Age and sex independently influence safety learning in mice

Difficulty discriminating between threat and safety cues is a hallmark symptom of many fear-related disorders including anxiety, which are highly prevalent during adolescence. Moreover, females are disproportionately affected by fear and anxiety disorders. Correspondingly, growing evidence suggests that sex differences in fear responding and regulation are readily apparent in adults. Yet, it remains unclear when these differences emerge throughout development and what behavioral factors may drive them. Using adult (postnatal day/PND 69-128) and adolescent (PND 29-34) mice, we set out to study age and sex differences in learning about cues explicitly indicating safety (i.e., safety learning). Our results revealed that female mice of both ages froze more and discriminated less than males of both ages during initial discriminative conditioning. All mice showed evidence of conditioned inhibition during a summation test, though the magnitude of suppression was smaller in females. During an expanded summation test that included novel cues and different combinations of stimulus parameters, females of both ages exhibited higher fear generalization to novel cues. In addition, adolescents of both sexes failed to inhibit fear using a safety cue, suggesting that adolescent learning may be disrupted by complex experimental design. Finally, neural activity (cFos expression) was greater in the prefrontal cortex and ventral hippocampus of adolescents relative to adults, and the retrosplenial cortex and ventral hippocampus of males relative to females. Together, these findings illustrate the potential to take age and sex into account in the identification and treatment of fear and anxiety disorders.