When asked to imagine an alien, you might conjure up something with a large head, big eyes, and maybe some tentacles. But this description also fits a creature that lives here on earth, the octopus! Many people are quick to label octopuses as alien and strange, and for good reason. Octopuses look, act and even appear to think so differently from us. Although, by all accounts, you could not get any farther from humanity than this strange sea creature, scientists Eric Edsinger and Gül Dӧlen have recently determined that humans and octopuses actually have much more in common than meets the eye, at least when it comes to our brains.
Unlike the human brain, the octopus brain is made up of clumps of neurons called ganglia. The biggest clump, the one most closely resembling our brain, is nestled happily around the esophagus. But more than half of the octopus’ 500 million neurons extend into each of their eight limbs. This allows each of their limbs to act completely autonomously of each other. Even when completely detached from the body, the arms can perform basic movements on their own. Despite these anatomical differences, human and octopus brains have one major thing in common and that is the presence of the same chemicals that send signals in our brains, known as neurotransmitters, including one called serotonin. In primates like us, serotonin is well known for its role in social behaviors. For a notoriously solitary species like the octopus, it’s unclear what role this neurotransmitter plays.
In order to better understand whether serotonin plays a role in octopus sociality, Edsinger and Dӧlen first compared our serotonin systems to that of the octopus. First, they examined the genes that code for serotonin systems in the California two-spot octopus. They found one particular gene that was almost 100% identical between humans and octopuses. This gene generates the part of a protein that shuttles serotonin back from between two neurons, a place called the synaptic cleft, where it often sits as it waits to send a signal to the rest of the brain. This process, known as “reuptake,” is important as it allows the brain to better control the length and thereby also the strength of a neural signal. Reuptake is a well-established target for many medications that treat psychiatric diseases because by modulating this process we can change the way our brains get certain signals such as happiness or ease in social situations. The part of this gene that the researchers found was the most similar to ours is one that creates a part of the reuptake protein that is affected by a drug called MDMA, otherwise known as ecstasy. When MDMA interacts with this part of the protein, it blocks serotonin from being able to attach to the protein to be taken back up into the neuron before the synaptic cleft, causing more serotonin to be taken in by the next neuron, thereby increasing the length of time serotonin is sending its signal. It’s thought that this blocking of reuptake and increase in the serotonin signal helps make the people who take MDMA more socially disinhibited. Knowing octopuses have the same spot on their reuptake protein led the researchers to wonder whether or not MDMA would cause octopuses to forget their inhibitions and become socially liberated in the same manner as humans.
In order to test this, the authors set up an experiment. They put the octopus into a glass box with three chambers. The middle chamber was empty, one side had a fun object for the octopus to look at and the other had a new octopus that the octopus being tested had never met before. The test octopus was put in the middle to start, and the researchers recorded the time spent in each chamber of the glass box. Since octopuses are solitary, but very curious creatures, under normal circumstances they tend to prefer spending time looking at the object. However, once the octopus was given MDMA, they became much more interested in spending time on the side of the box with the new friend. This data suggested to the scientists that octopuses use serotonin to mediate their social behavior in a manner that is quite similar to us.
In the end, the researchers think this information might be a crucial step in better understanding octopus behaviors. Although they are normally completely introverted, they manage to turn this social hesitation off during their period of mating. The researchers posit that serotonin plays an important role in this change that is vital to the survival of the species. This study also shows how neural systems involved in social behavior have advanced similarly in species that developed differently from us across evolutionary time in almost every other way. This consistency across life indicates the importance and robustness of certain neural pathways. It also proves that these beautiful and unique creatures are not so alien from us after all.
Edsinger E, Dölen G. A conserved role for serotonergic neurotransmission in mediating social behavior in octopus. Current Biology. 2018 Oct 8;28(19):3136-42.
Edited by Arielle Keller