Differential hippocampal neuron density between inbred Roman high- (low anxious) and low-avoidance (high anxious) rats

The inbred Roman low- (RLA-I) and high-avoidance (RHA-I) rats used in this study were initially selected and bred for extremely poor vs. rapid acquisition of active two-way avoidance behavior in the shuttle box. As a result of the selection for divergent avoidance acquisition, clear behavioral differences have been found between RHA and RLA rats in a variety of tasks related to anxiety and conflict. In rats of these two strains/lines previous brain studies have been performed, specifically in the striatum, the mesencephalic dopaminergic areas and the prefrontal cortex, as these brain areas are the classical ones for their critical role in sensitization and may play a role in the well-characterized anxiety response. In this study we analyzed, in RHA and RLA groups (N = 5 each), the density of NeuN neurons counterstained with toluidine blue in the cingulate cortex (subdivision 1) and the hippocampus (CA1, CA2 and CA3). A statistical difference was found in the density of neurons of CA1 and CA2 (p = 0.047 in both) and in the total density of the hippocampus (p = 0.009). Contrary to our expectations, significant strain differences for the density of neurons in the cingulate cortex were not found. The relationship between those differences in the hippocampus and the between-strain differences in anxiety and in learning processes depending on anxiety are discussed.

► The Roman rat lines/strains (RHA and RLA) are psychogenetically selected.
► They show extreme differences in two-way active avoidance task.
► They differ in hippocampus-dependent anxiety responses and spatial learning/memory.
► RLA rats are more anxious and better spatial learners than RHA rats.
► Inbred RLA-I rats show higher hippocampal neuron density than RHA-I rats.