Functional behavioral profiling of multiple Cre-driver rat lines.

The Cre-loxP system has been widely used in neuroscience for spatial and temporal control of gene expression. Although it is considered a powerful genetic tool, increasing evidence suggests that the Cre recombinase itself, without targeting loxP sites, may affect brain function and behavior. In this study, we assessed behavioral phenotypes in several Cre-driver rat lines, each expressing Cre under a different promoter (CAG, Pvalb, TH, Drd2, Tac1, or Thy1) to determine whether Cre expression influences behavioral outcomes. Behavioral testing included open field (for locomotor activity), hot plate (for nociceptive responses), prepulse inhibition (for sensorimotor gating), and contextual and cued fear conditioning (for associative learning). The Drd2-Cre rats exhibited significantly increased locomotor activity and movement speed compared to wild-type controls, suggesting heightened baseline activity. The CAG-Cre rats spent more time in the center area of the open field and showed reduced freezing during fear conditioning, suggesting changes in emotional regulation or learning. In contrast, no significant differences in the nociceptive responses or prepulse inhibition were observed in any of the Cre-driver lines. Following behavioral testing, we conducted structural MRI scans and found no obvious abnormalities in brain morphology across any of the Cre-driver lines, suggesting that the observed behavioral changes may reflect subtle neural alterations. These results suggested that Cre expression can produce promoter-dependent behavioral alterations, particularly in emotion or cognition. Therefore, behavioral validation of Cre-driver lines is recommended in neural circuit studies.