Unreinforced spatial (latent) learning is mediated by a circuit that includes dorsal entorhinal cortex and fimbria fornix

Abstract

The relationship of the entorhinal cortex (EC) and fimbria-fornix (FF) in unreinforced spatial (latent) learning was studied using the conditioned-cue-preference task on an eight-arm radial maze. The maze was turned before every trial to eliminate the use of local cues. During three pre-exposure sessions, food-deprived rats explored the center platform and two adjacent arms of the maze. Since most of the same cues were visible from both arm locations, discriminating them required spatial learning. The rats were then alternately confined to the end of each arm over several days: one arm always contained food, the other was empty. Finally, the rats were allowed free access to both arms with no food present. Normal rats spent more time in their food-paired than in their unpaired arms showing that they learned to discriminate between the arm locations. Bilateral micro-injections of muscimol into the dorsal, but not into the ventral EC, given before the pre-exposure sessions only, impaired the discrimination. The discrimination was also impaired in rats with unilateral lesions of FF and contralateral injections of muscimol into the dorsal EC given before the pre-exposure sessions. Ipsilateral FF lesions and entorhinal inactivation had no effect. These results indicate that the acquisition of information during unreinforced exploration of a novel environment requires an intact circuit involving the dorsal EC and fimbria fornix. Together with previous reports, that this form of learning does not require a functional hippocampus, (Gaskin et al. (2005) Hippocampus 15:1085-1093) the findings also suggest that the acquisition of certain kinds of unreinforced information by this circuit is independent of the hippocampus.