Characterization of the antinociceptive effects of some adenosine analogues in the rat

Cellular Mechanisms for Antinociception Produced by Oxytocin and Orexins in the Rat Spinal Lamina II-Comparison with Those of Other Endogenous Pain Modulators

Much evidence indicates that hypothalamus-derived neuropeptides, oxytocin, orexins A and B, inhibit nociceptive transmission in the rat spinal dorsal horn. In order to unveil cellular mechanisms for this antinociception, the effects of the neuropeptides on synaptic transmission were examined in spinal lamina II neurons that play a crucial role in antinociception produced by various analgesics by using the whole-cell patch-clamp technique and adult rat spinal cord slices. Oxytocin had no effect on glutamatergic excitatory transmission while producing a membrane depolarization, γ-aminobutyric acid (GABA)-ergic and glycinergic spontaneous inhibitory transmission enhancement. On the other hand, orexins A and B produced a membrane depolarization and/or a presynaptic spontaneous excitatory transmission enhancement. Like oxytocin, orexin A enhanced both GABAergic and glycinergic transmission, whereas orexin B facilitated glycinergic but not GABAergic transmission. These inhibitory transmission enhancements were due to action potential production. Oxytocin, orexins A and B activities were mediated by oxytocin, orexin-1 and orexin-2 receptors, respectively. This review article will mention cellular mechanisms for antinociception produced by oxytocin, orexins A and B, and discuss similarity and difference in antinociceptive mechanisms among the hypothalamic neuropeptides and other endogenous pain modulators (opioids, nociceptin, adenosine, adenosine 5’-triphosphate (ATP), noradrenaline, serotonin, dopamine, somatostatin, cannabinoids, galanin, substance P, bradykinin, neuropeptide Y and acetylcholine) exhibiting a change in membrane potential, excitatory or inhibitory transmission in the spinal lamina II neurons.

Adenosine modulates excitatory synaptic transmission and suppresses neuronal death induced by ischaemia in rat spinal motoneurones

Although adenosine is an important neuromodulator, its role in modulating motor functions at the level of the spinal cord is poorly understood. In the present study, we investigated the effects of adenosine on excitatory synaptic transmission and neuronal death induced by experimental ischaemia by using whole-cell patch-clamp recordings from lamina IX neurones in spinal cord slices. Adenosine significantly decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) in almost all neurones examined that could be mimicked by an A(1) receptor agonist, N (6)-cyclopentyladenosine (CPA), and inhibited by an A(1) receptor antagonist, 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX). Interestingly, adenosine increased mEPSC frequency in the presence of DPCPX in a subpopulation of neurones. In these neurones, an A(2A) receptor agonist, 2-[4-(2-carbonylethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680), increased mEPSC frequency. Adenosine also induced an outward current that was blocked by the addition of Cs(+) and tetraethylammonium into the patch-pipette solution and inhibited in the presence of Ba(2+). The adenosine-induced outward current was mimicked by CPA, but not CGS21680, and inhibited by DPCPX. Moreover, superfusing with ischaemia simulating medium (ISM) generated an agonal inward current in all of the neurones tested. The latencies of the inward currents induced by ISM were significantly prolonged by adenosine or CPA, but not by CGS21680. These results suggest that adenosine receptors are functionally expressed in both the pre- and postsynaptic sites of lamina IX neurones and that their activation may exert multiple effects on motor function. Moreover, this study has provided a cellular basis for an involvement of A(1) receptors in the neuroprotective actions of adenosine.

Adenosine as a Non-Opioid Analgesic in the Perioperative Setting

Adenosine, a ubiquitous metabolic intermediate in the body, is involved in nearly every aspect of cell function, including neuromodulation and neurotransmission. Adenosine A(1) and A(2) receptors are widely distributed in the brain and spinal cord, and are a novel, non-opiate target for pain management. The potential of adenosine as a non-narcotic analgesic in anesthetized patients has been explored in clinical trials, including double-blind studies versus placebo and remifentanil infusion. These studies suggest that, compared to placebo or remifentanil, an intraoperative adenosine infusion stabilizes core hemodynamics and reduces the requirement for anesthesia during surgery. Further, adenosine improves postoperative recovery, as indicated by lower pain scores and less opioid consumption. The safety profile of adenosine has been well characterized based on use of currently approved adenosine products. The most common adverse events associated with its use include flushing, chest discomfort, dyspnea, headache, gastrointestinal discomfort, and lightheadedness. These effects are generally well tolerated and transient. Further studies are warranted to investigate the full potential of adenosine as a non-opioid analgesic in the perioperative setting.

Inhibition of adenosine kinase by phosphonate and bisphosphonate derivatives

The enzyme adenosine kinase (AK) plays a central role in regulating the intracellular and interstitial concentration of the purine nucleoside adenosine (Ado). In view of the beneficial effects of Ado in protecting tissues from ischemia and other stresses, there is much interest in developing AK inhibitors, which can regulate Ado concentration in a site- and event-specific manner. The catalytic activity of AK from different sources is dependent upon the presence of activators such as phosphate (Pi). In this work we describe several new phosphorylated compounds which either activate or inhibit AK. The compounds acetyl phosphate, carbamoyl phosphate, dihydroxyacetone phosphate and imidodiphosphate were found to stimulate AK activity in a dose-dependent manner comparable to that seen with Pi. In contrast, a number of phosphonate and bisphosphonate derivatives, which included clodronate and etidronate, were found to inhibit the activity of purified AK in the presence of Pi. These AK inhibitors (viz. clodronate, etidronate, phosphonoacetic acid, 2-carboxyethylphosphonic acid, N-(phosphonomethyl)-glycine and N-(phosphonomethyl)iminodiacetic acid), at concentrations at which they inhibited AK, were also shown to inhibit the uptake of (3)H-adenosine and its incorporation into macromolecules in cultured mammalian cells, indicating that they were also inhibiting AK in intact cells. The drug concentrations at which these effects were observed showed limited toxicity to the cultured cells, indicating that these effects are not caused by cellular toxicity. These results indicate that the enzyme AK provides an additional cellular target for the clinically widely used bisphosphonates and related compounds, which could possibly be exploited for a new therapeutic application. Our structure-activity studies on different AK activators and inhibitors also indicate that all of the AK activating compounds have a higher partial positive charge (delta(+)) on the central phosphorous atom in comparison to the inhibitors. This information should prove helpful in the design and synthesis of more potent inhibitors of AK.

Modulation of paracetamol antinociception by caffeine and by selective adenosine A2 receptor antagonists in mice

This study investigated the involvement of adenosine receptors in the interaction between paracetamol and caffeine in mice, using the adenosine A2A receptor antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH58261) and the adenosine A2B receptor antagonist 1-propyl-8-p-sulfophenylxanthine (PSB1115), in the tail immersion and hot-plate tests. Paracetamol (10-200 mg/kg) was antinociceptive in both tests, but, in contrast to previous studies, caffeine (10 mg/kg) was pronociceptive in the tail immersion test, and reduced the effects of paracetamol in both tests. SCH58261 (3 mg/kg) was antinociceptive in both tests and in its presence paracetamol (50 mg/kg) had no further effect. PSB1115 (10 mg/kg) had little effect alone but potentiated the effect of paracetamol (50 mg/kg) in the hot-plate test and abolished it in the tail immersion test. These results suggest that adenosine A2B receptors may be involved in the action of paracetamol in a pathway-dependent manner, and also support the existence of pronociceptive adenosine A2A receptors.

Adenosine modulation: a novel approach to analgesia and inflammation

Adenosine (ADO) is an endogenous purine nucleoside that functions as an extracellular signalling molecule. It is released locally at sites of cellular trauma, and acts on specific cell-surface purinergic receptors (termed P1 receptors) near its site of release to exert its effects. Four subtypes of the P1 family of G-protein-coupled receptors have been identified and cloned: A1, A2A, A2B and A3. A considerable body of evidence, including experimental animal data and preliminary clinical reports, indicates that ADO is involved in modulating endogenous antinociceptive processes in the brain and spinal cord. ADO analogues provide analgesic activity after systemic or spinal administration in a broad spectrum of animal pain models. In addition, iv. ADO infusion has shown benefit in human pain states. The spinal cord is a key site for ADO-mediated modulation of nociception. ADO is well known to act as an inhibitory neuromodulator in the central and peripheral nervous system, and it may act to control N-methyl-D-aspartate (NMDA)- and substance P-mediated events in nociception and central sensitisation at the spinal level. ADO is also released at sites of inflammation and it exerts anti-inflammatory effects via multiple mechanisms involving several cell types. These include effects on neutrophil function, endothelial cell permeability, in vivo and in vitro release of tumour necrosis factor (TNF-alpha and collagenase expression in synoviocytes. Accordingly, ADO analogues are effective in several animal models of inflammation, including the rat adjuvant arthritis model. Several therapeutic approaches to pain and inflammation, based on mimicking or modulating the effects of endogenous ADO, are currently under preclinical and clinical investigation. These include the use of ADO itself, the use of direct-acting ADO receptor agonists and the use of agents designed to modulate the levels and, therefore, the actions of ADO in the extracellular space (ADO kinase (AK) inhibitors). Data emerging in the next several years should indicate whether these strategies represent a therapeutically useful new approach to analgesia and inflammation.

Intrathecal adenosine increases spinal cord blood flow in the rat: measurements with the laser-Doppler flowmetry technique

BACKGROUND

Adenosine and adenosine analogues induce antinociception both after systemic and intrathecal (i.t.) administration in animal models. Further, patients with neuropathic pain have been treated successfully with i.t. adenosine. Prior to introducing new analgesic drugs for regular spinal use in humans, experimental studies must be undertaken to evaluate the risks of neurotoxicity. It is important to evaluate the possibility of cytotoxic effects and that antinociception may be due to decreased spinal cord blood flow (SCBF) and neural ischaemia. The present study evaluates whether adenosine or isotonic mannitol induces changes in SCBF as assessed by laser-Doppler flowmetry (LDF).

METHODS

After laminectomy and insertion of i.t. catheters, seven rats received adenosine 50 microg in isotonic mannitol 500 microg, six rats received isotonic mannitol 500 microg and eight rats received saline 0.9%. SCBF was registered by the LDF technique continuously for 3 h after injection. Arterial blood pressure was also assessed.

RESULTS

In the adenosine in mannitol group, SCBF increased up to 230% of baseline levels for almost 40 min, P = 0.044 and then declined. In the mannitol group, SCBF increased up to 180% of baseline (P < 0.011) before declining. At 60 min, SCBF had returned to saline levels and remained stable during the rest of the experiment.

CONCLUSION

Intrathecal administration of adenosine in mannitol and of mannitol both increased SCBF in rats, compared with saline. It is unlikely that the effects on SCBF induced by adenosine and mannitol could result in neurotoxicity of the spinal cord.

Population pharmacokinetic–pharmacodynamic modelling of the anti-hyperalgesic effect of 5′deoxy-N6-cylopentyladenosine in the mononeuropathic rat

The objective of this investigation was to characterise the pharmacokinetic-pharmacodynamic correlation of 5'-deoxy-N6-cyclopentyl-adenosine (5'dCPA) in the chronic constriction injury model of neuropathic pain. Following intravenous administration of 5'dCPA (0.30 or 0.75 mg kg(-1)), the time course of the drug concentration in plasma was determined in conjunction with the effect on (1) the mechanical paw pressure and (2) the Von Frey Hair monofilament withdrawal threshold. Population pharmacokinetic-pharmacodynamic analysis was applied to derive individual concentration-effect relationships. For mechanical paw pressure a composite model consisting of an Emax model for the anti-hyperalgesic effect in combination with a linear model for the anti-nociceptive effect accurately described the data. The EC50 for the anti-hyperalgesic effect was 178+/-51 ng ml(-1) and the slope of the anti-nociceptive effect 0.055+/-0.008 g ml ng(-1). For the Von Frey Hair monofilament withdrawal threshold responders and non-responders were observed. Typically, in responders, full pain relief was observed at concentrations exceeding 100 ng ml(-1). The high plasma concentrations required for the anti-hyperalgesic effect relative to the receptor affinity are consistent with restricted transport of 5'dCPA to the site of action in the spinal cord and/or the brain.

Regulation by equilibrative nucleoside transporter of adenosine outward currents in adult rat spinal dorsal horn neurons

A current response induced by superfusing adenosine was examined in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole-cell patch-clamp technique. In 78% of the neurons examined, adenosine induced an outward current at -70 mV [18.8 +/- 1.1 pA (n = 98) at 1mM] in a dose-dependent manner (EC(50) = 177 microM). A similar current was induced by A(1) agonist N(6)-cyclopentyladenosine (1 microM), whereas A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (1 microM) reversed the adenosine action. The adenosine current reversed its polarity at a potential being close to the equilibrium potential for K(+), and was attenuated by Ba(2+) (100 microM) and 4-aminopyridine (5mM) but not tetraethylammonium (5mM). The adenosine current was enhanced in duration by equilibrative nucleoside-transport (rENT1) inhibitor S-(4-nitrobenzyl)-6-thioinosine (1 microM) and adenosine deaminase (ADA) inhibitor erythro-9-(2-hydroxy-3-nonyl) adenine (1 microM), and slowed in falling phase by adenosine kinase (AK) inhibitor iodotubercidine (1 microM). We conclude that a Ba(2+)- and 4-aminopyridine-sensitive K(+) channel in SG neurons is opened via the activation of A(1) receptors by adenosine whose level is possibly regulated by rENT1, adenosine deaminase and adenosine kinase. Considering that intrathecally-administered adenosine analogues produce antinociception, the regulatory systems of adenosine may serve as targets for antinociceptive drugs.

Perioperative intravenous adenosine infusion to extend postoperative analgesia in brachial plexus block

BACKGROUND AND OBJECTIVE

Adenosine infusions have been shown to reduce requirements of anaesthetics, to decrease the need for postoperative analgesics and to attenuate hyperaesthesia related to neuropathic pain. We decided to investigate the effects, beneficial or otherwise, of an adenosine infusion administered during surgery. A brachial plexus block was used to produce anaesthesia for the surgery.

METHODS

Sixty adults undergoing upper extremity surgery were included in the study. Brachial plexus block was performed via an axillary approach with lidocaine 1.25% and epinephrine 1/200 000 (40 mL). Patients were randomly assigned to two groups. During surgery, saline (control) or adenosine 80 microg kg min was infused intravenously in a double-blind fashion for 1 h. Visual analogue scores every 4 h, analgesic consumption, time to first spontaneous pain sensation, time to first rescue analgesic and adverse effects were noted during the first 24 h.

RESULTS

Vital signs were stable in both groups throughout surgery. During the adenosine infusion, one patient fainted while another complained of palpitations and tightness of the chest; both patients were excluded from further analyses. The time to first sensation of pain was significantly longer in the adenosine group compared to the control group (438 +/- 387 vs. 290 +/- 227 min, P = 0.02). The time to first rescue analgesic, the visual analogue scale scores and analgesic consumption in the postoperative period were similar.

CONCLUSIONS

In patients undergoing surgery with an axillary plexus block, a perioperative adenosine infusion prolongs the duration of postoperative analgesia to some extent. However, the time to first rescue analgesic, total analgesic requirements and pain scores were unchanged; the risk of potentially serious adverse effects is high. This therapy cannot be recommended.

Combination of adenosine with prilocaine and lignocaine for brachial plexus block does not prolong postoperative analgesia

Adenosine analogues have been used by subarachnoid injection for the treatment of inflammatory and neuropathic pain. There is no data on the use of adenosine in peripheral nerve blocks. The aim of the present study was to determine the analgesic efficacy of adenosine in combination with a local anaesthetic solution for brachial plexus (BP) block. With local ethics committee approval, 50 consenting adult patients undergoing upper limb surgery were enrolled in this double-blind, prospective, randomized study. Patients with a history of bronchospastic disease were excluded. Patients were instructed not to take theophylline-containing drugs and beverages for at least one day before surgery or on the first postoperative day. A supraclavicular BP block was performed by injecting a mixture totalling 35 ml made up of prilocaine 1% 10 ml and lignocaine 2% 20 ml with adrenaline 1:200,000, and adenosine 10 mg in 5 ml saline (Group 1) or 5 ml saline (Group 2) as a placebo control group. Postoperative analgesia was assessed by time to first rescue analgesia, analgesic consumption in the first 24 hours, and VAS at rest at 4, 8, 12, 16, 20 and 24 hours. Side-effects were also noted. Vital signs were stable in both groups throughout the operation. There were no significant differences between the groups in onset of motor and sensory block. Time to first pain sensation from block was not significantly longer in the adenosine group (379 +/- 336 min) compared with controls (304 +/- 249 min, mean +/- SD, P = 0.14). Time to first analgesic requirements and analgesic consumption in the first 24 hours were also similar in both study groups. In the present study, the addition of adenosine to local anaesthetic in brachial plexus block did not significantly extend the duration of analgesia.

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.

Modulation by adenosine of aδ and c primary-afferent glutamatergic transmission in adult rat substantia gelatinosa neurons

The present study examined the actions of adenosine on monosynaptic Adelta and C primary-afferent excitatory postsynaptic currents (EPSCs) recorded from substantia gelatinosa (SG) neurons of an adult rat spinal cord slice. In 67% of the neurons examined, adenosine reversibly decreased the amplitude of the Adelta-fiber EPSC, while in 13% of the neurons the amplitude was reduced or unaffected, which was followed by its increase persisting for several minutes after adenosine washout. The remaining neurons did not exhibit a change in the amplitude. The reduction in Adelta-fiber EPSC amplitude by adenosine was dose-dependent with an effective concentration for half-inhibition (EC50) value of 217 microM. When examined by using a paired-pulse stimulus, a ratio of the second to first Adelta-fiber EPSC amplitude under the reduction was larger than that of EPSC amplitude in the control, suggesting a presynaptic action of adenosine. In 69% of the neurons tested, the C-fiber EPSC was reversibly decreased in amplitude by adenosine (100 microM) by an extent comparable to that of Adelta-fiber EPSC; the remaining neurons were without adenosine actions. Similar inhibitory actions of adenosine were also seen in neurons where both Adelta-fiber and C-fiber EPSCs were elicited. Similar reduction in the Adelta-fiber or C-fiber EPSC amplitude was induced by an A1 adenosine-receptor agonist, N6-cyclopentyladenosine (1 microM), and the adenosine-induced reduction was not observed in the presence of an A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (1 microM). An A2a agonist, CGS 21680 (1 microM), did not significantly affect the Adelta-fiber EPSC amplitude. It is concluded that adenosine presynaptically inhibits monosynaptic Adelta-fiber and C-fiber transmission by a similar extent through the activation of the A1 receptor in many but not all SG neurons; this could contribute to at least a part of antinociception by intrathecally administered adenosine analogues and probably by endogenous adenosine.

Antinociceptive Effects of Novel A2B Adenosine Receptor Antagonists

Caffeine, an adenosine A1, A2A, and A2B receptor antagonist, is frequently used as an adjuvant analgesic in combination with nonsteroidal anti-inflammatory drugs or opioids. In this study, we have examined the effects of novel specific adenosine receptor antagonists in an acute animal model of nociception. Several A2B-selective compounds showed antinociceptive effects in the hot-plate test. In contrast, A1- and A2A-selective compounds did not alter pain thresholds, and an A3 adenosine receptor antagonist produced thermal hyperalgesia. Evaluation of psychostimulant effects of these compounds in the open field showed only small effects of some antagonists at high doses. Coadministration of low, subeffective doses of A2B-selective antagonists with a low dose of morphine enhanced the efficacy of morphine. Our results indicate that analgesic effects of caffeine are mediated, at least in part, by A2B adenosine receptors.

Adenosine kinase inhibitors: polar 7-substitutent of pyridopyrimidine derivatives improving their locomotor selectivity

We have discovered that polar 7-substituents of pyridopyrimidine derivatives affect not only whole cell AK inhibitory potency, but also selectivity in causing locomotor side effects in vivo animal models. We have identified compound, 1o, which has potent whole cell AK inhibitory potency, analgesic activity and minimal reduction of locomotor activity.

Formalin-induced pain and mu-opioid receptor density in brain and spinal cord are modulated by A1 and A2a adenosine agonists in mice

The effects of adenosine analogues on pain have been shown to depend on the subtype receptor involved as well as on the nociceptive stimuli and on the route of administration. In the first experiment of the present study intraperitoneal administration of the A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) (0.015, 0.03, 0.09, 0.15, 0.21, 0.3 mg/kg) induced dose-dependent analgesia to formalin pain in both phases characterizing the test. The A(2a) receptor agonist 2-[p-2-(carbonyl-ethyl)-phenyethylamino]-5'-N-ethylcarboxaminoadenosine (CGS21680) (0.025, 0.05, 0.1, 0.15 mg/kg) significantly affected behavioral responses to formalin only during the early phase. In the second experiment the interaction between adenosine and the opioid system was investigated through both behavioral and neurochemical studies. The opioid antagonist naltrexone (0.1 mg/kg) did not affect the antinociception induced by CPA (0.21 mg/kg) and CGS21680 (0.05 mg/kg). Autoradiographic studies showed that formalin administration significantly modified mu-opioid receptor density in the superficial laminae of the spinal cord and in the paracentral thalamic nucleus, contralateral to the side of formalin injection. CPA and CGS21680 counteracted these effects induced by formalin. In conclusion the present study confirms and extends the role of A(1) and A(2a) adenosine receptors in the modulation of inflammatory pain and their interaction with the mu-opioid system, and suggests further investigation of these purinergic receptors from a therapeutic perspective.

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

Although it is well established that adenosine exerts antinociceptive effects at the spinal level in various species including human, the mechanisms responsible for such effects are still a matter of debate. We presently investigated whether adenosine-induced antinociception might possibly be related to an inhibitory influence of this neuromodulator on the spinal release of neuropeptides implicated in the transfer and/or control of nociceptive signals. For this purpose, the K(+)-evoked overflow of substance P-, calcitonin gene-related peptide (CGRP)- and cholecystokinin-like materials was measured from slices of the dorsal half of the rat lumbar enlargement superfused with an artificial cerebrospinal fluid supplemented with increasing concentrations of various adenosine receptor ligands. The data showed that stimulation of adenosine A(1) and (possibly) A(3) receptors, but not A(2A) receptors, exerted an inhibitory influence on the spinal release of CGRP-like material. In contrast, none of the adenosine A(1), A(2A) and A(3) receptor agonists tested within relevant ranges of concentrations significantly affected the release of substance P- and cholecystokinin-like materials. These results support the idea that adenosine-induced antinociception at the spinal level might possibly be caused, at least partly, by the stimulation of inhibitory adenosine A(1) receptors located presynaptically on primary afferent fibres containing CGRP but not substance P.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Analgesic and anti-inflammatory effects of A-286501, a novel orally active adenosine kinase inhibitor

Adenosine (ADO) is an inhibitory neuromodulator that can increase nociceptive thresholds in response to noxious stimulation. Inhibition of the ADO-metabolizing enzyme, adenosine kinase (AK) increases extracellular ADO concentrations at sites of tissue trauma and AK inhibitors may have therapeutic potential as analgesic and anti-inflammatory agents. N7-((1'R,2'S,3'R,4'S)-2',3'-dihydroxy-4'-amino-cyclopentyl)-4-amino-5-bromo-pyrrolo[2,3-a]pyrimidine (A-286501) is a novel and potent (IC50=0.47 nM) carbocyclic nucleoside AK inhibitor that has no significant activity (IC50 >100 microM) at other sites of ADO interaction (A1, A2A, A3 receptors, ADO transporter, and ADO deaminase) or other (IC50 value >10 microM) neurotransmitter and peptide receptors, ion channel proteins, neurotransmitter reuptake sites and enzymes, including cyclooxygenases-1 and -2. A-286501 showed equivalent potency to inhibit AK from several mammalian species and kinetic studies revealed that A-286501 was a reversible and competitive inhibitor with respect to ADO and non-competitive with respect to MgATP2-. A-286501 was orally effective to reduce nociception in animal models of acute (thermal), inflammatory (formalin and carrageenan), and neuropathic (L5/L6 nerve ligation and streptozotocin-induced diabetic) pain. A-286501 was particularly potent (ED50=1 micromol/kg, p.o.) to reduce carrageenan-induced inflammatory thermal hyperalgesia as compared to its analgesic actions in other pain models (acute and neuropathic) and its ability to alter hemodynamic function and motor performance. A-286501 was also effective to reduce carrageenan-induced paw edema and myeloperoxidase activity, a measure of neutrophil influx (ED50=10 micromol/kg, p.o.), in the injured paw. The anti-nociceptive effects of A-286501 in the L5/L6 nerve injury model of neuropathic pain (ED50=20 micromol/kg, p.o.) were not blocked by the opioid antagonist naloxone, but were blocked by the ADO receptor antagonist, theophylline. Following repeated administration, A-286501 showed less potential to produce tolerance as compared to morphine. Thus, A-286501 is a structurally novel AK inhibitor that effectively attenuates nociception by a non-opioid, non-non-steroidal anti-inflammatory drug ADO, receptor mediated mechanism.

Recent developments in the discovery of novel adenosine kinase inhibitors: mechanism of action and therapeutic potential

Adenosine (ADO) is an endogenous inhibitory neuromodulator that limits cellular excitability in response to tissue trauma and inflammation. Adenosine kinase (AK; EC 2.7.1.20) is the primary metabolic enzyme regulating intra- and extracellular concentrations of ADO. AK inhibitors have been shown to significantly increase ADO concentrations at sites of tissue injury and to provide effective antinociceptive, antiinflammatory, and anticonvulsant activity in animal models. Structurally novel nucleoside and non-nucleoside AK inhibitors that demonstrate high specificity for the AK enzyme compared with other ADO metabolic enzymes, transporters, and receptors have recently been synthesized. These compounds have also demonstrated improved cellular and tissue penetration compared with earlier tubercidin analogs. These compounds have been shown to exert beneficial effects in animal models of pain, inflammation and epilepsy with reduced cardiovascular side effects compared with direct acting ADO receptor (P1) agonists, thus supporting the hypothesis that AK inhibitors can enhance the actions of ADO in a site- and event-specific fashion.

Adenosine inhibits excitatory transmission to substantia gelatinosa neurons of the adult rat spinal cord through the activation of presynaptic A(1) adenosine receptor

Although intrathecal administration of adenosine analogues or A(1) adenosine receptor agonists is known to result in antinociception, this has not been examined yet at the cellular level. In the present study, we examined in pharmacology an action of adenosine on glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice; this was done under the condition where a postsynaptic action of adenosine was blocked. In 65% of the neurons examined (n=72), adenosine at a concentration of 100 microM depressed the frequency of mEPSC in a reversible manner; the remaining neurons exhibited an inhibition followed by potentiation of the frequency. When examined quantitatively in extent in some cells (n=25), the inhibition was 40+/-3% (n=25) while the potentiation was 42+/-8% (n=6). These actions were not accompanied by a change in mEPSC amplitude. The inhibitory action on mEPSC frequency was dose-dependent in a range of 10-500 microM with an EC(50) value of 277 microM. The inhibitory action of adenosine was mimicked by a selective A(1) adenosine receptor agonist, CPA (1 microM; depression: 54+/-9%, n=4); this action of adenosine (100 microM) was not observed in the presence of a specific A(1) adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (1 microM; 94+/-4% of control, n=3). The facilitatory action of adenosine (100 microM) was unaffected by an A(2a) antagonist, ZM 241385 (0.1 microM, n=3); an A(2a) agonist, CGS 21680 (0.1-10 microM; n=6), was without actions on mEPSC frequency. It is concluded that adenosine inhibits excitatory transmission to SG neurons through the activation of presynaptic A(1) adenosine receptor and that some of the inhibition is followed by a potentiation of the transmission. It remains to be examined which subtypes of adenosine receptors except for the A(1)- and A(2a)-subtypes are involved in the potentiating action. Considering that adenosine-like immunoreactivity and adenosine receptors are expressed at a high density in the SG, which is thought to play an important role in modulating nociceptive transmission from the periphery to the central nervous system, this inhibitory action of adenosine could contribute to a negative modulation of pain transmission.

Pyridopyrimidine analogues as novel adenosine kinase inhibitors

A novel series of pyridopyrimidine analogues 9 was identified as potent adenosine kinase inhibitors based on the SAR and computational studies. Substitution of the C7 position of the pyridopyrimidino core with C2' substituted pyridino moiety increased the in vivo potency and enhanced oral bioavailability of these adenosine kinase inhibitors.

Oscillation of pain intensity during adenosine infusion. Relationship to beta-endorphin and sympathetic tone

Adenosine is a neuromodulator with both excitatory and inhibitory effects dependent in part upon preconditions; it can act as an algesic or an analgesic agent. Previously we found variations of pain intensity during constant infusion of adenosine. We therefore quantified pain intensity during constant infusion of adenosine at a rate of 140 microg/kg/min intravenously in healthy volunteers, placebo controlled, double blind, and the relation to hemodynamic, vasomotor and sudomotor responses of the sympathetic nervous system and to the role of peripheral beta-endorphin response. The perceived chest pain during adenosine infusion showed an oscillatory pattern. Painful periods of about 30s were interrupted by painfree periods, and pain was always preceded by an increase in vasomotor sympathetic activity and by increased sudomotor activity. Plasma beta-endorphin values were heterogenous but exhibited an increase during infusion.

Intrathecal adenosine administration in abdominal hysterectomy lacks analgesic effect

BACKGROUND

Adenosine (Ado) is known, from studies in both animals and humans, to produce antinociception when administered systemically or intrathecally (IT). The current aim was to evaluate, in a placebo-controlled, randomised, double-blind study, whether IT adenosine given before surgery could reduce anaesthetic requirement and the need of opioids during 48 h after visceral surgery.

METHOD

Forty women (37-66 years, ASA I and II) scheduled for elective hysterectomy were included. Before inducing the standardised O2/N2O/isoflurane/fentanyl anaesthesia, the patients received an IT injection of either adenosine (500 microg in 1 ml volume) or placebo 1 ml (saline). Intraoperative anaesthetic drug doses and haemodynamics were recorded. Postoperative pain was assessed by visual analogue scale. For postoperative analgesia, cetobemidone was provided via intravenous patient-controlled analgesia (PCA).

RESULTS

During surgery, there were no differences between groups in anaesthetic requirement or haemodynamic parameters. Postoperative cetobemidone requirements were similar in both groups (median 48 mg for adenosine/50 mg for saline) during the first 48 postoperative hours.

CONCLUSION

IT adenosine did not influence the requirement of anaesthetic drug or postoperative analgesics after hysterectomy.

Therapeutic potential of adenosine kinase inhibitors

Adenosine kinase (AK; EC 2.7.1.20) is a key intracellular enzyme regulating intra and extracellular concentrations of adenosine (ADO), an endogenous modulator of intercellular signalling that reduces cell excitability during tissue stress and trauma. The inhibitory effects of ADO are mediated by interactions with specific cell-surface G-protein coupled receptors (GPCR), which regulate membrane cation flux, membrane polarisation and the release of excitatory neurotransmitters. Inhibition of AK potentiates local extracellular ADO levels at cell and tissue sites which are undergoing accelerated ADO release. Thus, AK inhibition represents a mechanism to selectively enhance the endogenous protective actions of ADO during cellular stress while potentially minimising the non-specific effects associated with the systemic administration of ADO receptor agonists. Novel, potent AK inhibitors have recently been synthesised that demonstrate high specificity for this particular enzyme as compared to other ADO metabolic enzymes, transporters and receptors. AK inhibitors have been shown to increase ADO concentrations in various systems in vitro, as well as in an in vivo model of neurotoxicity. In addition, AK inhibitors have demonstrated efficacy in animal models of epilepsy, cerebral ischaemia as well as pain and inflammation, thus suggesting their potential therapeutic utility for these conditions.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.

Adenosine receptor activation and nociception

Adenosine and ATP exert multiple influences on pain transmission at peripheral and spinal sites. At peripheral nerve terminals in rodents, adenosine A1 receptor activation produces antinociception by decreasing, while adenosine A1 receptor activation produces pronociceptive or pain enhancing properties by increasing, cyclic AMP levels in the sensory nerve terminal. Adenosine A3 receptor activation produces pain behaviours due to the release of histamine and 5-hydroxytryptamine from mast cells and subsequent actions on the sensory nerve terminal. In humans, the peripheral administration of adenosine produces pain responses resembling that generated under ischemic conditions and the local release of adenosine may contribute to ischemic pain. In the spinal cord, adenosine A receptor activation produces antinociceptive properties in acute nociceptive, inflammatory and neuropathic pain tests. This is seen at doses lower than those which produce motor effects. Antinociception results from the inhibition of intrinsic neurons by an increase in K+ conductance and presynaptic inhibition of sensory nerve terminals to inhibit the release of substance P and perhaps glutamate. There are observations suggesting some involvement of spinal adenosine A2 receptors in pain processing, but no data on any adenosine A3 receptor involvement. Endogenous adenosine systems contribute to antinociceptive properties of caffeine, opioids, noradrenaline, 5-hydroxytryptamine, tricyclic antidepressants and transcutaneous electrical nerve stimulation. Purinergic systems exhibit a significant potential for development as therapeutic agents. An understanding of the contribution of adenosine to pain processing is important for understanding how caffeine produces adjuvant analgesic properties in some situations, but might interfere with the optimal benefit to be derived from others.

Effect of adenosine receptor agonists and antagonists on the expression of opiate withdrawal in rats

The effects of the selective A1 adenosine receptor agonist N6-cyclopentyladenosine (CPA) and the selective A2a agonist 2-[p-(2-carboxethyl)phenylethyl-ethylamino]-5'-ethylcarboxamidoade nosine (CGS 21680) (each at 0.03, 0.1 and 0.3 mg/kg, SC) as well as the selective A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), non-selective antagonists 3-isobutyl-1-methylxanthine (IBMX), aminophylline, 3,7-dimethyl-1-propargyl-xanthine (DMPX) and 8(p-sulfophenyl)-theophylline (8-SPT) were investigated (each at 5, 10 and 30 mg/kg, SC) for their ability to alter the naloxone-precipitated opiate withdrawal syndrome in morphine-dependent rats. Effects of CPA and CGS 21680 on opiate withdrawal in the presence of aminophylline were also investigated. Both CPA and CGS 21680, caused a significant reduction in the incidence of body shakes, teeth chatter and paw shakes and decreased the amount of faecal matter produced. DPCPX, IBMX, DMPX, 8-SPT and aminophylline significantly increased the incidence of jumps and decreased the amount of faecal matter produced. The incidence of body shakes was significantly increased by DMPX, 8-SPT and IBMX. Neither CPA nor CGS 21680 were able to reverse the significant increase in the incidence of jumps caused by aminophylline. These data suggest that there is a role for endogenous adenosine in the modulation of the opiate abstinence syndrome and both A1 and A2a adenosine receptors are involved in this phenomenon.

Adenosine receptor activation suppresses tactile hypersensitivity and potentiates spinal cord stimulation in mononeuropathic rats

The aim of the present study was to investigate the intrathecal (i.t.) action of a selective A1-adenosine receptor agonist, R-phenylisopropyl adenosine (R-PIA), on tactile withdrawal thresholds in a rat model of mononeuropathy produced by sciatic chronic constriction injury (CCI). An additional aim was to examine whether adenosine receptor activation is involved in the effects of spinal cord stimulation (SCS), which activates low-threshold fibers and suppresses touch-evoked pain both in patients and in experimental animals with neuropathy. Animals presenting hindlimb withdrawal to von Frey filaments with a bending force of < 7.5 g on the lesioned side (compared to > or = 35 g in the normal limb), were considered as having tactile hypersensitivity ("allodynia'). R-PIA (1-10 nmol i.t.) induced a dose-dependent suppression of the tactile allodynia without producing impairment of motor function. The effect of R-PIA (3 nmol i.t.), a clearly submaximal dose, was abolished by concomitant treatment with the selective A1-adenosine receptor antagonist cyclopentylxanthine (10 nmol i.t.). In animals where SCS failed to influence tactile allodynia, concomitant i.t. administration of R-PIA (3 nmol) and SCS induced a clear-cut and long-lasting suppression of the hypersensitivity to tactile stimulation. In conclusion, adenosine receptor stimulation antagonizes tactile hypersensitivity in a CCI model of mononeuropathy and potentiates the action of spinal cord stimulation.

Altered sensory behaviors in mice following manipulation of endogenous spinal adenosine neurotransmission

Adenosine or adenosine analogs injected intrathecally (i.t.) induce significant antinociception. Recent studies support the existence of an endogenous spinal system that can modulate nociceptive input by releasing adenosine. Inhibition of adenosine metabolism by administration of an adenosine kinase inhibitor, in the present study, decreased behavior induced by putative pain neurotransmitters providing additional support for an endogenous purinergic system. Conversely, administration of high doses of methylxanthines (i.t.), adenosine receptor antagonists, induced behavior similar to that induced by pain neurotransmitters. Methylxanthine (i.t.)-induced behavior was partially inhibited by antagonists of receptors for pain neurotransmitters. These observations are consistent with the hypothesis that an endogenous purinergic system tonically modulates nociceptive input involving a variety of chemical mediators. Preliminary studies also revealed methylxanthine-induced allodynia and suggested spinal purinergic systems may have a broader role in discriminating sensory input.

Antinociception by adenosine analogs and an adenosine kinase inhibitor: dependence on formalin concentration

Spinal administration of adenosine analogs and an adenosine kinase inhibitor produces antinociception in thermal threshold tests. In the present study, we determined the effects of N6-cyclohexyladenosine (adenosine A1 receptor selective), 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethyl-carboxamidoadeno sine (CGS-21680) (adenosine A2A receptor selective), and 5'-N-ethylcarboxamidoadenosine (NECA) (non-selective), on formalin induced nociceptive responses (flinching/lifting and licking/biting) using two concentrations of formalin (2% and 5%). We also examined the antinociceptive effects of 5'-amino-5'-deoxyadenosine, an adenosine kinase inhibitor, and deoxycoformycin, an adenosine deaminase inhibitor, under these conditions. Adenosine A1 receptor agonists, but not the A2A selective agent, produced significant antinociception, as did 5'-amino-5'-deoxyadenosine, but not deoxycoformycin. The extent of antinociception produced was greater with the lower stimulus intensity. The effects of NECA and 5'-amino-5'-deoxyadenosine were inhibited by caffeine, indicating the involvement of cell surface adenosine receptors in their actions. We conclude (a) that the adenosine A1, but not the A2A, receptor is involved in spinally mediated antinociception, (b) that adenosine kinase is more important than adenosine deaminase in regulating endogenous adenosine levels in the spinal cord, and (c) that stimulus intensity is an important determinant of the efficacy of purines in the spinal cord.

Complex role of peripheral adenosine in the genesis of the response to subcutaneous formalin in the rat

When applied peripherally, adenosine has been shown to be pronociceptive in a number of animal and human models. Recent evidence has implicated adenosine as a significant mediator in the inflammatory process. In this study using rats, we have examined the effect of adenosine and of selective adenosine A1 and A2 receptor agonists and antagonists on the response to a subcutaneous injection of formalin into the rat hindpaw. Adenosine co-injected with formalin 0.5% significantly increased flinching in both phases in a dose-dependent manner. The highest dose of adenosine had no behavioral effect on its own. The adenosine A2 receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS-21680), at a dose of 1.5 nmol, increased flinching associated with 0.5% formalin injection but at higher doses produced depressant effects due to systemic absorption. The adenosine A1 receptor agonist N6-cyclohexyladenosine produced only systemic behavioral effects as determined by contralateral application. The flinching response to 2.5% formalin was significantly decreased by the adenosine A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine (DMPX). In contrast, 8-cyclopentyl-1,3-dimethylxanthine (CPT), the selective adenosine A1 receptor antagonist augmented the response to 2.5% formalin. The non-selective adenosine receptor antagonist caffeine had no significant effect over a wide range of doses. In summary, exogenous adenosine enhances nociception in the formalin test, probably via a peripheral A2 receptor-mediated action. Endogenous adenosine, acting at both A1 and A2 receptors, appears to be involved in the formalin-induced inflammatory response.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroperative adenosine infusion reduces the requirements for isoflurane and postoperative analgesics

The aims of this study were to investigate the influence of adenosine infusion, firstly, on postoperative analgesic requirements, and secondly, on peroperative isoflurane requirements. Seventy-five women, aged 18-70 yrs, ASA grades I and II, scheduled for breast surgery, were randomly assigned to peroperatively receive a double-blind intravenous infusion of either adenosine, 80 micrograms.kg-1.min-1, or placebo, during surgery under isoflurane/N2O/O2 anesthesia. The peroperative isoflurane requirements were significantly reduced at 30 and 90 min of surgery during adenosine treatment. The number of patients reporting pain when regaining consciousness after surgery was reduced by 57% in the adenosine group, 8/31 vs 19/32 (P < 0.02). Further, the postoperative 24-h opioid requirements were reduced by 27% in the adenosine group (P < 0.03). In conclusion, we found that a peroperative infusion of a small dose of adenosine during breast surgery, reduces the peroperative anesthetic requirements, and the demand for post-operative analgesics.

Systemic adenosine infusion: a new treatment modality to alleviate neuropathic pain

Adenosine, an endogenous antinociceptive compound acting in the central nervous system, was infused intravenously (50-70 micrograms/kg/min) to 2 patients with peripheral neuropathic pain. In 1 subject, spontaneous pain was alleviated, and tactile allodynia was essentially relieved during 40 min of infusion. Allodynia to warmth and touch were abolished in the other subject. In addition, hyperalgesia to pinprick was markedly attenuated as was pressure-induced allodynia. The reported effects lasted for hours after termination of the infusion. Our preliminary encouraging data call for further controlled studies of the potentially relieving effect of adenosine in painful neuropathic conditions.

Effects of morphine metabolites on micturition in normal, unanaesthetized rats

1. By means of continuous cystometry in normal, unanaesthetized rats, the effects on micturition of intrathecally (i.t.) administered morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), the two main metabolites of morphine, were studied and compared with those of i.t. morphine. 2. Both M6G (0.01, 0.1, and 0.5 microgram) and M3G (5 micrograms) were found to have significant effects on micturition. Like morphine (0.1, 0.5, and 10 micrograms), M6G was able to inhibit the micturition reflex, and produce urinary retention and dribbling incontinence in a dose-dependent manner. The potency of M6G for inhibiting micturition was approximately 10 times higher than that of morphine, and the duration of its effect was longer. All effects of M6G could be reversed by naloxone. 3. M3G (5 micrograms) facilitated the micturition reflex, resulting in decreases in bladder capacity and micturition volume, and an increase in spontaneous contractile activity. Pretreatment with naloxone (10 micrograms), which by itself had no effect on micturition, enhanced the facilitatory effects of M3G. In addition, M3G tended to counteract the inhibitory effects of both morphine and M6G on micturition. M3G (5 micrograms) also produced an excitatory behavioural syndrome. 4. It is concluded that in rats, i.t. M3G has excitatory effects on micturition and behaviour, probably not mediated via opioid receptors. I.t M6G has a potent inhibitory effect on micturition mediated by stimulation of opioid receptors. It may have effects on somatosensory afferent input in lower doses than those required for effects on micturition.

Restraint stress potentiates analgesia induced by 5'-N-ethylcarboxamidoadenosine: comparison with morphine

mu-Opioid receptor agonists, e.g. morphine, produce analgesia that can be potentiated by restraint stress. Adenosine receptor agonists, e.g. 5'-N-ethylcarboxamidoadenosine (NECA), also produce analgesia. To determine if adenosine-induced analgesia is also potentiated by stress, dose- and time-effect curves for NECA (0.01-0.1 mg/kg s.c.) were generated in adult male Sprague-Dawley rats either unrestrained or restrained in Plexiglas cylinders, using the tail-flick assay. Morphine (1.0-5.6 mg/kg s.c.) was also tested for comparison. Both compounds produced dose-dependent increases in tail-flick latencies. This effect of both drugs was potentiated in restrained rats. The opioid receptor antagonist, naltrexone (1.0 mg/kg s.c.) blocked completely the effect of morphine in both groups and attenuated the stress-induced potentiation of NECA-induced analgesia. The adenosine receptor antagonist, caffeine (10.0 mg/kg s.c.), blocked the analgesic effect of NECA but not that of morphine. These results indicate that adenosine-mediated analgesia is potentiated by restraint stress and suggest a role for endogenous opioids in the mediation of stress-induced potentiation of analgesia.

Adenosine inhibits action potential-dependent release of calcitonin gene-related peptide- and substance P-like immunoreactivities from primary afferents in rat spinal cord

Electrical field stimulation (5 Hz) evoked a prompt outflow of calcitonin gene-related peptide- and substance P-like immunoreactivities (CGRP-LI and SP-LI, respectively) from superfused slices of the dorsal but not ventral half of the rat spinal cord. The evoked outflow was abolished by tetrodotoxin, calcium-free medium or previous exposure to capsaicin, indicating that it is produced through action potentials invading the central terminals of capsaicin-sensitive primary afferents. Adenosine as well as gamma-aminobutyric acid (GABA) or the GABAB receptor agonist (-)-baclofen produced a concentration-dependent inhibition of the evoked CGRP-LI outflow. Adenosine also inhibited the evoked SP-LI outflow. These findings demonstrate that inhibition of transmitter release from primary afferent neurons should be considered as a possible mechanism of the antinociceptive action of adenosine and adenosine analogs.

Adenosine infusion during isoflurane-nitrous oxide anaesthesia: indications of perioperative analgesic effect

Adenosine, an endogenous compound with a known antinociceptive effect when administered into the CNS, was applied in nine patients (21-65 years) by the peripheral intravenous route (70-130 micrograms.kg-1.min-1) as a replacement for peroperative opioids during inhalation anaesthesia for surgical procedures not requiring muscle relaxation. Lorazepam was given as premedication, thiopentone was used for induction, and succinylcholine facilitated intubation of the trachea. Anaesthesia was maintained with isoflurane [initial surgery endtidal concentration (ET) 0.88% (range 0.7-1%)] and nitrous oxide (60-70%) in oxygen. Adenosine infusion was initiated 5-10 min prior to surgery, and stopped close to or when isoflurane was terminated at the end of surgery. The duration of anaesthesia, adenosine infusion, and surgery were 120 min (range 80-165), 90 min (range 70-145), and 90 min (range 60-135), respectively. Spontaneous unassisted ventilation was maintained in all patients. Mean heart rate increased 10 beats.min-1 (range 0-35) upon induction of surgery, while systolic blood pressure was unaffected at 105 mmHg (range 85-120) (14 kPa (range 11.3-16.0)). Spo2 and ETCO2 were in the normal range. The isoflurane concentration was gradually reduced in most cases [mid-surgery ET 0.63% (range 0.5-0.8) and end-surgery ET 0.57% (range 0.3-0.8)]. Extubation and verbal communication were rapidly achieved after anaesthesia. The mean postoperative (24 h) opioid requirement was 4 mg (range 0-10 mg). These pilot cases suggest that systemic adenosine infusion may replace opioids during inhalation anaesthesia, and that adequate spontaneous ventilation can be achieved.

Local antinociceptive and hyperalgesic effects in the formalin test after peripheral administration of adenosine analogues in mice

Adenosine administered to humans has been reported to induce pain after intravenous administration. On the other hand adenosine analogues have been shown to possess antinociceptive effects after peripheral and intrathecal administration in animals. The aim of the present study was to investigate the effect of peripheral administration of adenosine agonists with different affinities for the A1 and A2 adenosine receptors on a persistent pain stimulus using the formalin test. The drugs chosen were, R-phenylisopropyl-adenosine (R-PIA) with high affinity for the A1 receptor, N-ethylcarboxamide-adenosine (NECA) with almost equal affinity for the A1 and A2 receptor and 2-(2-aminoethylamino)-carbonylethylphenylethylamino-adenosin e (APEC) with high affinity for the A2 receptor. The drugs were mixed with formalin and administered subcutaneously into the dorsal hind paw in mice to study the local effects. They were also injected separately from the formalin solution in different paws to evaluate the systemic effect. The total time of licking the injected paw during the first 5 min. was recorded. In high doses all compounds reduced the licking activity, but a low dose of APEC (1 microM) injected together with the formalin solution had an algesic effect. All effects were antagonized by theophylline. These results suggests that A1 adenosine receptors mediate a local peripheral antinociceptive effect and the involvement of local peripheral A2 receptors in the enhancement of the algesic response.

Spinally-mediated antinociception is induced in mice by an adenosine kinase-, but not by an adenosine deaminase-, inhibitor

Relative involvement of adenosine deaminase and adenosine kinase in antinociception induced by endogenous adenosine was investigated. Antinociception induced by 5'-amino 5'-deoxyadenosine (5'-ADAdo; an adenosine kinase inhibitor) and deoxycoformycin (dCF; an adenosine deaminase inhibitor) administered i.t. was determined using the mouse tail-flick assay. Dose- and time-dependent antinociception was observed following i.t. administration of 5'-ADAdo, but not dCF. Antinociception induced by 5'-ADAdo was reversed by coadministration i.t. of theophylline, an adenosine receptor antagonist, in a dose-dependent manner. These data provide preliminary evidence that adenosine kinase plays a more significant physiological role than adenosine deaminase in the regulation of adenosine involved in spinally-mediated antinociception.

Antinociceptive and ventilatory effects of the morphine metabolites: morphine-6-glucuronide and morphine-3-glucuronide

Morphine and its major metabolites, morphine-3-glucuronide and morphine-6-glucuronide, were given intracerebroventricularly (i.c.v.) to rats. The antinociceptive effects were assessed in the tail-flick and hot-plate tests as well as the writhing test. Ventilatory effects were studied in halothane-anaesthetized rats. Based on calculated ED50 values, morphine-6-glucuronide was approximately 200 times more potent that morphine itself in the tail-flick and hot-plate tests. In the writhing test the difference in ED50 was approximately 9-fold. Morphine and morphine-6-glucuronide administered i.c.v. induced dose-related decreases in minute ventilation in the dose range 2.7 x 10(-9)-1.3 x 10(-7) mol. The dose-response curve for minute ventilation was steeper for morphine-6-glucuronide than for morphine. Morphine-6-glucuronide was approximately 10 times more potent than morphine in depressing minute ventilation. Morphine-6-glucuronide reduced both tidal volume and respiratory frequency, while morphine reduced only the tidal volume. Morphine-3-glucuronide, in contrast, increased both tidal volume and respiratory frequency, causing an increase in minute ventilation. Apnoea was elicited after the highest doses of morphine-6-glucuronide but not of morphine. The potency difference for depression of minute ventilation between morphine-6-glucuronide and morphine corresponded well to the difference in the writhing test but not to the potency difference in the tail-flick or hot-plate tests. The ventilatory depression induced by morphine and morphine-6-glucuronide was readily reversed by naloxone, while the hyperventilation caused by morphine-3-glucuronide was slightly potentiated by the opioid antagonist. Naloxone pretreatment completely blocked the ventilatory depression induced by morphine-6-glucuronide. These results show that the potent ventilatory depression induced by morphine-6-glucuronide is related to its antinociceptive effects in rats. Furthermore, the fact that morphine-3-glucuronide stimulated ventilation and that morphine had a more shallow ventilatory dose-response curve compared to morphine-6-glucuronide may indicate that morphine-3-glucuronide is a functional antagonist of the depressive effects of morphine and morphine-6-glucuronide on ventilation.