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

The selective adenosine A2A receptor antagonist SCH 58261 discriminates between two different binding sites for [3H]-CGS 21680 in the rat brain

We have used quantitative autoradiography to further examine two previously described binding sites for [3H]-CGS 21680 in cortical regions and in striatum, respectively. The striatal binding sites largely represent classical adenosine A2A receptors whereas the cortical sites show characteristics that differ from those of recognised adenosine receptors. A recently developed non-xanthine A2A receptor antagonist SCH 58261 displaced the binding of [3H]-CGS 21680 from the A2A receptors in striatum with an estimated Ki value of 2.4 nM, but was more than 1000-fold less potent in displacing its binding from cortex. Conversely, the adenosine analogue 2-chloro-NECA was found to be some 10 times more potent in displacing CGS 21680 from the cortical binding sites than from A2A receptors. The results provide additional evidence that CGS 21680 binds not only to classical A2A receptors, but also to sites that differ from defined adenosine receptors. They also suggest that effects of CGS 21680 observed in the presence of SCH 58261 might reveal the functional significance (if any) of these sites.

Augmented release of serotonin by adenosine A2a receptor activation and desensitization by CGS 21680 in the rat nucleus tractus solitarius

Rat dorsomedial medullary brain segments containing primarily nucleus tractus solitarius (NTS) were employed for slice superfusion studies of electrically evoked [3H]serotonin ([3H]5-HT) release. Individual slices loaded with [3H]5-HT were stimulated two times, S1 and S2, at 3 Hz, 25 mA, 2 ms pulses for 1 min. Control NTS slices had a S2/S1 ratio of 0.94 (+/- 0.02). Superfusion of tissue slices with 0.1 nM to 100 nM 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680), a selective adenosine A2a receptor agonist, for 5 min prior to the S2 stimulus produced a significant concentration-dependent increase in the S2/S1 fractional release ratio which was maximal (37.2% increase, P < 0.01) at 1.0 nM. However, superfusion of tissue slices with CGS 21680 over the same concentration range for 20 min prior to the S2 stimulus did not significantly alter the S2/S1 ratio from control release ratios. The augmented release of [3H]5-HT mediated by 1.0 nM CGS 21680 with 5 min tissue exposure was abolished by 1.0 nM 9-chloro-2-(2-furanyl)-5, 6-dihydro-[1,2,4]-triazolo[1,5-c]quinazolin-5-imine (CGS 15943) as well as by 100 nM 8-(3-chlorostyryl)caffeine (CSC), both A2a receptor antagonists, but not by 1.0 nM 8-cyclopentyl-1,3,-dipropylxanthine (DPCPX), the A1 receptor antagonist. These results indicate that CGS 21680 augmented the evoked release of [3H]5-HT in the NTS by way of activation of presynaptic adenosine A2a receptors. It was also apparent that this population of adenosine A2a receptors in the NTS desensitized within 20 min since the augmenting action of CGS 21680 on evoked transmitter release was not evident at the longer time interval.

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

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

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

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

Evidence for high-affinity binding sites for the adenosine A2A receptor agonist [3H] CGS 21680 in the rat hippocampus and cerebral cortex that are different from striatal A2A receptors

The binding of the adenosine A2A receptor selective agonist 2-[4-(2-p-carboxyethyl)phenylamino] -5'-N-ethylcarboxamidoadenosine (CGS 21680) to the rat hippocampal and cerebral cortical membranes was studied and compared with that to striatal membranes. [3H] CGS 21680, in the concentration range tested (0.2-200 nM), bound to a single site with a Kd of 58 nM and a Bmax of 353 fmol/mg protein in the hippocampus, and with a Kd of 58 nM and a Bmax of 264 fmol/mg protein in the cortex; in the striatum, the single high-affinity [3H] CGS 21680 binding site had a Kd of 17 nM and a Bmax of 419 fmol/mg protein. Both guanylylimidodiphosphate (100 microM) and Na+ (100 mM) reduced the affinity of [3H] CGS 21680 binding in the striatum by half and virtually abolished [3H] CGS 21680 binding in the hippocampus and cortex. The displacement curves of [3H] CGS 21680 binding with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), N6-cyclohexyladenosine (CHA), 5'-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine (CADO) were biphasic in the hippocampus and cortex as well as in the striatum. The predominant [3H]CGS 21680 binding site in the striatum (80%) had a pharmacological profile compatible with A2A receptors and was also present in the hippocampus and cortex, representing 10-25% of [3H]CGS 21680 binding. The predominant [3H]CGS 21680 binding site in the hippocampus and cortex had a pharmacological profile distinct from A2A receptors: the relative potency order of adenosine antagonists DPCPX, 1,3-dipropyl- 8-¿4-[(2-aminoethyl)amino]carbonylmethyl- oxyphenyl¿ xanthine (XAC), 8-(3-chlorostyryl)caffeine (CSC), and (E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)- methylxanthine (KF 17,837) as displacers of [3H] CGS 21680 (5 nM) binding in the hippocampus and cerebral cortex was DPCPX > XAC >> CSC approximately KF 17,837, and the relative potency order of adenosine agonists CHA, NECA, CADO, 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5'-N- ethylcarboxamidoadenosine (APEC), and 2-phenylaminoadenosine (CV 1808) was CHA approximately NECA > or = CADO > APEC approximately CV1808 > CGS 21680. In the presence of DPCPX (20 nM), [3H] CGS 21680 (0.2-200 nM) bound to a site (A2A-like) with a Kd of 20 nM and a Bmax of 56fmol/mg protein in the hippocampus and with a Kd of 22 nM and a Bmax of 63fmol/mg protein in the cortex. In the presence of CSC (200 nM), [3H]CGS 21680(0.2-200 nM) bound to a second high-affinity site with a Kd of 97 nM and a Bmax of 255 fmol/mg protein in the hippocampus and with a Kd of 112 nM and a Bmax of 221 fmol/mg protein in the cortex. Two pharmacologically distinct [3H]CGS 21680 binding sites were found in synaptosomal membranes of the hippocampus and cortex and in the striatum, one corresponding to A2A receptors and the other to the second high-affinity [3H]CGS 21680 binding site. In contrast, the pharmacology of [3H]CHA binding was similar in synaptosomal membranes of the three brain areas. The present results establish the existence of at least two high-affinity [3H]CGS 21680 binding sites in the CNS and demonstrate that the [3H]CGS 21680 binding site predominant in the hippocampus and cerebral cortex has different binding characteristics from the classic A2A adenosine receptor, which predominates in the striatum.

Adenosine A1 and A2A receptors and nitrobenzylthioinosine-sensitive transporters in gerbil brain: no changes following long-term treatment with the adenosine transport inhibitor propentofylline

There is evidence that adenosine is an endogenous neuroprotective substance in the gerbil and that propentofylline, a novel xanthine derivative that acts as a transport inhibitor, exerts part of its neuroprotective activity in this species by enhancing adenosine actions. Using autoradiography we have examined the distribution of adenosine A1 and A2A receptors and of equilibrative adenosine transporters in gerbil brain as well as the possible changes induced by repeated treatment with propentofylline. Nucleoside transporters, studied by [3H]NBMPR binding, were found to be widely distributed in the gerbil brain, with no clear relationship to the distribution of adenosine receptors. Adenosine A2A receptors, studied by [3H]CGS 21680 binding and by in situ hybridization, were found to be present in intrinsic neurons in the caudate putamen, nucleus accumbens and tuberculum olfactorium. Adenosine A1 receptors were studied by examining the binding of [3H]CHA, an agonist, and [3H]DPCPX, an antagonist. There was an overall similarity in the distribution of binding sites for these two ligands, and a similarity with the distribution in the rat. However, the antagonist was found to label certain structures, especially white matter structures, more than the agonist. It is argued that these binding sites for antagonists represent receptors that are in transit from the site of synthesis in the perikaryon to the destination in the nerve terminal, and are not coupled to G proteins. There were no differences in the binding of any of these ligands or in A2A mRNA following 2 weeks' treatment with propentofylline, indicating that the drug has minimal effects on adenosine mechanisms under basal physiological conditions. This also suggests that tolerance to adenosine-related effects of the drug is less likely to occur.

Adaptive changes in adenosine receptors following long-term treatment with the adenosine receptor agonist R-phenylisopropyl adenosine

Changes in brain A1 and A2A receptors and in the corresponding mRNA were studied using quantitative receptor autoradiography and in situ hybridisation. [3H]-DPCPX was used as an antagonist ligand at A1 receptors and [3H]-CGS 21680 as an agonist ligand at A2A receptors. Treatment of rats with the relatively A1 receptor selective adenosine analogue R-PIA (0.3 mg/kg) for 7 days in the presence of the peripherally acting antagonist 8-p-sulfophenyltheophylline (8-PST; 10 mg/kg) caused a decrease in the binding of the A1 receptor ligand, but not in that of the A2A receptor ligand. The effect on A1 receptors was also seen in the presence of 100 microM GTP that decreases agonist binding to insignificant levels. There was no change in either A1 or A2A receptor mRNA. No significant changes were detected following administration of either R-PIA or 8-PST alone. These results thus demonstrate an effect on brain A1 receptors after systemic administration of R-PIA in the presence of a peripherally acting adenosine antagonist, demonstrating that, under these conditions, the agonist reaches receptors in significant amounts.

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

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

Adenosine A2A receptors stimulate acetylcholine release from nerve terminals of the rat hippocampus

The nature of the adenosine receptors involved in the enhancement of acetylcholine release in the hippocampus was studied. The A2A agonist, CGS 21680, increased the veratridine-evoked release of [3H]acetylcholine from hippocampal synaptosomes. This presynaptic effect of CGS 21680 was greater at 3-30 nM than at 100 nM. The excitatory effect of CGS 21680 was antagonised by the A2 antagonist, DMPX (10 microM), and by the A2A antagonist, CSC (200 nM), but not by the A1 antagonist, DPCPX (20 nM). We also found co-expression of A2A and choline acetyltransferase mRNAs in the nucleus of the diagonal band and the medial septum, where the cholinergic cell bodies that project into the hippocampus are located. These results indicate that A2A adenosine receptors are present in cholinergic nerve terminals in the hippocampus and that activation of these receptors enhances acetylcholine release.

Purinoceptors in the nervous system

The purine nucleoside adenosine and the purine nucleotide ATP play different roles in the nervous system. Adenosine acts on a family of G protein coupled receptors, collectively called adenosine receptors or P1 purinoceptors. Four members of this family have been cloned and pharmacologically characterized: A1, A2A, A2B and A3. Their distribution, pharmacology and biological roles are briefly discussed. In particular, the evidence that adenosine acting at A1 receptors regulates the release of several neurotransmitters and that adenosine acting at A2A receptors modulates dopaminergic transmission is summarized. ATP acts on receptors called P2 purinoceptors, which appear to fall into at least two main families--G protein coupled receptors and intrinsic ion channels. Their subclassification is becoming clearer as receptors are cloned and new selective agonists and/or antagonists are becoming available. There is an interesting potential for development of drugs targeted at purines or their receptors.

Further characterization of the binding of the adenosine receptor agonist [3H]CGS 21680 to rat brain using autoradiography

2-[p-(2-carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadeno sine (CGS 21680) is considered a selective ligand for adenosine A2A receptors, which are known to be enriched in striatum and olfactory tubercle. We have investigated the characteristics of [3H]CGS 21680 binding in several brain regions using quantitative autoradiography. In agreement with previous data the radioligand was found to label the caudate-putamen, accumbens nucleus, olfactory tubercle and globus pallidus, but also many other structures, e.g. cerebral and cerebellar cortex, hippocampus, thalamus and some brainstem nuclei, were labelled. Cortical and striatal binding of [3H]CGS 21680 was unaltered by high concentrations of the adenosine transport inhibitor dipyridamole or the phosphodiesterase inhibitor rolipram but was displaced by 1,3-diethyl-8-phenylxanthine, the A2 selective adenosine antagonist CP 66,713, and the A2A selective agonist SHA 118. These three agents were approximately equipotent in striatum, cortex and hippocampus. The A2 selective agonist CV 1808 was a 4-5 times more potent displacer in cortex and hippocampus than in the striatum. [3H]CGS 21680 binding was strongly magnesium-dependent in all the studied brain regions, in contrast to the binding of adenosine A1 agonists. The binding of [3H]CGS 21680 to cerebral cortex and hippocampus, but not the binding to striatum, was displaced by the adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine in nanomolar concentrations. The present study provides evidence that in cerebral cortex and hippocampus, most of the [3H]CGS 21680 binds to a receptor site that is distinct from the striatal A2A receptor and the classical adenosine A1 receptor and may represent a hitherto unrecognized binding site.

Astra Award Lecture. Adenosine, adenosine receptors and the actions of caffeine

Of the known biochemical actions of caffeine, only inhibition of adenosine receptors occurs at concentrations achieved during normal human consumption of the drug. Under normal physiological conditions, adenosine is present in sufficient concentrations to activate A1 and A2a receptors. Via actions on A1 receptors, adenosine decreases neuronal firing and the release of neurotransmitters. The exact mechanisms are not known, but several possibilities are discussed. Via actions on A2a receptors, adenosine--and hence caffeine--can influence dopaminergic neurotransmission. Caffeine can induce rapid changes in gene expression and, somewhat later, marked adaptive changes. These include antiepileptic and neuroprotective changes. Thus, caffeine has a number of central effects directly or indirectly related to adenosine receptors. Some of these are potentially useful, and drug development based on the actions of caffeine should be interesting.

Further characterization of [3H]-CGS 21680 binding sites in the rat striatum and cortex

1. The putative high affinity binding site for the adenosine A2A receptor agonist 2-p-(2-carboxyethyl)phenethyl-amino-5'-N- ethylcarboxamidoadenosine (CGS 21680) in the rat cerebral cortex was characterized by use of a number of selective A1 and A2 adenosine receptor ligands, and compared to the characteristics of the more abundant striatal A2A receptor. 2. The binding of [3H]-CGS 21680 to cortical membranes was performed at pH 5.5, in order to increase the amount of specific binding. 3. Reduction of the pH from 7.4 to 5.5 increased the apparent affinity of the striatal binding side for both agonists and antagonists. The relative order of potencies of both groups of ligands were the same at both pH values, and were consistent with binding to the A2A receptor. There was no observable change in the Bmax, the values being 415 and 446 fmol mg-1 protein at pH 5.5 and 7.4 respectively. 4. The cortical binding site yielded a Bmax value of 117 fmol mg-1 protein. The relative order of potencies of the adenosine receptor ligands observed at this binding site were not the same as those observed in the striatum, exhibiting a profile with both A1 and A2 characteristics. 5. Further characterization of this cortical binding site in the presence of the A1 selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) revealed a more typical A2A profile. This indicated that under the conditions used there were two components of [3H]-CGS 21680 binding, approximately 20% of the A1 receptor and 80% to the A2A receptor. 6. It is concluded that in the cerebral cortex there is a CGS 21680 binding site showing the characteristic properties of the striatal A2A receptor, and no evidence was obtained for the existence of a novelA2A-like binding site.

Inhibition by KF17837 of adenosine A2A receptor-mediated modulation of striatal GABA and ACh release

1. The effect of the A2A adenosine receptor agonist, 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS 21680) on the potassium evoked release of [3H]-gamma-aminobutyric acid ([3H]-GABA) from nerve terminals derived from the caudate-putamen and the globus pallidus of the rat was compared. In both preparations CGS 21680 (1 nM) inhibited the [3H]-GABA release evoked by 15 mM KCl but had no effect on that evoked by 30 mM KCl. 2. The ability of CGS 21680 (1 nM) to inhibit the release of [3H]-GABA from striatal nerve terminals was unaffected by the presence of the GABA receptor antagonists, bicuculline (10 microM), phaclofen (100 microM) and 2-hydroxysaclofen (100 microM). Similarly the opioid receptor antagonist, naloxone (10 microM), the adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 40 nM), and the cholinoceptor antagonists, mecamylamine (10 microM) and atropine (100 nM) had no effect on this inhibition. 3. The ability of CGS 21680 (0.1 nM) to stimulate the release of [3H]-acetylcholine ([3H]-ACh) from striatal nerve terminals was unaffected by the presence of bicuculline (10 microM), 2-hydroxysaclofen (100 microM), phaclofen (100 microM), naloxone (10 microM) and DPCPX (4 nM). 4. The novel A2A receptor antagonist, (E)-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine (KF 17837), blocked the CGS 21680 (1 nM)-induced inhibition of [3H]-GABA efflux with an EC50 of approximately 30 nM and also antagonized the CGS 21680 (0.1 nM)-induced stimulation of [3H]-ACh release with an EC50 of approximately 0.3 nM. 5. It is concluded that the A2A adenosine receptor is present on both GABAergic and cholinergic nerve terminals of the rat striatum and that in both the caudate-putamen and the globus pallidus this receptor inhibits [3H]-GABA release. No evidence was seen for a difference in the ligand binding sites of this receptor in the two groups of nerve terminals.

Presynaptic adenosine A2a receptors on soma and central terminals of rat vagal afferent neurons

The dorsal vagal complex of the medulla oblongata is a key centre involved in the regulation of numerous autonomic functions, including cardiovascular control. Adenosine has been implicated as a potential neuromodulator of the baroreceptor reflex, and therefore the current study has investigated the presence and characteristics of adenosine receptors on rat vagal afferent neurons. In the nodose-vagal grease gap preparation, the adenosine A2a agonist CGS-21680 evoked a depolarisation only in the presence of the selective adenosine A1 antagonist PACPX. Autoradiography using [3H]NECA (4 nM) with suppression of A1 binding enabled the first visualisation of high affinity adenosine A2 receptors in the nucleus tractus solitarius (NTS). Unilateral nodose ganglionectomy resulted in over 90% reduction in binding in the lesioned (ipsilateral) NTS compared to a sham control. Furthermore, local administration of CGS-21680 increased evoked glutamate release in the NTS, as measured by in vivo microdialysis. These data suggest the presence of presynaptic adenosine A2a receptors on both the soma and central terminals of rat vagal afferent neurons, and thereby support the hypothesis that adenosine may have a modulatory role in the baroreceptor reflex.

Evidence for functionally important adenosine A2a receptors in the rat hippocampus

Adenosine A2a receptors are not confined to dopamine-rich areas of the brain, since thermocycling analysis shows that adenosine A2a receptor mRNA is expressed also in the hippocampus (CA1, CA3 and dentate gyrus) and cerebral cortex. The expression of A2a mRNA in three main areas of the hippocampus was confirmed by in situ hybridization; A2a mRNA expression was mainly localized in the pyramidal and granular cells, the same hippocampal regions that showed adenosine A1 receptor mRNA expression. Receptor autoradiographic studies with [3H]CGS 21680 (30 nM), a selective adenosine A2a receptor agonist, showed specific binding sites in the hippocampus. The density of [3H]CGS 21680 binding was greatest in the stratum radiatum of the CA1 area, followed by the stratum oriens of the cornu Ammonis, stratum radiatum of the CA3 are and supra-granular layer of the dentate gyrus. This anatomical distribution of [3H]CGS 21680 binding was similar to the pattern of [3H]CHA binding in the hippocampus. Electrophysiological studies in the Schaffer fibers/CA1 pyramids showed that upon activation of the A2a receptors with CGS 21680 (10 nM) the ability of the adenosine A1 receptor agonist, CPA, to inhibit neuronal activity was significantly attenuated. These results show functionally important co-expression and co-localization of adenosine A2a and A1 receptors in the hippocampus. The results also suggest that adenosine A2a receptor-mediated neuromodulation is not confined to the basal ganglia, but is more widespread throughout the nervous system.

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.

Activation of adenosine A1 receptors underlies anticonvulsant effect of CGS21680

Focal injection of the adenosine A2A receptor agonist CGS21680 (2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethyl-carboxamidoaden osine) in the rat prepiriform cortex produced a reduction in the severity of bicuculline methiodide-induced motor seizures. The anticonvulsant effect of CGS21680 exhibited dose-dependency and modest potency (ED50 = 605 +/- 47 pmol/rat). Pharmacological characterization of the anticonvulsant response in the prepiriform cortex revealed a significant correlation between the potency of adenosine analogs as anticonvulsants and their respective affinities for adenosine A1 receptors in vitro. These results indicate that the low affinity of CGS21680 for adenosine A1 receptors is sufficient to account for the anticonvulsant activity of this compound in the rat prepiriform cortex.