Regulation of the cyclic AMP response element binding protein (CREB) by group I metabotropic glutamate receptors
Initially, experiments investigated mGlu1/5 receptor signalling in recombinant CHO-lac cell lines, where receptor expression is under inducible control. Key differences in the signalling characteristics of the two receptor subtypes were apparent. Thus, whilst mGlula receptors activated a small, yet robust cyclic AMP response in the recombinant cell lines, mGlu5a receptors failed to stimulate significant cyclic AMP accumulation. Furthermore, in agreement with previous reports the patterns of Ca2+-mobilization activated by receptor stimulation also differed in these cells, with the mGlula receptor mediating a peak-and-plateau [Ca2+]I response, whilst mGlu5 receptor activation caused oscillatory changes in [Ca 2+]i.;Despite the differences observed at the level of receptor-stimulated cyclic AMP and Ca2+-signalling, both receptors stimulated CREB phosphorylated with similar time-courses and comparable agonist potencies in the CHO-lac cell lines. The mechanisms employed by the different receptor subtypes to couple to CREB phosphorylation were also similar, requiring sustained Ca2+ elevation and protein kinase C activation, but occurred independently of extracellular signal-regulated, cyclic AMP-dependent or Ca2+/calmodulin-depdnent protein kinases.;CREB regulation was also examined in primary cerebellar granular cells (CGCs). Under the culture conditions used here both the mGlul and M 3 muscarinic acetylcholine receptors elicited Ca 2+ responses in CGCs, however, neither receptor subtype caused a significant increase in CREB phosphorylation, either directly, or indirectly via crosslink during ionotropic receptor co-stimulation. In contrast, NMDA receptor stimulation activated a robust time- and concentration-dependent increase in CREB phosphorylation in the CGCs. It therefore appears, in cerebellar granule neurones at least, that CREB is regulated predominantly by ionotropic mechanisms.