New evidence for purinergic signaling in the olfactory bulb: A2A and P2Y1 receptors mediate intracellular calcium release in astrocytes.

Purinergic receptors play a key role in neuron-glia and glia-neuron interactions. In the present study, we have recorded cytosolic Ca(2+) responses using confocal imaging in astrocytes of acute olfactory bulb slices from mice (postnatal days 3-8). By application of agonists and antagonists, we identified two types of receptors, P2Y(1) and A(2A), that mediated Ca(2+) responses attributable to Ca(2+) release from intracellular stores in the astrocytes. Both receptor types were activated by application of ATP and ADP; however, when enzymatic ATP degradation was suppressed by the alkaline phosphatase inhibitor levamisole, ATP only activated MRS2179-sensitive P2Y(1) but not ZM241385-sensitive A(2A) receptors. The dose-response curve for A(2A) receptors activated by adenosine revealed an EC(50) of 0.3 microM, one order of magnitude smaller than the EC(50) of 5 microM determined for P2Y(1) receptors activated by ADP. Electrical stimulation of the olfactory nerve in the presence of glutamate receptor blockers to suppress excitation of postsynaptic neurons evoked Ca(2+) responses in most of the astrocytes, which were inhibited by blocking both P2Y(1) and A(2A) receptors. Our results indicate that olfactory nerve terminals release not only glutamate, but also ATP, which activates P2Y(1) receptors and, after degradation of ATP to adenosine, A(2A) receptors in astrocytes.