Common genetic factors likely contribute to multiple psychiatric diseases including mood and substance use disorders. Certain stable, heritable traits reflecting temperament, termed externalizing or internalizing, play a large role in modulating vulnerability to these disorders. To model these heritable tendencies, we selectively bred rats for high and low exploration in a novel environment [bred High Responders (bHR) vs. Low Responders (bLR)]. To identify genes underlying the response to selection, we phenotyped and genotyped 538 rats from an F2 cross between bHR and bLR. Several behavioral traits show high heritability, including the selection trait: exploratory locomotion (EL) in a novel environment. There were significant phenotypic and genetic correlations between tests that capture facets of EL and anxiety. There were also correlations with Pavlovian conditioned approach (PavCA) behavior despite the lower heritability of that trait. Ten significant and conditionally independent loci for six behavioral traits were identified. Five of the six traits reflect different facets of EL that were captured by three behavioral tests. Distance traveled measures from the open field and the elevated plus maze map onto different loci, thus may represent different aspects of novelty-induced locomotor activity. The sixth behavioral trait, number of fecal boli, is the only anxiety-related trait mapping to a significant locus on chromosome 18 within which the Pik3c3 gene is located. There were no significant loci for PavCA. We identified a missense variant in the Plekhf1 gene on the chromosome 1:95 Mb QTL and Fancf and Gas2 as potential candidate genes that may drive the chromosome 1:107 Mb QTL for EL traits. The identification of a locomotor activity-related QTL on chromosome 7 encompassing the Pkhd1l1 and Trhr genes is consistent with our previous finding of these genes being differentially expressed in the hippocampus of bHR vs. bLR rats. The strong heritability coupled with identification of several loci associated with exploratory locomotion and emotionality provide compelling support for this selectively bred rat model in discovering relatively large effect causal variants tied to elements of internalizing and externalizing behaviors inherent to psychiatric and substance use disorders.

Emotions are characterized not only by their valence but also by whether they are stable or labile. Yet, we do not understand the molecular or circuit mechanisms that control the dynamic nature of emotional responses. We have shown that glucocorticoid receptor overexpression in the forebrain (GRov) leads to a highly reactive mouse with increased anxiety behavior coupled with greater swings in emotional responses. This phenotype is established early in development and persists into adulthood. However, the neural circuitry mediating this lifelong emotional lability remains unknown. In the present study, optogenetic stimulation in ventral dentate gyrus (vDG) of GRov mice led to a greater range and a prolonged duration of anxiety behavior. cFos expression analysis showed that the amplified behavioral response to vDG activation in GRov mice is coupled to increased neuronal activity in specific brain regions. Relative to wild type mice, GRov mice displayed glutamatergic/GABAergic activation imbalance in ventral CA1 (vCA1) and selectively increased glutamatergic activation in the basal posterior amygdaloid complex. Moreover, forebrain GR overexpression led to increased activation of molecularly distinct subpopulations of neurons within the hippocampus and the posterior basolateral amygdala (pBLA) as evident from the increased cFos co-labeling in the calbindin1+ glutamatergic neurons in vCA1 and in the DARPP-32/Ppp1r1b+ glutamatergic neurons in pBLA. We propose that a molecularly distinct hippocampal-amygdala circuit is shaped by stress early in life and tunes the dynamics of emotional responses.

Two classes of peptide-producing neurons in the arcuate nucleus (Arc) of the hypothalamus are known to exert opposing actions on feeding: the anorexigenic neurons that express proopiomelanocortin (POMC) and the orexigenic neurons that express agoutirelated protein (AgRP) and neuropeptide Y (NPY). These neurons are thought to arise from a common embryonic progenitor, but our anatomical and functional understanding of the interplay of these two peptidergic systems that contribute to the control of feeding remains incomplete. The present study uses a combination of optogenetic stimulation with viral and transgenic approaches, coupled with neural activity mapping and brain transparency visualization to demonstrate the following: (i) selective activation of Arc POMC neurons inhibits food consumption rapidly in unsated animals; (ii) activation of Arc neurons arising from POMC-expressing progenitors, including POMC and a subset of AgRP neurons, triggers robust feeding behavior, even in the face of satiety signals from POMC neurons; (iii) the opposing effects on food intake are associated with distinct neuronal projection and activation patterns of adult hypothalamic POMC neurons versus Arc neurons derived from POMC-expressing lineages; and (iv) the increased food intake following the activation of orexigenic neurons derived from POMC-expressing progenitors engages an extensive neural network that involves the endogenous opioid system. Together, these findings shed further light on the dynamic balance between two peptidergic systems in the moment-tomoment regulation of feeding behavior.