The interaction of Ro 25–6981 with N-methyl-D-aspartate (NMDA) receptors was characterized by a variety of different tests in vitro. Ro 25–6981 inhibited 3 H-MK-801 binding to rat forebrain membranes in a biphasic manner with IC 50 values of 0.003 �M and 149 �M for high- (about 60%) and low-affinity sites, respectively. NMDA receptor subtypes expressed in Xenopus oocytes were blocked with IC 50 values of 0.009 �M and 52 �M for the subunit combinations NR1C & NR2B and NR1C & NR2A, respectively, which indicated a �5000-fold selectivity. Like ifenprodil, Ro 25–6981 blocked NMDA receptor subtypes in an activity-dependent manner. Ro 25–6981 protected cul-Functional NMDA receptors are composed of members from two subunit families, namely NR1 (eight different splice variants) and NR2A-D (reviewed by Mori and Mishina, 1995). The subunit family members show distinct distribution patterns in adult brain and during development (Kutsuwada et al., 1992; Laurie and Seeburg, 1994; Monyer et al., 1994; Mori and Mishina, 1995). Members of the NR1 family are expressed in all brain areas with a differential distribution of splice variants (Laurie and Seeburg, 1994; Nakanishi et al., 1992), whereas NR2 family members exhibit a more selective distribution. In cortex, NR2B subunits are expressed from late embryonic stages up to adulthood, whereas expression of NR2A subunits only becomes detectable at early postnatal stages and increases to adult levels within about 3 weeks of birth (Mori and Mishina, 1995; Portera-Cailliau et al., 1996; Wenzel et al., 1995; Williams et al., 1993). This expression pattern of NR2A and NR2B subunits is replicated during differentiation of cortical neurons i
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