Circadian Modulation of Acute Alcohol Sensitivity But Not Acute Tolerance inDrosophila

A Brief Overview of Ethanol Tolerance and Its Potential Association with Circadian Rhythm in Drosophila

Alcohol consumption and addiction remain global health concerns, with significant loss of productivity, morbidity, and mortality. Drosophila melanogaster, a widely used model organism, offers valuable insights into the genetic and neuronal mechanisms underlying ethanol-induced behaviors (EIBs) such as sedation, recovery, and tolerance. This narrative review focuses on studies in the Drosophila model system suggesting an association between circadian rhythm genes as modulators of ethanol tolerance. Mutations in these genes disrupt both the circadian cycle and tolerance, underscoring the interplay between circadian rhythm and ethanol processing although the exact mechanisms remain largely unknown. Additionally, genes involved in stress response, gene expression regulation, neurotransmission, and synaptic activity were implicated in ethanol tolerance modulation. At the neuronal level, recent studies have highlighted the involvement of corazonin (CRZ) and neuropeptide F (NPF) neurons in modulating EIBs. Understanding the temporal dynamics of tolerance development is crucial for describing the molecular basis of ethanol tolerance. Ultimately, insights gained from Drosophila studies hold promise for elucidating the neurobiological underpinnings of alcohol use disorders and addiction, contributing to more effective interventions and treatments.

Alcohol sensitivity and tolerance encoding in sleep regulatory circadian neurons in Drosophila

Alcohol tolerance is a simple form of behavioural and neural plasticity that occurs with the first drink. Neural plasticity in tolerance is likely a substrate for longer term adaptations that can lead to alcohol use disorder. Drosophila develop tolerance with characteristics similar to vertebrates, and it is a useful model for determining the molecular and circuit encoding mechanisms in detail. Rapid tolerance, measured after the first alcohol exposure is completely metabolized, is localized to specific brain regions that are not interconnected in an obvious way. We used a forward neuroanatomical screen to identify three new neural sites for rapid tolerance encoding. One of these was composed of two groups of neurons, the DN1a and DN1p glutamatergic neurons, that are part of the Drosophila circadian clock. We localized rapid tolerance to the two DN1a neurons that regulate arousal by light at night, temperature-dependent sleep timing, and night-time sleep. Two clock neurons that regulate evening activity, LNd6 and the 5th LNv, are postsynaptic to the DN1as, and they promote rapid tolerance via the metabotropic glutamate receptor. Thus, rapid tolerance to alcohol overlaps with sleep regulatory neural circuitry, suggesting a mechanistic link.

Alcohol tolerance encoding in sleep regulatory circadian neurons in Drosophila

Alcohol tolerance is a simple form of behavioral and neural plasticity that occurs with the first drink. Neural plasticity in tolerance is likely a substrate for longer term adaptations that can lead to alcohol use disorder. Drosophila develop tolerance with characteristics similar to vertebrates, and it is useful model for determining the molecular and circuit encoding mechanisms in detail. Rapid tolerance, measured after the first alcohol exposure is completely metabolized, is localized to specific brain regions that are not interconnected in an obvious way. We used a forward neuroanatomical screen to identify three new neural sites for rapid tolerance encoding. One of these was comprised of two groups of neurons, the DN1a and DN1p glutamatergic neurons, that are part of the Drosophila circadian clock. We localized rapid tolerance to the two DN1a neurons that regulate arousal by light at night, temperature-dependent sleep timing, and night-time sleep. Two clock neurons that regulate evening activity, LNd6 and the 5th LNv, are postsynaptic to the DN1as and they promote rapid tolerance via the metabotropic glutamate receptor. Thus, rapid tolerance to alcohol overlaps with sleep regulatory neural circuitry, suggesting a mechanistic link.

A simple high-throughput method for automated detection of Drosophila melanogaster light-dependent behaviours

BACKGROUND

Like most living organisms, the fruit fly Drosophila melanogaster exhibits strong and diverse behavioural reactions to light. Drosophila is a diurnal animal that displays both short- and long-term responses to light, important for, instance, in avoidance and light wavelength preference, regulation of eclosion, courtship, and activity, and provides an important model organism for understanding the regulation of circadian rhythms both at molecular and circuit levels. However, the assessment and comparison of light-based behaviours is still a challenge, mainly due to the lack of a standardised platform to measure behaviour and different protocols created across studies. Here, we describe the Drosophila Interactive System for Controlled Optical manipulations (DISCO), a low-cost, automated, high-throughput device that records the flies' activity using infrared beams while performing LED light manipulations.

RESULTS

To demonstrate the effectiveness of this tool and validate its potential as a standard platform, we developed a number of distinct assays, including measuring the locomotor response of flies exposed to sudden darkness (lights-off) stimuli. Both white-eyed and red-eyed wild-type flies exhibit increased activity after the application of stimuli, while no changes can be observed in Fmr1 null allele flies, a model of fragile X syndrome. Next, to demonstrate the use of DISCO in long-term protocols, we monitored the circadian rhythm of the flies for 48 h while performing an alcohol preference test. We show that increased alcohol consumption happens intermittently throughout the day, especially in the dark phases. Finally, we developed a feedback-loop algorithm to implement a place preference test based on the flies' innate aversion to blue light and preference for green light. We show that both white-eyed and red-eyed wild-type flies were able to learn to avoid the blue-illuminated zones.

CONCLUSIONS

Our results demonstrate the versatility of DISCO for a range of protocols, indicating that this platform can be used in a variety of ways to study light-dependent behaviours in flies.

Sleep Modulates Alcohol Toxicity in Drosophila

Alcohol abuse is a significant public health problem. While considerable research has shown that alcohol use affects sleep, little is known about the role of sleep deprivation in alcohol toxicity. We investigated sleep as a factor modulating alcohol toxicity using Drosophila melanogaster, a model for studies of sleep, alcohol, and aging. Following 24 h of sleep deprivation using a paradigm that similarly affects males and females and induces rebound sleep, flies were given binge-like alcohol exposures. Sleep deprivation increased mortality, with no sex-dependent differences. Sleep deprivation also abolished functional tolerance measured at 24 h after the initial alcohol exposure, although there was no effect on alcohol absorbance or clearance. We investigated the effect of chronic sleep deprivation using mutants with decreased sleep, insomniac and insulin-like peptide 2, finding increased alcohol mortality. Furthermore, we investigated whether pharmacologically inducing sleep prior to alcohol exposure using the GABA_A-receptor agonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) mitigated the effects of alcohol toxicity on middle-aged flies, flies with environmentally disrupted circadian clocks, and flies with short sleep. Pharmacologically increasing sleep prior to alcohol exposure decreased alcohol-induced mortality. Thus, sleep prior to binge-like alcohol exposure affects alcohol-induced mortality, even in vulnerable groups such as aging flies and those with circadian dysfunction.

Corazonin Neurons Contribute to Dimorphic Ethanol Sedation Sensitivity in Drosophila melanogaster

Exposure to alcohol has multiple effects on nervous system function, and organisms have evolved mechanisms to optimally respond to the presence of ethanol. Sex differences in ethanol-induced behaviors have been observed in several organisms, ranging from humans to invertebrates. However, the molecular mechanisms underlying the dimorphic regulation of ethanol-induced behaviors remain incompletely understood. Here, we observed sex differences in ethanol sedation sensitivity in Drosophila Genome Reference Panel (DGRP) lines of Drosophila melanogaster compared to the absence of dimorphism in standard laboratory wildtype and control lines. However, in dose response experiments, we were able to unmask dimorphic responses for the control mutant line w 1118 by lowering the testing ethanol concentration. Notably, feminization of the small population of Corazonin (Crz) neurons in males was sufficient to induce female-like sedation sensitivity. We also tested the role of the transcription factor apontic (apt) based on its known expression in Crz neurons and its regulation of sedation responses. Interestingly, loss of function apt mutations increased sedation times in both males and females as compared to controls. No significant difference between male and female apt mutants was observed, suggesting a possible role of apt in the regulation of dimorphic ethanol-induced responses. Thus, our results shed light into the mechanisms regulating sex-differences in ethanol-induced behaviors at the cellular and molecular level, suggesting that the genetic sex in a small neuronal population plays an important role in modulating sex differences in behavioral responses to ethanol.

Epigenetic Regulation of Circadian Clocks and Its Involvement in Drug Addiction

Based on studies describing an increased prevalence of addictive behaviours in several rare sleep disorders and shift workers, a relationship between circadian rhythms and addiction has been hinted for more than a decade. Although circadian rhythm alterations and molecular mechanisms associated with neuropsychiatric conditions are an area of active investigation, success is limited so far, and further investigations are required. Thus, even though compelling evidence connects the circadian clock to addictive behaviour and vice-versa, yet the functional mechanism behind this interaction remains largely unknown. At the molecular level, multiple mechanisms have been proposed to link the circadian timing system to addiction. The molecular mechanism of the circadian clock consists of a transcriptional/translational feedback system, with several regulatory loops, that are also intricately regulated at the epigenetic level. Interestingly, the epigenetic landscape shows profound changes in the addictive brain, with significant alterations in histone modification, DNA methylation, and small regulatory RNAs. The combination of these two observations raises the possibility that epigenetic regulation is a common plot linking the circadian clocks with addiction, though very little evidence has been reported to date. This review provides an elaborate overview of the circadian system and its involvement in addiction, and we hypothesise a possible connection at the epigenetic level that could further link them. Therefore, we think this review may further improve our understanding of the etiology or/and pathology of psychiatric disorders related to drug addiction.

Drosophila Corazonin Neurons as a Hub for Regulating Growth, Stress Responses, Ethanol-Related Behaviors, Copulation Persistence and Sexually Dimorphic Reward Pathways

The neuronal mechanisms by which complex behaviors are coordinated and timed often involve neuropeptidergic regulation of stress and reward pathways. Recent studies of the neuropeptide Corazonin (Crz), a homolog of the mammalian Gonadotrophin Releasing Hormone (GnRH), have suggested its crucial role in the regulation of growth, internal states and behavioral decision making. We focus this review on Crz neurons with the goal to (1) highlight the diverse roles of Crz neuron function, including mechanisms that may be independent of the Crz peptide, (2) emphasize current gaps in knowledge about Crz neuron functions, and (3) propose exciting ideas of novel research directions involving the use of Crz neurons. We describe the different developmental fates of distinct subsets of Crz neurons, including recent findings elucidating the molecular regulation of apoptosis. Crz regulates systemic growth, food intake, stress responses and homeostasis by interacting with the short Neuropeptide F (sNPF) and the steroid hormone ecdysone. Additionally, activation of Crz neurons is shown to be pleasurable by interacting with the Neuropeptide F (NPF) and regulates reward processes such as ejaculation and ethanol-related behaviors in a sexually dimorphic manner. Crz neurons are proposed to be a motivational switch regulating copulation duration using a CaMKII-dependent mechanism described as the first neuronal interval timer lasting longer than a few seconds. Lastly, we propose ideas to use Crz neuron-induced ejaculation to study the effects of fictive mating and sex addiction in flies, as well as to elucidate dimorphic molecular mechanisms underlying reward behaviors and feeding disorders.

Repeated ethanol intoxications of Drosophila melanogaster adults increases the resistance to ethanol of their progeny

BACKGROUND

For decades, Drosophila melanogaster has been used as a model organism to understand the genetics and neurobiology of ethanol intoxication and tolerance. Previous research has shown that acute and chronic pre-exposures to ethanol can trigger the development of functional ethanol tolerance in flies and has unveiled some of the genetic pathways involved in the process. To our knowledge, however, no previous work has systematically explored whether repeated intoxications of adult flies can affect the ethanol tolerance of their progeny.

METHODS

Adult flies were intoxicated several times (once daily, over several days), and their F1 and F2 progeny were subjected to a functional tolerance test in which flies are exposed to ethanol and video recorded twice within 5 hr. Their behavior was subsequently analyzed to determine how long it took them to become sedated during the first and second exposures. One- and 2-way ANOVAs were used to determine whether parental treatment had an effect on their progeny's baseline resistance and/or acquired functional tolerance to ethanol.

RESULTS

Parental flies that were intoxicated several times produced F1 and F2 progeny with a significantly higher resistance to ethanol than progeny from unexposed controls. Further, parental intoxications inconsistently increased the progeny's capacity to develop rapid functional tolerance upon re-exposure to ethanol. The transmission of increased ethanol resistance to progeny lasted several days after the last parental intoxication.

CONCLUSION

To our knowledge, this is the first demonstration that repeated parental daily intoxications affect the progeny's response to ethanol in fruit flies. Our findings support the use of D. melanogaster to explore conserved pathways underlying the transmission of ethanol tolerance and can help in the identificaton of novel strategies for managing alcohol use disorder.

Flying Together: Drosophila as a Tool to Understand the Genetics of Human Alcoholism

Alcohol use disorder (AUD) exacts an immense toll on individuals, families, and society. Genetic factors determine up to 60% of an individual's risk of developing problematic alcohol habits. Effective AUD prevention and treatment requires knowledge of the genes that predispose people to alcoholism, play a role in alcohol responses, and/or contribute to the development of addiction. As a highly tractable and translatable genetic and behavioral model organism, Drosophila melanogaster has proven valuable to uncover important genes and mechanistic pathways that have obvious orthologs in humans and that help explain the complexities of addiction. Vinegar flies exhibit remarkably strong face and mechanistic validity as a model for AUDs, permitting many advancements in the quest to understand human genetic involvement in this disease. These advancements occur via approaches that essentially fall into one of two categories: (1) discovering candidate genes via human genome-wide association studies (GWAS), transcriptomics on post-mortem tissue from AUD patients, or relevant physiological connections, then using reverse genetics in flies to validate candidate genes' roles and investigate their molecular function in the context of alcohol. (2) Utilizing flies to discover candidate genes through unbiased screens, GWAS, quantitative trait locus analyses, transcriptomics, or single-gene studies, then validating their translational role in human genetic surveys. In this review, we highlight the utility of Drosophila as a model for alcoholism by surveying recent advances in our understanding of human AUDs that resulted from these various approaches. We summarize the genes that are conserved in alcohol-related function between humans and flies. We also provide insight into some advantages and limitations of these approaches. Overall, this review demonstrates how Drosophila have and can be used to answer important genetic questions about alcohol addiction.

Investigating Alcohol Behavior and Physiology Using Drosophila melanogaster

Drosophila melanogaster, the fruit fly, is one of the most versatile models for biomedical studies due to the economical husbandry, rapid generation time, and the array of tools for spatial and temporal gene manipulation. The relatively short lifespan of Drosophila (60-80 days) and the high degree of molecular conservation across species make Drosophila ideal to study the complexities of aging. Alcohol is the most abused drug worldwide and alcohol use disorders represent a significant public health problem and economic burden to individuals and society. Stereotypical alcohol-induced behaviors and the underlying molecular mechanisms are conserved from flies to humans making Drosophila a practical model for investigating the development of alcohol-induced behaviors and alcohol pathologies. Here, we outline how to assemble an efficient and controlled alcohol vapor delivery system, the FlyBar, and review paradigms and protocols for the assessment of alcohol-induced behaviors and physiology in Drosophila including the loss-of-righting reflex, sedation, tolerance, alcohol metabolism, and gut permeability.

Resetting the Aging Clock: Implications for Managing Age-Related Diseases

Worldwide, individuals are living longer due to medical and scientific advances, increased availability of medical care and changes in public health policies. Consequently, increasing attention has been focused on managing chronic conditions and age-related diseases to ensure healthy aging. The endogenous circadian system regulates molecular, physiological and behavioral rhythms orchestrating functional coordination and processes across tissues and organs. Circadian disruption or desynchronization of circadian oscillators increases disease risk and appears to accelerate aging. Reciprocally, aging weakens circadian function aggravating age-related diseases and pathologies. In this review, we summarize the molecular composition and structural organization of the circadian system in mammals and humans, and evaluate the technological and societal factors contributing to the increasing incidence of circadian disorders. Furthermore, we discuss the adverse effects of circadian dysfunction on aging and longevity and the bidirectional interactions through which aging affects circadian function using examples from mammalian research models and humans. Additionally, we review promising methods for managing healthy aging through behavioral and pharmacological reinforcement of the circadian system. Understanding age-related changes in the circadian clock and minimizing circadian dysfunction may be crucial components to promote healthy aging.

Aging and the clock: Perspective from flies to humans

Endogenous circadian oscillators regulate molecular, cellular and physiological rhythms, synchronizing tissues and organ function to coordinate activity and metabolism with environmental cycles. The technological nature of modern society with round-the-clock work schedules and heavy reliance on personal electronics has precipitated a striking increase in the incidence of circadian and sleep disorders. Circadian dysfunction contributes to an increased risk for many diseases and appears to have adverse effects on aging and longevity in animal models. From invertebrate organisms to humans, the function and synchronization of the circadian system weakens with age aggravating the age-related disorders and pathologies. In this review, we highlight the impacts of circadian dysfunction on aging and longevity and the reciprocal effects of aging on circadian function with examples from Drosophila to humans underscoring the highly conserved nature of these interactions. Additionally, we review the potential for using reinforcement of the circadian system to promote healthy aging and mitigate age-related pathologies. Advancements in medicine and public health have significantly increased human life span in the past century. With the demographics of countries worldwide shifting to an older population, there is a critical need to understand the factors that shape healthy aging. Drosophila melanogaster, as a model for aging and circadian interactions, has the capacity to facilitate the rapid advancement of research in this area and provide mechanistic insights for targeted investigations in mammals.

Insights from intoxicated Drosophila

Our understanding of alcohol use disorder (AUD), particularly alcohol's effects on the nervous system, has unquestionably benefited from the use of model systems such as Drosophila melanogaster. Here, we briefly introduce the use of flies in alcohol research, and highlight the genetic accessibility and neurobiological contribution that flies have made to our understanding of AUD. Future fly research offers unique opportunities for addressing unresolved questions in the alcohol field, such as the neuromolecular and circuit basis for cravings and alcohol-induced neuroimmune dysfunction. This review strongly advocates for interdisciplinary approaches and translational collaborations with the united goal of confronting the major health problems associated with alcohol abuse and addiction.

Attentional and working memory performance following alcohol and energy drink: A randomised, double-blind, placebo-controlled, factorial design laboratory study

Alcohol mixed with energy drinks (AMED) studies have typically not shown antagonism of acute alcohol effects by energy drink (ED), particularly over relatively short time frames. This study investigated the effects of alcohol, ED, and AMED on attentional and working memory processes over a 3 h period. Twenty-four young adults took part in a randomised, double-blind, placebo-controlled, factorial, 4-arm study. They were administered 0.6g/kg alcohol and 250 ml ED (containing 80 mg caffeine), and matching placebos alone and in combination. A battery of attentional and working memory measures was completed at baseline then 45, 90 and 180 min post-treatment. Alcohol produced a characteristic shift in speed/accuracy trade-off, having little effect on reaction times while increasing errors on all attentional measures (4-choice Reaction Time, Number Pairs and Visual Search), as well as a composite Attentional error score and one working memory task (Serial Sevens). ED alone improved two working memory measures (Memory Scanning accuracy and Digit-Symbol reaction times) and improved speed of responding on a composite Working Memory score. There was no consistent pattern of AMED vs. alcohol effects; AMED produced more errors than alcohol alone on one attentional measure (Visual Search errors) at 45 min only whereas AMED resulted in fewer errors on the Serial Sevens task at 90 min and better Digit-Symbol accuracy and reaction time at 45 min. Alcohol consumption increases error rate across several attentional and working memory processes. Mutual antagonism between alcohol and ED showed no consistent pattern and likely reflects a complex interaction between caffeine and alcohol levels, phase of the blood alcohol limb, task domain and cognitive load.

Drinking in the morning versus evening: Time-dependent differential effects of acute alcohol administration on the behavior of zebrafish

Alcohol (ethanol) abuse remains to be a leading cause of medical, including mental, problems throughout the world. Whether alcohol consumption leads to chronic use, and subsequent alcohol dependency and abuse is known to be influenced by the acute effects of this drug. Numerous factors may influence how alcohol administered acutely affects the individual. For example, the mechanisms engaged by drugs of abuse, e.g. cocaine as well as alcohol, have been shown to overlap with those underlying circadian rhythm, and conversely, the effects of these drugs may be dependent upon the time of day of their consumption. To investigate the interaction between circadian rhythm and alcohol, here we employ a simple vertebrate model organism that was previously successfully utilized in other aspects of alcohol research, the zebrafish. We expose zebrafish to alcohol for 20 min in the morning or in the evening, and analyze the effects of this treatment by comparing 1% (vol/vol) alcohol-treated and control (alcohol naive) zebrafish. We record numerous swim path parameters, and report, for the first time, that the time of day of alcohol administration differentially affects certain behavioral parameters, enhancing some while blunting others. Our results suggest a complex interaction between circadian dependent and alcohol engaged mechanisms, findings that represent both practical complications as well as opportunities for understanding how alcohol affects brain function and behavior of vertebrates.

Effects of Ethanol on Sensory Inputs to the Medial Giant Interneurons of Crayfish

Crayfish are capable of two rapid, escape reflexes that are mediated by two pairs of giant interneurons, the lateral giants (LG) and the medial giants (MG), which respond to threats presented to the abdomen or head and thorax, respectively. The LG has been the focus of study for many decades and the role of GABAergic inhibition on the escape circuit is well-described. More recently, we demonstrated that the LG circuit is sensitive to the acute effects of ethanol and this sensitivity is likely mediated by interactions between ethanol and the GABAergic system. The MG neurons, however, which receive multi-modal sensory inputs and are located in the brain, have been less studied despite their established importance during many naturally occurring behaviors. Using a combination of electrophysiological and neuropharmacological techniques, we report here that the MG neurons are sensitive to ethanol and experience an increase in amplitudes of post-synaptic potentials following ethanol exposure. Moreover, they are affected by GABAergic mechanisms: the facilitatory effect of acute EtOH can be suppressed by pretreatment with a GABA receptor agonist whereas the inhibitory effects resulting from a GABA agonist can be occluded by ethanol exposure. Together, our findings suggest intriguing neurocellular interactions between alcohol and the crayfish GABAergic system. These results enable further exploration of potentially conserved neurochemical mechanisms underlying the interactions between alcohol and neural circuitry that controls complex behaviors.

Cue-Induced Ethanol Seeking in Drosophila melanogaster Is Dose-Dependent

Alcohol use disorder generates devastating social, medical and economic burdens, making it a major global health issue. The persistent nature of memories associated with intoxication experiences often induces cravings and triggers relapse in recovering individuals. Despite recent advances, the neural and molecular mechanisms underlying these memories are complex and not well understood. This makes finding effective pharmacological targets challenging. The investigation of persistent alcohol-associated memories in the fruit fly, Drosophila melanogaster, presents a unique opportunity to gain a comprehensive understanding of the memories for ethanol reward at the level of genes, molecules, neurons and circuits. Here we characterize the dose-dependent nature of ethanol on the expression of memory for an intoxication experience. We report that the concentration of ethanol, number of ethanol exposures, length of ethanol exposures, and timing between ethanol exposures are critical in determining whether ethanol is perceived as aversive or appetitive, and in how long the memory for the intoxicating properties of ethanol last. Our study highlights that fruit flies display both acute and persistent memories for ethanol-conditioned odor cues, and that a combination of parameters that determine the intoxication state of the fly influence the seemingly complex retention and expression of memories associated with intoxication. Our thorough behavioral characterization provides the opportunity to interrogate the biological underpinnings of these observed preference differences in future studies.

Drosophila: An Emergent Model for Delineating Interactions between the Circadian Clock and Drugs of Abuse

Endogenous circadian oscillators orchestrate rhythms at the cellular, physiological, and behavioral levels across species to coordinate activity, for example, sleep/wake cycles, metabolism, and learning and memory, with predictable environmental cycles. The 21st century has seen a dramatic rise in the incidence of circadian and sleep disorders with globalization, technological advances, and the use of personal electronics. The circadian clock modulates alcohol- and drug-induced behaviors with circadian misalignment contributing to increased substance use and abuse. Invertebrate models, such as Drosophila melanogaster, have proven invaluable for the identification of genetic and molecular mechanisms underlying highly conserved processes including the circadian clock, drug tolerance, and reward systems. In this review, we highlight the contributions of Drosophila as a model system for understanding the bidirectional interactions between the circadian system and the drugs of abuse, alcohol and cocaine, and illustrate the highly conserved nature of these interactions between Drosophila and mammalian systems. Research in Drosophila provides mechanistic insights into the corresponding behaviors in higher organisms and can be used as a guide for targeted inquiries in mammals.

Aging and circadian dysfunction increase alcohol sensitivity and exacerbate mortality in Drosophila melanogaster

Alcohol abuse is a rising problem in middle-aged and older individuals resulting in serious health, family and economic consequences. Effective treatment necessitates the identification of factors influencing alcohol toxicity with aging. We investigated the interaction between aging, alcohol toxicity and circadian function using Drosophila as a model system. We found as wild type flies age, sensitivity to alcohol increases and circadian regulation of alcohol-induced behaviors weakens. Decreased circadian modulation is correlated with significantly greater alcohol sensitivity during the subjective day. The circadian clock modulates alcohol-induced mortality in younger flies with increased mortality following alcohol exposure at night. Older flies exhibit significantly longer recovery times following alcohol-induced sedation and increased mortality following binge-like or chronic alcohol exposure. Flies rendered arrhythmic either genetically or environmentally exhibit significantly increased alcohol sensitivity, longer recovery times and increased mortality. We hypothesize that the circadian clock phase specifically buffers behavioral and cellular alcohol sensitivity with this protection diminishing as the circadian clock weakens with age.

Genetics and genomics of alcohol responses in Drosophila

Drosophila melanogaster has become a significant model organism for alcohol research. In flies, a rich variety of behaviors can be leveraged for identifying genes affecting alcohol responses and adaptations. Furthermore, almost all genes can be easily genetically manipulated. Despite the great evolutionary distance between flies and mammals, many of the same genes have been implicated in strikingly similar alcohol-induced behaviors. A major problem in medical research today is that it is difficult to extrapolate from any single model system to humans. Strong evolutionary conservation of a mechanistic response between distantly related organisms, such as flies and mammals, is a powerful predictor that conservation will continue all the way to humans. This review describes the state of the Drosophila alcohol research field. It describes common alcohol behavioral assays, the independent origins of resistance and tolerance, the results of classical genetic screens and candidate gene analysis, and the outcomes of recent genomics studies employing GWAS, transcriptome, miRNA, and genome-wide histone acetylation surveys. This article is part of the Special Issue entitled "Alcoholism".

Alcohol resistance in Drosophila is modulated by the Toll innate immune pathway

A growing body of evidence has shown that alcohol alters the activity of the innate immune system and that changes in innate immune system activity can influence alcohol-related behaviors. Here, we show that the Toll innate immune signaling pathway modulates the level of alcohol resistance in Drosophila. In humans, a low level of response to alcohol is correlated with increased risk of developing an alcohol use disorder. The Toll signaling pathway was originally discovered in, and has been extensively studied in Drosophila. The Toll pathway is a major regulator of innate immunity in Drosophila, and mammalian Toll-like receptor signaling has been implicated in alcohol responses. Here, we use Drosophila-specific genetic tools to test eight genes in the Toll signaling pathway for effects on the level of response to ethanol. We show that increasing the activity of the pathway increases ethanol resistance whereas decreasing the pathway activity reduces ethanol resistance. Furthermore, we show that gene products known to be outputs of innate immune signaling are rapidly induced following ethanol exposure. The interaction between the Toll signaling pathway and ethanol is rooted in the natural history of Drosophila melanogaster.

Shared neurocircuitry underlying feeding and drugs of abuse in Drosophila

The neural circuitry and molecules that control the rewarding properties of food and drugs of abuse appear to partially overlap in the mammalian brain. This has raised questions about the extent of the overlap and the precise role of specific circuit elements in reward and in other behaviors associated with feeding regulation and drug responses. The much simpler brain of invertebrates including the fruit fly Drosophila, offers an opportunity to make high-resolution maps of the circuits and molecules that govern behavior. Recent progress in Drosophila has revealed not only some common substrates for the actions of drugs of abuse and for the regulation of feeding, but also a remarkable level of conservation with vertebrates for key neuromodulatory transmitters. We speculate that Drosophila may serve as a model for distinguishing the neural mechanisms underlying normal and pathological motivational states that will be applicable to mammals.

Circadian Modulation of Alcohol-Induced Sedation and Recovery in Male and Female Drosophila

Delineating the factors that affect behavioral and neurological responses to alcohol is critical to facilitate measures for preventing or treating alcohol abuse. The high degree of conserved molecular and physiological processes makes Drosophila melanogaster a valuable model for investigating circadian interactions with alcohol-induced behaviors and examining sex-specific differences in alcohol sensitivity. We found that wild-type Drosophila exhibited rhythms in alcohol-induced sedation under light-dark and constant dark conditions with considerably greater alcohol exposure necessary to induce sedation during the late (subjective) day and peak sensitivity to alcohol occurring during the late (subjective) night. The circadian clock also modulated the recovery from alcohol-induced sedation with flies regaining motor control significantly faster during the late (subjective) day. As predicted, the circadian rhythms in sedation and recovery were absent in flies with a mutation in the circadian gene period or arrhythmic flies housed in constant light conditions. Flies lacking a functional circadian clock were more sensitive to the effects of alcohol with significantly longer recovery times. Similar to other animals and humans, Drosophila exhibit sex-specific differences in alcohol sensitivity. We investigated whether the circadian clock modulated the rhythms in the loss-of-righting reflex, alcohol-induced sedation, and recovery differently in males and females. We found that both sexes demonstrated circadian rhythms in the loss-of-righting reflex and sedation with the differences in alcohol sensitivity between males and females most pronounced during the late subjective day. Recovery of motor reflexes following alcohol sedation also exhibited circadian modulation in male and female flies, although the circadian clock did not modulate the difference in recovery times between the sexes. These studies provide a framework outlining how the circadian clock modulates alcohol-induced behaviors in Drosophila and identifies sexual dimorphisms in the circadian modulation of alcohol behaviors.

Mutations in the circadian gene period alter behavioral and biochemical responses to ethanol in Drosophila

Clock genes, such as period, which maintain an organism's circadian rhythm, can have profound effects on metabolic activity, including ethanol metabolism. In turn, ethanol exposure has been shown in Drosophila and mammals to cause disruptions of the circadian rhythm. Previous studies from our labs have shown that larval ethanol exposure disrupted the free-running period and period expression of Drosophila. In addition, a recent study has shown that arrhythmic flies show no tolerance to ethanol exposure. As such, Drosophila period mutants, which have either a shorter than wild-type free-running period (perS) or a longer one (perL), may also exhibit altered responses to ethanol due to their intrinsic circadian differences. In this study, we tested the initial sensitivity and tolerance of ethanol exposure on Canton-S, perS, and perL, and then measured their Alcohol Dehydrogenase (ADH) and body ethanol levels. We showed that perL flies had slower sedation rate, longer recovery from ethanol sedation, and generated higher tolerance for sedation upon repeated ethanol exposure compared to Canton-S wild-type flies. Furthermore, perL flies had lower ADH activity and had a slower ethanol clearance compared to wild-type flies. The findings of this study suggest that period mutations influence ethanol induced behavior and ethanol metabolism in Drosophila and that flies with longer circadian periods are more sensitive to ethanol exposure.

Drosophila and Caenorhabditis elegans as Discovery Platforms for Genes Involved in Human Alcohol Use Disorder

BACKGROUND

Despite the profound clinical significance and strong heritability of alcohol use disorder (AUD), we do not yet have a comprehensive understanding of the naturally occurring genetic variance within the human genome that drives its development. This lack of understanding is likely to be due in part to the large phenotypic and genetic heterogeneities that underlie human AUD. As a complement to genetic studies in humans, many laboratories are using the invertebrate model organisms (iMOs) Drosophila melanogaster (fruit fly) and Caenorhabditis elegans (nematode worm) to identify genetic mechanisms that influence the effects of alcohol (ethanol) on behavior. While these extremely powerful models have identified many genes that influence the behavioral responses to alcohol, in most cases it has remained unclear whether results from behavioral-genetic studies in iMOs are directly applicable to understanding the genetic basis of human AUD.

METHODS

In this review, we critically evaluate the utility of the fly and worm models for identifying genes that influence AUD in humans.

RESULTS

Based on results published through early 2015, studies in flies and worms have identified 91 and 50 genes, respectively, that influence 1 or more aspects of behavioral responses to alcohol. Collectively, these fly and worm genes correspond to 293 orthologous genes in humans. Intriguingly, 51 of these 293 human genes have been implicated in AUD by at least 1 study in human populations.

CONCLUSIONS

Our analyses strongly suggest that the Drosophila and C. elegans models have considerable utility for identifying orthologs of genes that influence human AUD.

Effects of alcohol and energy drink on mood and subjective intoxication: a double-blind, placebo-controlled, crossover study

OBJECTIVE

There is concern that combining energy drinks with alcohol may 'mask' subjective intoxication leading to greater alcohol consumption. This study examines the effects of alcohol alone and combined with energy drink on objective and subjective intoxication and mood over the course of 3 h.

METHOD

Using a double-blind, placebo-controlled, balanced, crossover design, 24 participants (mean age 22.23 years) were administered with double placebo, 0.6 g/kg alcohol (mean peak blood alcohol content of 0.051%), 250 ml energy drink and alcohol/energy drink, according to a Latin square design, with a washout of >48 h. On each visit, they were breathalysed and rated themselves on a comprehensive battery of mood items at baseline and then at 45, 90 and 180 min post-drink.

RESULTS

Blood alcohol and subjective intoxication were significantly increased following both alcohol alone and alcohol/energy drink. Both measures were statistically indistinguishable between alcohol conditions. In keeping with its (80 mg) caffeine content, the energy drink alone significantly increased self-rated 'alertness' and reduced 'depression-dejection' scores compared with the combined alcohol/energy drink. The alcohol/energy drink increased 'vigor' and 'contentment' at 45 min and decreased 'contentment' at 180 min.

CONCLUSIONS

The co-ingestion of an energy drink with alcohol does not differently influence blood alcohol content recordings or subjective intoxication compared with alcohol alone, although some mood items are differentially affected.

The FlyBar: administering alcohol to flies

Fruit flies (Drosophila melanogaster) are an established model for both alcohol research and circadian biology. Recently, we showed that the circadian clock modulates alcohol sensitivity, but not the formation of tolerance. Here, we describe our protocol in detail. Alcohol is administered to the flies using the FlyBar. In this setup, saturated alcohol vapor is mixed with humidified air in set proportions, and administered to the flies in four tubes simultaneously. Flies are reared under standardized conditions in order to minimize variation between the replicates. Three-day old flies of different genotypes or treatments are used for the experiments, preferably by matching flies of two different time points (e.g., CT 5 and CT 17) making direct comparisons possible. During the experiment, flies are exposed for 1 hr to the pre-determined percentage of alcohol vapor and the number of flies that exhibit the Loss of Righting reflex (LoRR) or sedation are counted every 5 min. The data can be analyzed using three different statistical approaches. The first is to determine the time at which 50% of the flies have lost their righting reflex and use an Analysis of the Variance (ANOVA) to determine whether significant differences exist between time points. The second is to determine the percentage flies that show LoRR after a specified number of minutes, followed by an ANOVA analysis. The last method is to analyze the whole times series using multivariate statistics. The protocol can also be used for non-circadian experiments or comparisons between genotypes.

Circadian genes differentially affect tolerance to ethanol in Drosophila

BACKGROUND

There is a strong relationship between circadian rhythms and ethanol (EtOH) responses. EtOH consumption has been shown to disrupt physiological and behavioral circadian rhythms in mammals (Alcohol Clin Exp Res 2005b, 29, 1550). The Drosophila central circadian pacemaker is composed of proteins encoded by the per, tim, cyc, and Clk genes. Using Drosophila mutant analysis, we asked whether these central components of the circadian clock make the equivalent contribution toward EtOH tolerance and whether rhythmicity itself is necessary for tolerance.

METHODS

We tested flies carrying mutations in core clock genes for the capacity to acquire EtOH tolerance. Tolerance was assayed by comparing the sedation curves of populations during their first and second sedation. Animals that had acquired tolerance sedated more slowly. Movement was also monitored as the flies breathe the EtOH vapor to determine if other facets of the EtOH response were affected by the mutations. Gas chromatography was used to measure internal EtOH concentration. Constant light was used to nongenetically destabilize the PER and TIM proteins.

RESULTS

A group of circadian mutations, all of which eliminate circadian rhythms, do not disrupt tolerance identically. Mutations in per, tim, and cyc completely block tolerance. However, a mutation in Clk does not interfere with tolerance. Constant light also disrupts the capacity to acquire tolerance. These lines did not differ in EtOH absorption.

CONCLUSIONS

Mutations affecting different parts of the intracellular circadian clock can block the capacity to acquire rapid EtOH tolerance. However, the role of circadian genes in EtOH tolerance is independent of their role in producing circadian rhythmicity. The interference in the capacity to acquire EtOH tolerance by some circadian mutations is not merely a downstream effect of a nonfunctional circadian clock; instead, these circadian genes play an independent role in EtOH tolerance.