Characterization of the depression of rat cerebral cortical neurons by selective adenosine agonists

Adaptive responses of vertebrate neurons to anoxia--matching supply to demand

Oxygen depleted environments are relatively common on earth and represent both a challenge and an opportunity to organisms that survive there. A commonly observed survival strategy to this kind of stress is a lowering of metabolic rate or metabolic depression. Whether metabolic rate is at a normal or a depressed level the supply of ATP (glycolysis and oxidative phosphorylation) must match the cellular demand for ATP (protein synthesis and ion pumping), a condition that must of course be met for long-term survival in hypoxic and anoxic environments. Underlying a decrease in metabolic rate is a corresponding decrease in both ATP supply and ATP demand pathways setting a new lower level for ATP turnover. Both sides of this equation can be actively regulated by second messenger pathways but it is less clear if they are regulated differentially or even sequentially with the onset of anoxia. The vertebrate brain is extremely sensitive to low oxygen levels yet some species can survive in oxygen depleted environments for extended periods and offer a working model of brain survival without oxygen. Hypoxia tolerant vertebrate brain will be the primary focus of this review; however, we will draw upon research involving hypoxia/ischemia tolerance mechanisms in liver and heart to offer clues to how brain can tolerate anoxia. The issue of regulating ATP supply or demand pathways will also be addressed with a focus on ion channel arrest being a significant mechanism to reduce ATP demand and therefore metabolic rate. Furthermore, mitochondria are ideally situated to serve as cellular oxygen sensors and mediator of protective mechanisms such as ion channel arrest. Therefore, we will also describe a mitochondria based mechanism of ion channel arrest involving ATP-sensitive mitochondrial K(+) channels, cytosolic calcium and reaction oxygen species concentrations.

Caffeine increases time to fatigue by maintaining force and not by altering firing rates during submaximal isometric contractions

Caffeine increases time to fatigue [limit of endurance (Tlim)] during submaximal isometric contractions without altering whole muscle activation or neuromuscular junction transmission. We used 10 male volunteers in a randomized, double-blind, repeated-measures experiment to examine single motor unit firing rates during intermittent submaximal contractions and to determine whether administering caffeine increased Tlim by maintaining higher firing rates. On 2 separate days, subjects performed intermittent 50% maximal voluntary contractions of the quadriceps to Tlim, 1 h after ingesting a caffeine (6 mg/kg) or placebo capsule. Average motor unit firing rates recorded with tungsten microelectrodes were constant for the duration of contractions. Caffeine increased average Tlim by 20.5 ± 8.1% ( P < 0.05) compared with placebo conditions. This increase was due to seven subjects, termed responders, who increased Tlim significantly. Two other subjects showed no response, and a third had a shorter Tlim. Neither the increased Tlim nor the responders' performance could be explained by alterations in firing rates or other neuromuscular variables. However, the amplitude of the evoked twitch and its maximal instantaneous rate of relaxation did not decline to the same degree in the caffeine trial of the responders; this resulted in values 20 and 30% higher at the time point matching the end of the placebo trial ( P < 0.05). The amplitude of the evoked twitch and the maximal instantaneous rate of relaxation were linearly correlated (caffeine r = 0.72, placebo r = 0.80, both P < 0.001), suggesting that the increase in Tlim may be partially explained by caffeine's effects on calcium reuptake and twitch force.

Acquired neuronal channelopathies in HIV-associated dementia

A gene expression profile of the human brain cortex was performed in people with HIV-1-associated dementia (HAD) using Affymetrix HG-U133 chips. Messenger RNA transcripts in middle frontal gyrus from subjects with HAD or milder neurocognitive dysfunction were compared to HIV-negative people. The analysis focused on ionic conductance carriers that control membrane excitation. Overexpressed ionic channel genes in brain cortex of subjects with dementia included (1) a calcium-driven K+ channel that prolongs afterhyperpolarization (AHP) current, (2) a leak type of K+ channel that prolongs the AHP, (3) an adenosine receptor that modulates cationic current via G proteins, (4) a G protein-coupled serotonin receptor that modulates cyclic AMP-linked current transduction, (5) a G protein-coupled dopamine receptor, (6) a GABA receptor subunit that conducts chloride current. Underexpressed current generators in the demented subjects included (1) two voltage-gated K+ channels that influence refractory periods and the onset of AHP, (2) a Na+ channel subunit that modifies current inactivation and the onset of the AHP, (3) a neuronal type of voltage-sensitive Ca+ channel that controls postsynaptic membrane excitability, (4) a metabotropic glutamate receptor that regulates cationic gating via G protein coupling, (5) A specific Galpha protein that transduces metabotropic cationic current, (6) an NMDA receptor subunit, (7) a glycine receptor subunit that modulates chloride current. These gene expression shifts probably occurred in neurons because they were not present in gyral white matter. Acquired neuronal channelopathies were not associated with a generalized shift of neuronal or glial cell markers, which suggest that they were not an artifact produced by neurodegeneration and/or glial cell proliferation. Channelopathies were not correlated with a generalized increase of inflammatory cell transcripts and were present in demented people without, and with HIV encephalitis (HIVE). We surveyed experimentally induced perturbations of these channels to determine the implications for brain function. Eleven experimental channelopathies produced decreased neuronal firing frequencies and pacemaker rates in model neurons; seven channelopathies increase neuronal firing rates experimentally. The implied disruption of neuronal excitability is consistent with some features of HAD, including its potential reversibility after HIV-1 replication is suppressed, the abnormal electroencephalographic recordings, the lack of clear-cut correlation with neurodegeneration and the lack of strict correlation with brain inflammation. The channelopathy concept may have wide relevance to the subcortical dementias.

The A3 adenosine receptor agonist 2-Cl-IB-MECA facilitates epileptiform discharges in the CA3 area of immature rat hippocampal slices

The effects of the A(3) adenosine receptor agonist 2-Cl-IB-MECA were tested on epileptiform field potentials recorded in the CA3 area of postnatal days 10-20 immature hippocampal slices, during perfusion with the GABA(A) receptor antagonist bicuculline (10 microM). Evoked potentials: 2-Cl-IB-MECA (1-50 microM, n = 17) had consistently excitatory effects, blocked by the A(3) receptor antagonist MRS 1220 (1 microM, n = 7), but not occluded in the presence of the A(1) antagonist DPCPX (1 microM, n = 12) or the A(2A) antagonist ZM-241385 (0.1 microM, n = 12). 2-Cl-IB-MECA reversed the inhibitory effects (n = 5) of the adenosine uptake blocker nitrobenzylthioinosine (NBTI, 50 microM), but did not increase its excitatory effects (n = 19). Spontaneous discharges: 2-Cl-IB-MECA (1 microM) induced them or increased their frequency in 14/30 slices, an effect reversed by MRS 1220 (n = 3), and observed also following pre-perfusion with DPCPX (n = 11), ZM-241385 (n = 11) or both (n = 10). In the presence of the A(1) antagonist DPCPX, NBTI increased the frequency of spontaneous discharges, an effect partially reversed by MRS 1220 (n = 8), thus suggesting that a rise in endogenous adenosine during disinhibition may activate A(3) receptors. In conclusion, these findings suggest strongly that activation of A(3) receptors, following a rise in endogenous adenosine (i.e. during seizures, hypoxia), facilitates excitation, thus limiting the known inhibitory and/or neuroprotective effects of adenosine in immature brain.

Administration of adenosine receptor agonists or antagonists after controlled cortical impact in mice: effects on function and histopathology

Adenosine is an endogenous neuroprotectant via anti-excitotoxic effects at A(1) receptors, and blood flow promoting and anti-inflammatory effects at A(2a) receptors. Previous studies showed improved motor function after fluid percussion injury (FPI) in rats treated with the broad-spectrum adenosine receptor agonist 2-chloroadenosine (2-CA). We studied the effects of 2-CA, a specific A(1) agonist (2-chloro-N(6)-cyclopentyladenosine, CCPA), and a specific A(1) antagonist (8-cyclopentyl-1,3-dipropylxanthine, DPCPX) on motor task and Morris water maze (MWM) performance, and histopathology (contusion volume, hippocampal cell counts) after controlled cortical impact (CCI) in mice. Each agent (12 nmol), or respective vehicle (saline or DMSO) was injected into dorsal hippocampus beneath the contusion immediately after CCI or craniotomy (sham). 2-CA treatment attenuated wire grip deficits after CCI (P<0.05 versus other treatments). DPCPX treatment exacerbated deficits on beam balance (P<0.05 versus sham). No treatment effect was seen on MWM performance, although there was a deleterious effect of the DMSO vehicle used for DPCPX. Contusion volume tended to be attenuated by 2-CA (P=0.08 versus saline) and increased after either DMSO or DPCPX (P<0.05 versus all groups). CA1 and CA3 counts were decreased in all groups versus sham. However, treatment with the selective A(1) agonist CCPA attenuated the CA3 cell loss (P<0.05 versus other treatment). We suggest that the beneficial effect of the broad spectrum adenosine receptor agonist 2-CA on motor function after CCI is not mediated solely by effects at the A(1) receptor.

Interstitial brain adenosine and xanthine increase during jugular venous oxygen desaturations in humans after traumatic brain injury

OBJECTIVE

Adenosine decreases the cerebral metabolic rate for oxygen and increases cerebral blood flow, and it may play an important role in cerebrometabolic and cerebrovascular responses to hypoperfusion after traumatic brain injury. Jugular venous oxygen saturation is monitored after traumatic brain injury to assess brain oxygen extraction, and desaturations may reflect secondary brain insults. We hypothesized that brain interstitial adenosine and related purine metabolites would be increased during jugular venous oxygen saturation desaturations (<50%) and determined associations between the purines, lactate, and glucose to assess the role of adenosine during secondary insults in humans.

DESIGN

Study of critically ill adults with severe traumatic brain injury.

SETTING

Adult neurointensive care unit.

PATIENTS

We prospectively defined periods of normal saturation and desaturation in six patients after severe traumatic brain injury.

INTERVENTIONS

During these periods, cerebral microdialysis samples of brain interstitial fluid were collected, and adenosine and purine metabolites were measured by high-pressure liquid chromatography.

MEASUREMENTS AND MAIN RESULTS

Adenosine increased 3.1-fold and xanthine increased 2.5-fold during desaturation periods (both p <.05 vs. normal saturation period, signed rank). Adenosine, xanthine, hypoxanthine, and cyclic-adenosine monophosphate correlated with lactate over both study periods (r(2) =.32,.14,.31,.07, and.26, respectively, all p <.05, Pearson product moment correlation).

CONCLUSION

The marked increases in interstitial brain adenosine that occur during jugular venous oxygen desaturations suggest that adenosine may play an important role during periods of secondary insults after traumatic brain injury. The correlation of these metabolites with lactate further suggests that adenosine is increased during periods of enhanced glycolytic metabolism.

Indirect modulation of dopamine D2 receptors as potential pharmacotherapy for schizophrenia: I. Adenosine agonists

OBJECTIVE

To review preclinical and clinical information related to pharmacologic modulation of dopamine D2 receptors as potential novel antipsychotic therapy. Specifically, to summarize the data that suggest a modulatory action of adenosine A2A receptors on dopamine D2 receptors and, therefore, a possible rational role of adenosine A2A agonists as novel antipsychotic agents.

DATA SOURCES

Primary and review articles were identified by MEDLINE search (from 1966 to May 1998) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All of the articles identified from the data sources were evaluated and all information deemed relevant was included in this review.

DATA SYNTHESIS

For all of the older and many of the newer antipsychotic agents, there is a strong correlation between clinical antipsychotic activity and affinity for dopamine D2 receptors. Unfortunately, dopamine D2 receptors are believed to also be involved in the adverse effect profile of these agents. The indirect modulation of dopamine D2 receptors, rather than direct block, might produce antipsychotic effects without the usual adverse reactions. Several lines of evidence from animal studies suggest that the use of selective A2A agonists might represent a novel approach to the treatment of psychoses.

CONCLUSIONS

Dopamine receptor modulation might represent a novel antipsychotic approach or adjunct therapy. The data regarding adenosine agonists (particularly selective A2A receptor agonists) are inconclusive at the present time. Direct clinical demonstration of effectiveness is required.

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

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

Inhibitory action of CGS 21680 on cerebral cortical neurons is antagonized by bicuculline and picrotoxin-is GABA involved?

The possibility of an involvement of endogenously released GABA in the inhibitory actions of A1 and A2a adenosine receptor agonists on rat cerebral cortical neurons discharges was examined using the GABAA antagonists bicuculline and picrotoxin. The A1 agonist N6-cyclopentyladenosine (CPA), the A2a agonist CGS 21680 and the non-selective receptor agonist, adenosine, depressed neuronal firing. Applications of bicuculline or picrotoxin enhanced the spontaneous firing rate of cortical neurons, indicating the presence of ongoing GABA-ergic inhibition. Antagonism of GABAA receptors blocked the depressant effects of CGS 21680 on neuronal firing; was without effect on CPA-evoked inhibitions and tended to reduce the duration of adenosine-evoked inhibitions. These results suggest that the depressant effects of A2a receptor activation are due to an increase in GABA-ergic inhibition, likely as a consequence of increased GABA release. GABA does not appear to be involved in adenosine A1 receptor-mediated inhibition of neuronal firing.

Protection against kainate-induced excitotoxicity by adenosine A2A receptor agonists and antagonists

The neuroprotective role of adenosine receptor agonists in various models of ischaemia and neuronal excitotoxicity has been attributed to adenosine A1 receptor activation. In this study we examine the role of the A2A receptor in the kainate model of excitotoxicity. Kainate (10 mg/kg) was administered systemically 10 min after the intraperitoneal injection of adenosine analogues. The A2A agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS21680) protected the hippocampus at concentrations of 0.1 and 0.01 mg/kg, but not at 2 microg/kg. The addition of the centrally acting adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine partially reduced protection only in the CA3a region, suggesting that only a small proportion of the protection was attributable to the A1 receptor. A less potent A2A agonist, N6-[2-(3,5-dimethyoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (1 mg/kg), provided only partial protection against kainate. 4-(2-[7-Amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl -amino]ethyl)phenol, a selective A2A antagonist, also showed protection against kainate-induced neuronal death, when administered alone or in combination with CGS21680. These results show that adenosine A2A receptor activation is protective against excitotoxicity. The protection is largely independent of A, receptor activation or blockade.

Hypercapnia-induced increases in cerebral blood flow: roles of adenosine, nitric oxide and cortical arousal

The roles of nitric oxide, adenosine and cortical arousal in the response to 7.5% CO2 inhalation were investigated by measuring cerebral blood flow bilaterally in the rat somatosensory cortices with laser-Doppler flow probes. Administration of N(omega)-nitro-L-arginine methyl ester (L-NAME; 20 mg/kg, i.v.) significantly attenuated the response to hypercapnia (mean decrease of 47%). This effect was partially reversed by a subsequent administration of L-arginine. Caffeine (10 mg/kg, i.v.) also significantly reduced hypercapnic responses (mean decrease of 44%). Caffeine administration was also associated with a tendency for animals to exhibit electrocorticographic signs of arousal; often associated with a reduction in the attenuation of the flow response to CO2 inhalation. 8-(3-Chlorostyryl) caffeine (CSC, 1.0 mg/kg), a selective antagonist at adenosine A2a striatal receptors failed to attenuate CO2-evoked responses, whereas CGS 15943, a less selective A2a receptor antagonist, significantly reduced CO2 responses. These data from the rat suggest (1) that both nitric oxide and adenosine may contribute to pial arteriolar vasodilatation during hypercapnia, and (2) that CO2 inhalation acts as a potent stimulus for cortical arousal, with enhanced neuronal activity contributing to the vascular response. The effects of administration of adenosine antagonists, such as the methylxanthines antagonists caffeine and theophylline, on CBF responses to hypercapnia can potentially be negated by the ability of these agents to facilitate CO2-induced cortical arousal.

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

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

Effects of selective adenosine A1 and A2 receptor agonists and antagonists on local rates of energy metabolism in the rat brain

The quantitative [14C]2-deoxyglucose autoradiographic technique was applied to the measurement of the cerebral metabolic effects of adenosine A1 and A2 receptor agonists and antagonists in adult rats. The adenosine A1 receptor agonist and antagonist, 2-chloro-N6-cyclopentyladenosine (CCPA) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as the adenosine A2 receptor agonist, 2-[p-(2-carboxyethyl)phenylethylamino]-5'-ethylcarboxamidoadenosin e (CGS 21680), were injected at the dose of 0.01 mg/kg. The adenosine A2 receptor antagonist, 3,7-dimethyl-1-proparglyxanthine (DMPX) was injected at the dose of 0.3 mg/kg. These doses were chosen in accordance with the known affinity of the drugs for their respective receptor and to avoid peripheral effects. The adenosine A1 receptor agonist, CCPA, induced decreases in glucose utilization in three brain areas, the globus pallidus and two hypothalamic nuclei. The adenosine A2 receptor agonist, CGS 21680, induced more general depressant effects on energy metabolism which were significant in 17 brain areas, such as cerebral cortex, hippocampal and white matter regions plus motor and limbic structures. The adenosine A2 receptor antagonist, DMPX, decreased glucose utilization in the globus pallidus while increasing energy metabolism in the cochlear nucleus. The adenosine A1 receptor antagonist, DPCPX, depressed glucose utilization in the globus pallidus and dentate gyrus, and increased rates of energy metabolism in six regions, mainly hypothalamic, thalamic areas and in the cochlear nucleus. There was a mismatch between cerebral metabolic consequences of adenosine A1 and A2 receptor agonists and the localization of corresponding adenosine receptors. The metabolic effects of the adenosine A2 receptor agonist and antagonist were consistent with the known involvement of that type of receptor in the control of locomotion and its effects on neuronal firing in the hippocampus and cerebral cortex. The effects of the adenosine A1 receptor agonist were very discrete and mostly related to the transient decrease in blood pressure induced by the drug. The increases in glucose utilization induced in limbic regions by the adenosine A1 receptor antagonist are probably linked to the regulation by adenosine of arousal and cardiorespiratory function. These results are in good agreement with the neuroregulatory function of the adenosine system as previously shown by other methods.

Excitatory effects of adenosine receptor agonists and antagonists on neurotransmission in guinea pig superior collicular slices

Adenosine has excitatory actions on neurotransmission in the superior colliculus. To investigate whether adenosine A1 or A2 receptors are involved in mediating these excitatory actions, the effect of A1 and A2 receptor agonists and antagonists on the evoked postsynaptic potentials (PSP) in the superficial grey layer were tested using slices of the superior colliculus. Application of both A1 agonists, such as CHA, R-PIA, and the A2 agonist, CGS-21680 increased the amplitude of the PSP. The increase in PSP amplitude occurred gradually over 20-30 min after application of these adenosine agonists. Application of the A1 antagonist 8-CPT, and the A2 antagonists, DMPX and CGS-15943, increased the amplitude of the PSP and could not antagonize the excitatory effect of adenosine. These results suggest that the mechanism of the excitatory effect of adenosine cannot be explained by the classical concept of A1 and A2 adenosine receptor subtypes which were identified by their effect on adenylate cyclase activity.

Characterization of adenosine receptors in a model of cultured neurons from rat forebrain

The neuromodulator adenosine is acting through specific receptors coupled to adenylate cyclase via G-proteins. The expression of both adenosine receptors A1 and A2 as well as forskolin binding sites was investigated by radioligand binding techniques in 8-day-old neurons isolated from fetal rat forebrain and cultured in chemically-defined medium. Adenosine A1 receptors were specifically labeled with [3H]chloro-N6-cyclopentyladenosine (CCPA), whereas [3H]CGS 21680 was used for the analysis of A2 receptors. Cultured neurons exhibited high affinity binding sites for CCPA (Bmax = 160 fmol/mg protein; Kd = 2.9 nM), and for CGS 21680 (Bmax = 14 fmol/mg protein; Kd = 1.7 nM). These data correlate well with those obtained in crude membranes isolated from the newborn rat forebrain. The incubation of culture membranes in the additional presence of guanylyl-5'-imidodiphosphate (Gpp(NH)p, a GTP analogue) led to significantly increased Kd-values, suggesting the association of adenosine receptors with G-proteins. Finally, cultured neurons also bound specifically [3H]forskolin with characteristics close to those found in the newborn brain, indicating that cultured neurons appear as an appropriate model for studying the neuromodulatory properties of adenosine.

Adenosine A2a receptor-mediated modulation of striatal [3H]GABA and [3H]acetylcholine release

The ability of adenosine agonists to modulate K(+)-evoked gamma-[3H]aminobutyric acid ([3H]GABA) and acetylcholine (ACh) release from rat striatal synaptosomes was investigated. The A2a receptor-selective agonist CGS 21680 inhibited Ca(2+)-dependent [3H]GABA release evoked by 15 mM KCl with a maximal inhibition of 29 +/- 4% (IC50 of approximately 4 x 10(-12) M). The relative order of potency of three agonists was CGS 21680 > or = 5'-N-ethylcarboxamidoadenosine > R-phenylisopropyladenosine (R-PIA), with the inhibition being blocked by A2a receptor-selective antagonists (CP 66,713 and CGS 15943A) but not by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). When release of [3H]GABA was evoked by 30 mM KCl, no significant inhibition was observed. In contrast, CGS 21680 stimulated the release of [3H]ACh evoked by 30 mM KCl, with a maximal stimulation of 26 +/- 5% (IC50 of approximately 10(-11) M). This effect was blocked by CP 66,713 but not by DPCPX. The A1 agonist R-PIA inhibited [3H]ACh release, an effect blocked by DPCPX. It is concluded that adenosine A2a receptors are present on both GABAergic and cholinergic striatal nerve terminals where they inhibit and stimulate transmitter release, respectively.

CGS 15943, an adenosine A2 receptor antagonist, reduces cerebral ischemic injury in the Mongolian gerbil

The adenosine A2 receptor antagonist CGS 15943 (0.1 mg/kg, i.p.) was tested for cerebroprotective activity in a gerbil stroke model. CGS 15943 markedly reduced stroke injury assessed by locomotor activity monitoring and by histopathological measurement of hippocampal CA1 pyramidal cell injury. It is proposed that a previously demonstrated reduction in the ischemia/reperfusion-evoked release of excitotoxic amino acids following CGS 15943 administration could account for its cerebroprotective actions.

The binding of the adenosine A2 receptor selective agonist [3H]CGS 21680 to rat cortex differs from its binding to rat striatum

The binding of the reportedly A2A selective agonist CGS 21680 (2-[p-(2-carboxyethyl)phenylethylamino]-5'N-ethylcarboxamidoadenos ine) to cortex and striatum was examined in parallel using quantitative receptor autoradiography. [3H]CGS 21680 bound to a single site in rat striatum with KD 2.3 nM and Bmax 320 fmol/mg grey matter. In addition [3H]CGS 21680 bound to a single site in the cerebral cortex with KD 47 nM and Bmax 100 fmol/mg grey matter. In cat cortex [3H]CGS 21680 (2 nM) binding was strong and particularly evident in the most superficial layers. The potency order for inhibition of 2 nM [3H]CGS 21680 binding to rat striatum was NECA (5'-N-ethylcarboxamidoadenosine; IC50 9.0 nM) > 2-CADO (2-chloroadenosine; 87 nM) > R-PIA (N6-(R)-phenylisopropyladenosine; 110 nM). The potency order for inhibition of 2 nM [3H]CGS 21680 binding to rat cortex was NECA (3.0 nM) > 2-CADO (14 nM) > or = R-PIA (16 nM). Gpp(NH)p (5'-guanylyl imidodiphosphate) inhibited [3H]CGS 21680 binding to both cortex and striatum, but more potently in cortex (IC50 100 nM vs. 470 nM). The present results show that there is a cortical binding site for [3H]CGS 21680 which appears to be different from the the striatal A2A receptor, the A2B receptor and the A1 receptor.

Effects of bivalent cations on adenosine sensitivity in the rat hippocampal slice

Ligand binding studies have demonstrated that bivalent cations can alter interactions between purinergic agonists and adenosine receptors in brain membranes. In the present study, we have investigated whether a similar interaction can be demonstrated in terms of a functional response to adenosine, which is the inhibition of synaptic transmission in the CA1 region of the rat hippocampus mediated via presynaptic A1 receptors. Our data suggest that alterations in cation concentrations do not significantly affect the sensitivity of these adenosine receptors, as long as changes are made in such a way as to leave presynaptic Ca2+ entry unaffected. The experimental results do not support the conclusion that there is a specific effect of either Ca2+ or Mg2+ on adenosine receptor sensitivity, such as has been described for agonist interactions with adenosine A1 receptors. We conclude that the effects of bivalent cations observed in ligand binding studies probably reflect an effect at an intracellular site, either on the receptor itself, or perhaps on associated GTP binding proteins.

Excitatory Transmitter Amino Acid Release from the Ischemic Rat Cerebral Cortex: Effects of Adenosine Receptor Agonists and Antagonists

The effects of selective adenosine receptor agonists [N6-cyclopentyladenosine (CPA) and N-ethylcarboxamidoadenosine (NECA)] and antagonists [8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c]quinazoline-5-im ine (CGS-15943A)] on aspartate and glutamate release from the ischemic rat cerebral cortex were studied with the cortical cup technique. Cerebral ischemia (for 20 min) was elicited by four-vessel occlusion. Excitatory amino acid releases were compared from control ischemic rats and drug-treated rats. Basal levels of aspartate and glutamate release were not greatly affected by pretreatment with the adenosine receptor agonists or antagonists. However, CPA (10(-10) M) and NECA (10(-9) M) significantly inhibited the ischemia-evoked release of aspartate and glutamate into cortical superfusates. The ability to block ischemia-evoked release of excitatory amino acids was not evident at higher concentrations of CPA (10(-6) M) or NECA (10(-5) M). The selective A1 receptor antagonist DPCPX also had no effect on release when administered at a low dosage (0.01 mg/kg, i.p.) but blocked the ischemia-evoked release of aspartate and glutamate at a higher dosage (0.1 mg/kg). Evoked release was inhibited by the selective A2 receptor antagonist CGS-15943A (0.1 mg/kg, i.p.). Thus, adenosine and its analogs may suppress ischemia-evoked release of excitatory neurotransmitter amino acids via high-affinity A1 receptors, whereas coactivation of lower-affinity A2 receptors may block (or reverse) the A1-mediated response.

Evidence for the presence of excitatory A2 adenosine receptors in the rat hippocampus

The A2 adenosine receptor agonist, CGS 21680 in nanomolar concentrations, reversibly increased in a concentration-dependent manner the amplitude of orthodromically-evoked population spikes recorded from the CA1 pyramidal cell layer of rat hippocampal slices. The adenosine receptor antagonist, 3,7-dimethyl-l-propargylxanthine (DMPX, 10 microM), which has selectivity for A2 adenosine receptors, prevented this excitatory effect of CGS 21680. These results suggest that A2 adenosine receptors are present in the rat hippocampus and that activation of these receptors enhance hippocampal excitability.