Autoradiographic visualization of A1 adenosine receptors in rat brain with [3H]8-cyclopentyl-1,3-dipropylxanthine

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Purinergic signalling and development of the autonomic nervous system

Most early studies of the role of nucleotides in development have evidenced their crucial importance as carriers of energy in all organisms. However, an increasing number of studies are now available to suggest that purines and pyrimidines, acting as extracellular ligands specifically on receptors of the plasma membrane, may play a pivotal role throughout pre- and postnatal development in a wide variety of organisms including amphibians, birds, and mammals. Purinergic receptor expression and functions have been studied in the development of many organs, including the autonomic nervous system (ANS). Nucleotide receptors can induce a multiplicity of cellular signalling pathways via crosstalk with bioactive molecules acting on growth factors and neurotransmitter receptors which are fundamental for the development of a mature and functional ANS. Purines and pyrimidines may influence all the stages of neuronal development, including neural cell proliferation, migration, differentiation and phenotype determination of differentiated cells. Indeed, the normal development of the ANS is disturbed by dysfunction of purinergic signalling in animal models. To establish the primitive and fundamental nature of purinergic neurotransmission in the ontogeny of the ANS, in this review the roles of purines and pyrimidines as signalling molecules during embryological and postnatal development are considered.

Adenosine A₁ receptors do not play a major role in the regulation of lipogenic gene expression in hepatocytes

Activation of adenosine A₁ receptors was reported to promote fatty acid synthesis in AML-12 cells, by increasing the expression of SREBP-(1c) (sterol regulatory binding protein 1c) and FAS (fatty acid synthase). Since these findings have important therapeutic implications for the discovery of adenosine A₁ receptor agonists, further studies were undertaken to determine the expression and functional relevance of adenosine A₁ receptor in the liver. To that end, we used two classes of distinct adenosine A₁ receptor agonists: CPA (N⁶-cyclopentyl-adenosine), a full agonist and GS-9667 (2-{6-[((1R,2R)-2-hydroxycyclopentyl)-amino]purin-9-yl}(4S,5S,2R,3R)-5-[(2-fluorophenylthio)methyl]-oxolane-3,4-diol), a partial agonist. Treatment of AML-12 cells, HepG2 cells and primary human hepatocytes with either CPA or GS-9667 did not increase the gene expression of SREBP-(1c) or FAS. Furthermore, in AML-12 and HepG2 cells, CPA did not antagonize forskolin-stimulated cAMP production, a characteristic of adenosine A₁ receptor activation, indicating that these cells lack adenosine A₁ receptor function. Consistent with this finding, adenosine A₁ receptor gene expression was found to be very low and adenosine A₁ receptor protein levels were hardly detectable by radioligand binding assays in hepatic cell lines such as AML-12 and HepG2 as well as in both mouse and human liver tissues. Finally, acute treatment with adenosine A₁ receptor agonist GS-9667 had no significant effect on gene expression of both SREBP-(1c) and FAS in livers of Sprague Dawley rats. Taken together, our data suggest that the expression of adenosine A₁ receptor is too low to play a major role in the regulation of lipogenic gene expression in hepatocytes.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Improvement of postural adjustment steps in hemiparkinsonian rats chronically treated with caffeine is mediated by concurrent blockade of A1 and A2A adenosine receptors

Chronic treatment with the non-selective adenosine receptor antagonist caffeine produces full recovery of the contralateral adjusting steps in hemiparkinsonian rats. In order to disclose which adenosine receptor subtype mediates this effect, a group of hemiparkinsonian rats (n=9) was treated with caffeine (5.15 mumol/kg/day), or equimolar doses of selective A1 (DPCPX) or A2A (ZM 241385) adenosine receptor antagonists, administered in a counterbalanced order over periods of 3 weeks, interspersed with equivalent washout intervals. Treatment with ZM 241385 caused full recovery (102+/-6%) of the contralateral forepaw stepping, while the maximal effect of DPCPX was only 73+/-7% of that produced by caffeine. The maximal effect of caffeine and ZM 241385 remained stable throughout the treatment period. The response to DPCPX showed more fluctuations, but tolerance did not develop. Stepping improvement was significantly faster with DPCPX than with ZM 241385, while caffeine had intermediate values. Stepping decrease after treatment interruption was faster with ZM 241385 than with caffeine, while DPCPX had intermediate values. In other experiments with the same rats, addition of the A2AR agonist CGS 21680 (5.15 mumol/kg) or the A1R agonist CCPA (2.71 mumol/kg) during the second week of caffeine treatment reversed the improvement of contralateral stepping by 59+/-4% and 30+/-3%, respectively. The combined treatment with CGS 21680 and CCPA caused complete reversal of the contralateral stepping recovery afforded by caffeine, which was more than additive (114+/-5%) compared with the sum of the maximal inhibition produced by either agonist administered alone (89+/-4%). In all cases, after interrupting the adenosine agonists, the effect of caffeine was fully restored. None of the aforementioned treatments induced significant changes in the stepping of the ipsilateral forepaw. Collectively, these results suggest that the improvement of postural adjustments induced by chronic treatment with low doses of caffeine in hemiparkinsonian rats is mediated by concurrent blockade of A1 and A2A adenosine receptors, with a larger involvement of the latter.

Rofecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor increases pentylenetetrazol seizure threshold in mice: possible involvement of adenosinergic mechanism

Multiple lines of investigations have explored the role of cyclooxygenases (COX) in epilepsy and related neuropsychiatric disorders. Cyclooxygenase particularly, COX-2 expression was found to increase in brain during seizure paradigms. The present study was carried out to investigate the effect of rofecoxib, a selective COX-2 inhibitor against pentylenetetrazol (PTZ i.v.) seizure threshold in mice. The study was further extended to elucidate the possible involvement of adenosinergic mechanism in mediating its anticonvulsant action. Minimal dose of PTZ (i.v., mg/kg) needed to induce different phases (myoclonic jerks, generalized clonus and tonic extension) of PTZ convulsions were noted as an index of seizure threshold. Acute administration of rofecoxib (4mg/kg, i.p.) before PTZ infusion produced an elevation of seizure threshold for all the phases of convulsions. A lower dose of rofecoxib (2mg/kg, i.p.) showed an increase in PTZ seizure threshold for the onset of myoclonic jerks and tonic extension phases but not for generalized clonus. A still lower dose of rofecoxib (1mg/kg, i.p.) failed to increase the threshold in any of the convulsive phases induced by PTZ i.v. infusion. Pretreatment with sub-effective dose of rofecoxib (1mg/kg, i.p.) enhanced the action of sub-protective doses of either adenosine (25mg/kg, i.p.) or 2-chloroadenosine (1 or 2mg/kg, i.p.) in increasing the seizure threshold. On the contrary, treatment with caffeine (100 or 200mg/kg, i.p.) or theophylline (50 or 100mg/kg, i.p.), both non-selective A(1)/A(2) adenosine receptor antagonists reversed the anticonvulsant effect of rofecoxib (4mg/kg, i.p.). Further, dipyridamole (5mg/kg, i.p.), an adenosine uptake inhibitor displayed an anticonvulsant effect with rofecoxib (1mg/kg, i.p.). The study for the first time demonstrated the possible involvement of adenosinergic system in the anticonvulsant effects of rofecoxib against PTZ i.v. seizure threshold paradigm in mice.

Consequences of Prolonged Caffeine Administration and Its Withdrawal on Pilocarpine- and Kainate-induced Seizures in Rats

PURPOSE

To investigate the consequences of caffeine consumption on epileptic seizures, we used the pilocarpine and the kainate models of epilepsy. We hypothesized that prolonged caffeine consumption or its withdrawal would alter adenosine levels and hence alter seizure susceptibility.

METHODS

We administered a 0.1% caffeine solution in the drinking water of adult male Wistar rats over a 2-week period. We challenged another group of animals with the same doses of pilocarpine or kainate 12 h after the withdrawal of the same caffeine-administration protocol.

RESULTS

This did not alter the threshold for the induction of seizures by a subconvulsant dose of pilocarpine (200 mg/kg, i.p.) or kainic acid (8 mg/kg, i.p.). Similarly, challenging another group of animals with the same doses of pilocarpine or kainate 12 h after the withdrawal of the same caffeine-administration protocol did not lead to any significant changes in seizures.

CONCLUSIONS

With the pilocarpine model of epilepsy, we were not able to find any significant difference in seizure profile that could stem from either caffeine administration or its withdrawal. Despite the extensive laboratory evidence on the convulsant properties of xanthine derivatives in animal models of epilepsy, such strong evidence is lacking in clinical settings. Our current findings with the administration of caffeine at doses similar to those of daily life both support and confirm the clinical experience.

Evaluation of radioiodinated 8-Cyclopentyl-3-[(E)-3-iodoprop-2-en-1-yl]-1-propylxanthine ([*I]CPIPX) as a new potential A1 adenosine receptor antagonist for SPECT

8-Cyclopentyl-3-[(E)-3-[(131)I]iodoprop-2-en-1-yl]-1-propylxanthine (2*) was generated by iododestannylation of the tributyl-stannyl-precursor with [(131)I]NaI and chloramine T. The radiochemical yield of 2* was 82 +/- 4%, and the purity exceeded 98%. The specific activity was 33 +/- 19 GBq/micromol. Affinities for rat, pig and human A(1) adenosine receptors (A(1)ARs) were in the low nanomolar range, but poor selectivity for the human A(1)AR over the A(2A)AR was found. Additionally, in vitro and ex vivo autoradiographic studies revealed high unspecific binding which makes this ligand unsuitable for SPECT imaging.

High level of adenosine A1 receptor-like immunoreactivity in the CA2/CA3a region of the adult rat hippocampus

We describe the immunocytochemical distribution of adenosine A1 receptors in the rat hippocampus. Adenosine A1 receptor-like immunoreactivity was seen on the cell soma and dendrites of pyramidal cells and the cell soma and proximal part of dendrites of granule cells, but not on glial cells. Developmentally, adenosine A1 receptor-like immunoreactivity was diffuse on postnatal day 7 and increased in intensity in individual cells by day 21. In the CA2/CA3a region, the adult pattern of A1 receptor distribution was established by day 28. In the adult rat hippocampus, rostrocaudal inspection revealed that immunoreactivity in CA2/CA3a was greatest. Confocal microscopy revealed differences in the staining patterns for the adenosine A receptor and synaptophysin, a marker of presynaptic terminals. This result suggests that the adenosine A1 receptor might have postsynaptic physiological functions. Double-labeling of adenosine A1 receptors and anterogradely-labeled fibers from the supramammillary nucleus showed that the fibers from the supramammillary nucleus terminate directly on the cell soma of the A1 receptor-immunopositive neurons in CA2/CA3a and the dentate gyrus. These results indicate that the adenosine A 1 receptor in CA2/CA3a and the dentate gyrus are in a position to regulate hippocampal theta activity and that resultant strong synaptic depression in CA2/CA3a could play a role in regulating the intrinsic signal flow between CA3 and CA1.

A1 adenosine receptor antagonism improves glucose tolerance in Zucker rats

The A1 adenosine receptor (A1ar) antagonist 1,3-dipropyl-8-(p-acrylic)-phenylxanthine (BW-1433) was administered to lean and obese Zucker rats to probe the influence of endogenously activated A1ars on whole body energy metabolism. The drug induced a transient increase in lipolysis as indicated by a rise in serum glycerol in obese rats. The disappearance of the response by day 7 of chronic studies was accompanied by an increase in A1ar numbers. Glucose tolerance tests were administered to rats treated with BW-1433. Peak serum insulin levels and areas under glucose curves (AUGs) were 34 and 41% lower in treated obese animals than in controls, respectively, and 19 and 39% lower in lean animals. With chronic administration (6 wk), AUGs decreased 47 and 33% in obese and lean animals, respectively. There was no effect of BW-1433 in either lean or obese rats on weight gain or percent body fat. Thus the major sustained influence of whole body A1ar antagonism in both lean and obese animals was an increase in whole body glucose tolerance at lower levels of insulin.

Distribution of adenosine receptors in the postmortem human brain: an extended autoradiographic study

Whole-hemisphere sections from six subjects were used in a quantitative autoradiographic study to characterize and to investigate the distribution of adenosine receptors, using [3H]DPCPX, [3H]CGS 21680, and [3H]SCH 58261 as radioligands. [3H]DPCPX-binding showed the pharmacology expected for adenosine A1 receptors and is therefore taken to mirror adenosine A1 receptors. Adenosine A1 receptors were widely distributed, with the highest densities in the stratum radiatum/pyramidale of the hippocampal region CA1. Adenosine A1 receptors were nonhomogeneously distributed in nucleus caudatus, globus pallidus, and cortical areas: In the cingulate and frontal cortex the deep layers showed the highest labeling, while in the occipital, parietal, temporal, and insular cortex it was highest in the superficial layers. In addition, we found very high levels of adenosine A1 receptors in structures known to be important for cholinergic transmission, especially the septal nuclei. The Bmax values and KD values for [3H]DPCPX-binding in stratum radiatum/pyramidale of CA1 and the superficial layer of insular cortex were 598 and 430 fmol/mg gray matter and 9.9 and 14.2 nM, respectively. [3H]CGS 21680-binding was multiphasic, but showed the pharmacology expected for adenosine A2A receptors and was taken to represent them. Adenosine A2A receptors were abundant in putamen, nucleus caudatus, nucleus accumbens, and globus pallidus pars lateralis. Specific [3H]CGS 21680-binding was also found in certain thalamic nuclei and throughout the cerebral cortex. The adenosine A2A receptor antagonist radioligand [3H]SCH 58261 was also found to label these extrastriatal structures. Thus, adenosine A2A receptors seem to be more widely distributed in the human brain than previously recognized.

Heteromultimerization of G-protein-gated inwardly rectifying K+ channel proteins GIRK1 and GIRK2 and their altered expression in weaver brain

The weaver (wv) gene (GIRK2) is a member of the G-protein-gated inwardly rectifying potassium (GIRK) channel family, known effectors in the signal transduction pathway of neurotransmitters such as acetylcholine, dopamine, opioid peptides, and substance P in modulation of neurotransmitter release and neuronal excitability. GIRK2 immunoreactivity is found in but not limited to brain regions known to be affected in wv mice, such as the cerebellar granule cells and dopaminergic neurons in the substantia nigra pars compacta. It is also observed in the ventral tegmental area, hippocampus, cerebral cortex, and thalamus. GIRK2 and GIRK1, a related family member, have overlapping yet distinct distributions in rat and mouse brains. In regions where both channel proteins are expressed, such as the cerebral cortex, hippocampus, and cerebellum, they can be co-immunoprecipitated, indicating that they interact to form heteromeric channels in vivo. In the brain of the wv mouse, GIRK2 expression is decreased dramatically. In regions where GIRK1 and GIRK2 distributions overlap, both GIRK1 and GIRK2 expressions are severely disrupted, probably because of their co-assembly. The expression patterns of these GIRK channel subunits provide a basis for consideration of the machinery for neuronal signaling as well as the differential effects of the wv mutation in various neurons.

Long-term treatment with some methylxanthines decreases the susceptibility to bicuculline- and pentylenetetrazol-induced seizures in mice. Relationship to c-fos expression and receptor binding

The effects of long-term oral administration of low doses of caffeine (0.3 g/l) and its metabolites theophylline, theobromine and paraxanthine (each at 0.5 g/l in drinking water) on bicuculline- and pentylenetetrazol (PTZ)-induced seizures and c-fos expression were studied in mice. In addition, adenosine and benzodiazepine receptor density was examined. The plasma levels of the methylxanthines were much higher during the active period at night than during the day. The maximal level of caffeine was 14 microM. Brain theophylline levels (8-13 nmol/g) tended to be higher and more constant than brain caffeine levels in caffeine-consuming mice. Clonic seizures induced by bicuculline (4 mg/kg i.p.) were significantly reduced in severity by 14 day caffeine treatment and mortality was also reduced. Long-term treatment with caffeine metabolites was less effective. The seizures induced by PTZ (60 mg/kg i.p.) were also significantly reduced by long-term caffeine treatment. After bicuculline or PTZ treatment, c-fos mRNA expression was weaker in the cerebral cortex in animals receiving caffeine, irrespective of whether the animals had seizures or not. No significant changes in the binding of adenosine receptor ligands or benzodiazepines were seen after long-term caffeine treatment. These results show that long-term treatment with caffeine in a dose that is commonly seen in humans decreases the seizures induced by bicuculline, and to a lesser extent, those induced by PTZ. This may be related to a decreased neuronal excitability. The effect is due to the combined effects of theophylline, to which caffeine is metabolized in brain, and caffeine itself, but could not be ascribed to changes in A1 and A2A adenosine or benzodiazepine receptors.

Tissue distribution of adenosine receptor mRNAs in the rat

1. A degree of ambiguity and uncertainty exists concerning the distribution of mRNAs encoding the four cloned adenosine receptors. In order to consolidate and extent current understanding in this area, the expression of the adenosine receptors has been examined in the rat by use of in situ hybridisation and the reverse transcription-polymerase chain reaction (RT-PCR). 2. In accordance with earlier studies, in situ hybridisation revealed that the adenosine A1 receptor was widely expressed in the brain, whereas A2A receptor mRNA was restricted to the striatum, nucleus accumbens and olfactory tubercle. In addition, A1 receptor mRNA was detected in large striatal cholinergic interneurones, 26% of these neurones were also found to express the A2A receptor gene. Central levels of mRNAs encoding adenosine A2B and A3 receptors were, however, below the detection limits of in situ hybridisation. 3. The more sensitive technique of RT-PCR was then employed to investigate the distribution of adenosine receptor mRNAs in the central nervous system (CNS) and a wide range of peripheral tissues. As a result, many novel sites of adenosine receptor gene expression were identified. A1 receptor expression has now been found in the heart, aorta, liver, kidney, eye and bladder. These observations are largely consistent with previous functional data. A2A receptor mRNA was detected in all brain regions tested, demonstrating that expression of this receptor is not restricted to the basal ganglia. In the periphery A2A receptor mRNA was also found to be more widely distributed than generally recognised. The ubiquitous distribution of the A2B receptor is shown for the first time, A2B mRNA was detected at various levels in all rat tissues studied. Expression of the gene encoding the adenosine A3 receptor was also found to be widespread in the rat, message detected throughout the CNS and in many peripheral tissues. This pattern of expression is similar to that observed in man and sheep, which had previously been perceived to possess distinct patterns of A3 receptor gene expression in comparison to the rat. 4. In summary, this work has comprehensively studied the expression of all the cloned adenosine receptors in the rat, and in so doing, resolves some of the uncertainty over where these receptors might act to control physiological processes mediated by adenosine.

Adenosine A1 receptors are located predominantly on axons in the rat hippocampal formation

The nucleoside adenosine exerts potent biological effects via specific receptors, including the inhibitory A1 adenosine receptor (A1AR). In the hippocampus A1ARs play an important role in regulating neuronal activity. However, the cellular sites of hippocampal A1ARs are undefined. Using in situ hybridization, receptor autoradiography, and single- and double-label immunocytochemistry techniques, we have characterized the cellular sites of A1AR expression in the rat hippocampus. In situ hybridization and receptor autoradiography studies revealed strikingly different patterns of labeling. In situ hybridization studies revealed heaviest labeling of cell bodies in the granular layer of the dentate gyrus and the pyramidal layers of Ammon's horn. In contrast, using [3H]DPCPX, we observed heavy specific labeling over the neuropil in the dentate hilus stratum moleculare, stratum lacunosum-moleculare, stratum radiatum, and stratum oriens, and little labeling over cell bodies. Using single-label immunocytochemistry, A1AR immunoreactivity was found to be heaviest over fibers in regions corresponding with heavy [3H]DPCPX labeling. Double-label florescent confocal microscopy was then used to determine the identity of labeled fibers. A1AR immunoreactivity was found to co-localize with SMI-31 that labels axons, but not with MAP2a,b that labels cell bodies and dendrites, or with synaptophysin that labels synapses. These data identify axons as the predominant site of A1AR expression in hippocampus. Activation of A1ARs may be a powerful mechanism by which adenosine alters axonal transmission to inhibit neurotransmitter release.

The ontogeny of cardiac and neural A1 adenosine receptor expression in rats

To provide insights into the sites and mechanisms of adenosine action during fetal life, the ontogeny of A1 adenosine receptor (A1AR) expression was studied in rats. Using in situ hybridization and receptor binding assays, A1AR expression was examined at gestational days (GD) 8, 11, 14, 17, and 21. At GD 8, A1AR mRNA expression was detected in the myocardium but not in other fetal structures. At GD 11, A1AR mRNA was present in the atria, but not in the ventricles or neural structures. At GD 14, A1AR mRNA was present in the atria and the pontine neuroectoderm, the thalamus, and the ventral horn of the spinal cord. At GD 14, A1ARs were first detectable using [3H]DPCPX (8-cyclopentyl-1,3-dipropylxanthine). Functional binding sites were highest in the atria and present at low levels in brain. When GD 17 was reached, patterns of A1AR expression in the brain were similar to those observed in adult animals. Sites of heavy labeling included the spinal cord, medulla, pons, midbrain, thalamus, and the hippocampus. When pre- and postnatal tissue concentrations of A1ARs were compared, cardiac A1AR receptor concentrations were similar. However, postnatal brain A1AR levels were considerably greater than in fetuses. These data identify the atria as a prominent site of fetal A1AR expression and show that the A1AR gene is one of the earliest expressed G protein-coupled receptor genes in the heart. A1AR expression in brain lags behind cardiac expression in early stages of gestation and exceeds cardiac A1AR expression with progressive maturation. A1ARs may therefore influence the heart and brain during critical periods of fetal development.

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.

Adenosine antagonists have differential effects on induction of long-term potentiation in hippocampal slices

How adenosine leakage and tetanic release might affect long-term potentiation (LTP) was investigated by applying adenosine antagonists 8(p-sulfophenyl)theophylline (8SPT) or 8-cyclopentyl-3,7-dihydro-1,3-dipropyl-1H-purine-2,6-dione (DPCPX) to slices, while recording CA1 field EPSPs and population spikes. In the first series of experiments, we applied weak double tetani (at 100 Hz, for 1 s) that were subliminal for evoking LTP in initial control runs. In the presence of 8SPT--at concentrations (10-50 microM) which block both A1 and A2 receptors--the same tetani consistently evoked LTP of population spikes but not of excitatory postsynaptic potentials (EPSPs), whereas DPCPX (50 nM), which blocks only A1 receptors, facilitated LTP of both EPSPs and population spikes. These results are consistent with previous evidence that tetanic adenosine release on the one hand depresses LTP via A1 receptors but on the other facilitates LTP via A2 receptors. In a second set of experiments, 8SPT (50-100 microM) did not prevent the induction of LTP of both EPSPs and population spikes by stronger tetanic stimulation. Therefore A2 receptor activation is not essential for the induction of LTP when stronger tetani are applied. Overall, the main effect of endogenous adenosine release is to oppose LTP induction.

Effects of neonatal exposure to caffeine on adenosine A1 receptor ontogeny using autoradiography

The ontogeny of adenosine A1 receptor density was assessed via autoradiographical analysis of [3H]cyclohexyladenosine ([3H]CHA) binding in brains of 14-31-day-old rats as a function of exposure to caffeine over postnatal days 2-6. This exposure period was analogous to the period during which human infants are administered caffeine as treatment for apnea of prematurity. [3H]CHA binding was greatest in CA1 and CA3 hippocampus in both caffeine-exposed and control rats across all ages. Within the anterior, ventral, lateral and medial regions of the thalamus of unmanipulated rats, [3H]CHA binding did not change with age. In caffeine-exposed rats, however, [3H]CHA binding increased significantly within these thalamic subregions as the rats aged. In addition, with age in both treatment groups, the molecular and granular layers of the cerebellum and the CA1 and dentate gyrus of the hippocampus displayed increasing [3H]CHA density. Furthermore, regardless of age, [3H]CHA binding was decreased in the molecular layer of neonatally caffeine-exposed animals as compared to controls. Thus, limited exposure to caffeine within the first postnatal week altered the subsequent expression of adenosine A1 receptors in most subregions of the thalamus and in the molecular layer of the cerebellum.

Caffeine augmentation of electroconvulsive seizures

Caffeine has been used clinically to increase seizure length in electroconvulsive treatment (ECT). The present study was designed to establish an animal model of caffeine-augmented seizures for further study of mechanisms and effects of pharmacological manipulation of seizure length. Increasing doses of caffeine (0-200 mg/kg, IP) were given before electroconvulsive stimulation (ECS) in rats and resulting seizure lengths were quantified by timing of classical tonic-clonic convulsive movements. With this paradigm, caffeine led to a dose-dependent increase in seizure duration. This proconvulsant action of caffeine was detectable within 1 min after dosing, persisted for at least 230 min and was reversible. The results suggest that seizure length is a practicable measure in pharmacological modification of electroconvulsive seizures. They also suggest that pharmacologically-modified ECS can be modeled effectively in animals.

1,3-Dipropyl-8-cyclopentyl xanthine (DPCPX): a useful tool for pharmacologists and physiologists?

There is now ample evidence in the literature to demonstrate the selectivity of action of DPCPX for adenosine A1 vs other adenosine receptor types in tissues derived from a wide range of species. However, care has to be exercised to ensure that its physiochemical properties do not result in the production of quantitatively misleading data. In experiments using canine tissues the still limited data available in the literature clearly and consistently demonstrate that DPCPX has a lower affinity than expected in preparations which would be anticipated to contain A1 receptors. A range of in vitro experiments also demonstrate that DPCPX is not always a "neutral" or "silent" antagonist. The mechanism underlying these additional effects is unclear, but may result from an ability of the compound to disrupt the normal interaction of the A1 receptor with Gi, or may be indicative of a lack of specificity of action. The limited evidence available suggests that the compound retains its selectivity and specificity of action in vivo, and early work indicates that the compound is proving to be a useful tool with which to explore the potential of activation of adenosine A1 receptors as an important mechanism in physiological and pathophysiological processes.

On the high affinity binding site for [3H]-1,3-dipropyl-8-cyclopentylxanthine in frog brain membranes

1. Radioligand binding properties of the adenosine receptor ligands, [3H]-1,3-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), and [3H]-R-phenylisopropyladenosine ([3H]-R-PIA) were investigated in frog brain membranes. 2. The specific binding of the adenosine antagonist, [3H]-DPCPX to frog brain membranes showed one binding site with Kd and Bmax values of 43.8 nM and 0.238 +/- 0.016 pmol mg-1 protein, respectively. Guanosine 5'-triphosphate (GTP, 100 microM) decreased to 72 +/- 7% and Mg2+ (8 mM) increased to 121 +/- 3% [3H]-DPCPX (40 nM) binding to frog brain membranes. 3. [3H]-DPCPX saturation binding experiments performed in the presence of Mg2+ (8 mM), or in the presence of GTP showed that Mg2+ ions decreased the Kd value of [3H]-DPCPX to 14 nM, and GTP increased this value to 65.6 nM. Bmax values were not significantly (P > 0.05) modified (0.261 +/- 0.018 pmol mg-1 protein, with Mg2+, and 0.266 +/- 0.026 pmol mg-1 protein, in presence of GTP) by the presence of Mg2+ or GTP. 4. The specific binding of [3H]-R-PIA (15 nM) was decreased to 37 +/- 6% by GTP (100 microM) and increased to 123 +/- 4% by Mg2+ (8 mM). [3H]-R-PIA saturation binding experiments performed in the presence of Mg2+ (8 mM) showed one binding site with Kd and Bmax values of 0.9 nM and 0.229 +/- 0.008 pmol mg-1 of protein, respectively. 5. The concentration-inhibition curves of adenosine agonists and antagonists versus [3H]-DPCPX binding showed the following order of potencies: CPA> R-PIA~ NECA> S-PIA> > CGS 21680, for the agonists, and XAC ~-DPCPX> > XCC> PACPX, for the antagonists.6. The present results suggest that the adenosine binding site in the frog brain membranes is G-protein coupled, but that the antagonist affinities and the pharmacological profile is different from the Al or A2 adenosine receptors.

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.

Magnesium-dependent enhancement of endogenous agonist binding to A1 adenosine receptors: a complicating factor in quantitative autoradiography

Quantitative autoradiography was used to investigate the effects of Mg2+ on agonist and antagonist binding to A1 receptors in rat striatum. A1 receptors were labelled with the selective agonist N6-[3H]cyclohexyladenosine ([3H]CHA) or the selective antagonist 1,3-[3H]dipropyl-8-cyclopentylxanthine ([3H]DPCPX). Mg2+ had no significant effect on equilibrium binding constants for [3H]CHA [control: KD (95% confidence interval) of 0.34 (0.15-0.80) nM and Bmax of 267 +/- 8 fmol/mg of gray matter; with 10 mM Mg2+: KD of 0.8 (0.13-4.9) nM and Bmax of 313 +/- 8.9 fmol/mg of gray matter] or [3H]DPCPX [control: KD of 0.54 (0.30-0.99) nM and Bmax of 256 +/- 2.3 fmol/mg of gray matter; with 10 mM Mg2+: KD of 1.54 (0.2-11.0) nM and Bmax of 269 +/- 35.7 fmol/mg of gray matter]. In contrast, Mg2+ slowed the apparent association rate for both ligands; this was observed as a shift from a one-component to a two-component model for [3H]DPCPX. Mg2+ also affected the dissociation rates of both ligands; for [3H]CHA, dissociation in the presence of Mg2+ was not detected. Mg2+ produced a concentration-dependent inhibition of [3H]CHA binding only prior to equilibrium. HPLC was performed on untreated sections, sections preincubated with adenosine deaminase (ADA), and sections preincubated with ADA and incubated with ADA in the absence or presence of Mg2+. Adenosine was found in measurable quantities under all conditions, and the concentration was not influenced by Mg2+ or by the inclusion of GTP in the preincubation medium. From these data, we conclude the following: (a) adenosine is present and may be produced continuously in brain sections; (b) ADA is not capable of completely eliminating the produced adenosine; (c) Mg2+ apparently does not influence adenosine production or elimination; (d) A1 receptor-guanine nucleotide binding protein coupling is maximal in this preparation; and (e) Mg2+ decreases the dissociation rate of bound endogenous adenosine from A1 receptors, thus limiting the access of [3H]CHA and [3H]DPCPX to the receptors. Thus, enhancement of endogenous adenosine binding to A1 receptors by Mg2+ is a complicating factor in receptor autoradiography and may be so in other preparations as well.

Adenosinergic inhibition in hippocampus is mediated by adenosine A1 receptors very similar to those of peripheral tissues

The amplitude of the orthodromically evoked population spike (PS) of CA1 neurons was used to investigate quantitatively adenosine receptor antagonism in guinea pig hippocampal slices. Increasing concentrations of the highly selective adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 3-100 nM) produced parallel, rightward shifts of the dose-response curve for the N6-cyclopentyladenosine (CPA)-induced decrease in PS amplitude. Schild plot analyses of the respective antagonism data obtained in both the presence and virtual absence of endogenous adenosine yielded apparent dissociation constants (KD) of DPCPX at the hippocampal A1 receptor of 3.3 and 3.6 nM, respectively. This indicates that the inhibitory tonus generated by endogenously produced adenosine is due to tonic activation of A1 receptors. The KD values agree well with the binding affinity of DPCPX to A1 receptors determined in brain tissue sections. Since, in our preparation, Schild plot analyses of DPCPX antagonism revealed KD values close to those reported for other tissues, it is concluded that the central A1 receptor mediating adenosinergic inhibition is pharmacologically not distinct from A1 receptors identified in peripheral tissues.

Adenosine receptors in the central nervous system

Two major subclasses of adenosine receptors have been distinguished in the central nervous system, termed A1 and A2. They are coupled to G-proteins and regulate the activity of adenylyl cyclase, potassium channels and several other effector systems. Autoradiographic studies have shown that A1 receptors are mainly found in the hippocampus and the cerebellum, whereas A2 receptors are almost exclusively located in the striatum and olfactory tubercle. Furthermore, a novel adenosine binding protein was identified in bovine striatum by radioligand binding with [3H]5'-N-ethylcarboxamidoadenosine ([3H]NECA). The pharmacological profile of this NECA binding protein has been determined in competition experiments with adenosine receptor ligands. It can be distinguished from that of A2 adenosine receptors and other adenosine binding proteins such as S-adenosylhomocysteine hydrolase and the adenosine transporter.

Adenosine receptor subtypes in the brainstem mediate distinct cardiovascular response patterns

A limited occipital craniotomy was conducted on urethane-chloralose anesthetized, spontaneously breathing rats to expose the caudal medulla in the region of the obex. Microinjections of highly selective agonists for adenosine receptor subtypes were made into the medial region of the caudal nucleus tractus solitarius (NTS) at the level of the posterior portion of the area postrema. Cardiorespiratory parameters were subsequently recorded for a 60-min test period following microinjection of drug or vehicle solutions. The following selective adenosine receptor agonists were used: the A1 agonist, N6-cyclopentyladenosine (CPA), which is 480-fold selective for A1 receptors in rat brain binding assays, and the A2 agonist, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680), which is 170-fold selective for A2 receptors in rat brain binding studies and over 1500-fold selective in functional assays. The results showed that distinct and converse cardiovascular response patterns were elicited by these selective agonists for adenosine receptor subtypes following microinjections into the caudal NTS. Specifically, CGS 21680 selectively elicited potent dose-related decreases in mean arterial blood pressure (ED50 = 0.064 nmol/kg) and dose-related decreases in pulse pressure (ED50= 0.058 nmol/kg). Conversely, CPA selectively elicited potent dose-related increases in mean arterial blood pressure (ED50 = 0.62 nmol/kg) and dose-related increases in pulse pressure (ED50 = 0.70 nmol/kg). Additionally, the overall agonist-mediated response patterns were dramatically different wherein the CGS agonist exhibited a considerably more rapid time course in eliciting its hypotensive responses whereas CPA exhibited a more delayed and substantially longer time course to exert its hypertensive responses. Additionally, these distinct and converse cardiovascular response patterns were further shown to be receptor-selective since the depressor responses elicited by the A2 receptor agonist, CGS 21680, and the pressor responses elicited by the A1 receptor agonist, CPA, were completely and selectively blocked, respectively, by the selective A2 receptor antagonist, CGS 15943A, and the selective A1 receptor antagonist, DPCPX. Taken together, these findings provide persuasive in vivo evidence showing that pharmacologic activation of adenosine receptor subtypes in the caudal NTS of rats elicits specific response patterns with selective and opposite actions on cardiorespiratory behavior. These data also indicate that separate physiologic responses are specifically mediated by A2 receptors in the intact nervous system and thereby lend additional support to the case for using in vivo models to assess the functional role of adenosine A2 receptors in brain function.