We have conducted pioneering studies demonstrating that endothelin-1 (ET) is a novel neuropeptide in the CNS. We have found that cerebellar granule cells not only synthesize and release ET upon stimulation, but also express ET-specific receptors coupled to phospholipase C. Stimulation of cerebellar neurons also leads to release of preloaded D-aspartate. Both ET-induced activities are Ca 2+ dependent and Na+ independent, but the release process requires ET-induced phosphoinositide hydrolysis and other receptor-mediated events. Because increasing evidence indicates that glial cells are a target for endothelin, we have characterized the effector responses mediated by ET receptors in C6 glioma cells. Stimulation with ET induces phosphoinositide hydrolysis to generate inositol trisphosphate and causes Ca 2+ influx through a receptor-gated channel. Both events are dependent on external Ca2+ and sensitive to inorganic Ca2+ channel blockers (Cd2+, La3+ , and Mn2+). The former response is mediated by a pertussis toxin-sensitive G protein, leading to intracellular Ca 2+ mobilization, while the latter is positively regulated by protein kinase C and results in further Ca2+ increase to sustain phosphoinositide turnover. The presence of Ca 2+ ionophores A23187 and ionomycin also potentiates the phosphoinositide response to ET and ATP. Conversely, KC1 in the range of 15 to 55 mM markedly inhibits ET and ATP-induced phosphoinositide breakdown and attenuates the intracellular Ca 2+ increase elicited by these two agonists. These results demonstrate that the phosphoinositide response mediated by these two types of receptors are tightly controlled by intracellular Ca 2+ levels. Based on the selectivity of adenine nucleotides, it can be concluded that P2y purinoceptors are expressed in C6 glioma cells. As in ET receptors, purinoceptors in glioma cells are coupled to both phospholipase C and Ca 2+ influx and show homologous desensitization via a protein kinase C-independent mechanism.
