Proliferation and apoptosis of male germ cells in captive Atlantic bluefin tuna ( Thunnus thynnus L.) treated with gonadotropin-releasing hormone agonist (GnRHa)

Abstract

The effects of administration of gonadotropin-releasing hormone agonist (GnRHa) on proliferation and apoptosis of male germ cells were evaluated on Atlantic bluefin tuna ( Thunnus thynnus L.) reared in captivity. Fish ( n = 19) were treated with a sustained-release delivery system loaded with GnRHa during the natural spawning season of 2004 and 2005 (June–July). Untreated Control fish ( n = 17) and adult wild spawners were used for comparison. Fish were sacrificed 2–8 d after GnRHa implantation and body weight and gonad weight were recorded, and gonads and blood were taken. Germ cell proliferation and apoptosis were evaluated through the immunohistochemical detection of proliferating cell nuclear antigen (PCNA) and the terminal deoxynucleotidyl transferase-mediated d’UTP nick end labelling (TUNEL) method, respectively. Plasma 11 ketotestosterone (11-KT) levels were measured using an ELISA method. Mean gonado-somatic index and seminiferous lobule diameter did not differ between GnRHa-treated and Control fish, and were significantly lower in captive-reared individuals than in wild spawners. Significant increases in 11-KT plasma levels and spermatogonial mitosis, along with a reduction of germ cell apoptosis were demonstrated in GnRHa-treated fish compared to Controls. The results suggest that GnRHa administration was effective in enhancing germ cell proliferation and reducing apoptosis in captive males through the stimulation of luteinizing hormone (LH) release and testicular 11-KT production.