Excitotoxic lesion of the hippocampus of Wistar rats disrupts the circadian control of the latent inhibition of taste aversion learning.

Previous experiments have shown that changes in the time of day between taste pre-exposure and conditioning prevent the latent inhibition of conditioning taste aversion. The effect of these changes in circadian context between pre-exposure and conditioning on the magnitude of the learned aversion appears to be similar to the effect of changes in spatial context on this type of learning. To elucidate the brain areas involved in this circadian dependence of latent inhibition of conditioning taste aversion, the effect of excitotoxic lesions of the hippocampus, a region related to spatial-contextual modulation in this learning process, was analyzed. The latent inhibition of conditioning taste aversion in animals with hippocampal lesions, that were pre-exposed and conditioned to the same or different time of day, was compared with the response of animals exposed to either conditions ("same" or "different") but had undergone amygdala lesions or sham lesions. The results showed that selective dorsal hippocampus lesion eliminated the circadian dependence of latent inhibition of taste aversion. A change in the time of day between pre-exposure and conditioning did not prevent latent inhibition in animals with hippocampal lesions. In contrast, this change prevented latent inhibition in the amygdala-lesioned and sham groups. These findings suggest that the hippocampus contains a selective mechanism that modulates the contextual dependency of the latent inhibition of conditioning taste aversion without interfering with the effect of taste pre-exposure itself. This study may help to understand the possible common involvement of the hippocampus in different types of contextual control of associative learning.