Dynamic microglial modulation of spatial learning and social behavior

Highlights

• Local hippocampal microglial depletion alters learning and social behavior.

• Systemic microglia depletion alters learning but not social behavior.

• These behavioral changes are reversed upon microglial repopulation.

Abstract

Microglia are active players in inflammation, but also have important supporting roles in CNS maintenance and function, including modulation of neuronal activity. We previously observed an increase in the frequency of excitatory postsynaptic current in organotypic brain slices after depletion of microglia using clodronate. Here, we describe that local hippocampal depletion of microglia by clodronate alters performance in tests of spatial memory and sociability. Global depletion of microglia by high-dose oral administration of a Csf1R inhibitor transiently altered spatial memory but produced no change in sociability behavior. Microglia depletion and behavior effects were both reversible, consistent with a dynamic role for microglia in the regulation of such behaviors.

Introduction

Microglia, the immunocompetent cells of the CNS, originate from yolk-sac precursors that populate the brain prior to the formation of the blood–brain barrier (Kierdorf et al., 2013). Once they fully colonize the brain parenchyma Csf-1, among other factors, contributes to their maintenance and self-renewal (Wegiel et al., 1998). Csf-1 and its receptor Csf1R are essential for microglial proliferation, differentiation, survival, and migration. Mice that lack Csf1R display severe microglia deficiency as well as other developmental defects (Erblich et al., 2011, Michaelson et al., 1996, Nandi et al., 2012). Conversely, overexpression of Csf-1 increases microglia numbers by increasing microglial proliferation (De et al., 2014).

Besides their known roles in mediating inflammation (Colton and Wilcock, 2010, Kraft and Harry, 2011), microglia play an important supporting role in the postnatal CNS (Nayak et al., 2014, Paolicelli et al., 2011, Schwartz et al., 2013). During early neurogenesis microglia phagocytose apoptotic cells and eliminate unwanted neuronal projections (Paolicelli et al., 2011, Schafer et al., 2012). They also control production of cortical neurons through phagocytosis of neural precursor cells (Cunningham, 2013).

Microglia provide trophic support for the formation of neuronal circuits and are indispensible for neuronal survival (Ueno et al., 2013). They also directly modulate synaptic activity by contacting synapses in a CR3/C3-dependent manner (Paolicelli et al., 2011, Schafer et al., 2012, Wake et al., 2009). Neuronal stimulation results in extension of microglial processes towards highly active neurons, which in turn reduces neuronal activity (Li et al., 2012). Previous work from our lab showed an increase in the frequency of excitatory postsynaptic currents in hippocampal slices (Ji et al., 2013) and the complementary effect in neuronal/microglial co-cultures. Together, these data support a role for microglia in the regulation of neuronal activity in the healthy brain by affecting the number of active synapses.

Due to their extensive functions as supporting cells of the CNS, microglia are now considered active regulators of neurological function. Specific depletion of brain microglia upon diphtheria toxin administration in CX3CR1^CreER mice significantly reduces motor-learning-dependent synapse formation as well as performance in learning and memory tasks including auditory-cued fear conditioning and novel object recognition (Parkhurst et al., 2013). Long term depletion of microglia via systemic inhibition of Csf1R improves spatial memory as assessed with the Barnes maze test (Elmore et al., 2014). Mice that lack the chemokine receptor CX3CR1 have reduced numbers of microglia along with deficits in social behavior and spatial learning (Rogers et al., 2011, Zhan et al., 2014). Finally, mice deficient in Fractalkine (CX3CL1), a chemokine that directs microglia towards developing synapses (Hoshiko et al., 2012), have reduced performance in contextual fear conditioning and cued fear conditioning (Rogers et al., 2011).

In this study, we investigate the functional significance of microglia depletion in vivo using two different methods to deplete microglia: bilateral microinjections of clodronate in the dorsal hippocampus, and systemic inhibition of Csf1R. The first method provides anatomical specificity, allowing us to examine the functional consequences of loss of microglia in hippocampal-related behaviors. The second allowed us to compare the results to a method that produced more widespread effects. We show that absence of microglia leads to alterations in spatial memory task performance and social behavior. These effects are reversed when microglia are allowed to repopulate the brain, supporting an active role for microglia in such physiological processes.