In a recent entry, I talked about animal models in biomedical research. I would like to share with you one of my favorite research subjects, a particular type of invertebrates generically called planarians (see the figure below). Planarians are non-parasitic flatworms which are widely distributed in nature; they can be found in the ocean or in fresh water. There are terrestrial species as well.
In evolutionary terms, planarians are the simplest example of organisms which display cephalization (the formation of a head, usually containing sensory organs), including a primitive “brain”. Their brain shares many traits with the brains of more “advanced” organisms including us.
These flatworms possess a very peculiar characteristic, an impressive capacity for regeneration. All organisms possess some degree of regenerative capacities. For example, if you get a small cut (while shaving for example), if you are healthy, with time your skin will heal, leaving a scar or even heal completely if the cut is small enough. On the other hand, when a person has brain damage, due to accident or illness, the healing process is much more difficult or even impossible, as the nervous system cells in vertebrates display very limited regeneration capacities. In contrast, if you cut a planarian in two pieces, first, you will not kill it. Second, each part will develop into a complete worm half the original size in about a week (give or take, depending on the species). Remarkably, the tail segment will regenerate a complete nervous system, including a fully-functional brain!
It gets better; if you cut one of them into 100 or even more than 200 pieces (the “record” seems to be 279 pieces), each tiny piece will eventually form a tiny complete worm, which will grow to its normal size (up to ¾ inches, again, depending on the species) if food is readily available. Based on these facts, it is no surprise that traditionally, flatworms have been used as an animal model in developmental biology and regeneration research. Can you imagine all the benefits that we could get if we learn the secrets of regeneration, particularly of the nervous system? Think about the people with spinal cord injury or brain damage. Research on this subject has the potential to result in treatment strategies for these and maybe other related conditions.
Part of my own research focuses on the study of behavioral pharmacology and neuropharmacology, using freshwater planarians as a model organism. Doing research with planarians has many distinct advantages. Working with them is technically easy and relative inexpensive; we do not need complex incubators or big tanks to keep them. A big plus is that they do not bite or can harm you in any way! Also, these organisms provide an excellent animal model for when experiments using vertebrate animals are ethically, experimentally or financially impractical.
Planarians also use every major neurotransmitter found in mammals, including humans (neurotransmitters are chemicals that control many of the functions of your nervous system and mine). Planarians are therefore becoming increasingly popular in neuropharmacology research, as they exhibit a variety of responses when exposed to psychoactive substances (see my previous post). Interestingly, these worms display many behavioral responses when exposed to the same type of drugs that are abused by humans. For example, if you treat them with nicotine, one of the most addictive and abused drugs, they will slow down (think of a human relaxing when smoking). They can even display seizure-like behaviors when exposed to high nicotine amounts (nicotine can be toxic to humans too). Even more interestingly, if you give them nicotine for a certain period of time and then take it away, they behave as if they were addicted to nicotine! They squirm, twitch, and move faster; behaviors that, in humans, would be interpreted as “nervousness”.
In summary, neuropharmacology research using planarians is exciting and show a lot of promise. It can potentially contribute to fundamental research in unexpected ways. I will expand this discussion in my next entries. Talk to you soon!