Excitotoxic lesions of the pre- and parasubiculum disrupt the place fields of hippocampal pyramidal cells

Abstract

To determine what influence the pre- and parasubiculum regions of the hippocampal formation have on neural representations within the dorsal hippocampus, single-unit recordings were made as rats with bilateral ibotenic acid lesions centered on the former regions (n = 4) or control surgeries (n = 3) foraged freely. Spatial firing specificity was measured using an information content procedure. Cells from lesioned animals (n = 57) provided significantly less spatial information than cells from control animals (n = 44). Whereas some degree of location-related activity (place fields) was observed in 98% of neurons recorded from control animals, it was observed in only 65% of the neurons from lesioned animals. The spatial resolution of the intact place fields appeared to be compromised in lesioned animals as a result of their having a higher firing rate outside the place field. These findings indicate that the pre- and parasubiculum regions have a major role in maintaining the specificity of the place field firing of hippocampal pyramidal cells. Since previous data indicate that these lesioned animals displayed delay-dependent deficits in spatial tasks, these findings also suggest that a disruption in place field activity may be a causal factor in this spatial memory deficit.