Defining protein kinase/phosphatase isoenzymic regulation of mGlu5 receptor‐stimulated phospholipase C and Ca2+ responses in astrocytes

BACKGROUND AND PURPOSE Cyclical phosphorylation and dephosphorylation of a key residue within the C‐terminal domain of the activated type 5 metabotropic glutamate (mGlu5) receptor is believed to cause the synchronous, oscillatory changes in inositol 1,4,5‐trisphosphate and Ca2+ levels observed in a variety of cell types. Here, we have attempted to better define the kinase and phosphatase enzymes involved in this modulation. EXPERIMENTAL APPROACH Ca2+ and [3H]inositol phosphate ([3H]IPx) measurements in astrocyte preparations have been used to evaluate the effects of pharmacological inhibition of protein kinase C (PKC) and protein phosphatase activities and small interfering RNA‐mediated specific PKC isoenzymic knock‐down on mGlu5 receptor signalling. KEY RESULTS Ca2+ oscillation frequency or [3H]IPx accumulation in astrocytes stimulated by mGlu5 receptors, was concentration‐dependently decreased by protein phosphatase‐1/2A inhibition or by PKC activation. PKC inhibition also increased [3H]IPx accumulation two‐ to threefold and changed the Ca2+ response into a peak‐plateau response. However, selective inhibition of conventional PKC isoenzymes or preventing changes in [Ca2+]i concentration by BAPTA‐AM loading was without effect on mGlu5 receptor‐stimulated [3H]IPx accumulation. Selective knock‐down of PKCδ was without effect on glutamate‐stimulated Ca2+ responses; however, selective PKCε knock‐down in astrocytes changed Ca2+ responses from oscillatory into peak‐plateau type. CONCLUSION AND IMPLICATIONS These data confirm the acute regulation of mGlu5 receptor signalling by protein kinases and protein phosphatases and provide novel data pinpointing the isoenzymic dependence of this regulation in the native mGlu5 receptor‐expressing rat cortical astrocyte. These data also highlight a potential alternative mechanism by which mGlu5 receptor signalling might be therapeutically manipulated.