Scaling of ventral hippocampal activity during anxiety

The hippocampus supports a multiplicity of functions, with the dorsal region contributing to spatial representations and memory, and the ventral hippocampus (vH) being primarily involved in emotional processing. While spatial encoding has been extensively investigated, how the vH activity is tuned to emotional states, e.g. to different anxiety levels, is not well understood. We developed an adjustable linear track maze (aLTM) for male mice with which we could induce a scaling of behavioral anxiety levels within the same spatial environment. Using in vivo single-unit recordings, optogenetic manipulations and the application of a convolutional non-linear classifier, we examined the changes and causal effects of vH activity at different anxiety levels. We found that anxiogenic experiences activated the vH and that this activity scaled with increasing anxiety levels. We identified two processes that contributed to this scaling of anxiety-related activity: increased tuning and successive remapping of neurons to the anxiogenic compartment. Moreover, optogenetic inhibition of the vH reduced anxiety across different levels, while anxiety-related activity scaling could be decoded using a convolutional non-linear classifier. Collectively, our findings position the vH as a critical limbic region that functions as an 'anxiometer' by scaling its activity based on perceived anxiety levels. Our discoveries go beyond the traditional theory of cognitive maps in the hippocampus underlying spatial navigation and memory, by identifying hippocampal mechanisms selectively regulating anxiety.Significant statement This study reveals how the ventral hippocampus (vH) functions as an "anxiometer", tuning its activity to different anxiety levels. Using an adjustable linear track maze (aLTM) for mice, we demonstrated that vH activity scales with increased anxiety. By recording single-neuron activity and performing optogenetic manipulation of vH during the aLTM task, we identified key neuronal mechanisms for neuronal scaling during anxiety. Additionally, a convolutional non-linear classifier was used to decode anxiety-related activity scaling. Our findings advance the understanding of hippocampal function beyond spatial navigation and memory, offering new insights into how the brain regulates anxiety at the neuronal level.