[Seminar blog] What do you do with a drunken crayfish?

written by: Jillian Stewart

What do you do with a drunken crayfish? If you’re in the Herberholz lab, you quantify how long it takes for the crayfish to get well and truly sloshed, and use it to model the impacts of social isolation on the nervous system.
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Crayfish are a valuable model for the nervous system, with which researchers are quite familiar. Their relatively large neurons are easy to work with and can survive outside the animal for several hours. The crayfish nervous system has been well mapped and the roles of individual nerves are often known (Venuti et al, 2021).

In addition to their nervous systems, crayfish exhibit simple social behaviours. Crayfish form linear social hierarchies by dueling with each other. These hierarchies last for about seven days, after which the crayfish forget who won and duke it out again. Dr. Herberholz and his lab at UMD’s Department of Psychology used these social systems to model how isolation affects the nervous system’s response to alcohol. They placed individual crayfish, some socially isolated for seven days and others communally housed, in a tank of water with alcohol in it.

Crayfish exhibit distinct phases of intoxication: for the first ten or fifteen minutes, they are unaffected. Then, they start to stand up tall on their tip-toes. Next, they perform an escape maneuver by flipping their tail repeatedly (image 1). Finally, the crayfish lands on its back and is unable to right itself. At this point, the crayfish is returned to fresh water and recovers in a couple hours. 

Image 1) Crayfish demonstrates an escape maneuver by flipping its tail. This maneuver is governed by the two lateral giant interneurons (LG) on either side of the animal. Image credit: Alexis Exum and Jens Herberholz/University of Maryland, College Park

Socially isolated crayfish exhibited lower sensitivity to alcohol than communally housed individuals. Isolated individuals took longer to show symptoms of intoxication. There was no apparent difference in sensitivity based on sex, only on social history (Swierzbinski et al, 2017). Additionally, both treatments of crayfish had similar concentrations of alcohol in their hemolymph, indicating that they uptook and eliminated alcohol at the same rate.

The next step was to determine why socially isolated crayfish are less sensitive to alcohol. The tail flip maneuver is governed by a specific neuronal circuit, called the lateral giant interneurons or the ‘LG’. This gave Dr. Herberholz’s lab a specific circuit to study. They had previously found that the GABAergic system is involved, but that system does not fully explain the differences in sensitivity (Venuti et al, 2021). In this new round of experiments, they studied the role of serotonin. Two receptors found on the LG have been characterized and found to be part of the serotonergic system: 5-HT1A and 5-HT2B. The former is inhibitory and the latter is excitatory. Both are targeted by alcohol. In socially isolated crayfish, 5-HT2B might be downregulated, so those crayfish would experience less ‘excitement’ after alcohol exposure than communally housed crayfish. When communally housed crayfish were depleted of 5-HT, they exhibited delays in intoxication similar to socially isolated crayfish.

This reduced sensitivity to alcohol could be a factor in why drinking often increases during periods of isolation. The nervous system requires more alcohol to experience the same intoxicating effects. In light of the isolation brought on by the COVID pandemic, research on isolation’s effects is urgently needed. Isolation has been linked to declines in mental and physical health, disease, drug use, and mortality (Venuti et al, 2021). In the future, Dr. Herberholz and his lab intend to test the persistence of decreased alcohol sensitivity by returning isolated crayfish to communal tanks. They hope to uncover mechanisms by which the nervous system can recover from the ravages of isolation.
 
Dr. Herberholz is not just studying the effects of alcohol on the nervous system. Recently, his lab has begun a project on how the gut-brain axis is affected by social isolation. They used two treatment groups for this study. One group was isolated for seven days, another was housed in pairs that could see and smell each other but not fight.
Male crayfish did not demonstrate any difference in their gut microbiome. However, females who had been isolated had significantly more disease-associated bacteria in their guts compared to pairs.
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The implications of this research are unclear for now. The gut and the brain are known to be linked, but exactly how they influence each other is not fully characterized. The gut-brain axis has been implicated in disorders like depression and irritable bowel syndrome. It may even play a role in central nervous system development (Margolis et al, 2021).
 
Citations:
 
Lucy S. Venuti, Norma L. Pena-Flores, Jens Herberholz. Cellular interactions between social experience, alcohol sensitivity, and GABAergic inhibition in a crayfish neural circuit. Journal of Neurophysiology 2021 125:1, 256-272. https://doi.org/10.1152/jn.00519.2020
 
Kara G. Margolis, John F. Cryan, Emeran A. Mayer. The Microbiota-Gut-Brain Axis: From Motility to Mood. Gastroenterology, Volume 160, Issue 5, 2021, Pages 1486-1501, ISSN 0016-5085, https://doi.org/10.1053/j.gastro.2020.10.066.
 
Swierzbinski ME, Lazarchik AR, Herberholz J. Prior social experience affects the behavioral and neural responses to acute alcohol in juvenile crayfish. J Exp Biol. 2017 Apr 15;220(Pt 8):1516-1523. doi: 10.1242/jeb.154419. PMID: 28424315.