Ferry B, Herbeaux K, Petoukhova-Traissard N, Galani R, Cassel J, Majchrzak M (2005) Facilitation of conditioned odor aversion by entorhinal cortex lesion in the rat is reversed by cholinergic lesion in the basal forebrain. Neuroscience 2005 Abstracts 881.6. Society for Neuroscience, Washington, DC.
Summary: In the rat, conditioned odor aversion (COA) corresponds to the avoidance of an odorized-tasteless solution (conditioned stimulus, CS) previously associated with toxicosis (unconditioned stimulus, US). Evidence suggests that the entorhinal cortex (EC) is part of the neural substrate involved in the acquisition of COA. Indeed, we showed that EC lesion facilitated CS-US association and rendered it resistant to lengthening of the interstimulus interval (ISI). This facilitation phenomenon might correspond to a lengthening of the olfactory CS memory trace, rendering the association with the subsequent US possible. Because i) all our EC-lesioned rats showed septo-hippocampal cholinergic sprouting, and ii) scopolamine infusions into the dentate gyrus reversed performance in EC-lesioned but not in sham-operated rats in a spontaneous olfactory preference test, we suggested that COA facilitation resulted from enhanced cholinergic activity in the hippocampus. In order to test this hypothesis, we studied the effect of a cholinergic basal forebrain lesion combined to an EC-lesion during COA. Male Long-Evans rats subjected to bilateral EC lesions and intraventricular infusions of the selective toxin 192 IgG-saporin received odor-US pairings with a short or long ISI. Results showed that sham-lesioned rats displayed COA with the short, but not the long ISI, whereas EC-lesioned rats showed COA with both ISI. More interestingly, rats with double lesions did not differ from controls, suggesting that the cholinergic lesion suppressed the effect of EC-lesions. These results strongly suggest that the facilitative effects observed in EC-lesioned animals during COA are due, at least in part, to the septo-hippocampal cholinergic sprouting elicited by the EC lesion. Moreover, they suggest that the hippocampal cholinergic system is involved in the control of memory processes underlying the association between the olfactory CS and the US during acquisition of COA.
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