The Rodent Hippocampus Is Essential for Nonspatial Object Memory

Elucidating the role of the rodent hippocampus in object recognition memory is critical for establishing the appropriateness of rodents as models of human memory and for their use in the development of memory disorder treatments. In mammals, spatial memory [1-6] and nonspatial memory [7, 8] depend upon the hippocampus and associated medial temporal lobe (MTL) structures. Although well established in humans [1, 9], the role of the rodent hippocampus in object memory remains highly debated due to conflicting findings across temporary and permanent hippocampal lesion studies [10-22] and evidence that the perirhinal cortex may support object memory [17, 23, 24]. In the current studies, we used intrahippocampal muscimol microinfusions to transiently inactivate the male C57BL/6J mouse hippocampus at distinct stages during the novel object recognition (NOR) task: during object memory encoding and consolidation, just consolidation, and/or retrieval. We also assessed the effect of temporary hippocampal inactivation when objects were presented in different contexts, thus eliminating the spatial or contextual components of the task. Lastly, we assessed extracellular dorsal hippocampal glutamate efflux and firing properties of hippocampal neurons while mice performed the NOR task. Our results reveal a clear and compelling role of the rodent hippocampus in nonspatial object memory.