Title

Investigations of limbic structures on a novel conditioned discrimination task: Lesion studies, hippocampal place cells, and the effects of age

Date of Completion

January 2004

Keywords

Biology, Neuroscience

Degree

Ph.D.

Abstract

A task was developed to simultaneously examine place learning and fear motivated context discrimination. Rats were trained to alternate along a high-walled, diamond shaped runway between two chambers for food. On half the trials, a tone signaled the fact that a fixed section at the apex of the runway was electrified. The animals were required to cross over this shock region to enter the successive goal chamber and gain reinforcement. The behavioral performance of rats with small lesions of the amygdala or dorsal hippocampus was compared to that of sham lesioned controls. All groups displayed significant discrimination, hesitating more on tone trials while in areas of the runway adjacent to the shock region. Animals with lesions to the hippocampus were similar to controls in tone discrimination, yet were delayed in learning the location of the shock region. Conversely, amygdala lesions did not affect place learning, however these animals were impaired in their suppression of the fear response following repeated unpaired trials. ^ Hippocampal unit activity was recorded from animals performing the task, and was analyzed to investigate the effect of discriminative fear conditioning on place field remapping. Rats of varying ages were used as subjects in order to investigate whether the nature of place cell re-mapping was different across the life span. The majority of neurons recorded were unaffected by the discriminative behavior, and did not change spatial firing patterns across conditions. This finding is in contrast with previous studies in which substantial proportions of cells displayed task-specific activity. In those previous experiments, the pathways traversed by the animals were not constrained, and in fact, the pathways taken in order to receive reinforcement changed between tasks. Therefore, the results presented in this dissertation seem to demonstrate that place field re-mapping (in dorsal regions of hippocampus) is driven primarily by changes in routes traveled, rather than by altered emotional or motivational context. Still, a small percentage of neurons displayed condition-specific activity, indicating that the hippocampus is not an exclusively “spatial”, and can also encode changes in context. Finally, there was no effect of age on the extent of re-mapping observed. ^