Adenosine A1 receptor-mediated excitation of nociceptive afferents innervating the normal and arthritic rat knee joint

Adenosine-A1 receptor agonist induced hyperalgesic priming type II

We have recently shown that repeated exposure of the peripheral terminal of the primary afferent nociceptor to the mu-opioid receptor (MOR) agonist DAMGO ([D-Ala, N-Me-Phe, Gly-ol]-enkephalin acetate salt) induces a model of transition to chronic pain that we have termed type II hyperalgesic priming. Similar to type I hyperalgesic priming, there is a markedly prolonged response to subsequent administration of proalgesic cytokines, prototypically prostaglandin E2 (PGE2). However, type II hyperalgesic priming differs from type I in being rapidly induced, protein kinase A (PKA), rather than PKCε dependent, not reversed by a protein translation inhibitor, occurring in female as well as in male rats, and isolectin B4-negative neuron dependent. We report that, as with the repeated injection of a MOR agonist, the repeated administration of an agonist at the A1-adenosine receptor, also a Gi-protein coupled receptor, N-cyclopentyladenosine (CPA), also produces priming similar to DAMGO-induced type II hyperalgesic priming. In this study, we demonstrate that priming induced by repeated exposure to this A1-adenosine receptor agonist shares the same mechanisms, as MOR-agonist induced priming. However, the prolongation of PGE2 hyperalgesia induced by repeated administration of CPA depends on G-protein αi subunit activation, differently from DAMGO-induced type II priming, in which it depends on the β/γ subunit. These data implicate a novel form of Gi-protein signaling pathway in the type II hyperalgesic priming induced by repeated administration of an agonist at A1-adenosine receptor to the peripheral terminal of the nociceptor.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Correlation of changes in pain intensity with synovial fluid adenosine triphosphate levels after treatment of patients with osteoarthritis of the knee with high-molecular-weight hyaluronic acid

We sought to determine whether a clinical association exists between osteoarthritis (OA)-associated knee pain and adenosine triphosphate (ATP) levels in synovial fluid (SF). A total of 28 patients with 28 primary OA knees were included. They routinely received intra-articular injection of high-molecular-weight hyaluronic acid (HA) once weekly for 5 weeks (treated group). Eight patients without knee pain who had undergone an operation for anterior or posterior cruciate ligament reconstruction 2 years ago were also examined (control group). SF and blood ATP concentrations, total amount of ATP, total SF volume, and Visual Analogue Scale (VAS) scores in all patients were measured and we compared pre-treatment values with those 1 week after the final treatment. We evaluated the correlation of change in total ATP (ΔATP) and change in VAS score (ΔVAS), ΔVAS and change in SF volume (ΔSF), and ATP concentration in SF and blood. In the treated group, SF ATP concentration, total amount of ATP, SF volume, and VAS score were all significantly lower post-treatment than pre-treatment (p = 0.0005, 0.0003, 0.0022, and < 0.0001, respectively). In treated group, ΔVAS was significantly associated with ΔATP (r = 0.56, p = 0.0032), ΔSF was significantly associated with ΔVAS (r = 0.78, p < 0.0001), and total amount of SF ATP and SF volume at pre-treatment were significantly higher than the control group (p < 0.0001, p < 0.0001) We demonstrated an association between SF ATP level changes and OA knee pain, which should facilitate a further understanding of OA pain mechanisms.

Adenosine-induced activation of esophageal nociceptors

Clinical studies implicate adenosine acting on esophageal nociceptive pathways in the pathogenesis of noncardiac chest pain originating from the esophagus. However, the effect of adenosine on esophageal afferent nerve subtypes is incompletely understood. We addressed the hypothesis that adenosine selectively activates esophageal nociceptors. Whole cell perforated patch-clamp recordings and single-cell RT-PCR analysis were performed on the primary afferent neurons retrogradely labeled from the esophagus in the guinea pig. Extracellular recordings were made from the isolated innervated esophagus. In patch-clamp studies, adenosine evoked activation (inward current) in a majority of putative nociceptive (capsaicin-sensitive) vagal nodose, vagal jugular, and spinal dorsal root ganglia (DRG) neurons innervating the esophagus. Single-cell RT-PCR analysis indicated that the majority of the putative nociceptive (transient receptor potential V1-positive) neurons innervating the esophagus express the adenosine receptors. The neural crest-derived (spinal DRG and vagal jugular) esophageal nociceptors expressed predominantly the adenosine A(1) receptor while the placodes-derived vagal nodose nociceptors expressed the adenosine A(1) and/or A(2A) receptors. Consistent with the studies in the cell bodies, adenosine evoked activation (overt action potential discharge) in esophageal nociceptive nerve terminals. Furthermore, the neural crest-derived jugular nociceptors were activated by the selective A(1) receptor agonist CCPA, and the placodes-derived nodose nociceptors were activated by CCPA and/or the selective adenosine A(2A) receptor CGS-21680. In contrast to esophageal nociceptors, adenosine failed to stimulate the vagal esophageal low-threshold (tension) mechanosensors. We conclude that adenosine selectively activates esophageal nociceptors. Our data indicate that the esophageal neural crest-derived nociceptors can be activated via the adenosine A(1) receptor while the placodes-derived esophageal nociceptors can be activated via A(1) and/or A(2A) receptors. Direct activation of esophageal nociceptors via adenosine receptors may contribute to the symptoms in esophageal diseases.

Adenosine receptor subtype-selective antagonists in inflammation and hyperalgesia

In this study, we examined the effects of systemic and local administration of the subtype-selective adenosine receptor antagonists PSB-36, PSB-1115, MSX-3, and PSB-10 on inflammation and inflammatory hyperalgesia. Pharmacological blockade of adenosine receptor subtypes after systemic application of antagonists generally led to a decreased edema formation after formalin injection and, with the exception of A(3) receptor antagonism, also after the carrageenan injection. The selective A(2B) receptor antagonist PSB-1115 showed a biphasic, dose-dependent effect in the carrageenan test, increasing edema formation at lower doses and reducing it at a high dose. A(1) and A(2B) antagonists diminished pain-related behaviors in the first phase of the formalin test, while the second, inflammatory phase was attenuated by A(2B) and A(3) antagonists. The A(2B) antagonist was particularly potent in reducing inflammatory pain dose-dependently reaching the maximum effect at a low dose of 3 mg/kg. Inflammatory hyperalgesia was totally eliminated by the A(2A) antagonist MSX-3 at a dose of 10 mg/kg. In contrast to the A(1) antagonist, the selective antagonists of A(2A), A(2B), and A(3) receptors were also active upon local administration. Our results demonstrate that the blockade of adenosine receptor subtypes can decrease the magnitude of inflammatory responses. Selective A(2A) antagonists may be useful for the treatment of inflammatory hyperalgesia, while A(2B) antagonists have potential as analgesic drugs for the treatment of inflammatory pain.

Adenosine A2A receptors in ventral striatum, hypothalamus and nociceptive circuitry implications for drug addiction, sleep and pain

Adenosine A2A receptors localized in the dorsal striatum are considered as a new target for the development of antiparkinsonian drugs. Co-administration of A2A receptor antagonists has shown a significant improvement of the effects of l-DOPA. The present review emphasizes the possible application of A2A receptor antagonists in pathological conditions other than parkinsonism, including drug addiction, sleep disorders and pain. In addition to the dorsal striatum, the ventral striatum (nucleus accumbens) contains a high density of A2A receptors, which presynaptically and postsynaptically regulate glutamatergic transmission in the cortical glutamatergic projections to the nucleus accumbens. It is currently believed that molecular adaptations of the cortico-accumbens glutamatergic synapses are involved in compulsive drug seeking and relapse. Here we review recent experimental evidence suggesting that A2A antagonists could become new therapeutic agents for drug addiction. Morphological and functional studies have identified lower levels of A2A receptors in brain areas other than the striatum, such as the ventrolateral preoptic area of the hypothalamus, where adenosine plays an important role in sleep regulation. Although initially believed to be mostly dependent on A1 receptors, here we review recent studies that demonstrate that the somnogenic effects of adenosine are largely mediated by hypothalamic A2A receptors. A2A)receptor antagonists could therefore be considered as a possible treatment for narcolepsy and other sleep-related disorders. Finally, nociception is another adenosine-regulated neural function previously thought to mostly involve A1 receptors. Although there is some conflicting literature on the effects of agonists and antagonists, which may partly be due to the lack of selectivity of available drugs, the studies in A2A receptor knockout mice suggest that A2A receptor antagonists might have some therapeutic potential in pain states, in particular where high intensity stimuli are prevalent.

Arthritis and pain. Neurogenic origin of joint pain

Arthritis pain affects millions of people worldwide yet we still have only a limited understanding of what makes our joints ache. This review examines the sensory innervation of diarthroidal joints and discusses the neurophysiological processes that lead to the generation of painful sensation. During inflammation, joint nerves become sensitized to mechanical stimuli through the actions of neuropeptides, eicosanoids, proteinase-activated receptors and ion channel ligands. The contribution of immunocytes to arthritis pain is also reviewed. Finally, the existence of an endogenous analgesic system in joints is considered and the reasons for its inability to control pain are postulated.

Adenosine receptor mediates nicotine-induced antinociception in formalin test

In this study, the effect of adenosine receptor agents on nicotine induced antinociception, in formalin test, has been investigated. Intraperitoneal (i.p.) administration of different doses of nicotine (0.1, 1, 10 and 100 microgkg(-1)) induced a dose-dependent antinociception in mice, in the both first and second phases of the test. Adenosine receptor antagonist, theophylline (5, 10, 20 and 80 mgkg(-1), i.p.) also induced antinociception in the both phases, while a dose of the drug (40 mgkg(-1), i.p.) did not induce any response. Theophylline reduced antinociception induced by nicotine in both phases of formalin test. The A(2) receptor agonist, 5'-N-ethylcarboxamide adenosine (NECA; 1 and 5 microgkg(-1), i.p.) also produced antinociception, which was reversed with different doses of theophylline (5, 10, 20 and 40 mgkg(-1), i.p.). But administration of the adenosine receptor agonist, NECA did not potentiate the response of nicotine. It is concluded that adenosine system may be involved in modulation of antinociception induced by nicotine.

Experimental pain by ischaemic contractions compared with pain by intramuscular infusions of adenosine and hypertonic saline

Deep tissue pain can be related to reduced muscle blood flow, which comprises the metabolic demand under muscle work. The tissues and receptors involved in nociception after ischaemic muscle contractions are not known. The concentration of adenosine is increased after ischaemic contractions and might act as an algesic substance. In 15 subjects, adenosine, hypertonic saline (algesic), and isotonic mannitol (placebo) were infused into the tibialis anterior muscle and compared with the pain caused by ischaemic contractions. The muscle pain intensity (visual analogue scale; VAS), distribution, and quality were assessed. Pressure pain thresholds were recorded to assess the deep tissue sensitivity. Adenosine did not induce more pain than the placebo. The maximal VAS score after hypertonic saline and ischaemic contractions was higher compared with adenosine/placebo infusions. The duration and area of pain were significantly increased after hypertonic saline infusions compared with ischaemic contractions. Higher scores on the McGill pain questionnaire were given to the "stabbing", "burning", "heavy", and "exhausting" word categories after ischaemic contractions, and "cramping" was rated higher during hypertonic saline-induced muscle pain compared with ischaemic contractions. During hypertonic saline infusions, the pressure pain threshold was decreased compared with before and immediately after the pain had vanished. The present study shows that pharmacological levels of adenosine in skeletal muscle did not induce pain. Excitation of muscle nociceptors by hypertonic saline evoked hyperalgesia, larger areas of pain, and a different quality of pain compared with ischaemic contractions, suggesting that the pain after ischaemic contractions is mediated by other populations of nociceptors in muscle and/or other tissues than excited by hypertonic saline.

Intravenous adenosine alleviates neuropathic pain: a double blind placebo controlled crossover trial using an enriched enrolment design

Adenosine analogs produce analgesic actions in nociceptive paradigms and alleviate manifestations of neuropathic pain in nerve injury models in rodents. In humans, previous work indicates an analgesic effect for adenosine administered intravenously in postoperative and neuropathic pain. In this double blind placebo controlled crossover trial, we used an enriched enrolment design to determine the effects of intravenous adenosine (50 microg/kg/min over 60min) on neuropathic pain. In Phase 1 of the trial, adenosine was administered in an open label manner, while in Phase 2 adenosine was administered in a double blind placebo controlled manner to 23 adenosine responders who had experienced a 30% or greater response in the open trial. Outcome measures included the McGill pain questionnaire (MPQ), which generates a pain rating index (PRI), and contains a visual analog scale (VAS) of pain intensity, the neuropathy pain scale (NPS), and a VAS for pain relief. Subjects also graded the degree of allodynia and hyperalgesia using a VAS. Adenosine led to a significant reduction in spontaneous pain according to the MPQ-PRI, the MPQ-VAS and the VAS for pain relief. The NPS showed a pattern similar to the MPQ-PRI, with statistically significant reductions in scales 1 (intensity), 3 (hot), 6 (sensitive), 7 (itchy) and 9 (unpleasant). Adenosine also led to a significant reduction in pinprick hyperalgesia, but not in allodynia. Three patients from Phase 1 of the trial experienced long term resolution of their pain following intravenous adenosine (5,16,25 months). The results of this study support previous reports that indicate intravenous adenosine alleviates neuropathic pain and hyperalgesia.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

Topical and peripherally acting analgesics

Acute nociceptive, inflammatory, and neuropathic pain all depend to some degree on the peripheral activation of primary sensory afferent neurons. The localized peripheral administration of drugs, such as by topical application, can potentially optimize drug concentrations at the site of origin of the pain, while leading to lower systemic levels and fewer adverse systemic effects, fewer drug interactions, and no need to titrate doses into a therapeutic range compared with systemic administration. Primary sensory afferent neurons can be activated by a range of inflammatory mediators such as prostanoids, bradykinin, ATP, histamine, and serotonin, and inhibiting their actions represents a strategy for the development of analgesics. Peripheral nerve endings also express a variety of inhibitory neuroreceptors such as opioid, alpha-adrenergic, cholinergic, adenosine and cannabinoid receptors, and agonists for these receptors also represent viable targets for drug development. At present, topical and other forms of peripheral administration of nonsteroidal anti-inflammatory drugs, opioids, capsaicin, local anesthetics, and alpha-adrenoceptor agonists are being used in a variety of clinical states. There also are some clinical data on the use of topical antidepressants and glutamate receptor antagonists. There are preclinical data supporting the potential for development of local formulations of adenosine agonists, cannabinoid agonists, cholinergic ligands, cytokine antagonists, bradykinin antagonists, ATP antagonists, biogenic amine antagonists, neuropeptide antagonists, and agents that alter the availability of nerve growth factor. Given that activation of sensory neurons involves multiple mediators, combinations of agents targeting different mechanisms may be particularly useful. Topical analgesics represent a promising area for future drug development.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

A pulsed DC electric field affects P2-purinergic receptor functions by altering the ATP levels in in vitro and in vivo systems

Recently it was shown that extracellular ATP, acting through purinergic receptors, has many physiological functions, including opening of Ca(2+)-ion channels, activation and mediation of signal transduction mechanisms as well as activation of the pain sensation. Since electrical stimulation is also known to affect many signal transduction processes as well as the alleviation of pain, we hypothesized that electric stimulation may affect the extracellular release of ATP. We investigated the effects of a small DC electric field (10(1)--10(2) V m(-1) range and with frequencies below 150 Hz) on the release of ATP in vitro (HeLa cells), and on the levels of ATP in vivo (the plasma of healthy volunteers). In HeLa cells ATP release was increased 50 fold, while the total amount of ATP in the cells was increased by 163%. In the plasma a significant decrease (P<0.05) in ATP concentration was seen after electrical stimulation, in all the volunteers. The small DC electric field also affected the cAMP signal transduction system in vitro (HeLa cells and human lymphocytes) and in vivo (human plasma). Decreased levels of cAMP (P<0.05) were seen in HeLa cells and increased levels of cAMP (P<0.05) in isolated human lymphocytes. The cAMP levels in the plasma of the electrically treated volunteers were lower than control values. These results show that the frequency, waveform and signal strength of the applied electric field are suitable for effecting measurable changes on signal transduction in vitro and in vivo.

The plasma protein extravasation induced by adenosine and its analogues in the rat dorsal skin: evidence for the involvement of capsaicin sensitive primary afferent neurones and mast cells

1. The contribution of sensory neurons and mast cells to the oedema evoked by adenosine A1 (N(6)-cyclopentyladenosine, CPA, 3 - 30 nmol site(-1)), A2 (5'N-ethylcarboxamidoadenosine, NECA, 1 - 10 nmol site(-1)) and A3 receptor agonists (N6-[3-iodobenzyl]-N-methyl-5'-carboxiamidoadenosine, IB-MECA, 0.01 - 3 nmol site(-1)) was investigated in the rat skin microvasculature, by the extravascular accumulation of intravenously-injected (i.v.) 125I-albumin. 2. Intradermal (i.d.) injection of adenosine and analogues induced increased microvascular permeability in a dose-dependent manner (IB-MECA > NECA > CPA > adenosine). The non-selective adenosine receptor antagonist theophylline (5 - 50 nmol site(-1)) markedly inhibited adenosine, CPA or NECA but not IB-MECA-induced plasma extravasation. The A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.3 - 3 micromol kg(-1), i.v.) significantly reduced CPA-induced plasma extravasation whereas responses to adenosine, NECA or IB-MECA were unchanged. The A2 receptor antagonist 3,7-dymethyl-1-proprargylxanthine (DMPX, 0.5 - 50 nmol site(-1)) significantly reduced NECA-induced plasma extravasation without affecting responses to adenosine, CPA and IB-MECA. 3. The tachykinin NK1 receptor antagonist (S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl) piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2]octane chloride (SR140333), but not the NK2 receptor antagonist (S)-N-methyl-N[4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl]-benzamide (SR48968), significantly inhibited the plasma extravasation evoked by higher doses of adenosine (100 nmol site(-1)), CPA (100 nmol site(-1)), NECA (1 nmol site(-1)) and IB-MECA (0.1 - 1 nmol site(-1)). In rats treated with capsaicin to destroy sensory neurons, the response to higher doses of adenosine, CPA and NECA, but not IB-MECA, was significantly inhibited. 4. The effects of adenosine and analogues were largely inhibited by histamine and 5-hydroxytryptamine (5-HT) antagonists and by compound 48/80 pretreatment. 5. In conclusion, our results provide evidence that adenosine A1 and A2, but not A3, receptor agonists may function as cutaneous neurogenic pro-inflammatory mediators; acting via microvascular permeability-increasing mechanisms that can, depending on dose of agonist and purine receptor under study, involve the tachykinin NK1 receptor and mast cell amines.

Novel purinergic sensitivity develops in injured sensory axons following sciatic nerve transection in rat

Teased fibers were made from 153 spontaneous A afferents ending in sciatic nerve end neuromas of 3-14 days standing, 21 A afferents from intact sensory endings in the contralateral sciatic nerve, and 50 intact A afferents from the sciatic nerve in intact rats. Ninety-two percent of the injured fibers responded to adenosine 5'-triphosphate (ATP) (i.v.). However, few fibers from the contralateral nerve or nerves from intact animals responded to ATP. P2 receptor antagonist suramin or reactive blue 2 blocked the ATP-induced response in 76% of the fibers tested, whereas the P1 receptor antagonist aminophylline blocked the ATP-evoked effect in only 18% of the fibers tested. Sympathectomy did not affect the ATP-induced effects in injured axons. Close-arterial injection of ATP caused similar results as i.v. injection of ATP. The present study suggests that a novel purinergic sensitivity is developed at the injury site after sciatic nerve transection in rats, which may play a role in neuropathic pain under some conditions such as sympathetic activation.

ATP release by mechanically loaded porcine chondrons in pellet culture

OBJECTIVE

To determine whether ATP is released from chondrocytes during mechanical stimulation and whether degradation of ATP generates inorganic pyrophosphate in chondron pellet cultures.

METHODS

Chondron pellets were formed from 1.6 x 10(6) cells that had been enzymatically isolated from porcine articular cartilage. ATP was measured in media from cultures at rest and during fluid movement and cyclic compression. ATP hydrolysis was examined by high-performance liquid chromatography following the addition of gamma32P-ATP to resting cultures.

RESULTS

Pellet cultures at rest maintained a steady-state concentration of 2-4 nM ATP in 2 ml of medium. The ATP concentration increased 5-12-fold with cyclic compression (7.5 and 15 kPa at 0.5 Hz), then decreased to preloading levels within 60 minutes despite continued loading. A subsequent increase in pressure stimulated a further increase in ATP release, suggesting that chondrocytes desensitize to load. Cell viability was similar for pellets at rest and up to 24 hours after compression. ATP released in response to mechanical stimulation was inhibited 50% by 0.5 mM octanol, suggesting a regulated mechanism for ATP release. Exogenous ATP was rapidly hydrolyzed to pyrophosphate in resting cultures.

CONCLUSION

The occurrence of basal levels of extracellular ATP in the presence of pyrophosphohydrolase activity indicates that ATP was continuously released by chondrocytes at rest. Considering that chondrocytes express purinoceptors that respond to ATP, we suggest a role for ATP in extracellular signaling by chondrocytes in response to mechanical load. ATP released by chondrocytes in response to mechanical load is a likely source of pyrophosphate in calcium pyrophosphate dihydrate crystal deposition diseases.

Involvement of mast cells, sensory afferents and sympathetic mechanisms in paw oedema induced by adenosine A(1) and A(2B/3) receptor agonists

Both the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine and the adenosine A(2B/3) receptor agonist N(6)-benzyl-5'-N-ethylcarboxamido adenosine (N(6)-B-NECA) produce an acute paw oedema response following local s.c. injection into the rat hindpaw. This study characterized aspects of the mechanisms by which these responses occur by determining the effect of compound 48/80 (mast cell depleting agent), capsaicin (sensory neurotoxin) and 6-hydroxydopamine (sympathetic nervous system neurotoxin) on the paw oedema response produced by these agents. Compound 48/80 markedly reduced the increase in paw volume produced by both N(6)-cyclopentyladenosine and N(6)-B-NECA. Capsaicin significantly reduced paw oedema induced by N(6)-cyclopentyladenosine but not N(6)-B-NECA. In contrast, 6-hydroxydopamine reduced paw oedema induced by N(6)-B-NECA but not N(6)-cyclopentyladenosine. These results indicate an involvement of mast cells in paw oedema produced by both adenosine A(1) and A(2B/3) receptor agonists. For N(6)-cyclopentyladenosine, this involvement may be a secondary involvement due to activation of a neurogenic mechanism, but for N(6)-B-NECA, it may be a direct effect on mast cells. The nature of the involvement of the sympathetic nervous system in the action of N(6)-B-NECA is not entirely clear.

Acute paw oedema induced by local injection of adenosine A(1), A(2) and A(3) receptor agonists

The present study used plethysmometry to examine oedema following local injection of selective adenosine A(1), A(2) and A(3) receptor agonists and inhibitors of adenosine metabolism into the hindpaw of the rat. N(6)-Cyclopentyladenosine and L-N(6)-phenylisopropyladenosine (A(1)), 2-[p(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS21680) (A(2A)) and N(6)-benzyl-5'-N-ethylcarboxamido adenosine (N(6)-B-NECA) (A(3)) all produced an increase in paw volume (N(6)N(6)-cyclopentyladenosine, L-N(6)CGS21680). At the highest dose, each agent also produced a systemically mediated suppression of oedema. Oedema by N(6)-cyclopentyladenosine was blocked by caffeine, 8-cyclopentyl-1,3-dimethylxanthine and enprofylline. Oedema by CGS21680 was blocked by caffeine and 8-cyclopentyl-1, 3-dimethylxanthine. Oedema by N(6)-B-NECA was blocked by enprofylline, but not by caffeine or 8-cyclopentyl-1, 3-dimethylxanthine, or by systemic administration of MRS 1191. Oedema by both N(6)-cyclopentyladenosine and N(6)-B-NECA was blocked by mepyramine, ketanserin and phentolamine, but that by CGS21680 was not. The adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine and the adenosine deaminase inhibitor 2'-deoxycoformycin produced only a limited increase in paw volume, and this was blocked by caffeine. This study demonstrates an acute paw oedema response following local administration of adenosine A(1), A(2) and A(3) receptor agonists, which likely results from different mechanisms of action in each case.

Excitatory effect of P2X receptor activation on mesenteric afferent nerves in the anaesthetised rat

1. We examined the effects of P2X purinoceptor agonists and P2 purinoceptor antagonists on mesenteric afferent nerves supplying the jejunum in the pentobarbitone sodium-anaesthetised rat. 2. ATP (0. 01-10 mg kg-1, i.a.) and alpha,beta-methylene-ATP (1-30 microg kg-1, i.a.) each induced dose-dependent increases in afferent nerve discharge and intrajejunal pressure. The effect on afferent nerves comprised an early (< 2 s after administration) intense burst of activity followed by a later increase (> 2 s after administration), less pronounced in comparison, which coincided with elevated intrajejunal pressure. 3. Pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (20 mg kg-1, i.v.) and suramin (80 mg kg-1, i.v. ) each antagonised both the early and later increases in afferent nerve discharge elicited by alpha,beta-methylene-ATP (30 microg kg-1, i.a.). 4. Co-administration of omega-conotoxin MVIIA and omega-conotoxin SVIB (each at 25 microg kg-1, i.v.), or treatment with the selective 5-HT3 receptor antagonist alosetron (30 microg kg-1, i.v.), did not affect the rapid burst of afferent nerve activity elicited by alpha,beta-methylene-ATP (30 microg kg-1, i.a.). However, toxin treatment did attenuate the elevations in intrajejunal pressure and the corresponding later phases of evoked afferent discharge, while alosetron inhibited basal afferent nerve activity. 5. In summary, ATP and alpha,beta-methylene-ATP each evoke excitation of mesenteric afferent nerves in the anaesthetised rat. We propose that the early increase in mesenteric afferent nerve activity represents a direct effect on the nerve ending, mediated by P2X receptors, whereas the later increase reflects activation of mechanosensitive fibres secondary to elevated intrajejunal pressure.

Effects of dexamethasone on airway hyper-responsiveness to the adenosine A1 receptor agonist cyclo-pentyl adenosine in an allergic rabbit model

1. New Zealand White (NZW) rabbits were immunized within 24 h of birth with Alternaria tenuis in aluminium hydroxide (Al (OH)3) (i.p.) or sham immunized (saline plus Al (OH)3 i.p.) and subsequently injected with the allergen (i.p.) or sham-immunized for the next 3 months. At 3 months of age, baseline airway responsiveness was assessed using cyclo-pentyl adenosine (CPA). Bronchoalveolar lavage (BAL) was performed in all animals and samples of peripheral blood were collected from some animals for estimation of dexamethasone levels. In some animals, blood was collected at the end of the experiment and cellular function was assessed by measurement of ex vivo proliferation of mononuclear cells in response to phytohaemagglutinin (PHA). 2. Allergen immunization significantly increased baseline airway responsiveness to inhaled CPA (P<0.05) in comparison with sham-immunized animals, at 3 months after immunization. Dexamethasone (0.5 mg kg(-1) day(-1)) treatment for 1 month did not modify this established airway hyper-responsiveness to CPA. Dexamethasone treatment did not affect either total or differential cell numbers in BAL fluid during the 4 week period, although significant plasma levels of dexamethasone were achieved in dexamethasone treated animals. 3. Treatment of rabbits with dexamethasone (0.1 mg kg(-1) i.p.), 6 h prior to each allergen injection from the neonatal stage, significantly reduced baseline airway hyper-responsiveness to CPA measured at 3 months (P<0.05). There was no significant difference in either total or differential cell numbers in BAL fluid, or any difference in mitogen-induced proliferation of mononuclear cells between dexamethasone and vehicle treated rabbits. 4. These results suggest that introduction of glucocorticosteroids in early life can prevent baseline airway hyper-responsiveness to inhaled CPA in allergic rabbits. However, once established, such underlying airway hyper-responsiveness is difficult to resolve, even with prolonged treatment with glucocorticosteroids.