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

Comparison of the effects of adrafinil, propentofylline, and nicergoline on behavior in aged dogs

OBJECTIVE

To compare the efficacy of adrafinil, propentofylline, and nicergoline for enhancing behavior of aged dogs.

ANIMALS

36 Beagles between 9 and 16 years old.

PROCEDURE

Dogs were randomly assigned to receive adrafinil (20 mg/kg of body weight, PO, q 24 h; n = 12), propentofylline (5 mg/kg, PO, q 12 h; 12), or nicergoline (0.5 mg/kg, PO, q 24 h; 12) for 33 days. Baseline behaviors in an open field and in kennels (home cage) were recorded before treatment. After treatment, behaviors in the open field were recorded 2 hours after drug administration on days 2, 15, and 28, and 10 hours after administration on days 7, 20, and 33. Behaviors in the home cage were recorded 2 and 7 hours after drug administration on days 4, 17, and 30.

RESULTS

Treatment with adrafinil resulted in a significant increase in locomotion in each of the open-field tests and an increase in locomotion in the home cage. This latter increase was smaller and more variable than that in the open field. Locomotion was not affected by treatment with propentofylline or nicergoline. In the open field, sniffing decreased over time in all 3 groups, but the largest decline was observed in the propentofylline group.

CONCLUSIONS AND CLINICAL RELEVANCE

Treatment with adrafinil may improve the quality of life of aged dogs by increasing exploratory behavior and alertness.

Age-dependent changes in adenosine A1 receptor and uptake site binding in the mouse brain: an autoradiographic study

Ageing is a multifactorial, inevitable event of life span, which affects neurotransmission in the CNS. Since adenosine is a major neuromodulator of the synaptic activity, it was of interest to investigate the possible modification of the adenosinergic system in the brain during ageing. Using "in vitro" quantitative autoradiography and the radioactive ligands [(3)H]Cyclohexyladenosine and [(3)H]Nitrobenzylthioinosine, we have studied the distribution of A1 adenosine receptors and adenosine uptake sites in the aged mice (26 months) compared to the young ones (3 months). Our results showed a widespread reduction in A1 receptor binding in the aged animals, which was brain area-specific, occurring in areas where adenosine plays a significant neuromodulatory role such as the hippocampus, cortex, basal ganglia, and thalamus. Interestingly, the significant reduction in NBI-sensitive adenosine uptake sites was restricted to few areas of the aged brain, mainly in thalamic nuclei. Since the alterations in the density of A1 receptors and adenosine uptake sites showed no regional correlation and since no significant changes in either neuronal or glial cell number are observed, at least in hippocampus and cortex in this mouse strain during ageing, our findings could be explained by a selective age-dependent reduction of these adenosinergic components rather than by a general neuronal cell degeneration. As adenosine depresses electrical activity in hippocampus, a downregulation of adenosinergic function could probably be related to enhanced excitability seen in hippocampal neurons of the CA1 subregion and dentate gyrus of aged animals.

Chronic effects of xanthines on levels of central receptors in mice

1. Chronic ingestion of caffeine causes a significant increase in levels of A1-adenosine, nicotinic and muscarinic receptors, serotonergic receptors, GABAA receptors and L-type calcium channels in cerebral cortical membranes from mice NIH Swiss strain mice. 2. Chronic theophylline and paraxanthine had effects similar to those of caffeine except that levels of L-type channels were unchanged. Chronic theobromine, a weak adenosine antagonist, and 1-isobutyl-3-methylxanthine (IBMX), a potent adenosine antagonist and phosphodiesterase inhibitor, caused only an increase in levels of A1-adenosine receptors. A combination of chronic caffeine and IBMX had the same effects on receptors as caffeine alone. Chronic 3,7-dimethyl-1-propargylxanthine (DMPX), a somewhat selective A2A-antagonist, caused only an increase in levels of A1-adenosine receptors. Pentoxifylline, an adenosine-uptake inhibitor inactive at adenosine receptors, had no effect on receptor levels or calcium channels. 3. A comparison of plasma and brain levels of xanthines indicated that caffeine penetrated more readily and attained somewhat higher brain levels than theophylline or theobromine. Penetration and levels were even lower for IBMX, paraxanthine, DMPX, and pentoxyfylline. 4. The results suggest that effective blockade of both A1 and A2A-adenosine receptors is necessary for the full spectrum of biochemical changes elicited by chronic ingestion of xanthines, such as caffeine, theophylline, and paraxanthine.

Expression of adenosine A2a receptors gene in the olfactory bulb and spinal cord of rat and mouse

The expression of adenosine A2a receptors (A2aR) in the mammalian striatum is well known. In contrast the exact distribution of A2aR in other regions of the central nervous system remains unclear. The aim of this study was to investigate the A2aR gene expression in the rat olfactory bulb and spinal cord, two regions which are seldom included in mapping studies. Secondly, we compared the A2aR expression in the rat and in the mouse brain. Hybridization histochemistry was performed with an S35-labelled radioactive oligonucleotide probe. The results show strong expression of A2aR in the mouse and rat striatum in accordance with previous reports. In the olfactory bulb a weak but specific expression of A2aR was found in the granular cell layer in both species. In contrast, no significant expression of the A2aR gene was observed in other parts of the brain or the rat spinal cord. The presence of the A2aR in the mammalian olfactory bulb suggests a functional role for this receptor in olfaction.

Adenosine A2A receptors and neuroprotection

The adenosine A2A receptor subtype is one of the four adenosine receptors that have been identified in the mammalian organism. In addition to being found in blood vessels, platelets and polymorphonuclear leukocytes, the A2A receptors are abundant in the central nervous system, especially in the striatum. The recent development of selective A2A receptor ligands, in particular of receptor antagonists, makes it possible to elucidate the function of A2A receptors in normal and altered conditions. Pharmacological studies have shown that A2A receptor antagonists are potentially effective for treatment of neurodegenerative processes such as Parkinson's disease. Their activity is attributed to the close anatomical and functional links between A2A receptors and dopaminergic pathways in the basal ganglia. More recently, A2A receptor antagonists have proved to be active in models of cerebral ischemia. While the mechanisms underlying the role of A2A receptors in the hypoxia/ ischemia processes remains to be clarified, it is recognized that A2A receptor antagonists counteract the effects of excitatory aminoacids, which are massively released after cerebral ischemia. Another function of A2A receptors is related to protection from seizures, but further studies are needed to elucidate their specific interaction, if any, with neuronal excitability. Altogether, the great advance recently made with the discovery of selective A2A receptor ligands provides increasing information on the function of A2A receptors and opens new perspectives for treatment of neurological disorders.

A1 and A2A adenosine receptors and A1 mRNA in mouse brain: effect of long-term caffeine treatment

The effect of oral treatment with caffeine, in doses that are known to produce marked adaptive effects, was investigated on A1 and A2A receptors in the mouse brain. Caffeine (0.1, 0.3 or 1 g/l) was added to the drinking water and the animals were sacrificed after a 14-day treatment period. Ligand binding to A1 receptors was studied, using quantitative autoradiography, with the agonist [3H]cyclohexyladenosine (CHA) and the antagonist [3H]1,3-dipropyl-8-cyclopentyl xanthine (DPCPX). Caffeine did not remain in the sections during the autoradiography experiments. Caffeine treatment (1 g/l, but not 0.1 or 0.3 g/l) tended to increase [3H]CHA binding to the CA3 subfield of the hippocampus, but in no other region studied. There was no change in the number of A1 receptors since [3H]DPCPX binding to the CA3, cerebral and cerebellar cortex was not influenced by caffeine treatment. There was similarly no change in the ability of CHA to displace [3H]DPCPX binding, suggesting that there are no major changes in the proportion of A1 receptors that are coupled to G-proteins. mRNA for the A1 receptor, measured by in situ hybridization, did not differ significantly between caffeine-treated and control mice in the structures examined. Thus, higher doses of caffeine can cause an increase in A1 agonist binding without a corresponding change in A1 mRNA or in A1 antagonist binding, suggesting that the adaptive changes seen upon prolonged caffeine treatment may be in sites different from A1 receptors. Caffeine (1 g/l) increased A2A receptors in the striatum measured as binding of the agonist [3H]CGS 21680 suggesting that up-regulation of A2A receptors may be an adaptive effect of caffeine intake.

In vivo biodistribution of [N-11C-methyl]KF 17837 using 3-D-PET: evaluation as a ligand for the study of adenosine A2A receptors

(KF 17837, (E)-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine, was 11C-labelled by methylation at N-7 of the nor-compound, KF 17440, using [11C]methyl iodide. Radiochemical conversions of 50% or 70-80% were obtained using sodium hydride or potassium carbonate, respectively, as base. Total synthesis time was 40-45 min, including isolation by semipreparative liquid chromatography. Cerebral uptake of [N-11C-methyl]KF 17837 in Cynomolgus monkeys, evaluated using positron emission tomography (PET), was so low that regional differences in distribution kinetics were revealed first after increasing injected dose 3-fold and using 3-D mode of data acquisition. At all times, the relative regional retention (maximum striatum:cerebellum: cortex approximately 1.1:1:0.8 at 20 min) was considerably different from the known relative density of A2A receptors in these regions. Radioactivity decreased more rapidly in the cortex than in the striatum and cerebellum (by 20% vs. 3-7%, respectively, between 5 and 50 min). Addition of carrier to [N-11C-methyl]KF 17837 only marginally affected the cerebral radiotracer uptake. By contrast, in the heart the initial tracer uptake was high and the elimination kinetics was enhanced by adding unlabelled carrier. We have thus shown that KF 17837 passes the blood-brain barrier, though to a very low extent. This fact and the apparently high nonspecific binding in vivo of [N-11C-methyl]KF 17837 in regions with low receptor densities limits its usefulness as a ligand for quantification of the adenosine A2A receptors in the primate brain.

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.

Chronic dipyridamole administration downregulates [3H]nitrobenzylthioinosine binding site affinity in guinea pig kidney but not heart and brain

Specific binding of the nucleoside transporter probe, [3H]nitrobenzylthioinosine, ([3H]NBMPR) was measured in washed guinea pig cardiac, renal and forebrain membranes after 14 days of treatment with dipyridamole (37.5 mg/kg, s.c., b.i.d.) or vehicle. When compared to values in vehicle-treated animals, a 100 percent increase in equilibrium dissociation constant (Kd) was observed in the kidney of dipyridamole-treated animals (0.51 +/- 0.04 to 1.0 +/- 0.06, p < 0.01). The maximal binding capacity (Bmax) was unaltered. No changes were observed in the heart or forebrain. The increase in Kd suggests that chronic dipyridamole treatment decreases the apparent binding affinity of NBMPR for kidney nucleoside transporters. Cardiac and brain nucleoside transporters may be either less susceptible to chronic dipyridamole administration or have a different adaptive mechanism.