Glutamate-mediated cytosolic calcium oscillations regulate a pulsatile prostaglandin release from cultured rat astrocytes

The synaptic release of glutamate evokes in astrocytes periodic increases in [Ca2+](i), due to the activation of metabotropic glutamate receptors (mGluRs). The frequency of these [Ca2+](i) oscillations is controlled by the level of neuronal activity, indicating that they represent a specific, frequency-coded signalling system of neuron-to-astrocyte communication. We recently found that neuronal activity-dependent [Ca2+](i) oscillations in astrocytes are the main signal that regulates the coupling between neuronal activity and blood flow, the so-called functional hyperaemia. Prostaglandins play a major role in this fundamental phenomenon in brain function, but little is known about a possible link between [Ca2+](i) oscillations and prostaglandin release from astrocytes. To investigate whether [Ca2+](i) oscillations regulate the release of vasoactive prostaglandins, such as the potent vasodilator prostaglandin E-2 (PGE(2)), from astrocytes, we plated wild-type human embryonic kidney (HEK)293 cells, which respond constitutively to PGE(2) with [Ca2+](i) elevations, onto cultured astrocytes, and used them as biosensors of prostaglandin release. After loading the astrocyte-HEK cell co-cultures with the calcium indicator Indo-1, confocal microscopy revealed that mGluR-mediated [Ca2+](i) oscillations triggered spatially and temporally coordinated [Ca2+](i) increases in the sensor cells. This response was absent in a clone of HEK cells that are unresponsive to PGE(2) and recovered after transfection with the InsP(3)-Iinked prostanoid receptor EP1. We conclude that [Ca2+](i) oscillations in astrocytes regulate prostaglandin releases that retain the oscillatory behaviour of the [Ca2+](i) changes. This finely tuned release of PGE(2) from astrocytes provides a coherent mechanistic background for the role of these glial cells in functional hyperaernia.