Structural, Functional, and Transcriptional Responses of Nucleus Accumbens Microglia to Long-access Cocaine Self-Administration

Cocaine use disorder (CUD) is a persistent public health concern in the United States with no current FDA-approved pharmaceutical therapeutics. Individuals experiencing CUD are at substantial risk for relapse during periods of abstinence. A deeper understanding of the cellular and molecular underpinnings of CUD is essential for developing therapeutics and relapse prevention. While preclinical animal research has historically focused on neuronal mechanisms of cocaine action in the brain, evidence indicates that neuroglia contribute to mechanisms of dependence and relapse as well. Published research from the Reissner lab illustrates substantial decreases in astrocyte morphology, complexity, and synaptic colocalization in the nucleus accumbens (NAc) core following long-access (LgA, 6 h/day, 10 days) rat cocaine self-administration and a 45-day abstinence period, mediated by microglia pruning. These observations prompt questions regarding microglia responses to cocaine self-administration, which this dissertation explores. The first aim examined the structural and functional responses of microglia on days 1, 30, and 45 of abstinence. Results demonstrate no significant morphological adaptations in microglia at any time point during abstinence. However, colocalization analyses confirmed increased colocalization of microglia with astrocyte inclusions, complement component C3, and lysosomal CD68 at abstinence day 45, suggesting increased microglial phagocytic activity and astrocyte pruning. Accordingly, the second aim assessed how inhibiting complement-mediated phagocytosis affects astrocyte structure and synaptic colocalization following abstinence. Results show that inhibiting microglial phagocytosis during abstinence rescues astrocyte structure and synaptic colocalization in the NAc, and also reduces cocaine-seeking. Additional experiments aimed to identify the signaling mechanism(s) driving microglial pruning of astrocytes by examining gene expression of “eat-me” and “don’t-eat-me” markers in isolated microglia and astrocytes from the NAc. Finally, the last aim examined transcriptional adaptations in microglia at abstinence day 45, in male and female rats. There is an increase in literature demonstrating sex differences in glial responses to cocaine self-administration. Furthermore, previous research demonstrates sex-specific glial responses to cocaine self-administration. Accordingly, mRNA levels of genes associated with pro-inflammatory responses, phagocytosis, and homeostasis were analyzed in isolated NAc microglia. In summary, the research presented in this dissertation examined structural, functional, and transcriptional responses of microglia in the NAc following LgA cocaine self-administration.