RELEASE AND ACTIONS OF ADENOSINE IN THE CENTRAL-NERVOUS-SYSTEM

Adenosine is released from active neurons into the extracellular fluid at a concentration of about 1 mu mol/l. Neither the precise cellular origin nor the biochemical form of release has been firmly established , though the nucleotide is probably released partly directly, as a res ult of raised intracellular levels, and partly as nucleotides, which a re subsequently hydrolysed. Once in the extracellular medium, adenosin e markedly inhibits the release of excitatory neurotransmitters and mo dulatory peptides and has direct inhibitory effects on postsynaptic ex citability via A(1) receptors. A population of A(2) receptors may medi ate depolarization and enhanced transmitter release. Adenosine also mo dulates neuronal sensitivity to acetylcholine and catecholamines, all these effects probably contributing to the behavioural changes observe d in conscious animals. As a result of their many actions, adenosine a nalogues are being intensively investigated for use as anticonvulsant, anxiolytic, and neuroprotective agents.