Many species use social cues or signals to guide the expression of contextually appropriate behavior, yet little is known about how the brain processes such information. We are currently investigating this question by exposing mice to social cues and analyzing neural excitation and the expression of other receptors and neurotransmitters in the brain using various histological techniques.
Chemical signals, such as pheromones and urinary proteins, are perhaps the most common mode of communication used by social species to convey identifying information including sex, species, status, and individual identity. The urine of mice contains various chemicals including Major Urinary Proteins (MUPs), some of which are crucial for territory marking and are vastly more prevalent in the urine of dominant mice.
In one study (Lee et al. 2021), we exposed mice of dominant or subordinate status to the urine of familiar (from their home-cage) or unfamiliar (from a different home-cage) mice of dominant or subordinate social status and subsequently examined the immunoreactivity of the Immediate Early Gene (IEG) cFos in their brain tissue. IEGs are expressed upon depolarization of a neuron’s cell membrane, and thus they serve as a proxy for neuronal excitation. Briefly, we found that several brain regions respond with more or less excitation depending on the subject animal’s own social status as well as their familiarity with the animal who provided the urine and that animal’s social status.
We are further investigating how the brain processes social information using a technique called cellular Compartment Analysis of Temporal activity by Fluorescence In Situ Hybridization (catFISH) to trace back when neurons have been activated at multiple time points. By targeting the expression of different IEGs with non-overlapping expression timescales we can determine which neurons were activated by two temporally distinct social stimulus events.
Subdominant male mice are able to rapidly respond to the emergence of power vacuums. When an alpha male is removed from a hierarchy, subdominant males rapidly (within 3 minutes) recognize that there exists a social opportunity and they aggressively exert their own dominance over all other animals in the group (see Williamson et al 2017 ). These males socially ascend to become the new alpha males and are able to stay at the top of the hierarchy. This demonstrates great social competence on behalf of these males to be able to so quickly respond to a change in the social context of the group. We have also found that this change in social context leads to changes in neural gene expression and activity. When the alpha male is removed, we observe that subdominant and subordinate males show increased GnRH mRNA expression compared to when the alpha male has not been removed.
Notably, we see increased cFos immunoreactivity throughout several regions of the forebrain and midbrain of socially ascending sub-dominant individuals. In particular, ascenders have profoundly increased cFos in the frontal cortices (infralimbic, prelimbic, retrosplenial) as well the CA1 and dentate gyrus subregions of the hippocampus. Future work will identify the functional role of this increased immunoreactivity in enabling mice to make social decisions to ascend a hierarchy.
We are also actively exploring the global transcriptomic changes that occur as a function of social ascension and descension to support transitions of social status.
Animals of dominant, sub-dominant and subordinate status exhibit different neurobiological features. These differences likely are related to the differential requirements of animals of each rank in behaviors including social cognition, spatial cognition, feeding, drinking, activity, sleep, aggression, social reward etc. An important step to understanding how the brain facilitates status-specific behavior is to characterize how variation in key neurobiological markers is associated with social status.
In one study, we used autoradiography to examine whether variation in social status was associated with levels of oxytocin (OTR) and vasopressin 1a (V1aR) receptor binding in socially relevant brain regions.
We found that patterns of OTR and V1aR binding differed between mice of high and low social status. In particular, we observed that dominant males have markedly increased levels of OTR binding in the Nucleus Accumbens compared to subordinate individuals. Differences in receptor density in social brain regions may underlie behavioral differences that promote or inhibit the acquisition of social status. Alternatively, the different social experiences of dominant and subordinate animals shift receptor expression, potentially facilitating the expression of adaptive social behaviors.
We also use candidate gene approaches to study individual differences in the brain associated with social status. Using quantitative real-time PCR we identified that mRNA levels of two neural plasticity genes, DNA methyltransferase 1 and 3a (DNMT1 and DNMT3a) , in the hippocampus were negatively correlated with measures of social dominance and power. DNA methylation via DNMT is one epigenetic mechanism that cells use to inhibit gene expression. Higher levels of DNMT1 in more subordinate mice may suggest that these mice are experiencing a social suppression of gene expression in the hippocampus. Differential gene expression between more and less dominant individuals in specific brain regions may enable status-specific contextually appropriate behaviors.
In our current work, we are taking a more explorative approach to investigate brain gene expression profiles of mice varying in social status. Using Tag-based RNA-Sequencing (Tag-Seq) we have identified genes and gene networks that are differentially regulated between dominant and subordinate male mice in both the forebrain and midbrain.