Microglia are the resident macrophages of the central nervous system. Microglia possess varied morphologies and functions. Under normal physiological conditions, microglia mainly exist in a resting state and constantly monitor their microenvironment and survey neuronal and synaptic activity. Through the C1q, C3 and CR3 “Eat Me” and CD47 and SIRPα “Don’t Eat Me” complement pathways, as well as other pathways such as CX3CR1 signaling, resting microglia regulate synaptic pruning, a process crucial for the promotion of synapse formation and the regulation of neuronal activity and synaptic plasticity. By mediating synaptic pruning, resting microglia play an important role in the regulation of experience-dependent plasticity in the barrel cortex and visual cortex after whisker removal or monocular deprivation, and also in the regulation of learning and memory, including the modulation of memory strength, forgetfulness, and memory quality. As a response to brain injury, infection or neuroinflammation, microglia become activated and increase in number. Activated microglia change to an amoeboid shape, migrate to sites of inflammation and secrete proteins such as cytokines, chemokines and reactive oxygen species. These molecules released by microglia can lead to synaptic plasticity and learning and memory deficits associated with aging, Alzheimer’s disease, traumatic brain injury, HIV-associated neurocognitive disorder, and other neurological or mental disorders such as autism, depression and post-traumatic stress disorder. With a focus mainly on recently published literature, here we reviewed the studies investigating the role of resting microglia in synaptic plasticity and learning and memory, as well as how activated microglia modulate disease-related plasticity and learning and memory deficits. By summarizing the function of microglia in these processes, we aim to provide an overview of microglia regulation of synaptic plasticity and learning and memory, and to discuss the possibility of microglia manipulation as a therapeutic to ameliorate cognitive deficits associated with aging, Alzheimer’s disease, traumatic brain injury, HIV-associated neurocognitive disorder, and mental disorders.

The excitatory neurotransmitter glutamate (100 μM) induces intracellular calcium transients in cultured hippocampal astrocytes that can be imaged using the calcium indicator (Fluo3AM) and time-lapse microscopy. In response to glutamate (Fig. 1A), cultured astrocytes exhibit distinct patterns of intracellular Ca2+ oscillations and long-distance intercellular waves. Two distinct types of intercellular Ca2+ waves are attributed to excitation by different agonists of the glutamate receptor subtypes. A long-distance regenerative intercellular wave is induced by the ionotropic glutamate receptor, kainate (Fig. 1B). This is a true wave lasting for 50-125 sec with a constant velocity of 10-20 μm/sec. This wave requires extracellular Ca2+ and Na+ and is driven by the Na+/Ca2+ exchanger. A fast Ca2+ wave which travels at speeds from 10 to 200 μm/sec is dependent upon the metabotropic glutamate receptor, is inducible by t-ACPD and is dependent upon cytoplasmic release of Ca2+ regulated by IP3 (Fig. 1C). This wave is not dependent upon extracellular Ca2+ and is stopped by MCPG, a specific inhibitor of IP3-mediated intracellular Ca2+ release.