Histamine alters environmental place preference in planaria

Although histamine functions as a punisher in vertebrate models, its potential aversive effects in invertebrates has gone largely unexamined. We investigated if planaria would develop conditioned place aversions to histamine. In the absence of any training, planaria readily move away from a light source. However, planaria will develop conditioned place preferences for lighted areas if those areas are paired with many of the same psychoactive drugs that produce conditioned place preferences in vertebrates. We confined planaria in the darkened side of a petri dish containing histamine, and they subsequently spent more time in the light than planaria confined initially to the darkened side of a petri dish containing fresh water only. This occurred whether we tested planaria individually or in groups. Pairing histamine with the light side of the petri dish had inconsistent effects, and histamine did not affect behavior under a motility assay. Although histamine altered the planaria behavior, it did not completely overcome the planaria's innate preference for darkened areas. Results add to a growing body of research showing continuity between planaria and vertebrates in the behavioral effects of psychoactive chemicals.