Structure and alteration of gonadotropin releasing hormone–1 peptidergic neuronal system during breeding season of short mackerel Rastrelliger brachysoma (Bleeker, 1851) from Samut Songkhram province

Abstract

The Short mackerel (Rastrelligerbrachysoma) is one of the most economically important and a mariculture candidate fish in Thailand. Nevertheless, it is plagued by reproductive problems that cause low production of gamete in the captivity. An understanding about the neuroendocrine system which involves with the reproductive activity of the wild and captive R. brachysoma is needed in order to clarify the situation. Based on their histological characteristics, the ovary of R. brachysoma is considered as an asynchronous developmental type. Oogenic cells was found in this type. These cells could be classified into six stages i.e. oogonium, chromatin nucleolar, perinucleolarnucleolar, lipid droplet and cortical alveolar, early vitellogenic, late vitellogenic and post vitellogenic stages. The testicular parenchyma of this fish species was a lobular organ, which is considered as an unrestricted spermatogonial type. Based on the pattern of chromatin condensation and other characterizations, the spermatogenetic stage could also be divided into six stages including primary spermatogonium, secondary spermatogonia, primary spermatocytes, secondary spermatocyte, spermatids and spermatozoa stages. The results revealed that a detailed anatomy and histological structure of the brain consisted of telencephalon, mesencephalon, diencephalon, metencephalon and myelencephalon. In female R. brachysoma brain, the distribution pattern of sbGnRH-immunoreactive neurons was structurally present in the nucleus preopticus-periventricularis in three areas including: nucleus periventricularis, nucleus preopticus and nucleus lateralistuberis. Additionally, the sbGnRH neuronal immunoreactive fibers protruded into the proximal par distalis where these cells positively reacted with GtHsimmunoreactivty (ir) (FSHβ – ir and LHβ – ir). The number of sbGnRH and GtHs in both number and mRNA level from the breeding season was significantly higher (P<0.05) than the fish from the non-breeding season. Only the mRNA level of the sbGnRH from the non-breeding season fish was significantly higher (P<0.05) than the fish from the breeding season. Moreover, the number and mRNA levels in all hormones (sbGnRH, FSHβ and LHβ) from the wild fish were significantly higher (P<0.05) than the cultured broodstock. In R. brachysoma male, the distribution of the sbGnRH-immunoreactive neurons and GtHs immnoreactivity in brain were similar to that of female fish. This study also suggested that the wild fish tended to have better reproductive system than hatchery fishes. This could be related to the endocrinological dysfunction and the reproductive failure in the hatchery condition.