Shaping Adolescent Behavior: The Role of Brain Immune Cells

Do you remember what it was like to be a teenager? It is likely that as you transitioned into a teenager, you spent more time exploring, taking risks, and hanging out with friends. Socializing itself is a highly rewarding and motivating experience that involves the dopamine reward circuitry, which are pathways in the brain that get activated when we feel pleasure. When we are exposed to something that is rewarding, the brain releases an increased amount of dopamine, the main neurotransmitter associated with pleasure and reward. The nucleus accumbens (NAc) is a region in the brain that is important for motivation and reward. Past research has shown that dopamine D1 receptors (D1r) at synapses in the NAc are required for normal social play behavior in adolescent rats. However, how the dopamine system in the NAc develops during adolescence and influences social behavior was unknown until now.

Microglia, immune cells that live in the brain, are important for normal brain development. These cells are key regulators of neural circuit development through the regulation of cell death and synapse elimination, which ultimately shape behavior. Microglia are able to engulf or eliminate synapses through a process called synaptic pruning, resulting in the remodeling of neural circuitry.

This study by Kopec A, Smith C, et al. sought to determine if microglia participate in dopamine reward circuitry development via D1r elimination in the NAc during adolescence and whether this plays a causal role in developmental changes in social behavior.

To answer this, the authors first used immunohistochemistry (IHC), a technique to visualize different cells in the brain with fluorescence, to investigate changes in dopamine circuitry and microglia during normal development in male and female rats at four different ages: postnatal day 20 (P20) representing pre-adolescent animals, P30 representing early adolescence, P38 representing mid-adolescence, and P54 representing late adolescence. In males, they found an increase in microglial pruning of synaptic material at P30, followed by a decrease in number of D1rs by P38. This pattern was sex-specific and did not hold true for female rats. The findings suggest that microglia may be regulating D1r levels in male rats during adolescence, but the interactions between D1rs and microglia are unclear in females.  

The complement system is a collection of proteins that are important for microglial engulfment of synapses. In particular, a complement protein called C3 tags synapses to signal that they are ready to be eaten by microglia. The microglia then use a receptor called C3R to bind C3 and engulf the synapse. To test whether C3 is involved in microglial synaptic pruning of D1rs, the authors used a peptide called neutrophil inhibitor factor (NIF), which prevents microglia from binding to C3. The authors injected NIF into the NAc region of rats and found that NIF prevented the microglia from eliminating D1rs in male rats. NIF had no effect on the female rats. These results demonstrated a sex-specific immune mechanism involving C3 regulating dopaminergic NAc development during adolescence in rats.

Finally, the authors sought to address whether normal changes in social behaviors during adolescence require microglial pruning. They first implemented a social play and social exploration behavioral task to determine normal developmental changes in social behavior in males and females. In this task, rats had the opportunity to play with a novel rat for ten minutes. Male social play behavior peaked at P30 and then decreased over time at P38, mimicking the time course of microglia synaptic pruning of D1rs. Play levels did not change significantly over time in females. To determine whether male immune mediated D1r synapse elimination caused the changes observed in social play behavior during development, the authors injected NIF (to disrupt microglia-complement signaling) or a control solution into the NAc of males and females and assessed social behavior. Male social play behavior increased in NIF-treated compared to control-treated males. NIF-treated females also demonstrated a modest increase in social play behavior. These data suggested that microglial synaptic pruning may regulate adolescent social behavior in both male and female rats. This was surprising since previous results had not found a link between microglia and D1r developmental regulation in females.

Ultimately, this paper demonstrated for the first time a role for microglia and immune signaling in developmental changes in social behavior, with microglia-specific complement regulation of D1rs in the NAc differentially mediating changes in social play behavior in male versus female rats (Figure 1). Understanding the role that microglia play in shaping normal development can help us develop novel therapies targeting microglia when development goes awry.

Figure 1: In male rats, microglia C3R recognize dopaminergic D1r synapses tagged with the complement protein C3 and eliminate them. The elimination of dopaminergic synapses during adolescence results in a decrease in social play behavior in male rats. 

References:

Kopec, Ashley M., et al. "Microglial dopamine receptor elimination defines sex-specific nucleus accumbens development and social behavior in adolescent rats." Nature communications 9.1 (2018): 3769. https://doi.org/10.1038/s41467-018-06118-z

Edited by Lindsey Mehl