Social Dominance Modulates Stress-induced Neural Activity in Medial Prefrontal Cortex Projections to the Basolateral Amygdala

Acetylcholine muscarinic M1 receptors in the rodent prefrontal cortex modulate cognitive abilities to establish social hierarchy

In most social species, the attainment of social dominance is strongly affected by personality traits. Dominant individuals show better cognitive abilities, however, whether an individual's cognition can determine its social status has remained inconclusive. We found that mice show better cognitive abilities tend to possess a higher social rank after cohousing. The dynamic release of acetylcholine (ACh) in the prelimbic cortex (PL) is correlated with mouse dominance behavior. ACh enhanced the excitability of the PL neurons via acetylcholine muscarinic M1 receptors (M1). Inhibition of M1 impaired mice cognitive performance and induced losing in social competition. Mice with M1 deficiency in the PL performed worse on cognitive behavioral tests, and exhibited lower status when re-grouped with others. Elevating ACh level in the PL of subordinate mice induced winning. These results provide direct evidence for the involvement of M1 in social hierarchy and suggest that social rank can be tuned by altering cognition through cholinergic system.

Distinct transcriptional programs define a heterogeneous neuronal ensemble for social interaction

Reliable representations of information regarding complex behaviors including social interactions require the coordinated activity of heterogeneous cell types within distributed brain regions. Activity in the medial prefrontal cortex is critical in regulating social behavior, but our understanding of the specific cell types which comprise the social ensemble has been limited by available mouse lines and molecular tagging strategies which rely on the expression of a single marker gene. Here we sought to quantify the heterogeneous neuronal populations which are recruited during social interaction in parallel in a non-biased manner and determine how distinct cell types are differentially active during social interactions. We identify distinct populations of prefrontal neurons activated by social interaction by quantification of immediate early gene (IEG) expression in transcriptomically clustered neurons. This approach revealed variability in the recruitment of different excitatory and inhibitory populations within the medial prefrontal cortex. Furthermore, evaluation of the populations of IEGs recruited following social interaction revealed both cell-type and region-specific transcriptional programs, suggesting that reliance on a single molecular marker is insufficient to quantify activation across all cell types. Our findings provide a comprehensive description of cell-type specific transcriptional programs invoked by social interactions and reveal new insights into the heterogeneous neuronal populations which compose the social ensemble.

Dominance status modulates activity in medial amygdala cells with projections to the bed nucleus of the stria terminalis

The medial amygdala (MeA) controls several types of social behavior via its projections to other limbic regions. Cells in the posterior dorsal and posterior ventral medial amygdala (MePD and MePV, respectively) project to the bed nucleus of the stria terminalis (BNST) and these pathways respond to chemosensory cues and regulate aggressive and defensive behavior. Because the BNST is also essential for the display of stress-induced anxiety, a MePD/MePV-BNST pathway may modulate both aggression and responses to stress. In this study we tested the hypothesis that dominant animals would show greater neural activity than subordinates in BNST-projecting MePD and MePV cells after winning a dominance encounter as well as after losing a social defeat encounter. We created dominance relationships in male and female Syrian hamsters (Mesocricetus auratus), used cholera toxin b (CTB) as a retrograde tracer to label BNST-projecting cells, and collected brains for c-Fos staining in the MePD and MePV. We found that c-Fos immunoreactivity in the MePD and MePV was positively associated with aggression in males, but not in females. Also, dominant males showed a greater proportion of c-Fos+ /CTB+ double-labeled cells compared to their same-sex subordinate counterparts. Another set of animals received social defeat stress after acquiring a dominant or subordinate social status and we stained for stress-induced c-Fos expression in the MePD and MePV. We found that dominant males showed a greater proportion of c-Fos+ /CTB+ double-labeled cells in the MePD after social defeat stress compared to subordinates. Also, dominants showed a longer latency to submit during social defeat than subordinates. Further, in males, latency to submit was positively associated with the proportion of c-Fos+ /CTB+ double-labeled cells in the MePD and MePV. These findings indicate that social dominance increases neural activity in BNST-projecting MePD and MePV cells and activity in this pathway is also associated with proactive responses during social defeat stress. In sum, activity in a MePD/MePV-BNST pathway contributes to status-dependent differences in stress coping responses and may underlie experience-dependent changes in stress resilience.

Comparing the ontogeny, neurobiology, and function of social play in hamsters and rats

Syrian hamsters show complex social play behavior and provide a valuable animal model for delineating the neurobiological mechanisms and functions of social play. In this review, we compare social play behavior of hamsters and rats and underlying neurobiological mechanisms. Juvenile rats play by competing for opportunities to pin one another and attack their partner's neck. A broad set of cortical, limbic, and striatal regions regulate the display of social play in rats. In hamsters, social play is characterized by attacks to the head in early puberty, which gradually transitions to the flanks in late puberty. The transition from juvenile social play to adult hamster aggression corresponds with engagement of neural ensembles controlling aggression. Play deprivation in rats and hamsters alters dendritic morphology in mPFC neurons and impairs flexible, context-dependent behavior in adulthood, which suggests these animals may have converged on a similar function for social play. Overall, dissecting the neurobiology of social play in hamsters and rats can provide a valuable comparative approach for evaluating the function of social play.

The interaction between social hierarchy and depression/anxiety: Involvement of glutamatergic pyramidal neurons in the medial prefrontal cortex (mPFC)

Social hierarchy greatly impacts physical and mental health, but the relationship between social hierarchy and depression/anxiety and the underlying neural mechanism remain unclear. The present study used the tube test to determine the social hierarchy status of mice and then performed several behavioral tests to evaluate depression-like and anxiety-like behaviors. Electrophysiological techniques were used to record the firing activities of glutamatergic pyramidal neurons and local field potentials in the medial prefrontal cortex (mPFC). The results suggested that the mice in each cage (4 per cage) established a stable social hierarchy after 2 weeks. Subordinate mice displayed significantly fewer pushing and advancing behaviors, and more retreat behaviors compared with dominant mice. Furthermore, subordinate mice had significantly more immobility durations in the TST, but significantly fewer distances, entries, and time into the center in the OFT, as well as significantly less percent of distances, entries, and time into the open arms in the EPMT, compared with dominant mice, which indicated that subordinate mice displayed depression- and anxiety-like behaviors. In addition, chronic restraint stress (CRS) significantly induced depression- and anxiety-like behaviors in mice and altered social dominance behaviors in the tube test. CRS mice displayed significantly fewer pushing and advancing behaviors, and more retreat behaviors compared with control mice. Furthermore, low social rank and CRS significantly decreased the firing of pyramidal neurons and γ-oscillation activity in the mPFC. Taken together, the present study revealed an inverse relationship between social hierarchy and depression/anxiety, and the neural basis underlying this association might be the excitability of pyramidal neurons and γ oscillation in the mPFC. These findings established an important foundation for a depression/anxiety model based on social hierarchy and provided a new avenue for the development of therapies for stress-related mood disorders.

Insulin and fluoxetine produce opposite actions on lateral septal nucleus-infralimbic region connection responsivity

Some diabetes patients develop depression, the main treatment for which is antidepressants. Pharmacological interactions between insulin and antidepressants (e.g., fluoxetine) are controversial in the literature. Some authors reported hypoglycemic actions of fluoxetine, whereas others reported antidepressant-like actions. In healthy rats, insulin produces an antidespair-like action in rats through an increase in locomotor and exploratory activity, but differences in actions of insulin and fluoxetine on neuronal activity are unknown. The present study evaluated Wistar healthy rats that were treated with saline, insulin, fluoxetine, or fluoxetine + insulin for 3 days (short-term) or 21 days (long-term). The model consisted of electrical stimulation of the lateral septal nucleus (LSN) while we performed single-unit extracellular response recordings in the prelimbic cortex (PL) and infralimbic cortex (IL) subregions of the medial prefrontal cortex (mPFC). Stimulation of the LSN produced an initial brief excitatory paucisynaptic response and then a long-lasting inhibitory afterdischarge in the PL and IL. Treatment with saline and fluoxetine, but not insulin, minimally affected the paucisynaptic response. Differences were found in LSN-IL responsivity. The inhibitory afterdischarge was clearly enhanced in the long-term fluoxetine group but not by insulin alone or fluoxetine + insulin. These findings suggest that insulin produces some actions that are opposite to fluoxetine on LSN-mPFC connection responsivity, with no synergistic actions between the actions of insulin and fluoxetine.

Neural Circuits for Social Interactions: From Microcircuits to Input-Output Circuits

Social behaviors entail responses to social information and requires the perception and integration of social cues through a complex cognition process that involves attention, memory, motivation, and emotion. Neurobiological and molecular mechanisms underlying social behavior are highly conserved across species, and inter- and intra-specific variability observed in social behavior can be explained to large extent by differential activity of a conserved neural network. However, neural microcircuits and precise networks involved in social behavior remain mysterious. In this review, we summarize the microcircuits and input-output circuits on the molecular, cellular, and network levels of different social interactions, such as social exploration, social hierarchy, social memory, and social preference. This review provides a broad view of how multiple microcircuits and input-output circuits converge on the medial prefrontal cortex, hippocampus, and amygdala to regulate complex social behaviors, as well as a potential novel view for better control over pathological development.

Using social rank as the lens to focus on the neural circuitry driving stress coping styles

Social hierarchy position in humans is negatively correlated with stress-related psychiatric disease risk. Animal models have largely corroborated human studies, showing that social rank can impact stress susceptibility and is considered to be a major risk factor in the development of psychiatric illness. Differences in stress coping style is one of several factors that mediate this relationship between social rank and stress susceptibility. Coping styles encompass correlated groupings of behaviors associated with differential physiological stress responses. Here, we discuss recent insights from animal models that highlight several neural circuits that can contribute to social rank-associated differences in coping style.

Delineating the organization of projection neuron subsets in primary visual cortex with multiple fluorescent rabies virus tracing

The impressive functions of the brain rely on an extensive connectivity matrix between specific neurons, the architecture of which is frequently characterized by one brain nucleus/region connecting to multiple targets, either via collaterals of the same projection neuron or several, differentially specified neurons. Delineating the fine architecture of projection neuron subsets in a specific brain region could greatly facilitate its circuit, computational, and functional resolution. Here, we developed multiple fluorescent rabies viruses (RV) to delineate the fine organization of corticothalamic projection neuron subsets in the primary visual cortex (V1). By simultaneously retrograde labeling multiple distinct subsets of corticothalamic projection neurons in V1 from their target nuclei in thalamus (dLGN, LP, LD), we observed that V1-dLGN corticothalamic projection neurons were densely concentrated in layer VI, except for several sparsely scattered neurons in layer V, while V1-LP and V1-LD corticothalamic projection neurons were localized to both layers V and VI. Meanwhile, we observed a fraction of V1 corticothalamic projection neurons targeting two thalamic nuclei, which was further confirmed by fMOST whole-brain imaging. The multiple fluorescent RV tracing tools can be extensively applied to resolve the architecture of projection neuron subsets in certain brain regions, with a strong potential to delineate the computational and functional organization of these brain regions.

Activity of a vmPFC-DRN Pathway Corresponds With Resistance to Acute Social Defeat Stress

The ventromedial prefrontal cortex (vmPFC) plays a critical role in stress resilience through top-down inhibition of key stress-sensitive limbic and hindbrain structures, including the dorsal raphe nucleus (DRN). In a model of experience-dependent stress resistance, socially dominant Syrian hamsters display fewer signs of anxiety following acute social defeat when compared to subordinate or control counterparts. Further, dominants activate vmPFC neurons to a greater degree during stress than do subordinates and become stress-vulnerable following pharmacological inhibition of the vmPFC. Dominants also display fewer stress-activated DRN neurons than subordinates do, suggesting that dominance experience gates activation of vmPFC neurons that inhibit the DRN during social defeat stress. To test whether social dominance alters stress-induced activity of a vmPFC-DRN pathway, we injected a retrograde tracer, cholera toxin B (CTB), into the DRN of dominant, subordinate, and control hamsters and used a dual-label immunohistochemical approach to identify vmPFC neurons co-labeled with CTB and the defeat-induced expression of an immediate early gene, cFos. Results indicate that dominant hamsters display more cFos+ and dual-labeled cells in layers V/VI of infralimbic and prelimbic subregions of the vmPFC compared to other animals. Furthermore, vmPFC-DRN activation corresponded directly with proactive behavioral strategies during defeat, which is indicative of stress resilience. Together, results suggest that recruiting the vmPFC-DRN pathway during acute stress corresponds with resistance to the effects of social defeat in dominant hamsters. Overall, these findings indicate that a monosynaptic vmPFC-DRN pathway can be engaged in an experience-dependent manner, which has implications for behavioral interventions aimed at alleviating stress-related psychopathologies.

Molecular and neurocircuitry mechanisms of social avoidance

Humans and animals live in social relationships shaped by actions of approach and avoidance. Both are crucial for normal physical and mental development, survival, and well-being. Active withdrawal from social interaction is often induced by the perception of threat or unpleasant social experience and relies on adaptive mechanisms within neuronal networks associated with social behavior. In case of confrontation with overly strong or persistent stressors and/or dispositions of the affected individual, maladaptive processes in the neuronal circuitries and its associated transmitters and modulators lead to pathological social avoidance. This review focuses on active, fear-driven social avoidance, affected circuits within the mesocorticolimbic system and associated regions and a selection of molecular modulators that promise translational potential. A comprehensive review of human research in this field is followed by a reflection on animal studies that offer a broader and often more detailed range of analytical methodologies. Finally, we take a critical look at challenges that could be addressed in future translational research on fear-driven social avoidance.

Chemogenetic activation of an infralimbic cortex to basolateral amygdala projection promotes resistance to acute social defeat stress

Tremendous individual differences exist in stress responsivity and social defeat stress is a key approach for identifying cellular mechanisms of stress susceptibility and resilience. Syrian hamsters show reliable territorial aggression, but after social defeat they exhibit a conditioned defeat (CD) response characterized by increased submission and an absence of aggression in future social interactions. Hamsters that achieve social dominance prior to social defeat exhibit greater defeat-induced neural activity in infralimbic (IL) cortex neurons that project to the basolateral amygdala (BLA) and reduced CD response compared to subordinate hamsters. Here, we hypothesize that chemogenetic activation of an IL-to-BLA neural projection during acute social defeat will reduce the CD response in subordinate hamsters and thereby produce dominant-like behavior. We confirmed that clozapine-N-oxide (CNO) itself did not alter the CD response and validated a dual-virus, Cre-dependent, chemogenetic approach by showing that CNO treatment increased c-Fos expression in the IL and decreased it in the BLA. We found that CNO treatment during social defeat reduced the acquisition of CD in subordinate, but not dominant, hamsters. This project extends our understanding of the neural circuits underlying resistance to acute social stress, which is an important step toward delineating circuit-based approaches for the treatment of stress-related psychopathologies.

Odorants: a tool to provide nonpharmacological intervention to reduce anxiety during normal and pathological aging

Anxiety disorders represent 1 of the most common classes of psychiatric disorders. In the aging population and for patients with age-related pathology, the percentage of people suffering of anxiety is significantly elevated. Furthermore, anxiety carries with it an increased risk for a variety of age-related medical conditions, including cardiovascular disease, stroke, cognitive decline, and increased severity of motor symptoms in Parkinson's disease. A variety of anxiolytic compounds are available but often carry with them disturbing side effects that impact quality of life. Among nonmedicinal approaches to reducing anxiety, odor diffusion and aromatherapy are the most popular. In this review, we highlight the emerging perspective that the use of odorants may reduce anxiety symptoms or at least potentiate the effect of other anxiolytic approaches and may serve as an alternative form of therapy to deal with anxiety symptoms. Such approaches may be particularly beneficial in aging populations with elevated risk for these disorders. We also discuss potential neural mechanisms underlying the anxiolytic effects of odorants based on work in animal models.

Development of social play in hamsters: Sex differences and their possible functions

In several rodent species social play appears to be necessary for proper deployment of species-specific patterns of aggressive and reproductive behavior. Specifically, in male Syrian hamsters (Mesocricetus auratus), play has been linked to the development of adult aggression. We quantified several types of social play behavior in same-sex peer groups of Syrian hamsters three times per week for three consecutive weeks after weaning, which included postnatal days 22-42 (PD22 to PD42). Male hamsters increased playful contact during PD36-PD42, whereas females showed peak playful contact during PD29-PD35. These findings suggest that the motivation for social play increases during mid-adolescence in males, but dissipates in females. To investigate the effects of social play deprivation, one hamster per litter remained pair-housed with its mother forthree weeks after weaning its littermates. In adulthood, both play-deprived and play-exposed animals received acute social defeat stress followed by social interaction testing. Play deprivation led to increased defeat-induced social avoidance in both males and females. In males, play deprivation increased fighting back during social defeat stress, whereas in females it reduced aggressive behavior during conditioned defeat testing. We suggest that social play deprivation disrupts neural circuits regulating aggression in a sex-specific manner, perhaps related to sex differences in territorial defense, but has similar effects on neural circuits regulating stress responsivity. Overall, these findings suggest that juvenile social play functions to promote coping with stress and appropriate social behavior in adulthood.

Differences in neurotropism and neurotoxicity among retrograde viral tracers

Background

Neurotropic virus-based tracers have been extensively applied in mapping and manipulation of neural circuits. However, their neurotropic and neurotoxic properties remain to be fully characterized.

Methods

Through neural circuit tracing, we systematically compared the neurotropism discrepancy among different multi-trans-synaptic and mono-synaptic retrograde viral tracers including pseudorabies virus (PRV), rabies virus (RV), and the newly engineered retro adeno-associated virus (rAAV2-retro) tracers. The (single-cell) RNA sequencing analysis was utilized for seeking possible attribution to neurotropism discrepancy and comparing cell toxicity caused by viral infection between glycoprotein-deleted RV (RV-∆G) and rAAV2-retro. Viral toxicity induced microglia activation and neuronal protein change were evaluated by immunohistochemistry.

Results

Multi-trans-synaptic retrograde viral tracers, PRV and RV, exhibit differential neurotropism when they were used for central neural circuit tracing from popliteal lymph nodes. Mono-synaptic retrograde tracers, including RV-∆G and rAAV2-retro, displayed discrepant neurotropic property, when they were applied to trace the inputs of lateral hypothalamic area and medial preoptic nucleus. rAAV2-retro demonstrated preference in cerebral cortex, whereas RV-∆G prefers to label basal ganglia and hypothalamus. Remarkably, we detected a distinct preference for specific cortical layer of rAAV2-retro in layer 5 and RV-∆G in layer 6 when they were injected into dorsal lateral geniculate nucleus to label corticothalamic neurons in primary visual cortex. Complementation of TVA receptor gene in RV-resistant neurons enabled EnvA-pseudotyped RV infection, supporting receptors attribution to viral neurotropism. Furthermore, both RV-∆G and rAAV2-retro exerted neurotoxic influence at the injection sites and retrogradely labeled sites, while the changes were more profound for RV-∆G infection. Finally, we demonstrated a proof-of-concept strategy for more comprehensive high-order circuit tracing of a specific target nucleus by combining rAAV2-retro, RV, and rAAV tracers.

Conclusions

Different multi-trans-synaptic and mono-synaptic retrograde viral tracers exhibited discrepant neurotropism within certain brain regions, even cortical layer preference. More neurotoxicity was observed under RV-∆G infection as compared with rAAV2-retro. By combining rAAV2-retro, RV, and rAAV tracers, high-order circuit tracing can be achieved. Our findings provide important reference for appropriate application of viral tracers to delineate the landscape and dissect the function of neural network.

Electronic supplementary material

The online version of this article (10.1186/s13024-019-0308-6) contains supplementary material, which is available to authorized users.