Functional and Structural Correlates of Impaired Enrichment-Mediated Adult Hippocampal Neurogenesis in a Mouse Model of Prenatal Alcohol Exposure

Impaired cognitive function and altered dendritic morphology of hippocampal neurons in a mouse model of fetal alcohol spectrum disorder

Prenatal ethanol exposure is a leading preventable cause of neurodevelopmental disability, clinically categorized under fetal alcohol spectrum disorders (FASD). This study explores how developmental alcohol exposure affects the dendritic morphology of hippocampal pyramidal neurons, focusing on the actin cytoskeleton's dynamics essential for neuronal structure and synaptic function. Within this context, we hypothesized that developmental alcohol exposure disrupts actin cytoskeleton dynamics, leading to cognitive deficits and dendritic remodeling in the hippocampus. Neonatal mice (C57BL/6 J) were administered ethanol (5.0 g/kg) intraperitoneally from postnatal day 2-8, establishing a third trimester-equivalent alcohol exposure FASD model. At postnatal day 28, cognitive performance was evaluated using novel location recognition (NLR), novel object recognition (NOR), and the Morris water maze (MWM). Golgi staining assessed dendritic morphology in the hippocampal CA1 region, and the ratio of polymerized (F-actin) to globular actin (G-actin) was measured using a biochemical assay. The results revealed that developmental alcohol exposure significantly impaired recognition and spatial memory, as evidenced by decreased performances in the NOR and MWM tests across both sexes. Golgi staining revealed reduced dendritic arborization complexity and spine density in the CA1 region of the hippocampal pyramidal neurons of both male and female juvenile mice. Biochemical analyses further revealed decresed hipocampal F-actin/G-actin ratios and decreased levels of polymerized F-actin in both sexes. These findings underscore the critical role of cytoskeletal integrity in cognitive development and highlight potential targets for therapeutic intervention in FASD.

Adult Hippocampal Neurogenesis as a Therapeutic Target in Fetal Alcohol Spectrum Disorder

This review is focused on adult hippocampal neurogenesis as a potential therapeutic target in fetal alcohol spectrum disorder (FASD). Adult hippocampal neurogenesis refers to the production of new hippocampal dentate granule cells (DGCs) from a replenishable pool of neural stem and progenitor cells throughout life. Adult-generated DGCs have been shown to exert a profound influence on hippocampal network activity in experimental animals and have been implicated in the regulation of many hippocampal-dependent behaviors and emotional states, including certain forms of learning and memory, anxiety, mood, and stress resilience. While adult hippocampal neurogenesis in humans remains controversial, many studies support its existence and impact on hippocampal function in human health and disease. Here, we review mechanisms of adult hippocampal neurogenesis under physiological conditions, as described primarily in rodent brain, its impact on network activity and behavior, and the negative effects of developmental alcohol exposure on this process. We then explore hippocampal neurogenesis as a potential target for FASD therapy using pharmacological and neurophysiological approaches known to stimulate adult hippocampal neurogenesis, currently available for clinical use in FASD patients.

Persistent myelin abnormalities in a third trimester-equivalent mouse model of fetal alcohol spectrum disorder

BACKGROUND

Abnormal diffusion within white matter (WM) tracts has been linked to cognitive impairment in children with fetal alcohol spectrum disorder. Whether changes to myelin organization and structure underlie the observed abnormal diffusion patterns remains unknown. Using a third trimester-equivalent mouse model of alcohol exposure, we previously demonstrated acute loss of oligodendrocyte lineage cells with persistent loss of myelin basic protein and lower fractional anisotropy (FA) in the corpus callosum (CC). Here, we tested whether these WM deficits are accompanied by changes in: (i) axial diffusion (AD) and radial diffusion (RD), (ii) myelin ultrastructure, or (iii) structural components of the node of Ranvier.

METHODS

Mouse pups were exposed to alcohol or air vapor for 4 h daily from postnatal day (P)3 to P15 (BEC: 160.4 ± 12.0 mg/dl; range = 128.2 to 185.6 mg/dl). Diffusion tensor imaging (DTI) and histological analyses were performed on brain tissue isolated at P50. Diffusion parameters were measured with Paravision™ 5.1 software (Bruker) following ex vivo scanning in a 7.0 T MRI. Nodes of Ranvier were identified using high-resolution confocal imaging of immunofluorescence for Nav 1.6 (nodes) and Caspr (paranodes) and measured using Imaris™ imaging software (Bitplane). Myelin ultrastructure was evaluated by calculating the G-ratio (axonal diameter/myelinated fiber diameter) on images acquired using transmission electron microscopy.

RESULTS

Consistent with our previous study, high resolution DTI at P50 showed lower FA in the CC of alcohol-exposed mice (p = 0.0014). Here, we show that while AD (diffusion parallel to CC axons) was similar between treatment groups (p = 0.30), RD (diffusion perpendicular to CC axons) in alcohol-exposed subjects was significantly higher than in controls (p = 0.0087). In the posterior CC, where we identified the highest degree of abnormal diffusion, node of Ranvier length did not differ between treatment groups (p = 0.41); however, the G-ratio of myelinated axons was significantly higher in alcohol-exposed animals than controls (p = 0.023).

CONCLUSIONS

High resolution DTI revealed higher RD at P50 in the CC of alcohol-exposed animals, suggesting less myelination of axons, particularly in the posterior regions. In agreement with these findings, ultrastructural analysis of myelinated axons in the posterior CC showed reduced myelin thickness in alcohol-exposed animals, evidenced by a higher G-ratio.