Fig. 1. Hippocampal granule cell degeneration in adult MR^–/– mice. In Nissl-stained sections, adult MR^–/– mice, but not GR^NesCre mice, show a significant reduction of granule cells in the hippocampus, most pronounced in the upper blade of the dentate gyrus (arrowheads). In the hilus of the dentate gyrus (asterisk), an increase in the number of neurons is seen in MR^–/– mice. In the same areas of dentate gyrus in MR^–/– mice where the granule cell number is reduced, an increase of activated astrocytes is found by immunostaining for activated MAP-kinase (MAPK). Increased cell density in the hilus is also demonstrated by immunostaining for calretinin, a marker for a subset of GABAergic hilus neurons.