Microglia have crucial roles in sculpting synapses and maintaining neural circuits during development. To test the hypothesis that microglia continue to regulate neural circuit connectivity in adult brain, we have investigated the effects of chronic microglial depletion, via CSF1R inhibition, on synaptic connectivity in the visual cortex in adult mice of both sexes. We find that the absence of microglia dramatically increases both excitatory and inhibitory synaptic connections to excitatory cortical neurons assessed with functional circuit mapping experiments in acutely prepared adult brain slices. Microglia depletion leads to increased densities and intensities of perineuronal nets. Furthermore, in vivo calcium imaging across large populations of visual cortical neurons reveals enhanced neural activities of both excitatory neurons and parvalbumin-expressing interneurons in the visual cortex following microglia depletion. These changes recover following adult microglia repopulation. In summary, our new results demonstrate a prominent role of microglia in sculpting neuronal circuit connectivity and regulating subsequent functional activity in adult cortex.

SIGNIFICANCE STATEMENT Microglia are the primary immune cell of the brain, but recent evidence supports that microglia play an important role in synaptic sculpting during development. However, it remains unknown whether and how microglia regulate synaptic connectivity in adult brain. Our present work shows chronic microglia depletion in adult visual cortex induces robust increases in perineuronal nets, and enhances local excitatory and inhibitory circuit connectivity to excitatory neurons. Microglia depletion increases in vivo neural activities of both excitatory neurons and parvalbumin inhibitory neurons. Our new results reveal new potential avenues to modulate adult neural plasticity by microglia manipulation to better treat brain disorders, such as Alzheimer's disease.

在发育过程中，小胶质细胞在塑造突触和维持神经回路方面发挥着至关重要的作用。为了验证小胶质细胞继续调节成年大脑中神经回路连接的假设，我们研究了通过 CSF1R 抑制慢性小胶质细胞耗竭对两性成年小鼠视觉皮层突触连接的影响。我们发现小胶质细胞的缺失显着增加了兴奋性和抑制性突触与兴奋性皮层神经元的连接，这些神经元在急性准备的成人脑切片中通过功能电路映射实验进行评估。小胶质细胞耗竭导致神经周围网的密度和强度增加。此外，在体内大量视觉皮层神经元的钙成像揭示了小胶质细胞耗竭后视觉皮层中兴奋性神经元和表达小白蛋白的中间神经元的神经活动增强。这些变化在成年小胶质细胞重新增殖后恢复。总之，我们的新结果证明了小胶质细胞在塑造神经元回路连接和调节成人皮层后续功能活动方面的突出作用。

意义声明小胶质细胞是大脑的主要免疫细胞，但最近的证据支持小胶质细胞在发育过程中在突触塑造中发挥重要作用。然而，小胶质细胞是否以及如何调节成人大脑中的突触连接仍然未知。我们目前的工作表明，成人视觉皮层中的慢性小胶质细胞耗竭会导致神经周围网络的强烈增加，并增强局部兴奋性和抑制性电路与兴奋性神经元的连接。小胶质细胞耗竭增加了兴奋性神经元和小白蛋白抑制性神经元的体内神经活动。我们的新结果揭示了通过小胶质细胞操纵来调节成人神经可塑性的新潜在途径，以更好地治疗阿尔茨海默病等脑部疾病。