Effects of Prolonged Exposure to Manganese Chloride on the Brain Serotonin Metabolism and Serotonin-Regulated Behavior in Zebrafish

Manganese ions are toxic for the central nervous system and cause motor impairments. Zebrafish (Danio rerio) are widely used in neuroscience, psychopharmacology, and toxicology. The study was aimed to investigate the effect of prolonged exposure to Mn ions on the serotonin (5-HT) system of the brain and the zebrafish behavior controlled by this system. The studies were performed with males and females of zebrafish of AB strain, which were divided into four groups: control and exposed to 0.1, 0.2, and 0.5 mM MnCl2 for 10 days (the drug was added to the aquarium water). Throughout the exposure, the locomotion of fish was continuously recorded and analyzed using DanioStudio software. On the 11th day of exposure, the behavior of fish was studied in the novel tank diving test, then the levels of 5-HT, 5-hydroxyindoleacetic acid (5‑HIAA), the activity of key enzymes of the synthesis and destruction of 5-HT, tryptophan hydroxylase (TPH), and monoamine oxidase (MAO), respectively, were determined in their brain by HPLC. Prolonged exposure to MnCl2 did not affect body mass, locomotor activity, time in the lower and upper thirds of the home aquarium, as well as locomotor and exploration activities, time in the lower and upper thirds in the novel tank diving test. Moreover, the prolonged exposure to MnCl2 did not affect 5-HT, 5-HIAA levels, and MAO activity in fish brain. However, TPH activity was significantly increased in fish kept at 0.2 and 0.5 mM MnCl2. In an additional experiment, Mn ions were shown to increase the thermal stability of the TPH molecule in vitro. This stabilizing (chaperone) activity of Mn ions was demonstrated for the first time. The discovery of the chaperone activity of Mn ions will help to reveal the fundamental molecular principles and mechanisms of action of pharmacological chaperones.