 In humans , this signalling role is important in both the central and peripheral nervous system . Activity @-@ dependent release of ATP from synapses , axons and glia activates purinergic membrane receptors known as P2 . The P2Y receptors are metabotropic , i.e. G protein @-@ coupled and modulate mainly intracellular calcium and sometimes cyclic AMP levels . Though named between <unk> and <unk> , only nine members of the P2Y family have been cloned , and some are only related through weak homology and several ( <unk> , <unk> , <unk> , <unk> ) do not function as receptors that raise cytosolic calcium . The <unk> <unk> receptor subgroup comprises seven members ( <unk> – <unk> ) , which are ligand @-@ gated Ca2 + <unk> ion channels that open when bound to an extracellular purine nucleotide . In contrast to P2 receptors ( agonist order ATP > ADP > AMP > ADO ) , purinergic nucleoside triphosphates like ATP are not strong agonists of P1 receptors , which are strongly activated by adenosine and other nucleosides ( ADO > AMP > ADP > ATP ) . P1 receptors have A1 , A2a , A2b , and A3 subtypes ( " A " as a remnant of old nomenclature of adenosine receptor ) , all of which are G protein @-@ coupled receptors , A1 and A3 being coupled to Gi , and A2a and A2b being coupled to Gs . All adenosine receptors were shown to activate at least one subfamily of mitogen @-@ activated protein kinases . The actions of adenosine are often antagonistic or synergistic to the actions of ATP . In the CNS , adenosine has multiple functions , such as modulation of neural development , neuron and glial signalling and the control of innate and adaptive immune systems . 
