Project Summary The ability to recognize dangerous situations and environments is crucial for survival, but overvaluing risk can lead to pathological avoidance of normal activities, potentially leading to anxiety disorders. Many studies over the past several decades have begun to identify the brain regions underlying threat detection and anxiety behavior. In particular, the ventral hippocampus has emerged as a critical structure for emotional behaviors, including innate anxiety. Recently work from our lab and others has shown that neurons in the ventral dentate gyrus and CA1 encode information about anxiety, and these CA1 neurons preferentially target downstream structures such as hypothalamus and medial prefrontal cortex. The mechanism by which this anxiety representation arises in the ventral hippocampus is still unknown, but it is likely that interneurons play a critical role in shaping this activity. In this proposal, I will test my hypothesis that excitatory interneurons in the ventral dentate gyrus called mossy cells are preferentially active during exploration of anxiogenic contexts and are critical for anxiety-related behavior. I will test this hypothesis by combining freely-moving calcium imaging and optogenetics in a mossy cell-specific Cre-driver line. In Aim 1 I will record calcium activity in ventral mossy cells during behavioral tests of anxiety in rodents. In Aim 2 I will use bilateral optogenetic inhibition to test whether this activity is necessary for normal avoidance behavior. To determine how mossy cells influence the output of the dentate gyrus, in Aim 3 I will simultaneously record calcium activity in granule cells while optogenetically inhibiting mossy cell activity during anxiety behavior. These experiments will shed light on a poorly understood component of the hippocampus circuit and provide insight into circuit mechanisms of anxiety representation in the mammalian brain.