Conditioned Place Preference reveals tonic pain in Octopus

Abstract

Tonic pain is an ongoing, negative affective state arising from tissue damage or inflammation (1). Because pain is aversive and its relief is innately rewarding, mammals learn to avoid a context in which pain is experienced, and prefer one where pain relief occurs(2, 3). It is generally accepted that vertebrate animals experience pain(4), however, there is currently no compelling evidence that pain occurs in any invertebrate(5). Here we show that octopuses exhibit tonic pain behavior after subcutaneous injection of dilute acetic acid. In conditioned place preference assays, octopuses avoid contexts in which pain was experienced, prefer a location in which they experienced tonic pain relief, and show no conditioned preference in pains absence. Octopuses are thus the first invertebrate shown to experience pain. One sentence summaryA cognitive test demonstrating the emotional component of pain in mammals reveals the first example of pain in any invertebrate.