M2R+ structures labeled using Ni-DAB as a chromogen. (A) An M2R+ terminal (M2t) forms a symmetrical synapse with a lightly-immunoreactive axon initial segment (M2ais) identified on the basis of its decorated fasciculated microtubules. The M2ais also receives a symmetrical synapse from an unlabeled terminal (Ut, arrowheads). Two nearby M2R+ spines (M2sp, top and bottom right) receive asymmetrical synapses from unlabeled terminals (Ut, asterisks). (B) A lightly-immunoreactive M2R+ dendrite (M2d) receives symmetrical synaptic input from an M2R+ terminal (M2t, arrowhead) and asymmetrical synaptic input from an adjacent unlabeled terminal (Ut, arrow). Scale bars =0.5 μm.