The neuroimmune system - Where aging and excess alcohol intersect

Neuroimmune regulation of the prefrontal cortex tetrapartite synapse

The prefrontal cortex (PFC) is an essential driver of cognitive, affective, and motivational behavior. There is clear evidence that the neuroimmune system directly influences PFC synapses, in addition to its role as the first line of defense against toxins and pathogens. In this review, we first describe the core structures that form the tetrapartite PFC synapse, focusing on the signaling microdomain created by astrocytic cradling of the synapse as well as the emerging role of the extracellular matrix in synaptic organization and plasticity. Neuroimmune signals (e.g. pro-inflammatory interleukin 1β) can impact the function of each core structure within the tetrapartite synapse, as well as promote intra-synaptic crosstalk, and we will provide an overview of recent advances in this field. Finally, evidence from post mortem human brain tissue and preclinical studies indicate that inflammation may be a key contributor to PFC dysfunction. Therefore, we conclude with a mechanistic discussion of neuroimmune-mediated maladaptive plasticity in neuropsychiatric disorders, with a focus on alcohol use disorder (AUD). Growing recognition of the neuroimmune system's role as a critical regulator of the PFC tetrapartite synapse provides strong support for targeting the neuroimmune system to develop new pharmacotherapeutics.

Menstrual cycle phase affects alcohol impairment of working memory

BACKGROUND

Behavioral endocrinology studies in women suggest that higher circulating levels of the ovarian sex hormone estradiol (E2) may be linked to better working memory performance, especially under conditions of cognitive impairment (e.g., age-related cognitive decline). Phases of the menstrual cycle characterized by different levels of E2 may therefore influence the degree to which women are vulnerable to the acute impairing effect of alcohol on working memory.

METHODS

This study used a within-subjects design to test the hypothesis that women are less sensitive to acute alcohol-induced impairment of working memory during the late follicular phase of the menstrual cycle (when E2 is elevated) compared to the early follicular phase (when E2 is low). A sample of 75 premenopausal women completed two placebo-controlled alcohol administration sessions during the early and late follicular phases, respectively. At both sessions, participants completed an N-Back visual letter task of working memory first following placebo, then again 60 min after consuming a controlled dose of 0.6 g/kg alcohol.

RESULTS

Working memory performance was impaired under alcohol relative to placebo at both the early and late follicular phases of the menstrual cycle. However, as predicted, the magnitude of this impairment was significantly less pronounced during the late versus early follicular phases.

CONCLUSIONS

Women are less vulnerable to the acute impairing effect of alcohol at the late follicular phase of the menstrual cycle when ovulation occurs, possibly as a function of heightened levels of circulating E2. Considered in the context of the broader literature, these findings provide novel evidence to suggest that specific phases of the menstrual cycle may differentially affect women's sensitivity to the acute effects of alcohol on particular cognitive functions.

Greater alcohol intake predicts accelerated brain aging in humans, which mediates the relationship between alcohol intake and behavioral inflexibility

BACKGROUND

Hazardous use of alcohol is associated with cognitive-behavioral impairments and accelerated aging. To date, however, accelerated brain aging has not been tested as a mediating factor between alcohol use and associated task-based behavioral deficits, such as behavioral inflexibility. Here, we evaluated hazardous alcohol use as a predictor of machine learning-derived brain aging and tested if this measure accounted for the relationship between hazardous alcohol use and a task-based measure of behavioral flexibility.

METHODS

In this secondary analysis, we applied brainageR, a machine learning algorithm, to anatomical T1-weighted magnetic resonance imaging (MRI) images to estimate brain age for a sample of healthy adults (ages 22-40) who self-reported alcohol use with the alcohol use disorder identification test (AUDIT) and performed the hidden association between images task (HABIT), a behavioral flexibility task. Behavioral inflexibility was quantified as the proportion of perseverative errors performed on the HABIT as a measure of habitual action selection. We then analyzed AUDIT score as a predictor of brain aging, and brain aging as a predictor of behavioral inflexibility. Lastly, we conducted a mediation analysis to evaluate brain aging as a mediator between alcohol use and behavioral inflexibility.

RESULTS

Controlling for chronological age and sex, a higher AUDIT score predicted significantly more accelerated brain aging, which was further associated with more perseverative errors on the HABIT. Moreover, brain aging significantly mediated the association between AUDIT scores and behavioral inflexibility.

CONCLUSIONS

Our findings demonstrate that alcohol use is a significant predictor of accelerated brain aging, even in young adulthood. In addition, our findings suggest that such brain changes may mechanistically link more hazardous alcohol use to impaired behavioral flexibility. Future studies should also explore factors, such as other lifestyle behaviors, that may mitigate alcohol- and age-related processes.

Central amygdala neuroimmune signaling in alcohol use disorder

Alcohol Use Disorder (AUD) is a prevalent and debilitating condition characterized by an inability to control alcohol consumption despite adverse consequences. Current treatments for AUD, including FDA-approved medications such as naltrexone and acamprosate, have limited efficacy and compliance, underscoring the need for novel therapeutic approaches. The central amygdala (CeA) plays a crucial role in the development and maintenance of AUD, particularly aspects associated with stress and binge behaviors. Recent research indicates neuroimmune signaling in the CeA is emerging as a key factor in this process. Chronic alcohol consumption disrupts neuroimmune signaling, leading to altered cytokine expression and activation of glial cells, including astrocytes and microglia. These changes contribute to the dysregulation of neural circuits involved in reward and stress, perpetuating alcohol-seeking behavior and relapse. This review delves into how chronic alcohol exposure affects neuroimmune signaling in the CeA, contributing to the pathophysiology of AUD. By focusing on the impact of cytokine expression and glial cell activation, this review aims to elucidate the mechanisms by which neuroinflammation in the CeA influences alcohol-related behaviors. By providing a comprehensive overview of the current state of research, this review identifies potential therapeutic targets for AUD. Understanding the complex interplay between neuroimmune signaling and alcohol-related behaviors may pave the way for more effective treatments and improved outcomes for individuals struggling with AUD.

Psychopharmacological and Neuroregenerative Effects of a NF-κB Inhibitor under Conditions of Modeled Ethanol-Induced Encephalopathy

We studied the psychopharmacological effects of a NF-κB inhibitor, a stimulator of the functions of progenitor cells of the nervous tissue, under conditions of modeled ethanol-induced encephalopathy. The pharmacological agent improved indicators of the orientation and exploratory behavior and reproducibility of the conditioned passive avoidance reflex in experimental animals. These effects developed against the background of a significant increase in the content and proliferative activity of neural stem cells in the subventricular zone of the brain and intensification of their specialization. The results indicate the prospects of developing fundamentally new agents with regenerative activity based on NF-κB blockers for the treatment of alcoholic encephalopathy.

Adolescent Alcohol and the Spectrum of Cognitive Dysfunction in Aging

Among the many changes associated with aging, inflammation in the central nervous system (CNS) and throughout the body likely contributes to the constellation of health-related maladies associated with aging. Genetics, lifestyle factors, and environmental experiences shape the trajectory of aging-associated inflammation, including the developmental timing, frequency, and intensity of alcohol consumption. This chapter posits that neuroinflammatory processes form a critical link between alcohol exposure and the trajectory of healthy aging, at least in part through direct or indirect interactions with cholinergic circuits that are crucial to cognitive integrity. In this chapter, we begin with a discussion of how inflammation changes from early development through late aging; discuss the role of inflammation and alcohol in the emergence of mild cognitive impairment (MCI); elaborate on critical findings on the contribution of alcohol-related thiamine deficiency to the loss of cholinergic function and subsequent development of Wernicke-Korsakoff syndrome (WKS); and present emerging findings at the intersection of alcohol and Alzheimer's disease and related dementias (ADRD). In doing so, our analysis points toward inflammation-mediated compromise of basal forebrain cholinergic function as a key culprit in cognitive dysfunction associated with chronic alcohol exposure, effects that may be rescuable through either pharmacological or behavioral approaches. Furthermore, our chapter reveals an interesting dichotomy in the effects of alcohol on neuropathological markers of ADRD that depend upon both biological sex and genetic vulnerability.

The mechanisms, hallmarks, and therapies for brain aging and age-related dementia

Age-related cognitive decline and dementia are significant manifestations of brain aging. As the elderly population grows rapidly, the health and socio-economic impacts of cognitive dysfunction have become increasingly significant. Although clinical treatment of dementia has faced considerable challenges over the past few decades, with limited breakthroughs in slowing its progression, there has been substantial progress in understanding the molecular mechanisms and hallmarks of age-related dementia (ARD). This progress brings new hope for the intervention and treatment of this disease. In this review, we categorize the latest findings in ARD biomarkers into four stages based on disease progression: Healthy brain, pre-clinical, mild cognitive impairment, and dementia. We then systematically summarize the most promising therapeutic approaches to prevent or slow ARD at four levels: Genome and epigenome, organelle, cell, and organ and organism. We emphasize the importance of early prevention and detection, along with the implementation of combined treatments as multimodal intervention strategies, to address brain aging and ARD in the future.

Promising strategies for the prevention of alcohol-related brain damage through optimised management of acute alcohol withdrawal: A focussed literature review

There is an increasing awareness of the link between chronic alcohol consumption and the development of cognitive, behavioural and functional deficits. Currently, preventative strategies are limited and require engagement in dedicated long-term rehabilitation and sobriety services, the availability of which is low. The acute alcohol withdrawal syndrome is an episode of neurochemical imbalance leading to autonomic dysregulation, increased seizure risk and cognitive disorientation. In addition to harm from symptoms of alcohol withdrawal (e.g. seizures), the underpinning neurochemical changes may also lead to cytotoxicity through various cellular mechanisms, which long-term, may translate to some of the cognitive impairments observed in Alcohol-Related Brain Damage (ARBD). Here we review some of the pharmacological and neurochemical mechanisms underpinning alcohol withdrawal. We discuss the cellular and pharmacological basis of various potential neuroprotective strategies that warrant further exploration in clinical populations with a view to preventing the development of ARBD. Such strategies, when integrated into the clinical management of acute alcohol withdrawal, may impact large populations of individuals, who currently face limited dedicated service delivery and healthcare resource.

Increased anxiolytic effect in aged female rats and increased motoric behavior in aged male rats to acute alcohol administration: Comparison to younger animals

The population of most countries in the world is increasing and understanding risk factors that can influence the health of the older population is critical. Older adults consume alcohol often in a risky, binge manner. Previous work has demonstrated that aged rats are more sensitive to many of the effects of acute ethanol. In the current project aged, adult, and adolescent female and male rats were tested on the elevated plus maze and open field following either a 1.0 g/kg alcohol injection or a saline injection. We report sex- and age-dependent effects whereas aged female rats, but not aged male rats, showed an increased anxiolytic effect of alcohol in the elevated plus maze while aged male rats, but not aged female rats, showed increased stimulatory movement in the open field. In addition, significant age effects were found for both female and male rats. It is proposed that the sex- and age-dependent effects reported in the current studies may be due to differential levels of alcohol-induced allopregnanolone for the anxiolytic effects and differential levels of alcohol-induced dopamine for the stimulatory effects. The current work provides insights into factors influencing alcohol consumption in older adults.

Clinical and Preclinical Evidence for Gut Microbiome Mechanisms in Substance Use Disorders

Substance use disorders are a set of recalcitrant neuropsychiatric conditions that cause tremendous morbidity and mortality and are among the leading causes of loss of disability-adjusted life years worldwide. While each specific substance use disorder is driven by problematic use of a different substance, they all share a similar pattern of escalating and out-of-control substance use, continued use despite negative consequences, and a remitting/relapsing pattern over time. Despite significant advances in our understanding of the neurobiology of these conditions, current treatment options remain few and are ineffective for too many individuals. In recent years, there has been a rapidly growing body of literature demonstrating that the resident population of microbes in the gastrointestinal tract, collectively called the gut microbiome, plays an important role in modulating brain and behavior in preclinical and clinical studies of psychiatric disease. While these findings have not yet been translated into clinical practice, this remains an important and exciting avenue for translational research. In this review, we highlight the current state of microbiome-brain research within the substance use field with a focus on both clinical and preclinical studies. We also discuss potential neurobiological mechanisms underlying microbiome effects on models of substance use disorder and propose future directions to bring these findings from bench to bedside.

Impact of Elevated Brain IL-6 in Transgenic Mice on the Behavioral and Neurochemical Consequences of Chronic Alcohol Exposure

Alcohol consumption activates the neuroimmune system of the brain, a system in which brain astrocytes and microglia play dominant roles. These glial cells normally produce low levels of neuroimmune factors, which are important signaling factors and regulators of brain function. Alcohol activation of the neuroimmune system is known to dysregulate the production of neuroimmune factors, such as the cytokine IL-6, thereby changing the neuroimmune status of the brain, which could impact the actions of alcohol. The consequences of neuroimmune-alcohol interactions are not fully known. In the current studies we investigated this issue in transgenic (TG) mice with altered neuroimmune status relative to IL-6. The TG mice express elevated levels of astrocyte-produced IL-6, a condition known to occur with alcohol exposure. Standard behavioral tests of alcohol drinking and negative affect/emotionality were carried out in homozygous and heterozygous TG mice and control mice to assess the impact of neuroimmune status on the actions of chronic intermittent alcohol (ethanol) (CIE) exposure on these behaviors. The expressions of signal transduction and synaptic proteins were also assessed by Western blot to identify the impact of alcohol-neuroimmune interactions on brain neurochemistry. The results from these studies show that neuroimmune status with respect to IL-6 significantly impacts the effects of alcohol on multiple levels.

Sex-dependent factors of alcohol and neuroimmune mechanisms

Excessive alcohol use disrupts neuroimmune signaling across various cell types, including neurons, microglia, and astrocytes. The present review focuses on recent, albeit limited, evidence of sex differences in biological factors that mediate neuroimmune responses to alcohol and underlying neuroimmune systems that may influence alcohol drinking behaviors. Females are more vulnerable than males to the neurotoxic and negative consequences of chronic alcohol drinking, reflected by elevations of pro-inflammatory cytokines and inflammatory mediators. Differences in cytokine, microglial, astrocytic, genomic, and transcriptomic evidence suggest females are more reactive than males to neuroinflammatory changes after chronic alcohol exposure. The growing body of evidence supports that innate immune factors modulate synaptic transmission, providing a mechanistic framework to examine sex differences across neurocircuitry. Targeting neuroimmune signaling may be a viable strategy for treating AUD, but more research is needed to understand sex-specific differences in alcohol drinking and neuroimmune mechanisms.

Chronic intermittent ethanol exposure during adolescence produces sex- and age-dependent changes in anxiety and cognition without changes in microglia reactivity late in life

Binge-like ethanol exposure during adolescence has been shown to produce long lasting effects in animal models including anxiety-like behavior that can last into young adulthood and impairments in cognition that can last throughout most of the lifespan. However, little research has investigated if binge-like ethanol exposure during adolescence produces persistent anxiety-like behavior and concomitantly impairs cognition late in life. Furthermore, few studies have investigated such behavioral effects in both female and male rats over the lifespan. Finally, it is yet to be determined if binge-like ethanol exposure during adolescence alters microglia activation in relevant brain regions late in life. In the present study female and male adolescent rats were exposed to either 3.0 or 5.0 g/kg ethanol, or water control, in a chronic intermittent pattern before being tested in the elevated plus maze and open field task over the next ∼18 months. Animals were then trained in a spatial reference task via the Morris water maze before having their behavioral flexibility tested. Finally, brains were removed, sectioned and presumptive microglia activation determined using autoradiography for [3H]PK11195 binding. Males, but not females, displayed an anxiety-like phenotype initially following the chronic intermittent ethanol exposure paradigm which resolved in adulthood. Further, males but not females had altered spatial reference learning and impaired behavioral flexibility late in life. Conversely, [3H]PK11195 binding was significantly elevated in females compared to males late in life and the level of microglia activation interacted as a function of sex and brain regions, but there was no long-term outcome related to adolescent alcohol exposure. These data further confirm that binge-like ethanol exposure during adolescence produces alterations in behavior that can last throughout the lifespan. In addition, the data suggest that microglia activation late in life is not exacerbated by prior binge-like ethanol exposure during adolescence but the expression is sex- and brain region-dependent across the lifespan.