Adenosine: The prototypic neuromodulator

Purinergic signaling in diabetes and metabolism

Purinergic signaling, a concept originally formulated by the late Geoffrey Burnstock (1929-2020), was found to modulate pathways in every physiological system. In metabolic disorders there is a role for both adenosine receptors and P2 (nucleotide) receptors, of which there are two classes, i.e. P2Y metabotropic and P2X ionotropic receptors. The individual roles of the 19 receptors encompassed by this family have been dissected - and in many cases the effects associated with specific cell types, including adipocytes, skeletal muscle, liver cells and immune cells. It is suggested that ligands selective for each of the four adenosine receptors (A₁, A_2A, A_2B and A₃), and several of the P2 subtypes (e.g. P2Y₆ or P2X7 antagonists) might have therapeutic potential for treating diabetes and obesity. This is a developing story with some conflicting conclusions relevant to drug discovery, which we summarize here.

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

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

Neurobiology of REM and NREM sleep

This paper presents an overview of the current knowledge of the neurophysiology and cellular pharmacology of sleep mechanisms. It is written from the perspective that recent years have seen a remarkable development of knowledge about sleep mechanisms, due to the capability of current cellular neurophysiological, pharmacological and molecular techniques to provide focused, detailed, and replicable studies that have enriched and informed the knowledge of sleep phenomenology and pathology derived from electroencephalographic (EEG) analysis. This chapter has a cellular and neurophysiological/neuropharmacological focus, with an emphasis on rapid eye movement (REM) sleep mechanisms and non-REM (NREM) sleep phenomena attributable to adenosine. The survey of neuronal and neurotransmitter-related brainstem mechanisms of REM includes monoamines, acetylcholine, the reticular formation, a new emphasis on GABAergic mechanisms and a discussion of the role of orexin/hypcretin in diurnal consolidation of REM sleep. The focus of the NREM sleep discussion is on the basal forebrain and adenosine as a mediator of homeostatic control. Control is through basal forebrain extracellular adenosine accumulation during wakefulness and inhibition of wakefulness-active neurons. Over longer periods of sleep loss, there is a second mechanism of homeostatic control through transcriptional modification. Adenosine acting at the A1 receptor produces an up-regulation of A1 receptors, which increases inhibition for a given level of adenosine, effectively increasing the gain of the sleep homeostat. This second mechanism likely occurs in widespread cortical areas as well as in the basal forebrain. Finally, the results of a new series of experimental paradigms in rodents to measure the neurocognitive effects of sleep loss and sleep interruption (modeling sleep apnea) provide animal model data congruent with those in humans.

Identification and Biochemical Studies on Novel Non-Nucleoside Inhibitors of the Enzyme Adenosine Kinase

The enzyme adenosine kinase (AK) plays a key role in the regulation of intracellular and extracellular concentration of adenosine (Ado), which exhibits potent hormonal activity in cardiovascular, nervous and immune systems. In view of the pharmacological effects of Ado, there is much interest in identifying inhibitors of AK, which can augment its tissue-protective effects. In this study, we have screened 1040 compounds from a chemical library of putative kinase inhibitors for their effect on purified human recombinant AK. These studies have identified 8 novel, non-nucleoside AK inhibitors. Four of these compounds (viz. 2-tert-butyl-4H-benzo[1,2,4]thiadiazine-3-thione (2759-0749); N-(5,6-diphenyl-furo[2,3-d]pyrimidin-4-yl)-propionamide (3998-0118); 3-[5,6-Bis-(4-methoxy-phenyl)-furo[2,3-d]pyrimidin-4-ylamino]-propan-1-ol (4072-2732); and 2-[2-(3,4-dihydroxy-phenyl)-5-phenyl-1H-imidazol-4-yl]-fluoren-9-one (8008-6198)), which inhibited human AK in a concentration-dependent manner in a low micromolar range (IC(50) = 0.38 approximately 1.98 microM) were further studied. Kinetic and structural studies on these compounds provide evidence that inhibition of AK by these compounds was competitive with respect to Ado and non-competitive for ATP. All of these compounds also inhibited uptake of Ado and its metabolism in cultured mammalian cells at comparable concentrations indicating their efficient cellular penetrability. These AK inhibitors, whose chemical structures differ significantly from all previously known inhibitors, provide useful lead compounds for identification of more potent but less toxic AK inhibitors that may prove useful for therapeutic purposes.

Adenosine treatment delays postischemic hippocampal CA1 loss after cardiac arrest and resuscitation in rats

Resuscitation from cardiac arrest results in reperfusion injury that leads to increased postresuscitation mortality and delayed neuronal death. One of the many consequences of resuscitation from cardiac arrest is a derangement of energy metabolism and the loss of adenylates, impairing the tissue's ability to regain proper energy balance. In this study, we investigated the effects of adenosine (ADO) on the recovery of the brain from 12 min of ischemia using a rat model of cardiac arrest and resuscitation. Compared to the untreated group, treatment with adenosine (7.2 mg/kg) initiated immediately after resuscitation increased the proportion of rats surviving to 4 days and significantly delayed hippocampal CA1 neuronal loss. Brain blood flow was increased significantly in the adenosine-treated rats 1 h after cardiac arrest and resuscitation. Adenosine-treated rats exhibited less edema in cortex, brainstem and hippocampus during the first 48 h of recovery. Adenosine treatment significantly lowered brain temperature during recovery, and a part of the neuroprotective effects of adenosine treatment could be ascribed to adenosine-induced hypothermia. With this dose, adenosine may have a delayed transient effect on the restoration of the adenylate pool (AXP = ATP + ADP + AMP) 24 h after cardiac arrest and resuscitation. Our findings suggested that improved postischemic brain blood flow and ADO-induced hypothermia, rather than adenylate supplementation, may be the two major contributors to the neuroprotective effects of adenosine following cardiac arrest and resuscitation. Although adenosine did not prevent eventual CA1 neuronal loss in the long term, it did delay neuronal loss and promoted long-term survival. Thus, adenosine or specific agonists of adenosine receptors should be evaluated as adjuncts to broaden the window of opportunity in the treatment of the reperfusion injury following cardiac arrest and resuscitation.

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.

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.

Adenosine A3 receptor-induced CCL2 synthesis in cultured mouse astrocytes

During neuropathological conditions, high concentrations of adenosine are released, stimulating adenosine receptors in neurons and glial cells. It has recently been shown that stimulation of adenosine receptors in glial cells induces the release of neuroprotective substances such as NGF, S-100beta, and interleukin-6 (IL-6). It has therefore been suggested that glial adenosine receptors are involved in neuroprotection. Since recently neuroprotective effects of the chemokine CCL2 (formerly known as MCP-1) have been reported, we investigated the possible effect of adenosine receptor stimulation on glial CCL2 synthesis. Here we show that stimulation of cultured murine astrocytes with the selective adenosine A3 receptor agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (CL-IB-MECA) induced the release of CCL2. Specific ligands for adenosine A1 or A2 receptors did not affect CCL2 release. Furthermore, CL-IB-MECA-induced CCL2 synthesis was inhibited by adenosine A3 receptor antagonists. These results show that stimulation of adenosine A3 receptors in astrocytes induced the release of CCL2, thus supporting the assumption that adenosine receptors in glial cells regulate the synthesis of neuroprotective substances.

The platelet window: examining receptor regulated second messenger processes in psychosis and depression

Peripheral markers of psychiatric illness provide a potentially important window into the pathophysiology of a number of psychiatric illnesses. Direct access to pathophysiological processes is fraught with difficulty. However, receptor-regulated second messenger-mediated calcium shifts are an accessible and practical method by which to examine changes in a clinical population. This is possible because platelets and neurons share some physiological features. The platelet intracellular calcium response to receptor stimulation has previously been used as a peripheral marker of psychiatric illness across a range of neurotransmitters, including serotonin, dopamine and glutamate. This review considers the specificity and selectivity of this response and its use in psychotic and mood disorders.

Characterization of the analgesic actions of adenosine: comparison of adenosine and remifentanil infusions in patients undergoing major surgical procedures

Perioperative pain is still a major problem, and new pharmacological means should be explored to mitigate such pain. Adenosine is an ubiquitous endogenous substance; when exogenously administered, it provides a number of salutary effects including neuromodulation, antinociception, and cytoprotective actions. The aim of this study was to characterize the perioperative antinociceptive-analgesic effects of intraoperative adenosine infusion and determine the duration of actions in the postoperative period, and compare them to those of remifentanil in patients undergoing major surgical procedures in a double-blind study.Sixty-two patients were randomly assigned to one of the two treatments. After standard induction of anesthesia, the lungs were mechanically ventilated. Anesthesia was maintained with a constant alveolar concentration of inhaled anesthetics (3% desflurane and 65% nitrous oxide in oxygen). A variable-rate of intravenous infusion of adenosine (50-500 microg kg(-1) x min(-1)) or remifentanil (0.05-0.5 microg kg(-1) x min(-1)) was initiated 5 min before the skin incision and was titrated to maintain systolic blood pressure and heart rate within 20% of baseline values during surgery. Postoperative evaluations included the level of sedation, degree of pain severity, opioid analgesic (fentanyl, morphine) consumption, and cardiorespiratory variables for 48 h. Intraoperative inhibition of the cardiovascular responses to surgical stimulation could be equally achieved by adenosine or remifentanil, and both could maintain excellent hemodynamic stability. Postoperatively, however, there were striking differences: (1). initial pain score was reduced by 60% (P<0.001) in the adenosine group compared to the remifentanil group and it remained lower throughout the 48 h recovery period; (2). postoperative morphine requirements during the first 0.25, 2 and 48 h were consistently lower in the adenosine group as compared to the remifentanil group (78, 71 and 42%, P<0.001, respectively); (3). adenosine patients remained significantly less sedated at all evaluations; (4) the end-tidal and arterial carbon dioxide values in the remifentanil group were significantly higher when patients were admitted to the postanesthesia care unit. No adverse effect of adenosine was observed at any time. Intraoperative adenosine infusion provided a salutary recovery from anesthesia associated with a pronounced and sustained postoperative pain relief. Compared to remifentanil, adenosine significantly reduced the opioid requirements and minimized the side effects including protracted sedation, cardiorespiratory instability, nausea, and vomiting in the postoperative recovery period.

New irreversible adenosine A(1) antagonists based on FSCPX

FSCPX (1) and its amide analogue (2) have been reported to exhibit potent and selective irreversible antagonism of the A(1) adenosine receptor (A(1)AR) when used in in vitro biological preparations. In order to obtain an irreversible A(1)AR antagonist with improved stability, analogues of FSCPX incorporating the chemoreactive 4-(fluorosulfonyl)phenyl moiety separated from the xanthine pharmacophore by a ketone linkage were explored. Compounds 4a-c exhibited improved affinity for the A(1)AR and concentration-dependent irreversible binding to the A(1)AR.

Synthesis and biological data of 4-amino-1-(2-chloro-2-phenylethyl)-1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid ethyl esters, a new series of A1-adenosine receptor (A1AR) ligands

The synthesis of a new family of A1-adenosine receptor (A1AR) ligands 3a-n has been performed in a straightforward way. Affinity data at A1AR, A2AAR and A3AR in bovine membranes show that these new compounds bind the A1AR in a selective way over A2AAR and A3AR and one of them (3j) presents a very high affinity, probably due to the phenethylamine substituent at C-4.

Effects of angiotensin III and angiotensin IV on pentylenetetrazol seizure susceptibility (threshold and kindling): interaction with adenosine A(1) receptors

The effects of angiotensin (ANG) III and ANG IV on pentylenetetrazol (PTZ) seizure susceptibility--threshold and kindling in mice--as well as the influence of adenosine A(1) receptor agents (agonist and antagonist) on these effects were studied. It was found that ANG III and ANG IV increased dose-dependently the PTZ seizure threshold and decreased the seizure intensity in PTZ kindled mice. Cyclohexyladenosine (CHA), an adenosine A(1) receptor agonist, potentiated the effects of ANG III and ANG IV on the seizure threshold and kindling, whereas DPCPX (an A(1) receptor antagonist) reversed peptide-induced effects on the PTZ kindling. Taken together, ANG III and ANG IV decrease the PTZ seizure susceptibility. We could suggest that these effects are realized in part through interaction with adenosine A(1) receptors.

Adenosine A(1) receptor agonism in the immature rat brain and heart

We examined if the adenosine A(1) receptor agonist adenosine amine congener (ADAC, 100 microg/kg i.p.) is neuroprotective in 7-day-old rats subjected to hypoxic ischemia. Brain damage, evaluated as weight deficit and gross morphology, was not affected by ADAC treatment. Nonetheless, ADAC (100 microg/kg i.p.) reduced heart rate by 44% (p<0.0001), indicating that the dose given was pharmacologically active. Adenosine A(1) receptors were determined by [(3)H] 1,3-dipropyl-8-cyclopentylxanthine (DPCPX)-binding and levels were 23% of the adult levels. GTP did not affect [(3)H] DPCPX-binding in the cerebral cortex at postnatal day 7 whereas there was strong enhancement of [(3)H] DPCPX-binding in the heart. This suggested a poor G-protein coupling at postnatal day 7 in the brain, which also was confirmed using GTP [gamma-(35)S]-binding in the presence of an adenosine A(1) receptor agonist. Thus, the lack of a neuroprotective effect of ADAC may be explained by the fact that adenosine A(1) receptors are not part of a functional unit in the 7-day-old rat brain.

Blunted adenosine A2a receptor function in platelets in patients with major depression

There is provisional evidence of involvement of adenosine in depression. In this study, the second messenger intracellular calcium response in platelets was measured in patients with major depression and controls using spectrofluorometry. The primary result of this study was a statistically significantly blunted second messenger response to agonist stimulation in the depressed group compared to the control group at the 50 and 100 nM and 1 microM dosage levels. This suggests that dysregulation of the adenosine A2a receptor may be present in depression.

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.

Striatum adenosine A2 receptors are modified during seizure: effect of cyclopentyladenosine administration

Rat CNS adenosine A2A receptors were studied after administration of the convulsant drug 3-mercaptopropionic acid (MP) and the adenosine analogue cyclopentyladenosine (CPA) by means of a quantitative autoradiographic method. Specific binding was quantified in striatum only. The highest density was found in caudate-putamen (2.50 fmol/mm2), followed by nuclei accumbens (1.85 fmol/mm2) and the lowest values in the olfactory tubercle (1.26 fmol/mm2). These differences were statistically significant. MP administration (150 mg/kg) caused significant increases (12-18%) in caudate-putamen and nuclei accumbens in both stages: seizure and postseizure and no changes in the olfactory tubercle. CPA administration (2 mg/kg) originated a rise of 16% in nuclei accumbens but no change in the other two regions. When CPA was injected 30 minutes before MP, an increase (18 to 45%) in caudate-putamen and nuclei accumbens at seizure and postseizure stages was observed. Saturation results, in striatal membrane fraction, indicate that receptor sites increased their maximal binding capacity (Bmax) while the apparent dissociation constant (Kd) remained unchanged. These results suggest the involvement of the adenosine A2A receptors in convulsant activity and that CPA administration at the dose selected brings about a rise in neuronal excitability in this area.

Differential effects of the adenosine A(1) receptor allosteric enhancer PD 81,723 on agonist binding to brain and adipocyte membranes

The benzoylthiophene analog, PD 81,723, has been shown to allosterically enhance agonist binding and functional activation of the mammalian adenosine (ADO) A(1) receptor subtype by putatively maintaining the receptor in a high affinity state. The present studies were conducted to evaluate the ability of PD 81,723 to enhance the binding of [3H]cyclohexyladenosine ([3H]CHA) to A(1) receptors of neural (cerebral cortex) and non-neural (adipocyte) origin in three different species; rat, guinea pig and dog. PD 81, 723 (0.3-100 microM) produced a concentration-dependent enhancement of [3H]CHA binding to rat brain A(1) receptors. These effects were also species-dependent with larger enhancements (150-200% of control) observed in guinea pig and dog brain membranes as compared to the rat (120% of control). In contrast, PD 81,723 did not produce any enhancement of [3H]CHA binding to A(1) receptors in adipocyte membranes from any of the species examined. Additional binding studies were conducted using pharmacological manipulations that have previously been shown to enhance the allosteric effects of PD 81,723. In the presence of 1 mM GTP, the allosteric effects of PD 81,723 (15 microM) were increased in rat, guinea pig and dog brain membranes, however, in adipocyte membranes from each species, no significant alteration in agonist binding was observed. Similarly, the A(1) receptor selective antagonist 8-cyclopentyl-1, 3-dipropylxanthine (added to effectively reduce the intrinsic antagonist properties of PD 81,723) was found to enhance the allosteric effects of PD 81,723 (15 microM) in brain, but produce no alteration of agonist binding in adipocyte membranes from each species. Examination of the dissociation kinetics of [3H]CHA binding from rat brain and adipocyte membranes revealed that PD 81,723 (15 microM) differentially slowed agonist dissociation from brain, but not adipocyte, membranes. Taken together, the present data support the hypothesis that in tissues that are sensitive to PD 81,723, this benzyolthiophene functions to maintain the A(1) receptor in a high-affinity state and that the relative proportions of high-affinity A(1) receptors present in specific tissues may contribute, at least in part, to the apparent differential effects of PD 81,723 on agonist binding. The tissue specific modulation of A(1) receptor function by PD 81,723 also illustrates the possibility that the locus of allosteric modulation by PD 81,723 may be manifest via a specific, but indirect and tissue-dependent, interaction with the A(1) receptor.

Medium- and long-term alterations of brain A1 and A2A adenosine receptor characteristics following repeated seizures in developing rats

In order to assess long-lasting consequences of recurrent seizures during development, the effects of repeated seizures in developing rats were investigated on brain adenosine A1 and A2A receptors. The characteristics of A1 and A2A receptors were analyzed by measuring the binding of the selective agonists [3H]CHA (N6-cyclohexyladenosine) and [3H]CGS 21680 (2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamido adenosine), respectively, on cerebral membrane preparations, whereas receptor coupling to G-proteins was examined by using a GTP analogue (Gpp(NH)p; guanylyl-5'-imidodiphosphate). Seizures were induced by bicuculline once a day at two different developmental stages: either from postnatal day 5 to postnatal day 7 (P5-P7) or from P15 to P17. Adenosine receptors were then studied at P15, P25 and P60. P5-P7 seizures led to an increase in A1 receptor density at P60 and to a decrease in their coupling to G-proteins at P15, but they did not affect A2A receptors. P15-P17 seizures decreased the coupling of A1 receptors to G-proteins at P25 and P60, reduced the density of A2A receptors at P25 and increased their affinity at P60. These results depict a persistent sensitivity of both A1 and A2A brain adenosine receptors to repeated seizures, with selective receptor alterations according to the cerebral maturational stage when seizures occur. In respect to the neuromodulatory and anticonvulsant properties of adenosine, such changes might be implicated in long-term functional brain reorganization after early seizures and future susceptibility to convulsive disorders.

Effects of adenosinergic agents on the vascular resistance and on the optic nerve response in the perfused cat eye

The function of A1- and A2a-adenosine receptors in the control of vascular resistance and in the modulation of light-evoked neuronal activity was investigated in the isolated perfused cat eye. The A1 agonist CCPA, the A1 antagonist CPT, the A2a agonist CGS 21680 and the A2 antagonist DMPX were used. The agents were applied intra-arterially at concentrations in the low nanomolar to micromolar range during rod-selective photic stimulation. The flow rate of perfusate, reflecting vascular resistance and the light-evoked optic nerve response (ONR) were recorded. Our results show a vasodilating effect of both A1 and A2 agonists and a vasoconstricting effect of the respective antagonists. The dose-effect relationships are suggestive, however, of an A2a receptor-mediated mechanism. The amplitude of the ONR-ON component was decreased during application of both adenosine-agonists. Analysis of the dose-effect relationships and the blockade of the CCPA-induced decrease by CPT suggests that inhibition is mediated by A1 receptors. However, CGS 21680-mediated inhibition cannot be explained by unspecific binding at A1 receptors alone and suggests the involvement of inhibitory A2a receptors.

Adenosine receptors: new opportunities for future drugs

This review summarises current knowledge on adenosine receptors, an important G protein-coupled receptor. The four known adenosine receptor subtypes A1, A2A, A2B, and A3 are discussed with special reference to the opportunities for drug development.

Adenosine receptor-mediated inhibition of neurite outgrowth from cultured sensory neurons is via an A1 receptor and is reduced by nerve growth factor

Adult dorsal root ganglion (DRG) cells are capable of neurite outgrowth in vitro as well as in vivo. We have investigated the influence of adenosine and analogs on the potential of cultured adult mouse DRG neurons to produce neurites in the presence and absence of nerve growth factor (NGF) which is a well-established trophic factor of sympathetic and sensory neurons during development. It is also believed to be essential for the maintenance or regulation of differentiated phenotypes of mature peripheral neurons. The results demonstrate that DRG neurons are modulated by purines in the absence of exogenous NGF. The addition of 100 microM adenosine to neurite-bearing DRG neurons inhibited neurite growth by 47% after 2-day exposures in vitro and by 50% after 5 days whereas in the presence of NGF this inhibition was reduced to 28% and 32%, respectively. 100 microM CHA (N(6)-cyclohexyl adenosine) alone reduced neurite total length by 47% after 2 days and by 48% after 5 days. 100 microM CGS21680 (2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride) alone also reduced neurite total length by 46% after 2 days and by 58% after 5 days which was reduced to 21% and 37%, respectively, in the presence of 100 ng/ml NGF. The antagonist studies revealed that activation of A1 adenosine receptors is primarily responsible for the effect on neuritogenesis since the inclusion of 1 or 10 microM CPX (8-cyclopentyl-1,3-dipropyl xanthine) fully prevented the inhibitory activity of adenosine or CHA whereas DMPX (3,7-dimethyl-1-propargyl xanthine) did not prevent inhibition by CHA. The converse experiment yielded the consistent result that inhibition by the A2 receptor agonist CGS21680 could be prevented by CPX, but not DMPX.

Non-selective effects of adenosine A1 receptor ligands on energy metabolism and macromolecular biosynthesis in cultured central neurons

To investigate the effects of adenosine A1 receptor activation on energy metabolism and RNA and protein biosynthesis in central neurons, cultured neurons from the rat forebrain were exposed for 1 hr to 72 hr to various concentrations (10 nM-100 microM) of the selective A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) or the A1 receptor antagonist 8-cyclopentyltheophylline (CPT). At all concentrations tested, the adenosinergic compounds did not affect cell viability within 72 hr of treatment, except for CPT, which reduced viability by 19.7% when used at the concentration of 100 microM. Energy metabolism was analysed by studying the specific uptake of 2-D-[3H]deoxyglucose ([3H]2DG). Rates of RNA and protein biosynthesis were assessed by the measurement of [3H]uridine and [3H]leucine incorporation, respectively. Neuronal [3H]2DG uptake was increased by 16% (P < 0.01) after 8 hr in the presence of 100 microM CCPA, whereas 100 microM CPT for 24 hr also increased [3H]2DG uptake (8%, P < 0.01). At these concentrations, both ligands inhibited [3H]uridine incorporation after a 3-hr treatment by 92% and 30%, respectively. CCPA never altered [3H]leucine incorporation when compared to controls, and CPT significantly inhibited protein synthesis only at 10-100 microM. Additional experiments to analyse the influence of A1 ligands on the transport of [3H]2DG, [3H]leucine and [3H]uridine suggested that CCPA and CPT, which interact functionally with adenosine receptors by regulating cyclic AMP production in this model, are able to alter energy metabolism and RNA synthesis in central neurons in a nonspecific manner by interacting with glucose and uridine transporters.

Use of a newly developed technique to isolate rat pinealocytes and study the effects of adenosine agonists on melatonin production

Recent studies have suggested a role for adenosine in the regulation of rat pineal melatonin synthesis. The data, however, are conflicting and therefore the aim of this study was to characterize adenosine receptors more fully in vitro by using a range of selective adenosine agonists and the adenosine antagonist 8-sulphophenyltheophylline (8-SPT). A simple method for the mechanical separation of rat pinealocytes was developed. Pinealocytes were briefly (15 min) incubated with drugs followed by a 4 hr drug-free incubation period after which melatonin concentrations in the incubation medium were measured by radioimmunoassay. The beta-adrenoceptor agonist isoprenaline gave a dose-related increase in melatonin production, demonstrating that this pinealocyte preparation technique is suitable to evaluate the effect of drugs on pineal melatonin synthesis. Our results show that adenosine, N6-(phenylisopropyl)adenosine (R-PIA) and 2-p-(2-carboxethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS21680) did not affect melatonin synthesis alone or in combination with isoprenaline. However 5'-N-ethylcarboxamidoadenosine (NECA) (100 microM) potentiated the stimulatory effect of isoprenaline (3 microM) on pineal melatonin production and this effect appeared to be antagonized by 8-SPT (50 microM). These results are consistent with activation by NECA of an A2b adenosine receptor.

Effect of adenosine and some of its structural analogues on the conductance of NMDA receptor channels in a subset of rat neostriatal neurones

1. In order to investigate the modulatory effects of adenosine on excitatory amino acid projections onto striatal medium spiny neurons, whole-cell patch clamp experiments were carried out in rat brain slices. The effects of various agonists for P1 (adenosine) and P2 (ATP) purinoceptors and their antagonists were investigated. The A2A receptor agonist 2-p-(2-carboxyethyl)phenythylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 0.1 microM), the A1 receptor agonist 2-chloro-N6-cyclcopentyladenosine (CCPA; 10 microM) and the non-selective P1 purinoceptor antagonist 8-(p sulphophenyl)-theophylline (8-SPT; 100 microM) did not alter the resting membrane potential, the threshold current necessary to elicit an action potential, the amplitude of spikes, their rise time, the amplitude of the afterhyperpolarization (AHP) and the time to peak of the AHP. 2. N-methyl-D-aspartate (NMDA; 1-1000 microM) caused a concentration-dependent inward current which was larger in the absence than in the presence of Mg2+ (1.3 mM). In a subset of striatal neurones, the current response to NMDA (10 microM) and the accompanying increase in conductance were both inhibited by CGS 21680 (0.01-1 microM). The effect of CGS 21680 (0.1 microM) persisted in the presence of tetrodotoxin (0.5 microM) or in a Ca(2+)-free medium, under conditions when synaptically mediated influences may be negligible. 3. The A3 receptor agonist N6-2-(4-aminophenyl)ethyladenosine (APNEA; 0.1-10 microM) also diminished the effect of NMDA (10 microM), while the A1 receptor agonists CCPA (0.1-10 microM) and (2S)-N6-[2-endonorbornyl] adenosine [S(-)-ENBA; 10 microM] as well as the endogenous, non-selective P1 purinoceptor agonist adenosine (100 microM) were inactive. The endogenous non-selective P2 purinoceptor agonist ATP (1000 microM) also failed to alter the current response to NMDA (10 microM). Adenosine (100 microM), but not ATP (1000 microM) became inhibitory after blockade of nucleoside uptake by S[4-nitrobenzyl)-6-thioguanosine (NBTG; 30 microM). 4. 8-(p-Sulphophenyl)-theophylline (8-SPT; 100 microM), as well as the A2A receptor antagonist 8-(3-chlorostyryl) caffeine (CSC; 1 microM) and the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) at 0.03, but not 0.003 microM abolished the inhibitory action of CGS 21,680 (0.1 microM). None of these compounds altered the effect of NMDA (10 microM) by itself. DPCPX (0.03 microM) prevented the inhibition of APNEA (10 microM). 5. There was no effect of CGS 21,680 (0.1 microM), when guanosine 5'-O-(3-thiodiphosphate (GDP-beta-S; 300 microM) was included in the pipette solution in order to block G protein-mediated reactions. 6. In conclusion, adenosine receptors, probably of the A2A-subtype, inhibit the conductance of NMDA receptor channels in a subset of medium spiny neurones of the rat striatum by a transduction mechanism which involves a G protein.

The adenosine A1 receptor antagonist BIIP 20 counteracts scopolamine-induced behavioral deficits in the passive avoidance task in the rat

The present study investigated the effects of the putative adenosine A1 receptor antagonist BIIP 20 ((S)-(-)-8-(3-oxocyclopentyl)-1,3-dipropyl-7H-purine-2,6-dione) on counteracting scopolamine-induced behavioral deficits in the rat in the passive avoidance paradigm. A single oral application of BIIP 20 (1 and 3 mg/kg) 90 min before the rats received the noxious stimulus significantly attenuated the scopolamine-induced deficits observed during the retention trial of this task.

In vivo amino acid release from the striatum of aging rats: adenosine modulation

The release of glutamate, aspartate, GABA, and taurine from the striatum of young (3 months), mature (12 months), and old (22 months), freely moving male rats was investigated by using a microdialysis fiber inserted transversally in the striatum. In old rats basal extracellular glutamate and aspartate levels were decreased vs. young rats (-38 and -49%, respectively). GABA and taurine levels were unmodified by age. In the presence of the adenosine receptor antagonist 8-phenyltheophilline (8-pT) at the concentration of 50 microM, both K(+)-evoked releases of glutamate and aspartate were more than doubled in young, but not in mature and old rats. 8-pT at the concentration of 500 microM significantly decreased glutamate basal levels and K(+)-evoked aspartate release in old rats only. GABA and taurine releases were not affected by 8-pT at either dose. Our findings indicate a modified adenosine modulation on glutamate and aspartate release in aged rats, that could result from a change in the balance between A1 and A2a adenosine receptor density or an alteration of A1 and A2a receptor-effector coupling.

Determination of the effects of caffeine and carbamazepine on striatal dopamine release by in vivo microdialysis

The effects of carbamazepine and caffeine on adenosine receptor subtypes were determined using in vivo microdialysis in an attempt to elucidate their different psychotropic mechanisms of action. Adenosine and a selective adenosine A1 receptor agonist decreased the striatal extracellular dopamine level, whereas caffeine, carbamazepine and a selective adenosine A1 receptor antagonist increased it, but neither an adenosine A2 receptor agonist nor an antagonist affected it. Under conditions of adenosine A1 receptor blockade, adenosine, carbamazepine and a selective adenosine A2 receptor agonist increased the striatal extracellular dopamine level, whereas caffeine and a selective adenosine A2 receptor antagonist decreased it. These results suggest that adenosine A1 receptor stimulation reduces the striatal extracellular dopamine level, and that adenosine A2 receptor stimulation under conditions of adenosine A1 receptor blockade increases it. Therefore, caffeine is an antagonist of both adenosine A1 and A2 receptor subtypes, and carbamazepine is an adenosine A1 receptor antagonist as well as an adenosine A2 receptor agonist. These properties support the hypothesis that the central actions of both carbamazepine and caffeine result from effects on both adenosine A1 and A2 receptors.

Role of cysteine residues of rat A2a adenosine receptors in agonist binding

In the present study, we investigated the role of disulfide bridges and sulfhydryl groups in A2a adenosine receptor binding of the agonist 2-p-(2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxamidoadenosi ne (CGS 21680). To evaluate the presence of essential disulfide bridges, rat striatal membranes were incubated with [3H]CGS 21680 in the presence of dithiothreitol and binding of the agonist to membranes was measured. The amount of [3H]CGS 21680 which specifically bound, decreased progressively upon pretreatment of membranes with increasing concentrations of dithiothreitol. Pretreatment of rat striatal membranes with 12.5 mM dithiothreitol for 15 min at 25 degrees C resulted in a 2-fold decrease of A2a adenosine receptor affinity for [3H]CGS 21680, and a reduction in the maximal number of binding sites. The presence of agonist or antagonist ligands protected the A2a adenosine receptor sites from the effect of dithiothreitol. We also examined the susceptibility of A2a adenosine receptors to inactivation by the sulfhydryl alkylating reagent, N-ethylmaleimide. When rat striatal membranes were pretreated with N-ethylmaleimide for 30 minutes at 37 degrees C, a decrease in specific [3H]CGS 21680 binding was observed. Pretreatment of membranes with 1 mM N-ethylmaleimide also resulted in a 2-fold reduction of A2a adenosine receptor affinity for [3H]CGS 21680, as well as a slight decrease in the maximal number of binding sites. Neither agonist nor antagonist ligands were effective in protecting the receptor sites from inactivation by N-ethylmaleimide. In contrast, addition of 100 microM guanosine-5'-O-(3-thiotriphosphate) or 5'-guanylylimidodiphosphate were both effective in protecting the receptor sites from inactivation by N-ethylmaleimide. This protective effect was significant but not complete. Our data suggest that disulfide bridges play a role in the structural integrity of the A2a adenosine receptor, furthermore, reduced sulfhydryl groups appear to be important but we do not yet know if they are on the receptor or on the Gs alpha subunit.

Postischemic administration of adenosine amine congener (ADAC): analysis of recovery in gerbils

Although adenosine receptor-based treatment of cerebral ischemia and other neurodegenerative disorders has been frequently advocated, cardiovascular side effects and an uncertain therapeutic time window of such treatment have constituted major obstacles to clinical implementation. Therefore, we have investigated the neuroprotective effects of the adenosine A1 receptor agonist adenosine amine congener (ADAC) injected after either 5 or 10 min ischemia at 100 micrograms/kg. When the drug was administered at either 6 or 12 h following 5 min forebrain ischemia, all animals were still alive on the 14th day after the occlusion. In both ADAC treated groups neuronal survival was approximately 85% vs. 50% in controls. Administration of a single dose of ADAC at times 15 min to 12 h after 10 min ischemia resulted in a significant improvement of survival in animals injected either at 15 or 30 min, or at 1, 2, or 3 h after the insult. In all 10 min ischemia groups, administration of ADAC resulted in a significant protection of neuronal morphology and preservation of microtubule associated protein 2 (MAP-2). However, postischemic Morris' water maze tests revealed full preservation of spatial memory and learning ability in animals injected at 6 h. On the other hand, the performance of gerbils treated at 12 h postischemia was indistinguishable from that of the controls. Administration of ADAC at 100 micrograms/kg in non-ischemic animals did not result in bradycardia, hypotension, or hypothermia. The data indicate that when ADAC is used postischemically, the most optimal level of protection is obtained when drugs are given at 30 min to 6 h after the insult. Although the mechanisms involved in neuroprotective effects of adenosine A1 receptor agonists require further studies, the present results demonstrate the feasibility of their clinical applications.

Kindled seizures do not affect adenosinergic inhibition of DA or ACh release in rat accumbens or PFC

Epileptic seizures are thought to terminate largely as a result of the extracellular accumulation of the purinergic neuromodulator, adenosine, released by discharging neurons. However, the postictal surge in extracellular adenosine and its widespread inhibitory effects are limited in time to only a few minutes and cannot directly account for increased resistance to seizures and the complex behavioural and motivational effects that may persist for hours or days after a seizure. The present study examined whether kindled seizures might alter the sensitivity or efficacy of inhibitory presynaptic adenosine receptors, and thereby induce more enduring changes in downstream transmitter systems. Rats were kindled in the amygdala of the dominant cerebral hemisphere, contralateral to the preferred direction of rotation, and their brains were removed either 2 h or 28 days after completion of kindling. Inhibition of electrically stimulated release of dopamine (DA) and acetylcholine (ACh) by the A1 adenosine-receptor agonist, R-phenylisopropyladenosine (R-PIA) was then measured in the prefrontal cortex (PFC) and nucleus accumbens. R-PIA (1.0 microM) inhibited [1H]DA release from PFC and nucleus accumbens tissue, and [14C]ACh release from nucleus accumbens tissue, but release was unaffected by prior kindling, regardless of the intervening interval. These results do not support suggestions that DA or ACh might mediate the effects of seizure-induced changes in purinergic inhibitory tone so as to cause long-term shifts in seizure threshold and postictal behavior.

Purinoceptors in the Central Nervous System

New exciting developments on the occurrence and functional role of purinoceptors in mammalian brain were presented at the session "Purinoceptors in the central nervous system" chaired by Flaminio Cattabeni and Tom Dunwiddie at the Purines '96 international conference. The focus of the session were topics of recent interest, including the sources and mechanisms involved in ATP and adenosine release during physiological neurotransmission in hippocampus, the brain expression of the recently cloned P2 receptors, and the role of the various adenosine receptor subtypes in brain protection from neurodegeneration associated with trauma-, ischemia-and excessive excitatory amino acid neurotransmission. New important insights into the mechanisms responsible for the formation and release of adenosine into the extracellular space were provided by data obtained by Dunwiddie and coworkers in hippocampal pyramidal neurons. These data may have functional implications for the role of purines in modulation of synaptic plasticity and long-term potentiation in this brain area, and hence in cognitive functions. Buell provided an updated overview on the cloning, molecular characteristics and brain expression of various ligand-gated P2X purinoceptors; although the functional role of these receptors in mammalian brain still awaits elucidation, their widespread distribution in the nervous system strongly suggests that ATP-mediated events are more prevalent and important in brain than expected. Pedata presented data on the functional interrelationships between adenosine and glutamate in the brain, and also provided evidence for alterations of the reciprocal regulation between these two systems in aged brain, which may have important implications for both ischemia-and trauma-associated neurodegenerative events and senescence-associated cognitive impairment. Finally, von Lubitz provided novel data on the molecular mechanisms likely to be at the basis of the brain protective effects associated with the chronic stimulation of the adenosine A3 receptor, further confirming that this receptor represents a crucial target for the development of new antiischemic and antineurodegenerative therapeutic agents.

A2 adenosine receptors: their presence and neuromodulatory role in the central nervous system

1. Adenosine is an endogenous neuromodulator that exerts its depressant effect on neurons by acting on the A1 adenosine receptor subtype. Excitatory actions of adenosine, mediated by the activation of the A2 adenosine receptor subtype, have also been shown in the central nervous system. 2. Adenosine A2a receptors are highly localized in the striatum, as demonstrated by the binding assay of the A2a selective agonist, CGS2680, and by analysis of the A2 receptor mRNA localization with in situ hybridization histochemistry. However, adenosine A2a, receptors, albeit at lower levels, are also localized in other brain regions, such as the cortex and the hippocampus. 3. In the striatum, adenosine A2a, receptors are implicated in the control of motor activity. Evidences exists of an antagonistic interaction between adenosine A2a and dopamine D2 receptors. 4. Utilizing selective agonists and antagonists for adenosine A2a receptors, their role in the modulation of the release of several neurotransmitters (acetylcholine, dopamine, glutamate, GABA) has been extensively studied in the brain (striatum, cortex, hippocampus). Controversial results have been obtained and, because the overall effect of endogenous adenosine in the brain is that of an inhibitory tonus, the physiological meaning of the excitatory A2 receptor remains to be clarified.

Temporal variations of adenosine metabolism in human blood

Eight diurnally active (06:00-23:00 h) subjects were adapted for 2 days to the room conditions where the experiments were performed. Blood sampling for adenosine metabolites and metabolizing enzymes was done hourly during the activity span and every 30 min during sleep. The results showed that adenosine and its catabolites (inosine, hypoxanthine, and uric acid), adenosine synthesizing (S-adenosylhomocysteine hydrolase and 5'-nucleotidase), degrading (adenosine deaminase) and nucleotide-forming (adenosine kinase) enzymes as well as adenine nucleotides (AMP, ADP, and ATP) undergo statistically significant fluctuations (ANOVA) during the 24 h. However, energy charge was invariable. Glucose and lactate chronograms were determined as metabolic indicators. The same data analyzed by the chi-square periodogram and Fourier series indicated ultradian oscillatory periods for all the metabolites and enzymatic activities determined, and 24-h oscillatory components for inosine, hypoxanthine, adenine nucleotides, glucose, and the activities of SAH-hydrolase, 5'-nucleotidase, and adenosine kinase. The single cosinor method showed significant oscillatory components exclusively for lactate. As a whole, these results suggest that adenosine metabolism may play a role as a biological oscillator coordinating and/or modulating the energy homeostasis and physiological status of erythrocytes in vivo and could be an important factor in the distribution of purine rings for the rest of the organism.

Sensorineural deafness in siblings with adenosine deaminase deficiency

Two siblings with adenosine deaminase deficiency were successfully treated with allogeneic bone marrow transplantation without conditioning. Although the patients were free from infections after immunologic reconstitution, both showed sensorineural deafness at 1 year of age. Because there were no structural abnormalities in the inner and middle ears, no evidence of prenatal infections of rubella, cytomegalovirus or toxoplasma, and no postnatal infection of mumps in the siblings, sensorineural deafness might be one of the neurologic problems associated with adenosine deaminase deficiency.

Magnesium ion augmentation of inhibitory effects of adenosine on dopamine release in the rat striatum

The effects of adenosine and magnesium ion (Mg2+) on striatal dopamine release were studied in awake rats by in vivo microdialysis. The mean striatal basal levels of dopamine release at Mg2+ free perfusate were 56.95 +/- 5.30 fmol/sample (for 20 min). By varying the Mg2+ levels in perfusate from 0 mmol/L to 1, 10 or 40 mmol/L, the dopamine release was inhibited by Mg2+ in a level-dependent manner. Perfusion with modified Ringer's solution containing zero Mg2+ and from 5 to 50 mumol/L adenosine, non-selective adenosine agonist, as well as 0.1 mumol/L 2-chloro-N6-cyclopentyladenosine (CCPA), selective adenosine A1 agonist, showed no effect on dopamine release. However, from 5 to 50 mumol/L adenosine and from 0.1 to 1 mumol/L CCPA plus Mg2+ (1 and 40 mumol/L) perfusion decreased the dopamine release. This inhibitory effect of adenosine and CCPA on striatal dopamine release was enhanced by an increase in extracellular Mg2+ levels. Levels of 50 mumol/L of 8-cyclopentyl-1,3-dimethylxanthine (CPT), a selective adenosine A1 receptor antagonist, in perfusate increased the dopamine release under conditions both with and without Mg2+. This stimulatory effect of CPT on striatal dopamine release was reduced by an increase in extracellular Mg2+ levels. As a result, CPT antagonized the inhibitory effects of adenosine and CCPA on dopamine release under conditions of the presence and absence of Mg2+. These results suggest that the inhibition of striatal dopamine release by adenosine was mediated by adenosine A1 receptor. This inhibition was intensified by Mg2+. This study also revealed that the concentrations of Mg2+, which ranged from physiological to supraphysiological, reduced the striatal dopamine release; furthermore it was found that the physiological concentration of Mg2+ potentiated the effects of adenosine agonists, but inhibited adenosine antagonist. Thus, the present study, using in vivo microdialysis preparations, suggests Mg2+ inhibits the calcium ion channels and enhances the adenosinergic function in the central nervous system.

Brain maturation of high-affinity adenosine A2 receptors and their coupling to G-proteins

The neuromodulator adenosine is acting through specific receptors, A1 and A2, coupled to their effector systems via G-proteins. The regulatory effects of adenosine on locomotor activity have been attributed to an interaction with A2 striatal receptors. The postnatal development of adenosine A2a receptors was analysed in rat striatal membranes and by quantitative autoradiography in brain sections using [3H]CGS 21680 as specific probe. At the concentration of radioligand used (5 nM), A2a sites were concentrated in the striatum at all ages, with minor developmental alterations in the expression pattern within the striatal regions. In membrane preparations, Scatchard analysis showed that the density of CGS 21680 binding sites was low at birth, around 3% of the adult value, and then increased, mostly between birth and 5 days and then from 15 days to adulthood. Concomitantly, the receptor affinity decreased sharply during brain development, Kd values varying from 2 to 15.5 nM. The addition of a GTP analogue, guanylyl-5'-imidodiphosphate (Gpp(NH)p, 10 microM), to the assay medium reduced significantly the receptor affinity throughout the postnatal development, reflecting a coupling to G-proteins at all ages, but it also suggested a weaker association at birth. These data show that the developmental properties of A2a receptors contrast with those of A1 receptors, and emphasize the role played by adenosine through its A2 receptors in the maturation of striatum-related cerebral pathways.

Hypothermia in hypoxic animals: mechanisms, mediators, and functional significance

A basic tenet of biology is that body temperature (Tb) has a marked effect on oxygen uptake of resting animals. For most animals, the temperature coefficient (Q10) is >> 2.5; e.g., resting oxygen uptake changes about 11% per degree C change in Tb. An important consequence of this dependence is that hyperthermia could be deleterious for hypoxic animals, particularly for oxygen sensitive organs, e.g., heart and brain. Conversely, a moderate degree of hypothermia could be beneficial during hypoxia. This concept is not new. Forced hypothermia is sometimes used in surgical procedures, particularly for heart and brain surgery. However, in many situations where hypothermia might have benefits, e.g., pediatric intensive care, it is not permitted. This is due in part to dogma and in part to the real and potential disadvantages of hypothermia, even in severely hypoxic animals. Among these in ventricular fibrillation. This is apparently preventable if blood pH is allowed to rise following the "Buffalo Curve." Another important disadvantage, were it to occur, is elevation of oxygen demand due to a thermogenic responses. However, at least in some species, the thermogenic response is blunted during hypoxia; e.g., in young rats. Furthermore, even if a thermogenic response occurs, this takes place primarily in muscles (shivering) and brown fat (non-shivering) and not in the O2-sensitive organs, heart and brain. A third disadvantage, for prolonged hypothermia, might be impairment of the immune response, a serious problem if hypoxia is combined with infection. This paper will review four aspects of behavioral fever and hypothermia: the occurrence among animals, the mechanisms and mediators that might trigger behavioral responses, and the functional significance.

The effects of selective A1 and A2a adenosine receptor antagonists on cerebral ischemic injury in the gerbil

Cerebral ischemia of 5 min duration was induced in unanesthetized Mongolian gerbils by bilateral occlusion of the carotid arteries. The extent of ischemic injury was assessed behaviorally by measuring the increases in locomotor activity following ischemia and by a histopathological assessment of the extent of CA1 hippocampal pyramidal cell injury and loss 5 days after ischemia. The A2a adenosine receptor selective antagonists 8-(3-chlorostyryl) caffeine (CSC; 0.1 mg/kg i.p.) and 4-amino-1-phenyl[1,2,4]-triazolo[4,3-a] quinoxaline (CP 66,713; 0.1 mg/kg i.p.) reduced the extent of ischemia-induced injury. An A1 selective receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1.0 mg/kg i.p.), enhanced ischemia-evoked injury. These results suggest that adenosine A2a receptor antagonists may be useful for the prevention of cerebral injuries resulting from stroke or cardiac arrest.

Adenosine A2A receptor stimulation enhances striatal extracellular glutamate levels in rats

The influence of CGS 21680, an adenosine A2A receptor agonist, on striatal glutamate extracellular levels was tested in a microdialysis study in rats. CGS 21680 (10 mu M), infused intrastriatally through the microdialysis probe, greatly enhanced glutamate extracellular levels. These results show that striatal adenosine A2A receptors are involved in the regulation of striatal glutamate extracellular levels. They also suggest that adenosine A2A receptor antagonists may possess neuroprotective effects in models of striatal excitotoxicity.