Forebrain glucocorticoid receptor overexpression alters behavioral encoding of hippocampal CA1 pyramidal cells in mice
Stress hormone signaling via the glucocorticoid receptor (GR) modulates vulnerability to stress-related disorders, but whether GR influences how the brain encodes contextual experience is unknown. Mice with lifelong GR overexpression in forebrain glutamatergic neurons (GRov) show increased sensitivity to environmental stimuli. This phenotype is developmentally programmed and associated with profound changes in hippocampal gene expression. We hypothesized that GR overexpression influences hippocampal encoding of experiences. To test our hypothesis, we performed in vivo microendoscopic calcium imaging of 1359 dorsal CA1 pyramidal cells in freely behaving male and female WT and GRov mice during exploration of a novel open field. We compared calcium amplitude and event rate as well as sensitivity to center location and mobility between genotypes. GRov neurons exhibited higher average calcium activity than WT neurons in the novel open field. While most neurons showed sensitivity to center location and/or mobility, GRov neurons were more likely to be sensitive to center location and less likely to be sensitive to mobility, as compared to WT neurons. More than one-third of behavior-selective GRov neurons were uniquely sensitive to location without mobility sensitivity; these uniquely center-sensitive neurons were rare in WT. We conclude that dorsal CA1 pyramidal cells in GRov mice show increased activity in a novel environment and preferentially encode emotionally salient behavior. This heightened sensitivity to a novel environment and preferential encoding of emotionally salient elements of experience could underlie differential stress vulnerability in humans with increased glucocorticoid sensitivity. Significance Statement (120 words maximum)Endogenous stress hormones, glucocorticoids, are known to alter vulnerability to stress-related disorders. Here, we find that increased sensitivity to glucocorticoid via lifelong overexpression of glucocorticoid receptor in forebrain neurons (GRov) alters the encoding of environmental experiences in the hippocampus. GRov mice showed increased activity of dorsal CA1 hippocampal pyramidal cells during exploration of a novel environment and more sensitivity to emotionally relevant behaviors. These changes in how hippocampal neurons encode environmental experiences could underlie the differences in stress vulnerability in humans with genetic and epigenetic differences in glucocorticoid receptor signaling.