Houston, Texas, USA : When people take MDMA, the drug popularly known as ecstasy, a rush of serotonin, dopamine, and oxytocin produces feelings of emotional closeness and euphoria, making people more interested than they would normally be in connecting and sharing with other people. Now, researchers reporting in Current Biology on September 20 have made the surprising discovery that a species of octopus considered to be primarily solitary and asocial responds to MDMA in a similar manner: by becoming much more interested than usual in engaging with one other.
“Despite anatomical differences between octopus and human brain, we’ve shown that there are molecular similarities in the serotonin transporter gene,” says Gul Dolen of Johns Hopkins University, noting that the gene encodes a transmembrane protein that serves as the primary binding site for MDMA. “These molecular similarities are sufficient to enable MDMA to induce prosocial behaviors in octopuses.”
“The brains of octopuses are more similar to those of snails than humans, but our studies add to evidence that they can exhibit some of the same behaviors that we can. What our studies suggest is that certain brain chemicals, or neurotransmitters, that send signals between neurons required for these social behaviors are evolutionarily conserved.”
Octopuses, says Dölen, are well-known to be clever creatures. They can trick prey to come into their clutches, and Dölen says there is some evidence they also learn by observation and have episodic memory. The gelatinous invertebrates (animals without backbones) are further notorious for escaping from their tank, eating other animals’ food, eluding caretakers and sneaking around.
But most octopuses are asocial animals and avoid others, including other octopuses. But because of some of their behaviors, Dölen still thought there may be a link between the genetics that guide social behavior in them and humans. One place to look was in the genomics that guide neurotransmitters, the signals that neurons pass between each other to communicate.
Dolen and the study’s first author, Eric Edsinger, chose to study Octopus bimaculoides because it’s possible to breed and study their behavior in the lab. It’s also the only octopus to have its genome fully sequenced. That allowed the team to make comparisons between the genes in octopuses and humans.
Specifically, in the gene regions that control how neurons hook neurotransmitters to their membrane, Dölen and Edsinger found that octopuses and humans had nearly identical genomic codes for the transporter that binds the neurotransmitter serotonin to the neuron’s membrane. Serotonin is a well-known regulator of mood and closely linked to certain kinds of depression.
The serotonin-binding transporter is also known to be the place where the drug MDMA binds to brain cells and alters mood. So, the researchers set out to see if and/or how octopuses react to the drug, which also produces so-called pro-social behaviors in humans, mice and other vertebrates.
Octopus and human lineages are separated by more than 500 million years of evolution, and yet their genomic analyses showed that O. bimaculoides has the serotonin transporter gene known to serve as the principle binding site of MDMA. The findings added to evidence that ancient neurotransmitter systems are shared across vertebrate and invertebrate species. The discovery also suggested that the octopuses had the molecular components needed to sense and potentially respond to MDMA.
But was it possible that the relatively asocial octopuses actually might show a behavioral response to MDMA similar to people? To find out, Dolen and Edsinger tested the octopuses’ interest in other octopuses compared to novel objects under normal circumstances. Those studies showed the octopuses actually do have more interest in each other, and particularly in other females, than had often been thought.
Next, they tested the octopuses’ interest in each other again while under the influence of MDMA. And what they saw surprised them. The octopuses not only spent more time with other octopus individuals including other males while on the drug, but they also engaged in extensive ventral surface contact. That unusual physical contact between individuals appeared exploratory, not aggressive, in nature.
The findings show that despite being evolutionarily distant from invertebrate species like octopuses, humans share a common evolutionary heritage that enables serotonin to encode social behaviors, the researchers say. They add that the octopuses may rely on common pathways to behave socially at certain times, such as during mating season.
The researchers are now in the process of sequencing the genomes of two other species of octopus, which are closely related to each other but differ in their behaviors. By comparing the genomes of those species, they hope to gain more insight into the evolution of social behavior.
This research was supported by grants from Kinship Foundation, Hartwell Foundation, Klingenstein-Simons Foundation, and the Vetlesen Foundation.
Current Biology, Edsinger and Dolen: “A Conserved Role for Serotonergic Neurotransmission in Mediating Social Behavior in Octopus”
Zachary Mainen, a neuroscientist at the Champalimaud Centre for the Unknown in Portugal, points out that some research done decades ago showed that giving extra serotonin to lobsters can alter their social behavior.
“Specifically,” Mainen explains, “if you give them more serotonin, they become more dominant. A small lobster given serotonin will become a more aggressive, socially dominant lobster.”
He says MDMA, which affects the serotonin system, clearly effects the octopuses’ social behavior, but it’s not clear to him if it’s really inducing greater love for another creature.
“Is it really affection? How would we know?” Mainen asks. “It’s totally fascinating and super-suggestive, but I am not a hundred percent convinced that this is doing the same thing in octopus and in human.”
Still, he thinks it’s amazing that this drug has somewhat similar behavioral effects.
“I like the fact that they’ve shown that a species so alien to us as an octopus has a molecule, a serotonin transporter, which can be affected by a drug which affects us, and that the drug has a somewhat similar effect on the octopus,” Mainen says.
“It just shows us how much we don’t know and how much there is out there to understand.”
Image : This image shows a California two-spot octopus (O. bimaculoides). By studying the genome of a kind of octopus not known for its friendliness toward its peers, then testing its behavioral reaction to a popular mood-altering drug called MDMA or ‘ecstasy,’ scientists say they have found preliminary evidence of an evolutionary link between the social behaviors of the sea creature and humans, species separated by 500 million years on the evolutionary tree.
Image credit : Thomas Kleindinst