Spatial Working Memory Performance on a
Radial Arm Maze After Bilateral Fimbria-Fornix Transection in Rats
The fimbria-fornix (FF) is a major fiber bundle connecting the hippocampal formation to the septum, hypothalamus, mammillary bodies, and other cortical and subcortical structures. Although lesions of the FF have produced impairments in spatial working memory, it is unclear which of the damaged connections and neurotransmitter systems are critical for this type of memory. In fact, the majority of studies involving bilateral lesions of the FF have produced significant damage to the hippocampal commissure and other structures surrounding the FF. In the present study, rats underwent bilateral FF transection by a procedure that minimizes damage to hippocampal commissural pathways. For six weeks, spatial working memory performance of animals with transections or sham craniotomies was assessed on the radial arm maze (RAM), using a win-shift task with 30s confinement in each arm. Optical densitometry analysis, performed blind to surgery group, quantified acetylcholinesterase (AChE) staining density in the outer molecular layer and supragranular zone of the dentate gyrus. Transection animals with at least 25% staining reduction from the sham average, both dorsally and ventrally, committed significantly more spatial working memory errors than did sham animals across the 6 weeks of testing; they also took longer to acquire the task. Transection animals were no more likely than sham animals to use non-spatial strategies to solve the task. These findings suggest that signals passing through the FF, rather than the hippocampal commissure, are necessary both for unimpaired spatial working memory and spatial task acquisition.