Open field motor patterns and object marking, but not object sniffing, arealtered by ibotenate lesions of the hippocampus

Object marking and object sniffing were assessed in an open field during si x 5-min trials in unoperated Long-Evans rats and rats with ibotenate lesion s of the hippocampus and/or neocortex. Object marking was higher in hippoca mpally lesioned rats than in unoperated rats. Object marking did not differ between neocortically lesioned rats and unoperated rats. Object sniffing d urations and visits did not differ between unoperated and hippocampally les ioned rats nor between unoperated and neocortically lesioned rats. A new ob ject elicited longer sniffing by both unoperated and hippocampally lesioned rats. Neocortically lesioned rats did not show this effect. There were no effects of the new object on marking. Computerized tracing of open field pa ths revealed a smaller perimeter track for hippocampally lesioned rats than for unoperated rats. This difference reflected distinct ambulatory pattern s. Hippocampally lesioned rats made stereotyped hind-limb pivots at each co rner, while normal rats used forelimb pivots or reared and reoriented adjac ent to the wall. Bearing was lower in hippocampally lesioned rats, and high er in neocortically lesioned rats, than in unoperated rats. These data indi cate that investigative object behavior (sniffing) is resistant to the effe cts of hippocampal damage, while object-elicited marking and motoric output may be profoundly altered. The data on sniffing suggest either;that (I) th e noticeability of the objects used elicited investigative behaviors in hip pocampally damaged rats comparable to those of novelty-induced exploration in normal rats or (2) object exploration is not used to create a spatial ma p, and, hence, not disturbed by hippocampal lass. Object marking may requir e spatial locale information to be exercised normally, or may index the med iation of an olfactory-modulated behavioral pattern through the hippocampal system. (C) 1999 Academic Press.