Fig. 3. Characterization of NMDA-induced morphological changes in dendrites. (A) Immediately after 10 minutes exposure to NMDA at concentrations in the range of 1-100 µM, slices were fixed with 4% paraformaldehyde and the dendrites were labeled with DiI. The density of spines along the length of the dendrites (a), the percentage of dendrites bearing varicosities (b), the density of varicosities along the dendrites (c) and the average size of varicosities (d) were measured. NMDA treatment resulted in a reduction in the spine density and simultaneously caused an increase in the number and size of varicosities in a concentration-dependent manner. No regional difference in these morphological changes was detected among the CA1 region (circle), the CA3 region (triangle) and the DG (square). (B) Slice cultures were fixed after 4, 6 and 10 minutes of NMDA exposure, or at 6, 12, 24, 48, 96 and 144 hours after 10 minute exposure to 3 µM NMDA and the ratio of varicosity-bearing dendrites was measured. The dendritic varicosities appeared within 4 minutes of exposure and were observed in about 80% of dendrites by 6 minutes of exposure, but gradually decreased until 144 hours after removal of NMDA. (C) NMDA (3 or 30 µM) was applied for 10 minutes in the absence or presence of 10 µM MK-801. Immediately, the cultures were fixed and the ratio of dendrites with varicosities was measured in the CA1 region. MK-801 was added to culture medium 30 minutes before NMDA exposure. NMDA-induced varicosity formation was almost completely prevented by MK-801. **P<0.01 versus Control, ^##P<0.01 versus slices treated with corresponding doses of NMDA. Data are the means±s.e.m. of 9-12 slices.