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

Abstract

It is known that ATP acts as an extracellular messenger mediating Ca2+signalling in glial cells. Here, the mechanisms involved in the ATP-evoked increase in glial [Ca2+]iwere studied in situ, in the acutely isolated rat optic nerve. ATP and agonists for P2X (a,b-metATP) and P2Y (2MeSATP) purinoreceptors triggered raised glial [Ca2+]i, and there was no significant difference between cells identified morphologically as astrocytes and oligodendrocytes. Dose–response curves indicated that P2Y receptors were activated at nanomolar concentrations, whereas P2X purinoreceptors were only act ivated above 10 μM. The rank order of potency for several agonists indicated optic nerve glia expressed heterogeneous purinoreceptors, with P2Y1≤ P2Y2/4≤ P2X. The ATP evoked increase in [Ca2+]iwas reversibly blocked by the P2X/Y purinoreceptor antagonist suramin (100 μM) and markedly reduced by thapsigargin (10μM), which blocks IP3-dependent release of Ca2+from intracellular stores. Removal of extracellular Ca2+reduced the ATP evoked increase in [Ca2+]iand completely blocked its recovery, indicating that refilling of intracellular stores was ultimately dependent on Ca2+influx from the extracellular milieu. The results implicate ATP as an important signalin CNS white matter astrocytes and oligodendrocytes in situ, and indicate 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 ATP, such as occur following CNS injury.