receptor. 5-HT6 receptor immunoreactive cells in the prefrontal cortex of normal aging (a, c, d) and Alzheimer patients (b, e), cresyl violet counterstain. Label is observed in large pyramidal cells (arrowhead) and small interneurons (arrow). Note positive cells and dense immunoreactive fibers in the molecular layer (d). In the cortex of the Alzheimer patient (b, e), significantly fewer cells are labeled than in that of the normal aging patient (a, c, d). a, b: x200; c, d, e; x400.