We previously reported that the metabotropic glutamate receptor R1alpha (mGluR1alpha) can be activated not only by applying glutamate but also by raising extracellular Ca2+ (Ca2+o) concentration, and that the constant stimulation by Ca2+o causes morphological change of transfected Chinese Hamster Ovary (CHO) cells (Kubo Y Miyashita T and Murata Y (1998) Science 279, 1722-1725). The physiological role of the Ca2+o-sensing function of mGluR1alpha, however, is not fully clear yet, especially because Ca2+ is constitutively present in the extracellular space unlike other neurotransmitters. In this work, we aimed to elucidate the physiological significance of the Ca2+o-sensing function of mGluR1alpha. The effect of mGluR1alpha activation by Ca2+o on the morphological change of CHO cells was mimicked by forskolin. The effect of mGluR1alpha activation on the morphological change was suppressed by the inhibitors of adenylate cyclase, protein kinase A (PKA) and MAP kinase kinase (MAPKK), and the effect of forskolin was also decreased by the inhibitors of PKA and MAPKK. These results demonstrate the involvement of cAMP, PKA, MAPKK, MAPK pathway in the morphological change. We actually confirmed that the Ca2+o stimulation of mGluR1alpha increased the basal cAMP level of transfected CHO cells. This increase in cAMP was observed even when only the membrane fraction of mGluR1alpha transfected CHO cells were used, and the increase was inhibited by anti-Gs alpha antibody. Taken together, we concluded that the Ca2+o-sensing function of mGluR1alpha and the continuous stimulation by Ca2+o caused the increase in the basal cAMP level by direct coupling with Gs, and triggered the subsequent activation of PKA, MAPKK, and MAPK cascade which resulted in the morphological change of transfected CHO cells.