Noradrenaline-ATP co-transmission in the sympathetic nervous system

ATP release in human kidney cortex and its mitogenic effects in visceral glomerular epithelial cells

BACKGROUND

In chronic renal failure the sympathetic nervous system is activated. Sympathetic cotransmitters released within the kidney may contribute to the progression of renal disease through receptor-mediated proliferative mechanisms.

METHODS

In human renal cortex electrical stimulation induced adenosine 5'-triphosphate (ATP; luciferin-luciferase-assay) and norepinephrine (HPLC) release was measured. ATP release also was induced by alpha1- and alpha2-adrenergic agonists. [3H]-thymidine uptake was tested in human visceral glomerular epithelial cells (vGEC) and mitogen-activated protein kinase (MAPK42/44) activation in vGEC and kidney cortex. The involved P2-receptors were characterized pharmacologically and by RT-PCR.

RESULTS

Sympathetic nerve stimulation and alpha-adrenergic agonists induced release of ATP from human kidney cortex. Seventy-five percent of the ATP released originated from non-neuronal sources, mainly through activation of alpha2-adrenergic receptors. ATP (1 to 100 micromol/L) and related nucleotides (1 to 100 micromol/L) increased [3H]-thymidine uptake. The adenine nucleotides ATP, ATPgammaS, ADP and ADPbetaS were about equally potent. UTP, UDP and alpha,beta-methylene ATP had no effect. ATP, ADPbetaS but not alpha,beta-methylene ATP activated MAPK42/44. ATP induced MAPK42/44 activation, and [3H]-thymidine uptake was abolished in the presence of the MAPK inhibitor PD 98059 (100 micromol/L). mRNA for P2X4,5,6,7 and P2Y1,2,4,6,11 were detected in human vGEC by RT-PCR.

CONCLUSIONS

In human renal cortex, adrenergic stimulation releases ATP from neuronal and non-neuronal sources. ATP has mitogenic effects in vGEC and therefore the potential to contribute to progression in chronic renal disease. The pattern of purinoceptor agonist effects on DNA synthesis together with the mRNA expression suggests a major contribution of a P2Y1-like receptor.

Multiple regulation by external ATP of nifedipine-insensitive, high voltage-activated Ca(2+) current in guinea-pig mesenteric terminal arteriole

We investigated the receptor-mediated regulation of nifedipine-insensitive, high voltage-activated Ca(2+) currents in guinea-pig terminal mesenteric arterioles (I(mVDCC)) using the whole-cell clamp technique. Screening of various vasoactive substances revealed that ATP, histamine and substance P exert modulatory effects on I(mVDCC). The effects of ATP on I(mVDCC) after complete P2X receptor desensitization exhibited a complex concentration dependence. With 5 mM Ba(2+), ATP potentiated I(mVDCC) at low concentrations (approximately 1-100 microM), but inhibited it at higher concentrations (>100 microM). The potentiating effects of ATP were abolished by suramin (100 microM) and PPADS (10 microM) and by intracellular application of GDPbetaS (500 microM), whereas a substantial part of I(mVDCC) inhibition by milimolar concentrations of ATP remained unaffected; due probably to its divalent cation chelating actions. In divalent cation-free solution, I(mVDCC) was enlarged and underwent biphasic effects by ATPgammaS and ADP, while 2-methylthio ATP (2MeSATP) exerted only inhibition, and pyrimidines such as UTP and UDP were ineffective. ATP-induced I(mVDCC) potentiation was selectively inhibited by anti-Galpha(s) antibodies or protein kinase A (PKA) inhibitory peptides and mimicked by dibutyryl cAMP. In contrast, ATP-induced inhibition was selectively inhibited by Galpha(q/11) antibodies or protein kinase C (PKC) inhibitory peptides and mimicked by PDBu. Pretreatment with pertussis toxin was ineffective. The apparent efficacy for I(mVDCC) potentiation with PKC inhibitors was: ATPgammaS > ATP>/=ADP and for inhibition with PKA inhibitors was: 2MeSATP > ATPgammaS > ATP > ADP. Neither I(mVDCC) potentiation nor inhibition showed voltage dependence. These results suggest that I(mVDCC) is multi-phasically regulated by external ATP via P2Y(11)-resembling receptor/G(s)/PKA pathway, P2Y(1)-like receptor/G(q/11)/PKC pathway, and metal chelation.

Interaction between P2Y and NMDA receptors in layer V pyramidal neurons of the rat prefrontal cortex

In the first part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex were evoked by electrical stimulation of layer I. Recordings by intracellular sharp microelectrodes showed that EPSPs were concentration-dependently facilitated by the P2 receptor antagonistic ATP analogue 2-methylthio ATP (2-MeSATP), while ATP itself depressed the synaptic potentials. The inhibitory effect of ATP turned into facilitation in the presence of the adenosine A(1) receptor antagonist DPCPX. The 2-MeSATP-induced potentiation of EPSP amplitudes were prevented by the P2 receptor antagonists PPADS and Suramin. The EPSP was almost abolished by coapplication of the NMDA receptor antagonist AP-5 and the AMPA/kainate receptor antagonist CNQX. After blockade of the NMDA receptor-mediated part of the EPSP by AP-5, the stimulatory effect of 2-MeSATP disappeared. When NMDA or AMPA were pressure-applied onto pyramidal cells, only the NMDA-induced depolarization was potentiated by 2-MeSATP. In the second part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ATP, 2-MeSATP, UDP and UTP potentiated the response to NMDA, while ADP-beta-S was inactive. PPADS antagonized the effect of ATP. Synaptic isolation of pyramidal neurons by a Ca(2+)-free medium or tetrodotoxin did not alter the effect of ATP which, however, was markedly depressed when GTP in the micropipette was replaced by GDP-beta-S. These observations suggest that in layer V pyramidal neurons of the prefrontal cortex postsynaptically localized P2Y receptors interact with NMDA receptor-channels.

Bradykinin enhances sympathetic neurotransmission in rat blood vessels

Bradykinin evokes endothelium-dependent relaxation in some vascular beds; on the other hand, the possibility has been demonstrated that in certain organs, such as the adrenal medulla or atria, bradykinin may enhance transmitter release from the sympathetic nerves. We hypothesized that bradykinin may also enhance postganglionic sympathetic neurotransmission in blood vessels. To test this hypothesis, we recorded excitatory junction potentials (EJPs), a measure of sympathetic purinergic neurotransmission, in rat mesenteric resistance arteries with a conventional microelectrode technique. EJPs were elicited by repetitive perivascular nerve stimulation (1 Hz, 20 to 50 V, 30 to 60 micros, 11 pulses). In this preparation, bradykinin (10(-7) or 10(-6) mol/L) significantly enhanced the amplitude of EJPs without altering the resting membrane potential. This effect of bradykinin was blocked by Hoe 140, a bradykinin B2 receptor antagonist, but not by des-Arg(9),[Leu(8)]-bradykinin, a bradykinin B1 receptor antagonist. The cyclooxygenase inhibitor indomethacin or NO synthase inhibitor N(G)-nitro-L-arginine did not alter the effect of bradykinin. Captopril, an ACE inhibitor, but not candesartan, an angiotensin II type 1 receptor antagonist, enhanced the action of a low concentration (10(-8) mol/L) of bradykinin on EJPs. These findings suggest that in rat mesenteric resistance arteries, bradykinin enhances sympathetic purinergic neurotransmission, presumably through presynaptic bradykinin B2 receptors. The clinical relevance of the present findings remains unclear; however, the fact that the ACE inhibitor, but not the angiotensin II type 1 receptor antagonist, enhanced the action of bradykinin on sympathetic neurotransmission may warrant further investigation.

Nucleotide-evoked relaxation of rat vas deferens--a possible role for endogenous ATP released upon alpha(1)-adrenoceptor stimulation

The possibility was tested that endogenous ATP released upon alpha(1)-adrenoceptor activation causes relaxation of the rat vas deferens smooth muscle. ATP, 2-methylthio ATP and adenosine relaxed the vas deferens precontracted with 80 mM K(+). The metabolically stable P2 receptor agonists alpha,beta-methylene ATP (alpha,beta-MeATP) and adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS) had little or no effect. The adenosine P1 receptor antagonist 8-(para-sulfophenyl)theophylline did not significantly affect the response to ATP. The P2 receptor antagonist reactive blue 2 markedly reduced the relaxation (by up to 73%); suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and acid blue 129 caused no change. ATP, but not alpha,beta-MeATP, also attenuated contractions elicited by noradrenaline at resting tension; reactive blue 2 blocked the inhibitory effect of ATP. Reactive blue 2, by itself, enhanced the response to noradrenaline (by up to 36%); suramin, PPADS and acid blue 129 caused no change. In the presence of the ATP-degrading enzymes apyrase and nucleotide pyrophosphatase, the facilitatory effect of reactive blue 2 was lost. Apyrase, by itself, enhanced the response to noradrenaline (by 13%). The results indicate that endogenous ATP, released from rat vas deferens smooth muscle upon alpha(1)-adrenoceptor stimulation, causes relaxation. The site of action of ATP is not a typical smooth muscle P2Y receptor.

Co-transmitter function of ATP in central catecholaminergic neurons of the rat

Intracellular recordings were made in a mid-pontine slice preparation of the rat brain containing the nucleus locus coeruleus. Focal electrical stimulation evoked biphasic synaptic potentials consisting of early depolarizing (d.p.s.p.) and late hyperpolarizing (i.p.s.p.) components. The alpha(2)-adrenoceptor antagonist idazoxan inhibited the i.p.s.p. without altering the d.p.s.p. All of the following experiments were carried out in the presence of kynurenic acid and picrotoxin to block the glutamatergic and GABAergic fractions of the d.p.s.p., respectively. Guanethidine, which is known to inhibit noradrenaline and ATP release from nerve terminals of postganglionic sympathetic nerves, depressed both the d.p.s.p. and the i.p.s.p. in a concentration-dependent manner. Damage of catecholaminergic nerve terminals by 6-hydroxydopamine also decreased both the d.p.s.p. and the i.p.s.p. The P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) depressed the d.p.s.p., whereas the i.p.s.p. remained unaffected. The further application of PPADS did not increase the depression of the d.p.s.p. by guanethidine. Superfusion with the mixed alpha-adrenoceptor agonist noradrenaline or the selective P2 receptor agonist adenosine 5'-O-(2-thiodiphosphate) inhibited both the d.p.s.p. and the i.p.s.p. The inhibitory effects of these agonists were prevented by the respective antagonists idazoxan or suramin. In the presence of suramin noradrenaline failed to inhibit the residual d.p.s.p. Superfused noradrenaline potentiated rather than inhibited responses to pressure-applied alpha,beta-methylene-ATP; superfused adenosine 5'-O-(2-thiodiphosphate) did not interact with pressure-applied noradrenaline. In conclusion, we present electrophysiological evidence for the co-release of ATP and catecholamines in the CNS. At the cell somata of neurons in the locus coeruleus, noradrenaline and ATP activate inhibitory alpha(2)-adrenoceptors and excitatory P2 receptors, respectively. In addition, inhibitory presynaptic autoreceptors of the alpha(2) and P2 types appear to regulate release of the two co-transmitters.

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.

Differential effects of omega-conotoxin GVIA, tetrodotoxin and prolonged cold storage on purinergic and adrenergic transmission in isolated canine splenic artery

Double-peaked vasoconstrictions (biphases of vasoconstrictions) were readily induced in the conditions of 30 s trains of pulses at 1 Hz in the isolated, perfused canine splenic artery. P2X purinoceptors have previously been shown to be involved mainfy in the first-peaked response and alpha1-adrenoceptors mostly in the second. The treatment with 10 nM omega-conotoxin GVIA (omega-CTX) produced a parallel inhibitory effect on the first- and second-peaked vasoconstrictor responses to nerve stimulation. A submaximal concentration of tetrodotoxin (TTX) (3 nM) did not affect the first peak of constriction, but strongly inhibited the second peak, although a larger dose of TTX (30 nM) abolished either the first- or second-peaked response. On the other hand, after cold storage at 4 degrees C for 7 days, the first-peaked vasoconstriction markedly decreased, whereas the second-peaked response was not significantly modified.

IN CONCLUSION

(1) omega-CTX-sensitive calcium channels may produce a parallel modulation of purinergic and adrenergic components of sympathetic cotransmission; (2) TTX-sensitive sodium channels may have a more important role in controlling the adrenergic rather than purinergic transmission; and (3) the function of purinergic transmission of sympathetic nerve might be affected more strongly than that of adrenergic transmission in the cold-stored canine splenic artery.

Mechanisms of adenosine 5'-triphosphate-induced dopamine release in the rat nucleus accumbens in vivo

The endogenous mechanisms modulating ATP-induced dopamine release in the nucleus accumbens (NAc) were studied by microdialysis in freely moving rats. The ATP analog 2-Methylthio ATP (2-MeSATP) facilitated the release of dopamine in a manner sensitive to pertussis toxin and tetrodotoxin. It is suggested that G-protein-coupled P2Y receptors and voltage-gated sodium channels are involved in this process. N-methyl-D-aspartate (NMDA) applied in a concentration of 100 microM decreased the extracellular dopamine level, whereas 1 and 10 mM NMDA enhanced it. The endogenous agonist glutamate (10 microM) inhibited the basal and facilitated release of dopamine. Infusion with a combination of the ionotropic glutamate receptor antagonists (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), as well as with the metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG) increased the basal level of dopamine and potentiated the 2-MeSATP-facilitated dopamine release, suggesting an ATP-mediated glutamate release. The GABA(A) receptor antagonist bicuculline infused into the NAc also enhanced the basal level of dopamine; however, the application of 2-MeSATP in the presence of bicuculline caused an early decrease and a subsequent increase of dopamine release. The facilitatory phase of the 2-MeSATP effect was comparable with that measured in the absence of bicuculline. By contrast, when bicuculline was infused into the ventral tegmental area (VTA) it elevated the accumbal basal dopamine level and in addition facilitated the 2-MeSATP- and the glutamate-induced dopamine release above that measured in the absence of bicuculline. These results suggest that ATP in the NAc has a physiologically relevant function in modulating dopaminergic transmission depending on the mesolimbic neuronal activity. The first component of the ATP effect involves a direct stimulation of the terminals of VTA neurons, while the second inhibitory component involves a sequential activation of glutamate and, finally, via ionotropic and metabotropic glutamate receptors, of GABA neurons projecting to the VTA.

P2Y receptor-mediated inhibition of voltage-activated Ca(2+) currents in PC12 cells

To search for inhibitory nucleotide receptors in the sympathoadrenal cell lineage of the rat, voltage-activated Ca(2+) currents were recorded in PC12 cells after differentiation with nerve growth factor. ADP and ATP, but not uridine nucleotides, reduced Ca(2+) current amplitudes and slowed activation kinetics. This effect was mediated by GTP binding proteins, as it was abolished by intracellular GDP beta S and after treatment with pertussis toxin. Furthermore, depolarizations preceding the activation of Ca(2+) currents abolished the ADP-induced slowing of activation kinetics and attenuated its inhibitory action on current amplitudes. The modulatory effect of ADP was neither altered in the presence of adenosine receptor antagonists, nor mimicked by agonists at these receptors. In addition, the action of ADP was antagonized by reactive blue 2, but not by suramin or PPADS. Nucleotides tested for their inhibitory action on Ca(2+) currents displayed the following rank order of potency: 2-methylthio-ADP > or = 2-methylthio-ATP >> ADP beta S > ADP = ATP. When P2X receptors were blocked, the P2X agonists ATP and 2-methylthio-ATP still reduced Ca(2+) currents. The P2Y1 receptor antagonists adenosine-2'-phosphate-5'-phosphate and adenosine-3'-phosphate-5'-phosphate did not alter the inhibitory action of ADP, whereas the Sp-isomer of adenosine-5'-O-(1-thiotriphosphate) and 2'- and 3'-O-(4-benzoylbenzoyl)-ATP showed significant antagonistic activity. These results demonstrate that PC12 cells express an as yet unidentified P2Y receptor with pharmacological characteristics similar to those of P2Y1. As receptor-dependent modulation of Ca(2+) channels is a key event in presynaptic inhibition, this receptor may correspond to previously described presynaptic nucleotide receptors mediating autoinhibition of sympathetic transmitter release.

Purinergic and adrenergic agonists synergize in stimulating vasopressin and oxytocin release

The A1 catecholamine neurons of the caudal ventrolateral medulla transmit hemodynamic information to the vasopressin (VP) neurons in the hypothalamus. These neurons corelease ATP with norepinephrine. Perifused explants of the hypothalamoneurohypophyseal system were used to investigate the role of these substances on VP release. ATP (100 micrometer) increased VP release 1.5-fold (p = 0.027). The response was rapid but unsustained. It was blocked by the P(2) receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). The alpha(1)-adrenergic agonist phenylephrine (PE; 100 micrometer) also increased VP release by 1.5-fold (p = 0.014). Again, the response was rapid and unsustained. However, simultaneous perifusion of explants with ATP (100 micrometer) and PE (100 micrometer) resulted in a threefold to fourfold increase in VP release, which was sustained for as long as 4 hr. There was a similar synergistic effect of ATP and PE on oxytocin release. Interestingly, the synergistic response was delayed approximately 40 min relative to the response to either agent alone. Several experiments were performed to elucidate the cellular mechanisms of this synergism. The effect was blocked by PPADS, a protein kinase C inhibitor (bisindolylmaleimide I HCl), and actinomycin, an inhibitor of gene transcription. These data suggest that P(2X) receptor activation, PKC-mediated phosphorylation, and gene transcription are required for the synergistic response. The marked synergism of these coreleased agents is probably important to achieve sustained increases in plasma VP in response to prolonged hypotension. These observations may also have broad applications to CNS function, because ATP may be coreleased at noradrenergic synapses throughout the CNS.

Distribution of P2X1, P2X2, and P2X3 receptor subunits in rat primary afferents: relation to population markers and specific cell types

We determined the co-expression of immunoreactivity (IR) for ATP-receptor subunits (P2X1, P2X2, and P2X3), neuropeptides, neurofilament (NF), and binding of the isolectin B(4) from Griffonia simplicifolia type one (GS-I-B(4)) in adult dorsal root ganglion neurons. P2X1-IR was expressed primarily in small DRG neurons. Most P2X1-IR neurons expressed neuropeptides and/or GS-I-B(4)-binding, but lacked NF-IR. P2X1-IR overlapped with P2X3-IR, though each was also found alone. P2X2-IR was expressed in many P2X3-IR small neurons, as well as a group of medium to large neurons that lacked either P2X3-IR or GS-I-B(4)-binding. A novel visible four-channel fluorescence technique revealed a unique population of P2X2/3-IR neurons that lacked GS-I-B(4)-binding but expressed NF-IR. Co-expression of P2X1, and P2X3 in individual neurons was also demonstrated. We examined P2X subunit-IR on individual recorded neurons that had been classified by current signature in vitro. Types 1, 2, 4 5, and 7 expressed distinct patterns of P2X-IR that corresponded to patterns identified in DRG sections, and had distinct responses to ATP. Types with rapid ATP currents (types 2, 5, and 7) displayed P2X3-IR and/or P2X1-IR. Types with slow ATP currents (types 1 and 4) displayed P2X2/3-IR. Type 1 neurons also displayed P2X1-IR. This study demonstrates that the correlation between physiological responses to ATP and the expression of particular P2X receptor subunits derived from expression systems is also present in native neurons, and also suggests that novel functional subunit combinations likely exist.

P2X2 receptor expression by interstitial cells of Cajal in vas deferens implicated in semen emission

Male reproduction is dependent upon seminal emission mediated by vas deferens contraction. This drives spermatic fluid to the prostatic urethra during ejaculation. We localize interstitial cells of Cajal (ICC), which express P2X2 receptor, subunits of ATP-gated ion channels, to rat, mouse and guinea-pig vas deferens submucosa. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of rat vas deferens resolved two functional splice variant transcripts of the P2X2 receptor subunit. The P2X2 receptor mRNA was localized principally within the lamina propria (submucosal) region of the rat vas deferens using in situ hybridization (ISH) and in situ RT-PCR-ISH. Immunohistochemistry using rat, mouse and guinea-pig vas deferens tissues confirmed expression of P2X2 receptor protein within the lamina propria, particularly within a dense column of small spindle-shaped cells adjacent to the columnar epithelial cells which line the lumen. This immunoreactivity was co-localized with neurone-specific enolase (NSE) and c-Kit protein expression, gene markers for ICC. Mucosal mast cells were distinguished from ICC by toluidine blue staining. Choline acetyltransferase immunoreactivity, a marker for post-ganglionic parasympathetic innervation, occurred on the lateral margin of the lamina propria and extended into the inner longitudinal muscle layer. P2X1 receptor immunolabelling was associated with sympathetic innervation of the smooth muscle in the outer longitudinal and circular muscle layers, but not the inner longitudinal layer. The physiological significance of the vas deferens ICC which express P2X2 receptors remains to be established. Possible roles include regulation of smooth muscle activity or mucosal secretion utilizing local ATP signaling, both of which would affect semen transport.

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.

Functional properties of heteromeric P2X(1/5) receptors expressed in HEK cells and excitatory junction potentials in guinea-pig submucosal arterioles

P2X receptors are ATP-gated cation channels; they form as homomers or heteromers from a family of seven related subunits. In particular, heteromeric channels comprising P2X(2) and P2X(3) subunits, or P2X(1) and P2X(5) subunits, show distinctive physiological and pharmacological properties in heterologous expression systems. There is substantial evidence that one of the native P2X receptors in sensory neurones corresponds to the P2X(2/3) heteromer, but there is no evidence for P2X(1/5) heteromers in native tissue. We recorded currents in response to activation of heteromeric P2X(1/5) receptors expressed in HEK293 cells to characterize further their functional properties. The ATP concentration-response curve had a threshold concentration of 1 nM, and a Hill slope of one. TNP-ATP was a weak partial agonist, and a non-competitive antagonist which inhibited maximal ATP currents by 60%. Increasing or decreasing pH from 7.3 shifted the ATP concentration-response curves to the right by fivefold and decreased the maximum current by 40%. Calcium permeability was lower than that observed for other P2X receptors (P(Ca)/P(Na) ratio=1.1). The nanomolar sensitivity of this receptor revealed a steady release of ATP from HEK293 cells, providing an extracellular concentration which ranged from 3 to 300 nM. Noradrenaline (0.3-30 microM) increased ATP-evoked currents by 35%; this facilitation occurred within 20 ms. We also recorded excitatory junction potentials (EJPs) from guinea-pig submucosal arterioles. EJPs were inhibited by suramin and PPADS (IC(50)s of 0.2 microM and 20 microM) but TNP-ATP (0.1-10 microM) inhibited EJPs by <30%. Noradrenaline (0.3-30 microM in the presence of phentolamine and propranolol) decreased EJPs in control preparations but facilitated EJPs by 5-20% in submucosal arterioles from reserpinized guinea-pigs. These properties are discussed in relation to P2X receptors underlying EJPs at autonomic neuroeffector junctions.

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.

Differential effects of drugs interacting with autonomic transmitters on responses of rat vas deferens to field stimulation

1. Frequency-response curves (0.1-30 Hz) were obtained in the epididymal portion of rat vas deferens. At low frequencies (0.1-1 Hz), the parameters evaluated were the first twitch and the fourth twitch at each frequency. The responses to trains of stimuli at intermediate (2-5 Hz) and high (10-30 Hz) frequencies were biphasic consisting of phase I (the first rapid phase of tetanus) and of phase II (the secondary slowly developing one). 2. Prazosin inhibited the first and the fourth twitch but not when the frequency was < 1 Hz. Suramin inhibited the first twitch while substantially depressing the fourth one. The combination of prazosin and suramin almost completely abolished all the twitches evoked by a train of stimuli at low frequencies. Nifedipine left almost unaltered the first twitch while markedly depressing the fourth one, especially at relatively high frequency (1 Hz). Verapamil was devoid of any inhibitory action. Papaverine depressed the first twitch while only at the highest concentration used (1 x 10(-4) M) markedly inhibited the fourth one. Chloroethylclonidine (CEC) depressed the first twitch and increased the fourth. 3. When intermediate (2-5 Hz) and high (10-30 Hz) frequencies are considered, prazosin and suramin partially inhibited both phase I and phase II, while in combination they almost completely abolished both phases. Nifedipine and verapamil selectively suppressed phase II, leaving phase I unaffected. Papaverine completely abolished both phase I and phase II. CEC was able to completely abolish phase I but increased phase II. 4. These results suggest that the response to the first twitch of a train at low frequency is prevailingly noradrenergic, prazosin-sensitive, while when the twitches are close enough (i.e. at 1 Hz) a summation of stimuli takes place and a predominant purinergic component, both suramin- and nifedipine-sensitive, becomes evident. 5. At high frequencies, both phases are due to the concomitant release of noradrenaline and adenosine triphosphate (ATP). The noradrenergic component of phase I is nifedipine-insensitive and CEC-sensitive, resembling the pharmacological profile of the endogenously released noradrenaline by single pulse, while that of phase II, nifedipine-sensitive and CEC-insensitive, is similar to that produced by exogenously applied noradrenaline.

Effects of adrenomedullin and PAMP on membrane potential and neurotransmission

The effects of adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP) on membrane potential and sympathetic neurotransmission were studied in rat mesenteric arteries by using microelectrodes. AM (10(-7) M) but not PAMP (10(-6) M) produced membrane hyperpolarization, which was abolished by high K solution or by glibenclamide, an ATP-sensitive K(+) (KATP) channel blocker. Neither AM nor PAMP affected excitatory junction potentials, a measure of sympathetic, purinergic neurotransmission. These findings suggest that AM hyperpolarizes the membrane via activation of KATP channels, which may contribute to the vasodilatory action of AM, whereas the mechanisms of the vasodepressor action of PAMP remain unclear.

Perivascular purinergic nerve-induced vasoconstrictions in canine isolated splenic arteries

We tried to induce selective perivascular purinergic nerve stimulation in isolated canine splenic arterial preparations, using the cannula insertion method. Under the conditions of periarterial electrical stimulation (ES), i.e., trains of 1, 3 and 10 pulses, 1-ms pulse duration and 10-V amplitude at 1 Hz, monophasic vasoconstriction was consistently induced. The ES-induced vasoconstriction was not influenced by prazosin in doses that completely inhibited noradrenaline-induced vasoconstrictions, but it was suppressed by alpha,beta-methylene ATP, a P2X purinoceptor desensitizer. Thus, it is indicated that a selective purinergic transmitter release is readily obtained in the isolated splenic arterial preparation.

Pharmacological analysis of vasoconstrictor responses to periarterial purinergic nerve stimulation

1. Periarterial electrical nerve stimulation at a low frequency (1 Hz) readily induced a vasoconstrictor response of the canine splenic artery in a pulse number-related manner (1-30 pulses of trains). The vasoconstrictor response to trains of up to 10 pulses at 1 Hz of stimulation appeared to be monophasic, whereas it became clearly distinguished into two phases at a longer train of 30 pulses. 2. The monophasic vasoconstrictor responses to trains of 1, 3 or 10 pulses were not modified by an alpha1-adrenoceptor blocking agent, prazosin (0.1 microM), but were completely inhibited by the P2X receptor desensitization with alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-methylene ATP; 1 microM). The 1st phase of vasoconstriction induced by a train length of 30 pulses was not influenced by the treatment with prazosin, but was abolished by alpha,beta-methylene ATP. The 2nd phase response was markedly inhibited by prazosin, and the remaining response of this phase was blocked by alpha,beta-methylene ATP. 3. Rauwolscine (0.3 microM), an alpha2-adrenoceptor antagonist, enhanced the vasoconstrictor responses to trains of 1, 3 or 10 pulses. Particularly at 10 pulses of electrical stimulation, the vasoconstrictor responses were significantly potentiated. The blockade of neuronal uptake of noradrenaline with imipramine (1 microM) did not affect the vasoconstrictor responses to trains of 1, 3 or 10 pulses. 4. It is concluded that short pulse trains of stimulation at a low frequency may selectively activate a purinergic component of sympathetic cotransmission, and the prejunctional alpha2-adrenergic feedback mechanism may tonically participate into the modulation of ATP release. Imipramine-sensitive neuronal uptake mechanism may not play an important role in regulating vascular responses to periarterial purinergic nerve stimulation.

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.

Adrenergic and purinergic components in bisected vas deferens from spontaneously hypertensive rats

1. Purinergic and adrenergic components of the contractile response to electrical field stimulation (EFS) have been investigated in epididymal and prostatic portions of Wystar Kyoto (WKY) and spontaneously hypertensive rat (SHR) vas deferens. 2. In both halves of SHR and WKY vas deferens, EFS (40 V, 0.5 ms for 30 s, 0.5-32 Hz) evoked frequency-related contractions. The neurogenic responses were biphasic, consisting of a rapid non-adrenergic response, dominant in the prostatic portion, followed by a slow tonic adrenergic component, dominant in the epididymal half. 3. Phasic and tonic components of the frequency-response curves evoked by EFS were significantly higher in the epididymal but not in the prostatic portion of vas deferens from SHR compared to WKY rats. 4. The alpha1-adrenoceptor antagonist prazosin (0.1 microM) was more effective against both components of the contractile response in the epididymal end of SHR than in WKY rats. 5. Inhibition by alpha, beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP 3 and 30 microM) was higher in both components of the contractile responses in WKY preparations than in SHR. 6. Combined alpha1-adrenoceptor and P2x-purinoceptor antagonism virtually abolished the EFS-evoked contractile response in both strains. The degree of inhibition by prazosin (0.1 microM) after P2x-purinoceptor blockade was higher in SHR than in WKY rats. 7. These results demonstrate a modification in the purinergic and noradrenergic contribution to neurogenic responses in SHR and WKY animals besides a co-participation of ATP and noradrenaline in both contractile components of the response to EFS.

Endothelium-released adenosine triphosphate contributes to vasoconstrictor responses to periarterial nerve stimulation in isolated, perfused canine splenic arteries

P2X-Purinoceptors and alpha1-adrenoceptors have previously been shown to be involved in double-peaked vasoconstrictor responses to periarterial electrical nerve stimulation in the isolated and perfused canine splenic artery. The present study was designed to investigate the influence of endothelium removal on vasoconstrictor responses to periarterial nerve stimulation, tyramine, noradrenaline, and adenosine triphosphate (ATP) in the isolated canine splenic artery. Intraluminal administration of saponin completely abolished the acetylcholine-induced vasodilatation and potentiated the vasoconstrictor response to KCl. Double-peaked vasoconstriction (two phases of vasoconstriction) was readily induced by periarterial electrical nerve stimulation in the canine splenic arterial preparation with or without endothelium and was consistently abolished by treatment with tetrodotoxin. Removal of endothelium slightly but significantly decreased the first-phase vasoconstrictor responses to stimulation of 1 or 10 Hz, and did not affect the second-phase. The vasoconstrictor responses to tyramine, noradrenaline, and ATP were not modified by endothelium removal. From these results, it is postulated that ATP released from endothelium, as a modulator of sympathetic nerve cotransmission, may partially contribute to the purinergic constriction component in the canine splenic artery.

Dissociation of inhibitory effects of guanethidine on adrenergic and on purinergic transmission in isolated canine splenic artery

The aim of this study was both to investigate the effects of progressive inhibition of adrenergic neurons by increasing concentrations of guanethidine (0.1-10 microM) on the double-peaked vasoconstrictor responses to electrical periarterial nerve stimulation in the isolated and perfused canine splenic artery, and to clarify whether release of noradrenaline is presynaptically separate from release of adenosine 5'-triphosphate (ATP). Double-peaked vasoconstrictions (biphases of vasoconstrictions) were consistently observed under the conditions of 30-s trains of pulses at 1-10 Hz frequencies. Guanethidine, at a lower concentration (0.1 microM) did not modify the first (1st) phase vasoconstriction at low frequencies (1-2 Hz), but markedly inhibited the second (2nd) responses. On the other hand, it slightly but significantly inhibited the double-peaked vasoconstrictor responses at high frequencies (6-10 Hz). Furthermore, a 10-fold increase of concentration of guanethidine (1 microM) almost completely inhibited the 2nd phase responses at any frequencies used but did not completely inhibit the 1st phase response. A further increased concentration of guanethidine (10 microM) failed to enhance the 1 microM guanethidine-induced inhibition. The 1 microM guanethidine-resistant 1st phase responses at any frequencies used (1-10 Hz) were sensitive to tetrodotoxin (30 nM). Treatment with 0.1 microM prazosin did not modify the 1st phase response at any frequencies used in the 1 microM guanethidine-treated preparation. The responses remaining after 1 microM guanethidine and 0.1 microM prazosin were completely suppressed by a subsequent application of 1 microM alpha,beta-methylene ATP at any frequencies used. The results indicated that guanethidine, an adrenergic neuron blocker, may exert a dominant inhibitory effect on adrenergic rather than on purinergic components of sympathetic nerve co-transmission, indicating that guanethidine-sensitive mechanisms may mainly contribute to determine noradrenaline secretion from neurosecretory vesicles rather than ATP secretion.

Abnormal regulation of the sympathetic nervous system in alpha2A-adrenergic receptor knockout mice

alpha2-Adrenergic receptors (ARs) play a key role in regulating neurotransmitter release in the central and peripheral sympathetic nervous systems. To date, three subtypes of alpha2-ARs have been cloned (alpha2A, alpha2B, and alpha2C). Here we describe the physiological consequences of disrupting the gene for the alpha2A-AR. Mice lacking functional alpha2A subtypes were compared with wild-type (WT) mice, with animals lacking the alpha2B or alpha2C subtypes, and with mice carrying a point mutation in the alpha2A-AR gene (alpha2AD79N). Deletion of the alpha2A subtype led to an increase in sympathetic activity with resting tachycardia (knockout, 581 +/- 21 min-1; WT, 395 +/- 21 min-1), depletion of cardiac tissue norepinephrine concentration (knockout, 676 +/- 31 pg/mg protein; WT, 1178 +/- 98 pg/mg protein), and down-regulation of cardiac beta-ARs (Bmax: knockout, 23 +/- 1 fmol/mg protein; WT, 31 +/- 2 fmol/mg protein). The hypotensive effect of alpha2 agonists was completely absent in alpha2A-deficient mice. Presynaptic alpha2-AR function was tested in two isolated vas deferens preparations. The nonsubtype-selective alpha2 agonist dexmedetomidine completely blocked the contractile response to electrical stimulation in vas deferens from alpha2B-AR knockout, alpha2C-AR knockout, alpha2AD79N mutant, and WT mice. The maximal inhibition of vas deferens contraction by the alpha2 agonist in alpha2A-AR knockout mice was only 42 +/- 9%. [3H]Norepinephrine release studies performed in vas deferens confirmed these findings. The results indicate that the alpha2A-AR is a major presynaptic receptor subtype regulating norepinephrine release from sympathetic nerves; however, the residual alpha2-mediated effect in the alpha2A-AR knockout mice suggests that a second alpha2 subtype (alpha2B or alpha2C) also functions as a presynaptic autoreceptor to inhibit transmitter release.

Effects of activation and blockade of P2x receptors in the ventrolateral medulla on arterial pressure and sympathetic activity

Sympathoexcitatory and sympathoinhibitory neurons in the rostral and caudal ventrolateral medulla (VLM) play a crucial role in the tonic and reflex control of sympathetic vasomotor activity. Recent evidence also indicates that the VLM contains a high density of P2x purinoceptors. In this study, we investigated the cardiovascular effects of selective activation of P2x purinoceptors in the rostral and caudal VLM, and the effects of blockade of P2x purinoceptors in the rostral VLM on the tonic and reflex control of sympathetic vasomotor activity. In anesthetized barodenervated rabbits, microinjection into the rostral and caudal VLM of the P2x purinoceptor agonist, alpha,beta-methylene adenosine triphosphate (alpha,beta-meATP) (4-400 pmol) elicited dose-dependent increases and decreases, respectively, in arterial pressure (AP), heart rate (HR) and renal sympathetic nerve activity (RSNA). The response evoked by alpha,beta-meATP in the rostral VLM was blocked by prior injection into the same site of the P2 purinoceptor antagonist suramin but not by the ionotropic glutamate receptor antagonist kynurenic acid. Bilateral injections of suramin into the rostral VLM sympathoexcitatory region had no significant effect on resting cardiovascular variables, nor on the reflex increase in RSNA evoked by sciatic nerve stimulation (which is known to be mediated by the rostral VLM sympathoexcitatory neurons). The results demonstrate that: (1) activation of P2x purinoceptors in the VLM are capable of producing marked excitation of both sympathoexcitatory and sympathoinhibitory neurons; (2) these effects are not due to modulation of glutamatergic inputs to these neurons; and (3) P2x purinoceptors do not play a significant role in maintaining the tonic activity of rostral VLM sympathoexcitatory neurons or in modulating their responses to excitatory synaptic inputs evoked by stimulation of sciatic nerve afferents.

Ca2+-permeable P2X receptor channels in cultured rat retinal ganglion cells

ATP has been identified as an excitatory neurotransmitter in both the CNS and peripheral nervous system; however, little is known about the functional properties of ATP-gated channels in central neurons. Here we used a culture preparation of the postnatal rat retina to test the responsiveness of identified retinal ganglion cells (RGCs) and putative amacrines to exogenous ATP and other purinoceptor agonists. Rapidly activating ATP-induced currents (IATP) were exclusively generated in a subpopulation (approximately 65%) of RGCs. The latter were identified by Thy1.1 immunostaining, repetitive firing patterns, and activation of glutamatergic autaptic currents. None of the putative amacrine cells was ATP-sensitive. IATP could be induced with ATP, ADP, and alpha,beta-mATP but not with adenosine. It was antagonized by suramin. The current-voltage relationship of IATP showed marked inward rectification. Dose-response analysis yielded an EC50 of 14.5 microM, with a Hill coefficient of 0.9. Noise analysis of IATP suggested a mean single channel conductance of 2.3 pS. Retinal P2X purinoceptor channels exhibited a high permeability for Ca2+. PCa/PCs obtained from reversal potentials of IATP under bi-ionic conditions amounted to 2. 2 +/- 0.7. In the majority of cells, the decay of IATP was biphasic. The degree of current inactivation during the first 2 sec of agonist application was highly variable. Heterogeneity was also found with respect to the sensitivity to ADP and alpha,beta-mATP and the blocking action of suramin, suggesting expression of multiple P2X receptor subtypes. Our results indicate that activation of P2X receptor channels represents an important pathway for Ca2+ influx in postnatal RGCs.

Extracellular ATP increases [Ca2+]i in primarily cultured pig coronary smooth muscle cells via a P2Y purinoceptor subtype

In primarily cultured pig coronary smooth muscle cells, extracellular adenosine triphosphate (ATP; 10(-9) to 10(-3) M) dose-dependently increases intracellular calcium ([Ca2+]i). The [Ca2+]i transients measured by fura-2 fluorescence consist of peak and plateau phases with [Ca2+]i values of 191.84 +/- 5.67 nM (n = 10) and 91.67 +/- 1.89 nM, respectively. In Ca(2+)-free solution, the peak phases persisted, but there was a loss of the plateau response, indicating an initial ATP-stimulated intracellular Ca2+ release and a subsequent transarcolemmal Ca2+ entry. Various agonists have been used to characterize the P2 purinoceptor subtype involved in the ATP-induced Ca2+ transients. The rank order of potency was uridine triphosphate (UTP) > ATP >> 2-meSATP > beta,gamma-meATP = alpha,beta-meATP = adenosine = 0. To examine the refilling of ATP-sensitive stores, four repetitive 60-s ATP responses were produced throughout with a 5-min recovery period in between. Now the ATP peaks gradually declined in Ca(2+)-free solution, indicating the emptying of the stores. If, however, Ca2+ entry was allowed in the "refilling period" (i.e., between the ATP pulses), the Ca2+ peaks could be maintained or restored, respectively. The data suggest that the ATP-dependent [Ca2+]i transients may be mediated via a UTP > ATP-activated P2Y purinoceptor subtype, mediating both an intracellular Ca2+ release and a transarcolemmal Ca2+ influx. The refilling of Ca2+ stores may occur through the unstimulated membrane after agonist stimulation. A putative pathway may be a "capacitative" Ca2+ entry induced on depletion of intracellular Ca2+ stores.

Adenosine 5'-triphosphate-induced dopamine release in the rat nucleus accumbens in vivo

Microdialysis experiments were used to investigate the influence of locally applied 2-methylthioadenosine 5'-triphosphate (2-MeSATP) on extracellular dopamine concentrations in the rat nucleus accumbens (NAc). 2-MeSATP (0.1, 1, 10 mM) infused via the microdialysis probe caused a concentration-dependent stimulation of dopamine release. The P2 receptor antagonists reactive blue 2 and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (30 microM each) depressed the basal release of dopamine when given alone and in addition counteracted the stimulatory effect of 2-MeSATP (1 mM). In contrast, a combination of the excitatory amino acid receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 300 microM) and 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 100 microM) increased the basal release of dopamine by themselves and facilitated the effect of 2-MeSATP (1 mM). The results suggest a physiologically relevant regulation of tonic dopamine release in the NAc by endogenous ATP via P2 receptors. This is due to the combination of a direct and an indirect (via glutamate release) effect of ATP on mesolimbic dopaminergic neurons.

Effects of A1-adenosine receptor antagonists on purinergic transmission in the guinea-pig vas deferens in vitro

1. Intracellularly recorded excitatory junction potentials (ej.ps) were used to study the effects of adenosine receptor antagonists on neurotransmitter release from postganglionic sympathetic nerve terminals in the guinea-pig vas deferens in vitro. 2. The A1 adenosine receptor antagonists, 8-phenyltheophylline (10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM), increased the amplitude of e.j.ps evoked during trains of 20 stimuli at 1 Hz in the presence, but not in the absence, of the alpha2-adrenoceptor antagonist, yohimbine (1 microM) or the non-selective alpha-adrenoceptor antagonist, phentolamine (1 microM). 3. Adenosine (100 microM) reduced the amplitude of e.j.ps, both in the presence and in the absence of phentolamine (1 microM). This inhibitory effect of adenosine is most likely caused by a reduction in transmitter release as there was no detectable change in spontaneous ej.p. amplitudes. 4. In the presence of phentolamine, application of the adenosine uptake inhibitor, S-(p-nitrobenzyl)-6-thioinosine (0.1 microM), had no effect on ej.p. amplitudes. 5. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (100 microM), significantly increased the amplitudes of all e.j.ps evoked during trains of 20 stimuli at 1 Hz, both in the presence and in the absence of phentolamine (1 microM). 6. These results suggest that endogenous adenosine modulates neurotransmitter release by an action at prejunctional A1 adenosine receptors only when alpha2-adrenoceptors are blocked.

Facilitation and depression of ATP and noradrenaline release from sympathetic nerves of rat tail artery

1. Excitatory junction currents (EJCs) were used to measure ATP release; noradrenaline (NA) oxidation currents and fractional overflow of labelled NA, [3H]NA, were used to monitor the release of endogenous and exogenous NA, respectively, from post-ganglionic sympathetic nerves of rat tail artery. 2. During nerve stimulation with 100 pulses at 5-20 Hz the EJCs initially grew in size (maximally by 23 %, at 2-10 Hz), and then depressed, maximally by 68 % at 20 Hz. 3. The peak amplitude of NA oxidation currents in response to nerve stimulation with 100 pulses at 2-20 Hz grew in size with frequency, while the area was independent of frequency and roughly constant. 4. The size of the NA oxidation currents evoked by nerve stimulation with 4-100 pulses at 20 Hz grew linearly with train length between pulses 4-16. Between pulses 20-100 there was a train length-dependent depression of the signal. 5. Fractional overflow of [3H]NA in response to nerve stimulation with 5-100 pulses at 20 Hz behaved similarly to the EJCs. It initially grew roughly linearly between pulses 5-25, and then showed a dramatic depression similar to that of the EJCs. 6. The alpha2-adrenoceptor antagonists rauwolscine and yohimbine increased the overflow of [3H]NA and the amplitude of NA oxidation currents, but not that of the EJCs. 7. It is concluded that during high-frequency stimulation (i) the release of ATP and NA is first briefly facilitated then markedly depressed, (ii) facilitation and depression of the two transmitters are similar in magnitude and time course, and (iii) alpha2-adrenoceptor antagonists differentially modify EJCs and the NA signals. The results obtained in the absence of drugs are compatible with the hypothesis that ATP and NA are released in parallel, while the effects of alpha2-adrenoceptor antagonists seem to suggest dissociated release.

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.

Extracellular ATP: a further modulator in neuroendocrine control of the thymus

It is well established that the process of thymocyte differentiation and maturation occurs in the thymus, where cell-to-cell communication is essential for providing the messages to T-cell precursors. At least two pathways are important for such communication: one via membrane surface molecules and the other via soluble mediators such as cytokines and some hormones. Recently, the presence of receptors for extracellular ATP has been demonstrated on thymocytes and microenvironment cells, and putative functions for this molecule have been proposed. Herein we focus on the recent evidence which supports the view of extracellular ATP and some related nucleotides as novel intrathymic signal molecules. In addition, we discuss the possible physiological implications of such purinergic receptors for the physiology of the thymus.

Pharmacological analysis of the double peaked vasoconstrictor responses to periarterial electric stimulation

Vasoconstrictor responses to periarterial nerve stimulation were studied with the use of prazosin (an alpha1-adrenoceptor antagonist) and alpha,beta-methylene ATP (a P2X receptor desensitizer) in canine isolated, perfused splenic arterial preparations. Double peaked responses (two phases of the constriction) were readily induced with the conditions of 30 s trains of pulses at 10 V amplitude, 1 ms duration in a frequency-related manner. At low frequencies (1-3 Hz), the 1st phase might contain only a purinergic component which was mostly inhibited by treatment with alpha,beta-methylene ATP. At high frequencies (4-10 Hz), the 1st phase was in part inhibited by prazosin and the remaining component of this phase was abolished by alpha,beta-methylene ATP. The 2nd phase response was markedly inhibited by prazosin, and remaining component of this phase was abolished by alpha,beta-methylene ATP at all frequencies used. It is concluded that (1) the 1st phase is probably induced via an activation of P2X receptors at low frequencies, and at high frequencies via both P2X and alpha1-adrenoceptors, and (2) the 2nd phase probably involves mostly alpha1-adrenoceptors and partially P2X receptors.

Effect of hibernation on responses of hamster vas deferens to sympathetic nerve stimulation and exogenous neurotransmitters

The present study investigated the responses of the vas deferens to sympathetic nerve stimulation and exogenous neurotransmitters taken from golden hamsters which had undergone 8 weeks of hibernation, 2 h of arousal from hibernation, those exposed to the cold but which failed to hibernate and age-matched control animals. Electrical field stimulation (EFS) of the vas deferens from each group produced frequency-dependent, tetrodotoxin-sensitive contractions. Contractions elicited by low frequencies of EFS in the hibernating group were significantly greater than in the other groups in the absence of any blocking agents. In the presence of the alpha1-adrenoceptor antagonist prazosin (3 microM) responses from all groups were reduced by approximately 40%, with the residual responses from the hibernating group being somewhat increased compared to the other groups. In the presence of the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2'4'-disulphonic acid (30 microM), there was no significant difference in responses from all 4 groups. Exogenously applied beta, gamma-methylene ATP (beta,gamma-meATP; 0.1-300 microM), a P2X receptor agonist, and noradrenaline (NA; 30 nM(-1) mM) both caused transient concentration-dependent contractions in all groups of animals. Contractions to beta,gamma-meATP at concentrations above 0.3 microM, and NA above 0.3 microM in the hibernating animals were statistically significantly greater than the cold- and age-matched control groups, although not significantly different from the aroused group. This study has shown postjunctional increases in responses to beta,gamma-meATP and NA as a result of hibernation, possible explanations for these increases are discussed.

Purinoceptors mediate renal vasodilation by nitric oxide dependent and independent mechanisms

BACKGROUND

Adenosine triphosphate (ATP) and its metabolites including adenosine modulate renal vascular tone under physiological and pathophysiological conditions. Their effects are brought about by activation of membrane bound P1- and P2-purinoceptors located on smooth muscle and endothelial cells. In this study we analyzed the purinoceptor mediated dilation of rabbit and human renal arteries, and evaluated the possible involvement of endothelium-derived relaxing factors.

METHODS

Segments of rabbit and human renal arteries were incubated and perfused with medium containing indomethacin. After preconstriction, drug induced changes in the vessel diameters were measured by a photoelectric device.

RESULTS

ATP (EC50 = 1 mumol/liter), added intraluminally, caused maximal vasodilation of 80 to 100% of the preconstriction response in both species. This effect was inhibited by the P1-purinoceptor antagonist 8-p-(sulphophenyl)theophylline (100 mumol/liter), suggesting that it was in part due to breakdown of ATP to adenosine. The nature of purinoceptor mediated renal vasodilation was studied further in rabbit renal arteries. Adenosine (EC50 = 1 mumol/liter) as well as the P2Y-receptor agonists ADP beta S (EC50 = 0.4 mumol/liter) and 2-MeSATP (EC50 = 0.2 mumol/liter) dilated the arteries by 80 to 100%. The effects of 2-MeSATP, which were to a much lesser extent that of ADP beta S but not that of adenosine, were attenuated by the P2Y-antagonist reactive blue 2 (3 mumol/liter). Removal of the endothelium almost abolished the vasodilation induced by adenosine and ATP. In contrast, these dilator response were only slightly attenuated by the nitric oxide synthase blockers NG-nitro-L-arginine methyl ester and NG-nitro-L-arginine (300 mumol/liter each), whereas acetylcholine and 2-MeSATP induced dilation was markedly reduced by NG-nitro-L-arginine methyl ester.

CONCLUSIONS

P1-purinoceptors activated by adenosine dilate rabbit renal arteries by an endothelium-derived relaxing factor that appears to be distinct from nitric oxide. In contrast, P2Y-purinoceptor induced renal dilation is mediated by nitric oxide. ATP, the physiological activator of P2Y-purinoceptors, is rapidly broken down to adenosine in rabbit and human renal arteries. Therefore, in rabbit and human renal arteries the vasodilatory effect of exogenous ATP mainly results from P1-purinoceptor activation probably through its breakdown product, adenosine.

Effect of extracellular adenosine 5'-triphosphate on principal neurons of the rat ventral tegmental area

Intracellular recordings were made in a midbrain slice preparation of the rat brain containing the ventral tegmental area (VTA). Dopaminergic principal cells were identified by their electrophysiological properties and their hyperpolarizing responses to dopamine. Superfusion with dopamine (100 microM) caused hyperpolarization and a decrease of the apparent input resistance. By contrast, two structural analogues of ATP, 2-methylthio ATP (2-MeSATP; 10 microM) and alpha,beta-methylene ATP (alpha, beta-meATP; 30 microM) had no effect, when added to the superfusion medium. Pressure applied dopamine also hyperpolarized the membrane, while both 2-MeSATP and alpha,beta-meATP were ineffective. Hence, dopaminergic principal neurons of the VTA do not possess somatic P2 purinoceptors present on peripheral and central noradrenergic neurons.

P2 purinoceptor localization along rat nephron and evidence suggesting existence of subtypes P2Y1 and P2Y2

Effects of extracellular ATP on intracellular free calcium concentration([Ca2+]i) were examined in rat single nephron segments using the fura 2-AM. ATP (10 ¿M) induced a significant transient increase in [Ca2+]i in the glomerulus, the early proximal convoluted tubule (S1), the cortical collecting tubule (CCT), and the outer medullary collecting tubule (OMCT). The magnitude of the response was the greatest in the OMCT among four segments. ATP induced an increase in the [Ca2+]i in a dose-dependent manner in S1 and OMCT. In the OMCT, ATP caused a biphasic increase in [Ca2+]i consisting of an initial rapid rise and a sustained phase. Removal of calcium from the medium resulted in an attenuation of the sustained phase of [Ca2+]i and an approximately 30% reduction in the height of the initial [Ca2+]i peak in response to 10 ¿M ATP. Effects of ATP, its analogs, and its metabolites were tested in the S1 and OMCT. ATP, 2-methylthio-ATP (2-MeS-ATP), ADP, and UTP increased [Ca2+]i dose dependently. AMP and adenosine did not affect [Ca2+]i in the S1 and OMCT. The ATP- or 2-MeS-ATP-induced [Ca2+]i increase was inhibited by the pretreatment of the S1 and OMCT with suramin or reactive blue 2. Neomycin, a phospholipase C inhibitor, attenuated the ATP-induced [Ca2+]i increase. To investigate the hormonelike action of ATP in OMCT, a heterologous cross desensitization was performed. The pretreatment of OMCT with ATP inhibited increases in vasopressin-, ANG II-, endothelin-1-, or bradykinin-induced[Ca2+]i increase. These findings suggest that ATP might affect the above peptidyl agonist-activated calcium mobilizations.

Pharmacological evidence for a receptor mediating sustained nucleotide-evoked contractions of rat aorta in the presence of UTP

The contractile effect of ATP given alone or in the presence of other nucleotides was studied in rat aortic strips. A sustained contraction in response to ATP (30 microM to 10 mM) was observed during UTP exposure instead of the fast transient contraction produced via P2x purinoceptor activation in the absence of UTP, and contrary to the relaxation elicited when the tone had been raised by noradrenaline and KCl. This sustained ATP effect was produced in the smooth muscle and not via the same mechanism through which UTP elicited contraction, since the contractions in response to UTP and ATP were additive. They were also coupled to different transduction pathways: the effect of UTP but not that of ATP was pertussis toxin-sensitive. In contrast to the fast transient ATP contraction during basal tone, the sustained response was not desensitized by alpha,beta-methylene ATP exposure (30 microM), but was inhibited by reactive blue 2 (10 and 30 microM). Among the nucleotides assayed, UDP and ATPgammaS also enabled ATP to elicit a sustained contraction. ADP, AMP, dATP, 2-methylthio ATP, alpha,beta-methylene ATP, GTP, GDP, GMP, CTP and ITP also induced a sustained contraction in the presence of UTP. However, adenosine (1 mM) and adenine (0.3 to 3 mM) induced relaxation when the tone had been raised by UTP. According to these results a non-selective nucleotide receptor, different from the P2 purinoceptors functionally characterized so far, seems to mediate sustained contractions in rat aortic strips in the presence of UTP, UDP or ATPgammaS.

ATP-activated cation currents in single guinea-pig hepatocytes

1. Responses of single guinea-pig liver cells to the application of external ATP were studied using the whole-cell voltage clamp technique. 2. When the cells were loaded with 5 mM EGTA in the absence of K+ and Cl- in both internal and external solutions, application of ATP (0.03-100 microM) elicited a large cation-selective inward current at negative holding potentials. The current densities at the peak of the response to 100 microM ATP were 4.5 +/- 0.5 pA pF-1 (mean +/- s.e.m., n = 18) in the presence of Na+ and Ca2+ in the external medium and 3.3 +/- 0.7 pA pF-1 (n = 6) with Ca2+ as the major permeant ion. 3. Divalent cations, when added during the response to ATP in the presence of Na+ and Ca2+, exerted different effects: CdSO4 (2 mM) totally and NiSO4 (2 mM) partially blocked the inward current whereas MnSO4 (2 mM) did not block it. The ATP-activated conductance was permeable to all the divalent cations tested in this study, i.e. Ca2+, Cd2+, Ni2+, Mn2+ and Mg2+. No response to ATP was observed in the absence of external cations. 4. The activation of the inward current was not maintained in the continuous presence of ATP. The effect of Ca2+ ions on the desensitization of the response was studied in different external solutions. The decline in the amplitude of the inward current after the peak was fitted with a single exponential with a time constant of about 2 s for pure Ca2+, Cd2+ or Ni2+ currents, 3 s for Mg2+ or Mn2+ and 4 s in the presence of both Na+ and Ca2+. 5. Under more physiological conditions, the entry of Ca2+ evoked after the stimulation of P2X purinoceptors was associated with an increase in fluo-3 fluorescence and a marked reduction in the delay before the mobilization of internal Ca2+ stores following the activation of P2Y purinoceptors.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.