Third trimester-equivalent ethanol exposure does not alter complex spikes and climbing fiber long-term depression in cerebellar Purkinje neurons from juvenile rats

Recent breakthroughs in understanding the cerebellum's role in fetal alcohol spectrum disorder: A systematic review

Exposure to alcohol during fetal development can lead to structural and functional abnormalities in the cerebellum, a brain region responsible for motor coordination, balance, and specific cognitive functions. In this systematic review, we comprehensively analyze a vast body of research conducted on vertebrate animals and humans over the past 13 years. We identified studies through PubMed and screened them following PRISMA guidelines. Data extraction and quality analysis were conducted using Covidence systematic review software. A total of 108 studies met our inclusion criteria, with the majority (79 studies) involving vertebrate animal models and 29 studies focusing on human subjects. Animal models included zebrafish, mice, rats, sheep, and non-human primates, investigating the impact of ethanol on cerebellar structure, gene/protein expression, physiology, and cerebellar-dependent behaviors. Additionally, some animal studies explored potential therapeutic interventions against ethanol-induced cerebellar damage. The human studies predominantly adopted cohort designs, exploring the effects of prenatal alcohol exposure on cerebellar structure and function. Certain human studies delved into innovative cerebellar-based diagnostic approaches for fetal alcohol spectrum disorder (FASD). The collective findings from these studies clearly indicate that the cerebellum is involved in various neurophysiological deficits associated with FASD, emphasizing the importance of evaluating both cerebellar structure and function in the diagnostic process for this condition. Moreover, this review sheds light into potential therapeutic strategies that can mitigate prenatal alcohol exposure-induced cerebellar damage.

Developmental Ethanol Exposure Impacts Purkinje Cells but Not Microglia in the Young Adult Cerebellum

Fetal alcohol spectrum disorders (FASD) caused by developmental ethanol exposure lead to cerebellar impairments, including motor problems, decreased cerebellar weight, and cell death. Alterations in the sole output of the cerebellar cortex, Purkinje cells, and central nervous system immune cells, microglia, have been reported in animal models of FASD. To determine how developmental ethanol exposure affects adult cerebellar microglia and Purkinje cells, we used a human third-trimester binge exposure model in which mice received ethanol or saline from postnatal (P) days 4-9. In adolescence, cerebellar cranial windows were implanted and mice were aged to young adulthood for examination of microglia and Purkinje cells in vivo with two-photon imaging or in fixed tissue. Ethanol had no effect on microglia density, morphology, dynamics, or injury response. However, Purkinje cell linear frequency was reduced by ethanol. Microglia-Purkinje cell interactions in the Purkinje Cell Layer were altered in females compared to males. Overall, developmental ethanol exposure had few effects on cerebellar microglia in young adulthood and Purkinje cells appeared to be more susceptible to its effects.

Developmental ethanol exposure has minimal impact on cerebellar microglial dynamics, morphology, and interactions with Purkinje cells during adolescence

Introduction

Fetal alcohol spectrum disorders (FASD) are the most common cause of non-heritable, preventable mental disability, occurring in almost 5% of births in the United States. FASD lead to physical, behavioral, and cognitive impairments, including deficits related to the cerebellum. There is no known cure for FASD and their mechanisms remain poorly understood. To better understand these mechanisms, we examined the cerebellum on a cellular level by studying microglia, the principal immune cells of the central nervous system, and Purkinje cells, the sole output of the cerebellum. Both cell types have been shown to be affected in models of FASD, with increased cell death, immune activation of microglia, and altered firing in Purkinje cells. While ethanol administered in adulthood can acutely depress the dynamics of the microglial process arbor, it is unknown how developmental ethanol exposure impacts microglia dynamics and their interactions with Purkinje cells in the long term.

Methods

To address this question, we used a mouse model of human 3rd trimester exposure, whereby L7cre/Ai9+/-/Cx3cr1G/+ mice (with fluorescently labeled microglia and Purkinje cells) of both sexes were subcutaneously treated with a binge-level dose of ethanol (5.0 g/kg/day) or saline from postnatal days 4-9. Cranial windows were implanted in adolescent mice above the cerebellum to examine the long-term effects of developmental ethanol exposure on cerebellar microglia and Purkinje cell interactions using in vivo two-photon imaging.

Results

We found that cerebellar microglia dynamics and morphology were not affected after developmental ethanol exposure. Microglia dynamics were also largely unaltered with respect to how they interact with Purkinje cells, although subtle changes in these interactions were observed in females in the molecular layer of the cerebellum.

Discussion

This work suggests that there are limited in vivo long-term effects of ethanol exposure on microglia morphology, dynamics, and neuronal interactions, so other avenues of research may be important in elucidating the mechanisms of FASD.

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.

Age, but Not Sex, Modulates Foxp3 Expression in the Rat Brain across Development

The interconnectivity between brain development and the immune system has become an area of interest for many neuroscientists. However, to date, a limited number of known immune mediators of the peripheral nervous system (PNS) have been found to influence the development of the central nervous system (CNS). FOXP3 is a well-established mediator of regulatory T-cells in the PNS. However, the expression pattern of FOXP3 in the CNS and the PNS throughout development is unknown. To fill this void, we have characterized, in several brain regions, the developmental profile of Foxp3 for both sexes using rats. We found different patterns of Foxp3 in the CNS and PNS. In the CNS, we found Foxp3 was ubiquitously expressed, with the levels of Foxp3 varying by brain region. We also found both Foxp3 mRNA and protein levels peak during embryonic development and then steadily decrease with a peak increase during adulthood. In adulthood, the protein but not mRNA increases to the equivalent levels found at the embryonic stage of life. In the PNS, Foxp3 protein levels were low embryonically and increased steadily over the life of the animal with maximal levels reached in adulthood. Patterns observed for both the PNS and CNS were similar in males and females across all developmental timepoints. Our novel findings have implications for understanding how the neural immune system impacts neurodevelopmental disorders such as autism and schizophrenia.

Gestational alcohol exposure disrupts cognitive function and striatal circuits in adult offspring

Fetal alcohol exposure (FAE) is the leading preventable developmental cause of cognitive dysfunction. Even in the absence of binge drinking, alcohol consumption during pregnancy can leave offspring deficient. However, the mechanisms underlying these deficiencies are unknown. Using a mouse model of gestational ethanol exposure (GEE), we show increased instrumental lever-pressing and disruption of efficient habitual actions in adults, indicative of disrupted cognitive function. In vivo electrophysiology reveals disrupted action encoding in dorsolateral striatum (DLS) associated with altered habit learning. GEE mice exhibit decreased GABAergic transmission onto DLS projection neurons, including inputs from parvalbumin interneurons, and increased endocannabinoid tone. Chemogenetic activation of DLS parvalbumin interneurons reduces the elevated lever pressing of GEE mice. Pharmacologically increasing endocannabinoid tone mimics GEE effects on cognition and synaptic transmission. These findings show GEE induces long-lasting deficits in cognitive function that may contribute to human FAE, and identify potential mechanisms for future therapeutic targeting.

Alcohol is the leading cause of preventable birth defects in the US, collectively referred to as Fetal Alcohol Spectrum Disorder. Here, the authors show that fetal alcohol exposure induces lasting neurophysiological changes in dorsal striatum that contribute to less efficient decision making.

Intravenous Anesthetic, Propofol Affects Synaptic Responses in Cerebellar Purkinje Cells

Objective

Propofol is an intravenously administered anesthetic that enhances γ-aminobutyric acid-mediated inhibition in the central nerve system. Other mechanisms may also be involved in general anesthesia. Propofol has been implicated in movement disorders. The cerebellum is important for motor coordination and motor learning. The aim of the present study was to investigate the propofol effect on excitatory synaptic transmissions in cerebellar cortex.

Methods

Excitatory postsynaptic currents by parallel fiber stimulation and complex spikes by climbing fiber stimulation were monitored in Purkinje cells of Wister rat cerebellar slice using whole-cell patch-clamp techniques.

Results

Decay time, rise time and amplitude of excitatory postsynaptic currents at parallel fiber Purkinje cell synapses and area of complex spikes at climbing fiber Purkinje cell synapses were significantly increased by propofol administration.

Conclusion

The detected changes of glutamatergic synaptic transmission in cerebellar Purkinje cell, which determine cerebellar motor output, could explain cerebellar mechanism of motor deficits induced by propofol.

Acute oligodendrocyte loss with persistent white matter injury in a third trimester equivalent mouse model of fetal alcohol spectrum disorder

Alcohol exposure during central nervous system (CNS) development can lead to fetal alcohol spectrum disorder (FASD). Human imaging studies have revealed significant white matter (WM) abnormalities linked to cognitive impairment in children with FASD; however, the underlying mechanisms remain unknown. Here, we evaluated both the acute and long-term impacts of alcohol exposure on oligodendrocyte number and WM integrity in a third trimester-equivalent mouse model of FASD, in which mouse pups were exposed to alcohol during the first 2 weeks of postnatal development. Our results demonstrate a 58% decrease in the number of mature oligodendrocytes (OLs) and a 75% decrease in the number of proliferating oligodendrocyte progenitor cells (OPCs) within the corpus callosum of alcohol-exposed mice at postnatal day 16 (P16). Interestingly, neither mature OLs nor OPCs derived from the postnatal subventricular zone (SVZ) were numerically affected by alcohol exposure, indicating heterogeneity in susceptibility based on OL ontogenetic origin. Although mature OL and proliferating OPC numbers recovered by postnatal day 50 (P50), abnormalities in myelin protein expression and microstructure within the corpus callosum of alcohol-exposed subjects persisted, as assessed by western immunoblotting of myelin basic protein (MBP; decreased expression) and MRI diffusion tensor imaging (DTI; decreased fractional anisotropy). These results indicate that third trimester-equivalent alcohol exposure leads to an acute, albeit recoverable, decrease in OL lineage cell numbers, accompanied by enduring WM injury. Additionally, our finding of heterogeneity in alcohol susceptibility based on the developmental origin of OLs may have therapeutic implications in FASD and other disorders of WM development.

Electrophysiological and Immunohistochemical Evidence for an Increase in GABAergic Inputs and HCN Channels in Purkinje Cells that Survive Developmental Ethanol Exposure

Ethanol exposures during the early postnatal period of the rat result in significant death of Purkinje cells (PCs). The magnitude, time-course, and lobular specificity of PC death have been well characterized in several studies. Additionally, significant reduction of climbing fiber inputs to the surviving PCs has been characterized. This study investigates whether further alterations to the cerebellar cortical circuits might occur as a result of developmental ethanol exposures. We first examined the firing pattern of PCs in acute slice preparations on postnatal days 13-15. While the basic firing frequency was not significantly altered, PCs from rat pups treated with ethanol on postnatal days 4-6 showed a significantly increased number of inhibitory postsynaptic potentials (IPSCs) and a larger Ih current. We conducted immunofluorescent studies to identify the probable cause of the increased IPSCs. We found a significant 21 % increase in the number of basket cells per PC and a near doubling of the volume of co-localized basket cell axonal membrane with PC. In addition, we identified a significant (~147 %) increase in HCN1 channel volume co-localized to PC volume. Therefore, the cerebellar cortex that survives targeted postnatal ethanol exposure is dramatically altered in development subsequent to PC death. The cerebellar cortical circuit that results is one that operates under a significant degree of increased resting inhibition. The alterations in the development of cerebellar circuitry following ethanol exposure, and the significant loss of PCs, could result in modifications of the structure and function of other brain regions that receive cerebellar inputs.

Effect of repeated alcohol exposure during the third trimester-equivalent on messenger RNA levels for interleukin-1β, chemokine (C-C motif) ligand 2, and interleukin 10 in the developing rat brain after injection of lipopolysaccharide

Microglia undergo maturation during the third trimester of human development (equivalent to the first 1-2 weeks of postnatal life in rodents), during which these cells may be particularly sensitive to insult. Alcohol exposure during this period can activate the neuroimmune system, an effect that may contribute to the pathophysiology of fetal alcohol spectrum disorders. Here, we investigated whether repeated alcohol exposure during the third trimester-equivalent in rats has a priming effect on the neuroimmune response to injection of bacterial lipopolysaccharide (LPS). Pups were exposed to alcohol in vapor chambers for 4 h daily from postnatal day (PD)2 to PD16 (peak blood alcohol concentrations ∼150 mg/dL). On PD17, rats were injected with either saline or LPS (50 μg/kg) and the frontal cortex, cerebellar vermis, and dentate gyrus were collected 2 h later. Messenger RNA (mRNA) levels for the pro-inflammatory agents interleukin 1β (IL-1β) and chemokine (C-C) motif ligand 2 (CCL2), as well as levels of the anti-inflammatory cytokine interleukin 10 (IL-10), were measured using reverse transcriptase polymerase chain reaction. LPS consistently increased IL-1β and CCL2 mRNA levels in the dentate gyrus, frontal cortex, and cerebellum of both male and female rats. Furthermore, the LPS-induced increase of IL-1β mRNA levels was significantly blunted in the frontal cortex of alcohol-exposed female rats. Conversely, LPS only minimally affected IL-10 mRNA expression and there were no significant differences between air- and alcohol-exposed rats. Taken together with the literature regarding the effect of third-trimester alcohol exposure on the neuroimmune system, our findings suggest that chronic exposure to lower levels is less disruptive to the neuroimmune system than binge-like exposure to high doses of alcohol.

Moderate Alcohol Exposure during the Rat Equivalent to the Third Trimester of Human Pregnancy Alters Regulation of GABAA Receptor-Mediated Synaptic Transmission by Dopamine in the Basolateral Amygdala

Fetal ethanol (EtOH) exposure leads to a range of neurobehavioral alterations, including deficits in emotional processing. The basolateral amygdala (BLA) plays a critical role in modulating emotional processing, in part, via dopamine (DA) regulation of GABA transmission. This BLA modulatory system is acquired during the first 2 weeks of postnatal life in rodents (equivalent to the third trimester of human pregnancy) and we hypothesized that it could be altered by EtOH exposure during this period. We found that exposure of rats to moderate levels of EtOH vapor during the third trimester-equivalent [postnatal days (P) 2-12] alters DA modulation of GABAergic transmission in BLA pyramidal neurons during periadolescence. Specifically, D1R-mediated potentiation of spontaneous inhibitory postsynaptic currents (IPSCs) was significantly attenuated in EtOH-exposed animals. However, this was associated with a compensatory decrease in D3R-mediated suppression of miniature IPSCs. Western blot analysis revealed that these effects were not a result of altered D1R or D3R levels. BLA samples from EtOH-exposed animals also had significantly lower levels of the DA precursor (L-3,4-dihydroxyphenylalanine) but DA levels were not affected. This is likely a consequence of reduced catabolism of DA, as indicated by reduced levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid in the BLA samples. Anxiety-like behavior was not altered in EtOH-exposed animals. This is the first study to demonstrate that the modulatory actions of DA in the BLA are altered by developmental EtOH exposure. Although compensatory adaptations were engaged in our moderate EtOH exposure paradigm, it is possible that these are not able to restore homeostasis and correct anxiety-like behaviors under conditions of heavier EtOH exposure. Therefore, future studies should investigate the potential role of alterations in the modulatory actions of DA in the pathophysiology of fetal alcohol spectrum disorders.