Transcriptional Profiling of the Hippocampus in an F2 Cross of a Genetic Rat Model of Internalizing vs. Externalizing Behavior and Addiction Liability

In humans, differences in temperament are highly predictive of classes of psychopathology classified as externalizing versus internalizing disorders. To better understand the genetic and neural causes of temperamental differences, we have selectively bred two lines of rats based on their exploration of a novel environment. The bred High Responder (bHR) and bred Low Responder (bLR) rats show contrasting heritable behaviors that map onto human temperamental differences and are associated with two paths to drug abuse—sensation seeking and reactivity to psychosocial stress. To elucidate the genes that underlie the divergent behavior of bHR/bLR rats, we created a bHRxbLR F0-F1-F2 cross and performed behavioral testing, transcriptional profiling, and genetic sequencing. We used RNA-Seq to characterize hippocampal tissue in F0s (n=24, n=6 per phenotype /sex) and F2s (n=250, n=125 per sex) to identify differentially expressed genes related to bHR/bLR lineage and phenotypical behaviors: locomotor response to novelty, anxiety, and Pavlovian Conditioned Approach (PavCA). We found that bHR/bLR phenotypical behaviors remained correlated in the F2s, implying a shared genetic basis. We also found robust differences in hippocampal transcriptional profiles associated with bHR/bLR lineage in the F0s which surpassed the differences associated with sex. These distinct expression profiles were predictive of gene expression patterns associated with F2 bHR/bLR behavior. We then prioritized bHR/bLR differentially expressed genes identified in our current and previous studies as candidates for mediating bHR/bLR phenotypical behaviors in F2s. Seventeen genes were differentially expressed in association with locomotor response to novelty, many of which also had nominal relationships with PavCA behavior. Seven of these genes were located near (+/−1MB) quantitative trait loci for bHR/bLR phenotypical behavior identified in our previous exome sequencing or genome wide association studies: AABR07071904, Ucp2, Ttc30a1, Fzd6, Spg7, Vps9d1, and Afg3l1. We also identified convergence between our model and other genetic rat models targeting internalizing behaviors, with 26 hippocampal genes showing shared patterns of differential expression, including Tmem144 and Mfge8. Our findings provide strong candidates for elucidating genetic and neurobiological factors that may shape differences in temperament and modulate vulnerability to psychiatric and addictive disorders. ### Competing Interest Statement The authors have declared no competing interest.