Fig. 1.

Induction of Arc expression after different behavioral experiences. The photomicrographs illustrate Arc mRNA distribution as revealed by nonisotopic in situhybridization. A, Control animals left undisturbed in their home cage (isolated controls). B, After 1 hr of exploration of a novel complex environment. C, One hour after four trials (1 session) of training in a Morris water maze.D, One hour after the last of 4 d of training (4 trials per day) in a Morris water maze. Note the experience-induced increases in labeling in both the cell layers and dendritic layers of the CA1 region of the hippocampus and the increase in the number of dentate granule cells exhibiting high levels of labeling.CA1 and CA3 indicate the respective divisions of the hippocampus; DG refers to granule cell layer, in which heavily labeled granule cells can be seen. Short arrows in B indicate labeled dendrites.Long arrows indicate the boundary of dendritic labeling at the CA1/CA2 junction. E, The graph plots the average numbers of Arc-positive dentate granule cells per square millimeter in the different groups. Note the substantial increases in the number of labeled granule cells in the dorsal blade in each of the experimental groups.