Changes in the levels of serotonin and dopamine in the central nervous system and ovary, and their possible roles in the ovarian development in the giant freshwater prawn, Macrobrachium rosenbergii

Serotonin or 5-hydroxytryptamine (5-HT) and dopamine (DA) are the two key neurotransmitters that control gonadal development in decapod crustaceans. This study investigated changes in the levels of 5-HT and DA in the CNS and ovary during different phases of the ovarian cycle of the freshwater prawn, Macrobrachium rosenbergii. The levels of 5-HT and DA were quantified by using High Performance Liquid Chromatography with electrochemical detection (HPLC-ECD). Moreover, changes of vitellogenin (Vg) concentrations in the hemolymph after treatment with 5-HT and DA (at doses of 2.5 × 10−6 and 2.5 × 10−7 mol per prawn) were also examined. 5-HT exhibited a gradual increase in concentration in the brain and thoracic ganglia from ovarian stage I (0.12 ± 0.01 nmol/mg, 0.22 ± 0.01 nmol/mg, respectively) to reach a maximum (0.66 ± 0.03 nmol/mg, 1.48 ± 0.03 nmol/mg, respectively) at ovarian stage IV. In contrast, DA in the brain and thoracic ganglia showed the highest concentrations at ovarian stage II (0.20 ± 0.01 nmol/mg, 1.27 ± 0.06 nmol/mg, respectively) and then decreased to the lowest concentrations (0.06 ± 0.01 nmol/mg, 0.28 ± 0.04 nmol/mg, respectively) at ovarian stage IV. The ovarian concentration of 5-HT was 0.53 ± 0.11 nmol/mg at ovarian stage I and gradually increased to 1.63 ± 0.16 nmol/mg at ovarian stage IV. In contrast, the concentration of DA was highest at ovarian stage I (29.05 ± 1.31 nmol/mg), and lowest at the ovarian stage IV (11.43 ± 0.74 nmol/mg). Injecting 5-HT into prawns significantly increased Vg concentration in the hemolymph at ovarian stage IV compared to control groups, and injecting DA into prawns had the opposite effect. The inverse relationship between 5-HT and DA levels in neural ganglia and ovaries, and their opposing effects on hemolymph Vg levels suggest that these two transmitters play opposite regulatory roles in controlling ovarian maturation and oocyte development in this species.