Facilitation of olfactory recognition by lateral entorhinal cortex lesion in rats

An original olfactory recognition task was developed in order to examine the effect of lateral entorhinal cortex (LEC) lesion on olfactory mnesic processes. The task was based on the spontaneous exploratory behavior of rats toward odor sources. It consisted of a learning phase during which an odor was presented twice and in a recognition test, during which the same odor plus a new one was presented. The time rats spent sniffing the odor sources was measured. Olfactory recognition was identified by a short investigatory duration for the familiar odor as compared to a normal investigatory duration for the new odor during the test. The first three experiments aimed to validate the procedure. Experiment 1 was designed to show the decay of investigatory behavior caused by repeated exposure of the rats to one odor. Experiment 2 showed that normal rats display recognition when a short (5 or 40 min) pre-test delay was used, but not when a long pre-test delay (120 min) was used. Experiment 3 showed that FG7142, a well-known promnesic drug, enhanced the performance of the rats in this test as it allowed recognition at longer pre-test delays. The last experiment aimed at testing the effects of aspirative lesion of the LEC. Therefore, LEC-lesioned and sham-lesioned rats were submitted to variable pre-test delays. The experiment showed that an entorhinal lesion did not produce an impairment, but on the contrary facilitated olfactory recognition, as lesioned rats displayed recognition for delays at which sham-operated rats did not. These results show that LEC lesion apparently prolongs the duration of the olfactory mnesic trace. This effect might result from a modification of the functioning of structures innervated by the LEC. In this regard, it is noteworthy that LEC lesion produced a sprouting of septo-hippocampal fibers in the dentate gyrus of the hippocampus as assessed by acetylcholinesterase staining. Although the functional significance of this regrowth is not fully understood, the possible role of this sprouting should be considered.