Nitric oxide-mediated modulation of synaptic activity by astrocytic P2Y receptors

We investigated the mechanism of synaptic suppression by P2Y receptors in mixed hippocampal cultures wherein networked neurons exhibit synchronized Ca2+ oscillations (SCO) due to spontaneous glutamatergic synaptic transmission. Pharmacological studies suggested that SCO suppression was mediated by P2Y2/P2Y4 receptors. Immuno-staining studies and characterization of ATP/UTP-stimulated Ca2+ responses in solitary neurons and astrocytes revealed that the SCO attenuation was effectuated by astrocytes. We demonstrate that nitric oxide released from activated astrocytes causes synaptic suppression by inhibiting neurotransmitter release. Physiological concentrations of ATP and UTP evoked NO production in astrocytes. SCO suppression was considerably diminished by removal of extracellular NO by membrane-impermeable scavenger c-PTIO or by pretreatment of cells with nitric oxide synthase inhibitor L-NAME. The nitric oxide donor DETA/NO effectively suppressed the SCO. ATP/UTP inhibited KCl-induced exocytosis at presynaptic terminals in an NO-dependent manner. In the absence of exogenously added ATP/UTP, both the NO scavenger and NOS inhibitor enhanced the frequency of SCO, implying that astrocytes release NO during spontaneous synaptic activity and exert a suppressive effect. We report for the first time that under physiological conditions astrocytes use NO as a messenger molecule to modulate the synaptic strength in the networked neurons.