The OCT3 transporter is highly expressed in several CVOs and found throughout the brain. Sections of rat brain were cut sagittally, except in K, which was cut frontally, stained with OCT3 antibodies, and revealed by immunofluorescence. High expression of OCT3 is shown in AP (A), in SFO (B), in neurons of this structure, as shown by double-labeling with OCT3 antibodies (red) and neuronal marker NeuN antibodies (green) (C), in SCO (D), in the pineal gland (E), and in pinealocytes of this structure as shown by double-labeling of OCT3 (green) with pinealocyte marker VGLUT1 (red) (F). Expression of OCT3 was also seen in frontal cortex (G, H), PVN (lateral magnocellular division) (I), MPO (J), SON (K), and SOL (L). Colocalization of OCT3 (red) with NeuN (G), but not with astrocyte marker GFAP (H) (green), is shown for frontal cortex. For double-labeling experiments, merged pictures are shown on the right. Scale bars: A, B, E, 100 μm ; D, J, 20 μm; C, F–I, K, L, 10 μm.