Nucleotides and nucleosides are primitive biological molecules that were utilized early in evolution both as intracellular energy sources and as extracellular signalling molecules. ATP was first identified as a neurotransmitter and later as a co-transmitter with all the established neurotransmitters in both peripheral and central nervous systems. Four subtypes of P1 (adenosine) receptors| 7 subtypes of P2X ion channel receptors and 8 subtypes of P2Y G protein-coupled receptors have currently been identified. Since P2 receptors were first cloned in the early 1990’s| there is clear evidence for the widespread distribution of both P1 and P2 receptor subtypes in neuronal and non-neuronal cells| including glial| immune| bone| muscle| endothelial| epithelial and endocrine cells.
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