Disturbances to neurotransmitter levels and their metabolic enzyme activity in a freshwater planarian exposed to cadmium.

•We demonstrated the occurrence of 5-hydroxyindole-3-acetic acid in a planarian.•The substrate- and inhibitor-specificity of planarian MAO activity was observed.•Catechol-O-methyl transferase activity in the planarian was undetectable.•Altered dopamine and serotonin levels were observed in Cd-treated planarians.•Altered MAO-A and -B activities were also observed in Cd-treated planarians.