Increased formation of MISs by the NO donor DETA NONOate and the cell-permeable cGMP analogue 8-Br-cGMP. (A) EM sections illustrating the presence of MISs (asterisks) in slice cultures treated for 2 d with 150 μM DETA NONOate (left) or 5 mM 8-Br-cGMP (right). (B) 3D reconstruction of the two MISs illustrated in A, the left and right spines making synaptic contacts with five and four different axons, respectively. (C) Proportion of MISs observed in randomly selected volume samples of CA1 stratum radiatum in the control situation (ctrl; three slices and 615 spines analyzed), in DETA NONOate–treated cultures (DETA; three slices and 583 spines analyzed), and in 8-Br-cGMP–treated slices (cGMP; three slices and 963 spines analyzed) compared with the analysis of 234 spines in PSD-95–transfected cells (seven slices). Data are mean ± SEM (error bars; *, P < 0.01). (D) Size of individual PSDs on spines from control slices and on MISs of PSD-95–transfected cells and of DETA NONOate– or of 8-Br-cGMP–treated cultures. Data are mean ± SEM (error bars) of 114–269 reconstructed PSDs (*, P < 0.01). Bars, 0.5 μm.