Adenosine Neuromodulation and Neuroprotection

Abstract

The adenosine moiety fulfils an important intracellular homeostatic role since it is part of molecules that play key roles in defining the status of all cells, namely energy charge (ATP), redox status (NADH), and cell division (SAH/SAM). But the signaling role of the adenosine molecule itself is restricted to a paracrine role, signaling metabolic imbalance of cells within a tissue. Apart from this general role common to most tissues in mammals, adenosine fulfils a particular role as a neuromodulator in the nervous system. This involves a predominant inhibitory effect operated by adenosine A1 receptors, which results from a combined presynaptic inhibition of the release of excitatory neurotransmitters together with a postsynaptic action leading to neuronal hyperpolarization and an ability to depress plasticity by inhibition of NMDA receptors and voltage-sensitive calcium channels. Overall, this A1 receptor-mediated inhibition is aimed at decreasing the noise of excitatory transmission in brain circuits. Adenosine can also activate facilitatory A2A receptors, which only come into play at higher frequency of nerve stimulation and are directed at selectively shutting down A1 receptor inhibition in stimulated synapses to aid implementing changes in synaptic efficiency. Therefore, the combined role of A1 and A2A receptors is designed to increase salience of information in brain circuits. Apart from this main physiological role, adenosine also plays a relevant role in controlling the demise of damage in noxious brain conditions. In fact, the inhibitory A1 receptors are able to curtail brain damage. They play a role at the onset of brain damage and function as a hurl that needs to be overcome to allow the development of brain damage. In parallel, in chronic noxious brain conditions, A2A receptors contribute for brain damage, especially when insidious damage to synapses initiates neurodegeneration. Hence, A2A receptor antagonists are now being explored as novel neuroprotective strategies to interfere with the initial processes of neurodegenerative conditions, such as Parkinson's and Alzheimer's diseases. [ABSTRACT FROM AUTHOR]