Joaquim A. Ribeiro

Adenosine receptors in the nervous system: pathophysiological implications

Adenosine is a ubiquitous homeostatic substance released from most cells, including neurones and glia. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)) that can inhibit (A(1)) or enhance (A(2)) neuronal communication. Interactions between adenosine receptors and other G-protein-coupled receptors, ionotropic receptors and receptors for neurotrophins also occur, and this might contribute to a fine-tuning of neuronal function. Manipulations of adenosine receptors influence sleep and arousal, cognition and memory, neuronal damage and degeneration, as well as neuronal maturation. These actions might have therapeutic implications for neurodegenerative diseases such as Parkinsons disease, Alzheimers disease, as well as for other neurological situations such as epilepsy, idiopathic pain or even drug addition. Peripheral side effects associated with adenosine receptor agonists limit their usefulness in therapeutics; in contrast, adenosine receptor antagonists appear to have less side effects as it is the case of the well-known non-selective antagonists theophylline (present in tea) or caffeine (abundant in coffee and tea), and their emerging beneficial actions in Parkinsons disease and Alzheimers disease are encouraging. A(1) receptor antagonism may also be useful to enhance cognition and facilitate arousal, as well as in the periphery when deficits of neurotransmitter release occur (e.g. myasthenic syndromes). Enhancement of extracellular adenosine levels through drugs that influence its metabolism might prove useful approaches in situations such as neuropathic pain, where enhanced activation of inhibitory adenosine A(1) receptors is beneficial. One might then consider adenosine as a fine-tuning modulator of neuronal activity, which via subtle effects causes harmonic actions on neuronal activity. Whenever this homeostasis is disrupted, pathology may be installed and selective receptor antagonism or agonism required.

Adenosine : does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

Caffeine and Adenosine

Caffeine causes most of its biological effects via antagonizing all types of adenosine receptors (ARs): A1, A2A, A3, and A2B and, as does adenosine, exerts effects on neurons and glial cells of all brain areas. In consequence, caffeine, when acting as an AR antagonist, is doing the opposite of activation of adenosine receptors due to removal of endogenous adenosinergic tonus. Besides AR antagonism, xanthines, including caffeine, have other biological actions: they inhibit phosphodiesterases (PDEs) (e.g., PDE1, PDE4, PDE5), promote calcium release from intracellular stores, and interfere with GABA-A receptors. Caffeine, through antagonism of ARs, affects brain functions such as sleep, cognition, learning, and memory, and modifies brain dysfunctions and diseases: Alzheimers disease, Parkinsons disease, Huntingtons disease, Epilepsy, Pain/Migraine, Depression, Schizophrenia. In conclusion, targeting approaches that involve ARs will enhance the possibilities to correct brain dysfunctions, via the universally consumed substance that is caffeine.

ATP as a presynaptic modulator

There is considerable evidence that ATP acts as a fast transmitter or co-transmitter in autonomic and sensory nerves mostly through activation of ionotropic P2X receptors but also through metabotropic P2Y receptors. By analogy, the observations that ATP is released from stimulated central nervous system (CNS) nerve terminals and that responses to exogenously added ATP can be recorded in central neurons, lead to the proposal that ATP might also be a fast transmitter in the CNS. However, in spite of the robust expression of P2 receptor mRNA and binding to P2 receptors in the CNS, the demonstration of central purinergic transmission has mostly remained elusive. We now review evidence to suggest that ATP may also act presynaptically rather than solely postsynaptically in the nervous system.

THE EFFECTS OF ADENOSINE TRIPHOSPHATE AND ADENOSINE DIPHOSPHATE ON TRANSMISSION AT THE RAT AND FROG NEUROMUSCULAR JUNCTIONS

1 The effects of adenosine triphosphate (ATP) and adenosine diphosphate (ADP) were investigated on evoked end‐plate potentials (e.p.ps) and on miniature end‐plate potentials (min. e.p.ps) recorded from muscle fibres of the rat diaphragm and the frog sartorius. 2 ATP and ADP decreased the quantum content of the e.p.ps and the frequency of the min. e.p.ps. The maximum effects produced by the two substances were similar. 3 The potency of ATP was found to be similar to that of adenosine. In the presence of adenosine, in a concentration producing its maximum effect, the addition of ATP had no further effect. This is compatible with the idea that ATP acts in the same way as adenosine.

Adenosine A2A receptor facilitation of hippocampal synaptic transmission is dependent on tonic A1 receptor inhibition

Abstract Adenosine tonically inhibits synaptic transmission through actions at A 1 receptors. It also facilitates synaptic transmission, but it is unclear if this facilitation results from pre- and/or postsynaptic A 2A receptor activation or from indirect control of inhibitory GABAergic transmission. The A 2A receptor agonist, CGS 21680 (10 nM), facilitated synaptic transmission in the CA1 area of rat hippocampal slices (by 14%), independent of whether or not GABAergic transmission was blocked by the GABA A and GABA B receptor antagonists, picrotoxin (50 μM) and CGP 55845 (1 μM), respectively. CGS 21680 (10 nM) also inhibited paired-pulse facilitation by 12%, an effect prevented by the A 2A receptor antagonist, ZM 241385 (20 nM). These effects of CGS 21680 (10 nM) were occluded by adenosine deaminase (2 U/ml) and were made to reappear upon direct activation of A 1 receptors with N 6 -cyclopentyladenosine (CPA, 6 nM). CGS 21680 (10 nM) only facilitated (by 17%) the K + -evoked release of glutamate from superfused hippocampal synaptosomes in the presence of 100 nM CPA. This effect of CGS 21680 (10 nM), in contrast to the isoproterenol (30 μM) facilitation of glutamate release, was prevented by the protein kinase C inhibitors, chelerythrine (6 μM) and bisindolylmaleimide (1 μM), but not by the protein kinase A inhibitor, H-89 (1 μM). Isoproterenol (30 μM), but not CGS 21680 (10–300 nM), enhanced synaptosomal cAMP levels, indicating that the CGS 21680-induced facilitation of glutamate release involves a cAMP-independent protein kinase C activation. To discard any direct effect of CGS 21680 on adenosine A 1 receptor, we also show that in autoradiography experiments CGS 21680 only displaced the adenosine A 1 receptor antagonist, 1,3-dipropyl-8-cyclopentyladenosine ([ 3 H]DPCPX, 0.5 nM) with an EC 50 of 1 μM in all brain areas studied and CGS 21680 (30 nM) failed to change the ability of CPA to displace DPCPX (1 nM) binding to CHO cells stably transfected with A 1 receptors. Our results suggest that A 2A receptor agonists facilitate hippocampal synaptic transmission by attenuating the tonic effect of inhibitory presynaptic A 1 receptors located in glutamatergic nerve terminals. This might be a fine-tuning role for adenosine A 2A receptors to allow frequency-dependent plasticity phenomena without compromising the A 1 receptor-mediated neuroprotective role of adenosine.

Activation of Adenosine A2A Receptor Facilitates Brain-Derived Neurotrophic Factor Modulation of Synaptic Transmission in Hippocampal Slices

Both brain-derived neurotrophic factor (BDNF) and adenosine influence neuronal plasticity. We now investigated how adenosine influences the action of BDNF on synaptic transmission in the CA1 area of the rat hippocampal slices. Alone, BDNF (20–100 ng/ml) did not significantly affect field EPSPs (fEPSPs). However, a 2 min pulse of high-K+ (10 mm) 46 min before the application of BDNF (20 ng/ml) triggered an excitatory action, an effect blocked by the TrkB receptor inhibitor K252a (200 nm), by the adenosine A2A receptor antagonist ZM 241385 (50 nm), and by the protein kinase A inhibitor H-89 (1 μm). Presynaptic, rather than postsynaptic depolarization was required to trigger the BDNF action because after K+ depolarization BDNF also increased EPSCs recorded from pyramidal neurons voltage-clamped at –60 mV, and transient postsynaptic depolarization was unable to unmask the BDNF action. A weak theta burst stimulation of the afferents could elicit potentiation of synaptic transmission only when applied in the presence of BDNF. Activation of adenosine A2A receptors with CGS 21680 (10 nm), or the increase in extracellular adenosine levels induced by 5-iodotubercidin (100 nm) triggered the excitatory action of BDNF, a process prevented by ZM 241385 and by H-89. In the presence of dibutyryl-cAMP (0.5 mm), BDNF also increased fEPSPs but postsynaptic cAMP (0.5 mm) was unable to trigger the BDNF action. It is concluded that presynaptic activity-dependent release of adenosine, through activation of A2A receptors, facilitates BDNF modulation of synaptic transmission at hippocampal synapses.

Purinergic modulation of [3H]GABA release from rat hippocampal nerve terminals

The hippocampal GABAergic system is assumed not to be a target for purine modulation. We have now confirmed that neither adenosine A(1) and A(3) receptor nor nucleotide P(2) or P(4) receptor activation modified the K(+)-evoked [(3)H]GABA release from hippocampal synaptosomes. However, activation of adenosine A(2A) receptors with CGS 21680 (10 nM) or HENECA (30 nM) facilitated GABA release by 32% and 21%, respectively. These effects were prevented by the A(2A) antagonist, ZM 241385 (20 nM). A(2A) receptors may activate adenylate cyclase and protein kinase A since CGS 21680 (10 nM) facilitation was partially prevented by 8-bromo-cAMP (1 mM), forskolin (10 microM) and HA-1004 (10 microM). Protein kinase C may also be recruited, since chelerythrine (6 microM) and phorbol-12, 13-didecanoate (250 nM) attenuated CGS 21680 (10 nM) facilitation of [(3)H]GABA release. Omega-agatoxin-IVA (200 nM) occluded CGS 21680 facilitation suggesting the involvement of P-type calcium channels. Thus, the adenosine A(2A) receptor system appears to be one of the first presynaptic neuromodulatory systems able to enhance the evoked release of GABA from hippocampal nerve terminals.

Effect of adenosine on carotid chemoreceptor activity in the cat.

1 The effects of intracarotid (i.e.) injections or infusions of adenosine on chemoreceptor activity recorded from the peripheral end of a sectioned carotid sinus nerve have been studied in cats anaesthetized with pentobarbitone 2 Adenosine injections (0.1–100 μg) caused a rapid and marked increase of spontaneous chemoreceptor discharge, the intensity, duration and onset of which was dose‐dependent. Infusion of adenosine, 50 μg/min, also evoked an increase in discharge which persisted for the duration of the infusion 3 Both theophylline (1 mg i.e.) and aminophylline (1 mg i.e.) caused short‐lasting decreases in spontaneous discharge but did not prevent the excitatory effect of adenosine. Theophylline increased the excitatory action of adenosine 4 Naloxone (400 μg i.e.) antagonized the depressant effect of morphine on chemoreceptor discharge but not the excitatory action of adenosine 5 It is concluded that exogenous adenosine can excite the cat carotid chemoreceptors, an effect which is not prevented by theophylline in the doses studied. The physiological significance of the findings is discussed.

Adenosine and neuronal plasticity.

Adenosine is considered an important neuromodulator of the nervous system acting at pre-, post- and non-synaptic levels. In the present review we describe how adenosine modifies paired-pulse facilitation (PPF), posttetanic depression (PTD), long-term potentiation (LTP), long-term depression (LTD) and depotentiation at the hippocampus, and therefore how this nucleoside modulates synaptic plasticity.