A Role for Microglia as Integrators of Social Experience: Impact of Adolescent Social Isolation on Dopamine System Development

Abstract

During the COVID-19 pandemic, hundreds of millions of Americans experienced social isolation. During the pandemic, domestic violence rates rose while access to social supports diminished. Adverse childhood experiences, including isolation and domestic violence, can have lasting consequences on physical and mental health. Alterations in neuroimmune signaling as a result of early life stress contribute to the development of schizophrenia and autism spectrum disorders. Altered synaptic connectivity in schizophrenia and ASD are thought to drive the etiology of these disorders. Adolescence itself is a period of increased peer oriented social, risk and reward seeking behaviors. The dopamine system in the brain regulates these behaviors and is under remodeling during adolescence, making these circuits sensitive to environmental perturbations.Dopamine projections from the ventral tegmental area (VTA) are received by the Nucleus Accumbens (NAc). These projections are refined over development and in response to social experience. Using rodent models, we can test with specificity the role of experience on dopamine system development and behavior. Rats are useful to study social behavior as they exhibit adolescent specific play behavior. Microglia, immune cells in the brain, prune dopamine D1 receptors (D1r) in the NAc of males. Microglial pruning of D1r specifically terminates adolescent play behavior. We sought to test how a lack of social experience during the adolescent period alters microglial D1r pruning, shaping behavior. In this model we characterized the impacts of adolescent specific isolation on development, as most current models are persistent isolation into adulthood. In these models there are increases in dopamine signaling and addiction liability. Understanding the role microglia may have in sculpting behavioral differences due to adolescent specific isolation may yield novel insights into the development of mental health disorders. We isolated animals across the peak window of D1r pruning, from P21-38. We collected brain samples for immunohistochemical analyses across P30, P38 and P54 in males and females. SI animals were housed together again at P38 if kept for a P54 timepoint. Using antibody based fluorescent staining we saw changes in TH+ fiber density in the NAc of males and females as a result of SI. There were D1r changes in the NAc core and shell of SI males, with no changes in D1r observed in females. There were minimal differences in microglia gene expressions a result of SI in males at P38. There were significant increases in proinflammatory and complement signaling at P54. There were few differences in SI female microglia gene expression patterns in P38 and P54. We found increases of activity dependent marker FosB in neurons and microglia of the NAc core of females, and not males. Behaviorally SI males and females play significantly more than their group housed counterparts. We found SI increases in behaviors like pinning in both males and females, chasing in just males and allogrooming only in SI females. SI behavioral complexity was significantly increased overall when compared to controls. In a P54 social preference and novelty task, SI males showed increased novelty preference. Females showed no change in P54 social preference or novelty as a result of SI. We found SI microglia in males at P30 and P38 engulf more D1r in the NAc. We did not investigate this in females given we know their mechanisms are different and saw no change in D1r. To understand the role microglia have in shaping SI induced play in males we blocked complement mediated engulfment by microglia at P30 using neutrophil inhibitory factor (NIF). We found in NIF SI animals an exacerbation in non-reciprocal assertive play, as well as allogrooming behavior. In a cohort of animals that underwent play, we collected their brains on day P38 and correlated D1r level to play. We found a significant correlation in control PBS animals that is lost in all other groups. This shows D1r remains coupled to play behavior in typical conditions, and when perturbed by SI or NIF it becomes uncoupled. This demonstrates microglia have a key role in regulating social behavior development. Adverse childhood experiences increase addiction liability. We used a self-administration paradigm to understand how drug seeking behavior is altered in SI males and females. There is evidence for increased drug seeking behavior, particularly stimulants in the context of SI. There is evidence for increased alcohol consumption, though these studies are in persistently isolated animals. We tested for remifentanil self-administration due to the ongoing opioid epidemic, and evidence for immune intervention in remifentanil self-administration. We interestingly found that SI animals overall take less drug. We found no differences in overall propensity to seek a reward, or gross locomotor activity as a result of social isolation. Collectively these data show that adolescent social isolation robustly impacts the dopamine system. Isolation changes microglia dopamine interactions that can drive behavior. Interestingly, motivation for opioid drugs is decreased; thus, questions about drug dose and aversion remain in these animals. Social experience and immune system functioning are both critical for development. Developmental disorders are in part driven by microglial mechanisms of brain organization, responding to altered experiences. There are sex differences in the lasting inflammatory impacts of social isolation. All these factors are equally important to understanding the lasting impacts microglia have on the development of social expression.