Florey Department of Neuroscience and Mental Health - Theses
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ItemSex differences in fear learning in juvenile rats( 2020)The present thesis examined the sex differences in fear learning in the developing rat. Traditionally, it has been widely assumed that pre-pubertal sex differences are negligible in developmental studies. However, epidemiological studies and sexual dimorphism in the brain prior to puberty, indicate that sex differences in fear learning may emerge early in life. In chapter 2, I assessed renewal, reinstatement and spontaneous recovery of extinguished fear (i.e. relapse of extinguished fear) in juvenile male and female rats. I found that P18 females showed all three fear relapse behaviors while P18 males did not. This finding implied that P18 female rats were able to form the contextual memories while P18 males could not. However, whether the ability to form a context-shock association contributes to fear extinction in juvenile rats, has not been explicitly tested. Therefore in chapter 3a and 3b, I directly compared renewal with context fear, using identical fear conditioning parameters in the developing rats. In chapter 3a, I found that P18 male rats did not show renewal while P25 male rats did. P18 and P25 male rats displayed comparable contextual freezing immediately after conditioning and after 24 hours delay. In chapter 3b, both P18 and P25 female rats displayed renewal. I also found that P18 female rats displayed a developmental deficit in context fear learning compared to P25 female rats. Together, findings from chapter 2, 3a and 3b strongly suggested that the emergence of context memory may be sex-dependent, where context-specific extinction emerges earlier in females while context fear learning emerges first in males. Given the critical role of the hippocampus in context-dependent learning, I thought the sex differences in the hippocampal function may underlie the observed sex differences in fear learning. In chapter 4, I examined the role of dHPC and vHPC in fear extinction in P18 male and female rats. I showed that temporary inactivation of the dHPC prior to extinction accelerated extinction acquisition in both male and female rats. I also showed that pre extinction inactivation of the vHPC reduced freezing during extinction and impaired extinction recall, regardless of sex. Collectively, these findings strongly suggest sexually dimorphism in fear learning emerges early in life and emphasize the importance of sex as a factor in the field of developmental learning and memory.