Excitatory effects of adenosine 5'-triphosphate on rat locus coeruleus neurones

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

Potentiation of the glutamatergic synaptic input to rat locus coeruleus neurons by P2X7 receptors

Locus coeruleus (LC) neurons in a rat brain slice preparation were superfused with a Mg(2+)-free and bicuculline-containing external medium. Under these conditions, glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) were recorded by means of the whole-cell patch-clamp method. ATP, as well as its structural analogue 2-methylthio ATP (2-MeSATP), both caused transient inward currents, which were outlasted by an increase in the frequency but not the amplitude of the sEPSCs. PPADS, but not suramin or reactive blue 2 counteracted both effects of 2-MeSATP. By contrast, α,β-methylene ATP (α,β-meATP), UTP and BzATP did not cause an inward current response. Of these latter agonists, only BzATP slightly facilitated the sEPSC amplitude and strongly potentiated its frequency. PPADS and Brilliant Blue G, as well as fluorocitric acid and aminoadipic acid prevented the activity of BzATP. Furthermore, BzATP caused a similar facilitation of the miniature (m)EPSC (recorded in the presence of tetrodotoxin) and sEPSC frequencies (recorded in its absence). Eventually, capsaicin augmented the frequency of the sEPSCs in a capsazepine-, but not PPADS-antagonizable, manner. In conclusion, the stimulation of astrocytic P2X7 receptors appears to lead to the outflow of a signalling molecule, which presynaptically increases the spontaneous release of glutamate onto LC neurons from their afferent fibre tracts. It is suggested, that the two algogenic compounds ATP and capsaicin utilise separate receptor systems to potentiate the release of glutamate and in consequence to increase the excitability of LC neurons.

The diadenosine polyphosphate receptors: P2D purinoceptors

Diadenosine polyphosphates-Ap4A, Ap5A and Ap6A-are co-stored in neurosecretory vesicles together with ATP and aminergic compounds. They are released from neural cells and synaptic terminals in a Ca(2+)-dependent process. Ligand binding and displacement experiments carried out with [3H]Ap4A on isolated chromaffin cells and synaptosomal preparations result in curvilinear Scatchard plots with Kd values close to 0.1 nM for the high-affinity binding sites. Displacement curves with two steps are obtained for homologous and heterologous nucleotide ligands; the lowest-affinity step exhibits Ki values in the micromolar range for ApnA compounds. The high-affinity binding sites were named P2D purinoceptors on the basis of their binding characteristics. Single-cell studies in neurochromaffin cells indicate the presence of P2X purinoceptors in noradrenergic cells that do not respond to Ap4A and in which noradrenaline secretion can be induced by influx of extracellular Ca2+. P2Y receptors that respond to ATP analogues and ApnAs are present in endothelial cells from adrenal medulla. Those cells that express P2U purinoceptors are unresponsive to ApnAs. Ectodiadenosine polyphosphate hydrolases with Km values of 0.3 to 2 microM are present in both neural and endothelial cells from adrenal medulla. In midbrain synaptic terminals diadenosine polyphosphates induce Ca2+ entry from the extracellular medium. The fact that the synaptic response is not cross-desensitized by ATP and its non-hydrolysable analogues, the non-blocking effect of suramin, and the differential effect of Ca2+ channel blockers, together suggest that there are different receptors for nucleotides and dinucleotides in rat brain synaptosomes, which we have called P4 purinoceptors on the basis of functional studies.

P2 purinoceptors and pyrimidinoceptors of catecholamine-producing cells and immunocytes

ATP is a neuronal (co)transmitter. In addition, both ATP and UTP may exit damaged cells and thereby function as extracellular signal molecules. The targets of signalling may be the P2 (for ATP and UTP) and P1 (for the degradation product adenosine) receptors of, for instance, neurons and immunocytes. UTP may also act at separate pyrimidinoceptors. Catecholamine-producing cells (adrenal chromaffin cells and peripheral and central noradrenergic neurons) possess P2X and P2Y purinoceptors. ATP appears to be a fast excitatory neuro-neuronal transmitter of the noradrenergic coeliac and locus coeruleus neurons. This effect is mediated by P2X purinoceptors. P2Y purinoceptor-mediated slow excitatory synaptic potentials have not yet been demonstrated either in the peripheral or central nervous system. On the other hand, after neuronal injury microglial cells (brain immunocytes) are engaged in a process called 'synaptic stripping', i.e. the displacement of synaptic boutons from the neuronal surface. During this process microglial cells are in direct contact with the (co)transmitter ATP. Activation of P2X, P2Z and P2Y purinoceptors results in an elevated intracellular Ca2+ concentration in microglia and macrophages. Various functions of these cells are regulated by intracellular Ca2+ (e.g. cytokine production, phagocytosis) and may therefore be modulated by nucleotides. Since neuronal damage leads to the transformation of microglial cells to macrophages and, at the same time, to the efflux of nucleotides from the damaged cells, the requirements for a modulatory interaction are fulfilled.

Neuroprotective effects of the P2 receptor antagonist PPADS on focal cerebral ischaemia-induced injury in rats

After acute injury of the central nervous system extracellular adenosine 5'-triphosphate (ATP) can reach high concentrations as a result of cell damage and subsequent increase in membrane permeability. Released ATP may act as a toxic agent, which causes cellular degeneration and death, mediated through P2X and P2Y receptors. Mechanisms underlying the various effects of purinoceptor modulators in models of cerebral damage are still uncertain. In the present study the effect of P2 receptor inhibition after permanent middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats was investigated. Rats received either the non-selective P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) or artificial cerebrospinal fluid (ACSF) as control by the intracerebroventricular route. First, these treatments were administered 10 min before MCAO and subsequently twice daily for 1 or 7 days after MCAO. The functional recovery of motor and cognitive deficits was tested at an elevated T-labyrinth. The PPADS-treated group showed a significant reduction of paresis-induced sideslips compared with ACSF-treated animals. Infarct volume was reduced in the PPADS group in comparison with the ACSF group. A significant decrease in intermediately and profoundly injured cells in favour of intact cells in the PPADS group was revealed by quantification of celestine blue/acid fuchsin-stained cells in the peri-infarct area. The data provide further evidence for the involvement of P2 receptors in the pathophysiology of cerebral ischaemia in vivo. The inhibition of P2 receptors at least partially reduces functional and morphological deficits after an acute cerebral ischaemic event.

Purinergic modulation of cardiovascular function in the rat locus coeruleus

1 The purpose of the present study was to determine whether purines exerted a physiological role in central cardiovascular modulation at the level of the locus coeruleus (LC). 2 In pentobarbitone-anaesthetised Wistar-Kyoto rats, unilateral microinjection of ATP or alpha,beta-methyleneATP into the LC elicited dose-related decreases in blood pressure and heart rate. Unilateral microinjection of the P2 purinoceptor antagonists suramin and PPADS, caused pressor and tachycardic responses. Administration of the selective P2X(1) receptor antagonist NF-279 had no effect. While both ATP and L-glutamate (L-GLU) resulted in depressor responses after intra-LC microinjection, following intra-LC microinjection of P2 purinoceptor antagonists into the LC, the effects of subsequent administration of either ATP or L-GLU were functionally reversed, such that a pressor response ensued. 3 Microinjection of noradrenaline into the LC caused an increase in blood pressure and heart rate; however, the alpha(2)-adrenoceptor antagonist idazoxan had no cardiovascular effects, but did prevent the pressor response to PPADS or suramin. In addition, coinjection of idazoxan with either suramin or PPADS abolished the ATP and L-GLU mediated pressor responses observed following either suramin or PPADS administration. 4 The present data suggest that firstly, purines are capable of acting within the LC to ultimately modulate the cardiovascular system and secondly, that there is apparently a functional interaction between tonically active purinergic and noradrenergic systems within the LC of the rat.

P2X2 and P2Y1 immunofluorescence in rat neostriatal medium-spiny projection neurones and cholinergic interneurones is not linked to respective purinergic receptor function

1. The presence of ionotropic P2X receptors, targets of ATP in fast synaptic transmission, as well as metabotropic P2Y receptors, known to activate K(+) currents in cultured neostriatal neurones, was investigated in medium-spiny neurones and cholinergic interneurones contained in neostriatal brain slices from 5-26-day-old rats. 2. In these cells, adenosine-5'-triphosphate (ATP) (100-1000 microm), 2-methylthioadenosine-5'-triphosphate (2MeSATP), alpha,beta-methyleneadenosine-5'-triphosphate (alpha,betameATP, 30-300 microm, each) and adenosine-5'-O-(3-thiotriphosphate (ATPgammaS) (100 microm) failed to evoke P2X receptor currents even when 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 microm), apyrase (10 U ml(-1)) or intracellular Cs(+) was used to prevent occluding effects of the ATP breakdown product adenosine, desensitisation of P2X receptors by endogenous ATP and an interference with the activation of K(+) channels, respectively. P2X receptor agonists were also ineffective in outside-out patches withdrawn from the brain slice tissue. Muscimol (10 microm) evoked GABA(A) receptor-mediated currents under all these conditions. 3. When used as a control, locus coeruleus neurones responded with P2X receptor-mediated currents to ATP (300 microm), 2MeSATP and alpha,betameATP (100 microm, each). 4. ATP and adenosine-5'-diphosphate (ADP) (100 microm, each) did not activate K(+) currents in the neostriatal neurones. 5. Despite the observed lack of function, P2X(2) and P2Y(1) immunofluorescence was found in roughly 50% of the medium-spiny neurones and cholinergic interneurones. 6. A role of ATP in synaptic transmission to striatal medium-spiny neurones and cholinergic interneurones appears unlikely, however, the otherwise silent P2X and P2Y receptors may gain functionality under certain yet unknown conditions.

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.

Reduction by sevoflurane of adenosine 5'-triphosphate-activated inward current of locus coeruleus neurons in pontine slices of rats

Increasing evidence indicates that volatile general anesthetics exert their effects by affecting various types of membrane conductance expressed in the central nervous system (CNS), such as ligand-gated receptor-channels. The most recently identified family of the receptor-channels in the CNS are the extracellular ATP-gated channels (P2X purinoceptors). In the present study, we tested whether volatile anesthetics can affect P2X receptor function in the CNS network. We recorded whole-cell currents of locus coeruleus (LC) neurons in pontine slices from young rats. Adenosine 5'-triphosphate (ATP) sodium (0.03-3 mM) evoked a rapidly rising and moderately desensitizing inward current (50-200 pA) in a dose-dependent manner in LC neurons at a holding potential of -80 mV. Perfusion with clinically relevant concentration of sevoflurane (0.1-0.5 mM) reduced the ATP-induced inward current in a dose-dependent manner (to 56.8+/-5.9% of control with 0.5 mM sevoflurane; mean+/-S.E.M., n=13). Estimated IC(50) of sevoflurane was 0.59 mM. We conclude that the attenuation of extracellular ATP-mediated signaling in the central nervous system might be one of the multiple actions of volatile anesthetics.

The acute effect of valproate on cerebral energy metabolism in mice

Sodium valproate (VPA) is used in the acute treatment of status epilepticus and mania. We studied the acute effect of VPA on cerebral energy metabolism in awake mice that received VPA 400 mg kg(-1) and [1-(13)C]glucose or [2-(13)C]acetate. At 25 min, (13)C NMR spectroscopy of brain extracts indicated inhibition of the tricarboxylic acid (TCA) cycle, as could be seen from the accumulation of [4-(13)C]glutamate and reduction in [(13)C]aspartate formation. Concomitantly, the level of ATP was reduced by 40%. To identify the enzymatic step at which the TCA cycle was inhibited [U-(14)C]alpha-ketoglutarate was injected intracerebrally. Inhibition of alpha-ketoglutarate dehydrogenase was evident at 25 min, as shown by accumulation of [(14)C]glutamate. At 45 min the inhibition of alpha-ketoglutarate dehydrogenase was reversed, shown by both (13)C- and (14)C-labeling, and the ATP level was normalized. The study shows for the first time that acute administration of VPA causes inhibition of the TCA cycle activity in vivo. The reduction in brain ATP would be expected to reduce neuronal excitability through modulation of sodium channels which may be clinically advantageous in the initial phase of VPA treatment.

Effect on urinary bladder function and arterial blood pressure of the activation of putative purine receptors in brainstem areas

The effect on bladder function and arterial blood pressure of adenosine-5'-triphosphate (ATP) and its synthetic analogue, alpha,beta-methylene ATP (alpha,beta-meATP) applied by microinjection to brainstem areas was assessed in the anaesthetised, paralysed and artificially ventilated female rat. Recordings of bladder pressure, changes in the pelvic nerve activity, arterial blood pressure and heart rate were evaluated. The purinergic drugs were microinjected into two brainstem areas the periaqueductal grey matter (PAG) and the area of the Barrington nucleus/locus coeruleus (LC) - only after electrical stimulation (50 Hz, 1 ms, 30-50 microA; n(PAG) = 17; n(LC) = 18) and the microinjection of glutamate (2 mM, pH 7.4+/-0.1; n(PAG) = 16; n(LC) = 16) had shown increases of bladder pressure and/or rate of bladder contractions and/or pelvic nerve activity at specific sites. Electrical and glutamate activation of PAG evoked an increase of arterial blood pressure. Microinjections of ATP (20 mM, pH 7.4+/-0.1; n(PAG) = 11; n(LC) = 11) and alpha,beta-meATP (2 mM, pH 7.4+/-0.1; n(PAG) = 10; n(LC) = 9) both evoked consistent increases of bladder pressure and/or pelvic nerve activity. Stimulation with ATP elicited a biphasic change of arterial blood pressure characterised by an increase followed by a decrease which was accompanied by a rise of heart rate. Microinjection of alpha,beta-meATP into PAG did not elicit a consistent response: a decrease of arterial blood pressure was evoked in five rats, while in two other rats an increase occurred. Electrical stimulation and glutamate activation of Barrington's nucleus/LC evoked an increase of arterial blood pressure, but a decrease was observed after microinjection of both ATP and alpha,beta-meATP. At some sites (n = 8) the effect of alpha,beta-meATP after a pre-injection at the same site of the P2 purino receptor antagonist, suramin (20 mM, pH 7.4+/-0.1) was smaller than the control. At three sites within PAG and two within LC located more medially to sites where an excitatory response had been observed, electrical stimulation evoked a small decrease or no change in bladder pressure. Following the stimulus, a rise in bladder pressure was preceded by an increase of pelvic nerve activity. A similar effect of glutamate was observed in one case. These data suggest that activation of P2 purine receptors in both PAG and Barrington's nucleus/LC is implicated in the neuronal mechanisms that generate patterns of activity in the parasympathetic innervation of the bladder and that purines also act at this level to modify sympathetic outflow to the cardiovascular system.

Characterisation of ATP-induced facilitation of transmission in rat hippocampus

Superfusion of rat hippocampal slices with ATP induces a form of facilitation that has been poorly characterised. The present study has confirmed that at low concentrations of ATP (10 microM or less), an initial depression of evoked potential size is followed by a rebound facilitation which is not reproduced by alphabeta-methyleneATP, betagamma-methyleneATP, or the dinucleotide P1,P6-diadenosine hexaphosphate. The post-ATP facilitation could be prevented by the adenosine A1 receptor antagonists 8-phenyltheophylline or 1,3-dipropyl-8-cyclopentyltheophylline (50 nM), or superfusion of adenosine deaminase. The adenosine A2A receptor antagonist 8-(chlorostyryl)-caffeine did not affect the inhibition but prevented the post-ATP facilitation. The NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid prevented the establishment of post-ATP facilitation. The post-ATP facilitation was also blocked by suramin at a concentration (50 microM) that does not block glutamate receptors. Suramin prevented the induction but not the maintenance phase of the post-ATP facilitation. The repeated induction of post-ATP facilitation by bursts of electrical stimulation designed to saturate the normal mechanisms of long-term potentiation prevented the induction of post-ATP facilitation. However, repeated applications of ATP to achieve saturation of its receptor did not prevent the subsequent induction of electrically evoked long-term potentiation. It is concluded that ATP can induce a form of synaptic facilitation which resembles only partially that induced by electrical stimulation and which may require the simultaneous activation of P1 and P2 receptors.

Diadenosine polyphosphate receptors. from rat and guinea-pig brain to human nervous system

Diadenosine polyphosphates are a family of naturally occurring nucleotidic compounds present in secretory vesicles together with other chemical messengers. The exocytotic release of these compounds permits them to stimulate receptors termed "purinoceptors" or "ATP receptors." Purinoceptors for nucleotides are named P2 in contrast with those sensitive to nucleosides (P1). P2 receptors are further subdivided into metabotropic P2Y receptors, further divided into 5 subtypes, and ionotropic P2X receptors, with 7 different subtypes. Diadenosine polyphosphates can activate recombinant P2Y(1), P2Y(2), and P2Y(4) and recombinant homomeric P2X(1), P2X(2), P2X(3), P2X(4), and P2X(6). Heteromeric P2X receptors change their sensitivity to diadenosine polyphosphates when co-assembly between different subunits occurs. Diadenosine polyphosphates can activate specific receptors termed dinucleotide receptors or P4 receptors, which are insensitive to other nucleosides or nucleotides. The P4 receptor is a receptor-operated Ca(2)+ channel present in rat brain synaptic terminals, stimulated by diadenosine pentaphosphate and diadenosine tetraphosphate. This receptor is strongly modulated by protein kinases A and C and protein phosphatases. The dinucleotide receptor is present in different brain areas, such as midbrain (in rat and guinea-pig), cerebellum (in guinea-pig), and cortex (in human).

Presynaptic P2 receptors?

Although the emphasis in ATP research has been on postjunctional receptors, there is also evidence for presynaptic receptors regulating transmitter release in the autonomic nervous system. Recent work has attempted to identify similar mechanisms in the central nervous system. Some of the existing results can be explained by the metabolism of nucleotides to adenosine or adenosine 5'-monophosphate (AMP). However, studies of presynaptic effects using sensitive electrophysiological tests such as paired-pulse interactions indicate that nucleotides can act at presynaptic sites, but that their effects may be mediated by a release of adenosine. Results are also described which indicate that, under some conditions, nucleotides can mediate phenomena such as long-term potentiation, which probably involves a significant presynaptic element. In part these effects may involve a nucleotide-induced release of adenosine and the simultaneous activation of P1 and P2 receptors.

Inhibition by ethanol of rat P2X(4) receptors expressed in Xenopus oocytes

1. The effect of ethanol on the function of P2X(4) receptors expressed in Xenopus oocytes was studied using two-electrode voltage-clamp recording. 2. The amplitude of current activated by 1 microM ATP was decreased by ethanol in a concentration-dependent manner over the concentration range 1 - 500 mM. The concentration of ethanol that produced 50% inhibition (IC(50)) of current activated by 1 microM ATP was 58 mM. 3. Ethanol inhibition of ATP-activated current was not dependent on membrane potential from -60 to +20 mV, and ethanol did not change the reversal potential of ATP-activated current. 4. Ethanol, 50 mM, shifted the ATP concentration-response curve to the right, increasing the EC(50) for ATP from 9.1 to 16.0 microM, but did not reduce the maximal response to ATP. 5. The results suggest that ethanol may inhibit P2X(4) receptors by decreasing the apparent affinity of the binding site for ATP. 6. Since the P2X(4) receptor is the most abundant P2X subunit in the brain, these receptors could be important effectors of ethanol action in the central nervous system.

Expression of purinergic P2X2 receptor-channels and their role in calcium signaling in pituitary cells

Pituitary cells express purinergic receptor-channels (P2XR), the activation of which by ATP is associated with the facilitation of Ca2+ influx. Pharmacological, RT-PCR, and nucleotide sequence analyses confirm the presence of a wild type P2X2aR and a spliced isoform P2X2bR, which lacks a portion of carboxyl terminal amino acids. Wild type and spliced isoform receptors have a similar EC50 for ATP and time-course for activation, but the spliced isoform exhibits rapid and complete desensitization, whereas the wild type channel desensitizes slowly and incompletely. Deletion and insertion studies have revealed that a 6 residue sequence located in carboxyl tail (Arg371-Pro376) is required for sustained Ca2+ influx through wild type receptors. When co-expressed, the wild type and spliced channels form functional heteropolymeric channels. The patterns of Ca2+ signaling in the majority of pituitary cells expressing ATP-gated receptor-channels are highly comparable to those observed in cells co-transfected with P2X2aR and P2X2bR. ATP-induced [Ca2+]i response in pituitary cells is partially inhibited by nifedipine, a blocker of voltage-gated L-type Ca2+ channels, suggesting that P2X2R not only drive Ca2+ into the cell, but also activate voltage-gated Ca2+ entry. Our results indicate that ATP represents a paracrine and (or) autocrine factor in the regulation of Ca2+ signaling, and that its actions are mediated in part by heteropolymeric P2X2R.

Key words: ATP-gated channels, gonadotrophs, lactotrophs, somatotrophs.

Complex hippocampal responses to ATP: fade due to nucleotidase inhibition and P2-receptor-mediated adenosine release

When ATP or the related stable analogue, betagamma-imidoATP, were applied to rat hippocampal slices showing population spikes larger than 5 mV peak-to-peak amplitude, a depression of spike size was obtained, which showed a marked fade during the 10-min period of superfusion. The inhibitory responses were prevented by adenosine deaminase or 8-phenyltheophylline. Adenosine responses showed no fade. alphabeta-MethyleneADP enhanced the fade, while suramin at 50 micrometer prevented the early component of the responses. The results suggest that in slices with large population spikes, inhibitory responses to nucleotides are partly due to their conversion to adenosine, and partly due to the activation of P2 receptors which trigger the release of endogenous adenosine.

Identification and localisation of ATP P2X receptors in rat midbrain

P2X receptors that are gated by extracellular ATP are among the few known examples of ligand-gated cation-selective channels. There have been seven cloned proteins identified to date as members of the P2X receptor family in a wide range of tissues from the peripheral and central nervous systems and from many species. To determine the distribution of the P2X subtypes in the rat midbrain, sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE)/Western blotting was combined with immunolocalisation using confocal microscopy. Subtypes P2X1-6 were detected in the periaqueductal gray area and the ependymal layer bordering the ventricle with a widespread distribution.

Antagonistic properties of the suramin analogue NF023 at heterologously expressed P2X receptors

The suramin analogue 8,8'-(carbonylbis(imino-3,1-phenylene carbonylimino)bis(1,3,5-naphthalenetrisulfonic acid) (NF023) antagonizes in a competitive fashion P2X receptor-mediated responses in certain vascular and visceral smooth muscles. In the present study, the effect of NF023 on voltage-clamped Xenopus oocytes heterologously expressing homomultimeric P2X1-P2X4 as well as heteromultimeric P2X2/P2X3 receptors has been characterized. P2X1 receptors were most sensitive to inhibition by NF023 with IC50 values of 0.24 and 0.21 microM for the rat and human homologue, respectively. P2X3 receptors have an intermediate sensitivity with IC50 values of 8.5 and 28.9 microM for rat and human subtypes, respectively and P2X2 was the least sensitive subtype (IC50 > 50 microM). P2X4 receptors were insensitive to NF023 at concentrations up to 100 microM. Coexpression of rat P2X3 with rat P2X2 resulted in receptors whose sensitivity to NF023 was identical to that obtained for homomultimeric rat P2X3 receptors (alphabeta meATP as agonist; IC50 = 1.4 and 1.6 microM, respectively). NF023 inhibited P2X1 receptors in a voltage-insensitive manner. In addition, NF023 (5 and 30 microM) caused a shift of the concentration-response curve to the right without affecting the maximal response to ATP (K(B) = 1.1 +/- 0.2 microM). Our results indicate that NF023 is a subtype-selective and surmountable antagonist at P2X1 receptors heterologously expressed in Xenopus oocytes.

Impaired arterial baroreflex regulation of heart rate after blockade of P2-purinoceptors in the nucleus tractus solitarius

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of ATP mimics baroreflex responses (bradycardia, hypotension); however, the physiological role of these receptors in cardiovascular control remains unclear. We tested whether blockade of these receptors attenuates arterial baroreflex control of heart rate (HR). Baroreflex-induced changes in HR (via graded i.v. infusion of phenylephrine and nitroprusside) were observed in seven alpha-chloralose/urethane anesthetized male Sprague-Dawley rats before and after microinjection of the purinergic P2 receptor antagonist suramin (0.5 nmol in 50 nL) into the subpostremal NTS. Before suramin, typical baroreflex changes in HR were observed (maximum gain, Gmax = 2.94 +/- 0.54 bpm/mmHg). Suramin markedly impaired baroreflex-induced changes in HR (gain = 0.02 +/- 0.08 and 0.18 +/- 0.09 bpm/mmHg for increases and decreases in mean arterial blood pressure, respectively); however, after 90-130 min, HR and baroreflex reactivity returned to control levels. Microinjections of vehicle into the same area did not alter baroreflex function. In addition, suramin did not alter the depressor responses to microinjections of glutamate into the same site of the NTS. We conclude that normal P2x-purinoceptor function in subpostremal NTS may be necessary for baroreflex regulation of HR.

Evidence that P2X purinoceptors mediate the excitatory effects of alphabetamethylene-ADP in rat locus coeruleus neurones

Extracellular and whole-cell patch clamp recordings were used to study the excitatory responses elicited by purine nucleotides in pontine slices of the rat brain containing the locus coeruleus (LC). The P2 purinoceptor agonists, alphabeta-methyleneadenosine 5'-triphosphate (alphabetameATP) and adenosine 5'-O-(2-thiodiphosphate) (ADPalphabetaS), and a novel purinoceptor agonist, alphabeta-methyleneadenosine 5'-diphosphate (alphabetameADP), elicited concentration-dependent increases in the spontaneous firing rate over the concentration range (1-300 microM). On vagus nerve or dorsal root preparations alphabetameADP (100 microM) had no agonist activity. In the presence of both alphabetameATP (300 microM), ADPbetaS (300 microM) elicited a further and significant increase in the firing rate of the LC neurones, whilst neither alphabetameATP nor alphabetameADP (300 microM) elicited a further response. The P2 purinoceptor antagonists, suramin (100 microM) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 30 microM), markedly attenuated responses to all three agonists. Whole-cell recording of membrane current showed that, at - 60 mV, alphabetameATP and alphabetameADP (both 100 microM) elicited inward currents of a similar magnitude, whilst the inward currents elicited by a lower concentration of ADPbetaS (30 microM) were larger and faded in the presence of this agonist. In the presence of tetrodotoxin and a combination of other neurotransmission blockers, both alphabetameATP and alphabetameADP still produced inward currents. Based on the known selectivity of the agonists used in this study, there appear to be two distinct P2 purinoceptor types present on neurones in the LC, which correspond to the P2X and P2Y types. The responses elicited by alphabetameADP appear to be mediated through a putative P2X purinoceptor, although further work is required to determine which P2X receptor subtype(s) are involved.

Modulation by adenine nucleotides of epileptiform activity in the CA3 region of rat hippocampal slices

1. Hippocampal slices (450 microm) generate epileptiform bursts of an interictal nature when perfused with a zero magnesium medium containing 4-aminopyridine (50 microM). The effect of adenine nucleotides on this activity was investigated. 2. ATP and adenosine depressed this epileptiform activity in a concentration-dependent manner, with both purines being equipotent at concentrations above 10 microM. 3. Adenosine deaminase 0.2 u ml(-1), a concentration that annuls the effect of adenosine (50 microM), did not significantly alter the depression of activity caused by ATP (50 microM). 4. 8-Cyclopentyl-1,3-dimethylxanthine (CPT), an A1 receptor antagonist, enhanced the discharge rate significantly and inhibited the depressant effect of both ATP and adenosine such that the net effect of ATP or adenosine plus CPT was excitatory. 5. Several ATP analogues were also tested: alpha, beta-methyleneATP (alpha, beta-meATP), 2-methylthioATP (2-meSATP) and uridine triphosphate (UTP). Only alpha, beta-meATP (10 microM) produced an increase in the frequency of spontaneous activity which suggests a lack of involvement of P2Y or P2U receptors. 6. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), P2 receptor antagonists, failed to inhibit the depression produced by ATP (50 microM). The excitatory effect of alpha, beta-meATP (10 microM) was inhibited by suramin (50 microM) and PPADS (5 microM). 7. ATP therefore depresses epileptiform activity in this model in a manner which is not consistent with the activation of known P1 or P2 receptors, suggesting the involvement of a xanthine-sensitive nucleotide receptor. The results are also indicative of an excitatory P2X receptor existing in the hippocampal CA3 region.

P2-receptor-mediated inhibition of serotonin release in the rat brain cortex

The possibility of a P2-receptor-mediated modulation of the release of serotonin in the rat brain cortex was investigated in occipito-parietal slices preincubated with [3H]serotonin and then superfused and stimulated electrically (10 pulses, 1 Hz). Adenosine receptor agonists decreased the stimulation-evoked overflow of tritium at best slightly; the selective A1 agonist N6-cyclopentyl-adenosine caused no change. Several nucleotides had more marked effects: ATP (3-1000 microM), adenosine-5'-O-(3-thiotriphosphate) (3-300 microM) and P1,P5-di(adenosine-5')-pentaphosphate (3-300 microM) decreased the evoked overflow by up to ca 35%. AMP, alpha,beta-methylene-ATP and UTP produced smaller decreases and 2-methylthio-ATP and UMP caused no change. The inhibition by ATP was attenuated both by the P1-receptor antagonist 8-(p-sulphophenyl)-theophylline (100 microM) and by the P2-receptor antagonist suramin (300 microM) but was not changed by indomethacin (10 microM) and NG-nitro-L-arginine (10 microM). We conclude that the release of serotonin in the rat brain cortex is inhibited through presynaptic P1-receptors (which are not A1) as well as P2-receptors. Inhibition of release via P2-receptors has been previously shown for noradrenaline (brain cortex and hippocampus) and dopamine (neostriatum) and, hence, may be widespread. Differences between transmitter systems exist, however, in the degree of their sensitivity to presynaptic P2-receptor-mediated modulation.

P2 receptor excitation of rodent hypoglossal motoneuron activity in vitro and in vivo: a molecular physiological analysis

The role of P2 receptors in controlling hypoglossal motoneuron (XII MN) output was examined (1) electrophysiologically, via application of ATP to the hypoglossal nucleus of rhythmically active mouse medullary slices and anesthetized adult rats; (2) immunohistochemically, using an antiserum against the P2X2 receptor subunit; and (3) using PCR to identify expression of P2X2 receptor subunits in micropunches of tissue taken from the XII motor nucleus. Application of ATP to the hypoglossal nucleus of mouse medullary slices and anesthetized rats produced a suramin-sensitive excitation of hypoglossal nerve activity. Additional in vitro effects included potentiation of inspiratory hypoglossal nerve output via a suramin- and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS)-sensitive mechanism, XII MN depolarization via activation of a suramin-sensitive inward current, decreased neuronal input resistance, and a slow-onset theophylline-sensitive reduction of inspiratory output likely resulting from hydrolysis of extracellular ATP to adenosine and activation of P1 receptors. Immunohistochemically, P2X2 receptors were detected in inspiratory XII MNs that were labeled with Lucifer yellow. These data, combined with identification of mRNA for three P2X2 receptor subunit isoforms within the hypoglossal nucleus (two of which have not been localized previously in brain) and the previous demonstration that P2X receptors are ubiquitously expressed in cranial and spinal motoneuron pools, support not only a role of P2 receptors in modulating inspiratory hypoglossal activity but a general role of P2 receptors in modulating motor outflow from the CNS.

Structural motif and characteristics of the extracellular domain of P2X receptors

Seven sequences of the proteins recently cloned from rat tissues and identified as P2X receptors are examined. Their putative transmembrane and extracellular domains and the most probable locations of disulphide bridges are discussed. Some surface accessible regions are identified and the structure of the putative ATP binding domain is discussed in the light of known selectivities and physiological characteristics of individual P2X subunits. The summarised information and the theoretical considerations regarding the protein structure of P2X receptors have been used to select amino acid sequences for raising antibodies against the P2X1 receptor subunit. Initial results suggest that, in rat brainstem, P2X1 receptor subunits are found on a distinct subpopulation of neuronal perikarya in distributions consistent with distributions of known alpha,beta-methylene-ATP autoradiography and physiological effects of purinoceptor activation.

Activation of P2x-purinoceptors in the nucleus tractus solitarius elicits differential inhibition of lumbar and renal sympathetic nerve activity

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of alpha,beta-methylene ATP (alpha,beta-MeATP) elicits large dose-dependent decreases in mean arterial pressure (MAP) and heart rate (HR) and preferential dilation of the iliac vascular bed in comparison to renal and mesenteric vascular beds. We investigated whether sympathoinhibition contributes to the depressor responses and whether differential changes in regional sympathetic output occur. In 43 chloralose/urethane anesthetized male Sprague-Dawley rats, MAP, HR, renal (RSNA) and lumbar sympathetic nerve activity (LSNA) were recorded. Data were analyzed as both the maximum decrease and the integral of the decrease over the duration of the depressor response. Microinjection of alpha,beta-MeATP (25 and 100 pmol in 50 nl volume) into the subpostremal NTS caused significant and dose-dependent decreases in MAP, HR, RSNA and LSNA. However, the changes in RSNA were significantly greater than those observed in LSNA for both doses and both methods of analysis of data (maximum responses in delta %: 84 +/- 3 vs 62 +/- 4, and 93 +/- 3 vs 74 +/- 4 for low and high dose of alpha,beta-MeATP, respectively; integral responses in delta % x min: 32 +/- 4 vs 18 +/- 3 and 179 +/- 7 vs 134 +/- 14 for low and high dose of alpha,beta-MeATP, respectively). Blockade of P2-purinoceptors in the NTS by the specific P2-receptor antagonist suramin abolished responses to 100 pmol alpha,beta-MeATP and microinjections of vehicle did not alter neural nor hemodynamic parameters. We conclude that activation of P2x-purinoceptors in the NTS inhibits sympathetic nerve activity and evokes differential regional sympathetic responses. However, differential sympathoinhibition does not explain differential vascular responses to the activation of P2x-purinoceptors in the NTS.

Electrophysiological effects of ATP on brain neurones

1. The electrophysiological effects of ATP on brain neurones are either due to the direct activation of P2 purinoceptors by the unmetabolized nucleotide or to the indirect activation of P1. purinoceptors by the degradation product adenosine. 2. Two subtypes of P2 purinoceptors are involved, a ligand-activated ion channel (P2X) and a G protein-coupled receptor (P2Y). Hence, the stimulation of P2X purinoceptors leads to a cationic conductance increase, while the stimulation of P2Y purinoceptors leads to a G protein-mediated opening or closure of potassium channels. 3. ATP may induce a calcium-dependent potassium current by increasing the intracellular Ca2+ concentration. This is due either to the entry of Ca2+ via P2X purinoceptors or to the activation of metabotropic P2Y purinoceptors followed by signaling via the G protein/phospholipase C/inositol 1,4,5-trisphosphate (IP3) cascade. Eventually, IP3 releases Ca2+ from its intracellular pools. 4. There is no convincing evidence for the presence of P2U purinoceptors sensitive to both ATP and UTP, or pyrimidinoceptors sensitive to UTP only, in the central nervous system (CNS). 5. ATP-sensitive P2X and P2Y purinoceptors show a wide distribution in the CNS and appear to regulate important neuronal functions.

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.

Purinoceptors in the Central Nervous System

New exciting developments on the occurrence and functional role of purinoceptors in mammalian brain were presented at the session "Purinoceptors in the central nervous system" chaired by Flaminio Cattabeni and Tom Dunwiddie at the Purines '96 international conference. The focus of the session were topics of recent interest, including the sources and mechanisms involved in ATP and adenosine release during physiological neurotransmission in hippocampus, the brain expression of the recently cloned P2 receptors, and the role of the various adenosine receptor subtypes in brain protection from neurodegeneration associated with trauma-, ischemia-and excessive excitatory amino acid neurotransmission. New important insights into the mechanisms responsible for the formation and release of adenosine into the extracellular space were provided by data obtained by Dunwiddie and coworkers in hippocampal pyramidal neurons. These data may have functional implications for the role of purines in modulation of synaptic plasticity and long-term potentiation in this brain area, and hence in cognitive functions. Buell provided an updated overview on the cloning, molecular characteristics and brain expression of various ligand-gated P2X purinoceptors; although the functional role of these receptors in mammalian brain still awaits elucidation, their widespread distribution in the nervous system strongly suggests that ATP-mediated events are more prevalent and important in brain than expected. Pedata presented data on the functional interrelationships between adenosine and glutamate in the brain, and also provided evidence for alterations of the reciprocal regulation between these two systems in aged brain, which may have important implications for both ischemia-and trauma-associated neurodegenerative events and senescence-associated cognitive impairment. Finally, von Lubitz provided novel data on the molecular mechanisms likely to be at the basis of the brain protective effects associated with the chronic stimulation of the adenosine A3 receptor, further confirming that this receptor represents a crucial target for the development of new antiischemic and antineurodegenerative therapeutic agents.

Expression of purinergic receptor channels and their role in calcium signaling and hormone release in pituitary gonadotrophs. Integration of P2 channels in plasma membrane- and endoplasmic reticulum-derived calcium oscillations

The role of ATP as a positive feedback element in Ca2+ signaling and secretion was examined in female rat pituitary gonadotrophs. ATP and ADP, but not AMP or adenosine, induced a dose- and extracellular Ca2+-dependent rise in [Ca2+]i in identified gonadotrophs in a Mg2+- and suramin-sensitive manner. ATP, adenosine-5'-O-(3-thiotriphosphate), adenosine-5'-O-(1-thiotriphosphate), 2-methylthio-ATP, and 3'-O-(4-benzoyl)benzoyl-ATP were roughly equipotent in rising [Ca2+]i in gonadotrophs, while ADP was effective only at submillimolar concentration range, and none of these compounds permeabilized the cells. On the other hand, alpha,beta-methylene-ATP, beta,gamma-methylene-ATP, and UTP were unable to induce any rise in [Ca2+]i. This pharmacological profile is consistent with expression of P2X2 and/or P2X5 purinergic receptor channels. Patch-clamp experiments showed that ATP induced an inward depolarizing current in gonadotrophs clamped at -90 mV, associated with an increase in [Ca2+]i. The ATP-induced [Ca2+]i response was partially inhibited by nifedipine, a blocker of voltage-sensitive Ca2+ channels (VSCC), but was not affected by tetrodotoxin, a blocker of voltage-sensitive Na+ channels. Thus, the P2-depolarizing current itself drives Ca2+ into the cell, but also activates Ca2+ entry through VSCC. In accord with this, low [ATP] induced plasma membrane-dependent [Ca2+]i oscillations in quiescent cells, and increased the frequency of spiking in spontaneously active cells. ATP-induced Ca2+ influx also affected agonist-induced and InsP3-dependent [Ca2+]i oscillations by increasing the frequency, base line, and duration of Ca2+ spiking. In addition, ATP stimulated gonadotropin secretion and enhanced agonist-induced gonadotropin release. ATP was found to be secreted by pituitary cells during agonist stimulation and was promptly degraded by ectonucleotidase to adenosine. These observations indicate that ATP represents a paracrine/autocrine factor in the regulation of Ca2+ signaling and secretion in gonadotrophs, and that these actions are mediated by P2 receptor channels.

Purinoceptor modulation of noradrenaline release in rat tail artery: tonic modulation mediated by inhibitory P2Y- and facilitatory A2A-purinoceptors

1. The effects of analogues of adenosine and ATP on noradrenaline release elicited by electrical stimulation (5 Hz, 2700 pulses) were studied in superfused preparations of rat tail artery. The effects of purinoceptor antagonists, of adenosine deaminase and of adenosine uptake blockade were also examined. Noradrenaline was measured by h.p.l.c. electrochemical detection. 2. The A1-adenosine receptor agonist, N6-cyclopentyladenosine (CPA; 0.1-100 nM) reduced, whereas the A2A-receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 3-30 nM) increased evoked noradrenaline overflow. These effects were antagonized by the A1-adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 20 nM) and the A2-adenosine receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX; 100 nM), respectively. The P2Y-purinoceptor agonist, 2-methylthio-ATP (1-100 microM) reduced noradrenaline overflow, an effect prevented by the P2-purinoceptor antagonist, cibacron blue 3GA (100 microM) and suramin (100 microM). 3. Adenosine deaminase (2 u ml-1), DMPX (100 nM) and inhibition of adenosine uptake with S-(p-nitrobenzyl)-6-thioinosine (NBTI; 50 nM) decreased evoked noradrenaline overflow. DPCPX alone did not change noradrenaline overflow but prevented the inhibition caused by NBTI. The P2Y-purinoceptor antagonist, cibacron blue 3GA (100 microM) increased evoked noradrenaline overflow as did suramin, a non-selective P2-antagonist. 4. It is concluded that, in rat tail artery, inhibitory (A1 and P2Y) and facilitatory (A2A) purinoceptors are present and modulate noradrenaline release evoked by electrical stimulation. Endogenous purines tonically modulate noradrenaline release through activation of inhibitory P2Y and facilitatory A2A purinoceptors, whereas a tonic activation of inhibitory A1 purinoceptors seems to be prevented by adenosine uptake.

Colocalization of ATP and nicotinic ACh receptors in the identified vagal preganglionic neurone of rat

1. Effects of exogenous adenosine 5'-triphosphate (ATP) and acetylcholine (ACh) were investigated on acutely dissociated preganglionic neurones in the dorsal motor nucleus of vagus (DMV) of rats using whole-cell patch clamp recording methods. 2. The DMV neurones identified by retrograde transport of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) fixed onto the cervical vagal nerve bundle were large in size (25-35 microns diameter) and bipolar or tripolar in shape. 3. About 90% of DiI labelled DMV neurones responded to both ATP (10(-4) M) and ACh (10(-4) M) with inward currents at a holding potential (Vh) of -40 mV. 4. The ATP-induced current (IATP) and the ACh-induced current (IACh) reversed their polarities at membrane potentials between +5 and +15 mV, indicating that ATP and ACh increase the membrane permeability to cations. 5. The inhibitory potency of Reactive Blue on 5 x 10(-4) M IATP is more effective (concentration for half-inhibition (IC50), 4.4 x 10(-7) M) than suramin (IC50, 6.0 x 10(-6) M). In addition, alpha,beta-methylene ATP up to 10(-4) M could not induce any current. As intracellular application of guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) did not block the IATP, the IATP was mediated not by guanosine triphosphate (GTP) binding protein, but rather by ligand-gated ionic channels, presumably via P2X receptors. 6. Currents produced by ACh were due to activation of nicotinic receptors because they were mimicked by nicotine and carbachol, and blocked by hexamethonium. In addition, muscarine evoked no response. 7. Only 25% of nucleus tractus solitarii (NTS) neurones and no hypoglossal neurones responded to the exogenous application of ATP. 8. These results suggest that vagal preganglionic neurones colocalize functionally nicotinic and P2X purinergic receptors.

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.

Purinoceptors in the nervous system

The purine nucleoside adenosine and the purine nucleotide ATP play different roles in the nervous system. Adenosine acts on a family of G protein coupled receptors, collectively called adenosine receptors or P1 purinoceptors. Four members of this family have been cloned and pharmacologically characterized: A1, A2A, A2B and A3. Their distribution, pharmacology and biological roles are briefly discussed. In particular, the evidence that adenosine acting at A1 receptors regulates the release of several neurotransmitters and that adenosine acting at A2A receptors modulates dopaminergic transmission is summarized. ATP acts on receptors called P2 purinoceptors, which appear to fall into at least two main families--G protein coupled receptors and intrinsic ion channels. Their subclassification is becoming clearer as receptors are cloned and new selective agonists and/or antagonists are becoming available. There is an interesting potential for development of drugs targeted at purines or their receptors.

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.

Evidence for P2-purinoceptor-mediated inhibition of noradrenaline release in rat brain cortex

1. Some postganglionic sympathetic axons possess P2Y-like P2-purinoceptors which, when activated, decrease the release of noradrenaline. We examined the question of whether such receptors also occur at the noradrenergic axons in the rat brain cortex. Slices of the brain cortex were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine (1 microM) and stimulated electrically, in most experiments by trains of 4 pulses/100 Hz. 2. The selective adenosine A1-receptor agonist, N6-cyclopentyl-adenosine (CPA; 0.03-3 microM) as well as the non-subtype-selective agonist 5'-N-ethylcarboxamido-adenosine (NECA; 0.3-3 microM) reduced the evoked overflow of tritium, whereas the adenosine A2a-receptor agonist, 2-p-(2-carbonylethyl)-phenethylamino-5'-N-ethylcarboxamido-a denosine (CGS-21680; 0.003-30 microM) and the adenosine A3-receptor agonist N6-2-(4-aminophenyl)ethyl-adenosine (APNEA; 0.03-3 microM) caused no change. Of the nucleotides tested, ATP (30-300 microM), adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S; 30-300 microM), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S; 30-300 microM), P1,P4-di(adenosine-5')-tetraphosphate (Ap4A; 30-300 microM) and the preferential P2Y-purinoceptor agonist, 2-methylthio-ATP (300 microM) decreased the evoked overflow of tritium. The P2X-purinoceptor agonist, alpha,beta-methylene-ATP (3-300 microM) caused no change. 3. The A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM) attenuated the effects of the nucleosides CPA (apparent pKB value 9.8) and NECA as well as of the nucleotides ATP (apparent pKB 9.3), ATP gamma S (apparent pKB 9.2) and ADP beta S (apparent pKB 8.7). CGS-21680 and APNEA were ineffective also in the presence of DPCPX. The A2-selective antagonist 1,3-dipropyl-8-(3,4-dimethoxystyryl)-7-methylxanthine (KF-17837) reduced the effects of CPA, NECA and ATP gamma S only when given at a concentration of 300 nM but not at 1O nM.4. The P2-purinoceptor antagonists, suramin (300 micro M), reactive blue 2 (30 micro M) and cibacron blue 3GA(30 micro M) did not change the effect of CPA. Suramin and cibacron blue 3GA shifted the concentration response curve of ATP gamma S to the right (apparent pKB values 3.7 and 5.0, respectively). Reactive blue 2 also attenuated the effect of ATPyS, and cibacron blue 3GA attenuated the effect of ATP, but in these cases the agonist concentration-response curves were not shifted to the right. There was no antagonistic effect of suramin against ATP and ADP beta S.5. The results indicate that rat cerebrocortical noradrenergic axons possess, in addition to the knownadenosine Al-receptor, a separate purinoceptor for nucleotides (P2) which, in contrast to the Al-receptor,is blocked by suramin, reactive blue 2 and cibacron blue 3GA. Nucleotides such as ATP and ATP gamma S activate both receptors. Inconsistencies in antagonist effects against nucleotides are probably due to this activation of two receptors. The presynaptic P2-purinoceptor is P2Y-like, as it is in the peripheral sympathetic nervous system.

Activation of P2-purinoceptors in the nucleus tractus solitarius mediate depressor responses

The purpose of the study was to examine the role of P2 purinergic receptors in mechanisms of cardiovascular control mediated by the nucleus tractus solitarius (NTS), a major integrative site in the brainstem involved in the reflex coordination of cardiorespiratory and visceral response patterns. Microinjections of ATP and its analogues were made into the subpostremal NTS of anesthetized, spontaneously breathing rats. ATP, alpha,beta-methylene ATP (alpha beta-meATP) and 2-methylthio-ATP (2-meSATP) produced significant dose-related reductions in arterial blood pressure. alpha beta-meATP was slightly more potent than ATP and 2-meSATP. Pretreatment with the P2 receptor antagonist, suramin (0.5 nmol/rat), into the same NTS site 10 min prior to agonist administration completely blocked pronounced depressor response pattern elicited by the highest dose of alpha beta-meATP (0.1 nmol/rat). The present findings suggest that endogenous ATP may serve as a fast transmitter substance in NTS-mediated mechanisms of cardiovascular control.

Activation of specific ATP receptors induces a rapid increase in intracellular calcium ions in rat hypothalamic neurons

We have used real-time dynamic video imaging of Fura-2 fluorescence to study the acute effects of external ATP on [Ca2+]i in cultured rat hypothalamic neurons. The addition of ATP at microM concentrations, but not adenosine, AMP, ADP or GTP, produced a rapid, dose-dependent increase in cytosolic Ca2+. The hydrolysis-resistant ATP analogues 3-thio-ATP and beta,gamma-imido-ATP produced a similar response but alpha,beta-methylene ATP had much lower efficacy. The ATP response was inhibited by 10 microM nifedipine, abolished by 50 microM cadmium and by the absence of extracellular Ca2+, but was unaffected by ryanodine or omega-conotoxin GVIA. The P2-purinoceptor antagonist suramin reversibly and selectively inhibited the ATP response but had no effect on other neurotransmitter-induced Cai2+ responses. Antagonists to muscarinic, nicotinic, NMDA, non-NMDA, GABA, 5-HT and adenosine receptors had no effect on the ATP response. Thus the Ca2+ response of hypothalamic neurons to ATP is mediated by specific suramin-sensitive ATP-receptors, activation of which is independent of ATP hydrolysis and results in an influx of extracellular Ca2+ largely through high voltage-gated Ca2+ channels. These findings support the assertion that ATP acts in the CNS as an excitatory neurotransmitter.

Modulation of locus coeruleus neurons by extra- and intracellular adenosine 5'-triphosphate

The cell membrane of rat locus coeruleus (LC) neurons is sensitive to both extra- and intracellular ATP. Extracellular ATP or its enzymatically stable analogues activate membrane receptors of the P2 type. These receptors inhibit a persistent potassium current and simultaneously activate a nonselective cationic conductance. The resulting depolarization increases the spontaneous firing rate. A decrease in the concentration of intracellular ATP during hypoxia or hypoglycemia opens ATP-sensitive K+ (KATP) channels of LC neurons. The resulting hyperpolarization depresses the discharge of action potentials and conserved energy. The hypoxia-induced hyperpolarization is additionally due to the release of adenosine from neighboring neurons or glial cells. A certain class of compounds, termed potassium channel openers, also decrease the firing, while sulphonylurea antidiabetics known to block KATP channels increase it. Sulphonylurea antidiabetics antagonize the excitability decrease induced both by potassium channel openers and metabolic damage.