Do planarians need their brain to react to abused drugs? Well, it depends on the drug…

This is my first post specifically on one of the papers from my laboratory. As an introduction, I want to remind you that I use planarians in pharmacology research.

The title of the paper essentially answers the question that is serving as the title of this post: “Planarians require an intact brain to behaviorally react to cocaine, but not to react to nicotine.” It appeared in the journal “Neuroscience”, published by the International Brain Research Organization (IBRO). I will put the complete reference at the end.

Anyway, probably the main spirit of the paper is summarized by the following sentence, taken from the introduction:

“Given the power of modern molecular techniques, it is easy to forget that the ultimate objective of physiological discoveries is to find their possible significance within the context of the whole organism, particularly its behavior.”

I truly think so because frequently many scientists (and I have been caught in that myself; I am as much a sinner as the next scientist) fail to see the metaphorical forest because of the fascination with a particular tree. In other words, sometimes we get lost in the details and forget that an organism is an unified entity.

Anyway, several years ago I had the opportunity to meet Dr. Kiyokazu Agata, one of the most preeminent researchers in the area of animal regeneration (and a really, and I mean REALLY nice guy). He mentioned that he used to determine whether a particular substance was psychoactive in planarians by testing it in decapitated worms, and we decided to use this method to test the type of substances that we work with in my lab. We actually acknowledged him in the paper for his suggestion… (:-)…

The type of behavior that I decided to use in our paper was the induction of seizure-like behavior. In a previous post I included a link to a short video showing this behavior. This behavior is easily quantifiable and many types of substances induce it, so, despite being pretty non-specific is very useful to screen for potentially neuroactive drugs. We use other types of behavior to study drugs; I will talk about them in a future post.
The two drugs that we used were nicotine and cocaine. Before going any further, let me give you an idea of the amounts that we use on the worms. Also, let me tell you that we do not store any drugs, legal or otherwise in my laboratory; as you may imagine, some of those are highly regulated. In our experiments we use a maximal amount of about 35 micrograms (ug) per worm. Let me give you some perspective on how tiny this amount is. Below there is a picture of a ball point pen and its cap, besides a penny. A penny weighs about 3 grams and you will need three pen caps to match that weight. Therefore a ballpoint pen cap weighs about a gram.

Picture credit; Wikimedia commons and Baldscientist.

Bear with me, I like explaining things…

Now, if you take a very sharp scalpel and cut that cap into a thousand pieces, each one of those will be about a milligram (mg), and if you take one of those small pieces and cut it in yet another thousand pieces, you will have a microgram (ug). Now you should have an idea of how small is the amount of the compounds that we use in our experiments. A ug is 0.000001 the weight of a ballpoint pen cap.

Anyway, we tested close to 15 compounds in our initial screening; this is being described in a couple of papers in preparation (note: one of these papers will be published in Sept 2020!). The two compounds that I decided to publish first were nicotine and cocaine. Both compounds are able to induce the seizure-like movements in planarians so we thought that if we tested them in decapitated planarians (I should have posted this on Halloween…) they would not display any reaction. At least that was the idea…

The worms exposed to cocaine did not react, as expected. However, the ones exposed to nicotine, did not care, they twitched the same whether they had a head or not! This made things really interesting and made us think about the role of the brain on the worm’s reaction to cocaine.

Please remember that the type of planarians that we work with are the kind that regenerate after injury; they are able to grow a new head and therefore a new brain. I say this just in case you were worried about the little wormies… No, they did not die…
The first thing that we did was to follow up the recovery of their sensitivity to cocaine as a function of head regeneration over time. Here’s what we got:

Picture credit: Modified from Pagán et al., (2013).

It gets better; our results showed that they began to recover their behavior at day 4, which correlated with the experiments of others showing that the two hemispheres of the planarian brain start regaining their connectivity at roughly the 4^th day post-decapitation. This is one of the exciting things about science, when results done a continent away are consistent with your own…

Anyway, we then went nuts with the scalpel. We tested the planarian behavioral reaction to cocaine upon the following conditions:

Picture credit: Baldscientist.

Basically, they completely lost their reaction to cocaine upon decapitation, the behavior significantly decreased upon any cut near the head and they did not lose the behavior upon losing their tail tip…

So what does these results mean? At first glance they seem to indicate that the organization of the nervous system (including the neuromuscular system) in planarians is similar to the arrangement found in vertebrates, where many receptors for nicotine are located outside of the brain and nervous system; in fact, by sheer amount there are more nicotine receptors in vertebrate muscle cells that in the brain itself. These receptors control muscle contraction. In contrast, there do no seem to be any cocaine receptors capable of inducing significant changes in muscle physiology. This is intriguing and exciting, because it validates the usefulness of planarians as a pharmacological model…

Now I am planning even more interesting things, like going molecular to try to “dissect” the behavior… stay tuned!

By the way, our paper made the cover of the journal… 😀…

Picture credit: IBRO.

If you want to know more
Fraguas S et al. (2012) Regeneration of neuronal cell types in Schmidtea mediterranea: an immunohistochemical and expression study. Int J Dev Biol 56(1-3):143-53.

Pagán OR et al. (2013) Planarians require an intact brain to behaviorally react to cocaine, but not to react to nicotine. Neuroscience. 246:265-70.

Pagán OR et al. (2009) A cembranoid from tobacco prevents the expression of nicotine-induced withdrawal behavior in planarian worms. Eur J Pharmacol. 615(1-3):118-24.

Rawls SM et al. (2011) Nicotine Behavioral Pharmacology: clues from planarians. Drug Alcohol Depend. 2011 Nov 1;118(2-3):274-9.

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