Trypsin sensitivity of the heterodimer GluN1R/GluN2BR with spermine or ifenprodil and of GluN1R or GluN2BR monomer. The heterodimer GluN1R/GluN2BR with and without spermine (100 μM) or ifenprodil (1 μM) was treated with trypsin, and the level of GluN1R and GluN2BR was measured as described under Materials and Methods. Complexes were immunoprecipitated with anti-GluN1 antibody and detected by Western blot analysis with anti-GluN1 or anti-GluN2B antibodies. In the right panel, GluN1R or GluN2BR monomer was incubated with trypsin and detected by Western blot analysis with anti-GluN1 or anti-GluN2B antibodies. The level of protein was quantified with a LAS-3000 luminescent image analyzer (Fuji Film, Tokyo, Japan). Relative amounts are shown as mean ± S.E. of triplicate determinations. Student's t test was performed for the control values versus the values obtained with trypsin treatment. **, p < 0.01; ***, p < 0.001.