Volumetric brain differences between the Roman rat strains: Neonatal handling effects, sensorimotor gating and working memory

Abstract

The present study was aimed at evaluating whether the differences between the Roman high- (RHA) and low-avoidance (RLA) rat strains in novelty-induced behavioural inhibition/disinhibition, sensorimotor gating (i.e., prepulse inhibition, PPI) and spatial learning/memory parallel differences in the volume of brain areas related to those behavioural phenotypes. To this aim, we conducted two experiments. In Experiment 1, we evaluated the performance of adult rats from both strains, either untreated (controls) or treated with neonatal handling (NH; administered during the first 21 days of life), in a novel object exploration test (NOE), in the elevated zero-maze test (ZM) of anxiety, and in a PPI test; moreover, magnetic resonance imaging (MRI) was used to measure the volume of limbic and cortical brain regions (amygdala -Am-, hippocampus -Hc-, striatum -St-, medial prefrontal cortex -mPFc-, anterior cingulate cortex -ACC-, nucleus accumbens -NAc-) and lateral ventricles -LV-. In Experiment 2, adult rats neonatally exposed to NH and their naïve controls were submitted to the NOE and PPI tests, and to several spatial learning/memory tasks using the Morris water maze. It was found that, compared with their RLA counterparts, RHA rats show increased exploration of the novel object in the NOE test, lowered anxiety in the ZM and impaired PPI, whereas RLAs display better spatial reference learning and memory and better cognitive flexibility in a reversal task. Furthermore, MRI measurements revealed that the volume of Hc, Am and mPFc is larger in RLA vs. RHA rats, whereas the latter have dramatically enlarged lateral ventricles. NH treatment markedly enhanced exploration in the NOE test in RLA rats, improved PPI in RHA rats but impaired it in their RLA counterparts, and produced beneficial effects on spatial working memory mainly in RHA rats. Finally, exposure to NH decreased the volume of Hc and Am in the RLA strain. The results are discussed in terms of the possible relationships between strain-related volumetric brain differences and the behavioral (anxiety-related and schizophrenia-relevant) traits that distinguish RHA from RLA rats, and highlighting the finding that, in RLA rats, NH is for the first time shown to enduringly reduce the volume of Hc and Am in parallel to the decrease of anxiety and the impairment of sensorimotor gating.

Keywords: Anxiety; MRI; Neonatal handling; Roman rat strains; Sensorimotor gating; Spatial learning.