors for adenosine, the degradation product of ATP, were detected.

Ameliorative effect of adenosine on hypoxia-reoxygenation injury in LLC-PK1, a porcine kidney cell line

We studied the effects of adenosine on injury caused by hypoxia and reoxygenation in LLC-PK1 cells. Lactate dehydrogenase and gamma-glutamyltranspeptidase were released from cells exposed to hypoxia for 6 hr and then reoxygenation for 1 hr. The addition of adenosine at 100 microM to the medium before hypoxia began significantly decreased enzyme leakage into medium during both hypoxia and reoxygenation. The adenosine A1-receptor agonist, R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA), at the concentration of 100 microM, did not affect enzyme release, but the adenosine A2-receptor agonist 2-p-[2-car-boxyethyl]phenethyl-amino-5'-N-ethylcarboxamido-adenosi ne hydrochloride (CGS 21680) at the concentration of 100 nM, suppressed the injury caused by hypoxia and reoxygenation. There were decreases in cAMP contents and ATP levels in LLC-PK1 cells injured by hypoxia and reoxygenation. Adenosine (100 microM) restored ATP levels in the cells during reoxygenation. With adenosine, the intracellular cAMP level was increased prominently during reoxygenation. These results suggest that adenosine protects LLC-PK1 cells from injury caused by hypoxia and reoxygenation by increasing the intracellular cAMP level via adenosine A2 receptor.

Modulation of spontaneous and stimulation-evoked transmitter release from rat sympathetic neurons by the cognition enhancer linopirdine: insights into its mechanisms of action

The mechanisms by which the cognition enhancer linopirdine may affect transmitter release were investigated in cultures of rat superior cervical ganglion neurons. Overflow of previously incorporated [3H]noradrenaline evoked by 10 microM UTP or 0.1 microM bradykinin was enhanced by linopirdine at > or =3 microM, overflow evoked by 25 mM K(-), 100 microM nicotine, or 300 microM ATP was enhanced by linopirdine at > or =10 microM, and overflow due to 40 mM K+ or electrical field stimulation was not altered by linopirdine. Ba2+ (0.3 mM) augmented the same types of stimulation-evoked overflow to a similar extent as linopirdine. K+ (25 mM), nicotine (100 microM), and ATP (300 microM) triggered transmitter release in a partially tetrodotoxin-resistant manner, and the release-enhancing action of linopirdine was lost in the presence of tetrodotoxin (1 microM). Linopirdine (10 microM) raised spontaneous tritium outflow and reduced currents through muscarinic K+ (K(M)) channels with a similar time course. The secretagogue action of linopirdine was concentration- and Ca2(+)-dependent and abolished by tetrodotoxin (1 microM) or Cd2+ (100 microM). Linopirdine (10 microM) added to the partial inhibition of K(M) channels by 1 or 3 mM Ba2(+) but not to the complete inhibition by 10 mM Ba2(+). Likewise, the secretagogue action of 1 and 3 mM, but not that of 10 mM, Ba2+ was enhanced by linopirdine. These results indicate that linopirdine facilitates and triggers transmitter release via blockade of K(M) channels and suggest that these K+ channels are located at neuronal somata rather than at presynaptic sites.

ATP stimulates sympathetic transmitter release via presynaptic P2X purinoceptors

ATP is a fast transmitter in sympathetic ganglia and at the sympathoeffector junction. In primary cultures of dissociated rat superior cervical ganglion neurons, ATP elicits noradrenaline release in an entirely Ca2+-dependent manner. Nevertheless, ATP-evoked noradrenaline release was only partially reduced (by approximately 50%) when either Na+ or Ca2+ channels were blocked, which indicates that ATP receptors themselves mediated transmembrane Ca2+ entry. An "axonal" preparation was obtained by removing ganglia from explant cultures, which left a network of neurites behind; immunostaining for axonal and dendritic markers revealed that all of these neurites were axons. In this preparation, ATP raised intraaxonal Ca2+ and triggered noradrenaline release, and these actions were not altered when Ca2+ channels were blocked by Cd2+. Hence, Ca2+-permeable ATP-gated ion channels, i.e., P2X purinoceptors, are located at presynaptic sites and directly mediate Ca2+-dependent transmitter release. These presynaptic P2X receptors displayed a rank order of agonist potency of ATP >/= 2-methylthio-ATP > ATPgammaS >> alpha,beta-methylene-ATP approximately beta,gamma-methylene-L-ATP and were blocked by suramin or PPADS. ATP, 2-methylthio-ATP, and ATPgammaS also evoked inward currents measured at neuronal somata, but there these agonists were equipotent. Hence, presynaptic P2X receptors resemble the cloned P2X2 subtype, but they appear to differ from somatodendritic P2X receptors in terms of agonist sensitivity. Suramin reduced depolarization-evoked noradrenaline release by up to 20%, when autoinhibitory mechanisms were inactivated by pertussis toxin. These results indicate that presynaptic P2X purinoceptors mediate a positive, whereas G-protein-coupled P2Y purinoceptors mediate a negative, feedback modulation of sympathetic transmitter release.

Selective inhibition of M-type potassium channels in rat sympathetic neurons by uridine nucleotide preferring receptors

1. UTP and UDP depolarize rat superior cervical ganglion neurons and trigger noradrenaline release from these cells. The present study investigated the mechanisms underlying this excitatory action of uridine nucleotides by measuring whole-cell voltage-dependent K+ and Ca2+ currents. 2. Steady-state outward (holding) currents measured in the amphotericin B perforated-patch configuration at a potential of -30 mV were reduced by 10 microM UTP in a reversible manner, but steady-state inward (holding) currents at -70 mV were not affected. This action of UTP was shared by the muscarinic agonist oxotremorine-M. In current-voltage curves between -20 and -100 mV, UTP diminished primarily the outwardly rectifying current components arising at potentials positive to -60 mV. 3. Slow relaxations of muscarinic K+ currents (IM) evoked by hyperpolarizations from -30 to -55 mV were also reduced by 10 microM UTP (37% inhibition) and oxotremorine-M (81% inhibition). In contrast, transient K+-currents, delayed rectifier currents, fast and slow Ca2+-dependent K+ currents, as well as voltage-dependent Ca2+ currents were not altered by UTP. 4. In conventional (open-tip) whole-cell recordings, replacement of GTP in the pipette by GDPbetaS abolished the UTP-induced inhibition of IM, whereas replacement by GTPgammaS rendered it irreversible. 5. The UTP-induced reduction of IM was half maximal at 1.5 microM with a maximum of 37% inhibition; UDP was equipotent and equieffective, while ADP was less potent (half maximal inhibition at 29 microM). ATP had no effect at < or = 30 microM. 6. The inhibition of IM induced by 10 microM UTP was antagonized by pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) at > or = 30 microM and by reactive blue 2 at > or = 10 microM, but not by suramin at concentrations up to 30 microM. 7. These results show that rat superior cervical ganglion neurons possess uridine nucleotide preferring P2Y receptors which inhibit KM channels. This effect presumably forms the basis of the excitatory action of uridine nucleotides in rat sympathetic neurons.

Noradrenaline release from rat sympathetic neurones triggered by activation of B2 bradykinin receptors

1. The role of bradykinin receptors in the regulation of sympathetic transmitter release was investigated in primary cultures of neurones dissociated from superior cervical ganglia of neonatal rats. These cultures were loaded with [3H]-noradrenaline and the outflow of radioactivity was determined under continuous superfusion. 2. Bradykinin (100 nmol l[-1] applied for 10 min) caused a transient increase in tritium outflow that reached a peak within four minutes after the beginning of the application and then declined towards the baseline, despite the continuing presence of the peptide. ATP (100 micromol l[-1]) and nicotine (10 micromol l[-1]) caused elevations in 3H outflow with similar kinetics, whereas outflow remained elevated during a 10 min period of electrical field stimulation (0.5 ms, 50 mA, 50 V cm[-1], 1.0 Hz). 3. When bradykinin was applied for periods of 2 min, the evoked 3H overflow was half-maximal at 12 nmol l(-1) and reached a maximum of 2.3% of cellular radioactivity. The preferential B1 receptor agonist des-Arg9-bradykinin failed to alter 3H outflow. The B2 receptor antagonists, [D-Phe7]-bradykinin (1 micromol l[-1]) and Hoe 140 (10 nmol l[-1]), per se did not alter 3H outflow, but shifted the concentration-response curve for bradykinin-evoked 3H overflow to the right by a factor of 7.9 and 4.3, respectively. 4. Bradykinin-induced overflow was abolished in the absence of extracellular Ca2+ and in the presence of either 1 micromol l(-1) tetrodotoxin or 300 micromol l(-1) Cd2+, as was electrically-induced overflow. Activation of alpha2-adrenoceptors by 1 micromol l(-1) UK 14,304 reduced both bradykinin- and electrically-triggered overflow. The Ca2+-ATPase inhibitor thapsigargin (0.3 micromol l[-1]) failed to alter either type of stimulated overflow. Caffeine (10 mmol l[-1]) enhanced bradykinin-induced overflow, but reduced overflow triggered by electrical field stimulation. 5. Inclusion of Ba2+ (0.1 to 1 mmol l[-1]) in the superfusion medium enhanced electrically induced overflow by approximately 100% and potentiated bradykinin-triggered overflow by almost 400%. Application of 1 mmol l(-1) Ba2+ for periods of 2 min triggered 3H overflow, and this overflow was abolished by 1 micromol l(-1) tetrodotoxin and enhanced by 10 mmol l(-1) caffeine. In contrast, inclusion of tetraethylammonium (0.1 to 1 mmol l[-1]) in the superfusion buffer caused similar increases of bradykinin- and electrically evoked 3H overflow (by about 100%), and tetraethylammonium, when applied for 2 min, failed to alter 3H outflow. 6. Treatment of cultures with 100 ng ml(-1) pertussis toxin caused a significant increase in bradykinin-, but not in electrically-, evoked tritium overflow. Treatment with 100 ng ml(-1) cholera toxin reduced both types of stimulated 3H overflow. 7. These data reveal bradykinin as a potent stimulant of action potential-mediated and Ca2+-dependent transmitter release from rat sympathetic neurones in primary cell culture. This neurosecretory effect of bradykinin involves activation of B2-receptors, presumably linked to pertussis- and cholera toxin-insensitive G proteins, most likely members of the Gq family. Results obtained with inhibitors of muscarinic K+ (KM) channels, like caffeine and Ba2+, indicate that the secretagogue action of bradykinin probably involves inhibition of these K+ channels.

Receptors controlling transmitter release from sympathetic neurons in vitro

Primary cultures of postganglionic sympathetic neurons were established more than 30 years ago. More recently, these cultures have been used to characterize various neurotransmitter receptors that govern sympathetic transmitter release. These receptors may be categorized into at least three groups: (1) receptors which evoke transmitter release: (2) receptors which facilitate; (3) receptors which inhibit, depolarization-evoked release. Group (1) comprises nicotinic and muscarinic acetylcholine receptors, P2X purinoceptors and pyrimidinoceptors. Group (2) currently harbours beta-adrenoceptors, P2 purinoceptors, receptors for PACAP and VIP, as well as prostanoid EP1 receptors. In group (3), muscarinic cholinoceptors, alpha 2- and beta-adrenoceptors, P2 purinoceptors, and receptors for the neuropeptides NPY, somatostatin (SRIF1) and LHRH, as well as opioid (delta and kappa) receptors can be found. Receptors which regulate transmitter release from neurons in cell culture may be located either at the somatodendritic region or at the sites of exocytosis, i.e. the presynaptic specializations of axons. Most of the receptors that evoke release are located at the soma. There ionotropic receptors cause depolarizations to generate action potentials which then trigger Ca(2+)-dependent exocytosis at axon terminals. The signalling mechanisms of metabotropic receptors which evoke release still remain to be identified. Receptors which facilitate depolarization-evoked release appear to be located preferentially at presynaptic sites and presumably act via an increase in cyclic AMP. Receptors which inhibit stimulation evoked release are also presynaptic origin and most commonly rely on a G protein-mediated blockade of voltage-gated Ca2+ channels. Results obtained with primary cell cultures of postganglionic sympathetic neurons have now supplemented previous data about neurotransmitter receptors involved in the regulation of ganglionic as well as sympatho-effector transmission. In the future, this technique may prove useful to identify yet unrecognized receptors which control the output of the sympathetic nervous system and to elucidate underlying signalling mechanisms.

P2-purinoceptors on postganglionic sympathetic neurones

1. Postganglionic sympathetic neurones possess both excitatory and inhibitory P2-purinoceptors. 2. The mechanisms of action of excitatory P2-purinoceptors have recently been studied on cultured sympathetic neurones of the rat. The receptors mediate fast increases in intracellular Ca2+ levels and a release of noradrenaline. They are likely to belong to the neuronal types of P2X-purinoceptors and to be located on the sympathetic nerve cell bodies or their dendrites. 3. Inhibitory P2-purinoceptors have been shown to operate at sympathetic axon terminals in isolated tissues. Adenine nucleotides decreased the stimulation-evoked release of noradrenaline by activation of these receptors. The receptors are likely to belong to the group of G-protein-coupled P2Y-purinoceptors. They mediate a negative feedback in which co-transmitter ATP inhibits subsequent sympathetic transmitter release.

Pharmacological characterization of P2 purinoceptor types in rat locus coeruleus neurons

The frequency of spontaneous action potentials of locus coeruleus neurons was recorded extracellularly in pontine slices of the rat brain. The adenosine 5'-triphosphate (ATP) analogues alpha,beta-methylene ATP (alpha,beta-meATP) and 2-methylthio ATP increased the firing rate with a similar potency, while uridine 5'-triphosphate (UTP) was inactive. Diadenosine 5'-pentaphosphate (Ap5A), diadenosine 5'-tetraphosphate (Ap4A) and diadenosine 5'-triphosphate (Ap3A) all facilitated the firing. When equimolar concentrations were compared, Ap5A had the largest effect followed by Ap4A and Ap3A. Suramin markedly inhibited responses to alpha,beta-meATP and 2-methylthio ATP; the effect of Ap4A was only slightly depressed by suramin. Pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS) strongly antagonized alpha, beta-meATP, but failed to alter the effects of 2-methylthio ATP and Ap4A. Reactive blue 2 weakly antagonized alpha,beta-meATP and did not interfere with 2-methylthio ATP and Ap4A. Moreover, suramin depressed responses to (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA), but not to substance P. PPADS failed to affect the AMPA- and NMDA-induced increases in firing. Hence, locus coeruleus neurons may possess receptors for adenosine nucleotides (P2X and P2Y purinoceptors) and dinucleotides (P2D purinoceptors); receptors for uridine nucleotides (P2U purinoceptors or pyrimidinoceptors) are probably absent.

A somatostatin receptor inhibits noradrenaline release from chick sympathetic neurons through pertussis toxin-sensitive mechanisms: comparison with the action of alpha 2-adrenoceptors

The effects of somatostatin and analogues were investigated in cultures of chick sympathetic neurons. Electrically evoked tritium overflow from cultures labelled with [3H]noradrenaline was reduced by somatostatin-14 in a concentration-dependent manner, with half maximal effects at 0.3 nM and a maximum of 45% inhibition. Somatostatin-28 was equipotent to somatostatin-14 (half maximal concentration at 0.5 nM), and seglitide was less potent, the effects being half maximal at 4.2 nM. The inhibitory action of somatostatin-14 on stimulation-evoked overflow desensitized within minutes at 100 nM, but not at 10 nM, and was abolished by a pretreatment of neurons with pertussis toxin. All somatostatin analogues reduced voltage-activated Ca2+ currents recorded in the whole-cell configuration of the patch-clamp technique, with somatostatin-14 being equipotent to somatostatin-28, but more potent than seglitide. However, the inhibition of Ca2+ currents occurred at concentrations more than ten-fold higher than those required for the reduction of stimulation evoked 3H overflow. The action of somatostatin upon Ca2+ currents was also abolished by pertussis toxin and desensitized within minutes. In preceding experiments, alpha 2-adrenoceptor activation had been found to reduce transmitter release and Ca2+ currents of chick sympathetic neurons through a pertussis toxin-sensitive mechanism. In the present study, the alpha 2-adrenergic agonist UK 14,304 completely occluded the inhibition of Ca2+ currents and of electrically evoked overflow by somatostatin-14. Neither UK 14,304 nor somatostatin affected the resting membrane potential or voltage-dependent K+ currents. These results demonstrate that chick sympathetic neurons possess SRIF1 type somatostatin receptors which control transmitter release. This effect is mediated by pertussis toxin-sensitive GTP binding proteins and apparently involves an inhibition of voltage-activated Ca2+ channels, but not a modulation of K+ channels. Since alpha 2-adrenergic agonists share all of these actions and occlude the effects of somatostatin, alpha 2-adrenoceptors and SRIF1 receptors seem to regulate sympathetic transmitter release via common signalling mechanisms.

UTP- and ATP-triggered transmitter release from rat sympathetic neurones via separate receptors

In rat cultured sympathetic neurones, UDP, UTP and ATP at micromolar concentrations triggered Ca(2+)-dependent and tetrodotoxin-sensitive [3H]-noradrenaline release. The overflow evoked by UTP or ATP was similar at 100 mumol l-1, the concentration used in all subsequent experiments. Pre-exposure of the neurones to 100 mumol l-1 UTP significantly reduced ensuing secretory effects of UTP but not of ATP. Conversely, pre-exposure to ATP diminished the overflow due to ATP but not that due to UTP. In the presence of 10 mumol l-1 pyridoxal-5'-phosphate or 30 mumol l-1 suramin, the secretory response to ATP was reduced, but the effect of UTP was unaltered. Zn2+ (10 mumol l-1) reduced the overflow triggered by UTP, but increased the overflow due to ATP. These results indicate the presence of separate receptors for pyrimidine nucleotides and for purine nucleotides which both trigger transmitter release.

alpha 2-Adrenoreceptor-mediated inhibition of acetylcholine-induced noradrenaline release from rat sympathetic neurons: an action at voltage-gated Ca2+ channels

[3H]Noradrenaline release was studied in cultured sympathetic neurons derived from superior cervical ganglia of neonatal rats. Acetylcholine elicited a concentration- and time-dependent increase in 3H outflow which was half-maximal at about 300 microM and within 5 s. The overflow induced by 10 s exposure to 300 micro A acetylcholine was reduced by the nicotinic antagonist hexamethonium, but increased by the muscarinic antagonist atropine. Cd2+ (300 microM) prevented the overflow evoked by electrical field stimulation, but reduced acetylcholine-induced overflow by less than 50%. Removal of extracellular Ca2+ abolished stimulation-evoked tritium overflow irrespective of the stimulus. The selective alpha2-adrenoceptor agonist UK 14,304 inhibited acetylcholine-evoked overflow to a significantly smaller extent (approximately 25% maximal inhibition) than electrically induced overflow ( > or = 45% maximal inhibition). These inhibitory effects were antagonized by the alpha2-adrenoceptor antagonist yohimbine. Noradrenaline (0.1 microM) reduced acetylcholine-evoked overflow to the same extent as did UK 14,304 (0.1 microM). UK 14,304 had no effect when 3H overflow was evoked by acetylcholine in the presence of 300 microM Cd2+. Currents through nicotinic acetylcholine receptors and voltage-activated Ca2+ currents were studied with the whole-cell variant of teh patch-clamp technique. UK 14,304 reduced nicotinic acetylcholine receptor currents and voltage-activated Ca2+ currents with similar potency and efficacy. Yohimbine, however, antagonized only the inhibition of voltage-activated Ca2+ currents, but not the effects of UK 14,304 on nicotinic receptor currents. Furthermore, yohimbine per se reduced currents through nicotinic receptors. Noradrenaline (10 microM) inhibited voltage-dependent Ca2+ currents just as did UK 14,304 (10 microM), but failed to reduce currents through nicotinic acetylcholine receptor channels. Cd2+ (300 microM) abolished voltage-activated Ca2+ currents and reduced nicotinic acetylcholine receptor currents by 65%. These results indicate that acetylcholine evokes noradrenaline release from rat sympathetic neurons by activation of nicotinic receptors and restricts this release via muscarinic receptors. The acetylcholine-induced transmitter release is based on two mechanisms, one involving and the other one bypassing voltage-dependent Ca2+ channels. alpha2-Adrenoceptor activation reduces voltage-activated Ca2+ currents and effects exclusively the component of acetylcholine-induced release which involves voltage-dependent Ca2+ channels. These results support the hypothesis that voltage-activated Ca2+ channels are the sole site of autoinhibitory alpha2-adrenergic effects on transmitter release from rat sympathetic neurons. The inhibitory effects of alpha2-adrenoceptor agonists and antagonists on currents through nicotinic acetylcholine receptors are not mediated by an alpha2-adrenoceptor.

Cultured chick sympathetic neurons: ATP-induced noradrenaline release and its blockade by nicotinic receptor antagonists

The ATP-induced increase in tritium outflow from cultured chick sympathetic neurons prelabelled with [3H]-noradrenaline was investigated. Seven days-old dissociated cell cultures of embryonic paravertebral ganglia, loaded with [3H]-noradrenaline (0.05 microM), were superfused in the presence of (+)-oxaprotiline and exposed to ATP, ATP-analogues, or 1,1-dimethyl-4-piperazinium (DMPP) for 2 min. ATP (3 microM-3 mM), 2-methylthio-ATP (3-100 microM), as well as DMPP (10 and 100 microM) induced a significant overflow of tritium. The EC50-value of ATP was 20 microM. Both the ATP-induced and the DMPP-induced tritium overflow was Ca(2+)-dependent and sensitive to tetrodotoxin (0.3 microM) and omega-conotoxin (0.1 microM); in addition, it was inhibited by the alpha 2-adrenoceptor agonist 5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline (UK-14,304; 1 microM). The effects of ATP and DMPP were not additive. The ATP-induced as well as the DMPP-induced overflow of tritium was diminished by the P2-purinoceptor antagonists suramin (300 microM) and reactive blue 2 (3 microM); in all 4 cases, the inhibition amouted to approximately 40%. The tritium overflow induced by ATP or DMPP was almost abolished by the nicotinic receptor antagonist mecamylamine (10 microM) and markedly inhibited by hexamethonium (100 microM). Neither ATP nor electrical stimulation caused an overflow of tritium from cultures loaded with [3H]-choline. The results suggest that ATP at mumolar concentrations induces noradrenaline release from cultured chick sympathetic neurons via an action on a subclass of the nicotinic cholinoceptor.

Rapid, agonist-induced desensitization of alpha 2-autoreceptors modulating transmitter release

1. The release of previously incorporated [3H]-noradrenaline was investigated in cultures of dissociated chick or rat sympathetic neurones and in cerebrocortical slices from neonatal or adult rats. Noradrenaline, in the presence of 10 mumol l-1 of the uptake inhibitor, cocaine, or the selective alpha 2-adrenoceptor agonist, 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 4,304), was applied for different periods of time in order to detect a possible time-dependence of the alpha 2-adrenoceptor-mediated inhibition of electrically evoked tritium outflow. 2. In chick sympathetic neurones, stimulation-evoked overflow was reduced to 30%, 42%, or 56% of control when noradrenaline (1 mumol l-1) was present for 2, 8, or 16 min, respectively. Likewise, UK 14,304 (1 mumol l-1) present for these periods of time reduced 3H overflow to 35%, 51%, and 53% of control, respectively. Addition of 1 nmol l-1 to 10 mumol l-1 UK 14,304 for either 2 or 16 min did not produce significantly different IC50 values, but the inhibitory effects were smaller with 16 min as compared to 2 min exposure at concentrations > or = 10 nmol l-1. 3. In rat sympathetic neurones, noradrenaline (100 nmol l-1) reduced stimulation-evoked overflow to 33%, 56%, or 57% of control, when present for 2, 8, or 16 min, respectively. Addition of UK 14,304 (1 mumol l-1) for these periods of time caused inhibition to 11%, 41%, and 46% of control. Applying UK14,304 for either 2 or 16 min did not result in significantly different IC5o values, but the inhibition induced by 16 min as compared to 2 min exposure was smaller at concentrations > 10 nmol 1-1.4. In cerebrocortical slices from either neonatal or adult rats, exposure to 0.1 to 1.0 micromol 1-1 UK14,304 for 16 min never caused a smaller inhibition than a corresponding 3 min exposure, although various experimental conditions were investigated.5 The results demonstrate that alpha 2-adrenoceptors which regulate noradrenaline release from sympathetic neurones undergo agonist-induced desensitization within minutes. Such rapid desensitization of alpha 2-autoreceptors was not detected in brain slice preparations.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.