Temporal effects of discrete lesions in the olfactory and limbic systems on open-field behavior and dyadic encounters in male hooded rats

Abstract

The spontaneous and dyadic behavior patterns of five matched groups of male hooded rats were compared to SHAMs following selective lesions of the olfactory and limbic system. One group received bilateral lateral olfactory tract lesions (LOT); a second bilateral anterior olfactory nucleus lesions (AON); a third sham operations (SHAM); a fourth bilateral AON and LOT lesions combined (ANLT); and a fifth cortico-medial amygdalar lesions (AMYG). When tested alone in the open field, LOT, ANLT, and, to some extent, AON lesioned animals were hyperactive. Activity scores for the AMYGs were not significantly different from that of the SHAMs. High rearing time scores were seen in the LOT, ANLT, and AMYG groups while high sniffing frequency and rearing frequency scores were associated with LOT and ANLT lesions. AONs had significantly higher and LOTs significantly lower sniffing time scores than any other groups. The AMYGs had the highest and LOTs the lowest sniffing bout lengths. Sniffing time for the ANLTs did not differ significantly from that of the SHAMs. Freezing was reduced in all lesioned groups as compared to SHAMs. These differences did not become clearly apparent until 7 to 10 days following surgery and were evident 40 days postsurgically. All lesioned animals were hyperirritable and difficult to handle following surgery with the LOTs, ANLTs, and AMYGs particularly so. This irritability was evident on the first day following surgery and persisted through postoperative day 40. When tested in pairs in a circular open field, all animals spent a large proportion of their time in direct contact. All lesioned animals, however, spent more time in aggressive interactions than did the SHAMs. The results were discussed against a background of other olfactory and limbic system lesions with particular reference to possible functional and anatomical substrates mediating behavioral changes typically reported following olfactory system lesions.