We have conducted pioneering studies demonstrating that endothelin-1 (ET) is a novel neuropeptide in the CNS. We have found that cerebellar granule cells express ET-specific receptors coupled to phospholipase C. Stimulation of cerebellar neurons also leads to release of preloaded D- aspartate. Because increasing evidence indicates that glial cells are a target for endothelin, we have characterized the effector responses mediated by ET receptors in C-6 glioma cells. Stimulation with ET induces phosphoinositide hydrolysis to generate inositol trisphosphate and causes Ca-2+ influx through a receptor-gated channel. 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 Ca-2+ increase to sustain phosphoinositide turnover. ET and ATP also inhibit adenylyl cyclase (AC) activity in C-6 glioma cells. This inhibition is primarily due to interaction with a pertussis toxin-sensitive G-i protein and partially mediated by elevation of intracellular Ca-2+. Protein kinase C is not responsible for agonist-induced inhibition of AC, but appears to uncouple the G-i/AC system elicited by ET or ATP. The rank order of 15 nucleotide analogues for stimulating phospholipase C is clearly different from the rank order of potency for inhibiting AC. We suggest that the nucleotide receptor is linked to phospholipase C via pertussis toxin-insensitive G-p protein and the P-2y receptor is coupled to AC via G-i protein.
