Dynamics of microglia and dendritic spines in early adolescent cortex after developmental alcohol exposure.

Fetal alcohol spectrum disorder patients suffer from many cognitive disabilities. These include impaired auditory, visual, and tactile sensory information processing, making it more difficult for these patients to learn to navigate social scenarios. Rodent studies have shown that alcohol exposure during the brain growth spurt (BGS) can lead to acute neuronal apoptosis and an immunological response by microglia in the somatosensory cortex. Since microglia have critical physiological functions, including the support of excitatory synapse remodeling via interactions with dendritic spines, we sought to understand whether BGS alcohol exposure has long-term effects on microglial or dendritic spine dynamics. Using in vivo two-photon microscopy in 4-5 week old mice, we evaluated microglial functions such as process motility, the response to tissue injury, and the dynamics of physical interactions between microglial processes and dendritic spines. We also investigated potential differences in the morphology, density, or dynamics of dendritic spines in layer I/II primary sensory cortex of control and BGS alcohol exposed mice. We found that microglial process motility and contact with dendritic spines were not altered after BGS alcohol exposure. While the response of microglial processes toward tissue injury was not significantly altered by prior alcohol exposure, there was a trend suggesting that alcohol early in life may prime microglia to respond more quickly to secondary injury. Spine density, morphology, stability, and remodeling over time were not perturbed after BGS alcohol exposure. We demonstrate that after BGS alcohol exposure, the physiological functions of microglia and excitatory neurons remain intact in early adolescence.

Developmental alcohol exposure impairs synaptic plasticity without overtly altering microglial function in mouse visual cortex.

Fetal alcohol spectrum disorder (FASD), caused by gestational ethanol (EtOH) exposure, is one of the most common causes of non-heritable and life-long mental disability worldwide, with no standard treatment or therapy available. While EtOH exposure can alter the function of both neurons and glia, it is still unclear how EtOH influences brain development to cause deficits in sensory and cognitive processing later in life. Microglia play an important role in shaping synaptic function and plasticity during neural circuit development and have been shown to mount an acute immunological response to EtOH exposure in certain brain regions. Therefore, we hypothesized that microglial roles in the healthy brain could be permanently altered by early EtOH exposure leading to deficits in experience-dependent plasticity. We used a mouse model of human third trimester high binge EtOH exposure, administering EtOH twice daily by subcutaneous injections from postnatal day 4 through postnatal day 9 (P4-:P9). Using a monocular deprivation model to assess ocular dominance plasticity, we found an EtOH-induced deficit in this type of visually driven experience-dependent plasticity. However, using a combination of immunohistochemistry, confocal microscopy, and in vivo two-photon microscopy to assay microglial morphology and dynamics, as well as fluorescence activated cell sorting (FACS) and RNA-seq to examine the microglial transcriptome, we found no evidence of microglial dysfunction in early adolescence. We also found no evidence of microglial activation in visual cortex acutely after early ethanol exposure, possibly because we also did not observe EtOH-induced neuronal cell death in this brain region. We conclude that early EtOH exposure caused a deficit in experience-dependent synaptic plasticity in the visual cortex that was independent of changes in microglial phenotype or function. This demonstrates that neural plasticity can remain impaired by developmental ethanol exposure even in a brain region where microglia do not acutely assume nor maintain an activated phenotype.

Cerebellar microglia are dynamically unique and survey Purkinje neurons in vivo.

Microglia are the innate immune cells of the central nervous system and are also important participants in normal development and synaptic plasticity. Here, we demonstrate that the microglia of the mouse cerebellum represent a unique population compared to cortical microglia. Microglia are more sparsely distributed within the cerebellum and have a markedly less ramified morphology compared to their cortical counterparts. Using time-lapse in vivo imaging, we found that these differences in distribution and morphology ultimately lead to decreased parenchymal surveillance by cerebellar microglia. We also observed a novel form of somal motility in cerebellar microglia in vivo, which has not been described in cortical populations. We captured microglial interactions with Purkinje neurons in vivo. Cerebellar microglia interact dynamically with both the dendritic arbors and somas of Purkinje neurons. These findings suggest that cerebellar microglia are physiologically distinct from cortical populations and that these differences may ultimately alter how they could contribute to plasticity and disease processes in the cerebellum. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 627-644, 2018.

What the Spectrum of Microglial Functions Can Teach us About Fetal Alcohol Spectrum Disorder.

Alcohol exposure during gestation can lead to severe defects in brain development and lifelong physical, behavioral and learning deficits that are classified under the umbrella term fetal alcohol spectrum disorder (FASD). Sadly, FASD is diagnosed at an alarmingly high rate, affecting 2%-5% of live births in the United States, making it the most common non-heritable cause of mental disability. Currently, no standard therapies exist that are effective at battling FASD symptoms, highlighting a pressing need to better understand the underlying mechanisms by which alcohol affects the developing brain. While it is clear that sensory and cognitive deficits are driven by inappropriate development and remodeling of the neural circuits that mediate these processes, alcohol's actions acutely and long-term on the brain milieu are diverse and complex. Microglia, the brain's immune cells, have been thought to be a target for alcohol during development because of their exquisite ability to rapidly detect and respond to perturbations affecting the brain. Additionally, our view of these immune cells is rapidly changing, and recent studies have revealed a myriad of microglial physiological functions critical for normal brain development and long-term function. A clear and complete understanding of how microglial roles on this end of the spectrum may be altered in FASD is currently lacking. Such information could provide important insights toward novel therapeutic targets for FASD treatment. Here we review the literature that links microglia to neural circuit remodeling and provide a discussion of the current understanding of how developmental alcohol exposure affects microglial behavior in the context of developing brain circuits.

Effects of Developmental Alcohol Exposure on Potentiation and Depression of Visual Cortex Responses.

BACKGROUND: Neuronal plasticity deficits are thought to underlie abnormal neurodevelopment in fetal alcohol spectrum disorders and in animal models of this condition. Previously, we found that alcohol exposure during a period that is similar to the last months of gestation in humans disrupts ocular dominance plasticity (ODP), as measured in superficial cortical layers. We hypothesize that exposure to alcohol can differentially affect the potentiation and depression of responses that are necessary for activity-dependent sprouting and pruning of neuronal networks. ODP is an established paradigm that allows the assessment of activity-dependent depression and potentiation of responses in vivo. METHODS: Mouse pups were exposed to 3.6 to 5 g/kg of ethanol in saline daily or every other day between postnatal days 4 and 9. Visual cortex plasticity was then assessed during the critical period for ODP using 2 techniques that separately record in layers 4 (visually evoked potentials [VEPs]) and 2/3 (optical imaging of intrinsic signals [OI]). RESULTS: We discovered a layer-specific effect of early alcohol exposure. Recording of VEPs from layer 4 showed that while the potentiation component of ODP was disrupted in animals treated with alcohol when compared with saline controls, the depression component of ODP (Dc-ODP) was unaltered. In contrast, OI from layers 2/3 showed that Dc-ODP was markedly disrupted in alcohol-treated animals when compared with controls. CONCLUSIONS: Combined with our previous work, these findings strongly suggest that developmental alcohol exposure has a distinct and layer-specific effect on the potentiation and depression of cortical responses after monocular deprivation.