Inhibition of brain adenylate cyclase by A1 adenosine receptors: pharmacological characteristics and locations

Thyrotropin Causes Dose-dependent Biphasic Regulation of cAMP Production Mediated by Gs and Gi/o Proteins

The thyrotropin (TSH) receptor (TSHR) signals via G proteins of all four classes and β-arrestin 1. Stimulation of TSHR leads to increasing cAMP production that has been reported as a monotonic dose-response curve that plateaus at high TSH doses. In HEK 293 cells overexpressing TSHRs (HEK-TSHR cells), we found that TSHR activation exhibits an "inverted U-shaped dose-response curve" with increasing cAMP production at low doses of TSH and decreased cAMP production at high doses (>1 mU/ml). Since protein kinase A inhibition by H-89 and knockdown of β-arrestin 1 or β-arrestin 2 did not affect the decreased cAMP production at high TSH doses, we studied the roles of TSHR downregulation and of Gi/Go proteins. A high TSH dose (100 mU/ml) caused a 33% decrease in cell-surface TSHR. However, because inhibiting TSHR downregulation with combined expression of a dominant negative dynamin 1 and β-arrestin 2 knockdown had no effect, we concluded that downregulation is not involved in the biphasic cAMP response. Pertussis toxin, which inhibits activation of Gi/Go, abolished the biphasic response with no statistically significant difference in cAMP levels at 1 and 100 mU/ml TSH. Concordantly, co-knockdown of Gi/Go proteins increased cAMP levels stimulated by 100 mU/ml TSH from 55% to 73% of the peak level. These data show that biphasic regulation of cAMP production is mediated by Gs and Gi/Go at low and high TSH doses, respectively, which may represent a mechanism to prevent overstimulation in TSHR-expressing cells. SIGNIFICANCE STATEMENT: We demonstrate biphasic regulation of TSH-mediated cAMP production involving coupling of the TSH receptor (TSHR) to Gs at low TSH doses and to Gi/o at high TSH doses. We suggest that this biphasic cAMP response allows the TSHR to mediate responses at lower levels of TSH and that decreased cAMP production at high doses may represent a mechanism to prevent overstimulation of TSHR-expressing cells. This mechanism could prevent chronic stimulation of thyroid gland function.

Antidepressant-Like Activity of Typical Antidepressant Drugs in the Forced Swim Test and Tail Suspension Test in Mice Is Augmented by DMPX, an Adenosine A2A Receptor Antagonist

Unsatisfactory therapeutic effects of currently used antidepressants force to search for new pharmacological treatment strategies. Recent research points to the relationship between depressive disorders and the adenosinergic system. Therefore, the main goal of our studies was to evaluate the effects of DMPX (3 mg/kg, i.p.), which possesses selectivity for adenosine A_2A receptors versus A₁ receptors, on the activity of imipramine (15 mg/kg, i.p.), escitalopram (2.5 mg/kg, i.p.), and reboxetine (2 mg/kg, i.p.) given in subtherapeutic doses. The studies carried out using the forced swim and tail suspension tests in mice showed that DMPX at a dose of 6 and 12 mg/kg exerts antidepressant-like effect and does not affect the locomotor activity. Co-administration of DMPX at a dose of 3 mg/kg with the studied antidepressant drugs caused the reduction of immobility time in both behavioral tests. The observed effect was not associated with an increase in the locomotor activity. To evaluate whether the observed effects were due to a pharmacokinetic/pharmacodynamic interaction, the levels of the antidepressants in blood and brain were measured using high-performance liquid chromatography. It can be assumed that the interaction between DMPX and imipramine was exclusively pharmacodynamic in nature, whereas an increased antidepressant activity of escitalopram and reboxetine was at least partly related to its pharmacokinetic interaction with DMPX.

An essential role for adenosine signaling in alcohol abuse

In the central nervous system (CNS), adenosine plays an important role in regulating neuronal activity and modulates signaling by other neurotransmitters, including GABA, glutamate, and dopamine. Adenosine suppresses neurotransmitter release, reduces neuronal excitability, and regulates ion channel function through activation of four classes of G protein-coupled receptors, A(1), A(2A), A(2B), and A(3). Central adenosine are largely controlled by nucleoside transporters, which transport adenosine levels across the plasma membrane. Adenosine has been shown to modulate cortical glutamate signaling and ventral-tegmental dopaminergic signaling, which are involved in several aspects of alcohol use disorders. Acute ethanol elevates extracellular adenosine levels by selectively inhibiting the type 1 equilibrative nucleoside transporter, ENT1. Raised adenosine levels mediate the ataxic and sedative/hypnotic effects of ethanol through activation of A(1) receptors in the cerebellum, striatum, and cerebral cortex. Recently, we have shown that pharmacological inhibition or genetic deletion of ENT1 reduces the expression of excitatory amino acid transporter 2 (EAAT2), the primary regulator of extracellular glutamate, in astrocytes. These lines of evidence support a central role for adenosine-mediated glutamate signaling and the involvement of astrocytes in regulating ethanol intoxication and preference. In this paper, we discuss recent findings on the implication of adenosine signaling in alcohol use disorders.

Adenosine modulates transmission at the hippocampal mossy fibre synapse via direct inhibition of presynaptic calcium channels

The modulation of synaptic transmission by presynaptic ionotropic and metabotropic receptors is an important means to control and dynamically adjust synaptic strength. Even though synaptic transmission and plasticity at the hippocampal mossy fibre synapse are tightly controlled by presynaptic receptors, little is known about the downstream signalling mechanisms and targets of the different receptor systems. In the present study, we identified the cellular signalling cascade by which adenosine modulates mossy fibre synaptic transmission. By means of electrophysiological and optical recording techniques, we found that adenosine activates presynaptic A1 receptors and reduces Ca2+ influx into mossy fibre terminals. Ca2+ currents are directly modulated via a membrane-delimited pathway and the reduction of presynaptic Ca2+ influx can explain the inhibition of synaptic transmission. Specifically, we found that adenosine modulates both P/Q- and N-type presynaptic voltage-dependent Ca2+ channels and thereby controls transmitter release at the mossy fibre synapse.

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

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

Presynaptic inhibition of spontaneous acetylcholine release induced by adenosine at the mouse neuromuscular junction

1. At the mouse neuromuscular junction, adenosine (AD) and the A(1) agonist 2-chloro-N(6)-cyclopentyl-adenosine (CCPA) induce presynaptic inhibition of spontaneous acetylcholine (ACh) release by activation of A(1) AD receptors through a mechanism that is still unknown. To evaluate whether the inhibition is mediated by modulation of the voltage-dependent calcium channels (VDCCs) associated with tonic secretion (L- and N-type VDCCs), we measured the miniature end-plate potential (mepp) frequency in mouse diaphragm muscles. 2. Blockade of VDCCs by Cd(2+) prevented the effect of the CCPA. Nitrendipine (an L-type VDCC antagonist) but not omega-conotoxin GVIA (an N-type VDCC antagonist) blocked the action of CCPA, suggesting that the decrease in spontaneous mepp frequency by CCPA is associated with an action on L-type VDCCs only. 3. As A(1) receptors are coupled to a G(i/o) protein, we investigated whether the inhibition of PKA or the activation of PKC is involved in the presynaptic inhibition mechanism. Neither N-(2[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide (H-89, a PKA inhibitor), nor 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine (H-7, a PKC antagonist), nor phorbol 12-myristate 13-acetate (PHA, a PKC activator) modified CCPA-induced presynaptic inhibition, suggesting that these second messenger pathways are not involved. 4. The effect of CCPA was eliminated by the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-acetoxymethyl ester epsilon6TDelta-BM, which suggests that the action of CCPA to modulate L-type VDCCs may involve Ca(2+)-calmodulin. 5. To investigate the action of CCPA on diverse degrees of nerve terminal depolarization, we studied its effect at different external K(+) concentrations. The effect of CCPA on ACh secretion evoked by 10 mm K(+) was prevented by the P/Q-type VDCC antagonist omega-agatoxin IVA. 6. CCPA failed to inhibit the increases in mepp frequency evoked by 15 and 20 mm K(+). We demonstrated that, at high K(+) concentrations, endogenous AD occupies A1 receptors, impairing the action of CCPA, since incubation with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A(1) receptor antagonist) and adenosine deaminase (ADA), which degrades AD into the inactive metabolite inosine, increased mepp frequency compared with that obtained in 15 and 20 mm K(+) in the absence of the drugs. Moreover, CCPA was able to induce presynaptic inhibition in the presence of ADA. It is concluded that, at high K(+) concentrations, the activation of A(1) receptors by endogenous AD prevents excessive neurotransmitter release.

Nonmonotonic Dose-Response Relationships: Mechanistic Basis, Kinetic Modeling, and Implications for Risk Assessment

Dose-response curves for the first interaction of a chemical with a biochemical target molecule are usually monotonic; i.e., they increase or decrease over the entire dose range. However, for reactions of a complex biological system to a toxicant, nonmonotonic (biphasic) dose-effect relationships can be observed, showing a decrease at low dose followed by an increase at high dose, or vice versa. We present four examples to demonstrate that nonmonotonic dose-response relationships can result from superimposition of monotonic dose responses of component biological reactions. Examples include (i) a membrane-receptor model with receptor subtypes of different ligand affinity and opposing downstream effects (adenosine receptors A1 vs. A2), (ii) androgen receptor-mediated gene expression driven by homodimers, but not mixed-ligand dimers, (iii) repair of background DNA damage by enzymatic activity induced by adducts formed by a xenobiotic, (iv) rate of mutation as a consequence of DNA damage times rate of cell division, the latter being modulated by cell-cycle delay at low-level DNA damage, and cell-cycle acceleration due to regenerative hyperplasia at cytotoxic dose levels. Quantitative analyses based on biological models are shown, and factors that affect the degree of nonmonotonicity are identified. It is noted that threshold-type dose-response curves could in fact be nonmonotonic. Our analysis should promote a scientific discussion of biphasic dose responses and the concept termed "hormesis," and of default procedures for low-dose extrapolation in toxicological risk assessment.

Adenosine A1 receptor-mediated presynaptic inhibition of GABAergic transmission in immature rat hippocampal CA1 neurons

In the mechanically dissociated rat hippocampal CA1 neurons with native presynaptic nerve endings, namely "synaptic bouton" preparation, the purinergic modulation of spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) was investigated using whole-cell recording mode under the voltage-clamp conditions. In immature neurons, adenosine (10 microM) reversibly decreased GABAergic mIPSC frequency without affecting the mean current amplitude. The inhibitory effect of adenosine transmission was completely blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM), a selective Alpha(1) receptor antagonist, and was mimicked by N(6)-cyclopentyladenosine (CPA, 1 microM), a selective Alpha(1) receptor agonist. However, CPA had no effect on GABAergic mIPSC frequency in postnatal 30 day neurons. N-ethylmaleimide (10 microM), a guanosine 5'-triphosphate binding protein uncoupler, and Ca(2+)-free external solution removed the CPA-induced inhibition of mIPSC frequency. K(+) channel blockers, 4-aminopyridine (100 microM) and Ba(2+) (1 mM), had no effect on the inhibitory effect of CPA on GABAergic mIPSC frequency. Stimulation of adenylyl cyclase with forskolin (10 microM) prevented the CPA action on GABAergic mIPSC frequency. Rp-cAMPS (100 microM), a selective PKA inhibitor, also blocked the CPA action. It was concluded that the activation of presynaptic Alpha(1) receptors modulates the probability of spontaneous GABA release via cAMP- and protein kinase A dependent pathway. This Alpha(1) receptor-mediated modulation of GABAergic transmission may play an important role in the regulation of excitability of immature hippocampal CA1 neurons.

Pyrazolopyridine derivatives act as competitive antagonists of brain adenosine A1 receptors: [35S]GTPgammaS binding studies

The effects of adenosine receptor ligands and three novel pyrazolopyridine derivatives on guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding to rat cerebral cortical membranes were examined. [35S]GTPgammaS binding was stimulated in a concentration dependent manner by several adenosine receptor agonists. The adenosine A2a receptor selective agonist, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680), was ineffective confirming specificity for adenosine A1 receptor activation. 2-Chloro-N6-cyclopentyladenosine (CCPA; 10(-7) M)-stimulated [35S]GTPgammaS binding was inhibited by xanthine and pyrazolopyridine based adenosine receptor antagonists. The concentration-response curve for CCPA-stimulated [35S]GTPgammaS binding was shifted to the right with increasing concentrations of antagonist without significant changes in maximal response. Schild analyses determined pK(B) values of 8.97, 8.88, 8.21, 8.16, 7.79 and 7.65 for 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5a]pyridin-3-yl) acryloyl]-2-piperidine ethanol (FK453), 6-oxo-3-(2-phenylpyrazolo[1,5a]pyridin-3-yl)-1(6H)-pyridazinebutyric+ ++ acid (FK838), 9-chloro-2-(2-furyl)[1,2,4]triazolo-[1,5c]quinazolin-5-amine (CGS 15943), 8-cyclopentyl-1,3-methylxanthine (CPT) and (R)-1-[(E)-3-(2-phenylpyrazolo[1,5a]pyridin-3-yl) acryloyl]-piperidin-2-yl acetic acid (FK352), respectively. Schild slopes were close to unity, confirming that these novel pyrazolopyridine derivatives act as competitive antagonists at rat brain adenosine A1 receptors.

Exogenous adenosine potentiates hypnosis induced by intravenous anaesthetics

We investigated the effect of adenosine on hypnosis induced by thiopentone, propofol and midazolam in mice. The onset and duration of hypnosis were determined by the loss of righting reflex. Adenosine and 2-chloroadenosine caused a significant shortening of onset of sleep-time and prolongation of duration of sleep-time in all groups (p < 0.05). Dipyridamole administration before combined intravenous anaesthetic-adenosine or intravenous anaesthetic-2-chloroadenosine administration produced similar effects to adenosine (p < 0.05). The adenosine antagonist theophylline, given before intravenous anaesthetic-adenosine or intravenous anaesthetic-2-chloroadenosine administration caused a significant delay in onset of sleep-time and shortening in the duration of sleep-time (p < 0.05). We conclude that central excitatory noradrenergic neurones play an important role in adenosine, 2-chloroadenosine and dipyridamole-induced hypnotic responses to intravenous anaesthetics and their inhibition by adenosine antagonists.

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.

Stimulation of the hypothalamo-pituitary-adrenal axis in the rat by three selective type-4 phosphodiesterase inhibitors: in vitro and in vivo studies

1. Previous studies in our laboratory have shown that the synthetic xanthine analogue denbufylline, a selective type 4 phosphodiesterase (PDE-4) inhibitor, is a potent activator of the hypothalamo-pituitary-adrenal (HPA) axis when given orally or intraperitoneally (i.p.) to adult male rats. This paper describes the results of experiments in which well established in vivo and in vitro methods were used to compare the effects of denbufylline on HPA function with those of two other selective PDE-4 inhibitors, rolipram and BRL 61063 (1,3-dicyclopropylmethyl-8-amino-xanthine). For comparison, parallel measurements of the immunoreactive- (ir-) luteinising hormone (LH) were made where appropriate. 2. When injected intraperitoneally, rolipram (40 and 200 micrograms kg-1, P < 0.005), denbufylline (0.07-0.6 microgram kg-1, P < 0.05) and BRL 61063 (30 micrograms kg-1, P < 0.005) each produced marked rises in the serum ir-corticosterone concentrations. However, lower doses of rolipram (1.6 and 8 micrograms kg-1) and BRL 61063 (0.25-6 micrograms kg-1) were without effect (P > 0.05). By contrast, intracerebroventricular (i.c.v.) injection of rolipram (8 ng-1 micrograms kg-1) or denbufylline (50 ng-1 microgram kg-1) failed to influence the serum ir-corticosterone concentration. BRL 61063 (8-120 ng kg-1, i.c.v.) was also ineffective in this regard although at a higher dose (1 microgram kg-1, i.c.v.) it produced a small but significant (P < 0.05) increase in ir-corticosterone release. Denbufylline also increased the serum ir-LH concentration when given peripherally (0.2-0.6 microgram kg-1, i.p., P < 0.05) or centrally (100 ng kg-1, i.c.v., P < 0.05) but rolipram (1.6-200 micrograms kg-1, i.p. or 8 ng-1 microgram kg-1, i.c.v.) and BRL 61063 (0.25-30 micrograms kg-1, i.p. or 1 ng-1 microgram kg-1, i.c.v.) did not (P > 0.05). 3. In vitro rolipram (10 microM, P < 0.01), denbufylline (1 mM, P < 0.001) and BRL 61063 (1 and 10 microM, P < 0.05) stimulated the release of corticotrophin releasing hormone (ir-CRH-41) but lower concentrations of the drugs were without effect as also was BRL 61063 at 100 microM (P > 0.05); the rank order of potency was thus BRL 61063 > rolipram > denbufylline. The adenylyl cyclase activator forskolin (100 microM, P < 0.01) also stimulated the release of ir-CRH-41, producing effects which were additive with those of rolipram and denbufylline but not with those of BRL 61063. The secretory responses to forskolin (100 microM) were accompanied by a highly significant increase in the cyclic AMP content of the hypothalamic tissue (P < 0.005). Rolipram (10 microM) also significantly (P < 0.05) elevated the hypothalamic cyclic AMP but denbufylline (10 mM) and BRL 61063 (10 microM) did not. However, all three PDE-inhibitors potentiated the rise in cyclic AMP induced by forskolin (P < 0.05). None of the drugs tested, alone or in combination, modified the release of arginine vasopressin (ir-AVP) from the hypothalamus. 4. Rolipram (100 microM), denbufylline (100 microM) and BRL 61063 (100 microM) stimulated the release of corticotrophin (ir-ACTH) from pituitary tissue in vitro (P < 0.05) but in lower concentrations they were without significant effect. In addition, rolipram (10 microM, P < 0.05), denbufylline (0.1 microM, P < 0.05) and BRL 61063 (10 microM, P < 0.05) potentiated the significant (P < 0.05) rises in ir-ACTH secretion induced by forskolin (100 microM). Forskolin (100 microM) also produced a highly significant increase (P < 0.01) in the tissue cyclic AMP content which was further potentiated by rolipram (10 microM), denbufylline (10 microM) and BRL 61063 (10 microM) which, alone did not affect the cyclic AMP content of the tissue. 5. Since both denbufylline and BRL 61063 possess significant adenosine A1 receptor blocking activity, further studies examined the potential influence of these receptors on the secretion in vitro of CRH-41, AVP and ACTH. The release of ir-CRH-41 was increased significantly by adenosine deaminase (ADA, 5microml-1, P<0.05) and the A1-receptor antagonist, 1,3-dicyclopropyl-8-cyclopentylxanthine (DPCPX, 0.1-10nM, P<0.05). The responses to ADA were abolished by the A1 receptor agonist N6-cyclo-hexyladenosine (CHA, 100nM, P<0.05) which alone had no significant effect on ir-CRH-41 release. ADA (0.1-10microml-1) and DPCPX (1nM) had weak stimulant and inhibitory effects, respectively, on the release of ir-ACTH from the pituitary gland while CHA (0.1-10nM) was without effect. Ligand binding studies with [3H]-DPCPX as a probe demonstrated the presence of specific high affinity A1 binding sites in the hypothalamus (Kd=0.7nM; Bmax=367+/-32fmolmg-1 protein) and in the hippocampus (Kd=1nM; Bmax=1165 +/-145fmolmg-1 protein). In both tissues binding of the ligand was displaced by CHA (IC50=1nM (hypothalamus) and 2nM (hippocampus)), BRL 61063 (IC50=80nM (hypothalamus) and 100nM (hippocampus)) and denbufylline (IC50=5microM (hypothalamus) and 9microM(hippocampus)) but not by rolipram. 6.The results suggest that rolipram, denblufylline and BRL 61063 stimulate the HPA axis in the rat, acting at the levels of both the hypothalamus and the pituitary gland. Their actions may be explained, at least in part, by inhibition of PDE-4 but additional actions including blockade of hypothalamic adenosine A1 receptors by denbufylline and BRL 61063 cannot be excluded.

Chronic ethanol intoxication enhances [3H]CCPA binding and does not reduce A1 adenosine receptor function in rat cerebellum

The effects of acute and chronic treatment with ethanol on the function of A1 adenosine receptor in the rat cerebellar cortex were investigated. Acute administration of ethanol (0.5-5 g/kg) had no effect on the binding of the A1-receptor agonist [3H]2-chloro-N6-cyclopentyladenosine ([3H]CCPA) or that the antagonist [3H]8-cyclopentyl-1-3-dipropylxanthine ([3H]DPCPX) in rat cerebellar cortical membranes. Rats were rendered ethanol dependent by repeated forced oral administration of ethanol (12-18 g/kg per day) for 6 days. [3H]CCPA binding was increased by 23% in cerebellar cortical membranes prepared from rats killed 3 h after ethanol withdrawal compared with saline-treated animals. The increase in [3H]CCPA binding was still apparent 12-24 h after the last ethanol administration, but was no longer detectable 3-6 days after ethanol withdrawal. In contrast, the binding of [3H]DPCPX was not modified in the cerebellar cortex of rats killed at various times after ethanol withdrawal. The acute administration of CCPA [0.25-1 mg/kg, intraperitoneally (IP)] suppressed the tremors and audiogenic seizures apparent 24 h after ethanol withdrawal. Moreover, repeated coadministration of CCPA (0.5 mg/kg, IP, four times daily) and ethanol did not prevent the generation of audiogenic seizures during withdrawal but completely prevented mortality. Finally, CCPA antagonized with similar potencies and efficacies the isoniazid-induced convulsions observed in control and ethanol-withdrawn rats. These results indicate that long-term treatment with intoxicating doses of ethanol enhances [3H]CCPA binding but does not reduce the anticonvulsant efficacy of CCPA or the function of A1 adenosine receptors.

Purinergic inhibition of neurotransmitter release in the central nervous system

Neurotransmitter release and the role of adenosine in its regulation has been investigated for more than twenty years, and it is now widely accepted that adenosine tonically inhibits the release of excitatory neurotransmitters. This effect of adenosine is operated by an A1 adenosine receptor. Since activation of this receptor could inhibit Ca2+ conductance, increase K+ conductance, inhibit adenylate cyclase or phospholipase C, it is not clear if there is only one mechanism or several mechanisms operated by adenosine to inhibit neurotransmitter release, and in that case, what is the relative importance of each mechanism. The mechanism by which adenosine inhibits evoked synchronous transmitter release might be different from that used by the nucleoside to inhibit spontaneous asynchronous release. In some systems adenosine triphosphate per se acts like adenosine and inhibits neurotransmitter release. However, in most cases the inhibitory effect of this adenine nucleotide depends upon its hydrolysis into adenosine by a cascade of ectoenzymes, the last step being mediated by ecto-5'-nucleotidase.

Functional coupling of dopamine D2 and muscarinic cholinergic receptors to their respective G proteins assessed by agonist-induced activation of high-affinity GTPase activity in rat striatal membranes

Agonist-induced high-affinity GTPase activity was investigated using the crude membrane preparation from rat striatum. High-affinity GTPase activity was stimulated by dopamine and carbachol in a Mg(2+)-dependent manner and with possible optimum NaCl concentrations of 50-100 mM to detect the percent increase induced by each agonist. Dopamine and selective (as well as non-selective) D2 receptor agonists, but not selective D1 receptor agonists, stimulated activity in a concentration-dependent manner, with affinities which were significantly correlated with those for adenylate cyclase inhibition as previously reported in the literature. Maximal percent stimulation above basal high-affinity GTPase activity was 9.8 +/- 0.6% and 4.4-7.6% for dopamine and other synthetic dopamine D2 receptor agonists, respectively. Dopamine-stimulated activity was inhibited by several dopamine receptor antagonists with the following rank order of potency: (+)-butaclamol > spiperone > raclopride > S(-)-sulpiride; but not by (-)-butaclamol or SCH 23390. High-affinity GTPase activity was also stimulated by carbachol and acetylcholine through the pirenzepine-insensitive muscarinic receptors. Preincubation of the membranes with AS/7, a specific antiserum to Gi1 and Gi2, appeared to attenuate dopamine-sensitive activity, suggesting that Gi1 and/or Gi2 may be at least partially involved. These results indicate that high-affinity GTPase activity in rat striatal membranes is activated through dopamine D2-like receptors and pirenzepine-insensitive muscarinic receptors, both of which are negatively coupled to adenylate cyclase via Gi proteins.

Adenosine A1 receptor-mediated changes in basal and histamine-stimulated levels of intracellular calcium in primary rat astrocytes

1. The effects of adenosine A1 receptor stimulation on basal and histamine-stimulated levels of intracellular free calcium ion concentration ([Ca2+]i) have been investigated in primary astrocyte cultures derived from neonatal rat forebrains. 2. Histamine (0.1 microM-1 mM) caused rapid, concentration-dependent increases in [Ca2+]i over basal levels in single type-2 astrocytes in the presence of extracellular calcium. A maximum mean increase of 1,468 +/- 94 nM over basal levels was recorded in 90% of type-2 cells treated with 1 mM histamine (n = 49). The percentage of type-2 cells exhibiting calcium increases in response to histamine appeared to vary in a concentration-dependent manner. However, the application of 1 mM histamine to type-1 astrocytes had less effect, eliciting a mean increase in [Ca2+]i of 805 +/- 197 nM over basal levels in only 30% of the cells observed (n = 24). 3. In the presence of extracellular calcium, the A1 receptor-selective agonist, N6-cyclopentyladenosine (CPA, 10 microM), caused a maximum mean increase in [Ca2+]i of 1,110 +/- 181 nM over basal levels in 30% of type-2 astrocytes observed (n = 53). The size of this response was concentration-dependent; however, the percentage of type-2 cells exhibiting calcium increases in response to CPA did not appear to vary in a concentration-dependent manner. A mean calcium increase of 605 +/- 89 nM over basal levels was also recorded in 23% of type-1 astrocytes treated with 10 microM CPA (n = 30). 4. In the absence of extracellular calcium, in medium containing 0.1 mM EGTA, a mean increase in [Ca2+]i of 504 +/- 67 nM over basal levels was recorded in 41% of type-2 astrocytes observed (n = 41) after stimulation with 1 microM CPA. However, in the presence of extracellular calcium, pretreatment with the A1 receptor-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine, for 5-10 min before stimulation with 1 microM CPA, completely antagonized the response in 100% of the cells observed. 5. In type-2 astrocytes, prestimulation with 10 nM CPA significantly increased the size of the calcium response produced by 0.1 microM histamine and the percentage of responding cells. Treatment with 0.1 microM histamine alone caused a mean calcium increase of 268 +/- 34 nM in 41% of the cells observed (n = 34). After treatment with 10 nM CPA, mean calcium increase of 543 +/- 97 nM was recorded in 100% of the cells observed (n = 33). 6. These data indicate that adenosine Al receptors couple to intracellular calcium mobilization and extracellular calcium influx in type-1 and type-2 astrocytes in primary culture. In addition, the simultaneous activation of adenosine Al receptors on type-2 astrocytes results in an augmentation of the calcium response to histamine H1 receptor stimulation.

Adenylyl cyclase activity in Alzheimer's disease brain: stimulatory and inhibitory signal transduction pathways are differently affected

Adenylyl cyclase (AC) activity was studied in post mortem hippocampus and cerebellum from eight patients with Alzheimer's disease/senile dementia of the Alzheimer type (AD/SDAT) and seven non-demented control patients. AC was stimulated via stimulatory guanine nucleotide binding proteins (Gs) using guanosine triphosphate (GTP) and GppNHp (both 10(-4) M) or directly with either forskolin (10(-4) M) or Mn2+ (10(-2) M). Inhibition of AC via A1-receptors was performed with N6-cyclohexyladenosine (CHA) under basal conditions and in the presence of forskolin (10(-5) M). In both brain regions AC activity was significantly reduced in AD/SDAT when compared to controls. Under basal conditions and after stimulation via Gs mean reduction in hippocampus and cerebellum was 47.7% and 58.2%, respectively. The reduction was less pronounced after direct activation of the AC, amounting to 21.8% in hippocampus and 28.1% in cerebellum. CHA inhibited basal and forskolin-stimulated AC concentration-dependently by about 20% (basal) and 30% (forskolin). Inhibition by CHA was similar in hippocampus and cerebellum and tended to be more pronounced in AD/SDAT than in controls. Since the reduction of AC activity in AD/SDAT is greater after stimulation via Gs than after direct activation of the catalytic subunit, we suggest that both Gs and the catalytic subunit seem to be impaired. The fact that CHA-mediated inhibition of AC is not significantly different in AD/SDAT and controls, indicates that in contrast to Gs-, inhibitory G-proteins (Gi) coupling to AC remains intact in Alzheimer's disease.

Synergistic regulation of cytosolic Ca2+ concentration by somatostatin and alpha 1-adrenergic agonists in mouse astrocytes

The effects of somatostatin and alpha 1-adrenergic receptor agonists on cytosolic Ca2+ in striatal astrocytes from the embryonic mouse in primary culture have been investigated by microfluorimetry. Methoxamine or somatostatin induced a transitory increase in cytosolic Ca2+, but their combined addition led to a sustained increase in cytosolic Ca2+ which seems to be due to a Ca2+ influx since it was not observed in the absence of external Ca2+. Voltage-independent Ca2+ channels contribute to this process. Indeed, voltage-operated calcium channels are not involved since neither dihydropyridines nor La3+ were effective in suppressing the sustained cytosolic Ca2+ elevation. Moreover, depolarization by 50 mM KCl, which was ineffective alone, suppressed the effect of somatostatin observed in the presence of the alpha 1 agonist, methoxamine. The implication of arachidonic acid in the observed potentiation is suggested by the following observations: 1) arachidonic acid induced a sustained elevation of cytosolic Ca2+ similar to that evoked by the co-application of methoxamine and somatostatin; 2) the addition of ETYA, an inactive and non-metabolizable analogue of arachidonic acid suppressed the calcium plateau produced by the agonists. In addition, direct activation of PKC by an exogeneous diacylglycerol analogue allowed somatostatin alone to evoke a sustained elevation of cytosolic Ca2+. Therefore, methoxamine through the successive activation of PLC and PKC could allow a lipase, probably PLA2, to be stimulated by somatostatin. Since arachidonic acid has already been shown to trigger the opening of K+ channels and the formation of inositol phosphates, somatostatin, through the arachidonic acid-mediated hyperpolarization could increase the Ca2+ driving force and thus improve Ca2+ influx through the inositol phosphate gated channels.

Adenosine A2B-receptor-mediated cyclic AMP accumulation in primary rat astrocytes

1. The effects of adenosine receptor agonists and antagonists on the accumulation of cyclic AMP have been investigated in primary cultures of rat astrocytes. 2. Adenosine A2-receptor stimulation caused a concentration-dependent increase in the accumulation of [3H]-cyclic AMP in cells prelabelled with [3H]-adenine. The rank order of agonist potencies was 5'-N-ethylcarboxamidoadenosine (NECA; EC50 = 1 microM) > adenosine (EC50 = 5 microM) > 2-chloroadenosine (EC50 = 20 microM) >> CGS 21680 (EC50 > 10 microM). The presence of 0.5 microM dipyridamole, an adenosine uptake blocker, had no effect on the potency of adenosine. 3. The response to 10 microM NECA was antagonized in a concentration-dependent manner by the non-selective adenosine receptor antagonists, xanthine amine congener (apparent KD = 12 nM), PD 115,199 (apparent KD = 134 nM) and 8-phenyltheophylline (apparent KD = 126 nM). However, the A1-receptor-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine, had no significant effect on the responses to NECA or 2-chloroadenosine at concentrations up to 1 microM. 4. Stimulation of A1-receptors with the selective agonist, N6-cyclopentyladenosine, did not alter the basal accumulation of [3H]-cyclic AMP but inhibited a forskolin-mediated elevation of [3H]-cyclic AMP accumulation by a maximal value of 42%. This inhibition was fully reversed in the presence of 0.1 microM, 8-cyclopentyl-1,3-dipropylxanthine. 5. The time course for NECA-mediated [3H]-cyclic AMP accumulation was investigated. The results suggest that there is a substantial efflux of cyclic AMP from the cells in addition to the rapid and sustained elevation of intracellular cyclic AMP (5 fold over basal) which was also observed. 6. These data indicate that rat astrocytes in primary culture express an A2B-adenosine receptor coupled positively to adenylyl cyclase. Furthermore, the presence of A1-receptors negatively coupled to adenylyl cyclase appears to have no significant effect on the A2B-receptor-mediated cyclic AMP responses to NECA and 2-chloroadenosine.

A comparison of A2 adenosine receptor-induced cyclic AMP generation in cerebral cortex and relaxation of pre-contracted aorta

1. A comparative study was carried out between the adenosine receptor mediating a stimulation of cyclic AMP formation in guinea-pig cerebral cortical slices with the adenosine receptor mediating relaxation of phenylephrine precontracted guinea-pig aortic rings. 2. [3H]-cyclic AMP accumulation in [3H]-adenine-prelabelled guinea-pig cerebral cortical slices was stimulated by adenosine and its analogues with the following EC50 values (microM): 5'-N-ethylcarboxamidoadenosine (3.1 +/- 0.3) > 2-chloroadenosine (10 +/- 2) > adenosine (109 +/- 15). 3. 2-Chloroadenosine and adenosine elicited maximal responses for [3H]-cyclic AMP accumulation that were 100 +/- 7 and 71 +/- 6% of the maximal response to 5'-N-ethylcarboxamidoadenosine, respectively. CGS 21680 (100 microM) and DPMA (100 microM) elicited -2 +/- 2 and 12 +/- 3% of the response to 100 microM 5'-N-ethylcarboxamidoadenosine. 4. Estimation of antagonist potencies at the A2 adenosine receptor of cerebral cortex showed a rank order of potency (K1, nM): xanthine amino congener (35 +/- 3) > 8-cyclopentyl-1,3-dipropylxanthine (130 +/- 22) > PD 115,199 (407 +/- 82) > 3,7-dimethyl-1-propargylxanthine (13 +/- 2 microM). 5. Adenosine analogues produced long-lasting relaxation of phenylephrine-precontracted aortic rings with the following rank order of potency (EC50 values, microM): 5'-N-ethylcarboxamidoadenosine (0.68 +/- 0.06) > 2-chloroadenosine (4.3 +/- 0.6) > adenosine (104 +/- 13). Maximal relaxations elicited by these agents were 71 +/- 3, 98 +/- 1, and 100 +/- 1%, respectively. CGS 21680 and DPMA at 100 microM elicited smaller relaxations of the precontracted tissues (12 +/- 2 and 43 +/- 15%, respectively). 6. Antagonism by xanthine derivatives of the 5'-N-ethylcarboxamidoadenosine-induced relaxation of aortic rings showed the following rank order of potency (Ki, nM): xanthine amino congener (17 +/- 4) > 8-cyclopentyl-1,3-dipropylxanthine (171 +/- 36) > PD 115,199 (341 +/- 64) > 3,7-dimethyl-1-propargylxanthine (5520 +/- 820). 7. We conclude that the A2 adenosine receptor mediating relaxation of phenylephrine-contracted aortic rings is an A2b adenosine receptor which exhibits certain minor differences from the A2b receptor which stimulates cyclic AMP accumulation in cerebral cortical slices.

Status epilepticus may be caused by loss of adenosine anticonvulsant mechanisms

The inhibitory neuromodulator adenosine is an endogenous anticonvulsant that terminates brief seizures in the brain and it has been proposed that loss of adenosine or adenosine-mediating systems may play a major role in the development of status epilepticus, a seizure condition characterized by prolonged and/or recurrent seizures that last by definition, at least 20 min. In this study, the effect of specific A1-adenosine agonists and antagonists were tested for their ability to prevent and cause status epilepticus in two electrical stimulation models in rats. In a recurrent electrical stimulation model, whereas no vehicle-treated animals developed status epilepticus after 20 recurrent electrical stimulations, rats injected with 10 mg/kg of the specific A1-adenosine antagonist 8-cyclopentyl-1,3-dimethylxanthine intraperitoneally developed status epilepticus after stimulation. 8-(p-Sulphophenyl)-theophylline, which has limited penetrability into the brain when administered peripherally, did not cause status epilepticus when injected intraperitoneally. However, when 200 micrograms of 8-(p-sulphophenyl)-theophylline were administered intracerebroventricularly, status epilepticus developed in all animals, suggesting status epilepticus developed as a result of central adenosine receptor antagonism. In the second study, whereas all vehicle-treated animals developed status epilepticus after constant electrical stimulation, administration of N6-cyclohexyladenosine and N6-cyclopentyladenosine prior to stimulation suppressed the development of status epilepticus. N6-Cyclohexyladenosine was also effective in terminating status epilepticus after it had progressed for 20 min. The effects of a selective A2-agonist was also tested on both stimulation models and had no anticonvulsant effects. An electrical stimulus given to rats pretreated three days prior to stimulation with pertussis toxin, a compound which inactivates Gi-proteins, also resulted in generalized status epilepticus, suggesting that impairment of G-protein-linked receptors is involved in the development of status epilepticus. The effects of a GABAB antagonist, phaclofen, and a GABAB agonist, baclofen, were also tested in the recurrent stimulation model, as GABAB receptors are also coupled to the same subset of K+ channels as the A1-receptor. Rats given phaclofen did not develop status epilepticus after recurrent electrical stimulation, although baclofen was effective at preventing the induction of status epilepticus in the constant stimulation model. These results, together with some preliminary data obtained showing that the GABAA antagonist picrotoxin did not cause status epilepticus after recurrent stimulation, suggest that loss of GABAergic inhibition only has a minor role in status epilepticus development in our models. Brains from all animals were also assessed for brain injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine, a "retaliatory" metabolite, promotes anoxia tolerance in turtle brain

Contrary to what is found in most vertebrates, the brains of certain turtle species maintain ATP levels and ion homeostasis and survive prolonged anoxia. The hypothesis tested here is that the release of adenosine and its binding to A1 receptors are essential for this anoxic tolerance. Studies were conducted in the isolated turtle cerebellum, which did release adenosine to the extracellular space during anoxia. When adenosine receptor antagonists [theophylline, 8-cyclopentyltheophylline (CPT), or 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)] were added to the superfusate under control conditions, they had no effect on extracellular potassium ion activity ([K+]o). During anoxia, however, these antagonists provoked maximal efflux of K+ (anoxic depolarization). Anoxic depolarization occurred earlier during anoxia with theophylline (a nonspecific adenosine receptor antagonist) than with CPT or DPCPX, which specifically block A1 receptors. Therefore, adenosine release and effects mediated by A1 receptors are essential to anoxia tolerance in turtle brain.

Attenuation of potassium cyanide-mediated neuronal cell death by adenosine

Glutamate has been shown to play an important role in delayed neuronal cell death occurring due to ischemia. Attenuation of synaptically released glutamate can be accomplished by modulators such as adenosine and baclofen. This study focused on the ability of adenosine to attenuate the excitotoxicity secondary to glutamate receptor activation in vitro after exposure to potassium cyanide (KCN) in hippocampal neuronal cell cultures. For this study, hippocampal cell cultures were obtained from 1-day-old rats and trypan blue staining was used for assessment of cell viability. It was found that the N-methyl-D-aspartate-specific antagonist MK801 (10 microM) attenuated neuronal cell death resulting from exposure to 1 mM KCN for 60 minutes. Adenosine (10 to 1000 microM) decreased neuronal cell death secondary to the same concentration of KCN in a dose-dependent manner. This same neuroprotective effect is mimicked by the adenosine A1-specific receptor agonist N6-cyclopentyladenosine (10 microM). The A1-specific receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (10 to 1000 nM) blocked the neuroprotective effect of adenosine in a dose-dependent manner. Therefore, neuronal cell death produced by KCN in the experimental model described was mediated at least in part by glutamate. This neuronal cell death was attenuated by adenosine via the A1-specific mechanism.

Reduced density of adenosine A1 receptors and preserved coupling of adenosine A1 receptors to G proteins in Alzheimer hippocampus: a quantitative autoradiographic study

Binding to adenosine A1 receptors and the status of their coupling to G proteins were studied in the hippocampus and parahippocampal gyrus of Alzheimer individuals and age-matched controls. The binding to A1 receptors was compared with binding to the N-methyl-D-aspartate receptor complex channel-associated sites (labeled with (+)-[3H]5-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine maleate). In vitro quantitative autoradiography demonstrated a similar anatomical distribution of A1 receptors labeled either with an agonist ((-)-[3H]phenylisopropyladenosine) or antagonist ([3H]8-cyclopentyl-1,3-dipropylxanthine) in the brains of elderly controls. In Alzheimer patients, significant decreases in the density of both agonist and antagonist binding sites were found in the molecular layer of the dentate gyrus. Decreased A1 agonist binding was also observed in the CA1 stratum oriens and outer layers of the parahippocampal gyrus, while reduced antagonist binding was found in the subiculum and CA3 region. Reduced density of the N-methyl-D-aspartate receptor channel sites was found in the CA1 region and parahippocampal gyrus. The reductions in binding to adenosine A1 and N-methyl-D-aspartate receptors were due to a decrease in the density of binding sites (Bmax), and not changes in receptor affinity (KD). In both elderly control and Alzheimer subjects, GTP substantially reduced the density of A1 agonist binding sites with a concomitant increase in the KD values, whereas antagonist binding was unaffected by GTP. The results suggest that adenosine A1 receptor agonists and antagonists recognize overlapping populations of binding sites. Reduced density of A1 receptors in the molecular layer of the dentate gyrus most probably reflects damage of the perforant path input in Alzheimer's disease, while altered binding in the CA1 and CA3 regions is probably due to loss of intrinsic neurons. Similar effects of GTP on binding to A1 receptors in control and Alzheimer individuals suggest lack of alterations in coupling of A1 receptors to G proteins in Alzheimer's disease, thus supporting the notion of normal receptor coupling to their effector systems in Alzheimer's disease.

A monoclonal anti-idiotypic 'internal image' antibody that recognizes the A1 adenosine receptor potentiates the alpha 1-adrenergic activation of phospholipase C in primary cultures of mouse striatal astrocytes

To determine which subtype of adenosine receptor mediates the potentiating effect of 2-chloroadenosine on the noradrenaline-induced inositol-phosphate formation, we used the monoclonal anti-idiotypic antibody AA1 that acts as an 'internal image' of adenosine and specifically recognizes the A1 adenosine receptor. In cultured mouse striatal astrocytes, AA1 increased the noradrenaline-evoked inositol phosphate (IP) accumulation, thus demonstrating a biological activity of an anti-idiotypic antibody. This effect was inhibited by PACPX, a selective A1 antagonist. Inhibitors of phospholipase A2 activity prevented the potentiation. These results establish the involvement of A1 adenosine receptors in the modulation of phospholipase C activity.

P2 purinoceptor-mediated cyclic AMP accumulation in bovine vascular smooth muscle cells

Extracellular ATP has been shown to induce intracellular Ca2+ mobilization and adenylate cyclase inhibition via P2 purinoceptors in several species of cells. Now we found that in calf vascular smooth muscle cells the addition of ATP to the medium did not induce inhibition but stimulation of cyclic AMP accumulation, in addition to stimulation of inositol phosphate production. Adenosine and AMP also induced cyclic AMP accumulation but their efficacy was much less than that of ATP. The ATP action was not influenced by the presence of either adenosine deaminase or of an ATP regenerating system, whereas the AMP action was increased by the regenerating system. The results indicate that the cyclic AMP accumulation by ATP is due to ATP itself but neither to adenosine nor to AMP, both of which are produced from ATP. ATP receptor coupled to the cyclic AMP generation was shown to be different from that coupled to phospholipase C based on the difference in the potency order of the receptor agonists and in the sensitivity of P2 receptor agonists to 8-cyclopentyl-1,3-dipropylxanthine (CPX)- and suramin-induced antagonism. We conclude that in the aortic smooth muscle cells a novel P2-type receptor directly coupled to adenylate cyclase activation exists in addition to the previously known P2 receptor linked to phospholipase C activation.

Inhibition of quantal transmitter release in the absence of calcium influx by a G protein-linked adenosine receptor at hippocampal synapses

Spontaneous miniature excitatory postsynaptic currents (MEPSCs) were recorded by whole-cell voltage-clamp techniques in cultured rat hippocampal pyramidal neurons. The specific adenosine A1 receptor agonist cyclopentyladenosine (CPA) reduced the frequency of MEPSCs without affecting their amplitude distribution or kinetic properties. This action was blocked by pretreatment of the cells with pertussis toxin. In the presence of divalent cation Ca2+ channel blockers, CPA was still effective in reducing the frequency of MEPSCs. It was shown that this effect cannot be explained by changes in basal Ca2+ influx. These results suggest that neurotransmitters that produce presynaptic inhibition at hippocampal synapses utilize several mechanisms, one of which may involve inhibition of some component of the quantal release apparatus that occurs independently of inhibition of Ca2+ influx.

Synergistic regulation of cytosolic Ca2+ concentration in mouse astrocytes by NK1 tachykinin and adenosine agonists

The effects on cytosolic Ca2+ concentration of 2-chloroadenosine and [L-Pro9]-substance P, a selective agonist of NK1 receptors, were investigated on astrocytes from embryonic mice in primary culture. Cells responded to [L-Pro9]-substance P with a transitory increase in cytosolic Ca2+ which was of shorter duration when external Ca2+ was removed. A transient response to 2-chloroadenosine alone occurred. When simultaneously applied, [L-Pro9]-substance P and 2-chloroadenosine evoked a prolonged elevation of cytosolic Ca2+ (up to 30 min). This phenomenon was dependent on the presence of extracellular Ca2+, but insensitive to dihydropyridines, La3+, and Co2+, excluding the implication of voltage-operated Ca2+ channels. Arachidonic acid also induced a sustained elevation of cytosolic Ca2+, but did not increase further the response evoked by [L-Pro9]-substance P and 2-chloroadenosine. The activation of protein kinase C by a diacylglycerol analogue mimicked the effect of [L-Pro9]-substance P in potentiating the 2-chloroadenosine-evoked response. Like 2-chloroadenosine, pinacidil, which hyperpolarizes the cells by opening K+ channels, prolonged the elevation of cytosolic Ca2+ concentration induced by [L-Pro9]-substance P. Conversely, depolarization with 50 mM KCl canceled the effects of either pinacidil or 2-chloroadenosine applied with [L-Pro9]-substance P. Pertussis toxin pretreatment suppressed all the effects induced by 2-chloroadenosine.

Adenosine-dependent regulation of cyclic AMP accumulation in primary cultures of rat astrocytes and neurons

The regulation of intracellular cyclic AMP (cAMP) formation by adenosine (Ado) and its analogues has been examined in primary cultures of rat-brain astrocytes and neurons. In the presence of the phosphodiesterase inhibitor, Ro 20-1724, basal levels of cAMP ranged from 40-120 pmol/mg protein in both cell types. Levels were not altered by treating the cells with Ado deaminase, which suggested that they did not produce appreciable amounts of endogenous Ado under standard culture conditions. In the astrocytes, microM quantities of agonists increased cAMP up to 30-fold higher than basal values; the relative potencies were typical of an A2 Ado receptor (NECA greater than Ado greater than R-PIA). Neuron-enriched cultures exhibited a maximum fourfold increase in cAMP in response to NECA; this was decreased a further eightfold when the cultures had prolonged exposure to the antimitotic agent, c-Ara, to eliminate greater than 98% of the nonneuronal cells. Low (nM) amounts of the Ado agonists inhibited cAMP formation in both cell types. In the astrocytes, the order of potency of inhibition of isoproterenol-stimulated cAMP formation was typical of an A1 receptor (R-PIA greater than Ado greater than NECA); maximum inhibition was 55-65%. Isoproterenol did not increase cAMP in the neuronal cultures. However, forskolin-stimulated formation was effectively (approximately 50%) inhibited by A1 Ado agonists; inhibition was not affected by prolonged treatment with c-Ara. From this study we tentatively concluded that rat astrocytes and neurons both contain inhibitory A1 Ado receptors, but that the stimulatory "A2" subtype is localized mainly on astrocytes.

Somatostatin potentiates the alpha 1-adrenergic activation of phospholipase C in striatal astrocytes through a mechanism involving arachidonic acid and glutamate

As previously shown with adenosine, somatostatin, which is ineffective alone, enhanced the alpha 1-adrenergic-agonist-stimulated production of inositol phosphates in cultured striatal astrocytes. This effect was suppressed in cells pretreated with pertussis toxin. It required external calcium and was selectively antagonized by both mepacrine, an inhibitor of phospholipase A2, and 5,8,11,14-eicosatetraynoic acid, a nonmetabolizable analog of arachidonic acid. In addition, a long-lasting elevation of cytosolic calcium and a release of arachidonic acid were observed only under the combined stimulation of somatostatin and alpha 1-adrenergic receptors. Arachidonic acid could in turn inhibit glutamate uptake into astrocytes, and the resulting external accumulation of glutamate could account for the somatostatin-evoked amplification of the alpha 1-adrenergic-agonist-stimulated hydrolysis of inositol-phospholipids. The effect of somatostatin was indeed reproduced by glutamate or glutamate uptake inhibitors and suppressed by enzymatic removal of external glutamate. Thus, astrocytes may contribute to long-term plasticity events in glutamatergic synapses through regulation of external glutamate levels.

Synergistic Regulation of Cytosolic Ca2+ Concentration by Adenosine and alpha1-Adrenergic Agonists in Mouse Striatal Astrocytes

Adenosine has a broad array of actions on neurons but astrocytes also possess adenosine receptors. We have previously shown that adenosine, by acting on astrocytes in the striatum, can modulate neuronal responses mediated by receptors coupled to phospholipase C through an astrocyto - neuronal interaction. In addition, adenosine was found to potentiate the alpha1-adrenergic production of inositol phosphates in astrocytes. The mechanism involved in this potentiation was further investigated by examining the effects of adenosine and alpha1-adrenergic receptor agonists on cytosolic Ca2+ in cultured striatal astrocytes from the embryonic mouse in primary culture. When used alone, methoxamine, a selective agonist of alpha-adrenergic receptors or 2-chloroadenosine, a stable analogue of adenosine, induced a transitory increase in cytosolic Ca2+, but their combined addition led to a sustained increase in cytosolic Ca2+, which seems to be due to a Ca2+ influx, because it was not observed in the absence of external Ca2+. Voltage independent Ca2+ channels contribute to this process and different blockers of voltage-operated calcium channels, such as dihydropyridines, phenylalkylamines, La3+ or Co2+ were ineffective in suppressing the sustained cytosolic Ca2+ elevation. Three observations suggest the implication of arachidonic acid in the observed potentiation: (i) arachidonic acid induced a sustained elevation of cytosolic Ca2+ similar to that evoked by the coapplication of methoxamine and 2-chloroadenosine; (ii) the addition of arachidonic acid during the calcic plateau produced by the combined application of the agonists did not increase further cytosolic Ca2+ levels; (iii) in the presence of methoxamine, 2-chloroadenosine induced a release of arachidonic acid. The stimulation of phospholipase C and the resulting activation of protein kinase C induced by methoxamine seem to be required for the potentiating effect of 2-chloroadenosine on cytosolic Ca2+. In fact, the direct activation of protein kinase C by an exogenous diacylglycerol analogue mimicked the effect of methoxamine because, in this condition, 2-chloroadenosine alone evoked a sustained elevation of cytosolic Ca2+. Therefore, methoxamine, through the successive activation of phospholipase C and protein kinase C, could allow a lipase, probably phospholipase A2, to be stimulated by 2-chloroadenosine. Arachidonic acid has already been shown to trigger the opening of K+ channels and the formation of inositol phosphates in other cell types. Therefore, in striatal astrocytes, 2-chloroadenosine, through an arachidonic acid-mediated hyperpolarization, could increase the Ca2+ driving force and thus improve Ca2+ influx through inositol phosphate-gated channels. This hypothesis is further supported by the suppressing effect of a 50 mM KCI-induced depolarization on the long lasting elevation of cytosolic Ca2+ seen in the combined presence of 2-chloroadenosine and methoxamine.

Binding of the adenosine A2 receptor ligand [3H]CGS 21680 to human and rat brain: evidence for multiple affinity sites

A new radiolabeled adenosine receptor agonist, 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadeno sin e (CGS 21680), apparently specific for high-affinity binding sites of the A2 subtype in rat brain, was used to identify and pharmacologically characterize adenosine receptors in human brain. The binding of [3H]CGS 21680, as determined by standard radioligand binding technique in the presence of exogenously added adenosine deaminase, reached equilibrium after 40 min at 25 degrees C. In saturation studies, a single class of high-affinity binding sites with values for KD of 22 +/- 0.5 nM and Bmax of 444 +/- 63 fmol/mg of protein were observed. Similar binding characteristics were observed regardless of whether rapid filtration or centrifugation was used to separate bound versus free ligand. Of the 14 brain regions examined, [3H]CGS 21680 binding was highest in putamen, followed by globus pallidus and caudate nucleus. The level of [3H]CGS 21680 binding in these areas of basal ganglia was identical to 5'-N-[3H]ethylcarboxamidoadenosine ([3H]NECA) binding in the presence of 50 nM N6-cyclopentyladenosine (CPA). The rank order of agonist potencies as determined by a series of competition experiments was NECA greater than or equal to CGS 21680 greater than 2-chloroadenosine greater than N6-(R)-phenylisopropyladenosine greater than N6-cyclohexyladenosine greater than N6-(S)-phenylisopropyladenosine. This potency order was the same for the binding of [3H]CGS 21680 to rat, and of [3H]NECA in the presence of 50 nM CPA to rat and human, brain membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Both A1 and A2a purine receptors regulate striatal acetylcholine release

The receptors responsible for the adenosine-mediated control of acetylcholine release from immunoaffinity-purified rat striatal cholinergic nerve terminals have been characterized. The relative affinities of three analogues for the inhibitory receptor were (R)-phenylisopropyladenosine greater than cyclohexyladenosine greater than N-ethylcarboxamidoadenosine (NECA), with binding being dependent of the presence of Mg2+ and inhibited by 5'-guanylylimidodiphosphate [Gpp(NH)p] and adenosine receptor antagonists. Adenosine A1 receptor agonists inhibited forskolin-stimulated cholinergic adenylate cyclase activity, with an IC50 of 0.5 nM for (R)-phenylisopropyladenosine and 500 nM for (S)-phenylisopropyladenosine. A1 agonists inhibited acetylcholine release at concentrations approximately 10% of those required to inhibit the cholinergic adenylate cyclase. High concentrations (1 microM) of adenosine A1 agonists were less effective in inhibiting both adenylate cyclase and acetylcholine release, due to the presence of a lower affinity stimulatory A2 receptor. Blockade of the A1 receptor with 8-cyclopentyl-1,3-dipropylxanthine revealed a half-maximal stimulation by NECA of the adenylate cyclase at 10 nM, and of acetylcholine release at approximately 100 nM. NECA-stimulated adenylate cyclase activity copurified with choline acetyltransferase in the preparation of the cholinergic nerve terminals, suggesting that the striatal A2 receptor is localized to cholinergic neurones. The possible role of feedback inhibitory and stimulatory receptors on cholinergic nerve terminals is discussed.

Adenosine depresses spontaneous transmitter release from frog motor nerve terminals by acting at an A1-like receptor

Adenosine (1 microM to 1 mM) depressed spontaneous transmitter release from frog motor nerve terminals without producing any observable postsynaptic effects. Since this action of adenosine was blocked by 20 microM theophylline and 1 microM 8-phenyltheophylline, adenosine probably acts at a specific receptor on motor nerve terminals to reduce spontaneous transmitter output. The effects of the adenosine analogs, L-N6-phenylisopropyladenosine (L-PIA, 100 pM to 1 microM), D-PIA (100 nM to 100 microM), and 5'-N-ethylcarboxamidoadenosine (NECA, 10nM to 100 microM), were tested on spontaneous transmitter release at the frog neuromuscular junction. L-PIA depressed mepp frequency at a threshold concentration of about 1 nM, was thirteen times more potent than NECA, and was 294 times more effective than D-PIA. The rank-order potency of these analogs indicates that adenosine acts at an A1-like receptor to depress spontaneous transmitter release. Inhibitory actions of maximally effective concentrations of adenosine and L-PIA were also blocked by the A1-specific antagonist, 1-3-dipropyl-8-cyclopentylxanthine (DPCPX) at a concentration of 100 nM. Micromolar concentrations of NECA, an agonist with approximately equal affinity for the A1 and A2 receptors, produced biphasic effects on mepp frequency. Thus, a second adenosine receptor, perhaps of the A2 subtype, may be present on motor nerve terminals and may mediate an increase in spontaneous transmitter release.

Direct cutaneous hyperalgesia induced by adenosine

The intradermal injection of adenosine produces a dose-dependent decrease in mechanical nociceptive threshold in the hindpaw of the rat that is not attenuated by elimination of indirect pathways for the production of hyperalgesia. Adenosine-induced hyperalgesia is mimicked by the A2-agonists, 5'-(N-ethyl)-carboxamido-adenosine and 2-phenylaminoadenosine but not by the A1-agonist, N6-cyclopentyladenosine and antagonized by the adenosine A2-receptor antagonist, PD 081360-0002 but not by the A1-antagonist, 1,3-dipropyl-8-(2-amino-4-chlorophenyl)xanthine. The latency to onset of adenosine and 2-phenylaminoadenosine hyperalgesia is similar to that produced by prostaglandin E2, a directly acting hyperalgesic agent but shorter than that produced by leukotriene B4, which acts indirectly. 2-Phenylaminoadenosine hyperalgesia is prolonged by rolipram, a phosphodiesterase inhibitor. Both 2-phenylaminoadenosine and prostaglandin E2 hyperalgesia are antagonized by the A1-agonist N6-cyclopentyladenosine and the mu-agonist, [D-Ala2, NMe-Phe4, Gly-ol]enkephalin. However, 1-acetyl-2-(8-chloro-10,11-dihydrodibenz[b,f]oxazepine-10-ca rbonyl) hydrazine, a prostaglandin-receptor antagonist, inhibits prostaglandin E2 (Taiwo and Levine, Brain Res. 458, 402-406, 1988) but not 2-phenylamino-adenosine hyperalgesia and PD 081360-0002, the adenosine receptor antagonist, inhibits 2-phenylamino-adenosine but not prostaglandin E2 hyperalgesia. These data suggest that adenosine is a directly acting agent that produces hyperalgesia by an action at the A2-receptor and that this hyperalgesia is mediated by the cAMP second messenger.

Activation of adenosine A1 receptor by N6-(R)-phenylisopropyladenosine (R-PIA) inhibits forskolin-stimulated tyrosine hydroxylase activity in rat striatal synaptosomes

We investigated the effect of the relatively selective A1 adenosine receptor agonist N6-(R)-phenylisopropyladenosine (R-PIA) on tyrosine hydroxylase activity (TH) of synaptosomes obtained from rat striatum. TH activity was assayed in supernatant obtained following sonication and centrifugation of the tissue preincubated with the test compounds. R-PIA produced a modest decrease of basal enzyme activity, but significantly reduced the activation of the enzyme by submaximal (0.1-0.5 microM) concentrations of forskolin (FSK) a stimulator of adenylate cyclase. The IC 50 value of R-PIA was 17 nM and the maximal inhibition corresponded to 30-40% decrease of the enzyme activity stimulated by FSK. The S-isomer of PIA failed to affect TH activity under control and stimulated conditions. Moreover, the inhibitory effect of R-PIA was completely antagonized by 8-cyclopentyl- 1,3 -dimethylxanthine, an adenosine receptor blocker. R-PIA inhibited both basal and FSK-stimulated adenylate cyclase activity. These results indicate that in striatal dopaminergic terminals TH activity can be modulated in an inhibitory manner by activation of presynaptic A1 adenosine receptors.

Inhibition of adenylate cyclase in rat brain synaptosomal membranes by GTP and phenylisopropyladenosine is enhanced in hypothyroidism

1. Synaptosomal membranes were isolated from rats made hypothyroid by treatment with propylthiouracil and a low iodine diet. 2. When assayed in the presence of 100 mM-Na+, inhibition of forskolin-stimulated adenylate cyclase by GTP was enhanced in membranes from hypothyroid animals. 3. Hypothyroidism also enhanced inhibition of adenylate cyclase by phenylisopropyladenosine (with 100 mM-Na+ and 10 microM-GTP present). 4. Hypothyroidism did not increase binding of the A1 adenosine receptor agonist phenylisopropyladenosine to synaptosomal membranes; rather, the maximum binding was slightly decreased without any change in the KD. 5. The effect of GTP in modifying the displacement of the antagonist [3H]diethylphenylxanthine from synaptosomal membranes by unlabelled phenylisopropyladenosine was more pronounced in the hypothyroid state. 6. These findings are consistent with hypothyroidism causing modification of the brain adenylate cyclase system at the level of the coupling protein Gi.

Regulation of cyclic AMP formation in cultures of human foetal astrocytes by beta 2-adrenergic and adenosine receptors

Two cell cultures, NEP2 and NEM2, isolated from human foetal brain have been maintained through several passages and found to express some properties of astrocytes. Both cell cultures contain adenylate cyclase stimulated by catecholamines with a potency order of isoprenaline greater than adrenaline greater than salbutamol much greater than noradrenaline, which is consistent with the presence of beta 2-adrenergic receptors. This study reports that the beta 2-adrenergic-selective antagonist ICI 118,551 is approximately 1,000 times more potent at inhibiting isoprenaline stimulation of cyclic AMP (cAMP) formation in both NEP2 and NEM2 than the beta 1-adrenergic-selective antagonist practolol. This observation confirms the presence of beta 2-adrenergic receptors in these cell cultures. The formation of cAMP in NEP2 is also stimulated by 5'-(N-ethylcarboxamido)adenosine (NECA) more potently than by either adenosine or N6-(L-phenylisopropyl)adenosine (L-PIA), which suggests that this foetal astrocyte expresses adenosine A2 receptors. Furthermore, L-PIA and NECA inhibit isoprenaline stimulation of cAMP formation, a result suggesting the presence of adenosine A1 receptors on NEP2. The presence of A1 receptors is confirmed by the observation that the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine reverses the inhibition of isoprenaline stimulation of cAMP formation by L-PIA and NECA. Additional evidence that NEP2 expresses adenosine receptors linked to the adenylate cyclase-inhibitory GTP-binding protein is provided by the finding that pretreatment of these cells with pertussis toxin reverses the adenosine inhibition of cAMP formation stimulated by either isoprenaline or forskolin.

The cellular localization of adenosine receptors in rat neostriatum

Using quantitative autoradiography of ligand binding sites combined with lesions of specific neuronal pathways, the cellular locations of A1 and A2 adenosine receptors, as well as a third binding site for the adenosine receptor ligand, [3H]N-ethylcarboxamidoadenosine, and a nucleoside transporter were investigated in rat neostriatum. Intrastriatal kainic acid administration resulted in the loss of 50% of A1 adenosine receptors and virtually abolished ligand binding to A2 receptors. A small reduction in [3H]cyclohexyladenosine binding to striatal A1 receptors was found after lesioning the corticostriatal input. A2 receptor sites were unaffected by this treatment. Destruction of dopaminergic neurons using 6-hydroxydopamine or the raphestriatal serotoninergic input using 5,7-dihydroxytryptamine affected neither A1 nor A2 binding sites. These results indicate the localization of both A1 and A2 adenosine receptors on neurons intrinsic to the neostriatum and probably postsynaptic to the dopaminergic input. In addition, a binding site for [3H]N-ethylcarboxamidoadenosine which is not affected by the adenosine receptor agonist, R-phenylisopropyladenosine, was also partly abolished after kainic acid injection. In contrast, no significant change in the binding of the nucleoside transporter ligand, [3H]nitrobenzylthioinosine, was observed after any lesions, indicating the widespread association of this site with various cell types.

Comparative effects of caffeine and L-phenylisopropyladenosine on local cerebral glucose utilization in the rat

The quantitative [14C]2-deoxyglucose autoradiographic method was used to study the effects of the acute administration of 10 mg/kg of caffeine and 0.1 mg/kg of L-phenylisopropyladenosine (LPIA) given separately or in combination. After an injection of caffeine, the local cerebral metabolic rates for glucose (LCMRglu) were increased in 34 out of 61 structures studied, mainly in monoaminergic cell groupings, the thalamus and structures belonging to the extrapyramidal motor system. The administration of LPIA decreased the LCMRglu in 30 out of 61 structures studied. These areas were essentially monoaminergic cell groupings, structures of the extrapyramidal motor system and all hypothalamic areas. After a combined injection of caffeine and LPIA, the LCMRglu was either increased or not changed in structures affected by caffeine or LPIA given alone. The results of the present study indicate that, even if caffeine and LPIA compete for the occupation of adenosine receptors, the effects of these two substances on the energy metabolism of the brain are partly mediated by peripheral mechanisms.

Autoradiographic studies on the uptake of adenosine and on binding of adenosine analogues in neurons and astrocytes of cultured rat cerebellum and spinal cord

The cellular localization of the uptake of [3H]adenosine and of binding of labelled adenosine analogues was studied in explant cultures of rat cerebellum and spinal cord by means of autoradiography. [3H]Adenosine was taken up by many neurons and astrocytes in both cerebellar and spinal cord cultures. The uptake of adenosine was inhibited in the absence of sodium or at 0 degrees C, suggesting an active transport mechanism. In both types of cultures, a great number of neurons showed binding sites for the A1-receptor agonist [3H]R-N6-phenylisopropyladenosine and for the mixed A1/A2-agonist [3H]N(ethyl)carboxamidoadenosine. Binding sites for both radioligands were also found on astrocytes, suggesting that these cells have receptors for the purinergic neurotransmitter adenosine. This suggestion is further supported by recent electrophysiological studies from our laboratory demonstrating that adenosine and its analogues produce hyperpolarizations of astrocytes which are blocked by the adenosine antagonist theophylline.

Purinergic receptors in the brainstem mediate hypotension and bradycardia

Adenosine acts at many sites to modulate neuronal activity. The purpose of this study was to investigate a possible role for adenosine as a neuromodulator of brainstem cardiovascular control. Microinjections of adenosine (0-2.3 nmol) were made stereotaxically into various brainstem sites. Injection of adenosine into the nucleus tractus solitarii (NTS) produced dose-related decreases in heart rate and systolic and diastolic blood pressures. Maximal changes occurred 90 seconds after injection. Injection into the area postrema also produced decreased heart rate and systolic and diastolic blood pressures. No significant effect occurred following injection into the C1 area. Adenosine 5'-triphosphate and its analogue, beta, gamma-methylene adenosine 5'-triphosphate also produced dose-related and potent vasodepressor and bradycardia effects in the NTS. Injection of 1,3-dipropyl-8-p-sulfophenylxanthine (0.92 nmol), a potent adenosine receptor antagonist, produced no effect itself, but abolished for 45 minutes the actions of further injections of adenosine and adenosine 5'-triphosphate (but not L-glutamate) in both the NTS and area postrema. Thus, NTS and area postrema injections of adenosine decrease blood pressure and heart rate in anesthetized normotensive rats through adenosine receptors located in these areas. These findings support a role for endogenous adenosine as a central modulator in cardiovascular control.

Differential effects of adenosine analogs on amygdala, hippocampus, and caudate nucleus kindled seizures

This study explored the anticonvulsant effects of adenosine analogs at the focus of seizures kindled from various brain structures. Chemitrodes were implanted in the amygdala (AM), hippocampus (HIPP), or caudate nucleus (CN) of Long-Evans rats and electrically stimulated once daily until fully generalized seizures appeared (i.e., kindled). Once kindled, various doses (0.001-0.5 microgram/0.5 microliter) of the adenosine analogs, L-phenylisopropyladenosine (L-PIA), N-ethylcarboximidoadenosine (NECA) or vehicle were injected into the seizure focus 5 min prior to electrical stimulation. The afterdischarge (AD) and behavioral seizure stages were measured. L-PIA had potent anticonvulsant effects when injected directly into the kindled seizure focus in the AM, HIPP, or CN. NECA effects were statistically significant only in CN-kindled seizures. The regional differences in efficacy of the two adenosine analogs suggest that L-PIA, an A1 adenosine subtype agonist, may exert its effects through A1 adenosine receptors in the AM, HIPP, and CN, where A1 binding has been demonstrated, whereas NECA, an A2 adenosine receptor agonist, may only be maximally effective in the CN where A2 adenosine binding sites are located.

Electrophysiological evidence for adenosine receptors on astrocytes of cultured rat central nervous system

The actions of adenosine, R-N6-phenylisopropyladenosine (R-PIA) and N-(ethyl)carboxamidoadenosine (NECA) were tested on the membrane potential of astrocytes of cultured rat spinal cord and cerebellum. All 3 compounds hyperpolarized the majority of astrocytes studied. A considerable number of cells did, however, not respond to adenosine and its analogues, suggesting that only a certain type of astrocyte possesses adenosine receptors. The hyperpolarizations by adenosine, R-PIA and NECA were reversibly blocked by their antagonist 8-phenyltheophylline indicating that these compounds activate specific adenosine receptors. In agreement with biochemical and autoradiographic binding studies, our electrophysiological data strongly suggest the existence of adenosine receptors on astrocytes.