Influence of hippocampectomy on habituation, exploratory behavior, and spatial memory in rats

Two frequently cited functions of the hippocampus are mediation of spatial memories and habituation. The present investigation employed head-shake response (HSR) as the habituated behavior in intact and bilaterally hippocampectomized rats. This HSR appears to be minimally influenced by spatial cues. These rats were further tested on two behavioral paradigms that make use of spatial cues, namely open field object exploration, and the Morris water maze. The results indicate that hippocampectomized rats revealed habituation of the HSR, but not to objects within the open field. In agreement with previous reports, hippocampectomized rats were severely impaired both in acquiring and recalling the location of the submerged platform in the Morris water maze task. In a separate experiment independent groups of rats were trained on one of these three paradigms, and tissues were collected from hippocampal, prefrontal, and piriform cortices for the measurement of matrix metalloproteinases (MMPs) as markers of neural plasticity. There were significant MMP-9 elevations in the prefrontal and piriform cortices of rats tested using the object exploration task, in the prefrontal and hippocampal cortices of rats that solved the Morris water maze task, but minimal MMP changes in any tissues taken from HSR habituated rats. These results question the hypothesis that habituation is solely mediated by the hippocampus in favor of a process that utilizes different brain structures and degrees of neural plasticity dependent upon task requirements.