Intact Female Mice Acquire Trace-Eyeblink Conditioning Faster Than Male and Ovariectomized Female Mice

Abstract

Female subjects have been widely excluded from past neuroscience work due to a number of biases, including the notion that cycling sex hormones increase variability. However, it is necessary to conduct behavioral research in mice that includes both sexes as mice are typically used for developing and evaluating future therapeutics. Understanding sex differences in learning is fundamental for the development of targeted therapies for numerous neurological and neurodegenerative disorders, including Alzheimer’s disease, which is more prevalent in females than males. This study set out to confirm the role of sex and necessity of circulating ovarian hormones in the acquisition of the temporal associative memory task trace eyeblink conditioning in C57BL/6J mice. We present evidence that sex and ovarian hormones are important factors in learning. Specifically, intact female mice learn significantly faster than both male and ovariectomized female mice. Data from pseudoconditioned control mice indicate that sex differences are due to differences in learned associations, not sensitization or spontaneous blink rate. This study strengthens the idea that ovarian hormones such as estrogen and progesterone significantly influence learning and memory and that further research is needed to determine the underlying mechanisms behind their effects. Overall, our findings emphasize the necessity of including both sexes in future behavioral studies.

SIGNIFICANCE STATEMENT Preclinical research commonly employs mice and it is imperative to understand differences between females and males that may impact the success of future therapies. Our study found that intact female mice learned at a faster rate than male and ovariectomized female mice in trace-eyeblink conditioning, a temporal associative memory task. While all mice successfully acquired the task, ovariectomized females were impaired compared to intact females throughout the course of training, including during the final day of training. These differences suggest additional research is needed on the role of ovarian hormones and the mechanisms underlying their effects on learning and memory.