The role and regulation of adenosine in the central nervous system

Adenosine is a modulator that has a pervasive and generally inhibitory effect on neuronal activity. Tonic activation of adenosine receptors by adenosine that is normally present in the extracellular space in brain tissue leads to inhibitory effects that appear to be mediated by both adenosine A1 and A2A receptors. Relief from this tonic inhibition by receptor antagonists such as caffeine accounts for the excitatory actions of these agents. Characterization of the effects of adenosine receptor agonists and antagonists has led to numerous hypotheses concerning the role of this nucleoside. Previous work has established a role for adenosine in a diverse array of neural phenomena, which include regulation of sleep and the level of arousal, neuroprotection, regulation of seizure susceptibility, locomotor effects, analgesia, mediation of the effects of ethanol, and chronic drug use.

Adenosine receptors as therapeutic targets

Adenosine receptors are major targets of caffeine, the most commonly consumed drug in the world. There is growing evidence that they could also be promising therapeutic targets in a wide range of conditions, including cerebral and cardiac ischaemic diseases, sleep disorders, immune and inflammatory disorders and cancer. After more than three decades of medicinal chemistry research, a considerable number of selective agonists and antagonists of adenosine receptors have been discovered, and some have been clinically evaluated, although none has yet received regulatory approval. However, recent advances in the understanding of the roles of the various adenosine receptor subtypes, and in the development of selective and potent ligands, as discussed in this review, have brought the goal of therapeutic application of adenosine receptor modulators considerably closer.

Role of G-protein-coupled adenosine receptors in downregulation of inflammation and protection from tissue damage

Inappropriate or prolonged inflammation is the main cause of many diseases; for this reason it is important to understand the physiological mechanisms that terminate inflammation in vivo. Agonists for several Gs-protein-coupled receptors, including cell-surface adenosine purinergic receptors, can increase levels of immunosuppressive cyclic AMP in immune cells; however, it was unknown whether any of these receptors regulates inflammation in vivo. Here we show that A2a adenosine receptors have a non-redundant role in the attenuation of inflammation and tissue damage in vivo. Sub-threshold doses of an inflammatory stimulus that caused minimal tissue damage in wild-type mice were sufficient to induce extensive tissue damage, more prolonged and higher levels of pro-inflammatory cytokines, and death of male animals deficient in the A2a adenosine receptor. Similar observations were made in studies of three different models of inflammation and liver damage as well as during bacterial endotoxin-induced septic shock. We suggest that A2a adenosine receptors are a critical part of the physiological negative feedback mechanism for limitation and termination of both tissue-specific and systemic inflammatory responses.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

Aggressiveness, hypoalgesia and high blood pressure in mice lacking the adenosine A2a receptor

Adenosine is released from metabolically active cells by facilitated diffusion, and is generated extracellularly by degradation of released ATP. It is a potent biological mediator that modulates the activity of numerous cell types, including various neuronal populations, platelets, neutrophils and mast cells, and smooth muscle cells in bronchi and vasculature. Most of these effects help to protect cells and tissues during stress conditions such as ischaemia. Adenosine mediates its effects through four receptor subtypes: the A1, A2a, A2b and A3 receptors. The A2a receptor (A2aR) is abundant in basal ganglia, vasculature and platelets, and stimulates adenylyl cyclase. It is a major target of caffeine, the most widely used psychoactive drug. Here we investigate the role of the A2a receptor by disrupting the gene in mice. We found that A2aR-knockout (A2aR-/-) mice were viable and bred normally. Their exploratory activity was reduced, whereas caffeine, which normally stimulates exploratory behaviour, became a depressant of exploratory activity. Knockout animals scored higher in anxiety tests, and male mice were much more aggressive towards intruders. The response of A2aR-/- mice to acute pain stimuli was slower. Blood pressure and heart rate were increased, as well as platelet aggregation. The specific A2a agonist CGS 21680 lost its biological activity in all systems tested.

Retrograde inhibition of presynaptic calcium influx by endogenous cannabinoids at excitatory synapses onto Purkinje cells

Brief depolarization of cerebellar Purkinje cells was found to inhibit parallel fiber and climbing fiber EPSCs for tens of seconds. This depolarization-induced suppression of excitation (DSE) is accompanied by altered paired-pulse plasticity, suggesting a presynaptic locus. Fluorometric imaging revealed that postsynaptic depolarization also reduces presynaptic calcium influx. The inhibition of both presynaptic calcium influx and EPSCs is eliminated by buffering postsynaptic calcium with BAPTA. The cannabinoid CB1 receptor antagonist AM251 prevents DSE, and the agonist WIN 55,212-2 occludes DSE. These findings suggest that Purkinje cells release endogenous cannabinoids in response to elevated calcium, thereby inhibiting presynaptic calcium entry and suppressing transmitter release. DSE may provide a way for cells to use their firing rate to dynamically regulate synaptic inputs. Together with previous studies, these findings suggest a widespread role for endogenous cannabinoids in retrograde synaptic inhibition.

Adenosine-dopamine receptor-receptor interactions as an integrative mechanism in the basal ganglia

Increasing evidence suggests that antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the basal ganglia are involved in the motor depressant effects of adenosine receptor agonists and the motor stimulant effects of adenosine receptor antagonists, such as caffeine. The GABAergic striatopallidal neurons are regulated by interacting adenosine A2A and dopamine D2 receptors. On the other hand, the GABAergic striatonigral and striatoentopeduncular neurons seem to be regulated by interacting adenosine A1 and dopamine D1 receptors. Furthermore, behavioural studies have revealed interactions between adenosine A2A and dopamine D1 receptors that occur at the network level. These adenosine-dopamine receptor-receptor interactions might offer new therapeutic leads for basal ganglia disorders.

Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection

Adenosine accumulation during ischemia and inflammation protects tissues from injury. In ischemic tissues adenosine accumulates due to inhibition of adenosine kinase, and in inflamed tissues adenosine is formed from adenine nucleotides that are released from many cells including platelets, mast cells, nerves, and endothelium. Nucleotides are rapidly converted to adenosine by a family of ecto-nucleotidases including CD39 and CD73. Activation of A(1) and possibly A(3) adenosine receptors (ARs) protects heart and other tissues by preconditioning through a pathway including protein kinase C and mitochondrial K(ATP) channels. Activation of A(2A) receptors limits reperfusion injury by inhibiting inflammatory processes in neutrophils, platelets, macrophages and T cells. Adenosine produces proinflammatory responses mediated by receptors that vary among species; A(3) and A(2B) receptors mediate degranulation of rodent and human or canine mast cells, respectively. Novel adenosine receptor subtype-selective ligands have recently been developed. These include MRS1754 (A(2B) blocker), MRS1220 (A(3) blocker), MRE 3008F20 (human A(3) blocker), MRS1523 (rat A(3) blocker), and ATL146e (A(2A) agonist). These new pharmacologic tools will help investigators to sort out how adenosine protects tissues from injury and to identify new therapeutic agents that hold promise for the treatment of inflammatory and ischemic diseases.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

Adenosine, an endogenous anti-inflammatory agent

Adenosine receptors are present on most cells and organs, yet, although the physiological effects of adenosine were first described over 60 years ago, the potential therapeutic uses of adenosine have only been recognized and realized recently. A decade ago the potent anti-inflammatory effects of adenosine were first described; adenosine, acting at specific A2 receptors, inhibits some, but not all, neutrophil functions. Adenosine inhibits phagocytosis, generation of toxic oxygen metabolites, and adhesion (to some surfaces and to endothelial cells) but does not inhibit degranulation or chemotaxis. Occupancy of adenosine A2 receptors modulates leukocyte function by a novel mechanism. Although adenosine A2 receptors are classically linked to heterotrimeric GS signaling proteins and stimulation of adenylate cyclase, adenosine 3',5'-cyclic monophosphate does not act as the second messenger for inhibition of leukocyte function. By a mechanism that still remains obscure, occupancy of adenosine A2 receptors on neutrophils "uncouples" chemoattractant receptors from their stimulus-transduction proteins. The concentrations of adenosine that inhibit inflammatory cell function are similar to those observed in vivo and suggest a role for adenosine in the modulation of inflammation in vivo. Indeed, recent studies indicate that nonmetabolized adenosine receptor agonists are potent anti-inflammatory agents, and other studies indicate that methotrexate, a commonly used anti-inflammatory agent, diminishes inflammation by increasing adenosine release at inflamed sites. The observations reviewed here suggest that adenosine and agents that act through adenosine are excellent candidates for development as anti-inflammatory agents.

Essential role of adenosine, adenosine A1 receptors, and ATP-sensitive K+ channels in cerebral ischemic preconditioning

Preconditioning with sublethal ischemia protects against neuronal damage after subsequent lethal ischemic insults in hippocampal neurons. A pharmacological approach using agonists and antagonists at the adenosine A1 receptor as well as openers and blockers of ATP-sensitive K+ channels has been combined with an analysis of neuronal death and gene expression of subunits of glutamate and gamma-aminobutyric acid receptors, HSP70, c-fos, c-jun, and growth factors. It indicates that the mechanism of ischemic tolerance involves a cascade of events including liberation of adenosine, stimulation of adenosine A1 receptors, and, via these receptors, opening of sulfonylurea-sensitive ATP-sensitive K+ channels.

Structure and function of adenosine receptors and their genes

Four adenosine receptors have been cloned from many mammalian and some non-mammalian species. In each case the translated part of the receptor is encoded by two separate exons. Two separate promoters regulate the A1 receptor expression, and a similar situation may pertain also for the other receptors. The receptors are expressed in a cell and tissue specific manner, even though A1 and A2B receptors are found in many different cell types. Emerging data indicate that the receptor protein is targeted to specific parts of the cell. A1 and A3 receptors activate the Gi family of G proteins, whereas A2A and A2B receptors activate the Gs family. However, other G proteins can also be activated even though the physiological significance of this is unknown. Following the activation of G proteins several cellular effector pathways can be affected. Signaling via adenosine receptors is also known to interact in functionally important ways with signaling initiated via other receptors.

Hyperalgesia, anxiety, and decreased hypoxic neuroprotection in mice lacking the adenosine A1 receptor

Caffeine is believed to act by blocking adenosine A(1) and A(2A) receptors (A(1)R, A(2A)R), indicating that some A(1) receptors are tonically activated. We generated mice with a targeted disruption of the second coding exon of the A(1)R (A(1)R(-/-)). These animals bred and gained weight normally and had a normal heart rate, blood pressure, and body temperature. In most behavioral tests they were similar to A(1)R(+/+) mice, but A(1)R(-/-) mice showed signs of increased anxiety. Electrophysiological recordings from hippocampal slices revealed that both adenosine-mediated inhibition and theophylline-mediated augmentation of excitatory glutamatergic neurotransmission were abolished in A(1)R(-/-) mice. In A(1)R(+/-) mice the potency of adenosine was halved, as was the number of A(1)R. In A(1)R(-/-) mice, the analgesic effect of intrathecal adenosine was lost, and thermal hyperalgesia was observed, but the analgesic effect of morphine was intact. The decrease in neuronal activity upon hypoxia was reduced both in hippocampal slices and in brainstem, and functional recovery after hypoxia was attenuated. Thus A(1)Rs do not play an essential role during development, and although they significantly influence synaptic activity, they play a nonessential role in normal physiology. However, under pathophysiological conditions, including noxious stimulation and oxygen deficiency, they are important.

Coordinated adenine nucleotide phosphohydrolysis and nucleoside signaling in posthypoxic endothelium: role of ectonucleotidases and adenosine A2B receptors

Limited oxygen delivery to tissues (hypoxia) is common in a variety of disease states. A number of parallels exist between hypoxia and acute inflammation, including the observation that both influence vascular permeability. As such, we compared the functional influence of activated polymorphonuclear leukocytes (PMN) on normoxic and posthypoxic endothelial cells. Initial studies indicated that activated PMN preferentially promote endothelial barrier function in posthypoxic endothelial cells (>60% increase over normoxia). Extension of these findings identified at least one soluble mediator as extracellular adenosine triphosphate (ATP). Subsequent studies revealed that ATP is coordinately hydrolyzed to adenosine at the endothelial cell surface by hypoxia-induced CD39 and CD73 (>20-and >12-fold increase in mRNA, respectively). Studies in vitro and in cd39-null mice identified these surface ecto-enzymes as critical control points for posthypoxia-associated protection of vascular permeability. Furthermore, insight gained through microarray analysis revealed that the adenosine A2B receptor (AdoRA2B) is selectively up-regulated by hypoxia (>5-fold increase in mRNA), and that AdoRA2B antagonists effectively neutralize ATP-mediated changes in posthypoxic endothelial permeability. Taken together, these results demonstrate transcription coordination of adenine nucleotide and nucleoside signaling at the vascular interface during hypoxia.

Regulation of immune cells by local-tissue oxygen tension: HIF1 alpha and adenosine receptors

Immune cells are often exposed to low oxygen tensions, which markedly affect cellular metabolism. We describe how activated T cells adapt to the changing energy supplies in hypoxic areas of inflamed tissues by using hypoxia-inducible factor 1 (HIF1) to switch to glycolysis as the main source of energy and by signalling through extracellular-adenosine receptors. This hypoxic regulation might alter the balance between T helper 1 cells and T helper 2 cells and might alter the activities of cells of the innate immune system, thereby qualitatively and quantitatively affecting immune responses. This regulatory mechanism should be taken into account in the design and interpretation of in vitro and in vivo studies of immune-cell effector functions.

Distribution, biochemistry and function of striatal adenosine A2A receptors

It is well known that the nucleoside adenosine exerts a modulatory influence in the central nervous system by activating G protein coupled receptors. Adenosine A2A receptors, the subject of the present review, are predominantly expressed in striatum, the major area of the basal ganglia. Activation of A2A receptors interferes with effects mediated by most of the principal neurotransmitters in striatum. In particular, the inhibitory interactions between adenosine acting on A2A receptors and dopamine acting on D2 receptors have been well examined and there is much evidence that A2A receptors may be a possible target for future development of drugs for treatment of Parkinson's disease, schizophrenia and affective disorders. Our understanding of the role of striatal A2A receptors has increased dramatically over the last few years. New selective antibodies, antagonist radioligands and optimized in situ hybridization protocols have provided detailed information on the distribution of A2A receptors in rodent as well as primate striatum. Studies on the involvement of A2A receptors in the regulation of DARPP-32 and the expression of immediate early genes, such as nerve growth factor-induced clone A and c-fos, have pointed out an important role for these receptors in regulating striatopallidal neurotransmission. Moreover, by using novel selective antagonists for A2A receptors and transgenic mice lacking functional A2A receptors, crucial information on the behavioral role of striatal A2A receptors has been provided, especially concerning their involvement in the stimulatory action of caffeine and the anti-Parkinsonian properties of A2A receptor antagonists. In the present review, current knowledge on the distribution, biochemistry and function of striatal A2A receptors is summarized.

Nucleotide receptors

Adenosine 5'-triphosphate (ATP) and/or related nucleotides act at both ionotropic (P2X) and metabotropic (P2Y) receptors. P2X receptor subunits (P2X1-P2X7) form ligand-gated cation channels, as homomultimers or heteromultimers. Recent work indicates that P2X3 subunits participate in channels expressed by nociceptive sensory neurons, and that the second of the two transmembrane domains of each subunit contributes to the ion permeation pathway. P2X7 subunits form large cytolytic pores in addition to cation channels; they have been found in macrophages and brain microglia. P2Y receptors form a distinct subset of G-protein-coupled receptors; most couple through G proteins to phospholipase C, but inhibition of adenylate cyclase and N-type Ca2+ channels, and activation of K+ channels also occurs. Expressed P2Y receptors have generally been distinguished pharmacologically by the rank order of effectiveness of agonists; some prefer pyrimidines to purines. Recent studies suggest that it is important to use purified nucleotides in such classifications. Several P2Y receptors have a very widespread tissue distribution.

Ischemic preconditioning: from adenosine receptor to KATP channel

Ischemic preconditioning is a phenomenon whereby exposure of the myocardium to a brief episode of ischemia and reperfusion markedly reduces tissue necrosis induced by a subsequent prolonged ischemia. It is hoped that elucidation of the mechanism for preconditioning will yield therapeutic strategies capable of reducing myocardial infarction. In the rabbit, the brief period of preconditioning ischemia and reperfusion releases adenosine, bradykinin, opioids, and oxygen radicals. The combined effect of the release of these substances on G proteins and the cell's phospholipases induces the translocation and activation of the epsilon isozyme of protein kinase C. Protein kinase C appears to be the first element of a complex kinase cascade that is activated during the prolonged ischemia in preconditioned hearts. Current evidence indicates that this cascade contains at least one tyrosine kinase and ultimately leads to the activation of p38 mitogen-activated protein kinase. p38 Mitogen-activated protein kinase phosphorylates mitogen-activated protein kinase-activated protein kinase 2. Mitogen-activated protein kinase-activated protein kinase 2 phosphorylates HSP27, a 27-kDa heat shock protein that controls actin filament polymerization, and, therefore, affects the integrity of the cytoskeleton. Finally, mitochondrial adenosine 5'-triphosphate-sensitive K+ channels open, and the latter may be the final mediator of protection for ischemic preconditioning. The protective pathway has many built-in redundancies, perhaps creating a safety factor. These redundancies may also explain some of the species-related differences seen in ischemic preconditioning in which one redundant pathway may predominate over another.

Adenosine and sleep–wake regulation

This review addresses three principal questions about adenosine and sleep-wake regulation: (1) Is adenosine an endogenous sleep factor? (2) Are there specific brain regions/neuroanatomical targets and receptor subtypes through which adenosine mediates sleepiness? (3) What are the molecular mechanisms by which adenosine may mediate the long-term effects of sleep loss? Data suggest that adenosine is indeed an important endogenous, homeostatic sleep factor, likely mediating the sleepiness that follows prolonged wakefulness. The cholinergic basal forebrain is reviewed in detail as an essential area for mediating the sleep-inducing effects of adenosine by inhibition of wake-promoting neurons via the A1 receptor. The A2A receptor in the subarachnoid space below the rostral forebrain may play a role in the prostaglandin D2-mediated somnogenic effects of adenosine. Recent evidence indicates that a cascade of signal transduction induced by basal forebrain adenosine A1 receptor activation in cholinergic neurons leads to increased transcription of the A1 receptor; this may play a role in mediating the longer-term effects of sleep deprivation, often called sleep debt.

Signalling from adenosine receptors to mitogen-activated protein kinases

The purine nucleoside adenosine acts via four distinct adenosine receptor subtypes: the adenosine A(1), A(2A), A(2B), and A(3) receptor. They are all G protein-coupled receptors (GPCR) coupling to classical second messenger pathways such as modulation of cAMP production or the phospholipase C (PLC) pathway. In addition, they couple to mitogen-activated protein kinases (MAPK), which could give them a role in cell growth, survival, death and differentiation. Although each of the adenosine receptors can activate one or more of the MAPKs, the mechanisms appear to differ substantially, both between receptor subtypes in the same cell type and between the same receptor in different cell types.

Dopamine D1 and adenosine A1 receptors form functionally interacting heteromeric complexes

The possible molecular basis for the previously described antagonistic interactions between adenosine A(1) receptors (A(1)R) and dopamine D(1) receptors (D(1)R) in the brain have been studied in mouse fibroblast Ltk(-) cells cotransfected with human A(1)R and D(1)R cDNAs or with human A(1)R and dopamine D(2) receptor (long-form) (D(2)R) cDNAs and in cortical neurons in culture. A(1)R and D(1)R, but not A(1)R and D(2)R, were found to coimmunoprecipitate in cotransfected fibroblasts. This selective A(1)R/D(1)R heteromerization disappeared after pretreatment with the D(1)R agonist, but not after combined pretreatment with D(1)R and A(1)R agonists. A high degree of A(1)R and D(1)R colocalization, demonstrated in double immunofluorescence experiments with confocal laser microscopy, was found in both cotransfected fibroblast cells and cortical neurons in culture. On the other hand, a low degree of A(1)R and D(2)R colocalization was observed in cotransfected fibroblasts. Pretreatment with the A(1)R agonist caused coclustering (coaggregation) of A(1)R and D(1)R, which was blocked by combined pretreatment with the D(1)R and A(1)R agonists in both fibroblast cells and in cortical neurons in culture. Combined pretreatment with D(1)R and A(1)R agonists, but not with either one alone, substantially reduced the D(1)R agonist-induced accumulation of cAMP. The A(1)R/D(1)R heteromerization may be one molecular basis for the demonstrated antagonistic modulation of A(1)R of D(1)R receptor signaling in the brain. The persistence of A(1)R/D(1)R heteromerization seems to be essential for the blockade of A(1)R agonist-induced A(1)R/D(1)R coclustering and for the desensitization of the D(1)R agonist-induced cAMP accumulation seen on combined pretreatment with D(1)R and A(1)R agonists, which indicates a potential role of A(1)R/D(1)R heteromers also in desensitization mechanisms and receptor trafficking.

Mediation of tubuloglomerular feedback by adenosine: evidence from mice lacking adenosine 1 receptors

Adenosine is a determinant of metabolic control of organ function increasing oxygen supply through the A2 class of adenosine receptors and reducing oxygen demand through A1 adenosine receptors (A1AR). In the kidney, activation of A1AR in afferent glomerular arterioles has been suggested to contribute to tubuloglomerular feedback (TGF), the vasoconstriction elicited by elevations in [NaCl] in the macula densa region of the nephron. To further elucidate the role of A1AR in TGF, we have generated mice in which the entire A1AR coding sequence was deleted by homologous recombination. Homozygous A1AR mutants that do not express A1AR mRNA transcripts and do not respond to A1AR agonists are viable and without gross anatomical abnormalities. Plasma and urinary electrolytes were not different between genotypes. Likewise, arterial blood pressure, heart rates, and glomerular filtration rates were indistinguishable between A1AR(+/+), A1AR(+/-), and A1AR(-/-) mice. TGF responses to an increase in loop of Henle flow rate from 0 to 30 nl/min, whether determined as change of stop flow pressure or early proximal flow rate, were completely abolished in A1AR(-/-) mice (stop flow pressure response, -6.8 +/- 0.55 mmHg and -0.4 +/- 0.2 in A1AR(+/+) and A1AR(-/-) mice; early proximal flow rate response, -3.4 +/- 0.4 nl/min and +0.02 +/- 0.3 nl/min in A1AR(+/+) and A1AR(-/-) mice). Absence of TGF responses in A1AR-deficient mice suggests that adenosine is a required constituent of the juxtaglomerular signaling pathway. A1AR null mutant mice are a promising tool to study the functional role of A1AR in different target tissues.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

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.