Using fos imaging in the rat to reveal the anatomical extent of the disruptive effects of fornix lesions

Activity of the immediate early gene c-fos was compared across hemispheres in rats with unilateral fornix lesions. To engage Fos production, rats first performed a radial arm maze task that is severely disrupted by bilateral fornix lesions. Using immunohis-tochemical techniques, Fos-positive cells were visualized and counted in 39 sites in both hemispheres. Fornix lesions led to a significant reduction in Fos in all ipsilateral hippocampal sub-fields, as well as the entorhinal cortex and most of the subicular complex. Other sites that showed reduced activity included the ipsilateral retrosplenial, anterior cingulate, and postrhinal corti-ces. Subcortical regions showing significant Fos decreases in-cluded the anterior thalamic nuclei, supramammillary nucleus, diagonal band of Broca, and lateral septum. Thus, the effects of fornix lesions extended beyond the hippocampal formation and included sites not directly innervated by the tract. These change