Changes in brain and pituitary GnRH levels during a reproductive cycle in wild female Asian sea bass, Lates calcarifer (Bloch, 1790)

Changes in the peptide levels of sea bream (Sparus aurata) GnRH (sbGnRH) in the olfactory bulb, telencephalon including the preoptic area, hypothalamus, optic tectum-thalamus and pituitary during a reproductive cycle were investigated in addition to plasma levels of testosterone (T), 17β- estradiol (E2) and 17α, 20b-dihydroxy-4-pregnen-3-one (DHP) in wild female Asian sea bass Lates calcarifer in Nha Phu lagoon, Vietnam from March to September, 2010. Ovaries were classified into five stages: II Recovering spent (March -April), III Maturing (May-June), IV Mature (June), V Ripe and ovulating (July- August) and VI Spent (September). The gonadosomatic index increased significantly from the stage II (1.62%±0.12) to stage V (8.48%±0.23) and dropped thereafter in stage VI (2.05%±0.25). In the olfactory bulb and telencephalon, sbGnRH levels in stage IV and stage V were slightly higher than those in the other stages, but no significant differences in sbGnRH levels were found among the ovarian stages. In the hypothalamus and optic tectum-thalamus, although significant differences in sbGnRH level were not found among the ovarian stages, the highest level of sbGnRH was determined in stage III (594.13±152.12 pg tissue^-1 and 501.37±124.35 pg tissue^-1, respectively). In the pituitary, sbGnRH level in stage III (6927.03±218.92 pg tissue^-1) was significantly higher than that in the other stages. In addition, plasma T, E2 and DHP levels increased significantly during ovarian maturation or ovulation with the highest level of T (2.91±0.31 ng ml^-1) and E2 (4.98±1.43 ng ml^-1) recorded in stage IV and DHP (389.6 ± 136.03 pg ml^-1) in stage V. The results showed that sbGnRH in the anterior brain and pituitary is involved in initiation and ovarian maturation in wild Asian sea bass.