Sublethal effects of L-tryptophan and caffeine on planarian physiology: Insights into serotonergic and purinergic disruption in freshwater ecosystems

L-tryptophan (Trp) and caffeine (Caf) are persistent neuroactive contaminants in aquatic systems. We investigated the sublethal toxicity of Trp and Caf to the freshwater planarian Girardia tigrina by assessing reproduction, regeneration, energy metabolism, and oxidative stress. Trp exposure (10–100 μM) significantly reduced fecundity in a concentration-dependent manner (number of cocoons) and fertility (hatching success) by up to 67 % and 100 %, respectively, and delayed hatching. Caf showed a biphasic effect on fertility (stimulation at 10 μM, inhibition at higher doses) while reducing fecundity by 22–59 %. Head regeneration was differentially impaired: Trp caused transient stimulation followed by inhibition, whereas Caf induced early and persistent suppression (up to 72 % inhibition). Both compounds depleted glycogen reserves, indicating disrupted energy metabolism. The antioxidant responses differed markedly between the two compounds: Trp caused an initial increase in catalase (CAT) activity followed by a suppression of superoxide dismutase (SOD) and CAT, suggesting a collapse of redox defenses. In contrast, Caf prompted a sustained increase in both enzymes, but an imbalanced SOD/CAT ratio at high doses indicated potential dysregulation. These results reveal distinct, temporally dynamic toxicity profiles for Trp and Caf, disrupting key physiological processes at environmentally relevant concentrations. The findings suggest Trp's effects may progress from serotonergic modulation to metabolic oxidative stress, while Caf acts via sustained purinergic signaling disruption. This study underscores the sensitivity of freshwater invertebrates to neuroactive pollutants and validates G. tigrina as a model for elucidating complex toxicological mechanisms.