Glutamatergic Projections from the Posterior Complex of the Anterior Olfactory Nucleus to the Amygdala Complexes

Capturing dynamic neuronal responses to dominant and subordinate social hierarchy members with catFISH

Dominance hierarchies are key to social organization in group-living species, requiring individuals to recognize their own and others' ranks. This is particularly complex for mid-ranking animals, who navigate interactions with higher- and lower-ranking individuals. Using in situ hybridization, we examined how mid-ranked mice's brains respond to dominant and subordinate stimuli by labeling activity-induced immediate early genes and neuronal markers. We show that distinct neuronal populations in the amygdala and hippocampus respond differentially across social contexts. In the basolateral amygdala and dorsal endopiriform, glutamatergic Slc17a7+ neurons, particularly dopamine-receptive Slc17a7+Drd1+ neurons, show elevated IEG expression in response to social stimuli, with a higher response to dominant over subordinate animals. Similar response patterns are observed among GABAergic Slc32a1+Oxtr+ neurons in the medial amygdala. We also identified distinct neural ensembles selectively active in response to dominant and subordinate animals by examining cell reactivation over repeated stimulus presentations. We find a higher degree of reactivation among Slc17a7+Oxtr+ ensembles in the endopiriform when the same individual was presented twice in succession. A similar pattern was observed among Oxtr+ neurons in the dentate gyrus hilus, while the inverse was observed among Slc17a7+Avrp1b+Oxtr+ neurons in distal CA2CA3, suggesting distinct encoding or recollection mechanisms across hippocampal subregions. We also highlight methodological advances showing that IEG responses are shaped by stimulus duration and the identity of the IEG and timepoint at which expression is measured. This work lays the foundation for further precise, cell type-resolved investigation into how the brain processes social information.

Rapid changes in plasma corticosterone and medial amygdala transcriptome profiles during social status change reveal molecular pathways associated with a major life history transition in mouse dominance hierarchies

Social hierarchies are a common form of social organization across species. Although hierarchies are largely stable across time, animals may socially ascend or descend within hierarchies depending on environmental and social challenges. Here, we develop a novel paradigm to study social ascent and descent within male CD-1 mouse social hierarchies. We show that mice of all social ranks rapidly establish new stable social hierarchies when placed in novel social groups with animals of equivalent social status. Seventy minutes following social hierarchy formation, males that were socially dominant prior to being placed into new social hierarchies exhibit higher increases in plasma corticosterone and vastly greater transcriptional changes in the medial amygdala (MeA), which is central to the regulation of social behavior, compared to males who were socially subordinate prior to being placed into a new hierarchy. Specifically, the loss of social status in a new hierarchy (social descent) is associated with reductions in MeA expression of myelination and oligodendrocyte differentiation genes. Maintaining high social status is associated with high expression of genes related to cholinergic signaling in the MeA. Conversely, gaining social status in a new hierarchy (social ascent) is related to relatively few unique rapid changes in the MeA. We also identify novel genes associated with social transition that show common changes in expression when animals undergo either social descent or social ascent compared to maintaining their status. Two genes, Myosin binding protein C1 (Mybpc1) and μ-Crystallin (Crym), associated with vasoactive intestinal polypeptide (VIP) and thyroid hormone pathways respectively, are highly upregulated in socially transitioning individuals. Further, increases in genes associated with synaptic plasticity, excitatory glutamatergic signaling and learning and memory pathways were observed in transitioning animals suggesting that these processes may support rapid social status changes.

Naloxone increases conditioned fear responses during social buffering in male rats

Social buffering is the phenomenon in which the presence of an affiliative conspecific mitigates stress responses. We previously demonstrated that social buffering completely ameliorates conditioned fear responses in rats. However, the neuromodulators involved in social buffering are poorly understood. Given that opioids, dopamine, oxytocin and vasopressin play an important role in affiliative behaviour, here, we assessed the effects of the most well-known antagonists, naloxone (opioid receptor antagonist), haloperidol (dopamine D2 receptor antagonist), atosiban (oxytocin receptor antagonist) and SR49059 (vasopressin V1a receptor antagonist), on social buffering. In Experiment 1, fear-conditioned male subjects were intraperitoneally administered one of the four antagonists 25 min prior to exposure to a conditioned stimulus with an unfamiliar non-conditioned rat. Naloxone, but not the other three antagonists, increased freezing and decreased walking and investigation as compared with saline administration. In Experiment 2, identical naloxone administration did not affect locomotor activity, anxiety-like behaviour or freezing in an open-field test. In Experiment 3, after confirming that the same naloxone administration again increased conditioned fear responses, as done in Experiment 1, we measured Fos expression in 16 brain regions. Compared with saline, naloxone increased Fos expression in the paraventricular nucleus of the hypothalamus and decreased Fos expression in the nucleus accumbens shell, anterior cingulate cortex and insular cortex and tended to decrease Fos expression in the nucleus accumbens core. Based on these results, we suggest that naloxone blocks social buffering of conditioned fear responses in male rats.