[3H]N6-(L-Phenylisopropyl) adenosine binding in brains from young and old rats

Functional coupling between adenosine A1 receptors and G-proteins in rat and postmortem human brain membranes determined with conventional guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding or [35S]GTPγS/immunoprecipitation assay

Adenosine signaling plays a complex role in multiple physiological processes in the brain, and its dysfunction has been implicated in pathophysiology of neuropsychiatric diseases such as schizophrenia and affective disorders. In the present study, the coupling between adenosine A₁ receptor and G-protein was assessed by means of two [³⁵S]GTPγS binding assays, i.e., conventional filtration method and [³⁵S]GTPγS binding/immunoprecipitation in rat and human brain membranes. The latter method provides information about adenosine A₁ receptor-mediated Gα_i-3 activation in rat as well as human brain membranes. On the other hand, adenosine-stimulated [³⁵S]GTPγS binding determined with conventional assay derives from functional activation of Gα_i/o proteins (not restricted only to Gα_i-3) coupled to adenosine A₁ receptors. The determination of adenosine concentrations in the samples used in the present study indicates the possibility that the assay mixture under our experimental conditions contains residual endogenous adenosine at nanomolar concentrations, which was also suggested by the results on the effects of adenosine receptor antagonists on basal [³⁵S]GTPγS binding level. The effects of adenosine deaminase (ADA) on basal binding also support the presence of adenosine. Nevertheless, the varied patterns of ADA discouraged us from adding ADA into assay medium routinely. The concentration-dependent increases elicited by adenosine were determined in 40 subjects without any neuropsychiatric disorders. The increases in %E_max values determined by conventional assay according to aging and postmortem delay should be taken into account in future studies focusing on the effects of psychiatric disorders on adenosine A₁ receptor/G-protein interaction in postmortem human brain tissue.

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.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Effect of aging on cerebral A1 adenosine receptors: A [18F]CPFPX PET study in humans

Cerebral A(1) adenosine receptors (A(1)AR) fulfill important neuromodulatory and homeostatic functions. The present study examines possible age-related A(1)AR changes in living humans by positron emission tomography (PET) and the A(1)AR ligand [(18)F]CPFPX. Thirty-six healthy volunteers aged 22-74 years were included. The apparent binding potential (BP'2) of [(18)F]CPFPX in various cerebral regions was calculated non-invasively using the cerebellum as reference region. In addition, the total distribution volume (DV't) was assessed in 10 subjects undergoing arterial blood sampling. There was no significant association between regional DV't and age, gender, caffeine consumption or sleep duration. BP'2 showed a significant age-dependent decrease in all regions except cingulate gyrus (p=0.062). The BP'2 decline ranged from -17% (striatum) to -34% (postcentral gyrus), the average cortical decline being -23%. There was no significant effect of gender, caffeine consumption and sleep duration on BP'2. In line with in vitro animal studies, the present in vivo PET study detected an age-dependent A(1)AR loss in humans that may be of pathophysiological importance in various neurological diseases associated with aging. Because of the discrepant results of the invasive (DV't) and the non-invasive (BP'2) analyses the present study needs further validation.

Ageing-related decline in adenosine A1 receptor binding in the rat brain: An autoradiographic study

The adenosine system has important neuromodulatory and neuroprotective functions in the brain. Several lines of evidence suggest that ageing is associated with major alterations in the adenosine system, which may be partially responsible for changes in sleep, mood, and cognition. In the present study, we examined adenosine A1 receptor density in the rat brain by means of quantitative autoradiography to obtain a detailed anatomical overview of the changes during ageing. A1 receptor binding was assessed in young, old, and senescent animals of 3, 24, and 30 months old, respectively. There was a clear age-dependent reduction in adenosine A1 receptors in most of the brain areas examined, but the magnitude of this reduction varied greatly among regions. Also, whereas some regions displayed a gradual decline in A1 binding sites across the three age classes, other regions showed a particularly strong decrease between the ages of 24 and 30 months. For example, whereas the hippocampus and thalamus showed a gradual decline in A1 binding, some cortical and septal regions showed a more abrupt decline after the age of 24 months. Since particularly in rats many studies have used animals at the age of 24 months or even less, the ageing-related decline in adenosine A1 signaling might have been underestimated.

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.

Age-dependent changes in adenosine receptors are not modified by life-long intermittent alcohol administration

Autoradiography and in situ hybridisation were used to examine age-dependent changes in adenosine receptors in male rats and to determine if life-long (94 weeks) intermittent ethanol consumption had any additional effect. Adenosine A2A receptors in striatum, as assessed by [3H]CGS 21680 binding, decreased by approximately 20% between the ages 6 and 99 weeks. Since dopamine D2 receptors and the mRNA for preproenkephalin also decreased there appears to be a loss of A2A-D2 receptor-bearing striatopallidal cells. Life-long ethanol consumption had no additional effect. Adenosine A1 receptors, as determined by [3H]DPCPX binding, did not decrease with age in any region of the brain, but increased slightly in the cerebellum. In substantia nigra, the increase in [3H]DPCPX binding upon addition of GTP was eliminated. Surprisingly, the amount of A1 receptor mRNA decreased significantly with age in most of the examined regions, including the cerebellum. There was no additional effect of ethanol treatment. It is suggested that age alters the number of cells that express A2A receptors, the turnover of A1 receptors, and in some regions their coupling to G proteins, but that life-long intermittent ethanol exposure has little additional effect.

Age-dependent changes of presynaptic neuromodulation via A1-adenosine receptors in rat hippocampal slices

The presynaptic neuromodulation of stimulation-evoked release of [3H]-acetylcholine by endogenous adenosine, via A1-adenosine receptors, was studied in superfused hippocampal slices taken from 4-, 12- and 24-month-old rats. 8-Cyclopentyl-1,3-dimethylxanthine (0.25 microM), a selective A1-receptor antagonist, increased significantly the electrical field stimulation-induced release of [3H]-acetylcholine in slices prepared from 4- and 12-month-old rats, showing a tonic inhibitory action of endogenous adenosine via stimulation of presynaptic A1-adenosine receptors. In contrast, 8-cyclopentyl-1,3-dimethylxanthine had no effect in 24-month-old rats. 2-Chloroadenosine (10 microM), an adenosine receptor agonist decreased the release of [3H]-acetylcholine in slices taken from 4- and 12-month-old rats, and no significant change was observed in slices taken from 24-month-old rats. In order to show whether the number/or affinity of the A1-receptors was affected in aged rats, [3H]-8-cyclopentyl-1,3-dimethylxanthine binding was studied in hippocampal membranes prepared from rats of different ages. Whereas the Bmax value was significantly lower in 2-year-old rats than in younger counterparts, the dissociation constant (Kd) was not affected by aging, indicating that the density rather than the affinity of adenosine receptors was altered. Endogenous adenosine levels present in the extracellular space were also measured in the superfusate by high performance liquid chromatography (HPLC) coupled with ultraviolet detection, and an age-related increase in the adenosine level was found. In summary, our results indicate that during aging the level of adenosine in the extracellular fluid is increased in the hippocampus. There is a downregulation and reduced responsiveness of presynaptic adenosine A1-receptors, and it seems likely that these changes are due to the enhanced adenosine level in the extracellular space.

In vivo amino acid release from the striatum of aging rats: adenosine modulation

The release of glutamate, aspartate, GABA, and taurine from the striatum of young (3 months), mature (12 months), and old (22 months), freely moving male rats was investigated by using a microdialysis fiber inserted transversally in the striatum. In old rats basal extracellular glutamate and aspartate levels were decreased vs. young rats (-38 and -49%, respectively). GABA and taurine levels were unmodified by age. In the presence of the adenosine receptor antagonist 8-phenyltheophilline (8-pT) at the concentration of 50 microM, both K(+)-evoked releases of glutamate and aspartate were more than doubled in young, but not in mature and old rats. 8-pT at the concentration of 500 microM significantly decreased glutamate basal levels and K(+)-evoked aspartate release in old rats only. GABA and taurine releases were not affected by 8-pT at either dose. Our findings indicate a modified adenosine modulation on glutamate and aspartate release in aged rats, that could result from a change in the balance between A1 and A2a adenosine receptor density or an alteration of A1 and A2a receptor-effector coupling.

Effects of aging on signal transmission and transduction systems in the gerbil brain: Morphological and autoradiographic study

The Mongolian gerbil was used as a model of aging because of its relatively short lifespan, genetic homogeneity and the fact that data had been collected previously. Furthermore, gerbils have been widely used in biomedical investigations of stroke and epilepsy. Age-related differences in signal transmission and transduction systems were investigated in brains of three-, 11- and 21-month-old gerbils by morphological and in vitro receptor autoradiographic studies. Morphometric analysis revealed a decreased number of neurons in layer III of the occipital cortex and also a decrease in cerebellar Purkinje cells in 21-month-old animals. However, no statistical differences were observed in the hippocampal formation, the dorsolateral striatum and layer III of the frontal cortex. Autoradiography was used to map muscarinic cholinergic (labeled with [3H]quinuclidinyl benzilate), serotonin2 ([3H]spiperone), dopamine D2 ([3H]spiperone), adenosine A1 ([3H]cyclohexyladenosine), GABAA ([3H]muscimol), naloxone ([3H]naloxone), protein kinase C ([3H]phorbol 12,13-dibutyrate), adenylate cyclase ([3H]forskolin), cyclic AMP ([3H]cyclic AMP) and L-type Ca2+ channels ([3H]PN200-110). Muscarinic cholinergic receptor and protein kinase C, cyclic AMP and L-type Ca2+ channels were significantly decreased in the cerebral cortex and/or in the CA1 subfield of the hippocampus in the 21-month-old group. Muscarinic cholinergic receptor and L-type Ca2+ channel binding sites were significantly reduced in the dentate gyrus. In contrast, protein kinase C was increased in this area in the 21-month-old group. Also, naloxone binding sites were increased in the CA3 subfield, hilus, dentate gyrus and molecular layer of the cerebellum in the 11- and 21-month-old groups. Muscarinic cholinergic, serotonin2 and dopamine D2 receptors and adenylate cyclase were significantly decreased in the striatum. On the other hand, adenosine A1 and GABAA receptors remained unchanged in the 21-month-old group. Although age-related histopathological abnormalities were only observed in the occipital cortex and in the cerebellum, alterations of signal transmission and transduction systems were noticed in all areas examined (e.g. cerebral cortex, CA1 subfield, dentate gyrus and striatum). These data indicate that changes in these receptors and binding sites may be related to dysfunction of learning and memory and to the loss of motor function. The aged gerbil model is a good system for studying aging and is of value for simulating aging after epilepsy and stroke.

A method for binding parameters estimation of A1 adenosine receptor subtype: a practical approach

Working with pig brain striatum in which A1 and A2 adenosine receptor subtypes coexist, we describe an uncomplicated method for unequivocally obtaining the equilibrium parameters (KD and binding capacity) of A1 receptor without interference from ligand binding to A2 receptor. Also, the equilibrium parameter estimation method we propose avoids the experimental determination of nonspecific binding by the inclusion of the corresponding unknown parameter in the function. This not only saves time but also avoids the use of expensive radioligands in saturation experiments. The method is suitable for any system with two different receptor subtypes for the same physiological ligand, and good estimates of the equilibrium parameters corresponding to the subtype displaying the higher affinity for the ligand can be obtained.