Adenosine A2 receptor-mediated excitatory actions on the nervous system.

The distribution, molecular structure and role of adenosine A2 receptors in the nervous system, is reviewed. The adenosine A2a receptor subtype, identified in the nervous system with ligand binding, functional studies or genetic molecular techniques, has been demonstrated in the striatum and other basal ganglia structures, in the hippocampus, in the cerebral cortex, in the nucleus tractus solitarius, in motor nerve terminals, in noradrenergic terminals in the vas deferens, in myenteric neurones of the ileum, in the retina and in the carotid body. The A2b receptors have been identified in glial and neuronal cells, and may have a widespread distribution in the brain. Activation of adenosine A2a receptors can enhance the release of several neurotransmitters, such as acetylcholine, glutamate, and noradrenaline. The release of GABA might be either enhanced or inhibited by A2a receptor activation. The A2 receptor activation also modulates neuronal excitability, synaptic plasticity, as well as locomotor activity and behaviour. The ability of A2 receptors to interact with other receptors for neurotransmitters/neuromodulators, such as dopamine D2 and D1 receptors, adenosine A1 receptors, CGRP receptors, metabotropic glutamate receptors and nicotinic autofacilitatory receptors, expands the range of possibilities used by adenosine to interfere with neuronal function and communication. These A2 receptor-mediated adenosine actions might have potential therapeutic interest, in particular in movement disorders such as Parkinson's disease and Huntington's chorea, as well as in schizophrenia, Alzheimer's disease, myasthenia gravis and myasthenic syndromes.