Adenosine receptor-mediated control of in vitro release of pain-related neuropeptides from the rat spinal cord

Glial A2B Adenosine Receptors Modulate Abnormal Tachykininergic Responses and Prevent Enteric Inflammation Associated with High Fat Diet-Induced Obesity

The role played by adenosine A_2B receptors (A_2BRs) in the regulation of enteric glial cell (EGC) functions remains unclear. This study was aimed at investigating the involvement of A_2BRs in the control of EGC functions in a model of obesity. C57BL/6 mice were fed with standard diet (SD) or high fat diet (HFD) for eight weeks. Colonic tachykininergic contractions were recorded in the presence of BAY60-6583 (A_2BRs agonist), MRS1754 (A_2BRs antagonist), and the gliotoxin fluorocitrate. Immunofluorescence distribution of HuC/D, S100β, and A_2BRs was assessed in whole mount preparations of colonic myenteric plexus. To mimic HFD, EGCs were incubated in vitro with palmitate (PA) and lipopolysaccharide (LPS), in the absence or in the presence of A_2BR ligands. Toll-like receptor 4 (TLR4) expression was assessed by Western blot analysis. Interleukin-1β (IL-1β), substance P (SP), and glial cell derived neurotrophic factor (GDNF) release were determined by enzyme-linked immunosorbent assay (ELISA) assays. MRS1754 enhanced electrically evoked tachykininergic contractions of colonic preparations from HFD mice. BAY60-6583 decreased the evoked tachykininergic contractions, with higher efficacy in HFD mice. Such effects were blunted upon incubation with fluorocitrate. In in vitro experiments on EGCs, PA and LPS increased TLR4 expression as well as IL-1β, GDNF, and SP release. Incubation with BAY60-6583 reduced TLR4 expression as well as IL-1β, GDNF, and SP release. Such effects were blunted by MRS1754. The present results suggest that A_2BRs, expressed on EGCs, participate in the modulation of enteric inflammation and altered tachykininergic responses associated with obesity, thus representing a potential therapeutic target.

Caffeine inhibits hypothalamic A1R to excite oxytocin neuron and ameliorate dietary obesity in mice

Caffeine, an antagonist of the adenosine receptor A₁R, is used as a dietary supplement to reduce body weight, although the underlying mechanism is unclear. Here, we report that adenosine level in the cerebrospinal fluid, and hypothalamic expression of A₁R, are increased in the diet-induced obesity (DIO) mouse. We find that mice with overexpression of A₁R in the neurons of paraventricular nucleus (PVN) of the hypothalamus are hyperphagic, have glucose intolerance and high body weight. Central or peripheral administration of caffeine reduces the body weight of DIO mice by the suppression of appetite and increasing of energy expenditure. We also show that caffeine excites oxytocin expressing neurons, and blockade of the action of oxytocin significantly attenuates the effect of caffeine on energy balance. These data suggest that caffeine inhibits A₁Rs expressed on PVN oxytocin neurons to negatively regulate energy balance in DIO mice.

Spinal inhibitory neurotransmission in neuropathic pain

Nerve injury increases the spinal cord expression and/or activity of voltage- and ligand-gated ion channels, peptide receptors, and neuroimmune factors, which then drive dorsal horn neuron hyperexcitability. The intensity and duration of this central sensitization is determined by the net activity of local excitatory and inhibitory neurotransmitter systems, together with ongoing/evoked primary afferent activity and descending supraspinal control. Spinal endogenous inhibitory systems serve as opposing compensatory influences and are gaining recognition for their powerful capacity to restrain allodynia and hyperalgesia. These include numerous G protein-coupled receptors (mu- and delta-opioid, alpha(2)-adrenergic, purinergic A1, neuropeptide Y1 and Y2, cannabinoid CB1 and CB2, muscarinic M2, gamma-amino-butyric acid type B, metabotropic glutamate type II-III, somatostatin) and perhaps nuclear receptors (peroxisome proliferator-activated receptor gamma). Excessive downregulation or defective compensatory upregulation of these systems may contribute to the maintenance of neuropathic pain. An increasing number of pharmacotherapeutic strategies for neuropathic pain are emerging that mimic and enhance inhibitory neurotransmission in the dorsal horn.

Fructose-1,6-bisphosphate reduces inflammatory pain-like behaviour in mice: role of adenosine acting on A1 receptors

BACKGROUND AND PURPOSE

D-Fructose-1,6-bisphosphate (FBP) is an intermediate in the glycolytic pathway, exerting pharmacological actions on inflammation by inhibiting cytokine production or interfering with adenosine production. Here, the possible antinociceptive effect of FBP and its mechanism of action in the carrageenin paw inflammation model in mice were addressed, focusing on the two mechanisms described above.

EXPERIMENTAL APPROACH

Mechanical hyperalgesia (decrease in the nociceptive threshold) was evaluated by the electronic pressure-metre test; cytokine levels were measured by elisa and adenosine was determined by high performance liquid chromatography.

KEY RESULTS

Pretreatment of mice with FBP reduced hyperalgesia induced by intraplantar injection of carrageenin (up to 54%), tumour necrosis factor alpha (40%), interleukin-1 beta (46%), CXCL1 (33%), prostaglandin E(2) (41%) or dopamine (55%). However, FBP treatment did not alter carrageenin-induced cytokine (tumour necrosis factor alpha and interleukin-1 beta) or chemokine (CXCL1) production. On the other hand, the antinociceptive effect of FBP was prevented by systemic and intraplantar treatment with an adenosine A(1) receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine), suggesting that the FBP effect is mediated by peripheral adenosine acting on A(1) receptors. Giving FBP to mice increased adenosine levels in plasma, and adenosine treatment of paw inflammation presented a similar antinociceptive mechanism to that of FBP.

CONCLUSIONS AND IMPLICATIONS

In addition to anti-inflammatory action, FBP also presents an antinociceptive effect upon inflammatory hyperalgesia. Its mechanism of action seems dependent on adenosine production but not on modulation of hyperalgesic cytokine/chemokine production. In turn, adenosine acts peripherally on its A(1) receptor inhibiting hyperalgesia. FBP may have possible therapeutic applications in reducing inflammatory pain.

Evidence for adenosine- and serotonin-mediated antihyperalgesic effects of cizolirtine in rats suffering from diabetic neuropathy

Cizolirtine is a novel non-opioid drug which demonstrated antinociceptive activity in numerous pain models in rodents. Yet, its mechanism of action remains unknown. Several lines of evidence support the idea that adenosine (ADO) and serotonin (5-HT) modulate nociceptive signaling. Our study aimed at investigating whether these neuroactive molecules could be implicated in the mechanism of action of cizolirtine. Cizolirtine-induced antihyperalgesia was compared before and after pretreatment with ADO A(1)-A(2A) and 5-HT(1B/1D) receptor ligands in rats rendered diabetic by streptozotocin pretreatment and suffering from neuropathic pain. Cizolirtine alone (30-80 mg/kg, i.p.) significantly increased mechanical nociceptive thresholds. Acute pretreatment with the A(1)-A(2A) receptor antagonist caffeine (5 mg/kg, i.p.) or the 5-HT(1B/1D) receptor antagonist GR-127,935 (3 mg/kg, i.p.) significantly reduced the antihyperalgesic effects of cizolirtine. Conversely, cizolirtine-induced antihyperalgesia was promoted by pretreatment with either the selective A(1) receptor agonist CPA (0.3 mg/kg, i.p.) or the selective 5-HT(1B) receptor agonist CP-94,253 (3mg/kg, i.p.), and this potentiation was totally prevented by acute pretreatment with respective antagonists. Interestingly, A(1) receptor blockade by DPCPX inhibited the promoting effect of CP-94,253 on cizolirtine-induced antihyperalgesia, suggesting that the adenosine A(1)-mediated step takes place downstream the serotonin 5-HT(1B)-mediated step in the neurobiological mechanisms underlying cizolirtine action.

Adenosine and ATP receptors

Adenosine and ATP, via P1 and P2 receptors respectively, can modulate pain transmission under physiological, inflammatory, and neuropathic pain conditions. Such influences reflect peripheral and central actions and effects on neurons as well as other cell types. In general, adenosine A1 receptors produce inhibitory effects on pain in a number of preclinical models and are a focus of attention. In humans, i.v. infusions of adenosine reduce some aspects of neuropathic pain and can reduce postoperative pain. For P2X receptors, there is a significant body of information indicating that inhibition of P2X3 receptors may be useful for relieving inflammatory and neuropathic pain. More recently, data have begun to emerge implicating P2X4, P2X7 and P2Y receptors in aspects of pain transmission. Both P1 and P2 receptors may represent novel targets for pain relief.

Roles of Adenosine Receptor Subtypes in the Antinociceptive Effect of Intrathecal Adenosine in a Rat Formalin Test

The contributions of adenosine receptor subtypes to antinociception produced by adenosine were determined at the spinal level. There are 4 types of adenosine receptors, namely A1, A(2A), A(2B) and A3. The authors investigated the properties of the subtypes of spinal adenosine receptors in terms of nociceptive modulation. The nociceptive state was induced by subcutaneously injecting formalin solution (5%, 50 microl) into the hind paws of male Sprague-Dawley rats. After observing the effect of intrathecal adenosine during the formalin test, the effects of intrathecal adenosine A1 (CPT), A(2A) (CSC), A(2B) (alloxazine) and A3 (MRS 1220) receptor antagonists on the action of adenosine were examined. Intrathecal adenosine inhibited phase 2 flinching response without affecting phase 1 response. CPT, CSC, alloxazine and MRS 1220 antagonized the antinociceptive action of adenosine during phase 2 of the formalin test. These results suggest that spinal adenosine A1, A(2A), A(2B) and A3 receptors may play an important role in the antinociception of adenosine in the formalin-induced facilitated state.

Antinociception of intrathecal adenosine receptor subtype agonists in rat formalin test

Adenosine has shown antinociceptive action via spinal adenosine receptors. There are four types of adenosine receptors: A1, A2A, A2B, and A3. We characterized the nature of types of adenosine receptors for the control of nociception at the spinal level. For nociception, formalin solution (5%, 50 microL) was injected into the hindpaw of male Sprague-Dawley rats. The effects of intrathecal adenosine A1 (CPA), A2A (DPMA), and A3 (IB-MECA) receptor agonists were examined. CPA and IB-MECA produced limited or no effect on the early phase response of the formalin test, respectively, but the two drugs depressed the late phase response. DPMA suppressed both phase responses. CPA was the most potent drug among the three in the late phase. These results suggest that spinal adenosine A1 and A2A receptors may be involved in the modulation of the early and the late phase responses of the formalin test, whereas adenosine A3 receptor may be involved in the regulation of the late phase response.

The role of intraspinal adenosine A1 receptors in sympathetic regulation

Using a splanchnic nerve-spinal cord preparation in vitro, we have previously demonstrated that tonic sympathetic activity is generated from the thoracic spinal cord. Here, we sought to determine if adenosine receptors play a role in modulating this spinally generated sympathetic activity. Various adenosine analogs were applied. N6-Cyclopentyladenosine (CPA, adenosine A1 receptor agonist) and 5'-N-ethylcarboxamidoadenosine (NECA, adenosine A1/A2 receptor agonist) reduced, while N6-[2-(4-aminophenyl)ethyl]adenosine (APNEA, non-selective adenosine A3 receptor agonist) did not alter sympathetic activity. The inhibitory effect of CPA or NECA on sympathetic activity was reversed by 8-cyclopentyltheophylline (CPT, adenosine A1 receptor antagonist) or abolished by CPT pretreatment. In the presence of 3,7-dimethyl-1-propargylxanthine (DMPX, adenosine A2 receptor antagonist), sympathetic activity was still reduced by CPA or NECA. Sympathetic activities were not changed by applications of the more selective adenosine A2 or A3 receptor agonists or antagonists, including 4-[2-[[6-amino-9-(N-ethyl-beta-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid (CGS21680), 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385), 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Chloro-IB-MECA), and 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191). These findings exclude a possible involvement of A2 or A3 receptors in sympathetic regulation at the spinal levels. Interestingly, CPT alone did not affect sympathetic activity, suggesting that adenosine A1 receptors are endogenously quiescent under our experimental conditions. We conclude that intraspinal adenosine A1 receptors may down-regulate sympathetic outflow and serve as a part of the scheme for neuroprotection.

Distribution of antinociceptive adenosine A1 receptors in the spinal cord dorsal horn, and relationship to primary afferents and neuronal subpopulations

Adenosine can reduce pain and allodynia in animals and man, probably via spinal adenosine A1 receptors. In the present study, we investigate the distribution of the adenosine A1 receptor in the rat spinal cord dorsal horn using immunohistochemistry, in situ hybridization, radioligand binding, and confocal microscopy. In the lumbar cord dorsal horn, dense immunoreactivity was seen in the inner part of lamina II. This was unaltered by dorsal root section or thoracic cord hemisection. Confocal microscopy of the dorsal horn revealed close anatomical relationships but no or only minor overlap between A1 receptors and immunoreactivity for markers associated with primary afferent central endings: calcitonin gene-related peptide, or isolectin B4, or with neuronal subpopulations: mu-opioid receptor, neuronal nitric oxide synthase, met-enkephalin, parvalbumin, or protein kinase Cgamma, or with glial cells: glial fibrillary acidic protein. A few adenosine A1 receptor positive structures were double-labeled with alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionic acid glutamate receptor subunits 1 and 2/3. The results indicate that most of the adenosine A1 receptors in the dorsal horn are located in inner lamina II postsynaptic neuronal cell bodies and processes whose functional and neurochemical identity is so far unknown. Many adenosine A1 receptor positive structures are in close contact with isolectin B4 positive C-fiber primary afferents and/or postsynaptic structures containing components of importance for the modulation of nociceptive information.

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.