Distinct Transcriptomic Profiles in the Dorsal Hippocampus and Prelimbic Cortex Are Transiently Regulated following Episodic Learning

Abstract

A fundamental, evolutionarily conserved biological mechanism required for long-term memory formation is rapid induction of gene transcription upon learning in relevant brain areas. For episodic types of memories, two regions undergoing this transcription are the dorsal hippocampus (dHC) and prelimbic (PL) cortex. Whether and to what extent these regions regulate similar or distinct transcriptomic profiles upon learning remain to be understood. Here, we used RNA sequencing in the dHC and PL cortex of male rats to profile their transcriptomes in untrained conditions (baseline) and at 1 h and 6 d after inhibitory avoidance learning. We found that, of 33,713 transcripts, >14,000 were significantly expressed at baseline in both regions and ∼3000 were selectively enriched in each region. Gene Ontology biological pathway analyses indicated that commonly expressed pathways included synapse organization, regulation of membrane potential, and vesicle localization. The enriched pathways in the dHC were gliogenesis, axon development, and lipid modification, while in the PL cortex included vesicle localization and synaptic vesicle cycle. At 1 h after learning, 135 transcripts changed significantly in the dHC and 478 in the PL cortex; of these, only 34 were shared. Biological pathways most significantly regulated by learning in the dHC were protein dephosphorylation, glycogen and glucan metabolism, while in the PL cortex were axon development and axonogenesis. The transcriptome profiles returned to baseline by 6 d after training. Thus, a significant portion of dHC and PL cortex transcriptomic profiles is divergent, and their regulation upon learning is largely distinct and transient.SIGNIFICANCE STATEMENT Long-term episodic memory formation requires gene transcription in several brain regions, including the hippocampus and PFC. The comprehensive profiles of the dynamic mRNA changes that occur in these regions following learning are not well understood. Here, we performed RNA sequencing in the dorsal hippocampus and prelimbic cortex, a PFC subregion, at baseline, 1 h, and 6 d after episodic learning in rats. We found that, at baseline, dorsal hippocampus and prelimbic cortex differentially express a significant portion of mRNAs. Moreover, learning produces a transient regulation of region-specific profiles of mRNA, indicating that unique biological programs in different brain regions underlie memory formation.

Keywords: RNAseq; hippocampus; memory; prelimbic cortex; transcriptomic.