Decreased inhibitory synapse density in the Lrfn2^−/− hippocampus. A, B, Decreased density of inhibitory, but not excitatory, synapses in the stratum radiatum region of the CA1 region in Lrfn2^−/− mice (P21), as determined by EM analysis. Excitatory synapses were defined by PSD structures apposed to axon terminals (arrows and double arrowheads for nonperforated and perforated PSD, respectively) and inhibitory synapses were defined by PSDs apposed to presynaptic axon terminals with immunopositive GABA signals (arrows). Asterisks, GABA immunopositive axon terminals; d, postsynaptic dendrites; single arrowhead, GABA-immunopositive axon. Scale bar, 500 nm. n = 3 mice for WT and KO. **p < 0.01, NS, Student's t test.