Thesis title: Exploring the role of microglia in the development of early life stress-induced behavioural sensitivity to addictive drugs
Exposure to early life psychological stress (ELS) can alter trajectories of brain maturation and induce susceptibility to develop substance use disorder (SUD) later in life. However, the mechanisms underlying these processes are still not clear. ELS produces dysfunction in the dopamine (DA) system, whose alteration has been associated with SUD. ELS can also perturb microglia, the resident immune cells of the brain, altering their activity in the short term and priming their responses to subsequent challenges later in life. Microglia play a major role in the shaping and refinement of neural circuits during development and directly contribute to the correct maturation of the DA system and DA-dependent behaviour. Moreover, microglia in DA centres of the brain respond to psychostimulants and mediate the behavioural effects of cocaine. Crucially, our lab has previously demonstrated that ELS-induced microglia activation is responsible for cocaine behavioural susceptibility in a mouse model of periadolescent social stress (S-S).
The aim of my PhD thesis was to test two possible hypotheses on the mechanisms underlying microglia contribution to cocaine sensitivity in S-S mice.
First, I hypothesised that S-S primed microglia responses to future cocaine exposure, and that heightened microglia responses to the drug in reward/DA centres mediated behavioural susceptibility to the substance. Immunofluorescence and confocal microscopy analyses demonstrated long-term microglia alteration in Ventral Tegmental Area (VTA) from adult S-S mice. However, pharmacological inhibition of microglial/macrophages responses to cocaine through GW2580 treatment did not prevent behavioural susceptibility to the substance.
Then, I hypothesised that activated microglia in S-S mice mediated an aberrant development of the reward/DA system, that may underlie behavioural susceptibility to the effects of cocaine. I performed experiments at two different time points: 1) after the stress, to evaluate DA system changes upon microglia activation; 2) early adulthood, to evaluate the adaptive responses of the dopaminergic circuit to stress and microglia activation. In pups, S-S altered developmental programming of the VTA, as demonstrated by RNA-sequencing, immunohistochemistry, stereology, confocal microscopy, and electrophysiology analyses. In adulthood, S-S mice presented altered gene expression of mediators of dopaminergic transmission, such as metabolic and catabolic enzymes, and dopamine transporter and receptors in key areas of the mesocorticolimbic DA system. Inhibition of S-S-induced microglia activation through developmental minocycline treatment normalised the dopaminergic alterations in pups and adult mice.
These findings demonstrated for the first time a role for microglia in the mediation of aberrant dopaminergic development following early life social stress. Moreover, they suggest a possible mechanism underlying the interaction between early life traumatic experiences and subsequent susceptibility to SUD. Further experiments are necessary to elucidate the pathways by which microglia contribute to alteration in the DA system following ELS and whether/how this alteration mediates susceptibility to cocaine observed in adulthood.
Results from this PhD project provide further information to understand mechanisms of altered brain maturation in the context of ELS, with the long-term goal of discovering possible therapeutic targets for maltreated children/adolescents that prevent the emergence of psychopathological behaviours.