The adenosine modulation system mostly operates through inhibitory A 1 (A 1 R) and facilitatory A 2A receptors (A 2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A 1 R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: highfrequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A 2A R; A 2A R switch off A 1 R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A 1 R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A 1 R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A 2A R, probably to bolster adaptive changes, but this heightens brain damage since A 2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A 2A R, whereas astrocytic and microglia A 2A R might control the spread of damage. The A 2A R signaling mechanisms are largely unknown since A 2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A 1 R preconditioning and preventing excessive A 2A R function might afford maximal neuroprotection. Keywords: A 1 receptor, A 2A receptor, astrocyte, microglia, synaptic plasticity, synaptotoxicity. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".
Relevance of modulation systems to tune information flow in brain circuitsThe goal of understanding the flow of information in neuronal circuits has traditionally been centered in studying the key processes sustaining synaptic transmission and plasticity -i.e. the role of glutamate and glutamate receptors, as well as to a lesser extent the role of GABA and its receptors. If one considers an analogy to the experience of watching TV, this corresponds to studying how the ON/OFF button impacts all Received April 4, 2016; revised manuscript received May 23, 2016; accepted June 23, 2016. Address correspondence and reprint requests to R. A. Cunha, Center for Neurosciences and Cell Biology, University of Coimbra, 3004-517 Coimbra, Portugal. E-mail: cunharod@gmail.com Abbreviations used: A 1 R, adenosine A 1 receptor; A 2A R, adenosine A 2A receptor; ADHD, attention deficit and hyperactivity disorder; BDNF, brain-derived neurotrophi...