How do you study the neuroscience of a creature whose brain bears little resemblance to that of humans? One approach, detailed yesterday (September 20) in Current Biology, is to see how the animal reacts to MDMA—also known as the street drug ecstasy. Johns Hopkins University researchers Gül Dölen and Eric Edsinger report that the drug induced social behavior in California two-spot octopuses (Octopus bimaculoides), much as it does in humans, suggesting similar roles for serotonin signaling in intelligent life throughout the animal kingdom.
“I was absolutely shocked that it had this effect,” Judit Pungor, a neuroscientist at the University of Oregon who was not involved in the research, tells NPR.
Octopuses are more closely related to snails than to humans, and their brains are organized very differently—they lack a cerebral cortex, for example. “It’s a little bit like studying alien intelligence,” Dölen tells National Geographic. “It can potentially tell us a lot about the ‘rules’ for building a nervous system that supports complex cognitive behaviors, without getting bogged down in the incidental organization of brains.”
The genome for the California two-spot octopus was sequenced a few years ago, and in this study, Dölen and Edsinger found that it contains a gene similar to SLC6A4, which in humans encodes a serotonin transporter that binds to MDMA. That docking stimulates the release of serotonin, a neurotransmitter linked, among other things, to mood and social behavior.
To explore how the octopus version of the receptor functions, the researchers borrowed seven of the animals from Woods Hole Marine Biological Laboratory and devised a behavioral test for them. After dunking them in either a saltwater bath or a solution of MDMA, they put the octopuses in an enclosure with a toy at one end and another, caged octopus at the other.
Octopuses are not social animals, and before they received the drug, “[t]hey mashed themselves against one wall, very slowly extended one arm, touched the [other animal], and went back to the other side,” Dölen tells The Atlantic. But after the MDMA bath they spent more time near the other animal, and, she tells National Geographic, "they tended to hug the cage and put their mouth parts on the cage. . . . This is very similar to how humans react to MDMA; they touch each other frequently."
One takeaway, Dölen tells NPR, is that "serotonin has been encoding social functions for a very, very long time. At least 500 million years ago, it started doing this function."
It’s possible the behavior could be explained by something other than social impulses, the University of Lethbridge’s Jennifer Mather, who was not involved in the study, tells The Atlantic. She suggests the drug could have affected the animals’ perception of chemical cues. “There’s no proof that it is anything more than attraction.”
“Octopuses really are the best example we have on Earth of a second intelligence,” San Francisco State University neuroscientist Robyn Crook tells the magazine. The similarities they show with humans in the new study indicate, she says, that “there are only so many ways to make an intelligent brain.”
Correction (September 21): The original version of this article described the protein coded for by SLC6A4 as a receptor; it is in fact a transporter. The Scientist regrets the error.