GABAB heterodimeric receptors promote Ca2+ influx via store-operated channels in rat cortical neurons and transfected Chinese hamster ovary cells.
The GABAB receptors are generally considered to be classical Gi-coupled receptors that lack the ability to mobilize intracellular Ca2+ without the aid of promiscuous G proteins. Here, we report the ability of GABAB receptors to promote calcium influx into primary cultures of rat cortical neurons and transfected Chinese hamster ovary cells. Chinese hamster ovary cells were transfected with GABAB1(a) or GABAB1(b) subunits along with GABAB2 subunits. In experiments using the fluorometric imaging plate reader platform, GABA and selective agonists promoted increases in intracellular Ca2+ levels in transfected Chinese hamster ovary cells and cortical neurons with the expected order of potency. These effects were fully antagonized by selective GABAB receptor antagonists. To investigate the intracellular pathways responsible for mediating these effects we employed several pharmacological inhibitors. Pertussis toxin abolished GABAB mediated Ca2+ increases, as did the phospholipase Cbeta inhibitor U73122. Inhibitor 2-aminethoxydiphenyl borane acts as an antagonist at inositol 1,4,5-trisphosphate receptors and at store-operated channels. In all cell types, 2-aminethoxydiphenyl borane prevented Ca2+ mobilization. The selective store-operated channel inhibitor 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy]ethyl-1H-imidazole hydrochloride prevented increases in intracellular Ca2+ levels as did performing the assays in Ca2+ free buffers. In conclusion, GABAB receptors expressed in Chinese hamster ovary cells and endogenously expressed in rat cortical neurons promote Ca2+ entry into the cell via the activation of store-operated channels, using a mechanism that is dependent on Gi/o heterotrimeric proteins and phospholipase Cbeta. These findings suggest that the neuronal effects mediated by GABAB receptors may, in part, rely on the receptor's ability to promote Ca2+ influx.