Objectmarking and object sniffing were assessed in an open field during six 5-min trials in unoperated Long-Evans rats and rats with ibotenate lesions of the hippocampus and/or neocortex. Object marking was higher in hippocampally lesioned rats than in unoperated rats. Object marking did not differ between neocortically lesioned rats and unoperated rats. Object sniffing durations and visits did not differ between unoperated and hippocampally lesioned rats nor between unoperated and neocortically lesioned rats. A new object 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 paths revealed a smaller perimeter track for hippocampally lesioned rats than for unoperated rats. This difference reflected distinct ambulatory patterns. Hippocampally lesioned rats made stereotyped hind-limb pivots at each corner, while normal rats used forelimb pivots or reared and reoriented adjacent to the wall. Rearing was lower in hippocampally lesioned rats, and higher in neocortically lesioned rats, than in unoperated rats. These data indicate that investigative object behavior (sniffing) is resistant to the effects of hippocampal damage, while object-elicited marking and motoric output may be profoundly altered. The data on sniffing suggest either that (1) the noticeability of the objects used elicited investigative behaviors in hippocampally damaged rats comparable to those of novelty-induced exploration in normal rats or (2) object exploration is not used to create a spatial map, and, hence, not disturbed by hippocampal loss. Object marking may require spatial locale information to be exercised normally, or may index the mediation of an olfactory-modulated behavioral pattern through the hippocampal system.
Copyright 1999 Academic Press.