Alcohol potentiates a pheromone signal in flies

The Promise of an Evolutionary Perspective of Alcohol Consumption

The urgent need for medical treatments of alcohol use disorders has motivated the search for novel molecular targets of alcohol response. Most studies exploit the strengths of lab animals without considering how these and other species may have adapted to respond to alcohol in an ecological context. Here, we provide an evolutionary perspective on the molecular and genetic underpinnings of alcohol consumption by reviewing evidence that alcohol metabolic enzymes have undergone adaptive evolution at 2 evolutionary junctures: first, to enable alcohol consumption accompanying the advent of a frugivorous diet in a primate ancestor, and second, to decrease the likelihood of excessive alcohol consumption concurrent with the spread of agriculture and fermentation in East Asia. By similarly considering how diverse vertebrate and invertebrate species have undergone natural selection for alcohol responses, novel conserved molecular targets of alcohol are likely be discovered that may represent promising therapeutic targets.

From Natural Behavior to Drug Screening: Invertebrates as Models to Study Mechanisms Associated with Alcohol Use Disorders

Humans consume ethanol-containing beverages, which may cause an uncontrollable or difficult-to-control intake of ethanol-containing liquids and may result in alcohol use disorders. How the transition at the molecular level from "normal" ethanol-associated behaviors to addictive behaviors occurs is still unknown. One problem is that the components contributing to normal ethanol intake and their underlying molecular adaptations, especially in neurons that regulate behavior, are not clear. The fruit fly Drosophila melanogaster and the earthworm Caenorhabditis elegans show behavioral similarities to humans such as signs of intoxication, tolerance, and withdrawal. Underlying the phenotypic similarities, invertebrates and vertebrates share mechanistic similarities. For example in Drosophila melanogaster, the dopaminergic neurotransmitter system regulates the positive reinforcing properties of ethanol and in Caenorhabditis elegans, serotonergic neurons regulate feeding behavior. Since these mechanisms are fundamental molecular mechanisms and are highly conserved, invertebrates are good models for uncovering the basic principles of neuronal adaptation underlying the behavioral response to ethanol. This review will focus on the following aspects that might shed light on the mechanisms underlying normal ethanol-associated behaviors. First, the current status of what is required at the behavioral and cellular level to respond to naturally occurring levels of ethanol is summarized. Low levels of ethanol delay the development and activate compensatory mechanisms that in turn might be beneficial for some aspects of the animal's physiology. Repeated exposure to ethanol however might change brain structures involved in mediating learning and memory processes. The smell of ethanol is already a key component in the environment that is able to elicit behavioral changes and molecular programs. Minimal networks have been identified that regulate normal ethanol consumption. Other environmental factors that influence ethanol-induced behaviors include the diet, dietary supplements, and the microbiome. Second, the molecular mechanisms underlying neuronal adaptation to the cellular stressor ethanol are discussed. Components of the heat shock and oxidative stress pathways regulate adaptive responses to low levels of ethanol and in turn change behavior. The adaptive potential of the brain cells is challenged when the organism encounters additional cellular stressors caused by aging, endosymbionts or environmental toxins or excessive ethanol intake. Finally, to underline the conserved nature of these mechanisms between invertebrates and higher organisms, recent approaches to identify drug targets for ethanol-induced behaviors are provided. Already approved drugs regulate ethanol-induced behaviors and they do so in part by interfering with cellular stress pathways. In addition, invertebrates have been used to identify new compounds targeting molecules involved in the regulation in ethanol withdrawal-like symptoms. This review primarily highlights the advances of the last 5 years concerning Drosophila melanogaster, but also provides intriguing examples of Caenorhabditis elegans and Apis mellifera in support.

Neuroecology of Alcohol Preference in Drosophila

In this review, we highlight sources of alcohols in nature, as well as the behavioral and ecological roles that these fermentation cues play in the short lifespan of Drosophila melanogaster. With a focus on neuroethology, we describe the olfactory detection of alcohol as well as ensuing neural signaling within the brain of the fly. We proceed to explain the plethora of behaviors related to alcohol, including attraction, feeding, and oviposition, as well as general effects on aggression and courtship. All of these behaviors are shaped by physiological state and social contexts. In a comparative perspective, we also discuss inter- and intraspecies differences related to alcohol tolerance and metabolism. Lastly, we provide corollaries with other dipteran and coleopteran insect species that also have olfactory systems attuned to ethanol detection and describe ecological and evolutionary directions for further studies of the natural history of alcohol and the fly.

Alcohol-induced Aggression

Intraspecies aggression is commonly focused on securing reproductive resources such as food, territory, and mates, and it is often males who do the fighting. In humans, individual acts of overt physical aggression seem maladaptive and probably represent dysregulation of the pathways underlying aggression. Such acts are often associated with ethanol consumption. The Drosophila melanogaster model system, which has long been used to study how ethanol affects the nervous system and behavior, has also been used to study the molecular origins of aggression. In addition, ethanol-induced aggression has been demonstrated in flies. Recent publications show that ethanol stimulates Drosophila aggression in 2 ways: the odor of ethanol and the consumption of ethanol both make males more aggressive. These ethanol effects occur at concentrations that flies likely experience in the wild. A picture emerges of males arriving on their preferred reproductive site-fermenting plant matter-and being stimulated by ethanol to fight harder to secure the site for their own use. Fly fighting assays appear to be a suitable bioassay for studying how low doses of ethanol reshape neural signaling.

Alcohol-induced aggression in Drosophila

Alcohol-induced aggression is a destructive and widespread phenomenon associated with violence and sexual assault. However, little is understood concerning its mechanistic origin. We have developed a Drosophila melanogaster model to genetically dissect and understand the phenomenon of sexually dimorphic alcohol-induced aggression. Males with blood alcohol levels of 0.04-mg/ml BAC were less aggressive than alcohol-naive males, but when the BAC had dropped to ~0.015 mg/ml, the alcohol-treated males showed an increase in aggression toward other males. This aggression-promoting treatment is referred to as the post-ethanol aggression (PEA) treatment. Females do not show increased aggression after the same treatment. PEA-treated males also spend less time courting and attempt to copulate earlier than alcohol-naive flies. PEA treatment induces expression of the FruM transcription factor (encoded by a male-specific transcript from the fruitless gene), whereas sedating doses of alcohol reduce FruM expression and reduce male aggression. Transgenic suppression of FruM induction also prevents alcohol-induced aggression. In male flies, alcohol-induced aggression is dependent on the male isoform of the fruitless transcription factor (FruM). Low-dose alcohol induces FruM expression and promotes aggression, whereas higher doses of alcohol suppress FruM and suppress aggression.

Receptors and Channels Associated with Alcohol Use: Contributions from Drosophila

Alcohol Use Disorder (AUD) is a debilitating disorder that manifests as problematic patterns of alcohol use. At the core of AUD's behavioral manifestations are the profound structural, physiological, cellular, and molecular effects of alcohol on the brain. While the field has made considerable progress in understanding the neuromolecular targets of alcohol we still lack a comprehensive understanding of alcohol's actions and effective treatment strategies. Drosophila melanogaster is a powerful model for investigating the neuromolecular targets of alcohol because flies model many of the core behavioral elements of AUD and offer a rich genetic toolkit to precisely reveal the in vivo molecular actions of alcohol. In this review, we focus on receptors and channels that are often targeted by alcohol within the brain. We discuss the general roles of these proteins, their role in alcohol-associated behaviors across species, and propose ways in which Drosophila models can help advance the field.