The Nature of Neural Activity in Hippocampus Evoked by an Object in Place Task

Meghan Nicole Pavelka, Ricardo Salinas

Abstract


The interaction with novel objects and same-sex conspecifics, known as environmental enrichment (EE), affects neural development, preference, and exploratory behavior. Specifically, EE can impact neural networks associated with spatial memory in adolescent rats. In this study, the influence of EE on neural activation in the hippocampal formation (HF) excitatory pathway was investigated. An object place preference (OPP) task, in which rats interacted with newly placed familiar objects in an open field, was used to stimulate hippocampal activity. A group of non-enriched Long Evans rats (n=14), housed in shoebox cages, underwent an OPP task and 60 minutes of quiet/dark prior to being sacrificed. Enriched rats (n=16), housed similarly, experienced the same task after 18, 90-min EE sessions between postnatal days 25 and 48. Trial 2 of the two-trial OPP task showed how exposure to an environment with consistently and newly placed familiar objects evoked activity within major input and output regions of the HF, including CA1, CA3, dentate gyrus (DG), and subiculum. Brain tissue was processed using immunohistochemistry to visualize c-fos protein expression, a marker of neural activation, and neural densities were quantified via microscopy. Considering the HF as a whole, a history of EE reduced neural activation. In one primary input area, DG (30.9% reduction, p < 0.09), and internal processing area CA3 (-18.0%, p < 0.21), activity change from controls was not statistically significant. However, in both CA1 (-30.6%, p < 0.02) and the subiculum (-34.7%, p < 0.02), a primary output zone, EE did reduce neural activation. The results suggest that an EE history suppresses activation as signal moves sequentially throughout the HF. Because EE rats were routinely exposed to novel placement of familiar objects, habituation to such experience may be responsible for the observed forward moving reduction in neural activity in the HF.

Keywords


Adolescence; Environmental Enrichment; Novelty Preference

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