2؉ mobilization, but it was blocked upon phospholipase C inhibition. These results suggest a novel mechanism wherein, upon DAMGO binding, CaM is released from the receptor and activates phospholipase C. Subsequently, phospholipase C generates diacylglycerides that activate PKC⑀. In contrast, U69,593 appears to act via phosphoinositide 3-kinase, PKC, and Ca 2؉ mobilization. These signaling components were implicated based on studies with specific inhibitors and a dominant negative mutant of PKC. Collectively, our findings on acute opioid effects suggest that differences in their mechanism of signaling may contribute to the distinct outcomes on ERK modulation induced by chronic and opioids.

Mitogenic signaling of G protein-coupled receptors (GPCRs) can proceed via sequential epidermal growth factor receptor (EGFR) transactivation and extracellular signal-regulated kinase (ERK) phosphorylation. Although the -opioid receptor (MOR) mediates stimulation of ERK via EGFR transactivation in human embryonic kidney 293 cells, the mechanism of acute MOR signaling to ERK has not been characterized in rat C6 glioma cells that seem to contain little EGFR. Herein, we describe experiments that implicate fibroblast growth factor (FGF) receptor (FGFR) transactivation in the convergence of MOR and growth factor signaling pathways in C6 cells. MOR agonists, endomorphin-1 and morphine, induced a rapid (3-min) increase of ERK phosphorylation that was abolished by MOR antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH 2 . By using selective inhibitors and overexpression of dominant negative mutants, data were obtained to suggest that MOR signaling to ERK is transduced by G␤␥ and entails Ca 2ϩ -and protein kinase Cmediated steps, whereas the FGFR branch of the pathway is Ras-dependent. An intermediary role of FGFR1 transactivation was suggested by MOR-but not -opioid receptor (KOR)-induced FGFR1 tyrosine phosphorylation. A dominant negative mutant of FGFR1 attenuated MOR-but not KOR-induced ERK phosphorylation. Thus, a novel transactivation mechanism entailing secreted endogenous FGF may link the GPCR and growth factor pathways involved in MOR activation of ERK in C6 cells.