A1 adenosine receptors mediate hypoxia-induced ventriculomegaly

Periventricular leukomalacia is characterized by a reduction in brain matter and secondary ventriculomegaly and is a major cause of developmental delay and cerebral palsy in prematurely born infants. Currently, our understanding of the pathogenesis of this condition is limited. In animal models, features of periventricular leukomalacia can be induced by hypoxia and activation of A1 adenosine receptors (A1ARs). Using mice that are deficient in the A1AR gene (A1AR-/-), we show that A1ARs play a prominent role in the development of hypoxia-induced ventriculomegaly in neonates. Supporting a role for adenosine in the pathogenesis of developmental brain injury, ventriculomegaly was also observed in mice lacking the enzyme adenosine deaminase, which degrades adenosine. Thus, adenosine acting on A1ARs appears to mediate hypoxia-induced brain injury ventriculomegaly during early postnatal development.

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.

Evidence for functional adenosine A3 receptors in microglia cells

Adenosine exerts its effects through four subtypes of G-protein-coupled receptors (GPCRs): adenosine A1 and A3 receptors (A3R), which generally couple to Gi proteins and adenosine A2A and A2B receptors that activate Gs proteins. Though there is evidence for the expression of mRNA for the A3R in the central nervous system, evidence for functional receptors has depended on drugs with uncertain specificity. Here, we show that A3Rs mediating functional responses are present in microglia cells. By selectively stimulating the A3R in both primary mouse microglia cells and the N13 microglia cell line with the agonist Cl-IB-MECA, we have found a biphasic, partly Gi protein-dependent influence on the phosphorylation of the extracellular signal-regulated protein kinase 1/2 (ERK1/2). ERK1/2 activation was assessed by immunoblotting with phospho-specific antibodies. The involvement of the A3R in Cl-IB-MECA-induced ERK1/2 phosphorylation was confirmed by demonstrating that those effects are absent in primary mouse microglia cells isolated from mice lacking the gene for the A3R.

Adenosine A 2B receptors modulate cAMP levels and induce CREB but not ERK1/2 and p38 phosphorylation in rat skeletal muscle cells

The present study examined the existence of the adenosine A(1),A(2A), and A(2B) receptors and the effect of receptor activation on cAMP accumulation and protein phosphorylation in primary rat skeletal muscle cells. Presence of mRNA and protein for all three receptors was demonstrated in both cultured and adult rat skeletal muscle. NECA (10(-9)-10(-4)M) increased the cAMP concentration in cultured muscle cells with an EC(50) of (95% confidence interval)=15 (5.9-25.1) micro M, whereas CGS 21680 (10(-9)-10(-4)M) had no effect on cAMP accumulation. Concentrations of [R]-PIA below 10(-6)M had no effect on cAMP accumulation induced by either isoproterenol or forskolin. NECA resulted in phosphorylation of CREB with an EC(50) of (95% confidence interval)=1.7 (0.40-7.02) micro M, whereas ERK1/2 and p38 phosphorylation was unchanged. The results show that, although the A(1),A(2A), and A(2B) receptors are all present in skeletal muscle cells, the effect of adenosine on adenylyl cyclase activation and phosphorylation of CREB is mainly mediated via the adenosine A(2B) receptor.

Pharmacology of adenosine A2A receptors and therapeutic applications

Adenosine A(2A) receptors were cloned about ten years ago and are known to be well conserved among mammals. Rather selective agonists and antagonists are available. In addition, two different knock-out mice have been prepared and extensively characterized. A(2A) receptors are highly enriched in the basal ganglia and on cells involved in inflammatory reactions. At these sites they are likely to play physiologically important roles. Efforts to develop new therapies based on A(2A) receptors have focused on these topics. However, A(2A) receptors are found on many other cell types and on them as well agonists can exert effect.

Synergistic effects of adenosine A1 and P2Y receptor stimulation on calcium mobilization and PKC translocation in DDT1 MF-2 cells

1. The effect of adenosine analogues and of nucleotides, alone or in combination, on intracellular calcium, accumulation of inositol (1,4,5) trisphosphate (InsP3), and on activation of protein kinase C (PKC) was studied in DDT1 MF2 cells derived from a Syrian hamster myosarcoma. These cells were found to express mRNA for A1 and some as yet unidentified P2Y receptor(s). 2. Activation of either receptor type stimulated the production of InsP3 and raised intracellular calcium in DDT1 MF2 cells. Similarly, the A1 selective agonist N6-cyclopentyladenosine (CPA) increased PKC-dependent phosphorylation of the substrate MBP(4-14) and induced a PKC translocation to the plasma membrane as determined using [3H]-phorbol dibutyrate (PDBu) binding in DDT1 MF-2 cells. However, neither adenosine nor CPA induced a significant translocation of transiently transfected gamma-PKC-GFP from the cytosol to the cell membrane. In contrast to adenosine analogues, ATP and UTP also caused a rapid but transient translocation of gamma-PKC-GFP and activation of PKC. 3. Doses of the A1 agonist CPA and of ATP or UTP per se caused barely detectable increases in intracellular Ca2+ but when combined, they caused an almost maximal stimulation. Similarly, adenosine (0.6 microM) and UTP (or ATP, 2.5 microM), which per se caused no detectable translocation of either gamma- or epsilon-PKC-GFP, caused when combined a very clear-cut translocation of both PKC subforms, albeit with different time courses. These results show that simultaneous activation of P2Y and adenosine A1 receptors synergistically increases Ca2+ transients and translocation of PKC in DDT1 MF-2 cells. Since adenosine is rapidly formed by breakdown of extracellular ATP, such interactions may be biologically important.

Adenosine receptors as targets for drug development

Adenosine is a ubiquitous autacoid that acts on four defined receptors, named A(1), A(2A), A(2B) and A(3). Although the biological activity of adenosine has been known for more than 70 years and the existence of specific receptors for more than 25 years, it is only now that the full potential for drug development is becoming clear. Among some of the conditions for which adenosine receptor-based therapy might be used are Parkinson's disease, hypoxia/ischemia, epilepsy, kidney disease and asthma.

Aggravated brain damage after hypoxic ischemia in immature adenosine A2A knockout mice

BACKGROUND AND PURPOSE

Cerebral hypoxic ischemia (HI) is an important cause of brain injury in the newborn infant. Adenosine is believed to protect against HI brain damage. However, the roles of the different adenosine receptors are unclear, particularly in young animals. We examined the role of adenosine A2A receptors (A2AR) using 7-day-old A2A knockout (A2AR(-/-)) mice in a model of HI.

METHODS

HI was induced in 7-day-old CD1 mice by exposure to 8% oxygen for 30 minutes after occlusion of the left common carotid artery. The resulting unilateral focal lesion was evaluated with the use of histopathological scoring and measurements of residual brain areas at 5 days, 3 weeks, and 3 months after HI. Behavioral evaluation of brain injury by locomotor activity, rotarod, and beam-walking test was made 3 weeks and 3 months after HI. Cortical cerebral blood flow, assessed by laser-Doppler flowmetry, and rectal temperature were measured during HI.

RESULTS

Reduction in cortical cerebral blood flow during HI and rectal temperature did not differ between wild-type (A2AR(+/+)) and knockout mice. In the A2AR(-/-) animals, brain injury was aggravated compared with wild-type mice. The A2AR(-/-) mice subjected to HI displayed increased forward locomotion and impaired rotarod performance in adulthood compared with A2AR(+/+) mice subjected to HI, whereas beam-walking performance was similarly defective in both groups.

CONCLUSIONS

These results suggest that, in contrast to the situation in adult animals, A2AR play an important protective role in neonatal HI brain injury.

Decreased inflammatory pain due to reduced carrageenan-induced inflammation in mice lacking adenosine A3 receptors

Mice with a targeted disruption of adenosine A(3) receptor (A(3)AR) gene were assessed for their nociceptive threshold and for their localized inflammatory response following carrageenan injected into the hindpaw. Under basal conditions no difference was seen between A(3)AR knock-out (A(3)AR(-/-)) and wild-type (A(3)AR(+/+)) mice in nociceptive response to mechanical or heat stimuli. The antinociceptive response to the intrathecal adenosine analogue R-phenylisopropyl adenosine (R-PIA) was also unchanged in the A(3)AR(-/-) mice. In contrast, heat hyperalgesia, plasma extravasation and edema following carrageenan-induced inflammation in the hind paw were significantly reduced in A(3)AR(-/-) mice compared to the A(3)AR(+/+) controls. Thus, mice lacking A(3)AR had deficits in generating the localized inflammatory response to carrageenan, supporting a pro-inflammatory role of A(3)AR in peripheral tissues. However, no evidence for a role of A(3)AR in nociception and the antinociceptive effect of R-PIA was found.

Signaling pathway from the human adenosine A(3) receptor expressed in Chinese hamster ovary cells to the extracellular signal-regulated kinase 1/2

Adenosine activates four different receptors, the A(1), A(2A), A(2B), and the A(3) receptors, all of which are G protein-coupled. We have previously shown that stimulation of the human adenosine A(3) receptor can induce phosphorylation of extracellular signal-regulated kinase (ERK1/2). Here we show that the adenosine receptor agonist 5' N-ethylcarboxamidoadenosine (NECA) induces phosphorylation and activation of ERK1/2 in Chinese hamster ovary (CHO) cells expressing the human adenosine A(3) receptor (CHO A(3) cells) with the same potency. Pretreatment with pertussis toxin abolished the effect, which also could be blunted by overexpressing the betagamma-sequestering peptide beta-adrenergic receptor kinase-ct, implicating the involvement of betagamma subunits released from G(i/o) proteins. Activation of phosphatidylinositol-3-kinase (PI3K) by adenosine A(3) receptors is inferred from a dose-dependent Ser-phosphorylation of the protein kinase B (Akt). Furthermore the ERK1/2 phosphorylation was sensitive to the PI3K inhibitors wortmannin and LY294002 (2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride) and the MEK inhibitor PD98059 (2'-amino-3'-methoxyflavone), whereas chelation of Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) and long-term treatment with phorboldibutyrate did not decrease the adenosine A(3) receptor-mediated ERK1/2 phosphorylation. Thus, Ca(2+) mobilization and conventional and novel protein kinase C (PKC) isoforms are not involved in this pathway. The atypical PKCzeta was not activated by NECA and thus not involved in the A(3) receptor-mediated ERK1/2 phosphorylation. NECA stimulation of CHO A(3) cells activated the small G protein Ras and the dominant negative mutant RasS17N prevented the phosphorylation of ERK1/2. In conclusion, the adenosine A(3) receptor recruits a pathway that involves betagamma release from G(i/o), PI3K, Ras, and MEK to induce ERK1/2 phosphorylation and activation, whereas signaling is independent of Ca(2+), PKC, and c-Src.

Modulation of hippocampal glutamatergic transmission by ATP is dependent on adenosine a(1) receptors

Excitatory glutamatergic synapses in the hippocampal CA1 region of rats are potently inhibited by purines, including adenosine, ATP, and ATP analogs. Adenosine A(1) receptors are known to mediate at least part of the response to adenine nucleotides, either because adenine nucleotides activate A(1) receptors directly, or activate them secondarily upon the nucleotides' conversion to adenosine. In the present studies, the inhibitory effects of adenosine, ATP, the purportedly stable ATP analog adenosine-5'-O-(3-thio)triphosphate (ATPgammaS), and cyclic AMP were examined in mice with a null mutation in the adenosine A(1) receptor gene. ATPgammaS displaced the binding of A(1)-selective ligands to intact brain sections and brain homogenates from adenosine A(1) receptor wild-type animals. In homogenates, but not in intact brain sections, this displacement was abolished by adenosine deaminase. In hippocampal slices from wild-type mice, purines abolished synaptic responses, but slices from mice lacking functional A(1) receptors showed no synaptic modulation by adenosine, ATP, cAMP, or ATPgammaS. In slices from heterozygous mice the dose-response curve for both adenosine and ATP was shifted to the right. In all cases, inhibition of synaptic responses by purines could be blocked by prior treatment with the competitive adenosine A(1) receptor antagonist 8-cyclopentyltheophylline. Taken together, these results show that even supposedly stable adenine nucleotides are rapidly converted to adenosine at sites close to the A(1) receptor, and that inhibition of synaptic transmission by purine nucleotides is mediated exclusively by A(1) receptors.

Adenosine A(2A) receptor facilitation of hippocampal synaptic transmission is dependent on tonic A(1) receptor inhibition

Adenosine tonically inhibits synaptic transmission through actions at A(1) receptors. It also facilitates synaptic transmission, but it is unclear if this facilitation results from pre- and/or postsynaptic A(2A) receptor activation or from indirect control of inhibitory GABAergic transmission. The A(2A) receptor agonist, CGS 21680 (10 nM), facilitated synaptic transmission in the CA1 area of rat hippocampal slices (by 14%), independent of whether or not GABAergic transmission was blocked by the GABA(A) and GABA(B) receptor antagonists, picrotoxin (50 microM) and CGP 55845 (1 microM), respectively. CGS 21680 (10 nM) also inhibited paired-pulse facilitation by 12%, an effect prevented by the A(2A) receptor antagonist, ZM 241385 (20 nM). These effects of CGS 21680 (10 nM) were occluded by adenosine deaminase (2 U/ml) and were made to reappear upon direct activation of A(1) receptors with N(6)-cyclopentyladenosine (CPA, 6 nM). CGS 21680 (10 nM) only facilitated (by 17%) the K(+)-evoked release of glutamate from superfused hippocampal synaptosomes in the presence of 100 nM CPA. This effect of CGS 21680 (10 nM), in contrast to the isoproterenol (30 microM) facilitation of glutamate release, was prevented by the protein kinase C inhibitors, chelerythrine (6 microM) and bisindolylmaleimide (1 microM), but not by the protein kinase A inhibitor, H-89 (1 microM). Isoproterenol (30 microM), but not CGS 21680 (10-300 nM), enhanced synaptosomal cAMP levels, indicating that the CGS 21680-induced facilitation of glutamate release involves a cAMP-independent protein kinase C activation. To discard any direct effect of CGS 21680 on adenosine A(1) receptor, we also show that in autoradiography experiments CGS 21680 only displaced the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentyladenosine ([(3)H]DPCPX, 0.5 nM) with an EC(50) of 1 microM in all brain areas studied and CGS 21680 (30 nM) failed to change the ability of CPA to displace DPCPX (1 nM) binding to CHO cells stably transfected with A(1) receptors. Our results suggest that A(2A) receptor agonists facilitate hippocampal synaptic transmission by attenuating the tonic effect of inhibitory presynaptic A(1) receptors located in glutamatergic nerve terminals. This might be a fine-tuning role for adenosine A(2A) receptors to allow frequency-dependent plasticity phenomena without compromising the A(1) receptor-mediated neuroprotective role of adenosine.

Involvement of DARPP-32 phosphorylation in the stimulant action of caffeine

Caffeine has been imbibed since ancient times in tea and coffee, and more recently in colas. Caffeine owes its psychostimulant action to a blockade of adenosine A(2A) receptors, but little is known about its intracellular mechanism of action. Here we show that the stimulatory effect of caffeine on motor activity in mice was greatly reduced following genetic deletion of DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein of relative molecular mass 32,000). Results virtually identical to those seen with caffeine were obtained with the selective A(2A) antagonist SCH 58261. The depressant effect of the A(2A) receptor agonist, CGS 21680, on motor activity was also greatly attenuated in DARPP-32 knockout mice. In support of a role for DARPP-32 in the action of caffeine, we found that, in striata of intact mice, caffeine increased the state of phosphorylation of DARPP-32 at Thr 75. Caffeine increased Thr 75 phosphorylation through inhibition of PP-2A-catalysed dephosphorylation, rather than through stimulation of cyclin-dependent kinase 5 (Cdk5)-catalysed phosphorylation, of this residue. Together, these studies demonstrate the involvement of DARPP-32 and its phosphorylation/dephosphorylation in the stimulant action of caffeine.

Mice lacking the adenosine A1 receptor are anxious and aggressive, but are normal learners with reduced muscle strength and survival rate

Behavioural assessment of mice lacking adenosine A1 receptors (A1Rs) showed reduced activity in some phases of the light-dark cycle, reduced exploratory behaviour in the open-field and in the hole-board, increased anxiety in the plus maze and dark-light box and increased aggressiveness in the resident-intruder test. No differences were found in spatial reference and working memory in several Morris water maze tasks. Both mutant mice had reduced muscle strength and survival rate. These results confirm the involvement of adenosine in motor activity, exploratory behaviour, anxiety and aggressiveness. A1Rs also appear to play a critical role in ageing-related deterioration.

MRI evaluation and functional assessment of brain injury after hypoxic ischemia in neonatal mice

BACKGROUND AND PURPOSE

Severe perinatal asphyxia is an important cause of brain injury in the newborn infant. We examined early events after hypoxic ischemia (HI) in the 7-day-old mouse brain by MRI and related them to long-term functional effects and histopathology in the same animals at 4 to 5 weeks of age.

METHODS

HI was induced in 7-day-old CD1 mice by exposure to 8% oxygen for 30 minutes after occlusion of the left common carotid artery. The resulting unilateral focal lesion was evaluated in vivo by MRI (T2 maps and apparent diffusion coefficient maps) at 3, 6, and 24 hours and 5 days after hypoxia. Locomotion and sensorimotor function were analyzed after 3 weeks. Four weeks after HI, the mice were killed, and cresyl violet-stained brain sections were examined morphologically.

RESULTS

A decrease in apparent diffusion coefficient values in cortex on the affected side was found at 3 hours after HI. T2 values were significantly increased after 6 hours and remained so for 5 days. Maximal size of the lesion was attained at 3 to 6 hours after HI and declined thereafter. Animals with MRI-detected lesions had decreased forward locomotion, performed worse than controls in the beam-walking test, and showed a unilateral hypotrophy in the cresyl violet-stained brain sections 4 weeks later.

CONCLUSIONS

The temporal progression of the damage after HI in 7-day-old mice differs from that of the adult brain as judged by MRI. The early lesions detected by MRI were related to functional impairments for these mice in near-adult life.

The role of pharmacology in drug discovery

The International Union of Pharmacology (IUPHAR) enthusiastically welcomes the decision by the Nature Publishing Group to launch its new journal, Nature Reviews Drug Discovery. The title of the new journal poses interesting questions for pharmacologists. Why 'Drug Discovery'? Would we have preferred 'Pharmacology'? And do these distinctions even matter, as aren't all pharmacologists involved somehow in drug discovery?

Anoxia redistributes adenosine A(2A) receptors in PC12 cells and increases receptor-mediated formation of cAMP

Abstract. Undifferentiated and NGF-treated PC12 cells were subjected to anoxia for up to 24 h. The adenosine A(2A) receptor antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4 triazolo[1,5-c]pyrimidine (SCH 58261) decreased viability of undifferentiated, but not NGF-treated PC12 cells after 6 h of anoxia. Anoxia also transiently enhanced cAMP responses induced via activation of adenosine A(2A) receptors in undifferentiated PC12 cells (20-fold decrease in the EC(50) value for the agonist 2-[ p-(2-carbonylethyl) phenylethylamino]-5'- N-ethylcarboxamidoadenosine, CGS 21680). In NGF-treated PC12 cells, by contrast, anoxia decreased both the maximal response to and the potency of CGS 21680. In undifferentiated PC12 cells subjected to anoxia a very modest increase of A(2A) receptor mRNA was detected in cells by Northern blotting, but no changes in the amount of the receptor protein could be seen by Western blotting. However, surface biotinylation of the cells followed by avidin pull-down showed that the A(2A) receptor at the cell membrane was increased after anoxia. This was supported by immunolabelling of the A(2A) receptors: much of the receptor protein was present in the cytoplasm of normoxic cells, but in cells subjected to anoxia the A(2A) receptor immunolabelling at the cell membrane was more pronounced, indicating redistribution of the receptors from intracellular pools to the cell membrane during anoxia.

Diverse inhibitors of intracellular signalling act as adenosine receptor antagonists

Inhibitors of intracellular signalling events, including enzyme inhibitors, are often used to investigate signal transduction pathways. We examined whether some inhibitors that act on the ATP site of enzymes are also potent adenosine receptor antagonists. Competitive radioligand binding assays in membranes or brain sections show that genistein, chelerythrine, and SQ22536 [9-(tetrahydro-2'-furyl) adenine] block A(1), A(2A), and A(3) adenosine receptors in concentrations of these drugs commonly used to examine cellular signalling (K(i) of [(3)H]-DPCPX (1,3-dipropyl-8-cyclopentylxanthine) competition mean (95% confidence interval): 2.6 (1.5-4.8) microM, 5.7 (2.1-15.8) microM, 59.4 (17.3-203.8) microM; K(i) of [(3)H]-SCH58261 [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine] competition: 15.3 (8.1-28.8) microM, 37.6 (10.3-137.4) microM, 16.7 (11.5-24.3) microM for genistein, chelerythrine, and SQ22536, respectively). Given that adenosine receptors are present on most cells, that adenosine is often present, and that adenosine receptors interact functionally with several signalling pathways, these results may be of significance also when studying signalling via other receptors.

International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors

Four adenosine receptors have been cloned and characterized from several mammalian species. The receptors are named adenosine A(1), A(2A), A(2B), and A(3). The A(2A) and A(2B) receptors preferably interact with members of the G(s) family of G proteins and the A(1) and A(3) receptors with G(i/o) proteins. However, other G protein interactions have also been described. Adenosine is the preferred endogenous agonist at all these receptors, but inosine can also activate the A(3) receptor. The levels of adenosine seen under basal conditions are sufficient to cause some activation of all the receptors, at least where they are abundantly expressed. Adenosine levels during, e.g., ischemia can activate all receptors even when expressed in low abundance. Accordingly, experiments with receptor antagonists and mice with targeted disruption of adenosine A(1), A(2A), and A(3) expression reveal roles for these receptors under physiological and particularly pathophysiological conditions. There are pharmacological tools that can be used to classify A(1), A(2A), and A(3) receptors but few drugs that interact selectively with A(2B) receptors. Testable models of the interaction of these drugs with their receptors have been generated by site-directed mutagenesis and homology-based modelling. Both agonists and antagonists are being developed as potential drugs.

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.

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.

Opposite changes in adenosine A1 and A2A receptor mRNA in the rat following sleep deprivation

Extracellular levels of adenosine increase in basal forebrain following prolonged wakefulness. Moreover, perfusion of adenosine into basal forebrain increases sleep. In this study we have examined the adenosine receptor subtypes, A1 and A2A, for changes in the levels of mRNA using RT-PCR and in situ hybridization and the receptor ligand binding efficiency using autoradiography following 3 and 6 h of sleep deprivation. We observed that A1 receptor mRNA levels increased in basal forebrain with no changes in other forebrain areas examined. A1 receptor binding was not affected. A2A receptor mRNA and ligand binding were undetectable in basal forebrain. However, in the olfactory tubercle, A2A mRNA and receptor binding decreased significantly. Based on the significant increase in the A1 but not in A2A receptor, we hypothesize that the effects of sleep deprivation-induced increased adenosine are mediated by A1 receptor in basal forebrain of rats.