Protective role of melatonin against allethrin-induced oxidative and histopathological alterations in the medicinal leech Hirudo troctina Johnson, 1816 (Annelida, Clitellata, Hirudinida)

Abstract

Pesticides such as allethrin, a synthetic pyrethroid widely used in agriculture and domestic pest control, can enter aquatic environments and exert toxic effects on non-target organisms. This study aimed to evaluate the impact of allethrin on the medicinal leech Hirudo troctina, focusing on histopathological and biochemical alterations in the body wall and botryoidal tissue, and to investigate the potential protective effect of melatonin. Leeches were divided into four groups: a control, two allethrin-exposed groups (0.4 and 0.8 µg/L), and one co-treated with allethrin (0.8 µg/L) and melatonin (50 µg/L) for 21 days. Histological observations revealed dose-dependent tissue alterations in the body wall and botryoidal tissue, including cuticle detachment, vacuolization, cellular disorganization, and muscle fiber fragmentation. Co-treatment with melatonin markedly improved tissue integrity and reduced degenerative changes. Biochemical assays demonstrated a significant increase in lipid peroxidation (MDA) and a decrease in antioxidant enzyme activities (SOD, CAT, GPx) following allethrin exposure, confirming oxidative stress induction. Melatonin supplementation restored antioxidant activities and reduced MDA levels, indicating strong cytoprotective and antioxidative effects. The combined histological and biochemical findings highlight that allethrin induces oxidative and structural damage in H. troctina, while melatonin effectively mitigates these effects. These results underscore the value of H. troctina as a sensitive bioindicator for aquatic pollution and suggest melatonin as a promising agent for counteracting pyrethroid-induced toxicity.