Involvement of dopamine D3 receptor in impulsive choice decision-making in male rats.

Impulsive decision-making has been linked to impulse control disorders and substance use disorders. However, the neural mechanisms underlying impulsive choice are not fully understood. While previous PET imaging and autoradiography studies have shown involvement of dopamine and D2/3 receptors in impulsive behavior, the roles of distinct D1, D2, and D3 receptors in impulsive decision-making remain unclear. In this study, we used a food reward delay-discounting task (DDT) to identify low- and high-impulsive rats, in which low-impulsive rats exhibited preference for large delayed reward over small immediate rewards, while high-impulsive rats showed the opposite preference. We then examined D1, D2, and D3 receptor gene expression using RNAscope in situ hybridization assays. We found that high-impulsive male rats exhibited lower levels of D2 and D3, and particularly D3, receptor expression in the nucleus accumbens (NAc), with no significant changes in the insular, prelimbic, and infralimbic cortices. Based on these findings, we further explored the role of the D3 receptor in impulsive decision-making. Systemic administration of a selective D3 receptor agonist (FOB02-04) significantly reduced impulsive choices in high-impulsive rats but had no effects in low-impulsive rats. Conversely, a selective D3 receptor antagonist (VK4-116) produced increased both impulsive and omission choices in both groups of rats. These findings suggest that impulsive decision-making is associated with a reduction in D3 receptor expression in the NAc. Selective D3 receptor agonists, but not antagonists, may hold therapeutic potentials for mitigating impulsivity in high-impulsive subjects.

Chronic Cortical Inflammation, Cognitive Impairment, and Immune Reactivity Associated with Diffuse Brain Injury Are Ameliorated by Forced Turnover of Microglia.

Traumatic brain injury (TBI) is associated with an increased risk of cognitive, psychiatric, and neurodegenerative complications that may develop after injury. Increased microglial reactivity following TBI may underlie chronic neuroinflammation, neuropathology, and exaggerated responses to immune challenges. Therefore, the goal of this study was to force turnover of trauma-associated microglia that develop after diffuse TBI and determine whether this alleviated chronic inflammation, improved functional recovery and attenuated reduced immune reactivity to lipopolysaccharide (LPS) challenge. Male mice received a midline fluid percussion injury (mFPI) and 7 d later were subjected to a forced microglia turnover paradigm using CSF1R antagonism (PLX5622). At 30 d postinjury (dpi), cortical gene expression, dendritic complexity, myelin content, neuronal connectivity, cognition, and immune reactivity were assessed. Myriad neuropathology-related genes were increased 30 dpi in the cortex, and 90% of these gene changes were reversed by microglial turnover. Reduced neuronal connectivity was evident 30 dpi and these deficits were attenuated by microglial turnover. TBI-associated dendritic remodeling and myelin alterations, however, remained 30 dpi independent of microglial turnover. In assessments of functional recovery, increased depressive-like behavior, and cognitive impairment 30 dpi were ameliorated by microglia turnover. To investigate microglial priming and reactivity 30 dpi, mice were injected intraperitoneally with LPS. This immune challenge caused prolonged lethargy, sickness behavior, and microglial reactivity in the TBI mice. These extended complications with LPS in TBI mice were prevented by microglia turnover. Collectively, microglial turnover 7 dpi alleviated behavioral and cognitive impairments associated with microglial priming and immune reactivity 30 dpi.SIGNIFICANCE STATEMENT A striking feature of traumatic brain injury (TBI), even mild injuries, is that over 70% of individuals have long-term neuropsychiatric complications. Chronic inflammatory processes are implicated in the pathology of these complications and these issues can be exaggerated by immune challenge. Therefore, our goal was to force the turnover of microglia 7 d after TBI. This subacute 7 d postinjury (dpi) time point is a critical transitional period in the shift toward chronic inflammatory processes and microglia priming. This forced microglia turnover intervention in mice attenuated the deficits in behavior and cognition 30 dpi. Moreover, microglia priming and immune reactivity after TBI were also reduced with microglia turnover. Therefore, microglia represent therapeutic targets after TBI to reduce persistent neuroinflammation and improve recovery.

Astrocyte immunosenescence and deficits in interleukin 10 signaling in the aged brain disrupt the regulation of microglia following innate immune activation.

Microglia, the innate immune cells of the brain, develops a pro-inflammatory, "primed" profile with age. Using single-cell RNA-sequencing, we confirmed hippocampal microglia of aged mice (18 m.o.) had an amplified (4 h) and prolonged (24 h) neuroinflammatory response to peripheral lipopolysaccharide (LPS) challenge compared to adults (2 m.o.). Overall, there were several unique cell-, age-, and time-dependent differences in the clusters of microglia identified. Analysis of upstream regulators and canonical pathways revealed impaired regulation of an activated, neuroinflammatory state within microglia. Moreover, microglia in the aged hippocampus failed to turn over during the resolving phase of neuroinflammation. Concomitantly, astrocytes in the aged hippocampus were "immunosenescent" both 4 and 24 h after LPS challenge. For example, aged astrocytes had reduced anti-inflammatory signaling and cholesterol biosynthesis, two pathways by which astrocytes regulate the inflammatory profile of microglia. One of the pathways reduced in the aged hippocampus was interleukin (IL)-10 signaling. This pathway increases astrocytic expression of transforming growth factor (TGF)-ß, an anti-inflammatory cytokine with abundant receptor expression on microglia. Therefore, transgenic astrocytic Il10raKO mice were generated to determine if impaired IL-10R/TGFß signaling within astrocytes caused an amplified microglial neuroinflammatory response. Astrocytic Il10raKO caused exaggerated sickness behavior and a prolonged neuroinflammatory response to peripheral LPS, including increased social avoidance with amplified microglial Il1b and Tnf mRNA expression. In summary, astrocytes had an immunosenescent profile with age and, in response to peripheral LPS, had IL-10R signaling deficits and a lack of cholesterol biosynthesis, both leading to the inability to resolve microglial activation.