Entorhinal but not hippocampal or subicular lesions disrupt latent inhibition in rats

Latent inhibition (LI) is the deficit of conditioning resulting from repeat ed nonreinforced preexposure to a conditioned stimulus before its pairing w ith an unconditioned stimulus. There are cumulative data showing that large lesions of the hippocampal formation disrupt LI. However, the effects of s elective lesions of the different components of the hippocampal formation h ave never been directly addressed in the same study and conditioning paradi gm. The first experiment of the present study aimed at investigating the ef fects of excitotoxic lesions of the hippocampus, subiculum, or entorhinal c ortex on LI in an "off-baseline"-conditioned emotional response procedure. Hippocampus or subiculum lesions had no effect on either LI or conditioning . In contrast, entorhinal cortex lesions disrupted LI without modifying con ditioning. In Experiment 2, locomotor activity in a novel environment was a ssessed in the same rats. Whereas lesions of hippocampus increased locomoto r activity, lesions of the subiculum or the entorhinal cortex were devoid o f effect. Although both LI and habituation to novel environmental cues are thought to involve interactions between the hippocampal formation and the m esolimbic pathway, these results indicate a functional dissociation between the hippocampus and the entorhinal cortex. (C) 1999 Academic Press.