FACILITATION OF OLFACTORY RECOGNITION BY LATERAL ENTORHINAL CORTEX LESION IN RATS

An original olfactory recognition task was developed in order to exami ne the effect of lateral entorhinal cortex (LEG) lesion on olfactory m nesic processes. The task was based on the spontaneous exploratory beh avior of rats toward odor sources. It consisted of a learning phase du ring which an odor was presented twice and in a recognition test, duri ng which the same odor plus a new one was presented. The time rats spe nt sniffing the odor sources was measured. Olfactory recognition was i dentified by a short investigatory duration for the familiar odor as c ompared to a normal investigatory duration for the new odor during the test. The first three experiments aimed to validate the procedure. Ex periment 1 was designed to show the decay of investigatory behavior ca used 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-t est delay was used, but not when a long pre-test delay (120 min) was u sed. Experiment 3 showed that FG7142, a well-known promnesic drug, enh anced the performance of the rats in this test as it allowed recogniti on at longer pre-test delays. The last experiment aimed at testing the effects of aspirative lesion of the LEG. Therefore, LEC-lesioned and sham-lesioned rats were submitted to variable pre-test delays. The exp eriment showed that an entorhinal lesion did not produce an impairment , but on the contrary facilitated olfactory recognition, as lesioned r ats displayed recognition for delays at which sham-operated rats did n ot. These results show that LEC lesion apparently prolongs the duratio n of the olfactory mnesic trace. This effect might result from a modif ication of the functioning of structures innervated by the LEG. In thi s regard, it is noteworthy that LEC lesion produced a sprouting of sep to-hippocampal fibers in the dentate gyrus of the hippocampus as asses sed by acetylcholinesterase staining. Although the functional signific ance of this regrowth is not fully understood, the possible role of th is sprouting should be considered. (C) 1998 Elsevier Science B.V. All rights reserved.