Expression and regulation of types I and II inositol 1,4,5-trisphosphate receptors in rat cerebellar granule cell preparations.

Previous studies have shown that as rat cerebellar granule cell cultures differentiate in the presence of 25 mM KCl, they "up-regulate" their ability to form inositol phosphates and release Ca2+ from internal stores in response to the activation of phosphoinositidase C-linked muscarinic and metabotropic receptors. Here we show that they simultaneously up-regulate their ability to respond to inositol 1,4,5-trisphosphate (InsP3) by increasing InsP3 receptor (InsP3R) expression. In contrast, if granule cells are maintained at the more physiological KCl concentration of 5 mM, most cells undergo apoptosis, although a significant number survive. The surviving cells, however, express few InsP3Rs, suggesting that an influx of Ca2+ through voltage-dependent channels is required for InsP3R up-regulation. In addition, we have determined that these cultures express two genetically distinct InsP3R types, but that only one, the type I receptor, is expressed in granule cells. Type II receptors are also present but are found exclusively in astrocytes, which are a minor contaminant of granule cell cultures. This segregation of InsP3R types explains a previous observation, showing that the muscarinic agonist carbachol causes the reduction or "down-regulation" of type I but not type II InsP3Rs.