Insulin receptors are expressed by axons in hippocampus and do not localize to the same cellular compartments as MHCI, consistent with non-cell-autonomous interactions. A, B, Double-label immunohistochemistry for IRβ (green) and the dendritic marker MAP2 (red) or the axonal marker TAU (red). IRβ does not colocalize with MAP2 (yellow, merge in A), but colocalizes with TAU (yellow, merge in B) in area CA3 stratum lucidum of WT hippocampus. Scale bars, 20 μm. C, Double labeling for IRβ (green) and the dentate granule cell marker calbindin (red). IRβ and calbindin colocalize extensively in dentate granule cell axons (mossy fibers). Scale bar, 100 μm. D, Double-label cytochemistry for IRβ (green) and calbindin (red) in WT hippocampal cultures at 18 DIV. IRβ labeling colocalizes with calbindin-positive neurons (yellow, merge). Scale bar, 10 μm. E, P30 mouse hippocampus immunostained for IRβ (green) and MHCI (red). Boxed region (top right) is magnified in bottom. Punctate MHCI labeling is detected in processes in apposition to IRβ-positive axons in stratum lucidum of hippocampus (merge). sp, Stratum pyramidale; sl, stratum lucidum. Scale bars: top, 100 μm; bottom, 10 μm.