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

Abstract

The synaptic release of glutamate evokes in astrocytes periodic increases in [Ca 2+]i, due to the activation of metabotropic glutamate receptors (mGluRs). The frequency of these [Ca 2+]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 [Ca 2+]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 [Ca 2+]i oscillations and prostaglandin release from astrocytes. To investigate whether [Ca 2+]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 [Ca 2+]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 [Ca 2+]i oscillations triggered spatially and temporally coordinated [Ca 2+]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 Ins P3‐linked prostanoid receptor EP 1. We conclude that [Ca 2+]i oscillations in astrocytes regulate prostaglandin releases that retain the oscillatory behaviour of the [Ca 2+]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 hyperaemia.