Effects of exercise and environmental complexity on deficits in trace and contextual fear conditioning produced by neonatal alcohol exposure in rats

The Fetal Alcohol Spectrum Disorders-An Overview of Experimental Models, Therapeutic Strategies, and Future Research Directions

Since the establishment of a clear link between maternal alcohol consumption during pregnancy and certain birth defects, the research into the treatment of FASD has become increasingly sophisticated. The field has begun to explore the possibility of intervening at different levels, and animal studies have provided valuable insights into the pathophysiology of the disease, forming the basis for implementing potential therapies with increasingly precise mechanisms. The recent reports suggest that compounds that reduce the severity of neurodevelopmental deficits, including glial cell function and myelination, and/or target oxidative stress and inflammation may be effective in treating FASD. Our goal in writing this article was to analyze and synthesize current experimental therapeutic interventions for FASD, elucidating their potential mechanisms of action, translational relevance, and implications for clinical application. This review exclusively focuses on animal models and the interventions used in these models to outline the current direction of research. We conclude that given the complexity of the underlying mechanisms, a multifactorial approach combining nutritional supplementation, pharmacotherapy, and behavioral techniques tailored to the stage and severity of the disease may be a promising avenue for further research in humans.

Exercise in Adolescence Enhances Callosal White Matter Refinement in the Female Brain in a Rat Model of Fetal Alcohol Spectrum Disorders

A total of 1 in 20 infants born annually are exposed to alcohol prenatally, which disrupts neurodevelopment and results in several disorders categorized under the umbrella term Fetal Alcohol Spectrum Disorders (FASD). Children and adolescents affected by FASD exhibit delayed maturation of cerebral white matter, which contributes to deficits in executive function, visuospatial processing, sensory integration, and interhemispheric communication. Research using animal models of FASD have uncovered that oligoglia proliferation, differentiation, and survival are vulnerable to alcohol teratogenesis in the male brain due in part to the activation of the neuroimmune system during gestation and infancy. A comprehensive investigation of prenatal alcohol exposure on white matter development in the female brain is limited. This study demonstrated that the number of mature oligodendrocytes and the production of myelin basic protein were reduced first in the female corpus callosum following alcohol exposure in a rat model of FASD. Analysis of myelin-related genes confirmed that myelination occurs earlier in the female corpus callosum compared to their counterparts, irrespective of postnatal treatment. Moreover, dysregulated oligodendrocyte number and myelin basic protein production was observed in the male and female FASD brain in adolescence. Targeted interventions that support white matter development in FASD-affected youth are nonexistent. The capacity for an adolescent exercise intervention to upregulate corpus callosum myelination was evaluated: we discovered that volunteer exercise increases the number of mature oligodendrocytes in alcohol-exposed female rats. This study provides critical evidence that oligoglia differentiation is difficult but not impossible to induce in the female FASD brain in adolescence following a behavioral intervention.

Midline Thalamic Damage Associated with Alcohol-Use Disorders: Disruption of Distinct Thalamocortical Pathways and Function

The thalamus, a significant part of the diencephalon, is a symmetrical and bilateral central brain structure. The thalamus is subdivided into three major groups of nuclei based on their function: sensorimotor nuclei (or principal/relay nuclei), limbic nuclei and nuclei bridging these two domains. Anatomically, nuclei within the thalamus are described by their location, such as anterior, medial, lateral, ventral, and posterior. In this review, we summarize the role of medial and midline thalamus in cognition, ranging from learning and memory to flexible adaptation. We focus on the discoveries in animal models of alcohol-related brain damage, which identify the loss of neurons in the medial and midline thalamus as drivers of cognitive dysfunction associated with alcohol use disorders. Models of developmental ethanol exposure and models of adult alcohol-related brain damage and are compared and contrasted, and it was revealed that there are similar (anterior thalamus) and different (intralaminar [adult exposure] versus ventral midline [developmental exposure]) thalamic pathology, as well as disruptions of thalamo-hippocampal and thalamo-cortical circuits. The final part of the review summarizes approaches to recover alcohol-related brain damage and cognitive and behavioral outcomes. These approaches include pharmacological, nutritional and behavioral interventions that demonstrated the potential to mitigate alcohol-related damage. In summary, the medial/midline thalamus is a significant contributor to cognition function, which is also sensitive to alcohol-related brain damage across the life span, and plays a role in alcohol-related cognitive dysfunction.

Post-exposure environment modulates long-term developmental ethanol effects on behavior, neuroanatomy, and cortical oscillations

Developmental exposure to ethanol has a wide range of anatomical, cellular, physiological and behavioral impacts that can last throughout life. In humans, this cluster of effects is termed fetal alcohol spectrum disorder and is highly prevalent in western cultures. The ultimate expression of the effects of developmental ethanol exposure however can be influenced by post-exposure experience. Here we examined the effects of developmental binge exposure to ethanol (postnatal day 7) in C57BL/6By mice on a specific cohort of inter-related long-term outcomes including contextual memory, hippocampal parvalbumin-expressing neuron density, frontal cortex oscillations related to sleep-wake cycling including delta oscillation amplitude and sleep spindle density, and home-cage behavioral activity. When assessed in adults that were raised in standard housing, all of these factors were altered by early ethanol exposure compared to saline controls except home-cage activity. However, exposure to an enriched environment and exercise from weaning to postnatal day 90 reversed most of these ethanol-induced impairments including memory, CA1 but not dentate gyrus PV+ cell density, delta oscillations and sleep spindles, and enhanced home-cage behavioral activity in Saline- but not EtOH-treated mice. The results are discussed in terms of the inter-dependence of diverse developmental ethanol outcomes and potential mechanisms of post-exposure experiences to regulate those outcomes.

Neonatal ethanol exposure impairs long-term context memory formation and prefrontal immediate early gene expression in adolescent rats

Fetal alcohol exposure leads to severe disruptions in learning and memory involving the hippocampus and prefrontal cortex in humans. Animal model research on FASD has documented impairment of hippocampal neuroanatomy and function but animal studies of cognition involving the prefrontal cortex are sparse. We have found that a variant of contextual fear conditioning in which both the hippocampus and prefrontal cortex is required, the Context Preexposure Facilitation Effect (CPFE), is particularly sensitive to neurobehavioral disruption caused by neonatal ethanol exposure during the third trimester equivalent of human pregnancy in the rat (i.e., PD4-9). In the CPFE, learning about the context, acquiring a context-shock association, and retrieving contextual fear are temporally separated across three days. The current study asked whether neonatal alcohol exposure impairs context learning, consolidation, or retrieval and examined prefrontal and hippocampal molecular signaling as correlates of this impairment. Long-Evans rats that received oral intubation of ethanol (AE; 5.25 g/kg/day, split into two doses) or underwent sham-intubation (SI) from PND4-9 were tested on the CPFE on PD31-33. Extending our previous reports, ethanol abolished both post-shock and retention test freezing in the CPFE. Assays (qPCR) of immediate early gene expression revealed that ethanol disrupted prefrontal but not hippocampal expression of c-Fos, Arc, Egr-1, and Npas4 during context learning. Finally, ethanol-exposed animals were unimpaired in a standard contextual fear conditioning procedure in which learning about the context and acquiring a context-shock association occurs concurrently. These findings implicate impaired prefrontal function in cognitive deficits arising from 3rd-trimester equivalent alcohol exposure in the rat.

Impairment of the context preexposure facilitation effect in juvenile rats by neonatal alcohol exposure is associated with decreased Egr-1 mRNA expression in the prefrontal cortex

The context preexposure facilitation effect (CPFE) is a variant of contextual fear conditioning in which learning about the context (preexposure) and associating the context with a shock (training) occur on separate occasions. The CPFE is sensitive to a range of neonatal alcohol doses (Murawski & Stanton, 2011). The current study examined the impact of neonatal alcohol on Egr-1 mRNA expression in the infralimbic (IL) and prelimbic (PL) subregions of the mPFC, the CA1 of dorsal hippocampus (dHPC), and the lateral nucleus of the amygdala (LA), following the preexposure and training phases of the CPFE. Rat pups were exposed to a 5.25 g/kg/day single binge-like dose of alcohol (Group EtOH) or were sham intubated (SI; Group SI) over postnatal days (PD) 7-9. In behaviorally tested rats, alcohol administration disrupted freezing. Following context preexposure, Egr-1 mRNA was elevated in both EtOH and SI groups compared with baseline control animals in all regions analyzed. Following both preexposure and training, Group EtOH displayed a significant decrease in mPFC Egr-1 mRNA expression compared with Group SI. However, this decrease was greatest after training. Training day decreases in Egr-1 expression were not found in LA or CA1 in Group EtOH compared with Group SI. A second experiment confirmed that the EtOH-induced training-day deficits in mPFC Egr-1 mRNA expression were specific to groups which learned contextual fear (vs. nonassociative controls). Thus, memory processes that engage the mPFC during the context-shock association may be most susceptible to the teratogenic effects of neonatal alcohol. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Differential expression of the immediate early genes c-Fos, Arc, Egr-1, and Npas4 during long-term memory formation in the context preexposure facilitation effect (CPFE)

The context preexposure facilitation effect (CPFE) is a contextual fear conditioning paradigm in which learning about the context, acquiring the context-shock association, and retrieving/expressing contextual fear are temporally dissociated into three distinct phases (context preexposure, immediate-shock training, and retention). The current study examined changes in the expression of plasticity-associated immediate early genes (IEGs) during context and contextual fear memory formation on the preexposure and training days of the CPFE, respectively. Using adolescent Long-Evans rats, preexposure and training day expression of the IEGs c-Fos, Arc, Egr-1, and Npas4 in the medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and basolateral amygdala (BLA) was analyzed using qPCR as an extension of previous studies from our lab examining Egr-1 via in situ hybridization (Asok, Schreiber, Jablonski, Rosen, & Stanton, 2013; Schreiber, Asok, Jablonski, Rosen, & Stanton, 2014). In Expt. 1, context preexposure induced expression of c-Fos, Arc, Egr-1 and Npas4 significantly above that of home-cage (HC) controls in all three regions. In Expt. 2, immediate-shock was followed by a post-shock freezing test, resulting in increased mPFC c-Fos expression in a group preexposed to the training context but not a control group preexposed to an alternate context, indicating expression related to associative learning. This was not seen with other IEGs in mPFC or with any IEG in dHPC or BLA. Finally, when the post-shock freezing test was omitted in Expt. 3, training-related increases were observed in prefrontal c-Fos, Arc, Egr-1, and Npas4, hippocampal c-Fos, and amygdalar Egr-1 expression. These results indicate that context exposure in a post-shock freezing test re-engages IEG expression that may obscure associatively-induced expression during contextual fear conditioning. Additionally, these studies suggest a key role for long-term synaptic plasticity in the mPFC in supporting the CPFE.

Impact of exercise and a complex environment on hippocampal dendritic morphology, Bdnf gene expression, and DNA methylation in male rat pups neonatally exposed to alcohol

Alcohol exposure in utero can result in Fetal Alcohol Spectrums Disorders (FASD). Measures of hippocampal neuroplasticity, including long-term potentiation, synaptic and dendritic organization, and adult neurogenesis, are consistently disrupted in rodent models of FASD. The current study investigated whether third trimester-equivalent binge-like alcohol exposure (AE) [postnatal days (PD) 4-9] affects dendritic morphology of immature dentate gyrus granule cells, and brain-derived neurotrophic factor (Bdnf) gene expression and DNA methylation in hippocampal tissue in adult male rats. To understand immediate impact of alcohol, DNA methylation was measured in the PD10 hippocampus. In addition, two behavioral interventions, wheel running (WR) and environmental complexity (EC), were utilized as rehabilitative therapies for alcohol-induced deficits. AE significantly decreased dendritic complexity of the immature neurons, demonstrating the long-lasting impact of neonatal alcohol exposure on dendritic morphology of immature neurons in the hippocampus. Both housing conditions robustly enhanced dendritic complexity in the AE animals. While Bdnf exon I DNA methylation was lower in the AE and sham-intubated animals compared with suckle controls on PD10, alterations to Bdnf DNA methylation and gene expression levels were not present at PD72. In control animals, exercise, but not exercise followed by housing in EC, resulted in higher levels of hippocampal Bdnf gene expression and lower DNA methylation. These studies demonstrate the long-lasting negative impact of developmental alcohol exposure on hippocampal dendritic morphology and support the implementation of exercise and complex environments as therapeutic interventions for individuals with FASD. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 708-725, 2017.

Neonatal binge alcohol exposure increases microglial activation in the developing rat hippocampus

Aberrant activation of the developing immune system can have long-term negative consequences on cognition and behavior. Teratogens, such as alcohol, activate microglia, the brain's resident immune cells, which could contribute to the lifelong deficits in learning and memory observed in humans with Fetal Alcohol Spectrum Disorders (FASD) and in rodent models of FASD. The current study investigates the microglial response of the brain 24 h following neonatal alcohol exposure (postnatal days (PDs) 4-9, 5.25 g/kg/day). On PD10, microglial cell counts and area of cell territory were assessed using unbiased stereology in the hippocampal subfields CA1, CA3 and dentate gyrus (DG), and hippocampal expression of pro- and anti-inflammatory genes was analyzed. A significant decrease in microglial cell counts in CA1 and DG was found in alcohol-exposed and sham-intubated (SI) animals compared to undisturbed suckle controls (SCs), suggesting overlapping effects of alcohol exposure and intubation alone on the neuroimmune response. Cell territory was decreased in alcohol-exposed animals in CA1, CA3, and DG compared to controls, suggesting the microglia have shifted to a more activated state following alcohol treatment. Furthermore, both alcohol-exposed and SI animals had increased levels of pro-inflammatory cytokines IL-1β, TNF-α, CD11b, and CCL4; in addition, CCL4 was significantly increased in alcohol-exposed animals compared to SI as well. Alcohol-exposed animals also showed increased levels of anti-inflammatory cytokine TGF-β compared to both SI and SCs. In summary, the number and activation of microglia in the neonatal hippocampus are both affected in a rat model of FASD, along with increased gene expression of pro- and anti-inflammatory cytokines. This study shows that alcohol exposure during development induces a neuroimmune response, potentially contributing to long-term alcohol-related changes to cognition, behavior and immune function.

The Benefits of Exercise on Structural and Functional Plasticity in the Rodent Hippocampus of Different Disease Models

In this review, the benefits of physical exercise on structural and functional plasticity in the hippocampus are discussed. The evidence is clear that voluntary exercise in rats and mice can lead to increases in hippocampal neurogenesis and enhanced synaptic plasticity which ultimately result in improved performance in hippocampal-dependent tasks. Furthermore, in models of neurological disorders, including fetal alcohol spectrum disorders, traumatic brain injury, stroke, and neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's disease exercise can also elicit beneficial effects on hippocampal function. Ultimately this review highlights the multiple benefits of exercise on hippocampal function in both the healthy and the diseased brain.

An animal model of fetal alcohol spectrum disorder: Trace conditioning as a window to inform memory deficits and intervention tactics

Animal models of Fetal Alcohol Spectrum Disorders (FASD) afford the unique capacity to precisely control timing of alcohol exposure and alcohol exposure amounts in the developing animal. These models have powerfully informed neurophysiological alterations associated with fetal and perinatal alcohol. In two experiments presented here we expand use of the Pavlovian Trace Conditioning procedure to examine cognitive deficits and intervention strategies in a rat model of FASD. Rat pups were exposed to 5g/kg/day ethanol on postnatal days (PD) 4-9, simulating alcohol exposure in the third trimester in humans. During early adolescence, approximately PD 30, the rats were trained in the trace conditioning task in which a light conditioned stimulus (CS) and shock unconditioned stimulus (US) were paired but separated by a 10-s stimulus free trace interval. Learning was assessed in freezing behavior during shock-free tests. Experiment 1 revealed that neonatal ethanol exposure significantly impaired hippocampus-dependent trace conditioning relative to controls. In Experiment 2 a serial compound conditioning procedure known as 'gap filling' completely reversed the ethanol-induced deficit in trace conditioning. We also discuss prior data regarding the beneficial effects of supplemental choline and novel preliminary data regarding the pharmacological cognitive enhancer physostigmine, both of which mitigate the alcohol-induced cognitive deficit otherwise seen in trace conditioning controls. We suggest trace conditioning as a useful tool for characterizing some of the core cognitive deficits seen in FASD, and as a model for developing effective environmental as well as nutritional and pharmacological interventions.

Activity and social behavior in a complex environment in rats neonatally exposed to alcohol

Environmental complexity (EC) is a powerful, stimulating paradigm that engages animals through a variety of sensory and motor pathways. Exposure to EC (30 days) following 12 days of wheel running preserves hippocampal neuroplasticity in male rats neonatally exposed to alcohol during the third-trimester equivalent (binge-like exposure on postnatal days [PD] 4-9). The current experiment investigates the importance of various components of EC (physical activity, exploration, social interaction, novelty) and examines whether neonatal alcohol exposure affects how male rats interact with their environment and other male rats. Male pups were assigned to 1 of 3 neonatal conditions from PD 4-9: suckle control (SC), sham-intubated (SI), or alcohol-exposed (AE, 5.25 g/kg/day). From PD 30-42 animals were housed with 24-h access to a voluntary running wheel. The animals were then placed in EC from PD 42-72 (9 animals/cage, counterbalanced by neonatal condition). During EC, the animals were filmed for five 30-min sessions (PD 42, 48, 56, 64, 68). For the first experiment, the videos were coded for distance traveled in the cage, overall locomotor activity, time spent near other animals, and interaction with toys. For the second experiment, the videos were analyzed for wrestling, mounting, boxing, grooming, sniffing, and crawling over/under. AE animals were found to be less active and exploratory and engaged in fewer mounting behaviors compared to control animals. Results suggest that after exposure to wheel running, AE animals still have deficits in activity and social behaviors while housed in EC compared to control animals with the same experience.

Exercise and environment as an intervention for neonatal alcohol effects on hippocampal adult neurogenesis and learning

Neonatal alcohol exposure impairs cognition and learning in adulthood and permanently damages the hippocampus. Wheel running (WR) improves hippocampus-associated learning and memory and increases the genesis and survival of newly generated neurons in the hippocampal dentate gyrus. WR significantly increases proliferation of newly generated dentate granule cells in alcohol-exposed (AE) and control rats on Postnatal Day (PD) 42 but only control rats show an increased number of surviving cells thirty days after WR (Helfer et al., 2009b). The present studies examined whether proliferation-promoting WR followed by survival-enhancing environmental complexity (EC) during adolescence could increase survival of new neurons in AE rats. On PD 4-9, pups were intubated with alcohol in a binge-like manner (5.25g/kg/day, AE), were sham-intubated (SI), or were reared normally (suckle control, SC). On PD 30 animals were assigned to WR (PD 30-42) followed by EC (PD 42-72; WR/EC) or were socially housed (SH/SH) for the duration of the experiment. All animals were injected with 200mg/kg bromodeoxyuridine (BrdU) on PD 41. In Experiment 1, survival of newly generated cells was significantly enhanced in the AE-WR/EC group in comparison with AE-SH/SH group. Experiment 2A examined trace eyeblink conditioning. In the SH/SH condition, AE impaired trace eyeblink conditioning relative to SI and SC controls. In the WR/EC condition, AE rats performed as well as controls. In Experiment 2B, the same intervention was examined using the context preexposure facilitation effect (CPFE); a hippocampus-dependent variant of contextual fear conditioning. Again, the WR/EC intervention reversed the deficit in conditioned fear to the context that was evident in the SH/SH condition. Post-weaning environmental manipulations promote cell survival and reverse learning deficits in rats that were exposed to alcohol during development. These manipulations may provide a basis for developing interventions that ameliorate learning impairments associated with human fetal alcohol spectrum disorders.

Long-term consequences of developmental alcohol exposure on brain structure and function: therapeutic benefits of physical activity

Developmental alcohol exposure both early in life and during adolescence can have a devastating impact on normal brain structure and functioning, leading to behavioral and cognitive impairments that persist throughout the lifespan. This review discusses human work as well as animal models used to investigate the effect of alcohol exposure at various time points during development, as well as specific behavioral and neuroanatomical deficits caused by alcohol exposure. Further, cellular and molecular mediators contributing to these alcohol-induced changes are examined, such as neurotrophic factors and apoptotic markers. Next, this review seeks to support the use of aerobic exercise as a potential therapeutic intervention for alcohol-related impairments. To date, few interventions, behavioral or pharmacological, have been proven effective in mitigating some alcohol-related deficits. Exercise is a simple therapy that can be used across species and also across socioeconomic status. It has a profoundly positive influence on many measures of learning and neuroplasticity; in particular, those measures damaged by alcohol exposure. This review discusses current evidence that exercise may mitigate damage caused by developmental alcohol exposure and is a promising therapeutic target for future research and intervention strategies.