Modulation of cellular differentiation by N-methyl-D-aspartate receptors in osteoblasts.

N-methyl-D-aspartate (NMDA) receptors for the central neurotransmitter l-glutamate (Glu) have been shown to be present in both osteoblasts and osteoclasts. Sustained exposure to the NMDA channel antagonist dizocilpine (MK-801) significantly prevented increases in both alkaline phosphatase activity and Ca2+ accumulation in a concentration-dependent manner in osteoblasts cultured for 7-28 days in vitro (DIV), without significantly affecting cell survivability. Osteocalcin expression was markedly reduced in the presence of MK-801 in osteoblasts cultured for 28 DIV. Both an NMDA domain antagonist and a glycine domain antagonist similarly prevented Ca2+ accumulation in osteoblasts exposed for 28 consecutive DIV. MK-801 was effective in significantly inhibiting Ca2+ accumulation determined at 28 DIV in osteoblasts exposed before 7 DIV but was ineffective in cells exposed after 11-21 DIV. Sustained exposure to MK-801 significantly inhibited DNA binding activity and expression of core binding factor alpha-1 (CBFA1) in osteoblasts exposed after 7 DIV up to 28 DIV, but not in those exposed before 7 DIV. These results suggest that heteromeric NMDA receptor channels may be functionally expressed to regulate mechanisms underlying cellular differentiation rather than proliferation and/or maturation through modulation of expression of CBFA1 in cultured rat calvarial osteoblasts.