The ATP, discovered in 1929 by Karl Lohman, Cyrus Hartwell Fiske and Yellagaprada SubbaRow, acts as an important extracellular signalling molecule. Purinoreceptors, represented by ionotropic P2X and metabotropic P2Y receptors are arguably the most abundant receptors in living tissues and are expressed in neural cells in both peripheral and central nervous system. In the CNS, ATP can be released from synaptic terminals, either on its own or together with other neurotransmitters. Furthermore, ATP also acts as an important mediator in neuronal-glial communications, as glial cells are endowed with numerous ATP receptors, which trigger Ca²⁺ signalling events and membrane currents in both macro- and microglia. In addition ATP can be released from astroglial cells thereby acting as a mediator of glial-glial and glial-neuronal signalling.