Impaired Bidirectional Synaptic Plasticity in Juvenile Offspring Following Prenatal Ethanol Exposure

Developmental ethanol exposure produces deficits in long-term potentiation in vivo that persist following postnatal choline supplementation

BACKGROUND

Fetal alcohol spectrum disorder (FASD) is one of the leading causes of neurodevelopmental disorder for which there is a pressing need for an effective treatment. Recent studies have investigated the essential nutrient choline as a postnatal treatment option. Supplementation with choline has produced improvements in behavioral tasks related to learning and memory and reverted changes in methylation signature following third-trimester equivalent ethanol exposure. We examined whether there are related improvements in hippocampal synaptic plasticity in vivo.

METHODS

Sprague-Dawley offspring were administered binge-levels of ethanol from postnatal day (PND) 4 to 9, then treated with choline chloride (100 mg/kg/day) from PND 10 to 30. In vivo electrophysiology was performed on male and female offspring from PND 55 to 70. Long-term potentiation (LTP) was induced in the medial perforant pathway of the dentate gyrus using a theta-burst stimulation (TBS) protocol, and field-evoked postsynaptic potentials (EPSPs) were evoked for 60 min following the conditioning stimulus.

RESULTS

Developmental ethanol exposure caused long-lasting deficits in LTP of the slope of the evoked responses and in the amplitude of the population spike potentiation. Neither deficit was rescued by postnatal choline supplementation.

CONCLUSIONS

In contrast to our prior findings that choline can improve hippocampal plasticity (Nutrients, 2022, 14, 2004), here we found that deficits in hippocampal synaptic plasticity due to developmental ethanol exposure persisted into adulthood despite adolescent choline supplementation. Future research should examine more subtle changes in synaptic plasticity to identify synaptic changes that mirror behavioral improvements.

Unraveling the complex relationship between prenatal alcohol exposure, hippocampal LTP, and learning and memory

Prenatal alcohol exposure (PAE) has been extensively studied for its profound impact on neurodevelopment, synaptic plasticity, and cognitive outcomes. While PAE, particularly at moderate levels, has long-lasting cognitive implications for the exposed individuals, there remains a substantial gap in our understanding of the precise mechanisms underlying these deficits. This review provides a framework for comprehending the neurobiological basis of learning and memory processes that are negatively impacted by PAE. Sex differences, diverse PAE protocols, and the timing of exposure are explored as potential variables influencing the diverse outcomes of PAE on long-term potentiation (LTP). Additionally, potential interventions, both pharmacological and non-pharmacological, are reviewed, offering promising avenues for mitigating the detrimental effects of PAE on cognitive processes. While significant progress has been made, further research is required to enhance our understanding of how prenatal alcohol exposure affects neural plasticity and cognitive functions and to develop effective therapeutic interventions for those impacted. Ultimately, this work aims to advance the comprehension of the consequences of PAE on the brain and cognitive functions.

Synaptic Plasticity Abnormalities in Fetal Alcohol Spectrum Disorders

The brain's ability to strengthen or weaken synaptic connections is often termed synaptic plasticity. It has been shown to function in brain remodeling following different types of brain damage (e.g., drugs of abuse, alcohol use disorders, neurodegenerative diseases, and inflammatory conditions). Although synaptic plasticity mechanisms have been extensively studied, how neural plasticity can influence neurobehavioral abnormalities in alcohol use disorders (AUDs) is far from being completely understood. Alcohol use during pregnancy and its harmful effects on the developing offspring are major public health, social, and economic challenges. The significant attribute of prenatal alcohol exposure on offspring is damage to the central nervous system (CNS), causing a range of synaptic structural, functional, and behavioral impairments, collectively called fetal alcohol spectrum disorder (FASD). Although the synaptic mechanisms in FASD are limited, emerging evidence suggests that FASD pathogenesis involves altering a set of molecules involved in neurotransmission, myelination, and neuroinflammation. These studies identify several immediate and long-lasting changes using many molecular approaches that are essential for synaptic plasticity and cognitive function. Therefore, they can offer potential synaptic targets for the many neurobehavioral abnormalities observed in FASD. In this review, we discuss the substantial research progress in different aspects of synaptic and molecular changes that can shed light on the mechanism of synaptic dysfunction in FASD. Increasing our understanding of the synaptic changes in FASD will significantly advance our knowledge and could provide a basis for finding novel therapeutic targets and innovative treatment strategies.

Autism Spectrum Disorder: Neurodevelopmental Risk Factors, Biological Mechanism, and Precision Therapy

Autism spectrum disorder (ASD) is a heterogeneous, behaviorally defined neurodevelopmental disorder. Over the past two decades, the prevalence of autism spectrum disorders has progressively increased, however, no clear diagnostic markers and specifically targeted medications for autism have emerged. As a result, neurobehavioral abnormalities, neurobiological alterations in ASD, and the development of novel ASD pharmacological therapy necessitate multidisciplinary collaboration. In this review, we discuss the development of multiple animal models of ASD to contribute to the disease mechanisms of ASD, as well as new studies from multiple disciplines to assess the behavioral pathology of ASD. In addition, we summarize and highlight the mechanistic advances regarding gene transcription, RNA and non-coding RNA translation, abnormal synaptic signaling pathways, epigenetic post-translational modifications, brain-gut axis, immune inflammation and neural loop abnormalities in autism to provide a theoretical basis for the next step of precision therapy. Furthermore, we review existing autism therapy tactics and limits and present challenges and opportunities for translating multidisciplinary knowledge of ASD into clinical practice.

Binge-like Prenatal Ethanol Exposure Causes Impaired Cellular Differentiation in the Embryonic Forebrain and Synaptic and Behavioral Defects in Adult Mice

An embryo’s in-utero exposure to ethanol due to a mother’s alcohol drinking results in a range of deficits in the child that are collectively termed fetal alcohol spectrum disorders (FASDs). Prenatal ethanol exposure is one of the leading causes of preventable intellectual disability. Its neurobehavioral underpinnings warrant systematic research. We investigated the immediate effects on embryos of acute prenatal ethanol exposure during gestational days (GDs) and the influence of such exposure on persistent neurobehavioral deficits in adult offspring. We administered pregnant C57BL/6J mice with ethanol (1.75 g/kg) (GDE) or saline (GDS) intraperitoneally (i.p.) at 0 h and again at 2 h intervals on GD 8 and GD 12. Subsequently, we assessed apoptosis, differentiation, and signaling events in embryo forebrains (E13.5; GD13.5). Long-lasting effects of GDE were evaluated via a behavioral test battery. We also determined the long-term potentiation and synaptic plasticity-related protein expression in adult hippocampal tissue. GDE caused apoptosis, inhibited differentiation, and reduced pERK and pCREB signaling and the expression of transcription factors Pax6 and Lhx2. GDE caused persistent spatial and social investigation memory deficits compared with saline controls, regardless of sex. Interestingly, GDE adult mice exhibited enhanced repetitive and anxiety-like behavior, irrespective of sex. GDE reduced synaptic plasticity-related protein expression and caused hippocampal synaptic plasticity (LTP and LTD) deficits in adult offspring. These findings demonstrate that binge-like ethanol exposure at the GD8 and GD12 developmental stages causes defects in pERK–pCREB signaling and reduces the expression of Pax6 and Lhx2, leading to impaired cellular differentiation during the embryonic stage. In the adult stage, binge-like ethanol exposure caused persistent synaptic and behavioral abnormalities in adult mice. Furthermore, the findings suggest that combining ethanol exposure at two sensitive stages (GD8 and GD12) causes deficits in synaptic plasticity-associated proteins (Arc, Egr1, Fgf1, GluR1, and GluN1), leading to persistent FASD-like neurobehavioral deficits in mice.

Postnatal Choline Supplementation Rescues Deficits in Synaptic Plasticity Following Prenatal Ethanol Exposure

Prenatal ethanol exposure (PNEE) is a leading cause of neurodevelopmental impairments, yet treatments for individuals with PNEE are limited. Importantly, postnatal supplementation with the essential nutrient choline can attenuate some adverse effects of PNEE on cognitive development; however, the mechanisms of action for choline supplementation remain unclear. This study used an animal model to determine if choline supplementation could restore hippocampal synaptic plasticity that is normally impaired by prenatal alcohol. Throughout gestation, pregnant Sprague Dawley rats were fed an ethanol liquid diet (35.5% ethanol-derived calories). Offspring were injected with choline chloride (100 mg/kg/day) from postnatal days (PD) 10-30, and then used for in vitro electrophysiology experiments as juveniles (PD 31-35). High-frequency conditioning stimuli were used to induce long-term potentiation (LTP) in the medial perforant path input to the dentate gyrus of the hippocampus. PNEE altered synaptic transmission in female offspring by increasing excitability, an effect that was mitigated with choline supplementation. In contrast, PNEE juvenile males had decreased LTP compared to controls, and this was rescued by choline supplementation. These data demonstrate sex-specific changes in plasticity following PNEE, and provide evidence that choline-related improvements in cognitive functioning may be due to its positive impact on hippocampal synaptic physiology.

Effects of prenatal ethanol exposure on choline-induced long-term depression in the hippocampus

Choline is an essential nutrient under evaluation as a cognitive enhancing treatment for fetal alcohol spectrum disorders (FASD) in clinical trials. As a result, there is increased pressure to identify therapeutic mechanism(s) of action. Choline is not only a precursor for several essential cell membrane components and signaling molecules but also has the potential to directly affect synaptic mechanisms that are believed important for cognitive processes. In the current work, we study how the direct application of choline can affect synaptic transmission in the dentate gyrus (DG) of hippocampal slices obtained from adolescent (postnatal days 21-28) Sprague-Dawley rats (Rattus norvegicus). The acute administration of choline chloride (2 mM) reliably induced a long-term depression (LTD) of field excitatory postsynaptic potentials (fEPSPs) in the DG in vitro. The depression required the involvement of M1 receptors, and the magnitude of the effect was similar in slices obtained from male and female animals. To further study the impact of choline in an animal model of FASD, we examined offspring from dams fed an ethanol-containing diet (35.5% ethanol-derived calories) throughout gestation. In slices from the adolescent animals that experienced prenatal ethanol exposure (PNEE), we found that the choline induced an LTD that uniquely involved the activation of N-methyl-d-aspartate (NMDA) and M1 receptors. This study provides a novel insight into how choline can modulate hippocampal transmission at the level of the synapse and that it can have unique effects following PNEE.NEW & NOTEWORTHY Choline supplementation is a nutraceutical therapy with significant potential for a variety of developmental disorders; however, the mechanisms involved in its therapeutic effects remain poorly understood. Our research shows that choline directly impacts synaptic communication in the brain, inducing a long-term depression of synaptic efficacy in brain slices. The depression is equivalent in male and female animals, involves M1 receptors in control animals, but uniquely involves NMDA receptors in a model of FASD.

Exercise hormone irisin is a critical regulator of cognitive function

Identifying secreted mediators that drive the cognitive benefits of exercise holds great promise for the treatment of cognitive decline in ageing or Alzheimer's disease (AD). Here, we show that irisin, the cleaved and circulating form of the exercise-induced membrane protein FNDC5, is sufficient to confer the benefits of exercise on cognitive function. Genetic deletion of Fndc5/irisin (global Fndc5 knock-out (KO) mice; F5KO) impairs cognitive function in exercise, ageing and AD. Diminished pattern separation in F5KO mice can be rescued by delivering irisin directly into the dentate gyrus, suggesting that irisin is the active moiety. In F5KO mice, adult-born neurons in the dentate gyrus are morphologically, transcriptionally and functionally abnormal. Importantly, elevation of circulating irisin levels by peripheral delivery of irisin via adeno-associated viral overexpression in the liver results in enrichment of central irisin and is sufficient to improve both the cognitive deficit and neuropathology in AD mouse models. Irisin is a crucial regulator of the cognitive benefits of exercise and is a potential therapeutic agent for treating cognitive disorders including AD.

Rapamycin Improves Recognition Memory and Normalizes Amino-Acids and Amines Levels in the Hippocampal Dentate Gyrus in Adult Rats Exposed to Ethanol during the Neonatal Period

The mammalian target of rapamycin (mTOR), a serine/ threonine kinase, is implicated in synaptic plasticity by controlling protein synthesis. Research suggests that ethanol exposure during pregnancy alters the mTOR signaling pathway in the fetal hippocampus. Thus, we investigated the influence of pre-treatment with rapamycin, an mTORC1 inhibitor, on the development of recognition memory deficits in adult rats that were neonatally exposed to ethanol. In the study, male and female rat pups received ethanol (5 g/kg/day) by intragastric intubation at postanatal day (PND 4-9), an equivalent to the third trimester of human pregnancy. Rapamycin (3 and 10 mg/kg) was given intraperitoneally before every ethanol administration. Short- and long-term recognition memory was assessed in the novel object recognition (NOR) task in adult (PND 59/60) rats. Locomotor activity and anxiety-like behavior were also evaluated to exclude the influence of such behavior on the outcome of the memory task. Moreover, the effects of rapamycin pre-treatment during neonatal ethanol exposure on the content of amino-acids and amines essential for the proper development of cognitive function in the dentate gyrus (DG) of the hippocampus was evaluated using proton magnetic resonance spectroscopy (1H MRS) in male adult (PND 60) rats. Our results show the deleterious effect of ethanol given to neonatal rats on long-term recognition memory in adults. The effect was more pronounced in male rather than female rats. Rapamycin reversed this ethanol-induced memory impairment and normalized the levels of amino acids and amines in the DG. This suggests the involvement of mTORC1 in the deleterious effect of ethanol on the developing brain.

Endocannabinoid receptors contribute significantly to multiple forms of long-term depression in the rat dentate gyrus

Cannabinoid receptors are widely expressed throughout the hippocampal formation, but are particularly dense in the dentate gyrus (DG) subregion. We, and others, have shown in mice that cannabinoid type 1 receptors (CB1Rs) are involved in a long-term depression (LTD) that can be induced by prolonged 10 Hz stimulation of the medial perforant path (MPP)-granule cell synaptic input to the DG. Here, we extend this work to examine the involvement of CB1Rs in other common forms of LTD in the hippocampus of juvenile male and female Sprague–Dawley rats (Rattus norvegicus). We found, as in mice, that prolonged 10 Hz stimulation (6000 pulses) could reliably induce a form of LTD that was dependent upon CB1R activation. In addition, we also discovered a role for both CB1R and mGluR proteins in LTD induced with 1 Hz low-frequency stimulation (1 Hz-LTD; 900 pulses) and in LTD induced by bath application of the group I mGluR agonist (RS)-3,5-Dihydroxyphenylglycine (DHPG; DHPG-LTD). This study elucidates an essential role for endocannabinoid receptors in a number of forms of LTD in the rat DG, and identifies a novel role for CB1Rs as potential therapeutic targets for conditions that involve impaired LTD in the DG.

Murine Models for the Study of Fetal Alcohol Spectrum Disorders: An Overview

Prenatal alcohol exposure is associated to different physical, behavioral, cognitive, and neurological impairments collectively known as fetal alcohol spectrum disorder. The underlying mechanisms of ethanol toxicity are not completely understood. Experimental studies during human pregnancy to identify new diagnostic biomarkers are difficult to carry out beyond genetic or epigenetic analyses in biological matrices. Therefore, animal models are a useful tool to study the teratogenic effects of alcohol on the central nervous system and analyze the benefits of promising therapies. Animal models of alcohol spectrum disorder allow the analysis of key variables such as amount, timing and frequency of ethanol consumption to describe the harmful effects of prenatal alcohol exposure. In this review, we aim to synthetize neurodevelopmental disabilities in rodent fetal alcohol spectrum disorder phenotypes, considering facial dysmorphology and fetal growth restriction. We examine the different neurodevelopmental stages based on the most consistently implicated epigenetic mechanisms, cell types and molecular pathways, and assess the advantages and disadvantages of murine models in the study of fetal alcohol spectrum disorder, the different routes of alcohol administration, and alcohol consumption patterns applied to rodents. Finally, we analyze a wide range of phenotypic features to identify fetal alcohol spectrum disorder phenotypes in murine models, exploring facial dysmorphology, neurodevelopmental deficits, and growth restriction, as well as the methodologies used to evaluate behavioral and anatomical alterations produced by prenatal alcohol exposure in rodents.

N-Methyl-d-Aspartate Receptor Modulation by Nicotinamide Adenine Dinucleotide Phosphate Oxidase Type 2 Drives Synaptic Plasticity and Spatial Memory Impairments in Rats Exposed Pre- and Postnatally to Ethanol

Aims: Pre- and/or early postnatal ethanol exposure (prenatal alcohol exposure [PAE]) impairs synaptic plasticity as well as memory formation, but the mechanisms underlying these effects remain unclear. Both long-term potentiation (LTP) and spatial memory formation in the hippocampus involve the nicotinamide adenine dinucleotide phosphate oxidase type 2 (NOX2) enzyme. Previous studies have reported that N-methyl-d-aspartate receptor (NMDAR) activation increases NOX2-mediated superoxide generation, resulting in inhibition of NMDAR function, but whether NOX2 impacts NMDAR function in PAE animals leading to impaired LTP and memory formation remains unknown. We aim to evaluate whether the NOX2-NMDAR complex is involved in the long-lasting deleterious effects of PAE on hippocampal LTP and memory formation. Results: Here we provide novel evidence that PAE animals display impaired NMDAR-dependent LTP in the cornus ammonis field 1 (CA1) and NMDAR-mediated LTP in the dentate gyrus (DG). Moreover, PAE rats displayed increased NMDAR-mediated transmission in both hippocampal areas. Interestingly, NOX2 pharmacological inhibition restored NMDAR-mediated transmission and LTP in the CA1, but not in the DG. PAE also induced overexpression of NOX2 and CaMKII isoforms, but did not modify the content or the redox state of the N-methyl-d-aspartate receptor subunit-1 (NR1) subunit of NMDAR in both areas of the hippocampus. In addition, adolescent PAE rats orally fed the antioxidant and free radical scavenger apocynin exhibited significantly improved spatial memory acquisition. Innovation and Conclusion: By showing in PAE animals NOX2 overexpression and increased NMDAR-mediated transmission, which might lead to impaired synaptic plasticity and memory formation in a region-specific manner, we provide an important advance to our current understanding of the cellular mechanisms underlying PAE-dependent defective hippocampal function.

Effect of Alcohol on Hippocampal-Dependent Plasticity and Behavior: Role of Glutamatergic Synaptic Transmission

Problematic alcohol drinking and alcohol dependence are an increasing health problem worldwide. Alcohol abuse is responsible for approximately 5% of the total deaths in the world, but addictive consumption of it has a substantial impact on neurological and memory disabilities throughout the population. One of the better-studied brain areas involved in cognitive functions is the hippocampus, which is also an essential brain region targeted by ethanol. Accumulated evidence in several rodent models has shown that ethanol treatment produces cognitive impairment in hippocampal-dependent tasks. These adverse effects may be related to the fact that ethanol impairs the cellular and synaptic plasticity mechanisms, including adverse changes in neuronal morphology, spine architecture, neuronal communication, and finally an increase in neuronal death. There is evidence that the damage that occurs in the different brain structures is varied according to the stage of development during which the subjects are exposed to ethanol, and even much earlier exposure to it would cause damage in the adult stage. Studies on the cellular and cognitive deficiencies produced by alcohol in the brain are needed in order to search for new strategies to reduce alcohol neuronal toxicity and to understand its consequences on memory and cognitive performance with emphasis on the crucial stages of development, including prenatal events to adulthood.

Early exposure to environmental enrichment modulates the effects of prenatal ethanol exposure upon opioid gene expression and adolescent ethanol intake

Prenatal ethanol exposure (PEE) promotes ethanol consumption in the adolescent offspring accompanied by the transcriptional regulation of kappa opioid receptor (KOR) system genes. This study analysed if environmental enrichment (EE, from gestational day 20 to postnatal day 26) exerts protective effects upon PEE-modulation of gene expression, ethanol intake and anxiety responses. Pregnant rats were exposed to PEE (0.0 or 2.0 g/kg ethanol, gestational days 17-20) and subsequently the dam and offspring were reared under EE or standard conditions. PEE upregulated KOR mRNA levels in amygdala (AMY) and prodynorphin (PDYN) mRNA levels in ventral tegmental area (VTA) with the latter effect associated with lower DNA methylation at the gene promoter. These effects were normalized by exposure to EE. PEE modulated BDNF mRNA levels in VTA and Nucleus accumbens (AcbN), and EE mitigated the changes in AcbN. EE induced a protective effect on ethanol intake and preference, an effect more noticeable in males than in females, and in prenatal vehicle-treated (PV) than in PEE rats. The male offspring drank significantly less ethanol than the female offspring. The latter result suggests that the protective effect of EE on ethanol drinking may only emerge at lower levels of drinking. In the dams, PEE induced an upregulation of PDYN and KOR in AcbN. PDYN gene expression was normalized by exposure to EE. These results suggest that EE is a promising treatment to inhibit the effects of PEE. The results confirm that PEE effects are mediated by alterations in the transcriptional regulation of KOR system genes.