Disconnecting hippocampal projections to the anterior thalamus produces deficits on tests of spatial memory in rats

A disconnection procedure was used to test whether projections from the hip pocampus to the anterior thalamic nuclei (AT), via the fimbria-fornix (FX), form functional components of a spatial memory system. The behavioural eff ects of combined unilateral lesions in the AT and FX were compared when the y were either in contralateral hemispheres (AT-FX Contra) or the same hemis phere (AT-FX Ipsi). Other groups received bilateral FX lesions and Sham sur geries. Expt 1 demonstrated that none of these lesions affected performance of an object recognition task, while performance of an object location tas k, which tests the subjects' preference for an object that has changed loca tion, was impaired in the AT-FX Contra and FX groups. In a T-maze alternati on task, however, the FX group was severely impaired while both the AT-FX I psi and AT-FX Contra lesion groups showed only a mild impairment. In order to test whether spared crossed projections might support spatial performanc e in the AT-FX Contra group we then examined the effects of a combined AT-F X Contra lesion coupled with transection of the hippocampal commissure. Thi s combination of lesions produced a severe disruption in spatial memory per formance in the water maze, radial arm maze and T-maze, which was significa ntly greater than that produced by ipsilateral and contralateral AT-FX lesi ons alone. These results support the notion that disconnection of the AT fr om their hippocampal inputs produces impairments on a range of spatial memo ry tasks, but indicate that there are an array of different routes that can subserve this function.