Comparative Analysis of Excitatory and Inhibitory Hippocampal Neurons on Retrieval of Associative Contextual Memory

We report here an analysis of the differential involvement of interneurons and pyramidal neurons of different hippocampal areas in the retrieval of associative aversive contextual memory. This was addressed using a model of associative learning in mice, where formation of a memory of a neutral situation and the subsequent association between short-term presentation of the context and an unconditioned stimulus in the form of footshock (FS) were significantly separated in time. Activation of neurons in different areas of the hippocampus during retrieval of the resulting associative contextual memory was studied by immunohistochemical detection of the protein product of the immediate early gene c-fos. Retrieval of associative memory of a context was accompanied by an increase in the number of c-Fos-positive cells in field CA1 but not in field CA3 or the dentate fascia of the hippocampus. Next, a protein marker – the product of the homeobox-containing gene Emx1 – was used for the specific detection of excitatory neurons and glutamate decarboxylase (GAD) – the product of the genes GAD1 and GAD2 – was used as a marker for the specific detection of inhibitory neurons. Results obtained by double staining for cellular markers and c-Fos protein showed that extraction of the associative aversive memory of the context led to activation of both excitatory neurons positive for the Emx1 marker and, to a lesser extent, inhibitory interneurons positive for the GAD marker in hippocampal field CA1. Independently of the type of behavioral impact (retrieval of associative memory of the context, non-associative memory of the context, or exploration of the context combined with FS but not remembered by the animals), the proportions of activated excitatory and inhibitory neurons remained constant, only the numbers of activated cells of each type changing. These results indicate a specific involvement of neurons in hippocampal field CA1 in associative contextual memory and also demonstrate that both excitatory and inhibitory neurons are involved in encoding this type of memory.