P2X and P2Y purinoreceptors mediate ATP-evoked calcium signalling in opticnerve glia in situ

It is known that ATP acts as an extracellular messenger mediating Ca2+ sign alling in glial cells. Here, the mechanisms involved in the ATP-evoked incr ease in glial [Ca2+](i) were studied in situ, in the acutely isolated rat o ptic nerve. ATP and agonists for P2X (alpha,beta -metATP) and P2Y (2MeSATP) purinoreceptors triggered raised glial [Ca2+](i), and there was no signifi cant difference between cells identified morphologically as astrocytes and oligodendrocytes. Dose-response curves indicated that P2Y receptors were ac tivated at nanomolar concentrations, whereas P2X purinoreceptors were only activated above 10 muM. The rank order of potency for several agonists. ind icated optic nerve glia expressed heterogeneous purinoreceptors, with P2Y(1 )>P2Y(2/4)>P2X. The ATP evoked increase in [Ca2+](i) was reversibly blocked by the P2X/Y purinoreceptor antagonist suramin (100 muM) and markedly redu ced by thapsigargin (10 muM), which blocks IP3-dependent release of Ca2+ fr om intracellular stores. Removal of extracellular Ca2+ reduced the ATP evok ed increase in [Ca2+](i) and completely blocked its recovery, indicating th at refilling of intracellular stores was ultimately dependent on Ca2+ influ x from the extracellular milieu. The results implicate ATP as an important signal in CNS white matter astrocytes and oligodendrocytes in situ, and ind icate that metabotropic P2Y purinoreceptors mobilize intracellular Ca2+ at physiological concentrations of ATP, whereas ionotropic P2X purinoreceptors induce Ca2+ influx across the plasmalemma only at high concentrations of A TP, such as occur following CNS injury. (C) 2001 Harcourt Publishers Ltd.