Sexually divergent expression of active and passive conditioned fear responses in rats

Animals can respond to fear in a variety of ways, such as by standing still (freezing), or rapidly escaping from an apparent threat. Freezing is the most common measure of fear that has been used in research studies. However, since the vast majority of these experiments have used male animals, it is not clear if freezing is a sufficient measure of fear in females. To address this question, Gruene et al. analyzed different types of fear responses in large groups of male and female rats. The experiments used a technique called cued fear conditioning, which pairs a sound with a mild electrical shock to a foot. When rats learn that the sound predicts the shock, the sound alone causes them to produce a fear response. However, if the sound is then played repeatedly without a footshock, the rats learn to become less fearful of the sound in another learning process called “extinction”. The experiments found that females were four times more likely than males to display fear in the form of rapid movements (referred to as “darting”). Animals that displayed darting were also less likely to freeze in response to the sound cue, which suggests that darting may represent an alternative fear strategy that is more common in females. During a subsequent extinction test, females that darted also displayed quicker reductions in both types of fear responses, which suggests that darting might be an active coping response that promotes long term reductions in fear. Gruene et al.’s findings suggest that there are differences in the ways that males and females respond in fear of a threatening stimulus, and highlight the importance of analyzing a variety of fear responses in experiments. The next steps will be to understand the biological basis of the darting response in female rats.