Melatonin inhibits reproductive activity through changes of serotonergic activity in the brain of freshwater catfish (Mystus cavasius)

Fish perceive environmental cues through sensory organs like the pineal gland and retina, and transduce them as internal stimuli to activate an endocrine network known as the brain-pituitary-gonadal (BPG) axis to initiate and terminate gonadal development. In the present study, we examined the influences of photoperiod and melatonin on serotonergic activity and gonadal development in an Indian the freshwater catfish, known as gulsha (Mystus cavasius). When fish were reared under a neutral photoperiod (12 L:12D), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of 5-HT) and 5-HIAA/5-HT increased during the day and decreased at night. Similar changes were observed when fish acclimated to constant light (LL) conditions, whereas almost no changes occurred when fish were acclimated to constant darkness (DD). When fish were kept in water containing melatonin, a significant reduction of 5-HT, 5-HIAA and 5-HIAA/5-HT occurred in the whole brain within 6 h. Melatonin-containing feed treatment significantly lowered the gonadosomatic index and serotonergic activity of fish. Well-developed vitellogenic oocytes were observed in the ovaries of control fish, but not in melatonin-treated fish. These results indicate that photoperiod alters serotonergic activity in the brain of gulsha fish through the oscillations in melatonin caused by external photic stimuli. In addition, inhibitory effects on ovarian development are controlled by functional interactions of the serotonin-melatonin system.