Hippocampal structure and function in the albino rat

Abstract

Theories concerning the function of the hippocampus in behavior have treated this structure as being homogenous in function. Inconsistencies in prior research have sometimes been attributed to differences in procedures or to extra-hippocampal damage resulting from the manipulation. While these factors have been shown to be potent variables, several writers have suggested that lesion location within the hippocampus is important and that smaller lesions are to be preferred to the generally large ones used in most earlier work. Recent work involving the organization of afferent and efferent pathways of the hippocampus has shown these to be well organized in terms of the various "fields" of the structure as distinguished by early anatomists. In the present experiment 14 control operated (HC), 14 hippocampally lesioned (BE), 16 dorsal fornix lesioned (DF) and 16 lateral fimbria lesioned (LF) albino rats were tested in a series of five behavioral tasks in order to assess the relative behavioral effects of gross hippocampal lesions and lesions of the hippocampal efferent pathways. No group differences were found in open field activity levels, or in number of approaches to novel ob.1ects placed in an open field. In an orienting shift task with subjects under 23-hour water deprivation, HE, LF and DF groups had shorter initial latencies to drink than HC subjects, and the HE group made fewer orientations to light stimuli but not to tone stimuli. In a passive avoidance task the HE and LF groups made more errors than the HC. group. In addition the HE group also made more errors than the LF and DF groups. HE and LF groups fought less than the DF and HC groups in a reflexive fighting task. The LF group also demonstrated shorter boxing-posture maintenance relative to the HC and Dr groups at the end of the reflexive fighting session. All groups were similar to each other in a shock threshold rating situation, but the HE group had a lower jump threshold than the HC group. The effects of the gross lesions were similar to those obtained in previous studies. Response perseveration, internal inhibition, and stimulus gating interpretations of hippocampal function were discussed and it was concluded that the gross lesion effects were best explained on the basis of an involvement of anxiety-defensive immobility mechanisms. The pattern of results for the behavioral tasks changed markedly as a function of the efferent path lesion location. Unitary explanations were rejected, and it was concluded that anatomical differences in hippocampal projections are accompanied by separation of function. Two multi-functional interpretations were offered to account for the differences observed.