Cogito ergo sum esurientem

(AUTHOR’S NOTE: ha ha, made you translate)

This would’ve been appropriate if I found it last week during the Thanksgiving holiday, but various December holidays are coming up and there’s a lot of socializing involved, which means there’s probably a lot of eating involved, too. And especially in the Northern Hemisphere, where December is winter (especially north of the Tropic of Cancer), you really know what’s coming:

Gingerbread cookies
Public Domain/Flickr (Author: PIVISO, 2016)
Public Domain (Author: tookapic, 2015)
Public Domain (Author: vivienviv0, 2014)
Public Domain (Author: Gerd Altmann, 2006)

I bring up this week’s Featured Article as the authors, based on some previous work, of course, wanted to look for some kind of biological ‘switch’ that can determine whether an organism finds food valuable enough to eat or not, regardless of hunger. Before we dive into this much further, please note that this ‘switch’ was not found in humans; rather, it was found in a species of Lymnaea. I’m sure everyone has their opinions on animal testing (that’s far beyond the scope of this casual science blog), but if you’ll allow me a bit of snark: is anyone standing up for the snails?

Anyway, the authors were looking for two particular snail reflexes: ingestion, which is taking food into its body so it may be digested, and egestion, which is removing undigested food from the body. This is not the same as excretion, which is simply waste removal. I suppose that this was what made Lymnaea a good candidate for this study (as opposed to caterpillars, regardless of what our inner or literal children might expect), as mullusks ingest and egest (but not excrete) through the mouth because of their unique digestive systems, so they can limit their study into only one set of muscles and associated nerves. Also, these behaviors are mutual in that snails don’t ingest and egest at the same time, so one reflex implies one mental ‘hunger’ state at a time. Before we get on with this week’s post, stop thinking snails and mollusks are “pooping” through the mouth, OK? There are already enough organisms that are capable of that anyway.

These tests were applied into two groups of snails: a ‘fed’ group that had food available, and a ‘food deprived’ group that had not eaten in four days. Several tests were applied, including monitoring reflex kicked off when snail lips touched lettuce, when the esophagus was stimulated, and how they respond to sucrose. To trip up the ‘switch,’ the authors added a dose of sulpiride to see if it affects feeding choice. This makes sense, as sulpiride acts on dopamine receptors in the brain, which means that one feels less satisfied/rewarded for having eaten some time ago, so the little snail would feel that must be feeding time.

Wikimedia Commons/Public Domain (Author: United States Fish and Wildlife Service, 2013)

(AUTHOR’S NOTE: I am very well aware that these are not snails. However, they are eating, and for some reason they remind me of Mario and Luigi, just saying)

So what did Crossley et al. find out about that ‘switch’? As one would expect, the ‘fed’ snails had more egestive responses to food while the hungry ones had more ingestive responses. After all, you’re not out looking for food when you’re not hungry – think about that the next time you do your weekly groceries if you don’t know already. They also found a neural ‘switch’ of sorts that they called ‘pattern reversing neurons’ or PRNs, and these were the neurons they wanted to trip with sulpiride to see if cutting off dopamine would indeed reverse the snails’ behavior, especially those who have been fed. And that’s indeed what happened – the fed snails had ingestive (instead of egestive) responses when exposed to table sugar.

Alright, I know what you’re thinking, it’s fascinating and all, but we already know that about ourselves, we don’t go out eating more when we’re full and satisfied thanks to the balance between leptin and ghrelin, and we’re not us when we’re hungry, so how exactly is this new, Jonny? Well, bear in mind that we’re thinking about our own behaviors as a complicated mess of signals firing off and showing up as various kinds of psychological phenomena that we know, like Pavlovian responses or eating disorders, but there hasn’t been enough research into the finer points of brain biology. We know what receptors are involved, we may even know what region of the brain may be lighting up, thanks to fMRIs, but what specifically in those regions are firing off? If we know, then we have an idea what could happen if that part of the brain is injured, how it could be fixed, and so on.

For now, it’s a good theory that explains why snails do as they do, but how it scales up to more complicated creatures like humans is another thing that requires more testing, but at least it’s a good bit of research to build upon. Sure, willpower and all that, but there are some nerves involved that will shut down you wanting to eat because, really, you’re just full. Then you can put the plate away to go do something else (provided you’re not tuckered out).

Public Domain (Author: Jill Wellington, 2013)
Public Domain (Author: Anton Porsche, 2016)
Public Domain (Author: William Adams, 2007)

What can I say – you’re not you, etc.

Anyway, thoughts? Comments? Let me know in the space below, throw a like if you liked it, and you don’t need to be a WordPress member to do it! If you’re not doing so already, please follow (if you’re into scientific research with snarky commentary anyway) and thanks again for stopping by. Hope I didn’t put you off your appetite because you’re more conscious that you’re eating, or that this study had snails gobbing up stuff and so on…

Featured Article: Crossley M, Staras K, Kemenes G. (2018). A central control circuit for encoding perceived food value. Science Advances 4:eaau9180. DOI: 10.1126/sciadv.aau9180.

Featured Image: Flickr (CC-BY-NC-ND-20, Author: Zenely Martin Rios, 2015)

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