Fig. 4.

Silver-enhanced immunogold electron microscopy demonstrating heavy immunoparticle labeling for CB₁ in presynaptic terminals forming invaginating synapses on BA pyramidal neurons. (A–D) CB₁ labeling is detected intensely on terminals forming invaginating perisomatic synapses (Iv; A and B) and weakly on those forming axospinous synapses (C and D) in pyramidal neurons (Py). No significant labeling is seen for terminals forming flat perisomatic synapses (Fl, B). (E) Labeling density for CB₁ in the presynaptic membrane of invaginating, flat, or axospinous synapses (colored portions for quantitative measurement) and in the postsynaptic membrane of inhibitory (symmetrical) or axospinous synapses (gray portions). Using 19 electron micrographs taken from three mice, we analyzed the total number of 13, 11, 56, 22, and 61 synaptic elements. *P < 0.01 compared with all other labelings; NS, P > 0.05 compared with postsynaptic labeling at inhibitory synapses; #P < 0.05 compared with presynaptic labeling at flat synapses and postsynaptic labeling at axospinous synapses, Mann–Whitney U test. (F) Parvalbumin (PV) is detected in presynaptic terminals forming flat perisomatic synapses, but not invaginating synapses. Sp, spine; Te, terminal. (Scale bars: 500 μm.)