A Conserved Role for Serotonergic Neurotransmission in Mediating Social Behavior in Octopus

Human and octopus lineages are separated by over 500 million years of evolution [1
, 2
] and show divergent anatomical patterns of brain organization [3
, 4
]. Despite these differences, growing evidence suggests that ancient neurotransmitter systems are shared across vertebrate and invertebrate species and in many cases enable overlapping functions [5
]. Sociality is widespread across the animal kingdom, with numerous examples in both invertebrate (e.g., bees, ants, termites, and shrimps) and vertebrate (e.g., fishes, birds, rodents, and primates) lineages [6
]. Serotonin is an evolutionarily ancient molecule [7
] that has been implicated in regulating both invertebrate [8
] and vertebrate [9
] social behaviors, raising the possibility that this neurotransmitter’s prosocial functions may be conserved across evolution. Members of the order Octopoda are predominantly asocial and solitary [10
]. Although at this time it is unknown whether serotonergic signaling systems are functionally conserved in octopuses, ethological studies indicate that agonistic behaviors are suspended during mating [11
, 12
, 13
], suggesting that neural mechanisms subserving social behaviors exist in octopuses but are suppressed outside the reproductive period. Here we provide evidence that, as in humans, the phenethylamine (+/−)-3,4-methylendioxymethamphetamine (MDMA) enhances acute prosocial behaviors in Octopus bimaculoides. This finding is paralleled by the evolutionary conservation of the serotonin transporter (SERT, encoded by the Slc6A4 gene) binding site of MDMA in the O. bimaculoides genome. Taken together, these data provide evidence that the neural mechanisms subserving social behaviors exist in O. bimaculoides and indicate that the role of serotonergic neurotransmission in regulating social behaviors is evolutionarily conserved.

DMT Models the Near-Death Experience

Near-death experiences (NDEs) are complex subjective experiences, which have been previously associated with the psychedelic experience and more specifically with the experience induced by the potent serotonergic, N,N-Dimethyltryptamine (DMT). Potential similarities between both subjective states have been noted previously, including the subjective feeling of transcending one’s body and entering an alternative realm, perceiving and communicating with sentient ‘entities’ and themes related to death and dying. In this within-subjects placebo-controled study we aimed to test the similarities between the DMT state and NDEs, by administering DMT and placebo to 13 healthy participants, who then completed a validated and widely used measure of NDEs. Results revealed significant increases in phenomenological features associated with the NDE, following DMT administration compared to placebo. Also, we found significant relationships between the NDE scores and DMT-induced ego-dissolution and mystical-type experiences, as well as a significant association between NDE scores and baseline trait ‘absorption’ and delusional ideation measured at baseline. Furthermore, we found a significant overlap in nearly all of the NDE phenomenological features when comparing DMT-induced NDEs with a matched group of ‘actual’ NDE experiencers. These results reveal a striking similarity between these states that warrants further investigation.